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+The Project Gutenberg EBook of The inventions, researches and writings of
+Nikola Tesla, by Thomas Commerford Martin
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: The inventions, researches and writings of Nikola Tesla
+       With special reference to his work in polyphase currents
+       and high potential lighting
+
+Author: Thomas Commerford Martin
+
+Release Date: March 26, 2012 [EBook #39272]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES ***
+
+
+
+
+Produced by Anna Hall, Albert László and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+
+
+THE INVENTIONS
+
+RESEARCHES AND WRITINGS
+
+OF
+
+NIKOLA TESLA
+
+
+
+TO HIS COUNTRYMEN
+
+  IN EASTERN EUROPE THIS RECORD OF
+  THE WORK ALREADY ACCOMPLISHED BY
+
+  NIKOLA TESLA
+
+  IS RESPECTFULLY DEDICATED
+
+
+
+[Illustration: Nikola Tesla]
+
+
+
+  THE INVENTIONS
+  RESEARCHES AND WRITINGS
+
+  OF
+
+  NIKOLA TESLA
+
+
+  WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE
+  CURRENTS AND HIGH POTENTIAL LIGHTING
+
+
+  BY
+
+  THOMAS COMMERFORD MARTIN
+
+  Editor THE ELECTRICAL ENGINEER; Past-President American Institute
+  Electrical Engineers
+
+
+  1894
+  THE ELECTRICAL ENGINEER
+  NEW YORK
+
+  D. VAN NOSTRAND COMPANY,
+  NEW YORK.
+
+
+
+  Entered according to Act of Congress in the year 1893 by
+  T. C. MARTIN
+  in the office of the Librarian of Congress at Washington
+
+
+  Press of McIlroy & Emmet, 36 Cortlandt St., N. Y.
+
+
+
+
+PREFACE.
+
+
+The electrical problems of the present day lie largely in the economical
+transmission of power and in the radical improvement of the means and
+methods of illumination. To many workers and thinkers in the domain of
+electrical invention, the apparatus and devices that are familiar,
+appear cumbrous and wasteful, and subject to severe limitations. They
+believe that the principles of current generation must be changed, the
+area of current supply be enlarged, and the appliances used by the
+consumer be at once cheapened and simplified. The brilliant successes of
+the past justify them in every expectancy of still more generous
+fruition.
+
+The present volume is a simple record of the pioneer work done in such
+departments up to date, by Mr. Nikola Tesla, in whom the world has
+already recognized one of the foremost of modern electrical
+investigators and inventors. No attempt whatever has been made here to
+emphasize the importance of his researches and discoveries. Great ideas
+and real inventions win their own way, determining their own place by
+intrinsic merit. But with the conviction that Mr. Tesla is blazing a
+path that electrical development must follow for many years to come, the
+compiler has endeavored to bring together all that bears the impress of
+Mr. Tesla's genius, and is worthy of preservation. Aside from its value
+as showing the scope of his inventions, this volume may be of service as
+indicating the range of his thought. There is intellectual profit in
+studying the push and play of a vigorous and original mind.
+
+Although the lively interest of the public in Mr. Tesla's work is
+perhaps of recent growth, this volume covers the results of full ten
+years. It includes his lectures, miscellaneous articles and
+discussions, and makes note of all his inventions thus far known,
+particularly those bearing on polyphase motors and the effects obtained
+with currents of high potential and high frequency. It will be seen that
+Mr. Tesla has ever pressed forward, barely pausing for an instant to
+work out in detail the utilizations that have at once been obvious to
+him of the new principles he has elucidated. Wherever possible his own
+language has been employed.
+
+It may be added that this volume is issued with Mr. Tesla's sanction and
+approval, and that permission has been obtained for the re-publication
+in it of such papers as have been read before various technical
+societies of this country and Europe. Mr. Tesla has kindly favored the
+author by looking over the proof sheets of the sections embodying his
+latest researches. The work has also enjoyed the careful revision of the
+author's friend and editorial associate, Mr. Joseph Wetzler, through
+whose hands all the proofs have passed.
+
+DECEMBER, 1893.
+
+                                                              T. C. M.
+
+
+
+
+CONTENTS.
+
+
+PART I.
+
+POLYPHASE CURRENTS.
+
+  CHAPTER I.
+  BIOGRAPHICAL AND INTRODUCTORY.                                       3
+
+  CHAPTER II.
+  A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.         7
+
+  CHAPTER III.
+  THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
+  CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.     9
+
+  CHAPTER IV.
+  MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.        26
+
+  CHAPTER V.
+  UTILIZING FAMILIAR TYPES OF GENERATORS OF THE CONTINUOUS CURRENT
+  TYPE.                                                               31
+
+  CHAPTER VI.
+  METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.            36
+
+  CHAPTER VII.
+  REGULATOR FOR ROTARY CURRENT MOTORS.                                45
+
+  CHAPTER VIII.
+  SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.                 50
+
+  CHAPTER IX.
+  CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTORS.                56
+
+  CHAPTER X.
+  MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.                      58
+
+  CHAPTER XI.
+  ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING
+  MOTOR.                                                              62
+
+  CHAPTER XII.
+  "MAGNETIC LAG" MOTOR.                                               67
+
+  CHAPTER XIII.
+  METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.      71
+
+  CHAPTER XIV.
+  TYPE OF TESLA SINGLE-PHASE MOTOR.                                   76
+
+  CHAPTER XV.
+  MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.                       79
+
+  CHAPTER XVI.
+  MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.           81
+
+  CHAPTER XVII.
+  MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND
+  FIELD.                                                              83
+
+  CHAPTER XVIII.
+  MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF
+  THE INNER AND OUTER PARTS OF AN IRON CORE.                          88
+
+  CHAPTER XIX.
+  ANOTHER TYPE OF TESLA INDUCTION MOTOR.                              92
+
+  CHAPTER XX.
+  COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.               95
+
+  CHAPTER XXI.
+  MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.                    101
+
+  CHAPTER XXII.
+  MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.                 106
+
+  CHAPTER XXIII.
+  TESLA POLYPHASE TRANSFORMER.                                       109
+
+  CHAPTER XXIV.
+  A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN
+  COILS OF PRIMARY AND SECONDARY.                                    113
+
+
+PART II.
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.
+
+  CHAPTER XXV.
+  INTRODUCTORY.--THE SCOPE OF THE TESLA LECTURES.                    119
+
+  CHAPTER XXVI.
+  THE NEW YORK LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY
+  HIGH FREQUENCY, AND THEIR APPLICATION TO METHODS OF ARTIFICIAL
+  ILLUMINATION, MAY 20, 1891.                                        145
+
+  CHAPTER XXVII.
+  THE LONDON LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH
+  POTENTIAL AND HIGH FREQUENCY, FEBRUARY 3, 1892.                    198
+
+  CHAPTER XXVIII.
+  THE PHILADELPHIA AND ST. LOUIS LECTURE. ON LIGHT AND OTHER HIGH
+  FREQUENCY PHENOMENA, FEBRUARY AND MARCH, 1893.                     294
+
+  CHAPTER XXIX.
+  TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY.           374
+
+  CHAPTER XXX.
+  ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.               392
+
+  CHAPTER XXXI.
+  "MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.                         394
+
+  CHAPTER XXXII.
+  ELECTRIC DISCHARGE IN VACUUM TUBES.                                396
+
+
+PART III.
+
+MISCELLANEOUS INVENTIONS AND WRITINGS.
+
+  CHAPTER XXXIII.
+  METHOD OF OBTAINING DIRECT FROM ALTERNATING CURRENTS.              409
+
+  CHAPTER XXXIV.
+  CONDENSERS WITH PLATES IN OIL.                                     418
+
+  CHAPTER XXXV.
+  ELECTROLYTIC REGISTERING METER.                                    420
+
+  CHAPTER XXXVI.
+  THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.               424
+
+  CHAPTER XXXVII.
+  ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.                         432
+
+  CHAPTER XXXVIII.
+  AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.              438
+
+  CHAPTER XXXIX.
+  IMPROVEMENT IN DYNAMO AND MOTOR CONSTRUCTION.                      448
+
+  CHAPTER XL.
+  TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.                          451
+
+  CHAPTER XLI.
+  IMPROVEMENT IN UNIPOLAR GENERATORS.                                465
+
+
+PART IV.
+
+APPENDIX: EARLY PHASE MOTORS AND THE TESLA OSCILLATORS.
+
+  CHAPTER XLII.
+  MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.                  477
+
+  CHAPTER XLIII.
+  THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.                   486
+
+
+
+
+PART I.
+
+POLYPHASE CURRENTS.
+
+
+
+
+CHAPTER I.
+
+BIOGRAPHICAL AND INTRODUCTORY.
+
+
+As an introduction to the record contained in this volume of Mr. Tesla's
+investigations and discoveries, a few words of a biographical nature
+will, it is deemed, not be out of place, nor other than welcome.
+
+Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland region of
+Austro-Hungary, of the Serbian race, which has maintained against Turkey
+and all comers so unceasing a struggle for freedom. His family is an old
+and representative one among these Switzers of Eastern Europe, and his
+father was an eloquent clergyman in the Greek Church. An uncle is to-day
+Metropolitan in Bosnia. His mother was a woman of inherited ingenuity,
+and delighted not only in skilful work of the ordinary household
+character, but in the construction of such mechanical appliances as
+looms and churns and other machinery required in a rural community.
+Nikola was educated at Gospich in the public school for four years, and
+then spent three years in the Real Schule. He was then sent to Carstatt,
+Croatia, where he continued his studies for three years in the Higher
+Real Schule. There for the first time he saw a steam locomotive. He
+graduated in 1873, and, surviving an attack of cholera, devoted himself
+to experimentation, especially in electricity and magnetism. His father
+would have had him maintain the family tradition by entering the Church,
+but native genius was too strong, and he was allowed to enter the
+Polytechnic School at Gratz, to finish his studies, and with the object
+of becoming a professor of mathematics and physics. One of the machines
+there experimented with was a Gramme dynamo, used as a motor. Despite
+his instructor's perfect demonstration of the fact that it was
+impossible to operate a dynamo without commutator or brushes, Mr. Tesla
+could not be convinced that such accessories were necessary or
+desirable. He had already seen with quick intuition that a way could be
+found to dispense with them; and from that time he may be said to have
+begun work on the ideas that fructified ultimately in his rotating field
+motors.
+
+In the second year of his Gratz course, Mr. Tesla gave up the notion of
+becoming a teacher, and took up the engineering curriculum. His studies
+ended, he returned home in time to see his father die, and then went to
+Prague and Buda-Pesth to study languages, with the object of qualifying
+himself broadly for the practice of the engineering profession. For a
+short time he served as an assistant in the Government Telegraph
+Engineering Department, and then became associated with M. Puskas, a
+personal and family friend, and other exploiters of the telephone in
+Hungary. He made a number of telephonic inventions, but found his
+opportunities of benefiting by them limited in various ways. To gain a
+wider field of action, he pushed on to Paris and there secured
+employment as an electrical engineer with one of the large companies in
+the new industry of electric lighting.
+
+It was during this period, and as early as 1882, that he began serious
+and continued efforts to embody the rotating field principle in
+operative apparatus. He was enthusiastic about it; believed it to mark a
+new departure in the electrical arts, and could think of nothing else.
+In fact, but for the solicitations of a few friends in commercial
+circles who urged him to form a company to exploit the invention, Mr.
+Tesla, then a youth of little worldly experience, would have sought an
+immediate opportunity to publish his ideas, believing them to be worthy
+of note as a novel and radical advance in electrical theory as well as
+destined to have a profound influence on all dynamo electric machinery.
+
+At last he determined that it would be best to try his fortunes in
+America. In France he had met many Americans, and in contact with them
+learned the desirability of turning every new idea in electricity to
+practical use. He learned also of the ready encouragement given in the
+United States to any inventor who could attain some new and valuable
+result. The resolution was formed with characteristic quickness, and
+abandoning all his prospects in Europe, he at once set his face
+westward.
+
+Arrived in the United States, Mr. Tesla took off his coat the day he
+arrived, in the Edison Works. That place had been a goal of his
+ambition, and one can readily imagine the benefit and stimulus derived
+from association with Mr. Edison, for whom Mr. Tesla has always had the
+strongest admiration. It was impossible, however, that, with his own
+ideas to carry out, and his own inventions to develop, Mr. Tesla could
+long remain in even the most delightful employ; and, his work now
+attracting attention, he left the Edison ranks to join a company
+intended to make and sell an arc lighting system based on some of his
+inventions in that branch of the art. With unceasing diligence he
+brought the system to perfection, and saw it placed on the market. But
+the thing which most occupied his time and thoughts, however, all
+through this period, was his old discovery of the rotating field
+principle for alternating current work, and the application of it in
+motors that have now become known the world over.
+
+Strong as his convictions on the subject then were, it is a fact that
+he stood very much alone, for the alternating current had no well
+recognized place. Few electrical engineers had ever used it, and the
+majority were entirely unfamiliar with its value, or even its essential
+features. Even Mr. Tesla himself did not, until after protracted effort
+and experimentation, learn how to construct alternating current
+apparatus of fair efficiency. But that he had accomplished his purpose
+was shown by the tests of Prof. Anthony, made in the of winter 1887-8,
+when Tesla motors in the hands of that distinguished expert gave an
+efficiency equal to that of direct current motors. Nothing now stood in
+the way of the commercial development and introduction of such motors,
+except that they had to be constructed with a view to operating on the
+circuits then existing, which in this country were all of high
+frequency.
+
+The first full publication of his work in this direction--outside his
+patents--was a paper read before the American Institute of Electrical
+Engineers in New York, in May, 1888 (read at the suggestion of Prof.
+Anthony and the present writer), when he exhibited motors that had been
+in operation long previous, and with which his belief that brushes and
+commutators could be dispensed with, was triumphantly proved to be
+correct. The section of this volume devoted to Mr. Tesla's inventions in
+the utilization of polyphase currents will show how thoroughly from the
+outset he had mastered the fundamental idea and applied it in the
+greatest variety of ways.
+
+Having noted for years the many advantages obtainable with alternating
+currents, Mr. Tesla was naturally led on to experiment with them at
+higher potentials and higher frequencies than were common or approved
+of. Ever pressing forward to determine in even the slightest degree the
+outlines of the unknown, he was rewarded very quickly in this field
+with results of the most surprising nature. A slight acquaintance with
+some of these experiments led the compiler of this volume to urge Mr.
+Tesla to repeat them before the American Institute of Electrical
+Engineers. This was done in May, 1891, in a lecture that marked, beyond
+question, a distinct departure in electrical theory and practice, and
+all the results of which have not yet made themselves fully apparent.
+The New York lecture, and its successors, two in number, are also
+included in this volume, with a few supplementary notes.
+
+Mr. Tesla's work ranges far beyond the vast departments of polyphase
+currents and high potential lighting. The "Miscellaneous" section of
+this volume includes a great many other inventions in arc lighting,
+transformers, pyro-magnetic generators, thermo-magnetic motors,
+third-brush regulation, improvements in dynamos, new forms of
+incandescent lamps, electrical meters, condensers, unipolar dynamos, the
+conversion of alternating into direct currents, etc. It is needless to
+say that at this moment Mr. Tesla is engaged on a number of interesting
+ideas and inventions, to be made public in due course. The present
+volume deals simply with his work accomplished to date.
+
+
+
+
+CHAPTER II.
+
+A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.
+
+
+The present section of this volume deals with polyphase currents, and
+the inventions by Mr. Tesla, made known thus far, in which he has
+embodied one feature or another of the broad principle of rotating field
+poles or _resultant attraction_ exerted on the armature. It is needless
+to remind electricians of the great interest aroused by the first
+enunciation of the rotating field principle, or to dwell upon the
+importance of the advance from a single alternating current, to methods
+and apparatus which deal with more than one. Simply prefacing the
+consideration here attempted of the subject, with the remark that in
+nowise is the object of this volume of a polemic or controversial
+nature, it may be pointed out that Mr. Tesla's work has not at all been
+fully understood or realized up to date. To many readers, it is
+believed, the analysis of what he has done in this department will be a
+revelation, while it will at the same time illustrate the beautiful
+flexibility and range of the principles involved. It will be seen that,
+as just suggested, Mr. Tesla did not stop short at a mere rotating
+field, but dealt broadly with the shifting of the resultant attraction
+of the magnets. It will be seen that he went on to evolve the
+"multiphase" system with many ramifications and turns; that he showed
+the broad idea of motors employing currents of differing phase in the
+armature with direct currents in the field; that he first described and
+worked out the idea of an armature with a body of iron and coils closed
+upon themselves; that he worked out both synchronizing and torque
+motors; that he explained and illustrated how machines of ordinary
+construction might be adapted to his system; that he employed condensers
+in field and armature circuits, and went to the bottom of the
+fundamental principles, testing, approving or rejecting, it would
+appear, every detail that inventive ingenuity could hit upon.
+
+Now that opinion is turning so emphatically in favor of lower
+frequencies, it deserves special note that Mr. Tesla early recognized
+the importance of the low frequency feature in motor work. In fact his
+first motors exhibited publicly--and which, as Prof. Anthony showed in
+his tests in the winter of 1887-8, were the equal of direct current
+motors in efficiency, output and starting torque--were of the low
+frequency type. The necessity arising, however, to utilize these motors
+in connection with the existing high frequency circuits, our survey
+reveals in an interesting manner Mr. Tesla's fertility of resource in
+this direction. But that, after exhausting all the possibilities of this
+field, Mr. Tesla returns to low frequencies, and insists on the
+superiority of his polyphase system in alternating current distribution,
+need not at all surprise us, in view of the strength of his convictions,
+so often expressed, on this subject. This is, indeed, significant, and
+may be regarded as indicative of the probable development next to be
+witnessed.
+
+Incidental reference has been made to the efficiency of rotating field
+motors, a matter of much importance, though it is not the intention to
+dwell upon it here. Prof. Anthony in his remarks before the American
+Institute of Electrical Engineers, in May, 1888, on the two small Tesla
+motors then shown, which he had tested, stated that one gave an
+efficiency of about 50 per cent. and the other a little over sixty per
+cent. In 1889, some tests were reported from Pittsburgh, made by Mr.
+Tesla and Mr. Albert Schmid, on motors up to 10 H. P. and weighing about
+850 pounds. These machines showed an efficiency of nearly 90 per cent.
+With some larger motors it was then found practicable to obtain an
+efficiency, with the three wire system, up to as high as 94 and 95 per
+cent. These interesting figures, which, of course, might be supplemented
+by others more elaborate and of later date, are cited to show that the
+efficiency of the system has not had to wait until the present late day
+for any demonstration of its commercial usefulness. An invention is none
+the less beautiful because it may lack utility, but it must be a
+pleasure to any inventor to know that the ideas he is advancing are
+fraught with substantial benefits to the public.
+
+
+
+
+CHAPTER III.
+
+THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
+CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.
+
+
+The best description that can be given of what he attempted, and
+succeeded in doing, with the rotating magnetic field, is to be found in
+Mr. Tesla's brief paper explanatory of his rotary current, polyphase
+system, read before the American Institute of Electrical Engineers, in
+New York, in May, 1888, under the title "A New System of Alternate
+Current Motors and Transformers." As a matter of fact, which a perusal
+of the paper will establish, Mr. Tesla made no attempt in that paper to
+describe all his work. It dealt in reality with the few topics
+enumerated in the caption of this chapter. Mr. Tesla's reticence was no
+doubt due largely to the fact that his action was governed by the wishes
+of others with whom he was associated, but it may be worth mention that
+the compiler of this volume--who had seen the motors running, and who
+was then chairman of the Institute Committee on Papers and Meetings--had
+great difficulty in inducing Mr. Tesla to give the Institute any paper
+at all. Mr. Tesla was overworked and ill, and manifested the greatest
+reluctance to an exhibition of his motors, but his objections were at
+last overcome. The paper was written the night previous to the meeting,
+in pencil, very hastily, and under the pressure just mentioned.
+
+In this paper casual reference was made to two special forms of motors
+not within the group to be considered. These two forms were: 1. A motor
+with one of its circuits in series with a transformer, and the other in
+the secondary of the transformer. 2. A motor having its armature circuit
+connected to the generator, and the field coils closed upon themselves.
+The paper in its essence is as follows, dealing with a few leading
+features of the Tesla system, namely, the rotating magnetic field,
+motors with closed conductors, synchronizing motors, and rotating field
+transformers:--
+
+The subject which I now have the pleasure of bringing to your notice is
+a novel system of electric distribution and transmission of power by
+means of alternate currents, affording peculiar advantages, particularly
+in the way of motors, which I am confident will at once establish the
+superior adaptability of these currents to the transmission of power and
+will show that many results heretofore unattainable can be reached by
+their use; results which are very much desired in the practical
+operation of such systems, and which cannot be accomplished by means of
+continuous currents.
+
+Before going into a detailed description of this system, I think it
+necessary to make a few remarks with reference to certain conditions
+existing in continuous current generators and motors, which, although
+generally known, are frequently disregarded.
+
+In our dynamo machines, it is well known, we generate alternate currents
+which we direct by means of a commutator, a complicated device and, it
+may be justly said, the source of most of the troubles experienced in
+the operation of the machines. Now, the currents so directed cannot be
+utilized in the motor, but they must--again by means of a similar
+unreliable device--be reconverted into their original state of alternate
+currents. The function of the commutator is entirely external, and in no
+way does it affect the internal working of the machines. In reality,
+therefore, all machines are alternate current machines, the currents
+appearing as continuous only in the external circuit during their
+transit from generator to motor. In view simply of this fact, alternate
+currents would commend themselves as a more direct application of
+electrical energy, and the employment of continuous currents would only
+be justified if we had dynamos which would primarily generate, and
+motors which would be directly actuated by, such currents.
+
+But the operation of the commutator on a motor is twofold; first, it
+reverses the currents through the motor, and secondly, it effects
+automatically, a progressive shifting of the poles of one of its
+magnetic constituents. Assuming, therefore, that both of the useless
+operations in the systems, that is to say, the directing of the
+alternate currents on the generator and reversing the direct currents on
+the motor, be eliminated, it would still be necessary, in order to cause
+a rotation of the motor, to produce a progressive shifting of the poles
+of one of its elements, and the question presented itself--How to
+perform this operation by the direct action of alternate currents? I
+will now proceed to show how this result was accomplished.
+
+[Illustration: FIG. 1.]
+
+[Illustration: FIG. 1a.]
+
+[Illustration: FIG. 2.]
+
+[Illustration: FIG. 2a.]
+
+In the first experiment a drum-armature was provided with two coils at
+right angles to each other, and the ends of these coils were connected
+to two pairs of insulated contact-rings as usual. A ring was then made
+of thin insulated plates of sheet-iron and wound with four coils, each
+two opposite coils being connected together so as to produce free poles
+on diametrically opposite sides of the ring. The remaining free ends of
+the coils were then connected to the contact-rings of the generator
+armature so as to form two independent circuits, as indicated in Fig. 9.
+It may now be seen what results were secured in this combination, and
+with this view I would refer to the diagrams, Figs. 1 to 8_a_. The field
+of the generator being independently excited, the rotation of the
+armature sets up currents in the coils C C_{1}, varying in strength and
+direction in the well-known manner. In the position shown in Fig. 1, the
+current in coil C is nil, while coil C_{1} is traversed by its maximum
+current, and the connections may be such that the ring is magnetized by
+the coils c_{1} c_{1}, as indicated by the letters N S in Fig. 1_a_,
+the magnetizing effect of the coils c c being nil, since these coils
+are included in the circuit of coil C.
+
+[Illustration: FIG. 3.]
+
+[Illustration: FIG. 3a.]
+
+In Fig. 2, the armature coils are shown in a more advanced position,
+one-eighth of one revolution being completed. Fig. 2_a_ illustrates the
+corresponding magnetic condition of the ring. At this moment the coil
+C_{1} generates a current of the same direction as previously, but
+weaker, producing the poles n_{1} s_{1} upon the ring; the coil C also
+generates a current of the same direction, and the connections may be
+such that the coils c c produce the poles n s, as shown in Fig. 2_a_.
+The resulting polarity is indicated by the letters N S, and it will be
+observed that the poles of the ring have been shifted one-eighth of the
+periphery of the same.
+
+[Illustration: FIG. 4.]
+
+[Illustration: FIG. 4a.]
+
+In Fig. 3 the armature has completed one quarter of one revolution. In
+this phase the current in coil C is a maximum, and of such direction as
+to produce the poles N S in Fig. 3_a_, whereas the current in coil C_{1}
+is nil, this coil being at its neutral position. The poles N S in Fig.
+3_a_ are thus shifted one quarter of the circumference of the ring.
+
+Fig. 4 shows the coils C C in a still more advanced position, the
+armature having completed three-eighths of one revolution. At that
+moment the coil C still generates a current of the same direction as
+before, but of less strength, producing the comparatively weaker poles
+n s in Fig. 4_a_. The current in the coil C_{1} is of the same strength,
+but opposite direction. Its effect is, therefore, to produce upon the
+ring the poles n_{1} s_{1}, as indicated, and a polarity, N S, results,
+the poles now being shifted three-eighths of the periphery of the ring.
+
+[Illustration: FIG. 5.]
+
+[Illustration: FIG. 5a.]
+
+In Fig. 5 one half of one revolution of the armature is completed, and
+the resulting magnetic condition of the ring is indicated in Fig. 5_a_.
+Now the current in coil C is nil, while the coil C_{1} yields its
+maximum current, which is of the same direction as previously; the
+magnetizing effect is, therefore, due to the coils, c_{1} c_{1} alone,
+and, referring to Fig. 5_a_, it will be observed that the poles N S are
+shifted one half of the circumference of the ring. During the next half
+revolution the operations are repeated, as represented in the Figs. 6 to
+8_a_.
+
+[Illustration: FIG. 6.]
+
+[Illustration: FIG. 6a.]
+
+A reference to the diagrams will make it clear that during one
+revolution of the armature the poles of the ring are shifted once around
+its periphery, and, each revolution producing like effects, a rapid
+whirling of the poles in harmony with the rotation of the armature is
+the result. If the connections of either one of the circuits in the ring
+are reversed, the shifting of the poles is made to progress in the
+opposite direction, but the operation is identically the same. Instead
+of using four wires, with like result, three wires may be used, one
+forming a common return for both circuits.
+
+[Illustration: FIG. 7.]
+
+[Illustration: FIG. 7_a_.]
+
+This rotation or whirling of the poles manifests itself in a series of
+curious phenomena. If a delicately pivoted disc of steel or other
+magnetic metal is approached to the ring it is set in rapid rotation,
+the direction of rotation varying with the position of the disc. For
+instance, noting the direction outside of the ring it will be found that
+inside the ring it turns in an opposite direction, while it is
+unaffected if placed in a position symmetrical to the ring. This is
+easily explained. Each time that a pole approaches, it induces an
+opposite pole in the nearest point on the disc, and an attraction is
+produced upon that point; owing to this, as the pole is shifted further
+away from the disc a tangential pull is exerted upon the same, and the
+action being constantly repeated, a more or less rapid rotation of the
+disc is the result. As the pull is exerted mainly upon that part which
+is nearest to the ring, the rotation outside and inside, or right and
+left, respectively, is in opposite directions, Fig. 9. When placed
+symmetrically to the ring, the pull on the opposite sides of the disc
+being equal, no rotation results. The action is based on the magnetic
+inertia of iron; for this reason a disc of hard steel is much more
+affected than a disc of soft iron, the latter being capable of very
+rapid variations of magnetism. Such a disc has proved to be a very
+useful instrument in all these investigations, as it has enabled me to
+detect any irregularity in the action. A curious effect is also produced
+upon iron filings. By placing some upon a paper and holding them
+externally quite close to the ring, they are set in a vibrating motion,
+remaining in the same place, although the paper may be moved back and
+forth; but in lifting the paper to a certain height which seems to be
+dependent on the intensity of the poles and the speed of rotation, they
+are thrown away in a direction always opposite to the supposed movement
+of the poles. If a paper with filings is put flat upon the ring and the
+current turned on suddenly, the existence of a magnetic whirl may easily
+be observed.
+
+To demonstrate the complete analogy between the ring and a revolving
+magnet, a strongly energized electro-magnet was rotated by mechanical
+power, and phenomena identical in every particular to those mentioned
+above were observed.
+
+Obviously, the rotation of the poles produces corresponding inductive
+effects and may be utilized to generate currents in a closed conductor
+placed within the influence of the poles. For this purpose it is
+convenient to wind a ring with two sets of superimposed coils forming
+respectively the primary and secondary circuits, as shown in Fig. 10. In
+order to secure the most economical results the magnetic circuit should
+be completely closed, and with this object in view the construction may
+be modified at will.
+
+[Illustration: FIG. 8.]
+
+[Illustration: FIG. 8_a_.]
+
+The inductive effect exerted upon the secondary coils will be mainly due
+to the shifting or movement of the magnetic action; but there may also
+be currents set up in the circuits in consequence of the variations in
+the intensity of the poles. However, by properly designing the generator
+and determining the magnetizing effect of the primary coils, the latter
+element may be made to disappear. The intensity of the poles being
+maintained constant, the action of the apparatus will be perfect, and
+the same result will be secured as though the shifting were effected by
+means of a commutator with an infinite number of bars. In such case the
+theoretical relation between the energizing effect of each set of
+primary coils and their resultant magnetizing effect may be expressed by
+the equation of a circle having its centre coinciding with that of an
+orthogonal system of axes, and in which the radius represents the
+resultant and the co-ordinates both of its components. These are then
+respectively the sine and cosine of the angle _a_ between the radius and
+one of the axes (_OX_). Referring to Fig. 11, we have r^2 = x^2 + y^2;
+where x = r cos _a_, and y = r sin _a_.
+
+Assuming the magnetizing effect of each set of coils in the transformer
+to be proportional to the current--which may be admitted for weak
+degrees of magnetization--then x = Kc and y = Kc^1, where K is a
+constant and c and c^1 the current in both sets of coils respectively.
+Supposing, further, the field of the generator to be uniform, we have
+for constant speed
+
+  c^1 = K^1 sin _a_ and
+  c = K^1 sin (90° + _a_) = K^1 cos _a_,
+
+where K^1 is a constant. See Fig. 12.
+
+Therefore,
+
+  x = Kc = K K^1 cos _a_;
+  y = Kc^1 = K K^1 sin _a_; and
+  K K^1 = r.
+
+[Illustration: FIG. 9.]
+
+That is, for a uniform field the disposition of the two coils at right
+angles will secure the theoretical result, and the intensity of the
+shifting poles will be constant. But from r^2 = x^2 + y^2 it follows
+that for y = 0, r = x; it follows that the joint magnetizing effect
+of both sets of coils should be equal to the effect of one set when at
+its maximum action. In transformers and in a certain class of motors the
+fluctuation of the poles is not of great importance, but in another
+class of these motors it is desirable to obtain the theoretical result.
+
+In applying this principle to the construction of motors, two typical
+forms of motor have been developed. First, a form having a comparatively
+small rotary effort at the start but maintaining a perfectly uniform
+speed at all loads, which motor has been termed synchronous. Second, a
+form possessing a great rotary effort at the start, the speed being
+dependent on the load.
+
+These motors may be operated in three different ways: 1. By the
+alternate currents of the source only. 2. By a combined action of these
+and of induced currents. 3. By the joint action of alternate and
+continuous currents.
+
+[Illustration: FIG. 10.]
+
+The simplest form of a synchronous motor is obtained by winding a
+laminated ring provided with pole projections with four coils, and
+connecting the same in the manner before indicated. An iron disc having
+a segment cut away on each side may be used as an armature. Such a motor
+is shown in Fig. 9. The disc being arranged to rotate freely within the
+ring in close proximity to the projections, it is evident that as the
+poles are shifted it will, owing to its tendency to place itself in such
+a position as to embrace the greatest number of the lines of force,
+closely follow the movement of the poles, and its motion will be
+synchronous with that of the armature of the generator; that is, in the
+peculiar disposition shown in Fig. 9, in which the armature produces by
+one revolution two current impulses in each of the circuits. It is
+evident that if, by one revolution of the armature, a greater number of
+impulses is produced, the speed of the motor will be correspondingly
+increased. Considering that the attraction exerted upon the disc is
+greatest when the same is in close proximity to the poles, it follows
+that such a motor will maintain exactly the same speed at all loads
+within the limits of its capacity.
+
+To facilitate the starting, the disc may be provided with a coil closed
+upon itself. The advantage secured by such a coil is evident. On the
+start the currents set up in the coil strongly energize the disc and
+increase the attraction exerted upon the same by the ring, and currents
+being generated in the coil as long as the speed of the armature is
+inferior to that of the poles, considerable work may be performed by
+such a motor even if the speed be below normal. The intensity of the
+poles being constant, no currents will be generated in the coil when the
+motor is turning at its normal speed.
+
+Instead of closing the coil upon itself, its ends may be connected to
+two insulated sliding rings, and a continuous current supplied to these
+from a suitable generator. The proper way to start such a motor is to
+close the coil upon itself until the normal speed is reached, or nearly
+so, and then turn on the continuous current. If the disc be very
+strongly energized by a continuous current the motor may not be able to
+start, but if it be weakly energized, or generally so that the
+magnetizing effect of the ring is preponderating, it will start and
+reach the normal speed. Such a motor will maintain absolutely the same
+speed at all loads. It has also been found that if the motive power of
+the generator is not excessive, by checking the motor the speed of the
+generator is diminished in synchronism with that of the motor. It is
+characteristic of this form of motor that it cannot be reversed by
+reversing the continuous current through the coil.
+
+[Illustration: FIG. 11.]
+
+[Illustration: FIG. 12.]
+
+The synchronism of these motors may be demonstrated experimentally in a
+variety of ways. For this purpose it is best to employ a motor
+consisting of a stationary field magnet and an armature arranged to
+rotate within the same, as indicated in Fig. 13. In this case the
+shifting of the poles of the armature produces a rotation of the latter
+in the opposite direction. It results therefrom that when the normal
+speed is reached, the poles of the armature assume fixed positions
+relatively to the field magnet, and the same is magnetized by
+induction, exhibiting a distinct pole on each of the pole-pieces. If a
+piece of soft iron is approached to the field magnet, it will at the
+start be attracted with a rapid vibrating motion produced by the
+reversals of polarity of the magnet, but as the speed of the armature
+increases, the vibrations become less and less frequent and finally
+entirely cease. Then the iron is weakly but permanently attracted,
+showing that synchronism is reached and the field magnet energized by
+induction.
+
+The disc may also be used for the experiment. If held quite close to the
+armature it will turn as long as the speed of rotation of the poles
+exceeds that of the armature; but when the normal speed is reached, or
+very nearly so, it ceases to rotate and is permanently attracted.
+
+[Illustration: FIG. 13.]
+
+A crude but illustrative experiment is made with an incandescent lamp.
+Placing the lamp in circuit with the continuous current generator and in
+series with the magnet coil, rapid fluctuations are observed in the
+light in consequence of the induced currents set up in the coil at the
+start; the speed increasing, the fluctuations occur at longer intervals,
+until they entirely disappear, showing that the motor has attained its
+normal speed. A telephone receiver affords a most sensitive instrument;
+when connected to any circuit in the motor the synchronism may be easily
+detected on the disappearance of the induced currents.
+
+In motors of the synchronous type it is desirable to maintain the
+quantity of the shifting magnetism constant, especially if the magnets
+are not properly subdivided.
+
+To obtain a rotary effort in these motors was the subject of long
+thought. In order to secure this result it was necessary to make such a
+disposition that while the poles of one element of the motor are shifted
+by the alternate currents of the source, the poles produced upon the
+other elements should always be maintained in the proper relation to the
+former, irrespective of the speed of the motor. Such a condition exists
+in a continuous current motor; but in a synchronous motor, such as
+described, this condition is fulfilled only when the speed is normal.
+
+[Illustration: FIG. 14.]
+
+The object has been attained by placing within the ring a properly
+subdivided cylindrical iron core wound with several independent coils
+closed upon themselves. Two coils at right angles as in Fig. 14, are
+sufficient, but a greater number may be advantageously employed. It
+results from this disposition that when the poles of the ring are
+shifted, currents are generated in the closed armature coils. These
+currents are the most intense at or near the points of the greatest
+density of the lines of force, and their effect is to produce poles upon
+the armature at right angles to those of the ring, at least
+theoretically so; and since this action is entirely independent of the
+speed--that is, as far as the location of the poles is concerned--a
+continuous pull is exerted upon the periphery of the armature. In many
+respects these motors are similar to the continuous current motors. If
+load is put on, the speed, and also the resistance of the motor, is
+diminished and more current is made to pass through the energizing
+coils, thus increasing the effort. Upon the load being taken off, the
+counter-electromotive force increases and less current passes through
+the primary or energizing coils. Without any load the speed is very
+nearly equal to that of the shifting poles of the field magnet.
+
+[Illustration: FIG. 15.]
+
+[Illustration: FIG. 16.]
+
+[Illustration: FIG. 17.]
+
+It will be found that the rotary effort in these motors fully equals
+that of the continuous current motors. The effort seems to be greatest
+when both armature and field magnet are without any projections; but as
+in such dispositions the field cannot be concentrated, probably the best
+results will be obtained by leaving pole projections on one of the
+elements only. Generally, it may be stated the projections diminish the
+torque and produce a tendency to synchronism.
+
+A characteristic feature of motors of this kind is their property of
+being very rapidly reversed. This follows from the peculiar action of
+the motor. Suppose the armature to be rotating and the direction of
+rotation of the poles to be reversed. The apparatus then represents a
+dynamo machine, the power to drive this machine being the momentum
+stored up in the armature and its speed being the sum of the speeds of
+the armature and the poles.
+
+[Illustration: FIG. 18.]
+
+[Illustration: FIG. 19.]
+
+[Illustration: FIG. 20.]
+
+[Illustration: FIG. 21.]
+
+If we now consider that the power to drive such a dynamo would be very
+nearly proportional to the third power of the speed, for that reason
+alone the armature should be quickly reversed. But simultaneously with
+the reversal another element is brought into action, namely, as the
+movement of the poles with respect to the armature is reversed, the
+motor acts like a transformer in which the resistance of the secondary
+circuit would be abnormally diminished by producing in this circuit an
+additional electromotive force. Owing to these causes the reversal is
+instantaneous.
+
+If it is desirable to secure a constant speed, and at the same time a
+certain effort at the start, this result may be easily attained in a
+variety of ways. For instance, two armatures, one for torque and the
+other for synchronism, may be fastened on the same shaft and any desired
+preponderance may be given to either one, or an armature may be wound
+for rotary effort, but a more or less pronounced tendency to synchronism
+may be given to it by properly constructing the iron core; and in many
+other ways.
+
+As a means of obtaining the required phase of the currents in both the
+circuits, the disposition of the two coils at right angles is the
+simplest, securing the most uniform action; but the phase may be
+obtained in many other ways, varying with the machine employed. Any of
+the dynamos at present in use may be easily adapted for this purpose by
+making connections to proper points of the generating coils. In closed
+circuit armatures, such as used in the continuous current systems, it is
+best to make four derivations from equi-distant points or bars of the
+commutator, and to connect the same to four insulated sliding rings on
+the shaft. In this case each of the motor circuits is connected to two
+diametrically opposite bars of the commutator. In such a disposition the
+motor may also be operated at half the potential and on the three-wire
+plan, by connecting the motor circuits in the proper order to three of
+the contact rings.
+
+In multipolar dynamo machines, such as used in the converter systems,
+the phase is conveniently obtained by winding upon the armature two
+series of coils in such a manner that while the coils of one set or
+series are at their maximum production of current, the coils of the
+other will be at their neutral position, or nearly so, whereby both sets
+of coils may be subjected simultaneously or successively to the inducing
+action of the field magnets.
+
+Generally the circuits in the motor will be similarly disposed, and
+various arrangements may be made to fulfill the requirements; but the
+simplest and most practicable is to arrange primary circuits on
+stationary parts of the motor, thereby obviating, at least in certain
+forms, the employment of sliding contacts. In such a case the magnet
+coils are connected alternately in both the circuits; that is, 1, 3,
+5 ... in one, and 2, 4, 6 ... in the other, and the coils of each set
+of series may be connected all in the same manner, or alternately in
+opposition; in the latter case a motor with half the number of poles
+will result, and its action will be correspondingly modified. The Figs.
+15, 16, and 17, show three different phases, the magnet coils in each
+circuit being connected alternately in opposition. In this case there
+will be always four poles, as in Figs. 15 and 17; four pole projections
+will be neutral; and in Fig. 16 two adjacent pole projections will have
+the same polarity. If the coils are connected in the same manner there
+will be eight alternating poles, as indicated by the letters n' s'
+in Fig. 15.
+
+The employment of multipolar motors secures in this system an advantage
+much desired and unattainable in the continuous current system, and that
+is, that a motor may be made to run exactly at a predetermined speed
+irrespective of imperfections in construction, of the load, and, within
+certain limits, of electromotive force and current strength.
+
+In a general distribution system of this kind the following plan should
+be adopted. At the central station of supply a generator should be
+provided having a considerable number of poles. The motors operated from
+this generator should be of the synchronous type, but possessing
+sufficient rotary effort to insure their starting. With the observance
+of proper rules of construction it may be admitted that the speed of
+each motor will be in some inverse proportion to its size, and the
+number of poles should be chosen accordingly. Still, exceptional demands
+may modify this rule. In view of this, it will be advantageous to
+provide each motor with a greater number of pole projections or coils,
+the number being preferably a multiple of two and three. By this means,
+by simply changing the connections of the coils, the motor may be
+adapted to any probable demands.
+
+If the number of the poles in the motor is even, the action will be
+harmonious and the proper result will be obtained; if this is not the
+case, the best plan to be followed is to make a motor with a double
+number of poles and connect the same in the manner before indicated, so
+that half the number of poles result. Suppose, for instance, that the
+generator has twelve poles, and it would be desired to obtain a speed
+equal to 12/7 of the speed of the generator. This would require a motor
+with seven pole projections or magnets, and such a motor could not be
+properly connected in the circuits unless fourteen armature coils would
+be provided, which would necessitate the employment of sliding
+contacts. To avoid this, the motor should be provided with fourteen
+magnets and seven connected in each circuit, the magnets in each circuit
+alternating among themselves. The armature should have fourteen closed
+coils. The action of the motor will not be quite as perfect as in the
+case of an even number of poles, but the drawback will not be of a
+serious nature.
+
+However, the disadvantages resulting from this unsymmetrical form will
+be reduced in the same proportion as the number of the poles is
+augmented.
+
+If the generator has, say, n, and the motor n_{1} poles, the speed of
+the motor will be equal to that of the generator multiplied by n/n_{1}.
+
+The speed of the motor will generally be dependent on the number of the
+poles, but there may be exceptions to this rule. The speed may be
+modified by the phase of the currents in the circuit or by the character
+of the current impulses or by intervals between each or between groups
+of impulses. Some of the possible cases are indicated in the diagrams,
+Figs. 18, 19, 20 and 21, which are self-explanatory. Fig. 18 represents
+the condition generally existing, and which secures the best result. In
+such a case, if the typical form of motor illustrated in Fig. 9 is
+employed, one complete wave in each circuit will produce one revolution
+of the motor. In Fig. 19 the same result will be effected by one wave in
+each circuit, the impulses being successive; in Fig. 20 by four, and in
+Fig. 21 by eight waves.
+
+By such means any desired speed may be attained, that is, at least
+within the limits of practical demands. This system possesses this
+advantage, besides others, resulting from simplicity. At full loads the
+motors show an efficiency fully equal to that of the continuous current
+motors. The transformers present an additional advantage in their
+capability of operating motors. They are capable of similar
+modifications in construction, and will facilitate the introduction of
+motors and their adaptation to practical demands. Their efficiency
+should be higher than that of the present transformers, and I base my
+assertion on the following:
+
+In a transformer, as constructed at present, we produce the currents in
+the secondary circuit by varying the strength of the primary or exciting
+currents. If we admit proportionality with respect to the iron core the
+inductive effect exerted upon the secondary coil will be proportional
+to the numerical sum of the variations in the strength of the exciting
+current per unit of time; whence it follows that for a given variation
+any prolongation of the primary current will result in a proportional
+loss. In order to obtain rapid variations in the strength of the
+current, essential to efficient induction, a great number of undulations
+are employed; from this practice various disadvantages result. These
+are: Increased cost and diminished efficiency of the generator; more
+waste of energy in heating the cores, and also diminished output of the
+transformer, since the core is not properly utilized, the reversals
+being too rapid. The inductive effect is also very small in certain
+phases, as will be apparent from a graphic representation, and there may
+be periods of inaction, if there are intervals between the succeeding
+current impulses or waves. In producing a shifting of the poles in a
+transformer, and thereby inducing currents, the induction is of the
+ideal character, being always maintained at its maximum action. It is
+also reasonable to assume that by a shifting of the poles less energy
+will be wasted than by reversals.
+
+
+
+
+CHAPTER IV.
+
+MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.
+
+
+In his earlier papers and patents relative to polyphase currents, Mr.
+Tesla devoted himself chiefly to an enunciation of the broad lines and
+ideas lying at the basis of this new work; but he supplemented this
+immediately by a series of other striking inventions which may be
+regarded as modifications and expansions of certain features of the
+Tesla systems. These we shall now proceed to deal with.
+
+In the preceding chapters we have thus shown and described the Tesla
+electrical systems for the transmission of power and the conversion and
+distribution of electrical energy, in which the motors and the
+transformers contain two or more coils or sets of coils, which were
+connected up in independent circuits with corresponding coils of an
+alternating current generator, the operation of the system being brought
+about by the co-operation of the alternating currents in the independent
+circuits in progressively moving or shifting the poles or points of
+maximum magnetic effect of the motors or converters. In these systems
+two independent conductors are employed for each of the independent
+circuits connecting the generator with the devices for converting the
+transmitted currents into mechanical energy or into electric currents of
+another character. This, however, is not always necessary. The two or
+more circuits may have a single return path or wire in common, with a
+loss, if any, which is so extremely slight that it may be disregarded
+entirely. For the sake of illustration, if the generator have two
+independent coils and the motor two coils or two sets of coils in
+corresponding relations to its operative elements one terminal of each
+generator coil is connected to the corresponding terminals of the motor
+coils through two independent conductors, while the opposite terminals
+of the respective coils are both connected to one return wire. The
+following description deals with the modification. Fig. 22 is a
+diagrammatic illustration of a generator and single motor constructed
+and electrically connected in accordance with the invention. Fig. 23 is
+a diagram of the system as it is used in operating motors or converters,
+or both, in parallel, while Fig. 24 illustrates diagrammatically the
+manner of operating two or more motors or converters, or both, in
+series. Referring to Fig. 22, A A designate the poles of the field
+magnets of an alternating-current generator, the armature of which,
+being in this case cylindrical in form and mounted on a shaft, C, is
+wound longitudinally with coils B B'. The shaft C carries three
+insulated contact-rings, _a b c_, to two of which, as _b c_, one
+terminal of each coil, as _e d_, is connected. The remaining terminals,
+_f g_, are both connected to the third ring, _a_.
+
+[Illustration: FIG. 22.]
+
+[Illustration: FIG. 24.]
+
+A motor in this case is shown as composed of a ring, H, wound with four
+coils, I I J J, electrically connected, so as to co-operate in pairs,
+with a tendency to fix the poles of the ring at four points ninety
+degrees apart. Within the magnetic ring H is a disc or cylindrical core
+wound with two coils, G G', which may be connected to form two closed
+circuits. The terminals _j k_ of the two sets or pairs of coils are
+connected, respectively, to the binding-posts E' F', and the other
+terminals, _h i_, are connected to a single binding-post, D'. To operate
+the motor, three line-wires are used to connect the terminals of the
+generator with those of the motor.
+
+[Illustration: FIG. 23.]
+
+So far as the apparent action or mode of operation of this arrangement
+is concerned, the single wire D, which is, so to speak, a common
+return-wire for both circuits, may be regarded as two independent wires.
+In the illustration, with the order of connection shown, coil B' of the
+generator is producing its maximum current and coil B its minimum; hence
+the current which passes through wire e, ring b, brush b', line-wire E,
+terminal E', wire j, coils I I, wire or terminal D', line-wire D, brush
+_a'_, ring _a_, and wire _f_, fixes the polar line of the motor midway
+between the two coils I I; but as the coil B' moves from the position
+indicated it generates less current, while coil B, moving into the
+field, generates more. The current from coil B passes through the
+devices and wires designated by the letters _d_, _c_, C' F, F' _k_, J J,
+_i_, D', D, _a'_, _a_, and _g_, and the position of the poles of the
+motor will be due to the resultant effect of the currents in the two
+sets of coils--that is, it will be advanced in proportion to the advance
+or forward movement of the armature coils. The movement of the
+generator-armature through one-quarter of a revolution will obviously
+bring coil B' into its neutral position and coil B into its position of
+maximum effect, and this shifts the poles ninety degrees, as they are
+fixed solely by coils B. This action is repeated for each quarter of a
+complete revolution.
+
+When more than one motor or other device is employed, they may be run
+either in parallel or series. In Fig. 23 the former arrangement is
+shown. The electrical device is shown as a converter, L, of which the
+two sets of primary coils _p r_ are connected, respectively, to the
+mains F E, which are electrically connected with the two coils of the
+generator. The cross-circuit wires _l m_, making these connections, are
+then connected to the common return-wire D. The secondary coils _p' p''_
+are in circuits _n o_, including, for example, incandescent lamps. Only
+one converter is shown entire in this figure, the others being
+illustrated diagrammatically.
+
+When motors or converters are to be run in series, the two wires E F are
+led from the generator to the coils of the first motor or converter,
+then continued on to the next, and so on through the whole series, and
+are then joined to the single wire D, which completes both circuits
+through the generator. This is shown in Fig. 24, in which J I represent
+the two coils or sets of coils of the motors.
+
+There are, of course, other conditions under which the same idea may be
+carried out. For example, in case the motor and generator each has three
+independent circuits, one terminal of each circuit is connected to a
+line-wire, and the other three terminals to a common return-conductor.
+This arrangement will secure similar results to those attained with a
+generator and motor having but two independent circuits, as above
+described.
+
+When applied to such machines and motors as have three or more induced
+circuits with a common electrical joint, the three or more terminals of
+the generator would be simply connected to those of the motor. Mr.
+Tesla states, however, that the results obtained in this manner show a
+lower efficiency than do the forms dwelt upon more fully above.
+
+
+
+
+CHAPTER V.
+
+UTILIZING FAMILIAR TYPES OF GENERATOR OF THE CONTINUOUS CURRENT TYPE.
+
+
+The preceding descriptions have assumed the use of alternating current
+generators in which, in order to produce the progressive movement of the
+magnetic poles, or of the resultant attraction of independent field
+magnets, the current generating coils are independent or separate. The
+ordinary forms of continuous current dynamos may, however, be employed
+for the same work, in accordance with a method of adaptation devised by
+Mr. Tesla. As will be seen, the modification involves but slight changes
+in their construction, and presents other elements of economy.
+
+On the shaft of a given generator, either in place of or in addition to
+the regular commutator, are secured as many pairs of insulated
+collecting-rings as there are circuits to be operated. Now, it will be
+understood that in the operation of any dynamo electric generator the
+currents in the coils in their movement through the field of force
+undergo different phases--that is to say, at different positions of the
+coils the currents have certain directions and certain strengths--and
+that in the Tesla motors or transformers it is necessary that the
+currents in the energizing coils should undergo a certain order of
+variations in strength and direction. Hence, the further step--viz., the
+connection between the induced or generating coils of the machine and
+the contact-rings from which the currents are to be taken off--will be
+determined solely by what order of variations of strength and direction
+in the currents is desired for producing a given result in the
+electrical translating device. This may be accomplished in various ways;
+but in the drawings we give typical instances only of the best and most
+practicable ways of applying the invention to three of the leading types
+of machines in widespread use, in order to illustrate the principle.
+
+Fig. 25 is a diagram illustrative of the mode of applying the invention
+to the well-known type of "closed" or continuous circuit machines. Fig.
+26 is a similar diagram embodying an armature with separate coils
+connected diametrically, or what is generally called an "open-circuit"
+machine. Fig. 27 is a diagram showing the application of the invention
+to a machine the armature-coils of which have a common joint.
+
+[Illustration: FIG. 25.]
+
+Referring to Fig. 25, let A represent a Tesla motor or transformer
+which, for convenience, we will designate as a "converter." It consists
+of an annular core, B, wound with four independent coils, C and D, those
+diametrically opposite being connected together so as to co-operate in
+pairs in establishing free poles in the ring, the tendency of each pair
+being to fix the poles at ninety degrees from the other. There may be an
+armature, E, within the ring, which is wound with coils closed upon
+themselves. The object is to pass through coils C D currents of such
+relative strength and direction as to produce a progressive shifting or
+movement of the points of maximum magnetic effect around the ring, and
+to thereby maintain a rotary movement of the armature. There are
+therefore secured to the shaft F of the generator, four insulated
+contact-rings, _a b c d_, upon which bear the collecting-brushes
+_a' b' c' d'_, connected by wires G G H H, respectively, with the
+terminals of coils C and D.
+
+Assume, for sake of illustration, that the coils D D are to receive the
+maximum and coils C C at the same instant the minimum current, so that
+the polar line may be midway between the coils D D. The rings _a b_
+would therefore be connected to the continuous armature-coil at its
+neutral points with respect to the field, or the point corresponding
+with that of the ordinary commutator brushes, and between which exists
+the greatest difference of potential; while rings _c d_ would be
+connected to two points in the coil, between which exists no difference
+of potential. The best results will be obtained by making these
+connections at points equidistant from one another, as shown. These
+connections are easiest made by using wires L between the rings and the
+loops or wires J, connecting the coil I to the segments of the
+commutator K. When the converters are made in this manner, it is evident
+that the phases of the currents in the sections of the generator coil
+will be reproduced in the converter coils. For example, after turning
+through an arc of ninety degrees the conductors L L, which before
+conveyed the maximum current, will receive the minimum current by reason
+of the change in the position of their coils, and it is evident that for
+the same reason the current in these coils has gradually fallen from the
+maximum to the minimum in passing through the arc of ninety degrees. In
+this special plan of connections, the rotation of the magnetic poles of
+the converter will be synchronous with that of the armature coils of the
+generator, and the result will be the same, whether the energizing
+circuits are derivations from a continuous armature coil or from
+independent coils, as in Mr. Tesla's other devices.
+
+In Fig. 25, the brushes M M are shown in dotted lines in their proper
+normal position. In practice these brushes may be removed from the
+commutator and the field of the generator excited by an external source
+of current; or the brushes may be allowed to remain on the commutator
+and to take off a converted current to excite the field, or to be used
+for other purposes.
+
+In a certain well-known class of machines known as the "open circuit,"
+the armature contains a number of coils the terminals of which connect
+to commutator segments, the coils being connected across the armature in
+pairs. This type of machine is represented in Fig. 26. In this machine
+each pair of coils goes through the same phases as the coils in some of
+the generators already shown, and it is obviously only necessary to
+utilize them in pairs or sets to operate a Tesla converter by extending
+the segments of the commutators belonging to each pair of coils and
+causing a collecting brush to bear on the continuous portion of each
+segment. In this way two or more circuits may be taken off from the
+generator, each including one or more pairs or sets of coils as may be
+desired.
+
+[Illustration: FIG. 26.]
+
+[Illustration: FIG. 27.]
+
+In Fig. 26 I I represent the armature coils, T T the poles of the field
+magnet, and F the shaft carrying the commutators, which are extended to
+form continuous portions _a b c d_. The brushes bearing on the
+continuous portions for taking off the alternating currents are
+represented by _a' b' c' d'_. The collecting brushes, or those which may
+be used to take off the direct current, are designated by M M. Two pairs
+of the armature coils and their commutators are shown in the figure as
+being utilized; but all may be utilized in a similar manner.
+
+There is another well-known type of machine in which three or more
+coils, A' B' C', on the armature have a common joint, the free ends
+being connected to the segments of a commutator. This form of generator
+is illustrated in Fig. 27. In this case each terminal of the generator
+is connected directly or in derivation to a continuous ring, _a b c_,
+and collecting brushes, _a' b' c'_, bearing thereon, take off the
+alternating currents that operate the motor. It is preferable in this
+case to employ a motor or transformer with three energizing coils, A''
+B'' C'', placed symmetrically with those of the generator, and the
+circuits from the latter are connected to the terminals of such coils
+either directly--as when they are stationary--or by means of brushes
+_e'_ and contact rings _e_. In this, as in the other cases, the ordinary
+commutator may be used on the generator, and the current taken from it
+utilized for exciting the generator field-magnets or for other
+purposes.
+
+
+
+
+CHAPTER VI.
+
+METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.
+
+
+With the object of obtaining the desired speed in motors operated by
+means of alternating currents of differing phase, Mr. Tesla has devised
+various plans intended to meet the practical requirements of the case,
+in adapting his system to types of multipolar alternating current
+machines yielding a large number of current reversals for each
+revolution.
+
+For example, Mr. Tesla has pointed out that to adapt a given type of
+alternating current generator, you may couple rigidly two complete
+machines, securing them together in such a way that the requisite
+difference in phase will be produced; or you may fasten two armatures to
+the same shaft within the influence of the same field and with the
+requisite angular displacement to yield the proper difference in phase
+between the two currents; or two armatures may be attached to the same
+shaft with their coils symmetrically disposed, but subject to the
+influence of two sets of field magnets duly displaced; or the two sets
+of coils may be wound on the same armature alternately or in such manner
+that they will develop currents the phases of which differ in time
+sufficiently to produce the rotation of the motor.
+
+Another method included in the scope of the same idea, whereby a single
+generator may run a number of motors either at its own rate of speed or
+all at different speeds, is to construct the motors with fewer poles
+than the generator, in which case their speed will be greater than that
+of the generator, the rate of speed being higher as the number of their
+poles is relatively less. This may be understood from an example, taking
+a generator that has two independent generating coils which revolve
+between two pole pieces oppositely magnetized; and a motor with
+energizing coils that produce at any given time two magnetic poles in
+one element that tend to set up a rotation of the motor. A generator
+thus constructed yields four reversals, or impulses, in each
+revolution, two in each of its independent circuits; and the effect upon
+the motor is to shift the magnetic poles through three hundred and sixty
+degrees. It is obvious that if the four reversals in the same order
+could be produced by each half-revolution of the generator the motor
+would make two revolutions to the generator's one. This would be readily
+accomplished by adding two intermediate poles to the generator or
+altering it in any of the other equivalent ways above indicated. The
+same rule applies to generators and motors with multiple poles. For
+instance, if a generator be constructed with two circuits, each of which
+produces twelve reversals of current to a revolution, and these currents
+be directed through the independent energizing-coils of a motor, the
+coils of which are so applied as to produce twelve magnetic poles at all
+times, the rotation of the two will be synchronous; but if the
+motor-coils produce but six poles, the movable element will be rotated
+twice while the generator rotates once; or if the motor have four poles,
+its rotation will be three times as fast as that of the generator.
+
+[Illustration: FIG. 28.]
+
+[Illustration: FIG. 29.]
+
+These features, so far as necessary to an understanding of the
+principle, are here illustrated. Fig. 28 is a diagrammatic illustration
+of a generator constructed in accordance with the invention. Fig. 29 is
+a similar view of a correspondingly constructed motor. Fig. 30 is a
+diagram of a generator of modified construction. Fig. 31 is a diagram of
+a motor of corresponding character. Fig. 32 is a diagram of a system
+containing a generator and several motors adapted to run at various
+speeds.
+
+In Fig. 28, let C represent a cylindrical armature core wound
+longitudinally with insulated coils A A, which are connected up in
+series, the terminals of the series being connected to collecting-rings
+_a a_ on the shaft G. By means of this shaft the armature is mounted to
+rotate between the poles of an annular field-magnet D, formed with polar
+projections wound with coils E, that magnetize the said projections. The
+coils E are included in the circuit of a generator F, by means of which
+the field-magnet is energized. If thus constructed, the machine is a
+well-known form of alternating-current generator. To adapt it to his
+system, however, Mr. Tesla winds on armature C a second set of coils B B
+intermediate to the first, or, in other words, in such positions that
+while the coils of one set are in the relative positions to the poles of
+the field-magnet to produce the maximum current, those of the other set
+will be in the position in which they produce the minimum current. The
+coils B are connected, also, in series and to two connecting-rings,
+secured generally to the shaft at the opposite end of the armature.
+
+[Illustration: FIG. 30.]
+
+[Illustration: FIG. 31.]
+
+The motor shown in Fig. 29 has an annular field-magnet H, with four
+pole-pieces wound with coils I. The armature is constructed similarly to
+the generator, but with two sets of two coils in closed circuits to
+correspond with the reduced number of magnetic poles in the field. From
+the foregoing it is evident that one revolution of the armature of the
+generator producing eight current impulses in each circuit will produce
+two revolutions of the motor-armature.
+
+The application of the principle of this invention is not, however,
+confined to any particular form of machine. In Figs. 30 and 31 a
+generator and motor of another well-known type are shown. In Fig. 30, J
+J are magnets disposed in a circle and wound with coils K, which are in
+circuit with a generator which supplies the current that maintains the
+field of force. In the usual construction of these machines the
+armature-conductor L is carried by a suitable frame, so as to be rotated
+in face of the magnets J J, or between these magnets and another similar
+set in front of them. The magnets are energized so as to be of
+alternately opposite polarity throughout the series, so that as the
+conductor C is rotated the current impulses combine or are added to one
+another, those produced by the conductor in any given position being all
+in the same direction. To adapt such a machine to his system, Mr. Tesla
+adds a second set of induced conductors M, in all respects similar to
+the first, but so placed in reference to it that the currents produced
+in each will differ by a quarter-phase. With such relations it is
+evident that as the current decreases in conductor L it increases in
+conductor M, and conversely, and that any of the forms of Tesla motor
+invented for use in this system may be operated by such a generator.
+
+Fig. 31 is intended to show a motor corresponding to the machine in Fig.
+30. The construction of the motor is identical with that of the
+generator, and if coupled thereto it will run synchronously therewith.
+J' J' are the field-magnets, and K' the coils thereon. L' is one of the
+armature-conductors and M' the other.
+
+Fig. 32 shows in diagram other forms of machine. The generator N in this
+case is shown as consisting of a stationary ring O, wound with
+twenty-four coils P P', alternate coils being connected in series in two
+circuits. Within this ring is a disc or drum Q, with projections Q'
+wound with energizing-coils included in circuit with a generator R. By
+driving this disc or cylinder alternating currents are produced in the
+coils P and P', which are carried off to run the several motors.
+
+The motors are composed of a ring or annular field-magnet S, wound with
+two sets of energizing-coils T T', and armatures U, having projections
+U' wound with coils V, all connected in series in a closed circuit or
+each closed independently on itself.
+
+Suppose the twelve generator-coils P are wound alternately in opposite
+directions, so that any two adjacent coils of the same set tend to
+produce a free pole in the ring O between them and the twelve coils P'
+to be similarly wound. A single revolution of the disc or cylinder Q,
+the twelve polar projections of which are of opposite polarity, will
+therefore produce twelve current impulses in each of the circuits W W'.
+Hence the motor X, which has sixteen coils or eight free poles, will
+make one and a half turns to the generator's one. The motor Y, with
+twelve coils or six poles, will rotate with twice the speed of the
+generator, and the motor Z, with eight coils or four poles, will revolve
+three times as fast as the generator. These multipolar motors have a
+peculiarity which may be often utilized to great advantage. For example,
+in the motor X, Fig. 32, the eight poles may be either alternately
+opposite or there may be at any given time alternately two like and two
+opposite poles. This is readily attained by making the proper electrical
+connections. The effect of such a change, however, would be the same as
+reducing the number of poles one-half, and thereby doubling the speed
+of any given motor.
+
+[Illustration: FIG. 32.]
+
+It is obvious that the Tesla electrical transformers which have
+independent primary currents may be used with the generators described.
+It may also be stated with respect to the devices we now describe that
+the most perfect and harmonious action of the generators and motors is
+obtained when the numbers of the poles of each are even and not odd. If
+this is not the case, there will be a certain unevenness of action which
+is the less appreciable as the number of poles is greater; although this
+may be in a measure corrected by special provisions which it is not here
+necessary to explain. It also follows, as a matter of course, that if
+the number of the poles of the motor be greater than that of the
+generator the motor will revolve at a slower speed than the generator.
+
+In this chapter, we may include a method devised by Mr. Tesla for
+avoiding the very high speeds which would be necessary with large
+generators. In lieu of revolving the generator armature at a high rate
+of speed, he secures the desired result by a rotation of the magnetic
+poles of one element of the generator, while driving the other at a
+different speed. The effect is the same as that yielded by a very high
+rate of rotation.
+
+In this instance, the generator which supplies the current for operating
+the motors or transformers consists of a subdivided ring or annular core
+wound with four diametrically-opposite coils, E E', Fig. 33. Within the
+ring is mounted a cylindrical armature-core wound longitudinally with
+two independent coils, F F', the ends of which lead, respectively, to
+two pairs of insulated contact or collecting rings, D D' G G', on the
+armature shaft. Collecting brushes _d d' g g'_ bear upon these rings,
+respectively, and convey the currents through the two independent
+line-circuits M M'. In the main line there may be included one or more
+motors or transformers, or both. If motors be used, they are of the
+usual form of Tesla construction with independent coils or sets of coils
+J J', included, respectively, in the circuits M M'. These
+energizing-coils are wound on a ring or annular field or on pole pieces
+thereon, and produce by the action of the alternating currents passing
+through them a progressive shifting of the magnetism from pole to pole.
+The cylindrical armature H of the motor is wound with two coils at right
+angles, which form independent closed circuits.
+
+If transformers be employed, one set of the primary coils, as N N, wound
+on a ring or annular core is connected to one circuit, as M', and the
+other primary coils, N N', to the circuit M. The secondary coils K K'
+may then be utilized for running groups of incandescent lamps P P'.
+
+[Illustration: FIG. 33.]
+
+With this generator an exciter is employed. This consists of two poles,
+A A, of steel permanently magnetized, or of iron excited by a battery or
+other generator of continuous currents, and a cylindrical armature core
+mounted on a shaft, B, and wound with two longitudinal coils, C C'. One
+end of each of these coils is connected to the collecting-rings _b c_,
+respectively, while the other ends are both connected to a ring, _a_.
+Collecting-brushes _b' c'_ bear on the rings _b c_, respectively, and
+conductors L L convey the currents therefrom through the coils E and E
+of the generator. L' is a common return-wire to brush _a'_. Two
+independent circuits are thus formed, one including coils C of the
+exciter and E E of the generator, the other coils C' of the exciter and
+E' E' of the generator. It results from this that the operation of the
+exciter produces a progressive movement of the magnetic poles of the
+annular field-core of the generator, the shifting or rotary movement of
+the poles being synchronous with the rotation of the exciter armature.
+Considering the operative conditions of a system thus established, it
+will be found that when the exciter is driven so as to energize the
+field of the generator, the armature of the latter, if left free to
+turn, would rotate at a speed practically the same as that of the
+exciter. If under such conditions the coils F F' of the generator
+armature be closed upon themselves or short-circuited, no currents, at
+least theoretically, will be generated in these armature coils. In
+practice the presence of slight currents is observed, the existence of
+which is attributable to more or less pronounced fluctuations in the
+intensity of the magnetic poles of the generator ring. So, if the
+armature-coils F F' be closed through the motor, the latter will not be
+turned as long as the movement of the generator armature is synchronous
+with that of the exciter or of the magnetic poles of its field. If, on
+the contrary, the speed of the generator armature be in any way checked,
+so that the shifting or rotation of the poles of the field becomes
+relatively more rapid, currents will be induced in the armature coils.
+This obviously follows from the passing of the lines of force across the
+armature conductors. The greater the speed of rotation of the magnetic
+poles relatively to that of the armature the more rapidly the currents
+developed in the coils of the latter will follow one another, and the
+more rapidly the motor will revolve in response thereto, and this
+continues until the armature generator is stopped entirely, as by a
+brake, when the motor, if properly constructed, runs at the speed with
+which the magnetic poles of the generator rotate.
+
+The effective strength of the currents developed in the armature coils
+of the generator is dependent upon the strength of the currents
+energizing the generator and upon the number of rotations per unit of
+time of the magnetic poles of the generator; hence the speed of the
+motor armature will depend in all cases upon the relative speeds of the
+armature of the generator and of its magnetic poles. For example, if the
+poles are turned two thousand times per unit of time and the armature is
+turned eight hundred, the motor will turn twelve hundred times, or
+nearly so. Very slight differences of speed may be indicated by a
+delicately balanced motor.
+
+Let it now be assumed that power is applied to the generator armature to
+turn it in a direction opposite to that in which its magnetic poles
+rotate. In such case the result would be similar to that produced by a
+generator the armature and field magnets of which are rotated in
+opposite directions, and by reason of these conditions the motor
+armature will turn at a rate of speed equal to the sum of the speeds of
+the armature and magnetic poles of the generator, so that a
+comparatively low speed of the generator armature will produce a high
+speed in the motor.
+
+It will be observed in connection with this system that on diminishing
+the resistance of the external circuit of the generator armature by
+checking the speed of the motor or by adding translating devices in
+multiple arc in the secondary circuit or circuits of the transformer the
+strength of the current in the armature circuit is greatly increased.
+This is due to two causes: first, to the great differences in the speeds
+of the motor and generator, and, secondly, to the fact that the
+apparatus follows the analogy of a transformer, for, in proportion as
+the resistance of the armature or secondary circuits is reduced, the
+strength of the currents in the field or primary circuits of the
+generator is increased and the currents in the armature are augmented
+correspondingly. For similar reasons the currents in the armature-coils
+of the generator increase very rapidly when the speed of the armature is
+reduced when running in the same direction as the magnetic poles or
+conversely.
+
+It will be understood from the above description that the
+generator-armature may be run in the direction of the shifting of the
+magnetic poles, but more rapidly, and that in such case the speed of the
+motor will be equal to the difference between the two rates.
+
+
+
+
+CHAPTER VII.
+
+REGULATOR FOR ROTARY CURRENT MOTORS.
+
+
+An interesting device for regulating and reversing has been devised by
+Mr. Tesla for the purpose of varying the speed of polyphase motors. It
+consists of a form of converter or transformer with one element capable
+of movement with respect to the other, whereby the inductive relations
+may be altered, either manually or automatically, for the purpose of
+varying the strength of the induced current. Mr. Tesla prefers to
+construct this device in such manner that the induced or secondary
+element may be movable with respect to the other; and the invention, so
+far as relates merely to the construction of the device itself,
+consists, essentially, in the combination, with two opposite magnetic
+poles, of an armature wound with an insulated coil and mounted on a
+shaft, whereby it may be turned to the desired extent within the field
+produced by the poles. The normal position of the core of the secondary
+element is that in which it most completely closes the magnetic circuit
+between the poles of the primary element, and in this position its coil
+is in its most effective position for the inductive action upon it of
+the primary coils; but by turning the movable core to either side, the
+induced currents delivered by its coil become weaker until, by a
+movement of the said core and coil through 90°, there will be no current
+delivered.
+
+Fig. 34 is a view in side elevation of the regulator. Fig. 35 is a
+broken section on line _x x_ of Fig. 34. Fig. 36 is a diagram
+illustrating the most convenient manner of applying the regulator to
+ordinary forms of motors, and Fig. 37 is a similar diagram illustrating
+the application of the device to the Tesla alternating-current motors.
+The regulator may be constructed in many ways to secure the desired
+result; but that which is, perhaps, its best form is shown in Figs. 34
+and 35.
+
+A represents a frame of iron. B B are the cores of the inducing or
+primary coils C C. D is a shaft mounted on the side bars, D', and on
+which is secured a sectional iron core, E, wound with an induced or
+secondary coil, F, the convolutions of which are parallel with the axis
+of the shaft. The ends of the core are rounded off so as to fit closely
+in the space between the two poles and permit the core E to be turned to
+and held at any desired point. A handle, G, secured to the projecting
+end of the shaft D, is provided for this purpose.
+
+[Illustration: FIG. 34.]
+
+[Illustration: FIG. 35.]
+
+In Fig. 36 let H represent an ordinary alternating current generator,
+the field-magnets of which are excited by a suitable source of current,
+I. Let J designate an ordinary form of electromagnetic motor provided
+with an armature, K, commutator L, and field-magnets M. It is well known
+that such a motor, if its field-magnet cores be divided up into
+insulated sections, may be practically operated by an alternating
+current; but in using this regulator with such a motor, Mr. Tesla
+includes one element of the motor only--say the armature-coils--in the
+main circuit of the generator, making the connections through the
+brushes and the commutator in the usual way. He also includes one of the
+elements of the regulator--say the stationary coils--in the same
+circuit, and in the circuit with the secondary or movable coil of the
+regulator he connects up the field-coils of the motor. He also prefers
+to use flexible conductors to make the connections from the secondary
+coil of the regulator, as he thereby avoids the use of sliding contacts
+or rings without interfering with the requisite movement of the core E.
+
+If the regulator be in its normal position, or that in which its
+magnetic circuit is most nearly closed, it delivers its maximum induced
+current, the phases of which so correspond with those of the primary
+current that the motor will run as though both field and armature were
+excited by the main current.
+
+[Illustration: FIG. 36.]
+
+To vary the speed of the motor to any rate between the minimum and
+maximum rates, the core E and coils F are turned in either direction to
+an extent which produces the desired result, for in its normal position
+the convolutions of coil F embrace the maximum number of lines of force,
+all of which act with the same effect upon the coil; hence it will
+deliver its maximum current; but by turning the coil F out of its
+position of maximum effect the number of lines of force embraced by it
+is diminished. The inductive effect is therefore impaired, and the
+current delivered by coil F will continue to diminish in proportion to
+the angle at which the coil F is turned until, after passing through an
+angle of ninety degrees, the convolutions of the coil will be at right
+angles to those of coils C C, and the inductive effect reduced to a
+minimum.
+
+Incidentally to certain constructions, other causes may influence the
+variation in the strength of the induced currents. For example, in the
+present case it will be observed that by the first movement of coil F a
+certain portion of its convolutions are carried beyond the line of the
+direct influence of the lines of force, and that the magnetic path or
+circuit for the lines is impaired; hence the inductive effect would be
+reduced. Next, that after moving through a certain angle, which is
+obviously determined by the relative dimensions of the bobbin or coil F,
+diagonally opposite portions of the coil will be simultaneously included
+in the field, but in such positions that the lines which produce a
+current-impulse in one portion of the coil in a certain direction will
+produce in the diagonally opposite portion a corresponding impulse in
+the opposite direction; hence portions of the current will neutralize
+one another.
+
+As before stated, the mechanical construction of the device may be
+greatly varied; but the essential conditions of the principle will be
+fulfilled in any apparatus in which the movement of the elements with
+respect to one another effects the same results by varying the inductive
+relations of the two elements in a manner similar to that described.
+
+[Illustration: FIG. 37.]
+
+It may also be stated that the core E is not indispensable to the
+operation of the regulator; but its presence is obviously beneficial.
+This regulator, however, has another valuable property in its capability
+of reversing the motor, for if the coil F be turned through a
+half-revolution, the position of its convolutions relatively to the two
+coils C C and to the lines of force is reversed, and consequently the
+phases of the current will be reversed. This will produce a rotation of
+the motor in an opposite direction. This form of regulator is also
+applied with great advantage to Mr. Tesla's system of utilizing
+alternating currents, in which the magnetic poles of the field of a
+motor are progressively shifted by means of the combined effects upon
+the field of magnetizing coils included in independent circuits, through
+which pass alternating currents in proper order and relations to each
+other.
+
+In Fig. 37, let P represent a Tesla generator having two independent
+coils, P' and P'', on the armature, and T a diagram of a motor having
+two independent energizing coils or sets of coils, R R'. One of the
+circuits from the generator, as S' S', includes one set, R' R', of the
+energizing coils of the motor, while the other circuit, as S S, includes
+the primary coils of the regulator. The secondary coil of the regulator
+includes the other coils, R R, of the motor.
+
+While the secondary coil of the regulator is in its normal position, it
+produces its maximum current, and the maximum rotary effect is imparted
+to the motor; but this effect will be diminished in proportion to the
+angle at which the coil F of the regulator is turned. The motor will
+also be reversed by reversing the position of the coil with reference to
+the coils C C, and thereby reversing the phases of the current produced
+by the generator. This changes the direction of the movement of the
+shifting poles which the armature follows.
+
+One of the main advantages of this plan of regulation is its economy of
+power. When the induced coil is generating its maximum current, the
+maximum amount of energy in the primary coils is absorbed; but as the
+induced coil is turned from its normal position the self-induction of
+the primary-coils reduces the expenditure of energy and saves power.
+
+It is obvious that in practice either coils C C or coil F may be used as
+primary or secondary, and it is well understood that their relative
+proportions may be varied to produce any desired difference or
+similarity in the inducing and induced currents.
+
+
+
+
+CHAPTER VIII.
+
+SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.
+
+
+In the first chapters of this section we have, bearing in mind the broad
+underlying principle, considered a distinct class of motors, namely,
+such as require for their operation a special generator capable of
+yielding currents of differing phase. As a matter of course, Mr. Tesla
+recognizing the desirability of utilizing his motors in connection with
+ordinary systems of distribution, addressed himself to the task of
+inventing various methods and ways of achieving this object. In the
+succeeding chapters, therefore, we witness the evolution of a number of
+ideas bearing upon this important branch of work. It must be obvious to
+a careful reader, from a number of hints encountered here and there,
+that even the inventions described in these chapters to follow do not
+represent the full scope of the work done in these lines. They might,
+indeed, be regarded as exemplifications.
+
+We will present these various inventions in the order which to us
+appears the most helpful to an understanding of the subject by the
+majority of readers. It will be naturally perceived that in offering a
+series of ideas of this nature, wherein some of the steps or links are
+missing, the descriptions are not altogether sequential; but any one who
+follows carefully the main drift of the thoughts now brought together
+will find that a satisfactory comprehension of the principles can be
+gained.
+
+As is well known, certain forms of alternating-current machines have the
+property, when connected in circuit with an alternating current
+generator, of running as a motor in synchronism therewith; but, while
+the alternating current will run the motor after it has attained a rate
+of speed synchronous with that of the generator, it will not start it.
+Hence, in all instances heretofore where these "synchronizing motors,"
+as they are termed, have been run, some means have been adopted to bring
+the motors up to synchronism with the generator, or approximately so,
+before the alternating current of the generator is applied to drive
+them. In some instances mechanical appliances have been utilized for
+this purpose. In others special and complicated forms of motor have been
+constructed. Mr. Tesla has discovered a much more simple method or plan
+of operating synchronizing motors, which requires practically no other
+apparatus than the motor itself. In other words, by a certain change in
+the circuit connections of the motor he converts it at will from a
+double circuit motor, or such as have been already described, and which
+will start under the action of an alternating current, into a
+synchronizing motor, or one which will be run by the generator only when
+it has reached a certain speed of rotation synchronous with that of the
+generator. In this manner he is enabled to extend very greatly the
+applications of his system and to secure all the advantages of both
+forms of alternating current motor.
+
+The expression "synchronous with that of the generator," is used here in
+its ordinary acceptation--that is to say, a motor is said to synchronize
+with the generator when it preserves a certain relative speed determined
+by its number of poles and the number of alternations produced per
+revolution of the generator. Its actual speed, therefore, may be faster
+or slower than that of the generator; but it is said to be synchronous
+so long as it preserves the same relative speed.
+
+In carrying out this invention Mr. Tesla constructs a motor which has a
+strong tendency to synchronism with the generator. The construction
+preferred is that in which the armature is provided with polar
+projections. The field-magnets are wound with two sets of coils, the
+terminals of which are connected to a switch mechanism, by means of
+which the line-current may be carried directly through these coils or
+indirectly through paths by which its phases are modified. To start such
+a motor, the switch is turned on to a set of contacts which includes in
+one motor circuit a dead resistance, in the other an inductive
+resistance, and, the two circuits being in derivation, it is obvious
+that the difference in phase of the current in such circuits will set up
+a rotation of the motor. When the speed of the motor has thus been
+brought to the desired rate the switch is shifted to throw the main
+current directly through the motor-circuits, and although the currents
+in both circuits will now be of the same phase the motor will continue
+to revolve, becoming a true synchronous motor. To secure greater
+efficiency, the armature or its polar projections are wound with coils
+closed on themselves.
+
+In the accompanying diagrams, Fig. 38 illustrates the details of the
+plan above set forth, and Figs. 39 and 40 modifications of the same.
+
+[Illustration: FIGS. 38, 39 and 40.]
+
+Referring to Fig. 38, let A designate the field-magnets of a motor, the
+polar projections of which are wound with coils B C included in
+independent circuits, and D the armature with polar projections wound
+with coils E closed upon themselves, the motor in these respects being
+similar in construction to those described already, but having on
+account of the polar projections on the armature core, or other similar
+and well-known features, the properties of a synchronizing-motor. L L'
+represents the conductors of a line from an alternating current
+generator G.
+
+Near the motor is placed a switch the action of which is that of the one
+shown in the diagrams, which is constructed as follows: F F' are two
+conducting plates or arms, pivoted at their ends and connected by an
+insulating cross-bar, H, so as to be shifted in parallelism. In the path
+of the bars F F' is the contact 2, which forms one terminal of the
+circuit through coils C, and the contact 4, which is one terminal of the
+circuit through coils B. The opposite end of the wire of coils C is
+connected to the wire L or bar F', and the corresponding end of coils B
+is connected to wire L' and bar F; hence if the bars be shifted so as to
+bear on contacts 2 and 4 both sets of coils B C will be included in the
+circuit L L' in multiple arc or derivation. In the path of the levers F
+F' are two other contact terminals, 1 and 3. The contact 1 is connected
+to contact 2 through an artificial resistance, I, and contact 3 with
+contact 4 through a self-induction coil, J, so that when the switch
+levers are shifted upon the points 1 and 3 the circuits of coils B and C
+will be connected in multiple arc or derivation to the circuit L L', and
+will include the resistance and self-induction coil respectively. A
+third position of the switch is that in which the levers F and F' are
+shifted out of contact with both sets of points. In this case the motor
+is entirely out of circuit.
+
+The purpose and manner of operating the motor by these devices are as
+follows: The normal position of the switch, the motor being out of
+circuit, is off the contact points. Assuming the generator to be
+running, and that it is desired to start the motor, the switch is
+shifted until its levers rest upon points 1 and 3. The two
+motor-circuits are thus connected with the generator circuit; but by
+reason of the presence of the resistance I in one and the self-induction
+coil J in the other the coincidence of the phases of the current is
+disturbed sufficiently to produce a progression of the poles, which
+starts the motor in rotation. When the speed of the motor has run up to
+synchronism with the generator, or approximately so, the switch is
+shifted over upon the points 2 and 4, thus cutting out the coils I and
+J, so that the currents in both circuits have the same phase; but the
+motor now runs as a synchronous motor.
+
+It will be understood that when brought up to speed the motor will run
+with only one of the circuits B or C connected with the main or
+generator circuit, or the two circuits may be connected in series. This
+latter plan is preferable when a current having a high number of
+alternations per unit of time is employed to drive the motor. In such
+case the starting of the motor is more difficult, and the dead and
+inductive resistances must take up a considerable proportion of the
+electromotive force of the circuits. Generally the conditions are so
+adjusted that the electromotive force used in each of the motor circuits
+is that which is required to operate the motor when its circuits are in
+series. The plan followed in this case is illustrated in Fig. 39. In
+this instance the motor has twelve poles and the armature has polar
+projections D wound with closed coils E. The switch used is of
+substantially the same construction as that shown in the previous
+figure. There are, however, five contacts, designated as 5, 6, 7, 8, and
+9. The motor-circuits B C, which include alternate field-coils, are
+connected to the terminals in the following order: One end of circuit C
+is connected to contact 9 and to contact 5 through a dead resistance, I.
+One terminal of circuit B is connected to contact 7 and to contact 6
+through a self-induction coil, J. The opposite terminals of both
+circuits are connected to contact 8.
+
+One of the levers, as F, of the switch is made with an extension, _f_,
+or otherwise, so as to cover both contacts 5 and 6 when shifted into the
+position to start the motor. It will be observed that when in this
+position and with lever F' on contact 8 the current divides between the
+two circuits B C, which from their difference in electrical character
+produce a progression of the poles that starts the motor in rotation.
+When the motor has attained the proper speed, the switch is shifted so
+that the levers cover the contacts 7 and 9, thereby connecting circuits
+B and C in series. It is found that by this disposition the motor is
+maintained in rotation in synchronism with the generator. This principle
+of operation, which consists in converting by a change of connections or
+otherwise a double-circuit motor, or one operating by a progressive
+shifting of the poles, into an ordinary synchronizing motor may be
+carried out in many other ways. For instance, instead of using the
+switch shown in the previous figures, we may use a temporary ground
+circuit between the generator and motor, in order to start the motor, in
+substantially the manner indicated in Fig. 40. Let G in this figure
+represent an ordinary alternating-current generator with, say, two
+poles, M M', and an armature wound with two coils, N N', at right angles
+and connected in series. The motor has, for example, four poles wound
+with coils B C, which are connected in series, and an armature with
+polar projections D wound with closed coils E E. From the common joint
+or union between the two circuits of both the generator and the motor an
+earth connection is established, while the terminals or ends of these
+circuits are connected to the line. Assuming that the motor is a
+synchronizing motor or one that has the capability of running in
+synchronism with the generator, but not of starting, it may be started
+by the above-described apparatus by closing the ground connection from
+both generator and motor. The system thus becomes one with a two-circuit
+generator and motor, the ground forming a common return for the currents
+in the two circuits L and L'. When by this arrangement of circuits the
+motor is brought to speed, the ground connection is broken between the
+motor or generator, or both, ground-switches P P' being employed for
+this purpose. The motor then runs as a synchronizing motor.
+
+In describing the main features which constitute this invention
+illustrations have necessarily been omitted of the appliances used in
+conjunction with the electrical devices of similar systems--such, for
+instance, as driving-belts, fixed and loose pulleys for the motor, and
+the like; but these are matters well understood.
+
+Mr. Tesla believes he is the first to operate electro-magnetic motors by
+alternating currents in any of the ways herein described--that is to
+say, by producing a progressive movement or rotation of their poles or
+points of greatest magnetic attraction by the alternating currents until
+they have reached a given speed, and then by the same currents producing
+a simple alternation of their poles, or, in other words, by a change in
+the order or character of the circuit connections to convert a motor
+operating on one principle to one operating on another.
+
+
+
+
+CHAPTER IX.
+
+CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTOR.
+
+
+A description is given elsewhere of a method of operating alternating
+current motors by first rotating their magnetic poles until they have
+attained synchronous speed, and then alternating the poles. The motor is
+thus transformed, by a simple change of circuit connections from one
+operated by the action of two or more independent energizing currents to
+one operated either by a single current or by several currents acting as
+one. Another way of doing this will now be described.
+
+At the start the magnetic poles of one element or field of the motor are
+progressively shifted by alternating currents differing in phase and
+passed through independent energizing circuits, and short circuit the
+coils of the other element. When the motor thus started reaches or
+passes the limit of speed synchronous with the generator, Mr. Tesla
+connects up the coils previously short-circuited with a source of direct
+current and by a change of the circuit connections produces a simple
+alternation of the poles. The motor then continues to run in synchronism
+with the generator. The motor here shown in Fig. 41 is one of the
+ordinary forms, with field-cores either laminated or solid and with a
+cylindrical laminated armature wound, for example, with the coils A B at
+right angles. The shaft of the armature carries three collecting or
+contact rings C D E. (Shown, for better illustration, as of different
+diameters.)
+
+One end of coil A connects to one ring, as C, and one end of coil B
+connects with ring D. The remaining ends are connected to ring E.
+Collecting springs or brushes F G H bear upon the rings and lead to the
+contacts of a switch, to be presently described. The field-coils have
+their terminals in binding-posts K K, and may be either closed upon
+themselves or connected with a source of direct current L, by means of a
+switch M. The main or controlling switch has five contacts _a b c d e_
+and two levers _f g_, pivoted and connected by an insulating cross-bar
+_h_, so as to move in parallelism. These levers are connected to the
+line wires from a source of alternating currents N. Contact _a_ is
+connected to brush G and coil B through a dead resistance R and wire P.
+Contact _b_ is connected with brush F and coil A through a
+self-induction coil S and wire O. Contacts _c_ and _e_ are connected to
+brushes G F, respectively, through the wires P O, and contact _d_ is
+directly connected with brush H. The lever _f_ has a widened end, which
+may span the contacts _a b_. When in such position and with lever _g_ on
+contact _d_, the alternating currents divide between the two
+motor-coils, and by reason of their different self-induction a
+difference of current-phase is obtained that starts the motor in
+rotation. In starting, the field-coils are short circuited.
+
+[Illustration: FIG. 41.]
+
+When the motor has attained the desired speed, the switch is shifted to
+the position shown in dotted lines--that is to say, with the levers _f
+g_ resting on points _c e_. This connects up the two armature coils in
+series, and the motor will then run as a synchronous motor. The
+field-coils are thrown into circuit with the direct current source when
+the main switch is shifted.
+
+
+
+
+CHAPTER X.
+
+MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.
+
+
+One of the general ways followed by Mr. Tesla in developing his rotary
+phase motors is to produce practically independent currents differing
+primarily in phase and to pass these through the motor-circuits. Another
+way is to produce a single alternating current, to divide it between the
+motor-circuits, and to effect artificially a lag in one of these
+circuits or branches, as by giving to the circuits different
+self-inductive capacity, and in other ways. In the former case, in which
+the necessary difference of phase is primarily effected in the
+generation of currents, in some instances, the currents are passed
+through the energizing coils of both elements of the motor--the field
+and armature; but a further result or modification may be obtained by
+doing this under the conditions hereinafter specified in the case of
+motors in which the lag, as above stated, is artificially secured.
+
+Figs. 42 to 47, inclusive, are diagrams of different ways in which the
+invention is carried out; and Fig. 48, a side view of a form of motor
+used by Mr. Tesla for this purpose.
+
+[Illustration: FIGS. 42, 43 and 44.]
+
+A B in Fig. 42 indicate the two energizing circuits of a motor, and C D
+two circuits on the armature. Circuit or coil A is connected in series
+with circuit or coil C, and the two circuits B D are similarly
+connected. Between coils A and C is a contact-ring _e_, forming one
+terminal of the latter, and a brush _a_, forming one terminal of the
+former. A ring _d_ and brush _c_ similarly connect coils B and D. The
+opposite terminals of the field-coils connect to one binding post _h_ of
+the motor, and those of the armature coils are similarly connected to
+the opposite binding post _i_ through a contact-ring _f_ and brush _g_.
+Thus each motor-circuit while in derivation to the other includes one
+armature and one field coil. These circuits are of different
+self-induction, and may be made so in various ways. For the sake
+of clearness, an artificial resistance R is shown in one of these
+circuits, and in the other a self-induction coil S. When an alternating
+current is passed through this motor it divides between its two
+energizing-circuits. The higher self-induction of one circuit produces a
+greater retardation or lag in the current therein than in the other. The
+difference of phase between the two currents effects the rotation or
+shifting of the points of maximum magnetic effect that secures the
+rotation of the armature. In certain respects this plan of including
+both armature and field coils in circuit is a marked improvement. Such a
+motor has a good torque at starting; yet it has also considerable
+tendency to synchronism, owing to the fact that when properly
+constructed the maximum magnetic effects in both armature and field
+coincide--a condition which in the usual construction of these motors
+with closed armature coils is not readily attained. The motor thus
+constructed exhibits too, a better regulation of current from no load to
+load, and there is less difference between the apparent and real energy
+expended in running it. The true synchronous speed of this form of motor
+is that of the generator when both are alike--that is to say, if the
+number of the coils on the armature and on the field is _x_, the motor
+will run normally at the same speed as a generator driving it if the
+number of field magnets or poles of the same be also _x_.
+
+[Illustration: FIGS. 45, 46 and 47.]
+
+Fig. 43 shows a somewhat modified arrangement of circuits. There is in
+this case but one armature coil E, the winding of which maintains
+effects corresponding to the resultant poles produced by the two
+field-circuits.
+
+Fig. 44 represents a disposition in which both armature and field are
+wound with two sets of coils, all in multiple arc to the line or main
+circuit. The armature coils are wound to correspond with the field-coils
+with respect to their self-induction. A modification of this plan is
+shown in Fig. 45--that is to say, the two field coils and two armature
+coils are in derivation to themselves and in series with one another.
+The armature coils in this case, as in the previous figure, are wound
+for different self-induction to correspond with the field coils.
+
+Another modification is shown in Fig. 46. In this case only one
+armature-coil, as D, is included in the line-circuit, while the other,
+as C, is short-circuited.
+
+In such a disposition as that shown in Fig. 43, or where only one
+armature-coil is employed, the torque on the start is somewhat reduced,
+while the tendency to synchronism is somewhat increased. In such a
+disposition as shown in Fig. 46, the opposite conditions would exist. In
+both instances, however, there is the advantage of dispensing with one
+contact-ring.
+
+[Illustration: FIG. 48.]
+
+In Fig. 46 the two field-coils and the armature-coil D are in multiple
+arc. In Fig. 47 this disposition is modified, coil D being shown in
+series with the two field-coils.
+
+Fig. 48 is an outline of the general form of motor in which this
+invention is embodied. The circuit connections between the armature and
+field coils are made, as indicated in the previous figures, through
+brushes and rings, which are not shown.
+
+
+
+
+CHAPTER XI.
+
+ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING MOTOR.
+
+
+In a preceding chapter we have described a method by which Mr. Tesla
+accomplishes the change in his type of rotating field motor from a
+torque to a synchronizing motor. As will be observed, the desired end is
+there reached by a change in the circuit connections at the proper
+moment. We will now proceed to describe another way of bringing about
+the same result. The principle involved in this method is as follows:--
+
+If an alternating current be passed through the field coils only of a
+motor having two energizing circuits of different self-induction and the
+armature coils be short-circuited, the motor will have a strong torque,
+but little or no tendency to synchronism with the generator; but if the
+same current which energizes the field be passed also through the
+armature coils the tendency to remain in synchronism is very
+considerably increased. This is due to the fact that the maximum
+magnetic effects produced in the field and armature more nearly
+coincide. On this principle Mr. Tesla constructs a motor having
+independent field circuits of different self-induction, which are joined
+in derivation to a source of alternating currents. The armature is wound
+with one or more coils, which are connected with the field coils through
+contact rings and brushes, and around the armature coils a shunt is
+arranged with means for opening or closing the same. In starting this
+motor the shunt is closed around the armature coils, which will
+therefore be in closed circuit. When the current is directed through the
+motor, it divides between the two circuits, (it is not necessary to
+consider any case where there are more than two circuits used), which,
+by reason of their different self-induction, secure a difference of
+phase between the two currents in the two branches, that produces a
+shifting or rotation of the poles. By the alternations of current, other
+currents are induced in the closed--or short-circuited--armature coils
+and the motor has a strong torque. When the desired speed is reached,
+the shunt around the armature-coils is opened and the current directed
+through both armature and field coils. Under these conditions the motor
+has a strong tendency to synchronism.
+
+[Illustration: FIGS. 49, 50 and 51.]
+
+In Fig. 49, A and B designate the field coils of the motor. As the
+circuits including these coils are of different self-induction, this is
+represented by a resistance coil R in circuit with A, and a
+self-induction coil S in circuit with B. The same result may of course
+be secured by the winding of the coils. C is the armature circuit, the
+terminals of which are rings _a b_. Brushes _c d_ bear on these rings
+and connect with the line and field circuits. D is the shunt or short
+circuit around the armature. E is the switch in the shunt.
+
+It will be observed that in such a disposition as is illustrated in
+Fig. 49, the field circuits A and B being of different self-induction,
+there will always be a greater lag of the current in one than the other,
+and that, generally, the armature phases will not correspond with
+either, but with the resultant of both. It is therefore important to
+observe the proper rule in winding the armature. For instance, if the
+motor have eight poles--four in each circuit--there will be four
+resultant poles, and hence the armature winding should be such as to
+produce four poles, in order to constitute a true synchronizing motor.
+
+[Illustration: FIG. 52.]
+
+The diagram, Fig. 50, differs from the previous one only in respect to
+the order of connections. In the present case the armature-coil, instead
+of being in series with the field-coils, is in multiple arc therewith.
+The armature-winding may be similar to that of the field--that is to
+say, the armature may have two or more coils wound or adapted for
+different self-induction and adapted, preferably, to produce the same
+difference of phase as the field-coils. On starting the motor the shunt
+is closed around both coils. This is shown in Fig. 51, in which the
+armature coils are F G. To indicate their different electrical
+character, there are shown in circuit with them, respectively, the
+resistance R' and the self-induction coil S'. The two armature coils are
+in series with the field-coils and the same disposition of the shunt or
+short-circuit D is used. It is of advantage in the operation of motors
+of this kind to construct or wind the armature in such manner that when
+short-circuited on the start it will have a tendency to reach a higher
+speed than that which synchronizes with the generator. For example, a
+given motor having, say, eight poles should run, with the armature coil
+short-circuited, at two thousand revolutions per minute to bring it up
+to synchronism. It will generally happen, however, that this speed is
+not reached, owing to the fact that the armature and field currents do
+not properly correspond, so that when the current is passed through the
+armature (the motor not being quite up to synchronism) there is a
+liability that it will not "hold on," as it is termed. It is preferable,
+therefore, to so wind or construct the motor that on the start, when the
+armature coils are short-circuited, the motor will tend to reach a speed
+higher than the synchronous--as for instance, double the latter. In such
+case the difficulty above alluded to is not felt, for the motor will
+always hold up to synchronism if the synchronous speed--in the case
+supposed of two thousand revolutions--is reached or passed. This may be
+accomplished in various ways; but for all practical purposes the
+following will suffice: On the armature are wound two sets of coils. At
+the start only one of these is short-circuited, thereby producing a
+number of poles on the armature, which will tend to run the speed up
+above the synchronous limit. When such limit is reached or passed, the
+current is directed through the other coil, which, by increasing the
+number of armature poles, tends to maintain synchronism.
+
+[Illustration: FIG. 53.]
+
+In Fig. 52, such a disposition is shown. The motor having, say, eight
+poles contains two field-circuits A and B, of different self-induction.
+The armature has two coils F and G. The former is closed upon itself,
+the latter connected with the field and line through contact-rings _a
+b_, brushes _c d_, and a switch E. On the start the coil F alone is
+active and the motor tends to run at a speed above the synchronous; but
+when the coil G is connected to the circuit the number of armature poles
+is increased, while the motor is made a true synchronous motor. This
+disposition has the advantage that the closed armature-circuit imparts
+to the motor torque when the speed falls off, but at the same time the
+conditions are such that the motor comes out of synchronism more
+readily. To increase the tendency to synchronism, two circuits may be
+used on the armature, one of which is short-circuited on the start and
+both connected with the external circuit after the synchronous speed is
+reached or passed. This disposition is shown in Fig. 53. There are three
+contact-rings _a b e_ and three brushes _c d f_, which connect the
+armature circuits with the external circuit. On starting, the switch H
+is turned to complete the connection between one binding-post P and the
+field-coils. This short-circuits one of the armature-coils, as G. The
+other coil F is out of circuit and open. When the motor is up to speed,
+the switch H is turned back, so that the connection from binding-post P
+to the field coils is through the coil G, and switch K is closed,
+thereby including coil F in multiple arc with the field coils. Both
+armature coils are thus active.
+
+From the above-described instances it is evident that many other
+dispositions for carrying out the invention are possible.
+
+
+
+
+CHAPTER XII.
+
+"MAGNETIC LAG" MOTOR.
+
+
+The following description deals with another form of motor, namely,
+depending on "magnetic lag" or hysteresis, its peculiarity being that in
+it the attractive effects or phases while lagging behind the phases of
+current which produce them, are manifested simultaneously and not
+successively. The phenomenon utilized thus at an early stage by Mr.
+Tesla, was not generally believed in by scientific men, and Prof. Ayrton
+was probably first to advocate it or to elucidate the reason of its
+supposed existence.
+
+Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a
+part-sectional view at right angles to Fig. 54. Fig. 56 is an end view
+in elevation and part section of a modification, and Fig. 57 is a
+similar view of another modification.
+
+In Figs. 54 and 55, A designates a base or stand, and B B the
+supporting-frame of the motor. Bolted to the supporting-frame are two
+magnetic cores or pole-pieces C C', of iron or soft steel. These may be
+subdivided or laminated, in which case hard iron or steel plates or bars
+should be used, or they should be wound with closed coils. D is a
+circular disc armature, built up of sections or plates of iron and
+mounted in the frame between the pole-pieces C C', curved to conform to
+the circular shape thereof. This disc may be wound with a number of
+closed coils E. F F are the main energizing coils, supported by the
+supporting-frame, so as to include within their magnetizing influence
+both the pole-pieces C C' and the armature D. The pole-pieces C C'
+project out beyond the coils F F on opposite sides, as indicated in the
+drawings. If an alternating current be passed through the coils F F,
+rotation of the armature will be produced, and this rotation is
+explained by the following apparent action, or mode of operation: An
+impulse of current in the coils F F establishes two polarities in the
+motor. The protruding end of pole-piece C, for instance, will be of one
+sign, and the corresponding end of pole-piece C' will be of the opposite
+sign. The armature also exhibits two poles at right angles to the coils
+F F, like poles to those in the pole-pieces being on the same side of
+the coils. While the current is flowing there is no appreciable tendency
+to rotation developed; but after each current impulse ceases or begins
+to fall, the magnetism in the armature and in the ends of the
+pole-pieces C C' lags or continues to manifest itself, which produces a
+rotation of the armature by the repellent force between the more closely
+approximating points of maximum magnetic effect. This effect is
+continued by the reversal of current, the polarities of field and
+armature being simply reversed. One or both of the elements--the
+armature or field--may be wound with closed induced coils to intensify
+this effect. Although in the illustrations but one of the fields is
+shown, each element of the motor really constitutes a field, wound with
+the closed coils, the currents being induced mainly in those
+convolutions or coils which are parallel to the coils F F.
+
+[Illustration: FIG. 54.]
+
+[Illustration: FIG. 55.]
+
+A modified form of this motor is shown in Fig. 56. In this form G is one
+of two standards that support the bearings for the armature-shaft. H H
+are uprights or sides of a frame, preferably magnetic, the ends C C' of
+which are bent in the manner indicated, to conform to the shape of the
+armature D and form field-magnet poles. The construction of the armature
+may be the same as in the previous figure, or it may be simply a
+magnetic disc or cylinder, as shown, and a coil or coils F F are
+secured in position to surround both the armature and the poles C C'.
+The armature is detachable from its shaft, the latter being passed
+through the armature after it has been inserted in position. The
+operation of this form of motor is the same in principle as that
+previously described and needs no further explanation.
+
+[Illustration: FIG. 56.]
+
+[Illustration: FIG. 57.]
+
+One of the most important features in alternating current motors is,
+however, that they should be adapted to and capable of running
+efficiently on the alternating circuits in present use, in which almost
+without exception the generators yield a very high number of
+alternations. Such a motor, of the type under consideration, Mr. Tesla
+has designed by a development of the principle of the motor shown in
+Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In
+the construction of this motor he employs an annular magnetic frame J,
+with inwardly-extending ribs or projections K, the ends of which all
+bend or turn in one direction and are generally shaped to conform to the
+curved surface of the armature. Coils F F are wound from one part K to
+the one next adjacent, the ends or loops of each coil or group of wires
+being carried over toward the shaft, so as to form U-shaped groups
+of convolutions at each end of the armature. The pole-pieces C C', being
+substantially concentric with the armature, form ledges, along which the
+coils are laid and should project to some extent beyond the the coils,
+as shown. The cylindrical or drum armature D is of the same construction
+as in the other motors described, and is mounted to rotate within the
+annular frame J and between the U-shaped ends or bends of the
+coils F. The coils F are connected in multiple or in series with a
+source of alternating currents, and are so wound that with a current or
+current impulse of given direction they will make the alternate
+pole-pieces C of one polarity and the other pole-pieces C' of the
+opposite polarity. The principle of the operation of this motor is the
+same as the other above described, for, considering any two pole-pieces
+C C', a current impulse passing in the coil which bridges them or is
+wound over both tends to establish polarities in their ends of opposite
+sign and to set up in the armature core between them a polarity of the
+same sign as that of the nearest pole-piece C. Upon the fall or
+cessation of the current impulse that established these polarities the
+magnetism which lags behind the current phase, and which continues to
+manifest itself in the polar projections C C' and the armature, produces
+by repulsion a rotation of the armature. The effect is continued by each
+reversal of the current. What occurs in the case of one pair of
+pole-pieces occurs simultaneously in all, so that the tendency to
+rotation of the armature is measured by the sum of all the forces
+exerted by the pole-pieces, as above described. In this motor also the
+magnetic lag or effect is intensified by winding one or both cores with
+closed induced coils. The armature core is shown as thus wound. When
+closed coils are used, the cores should be laminated.
+
+It is evident that a pulsatory as well as an alternating current might
+be used to drive or operate the motors above described.
+
+It will be understood that the degree of subdivision, the mass of the
+iron in the cores, their size and the number of alternations in the
+current employed to run the motor, must be taken into consideration in
+order to properly construct this motor. In other words, in all such
+motors the proper relations between the number of alternations and the
+mass, size, or quality of the iron must be preserved in order to secure
+the best results.
+
+
+
+
+CHAPTER XIII.
+
+METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.
+
+
+In that class of motors in which two or more sets of energizing magnets
+are employed, and in which by artificial means a certain interval of
+time is made to elapse between the respective maximum or minimum periods
+or phases of their magnetic attraction or effect, the interval or
+difference in phase between the two sets of magnets is limited in
+extent. It is desirable, however, for the economical working of such
+motors that the strength or attraction of one set of magnets should be
+maximum, at the time when that of the other set is minimum, and
+conversely; but these conditions have not heretofore been realized
+except in cases where the two currents have been obtained from
+independent sources in the same or different machines. Mr. Tesla has
+therefore devised a motor embodying conditions that approach more nearly
+the theoretical requirements of perfect working, or in other words, he
+produces artificially a difference of magnetic phase by means of a
+current from a single primary source sufficient in extent to meet the
+requirements of practical and economical working. He employs a motor
+with two sets of energizing or field magnets, each wound with coils
+connected with a source of alternating or rapidly-varying currents, but
+forming two separate paths or circuits. The magnets of one set are
+protected to a certain extent from the energizing action of the current
+by means of a magnetic shield or screen interposed between the magnet
+and its energizing coil. This shield is properly adapted to the
+conditions of particular cases, so as to shield or protect the main core
+from magnetization until it has become itself saturated and no longer
+capable of containing all the lines of force produced by the current. It
+will be seen that by this means the energizing action begins in the
+protected set of magnets a certain arbitrarily-determined period of time
+later than in the other, and that by this means alone or in conjunction
+with other means or devices heretofore employed a practical difference
+of magnetic phase may readily be secured.
+
+Fig. 58 is a view of a motor, partly in section, with a diagram
+illustrating the invention. Fig. 59 is a similar view of a modification
+of the same.
+
+[Illustration: FIG. 58.]
+
+[Illustration: FIG. 59.]
+
+In Fig. 58, which exhibits the simplest form of the invention, A A is
+the field-magnet of a motor, having, say, eight poles or
+inwardly-projecting cores B and C. The cores B form one set of magnets
+and are energized by coils D. The cores C, forming the other set are
+energized by coils E, and the coils are connected, preferably, in series
+with one another, in two derived or branched circuits, F G,
+respectively, from a suitable source of current. Each coil E is
+surrounded by a magnetic shield H, which is preferably composed of an
+annealed, insulated, or oxidized iron wire wrapped or wound on the coils
+in the manner indicated so as to form a closed magnetic circuit around
+the coils and between the same and the magnetic cores C. Between the
+pole pieces or cores B C is mounted the armature K, which, as is usual
+in this type of machines, is wound with coils L closed upon themselves.
+The operation resulting from this disposition is as follows: If a
+current impulse be directed through the two circuits of the motor, it
+will quickly energize the cores B, but not so the cores C, for the
+reason that in passing through the coils E there is encountered the
+influence of the closed magnetic circuits formed by the shields H. The
+first effect is to retard effectively the current impulse in circuit G,
+while at the same time the proportion of current which does pass does
+not magnetize the cores C, which are shielded or screened by the
+shields H. As the increasing electromotive force then urges more current
+through the coils E, the iron wire H becomes magnetically saturated and
+incapable of carrying all the lines of force, and hence ceases to
+protect the cores C, which becomes magnetized, developing their maximum
+effect after an interval of time subsequent to the similar manifestation
+of strength in the other set of magnets, the extent of which is
+arbitrarily determined by the thickness of the shield H, and other
+well-understood conditions.
+
+From the above it will be seen that the apparatus or device acts in two
+ways. First, by retarding the current, and, second, by retarding the
+magnetization of one set of the cores, from which its effectiveness will
+readily appear.
+
+Many modifications of the principle of this invention are possible. One
+useful and efficient application of the invention is shown in Fig. 59.
+In this figure a motor is shown similar in all respects to that above
+described, except that the iron wire H, which is wrapped around the
+coils E, is in this case connected in series with the coils D. The
+iron-wire coils H, are connected and wound, so as to have little or no
+self-induction, and being added to the resistance of the circuit F, the
+action of the current in that circuit will be accelerated, while in the
+other circuit G it will be retarded. The shield H may be made in many
+forms, as will be understood, and used in different ways, as appears
+from the foregoing description.
+
+As a modification of his type of motor with "shielded" fields, Mr. Tesla
+has constructed a motor with a field-magnet having two sets of poles or
+inwardly-projecting cores and placed side by side, so as practically to
+form two fields of force and alternately disposed--that is to say, with
+the poles of one set or field opposite the spaces between the other. He
+then connects the free ends of one set of poles by means of laminated
+iron bands or bridge-pieces of considerably smaller cross-section than
+the cores themselves, whereby the cores will all form parts of complete
+magnetic circuits. When the coils on each set of magnets are connected
+in multiple circuits or branches from a source of alternating currents,
+electromotive forces are set up in or impressed upon each circuit
+simultaneously; but the coils on the magnetically bridged or shunted
+cores will have, by reason of the closed magnetic circuits, a high
+self-induction, which retards the current, permitting at the beginning
+of each impulse but little current to pass. On the other hand, no such
+opposition being encountered in the other set of coils, the current
+passes freely through them, magnetizing the poles on which they are
+wound. As soon, however, as the laminated bridges become saturated and
+incapable of carrying all the lines of force which the rising
+electromotive force, and consequently increased current, produce, free
+poles are developed at the ends of the cores, which, acting in
+conjunction with the others, produce rotation of the armature.
+
+The construction in detail by which this invention is illustrated is
+shown in the accompanying drawings.
+
+[Illustration: FIG. 60.]
+
+[Illustration: FIG. 61.]
+
+Fig. 60 is a view in side elevation of a motor embodying the principle.
+Fig. 61 is a vertical cross-section of the motor. A is the frame of the
+motor, which should be built up of sheets of iron punched out to the
+desired shape and bolted together with insulation between the sheets.
+When complete, the frame makes a field-magnet with inwardly projecting
+pole-pieces B and C. To adapt them to the requirements of this
+particular case these pole-pieces are out of line with one another,
+those marked B surrounding one end of the armature and the others, as C,
+the opposite end, and they are disposed alternately--that is to say, the
+pole-pieces of one set occur in line with the spaces between those of
+the other sets.
+
+The armature D is of cylindrical form, and is also laminated in the
+usual way and is wound longitudinally with coils closed upon themselves.
+The pole-pieces C are connected or shunted by bridge-pieces E. These may
+be made independently and attached to the pole-pieces, or they may be
+parts of the forms or blanks stamped or punched out of sheet-iron. Their
+size or mass is determined by various conditions, such as the strength
+of the current to be employed, the mass or size of the cores to which
+they are applied, and other familiar conditions.
+
+Coils F surround the pole-pieces B, and other coils G are wound on the
+pole-pieces C. These coils are connected in series in two circuits,
+which are branches of a circuit from a generator of alternating
+currents, and they may be so wound, or the respective circuits in which
+they are included may be so arranged, that the circuit of coils G will
+have, independently of the particular construction described, a higher
+self-induction than the other circuit or branch.
+
+The function of the shunts or bridges E is that they shall form with the
+cores C a closed magnetic circuit for a current up to a predetermined
+strength, so that when saturated by such current and unable to carry
+more lines of force than such a current produces they will to no further
+appreciable extent interfere with the development, by a stronger
+current, of free magnetic poles at the ends of the cores C.
+
+In such a motor the current is so retarded in the coils G, and the
+manifestation of the free magnetism in the poles C is so delayed beyond
+the period of maximum magnetic effect in poles B, that a strong torque
+is produced and the motor operates with approximately the power
+developed in a motor of this kind energized by independently generated
+currents differing by a full quarter phase.
+
+
+
+
+CHAPTER XIV.
+
+TYPE OF TESLA SINGLE-PHASE MOTOR.
+
+
+Up to this point, two principal types of Tesla motors have been
+described: First, those containing two or more energizing circuits
+through which are caused to pass alternating currents differing from one
+another in phase to an extent sufficient to produce a continuous
+progression or shifting of the poles or points of greatest magnetic
+effect, in obedience to which the movable element of the motor is
+maintained in rotation; second, those containing poles, or parts of
+different magnetic susceptibility, which under the energizing influence
+of the same current or two currents coinciding in phase will exhibit
+differences in their magnetic periods or phases. In the first class of
+motors the torque is due to the magnetism established in different
+portions of the motor by currents from the same or from independent
+sources, and exhibiting time differences in phase. In the second class
+the torque results from the energizing effects of a current upon
+different parts of the motor which differ in magnetic susceptibility--in
+other words, parts which respond in the same relative degree to the
+action of a current, not simultaneously, but after different intervals
+of time.
+
+In another Tesla motor, however, the torque, instead of being solely the
+result of a time difference in the magnetic periods or phases of the
+poles or attractive parts to whatever cause due, is produced by an
+angular displacement of the parts which, though movable with respect to
+one another, are magnetized simultaneously, or approximately so, by the
+same currents. This principle of operation has been embodied practically
+in a motor in which the necessary angular displacement between the
+points of greatest magnetic attraction in the two elements of the
+motor--the armature and field--is obtained by the direction of the
+lamination of the magnetic cores of the elements.
+
+Fig. 62 is a side view of such a motor with a portion of its armature
+core exposed. Fig. 63 is an end or edge view of the same. Fig. 64 is a
+central cross-section of the same, the armature being shown mainly in
+elevation.
+
+[Illustration: FIG. 62.]
+
+[Illustration: FIG. 63.]
+
+[Illustration: FIG. 64.]
+
+Let A A designate two plates built up of thin sections or laminć of soft
+iron insulated more or less from one another and held together by bolts
+_a_ and secured to a base B. The inner faces of these plates contain
+recesses or grooves in which a coil or coils D are secured obliquely to
+the direction of the laminations. Within the coils D is a disc E,
+preferably composed of a spirally-wound iron wire or ribbon or a series
+of concentric rings and mounted on a shaft F, having bearings in the
+plates A A. Such a device when acted upon by an alternating current is
+capable of rotation and constitutes a motor, the operation of which may
+be explained in the following manner: A current or current-impulse
+traversing the coils D tends to magnetize the cores A A and E, all of
+which are within the influence of the field of the coils. The poles thus
+established would naturally lie in the same line at right angles to the
+coils D, but in the plates A they are deflected by reason of the
+direction of the laminations, and appear at or near the extremities of
+these plates. In the disc, however, where these conditions are not
+present, the poles or points of greatest attraction are on a line at
+right angles to the plane of the coils; hence there will be a torque
+established by this angular displacement of the poles or magnetic lines,
+which starts the disc in rotation, the magnetic lines of the armature
+and field tending toward a position of parallelism. This rotation is
+continued and maintained by the reversals of the current in coils D D,
+which change alternately the polarity of the field-cores A A. This
+rotary tendency or effect will be greatly increased by winding the disc
+with conductors G, closed upon themselves and having a radial direction,
+whereby the magnetic intensity of the poles of the disc will be greatly
+increased by the energizing effect of the currents induced in the coils
+G by the alternating currents in coils D.
+
+The cores of the disc and field may or may not be of different magnetic
+susceptibility--that is to say, they may both be of the same kind of
+iron, so as to be magnetized at approximately the same instant by the
+coils D; or one may be of soft iron and the other of hard, in order that
+a certain time may elapse between the periods of their magnetization. In
+either case rotation will be produced; but unless the disc is provided
+with the closed energizing coils it is desirable that the
+above-described difference of magnetic susceptibility be utilized to
+assist in its rotation.
+
+The cores of the field and armature may be made in various ways, as will
+be well understood, it being only requisite that the laminations in each
+be in such direction as to secure the necessary angular displacement of
+the points of greatest attraction. Moreover, since the disc may be
+considered as made up of an infinite number of radial arms, it is
+obvious that what is true of a disc holds for many other forms of
+armature.
+
+
+
+
+CHAPTER XV.
+
+MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.
+
+
+As has been pointed out elsewhere, the lag or retardation of the phases
+of an alternating current is directly proportional to the self-induction
+and inversely proportional to the resistance of the circuit through
+which the current flows. Hence, in order to secure the proper
+differences of phase between the two motor-circuits, it is desirable to
+make the self-induction in one much higher and the resistance much lower
+than the self-induction and resistance, respectively, in the other. At
+the same time the magnetic quantities of the two poles or sets of poles
+which the two circuits produce should be approximately equal. These
+requirements have led Mr. Tesla to the invention of a motor having the
+following general characteristics: The coils which are included in that
+energizing circuit which is to have the higher self-induction are made
+of coarse wire, or a conductor of relatively low resistance, and with
+the greatest possible length or number of turns. In the other set of
+coils a comparatively few turns of finer wire are used, or a wire of
+higher resistance. Furthermore, in order to approximate the magnetic
+quantities of the poles excited by these coils, Mr. Tesla employs in the
+self-induction circuit cores much longer than those in the other or
+resistance circuit.
+
+Fig. 65 is a part sectional view of the motor at right angles to the
+shaft. Fig. 66 is a diagram of the field circuits.
+
+In Fig. 66, let A represent the coils in one motor circuit, and B those
+in the other. The circuit A is to have the higher self-induction. There
+are, therefore, used a long length or a large number of turns of coarse
+wire in forming the coils of this circuit. For the circuit B, a smaller
+conductor is employed, or a conductor of a higher resistance than
+copper, such as German silver or iron, and the coils are wound with
+fewer turns. In applying these coils to a motor, Mr. Tesla builds up a
+field-magnet of plates C, of iron and steel, secured together in the
+usual manner by bolts D. Each plate is formed with four (more or less)
+long cores E, around which is a space to receive the coil and an equal
+number of short projections F to receive the coils of the
+resistance-circuit. The plates are generally annular in shape, having an
+open space in the centre for receiving the armature G, which Mr. Tesla
+prefers to wind with closed coils. An alternating current divided
+between the two circuits is retarded as to its phases in the circuit A
+to a much greater extent than in the circuit B. By reason of the
+relative sizes and disposition of the cores and coils the magnetic
+effect of the poles E and F upon the armature closely approximate.
+
+[Illustration: FIG. 65.]
+
+[Illustration: FIG. 66.]
+
+An important result secured by the construction shown here is that these
+coils which are designed to have the higher self-induction are almost
+completely surrounded by iron, and that the retardation is thus very
+materially increased.
+
+
+
+
+CHAPTER XVI.
+
+MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.
+
+
+Let it be assumed that the energy as represented in the magnetism in the
+field of a given rotating field motor is ninety and that of the armature
+ten. The sum of these quantities, which represents the total energy
+expended in driving the motor, is one hundred; but, assuming that the
+motor be so constructed that the energy in the field is represented by
+fifty, and that in the armature by fifty, the sum is still one hundred;
+but while in the first instance the product is nine hundred, in the
+second it is two thousand five hundred, and as the energy developed is
+in proportion to these products it is clear that those motors are the
+most efficient--other things being equal--in which the magnetic energies
+developed in the armature and field are equal. These results Mr. Tesla
+obtains by using the same amount of copper or ampere turns in both
+elements when the cores of both are equal, or approximately so, and the
+same current energizes both; or in cases where the currents in one
+element are induced to those of the other he uses in the induced coils
+an excess of copper over that in the primary element or conductor.
+
+[Illustration: FIG. 67.]
+
+The conventional figure of a motor here introduced, Fig. 67, will give
+an idea of the solution furnished by Mr. Tesla for the specific problem.
+Referring to the drawing, A is the field-magnet, B the armature, C the
+field coils, and D the armature-coils of the motor.
+
+Generally speaking, if the mass of the cores of armature and field be
+equal, the amount of copper or ampere turns of the energizing coils on
+both should also be equal; but these conditions will be modified in
+different forms of machine. It will be understood that these results are
+most advantageous when existing under the conditions presented where the
+motor is running with its normal load, a point to be well borne in
+mind.
+
+
+
+
+CHAPTER XVII.
+
+MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND FIELD.
+
+
+In this form of motor, Mr. Tesla's object is to design and build
+machines wherein the maxima of the magnetic effects of the armature and
+field will more nearly coincide than in some of the types previously
+under consideration. These types are: First, motors having two or more
+energizing circuits of the same electrical character, and in the
+operation of which the currents used differ primarily in phase; second,
+motors with a plurality of energizing circuits of different electrical
+character, in or by means of which the difference of phase is produced
+artificially, and, third, motors with a plurality of energizing
+circuits, the currents in one being induced from currents in another.
+Considering the structural and operative conditions of any one of
+them--as, for example, that first named--the armature which is mounted
+to rotate in obedience to the co-operative influence or action of the
+energizing circuits has coils wound upon it which are closed upon
+themselves and in which currents are induced by the energizing-currents
+with the object and result of energizing the armature-core; but under
+any such conditions as must exist in these motors, it is obvious that a
+certain time must elapse between the manifestations of an energizing
+current impulse in the field coils, and the corresponding magnetic state
+or phase in the armature established by the current induced thereby;
+consequently a given magnetic influence or effect in the field which is
+the direct result of a primary current impulse will have become more or
+less weakened or lost before the corresponding effect in the armature
+indirectly produced has reached its maximum. This is a condition
+unfavorable to efficient working in certain cases--as, for instance,
+when the progress of the resultant poles or points of maximum attraction
+is very great, or when a very high number of alternations is
+employed--for it is apparent that a stronger tendency to rotation will
+be maintained if the maximum magnetic attractions or conditions in both
+armature and field coincide, the energy developed by a motor being
+measured by the product of the magnetic quantities of the armature and
+field.
+
+To secure this coincidence of maximum magnetic effects, Mr. Tesla has
+devised various means, as explained below. Fig. 68 is a diagrammatic
+illustration of a Tesla motor system in which the alternating currents
+proceed from independent sources and differ primarily in phase.
+
+[Illustration: FIG. 68.]
+
+[Illustration: FIG. 69.]
+
+A designates the field-magnet or magnetic frame of the motor; B B,
+oppositely located pole-pieces adapted to receive the coils of one
+energizing circuit; and C C, similar pole-pieces for the coils of the
+other energizing circuit. These circuits are designated, respectively,
+by D E, the conductor D'' forming a common return to the generator G.
+Between these poles is mounted an armature--for example, a ring or
+annular armature, wound with a series of coils F, forming a closed
+circuit or circuits. The action or operation of a motor thus constructed
+is now well understood. It will be observed, however, that the magnetism
+of poles B, for example, established by a current impulse in the coils
+thereon, precedes the magnetic effect set up in the armature by the
+induced current in coils F. Consequently the mutual attraction between
+the armature and field-poles is considerably reduced. The same
+conditions will be found to exist if, instead of assuming the poles B or
+C as acting independently, we regard the ideal resultant of both acting
+together, which is the real condition. To remedy this, the motor field
+is constructed with secondary poles B' C', which are situated between
+the others. These pole-pieces are wound with coils D' E', the former in
+derivation to the coils D, the latter to coils E. The main or primary
+coils D and E are wound for a different self-induction from that of the
+coils D' and E', the relations being so fixed that if the currents in D
+and E differ, for example, by a quarter-phase, the currents in each
+secondary coil, as D' E', will differ from those in its appropriate
+primary D or E by, say, forty-five degrees, or one-eighth of a period.
+
+Now, assuming that an impulse or alternation in circuit or branch E is
+just beginning, while in the branch D it is just falling from maximum,
+the conditions are those of a quarter-phase difference. The ideal
+resultant of the attractive forces of the two sets of poles B C
+therefore may be considered as progressing from poles B to poles C,
+while the impulse in E is rising to maximum, and that in D is falling to
+zero or minimum. The polarity set up in the armature, however, lags
+behind the manifestations of field magnetism, and hence the maximum
+points of attraction in armature and field, instead of coinciding, are
+angularly displaced. This effect is counteracted by the supplemental
+poles B' C'. The magnetic phases of these poles succeed those of poles B
+C by the same, or nearly the same, period of time as elapses between the
+effect of the poles B C and the corresponding induced effect in the
+armature; hence the magnetic conditions of poles B' C' and of the
+armature more nearly coincide and a better result is obtained. As poles
+B' C' act in conjunction with the poles in the armature established by
+poles B C, so in turn poles C B act similarly with the poles set up by
+B' C', respectively. Under such conditions the retardation of the
+magnetic effect of the armature and that of the secondary poles will
+bring the maximum of the two more nearly into coincidence and a
+correspondingly stronger torque or magnetic attraction secured.
+
+In such a disposition as is shown in Fig. 68 it will be observed that
+as the adjacent pole-pieces of either circuit are of like polarity they
+will have a certain weakening effect upon one another. Mr. Tesla
+therefore prefers to remove the secondary poles from the direct
+influence of the others. This may be done by constructing a motor with
+two independent sets of fields, and with either one or two armatures
+electrically connected, or by using two armatures and one field. These
+modifications are illustrated further on.
+
+[Illustration: FIG. 70.]
+
+[Illustration: FIG. 71.]
+
+Fig. 69 is a diagrammatic illustration of a motor and system in which
+the difference of phase is artificially produced. There are two coils D
+D in one branch and two coils E E in another branch of the main circuit
+from the generator G. These two circuits or branches are of different
+self-induction, one, as D, being higher than the other. This is
+graphically indicated by making coils D much larger than coils E. By
+reason of the difference in the electrical character of the two
+circuits, the phases of current in one are retarded to a greater extent
+than the other. Let this difference be thirty degrees. A motor thus
+constructed will rotate under the action of an alternating current; but
+as happens in the case previously described the corresponding magnetic
+effects of the armature and field do not coincide owing to the time that
+elapses between a given magnetic effect in the armature and the
+condition of the field that produces it. The secondary or supplemental
+poles B' C' are therefore availed of. There being thirty degrees
+difference of phase between the currents in coils D E, the magnetic
+effect of poles B' C' should correspond to that produced by a current
+differing from the current in coils D or E by fifteen degrees. This we
+can attain by winding each supplemental pole B' C' with two coils H H'.
+The coils H are included in a derived circuit having the same
+self-induction as circuit D, and coils H' in a circuit having the same
+self-induction as circuit E, so that if these circuits differ by thirty
+degrees the magnetism of poles B' C' will correspond to that produced by
+a current differing from that in either D or E by fifteen degrees. This
+is true in all other cases. For example, if in Fig. 68 the coils D' E'
+be replaced by the coils H H' included in the derived circuits, the
+magnetism of the poles B' C' will correspond in effect or phase, if it
+may be so termed, to that produced by a current differing from that in
+either circuit D or E by forty-five degrees, or one-eighth of a period.
+
+This invention as applied to a derived circuit motor is illustrated in
+Figs. 70 and 71. The former is an end view of the motor with the
+armature in section and a diagram of connections, and Fig. 71 a vertical
+section through the field. These figures are also drawn to show one of
+the dispositions of two fields that may be adopted in carrying out the
+principle. The poles B B C C are in one field, the remaining poles in
+the other. The former are wound with primary coils I J and secondary
+coils I' J', the latter with coils K L. The primary coils I J are in
+derived circuits, between which, by reason of their different
+self-induction, there is a difference of phase, say, of thirty degrees.
+The coils I' K are in circuit with one another, as also are coils J' L,
+and there should be a difference of phase between the currents in coils
+K and L and their corresponding primaries of, say, fifteen degrees. If
+the poles B C are at right angles, the armature-coils should be
+connected directly across, or a single armature core wound from end to
+end may be used; but if the poles B C be in line there should be an
+angular displacement of the armature coils, as will be well understood.
+
+The operation will be understood from the foregoing. The maximum
+magnetic condition of a pair of poles, as B' B', coincides closely with
+the maximum effect in the armature, which lags behind the corresponding
+condition in poles B B.
+
+
+
+
+CHAPTER XVIII.
+
+MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF THE INNER
+AND OUTER PARTS OF AN IRON CORE.
+
+
+It is well known that if a magnetic core, even if laminated or
+subdivided, be wound with an insulated coil and a current of electricity
+be directed through the coil, the magnetization of the entire core does
+not immediately ensue, the magnetizing effect not being exhibited in all
+parts simultaneously. This may be attributed to the fact that the action
+of the current is to energize first those laminć or parts of the core
+nearest the surface and adjacent to the exciting-coil, and from thence
+the action progresses toward the interior. A certain interval of time
+therefore elapses between the manifestation of magnetism in the external
+and the internal sections or layers of the core. If the core be thin or
+of small mass, this effect may be inappreciable; but in the case of a
+thick core, or even of a comparatively thin one, if the number of
+alternations or rate of change of the current strength be very great,
+the time interval occurring between the manifestations of magnetism in
+the interior of the core and in those parts adjacent to the coil is more
+marked. In the construction of such apparatus as motors which are
+designed to be run by alternating or equivalent currents--such as
+pulsating or undulating currents generally--Mr. Tesla found it desirable
+and even necessary to give due consideration to this phenomenon and to
+make special provisions in order to obviate its consequences. With the
+specific object of taking advantage of this action or effect, and to
+render it more pronounced, he constructs a field magnet in which the
+parts of the core or cores that exhibit at different intervals of time
+the magnetic effect imparted to them by alternating or equivalent
+currents in an energizing coil or coils, are so placed with relation to
+a rotating armature as to exert thereon their attractive effect
+successively in the order of their magnetization. By this means he
+secures a result similar to that which he had previously attained in
+other forms or types of motor in which by means of one or more
+alternating currents he has produced the rotation or progression of the
+magnetic poles.
+
+This new mode of operation will now be described. Fig. 72 is a side
+elevation of such motor. Fig. 73 is a side elevation of a more
+practicable and efficient embodiment of the invention. Fig. 74 is a
+central vertical section of the same in the plane of the axis of
+rotation.
+
+[Illustration: FIGS. 72 and 73.]
+
+Referring to Fig. 72, let X represent a large iron core, which may be
+composed of a number of sheets or laminć of soft iron or steel.
+Surrounding this core is a coil Y, which is connected with a source E of
+rapidly varying currents. Let us consider now the magnetic conditions
+existing in this core at any point, as _b_, at or near the centre, and
+any other point, as _a_, nearer the surface. When a current impulse is
+started in the magnetizing coil Y, the section or part at _a_, being
+close to the coil, is immediately energized, while the section or part
+at _b_, which, to use a convenient expression, is "protected" by the
+intervening sections or layers between _a_ and _b_, does not at once
+exhibit its magnetism. However, as the magnetization of _a_ increases,
+_b_ becomes also affected, reaching finally its maximum strength some
+time later than _a_. Upon the weakening of the current the magnetization
+of _a_ first diminishes, while _b_ still exhibits its maximum strength;
+but the continued weakening of _a_ is attended by a subsequent weakening
+of _b_. Assuming the current to be an alternating one, _a_ will now be
+reversed, while _b_ still continues of the first imparted polarity. This
+action continues the magnetic condition of _b_, following that of _a_ in
+the manner above described. If an armature--for instance, a simple disc
+F, mounted to rotate freely on an axis--be brought into proximity to the
+core, a movement of rotation will be imparted to the disc, the direction
+depending upon its position relatively to the core, the tendency being
+to turn the portion of the disc nearest to the core from _a_ to _b_, as
+indicated in Fig. 72.
+
+[Illustration: FIG. 74.]
+
+This action or principle of operation has been embodied in a practicable
+form of motor, which is illustrated in Fig. 73. Let A in that figure
+represent a circular frame of iron, from diametrically opposite points
+of the interior of which the cores project. Each core is composed of
+three main parts B, B and C, and they are similarly formed with a
+straight portion or body _e_, around which the energizing coil is wound,
+a curved arm or extension _c_, and an inwardly projecting pole or end
+_d_. Each core is made up of two parts B B, with their polar extensions
+reaching in one direction, and a part C between the other two, and with
+its polar extension reaching in the opposite direction. In order to
+lessen in the cores the circulation of currents induced therein, the
+several sections are insulated from one another in the manner usually
+followed in such cases. These cores are wound with coils D, which are
+connected in the same circuit, either in parallel or series, and
+supplied with an alternating or a pulsating current, preferably the
+former, by a generator E, represented diagrammatically. Between the
+cores or their polar extensions is mounted a cylindrical or similar
+armature F, wound with magnetizing coils G, closed upon themselves.
+
+The operation of this motor is as follows: When a current impulse or
+alternation is directed through the coils D, the sections B B of the
+cores, being on the surface and in close proximity to the coils, are
+immediately energized. The sections C, on the other hand, are protected
+from the magnetizing influence of the coil by the interposed layers of
+iron B B. As the magnetism of B B increases, however, the sections C are
+also energized; but they do not attain their maximum strength until a
+certain time subsequent to the exhibition by the sections B B of their
+maximum. Upon the weakening of the current the magnetic strength of B B
+first diminishes, while the sections C have still their maximum
+strength; but as B B continue to weaken the interior sections are
+similarly weakened. B B may then begin to exhibit an opposite polarity,
+which is followed later by a similar change on C, and this action
+continues. B B and C may therefore be considered as separate
+field-magnets, being extended so as to act on the armature in the most
+efficient positions, and the effect is similar to that in the other
+forms of Tesla motor--viz., a rotation or progression of the maximum
+points of the field of force. Any armature--such, for instance, as a
+disc--mounted in this field would rotate from the pole first to exhibit
+its magnetism to that which exhibits it later.
+
+It is evident that the principle here described may be carried out in
+conjunction with other means for securing a more favorable or efficient
+action of the motor. For example, the polar extensions of the sections C
+may be wound or surrounded by closed coils. The effect of these coils
+will be to still more effectively retard the magnetization of the polar
+extensions of C.
+
+
+
+
+CHAPTER XIX.
+
+ANOTHER TYPE OF TESLA INDUCTION MOTOR.
+
+
+It will have been gathered by all who are interested in the advance of
+the electrical arts, and who follow carefully, step by step, the work of
+pioneers, that Mr. Tesla has been foremost to utilize inductive effects
+in permanently closed circuits, in the operation of alternating motors.
+In this chapter one simple type of such a motor is described and
+illustrated, which will serve as an exemplification of the principle.
+
+Let it be assumed that an ordinary alternating current generator is
+connected up in a circuit of practically no self-induction, such, for
+example, as a circuit containing incandescent lamps only. On the
+operation of the machine, alternating currents will be developed in the
+circuit, and the phases of these currents will theoretically coincide
+with the phases of the impressed electromotive force. Such currents may
+be regarded and designated as the "unretarded currents."
+
+It will be understood, of course, that in practice there is always more
+or less self-induction in the circuit, which modifies to a corresponding
+extent these conditions; but for convenience this may be disregarded in
+the consideration of the principle of operation, since the same laws
+apply. Assume next that a path of currents be formed across any two
+points of the above circuit, consisting, for example, of the primary of
+an induction device. The phases of the currents passing through the
+primary, owing to the self-induction of the same, will not coincide with
+the phases of the impressed electromotive force, but will lag behind,
+such lag being directly proportional to the self-induction and inversely
+proportional to the resistance of the said coil. The insertion of this
+coil will also cause a lagging or retardation of the currents traversing
+and delivered by the generator behind the impressed electromotive force,
+such lag being the mean or resultant of the lag of the current through
+the primary alone and of the "unretarded current" in the entire working
+circuit. Next consider the conditions imposed by the association in
+inductive relation with the primary coil, of a secondary coil. The
+current generated in the secondary coil will react upon the primary
+current, modifying the retardation of the same, according to the amount
+of self-induction and resistance in the secondary circuit. If the
+secondary circuit has but little self-induction--as, for instance, when
+it contains incandescent lamps only--it will increase the actual
+difference of phase between its own and the primary current, first, by
+diminishing the lag between the primary current and the impressed
+electromotive force, and, second, by its own lag or retardation behind
+the impressed electromotive force. On the other hand, if the secondary
+circuit have a high self-induction, its lag behind the current in the
+primary is directly increased, while it will be still further increased
+if the primary have a very low self-induction. The better results are
+obtained when the primary has a low self-induction.
+
+[Illustration: FIG. 75.]
+
+[Illustration: FIG. 76.]
+
+Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76
+is a similar diagram of a modification of the same. In Fig. 75 let A
+designate the field-magnet of a motor which, as in all these motors, is
+built up of sections or plates. B C are polar projections upon which the
+coils are wound. Upon one pair of these poles, as C, are wound primary
+coils D, which are directly connected to the circuit of an alternating
+current generator G. On the same poles are also wound secondary coils F,
+either side by side or over or under the primary coils, and these are
+connected with other coils E, which surround the poles B B. The
+currents in both primary and secondary coils in such a motor will be
+retarded or will lag behind the impressed electromotive force; but to
+secure a proper difference in phase between the primary and secondary
+currents themselves, Mr. Tesla increases the resistance of the circuit
+of the secondary and reduces as much as practicable its self-induction.
+This is done by using for the secondary circuit, particularly in the
+coils E, wire of comparatively small diameter and having but few turns
+around the cores; or by using some conductor of higher specific
+resistance, such as German silver; or by introducing at some point in
+the secondary circuit an artificial resistance R. Thus the
+self-induction of the secondary is kept down and its resistance
+increased, with the result of decreasing the lag between the impressed
+electro-motive force and the current in the primary coils and increasing
+the difference of phase between the primary and secondary currents.
+
+In the disposition shown in Fig. 76, the lag in the secondary is
+increased by increasing the self-induction of that circuit, while the
+increasing tendency of the primary to lag is counteracted by inserting
+therein a dead resistance. The primary coils D in this case have a low
+self-induction and high resistance, while the coils E F, included in the
+secondary circuit, have a high self-induction and low resistance. This
+may be done by the proper winding of the coils; or in the circuit
+including the secondary coils E F, we may introduce a self-induction
+coil S, while in the primary circuit from the generator G and including
+coils D, there may be inserted a dead resistance R. By this means the
+difference of phase between the primary and secondary is increased. It
+is evident that both means of increasing the difference of
+phase--namely, by the special winding as well as by the supplemental or
+external inductive and dead resistance--may be employed conjointly.
+
+In the operation of this motor the current impulses in the primary coils
+induce currents in the secondary coils, and by the conjoint action of
+the two the points of greatest magnetic attraction are shifted or
+rotated.
+
+In practice it is found desirable to wind the armature with closed coils
+in which currents are induced by the action thereon of the primaries.
+
+
+
+
+CHAPTER XX.
+
+COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.
+
+
+In the preceding descriptions relative to synchronizing motors and
+methods of operating them, reference has been made to the plan adopted
+by Mr. Tesla, which consists broadly in winding or arranging the motor
+in such manner that by means of suitable switches it could be started as
+a multiple-circuit motor, or one operating by a progression of its
+magnetic poles, and then, when up to speed, or nearly so, converted into
+an ordinary synchronizing motor, or one in which the magnetic poles were
+simply alternated. In some cases, as when a large motor is used and when
+the number of alternations is very high, there is more or less
+difficulty in bringing the motor to speed as a double or
+multiple-circuit motor, for the plan of construction which renders the
+motor best adapted to run as a synchronizing motor impairs its
+efficiency as a torque or double-circuit motor under the assumed
+conditions on the start. This will be readily understood, for in a large
+synchronizing motor the length of the magnetic circuit of the polar
+projections, and their mass, are so great that apparently considerable
+time is required for magnetization and demagnetization. Hence with a
+current of a very high number of alternations the motor may not respond
+properly. To avoid this objection and to start up a synchronizing motor
+in which these conditions obtain, Mr. Tesla has combined two motors, one
+a synchronizing motor, the other a multiple-circuit or torque motor, and
+by the latter he brings the first-named up to speed, and then either
+throws the whole current into the synchronizing motor or operates
+jointly both of the motors.
+
+This invention involves several novel and useful features. It will be
+observed, in the first place, that both motors are run, without
+commutators of any kind, and, secondly, that the speed of the torque
+motor may be higher than that of the synchronizing motor, as will be the
+case when it contains a fewer number of poles or sets of poles, so that
+the motor will be more readily and easily brought up to speed. Thirdly,
+the synchronizing motor may be constructed so as to have a much more
+pronounced tendency to synchronism without lessening the facility with
+which it is started.
+
+Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view
+of the synchronizing motor; Fig. 79 an end view and part section of the
+torque or double-circuit motor; Fig. 80 a diagram of the circuit
+connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of
+modified dispositions of the two motors.
+
+[Illustration: FIG. 77.]
+
+Inasmuch as neither motor is doing any work while the current is acting
+upon the other, the two armatures are rigidly connected, both being
+mounted upon the same shaft A, the field-magnets B of the synchronizing
+and C of the torque motor being secured to the same base D. The
+preferably larger synchronizing motor has polar projections on its
+armature, which rotate in very close proximity to the poles of the
+field, and in other respects it conforms to the conditions that are
+necessary to secure synchronous action. The pole-pieces of the armature
+are, however, wound with closed coils E, as this obviates the employment
+of sliding contacts. The smaller or torque motor, on the other hand,
+has, preferably, a cylindrical armature F, without polar projections and
+wound with closed coils G. The field-coils of the torque motor are
+connected up in two series H and I, and the alternating current from the
+generator is directed through or divided between these two circuits in
+any manner to produce a progression of the poles or points of maximum
+magnetic effect. This result is secured by connecting the two
+motor-circuits in derivation with the circuit from the generator,
+inserting in one motor circuit a dead resistance and in the other a
+self-induction coil, by which means a difference in phase between the
+two divisions of the current is secured. If both motors have the same
+number of field poles, the torque motor for a given number of
+alternations will tend to run at double the speed of the other, for,
+assuming the connections to be such as to give the best results, its
+poles are divided into two series and the number of poles is virtually
+reduced one-half, which being acted upon by the same number of
+alternations tend to rotate the armature at twice the speed. By this
+means the main armature is more easily brought to or above the required
+speed. When the speed necessary for synchronism is imparted to the main
+motor, the current is shifted from the torque motor into the other.
+
+[Illustration: FIG. 78.]
+
+[Illustration: FIG. 79.]
+
+A convenient arrangement for carrying out this invention is shown in
+Fig. 80, in which J J are the field coils of the synchronizing, and H I
+the field coils of the torque motor. L L' are the conductors of the main
+line. One end of, say, coils H is connected to wire L through a
+self-induction coil M. One end of the other set of coils I is connected
+to the same wire through a dead resistance N. The opposite ends of these
+two circuits are connected to the contact _m_ of a switch, the handle or
+lever of which is in connection with the line-wire L'. One end of the
+field circuit of the synchronizing motor is connected to the wire L. The
+other terminates in the switch-contact _n_. From the diagram it will be
+readily seen that if the lever P be turned upon contact _m_, the torque
+motor will start by reason of the difference of phase between the
+currents in its two energizing circuits. Then when the desired speed is
+attained, if the lever P be shifted upon contact _n_ the entire current
+will pass through the field coils of the synchronizing motor and the
+other will be doing no work.
+
+The torque motor may be constructed and operated in various ways, many
+of which have already been touched upon. It is not necessary that one
+motor be cut out of circuit while the other is in, for both may be acted
+upon by current at the same time, and Mr. Tesla has devised various
+dispositions or arrangements of the two motors for accomplishing this.
+Some of these arrangements are illustrated in Figs. 81 to 85.
+
+[Illustration: FIG. 80.]
+
+Referring to Fig. 81, let T designate the torque or multiple circuit
+motor and S the synchronizing motor, L L' being the line-wires from a
+source of alternating current. The two circuits of the torque motor of
+different degrees of self-induction, and designated by N M, are
+connected in derivation to the wire L. They are then joined and
+connected to the energizing circuit of the synchronizing motor, the
+opposite terminal of which is connected to wire L'. The two motors are
+thus in series. To start them Mr. Tesla short-circuits the synchronizing
+motor by a switch P', throwing the whole current through the torque
+motor. Then when the desired speed is reached the switch P' is opened,
+so that the current passes through both motors. In such an arrangement
+as this it is obviously desirable for economical and other reasons that
+a proper relation between the speeds of the two motors should be
+observed.
+
+In Fig. 82 another disposition is illustrated. S is the synchronizing
+motor and T the torque motor, the circuits of both being in parallel. W
+is a circuit also in derivation to the motor circuits and containing a
+switch P''. S' is a switch in the synchronizing motor circuit. On the
+start the switch S' is opened, cutting out the motor S. Then P'' is
+opened, throwing the entire current through the motor T, giving it a
+very strong torque. When the desired speed is reached, switch S' is
+closed and the current divides between both motors. By means of switch
+P'' both motors may be cut out.
+
+[Illustration: FIGS. 81, 82, 83, 84 and 85.]
+
+In Fig. 83 the arrangement is substantially the same, except that a
+switch T' is placed in the circuit which includes the two circuits of
+the torque motor. Fig. 84 shows the two motors in series, with a shunt
+around both containing a switch S T. There is also a shunt around the
+synchronizing motor S, with a switch P'. In Fig. 85 the same disposition
+is shown; but each motor is provided with a shunt, in which are switches
+P' and T'', as shown.
+
+
+
+
+CHAPTER XXI.
+
+MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.
+
+
+We now come to a new class of motors in which resort is had to
+condensers for the purpose of developing the required difference of
+phase and neutralizing the effects of self-induction. Mr. Tesla early
+began to apply the condenser to alternating apparatus, in just how many
+ways can only be learned from a perusal of other portions of this
+volume, especially those dealing with his high frequency work.
+
+Certain laws govern the action or effects produced by a condenser when
+connected to an electric circuit through which an alternating or in
+general an undulating current is made to pass. Some of the most
+important of such effects are as follows: First, if the terminals or
+plates of a condenser be connected with two points of a circuit, the
+potentials of which are made to rise and fall in rapid succession, the
+condenser allows the passage, or more strictly speaking, the
+transference of a current, although its plates or armatures may be so
+carefully insulated as to prevent almost completely the passage of a
+current of unvarying strength or direction and of moderate electromotive
+force. Second, if a circuit, the terminals of which are connected with
+the plates of the condenser, possess a certain self-induction, the
+condenser will overcome or counteract to a greater or less degree,
+dependent upon well-understood conditions, the effects of such
+self-induction. Third, if two points of a closed or complete circuit
+through which a rapidly rising and falling current flows be shunted or
+bridged by a condenser, a variation in the strength of the currents in
+the branches and also a difference of phase of the currents therein is
+produced. These effects Mr. Tesla has utilized and applied in a variety
+of ways in the construction and operation of his motors, such as by
+producing a difference in phase in the two energizing circuits of an
+alternating current motor by connecting the two circuits in derivation
+and connecting up a condenser in series in one of the circuits. A
+further development, however, possesses certain novel features of
+practical value and involves a knowledge of facts less generally
+understood. It comprises the use of a condenser or condensers in
+connection with the induced or armature circuit of a motor and certain
+details of the construction of such motors. In an alternating current
+motor of the type particularly referred to above, or in any other which
+has an armature coil or circuit closed upon itself, the latter
+represents not only an inductive resistance, but one which is
+periodically varying in value, both of which facts complicate and
+render difficult the attainment of the conditions best suited to the
+most efficient working conditions; in other words, they require, first,
+that for a given inductive effect upon the armature there should be the
+greatest possible current through the armature or induced coils, and,
+second, that there should always exist between the currents in the
+energizing and the induced circuits a given relation of phase. Hence
+whatever tends to decrease the self-induction and increase the current
+in the induced circuits will, other things being equal, increase the
+output and efficiency of the motor, and the same will be true of causes
+that operate to maintain the mutual attractive effect between the field
+magnets and armature at its maximum. Mr. Tesla secures these results by
+connecting with the induced circuit or circuits a condenser, in the
+manner described below, and he also, with this purpose in view,
+constructs the motor in a special manner.
+
+[Illustration: FIG. 86.]
+
+[Illustration: FIG. 88.]
+
+[Illustration: FIG. 89.]
+
+[Illustration: FIG. 87.]
+
+[Illustration: FIG. 90.]
+
+Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of
+an alternating current motor, in which the present principle is applied.
+Fig. 87 is a central section, in line with the shaft, of a special form
+of armature core. Fig. 88 is a similar section of a modification of the
+same. Fig. 89 is one of the sections of the core detached. Fig. 90 is a
+diagram showing a modified disposition of the armature or induced
+circuits.
+
+The general plan of the invention is illustrated in Fig. 86. A A in this
+figure represent the the frame and field magnets of an alternating
+current motor, the poles or projections of which are wound with coils B
+and C, forming independent energizing circuits connected either to the
+same or to independent sources of alternating currents, so that the
+currents flowing through the circuits, respectively, will have a
+difference of phase. Within the influence of this field is an armature
+core D, wound with coils E. In motors of this description heretofore
+these coils have been closed upon themselves, or connected in a closed
+series; but in the present case each coil or the connected series of
+coils terminates in the opposite plates of a condenser F. For this
+purpose the ends of the series of coils are brought out through the
+shaft to collecting rings G, which are connected to the condenser by
+contact brushes H and suitable conductors, the condenser being
+independent of the machine. The armature coils are wound or connected in
+such manner that adjacent coils produce opposite poles.
+
+The action of this motor and the effect of the plan followed in its
+construction are as follows: The motor being started in operation and
+the coils of the field magnets being traversed by alternating currents,
+currents are induced in the armature coils by one set of field coils, as
+B, and the poles thus established are acted upon by the other set, as C.
+The armature coils, however, have necessarily a high self-induction,
+which opposes the flow of the currents thus set up. The condenser F not
+only permits the passage or transference of these currents, but also
+counteracts the effects of self-induction, and by a proper adjustment of
+the capacity of the condenser, the self-induction of the coils, and the
+periods of the currents, the condenser may be made to overcome entirely
+the effect of self-induction.
+
+It is preferable on account of the undesirability of using sliding
+contacts of any kind, to associate the condenser with the armature
+directly, or make it a part of the armature. In some cases Mr. Tesla
+builds up the armature of annular plates K K, held by bolts L between
+heads M, which are secured to the driving shaft, and in the hollow space
+thus formed he places a condenser F, generally by winding the two
+insulated plates spirally around the shaft. In other cases he utilizes
+the plates of the core itself as the plates of the condenser. For
+example, in Figs. 88 and 89, N is the driving shaft, M M are the heads
+of the armature-core, and K K' the iron plates of which the core is
+built up. These plates are insulated from the shaft and from one
+another, and are held together by rods or bolts L. The bolts pass
+through a large hole in one plate and a small hole in the one next
+adjacent, and so on, connecting electrically all of plates K, as one
+armature of a condenser, and all of plates K' as the other.
+
+To either of the condensers above described the armature coils may be
+connected, as explained by reference to Fig. 86.
+
+In motors in which the armature coils are closed upon themselves--as,
+for example, in any form of alternating current motor in which one
+armature coil or set of coils is in the position of maximum induction
+with respect to the field coils or poles, while the other is in the
+position of minimum induction--the coils are best connected in one
+series, and two points of the circuit thus formed are bridged by a
+condenser. This is illustrated in Fig. 90, in which E represents one set
+of armature coils and E' the other. Their points of union are joined
+through a condenser F. It will be observed that in this disposition the
+self-induction of the two branches E and E' varies with their position
+relatively to the field magnet, and that each branch is alternately the
+predominating source of the induced current. Hence the effect of the
+condenser F is twofold. First, it increases the current in each of the
+branches alternately, and, secondly, it alters the phase of the currents
+in the branches, this being the well-known effect which results from
+such a disposition of a condenser with a circuit, as above described.
+This effect is favorable to the proper working of the motor, because it
+increases the flow of current in the armature circuits due to a given
+inductive effect, and also because it brings more nearly into
+coincidence the maximum magnetic effects of the coacting field and
+armature poles.
+
+It will be understood, of course, that the causes that contribute to the
+efficiency of condensers when applied to such uses as the above must be
+given due consideration in determining the practicability and efficiency
+of the motors. Chief among these is, as is well known, the periodicity
+of the current, and hence the improvements described are more
+particularly adapted to systems in which a very high rate of alternation
+or change is maintained.
+
+Although this invention has been illustrated in connection with a
+special form of motor, it will be understood that it is equally
+applicable to any other alternating current motor in which there is a
+closed armature coil wherein the currents are induced by the action of
+the field, and the feature of utilizing the plates or sections of a
+magnetic core for forming the condenser is applicable, generally, to
+other kinds of alternating current apparatus.
+
+
+
+
+CHAPTER XXII.
+
+MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.
+
+
+If the field or energizing circuits of a rotary phase motor be both
+derived from the same source of alternating currents and a condenser of
+proper capacity be included in one of the same, approximately, the
+desired difference of phase may be obtained between the currents flowing
+directly from the source and those flowing through the condenser; but
+the great size and expense of condensers for this purpose that would
+meet the requirements of the ordinary systems of comparatively low
+potential are particularly prohibitory to their employment.
+
+Another, now well-known, method or plan of securing a difference of
+phase between the energizing currents of motors of this kind is to
+induce by the currents in one circuit those in the other circuit or
+circuits; but as no means had been proposed that would secure in this
+way between the phases of the primary or inducing and the secondary or
+induced currents that difference--theoretically ninety degrees--that is
+best adapted for practical and economical working, Mr. Tesla devised a
+means which renders practicable both the above described plans or
+methods, and by which he is enabled to obtain an economical and
+efficient alternating current motor. His invention consists in placing a
+condenser in the secondary or induced circuit of the motor above
+described and raising the potential of the secondary currents to such a
+degree that the capacity of the condenser, which is in part dependent on
+the potential, need be quite small. The value of this condenser is
+determined in a well-understood manner with reference to the
+self-induction and other conditions of the circuit, so as to cause the
+currents which pass through it to differ from the primary currents by a
+quarter phase.
+
+Fig. 91 illustrates the invention as embodied in a motor in which the
+inductive relation of the primary and secondary circuits is secured by
+winding them inside the motor partly upon the same cores; but the
+invention applies, generally, to other forms of motor in which one of
+the energizing currents is induced in any way from the other.
+
+Let A B represent the poles of an alternating current motor, of which C
+is the armature wound with coils D, closed upon themselves, as is now
+the general practice in motors of this kind. The poles A, which
+alternate with poles B, are wound with coils of ordinary or coarse wire
+E in such direction as to make them of alternate north and south
+polarity, as indicated in the diagram by the characters N S. Over these
+coils, or in other inductive relation to the same, are wound long
+fine-wire coils F F, and in the same direction throughout as the coils
+E. These coils are secondaries, in which currents of very high potential
+are induced. All the coils E in one series are connected, and all the
+secondaries F in another.
+
+[Illustration: FIG. 91.]
+
+On the intermediate poles B are wound fine-wire energizing coils G,
+which are connected in series with one another, and also with the series
+of secondary coils F, the direction of winding being such that a
+current-impulse induced from the primary coils E imparts the same
+magnetism to the poles B as that produced in poles A by the primary
+impulse. This condition is indicated by the characters N' S'.
+
+In the circuit formed by the two sets of coils F and G is introduced a
+condenser H; otherwise this circuit is closed upon itself, while the
+free ends of the circuit of coils E are connected to a source of
+alternating currents. As the condenser capacity which is needed in any
+particular motor of this kind is dependent upon the rate of alternation
+or the potential, or both, its size or cost, as before explained, may be
+brought within economical limits for use with the ordinary circuits if
+the potential of the secondary circuit in the motor be sufficiently
+high. By giving to the condenser proper values, any desired difference
+of phase between the primary and secondary energizing circuits may be
+obtained.
+
+
+
+
+CHAPTER XXIII.
+
+TESLA POLYPHASE TRANSFORMER.
+
+
+Applying the polyphase principle to the construction of transformers as
+well to the motors already noticed, Mr. Tesla has invented some very
+interesting forms, which he considers free from the defects of earlier
+and, at present, more familiar forms. In these transformers he provides
+a series of inducing coils and corresponding induced coils, which are
+generally wound upon a core closed upon itself, usually a ring of
+laminated iron.
+
+The two sets of coils are wound side by side or superposed or otherwise
+placed in well-known ways to bring them into the most effective
+relations to one another and to the core. The inducing or primary coils
+wound on the core are divided into pairs or sets by the proper
+electrical connections, so that while the coils of one pair or set
+co-operate in fixing the magnetic poles of the core at two given
+diametrically opposite points, the coils of the other pair or
+set--assuming, for sake of illustration, that there are but two--tend to
+fix the poles ninety degrees from such points. With this induction
+device is used an alternating current generator with coils or sets of
+coils to correspond with those of the converter, and the corresponding
+coils of the generator and converter are then connected up in
+independent circuits. It results from this that the different electrical
+phases in the generator are attended by corresponding magnetic changes
+in the converter; or, in other words, that as the generator coils
+revolve, the points of greatest magnetic intensity in the converter will
+be progressively shifted or whirled around.
+
+Fig. 92 is a diagrammatic illustration of the converter and the
+electrical connections of the same. Fig. 93 is a horizontal central
+cross-section of Fig. 92. Fig. 94 is a diagram of the circuits of the
+entire system, the generator being shown in section.
+
+Mr. Tesla uses a core, A, which is closed upon itself--that is to say,
+of an annular cylindrical or equivalent form--and as the efficiency of
+the apparatus is largely increased by the subdivision of this core, he
+makes it of thin strips, plates, or wires of soft iron electrically
+insulated as far as practicable. Upon this core are wound, say, four
+coils, B B B' B', used as primary coils, and for which long lengths of
+comparatively fine wire are employed. Over these coils are then wound
+shorter coils of coarser wire, C C C' C', to constitute the induced or
+secondary coils. The construction of this or any equivalent form of
+converter may be carried further, as above pointed out, by inclosing
+these coils with iron--as, for example, by winding over the coils layers
+of insulated iron wire.
+
+[Illustration: FIGS. 92 and 93.]
+
+[Illustration: FIG. 94.]
+
+The device is provided with suitable binding posts, to which the ends of
+the coils are led. The diametrically opposite coils B B and B' B' are
+connected, respectively, in series, and the four terminals are connected
+to the binding posts. The induced coils are connected together in any
+desired manner. For example, as shown in Fig. 94, C C may be connected
+in multiple arc when a quantity current is desired--as for running a
+group of incandescent lamps--while C' C' may be independently connected
+in series in a circuit including arc lamps or the like. The generator in
+this system will be adapted to the converter in the manner illustrated.
+For example, in the present case there are employed a pair of ordinary
+permanent or electro-magnets, E E, between which is mounted a
+cylindrical armature on a shaft, F, and wound with two coils, G G'. The
+terminals of these coils are connected, respectively, to four insulated
+contact or collecting rings, H H H' H', and the four line circuit wires
+L connect the brushes K, bearing on these rings, to the converter in the
+order shown. Noting the results of this combination, it will be observed
+that at a given point of time the coil G is in its neutral position and
+is generating little or no current, while the other coil, G', is in a
+position where it exerts its maximum effect. Assuming coil G to be
+connected in circuit with coils B B of the converter, and coil G' with
+coils B' B', it is evident that the poles of the ring A will be
+determined by coils B' B' alone; but as the armature of the generator
+revolves, coil G develops more current and coil G' less, until G reaches
+its maximum and G' its neutral position. The obvious result will be to
+shift the poles of the ring A through one-quarter of its periphery. The
+movement of the coils through the next quarter of a turn--during which
+coil G' enters a field of opposite polarity and generates a current of
+opposite direction and increasing strength, while coil G, in passing
+from its maximum to its neutral position generates a current of
+decreasing strength and same direction as before--causes a further
+shifting of the poles through the second quarter of the ring. The second
+half-revolution will obviously be a repetition of the same action. By
+the shifting of the poles of the ring A, a powerful dynamic inductive
+effect on the coils C C' is produced. Besides the currents generated in
+the secondary coils by dynamo-magnetic induction, other currents will be
+set up in the same coils in consequence of many variations in the
+intensity of the poles in the ring A. This should be avoided by
+maintaining the intensity of the poles constant, to accomplish which
+care should be taken in designing and proportioning the generator and in
+distributing the coils in the ring A, and balancing their effect. When
+this is done, the currents are produced by dynamo-magnetic induction
+only, the same result being obtained as though the poles were shifted by
+a commutator with an infinite number of segments.
+
+The modifications which are applicable to other forms of converter are
+in many respects applicable to this, such as those pertaining more
+particularly to the form of the core, the relative lengths and
+resistances of the primary and secondary coils, and the arrangements for
+running or operating the same.
+
+
+
+
+CHAPTER XXIV.
+
+A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN COILS OF
+PRIMARY AND SECONDARY.
+
+
+Mr. Tesla has applied his principle of magnetic shielding of parts to
+the construction also of transformers, the shield being interposed
+between the primary and secondary coils. In transformers of the ordinary
+type it will be found that the wave of electromotive force of the
+secondary very nearly coincides with that of the primary, being,
+however, in opposite sign. At the same time the currents, both primary
+and secondary, lag behind their respective electromotive forces; but as
+this lag is practically or nearly the same in the case of each it
+follows that the maximum and minimum of the primary and secondary
+currents will nearly coincide, but differ in sign or direction, provided
+the secondary be not loaded or if it contain devices having the property
+of self-induction. On the other hand, the lag of the primary behind the
+impressed electromotive force may be diminished by loading the secondary
+with a non-inductive or dead resistance--such as incandescent
+lamps--whereby the time interval between the maximum or minimum periods
+of the primary and secondary currents is increased. This time interval,
+however, is limited, and the results obtained by phase difference in the
+operation of such devices as the Tesla alternating current motors can
+only be approximately realized by such means of producing or securing
+this difference, as above indicated, for it is desirable in such cases
+that there should exist between the primary and secondary currents, or
+those which, however produced, pass through the two circuits of the
+motor, a difference of phase of ninety degrees; or, in other words, the
+current in one circuit should be a maximum when that in the other
+circuit is a minimum. To attain to this condition more perfectly, an
+increased retardation of the secondary current is secured in the
+following manner: Instead of bringing the primary and secondary coils or
+circuits of a transformer into the closest possible relations, as has
+hitherto been done, Mr. Tesla protects in a measure the secondary from
+the inductive action or effect of the primary by surrounding either the
+primary or the secondary with a comparatively thin magnetic shield or
+screen. Under these modified conditions, as long as the primary current
+has a small value, the shield protects the secondary; but as soon as the
+primary current has reached a certain strength, which is arbitrarily
+determined, the protecting magnetic shield becomes saturated and the
+inductive action upon the secondary begins. It results, therefore, that
+the secondary current begins to flow at a certain fraction of a period
+later than it would without the interposed shield, and since this
+retardation may be obtained without necessarily retarding the primary
+current also, an additional lag is secured, and the time interval
+between the maximum or minimum periods of the primary and secondary
+currents is increased. Such a transformer may, by properly proportioning
+its several elements and determining the proper relations between the
+primary and secondary windings, the thickness of the magnetic shield,
+and other conditions, be constructed to yield a constant current at all
+loads.
+
+[Illustration: FIG. 95.]
+
+Fig. 95 is a cross-section of a transformer embodying this improvement.
+Fig. 96 is a similar view of a modified form of transformer, showing
+diagrammatically the manner of using the same.
+
+A A is the main core of the transformer, composed of a ring of soft
+annealed and insulated or oxidized iron wire. Upon this core is wound
+the secondary circuit or coil B B. This latter is then covered with a
+layer or layers of annealed and insulated iron wires C C, wound in a
+direction at right angles to the secondary coil. Over the whole is then
+wound the primary coil or wire D D. From the nature of this construction
+it will be obvious that as long as the shield formed by the wires C is
+below magnetic saturation the secondary coil or circuit is effectually
+protected or shielded from the inductive influence of the primary,
+although on open circuit it may exhibit some electromotive force. When
+the strength of the primary reaches a certain value, the shield C,
+becoming saturated, ceases to protect the secondary from inductive
+action, and current is in consequence developed therein. For similar
+reasons, when the primary current weakens, the weakening of the
+secondary is retarded to the same or approximately the same extent.
+
+[Illustration: FIG. 96.]
+
+The specific construction of the transformer is largely immaterial. In
+Fig. 96, for example, the core A is built up of thin insulated iron
+plates or discs. The primary circuit D is wound next the core A. Over
+this is applied the shield C, which in this case is made up of thin
+strips or plates of iron properly insulated and surrounding the primary,
+forming a closed magnetic circuit. The secondary B is wound over the
+shield C. In Fig. 96, also, E is a source of alternating or rapidly
+changing currents. The primary of the transformer is connected with the
+circuit of the generator. F is a two-circuit alternating current motor,
+one of the circuits being connected with the main circuit from the
+source E, and the other being supplied with currents from the secondary
+of the transformer.
+
+
+
+
+PART II.
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.
+
+
+
+
+CHAPTER XXV.
+
+INTRODUCTION.--THE SCOPE OF THE TESLA LECTURES.
+
+
+Before proceeding to study the three Tesla lectures here presented, the
+reader may find it of some assistance to have his attention directed to
+the main points of interest and significance therein. The first of these
+lectures was delivered in New York, at Columbia College, before the
+American Institute of Electrical Engineers, May 20, 1891. The urgent
+desire expressed immediately from all parts of Europe for an opportunity
+to witness the brilliant and unusual experiments with which the lecture
+was accompanied, induced Mr. Tesla to go to England early in 1892, when
+he appeared before the Institution of Electrical Engineers, and a day
+later, by special request, before the Royal Institution. His reception
+was of the most enthusiastic and flattering nature on both occasions. He
+then went, by invitation, to France, and repeated his novel
+demonstrations before the Société Internationale des Electriciens, and
+the Société Française de Physique. Mr. Tesla returned to America in the
+fall of 1892, and in February, 1893, delivered his third lecture before
+the Franklin Institute of Philadelphia, in fulfilment of a long standing
+promise to Prof. Houston. The following week, at the request of
+President James I. Ayer, of the National Electric Light Association, the
+same lecture was re-delivered in St. Louis. It had been intended to
+limit the invitations to members, but the appeals from residents in the
+city were so numerous and pressing that it became necessary to secure a
+very large hall. Hence it came about that the lecture was listened to by
+an audience of over 5,000 people, and was in some parts of a more
+popular nature than either of its predecessors. Despite this concession
+to the need of the hour and occasion, Mr. Tesla did not hesitate to show
+many new and brilliant experiments, and to advance the frontier of
+discovery far beyond any point he had theretofore marked publicly.
+
+We may now proceed to a running review of the lectures themselves. The
+ground covered by them is so vast that only the leading ideas and
+experiments can here be touched upon; besides, it is preferable that the
+lectures should be carefully gone over for their own sake, it being more
+than likely that each student will discover a new beauty or stimulus in
+them. Taking up the course of reasoning followed by Mr. Tesla in his
+first lecture, it will be noted that he started out with the recognition
+of the fact, which he has now experimentally demonstrated, that for the
+production of light waves, primarily, electrostatic effects must be
+brought into play, and continued study has led him to the opinion that
+all electrical and magnetic effects may be referred to electrostatic
+molecular forces. This opinion finds a singular confirmation in one of
+the most striking experiments which he describes, namely, the production
+of a veritable flame by the agitation of electrostatically charged
+molecules. It is of the highest interest to observe that this result
+points out a way of obtaining a flame which consumes no material and in
+which no chemical action whatever takes place. It also throws a light on
+the nature of the ordinary flame, which Mr. Tesla believes to be due to
+electrostatic molecular actions, which, if true, would lead directly to
+the idea that even chemical affinities might be electrostatic in their
+nature and that, as has already been suggested, molecular forces in
+general may be referable to one and the same cause. This singular
+phenomenon accounts in a plausible manner for the unexplained fact that
+buildings are frequently set on fire during thunder storms without
+having been at all struck by lightning. It may also explain the total
+disappearance of ships at sea.
+
+One of the striking proofs of the correctness of the ideas advanced by
+Mr. Tesla is the fact that, notwithstanding the employment of the most
+powerful electromagnetic inductive effects, but feeble luminosity is
+obtainable, and this only in close proximity to the source of
+disturbance; whereas, when the electrostatic effects are intensified,
+the same initial energy suffices to excite luminosity at considerable
+distances from the source. That there are only electrostatic effects
+active seems to be clearly proved by Mr. Tesla's experiments with an
+induction coil operated with alternating currents of very high
+frequency. He shows how tubes may be made to glow brilliantly at
+considerable distances from any object when placed in a powerful,
+rapidly alternating, electrostatic field, and he describes many
+interesting phenomena observed in such a field. His experiments open up
+the possibility of lighting an apartment by simply creating in it such
+an electrostatic field, and this, in a certain way, would appear to be
+the ideal method of lighting a room, as it would allow the illuminating
+device to be freely moved about. The power with which these exhausted
+tubes, devoid of any electrodes, light up is certainly remarkable.
+
+That the principle propounded by Mr. Tesla is a broad one is evident
+from the many ways in which it may be practically applied. We need only
+refer to the variety of the devices shown or described, all of which are
+novel in character and will, without doubt, lead to further important
+results at the hands of Mr. Tesla and other investigators. The
+experiment, for instance, of lighting up a single filament or block of
+refractory material with a single wire, is in itself sufficient to give
+Mr. Tesla's work the stamp of originality, and the numerous other
+experiments and effects which may be varied at will, are equally new and
+interesting. Thus, the incandescent filament spinning in an unexhausted
+globe, the well-known Crookes experiment on open circuit, and the many
+others suggested, will not fail to interest the reader. Mr. Tesla has
+made an exhaustive study of the various forms of the discharge presented
+by an induction coil when operated with these rapidly alternating
+currents, starting from the thread-like discharge and passing through
+various stages to the true electric flame.
+
+A point of great importance in the introduction of high tension
+alternating current which Mr. Tesla brings out is the necessity of
+carefully avoiding all gaseous matter in the high tension apparatus. He
+shows that, at least with very rapidly alternating currents of high
+potential, the discharge may work through almost any practicable
+thickness of the best insulators, if air is present. In such cases the
+air included within the apparatus is violently agitated and by molecular
+bombardment the parts may be so greatly heated as to cause a rupture of
+the insulation. The practical outcome of this is, that, whereas with
+steady currents, any kind of insulation may be used, with rapidly
+alternating currents oils will probably be the best to employ, a fact
+which has been observed, but not until now satisfactorily explained. The
+recognition of the above fact is of special importance in the
+construction of the costly commercial induction coils which are often
+rendered useless in an unaccountable manner. The truth of these views of
+Mr. Tesla is made evident by the interesting experiments illustrative
+of the behavior of the air between charged surfaces, the luminous
+streams formed by the charged molecules appearing even when great
+thicknesses of the best insulators are interposed between the charged
+surfaces. These luminous streams afford in themselves a very interesting
+study for the experimenter. With these rapidly alternating currents they
+become far more powerful and produce beautiful light effects when they
+issue from a wire, pinwheel or other object attached to a terminal of
+the coil; and it is interesting to note that they issue from a ball
+almost as freely as from a point, when the frequency is very high.
+
+From these experiments we also obtain a better idea of the importance of
+taking into account the capacity and self-induction in the apparatus
+employed and the possibilities offered by the use of condensers in
+conjunction with alternate currents, the employment of currents of high
+frequency, among other things, making it possible to reduce the
+condenser to practicable dimensions. Another point of interest and
+practical bearing is the fact, proved by Mr. Tesla, that for alternate
+currents, especially those of high frequency, insulators are required
+possessing a small specific inductive capacity, which at the same time
+have a high insulating power.
+
+Mr. Tesla also makes interesting and valuable suggestion in regard to
+the economical utilization of iron in machines and transformers. He
+shows how, by maintaining by continuous magnetization a flow of lines
+through the iron, the latter may be kept near its maximum permeability
+and a higher output and economy may be secured in such apparatus. This
+principle may prove of considerable commercial importance in the
+development of alternating systems. Mr. Tesla's suggestion that the same
+result can be secured by heating the iron by hysteresis and eddy
+currents, and increasing the permeability in this manner, while it may
+appear less practical, nevertheless opens another direction for
+investigation and improvement.
+
+The demonstration of the fact that with alternating currents of high
+frequency, sufficient energy may be transmitted under practicable
+conditions through the glass of an incandescent lamp by electrostatic or
+electromagnetic induction may lead to a departure in the construction of
+such devices. Another important experimental result achieved is the
+operation of lamps, and even motors, with the discharges of condensers,
+this method affording a means of converting direct or alternating
+currents. In this connection Mr. Tesla advocates the perfecting of
+apparatus capable of generating electricity of high tension from heat
+energy, believing this to be a better way of obtaining electrical energy
+for practical purposes, particularly for the production of light.
+
+While many were probably prepared to encounter curious phenomena of
+impedance in the use of a condenser discharged disruptively, the
+experiments shown were extremely interesting on account of their
+paradoxical character. The burning of an incandescent lamp at any candle
+power when connected across a heavy metal bar, the existence of nodes on
+the bar and the possibility of exploring the bar by means of an ordinary
+Cardew voltmeter, are all peculiar developments, but perhaps the most
+interesting observation is the phenomenon of impedance observed in the
+lamp with a straight filament, which remains dark while the bulb glows.
+
+Mr. Tesla's manner of operating an induction coil by means of the
+disruptive discharge, and thus obtaining enormous differences of
+potential from comparatively small and inexpensive coils, will be
+appreciated by experimenters and will find valuable application in
+laboratories. Indeed, his many suggestions and hints in regard to the
+construction and use of apparatus in these investigations will be highly
+valued and will aid materially in future research.
+
+The London lecture was delivered twice. In its first form, before the
+Institution of Electrical Engineers, it was in some respects an
+amplification of several points not specially enlarged upon in the New
+York lecture, but brought forward many additional discoveries and new
+investigations. Its repetition, in another form, at the Royal
+Institution, was due to Prof. Dewar, who with Lord Rayleigh, manifested
+a most lively interest in Mr. Tesla's work, and whose kindness
+illustrated once more the strong English love of scientific truth and
+appreciation of its votaries. As an indefatigable experimenter, Mr.
+Tesla was certainly nowhere more at home than in the haunts of Faraday,
+and as the guest of Faraday's successor. This Royal Institution lecture
+summed up the leading points of Mr. Tesla's work, in the high potential,
+high frequency field, and we may here avail ourselves of so valuable a
+summarization, in a simple form, of a subject by no means easy of
+comprehension until it has been thoroughly studied.
+
+In these London lectures, among the many notable points made was first,
+the difficulty of constructing the alternators to obtain the very high
+frequencies needed. To obtain the high frequencies it was necessary to
+provide several hundred polar projections, which were necessarily small
+and offered many drawbacks, and this the more as exceedingly high
+peripheral speeds had to be resorted to. In some of the first machines
+both armature and field had polar projections. These machines produced a
+curious noise, especially when the armature was started from the state
+of rest, the field being charged. The most efficient machine was found
+to be one with a drum armature, the iron body of which consisted of very
+thin wire annealed with special care. It was, of course, desirable to
+avoid the employment of iron in the armature, and several machines of
+this kind, with moving or stationary conductors were constructed, but
+the results obtained were not quite satisfactory, on account of the
+great mechanical and other difficulties encountered.
+
+The study of the properties of the high frequency currents obtained from
+these machines is very interesting, as nearly every experiment discloses
+something new. Two coils traversed by such a current attract or repel
+each other with a force which, owing to the imperfection of our sense of
+touch, seems continuous. An interesting observation, already noted under
+another form, is that a piece of iron, surrounded by a coil through
+which the current is passing appears to be continuously magnetized. This
+apparent continuity might be ascribed to the deficiency of the sense of
+touch, but there is evidence that in currents of such high frequencies
+one of the impulses preponderates over the other.
+
+As might be expected, conductors traversed by such currents are rapidly
+heated, owing to the increase of the resistance, and the heating effects
+are relatively much greater in the iron. The hysteresis losses in iron
+are so great that an iron core, even if finely subdivided, is heated in
+an incredibly short time. To give an idea of this, an ordinary iron wire
+1/16 inch in diameter inserted within a coil having 250 turns, with a
+current estimated to be five amperes passing through the coil, becomes
+within two seconds' time so hot as to scorch wood. Beyond a certain
+frequency, an iron core, no matter how finely subdivided, exercises a
+dampening effect, and it was easy to find a point at which the
+impedance of a coil was not affected by the presence of a core
+consisting of a bundle of very thin well annealed and varnished iron
+wires.
+
+Experiments with a telephone, a conductor in a strong magnetic field, or
+with a condenser or arc, seem to afford certain proof that sounds far
+above the usually accepted limit of hearing would be perceived if
+produced with sufficient power. The arc produced by these currents
+possesses several interesting features. Usually it emits a note the
+pitch of which corresponds to twice the frequency of the current, but if
+the frequency be sufficiently high it becomes noiseless, the limit of
+audition being determined principally by the linear dimensions of the
+arc. A curious feature of the arc is its persistency, which is due
+partly to the inability of the gaseous column to cool and increase
+considerably in resistance, as is the case with low frequencies, and
+partly to the tendency of such a high frequency machine to maintain a
+constant current.
+
+In connection with these machines the condenser affords a particularly
+interesting study. Striking effects are produced by proper adjustments
+of capacity and self-induction. It is easy to raise the electromotive
+force of the machine to many times the original value by simply
+adjusting the capacity of a condenser connected in the induced circuit.
+If the condenser be at some distance from the machine, the difference of
+potential on the terminals of the latter may be only a small fraction of
+that on the condenser.
+
+But the most interesting experiences are gained when the tension of the
+currents from the machine is raised by means of an induction coil. In
+consequence of the enormous rate of change obtainable in the primary
+current, much higher potential differences are obtained than with coils
+operated in the usual ways, and, owing to the high frequency, the
+secondary discharge possesses many striking peculiarities. Both the
+electrodes behave generally alike, though it appears from some
+observations that one current impulse preponderates over the other, as
+before mentioned.
+
+The physiological effects of the high tension discharge are found to be
+so small that the shock of the coil can be supported without any
+inconvenience, except perhaps a small burn produced by the discharge
+upon approaching the hand to one of the terminals. The decidedly smaller
+physiological effects of these currents are thought to be due either to
+a different distribution through the body or to the tissues acting as
+condensers. But in the case of an induction coil with a great many turns
+the harmlessness is principally due to the fact that but little energy
+is available in the external circuit when the same is closed through the
+experimenter's body, on account of the great impedance of the coil.
+
+In varying the frequency and strength of the currents through the
+primary of the coil, the character of the secondary discharge is greatly
+varied, and no less than five distinct forms are observed:--A weak,
+sensitive thread discharge, a powerful flaming discharge, and three
+forms of brush or streaming discharges. Each of these possesses certain
+noteworthy features, but the most interesting to study are the latter.
+
+Under certain conditions the streams, which are presumably due to the
+violent agitation of the air molecules, issue freely from all points of
+the coil, even through a thick insulation. If there is the smallest air
+space between the primary and secondary, they will form there and surely
+injure the coil by slowly warming the insulation. As they form even with
+ordinary frequencies when the potential is excessive, the air-space must
+be most carefully avoided. These high frequency streamers differ in
+aspect and properties from those produced by a static machine. The wind
+produced by them is small and should altogether cease if still
+considerably higher frequencies could be obtained. A peculiarity is that
+they issue as freely from surfaces as from points. Owing to this, a
+metallic vane, mounted in one of the terminals of the coil so as to
+rotate freely, and having one of its sides covered with insulation, is
+spun rapidly around. Such a vane would not rotate with a steady
+potential, but with a high frequency coil it will spin, even if it be
+entirely covered with insulation, provided the insulation on one side be
+either thicker or of a higher specific inductive capacity. A Crookes
+electric radiometer is also spun around when connected to one of the
+terminals of the coil, but only at very high exhaustion or at ordinary
+pressures.
+
+There is still another and more striking peculiarity of such a high
+frequency streamer, namely, it is hot. The heat is easily perceptible
+with frequencies of about 10,000, even if the potential is not
+excessively high. The heating effect is, of course, due to the molecular
+impacts and collisions. Could the frequency and potential be pushed far
+enough, then a brush could be produced resembling in every particular a
+flame and giving light and heat, yet without a chemical process taking
+place.
+
+The hot brush, when properly produced, resembles a jet of burning gas
+escaping under great pressure, and it emits an extraordinary strong
+smell of ozone. The great ozonizing action is ascribed to the fact that
+the agitation of the molecules of the air is more violent in such a
+brush than in the ordinary streamer of a static machine. But the most
+powerful brush discharges were produced by employing currents of much
+higher frequencies than it was possible to obtain by means of the
+alternators. These currents were obtained by disruptively discharging a
+condenser and setting up oscillations. In this manner currents of a
+frequency of several hundred thousand were obtained.
+
+Currents of this kind, Mr. Tesla pointed out, produce striking effects.
+At these frequencies, the impedance of a copper bar is so great that a
+potential difference of several hundred volts can be maintained between
+two points of a short and thick bar, and it is possible to keep an
+ordinary incandescent lamp burning at full candle power by attaching the
+terminals of the lamp to two points of the bar no more than a few inches
+apart. When the frequency is extremely high, nodes are found to exist on
+such a bar, and it is easy to locate them by means of a lamp.
+
+By converting the high tension discharges of a low frequency coil in
+this manner, it was found practicable to keep a few lamps burning on the
+ordinary circuit in the laboratory, and by bringing the undulation to a
+low pitch, it was possible to operate small motors.
+
+This plan likewise allows of converting high tension discharges of one
+direction into low tension unidirectional currents, by adjusting the
+circuit so that there are no oscillations. In passing the oscillating
+discharges through the primary of a specially constructed coil, it is
+easy to obtain enormous potential differences with only few turns of the
+secondary.
+
+Great difficulties were at first experienced in producing a successful
+coil on this plan. It was found necessary to keep all air, or gaseous
+matter in general, away from the charged surfaces, and oil immersion was
+resorted to. The wires used were heavily covered with gutta-percha and
+wound in oil, or the air was pumped out by means of a Sprengel pump. The
+general arrangement was the following:--An ordinary induction coil,
+operated from a low frequency alternator, was used to charge Leyden
+jars. The jars were made to discharge over a single or multiple gap
+through the primary of the second coil. To insure the action of the gap,
+the arc was blown out by a magnet or air blast. To adjust the potential
+in the secondary a small oil condenser was used, or polished brass
+spheres of different sizes were screwed on the terminals and their
+distance adjusted.
+
+When the conditions were carefully determined to suit each experiment,
+magnificent effects were obtained. Two wires, stretched through the
+room, each being connected to one of the terminals of the coil, emitted
+streams so powerful that the light from them allowed distinguishing the
+objects in the room; the wires became luminous even though covered with
+thick and most excellent insulation. When two straight wires, or two
+concentric circles of wire, are connected to the terminals, and set at
+the proper distance, a uniform luminous sheet is produced between them.
+It was possible in this way to cover an area of more than one meter
+square completely with the streams. By attaching to one terminal a large
+circle of wire and to the other terminal a small sphere, the streams are
+focused upon the sphere, produce a strongly lighted spot upon the same,
+and present the appearance of a luminous cone. A very thin wire glued
+upon a plate of hard rubber of great thickness, on the opposite side of
+which is fastened a tinfoil coating, is rendered intensely luminous when
+the coating is connected to the other terminal of the coil. Such an
+experiment can be performed also with low frequency currents, but much
+less satisfactorily.
+
+When the terminals of such a coil, even of a very small one, are
+separated by a rubber or glass plate, the discharge spreads over the
+plate in the form of streams, threads or brilliant sparks, and affords a
+magnificent display, which cannot be equaled by the largest coil
+operated in the usual ways. By a simple adjustment it is possible to
+produce with the coil a succession of brilliant sparks, exactly as with
+a Holtz machine.
+
+Under certain conditions, when the frequency of the oscillation is very
+great, white, phantom-like streams are seen to break forth from the
+terminals of the coil. The chief interesting feature about them is, that
+they stream freely against the outstretched hand or other conducting
+object without producing any sensation, and the hand may be approached
+very near to the terminal without a spark being induced to jump. This is
+due presumably to the fact that a considerable portion of the energy is
+carried away or dissipated in the streamers, and the difference of
+potential between the terminal and the hand is diminished.
+
+It is found in such experiments that the frequency of the vibration and
+the quickness of succession of the sparks between the knobs affect to a
+marked degree the appearance of the streams. When the frequency is very
+low, the air gives way in more or less the same manner as by a steady
+difference of potential, and the streams consist of distinct threads,
+generally mingled with thin sparks, which probably correspond to the
+successive discharges occurring between the knobs. But when the
+frequency is very high, and the arc of the discharge produces a sound
+which is loud and smooth (which indicates both that oscillation takes
+place and that the sparks succeed each other with great rapidity), then
+the luminous streams formed are perfectly uniform. They are generally of
+a purplish hue, but when the molecular vibration is increased by raising
+the potential, they assume a white color.
+
+The luminous intensity of the streams increases rapidly when the
+potential is increased; and with frequencies of only a few hundred
+thousand, could the coil be made to withstand a sufficiently high
+potential difference, there is no doubt that the space around a wire
+could be made to emit a strong light, merely by the agitation of the
+molecules of the air at ordinary pressure.
+
+Such discharges of very high frequency which render luminous the air at
+ordinary pressure we have very likely occasion to witness in the aurora
+borealis. From many of these experiments it seems reasonable to infer
+that sudden cosmic disturbances, such as eruptions on the sun, set the
+electrostatic charge of the earth in an extremely rapid vibration, and
+produce the glow by the violent agitation of the air in the upper and
+even in the lower strata. It is thought that if the frequency were low,
+or even more so if the charge were not at all vibrating, the lower dense
+strata would break down as in a lightning discharge. Indications of such
+breaking down have been repeatedly observed, but they can be attributed
+to the fundamental disturbances, which are few in number, for the
+superimposed vibration would be so rapid as not to allow a disruptive
+break.
+
+The study of these discharge phenomena has led Mr. Tesla to the
+recognition of some important facts. It was found, as already stated,
+that gaseous matter must be most carefully excluded from any dielectric
+which is subjected to great, rapidly changing electrostatic stresses.
+Since it is difficult to exclude the gas perfectly when solid insulators
+are used, it is necessary to resort to liquid dielectrics. When a solid
+dielectric is used, it matters little how thick and how good it is; if
+air be present, streamers form, which gradually heat the dielectric and
+impair its insulating power, and the discharge finally breaks through.
+Under ordinary conditions the best insulators are those which possess
+the highest specific inductive capacity, but such insulators are not the
+best to employ when working with these high frequency currents, for in
+most cases the higher specific inductive capacity is rather a
+disadvantage. The prime quality of the insulating medium for these
+currents is continuity. For this reason principally it is necessary to
+employ liquid insulators, such as oils. If two metal plates, connected
+to the terminals of the coil, are immersed in oil and set a distance
+apart, the coil may be kept working for any length of time without a
+break occurring, or without the oil being warmed, but if air bubbles are
+introduced, they become luminous; the air molecules, by their impact
+against the oil, heat it, and after some time cause the insulation to
+give way. If, instead of the oil, a solid plate of the best dielectric,
+even several times thicker than the oil intervening between the metal
+plates, is inserted between the latter, the air having free access to
+the charged surfaces, the dielectric invariably is warmed and breaks
+down.
+
+The employment of oil is advisable or necessary even with low
+frequencies, if the potentials are such that streamers form, but only in
+such cases, as is evident from the theory of the action. If the
+potentials are so low that streamers do not form, then it is even
+disadvantageous to employ oil, for it may, principally by confining the
+heat, be the cause of the breaking down of the insulation.
+
+The exclusion of gaseous matter is not only desirable on account of the
+safety of the apparatus, but also on account of economy, especially in a
+condenser, in which considerable waste of power may occur merely owing
+to the presence of air, if the electric density on the charged surfaces
+is great.
+
+In the course of these investigations a phenomenon of special scientific
+interest was observed. It may be ranked among the brush phenomena, in
+fact it is a kind of brush which forms at, or near, a single terminal in
+high vacuum. In a bulb with a conducting electrode, even if the latter
+be of aluminum, the brush has only a very short existence, but it can be
+preserved for a considerable length of time in a bulb devoid of any
+conducting electrode. To observe the phenomenon it is found best to
+employ a large spherical bulb having in its centre a small bulb
+supported on a tube sealed to the neck of the former. The large bulb
+being exhausted to a high degree, and the inside of the small bulb being
+connected to one of the terminals of the coil, under certain conditions
+there appears a misty haze around the small bulb, which, after passing
+through some stages, assumes the form of a brush, generally at right
+angles to the tube supporting the small bulb. When the brush assumes
+this form it may be brought to a state of extreme sensitiveness to
+electrostatic and magnetic influence. The bulb hanging straight down,
+and all objects being remote from it, the approach of the observer
+within a few paces will cause the brush to fly to the opposite side, and
+if he walks around the bulb it will always keep on the opposite side. It
+may begin to spin around the terminal long before it reaches that
+sensitive stage. When it begins to turn around, principally, but also
+before, it is affected by a magnet, and at a certain stage it is
+susceptible to magnetic influence to an astonishing degree. A small
+permanent magnet, with its poles at a distance of no more than two
+centimetres will affect it visibly at a distance of two metres, slowing
+down or accelerating the rotation according to how it is held relatively
+to the brush.
+
+When the bulb hangs with the globe down, the rotation is always
+clockwise. In the southern hemisphere it would occur in the opposite
+direction, and on the (magnetic) equator the brush should not turn at
+all. The rotation may be reversed by a magnet kept at some distance. The
+brush rotates best, seemingly, when it is at right angles to the lines
+of force of the earth. It very likely rotates, when at its maximum
+speed, in synchronism with the alternations, say, 10,000 times a second.
+The rotation can be slowed down or accelerated by the approach or
+recession of the observer, or any conducting body, but it cannot be
+reversed by putting the bulb in any position. Very curious experiments
+may be performed with the brush when in its most sensitive state. For
+instance, the brush resting in one position, the experimenter may, by
+selecting a proper position, approach the hand at a certain considerable
+distance to the bulb, and he may cause the brush to pass off by merely
+stiffening the muscles of the arm, the mere change of configuration of
+the arm and the consequent imperceptible displacement being sufficient
+to disturb the delicate balance. When it begins to rotate slowly, and
+the hands are held at a proper distance, it is impossible to make even
+the slightest motion without producing a visible effect upon the brush.
+A metal plate connected to the other terminal of the coil affects it at
+a great distance, slowing down the rotation often to one turn a second.
+
+Mr. Tesla hopes that this phenomenon will prove a valuable aid in the
+investigation of the nature of the forces acting in an electrostatic or
+magnetic field. If there is any motion which is measurable going on in
+the space, such a brush would be apt to reveal it. It is, so to speak, a
+beam of light, frictionless, devoid of inertia. On account of its
+marvellous sensitiveness to electrostatic or magnetic disturbances it
+may be the means of sending signals through submarine cables with any
+speed, and even of transmitting intelligence to a distance without
+wires.
+
+In operating an induction coil with these rapidly alternating currents,
+it is astonishing to note, for the first time, the great importance of
+the relation of capacity, self-induction, and frequency as bearing upon
+the general result. The combined effect of these elements produces many
+curious effects. For instance, two metal plates are connected to the
+terminals and set at a small distance, so that an arc is formed between
+them. This arc _prevents_ a strong current from flowing through the
+coil. If the arc be interrupted by the interposition of a glass plate,
+the capacity of the condenser obtained counteracts the self-induction,
+and a stronger current is made to pass. The effects of capacity are the
+most striking, for in these experiments, since the self-induction and
+frequency both are high, the critical capacity is very small, and need
+be but slightly varied to produce a very considerable change. The
+experimenter brings his body in contact with the terminals of the
+secondary of the coil, or attaches to one or both terminals insulated
+bodies of very small bulk, such as exhausted bulbs, and he produces a
+considerable rise or fall of potential on the secondary, and greatly
+affects the flow of the current through the primary coil.
+
+In many of the phenomena observed, the presence of the air, or,
+generally speaking, of a medium of a gaseous nature (using this term not
+to imply specific properties, but in contradistinction to homogeneity or
+perfect continuity) plays an important part, as it allows energy to be
+dissipated by molecular impact or bombardment. The action is thus
+explained:--When an insulated body connected to a terminal of the coil
+is suddenly charged to high potential, it acts inductively upon the
+surrounding air, or whatever gaseous medium there might be. The
+molecules or atoms which are near it are, of course, more attracted, and
+move through a greater distance than the further ones. When the nearest
+molecules strike the body they are repelled, and collisions occur at all
+distances within the inductive distance. It is now clear that, if the
+potential be steady, but little loss of energy can be caused in this
+way, for the molecules which are nearest to the body having had an
+additional charge imparted to them by contact, are not attracted until
+they have parted, if not with all, at least with most of the additional
+charge, which can be accomplished only after a great many collisions.
+This is inferred from the fact that with a steady potential there is but
+little loss in dry air. When the potential, instead of being steady, is
+alternating, the conditions are entirely different. In this case a
+rhythmical bombardment occurs, no matter whether the molecules after
+coming in contact with the body lose the imparted charge or not, and,
+what is more, if the charge is not lost, the impacts are all the more
+violent. Still, if the frequency of the impulses be very small, the loss
+caused by the impacts and collisions would not be serious unless the
+potential was excessive. But when extremely high frequencies and more or
+less high potentials are used, the loss may be very great. The total
+energy lost per unit of time is proportionate to the product of the
+number of impacts per second, or the frequency and the energy lost in
+each impact. But the energy of an impact must be proportionate to the
+square of the electric density of the body, on the assumption that the
+charge imparted to the molecule is proportionate to that density. It is
+concluded from this that the total energy lost must be proportionate to
+the product of the frequency and the square of the electric density; but
+this law needs experimental confirmation. Assuming the preceding
+considerations to be true, then, by rapidly alternating the potential of
+a body immersed in an insulating gaseous medium, any amount of energy
+may be dissipated into space. Most of that energy, then, is not
+dissipated in the form of long ether waves, propagated to considerable
+distance, as is thought most generally, but is consumed in impact and
+collisional losses--that is, heat vibrations--on the surface and in the
+vicinity of the body. To reduce the dissipation it is necessary to work
+with a small electric density--the smaller, the higher the frequency.
+
+The behavior of a gaseous medium to such rapid alternations of potential
+makes it appear plausible that electrostatic disturbances of the earth,
+produced by cosmic events, may have great influence upon the
+meteorological conditions. When such disturbances occur both the
+frequency of the vibrations of the charge and the potential are in all
+probability excessive, and the energy converted into heat may be
+considerable. Since the density must be unevenly distributed, either in
+consequence of the irregularity of the earth's surface, or on account of
+the condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable variations in
+the temperature and pressure of the atmosphere may in this manner be
+caused at any point of the surface of the earth. The variations may be
+gradual or very sudden, according to the nature of the original
+disturbance, and may produce rain and storms, or locally modify the
+weather in any way.
+
+From many experiences gathered in the course of these investigations it
+appears certain that in lightning discharges the air is an element of
+importance. For instance, during a storm a stream may form on a nail or
+pointed projection of a building. If lightning strikes somewhere in the
+neighborhood, the harmless static discharge may, in consequence of the
+oscillations set up, assume the character of a high-frequency streamer,
+and the nail or projection may be brought to a high temperature by the
+violent impact of the air molecules. Thus, it is thought, a building may
+be set on fire without the lightning striking it. In like manner small
+metallic objects may be fused and volatilized--as frequently occurs in
+lightning discharges--merely because they are surrounded by air. Were
+they immersed in a practically continuous medium, such as oil, they
+would probably be safe, as the energy would have to spend itself
+elsewhere.
+
+An instructive experience having a bearing on this subject is the
+following:--A glass tube of an inch or so in diameter and several inches
+long is taken, and a platinum wire sealed into it, the wire running
+through the center of the tube from end to end. The tube is exhausted to
+a moderate degree. If a steady current is passed through the wire it is
+heated uniformly in all parts and the gas in the tube is of no
+consequence. But if high frequency discharges are directed through the
+wire, it is heated more on the ends than in the middle portion, and if
+the frequency, or rate of charge, is high enough, the wire might as well
+be cut in the middle as not, for most of the heating on the ends is due
+to the rarefied gas. Here the gas might only act as a conductor of no
+impedance, diverting the current from the wire as the impedance of the
+latter is enormously increased, and merely heating the ends of the wire
+by reason of their resistance to the passage of the discharge. But it is
+not at all necessary that the gas in the tube should be conducting; it
+might be at an extremely low pressure, still the ends of the wire would
+be heated; however, as is ascertained by experience, only the two ends
+would in such case not be electrically connected through the gaseous
+medium. Now, what with these frequencies and potentials occurs in an
+exhausted tube, occurs in the lightning discharge at ordinary pressure.
+
+From the facility with which any amount of energy may be carried off
+through a gas, Mr. Tesla infers that the best way to render harmless a
+lightning discharge is to afford it in some way a passage through a
+volume of gas.
+
+The recognition of some of the above facts has a bearing upon
+far-reaching scientific investigations in which extremely high
+frequencies and potentials are used. In such cases the air is an
+important factor to be considered. So, for instance, if two wires are
+attached to the terminals of the coil, and the streamers issue from
+them, there is dissipation of energy in the form of heat and light, and
+the wires behave like a condenser of larger capacity. If the wires be
+immersed in oil, the dissipation of energy is prevented, or at least
+reduced, and the apparent capacity is diminished. The action of the air
+would seem to make it very difficult to tell, from the measured or
+computed capacity of a condenser in which the air is acted upon, its
+actual capacity or vibration period, especially if the condenser is of
+very small surface and is charged to a very high potential. As many
+important results are dependant upon the correctness of the estimation
+of the vibration period, this subject demands the most careful scrutiny
+of investigators.
+
+In Leyden jars the loss due to the presence of air is comparatively
+small, principally on account of the great surface of the coatings and
+the small external action, but if there are streamers on the top, the
+loss may be considerable, and the period of vibration is affected. In a
+resonator, the density is small, but the frequency is extreme, and may
+introduce a considerable error. It appears certain, at any rate, that
+the periods of vibration of a charged body in a gaseous and in a
+continuous medium, such as oil, are different, on account of the action
+of the former, as explained.
+
+Another fact recognized, which is of some consequence, is, that in
+similar investigations the general considerations of static screening
+are not applicable when a gaseous medium is present. This is evident
+from the following experiment:--A short and wide glass tube is taken and
+covered with a substantial coating of bronze powder, barely allowing the
+light to shine a little through. The tube is highly exhausted and
+suspended on a metallic clasp from the end of a wire. When the wire is
+connected with one of the terminals of the coil, the gas inside of the
+tube is lighted in spite of the metal coating. Here the metal evidently
+does not screen the gas inside as it ought to, even if it be very thin
+and poorly conducting. Yet, in a condition of rest the metal coating,
+however thin, screens the inside perfectly.
+
+One of the most interesting results arrived at in pursuing these
+experiments, is the demonstration of the fact that a gaseous medium,
+upon which vibration is impressed by rapid changes of electrostatic
+potential, is rigid. In illustration of this result an experiment made
+by Mr. Tesla may by cited:--A glass tube about one inch in diameter and
+three feet long, with outside condenser coatings on the ends, was
+exhausted to a certain point, when, the tube being suspended freely from
+a wire connecting the upper coating to one of the terminals of the coil,
+the discharge appeared in the form of a luminous thread passing through
+the axis of the tube. Usually the thread was sharply defined in the
+upper part of the tube and lost itself in the lower part. When a magnet
+or the finger was quickly passed near the upper part of the luminous
+thread, it was brought out of position by magnetic or electrostatic
+influence, and a transversal vibration like that of a suspended cord,
+with one or more distinct nodes, was set up, which lasted for a few
+minutes and gradually died out. By suspending from the lower condenser
+coating metal plates of different sizes, the speed of the vibration was
+varied. This vibration would seem to show beyond doubt that the thread
+possessed rigidity, at least to transversal displacements.
+
+Many experiments were tried to demonstrate this property in air at
+ordinary pressure. Though no positive evidence has been obtained, it is
+thought, nevertheless, that a high frequency brush or streamer, if the
+frequency could be pushed far enough, would be decidedly rigid. A small
+sphere might then be moved within it quite freely, but if thrown against
+it the sphere would rebound. An ordinary flame cannot possess rigidity
+to a marked degree because the vibration is directionless; but an
+electric arc, it is believed, must possess that property more or less. A
+luminous band excited in a bulb by repeated discharges of a Leyden jar
+must also possess rigidity, and if deformed and suddenly released should
+vibrate.
+
+From like considerations other conclusions of interest are reached. The
+most probable medium filling the space is one consisting of independent
+carriers immersed in an insulating fluid. If through this medium
+enormous electrostatic stresses are assumed to act, which vary rapidly
+in intensity, it would allow the motion of a body through it, yet it
+would be rigid and elastic, although the fluid itself might be devoid of
+these properties. Furthermore, on the assumption that the independent
+carriers are of any configuration such that the fluid resistance to
+motion in one direction is greater than in another, a stress of that
+nature would cause the carriers to arrange themselves in groups, since
+they would turn to each other their sides of the greatest electric
+density, in which position the fluid resistance to approach would be
+smaller than to receding. If in a medium of the above characteristics a
+brush would be formed by a steady potential, an exchange of the carriers
+would go on continually, and there would be less carriers per unit of
+volume in the brush than in the space at some distance from the
+electrode, this corresponding to rarefaction. If the potential were
+rapidly changing, the result would be very different; the higher the
+frequency of the pulses, the slower would be the exchange of the
+carriers; finally, the motion of translation through measurable space
+would cease, and, with a sufficiently high frequency and intensity of
+the stress, the carriers would be drawn towards the electrode, and
+compression would result.
+
+An interesting feature of these high frequency currents is that they
+allow of operating all kinds of devices by connecting the device with
+only one leading wire to the electric source. In fact, under certain
+conditions it may be more economical to supply the electrical energy
+with one lead than with two.
+
+An experiment of special interest shown by Mr. Tesla, is the running, by
+the use of only one insulated line, of a motor operating on the
+principle of the rotating magnetic field enunciated by Mr. Tesla. A
+simple form of such a motor is obtained by winding upon a laminated iron
+core a primary and close to it a secondary coil, closing the ends of the
+latter and placing a freely movable metal disc within the influence of
+the moving field. The secondary coil may, however, be omitted. When one
+of the ends of the primary coil of the motor is connected to one of the
+terminals of the high frequency coil and the other end to an insulated
+metal plate, which, it should be stated, is not absolutely necessary for
+the success of the experiment, the disc is set in rotation.
+
+Experiments of this kind seem to bring it within possibility to operate
+a motor at any point of the earth's surface from a central source,
+without any connection to the same except through the earth. If, by
+means of powerful machinery, rapid variations of the earth's potential
+were produced, a grounded wire reaching up to some height would be
+traversed by a current which could be increased by connecting the free
+end of the wire to a body of some size. The current might be converted
+to low tension and used to operate a motor or other device. The
+experiment, which would be one of great scientific interest, would
+probably best succeed on a ship at sea. In this manner, even if it were
+not possible to operate machinery, intelligence might be transmitted
+quite certainly.
+
+In the course of this experimental study special attention was devoted
+to the heating effects produced by these currents, which are not only
+striking, but open up the possibility of producing a more efficient
+illuminant. It is sufficient to attach to the coil terminal a thin wire
+or filament, to have the temperature of the latter perceptibly raised.
+If the wire or filament be enclosed in a bulb, the heating effect is
+increased by preventing the circulation of the air. If the air in the
+bulb be strongly compressed, the displacements are smaller, the impacts
+less violent, and the heating effect is diminished. On the contrary, if
+the air in the bulb be exhausted, an inclosed lamp filament is brought
+to incandescence, and any amount of light may thus be produced.
+
+The heating of the inclosed lamp filament depends on so many things of a
+different nature, that it is difficult to give a generally applicable
+rule under which the maximum heating occurs. As regards the size of the
+bulb, it is ascertained that at ordinary or only slightly differing
+atmospheric pressures, when air is a good insulator, the filament is
+heated more in a small bulb, because of the better confinement of heat
+in this case. At lower pressures, when air becomes conducting, the
+heating effect is greater in a large bulb, but at excessively high
+degrees of exhaustion there seems to be, beyond a certain and rather
+small size of the vessel, no perceptible difference in the heating.
+
+The shape of the vessel is also of some importance, and it has been
+found of advantage for reasons of economy to employ a spherical bulb
+with the electrode mounted in its centre, where the rebounding molecules
+collide.
+
+It is desirable on account of economy that all the energy supplied to
+the bulb from the source should reach without loss the body to be
+heated. The loss in conveying the energy from the source to the body may
+be reduced by employing thin wires heavily coated with insulation, and
+by the use of electrostatic screens. It is to be remarked, that the
+screen cannot be connected to the ground as under ordinary conditions.
+
+In the bulb itself a large portion of the energy supplied may be lost by
+molecular bombardment against the wire connecting the body to be heated
+with the source. Considerable improvement was effected by covering the
+glass stem containing the wire with a closely fitting conducting tube.
+This tube is made to project a little above the glass, and prevents the
+cracking of the latter near the heated body. The effectiveness of the
+conducting tube is limited to very high degrees of exhaustion. It
+diminishes the energy lost in bombardment for two reasons; first, the
+charge given up by the atoms spreads over a greater area, and hence the
+electric density at any point is small, and the atoms are repelled with
+less energy than if they would strike against a good insulator;
+secondly, as the tube is electrified by the atoms which first come in
+contact with it, the progress of the following atoms against the tube is
+more or less checked by the repulsion which the electrified tube must
+exert upon the similarly electrified atoms. This, it is thought,
+explains why the discharge through a bulb is established with much
+greater facility when an insulator, than when a conductor, is present.
+
+During the investigations a great many bulbs of different construction,
+with electrodes of different material, were experimented upon, and a
+number of observations of interest were made. Mr. Tesla has found that
+the deterioration of the electrode is the less, the higher the
+frequency. This was to be expected, as then the heating is effected by
+many small impacts, instead by fewer and more violent ones, which
+quickly shatter the structure. The deterioration is also smaller when
+the vibration is harmonic. Thus an electrode, maintained at a certain
+degree of heat, lasts much longer with currents obtained from an
+alternator, than with those obtained by means of a disruptive discharge.
+One of the most durable electrodes was obtained from strongly compressed
+carborundum, which is a kind of carbon recently produced by Mr. E. G.
+Acheson, of Monongahela City, Pa. From experience, it is inferred, that
+to be most durable, the electrode should be in the form of a sphere with
+a highly polished surface.
+
+In some bulbs refractory bodies were mounted in a carbon cup and put
+under the molecular impact. It was observed in such experiments that the
+carbon cup was heated at first, until a higher temperature was reached;
+then most of the bombardment was directed against the refractory body,
+and the carbon was relieved. In general, when different bodies were
+mounted in the bulb, the hardest fusible would be relieved, and would
+remain at a considerably lower temperature. This was necessitated by the
+fact that most of the energy supplied would find its way through the
+body which was more easily fused or "evaporated."
+
+Curiously enough it appeared in some of the experiments made, that a
+body was fused in a bulb under the molecular impact by evolution of less
+light than when fused by the application of heat in ordinary ways. This
+may be ascribed to a loosening of the structure of the body under the
+violent impacts and changing stresses.
+
+Some experiments seem to indicate that under certain conditions a body,
+conducting or nonconducting, may, when bombarded, emit light, which to
+all appearances is due to phosphorescence, but may in reality be caused
+by the incandescence of an infinitesimal layer, the mean temperature of
+the body being comparatively small. Such might be the case if each
+single rhythmical impact were capable of instantaneously exciting the
+retina, and the rhythm were just high enough to cause a continuous
+impression in the eye. According to this view, a coil operated by
+disruptive discharge would be eminently adapted to produce such a
+result, and it is found by experience that its power of exciting
+phosphorescence is extraordinarily great. It is capable of exciting
+phosphorescence at comparatively low degrees of exhaustion, and also
+projects shadows at pressures far greater than those at which the mean
+free path is comparable to the dimensions of the vessel. The latter
+observation is of some importance, inasmuch as it may modify the
+generally accepted views in regard to the "radiant state" phenomena.
+
+A thought which early and naturally suggested itself to Mr. Tesla, was
+to utilize the great inductive effects of high frequency currents to
+produce light in a sealed glass vessel without the use of leading in
+wires. Accordingly, many bulbs were constructed in which the energy
+necessary to maintain a button or filament at high incandescence, was
+supplied through the glass by either electrostatic or electrodynamic
+induction. It was easy to regulate the intensity of the light emitted by
+means of an externally applied condenser coating connected to an
+insulated plate, or simply by means of a plate attached to the bulb
+which at the same time performed the function of a shade.
+
+A subject of experiment, which has been exhaustively treated in England
+by Prof. J. J. Thomson, has been followed up independently by Mr. Tesla
+from the beginning of this study, namely, to excite by electrodynamic
+induction a luminous band in a closed tube or bulb. In observing the
+behavior of gases, and the luminous phenomena obtained, the importance
+of the electrostatic effects was noted and it appeared desirable to
+produce enormous potential differences, alternating with extreme
+rapidity. Experiments in this direction led to some of the most
+interesting results arrived at in the course of these investigations. It
+was found that by rapid alternations of a high electrostatic potential,
+exhausted tubes could be lighted at considerable distances from a
+conductor connected to a properly constructed coil, and that it was
+practicable to establish with the coil an alternating electrostatic
+field, acting through the whole room and lighting a tube wherever it was
+placed within the four walls. Phosphorescent bulbs may be excited in
+such a field, and it is easy to regulate the effect by connecting to the
+bulb a small insulated metal plate. It was likewise possible to maintain
+a filament or button mounted in a tube at bright incandescence, and, in
+one experiment, a mica vane was spun by the incandescence of a platinum
+wire.
+
+Coming now to the lecture delivered in Philadelphia and St. Louis, it
+may be remarked that to the superficial reader, Mr. Tesla's
+introduction, dealing with the importance of the eye, might appear as a
+digression, but the thoughtful reader will find therein much food for
+meditation and speculation. Throughout his discourse one can trace Mr.
+Tesla's effort to present in a popular way thoughts and views on the
+electrical phenomena which have in recent years captivated the
+scientific world, but of which the general public has even yet merely
+received an inkling. Mr. Tesla also dwells rather extensively on his
+well-known method of high-frequency conversion; and the large amount of
+detail information will be gratefully received by students and
+experimenters in this virgin field. The employment of apt analogies in
+explaining the fundamental principles involved makes it easy for all to
+gain a clear idea of their nature. Again, the ease with which, thanks to
+Mr. Tesla's efforts, these high-frequency currents may now be obtained
+from circuits carrying almost any kind of current, cannot fail to result
+in an extensive broadening of this field of research, which offers so
+many possibilities. Mr. Tesla, true philosopher as he is, does not
+hesitate to point out defects in some of his methods, and indicates the
+lines which to him seem the most promising. Particular stress is laid by
+him upon the employment of a medium in which the discharge electrodes
+should be immersed in order that this method of conversion may be
+brought to the highest perfection. He has evidently taken pains to give
+as much useful information as possible to those who wish to follow in
+his path, as he shows in detail the circuit arrangements to be adopted
+in all ordinary cases met with in practice, and although some of these
+methods were described by him two years before, the additional
+information is still timely and welcome.
+
+In his experiments he dwells first on some phenomena produced by
+electrostatic force, which he considers in the light of modern theories
+to be the most important force in nature for us to investigate. At the
+very outset he shows a strikingly novel experiment illustrating the
+effect of a rapidly varying electrostatic force in a gaseous medium, by
+touching with one hand one of the terminals of a 200,000 volt
+transformer and bringing the other hand to the opposite terminal. The
+powerful streamers which issued from his hand and astonished his
+audiences formed a capital illustration of some of the views advanced,
+and afforded Mr. Tesla an opportunity of pointing out the true reasons
+why, with these currents, such an amount of energy can be passed
+through the body with impunity. He then showed by experiment the
+difference between a steady and a rapidly varying force upon the
+dielectric. This difference is most strikingly illustrated in the
+experiment in which a bulb attached to the end of a wire in connection
+with one of the terminals of the transformer is ruptured, although all
+extraneous bodies are remote from the bulb. He next illustrates how
+mechanical motions are produced by a varying electrostatic force acting
+through a gaseous medium. The importance of the action of the air is
+particularly illustrated by an interesting experiment.
+
+Taking up another class of phenomena, namely, those of dynamic
+electricity, Mr. Tesla produced in a number of experiments a variety of
+effects by the employment of only a single wire with the evident intent
+of impressing upon his audience the idea that electric vibration or
+current can be transmitted with ease, without any return circuit; also
+how currents so transmitted can be converted and used for many practical
+purposes. A number of experiments are then shown, illustrating the
+effects of frequency, self-induction and capacity; then a number of ways
+of operating motive and other devices by the use of a single lead. A
+number of novel impedance phenomena are also shown which cannot fail to
+arouse interest.
+
+Mr. Tesla next dwelt upon a subject which he thinks of great importance,
+that is, electrical resonance, which he explained in a popular way. He
+expressed his firm conviction that by observing proper conditions,
+intelligence, and possibly even power, can be transmitted through the
+medium or through the earth; and he considers this problem worthy of
+serious and immediate consideration.
+
+Coming now to the light phenomena in particular, he illustrated the four
+distinct kinds of these phenomena in an original way, which to many must
+have been a revelation. Mr. Tesla attributes these light effects to
+molecular or atomic impacts produced by a varying electrostatic stress
+in a gaseous medium. He illustrated in a series of novel experiments the
+effect of the gas surrounding the conductor and shows beyond a doubt
+that with high frequency and high potential currents, the surrounding
+gas is of paramount importance in the heating of the conductor. He
+attributes the heating partially to a conduction current and partially
+to bombardment, and demonstrates that in many cases the heating may be
+practically due to the bombardment alone. He pointed out also that the
+skin effect is largely modified by the presence of the gas or of an
+atomic medium in general. He showed also some interesting experiments in
+which the effect of convection is illustrated. Probably one of the most
+curious experiments in this connection is that in which a thin platinum
+wire stretched along the axis of an exhausted tube is brought to
+incandescence at certain points corresponding to the position of the
+strić, while at others it remains dark. This experiment throws an
+interesting light upon the nature of the strić and may lead to important
+revelations.
+
+Mr. Tesla also demonstrated the dissipation of energy through an atomic
+medium and dwelt upon the behavior of vacuous space in conveying heat,
+and in this connection showed the curious behavior of an electrode
+stream, from which he concludes that the molecules of a gas probably
+cannot be acted upon directly at measurable distances.
+
+Mr. Tesla summarized the chief results arrived at in pursuing his
+investigations in a manner which will serve as a valuable guide to all
+who may engage in this work. Perhaps most interest will centre on his
+general statements regarding the phenomena of phosphorescence, the most
+important fact revealed in this direction being that when exciting a
+phosphorescent bulb a certain definite potential gives the most
+economical result.
+
+The lectures will now be presented in the order of their date of
+delivery.
+
+
+
+
+CHAPTER XXVI.
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY HIGH FREQUENCY AND THEIR
+APPLICATION TO METHODS OF ARTIFICIAL ILLUMINATION.[1]
+
+  [1] A lecture delivered before the American Institute of
+      Electrical Engineers, at Columbia College, N. Y.,
+      May 20, 1891.
+
+
+There is no subject more captivating, more worthy of study, than nature.
+To understand this great mechanism, to discover the forces which are
+active, and the laws which govern them, is the highest aim of the
+intellect of man.
+
+Nature has stored up in the universe infinite energy. The eternal
+recipient and transmitter of this infinite energy is the ether. The
+recognition of the existence of ether, and of the functions it performs,
+is one of the most important results of modern scientific research. The
+mere abandoning of the idea of action at a distance, the assumption of a
+medium pervading all space and connecting all gross matter, has freed
+the minds of thinkers of an ever present doubt, and, by opening a new
+horizon--new and unforeseen possibilities--has given fresh interest to
+phenomena with which we are familiar of old. It has been a great step
+towards the understanding of the forces of nature and their multifold
+manifestations to our senses. It has been for the enlightened student of
+physics what the understanding of the mechanism of the firearm or of the
+steam engine is for the barbarian. Phenomena upon which we used to look
+as wonders baffling explanation, we now see in a different light. The
+spark of an induction coil, the glow of an incandescent lamp, the
+manifestations of the mechanical forces of currents and magnets are no
+longer beyond our grasp; instead of the incomprehensible, as before,
+their observation suggests now in our minds a simple mechanism, and
+although as to its precise nature all is still conjecture, yet we know
+that the truth cannot be much longer hidden, and instinctively we feel
+that the understanding is dawning upon us. We still admire these
+beautiful phenomena, these strange forces, but we are helpless no
+longer; we can in a certain measure explain them, account for them, and
+we are hopeful of finally succeeding in unraveling the mystery which
+surrounds them.
+
+In how far we can understand the world around us is the ultimate thought
+of every student of nature. The coarseness of our senses prevents us
+from recognizing the ulterior construction of matter, and astronomy,
+this grandest and most positive of natural sciences, can only teach us
+something that happens, as it were, in our immediate neighborhood: of
+the remoter portions of the boundless universe, with its numberless
+stars and suns, we know nothing. But far beyond the limit of perception
+of our senses the spirit still can guide us, and so we may hope that
+even these unknown worlds--infinitely small and great--may in a measure
+become known to us. Still, even if this knowledge should reach us, the
+searching mind will find a barrier, perhaps forever unsurpassable, to
+the _true_ recognition of that which _seems_ to be, the mere
+_appearance_ of which is the only and slender basis of all our
+philosophy.
+
+Of all the forms of nature's immeasurable, all-pervading energy, which
+ever and ever changing and moving, like a soul animates the inert
+universe, electricity and magnetism are perhaps the most fascinating.
+The effects of gravitation, of heat and light we observe daily, and soon
+we get accustomed to them, and soon they lose for us the character of
+the marvelous and wonderful; but electricity and magnetism, with their
+singular relationship, with their seemingly dual character, unique among
+the forces in nature, with their phenomena of attractions, repulsions
+and rotations, strange manifestations of mysterious agents, stimulate
+and excite the mind to thought and research. What is electricity, and
+what is magnetism? These questions have been asked again and again. The
+most able intellects have ceaselessly wrestled with the problem; still
+the question has not as yet been fully answered. But while we cannot
+even to-day state what these singular forces are, we have made good
+headway towards the solution of the problem. We are now confident that
+electric and magnetic phenomena are attributable to ether, and we are
+perhaps justified in saying that the effects of static electricity are
+effects of ether under strain, and those of dynamic electricity and
+electro-magnetism effects of ether in motion. But this still leaves the
+question, as to what electricity and magnetism are, unanswered.
+
+First, we naturally inquire, What is electricity, and is there such a
+thing as electricity? In interpreting electric phenomena, we may speak
+of electricity or of an electric condition, state or effect. If we speak
+of electric effects we must distinguish two such effects, opposite in
+character and neutralizing each other, as observation shows that two
+such opposite effects exist. This is unavoidable, for in a medium of the
+properties of ether, we cannot possibly exert a strain, or produce a
+displacement or motion of any kind, without causing in the surrounding
+medium an equivalent and opposite effect. But if we speak of
+electricity, meaning a _thing_, we must, I think, abandon the idea of
+two electricities, as the existence of two such things is highly
+improbable. For how can we imagine that there should be two things,
+equivalent in amount, alike in their properties, but of opposite
+character, both clinging to matter, both attracting and completely
+neutralizing each other? Such an assumption, though suggested by many
+phenomena, though most convenient for explaining them, has little to
+commend it. If there _is_ such a thing as electricity, there can be only
+_one_ such thing, and excess and want of that one thing, possibly; but
+more probably its condition determines the positive and negative
+character. The old theory of Franklin, though falling short in some
+respects, is, from a certain point of view, after all, the most
+plausible one. Still, in spite of this, the theory of the two
+electricities is generally accepted, as it apparently explains electric
+phenomena in a more satisfactory manner. But a theory which better
+explains the facts is not necessarily true. Ingenious minds will invent
+theories to suit observation, and almost every independent thinker has
+his own views on the subject.
+
+It is not with the object of advancing an opinion, but with the desire
+of acquainting you better with some of the results, which I will
+describe, to show you the reasoning I have followed, the departures I
+have made--that I venture to express, in a few words, the views and
+convictions which have led me to these results.
+
+I adhere to the idea that there is a thing which we have been in the
+habit of calling electricity. The question is, What is that thing? or,
+What, of all things, the existence of which we know, have we the best
+reason to call electricity? We know that it acts like an incompressible
+fluid; that there must be a constant quantity of it in nature; that it
+can be neither produced nor destroyed; and, what is more important, the
+electro-magnetic theory of light and all facts observed teach us that
+electric and ether phenomena are identical. The idea at once suggests
+itself, therefore, that electricity might be called ether. In fact, this
+view has in a certain sense been advanced by Dr. Lodge. His interesting
+work has been read by everyone and many have been convinced by his
+arguments. His great ability and the interesting nature of the subject,
+keep the reader spellbound; but when the impressions fade, one realizes
+that he has to deal only with ingenious explanations. I must confess,
+that I cannot believe in two electricities, much less in a
+doubly-constituted ether. The puzzling behavior of the ether as a solid
+to waves of light and heat, and as a fluid to the motion of bodies
+through it, is certainly explained in the most natural and satisfactory
+manner by assuming it to be in motion, as Sir William Thomson has
+suggested; but regardless of this, there is nothing which would enable
+us to conclude with certainty that, while a fluid is not capable of
+transmitting transverse vibrations of a few hundred or thousand per
+second, it might not be capable of transmitting such vibrations when
+they range into hundreds of million millions per second. Nor can anyone
+prove that there are transverse ether waves emitted from an alternate
+current machine, giving a small number of alternations per second; to
+such slow disturbances, the ether, if at rest, may behave as a true
+fluid.
+
+Returning to the subject, and bearing in mind that the existence of two
+electricities is, to say the least, highly improbable, we must remember,
+that we have no evidence of electricity, nor can we hope to get it,
+unless gross matter is present. Electricity, therefore, cannot be called
+ether in the broad sense of the term; but nothing would seem to stand in
+the way of calling electricity ether associated with matter, or bound
+ether; or, in other words, that the so-called static charge of the
+molecule is ether associated in some way with the molecule. Looking at
+it in that light, we would be justified in saying, that electricity is
+concerned in all molecular actions.
+
+Now, precisely what the ether surrounding the molecules is, wherein it
+differs from ether in general, can only be conjectured. It cannot differ
+in density, ether being incompressible: it must, therefore, be under
+some strain or in motion, and the latter is the most probable. To
+understand its functions, it would be necessary to have an exact idea of
+the physical construction of matter, of which, of course, we can only
+form a mental picture.
+
+But of all the views on nature, the one which assumes one matter and one
+force, and a perfect uniformity throughout, is the most scientific and
+most likely to be true. An infinitesimal world, with the molecules and
+their atoms spinning and moving in orbits, in much the same manner as
+celestial bodies, carrying with them and probably spinning with them
+ether, or in other words, carrying with them static charges, seems to my
+mind the most probable view, and one which, in a plausible manner,
+accounts for most of the phenomena observed. The spinning of the
+molecules and their ether sets up the ether tensions or electrostatic
+strains; the equalization of ether tensions sets up ether motions or
+electric currents, and the orbital movements produce the effects of
+electro and permanent magnetism.
+
+About fifteen years ago, Prof. Rowland demonstrated a most interesting
+and important fact, namely, that a static charge carried around produces
+the effects of an electric current. Leaving out of consideration the
+precise nature of the mechanism, which produces the attraction and
+repulsion of currents, and conceiving the electrostatically charged
+molecules in motion, this experimental fact gives us a fair idea of
+magnetism. We can conceive lines or tubes of force which physically
+exist, being formed of rows of directed moving molecules; we can see
+that these lines must be closed, that they must tend to shorten and
+expand, etc. It likewise explains in a reasonable way, the most puzzling
+phenomenon of all, permanent magnetism, and, in general, has all the
+beauties of the Ampere theory without possessing the vital defect of the
+same, namely, the assumption of molecular currents. Without enlarging
+further upon the subject, I would say, that I look upon all
+electrostatic, current and magnetic phenomena as being due to
+electrostatic molecular forces.
+
+The preceding remarks I have deemed necessary to a full understanding of
+the subject as it presents itself to my mind.
+
+Of all these phenomena the most important to study are the current
+phenomena, on account of the already extensive and ever-growing use of
+currents for industrial purposes. It is now a century since the first
+practical source of current was produced, and, ever since, the phenomena
+which accompany the flow of currents have been diligently studied, and
+through the untiring efforts of scientific men the simple laws which
+govern them have been discovered. But these laws are found to hold good
+only when the currents are of a steady character. When the currents are
+rapidly varying in strength, quite different phenomena, often
+unexpected, present themselves, and quite different laws hold good,
+which even now have not been determined as fully as is desirable, though
+through the work, principally, of English scientists, enough knowledge
+has been gained on the subject to enable us to treat simple cases which
+now present themselves in daily practice.
+
+The phenomena which are peculiar to the changing character of the
+currents are greatly exalted when the rate of change is increased, hence
+the study of these currents is considerably facilitated by the
+employment of properly constructed apparatus. It was with this and other
+objects in view that I constructed alternate current machines capable of
+giving more than two million reversals of current per minute, and to
+this circumstance it is principally due, that I am able to bring to your
+attention some of the results thus far reached, which I hope will prove
+to be a step in advance on account of their direct bearing upon one of
+the most important problems, namely, the production of a practical and
+efficient source of light.
+
+The study of such rapidly alternating currents is very interesting.
+Nearly every experiment discloses something new. Many results may, of
+course, be predicted, but many more are unforeseen. The experimenter
+makes many interesting observations. For instance, we take a piece of
+iron and hold it against a magnet. Starting from low alternations and
+running up higher and higher we feel the impulses succeed each other
+faster and faster, get weaker and weaker, and finally disappear. We then
+observe a continuous pull; the pull, of course, is not continuous; it
+only appears so to us; our sense of touch is imperfect.
+
+We may next establish an arc between the electrodes and observe, as the
+alternations rise, that the note which accompanies alternating arcs gets
+shriller and shriller, gradually weakens, and finally ceases. The air
+vibrations, of course, continue, but they are too weak to be perceived;
+our sense of hearing fails us.
+
+We observe the small physiological effects, the rapid heating of the
+iron cores and conductors, curious inductive effects, interesting
+condenser phenomena, and still more interesting light phenomena with a
+high tension induction coil. All these experiments and observations
+would be of the greatest interest to the student, but their description
+would lead me too far from the principal subject. Partly for this
+reason, and partly on account of their vastly greater importance, I will
+confine myself to the description of the light effects produced by these
+currents.
+
+In the experiments to this end a high tension induction coil or
+equivalent apparatus for converting currents of comparatively low into
+currents of high tension is used.
+
+If you will be sufficiently interested in the results I shall describe
+as to enter into an experimental study of this subject; if you will be
+convinced of the truth of the arguments I shall advance--your aim will
+be to produce high frequencies and high potentials; in other words,
+powerful electrostatic effects. You will then encounter many
+difficulties, which, if completely overcome, would allow us to produce
+truly wonderful results.
+
+First will be met the difficulty of obtaining the required frequencies
+by means of mechanical apparatus, and, if they be obtained otherwise,
+obstacles of a different nature will present themselves. Next it will be
+found difficult to provide the requisite insulation without considerably
+increasing the size of the apparatus, for the potentials required are
+high, and, owing to the rapidity of the alternations, the insulation
+presents peculiar difficulties. So, for instance, when a gas is present,
+the discharge may work, by the molecular bombardment of the gas and
+consequent heating, through as much as an inch of the best solid
+insulating material, such as glass, hard rubber, porcelain, sealing wax,
+etc.; in fact, through any known insulating substance. The chief
+requisite in the insulation of the apparatus is, therefore, the
+exclusion of any gaseous matter.
+
+In general my experience tends to show that bodies which possess the
+highest specific inductive capacity, such as glass, afford a rather
+inferior insulation to others, which, while they are good insulators,
+have a much smaller specific inductive capacity, such as oils, for
+instance, the dielectric losses being no doubt greater in the former.
+The difficulty of insulating, of course, only exists when the potentials
+are excessively high, for with potentials such as a few thousand volts
+there is no particular difficulty encountered in conveying currents from
+a machine giving, say, 20,000 alternations per second, to quite a
+distance. This number of alternations, however, is by far too small for
+many purposes, though quite sufficient for some practical applications.
+This difficulty of insulating is fortunately not a vital drawback; it
+affects mostly the size of the apparatus, for, when excessively high
+potentials would be used, the light-giving devices would be located not
+far from the apparatus, and often they would be quite close to it. As
+the air-bombardment of the insulated wire is dependent on condenser
+action, the loss may be reduced to a trifle by using excessively thin
+wires heavily insulated.
+
+Another difficulty will be encountered in the capacity and
+self-induction necessarily possessed by the coil. If the coil be large,
+that is, if it contain a great length of wire, it will be generally
+unsuited for excessively high frequencies; if it be small, it may be
+well adapted for such frequencies, but the potential might then not be
+as high as desired. A good insulator, and preferably one possessing a
+small specific inductive capacity, would afford a two-fold advantage.
+First, it would enable us to construct a very small coil capable of
+withstanding enormous differences of potential; and secondly, such a
+small coil, by reason of its smaller capacity and self-induction, would
+be capable of a quicker and more vigorous vibration. The problem then of
+constructing a coil or induction apparatus of any kind possessing the
+requisite qualities I regard as one of no small importance, and it has
+occupied me for a considerable time.
+
+The investigator who desires to repeat the experiments which I will
+describe, with an alternate current machine, capable of supplying
+currents of the desired frequency, and an induction coil, will do well
+to take the primary coil out and mount the secondary in such a manner as
+to be able to look through the tube upon which the secondary is wound.
+He will then be able to observe the streams which pass from the primary
+to the insulating tube, and from their intensity he will know how far he
+can strain the coil. Without this precaution he is sure to injure the
+insulation. This arrangement permits, however, an easy exchange of the
+primaries, which is desirable in these experiments.
+
+The selection of the type of machine best suited for the purpose must be
+left to the judgment of the experimenter. There are here illustrated
+three distinct types of machines, which, besides others, I have used in
+my experiments.
+
+Fig. 97 represents the machine used in my experiments before this
+Institute. The field magnet consists of a ring of wrought iron with 384
+pole projections. The armature comprises a steel disc to which is
+fastened a thin, carefully welded rim of wrought iron. Upon the rim are
+wound several layers of fine, well annealed iron wire, which, when
+wound, is passed through shellac. The armature wires are wound around
+brass pins, wrapped with silk thread. The diameter of the armature wire
+in this type of machine should not be more than 1/6 of the thickness of
+the pole projections, else the local action will be considerable.
+
+[Illustration: FIG. 97.]
+
+Fig. 98 represents a larger machine of a different type. The field
+magnet of this machine consists of two like parts which either enclose
+an exciting coil, or else are independently wound. Each part has 480
+pole projections, the projections of one facing those of the other. The
+armature consists of a wheel of hard bronze, carrying the conductors
+which revolve between the projections of the field magnet. To wind the
+armature conductors, I have found it most convenient to proceed in the
+following manner. I construct a ring of hard bronze of the required
+size. This ring and the rim of the wheel are provided with the proper
+number of pins, and both fastened upon a plate. The armature conductors
+being wound, the pins are cut off and the ends of the conductors
+fastened by two rings which screw to the bronze ring and the rim of the
+wheel, respectively. The whole may then be taken off and forms a solid
+structure. The conductors in such a type of machine should consist of
+sheet copper, the thickness of which, of course, depends on the
+thickness of the pole projections; or else twisted thin wires should be
+employed.
+
+Fig. 99 is a smaller machine, in many respects similar to the former,
+only here the armature conductors and the exciting coil are kept
+stationary, while only a block of wrought iron is revolved.
+
+[Illustration: FIG. 98.]
+
+It would be uselessly lengthening this description were I to dwell more
+on the details of construction of these machines. Besides, they have
+been described somewhat more elaborately in _The Electrical Engineer_,
+of March 18, 1891. I deem it well, however, to call the attention of the
+investigator to two things, the importance of which, though self
+evident, he is nevertheless apt to underestimate; namely, to the local
+action in the conductors which must be carefully avoided, and to the
+clearance, which must be small. I may add, that since it is desirable to
+use very high peripheral speeds, the armature should be of very large
+diameter in order to avoid impracticable belt speeds. Of the several
+types of these machines which have been constructed by me, I have found
+that the type illustrated in Fig. 97 caused me the least trouble in
+construction, as well as in maintenance, and on the whole, it has been a
+good experimental machine.
+
+In operating an induction coil with very rapidly alternating currents,
+among the first luminous phenomena noticed are naturally those presented
+by the high-tension discharge. As the number of alternations per second
+is increased, or as--the number being high--the current through the
+primary is varied, the discharge gradually changes in appearance. It
+would be difficult to describe the minor changes which occur, and the
+conditions which bring them about, but one may note five distinct forms
+of the discharge.
+
+[Illustration: FIG. 99.]
+
+First, one may observe a weak, sensitive discharge in the form of a
+thin, feeble-colored thread. (Fig. 100a.) It always occurs when, the
+number of alternations per second being high, the current through the
+primary is very small. In spite of the excessively small current, the
+rate of change is great, and the difference of potential at the
+terminals of the secondary is therefore considerable, so that the arc is
+established at great distances; but the quantity of "electricity" set in
+motion is insignificant, barely sufficient to maintain a thin,
+threadlike arc. It is excessively sensitive and may be made so to such a
+degree that the mere act of breathing near the coil will affect it, and
+unless it is perfectly well protected from currents of air, it wriggles
+around constantly. Nevertheless, it is in this form excessively
+persistent, and when the terminals are approached to, say, one-third of
+the striking distance, it can be blown out only with difficulty. This
+exceptional persistency, when short, is largely due to the arc being
+excessively thin; presenting, therefore, a very small surface to the
+blast. Its great sensitiveness, when very long, is probably due to the
+motion of the particles of dust suspended in the air.
+
+[Illustration: FIG. 100a.]
+
+[Illustration: FIG. 100b.]
+
+When the current through the primary is increased, the discharge gets
+broader and stronger, and the effect of the capacity of the coil becomes
+visible until, finally, under proper conditions, a white flaming arc,
+Fig. 100 B, often as thick as one's finger, and striking across the
+whole coil, is produced. It develops remarkable heat, and may be further
+characterized by the absence of the high note which accompanies the
+less powerful discharges. To take a shock from the coil under these
+conditions would not be advisable, although under different conditions,
+the potential being much higher, a shock from the coil may be taken with
+impunity. To produce this kind of discharge the number of alternations
+per second must not be too great for the coil used; and, generally
+speaking, certain relations between capacity, self-induction and
+frequency must be observed.
+
+The importance of these elements in an alternate current circuit is now
+well-known, and under ordinary conditions, the general rules are
+applicable. But in an induction coil exceptional conditions prevail.
+First, the self-induction is of little importance before the arc is
+established, when it asserts itself, but perhaps never as prominently as
+in ordinary alternate current circuits, because the capacity is
+distributed all along the coil, and by reason of the fact that the coil
+usually discharges through very great resistances; hence the currents
+are exceptionally small. Secondly, the capacity goes on increasing
+continually as the potential rises, in consequence of absorption which
+takes place to a considerable extent. Owing to this there exists no
+critical relationship between these quantities, and ordinary rules would
+not seem to be applicable. As the potential is increased either in
+consequence of the increased frequency or of the increased current
+through the primary, the amount of the energy stored becomes greater and
+greater, and the capacity gains more and more in importance. Up to a
+certain point the capacity is beneficial, but after that it begins to be
+an enormous drawback. It follows from this that each coil gives the best
+result with a given frequency and primary current. A very large coil,
+when operated with currents of very high frequency, may not give as much
+as 1/8 inch spark. By adding capacity to the terminals, the condition
+may be improved, but what the coil really wants is a lower frequency.
+
+When the flaming discharge occurs, the conditions are evidently such
+that the greatest current is made to flow through the circuit. These
+conditions may be attained by varying the frequency within wide limits,
+but the highest frequency at which the flaming arc can still be
+produced, determines, for a given primary current, the maximum striking
+distance of the coil. In the flaming discharge the _eclat_ effect of the
+capacity is not perceptible; the rate at which the energy is being
+stored then just equals the rate at which it can be disposed of through
+the circuit. This kind of discharge is the severest test for a coil; the
+break, when it occurs, is of the nature of that in an overcharged Leyden
+jar. To give a rough approximation I would state that, with an ordinary
+coil of, say 10,000 ohms resistance, the most powerful arc would be
+produced with about 12,000 alternations per second.
+
+When the frequency is increased beyond that rate, the potential, of
+course, rises, but the striking distance may, nevertheless, diminish,
+paradoxical as it may seem. As the potential rises the coil attains more
+and more the properties of a static machine until, finally, one may
+observe the beautiful phenomenon of the streaming discharge, Fig. 101,
+which may be produced across the whole length of the coil. At that stage
+streams begin to issue freely from all points and projections. These
+streams will also be seen to pass in abundance in the space between the
+primary and the insulating tube. When the potential is excessively high
+they will always appear, even if the frequency be low, and even if the
+primary be surrounded by as much as an inch of wax, hard rubber, glass,
+or any other insulating substance. This limits greatly the output of the
+coil, but I will later show how I have been able to overcome to a
+considerable extent this disadvantage in the ordinary coil.
+
+Besides the potential, the intensity of the streams depends on the
+frequency; but if the coil be very large they show themselves, no matter
+how low the frequencies used. For instance, in a very large coil of a
+resistance of 67,000 ohms, constructed by me some time ago, they appear
+with as low as 100 alternations per second and less, the insulation of
+the secondary being 3/4 inch of ebonite. When very intense they produce
+a noise similar to that produced by the charging of a Holtz machine, but
+much more powerful, and they emit a strong smell of ozone. The lower the
+frequency, the more apt they are to suddenly injure the coil. With
+excessively high frequencies they may pass freely without producing any
+other effect than to heat the insulation slowly and uniformly.
+
+[Illustration: FIG. 101.]
+
+[Illustration: FIG. 102.]
+
+The existence of these streams shows the importance of constructing an
+expensive coil so as to permit of one's seeing through the tube
+surrounding the primary, and the latter should be easily exchangeable;
+or else the space between the primary and secondary should be completely
+filled up with insulating material so as to exclude all air. The
+non-observance of this simple rule in the construction of commercial
+coils is responsible for the destruction of many an expensive coil.
+
+At the stage when the streaming discharge occurs, or with somewhat
+higher frequencies, one may, by approaching the terminals quite nearly,
+and regulating properly the effect of capacity, produce a veritable
+spray of small silver-white sparks, or a bunch of excessively thin
+silvery threads (Fig. 102) amidst a powerful brush--each spark or thread
+possibly corresponding to one alternation. This, when produced under
+proper conditions, is probably the most beautiful discharge, and when an
+air blast is directed against it, it presents a singular appearance. The
+spray of sparks, when received through the body, causes some
+inconvenience, whereas, when the discharge simply streams, nothing at
+all is likely to be felt if large conducting objects are held in the
+hands to protect them from receiving small burns.
+
+If the frequency is still more increased, then the coil refuses to give
+any spark unless at comparatively small distances, and the fifth typical
+form of discharge may be observed (Fig. 103). The tendency to stream out
+and dissipate is then so great that when the brush is produced at one
+terminal no sparking occurs, even if, as I have repeatedly tried, the
+hand, or any conducting object, is held within the stream; and, what is
+more singular, the luminous stream is not at all easily deflected by the
+approach of a conducting body.
+
+[Illustration: FIG. 103.]
+
+[Illustration: FIG. 104.]
+
+At this stage the streams seemingly pass with the greatest freedom
+through considerable thicknesses of insulators, and it is particularly
+interesting to study their behavior. For this purpose it is convenient
+to connect to the terminals of the coil two metallic spheres which may
+be placed at any desired distance, Fig. 104. Spheres are preferable to
+plates, as the discharge can be better observed. By inserting dielectric
+bodies between the spheres, beautiful discharge phenomena may be
+observed. If the spheres be quite close and a spark be playing between
+them, by interposing a thin plate of ebonite between the spheres the
+spark instantly ceases and the discharge spreads into an intensely
+luminous circle several inches in diameter, provided the spheres are
+sufficiently large. The passage of the streams heats, and, after a
+while, softens, the rubber so much that two plates may be made to stick
+together in this manner. If the spheres are so far apart that no spark
+occurs, even if they are far beyond the striking distance, by inserting
+a thick plate of glass the discharge is instantly induced to pass from
+the spheres to the glass in the form of luminous streams. It appears
+almost as though these streams pass _through_ the dielectric. In reality
+this is not the case, as the streams are due to the molecules of the air
+which are violently agitated in the space between the oppositely charged
+surfaces of the spheres. When no dielectric other than air is present,
+the bombardment goes on, but is too weak to be visible; by inserting a
+dielectric the inductive effect is much increased, and besides, the
+projected air molecules find an obstacle and the bombardment becomes so
+intense that the streams become luminous. If by any mechanical means we
+could effect such a violent agitation of the molecules we could produce
+the same phenomenon. A jet of air escaping through a small hole under
+enormous pressure and striking against an insulating substance, such as
+glass, may be luminous in the dark, and it might be possible to produce
+a phosphorescence of the glass or other insulators in this manner.
+
+The greater the specific inductive capacity of the interposed
+dielectric, the more powerful the effect produced. Owing to this, the
+streams show themselves with excessively high potentials even if the
+glass be as much as one and one-half to two inches thick. But besides
+the heating due to bombardment, some heating goes on undoubtedly in the
+dielectric, being apparently greater in glass than in ebonite. I
+attribute this to the greater specific inductive capacity of the glass,
+in consequence of which, with the same potential difference, a greater
+amount of energy is taken up in it than in rubber. It is like connecting
+to a battery a copper and a brass wire of the same dimensions. The
+copper wire, though a more perfect conductor, would heat more by reason
+of its taking more current. Thus what is otherwise considered a virtue
+of the glass is here a defect. Glass usually gives way much quicker than
+ebonite; when it is heated to a certain degree, the discharge suddenly
+breaks through at one point, assuming then the ordinary form of an arc.
+
+The heating effect produced by molecular bombardment of the dielectric
+would, of course, diminish as the pressure of the air is increased, and
+at enormous pressure it would be negligible, unless the frequency would
+increase correspondingly.
+
+It will be often observed in these experiments that when the spheres are
+beyond the striking distance, the approach of a glass plate, for
+instance, may induce the spark to jump between the spheres. This occurs
+when the capacity of the spheres is somewhat below the critical value
+which gives the greatest difference of potential at the terminals of the
+coil. By approaching a dielectric, the specific inductive capacity of
+the space between the spheres is increased, producing the same effect as
+if the capacity of the spheres were increased. The potential at the
+terminals may then rise so high that the air space is cracked. The
+experiment is best performed with dense glass or mica.
+
+Another interesting observation is that a plate of insulating material,
+when the discharge is passing through it, is strongly attracted by
+either of the spheres, that is by the nearer one, this being obviously
+due to the smaller mechanical effect of the bombardment on that side,
+and perhaps also to the greater electrification.
+
+From the behavior of the dielectrics in these experiments, we may
+conclude that the best insulator for these rapidly alternating currents
+would be the one possessing the smallest specific inductive capacity and
+at the same time one capable of withstanding the greatest differences of
+potential; and thus two diametrically opposite ways of securing the
+required insulation are indicated, namely, to use either a perfect
+vacuum or a gas under great pressure; but the former would be
+preferable. Unfortunately neither of these two ways is easily carried
+out in practice.
+
+It is especially interesting to note the behavior of an excessively high
+vacuum in these experiments. If a test tube, provided with external
+electrodes and exhausted to the highest possible degree, be connected to
+the terminals of the coil, Fig. 105, the electrodes of the tube are
+instantly brought to a high temperature and the glass at each end of the
+tube is rendered intensely phosphorescent, but the middle appears
+comparatively dark, and for a while remains cool.
+
+When the frequency is so high that the discharge shown in Fig. 103 is
+observed, considerable dissipation no doubt occurs in the coil.
+Nevertheless the coil may be worked for a long time, as the heating is
+gradual.
+
+In spite of the fact that the difference of potential may be enormous,
+little is felt when the discharge is passed through the body, provided
+the hands are armed. This is to some extent due to the higher frequency,
+but principally to the fact that less energy is available externally,
+when the difference of potential reaches an enormous value, owing to the
+circumstance that, with the rise of potential, the energy absorbed in
+the coil increases as the square of the potential. Up to a certain point
+the energy available externally increases with the rise of potential,
+then it begins to fall off rapidly. Thus, with the ordinary high tension
+induction coil, the curious paradox exists, that, while with a given
+current through the primary the shock might be fatal, with many times
+that current it might be perfectly harmless, even if the frequency be
+the same. With high frequencies and excessively high potentials when the
+terminals are not connected to bodies of some size, practically all the
+energy supplied to the primary is taken up by the coil. There is no
+breaking through, no local injury, but all the material, insulating and
+conducting, is uniformly heated.
+
+[Illustration: FIG. 105.]
+
+[Illustration: FIG. 106.]
+
+To avoid misunderstanding in regard to the physiological effect of
+alternating currents of very high frequency, I think it necessary to
+state that, while it is an undeniable fact that they are incomparably
+less dangerous than currents of low frequencies, it should not be
+thought that they are altogether harmless. What has just been said
+refers only to currents from an ordinary high tension induction coil,
+which currents are necessarily very small; if received directly from a
+machine or from a secondary of low resistance, they produce more or less
+powerful effects, and may cause serious injury, especially when used in
+conjunction with condensers.
+
+The streaming discharge of a high tension induction coil differs in many
+respects from that of a powerful static machine. In color it has neither
+the violet of the positive, nor the brightness of the negative, static
+discharge, but lies somewhere between, being, of course, alternatively
+positive and negative. But since the streaming is more powerful when the
+point or terminal is electrified positively, than when electrified
+negatively, it follows that the point of the brush is more like the
+positive, and the root more like the negative, static discharge. In the
+dark, when the brush is very powerful, the root may appear almost white.
+The wind produced by the escaping streams, though it may be very
+strong--often indeed to such a degree that it may be felt quite a
+distance from the coil--is, nevertheless, considering the quantity of
+the discharge, smaller than that produced by the positive brush of a
+static machine, and it affects the flame much less powerfully. From the
+nature of the phenomenon we can conclude that the higher the frequency,
+the smaller must, of course, be the wind produced by the streams, and
+with sufficiently high frequencies no wind at all would be produced at
+the ordinary atmospheric pressures. With frequencies obtainable by means
+of a machine, the mechanical effect is sufficiently great to revolve,
+with considerable speed, large pin-wheels, which in the dark present a
+beautiful appearance owing to the abundance of the streams (Fig. 106).
+
+[Illustration: FIG. 107.]
+
+[Illustration: FIG. 108.]
+
+In general, most of the experiments usually performed with a static
+machine can be performed with an induction coil when operated with very
+rapidly alternating currents. The effects produced, however, are much
+more striking, being of incomparably greater power. When a small length
+of ordinary cotton covered wire, Fig. 107, is attached to one terminal
+of the coil, the streams issuing from all points of the wire may be so
+intense as to produce a considerable light effect. When the potentials
+and frequencies are very high, a wire insulated with gutta percha or
+rubber and attached to one of the terminals, appears to be covered with
+a luminous film. A very thin bare wire when attached to a terminal emits
+powerful streams and vibrates continually to and fro or spins in a
+circle, producing a singular effect (Fig. 108). Some of these
+experiments have been described by me in _The Electrical World_, of
+February 21, 1891.
+
+Another peculiarity of the rapidly alternating discharge of the
+induction coil is its radically different behavior with respect to
+points and rounded surfaces.
+
+If a thick wire, provided with a ball at one end and with a point at the
+other, be attached to the positive terminal of a static machine,
+practically all the charge will be lost through the point, on account of
+the enormously greater tension, dependent on the radius of curvature.
+But if such a wire is attached to one of the terminals of the induction
+coil, it will be observed that with very high frequencies streams issue
+from the ball almost as copiously as from the point (Fig. 109).
+
+It is hardly conceivable that we could produce such a condition to an
+equal degree in a static machine, for the simple reason, that the
+tension increases as the square of the density, which in turn is
+proportional to the radius of curvature; hence, with a steady potential
+an enormous charge would be required to make streams issue from a
+polished ball while it is connected with a point. But with an induction
+coil the discharge of which alternates with great rapidity it is
+different. Here we have to deal with two distinct tendencies. First,
+there is the tendency to escape which exists in a condition of rest, and
+which depends on the radius of curvature; second, there is the tendency
+to dissipate into the surrounding air by condenser action, which depends
+on the surface. When one of these tendencies is a maximum, the other is
+at a minimum. At the point the luminous stream is principally due to the
+air molecules coming bodily in contact with the point; they are
+attracted and repelled, charged and discharged, and, their atomic
+charges being thus disturbed, vibrate and emit light waves. At the ball,
+on the contrary, there is no doubt that the effect is to a great extent
+produced inductively, the air molecules not _necessarily_ coming in
+contact with the ball, though they undoubtedly do so. To convince
+ourselves of this we only need to exalt the condenser action, for
+instance, by enveloping the ball, at some distance, by a better
+conductor than the surrounding medium, the conductor being, of course,
+insulated; or else by surrounding it with a better dielectric and
+approaching an insulated conductor; in both cases the streams will break
+forth more copiously. Also, the larger the ball with a given frequency,
+or the higher the frequency, the more will the ball have the advantage
+over the point. But, since a certain intensity of action is required to
+render the streams visible, it is obvious that in the experiment
+described the ball should not be taken too large.
+
+In consequence of this two-fold tendency, it is possible to produce by
+means of points, effects identical to those produced by capacity. Thus,
+for instance, by attaching to one terminal of the coil a small length of
+soiled wire, presenting many points and offering great facility to
+escape, the potential of the coil may be raised to the same value as by
+attaching to the terminal a polished ball of a surface many times
+greater than that of the wire.
+
+[Illustration: FIG. 109.]
+
+[Illustration: FIG. 110.]
+
+An interesting experiment, showing the effect of the points, may be
+performed in the following manner: Attach to one of the terminals of the
+coil a cotton covered wire about two feet in length, and adjust the
+conditions so that streams issue from the wire. In this experiment the
+primary coil should be preferably placed so that it extends only about
+half way into the secondary coil. Now touch the free terminal of the
+secondary with a conducting object held in the hand, or else connect it
+to an insulated body of some size. In this manner the potential on the
+wire may be enormously raised. The effect of this will be either to
+increase, or to diminish, the streams. If they increase, the wire is too
+short; if they diminish, it is too long. By adjusting the length of the
+wire, a point is found where the touching of the other terminal does not
+at all affect the streams. In this case the rise of potential is exactly
+counteracted by the drop through the coil. It will be observed that
+small lengths of wire produce considerable difference in the magnitude
+and luminosity of the streams. The primary coil is placed sidewise for
+two reasons: First, to increase the potential at the wire; and, second,
+to increase the drop through the coil. The sensitiveness is thus
+augmented.
+
+There is still another and far more striking peculiarity of the brush
+discharge produced by very rapidly alternating currents. To observe this
+it is best to replace the usual terminals of the coil by two metal
+columns insulated with a good thickness of ebonite. It is also well to
+close all fissures and cracks with wax so that the brushes cannot form
+anywhere except at the tops of the columns. If the conditions are
+carefully adjusted--which, of course, must be left to the skill of the
+experimenter--so that the potential rises to an enormous value, one may
+produce two powerful brushes several inches long, nearly white at their
+roots, which in the dark bear a striking resemblance to two flames of a
+gas escaping under pressure (Fig. 110). But they do not only _resemble_,
+they _are_ veritable flames, for they are hot. Certainly they are not as
+hot as a gas burner, _but they would be so if the frequency and the
+potential would be sufficiently high_. Produced with, say, twenty
+thousand alternations per second, the heat is easily perceptible even if
+the potential is not excessively high. The heat developed is, of course,
+due to the impact of the air molecules against the terminals and against
+each other. As, at the ordinary pressures, the mean free path is
+excessively small, it is possible that in spite of the enormous initial
+speed imparted to each molecule upon coming in contact with the
+terminal, its progress--by collision with other molecules--is retarded
+to such an extent, that it does not get away far from the terminal, but
+may strike the same many times in succession. The higher the frequency,
+the less the molecule is able to get away, and this the more so, as for
+a given effect the potential required is smaller; and a frequency is
+conceivable--perhaps even obtainable--at which practically the same
+molecules would strike the terminal. Under such conditions the exchange
+of the molecules would be very slow, and the heat produced at, and very
+near, the terminal would be excessive. But if the frequency would go on
+increasing constantly, the heat produced would begin to diminish for
+obvious reasons. In the positive brush of a static machine the exchange
+of the molecules is very rapid, the stream is constantly of one
+direction, and there are fewer collisions; hence the heating effect must
+be very small. Anything that impairs the facility of exchange tends to
+increase the local heat produced. Thus, if a bulb be held over the
+terminal of the coil so as to enclose the brush, the air contained in
+the bulb is very quickly brought to a high temperature. If a glass tube
+be held over the brush so as to allow the draught to carry the brush
+upwards, scorching hot air escapes at the top of the tube. Anything held
+within the brush is, of course, rapidly heated, and the possibility of
+using such heating effects for some purpose or other suggests itself.
+
+When contemplating this singular phenomenon of the hot brush, we cannot
+help being convinced that a similar process must take place in the
+ordinary flame, and it seems strange that after all these centuries past
+of familiarity with the flame, now, in this era of electric lighting and
+heating, we are finally led to recognize, that since time immemorial we
+have, after all, always had "electric light and heat" at our disposal.
+It is also of no little interest to contemplate, that we have a possible
+way of producing--by other than chemical means--a veritable flame, which
+would give light and heat without any material being consumed, without
+any chemical process taking place, and to accomplish this, we only need
+to perfect methods of producing enormous frequencies and potentials. I
+have no doubt that if the potential could be made to alternate with
+sufficient rapidity and power, the brush formed at the end of a wire
+would lose its electrical characteristics and would become flamelike.
+The flame must be due to electrostatic molecular action.
+
+This phenomenon now explains in a manner which can hardly be doubted the
+frequent accidents occurring in storms. It is well known that objects
+are often set on fire without the lightning striking them. We shall
+presently see how this can happen. On a nail in a roof, for instance, or
+on a projection of any kind, more or less conducting, or rendered so by
+dampness, a powerful brush may appear. If the lightning strikes
+somewhere in the neighborhood the enormous potential may be made to
+alternate or fluctuate perhaps many million times a second. The air
+molecules are violently attracted and repelled, and by their impact
+produce such a powerful heating effect that a fire is started. It is
+conceivable that a ship at sea may, in this manner, catch fire at many
+points at once. When we consider, that even with the comparatively low
+frequencies obtained from a dynamo machine, and with potentials of no
+more than one or two hundred thousand volts, the heating effects are
+considerable, we may imagine how much more powerful they must be with
+frequencies and potentials many times greater; and the above explanation
+seems, to say the least, very probable. Similar explanations may have
+been suggested, but I am not aware that, up to the present, the heating
+effects of a brush produced by a rapidly alternating potential have been
+experimentally demonstrated, at least not to such a remarkable degree.
+
+[Illustration: FIG. 111.]
+
+By preventing completely the exchange of the air molecules, the local
+heating effect may be so exalted as to bring a body to incandescence.
+Thus, for instance, if a small button, or preferably a very thin wire or
+filament be enclosed in an unexhausted globe and connected with the
+terminal of the coil, it may be rendered incandescent. The phenomenon is
+made much more interesting by the rapid spinning round in a circle of
+the top of the filament, thus presenting the appearance of a luminous
+funnel, Fig. 111, which widens when the potential is increased. When the
+potential is small the end of the filament may perform irregular
+motions, suddenly changing from one to the other, or it may describe an
+ellipse; but when the potential is very high it always spins in a
+circle; and so does generally a thin straight wire attached freely to
+the terminal of the coil. These motions are, of course, due to the
+impact of the molecules, and the irregularity in the distribution of the
+potential, owing to the roughness and dissymmetry of the wire or
+filament. With a perfectly symmetrical and polished wire such motions
+would probably not occur. That the motion is not likely to be due to
+others causes is evident from the fact that it is not of a definite
+direction, and that in a very highly exhausted globe it ceases
+altogether. The possibility of bringing a body to incandescence in an
+exhausted globe, or even when not at all enclosed, would seem to afford
+a possible way of obtaining light effects, which, in perfecting methods
+of producing rapidly alternating potentials, might be rendered available
+for useful purposes.
+
+[Illustration: FIG. 112a.]
+
+In employing a commercial coil, the production of very powerful brush
+effects is attended with considerable difficulties, for when these high
+frequencies and enormous potentials are used, the best insulation is apt
+to give way. Usually the coil is insulated well enough to stand the
+strain from convolution to convolution, since two double silk covered
+paraffined wires will withstand a pressure of several thousand volts;
+the difficulty lies principally in preventing the breaking through from
+the secondary to the primary, which is greatly facilitated by the
+streams issuing from the latter. In the coil, of course, the strain is
+greatest from section to section, but usually in a larger coil there are
+so many sections that the danger of a sudden giving way is not very
+great. No difficulty will generally be encountered in that direction,
+and besides, the liability of injuring the coil internally is very much
+reduced by the fact that the effect most likely to be produced is simply
+a gradual heating, which, when far enough advanced, could not fail to
+be observed. The principal necessity is then to prevent the streams
+between the primary and the tube, not only on account of the heating and
+possible injury, but also because the streams may diminish very
+considerably the potential difference available at the terminals. A few
+hints as to how this may be accomplished will probably be found useful
+in most of these experiments with the ordinary induction coil.
+
+[Illustration: FIG. 112b.]
+
+One of the ways is to wind a short primary, Fig. 112a, so that the
+difference of potential is not at that length great enough to cause the
+breaking forth of the streams through the insulating tube. The length of
+the primary should be determined by experiment. Both the ends of the
+coil should be brought out on one end through a plug of insulating
+material fitting in the tube as illustrated. In such a disposition one
+terminal of the secondary is attached to a body, the surface of which is
+determined with the greatest care so as to produce the greatest rise in
+the potential. At the other terminal a powerful brush appears, which may
+be experimented upon.
+
+The above plan necessitates the employment of a primary of comparatively
+small size, and it is apt to heat when powerful effects are desirable
+for a certain length of time. In such a case it is better to employ a
+larger coil, Fig. 112b, and introduce it from one side of the tube,
+until the streams begin to appear. In this case the nearest terminal of
+the secondary may be connected to the primary or to the ground, which is
+practically the same thing, if the primary is connected directly to the
+machine. In the case of ground connections it is well to determine
+experimentally the frequency which is best suited under the conditions
+of the test. Another way of obviating the streams, more or less, is to
+make the primary in sections and supply it from separate, well insulated
+sources.
+
+In many of these experiments, when powerful effects are wanted for a
+short time, it is advantageous to use iron cores with the primaries. In
+such case a very large primary coil may be wound and placed side by side
+with the secondary, and, the nearest terminal of the latter being
+connected to the primary, a laminated iron core is introduced through
+the primary into the secondary as far as the streams will permit. Under
+these conditions an excessively powerful brush, several inches long,
+which may be appropriately called "St. Elmo's hot fire," may be caused
+to appear at the other terminal of the secondary, producing striking
+effects. It is a most powerful ozonizer, so powerful indeed, that only a
+few minutes are sufficient to fill the whole room with the smell of
+ozone, and it undoubtedly possesses the quality of exciting chemical
+affinities.
+
+For the production of ozone, alternating currents of very high frequency
+are eminently suited, not only on account of the advantages they offer
+in the way of conversion but also because of the fact, that the
+ozonizing action of a discharge is dependent on the frequency as well as
+on the potential, this being undoubtedly confirmed by observation.
+
+In these experiments if an iron core is used it should be carefully
+watched, as it is apt to get excessively hot in an incredibly short
+time. To give an idea of the rapidity of the heating, I will state, that
+by passing a powerful current through a coil with many turns, the
+inserting within the same of a thin iron wire for no more than one
+second's time is sufficient to heat the wire to something like 100° C.
+
+But this rapid heating need not discourage us in the use of iron cores
+in connection with rapidly alternating currents. I have for a long time
+been convinced that in the industrial distribution by means of
+transformers, some such plan as the following might be practicable. We
+may use a comparatively small iron core, subdivided, or perhaps not even
+subdivided. We may surround this core with a considerable thickness of
+material which is fire-proof and conducts the heat poorly, and on top of
+that we may place the primary and secondary windings. By using either
+higher frequencies or greater magnetizing forces, we may by hysteresis
+and eddy currents heat the iron core so far as to bring it nearly to its
+maximum permeability, which, as Hopkinson has shown, may be as much as
+sixteen times greater than that at ordinary temperatures. If the iron
+core were perfectly enclosed, it would not be deteriorated by the heat,
+and, if the enclosure of fire-proof material would be sufficiently
+thick, only a limited amount of energy could be radiated in spite of the
+high temperature. Transformers have been constructed by me on that plan,
+but for lack of time, no thorough tests have as yet been made.
+
+Another way of adapting the iron core to rapid alternations, or,
+generally speaking, reducing the frictional losses, is to produce by
+continuous magnetization a flow of something like seven thousand or
+eight thousand lines per square centimetre through the core, and then
+work with weak magnetizing forces and preferably high frequencies around
+the point of greatest permeability. A higher efficiency of conversion
+and greater output are obtainable in this manner. I have also employed
+this principle in connection with machines in which there is no reversal
+of polarity. In these types of machines, as long as there are only few
+pole projections, there is no great gain, as the maxima and minima of
+magnetization are far from the point of maximum permeability; but when
+the number of the pole projections is very great, the required rate of
+change may be obtained, without the magnetization varying so far as to
+depart greatly from the point of maximum permeability, and the gain is
+considerable.
+
+The above described arrangements refer only to the use of commercial
+coils as ordinarily constructed. If it is desired to construct a coil
+for the express purpose of performing with it such experiments as I have
+described, or, generally, rendering it capable of withstanding the
+greatest possible difference of potential, then a construction as
+indicated in Fig. 113 will be found of advantage. The coil in this case
+is formed of two independent parts which are wound oppositely, the
+connection between both being made near the primary. The potential in
+the middle being zero, there is not much tendency to jump to the primary
+and not much insulation is required. In some cases the middle point may,
+however, be connected to the primary or to the ground. In such a coil
+the places of greatest difference of potential are far apart and the
+coil is capable of withstanding an enormous strain. The two parts may be
+movable so as to allow a slight adjustment of the capacity effect.
+
+As to the manner of insulating the coil, it will be found convenient to
+proceed in the following way: First, the wire should be boiled in
+paraffine until all the air is out; then the coil is wound by running
+the wire through melted paraffine, merely for the purpose of fixing the
+wire. The coil is then taken off from the spool, immersed in a
+cylindrical vessel filled with pure melted wax and boiled for a long
+time until the bubbles cease to appear. The whole is then left to cool
+down thoroughly, and then the mass is taken out of the vessel and turned
+up in a lathe. A coil made in this manner and with care is capable of
+withstanding enormous potential differences.
+
+[Illustration: FIG. 113.]
+
+It may be found convenient to immerse the coil in paraffine oil or some
+other kind of oil; it is a most effective way of insulating, principally
+on account of the perfect exclusion of air, but it may be found that,
+after all, a vessel filled with oil is not a very convenient thing to
+handle in a laboratory.
+
+If an ordinary coil can be dismounted, the primary may be taken out of
+the tube and the latter plugged up at one end, filled with oil, and the
+primary reinserted. This affords an excellent insulation and prevents
+the formation of the streams.
+
+Of all the experiments which may be performed with rapidly alternating
+currents the most interesting are those which concern the production of
+a practical illuminant. It cannot be denied that the present methods,
+though they were brilliant advances, are very wasteful. Some better
+methods must be invented, some more perfect apparatus devised. Modern
+research has opened new possibilities for the production of an efficient
+source of light, and the attention of all has been turned in the
+direction indicated by able pioneers. Many have been carried away by
+the enthusiasm and passion to discover, but in their zeal to reach
+results, some have been misled. Starting with the idea of producing
+electro-magnetic waves, they turned their attention, perhaps, too much
+to the study of electro-magnetic effects, and neglected the study of
+electrostatic phenomena. Naturally, nearly every investigator availed
+himself of an apparatus similar to that used in earlier experiments. But
+in those forms of apparatus, while the electro-magnetic inductive
+effects are enormous, the electrostatic effects are excessively small.
+
+In the Hertz experiments, for instance, a high tension induction coil is
+short circuited by an arc, the resistance of which is very small, the
+smaller, the more capacity is attached to the terminals; and the
+difference of potential at these is enormously diminished. On the other
+hand, when the discharge is not passing between the terminals, the
+static effects may be considerable, but only qualitatively so, not
+quantitatively, since their rise and fall is very sudden, and since
+their frequency is small. In neither case, therefore, are powerful
+electrostatic effects perceivable. Similar conditions exist when, as in
+some interesting experiments of Dr. Lodge, Leyden jars are discharged
+disruptively. It has been thought--and I believe asserted--that in such
+cases most of the energy is radiated into space. In the light of the
+experiments which I have described above, it will now not be thought so.
+I feel safe in asserting that in such cases most of the energy is partly
+taken up and converted into heat in the arc of the discharge and in the
+conducting and insulating material of the jar, some energy being, of
+course, given off by electrification of the air; but the amount of the
+directly radiated energy is very small.
+
+When a high tension induction coil, operated by currents alternating
+only 20,000 times a second, has its terminals closed through even a very
+small jar, practically all the energy passes through the dielectric of
+the jar, which is heated, and the electrostatic effects manifest
+themselves outwardly only to a very weak degree. Now the external
+circuit of a Leyden jar, that is, the arc and the connections of the
+coatings, may be looked upon as a circuit generating alternating
+currents of excessively high frequency and fairly high potential, which
+is closed through the coatings and the dielectric between them, and from
+the above it is evident that the external electrostatic effects must be
+very small, even if a recoil circuit be used. These conditions make it
+appear that with the apparatus usually at hand, the observation of
+powerful electrostatic effects was impossible, and what experience has
+been gained in that direction is only due to the great ability of the
+investigators.
+
+But powerful electrostatic effects are a _sine qua non_ of light
+production on the lines indicated by theory. Electro-magnetic effects
+are primarily unavailable, for the reason that to produce the required
+effects we would have to pass current impulses through a conductor,
+which, long before the required frequency of the impulses could be
+reached, would cease to transmit them. On the other hand,
+electro-magnetic waves many times longer than those of light, and
+producible by sudden discharge of a condenser, could not be utilized, it
+would seem, except we avail ourselves of their effect upon conductors as
+in the present methods, which are wasteful. We could not affect by means
+of such waves the static molecular or atomic charges of a gas, cause
+them to vibrate and to emit light. Long transverse waves cannot,
+apparently, produce such effects, since excessively small
+electro-magnetic disturbances may pass readily through miles of air.
+Such dark waves, unless they are of the length of true light waves,
+cannot, it would seem, excite luminous radiation in a Geissler tube, and
+the luminous effects, which are producible by induction in a tube devoid
+of electrodes, I am inclined to consider as being of an electrostatic
+nature.
+
+To produce such luminous effects, straight electrostatic thrusts are
+required; these, whatever be their frequency, may disturb the molecular
+charges and produce light. Since current impulses of the required
+frequency cannot pass through a conductor of measurable dimensions, we
+must work with a gas, and then the production of powerful electrostatic
+effects becomes an imperative necessity.
+
+It has occurred to me, however, that electrostatic effects are in many
+ways available for the production of light. For instance, we may place a
+body of some refractory material in a closed, and preferably more or
+less exhausted, globe, connect it to a source of high, rapidly
+alternating potential, causing the molecules of the gas to strike it
+many times a second at enormous speeds, and in this manner, with
+trillions of invisible hammers, pound it until it gets incandescent; or
+we may place a body in a very highly exhausted globe, in a non-striking
+vacuum, and, by employing very high frequencies and potentials,
+transfer sufficient energy from it to other bodies in the vicinity, or
+in general to the surroundings, to maintain it at any degree of
+incandescence; or we may, by means of such rapidly alternating high
+potentials, disturb the ether carried by the molecules of a gas or their
+static charges, causing them to vibrate and to emit light.
+
+But, electrostatic effects being dependent upon the potential and
+frequency, to produce the most powerful action it is desirable to
+increase both as far as practicable. It may be possible to obtain quite
+fair results by keeping either of these factors small, provided the
+other is sufficiently great; but we are limited in both directions. My
+experience demonstrates that we cannot go below a certain frequency,
+for, first, the potential then becomes so great that it is dangerous;
+and, secondly, the light production is less efficient.
+
+I have found that, by using the ordinary low frequencies, the
+physiological effect of the current required to maintain at a certain
+degree of brightness a tube four feet long, provided at the ends with
+outside and inside condenser coatings, is so powerful that, I think, it
+might produce serious injury to those not accustomed to such shocks;
+whereas, with twenty thousand alternations per second, the tube may be
+maintained at the same degree of brightness without any effect being
+felt. This is due principally to the fact that a much smaller potential
+is required to produce the same light effect, and also to the higher
+efficiency in the light production. It is evident that the efficiency in
+such cases is the greater, the higher the frequency, for the quicker the
+process of charging and discharging the molecules, the less energy will
+be lost in the form of dark radiation. But, unfortunately, we cannot go
+beyond a certain frequency on account of the difficulty of producing and
+conveying the effects.
+
+I have stated above that a body inclosed in an unexhausted bulb may be
+intensely heated by simply connecting it with a source of rapidly
+alternating potential. The heating in such a case is, in all
+probability, due mostly to the bombardment of the molecules of the gas
+contained in the bulb. When the bulb is exhausted, the heating of the
+body is much more rapid, and there is no difficulty whatever in bringing
+a wire or filament to any degree of incandescence by simply connecting
+it to one terminal of a coil of the proper dimensions. Thus, if the
+well-known apparatus of Prof. Crookes, consisting of a bent platinum
+wire with vanes mounted over it (Fig. 114), be connected to one
+terminal of the coil--either one or both ends of the platinum wire being
+connected--the wire is rendered almost instantly incandescent, and the
+mica vanes are rotated as though a current from a battery were used. A
+thin carbon filament, or, preferably, a button of some refractory
+material (Fig. 115), even if it be a comparatively poor conductor,
+inclosed in an exhausted globe, may be rendered highly incandescent; and
+in this manner a simple lamp capable of giving any desired candle power
+is provided.
+
+The success of lamps of this kind would depend largely on the selection
+of the light-giving bodies contained within the bulb. Since, under the
+conditions described, refractory bodies--which are very poor conductors
+and capable of withstanding for a long time excessively high degrees of
+temperature--may be used, such illuminating devices may be rendered
+successful.
+
+[Illustration: FIG. 114.]
+
+[Illustration: FIG. 115.]
+
+It might be thought at first that if the bulb, containing the filament
+or button of refractory material, be perfectly well exhausted--that is,
+as far as it can be done by the use of the best apparatus--the heating
+would be much less intense, and that in a perfect vacuum it could not
+occur at all. This is not confirmed by my experience; quite the
+contrary, the better the vacuum the more easily the bodies are brought
+to incandescence. This result is interesting for many reasons.
+
+At the outset of this work the idea presented itself to me, whether two
+bodies of refractory material enclosed in a bulb exhausted to such a
+degree that the discharge of a large induction coil, operated in the
+usual manner, cannot pass through, could be rendered incandescent by
+mere condenser action. Obviously, to reach this result enormous
+potential differences and very high frequencies are required, as is
+evident from a simple calculation.
+
+But such a lamp would possess a vast advantage over an ordinary
+incandescent lamp in regard to efficiency. It is well-known that the
+efficiency of a lamp is to some extent a function of the degree of
+incandescence, and that, could we but work a filament at many times
+higher degrees of incandescence, the efficiency would be much greater.
+In an ordinary lamp this is impracticable on account of the destruction
+of the filament, and it has been determined by experience how far it is
+advisable to push the incandescence. It is impossible to tell how much
+higher efficiency could be obtained if the filament could withstand
+indefinitely, as the investigation to this end obviously cannot be
+carried beyond a certain stage; but there are reasons for believing that
+it would be very considerably higher. An improvement might be made in
+the ordinary lamp by employing a short and thick carbon; but then the
+leading-in wires would have to be thick, and, besides, there are many
+other considerations which render such a modification entirely
+impracticable. But in a lamp as above described, the leading in wires
+may be very small, the incandescent refractory material may be in the
+shape of blocks offering a very small radiating surface, so that less
+energy would be required to keep them at the desired incandescence; and
+in addition to this, the refractory material need not be carbon, but may
+be manufactured from mixtures of oxides, for instance, with carbon or
+other material, or may be selected from bodies which are practically
+non-conductors, and capable of withstanding enormous degrees of
+temperature.
+
+All this would point to the possibility of obtaining a much higher
+efficiency with such a lamp than is obtainable in ordinary lamps. In my
+experience it has been demonstrated that the blocks are brought to high
+degrees of incandescence with much lower potentials than those
+determined by calculation, and the blocks may be set at greater
+distances from each other. We may freely assume, and it is probable,
+that the molecular bombardment is an important element in the heating,
+even if the globe be exhausted with the utmost care, as I have done; for
+although the number of the molecules is, comparatively speaking,
+insignificant, yet on account of the mean free path being very great,
+there are fewer collisions, and the molecules may reach much higher
+speeds, so that the heating effect due to this cause may be
+considerable, as in the Crookes experiments with radiant matter.
+
+But it is likewise possible that we have to deal here with an increased
+facility of losing the charge in very high vacuum, when the potential is
+rapidly alternating, in which case most of the heating would be directly
+due to the surging of the charges in the heated bodies. Or else the
+observed fact may be largely attributable to the effect of the points
+which I have mentioned above, in consequence of which the blocks or
+filaments contained in the vacuum are equivalent to condensers of many
+times greater surface than that calculated from their geometrical
+dimensions. Scientific men still differ in opinion as to whether a
+charge should, or should not, be lost in a perfect vacuum, or in other
+words, whether ether is, or is not, a conductor. If the former were the
+case, then a thin filament enclosed in a perfectly exhausted globe, and
+connected to a source of enormous, steady potential, would be brought to
+incandescence.
+
+[Illustration: FIG. 116.]
+
+[Illustration: FIG. 117.]
+
+Various forms of lamps on the above described principle, with the
+refractory bodies in the form of filaments, Fig. 116, or blocks, Fig.
+117, have been constructed and operated by me, and investigations are
+being carried on in this line. There is no difficulty in reaching such
+high degrees of incandescence that ordinary carbon is to all appearance
+melted and volatilized. If the vacuum could be made absolutely perfect,
+such a lamp, although inoperative with apparatus ordinarily used, would,
+if operated with currents of the required character, afford an
+illuminant which would never be destroyed, and which would be far more
+efficient than an ordinary incandescent lamp. This perfection can, of
+course, never be reached, and a very slow destruction and gradual
+diminution in size always occurs, as in incandescent filaments; but
+there is no possibility of a sudden and premature disabling which occurs
+in the latter by the breaking of the filament, especially when the
+incandescent bodies are in the shape of blocks.
+
+With these rapidly alternating potentials there is, however, no
+necessity of enclosing two blocks in a globe, but a single block, as in
+Fig. 115, or filament, Fig. 118, may be used. The potential in this case
+must of course be higher, but is easily obtainable, and besides it is
+not necessarily dangerous.
+
+[Illustration: FIG. 118.]
+
+The facility with which the button or filament in such a lamp is brought
+to incandescence, other things being equal, depends on the size of the
+globe. If a perfect vacuum could be obtained, the size of the globe
+would not be of importance, for then the heating would be wholly due to
+the surging of the charges, and all the energy would be given off to the
+surroundings by radiation. But this can never occur in practice. There
+is always some gas left in the globe, and although the exhaustion may be
+carried to the highest degree, still the space inside of the bulb must
+be considered as conducting when such high potentials are used, and I
+assume that, in estimating the energy that may be given off from the
+filament to the surroundings, we may consider the inside surface of the
+bulb as one coating of a condenser, the air and other objects
+surrounding the bulb forming the other coating. When the alternations
+are very low there is no doubt that a considerable portion of the energy
+is given off by the electrification of the surrounding air.
+
+In order to study this subject better, I carried on some experiments
+with excessively high potentials and low frequencies. I then observed
+that when the hand is approached to the bulb,--the filament being
+connected with one terminal of the coil,--a powerful vibration is felt,
+being due to the attraction and repulsion of the molecules of the air
+which are electrified by induction through the glass. In some cases when
+the action is very intense I have been able to hear a sound, which must
+be due to the same cause.
+
+[Illustration: FIG. 119.]
+
+[Illustration: FIG. 120.]
+
+When the alternations are low, one is apt to get an excessively powerful
+shock from the bulb. In general, when one attaches bulbs or objects of
+some size to the terminals of the coil, one should look out for the rise
+of potential, for it may happen that by merely connecting a bulb or
+plate to the terminal, the potential may rise to many times its original
+value. When lamps are attached to the terminals, as illustrated in Fig.
+119, then the capacity of the bulbs should be such as to give the
+maximum rise of potential under the existing conditions. In this manner
+one may obtain the required potential with fewer turns of wire.
+
+The life of such lamps as described above depends, of course, largely on
+the degree of exhaustion, but to some extent also on the shape of the
+block of refractory material. Theoretically it would seem that a small
+sphere of carbon enclosed in a sphere of glass would not suffer
+deterioration from molecular bombardment, for, the matter in the globe
+being radiant, the molecules would move in straight lines, and would
+seldom strike the sphere obliquely. An interesting thought in connection
+with such a lamp is, that in it "electricity" and electrical energy
+apparently must move in the same lines.
+
+[Illustration: FIG. 121a.]
+
+[Illustration: FIG. 121b.]
+
+The use of alternating currents of very high frequency makes it possible
+to transfer, by electrostatic or electromagnetic induction through the
+glass of a lamp, sufficient energy to keep a filament at incandescence
+and so do away with the leading-in wires. Such lamps have been proposed,
+but for want of proper apparatus they have not been successfully
+operated. Many forms of lamps on this principle with continuous and
+broken filaments have been constructed by me and experimented upon. When
+using a secondary enclosed within the lamp, a condenser is
+advantageously combined with the secondary. When the transference is
+effected by electrostatic induction, the potentials used are, of course,
+very high with frequencies obtainable from a machine. For instance, with
+a condenser surface of forty square centimetres, which is not
+impracticably large, and with glass of good quality 1 mm. thick, using
+currents alternating twenty thousand times a second, the potential
+required is approximately 9,000 volts. This may seem large, but since
+each lamp may be included in the secondary of a transformer of very
+small dimensions, it would not be inconvenient, and, moreover, it would
+not produce fatal injury. The transformers would all be preferably in
+series. The regulation would offer no difficulties, as with currents of
+such frequencies it is very easy to maintain a constant current.
+
+In the accompanying engravings some of the types of lamps of this kind
+are shown. Fig. 120 is such a lamp with a broken filament, and Figs. 121
+A and 121 B one with a single outside and inside coating and a single
+filament. I have also made lamps with two outside and inside coatings
+and a continuous loop connecting the latter. Such lamps have been
+operated by me with current impulses of the enormous frequencies
+obtainable by the disruptive discharge of condensers.
+
+The disruptive discharge of a condenser is especially suited for
+operating such lamps--with no outward electrical connections--by means
+of electromagnetic induction, the electromagnetic inductive effects
+being excessively high; and I have been able to produce the desired
+incandescence with only a few short turns of wire. Incandescence may
+also be produced in this manner in a simple closed filament.
+
+Leaving now out of consideration the practicability of such lamps, I
+would only say that they possess a beautiful and desirable feature,
+namely, that they can be rendered, at will, more or less brilliant
+simply by altering the relative position of the outside and inside
+condenser coatings, or inducing and induced circuits.
+
+When a lamp is lighted by connecting it to one terminal only of the
+source, this may be facilitated by providing the globe with an outside
+condenser coating, which serves at the same time as a reflector, and
+connecting this to an insulated body of some size. Lamps of this kind
+are illustrated in Fig. 122 and Fig. 123. Fig. 124 shows the plan of
+connection. The brilliancy of the lamp may, in this case, be regulated
+within wide limits by varying the size of the insulated metal plate to
+which the coating is connected.
+
+It is likewise practicable to light with one leading wire lamps such as
+illustrated in Fig. 116 and Fig. 117, by connecting one terminal of the
+lamp to one terminal of the source, and the other to an insulated body
+of the required size. In all cases the insulated body serves to give off
+the energy into the surrounding space, and is equivalent to a return
+wire. Obviously, in the two last-named cases, instead of connecting the
+wires to an insulated body, connections may be made to the ground.
+
+The experiments which will prove most suggestive and of most interest to
+the investigator are probably those performed with exhausted tubes. As
+might be anticipated, a source of such rapidly alternating potentials is
+capable of exciting the tubes at a considerable distance, and the light
+effects produced are remarkable.
+
+[Illustration: FIG. 122.]
+
+[Illustration: FIG. 123.]
+
+During my investigations in this line I endeavored to excite tubes,
+devoid of any electrodes, by electromagnetic induction, making the tube
+the secondary of the induction device, and passing through the primary
+the discharges of a Leyden jar. These tubes were made of many shapes,
+and I was able to obtain luminous effects which I then thought were due
+wholly to electromagnetic induction. But on carefully investigating the
+phenomena I found that the effects produced were more of an
+electrostatic nature. It may be attributed to this circumstance that
+this mode of exciting tubes is very wasteful, namely, the primary
+circuit being closed, the potential, and consequently the electrostatic
+inductive effect, is much diminished.
+
+When an induction coil, operated as above described, is used, there is
+no doubt that the tubes are excited by electrostatic induction, and that
+electromagnetic induction has little, if anything, to do with the
+phenomena.
+
+[Illustration: FIG. 124.]
+
+This is evident from many experiments. For instance, if a tube be taken
+in one hand, the observer being near the coil, it is brilliantly lighted
+and remains so no matter in what position it is held relatively to the
+observer's body. Were the action electromagnetic, the tube could not be
+lighted when the observer's body is interposed between it and the coil,
+or at least its luminosity should be considerably diminished. When the
+tube is held exactly over the centre of the coil--the latter being wound
+in sections and the primary placed symmetrically to the secondary--it
+may remain completely dark, whereas it is rendered intensely luminous by
+moving it slightly to the right or left from the centre of the coil. It
+does not light because in the middle both halves of the coil neutralize
+each other, and the electric potential is zero. If the action were
+electromagnetic, the tube should light best in the plane through the
+centre of the coil, since the electromagnetic effect there should be a
+maximum. When an arc is established between the terminals, the tubes and
+lamps in the vicinity of the coil go out, but light up again when the
+arc is broken, on account of the rise of potential. Yet the
+electromagnetic effect should be practically the same in both cases.
+
+By placing a tube at some distance from the coil, and nearer to one
+terminal--preferably at a point on the axis of the coil--one may light
+it by touching the remote terminal with an insulated body of some size
+or with the hand, thereby raising the potential at that terminal nearer
+to the tube. If the tube is shifted nearer to the coil so that it is
+lighted by the action of the nearer terminal, it may be made to go out
+by holding, on an insulated support, the end of a wire connected to the
+remote terminal, in the vicinity of the nearer terminal, by this means
+counteracting the action of the latter upon the tube. These effects are
+evidently electrostatic. Likewise, when a tube is placed at a
+considerable distance from the coil, the observer may, standing upon an
+insulated support between coil and tube, light the latter by approaching
+the hand to it; or he may even render it luminous by simply stepping
+between it and the coil. This would be impossible with electro-magnetic
+induction, for the body of the observer would act as a screen.
+
+When the coil is energized by excessively weak currents, the
+experimenter may, by touching one terminal of the coil with the tube,
+extinguish the latter, and may again light it by bringing it out of
+contact with the terminal and allowing a small arc to form. This is
+clearly due to the respective lowering and raising of the potential at
+that terminal. In the above experiment, when the tube is lighted through
+a small arc, it may go out when the arc is broken, because the
+electrostatic inductive effect alone is too weak, though the potential
+may be much higher; but when the arc is established, the electrification
+of the end of the tube is much greater, and it consequently lights.
+
+If a tube is lighted by holding it near to the coil, and in the hand
+which is remote, by grasping the tube anywhere with the other hand, the
+part between the hands is rendered dark, and the singular effect of
+wiping out the light of the tube may be produced by passing the hand
+quickly along the tube and at the same time withdrawing it gently from
+the coil, judging properly the distance so that the tube remains dark
+afterwards.
+
+If the primary coil is placed sidewise, as in Fig. 112 B for instance,
+and an exhausted tube be introduced from the other side in the hollow
+space, the tube is lighted most intensely because of the increased
+condenser action, and in this position the strić are most sharply
+defined. In all these experiments described, and in many others, the
+action is clearly electrostatic.
+
+The effects of screening also indicate the electrostatic nature of the
+phenomena and show something of the nature of electrification through
+the air. For instance, if a tube is placed in the direction of the axis
+of the coil, and an insulated metal plate be interposed, the tube will
+generally increase in brilliancy, or if it be too far from the coil to
+light, it may even be rendered luminous by interposing an insulated
+metal plate. The magnitude of the effects depends to some extent on the
+size of the plate. But if the metal plate be connected by a wire to the
+ground, its interposition will always make the tube go out even if it be
+very near the coil. In general, the interposition of a body between the
+coil and tube, increases or diminishes the brilliancy of the tube, or
+its facility to light up, according to whether it increases or
+diminishes the electrification. When experimenting with an insulated
+plate, the plate should not be taken too large, else it will generally
+produce a weakening effect by reason of its great facility for giving
+off energy to the surroundings.
+
+If a tube be lighted at some distance from the coil, and a plate of hard
+rubber or other insulating substance be interposed, the tube may be made
+to go out. The interposition of the dielectric in this case only
+slightly increases the inductive effect, but diminishes considerably the
+electrification through the air.
+
+In all cases, then, when we excite luminosity in exhausted tubes by
+means of such a coil, the effect is due to the rapidly alternating
+electrostatic potential; and, furthermore, it must be attributed to the
+harmonic alternation produced directly by the machine, and not to any
+superimposed vibration which might be thought to exist. Such
+superimposed vibrations are impossible when we work with an alternate
+current machine. If a spring be gradually tightened and released, it
+does not perform independent vibrations; for this a sudden release is
+necessary. So with the alternate currents from a dynamo machine; the
+medium is harmonically strained and released, this giving rise to only
+one kind of waves; a sudden contact or break, or a sudden giving way of
+the dielectric, as in the disruptive discharge of a Leyden jar, are
+essential for the production of superimposed waves.
+
+In all the last described experiments, tubes devoid of any electrodes
+may be used, and there is no difficulty in producing by their means
+sufficient light to read by. The light effect is, however, considerably
+increased by the use of phosphorescent bodies such as yttria, uranium
+glass, etc. A difficulty will be found when the phosphorescent material
+is used, for with these powerful effects, it is carried gradually away,
+and it is preferable to use material in the form of a solid.
+
+Instead of depending on induction at a distance to light the tube, the
+same may be provided with an external--and, if desired, also with an
+internal--condenser coating, and it may then be suspended anywhere in
+the room from a conductor connected to one terminal of the coil, and in
+this manner a soft illumination may be provided.
+
+[Illustration: FIG. 125.]
+
+The ideal way of lighting a hall or room would, however, be to produce
+such a condition in it that an illuminating device could be moved and
+put anywhere, and that it is lighted, no matter where it is put and
+without being electrically connected to anything. I have been able to
+produce such a condition by creating in the room a powerful, rapidly
+alternating electrostatic field. For this purpose I suspend a sheet of
+metal a distance from the ceiling on insulating cords and connect it to
+one terminal of the induction coil, the other terminal being preferably
+connected to the ground. Or else I suspend two sheets as illustrated in
+Fig. 125, each sheet being connected with one of the terminals of the
+coil, and their size being carefully determined. An exhausted tube may
+then be carried in the hand anywhere between the sheets or placed
+anywhere, even a certain distance beyond them; it remains always
+luminous.
+
+In such an electrostatic field interesting phenomena may be observed,
+especially if the alternations are kept low and the potentials
+excessively high. In addition to the luminous phenomena mentioned, one
+may observe that any insulated conductor gives sparks when the hand or
+another object is approached to it, and the sparks may often be
+powerful. When a large conducting object is fastened on an insulating
+support, and the hand approached to it, a vibration, due to the
+rythmical motion of the air molecules is felt, and luminous streams may
+be perceived when the hand is held near a pointed projection. When a
+telephone receiver is made to touch with one or both of its terminals an
+insulated conductor of some size, the telephone emits a loud sound; it
+also emits a sound when a length of wire is attached to one or both
+terminals, and with very powerful fields a sound may be perceived even
+without any wire.
+
+How far this principle is capable of practical application, the future
+will tell. It might be thought that electrostatic effects are unsuited
+for such action at a distance. Electromagnetic inductive effects, if
+available for the production of light, might be thought better suited.
+It is true the electrostatic effects diminish nearly with the cube of
+the distance from the coil, whereas the electromagnetic inductive
+effects diminish simply with the distance. But when we establish an
+electrostatic field of force, the condition is very different, for then,
+instead of the differential effect of both the terminals, we get their
+conjoint effect. Besides, I would call attention to the effect, that in
+an alternating electrostatic field, a conductor, such as an exhausted
+tube, for instance, tends to take up most of the energy, whereas in an
+electromagnetic alternating field the conductor tends to take up the
+least energy, the waves being reflected with but little loss. This is
+one reason why it is difficult to excite an exhausted tube, at a
+distance, by electromagnetic induction. I have wound coils of very large
+diameter and of many turns of wire, and connected a Geissler tube to the
+ends of the coil with the object of exciting the tube at a distance; but
+even with the powerful inductive effects producible by Leyden jar
+discharges, the tube could not be excited unless at a very small
+distance, although some judgment was used as to the dimensions of the
+coil. I have also found that even the most powerful Leyden jar
+discharges are capable of exciting only feeble luminous effects in a
+closed exhausted tube, and even these effects upon thorough examination
+I have been forced to consider of an electrostatic nature.
+
+How then can we hope to produce the required effects at a distance by
+means of electromagnetic action, when even in the closest proximity to
+the source of disturbance, under the most advantageous conditions, we
+can excite but faint luminosity? It is true that when acting at a
+distance we have the resonance to help us out. We can connect an
+exhausted tube, or whatever the illuminating device may be, with an
+insulated system of the proper capacity, and so it may be possible to
+increase the effect qualitatively, and only qualitatively, for we would
+not get _more_ energy through the device. So we may, by resonance
+effect, obtain the required electromotive force in an exhausted tube,
+and excite faint luminous effects, but we cannot get enough energy to
+render the light practically available, and a simple calculation, based
+on experimental results, shows that even if all the energy which a tube
+would receive at a certain distance from the source should be wholly
+converted into light, it would hardly satisfy the practical
+requirements. Hence the necessity of directing, by means of a conducting
+circuit, the energy to the place of transformation. But in so doing we
+cannot very sensibly depart from present methods, and all we could do
+would be to improve the apparatus.
+
+From these considerations it would seem that if this ideal way of
+lighting is to be rendered practicable it will be only by the use of
+electrostatic effects. In such a case the most powerful electrostatic
+inductive effects are needed; the apparatus employed must, therefore, be
+capable of producing high electrostatic potentials changing in value
+with extreme rapidity. High frequencies are especially wanted, for
+practical considerations make it desirable to keep down the potential.
+By the employment of machines, or, generally speaking, of any
+mechanical apparatus, but low frequencies can be reached; recourse must,
+therefore, be had to some other means. The discharge of a condenser
+affords us a means of obtaining frequencies by far higher than are
+obtainable mechanically, and I have accordingly employed condensers in
+the experiments to the above end.
+
+When the terminals of a high tension induction coil, Fig. 126, are
+connected to a Leyden jar, and the latter is discharging disruptively
+into a circuit, we may look upon the arc playing between the knobs as
+being a source of alternating, or generally speaking, undulating
+currents, and then we have to deal with the familiar system of a
+generator of such currents, a circuit connected to it, and a condenser
+bridging the circuit. The condenser in such case is a veritable
+transformer, and since the frequency is excessive, almost any ratio in
+the strength of the currents in both the branches may be obtained. In
+reality the analogy is not quite complete, for in the disruptive
+discharge we have most generally a fundamental instantaneous variation
+of comparatively low frequency, and a superimposed harmonic vibration,
+and the laws governing the flow of currents are not the same for both.
+
+In converting in this manner, the ratio of conversion should not be too
+great, for the loss in the arc between the knobs increases with the
+square of the current, and if the jar be discharged through very thick
+and short conductors, with the view of obtaining a very rapid
+oscillation, a very considerable portion of the energy stored is lost.
+On the other hand, too small ratios are not practicable for many obvious
+reasons.
+
+As the converted currents flow in a practically closed circuit, the
+electrostatic effects are necessarily small, and I therefore convert
+them into currents or effects of the required character. I have effected
+such conversions in several ways. The preferred plan of connections is
+illustrated in Fig. 127. The manner of operating renders it easy to
+obtain by means of a small and inexpensive apparatus enormous
+differences of potential which have been usually obtained by means of
+large and expensive coils. For this it is only necessary to take an
+ordinary small coil, adjust to it a condenser and discharging circuit,
+forming the primary of an auxiliary small coil, and convert upward. As
+the inductive effect of the primary currents is excessively great, the
+second coil need have comparatively but very few turns. By properly
+adjusting the elements, remarkable results may be secured.
+
+In endeavoring to obtain the required electrostatic effects in this
+manner, I have, as might be expected, encountered many difficulties
+which I have been gradually overcoming, but I am not as yet prepared to
+dwell upon my experiences in this direction.
+
+I believe that the disruptive discharge of a condenser will play an
+important part in the future, for it offers vast possibilities, not only
+in the way of producing light in a more efficient manner and in the line
+indicated by theory, but also in many other respects.
+
+[Illustration: FIG. 126.]
+
+For years the efforts of inventors have been directed towards obtaining
+electrical energy from heat by means of the thermopile. It might seem
+invidious to remark that but few know what is the real trouble with the
+thermopile. It is not the inefficiency or small output--though these are
+great drawbacks--but the fact that the thermopile has its phylloxera,
+that is, that by constant use it is deteriorated, which has thus far
+prevented its introduction on an industrial scale. Now that all modern
+research seems to point with certainty to the use of electricity of
+excessively high tension, the question must present itself to many
+whether it is not possible to obtain in a practicable manner this form
+of energy from heat. We have been used to look upon an electrostatic
+machine as a plaything, and somehow we couple with it the idea of the
+inefficient and impractical. But now we must think differently, for now
+we know that everywhere we have to deal with the same forces, and that
+it is a mere question of inventing proper methods or apparatus for
+rendering them available.
+
+In the present systems of electrical distribution, the employment of the
+iron with its wonderful magnetic properties allows us to reduce
+considerably the size of the apparatus; but, in spite of this, it is
+still very cumbersome. The more we progress in the study of electric and
+magnetic phenomena, the more we become convinced that the present
+methods will be short-lived. For the production of light, at least, such
+heavy machinery would seem to be unnecessary. The energy required is
+very small, and if light can be obtained as efficiently as,
+theoretically, it appears possible, the apparatus need have but a very
+small output. There being a strong probability that the illuminating
+methods of the future will involve the use of very high potentials, it
+seems very desirable to perfect a contrivance capable of converting the
+energy of heat into energy of the requisite form. Nothing to speak of
+has been done towards this end, for the thought that electricity of some
+50,000 or 100,000 volts pressure or more, even if obtained, would be
+unavailable for practical purposes, has deterred inventors from working
+in this direction.
+
+[Illustration: FIG. 127.]
+
+In Fig. 126 a plan of connections is shown for converting currents of
+high, into currents of low, tension by means of the disruptive discharge
+of a condenser. This plan has been used by me frequently for operating a
+few incandescent lamps required in the laboratory. Some difficulties
+have been encountered in the arc of the discharge which I have been able
+to overcome to a great extent; besides this, and the adjustment
+necessary for the proper working, no other difficulties have been met
+with, and it was easy to operate ordinary lamps, and even motors, in
+this manner. The line being connected to the ground, all the wires could
+be handled with perfect impunity, no matter how high the potential at
+the terminals of the condenser. In these experiments a high tension
+induction coil, operated from a battery or from an alternate current
+machine, was employed to charge the condenser; but the induction coil
+might be replaced by an apparatus of a different kind, capable of giving
+electricity of such high tension. In this manner, direct or alternating
+currents may be converted, and in both cases the current-impulses may be
+of any desired frequency. When the currents charging the condenser are
+of the same direction, and it is desired that the converted currents
+should also be of one direction, the resistance of the discharging
+circuit should, of course, be so chosen that there are no oscillations.
+
+[Illustration: FIG. 128.]
+
+In operating devices on the above plan I have observed curious phenomena
+of impedance which are of interest. For instance if a thick copper bar
+be bent, as indicated in Fig. 128, and shunted by ordinary incandescent
+lamps, then, by passing the discharge between the knobs, the lamps may
+be brought to incandescence although they are short-circuited. When a
+large induction coil is employed it is easy to obtain nodes on the bar,
+which are rendered evident by the different degree of brilliancy of the
+lamps, as shown roughly in Fig. 128. The nodes are never clearly
+defined, but they are simply maxima and minima of potentials along the
+bar. This is probably due to the irregularity of the arc between the
+knobs. In general when the above-described plan of conversion from high
+to low tension is used, the behavior of the disruptive discharge may be
+closely studied. The nodes may also be investigated by means of an
+ordinary Cardew voltmeter which should be well insulated. Geissler
+tubes may also be lighted across the points of the bent bar; in this
+case, of course, it is better to employ smaller capacities. I have found
+it practicable to light up in this manner a lamp, and even a Geissler
+tube, shunted by a short, heavy block of metal, and this result seems at
+first very curious. In fact, the thicker the copper bar in Fig. 128, the
+better it is for the success of the experiments, as they appear more
+striking. When lamps with long slender filaments are used it will be
+often noted that the filaments are from time to time violently vibrated,
+the vibration being smallest at the nodal points. This vibration seems
+to be due to an electrostatic action between the filament and the glass
+of the bulb.
+
+[Illustration: FIG. 129.]
+
+In some of the above experiments it is preferable to use special lamps
+having a straight filament as shown in Fig. 129. When such a lamp is
+used a still more curious phenomenon than those described may be
+observed. The lamp may be placed across the copper bar and lighted, and
+by using somewhat larger capacities, or, in other words, smaller
+frequencies or smaller impulsive impedances, the filament may be brought
+to any desired degree of incandescence. But when the impedance is
+increased, a point is reached when comparatively little current passes
+through the carbon, and most of it through the rarefied gas; or perhaps
+it may be more correct to state that the current divides nearly evenly
+through both, in spite of the enormous difference in the resistance, and
+this would be true unless the gas and the filament behave differently.
+It is then noted that the whole bulb is brilliantly illuminated, and the
+ends of the leading-in wires become incandescent and often throw off
+sparks in consequence of the violent bombardment, but the carbon
+filament remains dark. This is illustrated in Fig. 129. Instead of the
+filament a single wire extending through the whole bulb may be used,
+and in this case the phenomenon would seem to be still more interesting.
+
+From the above experiment it will be evident, that when ordinary lamps
+are operated by the converted currents, those should be preferably taken
+in which the platinum wires are far apart, and the frequencies used
+should not be too great, else the discharge will occur at the ends of
+the filament or in the base of the lamp between the leading-in wires,
+and the lamp might then be damaged.
+
+In presenting to you these results of my investigation on the subject
+under consideration, I have paid only a passing notice to facts upon
+which I could have dwelt at length, and among many observations I have
+selected only those which I thought most likely to interest you. The
+field is wide and completely unexplored, and at every step a new truth
+is gleaned, a novel fact observed.
+
+How far the results here borne out are capable of practical applications
+will be decided in the future. As regards the production of light, some
+results already reached are encouraging and make me confident in
+asserting that the practical solution of the problem lies in the
+direction I have endeavored to indicate. Still, whatever may be the
+immediate outcome of these experiments I am hopeful that they will only
+prove a step in further development towards the ideal and final
+perfection. The possibilities which are opened by modern research are so
+vast that even the most reserved must feel sanguine of the future.
+Eminent scientists consider the problem of utilizing one kind of
+radiation without the others a rational one. In an apparatus designed
+for the production of light by conversion from any form of energy into
+that of light, such a result can never be reached, for no matter what
+the process of producing the required vibrations, be it electrical,
+chemical or any other, it will not be possible to obtain the higher
+light vibrations without going through the lower heat vibrations. It is
+the problem of imparting to a body a certain velocity without passing
+through all lower velocities. But there is a possibility of obtaining
+energy not only in the form of light, but motive power, and energy of
+any other form, in some more direct way from the medium. The time will
+be when this will be accomplished, and the time has come when one may
+utter such words before an enlightened audience without being considered
+a visionary. We are whirling through endless space with an
+inconceivable speed, all around us everything is spinning, everything is
+moving, everywhere is energy. There _must_ be some way of availing
+ourselves of this energy more directly. Then, with the light obtained
+from the medium, with the power derived from it, with every form of
+energy obtained without effort, from the store forever inexhaustible,
+humanity will advance with giant strides. The mere contemplation of
+these magnificent possibilities expands our minds, strengthens our hopes
+and fills our hearts with supreme delight.
+
+
+
+
+CHAPTER XXVII.
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH
+FREQUENCY.[2]
+
+  [2] Lecture delivered before the Institution of Electrical
+      Engineers, London, February, 1892.
+
+
+I cannot find words to express how deeply I feel the honor of addressing
+some of the foremost thinkers of the present time, and so many able
+scientific men, engineers and electricians, of the country greatest in
+scientific achievements.
+
+The results which I have the honor to present before such a gathering I
+cannot call my own. There are among you not a few who can lay better
+claim than myself on any feature of merit which this work may contain. I
+need not mention many names which are world-known--names of those among
+you who are recognized as the leaders in this enchanting science; but
+one, at least, I must mention--a name which could not be omitted in a
+demonstration of this kind. It is a name associated with the most
+beautiful invention ever made: it is Crookes!
+
+When I was at college, a good while ago, I read, in a translation (for
+then I was not familiar with your magnificent language), the description
+of his experiments on radiant matter. I read it only once in my
+life--that time--yet every detail about that charming work I can
+remember to this day. Few are the books, let me say, which can make such
+an impression upon the mind of a student.
+
+But if, on the present occasion, I mention this name as one of many your
+Institution can boast of, it is because I have more than one reason to
+do so. For what I have to tell you and to show you this evening
+concerns, in a large measure, that same vague world which Professor
+Crookes has so ably explored; and, more than this, when I trace back the
+mental process which led me to these advances--which even by myself
+cannot be considered trifling, since they are so appreciated by you--I
+believe that their real origin, that which started me to work in this
+direction, and brought me to them, after a long period of constant
+thought, was that fascinating little book which I read many years ago.
+
+And now that I have made a feeble effort to express my homage and
+acknowledge my indebtedness to him and others among you, I will make a
+second effort, which I hope you will not find so feeble as the first, to
+entertain you.
+
+Give me leave to introduce the subject in a few words.
+
+A short time ago I had the honor to bring before our American Institute
+of Electrical Engineers some results then arrived at by me in a novel
+line of work. I need not assure you that the many evidences which I have
+received that English scientific men and engineers were interested in
+this work have been for me a great reward and encouragement. I will not
+dwell upon the experiments already described, except with the view of
+completing, or more clearly expressing, some ideas advanced by me
+before, and also with the view of rendering the study here presented
+self-contained, and my remarks on the subject of this evening's lecture
+consistent.
+
+This investigation, then, it goes without saying, deals with alternating
+currents, and to be more precise, with alternating currents of high
+potential and high frequency. Just in how much a very high frequency is
+essential for the production of the results presented is a question
+which, even with my present experience, would embarrass me to answer.
+Some of the experiments may be performed with low frequencies; but very
+high frequencies are desirable, not only on account of the many effects
+secured by their use, but also as a convenient means of obtaining, in
+the induction apparatus employed, the high potentials, which in their
+turn are necessary to the demonstration of most of the experiments here
+contemplated.
+
+Of the various branches of electrical investigation, perhaps the most
+interesting and the most immediately promising is that dealing with
+alternating currents. The progress in this branch of applied science has
+been so great in recent years that it justifies the most sanguine hopes.
+Hardly have we become familiar with one fact, when novel experiences are
+met and new avenues of research are opened. Even at this hour
+possibilities not dreamed of before are, by the use of these currents,
+partly realized. As in nature all is ebb and tide, all is wave motion,
+so it seems that in all branches of industry alternating
+currents--electric wave motion--will have the sway.
+
+One reason, perhaps, why this branch of science is being so rapidly
+developed is to be found in the interest which is attached to its
+experimental study. We wind a simple ring of iron with coils; we
+establish the connections to the generator, and with wonder and delight
+we note the effects of strange forces which we bring into play, which
+allow us to transform, to transmit and direct energy at will. We arrange
+the circuits properly, and we see the mass of iron and wires behave as
+though it were endowed with life, spinning a heavy armature, through
+invisible connections, with great speed and power--with the energy
+possibly conveyed from a great distance. We observe how the energy of an
+alternating current traversing the wire manifests itself--not so much in
+the wire as in the surrounding space--in the most surprising manner,
+taking the forms of heat, light, mechanical energy, and, most surprising
+of all, even chemical affinity. All these observations fascinate us, and
+fill us with an intense desire to know more about the nature of these
+phenomena. Each day we go to our work in the hope of discovering,--in
+the hope that some one, no matter who, may find a solution of one of the
+pending great problems,--and each succeeding day we return to our task
+with renewed ardor; and even if we _are_ unsuccessful, our work has not
+been in vain, for in these strivings, in these efforts, we have found
+hours of untold pleasure, and we have directed our energies to the
+benefit of mankind.
+
+We may take--at random, if you choose--any of the many experiments which
+may be performed with alternating currents; a few of which only, and by
+no means the most striking, form the subject of this evening's
+demonstration; they are all equally interesting, equally inciting to
+thought.
+
+Here is a simple glass tube from which the air has been partially
+exhausted. I take hold of it; I bring my body in contact with a wire
+conveying alternating currents of high potential, and the tube in my
+hand is brilliantly lighted. In whatever position I may put it, wherever
+I move it in space, as far as I can reach, its soft, pleasing light
+persists with undiminished brightness.
+
+Here is an exhausted bulb suspended from a single wire. Standing on an
+insulated support, I grasp it, and a platinum button mounted in it is
+brought to vivid incandescence.
+
+Here, attached to a leading wire, is another bulb, which, as I touch its
+metallic socket, is filled with magnificent colors of phosphorescent
+light.
+
+Here still another, which by my fingers' touch casts a shadow--the
+Crookes shadow--of the stem inside of it.
+
+Here, again, insulated as I stand on this platform, I bring my body in
+contact with one of the terminals of the secondary of this induction
+coil--with the end of a wire many miles long--and you see streams of
+light break forth from its distant end, which is set in violent
+vibration.
+
+Here, once more, I attach these two plates of wire gauze to the
+terminals of the coil; I set them a distance apart, and I set the coil
+to work. You may see a small spark pass between the plates. I insert a
+thick plate of one of the best dielectrics between them, and instead of
+rendering altogether impossible, as we are used to expect, I _aid_ the
+passage of the discharge, which, as I insert the plate, merely changes
+in appearance and assumes the form of luminous streams.
+
+Is there, I ask, can there be, a more interesting study than that of
+alternating currents?
+
+In all these investigations, in all these experiments, which are so
+very, very interesting, for many years past--ever since the greatest
+experimenter who lectured in this hall discovered its principle--we have
+had a steady companion, an appliance familiar to every one, a plaything
+once, a thing of momentous importance now--the induction coil. There is
+no dearer appliance to the electrician. From the ablest among you, I
+dare say, down to the inexperienced student, to your lecturer, we all
+have passed many delightful hours in experimenting with the induction
+coil. We have watched its play, and thought and pondered over the
+beautiful phenomena which it disclosed to our ravished eyes. So well
+known is this apparatus, so familiar are these phenomena to every one,
+that my courage nearly fails me when I think that I have ventured to
+address so able an audience, that I have ventured to entertain you with
+that same old subject. Here in reality is the same apparatus, and here
+are the same phenomena, only the apparatus is operated somewhat
+differently, the phenomena are presented in a different aspect. Some of
+the results we find as expected, others surprise us, but all captivate
+our attention, for in scientific investigation each novel result
+achieved may be the centre of a new departure, each novel fact learned
+may lead to important developments.
+
+Usually in operating an induction coil we have set up a vibration of
+moderate frequency in the primary, either by means of an interrupter or
+break, or by the use of an alternator. Earlier English investigators, to
+mention only Spottiswoode and J. E. H. Gordon, have used a rapid break
+in connection with the coil. Our knowledge and experience of to-day
+enables us to see clearly why these coils under the conditions of the
+test did not disclose any remarkable phenomena, and why able
+experimenters failed to perceive many of the curious effects which have
+since been observed.
+
+In the experiments such as performed this evening, we operate the coil
+either from a specially constructed alternator capable of giving many
+thousands of reversals of current per second, or, by disruptively
+discharging a condenser through the primary, we set up a vibration in
+the secondary circuit of a frequency of many hundred thousand or
+millions per second, if we so desire; and in using either of these means
+we enter a field as yet unexplored.
+
+It is impossible to pursue an investigation in any novel line without
+finally making some interesting observation or learning some useful
+fact. That this statement is applicable to the subject of this lecture
+the many curious and unexpected phenomena which we observe afford a
+convincing proof. By way of illustration, take for instance the most
+obvious phenomena, those of the discharge of the induction coil.
+
+Here is a coil which is operated by currents vibrating with extreme
+rapidity, obtained by disruptively discharging a Leyden jar. It would
+not surprise a student were the lecturer to say that the secondary of
+this coil consists of a small length of comparatively stout wire; it
+would not surprise him were the lecturer to state that, in spite of
+this, the coil is capable of giving any potential which the best
+insulation of the turns is able to withstand; but although he may be
+prepared, and even be indifferent as to the anticipated result, yet the
+aspect of the discharge of the coil will surprise and interest him.
+Every one is familiar with the discharge of an ordinary coil; it need
+not be reproduced here. But, by way of contrast, here is a form of
+discharge of a coil, the primary current of which is vibrating several
+hundred thousand times per second. The discharge of an ordinary coil
+appears as a simple line or band of light. The discharge of this coil
+appears in the form of powerful brushes and luminous streams issuing
+from all points of the two straight wires attached to the terminals of
+the secondary. (Fig. 130.)
+
+[Illustration: FIG. 130.]
+
+[Illustration: FIG. 131.]
+
+Now compare this phenomenon which you have just witnessed with the
+discharge of a Holtz or Wimshurst machine--that other interesting
+appliance so dear to the experimenter. What a difference there is
+between these phenomena! And yet, had I made the necessary
+arrangements--which could have been made easily, were it not that they
+would interfere with other experiments--I could have produced with this
+coil sparks which, had I the coil hidden from your view and only two
+knobs exposed, even the keenest observer among you would find it
+difficult, if not impossible, to distinguish from those of an influence
+or friction machine. This may be done in many ways--for instance, by
+operating the induction coil which charges the condenser from an
+alternating-current machine of very low frequency, and preferably
+adjusting the discharge circuit so that there are no oscillations set up
+in it. We then obtain in the secondary circuit, if the knobs are of the
+required size and properly set, a more or less rapid succession of
+sparks of great intensity and small quantity, which possess the same
+brilliancy, and are accompanied by the same sharp crackling sound, as
+those obtained from a friction or influence machine.
+
+Another way is to pass through two primary circuits, having a common
+secondary, two currents of a slightly different period, which produce in
+the secondary circuit sparks occurring at comparatively long intervals.
+But, even with the means at hand this evening, I may succeed in
+imitating the spark of a Holtz machine. For this purpose I establish
+between the terminals of the coil which charges the condenser a long,
+unsteady arc, which is periodically interrupted by the upward current of
+air produced by it. To increase the current of air I place on each side
+of the arc, and close to it, a large plate of mica. The condenser
+charged from this coil discharges into the primary circuit of a second
+coil through a small air gap, which is necessary to produce a sudden
+rush of current through the primary. The scheme of connections in the
+present experiment is indicated in Fig. 131.
+
+G is an ordinarily constructed alternator, supplying the primary P of an
+induction coil, the secondary S of which charges the condensers or jars
+C C. The terminals of the secondary are connected to the inside coatings
+of the jars, the outer coatings being connected to the ends of the
+primary _p p_ of a second induction coil. This primary _p p_ has a small
+air gap _a b_.
+
+The secondary _s_ of this coil is provided with knobs or spheres K K of
+the proper size and set at a distance suitable for the experiment.
+
+A long arc is established between the terminals A B of the first
+induction coil. M M are the mica plates.
+
+Each time the arc is broken between A and B the jars are quickly charged
+and discharged through the primary _p p_, producing a snapping spark
+between the knobs K K. Upon the arc forming between A and B the
+potential falls, and the jars cannot be charged to such high potential
+as to break through the air gap _a b_ until the arc is again broken by
+the draught.
+
+In this manner sudden impulses, at long intervals, are produced in the
+primary _p p_, which in the secondary _s_ give a corresponding number of
+impulses of great intensity. If the secondary knobs or spheres, K K, are
+of the proper size, the sparks show much resemblance to those of a Holtz
+machine.
+
+But these two effects, which to the eye appear so very different, are
+only two of the many discharge phenomena. We only need to change the
+conditions of the test, and again we make other observations of
+interest.
+
+When, instead of operating the induction coil as in the last two
+experiments, we operate it from a high frequency alternator, as in the
+next experiment, a systematic study of the phenomena is rendered much
+more easy. In such case, in varying the strength and frequency of the
+currents through the primary, we may observe five distinct forms of
+discharge, which I have described in my former paper on the subject
+before the American Institute of Electrical Engineers, May 20, 1891.
+
+It would take too much time, and it would lead us too far from the
+subject presented this evening, to reproduce all these forms, but it
+seems to me desirable to show you one of them. It is a brush discharge,
+which is interesting in more than one respect. Viewed from a near
+position it resembles much a jet of gas escaping under great pressure.
+We know that the phenomenon is due to the agitation of the molecules
+near the terminal, and we anticipate that some heat must be developed by
+the impact of the molecules against the terminal or against each other.
+Indeed, we find that the brush is hot, and only a little thought leads
+us to the conclusion that, could we but reach sufficiently high
+frequencies, we could produce a brush which would give intense light and
+heat, and which would resemble in every particular an ordinary flame,
+save, perhaps, that both phenomena might not be due to the same
+agent--save, perhaps, that chemical affinity might not be _electrical_
+in its nature.
+
+As the production of heat and light is here due to the impact of the
+molecules, or atoms of air, or something else besides, and, as we can
+augment the energy simply by raising the potential, we might, even with
+frequencies obtained from a dynamo machine, intensify the action to such
+a degree as to bring the terminal to melting heat. But with such low
+frequencies we would have to deal always with something of the nature of
+an electric current. If I approach a conducting object to the brush, a
+thin little spark passes, yet, even with the frequencies used this
+evening, the tendency to spark is not very great. So, for instance, if I
+hold a metallic sphere at some distance above the terminal, you may see
+the whole space between the terminal and sphere illuminated by the
+streams without the spark passing; and with the much higher frequencies
+obtainable by the disruptive discharge of a condenser, were it not for
+the sudden impulses, which are comparatively few in number, sparking
+would not occur even at very small distances. However, with incomparably
+higher frequencies, which we may yet find means to produce efficiently,
+and provided that electric impulses of such high frequencies could be
+transmitted through a conductor, the electrical characteristics of the
+brush discharge would completely vanish--no spark would pass, no shock
+would be felt--yet we would still have to deal with an _electric_
+phenomenon, but in the broad, modern interpretation of the word. In my
+first paper, before referred to, I have pointed out the curious
+properties of the brush, and described the best manner of producing it,
+but I have thought it worth while to endeavor to express myself more
+clearly in regard to this phenomenon, because of its absorbing interest.
+
+When a coil is operated with currents of very high frequency, beautiful
+brush effects may be produced, even if the coil be of comparatively
+small dimensions. The experimenter may vary them in many ways, and, if
+it were for nothing else, they afford a pleasing sight. What adds to
+their interest is that they may be produced with one single terminal as
+well as with two--in fact, often better with one than with two.
+
+But of all the discharge phenomena observed, the most pleasing to the
+eye, and the most instructive, are those observed with a coil which is
+operated by means of the disruptive discharge of a condenser. The power
+of the brushes, the abundance of the sparks, when the conditions are
+patiently adjusted, is often amazing. With even a very small coil, if it
+be so well insulated as to stand a difference of potential of several
+thousand volts per turn, the sparks may be so abundant that the whole
+coil may appear a complete mass of fire.
+
+Curiously enough the sparks, when the terminals of the coil are set at a
+considerable distance, seem to dart in every possible direction as
+though the terminals were perfectly independent of each other. As the
+sparks would soon destroy the insulation, it is necessary to prevent
+them. This is best done by immersing the coil in a good liquid
+insulator, such as boiled-out oil. Immersion in a liquid may be
+considered almost an absolute necessity for the continued and successful
+working of such a coil.
+
+It is, of course, out of the question, in an experimental lecture, with
+only a few minutes at disposal for the performance of each experiment,
+to show these discharge phenomena to advantage, as, to produce each
+phenomenon at its best, a very careful adjustment is required. But even
+if imperfectly produced, as they are likely to be this evening, they are
+sufficiently striking to interest an intelligent audience.
+
+Before showing some of these curious effects I must, for the sake of
+completeness, give a short description of the coil and other apparatus
+used in the experiments with the disruptive discharge this evening.
+
+[Illustration: FIG. 132.]
+
+It is contained in a box B (Fig. 132) of thick boards of hard wood,
+covered on the outside with a zinc sheet Z, which is carefully soldered
+all around. It might be advisable, in a strictly scientific
+investigation, when accuracy is of great importance, to do away with the
+metal cover, as it might introduce many errors, principally on account
+of its complex action upon the coil, as a condenser of very small
+capacity and as an electrostatic and electromagnetic screen. When the
+coil is used for such experiments as are here contemplated, the
+employment of the metal cover offers some practical advantages, but
+these are not of sufficient importance to be dwelt upon.
+
+The coil should be placed symmetrically to the metal cover, and the
+space between should, of course, not be too small, certainly not less
+than, say, five centimetres, but much more if possible; especially the
+two sides of the zinc box, which are at right angles to the axis of the
+coil, should be sufficiently remote from the latter, as otherwise they
+might impair its action and be a source of loss.
+
+The coil consists of two spools of hard rubber R R, held apart at a
+distance of 10 centimetres by bolts C and nuts _n_, likewise of hard
+rubber. Each spool comprises a tube T of approximately 8 centimetres
+inside diameter, and 3 millimetres thick, upon which are screwed two
+flanges F F, 24 centimetres square, the space between the flanges being
+about 3 centimetres. The secondary, S S, of the best gutta
+percha-covered wire, has 26 layers, 10 turns in each, giving for each
+half a total of 260 turns. The two halves are wound oppositely and
+connected in series, the connection between both being made over the
+primary. This disposition, besides being convenient, has the advantage
+that when the coil is well balanced--that is, when both of its
+terminals T_{1}, T_{1}, are connected to bodies or devices of equal
+capacity--there is not much danger of breaking through to the primary,
+and the insulation between the primary and the secondary need not be
+thick. In using the coil it is advisable to attach to _both_ terminals
+devices of nearly equal capacity, as, when the capacity of the terminals
+is not equal, sparks will be apt to pass to the primary. To avoid this,
+the middle point of the secondary may be connected to the primary, but
+this is not always practicable.
+
+The primary P P is wound in two parts, and oppositely, upon a wooden
+spool w, and the four ends are led out of the oil through hard rubber
+tubes _t t_. The ends of the secondary T_{1} T_{1}, are also led out of
+the oil through rubber tubes t_{1} t_{1} of great thickness. The
+primary and secondary layers are insulated by cotton cloth, the
+thickness of the insulation, of course, bearing some proportion to the
+difference of potential between the turns of the different layers. Each
+half of the primary has four layers, 24 turns in each, this giving a
+total of 96 turns. When both the parts are connected in series, this
+gives a ratio of conversion of about 1:2.7, and with the primaries in
+multiple, 1:5.4; but in operating with very rapidly alternating currents
+this ratio does not convey even an approximate idea of the ratio of the
+E. M. F's. in the primary and secondary circuits. The coil is held in
+position in the oil on wooden supports, there being about 5 centimetres
+thickness of oil all round. Where the oil is not specially needed, the
+space is filled with pieces of wood, and for this purpose principally
+the wooden box B surrounding the whole is used.
+
+The construction here shown is, of course, not the best on general
+principles, but I believe it is a good and convenient one for the
+production of effects in which an excessive potential and a very small
+current are needed.
+
+In connection with the coil I use either the ordinary form of discharger
+or a modified form. In the former I have introduced two changes which
+secure some advantages, and which are obvious. If they are mentioned, it
+is only in the hope that some experimenter may find them of use.
+
+One of the changes is that the adjustable knobs A and B (Fig. 133), of
+the discharger are held in jaws of brass, J J, by spring pressure, this
+allowing of turning them successively into different positions, and so
+doing away with the tedious process of frequent polishing up.
+
+[Illustration: FIG. 133.]
+
+The other change consists in the employment of a strong electromagnet
+N S, which is placed with its axis at right angles to the line joining
+the knobs A and B, and produces a strong magnetic field between them.
+The pole pieces of the magnet are movable and properly formed so as to
+protrude between the brass knobs, in order to make the field as intense
+as possible; but to prevent the discharge from jumping to the magnet the
+pole pieces are protected by a layer of mica, M M, of sufficient
+thickness; s_{1} s_{1} and s_{2} s_{2} are screws for fastening the
+wires. On each side one of the screws is for large and the other for
+small wires. L L are screws for fixing in position the rods R R, which
+support the knobs.
+
+In another arrangement with the magnet I take the discharge between the
+rounded pole pieces themselves, which in such case are insulated and
+preferably provided with polished brass caps.
+
+The employment of an intense magnetic field is of advantage principally
+when the induction coil or transformer which charges the condenser is
+operated by currents of very low frequency. In such a case the number of
+the fundamental discharges between the knobs may be so small as to
+render the currents produced in the secondary unsuitable for many
+experiments. The intense magnetic field then serves to blow out the arc
+between the knobs as soon as it is formed, and the fundamental
+discharges occur in quicker succession.
+
+[Illustration: FIG. 134.]
+
+Instead of the magnet, a draught or blast of air may be employed with
+some advantage. In this case the arc is preferably established between
+the knobs A B, in Fig. 131 (the knobs _a b_ being generally joined, or
+entirely done away with), as in this disposition the arc is long and
+unsteady, and is easily affected by the draught.
+
+When a magnet is employed to break the arc, it is better to choose the
+connection indicated diagrammatically in Fig. 134, as in this case the
+currents forming the arc are much more powerful, and the magnetic field
+exercises a greater influence. The use of the magnet permits, however,
+of the arc being replaced by a vacuum tube, but I have encountered great
+difficulties in working with an exhausted tube.
+
+The other form of discharger used in these and similar experiments is
+indicated in Figs. 135 and 136. It consists of a number of brass pieces
+_c c_ (Fig. 135), each of which comprises a spherical middle portion _m_
+with an extension _e_ below--which is merely used to fasten the piece in
+a lathe when polishing up the discharging surface--and a column above,
+which consists of a knurled flange _f_ surmounted by a threaded stem _l_
+carrying a nut _n_, by means of which a wire is fastened to the column.
+The flange _f_ conveniently serves for holding the brass piece when
+fastening the wire, and also for turning it in any position when it
+becomes necessary to present a fresh discharging surface. Two stout
+strips of hard rubber R R, with planed grooves _g g_ (Fig. 136) to fit
+the middle portion of the pieces _c c_, serve to clamp the latter and
+hold them firmly in position by means of two bolts C C (of which only
+one is shown) passing through the ends of the strips.
+
+[Illustration: FIG. 135.]
+
+[Illustration: FIG. 136.]
+
+In the use of this kind of discharger I have found three principal
+advantages over the ordinary form. First, the dielectric strength of a
+given total width of air space is greater when a great many small air
+gaps are used instead of one, which permits of working with a smaller
+length of air gap, and that means smaller loss and less deterioration of
+the metal; secondly, by reason of splitting the arc up into smaller
+arcs, the polished surfaces are made to last much longer; and, thirdly,
+the apparatus affords some gauge in the experiments. I usually set the
+pieces by putting between them sheets of uniform thickness at a certain
+very small distance which is known from the experiments of Sir William
+Thomson to require a certain electromotive force to be bridged by the
+spark.
+
+It should, of course, be remembered that the sparking distance is much
+diminished as the frequency is increased. By taking any number of spaces
+the experimenter has a rough idea of the electromotive force, and he
+finds it easier to repeat an experiment, as he has not the trouble of
+setting the knobs again and again. With this kind of discharger I have
+been able to maintain an oscillating motion without any spark being
+visible with the naked eye between the knobs, and they would not show a
+very appreciable rise in temperature. This form of discharge also lends
+itself to many arrangements of condensers and circuits which are often
+very convenient and time-saving. I have used it preferably in a
+disposition similar to that indicated in Fig. 131, when the currents
+forming the arc are small.
+
+I may here mention that I have also used dischargers with single or
+multiple air gaps, in which the discharge surfaces were rotated with
+great speed. No particular advantage was, however, gained by this
+method, except in cases where the currents from the condenser were large
+and the keeping cool of the surfaces was necessary, and in cases when,
+the discharge not being oscillating of itself, the arc as soon as
+established was broken by the air current, thus starting the vibration
+at intervals in rapid succession. I have also used mechanical
+interrupters in many ways. To avoid the difficulties with frictional
+contacts, the preferred plan adopted was to establish the arc and rotate
+through it at great speed a rim of mica provided with many holes and
+fastened to a steel plate. It is understood, of course, that the
+employment of a magnet, air current, or other interrupter, produces no
+effect worth noticing, unless the self-induction, capacity and
+resistance are so related that there are oscillations set up upon each
+interruption.
+
+I will now endeavor to show you some of the most noteworthy of these
+discharge phenomena.
+
+I have stretched across the room two ordinary cotton covered wires, each
+about seven metres in length. They are supported on insulating cords at
+a distance of about thirty centimetres. I attach now to each of the
+terminals of the coil one of the wires, and set the coil in action.
+Upon turning the lights off in the room you see the wires strongly
+illuminated by the streams issuing abundantly from their whole surface
+in spite of the cotton covering, which may even be very thick. When the
+experiment is performed under good conditions, the light from the wires
+is sufficiently intense to allow distinguishing the objects in a room.
+To produce the best result it is, of course, necessary to adjust
+carefully the capacity of the jars, the arc between the knobs and the
+length of the wires. My experience is that calculation of the length of
+the wires leads, in such case, to no result whatever. The experimenter
+will do best to take the wires at the start very long, and then adjust
+by cutting off first long pieces, and then smaller and smaller ones as
+he approaches the right length.
+
+A convenient way is to use an oil condenser of very small capacity,
+consisting of two small adjustable metal plates, in connection with this
+and similar experiments. In such case I take wires rather short and at
+the beginning set the condenser plates at maximum distance. If the
+streams from the wires increase by approach of the plates, the length of
+the wires is about right; if they diminish, the wires are too long for
+that frequency and potential. When a condenser is used in connection
+with experiments with such a coil, it should be an oil condenser by all
+means, as in using an air condenser considerable energy might be wasted.
+The wires leading to the plates in the oil should be very thin, heavily
+coated with some insulating compound, and provided with a conducting
+covering--this preferably extending under the surface of the oil. The
+conducting cover should not be too near the terminals, or ends, of the
+wire, as a spark would be apt to jump from the wire to it. The
+conducting coating is used to diminish the air losses, in virtue of its
+action as an electrostatic screen. As to the size of the vessel
+containing the oil, and the size of the plates, the experimenter gains
+at once an idea from a rough trial. The size of the plates _in oil_ is,
+however, calculable, as the dielectric losses are very small.
+
+In the preceding experiment it is of considerable interest to know what
+relation the quantity of the light emitted bears to the frequency and
+potential of the electric impulses. My opinion is that the heat as well
+as light effects produced should be proportionate, under otherwise equal
+conditions of test, to the product of frequency and square of potential,
+but the experimental verification of the law, whatever it may be, would
+be exceedingly difficult. One thing is certain, at any rate, and that
+is, that in augmenting the potential and frequency we rapidly intensify
+the streams; and, though it may be very sanguine, it is surely not
+altogether hopeless to expect that we may succeed in producing a
+practical illuminant on these lines. We would then be simply using
+burners or flames, in which there would be no chemical process, no
+consumption of material, but merely a transfer of energy, and which
+would, in all probability, emit more light and less heat than ordinary
+flames.
+
+[Illustration: FIG. 137.]
+
+The luminous intensity of the streams is, of course, considerably
+increased when they are focused upon a small surface. This may be shown
+by the following experiment:
+
+I attach to one of the terminals of the coil a wire _w_ (Fig. 137), bent
+in a circle of about 30 centimetres in diameter, and to the other
+terminal I fasten a small brass sphere _s_, the surface of the wire
+being preferably equal to the surface of the sphere, and the centre of
+the latter being in a line at right angles to the plane of the wire
+circle and passing through its centre. When the discharge is established
+under proper conditions, a luminous hollow cone is formed, and in the
+dark one-half of the brass sphere is strongly illuminated, as shown in
+the cut.
+
+By some artifice or other it is easy to concentrate the streams upon
+small surfaces and to produce very strong light effects. Two thin wires
+may thus be rendered intensely luminous.
+
+In order to intensify the streams the wires should be very thin and
+short; but as in this case their capacity would be generally too small
+for the coil--at least for such a one as the present--it is necessary to
+augment the capacity to the required value, while, at the same time, the
+surface of the wires remains very small. This may be done in many ways.
+
+[Illustration: FIG. 138.]
+
+Here, for instance, I have two plates, R R, of hard rubber (Fig. 138),
+upon which I have glued two very thin wires _w w_, so as to form a name.
+The wires may be bare or covered with the best insulation--it is
+immaterial for the success of the experiment. Well insulated wires, if
+anything, are preferable. On the back of each plate, indicated by the
+shaded portion, is a tinfoil coating _t t_. The plates are placed in
+line at a sufficient distance to prevent a spark passing from one wire
+to the other. The two tinfoil coatings I have joined by a conductor C,
+and the two wires I presently connect to the terminals of the coil. It
+is now easy, by varying the strength and frequency of the currents
+through the primary, to find a point at which the capacity of the system
+is best suited to the conditions, and the wires become so strongly
+luminous that, when the light in the room is turned off the name formed
+by them appears in brilliant letters.
+
+It is perhaps preferable to perform this experiment with a coil operated
+from an alternator of high frequency, as then, owing to the harmonic
+rise and fall, the streams are very uniform, though they are less
+abundant than when produced with such a coil as the present one. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.
+
+[Illustration: FIG. 139.]
+
+When two wires, attached to the terminals of the coil, are set at the
+proper distance, the streams between them may be so intense as to
+produce a continuous luminous sheet. To show this phenomenon I have here
+two circles, C and _c_ (Fig. 139), of rather stout wire, one being about
+80 centimetres and the other 30 centimetres in diameter. To each of the
+terminals of the coil I attach one of the circles. The supporting wires
+are so bent that the circles may be placed in the same plane, coinciding
+as nearly as possible. When the light in the room is turned off and the
+coil set to work, you see the whole space between the wires uniformly
+filled with streams, forming a luminous disc, which could be seen from a
+considerable distance, such is the intensity of the streams. The outer
+circle could have been much larger than the present one; in fact, with
+this coil I have used much larger circles, and I have been able to
+produce a strongly luminous sheet, covering an area of more than one
+square metre, which is a remarkable effect with this very small coil. To
+avoid uncertainty, the circle has been taken smaller, and the area is
+now about 0.43 square metre.
+
+The frequency of the vibration, and the quickness of succession of the
+sparks between the knobs, affect to a marked degree the appearance of
+the streams. When the frequency is very low, the air gives way in more
+or less the same manner, as by a steady difference of potential, and the
+streams consist of distinct threads, generally mingled with thin sparks,
+which probably correspond to the successive discharges occurring between
+the knobs. But when the frequency is extremely high, and the arc of the
+discharge produces a very _loud_ and _smooth_ sound--showing both that
+oscillation takes place and that the sparks succeed each other with
+great rapidity--then the luminous streams formed are perfectly uniform.
+To reach this result very small coils and jars of small capacity should
+be used. I take two tubes of thick Bohemian glass, about 5 centimetres
+in diameter and 20 centimetres long. In each of the tubes I slip a
+primary of very thick copper wire. On the top of each tube I wind a
+secondary of much thinner gutta-percha covered wire. The two secondaries
+I connect in series, the primaries preferably in multiple arc. The tubes
+are then placed in a large glass vessel, at a distance of 10 to 15
+centimetres from each other, on insulating supports, and the vessel is
+filled with boiled-out oil, the oil reaching about an inch above the
+tubes. The free ends of the secondary are lifted out of the coil and
+placed parallel to each other at a distance of about ten centimetres.
+The ends which are scraped should be dipped in the oil. Two four-pint
+jars joined in series may be used to discharge through the primary. When
+the necessary adjustments in the length and distance of the wires above
+the oil and in the arc of discharge are made, a luminous sheet is
+produced between the wires which is perfectly smooth and textureless,
+like the ordinary discharge through a moderately exhausted tube.
+
+I have purposely dwelt upon this apparently insignificant experiment. In
+trials of this kind the experimenter arrives at the startling conclusion
+that, to pass ordinary luminous discharges through gases, no particular
+degree of exhaustion is needed, but that the gas may be at ordinary or
+even greater pressure. To accomplish this, a very high frequency is
+essential; a high potential is likewise required, but this is merely an
+incidental necessity. These experiments teach us that, in endeavoring to
+discover novel methods of producing light by the agitation of atoms, or
+molecules, of a gas, we need not limit our research to the vacuum tube,
+but may look forward quite seriously to the possibility of obtaining the
+light effects without the use of any vessel whatever, with air at
+ordinary pressure.
+
+Such discharges of very high frequency, which render luminous the air at
+ordinary pressures, we have probably occasion often to witness in
+Nature. I have no doubt that if, as many believe, the aurora borealis is
+produced by sudden cosmic disturbances, such as eruptions at the sun's
+surface, which set the electrostatic charge of the earth in an extremely
+rapid vibration, the red glow observed is not confined to the upper
+rarefied strata of the air, but the discharge traverses, by reason of
+its very high frequency, also the dense atmosphere in the form of a
+_glow_, such as we ordinarily produce in a slightly exhausted tube. If
+the frequency were very low, or even more so, if the charge were not at
+all vibrating, the dense air would break down as in a lightning
+discharge. Indications of such breaking down of the lower dense strata
+of the air have been repeatedly observed at the occurrence of this
+marvelous phenomenon; but if it does occur, it can only be attributed to
+the fundamental disturbances, which are few in number, for the vibration
+produced by them would be far too rapid to allow a disruptive break. It
+is the original and irregular impulses which affect the instruments; the
+superimposed vibrations probably pass unnoticed.
+
+When an ordinary low frequency discharge is passed through moderately
+rarefied air, the air assumes a purplish hue. If by some means or other
+we increase the intensity of the molecular, or atomic, vibration, the
+gas changes to a white color. A similar change occurs at ordinary
+pressures with electric impulses of very high frequency. If the
+molecules of the air around a wire are moderately agitated, the brush
+formed is reddish or violet; if the vibration is rendered sufficiently
+intense, the streams become white. We may accomplish this in various
+ways. In the experiment before shown with the two wires across the room,
+I have endeavored to secure the result by pushing to a high value both
+the frequency and potential; in the experiment with the thin wires glued
+on the rubber plate I have concentrated the action upon a very small
+surface--in other words, I have worked with a great electric density.
+
+[Illustration: FIG. 140.]
+
+A most curious form of discharge is observed with such a coil when the
+frequency and potential are pushed to the extreme limit. To perform the
+experiment, every part of the coil should be heavily insulated, and only
+two small spheres--or, better still, two sharp-edged metal discs (_d d_,
+Fig. 140) of no more than a few centimetres in diameter--should be
+exposed to the air. The coil here used is immersed in oil, and the ends
+of the secondary reaching out of the oil are covered with an air-tight
+cover of hard rubber of great thickness. All cracks, if there are any,
+should be carefully stopped up, so that the brush discharge cannot form
+anywhere except on the small spheres or plates which are exposed to the
+air. In this case, since there are no large plates or other bodies of
+capacity attached to the terminals, the coil is capable of an extremely
+rapid vibration. The potential may be raised by increasing, as far as
+the experimenter judges proper, the rate of change of the primary
+current. With a coil not widely differing from the present, it is best
+to connect the two primaries in multiple arc; but if the secondary
+should have a much greater number of turns the primaries should
+preferably be used in series, as otherwise the vibration might be too
+fast for the secondary. It occurs under these conditions that misty
+white streams break forth from the edges of the discs and spread out
+phantom-like into space. With this coil, when fairly well produced, they
+are about 25 to 30 centimetres long. When the hand is held against them
+no sensation is produced, and a spark, causing a shock, jumps from the
+terminal only upon the hand being brought much nearer. If the
+oscillation of the primary current is rendered intermittent by some
+means or other, there is a corresponding throbbing of the streams, and
+now the hand or other conducting object may be brought in still greater
+proximity to the terminal without a spark being caused to jump.
+
+Among the many beautiful phenomena which may be produced with such a
+coil, I have here selected only those which appear to possess some
+features of novelty, and lead us to some conclusions of interest. One
+will not find it at all difficult to produce in the laboratory, by means
+of it, many other phenomena which appeal to the eye even more than these
+here shown, but present no particular feature of novelty.
+
+Early experimenters describe the display of sparks produced by an
+ordinary large induction coil upon an insulating plate separating the
+terminals. Quite recently Siemens performed some experiments in which
+fine effects were obtained, which were seen by many with interest. No
+doubt large coils, even if operated with currents of low frequencies,
+are capable of producing beautiful effects. But the largest coil ever
+made could not, by far, equal the magnificent display of streams and
+sparks obtained from such a disruptive discharge coil when properly
+adjusted. To give an idea, a coil such as the present one will cover
+easily a plate of one metre in diameter completely with the streams. The
+best way to perform such experiments is to take a very thin rubber or a
+glass plate and glue on one side of it a narrow ring of tinfoil of very
+large diameter, and on the other a circular washer, the centre of the
+latter coinciding with that of the ring, and the surfaces of both being
+preferably equal, so as to keep the coil well balanced. The washer and
+ring should be connected to the terminals by heavily insulated thin
+wires. It is easy in observing the effect of the capacity to produce a
+sheet of uniform streams, or a fine network of thin silvery threads, or
+a mass of loud brilliant sparks, which completely cover the plate.
+
+Since I have advanced the idea of the conversion by means of the
+disruptive discharge, in my paper before the American Institute of
+Electrical Engineers at the beginning of the past year, the interest
+excited in it has been considerable. It affords us a means for producing
+any potentials by the aid of inexpensive coils operated from ordinary
+systems of distribution, and--what is perhaps more appreciated--it
+enables us to convert currents of any frequency into currents of any
+other lower or higher frequency. But its chief value will perhaps be
+found in the help which it will afford us in the investigations of the
+phenomena of phosphorescence, which a disruptive discharge coil is
+capable of exciting in innumerable cases where ordinary coils, even the
+largest, would utterly fail.
+
+Considering its probable uses for many practical purposes, and its
+possible introduction into laboratories for scientific research, a few
+additional remarks as to the construction of such a coil will perhaps
+not be found superfluous.
+
+It is, of course, absolutely necessary to employ in such a coil wires
+provided with the best insulation.
+
+Good coils may be produced by employing wires covered with several
+layers of cotton, boiling the coil a long time in pure wax, and cooling
+under moderate pressure. The advantage of such a coil is that it can be
+easily handled, but it cannot probably give as satisfactory results as a
+coil immersed in pure oil. Besides, it seems that the presence of a
+large body of wax affects the coil disadvantageously, whereas this does
+not seem to be the case with oil. Perhaps it is because the dielectric
+losses in the liquid are smaller.
+
+I have tried at first silk and cotton covered wires with oil immersions,
+but I have been gradually led to use gutta-percha covered wires, which
+proved most satisfactory. Gutta-percha insulation adds, of course, to
+the capacity of the coil, and this, especially if the coil be large, is
+a great disadvantage when extreme frequencies are desired; but, on the
+other hand, gutta-percha will withstand much more than an equal
+thickness of oil, and this advantage should be secured at any price.
+Once the coil has been immersed, it should never be taken out of the oil
+for more than a few hours, else the gutta-percha will crack up and the
+coil will not be worth half as much as before. Gutta-percha is probably
+slowly attacked by the oil, but after an immersion of eight to nine
+months I have found no ill effects.
+
+I have obtained two kinds of gutta-percha wire known in commerce: in one
+the insulation sticks tightly to the metal, in the other it does not.
+Unless a special method is followed to expel all air, it is much safer
+to use the first kind. I wind the coil within an oil tank so that all
+interstices are filled up with the oil. Between the layers I use cloth
+boiled out thoroughly in oil, calculating the thickness according to the
+difference of potential between the turns. There seems not to be a very
+great difference whatever kind of oil is used; I use paraffine or
+linseed oil.
+
+To exclude more perfectly the air, an excellent way to proceed, and
+easily practicable with small coils, is the following: Construct a box
+of hardwood of very thick boards which have been for a long time boiled
+in oil. The boards should be so joined as to safely withstand the
+external air pressure. The coil being placed and fastened in position
+within the box, the latter is closed with a strong lid, and covered with
+closely fitting metal sheets, the joints of which are soldered very
+carefully. On the top two small holes are drilled, passing through the
+metal sheet and the wood, and in these holes two small glass tubes are
+inserted and the joints made air-tight. One of the tubes is connected to
+a vacuum pump, and the other with a vessel containing a sufficient
+quantity of boiled-out oil. The latter tube has a very small hole at the
+bottom, and is provided with a stopcock. When a fairly good vacuum has
+been obtained, the stopcock is opened and the oil slowly fed in.
+Proceeding in this manner, it is impossible that any big bubbles, which
+are the principal danger, should remain between the turns. The air is
+most completely excluded, probably better than by boiling out, which,
+however, when gutta-percha coated wires are used, is not practicable.
+
+For the primaries I use ordinary line wire with a thick cotton coating.
+Strands of very thin insulated wires properly interlaced would, of
+course, be the best to employ for the primaries, but they are not to be
+had.
+
+In an experimental coil the size of the wires is not of great
+importance. In the coil here used the primary is No. 12 and the
+secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
+varied considerably. It would only imply different adjustments; the
+results aimed at would not be materially affected.
+
+I have dwelt at some length upon the various forms of brush discharge
+because, in studying them, we not only observe phenomena which please
+our eye, but also afford us food for thought, and lead us to conclusions
+of practical importance. In the use of alternating currents of very high
+tension, too much precaution cannot be taken to prevent the brush
+discharge. In a main conveying such currents, in an induction coil or
+transformer, or in a condenser, the brush discharge is a source of great
+danger to the insulation. In a condenser, especially, the gaseous matter
+must be most carefully expelled, for in it the charged surfaces are
+near each other, and if the potentials are high, just as sure as a
+weight will fall if let go, so the insulation will give way if a single
+gaseous bubble of some size be present, whereas, if all gaseous matter
+were carefully excluded, the condenser would safely withstand a much
+higher difference of potential. A main conveying alternating currents of
+very high tension may be injured merely by a blow hole or small crack in
+the insulation, the more so as a blowhole is apt to contain gas at low
+pressure; and as it appears almost impossible to completely obviate such
+little imperfections, I am led to believe that in our future
+distribution of electrical energy by currents of very high tension,
+liquid insulation will be used. The cost is a great drawback, but if we
+employ an oil as an insulator the distribution of electrical energy with
+something like 100,000 volts, and even more, becomes, at least with
+higher frequencies, so easy that it could be hardly called an
+engineering feat. With oil insulation and alternate current motors,
+transmissions of power can be affected with safety and upon an
+industrial basis at distances of as much as a thousand miles.
+
+A peculiar property of oils, and liquid insulation in general, when
+subjected to rapidly changing electric stresses, is to disperse any
+gaseous bubbles which may be present, and diffuse them through its mass,
+generally long before any injurious break can occur. This feature may be
+easily observed with an ordinary induction coil by taking the primary
+out, plugging up the end of the tube upon which the secondary is wound,
+and filling it with some fairly transparent insulator, such as paraffine
+oil. A primary of a diameter something like six millimetres smaller than
+the inside of the tube may be inserted in the oil. When the coil is set
+to work one may see, looking from the top through the oil, many luminous
+points--air bubbles which are caught by inserting the primary, and which
+are rendered luminous in consequence of the violent bombardment. The
+occluded air, by its impact against the oil, heats it; the oil begins to
+circulate, carrying some of the air along with it, until the bubbles are
+dispersed and the luminous points disappear. In this manner, unless
+large bubbles are occluded in such way that circulation is rendered
+impossible, a damaging break is averted, the only effect being a
+moderate warming up of the oil. If, instead of the liquid, a solid
+insulation, no matter how thick, were used, a breaking through and
+injury of the apparatus would be inevitable.
+
+The exclusion of gaseous matter from any apparatus in which the
+dielectric is subjected to more or less rapidly changing electric forces
+is, however, not only desirable in order to avoid a possible injury of
+the apparatus, but also on account of economy. In a condenser, for
+instance, as long as only a solid or only a liquid dielectric is used,
+the loss is small; but if a gas under ordinary or small pressure be
+present the loss may be very great. Whatever the nature of the force
+acting in the dielectric may be, it seems that in a solid or liquid the
+molecular displacement produced by the force is small: hence the product
+of force and displacement is insignificant, unless the force be very
+great; but in a gas the displacement, and therefore this product, is
+considerable; the molecules are free to move, they reach high speeds,
+and the energy of their impact is lost in heat or otherwise. If the gas
+be strongly compressed, the displacement due to the force is made
+smaller, and the losses are reduced.
+
+In most of the succeeding experiments I prefer, chiefly on account of
+the regular and positive action, to employ the alternator before
+referred to. This is one of the several machines constructed by me for
+the purpose of these investigations. It has 384 pole projections, and is
+capable of giving currents of a frequency of about 10,000 per second.
+This machine has been illustrated and briefly described in my first
+paper before the American Institute of Electrical Engineers, May 20th,
+1891, to which I have already referred. A more detailed description,
+sufficient to enable any engineer to build a similar machine, will be
+found in several electrical journals of that period.
+
+The induction coils operated from the machine are rather small,
+containing from 5,000 to 15,000 turns in the secondary. They are
+immersed in boiled-out linseed oil, contained in wooden boxes covered
+with zinc sheet.
+
+I have found it advantageous to reverse the usual position of the wires,
+and to wind, in these coils, the primaries on the top; thus allowing the
+use of a much larger primary, which, of course, reduces the danger of
+overheating and increases the output of the coil. I make the primary on
+each side at least one centimetre shorter than the secondary, to prevent
+the breaking through on the ends, which would surely occur unless the
+insulation on the top of the secondary be very thick, and this, of
+course, would be disadvantageous.
+
+When the primary is made movable, which is necessary in some
+experiments, and many times convenient for the purposes of adjustment, I
+cover the secondary with wax, and turn it off in a lathe to a diameter
+slightly smaller than the inside of the primary coil. The latter I
+provide with a handle reaching out of the oil, which serves to shift it
+in any position along the secondary.
+
+I will now venture to make, in regard to the general manipulation of
+induction coils, a few observations bearing upon points which have not
+been fully appreciated in earlier experiments with such coils, and are
+even now often overlooked.
+
+The secondary of the coil possesses usually such a high self-induction
+that the current through the wire is inappreciable, and may be so even
+when the terminals are joined by a conductor of small resistance. If
+capacity is added to the terminals, the self-induction is counteracted,
+and a stronger current is made to flow through the secondary, though its
+terminals are insulated from each other. To one entirely unacquainted
+with the properties of alternating currents nothing will look more
+puzzling. This feature was illustrated in the experiment performed at
+the beginning with the top plates of wire gauze attached to the
+terminals and the rubber plate. When the plates of wire gauze were close
+together, and a small arc passed between them, the arc _prevented_ a
+strong current from passing through the secondary, because it did away
+with the capacity on the terminals; when the rubber plate was inserted
+between, the capacity of the condenser formed counteracted the
+self-induction of the secondary, a stronger current passed now, the coil
+performed more work, and the discharge was by far more powerful.
+
+The first thing, then, in operating the induction coil is to combine
+capacity with the secondary to overcome the self-induction. If the
+frequencies and potentials are very high, gaseous matter should be
+carefully kept away from the charged surfaces. If Leyden jars are used,
+they should be immersed in oil, as otherwise considerable dissipation
+may occur if the jars are greatly strained. When high frequencies are
+used, it is of equal importance to combine a condenser with the primary.
+One may use a condenser connected to the ends of the primary or to the
+terminals of the alternator, but the latter is not to be recommended, as
+the machine might be injured. The best way is undoubtedly to use the
+condenser in series with the primary and with the alternator, and to
+adjust its capacity so as to annul the self-induction of both the
+latter. The condenser should be adjustable by very small steps, and for
+a finer adjustment a small oil condenser with movable plates may be used
+conveniently.
+
+I think it best at this juncture to bring before you a phenomenon,
+observed by me some time ago, which to the purely scientific
+investigator may perhaps appear more interesting than any of the results
+which I have the privilege to present to you this evening.
+
+It may be quite properly ranked among the brush phenomena--in fact, it
+is a brush, formed at, or near, a single terminal in high vacuum.
+
+[Illustration: FIG. 141.]
+
+[Illustration: FIG. 142.]
+
+In bulbs provided with a conducting terminal, though it be of aluminum,
+the brush has but an ephemeral existence, and cannot, unfortunately, be
+indefinitely preserved in its most sensitive state, even in a bulb
+devoid of any conducting electrode. In studying the phenomenon, by all
+means a bulb having no leading-in wire should be used. I have found it
+best to use bulbs constructed as indicated in Figs. 141 and 142.
+
+In Fig. 141 the bulb comprises an incandescent lamp globe _L_, in the
+neck of which is sealed a barometer tube _b_, the end of which is blown
+out to form a small sphere _s_. This sphere should be sealed as closely
+as possible in the centre of the large globe. Before sealing, a thin
+tube _t_, of aluminum sheet, may be slipped in the barometer tube, but
+it is not important to employ it.
+
+The small hollow sphere _s_ is filled with some conducting powder, and a
+wire _w_ is cemented in the neck for the purpose of connecting the
+conducting powder with the generator.
+
+The construction shown in Fig. 142 was chosen in order to remove from
+the brush any conducting body which might possibly affect it. The bulb
+consists in this case of a lamp globe _L_, which has a neck _n_,
+provided with a tube _b_ and small sphere _s_, sealed to it, so that two
+entirely independent compartments are formed, as indicated in the
+drawing. When the bulb is in use the neck _n_ is provided with a tinfoil
+coating, which is connected to the generator and acts inductively upon
+the moderately rarefied and highly conducted gas inclosed in the neck.
+From there the current passes through the tube _b_ into the small sphere
+_s_, to act by induction upon the gas contained in the globe _L_.
+
+It is of advantage to make the tube _t_ very thick, the hole through it
+very small, and to blow the sphere _s_ very thin. It is of the greatest
+importance that the sphere _s_ be placed in the centre of the globe _L_.
+
+[Illustration: FIG. 143.]
+
+Figs. 143, 144 and 145 indicate different forms, or stages, of the
+brush. Fig. 143 shows the brush as it first appears in a bulb provided
+with a conducting terminal; but, as in such a bulb it very soon
+disappears--often after a few minutes--I will confine myself to the
+description of the phenomenon as seen in a bulb without conducting
+electrode. It is observed under the following conditions:
+
+When the globe _L_ (Figs. 141 and 142) is exhausted to a very high
+degree, generally the bulb is not excited upon connecting the wire _w_
+(Fig. 141) or the tinfoil coating of the bulb (Fig. 142) to the
+terminal of the induction coil. To excite it, it is usually sufficient
+to grasp the globe _L_ with the hand. An intense phosphorescence then
+spreads at first over the globe, but soon gives place to a white, misty
+light. Shortly afterward one may notice that the luminosity is unevenly
+distributed in the globe, and after passing the current for some time
+the bulb appears as in Fig. 144. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 145, after some minutes, hours,
+days or weeks, according as the bulb is worked. Warming the bulb or
+increasing the potential hastens the transit.
+
+[Illustration: FIG. 144.]
+
+[Illustration: FIG. 145.]
+
+When the brush assumes the form indicated in Fig. 145, it may be brought
+to a state of extreme sensitiveness to electrostatic and magnetic
+influence. The bulb hanging straight down from a wire, and all objects
+being remote from it, the approach of the observer at a few paces from
+the bulb will cause the brush to fly to the opposite side, and if he
+walks around the bulb it will always keep on the opposite side. It may
+begin to spin around the terminal long before it reaches that sensitive
+stage. When it begins to turn around, principally, but also before, it
+is affected by a magnet, and at a certain stage it is susceptible to
+magnetic influence to an astonishing degree. A small permanent magnet,
+with its poles at a distance of no more than two centimetres, will
+affect it visibly at a distance of two metres, slowing down or
+accelerating the rotation according to how it is held relatively to the
+brush. I think I have observed that at the stage when it is most
+sensitive to magnetic, it is not most sensitive to electrostatic,
+influence. My explanation is, that the electrostatic attraction between
+the brush and the glass of the bulb, which retards the rotation, grows
+much quicker than the magnetic influence when the intensity of the
+stream is increased.
+
+When the bulb hangs with the globe _L_ down, the rotation is always
+clockwise. In the southern hemisphere it would occur in the opposite
+direction and on the equator the brush should not turn at all. The
+rotation may be reversed by a magnet kept at some distance. The brush
+rotates best, seemingly, when it is at right angles to the lines of
+force of the earth. It very likely rotates, when at its maximum speed,
+in synchronism with the alternations, say, 10,000 times a second. The
+rotation can be slowed down or accelerated by the approach or receding
+of the observer, or any conducting body, but it cannot be reversed by
+putting the bulb in any position. When it is in the state of the highest
+sensitiveness and the potential or frequency be varied, the
+sensitiveness is rapidly diminished. Changing either of these but little
+will generally stop the rotation. The sensitiveness is likewise affected
+by the variations of temperature. To attain great sensitiveness it is
+necessary to have the small sphere _s_ in the centre of the globe _L_,
+as otherwise the electrostatic action of the glass of the globe will
+tend to stop the rotation. The sphere _s_ should be small and of uniform
+thickness; any dissymmetry of course has the effect to diminish the
+sensitiveness.
+
+The fact that the brush rotates in a definite direction in a permanent
+magnetic field seems to show that in alternating currents of very high
+frequency the positive and negative impulses are not equal, but that one
+always preponderates over the other.
+
+Of course, this rotation in one direction may be due to the action of
+the two elements of the same current upon each other, or to the action
+of the field produced by one of the elements upon the other, as in a
+series motor, without necessarily one impulse being stronger than the
+other. The fact that the brush turns, as far as I could observe, in any
+position, would speak for this view. In such case it would turn at any
+point of the earth's surface. But, on the other hand, it is then hard to
+explain why a permanent magnet should reverse the rotation, and one must
+assume the preponderance of impulses of one kind.
+
+As to the causes of the formation of the brush or stream, I think it is
+due to the electrostatic action of the globe and the dissymmetry of the
+parts. If the small bulb _s_ and the globe _L_ were perfect concentric
+spheres, and the glass throughout of the same thickness and quality, I
+think the brush would not form, as the tendency to pass would be equal
+on all sides. That the formation of the stream is due to an irregularity
+is apparent from the fact that it has the tendency to remain in one
+position, and rotation occurs most generally only when it is brought out
+of this position by electrostatic or magnetic influence. When in an
+extremely sensitive state it rests in one position, most curious
+experiments may be performed with it. For instance, the experimenter
+may, by selecting a proper position, approach the hand at a certain
+considerable distance to the bulb, and he may cause the brush to pass
+off by merely stiffening the muscles of the arm. When it begins to
+rotate slowly, and the hands are held at a proper distance, it is
+impossible to make even the slightest motion without producing a visible
+effect upon the brush. A metal plate connected to the other terminal of
+the coil affects it at a great distance, slowing down the rotation often
+to one turn a second.
+
+I am firmly convinced that such a brush, when we learn how to produce it
+properly, will prove a valuable aid in the investigation of the nature
+of the forces acting in an electrostatic or magnetic field. If there is
+any motion which is measurable going on in the space, such a brush ought
+to reveal it. It is, so to speak, a beam of light, frictionless, devoid
+of inertia.
+
+I think that it may find practical applications in telegraphy. With such
+a brush it would be possible to send dispatches across the Atlantic, for
+instance, with any speed, since its sensitiveness may be so great that
+the slightest changes will affect it. If it were possible to make the
+stream more intense and very narrow, its deflections could be easily
+photographed.
+
+I have been interested to find whether there is a rotation of the stream
+itself, or whether there is simply a stress traveling around the bulb.
+For this purpose I mounted a light mica fan so that its vanes were in
+the path of the brush. If the stream itself was rotating the fan would
+be spun around. I could produce no distinct rotation of the fan,
+although I tried the experiment repeatedly; but as the fan exerted a
+noticeable influence on the stream, and the apparent rotation of the
+latter was, in this case, never quite satisfactory, the experiment did
+not appear to be conclusive.
+
+I have been unable to produce the phenomenon with the disruptive
+discharge coil, although every other of these phenomena can be well
+produced by it--many, in fact, much better than with coils operated from
+an alternator.
+
+It may be possible to produce the brush by impulses of one direction, or
+even by a steady potential, in which case it would be still more
+sensitive to magnetic influence.
+
+In operating an induction coil with rapidly alternating currents, we
+realize with astonishment, for the first time, the great importance of
+the relation of capacity, self-induction and frequency as regards the
+general results. The effects of capacity are the most striking, for in
+these experiments, since the self-induction and frequency both are high,
+the critical capacity is very small, and need be but slightly varied to
+produce a very considerable change. The experimenter may bring his body
+in contact with the terminals of the secondary of the coil, or attach to
+one or both terminals insulated bodies of very small bulk, such as
+bulbs, and he may produce a considerable rise or fall of potential, and
+greatly affect the flow of the current through the primary. In the
+experiment before shown, in which a brush appears at a wire attached to
+one terminal, and the wire is vibrated when the experimenter brings his
+insulated body in contact with the other terminal of the coil, the
+sudden rise of potential was made evident.
+
+I may show you the behavior of the coil in another manner which
+possesses a feature of some interest. I have here a little light fan of
+aluminum sheet, fastened to a needle and arranged to rotate freely in a
+metal piece screwed to one of the terminals of the coil. When the coil
+is set to work, the molecules of the air are rhythmically attracted and
+repelled. As the force with which they are repelled is greater than that
+with which they are attracted, it results that there is a repulsion
+exerted on the surfaces of the fan. If the fan were made simply of a
+metal sheet, the repulsion would be equal on the opposite sides, and
+would produce no effect. But if one of the opposing surfaces is
+screened, or if, generally speaking, the bombardment on this side is
+weakened in some way or other, there remains the repulsion exerted upon
+the other, and the fan is set in rotation. The screening is best
+effected by fastening upon one of the opposing sides of the fan
+insulated conducting coatings, or, if the fan is made in the shape of an
+ordinary propeller screw, by fastening on one side, and close to it, an
+insulated metal plate. The static screen may, however, be omitted, and
+simply a thickness of insulating material fastened to one of the sides
+of the fan.
+
+To show the behavior of the coil, the fan may be placed upon the
+terminal and it will readily rotate when the coil is operated by
+currents of very high frequency. With a steady potential, of course, and
+even with alternating currents of very low frequency, it would not turn,
+because of the very slow exchange of air and, consequently, smaller
+bombardment; but in the latter case it might turn if the potential were
+excessive. With a pin wheel, quite the opposite rule holds good; it
+rotates best with a steady potential, and the effort is the smaller the
+higher the frequency. Now, it is very easy to adjust the conditions so
+that the potential is normally not sufficient to turn the fan, but that
+by connecting the other terminal of the coil with an insulated body it
+rises to a much greater value, so as to rotate the fan, and it is
+likewise possible to stop the rotation by connecting to the terminal a
+body of different size, thereby diminishing the potential.
+
+Instead of using the fan in this experiment, we may use the "electric"
+radiometer with similar effect. But in this case it will be found that
+the vanes will rotate only at high exhaustion or at ordinary pressures;
+they will not rotate at moderate pressures, when the air is highly
+conducting. This curious observation was made conjointly by Professor
+Crookes and myself. I attribute the result to the high conductivity of
+the air, the molecules of which then do not act as independent carriers
+of electric charges, but act all together as a single conducting body.
+In such case, of course, if there is any repulsion at all of the
+molecules from the vanes, it must be very small. It is possible,
+however, that the result is in part due to the fact that the greater
+part of the discharge passes from the leading-in wire through the highly
+conducting gas, instead of passing off from the conducting vanes.
+
+In trying the preceding experiment with the electric radiometer the
+potential should not exceed a certain limit, as then the electrostatic
+attraction between the vanes and the glass of the bulb may be so great
+as to stop the rotation.
+
+A most curious feature of alternate currents of high frequencies and
+potentials is that they enable us to perform many experiments by the use
+of one wire only. In many respects this feature is of great interest.
+
+In a type of alternate current motor invented by me some years ago I
+produced rotation by inducing, by means of a single alternating current
+passed through a motor circuit, in the mass or other circuits of the
+motor, secondary currents, which, jointly with the primary or inducing
+current, created a moving field of force. A simple but crude form of
+such a motor is obtained by winding upon an iron core a primary, and
+close to it a secondary coil, joining the ends of the latter and placing
+a freely movable metal disc within the influence of the field produced
+by both. The iron core is employed for obvious reasons, but it is not
+essential to the operation. To improve the motor, the iron core is made
+to encircle the armature. Again to improve, the secondary coil is made
+to partly overlap the primary, so that it cannot free itself from a
+strong inductive action of the latter, repel its lines as it may. Once
+more to improve, the proper difference of phase is obtained between the
+primary and secondary currents by a condenser, self-induction,
+resistance or equivalent windings.
+
+I had discovered, however, that rotation is produced by means of a
+single coil and core; my explanation of the phenomenon, and leading
+thought in trying the experiment, being that there must be a true time
+lag in the magnetization of the core. I remember the pleasure I had
+when, in the writings of Professor Ayrton, which came later to my hand,
+I found the idea of the time lag advocated. Whether there is a true time
+lag, or whether the retardation is due to eddy currents circulating in
+minute paths, must remain an open question, but the fact is that a coil
+wound upon an iron core and traversed by an alternating current creates
+a moving field of force, capable of setting an armature in rotation. It
+is of some interest, in conjunction with the historical Arago
+experiment, to mention that in lag or phase motors I have produced
+rotation in the opposite direction to the moving field, which means that
+in that experiment the magnet may not rotate, or may even rotate in the
+opposite direction to the moving disc. Here, then, is a motor
+(diagrammatically illustrated in Fig. 146), comprising a coil and iron
+core, and a freely movable copper disc in proximity to the latter.
+
+[Illustration: FIG. 146.]
+
+To demonstrate a novel and interesting feature, I have, for a reason
+which I will explain, selected this type of motor. When the ends of the
+coil are connected to the terminals of an alternator the disc is set in
+rotation. But it is not this experiment, now well known, which I desire
+to perform. What I wish to show you is that this motor rotates with
+_one single_ connection between it and the generator; that is to say,
+one terminal of the motor is connected to one terminal of the
+generator--in this case the secondary of a high-tension induction
+coil--the other terminals of motor and generator being insulated in
+space. To produce rotation it is generally (but not absolutely)
+necessary to connect the free end of the motor coil to an insulated body
+of some size. The experimenter's body is more than sufficient. If he
+touches the free terminal with an object held in the hand, a current
+passes through the coil and the copper disc is set in rotation. If an
+exhausted tube is put in series with the coil, the tube lights
+brilliantly, showing the passage of a strong current. Instead of the
+experimenter's body, a small metal sheet suspended on a cord may be used
+with the same result. In this case the plate acts as a condenser in
+series with the coil. It counteracts the self-induction of the latter
+and allows a strong current to pass. In such a combination, the greater
+the self-induction of the coil the smaller need be the plate, and this
+means that a lower frequency, or eventually a lower potential, is
+required to operate the motor. A single coil wound upon a core has a
+high self-induction; for this reason, principally, this type of motor
+was chosen to perform the experiment. Were a secondary closed coil wound
+upon the core, it would tend to diminish the self-induction, and then
+it would be necessary to employ a much higher frequency and potential.
+Neither would be advisable, for a higher potential would endanger the
+insulation of the small primary coil, and a higher frequency would
+result in a materially diminished torque.
+
+It should be remarked that when such a motor with a closed secondary is
+used, it is not at all easy to obtain rotation with excessive
+frequencies, as the secondary cuts off almost completely the lines of
+the primary--and this, of course, the more, the higher the
+frequency--and allows the passage of but a minute current. In such a
+case, unless the secondary is closed through a condenser, it is almost
+essential, in order to produce rotation, to make the primary and
+secondary coils overlap each other more or less.
+
+But there is an additional feature of interest about this motor, namely,
+it is not necessary to have even a single connection between the motor
+and generator, except, perhaps, through the ground; for not only is an
+insulated plate capable of giving off energy into space, but it is
+likewise capable of deriving it from an alternating electrostatic field,
+though in the latter case the available energy is much smaller. In this
+instance one of the motor terminals is connected to the insulated plate
+or body located within the alternating electrostatic field, and the
+other terminal preferably to the ground.
+
+It is quite possible, however, that such "no wire" motors, as they might
+be called, could be operated by conduction through the rarefied air at
+considerable distances. Alternate currents, especially of high
+frequencies, pass with astonishing freedom through even slightly
+rarefied gases. The upper strata of the air are rarefied. To reach a
+number of miles out into space requires the overcoming of difficulties
+of a merely mechanical nature. There is no doubt that with the enormous
+potentials obtainable by the use of high frequencies and oil insulation,
+luminous discharges might be passed through many miles of rarefied air,
+and that, by thus directing the energy of many hundreds or thousands of
+horse-power, motors or lamps might be operated at considerable distances
+from stationary sources. But such schemes are mentioned merely as
+possibilities. We shall have no need to transmit power in this way. We
+shall have no need to _transmit_ power at all. Ere many generations
+pass, our machinery will be driven by a power obtainable at any point of
+the universe. This idea is not novel. Men have been led to it long ago
+by instinct or reason. It has been expressed in many ways, and in many
+places, in the history of old and new. We find it in the delightful myth
+of Antheus, who derives power from the earth; we find it among the
+subtle speculations of one of your splendid mathematicians, and in many
+hints and statements of thinkers of the present time. Throughout space
+there is energy. Is this energy static or kinetic? If static our hopes
+are in vain; if kinetic--and this we know it is, for certain--then it is
+a mere question of time when men will succeed in attaching their
+machinery to the very wheelwork of nature. Of all, living or dead,
+Crookes came nearest to doing it. His radiometer will turn in the light
+of day and in the darkness of the night; it will turn everywhere where
+there is heat, and heat is everywhere. But, unfortunately, this
+beautiful little machine, while it goes down to posterity as the most
+interesting, must likewise be put on record as the most inefficient
+machine ever invented!
+
+The preceding experiment is only one of many equally interesting
+experiments which may be performed by the use of only one wire with
+alternations of high potential and frequency. We may connect an
+insulated line to a source of such currents, we may pass an
+inappreciable current over the line, and on any point of the same we are
+able to obtain a heavy current, capable of fusing a thick copper wire.
+Or we may, by the help of some artifice, decompose a solution in any
+electrolytic cell by connecting only one pole of the cell to the line or
+source of energy. Or we may, by attaching to the line, or only bringing
+into its vicinity, light up an incandescent lamp, an exhausted tube, or
+a phosphorescent bulb.
+
+However impracticable this plan of working may appear in many cases, it
+certainly seems practicable, and even recommendable, in the production
+of light. A perfected lamp would require but little energy, and if wires
+were used at all we ought to be able to supply that energy without a
+return wire.
+
+It is now a fact that a body may be rendered incandescent or
+phosphorescent by bringing it either in single contact or merely in the
+vicinity of a source of electric impulses of the proper character, and
+that in this manner a quantity of light sufficient to afford a practical
+illuminant may be produced. It is, therefore, to say the least, worth
+while to attempt to determine the best conditions and to invent the best
+appliances for attaining this object.
+
+Some experiences have already been gained in this direction, and I will
+dwell on them briefly, in the hope that they might prove useful.
+
+The heating of a conducting body inclosed in a bulb, and connected to a
+source of rapidly alternating electric impulses, is dependent on so many
+things of a different nature, that it would be difficult to give a
+generally applicable rule under which the maximum heating occurs. As
+regards the size of the vessel, I have lately found that at ordinary or
+only slightly differing atmospheric pressures, when air is a good
+insulator, and hence practically the same amount of energy by a certain
+potential and frequency is given off from the body, whether the bulb be
+small or large, the body is brought to a higher temperature if enclosed
+in a small bulb, because of the better confinement of heat in this case.
+
+At lower pressures, when air becomes more or less conducting, or if the
+air be sufficiently warmed to become conducting, the body is rendered
+more intensely incandescent in a large bulb, obviously because, under
+otherwise equal conditions of test, more energy may be given off from
+the body when the bulb is large.
+
+At very high degrees of exhaustion, when the matter in the bulb becomes
+"radiant," a large bulb has still an advantage, but a comparatively
+slight one, over the small bulb.
+
+Finally, at excessively high degrees of exhaustion, which cannot be
+reached except by the employment of special means, there seems to be,
+beyond a certain and rather small size of vessel, no perceptible
+difference in the heating.
+
+These observations were the result of a number of experiments, of which
+one, showing the effect of the size of the bulb at a high degree of
+exhaustion, may be described and shown here, as it presents a feature of
+interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches
+diameter were taken, and in the centre of each was mounted an equal
+length of an ordinary incandescent lamp filament of uniform thickness.
+In each bulb the piece of filament was fastened to the leading-in wire
+of platinum, contained in a glass stem sealed in the bulb; care being
+taken, of course, to make everything as nearly alike as possible. On
+each glass stem in the inside of the bulb was slipped a highly polished
+tube made of aluminum sheet, which fitted the stem and was held on it by
+spring pressure. The function of this aluminum tube will be explained
+subsequently. In each bulb an equal length of filament protruded above
+the metal tube. It is sufficient to say now that under these conditions
+equal lengths of filament of the same thickness--in other words, bodies
+of equal bulk--were brought to incandescence. The three bulbs were
+sealed to a glass tube, which was connected to a Sprengel pump. When a
+high vacuum had been reached, the glass tube carrying the bulbs was
+sealed off. A current was then turned on successively on each bulb, and
+it was found that the filaments came to about the same brightness, and,
+if anything, the smallest bulb, which was placed midway between the two
+larger ones, may have been slightly brighter. This result was expected,
+for when either of the bulbs was connected to the coil the luminosity
+spread through the other two, hence the three bulbs constituted really
+one vessel. When all the three bulbs were connected in multiple arc to
+the coil, in the largest of them the filament glowed brightest, in the
+next smaller it was a little less bright, and in the smallest it only
+came to redness. The bulbs were then sealed off and separately tried.
+The brightness of the filaments was now such as would have been expected
+on the supposition that the energy given off was proportionate to the
+surface of the bulb, this surface in each case representing one of the
+coatings of a condenser. Accordingly, there was less difference between
+the largest and the middle sized than between the latter and the
+smallest bulb.
+
+An interesting observation was made in this experiment. The three bulbs
+were suspended from a straight bare wire connected to a terminal of a
+coil, the largest bulb being placed at the end of the wire, at some
+distance from it the smallest bulb, and at an equal distance from the
+latter the middle-sized one. The carbons glowed then in both the larger
+bulbs about as expected, but the smallest did not get its share by far.
+This observation led me to exchange the position of the bulbs, and I
+then observed that whichever of the bulbs was in the middle was by far
+less bright than it was in any other position. This mystifying result
+was, of course, found to be due to the electrostatic action between the
+bulbs. When they were placed at a considerable distance, or when they
+were attached to the corners of an equilateral triangle of copper wire,
+they glowed in about the order determined by their surfaces.
+
+As to the shape of the vessel, it is also of some importance, especially
+at high degrees of exhaustion. Of all the possible constructions, it
+seems that a spherical globe with the refractory body mounted in its
+centre is the best to employ. By experience it has been demonstrated
+that in such a globe a refractory body of a given bulk is more easily
+brought to incandescence than when differently shaped bulbs are used.
+There is also an advantage in giving to the incandescent body the shape
+of a sphere, for self-evident reasons. In any case the body should be
+mounted in the centre, where the atoms rebounding from the glass
+collide. This object is best attained in the spherical bulb; but it is
+also attained in a cylindrical vessel with one or two straight filaments
+coinciding with its axis, and possibly also in parabolical or spherical
+bulbs with refractory body or bodies placed in the focus or foci of the
+same; though the latter is not probable, as the electrified atoms should
+in all cases rebound normally from the surface they strike, unless the
+speed were excessive, in which case they _would_ probably follow the
+general law of reflection. No matter what shape the vessel may have, if
+the exhaustion be low, a filament mounted in the globe is brought to the
+same degree of incandescence in all parts; but if the exhaustion be high
+and the bulb be spherical or pear-shaped, as usual, focal points form
+and the filament is heated to a higher degree at or near such points.
+
+To illustrate the effect, I have here two small bulbs which are alike,
+only one is exhausted to a low and the other to a very high degree. When
+connected to the coil, the filament in the former glows uniformly
+throughout all its length; whereas in the latter, that portion of the
+filament which is in the centre of the bulb glows far more intensely
+than the rest. A curious point is that the phenomenon occurs even if two
+filaments are mounted in a bulb, each being connected to one terminal of
+the coil, and, what is still more curious, if they be very near
+together, provided the vacuum be very high. I noted in experiments with
+such bulbs that the filaments would give way usually at a certain point,
+and in the first trials I attributed it to a defect in the carbon. But
+when the phenomenon occurred many times in succession I recognized its
+real cause.
+
+In order to bring a refractory body inclosed in a bulb to incandescence,
+it is desirable, on account of economy, that all the energy supplied to
+the bulb from the source should reach without loss the body to be
+heated; from there, and from nowhere else, it should be radiated. It is,
+of course, out of the question to reach this theoretical result, but it
+is possible by a proper construction of the illuminating device to
+approximate it more or less.
+
+For many reasons, the refractory body is placed in the centre of the
+bulb, and it is usually supported on a glass stem containing the
+leading-in wire. As the potential of this wire is alternated, the
+rarefied gas surrounding the stem is acted upon inductively, and the
+glass stem is violently bombarded and heated. In this manner by far the
+greater portion of the energy supplied to the bulb--especially when
+exceedingly high frequencies are used--may be lost for the purpose
+contemplated. To obviate this loss, or at least to reduce it to a
+minimum, I usually screen the rarefied gas surrounding the stem from the
+inductive action of the leading-in wire by providing the stem with a
+tube or coating of conducting material. It seems beyond doubt that the
+best among metals to employ for this purpose is aluminum, on account of
+its many remarkable properties. Its only fault is that it is easily
+fusible, and, therefore, its distance from the incandescing body should
+be properly estimated. Usually, a thin tube, of a diameter somewhat
+smaller than that of the glass stem, is made of the finest aluminum
+sheet, and slipped on the stem. The tube is conveniently prepared by
+wrapping around a rod fastened in a lathe a piece of aluminum sheet of
+proper size, grasping the sheet firmly with clean chamois leather or
+blotting paper, and spinning the rod very fast. The sheet is wound
+tightly around the rod, and a highly polished tube of one or three
+layers of the sheet is obtained. When slipped on the stem, the pressure
+is generally sufficient to prevent it from slipping off, but, for
+safety, the lower edge of the sheet may be turned inside. The upper
+inside corner of the sheet--that is, the one which is nearest to the
+refractory incandescent body--should be cut out diagonally, as it often
+happens that, in consequence of the intense heat, this corner turns
+toward the inside and comes very near to, or in contact with, the wire,
+or filament, supporting the refractory body. The greater part of the
+energy supplied to the bulb is then used up in heating the metal tube,
+and the bulb is rendered useless for the purpose. The aluminum sheet
+should project above the glass stem more or less--one inch or so--or
+else, if the glass be too close to the incandescing body, it may be
+strongly heated and become more or less conducting, whereupon it may be
+ruptured, or may, by its conductivity, establish a good electrical
+connection between the metal tube and the leading-in wire, in which
+case, again, most of the energy will be lost in heating the former.
+Perhaps the best way is to make the top of the glass tube, for about an
+inch, of a much smaller diameter. To still further reduce the danger
+arising from the heating of the glass stem, and also with the view of
+preventing an electrical connection between the metal tube and the
+electrode, I preferably wrap the stem with several layers of thin mica,
+which extends at least as far as the metal tube. In some bulbs I have
+also used an outside insulating cover.
+
+The preceding remarks are only made to aid the experimenter in the first
+trials, for the difficulties which he encounters he may soon find means
+to overcome in his own way.
+
+To illustrate the effect of the screen, and the advantage of using it, I
+have here two bulbs of the same size, with their stems, leading-in wires
+and incandescent lamp filaments tied to the latter, as nearly alike as
+possible. The stem of one bulb is provided with an aluminum tube, the
+stem of the other has none. Originally the two bulbs were joined by a
+tube which was connected to a Sprengel pump. When a high vacuum had been
+reached, first the connecting tube, and then the bulbs, were sealed off;
+they are therefore of the same degree of exhaustion. When they are
+separately connected to the coil giving a certain potential, the carbon
+filament in the bulb provided with the aluminum screen is rendered
+highly incandescent, while the filament in the other bulb may, with the
+same potential, not even come to redness, although in reality the latter
+bulb takes generally more energy than the former. When they are both
+connected together to the terminal, the difference is even more
+apparent, showing the importance of the screening. The metal tube placed
+on the stem containing the leading-in wire performs really two distinct
+functions: First, it acts more or less as an electrostatic screen, thus
+economizing the energy supplied to the bulb; and, second, to whatever
+extent it may fail to act electrostatically, it acts mechanically,
+preventing the bombardment, and consequently intense heating and
+possible deterioration of the slender support of the refractory
+incandescent body, or of the glass stem containing the leading-in wire.
+I say _slender_ support, for it is evident that in order to confine the
+heat more completely to the incandescing body its support should be very
+thin, so as to carry away the smallest possible amount of heat by
+conduction. Of all the supports used I have found an ordinary
+incandescent lamp filament to be the best, principally because among
+conductors it can withstand the highest degree of heat.
+
+The effectiveness of the metal tube as an electrostatic screen depends
+largely on the degree of exhaustion.
+
+At excessively high degrees of exhaustion--which are reached by using
+great care and special means in connection with the Sprengel pump--when
+the matter in the globe is in the ultra-radiant state, it acts most
+perfectly. The shadow of the upper edge of the tube is then sharply
+defined upon the bulb.
+
+At a somewhat lower degree of exhaustion, which is about the ordinary
+"non-striking" vacuum, and generally as long as the matter moves
+predominantly in straight lines, the screen still does well. In
+elucidation of the preceding remark it is necessary to state that what
+is a "non-striking" vacuum for a coil operated as ordinarily, by
+impulses, or currents, of low frequency, is not so, by far, when the
+coil is operated by currents of very high frequency. In such case the
+discharge may pass with great freedom through the rarefied gas through
+which a low frequency discharge may not pass, even though the potential
+be much higher. At ordinary atmospheric pressures just the reverse rule
+holds good: the higher the frequency, the less the spark discharge is
+able to jump between the terminals, especially if they are knobs or
+spheres of some size.
+
+Finally, at very low degrees of exhaustion, when the gas is well
+conducting, the metal tube not only does not act as an electrostatic
+screen, but even is a drawback, aiding to a considerable extent the
+dissipation of the energy laterally from the leading-in wire. This, of
+course, is to be expected. In this case, namely, the metal tube is in
+good electrical connection with the leading-in wire, and most of the
+bombardment is directed upon the tube. As long as the electrical
+connection is not good, the conducting tube is always of some advantage,
+for although it may not greatly economize energy, still it protects the
+support of the refractory button, and is the means of concentrating more
+energy upon the same.
+
+To whatever extent the aluminum tube performs the function of a screen,
+its usefulness is therefore limited to very high degrees of exhaustion
+when it is insulated from the electrode--that is, when the gas as a
+whole is non-conducting, and the molecules, or atoms, act as independent
+carriers of electric charges.
+
+In addition to acting as a more or less effective screen, in the true
+meaning of the word, the conducting tube or coating may also act, by
+reason of its conductivity, as a sort of equalizer or dampener of the
+bombardment against the stem. To be explicit, I assume the action to be
+as follows: Suppose a rhythmical bombardment to occur against the
+conducting tube by reason of its imperfect action as a screen, it
+certainly must happen that some molecules, or atoms, strike the tube
+sooner than others. Those which come first in contact with it give up
+their superfluous charge, and the tube is electrified, the
+electrification instantly spreading over its surface. But this must
+diminish the energy lost in the bombardment, for two reasons: first, the
+charge given up by the atoms spreads over a great area, and hence the
+electric density at any point is small, and the atoms are repelled with
+less energy than they would be if they struck against a good insulator;
+secondly, as the tube is electrified by the atoms which first come in
+contact with it, the progress of the following atoms against the tube is
+more or less checked by the repulsion which the electrified tube must
+exert upon the similarly electrified atoms. This repulsion may perhaps
+be sufficient to prevent a large portion of the atoms from striking the
+tube, but at any rate it must diminish the energy of their impact. It is
+clear that when the exhaustion is very low, and the rarefied gas well
+conducting, neither of the above effects can occur, and, on the other
+hand, the fewer the atoms, with the greater freedom they move; in other
+words, the higher the degree of exhaustion, up to a limit, the more
+telling will be both the effects.
+
+[Illustration: FIG. 147.]
+
+[Illustration: FIG. 148.]
+
+What I have just said may afford an explanation of the phenomenon
+observed by Prof. Crookes, namely, that a discharge through a bulb is
+established with much greater facility when an insulator than when a
+conductor is present in the same. In my opinion, the conductor acts as a
+dampener of the motion of the atoms in the two ways pointed out; hence,
+to cause a visible discharge to pass through the bulb, a much higher
+potential is needed if a conductor, especially of much surface, be
+present.
+
+For the sake of elucidating of some of the remarks before made, I must
+now refer to Figs. 147, 148 and 149, which illustrate various
+arrangements with a type of bulb most generally used.
+
+Fig. 147 is a section through a spherical bulb L, with the glass stem
+_s_, contains the leading-in wire _w_, which has a lamp filament _l_
+fastened to it, serving to support the refractory button _m_ in the
+centre. M is a sheet of thin mica wound in several layers around the
+stem _s_, and _a_ is the aluminum tube.
+
+Fig. 148 illustrates such a bulb in a somewhat more advanced stage of
+perfection. A metallic tube S is fastened by means of some cement to the
+neck of the tube. In the tube is screwed a plug P, of insulating
+material, in the centre of which is fastened a metallic terminal _t_,
+for the connection to the leading-in wire _w_. This terminal must be
+well insulated from the metal tube S; therefore, if the cement used is
+conducting--and most generally it is sufficiently so--the space between
+the plug P and the neck of the bulb should be filled with some good
+insulating material, such as mica powder.
+
+
+Fig. 149 shows a bulb made for experimental purposes. In this bulb the
+aluminum tube is provided with an external connection, which serves to
+investigate the effect of the tube under various conditions. It is
+referred to chiefly to suggest a line of experiment followed.
+
+Since the bombardment against the stem containing the leading-in wire is
+due to the inductive action of the latter upon the rarefied gas, it is
+of advantage to reduce this action as far as practicable by employing a
+very thin wire, surrounded by a very thick insulation of glass or other
+material, and by making the wire passing through the rarefied gas as
+short as practicable. To combine these features I employ a large tube T
+(Fig. 150), which protrudes into the bulb to some distance, and carries
+on the top a very short glass stem _s_, into which is sealed the
+leading-in wire _w_, and I protect the top of the glass stem against the
+heat by a small aluminum tube _a_ and a layer of mica underneath the
+same, as usual. The wire _w_, passing through the large tube to the
+outside of the bulb, should be well insulated--with a glass tube, for
+instance--and the space between ought to be filled out with some
+excellent insulator. Among many insulating powders I have found that
+mica powder is the best to employ. If this precaution is not taken, the
+tube T, protruding into the bulb, will surely be cracked in consequence
+of the heating by the brushes which are apt to form in the upper part of
+the tube, near the exhausted globe, especially if the vacuum be
+excellent, and therefore the potential necessary to operate the lamp be
+very high.
+
+[Illustration: FIG. 149.]
+
+[Illustration: FIG. 150.]
+
+Fig. 151 illustrates a similar arrangement, with a large tube T
+protruding into the part of the bulb containing the refractory button
+_m_. In this case the wire leading from the outside into the bulb is
+omitted, the energy required being supplied through condenser coatings C
+C. The insulating packing P should in this construction be tightly
+fitting to the glass, and rather wide, or otherwise the discharge might
+avoid passing through the wire _w_, which connects the inside condenser
+coating to the incandescent button _m_.
+
+The molecular bombardment against the glass stem in the bulb is a source
+of great trouble. As an illustration I will cite a phenomenon only too
+frequently and unwillingly observed. A bulb, preferably a large one, may
+be taken, and a good conducting body, such as a piece of carbon, may be
+mounted in it upon a platinum wire sealed in the glass stem. The bulb
+may be exhausted to a fairly high degree, nearly to the point when
+phosphorescence begins to appear. When the bulb is connected with the
+coil, the piece of carbon, if small, may become highly incandescent at
+first, but its brightness immediately diminishes, and then the discharge
+may break through the glass somewhere in the middle of the stem, in the
+form of bright sparks, in spite of the fact that the platinum wire is in
+good electrical connection with the rarefied gas through the piece of
+carbon or metal at the top. The first sparks are singularly bright,
+recalling those drawn from a clear surface of mercury. But, as they heat
+the glass rapidly, they, of course, lose their brightness, and cease
+when the glass at the ruptured place becomes incandescent, or generally
+sufficiently hot to conduct. When observed for the first time the
+phenomenon must appear very curious, and shows in a striking manner how
+radically different alternate currents, or impulses, of high frequency
+behave, as compared with steady currents, or currents of low frequency.
+With such currents--namely, the latter--the phenomenon would of course
+not occur. When frequencies such as are obtained by mechanical means are
+used, I think that the rupture of the glass is more or less the
+consequence of the bombardment, which warms it up and impairs its
+insulating power; but with frequencies obtainable with condensers I have
+no doubt that the glass may give way without previous heating. Although
+this appears most singular at first, it is in reality what we might
+expect to occur. The energy supplied to the wire leading into the bulb
+is given off partly by direct action through the carbon button, and
+partly by inductive action through the glass surrounding the wire. The
+case is thus analogous to that in which a condenser shunted by a
+conductor of low resistance is connected to a source of alternating
+current. As long as the frequencies are low, the conductor gets the most
+and the condenser is perfectly safe; but when the frequency becomes
+excessive, the _role_ of the conductor may become quite insignificant.
+In the latter case the difference of potential at the terminals of the
+condenser may become so great as to rupture the dielectric,
+notwithstanding the fact that the terminals are joined by a conductor of
+low resistance.
+
+It is, of course, not necessary, when it is desired to produce the
+incandescence of a body inclosed in a bulb by means of these currents,
+that the body should be a conductor, for even a perfect non-conductor
+may be quite as readily heated. For this purpose it is sufficient to
+surround a conducting electrode with a non-conducting material, as, for
+instance, in the bulb described before in Fig. 150, in which a thin
+incandescent lamp filament is coated with a non-conductor, and supports
+a button of the same material on the top. At the start the bombardment
+goes on by inductive action through the non-conductor, until the same is
+sufficiently heated to become conducting, when the bombardment continues
+in the ordinary way.
+
+[Illustration: FIG. 151.]
+
+[Illustration: FIG. 152.]
+
+A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 152. In this instance a non-conductor _m_ is mounted
+in a piece of common arc light carbon so as to project some small
+distance above the latter. The carbon piece is connected to the
+leading-in wire passing through a glass stem, which is wrapped with
+several layers of mica. An aluminum tube _a_ is employed as usual for
+screening. It is so arranged that it reaches very nearly as high as the
+carbon and only the non-conductor _m_ projects a little above it. The
+bombardment goes at first against the upper surface of carbon, the lower
+parts being protected by the aluminum tube. As soon, however, as the
+non-conductor _m_ is heated it is rendered good conducting, and then it
+becomes the centre of the bombardment, being most exposed to the same.
+
+I have also constructed during these experiments many such single-wire
+bulbs with or without internal electrode, in which the radiant matter
+was projected against, or focused upon, the body to be rendered
+incandescent. Fig. 153 (page 263) illustrates one of the bulbs used. It
+consists of a spherical globe L, provided with a long neck _n_, on top,
+for increasing the action in some cases by the application of an
+external conducting coating. The globe L is blown out on the bottom into
+a very small bulb _b_, which serves to hold it firmly in a socket S of
+insulating material into which it is cemented. A fine lamp filament _f_,
+supported on a wire _w_, passes through the centre of the globe L. The
+filament is rendered incandescent in the middle portion, where the
+bombardment proceeding from the lower inside surface of the globe is
+most intense. The lower portion of the globe, as far as the socket S
+reaches, is rendered conducting, either by a tinfoil coating or
+otherwise, and the external electrode is connected to a terminal of the
+coil.
+
+The arrangement diagrammatically indicated in Fig. 153 was found to be
+an inferior one when it was desired to render incandescent a filament or
+button supported in the centre of the globe, but it was convenient when
+the object was to excite phosphorescence.
+
+In many experiments in which bodies of different kind were mounted in
+the bulb as, for instance, indicated in Fig. 152, some observations of
+interest were made.
+
+It was found, among other things, that in such cases, no matter where
+the bombardment began, just as soon as a high temperature was reached
+there was generally one of the bodies which seemed to take most of the
+bombardment upon itself, the other, or others, being thereby relieved.
+The quality appeared to depend principally on the point of fusion, and
+on the facility with which the body was "evaporated," or, generally
+speaking, disintegrated--meaning by the latter term not only the
+throwing off of atoms, but likewise of large lumps. The observation made
+was in accordance with generally accepted notions. In a highly exhausted
+bulb, electricity is carried off from the electrode by independent
+carriers, which are partly the atoms, or molecules, of the residual
+atmosphere, and partly the atoms, molecules, or lumps thrown off from
+the electrode. If the electrode is composed of bodies of different
+character, and if one of these is more easily disintegrated than the
+other, most of the electricity supplied is carried off from that body,
+which is then brought to a higher temperature than the others, and this
+the more, as upon an increase of the temperature the body is still more
+easily disintegrated.
+
+It seems to me quite probable that a similar process takes place in the
+bulb even with a homogeneous electrode, and I think it to be the
+principal cause of the disintegration. There is bound to be some
+irregularity, even if the surface is highly polished, which, of course,
+is impossible with most of the refractory bodies employed as electrodes.
+Assume that a point of the electrode gets hotter; instantly most of the
+discharge passes through that point, and a minute patch it probably
+fused and evaporated. It is now possible that in consequence of the
+violent disintegration the spot attacked sinks in temperature, or that a
+counter force is created, as in an arc; at any rate, the local tearing
+off meets with the limitations incident to the experiment, whereupon the
+same process occurs on another place. To the eye the electrode appears
+uniformly brilliant, but there are upon it points constantly shifting
+and wandering around, of a temperature far above the mean, and this
+materially hastens the process of deterioration. That some such thing
+occurs, at least when the electrode is at a lower temperature,
+sufficient experimental evidence can be obtained in the following
+manner: Exhaust a bulb to a very high degree, so that with a fairly high
+potential the discharge cannot pass--that is, not a _luminous_ one, for
+a weak invisible discharge occurs always, in all probability. Now raise
+slowly and carefully the potential, leaving the primary current on no
+more than for an instant. At a certain point, two, three, or half a
+dozen phosphorescent spots will appear on the globe. These places of the
+glass are evidently more violently bombarded than others, this being due
+to the unevenly distributed electric density, necessitated, of course,
+by sharp projections, or, generally speaking, irregularities of the
+electrode. But the luminous patches are constantly changing in position,
+which is especially well observable if one manages to produce very few,
+and this indicates that the configuration of the electrode is rapidly
+changing.
+
+From experiences of this kind I am led to infer that, in order to be
+most durable, the refractory button in the bulb should be in the form of
+a sphere with a highly polished surface. Such a small sphere could be
+manufactured from a diamond or some other crystal, but a better way
+would be to fuse, by the employment of extreme degrees of temperature,
+some oxide--as, for instance, zirconia--into a small drop, and then keep
+it in the bulb at a temperature somewhat below its point of fusion.
+
+Interesting and useful results can, no doubt, be reached in the
+direction of extreme degrees of heat. How can such high temperatures be
+arrived at? How are the highest degrees of heat reached in nature? By
+the impact of stars, by high speeds and collisions. In a collision any
+rate of heat generation may be attained. In a chemical process we are
+limited. When oxygen and hydrogen combine, they fall, metaphorically
+speaking, from a definite height. We cannot go very far with a blast,
+nor by confining heat in a furnace, but in an exhausted bulb we can
+concentrate any amount of energy upon a minute button. Leaving
+practicability out of consideration, this, then, would be the means
+which, in my opinion, would enable us to reach the highest temperature.
+But a great difficulty when proceeding in this way is encountered,
+namely, in most cases the body is carried off before it can fuse and
+form a drop. This difficulty exists principally with an oxide, such as
+zirconia, because it cannot be compressed in so hard a cake that it
+would not be carried off quickly. I have endeavored repeatedly to fuse
+zirconia, placing it in a cup of arc light carbon, as indicated in Fig.
+152. It glowed with a most intense light, and the stream of the
+particles projected out of the carbon cup was of a vivid white; but
+whether it was compressed in a cake or made into a paste with carbon, it
+was carried off before it could be fused. The carbon cup, containing
+zirconia, had to be mounted very low in the neck of a large bulb, as the
+heating of the glass by the projected particles of the oxide was so
+rapid that in the first trial the bulb was cracked almost in an instant,
+when the current was turned on. The heating of the glass by the
+projected particles was found to be always greater when the carbon cup
+contained a body which was rapidly carried off--I presume, because in
+such cases, with the same potential, higher speeds were reached, and
+also because, per unit of time, more matter was projected--that is, more
+particles would strike the glass.
+
+The before-mentioned difficulty did not exist, however, when the body
+mounted in the carbon cup offered great resistance to deterioration. For
+instance, when an oxide was first fused in an oxygen blast, and then
+mounted in the bulb, it melted very readily into a drop.
+
+Generally, during the process of fusion, magnificent light effects were
+noted, of which it would be difficult to give an adequate idea. Fig. 152
+is intended to illustrate the effect observed with a ruby drop. At first
+one may see a narrow funnel of white light projected against the top of
+the globe, where it produces an irregularly outlined phosphorescent
+patch. When the point of the ruby fuses, the phosphorescence becomes
+very powerful; but as the atoms are projected with much greater speed
+from the surface of the drop, soon the glass gets hot and "tired," and
+now only the outer edge of the patch glows. In this manner an intensely
+phosphorescent, sharply defined line, _l_, corresponding to the outline
+of the drop, is produced, which spreads slowly over the globe as the
+drop gets larger. When the mass begins to boil, small bubbles and
+cavities are formed, which cause dark colored spots to sweep across the
+globe. The bulb may be turned downward without fear of the drop falling
+off, as the mass possesses considerable viscosity.
+
+I may mention here another feature of some interest, which I believe to
+have noted in the course of these experiments, though the observations
+do not amount to a certitude. It _appeared_ that under the molecular
+impact caused by the rapidly alternating potential, the body was fused
+and maintained in that state at a lower temperature in a highly
+exhausted bulb than was the case at normal pressure and application of
+heat in the ordinary way--that is, at least, judging from the quantity
+of the light emitted. One of the experiments performed may be mentioned
+here by way of illustration. A small piece of pumice stone was stuck on
+a platinum wire, and first melted to it in a gas burner. The wire was
+next placed between two pieces of charcoal, and a burner applied, so as
+to produce an intense heat, sufficient to melt down the pumice stone
+into a small glass-like button. The platinum wire had to be taken of
+sufficient thickness, to prevent its melting in the fire. While in the
+charcoal fire, or when held in a burner to get a better idea of the
+degree of heat, the button glowed with great brilliancy. The wire with
+the button was then mounted in a bulb, and upon exhausting the same to a
+high degree, the current was turned on slowly, so as to prevent the
+cracking of the button. The button was heated to the point of fusion,
+and when it melted, it did not, apparently, glow with the same
+brilliancy as before, and this would indicate a lower temperature.
+Leaving out of consideration the observer's possible, and even probable,
+error, the question is, can a body under these conditions be brought
+from a solid to a liquid state with the evolution of _less_ light?
+
+When the potential of a body is rapidly alternated, it is certain that
+the structure is jarred. When the potential is very high, although the
+vibrations may be few--say 20,000 per second--the effect upon the
+structure may be considerable. Suppose, for example, that a ruby is
+melted into a drop by a steady application of energy. When it forms a
+drop, it will emit visible and invisible waves, which will be in a
+definite ratio, and to the eye the drop will appear to be of a certain
+brilliancy. Next, suppose we diminish to any degree we choose the energy
+steadily supplied, and, instead, supply energy which rises and falls
+according to a certain law. Now, when the drop is formed, there will be
+emitted from it three different kinds of vibrations--the ordinary
+visible, and two kinds of invisible waves: that is, the ordinary dark
+waves of all lengths, and, in addition, waves of a well defined
+character. The latter would not exist by a steady supply of the energy;
+still they help to jar and loosen the structure. If this really be the
+case, then the ruby drop will emit relatively less visible and more
+invisible waves than before. Thus it would seem that when a platinum
+wire, for instance, is fused by currents alternating with extreme
+rapidity, it emits at the point of fusion less light and more visible
+radiation than it does when melted by a steady current, though the total
+energy used up in the process of fusion is the same in both cases. Or,
+to cite another example, a lamp filament is not capable of withstanding
+as long with currents of extreme frequency as it does with steady
+currents, assuming that it be worked at the same luminous intensity.
+This means that for rapidly alternating currents the filament should be
+shorter and thicker. The higher the frequency--that is, the greater the
+departure from the steady flow--the worse it would be for the filament.
+But if the truth of this remark were demonstrated, it would be erroneous
+to conclude that such a refractory button as used in these bulbs would
+be deteriorated quicker by currents of extremely high frequency than by
+steady or low frequency currents. From experience I may say that just
+the opposite holds good: the button withstands the bombardment better
+with currents of very high frequency. But this is due to the fact that a
+high frequency discharge passes through a rarefied gas with much greater
+freedom than a steady or low frequency discharge, and this will mean
+that with the former we can work with a lower potential or with a less
+violent impact. As long, then, as the gas is of no consequence, a steady
+or low frequency current is better; but as soon as the action of the gas
+is desired and important, high frequencies are preferable.
+
+In the course of these experiments a great many trials were made with
+all kinds of carbon buttons. Electrodes made of ordinary carbon buttons
+were decidedly more durable when the buttons were obtained by the
+application of enormous pressure. Electrodes prepared by depositing
+carbon in well known ways did not show up well; they blackened the globe
+very quickly. From many experiences I conclude that lamp filaments
+obtained in this manner can be advantageously used only with low
+potentials and low frequency currents. Some kinds of carbon withstand so
+well that, in order to bring them to the point of fusion, it is
+necessary to employ very small buttons. In this case the observation is
+rendered very difficult on account of the intense heat produced.
+Nevertheless there can be no doubt that all kinds of carbon are fused
+under the molecular bombardment, but the liquid state must be one of
+great instability. Of all the bodies tried there were two which
+withstood best--diamond and carborundum. These two showed up about
+equally, but the latter was preferable for many reasons. As it is more
+than likely that this body is not yet generally known, I will venture to
+call your attention to it.
+
+It has been recently produced by Mr. E. G. Acheson, of Monongahela City,
+Pa., U. S. A. It is intended to replace ordinary diamond powder for
+polishing precious stones, etc., and I have been informed that it
+accomplishes this object quite successfully. I do not know why the name
+"carborundum" has been given to it, unless there is something in the
+process of its manufacture which justifies this selection. Through the
+kindness of the inventor, I obtained a short while ago some samples
+which I desired to test in regard to their qualities of phosphorescence
+and capability of withstanding high degrees of heat.
+
+Carborundum can be obtained in two forms--in the form of "crystals" and
+of powder. The former appear to the naked eye dark colored, but are very
+brilliant; the latter is of nearly the same color as ordinary diamond
+powder, but very much finer. When viewed under a microscope the samples
+of crystals given to me did not appear to have any definite form, but
+rather resembled pieces of broken up egg coal of fine quality. The
+majority were opaque, but there were some which were transparent and
+colored. The crystals are a kind of carbon containing some impurities;
+they are extremely hard, and withstand for a long time even an oxygen
+blast. When the blast is directed against them they at first form a
+cake of some compactness, probably in consequence of the fusion of
+impurities they contain. The mass withstands for a very long time the
+blast without further fusion; but a slow carrying off, or burning,
+occurs, and, finally, a small quantity of a glass-like residue is left,
+which, I suppose, is melted alumina. When compressed strongly they
+conduct very well, but not as well as ordinary carbon. The powder, which
+is obtained from the crystals in some way, is practically
+non-conducting. It affords a magnificent polishing material for stones.
+
+The time has been too short to make a satisfactory study of the
+properties of this product, but enough experience has been gained in a
+few weeks I have experimented upon it to say that it does possess some
+remarkable properties in many respects. It withstands excessively high
+degrees of heat, it is little deteriorated by molecular bombardment, and
+it does not blacken the globe as ordinary carbon does. The only
+difficulty which I have experienced in its use in connection with these
+experiments was to find some binding material which would resist the
+heat and the effect of the bombardment as successfully as carborundum
+itself does.
+
+I have here a number of bulbs which I have provided with buttons of
+carborundum. To make such a button of carborundum crystals I proceed in
+the following manner: I take an ordinary lamp filament and dip its point
+in tar, or some other thick substance or paint which may be readily
+carbonized. I next pass the point of the filament through the crystals,
+and then hold it vertically over a hot plate. The tar softens and forms
+a drop on the point of the filament, the crystals adhering to the
+surface of the drop. By regulating the distance from the plate the tar
+is slowly dried out and the button becomes solid. I then once more dip
+the button in tar and hold it again over a plate until the tar is
+evaporated, leaving only a hard mass which firmly binds the crystals.
+When a larger button is required I repeat the process several times, and
+I generally also cover the filament a certain distance below the button
+with crystals. The button being mounted in a bulb, when a good vacuum
+has been reached, first a weak and then a strong discharge is passed
+through the bulb to carbonize the tar and expel all gases, and later it
+is brought to a very intense incandescence.
+
+When the powder is used I have found it best to proceed as follows: I
+make a thick paint of carborundum and tar, and pass a lamp filament
+through the paint. Taking then most of the paint off by rubbing the
+filament against a piece of chamois leather, I hold it over a hot plate
+until the tar evaporates and the coating becomes firm. I repeat this
+process as many times as it is necessary to obtain a certain thickness
+of coating. On the point of the coated filament I form a button in the
+same manner.
+
+There is no doubt that such a button--properly prepared under great
+pressure--of carborundum, especially of powder of the best quality, will
+withstand the effect of the bombardment fully as well as anything we
+know. The difficulty is that the binding material gives way, and the
+carborundum is slowly thrown off after some time. As it does not seem to
+blacken the globe in the least, it might be found useful for coating the
+filaments of ordinary incandescent lamps, and I think that it is even
+possible to produce thin threads or sticks of carborundum which will
+replace the ordinary filaments in an incandescent lamp. A carborundum
+coating seems to be more durable than other coatings, not only because
+the carborundum can withstand high degrees of heat, but also because it
+seems to unite with the carbon better than any other material I have
+tried. A coating of zirconia or any other oxide, for instance, is far
+more quickly destroyed. I prepared buttons of diamond dust in the same
+manner as of carborundum, and these came in durability nearest to those
+prepared of carborundum, but the binding paste gave way much more
+quickly in the diamond buttons; this, however, I attributed to the size
+and irregularity of the grains of the diamond.
+
+It was of interest to find whether carborundum possesses the quality of
+phosphorescence. One is, of course, prepared to encounter two
+difficulties: first, as regards the rough product, the "crystals," they
+are good conducting, and it is a fact that conductors do not
+phosphoresce; second, the powder, being exceedingly fine, would not be
+apt to exhibit very prominently this quality, since we know that when
+crystals, even such as diamond or ruby, are finely powdered, they lose
+the property of phosphorescence to a considerable degree.
+
+The question presents itself here, can a conductor phosphoresce? What is
+there in such a body as a metal, for instance, that would deprive it of
+the quality of phosphoresence, unless it is that property which
+characterizes it as a conductor? For it is a fact that most of the
+phosphorescent bodies lose that quality when they are sufficiently
+heated to become more or less conducting. Then, if a metal be in a
+large measure, or perhaps entirely, deprived of that property, it should
+be capable of phosphoresence. Therefore it is quite possible that at
+some extremely high frequency, when behaving practically as a
+non-conductor, a metal or any other conductor might exhibit the quality
+of phosphoresence, even though it be entirely incapable of
+phosphorescing under the impact of a low-frequency discharge. There is,
+however, another possible way how a conductor might at least _appear_ to
+phosphoresce.
+
+Considerable doubt still exists as to what really is phosphorescence,
+and as to whether the various phenomena comprised under this head are
+due to the same causes. Suppose that in an exhausted bulb, under the
+molecular impact, the surface of a piece of metal or other conductor is
+rendered strongly luminous, but at the same time it is found that it
+remains comparatively cool, would not this luminosity be called
+phosphorescence? Now such a result, theoretically at least, is possible,
+for it is a mere question of potential or speed. Assume the potential of
+the electrode, and consequently the speed of the projected atoms, to be
+sufficiently high, the surface of the metal piece, against which the
+atoms are projected, would be rendered highly incandescent, since the
+process of heat generation would be incomparably faster than that of
+radiating or conducting away from the surface of the collision. In the
+eye of the observer a single impact of the atoms would cause an
+instantaneous flash, but if the impacts were repeated with sufficient
+rapidity, they would produce a continuous impression upon his retina. To
+him then the surface of the metal would appear continuously incandescent
+and of constant luminous intensity, while in reality the light would be
+either intermittent, or at least changing periodically in intensity. The
+metal piece would rise in temperature until equilibrium was
+attained--that is, until the energy continuously radiated would equal
+that intermittently supplied. But the supplied energy might under such
+conditions not be sufficient to bring the body to any more than a very
+moderate mean temperature, especially if the frequency of the atomic
+impacts be very low--just enough that the fluctuation of the intensity
+of the light emitted could not be detected by the eye. The body would
+now, owing to the manner in which the energy is supplied, emit a strong
+light, and yet be at a comparatively very low mean temperature. How
+should the observer name the luminosity thus produced? Even if the
+analysis of the light would teach him something definite, still he would
+probably rank it under the phenomena of phosphorescence. It is
+conceivable that in such a way both conducting and non-conducting bodies
+may be maintained at a certain luminous intensity, but the energy
+required would very greatly vary with the nature and properties of the
+bodies.
+
+These and some foregoing remarks of a speculative nature were made
+merely to bring out curious features of alternate currents or electric
+impulses. By their help we may cause a body to emit _more_ light, while
+at a certain mean temperature, than it would emit if brought to that
+temperature by a steady supply; and, again, we may bring a body to the
+point of fusion, and cause it to emit _less_ light than when fused by
+the application of energy in ordinary ways. It all depends on how we
+supply the energy, and what kind of vibrations we set up; in one case
+the vibrations are more, in the other less, adapted to affect our sense
+of vision.
+
+Some effects, which I had not observed before, obtained with carborundum
+in the first trials, I attributed to phosphorescence, but in subsequent
+experiments it appeared that it was devoid of that quality. The crystals
+possess a noteworthy feature. In a bulb provided with a single electrode
+in the shape of a small circular metal disc, for instance, at a certain
+degree of exhaustion the electrode is covered with a milky film, which
+is separated by a dark space from the glow filling the bulb. When the
+metal disc is covered with carborundum crystals, the film is far more
+intense, and snow-white. This I found later to be merely an effect of
+the bright surface of the crystals, for when an aluminum electrode was
+highly polished, it exhibited more or less the same phenomenon. I made a
+number of experiments with the samples of crystals obtained, principally
+because it would have been of special interest to find that they are
+capable of phosphorescence, on account of their being conducting. I
+could not produce phosphorescence distinctly, but I must remark that a
+decisive opinion cannot be formed until other experimenters have gone
+over the same ground.
+
+The powder behaved in some experiments as though it contained alumina,
+but it did not exhibit with sufficient distinctness the red of the
+latter. Its dead color brightens considerably under the molecular
+impact, but I am now convinced it does not phosphoresce. Still, the
+tests with the powder are not conclusive, because powdered carborundum
+probably does not behave like a phosphorescent sulphide, for example,
+which could be finely powdered without impairing the phosphorescence,
+but rather like powdered ruby or diamond, and therefore it would be
+necessary, in order to make a decisive test, to obtain it in a large
+lump and polish up the surface.
+
+If the carborundum proves useful in connection with these and similar
+experiments, its chief value will be found in the production of
+coatings, thin conductors, buttons, or other electrodes capable of
+withstanding extremely high degrees of heat.
+
+The production of a small electrode, capable of withstanding enormous
+temperatures, I regard as of the greatest importance in the manufacture
+of light. It would enable us to obtain, by means of currents of very
+high frequencies, certainly 20 times, if not more, the quantity of light
+which is obtained in the present incandescent lamp by the same
+expenditure of energy. This estimate may appear to many exaggerated, but
+in reality I think it is far from being so. As this statement might be
+misunderstood, I think it is necessary to expose clearly the problem
+with which, in this line of work, we are confronted, and the manner in
+which, in my opinion, a solution will be arrived at.
+
+Any one who begins a study of the problem will be apt to think that what
+is wanted in a lamp with an electrode is a very high degree of
+incandescence of the electrode. There he will be mistaken. The high
+incandescence of the button is a necessary evil, but what is really
+wanted is the high incandescence of the gas surrounding the button. In
+other words, the problem in such a lamp is to bring a mass of gas to the
+highest possible incandescence. The higher the incandescence, the
+quicker the mean vibration, the greater is the economy of the light
+production. But to maintain a mass of gas at a high degree of
+incandescence in a glass vessel, it will always be necessary to keep the
+incandescent mass away from the glass; that is, to confine it as much as
+possible to the central portion of the globe.
+
+In one of the experiments this evening a brush was produced at the end
+of a wire. The brush was a flame, a source of heat and light. It did not
+emit much perceptible heat, nor did it glow with an intense light; but
+is it the less a flame because it does not scorch my hand? Is it the
+less a flame because it does not hurt my eyes by its brilliancy? The
+problem is precisely to produce in the bulb such a flame, much smaller
+in size, but incomparably more powerful. Were there means at hand for
+producing electric impulses of a sufficiently high frequency, and for
+transmitting them, the bulb could be done away with, unless it were used
+to protect the electrode, or to economize the energy by confining the
+heat. But as such means are not at disposal, it becomes necessary to
+place the terminal in the bulb and rarefy the air in the same. This is
+done merely to enable the apparatus to perform the work which it is not
+capable of performing at ordinary air pressure. In the bulb we are able
+to intensify the action to any degree--so far that the brush emits a
+powerful light.
+
+The intensity of the light emitted depends principally on the frequency
+and potential of the impulses, and on the electric density on the
+surface of the electrode. It is of the greatest importance to employ the
+smallest possible button, in order to push the density very far. Under
+the violent impact of the molecules of the gas surrounding it, the small
+electrode is of course brought to an extremely high temperature, but
+around it is a mass of highly incandescent gas, a flame photosphere,
+many hundred times the volume of the electrode. With a diamond,
+carborundum or zirconia button the photosphere can be as much as one
+thousand times the volume of the button. Without much reflection one
+would think that in pushing so far the incandescence of the electrode it
+would be instantly volatilized. But after a careful consideration one
+would find that, theoretically, it should not occur, and in this
+fact--which, moreover, is experimentally demonstrated--lies principally
+the future value of such a lamp.
+
+At first, when the bombardment begins, most of the work is performed on
+the surface of the button, but when a highly conducting photosphere is
+formed the button is comparatively relieved. The higher the
+incandescence of the photosphere, the more it approaches in conductivity
+to that of the electrode, and the more, therefore, the solid and the gas
+form one conducting body. The consequence is that the further the
+incandescence is forced the more work, comparatively, is performed on
+the gas, and the less on the electrode. The formation of a powerful
+photosphere is consequently the very means for protecting the electrode.
+This protection, of course, is a relative one, and it should not be
+thought that by pushing the incandescence higher the electrode is
+actually less deteriorated. Still, theoretically, with extreme
+frequencies, this result must be reached, but probably at a temperature
+too high for most of the refractory bodies known. Given, then, an
+electrode which can withstand to a very high limit the effect of the
+bombardment and outward strain, it would be safe, no matter how much it
+was forced beyond that limit. In an incandescent lamp quite different
+considerations apply. There the gas is not at all concerned; the whole
+of the work is performed on the filament; and the life of the lamp
+diminishes so rapidly with the increase of the degree of incandescence
+that economical reasons compel us to work it at a low incandescence. But
+if an incandescent lamp is operated with currents of very high
+frequency, the action of the gas cannot be neglected, and the rules for
+the most economical working must be considerably modified.
+
+In order to bring such a lamp with one or two electrodes to a great
+perfection, it is necessary to employ impulses of very high frequency.
+The high frequency secures, among others, two chief advantages, which
+have a most important bearing upon the economy of the light production.
+First, the deterioration of the electrode is reduced by reason of the
+fact that we employ a great many small impacts, instead of a few violent
+ones, which quickly shatter the structure; secondly, the formation of a
+large photosphere is facilitated.
+
+In order to reduce the deterioration of the electrode to the minimum, it
+is desirable that the vibration be harmonic, for any suddenness hastens
+the process of destruction. An electrode lasts much longer when kept at
+incandescence by currents, or impulses, obtained from a high frequency
+alternator, which rise and fall more or less harmonically, than by
+impulses obtained from a disruptive discharge coil. In the latter case
+there is no doubt that most of the damage is done by the fundamental
+sudden discharges.
+
+One of the elements of loss in such a lamp is the bombardment of the
+globe. As the potential is very high, the molecules are projected with
+great speed; they strike the glass, and usually excite a strong
+phosphorescence. The effect produced is very pretty, but for economical
+reasons it would be perhaps preferable to prevent, or at least reduce to
+a minimum, the bombardment against the globe, as in such case it is, as
+a rule, not the object to excite phosphorescence, and as some loss of
+energy results from the bombardment. This loss in the bulb is
+principally dependent on the potential of the impulses and on the
+electric density on the surface of the electrode. In employing very high
+frequencies the loss of energy by the bombardment is greatly reduced,
+for, first, the potential needed to perform a given amount of work is
+much smaller; and, secondly, by producing a highly conducting
+photosphere around the electrode, the same result is obtained as though
+the electrode were much larger, which is equivalent to a smaller
+electric density. But be it by the diminution of the maximum potential
+or of the density, the gain is effected in the same manner, namely, by
+avoiding violent shocks, which strain the glass much beyond its limit of
+elasticity. If the frequency could be brought high enough, the loss due
+to the imperfect elasticity of the glass would be entirely negligible.
+The loss due to bombardment of the globe may, however, be reduced by
+using two electrodes instead of one. In such case each of the electrodes
+may be connected to one of the terminals; or else, if it is preferable
+to use only one wire, one electrode may be connected to one terminal and
+the other to the ground or to an insulated body of some surface, as, for
+instance, a shade on the lamp. In the latter case, unless some judgment
+is used, one of the electrodes might glow more intensely than the other.
+
+But on the whole I find it preferable, when using such high frequencies,
+to employ only one electrode and one connecting wire. I am convinced
+that the illuminating device of the near future will not require for its
+operation more than one lead, and, at any rate, it will have no
+leading-in wire, since the energy required can be as well transmitted
+through the glass. In experimental bulbs the leading-in wire is not
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 151, for example, there is some
+difficulty in fitting the parts, but these difficulties would not exist
+if a great many bulbs were manufactured; otherwise the energy can be
+conveyed through the glass as well as through a wire, and with these
+high frequencies the losses are very small. Such illustrating devices
+will necessarily involve the use of very high potentials, and this, in
+the eyes of practical men, might be an objectionable feature. Yet, in
+reality, high potentials are not objectionable--certainly not in the
+least so far as the safety of the devices is concerned.
+
+There are two ways of rendering an electric appliance safe. One is to
+use low potentials, the other is to determine the dimensions of the
+apparatus so that it is safe, no matter how high a potential is used. Of
+the two, the latter seems to me the better way, for then the safety is
+absolute, unaffected by any possible combination of circumstances which
+might render even a low-potential appliance dangerous to life and
+property. But the practical conditions require not only the judicious
+determination of the dimensions of the apparatus; they likewise
+necessitate the employment of energy of the proper kind. It is easy, for
+instance, to construct a transformer capable of giving, when operated
+from an ordinary alternate current machine of low tension, say 50,000
+volts, which might be required to light a highly exhausted
+phosphorescent tube, so that, in spite of the high potential, it is
+perfectly safe, the shock from it producing no inconvenience. Still such
+a transformer would be expensive, and in itself inefficient; and,
+besides, what energy was obtained from it would not be economically used
+for the production of light. The economy demands the employment of
+energy in the form of extremely rapid vibrations. The problem of
+producing light has been likened to that of maintaining a certain
+high-pitch note by means of a bell. It should be said a _barely audible_
+note; and even these words would not express it, so wonderful is the
+sensitiveness of the eye. We may deliver powerful blows at long
+intervals, waste a good deal of energy, and still not get what we want;
+or we may keep up the note by delivering frequent taps, and get nearer
+to the object sought by the expenditure of much less energy. In the
+production of light, as far as the illuminating device is concerned,
+there can be only one rule--that is, to use as high frequencies as can
+be obtained; but the means for the production and conveyance of impulses
+of such character impose, at present at least, great limitations. Once
+it is decided to use very high frequencies, the return wire becomes
+unnecessary, and all the appliances are simplified. By the use of
+obvious means the same result is obtained as though the return wire were
+used. It is sufficient for this purpose to bring in contact with the
+bulb, or merely in the vicinity of the same, an insulated body of some
+surface. The surface need, of course, be the smaller, the higher the
+frequency and potential used, and necessarily, also, the higher the
+economy of the lamp or other device.
+
+This plan of working has been resorted to on several occasions this
+evening. So, for instance, when the incandescence of a button was
+produced by grasping the bulb with the hand, the body of the
+experimenter merely served to intensify the action. The bulb used was
+similar to that illustrated in Fig. 148, and the coil was excited to a
+small potential, not sufficient to bring the button to incandescence
+when the bulb was hanging from the wire; and incidentally, in order to
+perform the experiment in a more suitable manner, the button was taken
+so large that a perceptible time had to elapse before, upon grasping the
+bulb, it could be rendered incandescent. The contact with the bulb was,
+of course, quite unnecessary. It is easy, by using a rather large bulb
+with an exceedingly small electrode, to adjust the conditions so that
+the latter is brought to bright incandescence by the mere approach of
+the experimenter within a few feet of the bulb, and that the
+incandescence subsides upon his receding.
+
+[Illustration: FIG. 153.]
+
+[Illustration: FIG. 154.]
+
+In another experiment, when phosphorescence was excited, a similar bulb
+was used. Here again, originally, the potential was not sufficient to
+excite phosphorescence until the action was intensified--in this case,
+however, to present a different feature, by touching the socket with a
+metallic object held in the hand. The electrode in the bulb was a carbon
+button so large that it could not be brought to incandescence, and
+thereby spoil the effect produced by phosphorescence.
+
+Again, in another of the early experiments, a bulb was used, as
+illustrated in Fig. 141. In this instance, by touching the bulb with one
+or two fingers, one or two shadows of the stem inside were projected
+against the glass, the touch of the finger producing the same results as
+the application of an external negative electrode under ordinary
+circumstances.
+
+In all these experiments the action was intensified by augmenting the
+capacity at the end of the lead connected to the terminal. As a rule, it
+is not necessary to resort to such means, and would be quite unnecessary
+with still higher frequencies; but when it _is_ desired, the bulb, or
+tube, can be easily adapted to the purpose.
+
+In Fig. 153, for example, an experimental bulb, L, is shown, which
+is provided with a neck, _n_, on the top, for the application of an
+external tinfoil coating, which may be connected to a body of larger
+surface. Such a lamp as illustrated in Fig. 154 may also be lighted by
+connecting the tinfoil coating on the neck _n_ to the terminal, and the
+leading-in wire, _w_, to an insulated plate. If the bulb stands in a
+socket upright, as shown in the cut, a shade of conducting material may
+be slipped in the neck, _n_, and the action thus magnified.
+
+A more perfected arrangement used in some of these bulbs is illustrated
+in Fig. 155. In this case the construction of the bulb is as shown and
+described before, when reference was made to Fig. 148. A zinc sheet, Z,
+with a tubular extension, T, is applied over the metallic socket, S.
+The bulb hangs downward from the terminal, _t_, the zinc sheet, Z,
+performing the double office of intensifier and reflector. The reflector
+is separated from the terminal, _t_, by an extension of the insulating
+plug, P.
+
+A similar disposition with a phosphorescent tube is illustrated in
+Fig. 156. The tube, T, is prepared from two short tubes of different
+diameter, which are sealed on the ends. On the lower end is placed an
+inside conducting coating, C, which connects to the wire _w_. The wire
+has a hook on the upper end for suspension, and passes through the
+centre of the inside tube, which is filled with some good and tightly
+packed insulator. On the outside of the upper end of the tube, T, is
+another conducting coating, C_{1}, upon which is slipped a metallic
+reflector Z, which should be separated by a thick insulation from the
+end of wire _w_.
+
+The economical use of such a reflector or intensifier would require that
+all energy supplied to an air condenser should be recoverable, or, in
+other words, that there should not be any losses, neither in the
+gaseous medium nor through its action elsewhere. This is far from being
+so, but, fortunately, the losses may be reduced to anything desired. A
+few remarks are necessary on this subject, in order to make the
+experiences gathered in the course of these investigations perfectly
+clear.
+
+[Illustration: FIG. 155.]
+
+Suppose a small helix with many well insulated turns, as in experiment
+Fig. 146, has one of its ends connected to one of the terminals of the
+induction coil, and the other to a metal plate, or, for the sake of
+simplicity, a sphere, insulated in space. When the coil is set to work,
+the potential of the sphere is alternated, and a small helix now behaves
+as though its free end were connected to the other terminal of the
+induction coil. If an iron rod be held within a small helix, it is
+quickly brought to a high temperature, indicating the passage of a
+strong current through the helix. How does the insulated sphere act in
+this case? It can be a condenser, storing and returning the energy
+supplied to it, or it can be a mere sink of energy, and the conditions
+of the experiment determine whether it is rather one than the other. The
+sphere being charged to a high potential, it acts inductively upon the
+surrounding air, or whatever gaseous medium there might be. The
+molecules, or atoms, which are near the sphere, are of course more
+attracted, and move through a greater distance than the farther ones.
+When the nearest molecules strike the sphere, they are repelled, and
+collisions occur at all distances within the inductive action of the
+sphere. It is now clear that, if the potential be steady, but little
+loss of energy can be caused in this way, for the molecules which are
+nearest to the sphere, having had an additional charge imparted to them
+by contact, are not attracted until they have parted, if not with all,
+at least with most of the additional charge, which can be accomplished
+only after a great many collisions. From the fact, that with a steady
+potential there is but little loss in dry air, one must come to such a
+conclusion. When the potential of a sphere, instead of being steady, is
+alternating, the conditions are entirely different. In this case a
+rhythmical bombardment occurs, no matter whether the molecules, after
+coming in contact with the sphere, lose the imparted charge or not; what
+is more, if the charge is not lost, the impacts are only the more
+violent. Still, if the frequency of the impulses be very small, the loss
+caused by the impacts and collisions would not be serious, unless the
+potential were excessive. But when extremely high frequencies and more
+or less high potentials are used, the loss may very great. The total
+energy lost per unit of time is proportionate to the product of the
+number of impacts per second, or the frequency and the energy lost in
+each impact. But the energy of an impact must be proportionate to the
+square of the electric density of the sphere, since the charge imparted
+to the molecule is proportionate to that density. I conclude from this
+that the total energy lost must be proportionate to the product of the
+frequency and the square of the electric density; but this law needs
+experimental confirmation. Assuming the preceding considerations to be
+true, then, by rapidly alternating the potential of a body immersed in
+an insulating gaseous medium, any amount of energy may be dissipated
+into space. Most of that energy then, I believe, is not dissipated in
+the form of long ether waves, propagated to considerable distance, as is
+thought most generally, but is consumed--in the case of an insulated
+sphere, for example--in impact and collisional losses--that is, heat
+vibrations--on the surface and in the vicinity of the sphere. To reduce
+the dissipation, it is necessary to work with a small electric
+density--the smaller, the higher the frequency.
+
+[Illustration: FIG. 156.]
+
+But since, on the assumption before made, the loss is diminished with
+the square of the density, and since currents of very high frequencies
+involve considerable waste when transmitted through conductors, it
+follows that, on the whole, it is better to employ one wire than two.
+Therefore, if motors, lamps, or devices of any kind are perfected,
+capable of being advantageously operated by currents of extremely high
+frequency, economical reasons will make it advisable to use only one
+wire, especially if the distances are great.
+
+When energy is absorbed in a condenser, the same behaves as though its
+capacity were increased. Absorption always exists more or less, but
+generally it is small and of no consequence as long as the frequencies
+are not very great. In using extremely high frequencies, and,
+necessarily in such case, also high potentials, the absorption--or, what
+is here meant more particularly by this term, the loss of energy due to
+the presence of a gaseous medium--is an important factor to be
+considered, as the energy absorbed in the air condenser may be any
+fraction of the supplied energy. This would seem to make it very
+difficult to tell from the measured or computed capacity of an air
+condenser its actual capacity or vibration period, especially if the
+condenser is of very small surface and is charged to a very high
+potential. As many important results are dependent upon the correctness
+of the estimation of the vibration period, this subject demands the most
+careful scrutiny of other investigators. To reduce the probable error as
+much as possible in experiments of the kind alluded to, it is advisable
+to use spheres or plates of large surface, so as to make the density
+exceedingly small. Otherwise, when it is practicable, an oil condenser
+should be used in preference. In oil or other liquid dielectrics there
+are seemingly no such losses as in gaseous media. It being impossible to
+exclude entirely the gas in condensers with solid dielectrics, such
+condensers should be immersed in oil, for economical reasons, if nothing
+else; they can then be strained to the utmost, and will remain cool. In
+Leyden jars the loss due to air is comparatively small, as the tinfoil
+coatings are large, close together, and the charged surfaces not
+directly exposed; but when the potentials are very high, the loss may be
+more or less considerable at, or near, the upper edge of the foil, where
+the air is principally acted upon. If the jar be immersed in boiled-out
+oil, it will be capable of performing four times the amount of work
+which it can for any length of time when used in the ordinary way, and
+the loss will be inappreciable.
+
+It should not be thought that the loss in heat in an air condenser is
+necessarily associated with the formation of _visible_ streams or
+brushes. If a small electrode, inclosed in an unexhausted bulb, is
+connected to one of the terminals of the coil, streams can be seen to
+issue from the electrode, and the air in the bulb is heated; if instead
+of a small electrode a large sphere is inclosed in the bulb, no streams
+are observed, still the air is heated.
+
+Nor should it be thought that the temperature of an air condenser would
+give even an approximate idea of the loss in heat incurred, as in such
+case heat must be given off much more quickly, since there is, in
+addition to the ordinary radiation, a very active carrying away of heat
+by independent carriers going on, and since not only the apparatus, but
+the air at some distance from it is heated in consequence of the
+collisions which must occur.
+
+Owing to this, in experiments with such a coil, a rise of temperature
+can be distinctly observed only when the body connected to the coil is
+very small. But with apparatus on a larger scale, even a body of
+considerable bulk would be heated, as, for instance, the body of a
+person; and I think that skilled physicians might make observations of
+utility in such experiments, which, if the apparatus were judiciously
+designed, would not present the slightest danger.
+
+A question of some interest, principally to meteorologists, presents
+itself here. How does the earth behave? The earth is an air condenser,
+but is it a perfect or a very imperfect one--a mere sink of energy?
+There can be little doubt that to such small disturbance as might be
+caused in an experiment, the earth behaves as an almost perfect
+condenser. But it might be different when its charge is set in vibration
+by some sudden disturbance occurring in the heavens. In such case, as
+before stated, probably only little of the energy of the vibrations set
+up would be lost into space in the form of long ether radiations, but
+most of the energy, I think, would spend itself in molecular impacts and
+collisions, and pass off into space in the form of short heat, and
+possibly light, waves. As both the frequency of the vibrations of the
+charge and the potential are in all probability excessive, the energy
+converted into heat may be considerable. Since the density must be
+unevenly distributed, either in consequence of the irregularity of the
+earth's surface, or on account of the condition of the atmosphere in
+various places, the effect produced would accordingly vary from place to
+place. Considerable variations in the temperature and pressure of the
+atmosphere may in this manner be caused at any point of the surface of
+the earth. The variations may be gradual or very sudden, according to
+the nature of the general disturbance, and may produce rain and storms,
+or locally modify the weather in any way.
+
+From the remarks before made, one may see what an important factor of
+loss the air in the neighborhood of a charged surface becomes when the
+electric density is great and the frequency of the impulses excessive.
+But the action, as explained, implies that the air is insulating--that
+is, that it is composed of independent carriers immersed in an
+insulating medium. This is the case only when the air is at something
+like ordinary or greater, or at extremely small, pressure. When the air
+is slightly rarefied and conducting, then true conduction losses occur
+also. In such case, of course, considerable energy may be dissipated
+into space even with a steady potential, or with impulses of low
+frequency, if the density is very great.
+
+When the gas is at very low pressure, an electrode is heated more
+because higher speeds can be reached. If the gas around the electrode is
+strongly compressed, the displacements, and consequently the speeds, are
+very small, and the heating is insignificant. But if in such case the
+frequency could be sufficiently increased, the electrode would be
+brought to a high temperature as well as if the gas were at very low
+pressure; in fact, exhausting the bulb is only necessary because we
+cannot produce, (and possibly not convey) currents of the required
+frequency.
+
+Returning to the subject of electrode lamps, it is obviously of
+advantage in such a lamp to confine as much as possible the heat to the
+electrode by preventing the circulation of the gas in the bulb. If a
+very small bulb be taken, it would confine the heat better than a large
+one, but it might not be of sufficient capacity to be operated from the
+coil, or, if so, the glass might get too hot. A simple way to improve in
+this direction is to employ a globe of the required size, but to place a
+small bulb, the diameter of which is properly estimated, over the
+refractory button contained in the globe. This arrangement is
+illustrated in Fig. 157.
+
+[Illustration: FIG. 157.]
+
+[Illustration: FIG. 158.]
+
+The globe L has in this case a large neck _n_, allowing the small bulb
+_b_ to slip through. Otherwise the construction is the same as shown in
+Fig. 147, for example. The small bulb is conveniently supported upon the
+stem _s_, carrying the refractory button _m_. It is separated from the
+aluminum tube _a_ by several layers of mica M, in order to prevent the
+cracking of the neck by the rapid heating of the aluminum tube upon a
+sudden turning on of the current. The inside bulb should be as small as
+possible when it is desired to obtain light only by incandescence of the
+electrode. If it is desired to produce phosphorescence, the bulb should
+be larger, else it would be apt to get too hot, and the phosphorescence
+would cease. In this arrangement usually only the small bulb shows
+phosphorescence, as there is practically no bombardment against the
+outer globe. In some of these bulbs constructed as illustrated in Fig.
+157, the small tube was coated with phosphorescent paint, and beautiful
+effects were obtained. Instead of making the inside bulb large, in order
+to avoid undue heating, it answers the purpose to make the electrode _m_
+larger. In this case the bombardment is weakened by reason of the
+smaller electric density.
+
+Many bulbs were constructed on the plan illustrated in Fig. 158. Here a
+small bulb _b_, containing the refractory button _m_, upon being
+exhausted to a very high degree was sealed in a large globe L, which was
+then moderately exhausted and sealed off. The principal advantage of
+this construction was that it allowed of reaching extremely high vacua,
+and, at the same time of using a large bulb. It was found, in the course
+of experiments with bulbs such as illustrated in Fig. 158, that it was
+well to make the stem _s_, near the seal at _e_, very thick, and the
+leading-in wire _w_ thin, as it occurred sometimes that the stem at _e_
+was heated and the bulb was cracked. Often the outer globe L was
+exhausted only just enough to allow the discharge to pass through, and
+the space between the bulbs appeared crimson, producing a curious
+effect. In some cases, when the exhaustion in globe L was very low, and
+the air good conducting, it was found necessary, in order to bring the
+button _m_ to high incandescence, to place, preferably on the upper part
+of the neck of the globe, a tinfoil coating which was connected to an
+insulated body, to the ground, or to the other terminal of the coil, as
+the highly conducting air weakened the effect somewhat, probably by
+being acted upon inductively from the wire _w_, where it entered the
+bulb at _e_. Another difficulty--which, however, is always present when
+the refractory button is mounted in a very small bulb--existed in the
+construction illustrated in Fig. 158, namely, the vacuum in the bulb _b_
+would be impaired in a comparatively short time.
+
+The chief idea in the two last described constructions was to confine
+the heat to the central portion of the globe by preventing the exchange
+of air. An advantage is secured, but owing to the heating of the inside
+bulb and slow evaporation of the glass, the vacuum is hard to maintain,
+even if the construction illustrated in Fig. 157 be chosen, in which
+both bulbs communicate.
+
+But by far the better way--the ideal way--would be to reach sufficiently
+high frequencies. The higher the frequency, the slower would be the
+exchange of the air, and I think that a frequency may be reached, at
+which there would be no exchange whatever of the air molecules around
+the terminal. We would then produce a flame in which there would be no
+carrying away of material, and a queer flame it would be, for it would
+be rigid! With such high frequencies the inertia of the particles would
+come into play. As the brush, or flame, would gain rigidity in virtue of
+the inertia of the particles, the exchange of the latter would be
+prevented. This would necessarily occur, for, the number of impulses
+being augmented, the potential energy of each would diminish, so that
+finally only atomic vibrations could be set up, and the motion of
+translation through measurable space would cease. Thus an ordinary gas
+burner connected to a source of rapidly alternating potential might have
+its efficiency augmented to a certain limit, and this for two
+reasons--because of the additional vibration imparted, and because of a
+slowing down of the process of carrying off. But the renewal being
+rendered difficult, a renewal being necessary to maintain the _burner_,
+a continued increase of the frequency of the impulses, assuming they
+could be transmitted to and impressed upon the flame, would result in
+the "extinction" of the latter, meaning by this term only the cessation
+of the chemical process.
+
+I think, however, that in the case of an electrode immersed in a fluid
+insulating medium, and surrounded by independent carriers of electric
+charges, which can be acted upon inductively, a sufficient high
+frequency of the impulses would probably result in a gravitation of the
+gas all around toward the electrode. For this it would be only necessary
+to assume that the independent bodies are irregularly shaped; they would
+then turn toward the electrode their side of the greatest electric
+density, and this would be a position in which the fluid resistance to
+approach would be smaller than that offered to the receding.
+
+The general opinion, I do not doubt, is that it is out of the question
+to reach any such frequencies as might--assuming some of the views
+before expressed to be true--produce any of the results which I have
+pointed out as mere possibilities. This may be so, but in the course of
+these investigations, from the observation of many phenomena, I have
+gained the conviction that these frequencies would be much lower than
+one is apt to estimate at first. In a flame we set up light vibrations
+by causing molecules, or atoms, to collide. But what is the ratio of the
+frequency of the collisions and that of the vibrations set up? Certainly
+it must be incomparably smaller than that of the strokes of the bell and
+the sound vibrations, or that of the discharges and the oscillations of
+the condenser. We may cause the molecules of the gas to collide by the
+use of alternate electric impulses of high frequency, and so we may
+imitate the process in a flame; and from experiments with frequencies
+which we are now able to obtain, I think that the result is producible
+with impulses which are transmissible through a conductor.
+
+In connection with thoughts of a similar nature, it appeared to me of
+great interest to demonstrate the rigidity of a vibrating gaseous
+column. Although with such low frequencies as, say 10,000 per second,
+which I was able to obtain without difficulty from a specially
+constructed alternator, the task looked discouraging at first, I made a
+series of experiments. The trials with air at ordinary pressure led to
+no result, but with air moderately rarefied I obtain what I think to be
+an unmistakable experimental evidence of the property sought for. As a
+result of this kind might lead able investigators to conclusions of
+importance, I will describe one of the experiments performed.
+
+It is well known that when a tube is slightly exhausted, the discharge
+may be passed through it in the form of a thin luminous thread. When
+produced with currents of low frequency, obtained from a coil operated
+as usual, this thread is inert. If a magnet be approached to it, the
+part near the same is attracted or repelled, according to the direction
+of the lines of force of the magnet. It occurred to me that if such a
+thread would be produced with currents of very high frequency, it should
+be more or less rigid, and as it was visible it could be easily studied.
+Accordingly I prepared a tube about one inch in diameter and one metre
+long, with outside coating at each end. The tube was exhausted to a
+point at which, by a little working, the thread discharge could be
+obtained. It must be remarked here that the general aspect of the tube,
+and the degree of exhaustion, are quite other than when ordinary low
+frequency currents are used. As it was found preferable to work with one
+terminal, the tube prepared was suspended from the end of a wire
+connected to the terminal, the tinfoil coating being connected to the
+wire, and to the lower coating sometimes a small insulated plate was
+attached. When the thread was formed, it extended through the upper part
+of the tube and lost itself in the lower end. If it possessed rigidity
+it resembled, not exactly an elastic cord stretched tight between two
+supports, but a cord suspended from a height with a small weight
+attached at the end. When the finger or a small magnet was approached to
+the upper end of the luminous thread, it could be brought locally out of
+position by electrostatic or magnetic action; and when the disturbing
+object was very quickly removed, an analogous result was produced, as
+though a suspended cord would be displaced and quickly released near the
+point of suspension. In doing this the luminous thread was set in
+vibration, and two very sharply marked nodes, and a third indistinct
+one, were formed. The vibration, once set up, continued for fully eight
+minutes, dying gradually out. The speed of the vibration often varied
+perceptibly, and it could be observed that the electrostatic attraction
+of the glass affected the vibrating thread; but it was clear that the
+electrostatic action was not the cause of the vibration, for the thread
+was most generally stationary, and could always be set in vibration by
+passing the finger quickly near the upper part of the tube. With a
+magnet the thread could be split in two and both parts vibrated. By
+approaching the hand to the lower coating of the tube, or insulation
+plate if attached, the vibration was quickened; also, as far as I could
+see, by raising the potential or frequency. Thus, either increasing the
+frequency or passing a stronger discharge of the same frequency
+corresponded to a tightening of the cord. I did not obtain any
+experimental evidence with condenser discharges. A luminous band excited
+in the bulb by repeated discharges of a Leyden jar must possess
+rigidity, and if deformed and suddenly released, should vibrate. But
+probably the amount of vibrating matter is so small that in spite of the
+extreme speed, the inertia cannot prominently assert itself. Besides,
+the observation in such a case is rendered extremely difficult on
+account of the fundamental vibration.
+
+The demonstration of the fact--which still needs better experimental
+confirmation--that a vibrating gaseous column possesses rigidity, might
+greatly modify the views of thinkers. When with low frequencies and
+insignificant potentials indications of that property may be noted, how
+must a gaseous medium behave under the influence of enormous
+electrostatic stresses which may be active in the interstellar space,
+and which may alternate with inconceivable rapidity? The existence of
+such an electrostatic, rhythmically throbbing force--of a vibrating
+electrostatic field--would show a possible way how solids might have
+formed from the ultra-gaseous uterus, and how transverse and all kinds
+of vibrations may be transmitted through a gaseous medium filling all
+space. Then, ether might be a true fluid, devoid of rigidity, and at
+rest, it being merely necessary as a connecting link to enable
+interaction. What determines the rigidity of a body? It must be the
+speed and the amount of motive matter. In a gas the speed maybe
+considerable, but the density is exceedingly small; in a liquid the
+speed would be likely to be small, though the density may be
+considerable; and in both cases the inertia resistance offered to
+displacement is practically _nil_. But place a gaseous (or liquid)
+column in an intense, rapidly alternating electrostatic field, set the
+particles vibrating with enormous speeds, then the inertia resistance
+asserts itself. A body might move with more or less freedom through the
+vibrating mass, but as a whole it would be rigid.
+
+There is a subject which I must mention in connection with these
+experiments: it is that of high vacua. This is a subject, the study of
+which is not only interesting, but useful, for it may lead to results of
+great practical importance. In commercial apparatus, such as
+incandescent lamps, operated from ordinary systems of distribution, a
+much higher vacuum than is obtained at present would not secure a very
+great advantage. In such a case the work is performed on the filament,
+and the gas is little concerned; the improvement, therefore, would be
+but trifling. But when we begin to use very high frequencies and
+potentials, the action of the gas becomes all important, and the degree
+of exhaustion materially modifies the results. As long as ordinary
+coils, even very large ones, were used, the study of the subject was
+limited, because just at a point when it became most interesting it had
+to be interrupted on account of the "non-striking" vacuum being reached.
+But at present we are able to obtain from a small disruptive discharge
+coil potentials much higher than even the largest coil was capable of
+giving, and, what is more, we can make the potential alternate with
+great rapidity. Both of these results enable us now to pass a luminous
+discharge through almost any vacua obtainable, and the field of our
+investigations is greatly extended. Think we as we may, of all the
+possible directions to develop a practical illuminant, the line of high
+vacua seems to be the most promising at present. But to reach extreme
+vacua the appliances must be much more improved, and ultimate perfection
+will not be attained until we shall have discharged the mechanical and
+perfected an _electrical_ vacuum pump. Molecules and atoms can be thrown
+out of a bulb under the action of an enormous potential: _this_ will be
+the principle of the vacuum pump of the future. For the present, we must
+secure the best results we can with mechanical appliances. In this
+respect, it might not be out of the way to say a few words about the
+method of, and apparatus for, producing excessively high degrees of
+exhaustion of which I have availed myself in the course of these
+investigations. It is very probable that other experimenters have used
+similar arrangements; but as it is possible that there may be an item of
+interest in their description, a few remarks, which will render this
+investigation more complete, might be permitted.
+
+[Illustration: FIG. 159.]
+
+The apparatus is illustrated in a drawing shown in Fig. 159. S
+represents a Sprengel pump, which has been specially constructed to
+better suit the work required. The stop-cock which is usually employed
+has been omitted, and instead of it a hollow stopper _s_ has been fitted
+in the neck of the reservoir R. This stopper has a small hole _h_,
+through which the mercury descends; the size of the outlet _o_ being
+properly determined with respect to the section of the fall tube _t_,
+which is sealed to the reservoir instead of being connected to it in the
+usual manner. This arrangement overcomes the imperfections and troubles
+which often arise from the use of the stopcock on the reservoir and the
+connections of the latter with the fall tube.
+
+The pump is connected through a U-shaped tube _t_ to a very large
+reservoir R_{1}. Especial care was taken in fitting the grinding
+surfaces of the stoppers p and p_{1}, and both of these and the mercury
+caps above them were made exceptionally long. After the U-shaped tube
+was fitted and put in place, it was heated, so as to soften and take
+off the strain resulting from imperfect fitting. The U-shaped tube was
+provided with a stopcock C, and two ground connections g and g_{1},--one
+for a small bulb _b_, usually containing caustic potash, and the other
+for the receiver _r_, to be exhausted.
+
+The reservoir R_{1}, was connected by means of a rubber tube to a
+slightly larger reservoir R_{2}, each of the two reservoirs being
+provided with a stopcock C_{1} and C_{2}, respectively. The reservoir
+R_{2} could be raised and lowered by a wheel and rack, and the range of
+its motion was so determined that when it was filled with mercury and
+the stopcock C_{2} closed, so as to form a Torricellian vacuum in it
+when raised, it could be lifted so high that the reservoir R_{1} would
+stand a little above stopcock C_{1}; and when this stopcock was closed
+and the reservoir R_{2} descended, so as to form a Torricellian vacuum
+in reservoir R_{1}, it could be lowered so far as to completely empty
+the latter, the mercury filling the reservoir R_{2} up to a little above
+stopcock C_{2}.
+
+The capacity of the pump and of the connections was taken as small as
+possible relatively to the volume of reservoir, R_{1}, since, of course,
+the degree of exhaustion depended upon the ratio of these quantities.
+
+With this apparatus I combined the usual means indicated by former
+experiments for the production of very high vacua. In most of the
+experiments it was most convenient to use caustic potash. I may venture
+to say, in regard to its use, that much time is saved and a more perfect
+action of the pump insured by fusing and boiling the potash as soon as,
+or even before, the pump settles down. If this course is not followed,
+the sticks, as ordinarily employed, may give off moisture at a certain
+very slow rate, and the pump may work for many hours without reaching a
+very high vacuum. The potash was heated either by a spirit lamp or by
+passing a discharge through it, or by passing a current through a wire
+contained in it. The advantage in the latter case was that the heating
+could be more rapidly repeated.
+
+Generally the process of exhaustion was the following:--At the start,
+the stop-cocks C and C_{1} being open, and all other connections closed,
+the reservoir R_{2} was raised so far that the mercury filled the
+reservoir R_{1} and a part of the narrow connecting U-shaped tube.
+When the pump was set to work, the mercury would, of course, quickly
+rise in the tube, and reservoir R_{2} was lowered, the experimenter
+keeping the mercury at about the same level. The reservoir R_{2} was
+balanced by a long spring which facilitated the operation, and the
+friction of the parts was generally sufficient to keep it in almost any
+position. When the Sprengel pump had done its work, the reservoir R_{2}
+was further lowered and the mercury descended in R_{1} and filled R_{2},
+whereupon stopcock C_{2} was closed. The air adhering to the walls of
+R_{1} and that absorbed by the mercury was carried off, and to free the
+mercury of all air the reservoir R_{2} was for a long time worked up and
+down. During this process some air, which would gather below stopcock
+C_{2}, was expelled from R_{2} by lowering it far enough and opening the
+stopcock, closing the latter again before raising the reservoir. When
+all the air had been expelled from the mercury, and no air would gather
+in R_{2} when it was lowered, the caustic potash was resorted to. The
+reservoir R_{2} was now again raised until the mercury in R_{1}, stood
+above stopcock C_{1}. The caustic potash was fused and boiled, and
+moisture partly carried off by the pump and partly re-absorbed; and this
+process of heating and cooling was repeated many times, and each time,
+upon the moisture being absorbed or carried off, the reservoir R_{2} was
+for a long time raised and lowered. In this manner all the moisture was
+carried off from the mercury, and both the reservoirs were in proper
+condition to be used. The reservoir R_{2} was then again raised to the
+top, and the pump was kept working for a long time. When the highest
+vacuum obtainable with the pump had been reached, the potash bulb was
+usually wrapped with cotton which was sprinkled with ether so as to keep
+the potash at a very low temperature, then the reservoir R_{2} was
+lowered, and upon reservoir R_{1} being emptied the receiver was quickly
+sealed up.
+
+When a new bulb was put on, the mercury was always raised above stopcock
+C_{1}, which was closed, so as to always keep the mercury and both the
+reservoirs in fine condition, and the mercury was never withdrawn from
+R_{1} except when the pump had reached the highest degree of exhaustion.
+It is necessary to observe this rule if it is desired to use the
+apparatus to advantage.
+
+By means of this arrangement I was able to proceed very quickly, and
+when the apparatus was in perfect order it was possible to reach the
+phosphorescent stage in a small bulb in less than fifteen minutes, which
+is certainly very quick work for a small laboratory arrangement
+requiring all in all about 100 pounds of mercury. With ordinary small
+bulbs the ratio of the capacity of the pump, receiver, and connections,
+and that of reservoir R was about 1 to 20, and the degrees of exhaustion
+reached were necessarily very high, though I am unable to make a precise
+and reliable statement how far the exhaustion was carried.
+
+What impresses the investigator most in the course of these experiences
+is the behavior of gases when subjected to great rapidly alternating
+electrostatic stresses. But he must remain in doubt as to whether the
+effects observed are due wholly to the molecules, or atoms, of the gas
+which chemical analysis discloses to us, or whether there enters into
+play another medium of a gaseous nature, comprising atoms, or molecules,
+immersed in a fluid pervading the space. Such a medium surely must
+exist, and I am convinced that, for instance, even if air were absent,
+the surface and neighborhood of a body in space would be heated by
+rapidly alternating the potential of the body; but no such heating of
+the surface or neighborhood could occur if all free atoms were removed
+and only a homogeneous, incompressible, and elastic fluid--such as ether
+is supposed to be--would remain, for then there would be no impacts, no
+collisions. In such a case, as far as the body itself is concerned, only
+frictional losses in the inside could occur.
+
+It is a striking fact that the discharge through a gas is established
+with ever-increasing freedom as the frequency of the impulses is
+augmented. It behaves in this respect quite contrarily to a metallic
+conductor. In the latter the impedance enters prominently into play as
+the frequency is increased, but the gas acts much as a series of
+condensers would; the facility with which the discharge passes through,
+seems to depend on the rate of change of potential. If it acts so, then
+in a vacuum tube even of great length, and no matter how strong the
+current, self-induction could not assert itself to any appreciable
+degree. We have, then, as far as we can now see, in the gas a conductor
+which is capable of transmitting electric impulses of any frequency
+which we may be able to produce. Could the frequency be brought high
+enough, then a queer system of electric distribution, which would be
+likely to interest gas companies, might be realized: metal pipes
+filled with gas--the metal being the insulator, the gas the
+conductor--supplying phosphorescent bulbs, or perhaps devices as yet
+uninvented. It is certainly possible to take a hollow core of copper,
+rarefy the gas in the same, and by passing impulses of sufficiently high
+frequency through a circuit around it, bring the gas inside to a high
+degree of incandescence; but as to the nature of the forces there would
+be considerable uncertainty, for it would be doubtful whether with such
+impulses the copper core would act as a static screen. Such paradoxes
+and apparent impossibilities we encounter at every step in this line of
+work, and therein lies, to a great extent, the charm of the study.
+
+I have here a short and wide tube which is exhausted to a high degree
+and covered with a substantial coating of bronze, the coating barely
+allowing the light to shine through. A metallic cap, with a hook for
+suspending the tube, is fastened around the middle portion of the
+latter, the clasp being in contact with the bronze coating. I now want
+to light the gas inside by suspending the tube on a wire connected to
+the coil. Any one who would try the experiment for the first time, not
+having any previous experience, would probably take care to be quite
+alone when making the trial, for fear that he might become the joke of
+his assistants. Still, the bulb lights in spite of the metal coating,
+and the light can be distinctly perceived through the latter. A long
+tube covered with aluminum bronze lights when held in one hand--the
+other touching the terminal of the coil--quite powerfully. It might be
+objected that the coatings are not sufficiently conducting; still, even
+if they were highly resistant, they ought to screen the gas. They
+certainly screen it perfectly in a condition of rest, but far from
+perfectly when the charge is surging in the coating. But the loss of
+energy which occurs within the tube, notwithstanding the screen, is
+occasioned principally by the presence of the gas. Were we to take a
+large hollow metallic sphere and fill it with a perfect, incompressible,
+fluid dielectric, there would be no loss inside of the sphere, and
+consequently the inside might be considered as perfectly screened,
+though the potential be very rapidly alternating. Even were the sphere
+filled with oil, the loss would be incomparably smaller than when the
+fluid is replaced by a gas, for in the latter case the force produces
+displacements; that means impact and collisions in the inside.
+
+No matter what the pressure of the gas may be, it becomes an important
+factor in the heating of a conductor when the electric density is great
+and the frequency very high. That in the heating of conductors by
+lightning discharges, air is an element of great importance, is almost
+as certain as an experimental fact. I may illustrate the action of the
+air by the following experiment: I take a short tube which is exhausted
+to a moderate degree and has a platinum wire running through the middle
+from one end to the other. I pass a steady or low frequency current
+through the wire, and it is heated uniformly in all parts. The heating
+here is due to conduction, or frictional losses, and the gas around the
+wire has--as far as we can see--no function to perform. But now let me
+pass sudden discharges, or high frequency currents, through the wire.
+Again the wire is heated, this time principally on the ends and least in
+the middle portion; and if the frequency of the impulses, or the rate of
+change, is high enough, the wire might as well be cut in the middle as
+not, for practically all heating is due to the rarefied gas. Here the
+gas might only act as a conductor of no impedance diverting the current
+from the wire as the impedance of the latter is enormously increased,
+and merely heating the ends of the wire by reason of their resistance to
+the passage of the discharge. But it is not at all necessary that the
+gas in the tube should be conducting; it might be at an extremely low
+pressure, still the ends of the wire would be heated--as, however, is
+ascertained by experience--only the two ends would in such case not be
+electrically connected through the gaseous medium. Now what with these
+frequencies and potentials occurs in an exhausted tube, occurs in the
+lightning discharges at ordinary pressure. We only need remember one of
+the facts arrived at in the course of these investigations, namely, that
+to impulses of very high frequency the gas at ordinary pressure behaves
+much in the same manner as though it were at moderately low pressure. I
+think that in lightning discharges frequently wires or conducting
+objects are volatilized merely because air is present, and that, were
+the conductor immersed in an insulating liquid, it would be safe, for
+then the energy would have to spend itself somewhere else. From the
+behavior of gases under sudden impulses of high potential, I am led to
+conclude that there can be no surer way of diverting a lightning
+discharge than by affording it a passage through a volume of gas, if
+such a thing can be done in a practical manner.
+
+There are two more features upon which I think it necessary to dwell in
+connection with these experiments--the "radiant state" and the
+"non-striking vacuum."
+
+Any one who has studied Crookes' work must have received the impression
+that the "radiant state" is a property of the gas inseparably connected
+with an extremely high degree of exhaustion. But it should be remembered
+that the phenomena observed in an exhausted vessel are limited to the
+character and capacity of the apparatus which is made use of. I think
+that in a bulb a molecule, or atom, does not precisely move in a
+straight line because it meets no obstacle, but because the velocity
+imparted to it is sufficient to propel it in a sensibly straight line.
+The mean free path is one thing, but the velocity--the energy associated
+with the moving body--is another, and under ordinary circumstances I
+believe that it is a mere question of potential or speed. A disruptive
+discharge coil, when the potential is pushed very far, excites
+phosphorescence and projects shadows, at comparatively low degrees of
+exhaustion. In a lightning discharge, matter moves in straight lines at
+ordinary pressure when the mean free path is exceedingly small, and
+frequently images of wires or other metallic objects have been produced
+by the particles thrown off in straight lines.
+
+I have prepared a bulb to illustrate by an experiment the correctness of
+these assertions. In a globe L, Fig. 160, I have mounted upon a lamp
+filament _f_ a piece of lime _l_. The lamp filament is connected with a
+wire which leads into the bulb, and the general construction of the
+latter is as indicated in Fig. 148, before described. The bulb being
+suspended from a wire connected to the terminal of the coil, and the
+latter being set to work, the lime piece _l_ and the projecting parts of
+the filament _f_ are bombarded. The degree of exhaustion is just such
+that with the potential the coil is capable of giving, phosphorescence
+of the glass is produced, but disappears as soon as the vacuum is
+impaired. The lime containing moisture, and moisture being given off as
+soon as heating occurs, the phosphorescence lasts only for a few
+moments. When the lime has been sufficiently heated, enough moisture has
+been given off to impair materially the vacuum of the bulb. As the
+bombardment goes on, one point of the lime piece is more heated than
+other points, and the result is that finally practically all the
+discharge passes through that point which is intensely heated, and a
+white stream of lime particles (Fig. 160) then breaks forth from that
+point. This stream is composed of "radiant" matter, yet the degree of
+exhaustion is low. But the particles move in straight lines because the
+velocity imparted to them is great, and this is due to three causes--to
+the great electric density, the high temperature of the small point, and
+the fact that the particles of the lime are easily torn and thrown
+off--far more easily than those of carbon. With frequencies such as we
+are able to obtain, the particles are bodily thrown off and projected to
+a considerable distance; but with sufficiently high frequencies no such
+thing would occur; in such case only a stress would spread or a
+vibration would be propagated through the bulb. It would be out of the
+question to reach any such frequency on the assumption that the atoms
+move with the speed of light; but I believe that such a thing is
+impossible; for this an enormous potential would be required. With
+potentials which we are able to obtain, even with a disruptive discharge
+coil, the speed must be quite insignificant.
+
+[Illustration: FIG. 160.]
+
+As to the "non-striking vacuum," the point to be noted is, that it can
+occur only with low frequency impulses, and it is necessitated by the
+impossibility of carrying off enough energy with such impulses in high
+vacuum, since the few atoms which are around the terminal upon coming in
+contact with the same, are repelled and kept at a distance for a
+comparatively long period of time, and not enough work can be performed
+to render the effect perceptible to the eye. If the difference of
+potential between the terminals is raised, the dielectric breaks down.
+But with very high frequency impulses there is no necessity for such
+breaking down, since any amount of work can be performed by continually
+agitating the atoms in the exhausted vessel, provided the frequency is
+high enough. It is easy to reach--even with frequencies obtained from an
+alternator as here used--a stage at which the discharge does not pass
+between two electrodes in a narrow tube, each of these being connected
+to one of the terminals of the coil, but it is difficult to reach a
+point at which a luminous discharge would not occur around each
+electrode.
+
+[Illustration: FIG. 161.]
+
+[Illustration: FIG. 162.]
+
+A thought which naturally presents itself in connection with high
+frequency currents, is to make use of their powerful electrodynamic
+inductive action to produce light effects in a sealed glass globe. The
+leading-in wire is one of the defects of the present incandescent lamp,
+and if no other improvement were made, that imperfection at least should
+be done away with. Following this thought, I have carried on
+experiments in various directions, of which some were indicated in my
+former paper. I may here mention one or two more lines of experiment
+which have been followed up.
+
+Many bulbs were constructed as shown in Fig. 161 and Fig. 162.
+
+In Fig. 161, a wide tube, T, was sealed to a smaller W
+shaped tube U, of phosphorescent glass. In the tube T, was placed a coil
+C, of aluminum wire, the ends of which were provided with small spheres,
+t and t_{1}, of aluminum, and reached into the U tube. The tube T
+was slipped into a socket containing a primary coil, through which
+usually the discharges of Leyden jars were directed, and the rarefied
+gas in the small U tube was excited to strong luminosity by the
+high-tension current induced in the coil C. When Leyden jar discharges
+were used to induce currents in the coil C, it was found necessary to
+pack the tube T tightly with insulating powder, as a discharge would
+occur frequently between the turns of the coil, especially when the
+primary was thick and the air gap, through which the jars discharged,
+large, and no little trouble was experienced in this way.
+
+In Fig. 162 is illustrated another form of the bulb constructed. In this
+case a tube T is sealed to a globe L. The tube contains a coil C, the
+ends of which pass through two small glass tubes t and t_{1}, which
+are sealed to the tube T. Two refractory buttons m and m_{1}, are
+mounted on lamp filaments which are fastened to the ends of the wires
+passing through the glass tubes t and t_{1}. Generally in bulbs made
+on this plan the globe L communicated with the tube T. For this purpose
+the ends of the small tubes t and t_{1} were heated just a trifle in
+the burner, merely to hold the wires, but not to interfere with the
+communication. The tube T, with the small tubes, wires through the same,
+and the refractory buttons m and m_{1}, were first prepared, and
+then sealed to globe L, whereupon the coil C was slipped in and the
+connections made to its ends. The tube was then packed with insulating
+powder, jamming the latter as tight as possible up to very nearly the
+end; then it was closed and only a small hole left through which the
+remainder of the powder was introduced, and finally the end of the tube
+was closed. Usually in bulbs constructed as shown in Fig. 162 an
+aluminum tube _a_ was fastened to the upper end _s_ of each of the tubes
+t and t_{1} in order to protect that end against the heat. The
+buttons m and m_{1} could be brought to any degree of incandescence
+by passing the discharges of Leyden jars around the coil C. In such
+bulbs with two buttons a very curious effect is produced by the
+formation of the shadows of each of the two buttons.
+
+Another line of experiment, which has been assiduously followed, was to
+induce by electro-dynamic induction a current or luminous discharge in
+an exhausted tube or bulb. This matter has received such able treatment
+at the hands of Prof. J. J. Thomson, that I could add but little to what
+he has made known, even had I made it the special subject of this
+lecture. Still, since experiments in this line have gradually led me to
+the present views and results, a few words must be devoted here to this
+subject.
+
+It has occurred, no doubt, to many that as a vacuum tube is made longer,
+the electromotive force per unit length of the tube, necessary to pass a
+luminous discharge through the latter, becomes continually smaller;
+therefore, if the exhausted tube be made long enough, even with low
+frequencies a luminous discharge could be induced in such a tube closed
+upon itself. Such a tube might be placed around a hall or on a ceiling,
+and at once a simple appliance capable of giving considerable light
+would be obtained. But this would be an appliance hard to manufacture
+and extremely unmanageable. It would not do to make the tube up of small
+lengths, because there would be with ordinary frequencies considerable
+loss in the coatings, and besides, if coatings were used, it would be
+better to supply the current directly to the tube by connecting the
+coatings to a transformer. But even if all objections of such nature
+were removed, with low frequencies the light conversion itself would be
+inefficient, as I have before stated. In using extremely high
+frequencies the length of the secondary--in other words, the size of the
+vessel--can be reduced as much as desired, and the efficiency of the
+light conversion is increased, provided that means are invented for
+efficiently obtaining such high frequencies. Thus one is led, from
+theoretical and practical considerations, to the use of high
+frequencies, and this means high electromotive forces and small currents
+in the primary. When one works with condenser charges--and they are the
+only means up to the present known for reaching these extreme
+frequencies--one gets to electromotive forces of several thousands of
+volts per turn of the primary. We cannot multiply the electro-dynamic
+inductive effect by taking more turns in the primary, for we arrive at
+the conclusion that the best way is to work with one single turn--though
+we must sometimes depart from this rule--and we must get along with
+whatever inductive effect we can obtain with one turn. But before one
+has long experimented with the extreme frequencies required to set up in
+a small bulb an electromotive force of several thousands of volts, one
+realizes the great importance of electrostatic effects, and these
+effects grow relatively to the electro-dynamic in significance as the
+frequency is increased.
+
+Now, if anything is desirable in this case, it is to increase the
+frequency, and this would make it still worse for the electrodynamic
+effects. On the other hand, it is easy to exalt the electrostatic action
+as far as one likes by taking more turns on the secondary, or combining
+self-induction and capacity to raise the potential. It should also be
+remembered that, in reducing the current to the smallest value and
+increasing the potential, the electric impulses of high frequency can be
+more easily transmitted through a conductor.
+
+These and similar thoughts determined me to devote more attention to the
+electrostatic phenomena, and to endeavor to produce potentials as high
+as possible, and alternating as fast as they could be made to alternate.
+I then found that I could excite vacuum tubes at considerable distance
+from a conductor connected to a properly constructed coil, and that I
+could, by converting the oscillatory current of a conductor to a higher
+potential, establish electrostatic alternating fields which acted
+through the whole extent of the room, lighting up a tube no matter where
+it was held in space. I thought I recognized that I had made a step in
+advance, and I have persevered in this line; but I wish to say that I
+share with all lovers of science and progress the one and only
+desire--to reach a result of utility to men in any direction to which
+thought or experiment may lead me. I think that this departure is the
+right one, for I cannot see, from the observation of the phenomena which
+manifest themselves as the frequency is increased, what there would
+remain to act between two circuits conveying, for instance, impulses of
+several hundred millions per second, except electrostatic forces. Even
+with such trifling frequencies the energy would be practically all
+potential, and my conviction has grown strong that, to whatever kind of
+motion light may be due, it is produced by tremendous electrostatic
+stresses vibrating with extreme rapidity.
+
+[Illustration: FIG. 163.]
+
+[Illustration: FIG. 164.]
+
+Of all these phenomena observed with currents, or electric impulses, of
+high frequency, the most fascinating for an audience are certainly those
+which are noted in an electrostatic field acting through considerable
+distance; and the best an unskilled lecturer can do is to begin and
+finish with the exhibition of these singular effects. I take a tube in
+my hand and move it about, and it is lighted wherever I may hold it;
+throughout space the invisible forces act. But I may take another tube
+and it might not light, the vacuum being very high. I excite it by means
+of a disruptive discharge coil, and now it will light in the
+electrostatic field. I may put it away for a few weeks or months, still
+it retains the faculty of being excited. What change have I produced in
+the tube in the act of exciting it? If a motion imparted to atoms, it is
+difficult to perceive how it can persist so long without being arrested
+by frictional losses; and if a strain exerted in the dielectric, such as
+a simple electrification would produce, it is easy to see how it may
+persist indefinitely, but very difficult to understand why such a
+condition should aid the excitation when we have to deal with potentials
+which are rapidly alternating.
+
+Since I have exhibited these phenomena for the first time, I have
+obtained some other interesting effects. For instance, I have produced
+the incandescence of a button, filament, or wire enclosed in a tube. To
+get to this result it was necessary to economize the energy which is
+obtained from the field, and direct most of it on the small body to be
+rendered incandescent. At the beginning the task appeared difficult, but
+the experiences gathered permitted me to reach the result easily. In
+Fig. 163 and Fig. 164, two such tubes are illustrated, which are
+prepared for the occasion. In Fig. 163 a short tube T_{1}, sealed to
+another long tube T, is provided with a stem _s_, with a platinum wire
+sealed in the latter. A very thin lamp filament _l_, is fastened to this
+wire and connection to the outside is made through a thin copper wire
+_w_. The tube is provided with outside and inside coatings, C and C_{1},
+respectively, and is filled as far as the coatings reach with
+conducting, and the space above with insulating, powder. These coatings
+are merely used to enable me to perform two experiments with the
+tube--namely, to produce the effect desired either by direct connection
+of the body of the experimenter or of another body to the wire _w_, or
+by acting inductively through the glass. The stem _s_ is provided with
+an aluminum tube _a_, for purposes before explained, and only a small
+part of the filament reaches out of this tube. By holding the tube T_{1}
+anywhere in the electrostatic field, the filament is rendered
+incandescent.
+
+A more interesting piece of apparatus is illustrated in Fig. 164. The
+construction is the same as before, only instead of the lamp filament a
+small platinum wire _p_, sealed in a stem _s_, and bent above it in a
+circle, is connected to the copper wire _w_, which is joined to an
+inside coating C. A small stem s_{1} is provided with a needle, on the
+point of which is arranged, to rotate very freely, a very light fan of
+mica _v_. To prevent the fan from falling out, a thin stem of glass _g_,
+is bent properly and fastened to the aluminum tube. When the glass tube
+is held anywhere in the electrostatic field the platinum wire becomes
+incandescent, and the mica vanes are rotated very fast.
+
+Intense phosphorescence may be excited in a bulb by merely connecting it
+to a plate within the field, and the plate need not be any larger than
+an ordinary lamp shade. The phosphorescence excited with these currents
+is incomparably more powerful than with ordinary apparatus. A small
+phosphorescent bulb, when attached to a wire connected to a coil, emits
+sufficient light to allow reading ordinary print at a distance of five
+to six paces. It was of interest to see how some of the phosphorescent
+bulbs of Professor Crookes would behave with these currents, and he has
+had the kindness to lend me a few for the occasion. The effects produced
+are magnificent, especially by the sulphide of calcium and sulphide of
+zinc. With the disruptive discharge coil they glow intensely merely by
+holding them in the hand and connecting the body to the terminal of the
+coil.
+
+To whatever results investigations of this kind may lead, the chief
+interest lies, for the present, in the possibilities they offer for the
+production of an efficient illuminating device. In no branch of electric
+industry is an advance more desired than in the manufacture of light.
+Every thinker, when considering the barbarous methods employed, the
+deplorable losses incurred in our best systems of light production, must
+have asked himself, What is likely to be the light of the future? Is it
+to be an incandescent solid, as in the present lamp, or an incandescent
+gas, or a phosphorescent body, or something like a burner, but
+incomparably more efficient?
+
+There is little chance to perfect a gas burner; not, perhaps, because
+human ingenuity has been bent upon that problem for centuries without a
+radical departure having been made--though the argument is not devoid of
+force--but because in a burner the highest vibrations can never be
+reached, except by passing through all the low ones. For how is a flame
+to proceed unless by a fall of lifted weights? Such process cannot be
+maintained without renewal, and renewal is repeated passing from low to
+high vibrations. One way only seems to be open to improve a burner, and
+that is by trying to reach higher degrees of incandescence. Higher
+incandescence is equivalent to a quicker vibration: that means more
+light from the same material, and that again, means more economy. In
+this direction some improvements have been made, but the progress is
+hampered by many limitations. Discarding, then, the burner, there
+remains the three ways first mentioned, which are essentially
+electrical.
+
+Suppose the light of the immediate future to be a solid, rendered
+incandescent by electricity. Would it not seem that it is better to
+employ a small button than a frail filament? From many considerations it
+certainly must be concluded that a button is capable of a higher
+economy, assuming, of course, the difficulties connected with the
+operation of such a lamp to be effectively overcome. But to light such
+a lamp we require a high potential; and to get this economically, we
+must use high frequencies.
+
+Such considerations apply even more to the production of light by the
+incandescence of a gas, or by phosphorescence. In all cases we require
+high frequencies and high potentials. These thoughts occurred to me a
+long time ago.
+
+Incidentally we gain, by the use of high frequencies, many advantages,
+such as higher economy in the light production, the possibility of
+working with one lead, the possibility of doing away with the leading-in
+wire, etc.
+
+The question is, how far can we go with frequencies? Ordinary conductors
+rapidly lose the facility of transmitting electric impulses when the
+frequency is greatly increased. Assume the means for the production of
+impulses of very great frequency brought to the utmost perfection, every
+one will naturally ask how to transmit them when the necessity arises.
+In transmitting such impulses through conductors we must remember that
+we have to deal with _pressure_ and _flow_, in the ordinary
+interpretation of these terms. Let the pressure increase to an enormous
+value, and let the flow correspondingly diminish, then such
+impulses--variations merely of pressure, as it were--can no doubt be
+transmitted through a wire even if their frequency be many hundreds of
+millions per second. It would, of course, be out of question to transmit
+such impulses through a wire immersed in a gaseous medium, even if the
+wire were provided with a thick and excellent insulation, for most of
+the energy would be lost in molecular bombardment and consequent
+heating. The end of the wire connected to the source would be heated,
+and the remote end would receive but a trifling part of the energy
+supplied. The prime necessity, then, if such electric impulses are to be
+used, is to find means to reduce as much as possible the dissipation.
+
+The first thought is, to employ the thinnest possible wire surrounded by
+the thickest practicable insulation. The next thought is to employ
+electrostatic screens. The insulation of the wire may be covered with a
+thin conducting coating and the latter connected to the ground. But this
+would not do, as then all the energy would pass through the conducting
+coating to the ground and nothing would get to the end of the wire. If a
+ground connection is made it can only be made through a conductor
+offering an enormous impedance, or through a condenser of extremely
+small capacity. This, however, does not do away with other difficulties.
+
+If the wave length of the impulses is much smaller than the length of
+the wire, then corresponding short waves will be set up in the
+conducting coating, and it will be more or less the same as though the
+coating were directly connected to earth. It is therefore necessary to
+cut up the coating in sections much shorter than the wave length. Such
+an arrangement does not still afford a perfect screen, but it is ten
+thousand times better than none. I think it preferable to cut up the
+conducting coating in small sections, even if the current waves be much
+longer than the coating.
+
+If a wire were provided with a perfect electrostatic screen, it would be
+the same as though all objects were removed from it at infinite
+distance. The capacity would then be reduced to the capacity of the wire
+itself, which would be very small. It would then be possible to send
+over the wire current vibrations of very high frequencies at enormous
+distances, without affecting greatly the character of the vibrations. A
+perfect screen is of course out of the question, but I believe that with
+a screen such as I have just described telephony could be rendered
+practicable across the Atlantic. According to my ideas, the gutta-percha
+covered wire should be provided with a third conducting coating
+subdivided in sections. On the top of this should be again placed a
+layer of gutta-percha and other insulation, and on the top of the whole
+the armor. But such cables will not be constructed, for ere long
+intelligence--transmitted without wires--will throb through the earth
+like a pulse through a living organism. The wonder is that, with the
+present state of knowledge and the experiences gained, no attempt is
+being made to disturb the electrostatic or magnetic condition of the
+earth, and transmit, if nothing else, intelligence.
+
+It has been my chief aim in presenting these results to point out
+phenomena or features of novelty, and to advance ideas which I am
+hopeful will serve as starting points of new departures. It has been my
+chief desire this evening to entertain you with some novel experiments.
+Your applause, so frequently and generously accorded, has told me that I
+have succeeded.
+
+In conclusion, let me thank you most heartily for your kindness and
+attention, and assure you that the honor I have had in addressing such
+a distinguished audience, the pleasure I have had in presenting these
+results to a gathering of so many able men--and among them also some of
+those in whose work for many years past I have found enlightenment and
+constant pleasure--I shall never forget.
+
+
+
+
+CHAPTER XXVIII.
+
+ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA.[3]
+
+  [3] A lecture delivered before the Franklin Institute, Philadelphia,
+      February, 1893, and before the National Electric Light
+      Association, St. Louis, March, 1893.
+
+
+INTRODUCTORY.--SOME THOUGHTS ON THE EYE.
+
+When we look at the world around us, on Nature, we are impressed with
+its beauty and grandeur. Each thing we perceive, though it may be
+vanishingly small, is in itself a world, that is, like the whole of the
+universe, matter and force governed by law,--a world, the contemplation
+of which fills us with feelings of wonder and irresistibly urges us to
+ceaseless thought and inquiry. But in all this vast world, of all
+objects our senses reveal to us, the most marvellous, the most appealing
+to our imagination, appears no doubt a highly developed organism, a
+thinking being. If there is anything fitted to make us admire Nature's
+handiwork, it is certainly this inconceivable structure, which performs
+its innumerable motions of obedience to external influence. To
+understand its workings, to get a deeper insight into this Nature's
+masterpiece, has ever been for thinkers a fascinating aim, and after
+many centuries of arduous research men have arrived at a fair
+understanding of the functions of its organs and senses. Again, in all
+the perfect harmony of its parts, of the parts which constitute the
+material or tangible of our being, of all its organs and senses, the eye
+is the most wonderful. It is the most precious, the most indispensable
+of our perceptive or directive organs, it is the great gateway through
+which all knowledge enters the mind. Of all our organs, it is the one,
+which is in the most intimate relation with that which we call
+intellect. So intimate is this relation, that it is often said, the very
+soul shows itself in the eye.
+
+It can be taken as a fact, which the theory of the action of the eye
+implies, that for each external impression, that is, for each image
+produced upon the retina, the ends of the visual nerves, concerned in
+the conveyance of the impression to the mind, must be under a peculiar
+stress or in a vibratory state. It now does not seem improbable that,
+when by the power of thought an image is evoked, a distinct reflex
+action, no matter how weak, is exerted upon certain ends of the visual
+nerves, and therefore upon the retina. Will it ever be within human
+power to analyze the condition of the retina when disturbed by thought
+or reflex action, by the help of some optical or other means of such
+sensitiveness, that a clear idea of its state might be gained at any
+time? If this were possible, then the problem of reading one's thoughts
+with precision, like the characters of an open book, might be much
+easier to solve than many problems belonging to the domain of positive
+physical science, in the solution of which many, if not the majority, of
+scientific men implicitly believe. Helmholtz, has shown that the fundi
+of the eye are themselves, luminous, and he was able to _see_, in total
+darkness, the movement of his arm by the light of his own eyes. This is
+one of the most remarkable experiments recorded in the history of
+science, and probably only a few men could satisfactorily repeat it, for
+it is very likely, that the luminosity of the eyes is associated with
+uncommon activity of the brain and great imaginative power. It is
+fluorescence of brain action, as it were.
+
+Another fact having a bearing on this subject which has probably been
+noted by many, since it is stated in popular expressions, but which I
+cannot recollect to have found chronicled as a positive result of
+observation is, that at times, when a sudden idea or image presents
+itself to the intellect, there is a distinct and sometimes painful
+sensation of luminosity produced in the eye, observable even in broad
+daylight.
+
+The saying then, that the soul shows itself in the eye, is deeply
+founded, and we feel that it expresses a great truth. It has a profound
+meaning even for one who, like a poet or artist, only following his
+inborn instinct or love for Nature, finds delight in aimless thoughts
+and in the mere contemplation of natural phenomena, but a still more
+profound meaning for one who, in the spirit of positive scientific
+investigation, seeks to ascertain the causes of the effects. It is
+principally the natural philosopher, the physicist, for whom the eye is
+the subject of the most intense admiration.
+
+Two facts about the eye must forcibly impress the mind of the physicist,
+notwithstanding he may think or say that it is an imperfect optical
+instrument, forgetting, that the very conception of that which is
+perfect or seems so to him, has been gained through this same
+instrument. First, the eye is, as far as our positive knowledge goes,
+the only organ which is _directly_ affected by that subtile medium,
+which as science teaches us, must fill all space; secondly, it is the
+most sensitive of our organs, incomparably more sensitive to external
+impressions than any other.
+
+The organ of hearing implies the impact of ponderable bodies, the organ
+of smell the transference of detached material particles, and the organs
+of taste, and of touch or force, the direct contact, or at least some
+interference of ponderable matter, and this is true even in those
+instances of animal organisms, in which some of these organs are
+developed to a degree of truly marvelous perfection. This being so, it
+seems wonderful that the organ of sight solely should be capable of
+being stirred by that, which all our other organs are powerless to
+detect, yet which plays an essential part in all natural phenomena,
+which transmits all energy and sustains all motion and, that most
+intricate of all, life, but which has properties such that even a
+scientifically trained mind cannot help drawing a distinction between it
+and all that is called matter. Considering merely this, and the fact
+that the eye, by its marvelous power, widens our otherwise very narrow
+range of perception far beyond the limits of the small world which is
+our own, to embrace myriads of other worlds, suns and stars in the
+infinite depths of the universe, would make it justifiable to assert,
+that it is an organ of a higher order. Its performances are beyond
+comprehension. Nature as far as we know never produced anything more
+wonderful. We can get barely a faint idea of its prodigious power by
+analyzing what it does and by comparing. When ether waves impinge upon
+the human body, they produce the sensations of warmth or cold, pleasure
+or pain, or perhaps other sensations of which we are not aware, and any
+degree or intensity of these sensations, which degrees are infinite in
+number, hence an infinite number of distinct sensations. But our sense
+of touch, or our sense of force, cannot reveal to us these differences
+in degree or intensity, unless they are very great. Now we can readily
+conceive how an organism, such as the human, in the eternal process of
+evolution, or more philosophically speaking, adaptation to Nature, being
+constrained to the use of only the sense of touch or force, for
+instance, might develop this sense to such a degree of sensitiveness or
+perfection, that it would be capable of distinguishing the minutest
+differences in the temperature of a body even at some distance, to a
+hundredth, or thousandth, or millionth part of a degree. Yet, even this
+apparently impossible performance would not begin to compare with that
+of the eye, which is capable of distinguishing and conveying to the mind
+in a single instant innumerable peculiarities of the body, be it in
+form, or color, or other respects. This power of the eye rests upon two
+things, namely, the rectilinear propagation of the disturbance by which
+it is effected, and upon its sensitiveness. To say that the eye is
+sensitive is not saying anything. Compared with it, all other organs are
+monstrously crude. The organ of smell which guides a dog on the trail of
+a deer, the organ of touch or force which guides an insect in its
+wanderings, the organ of hearing, which is affected by the slightest
+disturbances of the air, are sensitive organs, to be sure, but what are
+they compared with the human eye! No doubt it responds to the faintest
+echoes or reverberations of the medium; no doubt, it brings us tidings
+from other worlds, infinitely remote, but in a language we cannot as yet
+always understand. And why not? Because we live in a medium filled with
+air and other gases, vapors and a dense mass of solid particles flying
+about. These play an important part in many phenomena; they fritter away
+the energy of the vibrations before they can reach the eye; they too,
+are the carriers of germs of destruction, they get into our lungs and
+other organs, clog up the channels and imperceptibly, yet inevitably,
+arrest the stream of life. Could we but do away with all ponderable
+matter in the line of sight of the telescope, it would reveal to us
+undreamt of marvels. Even the unaided eye, I think, would be capable of
+distinguishing in the pure medium, small objects at distances measured
+probably by hundreds or perhaps thousands of miles.
+
+But there is something else about the eye which impresses us still more
+than these wonderful features which we observed, viewing it from the
+standpoint of a physicist, merely as an optical instrument,--something
+which appeals to us more than its marvelous faculty of being directly
+affected by the vibrations of the medium, without interference of gross
+matter, and more than its inconceivable sensitiveness and discerning
+power. It is its significance in the processes of life. No matter what
+one's views on nature and life may be, he must stand amazed when, for
+the first time in his thoughts, he realizes the importance of the eye in
+the physical processes and mental performances of the human organism.
+And how could it be otherwise, when he realizes, that the eye is the
+means through which the human race has acquired the entire knowledge it
+possesses, that it controls all our motions, more still, all our
+actions.
+
+There is no way of acquiring knowledge except through the eye. What is
+the foundation of all philosophical systems of ancient and modern times,
+in fact, of all the philosophy of man? _I am, I think; I think,
+therefore I am._ But how could I think and how would I know that I
+exist, if I had not the eye? For knowledge involves consciousness;
+consciousness involves ideas, conceptions; conceptions involve pictures
+or images, and images the sense of vision, and therefore the organ of
+sight. But how about blind men, will be asked? Yes, a blind man may
+depict in magnificent poems, forms and scenes from real life, from a
+world he physically does not see. A blind man may touch the keys of an
+instrument with unerring precision, may model the fastest boat, may
+discover and invent, calculate and construct, may do still greater
+wonders--but all the blind men who have done such things have descended
+from those who had seeing eyes. Nature may reach the same result in many
+ways. Like a wave in the physical world, in the infinite ocean of the
+medium which pervades all, so in the world of organisms, in life, an
+impulse started proceeds onward, at times, may be, with the speed of
+light, at times, again, so slowly that for ages and ages it seems to
+stay, passing through processes of a complexity inconceivable to men,
+but in all its forms, in all its stages, its energy ever and ever
+integrally present. A single ray of light from a distant star falling
+upon the eye of a tyrant in bygone times, may have altered the course of
+his life, may have changed the destiny of nations, may have transformed
+the surface of the globe, so intricate, so inconceivably complex are the
+processes in Nature. In no way can we get such an overwhelming idea of
+the grandeur of Nature, as when we consider, that in accordance with the
+law of the conservation of energy, throughout the infinite, the forces
+are in a perfect balance, and hence the energy of a single thought may
+determine the motion of a Universe. It is not necessary that every
+individual, not even that every generation or many generations, should
+have the physical instrument of sight, in order to be able to form
+images and to think, that is, form ideas or conceptions; but sometime or
+other, during the process of evolution, the eye certainly must have
+existed, else thought, as we understand it, would be impossible; else
+conceptions, like spirit, intellect, mind, call it as you may, could not
+exist. It is conceivable, that in some other world, in some other
+beings, the eye is replaced by a different organ, equally or more
+perfect, but these beings cannot be men.
+
+Now what prompts us all to voluntary motions and actions of any kind?
+Again the eye. If I am conscious of the motion, I must have an idea or
+conception, that is, an image, therefore the eye. If I am not precisely
+conscious of the motion, it is, because the images are vague or
+indistinct, being blurred by the superimposition of many. But when I
+perform the motion, does the impulse which prompts me to the action come
+from within or from without? The greatest physicists have not disdained
+to endeavor to answer this and similar questions and have at times
+abandoned themselves to the delights of pure and unrestrained thought.
+Such questions are generally considered not to belong to the realm of
+positive physical science, but will before long be annexed to its
+domain. Helmholtz has probably thought more on life than any modern
+scientist. Lord Kelvin expressed his belief that life's process is
+electrical and that there is a force inherent to the organism and
+determining its motions. Just as much as I am convinced of any physical
+truth I am convinced that the motive impulse must come from the outside.
+For, consider the lowest organism we know--and there are probably many
+lower ones--an aggregation of a few cells only. If it is capable of
+voluntary motion it can perform an infinite number of motions, all
+definite and precise. But now a mechanism consisting of a finite number
+of parts and few at that, cannot perform an infinite number of definite
+motions, hence the impulses which govern its movements must come from
+the environment. So, the atom, the ulterior element of the Universe's
+structure, is tossed about in space, eternally, a play to external
+influences, like a boat in a troubled sea. Were it to stop its motion
+_it would die_. Matter at rest, if such a thing could exist, would be
+matter dead. Death of matter! Never has a sentence of deeper
+philosophical meaning been uttered. This is the way in which Prof.
+Dewar forcibly expresses it in the description of his admirable
+experiments, in which liquid oxygen is handled as one handles water, and
+air at ordinary pressure is made to condense and even to solidify by the
+intense cold. Experiments, which serve to illustrate, in his language,
+the last feeble manifestations of life, the last quiverings of matter
+about to die. But human eyes shall not witness such death. There is no
+death of matter, for throughout the infinite universe, all has to move,
+to vibrate, that is, to live.
+
+I have made the preceding statements at the peril of treading upon
+metaphysical ground, in my desire to introduce the subject of this
+lecture in a manner not altogether uninteresting, I may hope, to an
+audience such as I have the honor to address. But now, then, returning
+to the subject, this divine organ of sight, this indispensable
+instrument for thought and all intellectual enjoyment, which lays open
+to us the marvels of this universe, through which we have acquired what
+knowledge we possess, and which prompts us to, and controls, all our
+physical and mental activity. By what is it affected? By light! What is
+light?
+
+We have witnessed the great strides which have been made in all
+departments of science in recent years. So great have been the advances
+that we cannot refrain from asking ourselves, Is this all true, or is it
+but a dream? Centuries ago men have lived, have thought, discovered,
+invented, and have believed that they were soaring, while they were
+merely proceeding at a snail's pace. So we too may be mistaken. But
+taking the truth of the observed events as one of the implied facts of
+science, we must rejoice in the immense progress already made and still
+more in the anticipation of what must come, judging from the
+possibilities opened up by modern research. There is, however, an
+advance which we have been witnessing, which must be particularly
+gratifying to every lover of progress. It is not a discovery, or an
+invention, or an achievement in any particular direction. It is an
+advance in all directions of scientific thought and experiment. I mean
+the generalization of the natural forces and phenomena, the looming up
+of a certain broad idea on the scientific horizon. It is this idea which
+has, however, long ago taken possession of the most advanced minds, to
+which I desire to call your attention, and which I intend to illustrate
+in a general way, in these experiments, as the first step in answering
+the question "What is light?" and to realize the modern meaning of this
+word.
+
+It is beyond the scope of my lecture to dwell upon the subject of light
+in general, my object being merely to bring presently to your notice a
+certain class of light effects and a number of phenomena observed in
+pursuing the study of these effects. But to be consistent in my remarks
+it is necessary to state that, according to that idea, now accepted by
+the majority of scientific men as a positive result of theoretical and
+experimental investigation, the various forms or manifestations of
+energy which were generally designated as "electric" or more precisely
+"electromagnetic" are energy manifestations of the same nature as those
+of radiant heat and light. Therefore the phenomena of light and heat and
+others besides these, may be called electrical phenomena. Thus
+electrical science has become the mother science of all and its study
+has become all important. The day when we shall know exactly what
+"electricity" is, will chronicle an event probably greater, more
+important than any other recorded in the history of the human race. The
+time will come when the comfort, the very existence, perhaps, of man
+will depend upon that wonderful agent. For our existence and comfort we
+require heat, light and mechanical power. How do we now get all these?
+We get them from fuel, we get them by consuming material. What will man
+do when the forests disappear, when the coal fields are exhausted? Only
+one thing, according to our present knowledge will remain; that is, to
+transmit power at great distances. Men will go to the waterfalls, to the
+tides, which are the stores of an infinitesimal part of Nature's
+immeasurable energy. There will they harness the energy and transmit the
+same to their settlements, to warm their homes by, to give them light,
+and to keep their obedient slaves, the machines, toiling. But how will
+they transmit this energy if not by electricity? Judge then, if the
+comfort, nay, the very existence, of man will not depend on electricity.
+I am aware that this view is not that of a practical engineer, but
+neither is it that of an illusionist, for it is certain, that power
+transmission, which at present is merely a stimulus to enterprise, will
+some day be a dire necessity.
+
+It is more important for the student, who takes up the study of light
+phenomena, to make himself thoroughly acquainted with certain modern
+views, than to peruse entire books on the subject of light itself, as
+disconnected from these views. Were I therefore to make these
+demonstrations before students seeking information--and for the sake of
+the few of those who may be present, give me leave to so assume--it
+would be my principal endeavor to impress these views upon their minds
+in this series of experiments.
+
+It might be sufficient for this purpose to perform a simple and
+well-known experiment. I might take a familiar appliance, a Leyden jar,
+charge it from a frictional machine, and then discharge it. In
+explaining to you its permanent state when charged, and its transitory
+condition when discharging, calling your attention to the forces which
+enter into play and to the various phenomena they produce, and pointing
+out the relation of the forces and phenomena, I might fully succeed in
+illustrating that modern idea. No doubt, to the thinker, this simple
+experiment would appeal as much as the most magnificent display. But
+this is to be an experimental demonstration, and one which should
+possess, besides instructive, also entertaining features and as such, a
+simple experiment, such as the one cited, would not go very far towards
+the attainment of the lecturer's aim. I must therefore choose another
+way of illustrating, more spectacular certainly, but perhaps also more
+instructive. Instead of the frictional machine and Leyden jar, I shall
+avail myself in these experiments, of an induction coil of peculiar
+properties, which was described in detail by me in a lecture before the
+London Institution of Electrical Engineers, in Feb., 1892. This
+induction coil is capable of yielding currents of enormous potential
+differences, alternating with extreme rapidity. With this apparatus I
+shall endeavor to show you three distinct classes of effects, or
+phenomena, and it is my desire that each experiment, while serving for
+the purposes of illustration, should at the same time teach us some
+novel truth, or show us some novel aspect of this fascinating science.
+But before doing this, it seems proper and useful to dwell upon the
+apparatus employed, and method of obtaining the high potentials and
+high-frequency currents which are made use of in these experiments.
+
+
+[Illustration: FIG. 165.]
+
+ON THE APPARATUS AND METHOD OF CONVERSION.
+
+These high-frequency currents are obtained in a peculiar manner. The
+method employed was advanced by me about two years ago in an
+experimental lecture before the American Institute of Electrical
+Engineers. A number of ways, as practiced in the laboratory, of
+obtaining these currents either from continuous or low frequency
+alternating currents, is diagramatically indicated in Fig. 165, which
+will be later described in detail. The general plan is to charge
+condensers, from a direct or alternate-current source, preferably of
+high-tension, and to discharge them disruptively while observing
+well-known conditions necessary to maintain the oscillations of the
+current. In view of the general interest taken in high-frequency
+currents and effects producible by them, it seems to me advisable to
+dwell at some length upon this method of conversion. In order to give
+you a clear idea of the action, I will suppose that a continuous-current
+generator is employed, which is often very convenient. It is desirable
+that the generator should possess such high tension as to be able to
+break through a small air space. If this is not the case, then auxiliary
+means have to be resorted to, some of which will be indicated
+subsequently. When the condensers are charged to a certain potential,
+the air, or insulating space, gives way and a disruptive discharge
+occurs. There is then a sudden rush of current and generally a large
+portion of accumulated electrical energy spends itself. The condensers
+are thereupon quickly charged and the same process is repeated in more
+or less rapid succession. To produce such sudden rushes of current it is
+necessary to observe certain conditions. If the rate at which the
+condensers are discharged is the same as that at which they are charged,
+then, clearly, in the assumed case the condensers do not come into play.
+If the rate of discharge be smaller than the rate of charging, then,
+again, the condensers cannot play an important part. But if, on the
+contrary, the rate of discharging is greater than that of charging, then
+a succession of rushes of current is obtained. It is evident that, if
+the rate at which the energy is dissipated by the discharge is very much
+greater than the rate of supply to the condensers, the sudden rushes
+will be comparatively few, with long-time intervals between. This always
+occurs when a condenser of considerable capacity is charged by means of
+a comparatively small machine. If the rates of supply and dissipation
+are not widely different, then the rushes of current will be in quicker
+succession, and this the more, the more nearly equal both the rates are,
+until limitations incident to each case and depending upon a number of
+causes are reached. Thus we are able to obtain from a continuous-current
+generator as rapid a succession of discharges as we like. Of course, the
+higher the tension of the generator, the smaller need be the capacity of
+the condensers, and for this reason, principally, it is of advantage to
+employ a generator of very high tension. Besides, such a generator
+permits the attaining of greater rates of vibration.
+
+The rushes of current may be of the same direction under the conditions
+before assumed, but most generally there is an oscillation superimposed
+upon the fundamental vibration of the current. When the conditions are
+so determined that there are no oscillations, the current impulses are
+unidirectional and thus a means is provided of transforming a continuous
+current of high tension, into a direct current of lower tension, which I
+think may find employment in the arts.
+
+This method of conversion is exceedingly interesting and I was much
+impressed by its beauty when I first conceived it. It is ideal in
+certain respects. It involves the employment of no mechanical devices of
+any kind, and it allows of obtaining currents of any desired frequency
+from an ordinary circuit, direct or alternating. The frequency of the
+fundamental discharges depending on the relative rates of supply and
+dissipation can be readily varied within wide limits, by simple
+adjustments of these quantities, and the frequency of the superimposed
+vibration by the determination of the capacity, self-induction and
+resistance of the circuit. The potential of the currents, again, may be
+raised as high as any insulation is capable of withstanding safely by
+combining capacity and self-induction or by induction in a secondary,
+which need have but comparatively few turns.
+
+As the conditions are often such that the intermittence or oscillation
+does not readily establish itself, especially when a direct current
+source is employed, it is of advantage to associate an interrupter with
+the arc, as I have, some time ago, indicated the use of an air-blast or
+magnet, or other such device readily at hand. The magnet is employed
+with special advantage in the conversion of direct currents, as it is
+then very effective. If the primary source is an alternate current
+generator, it is desirable, as I have stated on another occasion, that
+the frequency should be low, and that the current forming the arc be
+large, in order to render the magnet more effective.
+
+A form of such discharger with a magnet which has been found convenient,
+and adopted after some trials, in the conversion of direct currents
+particularly, is illustrated in Fig. 166. N S are the pole pieces of a
+very strong magnet which is excited by a coil C. The pole pieces are
+slotted for adjustment and can be fastened in any position by screws s
+s_{1}. The discharge rods d d_{1}, thinned down on the ends in order
+to allow a closer approach of the magnetic pole pieces, pass through the
+columns of brass b b_{1} and are fastened in position by screws s_{2}
+s_{2}. Springs r r_{1} and collars c c_{1} are slipped on the
+rods, the latter serving to set the points of the rods at a certain
+suitable distance by means of screws s_{3} s_{3}, and the former to
+draw the points apart. When it is desired to start the arc, one of the
+large rubber handles h h_{1} is tapped quickly with the hand, whereby
+the points of the rods are brought in contact but are instantly
+separated by the springs r r_{1}. Such an arrangement has been found
+to be often necessary, namely in cases when the E. M. F. was not large
+enough to cause the discharge to break through the gap, and also when it
+was desirable to avoid short circuiting of the generator by the metallic
+contact of the rods. The rapidity of the interruptions of the current
+with a magnet depends on the intensity of the magnetic field and on the
+potential difference at the end of the arc. The interruptions are
+generally in such quick succession as to produce a musical sound. Years
+ago it was observed that when a powerful induction coil is discharged
+between the poles of a strong magnet, the discharge produces a loud
+noise, not unlike a small pistol shot. It was vaguely stated that the
+spark was intensified by the presence of the magnetic field. It is now
+clear that the discharge current, flowing for some time, was interrupted
+a great number of times by the magnet, thus producing the sound. The
+phenomenon is especially marked when the field circuit of a large magnet
+or dynamo is broken in a powerful magnetic field.
+
+[Illustration: FIG. 166.]
+
+When the current through the gap is comparatively large, it is of
+advantage to slip on the points of the discharge rods pieces of very
+hard carbon and let the arc play between the carbon pieces. This
+preserves the rods, and besides has the advantage of keeping the air
+space hotter, as the heat is not conducted away as quickly through the
+carbons, and the result is that a smaller E. M. F. in the arc gap is
+required to maintain a succession of discharges.
+
+[Illustration: FIG. 167.]
+
+Another form of discharger, which may be employed with advantage in
+some cases, is illustrated in Fig. 167. In this form the discharge rods
+d d_{1} pass through perforations in a wooden box B, which is thickly
+coated with mica on the inside, as indicated by the heavy lines. The
+perforations are provided with mica tubes m m_{1} of some thickness,
+which are preferably not in contact with the rods d d_{1}. The box has
+a cover C which is a little larger and descends on the outside of the
+box. The spark gap is warmed by a small lamp _l_ contained in the box. A
+plate _p_ above the lamp allows the draught to pass only through the
+chimney _e_ of the lamp, the air entering through holes _o o_ in or near
+the bottom of the box and following the path indicated by the arrows.
+When the discharger is in operation, the door of the box is closed so
+that the light of the arc is not visible outside. It is desirable to
+exclude the light as perfectly as possible, as it interferes with some
+experiments. This form of discharger is simple and very effective when
+properly manipulated. The air being warmed to a certain temperature, has
+its insulating power impaired; it becomes dielectrically weak, as it
+were, and the consequence is that the arc can be established at much
+greater distance. The arc should, of course, be sufficiently insulating
+to allow the discharge to pass through the gap _disruptively_. The arc
+formed under such conditions, when long, may be made extremely
+sensitive, and the weak draught through the lamp chimney _c_ is quite
+sufficient to produce rapid interruptions. The adjustment is made by
+regulating the temperature and velocity of the draught. Instead of using
+the lamp, it answers the purpose to provide for a draught of warm air in
+other ways. A very simple way which has been practiced is to enclose the
+arc in a long vertical tube, with plates on the top and bottom for
+regulating the temperature and velocity of the air current. Some
+provision had to be made for deadening the sound.
+
+The air may be rendered dielectrically weak also by rarefaction.
+Dischargers of this kind have likewise been used by me in connection
+with a magnet. A large tube is for this purpose provided with heavy
+electrodes of carbon or metal, between which the discharge is made to
+pass, the tube being placed in a powerful magnetic field. The exhaustion
+of the tube is carried to a point at which the discharge breaks through
+easily, but the pressure should be more than 75 millimetres, at which
+the ordinary thread discharge occurs. In another form of discharger,
+combining the features before mentioned, the discharge was made to pass
+between two adjustable magnetic pole pieces, the space between them
+being kept at an elevated temperature.
+
+It should be remarked here that when such, or interrupting devices of
+any kind, are used and the currents are passed through the primary of a
+disruptive discharge coil, it is not, as a rule, of advantage to produce
+a number of interruptions of the current per second greater than the
+natural frequency of vibration of the dynamo supply circuit, which is
+ordinarily small. It should also be pointed out here, that while the
+devices mentioned in connection with the disruptive discharge are
+advantageous under certain conditions, they may be sometimes a source of
+trouble, as they produce intermittences and other irregularities in the
+vibration which it would be very desirable to overcome.
+
+There is, I regret to say, in this beautiful method of conversion a
+defect, which fortunately is not vital, and which I have been gradually
+overcoming. I will best call attention to this defect and indicate a
+fruitful line of work, by comparing the electrical process with its
+mechanical analogue. The process may be illustrated in this manner.
+Imagine a tank with a wide opening at the bottom, which is kept closed
+by spring pressure, but so that it snaps off _suddenly_ when the liquid
+in the tank has reached a certain height. Let the fluid be supplied to
+the tank by means of a pipe feeding at a certain rate. When the critical
+height of the liquid is reached, the spring gives way and the bottom of
+the tank drops out. Instantly the liquid falls through the wide opening,
+and the spring, reasserting itself, closes the bottom again. The tank is
+now filled, and after a certain time interval the same process is
+repeated. It is clear, that if the pipe feeds the fluid quicker than the
+bottom outlet is capable of letting it pass through, the bottom will
+remain off and the tank will still overflow. If the rates of supply are
+exactly equal, then the bottom lid will remain partially open and no
+vibration of the same and of the liquid column will generally occur,
+though it might, if started by some means. But if the inlet pipe does
+not feed the fluid fast enough for the outlet, then there will be always
+vibration. Again, in such case, each time the bottom flaps up or down,
+the spring and the liquid column, if the pliability of the spring and
+the inertia of the moving parts are properly chosen, will perform
+independent vibrations. In this analogue the fluid may be likened to
+electricity or electrical energy, the tank to the condenser, the spring
+to the dielectric, and the pipe to the conductor through which
+electricity is supplied to the condenser. To make this analogy quite
+complete it is necessary to make the assumption, that the bottom, each
+time it gives way, is knocked violently against a non-elastic stop, this
+impact involving some loss of energy; and that, besides, some
+dissipation of energy results due to frictional losses. In the preceding
+analogue the liquid is supposed to be under a steady pressure. If the
+presence of the fluid be assumed to vary rhythmically, this may be taken
+as corresponding to the case of an alternating current. The process is
+then not quite as simple to consider, but the action is the same in
+principle.
+
+It is desirable, in order to maintain the vibration economically, to
+reduce the impact and frictional losses as much as possible. As regards
+the latter, which in the electrical analogue correspond to the losses
+due to the resistance of the circuits, it is impossible to obviate them
+entirely, but they can be reduced to a minimum by a proper selection of
+the dimensions of the circuits and by the employment of thin conductors
+in the form of strands. But the loss of energy caused by the first
+breaking through of the dielectric--which in the above example
+corresponds to the violent knock of the bottom against the inelastic
+stop--would be more important to overcome. At the moment of the breaking
+through, the air space has a very high resistance, which is probably
+reduced to a very small value when the current has reached some
+strength, and the space is brought to a high temperature. It would
+materially diminish the loss of energy if the space were always kept at
+an extremely high temperature, but then there would be no disruptive
+break. By warming the space moderately by means of a lamp or otherwise,
+the economy as far as the arc is concerned is sensibly increased. But
+the magnet or other interrupting device does not diminish the loss in
+the arc. Likewise, a jet of air only facilitates the carrying off of the
+energy. Air, or a gas in general, behaves curiously in this respect.
+When two bodies charged to a very high potential, discharge disruptively
+through an air space, any amount of energy may be carried off by the
+air. This energy is evidently dissipated by bodily carriers, in impact
+and collisional losses of the molecules. The exchange of the molecules
+in the space occurs with inconceivable rapidity. A powerful discharge
+taking place between two electrodes, they may remain entirely cool, and
+yet the loss in the air may represent any amount of energy. It is
+perfectly practicable, with very great potential differences in the gap,
+to dissipate several horse-power in the arc of the discharge without
+even noticing a small increase in the temperature of the electrodes. All
+the frictional losses occur then practically in the air. If the exchange
+of the air molecules is prevented, as by enclosing the air hermetically,
+the gas inside of the vessel is brought quickly to a high temperature,
+even with a very small discharge. It is difficult to estimate how much
+of the energy is lost in sound waves, audible or not, in a powerful
+discharge. When the currents through the gap are large, the electrodes
+may become rapidly heated, but this is not a reliable measure of the
+energy wasted in the arc, as the loss through the gap itself may be
+comparatively small. The air or a gas in general is, at ordinary
+pressure at least, clearly not the best medium through which a
+disruptive discharge should occur. Air or other gas under great pressure
+is of course a much more suitable medium for the discharge gap. I have
+carried on long-continued experiments in this direction, unfortunately
+less practicable on account of the difficulties and expense in getting
+air under great pressure. But even if the medium in the discharge space
+is solid or liquid, still the same losses take place, though they are
+generally smaller, for just as soon as the arc is established, the solid
+or liquid is volatilized. Indeed, there is no body known which would not
+be disintegrated by the arc, and it is an open question among scientific
+men, whether an arc discharge could occur at all in the air itself
+without the particles of the electrodes being torn off. When the current
+through the gap is very small and the arc very long, I believe that a
+relatively considerable amount of heat is taken up in the disintegration
+of the electrodes, which partially on this account may remain quite
+cold.
+
+The ideal medium for a discharge gap should only _crack_, and the ideal
+electrode should be of some material which cannot be disintegrated. With
+small currents through the gap it is best to employ aluminum, but not
+when the currents are large. The disruptive break in the air, or more or
+less in any ordinary medium, is not of the nature of a crack, but it is
+rather comparable to the piercing of innumerable bullets through a mass
+offering great frictional resistances to the motion of the bullets, this
+involving considerable loss of energy. A medium which would merely crack
+when strained electrostatically--and this possibly might be the case
+with a perfect vacuum, that is, pure ether--would involve a very small
+loss in the gap, so small as to be entirely negligible, at least
+theoretically, because a crack may be produced by an infinitely small
+displacement. In exhausting an oblong bulb provided with two aluminum
+terminals, with the greatest care, I have succeeded in producing such a
+vacuum that the secondary discharge of a disruptive discharge coil would
+break disruptively through the bulb in the form of fine spark streams.
+The curious point was that the discharge would completely ignore the
+terminals and start far behind the two aluminum plates which served as
+electrodes. This extraordinary high vacuum could only be maintained for
+a very short while. To return to the ideal medium, think, for the sake
+of illustration, of a piece of glass or similar body clamped in a vice,
+and the latter tightened more and more. At a certain point a minute
+increase of the pressure will cause the glass to crack. The loss of
+energy involved in splitting the glass may be practically nothing, for
+though the force is great, the displacement need be but extremely small.
+Now imagine that the glass would possess the property of closing again
+perfectly the crack upon a minute diminution of the pressure. This is
+the way the dielectric in the discharge space should behave. But
+inasmuch as there would be always some loss in the gap, the medium,
+which should be continuous, should exchange through the gap at a rapid
+rate. In the preceding example, the glass being perfectly closed, it
+would mean that the dielectric in the discharge space possesses a great
+insulating power; the glass being cracked, it would signify that the
+medium in the space is a good conductor. The dielectric should vary
+enormously in resistance by minute variations of the E. M. F. across the
+discharge space. This condition is attained, but in an extremely
+imperfect manner, by warming the air space to a certain critical
+temperature, dependent on the E. M. F. across the gap, or by otherwise
+impairing the insulating power of the air. But as a matter of fact the
+air does never break down _disruptively_, if this term be rigorously
+interpreted, for before the sudden rush of the current occurs, there is
+always a weak current preceding it, which rises first gradually and then
+with comparative suddenness. That is the reason why the rate of change
+is very much greater when glass, for instance, is broken through, than
+when the break takes place through an air space of equivalent dielectric
+strength. As a medium for the discharge space, a solid, or even a
+liquid, would be preferable therefor. It is somewhat difficult to
+conceive of a solid body which would possess the property of closing
+instantly after it has been cracked. But a liquid, especially under
+great pressure, behaves practically like a solid, while it possesses the
+property of closing the crack. Hence it was thought that a liquid
+insulator might be more suitable as a dielectric than air. Following out
+this idea, a number of different forms of dischargers in which a variety
+of such insulators, sometimes under great pressure, were employed, have
+been experimented upon. It is thought sufficient to dwell in a few words
+upon one of the forms experimented upon. One of these dischargers is
+illustrated in Figs. 168_a_ and 168_b_.
+
+[Illustration: FIG. 168a.]
+
+[Illustration: FIG. 168b.]
+
+A hollow metal pulley P (Fig. 168_a_), was fastened upon an arbor _a_,
+which by suitable means was rotated at a considerable speed. On the
+inside of the pulley, but disconnected from the same, was supported a
+thin disc _h_ (which is shown thick for the sake of clearness), of hard
+rubber in which there were embedded two metal segments _s s_ with
+metallic extensions _e e_ into which were screwed conducting terminals
+_t t_ covered with thick tubes of hard rubber _t t_. The rubber disc _h_
+with its metallic segments _s s_, was finished in a lathe, and its
+entire surface highly polished so as to offer the smallest possible
+frictional resistance to the motion through a fluid. In the hollow of
+the pulley an insulating liquid such as a thin oil was poured so as to
+reach very nearly to the opening left in the flange _f_, which was
+screwed tightly on the front side of the pulley. The terminals _t t_,
+were connected to the opposite coatings of a battery of condensers so
+that the discharge occurred through the liquid. When the pulley was
+rotated, the liquid was forced against the rim of the pulley and
+considerable fluid pressure resulted. In this simple way the discharge
+gap was filled with a medium which behaved practically like a solid,
+which possessed the quality of closing instantly upon the occurrence of
+the break, and which moreover was circulating through the gap at a rapid
+rate. Very powerful effects were produced by discharges of this kind
+with liquid interrupters, of which a number of different forms were
+made. It was found that, as expected, a longer spark for a given length
+of wire was obtainable in this way than by using air as an interrupting
+device. Generally the speed, and therefore also the fluid pressure, was
+limited by reason of the fluid friction, in the form of discharger
+described, but the practically obtainable speed was more than sufficient
+to produce a number of breaks suitable for the circuits ordinarily used.
+In such instances the metal pulley P was provided with a few projections
+inwardly, and a definite number of breaks was then produced which could
+be computed from the speed of rotation of the pulley. Experiments were
+also carried on with liquids of different insulating power with the view
+of reducing the loss in the arc. When an insulating liquid is moderately
+warmed, the loss in the arc is diminished.
+
+A point of some importance was noted in experiments with various
+discharges of this kind. It was found, for instance, that whereas the
+conditions maintained in these forms were favorable for the production
+of a great spark length, the current so obtained was not best suited to
+the production of light effects. Experience undoubtedly has shown, that
+for such purposes a harmonic rise and fall of the potential is
+preferable. Be it that a solid is rendered incandescent, or
+phosphorescent, or be it that energy is transmitted by condenser coating
+through the glass, it is quite certain that a harmonically rising and
+falling potential produces less destructive action, and that the vacuum
+is more permanently maintained. This would be easily explained if it
+were ascertained that the process going on in an exhausted vessel is of
+an electrolytic nature.
+
+In the diagrammatical sketch, Fig. 165, which has been already referred
+to, the cases which are most likely to be met with in practice are
+illustrated. One has at his disposal either direct or alternating
+currents from a supply station. It is convenient for an experimenter in
+an isolated laboratory to employ a machine G, such as illustrated,
+capable of giving both kinds of currents. In such case it is also
+preferable to use a machine with multiple circuits, as in many
+experiments it is useful and convenient to have at one's disposal
+currents of different phases. In the sketch, D represents the direct and
+A the alternating circuit. In each of these, three branch circuits are
+shown, all of which are provided with double line switches _s s s s s
+s_. Consider first the direct current conversion; I_a_ represents the
+simplest case. If the E. M. F. of the generator is sufficient to break
+through a small air space, at least when the latter is warmed or
+otherwise rendered poorly insulating, there is no difficulty in
+maintaining a vibration with fair economy by judicious adjustment of the
+capacity, self-induction and resistance of the circuit L containing the
+devices _l l m_. The magnet N, S, can be in this case advantageously
+combined with the air space. The discharger _d d_ with the magnet may be
+placed either way, as indicated by the full or by the dotted lines. The
+circuit I_a_ with the connections and devices is supposed to possess
+dimensions such as are suitable for the maintenance of a vibration. But
+usually the E. M. F. on the circuit or branch I_a_ will be something
+like a 100 volts or so, and in this case it is not sufficient to break
+through the gap. Many different means may be used to remedy this by
+raising the E. M. F. across the gap. The simplest is probably to insert
+a large self-induction coil in series with the circuit L. When the arc
+is established, as by the discharger illustrated in Fig. 166, the magnet
+blows the arc out the instant it is formed. Now the extra current of the
+break, being of high E. M. F., breaks through the gap, and a path of low
+resistance for the dynamo current being again provided, there is a
+sudden rush of current from the dynamo upon the weakening or subsidence
+of the extra current. This process is repeated in rapid succession, and
+in this manner I have maintained oscillation with as low as 50 volts, or
+even less, across the gap. But conversion on this plan is not to be
+recommended on account of the too heavy currents through the gap and
+consequent heating of the electrodes; besides, the frequencies obtained
+in this way are low, owing to the high self-induction necessarily
+associated with the circuit. It is very desirable to have the E. M. F.
+as high as possible, first, in order to increase the economy of the
+conversion, and, secondly, to obtain high frequencies. The difference of
+potential in this electric oscillation is, of course, the equivalent of
+the stretching force in the mechanical vibration of the spring. To
+obtain very rapid vibration in a circuit of some inertia, a great
+stretching force or difference of potential is necessary. Incidentally,
+when the E. M. F. is very great, the condenser which is usually employed
+in connection with the circuit need but have a small capacity, and many
+other advantages are gained. With a view of raising the E. M. F. to a
+many times greater value than obtainable from ordinary distribution
+circuits, a rotating transformer _g_ is used, as indicated at II_a_,
+Fig. 165, or else a separate high potential machine is driven by means
+of a motor operated from the generator G. The latter plan is in fact
+preferable, as changes are easier made. The connections from the high
+tension winding are quite similar to those in branch I_a_ with the
+exception that a condenser C, which should be adjustable, is connected
+to the high tension circuit. Usually, also, an adjustable self-induction
+coil in series with the circuit has been employed in these experiments.
+When the tension of the currents is very high, the magnet ordinarily
+used in connection with the discharger is of comparatively small value,
+as it is quite easy to adjust the dimensions of the circuit so that
+oscillation is maintained. The employment of a steady E. M. F. in the
+high frequency conversion affords some advantages over the employment of
+alternating E. M. F., as the adjustments are much simpler and the action
+can be easier controlled. But unfortunately one is limited by the
+obtainable potential difference. The winding also breaks down easily in
+consequence of the sparks which form between the sections of the
+armature or commutator when a vigorous oscillation takes place. Besides,
+these transformers are expensive to build. It has been found by
+experience that it is best to follow the plan illustrated at III_a_. In
+this arrangement a rotating transformer _g_, is employed to convert the
+low tension direct currents into low frequency alternating currents,
+preferably also of small tension. The tension of the currents is then
+raised in a stationary transformer T. The secondary S of this
+transformer is connected to an adjustable condenser C which discharges
+through the gap or discharger _d d_, placed in either of the ways
+indicated, through the primary P of a disruptive discharge coil, the
+high frequency current being obtained from the secondary S of this coil,
+as described on previous occasions. This will undoubtedly be found the
+cheapest and most convenient way of converting direct currents.
+
+The three branches of the circuit A represent the usual cases met in
+practice when alternating currents are converted. In Fig. 1_b_ a
+condenser C, generally of large capacity, is connected to the circuit L
+containing the devices _l l_, _m m_. The devices _m m_ are supposed to
+be of high self-induction so as to bring the frequency of the circuit
+more or less to that of the dynamo. In this instance the discharger _d
+d_ should best have a number of makes and breaks per second equal to
+twice the frequency of the dynamo. If not so, then it should have at
+least a number equal to a multiple or even fraction of the dynamo
+frequency. It should be observed, referring to I_b_, that the conversion
+to a high potential is also effected when the discharger _d d_, which is
+shown in the sketch, is omitted. But the effects which are produced by
+currents which rise instantly to high values, as in a disruptive
+discharge, are entirely different from those produced by dynamo currents
+which rise and fall harmonically. So, for instance, there might be in a
+given case a number of makes and breaks at _d d_ equal to just twice the
+frequency of the dynamo, or in other words, there may be the same number
+of fundamental oscillations as would be produced without the discharge
+gap, and there might even not be any quicker superimposed vibration; yet
+the differences of potential at the various points of the circuit, the
+impedance and other phenomena, dependent upon the rate of change, will
+bear no similarity in the two cases. Thus, when working with currents
+discharging disruptively, the element chiefly to be considered is not
+the frequency, as a student might be apt to believe, but the rate of
+change per unit of time. With low frequencies in a certain measure the
+same effects may be obtained as with high frequencies, provided the rate
+of change is sufficiently great. So if a low frequency current is raised
+to a potential of, say, 75,000 volts, and the high tension current
+passed through a series of high resistance lamp filaments, the
+importance of the rarefied gas surrounding the filament is clearly
+noted, as will be seen later; or, if a low frequency current of several
+thousand amperes is passed through a metal bar, striking phenomena of
+impedance are observed, just as with currents of high frequencies. But
+it is, of course, evident that with low frequency currents it is
+impossible to obtain such rates of change per unit of time as with high
+frequencies, hence the effects produced by the latter are much more
+prominent. It is deemed advisable to make the preceding remarks,
+inasmuch as many more recently described effects have been unwittingly
+identified with high frequencies. Frequency alone in reality does not
+mean anything, except when an undisturbed harmonic oscillation is
+considered.
+
+In the branch III_b_ a similar disposition to that in I_b_ is
+illustrated, with the difference that the currents discharging through
+the gap _d d_ are used to induce currents in the secondary S of a
+transformer T. In such case the secondary should be provided with an
+adjustable condenser for the purpose of tuning it to the primary.
+
+II_b_ illustrates a plan of alternate current high frequency conversion
+which is most frequently used and which is found to be most convenient.
+This plan has been dwelt upon in detail on previous occasions and need
+not be described here.
+
+Some of these results were obtained by the use of a high frequency
+alternator. A description of such machines will be found in my original
+paper before the American Institute of Electrical Engineers, and in
+periodicals of that period, notably in THE ELECTRICAL ENGINEER of March
+18, 1891.
+
+I will now proceed with the experiments.
+
+
+ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.
+
+The first class of effects I intend to show you are effects produced by
+electrostatic force. It is the force which governs the the motion of the
+atoms, which causes them to collide and develop the life-sustaining
+energy of heat and light, and which causes them to aggregate in an
+infinite variety of ways, according to Nature's fanciful designs, and to
+form all these wondrous structures we perceive around us; it is, in
+fact, if our present views be true, the most important force for us to
+consider in Nature. As the term _electrostatic_ might imply a steady
+electric condition, it should be remarked, that in these experiments the
+force is not constant, but varies at a rate which may be considered
+moderate, about one million times a second, or thereabouts. This enables
+me to produce many effects which are not producible with an unvarying
+force.
+
+When two conducting bodies are insulated and electrified, we say that an
+electrostatic force is acting between them. This force manifests itself
+in attractions, repulsions and stresses in the bodies and space or
+medium without. So great may be the strain exerted in the air, or
+whatever separates the two conducting bodies, that it may break down,
+and we observe sparks or bundles of light or streamers, as they are
+called. These streamers form abundantly when the force through the air
+is rapidly varying. I will illustrate this action of electrostatic force
+in a novel experiment in which I will employ the induction coil before
+referred to. The coil is contained in a trough filled with oil, and
+placed under the table. The two ends of the secondary wire pass through
+the two thick columns of hard rubber which protrude to some height above
+the table. It is necessary to insulate the ends or terminals of the
+secondary heavily with hard rubber, because even dry wood is by far too
+poor an insulator for these currents of enormous potential differences.
+On one of the terminals of the coil, I have placed a large sphere of
+sheet brass, which is connected to a larger insulated brass plate, in
+order to enable me to perform the experiments under conditions, which,
+as you will see, are more suitable for this experiment. I now set the
+coil to work and approach the free terminal with a metallic object held
+in my hand, this simply to avoid burns. As I approach the metallic
+object to a distance of eight or ten inches, a torrent of furious sparks
+breaks forth from the end of the secondary wire, which passes through
+the rubber column. The sparks cease when the metal in my hand touches
+the wire. My arm is now traversed by a powerful electric current,
+vibrating at about the rate of one million times a second. All around me
+the electrostatic force makes itself felt, and the air molecules and
+particles of dust flying about are acted upon and are hammering
+violently against my body. So great is this agitation of the particles,
+that when the lights are turned out you may see streams of feeble light
+appear on some parts of my body. When such a streamer breaks out on any
+part of the body, it produces a sensation like the pricking of a needle.
+Were the potentials sufficiently high and the frequency of the vibration
+rather low, the skin would probably be ruptured under the tremendous
+strain, and the blood would rush out with great force in the form of
+fine spray or jet so thin as to be invisible, just as oil will when
+placed on the positive terminal of a Holtz machine. The breaking through
+of the skin though it may seem impossible at first, would perhaps occur,
+by reason of the tissues under the skin being incomparably better
+conducting. This, at least, appears plausible, judging from some
+observations.
+
+[Illustration: FIG. 169.]
+
+I can make these streams of light visible to all, by touching with the
+metallic object one of the terminals as before, and approaching my free
+hand to the brass sphere, which is connected to the second terminal of
+the coil. As the hand is approached, the air between it and the sphere,
+or in the immediate neighborhood, is more violently agitated, and you
+see streams of light now break forth from my finger tips and from the
+whole hand (Fig. 169). Were I to approach the hand closer, powerful
+sparks would jump from the brass sphere to my hand, which might be
+injurious. The streamers offer no particular inconvenience, except that
+in the ends of the finger tips a burning sensation is felt. They should
+not be confounded with those produced by an influence machine, because
+in many respects they behave differently. I have attached the brass
+sphere and plate to one of the terminals in order to prevent the
+formation of visible streamers on that terminal, also in order to
+prevent sparks from jumping at a considerable distance. Besides, the
+attachment is favorable for the working of the coil.
+
+The streams of light which you have observed issuing from my hand are
+due to a potential of about 200,000 volts, alternating in rather
+irregular intervals, sometimes like a million times a second. A
+vibration of the same amplitude, but four times as fast, to maintain
+which over 3,000,000 volts would be required, would be more than
+sufficient to envelop my body in a complete sheet of flame. But this
+flame would not burn me up; quite contrarily, the probability is that I
+would not be injured in the least. Yet a hundredth part of that energy,
+otherwise directed, would be amply sufficient to kill a person.
+
+The amount of energy which may thus be passed into the body of a person
+depends on the frequency and potential of the currents, and by making
+both of these very great, a vast amount of energy may be passed into the
+body without causing any discomfort, except perhaps, in the arm, which
+is traversed by a true conduction current. The reason why no pain in the
+body is felt, and no injurious effect noted, is that everywhere, if a
+current be imagined to flow through the body, the direction of its flow
+would be at right angles to the surface; hence the body of the
+experimenter offers an enormous section to the current, and the density
+is very small, with the exception of the arm, perhaps, where the density
+may be considerable. But if only a small fraction of that energy would
+be applied in such a way that a current would traverse the body in the
+same manner as a low frequency current, a shock would be received which
+might be fatal. A direct or low frequency alternating current is fatal,
+I think, principally because its distribution through the body is not
+uniform, as it must divide itself in minute streamlets of great density,
+whereby some organs are vitally injured. That such a process occurs I
+have not the least doubt, though no evidence might apparently exist, or
+be found upon examination. The surest to injure and destroy life, is a
+continuous current, but the most painful is an alternating current of
+very low frequency. The expression of these views, which are the result
+of long continued experiment and observation, both with steady and
+varying currents, is elicited by the interest which is at present taken
+in this subject, and by the manifestly erroneous ideas which are daily
+propounded in journals on this subject.
+
+I may illustrate an effect of the electrostatic force by another
+striking experiment, but before, I must call your attention to one or
+two facts. I have said before, that when the medium between two
+oppositely electrified bodies is strained beyond a certain limit it
+gives way and, stated in popular language, the opposite electric charges
+unite and neutralize each other. This breaking down of the medium occurs
+principally when the force acting between the bodies is steady, or
+varies at a moderate rate. Were the variation sufficiently rapid, such a
+destructive break would not occur, no matter how great the force, for
+all the energy would be spent in radiation, convection and mechanical
+and chemical action. Thus the _spark_ length, or greatest distance which
+a _spark_ will jump between the electrified bodies is the smaller, the
+greater the variation or time rate of change. But this rule may be taken
+to be true only in a general way, when comparing rates which are widely
+different.
+
+[Illustration: FIG. 170a.]
+
+[Illustration: FIG. 170b.]
+
+I will show you by an experiment the difference in the effect produced
+by a rapidly varying and a steady or moderately varying force. I have
+here two large circular brass plates _p p_ (Fig. 170_a_ and Fig.
+170_b_), supported on movable insulating stands on the table, connected
+to the ends of the secondary of a coil similar to the one used before. I
+place the plates ten or twelve inches apart and set the coil to work.
+You see the whole space between the plates, nearly two cubic feet,
+filled with uniform light, Fig. 170_a_. This light is due to the
+streamers you have seen in the first experiment, which are now much more
+intense. I have already pointed out the importance of these streamers in
+commercial apparatus and their still greater importance in some purely
+scientific investigations. Often they are too weak to be visible, but
+they always exist, consuming energy and modifying the action of the
+apparatus. When intense, as they are at present, they produce ozone in
+great quantity, and also, as Professor Crookes has pointed out, nitrous
+acid. So quick is the chemical action that if a coil, such as this one,
+is worked for a very long time it will make the atmosphere of a small
+room unbearable, for the eyes and throat are attacked. But when
+moderately produced, the streamers refresh the atmosphere wonderfully,
+like a thunder-storm, and exercises unquestionably a beneficial effect.
+
+In this experiment the force acting between the plates changes in
+intensity and direction at a very rapid rate. I will now make the rate
+of change per unit time much smaller. This I effect by rendering the
+discharges through the primary of the induction coil less frequent, and
+also by diminishing the rapidity of the vibration in the secondary. The
+former result is conveniently secured by lowering the E. M. F. over the
+air gap in the primary circuit, the latter by approaching the two brass
+plates to a distance of about three or four inches. When the coil is set
+to work, you see no streamers or light between the plates, yet the
+medium between them is under a tremendous strain. I still further
+augment the strain by raising the E. M. F. in the primary circuit, and
+soon you see the air give way and the hall is illuminated by a shower of
+brilliant and noisy sparks, Fig. 170_b_. These sparks could be produced
+also with unvarying force; they have been for many years a familiar
+phenomenon, though they were usually obtained from an entirely different
+apparatus. In describing these two phenomena so radically different in
+appearance, I have advisedly spoken of a "force" acting between the
+plates. It would be in accordance with accepted views to say, that there
+was an "alternating E. M. F," acting between the plates. This term is
+quite proper and applicable in all cases where there is evidence of at
+least a possibility of an essential inter-dependence of the electric
+state of the plates, or electric action in their neighborhood. But if
+the plates were removed to an infinite distance, or if at a finite
+distance, there is no probability or necessity whatever for such
+dependence. I prefer to use the term "electrostatic force," and to say
+that such a force is acting around each plate or electrified insulated
+body in general. There is an inconvenience in using this expression as
+the term incidentally means a steady electric condition; but a proper
+nomenclature will eventually settle this difficulty.
+
+I now return to the experiment to which I have already alluded, and with
+which I desire to illustrate a striking effect produced by a rapidly
+varying electrostatic force. I attach to the end of the wire, _l_ (Fig.
+171), which is in connection with one of the terminals of the secondary
+of the induction coil, an exhausted bulb _b_. This bulb contains a thin
+carbon filament _f_, which is fastened to a platinum wire _w_, sealed in
+the glass and leading outside of the bulb, where it connects to the wire
+_l_. The bulb may be exhausted to any degree attainable with ordinary
+apparatus. Just a moment before, you have witnessed the breaking down of
+the air between the charged brass plates. You know that a plate of
+glass, or any other insulating material, would break down in like
+manner. Had I therefore a metallic coating attached to the outside of
+the bulb, or placed near the same, and were this coating connected to
+the other terminal of the coil, you would be prepared to see the glass
+give way if the strain were sufficiently increased. Even were the
+coating not connected to the other terminal, but to an insulated plate,
+still, if you have followed recent developments, you would naturally
+expect a rupture of the glass.
+
+[Illustration: FIG. 171.]
+
+[Illustration: FIG. 172a.]
+
+[Illustration: FIG. 172b.]
+
+But it will certainly surprise you to note that under the action of the
+varying electrostatic force, the glass gives way when all other bodies
+are removed from the bulb. In fact, all the surrounding bodies we
+perceive might be removed to an infinite distance without affecting the
+result in the slightest. When the coil is set to work, the glass is
+invariably broken through at the seal, or other narrow channel, and the
+vacuum is quickly impaired. Such a damaging break would not occur with
+a steady force, even if the same were many times greater. The break is
+due to the agitation of the molecules of the gas within the bulb, and
+outside of the same. This agitation, which is generally most violent in
+the narrow pointed channel near the seal, causes a heating and rupture
+of the glass. This rupture, would, however, not occur, not even with a
+varying force, if the medium filling the inside of the bulb, and that
+surrounding it, were perfectly homogeneous. The break occurs much
+quicker if the top of the bulb is drawn out into a fine fibre. In bulbs
+used with these coils such narrow, pointed channels must therefore be
+avoided.
+
+When a conducting body is immersed in air, or similar insulating medium,
+consisting of, or containing, small freely movable particles capable of
+being electrified, and when the electrification of the body is made to
+undergo a very rapid change--which is equivalent to saying that the
+electrostatic force acting around the body is varying in intensity,--the
+small particles are attracted and repelled, and their violent impacts
+against the body may cause a mechanical motion of the latter. Phenomena
+of this kind are noteworthy, inasmuch as they have not been observed
+before with apparatus such as has been commonly in use. If a very light
+conducting sphere be suspended on an exceedingly fine wire, and charged
+to a steady potential, however high, the sphere will remain at rest.
+Even if the potential would be rapidly varying, provided that the small
+particles of matter, molecules or atoms, are evenly distributed, no
+motion of the sphere should result. But if one side of the conducting
+sphere is covered with a thick insulating layer, the impacts of the
+particles will cause the sphere to move about, generally in irregular
+curves, Fig. 172_a_. In like manner, as I have shown on a previous
+occasion, a fan of sheet metal, Fig. 172_b_, covered partially with
+insulating material as indicated, and placed upon the terminal of the
+coil so as to turn freely on it, is spun around.
+
+All these phenomena you have witnessed and others which will be shown
+later, are due to the presence of a medium like air, and would not occur
+in a continuous medium. The action of the air may be illustrated still
+better by the following experiment. I take a glass tube _t_, Fig. 173,
+of about an inch in diameter, which has a platinum wire _w_ sealed in
+the lower end, and to which is attached a thin lamp filament _f_. I
+connect the wire with the terminal of the coil and set the coil to work.
+The platinum wire is now electrified positively and negatively in rapid
+succession and the wire and air inside of the tube is rapidly heated by
+the impacts of the particles, which may be so violent as to render the
+filament incandescent. But if I pour oil in the tube, just as soon as
+the wire is covered with the oil, all action apparently ceases and there
+is no marked evidence of heating. The reason of this is that the oil is
+a practically continuous medium. The displacements in such a continuous
+medium are, with these frequencies, to all appearance incomparably
+smaller than in air, hence the work performed in such a medium is
+insignificant. But oil would behave very differently with frequencies
+many times as great, for even though the displacements be small, if the
+frequency were much greater, considerable work might be performed in the
+oil.
+
+[Illustration: FIG. 173.]
+
+[Illustration: FIG. 174.]
+
+The electrostatic attractions and repulsions between bodies of
+measurable dimensions are, of all the manifestations of this force, the
+first so-called _electrical_ phenomena noted. But though they have been
+known to us for many centuries, the precise nature of the mechanism
+concerned in these actions is still unknown to us, and has not been even
+quite satisfactorily explained. What kind of mechanism must that be? We
+cannot help wondering when we observe two magnets attracting and
+repelling each other with a force of hundreds of pounds with apparently
+nothing between them. We have in our commercial dynamos magnets capable
+of sustaining in mid-air tons of weight. But what are even these forces
+acting between magnets when compared with the tremendous attractions and
+repulsions produced by electrostatic force, to which there is apparently
+no limit as to intensity. In lightning discharges bodies are often
+charged to so high a potential that they are thrown away with
+inconceivable force and torn asunder or shattered into fragments. Still
+even such effects cannot compare with the attractions and repulsions
+which exist between charged molecules or atoms, and which are sufficient
+to project them with speeds of many kilometres a second, so that under
+their violent impact bodies are rendered highly incandescent and are
+volatilized. It is of special interest for the thinker who inquires into
+the nature of these forces to note that whereas the actions between
+individual molecules or atoms occur seemingly under any conditions, the
+attractions and repulsions of bodies of measurable dimensions imply a
+medium possessing insulating properties. So, if air, either by being
+rarefied or heated, is rendered more or less conducting, these actions
+between two electrified bodies practically cease, while the actions
+between the individual atoms continue to manifest themselves.
+
+An experiment may serve as an illustration and as a means of bringing
+out other features of interest. Some time ago I showed that a lamp
+filament or wire mounted in a bulb and connected to one of the terminals
+of a high tension secondary coil is set spinning, the top of the
+filament generally describing a circle. This vibration was very
+energetic when the air in the bulb was at ordinary pressure and became
+less energetic when the air in the bulb was strongly compressed. It
+ceased altogether when the air was exhausted so as to become
+comparatively good conducting. I found at that time that no vibration
+took place when the bulb was very highly exhausted. But I conjectured
+that the vibration which I ascribed to the electrostatic action between
+the walls of the bulb and the filament should take place also in a
+highly exhausted bulb. To test this under conditions which were more
+favorable, a bulb like the one in Fig. 174, was constructed. It
+comprised a globe _b_, in the neck of which was sealed a platinum wire
+_w_ carrying a thin lamp filament _f_. In the lower part of the globe a
+tube _t_ was sealed so as to surround the filament. The exhaustion was
+carried as far as it was practicable with the apparatus employed.
+
+This bulb verified my expectation, for the filament was set spinning
+when the current was turned on, and became incandescent. It also showed
+another interesting feature, bearing upon the preceding remarks, namely,
+when the filament had been kept incandescent some time, the narrow tube
+and the space inside were brought to an elevated temperature, and as the
+gas in the tube then became conducting, the electrostatic attraction
+between the glass and the filament became very weak or ceased, and the
+filament came to rest. When it came to rest it would glow far more
+intensely. This was probably due to its assuming the position in the
+centre of the tube where the molecular bombardment was most intense, and
+also partly to the fact that the individual impacts were more violent
+and that no part of the supplied energy was converted into mechanical
+movement. Since, in accordance with accepted views, in this experiment
+the incandescence must be attributed to the impacts of the particles,
+molecules or atoms in the heated space, these particles must therefore,
+in order to explain such action, be assumed to behave as independent
+carriers of electric charges immersed in an insulating medium; yet there
+is no attractive force between the glass tube and the filament because
+the space in the tube is, as a whole, conducting.
+
+It is of some interest to observe in this connection that whereas the
+attraction between two electrified bodies may cease owing to the
+impairing of the insulating power of the medium in which they are
+immersed, the repulsion between the bodies may still be observed. This
+may be explained in a plausible way. When the bodies are placed at some
+distance in a poorly conducting medium, such as slightly warmed or
+rarefied air, and are suddenly electrified, opposite electric charges
+being imparted to them, these charges equalize more or less by leakage
+through the air. But if the bodies are similarly electrified, there is
+less opportunity afforded for such dissipation, hence the repulsion
+observed in such case is greater than the attraction. Repulsive actions
+in a gaseous medium are however, as Prof. Crookes has shown, enhanced by
+molecular bombardment.
+
+
+ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.
+
+So far, I have considered principally effects produced by a varying
+electrostatic force in an insulating medium, such as air. When such a
+force is acting upon a conducting body of measurable dimensions, it
+causes within the same, or on its surface, displacements of the
+electricity and gives rise to electric currents, and these produce
+another kind of phenomena, some of which I shall presently endeavor to
+illustrate. In presenting this second class of electrical effects, I
+will avail myself principally of such as are producible without any
+return circuit, hoping to interest you the more by presenting these
+phenomena in a more or less novel aspect.
+
+It has been a long time customary, owing to the limited experience with
+vibratory currents, to consider an electric current as something
+circulating in a closed conducting path. It was astonishing at first to
+realize that a current may flow through the conducting path even if the
+latter be interrupted, and it was still more surprising to learn, that
+sometimes it may be even easier to make a current flow under such
+conditions than through a closed path. But that old idea is gradually
+disappearing, even among practical men, and will soon be entirely
+forgotten.
+
+[Illustration: FIG. 175.]
+
+If I connect an insulated metal plate P, Fig. 175, to one of the
+terminals T of the induction coil by means of a wire, though this plate
+be very well insulated, a current passes through the wire when the coil
+is set to work. First I wish to give you evidence that there _is_ a
+current passing through the connecting wire. An obvious way of
+demonstrating this is to insert between the terminal of the coil and the
+insulated plate a very thin platinum or german silver wire _w_ and bring
+the latter to incandescence or fusion by the current. This requires a
+rather large plate or else current impulses of very high potential and
+frequency. Another way is to take a coil C, Fig. 175, containing many
+turns of thin insulated wire and to insert the same in the path of the
+current to the plate. When I connect one of the ends of the coil to the
+wire leading to another insulated plate P_{1}, and its other end to the
+terminal T_{1} of the induction coil, and set the latter to work, a
+current passes through the inserted coil C and the existence of the
+current may be made manifest in various ways. For instance, I insert an
+iron core _i_ within the coil. The current being one of very high
+frequency, will, if it be of some strength, soon bring the iron core to
+a noticeably higher temperature, as the hysteresis and current losses
+are great with such high frequencies. One might take a core of some
+size, laminated or not, it would matter little; but ordinary iron wire
+1/16th or 1/8th of an inch thick is suitable for the purpose. While the
+induction coil is working, a current traverses the inserted coil and
+only a few moments are sufficient to bring the iron wire _i_ to an
+elevated temperature sufficient to soften the sealing-wax _s_, and cause
+a paper washer _p_ fastened by it to the iron wire to fall off. But with
+the apparatus such as I have here, other, much more interesting,
+demonstrations of this kind can be made. I have a secondary S, Fig 176,
+of coarse wire, wound upon a coil similar to the first. In the preceding
+experiment the current through the coil C, Fig. 175, was very small, but
+there being many turns a strong heating effect was, nevertheless,
+produced in the iron wire. Had I passed that current through a conductor
+in order to show the heating of the latter, the current might have been
+too small to produce the effect desired. But with this coil provided
+with a secondary winding, I can now transform the feeble current of high
+tension which passes through the primary P into a strong secondary
+current of low tension, and this current will quite certainly do what I
+expect. In a small glass tube (_t_, Fig. 176), I have enclosed a coiled
+platinum wire, _w_, this merely in order to protect the wire. On each
+end of the glass tube is sealed a terminal of stout wire to which one of
+the ends of the platinum wire _w_, is connected. I join the terminals of
+the secondary coil to these terminals and insert the primary _p_,
+between the insulated plate P_{1}, and the terminal T_{1}, of the
+induction coil as before. The latter being set to work, instantly the
+platinum wire _w_ is rendered incandescent and can be fused, even if it
+be very thick.
+
+[Illustration: FIG. 176.]
+
+Instead of the platinum wire I now take an ordinary 50-volt 16 C. P.
+lamp. When I set the induction coil in operation the lamp filament is
+brought to high incandescence. It is, however, not necessary to use the
+insulated plate, for the lamp (_l_, Fig. 177) is rendered incandescent
+even if the plate P_{1} be disconnected. The secondary may also be
+connected to the primary as indicated by the dotted line in Fig. 177, to
+do away more or less with the electrostatic induction or to modify the
+action otherwise.
+
+[Illustration: FIG. 177.]
+
+I may here call attention to a number of interesting observations with
+the lamp. First, I disconnect one of the terminals of the lamp from the
+secondary S. When the induction coil plays, a glow is noted which fills
+the whole bulb. This glow is due to electrostatic induction. It
+increases when the bulb is grasped with the hand, and the capacity of
+the experimenter's body thus added to the secondary circuit. The
+secondary, in effect, is equivalent to a metallic coating, which would
+be placed near the primary. If the secondary, or its equivalent, the
+coating, were placed symmetrically to the primary, the electrostatic
+induction would be nil under ordinary conditions, that is, when a
+primary return circuit is used, as both halves would neutralize each
+other. The secondary _is_ in fact placed symmetrically to the primary,
+but the action of both halves of the latter, when only one of its ends
+is connected to the induction coil, is not exactly equal; hence
+electrostatic induction takes place, and hence the glow in the bulb. I
+can nearly equalize the action of both halves of the primary by
+connecting the other, free end of the same to the insulated plate, as in
+the preceding experiment. When the plate is connected, the glow
+disappears. With a smaller plate it would not entirely disappear and
+then it would contribute to the brightness of the filament when the
+secondary is closed, by warming the air in the bulb.
+
+[Illustration: FIG. 178a.]
+
+[Illustration: FIG. 178b.]
+
+[Illustration: FIG. 179a.]
+
+[Illustration: FIG. 179b.]
+
+To demonstrate another interesting feature, I have adjusted the coils
+used in a certain way. I first connect both the terminals of the lamp to
+the secondary, one end of the primary being connected to the terminal
+T_{1} of the induction coil and the other to the insulated plate P_{1}
+as before. When the current is turned on, the lamp glows brightly, as
+shown in Fig. 178_b_, in which C is a fine wire coil and S a coarse wire
+secondary wound upon it. If the insulated plate P_{1} is disconnected,
+leaving one of the ends _a_ of the primary insulated, the filament
+becomes dark or generally it diminishes in brightness (Fig. 178_a_).
+Connecting again the plate P_{1} and raising the frequency of the
+current, I make the filament quite dark or barely red (Fig. 179_b_).
+Once more I will disconnect the plate. One will of course infer that
+when the plate is disconnected, the current through the primary will be
+weakened, that therefore the E. M. F. will fall in the secondary S, and
+that the brightness of the lamp will diminish. This might be the case
+and the result can be secured by an easy adjustment of the coils; also
+by varying the frequency and potential of the currents. But it is
+perhaps of greater interest to note, that the lamp increases in
+brightness when the plate is disconnected (Fig. 179_a_). In this case
+all the energy the primary receives is now sunk into it, like the charge
+of a battery in an ocean cable, but most of that energy is recovered
+through the secondary and used to light the lamp. The current traversing
+the primary is strongest at the end _b_ which is connected to the
+terminal T_{1} of the induction coil, and diminishes in strength towards
+the remote end _a_. But the dynamic inductive effect exerted upon the
+secondary S is now greater than before, when the suspended plate was
+connected to the primary. These results might have been produced by a
+number of causes. For instance, the plate P_{1} being connected, the
+reaction from the coil C may be such as to diminish the potential at the
+terminal T_{1} of the induction coil, and therefore weaken the current
+through the primary of the coil C. Or the disconnecting of the plate
+may diminish the capacity effect with relation to the primary of the
+latter coil to such an extent that the current through it is diminished,
+though the potential at the terminal T_{1} of the induction coil may be
+the same or even higher. Or the result might have been produced by the
+change of phase of the primary and secondary currents and consequent
+reaction. But the chief determining factor is the relation of the
+self-induction and capacity of coil C and plate P_{1} and the frequency
+of the currents. The greater brightness of the filament in Fig. 179_a_,
+is, however, in part due to the heating of the rarefied gas in the lamp
+by electrostatic induction, which, as before remarked, is greater when
+the suspended plate is disconnected.
+
+Still another feature of some interest I may here bring to your
+attention. When the insulated plate is disconnected and the secondary of
+the coil opened, by approaching a small object to the secondary, but
+very small sparks can be drawn from it, showing that the electrostatic
+induction is small in this case. But upon the secondary being closed
+upon itself or through the lamp, the filament glowing brightly, strong
+sparks are obtained from the secondary. The electrostatic induction is
+now much greater, because the closed secondary determines a greater flow
+of current through the primary and principally through that half of it
+which is connected to the induction coil. If now the bulb be grasped
+with the hand, the capacity of the secondary with reference to the
+primary is augmented by the experimenter's body and the luminosity of
+the filament is increased, the incandescence now being due partly to the
+flow of current through the filament and partly to the molecular
+bombardment of the rarefied gas in the bulb.
+
+The preceding experiments will have prepared one for the next following
+results of interest, obtained in the course of these investigations.
+Since I can pass a current through an insulated wire merely by
+connecting one of its ends to the source of electrical energy, since I
+can induce by it another current, magnetize an iron core, and, in short,
+perform all operations as though a return circuit were used, clearly I
+can also drive a motor by the aid of only one wire. On a former occasion
+I have described a simple form of motor comprising a single exciting
+coil, an iron core and disc. Fig. 180 illustrates a modified way of
+operating such an alternate current motor by currents induced in a
+transformer connected to one lead, and several other arrangements of
+circuits for operating a certain class of alternating motors founded on
+the action of currents of differing phase. In view of the present state
+of the art it is thought sufficient to describe these arrangements in a
+few words only. The diagram, Fig. 180 II., shows a primary coil P,
+connected with one of its ends to the line L leading from a high tension
+transformer terminal T_{1}. In inductive relation to this primary P is a
+secondary S of coarse wire in the circuit of which is a coil _c_. The
+currents induced in the secondary energize the iron core _i_, which is
+preferably, but not necessarily, subdivided, and set the metal disc _d_
+in rotation. Such a motor M_{2} as diagramatically shown in Fig. 180
+II., has been called a "magnetic lag motor," but this expression may be
+objected to by those who attribute the rotation of the disc to eddy
+currents circulating in minute paths when the core _i_ is finally
+subdivided. In order to operate such a motor effectively on the plan
+indicated, the frequencies should not be too high, not more than four or
+five thousand, though the rotation is produced even with ten thousand
+per second, or more.
+
+In Fig. 180 I., a motor M_{1} having two energizing circuits, A and B,
+is diagrammatically indicated. The circuit A is connected to the line L
+and in series with it is a primary P, which may have its free end
+connected to an insulated plate P_{1}, such connection being indicated
+by the dotted lines. The other motor circuit B is connected to the
+secondary S which is in inductive relation to the primary P. When the
+transformer terminal T_{1} is alternately electrified, currents traverse
+the open line L and also circuit A and primary P. The currents through
+the latter induce secondary currents in the circuit S, which pass
+through the energizing coil B of the motor. The currents through the
+secondary S and those through the primary P differ in phase 90 degrees,
+or nearly so, and are capable of rotating an armature placed in
+inductive relation to the circuits A and B.
+
+In Fig. 180 III., a similar motor M_{3} with two energizing circuits
+A_{1} and B_{1} is illustrated. A primary P, connected with one of its
+ends to the line L has a secondary S, which is preferably wound for a
+tolerably high E. M. F., and to which the two energizing circuits of the
+motor are connected, one directly to the ends of the secondary and the
+other through a condenser C, by the action of which the currents
+traversing the circuit A_{1} and B_{1} are made to differ in phase.
+
+[Illustration: FIG. 180.]
+
+[Illustration: FIG. 181.]
+
+[Illustration: FIG. 182.]
+
+In Fig. 180 IV., still another arrangement is shown. In this case two
+primaries P_{1} and P_{2} are connected to the line L, one through a
+condenser C of small capacity, and the other directly. The primaries are
+provided with secondaries S_{1} and S_{2} which are in series with the
+energizing circuits, A_{2} and B_{2} and a motor M_{3}, the condenser C
+again serving to produce the requisite difference in the phase of the
+currents traversing the motor circuits. As such phase motors with two or
+more circuits are now well known in the art, they have been here
+illustrated diagrammatically. No difficulty whatever is found in
+operating a motor in the manner indicated, or in similar ways; and
+although such experiments up to this day present only scientific
+interest, they may at a period not far distant, be carried out with
+practical objects in view.
+
+It is thought useful to devote here a few remarks to the subject of
+operating devices of all kinds by means of only one leading wire. It is
+quite obvious, that when high-frequency currents are made use of, ground
+connections are--at least when the E. M. F. of the currents is
+great--better than a return wire. Such ground connections are
+objectionable with steady or low frequency currents on account of
+destructive chemical actions of the former and disturbing influences
+exerted by both on the neighboring circuits; but with high frequencies
+these actions practically do not exist. Still, even ground connections
+become superfluous when the E. M. F. is very high, for soon a condition
+is reached, when the current may be passed more economically through
+open, than through closed, conductors. Remote as might seem an
+industrial application of such single wire transmission of energy to one
+not experienced in such lines of experiment, it will not seem so to
+anyone who for some time has carried on investigations of such nature.
+Indeed I cannot see why such a plan should not be practicable. Nor
+should it be thought that for carrying out such a plan currents of very
+high frequency are expressly required, for just as soon as potentials of
+say 30,000 volts are used, the single wire transmission may be effected
+with low frequencies, and experiments have been made by me from which
+these inferences are made.
+
+When the frequencies are very high it has been found in laboratory
+practice quite easy to regulate the effects in the manner shown in
+diagram Fig. 181. Here two primaries P and P_{1} are shown, each
+connected with one of its ends to the line L and with the other end to
+the condenser plates C and C, respectively. Near these are placed other
+condenser plates C_{1} and C_{1}, the former being connected to the line
+L and the latter to an insulated larger plate P_{2}. On the primaries
+are wound secondaries S and S_{1}, of coarse wire, connected to the
+devices _d_ and _l_ respectively. By varying the distances of the
+condenser plates C and C_{1}, and C and C_{1} the currents through the
+secondaries S and S_{1} are varied in intensity. The curious feature is
+the great sensitiveness, the slightest change in the distance of the
+plates producing considerable variations in the intensity or strength of
+the currents. The sensitiveness may be rendered extreme by making the
+frequency such, that the primary itself, without any plate attached to
+its free end, satisfies, in conjunction with the closed secondary, the
+condition of resonance. In such condition an extremely small change in
+the capacity of the free terminal produces great variations. For
+instance, I have been able to adjust the conditions so that the mere
+approach of a person to the coil produces a considerable change in the
+brightness of the lamps attached to the secondary. Such observations and
+experiments possess, of course, at present, chiefly scientific interest,
+but they may soon become of practical importance.
+
+Very high frequencies are of course not practicable with motors on
+account of the necessity of employing iron cores. But one may use sudden
+discharges of low frequency and thus obtain certain advantages of
+high-frequency currents without rendering the iron core entirely
+incapable of following the changes and without entailing a very great
+expenditure of energy in the core. I have found it quite practicable to
+operate with such low frequency disruptive discharges of condensers,
+alternating-current motors. A certain class of such motors which I
+advanced a few years ago, which contain closed secondary circuits, will
+rotate quite vigorously when the discharges are directed through the
+exciting coils. One reason that such a motor operates so well with these
+discharges is that the difference of phase between the primary and
+secondary currents is 90 degrees, which is generally not the case with
+harmonically rising and falling currents of low frequency. It might not
+be without interest to show an experiment with a simple motor of this
+kind, inasmuch as it is commonly thought that disruptive discharges are
+unsuitable for such purposes. The motor is illustrated in Fig. 182. It
+comprises a rather large iron core _i_ with slots on the top into which
+are embedded thick copper washers _c c_. In proximity to the core is a
+freely-movable metal disc D. The core is provided with a primary
+exciting coil C_{1} the ends _a_ and _b_ of which are connected to the
+terminals of the secondary S of an ordinary transformer, the primary P
+of the latter being connected to an alternating distribution circuit or
+generator G of low or moderate frequency. The terminals of the secondary
+S are attached to a condenser C which discharges through an air gap _d
+d_ which may be placed in series or shunt to the coil C_{1}. When the
+conditions are properly chosen the disc D rotates with considerable
+effort and the iron core _i_ does not get very perceptibly hot. With
+currents from a high-frequency alternator, on the contrary, the core
+gets rapidly hot and the disc rotates with a much smaller effort. To
+perform the experiment properly it should be first ascertained that the
+disc D is not set in rotation when the discharge is not occurring at _d
+d_. It is preferable to use a large iron core and a condenser of large
+capacity so as to bring the superimposed quicker oscillation to a very
+low pitch or to do away with it entirely. By observing certain
+elementary rules I have also found it practicable to operate ordinary
+series or shunt direct-current motors with such disruptive discharges,
+and this can be done with or without a return wire.
+
+
+IMPEDANCE PHENOMENA.
+
+Among the various current phenomena observed, perhaps the most
+interesting are those of impedance presented by conductors to currents
+varying at a rapid rate. In my first paper before the American Institute
+of Electrical Engineers, I have described a few striking observations of
+this kind. Thus I showed that when such currents or sudden discharges
+are passed through a thick metal bar there may be points on the bar only
+a few inches apart, which have a sufficient potential difference between
+them to maintain at bright incandescence an ordinary filament lamp. I
+have also described the curious behavior of rarefied gas surrounding a
+conductor, due to such sudden rushes of current. These phenomena have
+since been more carefully studied and one or two novel experiments of
+this kind are deemed of sufficient interest to be described here.
+
+Referring to Fig. 183_a_, B and B_{1} are very stout copper bars
+connected at their lower ends to plates C and C_{1}, respectively, of a
+condenser, the opposite plates of the latter being connected to the
+terminals of the secondary S of a high-tension transformer, the primary
+P of which is supplied with alternating currents from an ordinary
+low-frequency dynamo G or distribution circuit. The condenser
+discharges through an adjustable gap _d d_ as usual. By establishing a
+rapid vibration it was found quite easy to perform the following curious
+experiment. The bars B and B_{1} were joined at the top by a low-voltage
+lamp l_{3}; a little lower was placed by means of clamps _c c_, a
+50-volt lamp l_{2}; and still lower another 100-volt lamp l_{1}; and
+finally, at a certain distance below the latter lamp, an exhausted tube
+T. By carefully determining the positions of these devices it was found
+practicable to maintain them all at their proper illuminating power. Yet
+they were all connected in multiple arc to the two stout copper bars and
+required widely different pressures. This experiment requires of course
+some time for adjustment but is quite easily performed.
+
+[Illustration: FIGS. 183a, 183b and 183c.]
+
+In Figs. 183_b_ and 183_c_, two other experiments are illustrated which,
+unlike the previous experiment, do not require very careful adjustments.
+In Fig. 183_b_, two lamps, l_{1} and l_{2}, the former a 100-volt
+and the latter a 50-volt are placed in certain positions as indicated,
+the 100-volt lamp being below the 50-volt lamp. When the arc is playing
+at _d d_ and the sudden discharges are passed through the bars B B_{1},
+the 50-volt lamp will, as a rule, burn brightly, or at least this result
+is easily secured, while the 100-volt lamp will burn very low or remain
+quite dark, Fig. 183_b_. Now the bars B B_{1} may be joined at the top
+by a thick cross bar B_{2} and it is quite easy to maintain the 100-volt
+lamp at full candle-power while the 50-volt lamp remains dark, Fig.
+183_c_. These results, as I have pointed out previously, should not be
+considered to be due exactly to frequency but rather to the time rate of
+change which may be great, even with low frequencies. A great many other
+results of the same kind, equally interesting, especially to those who
+are only used to manipulate steady currents, may be obtained and they
+afford precious clues in investigating the nature of electric currents.
+
+In the preceding experiments I have already had occasion to show some
+light phenomena and it would now be proper to study these in particular;
+but to make this investigation more complete I think it necessary to
+make first a few remarks on the subject of electrical resonance which
+has to be always observed in carrying out these experiments.
+
+
+ON ELECTRICAL RESONANCE.
+
+The effects of resonance are being more and more noted by engineers and
+are becoming of great importance in the practical operation of apparatus
+of all kinds with alternating currents. A few general remarks may
+therefore be made concerning these effects. It is clear, that if we
+succeed in employing the effects of resonance practically in the
+operation of electric devices the return wire will, as a matter of
+course, become unnecessary, for the electric vibration may be conveyed
+with one wire just as well as, and sometimes even better than, with two.
+The question first to answer is, then, whether pure resonance effects
+are producible. Theory and experiment both show that such is impossible
+in Nature, for as the oscillation becomes more and more vigorous, the
+losses in the vibrating bodies and environing media rapidly increase and
+necessarily check the vibration which otherwise would go on increasing
+forever. It is a fortunate circumstance that pure resonance is not
+producible, for if it were there is no telling what dangers might not
+lie in wait for the innocent experimenter. But to a certain degree
+resonance is producible, the magnitude of the effects being limited by
+the imperfect conductivity and imperfect elasticity of the media or,
+generally stated, by frictional losses. The smaller these losses, the
+more striking are the effects. The same is the case in mechanical
+vibration. A stout steel bar may be set in vibration by drops of water
+falling upon it at proper intervals; and with glass, which is more
+perfectly elastic, the resonance effect is still more remarkable, for a
+goblet may be burst by singing into it a note of the proper pitch. The
+electrical resonance is the more perfectly attained, the smaller the
+resistance or the impedance of the conducting path and the more perfect
+the dielectric. In a Leyden jar discharging through a short stranded
+cable of thin wires these requirements are probably best fulfilled, and
+the resonance effects are therefore very prominent. Such is not the case
+with dynamo machines, transformers and their circuits, or with
+commercial apparatus in general in which the presence of iron cores
+complicates the action or renders it impossible. In regard to Leyden
+jars with which resonance effects are frequently demonstrated, I would
+say that the effects observed are often _attributed_ but are seldom
+_due_ to true resonance, for an error is quite easily made in this
+respect. This may be undoubtedly demonstrated by the following
+experiment. Take, for instance, two large insulated metallic plates or
+spheres which I shall designate A and B; place them at a certain small
+distance apart and charge them from a frictional or influence machine to
+a potential so high that just a slight increase of the difference of
+potential between them will cause the small air or insulating space to
+break down. This is easily reached by making a few preliminary trials.
+If now another plate--fastened on an insulating handle and connected by
+a wire to one of the terminals of a high tension secondary of an
+induction coil, which is maintained in action by an alternator
+(preferably high frequency)--is approached to one of the charged bodies
+A or B, so as to be nearer to either one of them, the discharge will
+invariably occur between them; at least it will, if the potential of the
+coil in connection with the plate is sufficiently high. But the
+explanation of this will soon be found in the fact that the approached
+plate acts inductively upon the bodies A and B and causes a spark to
+pass between them. When this spark occurs, the charges which were
+previously imparted to these bodies from the influence machine, must
+needs be lost, since the bodies are brought in electrical connection
+through the arc formed. Now this arc is formed whether there be
+resonance or not. But even if the spark would not be produced, still
+there is an alternating E. M. F. set up between the bodies when the
+plate is brought near one of them; therefore the approach of the plate,
+if it _does_ not always actually, will, at any rate, _tend_ to break
+down the air space by inductive action. Instead of the spheres or plates
+A and B we may take the coatings of a Leyden jar with the same result,
+and in place of the machine,--which is a high frequency alternator
+preferably, because it is more suitable for the experiment and also for
+the argument,--we may take another Leyden jar or battery of jars. When
+such jars are discharging through a circuit of low resistance the same
+is traversed by currents of very high frequency. The plate may now be
+connected to one of the coatings of the second jar, and when it is
+brought near to the first jar just previously charged to a high
+potential from an influence machine, the result is the same as before,
+and the first jar will discharge through a small air space upon the
+second being caused to discharge. But both jars and their circuits need
+not be tuned any closer than a basso profundo is to the note produced by
+a mosquito, as small sparks will be produced through the air space, or
+at least the latter will be considerably more strained owing to the
+setting up of an alternating E. M. F. by induction, which takes place
+when one of the jars begins to discharge. Again another error of a
+similar nature is quite easily made. If the circuits of the two jars are
+run parallel and close together, and the experiment has been performed
+of discharging one by the other, and now a coil of wire be added to one
+of the circuits whereupon the experiment does not succeed, the
+conclusion that this is due to the fact that the circuits are now not
+tuned, would be far from being safe. For the two circuits act as
+condenser coatings and the addition of the coil to one of them is
+equivalent to bridging them, at the point where the coil is placed, by a
+small condenser, and the effect of the latter might be to prevent the
+spark from jumping through the discharge space by diminishing the
+alternating E. M. F. acting across the same. All these remarks, and many
+more which might be added but for fear of wandering too far from the
+subject, are made with the pardonable intention of cautioning the
+unsuspecting student, who might gain an entirely unwarranted opinion of
+his skill at seeing every experiment succeed; but they are in no way
+thrust upon the experienced as novel observations.
+
+In order to make reliable observations of electric resonance effects it
+is very desirable, if not necessary, to employ an alternator giving
+currents which rise and fall harmonically, as in working with make and
+break currents the observations are not always trustworthy, since many
+phenomena, which depend on the rate of change, may be produced with
+widely different frequencies. Even when making such observations with an
+alternator one is apt to be mistaken. When a circuit is connected to an
+alternator there are an indefinite number of values for capacity and
+self-induction which, in conjunction, will satisfy the condition of
+resonance. So there are in mechanics an infinite number of tuning forks
+which will respond to a note of a certain pitch, or loaded springs which
+have a definite period of vibration. But the resonance will be most
+perfectly attained in that case in which the motion is effected with the
+greatest freedom. Now in mechanics, considering the vibration in the
+common medium--that is, air--it is of comparatively little importance
+whether one tuning fork be somewhat larger than another, because the
+losses in the air are not very considerable. One may, of course, enclose
+a tuning fork in an exhausted vessel and by thus reducing the air
+resistance to a minimum obtain better resonant action. Still the
+difference would not be very great. But it would make a great difference
+if the tuning fork were immersed in mercury. In the electrical vibration
+it is of enormous importance to arrange the conditions so that the
+vibration is effected with the greatest freedom. The magnitude of the
+resonance effect depends, under otherwise equal conditions, on the
+quantity of electricity set in motion or on the strength of the current
+driven through the circuit. But the circuit opposes the passage of the
+currents by reason of its impedance and therefore, to secure the best
+action it is necessary to reduce the impedance to a minimum. It is
+impossible to overcome it entirely, but merely in part, for the ohmic
+resistance cannot be overcome. But when the frequency of the impulses is
+very great, the flow of the current is practically determined by
+self-induction. Now self-induction can be overcome by combining it with
+capacity. If the relation between these is such, that at the frequency
+used they annul each other, that is, have such values as to satisfy the
+condition of resonance, and the greatest quantity of electricity is made
+to flow through the external circuit, then the best result is obtained.
+It is simpler and safer to join the condenser in series with the
+self-induction. It is clear that in such combinations there will be,
+for a given frequency, and considering only the fundamental vibration,
+values which will give the best result, with the condenser in shunt to
+the self-induction coil; of course more such values than with the
+condenser in series. But practical conditions determine the selection.
+In the latter case in performing the experiments one may take a small
+self-induction and a large capacity or a small capacity and a large
+self-induction, but the latter is preferable, because it is inconvenient
+to adjust a large capacity by small steps. By taking a coil with a very
+large self-induction the critical capacity is reduced to a very small
+value, and the capacity of the coil itself may be sufficient. It is
+easy, especially by observing certain artifices, to wind a coil through
+which the impedance will be reduced to the value of the ohmic resistance
+only; and for any coil there is, of course, a frequency at which the
+maximum current will be made to pass through the coil. The observation
+of the relation between self-induction, capacity and frequency is
+becoming important in the operation of alternate current apparatus, such
+as transformers or motors, because by a judicious determination of the
+elements the employment of an expensive condenser becomes unnecessary.
+Thus it is possible to pass through the coils of an alternating current
+motor under the normal working conditions the required current with a
+low E. M. F. and do away entirely with the false current, and the larger
+the motor, the easier such a plan becomes practicable; but it is
+necessary for this to employ currents of very high potential or high
+frequency.
+
+[Illustration: FIG. 184.]
+
+In Fig. 184 I. is shown a plan which has been followed in the study of
+the resonance effects by means of a high frequency alternator. C_{1} is
+a coil of many turns, which is divided into small separate sections for
+the purpose of adjustment. The final adjustment was made sometimes with
+a few thin iron wires (though this is not always advisable) or with a
+closed secondary. The coil C_{1} is connected with one of its ends to
+the line L from the alternator G and with the other end to one of the
+plates _c_ of a condenser c c_{1}, the plate (c_{1}) of the latter
+being connected to a much larger plate P_{1}. In this manner both
+capacity and self-induction were adjusted to suit the dynamo frequency.
+
+As regards the rise of potential through resonant action, of course,
+theoretically, it may amount to anything since it depends on
+self-induction and resistance and since these may have any value. But in
+practice one is limited in the selection of these values and besides
+these, there are other limiting causes. One may start with, say, 1,000
+volts and raise the E. M. F. to 50 times that value, but one cannot
+start with 100,000 and raise it to ten times that value because of the
+losses in the media which are great, especially if the frequency is
+high. It should be possible to start with, for instance, two volts from
+a high or low frequency circuit of a dynamo and raise the E. M. F. to
+many hundred times that value. Thus coils of the proper dimensions might
+be connected each with only one of its ends to the mains from a machine
+of low E. M. F., and though the circuit of the machine would not be
+closed in the ordinary acceptance of the term, yet the machine might be
+burned out if a proper resonance effect would be obtained. I have not
+been able to produce, nor have I observed with currents from a dynamo
+machine, such great rises of potential. It is possible, if not probable,
+that with currents obtained from apparatus containing iron the
+disturbing influence of the latter is the cause that these theoretical
+possibilities cannot be realized. But if such is the case I attribute it
+solely to the hysteresis and Foucault current losses in the core.
+Generally it was necessary to transform upward, when the E. M. F. was
+very low, and usually an ordinary form of induction coil was employed,
+but sometimes the arrangement illustrated in Fig. 184 II., has been
+found to be convenient. In this case a coil C is made in a great many
+sections, a few of these being used as a primary. In this manner both
+primary and secondary are adjustable. One end of the coil is connected
+to the line L_{1} from the alternator, and the other line L is connected
+to the intermediate point of the coil. Such a coil with adjustable
+primary and secondary will be found also convenient in experiments with
+the disruptive discharge. When true resonance is obtained the top of the
+wave must of course be on the free end of the coil as, for instance, at
+the terminal of the phosphorescence bulb B. This is easily recognized
+by observing the potential of a point on the wire _w_ near to the coil.
+
+In connection with resonance effects and the problem of transmission of
+energy over a single conductor which was previously considered, I would
+say a few words on a subject which constantly fills my thoughts and
+which concerns the welfare of all. I mean the transmission of
+intelligible signals or perhaps even power to any distance without the
+use of wires. I am becoming daily more convinced of the practicability
+of the scheme; and though I know full well that the great majority of
+scientific men will not believe that such results can be practically and
+immediately realized, yet I think that all consider the developments in
+recent years by a number of workers to have been such as to encourage
+thought and experiment in this direction. My conviction has grown so
+strong, that I no longer look upon this plan of energy or intelligence
+transmission as a mere theoretical possibility, but as a serious problem
+in electrical engineering, which must be carried out some day. The idea
+of transmitting intelligence without wires is the natural outcome of the
+most recent results of electrical investigations. Some enthusiasts have
+expressed their belief that telephony to any distance by induction
+through the air is possible. I cannot stretch my imagination so far, but
+I do firmly believe that it is practicable to disturb by means of
+powerful machines the electrostatic condition of the earth and thus
+transmit intelligible signals and perhaps power. In fact, what is there
+against the carrying out of such a scheme? We now know that electric
+vibration may be transmitted through a single conductor. Why then not
+try to avail ourselves of the earth for this purpose? We need not be
+frightened by the idea of distance. To the weary wanderer counting the
+mile-posts the earth may appear very large, but to that happiest of all
+men, the astronomer, who gazes at the heavens and by their standard
+judges the magnitude of our globe, it appears very small. And so I think
+it must seem to the electrician, for when he considers the speed with
+which an electric disturbance is propagated through the earth all his
+ideas of distance must completely vanish.
+
+A point of great importance would be first to know what is the capacity
+of the earth? and what charge does it contain if electrified? Though we
+have no positive evidence of a charged body existing in space without
+other oppositely electrified bodies being near, there is a fair
+probability that the earth is such a body, for by whatever process it
+was separated from other bodies--and this is the accepted view of its
+origin--it must have retained a charge, as occurs in all processes of
+mechanical separation. If it be a charged body insulated in space its
+capacity should be extremely small, less than one-thousandth of a farad.
+But the upper strata of the air are conducting, and so, perhaps, is the
+medium in free space beyond the atmosphere, and these may contain an
+opposite charge. Then the capacity might be incomparably greater. In any
+case it is of the greatest importance to get an idea of what quantity of
+electricity the earth contains. It is difficult to say whether we shall
+ever acquire this necessary knowledge, but there is hope that we may,
+and that is, by means of electrical resonance. If ever we can ascertain
+at what period the earth's charge, when disturbed, oscillates with
+respect to an oppositely electrified system or known circuit, we shall
+know a fact possibly of the greatest importance to the welfare of the
+human race. I propose to seek for the period by means of an electrical
+oscillator, or a source of alternating electric currents. One of the
+terminals of the source would be connected to earth as, for instance, to
+the city water mains, the other to an insulated body of large surface.
+It is possible that the outer conducting air strata, or free space,
+contain an opposite charge and that, together with the earth, they form
+a condenser of very large capacity. In such case the period of vibration
+may be very low and an alternating dynamo machine might serve for the
+purpose of the experiment. I would then transform the current to a
+potential as high as it would be found possible and connect the ends of
+the high tension secondary to the ground and to the insulated body. By
+varying the frequency of the currents and carefully observing the
+potential of the insulated body and watching for the disturbance at
+various neighboring points of the earth's surface resonance might be
+detected. Should, as the majority of scientific men in all probability
+believe, the period be extremely small, then a dynamo machine would not
+do and a proper electrical oscillator would have to be produced and
+perhaps it might not be possible to obtain such rapid vibrations. But
+whether this be possible or not, and whether the earth contains a charge
+or not, and whatever may be its period of vibration, it certainly is
+possible--for of this we have daily evidence--to produce some electrical
+disturbance sufficiently powerful to be perceptible by suitable
+instruments at any point of the earth's surface.
+
+[Illustration: FIG. 185.]
+
+Assume that a source of alternating current S be connected, as in Fig.
+185, with one of its terminals to earth (conveniently to the water
+mains) and with the other to a body of large surface P. When the
+electric oscillation is set up there will be a movement of electricity
+in and out of P, and alternating currents will pass through the earth,
+converging to, or diverging from, the point C where the ground
+connection is made. In this manner neighboring points on the earth's
+surface within a certain radius will be disturbed. But the disturbance
+will diminish with the distance, and the distance at which the effect
+will still be perceptible will depend on the quantity of electricity set
+in motion. Since the body P is insulated, in order to displace a
+considerable quantity, the potential of the source must be excessive,
+since there would be limitations as to the surface of P. The conditions
+might be adjusted so that the generator or source S will set up the same
+electrical movement as though its circuit were closed. Thus it is
+certainly practicable to impress an electric vibration at least of a
+certain low period upon the earth by means of proper machinery. At what
+distance such a vibration might be made perceptible can only be
+conjectured. I have on another occasion considered the question how the
+earth might behave to electric disturbances. There is no doubt that,
+since in such an experiment the electrical density at the surface could
+be but extremely small considering the size of the earth, the air would
+not act as a very disturbing factor, and there would be not much energy
+lost through the action of the air, which would be the case if the
+density were great. Theoretically, then, it could not require a great
+amount of energy to produce a disturbance perceptible at great distance,
+or even all over the surface of the globe. Now, it is quite certain that
+at any point within a certain radius of the source S a properly adjusted
+self-induction and capacity device can be set in action by resonance.
+But not only can this be done, but another source S_{1}, Fig. 185,
+similar to S, or any number of such sources, can be set to work in
+synchronism with the latter, and the vibration thus intensified and
+spread over a large area, or a flow of electricity produced to or from
+the source S_{1} if the same be of opposite phase to the source S. I
+think that beyond doubt it is possible to operate electrical devices in
+a city through the ground or pipe system by resonance from an electrical
+oscillator located at a central point. But the practical solution of
+this problem would be of incomparably smaller benefit to man than the
+realization of the scheme of transmitting intelligence, or perhaps
+power, to any distance through the earth or environing medium. If this
+is at all possible, distance does not mean anything. Proper apparatus
+must first be produced by means of which the problem can be attacked and
+I have devoted much thought to this subject. I am firmly convinced that
+it can be done and hope that we shall live to see it done.
+
+
+ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS OF HIGH
+POTENTIAL AND GENERAL REMARKS RELATING TO THE SUBJECT.
+
+Returning now to the light effects which it has been the chief object to
+investigate, it is thought proper to divide these effects into four
+classes: 1. Incandescence of a solid. 2. Phosphorescence. 3.
+Incandescence or phosphorescence of a rarefied gas; and 4. Luminosity
+produced in a gas at ordinary pressure. The first question is: How are
+these luminous effects produced? In order to answer this question as
+satisfactorily as I am able to do in the light of accepted views and
+with the experience acquired, and to add some interest to this
+demonstration, I shall dwell here upon a feature which I consider of
+great importance, inasmuch as it promises, besides, to throw a better
+light upon the nature of most of the phenomena produced by
+high-frequency electric currents. I have on other occasions pointed out
+the great importance of the presence of the rarefied gas, or atomic
+medium in general, around the conductor through which alternate currents
+of high frequency are passed, as regards the heating of the conductor by
+the currents. My experiments, described some time ago, have shown that,
+the higher the frequency and potential difference of the currents, the
+more important becomes the rarefied gas in which the conductor is
+immersed, as a factor of the heating. The potential difference, however,
+is, as I then pointed out, a more important element than the frequency.
+When both of these are sufficiently high, the heating may be almost
+entirely due to the presence of the rarefied gas. The experiments to
+follow will show the importance of the rarefied gas, or, generally, of
+gas at ordinary or other pressure as regards the incandescence or other
+luminous effects produced by currents of this kind.
+
+I take two ordinary 50-volt 16 C. P. lamps which are in every respect
+alike, with the exception, that one has been opened at the top and the
+air has filled the bulb, while the other is at the ordinary degree of
+exhaustion of commercial lamps. When I attach the lamp which is
+exhausted to the terminal of the secondary of the coil, which I have
+already used, as in experiments illustrated in Fig. 179_a_ for instance,
+and turn on the current, the filament, as you have before seen, comes to
+high incandescence. When I attach the second lamp, which is filled with
+air, instead of the former, the filament still glows, but much less
+brightly. This experiment illustrates only in part the truth of the
+statements before made. The importance of the filament's being immersed
+in rarefied gas is plainly noticeable but not to such a degree as might
+be desirable. The reason is that the secondary of this coil is wound for
+low tension, having only 150 turns, and the potential difference at the
+terminals of the lamp is therefore small. Were I to take another coil
+with many more turns in the secondary, the effect would be increased,
+since it depends partially on the potential difference, as before
+remarked. But since the effect likewise depends on the frequency, it
+maybe properly stated that it depends on the time rate of the variation
+of the potential difference. The greater this variation, the more
+important becomes the gas as an element of heating. I can produce a much
+greater rate of variation in another way, which, besides, has the
+advantage of doing away with the objections, which might be made in the
+experiment just shown, even if both the lamps were connected in series
+or multiple arc to the coil, namely, that in consequence of the
+reactions existing between the primary and secondary coil the
+conclusions are rendered uncertain. This result I secure by charging,
+from an ordinary transformer which is fed from the alternating current
+supply station, a battery of condensers, and discharging the latter
+directly through a circuit of small self-induction, as before
+illustrated in Figs. 183_a_, 183_b_, and 183_c_.
+
+[Illustration: FIG. 186a.]
+
+[Illustration: FIG. 186b.]
+
+[Illustration: FIG. 186c.]
+
+In Figs. 186_a_, 186_b_ and 186_c_, the heavy copper bars B B_{1}, are
+connected to the opposite coatings of a battery of condensers, or
+generally in such way, that the high frequency or sudden discharges are
+made to traverse them. I connect first an ordinary 50-volt incandescent
+lamp to the bars by means of the clamps _c c_. The discharges being
+passed through the lamp, the filament is rendered incandescent, though
+the current through it is very small, and would not be nearly sufficient
+to produce a visible effect under the conditions of ordinary use of the
+lamp. Instead of this I now attach to the bars another lamp exactly like
+the first, but with the seal broken off, the bulb being therefore filled
+with air at ordinary pressure. When the discharges are directed through
+the filament, as before, it does not become incandescent. But the result
+might still be attributed to one of the many possible reactions. I
+therefore connect both the lamps in multiple arc as illustrated in Fig.
+186_a_. Passing the discharges through both the lamps, again the
+filament in the exhausted lamp _l_ glows very brightly while that in the
+non-exhausted lamp l_{1} remains dark, as previously. But it should
+not be thought that the latter lamp is taking only a small fraction of
+the energy supplied to both the lamps; on the contrary, it may consume a
+considerable portion of the energy and it may become even hotter than
+the one which burns brightly. In this experiment the potential
+difference at the terminals of the lamps varies in sign theoretically
+three to four million times a second. The ends of the filaments are
+correspondingly electrified, and the gas in the bulbs is violently
+agitated and a large portion of the supplied energy is thus converted
+into heat. In the non-exhausted bulb, there being a few million times
+more gas molecules than in the exhausted one, the bombardment, which is
+most violent at the ends of the filament, in the neck of the bulb,
+consumes a large portion of the energy without producing any visible
+effect. The reason is that, there being many molecules, the bombardment
+is quantitatively considerable, but the individual impacts are not very
+violent, as the speeds of the molecules are comparatively small owing to
+the small free path. In the exhausted bulb, on the contrary, the speeds
+are very great, and the individual impacts are violent and therefore
+better adapted to produce a visible effect. Besides, the convection of
+heat is greater in the former bulb. In both the bulbs the current
+traversing the filaments is very small, incomparably smaller than that
+which they require on an ordinary low-frequency circuit. The potential
+difference, however, at the ends of the filaments is very great and
+might be possibly 20,000 volts or more, if the filaments were straight
+and their ends far apart. In the ordinary lamp a spark generally occurs
+between the ends of the filament or between the platinum wires outside,
+before such a difference of potential can be reached.
+
+It might be objected that in the experiment before shown the lamps,
+being in multiple arc, the exhausted lamp might take a much larger
+current and that the effect observed might not be exactly attributable
+to the action of the gas in the bulbs. Such objections will lose much
+weight if I connect the lamps in series, with the same result. When this
+is done and the discharges are directed through the filaments, it is
+again noted that the filament in the non-exhausted bulb l_{1}, remains
+dark, while that in the exhausted one (_l_) glows even more intensely
+than under its normal conditions of working, Fig. 186_b_. According to
+general ideas the current through the filaments should now be the same,
+were it not modified by the presence of the gas around the filaments.
+
+At this juncture I may point out another interesting feature, which
+illustrates the effect of the rate of change of potential of the
+currents. I will leave the two lamps connected in series to the bars
+B B_{1}, as in the previous experiment, Fig. 186_b_, but will presently
+reduce considerably the frequency of the currents, which was excessive
+in the experiment just before shown. This I may do by inserting a
+self-induction coil in the path of the discharges, or by augmenting the
+capacity of the condensers. When I now pass these low-frequency
+discharges through the lamps, the exhausted lamp _l_ again is as bright
+as before, but it is noted also that the non-exhausted lamp l_{1}
+glows, though not quite as intensely as the other. Reducing the current
+through the lamps, I may bring the filament in the latter lamp to
+redness, and, though the filament in the exhausted lamp _l_ is bright,
+Fig. 186_c_, the degree of its incandescence is much smaller than in
+Fig. 186_b_, when the currents were of a much higher frequency.
+
+In these experiments the gas acts in two opposite ways in determining
+the degree of the incandescence of the filaments, that is, by convection
+and bombardment. The higher the frequency and potential of the currents,
+the more important becomes the bombardment. The convection on the
+contrary should be the smaller, the higher the frequency. When the
+currents are steady there is practically no bombardment, and convection
+may therefore with such currents also considerably modify the degree of
+incandescence and produce results similar to those just before shown.
+Thus, if two lamps exactly alike, one exhausted and one not exhausted,
+are connected in multiple arc or series to a direct-current machine, the
+filament in the non-exhausted lamp will require a considerably greater
+current to be rendered incandescent. This result is entirely due to
+convection, and the effect is the more prominent the thinner the
+filament. Professor Ayrton and Mr. Kilgour some time ago published
+quantitative results concerning the thermal emissivity by radiation and
+convection in which the effect with thin wires was clearly shown. This
+effect may be strikingly illustrated by preparing a number of small,
+short, glass tubes, each containing through its axis the thinnest
+obtainable platinum wire. If these tubes be highly exhausted, a number
+of them may be connected in multiple arc to a direct-current machine and
+all of the wires may be kept at incandescence with a smaller current
+than that required to render incandescent a single one of the wires if
+the tube be not exhausted. Could the tubes be so highly exhausted that
+convection would be nil, then the relative amounts of heat given off by
+convection and radiation could be determined without the difficulties
+attending thermal quantitative measurements. If a source of electric
+impulses of high frequency and very high potential is employed, a still
+greater number of the tubes may be taken and the wires rendered
+incandescent by a current not capable of warming perceptibly a wire of
+the same size immersed in air at ordinary pressure, and conveying the
+energy to all of them.
+
+I may here describe a result which is still more interesting, and to
+which I have been led by the observation of these phenomena. I noted
+that small differences in the density of the air produced a considerable
+difference in the degree of incandescence of the wires, and I thought
+that, since in a tube, through which a luminous discharge is passed, the
+gas is generally not of uniform density, a very thin wire contained in
+the tube might be rendered incandescent at certain places of smaller
+density of the gas, while it would remain dark at the places of greater
+density, where the convection would be greater and the bombardment less
+intense. Accordingly a tube _t_ was prepared, as illustrated in Fig.
+187, which contained through the middle a very fine platinum wire _w_.
+The tube was exhausted to a moderate degree and it was found that when
+it was attached to the terminal of a high-frequency coil the platinum
+wire _w_ would indeed, become incandescent in patches, as illustrated in
+Fig. 187. Later a number of these tubes with one or more wires were
+prepared, each showing this result. The effect was best noted when the
+striated discharge occurred in the tube, but was also produced when the
+strić were not visible, showing that, even then, the gas in the tube was
+not of uniform density. The position of the strić was generally such,
+that the rarefactions corresponded to the places of incandescence or
+greater brightness on the wire _w_. But in a few instances it was noted,
+that the bright spots on the wire were covered by the dense parts of the
+striated discharge as indicated by _l_ in Fig. 187, though the effect
+was barely perceptible. This was explained in a plausible way by
+assuming that the convection was not widely different in the dense and
+rarefied places, and that the bombardment was greater on the dense
+places of the striated discharge. It is, in fact, often observed in
+bulbs, that under certain conditions a thin wire is brought to higher
+incandescence when the air is not too highly rarefied. This is the case
+when the potential of the coil is not high enough for the vacuum, but
+the result may be attributed to many different causes. In all cases this
+curious phenomenon of incandescence disappears when the tube, or rather
+the wire, acquires throughout a uniform temperature.
+
+[Illustration: FIG. 187.]
+
+[Illustration: FIG. 188.]
+
+Disregarding now the modifying effect of convection there are then two
+distinct causes which determine the incandescence of a wire or filament
+with varying currents, that is, conduction current and bombardment. With
+steady currents we have to deal only with the former of these two
+causes, and the heating effect is a minimum, since the resistance is
+least to steady flow. When the current is a varying one the resistance
+is greater, and hence the heating effect is increased. Thus if the rate
+of change of the current is very great, the resistance may increase to
+such an extent that the filament is brought to incandescence with
+inappreciable currents, and we are able to take a short and thick block
+of carbon or other material and bring it to bright incandescence with a
+current incomparably smaller than that required to bring to the same
+degree of incandescence an ordinary thin lamp filament with a steady or
+low frequency current. This result is important, and illustrates how
+rapidly our views on these subjects are changing, and how quickly our
+field of knowledge is extending. In the art of incandescent lighting, to
+view this result in one aspect only, it has been commonly considered as
+an essential requirement for practical success, that the lamp filament
+should be thin and of high resistance. But now we know that the
+resistance of the filament to the steady flow does not mean anything;
+the filament might as well be short and thick; for if it be immersed in
+rarefied gas it will become incandescent by the passage of a small
+current. It all depends on the frequency and potential of the currents.
+We may conclude from this, that it would be of advantage, so far as the
+lamp is considered, to employ high frequencies for lighting, as they
+allow the use of short and thick filaments and smaller currents.
+
+If a wire or filament be immersed in a homogeneous medium, all the
+heating is due to true conduction current, but if it be enclosed in an
+exhausted vessel the conditions are entirely different. Here the gas
+begins to act and the heating effect of the conduction current, as is
+shown in many experiments, may be very small compared with that of the
+bombardment. This is especially the case if the circuit is not closed
+and the potentials are of course very high. Suppose that a fine filament
+enclosed in an exhausted vessel be connected with one of its ends to the
+terminal of a high tension coil and with its other end to a large
+insulated plate. Though the circuit is not closed, the filament, as I
+have before shown, is brought to incandescence. If the frequency and
+potential be comparatively low, the filament is heated by the current
+passing _through it_. If the frequency and potential, and principally
+the latter, be increased, the insulated plate need be but very small, or
+may be done away with entirely; still the filament will become
+incandescent, practically all the heating being then due to the
+bombardment. A practical way of combining both the effects of conduction
+currents and bombardment is illustrated in Fig. 188, in which an
+ordinary lamp is shown provided with a very thin filament which has one
+of the ends of the latter connected to a shade serving the purpose of
+the insulated plate, and the other end to the terminal of a high tension
+source. It should not be thought that only rarefied gas is an important
+factor in the heating of a conductor by varying currents, but gas at
+ordinary pressure may become important, if the potential difference and
+frequency of the currents is excessive. On this subject I have already
+stated, that when a conductor is fused by a stroke of lightning, the
+current through it may be exceedingly small, not even sufficient to heat
+the conductor perceptibly, were the latter immersed in a homogeneous
+medium.
+
+From the preceding it is clear that when a conductor of high resistance
+is connected to the terminals of a source of high frequency currents of
+high potential, there may occur considerable dissipation of energy,
+principally at the ends of the conductor, in consequence of the action
+of the gas surrounding the conductor. Owing to this, the current through
+a section of the conductor at a point midway between its ends may be
+much smaller than through a section near the ends. Furthermore, the
+current passes principally through the outer portions of the conductor,
+but this effect is to be distinguished from the skin effect as
+ordinarily interpreted, for the latter would, or should, occur also in a
+continuous incompressible medium. If a great many incandescent lamps are
+connected in series to a source of such currents, the lamps at the ends
+may burn brightly, whereas those in the middle may remain entirely dark.
+This is due principally to bombardment, as before stated. But even if
+the currents be steady, provided the difference of potential is very
+great, the lamps at the end will burn more brightly than those in the
+middle. In such case there is no rhythmical bombardment, and the result
+is produced entirely by leakage. This leakage or dissipation into space
+when the tension is high, is considerable when incandescent lamps are
+used, and still more considerable with arcs, for the latter act like
+flames. Generally, of course, the dissipation is much smaller with
+steady, than with varying, currents.
+
+I have contrived an experiment which illustrates in an interesting
+manner the effect of lateral diffusion. If a very long tube is attached
+to the terminal of a high frequency coil, the luminosity is greatest
+near the terminal and falls off gradually towards the remote end. This
+is more marked if the tube is narrow.
+
+A small tube about one-half inch in diameter and twelve inches long
+(Fig. 189), has one of its ends drawn out into a fine fibre _f_ nearly
+three feet long. The tube is placed in a brass socket T which can be
+screwed on the terminal T_{1} of the induction coil. The discharge
+passing through the tube first illuminates the bottom of the same, which
+is of comparatively large section; but through the long glass fibre the
+discharge cannot pass. But gradually the rarefied gas inside becomes
+warmed and more conducting and the discharge spreads into the glass
+fibre. This spreading is so slow, that it may take half a minute or more
+until the discharge has worked through up to the top of the glass fibre,
+then presenting the appearance of a strongly luminous thin thread. By
+adjusting the potential at the terminal the light may be made to travel
+upwards at any speed. Once, however, the glass fibre is heated, the
+discharge breaks through its entire length instantly. The interesting
+point to be noted is that, the higher the frequency of the currents, or
+in other words, the greater relatively the lateral dissipation, at a
+slower rate may the light be made to propagate through the fibre. This
+experiment is best performed with a highly exhausted and freshly made
+tube. When the tube has been used for some time the experiment often
+fails. It is possible that the gradual and slow impairment of the vacuum
+is the cause. This slow propagation of the discharge through a very
+narrow glass tube corresponds exactly to the propagation of heat through
+a bar warmed at one end. The quicker the heat is carried away laterally
+the longer time it will take for the heat to warm the remote end. When
+the current of a low frequency coil is passed through the fibre from end
+to end, then the lateral dissipation is small and the discharge
+instantly breaks through almost without exception.
+
+[Illustration: FIG. 189.]
+
+[Illustration: FIG. 190.]
+
+After these experiments and observations which have shown the importance
+of the discontinuity or atomic structure of the medium and which will
+serve to explain, in a measure at least, the nature of the four kinds of
+light effects producible with these currents, I may now give you an
+illustration of these effects. For the sake of interest I may do this in
+a manner which to many of you might be novel. You have seen before that
+we may now convey the electric vibration to a body by means of a single
+wire or conductor of any kind. Since the human frame is conducting I
+may convey the vibration through my body.
+
+First, as in some previous experiments, I connect my body with one of
+the terminals of a high-tension transformer and take in my hand an
+exhausted bulb which contains a small carbon button mounted upon a
+platinum wire leading to the outside of the bulb, and the button is
+rendered incandescent as soon as the transformer is set to work (Fig.
+190). I may place a conducting shade on the bulb which serves to
+intensify the action, but is not necessary. Nor is it required that the
+button should be in conducting connection with the hand through a wire
+leading through the glass, for sufficient energy may be transmitted
+through the glass itself by inductive action to render the button
+incandescent.
+
+[Illustration: FIG. 191.]
+
+[Illustration: FIG. 192.]
+
+Next I take a highly exhausted bulb containing a strongly phosphorescent
+body, above which is mounted a small plate of aluminum on a platinum
+wire leading to the outside, and the currents flowing through my body
+excite intense phosphorescence in the bulb (Fig. 191). Next again I take
+in my hand a simple exhausted tube, and in the same manner the gas
+inside the tube is rendered highly incandescent or phosphorescent (Fig.
+192). Finally, I may take in my hand a wire, bare or covered with thick
+insulation, it is quite immaterial; the electrical vibration is so
+intense as to cover the wire with a luminous film (Fig. 193).
+
+[Illustration: FIG. 193.]
+
+[Illustration: FIG. 194.]
+
+[Illustration: FIG. 195.]
+
+A few words must now be devoted to each of these phenomena. In the first
+place, I will consider the incandescence of a button or of a solid in
+general, and dwell upon some facts which apply equally to all these
+phenomena. It was pointed out before that when a thin conductor, such as
+a lamp filament, for instance, is connected with one of its ends to the
+terminal of a transformer of high tension the filament is brought to
+incandescence partly by a conduction current and partly by bombardment.
+The shorter and thicker the filament the more important becomes the
+latter, and finally, reducing the filament to a mere button, all the
+heating must practically be attributed to the bombardment. So in the
+experiment before shown, the button is rendered incandescent by the
+rhythmical impact of freely movable small bodies in the bulb. These
+bodies may be the molecules of the residual gas, particles of dust or
+lumps torn from the electrode; whatever they are, it is certain that the
+heating of the button is essentially connected with the pressure of such
+freely movable particles, or of atomic matter in general in the bulb.
+The heating is the more intense the greater the number of impacts per
+second and the greater the energy of each impact. Yet the button would
+be heated also if it were connected to a source of a steady potential.
+In such a case electricity would be carried away from the button by the
+freely movable carriers or particles flying about, and the quantity of
+electricity thus carried away might be sufficient to bring the button to
+incandescence by its passage through the latter. But the bombardment
+could not be of great importance in such case. For this reason it would
+require a comparatively very great supply of energy to the button to
+maintain it at incandescence with a steady potential. The higher the
+frequency of the electric impulses the more economically can the button
+be maintained at incandescence. One of the chief reasons why this is so,
+is, I believe, that with impulses of very high frequency there is less
+exchange of the freely movable carriers around the electrode and this
+means, that in the bulb the heated matter is better confined to the
+neighborhood of the button. If a double bulb, as illustrated in Fig. 194
+be made, comprising a large globe B and a small one _b_, each containing
+as usual a filament _f_ mounted on a platinum wire w and w_{1}, it
+is found, that if the filaments _f f_ be exactly alike, it requires less
+energy to keep the filament in the globe _b_ at a certain degree of
+incandescence, than that in the globe B. This is due to the confinement
+of the movable particles around the button. In this case it is also
+ascertained, that the filament in the small globe _b_ is less
+deteriorated when maintained a certain length of time at incandescence.
+This is a necessary consequence of the fact that the gas in the small
+bulb becomes strongly heated and therefore a very good conductor, and
+less work is then performed on the button, since the bombardment becomes
+less intense as the conductivity of the gas increases. In this
+construction, of course, the small bulb becomes very hot and when it
+reaches an elevated temperature the convection and radiation on the
+outside increase. On another occasion I have shown bulbs in which this
+drawback was largely avoided. In these instances a very small bulb,
+containing a refractory button, was mounted in a large globe and the
+space between the walls of both was highly exhausted. The outer large
+globe remained comparatively cool in such constructions. When the large
+globe was on the pump and the vacuum between the walls maintained
+permanent by the continuous action of the pump, the outer globe would
+remain quite cold, while the button in the small bulb was kept at
+incandescence. But when the seal was made, and the button in the small
+bulb maintained incandescent some length of time, the large globe too
+would become warmed. From this I conjecture that if vacuous space (as
+Prof. Dewar finds) cannot convey heat, it is so merely in virtue of our
+rapid motion through space or, generally speaking, by the motion of the
+medium relatively to us, for a permanent condition could not be
+maintained without the medium being constantly renewed. A vacuum cannot,
+according to all evidence, be permanently maintained around a hot body.
+
+In these constructions, before mentioned, the small bulb inside would,
+at least in the first stages, prevent all bombardment against the outer
+large globe. It occurred to me then to ascertain how a metal sieve would
+behave in this respect, and several bulbs, as illustrated in Fig. 195,
+were prepared for this purpose. In a globe _b_, was mounted a thin
+filament _f_ (or button) upon a platinum wire _w_ passing through a
+glass stem and leading to the outside of the globe. The filament _f_ was
+surrounded by a metal sieve _s_. It was found in experiments with such
+bulbs that a sieve with wide meshes apparently did not in the slightest
+affect the bombardment against the globe _b_. When the vacuum was high,
+the shadow of the sieve was clearly projected against the globe and the
+latter would get hot in a short while. In some bulbs the sieve _s_ was
+connected to a platinum wire sealed in the glass. When this wire was
+connected to the other terminal of the induction coil (the E. M. F.
+being kept low in this case), or to an insulated plate, the bombardment
+against the outer globe _b_ was diminished. By taking a sieve with fine
+meshes the bombardment against the globe _b_ was always diminished, but
+even then if the exhaustion was carried very far, and when the potential
+of the transformer was very high, the globe _b_ would be bombarded and
+heated quickly, though no shadow of the sieve was visible, owing to the
+smallness of the meshes. But a glass tube or other continuous body
+mounted so as to surround the filament, did entirely cut off the
+bombardment and for a while the outer globe _b_ would remain perfectly
+cold. Of course when the glass tube was sufficiently heated the
+bombardment against the outer globe could be noted at once. The
+experiments with these bulbs seemed to show that the speeds of the
+projected molecules or particles must be considerable (though quite
+insignificant when compared with that of light), otherwise it would be
+difficult to understand how they could traverse a fine metal sieve
+without being affected, unless it were found that such small particles
+or atoms cannot be acted upon directly at measurable distances. In
+regard to the speed of the projected atoms, Lord Kelvin has recently
+estimated it at about one kilometre a second or thereabouts in an
+ordinary Crookes bulb. As the potentials obtainable with a disruptive
+discharge coil are much higher than with ordinary coils, the speeds
+must, of course, be much greater when the bulbs are lighted from such a
+coil. Assuming the speed to be as high as five kilometres and uniform
+through the whole trajectory, as it should be in a very highly exhausted
+vessel, then if the alternate electrifications of the electrode would be
+of a frequency of five million, the greatest distance a particle could
+get away from the electrode would be one millimetre, and if it could be
+acted upon directly at that distance, the exchange of electrode matter
+or of the atoms would be very slow and there would be practically no
+bombardment against the bulb. This at least should be so, if the action
+of an electrode upon the atoms of the residual gas would be such as upon
+electrified bodies which we can perceive. A hot body enclosed in an
+exhausted bulb produces always atomic bombardment, but a hot body has no
+definite rhythm, for its molecules perform vibrations of all kinds.
+
+If a bulb containing a button or filament be exhausted as high as is
+possible with the greatest care and by the use of the best artifices, it
+is often observed that the discharge cannot, at first, break through,
+but after some time, probably in consequence of some changes within the
+bulb, the discharge finally passes through and the button is rendered
+incandescent. In fact, it appears that the higher the degree of
+exhaustion the easier is the incandescence produced. There seem to be no
+other causes to which the incandescence might be attributed in such case
+except to the bombardment or similar action of the residual gas, or of
+particles of matter in general. But if the bulb be exhausted with the
+greatest care can these play an important part? Assume the vacuum in the
+bulb to be tolerably perfect, the great interest then centres in the
+question: Is the medium which pervades all space continuous or atomic?
+If atomic, then the heating of a conducting button or filament in an
+exhausted vessel might be due largely to ether bombardment, and then the
+heating of a conductor in general through which currents of high
+frequency or high potential are passed must be modified by the behavior
+of such medium; then also the skin effect, the apparent increase of the
+ohmic resistance, etc., admit, partially at least, of a different
+explanation.
+
+It is certainly more in accordance with many phenomena observed with
+high-frequency currents to hold that all space is pervaded with free
+atoms, rather than to assume that it is devoid of these, and dark and
+cold, for so it must be, if filled with a continuous medium, since in
+such there can be neither heat nor light. Is then energy transmitted by
+independent carriers or by the vibration of a continuous medium? This
+important question is by no means as yet positively answered. But most
+of the effects which are here considered, especially the light effects,
+incandescence, or phosphorescence, involve the presence of free atoms
+and would be impossible without these.
+
+In regard to the incandescence of a refractory button (or filament) in
+an exhausted receiver, which has been one of the subjects of this
+investigation, the chief experiences, which may serve as a guide in
+constructing such bulbs, may be summed up as follows: 1. The button
+should be as small as possible, spherical, of a smooth or polished
+surface, and of refractory material which withstands evaporation best.
+2. The support of the button should be very thin and screened by an
+aluminum and mica sheet, as I have described on another occasion. 3. The
+exhaustion of the bulb should be as high as possible. 4. The frequency
+of the currents should be as high as practicable. 5. The currents should
+be of a harmonic rise and fall, without sudden interruptions. 6. The
+heat should be confined to the button by inclosing the same in a small
+bulb or otherwise. 7. The space between the walls of the small bulb and
+the outer globe should be highly exhausted.
+
+Most of the considerations which apply to the incandescence of a solid
+just considered may likewise be applied to phosphorescence. Indeed, in
+an exhausted vessel the phosphorescence is, as a rule, primarily excited
+by the powerful beating of the electrode stream of atoms against the
+phosphorescent body. Even in many cases, where there is no evidence of
+such a bombardment, I think that phosphorescence is excited by violent
+impacts of atoms, which are not necessarily thrown off from the
+electrode but are acted upon from the same inductively through the
+medium or through chains of other atoms. That mechanical shocks play an
+important part in exciting phosphorescence in a bulb may be seen from
+the following experiment. If a bulb, constructed as that illustrated in
+Fig. 174, be taken and exhausted with the greatest care so that the
+discharge cannot pass, the filament _f_ acts by electrostatic induction
+upon the tube _t_ and the latter is set in vibration. If the tube _o_ be
+rather wide, about an inch or so, the filament may be so powerfully
+vibrated that whenever it hits the glass tube it excites
+phosphorescence. But the phosphorescence ceases when the filament comes
+to rest. The vibration can be arrested and again started by varying the
+frequency of the currents. Now the filament has its own period of
+vibration, and if the frequency of the currents is such that there is
+resonance, it is easily set vibrating, though the potential of the
+currents be small. I have often observed that the filament in the bulb
+is destroyed by such mechanical resonance. The filament vibrates as a
+rule so rapidly that it cannot be seen and the experimenter may at first
+be mystified. When such an experiment as the one described is carefully
+performed, the potential of the currents need be extremely small, and
+for this reason I infer that the phosphorescence is then due to the
+mechanical shock of the filament against the glass, just as it is
+produced by striking a loaf of sugar with a knife. The mechanical shock
+produced by the projected atoms is easily noted when a bulb containing a
+button is grasped in the hand and the current turned on suddenly. I
+believe that a bulb could be shattered by observing the conditions of
+resonance.
+
+In the experiment before cited it is, of course, open to say, that the
+glass tube, upon coming in contact with the filament, retains a charge
+of a certain sign upon the point of contact. If now the filament again
+touches the glass at the same point while it is oppositely charged, the
+charges equalize under evolution of light. But nothing of importance
+would be gained by such an explanation. It is unquestionable that the
+initial charges given to the atoms or to the glass play some part in
+exciting phosphorescence. So, for instance, if a phosphorescent bulb be
+first excited by a high frequency coil by connecting it to one of the
+terminals of the latter and the degree of luminosity be noted, and then
+the bulb be highly charged from a Holtz machine by attaching it
+preferably to the positive terminal of the machine, it is found that
+when the bulb is again connected to the terminal of the high frequency
+coil, the phosphorescence is far more intense. On another occasion I
+have considered the possibility of some phosphorescent phenomena in
+bulbs being produced by the incandescence of an infinitesimal layer on
+the surface of the phosphorescent body. Certainly the impact of the
+atoms is powerful enough to produce intense incandescence by the
+collisions, since they bring quickly to a high temperature a body of
+considerable bulk. If any such effect exists, then the best appliance
+for producing phosphorescence in a bulb, which we know so far, is a
+disruptive discharge coil giving an enormous potential with but few
+fundamental discharges, say 25-30 per second, just enough to produce a
+continuous impression upon the eye. It is a fact that such a coil
+excites phosphorescence under almost any condition and at all degrees of
+exhaustion, and I have observed effects which appear to be due to
+phosphorescence even at ordinary pressures of the atmosphere, when the
+potentials are extremely high. But if phosphorescent light is produced
+by the equalization of charges of electrified atoms (whatever this may
+mean ultimately), then the higher the frequency of the impulses or
+alternate electrifications, the more economical will be the light
+production. It is a long known and noteworthy fact that all the
+phosphorescent bodies are poor conductors of electricity and heat, and
+that all bodies cease to emit phosphorescent light when they are brought
+to a certain temperature. Conductors on the contrary do not possess this
+quality. There are but few exceptions to the rule. Carbon is one of
+them. Becquerel noted that carbon phosphoresces at a certain elevated
+temperature preceding the dark red. This phenomenon may be easily
+observed in bulbs provided with a rather large carbon electrode (say, a
+sphere of six millimetres diameter). If the current is turned on after a
+few seconds, a snow white film covers the electrode, just before it gets
+dark red. Similar effects are noted with other conducting bodies, but
+many scientific men will probably not attribute them to true
+phosphorescence. Whether true incandescence has anything to do with
+phosphorescence excited by atomic impact or mechanical shocks still
+remains to be decided, but it is a fact that all conditions, which tend
+to localize and increase the heating effect at the point of impact, are
+almost invariably the most favorable for the production of
+phosphorescence. So, if the electrode be very small, which is equivalent
+to saying in general, that the electric density is great; if the
+potential be high, and if the gas be highly rarefied, all of which
+things imply high speed of the projected atoms, or matter, and
+consequently violent impacts--the phosphorescence is very intense. If a
+bulb provided with a large and small electrode be attached to the
+terminal of an induction coil, the small electrode excites
+phosphorescence while the large one may not do so, because of the
+smaller electric density and hence smaller speed of the atoms. A bulb
+provided with a large electrode may be grasped with the hand while the
+electrode is connected to the terminal of the coil and it may not
+phosphoresce; but if instead of grasping the bulb with the hand, the
+same be touched with a pointed wire, the phosphorescence at once
+spreads through the bulb, because of the great density at the point of
+contact. With low frequencies it seems that gases of great atomic weight
+excite more intense phosphorescence than those of smaller weight, as for
+instance, hydrogen. With high frequencies the observations are not
+sufficiently reliable to draw a conclusion. Oxygen, as is well-known,
+produces exceptionally strong effects, which may be in part due to
+chemical action. A bulb with hydrogen residue seems to be most easily
+excited. Electrodes which are most easily deteriorated produce more
+intense phosphorescence in bulbs, but the condition is not permanent
+because of the impairment of the vacuum and the deposition of the
+electrode matter upon the phosphorescent surfaces. Some liquids, as
+oils, for instance, produce magnificent effects of phosphorescence (or
+fluorescence?), but they last only a few seconds. So if a bulb has a
+trace of oil on the walls and the current is turned on, the
+phosphorescence only persists for a few moments until the oil is carried
+away. Of all bodies so far tried, sulphide of zinc seems to be the most
+susceptible to phosphorescence. Some samples, obtained through the
+kindness of Prof. Henry in Paris, were employed in many of these bulbs.
+One of the defects of this sulphide is, that it loses its quality of
+emitting light when brought to a temperature which is by no means high.
+It can therefore, be used only for feeble intensities. An observation
+which might deserve notice is, that when violently bombarded from an
+aluminum electrode it assumes a black color, but singularly enough, it
+returns to the original condition when it cools down.
+
+The most important fact arrived at in pursuing investigations in this
+direction is, that in all cases it is necessary, in order to excite
+phosphorescence with a minimum amount of energy, to observe certain
+conditions. Namely, there is always, no matter what the frequency of the
+currents, degree of exhaustion and character of the bodies in the bulb,
+a certain potential (assuming the bulb excited from one terminal) or
+potential difference (assuming the bulb to be excited with both
+terminals) which produces the most economical result. If the potential
+be increased, considerable energy may be wasted without producing any
+more light, and if it be diminished, then again the light production is
+not as economical. The exact condition under which the best result is
+obtained seems to depend on many things of a different nature, and it is
+to be yet investigated by other experimenters, but it will certainly
+have to be observed when such phosphorescent bulbs are operated, if the
+best results are to be obtained.
+
+Coming now to the most interesting of these phenomena, the incandescence
+or phosphorescence of gases, at low pressures or at the ordinary
+pressure of the atmosphere, we must seek the explanation of these
+phenomena in the same primary causes, that is, in shocks or impacts of
+the atoms. Just as molecules or atoms beating upon a solid body excite
+phosphorescence in the same or render it incandescent, so when colliding
+among themselves they produce similar phenomena. But this is a very
+insufficient explanation and concerns only the crude mechanism. Light is
+produced by vibrations which go on at a rate almost inconceivable. If we
+compute, from the energy contained in the form of known radiations in a
+definite space the force which is necessary to set up such rapid
+vibrations, we find, that though the density of the ether be
+incomparably smaller than that of any body we know, even hydrogen, the
+force is something surpassing comprehension. What is this force, which
+in mechanical measure may amount to thousands of tons per square inch?
+It is electrostatic force in the light of modern views. It is impossible
+to conceive how a body of measurable dimensions could be charged to so
+high a potential that the force would be sufficient to produce these
+vibrations. Long before any such charge could be imparted to the body it
+would be shattered into atoms. The sun emits light and heat, and so does
+an ordinary flame or incandescent filament, but in neither of these can
+the force be accounted for if it be assumed that it is associated with
+the body as a whole. Only in one way may we account for it, namely, by
+identifying it with the atom. An atom is so small, that if it be charged
+by coming in contact with an electrified body and the charge be assumed
+to follow the same law as in the case of bodies of measurable
+dimensions, it must retain a quantity of electricity which is fully
+capable of accounting for these forces and tremendous rates of
+vibration. But the atom behaves singularly in this respect--it always
+takes the same "charge."
+
+It is very likely that resonant vibration plays a most important part in
+all manifestations of energy in nature. Throughout space all matter is
+vibrating, and all rates of vibration are represented, from the lowest
+musical note to the highest pitch of the chemical rays, hence an atom,
+or complex of atoms, no matter what its period, must find a vibration
+with which it is in resonance. When we consider the enormous rapidity
+of the light vibrations, we realize the impossibility of producing such
+vibrations directly with any apparatus of measurable dimensions, and we
+are driven to the only possible means of attaining the object of setting
+up waves of light by electrical means and economically, that is, to
+affect the molecules or atoms of a gas, to cause them to collide and
+vibrate. We then must ask ourselves--How can free molecules or atoms be
+affected?
+
+[Illustration: FIG. 196.]
+
+[Illustration: FIG. 197.]
+
+It is a fact that they can be affected by electrostatic force, as is
+apparent in many of these experiments. By varying the electrostatic
+force we can agitate the atoms, and cause them to collide accompanied by
+evolution of heat and light. It is not demonstrated beyond doubt that we
+can affect them otherwise. If a luminous discharge is produced in a
+closed exhausted tube, do the atoms arrange themselves in obedience to
+any other but to electrostatic force acting in straight lines from atom
+to atom? Only recently I investigated the mutual action between two
+circuits with extreme rates of vibration. When a battery of a few jars
+(_c c c c_, Fig. 196) is discharged through a primary P of low
+resistance (the connections being as illustrated in Figs. 183_a_, 183_b_
+and 183_c_), and the frequency of vibration is many millions there are
+great differences of potential between points on the primary not more
+than a few inches apart. These differences may be 10,000 volts per inch,
+if not more, taking the maximum value of the E. M. F. The secondary _s_
+is therefore acted upon by electrostatic induction, which is in such
+extreme cases of much greater importance than the electro-dynamic. To
+such sudden impulses the primary as well as the secondary are poor
+conductors, and therefore great differences of potential may be produced
+by electrostatic induction between adjacent points on the secondary.
+Then sparks may jump between the wires and streamers become visible in
+the dark if the light of the discharge through the spark gap _d d_ be
+carefully excluded. If now we substitute a closed vacuum tube for the
+metallic secondary _s_, the differences of potential produced in the
+tube by electrostatic induction from the primary are fully sufficient to
+excite portions of it; but as the points of certain differences of
+potential on the primary are not fixed, but are generally constantly
+changing in position, a luminous band is produced in the tube,
+apparently not touching the glass, as it should, if the points of
+maximum and minimum differences of potential were fixed on the primary.
+I do not exclude the possibility of such a tube being excited only by
+electro-dynamic induction, for very able physicists hold this view; but
+in my opinion, there is as yet no positive proof given that atoms of a
+gas in a closed tube may arrange themselves in chains under the action
+of an electromotive impulse produced by electro-dynamic induction in the
+tube. I have been unable so far to produce strić in a tube, however
+long, and at whatever degree of exhaustion, that is, strić at right
+angles to the supposed direction of the discharge or the axis of the
+tube; but I have distinctly observed in a large bulb, in which a wide
+luminous band was produced by passing a discharge of a battery through a
+wire surrounding the bulb, a circle of feeble luminosity between two
+luminous bands, one of which was more intense than the other.
+Furthermore, with my present experience I do not think that such a gas
+discharge in a closed tube can vibrate, that is, vibrate as a whole. I
+am convinced that no discharge through a gas can vibrate. The atoms of a
+gas behave very curiously in respect to sudden electric impulses. The
+gas does not seem to possess any appreciable inertia to such impulses,
+for it is a fact, that the higher the frequency of the impulses, with
+the greater freedom does the discharge pass through the gas. If the gas
+possesses no inertia then it cannot vibrate, for some inertia is
+necessary for the free vibration. I conclude from this that if a
+lightning discharge occurs between two clouds, there can be no
+oscillation, such as would be expected, considering the capacity of the
+clouds. But if the lightning discharge strike the earth, there is always
+vibration--in the earth, but not in the cloud. In a gas discharge each
+atom vibrates at its own rate, but there is no vibration of the
+conducting gaseous mass as a whole. This is an important consideration
+in the great problem of producing light economically, for it teaches us
+that to reach this result we must use impulses of very high frequency
+and necessarily also of high potential. It is a fact that oxygen
+produces a more intense light in a tube. Is it because oxygen atoms
+possess some inertia and the vibration does not die out instantly? But
+then nitrogen should be as good, and chlorine and vapors of many other
+bodies much better than oxygen, unless the magnetic properties of the
+latter enter prominently into play. Or, is the process in the tube of an
+electrolytic nature? Many observations certainly speak for it, the most
+important being that matter is always carried away from the electrodes
+and the vacuum in a bulb cannot be permanently maintained. If such
+process takes place in reality, then again must we take refuge in high
+frequencies, for, with such, electrolytic action should be reduced to a
+minimum, if not rendered entirely impossible. It is an undeniable fact
+that with very high frequencies, provided the impulses be of harmonic
+nature, like those obtained from an alternator, there is less
+deterioration and the vacua are more permanent. With disruptive
+discharge coils there are sudden rises of potential and the vacua are
+more quickly impaired, for the electrodes are deteriorated in a very
+short time. It was observed in some large tubes, which were provided
+with heavy carbon blocks B B_{1}, connected to platinum wires w w_{1}
+(as illustrated in Fig. 197), and which were employed in experiments
+with the disruptive discharge instead of the ordinary air gap, that the
+carbon particles under the action of the powerful magnetic field in
+which the tube was placed, were deposited in regular fine lines in the
+middle of the tube, as illustrated. These lines were attributed to the
+deflection or distortion of the discharge by the magnetic field, but why
+the deposit occurred principally where the field was most intense did
+not appear quite clear. A fact of interest, likewise noted, was that the
+presence of a strong magnetic field increases the deterioration of the
+electrodes, probably by reason of the rapid interruptions it produces,
+whereby there is actually a higher E. M. F. maintained between the
+electrodes.
+
+Much would remain to be said about the luminous effects produced in
+gases at low or ordinary pressures. With the present experiences before
+us we cannot say that the essential nature of these charming phenomena
+is sufficiently known. But investigations in this direction are being
+pushed with exceptional ardor. Every line of scientific pursuit has its
+fascinations, but electrical investigation appears to possess a
+peculiar attraction, for there is no experiment or observation of any
+kind in the domain of this wonderful science which would not forcibly
+appeal to us. Yet to me it seems, that of all the many marvelous things
+we observe, a vacuum tube, excited by an electric impulse from a distant
+source, bursting forth out of the darkness and illuminating the room
+with its beautiful light, is as lovely a phenomenon as can greet our
+eyes. More interesting still it appears when, reducing the fundamental
+discharges across the gap to a very small number and waving the tube
+about we produce all kinds of designs in luminous lines. So by way of
+amusement I take a straight long tube, or a square one, or a square
+attached to a straight tube, and by whirling them about in the hand, I
+imitate the spokes of a wheel, a Gramme winding, a drum winding, an
+alternate current motor winding, etc. (Fig. 198). Viewed from a distance
+the effect is weak and much of its beauty is lost, but being near or
+holding the tube in the hand, one cannot resist its charm.
+
+[Illustration: FIG. 198.]
+
+In presenting these insignificant results I have not attempted to
+arrange and co-ordinate them, as would be proper in a strictly
+scientific investigation, in which every succeeding result should be a
+logical sequence of the preceding, so that it might be guessed in
+advance by the careful reader or attentive listener. I have preferred to
+concentrate my energies chiefly upon advancing novel facts or ideas
+which might serve as suggestions to others, and this may serve as an
+excuse for the lack of harmony. The explanations of the phenomena have
+been given in good faith and in the spirit of a student prepared to find
+that they admit of a better interpretation. There can be no great harm
+in a student taking an erroneous view, but when great minds err, the
+world must dearly pay for their mistakes.
+
+
+
+
+CHAPTER XXIX.
+
+TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY, IN DETAIL.
+
+
+It has become a common practice to operate arc lamps by alternating or
+pulsating, as distinguished from continuous, currents; but an objection
+which has been raised to such systems exists in the fact that the arcs
+emit a pronounced sound, varying with the rate of the alternations or
+pulsations of current. This noise is due to the rapidly alternating
+heating and cooling, and consequent expansion and contraction, of the
+gaseous matter forming the arc, which corresponds with the periods or
+impulses of the current. Another disadvantageous feature is found in the
+difficulty of maintaining an alternating current arc in consequence of
+the periodical increase in resistance corresponding to the periodical
+working of the current. This feature entails a further disadvantage,
+namely, that small arcs are impracticable.
+
+Theoretical considerations have led Mr. Tesla to the belief that these
+disadvantageous features could be obviated by employing currents of a
+sufficiently high number of alternations, and his anticipations have
+been confirmed in practice. These rapidly alternating currents render it
+possible to maintain small arcs which, besides, possess the advantages
+of silence and persistency. The latter quality is due to the necessarily
+rapid alternations, in consequence of which the arc has no time to cool,
+and is always maintained at a high temperature and low resistance.
+
+At the outset of his experiments Mr. Tesla encountered great
+difficulties in the construction of high frequency machines. A generator
+of this kind is described here, which, though constructed quite some
+time ago, is well worthy of a detailed description. It may be mentioned,
+in passing, that dynamos of this type have been used by Mr. Tesla in his
+lighting researches and experiments with currents of high potential and
+high frequency, and reference to them will be found in his lectures
+elsewhere printed in this volume.[4]
+
+  [4] See pages 153-4 5.
+
+In the accompanying engravings, Figs. 199 and 200 show the machine,
+respectively, in side elevation and vertical cross-section; Figs. 201,
+202 and 203 showing enlarged details of construction. As will be seen, A
+is an annular magnetic frame, the interior of which is provided with a
+large number of pole-pieces D.
+
+Owing to the very large number and small size of the poles and the
+spaces between them, the field coils are applied by winding an insulated
+conductor F zigzag through the grooves, as shown in Fig. 203, carrying
+the wire around the annulus to form as many layers as is desired. In
+this way the pole-pieces D will be energized with alternately opposite
+polarity around the entire ring.
+
+For the armature, Mr. Tesla employs a spider carrying a ring J, turned
+down, except at its edges, to form a trough-like receptacle for a mass
+of fine annealed iron wires K, which are wound in the groove to form the
+core proper for the armature-coils. Pins L are set in the sides of the
+ring J and the coils M are wound over the periphery of the
+armature-structure and around the pins. The coils M are connected
+together in series, and these terminals N carried through the hollow
+shaft H to contact-rings P P, from which the currents are taken off by
+brushes O.
+
+[Illustration: FIG. 199.]
+
+In this way a machine with a very large number of poles may be
+constructed. It is easy, for instance, to obtain in this manner three
+hundred and seventy-five to four hundred poles in a machine that may be
+safely driven at a speed of fifteen hundred or sixteen hundred
+revolutions per minute, which will produce ten thousand or eleven
+thousand alternations of current per second. Arc lamps R R are shown in
+the diagram as connected up in series with the machine in Fig. 200. If
+such a current be applied to running arc lamps, the sound produced by or
+in the arc becomes practically inaudible, for, by increasing the rate of
+change in the current, and consequently the number of vibrations per
+unit of time of the gaseous material of the arc up to, or beyond, ten
+thousand or eleven thousand per second, or to what is regarded as the
+limit of audition, the sound due to such vibrations will not be audible.
+The exact number of changes or undulations necessary to produce this
+result will vary somewhat according to the size of the arc--that is to
+say, the smaller the arc, the greater the number of changes that will be
+required to render it inaudible within certain limits. It should also be
+stated that the arc should not exceed a certain length.
+
+[Illustration: FIGS. 200, 201, 202 and 203.]
+
+The difficulties encountered in the construction of these machines are
+of a mechanical as well as an electrical nature. The machines may be
+designed on two plans: the field may be formed either of alternating
+poles, or of polar projections of the same polarity. Up to about 15,000
+alternations per second in an experimental machine, the former plan may
+be followed, but a more efficient machine is obtained on the second
+plan.
+
+In the machine above described, which was capable of running two arcs of
+normal candle power, the field was composed of a ring of wrought iron
+32 inches outside diameter, and about 1 inch thick. The inside diameter
+was 30 inches. There were 384 polar projections. The wire was wound in
+zigzag form, but two wires were wound so as to completely envelop the
+projections. The distance between the projections is about 3/16 inch,
+and they are a little over 1/16 inch thick. The field magnet was made
+relatively small so as to adapt the machine for a constant current.
+There are 384 coils connected in two series. It was found impracticable
+to use any wire much thicker than No. 26 B. and S. gauge on account of
+the local effects. In such a machine the clearance should be as small as
+possible; for this reason the machine was made only 1-1/4 inch wide, so
+that the binding wires might be obviated. The armature wires must be
+wound with great care, as they are apt to fly off in consequence of the
+great peripheral speed. In various experiments this machine has been run
+as high as 3,000 revolutions per minute. Owing to the great speed it was
+possible to obtain as high as 10 amperes out of the machine. The
+electromotive force was regulated by means of an adjustable condenser
+within very wide limits, the limits being the greater, the greater the
+speed. This machine was frequently used to run Mr. Tesla's laboratory
+lights.
+
+[Illustration: FIG. 204.]
+
+The machine above described was only one of many such types constructed.
+It serves well for an experimental machine, but if still higher
+alternations are required and higher efficiency is necessary, then a
+machine on a plan shown in Figs. 204 to 207, is preferable. The
+principal advantage of this type of machine is that there is not much
+magnetic leakage, and that a field may be produced, varying greatly in
+intensity in places not much distant from each other.
+
+In these engravings, Figs. 204 and 205 illustrate a machine in which the
+armature conductor and field coils are stationary, while the field
+magnet core revolves. Fig. 206 shows a machine embodying the same plan
+of construction, but having a stationary field magnet and rotary
+armature.
+
+The conductor in which the currents are induced may be arranged in
+various ways; but Mr. Tesla prefers the following method: He employs an
+annular plate of copper D, and by means of a saw cuts in it radial slots
+from one edge nearly through to the other, beginning alternately from
+opposite edges. In this way a continuous zigzag conductor is formed.
+When the polar projections are 1/8 inch wide, the width of the conductor
+should not, under any circumstances, be more than 1/32 inch wide; even
+then the eddy effect is considerable.
+
+[Illustration: FIG. 205.]
+
+To the inner edge of this plate are secured two rings of non-magnetic
+metal E, which are insulated from the copper conductor, but held firmly
+thereto by means of the bolts F. Within the rings E is then placed an
+annular coil G, which is the energizing coil for the field magnet. The
+conductor D and the parts attached thereto are supported by means of the
+cylindrical shell or casting A A, the two parts of which are brought
+together and clamped to the outer edge of the conductor D.
+
+[Illustration: FIG. 206.]
+
+The core for the field magnet is built up of two circular parts H H,
+formed with annular grooves I, which, when the two parts are brought
+together, form a space for the reception of the energizing coil G. The
+hubs of the cores are trued off, so as to fit closely against one
+another, while the outer portions or flanges which form the polar faces
+J J, are reduced somewhat in thickness to make room for the conductor D,
+and are serrated on their faces. The number of serrations in the polar
+faces is arbitrary; but there must exist between them and the radial
+portions of the conductor D certain relation, which will be understood
+by reference to Fig. 207 in which N N represent the projections or
+points on one face of the core of the field, and S S the points of the
+other face. The conductor D is shown in this figure in section _a a'_
+designating the radial portions of the conductor, and _b_ the insulating
+divisions between them. The relative width of the parts _a a'_ and the
+space between any two adjacent points N N or S S is such that when the
+radial portions _a_ of the conductor are passing between the opposite
+points N S where the field is strongest, the intermediate radial
+portions _a'_ are passing through the widest spaces midway between such
+points and where the field is weakest. Since the core on one side is of
+opposite polarity to the part facing it, all the projections of one
+polar face will be of opposite polarity to those of the other face.
+Hence, although the space between any two adjacent points on the same
+face may be extremely small, there will be no leakage of the magnetic
+lines between any two points of the same name, but the lines of force
+will pass across from one set of points to the other. The construction
+followed obviates to a great degree the distortion of the magnetic lines
+by the action of the current in the conductor D, in which it will be
+observed the current is flowing at any given time from the centre toward
+the periphery in one set of radial parts _a_ and in the opposite
+direction in the adjacent parts _a'_.
+
+In order to connect the energizing coil G, Fig. 204, with a source of
+continuous current, Mr. Tesla utilizes two adjacent radial portions of
+the conductor D for connecting the terminals of the coil G with two
+binding posts M. For this purpose the plate D is cut entirely through,
+as shown, and the break thus made is bridged over by a short conductor
+C. The plate D is cut through to form two terminals _d_, which are
+connected to binding posts N. The core H H, when rotated by the driving
+pulley, generates in the conductors D an alternating current, which is
+taken off from the binding posts N.
+
+[Illustration: FIG. 207.]
+
+When it is desired to rotate the conductor between the faces of a
+stationary field magnet, the construction shown in Fig. 206, is adopted.
+The conductor D in this case is or may be made in substantially the same
+manner as above described by slotting an annular conducting-plate and
+supporting it between two heads O, held together by bolts _o_ and fixed
+to the driving-shaft K. The inner edge of the plate or conductor D is
+preferably flanged to secure a firmer union between it and the heads O.
+It is insulated from the head. The field-magnet in this case consists of
+two annular parts H H, provided with annular grooves I for the reception
+of the coils. The flanges or faces surrounding the annular groove are
+brought together, while the inner flanges are serrated, as in the
+previous case, and form the polar faces. The two parts H H are formed
+with a base R, upon which the machine rests. S S are non-magnetic
+bushings secured or set in the central opening of the cores. The
+conductor D is cut entirely through at one point to form terminals, from
+which insulated conductors T are led through the shaft to
+collecting-rings V.
+
+In one type of machine of this kind constructed by Mr. Tesla, the field
+had 480 polar projections on each side, and from this machine it was
+possible to obtain 30,000 alternations per second. As the polar
+projections must necessarily be very narrow, very thin wires or sheets
+must be used to avoid the eddy current effects. Mr. Tesla has thus
+constructed machines with a stationary armature and rotating field, in
+which case also the field-coil was supported so that the revolving part
+consisted only of a wrought iron body devoid of any wire and also
+machines with a rotating armature and stationary field. The machines may
+be either drum or disc, but Mr. Tesla's experience shows the latter to
+be preferable.
+
+       *       *       *       *       *
+
+In the course of a very interesting article contributed to the
+_Electrical World_ in February, 1891, Mr. Tesla makes some suggestive
+remarks on these high frequency machines and his experiences with them,
+as well as with other parts of the high frequency apparatus. Part of it
+is quoted here and is as follows:--
+
+The writer will incidentally mention that any one who attempts for the
+first time to construct such a machine will have a tale of woe to tell.
+He will first start out, as a matter of course, by making an armature
+with the required number of polar projections. He will then get the
+satisfaction of having produced an apparatus which is fit to accompany a
+thoroughly Wagnerian opera. It may besides possess the virtue of
+converting mechanical energy into heat in a nearly perfect manner. If
+there is a reversal in the polarity of the projections, he will get heat
+out of the machine; if there is no reversal, the heating will be less,
+but the output will be next to nothing. He will then abandon the iron in
+the armature, and he will get from the Scylla to the Charybdis. He will
+look for one difficulty and will find another, but, after a few trials,
+he may get nearly what he wanted.
+
+Among the many experiments which may be performed with such a machine,
+of not the least interest are those performed with a high-tension
+induction coil. The character of the discharge is completely changed.
+The arc is established at much greater distances, and it is so easily
+affected by the slightest current of air that it often wriggles around
+in the most singular manner. It usually emits the rhythmical sound
+peculiar to the alternate current arcs, but the curious point is that
+the sound may be heard with a number of alternations far above ten
+thousand per second, which by many is considered to be about the limit
+of audition. In many respects the coil behaves like a static machine.
+Points impair considerably the sparking interval, electricity escaping
+from them freely, and from a wire attached to one of the terminals
+streams of light issue, as though it were connected to a pole of a
+powerful Toepler machine. All these phenomena are, of course, mostly due
+to the enormous differences of potential obtained. As a consequence of
+the self-induction of the coil and the high frequency, the current is
+minute while there is a corresponding rise of pressure. A current
+impulse of some strength started in such a coil should persist to flow
+no less than four ten-thousandths of a second. As this time is greater
+than half the period, it occurs that an opposing electromotive force
+begins to act while the current is still flowing. As a consequence, the
+pressure rises as in a tube filled with liquid and vibrated rapidly
+around its axis. The current is so small that, in the opinion and
+involuntary experience of the writer, the discharge of even a very large
+coil cannot produce seriously injurious effects, whereas, if the same
+coil were operated with a current of lower frequency, though the
+electromotive force would be much smaller, the discharge would be most
+certainly injurious. This result, however, is due in part to the high
+frequency. The writer's experiences tend to show that the higher the
+frequency the greater the amount of electrical energy which may be
+passed through the body without serious discomfort; whence it seems
+certain that human tissues act as condensers.
+
+One is not quite prepared for the behavior of the coil when connected to
+a Leyden jar. One, of course, anticipates that since the frequency is
+high the capacity of the jar should be small. He therefore takes a very
+small jar, about the size of a small wine glass, but he finds that even
+with this jar the coil is practically short-circuited. He then reduces
+the capacity until he comes to about the capacity of two spheres, say,
+ten centimetres in diameter and two to four centimetres apart. The
+discharge then assumes the form of a serrated band exactly like a
+succession of sparks viewed in a rapidly revolving mirror; the
+serrations, of course, corresponding to the condenser discharges. In
+this case one may observe a queer phenomenon. The discharge starts at
+the nearest points, works gradually up, breaks somewhere near the top of
+the spheres, begins again at the bottom, and so on. This goes on so fast
+that several serrated bands are seen at once. One may be puzzled for a
+few minutes, but the explanation is simple enough. The discharge begins
+at the nearest points, the air is heated and carries the arc upward
+until it breaks, when it is re-established at the nearest points, etc.
+Since the current passes easily through a condenser of even small
+capacity, it will be found quite natural that connecting only one
+terminal to a body of the same size, no matter how well insulated,
+impairs considerably the striking distance of the arc.
+
+Experiments with Geissler tubes are of special interest. An exhausted
+tube, devoid of electrodes of any kind, will light up at some distance
+from the coil. If a tube from a vacuum pump is near the coil the whole
+of the pump is brilliantly lighted. An incandescent lamp approached to
+the coil lights up and gets perceptibly hot. If a lamp have the
+terminals connected to one of the binding posts of the coil and the hand
+is approached to the bulb, a very curious and rather unpleasant
+discharge from the glass to the hand takes place, and the filament may
+become incandescent. The discharge resembles to some extent the stream
+issuing from the plates of a powerful Toepler machine, but is of
+incomparably greater quantity. The lamp in this case acts as a
+condenser, the rarefied gas being one coating, the operator's hand the
+other. By taking the globe of a lamp in the hand, and by bringing the
+metallic terminals near to or in contact with a conductor connected to
+the coil, the carbon is brought to bright incandescence and the glass is
+rapidly heated. With a 100-volt 10 C. P. lamp one may without great
+discomfort stand as much current as will bring the lamp to a
+considerable brilliancy; but it can be held in the hand only for a few
+minutes, as the glass is heated in an incredibly short time. When a tube
+is lighted by bringing it near to the coil it may be made to go out by
+interposing a metal plate on the hand between the coil and tube; but if
+the metal plate be fastened to a glass rod or otherwise insulated, the
+tube may remain lighted if the plate be interposed, or may even
+increase in luminosity. The effect depends on the position of the plate
+and tube relatively to the coil, and may be always easily foretold by
+_assuming_ that conduction takes place from one terminal of the coil to
+the other. According to the position of the plate, it may either divert
+from or direct the current to the tube.
+
+In another line of work the writer has in frequent experiments
+maintained incandescent lamps of 50 or 100 volts burning at any desired
+candle power with both the terminals of each lamp connected to a stout
+copper wire of no more than a few feet in length. These experiments seem
+interesting enough, but they are not more so than the queer experiment
+of Faraday, which has been revived and made much of by recent
+investigators, and in which a discharge is made to jump between two
+points of a bent copper wire. An experiment may be cited here which may
+seem equally interesting. If a Geissler tube, the terminals of which are
+joined by a copper wire, be approached to the coil, certainly no one
+would be prepared to see the tube light up. Curiously enough, it does
+light up, and, what is more, the wire does not seem to make much
+difference. Now one is apt to think in the first moment that the
+impedance of the wire might have something to do with the phenomenon.
+But this is of course immediately rejected, as for this an enormous
+frequency would be required. This result, however, seems puzzling only
+at first; for upon reflection it is quite clear that the wire can make
+but little difference. It may be explained in more than one way, but it
+agrees perhaps best with observation to assume that conduction takes
+place from the terminals of the coil through the space. On this
+assumption, if the tube with the wire be held in any position, the wire
+can divert little more than the current which passes through the space
+occupied by the wire and the metallic terminals of the tube; through the
+adjacent space the current passes practically undisturbed. For this
+reason, if the tube be held in any position at right angles to the line
+joining the binding posts of the coil, the wire makes hardly any
+difference, but in a position more or less parallel with that line it
+impairs to a certain extent the brilliancy of the tube and its facility
+to light up. Numerous other phenomena may be explained on the same
+assumption. For instance, if the ends of the tube be provided with
+washers of sufficient size and held in the line joining the terminals of
+the coil, it will not light up, and then nearly the whole of the
+current, which would otherwise pass uniformly through the space between
+the washers, is diverted through the wire. But if the tube be inclined
+sufficiently to that line, it will light up in spite of the washers.
+Also, if a metal plate be fastened upon a glass rod and held at right
+angles to the line joining the binding posts, and nearer to one of them,
+a tube held more or less parallel with the line will light up instantly
+when one of the terminals touches the plate, and will go out when
+separated from the plate. The greater the surface of the plate, up to a
+certain limit, the easier the tube will light up. When a tube is placed
+at right angles to the straight line joining the binding posts, and then
+rotated, its luminosity steadily increases until it is parallel with
+that line. The writer must state, however, that he does not favor the
+idea of a leakage or current through the space any more than as a
+suitable explanation, for he is convinced that all these experiments
+could not be performed with a static machine yielding a constant
+difference of potential, and that condenser action is largely concerned
+in these phenomena.
+
+It is well to take certain precautions when operating a Ruhmkorff coil
+with very rapidly alternating currents. The primary current should not
+be turned on too long, else the core may get so hot as to melt the
+gutta-percha or paraffin, or otherwise injure the insulation, and this
+may occur in a surprisingly short time, considering the current's
+strength. The primary current being turned on, the fine wire terminals
+may be joined without great risk, the impedance being so great that it
+is difficult to force enough current through the fine wire so as to
+injure it, and in fact the coil may be on the whole much safer when the
+terminals of the fine wire are connected than when they are insulated;
+but special care should be taken when the terminals are connected to the
+coatings of a Leyden jar, for with anywhere near the critical capacity,
+which just counteracts the self-induction at the existing frequency, the
+coil might meet the fate of St. Polycarpus. If an expensive vacuum pump
+is lighted up by being near to the coil or touched with a wire connected
+to one of the terminals, the current should be left on no more than a
+few moments, else the glass will be cracked by the heating of the
+rarefied gas in one of the narrow passages--in the writer's own
+experience _quod erat demonstrandum_.[5]
+
+  [5] It is thought necessary to remark that, although the induction
+      coil may give quite a good result when operated with such
+      rapidly alternating currents, yet its construction, quite
+      irrespective of the iron core, makes it very unfit for such
+      high frequencies, and to obtain the best results the
+      construction should be greatly modified.
+
+There are a good many other points of interest which may be observed in
+connection with such a machine. Experiments with the telephone, a
+conductor in a strong field or with a condenser or arc, seem to afford
+certain proof that sounds far above the usual accepted limit of hearing
+would be perceived. A telephone will emit notes of twelve to thirteen
+thousand vibrations per second; then the inability of the core to follow
+such rapid alternations begins to tell. If, however, the magnet and core
+be replaced by a condenser and the terminals connected to the
+high-tension secondary of a transformer, higher notes may still be
+heard. If the current be sent around a finely laminated core and a small
+piece of thin sheet iron be held gently against the core, a sound may be
+still heard with thirteen to fourteen thousand alternations per second,
+provided the current is sufficiently strong. A small coil, however,
+tightly packed between the poles of a powerful magnet, will emit a sound
+with the above number of alternations, and arcs may be audible with a
+still higher frequency. The limit of audition is variously estimated. In
+Sir William Thomson's writings it is stated somewhere that ten thousand
+per second, or nearly so, is the limit. Other, but less reliable,
+sources give it as high as twenty-four thousand per second. The above
+experiments have convinced the writer that notes of an incomparably
+higher number of vibrations per second would be perceived provided they
+could be produced with sufficient power. There is no reason why it
+should not be so. The condensations and rarefactions of the air would
+necessarily set the diaphragm in a corresponding vibration and some
+sensation would be produced, whatever--within certain limits--the
+velocity of transmission to their nerve centres, though it is probable
+that for want of exercise the ear would not be able to distinguish any
+such high note. With the eye it is different; if the sense of vision is
+based upon some resonance effect, as many believe, no amount of increase
+in the intensity of the ethereal vibration could extend our range of
+vision on either side of the visible spectrum.
+
+The limit of audition of an arc depends on its size. The greater the
+surface by a given heating effect in the arc, the higher the limit of
+audition. The highest notes are emitted by the high-tension discharges
+of an induction coil in which the arc is, so to speak, all surface. If
+_R_ be the resistance of an arc, and _C_ the current, and the linear
+dimensions be _n_ times increased, then the resistance is _R_/_n_, and
+with the same current density the current would be _n_^2_C_; hence the
+heating effect is _n_^3 times greater, while the surface is only _n_^2
+times as great. For this reason very large arcs would not emit any
+rhythmical sound even with a very low frequency. It must be observed,
+however, that the sound emitted depends to some extent also on the
+composition of the carbon. If the carbon contain highly refractory
+material, this, when heated, tends to maintain the temperature of the
+arc uniform and the sound is lessened; for this reason it would seem
+that an alternating arc requires such carbons.
+
+With currents of such high frequencies it is possible to obtain
+noiseless arcs, but the regulation of the lamp is rendered extremely
+difficult on account of the excessively small attractions or repulsions
+between conductors conveying these currents.
+
+An interesting feature of the arc produced by these rapidly alternating
+currents is its persistency. There are two causes for it, one of which
+is always present, the other sometimes only. One is due to the character
+of the current and the other to a property of the machine. The first
+cause is the more important one, and is due directly to the rapidity of
+the alternations. When an arc is formed by a periodically undulating
+current, there is a corresponding undulation in the temperature of the
+gaseous column, and, therefore, a corresponding undulation in the
+resistance of the arc. But the resistance of the arc varies enormously
+with the temperature of the gaseous column, being practically infinite
+when the gas between the electrodes is cold. The persistence of the arc,
+therefore, depends on the inability of the column to cool. It is for
+this reason impossible to maintain an arc with the current alternating
+only a few times a second. On the other hand, with a practically
+continuous current, the arc is easily maintained, the column being
+constantly kept at a high temperature and low resistance. The higher the
+frequency the smaller the time interval during which the arc may cool
+and increase considerably in resistance. With a frequency of 10,000 per
+second or more in an arc of equal size excessively small variations of
+temperature are superimposed upon a steady temperature, like ripples on
+the surface of a deep sea. The heating effect is practically continuous
+and the arc behaves like one produced by a continuous current, with the
+exception, however, that it may not be quite as easily started, and that
+the electrodes are equally consumed; though the writer has observed
+some irregularities in this respect.
+
+The second cause alluded to, which possibly may not be present, is due
+to the tendency of a machine of such high frequency to maintain a
+practically constant current. When the arc is lengthened, the
+electromotive force rises in proportion and the arc appears to be more
+persistent.
+
+Such a machine is eminently adapted to maintain a constant current, but
+it is very unfit for a constant potential. As a matter of fact, in
+certain types of such machines a nearly constant current is an almost
+unavoidable result. As the number of poles or polar projections is
+greatly increased, the clearance becomes of great importance. One has
+really to do with a great number of very small machines. Then there is
+the impedance in the armature, enormously augmented by the high
+frequency. Then, again, the magnetic leakage is facilitated. If there
+are three or four hundred alternate poles, the leakage is so great that
+it is virtually the same as connecting, in a two-pole machine, the poles
+by a piece of iron. This disadvantage, it is true, may be obviated more
+or less by using a field throughout of the same polarity, but then one
+encounters difficulties of a different nature. All these things tend to
+maintain a constant current in the armature circuit.
+
+In this connection it is interesting to notice that even to-day
+engineers are astonished at the performance of a constant current
+machine, just as, some years ago, they used to consider it an
+extraordinary performance if a machine was capable of maintaining a
+constant potential difference between the terminals. Yet one result is
+just as easily secured as the other. It must only be remembered that in
+an inductive apparatus of any kind, if constant potential is required,
+the inductive relation between the primary or exciting and secondary or
+armature circuit must be the closest possible; whereas, in an apparatus
+for constant current just the opposite is required. Furthermore, the
+opposition to the current's flow in the induced circuit must be as small
+as possible in the former and as great as possible in the latter case.
+But opposition to a current's flow may be caused in more than one way.
+It may be caused by ohmic resistance or self-induction. One may make the
+induced circuit of a dynamo machine or transformer of such high
+resistance that when operating devices of considerably smaller
+resistance within very wide limits a nearly constant current is
+maintained. But such high resistance involves a great loss in power,
+hence it is not practicable. Not so self-induction. Self-induction does
+not necessarily mean loss of power. The moral is, use self-induction
+instead of resistance. There is, however, a circumstance which favors
+the adoption of this plan, and this is, that a very high self-induction
+may be obtained cheaply by surrounding a comparatively small length of
+wire more or less completely with iron, and, furthermore, the effect may
+be exalted at will by causing a rapid undulation of the current. To sum
+up, the requirements for constant current are: Weak magnetic connection
+between the induced and inducing circuits, greatest possible
+self-induction with the least resistance, greatest practicable rate of
+change of the current. Constant potential, on the other hand, requires:
+Closest magnetic connection between the circuits, steady induced
+current, and, if possible, no reaction. If the latter conditions could
+be fully satisfied in a constant potential machine, its output would
+surpass many times that of a machine primarily designed to give constant
+current. Unfortunately, the type of machine in which these conditions
+may be satisfied is of little practical value, owing to the small
+electromotive force obtainable and the difficulties in taking off the
+current.
+
+With their keen inventor's instinct, the now successful arc-light men
+have early recognized the desiderata of a constant current machine.
+Their arc light machines have weak fields, large armatures, with a great
+length of copper wire and few commutator segments to produce great
+variations in the current's strength and to bring self-induction into
+play. Such machines may maintain within considerable limits of variation
+in the resistance of the circuit a practically constant current. Their
+output is of course correspondingly diminished, and, perhaps with the
+object in view not to cut down the output too much, a simple device
+compensating exceptional variations is employed. The undulation of the
+current is almost essential to the commercial success of an arc-light
+system. It introduces in the circuit a steadying element taking the
+place of a large ohmic resistance, without involving a great loss in
+power, and, what is more important, it allows the use of simple clutch
+lamps, which with a current of a certain number of impulses per second,
+best suitable for each particular lamp, will, if properly attended to,
+regulate even better than the finest clock-work lamps. This discovery
+has been made by the writer--several years too late.
+
+It has been asserted by competent English electricians that in a
+constant-current machine or transformer the regulation is effected by
+varying the phase of the secondary current. That this view is erroneous
+may be easily proved by using, instead of lamps, devices each possessing
+self-induction and capacity or self-induction and resistance--that is,
+retarding and accelerating components--in such proportions as to not
+affect materially the phase of the secondary current. Any number of such
+devices may be inserted or cut out, still it will be found that the
+regulation occurs, a constant current being maintained, while the
+electromotive force is varied with the number of the devices. The change
+of phase of the secondary current is simply a result following from the
+changes in resistance, and, though secondary reaction is always of more
+or less importance, yet the real cause of the regulation lies in the
+existence of the conditions above enumerated. It should be stated,
+however, that in the case of a machine the above remarks are to be
+restricted to the cases in which the machine is independently excited.
+If the excitation be effected by commutating the armature current, then
+the fixed position of the brushes makes any shifting of the neutral line
+of the utmost importance, and it may not be thought immodest of the
+writer to mention that, as far as records go, he seems to have been the
+first who has successfully regulated machines by providing a bridge
+connection between a point of the external circuit and the commutator by
+means of a third brush. The armature and field being properly
+proportioned and the brushes placed in their determined positions, a
+constant current or constant potential resulted from the shifting of the
+diameter of commutation by the varying loads.
+
+In connection with machines of such high frequencies, the condenser
+affords an especially interesting study. It is easy to raise the
+electromotive force of such a machine to four or five times the value by
+simply connecting the condenser to the circuit, and the writer has
+continually used the condenser for the the purposes of regulation, as
+suggested by Blakesley in his book on alternate currents, in which he
+has treated the most frequently occurring condenser problems with
+exquisite simplicity and clearness. The high frequency allows the use of
+small capacities and renders investigation easy. But, although in most
+of the experiments the result may be foretold, some phenomena observed
+seem at first curious. One experiment performed three or four months ago
+with such a machine and a condenser may serve as an illustration. A
+machine was used giving about 20,000 alternations per second. Two bare
+wires about twenty feet long and two millimetres in diameter, in close
+proximity to each other, were connected to the terminals of the machine
+at the one end, and to a condenser at the other. A small transformer
+without an iron core, of course, was used to bring the reading within
+range of a Cardew voltmeter by connecting the voltmeter to the
+secondary. On the terminals of the condenser the electromotive force was
+about 120 volts, and from there inch by inch it gradually fell until at
+the terminals of the machine it was about 65 volts. It was virtually as
+though the condenser were a generator, and the line and armature circuit
+simply a resistance connected to it. The writer looked for a case of
+resonance, but he was unable to augment the effect by varying the
+capacity very carefully and gradually or by changing the speed of the
+machine. A case of pure resonance he was unable to obtain. When a
+condenser was connected to the terminals of the machine--the
+self-induction of the armature being first determined in the maximum and
+minimum position and the mean value taken--the capacity which gave the
+highest electromotive force corresponded most nearly to that which just
+counteracted the self-induction with the existing frequency. If the
+capacity was increased or diminished, the electromotive force fell as
+expected.
+
+With frequencies as high as the above mentioned, the condenser effects
+are of enormous importance. The condenser becomes a highly efficient
+apparatus capable of transferring considerable energy.
+
+       *       *       *       *       *
+
+In an appendix to this book will be found a description of the Tesla
+oscillator, which its inventor believes will among other great
+advantages give him the necessary high frequency conditions, while
+relieving him of the inconveniences that attach to generators of the
+type described at the beginning of this chapter.
+
+
+
+
+CHAPTER XXX.
+
+ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.[6]
+
+
+  [6] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
+      May 6, 1891.
+
+About a year and a half ago while engaged in the study of alternate
+currents of short period, it occurred to me that such currents could be
+obtained by rotating charged surfaces in close proximity to conductors.
+Accordingly I devised various forms of experimental apparatus of which
+two are illustrated in the accompanying engravings.
+
+[Illustration: FIG. 208.]
+
+In the apparatus shown in Fig. 208, A is a ring of dry shellacked hard
+wood provided on its inside with two sets of tin-foil coatings, _a_ and
+_b_, all the _a_ coatings and all the _b_ coatings being connected
+together, respectively, but independent from each other. These two sets
+of coatings are connected to two terminals, T. For the sake of
+clearness only a few coatings are shown. Inside of the ring A, and in
+close proximity to it there is arranged to rotate a cylinder B, likewise
+of dry, shellacked hard wood, and provided with two similar sets of
+coatings, _a^1_ and _b^1_, all the coatings _a^1_ being connected to one
+ring and all the others, _b^1_, to another marked + and -. These two
+sets, _a^1_ and _b^1_ are charged to a high potential by a Holtz or
+Wimshurst machine, and may be connected to a jar of some capacity. The
+inside of ring A is coated with mica in order to increase the induction
+and also to allow higher potentials to be used.
+
+[Illustration: FIG. 209.]
+
+When the cylinder B with the charged coatings is rotated, a circuit
+connected to the terminals T is traversed by alternating currents.
+Another form of apparatus is illustrated in Fig. 209. In this apparatus
+the two sets of tin-foil coatings are glued on a plate of ebonite, and a
+similar plate which is rotated, and the coatings of which are charged as
+in Fig. 208, is provided.
+
+The output of such an apparatus is very small, but some of the effects
+peculiar to alternating currents of short periods may be observed. The
+effects, however, cannot be compared with those obtainable with an
+induction coil which is operated by an alternate current machine of high
+frequency, some of which were described by me a short while ago.
+
+
+
+
+CHAPTER XXXI.
+
+"MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.[7]
+
+  [7] Article by Mr. Tesla in _The Electrical Engineer_ of Dec. 23d,
+      1891.
+
+I trust that the present brief communication will not be interpreted as
+an effort on my part to put myself on record as a "patent medicine" man,
+for a serious worker cannot despise anything more than the misuse and
+abuse of electricity which we have frequent occasion to witness. My
+remarks are elicited by the lively interest which prominent medical
+practitioners evince at every real advance in electrical investigation.
+The progress in recent years has been so great that every electrician
+and electrical engineer is confident that electricity will become the
+means of accomplishing many things that have been heretofore, with our
+existing knowledge, deemed impossible. No wonder then that progressive
+physicians also should expect to find in it a powerful tool and help in
+new curative processes. Since I had the honor to bring before the
+American Institute of Electrical Engineers some results in utilizing
+alternating currents of high tension, I have received many letters from
+noted physicians inquiring as to the physical effects of such currents
+of high frequency. It may be remembered that I then demonstrated that a
+body perfectly well insulated in air can be heated by simply connecting
+it with a source of rapidly alternating high potential. The heating in
+this case is due in all probability to the bombardment of the body by
+air, or possibly by some other medium, which is molecular or atomic in
+construction, and the presence of which has so far escaped our
+analysis--for according to my ideas, the true ether radiation with such
+frequencies as even a few millions per second must be very small. This
+body may be a good conductor or it may be a very poor conductor of
+electricity with little change in the result. The human body is, in such
+a case, a fine conductor, and if a person insulated in a room, or no
+matter where, is brought into contact with such a source of rapidly
+alternating high potential, the skin is heated by bombardment. It is a
+mere question of the dimensions and character of the apparatus to
+produce any degree of heating desired.
+
+It has occurred to me whether, with such apparatus properly prepared, it
+would not be possible for a skilled physician to find in it a means for
+the effective treatment of various types of disease. The heating will,
+of course, be superficial, that is, on the skin, and would result,
+whether the person operated on were in bed or walking around a room,
+whether dressed in thick clothes or whether reduced to nakedness. In
+fact, to put it broadly, it is conceivable that a person entirely nude
+at the North Pole might keep himself comfortably warm in this manner.
+
+Without vouching for all the results, which must, of course, be
+determined by experience and observation, I can at least warrant the
+fact that heating would occur by the use of this method of subjecting
+the human body to bombardment by alternating currents of high potential
+and frequency such I have long worked with. It is only reasonable to
+expect that some of the novel effects will be wholly different from
+those obtainable with the old familiar therapeutic methods generally
+used. Whether they would all be beneficial or not remains to be proved.
+
+
+
+
+CHAPTER XXXII.
+
+ELECTRIC DISCHARGE IN VACUUM TUBES.[8]
+
+  [8] Article by Mr. Tesla in _The Electrical Engineer_. N. Y.,
+      July 1, 1891.
+
+
+In _The Electrical Engineer_ of June 10 I have noted the description of
+some experiments of Prof. J. J. Thomson, on the "Electric Discharge in
+Vacuum Tubes," and in your issue of June 24 Prof. Elihu Thomson
+describes an experiment of the same kind. The fundamental idea in these
+experiments is to set up an electromotive force in a vacuum
+tube---preferably devoid of any electrodes--by means of electro-magnetic
+induction, and to excite the tube in this manner.
+
+As I view the subject I should, think that to any experimenter who had
+carefully studied the problem confronting us and who attempted to find a
+solution of it, this idea must present itself as naturally as, for
+instance, the idea of replacing the tinfoil coatings of a Leyden jar by
+rarefied gas and exciting luminosity in the condenser thus obtained by
+repeatedly charging and discharging it. The idea being obvious, whatever
+merit there is in this line of investigation must depend upon the
+completeness of the study of the subject and the correctness of the
+observations. The following lines are not penned with any desire on my
+part to put myself on record as one who has performed similar
+experiments, but with a desire to assist other experimenters by pointing
+out certain peculiarities of the phenomena observed, which, to all
+appearances, have not been noted by Prof. J. J. Thomson, who, however,
+seems to have gone about systematically in his investigations, and who
+has been the first to make his results known. These peculiarities noted
+by me would seem to be at variance with the views of Prof. J. J.
+Thomson, and present the phenomena in a different light.
+
+My investigations in this line occupied me principally during the winter
+and spring of the past year. During this time many different experiments
+were performed, and in my exchanges of ideas on this subject with Mr.
+Alfred S. Brown, of the Western Union Telegraph Company, various
+different dispositions were suggested which were carried out by me in
+practice. Fig. 210 may serve as an example of one of the many forms of
+apparatus used. This consisted of a large glass tube sealed at one end
+and projecting into an ordinary incandescent lamp bulb. The primary,
+usually consisting of a few turns of thick, well-insulated copper sheet
+was inserted within the tube, the inside space of the bulb furnishing
+the secondary. This form of apparatus was arrived at after some
+experimenting, and was used principally with the view of enabling me to
+place a polished reflecting surface on the inside of the tube, and for
+this purpose the last turn of the primary was covered with a thin silver
+sheet. In all forms of apparatus used there was no special difficulty in
+exciting a luminous circle or cylinder in proximity to the primary.
+
+[Illustration: FIG. 210.]
+
+As to the number of turns, I cannot quite understand why Prof. J. J.
+Thomson should think that a few turns were "quite sufficient," but lest
+I should impute to him an opinion he may not have, I will add that I
+have gained this impression from the reading of the published abstracts
+of his lecture. Clearly, the number of turns which gives the best result
+in any case, is dependent on the dimensions of the apparatus, and, were
+it not for various considerations, one turn would always give the best
+result.
+
+I have found that it is preferable to use in these experiments an
+alternate current machine giving a moderate number of alternations per
+second to excite the induction coil for charging the Leyden jar which
+discharges through the primary--shown diagrammatically in Fig. 211,--as
+in such case, before the disruptive discharge takes place, the tube or
+bulb is slightly excited and the formation of the luminous circle is
+decidedly facilitated. But I have also used a Wimshurst machine in some
+experiments.
+
+[Illustration: FIG. 211.]
+
+Prof. J. J. Thomson's view of the phenomena under consideration seems to
+be that they are wholly due to electro-magnetic action. I was, at one
+time, of the same opinion, but upon carefully investigating the subject
+I was led to the conviction that they are more of an electrostatic
+nature. It must be remembered that in these experiments we have to deal
+with primary currents of an enormous frequency or rate of change and of
+high potential, and that the secondary conductor consists of a rarefied
+gas, and that under such conditions electrostatic effects must play an
+important part.
+
+[Illustration: FIG. 212.]
+
+In support of my view I will describe a few experiments made by me. To
+excite luminosity in the tube it is not absolutely necessary that the
+conductor should be closed. For instance, if an ordinary exhausted tube
+(preferably of large diameter) be surrounded by a spiral of thick copper
+wire serving as the primary, a feebly luminous spiral may be induced in
+the tube, roughly shown in Fig. 212. In one of these experiments a
+curious phenomenon was observed; namely, two intensely luminous circles,
+each of them close to a turn of the primary spiral, were formed inside
+of the tube, and I attributed this phenomenon to the existence of nodes
+on the primary. The circles were connected by a faint luminous spiral
+parallel to the primary and in close proximity to it. To produce this
+effect I have found it necessary to strain the jar to the utmost. The
+turns of the spiral tend to close and form circles, but this, of course,
+would be expected, and does not necessarily indicate an electro-magnetic
+effect; Whereas the fact that a glow can be produced along the primary
+in the form of an open spiral argues for an electrostatic effect.
+
+[Illustration: FIG. 213.]
+
+In using Dr. Lodge's recoil circuit, the electrostatic action is
+likewise apparent. The arrangement is illustrated in Fig. 213. In his
+experiment two hollow exhausted tubes H H were slipped over the wires of
+the recoil circuit and upon discharging the jar in the usual manner
+luminosity was excited in the tubes.
+
+Another experiment performed is illustrated in Fig. 214. In this case an
+ordinary lamp-bulb was surrounded by one or two turns of thick copper
+wire P and the luminous circle L excited in the bulb by discharging the
+jar through the primary. The lamp-bulb was provided with a tinfoil
+coating on the side opposite to the primary and each time the tinfoil
+coating was connected to the ground or to a large object the luminosity
+of the circle was considerably increased. This was evidently due to
+electrostatic action.
+
+In other experiments I have noted that when the primary touches the
+glass the luminous circle is easier produced and is more sharply
+defined; but I have not noted that, generally speaking, the circles
+induced were very sharply defined, as Prof. J. J. Thomson has observed;
+on the contrary, in my experiments they were broad and often the whole
+of the bulb or tube was illuminated; and in one case I have observed an
+intensely purplish glow, to which Prof. J. J. Thomson refers. But the
+circles were always in close proximity to the primary and were
+considerably easier produced when the latter was very close to the
+glass, much more so than would be expected assuming the action to be
+electromagnetic and considering the distance; and these facts speak for
+an electrostatic effect.
+
+[Illustration: FIG. 214.]
+
+[Illustration: FIG. 215.]
+
+Furthermore I have observed that there is a molecular bombardment in the
+plane of the luminous circle at right angles to the glass--supposing the
+circle to be in the plane of the primary--this bombardment being
+evident from the rapid heating of the glass near the primary. Were the
+bombardment not at right angles to the glass the heating could not be so
+rapid. If there is a circumferential movement of the molecules
+constituting the luminous circle, I have thought that it might be
+rendered manifest by placing within the tube or bulb, radially to the
+circle, a thin plate of mica coated with some phosphorescent material
+and another such plate tangentially to the circle. If the molecules
+would move circumferentially, the former plate would be rendered more
+intensely phosphorescent. For want of time I have, however, not been
+able to perform the experiment.
+
+Another observation made by me was that when the specific inductive
+capacity of the medium between the primary and secondary is increased,
+the inductive effect is augmented. This is roughly illustrated in Fig.
+215. In this case luminosity was excited in an exhausted tube or bulb B
+and a glass tube T slipped between the primary and the bulb, when the
+effect pointed out was noted. Were the action wholly electromagnetic no
+change could possibly have been observed.
+
+I have likewise noted that when a bulb is surrounded by a wire closed
+upon itself and in the plane of the primary, the formation of the
+luminous circle within the bulb is not prevented. But if instead of the
+wire a broad strip of tinfoil is glued upon the bulb, the formation of
+the luminous band was prevented, because then the action was distributed
+over a greater surface. The effect of the closed tinfoil was no doubt of
+an electrostatic nature, for it presented a much greater resistance than
+the closed wire and produced therefore a much smaller electromagnetic
+effect.
+
+Some of the experiments of Prof. J. J. Thomson also would seem to show
+some electrostatic action. For instance, in the experiment with the bulb
+enclosed in a bell jar, I should think that when the latter is exhausted
+so far that the gas enclosed reaches the maximum conductivity, the
+formation of the circle in the bulb and jar is prevented because of the
+space surrounding the primary being highly conducting; when the jar is
+further exhausted, the conductivity of the space around the primary
+diminishes and the circles appear necessarily first in the bell jar, as
+the rarefied gas is nearer to the primary. But were the inductive effect
+very powerful, they would probably appear in the bulb also. If, however,
+the bell jar were exhausted to the highest degree they would very likely
+show themselves in the bulb only, that is, supposing the vacuous space
+to be non-conducting. On the assumption that in these phenomena
+electrostatic actions are concerned we find it easily explicable why the
+introduction of mercury or the heating of the bulb prevents the
+formation of the luminous band or shortens the after-glow; and also why
+in some cases a platinum wire may prevent the excitation of the tube.
+Nevertheless some of the experiments of Prof. J. J. Thomson would seem
+to indicate an electromagnetic effect. I may add that in one of my
+experiments in which a vacuum was produced by the Torricellian method, I
+was unable to produce the luminous band, but this may have been due to
+the weak exciting current employed.
+
+My principal argument is the following: I have experimentally proved
+that if the same discharge which is barely sufficient to excite a
+luminous band in the bulb when passed through the primary circuit be so
+directed as to exalt the electrostatic inductive effect--namely, by
+converting upwards--an exhausted tube, devoid of electrodes, may be
+excited at a distance of several feet.
+
+
+SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.[9]
+
+BY PROF. J. J. THOMSON, M.A., F.R.S.
+
+  [9] Abstract of a paper read before Physical Society of London.
+
+    [Illustration: FIG. 216.]
+
+    [Illustration: FIG. 217.]
+
+    [Illustration: FIG. 218.]
+
+    [Illustration: FIG. 219.]
+
+    The phenomena of vacuum discharges were, Prof. Thomson said,
+    greatly simplified when their path was wholly gaseous, the
+    complication of the dark space surrounding the negative electrode,
+    and the stratifications so commonly observed in ordinary vacuum
+    tubes, being absent. To produce discharges in tubes devoid of
+    electrodes was, however, not easy to accomplish, for the only
+    available means of producing an electromotive force in the
+    discharge circuit was by electro-magnetic induction. Ordinary
+    methods of producing variable induction were valueless, and
+    recourse was had to the oscillatory discharge of a Leyden jar,
+    which combines the two essentials of a current whose maximum value
+    is enormous, and whose rapidity of alternation is immensely great.
+    The discharge circuits, which may take the shape of bulbs, or of
+    tubes bent in the form of coils, were placed in close proximity to
+    glass tubes filled with mercury, which formed the path of the
+    oscillatory discharge. The parts thus corresponded to the windings
+    of an induction coil, the vacuum tubes being the secondary, and the
+    tubes filled with mercury the primary. In such an apparatus the
+    Leyden jar need not be large, and neither primary nor secondary
+    need have many turns, for this would increase the self-induction of
+    the former, and lengthen the discharge path in the latter.
+    Increasing the self-induction of the primary reduces the E. M. F.
+    induced in the secondary, whilst lengthening the secondary does not
+    increase the E. M. F. per unit length. The two or three turns, as
+    shown in Fig. 216, in each, were found to be quite sufficient, and,
+    on discharging the Leyden jar between two highly polished knobs in
+    the primary circuit, a plain uniform band of light was seen to pass
+    round the secondary. An exhausted bulb, Fig. 217, containing traces
+    of oxygen was placed within a primary spiral of three turns, and,
+    on passing the jar discharge, a circle of light was seen within the
+    bulb in close proximity to the primary circuit, accompanied by a
+    purplish glow, which lasted for a second or more. On heating the
+    bulb, the duration of the glow was greatly diminished, and it could
+    be instantly extinguished by the presence of an electro-magnet.
+    Another exhausted bulb, Fig. 218, surrounded by a primary spiral,
+    was contained in a bell-jar, and when the pressure of air in the
+    jar was about that of the atmosphere, the secondary discharge
+    occurred in the bulb, as is ordinarily the case. On exhausting the
+    jar, however, the luminous discharge grew fainter, and a point was
+    reached at which no secondary discharge was visible. Further
+    exhaustion of the jar caused the secondary discharge to appear
+    outside of the bulb. The fact of obtaining no luminous discharge,
+    either in the bulb or jar, the author could only explain on two
+    suppositions, viz.: that under the conditions then existing the
+    specific inductive capacity of the gas was very great, or that a
+    discharge could pass without being luminous. The author had also
+    observed that the conductivity of a vacuum tube without electrodes
+    increased as the pressure diminished, until a certain point was
+    reached, and afterwards diminished again, thus showing that the
+    high resistance of a nearly perfect vacuum is in no way due to the
+    presence of the electrodes. One peculiarity of the discharges was
+    their local nature, the rings of light being much more sharply
+    defined than was to be expected. They were also found to be most
+    easily produced when the chain of molecules in the discharge were
+    all of the same kind. For example, a discharge could be easily sent
+    through a tube many feet long, but the introduction of a small
+    pellet of mercury in the tube stopped the discharge, although the
+    conductivity of the mercury was much greater than that of the
+    vacuum. In some cases he had noticed that a very fine wire placed
+    within a tube, on the side remote from the primary circuit, would
+    prevent a luminous discharge in that tube.
+
+    Fig. 219 shows an exhausted secondary coil of one loop containing
+    bulbs; the discharge passed along the inner side of the bulbs, the
+    primary coils being placed within the secondary.
+
+
+[9]In _The Electrical Engineer_ of August 12, I find some remarks of
+Prof. J. J. Thomson, which appeared originally in the London
+_Electrician_ and which have a bearing upon some experiments described
+by me in your issue of July 1.
+
+  [9] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
+      August 26, 1891.
+
+I did not, as Prof. J. J. Thomson seems to believe, misunderstand his
+position in regard to the cause of the phenomena considered, but I
+thought that in his experiments, as well as in my own, electrostatic
+effects were of great importance. It did not appear, from the meagre
+description of his experiments, that all possible precautions had been
+taken to exclude these effects. I did not doubt that luminosity could be
+excited in a closed tube when electrostatic action is completely
+excluded. In fact, at the outset, I myself looked for a purely
+electrodynamic effect and believed that I had obtained it. But many
+experiments performed at that time proved to me that the electrostatic
+effects were generally of far greater importance, and admitted of a more
+satisfactory explanation of most of the phenomena observed.
+
+In using the term _electrostatic_ I had reference rather to the nature
+of the action than to a stationary condition, which is the usual
+acceptance of the term. To express myself more clearly, I will suppose
+that near a closed exhausted tube be placed a small sphere charged to a
+very high potential. The sphere would act inductively upon the tube, and
+by distributing electricity over the same would undoubtedly produce
+luminosity (if the potential be sufficiently high), until a permanent
+condition would be reached. Assuming the tube to be perfectly well
+insulated, there would be only one instantaneous flash during the act of
+distribution. This would be due to the electrostatic action simply.
+
+But now, suppose the charged sphere to be moved at short intervals with
+great speed along the exhausted tube. The tube would now be permanently
+excited, as the moving sphere would cause a constant redistribution of
+electricity and collisions of the molecules of the rarefied gas. We
+would still have to deal with an electrostatic effect, and in addition
+an electrodynamic effect would be observed. But if it were found that,
+for instance, the effect produced depended more on the specific
+inductive capacity than on the magnetic permeability of the
+medium--which would certainly be the case for speeds incomparably lower
+than that of light--then I believe I would be justified in saying that
+the effect produced was more of an electrostatic nature. I do not mean
+to say, however, that any similar condition prevails in the case of the
+discharge of a Leyden jar through the primary, but I think that such an
+action would be desirable.
+
+It is in the spirit of the above example that I used the terms "more of
+an electrostatic nature," and have investigated the influence of bodies
+of high specific inductive capacity, and observed, for instance, the
+importance of the quality of glass of which the tube is made. I also
+endeavored to ascertain the influence of a medium of high permeability
+by using oxygen. It appeared from rough estimation that an oxygen tube
+when excited under similar conditions--that is, as far as could be
+determined--gives more light; but this, of course, may be due to many
+causes.
+
+Without doubting in the least that, with the care and precautions taken
+by Prof. J. J. Thomson, the luminosity excited was due solely to
+electrodynamic action, I would say that in many experiments I have
+observed curious instances of the ineffectiveness of the screening, and
+I have also found that the electrification through the air is often of
+very great importance, and may, in some cases, determine the excitation
+of the tube.
+
+In his original communication to the _Electrician_, Prof. J. J. Thomson
+refers to the fact that the luminosity in a tube near a wire through
+which a Leyden jar was discharged was noted by Hittorf. I think that the
+feeble luminous effect referred to has been noted by many
+experimenters, but in my experiments the effects were much more powerful
+than those usually noted.
+
+The following is the communication[10] referred to:--
+
+  [10] Note by Prof. J. J. Thomson in the London _Electrician_,
+       July 24, 1891.
+
+    "Mr. Tesla seems to ascribe the effects he observed to
+    electrostatic action, and I have no doubt, from the description he
+    gives of his method of conducting his experiments, that in them
+    electrostatic action plays a very important part. He seems,
+    however, to have misunderstood my position with respect to the
+    cause of these discharges, which is not, as he implies, that
+    luminosity in tubes without electrodes cannot be produced by
+    electrostatic action, but that it can also be produced when this
+    action is excluded. As a matter of fact, it is very much easier to
+    get the luminosity when these electrostatic effects are operative
+    than when they are not. As an illustration of this I may mention
+    that the first experiment I tried with the discharge of a Leyden
+    jar produced luminosity in the tube, but it was not until after six
+    weeks' continuous experimenting that I was able to get a discharge
+    in the exhausted tube which I was satisfied was due to what is
+    ordinarily called electrodynamic action. It is advisable to have a
+    clear idea of what we mean by electrostatic action. If, previous to
+    the discharge of the jar, the primary coil is raised to a high
+    potential, it will induce over the glass of the tube a distribution
+    of electricity. When the potential of the primary suddenly falls,
+    this electrification will redistribute itself, and may pass through
+    the rarefied gas and produce luminosity in doing so. Whilst the
+    discharge of the jar is going on, it is difficult, and, from a
+    theoretical point of view, undesirable, to separate the effect into
+    parts, one of which is called electrostatic, the other
+    electromagnetic; what we can prove is that in this case the
+    discharge is not such as would be produced by electromotive forces
+    derived from a potential function. In my experiments the primary
+    coil was connected to earth, and, as a further precaution, the
+    primary was separated from the discharge tube by a screen of
+    blotting paper, moistened with dilute sulphuric acid, and connected
+    to earth. Wet blotting paper is a sufficiently good conductor to
+    screen off a stationary electrostatic effect, though it is not a
+    good enough one to stop waves of alternating electromotive
+    intensity. When showing the experiments to the Physical Society I
+    could not, of course, keep the tubes covered up, but, unless my
+    memory deceives me, I stated the precautions which had been taken
+    against the electrostatic effect. To correct misapprehension I may
+    state that I did not read a formal paper to the Society, my object
+    being to exhibit a few of the most typical experiments. The account
+    of the experiments in the _Electrician_ was from a reporter's note,
+    and was not written, or even read, by me. I have now almost
+    finished writing out, and hope very shortly to publish, an account
+    of these and a large number of allied experiments, including some
+    analogous to those mentioned by Mr. Tesla on the effect of
+    conductors placed near the discharge tube, which I find, in some
+    cases, to produce a diminution, in others an increase, in the
+    brightness of the discharge, as well as some on the effect of the
+    presence of substances of large specific inductive capacity. These
+    seem to me to admit of a satisfactory explanation, for which,
+    however, I must refer to my paper."
+
+
+
+
+PART III.
+
+MISCELLANEOUS INVENTIONS AND WRITINGS.
+
+
+
+
+CHAPTER XXXIII.
+
+METHOD OF OBTAINING DRIECT FROM ALTERNATING CURRENTS.
+
+
+This method consists in obtaining direct from alternating currents, or
+in directing the waves of an alternating current so as to produce direct
+or substantially direct currents by developing or producing in the
+branches of a circuit including a source of alternating currents, either
+permanently or periodically, and by electric, electro-magnetic, or
+magnetic agencies, manifestations of energy, or what may be termed
+active resistances of opposite electrical character, whereby the
+currents or current waves of opposite sign will be diverted through
+different circuits, those of one sign passing over one branch and those
+of opposite sign over the other.
+
+We may consider herein only the case of a circuit divided into two
+paths, inasmuch as any further subdivision involves merely an extension
+of the general principle. Selecting, then, any circuit through which is
+flowing an alternating current, Mr. Tesla divides such circuit at any
+desired point into two branches or paths. In one of these paths he
+inserts some device to create an electromotive force counter to the
+waves or impulses of current of one sign and a similar device in the
+other branch which opposes the waves of opposite sign. Assume, for
+example, that these devices are batteries, primary or secondary, or
+continuous current dynamo machines. The waves or impulses of opposite
+direction composing the main current have a natural tendency to divide
+between the two branches; but by reason of the opposite electrical
+character or effect of the two branches, one will offer an easy passage
+to a current of a certain direction, while the other will offer a
+relatively high resistance to the passage of the same current. The
+result of this disposition is, that the waves of current of one sign
+will, partly or wholly, pass over one of the paths or branches, while
+those of the opposite sign pass over the other. There may thus be
+obtained from an alternating current two or more direct currents without
+the employment of any commutator such as it has been heretofore
+regarded as necessary to use. The current in either branch may be
+used in the same way and for the same purposes as any other direct
+current--that is, it may be made to charge secondary batteries, energize
+electro-magnets, or for any other analogous purpose.
+
+Fig. 220 represents a plan of directing the alternating currents by
+means of devices purely electrical in character. Figs. 221, 222, 223,
+224, 225, and 226 are diagrams illustrative of other ways of carrying
+out the invention.
+
+[Illustration: FIG. 220.]
+
+In Fig. 220, A designates a generator of alternating currents, and B B
+the main or line circuit therefrom. At any given point in this circuit
+at or near which it is desired to obtain direct currents, the circuit B
+is divided into two paths or branches C D. In each of these branches is
+placed an electrical generator, which for the present we will assume
+produces direct or continuous currents. The direction of the current
+thus produced is opposite in one branch to that of the current in the
+other branch, or, considering the two branches as forming a closed
+circuit, the generators E F are connected up in series therein, one
+generator in each part or half of the circuit. The electromotive force
+of the current sources E and F may be equal to or higher or lower than
+the electromotive forces in the branches C D, or between the points X
+and Y of the circuit B B. If equal, it is evident that current waves of
+one sign will be opposed in one branch and assisted in the other to such
+an extent that all the waves of one sign will pass over one branch and
+those of opposite sign over the other. If, on the other hand, the
+electromotive force of the sources E F be lower than that between X and
+Y, the currents in both branches will be alternating, but the waves of
+one sign will preponderate. One of the generators or sources of current
+E or F may be dispensed with; but it is preferable to employ both, if
+they offer an appreciable resistance, as the two branches will be
+thereby better balanced. The translating or other devices to be acted
+upon by the current are designated by the letters G, and they are
+inserted in the branches C D in any desired manner; but in order to
+better preserve an even balance between the branches due regard should,
+of course, be had to the number and character of the devices.
+
+[Illustration: FIG. 221.]
+
+Figs. 221, 222, 223, and 224 illustrate what may termed
+"electro-magnetic" devices for accomplishing a similar result--that is
+to say, instead of producing directly by a generator an electromotive
+force in each branch of the circuit, Mr. Tesla establishes a field or
+fields of force and leads the branches through the same in such manner
+that an active opposition of opposite effect or direction will be
+developed therein by the passage, or tendency to pass, of the
+alternations of current. In Fig. 221, for example, A is the generator of
+alternating currents, B B the line circuit, and C D the branches over
+which the alternating currents are directed. In each branch is included
+the secondary of a transformer or induction coil, which, since they
+correspond in their functions to the batteries of the previous figure,
+are designated by the letters E F. The primaries H H' of the induction
+coils or transformers are connected either in parallel or series with a
+source of direct or continuous currents I, and the number of
+convolutions is so calculated for the strength of the current from I
+that the cores J J' will be saturated. The connections are such that the
+conditions in the two transformers are of opposite character--that is to
+say, the arrangement is such that a current wave or impulse
+corresponding in direction with that of the direct current in one
+primary, as H, is of opposite direction to that in the other primary H'.
+It thus results that while one secondary offers a resistance or
+opposition to the passage through it of a wave of one sign, the other
+secondary similarly opposes a wave of opposite sign. In consequence, the
+waves of one sign will, to a greater or less extent, pass by way of one
+branch, while those of opposite sign in like manner pass over the other
+branch.
+
+In lieu of saturating the primaries by a source of continuous current,
+we may include the primaries in the branches C D, respectively, and
+periodically short-circuit by any suitable mechanical devices--such as
+an ordinary revolving commutator--their secondaries. It will be
+understood, of course, that the rotation and action of the commutator
+must be in synchronism or in proper accord with the periods of the
+alternations in order to secure the desired results. Such a disposition
+is represented diagrammatically in Fig. 222. Corresponding to the
+previous figures, A is the generator of alternating currents, B B the
+line, and C D the two branches for the direct currents. In branch C are
+included two primary coils E E', and in branch D are two similar
+primaries F F' The corresponding secondaries for these coils and which
+are on the same subdivided cores J or J', are in circuits the terminals
+of which connect to opposite segments K K', and L L', respectively, of a
+commutator. Brushes _b b_ bear upon the commutator and alternately
+short-circuit the plates K and K', and L and L', through a connection
+_c_. It is obvious that either the magnets and commutator, or the
+brushes, may revolve.
+
+[Illustration: FIG. 222.]
+
+The operation will be understood from a consideration of the effects of
+closing or short-circuiting the secondaries. For example, if at the
+instant when a given wave of current passes, one set of secondaries be
+short-circuited, nearly all the current flows through the corresponding
+primaries; but the secondaries of the other branch being open-circuited,
+the self-induction in the primaries is highest, and hence little or no
+current will pass through that branch. If, as the current alternates,
+the secondaries of the two branches are alternately short-circuited, the
+result will be that the currents of one sign pass over one branch and
+those of the opposite sign over the other. The disadvantages of this
+arrangement, which would seem to result from the employment of sliding
+contacts, are in reality very slight, inasmuch as the electromotive
+force of the secondaries may be made exceedingly low, so that sparking
+at the brushes is avoided.
+
+[Illustration: FIG. 223.]
+
+Fig. 223 is a diagram, partly in section, of another plan of carrying
+out the invention. The circuit B in this case is divided, as before, and
+each branch includes the coils of both the fields and revolving
+armatures of two induction devices. The armatures O P are preferably
+mounted on the same shaft, and are adjusted relatively to one another in
+such manner that when the self-induction in one branch, as C, is
+maximum, in the other branch D it is minimum. The armatures are rotated
+in synchronism with the alternations from the source A. The winding or
+position of the armature coils is such that a current in a given
+direction passed through both armatures would establish in one, poles
+similar to those in the adjacent poles of the field, and in the other,
+poles unlike the adjacent field poles, as indicated by _n n s s_ in the
+diagram. If the like poles are presented, as shown in circuit D, the
+condition is that of a closed secondary upon a primary, or the position
+of least inductive resistance; hence a given alternation of current will
+pass mainly through D. A half revolution of the armatures produces an
+opposite effect and the succeeding current impulse passes through C.
+Using this figure as an illustration, it is evident that the fields N M
+may be permanent magnets or independently excited and the armatures O P
+driven, as in the present case, so as to produce alternate currents,
+which will set up alternately impulses of opposite direction in the two
+branches D C, which in such case would include the armature circuits and
+translating devices only.
+
+In Fig. 224 a plan alternative with that shown in Fig. 222 is
+illustrated. In the previous case illustrated, each branch C and D
+contained one or more primary coils, the secondaries of which were
+periodically short circuited in synchronism with the alternations of
+current from the main source A, and for this purpose a commutator was
+employed. The latter may, however, be dispensed with and an armature
+with a closed coil substituted.
+
+[Illustration: FIG. 224.]
+
+Referring to Fig. 224 in one of the branches, as C, are two coils M',
+wound on laminated cores, and in the other branches D are similar coils
+N'. A subdivided or laminated armature O', carrying a closed coil R', is
+rotatably supported between the coils M' N', as shown. In the position
+shown--that is, with the coil R' parallel with the convolutions of the
+primaries N' M'--practically the whole current will pass through branch
+D, because the self-induction in coils M' M' is maximum. If, therefore,
+the armature and coil be rotated at a proper speed relatively to the
+periods or alternations of the source A, the same results are obtained
+as in the case of Fig. 222.
+
+Fig. 225 is an instance of what may be called, in distinction to the
+others, a "magnetic" means of securing the result. V and W are two
+strong permanent magnets provided with armatures V' W', respectively.
+The armatures are made of thin laminć of soft iron or steel, and the
+amount of magnetic metal which they contain is so calculated that they
+will be fully or nearly saturated by the magnets. Around the armatures
+are coils E F, contained, respectively, in the circuits C and D. The
+connections and electrical conditions in this case are similar to those
+in Fig. 221, except that the current source of I, Fig. 221, is dispensed
+with and the saturation of the core of coils E F obtained from the
+permanent magnets.
+
+[Illustration: FIG. 225.]
+
+The previous illustrations have all shown the two branches or paths
+containing the translating or induction devices as in derivation one to
+the other; but this is not always necessary. For example, in Fig. 226, A
+is an alternating-current generator; B B, the line wires or circuit. At
+any given point in the circuit let us form two paths, as D D', and at
+another point two paths, as C C'. Either pair or group of paths is
+similar to the previous dispositions with the electrical source or
+induction device in one branch only, while the two groups taken together
+form the obvious equivalent of the cases in which an induction device or
+generator is included in both branches. In one of the paths, as D, are
+included the devices to be operated by the current. In the other branch,
+as D', is an induction device that opposes the current impulses of one
+direction and directs them through the branch D. So, also, in branch C
+are translating devices G, and in branch C' an induction device or its
+equivalent that diverts through C impulses of opposite direction to
+those diverted by the device in branch D'. The diagram shows a special
+form of induction device for this purpose. J J' are the cores, formed
+with pole-pieces, upon which are wound the coils M N. Between these
+pole-pieces are mounted at right angles to one another the magnetic
+armatures O P, preferably mounted on the same shaft and designed to be
+rotated in synchronism with the alternations of current. When one of the
+armatures is in line with the poles or in the position occupied by
+armature P, the magnetic circuit of the induction device is practically
+closed; hence there will be the greatest opposition to the passage of a
+current through coils N N. The alternation will therefore pass by way of
+branch D. At the same time, the magnetic circuit of the other induction
+device being broken by the position of the armature O, there will be
+less opposition to the current in coils M, which will shunt the current
+from branch C. A reversal of the current being attended by a shifting of
+the armatures, the opposite effect is produced.
+
+[Illustration: FIG. 226.]
+
+Other modifications of these methods are possible, but need not be
+pointed out. In all these plans, it will be observed, there is developed
+in one or all of these branches of a circuit from a source of
+alternating currents, an active (as distinguished from a dead)
+resistance or opposition to the currents of one sign, for the purpose of
+diverting the currents of that sign through the other or another path,
+but permitting the currents of opposite sign to pass without substantial
+opposition.
+
+Whether the division of the currents or waves of current of opposite
+sign be effected with absolute precision or not is immaterial, since it
+will be sufficient if the waves are only partially diverted or directed,
+for in such case the preponderating influence in each branch of the
+circuit of the waves of one sign secures the same practical results in
+many if not all respects as though the current were direct and
+continuous.
+
+An alternating and a direct current have been combined so that the waves
+of one direction or sign were partially or wholly overcome by the direct
+current; but by this plan only one set of alternations are utilized,
+whereas by the system just described the entire current is rendered
+available. By obvious applications of this discovery Mr. Tesla is
+enabled to produce a self-exciting alternating dynamo, or to operate
+direct current meters on alternating-current circuits or to run various
+devices--such as arc lamps--by direct currents in the same circuit with
+incandescent lamps or other devices operated by alternating currents.
+
+It will be observed that if an intermittent counter or opposing force be
+developed in the branches of the circuit and of higher electromotive
+force than that of the generator, an alternating current will result in
+each branch, with the waves of one sign preponderating, while a
+constantly or uniformly acting opposition in the branches of higher
+electromotive force than the generator would produce a pulsating
+current, which conditions would be, under some circumstances, the
+equivalent of those described.
+
+
+
+
+CHAPTER XXXIV.
+
+CONDENSERS WITH PLATES IN OIL.
+
+
+[Illustration: FIG. 227.]
+
+[Illustration: FIG. 228.]
+
+In experimenting with currents of high frequency and high potential, Mr.
+Tesla has found that insulating materials such as glass, mica, and in
+general those bodies which possess the highest specific inductive
+capacity, are inferior as insulators in such devices when currents of
+the kind described are employed compared with those possessing high
+insulating power, together with a smaller specific inductive capacity;
+and he has also found that it is very desirable to exclude all gaseous
+matter from the apparatus, or any access of the same to the electrified
+surfaces, in order to prevent heating by molecular bombardment and the
+loss or injury consequent thereon. He has therefore devised a method to
+accomplish these results and produce highly efficient and reliable
+condensers, by using oil as the dielectric[11]. The plan admits of a
+particular construction of condenser, in which the distance between the
+plates is adjustable, and of which he takes advantage.
+
+  [11] Mr. Tesla's experiments, as the careful reader of his three
+    lectures will perceive, have revealed a very important fact which
+    is taken advantage of in this invention. Namely, he has shown that
+    in a condenser a considerable amount of energy may be wasted, and
+    the condenser may break down merely because gaseous matter is
+    present between the surfaces. A number of experiments are described
+    in the lectures, which bring out this fact forcibly and serve as a
+    guide in the operation of high tension apparatus. But besides
+    bearing upon this point, these experiments also throw a light upon
+    investigations of a purely scientific nature and explain now the
+    lack of harmony among the observations of various investigators.
+    Mr. Tesla shows that in a fluid such as oil the losses are very
+    small as compared with those incurred in a gas.
+
+In the accompanying illustrations, Fig. 227 is a section of a condenser
+constructed in accordance with this principle and having stationary
+plates; and Fig. 228 is a similar view of a condenser with adjustable
+plates.
+
+Any suitable box or receptacle A may be used to contain the plates or
+armatures. These latter are designated by B and C and are connected,
+respectively, to terminals D and E, which pass out through the sides of
+the case. The plates ordinarily are separated by strips of porous
+insulating material F, which are used merely for the purpose of
+maintaining them in position. The space within the can is filled with
+oil G. Such a condenser will prove highly efficient and will not become
+heated or permanently injured.
+
+In many cases it is desirable to vary or adjust the capacity of a
+condenser, and this is provided for by securing the plates to adjustable
+supports--as, for example, to rods H--passing through stuffing boxes K
+in the sides of case A and furnished with nuts L, the ends of the rods
+being threaded for engagement with the nuts.
+
+It is well known that oils possess insulating properties, and it has
+been a common practice to interpose a body of oil between two conductors
+for purposes of insulation; but Mr. Tesla believes he has discovered
+peculiar properties in oils which render them very valuable in this
+particular form of device.
+
+
+
+
+CHAPTER XXXV.
+
+ELECTROLYTIC REGISTERING METER.
+
+
+An ingenious form of electrolytic meter attributable to Mr. Tesla is one
+in which a conductor is immersed in a solution, so arranged that metal
+may be deposited from the solution or taken away in such a manner that
+the electrical resistance of the conductor is varied in a definite
+proportion to the strength of the current the energy of which is to be
+computed, whereby this variation in resistance serves as a measure of
+the energy and also may actuate registering mechanism, whenever the
+resistance rises above or falls below certain limits.
+
+In carrying out this idea Mr. Tesla employs an electrolytic cell,
+through which extend two conductors parallel and in close proximity to
+each other. These conductors he connects in series through a resistance,
+but in such manner that there is an equal difference of potential
+between them throughout their entire extent. The free ends or terminals
+of the conductors are connected either in series in the circuit
+supplying the current to the lamps or other devices, or in parallel to a
+resistance in the circuit and in series with the current consuming
+devices. Under such circumstances a current passing through the
+conductors establishes a difference of potential between them which is
+proportional to the strength of the current, in consequence of which
+there is a leakage of current from one conductor to the other across the
+solution. The strength of this leakage current is proportional to the
+difference of potential, and, therefore, in proportion to the strength
+of the current passing through the conductors. Moreover, as there is a
+constant difference of potential between the two conductors throughout
+the entire extent that is exposed to the solution, the current density
+through such solution is the same at all corresponding points, and hence
+the deposit is uniform along the whole of one of the conductors, while
+the metal is taken away uniformly from the other. The resistance of one
+conductor is by this means diminished, while that of the other is
+increased, both in proportion to the strength of the current passing
+through the conductors. From such variation in the resistance of either
+or both of the conductors forming the positive and negative electrodes
+of the cell, the current energy expended may be readily computed. Figs.
+229 and 230 illustrate two forms of such a meter.
+
+[Illustration: FIG. 229.]
+
+In Fig. 229 G designates a direct-current generator. L L are the
+conductors of the circuit extending therefrom. A is a tube of glass, the
+ends of which are sealed, as by means of insulating plugs or caps B B. C
+C' are two conductors extending through the tube A, their ends passing
+out through the plugs B to terminals thereon. These conductors may be
+corrugated or formed in other proper ways to offer the desired
+electrical resistance. R is a resistance connected in series with the
+two conductors C C', which by their free terminals are connected up in
+circuit with one of the conductors L.
+
+The method of using this device and computing by means thereof the
+energy of the current will be readily understood. First, the resistances
+of the two conductors C C', respectively, are accurately measured and
+noted. Then a known current is passed through the instrument for a given
+time, and by a second measurement the increase and diminution of the
+resistances of the two conductors are respectively taken. From these
+data the constant is obtained--that is to say, for example, the
+increase of resistance of one conductor or the diminution of the
+resistance of the other per lamp hour. These two measurements evidently
+serve as a check, since the gain of one conductor should equal the loss
+of the other. A further check is afforded by measuring both wires in
+series with the resistance, in which case the resistance of the whole
+should remain constant.
+
+[Illustration: FIG. 230.]
+
+In Fig. 230 the conductors C C' are connected in parallel, the current
+device at X passing in one branch first through a resistance R' and then
+through conductor C, while on the other branch it passes first through
+conductor C', and then through resistance R''. The resistances R' R''
+are equal, as also are the resistances of the conductors C C'. It is,
+moreover, preferable that the respective resistances of the conductors C
+C' should be a known and convenient fraction of the coils or resistances
+R' R''. It will be observed that in the arrangement shown in Fig. 230
+there is a constant potential difference between the two conductors C C'
+throughout their entire length.
+
+It will be seen that in both cases illustrated, the proportionality of
+the increase or decrease of resistance to the current strength will
+always be preserved, for what one conductor gains the other loses, and
+the resistances of the conductors C C' being small as compared with the
+resistances in series with them. It will be understood that after each
+measurement or registration of a given variation of resistance in one or
+both conductors, the direction of the current should be changed or the
+instrument reversed, so that the deposit will be taken from the
+conductor which has gained and added to that which has lost. This
+principle is capable of many modifications. For instance, since there is
+a section of the circuit--to wit, the conductor C or C'--that varies in
+resistance in proportion to the current strength, such variation may be
+utilized, as is done in many analogous cases, to effect the operation of
+various automatic devices, such as registers. It is better, however, for
+the sake of simplicity to compute the energy by measurements of
+resistance.
+
+The chief advantages of this arrangement are, first, that it is possible
+to read off directly the amount of the energy expended by means of a
+properly constructed ohm-meter and without resorting to weighing the
+deposit; secondly it is not necessary to employ shunts, for the whole of
+the current to be measured may be passed through the instrument; third,
+the accuracy of the instrument and correctness of the indications are
+but slightly affected by changes in temperature. It is also said that
+such meters have the merit of superior economy and compactness, as well
+as of cheapness in construction. Electrolytic meters seem to need every
+auxiliary advantage to make them permanently popular and successful, no
+matter how much ingenuity may be shown in their design.
+
+
+
+
+CHAPTER XXXVI.
+
+THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.
+
+
+No electrical inventor of the present day dealing with the problems of
+light and power considers that he has done himself or his opportunities
+justice until he has attacked the subject of thermo-magnetism. As far
+back as the beginning of the seventeenth century it was shown by Dr.
+William Gilbert, the father of modern electricity, that a loadstone or
+iron bar when heated to redness loses its magnetism; and since that time
+the influence of heat on the magnetic metals has been investigated
+frequently, though not with any material or practical result.
+
+For a man of Mr. Tesla's inventive ability, the problems in this field
+have naturally had no small fascination, and though he has but glanced
+at them, it is to be hoped he may find time to pursue the study deeper
+and further. For such as he, the investigation must undoubtedly bear
+fruit. Meanwhile he has worked out one or two operative devices worthy
+of note.[12] He obtains mechanical power by a reciprocating action
+resulting from the joint operations of heat, magnetism, and a spring or
+weight or other force--that is to say he subjects a body magnetized by
+induction or otherwise to the action of heat until the magnetism is
+sufficiently neutralized to allow a weight or spring to give motion to
+the body and lessen the action of the heat, so that the magnetism may be
+sufficiently restored to move the body in the opposite direction, and
+again subject the same to the demagnetizing power of the heat.
+
+  [12] It will, of course, be inferred from the nature of these devices
+       that the vibration obtained in this manner is very slow owing to
+       the inability of the iron to follow rapid changes in temperature.
+       In an interview with Mr. Tesla on this subject, the compiler
+       learned of an experiment which will interest students. A simple
+       horseshoe magnet is taken and a piece of sheet iron bent in the
+       form of an L is brought in contact with one of the poles and
+       placed in such a position that it is kept in the attraction of
+       the opposite pole delicately suspended. A spirit lamp is placed
+       under the sheet iron piece and when the iron is heated to a
+       certain temperature it is easily set in vibration oscillating
+       as rapidly as 400 to 500 times a minute. The experiment is very
+       easily performed and is interesting principally on account of the
+       very rapid rate of vibration.
+
+Use is made of either an electro-magnet or a permanent magnet, and the
+heat is directed against a body that is magnetized by induction, rather
+than directly against a permanent magnet, thereby avoiding the loss of
+magnetism that might result in the permanent magnet by the action of
+heat. Mr. Tesla also provides for lessening the volume of the heat or
+for intercepting the same during that portion of the reciprocation in
+which the cooling action takes place.
+
+In the diagrams are shown some of the numerous arrangements that may be
+made use of in carrying out this idea. In all of these figures the
+magnet-poles are marked N S, the armature A, the Bunsen burner or other
+source of heat H, the axis of motion M, and the spring or the equivalent
+thereof--namely, a weight--is marked W.
+
+[Illustration: FIG. 232.]
+
+[Illustration: FIG. 231.]
+
+[Illustration: FIG. 233.]
+
+
+In Fig. 231 the permanent magnet N is connected with a frame, F,
+supporting the axis M, from which the arm P hangs, and at the lower end
+of which the armature A is supported. The stops 2 and 3 limit the extent
+of motion, and the spring W tends to draw the armature A away from the
+magnet N. It will now be understood that the magnetism of N is
+sufficient to overcome the spring W and draw the armature A toward the
+magnet N. The heat acting upon the armature A neutralizes its induced
+magnetism sufficiently for the spring W to draw the armature A away from
+the magnet N and also from the heat at H. The armature now cools, and
+the attraction of the magnet N overcomes the spring W and draws the
+armature A back again above the burner H, so that the same is again
+heated and the operations are repeated. The reciprocating movements thus
+obtained are employed as a source of mechanical power in any desired
+manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would
+be made use of, as indicated in Fig. 240.
+
+[Illustration: FIG. 234.]
+
+[Illustration: FIG. 236.]
+
+[Illustration: FIG. 235.]
+
+Fig. 232 represents the same parts as before described; but an
+electro-magnet is illustrated in place of a permanent magnet. The
+operations, however, are the same.
+
+In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they
+are differently arranged. The armature A, instead of swinging, is
+stationary and held by arm P', and the core N S of the electro-magnet is
+made to swing within the helix Q, the core being suspended by the arm P
+from the pivot M. A shield, R, is connected with the magnet-core and
+swings with it, so that after the heat has demagnetized the armature A
+to such an extent that the spring W draws the core N S away from the
+armature A, the shield R comes between the flame H and armature A,
+thereby intercepting the action of the heat and allowing the armature to
+cool, so that the magnetism, again preponderating, causes the movement
+of the core N S toward the armature A and the removal of the shield R
+from above the flame, so that the heat again acts to lessen or
+neutralize the magnetism. A rotary or other movement may be obtained
+from this reciprocation.
+
+Fig. 234 corresponds in every respect with Fig. 233, except that a
+permanent horseshoe-magnet, N S is represented as taking the place of
+the electro-magnet in Fig. 233.
+
+In Fig. 235 is shown a helix, Q, with an armature adapted to swing
+toward or from the helix. In this case there may be a soft-iron core in
+the helix, or the armature may assume the form of a solenoid core, there
+being no permanent core within the helix.
+
+[Illustration: FIG. 237.]
+
+[Illustration: FIG. 238.]
+
+[Illustration: FIG. 239.]
+
+
+Fig. 236 is an end view, and Fig. 237 a plan view, illustrating the
+method as applied to a swinging armature, A, and a stationary permanent
+magnet, N S. In this instance Mr. Tesla applies the heat to an auxiliary
+armature or keeper, T, which is adjacent to and preferably in direct
+contact with the magnet. This armature T, in the form of a plate of
+sheet-iron, extends across from one pole to the other and is of
+sufficient section to practically form a keeper for the magnet, so that
+when the armature T is cool nearly all the lines of force pass over the
+same and very little free magnetism is exhibited. Then the armature A,
+which swings freely on the pivots M in front of the poles N S, is very
+little attracted and the spring W pulls the same way from the poles into
+the position indicated in the diagram. The heat is directed upon the
+iron plate T at some distance from the magnet, so as to allow the magnet
+to keep comparatively cool. This heat is applied beneath the plate by
+means of the burners H, and there is a connection from the armature A or
+its pivot to the gas-cock 6, or other device for regulating the heat.
+The heat acting upon the middle portion of the plate T, the magnetic
+conductivity of the heated portion is diminished or destroyed, and a
+great number of the lines of force are deflected over the armature A,
+which is now powerfully attracted and drawn into line, or nearly so,
+with the poles N S. In so doing the cock 6 is nearly closed and the
+plate T cools, the lines of force are again deflected over the same, the
+attraction exerted upon the armature A is diminished, and the spring W
+pulls the same away from the magnet into the position shown by full
+lines, and the operations are repeated. The arrangement shown in Fig.
+236 has the advantages that the magnet and armature are kept cool and
+the strength of the permanent magnet is better preserved, as the
+magnetic circuit is constantly closed.
+
+In the plan view, Fig. 238, is shown a permanent magnet and keeper
+plate, T, similar to those in Figs. 236 and 237, with the burners H for
+the gas beneath the same; but the armature is pivoted at one end to one
+pole of the magnet and the other end swings toward and from the other
+pole of the magnet. The spring W acts against a lever arm that projects
+from the armature, and the supply of heat has to be partly cut off by a
+connection to the swinging armature, so as to lessen the heat acting
+upon the keeper plate when the armature A has been attracted.
+
+[Illustration: FIG. 240.]
+
+[Illustration: FIG. 241.]
+
+Fig. 239 is similar to Fig. 238, except that the keeper T is not made
+use of and the armature itself swings into and out of the range of the
+intense action of the heat from the burner H. Fig. 240 is a diagram
+similar to Fig. 231, except that in place of using a spring and stops,
+the armature is shown as connected by a link, to the crank of a
+fly-wheel, so that the fly-wheel will be revolved as rapidly as the
+armature can be heated and cooled to the necessary extent. A spring may
+be used in addition, as in Fig. 231. In Fig. 241 the armatures A A are
+connected by a link, so that one will be heating while the other is
+cooling, and the attraction exerted to move the cooled armature is
+availed of to draw away the heated armature instead of using a spring.
+
+Mr. Tesla has also devoted his attention to the development of a
+pyromagnetic generator of electricity[13] based upon the following laws:
+First, that electricity or electrical energy is developed in any
+conducting body by subjecting such body to a varying magnetic influence;
+and second, that the magnetic properties of iron or other magnetic
+substance may be partially or entirely destroyed or caused to disappear
+by raising it to a certain temperature, but restored and caused to
+reappear by again lowering its temperature to a certain degree. These
+laws may be applied in the production of electrical currents in many
+ways, the principle of which is in all cases the same, viz., to subject
+a conductor to a varying magnetic influence, producing such variations
+by the application of heat, or, more strictly speaking, by the
+application or action of a varying temperature upon the source of the
+magnetism. This principle of operation may be illustrated by a simple
+experiment: Place end to end, and preferably in actual contact, a
+permanently magnetized steel bar and a strip or bar of soft iron. Around
+the end of the iron bar or plate wind a coil of insulated wire. Then
+apply to the iron between the coil and the steel bar a flame or other
+source of heat which will be capable of raising that portion of the iron
+to an orange red, or a temperature of about 600° centigrade. When this
+condition is reached, the iron somewhat suddenly loses its magnetic
+properties, if it be very thin, and the same effect is produced as
+though the iron had been moved away from the magnet or the heated
+section had been removed. This change of position, however, is
+accompanied by a shifting of the magnetic lines, or, in other words, by
+a variation in the magnetic influence to which the coil is exposed, and
+a current in the coil is the result. Then remove the flame or in any
+other way reduce the temperature of the iron. The lowering of its
+temperature is accompanied by a return of its magnetic properties, and
+another change of magnetic conditions occurs, accompanied by a current
+in an opposite direction in the coil. The same operation may be
+repeated indefinitely, the effect upon the coil being similar to that
+which would follow from moving the magnetized bar to and from the end of
+the iron bar or plate.
+
+  [13] The chief point to be noted is that Mr. Tesla attacked this
+       problem in a way which was, from the standpoint of theory, and
+       that of an engineer, far better than that from which some
+       earlier trials in this direction started. The enlargement of
+       these ideas will be found in Mr. Tesla's work on the pyromagnetic
+       generator, treated in this chapter. The chief effort of the
+       inventor was to economize the heat, which was accomplished by
+       inclosing the iron in a source of heat well insulated, and by
+       cooling the iron by means of steam, utilizing the steam over
+       again. The construction also permits of more rapid magnetic
+       changes per unit of time, meaning larger output.
+
+The device illustrated below is a means of obtaining this result, the
+features of novelty in the invention being, first, the employment of an
+artificial cooling device, and, second, inclosing the source of heat and
+that portion of the magnetic circuit exposed to the heat and
+artificially cooling the heated part.
+
+These improvements are applicable generally to the generators
+constructed on the plan above described--that is to say, we may use an
+artificial cooling device in conjunction with a variable or varied or
+uniform source of heat.
+
+[Illustration: FIG. 242.]
+
+[Illustration: FIG. 243.]
+
+Fig. 242 is a central vertical longitudinal section of the complete
+apparatus and Fig. 243 is a cross-section of the magnetic armature-core
+of the generator.
+
+Let A represent a magnetized core or permanent magnet the poles of which
+are bridged by an armature-core composed of a casing or shell B
+inclosing a number of hollow iron tubes C. Around this core are wound
+the conductors E E', to form the coils in which the currents are
+developed. In the circuits of these coils are current-consuming devices,
+as F F'.
+
+D is a furnace or closed fire-box, through which the central portion of
+the core B extends. Above the fire is a boiler K, containing water. The
+flue L from the fire-box may extend up through the boiler.
+
+G is a water-supply pipe, and H is the steam-exhaust pipe, which
+communicates with all the tubes C in the armature B, so that steam
+escaping from the boiler will pass through the tubes.
+
+In the steam-exhaust pipe H is a valve V, to which is connected the
+lever I, by the movement of which the valve is opened or closed. In such
+a case as this the heat of the fire may be utilized for other purposes
+after as much of it as may be needed has been applied to heating the
+core B. There are special advantages in the employment of a cooling
+device, in that the metal of the core B is not so quickly oxidized.
+Moreover, the difference between the temperature of the applied heat and
+of the steam, air, or whatever gas or fluid be applied as the cooling
+medium, may be increased or decreased at will, whereby the rapidity of
+the magnetic changes or fluctuations may be regulated.
+
+
+
+
+CHAPTER XXXVII.
+
+ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.
+
+
+In direct current dynamos of great electromotive force--such, for
+instance, as those used for arc lighting--when one commutator bar or
+plate comes out of contact with the collecting-brush a spark is apt to
+appear on the commutator. This spark may be due to the break of the
+complete circuit, or to a shunt of low resistance formed by the brush
+between two or more commutator-bars. In the first case the spark is more
+apparent, as there is at the moment when the circuit is broken a
+discharge of the magnets through the field helices, producing a great
+spark or flash which causes an unsteady current, rapid wear of the
+commutator bars and brushes, and waste of power. The sparking may be
+reduced by various devices, such as providing a path for the current at
+the moment when the commutator segment or bar leaves the brush, by
+short-circuiting the field-helices, by increasing the number of the
+commutator-bars, or by other similar means; but all these devices are
+expensive or not fully available, and seldom attain the object desired.
+
+To prevent this sparking in a simple manner, Mr. Tesla some years ago
+employed with the commutator-bars and intervening insulating material,
+mica, asbestos paper or other insulating and incombustible material,
+arranged to bear on the surface of the commutator, near to and behind
+the brush.
+
+In the drawings, Fig. 244 is a section of a commutator with an asbestos
+insulating device; and Fig. 245 is a similar view, representing two
+plates of mica upon the back of the brush.
+
+In 244, C represents the commutator and intervening insulating material;
+B B, the brushes. _d d_ are sheets of asbestos paper or other suitable
+non-conducting material. _f f_ are springs, the pressure of which may be
+adjusted by means of the screws _g g_.
+
+In Fig. 245 a simple arrangement is shown with two plates of mica or
+other material. It will be seen that whenever one commutator segment
+passes out of contact with the brush, the formation of the arc will be
+prevented by the intervening insulating material coming in contact with
+the insulating material on the brush.
+
+[Illustration: FIG. 244.]
+
+[Illustration: FIG. 245.]
+
+Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia,
+or other suitable material, may be used, as the paper and cloth are
+soft, and serve at the same time to wipe and polish the commutator; but
+mica or any other suitable material can be employed, provided the
+material be an insulator or a bad conductor of electricity.
+
+A few years later Mr. Tesla turned his attention again to the same
+subject, as, perhaps, was very natural in view of the fact that the
+commutator had always been prominent in his thoughts, and that so much
+of his work was even aimed at dispensing with it entirely as an
+objectionable and unnecessary part of dynamos and motors. In these later
+efforts to remedy commutator troubles, Mr. Tesla constructs a commutator
+and the collectors therefor in two parts mutually adapted to one
+another, and, so far as the essential features are concerned, alike in
+mechanical structure. Selecting as an illustration a commutator of two
+segments adapted for use with an armature the coils or coil of which
+have but two free ends, connected respectively to the segments, the
+bearing-surface is the face of a disc, and is formed of two metallic
+quadrant segments and two insulating segments of the same dimensions,
+and the face of the disc is smoothed off, so that the metal and
+insulating segments are flush. The part which takes the place of the
+usual brushes, or the "collector," is a disc of the same character as
+the commutator and has a surface similarly formed with two insulating
+and two metallic segments. These two parts are mounted with their faces
+in contact and in such manner that the rotation of the armature causes
+the commutator to turn upon the collector, whereby the currents induced
+in the coils are taken off by the collector segments and thence
+conveyed off by suitable conductors leading from the collector segments.
+This is the general plan of the construction adopted. Aside from certain
+adjuncts, the nature and functions of which are set forth later, this
+means of commutation will be seen to possess many important advantages.
+In the first place the short-circuiting and the breaking of the armature
+coil connected to the commutator-segments occur at the same instant, and
+from the nature of the construction this will be done with the greatest
+precision; secondly, the duration of both the break and of the short
+circuit will be reduced to a minimum. The first results in a reduction
+which amounts practically to a suppression of the spark, since the break
+and the short circuit produce opposite effects in the armature-coil. The
+second has the effect of diminishing the destructive effect of a spark,
+since this would be in a measure proportional to the duration of the
+spark; while lessening the duration of the short circuit obviously
+increases the efficiency of the machine.
+
+[Illustration: FIG. 246.]
+
+[Illustration: FIG. 247.]
+
+The mechanical advantages will be better understood by referring to the
+accompanying diagrams, in which Fig. 246 is a central longitudinal
+section of the end of a shaft with the improved commutator carried
+thereon. Fig. 247 is a view of the inner or bearing face of the
+collector. Fig. 248 is an end view from the armature side of a modified
+form of commutator. Figs. 249 and 250 are views of details of Fig. 248.
+Fig. 251 is a longitudinal central section of another modification, and
+Fig. 252 is a sectional view of the same. A is the end of the
+armature-shaft of a dynamo-electric machine or motor. A' is a sleeve of
+insulating material around the shaft, secured in place by a screw, _a'_.
+
+[Illustration: FIG. 248.]
+
+[Illustration: FIG. 249.]
+
+[Illustration: FIG. 250.]
+
+The commutator proper is in the form of a disc which is made up of four
+segments D D' G G', similar to those shown in Fig. 248. Two of these
+segments, as D D', are of metal and are in electrical connection with
+the ends of the coils on the armature. The other two segments are of
+insulating material. The segments are held in place by a band, B, of
+insulating material. The disc is held in place by friction or by screws,
+_g' g'_, Fig. 248, which secure the disc firmly to the sleeve A'.
+
+The collector is made in the same form as the commutator. It is composed
+of the two metallic segments E E' and the two insulating segments F F',
+bound together by a band, C. The metallic segments E E' are of the same
+or practically the same width or extent as the insulating segments or
+spaces of the commutator. The collector is secured to a sleeve, B', by
+screws _g g_, and the sleeve is arranged to turn freely on the shaft A.
+The end of the sleeve B' is closed by a plate, _f_, upon which presses a
+pivot-pointed screw, _h_, adjustable in a spring, H, which acts to
+maintain the collector in close contact with the commutator and to
+compensate for the play of the shaft. The collector is so fixed that it
+cannot turn with the shaft. For example, the diagram shows a slotted
+plate, K, which is designed to be attached to a stationary support, and
+an arm extending from the collector and carrying a clamping screw, L, by
+which the collector may be adjusted and set to the desired position.
+
+Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit the
+insulating segments of both commutator and collector loosely and to
+provide some means--as, for example, light springs, _e e_, secured to
+the bands A' B', respectively, and bearing against the segments--to
+exert a light pressure upon them and keep them in close contact and to
+compensate for wear. The metal segments of the commutator may be moved
+forward by loosening the screw _a'_.
+
+The line wires are fed from the metal segments of the collector, being
+secured thereto in any convenient manner, the plan of connections being
+shown as applied to a modified form of the commutator in Fig. 251. The
+commutator and the collector in thus presenting two flat and smooth
+bearing surfaces prevent most effectually by mechanical action the
+occurrence of sparks.
+
+The insulating segments are made of some hard material capable of being
+polished and formed with sharp edges. Such materials as glass, marble,
+or soapstone may be advantageously used. The metal segments are
+preferably of copper or brass; but they may have a facing or edge of
+durable material--such as platinum or the like--where the sparks are
+liable to occur.
+
+[Illustration: FIG. 251.]
+
+[Illustration: FIG. 252.]
+
+In Fig. 248 a somewhat modified form of the invention is shown, a form
+designed to facilitate the construction and replacing of the parts. In
+this modification the commutator and collector are made in substantially
+the same manner as previously described, except that the bands B C are
+omitted. The four segments of each part, however, are secured to their
+respective sleeves by screws _g' g'_, and one edge of each segment is
+cut away, so that small plates _a b_ may be slipped into the spaces thus
+formed. Of these plates _a a_ are of metal, and are in contact with the
+metal segments D D', respectively. The other two, _b b_, are of glass or
+marble, and they are all better square, as shown in Figs. 249 and 250,
+so that they may be turned to present new edges should any edge become
+worn by use. Light springs _d_ bear upon these plates and press those in
+the commutator toward those in the collector, and insulating strips _c
+c_ are secured to the periphery of the discs to prevent the blocks from
+being thrown out by centrifugal action. These plates are, of course,
+useful at those edges of the segments only where sparks are liable to
+occur, and, as they are easily replaced, they are of great advantage. It
+is considered best to coat them with platinum or silver.
+
+In Figs. 251 and 252 is shown a construction where, instead of solid
+segments, a fluid is employed. In this case the commutator and collector
+are made of two insulating discs, S T, and in lieu of the metal segments
+a space is cut out of each part, as at R R', corresponding in shape and
+size to a metal segment. The two parts are fitted smoothly and the
+collector T held by the screw _h_ and spring H against the commutator S.
+As in the other cases, the commutator revolves while the collector
+remains stationary. The ends of the coils are connected to binding-posts
+_s s_, which are in electrical connection with metal plates _t t_ within
+the recesses in the two parts S T. These chambers or recesses are filled
+with mercury, and in the collector part are tubes W W, with screws _w
+w_, carrying springs X and pistons X', which compensate for the
+expansion and contraction of the mercury under varying temperatures, but
+which are sufficiently strong not to yield to the pressure of the fluid
+due to centrifugal action, and which serve as binding-posts.
+
+In all the above cases the commutators are adapted for a single coil,
+and the device is particularly suited to such purposes. The number of
+segments may be increased, however, or more than one commutator used
+with a single armature. Although the bearing-surfaces are shown as
+planes at right angles to the shaft or axis, it is evident that in this
+particular the construction may be very greatly modified.
+
+
+
+
+CHAPTER XXXVIII.
+
+AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.
+
+
+An interesting method devised by Mr. Tesla for the regulation of direct
+current dynamos, is that which has come to be known as the "third brush"
+method. In machines of this type, devised by him as far back as 1885, he
+makes use of two main brushes to which the ends of the field magnet
+coils are connected, an auxiliary brush, and a branch or shunt
+connection from an intermediate point of the field wire to the auxiliary
+brush.[14]
+
+  [14] The compiler has learned partially from statements made on
+       several occasions in journals and partially by personal inquiry
+       of Mr. Tesla, that a great deal of work in this interesting line
+       is unpublished. In these inventions as will be seen, the brushes
+       are automatically shifted, but in the broad method barely
+       suggested here the regulation is effected without any change in
+       the position of the brushes. This auxiliary brush invention, it
+       will be remembered, was very much discussed a few years ago, and
+       it may be of interest that this work of Mr. Tesla, then unknown
+       in this field, is now brought to light.
+
+The relative positions of the respective brushes are varied, either
+automatically or by hand, so that the shunt becomes inoperative when the
+auxiliary brush has a certain position upon the commutator; but when the
+auxiliary brush is moved in its relation to the main brushes, or the
+latter are moved in their relation to the auxiliary brush, the electric
+condition is disturbed and more or less of the current through the
+field-helices is diverted through the shunt or a current is passed over
+the shunt to the field-helices. By varying the relative position upon
+the commutator of the respective brushes automatically in proportion to
+the varying electrical conditions of the working-circuit, the current
+developed can be regulated in proportion to the demands in the
+working-circuit.
+
+Fig. 253 is a diagram illustrating the invention, showing one core of
+the field-magnets with one helix wound in the same direction throughout.
+Figs. 254 and 255 are diagrams showing one core of the field-magnets
+with a portion of the helices wound in opposite directions. Figs. 256
+and 257 are diagrams illustrating the electric devices that may be
+employed for automatically adjusting the brushes, and Fig. 258 is a
+diagram illustrating the positions of the brushes when the machine is
+being energized at the start.
+
+_a_ and _b_ are the positive and negative brushes of the main or
+working-circuit, and _c_ the auxiliary brush. The working-circuit D
+extends from the brushes _a_ and _b_, as usual, and contains electric
+lamps or other devices, D', either in series or in multiple arc.
+
+M M' represent the field-helices, the ends of which are connected to the
+main brushes _a_ and _b_. The branch or shunt wire _c'_ extends from the
+auxiliary brush _c_ to the circuit of the field-helices, and is
+connected to the same at an intermediate point, _x_.
+
+[Illustration: FIG. 253.]
+
+H represents the commutator, with the plates of ordinary construction.
+When the auxiliary brush _c_ occupies such a position upon the
+commutator that the electro-motive force between the brushes _a_ and _c_
+is to the electro-motive force between the brushes _c_ and _b_ as the
+resistance of the circuit _a_ M _c' c_ A is to the resistance of the
+circuit _b_ M' _c' c_ B, the potentials of the points _x_ and Y will be
+equal, and no current will flow over the auxiliary brush; but when the
+brush _c_ occupies a different position the potentials of the points _x_
+and Y will be different, and a current will flow over the auxiliary
+brush to and from the commutator, according to the relative position of
+the brushes. If, for instance, the commutator-space between the brushes
+_a_ and _c_, when the latter is at the neutral point, is diminished, a
+current will flow from the point Y over the shunt _c_ to the brush _b_,
+thus strengthening the current in the part M', and partly neutralizing
+the current in part M; but if the space between the brushes _a_ and _c_
+is increased, the current will flow over the auxiliary brush in an
+opposite direction, and the current in M will be strengthened, and in
+M', partly neutralized.
+
+By combining with the brushes _a_, _b_, and _c_ any usual automatic
+regulating mechanism, the current developed can be regulated in
+proportion to the demands in the working circuit. The parts M and M' of
+the field wire may be wound in the same direction. In this case they are
+arranged as shown in Fig. 253; or the part M may be wound in the
+opposite direction, as shown in Figs. 254 and 255.
+
+[Illustration: FIG. 254.]
+
+It will be apparent that the respective cores of the field-magnets are
+subjected to neutralizing or intensifying effects of the current in the
+shunt through _c'_, and the magnetism of the cores will be partially
+neutralized, or the points of greatest magnetism shifted, so that it
+will be more or less remote from or approaching to the armature, and
+hence the aggregate energizing actions of the field magnets on the
+armature will be correspondingly varied.
+
+In the form indicated in Fig. 253 the regulation is effected by shifting
+the point of greatest magnetism, and in Figs. 254 and 255 the same
+effect is produced by the action of the current in the shunt passing
+through the neutralizing helix.
+
+The relative positions of the respective brushes may be varied by moving
+the auxiliary brush, or the brush _c_ may remain stationary and the core
+P be connected to the main-brush holder A, so as to adjust the brushes
+_a b_ in their relation to the brush _c_. If, however, an adjustment is
+applied to all the brushes, as seen in Fig. 257, the solenoid should be
+connected to both _a_ and _c_, so as to move them toward or away from
+each other.
+
+There are several known devices for giving motion in proportion to an
+electric current. In Figs. 256 and 257 the moving cores are shown as
+convenient devices for obtaining the required extent of motion with very
+slight changes in the current passing through the helices. It is
+understood that the adjustment of the main brushes causes variations in
+the strength of the current independently of the relative position of
+those brushes to the auxiliary brush. In all cases the adjustment should
+be such that no current flows over the auxiliary brush when the dynamo
+is running with its normal load.
+
+In Figs. 256 and 257 A A indicate the main-brush holder, carrying the
+main brushes, and C the auxiliary-brush holder, carrying the auxiliary
+brush. These brush-holders are movable in arcs concentric with the
+centre of the commutator-shaft. An iron piston, P, of the solenoid S,
+Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is
+effected by means of a spring and screw or tightener.
+
+In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is
+shown. The piston of the coil is attached to both brush-holders A A and
+C. When the brushes are moved directly by electrical devices, as shown
+in Figs. 256 and 257, these are so constructed that the force exerted
+for adjusting is practically uniform through the whole length of motion.
+
+[Illustration: FIG. 255.]
+
+It is true that auxiliary brushes have been used in connection with the
+helices of the field-wire; but in these instances the helices receive
+the entire current through the auxiliary brush or brushes, and these
+brushes could not be taken off without breaking the circuit through the
+field. These brushes cause, moreover, heavy sparking at the commutator.
+In the present case the auxiliary brush causes very little or no
+sparking, and can be taken off without breaking the circuit through the
+field-helices. The arrangement has, besides, the advantage of
+facilitating the self-excitation of the machine in all cases where the
+resistance of the field-wire is very great comparatively to the
+resistance of the main circuit at the start--for instance, on arc-light
+machines. In this case the auxiliary brush _c_ is placed near to, or
+better still in contact with, the brush _b_, as shown in Fig. 258. In
+this manner the part M' is completely cut out, and as the part M has a
+considerably smaller resistance than the whole length of the field-wire
+the machine excites itself, whereupon the auxiliary brush is shifted
+automatically to its normal position.
+
+[Illustration: FIG. 256.]
+
+[Illustration: FIG. 257.]
+
+In a further method devised by Mr. Tesla, one or more auxiliary brushes
+are employed, by means of which a portion or the whole of the field
+coils is shunted. According to the relative position upon the commutator
+of the respective brushes more or less current is caused to pass through
+the helices of the field, and the current developed by the machine can
+be varied at will by varying the relative positions of the brushes.
+
+[Illustration: FIG. 258.]
+
+In Fig. 259, _a_ and _b_ are the positive and negative brushes of the
+main circuit, and _c_ an auxiliary brush. The main circuit D extends
+from the brushes _a_ and _b_, as usual, and contains the helices M of
+the field wire and the electric lamps or other working devices. The
+auxiliary brush _c_ is connected to the point _x_ of the main circuit by
+means of the wire _c'_. H is a commutator of ordinary construction. It
+will have been seen from what was said already that when the
+electro-motive force between the brushes _a_ and _c_ is to the
+electromotive force between the brushes _c_ and _b_ as the resistance of
+the circuit _a_ M _c' c_ A is to the resistance of the circuit _b_ C B
+_c c'_ D, the potentials of the points _x_ and _y_ will be equal, and no
+current will pass over the auxiliary brush _c_; but if that brush
+occupies a different position relatively to the main brushes the
+electric condition is disturbed, and current will flow either from _y_
+to _x_ or from _x_ to _y_, according to the relative position of the
+brushes. In the first case the current through the field-helices will be
+partly neutralized and the magnetism of the field magnets will be
+diminished. In the second case the current will be increased and the
+magnets gain strength. By combining with the brushes at _a b c_ any
+automatic regulating mechanism, the current developed can be regulated
+automatically in proportion to the demands of the working circuit.
+
+In Figs. 264 and 265 some of the automatic means are represented that
+maybe used for moving the brushes. The core P, Fig. 264, of the
+solenoid-helix S is connected with the brush _a_ to move the same, and
+in Fig. 265 the core P is shown as within the helix S, and connected
+with brushes _a_ and _c_, so as to move the same toward or from each
+other, according to the strength of the current in the helix, the helix
+being within an iron tube, S', that becomes magnetized and increases the
+action of the solenoid.
+
+In practice it is sufficient to move only the auxiliary brush, as shown
+in Fig. 264, as the regulation is very sensitive to the slightest
+changes; but the relative position of the auxiliary brush to the main
+brushes may be varied by moving the main brushes, or both main and
+auxiliary brushes may be moved, as illustrated in Fig. 265. In the
+latter two cases, it will be understood, the motion of the main brushes
+relatively to the neutral line of the machine causes variations in the
+strength of the current independently of their relative position to the
+auxiliary brush. In all cases the adjustment may be such that when the
+machine is running with the ordinary load, no current flows over the
+auxiliary brush.
+
+The field helices may be connected, as shown in Fig. 259, or a part of
+the field helices may be in the outgoing and the other part in the
+return circuit, and two auxiliary brushes may be employed as shown in
+Figs. 261 and 262. Instead of shunting the whole of the field helices, a
+portion only of such helices may be shunted, as shown in Figs. 260 and
+262.
+
+The arrangement shown in Fig. 262 is advantageous, as it diminishes the
+sparking upon the commutator, the main circuit being closed through the
+auxiliary brushes at the moment of the break of the circuit at the main
+brushes.
+
+[Illustration: FIG. 259.]
+
+[Illustration: FIG. 260.]
+
+[Illustration: FIG. 261.]
+
+[Illustration: FIG. 262.]
+
+[Illustration: FIG. 263.]
+
+The field helices may be wound in the same direction, or a part may be
+wound in opposite directions.
+
+The connection between the helices and the auxiliary brush or brushes
+may be made by a wire of small resistance, or a resistance may be
+interposed (R, Fig. 263,) between the point _x_ and the auxiliary brush
+or brushes to divide the sensitiveness when the brushes are adjusted.
+
+[Illustration: FIG. 264.]
+
+[Illustration: FIG. 265.]
+
+The accompanying sketches also illustrate improvements made by Mr. Tesla
+in the mechanical devices used to effect the shifting of the brushes, in
+the use of an auxiliary brush. Fig. 266 is an elevation of the regulator
+with the frame partly in section; and Fig. 267 is a section at the line
+_x x_, Fig. 266. C is the commutator; B and B', the brush-holders, B
+carrying the main brushes _a a'_, and B' the auxiliary or shunt brushes
+_b b_. The axis of the brush-holder B is supported by two pivot-screws,
+_p p_. The other brush-holder, B', has a sleeve, _d_, and is movable
+around the axis of the brush-holder B. In this way both brush-holders
+can turn very freely, the friction of the parts being reduced to a
+minimum. Over the brush-holders is mounted the solenoid S, which rests
+upon a forked column, _c_. This column also affords a support for the
+pivots _p p_, and is fastened upon a solid bracket or projection, P,
+which extends from the base of the machine, and is cast in one piece
+with the same. The brush-holders B B' are connected by means of the
+links _e e_ and the cross-piece F to the iron core I, which slides
+freely in the tube T of the solenoid. The iron core I has a screw, _s_,
+by means of which it can be raised and adjusted in its position
+relatively to the solenoid, so that the pull exerted upon it by the
+solenoid is practically uniform through the whole length of motion which
+is required to effect the regulation. In order to effect the adjustment
+with greater precision, the core I is provided with a small iron screw,
+_s'_. The core being first brought very nearly in the required position
+relatively to the solenoid by means of the screw _s_, the small screw
+_s'_ is then adjusted until the magnetic attraction upon the core is the
+same when the core is in any position. A convenient stop, _t_, serves to
+limit the upward movement of the iron core.
+
+To check somewhat the movement of the core I, a dash-pot, K, is used.
+The piston L of the dash-pot is provided with a valve, V, which opens by
+a downward pressure and allows an easy downward movement of the iron
+core I, but closes and checks the movement of the core when it is pulled
+up by the action of the solenoid.
+
+To balance the opposing forces, the weight of the moving parts, and the
+pull exerted by the solenoid upon the iron core, the weights W W may be
+used. The adjustment is such that when the solenoid is traversed by the
+normal current it is just strong enough to balance the downward pull of
+the parts.
+
+[Illustration: FIG. 266.]
+
+[Illustration: FIG. 267.]
+
+The electrical circuit-connections are substantially the same as
+indicated in the previous diagrams, the solenoid being in series with
+the circuit when the translating devices are in series, and in shunt
+when the devices are in multiple arc. The operation of the device is as
+follows: When upon a decrease of the resistance of the circuit or for
+some other reason, the current is increased, the solenoid S gains in
+strength and pulls up the iron core I, thus shifting the main brushes in
+the direction of rotation and the auxiliary brushes in the opposite way.
+This diminishes the strength of the current until the opposing forces
+are balanced and the solenoid is traversed by the normal current; but if
+from any cause the current in the circuit is diminished, then the weight
+of the moving parts overcomes the pull of the solenoid, the iron core I
+descends, thus shifting the brushes the opposite way and increasing the
+current to the normal strength. The dash-pot connected to the iron core
+I may be of ordinary construction; but it is better, especially in
+machines for arc lights, to provide the piston of the dash-pot with a
+valve, as indicated in the diagrams. This valve permits a comparatively
+easy downward movement of the iron core, but checks its movement when it
+is drawn up by the solenoid. Such an arrangement has the advantage that
+a great number of lights may be put on without diminishing the
+light-power of the lamps in the circuit, as the brushes assume at once
+the proper position. When lights are cut out, the dash-pot acts to
+retard the movement; but if the current is considerably increased the
+solenoid gets abnormally strong and the brushes are shifted instantly.
+The regulator being properly adjusted, lights or other devices may be
+put on or out with scarcely any perceptible difference. It is obvious
+that instead of the dash-pot any other retarding device may be used.
+
+
+
+
+CHAPTER XXXIX.
+
+IMPROVEMENT IN THE CONSTRUCTION OF DYNAMOS AND MOTORS.
+
+
+This invention of Mr. Tesla is an improvement in the construction of
+dynamo or magneto electric machines or motors, consisting in a novel
+form of frame and field magnet which renders the machine more solid and
+compact as a structure, which requires fewer parts, and which involves
+less trouble and expense in its manufacture. It is applicable to
+generators and motors generally, not only to those which have
+independent circuits adapted for use in the Tesla alternating current
+system, but to other continuous or alternating current machines of the
+ordinary type generally used.
+
+Fig. 268 shows the machine in side elevation. Fig. 269 is a vertical
+sectional view of the field magnets and frame and an end view of the
+armature; and Fig. 270 is a plan view of one of the parts of the frame
+and the armature, a portion of the latter being cut away.
+
+The field magnets and frame are cast in two parts. These parts are
+identical in size and shape, and each consists of the solid plates or
+ends A B, from which project inwardly the cores C D and the side bars or
+bridge pieces, E F. The precise shape of these parts is largely a matter
+of choice--that is to say, each casting, as shown, forms an
+approximately rectangular frame; but it might obviously be more or less
+oval, round, or square, without departure from the invention. It is also
+desirable to reduce the width of the side bars, E F, at the center and
+to so proportion the parts that when the frame is put together the
+spaces between the pole pieces will be practically equal to the arcs
+which the surfaces of the poles occupy.
+
+The bearings G for the armature shaft are cast in the side bars E F. The
+field coils are either wound on the pole pieces or on a form and then
+slipped on over the ends of the pole pieces. The lower part or casting
+is secured to the base after being finished off. The armature K on its
+shaft is then mounted in the bearings of the lower casting and the
+other part of the frame placed in position, dowel pins L or any other
+means being used to secure the two parts in proper position.
+
+[Illustration: FIG. 268.]
+
+[Illustration: FIG. 269.]
+
+[Illustration: FIG. 270.]
+
+In order to secure an easier fit, the side bars E F, and end pieces, A
+B, are so cast that slots M are formed when the two parts are put
+together.
+
+This machine possesses several advantages. For example, if we magnetize
+the cores alternately, as indicated by the characters N S, it will be
+seen that the magnetic circuit between the poles of each part of a
+casting is completed through the solid iron side bars. The bearings for
+the shaft are located at the neutral points of the field, so that the
+armature core is not affected by the magnetic condition of the field.
+
+The improvement is not restricted to the use of four pole pieces, as it
+is evident that each pole piece could be divided or more than four
+formed by the shape of the casting.
+
+
+
+
+CHAPTER XL.
+
+TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.
+
+
+At one time, soon after his arrival in America, Mr. Tesla was greatly
+interested in the subject of arc lighting, which then occupied public
+attention and readily enlisted the support of capital. He therefore
+worked out a system which was confided to a company formed for its
+exploitation, and then proceeded to devote his energies to the
+perfection of the details of his more celebrated "rotary field" motor
+system. The Tesla arc lighting apparatus appeared at a time when a great
+many other lamps and machines were in the market, but it commanded
+notice by its ingenuity. Its chief purpose was to lessen the
+manufacturing cost and simplify the processes of operation.
+
+We will take up the dynamo first. Fig. 271 is a longitudinal section,
+and Fig. 272 a cross section of the machine. Fig. 273 is a top view, and
+Fig. 274 a side view of the magnetic frame. Fig. 275 is an end view of
+the commutator bars, and Fig. 276 is a section of the shaft and
+commutator bars. Fig. 277 is a diagram illustrating the coils of the
+armature and the connections to the commutator plates.
+
+The cores _c c c c_ of the field-magnets are tapering in both
+directions, as shown, for the purposes of concentrating the magnetism
+upon the middle of the pole-pieces.
+
+The connecting-frame F F of the field-magnets is in the form indicated
+in the side view, Fig. 274, the lower part being provided with the
+spreading curved cast legs _e e_, so that the machine will rest firmly
+upon two base-bars, _r r_.
+
+To the lower pole, S, of the field-magnet M is fastened, by means of
+babbitt or other fusible diamagnetic material, the base B, which is
+provided with bearings _b_ for the armature-shaft H. The base B has a
+projection, P, which supports the brush-holders and the regulating
+devices, which are of a special character devised by Mr. Tesla.
+
+The armature is constructed with the view to reduce to a minimum the
+loss of power due to Foucault currents and to the change of polarity,
+and also to shorten as much as possible the length of the inactive wire
+wound upon the armature core.
+
+[Illustration: FIG. 271.]
+
+It is well known that when the armature is revolved between the poles of
+the field-magnets, currents are generated in the iron body of the
+armature which develop heat, and consequently cause a waste of power.
+Owing to the mutual action of the lines of force, the magnetic
+properties of iron, and the speed of the different portions of the
+armature core, these currents are generated principally on and near the
+surface of the armature core, diminishing in strength gradually toward
+the centre of the core. Their quantity is under some conditions
+proportional to the length of the iron body in the direction in which
+these currents are generated. By subdividing the iron core electrically
+in this direction, the generation of these currents can be reduced to a
+great extent. For instance, if the length of the armature-core is twelve
+inches, and by a suitable construction it is subdivided electrically, so
+that there are in the generating direction six inches of iron and six
+inches of intervening air-spaces or insulating material, the waste
+currents will be reduced to fifty per cent.
+
+As shown in the diagrams, the armature is constructed of thin iron discs
+D D D, of various diameters, fastened upon the armature-shaft in a
+suitable manner and arranged according to their sizes, so that a series
+of iron bodies, _i i i_, is formed, each of which diminishes in
+thickness from the centre toward the periphery. At both ends of the
+armature the inwardly curved discs _d d_, of cast iron, are fastened to
+the armature shaft.
+
+The armature core being constructed as shown, it will be easily seen
+that on those portions of the armature that are the most remote from the
+axis, and where the currents are principally developed, the length of
+iron in the generating direction is only a small fraction of the total
+length of the armature core, and besides this the iron body is
+subdivided in the generating direction, and therefore the Foucault
+currents are greatly reduced. Another cause of heating is the shifting
+of the poles of the armature core. In consequence of the subdivision of
+the iron in the armature and the increased surface for radiation, the
+risk of heating is lessened.
+
+The iron discs D D D are insulated or coated with some insulating-paint,
+a very careful insulation being unnecessary, as an electrical contact
+between several discs can only occur at places where the generated
+currents are comparatively weak. An armature core constructed in the
+manner described may be revolved between the poles of the field magnets
+without showing the slightest increase of temperature.
+
+[Illustration: FIG. 272.]
+
+[Illustration: FIG. 273.]
+
+The end discs, _d d_, which are of sufficient thickness and, for the
+sake of cheapness, of cast-iron, are curved inwardly, as indicated in
+the drawings. The extent of the curve is dependent on the amount of wire
+to be wound upon the armatures. In this machine the wire is wound upon
+the armature in two superimposed parts, and the curve of the end discs,
+_d d_, is so calculated that the first part--that is, practically half
+of the wire--just fills up the hollow space to the line _x x_; or, if
+the wire is wound in any other manner, the curve is such that when the
+whole of the wire is wound, the outside mass of wires, _w_, and the
+inside mass of wires, _w'_, are equal at each side of the plane _x x_.
+In this case the passive or electrically-inactive wires are of the
+smallest length practicable. The arrangement has further the advantage
+that the total lengths of the crossing wires at the two sides of the
+plane _x x_ are practically equal.
+
+[Illustration: FIG. 274.]
+
+To equalize further the armature coils at both sides of the plates that
+are in contact with the brushes, the winding and connecting up is
+effected in the following manner: The whole wire is wound upon the
+armature-core in two superimposed parts, which are thoroughly insulated
+from each other. Each of these two parts is composed of three separated
+groups of coils. The first group of coils of the first part of wire
+being wound and connected to the commutator-bars in the usual manner,
+this group is insulated and the second group wound; but the coils of
+this second group, instead of being connected to the next following
+commutator bars, are connected to the directly opposite bars of the
+commutator. The second group is then insulated and the third group
+wound, the coils of this group being connected to those bars to which
+they would be connected in the usual way. The wires are then thoroughly
+insulated and the second part of wire is wound and connected in the same
+manner.
+
+Suppose, for instance, that there are twenty-four coils--that is, twelve
+in each part--and consequently twenty-four commutator plates. There will
+be in each part three groups, each containing four coils, and the coils
+will be connected as follows:
+
+                        _Groups._   _Commutator Bars._
+                      { First            1--5
+  First part of wire  { Second          17--21
+                      { Third            9--13
+
+                      { First           13--17
+  Second part of wire { Second           5--9
+                      { Third           21--1
+
+In constructing the armature core and winding and connecting the coils
+in the manner indicated, the passive or electrically inactive wire is
+reduced to a minimum, and the coils at each side of the plates that are
+in contact with the brushes are practically equal. In this way the
+electrical efficiency of the machine is increased.
+
+[Illustration: FIG. 275.]
+
+[Illustration: FIG. 276.]
+
+The commutator plates _t_ are shown as outside the bearing _b_ of the
+armature shaft. The shaft H is tubular and split at the end portion, and
+the wires are carried through the same in the usual manner and connected
+to the respective commutator plates. The commutator plates are upon a
+cylinder, _u_, and insulated, and this cylinder is properly placed and
+then secured by expanding the split end of the shaft by a tapering screw
+plug, _v_.
+
+[Illustration: FIG. 277.]
+
+The arc lamps invented by Mr. Tesla for use on the circuits from the
+above described dynamo are those in which the separation and feed of the
+carbon electrodes or their equivalents is accomplished by means of
+electro-magnets or solenoids in connection with suitable clutch
+mechanism, and were designed for the purpose of remedying certain
+faults common to arc lamps.
+
+He proposed to prevent the frequent vibrations of the movable carbon
+"point" and flickering of the light arising therefrom; to prevent the
+falling into contact of the carbons; to dispense with the dash pot,
+clock work, or gearing and similar devices; to render the lamp extremely
+sensitive, and to feed the carbon almost imperceptibly, and thereby
+obtain a very steady and uniform light.
+
+In that class of lamps where the regulation of the arc is effected by
+forces acting in opposition on a free, movable rod or lever directly
+connected with the electrode, all or some of the forces being dependent
+on the strength of the current, any change in the electrical condition
+of the circuit causes a vibration and a corresponding flicker in the
+light. This difficulty is most apparent when there are only a few lamps
+in circuit. To lessen this difficulty lamps have been constructed in
+which the lever or armature, after the establishing of the arc, is kept
+in a fixed position and cannot vibrate during the feed operation, the
+feed mechanism acting independently; but in these lamps, when a clamp is
+employed, it frequently occurs that the carbons come into contact and
+the light is momentarily extinguished, and frequently parts of the
+circuit are injured. In both these classes of lamps it has been
+customary to use dash pot, clock work, or equivalent retarding devices;
+but these are often unreliable and objectionable, and increase the cost
+of construction.
+
+Mr. Tesla combines two electro-magnets--one of low resistance in the
+main or lamp circuit, and the other of comparatively high resistance in
+a shunt around the arc--a movable armature lever, and a special feed
+mechanism, the parts being arranged so that in the normal working
+position of the armature lever the same is kept almost rigidly in one
+position, and is not affected even by considerable changes in the
+electric circuit; but if the carbons fall into contact the armature will
+be actuated by the magnets so as to move the lever and start the arc,
+and hold the carbons until the arc lengthens and the armature lever
+returns to the normal position. After this the carbon rod holder is
+released by the action of the feed mechanism, so as to feed the carbon
+and restore the arc to its normal length.
+
+Fig. 278 is an elevation of the mechanism made use of in this arc lamp.
+Fig. 279 is a plan view. Fig. 280 is an elevation of the balancing lever
+and spring; Fig. 281 is a detached plan view of the pole pieces and
+armatures upon the friction clamp, and Fig. 282 is a section of the
+clamping tube.
+
+M is a helix of coarse wire in a circuit from the lower carbon holder to
+the negative binding screw -. N is a helix of fine wire in a shunt
+between the positive binding screw + and the negative binding screw -.
+The upper carbon holder S is a parallel rod sliding through the plates
+S' S^{2} of the frame of the lamp, and hence the electric current passes
+from the positive binding post + through the plate S^{2}, carbon holder
+S, and upper carbon to the lower carbon, and thence by the holder and a
+metallic connection to the helix M.
+
+[Illustration: FIG. 278.]
+
+[Illustration: FIG. 279.]
+
+[Illustration: FIG. 280.]
+
+[Illustration: FIG. 281.]
+
+[Illustration: FIG. 282.]
+
+The carbon holders are of the usual character, and to insure electric
+connections the springs _l_ are made use of to grasp the upper carbon
+holding rod S, but to allow the rod to slide freely through the same.
+These springs _l_ may be adjusted in their pressure by the screw _m_,
+and the spring _l_ maybe sustained upon any suitable support. They are
+shown as connected with the upper end of the core of the magnet N.
+
+Around the carbon-holding rod S, between the plates S' S^{2}, there is a
+tube, R, which forms a clamp. This tube is counter-bored, as seen in the
+section Fig. 282, so that it bears upon the rod S at its upper end and
+near the middle, and at the lower end of this tubular clamp R there are
+armature segments _r_ of soft iron. A frame or arm, _n_, extending,
+preferably, from the core N^{2}, supports the lever A by a fulcrum-pin,
+_o_. This lever A has a hole, through which the upper end of the tubular
+clamp R passes freely, and from the lever A is a link, _q_, to the lever
+_t_, which lever is pivoted at _y_ to a ring upon one of the columns
+S^{3}. This lever _t_ has an opening or bow surrounding the tubular
+clamp R, and there are pins or pivotal connections _w_ between the lever
+_t_ and this clamp R, and a spring, _r^{2}_, serves to support or
+suspend the weight of the parts and balance them, or nearly so. This
+spring is adjustable.
+
+At one end of the lever A is a soft-iron armature block, _a_, over the
+core M' of the helix M, and there is a limiting screw, _c_, passing
+through this armature block _a_, and at the other end of the lever A is
+a soft iron armature block, _b_, with the end tapering or wedge shaped,
+and the same comes close to and in line with the lateral projection _e_
+on the core N^{2}. The lower ends of the cores M' N^{2} are made with
+laterally projecting pole-pieces M^{3} N^{3}, respectively, and these
+pole-pieces are concave at their outer ends, and are at opposite sides
+of the armature segments _r_ at the lower end of the tubular clamp R.
+
+The operation of these devices is as follows: In the condition of
+inaction, the upper carbon rests upon the lower one, and when the
+electric current is turned on it passes freely, by the frame and spring
+_l_, through the rods and carbons to the coarse wire and helix M, and to
+the negative binding post V and the core M' thereby is energized. The
+pole piece M^{3} attracts the armature _r_, and by the lateral pressure
+causes the clamp R to grasp the rod S', and the lever A is
+simultaneously moved from the position shown by dotted lines, Fig. 278,
+to the normal position shown in full lines, and in so doing the link _q_
+and lever _t_ are raised, lifting the clamp R and S, separating the
+carbons and forming the arc. The magnetism of the pole piece _e_ tends
+to hold the lever A level, or nearly so, the core N^{2} being energized
+by the current in the shunt which contains the helix N. In this position
+the lever A is not moved by any ordinary variation in the current,
+because the armature _b_ is strongly attracted by the magnetism of _e_,
+and these parts are close to each other, and the magnetism of _e_ acts
+at right angles to the magnetism of the core M'. If, now, the arc
+becomes too long, the current through the helix M is lessened, and the
+magnetism of the core N^{3} is increased by the greater current passing
+through the shunt, and this core N^{3}, attracting the segmental
+armature _r_, lessens the hold of the clamp R upon the rod S, allowing
+the latter to slide and lessen the length of the arc, which instantly
+restores the magnetic equilibrium and causes the clamp R to hold the rod
+S. If it happens that the carbons fall into contact, then the magnetism
+of N^{2} is lessened so much that the attraction of the magnet M will be
+sufficient to move the armature _a_ and lever A so that the armature _b_
+passes above the normal position, so as to separate the carbons
+instantly; but when the carbons burn away, a greater amount of current
+will pass through the shunt until the attraction of the core N^{2} will
+overcome the attraction of the core M' and bring the armature lever A
+again into the normal horizontal position, and this occurs before the
+feed can take place. The segmental armature pieces _r_ are shown as
+nearly semicircular. They are square or of any other desired shape, the
+ends of the pole pieces M^{3}, N^{3} being made to correspond in shape.
+
+In a modification of this lamp, Mr. Tesla provided means for
+automatically withdrawing a lamp from the circuit, or cutting it out
+when, from a failure of the feed, the arc reached an abnormal length;
+and also means for automatically reinserting such lamp in the circuit
+when the rod drops and the carbons come into contact.
+
+Fig. 283 is an elevation of the lamp with the case in section. Fig. 284
+is a sectional plan at the line _x x_. Fig. 285 is an elevation, partly
+in section, of the lamp at right angles to Fig. 283. Fig. 286 is a
+sectional plan at the line _y y_ of Fig. 283. Fig. 287 is a section of
+the clamp in about full size. Fig. 288 is a detached section
+illustrating the connection of the spring to the lever that carries the
+pivots of the clamp, and Fig. 289 is a diagram showing the
+circuit-connections of the lamp.
+
+In Fig. 283, M represents the main and N the shunt magnet, both securely
+fastened to the base A, which with its side columns, S S, are cast in
+one piece of brass or other diamagnetic material. To the magnets are
+soldered or otherwise fastened the brass washers or discs _a a a a_.
+Similar washers, _b b_, of fibre or other insulating material, serve to
+insulate the wires from the brass washers.
+
+The magnets M and N are made very flat, so that their width exceeds
+three times their thickness, or even more. In this way a comparatively
+small number of convolutions is sufficient to produce the required
+magnetism, while a greater surface is offered for cooling off the wires.
+
+[Illustration: FIG. 286.]
+
+[Illustration: FIG. 283.]
+
+[Illustration: FIG. 285.]
+
+[Illustration: FIG. 284.]
+
+[Illustration: FIG. 287.]
+
+[Illustration: FIG. 288.]
+
+The upper pole pieces, _m n_, of the magnets are curved, as indicated in
+the drawings, Fig. 283. The lower pole pieces _m' n'_, are brought near
+together, tapering toward the armature _g_, as shown in Figs. 284 and
+286. The object of this taper is to concentrate the greatest amount of
+the developed magnetism upon the armature, and also to allow the pull to
+be exerted always upon the middle of the armature _g_. This armature _g_
+is a piece of iron in the shape of a hollow cylinder, having on each
+side a segment cut away, the width of which is equal to the width of the
+pole pieces _m' n'_.
+
+The armature is soldered or otherwise fastened to the clamp _r_, which
+is formed of a brass tube, provided with gripping-jaws _e e_, Fig. 287.
+These jaws are arcs of a circle of the diameter of the rod R, and are
+made of hardened German silver. The guides _f f_, through which the
+carbon-holding rod R slides, are made of the same material. This has the
+advantage of reducing greatly the wear and corrosion of the parts coming
+in frictional contact with the rod, which frequently causes trouble. The
+jaws _e e_ are fastened to the inside of the tube _r_, so that one is a
+little lower than the other. The object of this is to provide a greater
+opening for the passage of the rod when the same is released by the
+clamp. The clamp _r_ is supported on bearings _w w_, Figs. 283, 285 and
+287, which are just in the middle between the jaws _e e_. The bearings
+_w w_ are carried by a lever, _t_, one end of which rests upon an
+adjustable support, _q_, of the side columns, S, the other end being
+connected by means of the link _e'_ to the armature-lever L. The
+armature-lever L is a flat piece of iron in N shape, having its ends
+curved so as to correspond to the form of the upper pole-pieces of the
+magnets M and N. It is hung upon the pivots _v v_, Fig. 284, which are
+in the jaw _x_ of the top plate B. This plate B, with the jaw, is cast
+in one piece and screwed to the side columns, S S, that extend up from
+the base A. To partly balance the overweight of the moving parts, a
+spring, _s'_, Figs. 284 and 288, is fastened to the top plate, B, and
+hooked to the lever _t_. The hook _o_ is toward one side of the lever or
+bent a little sidewise, as seen in Fig. 288. By this means a slight
+tendency is given to swing the armature toward the pole-piece _m'_ of
+the main magnet.
+
+The binding-posts K K' are screwed to the base A. A manual switch, for
+short-circuiting the lamp when the carbons are renewed, is also fastened
+to the base. This switch is of ordinary character, and is not shown in
+the drawings.
+
+The rod R is electrically connected to the lamp-frame by means of a
+flexible conductor or otherwise. The lamp-case receives a removable
+cover, _s^{2}_, to inclose the parts.
+
+The electrical connections are as indicated diagrammatically in Fig.
+289. The wire in the main magnet consists of two parts, _x'_ and _p'_.
+These two parts may be in two separated coils or in one single helix,
+as shown in the drawings. The part _x'_ being normally in circuit, is,
+with the fine wire upon the shunt-magnet, wound and traversed by the
+current in the same direction, so as to tend to produce similar poles, N
+N or S S, on the corresponding pole-pieces of the magnets M and N. The
+part _p'_ is only in circuit when the lamp is cut out, and then the
+current being in the opposite direction produces in the main magnet,
+magnetism of the opposite polarity.
+
+The operation is as follows: At the start the carbons are to be in
+contact, and the current passes from the positive binding-post K to the
+lamp-frame, carbon-holder, upper and lower carbon, insulated return-wire
+in one of the side rods, and from there through the part _x'_ of the
+wire on the main magnet to the negative binding-post. Upon the passage
+of the current the main magnet is energized and attracts the
+clamping-armature _g_, swinging the clamp and gripping the rod by means
+of the gripping jaws _e e_. At the same time the armature lever L is
+pulled down and the carbons are separated. In pulling down the armature
+lever L the main magnet is assisted by the shunt-magnet N, the latter
+being magnetized by magnetic induction from the magnet M.
+
+[Illustration: FIG. 289.]
+
+It will be seen that the armatures L and _g_ are practically the keepers
+for the magnets M and N, and owing to this fact both magnets with either
+one of the armatures L and _g_ may be considered as one horseshoe
+magnet, which we might term a "compound magnet." The whole of the
+soft-iron parts M, _m'_, _g_, _n'_, N and L form a compound magnet.
+
+The carbons being separated, the fine wire receives a portion of the
+current. Now, the magnetic induction from the magnet M is such as to
+produce opposite poles on the corresponding ends of the magnet N; but
+the current traversing the helices tends to produce similar poles on the
+corresponding ends of both magnets, and therefore as soon as the fine
+wire is traversed by sufficient current the magnetism of the whole
+compound magnet is diminished.
+
+With regard to the armature _g_ and the operation of the lamp, the pole
+_m'_ may be considered as the "clamping" and the pole _n'_ as the
+"releasing" pole.
+
+As the carbons burn away, the fine wire receives more current and the
+magnetism diminishes in proportion. This causes the armature lever L to
+swing and the armature _g_ to descend gradually under the weight of the
+moving parts until the end _p_, Fig. 283, strikes a stop on the top
+plate, B. The adjustment is such that when this takes place the rod R is
+yet gripped securely by the jaws _e e_. The further downward movement of
+the armature lever being prevented, the arc becomes longer as the
+carbons are consumed, and the compound magnet is weakened more and more
+until the clamping armature _g_ releases the hold of the gripping-jaws
+_e e_ upon the rod R, and the rod is allowed to drop a little, thus
+shortening the arc. The fine wire now receiving less current, the
+magnetism increases, and the rod is clamped again and slightly raised,
+if necessary. This clamping and releasing of the rod continues until the
+carbons are consumed. In practice the feed is so sensitive that for the
+greatest part of the time the movement of the rod cannot be detected
+without some actual measurement. During the normal operation of the lamp
+the armature lever L remains practically stationary, in the position
+shown in Fig. 283.
+
+Should it happen that, owing to an imperfection in it, the rod and the
+carbons drop too far, so as to make the arc too short, or even bring the
+carbons in contact, a very small amount of current passes through the
+fine wire, and the compound magnet becomes sufficiently strong to act as
+at the start in pulling the armature lever L down and separating the
+carbons to a greater distance.
+
+It occurs often in practical work that the rod sticks in the guides. In
+this case the are reaches a great length, until it finally breaks. Then
+the light goes out, and frequently the fine wire is injured. To prevent
+such an accident Mr. Tesla provides this lamp with an automatic cut-out
+which operates as follows: When, upon a failure of the feed, the arc
+reaches a certain predetermined length, such an amount of current is
+diverted through the fine wire that the polarity of the compound magnet
+is reversed. The clamping armature _g_ is now moved against the shunt
+magnet N until it strikes the releasing pole _n'_. As soon as the
+contact is established, the current passes from the positive binding
+post over the clamp _r_, armature _g_, insulated shunt magnet, and the
+helix _p'_ upon the main magnet M to the negative binding post. In this
+case the current passes in the opposite direction and changes the
+polarity of the magnet M, at the same time maintaining by magnetic
+induction in the core of the shunt magnet the required magnetism without
+reversal of polarity, and the armature _g_ remains against the shunt
+magnet pole _n'_. The lamp is thus cut out as long as the carbons are
+separated. The cut out may be used in this form without any further
+improvement; but Mr. Tesla arranges it so that if the rod drops and the
+carbons come in contact the arc is started again. For this purpose he
+proportions the resistance of part _p'_ and the number of the
+convolutions of the wire upon the main magnet so that when the carbons
+come in contact a sufficient amount of current is diverted through the
+carbons and the part _x'_ to destroy or neutralize the magnetism of the
+compound magnet. Then the armature _g_, having a slight tendency to
+approach to the clamping pole _m'_, comes out of contact with the
+releasing pole _n'_. As soon as this happens, the current through the
+part _p'_ is interrupted, and the whole current passes through the part
+_x_. The magnet M is now strongly magnetized, the armature _g_ is
+attracted, and the rod clamped. At the same time the armature lever L is
+pulled down out of its normal position and the arc started. In this way
+the lamp cuts itself out automatically when the arc gets too long, and
+reinserts itself automatically in the circuit if the carbons drop
+together.
+
+
+
+
+CHAPTER XLI.
+
+IMPROVEMENT IN "UNIPOLAR" GENERATORS.
+
+
+Another interesting class of apparatus to which Mr. Tesla has directed
+his attention, is that of "unipolar" generators, in which a disc or a
+cylindrical conductor is mounted between magnetic poles adapted to
+produce an approximately uniform field. In the disc armature machines
+the currents induced in the rotating conductor flow from the centre to
+the periphery, or conversely, according to the direction of rotation or
+the lines of force as determined by the signs of the magnetic poles, and
+these currents are taken off usually by connections or brushes applied
+to the disc at points on its periphery and near its centre. In the case
+of the cylindrical armature machine, the currents developed in the
+cylinder are taken off by brushes applied to the sides of the cylinder
+at its ends.
+
+In order to develop economically an electromotive force available for
+practicable purposes, it is necessary either to rotate the conductor at
+a very high rate of speed or to use a disc of large diameter or a
+cylinder of great length; but in either case it becomes difficult to
+secure and maintain a good electrical connection between the collecting
+brushes and the conductor, owing to the high peripheral speed.
+
+It has been proposed to couple two or more discs together in series,
+with the object of obtaining a higher electro-motive force; but with the
+connections heretofore used and using other conditions of speed and
+dimension of disc necessary to securing good practicable results, this
+difficulty is still felt to be a serious obstacle to the use of this
+kind of generator. These objections Mr. Tesla has sought to avoid by
+constructing a machine with two fields, each having a rotary conductor
+mounted between its poles. The same principle is involved in the case of
+both forms of machine above described, but the description now given is
+confined to the disc type, which Mr. Tesla is inclined to favor for that
+machine. The discs are formed with flanges, after the manner of
+pulleys, and are connected together by flexible conducting bands or
+belts.
+
+The machine is built in such manner that the direction of magnetism or
+order of the poles in one field of force is opposite to that in the
+other, so that rotation of the discs in the same direction develops a
+current in one from centre to circumference and in the other from
+circumference to centre. Contacts applied therefore to the shafts upon
+which the discs are mounted form the terminals of a circuit the
+electro-motive force in which is the sum of the electro-motive forces of
+the two discs.
+
+It will be obvious that if the direction of magnetism in both fields be
+the same, the same result as above will be obtained by driving the discs
+in opposite directions and crossing the connecting belts. In this way
+the difficulty of securing and maintaining good contact with the
+peripheries of the discs is avoided and a cheap and durable machine made
+which is useful for many purposes--such as for an exciter for
+alternating current generators, for a motor, and for any other purpose
+for which dynamo machines are used.
+
+[Illustration: FIG. 290.]
+
+[Illustration: FIG. 291.]
+
+Fig. 290 is a side view, partly in section, of this machine. Fig. 291 is
+a vertical section of the same at right angles to the shafts.
+
+In order to form a frame with two fields of force, a support, A, is cast
+with two pole pieces B B' integral with it. To this are joined by bolts
+E a casting D, with two similar and corresponding pole pieces C C'. The
+pole pieces B B' are wound and connected to produce a field of force of
+given polarity, and the pole pieces C C' are wound so as to produce a
+field of opposite polarity. The driving shafts F G pass through the
+poles and are journaled in insulating bearings in the casting A D, as
+shown.
+
+H K are the discs or generating conductors. They are composed of copper,
+brass, or iron and are keyed or secured to their respective shafts. They
+are provided with broad peripheral flanges J. It is of course obvious
+that the discs may be insulated from their shafts, if so desired. A
+flexible metallic belt L is passed over the flanges of the two discs,
+and, if desired, may be used to drive one of the discs. It is better,
+however, to use this belt merely as a conductor, and for this purpose
+sheet steel, copper, or other suitable metal is used. Each shaft is
+provided with a driving pulley M, by which power is imparted from a
+driving shaft.
+
+N N are the terminals. For the sake of clearness they are shown as
+provided with springs P, that bear upon the ends of the shafts. This
+machine, if self-exciting, would have copper bands around its poles; or
+conductors of any kind--such as wires shown in the drawings--may be
+used.
+
+       *       *       *       *       *
+
+It is thought appropriate by the compiler to append here some notes on
+unipolar dynamos, written by Mr. Tesla, on a recent occasion.
+
+
+NOTES ON A UNIPOLAR DYNAMO.[15]
+
+  [15] Article by Mr. Tesla, contributed to _The Electrical Engineer_,
+       N. Y., Sept. 2, 1891.
+
+It is characteristic of fundamental discoveries, of great achievements
+of intellect, that they retain an undiminished power upon the
+imagination of the thinker. The memorable experiment of Faraday with a
+disc rotating between the two poles of a magnet, which has borne such
+magnificent fruit, has long passed into every-day experience; yet there
+are certain features about this embryo of the present dynamos and motors
+which even to-day appear to us striking, and are worthy of the most
+careful study.
+
+Consider, for instance, the case of a disc of iron or other metal
+revolving between the two opposite poles of a magnet, and the polar
+surfaces completely covering both sides of the disc, and assume the
+current to be taken off or conveyed to the same by contacts uniformly
+from all points of the periphery of the disc. Take first the case of a
+motor. In all ordinary motors the operation is dependent upon some
+shifting or change of the resultant of the magnetic attraction exerted
+upon the armature, this process being effected either by some mechanical
+contrivance on the motor or by the action of currents of the proper
+character. We may explain the operation of such a motor just as we can
+that of a water-wheel. But in the above example of the disc surrounded
+completely by the polar surfaces, there is no shifting of the magnetic
+action, no change whatever, as far as we know, and yet rotation ensues.
+Here, then, ordinary considerations do not apply; we cannot even give a
+superficial explanation, as in ordinary motors, and the operation will
+be clear to us only when we shall have recognized the very nature of the
+forces concerned, and fathomed the mystery of the invisible connecting
+mechanism.
+
+Considered as a dynamo machine, the disc is an equally interesting
+object of study. In addition to its peculiarity of giving currents of
+one direction without the employment of commutating devices, such a
+machine differs from ordinary dynamos in that there is no reaction
+between armature and field. The armature current tends to set up a
+magnetization at right angles to that of the field current, but since
+the current is taken off uniformly from all points of the periphery, and
+since, to be exact, the external circuit may also be arranged perfectly
+symmetrical to the field magnet, no reaction can occur. This, however,
+is true only as long as the magnets are weakly energized, for when the
+magnets are more or less saturated, both magnetizations at right angles
+seemingly interfere with each other.
+
+For the above reason alone it would appear that the output of such a
+machine should, for the same weight, be much greater than that of any
+other machine in which the armature current tends to demagnetize the
+field. The extraordinary output of the Forbes unipolar dynamo and the
+experience of the writer confirm this view.
+
+Again, the facility with which such a machine may be made to excite
+itself is striking, but this may be due--besides to the absence of
+armature reaction--to the perfect smoothness of the current and
+non-existence of self-induction.
+
+If the poles do not cover the disc completely on both sides, then, of
+course, unless the disc be properly subdivided, the machine will be very
+inefficient. Again, in this case there are points worthy of notice. If
+the disc be rotated and the field current interrupted, the current
+through the armature will continue to flow and the field magnets will
+lose their strength comparatively slowly. The reason for this will at
+once appear when we consider the direction of the currents set up in the
+disc.
+
+[Illustration: FIG. 292.]
+
+Referring to the diagram Fig. 292, _d_ represents the disc with the
+sliding contacts B B' on the shaft and periphery. N and S represent the
+two poles of a magnet. If the pole N be above, as indicated in the
+diagram, the disc being supposed to be in the plane of the paper, and
+rotating in the direction of the arrow D, the current set up in the disc
+will flow from the centre to the periphery, as indicated by the arrow A.
+Since the magnetic action is more or less confined to the space between
+the poles N S, the other portions of the disc may be considered
+inactive. The current set up will therefore not wholly pass through the
+external circuit F, but will close through the disc itself, and
+generally, if the disposition be in any way similar to the one
+illustrated, by far the greater portion of the current generated will
+not appear externally, as the circuit F is practically short-circuited
+by the inactive portions of the disc. The direction of the resulting
+currents in the latter may be assumed to be as indicated by the dotted
+lines and arrows _m_ and _n_; and the direction of the energizing field
+current being indicated by the arrows _a b c d_, an inspection of the
+figure shows that one of the two branches of the eddy current, that is,
+A B' _m_ B, will tend to demagnetize the field, while the other branch,
+that is, A B' _n_ B, will have the opposite effect. Therefore, the
+branch A B' _m_ B, that is, the one which is _approaching_ the field,
+will repel the lines of the same, while branch A B' _n_ B, that is, the
+one _leaving_ the field, will gather the lines of force upon itself.
+
+In consequence of this there will be a constant tendency to reduce the
+current flow in the path A B' _m_ B, while on the other hand no such
+opposition will exist in path A B' _n_ B, and the effect of the latter
+branch or path will be more or less preponderating over that of the
+former. The joint effect of both the assumed branch currents might be
+represented by that of one single current of the same direction as that
+energizing the field. In other words, the eddy currents circulating in
+the disc will energize the field magnet. This is a result quite contrary
+to what we might be led to suppose at first, for we would naturally
+expect that the resulting effect of the armature currents would be such
+as to oppose the field current, as generally occurs when a primary and
+secondary conductor are placed in inductive relations to each other. But
+it must be remembered that this results from the peculiar disposition in
+this case, namely, two paths being afforded to the current, and the
+latter selecting that path which offers the least opposition to its
+flow. From this we see that the eddy currents flowing in the disc partly
+energize the field, and for this reason when the field current is
+interrupted the currents in the disc will continue to flow, and the
+field magnet will lose its strength with comparative slowness and may
+even retain a certain strength as long as the rotation of the disc is
+continued.
+
+The result will, of course, largely depend on the resistance and
+geometrical dimensions of the path of the resulting eddy current and on
+the speed of rotation; these elements, namely, determine the retardation
+of this current and its position relative to the field. For a certain
+speed there would be a maximum energizing action; then at higher speeds,
+it would gradually fall off to zero and finally reverse, that is, the
+resultant eddy current effect would be to weaken the field. The reaction
+would be best demonstrated experimentally by arranging the fields N S,
+N' S', freely movable on an axis concentric with the shaft of the disc.
+If the latter were rotated as before in the direction of the arrow D,
+the field would be dragged in the same direction with a torque, which,
+up to a certain point, would go on increasing with the speed of
+rotation, then fall off, and, passing through zero, finally become
+negative; that is, the field would begin to rotate in opposite direction
+to the disc. In experiments with alternate current motors in which the
+field was shifted by currents of differing phase, this interesting
+result was observed. For very low speeds of rotation of the field the
+motor would show a torque of 900 lbs. or more, measured on a pulley 12
+inches in diameter. When the speed of rotation of the poles was
+increased, the torque would diminish, would finally go down to zero,
+become negative, and then the armature would begin to rotate in opposite
+direction to the field.
+
+To return to the principal subject; assume the conditions to be such
+that the eddy currents generated by the rotation of the disc strengthen
+the field, and suppose the latter gradually removed while the disc is
+kept rotating at an increased rate. The current, once started, may then
+be sufficient to maintain itself and even increase in strength, and then
+we have the case of Sir William Thomson's "current accumulator." But
+from the above considerations it would seem that for the success of the
+experiment the employment of a disc _not subdivided_[16] would be
+essential, for if there should be a radial subdivision, the eddy
+currents could not form and the self-exciting action would cease. If
+such a radially subdivided disc were used it would be necessary to
+connect the spokes by a conducting rim or in any proper manner so as to
+form a symmetrical system of closed circuits.
+
+  [16] Mr. Tesla here refers to an interesting article which appeared
+       in July, 1865, in the _Phil. Magazine_, by Sir W. Thomson, in
+       which Sir William, speaking of his "uniform electric current
+       accumulator," assumes that for self-excitation it is desirable
+       to subdivide the disc into an infinite number of infinitely thin
+       spokes, in order to prevent diffusion of the current. Mr. Tesla
+       shows that diffusion is absolutely necessary for the excitation
+       and that when the disc is subdivided no excitation can occur.
+
+The action of the eddy currents may be utilized to excite a machine of
+any construction. For instance, in Figs. 293 and 294 an arrangement is
+shown by which a machine with a disc armature might be excited. Here a
+number of magnets, N S, N S, are placed radially on each side of a metal
+disc D carrying on its rim a set of insulated coils, C C. The magnets
+form two separate fields, an internal and an external one, the solid
+disc rotating in the field nearest the axis, and the coils in the field
+further from it. Assume the magnets slightly energized at the start;
+they could be strengthened by the action of the eddy currents in the
+solid disc so as to afford a stronger field for the peripheral coils.
+Although there is no doubt that under proper conditions a machine might
+be excited in this or a similar manner, there being sufficient
+experimental evidence to warrant such an assertion, such a mode of
+excitation would be wasteful.
+
+But a unipolar dynamo or motor, such as shown in Fig. 292, may be
+excited in an efficient manner by simply properly subdividing the disc
+or cylinder in which the currents are set up, and it is practicable to
+do away with the field coils which are usually employed. Such a plan is
+illustrated in Fig. 295. The disc or cylinder D is supposed to be
+arranged to rotate between the two poles N and S of a magnet, which
+completely cover it on both sides, the contours of the disc and poles
+being represented by the circles _d_ and _d^{1}_ respectively, the upper
+pole being omitted for the sake of clearness. The cores of the magnet
+are supposed to be hollow, the shaft C of the disc passing through them.
+If the unmarked pole be below, and the disc be rotated screw fashion,
+the current will be, as before, from the centre to the periphery, and
+may be taken off by suitable sliding contacts, B B', on the shaft and
+periphery respectively. In this arrangement the current flowing through
+the disc and external circuit will have no appreciable effect on the
+field magnet.
+
+[Illustration: FIG. 293.]
+
+[Illustration: FIG. 294.]
+
+But let us now suppose the disc to be subdivided spirally, as indicated
+by the full or dotted lines, Fig. 295. The difference of potential
+between a point on the shaft and a point on the periphery will remain
+unchanged, in sign as well as in amount. The only difference will be
+that the resistance of the disc will be augmented and that there will be
+a greater fall of potential from a point on the shaft to a point on the
+periphery when the same current is traversing the external circuit. But
+since the current is forced to follow the lines of subdivision, we see
+that it will tend either to energize or de-energize the field, and this
+will depend, other things being equal, upon the direction of the lines
+of subdivision. If the subdivision be as indicated by the full lines in
+Fig. 295, it is evident that if the current is of the same direction as
+before, that is, from centre to periphery, its effect will be to
+strengthen the field magnet; Whereas, if the subdivision be as indicated
+by the dotted lines, the current generated will tend to weaken the
+magnet. In the former case the machine will be capable of exciting
+itself when the disc is rotated in the direction of arrow D; in the
+latter case the direction of rotation must be reversed. Two such discs
+may be combined, however, as indicated, the two discs rotating in
+opposite fields, and in the same or opposite direction.
+
+[Illustration: FIG. 295.]
+
+[Illustration: FIG. 296.]
+
+Similar disposition may, of course, be made in a type of machine in
+which, instead of a disc, a cylinder is rotated. In such unipolar
+machines, in the manner indicated, the usual field coils and poles may
+be omitted and the machine may be made to consist only of a cylinder or
+of two discs enveloped by a metal casting.
+
+Instead of subdividing the disc or cylinder spirally, as indicated in
+Fig. 295, it is more convenient to interpose one or more turns between
+the disc and the contact ring on the periphery, as illustrated in Fig.
+296.
+
+A Forbes dynamo may, for instance, be excited in such a manner. In the
+experience of the writer it has been found that instead of taking the
+current from two such discs by sliding contacts, as usual, a flexible
+conducting belt may be employed to advantage. The discs are in such case
+provided with large flanges, affording a very great contact surface. The
+belt should be made to bear on the flanges with spring pressure to take
+up the expansion. Several machines with belt contact were constructed by
+the writer two years ago, and worked satisfactorily; but for want of
+time the work in that direction has been temporarily suspended. A number
+of features pointed out above have also been used by the writer in
+connection with some types of alternating current motors.
+
+
+
+
+PART IV.
+
+APPENDIX.--EARLY PHASE MOTORS AND THE TESLA MECHANICAL AND ELECTRICAL
+OSCILLATOR.
+
+
+
+
+CHAPTER XLII.
+
+MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.
+
+While the exhibits of firms engaged in the manufacture of electrical
+apparatus of every description at the Chicago World's Fair, afforded the
+visitor ample opportunity for gaining an excellent knowledge of the
+state of the art, there were also numbers of exhibits which brought out
+in strong relief the work of the individual inventor, which lies at the
+foundation of much, if not all, industrial or mechanical achievement.
+Prominent among such personal exhibits was that of Mr. Tesla, whose
+apparatus occupied part of the space of the Westinghouse Company, in
+Electricity Building.
+
+This apparatus represented the results of work and thought covering a
+period of ten years. It embraced a large number of different alternating
+motors and Mr. Tesla's earlier high frequency apparatus. The motor
+exhibit consisted of a variety of fields and armatures for two, three
+and multiphase circuits, and gave a fair idea of the gradual evolution
+of the fundamental idea of the rotating magnetic field. The high
+frequency exhibit included Mr. Tesla's earlier machines and disruptive
+discharge coils and high frequency transformers, which he used in his
+investigations and some of which are referred to in his papers printed
+in this volume.
+
+Fig. 297 shows a view of part of the exhibits containing the motor
+apparatus. Among these is shown at A a large ring intended to exhibit
+the phenomena of the rotating magnetic field. The field produced was
+very powerful and exhibited striking effects, revolving copper balls and
+eggs and bodies of various shapes at considerable distances and at great
+speeds. This ring was wound for two-phase circuits, and the winding was
+so distributed that a practically uniform field was obtained. This ring
+was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician of
+the Westinghouse Electric and Manufacturing Company.
+
+[Illustration: FIG. 297.]
+
+A smaller ring, shown at B, was arranged like the one exhibited at A but
+designed especially to exhibit the rotation of an armature in a rotating
+field. In connection with these two rings there was an interesting
+exhibit shown by Mr. Tesla which consisted of a magnet with a coil, the
+magnet being arranged to rotate in bearings. With this magnet he first
+demonstrated the identity between a rotating field and a rotating
+magnet; the latter, when rotating, exhibited the same phenomena as the
+rings when they were energized by currents of differing phase. Another
+prominent exhibit was a model illustrated at C which is a two-phase
+motor, as well as an induction motor and transformer. It consists of a
+large outer ring of laminated iron wound with two superimposed,
+separated windings which can be connected in a variety of ways. This is
+one of the first models used by Mr. Tesla as an induction motor and
+rotating transformer. The armature was either a steel or wrought iron
+disc with a closed coil. When the motor was operated from a two phase
+generator the windings were connected in two groups, as usual. When used
+as an induction motor, the current induced in one of the windings of the
+ring was passed through the other winding on the ring and so the motor
+operated with only two wires. When used as a transformer the outer
+winding served, for instance, as a secondary and the inner as a primary.
+The model shown at D is one of the earliest rotating field motors,
+consisting of a thin iron ring wound with two sets of coils and an
+armature consisting of a series of steel discs partly cut away and
+arranged on a small arbor.
+
+At E is shown one of the first rotating field or induction motors used
+for the regulation of an arc lamp and for other purposes. It comprises a
+ring of discs with two sets of coils having different self-inductions,
+one set being of German silver and the other of copper wire. The
+armature is wound with two closed-circuited coils at right angles to
+each other. To the armature shaft are fastened levers and other devices
+to effect the regulation. At F is shown a model of a magnetic lag motor;
+this embodies a casting with pole projections protruding from two coils
+between which is arranged to rotate a smooth iron body. When an
+alternating current is sent through the two coils the pole projections
+of the field and armature within it are similarly magnetized, and upon
+the cessation or reversal of the current the armature and field repel
+each other and rotation is produced in this way. Another interesting
+exhibit, shown at G, is an early model of a two field motor energized by
+currents of different phase. There are two independent fields of
+laminated iron joined by brass bolts; in each field is mounted an
+armature, both armatures being on the same shaft. The armatures were
+originally so arranged as to be placed in any position relatively to
+each other, and the fields also were arranged to be connected in a
+number of ways. The motor has served for the exhibition of a number of
+features; among other things, it has been used as a dynamo for the
+production of currents of any frequency between wide limits. In this
+case the field, instead of being energized by direct current, was
+energized by currents differing in phase, which produced a rotation of
+the field; the armature was then rotated in the same or in opposite
+direction to the movement of the field; and so any number of
+alternations of the currents induced in the armature, from a small to a
+high number, determined by the frequency of the energizing field coils
+and the speed of the armature, was obtained.
+
+[Illustration: FIG. 298.]
+
+The models H, I, J, represent a variety of rotating field, synchronous
+motors which are of special value in long distance transmission work.
+The principle embodied in these motors was enunciated by Mr. Tesla in
+his lecture before the American Institute of Electrical Engineers, in
+May, 1888[17]. It involves the production of the rotating field in one
+of the elements of the motor by currents differing in phase and
+energizing the other element by direct currents. The armatures are of
+the two and three phase type. K is a model of a motor shown in an
+enlarged view in Fig. 298. This machine, together with that shown in
+Fig. 299, was exhibited at the same lecture, in May, 1888. They were the
+first rotating field motors which were independently tested, having for
+that purpose been placed in the hands of Prof. Anthony in the winter of
+1887-88. From these tests it was shown that the efficiency and output of
+these motors was quite satisfactory in every respect.
+
+  [17] See Part I, Chap. III, page 9.
+
+[Illustration: FIG. 299.]
+
+It was intended to exhibit the model shown in Fig. 299, but it was
+unavailable for that purpose owing to the fact that it was some time ago
+handed over to the care of Prof. Ayrton in England. This model was
+originally provided with twelve independent coils; this number, as Mr.
+Tesla pointed out in his first lecture, being divisible by two and
+three, was selected in order to make various connections for two and
+three-phase operations, and during Mr. Tesla's experiments was used in
+many ways with from two to six phases. The model, Fig. 298, consists of
+a magnetic frame of laminated iron with four polar projections between
+which an armature is supported on brass bolts passing through the frame.
+A great variety of armatures was used in connection with these two and
+other fields. Some of the armatures are shown in front on the table,
+Fig. 297, and several are also shown enlarged in Figs. 300 to 310. An
+interesting exhibit is that shown at L, Fig. 297. This is an armature of
+hardened steel which was used in a demonstration before the Society of
+Arts in Boston, by Prof. Anthony. Another curious exhibit is shown
+enlarged in Fig. 301. This consists of thick discs of wrought iron
+placed lengthwise, with a mass of copper cast around them. The discs
+were arranged longitudinally to afford an easier starting by reason of
+the induced current formed in the iron discs, which differed in phase
+from those in the copper. This armature would start with a single
+circuit and run in synchronism, and represents one of the earliest types
+of such an armature. Fig. 305 is another striking exhibit. This is one
+of the earliest types of an armature with holes beneath the periphery,
+in which copper conductors are imbedded. The armature has eight closed
+circuits and was used in many different ways. Fig. 304 is a type of
+synchronous armature consisting of a block of soft steel wound with a
+coil closed upon itself. This armature was used in connection with the
+field shown in Fig. 298 and gave excellent results.
+
+[Illustration: FIG. 300.]
+
+[Illustration: FIG. 301.]
+
+[Illustration: FIG. 302.]
+
+[Illustration: FIG. 303.]
+
+[Illustration: FIG. 304.]
+
+[Illustration: FIG. 305.]
+
+[Illustration: FIG. 306.]
+
+[Illustration: FIG. 307.]
+
+[Illustration: FIG. 308.]
+
+[Illustration: FIG. 309.]
+
+[Illustration: FIG. 310.]
+
+Fig. 302 represents a synchronous armature with a large coil around a
+body of iron. There is another very small coil at right angles to the
+first. This small coil was used for the purpose of increasing the
+starting torque and was found very effective in this connection. Figs.
+306 and 308 show a favorite construction of armature; the iron body is
+made up of two sets of discs cut away and placed at right angles to each
+other, the interstices being wound with coils. The one shown in Fig. 308
+is provided with an additional groove on each of the projections formed
+by the discs, for the purpose of increasing the starting torque by a
+wire wound in these projections. Fig. 307 is a form of armature
+similarly constructed, but with four independent coils wound upon the
+four projections. This armature was used to reduce the speed of the
+motor with reference to that of the generator. Fig. 300 is still another
+armature with a great number of independent circuits closed upon
+themselves, so that all the dead points on the armature are done away
+with, and the armature has a large starting torque. Fig. 303 is another
+type of armature for a four-pole motor but with coils wound upon a
+smooth surface. A number of these armatures have hollow shafts, as they
+have been used in many ways. Figs. 309 and 310 represent armatures to
+which either alternating or direct current was conveyed by means of
+sliding rings. Fig. 309 consists of a soft iron body with a single coil
+wound around it, the ends of the coil being connected to two sliding
+rings to which, usually, direct current was conveyed. The armature shown
+in Fig. 310 has three insulated rings on a shaft and was used in
+connection with two or three phase circuits.
+
+All these models shown represent early work, and the enlarged engravings
+are made from photographs taken early in 1888. There is a great number
+of other models which were exhibited, but which are not brought out
+sharply in the engraving, Fig. 297. For example at M is a model of a
+motor comprising an armature with a hollow shaft wound with two or three
+coils for two or three-phase circuits; the armature was arranged to be
+stationary and the generating circuits were connected directly to the
+generator. Around the armature is arranged to rotate on its shaft a
+casting forming six closed circuits. On the outside this casting was
+turned smooth and the belt was placed on it for driving with any desired
+appliance. This also is a very early model.
+
+On the left side of the table there are seen a large variety of models,
+N, O, P, etc., with fields of various shapes. Each of these models
+involves some distinct idea and they all represent gradual development
+chiefly interesting as showing Mr. Tesla's efforts to adapt his system
+to the existing high frequencies.
+
+On the right side of the table, at S, T, are shown, on separate
+supports, larger and more perfected armatures of commercial motors, and
+in the space around the table a variety of motors and generators
+supplying currents to them was exhibited.
+
+The high frequency exhibit embraced Mr. Tesla's first original apparatus
+used in his investigations. There was exhibited a glass tube with one
+layer of silk-covered wire wound at the top and a copper ribbon on the
+inside. This was the first disruptive discharge coil constructed by him.
+At U is shown the disruptive discharge coil exhibited by him in his
+lecture before the American Institute of Electrical Engineers, in May,
+1891.[18] At V and W are shown some of the first high frequency
+transformers. A number of various fields and armatures of small models
+of high frequency apparatus as shown at X and Y, and others not visible
+in the picture, were exhibited. In the annexed space the dynamo then
+used by Mr. Tesla at Columbia College was exhibited; also another form
+of high frequency dynamo used.
+
+  [18] See Part II, Chap. XXVI., page 145.
+
+[Illustration: FIG. 311.]
+
+In this space also was arranged a battery of Leyden jars and his large
+disruptive discharge coil which was used for exhibiting the light
+phenomena in the adjoining dark room. The coil was operated at only a
+small fraction of its capacity, as the necessary condensers and
+transformers could not be had and as Mr. Tesla's stay was limited to one
+week; notwithstanding, the phenomena were of a striking character. In
+the room were arranged two large plates placed at a distance of about
+eighteen feet from each other. Between them were placed two long tables
+with all sorts of phosphorescent bulbs and tubes; many of these were
+prepared with great care and marked legibly with the names which would
+shine with phosphorescent glow. Among them were some with the names of
+Helmholtz, Faraday, Maxwell, Henry, Franklin, etc. Mr. Tesla had also
+not forgotten the greatest living poet of his own country, Zmaj Jovan;
+two or three were prepared with inscriptions, like "Welcome,
+Electricians," and produced a beautiful effect. Each represented some
+phase of this work and stood for some individual experiment of
+importance. Outside the room was the small battery seen in Fig. 311, for
+the exhibition of some of the impedance and other phenomena of interest.
+Thus, for instance, a thick copper bar bent in arched form was provided
+with clamps for the attachment of lamps, and a number of lamps were kept
+at incandescence on the bar; there was also a little motor shown on the
+table operated by the disruptive discharge.
+
+As will be remembered by those who visited the Exposition, the
+Westinghouse Company made a line exhibit of the various commercial
+motors of the Tesla system, while the twelve generators in Machinery
+Hall were of the two-phase type constructed for distributing light and
+power. Mr. Tesla, also exhibited some models of his oscillators.
+
+
+
+
+CHAPTER XLIII.
+
+THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.
+
+
+On the evening of Friday, August 25, 1893, Mr. Tesla delivered a lecture
+on his mechanical and electrical oscillators, before the members of the
+Electrical Congress, in the hall adjoining the Agricultural Building, at
+the World's Fair, Chicago. Besides the apparatus in the room, he
+employed an air compressor, which was driven by an electric motor.
+
+Mr. Tesla was introduced by Dr. Elisha Gray, and began by stating that
+the problem he had set out to solve was to construct, first, a mechanism
+which would produce oscillations of a perfectly constant period
+independent of the pressure of steam or air applied, within the widest
+limits, and also independent of frictional losses and load. Secondly, to
+produce electric currents of a perfectly constant period independently
+of the working conditions, and to produce these currents with mechanism
+which should be reliable and positive in its action without resorting to
+spark gaps and breaks. This he successfully accomplished in his
+apparatus, and with this apparatus, now, scientific men will be provided
+with the necessaries for carrying on investigations with alternating
+currents with great precision. These two inventions Mr. Tesla called,
+quite appropriately, a mechanical and an electrical oscillator,
+respectively.
+
+The former is substantially constructed in the following way. There is a
+piston in a cylinder made to reciprocate automatically by proper
+dispositions of parts, similar to a reciprocating tool. Mr. Tesla
+pointed out that he had done a great deal of work in perfecting his
+apparatus so that it would work efficiently at such high frequency of
+reciprocation as he contemplated, but he did not dwell on the many
+difficulties encountered. He exhibited, however, the pieces of a steel
+arbor which had been actually torn apart while vibrating against a
+minute air cushion.
+
+With the piston above referred to there is associated in one of his
+models in an independent chamber an air spring, or dash pot, or else he
+obtains the spring within the chambers of the oscillator itself. To
+appreciate the beauty of this it is only necessary to say that in that
+disposition, as he showed it, no matter what the rigidity of the spring
+and no matter what the weight of the moving parts, in other words, no
+matter what the period of vibrations, the vibrations of the spring are
+always isochronous with the applied pressure. Owing to this, the results
+obtained with these vibrations are truly wonderful. Mr. Tesla provides
+for an air spring of tremendous rigidity, and he is enabled to vibrate
+big weights at an enormous rate, considering the inertia, owing to the
+recoil of the spring. Thus, for instance, in one of these experiments,
+he vibrates a weight of approximately 20 pounds at the rate of about 80
+per second and with a stroke of about 7/8 inch, but by shortening the
+stroke the weight could be vibrated many hundred times, and has been, in
+other experiments.
+
+To start the vibrations, a powerful blow is struck, but the adjustment
+can be so made that only a minute effort is required to start, and, even
+without any special provision it will start by merely turning on the
+pressure suddenly. The vibration being, of course, isochronous, any
+change of pressure merely produces a shortening or lengthening of the
+stroke. Mr. Tesla showed a number of very clear drawings, illustrating
+the construction of the apparatus from which its working was plainly
+discernible. Special provisions are made so as to equalize the pressure
+within the dash pot and the outer atmosphere. For this purpose the
+inside chambers of the dash pot are arranged to communicate with the
+outer atmosphere so that no matter how the temperature of the enclosed
+air might vary, it still retains the same mean density as the outer
+atmosphere, and by this means a spring of constant rigidity is obtained.
+Now, of course, the pressure of the atmosphere may vary, and this would
+vary the rigidity of the spring, and consequently the period of
+vibration, and this feature constitutes one of the great beauties of the
+apparatus; for, as Mr. Tesla pointed out, this mechanical system acts
+exactly like a string tightly stretched between two points, and with
+fixed nodes, so that slight changes of the tension do not in the least
+alter the period of oscillation.
+
+The applications of such an apparatus are, of course, numerous and
+obvious. The first is, of course, to produce electric currents, and by a
+number of models and apparatus on the lecture platform, Mr. Tesla showed
+how this could be carried out in practice by combining an electric
+generator with his oscillator. He pointed out what conditions must be
+observed in order that the period of vibration of the electrical system
+might not disturb the mechanical oscillation in such a way as to alter
+the periodicity, but merely to shorten the stroke. He combines a
+condenser with a self-induction, and gives to the electrical system the
+same period as that at which the machine itself oscillates, so that both
+together then fall in step and electrical and mechanical resonance is
+obtained, and maintained absolutely unvaried.
+
+Next he showed a model of a motor with delicate wheelwork, which was
+driven by these currents at a constant speed, no matter what the air
+pressure applied was, so that this motor could be employed as a clock.
+He also showed a clock so constructed that it could be attached to one
+of the oscillators, and would keep absolutely correct time. Another
+curious and interesting feature which Mr. Tesla pointed out was that,
+instead of controlling the motion of the reciprocating piston by means
+of a spring, so as to obtain isochronous vibration, he was actually able
+to control the mechanical motion by the natural vibration of the
+electro-magnetic system, and he said that the case was a very simple
+one, and was quite analogous to that of a pendulum. Thus, supposing we
+had a pendulum of great weight, preferably, which would be maintained in
+vibration by force, periodically applied; now that force, no matter how
+it might vary, although it would oscillate the pendulum, would have no
+control over its period.
+
+Mr. Tesla also described a very interesting phenomenon which he
+illustrated by an experiment. By means of this new apparatus, he is able
+to produce an alternating current in which the E. M. F. of the impulses
+in one direction preponderates over that of those in the other, so that
+there is produced the effect of a direct current. In fact he expressed
+the hope that these currents would be capable of application in many
+instances, serving as direct currents. The principle involved in this
+preponderating E. M. F. he explains in this way: Suppose a conductor is
+moved into the magnetic field and then suddenly withdrawn. If the
+current is not retarded, then the work performed will be a mere
+fractional one; but if the current is retarded, then the magnetic field
+acts as a spring. Imagine that the motion of the conductor is arrested
+by the current generated, and that at the instant when it stops to move
+into the field, there is still the maximum current flowing in the
+conductor; then this current will, according to Lenz's law, drive the
+conductor out of the field again, and if the conductor has no
+resistance, then it would leave the field with the velocity it entered
+it. Now it is clear that if, instead of simply depending on the current
+to drive the conductor out of the field, the mechanically applied force
+is so timed that it helps the conductor to get out of the field, then it
+might leave the field with higher velocity than it entered it, and thus
+one impulse is made to preponderate in E. M. F. over the other.
+
+With a current of this nature, Mr. Tesla energized magnets strongly, and
+performed many interesting experiments bearing out the fact that one of
+the current impulses preponderates. Among them was one in which he
+attached to his oscillator a ring magnet with a small air gap between
+the poles. This magnet was oscillated up and down 80 times a second. A
+copper disc, when inserted within the air gap of the ring magnet, was
+brought into rapid rotation. Mr. Tesla remarked that this experiment
+also seemed to demonstrate that the lines of flow of current through a
+metallic mass are disturbed by the presence of a magnet in a manner
+quite independently of the so-called Hall effect. He showed also a very
+interesting method of making a connection with the oscillating magnet.
+This was accomplished by attaching to the magnet small insulated steel
+rods, and connecting to these rods the ends of the energizing coil. As
+the magnet was vibrated, stationary nodes were produced in the steel
+rods, and at these points the terminals of a direct current source were
+attached. Mr. Tesla also pointed out that one of the uses of currents,
+such as those produced in his apparatus, would be to select any given
+one of a number of devices connected to the same circuit by picking out
+the vibration by resonance. There is indeed little doubt that with Mr.
+Tesla's devices, harmonic and synchronous telegraphy will receive a
+fresh impetus, and vast possibilities are again opened up.
+
+Mr. Tesla was very much elated over his latest achievements, and said
+that he hoped that in the hands of practical, as well as scientific men,
+the devices described by him would yield important results. He laid
+special stress on the facility now afforded for investigating the effect
+of mechanical vibration in all directions, and also showed that he had
+observed a number of facts in connection with iron cores.
+
+[Illustration: FIG. 312.]
+
+The engraving, Fig. 312, shows, in perspective, one of the forms of
+apparatus used by Mr. Tesla in his earlier investigations in this field
+of work, and its interior construction is made plain by the sectional
+view shown in Fig. 313. It will be noted that the piston P is fitted
+into the hollow of a cylinder C which is provided with channel ports
+O O, and _I_, extending all around the inside surface. In this
+particular apparatus there are two channels O O for the outlet of the
+working fluid and one, _I_, for the inlet. The piston P is provided with
+two slots S S' at a carefully determined distance, one from the other.
+The tubes T T which are screwed into the holes drilled into the piston,
+establish communication between the slots S S' and chambers on each side
+of the piston, each of these chambers connecting with the slot which is
+remote from it. The piston P is screwed tightly on a shaft A which
+passes through fitting boxes at the end of the cylinder C. The boxes
+project to a carefully determined distance into the hollow of the
+cylinder C, thus determining the length of the stroke.
+
+Surrounding the whole is a jacket J. This jacket acts chiefly to
+diminish the sound produced by the oscillator and as a jacket when the
+oscillator is driven by steam, in which case a somewhat different
+arrangement of the magnets is employed. The apparatus here illustrated
+was intended for demonstration purposes, air being used as most
+convenient for this purpose.
+
+A magnetic frame M M is fastened so as to closely surround the
+oscillator and is provided with energizing coils which establish two
+strong magnetic fields on opposite sides. The magnetic frame is made up
+of thin sheet iron. In the intensely concentrated field thus produced,
+there are arranged two pairs of coils H H supported in metallic frames
+which are screwed on the shaft A of the piston and have additional
+bearings in the boxes B B on each side. The whole is mounted on a
+metallic base resting on two wooden blocks.
+
+[Illustration: FIG. 313.]
+
+The operation of the device is as follows: The working fluid being
+admitted through an inlet pipe to the slot I and the piston being
+supposed to be in the position indicated, it is sufficient, though not
+necessary, to give a gentle tap on one of the shaft ends protruding
+from the boxes B. Assume that the motion imparted be such as to move the
+piston to the left (when looking at the diagram) then the air rushes
+through the slot S' and tube T into the chamber to the left. The
+pressure now drives the piston towards the right and, owing to its
+inertia, it overshoots the position of equilibrium and allows the air to
+rush through the slot S and tube T into the chamber to the right, while
+the communication to the left hand chamber is cut off, the air of the
+latter chamber escaping through the outlet O on the left. On the return
+stroke a similar operation takes place on the right hand side. This
+oscillation is maintained continuously and the apparatus performs
+vibrations from a scarcely perceptible quiver amounting to no more than
+1 of an inch, up to vibrations of a little over 3/8 of an inch,
+according to the air pressure and load. It is indeed interesting to see
+how an incandescent lamp is kept burning with the apparatus showing a
+scarcely perceptible quiver.
+
+To perfect the mechanical part of the apparatus so that oscillations are
+maintained economically was one thing, and Mr. Tesla hinted in his
+lecture at the great difficulties he had first encountered to accomplish
+this. But to produce oscillations which would be of constant period was
+another task of no mean proportions. As already pointed out, Mr. Tesla
+obtains the constancy of period in three distinct ways. Thus, he
+provides properly calculated chambers, as in the case illustrated, in
+the oscillator itself; or he associates with the oscillator an air
+spring of constant resilience. But the most interesting of all, perhaps,
+is the maintenance of the constancy of oscillation by the reaction of
+the electromagnetic part of the combination. Mr. Tesla winds his coils,
+by preference, for high tension and associates with them a condenser,
+making the natural period of the combination fairly approximating to the
+average period at which the piston would oscillate without any
+particular provision being made for the constancy of period under
+varying pressure and load. As the piston with the coils is perfectly
+free to move, it is extremely susceptible to the influence of the
+natural vibration set up in the circuits of the coils H H. The
+mechanical efficiency of the apparatus is very high owing to the fact
+that friction is reduced to a minimum and the weights which are moved
+are small; the output of the oscillator is therefore a very large one.
+
+Theoretically considered, when the various advantages which Mr. Tesla
+holds out are examined, it is surprising, considering the simplicity of
+the arrangement, that nothing was done in this direction before. No
+doubt many inventors, at one time or other, have entertained the idea of
+generating currents by attaching a coil or a magnetic core to the piston
+of a steam engine, or generating currents by the vibrations of a tuning
+fork, or similar devices, but the disadvantages of such arrangements
+from an engineering standpoint must be obvious. Mr. Tesla, however, in
+the introductory remarks of his lecture, pointed out how by a series of
+conclusions he was driven to take up this new line of work by the
+necessity of producing currents of constant period and as a result of
+his endeavors to maintain electrical oscillation in the most simple and
+economical manner.
+
+
+
+
+INDEX.
+
+
+Alternate Current Electrostatic Apparatus 392
+
+Alternating Current Generators for High Frequency 152, 374, 224
+
+Alternating Motors and Transformers 7
+
+American Institute Electrical Engineers Lecture 145
+
+Anthony, W. A., Tests of Tesla Motors 8
+
+Apparatus for Producing High Vacua 276
+
+Arc Lighting, Tesla Direct, System 451
+
+Auxiliary Brush Regulation 438
+
+
+Biography, Tesla 4
+
+Brush, Anti-Sparking 432
+
+Brush, Third, Regulation 438
+
+Brush, Phenomena in High Vacuum 226
+
+
+Carborundum Button for Tesla Lamps 140, 253
+
+Commutator, Anti-Sparking 432
+
+Combination of Synchronizing and Torque Motor 95
+
+Condensers with Plates in Oil 418
+
+Conversion with Disruptive Discharge 193, 204, 303
+
+Current or Dynamic Electricity Phenomena 327
+
+
+Direct Current Arc Lighting 451
+
+Dischargers, Forms of 305
+
+Disruptive Discharge Coil 207, 221
+
+Disruptive Discharge Phenomena 212
+
+Dynamos, Improved Direct Current 448
+
+
+Early Phase Motors 477
+
+Effects with High Frequency and High Potential Currents 119
+
+Electrical Congress Lecture, Chicago. 486
+
+Electric Resonance 340
+
+Electric Discharges in Vacuum Tubes 396
+
+Electrolytic Registering Meter 420
+
+Eye, Observations on the 294
+
+
+Flames, Electrostatic, Non-Consuming 166, 272
+
+Forbes Unipolar Generator 468, 474
+
+Franklin Institute Lecture 294
+
+
+Generators, Pyromagnetic 429
+
+
+High Potential, High Frequency:
+
+  Brush Phenomena in High Vacuum 226
+  Carborundum Buttons 140, 253
+  Disruptive Discharge Phenomena 212
+  Flames, Electrostatic, Non-Consuming 166, 272
+  Impedance, Novel Phenomena 194, 338
+  Lighting Lamps Through Body 359
+  Luminous Effects with Gases 368
+  "Massage" with Currents 394
+  Motor with Single Wire 234, 330
+  "No Wire" Motors 235
+  Oil Insulation of Induction Coils 173, 221
+  Ozone, Production of 171
+  Phosphorescence 367
+  Physiological Effects 162, 394
+  Resonance 340
+  Spinning Filament 168
+  Streaming Discharges of High Tension Coil 155, 163
+  Telegraphy without Wires 346
+
+
+Impedance, Novel Phenomena 194, 338
+
+Improvements in Unipolar Generators 465
+
+Improved Direct Current Dynamos and Motors 448
+
+Induction Motors 92
+
+Institution Electrical Engineers Lecture 198
+
+
+Lamps and Motor operated on a Single Wire 330
+
+Lamps with Single Straight Fiber 183
+
+Lamps containing only a Gas 188
+
+Lamps with Refractory Button 177, 239, 360
+
+Lamps for Simple Phosphorescence 187, 282, 364
+
+Lecture, Tesla before:
+
+ American Institute Electrical Engineers 145
+ Royal Institution 124
+ Institution Electrical Engineers 198
+ Franklin Institute and National Electric Light Association 294
+ Electrical Congress, Chicago 486
+
+Lighting Lamps Through the Body 359
+
+Light Phenomena with High Frequencies 349
+
+Luminous Effects with Gases at Low-Pressure 368
+
+
+"Magnetic Lag" Motor 67
+
+"Massage" with Currents of High Frequency 394
+
+Mechanical and Electrical Oscillators 486
+
+Method of obtaining Direct from Alternating currents 409
+
+Method of obtaining Difference of Phase by Magnetic Shielding 71
+
+Motors:
+
+  With Circuits of Different Resistance 79
+  With Closed Conductors 9
+  Combination of Synchronizing and Torque 95
+  With Condenser in Armature Circuit 101
+  With Condenser in one of the Field Circuits 106
+  With Coinciding Maxima of Magnetic Effect in Armature and Field 83
+  With "Current Lag" Artificially Secured 58
+  Early Phase 477
+  With Equal Magnetic Energies in Field and Armature 81
+  Or Generator, obtaining Desired Speed of 36
+  Improved Direct Current 448
+  Induction 92
+  "Magnetic Lag" 67
+  "No Wire" 235
+  With Phase Difference in Magnetization of Inner and Outer Parts
+          of Core 88
+  Regulator for Rotary Current 45
+  Single Circuit, Self-starting Synchronizing 50
+  Single Phase 76
+  With Single Wire to Generator 234, 330
+  Synchronizing 9
+  Thermo-Magnetic 424
+  Utilizing Continuous Current Generators 31
+
+
+National Electric Light Association Lecture 294
+
+"No Wire" Motor 235
+
+
+Observations on the Eye 294
+
+Oil, Condensers with Plates in 418
+
+Oil Insulation of Induction Coils 173, 221
+
+Oscillators, Mechanical and Electrical 486
+
+Ozone, Production of 171
+
+Phenomena Produced by Electrostatic Force 318
+
+Phosphorescence and Sulphide of Zinc 367
+
+Physiological Effects of High Frequency 162, 394
+
+Polyphase Systems 26
+
+Polyphase Transformer 109
+
+Pyromagnetic Generators 429
+
+Regulator for Rotary Current Motors 45
+
+Resonance, Electric, Phenomena of 340
+
+"Resultant Attraction" 7
+
+Rotating Field Transformers 9
+
+Rotating Magnetic Field 9
+
+Royal Institution Lecture 124
+
+Scope of Lectures 119
+
+Single Phase Motor 76
+
+Single Circuit, Self-Starting Synchronizing Motors 50
+
+Spinning Filament Effects 168
+
+Streaming Discharges of High Tension Coil 155, 163
+
+Synchronizing Motors 9
+
+Telegraphy without Wires 246
+
+Transformer with Shield between Primary and Secondary 113
+
+Thermo-Magnetic Motors 424
+
+Thomson, J. J., on Vacuum Tubes 397, 402, 406
+
+Thomson, Sir W., Current Accumulator 471
+
+Transformers:
+
+  Alternating 7
+  Magnetic Shield 113
+  Polyphase 109
+  Rotating Field 9
+
+Tubes:
+
+  Coated with Yttria, etc. 187
+  Coated with Sulphide of Zinc, etc. 290, 367
+
+Unipolar Generators 465
+
+Unipolar Generator, Forbes 468, 474
+
+Yttria, Coated Tubes 187
+
+Zinc, Tubes Coated with Sulphide of 367
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The inventions, researches and
+writings of Nikola Tesla, by Thomas Commerford Martin
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+      The Project Gutenberg eBook of The Inventions Researches and Writings of Nikola Tesla, by Thomas Commerford Martin.
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+
+The Project Gutenberg EBook of The inventions, researches and writings of
+Nikola Tesla, by Thomas Commerford Martin
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: The inventions, researches and writings of Nikola Tesla
+       With special reference to his work in polyphase currents
+              and high potential lighting
+
+Author: Thomas Commerford Martin
+
+Release Date: March 26, 2012 [EBook #39272]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES ***
+
+
+
+
+Produced by Anna Hall, Albert László and the Online
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+by The Internet Archive)
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+
+
+</pre>
+
+
+<p>&nbsp;</p>
+
+
+
+<h1>THE INVENTIONS</h1>
+
+<h1>RESEARCHES AND WRITINGS</h1>
+
+<h4>OF</h4>
+
+<h1>NIKOLA TESLA</h1>
+
+<p>&nbsp;</p><p>&nbsp;</p>
+
+<h2>TO HIS COUNTRYMEN</h2>
+<h4>IN EASTERN EUROPE THIS RECORD OF<br />
+THE WORK ALREADY ACCOMPLISHED BY</h4>
+<h2>NIKOLA TESLA</h2>
+
+<h4>IS RESPECTFULLY DEDICATED</h4>
+<p>&nbsp;</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_004.jpg" width="640" height="1024" alt="" title="" />
+</div>
+<p>&nbsp;</p>
+
+<h2>THE INVENTIONS</h2>
+<h2>RESEARCHES AND WRITINGS</h2>
+<h5>OF</h5>
+<h1><span class="smcap">Nikola Tesla</span></h1>
+
+<h4>WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE<br />
+CURRENTS AND HIGH POTENTIAL LIGHTING</h4>
+
+<h5>BY</h5>
+
+<h2>THOMAS COMMERFORD MARTIN</h2>
+<h4>Editor <span class="smcap">The Electrical Engineer</span>; Past-President American Institute Electrical Engineers</h4>
+<p>&nbsp;</p>
+
+<h4>1894<br />
+THE ELECTRICAL ENGINEER<br />
+<small>NEW YORK</small><br />
+<b>D. VAN NOSTRAND COMPANY,<br />
+<small>NEW YORK.</small></b>
+</h4>
+
+<p>&nbsp;</p>
+
+<hr style="width: 10%;" />
+<p>&nbsp;</p>
+<h4>Entered according to Act of Congress in the year 1893 by<br />
+T. C. MARTIN<br />
+in the office of the Librarian of Congress at Washington</h4>
+
+<p>&nbsp;</p>
+
+<p class="center">Press of McIlroy &amp; Emmet, 36 Cortlandt St., N. Y.</p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_v" id="Page_v">[Pg v]</a></span></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE.</h2>
+
+
+<p>The electrical problems of the present day lie largely in the
+economical transmission of power and in the radical improvement
+of the means and methods of illumination. To many
+workers and thinkers in the domain of electrical invention, the
+apparatus and devices that are familiar, appear cumbrous and
+wasteful, and subject to severe limitations. They believe that
+the principles of current generation must be changed, the area
+of current supply be enlarged, and the appliances used by the
+consumer be at once cheapened and simplified. The brilliant
+successes of the past justify them in every expectancy of still
+more generous fruition.</p>
+
+<p>The present volume is a simple record of the pioneer work
+done in such departments up to date, by Mr. Nikola Tesla, in
+whom the world has already recognized one of the foremost of
+modern electrical investigators and inventors. No attempt whatever
+has been made here to emphasize the importance of his
+researches and discoveries. Great ideas and real inventions win
+their own way, determining their own place by intrinsic merit.
+But with the conviction that Mr. Tesla is blazing a path that
+electrical development must follow for many years to come, the
+compiler has endeavored to bring together all that bears the impress
+of Mr. Tesla's genius, and is worthy of preservation. Aside
+from its value as showing the scope of his inventions, this
+volume may be of service as indicating the range of his thought.
+There is intellectual profit in studying the push and play of a
+vigorous and original mind.</p>
+
+<p>Although the lively interest of the public in Mr. Tesla's work
+is perhaps of recent growth, this volume covers the results of
+full ten years. It includes his lectures, miscellaneous articles<span class='pagenum'><a name="Page_vi" id="Page_vi">[Pg vi]</a></span>
+and discussions, and makes note of all his inventions thus far
+known, particularly those bearing on polyphase motors and the
+effects obtained with currents of high potential and high frequency.
+It will be seen that Mr. Tesla has ever pressed forward,
+barely pausing for an instant to work out in detail the utilizations
+that have at once been obvious to him of the new principles he
+has elucidated. Wherever possible his own language has been
+employed.</p>
+
+<p>It may be added that this volume is issued with Mr. Tesla's
+sanction and approval, and that permission has been obtained for
+the re-publication in it of such papers as have been read before
+various technical societies of this country and Europe. Mr.
+Tesla has kindly favored the author by looking over the proof
+sheets of the sections embodying his latest researches. The
+work has also enjoyed the careful revision of the author's
+friend and editorial associate, Mr. Joseph Wetzler, through
+whose hands all the proofs have passed.</p>
+
+<p><span class="smcap">December, 1893.</span></p>
+
+<p style='text-align: right'>T. C. M.</p>
+
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_vii" id="Page_vii">[Pg vii]</a></span></p>
+<h2>CONTENTS.</h2>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="80%">
+<tr><td colspan='2' class='center2'><a href="#PART_I">PART I.</a><br />
+<small>POLYPHASE CURRENTS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER I.</td></tr>
+<tr><td align='left'><span class="smcap">Biographical and Introductory.</span></td><td align='right'><a href="#Page_3">3</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER II.</td></tr>
+<tr><td align='left'><span class="smcap">A New System of Alternating Current Motors and Transformers.</span></td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER III.</td></tr>
+<tr><td align='left'><span class="smcap">The Tesla Rotating Magnetic Field.&mdash;Motors with Closed Conductors.&mdash;Synchronizing Motors.&mdash;Rotating Field Transformers.</span></td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER IV.</td></tr>
+<tr><td align='left'><span class="smcap">Modifications and Expansions of the Tesla Polyphase Systems.</span></td><td align='right'><a href="#Page_26">26</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER V.</td></tr>
+<tr><td align='left'><span class="smcap">Utilizing Familiar Types of Generators of the Continuous Current Type.</span></td><td align='right'><a href="#Page_31">31</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VI.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Desired Speed of Motor Or Generator.</span></td><td align='right'><a href="#Page_36">36</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_viii" id="Page_viii">[Pg viii]</a></span><span class="smcap">Regulator for Rotary Current Motors.</span></td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VIII.</td></tr>
+<tr><td align='left'><span class="smcap">Single Circuit, Self-starting Synchronizing Motors.</span></td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER IX.</td></tr>
+<tr><td align='left'><span class="smcap">Change from Double Current to Single Current Motors.</span></td><td align='right'><a href="#Page_56">56</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER X.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with "Current Lag" Artificially Secured.</span></td><td align='right'><a href="#Page_58">58</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XI.</td></tr>
+<tr><td align='left'><span class="smcap">Another Method of Transformation from a Torque to a Synchronizing Motor.</span></td><td align='right'><a href="#Page_62">62</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XII.</td></tr>
+<tr><td align='left'><span class="smcap">"Magnetic Lag" Motor.</span></td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIII.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Difference of Phase by Magnetic Shielding.</span></td><td align='right'><a href="#Page_71">71</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIV.</td></tr>
+<tr><td align='left'><span class="smcap">Type of Tesla Single-Phase Motor.</span></td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XV.</td></tr>
+<tr><td align='left'><span class="smcap">Motors With Circuits of Different Resistance.</span></td><td align='right'><a href="#Page_79">79</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVI.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with Equal Magnetic Energies in Field and Armature.</span></td><td align='right'><a href="#Page_81">81</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVII.</td></tr>
+<tr><td align='left'><span class="smcap">Motors with Coinciding Maxima of Magnetic Effect in Armature and Field.</span></td><td align='right'><a href="#Page_83">83</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVIII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_ix" id="Page_ix">[Pg ix]</a></span><span class="smcap">Motor Based on the Difference of Phase in the Magnetization of the Inner and Outer Parts of an Iron Core.</span></td><td align='right'><a href="#Page_88">88</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIX.</td></tr>
+<tr><td align='left'><span class="smcap">Another Type of Tesla Induction Motor.</span></td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XX.</td></tr>
+<tr><td align='left'><span class="smcap">Combinations of Synchronizing Motor and Torque Motor.</span></td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXI.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with a Condenser in the Armature Circuit.</span></td><td align='right'><a href="#Page_101">101</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXII.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with Condenser in One of the Field Circuits.</span></td><td align='right'><a href="#Page_106">106</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIII.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Polyphase Transformer.</span></td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIV.</td></tr>
+<tr><td align='left'><span class="smcap">A Constant Current Transformer with Magnetic Shield Between Coils of Primary and Secondary.</span></td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_II">PART II.</a><br />
+<small>THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXV.</td></tr>
+<tr><td align='left'><span class="smcap">Introductory.&mdash;The Scope of The Tesla Lectures.</span></td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVI.</td></tr>
+<tr><td align='left'><span class="smcap">The New York Lecture. Experiments with Alternate
+Currents of Very High Frequency, and Their
+Application to Methods of Artificial Illumination,
+May 20, 1891.</span></td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_x" id="Page_x">[Pg x]</a></span><span class="smcap">The London Lecture. Experiments with Alternate
+Currents of High Potential and High Frequency,
+February 3, 1892.</span></td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVIII.</td></tr>
+<tr><td align='left'><span class="smcap">The Philadelphia and St. Louis Lecture. On Light
+and Other High Frequency Phenomena, February
+and March, 1893.</span></td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIX.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Alternating Current Generators for High
+Frequency.</span></td><td align='right'><a href="#Page_374">374</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXX.</td></tr>
+<tr><td align='left'><span class="smcap">Alternate Current Electrostatic Induction Apparatus.</span></td><td align='right'><a href="#Page_392">392</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXI.</td></tr>
+<tr><td align='left'><span class="smcap">"Massage" with Currents of High Frequency.</span></td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXII.</td></tr>
+<tr><td align='left'><span class="smcap">Electric Discharge in Vacuum Tubes.</span></td><td align='right'><a href="#Page_396">396</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_III">PART III.</a><br />
+<small>MISCELLANEOUS INVENTIONS AND WRITINGS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIII.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Direct from Alternating Currents.</span></td><td align='right'><a href="#Page_409">409</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIV.</td></tr>
+<tr><td align='left'><span class="smcap">Condensers with Plates in Oil.</span></td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXV.</td></tr>
+<tr><td align='left'><span class="smcap">Electrolytic Registering Meter.</span></td><td align='right'><a href="#Page_420">420</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVI.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_xi" id="Page_xi">[Pg xi]</a></span><span class="smcap">Thermo-Magnetic Motors and Pyro-Magnetic Generators.</span></td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVII.</td></tr>
+<tr><td align='left'><span class="smcap">Anti-sparking Dynamo Brush and Commutator.</span></td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVIII.</td></tr>
+<tr><td align='left'><span class="smcap">Auxiliary Brush Regulation of Direct Current Dynamos.</span></td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIX.</td></tr>
+<tr><td align='left'><span class="smcap">Improvement in Dynamo and Motor Construction.</span></td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XL.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Direct Current Arc Lighting System.</span></td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLI.</td></tr>
+<tr><td align='left'><span class="smcap">Improvement in Unipolar Generators.</span></td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_IV">PART IV.</a><br />
+<small>APPENDIX: EARLY PHASE MOTORS AND THE TESLA OSCILLATORS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLII.</td></tr>
+<tr><td align='left'><span class="smcap">Mr. Tesla's Personal Exhibit at the World's Fair.</span></td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLIII.</td></tr>
+<tr><td align='left'><span class="smcap">The Tesla Mechanical and Electrical Oscillators.</span></td><td align='right'><a href="#Page_486">486</a></td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h1><small><a name="PART_I" id="PART_I"></a>PART I.</small><br /><br />
+
+POLYPHASE CURRENTS.</h1>
+<p><span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span></p>
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I.</h2>
+
+<h3><span class="smcap">Biographical and Introductory.</span></h3>
+
+
+<p>As an introduction to the record contained in this volume
+of Mr. Tesla's investigations and discoveries, a few words of a
+biographical nature will, it is deemed, not be out of place, nor
+other than welcome.</p>
+
+<p>Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland
+region of Austro-Hungary, of the Serbian race, which has maintained
+against Turkey and all comers so unceasing a struggle for
+freedom. His family is an old and representative one among
+these Switzers of Eastern Europe, and his father was an eloquent
+clergyman in the Greek Church. An uncle is to-day Metropolitan
+in Bosnia. His mother was a woman of inherited ingenuity,
+and delighted not only in skilful work of the ordinary household
+character, but in the construction of such mechanical appliances
+as looms and churns and other machinery required in a rural
+community. Nikola was educated at Gospich in the public
+school for four years, and then spent three years in the Real
+Schule. He was then sent to Carstatt, Croatia, where he continued
+his studies for three years in the Higher Real Schule.
+There for the first time he saw a steam locomotive. He graduated
+in 1873, and, surviving an attack of cholera, devoted himself
+to experimentation, especially in electricity and magnetism.
+His father would have had him maintain the family tradition by
+entering the Church, but native genius was too strong, and he
+was allowed to enter the Polytechnic School at Gratz, to finish
+his studies, and with the object of becoming a professor of mathematics
+and physics. One of the machines there experimented
+with was a Gramme dynamo, used as a motor. Despite his instructor's
+perfect demonstration of the fact that it was impossible
+to operate a dynamo without commutator or brushes, Mr. Tesla
+could not be convinced that such accessories were necessary or
+desirable. He had already seen with quick intuition that a way
+could be found to dispense with them; and from that time he may<span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+be said to have begun work on the ideas that fructified ultimately
+in his rotating field motors.</p>
+
+<p>In the second year of his Gratz course, Mr. Tesla gave up the
+notion of becoming a teacher, and took up the engineering curriculum.
+His studies ended, he returned home in time to see his
+father die, and then went to Prague and Buda-Pesth to study
+languages, with the object of qualifying himself broadly for the
+practice of the engineering profession. For a short time he
+served as an assistant in the Government Telegraph Engineering
+Department, and then became associated with M. Puskas, a
+personal and family friend, and other exploiters of the telephone
+in Hungary. He made a number of telephonic inventions, but
+found his opportunities of benefiting by them limited in various
+ways. To gain a wider field of action, he pushed on to Paris
+and there secured employment as an electrical engineer with one
+of the large companies in the new industry of electric lighting.</p>
+
+<p>It was during this period, and as early as 1882, that he began
+serious and continued efforts to embody the rotating field principle
+in operative apparatus. He was enthusiastic about it; believed
+it to mark a new departure in the electrical arts, and could
+think of nothing else. In fact, but for the solicitations of a few
+friends in commercial circles who urged him to form a company
+to exploit the invention, Mr. Tesla, then a youth of little worldly
+experience, would have sought an immediate opportunity to publish
+his ideas, believing them to be worthy of note as a novel and
+radical advance in electrical theory as well as destined to have
+a profound influence on all dynamo electric machinery.</p>
+
+<p>At last he determined that it would be best to try his fortunes
+in America. In France he had met many Americans, and in
+contact with them learned the desirability of turning every new
+idea in electricity to practical use. He learned also of the ready
+encouragement given in the United States to any inventor who
+could attain some new and valuable result. The resolution was
+formed with characteristic quickness, and abandoning all his
+prospects in Europe, he at once set his face westward.</p>
+
+<p>Arrived in the United States, Mr. Tesla took off his coat the
+day he arrived, in the Edison Works. That place had been a
+goal of his ambition, and one can readily imagine the benefit and
+stimulus derived from association with Mr. Edison, for whom
+Mr. Tesla has always had the strongest admiration. It was impossible,
+however, that, with his own ideas to carry out, and his<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span>
+own inventions to develop, Mr. Tesla could long remain in even
+the most delightful employ; and, his work now attracting attention,
+he left the Edison ranks to join a company intended to
+make and sell an arc lighting system based on some of his inventions
+in that branch of the art. With unceasing diligence he
+brought the system to perfection, and saw it placed on the market.
+But the thing which most occupied his time and thoughts, however,
+all through this period, was his old discovery of the rotating
+field principle for alternating current work, and the application
+of it in motors that have now become known the world over.</p>
+
+<p>Strong as his convictions on the subject then were, it is a fact
+that he stood very much alone, for the alternating current had
+no well recognized place. Few electrical engineers had ever
+used it, and the majority were entirely unfamiliar with its value,
+or even its essential features. Even Mr. Tesla himself did not,
+until after protracted effort and experimentation, learn how to
+construct alternating current apparatus of fair efficiency. But
+that he had accomplished his purpose was shown by the tests of
+Prof. Anthony, made in the of winter 1887-8, when Tesla motors
+in the hands of that distinguished expert gave an efficiency equal
+to that of direct current motors. Nothing now stood in the way
+of the commercial development and introduction of such motors,
+except that they had to be constructed with a view to operating
+on the circuits then existing, which in this country were all of
+high frequency.</p>
+
+<p>The first full publication of his work in this direction&mdash;outside
+his patents&mdash;was a paper read before the American Institute of
+Electrical Engineers in New York, in May, 1888 (read at the
+suggestion of Prof. Anthony and the present writer), when he
+exhibited motors that had been in operation long previous, and
+with which his belief that brushes and commutators could be
+dispensed with, was triumphantly proved to be correct. The
+section of this volume devoted to Mr. Tesla's inventions in the
+utilization of polyphase currents will show how thoroughly from
+the outset he had mastered the fundamental idea and applied it
+in the greatest variety of ways.</p>
+
+<p>Having noted for years the many advantages obtainable with
+alternating currents, Mr. Tesla was naturally led on to experiment
+with them at higher potentials and higher frequencies than
+were common or approved of. Ever pressing forward to determine
+in even the slightest degree the outlines of the unknown, he<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span>
+was rewarded very quickly in this field with results of the most
+surprising nature. A slight acquaintance with some of these
+experiments led the compiler of this volume to urge Mr. Tesla
+to repeat them before the American Institute of Electrical Engineers.
+This was done in May, 1891, in a lecture that marked,
+beyond question, a distinct departure in electrical theory and
+practice, and all the results of which have not yet made themselves
+fully apparent. The New York lecture, and its successors,
+two in number, are also included in this volume, with a
+few supplementary notes.</p>
+
+<p>Mr. Tesla's work ranges far beyond the vast departments of
+polyphase currents and high potential lighting. The "Miscellaneous"
+section of this volume includes a great many other inventions
+in arc lighting, transformers, pyro-magnetic generators,
+thermo-magnetic motors, third-brush regulation, improvements
+in dynamos, new forms of incandescent lamps, electrical meters,
+condensers, unipolar dynamos, the conversion of alternating into
+direct currents, etc. It is needless to say that at this moment
+Mr. Tesla is engaged on a number of interesting ideas and inventions,
+to be made public in due course. The present volume
+deals simply with his work accomplished to date.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II.</h2>
+
+<h3><span class="smcap">A New System of Alternating Current Motors and
+Transformers.</span></h3>
+
+
+<p>The present section of this volume deals with polyphase currents,
+and the inventions by Mr. Tesla, made known thus far, in
+which he has embodied one feature or another of the broad
+principle of rotating field poles or <i>resultant attraction</i> exerted on
+the armature. It is needless to remind electricians of the great
+interest aroused by the first enunciation of the rotating field
+principle, or to dwell upon the importance of the advance from
+a single alternating current, to methods and apparatus which deal
+with more than one. Simply prefacing the consideration here
+attempted of the subject, with the remark that in nowise is the
+object of this volume of a polemic or controversial nature, it
+may be pointed out that Mr. Tesla's work has not at all been
+fully understood or realized up to date. To many readers, it is
+believed, the analysis of what he has done in this department
+will be a revelation, while it will at the same time illustrate the
+beautiful flexibility and range of the principles involved. It
+will be seen that, as just suggested, Mr. Tesla did not stop short
+at a mere rotating field, but dealt broadly with the shifting of
+the resultant attraction of the magnets. It will be seen that he
+went on to evolve the "multiphase" system with many ramifications
+and turns; that he showed the broad idea of motors employing
+currents of differing phase in the armature with direct
+currents in the field; that he first described and worked out the
+idea of an armature with a body of iron and coils closed upon
+themselves; that he worked out both synchronizing and torque
+motors; that he explained and illustrated how machines of ordinary
+construction might be adapted to his system; that he employed
+condensers in field and armature circuits, and went to the
+bottom of the fundamental principles, testing, approving or rejecting,
+it would appear, every detail that inventive ingenuity could
+hit upon.<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span></p>
+
+<p>Now that opinion is turning so emphatically in favor of lower
+frequencies, it deserves special note that Mr. Tesla early recognized
+the importance of the low frequency feature in motor
+work. In fact his first motors exhibited publicly&mdash;and which, as
+Prof. Anthony showed in his tests in the winter of 1887-8, were
+the equal of direct current motors in efficiency, output and starting
+torque&mdash;were of the low frequency type. The necessity
+arising, however, to utilize these motors in connection with the
+existing high frequency circuits, our survey reveals in an interesting
+manner Mr. Tesla's fertility of resource in this direction.
+But that, after exhausting all the possibilities of this field, Mr.
+Tesla returns to low frequencies, and insists on the superiority of
+his polyphase system in alternating current distribution, need not
+at all surprise us, in view of the strength of his convictions, so
+often expressed, on this subject. This is, indeed, significant, and
+may be regarded as indicative of the probable development next
+to be witnessed.</p>
+
+<p>Incidental reference has been made to the efficiency of rotating
+field motors, a matter of much importance, though it is not the
+intention to dwell upon it here. Prof. Anthony in his remarks
+before the American Institute of Electrical Engineers, in May,
+1888, on the two small Tesla motors then shown, which he had
+tested, stated that one gave an efficiency of about 50 per cent.
+and the other a little over sixty per cent. In 1889, some tests
+were reported from Pittsburgh, made by Mr. Tesla and Mr.
+Albert Schmid, on motors up to 10 <span class="smcap">h.&nbsp;p.</span> and weighing about
+850 pounds. These machines showed an efficiency of nearly 90
+per cent. With some larger motors it was then found practicable
+to obtain an efficiency, with the three wire system, up to as
+high as 94 and 95 per cent. These interesting figures, which, of
+course, might be supplemented by others more elaborate and of
+later date, are cited to show that the efficiency of the system has
+not had to wait until the present late day for any demonstration
+of its commercial usefulness. An invention is none the less beautiful
+because it may lack utility, but it must be a pleasure to any
+inventor to know that the ideas he is advancing are fraught with
+substantial benefits to the public.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III.</h2>
+
+<h3><span class="smcap">The Tesla Rotating Magnetic Field.</span>&mdash;<span class="smcap">Motors With Closed
+Conductors.</span>&mdash;<span class="smcap">Synchronizing Motors.</span>&mdash;<span class="smcap">Rotating Field
+Transformers.</span></h3>
+
+
+<p>The best description that can be given of what he attempted,
+and succeeded in doing, with the rotating magnetic field, is to be
+found in Mr. Tesla's brief paper explanatory of his rotary current,
+polyphase system, read before the American Institute of
+Electrical Engineers, in New York, in May, 1888, under the
+title "A New System of Alternate Current Motors and Transformers."
+As a matter of fact, which a perusal of the paper
+will establish, Mr. Tesla made no attempt in that paper to describe
+all his work. It dealt in reality with the few topics enumerated
+in the caption of this chapter. Mr. Tesla's reticence
+was no doubt due largely to the fact that his action was governed
+by the wishes of others with whom he was associated, but
+it may be worth mention that the compiler of this volume&mdash;who
+had seen the motors running, and who was then chairman of the
+Institute Committee on Papers and Meetings&mdash;had great difficulty
+in inducing Mr. Tesla to give the Institute any paper at all.
+Mr. Tesla was overworked and ill, and manifested the greatest
+reluctance to an exhibition of his motors, but his objections were
+at last overcome. The paper was written the night previous to
+the meeting, in pencil, very hastily, and under the pressure
+just mentioned.</p>
+
+<p>In this paper casual reference was made to two special forms
+of motors not within the group to be considered. These two
+forms were: 1. A motor with one of its circuits in series with a
+transformer, and the other in the secondary of the transformer.
+2. A motor having its armature circuit connected to the generator,
+and the field coils closed upon themselves. The paper in
+its essence is as follows, dealing with a few leading features of
+the Tesla system, namely, the rotating magnetic field, motors<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span>
+with closed conductors, synchronizing motors, and rotating field
+transformers:&mdash;</p>
+
+<p>The subject which I now have the pleasure of bringing to
+your notice is a novel system of electric distribution and transmission
+of power by means of alternate currents, affording peculiar
+advantages, particularly in the way of motors, which I am
+confident will at once establish the superior adaptability of these
+currents to the transmission of power and will show that many
+results heretofore unattainable can be reached by their use; results
+which are very much desired in the practical operation of
+such systems, and which cannot be accomplished by means of
+continuous currents.</p>
+
+<p>Before going into a detailed description of this system, I think
+it necessary to make a few remarks with reference to certain conditions
+existing in continuous current generators and motors,
+which, although generally known, are frequently disregarded.</p>
+
+<p>In our dynamo machines, it is well known, we generate alternate
+currents which we direct by means of a commutator, a complicated
+device and, it may be justly said, the source of most of
+the troubles experienced in the operation of the machines. Now,
+the currents so directed cannot be utilized in the motor, but
+they must&mdash;again by means of a similar unreliable device&mdash;be
+reconverted into their original state of alternate currents.
+The function of the commutator is entirely external, and in no
+way does it affect the internal working of the machines. In
+reality, therefore, all machines are alternate current machines,
+the currents appearing as continuous only in the external circuit
+during their transit from generator to motor. In view simply of
+this fact, alternate currents would commend themselves as a more
+direct application of electrical energy, and the employment of
+continuous currents would only be justified if we had dynamos
+which would primarily generate, and motors which would be
+directly actuated by, such currents.</p>
+
+<p>But the operation of the commutator on a motor is twofold;
+first, it reverses the currents through the motor, and secondly,
+it effects automatically, a progressive shifting of the poles of one
+of its magnetic constituents. Assuming, therefore, that both of
+the useless operations in the systems, that is to say, the directing
+of the alternate currents on the generator and reversing the direct
+currents on the motor, be eliminated, it would still be necessary,
+in order to cause a rotation of the motor, to produce a progressive<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span>
+shifting of the poles of one of its elements, and the question
+presented itself&mdash;How to perform this operation by the direct
+action of alternate currents? I will now proceed to show how
+this result was accomplished.</p>
+
+<div class="figcenter" style="width: 640px;">
+<div class="figleft" style="width: 355px;">
+<img src="images/fig1.jpg" width="355" height="270" alt="Fig. 1." title="" />
+<span class="caption">Fig. 1.</span>
+</div>
+<div class="figright" style="width: 220px;">
+<img src="images/fig1a.jpg" width="220" height="270" alt="Fig. 1a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 1a.</span>
+</div>
+</div>
+
+
+<p>In the first experiment a drum-armature was provided with
+two coils at right angles to each other, and the ends of these coils
+were connected to two pairs of insulated contact-rings as usual.
+A ring was then made of thin insulated plates of sheet-iron and
+wound with four coils, each two opposite coils being connected
+together so as to produce free poles on diametrically opposite
+sides of the ring. The remaining free ends of the coils were then
+connected to the contact-rings of the generator armature so as
+to form two independent circuits, as indicated in Fig. 9. It
+may now be seen what results were secured in this combination,
+and with this view I would refer to the diagrams, Figs. 1 to 8<i>a</i>.
+The field of the generator being independently excited, the rotation
+of the armature sets up currents in the coils <small>C</small> <small>C<sub>1</sub></small>, varying in
+strength and direction in the well-known manner. In the position
+shown in Fig. 1, the current in coil <small>C</small> is nil, while coil <small>C<sub>1</sub></small> is
+traversed by its maximum current, and the connections may be
+such that the ring is magnetized by the coils <i>c</i><sub>1</sub> <i>c</i><sub>1</sub>, as indicated by
+the letters <small>N</small> <small>S</small> in Fig. 1<i>a</i>, the magnetizing effect of the coils
+<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span><i>c</i> <i>c</i> being nil, since these coils are included in the circuit of
+coil <small>C</small>.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 430px;">
+<img src="images/fig2.jpg" width="430" height="280" alt="Fig. 2." title="" />
+<span class="caption">Fig. 2.</span>
+</div>
+<div class="figright" style="width: 315px;">
+<img src="images/fig2a.jpg" width="315" height="280" alt="Fig. 2a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 2a.</span>
+</div>
+</div>
+
+<p>In Fig. 2, the armature coils are shown in a more advanced
+position, one-eighth of one revolution being completed. Fig.
+2<i>a</i> illustrates the corresponding magnetic condition of the ring.
+At this moment the coil <small>C<sub>1</sub></small> generates a current of the same direction
+as previously, but weaker, producing the poles <i>n</i><sub>1</sub> <i>s</i><sub>1</sub> upon
+the ring; the coil <small>C</small> also generates a current of the same direction,
+and the connections may be such that the coils <i>c</i> <i>c</i> produce
+the poles <i>n</i> <i>s</i>, as shown in Fig. 2<i>a</i>. The resulting polarity is
+indicated by the letters <small>N</small> <small>S</small>, and it will be observed that the
+poles of the ring have been shifted one-eighth of the periphery
+of the same.</p>
+
+
+<div class="figcenter" style="width: 720px;">
+<div class="figleft" style="width: 376px;">
+<img src="images/fig3.jpg" width="376" height="250" alt="Fig. 3." title="" />
+<span class="caption">Fig. 3.</span>
+</div>
+<div class="figright" style="width: 310px;">
+<img src="images/fig3a.jpg" width="310" height="250" alt="Fig. 3a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 3a.</span>
+</div>
+</div>
+
+
+<p>In Fig. 3 the armature has completed one quarter of one
+revolution. In this phase the current in coil <small>C</small> is a maximum, and
+of such direction as to produce the poles <small>N</small> <small>S</small> in Fig. 3<i>a</i>, whereas
+the current in coil <small>C<sub>1</sub></small> is nil, this coil being at its neutral position.
+The poles <small>N</small> <small>S</small> in Fig. 3<i>a</i> are thus shifted one quarter of the
+circumference of the ring.</p>
+
+<div class="figcenter" style="width: 680px;">
+<div class="figleft" style="width: 355px;">
+<img src="images/fig4.jpg" width="355" height="260" alt="Fig. 4." title="" />
+<span class="caption">Fig. 4.</span>
+</div>
+<div class="figright" style="width: 265px;">
+<img src="images/fig4a.jpg" width="265" height="260" alt="Fig. 4a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 4a.</span>
+</div>
+</div>
+
+<p>Fig. 4 shows the coils <small>C</small> <small>C</small> in a still more advanced position,
+the armature having completed three-eighths of one revolution.
+At that moment the coil <small>C</small> still generates a current of the same
+direction as before, but of less strength, producing the compar<span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span>atively
+weaker poles <i>n s</i> in Fig. 4<i>a</i>. The current in the coil <small>C<sub>1</sub></small>
+is of the same strength, but opposite direction. Its effect is,
+therefore, to produce upon the ring the poles <i>n</i><sub>1</sub> <i>s</i><sub>1</sub>, as indicated,
+and a polarity, <small>N S</small>, results, the poles now being shifted three-eighths
+of the periphery of the ring.</p>
+
+<div class="figcenter" style="width: 640px;">
+<div class="figleft" style="width: 345px;">
+<img src="images/fig5.jpg" width="345" height="270" alt="Fig. 5." title="" />
+<span class="caption">Fig. 5.</span>
+</div>
+<div class="figright" style="width: 235px;">
+<img src="images/fig5a.jpg" width="235" height="270" alt="Fig. 5a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 5a.</span>
+</div>
+</div>
+
+<p>In Fig. 5 one half of one revolution of the armature is completed,
+and the resulting magnetic condition of the ring is indicated
+in Fig. 5<i>a</i>. Now the current in coil <small>C</small> is nil, while the coil
+<small>C<sub>1</sub></small> yields its maximum current, which is of the same direction as
+previously; the magnetizing effect is, therefore, due to the coils,
+<i>c</i><sub>1</sub> <i>c</i><sub>1</sub> alone, and, referring to Fig. 5<i>a</i>, it will be observed that
+the poles <small>N S</small> are shifted one half of the circumference of the
+ring. During the next half revolution the operations are repeated,
+as represented in the Figs. 6 to 8<i>a</i>.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 435px;">
+<img src="images/fig6.jpg" width="435" height="270" alt="Fig. 6." title="" />
+<span class="caption">Fig. 6.</span>
+</div>
+<div class="figright" style="width: 265px;">
+<img src="images/fig6a.jpg" width="265" height="270" alt="Fig. 6a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 6a.</span>
+</div>
+</div>
+
+<p>A reference to the diagrams will make it clear that during one
+revolution of the armature the poles of the ring are shifted once
+around its periphery, and, each revolution producing like effects,
+a rapid whirling of the poles in harmony with the rotation of the
+armature is the result. If the connections of either one of the
+circuits in the ring are reversed, the shifting of the poles is made
+to progress in the opposite direction, but the operation is identi<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span>cally
+the same. Instead of using four wires, with like result,
+three wires may be used, one forming a common return for both
+circuits.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 410px;">
+<img src="images/fig7.jpg" width="410" height="270" alt="Fig. 7." title="" />
+<span class="caption">Fig. 7.</span>
+</div>
+<div class="figright" style="width: 330px;">
+<img src="images/fig7a.jpg" width="330" height="270" alt="Fig. 7a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 7a.</span>
+</div>
+</div>
+
+<p>This rotation or whirling of the poles manifests itself in a series
+of curious phenomena. If a delicately pivoted disc of steel or
+other magnetic metal is approached to the ring it is set in rapid
+rotation, the direction of rotation varying with the position of
+the disc. For instance, noting the direction outside of the ring
+it will be found that inside the ring it turns in an opposite direction,
+while it is unaffected if placed in a position symmetrical to
+the ring. This is easily explained. Each time that a pole approaches,
+it induces an opposite pole in the nearest point on the
+disc, and an attraction is produced upon that point; owing to this,
+as the pole is shifted further away from the disc a tangential pull
+is exerted upon the same, and the action being constantly repeated,
+a more or less rapid rotation of the disc is the result. As the
+pull is exerted mainly upon that part which is nearest to the
+ring, the rotation outside and inside, or right and left, respectively,
+is in opposite directions, Fig. 9. When placed symmetrically
+to the ring, the pull on the opposite sides of the disc being equal,
+no rotation results. The action is based on the magnetic inertia
+of iron; for this reason a disc of hard steel is much more affected
+than a disc of soft iron, the latter being capable of very
+rapid variations of magnetism. Such a disc has proved to be a
+very useful instrument in all these investigations, as it has enabled
+me to detect any irregularity in the action. A curious effect
+is also produced upon iron filings. By placing some upon a
+paper and holding them externally quite close to the ring, they
+are set in a vibrating motion, remaining in the same place, although
+the paper may be moved back and forth; but in lifting the paper
+to a certain height which seems to be dependent on the intensity
+of the poles and the speed of rotation, they are thrown away in<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span>
+a direction always opposite to the supposed movement of the
+poles. If a paper with filings is put flat upon the ring and the
+current turned on suddenly, the existence of a magnetic whirl
+may easily be observed.</p>
+
+<p>To demonstrate the complete analogy between the ring and a
+revolving magnet, a strongly energized electro-magnet was rotated
+by mechanical power, and phenomena identical in every particular
+to those mentioned above were observed.</p>
+
+<p>Obviously, the rotation of the poles produces corresponding
+inductive effects and may be utilized to generate currents in a
+closed conductor placed within the influence of the poles. For
+this purpose it is convenient to wind a ring with two sets of
+superimposed coils forming respectively the primary and secondary
+circuits, as shown in Fig. 10. In order to secure the most
+economical results the magnetic circuit should be completely
+closed, and with this object in view the construction may be
+modified at will.</p>
+
+<div class="figcenter" style="width: 720px;">
+<div class="figleft" style="width: 385px;">
+<img src="images/fig8.jpg" width="385" height="270" alt="Fig. 8." title="" />
+<span class="caption">Fig. 8.</span>
+</div>
+<div class="figright" style="width: 270px;">
+<img src="images/fig8a.jpg" width="270" height="270" alt="Fig. 8a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 8a.</span>
+</div>
+</div>
+
+<p>The inductive effect exerted upon the secondary coils will be
+mainly due to the shifting or movement of the magnetic action;
+but there may also be currents set up in the circuits in consequence
+of the variations in the intensity of the poles. However,
+by properly designing the generator and determining the magnetizing
+effect of the primary coils, the latter element may be made
+to disappear. The intensity of the poles being maintained constant,
+the action of the apparatus will be perfect, and the same
+result will be secured as though the shifting were effected by
+means of a commutator with an infinite number of bars. In such
+case the theoretical relation between the energizing effect of each
+set of primary coils and their resultant magnetizing effect may
+be expressed by the equation of a circle having its centre coinciding
+with that of an orthogonal system of axes, and in which
+the radius represents the resultant and the co-ordinates both<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+of its components. These are then respectively the sine and
+cosine of the angle &#945; between the radius and one of the axes
+(<i>O&nbsp;X</i>). Referring to Fig. 11, we have <i>r</i><sup>2</sup> = <i>x</i><sup>2</sup> + <i>y</i><sup>2</sup>; where
+<i>x</i> = <i>r</i> cos &#945;, and <i>y</i> = <i>r</i> sin &#945;.</p>
+
+<p>Assuming the magnetizing effect of each set of coils in the
+transformer to be proportional to the current&mdash;which may be
+admitted for weak degrees of magnetization&mdash;then <i>x</i> = <i>Kc</i> and
+<i>y</i> = <i>Kc</i><sup>1</sup>, where <i>K</i> is a constant and <i>c</i> and <i>c</i><sup>1</sup> the current in both
+sets of coils respectively. Supposing, further, the field of the
+generator to be uniform, we have for constant speed <i>c</i><sup>1</sup> = <i>K</i><sup>1</sup> sin &#945;
+and <i>c</i> = <i>K</i><sup>1</sup> sin (90&deg; + &#945;) = <i>K</i><sup>1</sup> cos &#945;, where <i>K</i><sup>1</sup> is a constant.
+See Fig. 12.</p>
+
+<p>Therefore,</p>
+
+<p class="blockquot">
+<i>x</i> = <i>K c</i> = <i>K K</i><sup>1</sup> cos &#945;;<br />
+<i>y</i> = <i>K c</i><sup>1</sup> = <i>K K</i><sup>1</sup> sin &#945;; and<br />
+<i>K K</i><sup>1</sup> = <i>r</i>.
+</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_030.jpg" width="640" height="335" alt="Fig. 9." title="" />
+<span class="caption">Fig. 9.</span>
+</div>
+
+<p>That is, for a uniform field the disposition of the two coils at
+right angles will secure the theoretical result, and the intensity
+of the shifting poles will be constant. But from <i>r</i><sup>2</sup> = <i>x</i><sup>2</sup> + <i>y</i><sup>2</sup> it
+follows that for <i>y</i> = 0, <i>r</i> = <i>x</i>; it follows that the joint magnetizing
+effect of both sets of coils should be equal to the effect of
+one set when at its maximum action. In transformers and in a
+certain class of motors the fluctuation of the poles is not of great
+importance, but in another class of these motors it is desirable to
+obtain the theoretical result.</p>
+
+<p>In applying this principle to the construction of motors, two
+typical forms of motor have been developed. First, a form having
+a comparatively small rotary effort at the start but maintaining
+a perfectly uniform speed at all loads, which motor has been
+termed synchronous. Second, a form possessing a great rotary
+effort at the start, the speed being dependent on the load.<span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+
+<p>These motors may be operated in three different ways: 1. By
+the alternate currents of the source only. 2. By a combined action
+of these and of induced currents. 3. By the joint action of
+alternate and continuous currents.</p>
+
+<div class="figcenter" style="width: 524px;">
+<img src="images/oi_031.jpg" width="524" height="480" alt="Fig. 10." title="" />
+<span class="caption">Fig. 10.</span>
+</div>
+
+<p>The simplest form of a synchronous motor is obtained by winding
+a laminated ring provided with pole projections with four
+coils, and connecting the same in the manner before indicated.
+An iron disc having a segment cut away on each side may be used
+as an armature. Such a motor is shown in Fig. 9. The disc
+being arranged to rotate freely within the ring in close proximity
+to the projections, it is evident that as the poles are shifted it
+will, owing to its tendency to place itself in such a position as to
+embrace the greatest number of the lines of force, closely follow
+the movement of the poles, and its motion will be synchronous
+with that of the armature of the generator; that is, in the peculiar
+disposition shown in Fig. 9, in which the armature produces by
+one revolution two current impulses in each of the circuits. It
+is evident that if, by one revolution of the armature, a greater
+number of impulses is produced, the speed of the motor will be
+correspondingly increased. Considering that the attraction exerted
+upon the disc is greatest when the same is in close proximity
+to the poles, it follows that such a motor will maintain exactly
+the same speed at all loads within the limits of its capacity.</p>
+
+<p>To facilitate the starting, the disc may be provided with a coil
+closed upon itself. The advantage secured by such a coil is evident.
+On the start the currents set up in the coil strongly ener<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span>gize
+the disc and increase the attraction exerted upon the same by
+the ring, and currents being generated in the coil as long as the
+speed of the armature is inferior to that of the poles, considerable
+work may be performed by such a motor even if the speed
+be below normal. The intensity of the poles being constant, no
+currents will be generated in the coil when the motor is turning
+at its normal speed.</p>
+
+<p>Instead of closing the coil upon itself, its ends may be connected
+to two insulated sliding rings, and a continuous current supplied
+to these from a suitable generator. The proper way to start such
+a motor is to close the coil upon itself until the normal speed is
+reached, or nearly so, and then turn on the continuous current.
+If the disc be very strongly energized by a continuous
+current the motor may not be able to start, but if it be weakly
+energized, or generally so that the magnetizing effect of the ring
+is preponderating, it will start and reach the normal speed. Such
+a motor will maintain absolutely the same speed at all loads. It
+has also been found that if the motive power of the generator is
+not excessive, by checking the motor the speed of the generator is
+diminished in synchronism with that of the motor. It is characteristic
+of this form of motor that it cannot be reversed by reversing
+the continuous current through the coil.</p>
+
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 336px;">
+<img src="images/oi_032a.jpg" width="336" height="346" alt="Fig. 11." title="" />
+<span class="caption">Fig. 11.</span>
+</div>
+
+<div class="figright" style="width: 400px;">
+<img src="images/oi_032.jpg" width="400" height="346" alt="Fig. 12." title="" />
+<span class="caption">Fig. 12.</span>
+</div>
+</div>
+
+
+<p>The synchronism of these motors may be demonstrated experimentally
+in a variety of ways. For this purpose it is best to
+employ a motor consisting of a stationary field magnet and an
+armature arranged to rotate within the same, as indicated in
+Fig. 13. In this case the shifting of the poles of the armature
+produces a rotation of the latter in the opposite direction. It
+results therefrom that when the normal speed is reached, the
+poles of the armature assume fixed positions relatively to the<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span>
+field magnet, and the same is magnetized by induction, exhibiting
+a distinct pole on each of the pole-pieces. If a piece of soft iron
+is approached to the field magnet, it will at the start be attracted
+with a rapid vibrating motion produced by the reversals of polarity
+of the magnet, but as the speed of the armature increases, the
+vibrations become less and less frequent and finally entirely cease.
+Then the iron is weakly but permanently attracted, showing that
+synchronism is reached and the field magnet energized by induction.</p>
+
+<p>The disc may also be used for the experiment. If held quite
+close to the armature it will turn as long as the speed of rotation
+of the poles exceeds that of the armature; but when the normal
+speed is reached, or very nearly so, it ceases to rotate and is permanently
+attracted.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_033.jpg" width="640" height="478" alt="Fig. 13." title="" />
+<span class="caption">Fig. 13.</span>
+</div>
+
+
+<p>A crude but illustrative experiment is made with an incandescent
+lamp. Placing the lamp in circuit with the continuous current
+generator and in series with the magnet coil, rapid fluctuations
+are observed in the light in consequence of the induced currents
+set up in the coil at the start; the speed increasing, the
+fluctuations occur at longer intervals, until they entirely disappear,
+showing that the motor has attained its normal speed. A
+telephone receiver affords a most sensitive instrument; when
+connected to any circuit in the motor the synchronism may be
+easily detected on the disappearance of the induced currents.</p>
+
+<p>In motors of the synchronous type it is desirable to maintain<span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span>
+the quantity of the shifting magnetism constant, especially if the
+magnets are not properly subdivided.</p>
+
+<p>To obtain a rotary effort in these motors was the subject of
+long thought. In order to secure this result it was necessary to
+make such a disposition that while the poles of one element of
+the motor are shifted by the alternate currents of the source, the
+poles produced upon the other elements should always be maintained
+in the proper relation to the former, irrespective of the
+speed of the motor. Such a condition exists in a continuous
+current motor; but in a synchronous motor, such as described,
+this condition is fulfilled only when the speed is normal.</p>
+
+<div class="figcenter" style="width: 335px;">
+<img src="images/oi_034.jpg" width="335" height="370" alt="Fig. 14." title="" />
+<span class="caption">Fig. 14.</span>
+</div>
+
+<p>The object has been attained by placing within the ring a properly
+subdivided cylindrical iron core wound with several independent
+coils closed upon themselves. Two coils at right angles as
+in Fig. 14, are sufficient, but a greater number may be advantageously
+employed. It results from this disposition that when
+the poles of the ring are shifted, currents are generated in the
+closed armature coils. These currents are the most intense at or
+near the points of the greatest density of the lines of force, and
+their effect is to produce poles upon the armature at right angles
+to those of the ring, at least theoretically so; and since this action
+is entirely independent of the speed&mdash;that is, as far as the location
+of the poles is concerned&mdash;a continuous pull is exerted upon the
+periphery of the armature. In many respects these motors are
+similar to the continuous current motors. If load is put on, the
+speed, and also the resistance of the motor, is diminished and
+more current is made to pass through the energizing coils, thus<span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span>
+increasing the effort. Upon the load being taken off, the
+counter-electromotive force increases and less current passes
+through the primary or energizing coils. Without any load the
+speed is very nearly equal to that of the shifting poles of the
+field magnet.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_035.jpg" width="640" height="199" alt="Fig. 15, 16, 17." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 15.</td><td class="caption">Fig. 16.</td><td class="caption">Fig. 17.</td></tr>
+</table>
+</div>
+
+<p>It will be found that the rotary effort in these motors fully
+equals that of the continuous current motors. The effort seems
+to be greatest when both armature and field magnet are without
+any projections; but as in such dispositions the field cannot be
+concentrated, probably the best results will be obtained by leaving
+pole projections on one of the elements only. Generally, it
+may be stated the projections diminish the torque and produce a
+tendency to synchronism.</p>
+
+<p>A characteristic feature of motors of this kind is their property
+of being very rapidly reversed. This follows from the peculiar
+action of the motor. Suppose the armature to be rotating and
+the direction of rotation of the poles to be reversed. The apparatus
+then represents a dynamo machine, the power to drive this
+machine being the momentum stored up in the armature and its
+speed being the sum of the speeds of the armature and the
+poles.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_035-1.jpg" width="640" height="132" alt="Fig. 18, 19, 20, 21." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 18.</td><td class="caption">Fig. 19.</td><td class="caption">Fig. 20.</td><td class="caption">Fig. 21.</td></tr>
+</table>
+</div>
+
+<p>If we now consider that the power to drive such a dynamo
+would be very nearly proportional to the third power of the
+speed, for that reason alone the armature should be quickly reversed.
+But simultaneously with the reversal another element is
+brought into action, namely, as the movement of the poles with
+respect to the armature is reversed, the motor acts like a transformer
+in which the resistance of the secondary circuit would be<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span>
+abnormally diminished by producing in this circuit an additional
+electromotive force. Owing to these causes the reversal is instantaneous.</p>
+
+<p>If it is desirable to secure a constant speed, and at the same
+time a certain effort at the start, this result may be easily attained
+in a variety of ways. For instance, two armatures, one for torque
+and the other for synchronism, may be fastened on the same shaft
+and any desired preponderance may be given to either one, or an
+armature may be wound for rotary effort, but a more or less pronounced
+tendency to synchronism may be given to it by properly
+constructing the iron core; and in many other ways.</p>
+
+<p>As a means of obtaining the required phase of the currents in
+both the circuits, the disposition of the two coils at right angles
+is the simplest, securing the most uniform action; but the phase
+may be obtained in many other ways, varying with the machine
+employed. Any of the dynamos at present in use may be easily
+adapted for this purpose by making connections to proper points
+of the generating coils. In closed circuit armatures, such as used
+in the continuous current systems, it is best to make four derivations
+from equi-distant points or bars of the commutator, and to
+connect the same to four insulated sliding rings on the shaft. In
+this case each of the motor circuits is connected to two diametrically
+opposite bars of the commutator. In such a disposition the
+motor may also be operated at half the potential and on the three-wire
+plan, by connecting the motor circuits in the proper order to
+three of the contact rings.</p>
+
+<p>In multipolar dynamo machines, such as used in the converter
+systems, the phase is conveniently obtained by winding upon the
+armature two series of coils in such a manner that while the coils
+of one set or series are at their maximum production of current,
+the coils of the other will be at their neutral position, or nearly
+so, whereby both sets of coils may be subjected simultaneously
+or successively to the inducing action of the field magnets.</p>
+
+<p>Generally the circuits in the motor will be similarly disposed,
+and various arrangements may be made to fulfill the requirements;
+but the simplest and most practicable is to arrange primary circuits
+on stationary parts of the motor, thereby obviating, at least
+in certain forms, the employment of sliding contacts. In such a
+case the magnet coils are connected alternately in both the circuits;
+that is, 1, 3, 5 ... in one, and 2, 4, 6 ... in the other, and
+the coils of each set of series may be connected all in the same<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span>
+manner, or alternately in opposition; in the latter case a motor
+with half the number of poles will result, and its action will be
+correspondingly modified. The Figs. 15, 16, and 17, show
+three different phases, the magnet coils in each circuit being connected
+alternately in opposition. In this case there will be always
+four poles, as in Figs. 15 and 17; four pole projections will be
+neutral; and in Fig. 16 two adjacent pole projections will have
+the same polarity. If the coils are connected in the same manner
+there will be eight alternating poles, as indicated by the letters
+<i>n'</i> <i>s'</i> in Fig. 15.</p>
+
+<p>The employment of multipolar motors secures in this system an
+advantage much desired and unattainable in the continuous current
+system, and that is, that a motor may be made to run exactly
+at a predetermined speed irrespective of imperfections in construction,
+of the load, and, within certain limits, of electromotive
+force and current strength.</p>
+
+<p>In a general distribution system of this kind the following plan
+should be adopted. At the central station of supply a generator
+should be provided having a considerable number of poles. The
+motors operated from this generator should be of the synchronous
+type, but possessing sufficient rotary effort to insure their starting.
+With the observance of proper rules of construction it may be
+admitted that the speed of each motor will be in some inverse
+proportion to its size, and the number of poles should be chosen
+accordingly. Still, exceptional demands may modify this rule.
+In view of this, it will be advantageous to provide each motor
+with a greater number of pole projections or coils, the number
+being preferably a multiple of two and three. By this means, by
+simply changing the connections of the coils, the motor may be
+adapted to any probable demands.</p>
+
+<p>If the number of the poles in the motor is even, the action will
+be harmonious and the proper result will be obtained; if this
+is not the case, the best plan to be followed is to make a
+motor with a double number of poles and connect the same in
+the manner before indicated, so that half the number of poles
+result. Suppose, for instance, that the generator has twelve poles,
+and it would be desired to obtain a speed equal to 12/7 of the speed
+of the generator. This would require a motor with seven pole
+projections or magnets, and such a motor could not be properly
+connected in the circuits unless fourteen armature coils would be
+provided, which would necessitate the employment of sliding<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span>
+contacts. To avoid this, the motor should be provided with fourteen
+magnets and seven connected in each circuit, the magnets
+in each circuit alternating among themselves. The armature
+should have fourteen closed coils. The action of the motor will
+not be quite as perfect as in the case of an even number of poles,
+but the drawback will not be of a serious nature.</p>
+
+<p>However, the disadvantages resulting from this unsymmetrical
+form will be reduced in the same proportion as the number of
+the poles is augmented.</p>
+
+<p>If the generator has, say, <i>n</i>, and the motor <i>n</i><sub>1</sub> poles, the speed
+of the motor will be equal to that of the generator multiplied by
+<i>n</i>/<i>n</i><sub>1</sub>.</p>
+
+<p>The speed of the motor will generally be dependent on the
+number of the poles, but there may be exceptions to this rule.
+The speed may be modified by the phase of the currents in the
+circuit or by the character of the current impulses or by intervals
+between each or between groups of impulses. Some of the
+possible cases are indicated in the diagrams, Figs. 18, 19, 20 and
+21, which are self-explanatory. Fig. 18 represents the condition
+generally existing, and which secures the best result. In
+such a case, if the typical form of motor illustrated in Fig. 9
+is employed, one complete wave in each circuit will produce one
+revolution of the motor. In Fig. 19 the same result will be
+effected by one wave in each circuit, the impulses being successive;
+in Fig. 20 by four, and in Fig. 21 by eight waves.</p>
+
+<p>By such means any desired speed may be attained, that is, at
+least within the limits of practical demands. This system possesses
+this advantage, besides others, resulting from simplicity.
+At full loads the motors show an efficiency fully equal to that of
+the continuous current motors. The transformers present an
+additional advantage in their capability of operating motors.
+They are capable of similar modifications in construction, and will
+facilitate the introduction of motors and their adaptation to practical
+demands. Their efficiency should be higher than that of
+the present transformers, and I base my assertion on the following:</p>
+
+<p>In a transformer, as constructed at present, we produce the
+currents in the secondary circuit by varying the strength of the
+primary or exciting currents. If we admit proportionality with
+respect to the iron core the inductive effect exerted upon the<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span>
+secondary coil will be proportional to the numerical sum of the
+variations in the strength of the exciting current per unit of time;
+whence it follows that for a given variation any prolongation of
+the primary current will result in a proportional loss. In order
+to obtain rapid variations in the strength of the current, essential
+to efficient induction, a great number of undulations are employed;
+from this practice various disadvantages result. These are:
+Increased cost and diminished efficiency of the generator; more
+waste of energy in heating the cores, and also diminished output
+of the transformer, since the core is not properly utilized, the
+reversals being too rapid. The inductive effect is also very small
+in certain phases, as will be apparent from a graphic representation,
+and there may be periods of inaction, if there are intervals
+between the succeeding current impulses or waves. In producing
+a shifting of the poles in a transformer, and thereby inducing
+currents, the induction is of the ideal character, being always
+maintained at its maximum action. It is also reasonable to assume
+that by a shifting of the poles less energy will be wasted
+than by reversals.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV.</h2>
+
+<h3><span class="smcap">Modifications and Expansions of the Tesla Polyphase
+Systems.</span></h3>
+
+
+<p>In his earlier papers and patents relative to polyphase currents,
+Mr. Tesla devoted himself chiefly to an enunciation of the broad
+lines and ideas lying at the basis of this new work; but he supplemented
+this immediately by a series of other striking inventions
+which may be regarded as modifications and expansions of
+certain features of the Tesla systems. These we shall now proceed
+to deal with.</p>
+
+<p>In the preceding chapters we have thus shown and described
+the Tesla electrical systems for the transmission of power and the
+conversion and distribution of electrical energy, in which the
+motors and the transformers contain two or more coils or sets of
+coils, which were connected up in independent circuits with
+corresponding coils of an alternating current generator, the operation
+of the system being brought about by the co-operation of
+the alternating currents in the independent circuits in progressively
+moving or shifting the poles or points of maximum magnetic
+effect of the motors or converters. In these systems two
+independent conductors are employed for each of the independent
+circuits connecting the generator with the devices for converting
+the transmitted currents into mechanical energy or into
+electric currents of another character. This, however, is not
+always necessary. The two or more circuits may have a single
+return path or wire in common, with a loss, if any, which is so
+extremely slight that it may be disregarded entirely. For the
+sake of illustration, if the generator have two independent coils
+and the motor two coils or two sets of coils in corresponding relations
+to its operative elements one terminal of each generator
+coil is connected to the corresponding terminals of the motor
+coils through two independent conductors, while the opposite
+terminals of the respective coils are both connected to one
+return wire. The following description deals with the modifica<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span>tion.
+Fig. 22 is a diagrammatic illustration of a generator and
+single motor constructed and electrically connected in accordance
+with the invention. Fig. 23 is a diagram of the system
+as it is used in operating motors or converters, or both, in parallel,
+while Fig. 24 illustrates diagrammatically the manner of operating
+two or more motors or converters, or both, in series. Referring
+to Fig. 22, <small>A A</small> designate the poles of the field magnets of
+an alternating-current generator, the armature of which, being in
+this case cylindrical in form and mounted on a shaft, <small>C</small>, is wound
+longitudinally with coils <small>B B'</small>. The shaft <small>C</small> carries three insulated
+contact-rings, <i>a b c</i>, to two of which, as <i>b c</i>, one terminal of each
+coil, as <i>e d</i>, is connected. The remaining terminals, <i>f g</i>, are both
+connected to the third ring, <i>a</i>.</p>
+
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 378px;">
+<img src="images/fig22.jpg" width="378" height="640" alt="Fig. 22." title="" />
+<span class="caption">Fig. 22.</span>
+</div>
+<div class="figright" style="width: 341px;">
+<img src="images/fig24.jpg" width="341" height="640" alt="Fig. 24." title="" />
+<span class="caption">Fig. 24.</span>
+</div>
+</div>
+
+<p>A motor in this case is shown as composed of a ring, <small>H</small>, wound
+with four coils, <small>I I J J</small>, electrically connected, so as to co-operate
+in pairs, with a tendency to fix the poles of the ring at four points
+ninety degrees apart. Within the magnetic ring <small>H</small> is a disc or
+cylindrical core wound with two coils, <small>G G'</small>, which may be con<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span>nected
+to form two closed circuits. The terminals <i>j k</i> of the two
+sets or pairs of coils are connected, respectively, to the binding-posts
+<small>E' F'</small>, and the other terminals, <i>h i</i>, are connected to a single
+binding-post, <small>D'</small>. To operate the motor, three line-wires are used
+to connect the terminals of the generator with those of the motor.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_042.jpg" width="600" height="675" alt="Fig. 23." title="" />
+<span class="caption">Fig. 23.</span>
+</div>
+
+
+<p>So far as the apparent action or mode of operation of this arrangement
+is concerned, the single wire <small>D</small>, which is, so to speak,
+a common return-wire for both circuits, may be regarded as two
+independent wires. In the illustration, with the order of connection
+shown, coil <small>B'</small> of the generator is producing its maximum
+current and coil <small>B</small> its minimum; hence the current which passes
+through wire <i>e</i>, ring <i>b</i>, brush <i>b'</i>, line-wire <small>E</small>, terminal <small>E'</small>, wire <i>j</i>,
+coils <small>I I</small>, wire or terminal <small>D'</small>, line-wire <small>D</small>, brush <i>a'</i>, ring <i>a</i>, and
+wire <i>f</i>, fixes the polar line of the motor midway between the<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+two coils <small>I I</small>; but as the coil <small>B'</small> moves from the position indicated
+it generates less current, while coil <small>B</small>, moving into the field, generates
+more. The current from coil <small>B</small> passes through the devices
+and wires designated by the letters <i>d</i>, <i>c</i>, <i>c'</i> <small>F</small>, <small>F'</small> <i>k</i>, <small>J J</small>, <i>i</i>, <small>D'</small>, <small>D</small>, <i>a'</i>,
+<i>a</i>, and <i>g</i>, and the position of the poles of the motor will be due
+to the resultant effect of the currents in the two sets of coils&mdash;that
+is, it will be advanced in proportion to the advance or forward
+movement of the armature coils. The movement of the
+generator-armature through one-quarter of a revolution will obviously
+bring coil <small>B'</small> into its neutral position and coil <small>B</small> into its
+position of maximum effect, and this shifts the poles ninety degrees,
+as they are fixed solely by coils <small>B</small>. This action is repeated
+for each quarter of a complete revolution.</p>
+
+<p>When more than one motor or other device is employed, they
+may be run either in parallel or series. In Fig. 23 the former
+arrangement is shown. The electrical device is shown as a converter,
+<small>L</small>, of which the two sets of primary coils <i>p r</i> are connected,
+respectively, to the mains <small>F E</small>, which are electrically connected
+with the two coils of the generator. The cross-circuit
+wires <i>l m</i>, making these connections, are then connected to the
+common return-wire <small>D</small>. The secondary coils <i>p' p''</i> are in circuits
+<i>n o</i>, including, for example, incandescent lamps. Only one converter
+is shown entire in this figure, the others being illustrated
+diagrammatically.</p>
+
+<p>When motors or converters are to be run in series, the two
+wires <small>E F</small> are led from the generator to the coils of the first
+motor or converter, then continued on to the next, and so on
+through the whole series, and are then joined to the single wire
+<small>D</small>, which completes both circuits through the generator. This is
+shown in Fig. 24, in which <small>J I</small> represent the two coils or sets of
+coils of the motors.</p>
+
+<p>There are, of course, other conditions under which the same
+idea may be carried out. For example, in case the motor and
+generator each has three independent circuits, one terminal of
+each circuit is connected to a line-wire, and the other three terminals
+to a common return-conductor. This arrangement will
+secure similar results to those attained with a generator and motor
+having but two independent circuits, as above described.</p>
+
+<p>When applied to such machines and motors as have three or
+more induced circuits with a common electrical joint, the three
+or more terminals of the generator would be simply connected<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span>
+to those of the motor. Mr. Tesla states, however, that the results
+obtained in this manner show a lower efficiency than do the
+forms dwelt upon more fully above.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V.</h2>
+
+<h3><span class="smcap">Utilizing Familiar Types of Generator of the Continuous
+Current Type.</span></h3>
+
+
+<p>The preceding descriptions have assumed the use of alternating
+current generators in which, in order to produce the progressive
+movement of the magnetic poles, or of the resultant attraction of
+independent field magnets, the current generating coils are independent
+or separate. The ordinary forms of continuous current
+dynamos may, however, be employed for the same work, in
+accordance with a method of adaptation devised by Mr. Tesla.
+As will be seen, the modification involves but slight changes in
+their construction, and presents other elements of economy.</p>
+
+<p>On the shaft of a given generator, either in place of or in addition
+to the regular commutator, are secured as many pairs of
+insulated collecting-rings as there are circuits to be operated.
+Now, it will be understood that in the operation of any dynamo
+electric generator the currents in the coils in their movement
+through the field of force undergo different phases&mdash;that is to
+say, at different positions of the coils the currents have certain
+directions and certain strengths&mdash;and that in the Tesla motors or
+transformers it is necessary that the currents in the energizing
+coils should undergo a certain order of variations in strength and
+direction. Hence, the further step&mdash;viz., the connection between
+the induced or generating coils of the machine and the contact-rings
+from which the currents are to be taken off&mdash;will be determined
+solely by what order of variations of strength and direction
+in the currents is desired for producing a given result in the
+electrical translating device. This may be accomplished in
+various ways; but in the drawings we give typical instances only
+of the best and most practicable ways of applying the invention
+to three of the leading types of machines in widespread use, in
+order to illustrate the principle.</p>
+
+<p>Fig. 25 is a diagram illustrative of the mode of applying the
+invention to the well-known type of "closed" or continuous cir<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span>cuit
+machines. Fig. 26 is a similar diagram embodying an armature
+with separate coils connected diametrically, or what is generally
+called an "open-circuit" machine. Fig. 27 is a diagram
+showing the application of the invention to a machine the armature-coils
+of which have a common joint.</p>
+
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_046.jpg" width="600" height="628" alt="Fig. 25." title="" />
+<span class="caption">Fig. 25.</span>
+</div>
+
+
+<p>Referring to Fig. 25, let <small>A</small> represent a Tesla motor or transformer
+which, for convenience, we will designate as a "converter."
+It consists of an annular core, <small>B</small>, wound with four independent
+coils, <small>C</small> and <small>D</small>, those diametrically opposite being connected
+together so as to co-operate in pairs in establishing free
+poles in the ring, the tendency of each pair being to fix the poles
+at ninety degrees from the other. There may be an armature,
+<small>E</small>, within the ring, which is wound with coils closed upon themselves.
+The object is to pass through coils <small>C D</small> currents of such
+relative strength and direction as to produce a progressive shifting
+or movement of the points of maximum magnetic effect
+around the ring, and to thereby maintain a rotary movement of
+the armature. There are therefore secured to the shaft <small>F</small> of the
+generator, four insulated contact-rings, <i>a b c d</i>, upon which bear<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span>
+the collecting-brushes <i>a' b' c' d'</i>, connected by wires <small>G G H H</small>, respectively,
+with the terminals of coils <small>C</small> and <small>D</small>.</p>
+
+<p>Assume, for sake of illustration, that the coils <small>D D</small> are to receive
+the maximum and coils <small>C C</small> at the same instant the minimum
+current, so that the polar line may be midway between the
+coils <small>D D</small>. The rings <i>a b</i> would therefore be connected to the
+continuous armature-coil at its neutral points with respect to the
+field, or the point corresponding with that of the ordinary commutator
+brushes, and between which exists the greatest difference
+of potential; while rings <i>c d</i> would be connected to two
+points in the coil, between which exists no difference of potential.
+The best results will be obtained by making these connections at
+points equidistant from one another, as shown. These connections
+are easiest made by using wires <small>L</small> between the rings and the
+loops or wires <small>J</small>, connecting the coil <small>I</small> to the segments of the
+commutator <small>K</small>. When the converters are made in this manner,
+it is evident that the phases of the currents in the sections of the
+generator coil will be reproduced in the converter coils. For
+example, after turning through an arc of ninety degrees the conductors
+<small>L L</small>, which before conveyed the maximum current, will
+receive the minimum current by reason of the change in the
+position of their coils, and it is evident that for the same reason
+the current in these coils has gradually fallen from the maximum
+to the minimum in passing through the arc of ninety degrees.
+In this special plan of connections, the rotation of the magnetic
+poles of the converter will be synchronous with that of the
+armature coils of the generator, and the result will be the same,
+whether the energizing circuits are derivations from a continuous
+armature coil or from independent coils, as in Mr. Tesla's
+other devices.</p>
+
+<p>In Fig. 25, the brushes <small>M M</small> are shown in dotted lines in their
+proper normal position. In practice these brushes may be removed
+from the commutator and the field of the generator
+excited by an external source of current; or the brushes may be
+allowed to remain on the commutator and to take off a converted
+current to excite the field, or to be used for other purposes.</p>
+
+<p>In a certain well-known class of machines known as the "open
+circuit," the armature contains a number of coils the terminals of
+which connect to commutator segments, the coils being connected
+across the armature in pairs. This type of machine is represented
+in Fig. 26. In this machine each pair of coils goes<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span>
+through the same phases as the coils in some of the generators
+already shown, and it is obviously only necessary to utilize them
+in pairs or sets to operate a Tesla converter by extending the
+segments of the commutators belonging to each pair of coils and
+causing a collecting brush to bear on the continuous portion of
+each segment. In this way two or more circuits may be taken
+off from the generator, each including one or more pairs or sets
+of coils as may be desired.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_048.jpg" width="638" height="480" alt="Fig. 26, 27." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 26.</td><td class="caption">Fig. 27.</td></tr>
+</table>
+</div>
+
+
+<p>In Fig. 26 <small>I I</small> represent the armature coils, <small>T T</small> the poles of the
+field magnet, and <small>F</small> the shaft carrying the commutators, which
+are extended to form continuous portions <i>a b c d</i>. The brushes
+bearing on the continuous portions for taking off the alternating
+currents are represented by <i>a' b' c' d'</i>. The collecting brushes,
+or those which may be used to take off the direct current, are
+designated by <small>M M</small>. Two pairs of the armature coils and their
+commutators are shown in the figure as being utilized; but all
+may be utilized in a similar manner.</p>
+
+<p>There is another well-known type of machine in which three
+or more coils, <small>A' B' C'</small>, on the armature have a common joint,
+the free ends being connected to the segments of a commutator.
+This form of generator is illustrated in Fig. 27. In this case each
+terminal of the generator is connected directly or in derivation
+to a continuous ring, <i>a b c</i>, and collecting brushes, <i>a' b' c'</i>, bearing<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span>
+thereon, take off the alternating currents that operate the motor.
+It is preferable in this case to employ a motor or transformer
+with three energizing coils, <small>A'' B'' C''</small>, placed symmetrically with
+those of the generator, and the circuits from the latter are connected
+to the terminals of such coils either directly&mdash;as when
+they are stationary&mdash;or by means of brushes <i>e'</i> and contact rings
+<i>e</i>. In this, as in the other cases, the ordinary commutator may
+be used on the generator, and the current taken from it utilized
+for exciting the generator field-magnets or for other purposes.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI.</h2>
+
+<h3><span class="smcap">Method of Obtaining Desired Speed of Motor or
+Generator.</span></h3>
+
+
+<p>With the object of obtaining the desired speed in motors
+operated by means of alternating currents of differing phase,
+Mr. Tesla has devised various plans intended to meet the practical
+requirements of the case, in adapting his system to types of
+multipolar alternating current machines yielding a large number
+of current reversals for each revolution.</p>
+
+<p>For example, Mr. Tesla has pointed out that to adapt a given
+type of alternating current generator, you may couple rigidly
+two complete machines, securing them together in such a way
+that the requisite difference in phase will be produced; or you
+may fasten two armatures to the same shaft within the influence
+of the same field and with the requisite angular displacement to
+yield the proper difference in phase between the two currents;
+or two armatures may be attached to the same shaft with their
+coils symmetrically disposed, but subject to the influence of two
+sets of field magnets duly displaced; or the two sets of coils
+may be wound on the same armature alternately or in such manner
+that they will develop currents the phases of which differ in
+time sufficiently to produce the rotation of the motor.</p>
+
+<p>Another method included in the scope of the same idea, whereby
+a single generator may run a number of motors either at its
+own rate of speed or all at different speeds, is to construct the
+motors with fewer poles than the generator, in which case their
+speed will be greater than that of the generator, the rate of speed
+being higher as the number of their poles is relatively less. This
+may be understood from an example, taking a generator that has
+two independent generating coils which revolve between two
+pole pieces oppositely magnetized; and a motor with energizing
+coils that produce at any given time two magnetic poles in one
+element that tend to set up a rotation of the motor. A generator
+thus constructed yields four reversals, or impulses, in each<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>
+revolution, two in each of its independent circuits; and the effect
+upon the motor is to shift the magnetic poles through three hundred
+and sixty degrees. It is obvious that if the four reversals
+in the same order could be produced by each half-revolution of
+the generator the motor would make two revolutions to the generator's
+one. This would be readily accomplished by adding two
+intermediate poles to the generator or altering it in any of the
+other equivalent ways above indicated. The same rule applies
+to generators and motors with multiple poles. For instance, if a
+generator be constructed with two circuits, each of which produces
+twelve reversals of current to a revolution, and these currents
+be directed through the independent energizing-coils of a
+motor, the coils of which are so applied as to produce twelve
+magnetic poles at all times, the rotation of the two will be synchronous;
+but if the motor-coils produce but six poles, the movable
+element will be rotated twice while the generator rotates once; or
+if the motor have four poles, its rotation will be three times as
+fast as that of the generator.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_051.jpg" width="640" height="405" alt="Fig. 28, 29." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 28.</td><td class="caption">Fig. 29.</td></tr>
+</table>
+</div>
+
+<p>These features, so far as necessary to an understanding of the
+principle, are here illustrated. Fig. 28 is a diagrammatic illustration
+of a generator constructed in accordance with the invention.
+Fig. 29 is a similar view of a correspondingly constructed
+motor. Fig. 30 is a diagram of a generator of modified construction.
+Fig. 31 is a diagram of a motor of corresponding
+character. Fig. 32 is a diagram of a system containing a generator
+and several motors adapted to run at various speeds.<span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+
+<p>In Fig. 28, let <small>C</small> represent a cylindrical armature core wound
+longitudinally with insulated coils <small>A A</small>, which are connected up
+in series, the terminals of the series being connected to collecting-rings
+<i>a a</i> on the shaft <small>G</small>. By means of this shaft the armature
+is mounted to rotate between the poles of an annular field-magnet
+<small>D</small>, formed with polar projections wound with coils <small>E</small>, that
+magnetize the said projections. The coils <small>E</small> are included in the
+circuit of a generator <small>F</small>, by means of which the field-magnet is
+energized. If thus constructed, the machine is a well-known
+form of alternating-current generator. To adapt it to his system,
+however, Mr. Tesla winds on armature <small>C</small> a second set of
+coils <small>B B</small> intermediate to the first, or, in other words, in such positions
+that while the coils of one set are in the relative positions
+to the poles of the field-magnet to produce the maximum current,
+those of the other set will be in the position in which they produce
+the minimum current. The coils <small>B</small> are connected, also, in
+series and to two connecting-rings, secured generally to the
+shaft at the opposite end of the armature.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 350px;">
+<img src="images/fig30.jpg" width="350" height="225" alt="Fig. 30." title="" />
+<span class="caption">Fig. 30.</span>
+</div>
+<div class="figright" style="width: 350px;">
+<img src="images/fig31.jpg" width="350" height="225" alt="Fig. 31." title="" />
+<span class="caption">Fig. 31.</span>
+</div>
+</div>
+
+<p>The motor shown in Fig. 29 has an annular field-magnet <small>H</small>,
+with four pole-pieces wound with coils <small>I</small>. The armature is constructed
+similarly to the generator, but with two sets of two
+coils in closed circuits to correspond with the reduced number of
+magnetic poles in the field. From the foregoing it is evident that
+one revolution of the armature of the generator producing eight
+current impulses in each circuit will produce two revolutions of
+the motor-armature.</p>
+
+<p>The application of the principle of this invention is not, however,
+confined to any particular form of machine. In Figs. 30
+and 31 a generator and motor of another well-known type are
+shown. In Fig. 30, <small>J J</small> are magnets disposed in a circle and
+wound with coils <small>K</small>, which are in circuit with a generator which<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+supplies the current that maintains the field of force. In the
+usual construction of these machines the armature-conductor <small>L</small> is
+carried by a suitable frame, so as to be rotated in face of the
+magnets <small>J J</small>, or between these magnets and another similar set
+in front of them. The magnets are energized so as to be of alternately
+opposite polarity throughout the series, so that as the
+conductor <small>C</small> is rotated the current impulses combine or are
+added to one another, those produced by the conductor in any
+given position being all in the same direction. To adapt such
+a machine to his system, Mr. Tesla adds a second set of induced
+conductors <small>M</small>, in all respects similar to the first, but so placed
+in reference to it that the currents produced in each will differ
+by a quarter-phase. With such relations it is evident that as the
+current decreases in conductor <small>L</small> it increases in conductor <small>M</small>, and
+conversely, and that any of the forms of Tesla motor invented
+for use in this system may be operated by such a generator.</p>
+
+<p>Fig. 31 is intended to show a motor corresponding to the machine
+in Fig. 30. The construction of the motor is identical with
+that of the generator, and if coupled thereto it will run synchronously
+therewith. <small>J' J'</small> are the field-magnets, and <small>K'</small> the
+coils thereon. <small>L'</small> is one of the armature-conductors and <small>M'</small> the
+other.</p>
+
+<p>Fig. 32 shows in diagram other forms of machine. The generator
+<small>N</small> in this case is shown as consisting of a stationary ring <small>O</small>,
+wound with twenty-four coils <small>P P'</small>, alternate coils being connected
+in series in two circuits. Within this ring is a disc or drum <small>Q</small>,
+with projections <small>Q'</small> wound with energizing-coils included in circuit
+with a generator <small>R</small>. By driving this disc or cylinder alternating
+currents are produced in the coils <small>P</small> and <small>P'</small>, which are
+carried off to run the several motors.</p>
+
+<p>The motors are composed of a ring or annular field-magnet <small>S</small>,
+wound with two sets of energizing-coils <small>T T'</small>, and armatures <small>U</small>,
+having projections <small>U'</small> wound with coils <small>V</small>, all connected in series
+in a closed circuit or each closed independently on itself.</p>
+
+<p>Suppose the twelve generator-coils <small>P</small> are wound alternately in
+opposite directions, so that any two adjacent coils of the same set
+tend to produce a free pole in the ring <small>O</small> between them and the
+twelve coils <small>P'</small> to be similarly wound. A single revolution of
+the disc or cylinder <small>Q</small>, the twelve polar projections of which are
+of opposite polarity, will therefore produce twelve current impulses
+in each of the circuits <small>W W'</small>. Hence the motor <small>X</small>, which<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span>
+has sixteen coils or eight free poles, will make one and a half turns
+to the generator's one. The motor <small>Y</small>, with twelve coils or six
+poles, will rotate with twice the speed of the generator, and the
+motor <small>Z</small>, with eight coils or four poles, will revolve three times
+as fast as the generator. These multipolar motors have a peculiarity
+which may be often utilized to great advantage. For example,
+in the motor <small>X</small>, Fig. 32, the eight poles may be either
+alternately opposite or there may be at any given time alternately
+two like and two opposite poles. This is readily attained by
+making the proper electrical connections. The effect of such a
+change, however, would be the same as reducing the number of
+<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span>poles one-half, and thereby doubling the speed of any given
+motor.</p>
+
+<div class="figcenter" style="width: 464px;">
+<img src="images/oi_054.jpg" width="464" height="640" alt="Fig. 32." title="" />
+<span class="caption">Fig. 32.</span>
+</div>
+
+
+<p>It is obvious that the Tesla electrical transformers which have
+independent primary currents may be used with the generators
+described. It may also be stated with respect to the devices
+we now describe that the most perfect and harmonious action
+of the generators and motors is obtained when the numbers of the
+poles of each are even and not odd. If this is not the case, there
+will be a certain unevenness of action which is the less appreciable
+as the number of poles is greater; although this may be in a
+measure corrected by special provisions which it is not here
+necessary to explain. It also follows, as a matter of course, that
+if the number of the poles of the motor be greater than that of
+the generator the motor will revolve at a slower speed than the
+generator.</p>
+
+<p>In this chapter, we may include a method devised by Mr.
+Tesla for avoiding the very high speeds which would be necessary
+with large generators. In lieu of revolving the generator
+armature at a high rate of speed, he secures the desired result by
+a rotation of the magnetic poles of one element of the generator,
+while driving the other at a different speed. The effect is the
+same as that yielded by a very high rate of rotation.</p>
+
+<p>In this instance, the generator which supplies the current for
+operating the motors or transformers consists of a subdivided
+ring or annular core wound with four diametrically-opposite
+coils, <small>E E'</small>, Fig. 33. Within the ring is mounted a cylindrical
+armature-core wound longitudinally with two independent coils,
+<small>F F'</small>, the ends of which lead, respectively, to two pairs of insulated
+contact or collecting rings, <small>D D' G G'</small>, on the armature shaft.
+Collecting brushes <i>d d' g g'</i> bear upon these rings, respectively,
+and convey the currents through the two independent line-circuits
+<small>M M'</small>. In the main line there may be included one or more
+motors or transformers, or both. If motors be used, they are of
+the usual form of Tesla construction with independent coils or
+sets of coils <small>J J'</small>, included, respectively, in the circuits <small>M M'</small>.
+These energizing-coils are wound on a ring or annular field or on
+pole pieces thereon, and produce by the action of the alternating
+currents passing through them a progressive shifting of the magnetism
+from pole to pole. The cylindrical armature <small>H</small> of the
+motor is wound with two coils at right angles, which form independent
+closed circuits.<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span></p>
+
+<p>If transformers be employed, one set of the primary coils, as
+<small>N N</small>, wound on a ring or annular core is connected to one circuit,
+as <small>M'</small>, and the other primary coils, <small>N N'</small>, to the circuit <small>M</small>. The
+secondary coils <small>K K'</small> may then be utilized for running groups of
+incandescent lamps <small>P P'</small>.</p>
+
+<div class="figcenter" style="width: 557px;">
+<img src="images/oi_056.jpg" width="557" height="800" alt="Fig. 33." title="" />
+<span class="caption">Fig. 33.</span>
+</div>
+
+<p>With this generator an exciter is employed. This consists of
+two poles, <small>A A</small>, of steel permanently magnetized, or of iron excited
+by a battery or other generator of continuous currents, and
+a cylindrical armature core mounted on a shaft, <small>B</small>, and wound
+with two longitudinal coils, <small>C C'</small>. One end of each of these coils
+is connected to the collecting-rings <i>b c</i>, respectively, while the<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span>
+other ends are both connected to a ring, <i>a</i>. Collecting-brushes
+<i>b' c'</i> bear on the rings <i>b c</i>, respectively, and conductors <small>L L</small> convey
+the currents therefrom through the coils <small>E</small> and <small>E</small> of the generator.
+<small>L'</small> is a common return-wire to brush <i>a'</i>. Two independent
+circuits are thus formed, one including coils <small>C</small> of the exciter
+and <small>E E</small> of the generator, the other coils <small>C'</small> of the exciter and <small>E'</small>
+<small>E'</small> of the generator. It results from this that the operation of
+the exciter produces a progressive movement of the magnetic
+poles of the annular field-core of the generator, the shifting or
+rotary movement of the poles being synchronous with the rotation
+of the exciter armature. Considering the operative conditions
+of a system thus established, it will be found that when
+the exciter is driven so as to energize the field of the generator,
+the armature of the latter, if left free to turn, would rotate at a
+speed practically the same as that of the exciter. If under such
+conditions the coils <small>F F'</small> of the generator armature be closed
+upon themselves or short-circuited, no currents, at least theoretically,
+will be generated in these armature coils. In practice
+the presence of slight currents is observed, the existence of which
+is attributable to more or less pronounced fluctuations in the intensity
+of the magnetic poles of the generator ring. So, if the
+armature-coils <small>F F'</small> be closed through the motor, the latter will
+not be turned as long as the movement of the generator armature
+is synchronous with that of the exciter or of the magnetic poles
+of its field. If, on the contrary, the speed of the generator armature
+be in any way checked, so that the shifting or rotation of
+the poles of the field becomes relatively more rapid, currents will
+be induced in the armature coils. This obviously follows from
+the passing of the lines of force across the armature conductors.
+The greater the speed of rotation of the magnetic poles relatively
+to that of the armature the more rapidly the currents developed
+in the coils of the latter will follow one another, and the more
+rapidly the motor will revolve in response thereto, and this continues
+until the armature generator is stopped entirely, as by a
+brake, when the motor, if properly constructed, runs at the speed
+with which the magnetic poles of the generator rotate.</p>
+
+<p>The effective strength of the currents developed in the armature
+coils of the generator is dependent upon the strength of the
+currents energizing the generator and upon the number of rotations
+per unit of time of the magnetic poles of the generator;
+hence the speed of the motor armature will depend in all cases<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span>
+upon the relative speeds of the armature of the generator and of
+its magnetic poles. For example, if the poles are turned two
+thousand times per unit of time and the armature is turned eight
+hundred, the motor will turn twelve hundred times, or nearly so.
+Very slight differences of speed may be indicated by a delicately
+balanced motor.</p>
+
+<p>Let it now be assumed that power is applied to the generator
+armature to turn it in a direction opposite to that in which its
+magnetic poles rotate. In such case the result would be similar
+to that produced by a generator the armature and field magnets
+of which are rotated in opposite directions, and by reason of these
+conditions the motor armature will turn at a rate of speed equal
+to the sum of the speeds of the armature and magnetic poles of
+the generator, so that a comparatively low speed of the generator
+armature will produce a high speed in the motor.</p>
+
+<p>It will be observed in connection with this system that on
+diminishing the resistance of the external circuit of the generator
+armature by checking the speed of the motor or by adding
+translating devices in multiple arc in the secondary circuit or circuits
+of the transformer the strength of the current in the armature
+circuit is greatly increased. This is due to two causes: first,
+to the great differences in the speeds of the motor and generator,
+and, secondly, to the fact that the apparatus follows the analogy
+of a transformer, for, in proportion as the resistance of the armature
+or secondary circuits is reduced, the strength of the currents
+in the field or primary circuits of the generator is increased and
+the currents in the armature are augmented correspondingly.
+For similar reasons the currents in the armature-coils of the
+generator increase very rapidly when the speed of the armature
+is reduced when running in the same direction as the magnetic
+poles or conversely.</p>
+
+<p>It will be understood from the above description that the
+generator-armature may be run in the direction of the shifting of
+the magnetic poles, but more rapidly, and that in such case the
+speed of the motor will be equal to the difference between the
+two rates.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII.</h2>
+
+<h3><span class="smcap">Regulator for Rotary Current Motors.</span></h3>
+
+
+<p>An interesting device for regulating and reversing has been
+devised by Mr. Tesla for the purpose of varying the speed of
+polyphase motors. It consists of a form of converter or transformer
+with one element capable of movement with respect to
+the other, whereby the inductive relations may be altered, either
+manually or automatically, for the purpose of varying the
+strength of the induced current. Mr. Tesla prefers to construct
+this device in such manner that the induced or secondary element
+may be movable with respect to the other; and the invention,
+so far as relates merely to the construction of the device itself,
+consists, essentially, in the combination, with two opposite
+magnetic poles, of an armature wound with an insulated coil and
+mounted on a shaft, whereby it may be turned to the desired
+extent within the field produced by the poles. The normal position
+of the core of the secondary element is that in which it
+most completely closes the magnetic circuit between the poles
+of the primary element, and in this position its coil is in its
+most effective position for the inductive action upon it of the
+primary coils; but by turning the movable core to either side,
+the induced currents delivered by its coil become weaker until,
+by a movement of the said core and coil through 90&deg;, there will
+be no current delivered.</p>
+
+<p>Fig. 34 is a view in side elevation of the regulator. Fig. 35 is
+a broken section on line <i>x x</i> of Fig. 34. Fig. 36 is a diagram
+illustrating the most convenient manner of applying the regulator
+to ordinary forms of motors, and Fig. 37 is a similar diagram illustrating
+the application of the device to the Tesla alternating-current
+motors. The regulator may be constructed in many
+ways to secure the desired result; but that which is, perhaps, its
+best form is shown in Figs. 34 and 35.</p>
+
+<p><small>A</small> represents a frame of iron. <small>B B</small> are the cores of the induc<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span>ing
+or primary coils <small>C C</small>. <small>D</small> is a shaft mounted on the side bars,
+<small>D'</small>, and on which is secured a sectional iron core, <small>E</small>, wound with
+an induced or secondary coil, <small>F</small>, the convolutions of which are
+parallel with the axis of the shaft. The ends of the core are
+rounded off so as to fit closely in the space between the two poles
+and permit the core <small>E</small> to be turned to and held at any desired
+point. A handle, <small>G</small>, secured to the projecting end of the shaft
+<small>D</small>, is provided for this purpose.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 310px;">
+<img src="images/fig34.jpg" width="310" height="427" alt="Fig. 34." title="" />
+<span class="caption">Fig. 34.</span>
+</div>
+<div class="figright" style="width: 380px;">
+<img src="images/fig35.jpg" width="380" height="427" alt="Fig. 35." title="" />
+<span class="caption">Fig. 35.</span>
+</div>
+</div>
+
+<p>In Fig. 36 let <small>H</small> represent an ordinary alternating current generator,
+the field-magnets of which are excited by a suitable
+source of current, <small>I</small>. Let <small>J</small> designate an ordinary form of electromagnetic
+motor provided with an armature, <small>K</small>, commutator <small>L</small>,
+and field-magnets <small>M</small>. It is well known that such a motor, if its
+field-magnet cores be divided up into insulated sections, may be
+practically operated by an alternating current; but in using this
+regulator with such a motor, Mr. Tesla includes one element of
+the motor only&mdash;say the armature-coils&mdash;in the main circuit of
+the generator, making the connections through the brushes and
+the commutator in the usual way. He also includes one of the
+elements of the regulator&mdash;say the stationary coils&mdash;in the same
+circuit, and in the circuit with the secondary or movable coil of
+the regulator he connects up the field-coils of the motor. He
+also prefers to use flexible conductors to make the connections
+from the secondary coil of the regulator, as he thereby avoids
+the use of sliding contacts or rings without interfering with the
+requisite movement of the core <small>E</small>.<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<p>If the regulator be in its normal position, or that in which its
+magnetic circuit is most nearly closed, it delivers its maximum
+induced current, the phases of which so correspond with those of
+the primary current that the motor will run as though both field
+and armature were excited by the main current.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_061.jpg" width="640" height="229" alt="Fig. 36." title="" />
+<span class="caption">Fig. 36.</span>
+</div>
+
+<p>To vary the speed of the motor to any rate between the minimum
+and maximum rates, the core <small>E</small> and coils <small>F</small> are turned in
+either direction to an extent which produces the desired result,
+for in its normal position the convolutions of coil <small>F</small> embrace the
+maximum number of lines of force, all of which act with the
+same effect upon the coil; hence it will deliver its maximum
+current; but by turning the coil <small>F</small> out of its position of maximum
+effect the number of lines of force embraced by it is diminished.
+The inductive effect is therefore impaired, and the current delivered
+by coil <small>F</small> will continue to diminish in proportion to the
+angle at which the coil <small>F</small> is turned until, after passing through
+an angle of ninety degrees, the convolutions of the coil will be
+at right angles to those of coils <small>C C</small>, and the inductive effect reduced
+to a minimum.</p>
+
+<p>Incidentally to certain constructions, other causes may influence
+the variation in the strength of the induced currents. For
+example, in the present case it will be observed that by the first
+movement of coil <small>F</small> a certain portion of its convolutions are carried
+beyond the line of the direct influence of the lines of force, and
+that the magnetic path or circuit for the lines is impaired; hence
+the inductive effect would be reduced. Next, that after moving
+through a certain angle, which is obviously determined by the
+relative dimensions of the bobbin or coil F, diagonally opposite
+portions of the coil will be simultaneously included in the field,
+but in such positions that the lines which produce a current-impulse
+in one portion of the coil in a certain direction will pro<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span>duce
+in the diagonally opposite portion a corresponding impulse
+in the opposite direction; hence portions of the current will
+neutralize one another.</p>
+
+<p>As before stated, the mechanical construction of the device
+may be greatly varied; but the essential conditions of the principle
+will be fulfilled in any apparatus in which the movement of
+the elements with respect to one another effects the same results
+by varying the inductive relations of the two elements in a manner
+similar to that described.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_062.jpg" width="640" height="462" alt="Fig. 37." title="" />
+<span class="caption">Fig. 37.</span>
+</div>
+
+
+<p>It may also be stated that the core <small>E</small> is not indispensable to the
+operation of the regulator; but its presence is obviously beneficial.
+This regulator, however, has another valuable property
+in its capability of reversing the motor, for if the coil <small>F</small> be turned
+through a half-revolution, the position of its convolutions relatively
+to the two coils <small>C C</small> and to the lines of force is reversed, and
+consequently the phases of the current will be reversed. This
+will produce a rotation of the motor in an opposite direction.
+This form of regulator is also applied with great advantage to
+Mr. Tesla's system of utilizing alternating currents, in which the
+magnetic poles of the field of a motor are progressively shifted
+by means of the combined effects upon the field of magnetizing
+coils included in independent circuits, through which pass alternating
+currents in proper order and relations to each other.</p>
+
+<p>In Fig. 37, let <small>P</small> represent a Tesla generator having two independent
+coils, <small>P'</small> and <small>P''</small>, on the armature, and <small>T</small> a diagram of a<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+motor having two independent energizing coils or sets of coils,
+<small>R R'</small>. One of the circuits from the generator, as <small>S' S'</small>, includes
+one set, <small>R' R'</small>, of the energizing coils of the motor, while the
+other circuit, as <small>S S</small>, includes the primary coils of the regulator.
+The secondary coil of the regulator includes the other coils, <small>R R</small>,
+of the motor.</p>
+
+<p>While the secondary coil of the regulator is in its normal position,
+it produces its maximum current, and the maximum rotary
+effect is imparted to the motor; but this effect will be diminished
+in proportion to the angle at which the coil <small>F</small> of the regulator is
+turned. The motor will also be reversed by reversing the position
+of the coil with reference to the coils <small>C C</small>, and thereby reversing
+the phases of the current produced by the generator. This
+changes the direction of the movement of the shifting poles which
+the armature follows.</p>
+
+<p>One of the main advantages of this plan of regulation is its
+economy of power. When the induced coil is generating its
+maximum current, the maximum amount of energy in the primary
+coils is absorbed; but as the induced coil is turned from its
+normal position the self-induction of the primary-coils reduces
+the expenditure of energy and saves power.</p>
+
+<p>It is obvious that in practice either coils <small>C C</small> or coil <small>F</small> may be
+used as primary or secondary, and it is well understood that their
+relative proportions may be varied to produce any desired difference
+or similarity in the inducing and induced currents.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII.</h2>
+
+<h3><span class="smcap">Single Circuit, Self-Starting Synchronizing Motors.</span></h3>
+
+
+<p>In the first chapters of this section we have, bearing in mind
+the broad underlying principle, considered a distinct class of motors,
+namely, such as require for their operation a special generator
+capable of yielding currents of differing phase. As a matter
+of course, Mr. Tesla recognizing the desirability of utilizing his
+motors in connection with ordinary systems of distribution, addressed
+himself to the task of inventing various methods and
+ways of achieving this object. In the succeeding chapters,
+therefore, we witness the evolution of a number of ideas bearing
+upon this important branch of work. It must be obvious to
+a careful reader, from a number of hints encountered here and
+there, that even the inventions described in these chapters to follow
+do not represent the full scope of the work done in these
+lines. They might, indeed, be regarded as exemplifications.</p>
+
+<p>We will present these various inventions in the order which
+to us appears the most helpful to an understanding of the subject
+by the majority of readers. It will be naturally perceived that
+in offering a series of ideas of this nature, wherein some of the
+steps or links are missing, the descriptions are not altogether sequential;
+but any one who follows carefully the main drift of
+the thoughts now brought together will find that a satisfactory
+comprehension of the principles can be gained.</p>
+
+<p>As is well known, certain forms of alternating-current machines
+have the property, when connected in circuit with an alternating
+current generator, of running as a motor in synchronism therewith;
+but, while the alternating current will run the motor after
+it has attained a rate of speed synchronous with that of the generator,
+it will not start it. Hence, in all instances heretofore
+where these "synchronizing motors," as they are termed, have
+been run, some means have been adopted to bring the motors up
+to synchronism with the generator, or approximately so, before
+the alternating current of the generator is applied to drive them.<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span>
+In some instances mechanical appliances have been utilized for
+this purpose. In others special and complicated forms of motor
+have been constructed. Mr. Tesla has discovered a much more
+simple method or plan of operating synchronizing motors, which
+requires practically no other apparatus than the motor itself. In
+other words, by a certain change in the circuit connections of the
+motor he converts it at will from a double circuit motor, or such
+as have been already described, and which will start under the
+action of an alternating current, into a synchronizing motor, or
+one which will be run by the generator only when it has reached
+a certain speed of rotation synchronous with that of the generator.
+In this manner he is enabled to extend very greatly the applications
+of his system and to secure all the advantages of both
+forms of alternating current motor.</p>
+
+<p>The expression "synchronous with that of the generator," is
+used here in its ordinary acceptation&mdash;that is to say, a motor is
+said to synchronize with the generator when it preserves a certain
+relative speed determined by its number of poles and the number
+of alternations produced per revolution of the generator. Its
+actual speed, therefore, may be faster or slower than that of the
+generator; but it is said to be synchronous so long as it preserves
+the same relative speed.</p>
+
+<p>In carrying out this invention Mr. Tesla constructs a motor
+which has a strong tendency to synchronism with the generator.
+The construction preferred is that in which the armature is provided
+with polar projections. The field-magnets are wound with
+two sets of coils, the terminals of which are connected to a switch
+mechanism, by means of which the line-current may be carried
+directly through these coils or indirectly through paths by
+which its phases are modified. To start such a motor, the switch
+is turned on to a set of contacts which includes in one motor
+circuit a dead resistance, in the other an inductive resistance, and,
+the two circuits being in derivation, it is obvious that the difference
+in phase of the current in such circuits will set up a rotation
+of the motor. When the speed of the motor has thus been
+brought to the desired rate the switch is shifted to throw the
+main current directly through the motor-circuits, and although
+the currents in both circuits will now be of the same phase the
+motor will continue to revolve, becoming a true synchronous
+motor. To secure greater efficiency, the armature or its polar
+projections are wound with coils closed on themselves.<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<p>In the accompanying diagrams, Fig. 38 illustrates the details
+of the plan above set forth, and Figs. 39 and 40 modifications
+of the same.</p>
+
+<div class="figcenter" style="width: 466px;">
+<img src="images/oi_066.jpg" width="466" height="640" alt="Fig. 38, 39 and 40." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 38, 39 and 40.</span>
+</div>
+
+<p>Referring to Fig. 38, let <small>A</small> designate the field-magnets of a
+motor, the polar projections of which are wound with coils <small>B C</small>
+included in independent circuits, and <small>D</small> the armature with polar
+projections wound with coils <small>E</small> closed upon themselves, the
+motor in these respects being similar in construction to those<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span>
+described already, but having on account of the polar projections
+on the armature core, or other similar and well-known features,
+the properties of a synchronizing-motor. <small>L L'</small> represents the
+conductors of a line from an alternating current generator <small>G</small>.</p>
+
+<p>Near the motor is placed a switch the action of which is that
+of the one shown in the diagrams, which is constructed as follows:
+<small>F F'</small> are two conducting plates or arms, pivoted at their
+ends and connected by an insulating cross-bar, <small>H</small>, so as to be
+shifted in parallelism. In the path of the bars <small>F F'</small> is the contact
+2, which forms one terminal of the circuit through coils <small>C</small>, and
+the contact 4, which is one terminal of the circuit through coils
+<small>B</small>. The opposite end of the wire of coils <small>C</small> is connected to the
+wire <small>L</small> or bar <small>F'</small>, and the corresponding end of coils <small>B</small> is connected
+to wire <small>L'</small> and bar <small>F</small>; hence if the bars be shifted so as to bear on
+contacts 2 and 4 both sets of coils <small>B C</small> will be included in the circuit
+<small>L L'</small> in multiple arc or derivation. In the path of the levers
+<small>F F'</small> are two other contact terminals, 1 and 3. The contact 1 is
+connected to contact 2 through an artificial resistance, <small>I</small>, and contact
+3 with contact 4 through a self-induction coil, <small>J</small>, so that when
+the switch levers are shifted upon the points 1 and 3 the circuits
+of coils <small>B</small> and <small>C</small> will be connected in multiple arc or derivation to
+the circuit <small>L L'</small>, and will include the resistance and self-induction
+coil respectively. A third position of the switch is that in which
+the levers <small>F</small> and <small>F'</small> are shifted out of contact with both sets of
+points. In this case the motor is entirely out of circuit.</p>
+
+<p>The purpose and manner of operating the motor by these devices
+are as follows: The normal position of the switch, the
+motor being out of circuit, is off the contact points. Assuming
+the generator to be running, and that it is desired to start the
+motor, the switch is shifted until its levers rest upon points 1 and
+3. The two motor-circuits are thus connected with the generator
+circuit; but by reason of the presence of the resistance <small>I</small> in one
+and the self-induction coil <small>J</small> in the other the coincidence of the
+phases of the current is disturbed sufficiently to produce a progression
+of the poles, which starts the motor in rotation. When
+the speed of the motor has run up to synchronism with the
+generator, or approximately so, the switch is shifted over upon
+the points 2 and 4, thus cutting out the coils <small>I</small> and <small>J</small>, so that the
+currents in both circuits have the same phase; but the motor
+now runs as a synchronous motor.</p>
+
+<p>It will be understood that when brought up to speed the mo<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span>tor
+will run with only one of the circuits <small>B</small> or <small>C</small> connected with
+the main or generator circuit, or the two circuits may be connected
+in series. This latter plan is preferable when a current
+having a high number of alternations per unit of time is employed
+to drive the motor. In such case the starting of the
+motor is more difficult, and the dead and inductive resistances
+must take up a considerable proportion of the electromotive
+force of the circuits. Generally the conditions are so adjusted
+that the electromotive force used in each of the motor circuits is
+that which is required to operate the motor when its circuits are
+in series. The plan followed in this case is illustrated in Fig.
+39. In this instance the motor has twelve poles and the armature
+has polar projections <small>D</small> wound with closed coils <small>E</small>. The
+switch used is of substantially the same construction as that
+shown in the previous figure. There are, however, five contacts,
+designated as 5, 6, 7, 8, and 9. The motor-circuits <small>B C</small>, which include
+alternate field-coils, are connected to the terminals in the
+following order: One end of circuit <small>C</small> is connected to contact 9
+and to contact 5 through a dead resistance, <small>I</small>. One terminal of
+circuit <small>B</small> is connected to contact 7 and to contact 6 through a
+self-induction coil, <small>J</small>. The opposite terminals of both circuits are
+connected to contact 8.</p>
+
+<p>One of the levers, as <small>F</small>, of the switch is made with an extension,
+<i>f</i>, or otherwise, so as to cover both contacts 5 and 6 when
+shifted into the position to start the motor. It will be observed
+that when in this position and with lever <small>F'</small> on contact 8 the current
+divides between the two circuits <small>B C</small>, which from their difference
+in electrical character produce a progression of the poles
+that starts the motor in rotation. When the motor has attained
+the proper speed, the switch is shifted so that the levers cover
+the contacts 7 and 9, thereby connecting circuits <small>B</small> and <small>C</small> in series.
+It is found that by this disposition the motor is maintained
+in rotation in synchronism with the generator. This principle
+of operation, which consists in converting by a change of connections
+or otherwise a double-circuit motor, or one operating by
+a progressive shifting of the poles, into an ordinary synchronizing
+motor may be carried out in many other ways. For instance,
+instead of using the switch shown in the previous figures, we
+may use a temporary ground circuit between the generator and
+motor, in order to start the motor, in substantially the manner
+indicated in Fig. 40. Let <small>G</small> in this figure represent an ordinary<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span>
+alternating-current generator with, say, two poles, <small>M M'</small>, and an
+armature wound with two coils, <small>N N'</small>, at right angles and connected
+in series. The motor has, for example, four poles wound
+with coils <small>B C</small>, which are connected in series, and an armature
+with polar projections <small>D</small> wound with closed coils <small>E E</small>. From the
+common joint or union between the two circuits of both the generator
+and the motor an earth connection is established, while
+the terminals or ends of these circuits are connected to the
+line. Assuming that the motor is a synchronizing motor or one
+that has the capability of running in synchronism with the generator,
+but not of starting, it may be started by the above-described
+apparatus by closing the ground connection from both
+generator and motor. The system thus becomes one with a two-circuit
+generator and motor, the ground forming a common return
+for the currents in the two circuits <small>L</small> and <small>L'</small>. When by
+this arrangement of circuits the motor is brought to speed, the
+ground connection is broken between the motor or generator, or
+both, ground-switches <small>P P'</small> being employed for this purpose.
+The motor then runs as a synchronizing motor.</p>
+
+<p>In describing the main features which constitute this invention
+illustrations have necessarily been omitted of the appliances used
+in conjunction with the electrical devices of similar systems&mdash;such,
+for instance, as driving-belts, fixed and loose pulleys for the
+motor, and the like; but these are matters well understood.</p>
+
+<p>Mr. Tesla believes he is the first to operate electro-magnetic
+motors by alternating currents in any of the ways herein described&mdash;that
+is to say, by producing a progressive movement or rotation
+of their poles or points of greatest magnetic attraction by
+the alternating currents until they have reached a given speed,
+and then by the same currents producing a simple alternation of
+their poles, or, in other words, by a change in the order or character
+of the circuit connections to convert a motor operating on
+one principle to one operating on another.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX.</h2>
+
+<h3><span class="smcap">Change From Double Current to Single Current Motor.</span></h3>
+
+
+<p>A description is given elsewhere of a method of operating alternating
+current motors by first rotating their magnetic poles
+until they have attained synchronous speed, and then alternating
+the poles. The motor is thus transformed, by a simple change
+of circuit connections from one operated by the action of two or
+more independent energizing currents to one operated either by
+a single current or by several currents acting as one. Another
+way of doing this will now be described.</p>
+
+<p>At the start the magnetic poles of one element or field of the
+motor are progressively shifted by alternating currents differing
+in phase and passed through independent energizing circuits, and
+short circuit the coils of the other element. When the motor
+thus started reaches or passes the limit of speed synchronous with
+the generator, Mr. Tesla connects up the coils previously short-circuited
+with a source of direct current and by a change of the circuit
+connections produces a simple alternation of the poles. The
+motor then continues to run in synchronism with the generator.
+The motor here shown in Fig. 41 is one of the ordinary forms, with
+field-cores either laminated or solid and with a cylindrical laminated
+armature wound, for example, with the coils <small>A B</small> at right angles.
+The shaft of the armature carries three collecting or contact rings
+<small>C D E</small>. (Shown, for better illustration, as of different diameters.)</p>
+
+<p>One end of coil <small>A</small> connects to one ring, as <small>C</small>, and one end of
+coil <small>B</small> connects with ring <small>D</small>. The remaining ends are connected
+to ring <small>E</small>. Collecting springs or brushes <small>F G H</small> bear upon the
+rings and lead to the contacts of a switch, to be presently described.
+The field-coils have their terminals in binding-posts <small>K
+K</small>, and may be either closed upon themselves or connected with
+a source of direct current <small>L</small>, by means of a switch <small>M</small>. The main
+or controlling switch has five contacts <i>a b c d e</i> and two levers <i>f
+g</i>, pivoted and connected by an insulating cross-bar <i>h</i>, so as to
+move in parallelism. These levers are connected to the line<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span>
+wires from a source of alternating currents <small>N</small>. Contact <i>a</i> is connected
+to brush <small>G</small> and coil <small>B</small> through a dead resistance <small>R</small> and
+wire <small>P</small>. Contact <i>b</i> is connected with brush <small>F</small> and coil <small>A</small> through
+a self-induction coil <small>S</small> and wire <small>O</small>. Contacts <i>c</i> and <i>e</i> are connected
+to brushes <small>G F</small>, respectively, through the wires <small>P O</small>, and contact
+<i>d</i> is directly connected with brush <small>H</small>. The lever <i>f</i> has a widened
+end, which may span the contacts <i>a b</i>. When in such position
+and with lever <i>g</i> on contact <i>d</i>, the alternating currents divide between
+the two motor-coils, and by reason of their different self-induction
+a difference of current-phase is obtained that starts the
+motor in rotation. In starting, the field-coils are short circuited.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_071.jpg" width="600" height="625" alt="Fig. 41." title="" />
+<span class="caption">Fig. 41.</span>
+</div>
+
+
+<p>When the motor has attained the desired speed, the switch is
+shifted to the position shown in dotted lines&mdash;that is to say, with
+the levers <i>f g</i> resting on points <i>c e</i>. This connects up the two
+armature coils in series, and the motor will then run as a synchronous
+motor. The field-coils are thrown into circuit with the
+direct current source when the main switch is shifted.</p>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span></p>
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X.</h2>
+
+<h3><span class="smcap">Motor With "Current Lag" Artificially Secured.</span></h3>
+
+
+<p>One of the general ways followed by Mr. Tesla in developing
+his rotary phase motors is to produce practically independent
+currents differing primarily in phase and to pass these through the
+motor-circuits. Another way is to produce a single alternating
+current, to divide it between the motor-circuits, and to effect
+artificially a lag in one of these circuits or branches, as by
+giving to the circuits different self-inductive capacity, and in
+other ways. In the former case, in which the necessary difference
+of phase is primarily effected in the generation of currents,
+in some instances, the currents are passed through the energizing
+coils of both elements of the motor&mdash;the field and armature; but
+a further result or modification may be obtained by doing this
+under the conditions hereinafter specified in the case of motors
+in which the lag, as above stated, is artificially secured.</p>
+
+<p>Figs. 42 to 47, inclusive, are diagrams of different ways in which
+the invention is carried out; and Fig. 48, a side view of a form
+of motor used by Mr. Tesla for this purpose.</p>
+
+<div class="figcenter" style="width: 431px;">
+<img src="images/oi_073.jpg" width="431" height="640" alt="Figs. 42, 43 and 44." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 42, 43 and 44.</span>
+</div>
+
+<p><small>A B</small> in Fig. 42 indicate the two energizing circuits of a motor,
+and <small>C D</small> two circuits on the armature. Circuit or coil <small>A</small> is connected
+in series with circuit or coil <small>C</small>, and the two circuits <small>B D</small> are
+similarly connected. Between coils <small>A</small> and <small>C</small> is a contact-ring <i>e</i>,
+forming one terminal of the latter, and a brush <i>a</i>, forming one
+terminal of the former. A ring <i>d</i> and brush <i>c</i> similarly connect
+coils <small>B</small> and <small>D</small>. The opposite terminals of the field-coils connect
+to one binding post <i>h</i> of the motor, and those of the armature
+coils are similarly connected to the opposite binding post <i>i</i> through
+a contact-ring <i>f</i> and brush <i>g</i>. Thus each motor-circuit while in
+derivation to the other includes one armature and one field coil.
+These circuits are of different self-induction, and may be made
+so in various ways. For the sake of clearness, an artificial resistance
+<small>R</small> is shown in one of these circuits, and in the other a
+self-induction coil <small>S</small>. When an alternating current is passed
+<span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span>through this motor it divides between its two energizing-circuits.
+The higher self-induction of one circuit produces a greater retardation
+or lag in the current therein than in the other. The
+difference of phase between the two currents effects the rotation
+or shifting of the points of maximum magnetic effect that secures
+the rotation of the armature. In certain respects this plan of including
+both armature and field coils in circuit is a marked improvement.
+Such a motor has a good torque at starting; yet it
+has also considerable tendency to synchronism, owing to the fact<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span>
+that when properly constructed the maximum magnetic effects in
+both armature and field coincide&mdash;a condition which in the usual
+construction of these motors with closed armature coils is not
+readily attained. The motor thus constructed exhibits too, a
+better regulation of current from no load to load, and there is
+less difference between the apparent and real energy expended
+in running it. The true synchronous speed of this form of motor
+is that of the generator when both are alike&mdash;that is to say, if
+the number of the coils on the armature and on the field is <i>x</i>, the
+motor will run normally at the same speed as a generator driving
+it if the number of field magnets or poles of the same be also <i>x</i>.</p>
+
+<div class="figcenter" style="width: 655px;">
+<img src="images/oi_074.jpg" width="655" height="600" alt="Figs. 45, 46 and 47." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 45, 46 and 47.</span>
+</div>
+
+<p>Fig. 43 shows a somewhat modified arrangement of circuits.
+There is in this case but one armature coil <small>E</small>, the winding of
+which maintains effects corresponding to the resultant poles produced
+by the two field-circuits.</p>
+
+<p>Fig. 44 represents a disposition in which both armature and
+field are wound with two sets of coils, all in multiple arc to the
+line or main circuit. The armature coils are wound to correspond
+with the field-coils with respect to their self-induction. A
+modification of this plan is shown in Fig. 45&mdash;that is to say, the<span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span>
+two field coils and two armature coils are in derivation to themselves
+and in series with one another. The armature coils in
+this case, as in the previous figure, are wound for different self-induction
+to correspond with the field coils.</p>
+
+<p>Another modification is shown in Fig. 46. In this case only
+one armature-coil, as <small>D</small>, is included in the line-circuit, while the
+other, as <small>C</small>, is short-circuited.</p>
+
+<p>In such a disposition as that shown in Fig. 43, or where only
+one armature-coil is employed, the torque on the start is somewhat
+reduced, while the tendency to synchronism is somewhat
+increased. In such a disposition as shown in Fig. 46, the opposite
+conditions would exist. In both instances, however, there
+is the advantage of dispensing with one contact-ring.</p>
+
+
+<div class="figcenter" style="width: 585px;">
+<img src="images/oi_075.jpg" width="585" height="480" alt="Fig. 48." title="" />
+<span class="caption">Fig. 48.</span>
+</div>
+
+
+<p>In Fig. 46 the two field-coils and the armature-coil <small>D</small> are in
+multiple arc. In Fig. 47 this disposition is modified, coil <small>D</small> being
+shown in series with the two field-coils.</p>
+
+<p>Fig. 48 is an outline of the general form of motor in which
+this invention is embodied. The circuit connections between
+the armature and field coils are made, as indicated in the previous
+figures, through brushes and rings, which are not shown.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span></p>
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI.</h2>
+
+<h3><span class="smcap">Another Method of Transformation from a Torque to a
+Synchronizing Motor.</span></h3>
+
+
+<p>In a preceding chapter we have described a method by which
+Mr. Tesla accomplishes the change in his type of rotating field
+motor from a torque to a synchronizing motor. As will be observed,
+the desired end is there reached by a change in the circuit
+connections at the proper moment. We will now proceed
+to describe another way of bringing about the same result. The
+principle involved in this method is as follows:&mdash;</p>
+
+<p>If an alternating current be passed through the field coils only
+of a motor having two energizing circuits of different self-induction
+and the armature coils be short-circuited, the motor will have
+a strong torque, but little or no tendency to synchronism with
+the generator; but if the same current which energizes the field
+be passed also through the armature coils the tendency to remain
+in synchronism is very considerably increased. This is due to
+the fact that the maximum magnetic effects produced in the field
+and armature more nearly coincide. On this principle Mr.
+Tesla constructs a motor having independent field circuits of
+different self-induction, which are joined in derivation to a
+source of alternating currents. The armature is wound with one
+or more coils, which are connected with the field coils through
+contact rings and brushes, and around the armature coils a shunt
+is arranged with means for opening or closing the same. In starting
+this motor the shunt is closed around the armature coils,
+which will therefore be in closed circuit. When the current is
+directed through the motor, it divides between the two circuits,
+(it is not necessary to consider any case where there are more
+than two circuits used), which, by reason of their different self-induction,
+secure a difference of phase between the two currents
+in the two branches, that produces a shifting or rotation of the
+poles. By the alternations of current, other currents are
+induced in the closed&mdash;or short-circuited&mdash;armature coils and the<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span>
+motor has a strong torque. When the desired speed is reached,
+the shunt around the armature-coils is opened and the current
+directed through both armature and field coils. Under these
+conditions the motor has a strong tendency to synchronism.</p>
+
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_077.jpg" width="600" height="667" alt="Figs. 49, 50 and 51." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 49, 50 and 51.</span>
+</div>
+
+<p>In Fig. 49, <small>A</small> and <small>B</small> designate the field coils of the motor. As
+the circuits including these coils are of different self-induction,
+this is represented by a resistance coil <small>R</small> in circuit with <small>A</small>, and a
+self-induction coil <small>S</small> in circuit with <small>B</small>. The same result may of
+course be secured by the winding of the coils. <small>C</small> is the armature
+circuit, the terminals of which are rings <i>a b</i>. Brushes <i>c d</i> bear
+on these rings and connect with the line and field circuits. <small>D</small> is
+the shunt or short circuit around the armature. <small>E</small> is the switch
+in the shunt.</p>
+
+<p>It will be observed that in such a disposition as is illustrated in<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span>
+Fig. 49, the field circuits <small>A</small> and <small>B</small> being of different self-induction,
+there will always be a greater lag of the current in one than the
+other, and that, generally, the armature phases will not correspond
+with either, but with the resultant of both. It is therefore
+important to observe the proper rule in winding the armature.
+For instance, if the motor have eight poles&mdash;four in each circuit&mdash;there
+will be four resultant poles, and hence the armature
+winding should be such as to produce four poles, in order to constitute
+a true synchronizing motor.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_078.jpg" width="640" height="307" alt="Fig. 52." title="" />
+<span class="caption">Fig. 52.</span>
+</div>
+
+
+<p>The diagram, Fig. 50, differs from the previous one only in
+respect to the order of connections. In the present case the armature-coil,
+instead of being in series with the field-coils, is in multiple
+arc therewith. The armature-winding may be similar to
+that of the field&mdash;that is to say, the armature may have two or
+more coils wound or adapted for different self-induction and
+adapted, preferably, to produce the same difference of
+phase as the field-coils. On starting the motor the shunt
+is closed around both coils. This is shown in Fig. 51, in
+which the armature coils are <small>F G</small>. To indicate their different
+electrical character, there are shown in circuit with them, respectively,
+the resistance <small>R'</small> and the self-induction coil <small>S'</small>. The two
+armature coils are in series with the field-coils and the same disposition
+of the shunt or short-circuit <small>D</small> is used. It is of advantage
+in the operation of motors of this kind to construct or wind
+the armature in such manner that when short-circuited on the
+start it will have a tendency to reach a higher speed than that
+which synchronizes with the generator. For example, a given
+motor having, say, eight poles should run, with the armature coil
+short-circuited, at two thousand revolutions per minute to bring
+it up to synchronism. It will generally happen, however, that<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span>
+this speed is not reached, owing to the fact that the armature
+and field currents do not properly correspond, so that when the
+current is passed through the armature (the motor not being
+quite up to synchronism) there is a liability that it will not "hold
+on," as it is termed. It is preferable, therefore, to so wind or
+construct the motor that on the start, when the armature coils
+are short-circuited, the motor will tend to reach a speed higher
+than the synchronous&mdash;as for instance, double the latter. In
+such case the difficulty above alluded to is not felt, for the motor
+will always hold up to synchronism if the synchronous speed&mdash;in
+the case supposed of two thousand revolutions&mdash;is reached or
+passed. This may be accomplished in various ways; but for all
+practical purposes the following will suffice: On the armature
+are wound two sets of coils. At the start only one of these is
+short-circuited, thereby producing a number of poles on the armature,
+which will tend to run the speed up above the synchronous
+limit. When such limit is reached or passed, the current is
+directed through the other coil, which, by increasing the number
+of armature poles, tends to maintain synchronism.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_079.jpg" width="640" height="332" alt="Fig. 53." title="" />
+<span class="caption">Fig. 53.</span>
+</div>
+
+<p>In Fig. 52, such a disposition is shown. The motor having,
+say, eight poles contains two field-circuits <small>A</small> and <small>B</small>, of different
+self-induction. The armature has two coils <small>F</small> and <small>G</small>. The former
+is closed upon itself, the latter connected with the field and line
+through contact-rings <i>a b</i>, brushes <i>c d</i>, and a switch <small>E</small>. On the
+start the coil <small>F</small> alone is active and the motor tends to run at a
+speed above the synchronous; but when the coil <small>G</small> is connected
+to the circuit the number of armature poles is increased, while
+the motor is made a true synchronous motor. This disposition<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span>
+has the advantage that the closed armature-circuit imparts to the
+motor torque when the speed falls off, but at the same time the
+conditions are such that the motor comes out of synchronism
+more readily. To increase the tendency to synchronism, two
+circuits may be used on the armature, one of which is short-circuited
+on the start and both connected with the external circuit
+after the synchronous speed is reached or passed. This disposition
+is shown in Fig. 53. There are three contact-rings <i>a b e</i>
+and three brushes <i>c d f</i>, which connect the armature circuits
+with the external circuit. On starting, the switch <small>H</small> is turned to
+complete the connection between one binding-post <small>P</small> and the field-coils.
+This short-circuits one of the armature-coils, as <small>G</small>. The
+other coil <small>F</small> is out of circuit and open. When the motor is up
+to speed, the switch <small>H</small> is turned back, so that the connection
+from binding-post <small>P</small> to the field coils is through the coil <small>G</small>, and
+switch <small>K</small> is closed, thereby including coil <small>F</small> in multiple arc with
+the field coils. Both armature coils are thus active.</p>
+
+<p>From the above-described instances it is evident that many
+other dispositions for carrying out the invention are possible.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII.</h2>
+
+<h3><span class="smcap">"Magnetic Lag" Motor.</span></h3>
+
+
+<p>The following description deals with another form of motor,
+namely, depending on "magnetic lag" or hysteresis, its peculiarity
+being that in it the attractive effects or phases while lagging
+behind the phases of current which produce them, are manifested
+simultaneously and not successively. The phenomenon
+utilized thus at an early stage by Mr. Tesla, was not generally
+believed in by scientific men, and Prof. Ayrton was probably
+first to advocate it or to elucidate the reason of its supposed existence.</p>
+
+<p>Fig. 54 is a side view of the motor, in elevation. Fig. 55 is
+a part-sectional view at right angles to Fig. 54. Fig. 56 is an
+end view in elevation and part section of a modification, and
+Fig. 57 is a similar view of another modification.</p>
+
+<p>In Figs. 54 and 55, <small>A</small> designates a base or stand, and B B
+the supporting-frame of the motor. Bolted to the supporting-frame
+are two magnetic cores or pole-pieces <small>C C'</small>, of iron or
+soft steel. These may be subdivided or laminated, in which
+case hard iron or steel plates or bars should be used, or they
+should be wound with closed coils. <small>D</small> is a circular disc armature,
+built up of sections or plates of iron and mounted in the
+frame between the pole-pieces <small>C C'</small>, curved to conform to the
+circular shape thereof. This disc may be wound with a number
+of closed coils <small>E</small>. <small>F F</small> are the main energizing coils, supported
+by the supporting-frame, so as to include within their magnetizing
+influence both the pole-pieces <small>C C'</small> and the armature <small>D</small>.
+The pole-pieces <small>C C'</small> project out beyond the coils <small>F F</small> on opposite
+sides, as indicated in the drawings. If an alternating
+current be passed through the coils <small>F F</small>, rotation of the armature
+will be produced, and this rotation is explained by the
+following apparent action, or mode of operation: An impulse
+of current in the coils <small>F F</small> establishes two polarities in the motor.
+The protruding end of pole-piece <small>C</small>, for instance, will be<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span>
+of one sign, and the corresponding end of pole-piece <small>C'</small> will be
+of the opposite sign. The armature also exhibits two poles at
+right angles to the coils <small>F F</small>, like poles to those in the pole-pieces
+being on the same side of the coils. While the current
+is flowing there is no appreciable tendency to rotation developed;
+but after each current impulse ceases or begins to fall,
+the magnetism in the armature and in the ends of the pole-pieces
+<small>C C'</small> lags or continues to manifest itself, which produces a
+rotation of the armature by the repellent force between the
+more closely approximating points of maximum magnetic effect.
+This effect is continued by the reversal of current, the polarities
+of field and armature being simply reversed. One or both
+of the elements&mdash;the armature or field&mdash;may be wound with
+closed induced coils to intensify this effect. Although in the
+illustrations but one of the fields is shown, each element of the
+motor really constitutes a field, wound with the closed coils,
+the currents being induced mainly in those convolutions or coils
+which are parallel to the coils <small>F F</small>.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_082.jpg" width="640" height="400" alt="Fig. 54, 55." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 54.</td><td class="caption">Fig. 55.</td></tr>
+</table>
+</div>
+
+<p>A modified form of this motor is shown in Fig. 56. In this
+form <small>G</small> is one of two standards that support the bearings for
+the armature-shaft. <small>H H</small> are uprights or sides of a frame, preferably
+magnetic, the ends <small>C C'</small> of which are bent in the manner
+indicated, to conform to the shape of the armature <small>D</small> and form
+field-magnet poles. The construction of the armature may be
+the same as in the previous figure, or it may be simply a magnetic
+disc or cylinder, as shown, and a coil or coils <small>F F</small> are se<span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span>cured
+in position to surround both the armature and the poles
+<small>C C'</small>. The armature is detachable from its shaft, the latter being
+passed through the armature after it has been inserted in position.
+The operation of this form of motor is the same in principle
+as that previously described and needs no further explanation.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 350px;">
+<img src="images/fig56.jpg" width="350" height="336" alt="Fig. 56." title="" />
+<span class="caption">Fig. 56.</span>
+</div>
+<div class="figright" style="width: 360px;">
+<img src="images/oi_083.jpg" width="360" height="360" alt="Fig. 57." title="" />
+<span class="caption">Fig. 57.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>One of the most important features in alternating current
+motors is, however, that they should be adapted to and capable
+of running efficiently on the alternating circuits in present use,
+in which almost without exception the generators yield a very
+high number of alternations. Such a motor, of the type under
+consideration, Mr. Tesla has designed by a development of the
+principle of the motor shown in Fig. 56, making a multipolar
+motor, which is illustrated in Fig. 57. In the construction of
+this motor he employs an annular magnetic frame <small>J</small>, with inwardly-extending
+ribs or projections <small>K</small>, the ends of which all
+bend or turn in one direction and are generally shaped to conform
+to the curved surface of the armature. Coils <small>F F</small> are wound
+from one part <small>K</small> to the one next adjacent, the ends or loops of
+each coil or group of wires being carried over toward the shaft,
+so as to form <big><b>U</b></big>-shaped groups of convolutions at each end of the
+armature. The pole-pieces <small>C C'</small>, being substantially concentric
+with the armature, form ledges, along which the coils are laid
+and should project to some extent beyond the the coils, as shown.
+The cylindrical or drum armature <small>D</small> is of the same construction
+as in the other motors described, and is mounted to rotate within
+the annular frame J and between the <big><b>U</b></big>-shaped ends or bends of<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span>
+the coils <small>F</small>. The coils <small>F</small> are connected in multiple or in series
+with a source of alternating currents, and are so wound that
+with a current or current impulse of given direction they will
+make the alternate pole-pieces <small>C</small> of one polarity and the other
+pole-pieces <small>C'</small> of the opposite polarity. The principle of the
+operation of this motor is the same as the other above described,
+for, considering any two pole-pieces <small>C C'</small>, a current
+impulse passing in the coil which bridges them or is wound
+over both tends to establish polarities in their ends of opposite
+sign and to set up in the armature core between them a polarity
+of the same sign as that of the nearest pole-piece <small>C</small>. Upon the
+fall or cessation of the current impulse that established these
+polarities the magnetism which lags behind the current phase,
+and which continues to manifest itself in the polar projections
+<small>C C'</small> and the armature, produces by repulsion a rotation of the
+armature. The effect is continued by each reversal of the current.
+What occurs in the case of one pair of pole-pieces occurs
+simultaneously in all, so that the tendency to rotation of the
+armature is measured by the sum of all the forces exerted by the
+pole-pieces, as above described. In this motor also the magnetic
+lag or effect is intensified by winding one or both cores
+with closed induced coils. The armature core is shown as thus
+wound. When closed coils are used, the cores should be laminated.</p>
+
+<p>It is evident that a pulsatory as well as an alternating current
+might be used to drive or operate the motors above described.</p>
+
+<p>It will be understood that the degree of subdivision, the mass
+of the iron in the cores, their size and the number of alternations
+in the current employed to run the motor, must be taken into
+consideration in order to properly construct this motor. In other
+words, in all such motors the proper relations between the number
+of alternations and the mass, size, or quality of the iron must
+be preserved in order to secure the best results.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span></p>
+<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII.</h2>
+
+<h3><span class="smcap">Method of Obtaining Difference of Phase by Magnetic
+Shielding.</span></h3>
+
+
+<p>In that class of motors in which two or more sets of energizing
+magnets are employed, and in which by artificial means a certain
+interval of time is made to elapse between the respective maximum
+or minimum periods or phases of their magnetic attraction
+or effect, the interval or difference in phase between the two sets
+of magnets is limited in extent. It is desirable, however, for the
+economical working of such motors that the strength or attraction
+of one set of magnets should be maximum, at the time when that
+of the other set is minimum, and conversely; but these conditions
+have not heretofore been realized except in cases where the two
+currents have been obtained from independent sources in the
+same or different machines. Mr. Tesla has therefore devised a
+motor embodying conditions that approach more nearly the theoretical
+requirements of perfect working, or in other words, he
+produces artificially a difference of magnetic phase by means of
+a current from a single primary source sufficient in extent to
+meet the requirements of practical and economical working. He
+employs a motor with two sets of energizing or field magnets,
+each wound with coils connected with a source of alternating or
+rapidly-varying currents, but forming two separate paths or
+circuits. The magnets of one set are protected to a certain extent
+from the energizing action of the current by means of a
+magnetic shield or screen interposed between the magnet and its
+energizing coil. This shield is properly adapted to the conditions
+of particular cases, so as to shield or protect the main core from
+magnetization until it has become itself saturated and no longer
+capable of containing all the lines of force produced by the current.
+It will be seen that by this means the energizing action
+begins in the protected set of magnets a certain arbitrarily-determined
+period of time later than in the other, and that by
+this means alone or in conjunction with other means or devices<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span>
+heretofore employed a practical difference of magnetic phase
+may readily be secured.</p>
+
+<p>Fig. 58 is a view of a motor, partly in section, with a diagram
+illustrating the invention. Fig. 59 is a similar view of a
+modification of the same.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_086.jpg" width="800" height="382" alt="Fig. 58, 59." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 58.</td><td class="caption">Fig. 59.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 58, which exhibits the simplest form of the invention,
+<small>A A</small> is the field-magnet of a motor, having, say, eight poles or
+inwardly-projecting cores <small>B</small> and <small>C</small>. The cores <small>B</small> form one set of
+magnets and are energized by coils <small>D</small>. The cores <small>C</small>, forming
+the other set are energized by coils <small>E</small>, and the coils are
+connected, preferably, in series with one another, in two derived
+or branched circuits, <small>F G</small>, respectively, from a suitable
+source of current. Each coil <small>E</small> is surrounded by a magnetic
+shield <small>H</small>, which is preferably composed of an annealed, insulated,
+or oxidized iron wire wrapped or wound on the coils in the manner
+indicated so as to form a closed magnetic circuit around the
+coils and between the same and the magnetic cores <small>C</small>. Between
+the pole pieces or cores <small>B C</small> is mounted the armature <small>K</small>,
+which, as is usual in this type of machines, is wound with coils
+<small>L</small> closed upon themselves. The operation resulting from this
+disposition is as follows: If a current impulse be directed
+through the two circuits of the motor, it will quickly energize
+the cores <small>B</small>, but not so the cores <small>C</small>, for the reason that in
+passing through the coils <small>E</small> there is encountered the influence
+of the closed magnetic circuits formed by the shields <small>H</small>. The
+first effect is to retard effectively the current impulse in circuit
+<small>G</small>, while at the same time the proportion of current which does
+pass does not magnetize the cores <small>C</small>, which are shielded or<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span>
+screened by the shields <small>H</small>. As the increasing electromotive
+force then urges more current through the coils <small>E</small>, the iron wire
+<small>H</small> becomes magnetically saturated and incapable of carrying all
+the lines of force, and hence ceases to protect the cores <small>C</small>, which
+becomes magnetized, developing their maximum effect after an
+interval of time subsequent to the similar manifestation of strength
+in the other set of magnets, the extent of which is arbitrarily
+determined by the thickness of the shield <small>H</small>, and other well-understood
+conditions.</p>
+
+<p>From the above it will be seen that the apparatus or device
+acts in two ways. First, by retarding the current, and, second,
+by retarding the magnetization of one set of the cores, from
+which its effectiveness will readily appear.</p>
+
+<p>Many modifications of the principle of this invention are possible.
+One useful and efficient application of the invention is
+shown in Fig. 59. In this figure a motor is shown similar in all
+respects to that above described, except that the iron wire <small>H</small>, which
+is wrapped around the coils <small>E</small>, is in this case connected in series
+with the coils <small>D</small>. The iron-wire coils <small>H</small>, are connected and wound,
+so as to have little or no self-induction, and being added to the
+resistance of the circuit <small>F</small>, the action of the current in that circuit
+will be accelerated, while in the other circuit <small>G</small> it will be
+retarded. The shield <small>H</small> may be made in many forms, as will be
+understood, and used in different ways, as appears from the
+foregoing description.</p>
+
+<p>As a modification of his type of motor with "shielded" fields,
+Mr. Tesla has constructed a motor with a field-magnet having
+two sets of poles or inwardly-projecting cores and placed side
+by side, so as practically to form two fields of force and alternately
+disposed&mdash;that is to say, with the poles of one set or field
+opposite the spaces between the other. He then connects the free
+ends of one set of poles by means of laminated iron bands or
+bridge-pieces of considerably smaller cross-section than the cores
+themselves, whereby the cores will all form parts of complete
+magnetic circuits. When the coils on each set of magnets are
+connected in multiple circuits or branches from a source of alternating
+currents, electromotive forces are set up in or impressed
+upon each circuit simultaneously; but the coils on the
+magnetically bridged or shunted cores will have, by reason of
+the closed magnetic circuits, a high self-induction, which retards
+the current, permitting at the beginning of each impulse but lit<span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span>tle
+current to pass. On the other hand, no such opposition being
+encountered in the other set of coils, the current passes freely
+through them, magnetizing the poles on which they are wound.
+As soon, however, as the laminated bridges become saturated
+and incapable of carrying all the lines of force which the rising
+electromotive force, and consequently increased current, produce,
+free poles are developed at the ends of the cores, which,
+acting in conjunction with the others, produce rotation of the
+armature.</p>
+
+<p>The construction in detail by which this invention is illustrated
+is shown in the accompanying drawings.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_088.jpg" width="800" height="340" alt="Fig. 60, 61." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 60.</td><td class="caption">Fig. 61.</td></tr>
+</table>
+</div>
+
+<p>Fig. 60 is a view in side elevation of a motor embodying the
+principle. Fig. 61 is a vertical cross-section of the motor. <small>A</small> is
+the frame of the motor, which should be built up of sheets of
+iron punched out to the desired shape and bolted together with
+insulation between the sheets. When complete, the frame makes
+a field-magnet with inwardly projecting pole-pieces <small>B</small> and <small>C</small>. To
+adapt them to the requirements of this particular case these pole-pieces
+are out of line with one another, those marked <small>B</small> surrounding
+one end of the armature and the others, as <small>C</small>, the opposite
+end, and they are disposed alternately&mdash;that is to say, the pole-pieces
+of one set occur in line with the spaces between those of the
+other sets.</p>
+
+<p>The armature <small>D</small> is of cylindrical form, and is also laminated in
+the usual way and is wound longitudinally with coils closed upon
+themselves. The pole-pieces <small>C</small> are connected or shunted by
+bridge-pieces <small>E</small>. These may be made independently and attached
+to the pole-pieces, or they may be parts of the forms or blanks
+stamped or punched out of sheet-iron. Their size or mass is
+de<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span>termined by various conditions, such as the strength of the current
+to be employed, the mass or size of the cores to which they
+are applied, and other familiar conditions.</p>
+
+<p>Coils <small>F</small> surround the pole-pieces <small>B</small>, and other coils <small>G</small> are wound
+on the pole-pieces <small>C</small>. These coils are connected in series in two
+circuits, which are branches of a circuit from a generator of alternating
+currents, and they may be so wound, or the respective
+circuits in which they are included may be so arranged, that the
+circuit of coils <small>G</small> will have, independently of the particular construction
+described, a higher self-induction than the other circuit
+or branch.</p>
+
+<p>The function of the shunts or bridges <small>E</small> is that they shall form
+with the cores <small>C</small> a closed magnetic circuit for a current up to a
+predetermined strength, so that when saturated by such current
+and unable to carry more lines of force than such a current produces
+they will to no further appreciable extent interfere with
+the development, by a stronger current, of free magnetic poles at
+the ends of the cores <small>C</small>.</p>
+
+<p>In such a motor the current is so retarded in the coils <small>G</small>, and
+the manifestation of the free magnetism in the poles <small>C</small> is so delayed
+beyond the period of maximum magnetic effect in poles <small>B</small>, that a
+strong torque is produced and the motor operates with approximately
+the power developed in a motor of this kind energized
+by independently generated currents differing by a full quarter
+phase.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span></p>
+<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV.</h2>
+
+<h3><span class="smcap">Type of Tesla Single-Phase Motor.</span></h3>
+
+
+<p>Up to this point, two principal types of Tesla motors have
+been described: First, those containing two or more energizing
+circuits through which are caused to pass alternating currents
+differing from one another in phase to an extent sufficient to
+produce a continuous progression or shifting of the poles or
+points of greatest magnetic effect, in obedience to which the
+movable element of the motor is maintained in rotation; second,
+those containing poles, or parts of different magnetic susceptibility,
+which under the energizing influence of the same current
+or two currents coinciding in phase will exhibit differences in
+their magnetic periods or phases. In the first class of motors
+the torque is due to the magnetism established in different portions
+of the motor by currents from the same or from independent
+sources, and exhibiting time differences in phase. In
+the second class the torque results from the energizing effects of
+a current upon different parts of the motor which differ in magnetic
+susceptibility&mdash;in other words, parts which respond in the
+same relative degree to the action of a current, not simultaneously,
+but after different intervals of time.</p>
+
+<p>In another Tesla motor, however, the torque, instead of being
+solely the result of a time difference in the magnetic periods or
+phases of the poles or attractive parts to whatever cause due, is
+produced by an angular displacement of the parts which, though
+movable with respect to one another, are magnetized simultaneously,
+or approximately so, by the same currents. This principle
+of operation has been embodied practically in a motor in which
+the necessary angular displacement between the points of greatest
+magnetic attraction in the two elements of the motor&mdash;the armature
+and field&mdash;is obtained by the direction of the lamination of
+the magnetic cores of the elements.</p>
+
+<p>Fig. 62 is a side view of such a motor with a portion of its
+armature core exposed. Fig. 63 is an end or edge view of the<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span>
+same. Fig. 64 is a central cross-section of the same, the armature
+being shown mainly in elevation.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_091.jpg" width="800" height="367" alt="Fig. 62, 63, 64." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 62.</td><td class="caption">Fig. 63.</td><td class="caption">Fig. 64.</td></tr>
+</table>
+</div>
+
+<p>Let <small>A A</small> designate two plates built up of thin sections or
+lamin&aelig; of soft iron insulated more or less from one another and
+held together by bolts <i>a</i> and secured to a base <small>B</small>. The inner
+faces of these plates contain recesses or grooves in which a coil
+or coils <small>D</small> are secured obliquely to the direction of the laminations.
+Within the coils <small>D</small> is a disc <small>E</small>, preferably composed of
+a spirally-wound iron wire or ribbon or a series of concentric
+rings and mounted on a shaft <small>F</small>, having bearings in the plates
+<small>A A</small>. Such a device when acted upon by an alternating current
+is capable of rotation and constitutes a motor, the operation of
+which may be explained in the following manner: A current or
+current-impulse traversing the coils <small>D</small> tends to magnetize the
+cores <small>A A</small> and <small>E</small>, all of which are within the influence of the
+field of the coils. The poles thus established would naturally
+lie in the same line at right angles to the coils <small>D</small>, but in the
+plates <small>A</small> they are deflected by reason of the direction of the
+laminations, and appear at or near the extremities of these plates.
+In the disc, however, where these conditions are not present, the
+poles or points of greatest attraction are on a line at right
+angles to the plane of the coils; hence there will be a torque established
+by this angular displacement of the poles or magnetic
+lines, which starts the disc in rotation, the magnetic lines of the
+armature and field tending toward a position of parallelism.
+This rotation is continued and maintained by the reversals of
+the current in coils <small>D D</small>, which change alternately the polarity of
+the field-cores <small>A A</small>. This rotary tendency or effect will be greatly<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span>
+increased by winding the disc with conductors <small>G</small>, closed upon
+themselves and having a radial direction, whereby the magnetic
+intensity of the poles of the disc will be greatly increased by
+the energizing effect of the currents induced in the coils <small>G</small> by the
+alternating currents in coils <small>D</small>.</p>
+
+<p>The cores of the disc and field may or may not be of different
+magnetic susceptibility&mdash;that is to say, they may both be of the
+same kind of iron, so as to be magnetized at approximately the
+same instant by the coils <small>D</small>; or one may be of soft iron and the
+other of hard, in order that a certain time may elapse between
+the periods of their magnetization. In either case rotation will
+be produced; but unless the disc is provided with the closed energizing
+coils it is desirable that the above-described difference of
+magnetic susceptibility be utilized to assist in its rotation.</p>
+
+<p>The cores of the field and armature may be made in various
+ways, as will be well understood, it being only requisite that the
+laminations in each be in such direction as to secure the necessary
+angular displacement of the points of greatest attraction.
+Moreover, since the disc may be considered as made up of an
+infinite number of radial arms, it is obvious that what is true of
+a disc holds for many other forms of armature.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV.</h2>
+
+<h3><span class="smcap">Motors with Circuits of Different Resistance.</span></h3>
+
+
+<p>As has been pointed out elsewhere, the lag or retardation of
+the phases of an alternating current is directly proportional to
+the self-induction and inversely proportional to the resistance of
+the circuit through which the current flows. Hence, in order
+to secure the proper differences of phase between the two motor-circuits,
+it is desirable to make the self-induction in one much
+higher and the resistance much lower than the self-induction and
+resistance, respectively, in the other. At the same time the
+magnetic quantities of the two poles or sets of poles which the
+two circuits produce should be approximately equal. These
+requirements have led Mr. Tesla to the invention of a motor
+having the following general characteristics: The coils which
+are included in that energizing circuit which is to have the
+higher self-induction are made of coarse wire, or a conductor of
+relatively low resistance, and with the greatest possible length
+or number of turns. In the other set of coils a comparatively
+few turns of finer wire are used, or a wire of higher resistance.
+Furthermore, in order to approximate the magnetic quantities of
+the poles excited by these coils, Mr. Tesla employs in the self-induction
+circuit cores much longer than those in the other or
+resistance circuit.</p>
+
+<p>Fig. 65 is a part sectional view of the motor at right angles to
+the shaft. Fig. 66 is a diagram of the field circuits.</p>
+
+<p>In Fig. 66, let <small>A</small> represent the coils in one motor circuit, and B
+those in the other. The circuit <small>A</small> is to have the higher self-induction.
+There are, therefore, used a long length or a large
+number of turns of coarse wire in forming the coils of this circuit.
+For the circuit <small>B</small>, a smaller conductor is employed, or a
+conductor of a higher resistance than copper, such as German
+silver or iron, and the coils are wound with fewer turns. In applying
+these coils to a motor, Mr. Tesla builds up a field-magnet of
+plates <small>C</small>, of iron and steel, secured together in the usual manner<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span>
+by bolts <small>D</small>. Each plate is formed with four (more or less) long
+cores <small>E</small>, around which is a space to receive the coil and an equal
+number of short projections <small>F</small> to receive the coils of the resistance-circuit.
+The plates are generally annular in shape, having an
+open space in the centre for receiving the armature <small>G</small>, which Mr.
+Tesla prefers to wind with closed coils. An alternating current
+divided between the two circuits is retarded as to its phases in
+the circuit <small>A</small> to a much greater extent than in the circuit <small>B</small>. By
+reason of the relative sizes and disposition of the cores and coils
+the magnetic effect of the poles <small>E</small> and <small>F</small> upon the armature closely
+approximate.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_094.jpg" width="800" height="334" alt="Fig. 65, 66." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 65.</td><td class="caption">Fig. 66.</td></tr>
+</table>
+</div>
+
+<p>An important result secured by the construction shown here
+is that these coils which are designed to have the higher self-induction
+are almost completely surrounded by iron, and that the
+retardation is thus very materially increased.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span></p>
+<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI.</h2>
+
+<h3><span class="smcap">Motor With Equal Magnetic Energies in Field and
+Armature.</span></h3>
+
+
+<p>Let it be assumed that the energy as represented in the magnetism
+in the field of a given rotating field motor is ninety and
+that of the armature ten. The sum of these quantities, which
+represents the total energy expended in driving the motor, is
+one hundred; but, assuming that the motor be so constructed
+that the energy in the field is represented by fifty, and that in
+the armature by fifty, the sum is still one hundred; but while in
+the first instance the product is nine hundred, in the second it is
+two thousand five hundred, and as the energy developed is in
+proportion to these products it is clear that those motors are the
+most efficient&mdash;other things being equal&mdash;in which the magnetic
+energies developed in the armature and field are equal. These
+results Mr. Tesla obtains by using the same amount of copper or
+ampere turns in both elements when the cores of both are equal,
+or approximately so, and the same current energizes both; or in
+cases where the currents in one element are induced to those of
+the other he uses in the induced coils an excess of copper over
+that in the primary element or conductor.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_095.jpg" width="640" height="430" alt="Fig. 67." title="" />
+<span class="caption">Fig. 67.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+
+<p>The conventional figure of a motor here introduced, Fig. 67,
+will give an idea of the solution furnished by Mr. Tesla for the
+specific problem. Referring to the drawing, <small>A</small> is the field-magnet,
+<small>B</small> the armature, <small>C</small> the field coils, and <small>D</small> the armature-coils of
+the motor.</p>
+
+<p>Generally speaking, if the mass of the cores of armature and
+field be equal, the amount of copper or ampere turns of the
+energizing coils on both should also be equal; but these conditions
+will be modified in different forms of machine. It will be
+understood that these results are most advantageous when existing
+under the conditions presented where the motor is running
+with its normal load, a point to be well borne in mind.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+<h2><a name="CHAPTER_XVII" id="CHAPTER_XVII"></a>CHAPTER XVII.</h2>
+
+<h3><span class="smcap">Motors With Coinciding Maxima of Magnetic Effect in
+Armature and Field.</span></h3>
+
+
+<p>In this form of motor, Mr. Tesla's object is to design and
+build machines wherein the maxima of the magnetic effects of
+the armature and field will more nearly coincide than in some of
+the types previously under consideration. These types are: First,
+motors having two or more energizing circuits of the same electrical
+character, and in the operation of which the currents used
+differ primarily in phase; second, motors with a plurality of
+energizing circuits of different electrical character, in or by
+means of which the difference of phase is produced artificially,
+and, third, motors with a plurality of energizing circuits, the
+currents in one being induced from currents in another. Considering
+the structural and operative conditions of any one of
+them&mdash;as, for example, that first named&mdash;the armature which is
+mounted to rotate in obedience to the co-operative influence or
+action of the energizing circuits has coils wound upon it which
+are closed upon themselves and in which currents are induced by
+the energizing-currents with the object and result of energizing
+the armature-core; but under any such conditions as must exist
+in these motors, it is obvious that a certain time must elapse
+between the manifestations of an energizing current impulse in
+the field coils, and the corresponding magnetic state or phase in
+the armature established by the current induced thereby; consequently
+a given magnetic influence or effect in the field which is
+the direct result of a primary current impulse will have become
+more or less weakened or lost before the corresponding effect in
+the armature indirectly produced has reached its maximum. This
+is a condition unfavorable to efficient working in certain cases&mdash;as,
+for instance, when the progress of the resultant poles or points
+of maximum attraction is very great, or when a very high number
+of alternations is employed&mdash;for it is apparent that a stronger<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span>
+tendency to rotation will be maintained if the maximum magnetic
+attractions or conditions in both armature and field coincide,
+the energy developed by a motor being measured by the product
+of the magnetic quantities of the armature and field.</p>
+
+<p>To secure this coincidence of maximum magnetic effects, Mr.
+Tesla has devised various means, as explained below. Fig. 68 is
+a diagrammatic illustration of a Tesla motor system in which the
+alternating currents proceed from independent sources and differ
+primarily in phase.</p>
+
+<div class="figcenter" style="width: 700px;">
+<img src="images/oi_098.jpg" width="700" height="600" alt="Fig. 68, 69." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 68.</td><td class="caption">Fig. 69.</td></tr>
+</table>
+</div>
+
+<p><small>A</small> designates the field-magnet or magnetic frame of the motor;
+<small>B B</small>, oppositely located pole-pieces adapted to receive the coils of
+one energizing circuit; and <small>C C</small>, similar pole-pieces for the coils
+of the other energizing circuit. These circuits are designated,
+respectively, by <small>D E</small>, the conductor <small>D''</small> forming a common return
+to the generator <small>G</small>. Between these poles is mounted an armature&mdash;for
+example, a ring or annular armature, wound with a series
+of coils <small>F</small>, forming a closed circuit or circuits. The action or
+operation of a motor thus constructed is now well understood.
+It will be observed, however, that the magnetism of poles <small>B</small>, for<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span>
+example, established by a current impulse in the coils thereon,
+precedes the magnetic effect set up in the armature by the induced
+current in coils <small>F</small>. Consequently the mutual attraction
+between the armature and field-poles is considerably reduced.
+The same conditions will be found to exist if, instead of assuming
+the poles <small>B</small> or <small>C</small> as acting independently, we regard the ideal resultant
+of both acting together, which is the real condition. To
+remedy this, the motor field is constructed with secondary poles
+<small>B' C'</small>, which are situated between the others. These pole-pieces
+are wound with coils <small>D' E'</small>, the former in derivation to the coils
+<small>D</small>, the latter to coils <small>E</small>. The main or primary coils <small>D</small> and <small>E</small> are
+wound for a different self-induction from that of the coils <small>D'</small> and
+<small>E'</small>, the relations being so fixed that if the currents in <small>D</small> and <small>E</small>
+differ, for example, by a quarter-phase, the currents in each
+secondary coil, as <small>D' E'</small>, will differ from those in its appropriate
+primary <small>D</small> or <small>E</small> by, say, forty-five degrees, or one-eighth of a
+period.</p>
+
+<p>Now, assuming that an impulse or alternation in circuit or
+branch <small>E</small> is just beginning, while in the branch <small>D</small> it is just falling
+from maximum, the conditions are those of a quarter-phase
+difference. The ideal resultant of the attractive forces of the two
+sets of poles <small>B C</small> therefore may be considered as progressing from
+poles <small>B</small> to poles <small>C</small>, while the impulse in <small>E</small> is rising to maximum,
+and that in <small>D</small> is falling to zero or minimum. The polarity set up
+in the armature, however, lags behind the manifestations of field
+magnetism, and hence the maximum points of attraction in armature
+and field, instead of coinciding, are angularly displaced.
+This effect is counteracted by the supplemental poles <small>B' C'</small>. The
+magnetic phases of these poles succeed those of poles <small>B C</small> by the
+same, or nearly the same, period of time as elapses between the
+effect of the poles <small>B C</small> and the corresponding induced effect in the
+armature; hence the magnetic conditions of poles <small>B' C'</small> and of
+the armature more nearly coincide and a better result is obtained.
+As poles <small>B' C'</small> act in conjunction with the poles in the armature
+established by poles <small>B C</small>, so in turn poles <small>C B</small> act similarly with
+the poles set up by <small>B' C'</small>, respectively. Under such conditions
+the retardation of the magnetic effect of the armature and that
+of the secondary poles will bring the maximum of the two more
+nearly into coincidence and a correspondingly stronger torque or
+magnetic attraction secured.</p>
+
+<p>In such a disposition as is shown in Fig. 68 it will be observed<span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span>
+that as the adjacent pole-pieces of either circuit are of like polarity
+they will have a certain weakening effect upon one another.
+Mr. Tesla therefore prefers to remove the secondary poles from
+the direct influence of the others. This may be done by constructing
+a motor with two independent sets of fields, and with
+either one or two armatures electrically connected, or by using
+two armatures and one field. These modifications are illustrated
+further on.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_100.jpg" width="800" height="564" alt="Fig. 70, 71." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 70.</td><td class="caption">Fig. 71.</td></tr>
+</table>
+</div>
+
+<p>Fig. 69 is a diagrammatic illustration of a motor and system in
+which the difference of phase is artificially produced. There are
+two coils <small>D D</small> in one branch and two coils <small>E E</small> in another branch
+of the main circuit from the generator <small>G</small>. These two circuits or
+branches are of different self-induction, one, as <small>D</small>, being higher
+than the other. This is graphically indicated by making coils <small>D</small>
+much larger than coils <small>E</small>. By reason of the difference in the
+electrical character of the two circuits, the phases of current in
+one are retarded to a greater extent than the other. Let this
+difference be thirty degrees. A motor thus constructed will
+rotate under the action of an alternating current; but as happens
+in the case previously described the corresponding magnetic effects
+of the armature and field do not coincide owing to the time
+that elapses between a given magnetic effect in the armature and<span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span>
+the condition of the field that produces it. The secondary or
+supplemental poles <small>B' C'</small> are therefore availed of. There being
+thirty degrees difference of phase between the currents in coils
+<small>D E</small>, the magnetic effect of poles <small>B' C'</small> should correspond to that
+produced by a current differing from the current in coils <small>D</small> or <small>E</small>
+by fifteen degrees. This we can attain by winding each supplemental
+pole <small>B' C'</small> with two coils <small>H H'</small>. The coils <small>H</small> are included
+in a derived circuit having the same self-induction as circuit <small>D</small>,
+and coils <small>H'</small> in a circuit having the same self-induction as circuit
+<small>E</small>, so that if these circuits differ by thirty degrees the magnetism
+of poles <small>B' C'</small> will correspond to that produced by a current differing
+from that in either <small>D</small> or <small>E</small> by fifteen degrees. This is true
+in all other cases. For example, if in Fig. 68 the coils <small>D' E'</small> be
+replaced by the coils <small>H H'</small> included in the derived circuits, the
+magnetism of the poles <small>B' C'</small> will correspond in effect or phase,
+if it may be so termed, to that produced by a current differing
+from that in either circuit <small>D</small> or <small>E</small> by forty-five degrees, or one-eighth
+of a period.</p>
+
+<p>This invention as applied to a derived circuit motor is illustrated
+in Figs. 70 and 71. The former is an end view of the motor
+with the armature in section and a diagram of connections, and
+Fig. 71 a vertical section through the field. These figures are
+also drawn to show one of the dispositions of two fields that may
+be adopted in carrying out the principle. The poles <small>B B C C</small> are
+in one field, the remaining poles in the other. The former are
+wound with primary coils <small>I J</small> and secondary coils <small>I' J'</small>, the latter
+with coils <small>K L</small>. The primary coils <small>I J</small> are in derived circuits, between
+which, by reason of their different self-induction, there is
+a difference of phase, say, of thirty degrees. The coils <small>I' K</small> are
+in circuit with one another, as also are coils <small>J' L</small>, and there should
+be a difference of phase between the currents in coils <small>K</small> and <small>L</small> and
+their corresponding primaries of, say, fifteen degrees. If the
+poles <small>B C</small> are at right angles, the armature-coils should be connected
+directly across, or a single armature core wound from end
+to end may be used; but if the poles <small>B C</small> be in line there should
+be an angular displacement of the armature coils, as will be well
+understood.</p>
+
+<p>The operation will be understood from the foregoing. The
+maximum magnetic condition of a pair of poles, as <small>B' B'</small>, coincides
+closely with the maximum effect in the armature, which lags behind
+the corresponding condition in poles <small>B B</small>.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span></p>
+<h2><a name="CHAPTER_XVIII" id="CHAPTER_XVIII"></a>CHAPTER XVIII.</h2>
+
+<h3><span class="smcap">Motor Based on the Difference of Phase in the Magnetization
+of the Inner and Outer Parts of an Iron Core.</span></h3>
+
+
+<p>It is well known that if a magnetic core, even if laminated or
+subdivided, be wound with an insulated coil and a current of
+electricity be directed through the coil, the magnetization of the
+entire core does not immediately ensue, the magnetizing effect
+not being exhibited in all parts simultaneously. This may be attributed
+to the fact that the action of the current is to energize
+first those lamin&aelig; or parts of the core nearest the surface and
+adjacent to the exciting-coil, and from thence the action progresses
+toward the interior. A certain interval of time therefore
+elapses between the manifestation of magnetism in the external
+and the internal sections or layers of the core. If the core be
+thin or of small mass, this effect may be inappreciable; but in
+the case of a thick core, or even of a comparatively thin one, if
+the number of alternations or rate of change of the current
+strength be very great, the time interval occurring between the
+manifestations of magnetism in the interior of the core and in
+those parts adjacent to the coil is more marked. In the construction
+of such apparatus as motors which are designed to be
+run by alternating or equivalent currents&mdash;such as pulsating or
+undulating currents generally&mdash;Mr. Tesla found it desirable and
+even necessary to give due consideration to this phenomenon and
+to make special provisions in order to obviate its consequences.
+With the specific object of taking advantage of this action or
+effect, and to render it more pronounced, he constructs a field
+magnet in which the parts of the core or cores that exhibit at
+different intervals of time the magnetic effect imparted to them
+by alternating or equivalent currents in an energizing coil or coils,
+are so placed with relation to a rotating armature as to exert
+thereon their attractive effect successively in the order of their
+magnetization. By this means he secures a result similar to that
+which he had previously attained in other forms or types of mo<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span>tor
+in which by means of one or more alternating currents he
+has produced the rotation or progression of the magnetic poles.</p>
+
+<p>This new mode of operation will now be described. Fig. 72
+is a side elevation of such motor. Fig. 73 is a side elevation of
+a more practicable and efficient embodiment of the invention.
+Fig. 74 is a central vertical section of the same in the plane of
+the axis of rotation.</p>
+
+<div class="figcenter" style="width: 668px;">
+<img src="images/oi_103.jpg" width="668" height="600" alt="Fig. 72 and 73." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 72 and 73.</span>
+</div>
+
+<p>Referring to Fig. 72, let <small>X</small> represent a large iron core, which
+may be composed of a number of sheets or lamin&aelig; of soft iron
+or steel. Surrounding this core is a coil <small>Y</small>, which is connected
+with a source <small>E</small> of rapidly varying currents. Let us consider now
+the magnetic conditions existing in this core at any point, as <i>b</i>,
+at or near the centre, and any other point, as <i>a</i>, nearer the surface.
+When a current impulse is started in the magnetizing coil
+<small>Y</small>, the section or part at <i>a</i>, being close to the coil, is immediately
+energized, while the section or part at <i>b</i>, which, to use a convenient
+expression, is "protected" by the intervening sections or
+layers between <i>a</i> and <i>b</i>, does not at once exhibit its magnetism.
+However, as the magnetization of <i>a</i> increases, <i>b</i> becomes also
+affected, reaching finally its maximum strength some time later
+than <i>a</i>. Upon the weakening of the current the magnetization
+of <i>a</i> first diminishes, while <i>b</i> still exhibits its maximum strength;<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span>
+but the continued weakening of <i>a</i> is attended by a subsequent
+weakening of <i>b</i>. Assuming the current to be an alternating one,
+<i>a</i> will now be reversed, while <i>b</i> still continues of the first imparted
+polarity. This action continues the magnetic condition of <i>b</i>, following
+that of <i>a</i> in the manner above described. If an armature&mdash;for
+instance, a simple disc <small>F</small>, mounted to rotate freely on an
+axis&mdash;be brought into proximity to the core, a movement of rotation
+will be imparted to the disc, the direction depending upon
+its position relatively to the core, the tendency being to turn the
+portion of the disc nearest to the core from <i>a</i> to <i>b</i>, as indicated
+in Fig. 72.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_104.jpg" width="336" height="443" alt="Fig. 74." title="" />
+<span class="caption">Fig. 74.</span>
+</div>
+
+<p>This action or principle of operation has been embodied in a
+practicable form of motor, which is illustrated in Fig. 73. Let <small>A</small>
+in that figure represent a circular frame of iron, from diametrically
+opposite points of the interior of which the cores project.
+Each core is composed of three main parts <small>B</small>, <small>B</small> and <small>C</small>, and they
+are similarly formed with a straight portion or body <i>e</i>, around
+which the energizing coil is wound, a curved arm or extension <i>c</i>,
+and an inwardly projecting pole or end <i>d</i>. Each core is made up
+of two parts <small>B B</small>, with their polar extensions reaching in one
+direction, and a part <small>C</small> between the other two, and with its polar
+extension reaching in the opposite direction. In order to lessen
+in the cores the circulation of currents induced therein, the several
+sections are insulated from one another in the manner usually<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span>
+followed in such cases. These cores are wound with coils <small>D</small>, which
+are connected in the same circuit, either in parallel or series, and
+supplied with an alternating or a pulsating current, preferably
+the former, by a generator <small>E</small>, represented diagrammatically. Between
+the cores or their polar extensions is mounted a cylindrical
+or similar armature <small>F</small>, wound with magnetizing coils <small>G</small>, closed
+upon themselves.</p>
+
+<p>The operation of this motor is as follows: When a current
+impulse or alternation is directed through the coils <small>D</small>, the sections
+<small>B B</small> of the cores, being on the surface and in close proximity to
+the coils, are immediately energized. The sections <small>C</small>, on the other
+hand, are protected from the magnetizing influence of the coil
+by the interposed layers of iron <small>B B</small>. As the magnetism of <small>B B</small>
+increases, however, the sections <small>C</small> are also energized; but they
+do not attain their maximum strength until a certain time subsequent
+to the exhibition by the sections <small>B B</small> of their maximum.
+Upon the weakening of the current the magnetic strength of <small>B B</small>
+first diminishes, while the sections <small>C</small> have still their maximum
+strength; but as <small>B B</small> continue to weaken the interior sections are
+similarly weakened. <small>B B</small> may then begin to exhibit an opposite
+polarity, which is followed later by a similar change on <small>C</small>, and
+this action continues. <small>B B</small> and <small>C</small> may therefore be considered as
+separate field-magnets, being extended so as to act on the armature
+in the most efficient positions, and the effect is similar to
+that in the other forms of Tesla motor&mdash;viz., a rotation or progression
+of the maximum points of the field of force. Any
+armature&mdash;such, for instance, as a disc&mdash;mounted in this field
+would rotate from the pole first to exhibit its magnetism to that
+which exhibits it later.</p>
+
+<p>It is evident that the principle here described may be carried
+out in conjunction with other means for securing a more favorable
+or efficient action of the motor. For example, the polar
+extensions of the sections <small>C</small> may be wound or surrounded by
+closed coils. The effect of these coils will be to still more
+effectively retard the magnetization of the polar extensions of <small>C</small>.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span></p>
+<h2><a name="CHAPTER_XIX" id="CHAPTER_XIX"></a>CHAPTER XIX.</h2>
+
+<h3><span class="smcap">Another Type of Tesla Induction Motor.</span></h3>
+
+
+<p>It will have been gathered by all who are interested in the
+advance of the electrical arts, and who follow carefully, step by
+step, the work of pioneers, that Mr. Tesla has been foremost to
+utilize inductive effects in permanently closed circuits, in the
+operation of alternating motors. In this chapter one simple type
+of such a motor is described and illustrated, which will serve as
+an exemplification of the principle.</p>
+
+<p>Let it be assumed that an ordinary alternating current generator
+is connected up in a circuit of practically no self-induction,
+such, for example, as a circuit containing incandescent lamps
+only. On the operation of the machine, alternating currents will
+be developed in the circuit, and the phases of these currents will
+theoretically coincide with the phases of the impressed electromotive
+force. Such currents may be regarded and designated as
+the "unretarded currents."</p>
+
+<p>It will be understood, of course, that in practice there is always
+more or less self-induction in the circuit, which modifies to
+a corresponding extent these conditions; but for convenience
+this may be disregarded in the consideration of the principle of
+operation, since the same laws apply. Assume next that a path
+of currents be formed across any two points of the above circuit,
+consisting, for example, of the primary of an induction device.
+The phases of the currents passing through the primary,
+owing to the self-induction of the same, will not coincide with
+the phases of the impressed electromotive force, but will lag
+behind, such lag being directly proportional to the self-induction
+and inversely proportional to the resistance of the said coil.
+The insertion of this coil will also cause a lagging or retardation
+of the currents traversing and delivered by the generator behind
+the impressed electromotive force, such lag being the mean or
+resultant of the lag of the current through the primary alone and
+of the "unretarded current" in the entire working circuit. Next<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span>
+consider the conditions imposed by the association in inductive
+relation with the primary coil, of a secondary coil. The current
+generated in the secondary coil will react upon the primary current,
+modifying the retardation of the same, according to the
+amount of self-induction and resistance in the secondary circuit.
+If the secondary circuit has but little self-induction&mdash;as, for instance,
+when it contains incandescent lamps only&mdash;it will increase
+the actual difference of phase between its own and the
+primary current, first, by diminishing the lag between the primary
+current and the impressed electromotive force, and, second,
+by its own lag or retardation behind the impressed electromotive
+force. On the other hand, if the secondary circuit have
+a high self-induction, its lag behind the current in the primary is
+directly increased, while it will be still further increased if the
+primary have a very low self-induction. The better results are
+obtained when the primary has a low self-induction.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_107.jpg" width="800" height="429" alt="Fig. 75, 76." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 75.</td><td class="caption">Fig. 76.</td></tr>
+</table>
+</div>
+
+<p>Fig. 75 is a diagram of a Tesla motor embodying this principle.
+Fig. 76 is a similar diagram of a modification of the same.
+In Fig. 75 let <small>A</small> designate the field-magnet of a motor which, as
+in all these motors, is built up of sections or plates. <small>B C</small> are polar
+projections upon which the coils are wound. Upon one pair
+of these poles, as <small>C</small>, are wound primary coils <small>D</small>, which are directly
+connected to the circuit of an alternating current generator
+<small>G</small>. On the same poles are also wound secondary coils <small>F</small>,
+either side by side or over or under the primary coils, and these
+are connected with other coils <small>E</small>, which surround the poles <small>B B</small>.<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+The currents in both primary and secondary coils in such a motor
+will be retarded or will lag behind the impressed electromotive
+force; but to secure a proper difference in phase between
+the primary and secondary currents themselves, Mr. Tesla increases
+the resistance of the circuit of the secondary and reduces
+as much as practicable its self-induction. This is done by using
+for the secondary circuit, particularly in the coils <small>E</small>, wire of comparatively
+small diameter and having but few turns around the
+cores; or by using some conductor of higher specific resistance,
+such as German silver; or by introducing at some point in the
+secondary circuit an artificial resistance <small>R</small>. Thus the self-induction
+of the secondary is kept down and its resistance increased,
+with the result of decreasing the lag between the impressed
+electro-motive force and the current in the primary coils and increasing
+the difference of phase between the primary and secondary
+currents.</p>
+
+<p>In the disposition shown in Fig. 76, the lag in the secondary
+is increased by increasing the self-induction of that circuit, while
+the increasing tendency of the primary to lag is counteracted by
+inserting therein a dead resistance. The primary coils <small>D</small> in this
+case have a low self-induction and high resistance, while the coils
+<small>E F</small>, included in the secondary circuit, have a high self-induction
+and low resistance. This may be done by the proper winding of
+the coils; or in the circuit including the secondary coils <small>E F</small>, we
+may introduce a self-induction coil <small>S</small>, while in the primary circuit
+from the generator <small>G</small> and including coils <small>D</small>, there may be inserted
+a dead resistance <small>R</small>. By this means the difference of
+phase between the primary and secondary is increased. It is evident
+that both means of increasing the difference of phase&mdash;namely,
+by the special winding as well as by the supplemental or
+external inductive and dead resistance&mdash;may be employed conjointly.</p>
+
+<p>In the operation of this motor the current impulses in the primary
+coils induce currents in the secondary coils, and by the conjoint
+action of the two the points of greatest magnetic attraction
+are shifted or rotated.</p>
+
+<p>In practice it is found desirable to wind the armature with
+closed coils in which currents are induced by the action thereon
+of the primaries.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span></p>
+<h2><a name="CHAPTER_XX" id="CHAPTER_XX"></a>CHAPTER XX.</h2>
+
+<h3><span class="smcap">Combinations of Synchronizing Motor and Torque Motor.</span></h3>
+
+
+<p>In the preceding descriptions relative to synchronizing motors
+and methods of operating them, reference has been made to the
+plan adopted by Mr. Tesla, which consists broadly in winding or
+arranging the motor in such manner that by means of suitable
+switches it could be started as a multiple-circuit motor, or one
+operating by a progression of its magnetic poles, and then, when
+up to speed, or nearly so, converted into an ordinary synchronizing
+motor, or one in which the magnetic poles were simply alternated.
+In some cases, as when a large motor is used and when
+the number of alternations is very high, there is more or less
+difficulty in bringing the motor to speed as a double or multiple-circuit
+motor, for the plan of construction which renders the
+motor best adapted to run as a synchronizing motor impairs its
+efficiency as a torque or double-circuit motor under the assumed
+conditions on the start. This will be readily understood, for in a
+large synchronizing motor the length of the magnetic circuit of
+the polar projections, and their mass, are so great that apparently
+considerable time is required for magnetization and demagnetization.
+Hence with a current of a very high number of alternations
+the motor may not respond properly. To avoid this objection
+and to start up a synchronizing motor in which these conditions
+obtain, Mr. Tesla has combined two motors, one a synchronizing
+motor, the other a multiple-circuit or torque motor, and by the
+latter he brings the first-named up to speed, and then either
+throws the whole current into the synchronizing motor or operates
+jointly both of the motors.</p>
+
+<p>This invention involves several novel and useful features. It
+will be observed, in the first place, that both motors are run,
+without commutators of any kind, and, secondly, that the speed
+of the torque motor may be higher than that of the synchronizing
+motor, as will be the case when it contains a fewer number of
+poles or sets of poles, so that the motor will be more readily and<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span>
+easily brought up to speed. Thirdly, the synchronizing motor
+may be constructed so as to have a much more pronounced tendency
+to synchronism without lessening the facility with which
+it is started.</p>
+
+<p>Fig. 77 is a part sectional view of the two motors; Fig. 78 an
+end view of the synchronizing motor; Fig. 79 an end view and
+part section of the torque or double-circuit motor; Fig. 80 a
+diagram of the circuit connections employed; and Figs. 81, 82,
+83, 84 and 85 are diagrams of modified dispositions of the two
+motors.</p>
+
+
+<div class="figcenter" style="width: 629px;">
+<img src="images/oi_110.jpg" width="629" height="480" alt="Fig. 77." title="" />
+<span class="caption">Fig. 77.</span>
+</div>
+
+<p>Inasmuch as neither motor is doing any work while the current
+is acting upon the other, the two armatures are rigidly connected,
+both being mounted upon the same shaft <small>A</small>, the field-magnets <small>B</small>
+of the synchronizing and <small>C</small> of the torque motor being secured to
+the same base <small>D</small>. The preferably larger synchronizing motor has
+polar projections on its armature, which rotate in very close proximity
+to the poles of the field, and in other respects it conforms
+to the conditions that are necessary to secure synchronous action.
+The pole-pieces of the armature are, however, wound with closed
+coils <small>E</small>, as this obviates the employment of sliding contacts. The
+smaller or torque motor, on the other hand, has, preferably, a
+cylindrical armature <small>F</small>, without polar projections and wound with
+closed coils <small>G</small>. The field-coils of the torque motor are connected
+up in two series <small>H</small> and <small>I</small>, and the alternating current from the
+generator is directed through or divided between these two circuits
+in any manner to produce a progression of the poles or
+points of maximum magnetic effect. This result is secured by
+connecting the two motor-circuits in derivation with the circuit<span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span>
+from the generator, inserting in one motor circuit a dead resistance
+and in the other a self-induction coil, by which means a
+difference in phase between the two divisions of the current is
+secured. If both motors have the same number of field poles,
+the torque motor for a given number of alternations will tend to
+run at double the speed of the other, for, assuming the connections
+to be such as to give the best results, its poles are divided
+into two series and the number of poles is virtually reduced one-half,
+which being acted upon by the same number of alternations
+tend to rotate the armature at twice the speed. By this means
+the main armature is more easily brought to or above the required
+speed. When the speed necessary for synchronism is imparted
+to the main motor, the current is shifted from the torque motor
+into the other.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_111.jpg" width="800" height="411" alt="Fig. 78, 79." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 78.</td><td class="caption">Fig. 79.</td></tr>
+</table>
+</div>
+
+<p>A convenient arrangement for carrying out this invention is
+shown in Fig. 80, in which <small>J J</small> are the field coils of the synchronizing,
+and <small>H I</small> the field coils of the torque motor. <small>L L'</small> are
+the conductors of the main line. One end of, say, coils <small>H</small> is connected
+to wire <small>L</small> through a self-induction coil <small>M</small>. One end of the
+other set of coils <small>I</small> is connected to the same wire through a dead
+resistance <small>N</small>. The opposite ends of these two circuits are connected
+to the contact <i>m</i> of a switch, the handle or lever of which
+is in connection with the line-wire <small>L'</small>. One end of the field circuit
+of the synchronizing motor is connected to the wire <small>L</small>. The
+other terminates in the switch-contact <i>n</i>. From the diagram it
+will be readily seen that if the lever <small>P</small> be turned upon contact <i>m</i>,
+the torque motor will start by reason of the difference of phase
+between the currents in its two energizing circuits. Then when
+the desired speed is attained, if the lever <small>P</small> be shifted upon con<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span>tact
+<i>n</i> the entire current will pass through the field coils of the
+synchronizing motor and the other will be doing no work.</p>
+
+<p>The torque motor may be constructed and operated in various
+ways, many of which have already been touched upon. It is not
+necessary that one motor be cut out of circuit while the other is
+in, for both may be acted upon by current at the same time, and
+Mr. Tesla has devised various dispositions or arrangements of the
+two motors for accomplishing this. Some of these arrangements
+are illustrated in Figs. 81 to 85.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_112.jpg" width="640" height="428" alt="Fig. 80." title="" />
+<span class="caption">Fig. 80.</span>
+</div>
+
+
+<p>Referring to Fig. 81, let <small>T</small> designate the torque or multiple
+circuit motor and <small>S</small> the synchronizing motor, <small>L L'</small> being the line-wires
+from a source of alternating current. The two circuits of
+the torque motor of different degrees of self-induction, and designated
+by <small>N M</small>, are connected in derivation to the wire L. They
+are then joined and connected to the energizing circuit of the
+synchronizing motor, the opposite terminal of which is connected
+to wire <small>L'</small>. The two motors are thus in series. To start them
+Mr. Tesla short-circuits the synchronizing motor by a switch <small>P'</small>,
+throwing the whole current through the torque motor. Then
+when the desired speed is reached the switch <small>P'</small> is opened, so
+that the current passes through both motors. In such an arrangement
+as this it is obviously desirable for economical and other
+reasons that a proper relation between the speeds of the two
+motors should be observed.</p>
+
+<p>In Fig. 82 another disposition is illustrated. <small>S</small> is the synchronizing
+motor and <small>T</small> the torque motor, the circuits of both being in
+parallel. <small>W</small> is a circuit also in derivation to the motor circuits
+and containing a switch <small>P''</small>. <small>S'</small> is a switch in the synchronizing
+motor circuit. On the start the switch <small>S'</small> is opened, cutting out
+the motor <small>S</small>. Then <small>P''</small> is opened, throwing the entire current<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span>
+through the motor <small>T</small>, giving it a very strong torque. When the
+desired speed is reached, switch <small>S'</small> is closed and the current divides
+between both motors. By means of switch <small>P''</small> both motors may
+be cut out.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/oi_113.jpg" width="500" height="800" alt="Fig. 81, 82, 83, 84 and 85." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 81, 82, 83, 84 and 85.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span></p>
+
+<p>In Fig. 83 the arrangement is substantially the same, except
+that a switch <small>T'</small> is placed in the circuit which includes the two circuits
+of the torque motor. Fig. 84 shows the two motors in
+series, with a shunt around both containing a switch <small>S T</small>. There
+is also a shunt around the synchronizing motor <small>S</small>, with a switch
+<small>P'</small>. In Fig. 85 the same disposition is shown; but each motor is
+provided with a shunt, in which are switches <small>P'</small> and <small>T''</small>, as shown.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span></p>
+
+<h2><a name="CHAPTER_XXI" id="CHAPTER_XXI"></a>CHAPTER XXI.</h2>
+
+<h3><span class="smcap">Motor with a Condenser in the Armature Circuit.</span></h3>
+
+
+<p>We now come to a new class of motors in which resort is had
+to condensers for the purpose of developing the required difference
+of phase and neutralizing the effects of self-induction. Mr.
+Tesla early began to apply the condenser to alternating apparatus,
+in just how many ways can only be learned from a perusal
+of other portions of this volume, especially those dealing with
+his high frequency work.</p>
+
+<p>Certain laws govern the action or effects produced by a condenser
+when connected to an electric circuit through which an
+alternating or in general an undulating current is made to pass.
+Some of the most important of such effects are as follows: First,
+if the terminals or plates of a condenser be connected with two
+points of a circuit, the potentials of which are made to rise and
+fall in rapid succession, the condenser allows the passage, or more
+strictly speaking, the transference of a current, although its
+plates or armatures may be so carefully insulated as to prevent
+almost completely the passage of a current of unvarying strength
+or direction and of moderate electromotive force. Second, if a
+circuit, the terminals of which are connected with the plates of
+the condenser, possess a certain self-induction, the condenser will
+overcome or counteract to a greater or less degree, dependent
+upon well-understood conditions, the effects of such self-induction.
+Third, if two points of a closed or complete circuit
+through which a rapidly rising and falling current flows be
+shunted or bridged by a condenser, a variation in the strength of
+the currents in the branches and also a difference of phase of the
+currents therein is produced. These effects Mr. Tesla has utilized
+and applied in a variety of ways in the construction and operation
+of his motors, such as by producing a difference in phase in the
+two energizing circuits of an alternating current motor by connecting
+the two circuits in derivation and connecting up a condenser
+in series in one of the circuits. A further development,
+<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span>however, possesses certain novel features of practical value and involves
+a knowledge of facts less generally understood. It comprises
+the use of a condenser or condensers in connection with the induced
+or armature circuit of a motor and certain details of the construction
+of such motors. In an alternating current motor of the
+type particularly referred to above, or in any other which has
+an armature coil or circuit closed upon itself, the latter represents
+not only an inductive resistance, but one which is period<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span>ically
+varying in value, both of which facts complicate and render
+difficult the attainment of the conditions best suited to the most
+efficient working conditions; in other words, they require, first,
+that for a given inductive effect upon the armature there should
+be the greatest possible current through the armature or induced
+coils, and, second, that there should always exist between the
+currents in the energizing and the induced circuits a given relation
+of phase. Hence whatever tends to decrease the self-induction
+and increase the current in the induced circuits will, other
+things being equal, increase the output and efficiency of the motor,
+and the same will be true of causes that operate to maintain
+the mutual attractive effect between the field magnets and armature
+at its maximum. Mr. Tesla secures these results by connecting
+with the induced circuit or circuits a condenser, in the
+manner described below, and he also, with this purpose in view,
+constructs the motor in a special manner.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_116.jpg" width="800" height="534" alt="Fig. 86." title="" />
+<span class="caption">Fig. 86.</span>
+</div>
+
+
+<div class="figcenter" style="width: 780px;">
+<img src="images/fig88-89.jpg" width="640" height="278" alt="Fig. 88, 89." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 88.</td><td class="caption">Fig. 89.</td></tr>
+</table>
+
+<div class="figleft" style="width: 276px;">
+<img src="images/fig87.jpg" width="276" height="448" alt="Fig. 87." title="" />
+<span class="caption">Fig. 87.</span>
+</div>
+
+<div class="figright" style="width: 448px;">
+<img src="images/fig90.jpg" width="448" height="295" alt="Fig. 90." title="" />
+<span class="caption">Fig. 90.</span>
+</div>
+</div>
+<div style="clear: both;"></div>
+
+<p>Referring to the drawings, Fig. 86, is a view, mainly diagrammatic,
+of an alternating current motor, in which the present
+principle is applied. Fig. 87 is a central section, in line with
+the shaft, of a special form of armature core. Fig. 88 is a similar
+section of a modification of the same. Fig. 89 is one of the
+sections of the core detached. Fig. 90 is a diagram showing a
+modified disposition of the armature or induced circuits.</p>
+
+<p>The general plan of the invention is illustrated in Fig. 86.
+<small>A A</small> in this figure represent the the frame and field magnets of
+an alternating current motor, the poles or projections of which
+are wound with coils <small>B</small> and <small>C</small>, forming independent energizing
+circuits connected either to the same or to independent sources
+of alternating currents, so that the currents flowing through the
+circuits, respectively, will have a difference of phase. Within
+the influence of this field is an armature core <small>D</small>, wound with coils
+<small>E</small>. In motors of this description heretofore these coils have been
+closed upon themselves, or connected in a closed series; but in
+the present case each coil or the connected series of coils terminates
+in the opposite plates of a condenser <small>F</small>. For this purpose
+the ends of the series of coils are brought out through the shaft
+to collecting rings <small>G</small>, which are connected to the condenser by
+contact brushes <small>H</small> and suitable conductors, the condenser being
+independent of the machine. The armature coils are wound or
+connected in such manner that adjacent coils produce opposite
+poles.<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span></p>
+
+<p>The action of this motor and the effect of the plan followed
+in its construction are as follows: The motor being started in
+operation and the coils of the field magnets being traversed by
+alternating currents, currents are induced in the armature coils
+by one set of field coils, as <small>B</small>, and the poles thus established are
+acted upon by the other set, as <small>C</small>. The armature coils, however,
+have necessarily a high self-induction, which opposes the flow of
+the currents thus set up. The condenser <small>F</small> not only permits the
+passage or transference of these currents, but also counteracts
+the effects of self-induction, and by a proper adjustment of the
+capacity of the condenser, the self-induction of the coils, and the
+periods of the currents, the condenser may be made to overcome
+entirely the effect of self-induction.</p>
+
+<p>It is preferable on account of the undesirability of using sliding
+contacts of any kind, to associate the condenser with the armature
+directly, or make it a part of the armature. In some cases Mr.
+Tesla builds up the armature of annular plates <small>K K</small>, held by bolts
+<small>L</small> between heads <small>M</small>, which are secured to the driving shaft, and
+in the hollow space thus formed he places a condenser <small>F</small>, generally
+by winding the two insulated plates spirally around the
+shaft. In other cases he utilizes the plates of the core itself
+as the plates of the condenser. For example, in Figs. 88 and 89,
+<small>N</small> is the driving shaft, <small>M M</small> are the heads of the armature-core,
+and <small>K K'</small> the iron plates of which the core is built up. These
+plates are insulated from the shaft and from one another, and are
+held together by rods or bolts <small>L</small>. The bolts pass through a large
+hole in one plate and a small hole in the one next adjacent, and
+so on, connecting electrically all of plates <small>K</small>, as one armature of a
+condenser, and all of plates <small>K'</small> as the other.</p>
+
+<p>To either of the condensers above described the armature coils
+may be connected, as explained by reference to Fig. 86.</p>
+
+<p>In motors in which the armature coils are closed upon themselves&mdash;as,
+for example, in any form of alternating current motor
+in which one armature coil or set of coils is in the position of
+maximum induction with respect to the field coils or poles, while
+the other is in the position of minimum induction&mdash;the coils are
+best connected in one series, and two points of the circuit
+thus formed are bridged by a condenser. This is illustrated in
+Fig. 90, in which <small>E</small> represents one set of armature coils and <small>E'</small>
+the other. Their points of union are joined through a condenser
+<small>F</small>. It will be observed that in this disposition the self<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span>-induction
+of the two branches <small>E</small> and <small>E'</small> varies with their position
+relatively to the field magnet, and that each branch is alternately
+the predominating source of the induced current. Hence the
+effect of the condenser <small>F</small> is twofold. First, it increases the current
+in each of the branches alternately, and, secondly, it alters
+the phase of the currents in the branches, this being the well-known
+effect which results from such a disposition of a condenser
+with a circuit, as above described. This effect is favorable
+to the proper working of the motor, because it increases the flow
+of current in the armature circuits due to a given inductive
+effect, and also because it brings more nearly into coincidence
+the maximum magnetic effects of the coacting field and armature
+poles.</p>
+
+<p>It will be understood, of course, that the causes that contribute
+to the efficiency of condensers when applied to such uses as
+the above must be given due consideration in determining the
+practicability and efficiency of the motors. Chief among these
+is, as is well known, the periodicity of the current, and hence the
+improvements described are more particularly adapted to systems
+in which a very high rate of alternation or change is maintained.</p>
+
+<p>Although this invention has been illustrated in connection
+with a special form of motor, it will be understood that it is
+equally applicable to any other alternating current motor in
+which there is a closed armature coil wherein the currents are
+induced by the action of the field, and the feature of utilizing
+the plates or sections of a magnetic core for forming the condenser
+is applicable, generally, to other kinds of alternating current
+apparatus.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+<h2><a name="CHAPTER_XXII" id="CHAPTER_XXII"></a>CHAPTER XXII.</h2>
+
+<h3><span class="smcap">Motor with Condenser in one of the Field Circuits.</span></h3>
+
+
+<p>If the field or energizing circuits of a rotary phase motor be
+both derived from the same source of alternating currents and a
+condenser of proper capacity be included in one of the same, approximately,
+the desired difference of phase may be obtained between
+the currents flowing directly from the source and those
+flowing through the condenser; but the great size and expense
+of condensers for this purpose that would meet the requirements
+of the ordinary systems of comparatively low potential are particularly
+prohibitory to their employment.</p>
+
+<p>Another, now well-known, method or plan of securing a difference
+of phase between the energizing currents of motors of this
+kind is to induce by the currents in one circuit those in the other
+circuit or circuits; but as no means had been proposed that
+would secure in this way between the phases of the primary or
+inducing and the secondary or induced currents that difference&mdash;theoretically
+ninety degrees&mdash;that is best adapted for practical
+and economical working, Mr. Tesla devised a means which renders
+practicable both the above described plans or methods, and
+by which he is enabled to obtain an economical and efficient alternating
+current motor. His invention consists in placing a
+condenser in the secondary or induced circuit of the motor above
+described and raising the potential of the secondary currents to
+such a degree that the capacity of the condenser, which is in
+part dependent on the potential, need be quite small. The value
+of this condenser is determined in a well-understood manner with
+reference to the self-induction and other conditions of the circuit,
+so as to cause the currents which pass through it to differ from
+the primary currents by a quarter phase.</p>
+
+<p>Fig. 91 illustrates the invention as embodied in a motor
+in which the inductive relation of the primary and secondary
+circuits is secured by winding them inside the motor partly
+upon the same cores; but the invention applies, generally, to<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span>
+other forms of motor in which one of the energizing currents is
+induced in any way from the other.</p>
+
+<p>Let <small>A B</small> represent the poles of an alternating current motor, of
+which <small>C</small> is the armature wound with coils <small>D</small>, closed upon themselves,
+as is now the general practice in motors of this kind. The
+poles <small>A</small>, which alternate with poles <small>B</small>, are wound with coils of
+ordinary or coarse wire <small>E</small> in such direction as to make them of
+alternate north and south polarity, as indicated in the diagram
+by the characters <small>N S</small>. Over these coils, or in other inductive relation
+to the same, are wound long fine-wire coils <small>F F</small>, and in the
+same direction throughout as the coils <small>E</small>. These coils are secondaries,
+in which currents of very high potential are induced. All
+the coils <small>E</small> in one series are connected, and all the secondaries <small>F</small>
+in another.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_121.jpg" width="600" height="609" alt="Fig. 91." title="" />
+<span class="caption">Fig. 91.</span>
+</div>
+
+<p>On the intermediate poles <small>B</small> are wound fine-wire energizing
+coils <small>G</small>, which are connected in series with one another, and also
+with the series of secondary coils <small>F</small>, the direction of winding being
+such that a current-impulse induced from the primary coils
+<small>E</small> imparts the same magnetism to the poles <small>B</small> as that produced<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span>
+in poles <small>A</small> by the primary impulse. This condition is indicated
+by the characters <small>N' S'</small>.</p>
+
+<p>In the circuit formed by the two sets of coils <small>F</small> and <small>G</small> is introduced
+a condenser <small>H</small>; otherwise this circuit is closed upon
+itself, while the free ends of the circuit of coils <small>E</small> are connected
+to a source of alternating currents. As the condenser capacity
+which is needed in any particular motor of this kind is dependent
+upon the rate of alternation or the potential, or both, its size
+or cost, as before explained, may be brought within economical
+limits for use with the ordinary circuits if the potential of the
+secondary circuit in the motor be sufficiently high. By giving
+to the condenser proper values, any desired difference of phase
+between the primary and secondary energizing circuits may be
+obtained.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span></p>
+<h2><a name="CHAPTER_XXIII" id="CHAPTER_XXIII"></a>CHAPTER XXIII.</h2>
+
+<h3><span class="smcap">Tesla Polyphase Transformer.</span></h3>
+
+
+<p>Applying the polyphase principle to the construction of transformers
+as well to the motors already noticed, Mr. Tesla has invented
+some very interesting forms, which he considers free
+from the defects of earlier and, at present, more familiar forms.
+In these transformers he provides a series of inducing coils and
+corresponding induced coils, which are generally wound upon a
+core closed upon itself, usually a ring of laminated iron.</p>
+
+<p>The two sets of coils are wound side by side or superposed or
+otherwise placed in well-known ways to bring them into the most
+effective relations to one another and to the core. The inducing
+or primary coils wound on the core are divided into pairs or sets
+by the proper electrical connections, so that while the coils of
+one pair or set co-operate in fixing the magnetic poles of the
+core at two given diametrically opposite points, the coils of the
+other pair or set&mdash;assuming, for sake of illustration, that there
+are but two&mdash;tend to fix the poles ninety degrees from such
+points. With this induction device is used an alternating current
+generator with coils or sets of coils to correspond with those of
+the converter, and the corresponding coils of the generator and
+converter are then connected up in independent circuits. It results
+from this that the different electrical phases in the generator
+are attended by corresponding magnetic changes in the converter;
+or, in other words, that as the generator coils revolve,
+the points of greatest magnetic intensity in the converter will be
+progressively shifted or whirled around.</p>
+
+<p>Fig. 92 is a diagrammatic illustration of the converter and the
+electrical connections of the same. Fig. 93 is a horizontal central
+cross-section of Fig. 92. Fig. 94 is a diagram of the circuits
+of the entire system, the generator being shown in section.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_124.jpg" width="600" height="663" alt="Fig. 92 and 93." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 92 and 93.</span>
+</div>
+
+<p>Mr. Tesla uses a core, <small>A</small>, which is closed upon itself&mdash;that is to
+say, of an annular cylindrical or equivalent form&mdash;and as the
+efficiency of the apparatus is largely increased by the subdivision<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span>
+of this core, he makes it of thin strips, plates, or wires of soft
+iron electrically insulated as far as practicable. Upon this core
+are wound, say, four coils, <small>B B B' B'</small>, used as primary coils, and for
+which long lengths of comparatively fine wire are employed.
+Over these coils are then wound shorter coils of coarser wire, <small>C C
+C' C'</small>, to constitute the induced or secondary coils. The construction
+of this or any equivalent form of converter may be carried
+further, as above pointed out, by inclosing these coils with iron&mdash;as,
+for example, by winding over the coils layers of insulated
+iron wire.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_125.jpg" width="800" height="469" alt="Fig. 94." title="" />
+<span class="caption">Fig. 94.</span>
+</div>
+
+<p>The device is provided with suitable binding posts, to which
+the ends of the coils are led. The diametrically opposite coils
+<small>B B</small> and <small>B' B'</small> are connected, respectively, in series, and the four
+terminals are connected to the binding posts. The induced
+coils are connected together in any desired manner. For example,
+as shown in Fig. 94, <small>C C</small> may be connected in multiple
+arc when a quantity current is desired&mdash;as for running a group
+of incandescent lamps&mdash;while <small>C' C'</small> may be independently connected
+in series in a circuit including arc lamps or the like. The
+generator in this system will be adapted to the converter in the
+<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span>manner illustrated. For example, in the present case there are
+employed a pair of ordinary permanent or electro-magnets, <small>E E</small>,
+between which is mounted a cylindrical armature on a shaft, <small>F</small>,
+and wound with two coils, <small>G G'</small>. The terminals of these coils are
+connected, respectively, to four insulated contact or collecting
+rings, <small>H H H' H'</small>, and the four line circuit wires <small>L</small> connect the
+brushes <small>K</small>, bearing on these rings, to the converter in the order
+shown. Noting the results of this combination, it will be observed
+that at a given point of time the coil <small>G</small> is in its neutral
+position and is generating little or no current, while the other
+coil, <small>G'</small>, is in a position where it exerts its maximum effect.
+Assuming coil <small>G</small> to be connected in circuit with coils <small>B B</small> of the
+converter, and coil <small>G'</small> with coils <small>B' B'</small>, it is evident that the poles
+of the ring <small>A</small> will be determined by coils <small>B' B'</small> alone; but as the
+armature of the generator revolves, coil <small>G</small> develops more current
+and coil <small>G'</small> less, until <small>G</small> reaches its maximum and <small>G'</small> its neutral
+position. The obvious result will be to shift the poles of the
+ring <small>A</small> through one-quarter of its periphery. The movement of
+the coils through the next quarter of a turn&mdash;during which coil
+<small>G'</small> enters a field of opposite polarity and generates a current of
+opposite direction and increasing strength, while coil <small>G</small>, in passing
+from its maximum to its neutral position generates a current of
+decreasing strength and same direction as before&mdash;causes a further
+shifting of the poles through the second quarter of the ring.
+The second half-revolution will obviously be a repetition of the
+same action. By the shifting of the poles of the ring <small>A</small>, a power<span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span>ful
+dynamic inductive effect on the coils <small>C C'</small> is produced. Besides
+the currents generated in the secondary coils by dynamo-magnetic
+induction, other currents will be set up in the same
+coils in consequence of many variations in the intensity of the
+poles in the ring <small>A</small>. This should be avoided by maintaining the
+intensity of the poles constant, to accomplish which care should
+be taken in designing and proportioning the generator and in
+distributing the coils in the ring <small>A</small>, and balancing their effect.
+When this is done, the currents are produced by dynamo-magnetic
+induction only, the same result being obtained as though
+the poles were shifted by a commutator with an infinite number
+of segments.</p>
+
+<p>The modifications which are applicable to other forms of converter
+are in many respects applicable to this, such as those pertaining
+more particularly to the form of the core, the relative
+lengths and resistances of the primary and secondary coils, and
+the arrangements for running or operating the same.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span></p>
+<h2><a name="CHAPTER_XXIV" id="CHAPTER_XXIV"></a>CHAPTER XXIV.</h2>
+
+<h3><span class="smcap">A Constant Current Transformer with Magnetic Shield
+Between Coils of Primary and Secondary.</span></h3>
+
+
+<p>Mr. Tesla has applied his principle of magnetic shielding of
+parts to the construction also of transformers, the shield being
+interposed between the primary and secondary coils. In transformers
+of the ordinary type it will be found that the wave of
+electromotive force of the secondary very nearly coincides with
+that of the primary, being, however, in opposite sign. At the same
+time the currents, both primary and secondary, lag behind their
+respective electromotive forces; but as this lag is practically or
+nearly the same in the case of each it follows that the maximum
+and minimum of the primary and secondary currents will nearly
+coincide, but differ in sign or direction, provided the secondary
+be not loaded or if it contain devices having the property of
+self-induction. On the other hand, the lag of the primary
+behind the impressed electromotive force may be diminished by
+loading the secondary with a non-inductive or dead resistance&mdash;such
+as incandescent lamps&mdash;whereby the time interval between
+the maximum or minimum periods of the primary and secondary
+currents is increased. This time interval, however, is limited,
+and the results obtained by phase difference in the operation of
+such devices as the Tesla alternating current motors can only be
+approximately realized by such means of producing or securing
+this difference, as above indicated, for it is desirable in such cases
+that there should exist between the primary and secondary currents,
+or those which, however produced, pass through the two
+circuits of the motor, a difference of phase of ninety degrees;
+or, in other words, the current in one circuit should be a maximum
+when that in the other circuit is a minimum. To attain
+to this condition more perfectly, an increased retardation of the
+secondary current is secured in the following manner: Instead
+of bringing the primary and secondary coils or circuits of a
+transformer into the closest possible relations, as has hitherto<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span>
+been done, Mr. Tesla protects in a measure the secondary from
+the inductive action or effect of the primary by surrounding
+either the primary or the secondary with a comparatively thin
+magnetic shield or screen. Under these modified conditions,
+as long as the primary current has a small value, the shield
+protects the secondary; but as soon as the primary current
+has reached a certain strength, which is arbitrarily determined,
+the protecting magnetic shield becomes saturated and the inductive
+action upon the secondary begins. It results, therefore, that
+the secondary current begins to flow at a certain fraction of a
+period later than it would without the interposed shield, and
+since this retardation may be obtained without necessarily retarding
+the primary current also, an additional lag is secured, and
+the time interval between the maximum or minimum periods of
+the primary and secondary currents is increased. Such a transformer
+may, by properly proportioning its several elements and
+determining the proper relations between the primary and
+secondary windings, the thickness of the magnetic shield, and
+other conditions, be constructed to yield a constant current at all
+loads.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_128.jpg" width="640" height="387" alt="Fig. 95." title="" />
+<span class="caption">Fig. 95.</span>
+</div>
+
+
+<p>Fig. 95 is a cross-section of a transformer embodying this improvement.
+Fig. 96 is a similar view of a modified form of
+transformer, showing diagrammatically the manner of using the
+same.</p>
+
+<p><small>A A</small> is the main core of the transformer, composed of a ring
+of soft annealed and insulated or oxidized iron wire. Upon this
+core is wound the secondary circuit or coil <small>B B</small>. This latter is
+then covered with a layer or layers of annealed and insulated
+iron wires <small>C C</small>, wound in a direction at right angles to the secondary<span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span>
+coil. Over the whole is then wound the primary coil or wire <small>D D</small>.
+From the nature of this construction it will be obvious that
+as long as the shield formed by the wires <small>C</small> is below magnetic
+saturation the secondary coil or circuit is effectually protected or
+shielded from the inductive influence of the primary, although
+on open circuit it may exhibit some electromotive force. When
+the strength of the primary reaches a certain value, the shield <small>C</small>,
+becoming saturated, ceases to protect the secondary from inductive
+action, and current is in consequence developed therein.
+For similar reasons, when the primary current weakens, the
+weakening of the secondary is retarded to the same or approximately
+the same extent.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_129.jpg" width="800" height="445" alt="Fig. 96." title="" />
+<span class="caption">Fig. 96.</span>
+</div>
+
+<p>The specific construction of the transformer is largely immaterial.
+In Fig. 96, for example, the core <small>A</small> is built up of thin
+insulated iron plates or discs. The primary circuit <small>D</small> is wound
+next the core <small>A</small>. Over this is applied the shield <small>C</small>, which in this
+case is made up of thin strips or plates of iron properly insulated
+and surrounding the primary, forming a closed magnetic circuit.
+The secondary <small>B</small> is wound over the shield <small>C</small>. In Fig. 96, also,
+<small>E</small> is a source of alternating or rapidly changing currents.
+The primary of the transformer is connected with the circuit of
+the generator. <small>F</small> is a two-circuit alternating current motor, one
+of the circuits being connected with the main circuit from the
+source <small>E</small>, and the other being supplied with currents from the
+secondary of the transformer.</p>
+<p><span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span></p>
+<h1><small><a name="PART_II" id="PART_II"></a>PART II.</small><br /><br />
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY<br />
+AND HIGH POTENTIAL CURRENTS.</h1>
+<p><span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span></p>
+<h2><a name="CHAPTER_XXV" id="CHAPTER_XXV"></a>CHAPTER XXV.</h2>
+
+<h3><span class="smcap">Introduction.&mdash;The Scope of the Tesla Lectures.</span></h3>
+
+
+<p>Before proceeding to study the three Tesla lectures here
+presented, the reader may find it of some assistance to have his
+attention directed to the main points of interest and significance
+therein. The first of these lectures was delivered in New York,
+at Columbia College, before the American Institute of Electrical
+Engineers, May 20, 1891. The urgent desire expressed immediately
+from all parts of Europe for an opportunity to witness the
+brilliant and unusual experiments with which the lecture was
+accompanied, induced Mr. Tesla to go to England early in 1892,
+when he appeared before the Institution of Electrical Engineers,
+and a day later, by special request, before the Royal Institution.
+His reception was of the most enthusiastic and flattering nature on
+both occasions. He then went, by invitation, to France, and repeated
+his novel demonstrations before the Soci&eacute;t&eacute; Internationale
+des Electriciens, and the Soci&eacute;t&eacute; Fran&ccedil;aise de Physique. Mr. Tesla
+returned to America in the fall of 1892, and in February, 1893, delivered
+his third lecture before the Franklin Institute of Philadelphia,
+in fulfilment of a long standing promise to Prof. Houston.
+The following week, at the request of President James I. Ayer,
+of the National Electric Light Association, the same lecture was
+re-delivered in St. Louis. It had been intended to limit the invitations
+to members, but the appeals from residents in the city
+were so numerous and pressing that it became necessary to secure
+a very large hall. Hence it came about that the lecture was
+listened to by an audience of over 5,000 people, and was in some
+parts of a more popular nature than either of its predecessors.
+Despite this concession to the need of the hour and occasion, Mr.
+Tesla did not hesitate to show many new and brilliant experiments,
+and to advance the frontier of discovery far beyond any
+point he had theretofore marked publicly.</p>
+
+<p>We may now proceed to a running review of the lectures themselves.
+The ground covered by them is so vast that only the<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span>
+leading ideas and experiments can here be touched upon; besides,
+it is preferable that the lectures should be carefully gone over for
+their own sake, it being more than likely that each student will
+discover a new beauty or stimulus in them. Taking up the
+course of reasoning followed by Mr. Tesla in his first lecture, it
+will be noted that he started out with the recognition of the fact,
+which he has now experimentally demonstrated, that for the production
+of light waves, primarily, electrostatic effects must be
+brought into play, and continued study has led him to the opinion
+that all electrical and magnetic effects may be referred to electrostatic
+molecular forces. This opinion finds a singular confirmation
+in one of the most striking experiments which he
+describes, namely, the production of a veritable flame by the
+agitation of electrostatically charged molecules. It is of the
+highest interest to observe that this result points out a way of
+obtaining a flame which consumes no material and in which no
+chemical action whatever takes place. It also throws a light on
+the nature of the ordinary flame, which Mr. Tesla believes to be
+due to electrostatic molecular actions, which, if true, would lead
+directly to the idea that even chemical affinities might be electrostatic
+in their nature and that, as has already been suggested,
+molecular forces in general may be referable to one and the same
+cause. This singular phenomenon accounts in a plausible manner
+for the unexplained fact that buildings are frequently set on
+fire during thunder storms without having been at all struck by
+lightning. It may also explain the total disappearance of ships
+at sea.</p>
+
+<p>One of the striking proofs of the correctness of the ideas advanced
+by Mr. Tesla is the fact that, notwithstanding the employment
+of the most powerful electromagnetic inductive effects, but
+feeble luminosity is obtainable, and this only in close proximity
+to the source of disturbance; whereas, when the electrostatic
+effects are intensified, the same initial energy suffices to excite
+luminosity at considerable distances from the source. That there
+are only electrostatic effects active seems to be clearly proved by
+Mr. Tesla's experiments with an induction coil operated with
+alternating currents of very high frequency. He shows how
+tubes may be made to glow brilliantly at considerable distances
+from any object when placed in a powerful, rapidly alternating,
+electrostatic field, and he describes many interesting phenomena
+observed in such a field. His experiments open up the possibility<span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span>
+of lighting an apartment by simply creating in it such an electrostatic
+field, and this, in a certain way, would appear to be the
+ideal method of lighting a room, as it would allow the illuminating
+device to be freely moved about. The power with which
+these exhausted tubes, devoid of any electrodes, light up is certainly
+remarkable.</p>
+
+<p>That the principle propounded by Mr. Tesla is a broad one is
+evident from the many ways in which it may be practically applied.
+We need only refer to the variety of the devices shown
+or described, all of which are novel in character and will, without
+doubt, lead to further important results at the hands of Mr.
+Tesla and other investigators. The experiment, for instance, of
+lighting up a single filament or block of refractory material with
+a single wire, is in itself sufficient to give Mr. Tesla's work the
+stamp of originality, and the numerous other experiments and
+effects which may be varied at will, are equally new and interesting.
+Thus, the incandescent filament spinning in an unexhausted
+globe, the well-known Crookes experiment on open circuit,
+and the many others suggested, will not fail to interest the
+reader. Mr. Tesla has made an exhaustive study of the various
+forms of the discharge presented by an induction coil when operated
+with these rapidly alternating currents, starting from the
+thread-like discharge and passing through various stages to the
+true electric flame.</p>
+
+<p>A point of great importance in the introduction of high tension
+alternating current which Mr. Tesla brings out is the necessity
+of carefully avoiding all gaseous matter in the high tension
+apparatus. He shows that, at least with very rapidly alternating
+currents of high potential, the discharge may work through almost
+any practicable thickness of the best insulators, if air is
+present. In such cases the air included within the apparatus is
+violently agitated and by molecular bombardment the parts may
+be so greatly heated as to cause a rupture of the insulation.
+The practical outcome of this is, that, whereas with steady currents,
+any kind of insulation may be used, with rapidly alternating
+currents oils will probably be the best to employ, a fact
+which has been observed, but not until now satisfactorily explained.
+The recognition of the above fact is of special importance
+in the construction of the costly commercial induction coils
+which are often rendered useless in an unaccountable manner.
+The truth of these views of Mr. Tesla is made evident by the in<span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span>teresting
+experiments illustrative of the behavior of the air between
+charged surfaces, the luminous streams formed by the
+charged molecules appearing even when great thicknesses of the
+best insulators are interposed between the charged surfaces.
+These luminous streams afford in themselves a very interesting
+study for the experimenter. With these rapidly alternating currents
+they become far more powerful and produce beautiful light
+effects when they issue from a wire, pinwheel or other object attached
+to a terminal of the coil; and it is interesting to note that
+they issue from a ball almost as freely as from a point, when the
+frequency is very high.</p>
+
+<p>From these experiments we also obtain a better idea of the
+importance of taking into account the capacity and self-induction
+in the apparatus employed and the possibilities offered by the
+use of condensers in conjunction with alternate currents, the employment
+of currents of high frequency, among other things,
+making it possible to reduce the condenser to practicable dimensions.
+Another point of interest and practical bearing is the
+fact, proved by Mr. Tesla, that for alternate currents, especially
+those of high frequency, insulators are required possessing a
+small specific inductive capacity, which at the same time have a
+high insulating power.</p>
+
+<p>Mr. Tesla also makes interesting and valuable suggestion in regard
+to the economical utilization of iron in machines and transformers.
+He shows how, by maintaining by continuous magnetization
+a flow of lines through the iron, the latter may be kept
+near its maximum permeability and a higher output and economy
+may be secured in such apparatus. This principle may prove of
+considerable commercial importance in the development of alternating
+systems. Mr. Tesla's suggestion that the same result can
+be secured by heating the iron by hysteresis and eddy currents,
+and increasing the permeability in this manner, while it may appear
+less practical, nevertheless opens another direction for investigation
+and improvement.</p>
+
+<p>The demonstration of the fact that with alternating currents
+of high frequency, sufficient energy may be transmitted under
+practicable conditions through the glass of an incandescent lamp
+by electrostatic or electromagnetic induction may lead to a departure
+in the construction of such devices. Another important
+experimental result achieved is the operation of lamps, and even
+motors, with the discharges of condensers, this method affording<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span>
+a means of converting direct or alternating currents. In this
+connection Mr. Tesla advocates the perfecting of apparatus capable
+of generating electricity of high tension from heat energy,
+believing this to be a better way of obtaining electrical energy
+for practical purposes, particularly for the production of light.</p>
+
+<p>While many were probably prepared to encounter curious
+phenomena of impedance in the use of a condenser discharged
+disruptively, the experiments shown were extremely interesting
+on account of their paradoxical character. The burning of an
+incandescent lamp at any candle power when connected across a
+heavy metal bar, the existence of nodes on the bar and the possibility
+of exploring the bar by means of an ordinary Cardew
+voltmeter, are all peculiar developments, but perhaps the most
+interesting observation is the phenomenon of impedance observed
+in the lamp with a straight filament, which remains dark while
+the bulb glows.</p>
+
+<p>Mr. Tesla's manner of operating an induction coil by means of
+the disruptive discharge, and thus obtaining enormous differences
+of potential from comparatively small and inexpensive coils, will
+be appreciated by experimenters and will find valuable application
+in laboratories. Indeed, his many suggestions and hints in
+regard to the construction and use of apparatus in these investigations
+will be highly valued and will aid materially in future
+research.</p>
+
+<p>The London lecture was delivered twice. In its first form,
+before the Institution of Electrical Engineers, it was in some
+respects an amplification of several points not specially enlarged
+upon in the New York lecture, but brought forward many additional
+discoveries and new investigations. Its repetition, in
+another form, at the Royal Institution, was due to Prof. Dewar,
+who with Lord Rayleigh, manifested a most lively interest in Mr.
+Tesla's work, and whose kindness illustrated once more the strong
+English love of scientific truth and appreciation of its votaries.
+As an indefatigable experimenter, Mr. Tesla was certainly nowhere
+more at home than in the haunts of Faraday, and as the
+guest of Faraday's successor. This Royal Institution lecture
+summed up the leading points of Mr. Tesla's work, in the high
+potential, high frequency field, and we may here avail ourselves
+of so valuable a summarization, in a simple form, of a subject by
+no means easy of comprehension until it has been thoroughly
+studied.<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span></p>
+
+<p>In these London lectures, among the many notable points made
+was first, the difficulty of constructing the alternators to obtain
+the very high frequencies needed. To obtain the high frequencies
+it was necessary to provide several hundred polar projections,
+which were necessarily small and offered many drawbacks,
+and this the more as exceedingly high peripheral speeds
+had to be resorted to. In some of the first machines both armature
+and field had polar projections. These machines produced
+a curious noise, especially when the armature was started from
+the state of rest, the field being charged. The most efficient
+machine was found to be one with a drum armature, the iron
+body of which consisted of very thin wire annealed with special
+care. It was, of course, desirable to avoid the employment of
+iron in the armature, and several machines of this kind, with
+moving or stationary conductors were constructed, but the results
+obtained were not quite satisfactory, on account of the
+great mechanical and other difficulties encountered.</p>
+
+<p>The study of the properties of the high frequency currents
+obtained from these machines is very interesting, as nearly every
+experiment discloses something new. Two coils traversed by
+such a current attract or repel each other with a force which,
+owing to the imperfection of our sense of touch, seems continuous.
+An interesting observation, already noted under another
+form, is that a piece of iron, surrounded by a coil through which
+the current is passing appears to be continuously magnetized.
+This apparent continuity might be ascribed to the deficiency of
+the sense of touch, but there is evidence that in currents of such
+high frequencies one of the impulses preponderates over the
+other.</p>
+
+<p>As might be expected, conductors traversed by such currents
+are rapidly heated, owing to the increase of the resistance, and
+the heating effects are relatively much greater in the iron.
+The hysteresis losses in iron are so great that an iron core,
+even if finely subdivided, is heated in an incredibly short time.
+To give an idea of this, an ordinary iron wire 1/16 inch in
+diameter inserted within a coil having 250 turns, with a current
+estimated to be five amperes passing through the coil, becomes
+within two seconds' time so hot as to scorch wood. Beyond a
+certain frequency, an iron core, no matter how finely subdivided,
+exercises a dampening effect, and it was easy to find a point at<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span>
+which the impedance of a coil was not affected by the presence
+of a core consisting of a bundle of very thin well annealed and
+varnished iron wires.</p>
+
+<p>Experiments with a telephone, a conductor in a strong magnetic
+field, or with a condenser or arc, seem to afford certain
+proof that sounds far above the usually accepted limit of hearing
+would be perceived if produced with sufficient power. The arc
+produced by these currents possesses several interesting features.
+Usually it emits a note the pitch of which corresponds to twice
+the frequency of the current, but if the frequency be sufficiently
+high it becomes noiseless, the limit of audition being determined
+principally by the linear dimensions of the arc. A curious feature
+of the arc is its persistency, which is due partly to the inability
+of the gaseous column to cool and increase considerably
+in resistance, as is the case with low frequencies, and partly to
+the tendency of such a high frequency machine to maintain a
+constant current.</p>
+
+<p>In connection with these machines the condenser affords a particularly
+interesting study. Striking effects are produced by
+proper adjustments of capacity and self-induction. It is easy to
+raise the electromotive force of the machine to many times the
+original value by simply adjusting the capacity of a condenser
+connected in the induced circuit. If the condenser be at some
+distance from the machine, the difference of potential on the
+terminals of the latter may be only a small fraction of that on
+the condenser.</p>
+
+<p>But the most interesting experiences are gained when the tension
+of the currents from the machine is raised by means of an
+induction coil. In consequence of the enormous rate of change
+obtainable in the primary current, much higher potential differences
+are obtained than with coils operated in the usual ways,
+and, owing to the high frequency, the secondary discharge possesses
+many striking peculiarities. Both the electrodes behave
+generally alike, though it appears from some observations that
+one current impulse preponderates over the other, as before
+mentioned.</p>
+
+<p>The physiological effects of the high tension discharge are
+found to be so small that the shock of the coil can be supported
+without any inconvenience, except perhaps a small burn produced
+by the discharge upon approaching the hand to one of the terminals.
+The decidedly smaller physiological effects of these cur<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span>rents
+are thought to be due either to a different distribution
+through the body or to the tissues acting as condensers. But in
+the case of an induction coil with a great many turns the harmlessness
+is principally due to the fact that but little energy is available
+in the external circuit when the same is closed through the
+experimenter's body, on account of the great impedance of the
+coil.</p>
+
+<p>In varying the frequency and strength of the currents through
+the primary of the coil, the character of the secondary discharge
+is greatly varied, and no less than five distinct forms are observed:&mdash;A
+weak, sensitive thread discharge, a powerful flaming
+discharge, and three forms of brush or streaming discharges.
+Each of these possesses certain noteworthy features, but the most
+interesting to study are the latter.</p>
+
+<p>Under certain conditions the streams, which are presumably
+due to the violent agitation of the air molecules, issue freely
+from all points of the coil, even through a thick insulation. If
+there is the smallest air space between the primary and secondary,
+they will form there and surely injure the coil by slowly warming
+the insulation. As they form even with ordinary frequencies
+when the potential is excessive, the air-space must be most carefully
+avoided. These high frequency streamers differ in aspect
+and properties from those produced by a static machine. The
+wind produced by them is small and should altogether cease if
+still considerably higher frequencies could be obtained. A peculiarity
+is that they issue as freely from surfaces as from points.
+Owing to this, a metallic vane, mounted in one of the terminals of
+the coil so as to rotate freely, and having one of its sides covered
+with insulation, is spun rapidly around. Such a vane would not
+rotate with a steady potential, but with a high frequency coil it
+will spin, even if it be entirely covered with insulation, provided
+the insulation on one side be either thicker or of a higher specific
+inductive capacity. A Crookes electric radiometer is also spun
+around when connected to one of the terminals of the coil, but
+only at very high exhaustion or at ordinary pressures.</p>
+
+<p>There is still another and more striking peculiarity of such a
+high frequency streamer, namely, it is hot. The heat is easily
+perceptible with frequencies of about 10,000, even if the potential
+is not excessively high. The heating effect is, of course, due
+to the molecular impacts and collisions. Could the frequency
+and potential be pushed far enough, then a brush could be pro<span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span>duced
+resembling in every particular a flame and giving light
+and heat, yet without a chemical process taking place.</p>
+
+<p>The hot brush, when properly produced, resembles a jet of
+burning gas escaping under great pressure, and it emits an extraordinary
+strong smell of ozone. The great ozonizing action is
+ascribed to the fact that the agitation of the molecules of the air
+is more violent in such a brush than in the ordinary streamer of
+a static machine. But the most powerful brush discharges were
+produced by employing currents of much higher frequencies than
+it was possible to obtain by means of the alternators. These
+currents were obtained by disruptively discharging a condenser
+and setting up oscillations. In this manner currents of a frequency
+of several hundred thousand were obtained.</p>
+
+<p>Currents of this kind, Mr. Tesla pointed out, produce striking
+effects. At these frequencies, the impedance of a copper bar is
+so great that a potential difference of several hundred volts can
+be maintained between two points of a short and thick bar, and
+it is possible to keep an ordinary incandescent lamp burning at
+full candle power by attaching the terminals of the lamp to two
+points of the bar no more than a few inches apart. When the
+frequency is extremely high, nodes are found to exist on such a
+bar, and it is easy to locate them by means of a lamp.</p>
+
+<p>By converting the high tension discharges of a low frequency
+coil in this manner, it was found practicable to keep a few lamps
+burning on the ordinary circuit in the laboratory, and by bringing
+the undulation to a low pitch, it was possible to operate small
+motors.</p>
+
+<p>This plan likewise allows of converting high tension discharges
+of one direction into low tension unidirectional currents, by adjusting
+the circuit so that there are no oscillations. In passing
+the oscillating discharges through the primary of a specially
+constructed coil, it is easy to obtain enormous potential differences
+with only few turns of the secondary.</p>
+
+<p>Great difficulties were at first experienced in producing a successful
+coil on this plan. It was found necessary to keep all air,
+or gaseous matter in general, away from the charged surfaces,
+and oil immersion was resorted to. The wires used were heavily
+covered with gutta-percha and wound in oil, or the air was pumped
+out by means of a Sprengel pump. The general arrangement
+was the following:&mdash;An ordinary induction coil, operated from
+a low frequency alternator, was used to charge Leyden jars. The<span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+jars were made to discharge over a single or multiple gap through
+the primary of the second coil. To insure the action of the gap,
+the arc was blown out by a magnet or air blast. To adjust the
+potential in the secondary a small oil condenser was used, or
+polished brass spheres of different sizes were screwed on the
+terminals and their distance adjusted.</p>
+
+<p>When the conditions were carefully determined to suit each
+experiment, magnificent effects were obtained. Two wires,
+stretched through the room, each being connected to one of the
+terminals of the coil, emitted streams so powerful that the light
+from them allowed distinguishing the objects in the room; the
+wires became luminous even though covered with thick and
+most excellent insulation. When two straight wires, or two concentric
+circles of wire, are connected to the terminals, and set at
+the proper distance, a uniform luminous sheet is produced between
+them. It was possible in this way to cover an area of
+more than one meter square completely with the streams. By
+attaching to one terminal a large circle of wire and to the other
+terminal a small sphere, the streams are focused upon the sphere,
+produce a strongly lighted spot upon the same, and present the
+appearance of a luminous cone. A very thin wire glued upon a
+plate of hard rubber of great thickness, on the opposite side of
+which is fastened a tinfoil coating, is rendered intensely luminous
+when the coating is connected to the other terminal of the coil.
+Such an experiment can be performed also with low frequency
+currents, but much less satisfactorily.</p>
+
+<p>When the terminals of such a coil, even of a very small one,
+are separated by a rubber or glass plate, the discharge spreads
+over the plate in the form of streams, threads or brilliant sparks,
+and affords a magnificent display, which cannot be equaled by
+the largest coil operated in the usual ways. By a simple adjustment
+it is possible to produce with the coil a succession of brilliant
+sparks, exactly as with a Holtz machine.</p>
+
+<p>Under certain conditions, when the frequency of the oscillation
+is very great, white, phantom-like streams are seen to break forth
+from the terminals of the coil. The chief interesting feature
+about them is, that they stream freely against the outstretched
+hand or other conducting object without producing any sensation,
+and the hand may be approached very near to the terminal
+without a spark being induced to jump. This is due presumably
+to the fact that a considerable portion of the energy is carried<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span>
+away or dissipated in the streamers, and the difference of potential
+between the terminal and the hand is diminished.</p>
+
+<p>It is found in such experiments that the frequency of the
+vibration and the quickness of succession of the sparks between
+the knobs affect to a marked degree the appearance of the
+streams. When the frequency is very low, the air gives way in
+more or less the same manner as by a steady difference of potential,
+and the streams consist of distinct threads, generally mingled
+with thin sparks, which probably correspond to the successive
+discharges occurring between the knobs. But when the frequency
+is very high, and the arc of the discharge produces a
+sound which is loud and smooth (which indicates both that oscillation
+takes place and that the sparks succeed each other with
+great rapidity), then the luminous streams formed are perfectly
+uniform. They are generally of a purplish hue, but when the
+molecular vibration is increased by raising the potential, they assume
+a white color.</p>
+
+<p>The luminous intensity of the streams increases rapidly when
+the potential is increased; and with frequencies of only a few
+hundred thousand, could the coil be made to withstand a sufficiently
+high potential difference, there is no doubt that the
+space around a wire could be made to emit a strong light,
+merely by the agitation of the molecules of the air at ordinary
+pressure.</p>
+
+<p>Such discharges of very high frequency which render luminous
+the air at ordinary pressure we have very likely occasion to
+witness in the aurora borealis. From many of these experiments
+it seems reasonable to infer that sudden cosmic disturbances,
+such as eruptions on the sun, set the electrostatic charge
+of the earth in an extremely rapid vibration, and produce the
+glow by the violent agitation of the air in the upper and even in
+the lower strata. It is thought that if the frequency were low,
+or even more so if the charge were not at all vibrating, the
+lower dense strata would break down as in a lightning discharge.
+Indications of such breaking down have been repeatedly observed,
+but they can be attributed to the fundamental disturbances,
+which are few in number, for the superimposed vibration
+would be so rapid as not to allow a disruptive break.</p>
+
+<p>The study of these discharge phenomena has led Mr. Tesla to
+the recognition of some important facts. It was found, as already
+stated, that gaseous matter must be most carefully excluded from<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span>
+any dielectric which is subjected to great, rapidly changing electrostatic
+stresses. Since it is difficult to exclude the gas perfectly
+when solid insulators are used, it is necessary to resort to liquid
+dielectrics. When a solid dielectric is used, it matters little how
+thick and how good it is; if air be present, streamers form,
+which gradually heat the dielectric and impair its insulating
+power, and the discharge finally breaks through. Under ordinary
+conditions the best insulators are those which possess the
+highest specific inductive capacity, but such insulators are not
+the best to employ when working with these high frequency
+currents, for in most cases the higher specific inductive capacity
+is rather a disadvantage. The prime quality of the insulating
+medium for these currents is continuity. For this reason principally
+it is necessary to employ liquid insulators, such as oils.
+If two metal plates, connected to the terminals of the coil, are
+immersed in oil and set a distance apart, the coil may be kept
+working for any length of time without a break occurring, or
+without the oil being warmed, but if air bubbles are introduced,
+they become luminous; the air molecules, by their impact
+against the oil, heat it, and after some time cause the insulation
+to give way. If, instead of the oil, a solid plate of the best
+dielectric, even several times thicker than the oil intervening
+between the metal plates, is inserted between the latter, the air
+having free access to the charged surfaces, the dielectric invariably
+is warmed and breaks down.</p>
+
+<p>The employment of oil is advisable or necessary even with low
+frequencies, if the potentials are such that streamers form, but
+only in such cases, as is evident from the theory of the action.
+If the potentials are so low that streamers do not form, then it
+is even disadvantageous to employ oil, for it may, principally by
+confining the heat, be the cause of the breaking down of the insulation.</p>
+
+<p>The exclusion of gaseous matter is not only desirable on account
+of the safety of the apparatus, but also on account of
+economy, especially in a condenser, in which considerable waste
+of power may occur merely owing to the presence of air, if the
+electric density on the charged surfaces is great.</p>
+
+<p>In the course of these investigations a phenomenon of special
+scientific interest was observed. It may be ranked among the
+brush phenomena, in fact it is a kind of brush which forms at, or
+near, a single terminal in high vacuum. In a bulb with a con<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span>ducting
+electrode, even if the latter be of aluminum, the brush
+has only a very short existence, but it can be preserved for a considerable
+length of time in a bulb devoid of any conducting electrode.
+To observe the phenomenon it is found best to employ a
+large spherical bulb having in its centre a small bulb supported
+on a tube sealed to the neck of the former. The large bulb being
+exhausted to a high degree, and the inside of the small bulb
+being connected to one of the terminals of the coil, under certain
+conditions there appears a misty haze around the small bulb,
+which, after passing through some stages, assumes the form of a
+brush, generally at right angles to the tube supporting the small
+bulb. When the brush assumes this form it may be brought to
+a state of extreme sensitiveness to electrostatic and magnetic influence.
+The bulb hanging straight down, and all objects being
+remote from it, the approach of the observer within a few paces
+will cause the brush to fly to the opposite side, and if he walks
+around the bulb it will always keep on the opposite side. It may
+begin to spin around the terminal long before it reaches that sensitive
+stage. When it begins to turn around, principally, but
+also before, it is affected by a magnet, and at a certain stage it is
+susceptible to magnetic influence to an astonishing degree. A
+small permanent magnet, with its poles at a distance of no more
+than two centimetres will affect it visibly at a distance of two metres,
+slowing down or accelerating the rotation according to how
+it is held relatively to the brush.</p>
+
+<p>When the bulb hangs with the globe down, the rotation is always
+clockwise. In the southern hemisphere it would occur in
+the opposite direction, and on the (magnetic) equator the brush
+should not turn at all. The rotation may be reversed by a magnet
+kept at some distance. The brush rotates best, seemingly,
+when it is at right angles to the lines of force of the earth. It
+very likely rotates, when at its maximum speed, in synchronism
+with the alternations, say, 10,000 times a second. The rotation
+can be slowed down or accelerated by the approach or recession
+of the observer, or any conducting body, but it cannot be reversed
+by putting the bulb in any position. Very curious experiments
+may be performed with the brush when in its most sensitive
+state. For instance, the brush resting in one position, the
+experimenter may, by selecting a proper position, approach the
+hand at a certain considerable distance to the bulb, and he may
+cause the brush to pass off by merely stiffening the muscles of<span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span>
+the arm, the mere change of configuration of the arm and the
+consequent imperceptible displacement being sufficient to disturb
+the delicate balance. When it begins to rotate slowly, and the
+hands are held at a proper distance, it is impossible to make even
+the slightest motion without producing a visible effect upon the
+brush. A metal plate connected to the other terminal of the coil
+affects it at a great distance, slowing down the rotation often to
+one turn a second.</p>
+
+<p>Mr. Tesla hopes that this phenomenon will prove a valuable
+aid in the investigation of the nature of the forces acting in an
+electrostatic or magnetic field. If there is any motion which is
+measurable going on in the space, such a brush would be apt to
+reveal it. It is, so to speak, a beam of light, frictionless, devoid
+of inertia. On account of its marvellous sensitiveness to electrostatic
+or magnetic disturbances it may be the means of sending
+signals through submarine cables with any speed, and even of
+transmitting intelligence to a distance without wires.</p>
+
+<p>In operating an induction coil with these rapidly alternating
+currents, it is astonishing to note, for the first time, the great
+importance of the relation of capacity, self-induction, and frequency
+as bearing upon the general result. The combined effect
+of these elements produces many curious effects. For instance,
+two metal plates are connected to the terminals and set at a small
+distance, so that an arc is formed between them. This arc <i>prevents</i>
+a strong current from flowing through the coil. If the arc
+be interrupted by the interposition of a glass plate, the capacity
+of the condenser obtained counteracts the self-induction, and a
+stronger current is made to pass. The effects of capacity are the
+most striking, for in these experiments, since the self-induction
+and frequency both are high, the critical capacity is very small,
+and need be but slightly varied to produce a very considerable
+change. The experimenter brings his body in contact with the
+terminals of the secondary of the coil, or attaches to one or both
+terminals insulated bodies of very small bulk, such as exhausted
+bulbs, and he produces a considerable rise or fall of potential on
+the secondary, and greatly affects the flow of the current through
+the primary coil.</p>
+
+<p>In many of the phenomena observed, the presence of the air,
+or, generally speaking, of a medium of a gaseous nature (using
+this term not to imply specific properties, but in contradistinction
+to homogeneity or perfect continuity) plays an important part,<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span>
+as it allows energy to be dissipated by molecular impact or bombardment.
+The action is thus explained:&mdash;When an insulated
+body connected to a terminal of the coil is suddenly charged to
+high potential, it acts inductively upon the surrounding air, or
+whatever gaseous medium there might be. The molecules or
+atoms which are near it are, of course, more attracted, and move
+through a greater distance than the further ones. When the
+nearest molecules strike the body they are repelled, and collisions
+occur at all distances within the inductive distance. It is now
+clear that, if the potential be steady, but little loss of energy can
+be caused in this way, for the molecules which are nearest to
+the body having had an additional charge imparted to them by
+contact, are not attracted until they have parted, if not with all,
+at least with most of the additional charge, which can be accomplished
+only after a great many collisions. This is inferred from
+the fact that with a steady potential there is but little loss in dry
+air. When the potential, instead of being steady, is alternating,
+the conditions are entirely different. In this case a rhythmical
+bombardment occurs, no matter whether the molecules after
+coming in contact with the body lose the imparted charge or
+not, and, what is more, if the charge is not lost, the impacts are
+all the more violent. Still, if the frequency of the impulses
+be very small, the loss caused by the impacts and collisions would
+not be serious unless the potential was excessive. But when
+extremely high frequencies and more or less high potentials are
+used, the loss may be very great. The total energy lost per unit
+of time is proportionate to the product of the number of impacts
+per second, or the frequency and the energy lost in each impact.
+But the energy of an impact must be proportionate to the square
+of the electric density of the body, on the assumption that the
+charge imparted to the molecule is proportionate to that density.
+It is concluded from this that the total energy lost must be proportionate
+to the product of the frequency and the square of the
+electric density; but this law needs experimental confirmation.
+Assuming the preceding considerations to be true, then, by rapidly
+alternating the potential of a body immersed in an insulating
+gaseous medium, any amount of energy may be dissipated
+into space. Most of that energy, then, is not dissipated in the
+form of long ether waves, propagated to considerable distance,
+as is thought most generally, but is consumed in impact and
+collisional losses&mdash;that is, heat vibrations&mdash;on the surface and in<span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span>
+the vicinity of the body. To reduce the dissipation it is necessary
+to work with a small electric density&mdash;the smaller, the
+higher the frequency.</p>
+
+<p>The behavior of a gaseous medium to such rapid alternations
+of potential makes it appear plausible that electrostatic disturbances
+of the earth, produced by cosmic events, may have
+great influence upon the meteorological conditions. When such
+disturbances occur both the frequency of the vibrations of the
+charge and the potential are in all probability excessive, and the
+energy converted into heat may be considerable. Since the
+density must be unevenly distributed, either in consequence of
+the irregularity of the earth's surface, or on account of the
+condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable
+variations in the temperature and pressure of the atmosphere
+may in this manner be caused at any point of the surface of the
+earth. The variations may be gradual or very sudden, according
+to the nature of the original disturbance, and may produce rain
+and storms, or locally modify the weather in any way.</p>
+
+<p>From many experiences gathered in the course of these investigations
+it appears certain that in lightning discharges the air is
+an element of importance. For instance, during a storm a
+stream may form on a nail or pointed projection of a building.
+If lightning strikes somewhere in the neighborhood, the harmless
+static discharge may, in consequence of the oscillations set
+up, assume the character of a high-frequency streamer, and the
+nail or projection may be brought to a high temperature by the
+violent impact of the air molecules. Thus, it is thought, a
+building may be set on fire without the lightning striking it. In
+like manner small metallic objects may be fused and volatilized&mdash;as
+frequently occurs in lightning discharges&mdash;merely because
+they are surrounded by air. Were they immersed in a practically
+continuous medium, such as oil, they would probably be
+safe, as the energy would have to spend itself elsewhere.</p>
+
+<p>An instructive experience having a bearing on this subject is
+the following:&mdash;A glass tube of an inch or so in diameter and
+several inches long is taken, and a platinum wire sealed into it,
+the wire running through the center of the tube from end to
+end. The tube is exhausted to a moderate degree. If a steady
+current is passed through the wire it is heated uniformly in all
+parts and the gas in the tube is of no consequence. But if high<span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span>
+frequency discharges are directed through the wire, it is heated
+more on the ends than in the middle portion, and if the frequency,
+or rate of charge, is high enough, the wire might as
+well be cut in the middle as not, for most of the heating on the
+ends is due to the rarefied gas. Here the gas might only act as
+a conductor of no impedance, diverting the current from the
+wire as the impedance of the latter is enormously increased, and
+merely heating the ends of the wire by reason of their resistance
+to the passage of the discharge. But it is not at all necessary that
+the gas in the tube should be conducting; it might be at an extremely
+low pressure, still the ends of the wire would be heated;
+however, as is ascertained by experience, only the two ends
+would in such case not be electrically connected through the
+gaseous medium. Now, what with these frequencies and potentials
+occurs in an exhausted tube, occurs in the lightning discharge
+at ordinary pressure.</p>
+
+<p>From the facility with which any amount of energy may be
+carried off through a gas, Mr. Tesla infers that the best way to
+render harmless a lightning discharge is to afford it in some way
+a passage through a volume of gas.</p>
+
+<p>The recognition of some of the above facts has a bearing upon
+far-reaching scientific investigations in which extremely high
+frequencies and potentials are used. In such cases the air is an
+important factor to be considered. So, for instance, if two wires
+are attached to the terminals of the coil, and the streamers issue
+from them, there is dissipation of energy in the form of heat
+and light, and the wires behave like a condenser of larger capacity.
+If the wires be immersed in oil, the dissipation of energy
+is prevented, or at least reduced, and the apparent capacity is
+diminished. The action of the air would seem to make it very
+difficult to tell, from the measured or computed capacity of a
+condenser in which the air is acted upon, its actual capacity or
+vibration period, especially if the condenser is of very small surface
+and is charged to a very high potential. As many important
+results are dependant upon the correctness of the estimation
+of the vibration period, this subject demands the most careful
+scrutiny of investigators.</p>
+
+<p>In Leyden jars the loss due to the presence of air is comparatively
+small, principally on account of the great surface of the
+coatings and the small external action, but if there are streamers
+on the top, the loss may be considerable, and the period of vibra<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span>tion
+is affected. In a resonator, the density is small, but the
+frequency is extreme, and may introduce a considerable error.
+It appears certain, at any rate, that the periods of vibration of a
+charged body in a gaseous and in a continuous medium, such
+as oil, are different, on account of the action of the former, as
+explained.</p>
+
+<p>Another fact recognized, which is of some consequence, is,
+that in similar investigations the general considerations of static
+screening are not applicable when a gaseous medium is present.
+This is evident from the following experiment:&mdash;A short and
+wide glass tube is taken and covered with a substantial coating of
+bronze powder, barely allowing the light to shine a little through.
+The tube is highly exhausted and suspended on a metallic clasp
+from the end of a wire. When the wire is connected with one
+of the terminals of the coil, the gas inside of the tube is lighted
+in spite of the metal coating. Here the metal evidently does
+not screen the gas inside as it ought to, even if it be very thin
+and poorly conducting. Yet, in a condition of rest the metal
+coating, however thin, screens the inside perfectly.</p>
+
+<p>One of the most interesting results arrived at in pursuing these
+experiments, is the demonstration of the fact that a gaseous medium,
+upon which vibration is impressed by rapid changes of
+electrostatic potential, is rigid. In illustration of this result an
+experiment made by Mr. Tesla may by cited:&mdash;A glass tube about
+one inch in diameter and three feet long, with outside condenser
+coatings on the ends, was exhausted to a certain point, when, the
+tube being suspended freely from a wire connecting the upper coating
+to one of the terminals of the coil, the discharge appeared in
+the form of a luminous thread passing through the axis of the tube.
+Usually the thread was sharply defined in the upper part of the
+tube and lost itself in the lower part. When a magnet or the
+finger was quickly passed near the upper part of the luminous
+thread, it was brought out of position by magnetic or electrostatic
+influence, and a transversal vibration like that of a suspended
+cord, with one or more distinct nodes, was set up, which
+lasted for a few minutes and gradually died out. By suspending
+from the lower condenser coating metal plates of different sizes,
+the speed of the vibration was varied. This vibration would
+seem to show beyond doubt that the thread possessed rigidity,
+at least to transversal displacements.</p>
+
+<p>Many experiments were tried to demonstrate this property in<span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span>
+air at ordinary pressure. Though no positive evidence has been
+obtained, it is thought, nevertheless, that a high frequency brush
+or streamer, if the frequency could be pushed far enough, would
+be decidedly rigid. A small sphere might then be moved within
+it quite freely, but if thrown against it the sphere would rebound.
+An ordinary flame cannot possess rigidity to a marked degree
+because the vibration is directionless; but an electric arc, it is
+believed, must possess that property more or less. A luminous
+band excited in a bulb by repeated discharges of a Leyden jar
+must also possess rigidity, and if deformed and suddenly released
+should vibrate.</p>
+
+<p>From like considerations other conclusions of interest are
+reached. The most probable medium filling the space is one
+consisting of independent carriers immersed in an insulating
+fluid. If through this medium enormous electrostatic stresses
+are assumed to act, which vary rapidly in intensity, it would
+allow the motion of a body through it, yet it would be rigid and
+elastic, although the fluid itself might be devoid of these properties.
+Furthermore, on the assumption that the independent
+carriers are of any configuration such that the fluid resistance to
+motion in one direction is greater than in another, a stress of
+that nature would cause the carriers to arrange themselves in
+groups, since they would turn to each other their sides of the
+greatest electric density, in which position the fluid resistance to
+approach would be smaller than to receding. If in a medium of
+the above characteristics a brush would be formed by a steady
+potential, an exchange of the carriers would go on continually,
+and there would be less carriers per unit of volume in the brush
+than in the space at some distance from the electrode, this corresponding
+to rarefaction. If the potential were rapidly changing,
+the result would be very different; the higher the frequency
+of the pulses, the slower would be the exchange of the carriers;
+finally, the motion of translation through measurable space would
+cease, and, with a sufficiently high frequency and intensity of the
+stress, the carriers would be drawn towards the electrode, and
+compression would result.</p>
+
+<p>An interesting feature of these high frequency currents is that
+they allow of operating all kinds of devices by connecting the device
+with only one leading wire to the electric source. In fact,
+under certain conditions it may be more economical to supply the
+electrical energy with one lead than with two.<span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span></p>
+
+<p>An experiment of special interest shown by Mr. Tesla, is the
+running, by the use of only one insulated line, of a motor operating
+on the principle of the rotating magnetic field enunciated
+by Mr. Tesla. A simple form of such a motor is obtained by
+winding upon a laminated iron core a primary and close to it a
+secondary coil, closing the ends of the latter and placing a freely
+movable metal disc within the influence of the moving field.
+The secondary coil may, however, be omitted. When one of the
+ends of the primary coil of the motor is connected to one of the
+terminals of the high frequency coil and the other end to an
+insulated metal plate, which, it should be stated, is not absolutely
+necessary for the success of the experiment, the disc is set in
+rotation.</p>
+
+<p>Experiments of this kind seem to bring it within possibility to
+operate a motor at any point of the earth's surface from a central
+source, without any connection to the same except through
+the earth. If, by means of powerful machinery, rapid variations
+of the earth's potential were produced, a grounded wire reaching
+up to some height would be traversed by a current which could
+be increased by connecting the free end of the wire to a body of
+some size. The current might be converted to low tension and
+used to operate a motor or other device. The experiment, which
+would be one of great scientific interest, would probably best
+succeed on a ship at sea. In this manner, even if it were not
+possible to operate machinery, intelligence might be transmitted
+quite certainly.</p>
+
+<p>In the course of this experimental study special attention was
+devoted to the heating effects produced by these currents, which
+are not only striking, but open up the possibility of producing a
+more efficient illuminant. It is sufficient to attach to the coil
+terminal a thin wire or filament, to have the temperature of the
+latter perceptibly raised. If the wire or filament be enclosed in
+a bulb, the heating effect is increased by preventing the circulation
+of the air. If the air in the bulb be strongly compressed,
+the displacements are smaller, the impacts less violent, and the
+heating effect is diminished. On the contrary, if the air in the
+bulb be exhausted, an inclosed lamp filament is brought to incandescence,
+and any amount of light may thus be produced.</p>
+
+<p>The heating of the inclosed lamp filament depends on so
+many things of a different nature, that it is difficult to give a
+generally applicable rule under which the maximum heating<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span>
+occurs. As regards the size of the bulb, it is ascertained that at
+ordinary or only slightly differing atmospheric pressures, when
+air is a good insulator, the filament is heated more in a small
+bulb, because of the better confinement of heat in this case. At
+lower pressures, when air becomes conducting, the heating effect
+is greater in a large bulb, but at excessively high degrees of
+exhaustion there seems to be, beyond a certain and rather small
+size of the vessel, no perceptible difference in the heating.</p>
+
+<p>The shape of the vessel is also of some importance, and it has
+been found of advantage for reasons of economy to employ a
+spherical bulb with the electrode mounted in its centre, where
+the rebounding molecules collide.</p>
+
+<p>It is desirable on account of economy that all the energy supplied
+to the bulb from the source should reach without loss the
+body to be heated. The loss in conveying the energy from the
+source to the body may be reduced by employing thin wires
+heavily coated with insulation, and by the use of electrostatic
+screens. It is to be remarked, that the screen cannot be connected
+to the ground as under ordinary conditions.</p>
+
+<p>In the bulb itself a large portion of the energy supplied may
+be lost by molecular bombardment against the wire connecting
+the body to be heated with the source. Considerable improvement
+was effected by covering the glass stem containing the wire
+with a closely fitting conducting tube. This tube is made to
+project a little above the glass, and prevents the cracking of the
+latter near the heated body. The effectiveness of the conducting
+tube is limited to very high degrees of exhaustion. It diminishes
+the energy lost in bombardment for two reasons; first, the
+charge given up by the atoms spreads over a greater area, and
+hence the electric density at any point is small, and the atoms
+are repelled with less energy than if they would strike against a
+good insulator; secondly, as the tube is electrified by the atoms
+which first come in contact with it, the progress of the following
+atoms against the tube is more or less checked by the repulsion
+which the electrified tube must exert upon the similarly electrified
+atoms. This, it is thought, explains why the discharge through
+a bulb is established with much greater facility when an insulator,
+than when a conductor, is present.</p>
+
+<p>During the investigations a great many bulbs of different construction,
+with electrodes of different material, were experimented
+upon, and a number of observations of interest were made. Mr.<span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span>
+Tesla has found that the deterioration of the electrode is the less,
+the higher the frequency. This was to be expected, as then the
+heating is effected by many small impacts, instead by fewer and
+more violent ones, which quickly shatter the structure. The deterioration
+is also smaller when the vibration is harmonic. Thus
+an electrode, maintained at a certain degree of heat, lasts much
+longer with currents obtained from an alternator, than with
+those obtained by means of a disruptive discharge. One of the
+most durable electrodes was obtained from strongly compressed
+carborundum, which is a kind of carbon recently produced by
+Mr. E. G. Acheson, of Monongahela City, Pa. From experience,
+it is inferred, that to be most durable, the electrode should
+be in the form of a sphere with a highly polished surface.</p>
+
+<p>In some bulbs refractory bodies were mounted in a carbon cup
+and put under the molecular impact. It was observed in
+such experiments that the carbon cup was heated at first, until a
+higher temperature was reached; then most of the bombardment
+was directed against the refractory body, and the carbon
+was relieved. In general, when different bodies were mounted
+in the bulb, the hardest fusible would be relieved, and would
+remain at a considerably lower temperature. This was necessitated
+by the fact that most of the energy supplied would find
+its way through the body which was more easily fused or "evaporated."</p>
+
+<p>Curiously enough it appeared in some of the experiments
+made, that a body was fused in a bulb under the molecular impact
+by evolution of less light than when fused by the application
+of heat in ordinary ways. This may be ascribed to a
+loosening of the structure of the body under the violent impacts
+and changing stresses.</p>
+
+<p>Some experiments seem to indicate that under certain conditions
+a body, conducting or nonconducting, may, when bombarded,
+emit light, which to all appearances is due to phosphorescence,
+but may in reality be caused by the incandescence of an
+infinitesimal layer, the mean temperature of the body being
+comparatively small. Such might be the case if each single
+rhythmical impact were capable of instantaneously exciting the
+retina, and the rhythm were just high enough to cause a continuous
+impression in the eye. According to this view, a coil operated
+by disruptive discharge would be eminently adapted to produce
+such a result, and it is found by experience that its power of<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span>
+exciting phosphorescence is extraordinarily great. It is capable
+of exciting phosphorescence at comparatively low degrees of
+exhaustion, and also projects shadows at pressures far greater
+than those at which the mean free path is comparable to the
+dimensions of the vessel. The latter observation is of some importance,
+inasmuch as it may modify the generally accepted views
+in regard to the "radiant state" phenomena.</p>
+
+<p>A thought which early and naturally suggested itself to Mr.
+Tesla, was to utilize the great inductive effects of high frequency
+currents to produce light in a sealed glass vessel without the use
+of leading in wires. Accordingly, many bulbs were constructed
+in which the energy necessary to maintain a button or filament
+at high incandescence, was supplied through the glass by either
+electrostatic or electrodynamic induction. It was easy to regulate
+the intensity of the light emitted by means of an externally
+applied condenser coating connected to an insulated plate, or
+simply by means of a plate attached to the bulb which at the
+same time performed the function of a shade.</p>
+
+<p>A subject of experiment, which has been exhaustively treated
+in England by Prof. J. J. Thomson, has been followed up independently
+by Mr. Tesla from the beginning of this study, namely,
+to excite by electrodynamic induction a luminous band in a closed
+tube or bulb. In observing the behavior of gases, and the
+luminous phenomena obtained, the importance of the electrostatic
+effects was noted and it appeared desirable to produce
+enormous potential differences, alternating with extreme rapidity.
+Experiments in this direction led to some of the most interesting
+results arrived at in the course of these investigations. It
+was found that by rapid alternations of a high electrostatic potential,
+exhausted tubes could be lighted at considerable distances
+from a conductor connected to a properly constructed coil, and
+that it was practicable to establish with the coil an alternating
+electrostatic field, acting through the whole room and lighting a
+tube wherever it was placed within the four walls. Phosphorescent
+bulbs may be excited in such a field, and it is easy to regulate
+the effect by connecting to the bulb a small insulated metal
+plate. It was likewise possible to maintain a filament or button
+mounted in a tube at bright incandescence, and, in one experiment,
+a mica vane was spun by the incandescence of a platinum
+wire.</p>
+
+<p>Coming now to the lecture delivered in Philadelphia and St.<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span>
+Louis, it may be remarked that to the superficial reader, Mr.
+Tesla's introduction, dealing with the importance of the eye, might
+appear as a digression, but the thoughtful reader will find therein
+much food for meditation and speculation. Throughout his discourse
+one can trace Mr. Tesla's effort to present in a popular
+way thoughts and views on the electrical phenomena which have
+in recent years captivated the scientific world, but of which the
+general public has even yet merely received an inkling. Mr.
+Tesla also dwells rather extensively on his well-known method of
+high-frequency conversion; and the large amount of detail information
+will be gratefully received by students and experimenters
+in this virgin field. The employment of apt analogies
+in explaining the fundamental principles involved makes it easy
+for all to gain a clear idea of their nature. Again, the ease with
+which, thanks to Mr. Tesla's efforts, these high-frequency currents
+may now be obtained from circuits carrying almost any
+kind of current, cannot fail to result in an extensive broadening
+of this field of research, which offers so many possibilities. Mr.
+Tesla, true philosopher as he is, does not hesitate to point out
+defects in some of his methods, and indicates the lines which to
+him seem the most promising. Particular stress is laid by him
+upon the employment of a medium in which the discharge
+electrodes should be immersed in order that this method of conversion
+may be brought to the highest perfection. He has evidently
+taken pains to give as much useful information as possible
+to those who wish to follow in his path, as he shows in detail the
+circuit arrangements to be adopted in all ordinary cases met with
+in practice, and although some of these methods were described
+by him two years before, the additional information is still timely
+and welcome.</p>
+
+<p>In his experiments he dwells first on some phenomena produced
+by electrostatic force, which he considers in the light of
+modern theories to be the most important force in nature for us
+to investigate. At the very outset he shows a strikingly novel
+experiment illustrating the effect of a rapidly varying electrostatic
+force in a gaseous medium, by touching with one hand one of
+the terminals of a 200,000 volt transformer and bringing the
+other hand to the opposite terminal. The powerful streamers
+which issued from his hand and astonished his audiences formed
+a capital illustration of some of the views advanced, and afforded
+Mr. Tesla an opportunity of pointing out the true reasons why,<span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span>
+with these currents, such an amount of energy can be passed
+through the body with impunity. He then showed by experiment
+the difference between a steady and a rapidly varying force
+upon the dielectric. This difference is most strikingly illustrated
+in the experiment in which a bulb attached to the end of a wire
+in connection with one of the terminals of the transformer is
+ruptured, although all extraneous bodies are remote from the
+bulb. He next illustrates how mechanical motions are produced
+by a varying electrostatic force acting through a gaseous medium.
+The importance of the action of the air is particularly illustrated
+by an interesting experiment.</p>
+
+<p>Taking up another class of phenomena, namely, those of dynamic
+electricity, Mr. Tesla produced in a number of experiments
+a variety of effects by the employment of only a single wire
+with the evident intent of impressing upon his audience the idea
+that electric vibration or current can be transmitted with ease,
+without any return circuit; also how currents so transmitted can
+be converted and used for many practical purposes. A number
+of experiments are then shown, illustrating the effects of frequency,
+self-induction and capacity; then a number of ways of
+operating motive and other devices by the use of a single lead.
+A number of novel impedance phenomena are also shown which
+cannot fail to arouse interest.</p>
+
+<p>Mr. Tesla next dwelt upon a subject which he thinks of great
+importance, that is, electrical resonance, which he explained in a
+popular way. He expressed his firm conviction that by observing
+proper conditions, intelligence, and possibly even power, can
+be transmitted through the medium or through the earth; and
+he considers this problem worthy of serious and immediate consideration.</p>
+
+<p>Coming now to the light phenomena in particular, he illustrated
+the four distinct kinds of these phenomena in an original way,
+which to many must have been a revelation. Mr. Tesla attributes
+these light effects to molecular or atomic impacts produced by a
+varying electrostatic stress in a gaseous medium. He illustrated
+in a series of novel experiments the effect of the gas surrounding
+the conductor and shows beyond a doubt that with high frequency
+and high potential currents, the surrounding gas is of
+paramount importance in the heating of the conductor. He
+attributes the heating partially to a conduction current and partially
+to bombardment, and demonstrates that in many cases the<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+heating may be practically due to the bombardment alone. He
+pointed out also that the skin effect is largely modified by the
+presence of the gas or of an atomic medium in general. He
+showed also some interesting experiments in which the effect of
+convection is illustrated. Probably one of the most curious experiments
+in this connection is that in which a thin platinum wire
+stretched along the axis of an exhausted tube is brought to incandescence
+at certain points corresponding to the position of
+the stri&aelig;, while at others it remains dark. This experiment
+throws an interesting light upon the nature of the stri&aelig; and may
+lead to important revelations.</p>
+
+<p>Mr. Tesla also demonstrated the dissipation of energy through
+an atomic medium and dwelt upon the behavior of vacuous
+space in conveying heat, and in this connection showed the curious
+behavior of an electrode stream, from which he concludes that
+the molecules of a gas probably cannot be acted upon directly
+at measurable distances.</p>
+
+<p>Mr. Tesla summarized the chief results arrived at in pursuing
+his investigations in a manner which will serve as a valuable
+guide to all who may engage in this work. Perhaps most interest
+will centre on his general statements regarding the phenomena
+of phosphorescence, the most important fact revealed in this direction
+being that when exciting a phosphorescent bulb a certain
+definite potential gives the most economical result.</p>
+
+<p>The lectures will now be presented in the order of their date
+of delivery.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span></p>
+<h2><a name="CHAPTER_XXVI" id="CHAPTER_XXVI"></a>CHAPTER XXVI.</h2>
+
+<h3><span class="smcap">Experiments With Alternate Currents of Very High Frequency
+and Their Application to Methods of Artificial
+Illumination.</span><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h3>
+
+
+<p>There is no subject more captivating, more worthy of study,
+than nature. To understand this great mechanism, to discover
+the forces which are active, and the laws which govern them, is
+the highest aim of the intellect of man.</p>
+
+<p>Nature has stored up in the universe infinite energy. The
+eternal recipient and transmitter of this infinite energy is the
+ether. The recognition of the existence of ether, and of the
+functions it performs, is one of the most important results of
+modern scientific research. The mere abandoning of the idea of
+action at a distance, the assumption of a medium pervading all
+space and connecting all gross matter, has freed the minds of
+thinkers of an ever present doubt, and, by opening a new horizon&mdash;new
+and unforeseen possibilities&mdash;has given fresh interest to
+phenomena with which we are familiar of old. It has been a
+great step towards the understanding of the forces of nature and
+their multifold manifestations to our senses. It has been for
+the enlightened student of physics what the understanding of
+the mechanism of the firearm or of the steam engine is for the
+barbarian. Phenomena upon which we used to look as wonders
+baffling explanation, we now see in a different light. The spark
+of an induction coil, the glow of an incandescent lamp, the manifestations
+of the mechanical forces of currents and magnets are
+no longer beyond our grasp; instead of the incomprehensible, as
+before, their observation suggests now in our minds a simple
+mechanism, and although as to its precise nature all is still conjecture,
+yet we know that the truth cannot be much longer hidden,
+and instinctively we feel that the understanding is dawning
+upon us. We still admire these beautiful phenomena, these<span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span>
+strange forces, but we are helpless no longer; we can in a certain
+measure explain them, account for them, and we are hopeful of
+finally succeeding in unraveling the mystery which surrounds
+them.</p>
+
+<p>In how far we can understand the world around us is the ultimate
+thought of every student of nature. The coarseness of our
+senses prevents us from recognizing the ulterior construction of
+matter, and astronomy, this grandest and most positive of natural
+sciences, can only teach us something that happens, as it were, in
+our immediate neighborhood: of the remoter portions of the
+boundless universe, with its numberless stars and suns, we know
+nothing. But far beyond the limit of perception of our senses
+the spirit still can guide us, and so we may hope that even these
+unknown worlds&mdash;infinitely small and great&mdash;may in a measure
+become known to us. Still, even if this knowledge should reach
+us, the searching mind will find a barrier, perhaps forever unsurpassable,
+to the <i>true</i> recognition of that which <i>seems</i> to be, the
+mere <i>appearance</i> of which is the only and slender basis of all
+our philosophy.</p>
+
+<p>Of all the forms of nature's immeasurable, all-pervading
+energy, which ever and ever changing and moving, like a soul
+animates the inert universe, electricity and magnetism are perhaps
+the most fascinating. The effects of gravitation, of heat
+and light we observe daily, and soon we get accustomed to
+them, and soon they lose for us the character of the marvelous
+and wonderful; but electricity and magnetism, with their singular
+relationship, with their seemingly dual character, unique among
+the forces in nature, with their phenomena of attractions, repulsions
+and rotations, strange manifestations of mysterious agents,
+stimulate and excite the mind to thought and research. What is
+electricity, and what is magnetism? These questions have been
+asked again and again. The most able intellects have ceaselessly
+wrestled with the problem; still the question has not as yet been
+fully answered. But while we cannot even to-day state what
+these singular forces are, we have made good headway towards
+the solution of the problem. We are now confident that
+electric and magnetic phenomena are attributable to ether, and
+we are perhaps justified in saying that the effects of static electricity
+are effects of ether under strain, and those of dynamic
+electricity and electro-magnetism effects of ether in motion. But
+this still leaves the question, as to what electricity and magnetism
+are, unanswered.<span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span></p>
+
+<p>First, we naturally inquire, What is electricity, and is there
+such a thing as electricity? In interpreting electric phenomena,
+we may speak of electricity or of an electric condition, state or
+effect. If we speak of electric effects we must distinguish two
+such effects, opposite in character and neutralizing each other, as
+observation shows that two such opposite effects exist. This is
+unavoidable, for in a medium of the properties of ether, we cannot
+possibly exert a strain, or produce a displacement or motion
+of any kind, without causing in the surrounding medium an
+equivalent and opposite effect. But if we speak of electricity,
+meaning a <i>thing</i>, we must, I think, abandon the idea of two
+electricities, as the existence of two such things is highly improbable.
+For how can we imagine that there should be two things,
+equivalent in amount, alike in their properties, but of opposite
+character, both clinging to matter, both attracting and completely
+neutralizing each other? Such an assumption, though suggested
+by many phenomena, though most convenient for explaining
+them, has little to commend it. If there <i>is</i> such a thing as electricity,
+there can be only <i>one</i> such thing, and excess and want
+of that one thing, possibly; but more probably its condition determines
+the positive and negative character. The old theory of
+Franklin, though falling short in some respects, is, from a certain
+point of view, after all, the most plausible one. Still, in spite
+of this, the theory of the two electricities is generally accepted,
+as it apparently explains electric phenomena in a more satisfactory
+manner. But a theory which better explains the facts is not
+necessarily true. Ingenious minds will invent theories to suit
+observation, and almost every independent thinker has his own
+views on the subject.</p>
+
+<p>It is not with the object of advancing an opinion, but with
+the desire of acquainting you better with some of the results,
+which I will describe, to show you the reasoning I have followed,
+the departures I have made&mdash;that I venture to express,
+in a few words, the views and convictions which have led me to
+these results.</p>
+
+<p>I adhere to the idea that there is a thing which we have been
+in the habit of calling electricity. The question is, What is that
+thing? or, What, of all things, the existence of which we know,
+have we the best reason to call electricity? We know that it acts
+like an incompressible fluid; that there must be a constant quantity
+of it in nature; that it can be neither produced nor destroyed;<span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span>
+and, what is more important, the electro-magnetic theory of light
+and all facts observed teach us that electric and ether phenomena
+are identical. The idea at once suggests itself, therefore, that
+electricity might be called ether. In fact, this view has in a certain
+sense been advanced by Dr. Lodge. His interesting work
+has been read by everyone and many have been convinced by
+his arguments. His great ability and the interesting nature of
+the subject, keep the reader spellbound; but when the impressions
+fade, one realizes that he has to deal only with ingenious
+explanations. I must confess, that I cannot believe in two electricities,
+much less in a doubly-constituted ether. The puzzling
+behavior of the ether as a solid to waves of light and heat, and
+as a fluid to the motion of bodies through it, is certainly explained
+in the most natural and satisfactory manner by assuming
+it to be in motion, as Sir William Thomson has suggested; but
+regardless of this, there is nothing which would enable us to
+conclude with certainty that, while a fluid is not capable of transmitting
+transverse vibrations of a few hundred or thousand per
+second, it might not be capable of transmitting such vibrations
+when they range into hundreds of million millions per second.
+Nor can anyone prove that there are transverse ether waves
+emitted from an alternate current machine, giving a small number
+of alternations per second; to such slow disturbances, the ether,
+if at rest, may behave as a true fluid.</p>
+
+<p>Returning to the subject, and bearing in mind that the existence
+of two electricities is, to say the least, highly improbable,
+we must remember, that we have no evidence of electricity, nor
+can we hope to get it, unless gross matter is present. Electricity,
+therefore, cannot be called ether in the broad sense of the term;
+but nothing would seem to stand in the way of calling electricity
+ether associated with matter, or bound ether; or, in other words,
+that the so-called static charge of the molecule is ether associated
+in some way with the molecule. Looking at it in that light, we
+would be justified in saying, that electricity is concerned in all
+molecular actions.</p>
+
+<p>Now, precisely what the ether surrounding the molecules is,
+wherein it differs from ether in general, can only be conjectured.
+It cannot differ in density, ether being incompressible:
+it must, therefore, be under some strain or in motion, and the
+latter is the most probable. To understand its functions, it
+would be necessary to have an exact idea of the physical con<span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span>struction
+of matter, of which, of course, we can only form a
+mental picture.</p>
+
+<p>But of all the views on nature, the one which assumes one
+matter and one force, and a perfect uniformity throughout, is
+the most scientific and most likely to be true. An infinitesimal
+world, with the molecules and their atoms spinning and moving
+in orbits, in much the same manner as celestial bodies, carrying
+with them and probably spinning with them ether, or in other
+words, carrying with them static charges, seems to my mind the
+most probable view, and one which, in a plausible manner, accounts
+for most of the phenomena observed. The spinning of
+the molecules and their ether sets up the ether tensions or electrostatic
+strains; the equalization of ether tensions sets up ether
+motions or electric currents, and the orbital movements produce
+the effects of electro and permanent magnetism.</p>
+
+<p>About fifteen years ago, Prof. Rowland demonstrated a most
+interesting and important fact, namely, that a static charge carried
+around produces the effects of an electric current. Leaving
+out of consideration the precise nature of the mechanism, which
+produces the attraction and repulsion of currents, and conceiving
+the electrostatically charged molecules in motion, this experimental
+fact gives us a fair idea of magnetism. We can conceive lines
+or tubes of force which physically exist, being formed of rows
+of directed moving molecules; we can see that these lines must be
+closed, that they must tend to shorten and expand, etc. It likewise
+explains in a reasonable way, the most puzzling phenomenon
+of all, permanent magnetism, and, in general, has all the beauties
+of the Ampere theory without possessing the vital defect of the
+same, namely, the assumption of molecular currents. Without
+enlarging further upon the subject, I would say, that I look upon
+all electrostatic, current and magnetic phenomena as being due
+to electrostatic molecular forces.</p>
+
+<p>The preceding remarks I have deemed necessary to a full
+understanding of the subject as it presents itself to my mind.</p>
+
+<p>Of all these phenomena the most important to study are the
+current phenomena, on account of the already extensive and ever-growing
+use of currents for industrial purposes. It is now a century
+since the first practical source of current was produced,
+and, ever since, the phenomena which accompany the flow of
+currents have been diligently studied, and through the untiring
+efforts of scientific men the simple laws which govern them have<span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span>
+been discovered. But these laws are found to hold good only
+when the currents are of a steady character. When the currents
+are rapidly varying in strength, quite different phenomena, often
+unexpected, present themselves, and quite different laws hold
+good, which even now have not been determined as fully as is
+desirable, though through the work, principally, of English scientists,
+enough knowledge has been gained on the subject to enable
+us to treat simple cases which now present themselves in daily
+practice.</p>
+
+<p>The phenomena which are peculiar to the changing character
+of the currents are greatly exalted when the rate of change is
+increased, hence the study of these currents is considerably facilitated
+by the employment of properly constructed apparatus.
+It was with this and other objects in view that I constructed
+alternate current machines capable of giving more than two
+million reversals of current per minute, and to this circumstance
+it is principally due, that I am able to bring to your attention
+some of the results thus far reached, which I hope will prove to
+be a step in advance on account of their direct bearing upon one
+of the most important problems, namely, the production of a
+practical and efficient source of light.</p>
+
+<p>The study of such rapidly alternating currents is very interesting.
+Nearly every experiment discloses something new. Many
+results may, of course, be predicted, but many more are unforeseen.
+The experimenter makes many interesting observations.
+For instance, we take a piece of iron and hold it against a magnet.
+Starting from low alternations and running up higher and higher
+we feel the impulses succeed each other faster and faster, get
+weaker and weaker, and finally disappear. We then observe a
+continuous pull; the pull, of course, is not continuous; it only
+appears so to us; our sense of touch is imperfect.</p>
+
+<p>We may next establish an arc between the electrodes and
+observe, as the alternations rise, that the note which accompanies
+alternating arcs gets shriller and shriller, gradually weakens, and
+finally ceases. The air vibrations, of course, continue, but they
+are too weak to be perceived; our sense of hearing fails us.</p>
+
+<p>We observe the small physiological effects, the rapid heating of
+the iron cores and conductors, curious inductive effects, interesting
+condenser phenomena, and still more interesting light phenomena
+with a high tension induction coil. All these experiments
+and observations would be of the greatest interest to the<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span>
+student, but their description would lead me too far from the
+principal subject. Partly for this reason, and partly on account
+of their vastly greater importance, I will confine myself to the
+description of the light effects produced by these currents.</p>
+
+<p>In the experiments to this end a high tension induction coil or
+equivalent apparatus for converting currents of comparatively
+low into currents of high tension is used.</p>
+
+<p>If you will be sufficiently interested in the results I shall describe
+as to enter into an experimental study of this subject; if you
+will be convinced of the truth of the arguments I shall advance&mdash;your
+aim will be to produce high frequencies and high potentials;
+in other words, powerful electrostatic effects. You will then encounter
+many difficulties, which, if completely overcome, would
+allow us to produce truly wonderful results.</p>
+
+<p>First will be met the difficulty of obtaining the required frequencies
+by means of mechanical apparatus, and, if they be obtained
+otherwise, obstacles of a different nature will present
+themselves. Next it will be found difficult to provide the requisite
+insulation without considerably increasing the size of the
+apparatus, for the potentials required are high, and, owing to the
+rapidity of the alternations, the insulation presents peculiar difficulties.
+So, for instance, when a gas is present, the discharge
+may work, by the molecular bombardment of the gas and consequent
+heating, through as much as an inch of the best solid
+insulating material, such as glass, hard rubber, porcelain, sealing
+wax, etc.; in fact, through any known insulating substance. The
+chief requisite in the insulation of the apparatus is, therefore, the
+exclusion of any gaseous matter.</p>
+
+<p>In general my experience tends to show that bodies which
+possess the highest specific inductive capacity, such as glass,
+afford a rather inferior insulation to others, which, while they are
+good insulators, have a much smaller specific inductive capacity,
+such as oils, for instance, the dielectric losses being no doubt
+greater in the former. The difficulty of insulating, of course,
+only exists when the potentials are excessively high, for with
+potentials such as a few thousand volts there is no particular difficulty
+encountered in conveying currents from a machine giving,
+say, 20,000 alternations per second, to quite a distance. This
+number of alternations, however, is by far too small for many
+purposes, though quite sufficient for some practical applications.
+This difficulty of insulating is fortunately not a vital drawback;<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span>
+it affects mostly the size of the apparatus, for, when excessively
+high potentials would be used, the light-giving devices would be
+located not far from the apparatus, and often they would be quite
+close to it. As the air-bombardment of the insulated wire is dependent
+on condenser action, the loss may be reduced to a trifle
+by using excessively thin wires heavily insulated.</p>
+
+<p>Another difficulty will be encountered in the capacity and self-induction
+necessarily possessed by the coil. If the coil be large,
+that is, if it contain a great length of wire, it will be generally
+unsuited for excessively high frequencies; if it be small, it may
+be well adapted for such frequencies, but the potential might
+then not be as high as desired. A good insulator, and preferably
+one possessing a small specific inductive capacity, would
+afford a two-fold advantage. First, it would enable us to construct
+a very small coil capable of withstanding enormous differences
+of potential; and secondly, such a small coil, by reason of
+its smaller capacity and self-induction, would be capable of a
+quicker and more vigorous vibration. The problem then of constructing
+a coil or induction apparatus of any kind possessing
+the requisite qualities I regard as one of no small importance,
+and it has occupied me for a considerable time.</p>
+
+<p>The investigator who desires to repeat the experiments which
+I will describe, with an alternate current machine, capable of
+supplying currents of the desired frequency, and an induction
+coil, will do well to take the primary coil out and mount the secondary
+in such a manner as to be able to look through the tube
+upon which the secondary is wound. He will then be able to
+observe the streams which pass from the primary to the insulating
+tube, and from their intensity he will know how far he can
+strain the coil. Without this precaution he is sure to injure
+the insulation. This arrangement permits, however, an easy
+exchange of the primaries, which is desirable in these experiments.</p>
+
+<p>The selection of the type of machine best suited for the purpose
+must be left to the judgment of the experimenter. There
+are here illustrated three distinct types of machines, which,
+besides others, I have used in my experiments.</p>
+
+<p>Fig. 97 represents the machine used in my experiments before
+this Institute. The field magnet consists of a ring of wrought
+iron with 384 pole projections. The armature comprises a steel
+disc to which is fastened a thin, carefully welded rim of wrought<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+iron. Upon the rim are wound several layers of fine, well
+annealed iron wire, which, when wound, is passed through
+shellac. The armature wires are wound around brass pins,
+wrapped with silk thread. The diameter of the armature wire
+in this type of machine should not be more than 1/6 of the thickness
+of the pole projections, else the local action will be considerable.</p>
+
+<div class="figcenter" style="width: 503px;">
+<img src="images/oi_167.jpg" width="503" height="480" alt="Fig. 97." title="" />
+<span class="caption">Fig. 97.</span>
+</div>
+
+
+<p>Fig. 98 represents a larger machine of a different type. The
+field magnet of this machine consists of two like parts which
+either enclose an exciting coil, or else are independently wound.
+Each part has 480 pole projections, the projections of one facing
+those of the other. The armature consists of a wheel of hard
+bronze, carrying the conductors which revolve between the projections
+of the field magnet. To wind the armature conductors,
+I have found it most convenient to proceed in the following
+manner. I construct a ring of hard bronze of the required size.
+This ring and the rim of the wheel are provided with the
+proper number of pins, and both fastened upon a plate. The
+armature conductors being wound, the pins are cut off and the
+ends of the conductors fastened by two rings which screw to the<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span>
+bronze ring and the rim of the wheel, respectively. The whole
+may then be taken off and forms a solid structure. The conductors
+in such a type of machine should consist of sheet copper,
+the thickness of which, of course, depends on the thickness of
+the pole projections; or else twisted thin wires should be employed.</p>
+
+<p>Fig. 99 is a smaller machine, in many respects similar to the
+former, only here the armature conductors and the exciting coil
+are kept stationary, while only a block of wrought iron is revolved.</p>
+
+<div class="figcenter" style="width: 567px;">
+<img src="images/oi_168.jpg" width="567" height="480" alt="Fig. 98." title="" />
+<span class="caption">Fig. 98.</span>
+</div>
+
+<p>It would be uselessly lengthening this description were I to
+dwell more on the details of construction of these machines.
+Besides, they have been described somewhat more elaborately in
+<i>The Electrical Engineer</i>, of March 18, 1891. I deem it well,
+however, to call the attention of the investigator to two things,
+the importance of which, though self evident, he is nevertheless
+apt to underestimate; namely, to the local action in the conductors
+which must be carefully avoided, and to the clearance,
+which must be small. I may add, that since it is desirable to use
+very high peripheral speeds, the armature should be of very
+large diameter in order to avoid impracticable belt speeds. Of<span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+the several types of these machines which have been constructed
+by me, I have found that the type illustrated in Fig. 97 caused
+me the least trouble in construction, as well as in maintenance,
+and on the whole, it has been a good experimental machine.</p>
+
+<p>In operating an induction coil with very rapidly alternating
+currents, among the first luminous phenomena noticed are naturally
+those presented by the high-tension discharge. As the number
+of alternations per second is increased, or as&mdash;the number
+being high&mdash;the current through the primary is varied, the discharge
+gradually changes in appearance. It would be difficult to
+describe the minor changes which occur, and the conditions which
+bring them about, but one may note five distinct forms of the
+discharge.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_169.jpg" width="640" height="437" alt="Fig. 99." title="" />
+<span class="caption">Fig. 99.</span>
+</div>
+
+<p>First, one may observe a weak, sensitive discharge in the form
+of a thin, feeble-colored thread. (Fig. 100a.) It always occurs
+when, the number of alternations per second being high, the current
+through the primary is very small. In spite of the excessively
+small current, the rate of change is great, and the difference
+of potential at the terminals of the secondary is therefore
+considerable, so that the arc is established at great distances; but
+the quantity of "electricity" set in motion is insignificant, barely
+sufficient to maintain a thin, threadlike arc. It is excessively
+sensitive and may be made so to such a degree that the mere act
+of breathing near the coil will affect it, and unless it is perfectly<span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span>
+well protected from currents of air, it wriggles around constantly.
+Nevertheless, it is in this form excessively persistent, and when
+the terminals are approached to, say, one-third of the striking
+distance, it can be blown out only with difficulty. This exceptional
+persistency, when short, is largely due to the arc being
+excessively thin; presenting, therefore, a very small surface
+to the blast. Its great sensitiveness, when very long, is probably
+due to the motion of the particles of dust suspended in the air.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_170.jpg" width="800" height="269" alt="Fig. 100a, 100b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 100a.</td><td class="caption1"><span class="smcap">Fig.</span> 100b.</td></tr>
+</table>
+</div>
+
+<p>When the current through the primary is increased, the discharge
+gets broader and stronger, and the effect of the capacity
+of the coil becomes visible until, finally, under proper conditions,
+a white flaming arc, Fig. 100<small>B</small>, often as thick as one's finger, and
+striking across the whole coil, is produced. It develops remarkable
+heat, and may be further characterized by the absence of
+the high note which accompanies the less powerful discharges.
+To take a shock from the coil under these conditions would not
+be advisable, although under different conditions, the potential
+being much higher, a shock from the coil may be taken with
+impunity. To produce this kind of discharge the number of
+alternations per second must not be too great for the coil used;
+and, generally speaking, certain relations between capacity, self-induction
+and frequency must be observed.</p>
+
+<p>The importance of these elements in an alternate current circuit
+is now well-known, and under ordinary conditions, the general
+rules are applicable. But in an induction coil exceptional
+conditions prevail. First, the self-induction is of little importance
+before the arc is established, when it asserts itself, but perhaps
+never as prominently as in ordinary alternate current circuits,
+because the capacity is distributed all along the coil, and by reason
+of the fact that the coil usually discharges through very great
+resistances; hence the currents are exceptionally small. Secondly,<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span>
+the capacity goes on increasing continually as the potential rises,
+in consequence of absorption which takes place to a considerable
+extent. Owing to this there exists no critical relationship between
+these quantities, and ordinary rules would not seem to be applicable.
+As the potential is increased either in consequence of the
+increased frequency or of the increased current through the
+primary, the amount of the energy stored becomes greater and
+greater, and the capacity gains more and more in importance.
+Up to a certain point the capacity is beneficial, but after that it
+begins to be an enormous drawback. It follows from this that
+each coil gives the best result with a given frequency and primary
+current. A very large coil, when operated with currents of very
+high frequency, may not give as much as 1/8 inch spark. By adding
+capacity to the terminals, the condition may be improved, but
+what the coil really wants is a lower frequency.</p>
+
+<p>When the flaming discharge occurs, the conditions are evidently
+such that the greatest current is made to flow through the
+circuit. These conditions may be attained by varying the frequency
+within wide limits, but the highest frequency at which
+the flaming arc can still be produced, determines, for a given
+primary current, the maximum striking distance of the coil. In
+the flaming discharge the <i>eclat</i> effect of the capacity is not perceptible;
+the rate at which the energy is being stored then just
+equals the rate at which it can be disposed of through the circuit.
+This kind of discharge is the severest test for a coil; the break,
+when it occurs, is of the nature of that in an overcharged Leyden
+jar. To give a rough approximation I would state that, with an
+ordinary coil of, say 10,000 ohms resistance, the most powerful
+arc would be produced with about 12,000 alternations per second.</p>
+
+<p>When the frequency is increased beyond that rate, the potential,
+of course, rises, but the striking distance may, nevertheless,
+diminish, paradoxical as it may seem. As the potential rises the
+coil attains more and more the properties of a static machine
+until, finally, one may observe the beautiful phenomenon of the
+streaming discharge, Fig. 101, which may be produced across the
+whole length of the coil. At that stage streams begin to issue
+freely from all points and projections. These streams will also be
+seen to pass in abundance in the space between the primary and
+the insulating tube. When the potential is excessively high they
+will always appear, even if the frequency be low, and even if the
+primary be surrounded by as much as an inch of wax, hard rub<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span>ber,
+glass, or any other insulating substance. This limits greatly
+the output of the coil, but I will later show how I have been able
+to overcome to a considerable extent this disadvantage in the
+ordinary coil.</p>
+
+<p>Besides the potential, the intensity of the streams depends on
+the frequency; but if the coil be very large they show themselves,
+no matter how low the frequencies used. For instance,
+in a very large coil of a resistance of 67,000 ohms, constructed
+by me some time ago, they appear with as low as 100 alternations
+per second and less, the insulation of the secondary being 3/4 inch
+of ebonite. When very intense they produce a noise similar to
+that produced by the charging of a Holtz machine, but much
+more powerful, and they emit a strong smell of ozone. The
+lower the frequency, the more apt they are to suddenly injure
+the coil. With excessively high frequencies they may pass freely
+without producing any other effect than to heat the insulation
+slowly and uniformly.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_172.jpg" width="800" height="252" alt="Fig. 101, 102." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 101.</td><td class="caption">Fig. 102.</td></tr>
+</table>
+</div>
+
+<p>The existence of these streams shows the importance of constructing
+an expensive coil so as to permit of one's seeing
+through the tube surrounding the primary, and the latter should
+be easily exchangeable; or else the space between the primary
+and secondary should be completely filled up with insulating
+material so as to exclude all air. The non-observance of this
+simple rule in the construction of commercial coils is responsible
+for the destruction of many an expensive coil.</p>
+
+<p>At the stage when the streaming discharge occurs, or with
+somewhat higher frequencies, one may, by approaching the terminals
+quite nearly, and regulating properly the effect of capacity,
+produce a veritable spray of small silver-white sparks, or a
+bunch of excessively thin silvery threads (Fig. 102) amidst a
+powerful brush&mdash;each spark or thread possibly corresponding<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span>
+to one alternation. This, when produced under proper conditions,
+is probably the most beautiful discharge, and when an air
+blast is directed against it, it presents a singular appearance.
+The spray of sparks, when received through the body, causes
+some inconvenience, whereas, when the discharge simply
+streams, nothing at all is likely to be felt if large conducting
+objects are held in the hands to protect them from receiving
+small burns.</p>
+
+<p>If the frequency is still more increased, then the coil refuses
+to give any spark unless at comparatively small distances, and the
+fifth typical form of discharge may be observed (Fig. 103). The
+tendency to stream out and dissipate is then so great that when
+the brush is produced at one terminal no sparking occurs, even
+if, as I have repeatedly tried, the hand, or any conducting object,
+is held within the stream; and, what is more singular, the luminous
+stream is not at all easily deflected by the approach of a
+conducting body.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_173.jpg" width="800" height="351" alt="Fig. 103, 104." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 103.</td><td class="caption">Fig. 104.</td></tr>
+</table>
+</div>
+
+<p>At this stage the streams seemingly pass with the greatest
+freedom through considerable thicknesses of insulators, and it is
+particularly interesting to study their behavior. For this purpose
+it is convenient to connect to the terminals of the coil two
+metallic spheres which may be placed at any desired distance,
+Fig. 104. Spheres are preferable to plates, as the discharge can
+be better observed. By inserting dielectric bodies between the
+spheres, beautiful discharge phenomena may be observed. If
+the spheres be quite close and a spark be playing between them, by
+interposing a thin plate of ebonite between the spheres the spark
+instantly ceases and the discharge spreads into an intensely luminous
+circle several inches in diameter, provided the spheres are<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span>
+sufficiently large. The passage of the streams heats, and, after
+a while, softens, the rubber so much that two plates may be
+made to stick together in this manner. If the spheres are so far
+apart that no spark occurs, even if they are far beyond the striking
+distance, by inserting a thick plate of glass the discharge is
+instantly induced to pass from the spheres to the glass in the
+form of luminous streams. It appears almost as though these
+streams pass <i>through</i> the dielectric. In reality this is not the
+case, as the streams are due to the molecules of the air which
+are violently agitated in the space between the oppositely charged
+surfaces of the spheres. When no dielectric other than air is
+present, the bombardment goes on, but is too weak to be visible;
+by inserting a dielectric the inductive effect is much increased,
+and besides, the projected air molecules find an obstacle and the
+bombardment becomes so intense that the streams become luminous.
+If by any mechanical means we could effect such a violent
+agitation of the molecules we could produce the same phenomenon.
+A jet of air escaping through a small hole under
+enormous pressure and striking against an insulating substance,
+such as glass, may be luminous in the dark, and it might be possible
+to produce a phosphorescence of the glass or other insulators
+in this manner.</p>
+
+<p>The greater the specific inductive capacity of the interposed
+dielectric, the more powerful the effect produced. Owing to
+this, the streams show themselves with excessively high potentials
+even if the glass be as much as one and one-half to two
+inches thick. But besides the heating due to bombardment,
+some heating goes on undoubtedly in the dielectric, being apparently
+greater in glass than in ebonite. I attribute this to the
+greater specific inductive capacity of the glass, in consequence of
+which, with the same potential difference, a greater amount of
+energy is taken up in it than in rubber. It is like connecting to
+a battery a copper and a brass wire of the same dimensions. The
+copper wire, though a more perfect conductor, would heat more
+by reason of its taking more current. Thus what is otherwise
+considered a virtue of the glass is here a defect. Glass usually
+gives way much quicker than ebonite; when it is heated to a certain
+degree, the discharge suddenly breaks through at one point,
+assuming then the ordinary form of an arc.</p>
+
+<p>The heating effect produced by molecular bombardment of
+the dielectric would, of course, diminish as the pressure of the<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span>
+air is increased, and at enormous pressure it would be negligible,
+unless the frequency would increase correspondingly.</p>
+
+<p>It will be often observed in these experiments that when the
+spheres are beyond the striking distance, the approach of a glass
+plate, for instance, may induce the spark to jump between the
+spheres. This occurs when the capacity of the spheres is somewhat
+below the critical value which gives the greatest difference
+of potential at the terminals of the coil. By approaching a dielectric,
+the specific inductive capacity of the space between the
+spheres is increased, producing the same effect as if the capacity
+of the spheres were increased. The potential at the terminals
+may then rise so high that the air space is cracked. The experiment
+is best performed with dense glass or mica.</p>
+
+<p>Another interesting observation is that a plate of insulating
+material, when the discharge is passing through it, is strongly
+attracted by either of the spheres, that is by the nearer one, this
+being obviously due to the smaller mechanical effect of the bombardment
+on that side, and perhaps also to the greater electrification.</p>
+
+<p>From the behavior of the dielectrics in these experiments, we
+may conclude that the best insulator for these rapidly alternating
+currents would be the one possessing the smallest specific inductive
+capacity and at the same time one capable of withstanding
+the greatest differences of potential; and thus two diametrically
+opposite ways of securing the required insulation are indicated,
+namely, to use either a perfect vacuum or a gas under great pressure;
+but the former would be preferable. Unfortunately neither
+of these two ways is easily carried out in practice.</p>
+
+<p>It is especially interesting to note the behavior of an excessively
+high vacuum in these experiments. If a test tube, provided
+with external electrodes and exhausted to the highest possible
+degree, be connected to the terminals of the coil, Fig. 105, the
+electrodes of the tube are instantly brought to a high temperature
+and the glass at each end of the tube is rendered intensely phosphorescent,
+but the middle appears comparatively dark, and for a
+while remains cool.</p>
+
+<p>When the frequency is so high that the discharge shown in
+Fig. 103 is observed, considerable dissipation no doubt occurs in
+the coil. Nevertheless the coil may be worked for a long time,
+as the heating is gradual.</p>
+
+<p>In spite of the fact that the difference of potential may be<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span>
+enormous, little is felt when the discharge is passed through the
+body, provided the hands are armed. This is to some extent due
+to the higher frequency, but principally to the fact that less energy
+is available externally, when the difference of potential
+reaches an enormous value, owing to the circumstance that, with
+the rise of potential, the energy absorbed in the coil increases as
+the square of the potential. Up to a certain point the energy
+available externally increases with the rise of potential, then it
+begins to fall off rapidly. Thus, with the ordinary high tension
+induction coil, the curious paradox exists, that, while with a given
+current through the primary the shock might be fatal, with many
+times that current it might be perfectly harmless, even if the
+frequency be the same. With high frequencies and excessively
+high potentials when the terminals are not connected to bodies
+of some size, practically all the energy supplied to the primary is
+taken up by the coil. There is no breaking through, no local injury,
+but all the material, insulating and conducting, is uniformly
+heated.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_176.jpg" width="800" height="336" alt="Fig. 105, 106." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 105.</td><td class="caption">Fig. 106.</td></tr>
+</table>
+</div>
+
+<p>To avoid misunderstanding in regard to the physiological
+effect of alternating currents of very high frequency, I think it
+necessary to state that, while it is an undeniable fact that they are
+incomparably less dangerous than currents of low frequencies,
+it should not be thought that they are altogether harmless.
+What has just been said refers only to currents from an ordinary
+high tension induction coil, which currents are necessarily very
+small; if received directly from a machine or from a secondary
+of low resistance, they produce more or less powerful effects, and
+may cause serious injury, especially when used in conjunction
+with condensers.<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span></p>
+
+<p>The streaming discharge of a high tension induction coil differs
+in many respects from that of a powerful static machine. In
+color it has neither the violet of the positive, nor the brightness
+of the negative, static discharge, but lies somewhere between,
+being, of course, alternatively positive and negative. But since
+the streaming is more powerful when the point or terminal is
+electrified positively, than when electrified negatively, it follows
+that the point of the brush is more like the positive, and the root
+more like the negative, static discharge. In the dark, when the
+brush is very powerful, the root may appear almost white. The
+wind produced by the escaping streams, though it may be very
+strong&mdash;often indeed to such a degree that it may be felt quite a
+distance from the coil&mdash;is, nevertheless, considering the quantity
+of the discharge, smaller than that produced by the positive
+brush of a static machine, and it affects the flame much less
+powerfully. From the nature of the phenomenon we can conclude
+that the higher the frequency, the smaller must, of course,
+be the wind produced by the streams, and with sufficiently high
+frequencies no wind at all would be produced at the ordinary
+atmospheric pressures. With frequencies obtainable by means
+of a machine, the mechanical effect is sufficiently great to revolve,
+with considerable speed, large pin-wheels, which in the dark
+present a beautiful appearance owing to the abundance of the
+streams (Fig. 106).</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_177.jpg" width="800" height="375" alt="Fig. 107, 108." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 107.</td><td class="caption">Fig. 108.</td></tr>
+</table>
+</div>
+
+<p>In general, most of the experiments usually performed with a
+static machine can be performed with an induction coil when
+operated with very rapidly alternating currents. The effects produced,
+however, are much more striking, being of incomparably<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span>
+greater power. When a small length of ordinary cotton covered
+wire, Fig. 107, is attached to one terminal of the coil, the streams
+issuing from all points of the wire may be so intense as to produce
+a considerable light effect. When the potentials and frequencies
+are very high, a wire insulated with gutta percha or rubber and
+attached to one of the terminals, appears to be covered with a
+luminous film. A very thin bare wire when attached to a terminal
+emits powerful streams and vibrates continually to and fro
+or spins in a circle, producing a singular effect (Fig. 108). Some
+of these experiments have been described by me in <i>The Electrical
+World</i>, of February 21, 1891.</p>
+
+<p>Another peculiarity of the rapidly alternating discharge of the
+induction coil is its radically different behavior with respect to
+points and rounded surfaces.</p>
+
+<p>If a thick wire, provided with a ball at one end and with a
+point at the other, be attached to the positive terminal of a static
+machine, practically all the charge will be lost through the point,
+on account of the enormously greater tension, dependent on the
+radius of curvature. But if such a wire is attached to one of the
+terminals of the induction coil, it will be observed that with very
+high frequencies streams issue from the ball almost as copiously
+as from the point (Fig. 109).</p>
+
+<p>It is hardly conceivable that we could produce such a condition
+to an equal degree in a static machine, for the simple reason,
+that the tension increases as the square of the density, which in
+turn is proportional to the radius of curvature; hence, with a
+steady potential an enormous charge would be required to make
+streams issue from a polished ball while it is connected with a
+point. But with an induction coil the discharge of which alternates
+with great rapidity it is different. Here we have to deal
+with two distinct tendencies. First, there is the tendency to
+escape which exists in a condition of rest, and which depends on
+the radius of curvature; second, there is the tendency to dissipate
+into the surrounding air by condenser action, which depends
+on the surface. When one of these tendencies is a maximum,
+the other is at a minimum. At the point the luminous
+stream is principally due to the air molecules coming bodily in
+contact with the point; they are attracted and repelled, charged
+and discharged, and, their atomic charges being thus disturbed,
+vibrate and emit light waves. At the ball, on the contrary, there
+is no doubt that the effect is to a great extent produced induc<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span>tively,
+the air molecules not <i>necessarily</i> coming in contact with
+the ball, though they undoubtedly do so. To convince ourselves
+of this we only need to exalt the condenser action, for instance,
+by enveloping the ball, at some distance, by a better conductor
+than the surrounding medium, the conductor being, of course,
+insulated; or else by surrounding it with a better dielectric and
+approaching an insulated conductor; in both cases the streams
+will break forth more copiously. Also, the larger the ball with
+a given frequency, or the higher the frequency, the more will
+the ball have the advantage over the point. But, since a certain
+intensity of action is required to render the streams visible, it is
+obvious that in the experiment described the ball should not be
+taken too large.</p>
+
+<p>In consequence of this two-fold tendency, it is possible to produce
+by means of points, effects identical to those produced by
+capacity. Thus, for instance, by attaching to one terminal of
+the coil a small length of soiled wire, presenting many points
+and offering great facility to escape, the potential of the coil
+may be raised to the same value as by attaching to the terminal
+a polished ball of a surface many times greater than that of the
+wire.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_179.jpg" width="800" height="329" alt="Fig. 109, 110." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 109.</td><td class="caption">Fig. 110.</td></tr>
+</table>
+</div>
+
+<p>An interesting experiment, showing the effect of the points,
+may be performed in the following manner: Attach to one of
+the terminals of the coil a cotton covered wire about two feet in
+length, and adjust the conditions so that streams issue from the
+wire. In this experiment the primary coil should be preferably
+placed so that it extends only about half way into the secondary
+coil. Now touch the free terminal of the secondary with a conducting
+object held in the hand, or else connect it to an insulated<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span>
+body of some size. In this manner the potential on the wire
+may be enormously raised. The effect of this will be either to
+increase, or to diminish, the streams. If they increase, the wire
+is too short; if they diminish, it is too long. By adjusting the
+length of the wire, a point is found where the touching of the
+other terminal does not at all affect the streams. In this case
+the rise of potential is exactly counteracted by the drop through
+the coil. It will be observed that small lengths of wire produce
+considerable difference in the magnitude and luminosity of the
+streams. The primary coil is placed sidewise for two reasons:
+First, to increase the potential at the wire; and, second, to increase
+the drop through the coil. The sensitiveness is thus augmented.</p>
+
+<p>There is still another and far more striking peculiarity of the
+brush discharge produced by very rapidly alternating currents.
+To observe this it is best to replace the usual terminals of the
+coil by two metal columns insulated with a good thickness of
+ebonite. It is also well to close all fissures and cracks with wax
+so that the brushes cannot form anywhere except at the tops of
+the columns. If the conditions are carefully adjusted&mdash;which,
+of course, must be left to the skill of the experimenter&mdash;so that
+the potential rises to an enormous value, one may produce two
+powerful brushes several inches long, nearly white at their roots,
+which in the dark bear a striking resemblance to two flames of
+a gas escaping under pressure (Fig. 110). But they do not only
+<i>resemble</i>, they <i>are</i> veritable flames, for they are hot. Certainly
+they are not as hot as a gas burner, <i>but they would be so if the
+frequency and the potential would be sufficiently high</i>. Produced
+with, say, twenty thousand alternations per second, the heat is
+easily perceptible even if the potential is not excessively high.
+The heat developed is, of course, due to the impact of the air
+molecules against the terminals and against each other. As, at
+the ordinary pressures, the mean free path is excessively small,
+it is possible that in spite of the enormous initial speed imparted
+to each molecule upon coming in contact with the terminal, its
+progress&mdash;by collision with other molecules&mdash;is retarded to such
+an extent, that it does not get away far from the terminal, but
+may strike the same many times in succession. The higher the
+frequency, the less the molecule is able to get away, and this the
+more so, as for a given effect the potential required is smaller;
+and a frequency is conceivable&mdash;perhaps even obtainable&mdash;at<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span>
+which practically the same molecules would strike the terminal.
+Under such conditions the exchange of the molecules would be
+very slow, and the heat produced at, and very near, the terminal
+would be excessive. But if the frequency would go on increasing
+constantly, the heat produced would begin to diminish for obvious
+reasons. In the positive brush of a static machine the exchange
+of the molecules is very rapid, the stream is constantly
+of one direction, and there are fewer collisions; hence the heating
+effect must be very small. Anything that impairs the facility
+of exchange tends to increase the local heat produced. Thus, if
+a bulb be held over the terminal of the coil so as to enclose the
+brush, the air contained in the bulb is very quickly brought to
+a high temperature. If a glass tube be held over the brush so
+as to allow the draught to carry the brush upwards, scorching hot
+air escapes at the top of the tube. Anything held within the
+brush is, of course, rapidly heated, and the possibility of using
+such heating effects for some purpose or other suggests itself.</p>
+
+<p>When contemplating this singular phenomenon of the hot
+brush, we cannot help being convinced that a similar process
+must take place in the ordinary flame, and it seems strange that
+after all these centuries past of familiarity with the flame, now,
+in this era of electric lighting and heating, we are finally led to
+recognize, that since time immemorial we have, after all, always
+had "electric light and heat" at our disposal. It is also of no
+little interest to contemplate, that we have a possible way of
+producing&mdash;by other than chemical means&mdash;a veritable flame,
+which would give light and heat without any material being
+consumed, without any chemical process taking place, and to
+accomplish this, we only need to perfect methods of producing
+enormous frequencies and potentials. I have no doubt that if
+the potential could be made to alternate with sufficient rapidity
+and power, the brush formed at the end of a wire would lose its
+electrical characteristics and would become flamelike. The flame
+must be due to electrostatic molecular action.</p>
+
+<p>This phenomenon now explains in a manner which can hardly
+be doubted the frequent accidents occurring in storms. It is well
+known that objects are often set on fire without the lightning
+striking them. We shall presently see how this can happen.
+On a nail in a roof, for instance, or on a projection of any kind,
+more or less conducting, or rendered so by dampness, a powerful
+brush may appear. If the lightning strikes somewhere in the<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span>
+neighborhood the enormous potential may be made to alternate
+or fluctuate perhaps many million times a second. The air
+molecules are violently attracted and repelled, and by their impact
+produce such a powerful heating effect that a fire is started.
+It is conceivable that a ship at sea may, in this manner, catch fire
+at many points at once. When we consider, that even with the
+comparatively low frequencies obtained from a dynamo machine,
+and with potentials of no more than one or two hundred thousand
+volts, the heating effects are considerable, we may imagine
+how much more powerful they must be with frequencies and potentials
+many times greater; and the above explanation seems, to
+say the least, very probable. Similar explanations may have been
+suggested, but I am not aware that, up to the present, the heating
+effects of a brush produced by a rapidly alternating potential
+have been experimentally demonstrated, at least not to such a
+remarkable degree.</p>
+
+<div class="figcenter" style="width: 568px;">
+<img src="images/oi_182.jpg" width="568" height="480" alt="Fig. 111." title="" />
+<span class="caption">Fig. 111.</span>
+</div>
+
+
+<p>By preventing completely the exchange of the air molecules,
+the local heating effect may be so exalted as to bring a body to
+incandescence. Thus, for instance, if a small button, or preferably
+a very thin wire or filament be enclosed in an unexhausted
+globe and connected with the terminal of the coil, it may be
+rendered incandescent. The phenomenon is made much more
+interesting by the rapid spinning round in a circle of the top of
+the filament, thus presenting the appearance of a luminous funnel,
+Fig. 111, which widens when the potential is increased.
+When the potential is small the end of the filament may perform
+irregular motions, suddenly changing from one to the other, or
+it may describe an ellipse; but when the potential is very
+high it always spins in a circle; and so does generally a thin<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span>
+straight wire attached freely to the terminal of the coil. These
+motions are, of course, due to the impact of the molecules, and
+the irregularity in the distribution of the potential, owing to the
+roughness and dissymmetry of the wire or filament. With a
+perfectly symmetrical and polished wire such motions would
+probably not occur. That the motion is not likely to be due to
+others causes is evident from the fact that it is not of a definite
+direction, and that in a very highly exhausted globe it ceases
+altogether. The possibility of bringing a body to incandescence
+in an exhausted globe, or even when not at all enclosed, would
+seem to afford a possible way of obtaining light effects, which,
+in perfecting methods of producing rapidly alternating potentials,
+might be rendered available for useful purposes.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_183.jpg" width="800" height="346" alt="Fig. 112a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 112a.</span>
+</div>
+
+
+<p>In employing a commercial coil, the production of very powerful
+brush effects is attended with considerable difficulties, for
+when these high frequencies and enormous potentials are used,
+the best insulation is apt to give way. Usually the coil is insulated
+well enough to stand the strain from convolution to convolution,
+since two double silk covered paraffined wires will withstand
+a pressure of several thousand volts; the difficulty lies
+principally in preventing the breaking through from the secondary
+to the primary, which is greatly facilitated by the streams
+issuing from the latter. In the coil, of course, the strain is greatest
+from section to section, but usually in a larger coil there are
+so many sections that the danger of a sudden giving way is not
+very great. No difficulty will generally be encountered in that
+direction, and besides, the liability of injuring the coil internally
+is very much reduced by the fact that the effect most likely to
+be produced is simply a gradual heating, which, when far enough<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span>
+advanced, could not fail to be observed. The principal necessity
+is then to prevent the streams between the primary and the tube,
+not only on account of the heating and possible injury, but also
+because the streams may diminish very considerably the potential
+difference available at the terminals. A few hints as to how
+this may be accomplished will probably be found useful in most
+of these experiments with the ordinary induction coil.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_184.jpg" width="640" height="369" alt="Fig. 112b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 112b.</span>
+</div>
+
+<p>One of the ways is to wind a short primary, Fig. 112a, so that
+the difference of potential is not at that length great enough to
+cause the breaking forth of the streams through the insulating
+tube. The length of the primary should be determined by experiment.
+Both the ends of the coil should be brought out on one
+end through a plug of insulating material fitting in the tube as
+illustrated. In such a disposition one terminal of the secondary
+is attached to a body, the surface of which is determined with the
+greatest care so as to produce the greatest rise in the potential.
+At the other terminal a powerful brush appears, which may be
+experimented upon.</p>
+
+<p>The above plan necessitates the employment of a primary of
+comparatively small size, and it is apt to heat when powerful effects
+are desirable for a certain length of time. In such a case it
+is better to employ a larger coil, Fig. 112b, and introduce it
+from one side of the tube, until the streams begin to appear. In
+this case the nearest terminal of the secondary may be connected
+to the primary or to the ground, which is practically the same
+thing, if the primary is connected directly to the machine. In the
+case of ground connections it is well to determine experimentally
+the frequency which is best suited under the conditions of the
+test. Another way of obviating the streams, more or less, is to<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span>
+make the primary in sections and supply it from separate, well
+insulated sources.</p>
+
+<p>In many of these experiments, when powerful effects are
+wanted for a short time, it is advantageous to use iron cores with
+the primaries. In such case a very large primary coil may be
+wound and placed side by side with the secondary, and, the nearest
+terminal of the latter being connected to the primary, a laminated
+iron core is introduced through the primary into the secondary
+as far as the streams will permit. Under these conditions
+an excessively powerful brush, several inches long, which may
+be appropriately called "St. Elmo's hot fire," may be caused to
+appear at the other terminal of the secondary, producing striking
+effects. It is a most powerful ozonizer, so powerful indeed, that
+only a few minutes are sufficient to fill the whole room with the
+smell of ozone, and it undoubtedly possesses the quality of exciting
+chemical affinities.</p>
+
+<p>For the production of ozone, alternating currents of very
+high frequency are eminently suited, not only on account of the
+advantages they offer in the way of conversion but also because
+of the fact, that the ozonizing action of a discharge is dependent
+on the frequency as well as on the potential, this being undoubtedly
+confirmed by observation.</p>
+
+<p>In these experiments if an iron core is used it should be carefully
+watched, as it is apt to get excessively hot in an incredibly
+short time. To give an idea of the rapidity of the heating, I
+will state, that by passing a powerful current through a coil with
+many turns, the inserting within the same of a thin iron wire for
+no more than one second's time is sufficient to heat the wire to
+something like 100&deg; C.</p>
+
+<p>But this rapid heating need not discourage us in the use
+of iron cores in connection with rapidly alternating currents.
+I have for a long time been convinced that in the industrial distribution
+by means of transformers, some such plan as the following
+might be practicable. We may use a comparatively small iron
+core, subdivided, or perhaps not even subdivided. We may surround
+this core with a considerable thickness of material which
+is fire-proof and conducts the heat poorly, and on top of that we
+may place the primary and secondary windings. By using either
+higher frequencies or greater magnetizing forces, we may by
+hysteresis and eddy currents heat the iron core so far as to bring
+it nearly to its maximum permeability, which, as Hopkinson has<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span>
+shown, may be as much as sixteen times greater than that at ordinary
+temperatures. If the iron core were perfectly enclosed,
+it would not be deteriorated by the heat, and, if the enclosure of
+fire-proof material would be sufficiently thick, only a limited
+amount of energy could be radiated in spite of the high temperature.
+Transformers have been constructed by me on that
+plan, but for lack of time, no thorough tests have as yet been
+made.</p>
+
+<p>Another way of adapting the iron core to rapid alternations,
+or, generally speaking, reducing the frictional losses, is to produce
+by continuous magnetization a flow of something like seven
+thousand or eight thousand lines per square centimetre through
+the core, and then work with weak magnetizing forces and preferably
+high frequencies around the point of greatest permeability.
+A higher efficiency of conversion and greater output are
+obtainable in this manner. I have also employed this principle
+in connection with machines in which there is no reversal of
+polarity. In these types of machines, as long as there are only
+few pole projections, there is no great gain, as the maxima and
+minima of magnetization are far from the point of maximum
+permeability; but when the number of the pole projections is
+very great, the required rate of change may be obtained, without
+the magnetization varying so far as to depart greatly from the
+point of maximum permeability, and the gain is considerable.</p>
+
+<p>The above described arrangements refer only to the use of
+commercial coils as ordinarily constructed. If it is desired to
+construct a coil for the express purpose of performing with it
+such experiments as I have described, or, generally, rendering it
+capable of withstanding the greatest possible difference of potential,
+then a construction as indicated in Fig. 113 will be found of
+advantage. The coil in this case is formed of two independent
+parts which are wound oppositely, the connection between both
+being made near the primary. The potential in the middle being
+zero, there is not much tendency to jump to the primary and not
+much insulation is required. In some cases the middle point
+may, however, be connected to the primary or to the ground. In
+such a coil the places of greatest difference of potential are far
+apart and the coil is capable of withstanding an enormous strain.
+The two parts may be movable so as to allow a slight adjustment
+of the capacity effect.</p>
+
+<p>As to the manner of insulating the coil, it will be found con<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span>venient
+to proceed in the following way: First, the wire should
+be boiled in paraffine until all the air is out; then the coil is
+wound by running the wire through melted paraffine, merely for
+the purpose of fixing the wire. The coil is then taken off from
+the spool, immersed in a cylindrical vessel filled with pure melted
+wax and boiled for a long time until the bubbles cease to appear.
+The whole is then left to cool down thoroughly, and then the
+mass is taken out of the vessel and turned up in a lathe. A coil
+made in this manner and with care is capable of withstanding
+enormous potential differences.</p>
+
+<div class="figcenter" style="width: 525px;">
+<img src="images/oi_187.jpg" width="525" height="480" alt="Fig. 113." title="" />
+<span class="caption">Fig. 113.</span>
+</div>
+
+
+<p>It may be found convenient to immerse the coil in paraffine oil
+or some other kind of oil; it is a most effective way of insulating,
+principally on account of the perfect exclusion of air, but it may
+be found that, after all, a vessel filled with oil is not a very convenient
+thing to handle in a laboratory.</p>
+
+<p>If an ordinary coil can be dismounted, the primary may be
+taken out of the tube and the latter plugged up at one end, filled
+with oil, and the primary reinserted. This affords an excellent
+insulation and prevents the formation of the streams.</p>
+
+<p>Of all the experiments which may be performed with rapidly
+alternating currents the most interesting are those which concern
+the production of a practical illuminant. It cannot be denied
+that the present methods, though they were brilliant advances,
+are very wasteful. Some better methods must be invented, some
+more perfect apparatus devised. Modern research has opened
+new possibilities for the production of an efficient source of light,
+and the attention of all has been turned in the direction indicated<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span>
+by able pioneers. Many have been carried away by the enthusiasm
+and passion to discover, but in their zeal to reach results, some
+have been misled. Starting with the idea of producing electro-magnetic
+waves, they turned their attention, perhaps, too much
+to the study of electro-magnetic effects, and neglected the study
+of electrostatic phenomena. Naturally, nearly every investigator
+availed himself of an apparatus similar to that used in earlier
+experiments. But in those forms of apparatus, while the electro-magnetic
+inductive effects are enormous, the electrostatic effects
+are excessively small.</p>
+
+<p>In the Hertz experiments, for instance, a high tension induction
+coil is short circuited by an arc, the resistance of which is
+very small, the smaller, the more capacity is attached to the terminals;
+and the difference of potential at these is enormously
+diminished. On the other hand, when the discharge is not passing
+between the terminals, the static effects may be considerable,
+but only qualitatively so, not quantitatively, since their rise and
+fall is very sudden, and since their frequency is small. In neither
+case, therefore, are powerful electrostatic effects perceivable.
+Similar conditions exist when, as in some interesting experiments
+of Dr. Lodge, Leyden jars are discharged disruptively. It has
+been thought&mdash;and I believe asserted&mdash;that in such cases
+most of the energy is radiated into space. In the light of the
+experiments which I have described above, it will now not be
+thought so. I feel safe in asserting that in such cases most of
+the energy is partly taken up and converted into heat in the arc
+of the discharge and in the conducting and insulating material of
+the jar, some energy being, of course, given off by electrification
+of the air; but the amount of the directly radiated energy is very
+small.</p>
+
+<p>When a high tension induction coil, operated by currents alternating
+only 20,000 times a second, has its terminals closed through
+even a very small jar, practically all the energy passes through
+the dielectric of the jar, which is heated, and the electrostatic
+effects manifest themselves outwardly only to a very weak degree.
+Now the external circuit of a Leyden jar, that is, the arc and the
+connections of the coatings, may be looked upon as a circuit generating
+alternating currents of excessively high frequency and
+fairly high potential, which is closed through the coatings and
+the dielectric between them, and from the above it is evident
+that the external electrostatic effects must be very small, even if a<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span>
+recoil circuit be used. These conditions make it appear that with
+the apparatus usually at hand, the observation of powerful electrostatic
+effects was impossible, and what experience has been
+gained in that direction is only due to the great ability of the
+investigators.</p>
+
+<p>But powerful electrostatic effects are a <i>sine qua non</i> of light
+production on the lines indicated by theory. Electro-magnetic
+effects are primarily unavailable, for the reason that to produce
+the required effects we would have to pass current impulses
+through a conductor, which, long before the required frequency
+of the impulses could be reached, would cease to transmit them.
+On the other hand, electro-magnetic waves many times longer
+than those of light, and producible by sudden discharge of a condenser,
+could not be utilized, it would seem, except we avail ourselves
+of their effect upon conductors as in the present methods,
+which are wasteful. We could not affect by means of such waves
+the static molecular or atomic charges of a gas, cause them to vibrate
+and to emit light. Long transverse waves cannot, apparently,
+produce such effects, since excessively small electro-magnetic
+disturbances may pass readily through miles of air. Such dark
+waves, unless they are of the length of true light waves, cannot,
+it would seem, excite luminous radiation in a Geissler tube, and
+the luminous effects, which are producible by induction in a tube
+devoid of electrodes, I am inclined to consider as being of an electrostatic
+nature.</p>
+
+<p>To produce such luminous effects, straight electrostatic thrusts
+are required; these, whatever be their frequency, may disturb
+the molecular charges and produce light. Since current impulses
+of the required frequency cannot pass through a conductor of
+measurable dimensions, we must work with a gas, and then the
+production of powerful electrostatic effects becomes an imperative
+necessity.</p>
+
+<p>It has occurred to me, however, that electrostatic effects are in
+many ways available for the production of light. For instance,
+we may place a body of some refractory material in a closed, and
+preferably more or less exhausted, globe, connect it to a source of
+high, rapidly alternating potential, causing the molecules of the
+gas to strike it many times a second at enormous speeds, and in
+this manner, with trillions of invisible hammers, pound it until it
+gets incandescent; or we may place a body in a very highly exhausted
+globe, in a non-striking vacuum, and, by employing very<span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span>
+high frequencies and potentials, transfer sufficient energy from it
+to other bodies in the vicinity, or in general to the surroundings,
+to maintain it at any degree of incandescence; or we may, by
+means of such rapidly alternating high potentials, disturb the
+ether carried by the molecules of a gas or their static charges,
+causing them to vibrate and to emit light.</p>
+
+<p>But, electrostatic effects being dependent upon the potential
+and frequency, to produce the most powerful action it is desirable
+to increase both as far as practicable. It may be possible to
+obtain quite fair results by keeping either of these factors small,
+provided the other is sufficiently great; but we are limited in
+both directions. My experience demonstrates that we cannot go
+below a certain frequency, for, first, the potential then becomes
+so great that it is dangerous; and, secondly, the light production
+is less efficient.</p>
+
+<p>I have found that, by using the ordinary low frequencies, the
+physiological effect of the current required to maintain at a certain
+degree of brightness a tube four feet long, provided at the
+ends with outside and inside condenser coatings, is so powerful
+that, I think, it might produce serious injury to those not accustomed
+to such shocks; whereas, with twenty thousand alternations
+per second, the tube may be maintained at the same degree
+of brightness without any effect being felt. This is due principally
+to the fact that a much smaller potential is required to produce
+the same light effect, and also to the higher efficiency in the
+light production. It is evident that the efficiency in such cases
+is the greater, the higher the frequency, for the quicker the process
+of charging and discharging the molecules, the less energy
+will be lost in the form of dark radiation. But, unfortunately,
+we cannot go beyond a certain frequency on account of the difficulty
+of producing and conveying the effects.</p>
+
+<p>I have stated above that a body inclosed in an unexhausted
+bulb may be intensely heated by simply connecting it with a
+source of rapidly alternating potential. The heating in such a
+case is, in all probability, due mostly to the bombardment of the
+molecules of the gas contained in the bulb. When the bulb is
+exhausted, the heating of the body is much more rapid, and there
+is no difficulty whatever in bringing a wire or filament to any
+degree of incandescence by simply connecting it to one terminal
+of a coil of the proper dimensions. Thus, if the well-known apparatus
+of Prof. Crookes, consisting of a bent platinum wire with<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span>
+vanes mounted over it (Fig. 114), be connected to one terminal of
+the coil&mdash;either one or both ends of the platinum wire being connected&mdash;the
+wire is rendered almost instantly incandescent, and
+the mica vanes are rotated as though a current from a battery
+were used. A thin carbon filament, or, preferably, a button of
+some refractory material (Fig. 115), even if it be a comparatively
+poor conductor, inclosed in an exhausted globe, may be rendered
+highly incandescent; and in this manner a simple lamp capable
+of giving any desired candle power is provided.</p>
+
+<p>The success of lamps of this kind would depend largely on the
+selection of the light-giving bodies contained within the bulb.
+Since, under the conditions described, refractory bodies&mdash;which
+are very poor conductors and capable of withstanding for a long
+time excessively high degrees of temperature&mdash;may be used,
+such illuminating devices may be rendered successful.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_191.jpg" width="800" height="321" alt="Fig. 114, 115." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 114.</td><td class="caption">Fig. 115.</td></tr>
+</table>
+</div>
+
+
+<p>It might be thought at first that if the bulb, containing the
+filament or button of refractory material, be perfectly well exhausted&mdash;that
+is, as far as it can be done by the use of the best
+apparatus&mdash;the heating would be much less intense, and that in
+a perfect vacuum it could not occur at all. This is not confirmed
+by my experience; quite the contrary, the better the vacuum
+the more easily the bodies are brought to incandescence. This
+result is interesting for many reasons.</p>
+
+<p>At the outset of this work the idea presented itself to me,
+whether two bodies of refractory material enclosed in a bulb exhausted
+to such a degree that the discharge of a large induction
+coil, operated in the usual manner, cannot pass through, could be
+rendered incandescent by mere condenser action. Obviously, to
+reach this result enormous potential differences and very high
+frequencies are required, as is evident from a simple calculation.<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span></p>
+
+<p>But such a lamp would possess a vast advantage over an ordinary
+incandescent lamp in regard to efficiency. It is well-known
+that the efficiency of a lamp is to some extent a function of the
+degree of incandescence, and that, could we but work a filament
+at many times higher degrees of incandescence, the efficiency
+would be much greater. In an ordinary lamp this is impracticable
+on account of the destruction of the filament, and it has been
+determined by experience how far it is advisable to push the incandescence.
+It is impossible to tell how much higher efficiency
+could be obtained if the filament could withstand indefinitely,
+as the investigation to this end obviously cannot be carried beyond
+a certain stage; but there are reasons for believing that it
+would be very considerably higher. An improvement might be
+made in the ordinary lamp by employing a short and thick carbon;
+but then the leading-in wires would have to be thick, and,
+besides, there are many other considerations which render such a
+modification entirely impracticable. But in a lamp as above described,
+the leading in wires may be very small, the incandescent
+refractory material may be in the shape of blocks offering a very
+small radiating surface, so that less energy would be required to
+keep them at the desired incandescence; and in addition to this,
+the refractory material need not be carbon, but may be manufactured
+from mixtures of oxides, for instance, with carbon or other
+material, or may be selected from bodies which are practically
+non-conductors, and capable of withstanding enormous degrees of
+temperature.</p>
+
+<p>All this would point to the possibility of obtaining a much
+higher efficiency with such a lamp than is obtainable in ordinary
+lamps. In my experience it has been demonstrated that the
+blocks are brought to high degrees of incandescence with much
+lower potentials than those determined by calculation, and the
+blocks may be set at greater distances from each other. We may
+freely assume, and it is probable, that the molecular bombardment
+is an important element in the heating, even if the globe
+be exhausted with the utmost care, as I have done; for although
+the number of the molecules is, comparatively speaking, insignificant,
+yet on account of the mean free path being very great,
+there are fewer collisions, and the molecules may reach much
+higher speeds, so that the heating effect due to this cause may
+be considerable, as in the Crookes experiments with radiant
+matter.<span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span></p>
+
+<p>But it is likewise possible that we have to deal here with an
+increased facility of losing the charge in very high vacuum, when
+the potential is rapidly alternating, in which case most of the
+heating would be directly due to the surging of the charges in
+the heated bodies. Or else the observed fact may be largely
+attributable to the effect of the points which I have mentioned
+above, in consequence of which the blocks or filaments contained
+in the vacuum are equivalent to condensers of many times
+greater surface than that calculated from their geometrical dimensions.
+Scientific men still differ in opinion as to whether a
+charge should, or should not, be lost in a perfect vacuum, or in
+other words, whether ether is, or is not, a conductor. If the
+former were the case, then a thin filament enclosed in a perfectly
+exhausted globe, and connected to a source of enormous, steady
+potential, would be brought to incandescence.</p>
+
+<div class="figcenter" style="width: 619px;">
+<img src="images/oi_193.jpg" width="619" height="480" alt="Fig. 116, 117." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 116.</td><td class="caption">Fig. 117.</td></tr>
+</table>
+</div>
+
+<p>Various forms of lamps on the above described principle, with
+the refractory bodies in the form of filaments, Fig. 116, or blocks,
+Fig. 117, have been constructed and operated by me, and investigations
+are being carried on in this line. There is no difficulty in
+reaching such high degrees of incandescence that ordinary carbon
+is to all appearance melted and volatilized. If the vacuum
+could be made absolutely perfect, such a lamp, although inoperative
+with apparatus ordinarily used, would, if operated with cur<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span>rents
+of the required character, afford an illuminant which would
+never be destroyed, and which would be far more efficient than
+an ordinary incandescent lamp. This perfection can, of course,
+never be reached, and a very slow destruction and gradual diminution
+in size always occurs, as in incandescent filaments; but there
+is no possibility of a sudden and premature disabling which occurs
+in the latter by the breaking of the filament, especially
+when the incandescent bodies are in the shape of blocks.</p>
+
+<p>With these rapidly alternating potentials there is, however, no
+necessity of enclosing two blocks in a globe, but a single block,
+as in Fig. 115, or filament, Fig. 118, may be used. The potential
+in this case must of course be higher, but is easily obtainable,
+and besides it is not necessarily dangerous.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_194.jpg" width="336" height="378" alt="Fig. 118." title="" />
+<span class="caption">Fig. 118.</span>
+</div>
+
+
+<p>The facility with which the button or filament in such a lamp
+is brought to incandescence, other things being equal, depends
+on the size of the globe. If a perfect vacuum could be obtained,
+the size of the globe would not be of importance, for then the
+heating would be wholly due to the surging of the charges, and
+all the energy would be given off to the surroundings by radiation.
+But this can never occur in practice. There is always
+some gas left in the globe, and although the exhaustion may be
+carried to the highest degree, still the space inside of the bulb
+must be considered as conducting when such high potentials are
+used, and I assume that, in estimating the energy that may be
+given off from the filament to the surroundings, we may consider<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span>
+the inside surface of the bulb as one coating of a condenser, the
+air and other objects surrounding the bulb forming the other
+coating. When the alternations are very low there is no doubt
+that a considerable portion of the energy is given off by the electrification
+of the surrounding air.</p>
+
+<p>In order to study this subject better, I carried on some experiments
+with excessively high potentials and low frequencies. I
+then observed that when the hand is approached to the bulb,&mdash;the
+filament being connected with one terminal of the coil,&mdash;a
+powerful vibration is felt, being due to the attraction and repulsion
+of the molecules of the air which are electrified by induction
+through the glass. In some cases when the action is very
+intense I have been able to hear a sound, which must be due to
+the same cause.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_195.jpg" width="800" height="375" alt="Fig. 119, 120." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 119.</td><td class="caption">Fig. 120.</td></tr>
+</table>
+</div>
+
+<p>When the alternations are low, one is apt to get an excessively
+powerful shock from the bulb. In general, when one attaches
+bulbs or objects of some size to the terminals of the coil, one
+should look out for the rise of potential, for it may happen that
+by merely connecting a bulb or plate to the terminal, the potential
+may rise to many times its original value. When lamps are
+attached to the terminals, as illustrated in Fig. 119, then the
+capacity of the bulbs should be such as to give the maximum
+rise of potential under the existing conditions. In this manner
+one may obtain the required potential with fewer turns of
+wire.</p>
+
+<p>The life of such lamps as described above depends, of course,
+largely on the degree of exhaustion, but to some extent also on
+the shape of the block of refractory material. Theoretically it<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span>
+would seem that a small sphere of carbon enclosed in a sphere of
+glass would not suffer deterioration from molecular bombardment,
+for, the matter in the globe being radiant, the molecules
+would move in straight lines, and would seldom strike the sphere
+obliquely. An interesting thought in connection with such a
+lamp is, that in it "electricity" and electrical energy apparently
+must move in the same lines.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_196.jpg" width="480" height="536" alt="Fig. 121a, 121b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 121a.</td><td class="caption1"><span class="smcap">Fig.</span> 121b.</td></tr>
+</table>
+</div>
+
+<p>The use of alternating currents of very high frequency makes
+it possible to transfer, by electrostatic or electromagnetic induction
+through the glass of a lamp, sufficient energy to keep a filament
+at incandescence and so do away with the leading-in wires.
+Such lamps have been proposed, but for want of proper apparatus
+they have not been successfully operated. Many forms of
+lamps on this principle with continuous and broken filaments
+have been constructed by me and experimented upon. When
+using a secondary enclosed within the lamp, a condenser is advantageously
+combined with the secondary. When the transference
+is effected by electrostatic induction, the potentials used are,
+of course, very high with frequencies obtainable from a machine.
+For instance, with a condenser surface of forty square centimetres,<span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span>
+which is not impracticably large, and with glass of good quality
+1 mm. thick, using currents alternating twenty thousand times
+a second, the potential required is approximately 9,000 volts.
+This may seem large, but since each lamp may be included
+in the secondary of a transformer of very small dimensions, it
+would not be inconvenient, and, moreover, it would not produce
+fatal injury. The transformers would all be preferably in series.
+The regulation would offer no difficulties, as with currents of such
+frequencies it is very easy to maintain a constant current.</p>
+
+<p>In the accompanying engravings some of the types of lamps of
+this kind are shown. Fig. 120 is such a lamp with a broken filament,
+and Figs. 121 <small>A</small> and 121 <small>B</small> one with a single outside and
+inside coating and a single filament. I have also made lamps
+with two outside and inside coatings and a continuous loop connecting
+the latter. Such lamps have been operated by me with
+current impulses of the enormous frequencies obtainable by the
+disruptive discharge of condensers.</p>
+
+<p>The disruptive discharge of a condenser is especially suited for
+operating such lamps&mdash;with no outward electrical connections&mdash;by
+means of electromagnetic induction, the electromagnetic inductive
+effects being excessively high; and I have been able to
+produce the desired incandescence with only a few short turns of
+wire. Incandescence may also be produced in this manner in a
+simple closed filament.</p>
+
+<p>Leaving now out of consideration the practicability of such
+lamps, I would only say that they possess a beautiful and desirable
+feature, namely, that they can be rendered, at will, more or
+less brilliant simply by altering the relative position of the outside
+and inside condenser coatings, or inducing and induced circuits.</p>
+
+<p>When a lamp is lighted by connecting it to one terminal only
+of the source, this may be facilitated by providing the globe with
+an outside condenser coating, which serves at the same time as a
+reflector, and connecting this to an insulated body of some size.
+Lamps of this kind are illustrated in Fig. 122 and Fig. 123.
+Fig. 124 shows the plan of connection. The brilliancy of the
+lamp may, in this case, be regulated within wide limits by varying
+the size of the insulated metal plate to which the coating is
+connected.</p>
+
+<p>It is likewise practicable to light with one leading wire lamps
+such as illustrated in Fig. 116 and Fig. 117, by connecting one<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span>
+terminal of the lamp to one terminal of the source, and the
+other to an insulated body of the required size. In all cases
+the insulated body serves to give off the energy into the surrounding
+space, and is equivalent to a return wire. Obviously,
+in the two last-named cases, instead of connecting the wires to
+an insulated body, connections may be made to the ground.</p>
+
+<p>The experiments which will prove most suggestive and of
+most interest to the investigator are probably those performed
+with exhausted tubes. As might be anticipated, a source of such
+rapidly alternating potentials is capable of exciting the tubes at
+a considerable distance, and the light effects produced are remarkable.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_198.jpg" width="800" height="503" alt="Fig. 122, 123." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 122.</td><td class="caption">Fig. 123.</td></tr>
+</table>
+</div>
+
+<p>During my investigations in this line I endeavored to excite
+tubes, devoid of any electrodes, by electromagnetic induction,
+making the tube the secondary of the induction device, and
+passing through the primary the discharges of a Leyden jar.
+These tubes were made of many shapes, and I was able to
+obtain luminous effects which I then thought were due wholly
+to electromagnetic induction. But on carefully investigating
+the phenomena I found that the effects produced were more
+of an electrostatic nature. It may be attributed to this circumstance
+that this mode of exciting tubes is very wasteful,
+namely, the primary circuit being closed, the potential, and
+consequently the electrostatic inductive effect, is much diminished.<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span></p>
+
+<p>When an induction coil, operated as above described, is used,
+there is no doubt that the tubes are excited by electrostatic induction,
+and that electromagnetic induction has little, if anything,
+to do with the phenomena.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_199.jpg" width="800" height="293" alt="Fig. 124." title="" />
+<span class="caption">Fig. 124.</span>
+</div>
+
+
+<p>This is evident from many experiments. For instance, if a
+tube be taken in one hand, the observer being near the coil, it is
+brilliantly lighted and remains so no matter in what position it is
+held relatively to the observer's body. Were the action electromagnetic,
+the tube could not be lighted when the observer's
+body is interposed between it and the coil, or at least its luminosity
+should be considerably diminished. When the tube is
+held exactly over the centre of the coil&mdash;the latter being wound
+in sections and the primary placed symmetrically to the secondary&mdash;it
+may remain completely dark, whereas it is rendered
+intensely luminous by moving it slightly to the right or left
+from the centre of the coil. It does not light because in the
+middle both halves of the coil neutralize each other, and the
+electric potential is zero. If the action were electromagnetic,
+the tube should light best in the plane through the centre of the
+coil, since the electromagnetic effect there should be a maximum.
+When an arc is established between the terminals, the tubes and
+lamps in the vicinity of the coil go out, but light up again
+when the arc is broken, on account of the rise of potential. Yet
+the electromagnetic effect should be practically the same in both
+cases.</p>
+
+<p>By placing a tube at some distance from the coil, and nearer to
+one terminal&mdash;preferably at a point on the axis of the coil&mdash;one
+may light it by touching the remote terminal with an insulated
+body of some size or with the hand, thereby raising the potential
+at that terminal nearer to the tube. If the tube is shifted nearer
+to the coil so that it is lighted by the action of the nearer termi<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span>nal,
+it may be made to go out by holding, on an insulated support,
+the end of a wire connected to the remote terminal, in the
+vicinity of the nearer terminal, by this means counteracting the
+action of the latter upon the tube. These effects are evidently
+electrostatic. Likewise, when a tube is placed at a considerable
+distance from the coil, the observer may, standing upon an insulated
+support between coil and tube, light the latter by approaching
+the hand to it; or he may even render it luminous by simply
+stepping between it and the coil. This would be impossible with
+electro-magnetic induction, for the body of the observer would
+act as a screen.</p>
+
+<p>When the coil is energized by excessively weak currents, the
+experimenter may, by touching one terminal of the coil with the
+tube, extinguish the latter, and may again light it by bringing it
+out of contact with the terminal and allowing a small arc to form.
+This is clearly due to the respective lowering and raising of the
+potential at that terminal. In the above experiment, when the
+tube is lighted through a small arc, it may go out when the arc is
+broken, because the electrostatic inductive effect alone is too
+weak, though the potential may be much higher; but when the
+arc is established, the electrification of the end of the tube is
+much greater, and it consequently lights.</p>
+
+<p>If a tube is lighted by holding it near to the coil, and in the
+hand which is remote, by grasping the tube anywhere with the
+other hand, the part between the hands is rendered dark, and the
+singular effect of wiping out the light of the tube may be produced
+by passing the hand quickly along the tube and at the
+same time withdrawing it gently from the coil, judging properly
+the distance so that the tube remains dark afterwards.</p>
+
+<p>If the primary coil is placed sidewise, as in Fig. 112 <small>B</small> for instance,
+and an exhausted tube be introduced from the other side
+in the hollow space, the tube is lighted most intensely because of
+the increased condenser action, and in this position the stri&aelig; are
+most sharply defined. In all these experiments described, and in
+many others, the action is clearly electrostatic.</p>
+
+<p>The effects of screening also indicate the electrostatic nature
+of the phenomena and show something of the nature of electrification
+through the air. For instance, if a tube is placed in the
+direction of the axis of the coil, and an insulated metal plate be
+interposed, the tube will generally increase in brilliancy, or if it
+be too far from the coil to light, it may even be rendered lumin<span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span>ous
+by interposing an insulated metal plate. The magnitude of
+the effects depends to some extent on the size of the plate. But if
+the metal plate be connected by a wire to the ground, its interposition
+will always make the tube go out even if it be very near the
+coil. In general, the interposition of a body between the coil and
+tube, increases or diminishes the brilliancy of the tube, or its
+facility to light up, according to whether it increases or diminishes
+the electrification. When experimenting with an insulated
+plate, the plate should not be taken too large, else it will generally
+produce a weakening effect by reason of its great facility for giving
+off energy to the surroundings.</p>
+
+<p>If a tube be lighted at some distance from the coil, and a plate
+of hard rubber or other insulating substance be interposed, the
+tube may be made to go out. The interposition of the dielectric
+in this case only slightly increases the inductive effect, but diminishes
+considerably the electrification through the air.</p>
+
+<p>In all cases, then, when we excite luminosity in exhausted
+tubes by means of such a coil, the effect is due to the rapidly
+alternating electrostatic potential; and, furthermore, it must be
+attributed to the harmonic alternation produced directly by the
+machine, and not to any superimposed vibration which might be
+thought to exist. Such superimposed vibrations are impossible
+when we work with an alternate current machine. If a spring be
+gradually tightened and released, it does not perform independent
+vibrations; for this a sudden release is necessary. So with
+the alternate currents from a dynamo machine; the medium is
+harmonically strained and released, this giving rise to only one
+kind of waves; a sudden contact or break, or a sudden giving
+way of the dielectric, as in the disruptive discharge of a Leyden
+jar, are essential for the production of superimposed waves.</p>
+
+<p>In all the last described experiments, tubes devoid of any electrodes
+may be used, and there is no difficulty in producing by
+their means sufficient light to read by. The light effect is, however,
+considerably increased by the use of phosphorescent bodies
+such as yttria, uranium glass, etc. A difficulty will be found
+when the phosphorescent material is used, for with these powerful
+effects, it is carried gradually away, and it is preferable to use
+material in the form of a solid.</p>
+
+<p>Instead of depending on induction at a distance to light the
+tube, the same may be provided with an external&mdash;and, if desired,
+also with an internal&mdash;condenser coating, and it may then<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span>
+be suspended anywhere in the room from a conductor connected
+to one terminal of the coil, and in this manner a soft illumination
+may be provided.</p>
+
+<div class="figcenter" style="width: 286px;">
+<img src="images/oi_202.jpg" width="286" height="640" alt="Fig. 125." title="" />
+<span class="caption">Fig. 125.</span>
+</div>
+
+
+<p>The ideal way of lighting a hall or room would, however, be
+to produce such a condition in it that an illuminating device
+could be moved and put anywhere, and that it is lighted, no matter
+where it is put and without being electrically connected to<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span>
+anything. I have been able to produce such a condition by creating
+in the room a powerful, rapidly alternating electrostatic
+field. For this purpose I suspend a sheet of metal a distance
+from the ceiling on insulating cords and connect it to one terminal
+of the induction coil, the other terminal being preferably connected
+to the ground. Or else I suspend two sheets as illustrated
+in Fig. 125, each sheet being connected with one of the terminals
+of the coil, and their size being carefully determined. An exhausted
+tube may then be carried in the hand anywhere between
+the sheets or placed anywhere, even a certain distance
+beyond them; it remains always luminous.</p>
+
+<p>In such an electrostatic field interesting phenomena may be
+observed, especially if the alternations are kept low and the potentials
+excessively high. In addition to the luminous phenomena
+mentioned, one may observe that any insulated conductor gives
+sparks when the hand or another object is approached to it, and
+the sparks may often be powerful. When a large conducting
+object is fastened on an insulating support, and the hand approached
+to it, a vibration, due to the rythmical motion of the
+air molecules is felt, and luminous streams may be perceived
+when the hand is held near a pointed projection. When a telephone
+receiver is made to touch with one or both of its terminals
+an insulated conductor of some size, the telephone emits a loud
+sound; it also emits a sound when a length of wire is attached to
+one or both terminals, and with very powerful fields a sound may
+be perceived even without any wire.</p>
+
+<p>How far this principle is capable of practical application, the
+future will tell. It might be thought that electrostatic effects
+are unsuited for such action at a distance. Electromagnetic inductive
+effects, if available for the production of light, might be
+thought better suited. It is true the electrostatic effects diminish
+nearly with the cube of the distance from the coil, whereas
+the electromagnetic inductive effects diminish simply with the
+distance. But when we establish an electrostatic field of force,
+the condition is very different, for then, instead of the differential
+effect of both the terminals, we get their conjoint effect.
+Besides, I would call attention to the effect, that in an alternating
+electrostatic field, a conductor, such as an exhausted tube,
+for instance, tends to take up most of the energy, whereas in an
+electromagnetic alternating field the conductor tends to take up
+the least energy, the waves being reflected with but little loss.<span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span>
+This is one reason why it is difficult to excite an exhausted tube,
+at a distance, by electromagnetic induction. I have wound coils
+of very large diameter and of many turns of wire, and connected
+a Geissler tube to the ends of the coil with the object of exciting
+the tube at a distance; but even with the powerful inductive
+effects producible by Leyden jar discharges, the tube could not
+be excited unless at a very small distance, although some judgment
+was used as to the dimensions of the coil. I have also
+found that even the most powerful Leyden jar discharges are
+capable of exciting only feeble luminous effects in a closed exhausted
+tube, and even these effects upon thorough examination
+I have been forced to consider of an electrostatic nature.</p>
+
+<p>How then can we hope to produce the required effects at a
+distance by means of electromagnetic action, when even in the
+closest proximity to the source of disturbance, under the most
+advantageous conditions, we can excite but faint luminosity? It
+is true that when acting at a distance we have the resonance to
+help us out. We can connect an exhausted tube, or whatever
+the illuminating device may be, with an insulated system of the
+proper capacity, and so it may be possible to increase the effect
+qualitatively, and only qualitatively, for we would not get <i>more</i>
+energy through the device. So we may, by resonance effect,
+obtain the required electromotive force in an exhausted tube, and
+excite faint luminous effects, but we cannot get enough energy to
+render the light practically available, and a simple calculation,
+based on experimental results, shows that even if all the energy
+which a tube would receive at a certain distance from the source
+should be wholly converted into light, it would hardly satisfy the
+practical requirements. Hence the necessity of directing, by
+means of a conducting circuit, the energy to the place of transformation.
+But in so doing we cannot very sensibly depart from
+present methods, and all we could do would be to improve the
+apparatus.</p>
+
+<p>From these considerations it would seem that if this ideal way
+of lighting is to be rendered practicable it will be only by the use
+of electrostatic effects. In such a case the most powerful electrostatic
+inductive effects are needed; the apparatus employed must,
+therefore, be capable of producing high electrostatic potentials
+changing in value with extreme rapidity. High frequencies are
+especially wanted, for practical considerations make it desirable
+to keep down the potential. By the employment of machines,<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span>
+or, generally speaking, of any mechanical apparatus, but low
+frequencies can be reached; recourse must, therefore, be had to
+some other means. The discharge of a condenser affords us a
+means of obtaining frequencies by far higher than are obtainable
+mechanically, and I have accordingly employed condensers in the
+experiments to the above end.</p>
+
+<p>When the terminals of a high tension induction coil, Fig. 126,
+are connected to a Leyden jar, and the latter is discharging disruptively
+into a circuit, we may look upon the arc playing between
+the knobs as being a source of alternating, or generally
+speaking, undulating currents, and then we have to deal with
+the familiar system of a generator of such currents, a circuit connected
+to it, and a condenser bridging the circuit. The condenser
+in such case is a veritable transformer, and since the frequency is
+excessive, almost any ratio in the strength of the currents in both
+the branches may be obtained. In reality the analogy is not quite
+complete, for in the disruptive discharge we have most generally
+a fundamental instantaneous variation of comparatively low frequency,
+and a superimposed harmonic vibration, and the laws
+governing the flow of currents are not the same for both.</p>
+
+<p>In converting in this manner, the ratio of conversion should
+not be too great, for the loss in the arc between the knobs increases
+with the square of the current, and if the jar be discharged
+through very thick and short conductors, with the view of obtaining
+a very rapid oscillation, a very considerable portion of the
+energy stored is lost. On the other hand, too small ratios are not
+practicable for many obvious reasons.</p>
+
+<p>As the converted currents flow in a practically closed circuit,
+the electrostatic effects are necessarily small, and I therefore convert
+them into currents or effects of the required character. I
+have effected such conversions in several ways. The preferred
+plan of connections is illustrated in Fig. 127. The manner of operating
+renders it easy to obtain by means of a small and inexpensive
+apparatus enormous differences of potential which have been
+usually obtained by means of large and expensive coils. For this
+it is only necessary to take an ordinary small coil, adjust to it a
+condenser and discharging circuit, forming the primary of an
+auxiliary small coil, and convert upward. As the inductive effect
+of the primary currents is excessively great, the second coil need
+have comparatively but very few turns. By properly adjusting
+the elements, remarkable results may be secured.<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span></p>
+
+<p>In endeavoring to obtain the required electrostatic effects in
+this manner, I have, as might be expected, encountered many
+difficulties which I have been gradually overcoming, but I am not
+as yet prepared to dwell upon my experiences in this direction.</p>
+
+<p>I believe that the disruptive discharge of a condenser will play
+an important part in the future, for it offers vast possibilities,
+not only in the way of producing light in a more efficient manner
+and in the line indicated by theory, but also in many other respects.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_206.jpg" width="640" height="230" alt="Fig. 126." title="" />
+<span class="caption">Fig. 126.</span>
+</div>
+
+<p>For years the efforts of inventors have been directed towards
+obtaining electrical energy from heat by means of the thermopile.
+It might seem invidious to remark that but few know
+what is the real trouble with the thermopile. It is not the inefficiency
+or small output&mdash;though these are great drawbacks&mdash;but
+the fact that the thermopile has its phylloxera, that is, that
+by constant use it is deteriorated, which has thus far prevented its
+introduction on an industrial scale. Now that all modern research
+seems to point with certainty to the use of electricity of excessively
+high tension, the question must present itself to many
+whether it is not possible to obtain in a practicable manner this
+form of energy from heat. We have been used to look upon
+an electrostatic machine as a plaything, and somehow we couple
+with it the idea of the inefficient and impractical. But now we
+must think differently, for now we know that everywhere we
+have to deal with the same forces, and that it is a mere question
+of inventing proper methods or apparatus for rendering them
+available.</p>
+
+<p>In the present systems of electrical distribution, the employment
+of the iron with its wonderful magnetic properties allows
+us to reduce considerably the size of the apparatus; but, in spite
+of this, it is still very cumbersome. The more we progress in
+the study of electric and magnetic phenomena, the more we be<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span>come
+convinced that the present methods will be short-lived. For
+the production of light, at least, such heavy machinery would
+seem to be unnecessary. The energy required is very small, and
+if light can be obtained as efficiently as, theoretically, it appears
+possible, the apparatus need have but a very small output.
+There being a strong probability that the illuminating methods
+of the future will involve the use of very high potentials, it seems
+very desirable to perfect a contrivance capable of converting the
+energy of heat into energy of the requisite form. Nothing to
+speak of has been done towards this end, for the thought that
+electricity of some 50,000 or 100,000 volts pressure or more, even
+if obtained, would be unavailable for practical purposes, has deterred
+inventors from working in this direction.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_207.jpg" width="640" height="229" alt="Fig. 127." title="" />
+<span class="caption">Fig. 127.</span>
+</div>
+
+
+<p>In Fig. 126 a plan of connections is shown for converting
+currents of high, into currents of low, tension by means of the
+disruptive discharge of a condenser. This plan has been used by
+me frequently for operating a few incandescent lamps required
+in the laboratory. Some difficulties have been encountered in the
+arc of the discharge which I have been able to overcome to a great
+extent; besides this, and the adjustment necessary for the proper
+working, no other difficulties have been met with, and it was easy
+to operate ordinary lamps, and even motors, in this manner.
+The line being connected to the ground, all the wires could be
+handled with perfect impunity, no matter how high the potential
+at the terminals of the condenser. In these experiments a high
+tension induction coil, operated from a battery or from an alternate
+current machine, was employed to charge the condenser; but
+the induction coil might be replaced by an apparatus of a different
+kind, capable of giving electricity of such high tension. In
+this manner, direct or alternating currents may be converted, and
+in both cases the current-impulses may be of any desired frequency.
+When the currents charging the condenser are of the<span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span>
+same direction, and it is desired that the converted currents
+should also be of one direction, the resistance of the discharging
+circuit should, of course, be so chosen that there are no
+oscillations.</p>
+
+<div class="figcenter" style="width: 324px;">
+<img src="images/oi_208.jpg" width="324" height="640" alt="Fig. 128." title="" />
+<span class="caption">Fig. 128.</span>
+</div>
+
+
+<p>In operating devices on the above plan I have observed curious
+phenomena of impedance which are of interest. For instance
+if a thick copper bar be bent, as indicated in Fig. 128, and shunted
+by ordinary incandescent lamps, then, by passing the discharge
+between the knobs, the lamps may be brought to incandescence
+although they are short-circuited. When a large induction coil
+is employed it is easy to obtain nodes on the bar, which are
+rendered evident by the different degree of brilliancy of the
+lamps, as shown roughly in Fig. 128. The nodes are never clearly
+defined, but they are simply maxima and minima of potentials
+along the bar. This is probably due to the irregularity of the arc
+between the knobs. In general when the above-described plan
+of conversion from high to low tension is used, the behavior of
+the disruptive discharge may be closely studied. The nodes may
+also be investigated by means of an ordinary Cardew voltmeter<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span>
+which should be well insulated. Geissler tubes may also be
+lighted across the points of the bent bar; in this case, of course,
+it is better to employ smaller capacities. I have found it practicable
+to light up in this manner a lamp, and even a Geissler
+tube, shunted by a short, heavy block of metal, and this result
+seems at first very curious. In fact, the thicker the copper bar
+in Fig. 128, the better it is for the success of the experiments, as
+they appear more striking. When lamps with long slender filaments
+are used it will be often noted that the filaments are from
+time to time violently vibrated, the vibration being smallest at
+the nodal points. This vibration seems to be due to an electrostatic
+action between the filament and the glass of the bulb.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_209.jpg" width="640" height="437" alt="Fig. 129." title="" />
+<span class="caption">Fig. 129.</span>
+</div>
+
+
+<p>In some of the above experiments it is preferable to use special
+lamps having a straight filament as shown in Fig. 129. When
+such a lamp is used a still more curious phenomenon than those
+described may be observed. The lamp may be placed across the
+copper bar and lighted, and by using somewhat larger capacities,
+or, in other words, smaller frequencies or smaller impulsive impedances,
+the filament may be brought to any desired degree of
+incandescence. But when the impedance is increased, a point is
+reached when comparatively little current passes through the
+carbon, and most of it through the rarefied gas; or perhaps it
+may be more correct to state that the current divides nearly
+evenly through both, in spite of the enormous difference in the
+resistance, and this would be true unless the gas and the filament
+behave differently. It is then noted that the whole bulb is brilliantly
+illuminated, and the ends of the leading-in wires become
+incandescent and often throw off sparks in consequence of the
+violent bombardment, but the carbon filament remains dark.
+This is illustrated in Fig. 129. Instead of the filament a single<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span>
+wire extending through the whole bulb may be used, and in this
+case the phenomenon would seem to be still more interesting.</p>
+
+<p>From the above experiment it will be evident, that when ordinary
+lamps are operated by the converted currents, those should
+be preferably taken in which the platinum wires are far apart,
+and the frequencies used should not be too great, else the discharge
+will occur at the ends of the filament or in the base of the
+lamp between the leading-in wires, and the lamp might then be
+damaged.</p>
+
+<p>In presenting to you these results of my investigation on the
+subject under consideration, I have paid only a passing notice to
+facts upon which I could have dwelt at length, and among many
+observations I have selected only those which I thought most
+likely to interest you. The field is wide and completely unexplored,
+and at every step a new truth is gleaned, a novel fact
+observed.</p>
+
+<p>How far the results here borne out are capable of practical
+applications will be decided in the future. As regards the production
+of light, some results already reached are encouraging
+and make me confident in asserting that the practical solution of
+the problem lies in the direction I have endeavored to indicate.
+Still, whatever may be the immediate outcome of these experiments
+I am hopeful that they will only prove a step in further
+development towards the ideal and final perfection. The possibilities
+which are opened by modern research are so vast that
+even the most reserved must feel sanguine of the future. Eminent
+scientists consider the problem of utilizing one kind of
+radiation without the others a rational one. In an apparatus designed
+for the production of light by conversion from any form
+of energy into that of light, such a result can never be reached,
+for no matter what the process of producing the required vibrations,
+be it electrical, chemical or any other, it will not be possible
+to obtain the higher light vibrations without going through
+the lower heat vibrations. It is the problem of imparting to a
+body a certain velocity without passing through all lower velocities.
+But there is a possibility of obtaining energy not only in
+the form of light, but motive power, and energy of any other
+form, in some more direct way from the medium. The time will
+be when this will be accomplished, and the time has come when
+one may utter such words before an enlightened audience without
+being considered a visionary. We are whirling through<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span>
+endless space with an inconceivable speed, all around us everything
+is spinning, everything is moving, everywhere is energy.
+There <i>must</i> be some way of availing ourselves of this energy
+more directly. Then, with the light obtained from the medium,
+with the power derived from it, with every form of energy
+obtained without effort, from the store forever inexhaustible,
+humanity will advance with giant strides. The mere contemplation
+of these magnificent possibilities expands our minds, strengthens
+our hopes and fills our hearts with supreme delight.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span></p>
+<h2><a name="CHAPTER_XXVII" id="CHAPTER_XXVII"></a>CHAPTER XXVII.</h2>
+
+<h3><span class="smcap">Experiments with Alternate Currents of High Potential
+and High Frequency.</span><a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a></h3>
+
+
+<p>I cannot find words to express how deeply I feel the honor of
+addressing some of the foremost thinkers of the present time,
+and so many able scientific men, engineers and electricians, of
+the country greatest in scientific achievements.</p>
+
+<p>The results which I have the honor to present before such a
+gathering I cannot call my own. There are among you not a
+few who can lay better claim than myself on any feature of
+merit which this work may contain. I need not mention many
+names which are world-known&mdash;names of those among you who
+are recognized as the leaders in this enchanting science; but one,
+at least, I must mention&mdash;a name which could not be omitted in
+a demonstration of this kind. It is a name associated with the
+most beautiful invention ever made: it is Crookes!</p>
+
+<p>When I was at college, a good while ago, I read, in a translation
+(for then I was not familiar with your magnificent language), the
+description of his experiments on radiant matter. I read it only
+once in my life&mdash;that time&mdash;yet every detail about that charming
+work I can remember to this day. Few are the books, let me
+say, which can make such an impression upon the mind of a
+student.</p>
+
+<p>But if, on the present occasion, I mention this name as one of
+many your Institution can boast of, it is because I have more
+than one reason to do so. For what I have to tell you and to
+show you this evening concerns, in a large measure, that same
+vague world which Professor Crookes has so ably explored; and,
+more than this, when I trace back the mental process which led
+me to these advances&mdash;which even by myself cannot be considered
+trifling, since they are so appreciated by you&mdash;I believe
+that their real origin, that which started me to work in this
+<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span>direction, and brought me to them, after a long period of constant
+thought, was that fascinating little book which I read many
+years ago.</p>
+
+<p>And now that I have made a feeble effort to express my
+homage and acknowledge my indebtedness to him and others
+among you, I will make a second effort, which I hope you will
+not find so feeble as the first, to entertain you.</p>
+
+<p>Give me leave to introduce the subject in a few words.</p>
+
+<p>A short time ago I had the honor to bring before our American
+Institute of Electrical Engineers some results then arrived
+at by me in a novel line of work. I need not assure you that
+the many evidences which I have received that English scientific
+men and engineers were interested in this work have been for
+me a great reward and encouragement. I will not dwell upon
+the experiments already described, except with the view of completing,
+or more clearly expressing, some ideas advanced by me
+before, and also with the view of rendering the study here presented
+self-contained, and my remarks on the subject of this
+evening's lecture consistent.</p>
+
+<p>This investigation, then, it goes without saying, deals with
+alternating currents, and to be more precise, with alternating
+currents of high potential and high frequency. Just in how
+much a very high frequency is essential for the production of
+the results presented is a question which, even with my present
+experience, would embarrass me to answer. Some of the experiments
+may be performed with low frequencies; but very high
+frequencies are desirable, not only on account of the many effects
+secured by their use, but also as a convenient means of obtaining,
+in the induction apparatus employed, the high potentials, which in
+their turn are necessary to the demonstration of most of the experiments
+here contemplated.</p>
+
+<p>Of the various branches of electrical investigation, perhaps the
+most interesting and the most immediately promising is that
+dealing with alternating currents. The progress in this branch
+of applied science has been so great in recent years that it justifies
+the most sanguine hopes. Hardly have we become familiar
+with one fact, when novel experiences are met and new avenues
+of research are opened. Even at this hour possibilities not
+dreamed of before are, by the use of these currents, partly realized.
+As in nature all is ebb and tide, all is wave motion, so it
+seems that in all branches of industry alternating currents&mdash;electric
+wave motion&mdash;will have the sway.<span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span></p>
+
+<p>One reason, perhaps, why this branch of science is being so
+rapidly developed is to be found in the interest which is attached
+to its experimental study. We wind a simple ring of iron with
+coils; we establish the connections to the generator, and with
+wonder and delight we note the effects of strange forces which
+we bring into play, which allow us to transform, to transmit and
+direct energy at will. We arrange the circuits properly, and we
+see the mass of iron and wires behave as though it were endowed
+with life, spinning a heavy armature, through invisible connections,
+with great speed and power&mdash;with the energy possibly conveyed
+from a great distance. We observe how the energy of an
+alternating current traversing the wire manifests itself&mdash;not so
+much in the wire as in the surrounding space&mdash;in the most surprising
+manner, taking the forms of heat, light, mechanical
+energy, and, most surprising of all, even chemical affinity. All
+these observations fascinate us, and fill us with an intense desire
+to know more about the nature of these phenomena. Each day
+we go to our work in the hope of discovering,&mdash;in the hope that
+some one, no matter who, may find a solution of one of the pending
+great problems,&mdash;and each succeeding day we return to our
+task with renewed ardor; and even if we <i>are</i> unsuccessful, our
+work has not been in vain, for in these strivings, in these efforts,
+we have found hours of untold pleasure, and we have directed
+our energies to the benefit of mankind.</p>
+
+<p>We may take&mdash;at random, if you choose&mdash;any of the many experiments
+which may be performed with alternating currents;
+a few of which only, and by no means the most striking, form
+the subject of this evening's demonstration; they are all equally
+interesting, equally inciting to thought.</p>
+
+<p>Here is a simple glass tube from which the air has been partially
+exhausted. I take hold of it; I bring my body in contact
+with a wire conveying alternating currents of high potential, and
+the tube in my hand is brilliantly lighted. In whatever position
+I may put it, wherever I move it in space, as far as I can reach,
+its soft, pleasing light persists with undiminished brightness.</p>
+
+<p>Here is an exhausted bulb suspended from a single wire.
+Standing on an insulated support, I grasp it, and a platinum button
+mounted in it is brought to vivid incandescence.</p>
+
+<p>Here, attached to a leading wire, is another bulb, which, as I
+touch its metallic socket, is filled with magnificent colors of phosphorescent
+light.<span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span></p>
+
+<p>Here still another, which by my fingers' touch casts a shadow&mdash;the
+Crookes shadow&mdash;of the stem inside of it.</p>
+
+<p>Here, again, insulated as I stand on this platform, I bring my
+body in contact with one of the terminals of the secondary of
+this induction coil&mdash;with the end of a wire many miles long&mdash;and
+you see streams of light break forth from its distant end, which
+is set in violent vibration.</p>
+
+<p>Here, once more, I attach these two plates of wire gauze to the
+terminals of the coil; I set them a distance apart, and I set the
+coil to work. You may see a small spark pass between the
+plates. I insert a thick plate of one of the best dielectrics between
+them, and instead of rendering altogether impossible, as
+we are used to expect, I <i>aid</i> the passage of the discharge, which,
+as I insert the plate, merely changes in appearance and assumes
+the form of luminous streams.</p>
+
+<p>Is there, I ask, can there be, a more interesting study than that
+of alternating currents?</p>
+
+<p>In all these investigations, in all these experiments, which are
+so very, very interesting, for many years past&mdash;ever since the
+greatest experimenter who lectured in this hall discovered its
+principle&mdash;we have had a steady companion, an appliance familiar
+to every one, a plaything once, a thing of momentous importance
+now&mdash;the induction coil. There is no dearer appliance to the
+electrician. From the ablest among you, I dare say, down to the
+inexperienced student, to your lecturer, we all have passed many
+delightful hours in experimenting with the induction coil. We
+have watched its play, and thought and pondered over the beautiful
+phenomena which it disclosed to our ravished eyes. So
+well known is this apparatus, so familiar are these phenomena to
+every one, that my courage nearly fails me when I think that I
+have ventured to address so able an audience, that I have ventured
+to entertain you with that same old subject. Here in reality
+is the same apparatus, and here are the same phenomena, only
+the apparatus is operated somewhat differently, the phenomena
+are presented in a different aspect. Some of the results we find
+as expected, others surprise us, but all captivate our attention, for
+in scientific investigation each novel result achieved may be the
+centre of a new departure, each novel fact learned may lead to
+important developments.</p>
+
+<p>Usually in operating an induction coil we have set up a vibration
+of moderate frequency in the primary, either by means of an<span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span>
+interrupter or break, or by the use of an alternator. Earlier
+English investigators, to mention only Spottiswoode and J. E. H.
+Gordon, have used a rapid break in connection with the coil.
+Our knowledge and experience of to-day enables us to see clearly
+why these coils under the conditions of the test did not disclose
+any remarkable phenomena, and why able experimenters failed
+to perceive many of the curious effects which have since been
+observed.</p>
+
+<p>In the experiments such as performed this evening, we operate
+the coil either from a specially constructed alternator capable of
+giving many thousands of reversals of current per second, or, by
+disruptively discharging a condenser through the primary, we set
+up a vibration in the secondary circuit of a frequency of many
+hundred thousand or millions per second, if we so desire; and in
+using either of these means we enter a field as yet unexplored.</p>
+
+<p>It is impossible to pursue an investigation in any novel line
+without finally making some interesting observation or learning
+some useful fact. That this statement is applicable to the subject
+of this lecture the many curious and unexpected phenomena
+which we observe afford a convincing proof. By way of illustration,
+take for instance the most obvious phenomena, those of the
+discharge of the induction coil.</p>
+
+<p>Here is a coil which is operated by currents vibrating with
+extreme rapidity, obtained by disruptively discharging a Leyden
+jar. It would not surprise a student were the lecturer to say
+that the secondary of this coil consists of a small length of comparatively
+stout wire; it would not surprise him were the lecturer
+to state that, in spite of this, the coil is capable of giving any
+potential which the best insulation of the turns is able to withstand;
+but although he may be prepared, and even be indifferent
+as to the anticipated result, yet the aspect of the discharge of the
+coil will surprise and interest him. Every one is familiar with
+the discharge of an ordinary coil; it need not be reproduced
+here. But, by way of contrast, here is a form of discharge of a
+coil, the primary current of which is vibrating several hundred
+thousand times per second. The discharge of an ordinary coil
+appears as a simple line or band of light. The discharge of this
+coil appears in the form of powerful brushes and luminous
+streams issuing from all points of the two straight wires attached
+to the terminals of the secondary. (Fig. 130.)</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_217.jpg" width="600" height="628" alt="Fig. 130, 131." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 130.</td><td class="caption">Fig. 131.</td></tr>
+</table>
+</div>
+
+<p>Now compare this phenomenon which you have just witnessed
+<span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span>with the discharge of a Holtz or Wimshurst machine&mdash;that other
+interesting appliance so dear to the experimenter. What a difference
+there is between these phenomena! And yet, had I made
+the necessary arrangements&mdash;which could have been made easily,
+were it not that they would interfere with other experiments&mdash;I
+could have produced with this coil sparks which, had I the coil
+hidden from your view and only two knobs exposed, even the
+keenest observer among you would find it difficult, if not impossible,
+to distinguish from those of an influence or friction machine.
+This may be done in many ways&mdash;for instance, by operating
+the induction coil which charges the condenser from an
+alternating-current machine of very low frequency, and preferably
+adjusting the discharge circuit so that there are no oscillations
+set up in it. We then obtain in the secondary circuit, if the
+knobs are of the required size and properly set, a more or less<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span>
+rapid succession of sparks of great intensity and small quantity,
+which possess the same brilliancy, and are accompanied by the
+same sharp crackling sound, as those obtained from a friction or
+influence machine.</p>
+
+<p>Another way is to pass through two primary circuits, having a
+common secondary, two currents of a slightly different period,
+which produce in the secondary circuit sparks occurring at comparatively
+long intervals. But, even with the means at hand
+this evening, I may succeed in imitating the spark of a Holtz
+machine. For this purpose I establish between the terminals of
+the coil which charges the condenser a long, unsteady arc, which
+is periodically interrupted by the upward current of air produced
+by it. To increase the current of air I place on each side of the
+arc, and close to it, a large plate of mica. The condenser charged
+from this coil discharges into the primary circuit of a second
+coil through a small air gap, which is necessary to produce a
+sudden rush of current through the primary. The scheme of
+connections in the present experiment is indicated in Fig. 131.</p>
+
+<p><small>G</small> is an ordinarily constructed alternator, supplying the primary
+<small>P</small> of an induction coil, the secondary <small>S</small> of which charges
+the condensers or jars <small>C C</small>. The terminals of the secondary are
+connected to the inside coatings of the jars, the outer coatings
+being connected to the ends of the primary <i>p p</i> of a second induction
+coil. This primary <i>p p</i> has a small air gap <i>a b</i>.</p>
+
+<p>The secondary <i>s</i> of this coil is provided with knobs or spheres
+<small>K K</small> of the proper size and set at a distance suitable for the experiment.</p>
+
+<p>A long arc is established between the terminals <small>A B</small> of the first
+induction coil. <small>M M</small> are the mica plates.</p>
+
+<p>Each time the arc is broken between <small>A</small> and <small>B</small> the jars are
+quickly charged and discharged through the primary <i>p p</i>, producing
+a snapping spark between the knobs <small>K K</small>. Upon the arc
+forming between <small>A</small> and <small>B</small> the potential falls, and the jars cannot
+be charged to such high potential as to break through the air
+gap <i>a b</i> until the arc is again broken by the draught.</p>
+
+<p>In this manner sudden impulses, at long intervals, are produced
+in the primary <i>p p</i>, which in the secondary <i>s</i> give a corresponding
+number of impulses of great intensity. If the secondary
+knobs or spheres, <small>K K</small>, are of the proper size, the sparks
+show much resemblance to those of a Holtz machine.</p>
+
+<p>But these two effects, which to the eye appear so very differ<span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span>ent,
+are only two of the many discharge phenomena. We only
+need to change the conditions of the test, and again we make
+other observations of interest.</p>
+
+<p>When, instead of operating the induction coil as in the last
+two experiments, we operate it from a high frequency alternator,
+as in the next experiment, a systematic study of the phenomena
+is rendered much more easy. In such case, in varying the
+strength and frequency of the currents through the primary, we
+may observe five distinct forms of discharge, which I have described
+in my former paper on the subject before the American
+Institute of Electrical Engineers, May 20, 1891.</p>
+
+<p>It would take too much time, and it would lead us too far
+from the subject presented this evening, to reproduce all these
+forms, but it seems to me desirable to show you one of them. It
+is a brush discharge, which is interesting in more than one respect.
+Viewed from a near position it resembles much a jet of
+gas escaping under great pressure. We know that the phenomenon
+is due to the agitation of the molecules near the terminal,
+and we anticipate that some heat must be developed by the impact
+of the molecules against the terminal or against each other.
+Indeed, we find that the brush is hot, and only a little thought
+leads us to the conclusion that, could we but reach sufficiently
+high frequencies, we could produce a brush which would give
+intense light and heat, and which would resemble in every particular
+an ordinary flame, save, perhaps, that both phenomena
+might not be due to the same agent&mdash;save, perhaps, that chemical
+affinity might not be <i>electrical</i> in its nature.</p>
+
+<p>As the production of heat and light is here due to the impact
+of the molecules, or atoms of air, or something else besides,
+and, as we can augment the energy simply by raising the
+potential, we might, even with frequencies obtained from
+a dynamo machine, intensify the action to such a degree as to
+bring the terminal to melting heat. But with such low frequencies
+we would have to deal always with something of the nature
+of an electric current. If I approach a conducting object to the
+brush, a thin little spark passes, yet, even with the frequencies
+used this evening, the tendency to spark is not very great. So,
+for instance, if I hold a metallic sphere at some distance above
+the terminal, you may see the whole space between the terminal
+and sphere illuminated by the streams without the spark passing;
+and with the much higher frequencies obtainable by the disrup<span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span>tive
+discharge of a condenser, were it not for the sudden impulses,
+which are comparatively few in number, sparking would not
+occur even at very small distances. However, with incomparably
+higher frequencies, which we may yet find means to produce
+efficiently, and provided that electric impulses of such high
+frequencies could be transmitted through a conductor, the electrical
+characteristics of the brush discharge would completely
+vanish&mdash;no spark would pass, no shock would be felt&mdash;yet we
+would still have to deal with an <i>electric</i> phenomenon, but in the
+broad, modern interpretation of the word. In my first paper, before
+referred to, I have pointed out the curious properties of the
+brush, and described the best manner of producing it, but I have
+thought it worth while to endeavor to express myself more clearly
+in regard to this phenomenon, because of its absorbing interest.</p>
+
+<p>When a coil is operated with currents of very high frequency,
+beautiful brush effects may be produced, even if the coil be of
+comparatively small dimensions. The experimenter may vary
+them in many ways, and, if it were for nothing else, they afford a
+pleasing sight. What adds to their interest is that they may be
+produced with one single terminal as well as with two&mdash;in fact,
+often better with one than with two.</p>
+
+<p>But of all the discharge phenomena observed, the most pleasing
+to the eye, and the most instructive, are those observed with
+a coil which is operated by means of the disruptive discharge of
+a condenser. The power of the brushes, the abundance of the
+sparks, when the conditions are patiently adjusted, is often amazing.
+With even a very small coil, if it be so well insulated as to
+stand a difference of potential of several thousand volts per turn,
+the sparks may be so abundant that the whole coil may appear
+a complete mass of fire.</p>
+
+<p>Curiously enough the sparks, when the terminals of the coil
+are set at a considerable distance, seem to dart in every possible
+direction as though the terminals were perfectly independent of
+each other. As the sparks would soon destroy the insulation, it
+is necessary to prevent them. This is best done by immersing
+the coil in a good liquid insulator, such as boiled-out oil. Immersion
+in a liquid may be considered almost an absolute necessity
+for the continued and successful working of such a coil.</p>
+
+<p>It is, of course, out of the question, in an experimental lecture,
+with only a few minutes at disposal for the performance of each
+experiment, to show these discharge phenomena to advantage,<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span>
+as, to produce each phenomenon at its best, a very careful adjustment
+is required. But even if imperfectly produced, as they are
+likely to be this evening, they are sufficiently striking to interest
+an intelligent audience.</p>
+
+<p>Before showing some of these curious effects I must, for the
+sake of completeness, give a short description of the coil and
+other apparatus used in the experiments with the disruptive discharge
+this evening.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_221.jpg" width="600" height="674" alt="Fig. 132." title="" />
+<span class="caption">Fig. 132.</span>
+</div>
+
+
+<p>It is contained in a box <small>B</small> (Fig. 132) of thick boards of hard
+wood, covered on the outside with a zinc sheet <small>Z</small>, which is carefully
+soldered all around. It might be advisable, in a strictly scientific
+investigation, when accuracy is of great importance, to do away
+with the metal cover, as it might introduce many errors, principally
+on account of its complex action upon the coil, as a condenser
+of very small capacity and as an electrostatic and electromagnetic
+screen. When the coil is used for such experiments as
+are here contemplated, the employment of the metal cover offers
+some practical advantages, but these are not of sufficient importance
+to be dwelt upon.</p>
+
+<p>The coil should be placed symmetrically to the metal cover,<span class='pagenum'><a name="Page_208" id="Page_208">[Pg 208]</a></span>
+and the space between should, of course, not be too small, certainly
+not less than, say, five centimetres, but much more if possible;
+especially the two sides of the zinc box, which are at right
+angles to the axis of the coil, should be sufficiently remote from
+the latter, as otherwise they might impair its action and be a
+source of loss.</p>
+
+<p>The coil consists of two spools of hard rubber <small>R R</small>, held apart
+at a distance of 10 centimetres by bolts <small>C</small> and nuts <i>n</i>, likewise of
+hard rubber. Each spool comprises a tube <small>T</small> of approximately 8
+centimetres inside diameter, and 3 millimetres thick, upon which
+are screwed two flanges <small>F F</small>, 24 centimetres square, the space between
+the flanges being about 3 centimetres. The secondary, <small>S S</small>,
+of the best gutta percha-covered wire, has 26 layers, 10 turns in
+each, giving for each half a total of 260 turns. The two halves
+are wound oppositely and connected in series, the connection between
+both being made over the primary. This disposition, besides
+being convenient, has the advantage that when the coil is
+well balanced&mdash;that is, when both of its terminals <small>T<sub>1</sub></small>, <small>T<sub>1</sub></small>, are connected
+to bodies or devices of equal capacity&mdash;there is not much
+danger of breaking through to the primary, and the insulation
+between the primary and the secondary need not be thick. In
+using the coil it is advisable to attach to <i>both</i> terminals devices of
+nearly equal capacity, as, when the capacity of the terminals is
+not equal, sparks will be apt to pass to the primary. To avoid
+this, the middle point of the secondary may be connected to the
+primary, but this is not always practicable.</p>
+
+<p>The primary <small>P P</small> is wound in two parts, and oppositely, upon
+a wooden spool w, and the four ends are led out of the oil through
+hard rubber tubes <i>t t</i>. The ends of the secondary <small>T<sub>1</sub> T<sub>1</sub></small>, are also
+led out of the oil through rubber tubes <i>t</i><sub>1</sub> <i>t</i><sub>1</sub> of great thickness.
+The primary and secondary layers are insulated by cotton cloth,
+the thickness of the insulation, of course, bearing some proportion
+to the difference of potential between the turns of the different
+layers. Each half of the primary has four layers, 24 turns
+in each, this giving a total of 96 turns. When both the parts
+are connected in series, this gives a ratio of conversion of about
+1:2.7, and with the primaries in multiple, 1:5.4; but in operating
+with very rapidly alternating currents this ratio does not convey
+even an approximate idea of the ratio of the <span class="smcap">e. m. f</span>'s. in the
+primary and secondary circuits. The coil is held in position in
+the oil on wooden supports, there being about 5 centimetres<span class='pagenum'><a name="Page_209" id="Page_209">[Pg 209]</a></span>
+thickness of oil all round. Where the oil is not specially needed,
+the space is filled with pieces of wood, and for this purpose
+principally the wooden box B surrounding the whole is used.</p>
+
+<p>The construction here shown is, of course, not the best on
+general principles, but I believe it is a good and convenient one
+for the production of effects in which an excessive potential and
+a very small current are needed.</p>
+
+<p>In connection with the coil I use either the ordinary form of
+discharger or a modified form. In the former I have introduced
+two changes which secure some advantages, and which are obvious.
+If they are mentioned, it is only in the hope that some
+experimenter may find them of use.</p>
+
+<p>One of the changes is that the adjustable knobs <small>A</small> and <small>B</small> (Fig.
+133), of the discharger are held in jaws of brass, <small>J J</small>, by spring
+pressure, this allowing of turning them successively into different
+positions, and so doing away with the tedious process of frequent
+polishing up.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_223.jpg" width="800" height="461" alt="Fig. 133." title="" />
+<span class="caption">Fig. 133.</span>
+</div>
+
+<p>The other change consists in the employment of a strong electromagnet
+<small>N S</small>, which is placed with its axis at right angles to
+the line joining the knobs <small>A</small> and <small>B</small>, and produces a strong magnetic
+field between them. The pole pieces of the magnet are
+movable and properly formed so as to protrude between the brass
+knobs, in order to make the field as intense as possible; but to
+prevent the discharge from jumping to the magnet the pole
+pieces are protected by a layer of mica, <small>M M</small>, of sufficient thickness;
+<i>s</i><sub>1</sub> <i>s</i><sub>1</sub> and <i>s</i><sub>2</sub> <i>s</i><sub>2</sub> are screws for fastening the wires. On each
+side one of the screws is for large and the other for small wires.
+<small>L L</small> are screws for fixing in position the rods <small>R R</small>, which support
+the knobs.<span class='pagenum'><a name="Page_210" id="Page_210">[Pg 210]</a></span></p>
+
+<p>In another arrangement with the magnet I take the discharge
+between the rounded pole pieces themselves, which in such
+case are insulated and preferably provided with polished brass
+caps.</p>
+
+<p>The employment of an intense magnetic field is of advantage
+principally when the induction coil or transformer which charges
+the condenser is operated by currents of very low frequency. In
+such a case the number of the fundamental discharges between
+the knobs may be so small as to render the currents produced in
+the secondary unsuitable for many experiments. The intense
+magnetic field then serves to blow out the arc between the knobs
+as soon as it is formed, and the fundamental discharges occur in
+quicker succession.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_224.jpg" width="800" height="331" alt="Fig. 134." title="" />
+<span class="caption">Fig. 134.</span>
+</div>
+
+<p>Instead of the magnet, a draught or blast of air may be employed
+with some advantage. In this case the arc is preferably
+established between the knobs <small>A B</small>, in Fig. 131 (the knobs <i>a b</i>
+being generally joined, or entirely done away with), as in this
+disposition the arc is long and unsteady, and is easily affected by
+the draught.</p>
+
+<p>When a magnet is employed to break the arc, it is better to
+choose the connection indicated diagrammatically in Fig. 134,
+as in this case the currents forming the arc are much more powerful,
+and the magnetic field exercises a greater influence. The
+use of the magnet permits, however, of the arc being replaced by
+a vacuum tube, but I have encountered great difficulties in working
+with an exhausted tube.</p>
+
+<p>The other form of discharger used in these and similar experiments
+is indicated in Figs. 135 and 136. It consists of a number
+of brass pieces <i>c c</i> (Fig. 135), each of which comprises a spherical
+middle portion <i>m</i> with an extension <i>e</i> below&mdash;which is merely used
+to fasten the piece in a lathe when polishing up the discharging<span class='pagenum'><a name="Page_211" id="Page_211">[Pg 211]</a></span>
+surface&mdash;and a column above, which consists of a knurled flange
+<i>f</i> surmounted by a threaded stem <i>l</i> carrying a nut <i>n</i>, by means
+of which a wire is fastened to the column. The flange <i>f</i> conveniently
+serves for holding the brass piece when fastening the
+wire, and also for turning it in any position when it becomes
+necessary to present a fresh discharging surface. Two stout
+strips of hard rubber <small>R R</small>, with planed grooves <i>g g</i> (Fig. 136) to fit
+the middle portion of the pieces <i>c c</i>, serve to clamp the latter
+and hold them firmly in position by means of two bolts <small>C C</small>
+(of which only one is shown) passing through the ends of the
+strips.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_225.jpg" width="800" height="312" alt="Fig. 135." title="" />
+<span class="caption">Fig. 135.</span>
+</div>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_225-1.jpg" width="800" height="506" alt="Fig. 136." title="" />
+<span class="caption">Fig. 136.</span>
+</div>
+
+<p>In the use of this kind of discharger I have found three principal
+advantages over the ordinary form. First, the dielectric
+strength of a given total width of air space is greater when a
+great many small air gaps are used instead of one, which permits
+of working with a smaller length of air gap, and that means
+smaller loss and less deterioration of the metal; secondly, by
+reason of splitting the arc up into smaller arcs, the polished
+surfaces are made to last much longer; and, thirdly, the appa<span class='pagenum'><a name="Page_212" id="Page_212">[Pg 212]</a></span>ratus
+affords some gauge in the experiments. I usually set the
+pieces by putting between them sheets of uniform thickness at a
+certain very small distance which is known from the experiments
+of Sir William Thomson to require a certain electromotive force
+to be bridged by the spark.</p>
+
+<p>It should, of course, be remembered that the sparking distance
+is much diminished as the frequency is increased. By taking
+any number of spaces the experimenter has a rough idea of the
+electromotive force, and he finds it easier to repeat an experiment,
+as he has not the trouble of setting the knobs again and
+again. With this kind of discharger I have been able to maintain
+an oscillating motion without any spark being visible with
+the naked eye between the knobs, and they would not show a
+very appreciable rise in temperature. This form of discharge
+also lends itself to many arrangements of condensers and circuits
+which are often very convenient and time-saving. I have used
+it preferably in a disposition similar to that indicated in Fig. 131,
+when the currents forming the arc are small.</p>
+
+<p>I may here mention that I have also used dischargers with
+single or multiple air gaps, in which the discharge surfaces were
+rotated with great speed. No particular advantage was, however,
+gained by this method, except in cases where the currents
+from the condenser were large and the keeping cool of the surfaces
+was necessary, and in cases when, the discharge not being
+oscillating of itself, the arc as soon as established was broken by
+the air current, thus starting the vibration at intervals in rapid
+succession. I have also used mechanical interrupters in many
+ways. To avoid the difficulties with frictional contacts, the preferred
+plan adopted was to establish the arc and rotate through
+it at great speed a rim of mica provided with many holes and
+fastened to a steel plate. It is understood, of course, that the
+employment of a magnet, air current, or other interrupter, produces
+no effect worth noticing, unless the self-induction, capacity
+and resistance are so related that there are oscillations set up
+upon each interruption.</p>
+
+<p>I will now endeavor to show you some of the most noteworthy
+of these discharge phenomena.</p>
+
+<p>I have stretched across the room two ordinary cotton covered
+wires, each about seven metres in length. They are supported
+on insulating cords at a distance of about thirty centimetres. I
+attach now to each of the terminals of the coil one of the wires,<span class='pagenum'><a name="Page_213" id="Page_213">[Pg 213]</a></span>
+and set the coil in action. Upon turning the lights off in the
+room you see the wires strongly illuminated by the streams issuing
+abundantly from their whole surface in spite of the cotton
+covering, which may even be very thick. When the experiment
+is performed under good conditions, the light from the wires is
+sufficiently intense to allow distinguishing the objects in a room.
+To produce the best result it is, of course, necessary to adjust
+carefully the capacity of the jars, the arc between the knobs and
+the length of the wires. My experience is that calculation of the
+length of the wires leads, in such case, to no result whatever. The
+experimenter will do best to take the wires at the start very long,
+and then adjust by cutting off first long pieces, and then smaller
+and smaller ones as he approaches the right length.</p>
+
+<p>A convenient way is to use an oil condenser of very small
+capacity, consisting of two small adjustable metal plates, in connection
+with this and similar experiments. In such case I take
+wires rather short and at the beginning set the condenser plates
+at maximum distance. If the streams from the wires increase by
+approach of the plates, the length of the wires is about right; if
+they diminish, the wires are too long for that frequency and potential.
+When a condenser is used in connection with experiments
+with such a coil, it should be an oil condenser by all means,
+as in using an air condenser considerable energy might be wasted.
+The wires leading to the plates in the oil should be very thin,
+heavily coated with some insulating compound, and provided
+with a conducting covering&mdash;this preferably extending under the
+surface of the oil. The conducting cover should not be too near
+the terminals, or ends, of the wire, as a spark would be apt to
+jump from the wire to it. The conducting coating is used to
+diminish the air losses, in virtue of its action as an electrostatic
+screen. As to the size of the vessel containing the oil, and the
+size of the plates, the experimenter gains at once an idea from a
+rough trial. The size of the plates <i>in oil</i> is, however, calculable,
+as the dielectric losses are very small.</p>
+
+<p>In the preceding experiment it is of considerable interest to
+know what relation the quantity of the light emitted bears to
+the frequency and potential of the electric impulses. My opinion
+is that the heat as well as light effects produced should be proportionate,
+under otherwise equal conditions of test, to the product
+of frequency and square of potential, but the experimental verification
+of the law, whatever it may be, would be exceedingly<span class='pagenum'><a name="Page_214" id="Page_214">[Pg 214]</a></span>
+difficult. One thing is certain, at any rate, and that is, that in
+augmenting the potential and frequency we rapidly intensify the
+streams; and, though it may be very sanguine, it is surely not
+altogether hopeless to expect that we may succeed in producing
+a practical illuminant on these lines. We would then be simply
+using burners or flames, in which there would be no chemical
+process, no consumption of material, but merely a transfer of
+energy, and which would, in all probability, emit more light and
+less heat than ordinary flames.</p>
+
+<div class="figcenter" style="width: 471px;">
+<img src="images/oi_228.jpg" width="471" height="640" alt="Fig. 137." title="" />
+<span class="caption">Fig. 137.</span>
+</div>
+
+<p>The luminous intensity of the streams is, of course, considerably
+increased when they are focused upon a small surface. This may
+be shown by the following experiment:</p>
+
+<p>I attach to one of the terminals of the coil a wire <i>w</i> (Fig. 137),
+bent in a circle of about 30 centimetres in diameter, and to the
+other terminal I fasten a small brass sphere <i>s</i>, the surface of the
+wire being preferably equal to the surface of the sphere, and the
+centre of the latter being in a line at right angles to the plane of
+the wire circle and passing through its centre. When the discharge
+is established under proper conditions, a luminous hollow
+cone is formed, and in the dark one-half of the brass sphere is
+strongly illuminated, as shown in the cut.</p>
+
+<p>By some artifice or other it is easy to concentrate the streams<span class='pagenum'><a name="Page_215" id="Page_215">[Pg 215]</a></span>
+upon small surfaces and to produce very strong light effects.
+Two thin wires may thus be rendered intensely luminous.</p>
+
+<p>In order to intensify the streams the wires should be very thin
+and short; but as in this case their capacity would be generally
+too small for the coil&mdash;at least for such a one as the present&mdash;it
+is necessary to augment the capacity to the required value, while,
+at the same time, the surface of the wires remains very small.
+This may be done in many ways.</p>
+
+<div class="figcenter" style="width: 550px;">
+<img src="images/oi_229.jpg" width="550" height="480" alt="Fig. 138." title="" />
+<span class="caption">Fig. 138.</span>
+</div>
+
+
+<p>Here, for instance, I have two plates, <small>R R</small>, of hard rubber (Fig.
+138), upon which I have glued two very thin wires <i>w w</i>, so as to
+form a name. The wires may be bare or covered with the best
+insulation&mdash;it is immaterial for the success of the experiment.
+Well insulated wires, if anything, are preferable. On the back
+of each plate, indicated by the shaded portion, is a tinfoil coating
+<i>t t</i>. The plates are placed in line at a sufficient distance to prevent
+a spark passing from one wire to the other. The two tinfoil
+coatings I have joined by a conductor <small>C</small>, and the two wires I
+presently connect to the terminals of the coil. It is now easy, by
+varying the strength and frequency of the currents through the
+primary, to find a point at which the capacity of the system is
+best suited to the conditions, and the wires become so strongly
+luminous that, when the light in the room is turned off the name
+formed by them appears in brilliant letters.</p>
+
+<p>It is perhaps preferable to perform this experiment with a
+coil operated from an alternator of high frequency, as then,<span class='pagenum'><a name="Page_216" id="Page_216">[Pg 216]</a></span>
+owing to the harmonic rise and fall, the streams are very uniform,
+though they are less abundant than when produced with such a
+coil as the present one. This experiment, however, may be performed
+with low frequencies, but much less satisfactorily.</p>
+
+<div class="figcenter" style="width: 406px;">
+<img src="images/oi_230.jpg" width="406" height="640" alt="Fig. 139." title="" />
+<span class="caption">Fig. 139.</span>
+</div>
+
+
+<p>When two wires, attached to the terminals of the coil, are set
+at the proper distance, the streams between them may be so intense
+as to produce a continuous luminous sheet. To show this
+phenomenon I have here two circles, <small>C</small> and <i>c</i> (Fig. 139), of rather
+stout wire, one being about 80 centimetres and the other 30 centimetres
+in diameter. To each of the terminals of the coil I
+attach one of the circles. The supporting wires are so bent that
+the circles may be placed in the same plane, coinciding as nearly
+as possible. When the light in the room is turned off and the
+coil set to work, you see the whole space between the wires uniformly
+filled with streams, forming a luminous disc, which could
+be seen from a considerable distance, such is the intensity of the
+streams. The outer circle could have been much larger than the
+present one; in fact, with this coil I have used much larger
+circles, and I have been able to produce a strongly luminous
+sheet, covering an area of more than one square metre, which is
+a remarkable effect with this very small coil. To avoid uncer<span class='pagenum'><a name="Page_217" id="Page_217">[Pg 217]</a></span>tainty,
+the circle has been taken smaller, and the area is now
+about 0.43 square metre.</p>
+
+<p>The frequency of the vibration, and the quickness of succession
+of the sparks between the knobs, affect to a marked degree
+the appearance of the streams. When the frequency is very
+low, the air gives way in more or less the same manner, as by a
+steady difference of potential, and the streams consist of distinct
+threads, generally mingled with thin sparks, which probably correspond
+to the successive discharges occurring between the
+knobs. But when the frequency is extremely high, and the arc
+of the discharge produces a very <i>loud</i> and <i>smooth</i> sound&mdash;showing
+both that oscillation takes place and that the sparks succeed
+each other with great rapidity&mdash;then the luminous streams
+formed are perfectly uniform. To reach this result very small
+coils and jars of small capacity should be used. I take two
+tubes of thick Bohemian glass, about 5 centimetres in diameter
+and 20 centimetres long. In each of the tubes I slip a primary
+of very thick copper wire. On the top of each tube I wind a
+secondary of much thinner gutta-percha covered wire. The two
+secondaries I connect in series, the primaries preferably in multiple
+arc. The tubes are then placed in a large glass vessel, at a distance
+of 10 to 15 centimetres from each other, on insulating supports,
+and the vessel is filled with boiled-out oil, the oil reaching
+about an inch above the tubes. The free ends of the secondary
+are lifted out of the coil and placed parallel to each other at a
+distance of about ten centimetres. The ends which are scraped
+should be dipped in the oil. Two four-pint jars joined in series
+may be used to discharge through the primary. When the necessary
+adjustments in the length and distance of the wires above
+the oil and in the arc of discharge are made, a luminous sheet is
+produced between the wires which is perfectly smooth and textureless,
+like the ordinary discharge through a moderately exhausted
+tube.</p>
+
+<p>I have purposely dwelt upon this apparently insignificant experiment.
+In trials of this kind the experimenter arrives at the
+startling conclusion that, to pass ordinary luminous discharges
+through gases, no particular degree of exhaustion is needed, but
+that the gas may be at ordinary or even greater pressure. To
+accomplish this, a very high frequency is essential; a high potential
+is likewise required, but this is merely an incidental necessity.
+These experiments teach us that, in endeavoring to dis<span class='pagenum'><a name="Page_218" id="Page_218">[Pg 218]</a></span>cover
+novel methods of producing light by the agitation of atoms,
+or molecules, of a gas, we need not limit our research to the
+vacuum tube, but may look forward quite seriously to the possibility
+of obtaining the light effects without the use of any vessel
+whatever, with air at ordinary pressure.</p>
+
+<p>Such discharges of very high frequency, which render luminous
+the air at ordinary pressures, we have probably occasion often to
+witness in Nature. I have no doubt that if, as many believe, the
+aurora borealis is produced by sudden cosmic disturbances, such
+as eruptions at the sun's surface, which set the electrostatic charge
+of the earth in an extremely rapid vibration, the red glow observed
+is not confined to the upper rarefied strata of the air, but
+the discharge traverses, by reason of its very high frequency,
+also the dense atmosphere in the form of a <i>glow</i>, such as we ordinarily
+produce in a slightly exhausted tube. If the frequency
+were very low, or even more so, if the charge were not at all
+vibrating, the dense air would break down as in a lightning discharge.
+Indications of such breaking down of the lower dense
+strata of the air have been repeatedly observed at the occurrence
+of this marvelous phenomenon; but if it does occur, it can only
+be attributed to the fundamental disturbances, which are few in
+number, for the vibration produced by them would be far too
+rapid to allow a disruptive break. It is the original and irregular
+impulses which affect the instruments; the superimposed vibrations
+probably pass unnoticed.</p>
+
+<p>When an ordinary low frequency discharge is passed through
+moderately rarefied air, the air assumes a purplish hue. If by
+some means or other we increase the intensity of the molecular,
+or atomic, vibration, the gas changes to a white color. A similar
+change occurs at ordinary pressures with electric impulses of very
+high frequency. If the molecules of the air around a wire are
+moderately agitated, the brush formed is reddish or violet; if
+the vibration is rendered sufficiently intense, the streams become
+white. We may accomplish this in various ways. In the experiment
+before shown with the two wires across the room, I have
+endeavored to secure the result by pushing to a high value both
+the frequency and potential; in the experiment with the thin
+wires glued on the rubber plate I have concentrated the action
+upon a very small surface&mdash;in other words, I have worked with
+a great electric density.</p>
+
+<div class="figcenter" style="width: 619px;">
+<img src="images/oi_233.jpg" width="619" height="600" alt="Fig. 140." title="" />
+<span class="caption">Fig. 140.</span>
+</div>
+
+
+<p>A most curious form of discharge is observed with such a coil
+<span class='pagenum'><a name="Page_219" id="Page_219">[Pg 219]</a></span>when the frequency and potential are pushed to the extreme
+limit. To perform the experiment, every part of the coil should
+be heavily insulated, and only two small spheres&mdash;or, better still,
+two sharp-edged metal discs (<i>d d</i>, Fig. 140) of no more than
+a few centimetres in diameter&mdash;should be exposed to the air.
+The coil here used is immersed in oil, and the ends of the
+secondary reaching out of the oil are covered with an air-tight
+cover of hard rubber of great thickness. All cracks, if there
+are any, should be carefully stopped up, so that the brush discharge
+cannot form anywhere except on the small spheres or
+plates which are exposed to the air. In this case, since there
+are no large plates or other bodies of capacity attached to the
+terminals, the coil is capable of an extremely rapid vibration.
+The potential may be raised by increasing, as far as the experimenter
+judges proper, the rate of change of the primary current.
+With a coil not widely differing from the present, it is
+best to connect the two primaries in multiple arc; but if the
+secondary should have a much greater number of turns the
+primaries should preferably be used in series, as otherwise the
+vibration might be too fast for the secondary. It occurs under
+these conditions that misty white streams break forth from the
+edges of the discs and spread out phantom-like into space.
+With this coil, when fairly well produced, they are about 25 to
+30 centimetres long. When the hand is held against them no
+sensation is produced, and a spark, causing a shock, jumps from<span class='pagenum'><a name="Page_220" id="Page_220">[Pg 220]</a></span>
+the terminal only upon the hand being brought much nearer.
+If the oscillation of the primary current is rendered intermittent
+by some means or other, there is a corresponding throbbing of
+the streams, and now the hand or other conducting object may
+be brought in still greater proximity to the terminal without a
+spark being caused to jump.</p>
+
+<p>Among the many beautiful phenomena which may be produced
+with such a coil, I have here selected only those which appear
+to possess some features of novelty, and lead us to some
+conclusions of interest. One will not find it at all difficult to
+produce in the laboratory, by means of it, many other phenomena
+which appeal to the eye even more than these here shown, but
+present no particular feature of novelty.</p>
+
+<p>Early experimenters describe the display of sparks produced by
+an ordinary large induction coil upon an insulating plate separating
+the terminals. Quite recently Siemens performed some experiments
+in which fine effects were obtained, which were seen
+by many with interest. No doubt large coils, even if operated
+with currents of low frequencies, are capable of producing
+beautiful effects. But the largest coil ever made could not, by
+far, equal the magnificent display of streams and sparks obtained
+from such a disruptive discharge coil when properly adjusted.
+To give an idea, a coil such as the present one will cover easily
+a plate of one metre in diameter completely with the streams.
+The best way to perform such experiments is to take a very thin
+rubber or a glass plate and glue on one side of it a narrow ring
+of tinfoil of very large diameter, and on the other a circular
+washer, the centre of the latter coinciding with that of the ring,
+and the surfaces of both being preferably equal, so as to keep
+the coil well balanced. The washer and ring should be connected
+to the terminals by heavily insulated thin wires. It is easy in
+observing the effect of the capacity to produce a sheet of uniform
+streams, or a fine network of thin silvery threads, or a
+mass of loud brilliant sparks, which completely cover the plate.</p>
+
+<p>Since I have advanced the idea of the conversion by means of
+the disruptive discharge, in my paper before the American Institute
+of Electrical Engineers at the beginning of the past year,
+the interest excited in it has been considerable. It affords us a
+means for producing any potentials by the aid of inexpensive
+coils operated from ordinary systems of distribution, and&mdash;what
+is perhaps more appreciated&mdash;it enables us to convert currents of<span class='pagenum'><a name="Page_221" id="Page_221">[Pg 221]</a></span>
+any frequency into currents of any other lower or higher frequency.
+But its chief value will perhaps be found in the help
+which it will afford us in the investigations of the phenomena
+of phosphorescence, which a disruptive discharge coil is capable
+of exciting in innumerable cases where ordinary coils, even the
+largest, would utterly fail.</p>
+
+<p>Considering its probable uses for many practical purposes, and
+its possible introduction into laboratories for scientific research,
+a few additional remarks as to the construction of such a coil
+will perhaps not be found superfluous.</p>
+
+<p>It is, of course, absolutely necessary to employ in such a coil
+wires provided with the best insulation.</p>
+
+<p>Good coils may be produced by employing wires covered with
+several layers of cotton, boiling the coil a long time in pure wax,
+and cooling under moderate pressure. The advantage of such a
+coil is that it can be easily handled, but it cannot probably give
+as satisfactory results as a coil immersed in pure oil. Besides, it
+seems that the presence of a large body of wax affects the coil
+disadvantageously, whereas this does not seem to be the case with
+oil. Perhaps it is because the dielectric losses in the liquid are
+smaller.</p>
+
+<p>I have tried at first silk and cotton covered wires with oil immersions,
+but I have been gradually led to use gutta-percha
+covered wires, which proved most satisfactory. Gutta-percha
+insulation adds, of course, to the capacity of the coil, and this,
+especially if the coil be large, is a great disadvantage when extreme
+frequencies are desired; but, on the other hand, gutta-percha
+will withstand much more than an equal thickness of oil,
+and this advantage should be secured at any price. Once the
+coil has been immersed, it should never be taken out of the oil
+for more than a few hours, else the gutta-percha will crack up
+and the coil will not be worth half as much as before. Gutta-percha
+is probably slowly attacked by the oil, but after an immersion
+of eight to nine months I have found no ill effects.</p>
+
+<p>I have obtained two kinds of gutta-percha wire known in commerce:
+in one the insulation sticks tightly to the metal, in the
+other it does not. Unless a special method is followed to expel all
+air, it is much safer to use the first kind. I wind the coil within
+an oil tank so that all interstices are filled up with the oil. Between
+the layers I use cloth boiled out thoroughly in oil,
+calculating the thickness according to the difference of potential<span class='pagenum'><a name="Page_222" id="Page_222">[Pg 222]</a></span>
+between the turns. There seems not to be a very great difference
+whatever kind of oil is used; I use paraffine or linseed oil.</p>
+
+<p>To exclude more perfectly the air, an excellent way to proceed,
+and easily practicable with small coils, is the following:
+Construct a box of hardwood of very thick boards which have
+been for a long time boiled in oil. The boards should be so
+joined as to safely withstand the external air pressure. The coil
+being placed and fastened in position within the box, the latter
+is closed with a strong lid, and covered with closely fitting metal
+sheets, the joints of which are soldered very carefully. On the
+top two small holes are drilled, passing through the metal sheet
+and the wood, and in these holes two small glass tubes are inserted
+and the joints made air-tight. One of the tubes is connected
+to a vacuum pump, and the other with a vessel containing a
+sufficient quantity of boiled-out oil. The latter tube has a very
+small hole at the bottom, and is provided with a stopcock.
+When a fairly good vacuum has been obtained, the stopcock is
+opened and the oil slowly fed in. Proceeding in this manner,
+it is impossible that any big bubbles, which are the principal
+danger, should remain between the turns. The air is most completely
+excluded, probably better than by boiling out, which,
+however, when gutta-percha coated wires are used, is not practicable.</p>
+
+<p>For the primaries I use ordinary line wire with a thick cotton
+coating. Strands of very thin insulated wires properly interlaced
+would, of course, be the best to employ for the primaries,
+but they are not to be had.</p>
+
+<p>In an experimental coil the size of the wires is not of great
+importance. In the coil here used the primary is No. 12 and the
+secondary No. 24 Brown &amp; Sharpe gauge wire; but the sections
+may be varied considerably. It would only imply different adjustments;
+the results aimed at would not be materially affected.</p>
+
+<p>I have dwelt at some length upon the various forms of brush
+discharge because, in studying them, we not only observe phenomena
+which please our eye, but also afford us food for thought,
+and lead us to conclusions of practical importance. In the use
+of alternating currents of very high tension, too much precaution
+cannot be taken to prevent the brush discharge. In a main conveying
+such currents, in an induction coil or transformer, or in a
+condenser, the brush discharge is a source of great danger to the
+insulation. In a condenser, especially, the gaseous matter must<span class='pagenum'><a name="Page_223" id="Page_223">[Pg 223]</a></span>
+be most carefully expelled, for in it the charged surfaces are near
+each other, and if the potentials are high, just as sure as a weight
+will fall if let go, so the insulation will give way if a single
+gaseous bubble of some size be present, whereas, if all gaseous
+matter were carefully excluded, the condenser would safely
+withstand a much higher difference of potential. A main conveying
+alternating currents of very high tension may be injured
+merely by a blow hole or small crack in the insulation, the more
+so as a blowhole is apt to contain gas at low pressure; and as it
+appears almost impossible to completely obviate such little imperfections,
+I am led to believe that in our future distribution of
+electrical energy by currents of very high tension, liquid insulation
+will be used. The cost is a great drawback, but if we employ
+an oil as an insulator the distribution of electrical energy
+with something like 100,000 volts, and even more, becomes, at
+least with higher frequencies, so easy that it could be hardly
+called an engineering feat. With oil insulation and alternate current
+motors, transmissions of power can be affected with safety
+and upon an industrial basis at distances of as much as a thousand
+miles.</p>
+
+<p>A peculiar property of oils, and liquid insulation in general,
+when subjected to rapidly changing electric stresses, is to disperse
+any gaseous bubbles which may be present, and diffuse them
+through its mass, generally long before any injurious break can
+occur. This feature may be easily observed with an ordinary induction
+coil by taking the primary out, plugging up the end of
+the tube upon which the secondary is wound, and filling it with
+some fairly transparent insulator, such as paraffine oil. A primary
+of a diameter something like six millimetres smaller than the
+inside of the tube may be inserted in the oil. When the coil is
+set to work one may see, looking from the top through the oil,
+many luminous points&mdash;air bubbles which are caught by inserting
+the primary, and which are rendered luminous in consequence
+of the violent bombardment. The occluded air, by its impact
+against the oil, heats it; the oil begins to circulate, carrying some
+of the air along with it, until the bubbles are dispersed and the
+luminous points disappear. In this manner, unless large bubbles
+are occluded in such way that circulation is rendered impossible,
+a damaging break is averted, the only effect being a moderate
+warming up of the oil. If, instead of the liquid, a solid insulation,
+no matter how thick, were used, a breaking through and injury
+of the apparatus would be inevitable.<span class='pagenum'><a name="Page_224" id="Page_224">[Pg 224]</a></span></p>
+
+<p>The exclusion of gaseous matter from any apparatus in which
+the dielectric is subjected to more or less rapidly changing electric
+forces is, however, not only desirable in order to avoid a
+possible injury of the apparatus, but also on account of economy.
+In a condenser, for instance, as long as only a solid or only a
+liquid dielectric is used, the loss is small; but if a gas under ordinary
+or small pressure be present the loss may be very great.
+Whatever the nature of the force acting in the dielectric may be,
+it seems that in a solid or liquid the molecular displacement produced
+by the force is small: hence the product of force and
+displacement is insignificant, unless the force be very great; but
+in a gas the displacement, and therefore this product, is considerable;
+the molecules are free to move, they reach high speeds, and
+the energy of their impact is lost in heat or otherwise. If the
+gas be strongly compressed, the displacement due to the force is
+made smaller, and the losses are reduced.</p>
+
+<p>In most of the succeeding experiments I prefer, chiefly on
+account of the regular and positive action, to employ the alternator
+before referred to. This is one of the several machines
+constructed by me for the purpose of these investigations. It has
+384 pole projections, and is capable of giving currents of a frequency
+of about 10,000 per second. This machine has been illustrated
+and briefly described in my first paper before the American
+Institute of Electrical Engineers, May 20th, 1891, to which I have
+already referred. A more detailed description, sufficient to enable
+any engineer to build a similar machine, will be found in
+several electrical journals of that period.</p>
+
+<p>The induction coils operated from the machine are rather small,
+containing from 5,000 to 15,000 turns in the secondary. They
+are immersed in boiled-out linseed oil, contained in wooden boxes
+covered with zinc sheet.</p>
+
+<p>I have found it advantageous to reverse the usual position of
+the wires, and to wind, in these coils, the primaries on the top;
+thus allowing the use of a much larger primary, which, of course,
+reduces the danger of overheating and increases the output of
+the coil. I make the primary on each side at least one centimetre
+shorter than the secondary, to prevent the breaking through on the
+ends, which would surely occur unless the insulation on the top
+of the secondary be very thick, and this, of course, would be disadvantageous.</p>
+
+<p>When the primary is made movable, which is necessary in<span class='pagenum'><a name="Page_225" id="Page_225">[Pg 225]</a></span>
+some experiments, and many times convenient for the purposes
+of adjustment, I cover the secondary with wax, and turn it off
+in a lathe to a diameter slightly smaller than the inside of the
+primary coil. The latter I provide with a handle reaching out
+of the oil, which serves to shift it in any position along the
+secondary.</p>
+
+<p>I will now venture to make, in regard to the general manipulation
+of induction coils, a few observations bearing upon points
+which have not been fully appreciated in earlier experiments
+with such coils, and are even now often overlooked.</p>
+
+<p>The secondary of the coil possesses usually such a high self-induction
+that the current through the wire is inappreciable, and
+may be so even when the terminals are joined by a conductor of
+small resistance. If capacity is added to the terminals, the self-induction
+is counteracted, and a stronger current is made to flow
+through the secondary, though its terminals are insulated from
+each other. To one entirely unacquainted with the properties of
+alternating currents nothing will look more puzzling. This feature
+was illustrated in the experiment performed at the beginning
+with the top plates of wire gauze attached to the terminals and
+the rubber plate. When the plates of wire gauze were close together,
+and a small arc passed between them, the arc <i>prevented</i> a
+strong current from passing through the secondary, because it
+did away with the capacity on the terminals; when the rubber
+plate was inserted between, the capacity of the condenser formed
+counteracted the self-induction of the secondary, a stronger current
+passed now, the coil performed more work, and the discharge
+was by far more powerful.</p>
+
+<p>The first thing, then, in operating the induction coil is to combine
+capacity with the secondary to overcome the self-induction.
+If the frequencies and potentials are very high, gaseous matter
+should be carefully kept away from the charged surfaces. If
+Leyden jars are used, they should be immersed in oil, as otherwise
+considerable dissipation may occur if the jars are greatly
+strained. When high frequencies are used, it is of equal importance
+to combine a condenser with the primary. One may
+use a condenser connected to the ends of the primary or to the
+terminals of the alternator, but the latter is not to be recommended,
+as the machine might be injured. The best way is
+undoubtedly to use the condenser in series with the primary and
+with the alternator, and to adjust its capacity so as to annul the<span class='pagenum'><a name="Page_226" id="Page_226">[Pg 226]</a></span>
+self-induction of both the latter. The condenser should be adjustable
+by very small steps, and for a finer adjustment a small
+oil condenser with movable plates may be used conveniently.</p>
+
+<p>I think it best at this juncture to bring before you a phenomenon,
+observed by me some time ago, which to the purely
+scientific investigator may perhaps appear more interesting than
+any of the results which I have the privilege to present to you
+this evening.</p>
+
+<p>It may be quite properly ranked among the brush phenomena&mdash;in
+fact, it is a brush, formed at, or near, a single terminal
+in high vacuum.</p>
+
+<div class="figcenter" style="width: 748px;">
+<img src="images/oi_240.jpg" width="748" height="600" alt="Fig. 141, 142." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 141.</td><td class="caption">Fig. 142.</td></tr>
+</table>
+</div>
+
+<p>In bulbs provided with a conducting terminal, though it be of
+aluminum, the brush has but an ephemeral existence, and cannot,
+unfortunately, be indefinitely preserved in its most sensitive
+state, even in a bulb devoid of any conducting electrode.
+In studying the phenomenon, by all means a bulb having no
+leading-in wire should be used. I have found it best to use
+bulbs constructed as indicated in Figs. 141 and 142.</p>
+
+<p>In Fig. 141 the bulb comprises an incandescent lamp globe <i>L</i>,
+in the neck of which is sealed a barometer tube <i>b</i>, the end of which
+is blown out to form a small sphere <i>s</i>. This sphere should be
+sealed as closely as possible in the centre of the large globe.
+Before sealing, a thin tube <i>t</i>, of aluminum sheet, may be slipped
+in the barometer tube, but it is not important to employ it.<span class='pagenum'><a name="Page_227" id="Page_227">[Pg 227]</a></span></p>
+
+<p>The small hollow sphere <i>s</i> is filled with some conducting
+powder, and a wire <i>w</i> is cemented in the neck for the purpose of
+connecting the conducting powder with the generator.</p>
+
+<p>The construction shown in Fig. 142 was chosen in order to
+remove from the brush any conducting body which might possibly
+affect it. The bulb consists in this case of a lamp globe <i>L</i>,
+which has a neck <i>n</i>, provided with a tube <i>b</i> and small sphere <i>s</i>,
+sealed to it, so that two entirely independent compartments are
+formed, as indicated in the drawing. When the bulb is in use
+the neck <i>n</i> is provided with a tinfoil coating, which is connected
+to the generator and acts inductively upon the moderately rarefied
+and highly conducted gas inclosed in the neck. From there
+the current passes through the tube <i>b</i> into the small sphere <i>s</i>, to
+act by induction upon the gas contained in the globe <i>L</i>.</p>
+
+<p>It is of advantage to make the tube <i>t</i> very thick, the hole
+through it very small, and to blow the sphere <i>s</i> very thin. It is
+of the greatest importance that the sphere <i>s</i> be placed in the
+centre of the globe <i>L</i>.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_241.jpg" width="480" height="548" alt="Fig. 143." title="" />
+<span class="caption">Fig. 143.</span>
+</div>
+
+
+<p>Figs. 143, 144 and 145 indicate different forms, or stages, of
+the brush. Fig. 143 shows the brush as it first appears in a bulb
+provided with a conducting terminal; but, as in such a bulb it
+very soon disappears&mdash;often after a few minutes&mdash;I will confine
+myself to the description of the phenomenon as seen in a bulb
+without conducting electrode. It is observed under the following
+conditions:</p>
+
+<p>When the globe <i>L</i> (Figs. 141 and 142) is exhausted to a very
+high degree, generally the bulb is not excited upon connecting
+the wire <i>w</i> (Fig. 141) or the tinfoil coating of the bulb (Fig.<span class='pagenum'><a name="Page_228" id="Page_228">[Pg 228]</a></span>
+142) to the terminal of the induction coil. To excite it, it is
+usually sufficient to grasp the globe <i>L</i> with the hand. An intense
+phosphorescence then spreads at first over the globe, but
+soon gives place to a white, misty light. Shortly afterward one
+may notice that the luminosity is unevenly distributed in the
+globe, and after passing the current for some time the bulb appears
+as in Fig. 144. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 145, after some minutes,
+hours, days or weeks, according as the bulb is worked. Warming
+the bulb or increasing the potential hastens the transit.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_242.jpg" width="800" height="530" alt="Fig. 144, 145." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 144.</td><td class="caption">Fig. 145.</td></tr>
+</table>
+</div>
+
+<p>When the brush assumes the form indicated in Fig. 145, it may
+be brought to a state of extreme sensitiveness to electrostatic
+and magnetic influence. The bulb hanging straight down from
+a wire, and all objects being remote from it, the approach of the
+observer at a few paces from the bulb will cause the brush to fly
+to the opposite side, and if he walks around the bulb it will
+always keep on the opposite side. It may begin to spin around
+the terminal long before it reaches that sensitive stage. When
+it begins to turn around, principally, but also before, it is affected
+by a magnet, and at a certain stage it is susceptible to magnetic
+influence to an astonishing degree. A small permanent magnet,
+with its poles at a distance of no more than two centimetres, will
+affect it visibly at a distance of two metres, slowing down or accelerating
+the rotation according to how it is held relatively to<span class='pagenum'><a name="Page_229" id="Page_229">[Pg 229]</a></span>
+the brush. I think I have observed that at the stage when it is
+most sensitive to magnetic, it is not most sensitive to electrostatic,
+influence. My explanation is, that the electrostatic attraction
+between the brush and the glass of the bulb, which retards the
+rotation, grows much quicker than the magnetic influence when
+the intensity of the stream is increased.</p>
+
+<p>When the bulb hangs with the globe <i>L</i> down, the rotation is
+always clockwise. In the southern hemisphere it would occur
+in the opposite direction and on the equator the brush should
+not turn at all. The rotation may be reversed by a magnet kept
+at some distance. The brush rotates best, seemingly, when it is
+at right angles to the lines of force of the earth. It very likely
+rotates, when at its maximum speed, in synchronism with the
+alternations, say, 10,000 times a second. The rotation can be
+slowed down or accelerated by the approach or receding of the
+observer, or any conducting body, but it cannot be reversed by
+putting the bulb in any position. When it is in the state of the
+highest sensitiveness and the potential or frequency be varied,
+the sensitiveness is rapidly diminished. Changing either of
+these but little will generally stop the rotation. The sensitiveness
+is likewise affected by the variations of temperature. To
+attain great sensitiveness it is necessary to have the small sphere
+<i>s</i> in the centre of the globe <i>L</i>, as otherwise the electrostatic
+action of the glass of the globe will tend to stop the rotation.
+The sphere <i>s</i> should be small and of uniform thickness; any dissymmetry
+of course has the effect to diminish the sensitiveness.</p>
+
+<p>The fact that the brush rotates in a definite direction in a permanent
+magnetic field seems to show that in alternating currents
+of very high frequency the positive and negative impulses are
+not equal, but that one always preponderates over the other.</p>
+
+<p>Of course, this rotation in one direction may be due to the
+action of the two elements of the same current upon each other,
+or to the action of the field produced by one of the elements
+upon the other, as in a series motor, without necessarily one impulse
+being stronger than the other. The fact that the brush
+turns, as far as I could observe, in any position, would speak for
+this view. In such case it would turn at any point of the earth's
+surface. But, on the other hand, it is then hard to explain why
+a permanent magnet should reverse the rotation, and one must
+assume the preponderance of impulses of one kind.</p>
+
+<p>As to the causes of the formation of the brush or stream, I<span class='pagenum'><a name="Page_230" id="Page_230">[Pg 230]</a></span>
+think it is due to the electrostatic action of the globe and the
+dissymmetry of the parts. If the small bulb <i>s</i> and the globe <i>L</i>
+were perfect concentric spheres, and the glass throughout of the
+same thickness and quality, I think the brush would not form,
+as the tendency to pass would be equal on all sides. That the
+formation of the stream is due to an irregularity is apparent from
+the fact that it has the tendency to remain in one position, and
+rotation occurs most generally only when it is brought out of
+this position by electrostatic or magnetic influence. When in an
+extremely sensitive state it rests in one position, most curious experiments
+may be performed with it. For instance, the experimenter
+may, by selecting a proper position, approach the hand
+at a certain considerable distance to the bulb, and he may cause
+the brush to pass off by merely stiffening the muscles of the arm.
+When it begins to rotate slowly, and the hands are held at a
+proper distance, it is impossible to make even the slightest motion
+without producing a visible effect upon the brush. A metal
+plate connected to the other terminal of the coil affects it at a
+great distance, slowing down the rotation often to one turn a
+second.</p>
+
+<p>I am firmly convinced that such a brush, when we learn how
+to produce it properly, will prove a valuable aid in the investigation
+of the nature of the forces acting in an electrostatic or
+magnetic field. If there is any motion which is measurable going
+on in the space, such a brush ought to reveal it. It is, so to
+speak, a beam of light, frictionless, devoid of inertia.</p>
+
+<p>I think that it may find practical applications in telegraphy.
+With such a brush it would be possible to send dispatches across
+the Atlantic, for instance, with any speed, since its sensitiveness
+may be so great that the slightest changes will affect it. If it
+were possible to make the stream more intense and very narrow,
+its deflections could be easily photographed.</p>
+
+<p>I have been interested to find whether there is a rotation of
+the stream itself, or whether there is simply a stress traveling
+around the bulb. For this purpose I mounted a light mica fan
+so that its vanes were in the path of the brush. If the stream
+itself was rotating the fan would be spun around. I could produce
+no distinct rotation of the fan, although I tried the experiment
+repeatedly; but as the fan exerted a noticeable influence
+on the stream, and the apparent rotation of the latter was, in this
+case, never quite satisfactory, the experiment did not appear to
+be conclusive.<span class='pagenum'><a name="Page_231" id="Page_231">[Pg 231]</a></span></p>
+
+<p>I have been unable to produce the phenomenon with the disruptive
+discharge coil, although every other of these phenomena
+can be well produced by it&mdash;many, in fact, much better than
+with coils operated from an alternator.</p>
+
+<p>It may be possible to produce the brush by impulses of one
+direction, or even by a steady potential, in which case it would
+be still more sensitive to magnetic influence.</p>
+
+<p>In operating an induction coil with rapidly alternating currents,
+we realize with astonishment, for the first time, the great importance
+of the relation of capacity, self-induction and frequency as
+regards the general results. The effects of capacity are the most
+striking, for in these experiments, since the self-induction and
+frequency both are high, the critical capacity is very small, and
+need be but slightly varied to produce a very considerable change.
+The experimenter may bring his body in contact with the terminals
+of the secondary of the coil, or attach to one or both terminals
+insulated bodies of very small bulk, such as bulbs, and he
+may produce a considerable rise or fall of potential, and greatly
+affect the flow of the current through the primary. In the experiment
+before shown, in which a brush appears at a wire
+attached to one terminal, and the wire is vibrated when the experimenter
+brings his insulated body in contact with the other
+terminal of the coil, the sudden rise of potential was made evident.</p>
+
+<p>I may show you the behavior of the coil in another manner
+which possesses a feature of some interest. I have here a little light
+fan of aluminum sheet, fastened to a needle and arranged to
+rotate freely in a metal piece screwed to one of the terminals of
+the coil. When the coil is set to work, the molecules of the air
+are rhythmically attracted and repelled. As the force with
+which they are repelled is greater than that with which they are
+attracted, it results that there is a repulsion exerted on the surfaces
+of the fan. If the fan were made simply of a metal sheet,
+the repulsion would be equal on the opposite sides, and would
+produce no effect. But if one of the opposing surfaces is screened,
+or if, generally speaking, the bombardment on this side is
+weakened in some way or other, there remains the repulsion exerted
+upon the other, and the fan is set in rotation. The screening
+is best effected by fastening upon one of the opposing sides
+of the fan insulated conducting coatings, or, if the fan is made
+in the shape of an ordinary propeller screw, by fastening on one<span class='pagenum'><a name="Page_232" id="Page_232">[Pg 232]</a></span>
+side, and close to it, an insulated metal plate. The static screen
+may, however, be omitted, and simply a thickness of insulating
+material fastened to one of the sides of the fan.</p>
+
+<p>To show the behavior of the coil, the fan may be placed upon
+the terminal and it will readily rotate when the coil is operated
+by currents of very high frequency. With a steady potential,
+of course, and even with alternating currents of very low frequency,
+it would not turn, because of the very slow exchange of
+air and, consequently, smaller bombardment; but in the latter
+case it might turn if the potential were excessive. With a pin
+wheel, quite the opposite rule holds good; it rotates best with
+a steady potential, and the effort is the smaller the higher the
+frequency. Now, it is very easy to adjust the conditions so that
+the potential is normally not sufficient to turn the fan, but that
+by connecting the other terminal of the coil with an insulated
+body it rises to a much greater value, so as to rotate the fan, and
+it is likewise possible to stop the rotation by connecting to the
+terminal a body of different size, thereby diminishing the potential.</p>
+
+<p>Instead of using the fan in this experiment, we may use the
+"electric" radiometer with similar effect. But in this case it will
+be found that the vanes will rotate only at high exhaustion or at
+ordinary pressures; they will not rotate at moderate pressures,
+when the air is highly conducting. This curious observation was
+made conjointly by Professor Crookes and myself. I attribute
+the result to the high conductivity of the air, the molecules of
+which then do not act as independent carriers of electric charges,
+but act all together as a single conducting body. In such case,
+of course, if there is any repulsion at all of the molecules from
+the vanes, it must be very small. It is possible, however, that
+the result is in part due to the fact that the greater part of the
+discharge passes from the leading-in wire through the highly conducting
+gas, instead of passing off from the conducting vanes.</p>
+
+<p>In trying the preceding experiment with the electric radiometer
+the potential should not exceed a certain limit, as then the electrostatic
+attraction between the vanes and the glass of the bulb
+may be so great as to stop the rotation.</p>
+
+<p>A most curious feature of alternate currents of high frequencies
+and potentials is that they enable us to perform many experiments
+by the use of one wire only. In many respects this feature
+is of great interest.<span class='pagenum'><a name="Page_233" id="Page_233">[Pg 233]</a></span></p>
+
+<p>In a type of alternate current motor invented by me some years
+ago I produced rotation by inducing, by means of a single alternating
+current passed through a motor circuit, in the mass or other
+circuits of the motor, secondary currents, which, jointly with the
+primary or inducing current, created a moving field of force. A
+simple but crude form of such a motor is obtained by winding
+upon an iron core a primary, and close to it a secondary coil, joining
+the ends of the latter and placing a freely movable metal disc
+within the influence of the field produced by both. The iron core
+is employed for obvious reasons, but it is not essential to the
+operation. To improve the motor, the iron core is made to encircle
+the armature. Again to improve, the secondary coil is
+made to partly overlap the primary, so that it cannot free itself
+from a strong inductive action of the latter, repel its lines as it
+may. Once more to improve, the proper difference of phase is
+obtained between the primary and secondary currents by a condenser,
+self-induction, resistance or equivalent windings.</p>
+
+<p>I had discovered, however, that rotation is produced by means
+of a single coil and core; my explanation of the phenomenon, and
+leading thought in trying the experiment, being that there must
+be a true time lag in the magnetization of the core. I remember
+the pleasure I had when, in the writings of Professor Ayrton,
+which came later to my hand, I found the idea of the time lag
+advocated. Whether there is a true time lag, or whether the retardation
+is due to eddy currents circulating in minute paths, must
+remain an open question, but the fact is that a coil wound upon
+an iron core and traversed by an alternating current creates a
+moving field of force, capable of setting an armature in rotation.
+It is of some interest, in conjunction with the historical Arago
+experiment, to mention that in lag or phase motors I have produced
+rotation in the opposite direction to the moving field, which
+means that in that experiment the magnet may not rotate, or may
+even rotate in the opposite direction to the moving disc. Here,
+then, is a motor (diagrammatically illustrated in Fig. 146), comprising
+a coil and iron core, and a freely movable copper disc in
+proximity to the latter.</p>
+
+<div class="figcenter" style="width: 613px;">
+<img src="images/oi_248.jpg" width="613" height="600" alt="Fig. 146." title="" />
+<span class="caption">Fig. 146.</span>
+</div>
+
+
+<p>To demonstrate a novel and interesting feature, I have, for a
+reason which I will explain, selected this type of motor. When
+the ends of the coil are connected to the terminals of an alternator
+the disc is set in rotation. But it is not this experiment,
+now well known, which I desire to perform. What I wish to
+<span class='pagenum'><a name="Page_234" id="Page_234">[Pg 234]</a></span>show you is that this motor rotates with <i>one single</i> connection between
+it and the generator; that is to say, one terminal of the
+motor is connected to one terminal of the generator&mdash;in this case
+the secondary of a high-tension induction coil&mdash;the other terminals
+of motor and generator being insulated in space. To produce
+rotation it is generally (but not absolutely) necessary to
+connect the free end of the motor coil to an insulated body of
+some size. The experimenter's body is more than sufficient. If
+he touches the free terminal with an object held in the hand, a
+current passes through the coil and the copper disc is set in rotation.
+If an exhausted tube is put in series with the coil, the tube
+lights brilliantly, showing the passage of a strong current. Instead
+of the experimenter's body, a small metal sheet suspended
+on a cord may be used with the same result. In this case the
+plate acts as a condenser in series with the coil. It counteracts
+the self-induction of the latter and allows a strong current to
+pass. In such a combination, the greater the self-induction of
+the coil the smaller need be the plate, and this means that a lower
+frequency, or eventually a lower potential, is required to operate
+the motor. A single coil wound upon a core has a high self-induction;
+for this reason, principally, this type of motor was
+chosen to perform the experiment. Were a secondary closed
+coil wound upon the core, it would tend to diminish the
+self-<span class='pagenum'><a name="Page_235" id="Page_235">[Pg 235]</a></span>induction, and then it would be necessary to employ a much
+higher frequency and potential. Neither would be advisable, for
+a higher potential would endanger the insulation of the small
+primary coil, and a higher frequency would result in a materially
+diminished torque.</p>
+
+<p>It should be remarked that when such a motor with a
+closed secondary is used, it is not at all easy to obtain rotation
+with excessive frequencies, as the secondary cuts off
+almost completely the lines of the primary&mdash;and this, of
+course, the more, the higher the frequency&mdash;and allows the passage
+of but a minute current. In such a case, unless the secondary
+is closed through a condenser, it is almost essential, in order
+to produce rotation, to make the primary and secondary coils
+overlap each other more or less.</p>
+
+<p>But there is an additional feature of interest about this motor,
+namely, it is not necessary to have even a single connection between
+the motor and generator, except, perhaps, through the
+ground; for not only is an insulated plate capable of giving off
+energy into space, but it is likewise capable of deriving it from
+an alternating electrostatic field, though in the latter case the
+available energy is much smaller. In this instance one of the
+motor terminals is connected to the insulated plate or body
+located within the alternating electrostatic field, and the other
+terminal preferably to the ground.</p>
+
+<p>It is quite possible, however, that such "no wire" motors, as
+they might be called, could be operated by conduction through
+the rarefied air at considerable distances. Alternate currents,
+especially of high frequencies, pass with astonishing freedom
+through even slightly rarefied gases. The upper strata of the air
+are rarefied. To reach a number of miles out into space requires
+the overcoming of difficulties of a merely mechanical nature.
+There is no doubt that with the enormous potentials obtainable by
+the use of high frequencies and oil insulation, luminous discharges
+might be passed through many miles of rarefied air, and that, by
+thus directing the energy of many hundreds or thousands of horse-power,
+motors or lamps might be operated at considerable
+distances from stationary sources. But such schemes are mentioned
+merely as possibilities. We shall have no need to transmit
+power in this way. We shall have no need to <i>transmit</i> power
+at all. Ere many generations pass, our machinery will be driven
+by a power obtainable at any point of the universe. This idea is<span class='pagenum'><a name="Page_236" id="Page_236">[Pg 236]</a></span>
+not novel. Men have been led to it long ago by instinct or reason.
+It has been expressed in many ways, and in many places, in the
+history of old and new. We find it in the delightful myth of
+Antheus, who derives power from the earth; we find it among
+the subtle speculations of one of your splendid mathematicians,
+and in many hints and statements of thinkers of the present time.
+Throughout space there is energy. Is this energy static or kinetic?
+If static our hopes are in vain; if kinetic&mdash;and this we know it
+is, for certain&mdash;then it is a mere question of time when men will
+succeed in attaching their machinery to the very wheelwork of
+nature. Of all, living or dead, Crookes came nearest to doing it.
+His radiometer will turn in the light of day and in the darkness
+of the night; it will turn everywhere where there is heat, and
+heat is everywhere. But, unfortunately, this beautiful little
+machine, while it goes down to posterity as the most interesting,
+must likewise be put on record as the most inefficient machine
+ever invented!</p>
+
+<p>The preceding experiment is only one of many equally interesting
+experiments which may be performed by the use of only
+one wire with alternations of high potential and frequency. We
+may connect an insulated line to a source of such currents, we
+may pass an inappreciable current over the line, and on any
+point of the same we are able to obtain a heavy current, capable
+of fusing a thick copper wire. Or we may, by the help of some
+artifice, decompose a solution in any electrolytic cell by connecting
+only one pole of the cell to the line or source of energy.
+Or we may, by attaching to the line, or only bringing into its
+vicinity, light up an incandescent lamp, an exhausted tube, or a
+phosphorescent bulb.</p>
+
+<p>However impracticable this plan of working may appear in
+many cases, it certainly seems practicable, and even recommendable,
+in the production of light. A perfected lamp would require
+but little energy, and if wires were used at all we ought to be able
+to supply that energy without a return wire.</p>
+
+<p>It is now a fact that a body may be rendered incandescent or
+phosphorescent by bringing it either in single contact or merely
+in the vicinity of a source of electric impulses of the proper
+character, and that in this manner a quantity of light sufficient
+to afford a practical illuminant may be produced. It is, therefore,
+to say the least, worth while to attempt to determine the
+best conditions and to invent the best appliances for attaining
+this object.<span class='pagenum'><a name="Page_237" id="Page_237">[Pg 237]</a></span></p>
+
+<p>Some experiences have already been gained in this direction,
+and I will dwell on them briefly, in the hope that they might
+prove useful.</p>
+
+<p>The heating of a conducting body inclosed in a bulb, and connected
+to a source of rapidly alternating electric impulses, is
+dependent on so many things of a different nature, that it would
+be difficult to give a generally applicable rule under which the
+maximum heating occurs. As regards the size of the vessel, I
+have lately found that at ordinary or only slightly differing
+atmospheric pressures, when air is a good insulator, and hence
+practically the same amount of energy by a certain potential and
+frequency is given off from the body, whether the bulb be small
+or large, the body is brought to a higher temperature if enclosed
+in a small bulb, because of the better confinement of heat in this
+case.</p>
+
+<p>At lower pressures, when air becomes more or less conducting,
+or if the air be sufficiently warmed to become conducting, the
+body is rendered more intensely incandescent in a large bulb,
+obviously because, under otherwise equal conditions of test, more
+energy may be given off from the body when the bulb is large.</p>
+
+<p>At very high degrees of exhaustion, when the matter in the
+bulb becomes "radiant," a large bulb has still an advantage, but
+a comparatively slight one, over the small bulb.</p>
+
+<p>Finally, at excessively high degrees of exhaustion, which cannot
+be reached except by the employment of special means, there
+seems to be, beyond a certain and rather small size of vessel, no
+perceptible difference in the heating.</p>
+
+<p>These observations were the result of a number of experiments,
+of which one, showing the effect of the size of the bulb at a high
+degree of exhaustion, may be described and shown here, as it
+presents a feature of interest. Three spherical bulbs of 2 inches,
+3 inches and 4 inches diameter were taken, and in the centre of
+each was mounted an equal length of an ordinary incandescent
+lamp filament of uniform thickness. In each bulb the piece of
+filament was fastened to the leading-in wire of platinum, contained
+in a glass stem sealed in the bulb; care being taken, of
+course, to make everything as nearly alike as possible. On each
+glass stem in the inside of the bulb was slipped a highly polished
+tube made of aluminum sheet, which fitted the stem and was held
+on it by spring pressure. The function of this aluminum tube will
+be explained subsequently. In each bulb an equal length of fila<span class='pagenum'><a name="Page_238" id="Page_238">[Pg 238]</a></span>ment
+protruded above the metal tube. It is sufficient to say now
+that under these conditions equal lengths of filament of the same
+thickness&mdash;in other words, bodies of equal bulk&mdash;were brought
+to incandescence. The three bulbs were sealed to a glass tube,
+which was connected to a Sprengel pump. When a high vacuum
+had been reached, the glass tube carrying the bulbs was sealed
+off. A current was then turned on successively on each bulb,
+and it was found that the filaments came to about the same
+brightness, and, if anything, the smallest bulb, which was placed
+midway between the two larger ones, may have been slightly
+brighter. This result was expected, for when either of the bulbs
+was connected to the coil the luminosity spread through the
+other two, hence the three bulbs constituted really one vessel.
+When all the three bulbs were connected in multiple arc to the
+coil, in the largest of them the filament glowed brightest, in the
+next smaller it was a little less bright, and in the smallest it only
+came to redness. The bulbs were then sealed off and separately
+tried. The brightness of the filaments was now such as would
+have been expected on the supposition that the energy given off
+was proportionate to the surface of the bulb, this surface in each
+case representing one of the coatings of a condenser. Accordingly,
+there was less difference between the largest and the
+middle sized than between the latter and the smallest bulb.</p>
+
+<p>An interesting observation was made in this experiment. The
+three bulbs were suspended from a straight bare wire connected
+to a terminal of a coil, the largest bulb being placed at the end
+of the wire, at some distance from it the smallest bulb, and at an
+equal distance from the latter the middle-sized one. The carbons
+glowed then in both the larger bulbs about as expected, but the
+smallest did not get its share by far. This observation led me to
+exchange the position of the bulbs, and I then observed that
+whichever of the bulbs was in the middle was by far less bright
+than it was in any other position. This mystifying result was,
+of course, found to be due to the electrostatic action between the
+bulbs. When they were placed at a considerable distance, or
+when they were attached to the corners of an equilateral triangle
+of copper wire, they glowed in about the order determined by
+their surfaces.</p>
+
+<p>As to the shape of the vessel, it is also of some importance, especially
+at high degrees of exhaustion. Of all the possible constructions,
+it seems that a spherical globe with the refractory body<span class='pagenum'><a name="Page_239" id="Page_239">[Pg 239]</a></span>
+mounted in its centre is the best to employ. By experience it
+has been demonstrated that in such a globe a refractory body of
+a given bulk is more easily brought to incandescence than when
+differently shaped bulbs are used. There is also an advantage in
+giving to the incandescent body the shape of a sphere, for self-evident
+reasons. In any case the body should be mounted in the
+centre, where the atoms rebounding from the glass collide. This
+object is best attained in the spherical bulb; but it is also attained
+in a cylindrical vessel with one or two straight filaments
+coinciding with its axis, and possibly also in parabolical or spherical
+bulbs with refractory body or bodies placed in the focus or
+foci of the same; though the latter is not probable, as the electrified
+atoms should in all cases rebound normally from the
+surface they strike, unless the speed were excessive, in which
+case they <i>would</i> probably follow the general law of reflection.
+No matter what shape the vessel may have, if the exhaustion be
+low, a filament mounted in the globe is brought to the same
+degree of incandescence in all parts; but if the exhaustion be
+high and the bulb be spherical or pear-shaped, as usual, focal
+points form and the filament is heated to a higher degree at or
+near such points.</p>
+
+<p>To illustrate the effect, I have here two small bulbs which are
+alike, only one is exhausted to a low and the other to a very high
+degree. When connected to the coil, the filament in the former
+glows uniformly throughout all its length; whereas in the latter,
+that portion of the filament which is in the centre of the bulb
+glows far more intensely than the rest. A curious point is that
+the phenomenon occurs even if two filaments are mounted in a
+bulb, each being connected to one terminal of the coil, and, what
+is still more curious, if they be very near together, provided the
+vacuum be very high. I noted in experiments with such bulbs
+that the filaments would give way usually at a certain point, and
+in the first trials I attributed it to a defect in the carbon. But
+when the phenomenon occurred many times in succession I
+recognized its real cause.</p>
+
+<p>In order to bring a refractory body inclosed in a bulb to incandescence,
+it is desirable, on account of economy, that all the
+energy supplied to the bulb from the source should reach without
+loss the body to be heated; from there, and from nowhere else,
+it should be radiated. It is, of course, out of the question to
+reach this theoretical result, but it is possible by a proper construction
+of the illuminating device to approximate it more or less.<span class='pagenum'><a name="Page_240" id="Page_240">[Pg 240]</a></span></p>
+
+<p>For many reasons, the refractory body is placed in the centre
+of the bulb, and it is usually supported on a glass stem containing
+the leading-in wire. As the potential of this wire is alternated,
+the rarefied gas surrounding the stem is acted upon inductively,
+and the glass stem is violently bombarded and heated. In this
+manner by far the greater portion of the energy supplied to the
+bulb&mdash;especially when exceedingly high frequencies are used&mdash;may
+be lost for the purpose contemplated. To obviate this loss,
+or at least to reduce it to a minimum, I usually screen the rarefied
+gas surrounding the stem from the inductive action of the leading-in
+wire by providing the stem with a tube or coating of conducting
+material. It seems beyond doubt that the best among metals to
+employ for this purpose is aluminum, on account of its many remarkable
+properties. Its only fault is that it is easily fusible,
+and, therefore, its distance from the incandescing body should be
+properly estimated. Usually, a thin tube, of a diameter somewhat
+smaller than that of the glass stem, is made of the finest
+aluminum sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a
+piece of aluminum sheet of proper size, grasping the sheet firmly
+with clean chamois leather or blotting paper, and spinning the
+rod very fast. The sheet is wound tightly around the rod, and a
+highly polished tube of one or three layers of the sheet is obtained.
+When slipped on the stem, the pressure is generally sufficient to
+prevent it from slipping off, but, for safety, the lower edge of
+the sheet may be turned inside. The upper inside corner of the
+sheet&mdash;that is, the one which is nearest to the refractory incandescent
+body&mdash;should be cut out diagonally, as it often happens
+that, in consequence of the intense heat, this corner turns toward
+the inside and comes very near to, or in contact with, the wire, or
+filament, supporting the refractory body. The greater part of
+the energy supplied to the bulb is then used up in heating the
+metal tube, and the bulb is rendered useless for the purpose.
+The aluminum sheet should project above the glass stem more or
+less&mdash;one inch or so&mdash;or else, if the glass be too close to the incandescing
+body, it may be strongly heated and become more or
+less conducting, whereupon it may be ruptured, or may, by its
+conductivity, establish a good electrical connection between the
+metal tube and the leading-in wire, in which case, again, most of
+the energy will be lost in heating the former. Perhaps the best
+way is to make the top of the glass tube, for about an inch, of a<span class='pagenum'><a name="Page_241" id="Page_241">[Pg 241]</a></span>
+much smaller diameter. To still further reduce the danger
+arising from the heating of the glass stem, and also with the view
+of preventing an electrical connection between the metal tube
+and the electrode, I preferably wrap the stem with several layers
+of thin mica, which extends at least as far as the metal tube. In
+some bulbs I have also used an outside insulating cover.</p>
+
+<p>The preceding remarks are only made to aid the experimenter
+in the first trials, for the difficulties which he encounters he may
+soon find means to overcome in his own way.</p>
+
+<p>To illustrate the effect of the screen, and the advantage of
+using it, I have here two bulbs of the same size, with their stems,
+leading-in wires and incandescent lamp filaments tied to the latter,
+as nearly alike as possible. The stem of one bulb is provided
+with an aluminum tube, the stem of the other has none. Originally
+the two bulbs were joined by a tube which was connected
+to a Sprengel pump. When a high vacuum had been reached,
+first the connecting tube, and then the bulbs, were sealed off;
+they are therefore of the same degree of exhaustion. When they
+are separately connected to the coil giving a certain potential, the
+carbon filament in the bulb provided with the aluminum screen
+is rendered highly incandescent, while the filament in the other
+bulb may, with the same potential, not even come to redness,
+although in reality the latter bulb takes generally more energy
+than the former. When they are both connected together to the
+terminal, the difference is even more apparent, showing the importance
+of the screening. The metal tube placed on the stem containing
+the leading-in wire performs really two distinct functions: First,
+it acts more or less as an electrostatic screen, thus economizing
+the energy supplied to the bulb; and, second, to whatever extent
+it may fail to act electrostatically, it acts mechanically, preventing
+the bombardment, and consequently intense heating and
+possible deterioration of the slender support of the refractory incandescent
+body, or of the glass stem containing the leading-in
+wire. I say <i>slender</i> support, for it is evident that in order to
+confine the heat more completely to the incandescing body its support
+should be very thin, so as to carry away the smallest possible
+amount of heat by conduction. Of all the supports used I have
+found an ordinary incandescent lamp filament to be the best,
+principally because among conductors it can withstand the highest
+degree of heat.</p>
+
+<p>The effectiveness of the metal tube as an electrostatic screen
+depends largely on the degree of exhaustion.<span class='pagenum'><a name="Page_242" id="Page_242">[Pg 242]</a></span></p>
+
+<p>At excessively high degrees of exhaustion&mdash;which are reached
+by using great care and special means in connection with the
+Sprengel pump&mdash;when the matter in the globe is in the ultra-radiant
+state, it acts most perfectly. The shadow of the upper
+edge of the tube is then sharply defined upon the bulb.</p>
+
+<p>At a somewhat lower degree of exhaustion, which is about the
+ordinary "non-striking" vacuum, and generally as long as the
+matter moves predominantly in straight lines, the screen still
+does well. In elucidation of the preceding remark it is necessary
+to state that what is a "non-striking" vacuum for a coil operated
+as ordinarily, by impulses, or currents, of low frequency, is not
+so, by far, when the coil is operated by currents of very high frequency.
+In such case the discharge may pass with great freedom
+through the rarefied gas through which a low frequency discharge
+may not pass, even though the potential be much higher.
+At ordinary atmospheric pressures just the reverse rule holds
+good: the higher the frequency, the less the spark discharge is
+able to jump between the terminals, especially if they are knobs
+or spheres of some size.</p>
+
+<p>Finally, at very low degrees of exhaustion, when the gas is well
+conducting, the metal tube not only does not act as an electrostatic
+screen, but even is a drawback, aiding to a considerable
+extent the dissipation of the energy laterally from the leading-in
+wire. This, of course, is to be expected. In this case, namely,
+the metal tube is in good electrical connection with the leading-in
+wire, and most of the bombardment is directed upon the tube.
+As long as the electrical connection is not good, the conducting
+tube is always of some advantage, for although it may not greatly
+economize energy, still it protects the support of the refractory
+button, and is the means of concentrating more energy upon the
+same.</p>
+
+<p>To whatever extent the aluminum tube performs the function
+of a screen, its usefulness is therefore limited to very high degrees
+of exhaustion when it is insulated from the electrode&mdash;that
+is, when the gas as a whole is non-conducting, and the molecules,
+or atoms, act as independent carriers of electric charges.</p>
+
+<p>In addition to acting as a more or less effective screen, in the
+true meaning of the word, the conducting tube or coating may
+also act, by reason of its conductivity, as a sort of equalizer or
+dampener of the bombardment against the stem. To be explicit,
+I assume the action to be as follows: Suppose a rhythmical bom<span class='pagenum'><a name="Page_243" id="Page_243">[Pg 243]</a></span>bardment
+to occur against the conducting tube by reason of its
+imperfect action as a screen, it certainly must happen that some
+molecules, or atoms, strike the tube sooner than others. Those
+which come first in contact with it give up their superfluous
+charge, and the tube is electrified, the electrification instantly
+spreading over its surface. But this must diminish the energy
+lost in the bombardment, for two reasons: first, the charge given
+up by the atoms spreads over a great area, and hence the electric
+density at any point is small, and the atoms are repelled with less
+energy than they would be if they struck against a good insulator;
+secondly, as the tube is electrified by the atoms which first
+come in contact with it, the progress of the following atoms
+against the tube is more or less checked by the repulsion which
+the electrified tube must exert upon the similarly electrified
+atoms. This repulsion may perhaps be sufficient to prevent a
+large portion of the atoms from striking the tube, but at any rate
+it must diminish the energy of their impact. It is clear that
+when the exhaustion is very low, and the rarefied gas well conducting,
+neither of the above effects can occur, and, on the other
+hand, the fewer the atoms, with the greater freedom they move;
+in other words, the higher the degree of exhaustion, up to a
+limit, the more telling will be both the effects.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_257.jpg" width="800" height="475" alt="Fig. 147, 148." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 147.</td><td class="caption">Fig. 148.</td></tr>
+</table>
+</div>
+
+<p>What I have just said may afford an explanation of the phenomenon
+observed by Prof. Crookes, namely, that a discharge
+through a bulb is established with much greater facility when an<span class='pagenum'><a name="Page_244" id="Page_244">[Pg 244]</a></span>
+insulator than when a conductor is present in the same. In my
+opinion, the conductor acts as a dampener of the motion of the
+atoms in the two ways pointed out; hence, to cause a visible discharge
+to pass through the bulb, a much higher potential is
+needed if a conductor, especially of much surface, be present.</p>
+
+<p>For the sake of elucidating of some of the remarks before made,
+I must now refer to Figs. 147, 148 and 149, which illustrate
+various arrangements with a type of bulb most generally used.</p>
+
+<p>Fig. 147 is a section through a spherical bulb <small>L</small>, with the glass
+stem <i>s</i>, contains the leading-in wire <i>w</i>, which has a lamp filament
+<i>l</i> fastened to it, serving to support the refractory button <i>m</i> in the
+centre. <small>M</small> is a sheet of thin mica wound in several layers around
+the stem <i>s</i>, and <i>a</i> is the aluminum tube.</p>
+
+<p>Fig. 148 illustrates such a bulb in a somewhat more advanced
+stage of perfection. A metallic tube <small>S</small> is fastened by means of
+some cement to the neck of the tube. In the tube is screwed a
+plug <small>P</small>, of insulating material, in the centre of which is fastened
+a metallic terminal <i>t</i>, for the connection to the leading-in wire <i>w</i>.
+This terminal must be well insulated from the metal tube <small>S</small>;
+therefore, if the cement used is conducting&mdash;and most generally
+it is sufficiently so&mdash;the space between the plug <small>P</small> and the neck
+of the bulb should be filled with some good insulating material,
+such as mica powder.</p>
+
+
+<p>Fig. 149 shows a bulb made for experimental purposes. In this
+bulb the aluminum tube is provided with an external connection,
+which serves to investigate the effect of the tube under various
+conditions. It is referred to chiefly to suggest a line of experiment
+followed.</p>
+
+<p>Since the bombardment against the stem containing the leading-in
+wire is due to the inductive action of the latter upon the
+rarefied gas, it is of advantage to reduce this action as far as
+practicable by employing a very thin wire, surrounded by a very
+thick insulation of glass or other material, and by making the
+wire passing through the rarefied gas as short as practicable. To
+combine these features I employ a large tube <small>T</small> (Fig. 150), which
+protrudes into the bulb to some distance, and carries on the top a
+very short glass stem <i>s</i>, into which is sealed the leading-in wire
+<i>w</i>, and I protect the top of the glass stem against the heat by a
+small aluminum tube <i>a</i> and a layer of mica underneath the same,
+as usual. The wire <i>w</i>, passing through the large tube to the
+outside of the bulb, should be well insulated&mdash;with a glass tube,<span class='pagenum'><a name="Page_245" id="Page_245">[Pg 245]</a></span>
+for instance&mdash;and the space between ought to be filled out with
+some excellent insulator. Among many insulating powders I
+have found that mica powder is the best to employ. If this precaution
+is not taken, the tube <small>T</small>, protruding into the bulb, will
+surely be cracked in consequence of the heating by the brushes
+which are apt to form in the upper part of the tube, near the exhausted
+globe, especially if the vacuum be excellent, and therefore
+the potential necessary to operate the lamp be very high.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_259.jpg" width="800" height="500" alt="Fig. 149, 150." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 149.</td><td class="caption">Fig. 150.</td></tr>
+</table>
+</div>
+
+<p>Fig. 151 illustrates a similar arrangement, with a large tube <small>T</small>
+protruding into the part of the bulb containing the refractory
+button <i>m</i>. In this case the wire leading from the outside into
+the bulb is omitted, the energy required being supplied through
+condenser coatings <small>C C</small>. The insulating packing <small>P</small> should in
+this construction be tightly fitting to the glass, and rather wide,
+or otherwise the discharge might avoid passing through the wire
+<i>w</i>, which connects the inside condenser coating to the incandescent
+button <i>m</i>.</p>
+
+<p>The molecular bombardment against the glass stem in the bulb
+is a source of great trouble. As an illustration I will cite a phenomenon
+only too frequently and unwillingly observed. A bulb,
+preferably a large one, may be taken, and a good conducting
+body, such as a piece of carbon, may be mounted in it upon a platinum
+wire sealed in the glass stem. The bulb may be exhausted
+to a fairly high degree, nearly to the point when phosphorescence<span class='pagenum'><a name="Page_246" id="Page_246">[Pg 246]</a></span>
+begins to appear. When the bulb is connected with the coil, the
+piece of carbon, if small, may become highly incandescent at
+first, but its brightness immediately diminishes, and then the discharge
+may break through the glass somewhere in the middle of
+the stem, in the form of bright sparks, in spite of the fact that
+the platinum wire is in good electrical connection with the rarefied
+gas through the piece of carbon or metal at the top. The
+first sparks are singularly bright, recalling those drawn from a
+clear surface of mercury. But, as they heat the glass rapidly,
+they, of course, lose their brightness, and cease when the glass at
+the ruptured place becomes incandescent, or generally sufficiently
+hot to conduct. When observed for the first time the phenomenon
+must appear very curious, and shows in a striking manner
+how radically different alternate currents, or impulses, of high
+frequency behave, as compared with steady currents, or currents
+of low frequency. With such currents&mdash;namely, the latter&mdash;the
+phenomenon would of course not occur. When frequencies such
+as are obtained by mechanical means are used, I think that the rupture
+of the glass is more or less the consequence of the bombardment,
+which warms it up and impairs its insulating power; but
+with frequencies obtainable with condensers I have no doubt
+that the glass may give way without previous heating. Although
+this appears most singular at first, it is in reality what we might
+expect to occur. The energy supplied to the wire leading into
+the bulb is given off partly by direct action through the carbon
+button, and partly by inductive action through the glass surrounding
+the wire. The case is thus analogous to that in which a condenser
+shunted by a conductor of low resistance is connected to
+a source of alternating current. As long as the frequencies are
+low, the conductor gets the most and the condenser is perfectly
+safe; but when the frequency becomes excessive, the <i>role</i> of the
+conductor may become quite insignificant. In the latter case the
+difference of potential at the terminals of the condenser may become
+so great as to rupture the dielectric, notwithstanding the
+fact that the terminals are joined by a conductor of low resistance.</p>
+
+<p>It is, of course, not necessary, when it is desired to produce
+the incandescence of a body inclosed in a bulb by means of these
+currents, that the body should be a conductor, for even a perfect
+non-conductor may be quite as readily heated. For this purpose
+it is sufficient to surround a conducting electrode with a non-con<span class='pagenum'><a name="Page_247" id="Page_247">[Pg 247]</a></span>ducting
+material, as, for instance, in the bulb described before in
+Fig. 150, in which a thin incandescent lamp filament is coated
+with a non-conductor, and supports a button of the same material
+on the top. At the start the bombardment goes on by inductive
+action through the non-conductor, until the same is sufficiently
+heated to become conducting, when the bombardment continues
+in the ordinary way.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_261.jpg" width="800" height="556" alt="Fig. 151, 152." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 151.</td><td class="caption">Fig. 152.</td></tr>
+</table>
+</div>
+
+<p>A different arrangement used in some of the bulbs constructed
+is illustrated in Fig. 152. In this instance a non-conductor <i>m</i> is
+mounted in a piece of common arc light carbon so as to project
+some small distance above the latter. The carbon piece is connected
+to the leading-in wire passing through a glass stem, which
+is wrapped with several layers of mica. An aluminum tube <i>a</i> is
+employed as usual for screening. It is so arranged that it reaches
+very nearly as high as the carbon and only the non-conductor <i>m</i>
+projects a little above it. The bombardment goes at first against
+the upper surface of carbon, the lower parts being protected by
+the aluminum tube. As soon, however, as the non-conductor <i>m</i>
+is heated it is rendered good conducting, and then it becomes the
+centre of the bombardment, being most exposed to the same.</p>
+
+<p>I have also constructed during these experiments many such
+single-wire bulbs with or without internal electrode, in which the
+radiant matter was projected against, or focused upon, the body<span class='pagenum'><a name="Page_248" id="Page_248">[Pg 248]</a></span>
+to be rendered incandescent. Fig. 153 (page 263) illustrates one
+of the bulbs used. It consists of a spherical globe <small>L</small>, provided
+with a long neck <i>n</i>, on top, for increasing the action in some cases
+by the application of an external conducting coating. The globe <small>L</small>
+is blown out on the bottom into a very small bulb <i>b</i>, which serves
+to hold it firmly in a socket <small>S</small> of insulating material into which it
+is cemented. A fine lamp filament <i>f</i>, supported on a wire <i>w</i>,
+passes through the centre of the globe <small>L</small>. The filament is rendered
+incandescent in the middle portion, where the bombardment
+proceeding from the lower inside surface of the globe is
+most intense. The lower portion of the globe, as far as the
+socket <small>S</small> reaches, is rendered conducting, either by a tinfoil coating
+or otherwise, and the external electrode is connected to a
+terminal of the coil.</p>
+
+<p>The arrangement diagrammatically indicated in Fig. 153 was
+found to be an inferior one when it was desired to render incandescent
+a filament or button supported in the centre of the globe,
+but it was convenient when the object was to excite phosphorescence.</p>
+
+<p>In many experiments in which bodies of different kind were
+mounted in the bulb as, for instance, indicated in Fig. 152, some
+observations of interest were made.</p>
+
+<p>It was found, among other things, that in such cases, no matter
+where the bombardment began, just as soon as a high temperature
+was reached there was generally one of the bodies
+which seemed to take most of the bombardment upon itself, the
+other, or others, being thereby relieved. The quality appeared
+to depend principally on the point of fusion, and on the facility
+with which the body was "evaporated," or, generally speaking,
+disintegrated&mdash;meaning by the latter term not only the throwing
+off of atoms, but likewise of large lumps. The observation made
+was in accordance with generally accepted notions. In a highly
+exhausted bulb, electricity is carried off from the electrode by
+independent carriers, which are partly the atoms, or molecules,
+of the residual atmosphere, and partly the atoms, molecules, or
+lumps thrown off from the electrode. If the electrode is composed
+of bodies of different character, and if one of these is more
+easily disintegrated than the other, most of the electricity supplied
+is carried off from that body, which is then brought to a
+higher temperature than the others, and this the more, as upon
+an increase of the temperature the body is still more easily disintegrated.</p>
+<p><span class='pagenum'><a name="Page_249" id="Page_249">[Pg 249]</a></span></p>
+<p>It seems to me quite probable that a similar process takes place
+in the bulb even with a homogeneous electrode, and I think it
+to be the principal cause of the disintegration. There is bound
+to be some irregularity, even if the surface is highly polished,
+which, of course, is impossible with most of the refractory bodies
+employed as electrodes. Assume that a point of the electrode
+gets hotter; instantly most of the discharge passes through that
+point, and a minute patch it probably fused and evaporated. It
+is now possible that in consequence of the violent disintegration
+the spot attacked sinks in temperature, or that a counter force is
+created, as in an arc; at any rate, the local tearing off meets with
+the limitations incident to the experiment, whereupon the same
+process occurs on another place. To the eye the electrode appears
+uniformly brilliant, but there are upon it points constantly
+shifting and wandering around, of a temperature far above the
+mean, and this materially hastens the process of deterioration.
+That some such thing occurs, at least when the electrode is at a lower
+temperature, sufficient experimental evidence can be obtained in
+the following manner: Exhaust a bulb to a very high degree, so
+that with a fairly high potential the discharge cannot pass&mdash;that
+is, not a <i>luminous</i> one, for a weak invisible discharge occurs
+always, in all probability. Now raise slowly and carefully the
+potential, leaving the primary current on no more than for an
+instant. At a certain point, two, three, or half a dozen phosphorescent
+spots will appear on the globe. These places of the
+glass are evidently more violently bombarded than others, this
+being due to the unevenly distributed electric density, necessitated,
+of course, by sharp projections, or, generally speaking, irregularities
+of the electrode. But the luminous patches are
+constantly changing in position, which is especially well observable
+if one manages to produce very few, and this indicates that
+the configuration of the electrode is rapidly changing.</p>
+
+<p>From experiences of this kind I am led to infer that, in order
+to be most durable, the refractory button in the bulb should be
+in the form of a sphere with a highly polished surface. Such a
+small sphere could be manufactured from a diamond or some
+other crystal, but a better way would be to fuse, by the employment
+of extreme degrees of temperature, some oxide&mdash;as, for
+instance, zirconia&mdash;into a small drop, and then keep it in the
+bulb at a temperature somewhat below its point of fusion.</p>
+
+<p>Interesting and useful results can, no doubt, be reached in the<span class='pagenum'><a name="Page_250" id="Page_250">[Pg 250]</a></span>
+direction of extreme degrees of heat. How can such high temperatures
+be arrived at? How are the highest degrees of heat
+reached in nature? By the impact of stars, by high speeds and
+collisions. In a collision any rate of heat generation may be
+attained. In a chemical process we are limited. When oxygen
+and hydrogen combine, they fall, metaphorically speaking, from
+a definite height. We cannot go very far with a blast, nor by
+confining heat in a furnace, but in an exhausted bulb we can
+concentrate any amount of energy upon a minute button. Leaving
+practicability out of consideration, this, then, would be the
+means which, in my opinion, would enable us to reach the highest
+temperature. But a great difficulty when proceeding in this way
+is encountered, namely, in most cases the body is carried off before
+it can fuse and form a drop. This difficulty exists principally
+with an oxide, such as zirconia, because it cannot be compressed
+in so hard a cake that it would not be carried off quickly.
+I have endeavored repeatedly to fuse zirconia, placing it in a cup of
+arc light carbon, as indicated in Fig. 152. It glowed with a most
+intense light, and the stream of the particles projected out of the
+carbon cup was of a vivid white; but whether it was compressed
+in a cake or made into a paste with carbon, it was carried off
+before it could be fused. The carbon cup, containing zirconia,
+had to be mounted very low in the neck of a large bulb, as the
+heating of the glass by the projected particles of the oxide was
+so rapid that in the first trial the bulb was cracked almost in an
+instant, when the current was turned on. The heating of the
+glass by the projected particles was found to be always greater
+when the carbon cup contained a body which was rapidly carried
+off&mdash;I presume, because in such cases, with the same potential,
+higher speeds were reached, and also because, per unit of time,
+more matter was projected&mdash;that is, more particles would strike
+the glass.</p>
+
+<p>The before-mentioned difficulty did not exist, however, when
+the body mounted in the carbon cup offered great resistance to
+deterioration. For instance, when an oxide was first fused in
+an oxygen blast, and then mounted in the bulb, it melted very
+readily into a drop.</p>
+
+<p>Generally, during the process of fusion, magnificent light
+effects were noted, of which it would be difficult to give an adequate
+idea. Fig. 152 is intended to illustrate the effect observed
+with a ruby drop. At first one may see a narrow funnel of<span class='pagenum'><a name="Page_251" id="Page_251">[Pg 251]</a></span>
+white light projected against the top of the globe, where it
+produces an irregularly outlined phosphorescent patch. When the
+point of the ruby fuses, the phosphorescence becomes very powerful;
+but as the atoms are projected with much greater speed
+from the surface of the drop, soon the glass gets hot and "tired,"
+and now only the outer edge of the patch glows. In this manner
+an intensely phosphorescent, sharply defined line, <i>l</i>, corresponding
+to the outline of the drop, is produced, which spreads slowly
+over the globe as the drop gets larger. When the mass begins
+to boil, small bubbles and cavities are formed, which cause dark
+colored spots to sweep across the globe. The bulb may be
+turned downward without fear of the drop falling off, as the
+mass possesses considerable viscosity.</p>
+
+<p>I may mention here another feature of some interest, which
+I believe to have noted in the course of these experiments,
+though the observations do not amount to a certitude. It <i>appeared</i>
+that under the molecular impact caused by the rapidly
+alternating potential, the body was fused and maintained in that
+state at a lower temperature in a highly exhausted bulb than
+was the case at normal pressure and application of heat in the
+ordinary way&mdash;that is, at least, judging from the quantity of the
+light emitted. One of the experiments performed may be mentioned
+here by way of illustration. A small piece of pumice
+stone was stuck on a platinum wire, and first melted to it in a
+gas burner. The wire was next placed between two pieces of
+charcoal, and a burner applied, so as to produce an intense heat,
+sufficient to melt down the pumice stone into a small glass-like
+button. The platinum wire had to be taken of sufficient thickness,
+to prevent its melting in the fire. While in the charcoal
+fire, or when held in a burner to get a better idea of the degree
+of heat, the button glowed with great brilliancy. The wire with
+the button was then mounted in a bulb, and upon exhausting the
+same to a high degree, the current was turned on slowly, so as to
+prevent the cracking of the button. The button was heated to
+the point of fusion, and when it melted, it did not, apparently,
+glow with the same brilliancy as before, and this would indicate
+a lower temperature. Leaving out of consideration the observer's
+possible, and even probable, error, the question is, can a body
+under these conditions be brought from a solid to a liquid state
+with the evolution of <i>less</i> light?</p>
+
+<p>When the potential of a body is rapidly alternated, it is certain<span class='pagenum'><a name="Page_252" id="Page_252">[Pg 252]</a></span>
+that the structure is jarred. When the potential is very high,
+although the vibrations may be few&mdash;say 20,000 per second&mdash;the
+effect upon the structure may be considerable. Suppose, for example,
+that a ruby is melted into a drop by a steady application
+of energy. When it forms a drop, it will emit visible and invisible
+waves, which will be in a definite ratio, and to the eye the
+drop will appear to be of a certain brilliancy. Next, suppose we
+diminish to any degree we choose the energy steadily supplied,
+and, instead, supply energy which rises and falls according to a
+certain law. Now, when the drop is formed, there will be emitted
+from it three different kinds of vibrations&mdash;the ordinary
+visible, and two kinds of invisible waves: that is, the ordinary
+dark waves of all lengths, and, in addition, waves of a well defined
+character. The latter would not exist by a steady supply
+of the energy; still they help to jar and loosen the structure. If
+this really be the case, then the ruby drop will emit relatively
+less visible and more invisible waves than before. Thus it would
+seem that when a platinum wire, for instance, is fused by currents
+alternating with extreme rapidity, it emits at the point of fusion
+less light and more visible radiation than it does when melted by
+a steady current, though the total energy used up in the process
+of fusion is the same in both cases. Or, to cite another example,
+a lamp filament is not capable of withstanding as long with currents
+of extreme frequency as it does with steady currents,
+assuming that it be worked at the same luminous intensity. This
+means that for rapidly alternating currents the filament should
+be shorter and thicker. The higher the frequency&mdash;that is, the
+greater the departure from the steady flow&mdash;the worse it would
+be for the filament. But if the truth of this remark were demonstrated,
+it would be erroneous to conclude that such a refractory
+button as used in these bulbs would be deteriorated quicker
+by currents of extremely high frequency than by steady or low
+frequency currents. From experience I may say that just the
+opposite holds good: the button withstands the bombardment
+better with currents of very high frequency. But this is due to
+the fact that a high frequency discharge passes through a rarefied
+gas with much greater freedom than a steady or low frequency
+discharge, and this will mean that with the former we can work
+with a lower potential or with a less violent impact. As long,
+then, as the gas is of no consequence, a steady or low frequency
+current is better; but as soon as the action of the gas is desired
+and important, high frequencies are preferable.<span class='pagenum'><a name="Page_253" id="Page_253">[Pg 253]</a></span></p>
+
+<p>In the course of these experiments a great many trials were
+made with all kinds of carbon buttons. Electrodes made of ordinary
+carbon buttons were decidedly more durable when the
+buttons were obtained by the application of enormous pressure.
+Electrodes prepared by depositing carbon in well known ways
+did not show up well; they blackened the globe very quickly.
+From many experiences I conclude that lamp filaments obtained
+in this manner can be advantageously used only with low potentials
+and low frequency currents. Some kinds of carbon withstand
+so well that, in order to bring them to the point of fusion, it is
+necessary to employ very small buttons. In this case the observation
+is rendered very difficult on account of the intense heat
+produced. Nevertheless there can be no doubt that all kinds of
+carbon are fused under the molecular bombardment, but the
+liquid state must be one of great instability. Of all the bodies
+tried there were two which withstood best&mdash;diamond and carborundum.
+These two showed up about equally, but the latter
+was preferable for many reasons. As it is more than likely that
+this body is not yet generally known, I will venture to call your
+attention to it.</p>
+
+<p>It has been recently produced by Mr. E. G. Acheson, of
+Monongahela City, Pa., U. S. A. It is intended to replace ordinary
+diamond powder for polishing precious stones, etc., and I
+have been informed that it accomplishes this object quite successfully.
+I do not know why the name "carborundum" has
+been given to it, unless there is something in the process of its
+manufacture which justifies this selection. Through the kindness
+of the inventor, I obtained a short while ago some samples which
+I desired to test in regard to their qualities of phosphorescence
+and capability of withstanding high degrees of heat.</p>
+
+<p>Carborundum can be obtained in two forms&mdash;in the form of
+"crystals" and of powder. The former appear to the naked eye
+dark colored, but are very brilliant; the latter is of nearly the
+same color as ordinary diamond powder, but very much finer.
+When viewed under a microscope the samples of crystals given
+to me did not appear to have any definite form, but rather resembled
+pieces of broken up egg coal of fine quality. The
+majority were opaque, but there were some which were transparent
+and colored. The crystals are a kind of carbon containing
+some impurities; they are extremely hard, and withstand for a
+long time even an oxygen blast. When the blast is directed<span class='pagenum'><a name="Page_254" id="Page_254">[Pg 254]</a></span>
+against them they at first form a cake of some compactness, probably
+in consequence of the fusion of impurities they contain. The
+mass withstands for a very long time the blast without further
+fusion; but a slow carrying off, or burning, occurs, and, finally,
+a small quantity of a glass-like residue is left, which, I suppose,
+is melted alumina. When compressed strongly they conduct very
+well, but not as well as ordinary carbon. The powder, which is
+obtained from the crystals in some way, is practically non-conducting.
+It affords a magnificent polishing material for stones.</p>
+
+<p>The time has been too short to make a satisfactory study of
+the properties of this product, but enough experience has been
+gained in a few weeks I have experimented upon it to say that
+it does possess some remarkable properties in many respects. It
+withstands excessively high degrees of heat, it is little deteriorated
+by molecular bombardment, and it does not blacken the globe as
+ordinary carbon does. The only difficulty which I have experienced
+in its use in connection with these experiments was to find some
+binding material which would resist the heat and the effect of the
+bombardment as successfully as carborundum itself does.</p>
+
+<p>I have here a number of bulbs which I have provided with
+buttons of carborundum. To make such a button of carborundum
+crystals I proceed in the following manner: I take an ordinary
+lamp filament and dip its point in tar, or some other
+thick substance or paint which may be readily carbonized. I
+next pass the point of the filament through the crystals, and then
+hold it vertically over a hot plate. The tar softens and forms a
+drop on the point of the filament, the crystals adhering to the
+surface of the drop. By regulating the distance from the plate
+the tar is slowly dried out and the button becomes solid. I then
+once more dip the button in tar and hold it again over a plate
+until the tar is evaporated, leaving only a hard mass which firmly
+binds the crystals. When a larger button is required I repeat
+the process several times, and I generally also cover the filament
+a certain distance below the button with crystals. The button
+being mounted in a bulb, when a good vacuum has been reached,
+first a weak and then a strong discharge is passed through the
+bulb to carbonize the tar and expel all gases, and later it is brought
+to a very intense incandescence.</p>
+
+<p>When the powder is used I have found it best to proceed as
+follows: I make a thick paint of carborundum and tar, and pass
+a lamp filament through the paint. Taking then most of the<span class='pagenum'><a name="Page_255" id="Page_255">[Pg 255]</a></span>
+paint off by rubbing the filament against a piece of chamois
+leather, I hold it over a hot plate until the tar evaporates and the
+coating becomes firm. I repeat this process as many times as it
+is necessary to obtain a certain thickness of coating. On the
+point of the coated filament I form a button in the same
+manner.</p>
+
+<p>There is no doubt that such a button&mdash;properly prepared under
+great pressure&mdash;of carborundum, especially of powder of the best
+quality, will withstand the effect of the bombardment fully as
+well as anything we know. The difficulty is that the binding
+material gives way, and the carborundum is slowly thrown off
+after some time. As it does not seem to blacken the globe in the
+least, it might be found useful for coating the filaments of ordinary
+incandescent lamps, and I think that it is even possible to produce
+thin threads or sticks of carborundum which will replace the ordinary
+filaments in an incandescent lamp. A carborundum coating
+seems to be more durable than other coatings, not only
+because the carborundum can withstand high degrees of heat, but
+also because it seems to unite with the carbon better than any
+other material I have tried. A coating of zirconia or any other
+oxide, for instance, is far more quickly destroyed. I prepared
+buttons of diamond dust in the same manner as of carborundum,
+and these came in durability nearest to those prepared of carborundum,
+but the binding paste gave way much more quickly
+in the diamond buttons; this, however, I attributed to the size
+and irregularity of the grains of the diamond.</p>
+
+<p>It was of interest to find whether carborundum possesses the
+quality of phosphorescence. One is, of course, prepared to encounter
+two difficulties: first, as regards the rough product, the
+"crystals," they are good conducting, and it is a fact that conductors
+do not phosphoresce; second, the powder, being exceedingly
+fine, would not be apt to exhibit very prominently this
+quality, since we know that when crystals, even such as diamond
+or ruby, are finely powdered, they lose the property of phosphorescence
+to a considerable degree.</p>
+
+<p>The question presents itself here, can a conductor phosphoresce?
+What is there in such a body as a metal, for instance, that
+would deprive it of the quality of phosphoresence, unless it is
+that property which characterizes it as a conductor? For it is a
+fact that most of the phosphorescent bodies lose that quality when
+they are sufficiently heated to become more or less conducting.<span class='pagenum'><a name="Page_256" id="Page_256">[Pg 256]</a></span>
+Then, if a metal be in a large measure, or perhaps entirely, deprived
+of that property, it should be capable of phosphoresence.
+Therefore it is quite possible that at some extremely high frequency,
+when behaving practically as a non-conductor, a metal
+or any other conductor might exhibit the quality of phosphoresence,
+even though it be entirely incapable of phosphorescing
+under the impact of a low-frequency discharge. There is, however,
+another possible way how a conductor might at least <i>appear</i>
+to phosphoresce.</p>
+
+<p>Considerable doubt still exists as to what really is phosphorescence,
+and as to whether the various phenomena comprised
+under this head are due to the same causes. Suppose that in an
+exhausted bulb, under the molecular impact, the surface of a
+piece of metal or other conductor is rendered strongly luminous,
+but at the same time it is found that it remains comparatively
+cool, would not this luminosity be called phosphorescence? Now
+such a result, theoretically at least, is possible, for it is a mere
+question of potential or speed. Assume the potential of the
+electrode, and consequently the speed of the projected atoms, to
+be sufficiently high, the surface of the metal piece, against which
+the atoms are projected, would be rendered highly incandescent,
+since the process of heat generation would be incomparably faster
+than that of radiating or conducting away from the surface of
+the collision. In the eye of the observer a single impact of the
+atoms would cause an instantaneous flash, but if the impacts were
+repeated with sufficient rapidity, they would produce a continuous
+impression upon his retina. To him then the surface of the
+metal would appear continuously incandescent and of constant
+luminous intensity, while in reality the light would be either
+intermittent, or at least changing periodically in intensity. The
+metal piece would rise in temperature until equilibrium was
+attained&mdash;that is, until the energy continuously radiated would
+equal that intermittently supplied. But the supplied energy
+might under such conditions not be sufficient to bring the body
+to any more than a very moderate mean temperature, especially
+if the frequency of the atomic impacts be very low&mdash;just enough
+that the fluctuation of the intensity of the light emitted could
+not be detected by the eye. The body would now, owing to the
+manner in which the energy is supplied, emit a strong light, and
+yet be at a comparatively very low mean temperature. How
+should the observer name the luminosity thus produced? Even if<span class='pagenum'><a name="Page_257" id="Page_257">[Pg 257]</a></span>
+the analysis of the light would teach him something definite, still
+he would probably rank it under the phenomena of phosphorescence.
+It is conceivable that in such a way both conducting
+and non-conducting bodies may be maintained at a certain luminous
+intensity, but the energy required would very greatly vary
+with the nature and properties of the bodies.</p>
+
+<p>These and some foregoing remarks of a speculative nature
+were made merely to bring out curious features of alternate
+currents or electric impulses. By their help we may cause a body
+to emit <i>more</i> light, while at a certain mean temperature, than it
+would emit if brought to that temperature by a steady supply;
+and, again, we may bring a body to the point of fusion, and cause
+it to emit <i>less</i> light than when fused by the application of energy
+in ordinary ways. It all depends on how we supply the energy,
+and what kind of vibrations we set up; in one case the vibrations
+are more, in the other less, adapted to affect our sense of vision.</p>
+
+<p>Some effects, which I had not observed before, obtained with
+carborundum in the first trials, I attributed to phosphorescence,
+but in subsequent experiments it appeared that it was devoid of
+that quality. The crystals possess a noteworthy feature. In a
+bulb provided with a single electrode in the shape of a small
+circular metal disc, for instance, at a certain degree of exhaustion
+the electrode is covered with a milky film, which is separated by
+a dark space from the glow filling the bulb. When the metal
+disc is covered with carborundum crystals, the film is far more
+intense, and snow-white. This I found later to be merely an
+effect of the bright surface of the crystals, for when an aluminum
+electrode was highly polished, it exhibited more or less the same
+phenomenon. I made a number of experiments with the samples
+of crystals obtained, principally because it would have been of
+special interest to find that they are capable of phosphorescence,
+on account of their being conducting. I could not produce phosphorescence
+distinctly, but I must remark that a decisive opinion
+cannot be formed until other experimenters have gone over the
+same ground.</p>
+
+<p>The powder behaved in some experiments as though it contained
+alumina, but it did not exhibit with sufficient distinctness
+the red of the latter. Its dead color brightens considerably under
+the molecular impact, but I am now convinced it does not
+phosphoresce. Still, the tests with the powder are not conclusive,
+because powdered carborundum probably does not behave like a<span class='pagenum'><a name="Page_258" id="Page_258">[Pg 258]</a></span>
+phosphorescent sulphide, for example, which could be finely
+powdered without impairing the phosphorescence, but rather like
+powdered ruby or diamond, and therefore it would be necessary,
+in order to make a decisive test, to obtain it in a large lump and
+polish up the surface.</p>
+
+<p>If the carborundum proves useful in connection with these
+and similar experiments, its chief value will be found in the
+production of coatings, thin conductors, buttons, or other electrodes
+capable of withstanding extremely high degrees of heat.</p>
+
+<p>The production of a small electrode, capable of withstanding
+enormous temperatures, I regard as of the greatest importance
+in the manufacture of light. It would enable us to obtain, by
+means of currents of very high frequencies, certainly 20 times, if
+not more, the quantity of light which is obtained in the present
+incandescent lamp by the same expenditure of energy. This
+estimate may appear to many exaggerated, but in reality I think
+it is far from being so. As this statement might be misunderstood,
+I think it is necessary to expose clearly the problem with
+which, in this line of work, we are confronted, and the manner
+in which, in my opinion, a solution will be arrived at.</p>
+
+<p>Any one who begins a study of the problem will be apt to
+think that what is wanted in a lamp with an electrode is a very
+high degree of incandescence of the electrode. There he will be
+mistaken. The high incandescence of the button is a necessary
+evil, but what is really wanted is the high incandescence of the
+gas surrounding the button. In other words, the problem in
+such a lamp is to bring a mass of gas to the highest possible incandescence.
+The higher the incandescence, the quicker the
+mean vibration, the greater is the economy of the light production.
+But to maintain a mass of gas at a high degree of incandescence
+in a glass vessel, it will always be necessary to keep the incandescent
+mass away from the glass; that is, to confine it as much as
+possible to the central portion of the globe.</p>
+
+<p>In one of the experiments this evening a brush was produced
+at the end of a wire. The brush was a flame, a source of heat
+and light. It did not emit much perceptible heat, nor did it
+glow with an intense light; but is it the less a flame because it
+does not scorch my hand? Is it the less a flame because it does
+not hurt my eyes by its brilliancy? The problem is precisely to
+produce in the bulb such a flame, much smaller in size, but incomparably
+more powerful. Were there means at hand for<span class='pagenum'><a name="Page_259" id="Page_259">[Pg 259]</a></span>
+producing electric impulses of a sufficiently high frequency, and
+for transmitting them, the bulb could be done away with, unless
+it were used to protect the electrode, or to economize the energy
+by confining the heat. But as such means are not at disposal, it
+becomes necessary to place the terminal in the bulb and rarefy
+the air in the same. This is done merely to enable the apparatus
+to perform the work which it is not capable of performing at ordinary
+air pressure. In the bulb we are able to intensify the
+action to any degree&mdash;so far that the brush emits a powerful
+light.</p>
+
+<p>The intensity of the light emitted depends principally on the
+frequency and potential of the impulses, and on the electric density
+on the surface of the electrode. It is of the greatest importance
+to employ the smallest possible button, in order to push
+the density very far. Under the violent impact of the molecules
+of the gas surrounding it, the small electrode is of course brought
+to an extremely high temperature, but around it is a mass of
+highly incandescent gas, a flame photosphere, many hundred
+times the volume of the electrode. With a diamond, carborundum
+or zirconia button the photosphere can be as much as one
+thousand times the volume of the button. Without much reflection
+one would think that in pushing so far the incandescence
+of the electrode it would be instantly volatilized. But after a
+careful consideration one would find that, theoretically, it should
+not occur, and in this fact&mdash;which, moreover, is experimentally
+demonstrated&mdash;lies principally the future value of such a lamp.</p>
+
+<p>At first, when the bombardment begins, most of the work is
+performed on the surface of the button, but when a highly conducting
+photosphere is formed the button is comparatively relieved.
+The higher the incandescence of the photosphere, the
+more it approaches in conductivity to that of the electrode, and
+the more, therefore, the solid and the gas form one conducting
+body. The consequence is that the further the incandescence is
+forced the more work, comparatively, is performed on the gas,
+and the less on the electrode. The formation of a powerful
+photosphere is consequently the very means for protecting the
+electrode. This protection, of course, is a relative one, and it
+should not be thought that by pushing the incandescence higher
+the electrode is actually less deteriorated. Still, theoretically,
+with extreme frequencies, this result must be reached, but probably
+at a temperature too high for most of the refractory bodies<span class='pagenum'><a name="Page_260" id="Page_260">[Pg 260]</a></span>
+known. Given, then, an electrode which can withstand to a very
+high limit the effect of the bombardment and outward strain, it
+would be safe, no matter how much it was forced beyond that
+limit. In an incandescent lamp quite different considerations
+apply. There the gas is not at all concerned; the whole of the
+work is performed on the filament; and the life of the lamp
+diminishes so rapidly with the increase of the degree of incandescence
+that economical reasons compel us to work it at a low
+incandescence. But if an incandescent lamp is operated with
+currents of very high frequency, the action of the gas cannot be
+neglected, and the rules for the most economical working must
+be considerably modified.</p>
+
+<p>In order to bring such a lamp with one or two electrodes to a
+great perfection, it is necessary to employ impulses of very high
+frequency. The high frequency secures, among others, two chief
+advantages, which have a most important bearing upon the
+economy of the light production. First, the deterioration of the
+electrode is reduced by reason of the fact that we employ a great
+many small impacts, instead of a few violent ones, which quickly
+shatter the structure; secondly, the formation of a large photosphere
+is facilitated.</p>
+
+<p>In order to reduce the deterioration of the electrode to the
+minimum, it is desirable that the vibration be harmonic, for any
+suddenness hastens the process of destruction. An electrode lasts
+much longer when kept at incandescence by currents, or impulses,
+obtained from a high frequency alternator, which rise and fall
+more or less harmonically, than by impulses obtained from a disruptive
+discharge coil. In the latter case there is no doubt that
+most of the damage is done by the fundamental sudden discharges.</p>
+
+<p>One of the elements of loss in such a lamp is the bombardment
+of the globe. As the potential is very high, the molecules
+are projected with great speed; they strike the glass, and usually excite
+a strong phosphorescence. The effect produced is very pretty,
+but for economical reasons it would be perhaps preferable to prevent,
+or at least reduce to a minimum, the bombardment against
+the globe, as in such case it is, as a rule, not the object to excite
+phosphorescence, and as some loss of energy results from the
+bombardment. This loss in the bulb is principally dependent
+on the potential of the impulses and on the electric density on
+the surface of the electrode. In employing very high frequen<span class='pagenum'><a name="Page_261" id="Page_261">[Pg 261]</a></span>cies
+the loss of energy by the bombardment is greatly reduced,
+for, first, the potential needed to perform a given amount of work
+is much smaller; and, secondly, by producing a highly conducting
+photosphere around the electrode, the same result is obtained
+as though the electrode were much larger, which is equivalent to
+a smaller electric density. But be it by the diminution of the
+maximum potential or of the density, the gain is effected in the
+same manner, namely, by avoiding violent shocks, which strain
+the glass much beyond its limit of elasticity. If the frequency
+could be brought high enough, the loss due to the imperfect
+elasticity of the glass would be entirely negligible. The loss due
+to bombardment of the globe may, however, be reduced by using
+two electrodes instead of one. In such case each of the electrodes
+may be connected to one of the terminals; or else, if it is
+preferable to use only one wire, one electrode may be connected
+to one terminal and the other to the ground or to an insulated
+body of some surface, as, for instance, a shade on the lamp. In
+the latter case, unless some judgment is used, one of the electrodes
+might glow more intensely than the other.</p>
+
+<p>But on the whole I find it preferable, when using such high
+frequencies, to employ only one electrode and one connecting
+wire. I am convinced that the illuminating device of the near
+future will not require for its operation more than one lead, and,
+at any rate, it will have no leading-in wire, since the energy required
+can be as well transmitted through the glass. In experimental
+bulbs the leading-in wire is not generally used on account
+of convenience, as in employing condenser coatings in the manner
+indicated in Fig. 151, for example, there is some difficulty in
+fitting the parts, but these difficulties would not exist if a great
+many bulbs were manufactured; otherwise the energy can be
+conveyed through the glass as well as through a wire, and with
+these high frequencies the losses are very small. Such illustrating
+devices will necessarily involve the use of very high
+potentials, and this, in the eyes of practical men, might be an objectionable
+feature. Yet, in reality, high potentials are not
+objectionable&mdash;certainly not in the least so far as the safety of
+the devices is concerned.</p>
+
+<p>There are two ways of rendering an electric appliance safe.
+One is to use low potentials, the other is to determine the dimensions
+of the apparatus so that it is safe, no matter how high a
+potential is used. Of the two, the latter seems to me the better<span class='pagenum'><a name="Page_262" id="Page_262">[Pg 262]</a></span>
+way, for then the safety is absolute, unaffected by any possible
+combination of circumstances which might render even a low-potential
+appliance dangerous to life and property. But the practical
+conditions require not only the judicious determination of the
+dimensions of the apparatus; they likewise necessitate the employment
+of energy of the proper kind. It is easy, for instance,
+to construct a transformer capable of giving, when operated from
+an ordinary alternate current machine of low tension, say 50,000
+volts, which might be required to light a highly exhausted phosphorescent
+tube, so that, in spite of the high potential, it is
+perfectly safe, the shock from it producing no inconvenience.
+Still such a transformer would be expensive, and in itself inefficient;
+and, besides, what energy was obtained from it would not
+be economically used for the production of light. The economy
+demands the employment of energy in the form of extremely rapid
+vibrations. The problem of producing light has been likened to
+that of maintaining a certain high-pitch note by means of a bell.
+It should be said a <i>barely audible</i> note; and even these words
+would not express it, so wonderful is the sensitiveness of the eye.
+We may deliver powerful blows at long intervals, waste a good
+deal of energy, and still not get what we want; or we may keep
+up the note by delivering frequent taps, and get nearer to the
+object sought by the expenditure of much less energy. In the
+production of light, as far as the illuminating device is concerned,
+there can be only one rule&mdash;that is, to use as high frequencies as
+can be obtained; but the means for the production and conveyance
+of impulses of such character impose, at present at least,
+great limitations. Once it is decided to use very high frequencies,
+the return wire becomes unnecessary, and all the appliances
+are simplified. By the use of obvious means the same result is
+obtained as though the return wire were used. It is sufficient for
+this purpose to bring in contact with the bulb, or merely in the
+vicinity of the same, an insulated body of some surface. The
+surface need, of course, be the smaller, the higher the frequency
+and potential used, and necessarily, also, the higher the economy
+of the lamp or other device.</p>
+
+<p>This plan of working has been resorted to on several occasions
+this evening. So, for instance, when the incandescence of a
+button was produced by grasping the bulb with the hand, the
+body of the experimenter merely served to intensify the action.
+The bulb used was similar to that illustrated in Fig. 148, and<span class='pagenum'><a name="Page_263" id="Page_263">[Pg 263]</a></span>
+the coil was excited to a small potential, not sufficient to bring
+the button to incandescence when the bulb was hanging from
+the wire; and incidentally, in order to perform the experiment
+in a more suitable manner, the button was taken so large that a
+perceptible time had to elapse before, upon grasping the bulb, it
+could be rendered incandescent. The contact with the bulb was,
+of course, quite unnecessary. It is easy, by using a rather large
+bulb with an exceedingly small electrode, to adjust the conditions
+so that the latter is brought to bright incandescence by the mere
+approach of the experimenter within a few feet of the bulb, and
+that the incandescence subsides upon his receding.</p>
+
+<div class="figcenter" style="width: 710px;">
+<img src="images/oi_277.jpg" width="710" height="600" alt="Fig. 153, 154." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 153.</td><td class="caption">Fig. 154.</td></tr>
+</table>
+</div>
+
+<p>In another experiment, when phosphorescence was excited, a
+similar bulb was used. Here again, originally, the potential was
+not sufficient to excite phosphorescence until the action was intensified&mdash;in
+this case, however, to present a different feature, by
+touching the socket with a metallic object held in the hand. The
+electrode in the bulb was a carbon button so large that it could
+not be brought to incandescence, and thereby spoil the effect
+produced by phosphorescence.</p>
+
+<p>Again, in another of the early experiments, a bulb was used,<span class='pagenum'><a name="Page_264" id="Page_264">[Pg 264]</a></span>
+as illustrated in Fig. 141. In this instance, by touching the bulb
+with one or two fingers, one or two shadows of the stem inside
+were projected against the glass, the touch of the finger producing
+the same results as the application of an external negative electrode
+under ordinary circumstances.</p>
+
+<p>In all these experiments the action was intensified by augmenting
+the capacity at the end of the lead connected to the terminal.
+As a rule, it is not necessary to resort to such means, and would
+be quite unnecessary with still higher frequencies; but when it
+<i>is</i> desired, the bulb, or tube, can be easily adapted to the purpose.</p>
+
+<p>In Fig. 153, for example, an experimental bulb, <small>L</small>, is shown,
+which is provided with a neck, <i>n</i>, on the top, for the application
+of an external tinfoil coating, which may be connected to a body
+of larger surface. Such a lamp as illustrated in Fig. 154 may
+also be lighted by connecting the tinfoil coating on the neck <i>n</i>
+to the terminal, and the leading-in wire, <i>w</i>, to an insulated plate.
+If the bulb stands in a socket upright, as shown in the cut, a
+shade of conducting material may be slipped in the neck, <i>n</i>, and
+the action thus magnified.</p>
+
+<p>A more perfected arrangement used in some of these bulbs is
+illustrated in Fig. 155. In this case the construction of the bulb
+is as shown and described before, when reference was made to
+Fig. 148. A zinc sheet, <small>Z</small>, with a tubular extension, <small>T</small>, is applied
+over the metallic socket, <small>S</small>. The bulb hangs downward from the
+terminal, <i>t</i>, the zinc sheet, <small>Z</small>, performing the double office of intensifier
+and reflector. The reflector is separated from the terminal,
+<i>t</i>, by an extension of the insulating plug, <small>P</small>.</p>
+
+<p>A similar disposition with a phosphorescent tube is illustrated
+in Fig. 156. The tube, <small>T</small>, is prepared from two short tubes of
+different diameter, which are sealed on the ends. On the lower
+end is placed an inside conducting coating, <small>C</small>, which connects to
+the wire <i>w</i>. The wire has a hook on the upper end for suspension,
+and passes through the centre of the inside tube, which is
+filled with some good and tightly packed insulator. On the outside
+of the upper end of the tube, <small>T</small>, is another conducting coating,
+<small>C<sub>1</sub></small>, upon which is slipped a metallic reflector <small>Z</small>, which should
+be separated by a thick insulation from the end of wire <i>w</i>.</p>
+
+<p>The economical use of such a reflector or intensifier would require
+that all energy supplied to an air condenser should be recoverable,
+or, in other words, that there should not be any losses,<span class='pagenum'><a name="Page_265" id="Page_265">[Pg 265]</a></span>
+neither in the gaseous medium nor through its action elsewhere.
+This is far from being so, but, fortunately, the losses may be reduced
+to anything desired. A few remarks are necessary on
+this subject, in order to make the experiences gathered in the
+course of these investigations perfectly clear.</p>
+
+<div class="figcenter" style="width: 670px;">
+<img src="images/oi_279.jpg" width="670" height="600" alt="Fig. 155." title="" />
+<span class="caption">Fig. 155.</span>
+</div>
+
+
+<p>Suppose a small helix with many well insulated turns, as in
+experiment Fig. 146, has one of its ends connected to one of the
+terminals of the induction coil, and the other to a metal plate,
+or, for the sake of simplicity, a sphere, insulated in space. When
+the coil is set to work, the potential of the sphere is alternated,
+and a small helix now behaves as though its free end were connected
+to the other terminal of the induction coil. If an iron
+rod be held within a small helix, it is quickly brought to a high
+temperature, indicating the passage of a strong current through
+the helix. How does the insulated sphere act in this case? It
+can be a condenser, storing and returning the energy supplied to
+it, or it can be a mere sink of energy, and the conditions of the
+experiment determine whether it is rather one than the other.
+The sphere being charged to a high potential, it acts inductively
+upon the surrounding air, or whatever gaseous medium there might
+be. The molecules, or atoms, which are near the sphere, are of
+course more attracted, and move through a greater distance than
+the farther ones. When the nearest molecules strike the sphere,
+they are repelled, and collisions occur at all distances within the
+inductive action of the sphere. It is now clear that, if the poten<span class='pagenum'><a name="Page_266" id="Page_266">[Pg 266]</a></span>tial
+be steady, but little loss of energy can be caused in this way,
+for the molecules which are nearest to the sphere, having had an
+additional charge imparted to them by contact, are not attracted
+until they have parted, if not with all, at least with most of the
+additional charge, which can be accomplished only after a great
+many collisions. From the fact, that with a steady potential
+there is but little loss in dry air, one must come to such a conclusion.
+When the potential of a sphere, instead of being steady,
+is alternating, the conditions are entirely different. In this case
+a rhythmical bombardment occurs, no matter whether the molecules,
+after coming in contact with the sphere, lose the imparted
+charge or not; what is more, if the charge is not lost, the impacts
+are only the more violent. Still, if the frequency of the impulses
+be very small, the loss caused by the impacts and collisions
+would not be serious, unless the potential were excessive. But
+when extremely high frequencies and more or less high potentials
+are used, the loss may very great. The total energy lost per unit
+of time is proportionate to the product of the number of impacts
+per second, or the frequency and the energy lost in each impact.
+But the energy of an impact must be proportionate to the square
+of the electric density of the sphere, since the charge imparted
+<span class='pagenum'><a name="Page_267" id="Page_267">[Pg 267]</a></span>to the molecule is proportionate to that density. I conclude from
+this that the total energy lost must be proportionate to the product
+of the frequency and the square of the electric density; but
+this law needs experimental confirmation. Assuming the preceding
+considerations to be true, then, by rapidly alternating the
+potential of a body immersed in an insulating gaseous medium,
+any amount of energy may be dissipated into space. Most of
+that energy then, I believe, is not dissipated in the form of long
+ether waves, propagated to considerable distance, as is thought
+most generally, but is consumed&mdash;in the case of an insulated
+sphere, for example&mdash;in impact and collisional losses&mdash;that is,
+heat vibrations&mdash;on the surface and in the vicinity of the sphere.
+To reduce the dissipation, it is necessary to work with a small
+electric density&mdash;the smaller, the higher the frequency.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_280.jpg" width="600" height="669" alt="Fig. 156." title="" />
+<span class="caption">Fig. 156.</span>
+</div>
+
+<p>But since, on the assumption before made, the loss is diminished
+with the square of the density, and since currents of very
+high frequencies involve considerable waste when transmitted
+through conductors, it follows that, on the whole, it is better to
+employ one wire than two. Therefore, if motors, lamps, or devices
+of any kind are perfected, capable of being advantageously
+operated by currents of extremely high frequency, economical
+reasons will make it advisable to use only one wire, especially if
+the distances are great.</p>
+
+<p>When energy is absorbed in a condenser, the same behaves as
+though its capacity were increased. Absorption always exists
+more or less, but generally it is small and of no consequence as
+long as the frequencies are not very great. In using extremely
+high frequencies, and, necessarily in such case, also high potentials,
+the absorption&mdash;or, what is here meant more particularly
+by this term, the loss of energy due to the presence of a gaseous
+medium&mdash;is an important factor to be considered, as the energy
+absorbed in the air condenser may be any fraction of the supplied
+energy. This would seem to make it very difficult to tell from
+the measured or computed capacity of an air condenser its actual
+capacity or vibration period, especially if the condenser is of very
+small surface and is charged to a very high potential. As many
+important results are dependent upon the correctness of the
+estimation of the vibration period, this subject demands the most
+careful scrutiny of other investigators. To reduce the probable
+error as much as possible in experiments of the kind alluded to,
+it is advisable to use spheres or plates of large surface, so as to<span class='pagenum'><a name="Page_268" id="Page_268">[Pg 268]</a></span>
+make the density exceedingly small. Otherwise, when it is
+practicable, an oil condenser should be used in preference. In
+oil or other liquid dielectrics there are seemingly no such losses
+as in gaseous media. It being impossible to exclude entirely the
+gas in condensers with solid dielectrics, such condensers should
+be immersed in oil, for economical reasons, if nothing else; they
+can then be strained to the utmost, and will remain cool. In
+Leyden jars the loss due to air is comparatively small, as the tinfoil
+coatings are large, close together, and the charged surfaces
+not directly exposed; but when the potentials are very high, the
+loss may be more or less considerable at, or near, the upper edge
+of the foil, where the air is principally acted upon. If the jar
+be immersed in boiled-out oil, it will be capable of performing
+four times the amount of work which it can for any length of
+time when used in the ordinary way, and the loss will be inappreciable.</p>
+
+<p>It should not be thought that the loss in heat in an air condenser
+is necessarily associated with the formation of <i>visible</i>
+streams or brushes. If a small electrode, inclosed in an unexhausted
+bulb, is connected to one of the terminals of the coil,
+streams can be seen to issue from the electrode, and the air in the
+bulb is heated; if instead of a small electrode a large sphere is
+inclosed in the bulb, no streams are observed, still the air is
+heated.</p>
+
+<p>Nor should it be thought that the temperature of an air condenser
+would give even an approximate idea of the loss in heat
+incurred, as in such case heat must be given off much more
+quickly, since there is, in addition to the ordinary radiation, a
+very active carrying away of heat by independent carriers going
+on, and since not only the apparatus, but the air at some distance
+from it is heated in consequence of the collisions which must
+occur.</p>
+
+<p>Owing to this, in experiments with such a coil, a rise of temperature
+can be distinctly observed only when the body connected
+to the coil is very small. But with apparatus on a larger scale,
+even a body of considerable bulk would be heated, as, for instance,
+the body of a person; and I think that skilled physicians might
+make observations of utility in such experiments, which, if the
+apparatus were judiciously designed, would not present the slightest
+danger.</p>
+
+<p>A question of some interest, principally to meteorologists,<span class='pagenum'><a name="Page_269" id="Page_269">[Pg 269]</a></span>
+presents itself here. How does the earth behave? The earth is
+an air condenser, but is it a perfect or a very imperfect one&mdash;a
+mere sink of energy? There can be little doubt that to such
+small disturbance as might be caused in an experiment, the earth
+behaves as an almost perfect condenser. But it might be different
+when its charge is set in vibration by some sudden disturbance
+occurring in the heavens. In such case, as before stated,
+probably only little of the energy of the vibrations set up would
+be lost into space in the form of long ether radiations, but most
+of the energy, I think, would spend itself in molecular impacts
+and collisions, and pass off into space in the form of short heat,
+and possibly light, waves. As both the frequency of the vibrations
+of the charge and the potential are in all probability excessive,
+the energy converted into heat may be considerable. Since
+the density must be unevenly distributed, either in consequence
+of the irregularity of the earth's surface, or on account of the
+condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable variations
+in the temperature and pressure of the atmosphere may in
+this manner be caused at any point of the surface of the earth.
+The variations may be gradual or very sudden, according to the
+nature of the general disturbance, and may produce rain and
+storms, or locally modify the weather in any way.</p>
+
+<p>From the remarks before made, one may see what an important
+factor of loss the air in the neighborhood of a charged surface
+becomes when the electric density is great and the frequency of
+the impulses excessive. But the action, as explained, implies
+that the air is insulating&mdash;that is, that it is composed of independent
+carriers immersed in an insulating medium. This is the case
+only when the air is at something like ordinary or greater, or at
+extremely small, pressure. When the air is slightly rarefied and
+conducting, then true conduction losses occur also. In such case,
+of course, considerable energy may be dissipated into space even
+with a steady potential, or with impulses of low frequency, if the
+density is very great.</p>
+
+<p>When the gas is at very low pressure, an electrode is heated
+more because higher speeds can be reached. If the gas around
+the electrode is strongly compressed, the displacements, and
+consequently the speeds, are very small, and the heating is insignificant.
+But if in such case the frequency could be sufficiently
+increased, the electrode would be brought to a high tem<span class='pagenum'><a name="Page_270" id="Page_270">[Pg 270]</a></span>perature
+as well as if the gas were at very low pressure; in fact,
+exhausting the bulb is only necessary because we cannot produce,
+(and possibly not convey) currents of the required frequency.</p>
+
+<p>Returning to the subject of electrode lamps, it is obviously of
+advantage in such a lamp to confine as much as possible the heat
+to the electrode by preventing the circulation of the gas in the
+bulb. If a very small bulb be taken, it would confine the heat
+better than a large one, but it might not be of sufficient capacity
+to be operated from the coil, or, if so, the glass might get too
+hot. A simple way to improve in this direction is to employ a
+globe of the required size, but to place a small bulb, the diameter
+of which is properly estimated, over the refractory button contained
+in the globe. This arrangement is illustrated in Fig. 157.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_284.jpg" width="800" height="554" alt="Fig. 157, 158." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 157.</td><td class="caption">Fig. 158.</td></tr>
+</table>
+</div>
+
+<p>The globe <small>L</small> has in this case a large neck <i>n</i>, allowing the small
+bulb <i>b</i> to slip through. Otherwise the construction is the same
+as shown in Fig. 147, for example. The small bulb is conveniently
+supported upon the stem <i>s</i>, carrying the refractory button
+<i>m</i>. It is separated from the aluminum tube <i>a</i> by several layers
+of mica <small>M</small>, in order to prevent the cracking of the neck by the
+rapid heating of the aluminum tube upon a sudden turning on
+of the current. The inside bulb should be as small as possible
+when it is desired to obtain light only by incandescence of the
+electrode. If it is desired to produce phosphorescence, the bulb<span class='pagenum'><a name="Page_271" id="Page_271">[Pg 271]</a></span>
+should be larger, else it would be apt to get too hot, and the
+phosphorescence would cease. In this arrangement usually only
+the small bulb shows phosphorescence, as there is practically no
+bombardment against the outer globe. In some of these bulbs
+constructed as illustrated in Fig. 157, the small tube was coated
+with phosphorescent paint, and beautiful effects were obtained.
+Instead of making the inside bulb large, in order to avoid undue
+heating, it answers the purpose to make the electrode <i>m</i> larger.
+In this case the bombardment is weakened by reason of the
+smaller electric density.</p>
+
+<p>Many bulbs were constructed on the plan illustrated in Fig.
+158. Here a small bulb <i>b</i>, containing the refractory button <i>m</i>,
+upon being exhausted to a very high degree was sealed in a large
+globe <small>L</small>, which was then moderately exhausted and sealed off.
+The principal advantage of this construction was that it allowed
+of reaching extremely high vacua, and, at the same time of using a
+large bulb. It was found, in the course of experiments with
+bulbs such as illustrated in Fig. 158, that it was well to make
+the stem <i>s</i>, near the seal at <i>e</i>, very thick, and the leading-in wire
+<i>w</i> thin, as it occurred sometimes that the stem at <i>e</i> was heated
+and the bulb was cracked. Often the outer globe <small>L</small> was exhausted
+only just enough to allow the discharge to pass through, and the
+space between the bulbs appeared crimson, producing a curious
+effect. In some cases, when the exhaustion in globe <small>L</small> was very
+low, and the air good conducting, it was found necessary, in order
+to bring the button <i>m</i> to high incandescence, to place, preferably
+on the upper part of the neck of the globe, a tinfoil coating which
+was connected to an insulated body, to the ground, or to the
+other terminal of the coil, as the highly conducting air weakened
+the effect somewhat, probably by being acted upon inductively
+from the wire <i>w</i>, where it entered the bulb at <i>e</i>. Another difficulty&mdash;which,
+however, is always present when the refractory
+button is mounted in a very small bulb&mdash;existed in the construction
+illustrated in Fig. 158, namely, the vacuum in the bulb <i>b</i>
+would be impaired in a comparatively short time.</p>
+
+<p>The chief idea in the two last described constructions was to
+confine the heat to the central portion of the globe by preventing
+the exchange of air. An advantage is secured, but owing to the
+heating of the inside bulb and slow evaporation of the glass, the
+vacuum is hard to maintain, even if the construction illustrated
+in Fig. 157 be chosen, in which both bulbs communicate.<span class='pagenum'><a name="Page_272" id="Page_272">[Pg 272]</a></span></p>
+
+<p>But by far the better way&mdash;the ideal way&mdash;would be to reach
+sufficiently high frequencies. The higher the frequency, the
+slower would be the exchange of the air, and I think that a frequency
+may be reached, at which there would be no exchange
+whatever of the air molecules around the terminal. We would
+then produce a flame in which there would be no carrying away
+of material, and a queer flame it would be, for it would be rigid!
+With such high frequencies the inertia of the particles would come
+into play. As the brush, or flame, would gain rigidity in virtue
+of the inertia of the particles, the exchange of the latter would
+be prevented. This would necessarily occur, for, the number of
+impulses being augmented, the potential energy of each would
+diminish, so that finally only atomic vibrations could be set up,
+and the motion of translation through measurable space would
+cease. Thus an ordinary gas burner connected to a source of
+rapidly alternating potential might have its efficiency augmented
+to a certain limit, and this for two reasons&mdash;because of the additional
+vibration imparted, and because of a slowing down of the
+process of carrying off. But the renewal being rendered difficult,
+a renewal being necessary to maintain the <i>burner</i>, a continued
+increase of the frequency of the impulses, assuming they could
+be transmitted to and impressed upon the flame, would result in
+the "extinction" of the latter, meaning by this term only the
+cessation of the chemical process.</p>
+
+<p>I think, however, that in the case of an electrode immersed in
+a fluid insulating medium, and surrounded by independent carriers
+of electric charges, which can be acted upon inductively, a
+sufficient high frequency of the impulses would probably result
+in a gravitation of the gas all around toward the electrode. For
+this it would be only necessary to assume that the independent
+bodies are irregularly shaped; they would then turn toward the
+electrode their side of the greatest electric density, and this
+would be a position in which the fluid resistance to approach
+would be smaller than that offered to the receding.</p>
+
+<p>The general opinion, I do not doubt, is that it is out of the
+question to reach any such frequencies as might&mdash;assuming some
+of the views before expressed to be true&mdash;produce any of the results
+which I have pointed out as mere possibilities. This may be
+so, but in the course of these investigations, from the observation
+of many phenomena, I have gained the conviction that these frequencies
+would be much lower than one is apt to estimate at<span class='pagenum'><a name="Page_273" id="Page_273">[Pg 273]</a></span>
+first. In a flame we set up light vibrations by causing molecules,
+or atoms, to collide. But what is the ratio of the frequency of
+the collisions and that of the vibrations set up? Certainly it
+must be incomparably smaller than that of the strokes of the bell
+and the sound vibrations, or that of the discharges and the oscillations
+of the condenser. We may cause the molecules of the
+gas to collide by the use of alternate electric impulses of high
+frequency, and so we may imitate the process in a flame; and
+from experiments with frequencies which we are now able to
+obtain, I think that the result is producible with impulses which
+are transmissible through a conductor.</p>
+
+<p>In connection with thoughts of a similar nature, it appeared to
+me of great interest to demonstrate the rigidity of a vibrating gaseous
+column. Although with such low frequencies as, say 10,000
+per second, which I was able to obtain without difficulty from a
+specially constructed alternator, the task looked discouraging at
+first, I made a series of experiments. The trials with air at ordinary
+pressure led to no result, but with air moderately rarefied I
+obtain what I think to be an unmistakable experimental evidence
+of the property sought for. As a result of this kind might lead
+able investigators to conclusions of importance, I will describe
+one of the experiments performed.</p>
+
+<p>It is well known that when a tube is slightly exhausted, the
+discharge may be passed through it in the form of a thin luminous
+thread. When produced with currents of low frequency,
+obtained from a coil operated as usual, this thread is inert. If a
+magnet be approached to it, the part near the same is attracted
+or repelled, according to the direction of the lines of force of the
+magnet. It occurred to me that if such a thread would be produced
+with currents of very high frequency, it should be more
+or less rigid, and as it was visible it could be easily studied.
+Accordingly I prepared a tube about one inch in diameter and
+one metre long, with outside coating at each end. The tube was
+exhausted to a point at which, by a little working, the thread discharge
+could be obtained. It must be remarked here that the
+general aspect of the tube, and the degree of exhaustion, are
+quite other than when ordinary low frequency currents are
+used. As it was found preferable to work with one terminal,
+the tube prepared was suspended from the end of a wire connected
+to the terminal, the tinfoil coating being connected to the
+wire, and to the lower coating sometimes a small insulated plate<span class='pagenum'><a name="Page_274" id="Page_274">[Pg 274]</a></span>
+was attached. When the thread was formed, it extended through
+the upper part of the tube and lost itself in the lower end. If it
+possessed rigidity it resembled, not exactly an elastic cord
+stretched tight between two supports, but a cord suspended from
+a height with a small weight attached at the end. When the
+finger or a small magnet was approached to the upper end of the
+luminous thread, it could be brought locally out of position by
+electrostatic or magnetic action; and when the disturbing object
+was very quickly removed, an analogous result was produced, as
+though a suspended cord would be displaced and quickly released
+near the point of suspension. In doing this the luminous thread
+was set in vibration, and two very sharply marked nodes, and a
+third indistinct one, were formed. The vibration, once set up,
+continued for fully eight minutes, dying gradually out. The
+speed of the vibration often varied perceptibly, and it could be
+observed that the electrostatic attraction of the glass affected the
+vibrating thread; but it was clear that the electrostatic action
+was not the cause of the vibration, for the thread was most generally
+stationary, and could always be set in vibration by passing
+the finger quickly near the upper part of the tube. With a
+magnet the thread could be split in two and both parts vibrated.
+By approaching the hand to the lower coating of the tube, or
+insulation plate if attached, the vibration was quickened; also, as
+far as I could see, by raising the potential or frequency. Thus,
+either increasing the frequency or passing a stronger discharge
+of the same frequency corresponded to a tightening of the cord.
+I did not obtain any experimental evidence with condenser discharges.
+A luminous band excited in the bulb by repeated discharges
+of a Leyden jar must possess rigidity, and if deformed
+and suddenly released, should vibrate. But probably the amount
+of vibrating matter is so small that in spite of the extreme speed,
+the inertia cannot prominently assert itself. Besides, the observation
+in such a case is rendered extremely difficult on account
+of the fundamental vibration.</p>
+
+<p>The demonstration of the fact&mdash;which still needs better experimental
+confirmation&mdash;that a vibrating gaseous column possesses
+rigidity, might greatly modify the views of thinkers.
+When with low frequencies and insignificant potentials indications
+of that property may be noted, how must a gaseous medium behave
+under the influence of enormous electrostatic stresses which
+may be active in the interstellar space, and which may alternate<span class='pagenum'><a name="Page_275" id="Page_275">[Pg 275]</a></span>
+with inconceivable rapidity? The existence of such an electrostatic,
+rhythmically throbbing force&mdash;of a vibrating electrostatic
+field&mdash;would show a possible way how solids might have formed
+from the ultra-gaseous uterus, and how transverse and all kinds
+of vibrations may be transmitted through a gaseous medium filling
+all space. Then, ether might be a true fluid, devoid of
+rigidity, and at rest, it being merely necessary as a connecting
+link to enable interaction. What determines the rigidity of a
+body? It must be the speed and the amount of motive matter.
+In a gas the speed maybe considerable, but the density is exceedingly
+small; in a liquid the speed would be likely to be small,
+though the density may be considerable; and in both cases the
+inertia resistance offered to displacement is practically <i>nil</i>. But
+place a gaseous (or liquid) column in an intense, rapidly alternating
+electrostatic field, set the particles vibrating with enormous
+speeds, then the inertia resistance asserts itself. A body might
+move with more or less freedom through the vibrating mass, but
+as a whole it would be rigid.</p>
+
+<p>There is a subject which I must mention in connection with
+these experiments: it is that of high vacua. This is a subject,
+the study of which is not only interesting, but useful, for it may
+lead to results of great practical importance. In commercial apparatus,
+such as incandescent lamps, operated from ordinary
+systems of distribution, a much higher vacuum than is obtained at
+present would not secure a very great advantage. In such a case
+the work is performed on the filament, and the gas is little concerned;
+the improvement, therefore, would be but trifling. But
+when we begin to use very high frequencies and potentials, the
+action of the gas becomes all important, and the degree of exhaustion
+materially modifies the results. As long as ordinary
+coils, even very large ones, were used, the study of the subject
+was limited, because just at a point when it became most interesting
+it had to be interrupted on account of the "non-striking"
+vacuum being reached. But at present we are able to obtain
+from a small disruptive discharge coil potentials much higher
+than even the largest coil was capable of giving, and, what is
+more, we can make the potential alternate with great rapidity.
+Both of these results enable us now to pass a luminous discharge
+through almost any vacua obtainable, and the field of our investigations
+is greatly extended. Think we as we may, of all the
+possible directions to develop a practical illuminant, the line of<span class='pagenum'><a name="Page_276" id="Page_276">[Pg 276]</a></span>
+high vacua seems to be the most promising at present. But to
+reach extreme vacua the appliances must be much more improved,
+and ultimate perfection will not be attained until we shall have
+discharged the mechanical and perfected an <i>electrical</i> vacuum
+pump. Molecules and atoms can be thrown out of a bulb under
+the action of an enormous potential: <i>this</i> will be the principle
+of the vacuum pump of the future. For the present, we must
+secure the best results we can with mechanical appliances. In
+this respect, it might not be out of the way to say a few words
+about the method of, and apparatus for, producing excessively
+high degrees of exhaustion of which I have availed myself in the
+course of these investigations. It is very probable that other experimenters
+have used similar arrangements; but as it is possible
+that there may be an item of interest in their description, a few
+remarks, which will render this investigation more complete,
+might be permitted.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_290.jpg" width="480" height="611" alt="Fig. 159." title="" />
+<span class="caption">Fig. 159.</span>
+</div>
+
+
+<p>The apparatus is illustrated in a drawing shown in Fig. 159.
+<small>S</small> represents a Sprengel pump, which has been specially constructed
+to better suit the work required. The stop-cock which<span class='pagenum'><a name="Page_277" id="Page_277">[Pg 277]</a></span>
+is usually employed has been omitted, and instead of it a hollow
+stopper s has been fitted in the neck of the reservoir <small>R</small>. This
+stopper has a small hole <i>h</i>, through which the mercury descends;
+the size of the outlet <i>o</i> being properly determined with respect
+to the section of the fall tube <i>t</i>, which is sealed to the reservoir
+instead of being connected to it in the usual manner. This
+arrangement overcomes the imperfections and troubles which
+often arise from the use of the stopcock on the reservoir and the
+connections of the latter with the fall tube.</p>
+
+<p>The pump is connected through a <big><b>U</b></big>-shaped tube <i>t</i> to a very
+large reservoir <small>R<sub>1</sub></small>. Especial care was taken in fitting the grinding
+surfaces of the stoppers <i>p</i> and <i>p</i><sub>1</sub>, and both of these and the
+mercury caps above them were made exceptionally long. After
+the <big><b>U</b></big>-shaped tube was fitted and put in place, it was heated, so
+as to soften and take off the strain resulting from imperfect
+fitting. The <big><b>U</b></big>-shaped tube was provided with a stopcock <small>C</small>,
+and two ground connections <i>g</i> and <i>g</i><sub>1</sub>,&mdash;one for a small bulb <i>b</i>,
+usually containing caustic potash, and the other for the receiver
+<i>r</i>, to be exhausted.</p>
+
+<p>The reservoir <small>R<sub>1</sub></small>, was connected by means of a rubber tube to
+a slightly larger reservoir <small>R<sub>2</sub></small>, each of the two reservoirs being
+provided with a stopcock <small>C<sub>1</sub></small> and <small>C<sub>2</sub></small>, respectively. The reservoir
+<small>R<sub>2</sub></small> could be raised and lowered by a wheel and rack, and the
+range of its motion was so determined that when it was filled
+with mercury and the stopcock <small>C<sub>2</sub></small> closed, so as to form a Torricellian
+vacuum in it when raised, it could be lifted so high that
+the reservoir <small>R<sub>1</sub></small> would stand a little above stopcock <small>C<sub>1</sub></small>; and when
+this stopcock was closed and the reservoir <small>R<sub>2</sub></small> descended, so as to
+form a Torricellian vacuum in reservoir <small>R<sub>1</sub></small>, it could be lowered
+so far as to completely empty the latter, the mercury filling the
+reservoir <small>R<sub>2</sub></small> up to a little above stopcock <small>C<sub>2</sub></small>.</p>
+
+<p>The capacity of the pump and of the connections was taken
+as small as possible relatively to the volume of reservoir, <small>R<sub>1</sub></small>,
+since, of course, the degree of exhaustion depended upon the
+ratio of these quantities.</p>
+
+<p>With this apparatus I combined the usual means indicated by
+former experiments for the production of very high vacua. In
+most of the experiments it was most convenient to use caustic
+potash. I may venture to say, in regard to its use, that much
+time is saved and a more perfect action of the pump insured by
+fusing and boiling the potash as soon as, or even before, the<span class='pagenum'><a name="Page_278" id="Page_278">[Pg 278]</a></span>
+pump settles down. If this course is not followed, the sticks, as
+ordinarily employed, may give off moisture at a certain very
+slow rate, and the pump may work for many hours without
+reaching a very high vacuum. The potash was heated either by
+a spirit lamp or by passing a discharge through it, or by passing
+a current through a wire contained in it. The advantage in the
+latter case was that the heating could be more rapidly repeated.</p>
+
+<p>Generally the process of exhaustion was the following:&mdash;At
+the start, the stop-cocks <small>C</small> and <small>C<sub>1</sub></small> being open, and all other connections
+closed, the reservoir <small>R<sub>2</sub></small> was raised so far that the mercury
+filled the reservoir <small>R<sub>1</sub></small> and a part of the narrow connecting
+<big><b>U</b></big>-shaped tube. When the pump was set to work, the mercury
+would, of course, quickly rise in the tube, and reservoir <small>R<sub>2</sub></small> was
+lowered, the experimenter keeping the mercury at about the
+same level. The reservoir <small>R<sub>2</sub></small> was balanced by a long spring
+which facilitated the operation, and the friction of the parts was
+generally sufficient to keep it in almost any position. When the
+Sprengel pump had done its work, the reservoir <small>R<sub>2</sub></small> was further lowered
+and the mercury descended in <small>R</small><sub>1</sub> and filled <small>R<sub>2</sub></small>, whereupon stopcock
+<small>C<sub>2</sub></small> was closed. The air adhering to the walls of <small>R<sub>1</sub></small> and that
+absorbed by the mercury was carried off, and to free the mercury
+of all air the reservoir <small>R<sub>2</sub></small> was for a long time worked up and
+down. During this process some air, which would gather below
+stopcock <small>C<sub>2</sub></small>, was expelled from <small>R<sub>2</sub></small> by lowering it far enough and
+opening the stopcock, closing the latter again before raising the
+reservoir. When all the air had been expelled from the mercury,
+and no air would gather in <small>R<sub>2</sub></small> when it was lowered, the caustic
+potash was resorted to. The reservoir <small>R<sub>2</sub></small> was now again raised
+until the mercury in <small>R<sub>1</sub></small>, stood above stopcock <small>C<sub>1</sub></small>. The caustic
+potash was fused and boiled, and moisture partly carried off by
+the pump and partly re-absorbed; and this process of heating
+and cooling was repeated many times, and each time, upon the
+moisture being absorbed or carried off, the reservoir <small>R<sub>2</sub></small> was for
+a long time raised and lowered. In this manner all the moisture
+was carried off from the mercury, and both the reservoirs were
+in proper condition to be used. The reservoir <small>R<sub>2</sub></small> was then again
+raised to the top, and the pump was kept working for a long
+time. When the highest vacuum obtainable with the pump had
+been reached, the potash bulb was usually wrapped with cotton
+which was sprinkled with ether so as to keep the potash at a
+very low temperature, then the reservoir <small>R<sub>2</sub></small> was lowered, and upon
+<span class='pagenum'><a name="Page_279" id="Page_279">[Pg 279]</a></span>reservoir
+<small>R<sub>1</sub></small> being emptied the receiver was quickly sealed up.</p>
+
+<p>When a new bulb was put on, the mercury was always raised
+above stopcock <small>C<sub>1</sub></small>, which was closed, so as to always keep the
+mercury and both the reservoirs in fine condition, and the mercury
+was never withdrawn from <small>R<sub>1</sub></small> except when the pump had
+reached the highest degree of exhaustion. It is necessary to observe
+this rule if it is desired to use the apparatus to advantage.</p>
+
+<p>By means of this arrangement I was able to proceed very
+quickly, and when the apparatus was in perfect order it was possible
+to reach the phosphorescent stage in a small bulb in less
+than fifteen minutes, which is certainly very quick work for a
+small laboratory arrangement requiring all in all about 100 pounds
+of mercury. With ordinary small bulbs the ratio of the capacity
+of the pump, receiver, and connections, and that of reservoir <small>R</small>
+was about 1 to 20, and the degrees of exhaustion reached were
+necessarily very high, though I am unable to make a precise and
+reliable statement how far the exhaustion was carried.</p>
+
+<p>What impresses the investigator most in the course of these
+experiences is the behavior of gases when subjected to great rapidly
+alternating electrostatic stresses. But he must remain in
+doubt as to whether the effects observed are due wholly to the
+molecules, or atoms, of the gas which chemical analysis discloses
+to us, or whether there enters into play another medium of a
+gaseous nature, comprising atoms, or molecules, immersed in a
+fluid pervading the space. Such a medium surely must exist,
+and I am convinced that, for instance, even if air were absent,
+the surface and neighborhood of a body in space would be heated
+by rapidly alternating the potential of the body; but no such
+heating of the surface or neighborhood could occur if all free
+atoms were removed and only a homogeneous, incompressible, and
+elastic fluid&mdash;such as ether is supposed to be&mdash;would remain, for
+then there would be no impacts, no collisions. In such a case,
+as far as the body itself is concerned, only frictional losses in the
+inside could occur.</p>
+
+<p>It is a striking fact that the discharge through a gas is established
+with ever-increasing freedom as the frequency of the
+impulses is augmented. It behaves in this respect quite contrarily
+to a metallic conductor. In the latter the impedance enters
+prominently into play as the frequency is increased, but the gas
+acts much as a series of condensers would; the facility with
+which the discharge passes through, seems to depend on the rate
+of change of potential. If it acts so, then in a vacuum tube even<span class='pagenum'><a name="Page_280" id="Page_280">[Pg 280]</a></span>
+of great length, and no matter how strong the current, self-induction
+could not assert itself to any appreciable degree. We
+have, then, as far as we can now see, in the gas a conductor
+which is capable of transmitting electric impulses of any frequency
+which we may be able to produce. Could the frequency be
+brought high enough, then a queer system of electric distribution,
+which would be likely to interest gas companies, might be realized:
+metal pipes filled with gas&mdash;the metal being the insulator,
+the gas the conductor&mdash;supplying phosphorescent bulbs, or perhaps
+devices as yet uninvented. It is certainly possible to take
+a hollow core of copper, rarefy the gas in the same, and by passing
+impulses of sufficiently high frequency through a circuit
+around it, bring the gas inside to a high degree of incandescence;
+but as to the nature of the forces there would be considerable
+uncertainty, for it would be doubtful whether with such impulses
+the copper core would act as a static screen. Such paradoxes and
+apparent impossibilities we encounter at every step in this line of
+work, and therein lies, to a great extent, the charm of the study.</p>
+
+<p>I have here a short and wide tube which is exhausted to a
+high degree and covered with a substantial coating of bronze, the
+coating barely allowing the light to shine through. A metallic
+cap, with a hook for suspending the tube, is fastened around the
+middle portion of the latter, the clasp being in contact with the
+bronze coating. I now want to light the gas inside by suspending
+the tube on a wire connected to the coil. Any one who
+would try the experiment for the first time, not having any previous
+experience, would probably take care to be quite alone
+when making the trial, for fear that he might become the joke of
+his assistants. Still, the bulb lights in spite of the metal coating,
+and the light can be distinctly perceived through the latter. A
+long tube covered with aluminum bronze lights when held in
+one hand&mdash;the other touching the terminal of the coil&mdash;quite
+powerfully. It might be objected that the coatings are not
+sufficiently conducting; still, even if they were highly resistant,
+they ought to screen the gas. They certainly screen it perfectly
+in a condition of rest, but far from perfectly when the charge
+is surging in the coating. But the loss of energy which occurs
+within the tube, notwithstanding the screen, is occasioned principally
+by the presence of the gas. Were we to take a large
+hollow metallic sphere and fill it with a perfect, incompressible,
+fluid dielectric, there would be no loss inside of the sphere, and<span class='pagenum'><a name="Page_281" id="Page_281">[Pg 281]</a></span>
+consequently the inside might be considered as perfectly screened,
+though the potential be very rapidly alternating. Even were
+the sphere filled with oil, the loss would be incomparably smaller
+than when the fluid is replaced by a gas, for in the latter case the
+force produces displacements; that means impact and collisions
+in the inside.</p>
+
+<p>No matter what the pressure of the gas may be, it becomes an
+important factor in the heating of a conductor when the electric
+density is great and the frequency very high. That in the heating
+of conductors by lightning discharges, air is an element of
+great importance, is almost as certain as an experimental fact. I
+may illustrate the action of the air by the following experiment:
+I take a short tube which is exhausted to a moderate degree and
+has a platinum wire running through the middle from one end
+to the other. I pass a steady or low frequency current through
+the wire, and it is heated uniformly in all parts. The heating
+here is due to conduction, or frictional losses, and the gas around
+the wire has&mdash;as far as we can see&mdash;no function to perform.
+But now let me pass sudden discharges, or high frequency currents,
+through the wire. Again the wire is heated, this time
+principally on the ends and least in the middle portion; and if
+the frequency of the impulses, or the rate of change, is high
+enough, the wire might as well be cut in the middle as not, for
+practically all heating is due to the rarefied gas. Here the gas
+might only act as a conductor of no impedance diverting the current
+from the wire as the impedance of the latter is enormously
+increased, and merely heating the ends of the wire by reason of
+their resistance to the passage of the discharge. But it is not
+at all necessary that the gas in the tube should be conducting; it
+might be at an extremely low pressure, still the ends of the wire
+would be heated&mdash;as, however, is ascertained by experience&mdash;only
+the two ends would in such case not be electrically connected
+through the gaseous medium. Now what with these frequencies
+and potentials occurs in an exhausted tube, occurs in the
+lightning discharges at ordinary pressure. We only need remember
+one of the facts arrived at in the course of these investigations,
+namely, that to impulses of very high frequency the gas
+at ordinary pressure behaves much in the same manner as though
+it were at moderately low pressure. I think that in lightning
+discharges frequently wires or conducting objects are volatilized
+merely because air is present, and that, were the conductor im<span class='pagenum'><a name="Page_282" id="Page_282">[Pg 282]</a></span>mersed
+in an insulating liquid, it would be safe, for then the
+energy would have to spend itself somewhere else. From the
+behavior of gases under sudden impulses of high potential, I am
+led to conclude that there can be no surer way of diverting a
+lightning discharge than by affording it a passage through a
+volume of gas, if such a thing can be done in a practical manner.</p>
+
+<p>There are two more features upon which I think it necessary
+to dwell in connection with these experiments&mdash;the "radiant
+state" and the "non-striking vacuum."</p>
+
+<p>Any one who has studied Crookes' work must have received
+the impression that the "radiant state" is a property of the gas
+inseparably connected with an extremely high degree of exhaustion.
+But it should be remembered that the phenomena
+observed in an exhausted vessel are limited to the character and
+capacity of the apparatus which is made use of. I think that in
+a bulb a molecule, or atom, does not precisely move in a straight
+line because it meets no obstacle, but because the velocity imparted
+to it is sufficient to propel it in a sensibly straight line.
+The mean free path is one thing, but the velocity&mdash;the energy
+associated with the moving body&mdash;is another, and under ordinary
+circumstances I believe that it is a mere question of potential or
+speed. A disruptive discharge coil, when the potential is pushed
+very far, excites phosphorescence and projects shadows, at comparatively
+low degrees of exhaustion. In a lightning discharge,
+matter moves in straight lines at ordinary pressure when the
+mean free path is exceedingly small, and frequently images of
+wires or other metallic objects have been produced by the particles
+thrown off in straight lines.</p>
+
+<p>I have prepared a bulb to illustrate by an experiment the
+correctness of these assertions. In a globe <small>L</small>, Fig. 160, I have
+mounted upon a lamp filament <i>f</i> a piece of lime <i>l</i>. The lamp
+filament is connected with a wire which leads into the bulb, and
+the general construction of the latter is as indicated in Fig. 148,
+before described. The bulb being suspended from a wire
+connected to the terminal of the coil, and the latter being set to
+work, the lime piece <i>l</i> and the projecting parts of the filament <i>f</i>
+are bombarded. The degree of exhaustion is just such that with
+the potential the coil is capable of giving, phosphorescence of the
+glass is produced, but disappears as soon as the vacuum is impaired.
+The lime containing moisture, and moisture being given
+off as soon as heating occurs, the phosphorescence lasts only for<span class='pagenum'><a name="Page_283" id="Page_283">[Pg 283]</a></span>
+a few moments. When the lime has been sufficiently heated,
+enough moisture has been given off to impair materially the
+vacuum of the bulb. As the bombardment goes on, one point
+of the lime piece is more heated than other points, and the result
+is that finally practically all the discharge passes through that
+point which is intensely heated, and a white stream of lime particles
+(Fig. 160) then breaks forth from that point. This stream
+is composed of "radiant" matter, yet the degree of exhaustion
+is low. But the particles move in straight lines because the
+velocity imparted to them is great, and this is due to three
+causes&mdash;to the great electric density, the high temperature of the
+small point, and the fact that the particles of the lime are easily
+torn and thrown off&mdash;far more easily than those of carbon. With
+frequencies such as we are able to obtain, the particles are bodily
+thrown off and projected to a considerable distance; but with
+sufficiently high frequencies no such thing would occur; in such
+case only a stress would spread or a vibration would be propagated
+through the bulb. It would be out of the question to
+reach any such frequency on the assumption that the atoms move
+with the speed of light; but I believe that such a thing is impossible;
+for this an enormous potential would be required.
+With potentials which we are able to obtain, even with a disruptive
+discharge coil, the speed must be quite insignificant.</p>
+
+<div class="figcenter" style="width: 467px;">
+<img src="images/oi_297.jpg" width="467" height="640" alt="Fig. 160." title="" />
+<span class="caption">Fig. 160.</span>
+</div>
+
+
+<p>As to the "non-striking vacuum," the point to be noted is,
+that it can occur only with low frequency impulses, and it is<span class='pagenum'><a name="Page_284" id="Page_284">[Pg 284]</a></span>
+necessitated by the impossibility of carrying off enough energy
+with such impulses in high vacuum, since the few atoms which
+are around the terminal upon coming in contact with the same,
+are repelled and kept at a distance for a comparatively long
+period of time, and not enough work can be performed to render
+the effect perceptible to the eye. If the difference of potential
+between the terminals is raised, the dielectric breaks down. But
+with very high frequency impulses there is no necessity for such
+breaking down, since any amount of work can be performed by
+continually agitating the atoms in the exhausted vessel, provided
+the frequency is high enough. It is easy to reach&mdash;even with
+frequencies obtained from an alternator as here used&mdash;a stage at
+which the discharge does not pass between two electrodes in a
+narrow tube, each of these being connected to one of the terminals
+of the coil, but it is difficult to reach a point at which a
+luminous discharge would not occur around each electrode.</p>
+
+<p>A thought which naturally presents itself in connection with
+high frequency currents, is to make use of their powerful electrodynamic
+inductive action to produce light effects in a sealed glass
+globe. The leading-in wire is one of the defects of the present
+incandescent lamp, and if no other improvement were made,
+that imperfection at least should be done away with. Following<span class='pagenum'><a name="Page_285" id="Page_285">[Pg 285]</a></span>
+this thought, I have carried on experiments in various directions,
+of which some were indicated in my former paper. I may here
+mention one or two more lines of experiment which have been
+followed up.</p>
+
+<p>Many bulbs were constructed as shown in Fig. 161 and Fig.
+162.</p>
+
+<div class="figcenter" style="width: 693px;">
+<img src="images/oi_298.jpg" width="693" height="600" alt="Fig. 161, 162." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 161.</td><td class="caption">Fig. 162.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 161, a wide tube, <small>T</small>, was sealed to a smaller <big><b>W</b></big> shaped
+tube <small>U</small>, of phosphorescent glass. In the tube <small>T</small>, was placed a coil
+<small>C</small>, of aluminum wire, the ends of which were provided with
+small spheres, <i>t</i> and <i>t</i><sub>1</sub>, of aluminum, and reached into the <small>U</small> tube.
+The tube <small>T</small> was slipped into a socket containing a primary coil,
+through which usually the discharges of Leyden jars were directed,
+and the rarefied gas in the small <small>U</small> tube was excited to
+strong luminosity by the high-tension current induced in the coil <small>C</small>.
+When Leyden jar discharges were used to induce currents in the
+coil <small>C</small>, it was found necessary to pack the tube <small>T</small> tightly with insulating
+powder, as a discharge would occur frequently between
+the turns of the coil, especially when the primary was thick and
+the air gap, through which the jars discharged, large, and no
+little trouble was experienced in this way.</p>
+
+<p>In Fig. 162 is illustrated another form of the bulb constructed.
+In this case a tube <small>T</small> is sealed to a globe <small>L</small>. The tube contains a
+coil <small>C</small>, the ends of which pass through two small glass tubes <i>t</i>
+and <i>t</i><sub>1</sub>, which are sealed to the tube <small>T</small>. Two refractory buttons
+<i>m</i> and <i>m</i><sub>1</sub>, are mounted on lamp filaments which are fastened to
+the ends of the wires passing through the glass tubes <i>t</i> and <i>t</i><sub>1</sub>.
+Generally in bulbs made on this plan the globe <small>L</small> communicated
+with the tube <small>T</small>. For this purpose the ends of the small tubes <i>t</i>
+and <i>t</i><sub>1</sub> were heated just a trifle in the burner, merely to hold the
+wires, but not to interfere with the communication. The tube <small>T</small>,
+with the small tubes, wires through the same, and the refractory
+buttons <i>m</i> and <i>m</i><sub>1</sub>, were first prepared, and then sealed to globe <small>L</small>,
+whereupon the coil <small>C</small> was slipped in and the connections made to
+its ends. The tube was then packed with insulating powder,
+jamming the latter as tight as possible up to very nearly the end;
+then it was closed and only a small hole left through which the
+remainder of the powder was introduced, and finally the end of
+the tube was closed. Usually in bulbs constructed as shown in
+Fig. 162 an aluminum tube <i>a</i> was fastened to the upper end <i>s</i>
+of each of the tubes <i>t</i> and <i>t</i><sub>1</sub> in order to protect that end against
+<span class='pagenum'><a name="Page_286" id="Page_286">[Pg 286]</a></span>the heat. The buttons <i>m</i> and <i>m</i><sub>1</sub> could be brought to any degree
+of incandescence by passing the discharges of Leyden jars
+around the coil <small>C</small>. In such bulbs with two buttons a very curious
+effect is produced by the formation of the shadows of each
+of the two buttons.</p>
+
+<p>Another line of experiment, which has been assiduously followed,
+was to induce by electro-dynamic induction a current or
+luminous discharge in an exhausted tube or bulb. This matter
+has received such able treatment at the hands of Prof. J. J.
+Thomson, that I could add but little to what he has made known,
+even had I made it the special subject of this lecture. Still,
+since experiments in this line have gradually led me to the present
+views and results, a few words must be devoted here to this
+subject.</p>
+
+<p>It has occurred, no doubt, to many that as a vacuum tube is
+made longer, the electromotive force per unit length of the tube,
+necessary to pass a luminous discharge through the latter, becomes
+continually smaller; therefore, if the exhausted tube be made
+long enough, even with low frequencies a luminous discharge
+could be induced in such a tube closed upon itself. Such a tube
+might be placed around a hall or on a ceiling, and at once a simple
+appliance capable of giving considerable light would be obtained.
+But this would be an appliance hard to manufacture
+and extremely unmanageable. It would not do to make the
+tube up of small lengths, because there would be with ordinary
+frequencies considerable loss in the coatings, and besides, if coatings
+were used, it would be better to supply the current directly
+to the tube by connecting the coatings to a transformer. But
+even if all objections of such nature were removed, with
+low frequencies the light conversion itself would be inefficient,
+as I have before stated. In using extremely high frequencies
+the length of the secondary&mdash;in other words, the size of the vessel&mdash;can
+be reduced as much as desired, and the efficiency of the
+light conversion is increased, provided that means are invented
+for efficiently obtaining such high frequencies. Thus one is led,
+from theoretical and practical considerations, to the use of high
+frequencies, and this means high electromotive forces and small
+currents in the primary. When one works with condenser
+charges&mdash;and they are the only means up to the present known
+for reaching these extreme frequencies&mdash;one gets to electromotive
+forces of several thousands of volts per turn of the primary. We
+cannot multiply the electro-dynamic inductive effect by taking<span class='pagenum'><a name="Page_287" id="Page_287">[Pg 287]</a></span>
+more turns in the primary, for we arrive at the conclusion that
+the best way is to work with one single turn&mdash;though we must
+sometimes depart from this rule&mdash;and we must get along with
+whatever inductive effect we can obtain with one turn. But before
+one has long experimented with the extreme frequencies required
+to set up in a small bulb an electromotive force of several
+thousands of volts, one realizes the great importance of electrostatic
+effects, and these effects grow relatively to the electro-dynamic
+in significance as the frequency is increased.</p>
+
+<p>Now, if anything is desirable in this case, it is to increase the
+frequency, and this would make it still worse for the electrodynamic
+effects. On the other hand, it is easy to exalt the electrostatic
+action as far as one likes by taking more turns on the
+secondary, or combining self-induction and capacity to raise the
+potential. It should also be remembered that, in reducing the
+current to the smallest value and increasing the potential,
+the electric impulses of high frequency can be more easily transmitted
+through a conductor.</p>
+
+<p>These and similar thoughts determined me to devote more attention
+to the electrostatic phenomena, and to endeavor to produce
+potentials as high as possible, and alternating as fast as
+they could be made to alternate. I then found that I could excite
+vacuum tubes at considerable distance from a conductor
+connected to a properly constructed coil, and that I could, by
+converting the oscillatory current of a conductor to a higher potential,
+establish electrostatic alternating fields which acted
+through the whole extent of the room, lighting up a tube no
+matter where it was held in space. I thought I recognized that
+I had made a step in advance, and I have persevered in this line;
+but I wish to say that I share with all lovers of science and progress
+the one and only desire&mdash;to reach a result of utility to men
+in any direction to which thought or experiment may lead me.
+I think that this departure is the right one, for I cannot see,
+from the observation of the phenomena which manifest themselves
+as the frequency is increased, what there would remain to
+act between two circuits conveying, for instance, impulses of
+several hundred millions per second, except electrostatic forces.
+Even with such trifling frequencies the energy would be practically
+all potential, and my conviction has grown strong that, to whatever
+kind of motion light may be due, it is produced by tremendous
+electrostatic stresses vibrating with extreme rapidity.<span class='pagenum'><a name="Page_288" id="Page_288">[Pg 288]</a></span></p>
+
+<div class="figcenter" style="width: 517px;">
+<img src="images/oi_302.jpg" width="517" height="800" alt="Fig. 163, 164." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 163.</td><td class="caption">Fig. 164.</td></tr>
+</table>
+</div>
+
+<p>Of all these phenomena observed with currents, or electric
+impulses, of high frequency, the most fascinating for an audience
+are certainly those which are noted in an electrostatic field
+acting through considerable distance; and the best an unskilled
+lecturer can do is to begin and finish with the exhibition of these
+singular effects. I take a tube in my hand and move it about,
+and it is lighted wherever I may hold it; throughout space the
+invisible forces act. But I may take another tube and it might
+not light, the vacuum being very high. I excite it by means of a
+disruptive discharge coil, and now it will light in the electrostatic
+field. I may put it away for a few weeks or months, still it retains
+the faculty of being excited. What change have I produced in the
+tube in the act of exciting it? If a motion imparted to atoms, it
+is difficult to perceive how it can persist so long without being
+arrested by frictional losses; and if a strain exerted in the dielectric,
+such as a simple electrification would produce, it is easy to
+see how it may persist indefinitely, but very difficult to understand
+why such a condition should aid the excitation when we
+have to deal with potentials which are rapidly alternating.<span class='pagenum'><a name="Page_289" id="Page_289">[Pg 289]</a></span></p>
+
+<p>Since I have exhibited these phenomena for the first time, I
+have obtained some other interesting effects. For instance, I
+have produced the incandescence of a button, filament, or wire
+enclosed in a tube. To get to this result it was necessary to
+economize the energy which is obtained from the field, and direct
+most of it on the small body to be rendered incandescent. At
+the beginning the task appeared difficult, but the experiences
+gathered permitted me to reach the result easily. In Fig. 163
+and Fig. 164, two such tubes are illustrated, which are prepared for
+the occasion. In Fig. 163 a short tube <small>T<sub>1</sub></small>, sealed to another long
+tube <small>T</small>, is provided with a stem <i>s</i>, with a platinum wire sealed in
+the latter. A very thin lamp filament <i>l</i>, is fastened to this wire
+and connection to the outside is made through a thin copper wire
+<i>w</i>. The tube is provided with outside and inside coatings, <small>C</small> and
+<small>C<sub>1</sub></small>, respectively, and is filled as far as the coatings reach with conducting,
+and the space above with insulating, powder. These
+coatings are merely used to enable me to perform two experiments
+with the tube&mdash;namely, to produce the effect desired either
+by direct connection of the body of the experimenter or of another
+body to the wire <i>w</i>, or by acting inductively through the
+glass. The stem <i>s</i> is provided with an aluminum tube <i>a</i>, for
+purposes before explained, and only a small part of the filament
+reaches out of this tube. By holding the tube <small>T<sub>1</sub></small> anywhere in
+the electrostatic field, the filament is rendered incandescent.</p>
+
+<p>A more interesting piece of apparatus is illustrated in Fig. 164.
+The construction is the same as before, only instead of the lamp
+filament a small platinum wire <i>p</i>, sealed in a stem <i>s</i>, and bent
+above it in a circle, is connected to the copper wire <i>w</i>, which is
+joined to an inside coating <small>C</small>. A small stem <i>s</i><sub>1</sub> is provided with
+a needle, on the point of which is arranged, to rotate very freely,
+a very light fan of mica <i>v</i>. To prevent the fan from falling out,
+a thin stem of glass <i>g</i>, is bent properly and fastened to the aluminum
+tube. When the glass tube is held anywhere in the electrostatic
+field the platinum wire becomes incandescent, and the
+mica vanes are rotated very fast.</p>
+
+<p>Intense phosphorescence may be excited in a bulb by merely
+connecting it to a plate within the field, and the plate need not
+be any larger than an ordinary lamp shade. The phosphorescence
+excited with these currents is incomparably more powerful
+than with ordinary apparatus. A small phosphorescent bulb,
+when attached to a wire connected to a coil, emits sufficient light<span class='pagenum'><a name="Page_290" id="Page_290">[Pg 290]</a></span>
+to allow reading ordinary print at a distance of five to six paces.
+It was of interest to see how some of the phosphorescent bulbs
+of Professor Crookes would behave with these currents, and he
+has had the kindness to lend me a few for the occasion. The
+effects produced are magnificent, especially by the sulphide of
+calcium and sulphide of zinc. With the disruptive discharge
+coil they glow intensely merely by holding them in the hand and
+connecting the body to the terminal of the coil.</p>
+
+<p>To whatever results investigations of this kind may lead, the
+chief interest lies, for the present, in the possibilities they offer
+for the production of an efficient illuminating device. In no
+branch of electric industry is an advance more desired than in
+the manufacture of light. Every thinker, when considering the
+barbarous methods employed, the deplorable losses incurred in
+our best systems of light production, must have asked himself,
+What is likely to be the light of the future? Is it to be an incandescent
+solid, as in the present lamp, or an incandescent gas,
+or a phosphorescent body, or something like a burner, but incomparably
+more efficient?</p>
+
+<p>There is little chance to perfect a gas burner; not, perhaps,
+because human ingenuity has been bent upon that problem for
+centuries without a radical departure having been made&mdash;though
+the argument is not devoid of force&mdash;but because in a
+burner the highest vibrations can never be reached, except by
+passing through all the low ones. For how is a flame to proceed
+unless by a fall of lifted weights? Such process cannot be maintained
+without renewal, and renewal is repeated passing from low
+to high vibrations. One way only seems to be open to improve
+a burner, and that is by trying to reach higher degrees of incandescence.
+Higher incandescence is equivalent to a quicker vibration:
+that means more light from the same material, and that
+again, means more economy. In this direction some improvements
+have been made, but the progress is hampered by many
+limitations. Discarding, then, the burner, there remains the
+three ways first mentioned, which are essentially electrical.</p>
+
+<p>Suppose the light of the immediate future to be a solid, rendered
+incandescent by electricity. Would it not seem that it is
+better to employ a small button than a frail filament? From
+many considerations it certainly must be concluded that a button
+is capable of a higher economy, assuming, of course, the difficulties
+connected with the operation of such a lamp to be effec<span class='pagenum'><a name="Page_291" id="Page_291">[Pg 291]</a></span>tively
+overcome. But to light such a lamp we require a high
+potential; and to get this economically, we must use high frequencies.</p>
+
+<p>Such considerations apply even more to the production of light
+by the incandescence of a gas, or by phosphorescence. In all
+cases we require high frequencies and high potentials. These
+thoughts occurred to me a long time ago.</p>
+
+<p>Incidentally we gain, by the use of high frequencies, many advantages,
+such as higher economy in the light production, the
+possibility of working with one lead, the possibility of doing away
+with the leading-in wire, etc.</p>
+
+<p>The question is, how far can we go with frequencies? Ordinary
+conductors rapidly lose the facility of transmitting electric
+impulses when the frequency is greatly increased. Assume the
+means for the production of impulses of very great frequency
+brought to the utmost perfection, every one will naturally ask
+how to transmit them when the necessity arises. In transmitting
+such impulses through conductors we must remember that we
+have to deal with <i>pressure</i> and <i>flow</i>, in the ordinary interpretation
+of these terms. Let the pressure increase to an enormous value,
+and let the flow correspondingly diminish, then such impulses&mdash;variations
+merely of pressure, as it were&mdash;can no doubt be
+transmitted through a wire even if their frequency be many
+hundreds of millions per second. It would, of course, be out of
+question to transmit such impulses through a wire immersed in a
+gaseous medium, even if the wire were provided with a thick
+and excellent insulation, for most of the energy would be lost in
+molecular bombardment and consequent heating. The end of
+the wire connected to the source would be heated, and the remote
+end would receive but a trifling part of the energy supplied.
+The prime necessity, then, if such electric impulses are
+to be used, is to find means to reduce as much as possible the
+dissipation.</p>
+
+<p>The first thought is, to employ the thinnest possible wire surrounded
+by the thickest practicable insulation. The next thought
+is to employ electrostatic screens. The insulation of the wire
+may be covered with a thin conducting coating and the latter
+connected to the ground. But this would not do, as then all the
+energy would pass through the conducting coating to the ground
+and nothing would get to the end of the wire. If a ground connection
+is made it can only be made through a conductor offer<span class='pagenum'><a name="Page_292" id="Page_292">[Pg 292]</a></span>ing
+an enormous impedance, or through a condenser of extremely
+small capacity. This, however, does not do away with
+other difficulties.</p>
+
+<p>If the wave length of the impulses is much smaller than the
+length of the wire, then corresponding short waves will be set
+up in the conducting coating, and it will be more or less the
+same as though the coating were directly connected to earth. It
+is therefore necessary to cut up the coating in sections much
+shorter than the wave length. Such an arrangement does not
+still afford a perfect screen, but it is ten thousand times better
+than none. I think it preferable to cut up the conducting coating
+in small sections, even if the current waves be much longer
+than the coating.</p>
+
+<p>If a wire were provided with a perfect electrostatic screen, it
+would be the same as though all objects were removed from it at
+infinite distance. The capacity would then be reduced to the
+capacity of the wire itself, which would be very small. It
+would then be possible to send over the wire current vibrations
+of very high frequencies at enormous distances, without affecting
+greatly the character of the vibrations. A perfect screen is of
+course out of the question, but I believe that with a screen such
+as I have just described telephony could be rendered practicable
+across the Atlantic. According to my ideas, the gutta-percha
+covered wire should be provided with a third conducting coating
+subdivided in sections. On the top of this should be again
+placed a layer of gutta-percha and other insulation, and on the
+top of the whole the armor. But such cables will not be constructed,
+for ere long intelligence&mdash;transmitted without wires&mdash;will
+throb through the earth like a pulse through a living organism.
+The wonder is that, with the present state of knowledge
+and the experiences gained, no attempt is being made to disturb
+the electrostatic or magnetic condition of the earth, and
+transmit, if nothing else, intelligence.</p>
+
+<p>It has been my chief aim in presenting these results to point
+out phenomena or features of novelty, and to advance ideas
+which I am hopeful will serve as starting points of new departures.
+It has been my chief desire this evening to entertain you
+with some novel experiments. Your applause, so frequently
+and generously accorded, has told me that I have succeeded.</p>
+
+<p>In conclusion, let me thank you most heartily for your kindness
+and attention, and assure you that the honor I have had in<span class='pagenum'><a name="Page_293" id="Page_293">[Pg 293]</a></span>
+addressing such a distinguished audience, the pleasure I have had
+in presenting these results to a gathering of so many able men&mdash;and
+among them also some of those in whose work for many
+years past I have found enlightenment and constant pleasure&mdash;I
+shall never forget.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_294" id="Page_294">[Pg 294]</a></span></p>
+<h2><a name="CHAPTER_XXVIII" id="CHAPTER_XXVIII"></a>CHAPTER XXVIII.</h2>
+
+<h3><span class="smcap">On Light and Other High Frequency Phenomena.</span><a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a></h3>
+
+<h5>INTRODUCTORY.&mdash;SOME THOUGHTS ON THE EYE.</h5>
+
+
+<p>When we look at the world around us, on Nature, we are impressed
+with its beauty and grandeur. Each thing we perceive,
+though it may be vanishingly small, is in itself a world, that is,
+like the whole of the universe, matter and force governed by
+law,&mdash;a world, the contemplation of which fills us with feelings
+of wonder and irresistibly urges us to ceaseless thought and inquiry.
+But in all this vast world, of all objects our senses reveal
+to us, the most marvellous, the most appealing to our
+imagination, appears no doubt a highly developed organism, a
+thinking being. If there is anything fitted to make us admire
+Nature's handiwork, it is certainly this inconceivable structure,
+which performs its innumerable motions of obedience to external
+influence. To understand its workings, to get a deeper insight
+into this Nature's masterpiece, has ever been for thinkers a fascinating
+aim, and after many centuries of arduous research men have
+arrived at a fair understanding of the functions of its organs and
+senses. Again, in all the perfect harmony of its parts, of the
+parts which constitute the material or tangible of our being, of all
+its organs and senses, the eye is the most wonderful. It is the
+most precious, the most indispensable of our perceptive or directive
+organs, it is the great gateway through which all knowledge
+enters the mind. Of all our organs, it is the one, which is in the
+<span class='pagenum'><a name="Page_295" id="Page_295">[Pg 295]</a></span>most intimate relation with that which we call intellect. So intimate
+is this relation, that it is often said, the very soul shows
+itself in the eye.</p>
+
+<p>It can be taken as a fact, which the theory of the action of the
+eye implies, that for each external impression, that is, for each
+image produced upon the retina, the ends of the visual nerves,
+concerned in the conveyance of the impression to the mind, must
+be under a peculiar stress or in a vibratory state. It now does
+not seem improbable that, when by the power of thought an image
+is evoked, a distinct reflex action, no matter how weak, is
+exerted upon certain ends of the visual nerves, and therefore
+upon the retina. Will it ever be within human power to analyze
+the condition of the retina when disturbed by thought or reflex
+action, by the help of some optical or other means of such sensitiveness,
+that a clear idea of its state might be gained at any
+time? If this were possible, then the problem of reading one's
+thoughts with precision, like the characters of an open book,
+might be much easier to solve than many problems belonging to
+the domain of positive physical science, in the solution of which
+many, if not the majority, of scientific men implicitly believe.
+Helmholtz, has shown that the fundi of the eye are themselves,
+luminous, and he was able to <i>see</i>, in total darkness, the movement
+of his arm by the light of his own eyes. This is one of the
+most remarkable experiments recorded in the history of science,
+and probably only a few men could satisfactorily repeat it, for it
+is very likely, that the luminosity of the eyes is associated with
+uncommon activity of the brain and great imaginative power. It
+is fluorescence of brain action, as it were.</p>
+
+<p>Another fact having a bearing on this subject which has probably
+been noted by many, since it is stated in popular expressions,
+but which I cannot recollect to have found chronicled as a positive
+result of observation is, that at times, when a sudden idea or
+image presents itself to the intellect, there is a distinct and sometimes
+painful sensation of luminosity produced in the eye, observable
+even in broad daylight.</p>
+
+<p>The saying then, that the soul shows itself in the eye, is deeply
+founded, and we feel that it expresses a great truth. It has a
+profound meaning even for one who, like a poet or artist, only
+following his inborn instinct or love for Nature, finds delight in
+aimless thoughts and in the mere contemplation of natural phenomena,
+but a still more profound meaning for one who, in the
+<span class='pagenum'><a name="Page_296" id="Page_296">[Pg 296]</a></span>spirit of positive scientific investigation, seeks to ascertain the
+causes of the effects. It is principally the natural philosopher,
+the physicist, for whom the eye is the subject of the most intense
+admiration.</p>
+
+<p>Two facts about the eye must forcibly impress the mind of the
+physicist, notwithstanding he may think or say that it is an
+imperfect optical instrument, forgetting, that the very conception
+of that which is perfect or seems so to him, has been gained
+through this same instrument. First, the eye is, as far as our
+positive knowledge goes, the only organ which is <i>directly</i> affected
+by that subtile medium, which as science teaches us, must fill all
+space; secondly, it is the most sensitive of our organs, incomparably
+more sensitive to external impressions than any other.</p>
+
+<p>The organ of hearing implies the impact of ponderable bodies,
+the organ of smell the transference of detached material particles,
+and the organs of taste, and of touch or force, the direct contact,
+or at least some interference of ponderable matter, and this is
+true even in those instances of animal organisms, in which some
+of these organs are developed to a degree of truly marvelous
+perfection. This being so, it seems wonderful that the organ of
+sight solely should be capable of being stirred by that, which all
+our other organs are powerless to detect, yet which plays an essential
+part in all natural phenomena, which transmits all energy
+and sustains all motion and, that most intricate of all, life, but
+which has properties such that even a scientifically trained mind
+cannot help drawing a distinction between it and all that is called
+matter. Considering merely this, and the fact that the eye, by
+its marvelous power, widens our otherwise very narrow range of
+perception far beyond the limits of the small world which is our
+own, to embrace myriads of other worlds, suns and stars in the
+infinite depths of the universe, would make it justifiable to assert,
+that it is an organ of a higher order. Its performances are beyond
+comprehension. Nature as far as we know never produced anything
+more wonderful. We can get barely a faint idea of its
+prodigious power by analyzing what it does and by comparing.
+When ether waves impinge upon the human body, they produce
+the sensations of warmth or cold, pleasure or pain, or perhaps other
+sensations of which we are not aware, and any degree or intensity
+of these sensations, which degrees are infinite in number, hence an
+infinite number of distinct sensations. But our sense of touch, or
+our sense of force, cannot reveal to us these differences in degree<span class='pagenum'><a name="Page_297" id="Page_297">[Pg 297]</a></span>
+or intensity, unless they are very great. Now we can readily conceive
+how an organism, such as the human, in the eternal process
+of evolution, or more philosophically speaking, adaptation to
+Nature, being constrained to the use of only the sense of touch or
+force, for instance, might develop this sense to such a degree of
+sensitiveness or perfection, that it would be capable of distinguishing
+the minutest differences in the temperature of a body even
+at some distance, to a hundredth, or thousandth, or millionth part
+of a degree. Yet, even this apparently impossible performance
+would not begin to compare with that of the eye, which is capable
+of distinguishing and conveying to the mind in a single
+instant innumerable peculiarities of the body, be it in form,
+or color, or other respects. This power of the eye rests upon
+two things, namely, the rectilinear propagation of the disturbance
+by which it is effected, and upon its sensitiveness.
+To say that the eye is sensitive is not saying anything. Compared
+with it, all other organs are monstrously crude. The organ of
+smell which guides a dog on the trail of a deer, the organ of touch
+or force which guides an insect in its wanderings, the organ of
+hearing, which is affected by the slightest disturbances of the air,
+are sensitive organs, to be sure, but what are they compared with
+the human eye! No doubt it responds to the faintest echoes or
+reverberations of the medium; no doubt, it brings us tidings from
+other worlds, infinitely remote, but in a language we cannot as
+yet always understand. And why not? Because we live in a
+medium filled with air and other gases, vapors and a dense mass
+of solid particles flying about. These play an important part in
+many phenomena; they fritter away the energy of the vibrations
+before they can reach the eye; they too, are the carriers of germs
+of destruction, they get into our lungs and other organs, clog up
+the channels and imperceptibly, yet inevitably, arrest the stream
+of life. Could we but do away with all ponderable matter in the
+line of sight of the telescope, it would reveal to us undreamt of
+marvels. Even the unaided eye, I think, would be capable of distinguishing
+in the pure medium, small objects at distances measured
+probably by hundreds or perhaps thousands of miles.</p>
+
+<p>But there is something else about the eye which impresses us
+still more than these wonderful features which we observed, viewing
+it from the standpoint of a physicist, merely as an optical
+instrument,&mdash;something which appeals to us more than its marvelous
+faculty of being directly affected by the vibrations of the<span class='pagenum'><a name="Page_298" id="Page_298">[Pg 298]</a></span>
+medium, without interference of gross matter, and more than its
+inconceivable sensitiveness and discerning power. It is its significance
+in the processes of life. No matter what one's views on
+nature and life may be, he must stand amazed when, for the first
+time in his thoughts, he realizes the importance of the eye in the
+physical processes and mental performances of the human organism.
+And how could it be otherwise, when he realizes, that the
+eye is the means through which the human race has acquired
+the entire knowledge it possesses, that it controls all our motions,
+more still, all our actions.</p>
+
+<p>There is no way of acquiring knowledge except through the eye.
+What is the foundation of all philosophical systems of ancient
+and modern times, in fact, of all the philosophy of man? <i>I am,
+I think; I think, therefore I am.</i> But how could I think and how
+would I know that I exist, if I had not the eye? For knowledge
+involves consciousness; consciousness involves ideas, conceptions;
+conceptions involve pictures or images, and images the sense of
+vision, and therefore the organ of sight. But how about blind
+men, will be asked? Yes, a blind man may depict in magnificent
+poems, forms and scenes from real life, from a world he physically
+does not see. A blind man may touch the keys of an instrument
+with unerring precision, may model the fastest boat, may discover
+and invent, calculate and construct, may do still greater wonders&mdash;but
+all the blind men who have done such things have descended
+from those who had seeing eyes. Nature may reach the same result
+in many ways. Like a wave in the physical world, in the infinite
+ocean of the medium which pervades all, so in the world of
+organisms, in life, an impulse started proceeds onward, at times,
+may be, with the speed of light, at times, again, so slowly that
+for ages and ages it seems to stay, passing through processes of a
+complexity inconceivable to men, but in all its forms, in all its
+stages, its energy ever and ever integrally present. A single ray
+of light from a distant star falling upon the eye of a tyrant in bygone
+times, may have altered the course of his life, may have
+changed the destiny of nations, may have transformed the surface
+of the globe, so intricate, so inconceivably complex are the
+processes in Nature. In no way can we get such an overwhelming
+idea of the grandeur of Nature, as when we consider, that in
+accordance with the law of the conservation of energy, throughout
+the infinite, the forces are in a perfect balance, and hence the
+energy of a single thought may determine the motion of a Uni<span class='pagenum'><a name="Page_299" id="Page_299">[Pg 299]</a></span>verse.
+It is not necessary that every individual, not even that
+every generation or many generations, should have the physical
+instrument of sight, in order to be able to form images and to
+think, that is, form ideas or conceptions; but sometime or other,
+during the process of evolution, the eye certainly must have existed,
+else thought, as we understand it, would be impossible;
+else conceptions, like spirit, intellect, mind, call it as you may,
+could not exist. It is conceivable, that in some other world, in
+some other beings, the eye is replaced by a different organ, equally
+or more perfect, but these beings cannot be men.</p>
+
+<p>Now what prompts us all to voluntary motions and actions of
+any kind? Again the eye. If I am conscious of the motion, I
+must have an idea or conception, that is, an image, therefore the
+eye. If I am not precisely conscious of the motion, it is, because
+the images are vague or indistinct, being blurred by the superimposition
+of many. But when I perform the motion, does the
+impulse which prompts me to the action come from within or from
+without? The greatest physicists have not disdained to endeavor
+to answer this and similar questions and have at times
+abandoned themselves to the delights of pure and unrestrained
+thought. Such questions are generally considered not to belong
+to the realm of positive physical science, but will before long be
+annexed to its domain. Helmholtz has probably thought more
+on life than any modern scientist. Lord Kelvin expressed his
+belief that life's process is electrical and that there is a force inherent
+to the organism and determining its motions. Just as
+much as I am convinced of any physical truth I am convinced
+that the motive impulse must come from the outside. For, consider
+the lowest organism we know&mdash;and there are probably
+many lower ones&mdash;an aggregation of a few cells only. If it is
+capable of voluntary motion it can perform an infinite number
+of motions, all definite and precise. But now a mechanism consisting
+of a finite number of parts and few at that, cannot perform
+an infinite number of definite motions, hence the impulses
+which govern its movements must come from the environment.
+So, the atom, the ulterior element of the Universe's structure, is
+tossed about in space, eternally, a play to external influences, like
+a boat in a troubled sea. Were it to stop its motion <i>it would die</i>.
+Matter at rest, if such a thing could exist, would be matter dead.
+Death of matter! Never has a sentence of deeper philosophical
+meaning been uttered. This is the way in which Prof. Dewar<span class='pagenum'><a name="Page_300" id="Page_300">[Pg 300]</a></span>
+forcibly expresses it in the description of his admirable experiments,
+in which liquid oxygen is handled as one handles water,
+and air at ordinary pressure is made to condense and even to
+solidify by the intense cold. Experiments, which serve to illustrate,
+in his language, the last feeble manifestations of life, the
+last quiverings of matter about to die. But human eyes shall
+not witness such death. There is no death of matter, for
+throughout the infinite universe, all has to move, to vibrate, that
+is, to live.</p>
+
+<p>I have made the preceding statements at the peril of treading
+upon metaphysical ground, in my desire to introduce the subject
+of this lecture in a manner not altogether uninteresting, I may
+hope, to an audience such as I have the honor to address. But
+now, then, returning to the subject, this divine organ of sight,
+this indispensable instrument for thought and all intellectual enjoyment,
+which lays open to us the marvels of this universe,
+through which we have acquired what knowledge we possess, and
+which prompts us to, and controls, all our physical and mental
+activity. By what is it affected? By light! What is light?</p>
+
+<p>We have witnessed the great strides which have been made in
+all departments of science in recent years. So great have been
+the advances that we cannot refrain from asking ourselves, Is
+this all true, or is it but a dream? Centuries ago men have
+lived, have thought, discovered, invented, and have believed that
+they were soaring, while they were merely proceeding at a snail's
+pace. So we too may be mistaken. But taking the truth of the
+observed events as one of the implied facts of science, we must
+rejoice in the immense progress already made and still more in the
+anticipation of what must come, judging from the possibilities
+opened up by modern research. There is, however, an advance
+which we have been witnessing, which must be particularly
+gratifying to every lover of progress. It is not a discovery, or
+an invention, or an achievement in any particular direction. It
+is an advance in all directions of scientific thought and experiment.
+I mean the generalization of the natural forces and phenomena,
+the looming up of a certain broad idea on the scientific
+horizon. It is this idea which has, however, long ago taken possession
+of the most advanced minds, to which I desire to call your
+attention, and which I intend to illustrate in a general way, in
+these experiments, as the first step in answering the question
+"What is light?" and to realize the modern meaning of this
+word.<span class='pagenum'><a name="Page_301" id="Page_301">[Pg 301]</a></span></p>
+
+<p>It is beyond the scope of my lecture to dwell upon the subject
+of light in general, my object being merely to bring presently to
+your notice a certain class of light effects and a number of phenomena
+observed in pursuing the study of these effects. But to
+be consistent in my remarks it is necessary to state that, according
+to that idea, now accepted by the majority of scientific men as a
+positive result of theoretical and experimental investigation, the
+various forms or manifestations of energy which were generally
+designated as "electric" or more precisely "electromagnetic" are
+energy manifestations of the same nature as those of radiant
+heat and light. Therefore the phenomena of light and heat and
+others besides these, may be called electrical phenomena. Thus
+electrical science has become the mother science of all and its
+study has become all important. The day when we shall know
+exactly what "electricity" is, will chronicle an event probably
+greater, more important than any other recorded in the history
+of the human race. The time will come when the comfort, the
+very existence, perhaps, of man will depend upon that wonderful
+agent. For our existence and comfort we require heat, light
+and mechanical power. How do we now get all these? We get
+them from fuel, we get them by consuming material. What
+will man do when the forests disappear, when the coal fields are
+exhausted? Only one thing, according to our present knowledge
+will remain; that is, to transmit power at great distances. Men
+will go to the waterfalls, to the tides, which are the stores of an
+infinitesimal part of Nature's immeasurable energy. There will
+they harness the energy and transmit the same to their settlements,
+to warm their homes by, to give them light, and to keep
+their obedient slaves, the machines, toiling. But how will they
+transmit this energy if not by electricity? Judge then, if the
+comfort, nay, the very existence, of man will not depend on electricity.
+I am aware that this view is not that of a practical
+engineer, but neither is it that of an illusionist, for it is certain,
+that power transmission, which at present is merely a stimulus to
+enterprise, will some day be a dire necessity.</p>
+
+<p>It is more important for the student, who takes up the study
+of light phenomena, to make himself thoroughly acquainted with
+certain modern views, than to peruse entire books on the subject
+of light itself, as disconnected from these views. Were I therefore
+to make these demonstrations before students seeking
+information&mdash;and for the sake of the few of those who may be<span class='pagenum'><a name="Page_302" id="Page_302">[Pg 302]</a></span>
+present, give me leave to so assume&mdash;it would be my principal
+endeavor to impress these views upon their minds in this series of
+experiments.</p>
+
+<p>It might be sufficient for this purpose to perform a simple and
+well-known experiment. I might take a familiar appliance, a
+Leyden jar, charge it from a frictional machine, and then discharge
+it. In explaining to you its permanent state when charged,
+and its transitory condition when discharging, calling your attention
+to the forces which enter into play and to the various phenomena
+they produce, and pointing out the relation of the forces
+and phenomena, I might fully succeed in illustrating that modern
+idea. No doubt, to the thinker, this simple experiment would
+appeal as much as the most magnificent display. But this is to
+be an experimental demonstration, and one which should possess,
+besides instructive, also entertaining features and as such, a simple
+experiment, such as the one cited, would not go very far towards
+the attainment of the lecturer's aim. I must therefore choose
+another way of illustrating, more spectacular certainly, but perhaps
+also more instructive. Instead of the frictional machine and
+Leyden jar, I shall avail myself in these experiments, of an induction
+coil of peculiar properties, which was described in detail by me
+in a lecture before the London Institution of Electrical Engineers,
+in Feb., 1892. This induction coil is capable of yielding currents of
+enormous potential differences, alternating with extreme rapidity.
+With this apparatus I shall endeavor to show you three distinct
+classes of effects, or phenomena, and it is my desire that each
+experiment, while serving for the purposes of illustration, should
+at the same time teach us some novel truth, or show us some
+novel aspect of this fascinating science. But before doing this, it
+seems proper and useful to dwell upon the apparatus employed,
+and method of obtaining the high potentials and high-frequency
+currents which are made use of in these experiments.</p>
+
+
+<p><span class='pagenum'><a name="Page_303" id="Page_303">[Pg 303]</a></span></p>
+<h5>ON THE APPARATUS AND METHOD OF CONVERSION.</h5>
+
+<p>These high-frequency currents are obtained in a peculiar manner.
+The method employed was advanced by me about two
+years ago in an experimental lecture before the American Institute
+of Electrical Engineers. A number of ways, as practiced in
+the laboratory, of obtaining these currents either from continuous
+or low frequency alternating currents, is diagramatically indicated
+in Fig. 165, which will be later described in detail. The general
+<span class='pagenum'><a name="Page_304" id="Page_304">[Pg 304]</a></span>
+plan is to charge condensers, from a direct or alternate-current
+source, preferably of high-tension, and to discharge them
+disruptively while observing well-known conditions necessary
+to maintain the oscillations of the current. In view of the
+general interest taken in high-frequency currents and effects producible
+by them, it seems to me advisable to dwell at some length
+upon this method of conversion. In order to give you a clear
+idea of the action, I will suppose that a continuous-current generator
+is employed, which is often very convenient. It is desirable
+that the generator should possess such high tension as to be able
+to break through a small air space. If this is not the case, then
+auxiliary means have to be resorted to, some of which will be indicated
+subsequently. When the condensers are charged to a
+certain potential, the air, or insulating space, gives way and a disruptive
+discharge occurs. There is then a sudden rush of current
+and generally a large portion of accumulated electrical energy
+spends itself. The condensers are thereupon quickly charged and
+the same process is repeated in more or less rapid succession.
+To produce such sudden rushes of current it is necessary to observe
+certain conditions. If the rate at which the condensers are
+discharged is the same as that at which they are charged, then,
+clearly, in the assumed case the condensers do not come into
+play. If the rate of discharge be smaller than the rate of charging,
+then, again, the condensers cannot play an important part.
+But if, on the contrary, the rate of discharging is greater than
+that of charging, then a succession of rushes of current is obtained.
+It is evident that, if the rate at which the energy is
+dissipated by the discharge is very much greater than the rate of
+supply to the condensers, the sudden rushes will be comparatively
+few, with long-time intervals between. This always occurs
+when a condenser of considerable capacity is charged by means
+of a comparatively small machine. If the rates of supply and
+dissipation are not widely different, then the rushes of current
+will be in quicker succession, and this the more, the more nearly
+equal both the rates are, until limitations incident to each case
+and depending upon a number of causes are reached. Thus we
+are able to obtain from a continuous-current generator as rapid a
+succession of discharges as we like. Of course, the higher the
+tension of the generator, the smaller need be the capacity of the
+condensers, and for this reason, principally, it is of advantage to
+employ a generator of very high tension. Besides, such a generator
+permits the attaining of greater rates of vibration.<span class='pagenum'><a name="Page_305" id="Page_305">[Pg 305]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_317.jpg" width="1024" height="686" alt="Fig. 165." title="" />
+<span class="caption">Fig. 165.</span>
+</div>
+
+<p>The rushes of current may be of the same direction under the
+conditions before assumed, but most generally there is an oscillation
+superimposed upon the fundamental vibration of the current.
+When the conditions are so determined that there are no oscillations,
+the current impulses are unidirectional and thus a means is
+provided of transforming a continuous current of high tension,
+into a direct current of lower tension, which I think may find
+employment in the arts.</p>
+
+<p>This method of conversion is exceedingly interesting and I
+was much impressed by its beauty when I first conceived it. It is
+ideal in certain respects. It involves the employment of no mechanical
+devices of any kind, and it allows of obtaining currents
+of any desired frequency from an ordinary circuit, direct or alternating.
+The frequency of the fundamental discharges depending
+on the relative rates of supply and dissipation can be readily
+varied within wide limits, by simple adjustments of these quantities,
+and the frequency of the superimposed vibration by the
+determination of the capacity, self-induction and resistance of the
+circuit. The potential of the currents, again, may be raised as
+high as any insulation is capable of withstanding safely by combining
+capacity and self-induction or by induction in a secondary,
+which need have but comparatively few turns.</p>
+
+<p>As the conditions are often such that the intermittence or oscillation
+does not readily establish itself, especially when a direct
+current source is employed, it is of advantage to associate an interrupter
+with the arc, as I have, some time ago, indicated the
+use of an air-blast or magnet, or other such device readily at
+hand. The magnet is employed with special advantage in the
+conversion of direct currents, as it is then very effective. If the
+primary source is an alternate current generator, it is desirable,
+as I have stated on another occasion, that the frequency should
+be low, and that the current forming the arc be large, in order
+to render the magnet more effective.</p>
+
+<p>A form of such discharger with a magnet which has been
+found convenient, and adopted after some trials, in the conversion
+of direct currents particularly, is illustrated in Fig. 166. <small>N S</small> are
+the pole pieces of a very strong magnet which is excited by a coil
+C. The pole pieces are slotted for adjustment and can be fastened
+in any position by screws <i>s s</i><sub>1</sub>. The discharge rods <i>d d</i><sub>1</sub>, thinned
+down on the ends in order to allow a closer approach of the magnetic
+pole pieces, pass through the columns of brass <i>b b</i><sub>1</sub> and are
+<span class='pagenum'><a name="Page_306" id="Page_306">[Pg 306]</a></span>fastened in position by screws <i>s</i><sub>2</sub> <i>s</i><sub>2</sub>. Springs <i>r r</i><sub>1</sub> and collars <i>c c</i><sub>1</sub>
+are slipped on the rods, the latter serving to set the points of the
+rods at a certain suitable distance by means of screws <i>s</i><sub>3</sub> <i>s</i><sub>3</sub>, and
+the former to draw the points apart. When it is desired to start
+the arc, one of the large rubber handles <i>h h</i><sub>1</sub> is tapped quickly
+with the hand, whereby the points of the rods are brought in
+contact but are instantly separated by the springs <i>r r</i><sub>1</sub>. Such an
+arrangement has been found to be often necessary, namely in
+cases when the <span class="smcap">e. m. f.</span> was not large enough to cause the discharge
+to break through the gap, and also when it was desirable to avoid
+short circuiting of the generator by the metallic contact of the
+rods. The rapidity of the interruptions of the current with a
+magnet depends on the intensity of the magnetic field and on the
+potential difference at the end of the arc. The interruptions are
+generally in such quick succession as to produce a musical sound.
+Years ago it was observed that when a powerful induction coil
+is discharged between the poles of a strong magnet, the discharge
+produces a loud noise, not unlike a small pistol shot. It was
+vaguely stated that the spark was intensified by the presence of
+the magnetic field. It is now clear that the discharge current,
+flowing for some time, was interrupted a great number of times
+by the magnet, thus producing the sound. The phenomenon is
+especially marked when the field circuit of a large magnet or
+dynamo is broken in a powerful magnetic field.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_320.jpg" width="800" height="535" alt="Fig. 166." title="" />
+<span class="caption">Fig. 166.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_307" id="Page_307">[Pg 307]</a></span></p>
+
+
+<p>When the current through the gap is comparatively large, it is
+of advantage to slip on the points of the discharge rods pieces of
+very hard carbon and let the arc play between the carbon pieces.
+This preserves the rods, and besides has the advantage of keeping
+the air space hotter, as the heat is not conducted away as
+quickly through the carbons, and the result is that a smaller
+<span class="smcap">e. m. f.</span> in the arc gap is required to maintain a succession of
+discharges.</p>
+
+<div class="figcenter" style="width: 695px;">
+<img src="images/oi_321.jpg" width="695" height="600" alt="Fig. 167." title="" />
+<span class="caption">Fig. 167.</span>
+</div>
+
+<p>Another form of discharger, which may be employed with advantage
+in some cases, is illustrated in Fig. 167. In this form
+the discharge rods <i>d d</i><sub>1</sub> pass through perforations in a wooden
+box <small>B</small>, which is thickly coated with mica on the inside, as indicated
+by the heavy lines. The perforations are provided with
+mica tubes <i>m m</i><sub>1</sub> of some thickness, which are preferably not in
+contact with the rods <i>d d</i><sub>1</sub>. The box has a cover <small>C</small> which is a
+little larger and descends on the outside of the box. The spark
+gap is warmed by a small lamp <i>l</i> contained in the box. A plate
+<i>p</i> above the lamp allows the draught to pass only through the
+chimney <i>e</i> of the lamp, the air entering through holes <i>o o</i> in or
+near the bottom of the box and following the path indicated by
+the arrows. When the discharger is in operation, the door of the
+box is closed so that the light of the arc is not visible outside.<span class='pagenum'><a name="Page_308" id="Page_308">[Pg 308]</a></span>
+It is desirable to exclude the light as perfectly as possible, as it
+interferes with some experiments. This form of discharger is simple
+and very effective when properly manipulated. The air
+being warmed to a certain temperature, has its insulating power
+impaired; it becomes dielectrically weak, as it were, and the consequence
+is that the arc can be established at much greater distance.
+The arc should, of course, be sufficiently insulating to
+allow the discharge to pass through the gap <i>disruptively</i>. The
+arc formed under such conditions, when long, may be made extremely
+sensitive, and the weak draught through the lamp
+chimney <i>c</i> is quite sufficient to produce rapid interruptions. The
+adjustment is made by regulating the temperature and velocity
+of the draught. Instead of using the lamp, it answers the purpose
+to provide for a draught of warm air in other ways. A
+very simple way which has been practiced is to enclose the arc
+in a long vertical tube, with plates on the top and bottom for
+regulating the temperature and velocity of the air current.
+Some provision had to be made for deadening the sound.</p>
+
+<p>The air may be rendered dielectrically weak also by rarefaction.
+Dischargers of this kind have likewise been used by me
+in connection with a magnet. A large tube is for this purpose
+provided with heavy electrodes of carbon or metal, between
+which the discharge is made to pass, the tube being placed in a
+powerful magnetic field. The exhaustion of the tube is carried
+to a point at which the discharge breaks through easily, but the
+pressure should be more than 75 millimetres, at which the ordinary
+thread discharge occurs. In another form of discharger,
+combining the features before mentioned, the discharge was
+made to pass between two adjustable magnetic pole pieces, the
+space between them being kept at an elevated temperature.</p>
+
+<p>It should be remarked here that when such, or interrupting
+devices of any kind, are used and the currents are passed through
+the primary of a disruptive discharge coil, it is not, as a rule, of
+advantage to produce a number of interruptions of the current
+per second greater than the natural frequency of vibration of the
+dynamo supply circuit, which is ordinarily small. It should also
+be pointed out here, that while the devices mentioned in connection
+with the disruptive discharge are advantageous under certain
+conditions, they may be sometimes a source of trouble, as
+they produce intermittences and other irregularities in the vibration
+which it would be very desirable to overcome.<span class='pagenum'><a name="Page_309" id="Page_309">[Pg 309]</a></span></p>
+
+<p>There is, I regret to say, in this beautiful method of conversion
+a defect, which fortunately is not vital, and which I have been
+gradually overcoming. I will best call attention to this defect
+and indicate a fruitful line of work, by comparing the electrical
+process with its mechanical analogue. The process may be illustrated
+in this manner. Imagine a tank with a wide opening at
+the bottom, which is kept closed by spring pressure, but so that
+it snaps off <i>suddenly</i> when the liquid in the tank has reached a
+certain height. Let the fluid be supplied to the tank by means
+of a pipe feeding at a certain rate. When the critical height of
+the liquid is reached, the spring gives way and the bottom of the
+tank drops out. Instantly the liquid falls through the wide opening,
+and the spring, reasserting itself, closes the bottom again.
+The tank is now filled, and after a certain time interval the same
+process is repeated. It is clear, that if the pipe feeds the fluid
+quicker than the bottom outlet is capable of letting it pass
+through, the bottom will remain off and the tank will still overflow.
+If the rates of supply are exactly equal, then the bottom lid will
+remain partially open and no vibration of the same and of the
+liquid column will generally occur, though it might, if started by
+some means. But if the inlet pipe does not feed the fluid fast
+enough for the outlet, then there will be always vibration.
+Again, in such case, each time the bottom flaps up or down, the
+spring and the liquid column, if the pliability of the spring and
+the inertia of the moving parts are properly chosen, will perform
+independent vibrations. In this analogue the fluid may be likened
+to electricity or electrical energy, the tank to the condenser,
+the spring to the dielectric, and the pipe to the conductor through
+which electricity is supplied to the condenser. To make this
+analogy quite complete it is necessary to make the assumption,
+that the bottom, each time it gives way, is knocked violently
+against a non-elastic stop, this impact involving some loss of energy;
+and that, besides, some dissipation of energy results due to
+frictional losses. In the preceding analogue the liquid is supposed
+to be under a steady pressure. If the presence of the fluid
+be assumed to vary rhythmically, this may be taken as corresponding
+to the case of an alternating current. The process is
+then not quite as simple to consider, but the action is the same in
+principle.</p>
+
+<p>It is desirable, in order to maintain the vibration economically,
+to reduce the impact and frictional losses as much as possible.<span class='pagenum'><a name="Page_310" id="Page_310">[Pg 310]</a></span>
+As regards the latter, which in the electrical analogue correspond
+to the losses due to the resistance of the circuits, it is impossible
+to obviate them entirely, but they can be reduced to a minimum
+by a proper selection of the dimensions of the circuits and by the
+employment of thin conductors in the form of strands. But
+the loss of energy caused by the first breaking through of the
+dielectric&mdash;which in the above example corresponds to the violent
+knock of the bottom against the inelastic stop&mdash;would be more important
+to overcome. At the moment of the breaking through,
+the air space has a very high resistance, which is probably reduced
+to a very small value when the current has reached some
+strength, and the space is brought to a high temperature. It
+would materially diminish the loss of energy if the space were
+always kept at an extremely high temperature, but then there
+would be no disruptive break. By warming the space moderately
+by means of a lamp or otherwise, the economy as far as the
+arc is concerned is sensibly increased. But the magnet or other
+interrupting device does not diminish the loss in the arc. Likewise,
+a jet of air only facilitates the carrying off of the energy.
+Air, or a gas in general, behaves curiously in this respect. When
+two bodies charged to a very high potential, discharge disruptively
+through an air space, any amount of energy may be carried
+off by the air. This energy is evidently dissipated by bodily
+carriers, in impact and collisional losses of the molecules. The
+exchange of the molecules in the space occurs with inconceivable
+rapidity. A powerful discharge taking place between two electrodes,
+they may remain entirely cool, and yet the loss in the
+air may represent any amount of energy. It is perfectly practicable,
+with very great potential differences in the gap, to dissipate
+several horse-power in the arc of the discharge without even
+noticing a small increase in the temperature of the electrodes.
+All the frictional losses occur then practically in the air. If the
+exchange of the air molecules is prevented, as by enclosing the air
+hermetically, the gas inside of the vessel is brought quickly to a
+high temperature, even with a very small discharge. It is difficult
+to estimate how much of the energy is lost in sound waves,
+audible or not, in a powerful discharge. When the currents
+through the gap are large, the electrodes may become rapidly
+heated, but this is not a reliable measure of the energy wasted in
+the arc, as the loss through the gap itself may be comparatively
+small. The air or a gas in general is, at ordinary pressure at least,<span class='pagenum'><a name="Page_311" id="Page_311">[Pg 311]</a></span>
+clearly not the best medium through which a disruptive discharge
+should occur. Air or other gas under great pressure is of
+course a much more suitable medium for the discharge gap. I
+have carried on long-continued experiments in this direction, unfortunately
+less practicable on account of the difficulties and expense
+in getting air under great pressure. But even if the
+medium in the discharge space is solid or liquid, still the same
+losses take place, though they are generally smaller, for just as
+soon as the arc is established, the solid or liquid is volatilized.
+Indeed, there is no body known which would not be disintegrated
+by the arc, and it is an open question among scientific men,
+whether an arc discharge could occur at all in the air itself without
+the particles of the electrodes being torn off. When the
+current through the gap is very small and the arc very long, I
+believe that a relatively considerable amount of heat is taken up
+in the disintegration of the electrodes, which partially on this account
+may remain quite cold.</p>
+
+<p>The ideal medium for a discharge gap should only <i>crack</i>, and
+the ideal electrode should be of some material which cannot be
+disintegrated. With small currents through the gap it is best to
+employ aluminum, but not when the currents are large. The disruptive
+break in the air, or more or less in any ordinary medium,
+is not of the nature of a crack, but it is rather comparable to the
+piercing of innumerable bullets through a mass offering great
+frictional resistances to the motion of the bullets, this involving
+considerable loss of energy. A medium which would merely
+crack when strained electrostatically&mdash;and this possibly might be
+the case with a perfect vacuum, that is, pure ether&mdash;would involve
+a very small loss in the gap, so small as to be entirely negligible,
+at least theoretically, because a crack may be produced by an
+infinitely small displacement. In exhausting an oblong bulb
+provided with two aluminum terminals, with the greatest care, I
+have succeeded in producing such a vacuum that the secondary
+discharge of a disruptive discharge coil would break disruptively
+through the bulb in the form of fine spark streams. The
+curious point was that the discharge would completely ignore the
+terminals and start far behind the two aluminum plates which
+served as electrodes. This extraordinary high vacuum could only
+be maintained for a very short while. To return to the ideal
+medium, think, for the sake of illustration, of a piece of glass or
+similar body clamped in a vice, and the latter tightened more and<span class='pagenum'><a name="Page_312" id="Page_312">[Pg 312]</a></span>
+more. At a certain point a minute increase of the pressure will
+cause the glass to crack. The loss of energy involved in splitting
+the glass may be practically nothing, for though the force is great,
+the displacement need be but extremely small. Now imagine
+that the glass would possess the property of closing again perfectly
+the crack upon a minute diminution of the pressure.
+This is the way the dielectric in the discharge space should
+behave. But inasmuch as there would be always some loss in the
+gap, the medium, which should be continuous, should exchange
+through the gap at a rapid rate. In the preceding example, the
+glass being perfectly closed, it would mean that the dielectric in
+the discharge space possesses a great insulating power; the glass
+being cracked, it would signify that the medium in the space is
+a good conductor. The dielectric should vary enormously in
+resistance by minute variations of the <span class="smcap">e. m. f.</span> across the
+discharge space. This condition is attained, but in an extremely
+imperfect manner, by warming the air space to a certain
+critical temperature, dependent on the <span class="smcap">e. m. f.</span> across the gap,
+or by otherwise impairing the insulating power of the air. But
+as a matter of fact the air does never break down <i>disruptively</i>,
+if this term be rigorously interpreted, for before the sudden
+rush of the current occurs, there is always a weak current
+preceding it, which rises first gradually and then with comparative
+suddenness. That is the reason why the rate of change is
+very much greater when glass, for instance, is broken through,
+than when the break takes place through an air space of equivalent
+dielectric strength. As a medium for the discharge space, a
+solid, or even a liquid, would be preferable therefor. It is somewhat
+difficult to conceive of a solid body which would possess the
+property of closing instantly after it has been cracked. But a
+liquid, especially under great pressure, behaves practically like a
+solid, while it possesses the property of closing the crack. Hence
+it was thought that a liquid insulator might be more suitable as a
+dielectric than air. Following out this idea, a number of different
+forms of dischargers in which a variety of such insulators, sometimes
+under great pressure, were employed, have been experimented
+upon. It is thought sufficient to dwell in a few words
+upon one of the forms experimented upon. One of these dischargers
+is illustrated in Figs. 168<i>a</i> and 168<i>b</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_327.jpg" width="800" height="327" alt="Fig. 168a, 168b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 168a.</td><td class="caption1"><span class="smcap">Fig.</span> 168b.</td></tr>
+</table>
+</div>
+
+<p>A hollow metal pulley <small>P</small> (Fig. 168<i>a</i>), was fastened upon an arbor
+<i>a</i>, which by suitable means was rotated at a considerable
+<span class='pagenum'><a name="Page_313" id="Page_313">[Pg 313]</a></span>speed. On the inside of the pulley, but disconnected from the
+same, was supported a thin disc <i>h</i> (which is shown thick for the
+sake of clearness), of hard rubber in which there were embedded
+two metal segments <i>s s</i> with metallic extensions <i>e e</i> into which
+were screwed conducting terminals <i>t t</i> covered with thick tubes
+of hard rubber <i>t t</i>. The rubber disc <i>h</i> with its metallic segments
+<i>s s</i>, was finished in a lathe, and its entire surface highly polished
+so as to offer the smallest possible frictional resistance to the motion
+through a fluid. In the hollow of the pulley an insulating
+liquid such as a thin oil was poured so as to reach very nearly to
+the opening left in the flange <i>f</i>, which was screwed tightly on the
+front side of the pulley. The terminals <i>t t</i>, were connected to the
+opposite coatings of a battery of condensers so that the discharge
+occurred through the liquid. When the pulley was rotated, the
+liquid was forced against the rim of the pulley and considerable
+fluid pressure resulted. In this simple way the discharge gap
+was filled with a medium which behaved practically like a solid,
+which possessed the quality of closing instantly upon the occurrence
+of the break, and which moreover was circulating through
+the gap at a rapid rate. Very powerful effects were produced by
+discharges of this kind with liquid interrupters, of which a number
+of different forms were made. It was found that, as expected,
+a longer spark for a given length of wire was obtainable
+in this way than by using air as an interrupting device. Generally
+the speed, and therefore also the fluid pressure, was limited
+by reason of the fluid friction, in the form of discharger described,
+but the practically obtainable speed was more than sufficient to
+produce a number of breaks suitable for the circuits ordinarily
+used. In such instances the metal pulley <small>P</small> was provided with a
+few projections inwardly, and a definite number of breaks was
+then produced which could be computed from the speed of<span class='pagenum'><a name="Page_314" id="Page_314">[Pg 314]</a></span>
+rotation of the pulley. Experiments were also carried on with
+liquids of different insulating power with the view of reducing
+the loss in the arc. When an insulating liquid is moderately
+warmed, the loss in the arc is diminished.</p>
+
+<p>A point of some importance was noted in experiments with
+various discharges of this kind. It was found, for instance, that
+whereas the conditions maintained in these forms were favorable
+for the production of a great spark length, the current so obtained
+was not best suited to the production of light effects. Experience
+undoubtedly has shown, that for such purposes a harmonic
+rise and fall of the potential is preferable. Be it that a
+solid is rendered incandescent, or phosphorescent, or be it that energy
+is transmitted by condenser coating through the glass, it is
+quite certain that a harmonically rising and falling potential produces
+less destructive action, and that the vacuum is more permanently
+maintained. This would be easily explained if it were
+ascertained that the process going on in an exhausted vessel is of
+an electrolytic nature.</p>
+
+<p>In the diagrammatical sketch, Fig. 165, which has been already
+referred to, the cases which are most likely to be met with in
+practice are illustrated. One has at his disposal either direct or
+alternating currents from a supply station. It is convenient for
+an experimenter in an isolated laboratory to employ a machine <small>G</small>,
+such as illustrated, capable of giving both kinds of currents. In
+such case it is also preferable to use a machine with multiple
+circuits, as in many experiments it is useful and convenient to
+have at one's disposal currents of different phases. In the
+sketch, <small>D</small> represents the direct and <small>A</small> the alternating circuit. In
+each of these, three branch circuits are shown, all of which are
+provided with double line switches <i>s s s s s s</i>. Consider first the
+direct current conversion; <small>I</small><i>a</i> represents the simplest case. If
+the <span class="smcap">e. m. f.</span> of the generator is sufficient to break through a small
+air space, at least when the latter is warmed or otherwise rendered
+poorly insulating, there is no difficulty in maintaining a
+vibration with fair economy by judicious adjustment of the
+capacity, self-induction and resistance of the circuit <small>L</small> containing
+the devices <i>l l m</i>. The magnet <small>N</small>, <small>S</small>, can be in this case advantageously
+combined with the air space. The discharger <i>d d</i> with
+the magnet may be placed either way, as indicated by the full or
+by the dotted lines. The circuit <small>I</small><i>a</i> with the connections and devices
+is supposed to possess dimensions such as are suitable for<span class='pagenum'><a name="Page_315" id="Page_315">[Pg 315]</a></span>
+the maintenance of a vibration. But usually the <span class="smcap">e. m. f.</span> on the
+circuit or branch <small>I</small><i>a</i> will be something like a 100 volts or so, and
+in this case it is not sufficient to break through the gap. Many
+different means may be used to remedy this by raising the <span class="smcap">e. m. f.</span>
+across the gap. The simplest is probably to insert a large self-induction
+coil in series with the circuit <small>L</small>. When the arc is
+established, as by the discharger illustrated in Fig. 166, the magnet
+blows the arc out the instant it is formed. Now the extra
+current of the break, being of high <span class="smcap">e. m. f.</span>, breaks through the
+gap, and a path of low resistance for the dynamo current being
+again provided, there is a sudden rush of current from the
+dynamo upon the weakening or subsidence of the extra current.
+This process is repeated in rapid succession, and in this manner I
+have maintained oscillation with as low as 50 volts, or even less,
+across the gap. But conversion on this plan is not to be recommended
+on account of the too heavy currents through the gap
+and consequent heating of the electrodes; besides, the frequencies
+obtained in this way are low, owing to the high self-induction
+necessarily associated with the circuit. It is very desirable
+to have the <span class="smcap">e. m. f.</span> as high as possible, first, in order to increase
+the economy of the conversion, and, secondly, to obtain high
+frequencies. The difference of potential in this electric oscillation
+is, of course, the equivalent of the stretching force in the
+mechanical vibration of the spring. To obtain very rapid vibration
+in a circuit of some inertia, a great stretching force or difference
+of potential is necessary. Incidentally, when the <span class="smcap">e. m. f.</span> is
+very great, the condenser which is usually employed in connection
+with the circuit need but have a small capacity, and many
+other advantages are gained. With a view of raising the <span class="smcap">e. m. f.</span>
+to a many times greater value than obtainable from ordinary
+distribution circuits, a rotating transformer <i>g</i> is used, as indicated
+at <small>II</small><i>a</i>, Fig. 165, or else a separate high potential machine
+is driven by means of a motor operated from the generator <small>G</small>.
+The latter plan is in fact preferable, as changes are easier made.
+The connections from the high tension winding are quite similar
+to those in branch <small>I</small><i>a</i> with the exception that a condenser <small>C</small>,
+which should be adjustable, is connected to the high tension
+circuit. Usually, also, an adjustable self-induction coil in series
+with the circuit has been employed in these experiments. When
+the tension of the currents is very high, the magnet ordinarily
+used in connection with the discharger is of comparatively small<span class='pagenum'><a name="Page_316" id="Page_316">[Pg 316]</a></span>
+value, as it is quite easy to adjust the dimensions of the circuit
+so that oscillation is maintained. The employment of a steady
+<span class="smcap">e. m. f.</span> in the high frequency conversion affords some advantages
+over the employment of alternating <span class="smcap">e. m. f.</span>, as the adjustments
+are much simpler and the action can be easier controlled.
+But unfortunately one is limited by the obtainable potential difference.
+The winding also breaks down easily in consequence
+of the sparks which form between the sections of the armature
+or commutator when a vigorous oscillation takes place. Besides,
+these transformers are expensive to build. It has been found by
+experience that it is best to follow the plan illustrated at <small>III</small><i>a</i>.
+In this arrangement a rotating transformer <i>g</i>, is employed to
+convert the low tension direct currents into low frequency alternating
+currents, preferably also of small tension. The tension
+of the currents is then raised in a stationary transformer <small>T</small>. The
+secondary <small>S</small> of this transformer is connected to an adjustable condenser
+<small>C</small> which discharges through the gap or discharger <i>d d</i>, placed
+in either of the ways indicated, through the primary <small>P</small> of a disruptive
+discharge coil, the high frequency current being obtained
+from the secondary <small>S</small> of this coil, as described on previous occasions.
+This will undoubtedly be found the cheapest and most convenient
+way of converting direct currents.</p>
+
+<p>The three branches of the circuit <small>A</small> represent the usual cases
+met in practice when alternating currents are converted. In
+Fig. 1<i>b</i> a condenser <small>C</small>, generally of large capacity, is connected to the
+circuit <small>L</small> containing the devices <i>l l</i>, <i>m m</i>. The devices <i>m m</i> are supposed
+to be of high self-induction so as to bring the frequency of
+the circuit more or less to that of the dynamo. In this instance
+the discharger <i>d d</i> should best have a number of makes and breaks
+per second equal to twice the frequency of the dynamo. If not
+so, then it should have at least a number equal to a multiple or
+even fraction of the dynamo frequency. It should be observed,
+referring to <small>I</small><i>b</i>, that the conversion to a high potential is also
+effected when the discharger <i>d d</i>, which is shown in the sketch, is
+omitted. But the effects which are produced by currents which
+rise instantly to high values, as in a disruptive discharge, are
+entirely different from those produced by dynamo currents which
+rise and fall harmonically. So, for instance, there might be in a
+given case a number of makes and breaks at <i>d d</i> equal to just
+twice the frequency of the dynamo, or in other words, there may
+be the same number of fundamental oscillations as would be pro<span class='pagenum'><a name="Page_317" id="Page_317">[Pg 317]</a></span>duced
+without the discharge gap, and there might even not be any
+quicker superimposed vibration; yet the differences of potential at
+the various points of the circuit, the impedance and other phenomena,
+dependent upon the rate of change, will bear no similarity in
+the two cases. Thus, when working with currents discharging disruptively,
+the element chiefly to be considered is not the frequency,
+as a student might be apt to believe, but the rate of change per
+unit of time. With low frequencies in a certain measure the same
+effects may be obtained as with high frequencies, provided the rate
+of change is sufficiently great. So if a low frequency current is
+raised to a potential of, say, 75,000 volts, and the high tension current
+passed through a series of high resistance lamp filaments, the
+importance of the rarefied gas surrounding the filament is clearly
+noted, as will be seen later; or, if a low frequency current of several
+thousand amperes is passed through a metal bar, striking phenomena
+of impedance are observed, just as with currents of high
+frequencies. But it is, of course, evident that with low frequency
+currents it is impossible to obtain such rates of change per unit of
+time as with high frequencies, hence the effects produced by the
+latter are much more prominent. It is deemed advisable to
+make the preceding remarks, inasmuch as many more recently
+described effects have been unwittingly identified with high
+frequencies. Frequency alone in reality does not mean anything,
+except when an undisturbed harmonic oscillation is considered.</p>
+
+<p>In the branch <small>III</small><i>b</i> a similar disposition to that in <small>I</small><i>b</i> is illustrated,
+with the difference that the currents discharging through the gap
+<i>d d</i> are used to induce currents in the secondary <small>S</small> of a transformer
+<small>T</small>. In such case the secondary should be provided with an
+adjustable condenser for the purpose of tuning it to the primary.</p>
+
+<p><small>II</small><i>b</i> illustrates a plan of alternate current high frequency
+conversion which is most frequently used and which is found to
+be most convenient. This plan has been dwelt upon in detail on
+previous occasions and need not be described here.</p>
+
+<p>Some of these results were obtained by the use of a high
+frequency alternator. A description of such machines will be
+found in my original paper before the American Institute of
+Electrical Engineers, and in periodicals of that period, notably
+in <span class="smcap">The Electrical Engineer</span> of March 18, 1891.</p>
+
+<p>I will now proceed with the experiments.<span class='pagenum'><a name="Page_318" id="Page_318">[Pg 318]</a></span></p>
+
+<h5>ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.</h5>
+
+<p>The first class of effects I intend to show you are effects produced
+by electrostatic force. It is the force which governs the
+the motion of the atoms, which causes them to collide and develop
+the life-sustaining energy of heat and light, and which
+causes them to aggregate in an infinite variety of ways, according
+to Nature's fanciful designs, and to form all these wondrous
+structures we perceive around us; it is, in fact, if our present
+views be true, the most important force for us to consider in Nature.
+As the term <i>electrostatic</i> might imply a steady electric
+condition, it should be remarked, that in these experiments the
+force is not constant, but varies at a rate which may be considered
+moderate, about one million times a second, or thereabouts.
+This enables me to produce many effects which are not producible
+with an unvarying force.</p>
+
+<p>When two conducting bodies are insulated and electrified,
+we say that an electrostatic force is acting between them. This
+force manifests itself in attractions, repulsions and stresses in the
+bodies and space or medium without. So great may be the strain
+exerted in the air, or whatever separates the two conducting
+bodies, that it may break down, and we observe sparks or bundles
+of light or streamers, as they are called. These streamers form
+abundantly when the force through the air is rapidly varying. I
+will illustrate this action of electrostatic force in a novel experiment
+in which I will employ the induction coil before referred
+to. The coil is contained in a trough filled with oil, and placed
+under the table. The two ends of the secondary wire pass
+through the two thick columns of hard rubber which protrude
+to some height above the table. It is necessary to insulate the
+ends or terminals of the secondary heavily with hard rubber, because
+even dry wood is by far too poor an insulator for these currents
+of enormous potential differences. On one of the terminals
+of the coil, I have placed a large sphere of sheet brass, which
+is connected to a larger insulated brass plate, in order to enable
+me to perform the experiments under conditions, which, as you
+will see, are more suitable for this experiment. I now set the
+coil to work and approach the free terminal with a metallic object
+held in my hand, this simply to avoid burns. As I approach the
+metallic object to a distance of eight or ten inches, a torrent of furious
+sparks breaks forth from the end of the secondary wire, which<span class='pagenum'><a name="Page_319" id="Page_319">[Pg 319]</a></span>
+passes through the rubber column. The sparks cease when the
+metal in my hand touches the wire. My arm is now traversed
+by a powerful electric current, vibrating at about the rate of one
+million times a second. All around me the electrostatic force
+makes itself felt, and the air molecules and particles of dust flying
+about are acted upon and are hammering violently against my
+body. So great is this agitation of the particles, that when the
+lights are turned out you may see streams of feeble light appear
+on some parts of my body. When such a streamer breaks out on
+any part of the body, it produces a sensation like the pricking of
+a needle. Were the potentials sufficiently high and the frequency
+of the vibration rather low, the skin would probably be ruptured
+under the tremendous strain, and the blood would rush out
+with great force in the form of fine spray or jet so thin as to be
+invisible, just as oil will when placed on the positive terminal of
+a Holtz machine. The breaking through of the skin though it
+may seem impossible at first, would perhaps occur, by reason of
+the tissues under the skin being incomparably better conducting.
+This, at least, appears plausible, judging from some observations.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_333.jpg" width="640" height="408" alt="Fig. 169." title="" />
+<span class="caption">Fig. 169.</span>
+</div>
+
+
+<p>I can make these streams of light visible to all, by touching
+with the metallic object one of the terminals as before, and
+approaching my free hand to the brass sphere, which is connected
+to the second terminal of the coil. As the hand is
+approached, the air between it and the sphere, or in the immediate
+neighborhood, is more violently agitated, and you see
+streams of light now break forth from my finger tips and
+from the whole hand (Fig. 169). Were I to approach the hand
+closer, powerful sparks would jump from the brass sphere to
+my hand, which might be injurious. The streamers offer no
+particular inconvenience, except that in the ends of the finger<span class='pagenum'><a name="Page_320" id="Page_320">[Pg 320]</a></span>
+tips a burning sensation is felt. They should not be confounded
+with those produced by an influence machine, because in many
+respects they behave differently. I have attached the brass sphere
+and plate to one of the terminals in order to prevent the formation
+of visible streamers on that terminal, also in order to prevent
+sparks from jumping at a considerable distance. Besides, the
+attachment is favorable for the working of the coil.</p>
+
+<p>The streams of light which you have observed issuing from my
+hand are due to a potential of about 200,000 volts, alternating in
+rather irregular intervals, sometimes like a million times a second.
+A vibration of the same amplitude, but four times as fast, to maintain
+which over 3,000,000 volts would be required, would be
+more than sufficient to envelop my body in a complete sheet of
+flame. But this flame would not burn me up; quite contrarily,
+the probability is that I would not be injured in the least. Yet a
+hundredth part of that energy, otherwise directed, would be amply
+sufficient to kill a person.</p>
+
+<p>The amount of energy which may thus be passed into the body
+of a person depends on the frequency and potential of the currents,
+and by making both of these very great, a vast amount of
+energy may be passed into the body without causing any discomfort,
+except perhaps, in the arm, which is traversed by a true
+conduction current. The reason why no pain in the body is felt,
+and no injurious effect noted, is that everywhere, if a current be
+imagined to flow through the body, the direction of its flow
+would be at right angles to the surface; hence the body of the
+experimenter offers an enormous section to the current, and the
+density is very small, with the exception of the arm, perhaps,
+where the density may be considerable. But if only a small
+fraction of that energy would be applied in such a way that a current
+would traverse the body in the same manner as a low frequency
+current, a shock would be received which might be fatal.
+A direct or low frequency alternating current is fatal, I think,
+principally because its distribution through the body is not
+uniform, as it must divide itself in minute streamlets of great
+density, whereby some organs are vitally injured. That such a
+process occurs I have not the least doubt, though no evidence
+might apparently exist, or be found upon examination. The
+surest to injure and destroy life, is a continuous current, but the
+most painful is an alternating current of very low frequency.
+The expression of these views, which are the result of long con<span class='pagenum'><a name="Page_321" id="Page_321">[Pg 321]</a></span>tinued
+experiment and observation, both with steady and varying
+currents, is elicited by the interest which is at present taken in
+this subject, and by the manifestly erroneous ideas which are
+daily propounded in journals on this subject.</p>
+
+<p>I may illustrate an effect of the electrostatic force by another
+striking experiment, but before, I must call your attention to one
+or two facts. I have said before, that when the medium between
+two oppositely electrified bodies is strained beyond a certain
+limit it gives way and, stated in popular language, the
+opposite electric charges unite and neutralize each other. This
+breaking down of the medium occurs principally when the force
+acting between the bodies is steady, or varies at a moderate rate.
+Were the variation sufficiently rapid, such a destructive break
+would not occur, no matter how great the force, for all the energy
+would be spent in radiation, convection and mechanical and
+chemical action. Thus the <i>spark</i> length, or greatest distance
+which a <i>spark</i> will jump between the electrified bodies is the
+smaller, the greater the variation or time rate of change. But
+this rule may be taken to be true only in a general way, when
+comparing rates which are widely different.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_335.jpg" width="800" height="346" alt="Fig. 170a, 170b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 170a.</td><td class="caption1"><span class="smcap">Fig.</span> 170b.</td></tr>
+</table>
+</div>
+
+<p>I will show you by an experiment the difference in the effect
+produced by a rapidly varying and a steady or moderately varying
+force. I have here two large circular brass plates <i>p p</i> (Fig.
+170<i>a</i> and Fig. 170<i>b</i>), supported on movable insulating stands on
+the table, connected to the ends of the secondary of a coil similar
+to the one used before. I place the plates ten or twelve inches
+apart and set the coil to work. You see the whole space between
+the plates, nearly two cubic feet, filled with uniform light, Fig.
+170<i>a</i>. This light is due to the streamers you have seen in the first
+experiment, which are now much more intense. I have already
+pointed out the importance of these streamers in commercial apparatus
+and their still greater importance in some purely scientific
+investigations. Often they are too weak to be visible, but<span class='pagenum'><a name="Page_322" id="Page_322">[Pg 322]</a></span>
+they always exist, consuming energy and modifying the action
+of the apparatus. When intense, as they are at present, they
+produce ozone in great quantity, and also, as Professor Crookes
+has pointed out, nitrous acid. So quick is the chemical action that
+if a coil, such as this one, is worked for a very long time it will
+make the atmosphere of a small room unbearable, for the eyes
+and throat are attacked. But when moderately produced, the
+streamers refresh the atmosphere wonderfully, like a thunder-storm,
+and exercises unquestionably a beneficial effect.</p>
+
+<p>In this experiment the force acting between the plates changes
+in intensity and direction at a very rapid rate. I will now make
+the rate of change per unit time much smaller. This I effect by
+rendering the discharges through the primary of the induction
+coil less frequent, and also by diminishing the rapidity of the vibration
+in the secondary. The former result is conveniently secured
+by lowering the <span class="smcap">e. m. f.</span> over the air gap in the primary
+circuit, the latter by approaching the two brass plates to a distance
+of about three or four inches. When the coil is set to work,
+you see no streamers or light between the plates, yet the medium
+between them is under a tremendous strain. I still further augment
+the strain by raising the <span class="smcap">e. m. f.</span> in the primary circuit, and
+soon you see the air give way and the hall is illuminated by a
+shower of brilliant and noisy sparks, Fig. 170<i>b</i>. These sparks could
+be produced also with unvarying force; they have been for many
+years a familiar phenomenon, though they were usually obtained
+from an entirely different apparatus. In describing these two
+phenomena so radically different in appearance, I have advisedly
+spoken of a "force" acting between the plates. It would be in
+accordance with accepted views to say, that there was an "alternating
+<span class="smcap">e. m. f,</span>" acting between the plates. This term is quite
+proper and applicable in all cases where there is evidence of at
+least a possibility of an essential inter-dependence of the electric
+state of the plates, or electric action in their neighborhood. But
+if the plates were removed to an infinite distance, or if at a finite
+distance, there is no probability or necessity whatever for such
+dependence. I prefer to use the term "electrostatic force," and
+to say that such a force is acting around each plate or electrified insulated
+body in general. There is an inconvenience in using this
+expression as the term incidentally means a steady electric condition;
+but a proper nomenclature will eventually settle this difficulty.<span class='pagenum'><a name="Page_323" id="Page_323">[Pg 323]</a></span></p>
+
+<p>I now return to the experiment to which I have already alluded,
+and with which I desire to illustrate a striking effect produced
+by a rapidly varying electrostatic force. I attach to the end
+of the wire, <i>l</i> (Fig. 171), which is in connection with one of the
+terminals of the secondary of the induction coil, an exhausted
+bulb <i>b</i>. This bulb contains a thin carbon filament <i>f</i>, which is
+fastened to a platinum wire <i>w</i>, sealed in the glass and leading
+outside of the bulb, where it connects to the wire <i>l</i>. The
+bulb may be exhausted to any degree attainable with ordinary
+apparatus. Just a moment before, you have witnessed the breaking
+down of the air between the charged brass plates. You know
+that a plate of glass, or any other insulating material, would break
+down in like manner. Had I therefore a metallic coating attached
+to the outside of the bulb, or placed near the same, and
+were this coating connected to the other terminal of the coil, you
+would be prepared to see the glass give way if the strain were
+sufficiently increased. Even were the coating not connected to
+the other terminal, but to an insulated plate, still, if you have
+followed recent developments, you would naturally expect a rupture
+of the glass.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_337.jpg" width="800" height="474" alt="Fig. 171, 172a, 172b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 171.</td><td class="caption1"><span class="smcap">Fig.</span> 172a.</td><td class="caption1"><span class="smcap">Fig.</span> 172b.</td></tr>
+</table>
+</div>
+
+
+<p>But it will certainly surprise you to note that under the action
+of the varying electrostatic force, the glass gives way when all
+other bodies are removed from the bulb. In fact, all the surrounding
+bodies we perceive might be removed to an infinite distance
+without affecting the result in the slightest. When the coil
+is set to work, the glass is invariably broken through at the seal,
+or other narrow channel, and the vacuum is quickly impaired.<span class='pagenum'><a name="Page_324" id="Page_324">[Pg 324]</a></span>
+Such a damaging break would not occur with a steady force, even
+if the same were many times greater. The break is due to the
+agitation of the molecules of the gas within the bulb, and outside
+of the same. This agitation, which is generally most violent in
+the narrow pointed channel near the seal, causes a heating and
+rupture of the glass. This rupture, would, however, not occur,
+not even with a varying force, if the medium filling the inside of
+the bulb, and that surrounding it, were perfectly homogeneous.
+The break occurs much quicker if the top of the bulb is drawn
+out into a fine fibre. In bulbs used with these coils such narrow,
+pointed channels must therefore be avoided.</p>
+
+<p>When a conducting body is immersed in air, or similar insulating
+medium, consisting of, or containing, small freely movable
+particles capable of being electrified, and when the electrification
+of the body is made to undergo a very rapid change&mdash;which is
+equivalent to saying that the electrostatic force acting around
+the body is varying in intensity,&mdash;the small particles are attracted
+and repelled, and their violent impacts against the body may
+cause a mechanical motion of the latter. Phenomena of this
+kind are noteworthy, inasmuch as they have not been observed
+before with apparatus such as has been commonly in use. If a
+very light conducting sphere be suspended on an exceedingly fine
+wire, and charged to a steady potential, however high, the sphere
+will remain at rest. Even if the potential would be rapidly
+varying, provided that the small particles of matter, molecules or
+atoms, are evenly distributed, no motion of the sphere should result.
+But if one side of the conducting sphere is covered with a
+thick insulating layer, the impacts of the particles will cause the
+sphere to move about, generally in irregular curves, Fig. 172<i>a</i>.
+In like manner, as I have shown on a previous occasion, a fan of
+sheet metal, Fig. 172<i>b</i>, covered partially with insulating material
+as indicated, and placed upon the terminal of the coil so as to turn
+freely on it, is spun around.</p>
+
+<p>All these phenomena you have witnessed and others which
+will be shown later, are due to the presence of a medium like
+air, and would not occur in a continuous medium. The action
+of the air may be illustrated still better by the following experiment.
+I take a glass tube <i>t</i>, Fig. 173, of about an inch in diameter,
+which has a platinum wire <i>w</i> sealed in the lower end,
+and to which is attached a thin lamp filament <i>f</i>. I connect the
+wire with the terminal of the coil and set the coil to work. The<span class='pagenum'><a name="Page_325" id="Page_325">[Pg 325]</a></span>
+platinum wire is now electrified positively and negatively
+in rapid succession and the wire and air inside of the tube
+is rapidly heated by the impacts of the particles, which may be
+so violent as to render the filament incandescent. But if I pour
+oil in the tube, just as soon as the wire is covered with the oil,
+all action apparently ceases and there is no marked evidence of
+heating. The reason of this is that the oil is a practically continuous
+medium. The displacements in such a continuous medium
+are, with these frequencies, to all appearance incomparably
+smaller than in air, hence the work performed in such a medium
+is insignificant. But oil would behave very differently with frequencies
+many times as great, for even though the displacements
+be small, if the frequency were much greater, considerable work
+might be performed in the oil.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_339.jpg" width="800" height="567" alt="Fig. 173, 174." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 173.</td><td class="caption">Fig. 174.</td></tr>
+</table>
+</div>
+
+<p>The electrostatic attractions and repulsions between bodies of
+measurable dimensions are, of all the manifestations of this force,
+the first so-called <i>electrical</i> phenomena noted. But though they
+have been known to us for many centuries, the precise nature of
+the mechanism concerned in these actions is still unknown to us,
+and has not been even quite satisfactorily explained. What kind
+of mechanism must that be? We cannot help wondering when
+we observe two magnets attracting and repelling each other with
+a force of hundreds of pounds with apparently nothing between
+them. We have in our commercial dynamos magnets capable of
+sustaining in mid-air tons of weight. But what are even these<span class='pagenum'><a name="Page_326" id="Page_326">[Pg 326]</a></span>
+forces acting between magnets when compared with the tremendous
+attractions and repulsions produced by electrostatic force, to
+which there is apparently no limit as to intensity. In lightning
+discharges bodies are often charged to so high a potential that
+they are thrown away with inconceivable force and torn asunder
+or shattered into fragments. Still even such effects cannot compare
+with the attractions and repulsions which exist between
+charged molecules or atoms, and which are sufficient to project
+them with speeds of many kilometres a second, so that under their
+violent impact bodies are rendered highly incandescent and are
+volatilized. It is of special interest for the thinker who inquires
+into the nature of these forces to note that whereas the actions
+between individual molecules or atoms occur seemingly under any
+conditions, the attractions and repulsions of bodies of measurable
+dimensions imply a medium possessing insulating properties. So,
+if air, either by being rarefied or heated, is rendered more or less
+conducting, these actions between two electrified bodies practically
+cease, while the actions between the individual atoms continue to
+manifest themselves.</p>
+
+<p>An experiment may serve as an illustration and as a means of
+bringing out other features of interest. Some time ago I showed
+that a lamp filament or wire mounted in a bulb and connected to
+one of the terminals of a high tension secondary coil is set spinning,
+the top of the filament generally describing a circle. This
+vibration was very energetic when the air in the bulb was at
+ordinary pressure and became less energetic when the air in the
+bulb was strongly compressed. It ceased altogether when the air
+was exhausted so as to become comparatively good conducting. I
+found at that time that no vibration took place when the bulb
+was very highly exhausted. But I conjectured that the vibration
+which I ascribed to the electrostatic action between the walls of
+the bulb and the filament should take place also in a highly
+exhausted bulb. To test this under conditions which were more
+favorable, a bulb like the one in Fig. 174, was constructed. It
+comprised a globe <i>b</i>, in the neck of which was sealed a platinum
+wire <i>w</i> carrying a thin lamp filament <i>f</i>. In the lower part of
+the globe a tube <i>t</i> was sealed so as to surround the filament. The
+exhaustion was carried as far as it was practicable with the apparatus
+employed.</p>
+
+<p>This bulb verified my expectation, for the filament was set
+spinning when the current was turned on, and became incandes<span class='pagenum'><a name="Page_327" id="Page_327">[Pg 327]</a></span>cent.
+It also showed another interesting feature, bearing upon
+the preceding remarks, namely, when the filament had been
+kept incandescent some time, the narrow tube and the space inside
+were brought to an elevated temperature, and as the gas in
+the tube then became conducting, the electrostatic attraction between
+the glass and the filament became very weak or ceased, and
+the filament came to rest. When it came to rest it would glow
+far more intensely. This was probably due to its assuming the
+position in the centre of the tube where the molecular bombardment
+was most intense, and also partly to the fact that the individual
+impacts were more violent and that no part of the supplied
+energy was converted into mechanical movement. Since, in accordance
+with accepted views, in this experiment the incandescence
+must be attributed to the impacts of the particles, molecules or
+atoms in the heated space, these particles must therefore, in order
+to explain such action, be assumed to behave as independent carriers
+of electric charges immersed in an insulating medium; yet
+there is no attractive force between the glass tube and the filament
+because the space in the tube is, as a whole, conducting.</p>
+
+<p>It is of some interest to observe in this connection that whereas
+the attraction between two electrified bodies may cease owing to
+the impairing of the insulating power of the medium in which
+they are immersed, the repulsion between the bodies may still be
+observed. This may be explained in a plausible way. When the
+bodies are placed at some distance in a poorly conducting medium,
+such as slightly warmed or rarefied air, and are suddenly electrified,
+opposite electric charges being imparted to them, these
+charges equalize more or less by leakage through the air. But if
+the bodies are similarly electrified, there is less opportunity afforded
+for such dissipation, hence the repulsion observed in such
+case is greater than the attraction. Repulsive actions in a gaseous
+medium are however, as Prof. Crookes has shown, enhanced
+by molecular bombardment.</p>
+
+
+<h5>ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.</h5>
+
+<p>So far, I have considered principally effects produced by a
+varying electrostatic force in an insulating medium, such as air.
+When such a force is acting upon a conducting body of measurable
+dimensions, it causes within the same, or on its surface,
+displacements of the electricity and gives rise to electric currents,
+and these produce another kind of phenomena, some of which I<span class='pagenum'><a name="Page_328" id="Page_328">[Pg 328]</a></span>
+shall presently endeavor to illustrate. In presenting this second
+class of electrical effects, I will avail myself principally of such
+as are producible without any return circuit, hoping to interest
+you the more by presenting these phenomena in a more or less
+novel aspect.</p>
+
+<p>It has been a long time customary, owing to the limited
+experience with vibratory currents, to consider an electric current
+as something circulating in a closed conducting path. It
+was astonishing at first to realize that a current may flow through
+the conducting path even if the latter be interrupted, and it
+was still more surprising to learn, that sometimes it may be
+even easier to make a current flow under such conditions
+than through a closed path. But that old idea is gradually disappearing,
+even among practical men, and will soon be entirely
+forgotten.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_342.jpg" width="800" height="332" alt="Fig. 175." title="" />
+<span class="caption">Fig. 175.</span>
+</div>
+
+
+<p>If I connect an insulated metal plate <small>P</small>, Fig. 175, to one of the
+terminals <small>T</small> of the induction coil by means of a wire, though this
+plate be very well insulated, a current passes through the
+wire when the coil is set to work. First I wish to give you
+evidence that there <i>is</i> a current passing through the connecting
+wire. An obvious way of demonstrating this is to insert between
+the terminal of the coil and the insulated plate a very thin platinum
+or german silver wire <i>w</i> and bring the latter to incandescence
+or fusion by the current. This requires a rather large plate
+or else current impulses of very high potential and frequency.
+Another way is to take a coil <small>C</small>, Fig. 175, containing many turns of
+thin insulated wire and to insert the same in the path of the current
+to the plate. When I connect one of the ends of the coil to the
+wire leading to another insulated plate <small>P<sub>1</sub></small>, and its other end to the
+terminal <small>T<sub>1</sub></small> of the induction coil, and set the latter to work, a current
+passes through the inserted coil <small>C</small> and the existence of the
+current may be made manifest in various ways. For instance, I<span class='pagenum'><a name="Page_329" id="Page_329">[Pg 329]</a></span>
+insert an iron core <i>i</i> within the coil. The current being one of
+very high frequency, will, if it be of some strength, soon bring the
+iron core to a noticeably higher temperature, as the hysteresis and
+current losses are great with such high frequencies. One might
+take a core of some size, laminated or not, it would matter little;
+but ordinary iron wire 1/16th or 1/8th of an inch thick is suitable
+for the purpose. While the induction coil is working, a current
+traverses the inserted coil and only a few moments are sufficient
+to bring the iron wire <i>i</i> to an elevated temperature sufficient to
+soften the sealing-wax <i>s</i>, and cause a paper washer <i>p</i> fastened by
+it to the iron wire to fall off. But with the apparatus such as I
+have here, other, much more interesting, demonstrations of this
+kind can be made. I have a secondary <small>S</small>, Fig 176, of coarse wire,
+wound upon a coil similar to the first. In the preceding experiment
+the current through the coil <small>C</small>, Fig. 175, was very small, but
+there being many turns a strong heating effect was, nevertheless,
+produced in the iron wire. Had I passed that current through a
+conductor in order to show the heating of the latter, the current
+might have been too small to produce the effect desired. But with
+this coil provided with a secondary winding, I can now transform
+the feeble current of high tension which passes through the primary
+<small>P</small> into a strong secondary current of low tension, and this
+current will quite certainly do what I expect. In a small glass
+tube (<i>t</i>, Fig. 176), I have enclosed a coiled platinum wire, <i>w</i>, this
+merely in order to protect the wire. On each end of the glass
+tube is sealed a terminal of stout wire to which one of the ends of
+the platinum wire <i>w</i>, is connected. I join the terminals of the
+secondary coil to these terminals and insert the primary <i>p</i>,
+between the insulated plate <small>P<sub>1</sub></small>, and the terminal <small>T<sub>1</sub></small>, of the induction
+coil as before. The latter being set to work, instantly the
+platinum wire <i>w</i> is rendered incandescent and can be fused, even
+if it be very thick.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_343.jpg" width="800" height="306" alt="Fig. 176." title="" />
+<span class="caption">Fig. 176.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_330" id="Page_330">[Pg 330]</a></span></p>
+
+<p>Instead of the platinum wire I now take an ordinary 50-volt
+16 <span class="smcap">c. p.</span> lamp. When I set the induction coil in operation the
+lamp filament is brought to high incandescence. It is, however,
+not necessary to use the insulated plate, for the lamp (<i>l</i>, Fig. 177)
+is rendered incandescent even if the plate <small>P<sub>1</sub></small> be disconnected.
+The secondary may also be connected to the primary as indicated
+by the dotted line in Fig. 177, to do away more or less with the
+electrostatic induction or to modify the action otherwise.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_344.jpg" width="800" height="463" alt="Fig. 177." title="" />
+<span class="caption">Fig. 177.</span>
+</div>
+
+<p>I may here call attention to a number of interesting observations
+with the lamp. First, I disconnect one of the terminals of
+the lamp from the secondary <small>S</small>. When the induction coil plays,
+a glow is noted which fills the whole bulb. This glow is due to
+electrostatic induction. It increases when the bulb is grasped
+with the hand, and the capacity of the experimenter's body thus
+added to the secondary circuit. The secondary, in effect, is equivalent
+to a metallic coating, which would be placed near the primary.
+If the secondary, or its equivalent, the coating, were placed
+symmetrically to the primary, the electrostatic induction would
+be nil under ordinary conditions, that is, when a primary return
+circuit is used, as both halves would neutralize each other. The
+secondary <i>is</i> in fact placed symmetrically to the primary, but the
+action of both halves of the latter, when only one of its ends is
+connected to the induction coil, is not exactly equal; hence electrostatic
+induction takes place, and hence the glow in the bulb. I
+can nearly equalize the action of both halves of the primary by
+connecting the other, free end of the same to the insulated plate,
+as in the preceding experiment. When the plate is connected,
+the glow disappears. With a smaller plate it would not entirely
+disappear and then it would contribute to the brightness of the
+filament when the secondary is closed, by warming the air in the
+bulb.<span class='pagenum'><a name="Page_331" id="Page_331">[Pg 331]</a></span></p>
+
+<div class="figcenter" style="width: 563px;">
+<img src="images/oi_345.jpg" width="563" height="800" alt="Fig. 178a, 178b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 178a. &nbsp; &nbsp; <span class="smcap">Fig.</span> 178b.</span>
+</div>
+
+<div class="figcenter" style="width: 519px;">
+<img src="images/oi_346.jpg" width="519" height="800" alt="Fig. 179a, 179b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 179a. &nbsp; &nbsp; <span class="smcap">Fig.</span> 179b.</span>
+</div>
+
+
+<p>To demonstrate another interesting feature, I have adjusted
+the coils used in a certain way. I first connect both the terminals
+of the lamp to the secondary, one end of the primary being connected
+to the terminal <small>T<sub>1</sub></small> of the induction coil and the other to
+the insulated plate <small>P<sub>1</sub></small> as before. When the current is turned on,
+the lamp glows brightly, as shown in Fig. 178<i>b</i>, in which <small>C</small> is a fine
+wire coil and <small>S</small> a coarse wire secondary wound upon it. If the
+insulated plate <small>P<sub>1</sub></small> is disconnected, leaving one of the ends <i>a</i> of the
+primary insulated, the filament becomes dark or generally it diminishes
+in brightness (Fig. 178<i>a</i>). Connecting again the plate <small>P<sub>1</sub></small>
+and raising the frequency of the current, I make the filament
+quite dark or barely red (Fig. 179<i>b</i>). Once more I will disconnect
+the plate. One will of course infer that when the plate is
+disconnected, the current through the primary will be weakened,
+that therefore the <span class="smcap">e. m. f.</span> will fall in the secondary <small>S</small>, and that
+the brightness of the lamp will diminish. This might be the
+case and the result can be secured by an easy adjustment of the
+<span class='pagenum'><a name="Page_332" id="Page_332">[Pg 332]</a></span>coils; also by varying the frequency and potential of the currents.
+But it is perhaps of greater interest to note, that the lamp
+increases in brightness when the plate is disconnected (Fig. 179<i>a</i>).
+In this case all the energy the primary receives is now sunk into
+it, like the charge of a battery in an ocean cable, but most of that
+energy is recovered through the secondary and used to light the
+lamp. The current traversing the primary is strongest at the end
+<i>b</i> which is connected to the terminal <small>T<sub>1</sub></small> of the induction coil, and
+diminishes in strength towards the remote end <i>a</i>. But the dynamic
+inductive effect exerted upon the secondary <small>S</small> is now greater
+than before, when the suspended plate was connected to the
+primary. These results might have been produced by a number
+of causes. For instance, the plate <small>P<sub>1</sub></small> being connected, the reaction
+from the coil <small>C</small> may be such as to diminish the potential at
+the terminal <small>T<sub>1</sub></small> of the induction coil, and therefore weaken the
+current through the primary of the coil <small>C</small>. Or the disconnecting<span class='pagenum'><a name="Page_333" id="Page_333">[Pg 333]</a></span>
+of the plate may diminish the capacity effect with relation to the
+primary of the latter coil to such an extent that the current
+through it is diminished, though the potential at the terminal <small>T<sub>1</sub></small>
+of the induction coil may be the same or even higher. Or the
+result might have been produced by the change of phase of the
+primary and secondary currents and consequent reaction. But
+the chief determining factor is the relation of the self-induction
+and capacity of coil <small>C</small> and plate <small>P<sub>1</sub></small> and the frequency of the currents.
+The greater brightness of the filament in Fig. 179<i>a</i>, is,
+however, in part due to the heating of the rarefied gas in the
+lamp by electrostatic induction, which, as before remarked, is
+greater when the suspended plate is disconnected.</p>
+
+<p>Still another feature of some interest I may here bring to your
+attention. When the insulated plate is disconnected and the secondary
+of the coil opened, by approaching a small object to the
+secondary, but very small sparks can be drawn from it, showing
+that the electrostatic induction is small in this case. But upon
+the secondary being closed upon itself or through the lamp, the
+filament glowing brightly, strong sparks are obtained from the
+secondary. The electrostatic induction is now much greater,
+because the closed secondary determines a greater flow of current
+through the primary and principally through that half of it which
+is connected to the induction coil. If now the bulb be grasped
+with the hand, the capacity of the secondary with reference to the
+primary is augmented by the experimenter's body and the luminosity
+of the filament is increased, the incandescence now being
+due partly to the flow of current through the filament and
+partly to the molecular bombardment of the rarefied gas in the
+bulb.</p>
+
+<p>The preceding experiments will have prepared one for the next
+following results of interest, obtained in the course of these investigations.
+Since I can pass a current through an insulated
+wire merely by connecting one of its ends to the source of electrical
+energy, since I can induce by it another current, magnetize
+an iron core, and, in short, perform all operations as though a return
+circuit were used, clearly I can also drive a motor by the aid
+of only one wire. On a former occasion I have described a simple
+form of motor comprising a single exciting coil, an iron core
+and disc. Fig. 180 illustrates a modified way of operating such
+an alternate current motor by currents induced in a transformer
+connected to one lead, and several other arrangements of circuits<span class='pagenum'><a name="Page_334" id="Page_334">[Pg 334]</a></span>
+for operating a certain class of alternating motors founded on the
+action of currents of differing phase. In view of the present
+state of the art it is thought sufficient to describe these arrangements
+in a few words only. The diagram, Fig. 180 II., shows
+a primary coil <small>P</small>, connected with one of its ends to the line <small>L</small> leading
+from a high tension transformer terminal <small>T<sub>1</sub></small>. In inductive
+relation to this primary <small>P</small> is a secondary <small>S</small> of coarse wire in the
+circuit of which is a coil <i>c</i>. The currents induced in the secondary
+energize the iron core <i>i</i>, which is preferably, but not necessarily,
+subdivided, and set the metal disc <i>d</i> in rotation. Such a
+motor <small>M<sub>2</sub></small> as diagramatically shown in Fig. 180 II., has been
+called a "magnetic lag motor," but this expression may be objected
+to by those who attribute the rotation of the disc to eddy
+currents circulating in minute paths when the core <i>i</i> is finally
+subdivided. In order to operate such a motor effectively on the
+plan indicated, the frequencies should not be too high, not more
+than four or five thousand, though the rotation is produced even
+with ten thousand per second, or more.</p>
+
+<p>In Fig. 180 I., a motor <small>M<sub>1</sub></small> having two energizing circuits, <small>A</small> and
+<small>B</small>, is diagrammatically indicated. The circuit <small>A</small> is connected to
+the line <small>L</small> and in series with it is a primary <small>P</small>, which may have its
+free end connected to an insulated plate <small>P<sub>1</sub></small>, such connection
+being indicated by the dotted lines. The other motor circuit <small>B</small>
+is connected to the secondary <small>S</small> which is in inductive relation to
+the primary <small>P</small>. When the transformer terminal <small>T<sub>1</sub></small> is alternately
+electrified, currents traverse the open line <small>L</small> and also circuit <small>A</small> and
+primary <small>P</small>. The currents through the latter induce secondary
+currents in the circuit <small>S</small>, which pass through the energizing coil
+<small>B</small> of the motor. The currents through the secondary <small>S</small> and those
+through the primary <small>P</small> differ in phase 90 degrees, or nearly so, and
+are capable of rotating an armature placed in inductive relation
+to the circuits <small>A</small> and <small>B</small>.</p>
+
+<p>In Fig. 180 III., a similar motor <small>M<sub>3</sub></small> with two energizing circuits
+<small>A<sub>1</sub></small> and <small>B<sub>1</sub></small> is illustrated. A primary <small>P</small>, connected with one
+of its ends to the line <small>L</small> has a secondary <small>S</small>, which is preferably
+wound for a tolerably high <span class="smcap">e. m. f.</span>, and to which the two energizing
+circuits of the motor are connected, one directly to the
+ends of the secondary and the other through a condenser <small>C</small>, by the
+action of which the currents traversing the circuit <small>A<sub>1</sub></small> and <small>B<sub>1</sub></small> are
+made to differ in phase.<span class='pagenum'><a name="Page_335" id="Page_335">[Pg 335]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_349-1.jpg" width="1024" height="281" alt="Fig. 180." title="" />
+<span class="caption">Fig. 180.</span>
+
+<img src="images/oi_349.jpg" width="1024" height="221" alt="Fig. 181, 182." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 181.</td><td class="caption">Fig. 182.</td></tr>
+</table>
+
+</div>
+
+<p>In Fig. 180 IV., still another arrangement is shown. In this
+case two primaries <small>P<sub>1</sub></small> and <small>P<sub>2</sub></small> are connected to the line <small>L</small>, one
+<span class='pagenum'><a name="Page_336" id="Page_336">[Pg 336]</a></span>
+through a condenser <small>C</small> of small capacity, and the other directly.
+The primaries are provided with secondaries <small>S<sub>1</sub></small> and <small>S<sub>2</sub></small> which are
+in series with the energizing circuits, <small>A<sub>2</sub></small> and <small>B<sub>2</sub></small> and a motor <small>M<sub>3</sub></small>,
+the condenser <small>C</small> again serving to produce the requisite difference
+in the phase of the currents traversing the motor circuits. As
+such phase motors with two or more circuits are now well known
+in the art, they have been here illustrated diagrammatically. No
+difficulty whatever is found in operating a motor in the manner
+indicated, or in similar ways; and although such experiments up
+to this day present only scientific interest, they may at a period
+not far distant, be carried out with practical objects in view.</p>
+
+<p>It is thought useful to devote here a few remarks to the subject
+of operating devices of all kinds by means of only one leading
+wire. It is quite obvious, that when high-frequency currents are
+made use of, ground connections are&mdash;at least when the <span class="smcap">e. m. f.</span>
+of the currents is great&mdash;better than a return wire. Such ground
+connections are objectionable with steady or low frequency currents
+on account of destructive chemical actions of the former
+and disturbing influences exerted by both on the neighboring circuits;
+but with high frequencies these actions practically do not
+exist. Still, even ground connections become superfluous when
+the <span class="smcap">e. m. f.</span> is very high, for soon a condition is reached, when the
+current may be passed more economically through open, than
+through closed, conductors. Remote as might seem an industrial
+application of such single wire transmission of energy to one not
+experienced in such lines of experiment, it will not seem so to
+anyone who for some time has carried on investigations of such
+nature. Indeed I cannot see why such a plan should not be
+practicable. Nor should it be thought that for carrying out such
+a plan currents of very high frequency are expressly required,
+for just as soon as potentials of say 30,000 volts are used, the
+single wire transmission may be effected with low frequencies,
+and experiments have been made by me from which these inferences
+are made.</p>
+
+<p>When the frequencies are very high it has been found in laboratory
+practice quite easy to regulate the effects in the manner
+shown in diagram Fig. 181. Here two primaries <small>P</small> and <small>P<sub>1</sub></small> are shown,
+each connected with one of its ends to the line <small>L</small> and with the
+other end to the condenser plates <small>C</small> and <small>C</small>, respectively. Near
+these are placed other condenser plates <small>C<sub>1</sub></small> and <small>C<sub>1</sub></small>, the former being
+connected to the line <small>L</small> and the latter to an insulated larger<span class='pagenum'><a name="Page_337" id="Page_337">[Pg 337]</a></span>
+plate <small>P<sub>2</sub></small>. On the primaries are wound secondaries <small>S</small> and <small>S<sub>1</sub></small>, of
+coarse wire, connected to the devices <i>d</i> and <i>l</i> respectively. By
+varying the distances of the condenser plates <small>C</small> and <small>C<sub>1</sub></small>, and <small>C</small> and
+<small>C<sub>1</sub></small> the currents through the secondaries <small>S</small> and <small>S<sub>1</sub></small> are varied in
+intensity. The curious feature is the great sensitiveness, the
+slightest change in the distance of the plates producing considerable
+variations in the intensity or strength of the currents. The
+sensitiveness may be rendered extreme by making the frequency
+such, that the primary itself, without any plate attached to its
+free end, satisfies, in conjunction with the closed secondary, the
+condition of resonance. In such condition an extremely small
+change in the capacity of the free terminal produces great variations.
+For instance, I have been able to adjust the conditions so
+that the mere approach of a person to the coil produces a considerable
+change in the brightness of the lamps attached to the
+secondary. Such observations and experiments possess, of course,
+at present, chiefly scientific interest, but they may soon become
+of practical importance.</p>
+
+<p>Very high frequencies are of course not practicable with
+motors on account of the necessity of employing iron cores. But
+one may use sudden discharges of low frequency and thus obtain
+certain advantages of high-frequency currents without rendering
+the iron core entirely incapable of following the changes and
+without entailing a very great expenditure of energy in the core.
+I have found it quite practicable to operate with such low frequency
+disruptive discharges of condensers, alternating-current
+motors. A certain class of such motors which I advanced a few
+years ago, which contain closed secondary circuits, will rotate
+quite vigorously when the discharges are directed through the
+exciting coils. One reason that such a motor operates so well
+with these discharges is that the difference of phase between the
+primary and secondary currents is 90 degrees, which is generally
+not the case with harmonically rising and falling currents of low
+frequency. It might not be without interest to show an experiment
+with a simple motor of this kind, inasmuch as it is commonly
+thought that disruptive discharges are unsuitable for such
+purposes. The motor is illustrated in Fig. 182. It comprises a
+rather large iron core <i>i</i> with slots on the top into which are embedded
+thick copper washers <i>c c</i>. In proximity to the core is
+a freely-movable metal disc <small>D</small>. The core is provided with a primary
+<span class='pagenum'><a name="Page_338" id="Page_338">[Pg 338]</a></span>exciting coil <small>C<sub>1</sub></small> the ends <i>a</i> and <i>b</i> of which are connected to
+the terminals of the secondary <small>S</small> of an ordinary transformer, the
+primary <small>P</small> of the latter being connected to an alternating distribution
+circuit or generator <small>G</small> of low or moderate frequency.
+The terminals of the secondary <small>S</small> are attached to a condenser <small>C</small>
+which discharges through an air gap <i>d d</i> which may be placed
+in series or shunt to the coil <small>C<sub>1</sub></small>. When the conditions are
+properly chosen the disc <small>D</small> rotates with considerable effort and the
+iron core <i>i</i> does not get very perceptibly hot. With currents from
+a high-frequency alternator, on the contrary, the core gets rapidly
+hot and the disc rotates with a much smaller effort. To perform
+the experiment properly it should be first ascertained that the
+disc <small>D</small> is not set in rotation when the discharge is not occurring
+at <i>d d</i>. It is preferable to use a large iron core and a condenser
+of large capacity so as to bring the superimposed quicker oscillation
+to a very low pitch or to do away with it entirely. By
+observing certain elementary rules I have also found it practicable
+to operate ordinary series or shunt direct-current motors
+with such disruptive discharges, and this can be done with or
+without a return wire.</p>
+
+
+<h5>IMPEDANCE PHENOMENA.</h5>
+
+<p>Among the various current phenomena observed, perhaps the
+most interesting are those of impedance presented by conductors
+to currents varying at a rapid rate. In my first paper before the
+American Institute of Electrical Engineers, I have described a
+few striking observations of this kind. Thus I showed that when
+such currents or sudden discharges are passed through a thick
+metal bar there may be points on the bar only a few inches apart,
+which have a sufficient potential difference between them to
+maintain at bright incandescence an ordinary filament lamp. I
+have also described the curious behavior of rarefied gas surrounding
+a conductor, due to such sudden rushes of current. These
+phenomena have since been more carefully studied and one or
+two novel experiments of this kind are deemed of sufficient interest
+to be described here.</p>
+
+<p>Referring to Fig. 183<i>a</i>, <small>B</small> and <small>B<sub>1</sub></small> are very stout copper bars
+connected at their lower ends to plates <small>C</small> and <small>C<sub>1</sub></small>, respectively, of a
+condenser, the opposite plates of the latter being connected to the
+terminals of the secondary <small>S</small> of a high-tension transformer, the
+primary <small>P</small> of which is supplied with alternating currents from an
+ordinary low-frequency dynamo <small>G</small> or distribution circuit. The<span class='pagenum'><a name="Page_339" id="Page_339">[Pg 339]</a></span>
+condenser discharges through an adjustable gap <i>d d</i> as usual. By
+establishing a rapid vibration it was found quite easy to perform
+the following curious experiment. The bars <small>B</small> and <small>B<sub>1</sub></small> were joined
+at the top by a low-voltage lamp <i>l</i><sub>3</sub>; a little lower was placed by
+means of clamps <i>c c</i>, a 50-volt lamp <i>l</i><sub>2</sub>; and still lower another 100-volt
+lamp <i>l</i><sub>1</sub>; and finally, at a certain distance below the latter
+lamp, an exhausted tube <small>T</small>. By carefully determining the positions
+of these devices it was found practicable to maintain them
+all at their proper illuminating power. Yet they were all connected
+in multiple arc to the two stout copper bars and required
+widely different pressures. This experiment requires of course
+some time for adjustment but is quite easily performed.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_353.jpg" width="600" height="767" alt="Fig. 183a, 183b and 183c." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 183a, 183b and 183c.</span>
+</div>
+
+<p>In Figs. 183<i>b</i> and 183<i>c</i>, two other experiments are illustrated
+which, unlike the previous experiment, do not require very careful
+<span class='pagenum'><a name="Page_340" id="Page_340">[Pg 340]</a></span>adjustments. In Fig. 183<i>b</i>, two lamps, <i>l</i><sub>1</sub> and <i>l</i><sub>2</sub>, the former a
+100-volt and the latter a 50-volt are placed in certain positions as
+indicated, the 100-volt lamp being below the 50-volt lamp. When
+the arc is playing at <i>d d</i> and the sudden discharges are passed
+through the bars <small>B B<sub>1</sub></small>, the 50-volt lamp will, as a rule, burn brightly,
+or at least this result is easily secured, while the 100-volt lamp
+will burn very low or remain quite dark, Fig. 183<i>b</i>. Now the
+bars <small>B B<sub>1</sub></small> may be joined at the top by a thick cross bar <small>B<sub>2</sub></small> and it
+is quite easy to maintain the 100-volt lamp at full candle-power
+while the 50-volt lamp remains dark, Fig. 183<i>c</i>. These results,
+as I have pointed out previously, should not be considered to be
+due exactly to frequency but rather to the time rate of change
+which may be great, even with low frequencies. A great many
+other results of the same kind, equally interesting, especially to
+those who are only used to manipulate steady currents, may be
+obtained and they afford precious clues in investigating the nature
+of electric currents.</p>
+
+<p>In the preceding experiments I have already had occasion to
+show some light phenomena and it would now be proper to study
+these in particular; but to make this investigation more complete
+I think it necessary to make first a few remarks on the
+subject of electrical resonance which has to be always observed
+in carrying out these experiments.</p>
+
+
+<h5>ON ELECTRICAL RESONANCE.</h5>
+
+<p>The effects of resonance are being more and more noted by engineers
+and are becoming of great importance in the practical operation
+of apparatus of all kinds with alternating currents. A few
+general remarks may therefore be made concerning these effects.
+It is clear, that if we succeed in employing the effects of resonance
+practically in the operation of electric devices the return wire will,
+as a matter of course, become unnecessary, for the electric vibration
+may be conveyed with one wire just as well as, and sometimes
+even better than, with two. The question first to answer is, then,
+whether pure resonance effects are producible. Theory and experiment
+both show that such is impossible in Nature, for as the
+oscillation becomes more and more vigorous, the losses in the vibrating
+bodies and environing media rapidly increase and necessarily
+check the vibration which otherwise would go on increasing
+forever. It is a fortunate circumstance that pure resonance is
+not producible, for if it were there is no telling what dangers
+might not lie in wait for the innocent experimenter. But to a<span class='pagenum'><a name="Page_341" id="Page_341">[Pg 341]</a></span>
+certain degree resonance is producible, the magnitude of the
+effects being limited by the imperfect conductivity and imperfect
+elasticity of the media or, generally stated, by frictional losses. The
+smaller these losses, the more striking are the effects. The same
+is the case in mechanical vibration. A stout steel bar may be set
+in vibration by drops of water falling upon it at proper intervals;
+and with glass, which is more perfectly elastic, the resonance
+effect is still more remarkable, for a goblet may be burst by
+singing into it a note of the proper pitch. The electrical resonance
+is the more perfectly attained, the smaller the resistance or the
+impedance of the conducting path and the more perfect the dielectric.
+In a Leyden jar discharging through a short stranded cable
+of thin wires these requirements are probably best fulfilled, and
+the resonance effects are therefore very prominent. Such is not
+the case with dynamo machines, transformers and their circuits,
+or with commercial apparatus in general in which the presence
+of iron cores complicates the action or renders it impossible.
+In regard to Leyden jars with which resonance effects are
+frequently demonstrated, I would say that the effects observed
+are often <i>attributed</i> but are seldom <i>due</i> to true resonance, for
+an error is quite easily made in this respect. This may be
+undoubtedly demonstrated by the following experiment. Take,
+for instance, two large insulated metallic plates or spheres which
+I shall designate <small>A</small> and <small>B</small>; place them at a certain small distance
+apart and charge them from a frictional or influence
+machine to a potential so high that just a slight increase of the
+difference of potential between them will cause the small air or
+insulating space to break down. This is easily reached by making
+a few preliminary trials. If now another plate&mdash;fastened on
+an insulating handle and connected by a wire to one of the terminals
+of a high tension secondary of an induction coil, which
+is maintained in action by an alternator (preferably high frequency)&mdash;is
+approached to one of the charged bodies <small>A</small> or <small>B</small>, so as
+to be nearer to either one of them, the discharge will invariably
+occur between them; at least it will, if the potential of the coil
+in connection with the plate is sufficiently high. But the explanation
+of this will soon be found in the fact that the approached
+plate acts inductively upon the bodies <small>A</small> and <small>B</small> and causes a spark
+to pass between them. When this spark occurs, the charges which
+were previously imparted to these bodies from the influence machine,
+must needs be lost, since the bodies are brought in electri<span class='pagenum'><a name="Page_342" id="Page_342">[Pg 342]</a></span>cal
+connection through the arc formed. Now this arc is formed
+whether there be resonance or not. But even if the spark would
+not be produced, still there is an alternating <span class="smcap">e. m. f.</span> set up between
+the bodies when the plate is brought near one of them; therefore
+the approach of the plate, if it <i>does</i> not always actually, will, at any
+rate, <i>tend</i> to break down the air space by inductive action. Instead
+of the spheres or plates <small>A</small> and <small>B</small> we may take the coatings of a Leyden
+jar with the same result, and in place of the machine,&mdash;which
+is a high frequency alternator preferably, because it is more suitable
+for the experiment and also for the argument,&mdash;we may take
+another Leyden jar or battery of jars. When such jars are discharging
+through a circuit of low resistance the same is traversed
+by currents of very high frequency. The plate may now be connected
+to one of the coatings of the second jar, and when it is
+brought near to the first jar just previously charged to a high
+potential from an influence machine, the result is the same as before,
+and the first jar will discharge through a small air space
+upon the second being caused to discharge. But both jars and
+their circuits need not be tuned any closer than a basso profundo
+is to the note produced by a mosquito, as small sparks will be produced
+through the air space, or at least the latter will be considerably
+more strained owing to the setting up of an alternating
+<span class="smcap">e. m. f.</span> by induction, which takes place when one of the jars begins
+to discharge. Again another error of a similar nature is quite
+easily made. If the circuits of the two jars are run parallel and
+close together, and the experiment has been performed of discharging
+one by the other, and now a coil of wire be added to one
+of the circuits whereupon the experiment does not succeed, the
+conclusion that this is due to the fact that the circuits are now
+not tuned, would be far from being safe. For the two circuits
+act as condenser coatings and the addition of the coil to one of
+them is equivalent to bridging them, at the point where the coil
+is placed, by a small condenser, and the effect of the latter might
+be to prevent the spark from jumping through the discharge space
+by diminishing the alternating <span class="smcap">e. m. f.</span> acting across the same.
+All these remarks, and many more which might be added but for
+fear of wandering too far from the subject, are made with the
+pardonable intention of cautioning the unsuspecting student, who
+might gain an entirely unwarranted opinion of his skill at seeing
+every experiment succeed; but they are in no way thrust upon
+the experienced as novel observations.<span class='pagenum'><a name="Page_343" id="Page_343">[Pg 343]</a></span></p>
+
+<p>In order to make reliable observations of electric resonance
+effects it is very desirable, if not necessary, to employ an alternator
+giving currents which rise and fall harmonically, as in
+working with make and break currents the observations are not
+always trustworthy, since many phenomena, which depend on
+the rate of change, may be produced with widely different frequencies.
+Even when making such observations with an alternator
+one is apt to be mistaken. When a circuit is connected to an
+alternator there are an indefinite number of values for capacity and
+self-induction which, in conjunction, will satisfy the condition of
+resonance. So there are in mechanics an infinite number of tuning
+forks which will respond to a note of a certain pitch, or loaded
+springs which have a definite period of vibration. But the resonance
+will be most perfectly attained in that case in which the motion
+is effected with the greatest freedom. Now in mechanics,
+considering the vibration in the common medium&mdash;that is, air&mdash;it
+is of comparatively little importance whether one tuning fork be
+somewhat larger than another, because the losses in the air are
+not very considerable. One may, of course, enclose a tuning fork
+in an exhausted vessel and by thus reducing the air resistance to
+a minimum obtain better resonant action. Still the difference
+would not be very great. But it would make a great difference if
+the tuning fork were immersed in mercury. In the electrical
+vibration it is of enormous importance to arrange the conditions
+so that the vibration is effected with the greatest freedom. The
+magnitude of the resonance effect depends, under otherwise equal
+conditions, on the quantity of electricity set in motion or on the
+strength of the current driven through the circuit. But the circuit
+opposes the passage of the currents by reason of its impedance
+and therefore, to secure the best action it is necessary to reduce
+the impedance to a minimum. It is impossible to overcome
+it entirely, but merely in part, for the ohmic resistance cannot be
+overcome. But when the frequency of the impulses is very great,
+the flow of the current is practically determined by self-induction.
+Now self-induction can be overcome by combining it with capacity.
+If the relation between these is such, that at the frequency
+used they annul each other, that is, have such values as to
+satisfy the condition of resonance, and the greatest quantity of
+electricity is made to flow through the external circuit, then the
+best result is obtained. It is simpler and safer to join the condenser
+in series with the self-induction. It is clear that in such<span class='pagenum'><a name="Page_344" id="Page_344">[Pg 344]</a></span>
+combinations there will be, for a given frequency, and considering
+only the fundamental vibration, values which will give the best
+result, with the condenser in shunt to the self-induction coil; of
+course more such values than with the condenser in series. But
+practical conditions determine the selection. In the latter case
+in performing the experiments one may take a small self-induction
+and a large capacity or a small capacity and a large self-induction,
+but the latter is preferable, because it is inconvenient to adjust
+a large capacity by small steps. By taking a coil with a very
+large self-induction the critical capacity is reduced to a very small
+value, and the capacity of the coil itself may be sufficient. It is
+easy, especially by observing certain artifices, to wind a coil
+through which the impedance will be reduced to the value of the
+ohmic resistance only; and for any coil there is, of course, a frequency
+at which the maximum current will be made to pass
+through the coil. The observation of the relation between self-induction,
+capacity and frequency is becoming important in the
+operation of alternate current apparatus, such as transformers or
+motors, because by a judicious determination of the elements the
+employment of an expensive condenser becomes unnecessary.
+Thus it is possible to pass through the coils of an alternating
+current motor under the normal working conditions the required
+current with a low <span class="smcap">e. m. f.</span> and do away entirely with the false
+current, and the larger the motor, the easier such a plan becomes
+practicable; but it is necessary for this to employ currents of very
+high potential or high frequency.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_358.jpg" width="800" height="267" alt="Fig. 184." title="" />
+<span class="caption">Fig. 184.</span>
+</div>
+
+
+<p>In Fig. 184 I. is shown a plan which has been followed in the
+study of the resonance effects by means of a high frequency alternator.
+<small>C<sub>1</sub></small> is a coil of many turns, which is divided into small
+separate sections for the purpose of adjustment. The final adjustment
+was made sometimes with a few thin iron wires (though
+this is not always advisable) or with a closed secondary. The coil<span class='pagenum'><a name="Page_345" id="Page_345">[Pg 345]</a></span>
+<small>C<sub>1</sub></small> is connected with one of its ends to the line <small>L</small> from the alternator
+<small>G</small> and with the other end to one of the plates <i>c</i> of a condenser
+<i>c c</i><sub>1</sub>, the plate (<i>c</i><sub>1</sub>) of the latter being connected to a much
+larger plate <small>P<sub>1</sub></small>. In this manner both capacity and self-induction
+were adjusted to suit the dynamo frequency.</p>
+
+<p>As regards the rise of potential through resonant action, of
+course, theoretically, it may amount to anything since it depends
+on self-induction and resistance and since these may have any
+value. But in practice one is limited in the selection of these
+values and besides these, there are other limiting causes. One
+may start with, say, 1,000 volts and raise the <span class="smcap">e. m. f.</span> to 50 times
+that value, but one cannot start with 100,000 and raise it to ten
+times that value because of the losses in the media which are
+great, especially if the frequency is high. It should be possible
+to start with, for instance, two volts from a high or low frequency
+circuit of a dynamo and raise the <span class="smcap">e. m. f.</span> to many hundred
+times that value. Thus coils of the proper dimensions
+might be connected each with only one of its ends to the
+mains from a machine of low <span class="smcap">e. m. f.</span>, and though the circuit of
+the machine would not be closed in the ordinary acceptance of the
+term, yet the machine might be burned out if a proper resonance
+effect would be obtained. I have not been able to produce, nor
+have I observed with currents from a dynamo machine, such
+great rises of potential. It is possible, if not probable, that with
+currents obtained from apparatus containing iron the disturbing
+influence of the latter is the cause that these theoretical possibilities
+cannot be realized. But if such is the case I attribute
+it solely to the hysteresis and Foucault current losses in the core.
+Generally it was necessary to transform upward, when the <span class="smcap">e. m. f.</span>
+was very low, and usually an ordinary form of induction coil
+was employed, but sometimes the arrangement illustrated in Fig.
+184 II., has been found to be convenient. In this case a coil <small>C</small> is
+made in a great many sections, a few of these being used as a
+primary. In this manner both primary and secondary are adjustable.
+One end of the coil is connected to the line <small>L<sub>1</sub></small> from
+the alternator, and the other line <small>L</small> is connected to the intermediate
+point of the coil. Such a coil with adjustable primary and
+secondary will be found also convenient in experiments with the
+disruptive discharge. When true resonance is obtained the top
+of the wave must of course be on the free end of the coil as, for
+instance, at the terminal of the phosphorescence bulb <small>B</small>. This is<span class='pagenum'><a name="Page_346" id="Page_346">[Pg 346]</a></span>
+easily recognized by observing the potential of a point on the
+wire <i>w</i> near to the coil.</p>
+
+<p>In connection with resonance effects and the problem of transmission
+of energy over a single conductor which was previously
+considered, I would say a few words on a subject which constantly
+fills my thoughts and which concerns the welfare of all. I mean
+the transmission of intelligible signals or perhaps even power to
+any distance without the use of wires. I am becoming daily
+more convinced of the practicability of the scheme; and though
+I know full well that the great majority of scientific men will
+not believe that such results can be practically and immediately
+realized, yet I think that all consider the developments in recent
+years by a number of workers to have been such as to encourage
+thought and experiment in this direction. My conviction has
+grown so strong, that I no longer look upon this plan of energy
+or intelligence transmission as a mere theoretical possibility, but as
+a serious problem in electrical engineering, which must be carried
+out some day. The idea of transmitting intelligence without
+wires is the natural outcome of the most recent results of electrical
+investigations. Some enthusiasts have expressed their belief
+that telephony to any distance by induction through the air
+is possible. I cannot stretch my imagination so far, but I do
+firmly believe that it is practicable to disturb by means of powerful
+machines the electrostatic condition of the earth and thus
+transmit intelligible signals and perhaps power. In fact, what is
+there against the carrying out of such a scheme? We now know
+that electric vibration may be transmitted through a single conductor.
+Why then not try to avail ourselves of the earth for
+this purpose? We need not be frightened by the idea of distance.
+To the weary wanderer counting the mile-posts the earth
+may appear very large, but to that happiest of all men, the astronomer,
+who gazes at the heavens and by their standard judges
+the magnitude of our globe, it appears very small. And so I
+think it must seem to the electrician, for when he considers the
+speed with which an electric disturbance is propagated through
+the earth all his ideas of distance must completely vanish.</p>
+
+<p>A point of great importance would be first to know what is the
+capacity of the earth? and what charge does it contain if electrified?
+Though we have no positive evidence of a charged body
+existing in space without other oppositely electrified bodies being
+near, there is a fair probability that the earth is such a body, for<span class='pagenum'><a name="Page_347" id="Page_347">[Pg 347]</a></span>
+by whatever process it was separated from other bodies&mdash;and this
+is the accepted view of its origin&mdash;it must have retained a charge,
+as occurs in all processes of mechanical separation. If it be a
+charged body insulated in space its capacity should be extremely
+small, less than one-thousandth of a farad. But the upper strata
+of the air are conducting, and so, perhaps, is the medium in free
+space beyond the atmosphere, and these may contain an opposite
+charge. Then the capacity might be incomparably greater. In
+any case it is of the greatest importance to get an idea of what
+quantity of electricity the earth contains. It is difficult to say
+whether we shall ever acquire this necessary knowledge, but there
+is hope that we may, and that is, by means of electrical resonance.
+If ever we can ascertain at what period the earth's charge, when
+disturbed, oscillates with respect to an oppositely electrified system
+or known circuit, we shall know a fact possibly of the greatest
+importance to the welfare of the human race. I propose to seek
+for the period by means of an electrical oscillator, or a source of
+alternating electric currents. One of the terminals of the source
+would be connected to earth as, for instance, to the city water
+mains, the other to an insulated body of large surface. It is possible
+that the outer conducting air strata, or free space, contain
+an opposite charge and that, together with the earth, they form a
+condenser of very large capacity. In such case the period of
+vibration may be very low and an alternating dynamo machine
+might serve for the purpose of the experiment. I would then
+transform the current to a potential as high as it would be found
+possible and connect the ends of the high tension secondary to the
+ground and to the insulated body. By varying the frequency of the
+currents and carefully observing the potential of the insulated body
+and watching for the disturbance at various neighboring points of
+the earth's surface resonance might be detected. Should, as the
+majority of scientific men in all probability believe, the period be
+extremely small, then a dynamo machine would not do and a
+proper electrical oscillator would have to be produced and perhaps
+it might not be possible to obtain such rapid vibrations. But
+whether this be possible or not, and whether the earth contains a
+charge or not, and whatever may be its period of vibration, it certainly
+is possible&mdash;for of this we have daily evidence&mdash;to produce
+some electrical disturbance sufficiently powerful to be perceptible
+by suitable instruments at any point of the earth's
+surface.<span class='pagenum'><a name="Page_348" id="Page_348">[Pg 348]</a></span></p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_362.jpg" width="800" height="94" alt="Fig. 185." title="" />
+<span class="caption">Fig. 185.</span>
+</div>
+
+
+<p>Assume that a source of alternating current <small>S</small> be connected, as
+in Fig. 185, with one of its terminals to earth (conveniently to the
+water mains) and with the other to a body of large surface <small>P</small>.
+When the electric oscillation is set up there will be
+a movement of electricity in and out of <small>P</small>, and alternating
+currents will pass through the earth, converging
+to, or diverging from, the point <small>C</small> where
+the ground connection is made. In this manner
+neighboring points on the earth's surface within a
+certain radius will be disturbed. But the disturbance
+will diminish with the distance, and the distance
+at which the effect will still be perceptible
+will depend on the quantity of electricity set in
+motion. Since the body <small>P</small> is insulated, in order to
+displace a considerable quantity, the potential of
+the source must be excessive, since there would be
+limitations as to the surface of <small>P</small>. The conditions
+might be adjusted so that the generator or source
+<small>S</small> will set up the same electrical movement as
+though its circuit were closed. Thus it is certainly
+practicable to impress an electric vibration at least
+of a certain low period upon the earth by means of
+proper machinery. At what distance such a vibration
+might be made perceptible can only be conjectured.
+I have on another occasion considered the
+question how the earth might behave to electric
+disturbances. There is no doubt that, since in such
+an experiment the electrical density at the surface
+could be but extremely small considering the size
+of the earth, the air would not act as a very disturbing
+factor, and there would be not much energy
+lost through the action of the air, which would be
+the case if the density were great. Theoretically,
+then, it could not require a great amount of energy
+to produce a disturbance perceptible at great distance,
+or even all over the surface of the globe.
+Now, it is quite certain that at any point within a
+certain radius of the source <small>S</small> a properly adjusted
+self-induction and capacity device can be set in action
+by resonance. But not only can this be done, but another source
+<span class='pagenum'><a name="Page_349" id="Page_349">[Pg 349]</a></span>
+<small>S<sub>1</sub></small>, Fig. 185, similar to <small>S</small>, or any number of such sources, can be set
+to work in synchronism with the latter, and the vibration thus
+intensified and spread over a large area, or a flow of electricity
+produced to or from the source <small>S<sub>1</sub></small> if the same be of
+opposite phase to the source <small>S</small>. I think that beyond doubt
+it is possible to operate electrical devices in a city through
+the ground or pipe system by resonance from an electrical
+oscillator located at a central point. But the practical solution
+of this problem would be of incomparably smaller benefit to man
+than the realization of the scheme of transmitting intelligence, or
+perhaps power, to any distance through the earth or environing
+medium. If this is at all possible, distance does not mean anything.
+Proper apparatus must first be produced by means of
+which the problem can be attacked and I have devoted much
+thought to this subject. I am firmly convinced that it can be
+done and hope that we shall live to see it done.</p>
+
+
+<h5>ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS
+OF HIGH POTENTIAL AND GENERAL REMARKS RELATING
+TO THE SUBJECT.</h5>
+
+<p>Returning now to the light effects which it has been the chief
+object to investigate, it is thought proper to divide these effects
+into four classes: 1. Incandescence of a solid. 2. Phosphorescence.
+3. Incandescence or phosphorescence of a rarefied gas; and
+4. Luminosity produced in a gas at ordinary pressure. The first
+question is: How are these luminous effects produced? In order
+to answer this question as satisfactorily as I am able to do in the
+light of accepted views and with the experience acquired, and to
+add some interest to this demonstration, I shall dwell here upon
+a feature which I consider of great importance, inasmuch as it
+promises, besides, to throw a better light upon the nature of most
+of the phenomena produced by high-frequency electric currents.
+I have on other occasions pointed out the great importance of the
+presence of the rarefied gas, or atomic medium in general, around
+the conductor through which alternate currents of high frequency
+are passed, as regards the heating of the conductor by the currents.
+My experiments, described some time ago, have shown
+that, the higher the frequency and potential difference of the currents,
+the more important becomes the rarefied gas in which the
+conductor is immersed, as a factor of the heating. The potential
+difference, however, is, as I then pointed out, a more im<span class='pagenum'><a name="Page_350" id="Page_350">[Pg 350]</a></span>portant
+element than the frequency. When both of these are
+sufficiently high, the heating may be almost entirely due to the
+presence of the rarefied gas. The experiments to follow will
+show the importance of the rarefied gas, or, generally, of gas at ordinary
+or other pressure as regards the incandescence or other
+luminous effects produced by currents of this kind.</p>
+
+<p>I take two ordinary 50-volt 16 <span class="smcap">c. p.</span> lamps which are in every
+respect alike, with the exception, that one has been opened at the
+top and the air has filled the bulb, while the other is at the ordinary
+degree of exhaustion of commercial lamps. When I attach
+the lamp which is exhausted to the terminal of the secondary of
+the coil, which I have already used, as in experiments illustrated
+in Fig. 179<i>a</i> for instance, and turn on the current, the filament, as
+you have before seen, comes to high incandescence. When I
+attach the second lamp, which is filled with air, instead of the
+former, the filament still glows, but much less brightly. This
+experiment illustrates only in part the truth of the statements
+before made. The importance of the filament's being immersed
+in rarefied gas is plainly noticeable but not to such a degree as
+might be desirable. The reason is that the secondary of this coil is
+wound for low tension, having only 150 turns, and the potential
+difference at the terminals of the lamp is therefore small. Were
+I to take another coil with many more turns in the secondary,
+the effect would be increased, since it depends partially on the
+potential difference, as before remarked. But since the effect
+likewise depends on the frequency, it maybe properly stated that
+it depends on the time rate of the variation of the potential difference.
+The greater this variation, the more important becomes
+the gas as an element of heating. I can produce a much greater
+rate of variation in another way, which, besides, has the advantage
+of doing away with the objections, which might be made in
+the experiment just shown, even if both the lamps were connected
+in series or multiple arc to the coil, namely, that in consequence
+of the reactions existing between the primary and
+secondary coil the conclusions are rendered uncertain. This result
+I secure by charging, from an ordinary transformer which is
+fed from the alternating current supply station, a battery of condensers,
+and discharging the latter directly through a circuit of
+small self-induction, as before illustrated in Figs. 183<i>a</i>, 183<i>b</i>,
+and 183<i>c</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_365.jpg" width="800" height="308" alt="Fig. 186a, 186b, 186c." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 186a.</td><td class="caption1"><span class="smcap">Fig.</span> 186b.</td><td class="caption1"><span class="smcap">Fig.</span> 186c.</td></tr>
+</table>
+</div>
+
+<p>In Figs. 186<i>a</i>, 186<i>b</i> and 186<i>c</i>, the heavy copper bars <small>B B<sub>1</sub></small>, are
+<span class='pagenum'><a name="Page_351" id="Page_351">[Pg 351]</a></span>connected to the opposite coatings of a battery of condensers,
+or generally in such way, that the high frequency or sudden
+discharges are made to traverse them. I connect first an
+ordinary 50-volt incandescent lamp to the bars by means of
+the clamps <i>c c</i>. The discharges being passed through the lamp,
+the filament is rendered incandescent, though the current
+through it is very small, and would not be nearly sufficient to
+produce a visible effect under the conditions of ordinary use of
+the lamp. Instead of this I now attach to the bars another
+lamp exactly like the first, but with the seal broken off, the bulb
+being therefore filled with air at ordinary pressure. When the
+discharges are directed through the filament, as before, it does
+not become incandescent. But the result might still be attributed
+to one of the many possible reactions. I therefore connect
+both the lamps in multiple arc as illustrated in Fig. 186<i>a</i>. Passing
+the discharges through both the lamps, again the filament in the
+exhausted lamp <i>l</i> glows very brightly while that in the non-exhausted
+lamp <i>l</i><sub>1</sub> remains dark, as previously. But it should not
+be thought that the latter lamp is taking only a small fraction of
+the energy supplied to both the lamps; on the contrary, it may
+consume a considerable portion of the energy and it may become
+even hotter than the one which burns brightly. In this experiment
+the potential difference at the terminals of the lamps varies
+in sign theoretically three to four million times a second. The
+ends of the filaments are correspondingly electrified, and the gas
+in the bulbs is violently agitated and a large portion of the supplied
+energy is thus converted into heat. In the non-exhausted
+bulb, there being a few million times more gas molecules than in
+the exhausted one, the bombardment, which is most violent at
+the ends of the filament, in the neck of the bulb, consumes a<span class='pagenum'><a name="Page_352" id="Page_352">[Pg 352]</a></span>
+large portion of the energy without producing any visible effect.
+The reason is that, there being many molecules, the bombardment
+is quantitatively considerable, but the individual impacts are
+not very violent, as the speeds of the molecules are comparatively
+small owing to the small free path. In the exhausted bulb, on
+the contrary, the speeds are very great, and the individual impacts
+are violent and therefore better adapted to produce a visible
+effect. Besides, the convection of heat is greater in the former
+bulb. In both the bulbs the current traversing the filaments is
+very small, incomparably smaller than that which they require on
+an ordinary low-frequency circuit. The potential difference,
+however, at the ends of the filaments is very great and might be
+possibly 20,000 volts or more, if the filaments were straight and
+their ends far apart. In the ordinary lamp a spark generally occurs
+between the ends of the filament or between the platinum
+wires outside, before such a difference of potential can be
+reached.</p>
+
+<p>It might be objected that in the experiment before shown the
+lamps, being in multiple arc, the exhausted lamp might take a
+much larger current and that the effect observed might not be
+exactly attributable to the action of the gas in the bulbs. Such
+objections will lose much weight if I connect the lamps in series,
+with the same result. When this is done and the discharges are
+directed through the filaments, it is again noted that the filament
+in the non-exhausted bulb <i>l</i><sub>1</sub>, remains dark, while that in the
+exhausted one (<i>l</i>) glows even more intensely than under its
+normal conditions of working, Fig. 186<i>b</i>. According to general
+ideas the current through the filaments should now be the same,
+were it not modified by the presence of the gas around the
+filaments.</p>
+
+<p>At this juncture I may point out another interesting feature,
+which illustrates the effect of the rate of change of potential
+of the currents. I will leave the two lamps connected in series
+to the bars <small>B B<sub>1</sub></small>, as in the previous experiment, Fig. 186<i>b</i>, but will
+presently reduce considerably the frequency of the currents,
+which was excessive in the experiment just before shown. This
+I may do by inserting a self-induction coil in the path of the discharges,
+or by augmenting the capacity of the condensers. When
+I now pass these low-frequency discharges through the lamps,
+the exhausted lamp <i>l</i> again is as bright as before, but it is noted
+<span class='pagenum'><a name="Page_353" id="Page_353">[Pg 353]</a></span>also that the non-exhausted lamp <i>l</i><sub>1</sub> glows, though not quite
+as intensely as the other. Reducing the current through the
+lamps, I may bring the filament in the latter lamp to redness, and,
+though the filament in the exhausted lamp <i>l</i> is bright, Fig. 186<i>c</i>,
+the degree of its incandescence is much smaller than in Fig. 186<i>b</i>,
+when the currents were of a much higher frequency.</p>
+
+<p>In these experiments the gas acts in two opposite ways in determining
+the degree of the incandescence of the filaments, that
+is, by convection and bombardment. The higher the frequency and
+potential of the currents, the more important becomes the bombardment.
+The convection on the contrary should be the smaller,
+the higher the frequency. When the currents are steady there is
+practically no bombardment, and convection may therefore with
+such currents also considerably modify the degree of incandescence
+and produce results similar to those just before shown. Thus, if
+two lamps exactly alike, one exhausted and one not exhausted,
+are connected in multiple arc or series to a direct-current machine,
+the filament in the non-exhausted lamp will require a considerably
+greater current to be rendered incandescent. This result is
+entirely due to convection, and the effect is the more prominent
+the thinner the filament. Professor Ayrton and Mr. Kilgour
+some time ago published quantitative results concerning the
+thermal emissivity by radiation and convection in which the effect
+with thin wires was clearly shown. This effect may be strikingly
+illustrated by preparing a number of small, short, glass tubes,
+each containing through its axis the thinnest obtainable platinum
+wire. If these tubes be highly exhausted, a number of them
+may be connected in multiple arc to a direct-current machine and
+all of the wires may be kept at incandescence with a smaller current
+than that required to render incandescent a single one of the
+wires if the tube be not exhausted. Could the tubes be so highly
+exhausted that convection would be nil, then the relative amounts
+of heat given off by convection and radiation could be determined
+without the difficulties attending thermal quantitative
+measurements. If a source of electric impulses of high frequency
+and very high potential is employed, a still greater number of
+the tubes may be taken and the wires rendered incandescent by a
+current not capable of warming perceptibly a wire of the same
+size immersed in air at ordinary pressure, and conveying the
+energy to all of them.</p>
+
+<p>I may here describe a result which is still more interesting,
+and to which I have been led by the observation of these phe<span class='pagenum'><a name="Page_354" id="Page_354">[Pg 354]</a></span>nomena.
+I noted that small differences in the density of the air
+produced a considerable difference in the degree of incandescence
+of the wires, and I thought that, since in a tube, through which
+a luminous discharge is passed, the gas is generally not of uniform
+density, a very thin wire contained in the tube might be
+rendered incandescent at certain places of smaller density of the
+gas, while it would remain dark at the places of greater density,
+where the convection would be greater and the bombardment less
+intense. Accordingly a tube <i>t</i> was prepared, as illustrated in Fig.
+187, which contained through the middle a very fine platinum wire
+<i>w</i>. The tube was exhausted to a moderate degree and it was found
+that when it was attached to the terminal of a high-frequency coil
+the platinum wire <i>w</i> would indeed, become incandescent in patches,
+as illustrated in Fig. 187. Later a number of these tubes with one
+or more wires were prepared, each showing this result. The effect
+was best noted when the striated discharge occurred in the
+tube, but was also produced when the stri&aelig; were not visible,
+showing that, even then, the gas in the tube was not of uniform
+density. The position of the stri&aelig; was generally such, that the
+rarefactions corresponded to the places of incandescence or greater
+brightness on the wire <i>w</i>. But in a few instances it was noted, that
+the bright spots on the wire were covered by the dense parts of
+the striated discharge as indicated by <i>l</i> in Fig. 187, though the effect
+was barely perceptible. This was explained in a plausible way
+by assuming that the convection was not widely different in the
+dense and rarefied places, and that the bombardment was greater
+on the dense places of the striated discharge. It is, in fact, often
+observed in bulbs, that under certain conditions a thin wire is
+brought to higher incandescence when the air is not too highly
+rarefied. This is the case when the potential of the coil is not
+high enough for the vacuum, but the result may be attributed to
+many different causes. In all cases this curious phenomenon of
+incandescence disappears when the tube, or rather the wire,
+acquires throughout a uniform temperature.</p>
+
+<div class="figcenter" style="width: 713px;">
+<img src="images/oi_369.jpg" width="713" height="600" alt="Fig. 187, 188." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 187.</td><td class="caption">Fig. 188.</td></tr>
+</table>
+</div>
+
+<p>Disregarding now the modifying effect of convection there are
+then two distinct causes which determine the incandescence of a
+wire or filament with varying currents, that is, conduction current
+and bombardment. With steady currents we have to deal
+only with the former of these two causes, and the heating effect
+is a minimum, since the resistance is least to steady flow. When
+the current is a varying one the resistance is greater, and hence
+<span class='pagenum'><a name="Page_355" id="Page_355">[Pg 355]</a></span>the heating effect is increased. Thus if the rate of change of
+the current is very great, the resistance may increase to such
+an extent that the filament is brought to incandescence with inappreciable
+currents, and we are able to take a short and thick
+block of carbon or other material and bring it to bright incandescence
+with a current incomparably smaller than that required
+to bring to the same degree of incandescence an ordinary thin
+lamp filament with a steady or low frequency current. This result
+is important, and illustrates how rapidly our views on these subjects
+are changing, and how quickly our field of knowledge is extending.
+In the art of incandescent lighting, to view this result
+in one aspect only, it has been commonly considered as an essential
+requirement for practical success, that the lamp filament
+should be thin and of high resistance. But now we know that
+the resistance of the filament to the steady flow does not mean
+anything; the filament might as well be short and thick; for if it
+be immersed in rarefied gas it will become incandescent by the
+passage of a small current. It all depends on the frequency and
+potential of the currents. We may conclude from this, that it
+<span class='pagenum'><a name="Page_356" id="Page_356">[Pg 356]</a></span>would be of advantage, so far as the lamp is considered, to employ
+high frequencies for lighting, as they allow the use of short
+and thick filaments and smaller currents.</p>
+
+<p>If a wire or filament be immersed in a homogeneous medium, all
+the heating is due to true conduction current, but if it be enclosed
+in an exhausted vessel the conditions are entirely different. Here
+the gas begins to act and the heating effect of the conduction current,
+as is shown in many experiments, may be very small compared
+with that of the bombardment. This is especially the case if
+the circuit is not closed and the potentials are of course very high.
+Suppose that a fine filament enclosed in an exhausted vessel be
+connected with one of its ends to the terminal of a high tension
+coil and with its other end to a large insulated plate. Though
+the circuit is not closed, the filament, as I have before shown, is
+brought to incandescence. If the frequency and potential be
+comparatively low, the filament is heated by the current passing
+<i>through it</i>. If the frequency and potential, and principally the
+latter, be increased, the insulated plate need be but very small, or
+may be done away with entirely; still the filament will become
+incandescent, practically all the heating being then due to the bombardment.
+A practical way of combining both the effects of
+conduction currents and bombardment is illustrated in Fig. 188,
+in which an ordinary lamp is shown provided with a very thin
+filament which has one of the ends of the latter connected to a
+shade serving the purpose of the insulated plate, and the other
+end to the terminal of a high tension source. It should not be
+thought that only rarefied gas is an important factor in the heating
+of a conductor by varying currents, but gas at ordinary pressure
+may become important, if the potential difference and frequency
+of the currents is excessive. On this subject I have already
+stated, that when a conductor is fused by a stroke of
+lightning, the current through it may be exceedingly small, not
+even sufficient to heat the conductor perceptibly, were the latter
+immersed in a homogeneous medium.</p>
+
+<p>From the preceding it is clear that when a conductor of high
+resistance is connected to the terminals of a source of high frequency
+currents of high potential, there may occur considerable
+dissipation of energy, principally at the ends of the conductor, in
+consequence of the action of the gas surrounding the conductor.
+Owing to this, the current through a section of the conductor at
+a point midway between its ends may be much smaller than
+<span class='pagenum'><a name="Page_357" id="Page_357">[Pg 357]</a></span>through a section near the ends. Furthermore, the current passes
+principally through the outer portions of the conductor, but this
+effect is to be distinguished from the skin effect as ordinarily interpreted,
+for the latter would, or should, occur also in a continuous
+incompressible medium. If a great many incandescent lamps
+are connected in series to a source of such currents, the lamps at
+the ends may burn brightly, whereas those in the middle may remain
+entirely dark. This is due principally to bombardment, as
+before stated. But even if the currents be steady, provided the
+difference of potential is very great, the lamps at the end will
+burn more brightly than those in the middle. In such case there
+is no rhythmical bombardment, and the result is produced entirely
+by leakage. This leakage or dissipation into space when
+the tension is high, is considerable when incandescent lamps are
+used, and still more considerable with arcs, for the latter act like
+flames. Generally, of course, the dissipation is much smaller
+with steady, than with varying, currents.</p>
+
+<p>I have contrived an experiment which illustrates in an interesting
+manner the effect of lateral diffusion. If a very long tube
+is attached to the terminal of a high frequency coil, the luminosity
+is greatest near the terminal and falls off gradually towards
+the remote end. This is more marked if the tube is narrow.</p>
+
+<p>A small tube about one-half inch in diameter and twelve
+inches long (Fig. 189), has one of its ends drawn out into a fine
+fibre <i>f</i> nearly three feet long. The tube is placed in a brass socket
+<small>T</small> which can be screwed on the terminal <small>T<sub>1</sub></small> of the induction coil.
+The discharge passing through the tube first illuminates the bottom
+of the same, which is of comparatively large section; but
+through the long glass fibre the discharge cannot pass. But
+gradually the rarefied gas inside becomes warmed and more conducting
+and the discharge spreads into the glass fibre. This spreading
+is so slow, that it may take half a minute or more until the
+discharge has worked through up to the top of the glass fibre,
+then presenting the appearance of a strongly luminous thin
+thread. By adjusting the potential at the terminal the light may
+be made to travel upwards at any speed. Once, however, the
+glass fibre is heated, the discharge breaks through its entire
+length instantly. The interesting point to be noted is that, the
+higher the frequency of the currents, or in other words, the
+greater relatively the lateral dissipation, at a slower rate may the
+light be made to propagate through the fibre. This experiment
+<span class='pagenum'><a name="Page_358" id="Page_358">[Pg 358]</a></span>is best performed with a highly exhausted and freshly made tube.
+When the tube has been used for some time the experiment
+often fails. It is possible that the gradual and slow impairment
+of the vacuum is the cause. This slow propagation of the discharge
+through a very narrow glass tube corresponds exactly to
+the propagation of heat through a bar warmed at one end. The
+quicker the heat is carried away laterally the longer time it will
+take for the heat to warm the remote end. When the current
+of a low frequency coil is passed through the fibre from end to
+end, then the lateral dissipation is small and the discharge instantly
+breaks through almost without exception.</p>
+
+<div class="figcenter" style="width: 636px;">
+<img src="images/oi_372.jpg" width="636" height="600" alt="Fig. 189, 190." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 189.</td><td class="caption">Fig. 190.</td></tr>
+</table>
+</div>
+
+<p>After these experiments and observations which have shown
+the importance of the discontinuity or atomic structure of the
+medium and which will serve to explain, in a measure at least,
+the nature of the four kinds of light effects producible with
+these currents, I may now give you an illustration of these
+effects. For the sake of interest I may do this in a manner
+which to many of you might be novel. You have seen before
+that we may now convey the electric vibration to a body by
+means of a single wire or conductor of any kind. Since the
+<span class='pagenum'><a name="Page_359" id="Page_359">[Pg 359]</a></span>human frame is conducting I may convey the vibration through
+my body.</p>
+
+<p>First, as in some previous experiments, I connect my body with
+one of the terminals of a high-tension transformer and take in my
+hand an exhausted bulb which contains a small carbon button
+mounted upon a platinum wire leading to the outside of the bulb,
+and the button is rendered incandescent as soon as the transformer
+is set to work (Fig. 190). I may place a conducting shade on the
+bulb which serves to intensify the action, but is not necessary.
+Nor is it required that the button should be in conducting connection
+with the hand through a wire leading through the glass,
+for sufficient energy may be transmitted through the glass itself
+by inductive action to render the button incandescent.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_373.jpg" width="800" height="508" alt="Fig. 191, 192." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 191.</td><td class="caption">Fig. 192.</td></tr>
+</table>
+</div>
+
+<p>Next I take a highly exhausted bulb containing a strongly
+phosphorescent body, above which is mounted a small plate of
+aluminum on a platinum wire leading to the outside, and the currents
+flowing through my body excite intense phosphorescence
+in the bulb (Fig. 191). Next again I take in my hand a simple
+exhausted tube, and in the same manner the gas inside the tube
+is rendered highly incandescent or phosphorescent (Fig. 192).
+Finally, I may take in my hand a wire, bare or covered with thick
+insulation, it is quite immaterial; the electrical vibration is so
+intense as to cover the wire with a luminous film (Fig. 193).<span class='pagenum'><a name="Page_360" id="Page_360">[Pg 360]</a></span></p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_375.jpg" width="800" height="453" alt="Fig. 193, 194, 195." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 193.</td><td class="caption">Fig. 194.</td><td class="caption">Fig. 195.</td></tr>
+</table>
+</div>
+
+<p>A few words must now be devoted to each of these phenomena.
+In the first place, I will consider the incandescence of a button or of
+a solid in general, and dwell upon some facts which apply equally
+to all these phenomena. It was pointed out before that when a
+thin conductor, such as a lamp filament, for instance, is connected
+with one of its ends to the terminal of a transformer of high
+tension the filament is brought to incandescence partly by a
+conduction current and partly by bombardment. The shorter
+and thicker the filament the more important becomes the latter,
+and finally, reducing the filament to a mere button, all the heating
+must practically be attributed to the bombardment. So in
+the experiment before shown, the button is rendered incandescent
+by the rhythmical impact of freely movable small bodies in the
+bulb. These bodies may be the molecules of the residual gas,
+particles of dust or lumps torn from the electrode; whatever they
+are, it is certain that the heating of the button is essentially connected
+with the pressure of such freely movable particles, or of
+atomic matter in general in the bulb. The heating is the more
+intense the greater the number of impacts per second and the
+greater the energy of each impact. Yet the button would
+be heated also if it were connected to a source of a steady potential.
+In such a case electricity would be carried away from
+the button by the freely movable carriers or particles flying
+about, and the quantity of electricity thus carried away might be
+sufficient to bring the button to incandescence by its passage
+through the latter. But the bombardment could not be of great
+importance in such case. For this reason it would require a comparatively
+very great supply of energy to the button to maintain
+it at incandescence with a steady potential. The higher the frequency
+of the electric impulses the more economically can the
+button be maintained at incandescence. One of the chief reasons
+why this is so, is, I believe, that with impulses of very high
+frequency there is less exchange of the freely movable carriers
+around the electrode and this means, that in the bulb the heated
+matter is better confined to the neighborhood of the button. If
+a double bulb, as illustrated in Fig. 194 be made, comprising a
+large globe <small>B</small> and a small one <i>b</i>, each containing as usual a filament
+<i>f</i> mounted on a platinum wire <i>w</i> and <i>w</i><sub>1</sub>, it is found, that if
+the filaments <i>f f</i> be exactly alike, it requires less energy to keep
+the filament in the globe <i>b</i> at a certain degree of incandescence,
+than that in the globe <small>B</small>. This is due to the confinement of the
+<span class='pagenum'><a name="Page_361" id="Page_361">[Pg 361]</a></span>movable particles around the button. In this case it is also ascertained,
+that the filament in the small globe <i>b</i> is less deteriorated
+when maintained a certain length of time at incandescence. This
+is a necessary consequence of the fact that the gas in the small
+bulb becomes strongly heated and therefore a very good conductor,
+and less work is then performed on the button, since the
+bombardment becomes less intense as the conductivity of the gas
+increases. In this construction, of course, the small bulb becomes
+very hot and when it reaches an elevated temperature the convection
+and radiation on the outside increase. On another occasion
+I have shown bulbs in which this drawback was largely
+avoided. In these instances a very small bulb, containing a refractory
+button, was mounted in a large globe and the space between
+the walls of both was highly exhausted. The outer large
+globe remained comparatively cool in such constructions. When
+the large globe was on the pump and the vacuum between the
+walls maintained permanent by the continuous action of the
+pump, the outer globe would remain quite cold, while the button
+in the small bulb was kept at incandescence. But when the seal
+was made, and the button in the small bulb maintained incandescent
+some length of time, the large globe too would become
+warmed. From this I conjecture that if vacuous space (as Prof.
+Dewar finds) cannot convey heat, it is so merely in virtue of our
+rapid motion through space or, generally speaking, by the motion
+of the medium relatively to us, for a permanent condition could
+<span class='pagenum'><a name="Page_362" id="Page_362">[Pg 362]</a></span>not be maintained without the medium being constantly renewed.
+A vacuum cannot, according to all evidence, be permanently
+maintained around a hot body.</p>
+
+<p>In these constructions, before mentioned, the small bulb inside
+would, at least in the first stages, prevent all bombardment
+against the outer large globe. It occurred to me then to ascertain
+how a metal sieve would behave in this respect, and several
+bulbs, as illustrated in Fig. 195, were prepared for this purpose.
+In a globe <i>b</i>, was mounted a thin filament <i>f</i> (or button) upon a
+platinum wire <i>w</i> passing through a glass stem and leading to the
+outside of the globe. The filament <i>f</i> was surrounded by a metal
+sieve <i>s</i>. It was found in experiments with such bulbs that a sieve
+with wide meshes apparently did not in the slightest affect the
+bombardment against the globe <i>b</i>. When the vacuum was high,
+the shadow of the sieve was clearly projected against the globe
+and the latter would get hot in a short while. In some bulbs the
+sieve <i>s</i> was connected to a platinum wire sealed in the glass.
+When this wire was connected to the other terminal of the induction
+coil (the <span class="smcap">e. m. f.</span> being kept low in this case), or to an insulated
+plate, the bombardment against the outer globe <i>b</i> was
+diminished. By taking a sieve with fine meshes the bombardment
+against the globe <i>b</i> was always diminished, but even then
+if the exhaustion was carried very far, and when the potential of
+the transformer was very high, the globe <i>b</i> would be bombarded
+and heated quickly, though no shadow of the sieve was visible,
+owing to the smallness of the meshes. But a glass tube or other
+continuous body mounted so as to surround the filament, did entirely
+cut off the bombardment and for a while the outer globe <i>b</i>
+would remain perfectly cold. Of course when the glass tube
+was sufficiently heated the bombardment against the outer globe
+could be noted at once. The experiments with these bulbs
+seemed to show that the speeds of the projected molecules or
+particles must be considerable (though quite insignificant when
+compared with that of light), otherwise it would be difficult to
+understand how they could traverse a fine metal sieve without
+being affected, unless it were found that such small particles or
+atoms cannot be acted upon directly at measurable distances.
+In regard to the speed of the projected atoms, Lord Kelvin has
+recently estimated it at about one kilometre a second or thereabouts
+in an ordinary Crookes bulb. As the potentials obtainable
+with a disruptive discharge coil are much higher than with or<span class='pagenum'><a name="Page_363" id="Page_363">[Pg 363]</a></span>dinary
+coils, the speeds must, of course, be much greater when
+the bulbs are lighted from such a coil. Assuming the speed to
+be as high as five kilometres and uniform through the whole
+trajectory, as it should be in a very highly exhausted vessel, then
+if the alternate electrifications of the electrode would be of a
+frequency of five million, the greatest distance a particle could
+get away from the electrode would be one millimetre, and if it
+could be acted upon directly at that distance, the exchange of
+electrode matter or of the atoms would be very slow and there
+would be practically no bombardment against the bulb. This at
+least should be so, if the action of an electrode upon the atoms
+of the residual gas would be such as upon electrified bodies which
+we can perceive. A hot body enclosed in an exhausted bulb
+produces always atomic bombardment, but a hot body has no
+definite rhythm, for its molecules perform vibrations of all kinds.</p>
+
+<p>If a bulb containing a button or filament be exhausted as high
+as is possible with the greatest care and by the use of the best artifices,
+it is often observed that the discharge cannot, at first,
+break through, but after some time, probably in consequence of
+some changes within the bulb, the discharge finally passes through
+and the button is rendered incandescent. In fact, it appears that
+the higher the degree of exhaustion the easier is the incandescence
+produced. There seem to be no other causes to which the incandescence
+might be attributed in such case except to the bombardment
+or similar action of the residual gas, or of particles of
+matter in general. But if the bulb be exhausted with the greatest
+care can these play an important part? Assume the vacuum
+in the bulb to be tolerably perfect, the great interest then centres
+in the question: Is the medium which pervades all space continuous
+or atomic? If atomic, then the heating of a conducting
+button or filament in an exhausted vessel might be due largely
+to ether bombardment, and then the heating of a conductor in
+general through which currents of high frequency or high potential
+are passed must be modified by the behavior of such medium;
+then also the skin effect, the apparent increase of the ohmic resistance,
+etc., admit, partially at least, of a different explanation.</p>
+
+<p>It is certainly more in accordance with many phenomena observed
+with high-frequency currents to hold that all space is pervaded
+with free atoms, rather than to assume that it is devoid of
+these, and dark and cold, for so it must be, if filled with a continuous
+medium, since in such there can be neither heat nor light.<span class='pagenum'><a name="Page_364" id="Page_364">[Pg 364]</a></span>
+Is then energy transmitted by independent carriers or by the
+vibration of a continuous medium? This important question is
+by no means as yet positively answered. But most of the effects
+which are here considered, especially the light effects, incandescence,
+or phosphorescence, involve the presence of free atoms and
+would be impossible without these.</p>
+
+<p>In regard to the incandescence of a refractory button (or filament)
+in an exhausted receiver, which has been one of the subjects
+of this investigation, the chief experiences, which may serve
+as a guide in constructing such bulbs, may be summed up as follows:
+1. The button should be as small as possible, spherical,
+of a smooth or polished surface, and of refractory material which
+withstands evaporation best. 2. The support of the button
+should be very thin and screened by an aluminum and mica sheet,
+as I have described on another occasion. 3. The exhaustion of
+the bulb should be as high as possible. 4. The frequency of the
+currents should be as high as practicable. 5. The currents should
+be of a harmonic rise and fall, without sudden interruptions. 6.
+The heat should be confined to the button by inclosing the same
+in a small bulb or otherwise. 7. The space between the walls of
+the small bulb and the outer globe should be highly exhausted.</p>
+
+<p>Most of the considerations which apply to the incandescence
+of a solid just considered may likewise be applied to phosphorescence.
+Indeed, in an exhausted vessel the phosphorescence is,
+as a rule, primarily excited by the powerful beating of the electrode
+stream of atoms against the phosphorescent body. Even in
+many cases, where there is no evidence of such a bombardment,
+I think that phosphorescence is excited by violent impacts of
+atoms, which are not necessarily thrown off from the electrode
+but are acted upon from the same inductively through the
+medium or through chains of other atoms. That mechanical
+shocks play an important part in exciting phosphorescence in a
+bulb may be seen from the following experiment. If a bulb,
+constructed as that illustrated in Fig. 174, be taken and exhausted
+with the greatest care so that the discharge cannot pass, the filament
+<i>f</i> acts by electrostatic induction upon the tube <i>t</i> and the
+latter is set in vibration. If the tube <i>o</i> be rather wide, about an
+inch or so, the filament may be so powerfully vibrated that whenever
+it hits the glass tube it excites phosphorescence. But the
+phosphorescence ceases when the filament comes to rest. The
+vibration can be arrested and again started by varying the<span class='pagenum'><a name="Page_365" id="Page_365">[Pg 365]</a></span>
+frequency of the currents. Now the filament has its own
+period of vibration, and if the frequency of the currents is such
+that there is resonance, it is easily set vibrating, though the potential
+of the currents be small. I have often observed that the
+filament in the bulb is destroyed by such mechanical resonance.
+The filament vibrates as a rule so rapidly that it cannot be seen
+and the experimenter may at first be mystified. When such an
+experiment as the one described is carefully performed, the potential
+of the currents need be extremely small, and for this
+reason I infer that the phosphorescence is then due to the
+mechanical shock of the filament against the glass, just as it is
+produced by striking a loaf of sugar with a knife. The mechanical
+shock produced by the projected atoms is easily noted when
+a bulb containing a button is grasped in the hand and the current
+turned on suddenly. I believe that a bulb could be shattered
+by observing the conditions of resonance.</p>
+
+<p>In the experiment before cited it is, of course, open to say,
+that the glass tube, upon coming in contact with the filament, retains
+a charge of a certain sign upon the point of contact. If
+now the filament again touches the glass at the same point while
+it is oppositely charged, the charges equalize under evolution of
+light. But nothing of importance would be gained by such an
+explanation. It is unquestionable that the initial charges given
+to the atoms or to the glass play some part in exciting phosphorescence.
+So, for instance, if a phosphorescent bulb be first excited
+by a high frequency coil by connecting it to one of the terminals
+of the latter and the degree of luminosity be noted, and then
+the bulb be highly charged from a Holtz machine by attaching
+it preferably to the positive terminal of the machine, it is found
+that when the bulb is again connected to the terminal of the high
+frequency coil, the phosphorescence is far more intense. On
+another occasion I have considered the possibility of some phosphorescent
+phenomena in bulbs being produced by the incandescence
+of an infinitesimal layer on the surface of the phosphorescent
+body. Certainly the impact of the atoms is powerful enough
+to produce intense incandescence by the collisions, since they bring
+quickly to a high temperature a body of considerable bulk. If any
+such effect exists, then the best appliance for producing phosphorescence
+in a bulb, which we know so far, is a disruptive discharge
+coil giving an enormous potential with but few fundamental discharges,
+say 25-30 per second, just enough to produce a continu<span class='pagenum'><a name="Page_366" id="Page_366">[Pg 366]</a></span>ous
+impression upon the eye. It is a fact that such a coil excites
+phosphorescence under almost any condition and at all degrees
+of exhaustion, and I have observed effects which appear to be due
+to phosphorescence even at ordinary pressures of the atmosphere,
+when the potentials are extremely high. But if phosphorescent
+light is produced by the equalization of charges of electrified
+atoms (whatever this may mean ultimately), then the higher the
+frequency of the impulses or alternate electrifications, the
+more economical will be the light production. It is a long
+known and noteworthy fact that all the phosphorescent bodies
+are poor conductors of electricity and heat, and that all bodies
+cease to emit phosphorescent light when they are brought to a
+certain temperature. Conductors on the contrary do not possess
+this quality. There are but few exceptions to the rule. Carbon
+is one of them. Becquerel noted that carbon phosphoresces at
+a certain elevated temperature preceding the dark red. This
+phenomenon may be easily observed in bulbs provided with a
+rather large carbon electrode (say, a sphere of six millimetres diameter).
+If the current is turned on after a few seconds, a snow
+white film covers the electrode, just before it gets dark red.
+Similar effects are noted with other conducting bodies, but many
+scientific men will probably not attribute them to true phosphorescence.
+Whether true incandescence has anything to do with
+phosphorescence excited by atomic impact or mechanical shocks
+still remains to be decided, but it is a fact that all conditions,
+which tend to localize and increase the heating effect at the point
+of impact, are almost invariably the most favorable for the production
+of phosphorescence. So, if the electrode be very small,
+which is equivalent to saying in general, that the electric density
+is great; if the potential be high, and if the gas be highly rarefied,
+all of which things imply high speed of the projected atoms,
+or matter, and consequently violent impacts&mdash;the phosphorescence
+is very intense. If a bulb provided with a large and small
+electrode be attached to the terminal of an induction coil, the
+small electrode excites phosphorescence while the large one may
+not do so, because of the smaller electric density and hence
+smaller speed of the atoms. A bulb provided with a large electrode
+may be grasped with the hand while the electrode is connected
+to the terminal of the coil and it may not phosphoresce;
+but if instead of grasping the bulb with the hand, the same be
+touched with a pointed wire, the phosphorescence at once spreads<span class='pagenum'><a name="Page_367" id="Page_367">[Pg 367]</a></span>
+through the bulb, because of the great density at the point of
+contact. With low frequencies it seems that gases of great
+atomic weight excite more intense phosphorescence than those
+of smaller weight, as for instance, hydrogen. With high frequencies
+the observations are not sufficiently reliable to draw a
+conclusion. Oxygen, as is well-known, produces exceptionally
+strong effects, which may be in part due to chemical action. A
+bulb with hydrogen residue seems to be most easily excited.
+Electrodes which are most easily deteriorated produce more
+intense phosphorescence in bulbs, but the condition is not permanent
+because of the impairment of the vacuum and the deposition
+of the electrode matter upon the phosphorescent surfaces.
+Some liquids, as oils, for instance, produce magnificent effects of
+phosphorescence (or fluorescence?), but they last only a few
+seconds. So if a bulb has a trace of oil on the walls and the
+current is turned on, the phosphorescence only persists for a few
+moments until the oil is carried away. Of all bodies so far tried,
+sulphide of zinc seems to be the most susceptible to phosphorescence.
+Some samples, obtained through the kindness of Prof.
+Henry in Paris, were employed in many of these bulbs. One of
+the defects of this sulphide is, that it loses its quality of emitting
+light when brought to a temperature which is by no means high.
+It can therefore, be used only for feeble intensities. An observation
+which might deserve notice is, that when violently bombarded
+from an aluminum electrode it assumes a black color, but
+singularly enough, it returns to the original condition when it
+cools down.</p>
+
+<p>The most important fact arrived at in pursuing investigations
+in this direction is, that in all cases it is necessary, in order to excite
+phosphorescence with a minimum amount of energy, to observe
+certain conditions. Namely, there is always, no matter what
+the frequency of the currents, degree of exhaustion and character
+of the bodies in the bulb, a certain potential (assuming the bulb
+excited from one terminal) or potential difference (assuming the
+bulb to be excited with both terminals) which produces the most
+economical result. If the potential be increased, considerable
+energy may be wasted without producing any more light, and if
+it be diminished, then again the light production is not as economical.
+The exact condition under which the best result is obtained
+seems to depend on many things of a different nature, and it is to
+be yet investigated by other experimenters, but it will certainly<span class='pagenum'><a name="Page_368" id="Page_368">[Pg 368]</a></span>
+have to be observed when such phosphorescent bulbs are operated,
+if the best results are to be obtained.</p>
+
+<p>Coming now to the most interesting of these phenomena, the
+incandescence or phosphorescence of gases, at low pressures or at
+the ordinary pressure of the atmosphere, we must seek the explanation
+of these phenomena in the same primary causes, that is,
+in shocks or impacts of the atoms. Just as molecules or atoms
+beating upon a solid body excite phosphorescence in the same or
+render it incandescent, so when colliding among themselves they
+produce similar phenomena. But this is a very insufficient explanation
+and concerns only the crude mechanism. Light is produced
+by vibrations which go on at a rate almost inconceivable.
+If we compute, from the energy contained in the form of known
+radiations in a definite space the force which is necessary to set
+up such rapid vibrations, we find, that though the density of the
+ether be incomparably smaller than that of any body we know,
+even hydrogen, the force is something surpassing comprehension.
+What is this force, which in mechanical measure may amount to
+thousands of tons per square inch? It is electrostatic force in the
+light of modern views. It is impossible to conceive how a body
+of measurable dimensions could be charged to so high a potential
+that the force would be sufficient to produce these vibrations.
+Long before any such charge could be imparted to the body it
+would be shattered into atoms. The sun emits light and heat, and
+so does an ordinary flame or incandescent filament, but in neither
+of these can the force be accounted for if it be assumed that it is
+associated with the body as a whole. Only in one way may we
+account for it, namely, by identifying it with the atom. An
+atom is so small, that if it be charged by coming in contact with
+an electrified body and the charge be assumed to follow the same
+law as in the case of bodies of measurable dimensions, it must
+retain a quantity of electricity which is fully capable of accounting
+for these forces and tremendous rates of vibration. But the
+atom behaves singularly in this respect&mdash;it always takes the same
+"charge."</p>
+
+<p>It is very likely that resonant vibration plays a most important
+part in all manifestations of energy in nature. Throughout space
+all matter is vibrating, and all rates of vibration are represented,
+from the lowest musical note to the highest pitch of the chemical
+rays, hence an atom, or complex of atoms, no matter what its
+period, must find a vibration with which it is in resonance.<span class='pagenum'><a name="Page_369" id="Page_369">[Pg 369]</a></span>
+When we consider the enormous rapidity of the light vibrations,
+we realize the impossibility of producing such vibrations directly
+with any apparatus of measurable dimensions, and we are driven
+to the only possible means of attaining the object of setting up
+waves of light by electrical means and economically, that is, to
+affect the molecules or atoms of a gas, to cause them to collide and
+vibrate. We then must ask ourselves&mdash;How can free molecules
+or atoms be affected?</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_384.jpg" width="800" height="338" alt="Fig. 196, 197." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 196.</td><td class="caption">Fig. 197.</td></tr>
+</table>
+</div>
+
+<p>It is a fact that they can be affected by electrostatic force, as is
+apparent in many of these experiments. By varying the electrostatic
+force we can agitate the atoms, and cause them to collide
+accompanied by evolution of heat and light. It is not demonstrated
+beyond doubt that we can affect them otherwise. If a luminous
+discharge is produced in a closed exhausted tube, do the atoms
+arrange themselves in obedience to any other but to electrostatic
+force acting in straight lines from atom to atom? Only recently
+I investigated the mutual action between two circuits with extreme
+rates of vibration. When a battery of a few jars (<i>c c c c</i>, Fig.
+196) is discharged through a primary <small>P</small> of low resistance (the connections
+being as illustrated in Figs. 183<i>a</i>, 183<i>b</i> and 183<i>c</i>), and the
+frequency of vibration is many millions there are great differences
+of potential between points on the primary not more than
+a few inches apart. These differences may be 10,000 volts per
+inch, if not more, taking the maximum value of the <span class="smcap">e. m. f.</span> The
+secondary <i>s</i> is therefore acted upon by electrostatic induction,
+which is in such extreme cases of much greater importance than
+the electro-dynamic. To such sudden impulses the primary as
+well as the secondary are poor conductors, and therefore great
+differences of potential may be produced by electrostatic induction
+between adjacent points on the secondary. Then sparks may
+jump between the wires and streamers become visible in the dark
+if the light of the discharge through the spark gap <i>d d</i> be carefully
+excluded. If now we substitute a closed vacuum tube for the
+metallic secondary <i>s</i>, the differences of potential produced in the
+tube by electrostatic induction from the primary are fully sufficient
+to excite portions of it; but as the points of certain differences
+of potential on the primary are not fixed, but are generally
+constantly changing in position, a luminous band is produced in
+the tube, apparently not touching the glass, as it should, if the
+points of maximum and minimum differences of potential were
+fixed on the primary. I do not exclude the possibility of such a
+<span class='pagenum'><a name="Page_370" id="Page_370">[Pg 370]</a></span>tube being excited only by electro-dynamic induction, for very
+able physicists hold this view; but in my opinion, there is as yet
+no positive proof given that atoms of a gas in a closed tube may
+arrange themselves in chains under the action of an electromotive
+impulse produced by electro-dynamic induction in the tube. I
+have been unable so far to produce stri&aelig; in a tube, however long,
+and at whatever degree of exhaustion, that is, stri&aelig; at right
+angles to the supposed direction of the discharge or the axis of
+the tube; but I have distinctly observed in a large bulb, in which
+a wide luminous band was produced by passing a discharge of a
+battery through a wire surrounding the bulb, a circle of feeble
+luminosity between two luminous bands, one of which was more
+intense than the other. Furthermore, with my present experience
+I do not think that such a gas discharge in a closed tube
+can vibrate, that is, vibrate as a whole. I am convinced that no
+discharge through a gas can vibrate. The atoms of a gas behave
+very curiously in respect to sudden electric impulses. The
+gas does not seem to possess any appreciable inertia to such
+impulses, for it is a fact, that the higher the frequency of
+the impulses, with the greater freedom does the discharge
+pass through the gas. If the gas possesses no inertia then
+it cannot vibrate, for some inertia is necessary for the free vibration.
+I conclude from this that if a lightning discharge occurs
+between two clouds, there can be no oscillation, such as would
+be expected, considering the capacity of the clouds. But if
+the lightning discharge strike the earth, there is always vibration&mdash;in
+the earth, but not in the cloud. In a gas discharge each
+atom vibrates at its own rate, but there is no vibration of the
+conducting gaseous mass as a whole. This is an important
+consideration in the great problem of producing light economi<span class='pagenum'><a name="Page_371" id="Page_371">[Pg 371]</a></span>cally,
+for it teaches us that to reach this result we must use
+impulses of very high frequency and necessarily also of high
+potential. It is a fact that oxygen produces a more intense
+light in a tube. Is it because oxygen atoms possess some inertia
+and the vibration does not die out instantly? But then nitrogen
+should be as good, and chlorine and vapors of many other bodies
+much better than oxygen, unless the magnetic properties of the
+latter enter prominently into play. Or, is the process in the tube
+of an electrolytic nature? Many observations certainly speak for
+it, the most important being that matter is always carried away
+from the electrodes and the vacuum in a bulb cannot be permanently
+maintained. If such process takes place in reality, then
+again must we take refuge in high frequencies, for, with such,
+electrolytic action should be reduced to a minimum, if not rendered
+entirely impossible. It is an undeniable fact that with very
+high frequencies, provided the impulses be of harmonic nature,
+like those obtained from an alternator, there is less deterioration
+and the vacua are more permanent. With disruptive discharge
+coils there are sudden rises of potential and the vacua are
+more quickly impaired, for the electrodes are deteriorated in a
+very short time. It was observed in some large tubes, which
+were provided with heavy carbon blocks <small>B B<sub>1</sub></small>, connected to platinum
+wires <i>w w</i><sub>1</sub> (as illustrated in Fig. 197), and which were employed
+in experiments with the disruptive discharge instead of the
+ordinary air gap, that the carbon particles under the action of the
+powerful magnetic field in which the tube was placed, were deposited
+in regular fine lines in the middle of the tube, as illustrated.
+These lines were attributed to the deflection or distortion
+of the discharge by the magnetic field, but why the deposit
+occurred principally where the field was most intense did not
+appear quite clear. A fact of interest, likewise noted, was
+that the presence of a strong magnetic field increases the deterioration
+of the electrodes, probably by reason of the rapid interruptions
+it produces, whereby there is actually a higher <span class="smcap">e. m. f.</span>
+maintained between the electrodes.</p>
+
+<p>Much would remain to be said about the luminous effects produced
+in gases at low or ordinary pressures. With the present
+experiences before us we cannot say that the essential nature of
+these charming phenomena is sufficiently known. But investigations
+in this direction are being pushed with exceptional ardor.
+Every line of scientific pursuit has its fascinations, but electrical<span class='pagenum'><a name="Page_372" id="Page_372">[Pg 372]</a></span>
+investigation appears to possess a peculiar attraction, for there is
+no experiment or observation of any kind in the domain of this
+wonderful science which would not forcibly appeal to us. Yet
+to me it seems, that of all the many marvelous things we observe,
+a vacuum tube, excited by an electric impulse from a distant
+source, bursting forth out of the darkness and illuminating the
+room with its beautiful light, is as lovely a phenomenon as can
+greet our eyes. More interesting still it appears when, reducing
+the fundamental discharges across the gap to a very small number
+and waving the tube about we produce all kinds of designs
+in luminous lines. So by way of amusement I take a straight
+long tube, or a square one, or a square attached to a straight tube,
+and by whirling them about in the hand, I imitate the spokes of
+a wheel, a Gramme winding, a drum winding, an alternate current
+motor winding, etc. (Fig. 198). Viewed from a distance the
+effect is weak and much of its beauty is lost, but being near or
+holding the tube in the hand, one cannot resist its charm.</p>
+
+<div class="figcenter" style="width: 617px;">
+<img src="images/oi_386.jpg" width="617" height="600" alt="Fig. 198." title="" />
+<span class="caption">Fig. 198.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_373" id="Page_373">[Pg 373]</a></span></p>
+
+<p>In presenting these insignificant results I have not attempted
+to arrange and co-ordinate them, as would be proper in a strictly
+scientific investigation, in which every succeeding result should
+be a logical sequence of the preceding, so that it might be guessed
+in advance by the careful reader or attentive listener. I have
+preferred to concentrate my energies chiefly upon advancing
+novel facts or ideas which might serve as suggestions to others,
+and this may serve as an excuse for the lack of harmony. The
+explanations of the phenomena have been given in good faith
+and in the spirit of a student prepared to find that they admit of
+a better interpretation. There can be no great harm in a student
+taking an erroneous view, but when great minds err, the world
+must dearly pay for their mistakes.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_374" id="Page_374">[Pg 374]</a></span></p>
+<h2><a name="CHAPTER_XXIX" id="CHAPTER_XXIX"></a>CHAPTER XXIX.</h2>
+
+<h3><span class="smcap">Tesla Alternating Current Generators for High Frequency,
+in Detail.</span></h3>
+
+
+<p>It has become a common practice to operate arc lamps by alternating
+or pulsating, as distinguished from continuous, currents;
+but an objection which has been raised to such systems exists in
+the fact that the arcs emit a pronounced sound, varying with the
+rate of the alternations or pulsations of current. This noise is
+due to the rapidly alternating heating and cooling, and consequent
+expansion and contraction, of the gaseous matter forming
+the arc, which corresponds with the periods or impulses of the
+current. Another disadvantageous feature is found in the difficulty
+of maintaining an alternating current arc in consequence of
+the periodical increase in resistance corresponding to the periodical
+working of the current. This feature entails a further disadvantage,
+namely, that small arcs are impracticable.</p>
+
+<p>Theoretical considerations have led Mr. Tesla to the belief
+that these disadvantageous features could be obviated by employing
+currents of a sufficiently high number of alternations, and his
+anticipations have been confirmed in practice. These rapidly
+alternating currents render it possible to maintain small arcs
+which, besides, possess the advantages of silence and persistency.
+The latter quality is due to the necessarily rapid alternations, in
+consequence of which the arc has no time to cool, and is always
+maintained at a high temperature and low resistance.</p>
+
+<p>At the outset of his experiments Mr. Tesla encountered great
+difficulties in the construction of high frequency machines. A
+generator of this kind is described here, which, though constructed
+quite some time ago, is well worthy of a detailed description.
+It may be mentioned, in passing, that dynamos of
+this type have been used by Mr. Tesla in his lighting researches and
+experiments with currents of high potential and high frequency,
+and reference to them will be found in his lectures
+elsewhere printed in this volume.<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a></p>
+<p><span class='pagenum'><a name="Page_375" id="Page_375">[Pg 375]</a></span></p>
+<p>In the accompanying engravings, Figs. 199 and 200 show the
+machine, respectively, in side elevation and vertical cross-section;
+Figs. 201, 202 and 203 showing enlarged details of construction.
+As will be seen, <small>A</small> is an annular magnetic frame, the interior of
+which is provided with a large number of pole-pieces <small>D</small>.</p>
+
+<p>Owing to the very large number and small size of the poles
+and the spaces between them, the field coils are applied by winding
+an insulated conductor <small>F</small> zigzag through the grooves, as shown
+in Fig. 203, carrying the wire around the annulus to form as
+many layers as is desired. In this way the pole-pieces <small>D</small> will be
+energized with alternately opposite polarity around the entire
+ring.</p>
+
+<p>For the armature, Mr. Tesla employs a spider carrying a ring
+<small>J</small>, turned down, except at its edges, to form a trough-like receptacle
+for a mass of fine annealed iron wires <small>K</small>, which are wound
+in the groove to form the core proper for the armature-coils.
+Pins <small>L</small> are set in the sides of the ring <small>J</small> and the coils <small>M</small> are wound
+over the periphery of the armature-structure and around the pins.
+The coils <small>M</small> are connected together in series, and these terminals
+<small>N</small> carried through the hollow shaft <small>H</small> to contact-rings <small>P P</small>, from
+which the currents are taken off by brushes <small>O</small>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_389.jpg" width="800" height="569" alt="Fig. 199." title="" />
+<span class="caption">Fig. 199.</span>
+</div>
+
+<p>In this way a machine with a very large number of poles may
+be constructed. It is easy, for instance, to obtain in this manner
+three hundred and seventy-five to four hundred poles in a machine
+that may be safely driven at a speed of fifteen hundred or sixteen
+hundred revolutions per minute, which will produce ten<span class='pagenum'><a name="Page_376" id="Page_376">[Pg 376]</a></span>
+thousand or eleven thousand alternations of current per second.
+Arc lamps <small>R R</small> are shown in the diagram as connected up in series
+with the machine in Fig. 200. If such a current be applied to
+running arc lamps, the sound produced by or in the arc becomes
+practically inaudible, for, by increasing the rate of change in the
+current, and consequently the number of vibrations per unit of
+time of the gaseous material of the arc up to, or beyond, ten
+thousand or eleven thousand per second, or to what is regarded
+as the limit of audition, the sound due to such vibrations will not
+be audible. The exact number of changes or undulations necessary
+to produce this result will vary somewhat according to the
+size of the arc&mdash;that is to say, the smaller the arc, the greater the
+number of changes that will be required to render it inaudible
+within certain limits. It should also be stated that the arc should
+not exceed a certain length.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_390.jpg" width="800" height="560" alt="Fig. 200, 201, 202 and 203." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 200, 201, 202 and 203.</span>
+</div>
+
+<p>The difficulties encountered in the construction of these
+machines are of a mechanical as well as an electrical nature.
+The machines may be designed on two plans: the field may be
+formed either of alternating poles, or of polar projections of the
+same polarity. Up to about 15,000 alternations per second in an
+experimental machine, the former plan may be followed, but a
+more efficient machine is obtained on the second plan.</p>
+
+<p>In the machine above described, which was capable of running
+two arcs of normal candle power, the field was composed of a<span class='pagenum'><a name="Page_377" id="Page_377">[Pg 377]</a></span>
+ring of wrought iron 32 inches outside diameter, and about 1
+inch thick. The inside diameter was 30 inches. There were 384
+polar projections. The wire was wound in zigzag form, but two
+wires were wound so as to completely envelop the projections.
+The distance between the projections is about 3/16 inch, and they
+are a little over 1/16 inch thick. The field magnet was made relatively
+small so as to adapt the machine for a constant current.
+There are 384 coils connected in two series. It was found impracticable
+to use any wire much thicker than No. 26 B. and S.
+gauge on account of the local effects. In such a machine the
+clearance should be as small as possible; for this reason the
+machine was made only 1&frac14; inch wide, so that the binding wires
+might be obviated. The armature wires must be wound with
+great care, as they are apt to fly off in consequence of the great
+peripheral speed. In various experiments this machine has been
+run as high as 3,000 revolutions per minute. Owing to the great
+speed it was possible to obtain as high as 10 amperes out of the
+machine. The electromotive force was regulated by means of
+an adjustable condenser within very wide limits, the limits
+being the greater, the greater the speed. This machine was
+frequently used to run Mr. Tesla's laboratory lights.</p>
+
+<div class="figcenter" style="width: 649px;">
+<img src="images/oi_391.jpg" width="649" height="600" alt="Fig. 204." title="" />
+<span class="caption">Fig. 204.</span>
+</div>
+
+<p>The machine above described was only one of many such
+types constructed. It serves well for an experimental machine,
+but if still higher alternations are required and higher efficiency
+is necessary, then a machine on a plan shown in Figs. 204 to<span class='pagenum'><a name="Page_378" id="Page_378">[Pg 378]</a></span>
+207, is preferable. The principal advantage of this type of
+machine is that there is not much magnetic leakage, and that a
+field may be produced, varying greatly in intensity in places not
+much distant from each other.</p>
+
+<p>In these engravings, Figs. 204 and 205 illustrate a machine in
+which the armature conductor and field coils are stationary, while
+the field magnet core revolves. Fig. 206 shows a machine
+embodying the same plan of construction, but having a stationary
+field magnet and rotary armature.</p>
+
+<p>The conductor in which the currents are induced may be
+arranged in various ways; but Mr. Tesla prefers the following
+method: He employs an annular plate of copper <small>D</small>, and by
+means of a saw cuts in it radial slots from one edge nearly
+through to the other, beginning alternately from opposite edges.
+In this way a continuous zigzag conductor is formed. When the
+polar projections are 1/8 inch wide, the width of the conductor
+should not, under any circumstances, be more than 1/32 inch wide;
+even then the eddy effect is considerable.</p>
+
+
+<div class="figcenter" style="width: 771px;">
+<img src="images/oi_392.jpg" width="771" height="600" alt="Fig. 205." title="" />
+<span class="caption">Fig. 205.</span>
+</div>
+
+<p>To the inner edge of this plate are secured two rings of non-magnetic
+metal <small>E</small>, which are insulated from the copper conductor,
+but held firmly thereto by means of the bolts <small>F</small>. Within the
+rings <small>E</small> is then placed an annular coil <small>G</small>, which is the energizing
+coil for the field magnet. The conductor <small>D</small> and the parts attached
+thereto are supported by means of the cylindrical shell or<span class='pagenum'><a name="Page_379" id="Page_379">[Pg 379]</a></span>
+casting <small>A A</small>, the two parts of which are brought together and
+clamped to the outer edge of the conductor <small>D</small>.</p>
+
+<div class="figcenter" style="width: 654px;">
+<img src="images/oi_393.jpg" width="654" height="600" alt="Fig. 206." title="" />
+<span class="caption">Fig. 206.</span>
+</div>
+
+
+<p>The core for the field magnet is built up of two circular parts
+<small>H H</small>, formed with annular grooves <small>I</small>, which, when the two parts
+are brought together, form a space for the reception of the energizing
+coil <small>G</small>. The hubs of the cores are trued off, so as to fit
+closely against one another, while the outer portions or flanges
+which form the polar faces <small>J J</small>, are reduced somewhat in thickness
+to make room for the conductor <small>D</small>, and are serrated on their
+faces. The number of serrations in the polar faces is arbitrary;
+but there must exist between them and the radial portions of
+the conductor <small>D</small> certain relation, which will be understood by
+reference to Fig. 207 in which <small>N N</small> represent the projections or
+points on one face of the core of the field, and <small>S S</small> the points of
+the other face. The conductor <small>D</small> is shown in this figure in section
+<i>a a'</i> designating the radial portions of the conductor, and <i>b</i> the
+insulating divisions between them. The relative width of the
+parts <i>a a'</i> and the space between any two adjacent points <small>N N</small> or
+<small>S S</small> is such that when the radial portions <i>a</i> of the conductor are
+passing between the opposite points <small>N S</small> where the field is strongest,
+the intermediate radial portions <i>a'</i> are passing through the<span class='pagenum'><a name="Page_380" id="Page_380">[Pg 380]</a></span>
+widest spaces midway between such points and where the field is
+weakest. Since the core on one side is of opposite polarity to
+the part facing it, all the projections of one polar face will be of
+opposite polarity to those of the other face. Hence, although
+the space between any two adjacent points on the same face may
+be extremely small, there will be no leakage of the magnetic
+lines between any two points of the same name, but the lines of
+force will pass across from one set of points to the other. The
+construction followed obviates to a great degree the distortion of
+the magnetic lines by the action of the current in the conductor
+<small>D</small>, in which it will be observed the current is flowing at any given
+time from the centre toward the periphery in one set of radial
+parts <i>a</i> and in the opposite direction in the adjacent parts <i>a'</i>.</p>
+
+<p>In order to connect the energizing coil <small>G</small>, Fig. 204, with a source
+of continuous current, Mr. Tesla utilizes two adjacent radial portions
+of the conductor <small>D</small> for connecting the terminals of the coil
+<small>G</small> with two binding posts <small>M</small>. For this purpose the plate <small>D</small> is cut
+entirely through, as shown, and the break thus made is bridged
+over by a short conductor <small>C</small>. The plate <small>D</small> is cut through to form
+two terminals <i>d</i>, which are connected to binding posts <small>N</small>. The
+core <small>H H</small>, when rotated by the driving pulley, generates in the conductors
+<small>D</small> an alternating current, which is taken off from the
+binding posts <small>N</small>.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_394.jpg" width="640" height="275" alt="Fig. 207." title="" />
+<span class="caption">Fig. 207.</span>
+</div>
+
+
+
+<p>When it is desired to rotate the conductor between the faces
+of a stationary field magnet, the construction shown in Fig.
+206, is adopted. The conductor <small>D</small> in this case is or may be
+made in substantially the same manner as above described by
+slotting an annular conducting-plate and supporting it between
+two heads <small>O</small>, held together by bolts <i>o</i> and fixed to the driving-shaft
+<small>K</small>. The inner edge of the plate or conductor <small>D</small> is preferably
+flanged to secure a firmer union between it and the heads <small>O</small>. It
+is insulated from the head. The field-magnet in this case consists
+of two annular parts <small>H H</small>, provided with annular grooves <small>I</small>
+for the reception of the coils. The flanges or faces surrounding<span class='pagenum'><a name="Page_381" id="Page_381">[Pg 381]</a></span>
+the annular groove are brought together, while the inner flanges
+are serrated, as in the previous case, and form the polar faces.
+The two parts <small>H H</small> are formed with a base <small>R</small>, upon which the
+machine rests. <small>S S</small> are non-magnetic bushings secured or set in
+the central opening of the cores. The conductor <small>D</small> is cut entirely
+through at one point to form terminals, from which insulated
+conductors <small>T</small> are led through the shaft to collecting-rings <small>V</small>.</p>
+
+<p>In one type of machine of this kind constructed by Mr. Tesla,
+the field had 480 polar projections on each side, and from this
+machine it was possible to obtain 30,000 alternations per second.
+As the polar projections must necessarily be very narrow, very
+thin wires or sheets must be used to avoid the eddy current
+effects. Mr. Tesla has thus constructed machines with a stationary
+armature and rotating field, in which case also the field-coil
+was supported so that the revolving part consisted only of a
+wrought iron body devoid of any wire and also machines with a
+rotating armature and stationary field. The machines may be
+either drum or disc, but Mr. Tesla's experience shows the latter
+to be preferable.</p>
+
+<hr style='width: 15%;' />
+
+<p>In the course of a very interesting article contributed to the
+<i>Electrical World</i> in February, 1891, Mr. Tesla makes some suggestive
+remarks on these high frequency machines and his experiences
+with them, as well as with other parts of the high
+frequency apparatus. Part of it is quoted here and is as
+follows:&mdash;</p>
+
+<p>The writer will incidentally mention that any one who attempts
+for the first time to construct such a machine will have a
+tale of woe to tell. He will first start out, as a matter of course,
+by making an armature with the required number of polar projections.
+He will then get the satisfaction of having produced
+an apparatus which is fit to accompany a thoroughly Wagnerian
+opera. It may besides possess the virtue of converting mechanical
+energy into heat in a nearly perfect manner. If there is a
+reversal in the polarity of the projections, he will get heat out of
+the machine; if there is no reversal, the heating will be less, but
+the output will be next to nothing. He will then abandon the
+iron in the armature, and he will get from the Scylla to the
+Charybdis. He will look for one difficulty and will find another,
+but, after a few trials, he may get nearly what he wanted.<span class='pagenum'><a name="Page_382" id="Page_382">[Pg 382]</a></span></p>
+
+<p>Among the many experiments which may be performed with
+such a machine, of not the least interest are those performed
+with a high-tension induction coil. The character of the discharge
+is completely changed. The arc is established at much
+greater distances, and it is so easily affected by the slightest current
+of air that it often wriggles around in the most singular
+manner. It usually emits the rhythmical sound peculiar to the
+alternate current arcs, but the curious point is that the sound
+may be heard with a number of alternations far above ten thousand
+per second, which by many is considered to be about the
+limit of audition. In many respects the coil behaves like a static
+machine. Points impair considerably the sparking interval, electricity
+escaping from them freely, and from a wire attached to
+one of the terminals streams of light issue, as though it were
+connected to a pole of a powerful Toepler machine. All these
+phenomena are, of course, mostly due to the enormous differences
+of potential obtained. As a consequence of the self-induction
+of the coil and the high frequency, the current is minute
+while there is a corresponding rise of pressure. A current impulse
+of some strength started in such a coil should persist to
+flow no less than four ten-thousandths of a second. As this time
+is greater than half the period, it occurs that an opposing electromotive
+force begins to act while the current is still flowing. As
+a consequence, the pressure rises as in a tube filled with liquid
+and vibrated rapidly around its axis. The current is so small
+that, in the opinion and involuntary experience of the writer, the
+discharge of even a very large coil cannot produce seriously injurious
+effects, whereas, if the same coil were operated with a
+current of lower frequency, though the electromotive force would
+be much smaller, the discharge would be most certainly injurious.
+This result, however, is due in part to the high frequency.
+The writer's experiences tend to show that the higher the frequency
+the greater the amount of electrical energy which may
+be passed through the body without serious discomfort; whence
+it seems certain that human tissues act as condensers.</p>
+
+<p>One is not quite prepared for the behavior of the coil when
+connected to a Leyden jar. One, of course, anticipates that since
+the frequency is high the capacity of the jar should be small. He
+therefore takes a very small jar, about the size of a small wine
+glass, but he finds that even with this jar the coil is practically
+short-circuited. He then reduces the capacity until he comes to<span class='pagenum'><a name="Page_383" id="Page_383">[Pg 383]</a></span>
+about the capacity of two spheres, say, ten centimetres in diameter
+and two to four centimetres apart. The discharge then assumes
+the form of a serrated band exactly like a succession of
+sparks viewed in a rapidly revolving mirror; the serrations, of
+course, corresponding to the condenser discharges. In this case
+one may observe a queer phenomenon. The discharge starts at
+the nearest points, works gradually up, breaks somewhere near
+the top of the spheres, begins again at the bottom, and so on.
+This goes on so fast that several serrated bands are seen at once.
+One may be puzzled for a few minutes, but the explanation is
+simple enough. The discharge begins at the nearest points, the air
+is heated and carries the arc upward until it breaks, when it is re-established
+at the nearest points, etc. Since the current passes
+easily through a condenser of even small capacity, it will be found
+quite natural that connecting only one terminal to a body of the
+same size, no matter how well insulated, impairs considerably the
+striking distance of the arc.</p>
+
+<p>Experiments with Geissler tubes are of special interest. An
+exhausted tube, devoid of electrodes of any kind, will light up at
+some distance from the coil. If a tube from a vacuum pump is
+near the coil the whole of the pump is brilliantly lighted. An
+incandescent lamp approached to the coil lights up and gets perceptibly
+hot. If a lamp have the terminals connected to one of
+the binding posts of the coil and the hand is approached to the
+bulb, a very curious and rather unpleasant discharge from the
+glass to the hand takes place, and the filament may become incandescent.
+The discharge resembles to some extent the stream
+issuing from the plates of a powerful Toepler machine, but is of
+incomparably greater quantity. The lamp in this case acts as a
+condenser, the rarefied gas being one coating, the operator's hand
+the other. By taking the globe of a lamp in the hand, and by
+bringing the metallic terminals near to or in contact with a conductor
+connected to the coil, the carbon is brought to bright incandescence
+and the glass is rapidly heated. With a 100-volt 10 <span class="smcap">c.
+p.</span> lamp one may without great discomfort stand as much current
+as will bring the lamp to a considerable brilliancy; but it can be
+held in the hand only for a few minutes, as the glass is heated in
+an incredibly short time. When a tube is lighted by bringing it
+near to the coil it may be made to go out by interposing a metal
+plate on the hand between the coil and tube; but if the metal
+plate be fastened to a glass rod or otherwise insulated, the tube<span class='pagenum'><a name="Page_384" id="Page_384">[Pg 384]</a></span>
+may remain lighted if the plate be interposed, or may even increase
+in luminosity. The effect depends on the position of the
+plate and tube relatively to the coil, and may be always easily
+foretold by <i>assuming</i> that conduction takes place from one terminal
+of the coil to the other. According to the position of the
+plate, it may either divert from or direct the current to the tube.</p>
+
+<p>In another line of work the writer has in frequent experiments
+maintained incandescent lamps of 50 or 100 volts burning at any
+desired candle power with both the terminals of each lamp connected
+to a stout copper wire of no more than a few feet in
+length. These experiments seem interesting enough, but they
+are not more so than the queer experiment of Faraday, which
+has been revived and made much of by recent investigators, and
+in which a discharge is made to jump between two points of a
+bent copper wire. An experiment may be cited here which may
+seem equally interesting. If a Geissler tube, the terminals of
+which are joined by a copper wire, be approached to the coil, certainly
+no one would be prepared to see the tube light up.
+Curiously enough, it does light up, and, what is more, the
+wire does not seem to make much difference. Now one is
+apt to think in the first moment that the impedance of the
+wire might have something to do with the phenomenon. But
+this is of course immediately rejected, as for this an enormous
+frequency would be required. This result, however, seems
+puzzling only at first; for upon reflection it is quite clear that
+the wire can make but little difference. It may be explained in
+more than one way, but it agrees perhaps best with observation
+to assume that conduction takes place from the terminals of the
+coil through the space. On this assumption, if the tube with the
+wire be held in any position, the wire can divert little more than
+the current which passes through the space occupied by the wire
+and the metallic terminals of the tube; through the adjacent
+space the current passes practically undisturbed. For this reason,
+if the tube be held in any position at right angles to the line
+joining the binding posts of the coil, the wire makes hardly any
+difference, but in a position more or less parallel with that line
+it impairs to a certain extent the brilliancy of the tube and its
+facility to light up. Numerous other phenomena may be explained
+on the same assumption. For instance, if the ends of the
+tube be provided with washers of sufficient size and held in the
+line joining the terminals of the coil, it will not light up, and
+then nearly the whole of the current, which would otherwise<span class='pagenum'><a name="Page_385" id="Page_385">[Pg 385]</a></span>
+pass uniformly through the space between the washers, is diverted
+through the wire. But if the tube be inclined sufficiently
+to that line, it will light up in spite of the washers. Also, if a
+metal plate be fastened upon a glass rod and held at right angles
+to the line joining the binding posts, and nearer to one of them,
+a tube held more or less parallel with the line will light up instantly
+when one of the terminals touches the plate, and will go
+out when separated from the plate. The greater the surface of
+the plate, up to a certain limit, the easier the tube will light up.
+When a tube is placed at right angles to the straight line joining
+the binding posts, and then rotated, its luminosity steadily increases
+until it is parallel with that line. The writer must state,
+however, that he does not favor the idea of a leakage or current
+through the space any more than as a suitable explanation, for he
+is convinced that all these experiments could not be performed with
+a static machine yielding a constant difference of potential, and
+that condenser action is largely concerned in these phenomena.</p>
+
+<p>It is well to take certain precautions when operating a Ruhmkorff
+coil with very rapidly alternating currents. The primary
+current should not be turned on too long, else the core may get
+so hot as to melt the gutta-percha or paraffin, or otherwise injure
+the insulation, and this may occur in a surprisingly short time,
+considering the current's strength. The primary current being
+turned on, the fine wire terminals may be joined without great
+risk, the impedance being so great that it is difficult to force
+enough current through the fine wire so as to injure it, and in
+fact the coil may be on the whole much safer when the terminals
+of the fine wire are connected than when they are insulated;
+but special care should be taken when the terminals are connected
+to the coatings of a Leyden jar, for with anywhere near
+the critical capacity, which just counteracts the self-induction at
+the existing frequency, the coil might meet the fate of St. Polycarpus.
+If an expensive vacuum pump is lighted up by being
+near to the coil or touched with a wire connected to one of the
+terminals, the current should be left on no more than a few
+moments, else the glass will be cracked by the heating of the
+rarefied gas in one of the narrow passages&mdash;in the writer's own
+experience <i>quod erat demonstrandum</i>.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
+
+<p><span class='pagenum'><a name="Page_386" id="Page_386">[Pg 386]</a></span></p>
+<p>There are a good many other points of interest which may be
+observed in connection with such a machine. Experiments with
+the telephone, a conductor in a strong field or with a condenser
+or arc, seem to afford certain proof that sounds far above the
+usual accepted limit of hearing would be perceived. A telephone
+will emit notes of twelve to thirteen thousand vibrations per
+second; then the inability of the core to follow such rapid alternations
+begins to tell. If, however, the magnet and core be
+replaced by a condenser and the terminals connected to the high-tension
+secondary of a transformer, higher notes may still be
+heard. If the current be sent around a finely laminated core
+and a small piece of thin sheet iron be held gently against the
+core, a sound may be still heard with thirteen to fourteen thousand
+alternations per second, provided the current is sufficiently
+strong. A small coil, however, tightly packed between the poles
+of a powerful magnet, will emit a sound with the above number
+of alternations, and arcs may be audible with a still higher frequency.
+The limit of audition is variously estimated. In Sir
+William Thomson's writings it is stated somewhere that ten
+thousand per second, or nearly so, is the limit. Other, but less
+reliable, sources give it as high as twenty-four thousand per
+second. The above experiments have convinced the writer that
+notes of an incomparably higher number of vibrations per second
+would be perceived provided they could be produced with sufficient
+power. There is no reason why it should not be so. The
+condensations and rarefactions of the air would necessarily set
+the diaphragm in a corresponding vibration and some sensation
+would be produced, whatever&mdash;within certain limits&mdash;the velocity
+of transmission to their nerve centres, though it is probable that
+for want of exercise the ear would not be able to distinguish any
+such high note. With the eye it is different; if the sense of
+vision is based upon some resonance effect, as many believe, no
+amount of increase in the intensity of the ethereal vibration
+could extend our range of vision on either side of the visible
+spectrum.</p>
+
+<p>The limit of audition of an arc depends on its size. The
+greater the surface by a given heating effect in the arc, the higher
+the limit of audition. The highest notes are emitted by the
+high-tension discharges of an induction coil in which the arc is,
+so to speak, all surface. If <i>R</i> be the resistance of an arc, and <i>C</i>
+the current, and the linear dimensions be <i>n</i> times increased, then<span class='pagenum'><a name="Page_387" id="Page_387">[Pg 387]</a></span>
+the resistance is <i>R</i>/<i>n</i>, and with the same current density the current
+would be <i>n</i><sup>2</sup><i>C</i>; hence the heating effect is <i>n</i><sup>3</sup> times greater,
+while the surface is only <i>n</i><sup>2</sup> times as great. For this reason very
+large arcs would not emit any rhythmical sound even with a very
+low frequency. It must be observed, however, that the sound
+emitted depends to some extent also on the composition of the
+carbon. If the carbon contain highly refractory material, this,
+when heated, tends to maintain the temperature of the arc uniform
+and the sound is lessened; for this reason it would seem
+that an alternating arc requires such carbons.</p>
+
+<p>With currents of such high frequencies it is possible to obtain
+noiseless arcs, but the regulation of the lamp is rendered extremely
+difficult on account of the excessively small attractions
+or repulsions between conductors conveying these currents.</p>
+
+<p>An interesting feature of the arc produced by these rapidly
+alternating currents is its persistency. There are two causes for
+it, one of which is always present, the other sometimes only.
+One is due to the character of the current and the other to a
+property of the machine. The first cause is the more important
+one, and is due directly to the rapidity of the alternations.
+When an arc is formed by a periodically undulating current,
+there is a corresponding undulation in the temperature of the
+gaseous column, and, therefore, a corresponding undulation in
+the resistance of the arc. But the resistance of the arc varies
+enormously with the temperature of the gaseous column, being
+practically infinite when the gas between the electrodes is cold.
+The persistence of the arc, therefore, depends on the inability of
+the column to cool. It is for this reason impossible to maintain
+an arc with the current alternating only a few times a second.
+On the other hand, with a practically continuous current, the arc
+is easily maintained, the column being constantly kept at a high
+temperature and low resistance. The higher the frequency the
+smaller the time interval during which the arc may cool and increase
+considerably in resistance. With a frequency of 10,000
+per second or more in an arc of equal size excessively small variations
+of temperature are superimposed upon a steady temperature,
+like ripples on the surface of a deep sea. The heating effect is
+practically continuous and the arc behaves like one produced by
+a continuous current, with the exception, however, that it may
+not be quite as easily started, and that the electrodes are equally<span class='pagenum'><a name="Page_388" id="Page_388">[Pg 388]</a></span>
+consumed; though the writer has observed some irregularities in
+this respect.</p>
+
+<p>The second cause alluded to, which possibly may not be present,
+is due to the tendency of a machine of such high frequency
+to maintain a practically constant current. When the arc is
+lengthened, the electromotive force rises in proportion and the
+arc appears to be more persistent.</p>
+
+<p>Such a machine is eminently adapted to maintain a constant
+current, but it is very unfit for a constant potential. As a matter
+of fact, in certain types of such machines a nearly constant current
+is an almost unavoidable result. As the number of poles or
+polar projections is greatly increased, the clearance becomes of
+great importance. One has really to do with a great number of
+very small machines. Then there is the impedance in the armature,
+enormously augmented by the high frequency. Then,
+again, the magnetic leakage is facilitated. If there are three or
+four hundred alternate poles, the leakage is so great that it is
+virtually the same as connecting, in a two-pole machine, the poles
+by a piece of iron. This disadvantage, it is true, may be obviated
+more or less by using a field throughout of the same polarity,
+but then one encounters difficulties of a different nature. All
+these things tend to maintain a constant current in the armature
+circuit.</p>
+
+<p>In this connection it is interesting to notice that even to-day
+engineers are astonished at the performance of a constant current
+machine, just as, some years ago, they used to consider it an extraordinary
+performance if a machine was capable of maintaining
+a constant potential difference between the terminals. Yet one
+result is just as easily secured as the other. It must only be
+remembered that in an inductive apparatus of any kind, if constant
+potential is required, the inductive relation between the
+primary or exciting and secondary or armature circuit must be
+the closest possible; whereas, in an apparatus for constant current
+just the opposite is required. Furthermore, the opposition
+to the current's flow in the induced circuit must be as small as
+possible in the former and as great as possible in the latter case.
+But opposition to a current's flow may be caused in more than
+one way. It may be caused by ohmic resistance or self-induction.
+One may make the induced circuit of a dynamo machine
+or transformer of such high resistance that when operating devices
+of considerably smaller resistance within very wide limits a<span class='pagenum'><a name="Page_389" id="Page_389">[Pg 389]</a></span>
+nearly constant current is maintained. But such high resistance
+involves a great loss in power, hence it is not practicable. Not
+so self-induction. Self-induction does not necessarily mean loss
+of power. The moral is, use self-induction instead of resistance.
+There is, however, a circumstance which favors the adoption of
+this plan, and this is, that a very high self-induction may be
+obtained cheaply by surrounding a comparatively small length
+of wire more or less completely with iron, and, furthermore, the
+effect may be exalted at will by causing a rapid undulation of the
+current. To sum up, the requirements for constant current
+are: Weak magnetic connection between the induced and
+inducing circuits, greatest possible self-induction with the
+least resistance, greatest practicable rate of change of the
+current. Constant potential, on the other hand, requires: Closest
+magnetic connection between the circuits, steady induced
+current, and, if possible, no reaction. If the latter conditions
+could be fully satisfied in a constant potential machine, its output
+would surpass many times that of a machine primarily designed
+to give constant current. Unfortunately, the type of machine
+in which these conditions may be satisfied is of little practical
+value, owing to the small electromotive force obtainable and the
+difficulties in taking off the current.</p>
+
+<p>With their keen inventor's instinct, the now successful arc-light
+men have early recognized the desiderata of a constant
+current machine. Their arc light machines have weak fields,
+large armatures, with a great length of copper wire and few
+commutator segments to produce great variations in the current's
+strength and to bring self-induction into play. Such machines
+may maintain within considerable limits of variation in the resistance
+of the circuit a practically constant current. Their output
+is of course correspondingly diminished, and, perhaps with
+the object in view not to cut down the output too much, a simple
+device compensating exceptional variations is employed.
+The undulation of the current is almost essential to the commercial
+success of an arc-light system. It introduces in the circuit a
+steadying element taking the place of a large ohmic resistance,
+without involving a great loss in power, and, what is more important,
+it allows the use of simple clutch lamps, which with a
+current of a certain number of impulses per second, best suitable
+for each particular lamp, will, if properly attended to, regulate
+even better than the finest clock-work lamps. This discovery
+has been made by the writer&mdash;several years too late.<span class='pagenum'><a name="Page_390" id="Page_390">[Pg 390]</a></span></p>
+
+<p>It has been asserted by competent English electricians that in a
+constant-current machine or transformer the regulation is effected
+by varying the phase of the secondary current. That this view
+is erroneous may be easily proved by using, instead of lamps, devices
+each possessing self-induction and capacity or self-induction
+and resistance&mdash;that is, retarding and accelerating components&mdash;in
+such proportions as to not affect materially the phase of the
+secondary current. Any number of such devices may be inserted
+or cut out, still it will be found that the regulation occurs, a constant
+current being maintained, while the electromotive force is
+varied with the number of the devices. The change of phase of
+the secondary current is simply a result following from the
+changes in resistance, and, though secondary reaction is always
+of more or less importance, yet the real cause of the regulation
+lies in the existence of the conditions above enumerated. It
+should be stated, however, that in the case of a machine the above
+remarks are to be restricted to the cases in which the machine is
+independently excited. If the excitation be effected by commutating
+the armature current, then the fixed position of the brushes
+makes any shifting of the neutral line of the utmost importance,
+and it may not be thought immodest of the writer to mention
+that, as far as records go, he seems to have been the first who has
+successfully regulated machines by providing a bridge connection
+between a point of the external circuit and the commutator by
+means of a third brush. The armature and field being properly
+proportioned and the brushes placed in their determined positions,
+a constant current or constant potential resulted from the
+shifting of the diameter of commutation by the varying loads.</p>
+
+<p>In connection with machines of such high frequencies, the
+condenser affords an especially interesting study. It is easy to
+raise the electromotive force of such a machine to four or five
+times the value by simply connecting the condenser to the circuit,
+and the writer has continually used the condenser for the
+the purposes of regulation, as suggested by Blakesley in his book
+on alternate currents, in which he has treated the most frequently
+occurring condenser problems with exquisite simplicity and clearness.
+The high frequency allows the use of small capacities and
+renders investigation easy. But, although in most of the experiments
+the result may be foretold, some phenomena observed seem
+at first curious. One experiment performed three or four months
+ago with such a machine and a condenser may serve as an il<span class='pagenum'><a name="Page_391" id="Page_391">[Pg 391]</a></span>lustration.
+A machine was used giving about 20,000 alternations
+per second. Two bare wires about twenty feet long and two
+millimetres in diameter, in close proximity to each other, were
+connected to the terminals of the machine at the one end, and
+to a condenser at the other. A small transformer without an
+iron core, of course, was used to bring the reading within range
+of a Cardew voltmeter by connecting the voltmeter to the
+secondary. On the terminals of the condenser the electromotive
+force was about 120 volts, and from there inch by inch it gradually
+fell until at the terminals of the machine it was about 65
+volts. It was virtually as though the condenser were a generator,
+and the line and armature circuit simply a resistance connected
+to it. The writer looked for a case of resonance, but he
+was unable to augment the effect by varying the capacity very
+carefully and gradually or by changing the speed of the machine.
+A case of pure resonance he was unable to obtain.
+When a condenser was connected to the terminals of the machine&mdash;the
+self-induction of the armature being first determined
+in the maximum and minimum position and the mean value taken&mdash;the
+capacity which gave the highest electromotive force corresponded
+most nearly to that which just counteracted the self-induction
+with the existing frequency. If the capacity was increased
+or diminished, the electromotive force fell as expected.</p>
+
+<p>With frequencies as high as the above mentioned, the condenser
+effects are of enormous importance. The condenser
+becomes a highly efficient apparatus capable of transferring
+considerable energy.</p>
+
+<hr style='width: 15%;' />
+
+<p>In an appendix to this book will be found a description of the
+Tesla oscillator, which its inventor believes will among other great
+advantages give him the necessary high frequency conditions,
+while relieving him of the inconveniences that attach to generators
+of the type described at the beginning of this chapter.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_392" id="Page_392">[Pg 392]</a></span></p>
+<h2><a name="CHAPTER_XXX" id="CHAPTER_XXX"></a>CHAPTER XXX.</h2>
+
+<h3><span class="smcap">Alternate Current Electrostatic Induction Apparatus.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a></span></h3>
+
+
+<p>About a year and a half ago while engaged in the study of
+alternate currents of short period, it occurred to me that such
+currents could be obtained by rotating charged surfaces in close
+proximity to conductors. Accordingly I devised various forms
+of experimental apparatus of which two are illustrated in the
+accompanying engravings.</p>
+
+<div class="figcenter" style="width: 639px;">
+<img src="images/oi_406.jpg" width="639" height="600" alt="Fig. 208." title="" />
+<span class="caption">Fig. 208.</span>
+</div>
+
+<p>In the apparatus shown in Fig. 208, <small>A</small> is a ring of dry shellacked
+hard wood provided on its inside with two sets of tin-foil
+coatings, <i>a</i> and <i>b</i>, all the <i>a</i> coatings and all the <i>b</i> coatings being
+connected together, respectively, but independent from each
+other. These two sets of coatings are connected to two termi<span class='pagenum'><a name="Page_393" id="Page_393">[Pg 393]</a></span>nals,
+<small>T</small>. For the sake of clearness only a few coatings are shown.
+Inside of the ring <small>A</small>, and in close proximity to it there is arranged
+to rotate a cylinder <small>B</small>, likewise of dry, shellacked hard wood, and
+provided with two similar sets of coatings, <i>a</i><sup>1</sup> and <i>b</i><sup>1</sup>, all the coatings
+<i>a</i><sup>1</sup> being connected to one ring and all the others, <i>b</i><sup>1</sup>, to
+another marked &#43; and &minus;. These two sets, <i>a</i><sup>1</sup> and <i>b</i><sup>1</sup> are charged
+to a high potential by a Holtz or Wimshurst machine, and may
+be connected to a jar of some capacity. The inside of ring <small>A</small> is
+coated with mica in order to increase the induction and also to
+allow higher potentials to be used.</p>
+
+<div class="figcenter" style="width: 627px;">
+<img src="images/oi_407.jpg" width="627" height="600" alt="Fig. 209." title="" />
+<span class="caption">Fig. 209.</span>
+</div>
+
+
+<p>When the cylinder <small>B</small> with the charged coatings is rotated, a
+circuit connected to the terminals <small>T</small> is traversed by alternating
+currents. Another form of apparatus is illustrated in Fig. 209.
+In this apparatus the two sets of tin-foil coatings are glued on a
+plate of ebonite, and a similar plate which is rotated, and the
+coatings of which are charged as in Fig. 208, is provided.</p>
+
+<p>The output of such an apparatus is very small, but some of
+the effects peculiar to alternating currents of short periods may
+be observed. The effects, however, cannot be compared with
+those obtainable with an induction coil which is operated by an
+alternate current machine of high frequency, some of which
+were described by me a short while ago.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_394" id="Page_394">[Pg 394]</a></span></p>
+<h2><a name="CHAPTER_XXXI" id="CHAPTER_XXXI"></a>CHAPTER XXXI.</h2>
+
+<h3><span class="smcap">"Massage" With Currents of High Frequency.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a></span></h3>
+
+
+<p>I trust that the present brief communication will not be interpreted
+as an effort on my part to put myself on record as a
+"patent medicine" man, for a serious worker cannot despise
+anything more than the misuse and abuse of electricity which we
+have frequent occasion to witness. My remarks are elicited by
+the lively interest which prominent medical practitioners evince
+at every real advance in electrical investigation. The progress
+in recent years has been so great that every electrician and electrical
+engineer is confident that electricity will become the means
+of accomplishing many things that have been heretofore, with
+our existing knowledge, deemed impossible. No wonder then
+that progressive physicians also should expect to find in it a
+powerful tool and help in new curative processes. Since I had
+the honor to bring before the American Institute of Electrical
+Engineers some results in utilizing alternating currents of high
+tension, I have received many letters from noted physicians inquiring
+as to the physical effects of such currents of high frequency.
+It may be remembered that I then demonstrated that
+a body perfectly well insulated in air can be heated by simply
+connecting it with a source of rapidly alternating high potential.
+The heating in this case is due in all probability to the bombardment
+of the body by air, or possibly by some other medium,
+which is molecular or atomic in construction, and the presence
+of which has so far escaped our analysis&mdash;for according to my
+ideas, the true ether radiation with such frequencies as even a
+few millions per second must be very small. This body may be
+a good conductor or it may be a very poor conductor of electricity
+with little change in the result. The human body is, in
+such a case, a fine conductor, and if a person insulated in a room,
+or no matter where, is brought into contact with such a source of
+<span class='pagenum'><a name="Page_395" id="Page_395">[Pg 395]</a></span>rapidly alternating high potential, the skin is heated by bombardment.
+It is a mere question of the dimensions and character
+of the apparatus to produce any degree of heating desired.</p>
+
+<p>It has occurred to me whether, with such apparatus properly
+prepared, it would not be possible for a skilled physician to find
+in it a means for the effective treatment of various types of disease.
+The heating will, of course, be superficial, that is, on the
+skin, and would result, whether the person operated on were in
+bed or walking around a room, whether dressed in thick clothes or
+whether reduced to nakedness. In fact, to put it broadly, it is
+conceivable that a person entirely nude at the North Pole might
+keep himself comfortably warm in this manner.</p>
+
+<p>Without vouching for all the results, which must, of course, be
+determined by experience and observation, I can at least warrant
+the fact that heating would occur by the use of this method of
+subjecting the human body to bombardment by alternating currents
+of high potential and frequency such I have long worked
+with. It is only reasonable to expect that some of the novel effects
+will be wholly different from those obtainable with the old
+familiar therapeutic methods generally used. Whether they
+would all be beneficial or not remains to be proved.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_396" id="Page_396">[Pg 396]</a></span></p>
+<h2><a name="CHAPTER_XXXII" id="CHAPTER_XXXII"></a>CHAPTER XXXII.</h2>
+
+<h3><span class="smcap">Electric Discharge in Vacuum Tubes.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></span></h3>
+
+
+<p>In <i>The Electrical Engineer</i> of June 10 I have noted the description
+of some experiments of Prof. J. J. Thomson, on the
+"Electric Discharge in Vacuum Tubes," and in your issue of June
+24 Prof. Elihu Thomson describes an experiment of the same
+kind. The fundamental idea in these experiments is to set up
+an electromotive force in a vacuum tube&mdash;-preferably devoid of
+any electrodes&mdash;by means of electro-magnetic induction, and to
+excite the tube in this manner.</p>
+
+<p>As I view the subject I should, think that to any experimenter
+who had carefully studied the problem confronting us and who
+attempted to find a solution of it, this idea must present itself as
+naturally as, for instance, the idea of replacing the tinfoil coatings
+of a Leyden jar by rarefied gas and exciting luminosity in
+the condenser thus obtained by repeatedly charging and discharging
+it. The idea being obvious, whatever merit there is in this
+line of investigation must depend upon the completeness of the
+study of the subject and the correctness of the observations. The
+following lines are not penned with any desire on my part to put
+myself on record as one who has performed similar experiments,
+but with a desire to assist other experimenters by pointing out
+certain peculiarities of the phenomena observed, which, to all appearances,
+have not been noted by Prof. J. J. Thomson, who,
+however, seems to have gone about systematically in his investigations,
+and who has been the first to make his results known.
+These peculiarities noted by me would seem to be at variance
+with the views of Prof. J. J. Thomson, and present the phenomena
+in a different light.</p>
+
+<p>My investigations in this line occupied me principally during
+the winter and spring of the past year. During this time many different
+experiments were performed, and in my exchanges of ideas
+<span class='pagenum'><a name="Page_397" id="Page_397">[Pg 397]</a></span>on this subject with Mr. Alfred S. Brown, of the Western Union
+Telegraph Company, various different dispositions were suggested
+which were carried out by me in practice. Fig. 210 may serve
+as an example of one of the many forms of apparatus used. This
+consisted of a large glass tube sealed at one end and projecting
+into an ordinary incandescent lamp bulb. The primary, usually
+consisting of a few turns of thick, well-insulated copper sheet was
+inserted within the tube, the inside space of the bulb furnishing
+the secondary. This form of apparatus was arrived at after some
+experimenting, and was used principally with the view of enabling
+me to place a polished reflecting surface on the inside of
+the tube, and for this purpose the last turn of the primary was
+covered with a thin silver sheet. In all forms of apparatus used
+there was no special difficulty in exciting a luminous circle or
+cylinder in proximity to the primary.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_411.jpg" width="336" height="448" alt="Fig. 210." title="" />
+<span class="caption">Fig. 210.</span>
+</div>
+
+<p>As to the number of turns, I cannot quite understand why
+Prof. J. J. Thomson should think that a few turns were "quite
+sufficient," but lest I should impute to him an opinion he may
+not have, I will add that I have gained this impression from the
+reading of the published abstracts of his lecture. Clearly, the
+number of turns which gives the best result in any case, is dependent
+on the dimensions of the apparatus, and, were it not for
+various considerations, one turn would always give the best
+result.</p>
+
+<p>I have found that it is preferable to use in these experiments
+an alternate current machine giving a moderate number of alter<span class='pagenum'><a name="Page_398" id="Page_398">[Pg 398]</a></span>nations
+per second to excite the induction coil for charging the
+Leyden jar which discharges through the primary&mdash;shown diagrammatically
+in Fig. 211,&mdash;as in such case, before the disruptive
+discharge takes place, the tube or bulb is slightly excited and
+the formation of the luminous circle is decidedly facilitated.
+But I have also used a Wimshurst machine in some experiments.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_412.jpg" width="640" height="180" alt="Fig. 211." title="" />
+<span class="caption">Fig. 211.</span>
+</div>
+
+<p>Prof. J. J. Thomson's view of the phenomena under consideration
+seems to be that they are wholly due to electro-magnetic
+action. I was, at one time, of the same opinion, but upon carefully
+investigating the subject I was led to the conviction that
+they are more of an electrostatic nature. It must be remembered
+that in these experiments we have to deal with primary
+currents of an enormous frequency or rate of change and of high
+potential, and that the secondary conductor consists of a rarefied
+gas, and that under such conditions electrostatic effects must play
+an important part.</p>
+
+<div class="figcenter" style="width: 588px;">
+<img src="images/oi_412-1.jpg" width="588" height="480" alt="Fig. 212." title="" />
+<span class="caption">Fig. 212.</span>
+</div>
+
+
+<p>In support of my view I will describe a few experiments made
+by me. To excite luminosity in the tube it is not absolutely
+necessary that the conductor should be closed. For instance, if<span class='pagenum'><a name="Page_399" id="Page_399">[Pg 399]</a></span>
+an ordinary exhausted tube (preferably of large diameter) be
+surrounded by a spiral of thick copper wire serving as the primary,
+a feebly luminous spiral may be induced in the tube, roughly
+shown in Fig. 212. In one of these experiments a curious phenomenon
+was observed; namely, two intensely luminous circles,
+each of them close to a turn of the primary spiral, were formed
+inside of the tube, and I attributed this phenomenon to the existence
+of nodes on the primary. The circles were connected by
+a faint luminous spiral parallel to the primary and in close proximity
+to it. To produce this effect I have found it necessary to
+strain the jar to the utmost. The turns of the spiral tend to
+close and form circles, but this, of course, would be expected,
+and does not necessarily indicate an electro-magnetic effect;
+Whereas the fact that a glow can be produced along the primary
+in the form of an open spiral argues for an electrostatic effect.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_413.jpg" width="640" height="334" alt="Fig. 213." title="" />
+<span class="caption">Fig. 213.</span>
+</div>
+
+<p>In using Dr. Lodge's recoil circuit, the electrostatic action is
+likewise apparent. The arrangement is illustrated in Fig. 213.
+In his experiment two hollow exhausted tubes <small>H H</small> were slipped
+over the wires of the recoil circuit and upon discharging the jar
+in the usual manner luminosity was excited in the tubes.</p>
+
+<p>Another experiment performed is illustrated in Fig. 214. In
+this case an ordinary lamp-bulb was surrounded by one or two
+turns of thick copper wire <small>P</small> and the luminous circle <small>L</small> excited
+in the bulb by discharging the jar through the primary. The
+lamp-bulb was provided with a tinfoil coating on the side opposite
+to the primary and each time the tinfoil coating was connected
+to the ground or to a large object the luminosity of the
+circle was considerably increased. This was evidently due to
+electrostatic action.</p>
+
+<p>In other experiments I have noted that when the primary
+touches the glass the luminous circle is easier produced and is<span class='pagenum'><a name="Page_400" id="Page_400">[Pg 400]</a></span>
+more sharply defined; but I have not noted that, generally speaking,
+the circles induced were very sharply defined, as Prof. J. J.
+Thomson has observed; on the contrary, in my experiments they
+were broad and often the whole of the bulb or tube was illuminated;
+and in one case I have observed an intensely purplish
+glow, to which Prof. J. J. Thomson refers. But the circles were
+always in close proximity to the primary and were considerably
+easier produced when the latter was very close to the glass, much
+more so than would be expected assuming the action to be electromagnetic
+and considering the distance; and these facts speak
+for an electrostatic effect.</p>
+
+<div class="figcenter" style="width: 534px;">
+<img src="images/oi_414.jpg" width="534" height="480" alt="Fig. 214." title="" />
+<span class="caption">Fig. 214.</span>
+</div>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_414-1.jpg" width="640" height="449" alt="Fig. 215." title="" />
+<span class="caption">Fig. 215.</span>
+</div>
+
+
+<p>Furthermore I have observed that there is a molecular bombardment
+in the plane of the luminous circle at right angles to
+the glass&mdash;supposing the circle to be in the plane of the primary<span class='pagenum'><a name="Page_401" id="Page_401">[Pg 401]</a></span>&mdash;this
+bombardment being evident from the rapid heating of the
+glass near the primary. Were the bombardment not at right
+angles to the glass the heating could not be so rapid. If there
+is a circumferential movement of the molecules constituting the
+luminous circle, I have thought that it might be rendered manifest
+by placing within the tube or bulb, radially to the circle, a
+thin plate of mica coated with some phosphorescent material and
+another such plate tangentially to the circle. If the molecules
+would move circumferentially, the former plate would be rendered
+more intensely phosphorescent. For want of time I have,
+however, not been able to perform the experiment.</p>
+
+<p>Another observation made by me was that when the specific
+inductive capacity of the medium between the primary and
+secondary is increased, the inductive effect is augmented. This
+is roughly illustrated in Fig. 215. In this case luminosity was
+excited in an exhausted tube or bulb <small>B</small> and a glass tube <small>T</small> slipped
+between the primary and the bulb, when the effect pointed out
+was noted. Were the action wholly electromagnetic no change
+could possibly have been observed.</p>
+
+<p>I have likewise noted that when a bulb is surrounded by a
+wire closed upon itself and in the plane of the primary, the formation
+of the luminous circle within the bulb is not prevented.
+But if instead of the wire a broad strip of tinfoil is glued upon
+the bulb, the formation of the luminous band was prevented, because
+then the action was distributed over a greater surface. The
+effect of the closed tinfoil was no doubt of an electrostatic nature,
+for it presented a much greater resistance than the closed wire
+and produced therefore a much smaller electromagnetic effect.</p>
+
+<p>Some of the experiments of Prof. J. J. Thomson also would
+seem to show some electrostatic action. For instance, in the experiment
+with the bulb enclosed in a bell jar, I should think
+that when the latter is exhausted so far that the gas enclosed
+reaches the maximum conductivity, the formation of the circle
+in the bulb and jar is prevented because of the space surrounding
+the primary being highly conducting; when the jar is further
+exhausted, the conductivity of the space around the primary
+diminishes and the circles appear necessarily first in the bell jar,
+as the rarefied gas is nearer to the primary. But were the inductive
+effect very powerful, they would probably appear in the
+bulb also. If, however, the bell jar were exhausted to the highest
+degree they would very likely show themselves in the bulb<span class='pagenum'><a name="Page_402" id="Page_402">[Pg 402]</a></span>
+only, that is, supposing the vacuous space to be non-conducting.
+On the assumption that in these phenomena electrostatic actions
+are concerned we find it easily explicable why the introduction
+of mercury or the heating of the bulb prevents the formation of
+the luminous band or shortens the after-glow; and also why in
+some cases a platinum wire may prevent the excitation of the
+tube. Nevertheless some of the experiments of Prof. J. J.
+Thomson would seem to indicate an electromagnetic effect. I
+may add that in one of my experiments in which a vacuum was
+produced by the Torricellian method, I was unable to produce
+the luminous band, but this may have been due to the weak exciting
+current employed.</p>
+
+<p>My principal argument is the following: I have experimentally
+proved that if the same discharge which is barely sufficient
+to excite a luminous band in the bulb when passed through the
+primary circuit be so directed as to exalt the electrostatic inductive
+effect&mdash;namely, by converting upwards&mdash;an exhausted tube,
+devoid of electrodes, may be excited at a distance of several feet.</p>
+
+<hr style='width: 15%;' />
+
+<h5>SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a></h5>
+
+<h5>BY PROF. J. J. THOMSON, M.A., F.R.S.</h5>
+
+
+
+<div  class="blockquot">
+<p>The phenomena of vacuum discharges were, Prof. Thomson said, greatly
+simplified when their path was wholly gaseous, the complication of the dark
+space surrounding the negative electrode, and the stratifications so commonly
+observed in ordinary vacuum tubes, being absent. To produce discharges in
+tubes devoid of electrodes was, however, not easy to accomplish, for the only
+available means of producing an electromotive force in the discharge circuit
+was by electro-magnetic induction. Ordinary methods of producing variable
+induction were valueless, and recourse was had to the oscillatory discharge of a
+<span class='pagenum'><a name="Page_403" id="Page_403">[Pg 403]</a></span>Leyden jar, which combines the two essentials of a current whose maximum
+value is enormous, and whose rapidity of alternation is immensely great. The
+discharge circuits, which may take the shape of bulbs, or of tubes bent in the
+form of coils, were placed in close proximity to glass tubes filled with mercury,
+which formed the path of the oscillatory discharge. The parts thus corresponded
+to the windings of an induction coil, the vacuum tubes being the secondary,
+and the tubes filled with mercury the primary. In such an apparatus
+the Leyden jar need not be large, and neither primary nor secondary need have
+many turns, for this would increase the self-induction of the former, and
+lengthen the discharge path in the latter. Increasing the self-induction of the
+primary reduces the <span class="smcap">e. m. f.</span> induced in the secondary, whilst lengthening the
+secondary does not increase the <span class="smcap">e. m. f.</span> per unit length. The two or three
+turns, as shown in Fig. 216, in each, were found to be quite sufficient, and, on
+discharging the Leyden jar between two highly polished knobs in the primary
+circuit, a plain uniform band of light was seen to pass round the secondary.
+An exhausted bulb, Fig. 217, containing traces of oxygen was placed within a
+primary spiral of three turns, and, on passing the jar discharge, a circle of light
+was seen within the bulb in close proximity to the primary circuit, accompanied
+by a purplish glow, which lasted for a second or more. On heating the
+bulb, the duration of the glow was greatly diminished, and it could be instantly
+extinguished by the presence of an electro-magnet. Another exhausted
+bulb, Fig. 218, surrounded by a primary spiral, was contained in a bell-jar,
+and when the pressure of air in the jar was about that of the atmosphere, the
+secondary discharge occurred in the bulb, as is ordinarily the case. On exhausting
+the jar, however, the luminous discharge grew fainter, and a point
+was reached at which no secondary discharge was visible. Further exhaustion
+of the jar caused the secondary discharge to appear outside of the bulb. The
+fact of obtaining no luminous discharge, either in the bulb or jar, the author<span class='pagenum'><a name="Page_404" id="Page_404">[Pg 404]</a></span>
+could only explain on two suppositions, viz.: that under the conditions then existing
+the specific inductive capacity of the gas was very great, or that a discharge
+could pass without being luminous. The author had also observed
+that the conductivity of a vacuum tube without electrodes increased as the pressure
+diminished, until a certain point was reached, and afterwards diminished
+again, thus showing that the high resistance of a nearly perfect vacuum is in
+no way due to the presence of the electrodes. One peculiarity of the discharges
+was their local nature, the rings of light being much more sharply defined than
+was to be expected. They were also found to be most easily produced when
+the chain of molecules in the discharge were all of the same kind. For example,
+a discharge could be easily sent through a tube many feet long, but the
+introduction of a small pellet of mercury in the tube stopped the discharge,
+although the conductivity of the mercury was much greater than that of the
+vacuum. In some cases he had noticed that a very fine wire placed within a
+tube, on the side remote from the primary circuit, would prevent a luminous
+discharge in that tube.</p>
+
+<p>Fig. 219 shows an exhausted secondary coil of one loop containing bulbs;
+the discharge passed along the inner side of the bulbs, the primary coils being
+placed within the secondary.</p>
+</div>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_416.jpg" width="800" height="456" alt="Fig. 216, 217." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 216.</td><td class="caption">Fig. 217.</td></tr>
+</table>
+</div>
+
+<div class="figcenter" style="width: 766px;">
+<img src="images/oi_417.jpg" width="766" height="600" alt="Fig. 218, 219." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 218.</td><td class="caption">Fig. 219.</td></tr>
+</table>
+</div>
+
+<hr style='width: 15%;' />
+
+<p><a name="FNanchor_9_10" id="FNanchor_9_10"></a><a href="#Footnote_9_10" class="fnanchor">[9]</a>In <i>The Electrical Engineer</i> of August 12, I find some remarks
+of Prof. J. J. Thomson, which appeared originally in the
+London <i>Electrician</i> and which have a bearing upon some experiments
+described by me in your issue of July 1.</p>
+
+<p>I did not, as Prof. J. J. Thomson seems to believe, misunderstand
+his position in regard to the cause of the phenomena
+considered, but I thought that in his experiments, as well as in
+my own, electrostatic effects were of great importance. It did
+not appear, from the meagre description of his experiments, that
+all possible precautions had been taken to exclude these effects.
+I did not doubt that luminosity could be excited in a closed tube
+when electrostatic action is completely excluded. In fact, at the
+outset, I myself looked for a purely electrodynamic effect and
+believed that I had obtained it. But many experiments performed
+at that time proved to me that the electrostatic effects
+were generally of far greater importance, and admitted of a more
+satisfactory explanation of most of the phenomena observed.</p>
+
+<p>In using the term <i>electrostatic</i> I had reference rather to the
+nature of the action than to a stationary condition, which is the
+usual acceptance of the term. To express myself more clearly,
+I will suppose that near a closed exhausted tube be placed a small
+sphere charged to a very high potential. The sphere would act
+inductively upon the tube, and by distributing electricity over
+<span class='pagenum'><a name="Page_405" id="Page_405">[Pg 405]</a></span>the same would undoubtedly produce luminosity (if the potential
+be sufficiently high), until a permanent condition would be
+reached. Assuming the tube to be perfectly well insulated,
+there would be only one instantaneous flash during the act of
+distribution. This would be due to the electrostatic action
+simply.</p>
+
+<p>But now, suppose the charged sphere to be moved at short intervals
+with great speed along the exhausted tube. The tube
+would now be permanently excited, as the moving sphere would
+cause a constant redistribution of electricity and collisions of the
+molecules of the rarefied gas. We would still have to deal with
+an electrostatic effect, and in addition an electrodynamic effect
+would be observed. But if it were found that, for instance, the
+effect produced depended more on the specific inductive capacity
+than on the magnetic permeability of the medium&mdash;which
+would certainly be the case for speeds incomparably lower than
+that of light&mdash;then I believe I would be justified in saying that
+the effect produced was more of an electrostatic nature. I do
+not mean to say, however, that any similar condition prevails in
+the case of the discharge of a Leyden jar through the primary,
+but I think that such an action would be desirable.</p>
+
+<p>It is in the spirit of the above example that I used the terms
+"more of an electrostatic nature," and have investigated the influence
+of bodies of high specific inductive capacity, and observed,
+for instance, the importance of the quality of glass of which the
+tube is made. I also endeavored to ascertain the influence of a
+medium of high permeability by using oxygen. It appeared
+from rough estimation that an oxygen tube when excited under
+similar conditions&mdash;that is, as far as could be determined&mdash;gives
+more light; but this, of course, may be due to many causes.</p>
+
+<p>Without doubting in the least that, with the care and precautions
+taken by Prof. J. J. Thomson, the luminosity excited was
+due solely to electrodynamic action, I would say that in many
+experiments I have observed curious instances of the ineffectiveness
+of the screening, and I have also found that the electrification
+through the air is often of very great importance, and may,
+in some cases, determine the excitation of the tube.</p>
+
+<p>In his original communication to the <i>Electrician</i>, Prof. J. J.
+Thomson refers to the fact that the luminosity in a tube near a
+wire through which a Leyden jar was discharged was noted by
+Hittorf. I think that the feeble luminous effect referred to has<span class='pagenum'><a name="Page_406" id="Page_406">[Pg 406]</a></span>
+been noted by many experimenters, but in my experiments the
+effects were much more powerful than those usually noted.</p>
+
+<p>The following is the communication<a name="FNanchor_10_11" id="FNanchor_10_11"></a><a href="#Footnote_10_11" class="fnanchor">[10]</a> referred to:&mdash;</p>
+
+<hr style='width: 15%;' />
+
+<div  class="blockquot">
+<p>"Mr. Tesla seems to ascribe the effects he observed to electrostatic action,
+and I have no doubt, from the description he gives of his method of conducting
+his experiments, that in them electrostatic action plays a very important
+part. He seems, however, to have misunderstood my position with respect to
+the cause of these discharges, which is not, as he implies, that luminosity in
+tubes without electrodes cannot be produced by electrostatic action, but that it
+can also be produced when this action is excluded. As a matter of fact, it is
+very much easier to get the luminosity when these electrostatic effects are
+operative than when they are not. As an illustration of this I may mention
+that the first experiment I tried with the discharge of a Leyden jar produced
+luminosity in the tube, but it was not until after six weeks' continuous experimenting
+that I was able to get a discharge in the exhausted tube which I was
+satisfied was due to what is ordinarily called electrodynamic action. It is advisable
+to have a clear idea of what we mean by electrostatic action. If,
+previous to the discharge of the jar, the primary coil is raised to a high potential,
+it will induce over the glass of the tube a distribution of electricity.
+When the potential of the primary suddenly falls, this electrification will redistribute
+itself, and may pass through the rarefied gas and produce luminosity
+in doing so. Whilst the discharge of the jar is going on, it is difficult, and,
+from a theoretical point of view, undesirable, to separate the effect into parts,
+one of which is called electrostatic, the other electromagnetic; what we can
+prove is that in this case the discharge is not such as would be produced by
+electromotive forces derived from a potential function. In my experiments the
+primary coil was connected to earth, and, as a further precaution, the primary
+was separated from the discharge tube by a screen of blotting paper, moistened
+with dilute sulphuric acid, and connected to earth. Wet blotting paper is a
+sufficiently good conductor to screen off a stationary electrostatic effect, though
+it is not a good enough one to stop waves of alternating electromotive intensity.
+When showing the experiments to the Physical Society I could not, of course,
+keep the tubes covered up, but, unless my memory deceives me, I stated the
+precautions which had been taken against the electrostatic effect. To correct
+misapprehension I may state that I did not read a formal paper to the Society,
+my object being to exhibit a few of the most typical experiments. The account
+of the experiments in the <i>Electrician</i> was from a reporter's note, and was
+not written, or even read, by me. I have now almost finished writing out, and
+hope very shortly to publish, an account of these and a large number of allied
+experiments, including some analogous to those mentioned by Mr. Tesla on the
+effect of conductors placed near the discharge tube, which I find, in some
+cases, to produce a diminution, in others an increase, in the brightness of the
+discharge, as well as some on the effect of the presence of substances of large
+specific inductive capacity. These seem to me to admit of a satisfactory explanation,
+for which, however, I must refer to my paper."</p>
+</div>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_407" id="Page_407">[Pg 407]</a></span></p>
+<h1><small><a name="PART_III" id="PART_III"></a>PART III.</small><br /><br />
+
+MISCELLANEOUS INVENTIONS AND<br />
+WRITINGS.</h1>
+<p><span class='pagenum'><a name="Page_408" id="Page_408">[Pg 408]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_409" id="Page_409">[Pg 409]</a></span></p>
+<h2><a name="CHAPTER_XXXIII" id="CHAPTER_XXXIII"></a>CHAPTER XXXIII.</h2>
+
+<h3><span class="smcap">Method of Obtaining Driect From Alternating Currents.</span></h3>
+
+
+<p>This method consists in obtaining direct from alternating
+currents, or in directing the waves of an alternating current so as
+to produce direct or substantially direct currents by developing
+or producing in the branches of a circuit including a source of alternating
+currents, either permanently or periodically, and by
+electric, electro-magnetic, or magnetic agencies, manifestations of
+energy, or what may be termed active resistances of opposite
+electrical character, whereby the currents or current waves of opposite
+sign will be diverted through different circuits, those of
+one sign passing over one branch and those of opposite sign over
+the other.</p>
+
+<p>We may consider herein only the case of a circuit divided into
+two paths, inasmuch as any further subdivision involves merely
+an extension of the general principle. Selecting, then, any circuit
+through which is flowing an alternating current, Mr. Tesla
+divides such circuit at any desired point into two branches or
+paths. In one of these paths he inserts some device to create
+an electromotive force counter to the waves or impulses of current
+of one sign and a similar device in the other branch which
+opposes the waves of opposite sign. Assume, for example, that
+these devices are batteries, primary or secondary, or continuous
+current dynamo machines. The waves or impulses of opposite
+direction composing the main current have a natural tendency to
+divide between the two branches; but by reason of the opposite
+electrical character or effect of the two branches, one will offer
+an easy passage to a current of a certain direction, while the other
+will offer a relatively high resistance to the passage of the same
+current. The result of this disposition is, that the waves of current
+of one sign will, partly or wholly, pass over one of the paths
+or branches, while those of the opposite sign pass over the other.
+There may thus be obtained from an alternating current two or
+more direct currents without the employment of any commutator<span class='pagenum'><a name="Page_410" id="Page_410">[Pg 410]</a></span>
+such as it has been heretofore regarded as necessary to use. The
+current in either branch may be used in the same way and for
+the same purposes as any other direct current&mdash;that is, it may be
+made to charge secondary batteries, energize electro-magnets, or
+for any other analogous purpose.</p>
+
+<p>Fig. 220 represents a plan of directing the alternating currents
+by means of devices purely electrical in character. Figs. 221,
+222, 223, 224, 225, and 226 are diagrams illustrative of other
+ways of carrying out the invention.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_424.jpg" width="640" height="300" alt="Fig. 220." title="" />
+<span class="caption">Fig. 220.</span>
+</div>
+
+
+<p>In Fig. 220, <small>A</small> designates a generator of alternating currents,
+and <small>B B</small> the main or line circuit therefrom. At any given point
+in this circuit at or near which it is desired to obtain direct currents,
+the circuit <b>B</b> is divided into two paths or branches <small>C D</small>. In
+each of these branches is placed an electrical generator, which
+for the present we will assume produces direct or continuous currents.
+The direction of the current thus produced is opposite in
+one branch to that of the current in the other branch, or, considering
+the two branches as forming a closed circuit, the generators
+<small>E F</small> are connected up in series therein, one generator in
+each part or half of the circuit. The electromotive force of the
+current sources <small>E</small> and <small>F</small> may be equal to or higher or lower than
+the electromotive forces in the branches <small>C D</small>, or between the points
+<small>X</small> and <small>Y</small> of the circuit <small>B B</small>. If equal, it is evident that current
+waves of one sign will be opposed in one branch and assisted in
+the other to such an extent that all the waves of one sign will
+pass over one branch and those of opposite sign over the other.
+If, on the other hand, the electromotive force of the sources <small>E F</small>
+be lower than that between <small>X</small> and <small>Y</small>, the currents in both
+branches will be alternating, but the waves of one sign will preponderate.
+One of the generators or sources of current <small>E</small> or <small>F</small>
+may be dispensed with; but it is preferable to employ both, if<span class='pagenum'><a name="Page_411" id="Page_411">[Pg 411]</a></span>
+they offer an appreciable resistance, as the two branches will be
+thereby better balanced. The translating or other devices to be
+acted upon by the current are designated by the letters <small>G</small>, and
+they are inserted in the branches <small>C D</small> in any desired manner; but
+in order to better preserve an even balance between the branches
+due regard should, of course, be had to the number and character
+of the devices.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_425.jpg" width="640" height="284" alt="Fig. 221." title="" />
+<span class="caption">Fig. 221.</span>
+</div>
+
+<p>Figs. 221, 222, 223, and 224 illustrate what may termed "electro-magnetic"
+devices for accomplishing a similar result&mdash;that is
+to say, instead of producing directly by a generator an electromotive
+force in each branch of the circuit, Mr. Tesla establishes
+a field or fields of force and leads the branches through the same
+in such manner that an active opposition of opposite effect or direction
+will be developed therein by the passage, or tendency to
+pass, of the alternations of current. In Fig. 221, for example, <small>A</small> is
+the generator of alternating currents, <small>B B</small> the line circuit, and <small>C D</small>
+the branches over which the alternating currents are directed. In
+each branch is included the secondary of a transformer or induction
+coil, which, since they correspond in their functions to the
+batteries of the previous figure, are designated by the letters <small>E F</small>.
+The primaries <small>H H'</small> of the induction coils or transformers are
+connected either in parallel or series with a source of direct or
+continuous currents <small>I</small>, and the number of convolutions is so calculated
+for the strength of the current from <small>I</small> that the cores <small>J J'</small> will be saturated. The connections are such that the conditions
+in the two transformers are of opposite character&mdash;that is to say,
+the arrangement is such that a current wave or impulse corresponding
+in direction with that of the direct current in one primary,
+as <small>H</small>, is of opposite direction to that in the other primary <small>H'</small>.
+It thus results that while one secondary offers a resistance or op<span class='pagenum'><a name="Page_412" id="Page_412">[Pg 412]</a></span>position
+to the passage through it of a wave of one sign, the other
+secondary similarly opposes a wave of opposite sign. In consequence,
+the waves of one sign will, to a greater or less extent, pass
+by way of one branch, while those of opposite sign in like manner
+pass over the other branch.</p>
+
+<p>In lieu of saturating the primaries by a source of continuous
+current, we may include the primaries in the branches <small>C D</small>, respectively,
+and periodically short-circuit by any suitable mechanical
+devices&mdash;such as an ordinary revolving commutator&mdash;their
+secondaries. It will be understood, of course, that the rotation
+and action of the commutator must be in synchronism or in
+proper accord with the periods of the alternations in order to
+secure the desired results. Such a disposition is represented
+diagrammatically in Fig. 222. Corresponding to the previous
+figures, <small>A</small> is the generator of alternating currents, <small>B B</small> the line,
+and <small>C D</small> the two branches for the direct currents. In branch <small>C</small>
+are included two primary coils <small>E E'</small>, and in branch <small>D</small> are two
+similar primaries <small>F F'</small> The corresponding secondaries for these
+coils and which are on the same subdivided cores <small>J</small> or <small>J'</small>, are in
+circuits the terminals of which connect to opposite segments
+<small>K K'</small>, and <small>L L'</small>, respectively, of a commutator. Brushes <i>b b</i> bear
+upon the commutator and alternately short-circuit the plates <small>K</small>
+and <small>K'</small>, and <small>L</small> and <small>L'</small>, through a connection <i>c</i>. It is obvious that
+either the magnets and commutator, or the brushes, may revolve.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_426.jpg" width="640" height="347" alt="Fig. 222." title="" />
+<span class="caption">Fig. 222.</span>
+</div>
+
+
+<p>The operation will be understood from a consideration of the
+effects of closing or short-circuiting the secondaries. For example,
+if at the instant when a given wave of current passes, one<span class='pagenum'><a name="Page_413" id="Page_413">[Pg 413]</a></span>
+set of secondaries be short-circuited, nearly all the current flows
+through the corresponding primaries; but the secondaries of the
+other branch being open-circuited, the self-induction in the
+primaries is highest, and hence little or no current will pass
+through that branch. If, as the current alternates, the secondaries
+of the two branches are alternately short-circuited, the
+result will be that the currents of one sign pass over one branch
+and those of the opposite sign over the other. The disadvantages
+of this arrangement, which would seem to result from the
+employment of sliding contacts, are in reality very slight, inasmuch
+as the electromotive force of the secondaries may be made
+exceedingly low, so that sparking at the brushes is avoided.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_427.jpg" width="640" height="269" alt="Fig. 223." title="" />
+<span class="caption">Fig. 223.</span>
+</div>
+
+
+<p>Fig. 223 is a diagram, partly in section, of another plan of
+carrying out the invention. The circuit <small>B</small> in this case is divided,
+as before, and each branch includes the coils of both the fields
+and revolving armatures of two induction devices. The armatures
+<small>O P</small> are preferably mounted on the same shaft, and are adjusted
+relatively to one another in such manner that when the
+self-induction in one branch, as <small>C</small>, is maximum, in the other branch
+<small>D</small> it is minimum. The armatures are rotated in synchronism with
+the alternations from the source <small>A</small>. The winding or position
+of the armature coils is such that a current in a given direction
+passed through both armatures would establish in one, poles similar
+to those in the adjacent poles of the field, and in the other,
+poles unlike the adjacent field poles, as indicated by <i>n n s s</i> in
+the diagram. If the like poles are presented, as shown in circuit
+<small>D</small>, the condition is that of a closed secondary upon a primary,
+or the position of least inductive resistance; hence a given alternation
+of current will pass mainly through <small>D</small>. A half revolution
+of the armatures produces an opposite effect and the succeeding<span class='pagenum'><a name="Page_414" id="Page_414">[Pg 414]</a></span>
+current impulse passes through <small>C</small>. Using this figure as an illustration,
+it is evident that the fields <small>N M</small> may be permanent magnets
+or independently excited and the armatures <small>O P</small> driven, as in
+the present case, so as to produce alternate currents, which will
+set up alternately impulses of opposite direction in the two
+branches <small>D C</small>, which in such case would include the armature circuits
+and translating devices only.</p>
+
+<p>In Fig. 224 a plan alternative with that shown in Fig. 222 is
+illustrated. In the previous case illustrated, each branch <small>C</small> and <small>D</small>
+contained one or more primary coils, the secondaries of which
+were periodically short circuited in synchronism with the alternations
+of current from the main source <small>A</small>, and for this purpose
+a commutator was employed. The latter may, however, be dispensed
+with and an armature with a closed coil substituted.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_428.jpg" width="640" height="261" alt="Fig. 224." title="" />
+<span class="caption">Fig. 224.</span>
+</div>
+
+
+<p>Referring to Fig. 224 in one of the branches, as <small>C</small>, are two coils
+<small>M'</small>, wound on laminated cores, and in the other branches <small>D</small> are
+similar coils <small>N'</small>. A subdivided or laminated armature <small>O'</small>, carrying
+a closed coil <small>R'</small>, is rotatably supported between the coils <small>M' N'</small>,
+as shown. In the position shown&mdash;that is, with the coil <small>R'</small> parallel
+with the convolutions of the primaries <small>N' M'</small>&mdash;practically the
+whole current will pass through branch <small>D</small>, because the self-induction
+in coils <small>M' M'</small> is maximum. If, therefore, the armature
+and coil be rotated at a proper speed relatively to the periods or
+alternations of the source <small>A</small>, the same results are obtained as in
+the case of Fig. 222.</p>
+
+<p>Fig. 225 is an instance of what may be called, in distinction to
+the others, a "magnetic" means of securing the result. <small>V</small> and
+<small>W</small> are two strong permanent magnets provided with armatures
+<small>V' W'</small>, respectively. The armatures are made of thin lamin&aelig; of
+soft iron or steel, and the amount of magnetic metal which they<span class='pagenum'><a name="Page_415" id="Page_415">[Pg 415]</a></span>
+contain is so calculated that they will be fully or nearly saturated
+by the magnets. Around the armatures are coils <small>E F</small>, contained,
+respectively, in the circuits <small>C</small> and <small>D</small>. The connections and electrical
+conditions in this case are similar to those in Fig. 221,
+except that the current source of <small>I</small>, Fig. 221, is dispensed with
+and the saturation of the core of coils <small>E F</small> obtained from the permanent
+magnets.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_429.jpg" width="640" height="286" alt="Fig. 225." title="" />
+<span class="caption">Fig. 225.</span>
+</div>
+
+<p>The previous illustrations have all shown the two branches or
+paths containing the translating or induction devices as in derivation
+one to the other; but this is not always necessary. For
+example, in Fig. 226, <small>A</small> is an alternating-current generator; <small>B B</small>,
+the line wires or circuit. At any given point in the circuit let
+us form two paths, as <small>D D'</small>, and at another point two paths, as <small>C</small>
+<small>C'</small>. Either pair or group of paths is similar to the previous dispositions
+with the electrical source or induction device in one
+branch only, while the two groups taken together form the
+obvious equivalent of the cases in which an induction device or
+generator is included in both branches. In one of the paths, as
+<small>D</small>, are included the devices to be operated by the current. In
+the other branch, as <small>D'</small>, is an induction device that opposes the
+current impulses of one direction and directs them through the
+branch <small>D</small>. So, also, in branch <small>C</small> are translating devices <small>G</small>, and in
+branch <small>C'</small> an induction device or its equivalent that diverts
+through <small>C</small> impulses of opposite direction to those diverted by the
+device in branch <small>D'</small>. The diagram shows a special form of induction
+device for this purpose. <small>J J'</small> are the cores, formed with
+pole-pieces, upon which are wound the coils <small>M N</small>. Between these
+pole-pieces are mounted at right angles to one another the magnetic
+armatures <small>O P</small>, preferably mounted on the same shaft and<span class='pagenum'><a name="Page_416" id="Page_416">[Pg 416]</a></span>
+designed to be rotated in synchronism with the alternations of
+current. When one of the armatures is in line with the poles or
+in the position occupied by armature <small>P</small>, the magnetic circuit of
+the induction device is practically closed; hence there will be
+the greatest opposition to the passage of a current through coils
+<small>N N</small>. The alternation will therefore pass by way of branch <small>D</small>.
+At the same time, the magnetic circuit of the other induction
+device being broken by the position of the armature <small>O</small>, there will
+be less opposition to the current in coils <small>M</small>, which will shunt the
+current from branch <small>C</small>. A reversal of the current being attended
+by a shifting of the armatures, the opposite effect is produced.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_430.jpg" width="640" height="370" alt="Fig. 226." title="" />
+<span class="caption">Fig. 226.</span>
+</div>
+
+
+<p>Other modifications of these methods are possible, but need
+not be pointed out. In all these plans, it will be observed, there
+is developed in one or all of these branches of a circuit from a
+source of alternating currents, an active (as distinguished from a
+dead) resistance or opposition to the currents of one sign, for the
+purpose of diverting the currents of that sign through the other
+or another path, but permitting the currents of opposite sign to
+pass without substantial opposition.</p>
+
+<p>Whether the division of the currents or waves of current of
+opposite sign be effected with absolute precision or not is immaterial,
+since it will be sufficient if the waves are only partially
+diverted or directed, for in such case the preponderating influence
+in each branch of the circuit of the waves of one sign secures
+the same practical results in many if not all respects as though
+the current were direct and continuous.<span class='pagenum'><a name="Page_417" id="Page_417">[Pg 417]</a></span></p>
+
+<p>An alternating and a direct current have been combined so that
+the waves of one direction or sign were partially or wholly overcome
+by the direct current; but by this plan only one set of alternations
+are utilized, whereas by the system just described the
+entire current is rendered available. By obvious applications of
+this discovery Mr. Tesla is enabled to produce a self-exciting alternating
+dynamo, or to operate direct current meters on alternating-current
+circuits or to run various devices&mdash;such as arc lamps&mdash;by
+direct currents in the same circuit with incandescent lamps
+or other devices operated by alternating currents.</p>
+
+<p>It will be observed that if an intermittent counter or opposing
+force be developed in the branches of the circuit and of higher
+electromotive force than that of the generator, an alternating
+current will result in each branch, with the waves of one sign
+preponderating, while a constantly or uniformly acting opposition
+in the branches of higher electromotive force than the
+generator would produce a pulsating current, which conditions
+would be, under some circumstances, the equivalent of those described.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_418" id="Page_418">[Pg 418]</a></span></p>
+<h2><a name="CHAPTER_XXXIV" id="CHAPTER_XXXIV"></a>CHAPTER XXXIV.</h2>
+
+<h3><span class="smcap">Condensers with Plates in Oil.</span></h3>
+
+<p>In experimenting with currents of high frequency and high
+potential, Mr. Tesla has found that insulating materials such as
+glass, mica, and in general those bodies which possess the highest
+specific inductive capacity, are inferior as insulators in such devices
+when currents of the kind described are employed compared
+with those possessing high insulating power, together with a smaller
+specific inductive capacity; and he has also found that it is very desirable
+to exclude all gaseous matter from the apparatus, or any access
+of the same to the electrified surfaces, in order to prevent heating
+by molecular bombardment and the loss or injury consequent
+thereon. He has therefore devised a method to accomplish these
+results and produce highly efficient and reliable condensers, by
+using oil as the dielectric<a name="FNanchor_11_12" id="FNanchor_11_12"></a><a href="#Footnote_11_12" class="fnanchor">[11]</a>. The plan admits of a particular
+con<span class='pagenum'><a name="Page_419" id="Page_419">[Pg 419]</a></span>struction of condenser, in which the distance between the plates
+is adjustable, and of which he takes advantage.</p>
+
+<div class="figcenter" style="width: 700px;">
+<div class="figleft" style="width: 407px;">
+<img src="images/fig227.jpg" width="407" height="220" alt="Fig. 227." title="" />
+<span class="caption">Fig. 227.</span>
+</div>
+<div class="figright" style="width: 231px;">
+<img src="images/oi_432.jpg" width="231" height="220" alt="Fig. 228." title="" />
+<span class="caption">Fig. 228.</span>
+</div>
+</div>
+
+<p>In the accompanying illustrations, Fig. 227 is a section of a
+condenser constructed in accordance with this principle and having
+stationary plates; and Fig. 228 is a similar view of a condenser
+with adjustable plates.</p>
+
+<p>Any suitable box or receptacle <small>A</small> may be used to contain the
+plates or armatures. These latter are designated by <small>B</small> and <small>C</small> and
+are connected, respectively, to terminals <small>D</small> and <small>E</small>, which pass out
+through the sides of the case. The plates ordinarily are separated
+by strips of porous insulating material <small>F</small>, which are used merely
+for the purpose of maintaining them in position. The space
+within the can is filled with oil <small>G</small>. Such a condenser will prove
+highly efficient and will not become heated or permanently injured.</p>
+
+<p>In many cases it is desirable to vary or adjust the capacity of
+a condenser, and this is provided for by securing the plates to adjustable
+supports&mdash;as, for example, to rods <small>H</small>&mdash;passing through
+stuffing boxes <small>K</small> in the sides of case <small>A</small> and furnished with nuts <small>L</small>,
+the ends of the rods being threaded for engagement with the
+nuts.</p>
+
+<p>It is well known that oils possess insulating properties, and it
+has been a common practice to interpose a body of oil between
+two conductors for purposes of insulation; but Mr. Tesla believes
+he has discovered peculiar properties in oils which render
+them very valuable in this particular form of device.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_420" id="Page_420">[Pg 420]</a></span></p>
+<h2><a name="CHAPTER_XXXV" id="CHAPTER_XXXV"></a>CHAPTER XXXV.</h2>
+
+<h3><span class="smcap">Electrolytic Registering Meter.</span></h3>
+
+
+<p>An ingenious form of electrolytic meter attributable to Mr.
+Tesla is one in which a conductor is immersed in a solution, so
+arranged that metal may be deposited from the solution or taken
+away in such a manner that the electrical resistance of the conductor
+is varied in a definite proportion to the strength of the
+current the energy of which is to be computed, whereby this
+variation in resistance serves as a measure of the energy and also
+may actuate registering mechanism, whenever the resistance
+rises above or falls below certain limits.</p>
+
+<p>In carrying out this idea Mr. Tesla employs an electrolytic
+cell, through which extend two conductors parallel and
+in close proximity to each other. These conductors he connects
+in series through a resistance, but in such manner that there is
+an equal difference of potential between them throughout their
+entire extent. The free ends or terminals of the conductors are
+connected either in series in the circuit supplying the current to
+the lamps or other devices, or in parallel to a resistance in the
+circuit and in series with the current consuming devices. Under
+such circumstances a current passing through the conductors
+establishes a difference of potential between them which is proportional
+to the strength of the current, in consequence of which
+there is a leakage of current from one conductor to the other
+across the solution. The strength of this leakage current is proportional
+to the difference of potential, and, therefore, in proportion
+to the strength of the current passing through the conductors.
+Moreover, as there is a constant difference of potential between
+the two conductors throughout the entire extent that is exposed
+to the solution, the current density through such solution is the
+same at all corresponding points, and hence the deposit is uniform
+along the whole of one of the conductors, while the metal
+is taken away uniformly from the other. The resistance of one
+conductor is by this means diminished, while that of the other is<span class='pagenum'><a name="Page_421" id="Page_421">[Pg 421]</a></span>
+increased, both in proportion to the strength of the current passing
+through the conductors. From such variation in the resistance
+of either or both of the conductors forming the positive
+and negative electrodes of the cell, the current energy expended
+may be readily computed. Figs. 229 and 230 illustrate two
+forms of such a meter.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_435.jpg" width="640" height="471" alt="Fig. 229." title="" />
+<span class="caption">Fig. 229.</span>
+</div>
+
+
+<p>In Fig. 229 <small>G</small> designates a direct-current generator. <small>L L</small> are
+the conductors of the circuit extending therefrom. <small>A</small> is a tube
+of glass, the ends of which are sealed, as by means of insulating
+plugs or caps <small>B B</small>. <small>C C'</small> are two conductors extending
+through the tube <small>A</small>, their ends passing out through the plugs <small>B</small> to
+terminals thereon. These conductors may be corrugated or
+formed in other proper ways to offer the desired electrical resistance.
+<small>R</small> is a resistance connected in series with the two conductors
+<small>C C'</small>, which by their free terminals are connected up in
+circuit with one of the conductors <small>L</small>.</p>
+
+<p>The method of using this device and computing by means
+thereof the energy of the current will be readily understood.
+First, the resistances of the two conductors <small>C C'</small>, respectively, are
+accurately measured and noted. Then a known current is passed
+through the instrument for a given time, and by a second measurement
+the increase and diminution of the resistances of the two
+conductors are respectively taken. From these data the constant is<span class='pagenum'><a name="Page_422" id="Page_422">[Pg 422]</a></span>
+obtained&mdash;that is to say, for example, the increase of resistance of
+one conductor or the diminution of the resistance of the other per
+lamp hour. These two measurements evidently serve as a check,
+since the gain of one conductor should equal the loss of the other.
+A further check is afforded by measuring both wires in series with
+the resistance, in which case the resistance of the whole should
+remain constant.</p>
+
+<div class="figcenter" style="width: 631px;">
+<img src="images/oi_436.jpg" width="631" height="480" alt="Fig. 230." title="" />
+<span class="caption">Fig. 230.</span>
+</div>
+
+<p>In Fig. 230 the conductors <small>C C'</small> are connected in parallel, the
+current device at <small>X</small> passing in one branch first through a resistance
+<small>R'</small> and then through conductor <small>C</small>, while on the other branch
+it passes first through conductor <small>C'</small>, and then through resistance
+<small>R''</small>. The resistances <small>R' R''</small> are equal, as also are the resistances of
+the conductors <small>C C'</small>. It is, moreover, preferable that the respective
+resistances of the conductors <small>C C'</small> should be a known and convenient
+fraction of the coils or resistances <small>R' R''</small>. It will be observed
+that in the arrangement shown in Fig. 230 there is a constant
+potential difference between the two conductors <small>C C'</small> throughout
+their entire length.</p>
+
+<p>It will be seen that in both cases illustrated, the proportionality
+of the increase or decrease of resistance to the current strength
+will always be preserved, for what one conductor gains the other
+loses, and the resistances of the conductors <small>C C'</small> being small as<span class='pagenum'><a name="Page_423" id="Page_423">[Pg 423]</a></span>
+compared with the resistances in series with them. It will be
+understood that after each measurement or registration of a given
+variation of resistance in one or both conductors, the direction of
+the current should be changed or the instrument reversed, so that
+the deposit will be taken from the conductor which has gained
+and added to that which has lost. This principle is capable of
+many modifications. For instance, since there is a section of the
+circuit&mdash;to wit, the conductor <small>C</small> or <small>C'</small>&mdash;that varies in resistance in
+proportion to the current strength, such variation may be utilized,
+as is done in many analogous cases, to effect the operation of
+various automatic devices, such as registers. It is better, however,
+for the sake of simplicity to compute the energy by measurements
+of resistance.</p>
+
+<p>The chief advantages of this arrangement are, first, that it is
+possible to read off directly the amount of the energy expended
+by means of a properly constructed ohm-meter and without resorting
+to weighing the deposit; secondly it is not necessary to
+employ shunts, for the whole of the current to be measured may
+be passed through the instrument; third, the accuracy of the instrument
+and correctness of the indications are but slightly affected
+by changes in temperature. It is also said that such meters
+have the merit of superior economy and compactness, as well as
+of cheapness in construction. Electrolytic meters seem to need
+every auxiliary advantage to make them permanently popular and
+successful, no matter how much ingenuity may be shown in their
+design.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_424" id="Page_424">[Pg 424]</a></span></p>
+<h2><a name="CHAPTER_XXXVI" id="CHAPTER_XXXVI"></a>CHAPTER XXXVI.</h2>
+
+<h3><span class="smcap">Thermo-Magnetic Motors and Pyro-Magnetic Generators.</span></h3>
+
+
+<p>No electrical inventor of the present day dealing with the
+problems of light and power considers that he has done himself
+or his opportunities justice until he has attacked the subject of
+thermo-magnetism. As far back as the beginning of the seventeenth
+century it was shown by Dr. William Gilbert, the father
+of modern electricity, that a loadstone or iron bar when heated
+to redness loses its magnetism; and since that time the influence
+of heat on the magnetic metals has been investigated frequently,
+though not with any material or practical result.</p>
+
+<p>For a man of Mr. Tesla's inventive ability, the problems in
+this field have naturally had no small fascination, and though he
+has but glanced at them, it is to be hoped he may find time to
+pursue the study deeper and further. For such as he, the investigation
+must undoubtedly bear fruit. Meanwhile he has
+worked out one or two operative devices worthy of note.<a name="FNanchor_12_13" id="FNanchor_12_13"></a><a href="#Footnote_12_13" class="fnanchor">[12]</a> He
+obtains mechanical power by a reciprocating action resulting
+from the joint operations of heat, magnetism, and a spring or
+weight or other force&mdash;that is to say he subjects a body magnetized
+by induction or otherwise to the action of heat until the
+magnetism is sufficiently neutralized to allow a weight or spring
+to give motion to the body and lessen the action of the heat, so
+that the magnetism may be sufficiently restored to move the
+<span class='pagenum'><a name="Page_425" id="Page_425">[Pg 425]</a></span>body in the opposite direction, and again subject the same to the
+demagnetizing power of the heat.</p>
+
+<p>Use is made of either an electro-magnet or a permanent magnet,
+and the heat is directed against a body that is magnetized
+by induction, rather than directly against a permanent magnet,
+thereby avoiding the loss of magnetism that might result in the
+permanent magnet by the action of heat. Mr. Tesla also provides
+for lessening the volume of the heat or for intercepting the same
+during that portion of the reciprocation in which the cooling
+action takes place.</p>
+
+<p>In the diagrams are shown some of the numerous arrangements
+that may be made use of in carrying out this idea. In all
+of these figures the magnet-poles are marked <small>N S</small>, the armature
+<small>A</small>, the Bunsen burner or other source of heat <small>H</small>, the axis of motion
+<small>M</small>, and the spring or the equivalent thereof&mdash;namely, a
+weight&mdash;is marked <small>W</small>.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_439.jpg" width="800" height="370" alt="Fig. 232, 231, 233." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 232.</td><td class="caption">Fig. 231.</td><td class="caption">Fig. 233.</td></tr>
+</table>
+</div>
+
+
+<p>In Fig. 231 the permanent magnet <small>N</small> is connected with a frame,
+<small>F</small>, supporting the axis <small>M</small>, from which the arm <small>P</small> hangs, and at the
+lower end of which the armature <small>A</small> is supported. The stops 2
+and 3 limit the extent of motion, and the spring <small>W</small> tends to draw
+the armature <small>A</small> away from the magnet <small>N</small>. It will now be understood
+that the magnetism of <small>N</small> is sufficient to overcome the
+spring <small>W</small> and draw the armature <small>A</small> toward the magnet <small>N</small>. The
+heat acting upon the armature <small>A</small> neutralizes its induced magnetism
+sufficiently for the spring <small>W</small> to draw the armature A away
+from the magnet <small>N</small> and also from the heat at <small>H</small>. The armature
+now cools, and the attraction of the magnet <small>N</small> overcomes the
+spring <small>W</small> and draws the armature <small>A</small> back again above the burner<span class='pagenum'><a name="Page_426" id="Page_426">[Pg 426]</a></span>
+<small>H</small>, so that the same is again heated and the operations are repeated.
+The reciprocating movements thus obtained are employed
+as a source of mechanical power in any desired manner.
+Usually a connecting-rod to a crank upon a fly-wheel shaft would
+be made use of, as indicated in Fig. 240.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_440.jpg" width="800" height="369" alt="Fig. 234, 236, 235." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 234.</td><td class="caption">Fig. 236.</td><td class="caption">Fig. 235.</td></tr>
+</table>
+</div>
+
+<p>Fig. 232 represents the same parts as before described; but an
+electro-magnet is illustrated in place of a permanent magnet.
+The operations, however, are the same.</p>
+
+<p>In Fig. 233 are shown the same parts as in Figs. 231 and 232,
+but they are differently arranged. The armature <small>A</small>, instead of
+swinging, is stationary and held by arm <small>P'</small>, and the core <small>N S</small> of
+the electro-magnet is made to swing within the helix <small>Q</small>, the
+core being suspended by the arm <small>P</small> from the pivot <small>M</small>. A shield,
+<small>R</small>, is connected with the magnet-core and swings with it, so
+that after the heat has demagnetized the armature <small>A</small> to such an
+extent that the spring <small>W</small> draws the core <small>N S</small> away from the armature
+<small>A</small>, the shield <small>R</small> comes between the flame <small>H</small> and armature <small>A</small>,
+thereby intercepting the action of the heat and allowing the armature
+to cool, so that the magnetism, again preponderating,
+causes the movement of the core <small>N S</small> toward the armature <small>A</small> and
+the removal of the shield <small>R</small> from above the flame, so that the heat
+again acts to lessen or neutralize the magnetism. A rotary or
+other movement may be obtained from this reciprocation.</p>
+
+<p>Fig. 234 corresponds in every respect with Fig. 233, except
+that a permanent horseshoe-magnet, <small>N S</small> is represented as taking
+the place of the electro-magnet in Fig. 233.</p>
+
+<p>In Fig. 235 is shown a helix, <small>Q</small>, with an armature adapted to
+swing toward or from the helix. In this case there may be a soft<span class='pagenum'><a name="Page_427" id="Page_427">[Pg 427]</a></span>-iron
+core in the helix, or the armature may assume the form of a
+solenoid core, there being no permanent core within the helix.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_441.jpg" width="800" height="323" alt="Fig. 237, 238, 239." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 237.</td><td class="caption">Fig. 238.</td><td class="caption">Fig. 239.</td></tr>
+</table>
+</div>
+
+<p>Fig. 236 is an end view, and Fig. 237 a plan view, illustrating
+the method as applied to a swinging armature, <small>A</small>, and a stationary
+permanent magnet, <small>N S</small>. In this instance Mr. Tesla applies the
+heat to an auxiliary armature or keeper, <small>T</small>, which is adjacent to
+and preferably in direct contact with the magnet. This armature
+<small>T</small>, in the form of a plate of sheet-iron, extends across from
+one pole to the other and is of sufficient section to practically
+form a keeper for the magnet, so that when the armature <small>T</small> is
+cool nearly all the lines of force pass over the same and very little
+free magnetism is exhibited. Then the armature <small>A</small>, which swings
+freely on the pivots <small>M</small> in front of the poles <small>N S</small>, is very little attracted
+and the spring <small>W</small> pulls the same way from the poles into
+the position indicated in the diagram. The heat is directed upon
+the iron plate <small>T</small> at some distance from the magnet, so as to allow
+the magnet to keep comparatively cool. This heat is applied beneath
+the plate by means of the burners <small>H</small>, and there is a connection
+from the armature <small>A</small> or its pivot to the gas-cock 6, or
+other device for regulating the heat. The heat acting upon the
+middle portion of the plate <small>T</small>, the magnetic conductivity of the
+heated portion is diminished or destroyed, and a great number of
+the lines of force are deflected over the armature <small>A</small>, which is now
+powerfully attracted and drawn into line, or nearly so, with the
+poles <small>N S</small>. In so doing the cock 6 is nearly closed and the plate
+<small>T</small> cools, the lines of force are again deflected over the same, the
+attraction exerted upon the armature <small>A</small> is diminished, and the
+spring <small>W</small> pulls the same away from the magnet into the position
+shown by full lines, and the operations are repeated. The ar<span class='pagenum'><a name="Page_428" id="Page_428">[Pg 428]</a></span>rangement
+shown in Fig. 236 has the advantages that the magnet
+and armature are kept cool and the strength of the permanent
+magnet is better preserved, as the magnetic circuit is
+constantly closed.</p>
+
+<p>In the plan view, Fig. 238, is shown a permanent magnet and
+keeper plate, <small>T</small>, similar to those in Figs. 236 and 237, with the
+burners <small>H</small> for the gas beneath the same; but the armature is
+pivoted at one end to one pole of the magnet and the other end
+swings toward and from the other pole of the magnet. The spring
+<small>W</small> acts against a lever arm that projects from the armature, and
+the supply of heat has to be partly cut off by a connection to the
+swinging armature, so as to lessen the heat acting upon the keeper
+plate when the armature <small>A</small> has been attracted.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_442.jpg" width="800" height="469" alt="Fig. 240, 241." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 240.</td><td class="caption">Fig. 241.</td></tr>
+</table>
+</div>
+
+<p>Fig. 239 is similar to Fig. 238, except that the keeper <small>T</small> is not
+made use of and the armature itself swings into and out of the
+range of the intense action of the heat from the burner <small>H</small>. Fig.
+240 is a diagram similar to Fig. 231, except that in place of using a
+spring and stops, the armature is shown as connected by a link,
+to the crank of a fly-wheel, so that the fly-wheel will be revolved
+as rapidly as the armature can be heated and cooled to the
+necessary extent. A spring may be used in addition, as in Fig.
+231. In Fig. 241 the armatures <small>A A</small> are connected by a link, so
+that one will be heating while the other is cooling, and the attraction
+exerted to move the cooled armature is availed of to draw
+away the heated armature instead of using a spring.<span class='pagenum'><a name="Page_429" id="Page_429">[Pg 429]</a></span></p>
+
+<p>Mr. Tesla has also devoted his attention to the development of
+a pyromagnetic generator of electricity<a name="FNanchor_13_14" id="FNanchor_13_14"></a><a href="#Footnote_13_14" class="fnanchor">[13]</a> based upon the following
+laws: First, that electricity or electrical energy is developed in
+any conducting body by subjecting such body to a varying magnetic
+influence; and second, that the magnetic properties of iron
+or other magnetic substance may be partially or entirely destroyed
+or caused to disappear by raising it to a certain temperature, but
+restored and caused to reappear by again lowering its temperature
+to a certain degree. These laws may be applied in the production
+of electrical currents in many ways, the principle of
+which is in all cases the same, viz., to subject a conductor to a
+varying magnetic influence, producing such variations by the application
+of heat, or, more strictly speaking, by the application or
+action of a varying temperature upon the source of the magnetism.
+This principle of operation may be illustrated by a simple
+experiment: Place end to end, and preferably in actual contact,
+a permanently magnetized steel bar and a strip or bar of soft iron.
+Around the end of the iron bar or plate wind a coil of insulated wire.
+Then apply to the iron between the coil and the steel bar a flame
+or other source of heat which will be capable of raising that portion
+of the iron to an orange red, or a temperature of about 600&deg;
+centigrade. When this condition is reached, the iron somewhat
+suddenly loses its magnetic properties, if it be very thin, and the
+same effect is produced as though the iron had been moved away
+from the magnet or the heated section had been removed. This
+change of position, however, is accompanied by a shifting of the
+magnetic lines, or, in other words, by a variation in the magnetic
+influence to which the coil is exposed, and a current in the coil
+is the result. Then remove the flame or in any other way reduce
+the temperature of the iron. The lowering of its temperature is
+accompanied by a return of its magnetic properties, and another
+change of magnetic conditions occurs, accompanied by a current
+in an opposite direction in the coil. The same operation may be
+<span class='pagenum'><a name="Page_430" id="Page_430">[Pg 430]</a></span>repeated indefinitely, the effect upon the coil being similar to
+that which would follow from moving the magnetized bar to and
+from the end of the iron bar or plate.</p>
+
+<p>The device illustrated below is a means of obtaining this
+result, the features of novelty in the invention being, first, the
+employment of an artificial cooling device, and, second, inclosing
+the source of heat and that portion of the magnetic circuit exposed
+to the heat and artificially cooling the heated part.</p>
+
+<p>These improvements are applicable generally to the generators
+constructed on the plan above described&mdash;that is to say, we may
+use an artificial cooling device in conjunction with a variable or
+varied or uniform source of heat.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_444.jpg" width="800" height="393" alt="Fig. 242, 243." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 242.</td><td class="caption">Fig. 243.</td></tr>
+</table>
+</div>
+
+<p>Fig. 242 is a central vertical longitudinal section of the complete
+apparatus and Fig. 243 is a cross-section of the magnetic
+armature-core of the generator.</p>
+
+<p>Let <small>A</small> represent a magnetized core or permanent magnet the
+poles of which are bridged by an armature-core composed of a
+casing or shell <small>B</small> inclosing a number of hollow iron tubes <small>C</small>.
+Around this core are wound the conductors <small>E E'</small>, to form the
+coils in which the currents are developed. In the circuits of
+these coils are current-consuming devices, as <small>F F'</small>.</p>
+
+<p><small>D</small> is a furnace or closed fire-box, through which the central
+portion of the core <small>B</small> extends. Above the fire is a boiler <small>K</small>, containing
+water. The flue <small>L</small> from the fire-box may extend up
+through the boiler.</p>
+
+<p><small>G</small> is a water-supply pipe, and <small>H</small> is the steam-exhaust pipe,
+which communicates with all the tubes <small>C</small> in the armature <small>B</small>, so
+that steam escaping from the boiler will pass through the tubes.<span class='pagenum'><a name="Page_431" id="Page_431">[Pg 431]</a></span></p>
+
+<p>In the steam-exhaust pipe <small>H</small> is a valve <small>V</small>, to which is connected
+the lever <small>I</small>, by the movement of which the valve is opened
+or closed. In such a case as this the heat of the fire may be
+utilized for other purposes after as much of it as may be needed
+has been applied to heating the core <small>B</small>. There are special advantages
+in the employment of a cooling device, in that the
+metal of the core <small>B</small> is not so quickly oxidized. Moreover, the
+difference between the temperature of the applied heat and of
+the steam, air, or whatever gas or fluid be applied as the cooling
+medium, may be increased or decreased at will, whereby the
+rapidity of the magnetic changes or fluctuations may be regulated.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_432" id="Page_432">[Pg 432]</a></span></p>
+<h2><a name="CHAPTER_XXXVII" id="CHAPTER_XXXVII"></a>CHAPTER XXXVII.</h2>
+
+<h3><span class="smcap">Anti-Sparking Dynamo Brush and Commutator.</span></h3>
+
+
+<p>In direct current dynamos of great electromotive force&mdash;such,
+for instance, as those used for arc lighting&mdash;when one commutator
+bar or plate comes out of contact with the collecting-brush a
+spark is apt to appear on the commutator. This spark may be
+due to the break of the complete circuit, or to a shunt of low
+resistance formed by the brush between two or more commutator-bars.
+In the first case the spark is more apparent, as there is
+at the moment when the circuit is broken a discharge of the
+magnets through the field helices, producing a great spark or
+flash which causes an unsteady current, rapid wear of the commutator
+bars and brushes, and waste of power. The sparking
+may be reduced by various devices, such as providing a path for
+the current at the moment when the commutator segment or bar
+leaves the brush, by short-circuiting the field-helices, by increasing
+the number of the commutator-bars, or by other similar
+means; but all these devices are expensive or not fully available,
+and seldom attain the object desired.</p>
+
+<p>To prevent this sparking in a simple manner, Mr. Tesla some
+years ago employed with the commutator-bars and intervening
+insulating material, mica, asbestos paper or other insulating and
+incombustible material, arranged to bear on the surface of the
+commutator, near to and behind the brush.</p>
+
+<p>In the drawings, Fig. 244 is a section of a commutator with
+an asbestos insulating device; and Fig. 245 is a similar view, representing
+two plates of mica upon the back of the brush.</p>
+
+<p>In 244, <small>C</small> represents the commutator and intervening
+insulating material; <small>B B</small>, the brushes. <i>d d</i> are sheets of asbestos
+paper or other suitable non-conducting material. <i>f f</i> are springs,
+the pressure of which may be adjusted by means of the screws
+<i>g g</i>.</p>
+
+<p>In Fig. 245 a simple arrangement is shown with two plates of
+mica or other material. It will be seen that whenever one com<span class='pagenum'><a name="Page_433" id="Page_433">[Pg 433]</a></span>mutator
+segment passes out of contact with the brush, the formation
+of the arc will be prevented by the intervening insulating
+material coming in contact with the insulating material on the
+brush.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_447.jpg" width="800" height="242" alt="Fig. 244, 245." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 244.</td><td class="caption">Fig. 245.</td></tr>
+</table>
+</div>
+
+<p>Asbestos paper or cloth impregnated with zinc-oxide, magnesia,
+zirconia, or other suitable material, may be used, as the
+paper and cloth are soft, and serve at the same time to wipe and
+polish the commutator; but mica or any other suitable material
+can be employed, provided the material be an insulator or a bad
+conductor of electricity.</p>
+
+<p>A few years later Mr. Tesla turned his attention again to the
+same subject, as, perhaps, was very natural in view of the fact
+that the commutator had always been prominent in his thoughts,
+and that so much of his work was even aimed at dispensing with
+it entirely as an objectionable and unnecessary part of dynamos
+and motors. In these later efforts to remedy commutator troubles,
+Mr. Tesla constructs a commutator and the collectors therefor in
+two parts mutually adapted to one another, and, so far as the essential
+features are concerned, alike in mechanical structure. Selecting
+as an illustration a commutator of two segments adapted
+for use with an armature the coils or coil of which have but two
+free ends, connected respectively to the segments, the bearing-surface
+is the face of a disc, and is formed of two metallic quadrant
+segments and two insulating segments of the same dimensions,
+and the face of the disc is smoothed off, so that the metal
+and insulating segments are flush. The part which takes the
+place of the usual brushes, or the "collector," is a disc of the
+same character as the commutator and has a surface similarly
+formed with two insulating and two metallic segments. These
+two parts are mounted with their faces in contact and in such
+manner that the rotation of the armature causes the commutator
+to turn upon the collector, whereby the currents induced in the<span class='pagenum'><a name="Page_434" id="Page_434">[Pg 434]</a></span>
+coils are taken off by the collector segments and thence conveyed
+off by suitable conductors leading from the collector segments.
+This is the general plan of the construction adopted. Aside from
+certain adjuncts, the nature and functions of which are set forth
+later, this means of commutation will be seen to possess many important
+advantages. In the first place the short-circuiting and the
+breaking of the armature coil connected to the commutator-segments
+occur at the same instant, and from the nature of the construction
+this will be done with the greatest precision; secondly, the
+duration of both the break and of the short circuit will be reduced
+to a minimum. The first results in a reduction which amounts
+practically to a suppression of the spark, since the break and
+the short circuit produce opposite effects in the armature-coil.
+The second has the effect of diminishing the destructive effect
+of a spark, since this would be in a measure proportional to the
+duration of the spark; while lessening the duration of the short
+circuit obviously increases the efficiency of the machine.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_448.jpg" width="800" height="510" alt="Fig. 246, 247." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 246.</td><td class="caption">Fig. 247.</td></tr>
+</table>
+</div>
+
+<p>The mechanical advantages will be better understood by referring
+to the accompanying diagrams, in which Fig. 246 is a
+central longitudinal section of the end of a shaft with the improved
+commutator carried thereon. Fig. 247 is a view of the
+inner or bearing face of the collector. Fig. 248 is an end view
+from the armature side of a modified form of commutator. Figs.<span class='pagenum'><a name="Page_435" id="Page_435">[Pg 435]</a></span>
+249 and 250 are views of details of Fig. 248. Fig. 251 is a longitudinal
+central section of another modification, and Fig. 252 is a
+sectional view of the same. <small>A</small> is the end of the armature-shaft
+of a dynamo-electric machine or motor. <small>A'</small> is a sleeve of insulating
+material around the shaft, secured in place by a screw, <i>a'</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_449.jpg" width="800" height="295" alt="Fig. 248, 249, 250." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 248.</td><td class="caption">Fig. 249. &nbsp; &nbsp; Fig. 250.</td></tr>
+</table>
+</div>
+
+
+<p>The commutator proper is in the form of a disc which is made
+up of four segments <small>D D'</small> <small>G G'</small>, similar to those shown in Fig. 248.
+Two of these segments, as <small>D D'</small>, are of metal and are in electrical
+connection with the ends of the coils on the armature. The
+other two segments are of insulating material. The segments are
+held in place by a band, <small>B</small>, of insulating material. The disc is
+held in place by friction or by screws, <i>g' g'</i>, Fig. 248, which
+secure the disc firmly to the sleeve <small>A'</small>.</p>
+
+<p>The collector is made in the same form as the commutator. It
+is composed of the two metallic segments <small>E E'</small> and the two insulating
+segments <small>F F'</small>, bound together by a band, <small>C</small>. The metallic
+segments <small>E E'</small> are of the same or practically the same width or
+extent as the insulating segments or spaces of the commutator.
+The collector is secured to a sleeve, <small>B'</small>, by screws <i>g g</i>, and the sleeve
+is arranged to turn freely on the shaft <small>A</small>. The end of the sleeve
+<small>B'</small> is closed by a plate, <i>f</i>, upon which presses a pivot-pointed
+screw, <i>h</i>, adjustable in a spring, <small>H</small>, which acts to maintain the
+collector in close contact with the commutator and to compensate
+for the play of the shaft. The collector is so fixed that it cannot
+turn with the shaft. For example, the diagram shows a slotted
+plate, <small>K</small>, which is designed to be attached to a stationary support,
+and an arm extending from the collector and carrying a clamping
+screw, <small>L</small>, by which the collector may be adjusted and set to the
+desired position.</p>
+
+<p>Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit<span class='pagenum'><a name="Page_436" id="Page_436">[Pg 436]</a></span>
+the insulating segments of both commutator and collector loosely
+and to provide some means&mdash;as, for example, light springs, <i>e e</i>,
+secured to the bands <small>A' B'</small>, respectively, and bearing against the
+segments&mdash;to exert a light pressure upon them and keep them in
+close contact and to compensate for wear. The metal segments
+of the commutator may be moved forward by loosening the
+screw <i>a'</i>.</p>
+
+<p>The line wires are fed from the metal segments of the collector,
+being secured thereto in any convenient manner, the plan of connections
+being shown as applied to a modified form of the commutator
+in Fig. 251. The commutator and the collector in thus
+presenting two flat and smooth bearing surfaces prevent most effectually
+by mechanical action the occurrence of sparks.</p>
+
+<p>The insulating segments are made of some hard material capable
+of being polished and formed with sharp edges. Such materials
+as glass, marble, or soapstone may be advantageously used.
+The metal segments are preferably of copper or brass; but they
+may have a facing or edge of durable material&mdash;such as platinum
+or the like&mdash;where the sparks are liable to occur.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_450.jpg" width="800" height="409" alt="Fig. 251, 252." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 251.</td><td class="caption">Fig. 252.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 248 a somewhat modified form of the invention is
+shown, a form designed to facilitate the construction and replacing
+of the parts. In this modification the commutator and collector
+are made in substantially the same manner as previously
+described, except that the bands <small>B C</small> are omitted. The four segments
+of each part, however, are secured to their respective sleeves
+by screws <i>g' g'</i>, and one edge of each segment is cut away, so that
+small plates <i>a b</i> may be slipped into the spaces thus formed. Of<span class='pagenum'><a name="Page_437" id="Page_437">[Pg 437]</a></span>
+these plates <i>a a</i> are of metal, and are in contact with the metal segments
+<small>D D'</small>, respectively. The other two, <i>b b</i>, are of glass or marble,
+and they are all better square, as shown in Figs. 249 and 250,
+so that they may be turned to present new edges should any edge
+become worn by use. Light springs <i>d</i> bear upon these plates
+and press those in the commutator toward those in the collector,
+and insulating strips <i>c c</i> are secured to the periphery of the discs
+to prevent the blocks from being thrown out by centrifugal action.
+These plates are, of course, useful at those edges of the segments
+only where sparks are liable to occur, and, as they are easily replaced,
+they are of great advantage. It is considered best to coat
+them with platinum or silver.</p>
+
+<p>In Figs. 251 and 252 is shown a construction where, instead of
+solid segments, a fluid is employed. In this case the commutator
+and collector are made of two insulating discs, <small>S T</small>, and in
+lieu of the metal segments a space is cut out of each part, as at
+<small>R R'</small>, corresponding in shape and size to a metal segment. The
+two parts are fitted smoothly and the collector <small>T</small> held by the
+screw <i>h</i> and spring <small>H</small> against the commutator <small>S</small>. As in the other
+cases, the commutator revolves while the collector remains stationary.
+The ends of the coils are connected to binding-posts
+<i>s s</i>, which are in electrical connection with metal plates <i>t t</i> within
+the recesses in the two parts <small>S T</small>. These chambers or recesses
+are filled with mercury, and in the collector part are tubes <small>W W</small>,
+with screws <i>w w</i>, carrying springs <small>X</small> and pistons <small>X'</small>, which compensate
+for the expansion and contraction of the mercury under
+varying temperatures, but which are sufficiently strong not to
+yield to the pressure of the fluid due to centrifugal action, and
+which serve as binding-posts.</p>
+
+<p>In all the above cases the commutators are adapted for a single
+coil, and the device is particularly suited to such purposes. The
+number of segments may be increased, however, or more than
+one commutator used with a single armature. Although the
+bearing-surfaces are shown as planes at right angles to the shaft
+or axis, it is evident that in this particular the construction may
+be very greatly modified.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_438" id="Page_438">[Pg 438]</a></span></p>
+<h2><a name="CHAPTER_XXXVIII" id="CHAPTER_XXXVIII"></a>CHAPTER XXXVIII.</h2>
+
+<h3><span class="smcap">Auxiliary Brush Regulation of Direct Current Dynamos.</span></h3>
+
+
+<p>An interesting method devised by Mr. Tesla for the regulation
+of direct current dynamos, is that which has come to be
+known as the "third brush" method. In machines of this type,
+devised by him as far back as 1885, he makes use of two main
+brushes to which the ends of the field magnet coils are connected,
+an auxiliary brush, and a branch or shunt connection from an intermediate
+point of the field wire to the auxiliary brush.<a name="FNanchor_14_15" id="FNanchor_14_15"></a><a href="#Footnote_14_15" class="fnanchor">[14]</a></p>
+
+<p>The relative positions of the respective brushes are varied,
+either automatically or by hand, so that the shunt becomes inoperative
+when the auxiliary brush has a certain position upon
+the commutator; but when the auxiliary brush is moved in its
+relation to the main brushes, or the latter are moved in their
+relation to the auxiliary brush, the electric condition is disturbed
+and more or less of the current through the field-helices is
+diverted through the shunt or a current is passed over the shunt
+to the field-helices. By varying the relative position upon the
+commutator of the respective brushes automatically in proportion
+to the varying electrical conditions of the working-circuit,
+the current developed can be regulated in proportion to the demands
+in the working-circuit.</p>
+
+<p>Fig. 253 is a diagram illustrating the invention, showing one
+core of the field-magnets with one helix wound in the same direction
+throughout. Figs. 254 and 255 are diagrams showing one
+core of the field-magnets with a portion of the helices wound in
+opposite directions. Figs. 256 and 257 are diagrams illustrating
+<span class='pagenum'><a name="Page_439" id="Page_439">[Pg 439]</a></span>the electric devices that may be employed for automatically
+adjusting the brushes, and Fig. 258 is a diagram illustrating the
+positions of the brushes when the machine is being energized at
+the start.</p>
+
+<p><i>a</i> and <i>b</i> are the positive and negative brushes of the main or
+working-circuit, and <i>c</i> the auxiliary brush. The working-circuit
+<small>D</small> extends from the brushes <i>a</i> and <i>b</i>, as usual, and contains electric
+lamps or other devices, <small>D'</small>, either in series or in multiple
+arc.</p>
+
+<p><small>M M'</small> represent the field-helices, the ends of which are connected
+to the main brushes <i>a</i> and <i>b</i>. The branch or shunt wire
+<i>c'</i> extends from the auxiliary brush <i>c</i> to the circuit of the field-helices,
+and is connected to the same at an intermediate point, <i>x</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_453.jpg" width="800" height="480" alt="Fig. 253." title="" />
+<span class="caption">Fig. 253.</span>
+</div>
+
+
+<p><small>H</small> represents the commutator, with the plates of ordinary construction.
+When the auxiliary brush <i>c</i> occupies such a position
+upon the commutator that the electro-motive force between the
+brushes <i>a</i> and <i>c</i> is to the electro-motive force between the brushes
+<i>c</i> and <i>b</i> as the resistance of the circuit <i>a</i> <small>M</small> <i>c' c</i> <small>A</small> is to the resistance
+of the circuit <i>b</i> <small>M'</small> <i>c' c</i> <small>B</small>, the potentials of the points <i>x</i> and <small>Y</small> will
+be equal, and no current will flow over the auxiliary brush; but
+when the brush <i>c</i> occupies a different position the potentials of
+the points <i>x</i> and <small>Y</small> will be different, and a current will flow over
+the auxiliary brush to and from the commutator, according to the
+relative position of the brushes. If, for instance, the commutator-space
+between the brushes <i>a</i> and <i>c</i>, when the latter is at the
+neutral point, is diminished, a current will flow from the point <small>Y</small>
+over the shunt <i>c</i> to the brush <i>b</i>, thus strengthening the current
+in the part <small>M'</small>, and partly neutralizing the current in part <small>M</small>; but
+if the space between the brushes <i>a</i> and <i>c</i> is increased, the cur<span class='pagenum'><a name="Page_440" id="Page_440">[Pg 440]</a></span>rent
+will flow over the auxiliary brush in an opposite direction,
+and the current in <small>M</small> will be strengthened, and in <small>M'</small>, partly neutralized.</p>
+
+<p>By combining with the brushes <i>a</i>, <i>b</i>, and <i>c</i> any usual automatic
+regulating mechanism, the current developed can be regulated in
+proportion to the demands in the working circuit. The parts <small>M</small>
+and <small>M'</small> of the field wire may be wound in the same direction.
+In this case they are arranged as shown in Fig. 253; or the part
+<small>M</small> may be wound in the opposite direction, as shown in Figs.
+254 and 255.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_454.jpg" width="800" height="366" alt="Fig. 254." title="" />
+<span class="caption">Fig. 254.</span>
+</div>
+
+<p>It will be apparent that the respective cores of the field-magnets
+are subjected to neutralizing or intensifying effects of the
+current in the shunt through <i>c'</i>, and the magnetism of the cores
+will be partially neutralized, or the points of greatest magnetism
+shifted, so that it will be more or less remote from or approaching
+to the armature, and hence the aggregate energizing actions
+of the field magnets on the armature will be correspondingly
+varied.</p>
+
+<p>In the form indicated in Fig. 253 the regulation is effected by
+shifting the point of greatest magnetism, and in Figs. 254 and
+255 the same effect is produced by the action of the current in
+the shunt passing through the neutralizing helix.</p>
+
+<p>The relative positions of the respective brushes may be varied
+by moving the auxiliary brush, or the brush <i>c</i> may remain stationary
+and the core <small>P</small> be connected to the main-brush holder <small>A</small>,
+so as to adjust the brushes <i>a b</i> in their relation to the brush <i>c</i>.
+If, however, an adjustment is applied to all the brushes, as seen
+in Fig. 257, the solenoid should be connected to both <i>a</i> and <i>c</i>, so
+as to move them toward or away from each other.</p>
+
+<p>There are several known devices for giving motion in propor<span class='pagenum'><a name="Page_441" id="Page_441">[Pg 441]</a></span>tion
+to an electric current. In Figs. 256 and 257 the moving
+cores are shown as convenient devices for obtaining the required
+extent of motion with very slight changes in the current passing
+through the helices. It is understood that the adjustment of
+the main brushes causes variations in the strength of the current
+independently of the relative position of those brushes to the
+auxiliary brush. In all cases the adjustment should be such that
+no current flows over the auxiliary brush when the dynamo is
+running with its normal load.</p>
+
+<p>In Figs. 256 and 257 <small>A A</small> indicate the main-brush holder,
+carrying the main brushes, and <small>C</small> the auxiliary-brush holder,
+carrying the auxiliary brush. These brush-holders are movable
+in arcs concentric with the centre of the commutator-shaft. An
+iron piston, <small>P</small>, of the solenoid <small>S</small>, Fig. 256, is attached to the auxiliary-brush
+holder <small>C</small>. The adjustment is effected by means of a
+spring and screw or tightener.</p>
+
+<p>In Fig. 257 instead of a solenoid, an iron tube inclosing a coil
+is shown. The piston of the coil is attached to both brush-holders
+<small>A A</small> and <small>C</small>. When the brushes are moved directly by
+electrical devices, as shown in Figs. 256 and 257, these are so
+constructed that the force exerted for adjusting is practically
+uniform through the whole length of motion.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_455.jpg" width="800" height="366" alt="Fig. 255." title="" />
+<span class="caption">Fig. 255.</span>
+</div>
+
+<p>It is true that auxiliary brushes have been used in connection
+with the helices of the field-wire; but in these instances the
+helices receive the entire current through the auxiliary brush or
+brushes, and these brushes could not be taken off without breaking
+the circuit through the field. These brushes cause, moreover,
+heavy sparking at the commutator. In the present
+case the auxiliary brush causes very little or no sparking, and
+can be taken off without breaking the circuit through the field<span class='pagenum'><a name="Page_442" id="Page_442">[Pg 442]</a></span>-helices.
+The arrangement has, besides, the advantage of facilitating
+the self-excitation of the machine in all cases where the resistance
+of the field-wire is very great comparatively to the resistance
+of the main circuit at the start&mdash;for instance, on arc-light
+machines. In this case the auxiliary brush <i>c</i> is placed near to, or
+better still in contact with, the brush <i>b</i>, as shown in Fig. 258.
+In this manner the part <small>M'</small> is completely cut out, and as the part
+<small>M</small> has a considerably smaller resistance than the whole length of
+the field-wire the machine excites itself, whereupon the auxiliary
+brush is shifted automatically to its normal position.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_456.jpg" width="800" height="281" alt="Fig. 256, 257." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 256.</td><td class="caption">Fig. 257.</td></tr>
+</table>
+</div>
+
+<p>In a further method devised by Mr. Tesla, one or more auxiliary
+brushes are employed, by means of which a portion or the
+whole of the field coils is shunted. According to the relative position
+upon the commutator of the respective brushes more or
+less current is caused to pass through the helices of the field, and
+the current developed by the machine can be varied at will by
+varying the relative positions of the brushes.</p>
+
+<div class="figcenter" style="width: 361px;">
+<img src="images/oi_456-1.jpg" width="361" height="336" alt="Fig. 258." title="" />
+<span class="caption">Fig. 258.</span>
+</div>
+
+<p>In Fig. 259, <i>a</i> and <i>b</i> are the positive and negative brushes of
+the main circuit, and <i>c</i> an auxiliary brush. The main circuit <small>D</small>
+extends from the brushes <i>a</i> and <i>b</i>, as usual, and contains the
+helices <small>M</small> of the field wire and the electric lamps or other working
+devices. The auxiliary brush <i>c</i> is connected to the point <i>x</i>
+of the main circuit by means of the wire <i>c'</i>. <small>H</small> is a commutator<span class='pagenum'><a name="Page_443" id="Page_443">[Pg 443]</a></span>
+of ordinary construction. It will have been seen from what was
+said already that when the electro-motive force between the brushes
+<i>a</i> and <i>c</i> is to the electromotive force between the brushes <i>c</i>
+and <i>b</i> as the resistance of the circuit <i>a</i> <small>M</small> <i>c' c</i> <small>A</small> is to the resistance
+of the circuit <i>b</i> <small>C B</small> <i>c c'</i> <small>D</small>, the potentials of the points <i>x</i> and <i>y</i>
+will be equal, and no current will pass over the auxiliary brush
+<i>c</i>; but if that brush occupies a different position relatively to the
+main brushes the electric condition is disturbed, and current
+will flow either from <i>y</i> to <i>x</i> or from <i>x</i> to <i>y</i>, according to the relative
+position of the brushes. In the first case the current through
+the field-helices will be partly neutralized and the magnetism of
+the field magnets will be diminished. In the second case the
+current will be increased and the magnets gain strength. By
+combining with the brushes at <i>a b c</i> any automatic regulating
+mechanism, the current developed can be regulated automatically
+in proportion to the demands of the working circuit.</p>
+
+<p>In Figs. 264 and 265 some of the automatic means are represented
+that maybe used for moving the brushes. The core <small>P</small>,
+Fig. 264, of the solenoid-helix <small>S</small> is connected with the brush <i>a</i> to
+move the same, and in Fig. 265 the core <small>P</small> is shown as within the
+helix <small>S</small>, and connected with brushes <i>a</i> and <i>c</i>, so as to move the
+same toward or from each other, according to the strength of the
+current in the helix, the helix being within an iron tube, <small>S'</small>, that
+becomes magnetized and increases the action of the solenoid.</p>
+
+<p>In practice it is sufficient to move only the auxiliary brush, as
+shown in Fig. 264, as the regulation is very sensitive to the
+slightest changes; but the relative position of the auxiliary brush
+to the main brushes may be varied by moving the main brushes,
+or both main and auxiliary brushes may be moved, as illustrated
+in Fig. 265. In the latter two cases, it will be understood, the
+motion of the main brushes relatively to the neutral line of the
+machine causes variations in the strength of the current independently
+of their relative position to the auxiliary brush. In
+all cases the adjustment may be such that when the machine is
+running with the ordinary load, no current flows over the auxiliary
+brush.</p>
+
+<p>The field helices may be connected, as shown in Fig. 259, or a
+part of the field helices may be in the outgoing and the other part
+in the return circuit, and two auxiliary brushes may be employed
+as shown in Figs. 261 and 262. Instead of shunting the whole
+of the field helices, a portion only of such helices may be shunted,
+as shown in Figs. 260 and 262.<span class='pagenum'><a name="Page_444" id="Page_444">[Pg 444]</a></span></p>
+
+<p>The arrangement shown in Fig. 262 is advantageous, as it diminishes
+the sparking upon the commutator, the main circuit being
+closed through the auxiliary brushes at the moment of the break
+of the circuit at the main brushes.</p>
+
+<div class="figcenter" style="width: 800px;">
+
+<img src="images/oi_458-1.jpg" width="800" height="321" alt="Fig. 259." title="" />
+<span class="caption">Fig. 259.</span>
+
+<img src="images/oi_458-2.jpg" width="800" height="270" alt="Fig. 260." title="" />
+<span class="caption">Fig. 260.</span>
+
+<img src="images/oi_458-3.jpg" width="800" height="283" alt="Fig. 261." title="" />
+<span class="caption">Fig. 261.</span>
+
+<img src="images/oi_458.jpg" width="800" height="277" alt="Fig. 262, 263." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 262.</td><td class="caption">Fig. 263.</td></tr>
+</table>
+
+</div>
+
+<p>The field helices may be wound in the same direction, or a part
+may be wound in opposite directions.</p>
+
+<p>The connection between the helices and the auxiliary brush or
+brushes may be made by a wire of small resistance, or a resistance
+may be interposed (<small>R</small>, Fig. 263,) between the point <i>x</i> and the<span class='pagenum'><a name="Page_445" id="Page_445">[Pg 445]</a></span>
+auxiliary brush or brushes to divide the sensitiveness when the
+brushes are adjusted.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_459.jpg" width="800" height="281" alt="Fig. 264, 265." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 264.</td><td class="caption">Fig. 265.</td></tr>
+</table>
+</div>
+
+<p>The accompanying sketches also illustrate improvements made
+by Mr. Tesla in the mechanical devices used to effect the shifting
+of the brushes, in the use of an auxiliary brush. Fig. 266 is
+an elevation of the regulator with the frame partly in section;
+and Fig. 267 is a section at the line <i>x x</i>, Fig. 266. <small>C</small> is the commutator;
+<small>B</small> and <small>B'</small>, the brush-holders, <small>B</small> carrying the main
+brushes <i>a a'</i>, and <small>B'</small> the auxiliary or shunt brushes <i>b b</i>. The
+axis of the brush-holder <small>B</small> is supported by two pivot-screws, <i>p p</i>.
+The other brush-holder, <small>B'</small>, has a sleeve, <i>d</i>, and is movable
+around the axis of the brush-holder <small>B</small>. In this way both brush-holders
+can turn very freely, the friction of the parts being
+reduced to a minimum. Over the brush-holders is mounted the
+solenoid <small>S</small>, which rests upon a forked column, <i>c</i>. This column
+also affords a support for the pivots <i>p p</i>, and is fastened upon a
+solid bracket or projection, <small>P</small>, which extends from the base of
+the machine, and is cast in one piece with the same. The
+brush-holders <small>B B'</small> are connected by means of the links <i>e e</i>
+and the cross-piece <small>F</small> to the iron core <small>I</small>, which slides freely in the
+tube <small>T</small> of the solenoid. The iron core <small>I</small> has a screw, <i>s</i>, by means
+of which it can be raised and adjusted in its position relatively
+to the solenoid, so that the pull exerted upon it by the solenoid
+is practically uniform through the whole length of motion which
+is required to effect the regulation. In order to effect the
+adjustment with greater precision, the core <small>I</small> is provided with a
+small iron screw, <i>s'</i>. The core being first brought very nearly
+in the required position relatively to the solenoid by means of
+the screw <i>s</i>, the small screw <i>s'</i> is then adjusted until the magnetic
+attraction upon the core is the same when the core is in any position.
+A convenient stop, <i>t</i>, serves to limit the upward movement
+of the iron core.<span class='pagenum'><a name="Page_446" id="Page_446">[Pg 446]</a></span></p>
+
+<p>To check somewhat the movement of the core <small>I</small>, a dash-pot, <small>K</small>,
+is used. The piston <small>L</small> of the dash-pot is provided with a valve,
+<small>V</small>, which opens by a downward pressure and allows an easy
+downward movement of the iron core <small>I</small>, but closes and checks
+the movement of the core when it is pulled up by the action
+of the solenoid.</p>
+
+<p>To balance the opposing forces, the weight of the moving
+parts, and the pull exerted by the solenoid upon the iron core,
+the weights <small>W W</small> may be used. The adjustment is such that
+when the solenoid is traversed by the normal current it is just
+strong enough to balance the downward pull of the parts.</p>
+
+<div class="figcenter" style="width: 733px;">
+<img src="images/oi_460.jpg" width="733" height="600" alt="Fig. 266, 267." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 266.</td><td class="caption">Fig. 267.</td></tr>
+</table>
+</div>
+
+<p>The electrical circuit-connections are substantially the same as
+indicated in the previous diagrams, the solenoid being in series
+with the circuit when the translating devices are in series, and in
+shunt when the devices are in multiple arc. The operation of
+the device is as follows: When upon a decrease of the resistance
+of the circuit or for some other reason, the current is
+increased, the solenoid <small>S</small> gains in strength and pulls up the iron
+core <small>I</small>, thus shifting the main brushes in the direction of rotation
+and the auxiliary brushes in the opposite way. This diminishes
+the strength of the current until the opposing forces are balanced
+and the solenoid is traversed by the normal current; but if from
+any cause the current in the circuit is diminished, then the weight
+of the moving parts overcomes the pull of the solenoid, the iron<span class='pagenum'><a name="Page_447" id="Page_447">[Pg 447]</a></span>
+core <small>I</small> descends, thus shifting the brushes the opposite way and
+increasing the current to the normal strength. The dash-pot
+connected to the iron core <small>I</small> may be of ordinary construction;
+but it is better, especially in machines for arc lights, to provide
+the piston of the dash-pot with a valve, as indicated in the diagrams.
+This valve permits a comparatively easy downward movement
+of the iron core, but checks its movement when it is drawn
+up by the solenoid. Such an arrangement has the advantage
+that a great number of lights may be put on without diminishing
+the light-power of the lamps in the circuit, as the brushes assume
+at once the proper position. When lights are cut out, the dash-pot
+acts to retard the movement; but if the current is considerably
+increased the solenoid gets abnormally strong and the brushes
+are shifted instantly. The regulator being properly adjusted,
+lights or other devices may be put on or out with scarcely any
+perceptible difference. It is obvious that instead of the dash-pot
+any other retarding device may be used.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_448" id="Page_448">[Pg 448]</a></span></p>
+<h2><a name="CHAPTER_XXXIX" id="CHAPTER_XXXIX"></a>CHAPTER XXXIX.</h2>
+
+<h3><span class="smcap">Improvement in the Construction of Dynamos and Motors.</span></h3>
+
+
+<p>This invention of Mr. Tesla is an improvement in the construction
+of dynamo or magneto electric machines or motors,
+consisting in a novel form of frame and field magnet which renders
+the machine more solid and compact as a structure, which
+requires fewer parts, and which involves less trouble and expense
+in its manufacture. It is applicable to generators and motors
+generally, not only to those which have independent circuits
+adapted for use in the Tesla alternating current system, but to
+other continuous or alternating current machines of the ordinary
+type generally used.</p>
+
+<p>Fig. 268 shows the machine in side elevation. Fig. 269 is a
+vertical sectional view of the field magnets and frame and an end
+view of the armature; and Fig. 270 is a plan view of one of
+the parts of the frame and the armature, a portion of the latter
+being cut away.</p>
+
+<p>The field magnets and frame are cast in two parts. These
+parts are identical in size and shape, and each consists of the solid
+plates or ends <small>A B</small>, from which project inwardly the cores <small>C D</small> and
+the side bars or bridge pieces, <small>E F</small>. The precise shape of these
+parts is largely a matter of choice&mdash;that is to say, each casting,
+as shown, forms an approximately rectangular frame; but it might
+obviously be more or less oval, round, or square, without departure
+from the invention. It is also desirable to reduce the
+width of the side bars, <small>E F</small>, at the center and to so proportion the
+parts that when the frame is put together the spaces between the
+pole pieces will be practically equal to the arcs which the surfaces
+of the poles occupy.</p>
+
+<p>The bearings <small>G</small> for the armature shaft are cast in the side bars
+<small>E F</small>. The field coils are either wound on the pole pieces or on a
+form and then slipped on over the ends of the pole pieces.
+The lower part or casting is secured to the base after being
+finished off. The armature <small>K</small> on its shaft is then mounted in<span class='pagenum'><a name="Page_449" id="Page_449">[Pg 449]</a></span>
+the bearings of the lower casting and the other part of the frame
+placed in position, dowel pins <small>L</small> or any other means being used to
+secure the two parts in proper position.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_463-1.jpg" width="800" height="407" alt="Fig. 268." title="" />
+<span class="caption">Fig. 268.</span>
+
+<img src="images/oi_463-2.jpg" width="800" height="426" alt="Fig. 269." title="" />
+<span class="caption">Fig. 269.</span>
+
+<img src="images/oi_463.jpg" width="800" height="513" alt="Fig. 270." title="" />
+<span class="caption">Fig. 270.</span>
+
+</div>
+
+<p>In order to secure an easier fit, the side bars <small>E F</small>, and end pieces,
+<small>A B</small>, are so cast that slots <small>M</small> are formed when the two parts are
+put together.<span class='pagenum'><a name="Page_450" id="Page_450">[Pg 450]</a></span></p>
+
+<p>This machine possesses several advantages. For example, if we
+magnetize the cores alternately, as indicated by the characters <small>N</small>
+<small>S</small>, it will be seen that the magnetic circuit between the poles of
+each part of a casting is completed through the solid iron side
+bars. The bearings for the shaft are located at the neutral points
+of the field, so that the armature core is not affected by the magnetic
+condition of the field.</p>
+
+<p>The improvement is not restricted to the use of four pole pieces,
+as it is evident that each pole piece could be divided or more than
+four formed by the shape of the casting.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_451" id="Page_451">[Pg 451]</a></span></p>
+<h2><a name="CHAPTER_XL" id="CHAPTER_XL"></a>CHAPTER XL.</h2>
+
+<h3><span class="smcap">Tesla Direct Current Arc Lighting System.</span></h3>
+
+
+<p>At one time, soon after his arrival in America, Mr. Tesla was
+greatly interested in the subject of arc lighting, which then occupied
+public attention and readily enlisted the support of capital.
+He therefore worked out a system which was confided to a company
+formed for its exploitation, and then proceeded to devote
+his energies to the perfection of the details of his more celebrated
+"rotary field" motor system. The Tesla arc lighting apparatus
+appeared at a time when a great many other lamps and machines
+were in the market, but it commanded notice by its ingenuity.
+Its chief purpose was to lessen the manufacturing cost and simplify
+the processes of operation.</p>
+
+<p>We will take up the dynamo first. Fig. 271 is a longitudinal
+section, and Fig. 272 a cross section of the machine. Fig. 273 is
+a top view, and Fig. 274 a side view of the magnetic frame. Fig.
+275 is an end view of the commutator bars, and Fig. 276 is a
+section of the shaft and commutator bars. Fig. 277 is a diagram
+illustrating the coils of the armature and the connections to the
+commutator plates.</p>
+
+<p>The cores <i>c c c c</i> of the field-magnets are tapering in both
+directions, as shown, for the purposes of concentrating the magnetism
+upon the middle of the pole-pieces.</p>
+
+<p>The connecting-frame <small>F F</small> of the field-magnets is in the form
+indicated in the side view, Fig. 274, the lower part being provided
+with the spreading curved cast legs <i>e e</i>, so that the machine
+will rest firmly upon two base-bars, <i>r r</i>.</p>
+
+<p>To the lower pole, <small>S</small>, of the field-magnet <small>M</small> is fastened, by
+means of babbitt or other fusible diamagnetic material, the base
+<small>B</small>, which is provided with bearings <i>b</i> for the armature-shaft <small>H</small>.
+The base <small>B</small> has a projection, <small>P</small>, which supports the brush-holders
+and the regulating devices, which are of a special character devised
+by Mr. Tesla.</p>
+
+<p>The armature is constructed with the view to reduce to a min<span class='pagenum'><a name="Page_452" id="Page_452">[Pg 452]</a></span>imum
+the loss of power due to Foucault currents and to the
+change of polarity, and also to shorten as much as possible the
+length of the inactive wire wound upon the armature core.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_466.jpg" width="640" height="301" alt="Fig. 271." title="" />
+<span class="caption">Fig. 271.</span>
+</div>
+
+<p>It is well known that when the armature is revolved between
+the poles of the field-magnets, currents are generated in the iron
+body of the armature which develop heat, and consequently cause
+a waste of power. Owing to the mutual action of the lines of
+force, the magnetic properties of iron, and the speed of the different
+portions of the armature core, these currents are generated
+principally on and near the surface of the armature core, diminishing
+in strength gradually toward the centre of the core.
+Their quantity is under some conditions proportional to the
+length of the iron body in the direction in which these currents
+are generated. By subdividing the iron core electrically in this
+direction, the generation of these currents can be reduced to a
+great extent. For instance, if the length of the armature-core is
+twelve inches, and by a suitable construction it is subdivided
+electrically, so that there are in the generating direction six inches
+of iron and six inches of intervening air-spaces or insulating material,
+the waste currents will be reduced to fifty per cent.</p>
+
+<p>As shown in the diagrams, the armature is constructed of thin
+iron discs <small>D D D</small>, of various diameters, fastened upon the armature-shaft
+in a suitable manner and arranged according to their
+sizes, so that a series of iron bodies, <i>i i i</i>, is formed, each of which
+diminishes in thickness from the centre toward the periphery.
+At both ends of the armature the inwardly curved discs <i>d d</i>, of
+cast iron, are fastened to the armature shaft.</p>
+
+<p>The armature core being constructed as shown, it will be easily
+seen that on those portions of the armature that are the most
+remote from the axis, and where the currents are principally developed,
+the length of iron in the generating direction is only a<span class='pagenum'><a name="Page_453" id="Page_453">[Pg 453]</a></span>
+small fraction of the total length of the armature core, and besides
+this the iron body is subdivided in the generating direction,
+and therefore the Foucault currents are greatly reduced. Another
+cause of heating is the shifting of the poles of the armature core.
+In consequence of the subdivision of the iron in the armature
+and the increased surface for radiation, the risk of heating is
+lessened.</p>
+
+<p>The iron discs <small>D D D</small> are insulated or coated with some insulating-paint,
+a very careful insulation being unnecessary, as an
+electrical contact between several discs can only occur at places
+where the generated currents are comparatively weak. An
+armature core constructed in the manner described may be revolved
+between the poles of the field magnets without showing
+the slightest increase of temperature.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_467.jpg" width="800" height="404" alt="Fig. 272, 273." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 272.</td><td class="caption">Fig. 273.</td></tr>
+</table>
+</div>
+
+<p>The end discs, <i>d d</i>, which are of sufficient thickness and, for
+the sake of cheapness, of cast-iron, are curved inwardly, as indicated
+in the drawings. The extent of the curve is dependent
+on the amount of wire to be wound upon the armatures. In this
+machine the wire is wound upon the armature in two superimposed
+parts, and the curve of the end discs, <i>d d</i>, is so calculated
+that the first part&mdash;that is, practically half of the wire&mdash;just fills
+up the hollow space to the line <i>x x</i>; or, if the wire is wound in
+any other manner, the curve is such that when the whole of the
+wire is wound, the outside mass of wires, <i>w</i>, and the inside mass
+of wires, <i>w'</i>, are equal at each side of the plane <i>x x</i>. In this case
+the passive or electrically-inactive wires are of the smallest
+length practicable. The arrangement has further the advantage<span class='pagenum'><a name="Page_454" id="Page_454">[Pg 454]</a></span>
+that the total lengths of the crossing wires at the two sides of
+the plane <i>x x</i> are practically equal.</p>
+
+<div class="figcenter" style="width: 617px;">
+<img src="images/oi_468.jpg" width="617" height="480" alt="Fig. 274." title="" />
+<span class="caption">Fig. 274.</span>
+</div>
+
+<p>To equalize further the armature coils at both sides of the
+plates that are in contact with the brushes, the winding and connecting
+up is effected in the following manner: The whole wire
+is wound upon the armature-core in two superimposed parts,
+which are thoroughly insulated from each other. Each of these
+two parts is composed of three separated groups of coils. The
+first group of coils of the first part of wire being wound and
+connected to the commutator-bars in the usual manner, this group
+is insulated and the second group wound; but the coils of this
+second group, instead of being connected to the next following
+commutator bars, are connected to the directly opposite bars of
+the commutator. The second group is then insulated and the
+third group wound, the coils of this group being connected to
+those bars to which they would be connected in the usual way.
+The wires are then thoroughly insulated and the second part of
+wire is wound and connected in the same manner.</p>
+
+<p>Suppose, for instance, that there are twenty-four coils&mdash;that is,
+twelve in each part&mdash;and consequently twenty-four commutator
+plates. There will be in each part three groups, each containing
+four coils, and the coils will be connected as follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="1" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='center'><i>Groups.</i> &nbsp; &nbsp;</td><td align='center'><i>Commutator Bars.</i></td></tr>
+<tr><td colspan='3'>&nbsp;</td></tr>
+<tr><td align='left'></td><td align='left'>First</td><td align='center'>1&mdash;5</td></tr>
+<tr><td align='left'>First part of wire</td><td align='left'>Second</td><td align='center'>17&mdash;21</td></tr>
+<tr><td align='left'></td><td align='left'>Third</td><td align='center'>9&mdash;13</td></tr>
+<tr><td colspan='3'>&nbsp;</td></tr>
+<tr><td align='left'></td><td align='left'>First</td><td align='center'>13&mdash;17</td></tr>
+<tr><td align='left'>Second part of wire &nbsp; &nbsp; </td><td align='left'>Second &nbsp; </td><td align='left'>5&mdash;9</td></tr>
+<tr><td align='left'></td><td align='left'>Third</td><td align='center'>21&mdash;1</td></tr>
+</table></div>
+
+<p>In constructing the armature core and winding and connecting
+the coils in the manner indicated, the passive or electrically in<span class='pagenum'><a name="Page_455" id="Page_455">[Pg 455]</a></span>active
+wire is reduced to a minimum, and the coils at each
+side of the plates that are in contact with the brushes are practically
+equal. In this way the electrical efficiency of the machine
+is increased.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_469.jpg" width="800" height="302" alt="Fig. 275, 276." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 275.</td><td class="caption">Fig. 276.</td></tr>
+</table>
+</div>
+
+<p>The commutator plates <i>t</i> are shown as outside the bearing <i>b</i> of
+the armature shaft. The shaft <small>H</small> is tubular and split at the end
+portion, and the wires are carried through the same in the usual
+manner and connected to the respective commutator plates. The
+commutator plates are upon a cylinder, <i>u</i>, and insulated, and this
+cylinder is properly placed and then secured by expanding the
+split end of the shaft by a tapering screw plug, <i>v</i>.</p>
+
+<div class="figcenter" style="width: 645px;">
+<img src="images/oi_469-1.jpg" width="645" height="600" alt="Fig. 277." title="" />
+<span class="caption">Fig. 277.</span>
+</div>
+
+
+<p>The arc lamps invented by Mr. Tesla for use on the circuits
+from the above described dynamo are those in which the separation
+and feed of the carbon electrodes or their equivalents is accomplished
+by means of electro-magnets or solenoids in connection
+with suitable clutch mechanism, and were designed for the purpose<span class='pagenum'><a name="Page_456" id="Page_456">[Pg 456]</a></span>
+of remedying certain faults common to arc lamps.</p>
+
+<p>He proposed to prevent the frequent vibrations of the movable
+carbon "point" and flickering of the light arising therefrom; to
+prevent the falling into contact of the carbons; to dispense with
+the dash pot, clock work, or gearing and similar devices; to render
+the lamp extremely sensitive, and to feed the carbon almost
+imperceptibly, and thereby obtain a very steady and uniform
+light.</p>
+
+<p>In that class of lamps where the regulation of the arc is effected
+by forces acting in opposition on a free, movable rod or lever directly
+connected with the electrode, all or some of the forces
+being dependent on the strength of the current, any change in
+the electrical condition of the circuit causes a vibration and a corresponding
+flicker in the light. This difficulty is most apparent
+when there are only a few lamps in circuit. To lessen this difficulty
+lamps have been constructed in which the lever or armature,
+after the establishing of the arc, is kept in a fixed position and
+cannot vibrate during the feed operation, the feed mechanism
+acting independently; but in these lamps, when a clamp is employed,
+it frequently occurs that the carbons come into contact
+and the light is momentarily extinguished, and frequently parts
+of the circuit are injured. In both these classes of lamps it has
+been customary to use dash pot, clock work, or equivalent retarding
+devices; but these are often unreliable and objectionable, and
+increase the cost of construction.</p>
+
+<p>Mr. Tesla combines two electro-magnets&mdash;one of low resistance
+in the main or lamp circuit, and the other of comparatively
+high resistance in a shunt around the arc&mdash;a movable armature
+lever, and a special feed mechanism, the parts being arranged so
+that in the normal working position of the armature lever the
+same is kept almost rigidly in one position, and is not affected
+even by considerable changes in the electric circuit; but if the
+carbons fall into contact the armature will be actuated by the
+magnets so as to move the lever and start the arc, and hold the
+carbons until the arc lengthens and the armature lever returns to
+the normal position. After this the carbon rod holder is released
+by the action of the feed mechanism, so as to feed the carbon and
+restore the arc to its normal length.</p>
+
+<p>Fig. 278 is an elevation of the mechanism made use of in
+this arc lamp. Fig. 279 is a plan view. Fig. 280 is an elevation
+of the balancing lever and spring; Fig. 281 is a de<span class='pagenum'><a name="Page_457" id="Page_457">[Pg 457]</a></span>tached
+plan view of the pole pieces and armatures upon the
+friction clamp, and Fig. 282 is a section of the clamping tube.</p>
+
+<p><small>M</small> is a helix of coarse wire in a circuit from the lower carbon
+holder to the negative binding screw &minus;. <small>N</small> is a helix of fine wire
+in a shunt between the positive binding screw &#43; and the
+negative binding screw &minus;. The upper carbon holder <small>S</small> is a parallel
+rod sliding through the plates <small>S' S<sup>2</sup></small> of the frame of the lamp,
+and hence the electric current passes from the positive binding
+post &#43; through the plate <small>S<sup>2</sup></small>, carbon holder <small>S</small>, and upper carbon
+to the lower carbon, and thence by the holder and a metallic
+connection to the helix <small>M</small>.</p>
+
+
+<div class="figcenter" style="width: 1024px;">
+<div class="figleft" style="width: 448px;">
+<img src="images/oi_471-1.jpg" width="414" height="640" alt="Fig. 278, 282." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 278.</td><td class="caption">Fig. 282.</td></tr>
+</table>
+</div>
+
+<div class="figright" style="width: 520px;">
+<img src="images/oi_471-2.jpg" width="520" height="480" alt="Fig. 279." title="" /><br />
+<span class="caption">Fig. 279.</span>
+</div>
+
+<div class="figright" style="width: 520px;">
+<img src="images/oi_471-4.jpg" width="520" height="480" alt="Fig. 280." title="" /><br />
+<span class="caption">Fig. 280.</span>
+</div>
+
+<div class="figleft" style="width: 448px;">
+<img src="images/oi_471-3.jpg" width="448" height="133" alt="Fig. 281." title="" /><br />
+<span class="caption">Fig. 281.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>The carbon holders are of the usual character, and to insure
+electric connections the springs <i>l</i> are made use of to grasp the
+upper carbon holding rod <small>S</small>, but to allow the rod to slide freely
+through the same. These springs <i>l</i> may be adjusted in their
+pressure by the screw <i>m</i>, and the spring <i>l</i> maybe sustained upon<span class='pagenum'><a name="Page_458" id="Page_458">[Pg 458]</a></span>
+any suitable support. They are shown as connected with the
+upper end of the core of the magnet <small>N</small>.</p>
+
+<p>Around the carbon-holding rod <small>S</small>, between the plates <small>S' S<sup>2</sup></small>,
+there is a tube, <small>R</small>, which forms a clamp. This tube is counter-bored,
+as seen in the section Fig. 282, so that it bears upon the
+rod <small>S</small> at its upper end and near the middle, and at the lower end of
+this tubular clamp <small>R</small> there are armature segments <i>r</i> of soft iron.
+A frame or arm, <i>n</i>, extending, preferably, from the core <small>N<sup>2</sup></small>, supports
+the lever <small>A</small> by a fulcrum-pin, <i>o</i>. This lever <small>A</small> has a hole,
+through which the upper end of the tubular clamp <small>R</small> passes
+freely, and from the lever <small>A</small> is a link, <i>q</i>, to the lever <i>t</i>, which
+lever is pivoted at <i>y</i> to a ring upon one of the columns <small>S<sup>3</sup></small>. This
+lever <i>t</i> has an opening or bow surrounding the tubular clamp
+<small>R</small>, and there are pins or pivotal connections <i>w</i> between the lever
+<i>t</i> and this clamp <small>R</small>, and a spring, <i>r</i><sup>2</sup>, serves to support or suspend
+the weight of the parts and balance them, or nearly so. This
+spring is adjustable.</p>
+
+<p>At one end of the lever <small>A</small> is a soft-iron armature block, <i>a</i>, over
+the core <small>M'</small> of the helix <small>M</small>, and there is a limiting screw, <i>c</i>, passing
+through this armature block <i>a</i>, and at the other end of the
+lever <small>A</small> is a soft iron armature block, <i>b</i>, with the end tapering or
+wedge shaped, and the same comes close to and in line with the
+lateral projection <i>e</i> on the core <small>N<sup>2</sup></small>. The lower ends of the cores
+<small>M' N<sup>2</sup></small> are made with laterally projecting pole-pieces <small>M<sup>3</sup> N<sup>3</sup></small>, respectively,
+and these pole-pieces are concave at their outer ends, and
+are at opposite sides of the armature segments <i>r</i> at the lower end
+of the tubular clamp <small>R</small>.</p>
+
+<p>The operation of these devices is as follows: In the condition
+of inaction, the upper carbon rests upon the lower one, and when
+the electric current is turned on it passes freely, by the frame
+and spring <i>l</i>, through the rods and carbons to the coarse wire and
+helix <small>M</small>, and to the negative binding post <small>V</small> and the core <small>M'</small> thereby
+is energized. The pole piece <small>M<sup>3</sup></small> attracts the armature <i>r</i>, and by
+the lateral pressure causes the clamp <small>R</small> to grasp the rod <small>S'</small>, and
+the lever <small>A</small> is simultaneously moved from the position shown by
+dotted lines, Fig. 278, to the normal position shown in full lines,
+and in so doing the link <i>q</i> and lever <i>t</i> are raised, lifting the clamp
+<small>R</small> and <small>S</small>, separating the carbons and forming the arc. The magnetism
+of the pole piece <i>e</i> tends to hold the lever <small>A</small> level, or
+nearly so, the core <small>N<sup>2</sup></small> being energized by the current in the shunt
+which contains the helix <small>N</small>. In this position the lever <small>A</small> is not<span class='pagenum'><a name="Page_459" id="Page_459">[Pg 459]</a></span>
+moved by any ordinary variation in the current, because the armature
+<i>b</i> is strongly attracted by the magnetism of <i>e</i>, and these
+parts are close to each other, and the magnetism of <i>e</i> acts at right
+angles to the magnetism of the core <small>M'</small>. If, now, the arc becomes
+too long, the current through the helix <small>M</small> is lessened, and the magnetism
+of the core <small>N<sup>3</sup></small> is increased by the greater current passing
+through the shunt, and this core <small>N<sup>3</sup></small>, attracting the segmental armature
+<i>r</i>, lessens the hold of the clamp <small>R</small> upon the rod <small>S</small>, allowing
+the latter to slide and lessen the length of the arc, which instantly
+restores the magnetic equilibrium and causes the clamp <small>R</small> to hold
+the rod <small>S</small>. If it happens that the carbons fall into contact, then
+the magnetism of <small>N<sup>2</sup></small> is lessened so much that the attraction of
+the magnet <small>M</small> will be sufficient to move the armature <i>a</i> and lever
+<small>A</small> so that the armature <i>b</i> passes above the normal position, so as
+to separate the carbons instantly; but when the carbons burn
+away, a greater amount of current will pass through the shunt
+until the attraction of the core <small>N<sup>2</sup></small> will overcome the attraction of
+the core <small>M'</small> and bring the armature lever <small>A</small> again into the normal
+horizontal position, and this occurs before the feed can take place.
+The segmental armature pieces <i>r</i> are shown as nearly semicircular.
+They are square or of any other desired shape, the ends of the
+pole pieces <small>M<sup>3</sup></small>, <small>N<sup>3</sup></small> being made to correspond in shape.</p>
+
+<p>In a modification of this lamp, Mr. Tesla provided means for
+automatically withdrawing a lamp from the circuit, or cutting
+it out when, from a failure of the feed, the arc reached an
+abnormal length; and also means for automatically reinserting
+such lamp in the circuit when the rod drops and the carbons
+come into contact.</p>
+
+<p>Fig. 283 is an elevation of the lamp with the case in section.
+Fig. 284 is a sectional plan at the line <i>x x</i>. Fig. 285 is an elevation,
+partly in section, of the lamp at right angles to Fig. 283.
+Fig. 286 is a sectional plan at the line <i>y y</i> of Fig. 283. Fig. 287
+is a section of the clamp in about full size. Fig. 288 is a detached
+section illustrating the connection of the spring to the
+lever that carries the pivots of the clamp, and Fig. 289 is a
+diagram showing the circuit-connections of the lamp.</p>
+
+<p>In Fig. 283, <small>M</small> represents the main and <small>N</small> the shunt magnet, both
+securely fastened to the base <small>A</small>, which with its side columns, <small>S S</small>,
+are cast in one piece of brass or other diamagnetic material. To
+the magnets are soldered or otherwise fastened the brass washers
+or discs <i>a a a a</i>. Similar washers, <i>b b</i>, of fibre or other insu<span class='pagenum'><a name="Page_460" id="Page_460">[Pg 460]</a></span>lating
+material, serve to insulate the wires from the brass washers.</p>
+
+<p>The magnets <small>M</small> and <small>N</small> are made very flat, so that their width
+exceeds three times their thickness, or even more. In this way
+a comparatively small number of convolutions is sufficient to produce
+the required magnetism, while a greater surface is offered
+for cooling off the wires.</p>
+
+<div class="figcenter" style="width: 1024px;">
+<div class="figleft" style="width: 480px;">
+<img src="images/oi_474-1.jpg" width="480" height="533" alt="Fig. 286, 283." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 286.</td><td class="caption">Fig. 283.</td></tr>
+</table>
+</div>
+
+<div class="figright" style="width: 480px;">
+<img src="images/oi_474-2.jpg" width="480" height="631" alt="Fig. 285." title="" /><br />
+<span class="caption">Fig. 285.</span>
+</div>
+
+<div class="figright" style="width: 336px;">
+<img src="images/oi_474-4.jpg" width="336" height="414" alt="Fig. 287, 288." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 287.</td><td class="caption">Fig. 288.</td></tr>
+</table>
+</div>
+
+<div class="figleft" style="width: 480px;">
+<img src="images/oi_474-3.jpg" width="480" height="480" alt="Fig. 284." title="" /><br />
+<span class="caption">Fig. 284.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>The upper pole pieces, <i>m n</i>, of the magnets are curved, as indicated
+in the drawings, Fig. 283. The lower pole pieces <i>m' n'</i>,
+are brought near together, tapering toward the armature <i>g</i>, as
+shown in Figs. 284 and 286. The object of this taper is to concentrate
+the greatest amount of the developed magnetism upon
+the armature, and also to allow the pull to be exerted always upon
+the middle of the armature <i>g</i>. This armature <i>g</i> is a piece of iron<span class='pagenum'><a name="Page_461" id="Page_461">[Pg 461]</a></span>
+in the shape of a hollow cylinder, having on each side a segment
+cut away, the width of which is equal to the width of the pole
+pieces <i>m' n'</i>.</p>
+
+<p>The armature is soldered or otherwise fastened to the clamp <i>r</i>,
+which is formed of a brass tube, provided with gripping-jaws <i>e e</i>,
+Fig. 287. These jaws are arcs of a circle of the diameter of the
+rod <small>R</small>, and are made of hardened German silver. The guides
+<i>f f</i>, through which the carbon-holding rod <small>R</small> slides, are made of
+the same material. This has the advantage of reducing greatly the
+wear and corrosion of the parts coming in frictional contact with
+the rod, which frequently causes trouble. The jaws <i>e e</i> are
+fastened to the inside of the tube <i>r</i>, so that one is a little lower
+than the other. The object of this is to provide a greater opening
+for the passage of the rod when the same is released by the
+clamp. The clamp <i>r</i> is supported on bearings <i>w w</i>, Figs. 283,
+285 and 287, which are just in the middle between the jaws <i>e e</i>.
+The bearings <i>w w</i> are carried by a lever, <i>t</i>, one end of which
+rests upon an adjustable support, <i>q</i>, of the side columns, <small>S</small>, the
+other end being connected by means of the link <i>e'</i> to the armature-lever
+<small>L</small>. The armature-lever <small>L</small> is a flat piece of iron in <big><b>N</b></big>
+shape, having its ends curved so as to correspond to the form of
+the upper pole-pieces of the magnets <small>M</small> and <small>N</small>. It is hung upon
+the pivots <i>v v</i>, Fig. 284, which are in the jaw <i>x</i> of the
+top plate <small>B</small>. This plate <small>B</small>, with the jaw, is cast in one piece
+and screwed to the side columns, <small>S S</small>, that extend up from the
+base <small>A</small>. To partly balance the overweight of the moving parts,
+a spring, <i>s'</i>, Figs. 284 and 288, is fastened to the top plate, <small>B</small>,
+and hooked to the lever <i>t</i>. The hook <i>o</i> is toward one side of the
+lever or bent a little sidewise, as seen in Fig. 288. By this means
+a slight tendency is given to swing the armature toward the
+pole-piece <i>m'</i> of the main magnet.</p>
+
+<p>The binding-posts <small>K K'</small> are screwed to the base <small>A</small>. A manual
+switch, for short-circuiting the lamp when the carbons are renewed,
+is also fastened to the base. This switch is of ordinary
+character, and is not shown in the drawings.</p>
+
+<p>The rod <small>R</small> is electrically connected to the lamp-frame by means
+of a flexible conductor or otherwise. The lamp-case receives a
+removable cover, <i>s</i><sup>2</sup>, to inclose the parts.</p>
+
+<p>The electrical connections are as indicated diagrammatically in
+Fig. 289. The wire in the main magnet consists of two parts,
+<i>x'</i> and <i>p'</i>. These two parts may be in two separated coils or in<span class='pagenum'><a name="Page_462" id="Page_462">[Pg 462]</a></span>
+one single helix, as shown in the drawings. The part <i>x'</i> being
+normally in circuit, is, with the fine wire upon the shunt-magnet,
+wound and traversed by the current in the same direction, so as
+to tend to produce similar poles, <small>N N</small> or <small>S S</small>, on the corresponding
+pole-pieces of the magnets <small>M</small> and <small>N</small>. The part <i>p'</i> is only in circuit
+when the lamp is cut out, and then the current being in the
+opposite direction produces in the main magnet, magnetism of
+the opposite polarity.</p>
+
+<p>The operation is as follows: At the start the carbons are to
+be in contact, and the current passes from the positive binding-post
+<small>K</small> to the lamp-frame, carbon-holder, upper and lower carbon,
+insulated return-wire in one of the side rods, and from there
+through the part <i>x'</i> of the wire on the main magnet to the negative
+binding-post. Upon the passage of the current the main
+magnet is energized and attracts the clamping-armature <i>g</i>, swinging
+the clamp and gripping the rod by means of the gripping
+jaws <i>e e</i>. At the same time the armature lever <small>L</small> is pulled down
+and the carbons are separated. In pulling down the armature lever
+<small>L</small> the main magnet is assisted by the shunt-magnet <small>N</small>, the latter
+being magnetized by magnetic induction from the magnet <small>M</small>.</p>
+
+<div class="figcenter" style="width: 616px;">
+<img src="images/oi_476.jpg" width="616" height="480" alt="Fig. 289." title="" />
+<span class="caption">Fig. 289.</span>
+</div>
+
+
+<p>It will be seen that the armatures <small>L</small> and <i>g</i> are practically the
+keepers for the magnets <small>M</small> and <small>N</small>, and owing to this fact both
+magnets with either one of the armatures <small>L</small> and <i>g</i> may be considered
+as one horseshoe magnet, which we might term a "compound
+magnet." The whole of the soft-iron parts <small>M</small>, <i>m'</i>, <i>g</i>, <i>n'</i>,
+<small>N</small> and <small>L</small> form a compound magnet.<span class='pagenum'><a name="Page_463" id="Page_463">[Pg 463]</a></span></p>
+
+<p>The carbons being separated, the fine wire receives a portion
+of the current. Now, the magnetic induction from the magnet
+<small>M</small> is such as to produce opposite poles on the corresponding ends
+of the magnet <small>N</small>; but the current traversing the helices tends to
+produce similar poles on the corresponding ends of both magnets,
+and therefore as soon as the fine wire is traversed by sufficient
+current the magnetism of the whole compound magnet is diminished.</p>
+
+<p>With regard to the armature <i>g</i> and the operation of the lamp,
+the pole <i>m'</i> may be considered as the "clamping" and the pole <i>n'</i>
+as the "releasing" pole.</p>
+
+<p>As the carbons burn away, the fine wire receives more current
+and the magnetism diminishes in proportion. This causes the
+armature lever <small>L</small> to swing and the armature <i>g</i> to descend gradually
+under the weight of the moving parts until the end <i>p</i>, Fig.
+283, strikes a stop on the top plate, <small>B</small>. The adjustment is such
+that when this takes place the rod <small>R</small> is yet gripped securely by
+the jaws <i>e e</i>. The further downward movement of the armature
+lever being prevented, the arc becomes longer as the carbons are
+consumed, and the compound magnet is weakened more and
+more until the clamping armature <i>g</i> releases the hold of the
+gripping-jaws <i>e e</i> upon the rod <small>R</small>, and the rod is allowed to drop
+a little, thus shortening the arc. The fine wire now receiving
+less current, the magnetism increases, and the rod is clamped
+again and slightly raised, if necessary. This clamping and releasing
+of the rod continues until the carbons are consumed. In
+practice the feed is so sensitive that for the greatest part of the
+time the movement of the rod cannot be detected without some
+actual measurement. During the normal operation of the lamp
+the armature lever <small>L</small> remains practically stationary, in the position
+shown in Fig. 283.</p>
+
+<p>Should it happen that, owing to an imperfection in it, the rod
+and the carbons drop too far, so as to make the arc too short, or
+even bring the carbons in contact, a very small amount of current
+passes through the fine wire, and the compound magnet
+becomes sufficiently strong to act as at the start in pulling the
+armature lever <small>L</small> down and separating the carbons to a greater
+distance.</p>
+
+<p>It occurs often in practical work that the rod sticks in the
+guides. In this case the are reaches a great length, until it finally
+breaks. Then the light goes out, and frequently the fine wire is<span class='pagenum'><a name="Page_464" id="Page_464">[Pg 464]</a></span>
+injured. To prevent such an accident Mr. Tesla provides this
+lamp with an automatic cut-out which operates as follows: When,
+upon a failure of the feed, the arc reaches a certain predetermined
+length, such an amount of current is diverted through
+the fine wire that the polarity of the compound magnet is reversed.
+The clamping armature <i>g</i> is now moved against the
+shunt magnet <small>N</small> until it strikes the releasing pole <i>n'</i>. As soon
+as the contact is established, the current passes from the positive
+binding post over the clamp <i>r</i>, armature <i>g</i>, insulated shunt magnet,
+and the helix <i>p'</i> upon the main magnet <small>M</small> to the negative
+binding post. In this case the current passes in the opposite direction
+and changes the polarity of the magnet <small>M</small>, at the same
+time maintaining by magnetic induction in the core of the shunt
+magnet the required magnetism without reversal of polarity, and
+the armature <i>g</i> remains against the shunt magnet pole <i>n'</i>. The
+lamp is thus cut out as long as the carbons are separated. The
+cut out may be used in this form without any further improvement;
+but Mr. Tesla arranges it so that if the rod drops and the
+carbons come in contact the arc is started again. For this purpose
+he proportions the resistance of part <i>p'</i> and the number of
+the convolutions of the wire upon the main magnet so that when
+the carbons come in contact a sufficient amount of current is diverted
+through the carbons and the part <i>x'</i> to destroy or neutralize
+the magnetism of the compound magnet. Then the armature
+<i>g</i>, having a slight tendency to approach to the clamping pole
+<i>m'</i>, comes out of contact with the releasing pole <i>n'</i>. As soon as
+this happens, the current through the part <i>p'</i> is interrupted, and
+the whole current passes through the part <i>x</i>. The magnet <small>M</small> is
+now strongly magnetized, the armature <i>g</i> is attracted, and the
+rod clamped. At the same time the armature lever <small>L</small> is pulled
+down out of its normal position and the arc started. In this way
+the lamp cuts itself out automatically when the arc gets too long,
+and reinserts itself automatically in the circuit if the carbons drop
+together.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_465" id="Page_465">[Pg 465]</a></span></p>
+<h2><a name="CHAPTER_XLI" id="CHAPTER_XLI"></a>CHAPTER XLI.</h2>
+
+<h3><span class="smcap">Improvement in "Unipolar" Generators.</span></h3>
+
+
+<p>Another interesting class of apparatus to which Mr. Tesla has
+directed his attention, is that of "unipolar" generators, in which a
+disc or a cylindrical conductor is mounted between magnetic
+poles adapted to produce an approximately uniform field. In
+the disc armature machines the currents induced in the rotating
+conductor flow from the centre to the periphery, or conversely,
+according to the direction of rotation or the lines of force as determined
+by the signs of the magnetic poles, and these currents
+are taken off usually by connections or brushes applied to the
+disc at points on its periphery and near its centre. In the case
+of the cylindrical armature machine, the currents developed in
+the cylinder are taken off by brushes applied to the sides of the
+cylinder at its ends.</p>
+
+<p>In order to develop economically an electromotive force available
+for practicable purposes, it is necessary either to rotate the
+conductor at a very high rate of speed or to use a disc of large
+diameter or a cylinder of great length; but in either case it becomes
+difficult to secure and maintain a good electrical connection
+between the collecting brushes and the conductor, owing to the
+high peripheral speed.</p>
+
+<p>It has been proposed to couple two or more discs together in
+series, with the object of obtaining a higher electro-motive force;
+but with the connections heretofore used and using other conditions
+of speed and dimension of disc necessary to securing good
+practicable results, this difficulty is still felt to be a serious
+obstacle to the use of this kind of generator. These objections
+Mr. Tesla has sought to avoid by constructing a machine with
+two fields, each having a rotary conductor mounted between its
+poles. The same principle is involved in the case of both forms
+of machine above described, but the description now given is
+confined to the disc type, which Mr. Tesla is inclined to favor for
+that machine. The discs are formed with flanges, after the<span class='pagenum'><a name="Page_466" id="Page_466">[Pg 466]</a></span>
+manner of pulleys, and are connected together by flexible conducting
+bands or belts.</p>
+
+<p>The machine is built in such manner that the direction of
+magnetism or order of the poles in one field of force is opposite
+to that in the other, so that rotation of the discs in the same direction
+develops a current in one from centre to circumference
+and in the other from circumference to centre. Contacts applied
+therefore to the shafts upon which the discs are mounted form
+the terminals of a circuit the electro-motive force in which is the
+sum of the electro-motive forces of the two discs.</p>
+
+<p>It will be obvious that if the direction of magnetism in both
+fields be the same, the same result as above will be obtained by
+driving the discs in opposite directions and crossing the connecting
+belts. In this way the difficulty of securing and maintaining
+good contact with the peripheries of the discs is avoided and a
+cheap and durable machine made which is useful for many purposes&mdash;such
+as for an exciter for alternating current generators,
+for a motor, and for any other purpose for which dynamo machines
+are used.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_480.jpg" width="640" height="600" alt="Fig. 290, 291." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 290.</td><td class="caption">Fig. 291.</td></tr>
+</table>
+</div>
+
+<p>Fig. 290 is a side view, partly in section, of this machine.
+Fig. 291 is a vertical section of the same at right angles to the
+shafts.<span class='pagenum'><a name="Page_467" id="Page_467">[Pg 467]</a></span></p>
+
+<p>In order to form a frame with two fields of force, a support,
+<small>A</small>, is cast with two pole pieces <small>B B'</small> integral with it. To this are
+joined by bolts <small>E</small> a casting <small>D</small>, with two similar and corresponding
+pole pieces <small>C C'</small>. The pole pieces <small>B B'</small> are wound and connected
+to produce a field of force of given polarity, and the pole
+pieces <small>C C'</small> are wound so as to produce a field of opposite polarity.
+The driving shafts <small>F G</small> pass through the poles and are
+journaled in insulating bearings in the casting <small>A D</small>, as shown.</p>
+
+<p><small>H K</small> are the discs or generating conductors. They are composed
+of copper, brass, or iron and are keyed or secured to their respective
+shafts. They are provided with broad peripheral flanges
+<small>J</small>. It is of course obvious that the discs may be insulated from their
+shafts, if so desired. A flexible metallic belt <small>L</small> is passed over the
+flanges of the two discs, and, if desired, may be used to drive one
+of the discs. It is better, however, to use this belt merely as a
+conductor, and for this purpose sheet steel, copper, or other suitable
+metal is used. Each shaft is provided with a driving pulley
+<small>M</small>, by which power is imparted from a driving shaft.</p>
+
+<p><small>N N</small> are the terminals. For the sake of clearness they are shown
+as provided with springs <small>P</small>, that bear upon the ends of the shafts.
+This machine, if self-exciting, would have copper bands around
+its poles; or conductors of any kind&mdash;such as wires shown in
+the drawings&mdash;may be used.</p>
+
+<hr style='width: 15%;' />
+
+<p>It is thought appropriate by the compiler to append here some
+notes on unipolar dynamos, written by Mr. Tesla, on a recent occasion.</p>
+
+
+
+<h5>NOTES ON A UNIPOLAR DYNAMO.<a name="FNanchor_15_16" id="FNanchor_15_16"></a><a href="#Footnote_15_16" class="fnanchor">[15]</a></h5>
+
+<p>It is characteristic of fundamental discoveries, of great achievements
+of intellect, that they retain an undiminished power upon
+the imagination of the thinker. The memorable experiment of
+Faraday with a disc rotating between the two poles of a magnet,
+which has borne such magnificent fruit, has long passed into
+every-day experience; yet there are certain features about this
+embryo of the present dynamos and motors which even to-day
+appear to us striking, and are worthy of the most careful study.</p>
+
+<p>Consider, for instance, the case of a disc of iron or other metal
+<span class='pagenum'><a name="Page_468" id="Page_468">[Pg 468]</a></span>revolving between the two opposite poles of a magnet, and the
+polar surfaces completely covering both sides of the disc, and
+assume the current to be taken off or conveyed to the same by
+contacts uniformly from all points of the periphery of the disc.
+Take first the case of a motor. In all ordinary motors the operation
+is dependent upon some shifting or change of the resultant
+of the magnetic attraction exerted upon the armature, this process
+being effected either by some mechanical contrivance on the
+motor or by the action of currents of the proper character. We
+may explain the operation of such a motor just as we can that of
+a water-wheel. But in the above example of the disc surrounded
+completely by the polar surfaces, there is no shifting of the magnetic
+action, no change whatever, as far as we know, and yet
+rotation ensues. Here, then, ordinary considerations do not
+apply; we cannot even give a superficial explanation, as in ordinary
+motors, and the operation will be clear to us only when we
+shall have recognized the very nature of the forces concerned,
+and fathomed the mystery of the invisible connecting mechanism.</p>
+
+<p>Considered as a dynamo machine, the disc is an equally interesting
+object of study. In addition to its peculiarity of giving
+currents of one direction without the employment of commutating
+devices, such a machine differs from ordinary dynamos in
+that there is no reaction between armature and field. The armature
+current tends to set up a magnetization at right angles to
+that of the field current, but since the current is taken off uniformly
+from all points of the periphery, and since, to be exact,
+the external circuit may also be arranged perfectly symmetrical
+to the field magnet, no reaction can occur. This, however, is
+true only as long as the magnets are weakly energized, for when
+the magnets are more or less saturated, both magnetizations at
+right angles seemingly interfere with each other.</p>
+
+<p>For the above reason alone it would appear that the output of
+such a machine should, for the same weight, be much greater
+than that of any other machine in which the armature current
+tends to demagnetize the field. The extraordinary output of the
+Forbes unipolar dynamo and the experience of the writer confirm
+this view.</p>
+
+<p>Again, the facility with which such a machine may be made to
+excite itself is striking, but this may be due&mdash;besides to the absence
+of armature reaction&mdash;to the perfect smoothness of the current
+and non-existence of self-induction.<span class='pagenum'><a name="Page_469" id="Page_469">[Pg 469]</a></span></p>
+
+<p>If the poles do not cover the disc completely on both sides,
+then, of course, unless the disc be properly subdivided, the
+machine will be very inefficient. Again, in this case there are
+points worthy of notice. If the disc be rotated and the field
+current interrupted, the current through the armature will continue
+to flow and the field magnets will lose their strength comparatively
+slowly. The reason for this will at once appear when
+we consider the direction of the currents set up in the disc.</p>
+
+<div class="figcenter" style="width: 613px;">
+<img src="images/oi_483.jpg" width="613" height="480" alt="Fig. 292." title="" />
+<span class="caption">Fig. 292.</span>
+</div>
+
+
+<p>Referring to the diagram Fig. 292, <i>d</i> represents the disc with
+the sliding contacts <small>B B'</small> on the shaft and periphery. <small>N</small> and <small>S</small>
+represent the two poles of a magnet. If the pole <small>N</small> be above, as
+indicated in the diagram, the disc being supposed to be in the
+plane of the paper, and rotating in the direction of the arrow <small>D</small>,
+the current set up in the disc will flow from the centre to the
+periphery, as indicated by the arrow <small>A</small>. Since the magnetic action
+is more or less confined to the space between the poles <small>N S</small>,
+the other portions of the disc may be considered inactive. The
+current set up will therefore not wholly pass through the external
+circuit <small>F</small>, but will close through the disc itself, and generally, if
+the disposition be in any way similar to the one illustrated, by far
+the greater portion of the current generated will not appear externally,
+as the circuit <small>F</small> is practically short-circuited by the inactive
+portions of the disc. The direction of the resulting currents
+in the latter may be assumed to be as indicated by the dotted<span class='pagenum'><a name="Page_470" id="Page_470">[Pg 470]</a></span>
+lines and arrows <i>m</i> and <i>n</i>; and the direction of the energizing
+field current being indicated by the arrows <i>a b c d</i>, an inspection of
+the figure shows that one of the two branches of the eddy current,
+that is, <small>A B'</small> <i>m</i> <small>B</small>, will tend to demagnetize the field, while the
+other branch, that is, <small>A B'</small> <i>n</i> <small>B</small>, will have the opposite effect.
+Therefore, the branch <small>A B'</small> <i>m</i> <small>B</small>, that is, the one which is <i>approaching</i>
+the field, will repel the lines of the same, while branch <small>A B'</small>
+<i>n</i> <small>B</small>, that is, the one <i>leaving</i> the field, will gather the lines of
+force upon itself.</p>
+
+<p>In consequence of this there will be a constant tendency to
+reduce the current flow in the path <small>A B'</small> <i>m</i> <small>B</small>, while on the other
+hand no such opposition will exist in path <small>A B'</small> <i>n</i> <small>B</small>, and the effect
+of the latter branch or path will be more or less preponderating
+over that of the former. The joint effect of both the assumed
+branch currents might be represented by that of one single current
+of the same direction as that energizing the field. In other
+words, the eddy currents circulating in the disc will energize the
+field magnet. This is a result quite contrary to what we might
+be led to suppose at first, for we would naturally expect that the
+resulting effect of the armature currents would be such as to
+oppose the field current, as generally occurs when a primary and
+secondary conductor are placed in inductive relations to each
+other. But it must be remembered that this results from the
+peculiar disposition in this case, namely, two paths being afforded
+to the current, and the latter selecting that path which offers the
+least opposition to its flow. From this we see that the eddy
+currents flowing in the disc partly energize the field, and for this
+reason when the field current is interrupted the currents in the
+disc will continue to flow, and the field magnet will lose its
+strength with comparative slowness and may even retain a certain
+strength as long as the rotation of the disc is continued.</p>
+
+<p>The result will, of course, largely depend on the resistance
+and geometrical dimensions of the path of the resulting eddy
+current and on the speed of rotation; these elements, namely,
+determine the retardation of this current and its position relative
+to the field. For a certain speed there would be a maximum
+energizing action; then at higher speeds, it would gradually fall
+off to zero and finally reverse, that is, the resultant eddy current
+effect would be to weaken the field. The reaction would be
+best demonstrated experimentally by arranging the fields <small>N S</small>,
+<small>N' S'</small>, freely movable on an axis concentric with the shaft of the<span class='pagenum'><a name="Page_471" id="Page_471">[Pg 471]</a></span>
+disc. If the latter were rotated as before in the direction of the
+arrow <small>D</small>, the field would be dragged in the same direction with a
+torque, which, up to a certain point, would go on increasing with
+the speed of rotation, then fall off, and, passing through zero,
+finally become negative; that is, the field would begin to rotate
+in opposite direction to the disc. In experiments with alternate
+current motors in which the field was shifted by currents of
+differing phase, this interesting result was observed. For very
+low speeds of rotation of the field the motor would show a
+torque of 900 lbs. or more, measured on a pulley 12 inches
+in diameter. When the speed of rotation of the poles was
+increased, the torque would diminish, would finally go down to
+zero, become negative, and then the armature would begin to
+rotate in opposite direction to the field.</p>
+
+<p>To return to the principal subject; assume the conditions to be
+such that the eddy currents generated by the rotation of the disc
+strengthen the field, and suppose the latter gradually removed
+while the disc is kept rotating at an increased rate. The current,
+once started, may then be sufficient to maintain itself and even
+increase in strength, and then we have the case of Sir William
+Thomson's "current accumulator." But from the above considerations
+it would seem that for the success of the experiment
+the employment of a disc <i>not subdivided</i><a name="FNanchor_16_17" id="FNanchor_16_17"></a><a href="#Footnote_16_17" class="fnanchor">[16]</a> would be essential,
+for if there should be a radial subdivision, the eddy currents
+could not form and the self-exciting action would cease. If
+such a radially subdivided disc were used it would be necessary
+to connect the spokes by a conducting rim or in any proper
+manner so as to form a symmetrical system of closed circuits.</p>
+
+<p>The action of the eddy currents may be utilized to excite a machine
+of any construction. For instance, in Figs. 293 and 294 an
+arrangement is shown by which a machine with a disc armature
+might be excited. Here a number of magnets, <small>N S</small>, <small>N S</small>, are
+placed radially on each side of a metal disc <small>D</small> carrying on its rim
+a set of insulated coils, <small>C C</small>. The magnets form two separate
+fields, an internal and an external one, the solid disc rotating in the
+<span class='pagenum'><a name="Page_472" id="Page_472">[Pg 472]</a></span>field nearest the axis, and the coils in the field further from it.
+Assume the magnets slightly energized at the start; they could be
+strengthened by the action of the eddy currents in the solid disc
+so as to afford a stronger field for the peripheral coils. Although
+there is no doubt that under proper conditions a machine might
+be excited in this or a similar manner, there being sufficient experimental
+evidence to warrant such an assertion, such a mode of
+excitation would be wasteful.</p>
+
+<p>But a unipolar dynamo or motor, such as shown in Fig. 292,
+may be excited in an efficient manner by simply properly subdividing
+the disc or cylinder in which the currents are set up, and
+it is practicable to do away with the field coils which are usually
+employed. Such a plan is illustrated in Fig. 295. The disc or
+cylinder <small>D</small> is supposed to be arranged to rotate between the two
+poles <small>N</small> and <small>S</small> of a magnet, which completely cover it on both
+sides, the contours of the disc and poles being represented by the
+circles <i>d</i> and <i>d</i><sup>1</sup> respectively, the upper pole being omitted for
+the sake of clearness. The cores of the magnet are supposed to
+be hollow, the shaft <small>C</small> of the disc passing through them. If the
+unmarked pole be below, and the disc be rotated screw fashion,
+the current will be, as before, from the centre to the periphery,
+and may be taken off by suitable sliding contacts, <small>B B'</small>, on the
+shaft and periphery respectively. In this arrangement the current
+flowing through the disc and external circuit will have no
+appreciable effect on the field magnet.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_486.jpg" width="800" height="557" alt="Fig. 293, 294." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 293.</td><td class="caption">Fig. 294.</td></tr>
+</table>
+</div>
+
+<p>But let us now suppose the disc to be subdivided spirally, as
+<span class='pagenum'><a name="Page_473" id="Page_473">[Pg 473]</a></span>indicated by the full or dotted lines, Fig. 295. The difference of
+potential between a point on the shaft and a point on the periphery
+will remain unchanged, in sign as well as in amount. The
+only difference will be that the resistance of the disc will be augmented
+and that there will be a greater fall of potential from a
+point on the shaft to a point on the periphery when the same current
+is traversing the external circuit. But since the current is
+forced to follow the lines of subdivision, we see that it will tend
+either to energize or de-energize the field, and this will depend,
+other things being equal, upon the direction of the lines of subdivision.
+If the subdivision be as indicated by the full lines in
+Fig. 295, it is evident that if the current is of the same direction
+as before, that is, from centre to periphery, its effect will be to
+strengthen the field magnet; Whereas, if the subdivision be as indicated
+by the dotted lines, the current generated will tend to
+weaken the magnet. In the former case the machine will be
+capable of exciting itself when the disc is rotated in the direction
+of arrow <small>D</small>; in the latter case the direction of rotation must be
+reversed. Two such discs may be combined, however, as indicated,
+the two discs rotating in opposite fields, and in the same
+or opposite direction.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_487.jpg" width="800" height="382" alt="Fig. 295, 296." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 295.</td><td class="caption">Fig. 296.</td></tr>
+</table>
+</div>
+
+<p>Similar disposition may, of course, be made in a type of
+machine in which, instead of a disc, a cylinder is rotated. In
+such unipolar machines, in the manner indicated, the usual field
+coils and poles may be omitted and the machine may be made to
+consist only of a cylinder or of two discs enveloped by a metal
+casting.</p>
+
+<p>Instead of subdividing the disc or cylinder spirally, as indicated
+in Fig. 295, it is more convenient to interpose one or more turns<span class='pagenum'><a name="Page_474" id="Page_474">[Pg 474]</a></span>
+between the disc and the contact ring on the periphery, as illustrated
+in Fig. 296.</p>
+
+<p>A Forbes dynamo may, for instance, be excited in such a manner.
+In the experience of the writer it has been found that instead
+of taking the current from two such discs by sliding
+contacts, as usual, a flexible conducting belt may be employed
+to advantage. The discs are in such case provided with large
+flanges, affording a very great contact surface. The belt should
+be made to bear on the flanges with spring pressure to take up
+the expansion. Several machines with belt contact were constructed
+by the writer two years ago, and worked satisfactorily;
+but for want of time the work in that direction has been temporarily
+suspended. A number of features pointed out above have
+also been used by the writer in connection with some types of
+alternating current motors.</p>
+
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_475" id="Page_475">[Pg 475]</a></span></p>
+<h1><small><a name="PART_IV" id="PART_IV"></a>PART IV.</small><br /><br />
+
+APPENDIX.&mdash;EARLY PHASE MOTORS AND THE<br />
+TESLA MECHANICAL AND ELECTRICAL<br />
+OSCILLATOR.</h1>
+<p><span class='pagenum'><a name="Page_476" id="Page_476">[Pg 476]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_477" id="Page_477">[Pg 477]</a></span></p>
+<h2><a name="CHAPTER_XLII" id="CHAPTER_XLII"></a>CHAPTER XLII.</h2>
+
+<h3><span class="smcap">Mr. Tesla's Personal Exhibit at the World's Fair.</span></h3>
+
+<p>While the exhibits of firms engaged in the manufacture of
+electrical apparatus of every description at the Chicago World's
+Fair, afforded the visitor ample opportunity for gaining an excellent
+knowledge of the state of the art, there were also numbers
+of exhibits which brought out in strong relief the work of the
+individual inventor, which lies at the foundation of much, if not
+all, industrial or mechanical achievement. Prominent among
+such personal exhibits was that of Mr. Tesla, whose apparatus
+occupied part of the space of the Westinghouse Company, in
+Electricity Building.</p>
+
+<p>This apparatus represented the results of work and thought
+covering a period of ten years. It embraced a large number of
+different alternating motors and Mr. Tesla's earlier high frequency
+apparatus. The motor exhibit consisted of a variety of
+fields and armatures for two, three and multiphase circuits, and
+gave a fair idea of the gradual evolution of the fundamental idea
+of the rotating magnetic field. The high frequency exhibit included
+Mr. Tesla's earlier machines and disruptive discharge coils
+and high frequency transformers, which he used in his investigations
+and some of which are referred to in his papers printed
+in this volume.</p>
+
+<p>Fig. 297 shows a view of part of the exhibits containing the
+motor apparatus. Among these is shown at A a large ring intended
+to exhibit the phenomena of the rotating magnetic field.
+The field produced was very powerful and exhibited striking
+effects, revolving copper balls and eggs and bodies of various
+shapes at considerable distances and at great speeds. This ring
+was wound for two-phase circuits, and the winding was so distributed
+that a practically uniform field was obtained. This ring
+was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician
+of the Westinghouse Electric and Manufacturing Company.</p>
+<p><span class='pagenum'><a name="Page_478" id="Page_478">[Pg 478]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_492.jpg" width="1024" height="487" alt="Fig. 297." title="" />
+<span class="caption">Fig. 297.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_479" id="Page_479">[Pg 479]</a></span></p>
+<p>A smaller ring, shown at <small>B</small>, was arranged like the one exhibited
+at <small>A</small> but designed especially to exhibit the rotation of an
+armature in a rotating field. In connection with these two
+rings there was an interesting exhibit shown by Mr. Tesla which
+consisted of a magnet with a coil, the magnet being arranged to
+rotate in bearings. With this magnet he first demonstrated the
+identity between a rotating field and a rotating magnet; the latter,
+when rotating, exhibited the same phenomena as the rings when
+they were energized by currents of differing phase. Another
+prominent exhibit was a model illustrated at <small>C</small> which is a two-phase
+motor, as well as an induction motor and transformer. It
+consists of a large outer ring of laminated iron wound with
+two superimposed, separated windings which can be connected
+in a variety of ways. This is one of the first models used by
+Mr. Tesla as an induction motor and rotating transformer. The
+armature was either a steel or wrought iron disc with a closed
+coil. When the motor was operated from a two phase generator
+the windings were connected in two groups, as usual. When
+used as an induction motor, the current induced in one of the
+windings of the ring was passed through the other winding on
+the ring and so the motor operated with only two wires. When
+used as a transformer the outer winding served, for instance, as
+a secondary and the inner as a primary. The model shown at
+D is one of the earliest rotating field motors, consisting of a thin
+iron ring wound with two sets of coils and an armature consisting
+of a series of steel discs partly cut away and arranged on a small
+arbor.</p>
+
+<p>At <small>E</small> is shown one of the first rotating field or induction motors
+used for the regulation of an arc lamp and for other purposes. It
+comprises a ring of discs with two sets of coils having different
+self-inductions, one set being of German silver and the other of
+copper wire. The armature is wound with two closed-circuited
+coils at right angles to each other. To the armature shaft are
+fastened levers and other devices to effect the regulation. At <small>F</small>
+is shown a model of a magnetic lag motor; this embodies a casting
+with pole projections protruding from two coils between
+which is arranged to rotate a smooth iron body. When an alternating
+current is sent through the two coils the pole projections
+of the field and armature within it are similarly magnetized, and
+upon the cessation or reversal of the current the armature and
+field repel each other and rotation is produced in this way.<span class='pagenum'><a name="Page_480" id="Page_480">[Pg 480]</a></span>
+Another interesting exhibit, shown at <small>G</small>, is an early model of a
+two field motor energized by currents of different phase. There
+are two independent fields of laminated iron joined by brass
+bolts; in each field is mounted an armature, both armatures being
+on the same shaft. The armatures were originally so arranged
+as to be placed in any position relatively to each other,
+and the fields also were arranged to be connected in a number
+of ways. The motor has served for the exhibition of a number
+of features; among other things, it has been used as a dynamo
+for the production of currents of any frequency between wide
+limits. In this case the field, instead of being energized by direct
+current, was energized by currents differing in phase, which
+produced a rotation of the field; the armature was then rotated
+in the same or in opposite direction to the movement of the field;
+and so any number of alternations of the currents induced in the
+armature, from a small to a high number, determined by the
+frequency of the energizing field coils and the speed of the armature,
+was obtained.</p>
+
+<div class="figcenter" style="width: 632px;">
+<img src="images/oi_494.jpg" width="632" height="480" alt="Fig. 298." title="" />
+<span class="caption">Fig. 298.</span>
+</div>
+
+<p>The models <small>H</small>, <small>I</small>, <small>J</small>, represent a variety of rotating field, synchronous
+motors which are of special value in long distance transmission
+work. The principle embodied in these motors was enunciated
+by Mr. Tesla in his lecture before the American Institute of
+Electrical Engineers, in May, 1888<a name="FNanchor_17_18" id="FNanchor_17_18"></a><a href="#Footnote_17_18" class="fnanchor">[17]</a>. It involves the production
+<span class='pagenum'><a name="Page_481" id="Page_481">[Pg 481]</a></span>of the rotating field in one of the elements of the motor by currents
+differing in phase and energizing the other element by
+direct currents. The armatures are of the two and three phase
+type. <small>K</small> is a model of a motor shown in an enlarged view in Fig.
+298. This machine, together with that shown in Fig. 299, was
+exhibited at the same lecture, in May, 1888. They were
+the first rotating field motors which were independently tested,
+having for that purpose been placed in the hands of Prof. Anthony
+in the winter of 1887-88. From these tests it was shown
+that the efficiency and output of these motors was quite satisfactory
+in every respect.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_495.jpg" width="640" height="410" alt="Fig. 299." title="" />
+<span class="caption">Fig. 299.</span>
+</div>
+
+
+<p>It was intended to exhibit the model shown in Fig. 299, but it
+was unavailable for that purpose owing to the fact that it was
+some time ago handed over to the care of Prof. Ayrton in England.
+This model was originally provided with twelve independent
+coils; this number, as Mr. Tesla pointed out in his first lecture,
+being divisible by two and three, was selected in order to make
+various connections for two and three-phase operations, and during
+Mr. Tesla's experiments was used in many ways with from two to
+six phases. The model, Fig. 298, consists of a magnetic frame of
+laminated iron with four polar projections between which an armature
+is supported on brass bolts passing through the frame. A
+great variety of armatures was used in connection with these two
+and other fields. Some of the armatures are shown in front on
+the table, Fig. 297, and several are also shown enlarged in Figs.
+300 to 310. An interesting exhibit is that shown at <small>L</small>, Fig. 297.
+This is an armature of hardened steel which was used in a demon<span class='pagenum'><a name="Page_482" id="Page_482">[Pg 482]</a></span>stration
+before the Society of Arts in Boston, by Prof. Anthony.
+Another curious exhibit is shown enlarged in Fig. 301. This
+consists of thick discs of wrought iron placed lengthwise, with a
+mass of copper cast around them. The discs were arranged
+longitudinally to afford an easier starting by reason of the induced
+current formed in the iron discs, which differed in phase from
+those in the copper. This armature would start with a single circuit
+and run in synchronism, and represents one of the earliest
+types of such an armature. Fig. 305 is another striking exhibit.
+This is one of the earliest types of an armature with holes beneath
+the periphery, in which copper conductors are imbedded. The
+armature has eight closed circuits and was used in many different
+ways. Fig. 304 is a type of synchronous armature consisting of
+a block of soft steel wound with a coil closed upon itself. This
+armature was used in connection with the field shown in Fig. 298
+and gave excellent results.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_496-1.jpg" width="800" height="139" alt="Fig. 300, 301, 302." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 300.</td><td class="caption">Fig. 301.</td><td class="caption">Fig. 302.</td></tr>
+</table>
+
+<img src="images/oi_496-2.jpg" width="800" height="172" alt="Fig. 303, 304, 305." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 303.</td><td class="caption">Fig. 304.</td><td class="caption">Fig. 305.</td></tr>
+</table>
+
+<img src="images/oi_496-3.jpg" width="800" height="150" alt="Fig. 306, 307, 308." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 306.</td><td class="caption">Fig. 307.</td><td class="caption">Fig. 308.</td></tr>
+</table>
+
+<img src="images/oi_496.jpg" width="800" height="139" alt="Fig. 309, 310." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 309.</td><td class="caption">Fig. 310.</td></tr>
+</table>
+</div>
+
+
+<p>Fig. 302 represents a synchronous armature with a large coil
+around a body of iron. There is another very small coil at right
+angles to the first. This small coil was used for the purpose of<span class='pagenum'><a name="Page_483" id="Page_483">[Pg 483]</a></span>
+increasing the starting torque and was found very effective in
+this connection. Figs. 306 and 308 show a favorite construction
+of armature; the iron body is made up of two sets of discs cut
+away and placed at right angles to each other, the interstices being
+wound with coils. The one shown in Fig. 308 is provided
+with an additional groove on each of the projections formed by
+the discs, for the purpose of increasing the starting torque by a
+wire wound in these projections. Fig. 307 is a form of armature
+similarly constructed, but with four independent coils wound upon
+the four projections. This armature was used to reduce the
+speed of the motor with reference to that of the generator. Fig.
+300 is still another armature with a great number of independent
+circuits closed upon themselves, so that all the dead points on
+the armature are done away with, and the armature has a large
+starting torque. Fig. 303 is another type of armature for a four-pole
+motor but with coils wound upon a smooth surface. A
+number of these armatures have hollow shafts, as they have been
+used in many ways. Figs. 309 and 310 represent armatures to
+which either alternating or direct current was conveyed by
+means of sliding rings. Fig. 309 consists of a soft iron body
+with a single coil wound around it, the ends of the coil being
+connected to two sliding rings to which, usually, direct current
+was conveyed. The armature shown in Fig. 310 has three insulated
+rings on a shaft and was used in connection with two or
+three phase circuits.</p>
+
+<p>All these models shown represent early work, and the enlarged
+engravings are made from photographs taken early in
+1888. There is a great number of other models which were exhibited,
+but which are not brought out sharply in the engraving,
+Fig. 297. For example at <small>M</small> is a model of a motor comprising
+an armature with a hollow shaft wound with two or three coils for
+two or three-phase circuits; the armature was arranged to be stationary
+and the generating circuits were connected directly to
+the generator. Around the armature is arranged to rotate on
+its shaft a casting forming six closed circuits. On the outside
+this casting was turned smooth and the belt was placed on it for
+driving with any desired appliance. This also is a very early
+model.</p>
+
+<p>On the left side of the table there are seen a large variety of
+models, <small>N</small>, <small>O</small>, <small>P</small>, etc., with fields of various shapes. Each of these
+models involves some distinct idea and they all represent gradual<span class='pagenum'><a name="Page_484" id="Page_484">[Pg 484]</a></span>
+development chiefly interesting as showing Mr. Tesla's efforts to
+adapt his system to the existing high frequencies.</p>
+
+<p>On the right side of the table, at <small>S</small>, <small>T</small>, are shown, on separate
+supports, larger and more perfected armatures of commercial
+motors, and in the space around the table a variety of motors and
+generators supplying currents to them was exhibited.</p>
+
+<p>The high frequency exhibit embraced Mr. Tesla's first original
+apparatus used in his investigations. There was exhibited a
+glass tube with one layer of silk-covered wire wound at the top
+and a copper ribbon on the inside. This was the first disruptive
+discharge coil constructed by him. At <small>U</small> is shown the disruptive
+discharge coil exhibited by him in his lecture before the American
+Institute of Electrical Engineers, in May, 1891.<a name="FNanchor_18_19" id="FNanchor_18_19"></a><a href="#Footnote_18_19" class="fnanchor">[18]</a> At <small>V</small> and <small>W</small>
+are shown some of the first high frequency transformers. A
+number of various fields and armatures of small models of high
+frequency apparatus as shown at <small>X</small> and <small>Y</small>, and others not visible
+in the picture, were exhibited. In the annexed space the dynamo
+then used by Mr. Tesla at Columbia College was exhibited; also
+another form of high frequency dynamo used.</p>
+
+<div class="figcenter" style="width: 611px;">
+<img src="images/oi_498.jpg" width="611" height="480" alt="Fig. 311." title="" />
+<span class="caption">Fig. 311.</span>
+</div>
+
+<p>In this space also was arranged a battery of Leyden jars and
+his large disruptive discharge coil which was used for exhibiting
+<span class='pagenum'><a name="Page_485" id="Page_485">[Pg 485]</a></span>the light phenomena in the adjoining dark room. The coil was
+operated at only a small fraction of its capacity, as the necessary
+condensers and transformers could not be had and as Mr. Tesla's
+stay was limited to one week; notwithstanding, the phenomena
+were of a striking character. In the room were arranged two
+large plates placed at a distance of about eighteen feet from each
+other. Between them were placed two long tables with all sorts
+of phosphorescent bulbs and tubes; many of these were prepared
+with great care and marked legibly with the names which would
+shine with phosphorescent glow. Among them were some with
+the names of Helmholtz, Faraday, Maxwell, Henry, Franklin,
+etc. Mr. Tesla had also not forgotten the greatest living poet of
+his own country, Zmaj Jovan; two or three were prepared with
+inscriptions, like "Welcome, Electricians," and produced a beautiful
+effect. Each represented some phase of this work and stood
+for some individual experiment of importance. Outside the room
+was the small battery seen in Fig. 311, for the exhibition of some
+of the impedance and other phenomena of interest. Thus, for
+instance, a thick copper bar bent in arched form was provided
+with clamps for the attachment of lamps, and a number of lamps
+were kept at incandescence on the bar; there was also a little motor
+shown on the table operated by the disruptive discharge.</p>
+
+<p>As will be remembered by those who visited the Exposition,
+the Westinghouse Company made a line exhibit of the various
+commercial motors of the Tesla system, while the twelve generators
+in Machinery Hall were of the two-phase type constructed
+for distributing light and power. Mr. Tesla, also exhibited
+some models of his oscillators.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_486" id="Page_486">[Pg 486]</a></span></p>
+<h2><a name="CHAPTER_XLIII" id="CHAPTER_XLIII"></a>CHAPTER XLIII.</h2>
+
+<h3><span class="smcap">The Tesla Mechanical and Electrical Oscillators.</span></h3>
+
+
+<p>On the evening of Friday, August 25, 1893, Mr. Tesla delivered
+a lecture on his mechanical and electrical oscillators, before
+the members of the Electrical Congress, in the hall adjoining
+the Agricultural Building, at the World's Fair, Chicago. Besides
+the apparatus in the room, he employed an air compressor,
+which was driven by an electric motor.</p>
+
+<p>Mr. Tesla was introduced by Dr. Elisha Gray, and began by
+stating that the problem he had set out to solve was to construct,
+first, a mechanism which would produce oscillations of a perfectly
+constant period independent of the pressure of steam or
+air applied, within the widest limits, and also independent of
+frictional losses and load. Secondly, to produce electric currents
+of a perfectly constant period independently of the working
+conditions, and to produce these currents with mechanism
+which should be reliable and positive in its action without resorting
+to spark gaps and breaks. This he successfully accomplished
+in his apparatus, and with this apparatus, now, scientific men will
+be provided with the necessaries for carrying on investigations
+with alternating currents with great precision. These two inventions
+Mr. Tesla called, quite appropriately, a mechanical and
+an electrical oscillator, respectively.</p>
+
+<p>The former is substantially constructed in the following way.
+There is a piston in a cylinder made to reciprocate automatically
+by proper dispositions of parts, similar to a reciprocating tool.
+Mr. Tesla pointed out that he had done a great deal of work in
+perfecting his apparatus so that it would work efficiently at such
+high frequency of reciprocation as he contemplated, but he did not
+dwell on the many difficulties encountered. He exhibited, however,
+the pieces of a steel arbor which had been actually torn
+apart while vibrating against a minute air cushion.</p>
+
+<p>With the piston above referred to there is associated in one of
+his models in an independent chamber an air spring, or dash pot,<span class='pagenum'><a name="Page_487" id="Page_487">[Pg 487]</a></span>
+or else he obtains the spring within the chambers of the oscillator
+itself. To appreciate the beauty of this it is only necessary to say
+that in that disposition, as he showed it, no matter what the
+rigidity of the spring and no matter what the weight of the moving
+parts, in other words, no matter what the period of vibrations,
+the vibrations of the spring are always isochronous with the applied
+pressure. Owing to this, the results obtained with these
+vibrations are truly wonderful. Mr. Tesla provides for an air
+spring of tremendous rigidity, and he is enabled to vibrate big
+weights at an enormous rate, considering the inertia, owing to the
+recoil of the spring. Thus, for instance, in one of these experiments,
+he vibrates a weight of approximately 20 pounds at the
+rate of about 80 per second and with a stroke of about 7/8 inch, but
+by shortening the stroke the weight could be vibrated many hundred
+times, and has been, in other experiments.</p>
+
+<p>To start the vibrations, a powerful blow is struck, but the adjustment
+can be so made that only a minute effort is required to
+start, and, even without any special provision it will start by
+merely turning on the pressure suddenly. The vibration being,
+of course, isochronous, any change of pressure merely produces a
+shortening or lengthening of the stroke. Mr. Tesla showed a
+number of very clear drawings, illustrating the construction of
+the apparatus from which its working was plainly discernible.
+Special provisions are made so as to equalize the pressure
+within the dash pot and the outer atmosphere. For this purpose
+the inside chambers of the dash pot are arranged to communicate
+with the outer atmosphere so that no matter how the temperature
+of the enclosed air might vary, it still retains the same mean
+density as the outer atmosphere, and by this means a spring of
+constant rigidity is obtained. Now, of course, the pressure of
+the atmosphere may vary, and this would vary the rigidity of the
+spring, and consequently the period of vibration, and this feature
+constitutes one of the great beauties of the apparatus; for, as Mr.
+Tesla pointed out, this mechanical system acts exactly like a
+string tightly stretched between two points, and with fixed nodes,
+so that slight changes of the tension do not in the least alter the
+period of oscillation.</p>
+
+<p>The applications of such an apparatus are, of course, numerous
+and obvious. The first is, of course, to produce electric
+currents, and by a number of models and apparatus on the lecture
+platform, Mr. Tesla showed how this could be carried out in<span class='pagenum'><a name="Page_488" id="Page_488">[Pg 488]</a></span>
+practice by combining an electric generator with his oscillator.
+He pointed out what conditions must be observed in order that
+the period of vibration of the electrical system might not disturb
+the mechanical oscillation in such a way as to alter the periodicity,
+but merely to shorten the stroke. He combines a condenser
+with a self-induction, and gives to the electrical system the same
+period as that at which the machine itself oscillates, so that both
+together then fall in step and electrical and mechanical resonance
+is obtained, and maintained absolutely unvaried.</p>
+
+<p>Next he showed a model of a motor with delicate wheelwork,
+which was driven by these currents at a constant speed, no matter
+what the air pressure applied was, so that this motor could
+be employed as a clock. He also showed a clock so constructed
+that it could be attached to one of the oscillators, and would
+keep absolutely correct time. Another curious and interesting
+feature which Mr. Tesla pointed out was that, instead of controlling
+the motion of the reciprocating piston by means of a
+spring, so as to obtain isochronous vibration, he was actually able
+to control the mechanical motion by the natural vibration of the
+electro-magnetic system, and he said that the case was a very
+simple one, and was quite analogous to that of a pendulum.
+Thus, supposing we had a pendulum of great weight, preferably,
+which would be maintained in vibration by force, periodically
+applied; now that force, no matter how it might vary, although
+it would oscillate the pendulum, would have no control over its
+period.</p>
+
+<p>Mr. Tesla also described a very interesting phenomenon which
+he illustrated by an experiment. By means of this new apparatus,
+he is able to produce an alternating current in which the
+<span class="smcap">e. m. f.</span> of the impulses in one direction preponderates over that
+of those in the other, so that there is produced the effect of a
+direct current. In fact he expressed the hope that these currents
+would be capable of application in many instances, serving
+as direct currents. The principle involved in this preponderating
+<span class="smcap">e. m. f.</span> he explains in this way: Suppose a conductor is
+moved into the magnetic field and then suddenly withdrawn. If
+the current is not retarded, then the work performed will be a
+mere fractional one; but if the current is retarded, then the
+magnetic field acts as a spring. Imagine that the motion of the
+conductor is arrested by the current generated, and that at the
+instant when it stops to move into the field, there is still the<span class='pagenum'><a name="Page_489" id="Page_489">[Pg 489]</a></span>
+maximum current flowing in the conductor; then this current
+will, according to Lenz's law, drive the conductor out of the field
+again, and if the conductor has no resistance, then it would leave
+the field with the velocity it entered it. Now it is clear that if,
+instead of simply depending on the current to drive the conductor
+out of the field, the mechanically applied force is so timed
+that it helps the conductor to get out of the field, then it might
+leave the field with higher velocity than it entered it, and
+thus one impulse is made to preponderate in <span class="smcap">e. m. f.</span> over the
+other.</p>
+
+<p>With a current of this nature, Mr. Tesla energized magnets
+strongly, and performed many interesting experiments bearing
+out the fact that one of the current impulses preponderates.
+Among them was one in which he attached to his oscillator a ring
+magnet with a small air gap between the poles. This magnet was
+oscillated up and down 80 times a second. A copper disc, when
+inserted within the air gap of the ring magnet, was brought into
+rapid rotation. Mr. Tesla remarked that this experiment also
+seemed to demonstrate that the lines of flow of current through
+a metallic mass are disturbed by the presence of a magnet in a
+manner quite independently of the so-called Hall effect. He
+showed also a very interesting method of making a connection
+with the oscillating magnet. This was accomplished by attaching
+to the magnet small insulated steel rods, and connecting to these
+rods the ends of the energizing coil. As the magnet was vibrated,
+stationary nodes were produced in the steel rods, and at these
+points the terminals of a direct current source were attached.
+Mr. Tesla also pointed out that one of the uses of currents, such
+as those produced in his apparatus, would be to select any given
+one of a number of devices connected to the same circuit by picking
+out the vibration by resonance. There is indeed little doubt
+that with Mr. Tesla's devices, harmonic and synchronous telegraphy
+will receive a fresh impetus, and vast possibilities are
+again opened up.</p>
+
+<p>Mr. Tesla was very much elated over his latest achievements,
+and said that he hoped that in the hands of practical, as well as
+scientific men, the devices described by him would yield important
+results. He laid special stress on the facility now afforded for
+investigating the effect of mechanical vibration in all directions,
+and also showed that he had observed a number of facts in connection
+with iron cores.<span class='pagenum'><a name="Page_490" id="Page_490">[Pg 490]</a></span></p>
+
+
+<div class="figcenter" style="width: 558px;">
+<img src="images/oi_504.jpg" width="558" height="480" alt="Fig. 312." title="" />
+<span class="caption">Fig. 312.</span>
+</div>
+
+
+<p>The engraving, Fig. 312, shows, in perspective, one of the
+forms of apparatus used by Mr. Tesla in his earlier investigations
+in this field of work, and its interior construction is made plain
+by the sectional view shown in Fig. 313. It will be noted that the
+piston <small>P</small> is fitted into the hollow of a cylinder <small>C</small> which is provided
+with channel ports <small>O O</small>, and <i>I</i>, extending all around the inside
+surface. In this particular apparatus there are two channels <small>O O</small>
+for the outlet of the working fluid and one, <i>I</i>, for the inlet.
+The piston <small>P</small> is provided with two slots <small>S S'</small> at a carefully determined
+distance, one from the other. The tubes <small>T T</small> which are
+screwed into the holes drilled into the piston, establish communication
+between the slots <small>S S'</small> and chambers on each side of the
+piston, each of these chambers connecting with the slot which is
+remote from it. The piston <small>P</small> is screwed tightly on a shaft <small>A</small>
+<span class='pagenum'><a name="Page_491" id="Page_491">[Pg 491]</a></span>
+which passes through fitting boxes at the end of the cylinder <small>C</small>.
+The boxes project to a carefully determined distance into the hollow
+of the cylinder <small>C</small>, thus determining the length of the stroke.</p>
+
+<p>Surrounding the whole is a jacket <small>J</small>. This jacket acts chiefly to
+diminish the sound produced by the oscillator and as a jacket when
+the oscillator is driven by steam, in which case a somewhat different
+arrangement of the magnets is employed. The apparatus here
+illustrated was intended for demonstration purposes, air being
+used as most convenient for this purpose.</p>
+
+<p>A magnetic frame <small>M M</small> is fastened so as to closely surround the
+oscillator and is provided with energizing coils which establish
+two strong magnetic fields on opposite sides. The magnetic frame
+is made up of thin sheet iron. In the intensely concentrated
+field thus produced, there are arranged two pairs of coils <small>H H</small> supported
+in metallic frames which are screwed on the shaft <b>A</b> of
+the piston and have additional bearings in the boxes <small>B B</small> on each
+side. The whole is mounted on a metallic base resting on two
+wooden blocks.</p>
+
+<div class="figcenter" style="width: 576px;">
+<img src="images/oi_505.jpg" width="576" height="480" alt="Fig. 313." title="" />
+<span class="caption">Fig. 313.</span>
+</div>
+
+<p>The operation of the device is as follows: The working fluid
+being admitted through an inlet pipe to the slot <small>I</small> and the piston
+being supposed to be in the position indicated, it is sufficient,
+though not necessary, to give a gentle tap on one of the shaft<span class='pagenum'><a name="Page_492" id="Page_492">[Pg 492]</a></span>
+ends protruding from the boxes <small>B</small>. Assume that the motion imparted
+be such as to move the piston to the left (when looking at
+the diagram) then the air rushes through the slot <small>S'</small> and tube <small>T</small>
+into the chamber to the left. The pressure now drives the piston
+towards the right and, owing to its inertia, it overshoots the
+position of equilibrium and allows the air to rush through the
+slot <small>S</small> and tube <small>T</small> into the chamber to the right, while the communication
+to the left hand chamber is cut off, the air of the
+latter chamber escaping through the outlet <small>O</small> on the left. On
+the return stroke a similar operation takes place on the right
+hand side. This oscillation is maintained continuously and the
+apparatus performs vibrations from a scarcely perceptible quiver
+amounting to no more than <small><sup>1</sup></small> of an inch, up to vibrations of a little
+over 3/8 of an inch, according to the air pressure and load. It is
+indeed interesting to see how an incandescent lamp is kept burning
+with the apparatus showing a scarcely perceptible quiver.</p>
+
+<p>To perfect the mechanical part of the apparatus so that oscillations
+are maintained economically was one thing, and Mr. Tesla
+hinted in his lecture at the great difficulties he had first encountered
+to accomplish this. But to produce oscillations which would
+be of constant period was another task of no mean proportions.
+As already pointed out, Mr. Tesla obtains the constancy of period
+in three distinct ways. Thus, he provides properly calculated
+chambers, as in the case illustrated, in the oscillator itself; or he associates
+with the oscillator an air spring of constant resilience. But
+the most interesting of all, perhaps, is the maintenance of the constancy
+of oscillation by the reaction of the electromagnetic part of
+the combination. Mr. Tesla winds his coils, by preference, for high
+tension and associates with them a condenser, making the natural
+period of the combination fairly approximating to the average period
+at which the piston would oscillate without any particular provision
+being made for the constancy of period under varying pressure
+and load. As the piston with the coils is perfectly free to move,
+it is extremely susceptible to the influence of the natural vibration
+set up in the circuits of the coils <small>H H</small>. The mechanical efficiency
+of the apparatus is very high owing to the fact that friction
+is reduced to a minimum and the weights which are moved are
+small; the output of the oscillator is therefore a very large one.</p>
+
+<p>Theoretically considered, when the various advantages which
+Mr. Tesla holds out are examined, it is surprising, considering
+the simplicity of the arrangement, that nothing was done in this<span class='pagenum'><a name="Page_493" id="Page_493">[Pg 493]</a></span>
+direction before. No doubt many inventors, at one time or
+other, have entertained the idea of generating currents by attaching
+a coil or a magnetic core to the piston of a steam engine,
+or generating currents by the vibrations of a tuning fork, or
+similar devices, but the disadvantages of such arrangements from
+an engineering standpoint must be obvious. Mr. Tesla, however,
+in the introductory remarks of his lecture, pointed out how by a
+series of conclusions he was driven to take up this new line of
+work by the necessity of producing currents of constant period
+and as a result of his endeavors to maintain electrical oscillation
+in the most simple and economical manner.</p>
+
+<hr style="width: 100%;" />
+<h3>FOOTNOTES</h3>
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> A lecture delivered before the American Institute of Electrical Engineers,
+at Columbia College, N. Y., May 20, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> Lecture delivered before the Institution of Electrical Engineers, London,
+February, 1892.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a>  A lecture delivered before the Franklin Institute, Philadelphia, February,
+1893, and before the National Electric Light Association, St. Louis, March,
+1893.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> See pages 153-4 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> It is thought necessary to remark that, although the induction coil may
+give quite a good result when operated with such rapidly alternating currents,
+yet its construction, quite irrespective of the iron core, makes it very unfit for
+such high frequencies, and to obtain the best results the construction should be
+greatly modified.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>, N. Y., May 6, 1891.</p></div>
+
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i> of Dec. 23d, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>. N. Y., July 1, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> Abstract of a paper read before Physical Society of London.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_10" id="Footnote_9_10"></a><a href="#FNanchor_9_10"><span class="label">[9]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>, N. Y., August 26, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_11" id="Footnote_10_11"></a><a href="#FNanchor_10_11"><span class="label">[10]</span></a> Note by Prof. J. J. Thomson in the London <i>Electrician</i>, July 24, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_12" id="Footnote_11_12"></a><a href="#FNanchor_11_12"><span class="label">[11]</span></a> Mr. Tesla's experiments, as the careful reader of his three lectures will
+perceive, have revealed a very important fact which is taken advantage of in
+this invention. Namely, he has shown that in a condenser a considerable
+amount of energy may be wasted, and the condenser may break down merely
+because gaseous matter is present between the surfaces. A number of experiments
+are described in the lectures, which bring out this fact forcibly and serve
+as a guide in the operation of high tension apparatus. But besides bearing
+upon this point, these experiments also throw a light upon investigations of a
+purely scientific nature and explain now the lack of harmony among the observations
+of various investigators. Mr. Tesla shows that in a fluid such as oil
+the losses are very small as compared with those incurred in a gas.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_13" id="Footnote_12_13"></a><a href="#FNanchor_12_13"><span class="label">[12]</span></a> It will, of course, be inferred from the nature of these devices that the
+vibration obtained in this manner is very slow owing to the inability of the
+iron to follow rapid changes in temperature. In an interview with Mr. Tesla
+on this subject, the compiler learned of an experiment which will interest
+students. A simple horseshoe magnet is taken and a piece of sheet iron bent in
+the form of an L is brought in contact with one of the poles and placed in
+such a position that it is kept in the attraction of the opposite pole delicately
+suspended. A spirit lamp is placed under the sheet iron piece and when the
+iron is heated to a certain temperature it is easily set in vibration oscillating as
+rapidly as 400 to 500 times a minute. The experiment is very easily performed
+and is interesting principally on account of the very rapid rate of
+vibration.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_14" id="Footnote_13_14"></a><a href="#FNanchor_13_14"><span class="label">[13]</span></a> The chief point to be noted is that Mr. Tesla attacked this problem in a
+way which was, from the standpoint of theory, and that of an engineer, far
+better than that from which some earlier trials in this direction started. The
+enlargement of these ideas will be found in Mr. Tesla's work on the pyromagnetic
+generator, treated in this chapter. The chief effort of the inventor was
+to economize the heat, which was accomplished by inclosing the iron in a source
+of heat well insulated, and by cooling the iron by means of steam, utilizing the
+steam over again. The construction also permits of more rapid magnetic
+changes per unit of time, meaning larger output.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_15" id="Footnote_14_15"></a><a href="#FNanchor_14_15"><span class="label">[14]</span></a> The compiler has learned partially from statements made on several
+occasions in journals and partially by personal inquiry of Mr. Tesla, that a
+great deal of work in this interesting line is unpublished. In these inventions
+as will be seen, the brushes are automatically shifted, but in the broad method
+barely suggested here the regulation is effected without any change in the
+position of the brushes. This auxiliary brush invention, it will be remembered,
+was very much discussed a few years ago, and it may be of interest that
+this work of Mr. Tesla, then unknown in this field, is now brought to light.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_16" id="Footnote_15_16"></a><a href="#FNanchor_15_16"><span class="label">[15]</span></a> Article by Mr. Tesla, contributed to <i>The Electrical Engineer</i>, N. Y.,
+Sept. 2, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_17" id="Footnote_16_17"></a><a href="#FNanchor_16_17"><span class="label">[16]</span></a> Mr. Tesla here refers to an interesting article which appeared in July,
+1865, in the <i>Phil. Magazine</i>, by Sir W. Thomson, in which Sir William,
+speaking of his "uniform electric current accumulator," assumes that for
+self-excitation it is desirable to subdivide the disc into an infinite number of infinitely
+thin spokes, in order to prevent diffusion of the current. Mr. Tesla
+shows that diffusion is absolutely necessary for the excitation and that when
+the disc is subdivided no excitation can occur.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_18" id="Footnote_17_18"></a><a href="#FNanchor_17_18"><span class="label">[17]</span></a> See Part I, Chap. III, page 9.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_19" id="Footnote_18_19"></a><a href="#FNanchor_18_19"><span class="label">[18]</span></a> See Part II, Chap. XXVI., page 145.</p></div>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_494" id="Page_494">[Pg 494]</a></span></p>
+<h2><a name="INDEX" id="INDEX"></a>INDEX.</h2>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Alternate Current Electrostatic Apparatus</td><td align='right'><a href="#Page_392">392</a></td></tr>
+<tr><td align='left'>Alternating Current Generators for High Frequency</td><td align='right'><a href="#Page_152">152</a>, <a href="#Page_374">374</a>, <a href="#Page_224">224</a></td></tr>
+<tr><td align='left'>Alternating Motors and Transformers</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'>American Institute Electrical Engineers Lecture</td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td align='left'>Anthony, W. A., Tests of Tesla Motors</td><td align='right'><a href="#Page_8">8</a></td></tr>
+<tr><td align='left'>Apparatus for Producing High Vacua</td><td align='right'><a href="#Page_276">276</a></td></tr>
+<tr><td align='left'>Arc Lighting, Tesla Direct, System</td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td align='left'>Auxiliary Brush Regulation</td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td align='left'>Biography, Tesla</td><td align='right'><a href="#Page_4">4</a></td></tr>
+<tr><td align='left'>Brush, Anti-Sparking</td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td align='left'>Brush, Third, Regulation</td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td align='left'>Brush, Phenomena in High Vacuum</td><td align='right'><a href="#Page_226">226</a></td></tr>
+<tr><td align='left'>Carborundum Button for Tesla Lamps</td><td align='right'><a href="#Page_140">140</a>, <a href="#Page_253">253</a></td></tr>
+<tr><td align='left'>Commutator, Anti-Sparking</td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td align='left'>Combination of Synchronizing and Torque Motor</td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td align='left'>Condensers with Plates in Oil</td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td align='left'>Conversion with Disruptive Discharge</td><td align='right'><a href="#Page_193">193</a>, <a href="#Page_204">204</a>, <a href="#Page_303">303</a></td></tr>
+<tr><td align='left'>Current or Dynamic Electricity Phenomena</td><td align='right'><a href="#Page_327">327</a></td></tr>
+<tr><td align='left'>Direct Current Arc Lighting</td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td align='left'>Dischargers, Forms of</td><td align='right'><a href="#Page_305">305</a></td></tr>
+<tr><td align='left'>Disruptive Discharge Coil</td><td align='right'><a href="#Page_207">207</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'>Disruptive Discharge Phenomena</td><td align='right'><a href="#Page_212">212</a></td></tr>
+<tr><td align='left'>Dynamos, Improved Direct Current</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'>Early Phase Motors</td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td align='left'>Effects with High Frequency and High Potential Currents</td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td align='left'>Electrical Congress Lecture, Chicago</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Electric Resonance</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'>Electric Discharges in Vacuum Tubes</td><td align='right'><a href="#Page_396">396</a></td></tr>
+<tr><td align='left'>Electrolytic Registering Meter</td><td align='right'><a href="#Page_420">420</a></td></tr>
+<tr><td align='left'>Eye, Observations on the</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Flames, Electrostatic, Non-Consuming</td><td align='right'><a href="#Page_166">166</a>, <a href="#Page_272">272</a></td></tr>
+<tr><td align='left'>Forbes Unipolar Generator</td><td align='right'><a href="#Page_468">468</a>, <a href="#Page_474">474</a></td></tr>
+<tr><td align='left'>Franklin Institute Lecture</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Generators, Pyromagnetic</td><td align='right'><a href="#Page_429">429</a></td></tr>
+<tr><td align='left'>High Potential, High Frequency:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Brush Phenomena in High Vacuum</td><td align='right'><a href="#Page_226">226</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Carborundum Buttons</td><td align='right'><a href="#Page_140">140</a>, <a href="#Page_253">253</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Disruptive Discharge Phenomena</td><td align='right'><a href="#Page_212">212</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Flames, Electrostatic, Non-Consuming</td><td align='right'><a href="#Page_166">166</a>, <a href="#Page_272">272</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Impedance, Novel Phenomena</td><td align='right'><a href="#Page_194">194</a>, <a href="#Page_338">338</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Lighting Lamps Through Body</td><td align='right'><a href="#Page_359">359</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Luminous Effects with Gases</td><td align='right'><a href="#Page_368">368</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "Massage" with Currents</td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Motor with Single Wire</td><td align='right'><a href="#Page_234">234</a>, <a href="#Page_330">330</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "No Wire" Motors</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Oil Insulation of Induction Coils</td><td align='right'><a href="#Page_173">173</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; <span class='pagenum'><a name="Page_495" id="Page_495">[Pg 495]</a></span>Ozone, Production of</td><td align='right'><a href="#Page_171">171</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Phosphorescence</td><td align='right'><a href="#Page_367">367</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Physiological Effects</td><td align='right'><a href="#Page_162">162</a>, <a href="#Page_394">394</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Resonance</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Spinning Filament</td><td align='right'><a href="#Page_168">168</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Streaming Discharges of High Tension Coil</td><td align='right'><a href="#Page_155">155</a>, <a href="#Page_163">163</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Telegraphy without Wires</td><td align='right'><a href="#Page_346">346</a></td></tr>
+<tr><td align='left'>Impedance, Novel Phenomena</td><td align='right'><a href="#Page_194">194</a>, <a href="#Page_338">338</a></td></tr>
+<tr><td align='left'>Improvements in Unipolar Generators</td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td align='left'>Improved Direct Current Dynamos and Motors</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'>Induction Motors</td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td align='left'>Institution Electrical Engineers Lecture</td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td align='left'>Lamps and Motor operated on a Single Wire</td><td align='right'><a href="#Page_330">330</a></td></tr>
+<tr><td align='left'>Lamps with Single Straight Fiber</td><td align='right'><a href="#Page_183">183</a></td></tr>
+<tr><td align='left'>Lamps containing only a Gas</td><td align='right'><a href="#Page_188">188</a></td></tr>
+<tr><td align='left'>Lamps with Refractory Button</td><td align='right'><a href="#Page_177">177</a>, <a href="#Page_239">239</a>, <a href="#Page_360">360</a></td></tr>
+<tr><td align='left'>Lamps for Simple Phosphorescence</td><td align='right'><a href="#Page_187">187</a>, <a href="#Page_282">282</a>, <a href="#Page_364">364</a></td></tr>
+<tr><td align='left'>Lecture, Tesla before:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; American Institute Electrical Engineers</td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Royal Institution</td><td align='right'><a href="#Page_124">124</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Institution Electrical Engineers</td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Franklin Institute and National Electric Light Association</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Electrical Congress, Chicago</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Lighting Lamps Through the Body</td><td align='right'><a href="#Page_359">359</a></td></tr>
+<tr><td align='left'>Light Phenomena with High Frequencies</td><td align='right'><a href="#Page_349">349</a></td></tr>
+<tr><td align='left'>Luminous Effects with Gases at Low-Pressure</td><td align='right'><a href="#Page_368">368</a></td></tr>
+<tr><td align='left'>"Magnetic Lag" Motor</td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td align='left'>"Massage" with Currents of High Frequency</td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td align='left'>Mechanical and Electrical Oscillators</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Method of obtaining Direct from Alternating currents</td><td align='right'><a href="#Page_409">409</a></td></tr>
+<tr><td align='left'>Method of obtaining Difference of Phase by Magnetic Shielding</td><td align='right'><a href="#Page_71">71</a></td></tr>
+<tr><td align='left'>Motors:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Circuits of Different Resistance</td><td align='right'><a href="#Page_79">79</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Closed Conductors</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Combination of Synchronizing and Torque</td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Condenser in Armature Circuit</td><td align='right'><a href="#Page_101">101</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Condenser in one of the Field Circuits</td><td align='right'><a href="#Page_106">106</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Coinciding Maxima of Magnetic Effect in Armature and Field</td><td align='right'><a href="#Page_83">83</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With "Current Lag" Artificially Secured</td><td align='right'><a href="#Page_58">58</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Early Phase</td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Equal Magnetic Energies in Field and Armature</td><td align='right'><a href="#Page_81">81</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Or Generator, obtaining Desired Speed of</td><td align='right'><a href="#Page_36">36</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Improved Direct Current</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Induction</td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "Magnetic Lag"</td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "No Wire"</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Phase Difference in Magnetization of Inner and Outer Parts of Core</td><td align='right'><a href="#Page_88">88</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Regulator for Rotary Current</td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Single Circuit, Self-starting Synchronizing</td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Single Phase</td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Single Wire to Generator</td><td align='right'><a href="#Page_234">234</a>, <a href="#Page_330">330</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Synchronizing</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Thermo-Magnetic</td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Utilizing Continuous Current Generators</td><td align='right'><a href="#Page_31">31</a></td></tr>
+<tr><td align='left'>National Electric Light Association Lecture</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>"No Wire" Motor</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'>Observations on the Eye</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Oil, Condensers with Plates in</td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td align='left'>Oil Insulation of Induction Coils</td><td align='right'><a href="#Page_173">173</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'>Oscillators, Mechanical and Electrical</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_496" id="Page_496">[Pg 496]</a></span>Ozone, Production of</td><td align='right'><a href="#Page_171">171</a></td></tr>
+<tr><td align='left'>Phenomena Produced by Electrostatic Force</td><td align='right'><a href="#Page_318">318</a></td></tr>
+<tr><td align='left'>Phosphorescence and Sulphide of Zinc</td><td align='right'><a href="#Page_367">367</a></td></tr>
+<tr><td align='left'>Physiological Effects of High Frequency</td><td align='right'><a href="#Page_162">162</a>, <a href="#Page_394">394</a></td></tr>
+<tr><td align='left'>Polyphase Systems</td><td align='right'><a href="#Page_26">26</a></td></tr>
+<tr><td align='left'>Polyphase Transformer</td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td align='left'>Pyromagnetic Generators</td><td align='right'><a href="#Page_429">429</a></td></tr>
+<tr><td align='left'>Regulator for Rotary Current Motors</td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td align='left'>Resonance, Electric, Phenomena of</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'>"Resultant Attraction"</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'>Rotating Field Transformers</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Rotating Magnetic Field</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Royal Institution Lecture</td><td align='right'><a href="#Page_124">124</a></td></tr>
+<tr><td align='left'>Scope of Lectures</td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td align='left'>Single Phase Motor</td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td align='left'>Single Circuit, Self-Starting Synchronizing Motors</td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td align='left'>Spinning Filament Effects</td><td align='right'><a href="#Page_168">168</a></td></tr>
+<tr><td align='left'>Streaming Discharges of High Tension Coil</td><td align='right'><a href="#Page_155">155</a>, <a href="#Page_163">163</a></td></tr>
+<tr><td align='left'>Synchronizing Motors</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Telegraphy without Wires</td><td align='right'><a href="#Page_346">346</a></td></tr>
+<tr><td align='left'>Transformer with Shield between Primary and Secondary</td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td align='left'>Thermo-Magnetic Motors</td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td align='left'>Thomson, J. J., on Vacuum Tubes</td><td align='right'><a href="#Page_397">397</a>, <a href="#Page_402">402</a>, <a href="#Page_406">406</a></td></tr>
+<tr><td align='left'>Thomson, Sir W., Current Accumulator</td><td align='right'><a href="#Page_471">471</a></td></tr>
+<tr><td align='left'>Transformers:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Alternating</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Magnetic Shield</td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Polyphase</td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Rotating Field</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Tubes:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Coated with Yttria, etc.</td><td align='right'><a href="#Page_187">187</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Coated with Sulphide of Zinc, etc.</td><td align='right'><a href="#Page_290">290</a>, <a href="#Page_367">367</a></td></tr>
+<tr><td align='left'>Unipolar Generators</td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td align='left'>Unipolar Generator, Forbes</td><td align='right'><a href="#Page_468">468</a>, <a href="#Page_474">474</a></td></tr>
+<tr><td align='left'>Yttria, Coated Tubes</td><td align='right'><a href="#Page_187">187</a></td></tr>
+<tr><td align='left'>Zinc, Tubes Coated with Sulphide of</td><td align='right'><a href="#Page_367">367</a></td></tr>
+</table></div>
+
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The inventions, researches and
+writings of Nikola Tesla, by Thomas Commerford Martin
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+</body>
+</html>
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+The Project Gutenberg EBook of The inventions, researches and writings of
+Nikola Tesla, by Thomas Commerford Martin
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: The inventions, researches and writings of Nikola Tesla
+       With special reference to his work in polyphase currents
+       and high potential lighting
+
+Author: Thomas Commerford Martin
+
+Release Date: March 26, 2012 [EBook #39272]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES ***
+
+
+
+
+Produced by Anna Hall, Albert Laszlo and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+
+
+THE INVENTIONS
+
+RESEARCHES AND WRITINGS
+
+OF
+
+NIKOLA TESLA
+
+
+
+TO HIS COUNTRYMEN
+
+  IN EASTERN EUROPE THIS RECORD OF
+  THE WORK ALREADY ACCOMPLISHED BY
+
+  NIKOLA TESLA
+
+  IS RESPECTFULLY DEDICATED
+
+
+
+[Illustration: Nikola Tesla]
+
+
+
+  THE INVENTIONS
+  RESEARCHES AND WRITINGS
+
+  OF
+
+  NIKOLA TESLA
+
+
+  WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE
+  CURRENTS AND HIGH POTENTIAL LIGHTING
+
+
+  BY
+
+  THOMAS COMMERFORD MARTIN
+
+  Editor THE ELECTRICAL ENGINEER; Past-President American Institute
+  Electrical Engineers
+
+
+  1894
+  THE ELECTRICAL ENGINEER
+  NEW YORK
+
+  D. VAN NOSTRAND COMPANY,
+  NEW YORK.
+
+
+
+  Entered according to Act of Congress in the year 1893 by
+  T. C. MARTIN
+  in the office of the Librarian of Congress at Washington
+
+
+  Press of McIlroy & Emmet, 36 Cortlandt St., N. Y.
+
+
+
+
+PREFACE.
+
+
+The electrical problems of the present day lie largely in the economical
+transmission of power and in the radical improvement of the means and
+methods of illumination. To many workers and thinkers in the domain of
+electrical invention, the apparatus and devices that are familiar,
+appear cumbrous and wasteful, and subject to severe limitations. They
+believe that the principles of current generation must be changed, the
+area of current supply be enlarged, and the appliances used by the
+consumer be at once cheapened and simplified. The brilliant successes of
+the past justify them in every expectancy of still more generous
+fruition.
+
+The present volume is a simple record of the pioneer work done in such
+departments up to date, by Mr. Nikola Tesla, in whom the world has
+already recognized one of the foremost of modern electrical
+investigators and inventors. No attempt whatever has been made here to
+emphasize the importance of his researches and discoveries. Great ideas
+and real inventions win their own way, determining their own place by
+intrinsic merit. But with the conviction that Mr. Tesla is blazing a
+path that electrical development must follow for many years to come, the
+compiler has endeavored to bring together all that bears the impress of
+Mr. Tesla's genius, and is worthy of preservation. Aside from its value
+as showing the scope of his inventions, this volume may be of service as
+indicating the range of his thought. There is intellectual profit in
+studying the push and play of a vigorous and original mind.
+
+Although the lively interest of the public in Mr. Tesla's work is
+perhaps of recent growth, this volume covers the results of full ten
+years. It includes his lectures, miscellaneous articles and
+discussions, and makes note of all his inventions thus far known,
+particularly those bearing on polyphase motors and the effects obtained
+with currents of high potential and high frequency. It will be seen that
+Mr. Tesla has ever pressed forward, barely pausing for an instant to
+work out in detail the utilizations that have at once been obvious to
+him of the new principles he has elucidated. Wherever possible his own
+language has been employed.
+
+It may be added that this volume is issued with Mr. Tesla's sanction and
+approval, and that permission has been obtained for the re-publication
+in it of such papers as have been read before various technical
+societies of this country and Europe. Mr. Tesla has kindly favored the
+author by looking over the proof sheets of the sections embodying his
+latest researches. The work has also enjoyed the careful revision of the
+author's friend and editorial associate, Mr. Joseph Wetzler, through
+whose hands all the proofs have passed.
+
+DECEMBER, 1893.
+
+                                                              T. C. M.
+
+
+
+
+CONTENTS.
+
+
+PART I.
+
+POLYPHASE CURRENTS.
+
+  CHAPTER I.
+  BIOGRAPHICAL AND INTRODUCTORY.                                       3
+
+  CHAPTER II.
+  A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.         7
+
+  CHAPTER III.
+  THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
+  CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.     9
+
+  CHAPTER IV.
+  MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.        26
+
+  CHAPTER V.
+  UTILIZING FAMILIAR TYPES OF GENERATORS OF THE CONTINUOUS CURRENT
+  TYPE.                                                               31
+
+  CHAPTER VI.
+  METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.            36
+
+  CHAPTER VII.
+  REGULATOR FOR ROTARY CURRENT MOTORS.                                45
+
+  CHAPTER VIII.
+  SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.                 50
+
+  CHAPTER IX.
+  CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTORS.                56
+
+  CHAPTER X.
+  MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.                      58
+
+  CHAPTER XI.
+  ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING
+  MOTOR.                                                              62
+
+  CHAPTER XII.
+  "MAGNETIC LAG" MOTOR.                                               67
+
+  CHAPTER XIII.
+  METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.      71
+
+  CHAPTER XIV.
+  TYPE OF TESLA SINGLE-PHASE MOTOR.                                   76
+
+  CHAPTER XV.
+  MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.                       79
+
+  CHAPTER XVI.
+  MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.           81
+
+  CHAPTER XVII.
+  MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND
+  FIELD.                                                              83
+
+  CHAPTER XVIII.
+  MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF
+  THE INNER AND OUTER PARTS OF AN IRON CORE.                          88
+
+  CHAPTER XIX.
+  ANOTHER TYPE OF TESLA INDUCTION MOTOR.                              92
+
+  CHAPTER XX.
+  COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.               95
+
+  CHAPTER XXI.
+  MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.                    101
+
+  CHAPTER XXII.
+  MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.                 106
+
+  CHAPTER XXIII.
+  TESLA POLYPHASE TRANSFORMER.                                       109
+
+  CHAPTER XXIV.
+  A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN
+  COILS OF PRIMARY AND SECONDARY.                                    113
+
+
+PART II.
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.
+
+  CHAPTER XXV.
+  INTRODUCTORY.--THE SCOPE OF THE TESLA LECTURES.                    119
+
+  CHAPTER XXVI.
+  THE NEW YORK LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY
+  HIGH FREQUENCY, AND THEIR APPLICATION TO METHODS OF ARTIFICIAL
+  ILLUMINATION, MAY 20, 1891.                                        145
+
+  CHAPTER XXVII.
+  THE LONDON LECTURE. EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH
+  POTENTIAL AND HIGH FREQUENCY, FEBRUARY 3, 1892.                    198
+
+  CHAPTER XXVIII.
+  THE PHILADELPHIA AND ST. LOUIS LECTURE. ON LIGHT AND OTHER HIGH
+  FREQUENCY PHENOMENA, FEBRUARY AND MARCH, 1893.                     294
+
+  CHAPTER XXIX.
+  TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY.           374
+
+  CHAPTER XXX.
+  ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.               392
+
+  CHAPTER XXXI.
+  "MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.                         394
+
+  CHAPTER XXXII.
+  ELECTRIC DISCHARGE IN VACUUM TUBES.                                396
+
+
+PART III.
+
+MISCELLANEOUS INVENTIONS AND WRITINGS.
+
+  CHAPTER XXXIII.
+  METHOD OF OBTAINING DIRECT FROM ALTERNATING CURRENTS.              409
+
+  CHAPTER XXXIV.
+  CONDENSERS WITH PLATES IN OIL.                                     418
+
+  CHAPTER XXXV.
+  ELECTROLYTIC REGISTERING METER.                                    420
+
+  CHAPTER XXXVI.
+  THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.               424
+
+  CHAPTER XXXVII.
+  ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.                         432
+
+  CHAPTER XXXVIII.
+  AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.              438
+
+  CHAPTER XXXIX.
+  IMPROVEMENT IN DYNAMO AND MOTOR CONSTRUCTION.                      448
+
+  CHAPTER XL.
+  TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.                          451
+
+  CHAPTER XLI.
+  IMPROVEMENT IN UNIPOLAR GENERATORS.                                465
+
+
+PART IV.
+
+APPENDIX: EARLY PHASE MOTORS AND THE TESLA OSCILLATORS.
+
+  CHAPTER XLII.
+  MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.                  477
+
+  CHAPTER XLIII.
+  THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.                   486
+
+
+
+
+PART I.
+
+POLYPHASE CURRENTS.
+
+
+
+
+CHAPTER I.
+
+BIOGRAPHICAL AND INTRODUCTORY.
+
+
+As an introduction to the record contained in this volume of Mr. Tesla's
+investigations and discoveries, a few words of a biographical nature
+will, it is deemed, not be out of place, nor other than welcome.
+
+Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland region of
+Austro-Hungary, of the Serbian race, which has maintained against Turkey
+and all comers so unceasing a struggle for freedom. His family is an old
+and representative one among these Switzers of Eastern Europe, and his
+father was an eloquent clergyman in the Greek Church. An uncle is to-day
+Metropolitan in Bosnia. His mother was a woman of inherited ingenuity,
+and delighted not only in skilful work of the ordinary household
+character, but in the construction of such mechanical appliances as
+looms and churns and other machinery required in a rural community.
+Nikola was educated at Gospich in the public school for four years, and
+then spent three years in the Real Schule. He was then sent to Carstatt,
+Croatia, where he continued his studies for three years in the Higher
+Real Schule. There for the first time he saw a steam locomotive. He
+graduated in 1873, and, surviving an attack of cholera, devoted himself
+to experimentation, especially in electricity and magnetism. His father
+would have had him maintain the family tradition by entering the Church,
+but native genius was too strong, and he was allowed to enter the
+Polytechnic School at Gratz, to finish his studies, and with the object
+of becoming a professor of mathematics and physics. One of the machines
+there experimented with was a Gramme dynamo, used as a motor. Despite
+his instructor's perfect demonstration of the fact that it was
+impossible to operate a dynamo without commutator or brushes, Mr. Tesla
+could not be convinced that such accessories were necessary or
+desirable. He had already seen with quick intuition that a way could be
+found to dispense with them; and from that time he may be said to have
+begun work on the ideas that fructified ultimately in his rotating field
+motors.
+
+In the second year of his Gratz course, Mr. Tesla gave up the notion of
+becoming a teacher, and took up the engineering curriculum. His studies
+ended, he returned home in time to see his father die, and then went to
+Prague and Buda-Pesth to study languages, with the object of qualifying
+himself broadly for the practice of the engineering profession. For a
+short time he served as an assistant in the Government Telegraph
+Engineering Department, and then became associated with M. Puskas, a
+personal and family friend, and other exploiters of the telephone in
+Hungary. He made a number of telephonic inventions, but found his
+opportunities of benefiting by them limited in various ways. To gain a
+wider field of action, he pushed on to Paris and there secured
+employment as an electrical engineer with one of the large companies in
+the new industry of electric lighting.
+
+It was during this period, and as early as 1882, that he began serious
+and continued efforts to embody the rotating field principle in
+operative apparatus. He was enthusiastic about it; believed it to mark a
+new departure in the electrical arts, and could think of nothing else.
+In fact, but for the solicitations of a few friends in commercial
+circles who urged him to form a company to exploit the invention, Mr.
+Tesla, then a youth of little worldly experience, would have sought an
+immediate opportunity to publish his ideas, believing them to be worthy
+of note as a novel and radical advance in electrical theory as well as
+destined to have a profound influence on all dynamo electric machinery.
+
+At last he determined that it would be best to try his fortunes in
+America. In France he had met many Americans, and in contact with them
+learned the desirability of turning every new idea in electricity to
+practical use. He learned also of the ready encouragement given in the
+United States to any inventor who could attain some new and valuable
+result. The resolution was formed with characteristic quickness, and
+abandoning all his prospects in Europe, he at once set his face
+westward.
+
+Arrived in the United States, Mr. Tesla took off his coat the day he
+arrived, in the Edison Works. That place had been a goal of his
+ambition, and one can readily imagine the benefit and stimulus derived
+from association with Mr. Edison, for whom Mr. Tesla has always had the
+strongest admiration. It was impossible, however, that, with his own
+ideas to carry out, and his own inventions to develop, Mr. Tesla could
+long remain in even the most delightful employ; and, his work now
+attracting attention, he left the Edison ranks to join a company
+intended to make and sell an arc lighting system based on some of his
+inventions in that branch of the art. With unceasing diligence he
+brought the system to perfection, and saw it placed on the market. But
+the thing which most occupied his time and thoughts, however, all
+through this period, was his old discovery of the rotating field
+principle for alternating current work, and the application of it in
+motors that have now become known the world over.
+
+Strong as his convictions on the subject then were, it is a fact that
+he stood very much alone, for the alternating current had no well
+recognized place. Few electrical engineers had ever used it, and the
+majority were entirely unfamiliar with its value, or even its essential
+features. Even Mr. Tesla himself did not, until after protracted effort
+and experimentation, learn how to construct alternating current
+apparatus of fair efficiency. But that he had accomplished his purpose
+was shown by the tests of Prof. Anthony, made in the of winter 1887-8,
+when Tesla motors in the hands of that distinguished expert gave an
+efficiency equal to that of direct current motors. Nothing now stood in
+the way of the commercial development and introduction of such motors,
+except that they had to be constructed with a view to operating on the
+circuits then existing, which in this country were all of high
+frequency.
+
+The first full publication of his work in this direction--outside his
+patents--was a paper read before the American Institute of Electrical
+Engineers in New York, in May, 1888 (read at the suggestion of Prof.
+Anthony and the present writer), when he exhibited motors that had been
+in operation long previous, and with which his belief that brushes and
+commutators could be dispensed with, was triumphantly proved to be
+correct. The section of this volume devoted to Mr. Tesla's inventions in
+the utilization of polyphase currents will show how thoroughly from the
+outset he had mastered the fundamental idea and applied it in the
+greatest variety of ways.
+
+Having noted for years the many advantages obtainable with alternating
+currents, Mr. Tesla was naturally led on to experiment with them at
+higher potentials and higher frequencies than were common or approved
+of. Ever pressing forward to determine in even the slightest degree the
+outlines of the unknown, he was rewarded very quickly in this field
+with results of the most surprising nature. A slight acquaintance with
+some of these experiments led the compiler of this volume to urge Mr.
+Tesla to repeat them before the American Institute of Electrical
+Engineers. This was done in May, 1891, in a lecture that marked, beyond
+question, a distinct departure in electrical theory and practice, and
+all the results of which have not yet made themselves fully apparent.
+The New York lecture, and its successors, two in number, are also
+included in this volume, with a few supplementary notes.
+
+Mr. Tesla's work ranges far beyond the vast departments of polyphase
+currents and high potential lighting. The "Miscellaneous" section of
+this volume includes a great many other inventions in arc lighting,
+transformers, pyro-magnetic generators, thermo-magnetic motors,
+third-brush regulation, improvements in dynamos, new forms of
+incandescent lamps, electrical meters, condensers, unipolar dynamos, the
+conversion of alternating into direct currents, etc. It is needless to
+say that at this moment Mr. Tesla is engaged on a number of interesting
+ideas and inventions, to be made public in due course. The present
+volume deals simply with his work accomplished to date.
+
+
+
+
+CHAPTER II.
+
+A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND TRANSFORMERS.
+
+
+The present section of this volume deals with polyphase currents, and
+the inventions by Mr. Tesla, made known thus far, in which he has
+embodied one feature or another of the broad principle of rotating field
+poles or _resultant attraction_ exerted on the armature. It is needless
+to remind electricians of the great interest aroused by the first
+enunciation of the rotating field principle, or to dwell upon the
+importance of the advance from a single alternating current, to methods
+and apparatus which deal with more than one. Simply prefacing the
+consideration here attempted of the subject, with the remark that in
+nowise is the object of this volume of a polemic or controversial
+nature, it may be pointed out that Mr. Tesla's work has not at all been
+fully understood or realized up to date. To many readers, it is
+believed, the analysis of what he has done in this department will be a
+revelation, while it will at the same time illustrate the beautiful
+flexibility and range of the principles involved. It will be seen that,
+as just suggested, Mr. Tesla did not stop short at a mere rotating
+field, but dealt broadly with the shifting of the resultant attraction
+of the magnets. It will be seen that he went on to evolve the
+"multiphase" system with many ramifications and turns; that he showed
+the broad idea of motors employing currents of differing phase in the
+armature with direct currents in the field; that he first described and
+worked out the idea of an armature with a body of iron and coils closed
+upon themselves; that he worked out both synchronizing and torque
+motors; that he explained and illustrated how machines of ordinary
+construction might be adapted to his system; that he employed condensers
+in field and armature circuits, and went to the bottom of the
+fundamental principles, testing, approving or rejecting, it would
+appear, every detail that inventive ingenuity could hit upon.
+
+Now that opinion is turning so emphatically in favor of lower
+frequencies, it deserves special note that Mr. Tesla early recognized
+the importance of the low frequency feature in motor work. In fact his
+first motors exhibited publicly--and which, as Prof. Anthony showed in
+his tests in the winter of 1887-8, were the equal of direct current
+motors in efficiency, output and starting torque--were of the low
+frequency type. The necessity arising, however, to utilize these motors
+in connection with the existing high frequency circuits, our survey
+reveals in an interesting manner Mr. Tesla's fertility of resource in
+this direction. But that, after exhausting all the possibilities of this
+field, Mr. Tesla returns to low frequencies, and insists on the
+superiority of his polyphase system in alternating current distribution,
+need not at all surprise us, in view of the strength of his convictions,
+so often expressed, on this subject. This is, indeed, significant, and
+may be regarded as indicative of the probable development next to be
+witnessed.
+
+Incidental reference has been made to the efficiency of rotating field
+motors, a matter of much importance, though it is not the intention to
+dwell upon it here. Prof. Anthony in his remarks before the American
+Institute of Electrical Engineers, in May, 1888, on the two small Tesla
+motors then shown, which he had tested, stated that one gave an
+efficiency of about 50 per cent. and the other a little over sixty per
+cent. In 1889, some tests were reported from Pittsburgh, made by Mr.
+Tesla and Mr. Albert Schmid, on motors up to 10 H. P. and weighing about
+850 pounds. These machines showed an efficiency of nearly 90 per cent.
+With some larger motors it was then found practicable to obtain an
+efficiency, with the three wire system, up to as high as 94 and 95 per
+cent. These interesting figures, which, of course, might be supplemented
+by others more elaborate and of later date, are cited to show that the
+efficiency of the system has not had to wait until the present late day
+for any demonstration of its commercial usefulness. An invention is none
+the less beautiful because it may lack utility, but it must be a
+pleasure to any inventor to know that the ideas he is advancing are
+fraught with substantial benefits to the public.
+
+
+
+
+CHAPTER III.
+
+THE TESLA ROTATING MAGNETIC FIELD.--MOTORS WITH CLOSED
+CONDUCTORS.--SYNCHRONIZING MOTORS.--ROTATING FIELD TRANSFORMERS.
+
+
+The best description that can be given of what he attempted, and
+succeeded in doing, with the rotating magnetic field, is to be found in
+Mr. Tesla's brief paper explanatory of his rotary current, polyphase
+system, read before the American Institute of Electrical Engineers, in
+New York, in May, 1888, under the title "A New System of Alternate
+Current Motors and Transformers." As a matter of fact, which a perusal
+of the paper will establish, Mr. Tesla made no attempt in that paper to
+describe all his work. It dealt in reality with the few topics
+enumerated in the caption of this chapter. Mr. Tesla's reticence was no
+doubt due largely to the fact that his action was governed by the wishes
+of others with whom he was associated, but it may be worth mention that
+the compiler of this volume--who had seen the motors running, and who
+was then chairman of the Institute Committee on Papers and Meetings--had
+great difficulty in inducing Mr. Tesla to give the Institute any paper
+at all. Mr. Tesla was overworked and ill, and manifested the greatest
+reluctance to an exhibition of his motors, but his objections were at
+last overcome. The paper was written the night previous to the meeting,
+in pencil, very hastily, and under the pressure just mentioned.
+
+In this paper casual reference was made to two special forms of motors
+not within the group to be considered. These two forms were: 1. A motor
+with one of its circuits in series with a transformer, and the other in
+the secondary of the transformer. 2. A motor having its armature circuit
+connected to the generator, and the field coils closed upon themselves.
+The paper in its essence is as follows, dealing with a few leading
+features of the Tesla system, namely, the rotating magnetic field,
+motors with closed conductors, synchronizing motors, and rotating field
+transformers:--
+
+The subject which I now have the pleasure of bringing to your notice is
+a novel system of electric distribution and transmission of power by
+means of alternate currents, affording peculiar advantages, particularly
+in the way of motors, which I am confident will at once establish the
+superior adaptability of these currents to the transmission of power and
+will show that many results heretofore unattainable can be reached by
+their use; results which are very much desired in the practical
+operation of such systems, and which cannot be accomplished by means of
+continuous currents.
+
+Before going into a detailed description of this system, I think it
+necessary to make a few remarks with reference to certain conditions
+existing in continuous current generators and motors, which, although
+generally known, are frequently disregarded.
+
+In our dynamo machines, it is well known, we generate alternate currents
+which we direct by means of a commutator, a complicated device and, it
+may be justly said, the source of most of the troubles experienced in
+the operation of the machines. Now, the currents so directed cannot be
+utilized in the motor, but they must--again by means of a similar
+unreliable device--be reconverted into their original state of alternate
+currents. The function of the commutator is entirely external, and in no
+way does it affect the internal working of the machines. In reality,
+therefore, all machines are alternate current machines, the currents
+appearing as continuous only in the external circuit during their
+transit from generator to motor. In view simply of this fact, alternate
+currents would commend themselves as a more direct application of
+electrical energy, and the employment of continuous currents would only
+be justified if we had dynamos which would primarily generate, and
+motors which would be directly actuated by, such currents.
+
+But the operation of the commutator on a motor is twofold; first, it
+reverses the currents through the motor, and secondly, it effects
+automatically, a progressive shifting of the poles of one of its
+magnetic constituents. Assuming, therefore, that both of the useless
+operations in the systems, that is to say, the directing of the
+alternate currents on the generator and reversing the direct currents on
+the motor, be eliminated, it would still be necessary, in order to cause
+a rotation of the motor, to produce a progressive shifting of the poles
+of one of its elements, and the question presented itself--How to
+perform this operation by the direct action of alternate currents? I
+will now proceed to show how this result was accomplished.
+
+[Illustration: FIG. 1.]
+
+[Illustration: FIG. 1a.]
+
+[Illustration: FIG. 2.]
+
+[Illustration: FIG. 2a.]
+
+In the first experiment a drum-armature was provided with two coils at
+right angles to each other, and the ends of these coils were connected
+to two pairs of insulated contact-rings as usual. A ring was then made
+of thin insulated plates of sheet-iron and wound with four coils, each
+two opposite coils being connected together so as to produce free poles
+on diametrically opposite sides of the ring. The remaining free ends of
+the coils were then connected to the contact-rings of the generator
+armature so as to form two independent circuits, as indicated in Fig. 9.
+It may now be seen what results were secured in this combination, and
+with this view I would refer to the diagrams, Figs. 1 to 8_a_. The field
+of the generator being independently excited, the rotation of the
+armature sets up currents in the coils C C_{1}, varying in strength and
+direction in the well-known manner. In the position shown in Fig. 1, the
+current in coil C is nil, while coil C_{1} is traversed by its maximum
+current, and the connections may be such that the ring is magnetized by
+the coils c_{1} c_{1}, as indicated by the letters N S in Fig. 1_a_,
+the magnetizing effect of the coils c c being nil, since these coils
+are included in the circuit of coil C.
+
+[Illustration: FIG. 3.]
+
+[Illustration: FIG. 3a.]
+
+In Fig. 2, the armature coils are shown in a more advanced position,
+one-eighth of one revolution being completed. Fig. 2_a_ illustrates the
+corresponding magnetic condition of the ring. At this moment the coil
+C_{1} generates a current of the same direction as previously, but
+weaker, producing the poles n_{1} s_{1} upon the ring; the coil C also
+generates a current of the same direction, and the connections may be
+such that the coils c c produce the poles n s, as shown in Fig. 2_a_.
+The resulting polarity is indicated by the letters N S, and it will be
+observed that the poles of the ring have been shifted one-eighth of the
+periphery of the same.
+
+[Illustration: FIG. 4.]
+
+[Illustration: FIG. 4a.]
+
+In Fig. 3 the armature has completed one quarter of one revolution. In
+this phase the current in coil C is a maximum, and of such direction as
+to produce the poles N S in Fig. 3_a_, whereas the current in coil C_{1}
+is nil, this coil being at its neutral position. The poles N S in Fig.
+3_a_ are thus shifted one quarter of the circumference of the ring.
+
+Fig. 4 shows the coils C C in a still more advanced position, the
+armature having completed three-eighths of one revolution. At that
+moment the coil C still generates a current of the same direction as
+before, but of less strength, producing the comparatively weaker poles
+n s in Fig. 4_a_. The current in the coil C_{1} is of the same strength,
+but opposite direction. Its effect is, therefore, to produce upon the
+ring the poles n_{1} s_{1}, as indicated, and a polarity, N S, results,
+the poles now being shifted three-eighths of the periphery of the ring.
+
+[Illustration: FIG. 5.]
+
+[Illustration: FIG. 5a.]
+
+In Fig. 5 one half of one revolution of the armature is completed, and
+the resulting magnetic condition of the ring is indicated in Fig. 5_a_.
+Now the current in coil C is nil, while the coil C_{1} yields its
+maximum current, which is of the same direction as previously; the
+magnetizing effect is, therefore, due to the coils, c_{1} c_{1} alone,
+and, referring to Fig. 5_a_, it will be observed that the poles N S are
+shifted one half of the circumference of the ring. During the next half
+revolution the operations are repeated, as represented in the Figs. 6 to
+8_a_.
+
+[Illustration: FIG. 6.]
+
+[Illustration: FIG. 6a.]
+
+A reference to the diagrams will make it clear that during one
+revolution of the armature the poles of the ring are shifted once around
+its periphery, and, each revolution producing like effects, a rapid
+whirling of the poles in harmony with the rotation of the armature is
+the result. If the connections of either one of the circuits in the ring
+are reversed, the shifting of the poles is made to progress in the
+opposite direction, but the operation is identically the same. Instead
+of using four wires, with like result, three wires may be used, one
+forming a common return for both circuits.
+
+[Illustration: FIG. 7.]
+
+[Illustration: FIG. 7_a_.]
+
+This rotation or whirling of the poles manifests itself in a series of
+curious phenomena. If a delicately pivoted disc of steel or other
+magnetic metal is approached to the ring it is set in rapid rotation,
+the direction of rotation varying with the position of the disc. For
+instance, noting the direction outside of the ring it will be found that
+inside the ring it turns in an opposite direction, while it is
+unaffected if placed in a position symmetrical to the ring. This is
+easily explained. Each time that a pole approaches, it induces an
+opposite pole in the nearest point on the disc, and an attraction is
+produced upon that point; owing to this, as the pole is shifted further
+away from the disc a tangential pull is exerted upon the same, and the
+action being constantly repeated, a more or less rapid rotation of the
+disc is the result. As the pull is exerted mainly upon that part which
+is nearest to the ring, the rotation outside and inside, or right and
+left, respectively, is in opposite directions, Fig. 9. When placed
+symmetrically to the ring, the pull on the opposite sides of the disc
+being equal, no rotation results. The action is based on the magnetic
+inertia of iron; for this reason a disc of hard steel is much more
+affected than a disc of soft iron, the latter being capable of very
+rapid variations of magnetism. Such a disc has proved to be a very
+useful instrument in all these investigations, as it has enabled me to
+detect any irregularity in the action. A curious effect is also produced
+upon iron filings. By placing some upon a paper and holding them
+externally quite close to the ring, they are set in a vibrating motion,
+remaining in the same place, although the paper may be moved back and
+forth; but in lifting the paper to a certain height which seems to be
+dependent on the intensity of the poles and the speed of rotation, they
+are thrown away in a direction always opposite to the supposed movement
+of the poles. If a paper with filings is put flat upon the ring and the
+current turned on suddenly, the existence of a magnetic whirl may easily
+be observed.
+
+To demonstrate the complete analogy between the ring and a revolving
+magnet, a strongly energized electro-magnet was rotated by mechanical
+power, and phenomena identical in every particular to those mentioned
+above were observed.
+
+Obviously, the rotation of the poles produces corresponding inductive
+effects and may be utilized to generate currents in a closed conductor
+placed within the influence of the poles. For this purpose it is
+convenient to wind a ring with two sets of superimposed coils forming
+respectively the primary and secondary circuits, as shown in Fig. 10. In
+order to secure the most economical results the magnetic circuit should
+be completely closed, and with this object in view the construction may
+be modified at will.
+
+[Illustration: FIG. 8.]
+
+[Illustration: FIG. 8_a_.]
+
+The inductive effect exerted upon the secondary coils will be mainly due
+to the shifting or movement of the magnetic action; but there may also
+be currents set up in the circuits in consequence of the variations in
+the intensity of the poles. However, by properly designing the generator
+and determining the magnetizing effect of the primary coils, the latter
+element may be made to disappear. The intensity of the poles being
+maintained constant, the action of the apparatus will be perfect, and
+the same result will be secured as though the shifting were effected by
+means of a commutator with an infinite number of bars. In such case the
+theoretical relation between the energizing effect of each set of
+primary coils and their resultant magnetizing effect may be expressed by
+the equation of a circle having its centre coinciding with that of an
+orthogonal system of axes, and in which the radius represents the
+resultant and the co-ordinates both of its components. These are then
+respectively the sine and cosine of the angle _a_ between the radius and
+one of the axes (_OX_). Referring to Fig. 11, we have r^2 = x^2 + y^2;
+where x = r cos _a_, and y = r sin _a_.
+
+Assuming the magnetizing effect of each set of coils in the transformer
+to be proportional to the current--which may be admitted for weak
+degrees of magnetization--then x = Kc and y = Kc^1, where K is a
+constant and c and c^1 the current in both sets of coils respectively.
+Supposing, further, the field of the generator to be uniform, we have
+for constant speed
+
+  c^1 = K^1 sin _a_ and
+  c = K^1 sin (90 deg. + _a_) = K^1 cos _a_,
+
+where K^1 is a constant. See Fig. 12.
+
+Therefore,
+
+  x = Kc = K K^1 cos _a_;
+  y = Kc^1 = K K^1 sin _a_; and
+  K K^1 = r.
+
+[Illustration: FIG. 9.]
+
+That is, for a uniform field the disposition of the two coils at right
+angles will secure the theoretical result, and the intensity of the
+shifting poles will be constant. But from r^2 = x^2 + y^2 it follows
+that for y = 0, r = x; it follows that the joint magnetizing effect
+of both sets of coils should be equal to the effect of one set when at
+its maximum action. In transformers and in a certain class of motors the
+fluctuation of the poles is not of great importance, but in another
+class of these motors it is desirable to obtain the theoretical result.
+
+In applying this principle to the construction of motors, two typical
+forms of motor have been developed. First, a form having a comparatively
+small rotary effort at the start but maintaining a perfectly uniform
+speed at all loads, which motor has been termed synchronous. Second, a
+form possessing a great rotary effort at the start, the speed being
+dependent on the load.
+
+These motors may be operated in three different ways: 1. By the
+alternate currents of the source only. 2. By a combined action of these
+and of induced currents. 3. By the joint action of alternate and
+continuous currents.
+
+[Illustration: FIG. 10.]
+
+The simplest form of a synchronous motor is obtained by winding a
+laminated ring provided with pole projections with four coils, and
+connecting the same in the manner before indicated. An iron disc having
+a segment cut away on each side may be used as an armature. Such a motor
+is shown in Fig. 9. The disc being arranged to rotate freely within the
+ring in close proximity to the projections, it is evident that as the
+poles are shifted it will, owing to its tendency to place itself in such
+a position as to embrace the greatest number of the lines of force,
+closely follow the movement of the poles, and its motion will be
+synchronous with that of the armature of the generator; that is, in the
+peculiar disposition shown in Fig. 9, in which the armature produces by
+one revolution two current impulses in each of the circuits. It is
+evident that if, by one revolution of the armature, a greater number of
+impulses is produced, the speed of the motor will be correspondingly
+increased. Considering that the attraction exerted upon the disc is
+greatest when the same is in close proximity to the poles, it follows
+that such a motor will maintain exactly the same speed at all loads
+within the limits of its capacity.
+
+To facilitate the starting, the disc may be provided with a coil closed
+upon itself. The advantage secured by such a coil is evident. On the
+start the currents set up in the coil strongly energize the disc and
+increase the attraction exerted upon the same by the ring, and currents
+being generated in the coil as long as the speed of the armature is
+inferior to that of the poles, considerable work may be performed by
+such a motor even if the speed be below normal. The intensity of the
+poles being constant, no currents will be generated in the coil when the
+motor is turning at its normal speed.
+
+Instead of closing the coil upon itself, its ends may be connected to
+two insulated sliding rings, and a continuous current supplied to these
+from a suitable generator. The proper way to start such a motor is to
+close the coil upon itself until the normal speed is reached, or nearly
+so, and then turn on the continuous current. If the disc be very
+strongly energized by a continuous current the motor may not be able to
+start, but if it be weakly energized, or generally so that the
+magnetizing effect of the ring is preponderating, it will start and
+reach the normal speed. Such a motor will maintain absolutely the same
+speed at all loads. It has also been found that if the motive power of
+the generator is not excessive, by checking the motor the speed of the
+generator is diminished in synchronism with that of the motor. It is
+characteristic of this form of motor that it cannot be reversed by
+reversing the continuous current through the coil.
+
+[Illustration: FIG. 11.]
+
+[Illustration: FIG. 12.]
+
+The synchronism of these motors may be demonstrated experimentally in a
+variety of ways. For this purpose it is best to employ a motor
+consisting of a stationary field magnet and an armature arranged to
+rotate within the same, as indicated in Fig. 13. In this case the
+shifting of the poles of the armature produces a rotation of the latter
+in the opposite direction. It results therefrom that when the normal
+speed is reached, the poles of the armature assume fixed positions
+relatively to the field magnet, and the same is magnetized by
+induction, exhibiting a distinct pole on each of the pole-pieces. If a
+piece of soft iron is approached to the field magnet, it will at the
+start be attracted with a rapid vibrating motion produced by the
+reversals of polarity of the magnet, but as the speed of the armature
+increases, the vibrations become less and less frequent and finally
+entirely cease. Then the iron is weakly but permanently attracted,
+showing that synchronism is reached and the field magnet energized by
+induction.
+
+The disc may also be used for the experiment. If held quite close to the
+armature it will turn as long as the speed of rotation of the poles
+exceeds that of the armature; but when the normal speed is reached, or
+very nearly so, it ceases to rotate and is permanently attracted.
+
+[Illustration: FIG. 13.]
+
+A crude but illustrative experiment is made with an incandescent lamp.
+Placing the lamp in circuit with the continuous current generator and in
+series with the magnet coil, rapid fluctuations are observed in the
+light in consequence of the induced currents set up in the coil at the
+start; the speed increasing, the fluctuations occur at longer intervals,
+until they entirely disappear, showing that the motor has attained its
+normal speed. A telephone receiver affords a most sensitive instrument;
+when connected to any circuit in the motor the synchronism may be easily
+detected on the disappearance of the induced currents.
+
+In motors of the synchronous type it is desirable to maintain the
+quantity of the shifting magnetism constant, especially if the magnets
+are not properly subdivided.
+
+To obtain a rotary effort in these motors was the subject of long
+thought. In order to secure this result it was necessary to make such a
+disposition that while the poles of one element of the motor are shifted
+by the alternate currents of the source, the poles produced upon the
+other elements should always be maintained in the proper relation to the
+former, irrespective of the speed of the motor. Such a condition exists
+in a continuous current motor; but in a synchronous motor, such as
+described, this condition is fulfilled only when the speed is normal.
+
+[Illustration: FIG. 14.]
+
+The object has been attained by placing within the ring a properly
+subdivided cylindrical iron core wound with several independent coils
+closed upon themselves. Two coils at right angles as in Fig. 14, are
+sufficient, but a greater number may be advantageously employed. It
+results from this disposition that when the poles of the ring are
+shifted, currents are generated in the closed armature coils. These
+currents are the most intense at or near the points of the greatest
+density of the lines of force, and their effect is to produce poles upon
+the armature at right angles to those of the ring, at least
+theoretically so; and since this action is entirely independent of the
+speed--that is, as far as the location of the poles is concerned--a
+continuous pull is exerted upon the periphery of the armature. In many
+respects these motors are similar to the continuous current motors. If
+load is put on, the speed, and also the resistance of the motor, is
+diminished and more current is made to pass through the energizing
+coils, thus increasing the effort. Upon the load being taken off, the
+counter-electromotive force increases and less current passes through
+the primary or energizing coils. Without any load the speed is very
+nearly equal to that of the shifting poles of the field magnet.
+
+[Illustration: FIG. 15.]
+
+[Illustration: FIG. 16.]
+
+[Illustration: FIG. 17.]
+
+It will be found that the rotary effort in these motors fully equals
+that of the continuous current motors. The effort seems to be greatest
+when both armature and field magnet are without any projections; but as
+in such dispositions the field cannot be concentrated, probably the best
+results will be obtained by leaving pole projections on one of the
+elements only. Generally, it may be stated the projections diminish the
+torque and produce a tendency to synchronism.
+
+A characteristic feature of motors of this kind is their property of
+being very rapidly reversed. This follows from the peculiar action of
+the motor. Suppose the armature to be rotating and the direction of
+rotation of the poles to be reversed. The apparatus then represents a
+dynamo machine, the power to drive this machine being the momentum
+stored up in the armature and its speed being the sum of the speeds of
+the armature and the poles.
+
+[Illustration: FIG. 18.]
+
+[Illustration: FIG. 19.]
+
+[Illustration: FIG. 20.]
+
+[Illustration: FIG. 21.]
+
+If we now consider that the power to drive such a dynamo would be very
+nearly proportional to the third power of the speed, for that reason
+alone the armature should be quickly reversed. But simultaneously with
+the reversal another element is brought into action, namely, as the
+movement of the poles with respect to the armature is reversed, the
+motor acts like a transformer in which the resistance of the secondary
+circuit would be abnormally diminished by producing in this circuit an
+additional electromotive force. Owing to these causes the reversal is
+instantaneous.
+
+If it is desirable to secure a constant speed, and at the same time a
+certain effort at the start, this result may be easily attained in a
+variety of ways. For instance, two armatures, one for torque and the
+other for synchronism, may be fastened on the same shaft and any desired
+preponderance may be given to either one, or an armature may be wound
+for rotary effort, but a more or less pronounced tendency to synchronism
+may be given to it by properly constructing the iron core; and in many
+other ways.
+
+As a means of obtaining the required phase of the currents in both the
+circuits, the disposition of the two coils at right angles is the
+simplest, securing the most uniform action; but the phase may be
+obtained in many other ways, varying with the machine employed. Any of
+the dynamos at present in use may be easily adapted for this purpose by
+making connections to proper points of the generating coils. In closed
+circuit armatures, such as used in the continuous current systems, it is
+best to make four derivations from equi-distant points or bars of the
+commutator, and to connect the same to four insulated sliding rings on
+the shaft. In this case each of the motor circuits is connected to two
+diametrically opposite bars of the commutator. In such a disposition the
+motor may also be operated at half the potential and on the three-wire
+plan, by connecting the motor circuits in the proper order to three of
+the contact rings.
+
+In multipolar dynamo machines, such as used in the converter systems,
+the phase is conveniently obtained by winding upon the armature two
+series of coils in such a manner that while the coils of one set or
+series are at their maximum production of current, the coils of the
+other will be at their neutral position, or nearly so, whereby both sets
+of coils may be subjected simultaneously or successively to the inducing
+action of the field magnets.
+
+Generally the circuits in the motor will be similarly disposed, and
+various arrangements may be made to fulfill the requirements; but the
+simplest and most practicable is to arrange primary circuits on
+stationary parts of the motor, thereby obviating, at least in certain
+forms, the employment of sliding contacts. In such a case the magnet
+coils are connected alternately in both the circuits; that is, 1, 3,
+5 ... in one, and 2, 4, 6 ... in the other, and the coils of each set
+of series may be connected all in the same manner, or alternately in
+opposition; in the latter case a motor with half the number of poles
+will result, and its action will be correspondingly modified. The Figs.
+15, 16, and 17, show three different phases, the magnet coils in each
+circuit being connected alternately in opposition. In this case there
+will be always four poles, as in Figs. 15 and 17; four pole projections
+will be neutral; and in Fig. 16 two adjacent pole projections will have
+the same polarity. If the coils are connected in the same manner there
+will be eight alternating poles, as indicated by the letters n' s'
+in Fig. 15.
+
+The employment of multipolar motors secures in this system an advantage
+much desired and unattainable in the continuous current system, and that
+is, that a motor may be made to run exactly at a predetermined speed
+irrespective of imperfections in construction, of the load, and, within
+certain limits, of electromotive force and current strength.
+
+In a general distribution system of this kind the following plan should
+be adopted. At the central station of supply a generator should be
+provided having a considerable number of poles. The motors operated from
+this generator should be of the synchronous type, but possessing
+sufficient rotary effort to insure their starting. With the observance
+of proper rules of construction it may be admitted that the speed of
+each motor will be in some inverse proportion to its size, and the
+number of poles should be chosen accordingly. Still, exceptional demands
+may modify this rule. In view of this, it will be advantageous to
+provide each motor with a greater number of pole projections or coils,
+the number being preferably a multiple of two and three. By this means,
+by simply changing the connections of the coils, the motor may be
+adapted to any probable demands.
+
+If the number of the poles in the motor is even, the action will be
+harmonious and the proper result will be obtained; if this is not the
+case, the best plan to be followed is to make a motor with a double
+number of poles and connect the same in the manner before indicated, so
+that half the number of poles result. Suppose, for instance, that the
+generator has twelve poles, and it would be desired to obtain a speed
+equal to 12/7 of the speed of the generator. This would require a motor
+with seven pole projections or magnets, and such a motor could not be
+properly connected in the circuits unless fourteen armature coils would
+be provided, which would necessitate the employment of sliding
+contacts. To avoid this, the motor should be provided with fourteen
+magnets and seven connected in each circuit, the magnets in each circuit
+alternating among themselves. The armature should have fourteen closed
+coils. The action of the motor will not be quite as perfect as in the
+case of an even number of poles, but the drawback will not be of a
+serious nature.
+
+However, the disadvantages resulting from this unsymmetrical form will
+be reduced in the same proportion as the number of the poles is
+augmented.
+
+If the generator has, say, n, and the motor n_{1} poles, the speed of
+the motor will be equal to that of the generator multiplied by n/n_{1}.
+
+The speed of the motor will generally be dependent on the number of the
+poles, but there may be exceptions to this rule. The speed may be
+modified by the phase of the currents in the circuit or by the character
+of the current impulses or by intervals between each or between groups
+of impulses. Some of the possible cases are indicated in the diagrams,
+Figs. 18, 19, 20 and 21, which are self-explanatory. Fig. 18 represents
+the condition generally existing, and which secures the best result. In
+such a case, if the typical form of motor illustrated in Fig. 9 is
+employed, one complete wave in each circuit will produce one revolution
+of the motor. In Fig. 19 the same result will be effected by one wave in
+each circuit, the impulses being successive; in Fig. 20 by four, and in
+Fig. 21 by eight waves.
+
+By such means any desired speed may be attained, that is, at least
+within the limits of practical demands. This system possesses this
+advantage, besides others, resulting from simplicity. At full loads the
+motors show an efficiency fully equal to that of the continuous current
+motors. The transformers present an additional advantage in their
+capability of operating motors. They are capable of similar
+modifications in construction, and will facilitate the introduction of
+motors and their adaptation to practical demands. Their efficiency
+should be higher than that of the present transformers, and I base my
+assertion on the following:
+
+In a transformer, as constructed at present, we produce the currents in
+the secondary circuit by varying the strength of the primary or exciting
+currents. If we admit proportionality with respect to the iron core the
+inductive effect exerted upon the secondary coil will be proportional
+to the numerical sum of the variations in the strength of the exciting
+current per unit of time; whence it follows that for a given variation
+any prolongation of the primary current will result in a proportional
+loss. In order to obtain rapid variations in the strength of the
+current, essential to efficient induction, a great number of undulations
+are employed; from this practice various disadvantages result. These
+are: Increased cost and diminished efficiency of the generator; more
+waste of energy in heating the cores, and also diminished output of the
+transformer, since the core is not properly utilized, the reversals
+being too rapid. The inductive effect is also very small in certain
+phases, as will be apparent from a graphic representation, and there may
+be periods of inaction, if there are intervals between the succeeding
+current impulses or waves. In producing a shifting of the poles in a
+transformer, and thereby inducing currents, the induction is of the
+ideal character, being always maintained at its maximum action. It is
+also reasonable to assume that by a shifting of the poles less energy
+will be wasted than by reversals.
+
+
+
+
+CHAPTER IV.
+
+MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE SYSTEMS.
+
+
+In his earlier papers and patents relative to polyphase currents, Mr.
+Tesla devoted himself chiefly to an enunciation of the broad lines and
+ideas lying at the basis of this new work; but he supplemented this
+immediately by a series of other striking inventions which may be
+regarded as modifications and expansions of certain features of the
+Tesla systems. These we shall now proceed to deal with.
+
+In the preceding chapters we have thus shown and described the Tesla
+electrical systems for the transmission of power and the conversion and
+distribution of electrical energy, in which the motors and the
+transformers contain two or more coils or sets of coils, which were
+connected up in independent circuits with corresponding coils of an
+alternating current generator, the operation of the system being brought
+about by the co-operation of the alternating currents in the independent
+circuits in progressively moving or shifting the poles or points of
+maximum magnetic effect of the motors or converters. In these systems
+two independent conductors are employed for each of the independent
+circuits connecting the generator with the devices for converting the
+transmitted currents into mechanical energy or into electric currents of
+another character. This, however, is not always necessary. The two or
+more circuits may have a single return path or wire in common, with a
+loss, if any, which is so extremely slight that it may be disregarded
+entirely. For the sake of illustration, if the generator have two
+independent coils and the motor two coils or two sets of coils in
+corresponding relations to its operative elements one terminal of each
+generator coil is connected to the corresponding terminals of the motor
+coils through two independent conductors, while the opposite terminals
+of the respective coils are both connected to one return wire. The
+following description deals with the modification. Fig. 22 is a
+diagrammatic illustration of a generator and single motor constructed
+and electrically connected in accordance with the invention. Fig. 23 is
+a diagram of the system as it is used in operating motors or converters,
+or both, in parallel, while Fig. 24 illustrates diagrammatically the
+manner of operating two or more motors or converters, or both, in
+series. Referring to Fig. 22, A A designate the poles of the field
+magnets of an alternating-current generator, the armature of which,
+being in this case cylindrical in form and mounted on a shaft, C, is
+wound longitudinally with coils B B'. The shaft C carries three
+insulated contact-rings, _a b c_, to two of which, as _b c_, one
+terminal of each coil, as _e d_, is connected. The remaining terminals,
+_f g_, are both connected to the third ring, _a_.
+
+[Illustration: FIG. 22.]
+
+[Illustration: FIG. 24.]
+
+A motor in this case is shown as composed of a ring, H, wound with four
+coils, I I J J, electrically connected, so as to co-operate in pairs,
+with a tendency to fix the poles of the ring at four points ninety
+degrees apart. Within the magnetic ring H is a disc or cylindrical core
+wound with two coils, G G', which may be connected to form two closed
+circuits. The terminals _j k_ of the two sets or pairs of coils are
+connected, respectively, to the binding-posts E' F', and the other
+terminals, _h i_, are connected to a single binding-post, D'. To operate
+the motor, three line-wires are used to connect the terminals of the
+generator with those of the motor.
+
+[Illustration: FIG. 23.]
+
+So far as the apparent action or mode of operation of this arrangement
+is concerned, the single wire D, which is, so to speak, a common
+return-wire for both circuits, may be regarded as two independent wires.
+In the illustration, with the order of connection shown, coil B' of the
+generator is producing its maximum current and coil B its minimum; hence
+the current which passes through wire e, ring b, brush b', line-wire E,
+terminal E', wire j, coils I I, wire or terminal D', line-wire D, brush
+_a'_, ring _a_, and wire _f_, fixes the polar line of the motor midway
+between the two coils I I; but as the coil B' moves from the position
+indicated it generates less current, while coil B, moving into the
+field, generates more. The current from coil B passes through the
+devices and wires designated by the letters _d_, _c_, C' F, F' _k_, J J,
+_i_, D', D, _a'_, _a_, and _g_, and the position of the poles of the
+motor will be due to the resultant effect of the currents in the two
+sets of coils--that is, it will be advanced in proportion to the advance
+or forward movement of the armature coils. The movement of the
+generator-armature through one-quarter of a revolution will obviously
+bring coil B' into its neutral position and coil B into its position of
+maximum effect, and this shifts the poles ninety degrees, as they are
+fixed solely by coils B. This action is repeated for each quarter of a
+complete revolution.
+
+When more than one motor or other device is employed, they may be run
+either in parallel or series. In Fig. 23 the former arrangement is
+shown. The electrical device is shown as a converter, L, of which the
+two sets of primary coils _p r_ are connected, respectively, to the
+mains F E, which are electrically connected with the two coils of the
+generator. The cross-circuit wires _l m_, making these connections, are
+then connected to the common return-wire D. The secondary coils _p' p''_
+are in circuits _n o_, including, for example, incandescent lamps. Only
+one converter is shown entire in this figure, the others being
+illustrated diagrammatically.
+
+When motors or converters are to be run in series, the two wires E F are
+led from the generator to the coils of the first motor or converter,
+then continued on to the next, and so on through the whole series, and
+are then joined to the single wire D, which completes both circuits
+through the generator. This is shown in Fig. 24, in which J I represent
+the two coils or sets of coils of the motors.
+
+There are, of course, other conditions under which the same idea may be
+carried out. For example, in case the motor and generator each has three
+independent circuits, one terminal of each circuit is connected to a
+line-wire, and the other three terminals to a common return-conductor.
+This arrangement will secure similar results to those attained with a
+generator and motor having but two independent circuits, as above
+described.
+
+When applied to such machines and motors as have three or more induced
+circuits with a common electrical joint, the three or more terminals of
+the generator would be simply connected to those of the motor. Mr.
+Tesla states, however, that the results obtained in this manner show a
+lower efficiency than do the forms dwelt upon more fully above.
+
+
+
+
+CHAPTER V.
+
+UTILIZING FAMILIAR TYPES OF GENERATOR OF THE CONTINUOUS CURRENT TYPE.
+
+
+The preceding descriptions have assumed the use of alternating current
+generators in which, in order to produce the progressive movement of the
+magnetic poles, or of the resultant attraction of independent field
+magnets, the current generating coils are independent or separate. The
+ordinary forms of continuous current dynamos may, however, be employed
+for the same work, in accordance with a method of adaptation devised by
+Mr. Tesla. As will be seen, the modification involves but slight changes
+in their construction, and presents other elements of economy.
+
+On the shaft of a given generator, either in place of or in addition to
+the regular commutator, are secured as many pairs of insulated
+collecting-rings as there are circuits to be operated. Now, it will be
+understood that in the operation of any dynamo electric generator the
+currents in the coils in their movement through the field of force
+undergo different phases--that is to say, at different positions of the
+coils the currents have certain directions and certain strengths--and
+that in the Tesla motors or transformers it is necessary that the
+currents in the energizing coils should undergo a certain order of
+variations in strength and direction. Hence, the further step--viz., the
+connection between the induced or generating coils of the machine and
+the contact-rings from which the currents are to be taken off--will be
+determined solely by what order of variations of strength and direction
+in the currents is desired for producing a given result in the
+electrical translating device. This may be accomplished in various ways;
+but in the drawings we give typical instances only of the best and most
+practicable ways of applying the invention to three of the leading types
+of machines in widespread use, in order to illustrate the principle.
+
+Fig. 25 is a diagram illustrative of the mode of applying the invention
+to the well-known type of "closed" or continuous circuit machines. Fig.
+26 is a similar diagram embodying an armature with separate coils
+connected diametrically, or what is generally called an "open-circuit"
+machine. Fig. 27 is a diagram showing the application of the invention
+to a machine the armature-coils of which have a common joint.
+
+[Illustration: FIG. 25.]
+
+Referring to Fig. 25, let A represent a Tesla motor or transformer
+which, for convenience, we will designate as a "converter." It consists
+of an annular core, B, wound with four independent coils, C and D, those
+diametrically opposite being connected together so as to co-operate in
+pairs in establishing free poles in the ring, the tendency of each pair
+being to fix the poles at ninety degrees from the other. There may be an
+armature, E, within the ring, which is wound with coils closed upon
+themselves. The object is to pass through coils C D currents of such
+relative strength and direction as to produce a progressive shifting or
+movement of the points of maximum magnetic effect around the ring, and
+to thereby maintain a rotary movement of the armature. There are
+therefore secured to the shaft F of the generator, four insulated
+contact-rings, _a b c d_, upon which bear the collecting-brushes
+_a' b' c' d'_, connected by wires G G H H, respectively, with the
+terminals of coils C and D.
+
+Assume, for sake of illustration, that the coils D D are to receive the
+maximum and coils C C at the same instant the minimum current, so that
+the polar line may be midway between the coils D D. The rings _a b_
+would therefore be connected to the continuous armature-coil at its
+neutral points with respect to the field, or the point corresponding
+with that of the ordinary commutator brushes, and between which exists
+the greatest difference of potential; while rings _c d_ would be
+connected to two points in the coil, between which exists no difference
+of potential. The best results will be obtained by making these
+connections at points equidistant from one another, as shown. These
+connections are easiest made by using wires L between the rings and the
+loops or wires J, connecting the coil I to the segments of the
+commutator K. When the converters are made in this manner, it is evident
+that the phases of the currents in the sections of the generator coil
+will be reproduced in the converter coils. For example, after turning
+through an arc of ninety degrees the conductors L L, which before
+conveyed the maximum current, will receive the minimum current by reason
+of the change in the position of their coils, and it is evident that for
+the same reason the current in these coils has gradually fallen from the
+maximum to the minimum in passing through the arc of ninety degrees. In
+this special plan of connections, the rotation of the magnetic poles of
+the converter will be synchronous with that of the armature coils of the
+generator, and the result will be the same, whether the energizing
+circuits are derivations from a continuous armature coil or from
+independent coils, as in Mr. Tesla's other devices.
+
+In Fig. 25, the brushes M M are shown in dotted lines in their proper
+normal position. In practice these brushes may be removed from the
+commutator and the field of the generator excited by an external source
+of current; or the brushes may be allowed to remain on the commutator
+and to take off a converted current to excite the field, or to be used
+for other purposes.
+
+In a certain well-known class of machines known as the "open circuit,"
+the armature contains a number of coils the terminals of which connect
+to commutator segments, the coils being connected across the armature in
+pairs. This type of machine is represented in Fig. 26. In this machine
+each pair of coils goes through the same phases as the coils in some of
+the generators already shown, and it is obviously only necessary to
+utilize them in pairs or sets to operate a Tesla converter by extending
+the segments of the commutators belonging to each pair of coils and
+causing a collecting brush to bear on the continuous portion of each
+segment. In this way two or more circuits may be taken off from the
+generator, each including one or more pairs or sets of coils as may be
+desired.
+
+[Illustration: FIG. 26.]
+
+[Illustration: FIG. 27.]
+
+In Fig. 26 I I represent the armature coils, T T the poles of the field
+magnet, and F the shaft carrying the commutators, which are extended to
+form continuous portions _a b c d_. The brushes bearing on the
+continuous portions for taking off the alternating currents are
+represented by _a' b' c' d'_. The collecting brushes, or those which may
+be used to take off the direct current, are designated by M M. Two pairs
+of the armature coils and their commutators are shown in the figure as
+being utilized; but all may be utilized in a similar manner.
+
+There is another well-known type of machine in which three or more
+coils, A' B' C', on the armature have a common joint, the free ends
+being connected to the segments of a commutator. This form of generator
+is illustrated in Fig. 27. In this case each terminal of the generator
+is connected directly or in derivation to a continuous ring, _a b c_,
+and collecting brushes, _a' b' c'_, bearing thereon, take off the
+alternating currents that operate the motor. It is preferable in this
+case to employ a motor or transformer with three energizing coils, A''
+B'' C'', placed symmetrically with those of the generator, and the
+circuits from the latter are connected to the terminals of such coils
+either directly--as when they are stationary--or by means of brushes
+_e'_ and contact rings _e_. In this, as in the other cases, the ordinary
+commutator may be used on the generator, and the current taken from it
+utilized for exciting the generator field-magnets or for other
+purposes.
+
+
+
+
+CHAPTER VI.
+
+METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR GENERATOR.
+
+
+With the object of obtaining the desired speed in motors operated by
+means of alternating currents of differing phase, Mr. Tesla has devised
+various plans intended to meet the practical requirements of the case,
+in adapting his system to types of multipolar alternating current
+machines yielding a large number of current reversals for each
+revolution.
+
+For example, Mr. Tesla has pointed out that to adapt a given type of
+alternating current generator, you may couple rigidly two complete
+machines, securing them together in such a way that the requisite
+difference in phase will be produced; or you may fasten two armatures to
+the same shaft within the influence of the same field and with the
+requisite angular displacement to yield the proper difference in phase
+between the two currents; or two armatures may be attached to the same
+shaft with their coils symmetrically disposed, but subject to the
+influence of two sets of field magnets duly displaced; or the two sets
+of coils may be wound on the same armature alternately or in such manner
+that they will develop currents the phases of which differ in time
+sufficiently to produce the rotation of the motor.
+
+Another method included in the scope of the same idea, whereby a single
+generator may run a number of motors either at its own rate of speed or
+all at different speeds, is to construct the motors with fewer poles
+than the generator, in which case their speed will be greater than that
+of the generator, the rate of speed being higher as the number of their
+poles is relatively less. This may be understood from an example, taking
+a generator that has two independent generating coils which revolve
+between two pole pieces oppositely magnetized; and a motor with
+energizing coils that produce at any given time two magnetic poles in
+one element that tend to set up a rotation of the motor. A generator
+thus constructed yields four reversals, or impulses, in each
+revolution, two in each of its independent circuits; and the effect upon
+the motor is to shift the magnetic poles through three hundred and sixty
+degrees. It is obvious that if the four reversals in the same order
+could be produced by each half-revolution of the generator the motor
+would make two revolutions to the generator's one. This would be readily
+accomplished by adding two intermediate poles to the generator or
+altering it in any of the other equivalent ways above indicated. The
+same rule applies to generators and motors with multiple poles. For
+instance, if a generator be constructed with two circuits, each of which
+produces twelve reversals of current to a revolution, and these currents
+be directed through the independent energizing-coils of a motor, the
+coils of which are so applied as to produce twelve magnetic poles at all
+times, the rotation of the two will be synchronous; but if the
+motor-coils produce but six poles, the movable element will be rotated
+twice while the generator rotates once; or if the motor have four poles,
+its rotation will be three times as fast as that of the generator.
+
+[Illustration: FIG. 28.]
+
+[Illustration: FIG. 29.]
+
+These features, so far as necessary to an understanding of the
+principle, are here illustrated. Fig. 28 is a diagrammatic illustration
+of a generator constructed in accordance with the invention. Fig. 29 is
+a similar view of a correspondingly constructed motor. Fig. 30 is a
+diagram of a generator of modified construction. Fig. 31 is a diagram of
+a motor of corresponding character. Fig. 32 is a diagram of a system
+containing a generator and several motors adapted to run at various
+speeds.
+
+In Fig. 28, let C represent a cylindrical armature core wound
+longitudinally with insulated coils A A, which are connected up in
+series, the terminals of the series being connected to collecting-rings
+_a a_ on the shaft G. By means of this shaft the armature is mounted to
+rotate between the poles of an annular field-magnet D, formed with polar
+projections wound with coils E, that magnetize the said projections. The
+coils E are included in the circuit of a generator F, by means of which
+the field-magnet is energized. If thus constructed, the machine is a
+well-known form of alternating-current generator. To adapt it to his
+system, however, Mr. Tesla winds on armature C a second set of coils B B
+intermediate to the first, or, in other words, in such positions that
+while the coils of one set are in the relative positions to the poles of
+the field-magnet to produce the maximum current, those of the other set
+will be in the position in which they produce the minimum current. The
+coils B are connected, also, in series and to two connecting-rings,
+secured generally to the shaft at the opposite end of the armature.
+
+[Illustration: FIG. 30.]
+
+[Illustration: FIG. 31.]
+
+The motor shown in Fig. 29 has an annular field-magnet H, with four
+pole-pieces wound with coils I. The armature is constructed similarly to
+the generator, but with two sets of two coils in closed circuits to
+correspond with the reduced number of magnetic poles in the field. From
+the foregoing it is evident that one revolution of the armature of the
+generator producing eight current impulses in each circuit will produce
+two revolutions of the motor-armature.
+
+The application of the principle of this invention is not, however,
+confined to any particular form of machine. In Figs. 30 and 31 a
+generator and motor of another well-known type are shown. In Fig. 30, J
+J are magnets disposed in a circle and wound with coils K, which are in
+circuit with a generator which supplies the current that maintains the
+field of force. In the usual construction of these machines the
+armature-conductor L is carried by a suitable frame, so as to be rotated
+in face of the magnets J J, or between these magnets and another similar
+set in front of them. The magnets are energized so as to be of
+alternately opposite polarity throughout the series, so that as the
+conductor C is rotated the current impulses combine or are added to one
+another, those produced by the conductor in any given position being all
+in the same direction. To adapt such a machine to his system, Mr. Tesla
+adds a second set of induced conductors M, in all respects similar to
+the first, but so placed in reference to it that the currents produced
+in each will differ by a quarter-phase. With such relations it is
+evident that as the current decreases in conductor L it increases in
+conductor M, and conversely, and that any of the forms of Tesla motor
+invented for use in this system may be operated by such a generator.
+
+Fig. 31 is intended to show a motor corresponding to the machine in Fig.
+30. The construction of the motor is identical with that of the
+generator, and if coupled thereto it will run synchronously therewith.
+J' J' are the field-magnets, and K' the coils thereon. L' is one of the
+armature-conductors and M' the other.
+
+Fig. 32 shows in diagram other forms of machine. The generator N in this
+case is shown as consisting of a stationary ring O, wound with
+twenty-four coils P P', alternate coils being connected in series in two
+circuits. Within this ring is a disc or drum Q, with projections Q'
+wound with energizing-coils included in circuit with a generator R. By
+driving this disc or cylinder alternating currents are produced in the
+coils P and P', which are carried off to run the several motors.
+
+The motors are composed of a ring or annular field-magnet S, wound with
+two sets of energizing-coils T T', and armatures U, having projections
+U' wound with coils V, all connected in series in a closed circuit or
+each closed independently on itself.
+
+Suppose the twelve generator-coils P are wound alternately in opposite
+directions, so that any two adjacent coils of the same set tend to
+produce a free pole in the ring O between them and the twelve coils P'
+to be similarly wound. A single revolution of the disc or cylinder Q,
+the twelve polar projections of which are of opposite polarity, will
+therefore produce twelve current impulses in each of the circuits W W'.
+Hence the motor X, which has sixteen coils or eight free poles, will
+make one and a half turns to the generator's one. The motor Y, with
+twelve coils or six poles, will rotate with twice the speed of the
+generator, and the motor Z, with eight coils or four poles, will revolve
+three times as fast as the generator. These multipolar motors have a
+peculiarity which may be often utilized to great advantage. For example,
+in the motor X, Fig. 32, the eight poles may be either alternately
+opposite or there may be at any given time alternately two like and two
+opposite poles. This is readily attained by making the proper electrical
+connections. The effect of such a change, however, would be the same as
+reducing the number of poles one-half, and thereby doubling the speed
+of any given motor.
+
+[Illustration: FIG. 32.]
+
+It is obvious that the Tesla electrical transformers which have
+independent primary currents may be used with the generators described.
+It may also be stated with respect to the devices we now describe that
+the most perfect and harmonious action of the generators and motors is
+obtained when the numbers of the poles of each are even and not odd. If
+this is not the case, there will be a certain unevenness of action which
+is the less appreciable as the number of poles is greater; although this
+may be in a measure corrected by special provisions which it is not here
+necessary to explain. It also follows, as a matter of course, that if
+the number of the poles of the motor be greater than that of the
+generator the motor will revolve at a slower speed than the generator.
+
+In this chapter, we may include a method devised by Mr. Tesla for
+avoiding the very high speeds which would be necessary with large
+generators. In lieu of revolving the generator armature at a high rate
+of speed, he secures the desired result by a rotation of the magnetic
+poles of one element of the generator, while driving the other at a
+different speed. The effect is the same as that yielded by a very high
+rate of rotation.
+
+In this instance, the generator which supplies the current for operating
+the motors or transformers consists of a subdivided ring or annular core
+wound with four diametrically-opposite coils, E E', Fig. 33. Within the
+ring is mounted a cylindrical armature-core wound longitudinally with
+two independent coils, F F', the ends of which lead, respectively, to
+two pairs of insulated contact or collecting rings, D D' G G', on the
+armature shaft. Collecting brushes _d d' g g'_ bear upon these rings,
+respectively, and convey the currents through the two independent
+line-circuits M M'. In the main line there may be included one or more
+motors or transformers, or both. If motors be used, they are of the
+usual form of Tesla construction with independent coils or sets of coils
+J J', included, respectively, in the circuits M M'. These
+energizing-coils are wound on a ring or annular field or on pole pieces
+thereon, and produce by the action of the alternating currents passing
+through them a progressive shifting of the magnetism from pole to pole.
+The cylindrical armature H of the motor is wound with two coils at right
+angles, which form independent closed circuits.
+
+If transformers be employed, one set of the primary coils, as N N, wound
+on a ring or annular core is connected to one circuit, as M', and the
+other primary coils, N N', to the circuit M. The secondary coils K K'
+may then be utilized for running groups of incandescent lamps P P'.
+
+[Illustration: FIG. 33.]
+
+With this generator an exciter is employed. This consists of two poles,
+A A, of steel permanently magnetized, or of iron excited by a battery or
+other generator of continuous currents, and a cylindrical armature core
+mounted on a shaft, B, and wound with two longitudinal coils, C C'. One
+end of each of these coils is connected to the collecting-rings _b c_,
+respectively, while the other ends are both connected to a ring, _a_.
+Collecting-brushes _b' c'_ bear on the rings _b c_, respectively, and
+conductors L L convey the currents therefrom through the coils E and E
+of the generator. L' is a common return-wire to brush _a'_. Two
+independent circuits are thus formed, one including coils C of the
+exciter and E E of the generator, the other coils C' of the exciter and
+E' E' of the generator. It results from this that the operation of the
+exciter produces a progressive movement of the magnetic poles of the
+annular field-core of the generator, the shifting or rotary movement of
+the poles being synchronous with the rotation of the exciter armature.
+Considering the operative conditions of a system thus established, it
+will be found that when the exciter is driven so as to energize the
+field of the generator, the armature of the latter, if left free to
+turn, would rotate at a speed practically the same as that of the
+exciter. If under such conditions the coils F F' of the generator
+armature be closed upon themselves or short-circuited, no currents, at
+least theoretically, will be generated in these armature coils. In
+practice the presence of slight currents is observed, the existence of
+which is attributable to more or less pronounced fluctuations in the
+intensity of the magnetic poles of the generator ring. So, if the
+armature-coils F F' be closed through the motor, the latter will not be
+turned as long as the movement of the generator armature is synchronous
+with that of the exciter or of the magnetic poles of its field. If, on
+the contrary, the speed of the generator armature be in any way checked,
+so that the shifting or rotation of the poles of the field becomes
+relatively more rapid, currents will be induced in the armature coils.
+This obviously follows from the passing of the lines of force across the
+armature conductors. The greater the speed of rotation of the magnetic
+poles relatively to that of the armature the more rapidly the currents
+developed in the coils of the latter will follow one another, and the
+more rapidly the motor will revolve in response thereto, and this
+continues until the armature generator is stopped entirely, as by a
+brake, when the motor, if properly constructed, runs at the speed with
+which the magnetic poles of the generator rotate.
+
+The effective strength of the currents developed in the armature coils
+of the generator is dependent upon the strength of the currents
+energizing the generator and upon the number of rotations per unit of
+time of the magnetic poles of the generator; hence the speed of the
+motor armature will depend in all cases upon the relative speeds of the
+armature of the generator and of its magnetic poles. For example, if the
+poles are turned two thousand times per unit of time and the armature is
+turned eight hundred, the motor will turn twelve hundred times, or
+nearly so. Very slight differences of speed may be indicated by a
+delicately balanced motor.
+
+Let it now be assumed that power is applied to the generator armature to
+turn it in a direction opposite to that in which its magnetic poles
+rotate. In such case the result would be similar to that produced by a
+generator the armature and field magnets of which are rotated in
+opposite directions, and by reason of these conditions the motor
+armature will turn at a rate of speed equal to the sum of the speeds of
+the armature and magnetic poles of the generator, so that a
+comparatively low speed of the generator armature will produce a high
+speed in the motor.
+
+It will be observed in connection with this system that on diminishing
+the resistance of the external circuit of the generator armature by
+checking the speed of the motor or by adding translating devices in
+multiple arc in the secondary circuit or circuits of the transformer the
+strength of the current in the armature circuit is greatly increased.
+This is due to two causes: first, to the great differences in the speeds
+of the motor and generator, and, secondly, to the fact that the
+apparatus follows the analogy of a transformer, for, in proportion as
+the resistance of the armature or secondary circuits is reduced, the
+strength of the currents in the field or primary circuits of the
+generator is increased and the currents in the armature are augmented
+correspondingly. For similar reasons the currents in the armature-coils
+of the generator increase very rapidly when the speed of the armature is
+reduced when running in the same direction as the magnetic poles or
+conversely.
+
+It will be understood from the above description that the
+generator-armature may be run in the direction of the shifting of the
+magnetic poles, but more rapidly, and that in such case the speed of the
+motor will be equal to the difference between the two rates.
+
+
+
+
+CHAPTER VII.
+
+REGULATOR FOR ROTARY CURRENT MOTORS.
+
+
+An interesting device for regulating and reversing has been devised by
+Mr. Tesla for the purpose of varying the speed of polyphase motors. It
+consists of a form of converter or transformer with one element capable
+of movement with respect to the other, whereby the inductive relations
+may be altered, either manually or automatically, for the purpose of
+varying the strength of the induced current. Mr. Tesla prefers to
+construct this device in such manner that the induced or secondary
+element may be movable with respect to the other; and the invention, so
+far as relates merely to the construction of the device itself,
+consists, essentially, in the combination, with two opposite magnetic
+poles, of an armature wound with an insulated coil and mounted on a
+shaft, whereby it may be turned to the desired extent within the field
+produced by the poles. The normal position of the core of the secondary
+element is that in which it most completely closes the magnetic circuit
+between the poles of the primary element, and in this position its coil
+is in its most effective position for the inductive action upon it of
+the primary coils; but by turning the movable core to either side, the
+induced currents delivered by its coil become weaker until, by a
+movement of the said core and coil through 90 deg., there will be no current
+delivered.
+
+Fig. 34 is a view in side elevation of the regulator. Fig. 35 is a
+broken section on line _x x_ of Fig. 34. Fig. 36 is a diagram
+illustrating the most convenient manner of applying the regulator to
+ordinary forms of motors, and Fig. 37 is a similar diagram illustrating
+the application of the device to the Tesla alternating-current motors.
+The regulator may be constructed in many ways to secure the desired
+result; but that which is, perhaps, its best form is shown in Figs. 34
+and 35.
+
+A represents a frame of iron. B B are the cores of the inducing or
+primary coils C C. D is a shaft mounted on the side bars, D', and on
+which is secured a sectional iron core, E, wound with an induced or
+secondary coil, F, the convolutions of which are parallel with the axis
+of the shaft. The ends of the core are rounded off so as to fit closely
+in the space between the two poles and permit the core E to be turned to
+and held at any desired point. A handle, G, secured to the projecting
+end of the shaft D, is provided for this purpose.
+
+[Illustration: FIG. 34.]
+
+[Illustration: FIG. 35.]
+
+In Fig. 36 let H represent an ordinary alternating current generator,
+the field-magnets of which are excited by a suitable source of current,
+I. Let J designate an ordinary form of electromagnetic motor provided
+with an armature, K, commutator L, and field-magnets M. It is well known
+that such a motor, if its field-magnet cores be divided up into
+insulated sections, may be practically operated by an alternating
+current; but in using this regulator with such a motor, Mr. Tesla
+includes one element of the motor only--say the armature-coils--in the
+main circuit of the generator, making the connections through the
+brushes and the commutator in the usual way. He also includes one of the
+elements of the regulator--say the stationary coils--in the same
+circuit, and in the circuit with the secondary or movable coil of the
+regulator he connects up the field-coils of the motor. He also prefers
+to use flexible conductors to make the connections from the secondary
+coil of the regulator, as he thereby avoids the use of sliding contacts
+or rings without interfering with the requisite movement of the core E.
+
+If the regulator be in its normal position, or that in which its
+magnetic circuit is most nearly closed, it delivers its maximum induced
+current, the phases of which so correspond with those of the primary
+current that the motor will run as though both field and armature were
+excited by the main current.
+
+[Illustration: FIG. 36.]
+
+To vary the speed of the motor to any rate between the minimum and
+maximum rates, the core E and coils F are turned in either direction to
+an extent which produces the desired result, for in its normal position
+the convolutions of coil F embrace the maximum number of lines of force,
+all of which act with the same effect upon the coil; hence it will
+deliver its maximum current; but by turning the coil F out of its
+position of maximum effect the number of lines of force embraced by it
+is diminished. The inductive effect is therefore impaired, and the
+current delivered by coil F will continue to diminish in proportion to
+the angle at which the coil F is turned until, after passing through an
+angle of ninety degrees, the convolutions of the coil will be at right
+angles to those of coils C C, and the inductive effect reduced to a
+minimum.
+
+Incidentally to certain constructions, other causes may influence the
+variation in the strength of the induced currents. For example, in the
+present case it will be observed that by the first movement of coil F a
+certain portion of its convolutions are carried beyond the line of the
+direct influence of the lines of force, and that the magnetic path or
+circuit for the lines is impaired; hence the inductive effect would be
+reduced. Next, that after moving through a certain angle, which is
+obviously determined by the relative dimensions of the bobbin or coil F,
+diagonally opposite portions of the coil will be simultaneously included
+in the field, but in such positions that the lines which produce a
+current-impulse in one portion of the coil in a certain direction will
+produce in the diagonally opposite portion a corresponding impulse in
+the opposite direction; hence portions of the current will neutralize
+one another.
+
+As before stated, the mechanical construction of the device may be
+greatly varied; but the essential conditions of the principle will be
+fulfilled in any apparatus in which the movement of the elements with
+respect to one another effects the same results by varying the inductive
+relations of the two elements in a manner similar to that described.
+
+[Illustration: FIG. 37.]
+
+It may also be stated that the core E is not indispensable to the
+operation of the regulator; but its presence is obviously beneficial.
+This regulator, however, has another valuable property in its capability
+of reversing the motor, for if the coil F be turned through a
+half-revolution, the position of its convolutions relatively to the two
+coils C C and to the lines of force is reversed, and consequently the
+phases of the current will be reversed. This will produce a rotation of
+the motor in an opposite direction. This form of regulator is also
+applied with great advantage to Mr. Tesla's system of utilizing
+alternating currents, in which the magnetic poles of the field of a
+motor are progressively shifted by means of the combined effects upon
+the field of magnetizing coils included in independent circuits, through
+which pass alternating currents in proper order and relations to each
+other.
+
+In Fig. 37, let P represent a Tesla generator having two independent
+coils, P' and P'', on the armature, and T a diagram of a motor having
+two independent energizing coils or sets of coils, R R'. One of the
+circuits from the generator, as S' S', includes one set, R' R', of the
+energizing coils of the motor, while the other circuit, as S S, includes
+the primary coils of the regulator. The secondary coil of the regulator
+includes the other coils, R R, of the motor.
+
+While the secondary coil of the regulator is in its normal position, it
+produces its maximum current, and the maximum rotary effect is imparted
+to the motor; but this effect will be diminished in proportion to the
+angle at which the coil F of the regulator is turned. The motor will
+also be reversed by reversing the position of the coil with reference to
+the coils C C, and thereby reversing the phases of the current produced
+by the generator. This changes the direction of the movement of the
+shifting poles which the armature follows.
+
+One of the main advantages of this plan of regulation is its economy of
+power. When the induced coil is generating its maximum current, the
+maximum amount of energy in the primary coils is absorbed; but as the
+induced coil is turned from its normal position the self-induction of
+the primary-coils reduces the expenditure of energy and saves power.
+
+It is obvious that in practice either coils C C or coil F may be used as
+primary or secondary, and it is well understood that their relative
+proportions may be varied to produce any desired difference or
+similarity in the inducing and induced currents.
+
+
+
+
+CHAPTER VIII.
+
+SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS.
+
+
+In the first chapters of this section we have, bearing in mind the broad
+underlying principle, considered a distinct class of motors, namely,
+such as require for their operation a special generator capable of
+yielding currents of differing phase. As a matter of course, Mr. Tesla
+recognizing the desirability of utilizing his motors in connection with
+ordinary systems of distribution, addressed himself to the task of
+inventing various methods and ways of achieving this object. In the
+succeeding chapters, therefore, we witness the evolution of a number of
+ideas bearing upon this important branch of work. It must be obvious to
+a careful reader, from a number of hints encountered here and there,
+that even the inventions described in these chapters to follow do not
+represent the full scope of the work done in these lines. They might,
+indeed, be regarded as exemplifications.
+
+We will present these various inventions in the order which to us
+appears the most helpful to an understanding of the subject by the
+majority of readers. It will be naturally perceived that in offering a
+series of ideas of this nature, wherein some of the steps or links are
+missing, the descriptions are not altogether sequential; but any one who
+follows carefully the main drift of the thoughts now brought together
+will find that a satisfactory comprehension of the principles can be
+gained.
+
+As is well known, certain forms of alternating-current machines have the
+property, when connected in circuit with an alternating current
+generator, of running as a motor in synchronism therewith; but, while
+the alternating current will run the motor after it has attained a rate
+of speed synchronous with that of the generator, it will not start it.
+Hence, in all instances heretofore where these "synchronizing motors,"
+as they are termed, have been run, some means have been adopted to bring
+the motors up to synchronism with the generator, or approximately so,
+before the alternating current of the generator is applied to drive
+them. In some instances mechanical appliances have been utilized for
+this purpose. In others special and complicated forms of motor have been
+constructed. Mr. Tesla has discovered a much more simple method or plan
+of operating synchronizing motors, which requires practically no other
+apparatus than the motor itself. In other words, by a certain change in
+the circuit connections of the motor he converts it at will from a
+double circuit motor, or such as have been already described, and which
+will start under the action of an alternating current, into a
+synchronizing motor, or one which will be run by the generator only when
+it has reached a certain speed of rotation synchronous with that of the
+generator. In this manner he is enabled to extend very greatly the
+applications of his system and to secure all the advantages of both
+forms of alternating current motor.
+
+The expression "synchronous with that of the generator," is used here in
+its ordinary acceptation--that is to say, a motor is said to synchronize
+with the generator when it preserves a certain relative speed determined
+by its number of poles and the number of alternations produced per
+revolution of the generator. Its actual speed, therefore, may be faster
+or slower than that of the generator; but it is said to be synchronous
+so long as it preserves the same relative speed.
+
+In carrying out this invention Mr. Tesla constructs a motor which has a
+strong tendency to synchronism with the generator. The construction
+preferred is that in which the armature is provided with polar
+projections. The field-magnets are wound with two sets of coils, the
+terminals of which are connected to a switch mechanism, by means of
+which the line-current may be carried directly through these coils or
+indirectly through paths by which its phases are modified. To start such
+a motor, the switch is turned on to a set of contacts which includes in
+one motor circuit a dead resistance, in the other an inductive
+resistance, and, the two circuits being in derivation, it is obvious
+that the difference in phase of the current in such circuits will set up
+a rotation of the motor. When the speed of the motor has thus been
+brought to the desired rate the switch is shifted to throw the main
+current directly through the motor-circuits, and although the currents
+in both circuits will now be of the same phase the motor will continue
+to revolve, becoming a true synchronous motor. To secure greater
+efficiency, the armature or its polar projections are wound with coils
+closed on themselves.
+
+In the accompanying diagrams, Fig. 38 illustrates the details of the
+plan above set forth, and Figs. 39 and 40 modifications of the same.
+
+[Illustration: FIGS. 38, 39 and 40.]
+
+Referring to Fig. 38, let A designate the field-magnets of a motor, the
+polar projections of which are wound with coils B C included in
+independent circuits, and D the armature with polar projections wound
+with coils E closed upon themselves, the motor in these respects being
+similar in construction to those described already, but having on
+account of the polar projections on the armature core, or other similar
+and well-known features, the properties of a synchronizing-motor. L L'
+represents the conductors of a line from an alternating current
+generator G.
+
+Near the motor is placed a switch the action of which is that of the one
+shown in the diagrams, which is constructed as follows: F F' are two
+conducting plates or arms, pivoted at their ends and connected by an
+insulating cross-bar, H, so as to be shifted in parallelism. In the path
+of the bars F F' is the contact 2, which forms one terminal of the
+circuit through coils C, and the contact 4, which is one terminal of the
+circuit through coils B. The opposite end of the wire of coils C is
+connected to the wire L or bar F', and the corresponding end of coils B
+is connected to wire L' and bar F; hence if the bars be shifted so as to
+bear on contacts 2 and 4 both sets of coils B C will be included in the
+circuit L L' in multiple arc or derivation. In the path of the levers F
+F' are two other contact terminals, 1 and 3. The contact 1 is connected
+to contact 2 through an artificial resistance, I, and contact 3 with
+contact 4 through a self-induction coil, J, so that when the switch
+levers are shifted upon the points 1 and 3 the circuits of coils B and C
+will be connected in multiple arc or derivation to the circuit L L', and
+will include the resistance and self-induction coil respectively. A
+third position of the switch is that in which the levers F and F' are
+shifted out of contact with both sets of points. In this case the motor
+is entirely out of circuit.
+
+The purpose and manner of operating the motor by these devices are as
+follows: The normal position of the switch, the motor being out of
+circuit, is off the contact points. Assuming the generator to be
+running, and that it is desired to start the motor, the switch is
+shifted until its levers rest upon points 1 and 3. The two
+motor-circuits are thus connected with the generator circuit; but by
+reason of the presence of the resistance I in one and the self-induction
+coil J in the other the coincidence of the phases of the current is
+disturbed sufficiently to produce a progression of the poles, which
+starts the motor in rotation. When the speed of the motor has run up to
+synchronism with the generator, or approximately so, the switch is
+shifted over upon the points 2 and 4, thus cutting out the coils I and
+J, so that the currents in both circuits have the same phase; but the
+motor now runs as a synchronous motor.
+
+It will be understood that when brought up to speed the motor will run
+with only one of the circuits B or C connected with the main or
+generator circuit, or the two circuits may be connected in series. This
+latter plan is preferable when a current having a high number of
+alternations per unit of time is employed to drive the motor. In such
+case the starting of the motor is more difficult, and the dead and
+inductive resistances must take up a considerable proportion of the
+electromotive force of the circuits. Generally the conditions are so
+adjusted that the electromotive force used in each of the motor circuits
+is that which is required to operate the motor when its circuits are in
+series. The plan followed in this case is illustrated in Fig. 39. In
+this instance the motor has twelve poles and the armature has polar
+projections D wound with closed coils E. The switch used is of
+substantially the same construction as that shown in the previous
+figure. There are, however, five contacts, designated as 5, 6, 7, 8, and
+9. The motor-circuits B C, which include alternate field-coils, are
+connected to the terminals in the following order: One end of circuit C
+is connected to contact 9 and to contact 5 through a dead resistance, I.
+One terminal of circuit B is connected to contact 7 and to contact 6
+through a self-induction coil, J. The opposite terminals of both
+circuits are connected to contact 8.
+
+One of the levers, as F, of the switch is made with an extension, _f_,
+or otherwise, so as to cover both contacts 5 and 6 when shifted into the
+position to start the motor. It will be observed that when in this
+position and with lever F' on contact 8 the current divides between the
+two circuits B C, which from their difference in electrical character
+produce a progression of the poles that starts the motor in rotation.
+When the motor has attained the proper speed, the switch is shifted so
+that the levers cover the contacts 7 and 9, thereby connecting circuits
+B and C in series. It is found that by this disposition the motor is
+maintained in rotation in synchronism with the generator. This principle
+of operation, which consists in converting by a change of connections or
+otherwise a double-circuit motor, or one operating by a progressive
+shifting of the poles, into an ordinary synchronizing motor may be
+carried out in many other ways. For instance, instead of using the
+switch shown in the previous figures, we may use a temporary ground
+circuit between the generator and motor, in order to start the motor, in
+substantially the manner indicated in Fig. 40. Let G in this figure
+represent an ordinary alternating-current generator with, say, two
+poles, M M', and an armature wound with two coils, N N', at right angles
+and connected in series. The motor has, for example, four poles wound
+with coils B C, which are connected in series, and an armature with
+polar projections D wound with closed coils E E. From the common joint
+or union between the two circuits of both the generator and the motor an
+earth connection is established, while the terminals or ends of these
+circuits are connected to the line. Assuming that the motor is a
+synchronizing motor or one that has the capability of running in
+synchronism with the generator, but not of starting, it may be started
+by the above-described apparatus by closing the ground connection from
+both generator and motor. The system thus becomes one with a two-circuit
+generator and motor, the ground forming a common return for the currents
+in the two circuits L and L'. When by this arrangement of circuits the
+motor is brought to speed, the ground connection is broken between the
+motor or generator, or both, ground-switches P P' being employed for
+this purpose. The motor then runs as a synchronizing motor.
+
+In describing the main features which constitute this invention
+illustrations have necessarily been omitted of the appliances used in
+conjunction with the electrical devices of similar systems--such, for
+instance, as driving-belts, fixed and loose pulleys for the motor, and
+the like; but these are matters well understood.
+
+Mr. Tesla believes he is the first to operate electro-magnetic motors by
+alternating currents in any of the ways herein described--that is to
+say, by producing a progressive movement or rotation of their poles or
+points of greatest magnetic attraction by the alternating currents until
+they have reached a given speed, and then by the same currents producing
+a simple alternation of their poles, or, in other words, by a change in
+the order or character of the circuit connections to convert a motor
+operating on one principle to one operating on another.
+
+
+
+
+CHAPTER IX.
+
+CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTOR.
+
+
+A description is given elsewhere of a method of operating alternating
+current motors by first rotating their magnetic poles until they have
+attained synchronous speed, and then alternating the poles. The motor is
+thus transformed, by a simple change of circuit connections from one
+operated by the action of two or more independent energizing currents to
+one operated either by a single current or by several currents acting as
+one. Another way of doing this will now be described.
+
+At the start the magnetic poles of one element or field of the motor are
+progressively shifted by alternating currents differing in phase and
+passed through independent energizing circuits, and short circuit the
+coils of the other element. When the motor thus started reaches or
+passes the limit of speed synchronous with the generator, Mr. Tesla
+connects up the coils previously short-circuited with a source of direct
+current and by a change of the circuit connections produces a simple
+alternation of the poles. The motor then continues to run in synchronism
+with the generator. The motor here shown in Fig. 41 is one of the
+ordinary forms, with field-cores either laminated or solid and with a
+cylindrical laminated armature wound, for example, with the coils A B at
+right angles. The shaft of the armature carries three collecting or
+contact rings C D E. (Shown, for better illustration, as of different
+diameters.)
+
+One end of coil A connects to one ring, as C, and one end of coil B
+connects with ring D. The remaining ends are connected to ring E.
+Collecting springs or brushes F G H bear upon the rings and lead to the
+contacts of a switch, to be presently described. The field-coils have
+their terminals in binding-posts K K, and may be either closed upon
+themselves or connected with a source of direct current L, by means of a
+switch M. The main or controlling switch has five contacts _a b c d e_
+and two levers _f g_, pivoted and connected by an insulating cross-bar
+_h_, so as to move in parallelism. These levers are connected to the
+line wires from a source of alternating currents N. Contact _a_ is
+connected to brush G and coil B through a dead resistance R and wire P.
+Contact _b_ is connected with brush F and coil A through a
+self-induction coil S and wire O. Contacts _c_ and _e_ are connected to
+brushes G F, respectively, through the wires P O, and contact _d_ is
+directly connected with brush H. The lever _f_ has a widened end, which
+may span the contacts _a b_. When in such position and with lever _g_ on
+contact _d_, the alternating currents divide between the two
+motor-coils, and by reason of their different self-induction a
+difference of current-phase is obtained that starts the motor in
+rotation. In starting, the field-coils are short circuited.
+
+[Illustration: FIG. 41.]
+
+When the motor has attained the desired speed, the switch is shifted to
+the position shown in dotted lines--that is to say, with the levers _f
+g_ resting on points _c e_. This connects up the two armature coils in
+series, and the motor will then run as a synchronous motor. The
+field-coils are thrown into circuit with the direct current source when
+the main switch is shifted.
+
+
+
+
+CHAPTER X.
+
+MOTOR WITH "CURRENT LAG" ARTIFICIALLY SECURED.
+
+
+One of the general ways followed by Mr. Tesla in developing his rotary
+phase motors is to produce practically independent currents differing
+primarily in phase and to pass these through the motor-circuits. Another
+way is to produce a single alternating current, to divide it between the
+motor-circuits, and to effect artificially a lag in one of these
+circuits or branches, as by giving to the circuits different
+self-inductive capacity, and in other ways. In the former case, in which
+the necessary difference of phase is primarily effected in the
+generation of currents, in some instances, the currents are passed
+through the energizing coils of both elements of the motor--the field
+and armature; but a further result or modification may be obtained by
+doing this under the conditions hereinafter specified in the case of
+motors in which the lag, as above stated, is artificially secured.
+
+Figs. 42 to 47, inclusive, are diagrams of different ways in which the
+invention is carried out; and Fig. 48, a side view of a form of motor
+used by Mr. Tesla for this purpose.
+
+[Illustration: FIGS. 42, 43 and 44.]
+
+A B in Fig. 42 indicate the two energizing circuits of a motor, and C D
+two circuits on the armature. Circuit or coil A is connected in series
+with circuit or coil C, and the two circuits B D are similarly
+connected. Between coils A and C is a contact-ring _e_, forming one
+terminal of the latter, and a brush _a_, forming one terminal of the
+former. A ring _d_ and brush _c_ similarly connect coils B and D. The
+opposite terminals of the field-coils connect to one binding post _h_ of
+the motor, and those of the armature coils are similarly connected to
+the opposite binding post _i_ through a contact-ring _f_ and brush _g_.
+Thus each motor-circuit while in derivation to the other includes one
+armature and one field coil. These circuits are of different
+self-induction, and may be made so in various ways. For the sake
+of clearness, an artificial resistance R is shown in one of these
+circuits, and in the other a self-induction coil S. When an alternating
+current is passed through this motor it divides between its two
+energizing-circuits. The higher self-induction of one circuit produces a
+greater retardation or lag in the current therein than in the other. The
+difference of phase between the two currents effects the rotation or
+shifting of the points of maximum magnetic effect that secures the
+rotation of the armature. In certain respects this plan of including
+both armature and field coils in circuit is a marked improvement. Such a
+motor has a good torque at starting; yet it has also considerable
+tendency to synchronism, owing to the fact that when properly
+constructed the maximum magnetic effects in both armature and field
+coincide--a condition which in the usual construction of these motors
+with closed armature coils is not readily attained. The motor thus
+constructed exhibits too, a better regulation of current from no load to
+load, and there is less difference between the apparent and real energy
+expended in running it. The true synchronous speed of this form of motor
+is that of the generator when both are alike--that is to say, if the
+number of the coils on the armature and on the field is _x_, the motor
+will run normally at the same speed as a generator driving it if the
+number of field magnets or poles of the same be also _x_.
+
+[Illustration: FIGS. 45, 46 and 47.]
+
+Fig. 43 shows a somewhat modified arrangement of circuits. There is in
+this case but one armature coil E, the winding of which maintains
+effects corresponding to the resultant poles produced by the two
+field-circuits.
+
+Fig. 44 represents a disposition in which both armature and field are
+wound with two sets of coils, all in multiple arc to the line or main
+circuit. The armature coils are wound to correspond with the field-coils
+with respect to their self-induction. A modification of this plan is
+shown in Fig. 45--that is to say, the two field coils and two armature
+coils are in derivation to themselves and in series with one another.
+The armature coils in this case, as in the previous figure, are wound
+for different self-induction to correspond with the field coils.
+
+Another modification is shown in Fig. 46. In this case only one
+armature-coil, as D, is included in the line-circuit, while the other,
+as C, is short-circuited.
+
+In such a disposition as that shown in Fig. 43, or where only one
+armature-coil is employed, the torque on the start is somewhat reduced,
+while the tendency to synchronism is somewhat increased. In such a
+disposition as shown in Fig. 46, the opposite conditions would exist. In
+both instances, however, there is the advantage of dispensing with one
+contact-ring.
+
+[Illustration: FIG. 48.]
+
+In Fig. 46 the two field-coils and the armature-coil D are in multiple
+arc. In Fig. 47 this disposition is modified, coil D being shown in
+series with the two field-coils.
+
+Fig. 48 is an outline of the general form of motor in which this
+invention is embodied. The circuit connections between the armature and
+field coils are made, as indicated in the previous figures, through
+brushes and rings, which are not shown.
+
+
+
+
+CHAPTER XI.
+
+ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A SYNCHRONIZING MOTOR.
+
+
+In a preceding chapter we have described a method by which Mr. Tesla
+accomplishes the change in his type of rotating field motor from a
+torque to a synchronizing motor. As will be observed, the desired end is
+there reached by a change in the circuit connections at the proper
+moment. We will now proceed to describe another way of bringing about
+the same result. The principle involved in this method is as follows:--
+
+If an alternating current be passed through the field coils only of a
+motor having two energizing circuits of different self-induction and the
+armature coils be short-circuited, the motor will have a strong torque,
+but little or no tendency to synchronism with the generator; but if the
+same current which energizes the field be passed also through the
+armature coils the tendency to remain in synchronism is very
+considerably increased. This is due to the fact that the maximum
+magnetic effects produced in the field and armature more nearly
+coincide. On this principle Mr. Tesla constructs a motor having
+independent field circuits of different self-induction, which are joined
+in derivation to a source of alternating currents. The armature is wound
+with one or more coils, which are connected with the field coils through
+contact rings and brushes, and around the armature coils a shunt is
+arranged with means for opening or closing the same. In starting this
+motor the shunt is closed around the armature coils, which will
+therefore be in closed circuit. When the current is directed through the
+motor, it divides between the two circuits, (it is not necessary to
+consider any case where there are more than two circuits used), which,
+by reason of their different self-induction, secure a difference of
+phase between the two currents in the two branches, that produces a
+shifting or rotation of the poles. By the alternations of current, other
+currents are induced in the closed--or short-circuited--armature coils
+and the motor has a strong torque. When the desired speed is reached,
+the shunt around the armature-coils is opened and the current directed
+through both armature and field coils. Under these conditions the motor
+has a strong tendency to synchronism.
+
+[Illustration: FIGS. 49, 50 and 51.]
+
+In Fig. 49, A and B designate the field coils of the motor. As the
+circuits including these coils are of different self-induction, this is
+represented by a resistance coil R in circuit with A, and a
+self-induction coil S in circuit with B. The same result may of course
+be secured by the winding of the coils. C is the armature circuit, the
+terminals of which are rings _a b_. Brushes _c d_ bear on these rings
+and connect with the line and field circuits. D is the shunt or short
+circuit around the armature. E is the switch in the shunt.
+
+It will be observed that in such a disposition as is illustrated in
+Fig. 49, the field circuits A and B being of different self-induction,
+there will always be a greater lag of the current in one than the other,
+and that, generally, the armature phases will not correspond with
+either, but with the resultant of both. It is therefore important to
+observe the proper rule in winding the armature. For instance, if the
+motor have eight poles--four in each circuit--there will be four
+resultant poles, and hence the armature winding should be such as to
+produce four poles, in order to constitute a true synchronizing motor.
+
+[Illustration: FIG. 52.]
+
+The diagram, Fig. 50, differs from the previous one only in respect to
+the order of connections. In the present case the armature-coil, instead
+of being in series with the field-coils, is in multiple arc therewith.
+The armature-winding may be similar to that of the field--that is to
+say, the armature may have two or more coils wound or adapted for
+different self-induction and adapted, preferably, to produce the same
+difference of phase as the field-coils. On starting the motor the shunt
+is closed around both coils. This is shown in Fig. 51, in which the
+armature coils are F G. To indicate their different electrical
+character, there are shown in circuit with them, respectively, the
+resistance R' and the self-induction coil S'. The two armature coils are
+in series with the field-coils and the same disposition of the shunt or
+short-circuit D is used. It is of advantage in the operation of motors
+of this kind to construct or wind the armature in such manner that when
+short-circuited on the start it will have a tendency to reach a higher
+speed than that which synchronizes with the generator. For example, a
+given motor having, say, eight poles should run, with the armature coil
+short-circuited, at two thousand revolutions per minute to bring it up
+to synchronism. It will generally happen, however, that this speed is
+not reached, owing to the fact that the armature and field currents do
+not properly correspond, so that when the current is passed through the
+armature (the motor not being quite up to synchronism) there is a
+liability that it will not "hold on," as it is termed. It is preferable,
+therefore, to so wind or construct the motor that on the start, when the
+armature coils are short-circuited, the motor will tend to reach a speed
+higher than the synchronous--as for instance, double the latter. In such
+case the difficulty above alluded to is not felt, for the motor will
+always hold up to synchronism if the synchronous speed--in the case
+supposed of two thousand revolutions--is reached or passed. This may be
+accomplished in various ways; but for all practical purposes the
+following will suffice: On the armature are wound two sets of coils. At
+the start only one of these is short-circuited, thereby producing a
+number of poles on the armature, which will tend to run the speed up
+above the synchronous limit. When such limit is reached or passed, the
+current is directed through the other coil, which, by increasing the
+number of armature poles, tends to maintain synchronism.
+
+[Illustration: FIG. 53.]
+
+In Fig. 52, such a disposition is shown. The motor having, say, eight
+poles contains two field-circuits A and B, of different self-induction.
+The armature has two coils F and G. The former is closed upon itself,
+the latter connected with the field and line through contact-rings _a
+b_, brushes _c d_, and a switch E. On the start the coil F alone is
+active and the motor tends to run at a speed above the synchronous; but
+when the coil G is connected to the circuit the number of armature poles
+is increased, while the motor is made a true synchronous motor. This
+disposition has the advantage that the closed armature-circuit imparts
+to the motor torque when the speed falls off, but at the same time the
+conditions are such that the motor comes out of synchronism more
+readily. To increase the tendency to synchronism, two circuits may be
+used on the armature, one of which is short-circuited on the start and
+both connected with the external circuit after the synchronous speed is
+reached or passed. This disposition is shown in Fig. 53. There are three
+contact-rings _a b e_ and three brushes _c d f_, which connect the
+armature circuits with the external circuit. On starting, the switch H
+is turned to complete the connection between one binding-post P and the
+field-coils. This short-circuits one of the armature-coils, as G. The
+other coil F is out of circuit and open. When the motor is up to speed,
+the switch H is turned back, so that the connection from binding-post P
+to the field coils is through the coil G, and switch K is closed,
+thereby including coil F in multiple arc with the field coils. Both
+armature coils are thus active.
+
+From the above-described instances it is evident that many other
+dispositions for carrying out the invention are possible.
+
+
+
+
+CHAPTER XII.
+
+"MAGNETIC LAG" MOTOR.
+
+
+The following description deals with another form of motor, namely,
+depending on "magnetic lag" or hysteresis, its peculiarity being that in
+it the attractive effects or phases while lagging behind the phases of
+current which produce them, are manifested simultaneously and not
+successively. The phenomenon utilized thus at an early stage by Mr.
+Tesla, was not generally believed in by scientific men, and Prof. Ayrton
+was probably first to advocate it or to elucidate the reason of its
+supposed existence.
+
+Fig. 54 is a side view of the motor, in elevation. Fig. 55 is a
+part-sectional view at right angles to Fig. 54. Fig. 56 is an end view
+in elevation and part section of a modification, and Fig. 57 is a
+similar view of another modification.
+
+In Figs. 54 and 55, A designates a base or stand, and B B the
+supporting-frame of the motor. Bolted to the supporting-frame are two
+magnetic cores or pole-pieces C C', of iron or soft steel. These may be
+subdivided or laminated, in which case hard iron or steel plates or bars
+should be used, or they should be wound with closed coils. D is a
+circular disc armature, built up of sections or plates of iron and
+mounted in the frame between the pole-pieces C C', curved to conform to
+the circular shape thereof. This disc may be wound with a number of
+closed coils E. F F are the main energizing coils, supported by the
+supporting-frame, so as to include within their magnetizing influence
+both the pole-pieces C C' and the armature D. The pole-pieces C C'
+project out beyond the coils F F on opposite sides, as indicated in the
+drawings. If an alternating current be passed through the coils F F,
+rotation of the armature will be produced, and this rotation is
+explained by the following apparent action, or mode of operation: An
+impulse of current in the coils F F establishes two polarities in the
+motor. The protruding end of pole-piece C, for instance, will be of one
+sign, and the corresponding end of pole-piece C' will be of the opposite
+sign. The armature also exhibits two poles at right angles to the coils
+F F, like poles to those in the pole-pieces being on the same side of
+the coils. While the current is flowing there is no appreciable tendency
+to rotation developed; but after each current impulse ceases or begins
+to fall, the magnetism in the armature and in the ends of the
+pole-pieces C C' lags or continues to manifest itself, which produces a
+rotation of the armature by the repellent force between the more closely
+approximating points of maximum magnetic effect. This effect is
+continued by the reversal of current, the polarities of field and
+armature being simply reversed. One or both of the elements--the
+armature or field--may be wound with closed induced coils to intensify
+this effect. Although in the illustrations but one of the fields is
+shown, each element of the motor really constitutes a field, wound with
+the closed coils, the currents being induced mainly in those
+convolutions or coils which are parallel to the coils F F.
+
+[Illustration: FIG. 54.]
+
+[Illustration: FIG. 55.]
+
+A modified form of this motor is shown in Fig. 56. In this form G is one
+of two standards that support the bearings for the armature-shaft. H H
+are uprights or sides of a frame, preferably magnetic, the ends C C' of
+which are bent in the manner indicated, to conform to the shape of the
+armature D and form field-magnet poles. The construction of the armature
+may be the same as in the previous figure, or it may be simply a
+magnetic disc or cylinder, as shown, and a coil or coils F F are
+secured in position to surround both the armature and the poles C C'.
+The armature is detachable from its shaft, the latter being passed
+through the armature after it has been inserted in position. The
+operation of this form of motor is the same in principle as that
+previously described and needs no further explanation.
+
+[Illustration: FIG. 56.]
+
+[Illustration: FIG. 57.]
+
+One of the most important features in alternating current motors is,
+however, that they should be adapted to and capable of running
+efficiently on the alternating circuits in present use, in which almost
+without exception the generators yield a very high number of
+alternations. Such a motor, of the type under consideration, Mr. Tesla
+has designed by a development of the principle of the motor shown in
+Fig. 56, making a multipolar motor, which is illustrated in Fig. 57. In
+the construction of this motor he employs an annular magnetic frame J,
+with inwardly-extending ribs or projections K, the ends of which all
+bend or turn in one direction and are generally shaped to conform to the
+curved surface of the armature. Coils F F are wound from one part K to
+the one next adjacent, the ends or loops of each coil or group of wires
+being carried over toward the shaft, so as to form U-shaped groups
+of convolutions at each end of the armature. The pole-pieces C C', being
+substantially concentric with the armature, form ledges, along which the
+coils are laid and should project to some extent beyond the the coils,
+as shown. The cylindrical or drum armature D is of the same construction
+as in the other motors described, and is mounted to rotate within the
+annular frame J and between the U-shaped ends or bends of the
+coils F. The coils F are connected in multiple or in series with a
+source of alternating currents, and are so wound that with a current or
+current impulse of given direction they will make the alternate
+pole-pieces C of one polarity and the other pole-pieces C' of the
+opposite polarity. The principle of the operation of this motor is the
+same as the other above described, for, considering any two pole-pieces
+C C', a current impulse passing in the coil which bridges them or is
+wound over both tends to establish polarities in their ends of opposite
+sign and to set up in the armature core between them a polarity of the
+same sign as that of the nearest pole-piece C. Upon the fall or
+cessation of the current impulse that established these polarities the
+magnetism which lags behind the current phase, and which continues to
+manifest itself in the polar projections C C' and the armature, produces
+by repulsion a rotation of the armature. The effect is continued by each
+reversal of the current. What occurs in the case of one pair of
+pole-pieces occurs simultaneously in all, so that the tendency to
+rotation of the armature is measured by the sum of all the forces
+exerted by the pole-pieces, as above described. In this motor also the
+magnetic lag or effect is intensified by winding one or both cores with
+closed induced coils. The armature core is shown as thus wound. When
+closed coils are used, the cores should be laminated.
+
+It is evident that a pulsatory as well as an alternating current might
+be used to drive or operate the motors above described.
+
+It will be understood that the degree of subdivision, the mass of the
+iron in the cores, their size and the number of alternations in the
+current employed to run the motor, must be taken into consideration in
+order to properly construct this motor. In other words, in all such
+motors the proper relations between the number of alternations and the
+mass, size, or quality of the iron must be preserved in order to secure
+the best results.
+
+
+
+
+CHAPTER XIII.
+
+METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC SHIELDING.
+
+
+In that class of motors in which two or more sets of energizing magnets
+are employed, and in which by artificial means a certain interval of
+time is made to elapse between the respective maximum or minimum periods
+or phases of their magnetic attraction or effect, the interval or
+difference in phase between the two sets of magnets is limited in
+extent. It is desirable, however, for the economical working of such
+motors that the strength or attraction of one set of magnets should be
+maximum, at the time when that of the other set is minimum, and
+conversely; but these conditions have not heretofore been realized
+except in cases where the two currents have been obtained from
+independent sources in the same or different machines. Mr. Tesla has
+therefore devised a motor embodying conditions that approach more nearly
+the theoretical requirements of perfect working, or in other words, he
+produces artificially a difference of magnetic phase by means of a
+current from a single primary source sufficient in extent to meet the
+requirements of practical and economical working. He employs a motor
+with two sets of energizing or field magnets, each wound with coils
+connected with a source of alternating or rapidly-varying currents, but
+forming two separate paths or circuits. The magnets of one set are
+protected to a certain extent from the energizing action of the current
+by means of a magnetic shield or screen interposed between the magnet
+and its energizing coil. This shield is properly adapted to the
+conditions of particular cases, so as to shield or protect the main core
+from magnetization until it has become itself saturated and no longer
+capable of containing all the lines of force produced by the current. It
+will be seen that by this means the energizing action begins in the
+protected set of magnets a certain arbitrarily-determined period of time
+later than in the other, and that by this means alone or in conjunction
+with other means or devices heretofore employed a practical difference
+of magnetic phase may readily be secured.
+
+Fig. 58 is a view of a motor, partly in section, with a diagram
+illustrating the invention. Fig. 59 is a similar view of a modification
+of the same.
+
+[Illustration: FIG. 58.]
+
+[Illustration: FIG. 59.]
+
+In Fig. 58, which exhibits the simplest form of the invention, A A is
+the field-magnet of a motor, having, say, eight poles or
+inwardly-projecting cores B and C. The cores B form one set of magnets
+and are energized by coils D. The cores C, forming the other set are
+energized by coils E, and the coils are connected, preferably, in series
+with one another, in two derived or branched circuits, F G,
+respectively, from a suitable source of current. Each coil E is
+surrounded by a magnetic shield H, which is preferably composed of an
+annealed, insulated, or oxidized iron wire wrapped or wound on the coils
+in the manner indicated so as to form a closed magnetic circuit around
+the coils and between the same and the magnetic cores C. Between the
+pole pieces or cores B C is mounted the armature K, which, as is usual
+in this type of machines, is wound with coils L closed upon themselves.
+The operation resulting from this disposition is as follows: If a
+current impulse be directed through the two circuits of the motor, it
+will quickly energize the cores B, but not so the cores C, for the
+reason that in passing through the coils E there is encountered the
+influence of the closed magnetic circuits formed by the shields H. The
+first effect is to retard effectively the current impulse in circuit G,
+while at the same time the proportion of current which does pass does
+not magnetize the cores C, which are shielded or screened by the
+shields H. As the increasing electromotive force then urges more current
+through the coils E, the iron wire H becomes magnetically saturated and
+incapable of carrying all the lines of force, and hence ceases to
+protect the cores C, which becomes magnetized, developing their maximum
+effect after an interval of time subsequent to the similar manifestation
+of strength in the other set of magnets, the extent of which is
+arbitrarily determined by the thickness of the shield H, and other
+well-understood conditions.
+
+From the above it will be seen that the apparatus or device acts in two
+ways. First, by retarding the current, and, second, by retarding the
+magnetization of one set of the cores, from which its effectiveness will
+readily appear.
+
+Many modifications of the principle of this invention are possible. One
+useful and efficient application of the invention is shown in Fig. 59.
+In this figure a motor is shown similar in all respects to that above
+described, except that the iron wire H, which is wrapped around the
+coils E, is in this case connected in series with the coils D. The
+iron-wire coils H, are connected and wound, so as to have little or no
+self-induction, and being added to the resistance of the circuit F, the
+action of the current in that circuit will be accelerated, while in the
+other circuit G it will be retarded. The shield H may be made in many
+forms, as will be understood, and used in different ways, as appears
+from the foregoing description.
+
+As a modification of his type of motor with "shielded" fields, Mr. Tesla
+has constructed a motor with a field-magnet having two sets of poles or
+inwardly-projecting cores and placed side by side, so as practically to
+form two fields of force and alternately disposed--that is to say, with
+the poles of one set or field opposite the spaces between the other. He
+then connects the free ends of one set of poles by means of laminated
+iron bands or bridge-pieces of considerably smaller cross-section than
+the cores themselves, whereby the cores will all form parts of complete
+magnetic circuits. When the coils on each set of magnets are connected
+in multiple circuits or branches from a source of alternating currents,
+electromotive forces are set up in or impressed upon each circuit
+simultaneously; but the coils on the magnetically bridged or shunted
+cores will have, by reason of the closed magnetic circuits, a high
+self-induction, which retards the current, permitting at the beginning
+of each impulse but little current to pass. On the other hand, no such
+opposition being encountered in the other set of coils, the current
+passes freely through them, magnetizing the poles on which they are
+wound. As soon, however, as the laminated bridges become saturated and
+incapable of carrying all the lines of force which the rising
+electromotive force, and consequently increased current, produce, free
+poles are developed at the ends of the cores, which, acting in
+conjunction with the others, produce rotation of the armature.
+
+The construction in detail by which this invention is illustrated is
+shown in the accompanying drawings.
+
+[Illustration: FIG. 60.]
+
+[Illustration: FIG. 61.]
+
+Fig. 60 is a view in side elevation of a motor embodying the principle.
+Fig. 61 is a vertical cross-section of the motor. A is the frame of the
+motor, which should be built up of sheets of iron punched out to the
+desired shape and bolted together with insulation between the sheets.
+When complete, the frame makes a field-magnet with inwardly projecting
+pole-pieces B and C. To adapt them to the requirements of this
+particular case these pole-pieces are out of line with one another,
+those marked B surrounding one end of the armature and the others, as C,
+the opposite end, and they are disposed alternately--that is to say, the
+pole-pieces of one set occur in line with the spaces between those of
+the other sets.
+
+The armature D is of cylindrical form, and is also laminated in the
+usual way and is wound longitudinally with coils closed upon themselves.
+The pole-pieces C are connected or shunted by bridge-pieces E. These may
+be made independently and attached to the pole-pieces, or they may be
+parts of the forms or blanks stamped or punched out of sheet-iron. Their
+size or mass is determined by various conditions, such as the strength
+of the current to be employed, the mass or size of the cores to which
+they are applied, and other familiar conditions.
+
+Coils F surround the pole-pieces B, and other coils G are wound on the
+pole-pieces C. These coils are connected in series in two circuits,
+which are branches of a circuit from a generator of alternating
+currents, and they may be so wound, or the respective circuits in which
+they are included may be so arranged, that the circuit of coils G will
+have, independently of the particular construction described, a higher
+self-induction than the other circuit or branch.
+
+The function of the shunts or bridges E is that they shall form with the
+cores C a closed magnetic circuit for a current up to a predetermined
+strength, so that when saturated by such current and unable to carry
+more lines of force than such a current produces they will to no further
+appreciable extent interfere with the development, by a stronger
+current, of free magnetic poles at the ends of the cores C.
+
+In such a motor the current is so retarded in the coils G, and the
+manifestation of the free magnetism in the poles C is so delayed beyond
+the period of maximum magnetic effect in poles B, that a strong torque
+is produced and the motor operates with approximately the power
+developed in a motor of this kind energized by independently generated
+currents differing by a full quarter phase.
+
+
+
+
+CHAPTER XIV.
+
+TYPE OF TESLA SINGLE-PHASE MOTOR.
+
+
+Up to this point, two principal types of Tesla motors have been
+described: First, those containing two or more energizing circuits
+through which are caused to pass alternating currents differing from one
+another in phase to an extent sufficient to produce a continuous
+progression or shifting of the poles or points of greatest magnetic
+effect, in obedience to which the movable element of the motor is
+maintained in rotation; second, those containing poles, or parts of
+different magnetic susceptibility, which under the energizing influence
+of the same current or two currents coinciding in phase will exhibit
+differences in their magnetic periods or phases. In the first class of
+motors the torque is due to the magnetism established in different
+portions of the motor by currents from the same or from independent
+sources, and exhibiting time differences in phase. In the second class
+the torque results from the energizing effects of a current upon
+different parts of the motor which differ in magnetic susceptibility--in
+other words, parts which respond in the same relative degree to the
+action of a current, not simultaneously, but after different intervals
+of time.
+
+In another Tesla motor, however, the torque, instead of being solely the
+result of a time difference in the magnetic periods or phases of the
+poles or attractive parts to whatever cause due, is produced by an
+angular displacement of the parts which, though movable with respect to
+one another, are magnetized simultaneously, or approximately so, by the
+same currents. This principle of operation has been embodied practically
+in a motor in which the necessary angular displacement between the
+points of greatest magnetic attraction in the two elements of the
+motor--the armature and field--is obtained by the direction of the
+lamination of the magnetic cores of the elements.
+
+Fig. 62 is a side view of such a motor with a portion of its armature
+core exposed. Fig. 63 is an end or edge view of the same. Fig. 64 is a
+central cross-section of the same, the armature being shown mainly in
+elevation.
+
+[Illustration: FIG. 62.]
+
+[Illustration: FIG. 63.]
+
+[Illustration: FIG. 64.]
+
+Let A A designate two plates built up of thin sections or laminae of soft
+iron insulated more or less from one another and held together by bolts
+_a_ and secured to a base B. The inner faces of these plates contain
+recesses or grooves in which a coil or coils D are secured obliquely to
+the direction of the laminations. Within the coils D is a disc E,
+preferably composed of a spirally-wound iron wire or ribbon or a series
+of concentric rings and mounted on a shaft F, having bearings in the
+plates A A. Such a device when acted upon by an alternating current is
+capable of rotation and constitutes a motor, the operation of which may
+be explained in the following manner: A current or current-impulse
+traversing the coils D tends to magnetize the cores A A and E, all of
+which are within the influence of the field of the coils. The poles thus
+established would naturally lie in the same line at right angles to the
+coils D, but in the plates A they are deflected by reason of the
+direction of the laminations, and appear at or near the extremities of
+these plates. In the disc, however, where these conditions are not
+present, the poles or points of greatest attraction are on a line at
+right angles to the plane of the coils; hence there will be a torque
+established by this angular displacement of the poles or magnetic lines,
+which starts the disc in rotation, the magnetic lines of the armature
+and field tending toward a position of parallelism. This rotation is
+continued and maintained by the reversals of the current in coils D D,
+which change alternately the polarity of the field-cores A A. This
+rotary tendency or effect will be greatly increased by winding the disc
+with conductors G, closed upon themselves and having a radial direction,
+whereby the magnetic intensity of the poles of the disc will be greatly
+increased by the energizing effect of the currents induced in the coils
+G by the alternating currents in coils D.
+
+The cores of the disc and field may or may not be of different magnetic
+susceptibility--that is to say, they may both be of the same kind of
+iron, so as to be magnetized at approximately the same instant by the
+coils D; or one may be of soft iron and the other of hard, in order that
+a certain time may elapse between the periods of their magnetization. In
+either case rotation will be produced; but unless the disc is provided
+with the closed energizing coils it is desirable that the
+above-described difference of magnetic susceptibility be utilized to
+assist in its rotation.
+
+The cores of the field and armature may be made in various ways, as will
+be well understood, it being only requisite that the laminations in each
+be in such direction as to secure the necessary angular displacement of
+the points of greatest attraction. Moreover, since the disc may be
+considered as made up of an infinite number of radial arms, it is
+obvious that what is true of a disc holds for many other forms of
+armature.
+
+
+
+
+CHAPTER XV.
+
+MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE.
+
+
+As has been pointed out elsewhere, the lag or retardation of the phases
+of an alternating current is directly proportional to the self-induction
+and inversely proportional to the resistance of the circuit through
+which the current flows. Hence, in order to secure the proper
+differences of phase between the two motor-circuits, it is desirable to
+make the self-induction in one much higher and the resistance much lower
+than the self-induction and resistance, respectively, in the other. At
+the same time the magnetic quantities of the two poles or sets of poles
+which the two circuits produce should be approximately equal. These
+requirements have led Mr. Tesla to the invention of a motor having the
+following general characteristics: The coils which are included in that
+energizing circuit which is to have the higher self-induction are made
+of coarse wire, or a conductor of relatively low resistance, and with
+the greatest possible length or number of turns. In the other set of
+coils a comparatively few turns of finer wire are used, or a wire of
+higher resistance. Furthermore, in order to approximate the magnetic
+quantities of the poles excited by these coils, Mr. Tesla employs in the
+self-induction circuit cores much longer than those in the other or
+resistance circuit.
+
+Fig. 65 is a part sectional view of the motor at right angles to the
+shaft. Fig. 66 is a diagram of the field circuits.
+
+In Fig. 66, let A represent the coils in one motor circuit, and B those
+in the other. The circuit A is to have the higher self-induction. There
+are, therefore, used a long length or a large number of turns of coarse
+wire in forming the coils of this circuit. For the circuit B, a smaller
+conductor is employed, or a conductor of a higher resistance than
+copper, such as German silver or iron, and the coils are wound with
+fewer turns. In applying these coils to a motor, Mr. Tesla builds up a
+field-magnet of plates C, of iron and steel, secured together in the
+usual manner by bolts D. Each plate is formed with four (more or less)
+long cores E, around which is a space to receive the coil and an equal
+number of short projections F to receive the coils of the
+resistance-circuit. The plates are generally annular in shape, having an
+open space in the centre for receiving the armature G, which Mr. Tesla
+prefers to wind with closed coils. An alternating current divided
+between the two circuits is retarded as to its phases in the circuit A
+to a much greater extent than in the circuit B. By reason of the
+relative sizes and disposition of the cores and coils the magnetic
+effect of the poles E and F upon the armature closely approximate.
+
+[Illustration: FIG. 65.]
+
+[Illustration: FIG. 66.]
+
+An important result secured by the construction shown here is that these
+coils which are designed to have the higher self-induction are almost
+completely surrounded by iron, and that the retardation is thus very
+materially increased.
+
+
+
+
+CHAPTER XVI.
+
+MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND ARMATURE.
+
+
+Let it be assumed that the energy as represented in the magnetism in the
+field of a given rotating field motor is ninety and that of the armature
+ten. The sum of these quantities, which represents the total energy
+expended in driving the motor, is one hundred; but, assuming that the
+motor be so constructed that the energy in the field is represented by
+fifty, and that in the armature by fifty, the sum is still one hundred;
+but while in the first instance the product is nine hundred, in the
+second it is two thousand five hundred, and as the energy developed is
+in proportion to these products it is clear that those motors are the
+most efficient--other things being equal--in which the magnetic energies
+developed in the armature and field are equal. These results Mr. Tesla
+obtains by using the same amount of copper or ampere turns in both
+elements when the cores of both are equal, or approximately so, and the
+same current energizes both; or in cases where the currents in one
+element are induced to those of the other he uses in the induced coils
+an excess of copper over that in the primary element or conductor.
+
+[Illustration: FIG. 67.]
+
+The conventional figure of a motor here introduced, Fig. 67, will give
+an idea of the solution furnished by Mr. Tesla for the specific problem.
+Referring to the drawing, A is the field-magnet, B the armature, C the
+field coils, and D the armature-coils of the motor.
+
+Generally speaking, if the mass of the cores of armature and field be
+equal, the amount of copper or ampere turns of the energizing coils on
+both should also be equal; but these conditions will be modified in
+different forms of machine. It will be understood that these results are
+most advantageous when existing under the conditions presented where the
+motor is running with its normal load, a point to be well borne in
+mind.
+
+
+
+
+CHAPTER XVII.
+
+MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN ARMATURE AND FIELD.
+
+
+In this form of motor, Mr. Tesla's object is to design and build
+machines wherein the maxima of the magnetic effects of the armature and
+field will more nearly coincide than in some of the types previously
+under consideration. These types are: First, motors having two or more
+energizing circuits of the same electrical character, and in the
+operation of which the currents used differ primarily in phase; second,
+motors with a plurality of energizing circuits of different electrical
+character, in or by means of which the difference of phase is produced
+artificially, and, third, motors with a plurality of energizing
+circuits, the currents in one being induced from currents in another.
+Considering the structural and operative conditions of any one of
+them--as, for example, that first named--the armature which is mounted
+to rotate in obedience to the co-operative influence or action of the
+energizing circuits has coils wound upon it which are closed upon
+themselves and in which currents are induced by the energizing-currents
+with the object and result of energizing the armature-core; but under
+any such conditions as must exist in these motors, it is obvious that a
+certain time must elapse between the manifestations of an energizing
+current impulse in the field coils, and the corresponding magnetic state
+or phase in the armature established by the current induced thereby;
+consequently a given magnetic influence or effect in the field which is
+the direct result of a primary current impulse will have become more or
+less weakened or lost before the corresponding effect in the armature
+indirectly produced has reached its maximum. This is a condition
+unfavorable to efficient working in certain cases--as, for instance,
+when the progress of the resultant poles or points of maximum attraction
+is very great, or when a very high number of alternations is
+employed--for it is apparent that a stronger tendency to rotation will
+be maintained if the maximum magnetic attractions or conditions in both
+armature and field coincide, the energy developed by a motor being
+measured by the product of the magnetic quantities of the armature and
+field.
+
+To secure this coincidence of maximum magnetic effects, Mr. Tesla has
+devised various means, as explained below. Fig. 68 is a diagrammatic
+illustration of a Tesla motor system in which the alternating currents
+proceed from independent sources and differ primarily in phase.
+
+[Illustration: FIG. 68.]
+
+[Illustration: FIG. 69.]
+
+A designates the field-magnet or magnetic frame of the motor; B B,
+oppositely located pole-pieces adapted to receive the coils of one
+energizing circuit; and C C, similar pole-pieces for the coils of the
+other energizing circuit. These circuits are designated, respectively,
+by D E, the conductor D'' forming a common return to the generator G.
+Between these poles is mounted an armature--for example, a ring or
+annular armature, wound with a series of coils F, forming a closed
+circuit or circuits. The action or operation of a motor thus constructed
+is now well understood. It will be observed, however, that the magnetism
+of poles B, for example, established by a current impulse in the coils
+thereon, precedes the magnetic effect set up in the armature by the
+induced current in coils F. Consequently the mutual attraction between
+the armature and field-poles is considerably reduced. The same
+conditions will be found to exist if, instead of assuming the poles B or
+C as acting independently, we regard the ideal resultant of both acting
+together, which is the real condition. To remedy this, the motor field
+is constructed with secondary poles B' C', which are situated between
+the others. These pole-pieces are wound with coils D' E', the former in
+derivation to the coils D, the latter to coils E. The main or primary
+coils D and E are wound for a different self-induction from that of the
+coils D' and E', the relations being so fixed that if the currents in D
+and E differ, for example, by a quarter-phase, the currents in each
+secondary coil, as D' E', will differ from those in its appropriate
+primary D or E by, say, forty-five degrees, or one-eighth of a period.
+
+Now, assuming that an impulse or alternation in circuit or branch E is
+just beginning, while in the branch D it is just falling from maximum,
+the conditions are those of a quarter-phase difference. The ideal
+resultant of the attractive forces of the two sets of poles B C
+therefore may be considered as progressing from poles B to poles C,
+while the impulse in E is rising to maximum, and that in D is falling to
+zero or minimum. The polarity set up in the armature, however, lags
+behind the manifestations of field magnetism, and hence the maximum
+points of attraction in armature and field, instead of coinciding, are
+angularly displaced. This effect is counteracted by the supplemental
+poles B' C'. The magnetic phases of these poles succeed those of poles B
+C by the same, or nearly the same, period of time as elapses between the
+effect of the poles B C and the corresponding induced effect in the
+armature; hence the magnetic conditions of poles B' C' and of the
+armature more nearly coincide and a better result is obtained. As poles
+B' C' act in conjunction with the poles in the armature established by
+poles B C, so in turn poles C B act similarly with the poles set up by
+B' C', respectively. Under such conditions the retardation of the
+magnetic effect of the armature and that of the secondary poles will
+bring the maximum of the two more nearly into coincidence and a
+correspondingly stronger torque or magnetic attraction secured.
+
+In such a disposition as is shown in Fig. 68 it will be observed that
+as the adjacent pole-pieces of either circuit are of like polarity they
+will have a certain weakening effect upon one another. Mr. Tesla
+therefore prefers to remove the secondary poles from the direct
+influence of the others. This may be done by constructing a motor with
+two independent sets of fields, and with either one or two armatures
+electrically connected, or by using two armatures and one field. These
+modifications are illustrated further on.
+
+[Illustration: FIG. 70.]
+
+[Illustration: FIG. 71.]
+
+Fig. 69 is a diagrammatic illustration of a motor and system in which
+the difference of phase is artificially produced. There are two coils D
+D in one branch and two coils E E in another branch of the main circuit
+from the generator G. These two circuits or branches are of different
+self-induction, one, as D, being higher than the other. This is
+graphically indicated by making coils D much larger than coils E. By
+reason of the difference in the electrical character of the two
+circuits, the phases of current in one are retarded to a greater extent
+than the other. Let this difference be thirty degrees. A motor thus
+constructed will rotate under the action of an alternating current; but
+as happens in the case previously described the corresponding magnetic
+effects of the armature and field do not coincide owing to the time that
+elapses between a given magnetic effect in the armature and the
+condition of the field that produces it. The secondary or supplemental
+poles B' C' are therefore availed of. There being thirty degrees
+difference of phase between the currents in coils D E, the magnetic
+effect of poles B' C' should correspond to that produced by a current
+differing from the current in coils D or E by fifteen degrees. This we
+can attain by winding each supplemental pole B' C' with two coils H H'.
+The coils H are included in a derived circuit having the same
+self-induction as circuit D, and coils H' in a circuit having the same
+self-induction as circuit E, so that if these circuits differ by thirty
+degrees the magnetism of poles B' C' will correspond to that produced by
+a current differing from that in either D or E by fifteen degrees. This
+is true in all other cases. For example, if in Fig. 68 the coils D' E'
+be replaced by the coils H H' included in the derived circuits, the
+magnetism of the poles B' C' will correspond in effect or phase, if it
+may be so termed, to that produced by a current differing from that in
+either circuit D or E by forty-five degrees, or one-eighth of a period.
+
+This invention as applied to a derived circuit motor is illustrated in
+Figs. 70 and 71. The former is an end view of the motor with the
+armature in section and a diagram of connections, and Fig. 71 a vertical
+section through the field. These figures are also drawn to show one of
+the dispositions of two fields that may be adopted in carrying out the
+principle. The poles B B C C are in one field, the remaining poles in
+the other. The former are wound with primary coils I J and secondary
+coils I' J', the latter with coils K L. The primary coils I J are in
+derived circuits, between which, by reason of their different
+self-induction, there is a difference of phase, say, of thirty degrees.
+The coils I' K are in circuit with one another, as also are coils J' L,
+and there should be a difference of phase between the currents in coils
+K and L and their corresponding primaries of, say, fifteen degrees. If
+the poles B C are at right angles, the armature-coils should be
+connected directly across, or a single armature core wound from end to
+end may be used; but if the poles B C be in line there should be an
+angular displacement of the armature coils, as will be well understood.
+
+The operation will be understood from the foregoing. The maximum
+magnetic condition of a pair of poles, as B' B', coincides closely with
+the maximum effect in the armature, which lags behind the corresponding
+condition in poles B B.
+
+
+
+
+CHAPTER XVIII.
+
+MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZATION OF THE INNER
+AND OUTER PARTS OF AN IRON CORE.
+
+
+It is well known that if a magnetic core, even if laminated or
+subdivided, be wound with an insulated coil and a current of electricity
+be directed through the coil, the magnetization of the entire core does
+not immediately ensue, the magnetizing effect not being exhibited in all
+parts simultaneously. This may be attributed to the fact that the action
+of the current is to energize first those laminae or parts of the core
+nearest the surface and adjacent to the exciting-coil, and from thence
+the action progresses toward the interior. A certain interval of time
+therefore elapses between the manifestation of magnetism in the external
+and the internal sections or layers of the core. If the core be thin or
+of small mass, this effect may be inappreciable; but in the case of a
+thick core, or even of a comparatively thin one, if the number of
+alternations or rate of change of the current strength be very great,
+the time interval occurring between the manifestations of magnetism in
+the interior of the core and in those parts adjacent to the coil is more
+marked. In the construction of such apparatus as motors which are
+designed to be run by alternating or equivalent currents--such as
+pulsating or undulating currents generally--Mr. Tesla found it desirable
+and even necessary to give due consideration to this phenomenon and to
+make special provisions in order to obviate its consequences. With the
+specific object of taking advantage of this action or effect, and to
+render it more pronounced, he constructs a field magnet in which the
+parts of the core or cores that exhibit at different intervals of time
+the magnetic effect imparted to them by alternating or equivalent
+currents in an energizing coil or coils, are so placed with relation to
+a rotating armature as to exert thereon their attractive effect
+successively in the order of their magnetization. By this means he
+secures a result similar to that which he had previously attained in
+other forms or types of motor in which by means of one or more
+alternating currents he has produced the rotation or progression of the
+magnetic poles.
+
+This new mode of operation will now be described. Fig. 72 is a side
+elevation of such motor. Fig. 73 is a side elevation of a more
+practicable and efficient embodiment of the invention. Fig. 74 is a
+central vertical section of the same in the plane of the axis of
+rotation.
+
+[Illustration: FIGS. 72 and 73.]
+
+Referring to Fig. 72, let X represent a large iron core, which may be
+composed of a number of sheets or laminae of soft iron or steel.
+Surrounding this core is a coil Y, which is connected with a source E of
+rapidly varying currents. Let us consider now the magnetic conditions
+existing in this core at any point, as _b_, at or near the centre, and
+any other point, as _a_, nearer the surface. When a current impulse is
+started in the magnetizing coil Y, the section or part at _a_, being
+close to the coil, is immediately energized, while the section or part
+at _b_, which, to use a convenient expression, is "protected" by the
+intervening sections or layers between _a_ and _b_, does not at once
+exhibit its magnetism. However, as the magnetization of _a_ increases,
+_b_ becomes also affected, reaching finally its maximum strength some
+time later than _a_. Upon the weakening of the current the magnetization
+of _a_ first diminishes, while _b_ still exhibits its maximum strength;
+but the continued weakening of _a_ is attended by a subsequent weakening
+of _b_. Assuming the current to be an alternating one, _a_ will now be
+reversed, while _b_ still continues of the first imparted polarity. This
+action continues the magnetic condition of _b_, following that of _a_ in
+the manner above described. If an armature--for instance, a simple disc
+F, mounted to rotate freely on an axis--be brought into proximity to the
+core, a movement of rotation will be imparted to the disc, the direction
+depending upon its position relatively to the core, the tendency being
+to turn the portion of the disc nearest to the core from _a_ to _b_, as
+indicated in Fig. 72.
+
+[Illustration: FIG. 74.]
+
+This action or principle of operation has been embodied in a practicable
+form of motor, which is illustrated in Fig. 73. Let A in that figure
+represent a circular frame of iron, from diametrically opposite points
+of the interior of which the cores project. Each core is composed of
+three main parts B, B and C, and they are similarly formed with a
+straight portion or body _e_, around which the energizing coil is wound,
+a curved arm or extension _c_, and an inwardly projecting pole or end
+_d_. Each core is made up of two parts B B, with their polar extensions
+reaching in one direction, and a part C between the other two, and with
+its polar extension reaching in the opposite direction. In order to
+lessen in the cores the circulation of currents induced therein, the
+several sections are insulated from one another in the manner usually
+followed in such cases. These cores are wound with coils D, which are
+connected in the same circuit, either in parallel or series, and
+supplied with an alternating or a pulsating current, preferably the
+former, by a generator E, represented diagrammatically. Between the
+cores or their polar extensions is mounted a cylindrical or similar
+armature F, wound with magnetizing coils G, closed upon themselves.
+
+The operation of this motor is as follows: When a current impulse or
+alternation is directed through the coils D, the sections B B of the
+cores, being on the surface and in close proximity to the coils, are
+immediately energized. The sections C, on the other hand, are protected
+from the magnetizing influence of the coil by the interposed layers of
+iron B B. As the magnetism of B B increases, however, the sections C are
+also energized; but they do not attain their maximum strength until a
+certain time subsequent to the exhibition by the sections B B of their
+maximum. Upon the weakening of the current the magnetic strength of B B
+first diminishes, while the sections C have still their maximum
+strength; but as B B continue to weaken the interior sections are
+similarly weakened. B B may then begin to exhibit an opposite polarity,
+which is followed later by a similar change on C, and this action
+continues. B B and C may therefore be considered as separate
+field-magnets, being extended so as to act on the armature in the most
+efficient positions, and the effect is similar to that in the other
+forms of Tesla motor--viz., a rotation or progression of the maximum
+points of the field of force. Any armature--such, for instance, as a
+disc--mounted in this field would rotate from the pole first to exhibit
+its magnetism to that which exhibits it later.
+
+It is evident that the principle here described may be carried out in
+conjunction with other means for securing a more favorable or efficient
+action of the motor. For example, the polar extensions of the sections C
+may be wound or surrounded by closed coils. The effect of these coils
+will be to still more effectively retard the magnetization of the polar
+extensions of C.
+
+
+
+
+CHAPTER XIX.
+
+ANOTHER TYPE OF TESLA INDUCTION MOTOR.
+
+
+It will have been gathered by all who are interested in the advance of
+the electrical arts, and who follow carefully, step by step, the work of
+pioneers, that Mr. Tesla has been foremost to utilize inductive effects
+in permanently closed circuits, in the operation of alternating motors.
+In this chapter one simple type of such a motor is described and
+illustrated, which will serve as an exemplification of the principle.
+
+Let it be assumed that an ordinary alternating current generator is
+connected up in a circuit of practically no self-induction, such, for
+example, as a circuit containing incandescent lamps only. On the
+operation of the machine, alternating currents will be developed in the
+circuit, and the phases of these currents will theoretically coincide
+with the phases of the impressed electromotive force. Such currents may
+be regarded and designated as the "unretarded currents."
+
+It will be understood, of course, that in practice there is always more
+or less self-induction in the circuit, which modifies to a corresponding
+extent these conditions; but for convenience this may be disregarded in
+the consideration of the principle of operation, since the same laws
+apply. Assume next that a path of currents be formed across any two
+points of the above circuit, consisting, for example, of the primary of
+an induction device. The phases of the currents passing through the
+primary, owing to the self-induction of the same, will not coincide with
+the phases of the impressed electromotive force, but will lag behind,
+such lag being directly proportional to the self-induction and inversely
+proportional to the resistance of the said coil. The insertion of this
+coil will also cause a lagging or retardation of the currents traversing
+and delivered by the generator behind the impressed electromotive force,
+such lag being the mean or resultant of the lag of the current through
+the primary alone and of the "unretarded current" in the entire working
+circuit. Next consider the conditions imposed by the association in
+inductive relation with the primary coil, of a secondary coil. The
+current generated in the secondary coil will react upon the primary
+current, modifying the retardation of the same, according to the amount
+of self-induction and resistance in the secondary circuit. If the
+secondary circuit has but little self-induction--as, for instance, when
+it contains incandescent lamps only--it will increase the actual
+difference of phase between its own and the primary current, first, by
+diminishing the lag between the primary current and the impressed
+electromotive force, and, second, by its own lag or retardation behind
+the impressed electromotive force. On the other hand, if the secondary
+circuit have a high self-induction, its lag behind the current in the
+primary is directly increased, while it will be still further increased
+if the primary have a very low self-induction. The better results are
+obtained when the primary has a low self-induction.
+
+[Illustration: FIG. 75.]
+
+[Illustration: FIG. 76.]
+
+Fig. 75 is a diagram of a Tesla motor embodying this principle. Fig. 76
+is a similar diagram of a modification of the same. In Fig. 75 let A
+designate the field-magnet of a motor which, as in all these motors, is
+built up of sections or plates. B C are polar projections upon which the
+coils are wound. Upon one pair of these poles, as C, are wound primary
+coils D, which are directly connected to the circuit of an alternating
+current generator G. On the same poles are also wound secondary coils F,
+either side by side or over or under the primary coils, and these are
+connected with other coils E, which surround the poles B B. The
+currents in both primary and secondary coils in such a motor will be
+retarded or will lag behind the impressed electromotive force; but to
+secure a proper difference in phase between the primary and secondary
+currents themselves, Mr. Tesla increases the resistance of the circuit
+of the secondary and reduces as much as practicable its self-induction.
+This is done by using for the secondary circuit, particularly in the
+coils E, wire of comparatively small diameter and having but few turns
+around the cores; or by using some conductor of higher specific
+resistance, such as German silver; or by introducing at some point in
+the secondary circuit an artificial resistance R. Thus the
+self-induction of the secondary is kept down and its resistance
+increased, with the result of decreasing the lag between the impressed
+electro-motive force and the current in the primary coils and increasing
+the difference of phase between the primary and secondary currents.
+
+In the disposition shown in Fig. 76, the lag in the secondary is
+increased by increasing the self-induction of that circuit, while the
+increasing tendency of the primary to lag is counteracted by inserting
+therein a dead resistance. The primary coils D in this case have a low
+self-induction and high resistance, while the coils E F, included in the
+secondary circuit, have a high self-induction and low resistance. This
+may be done by the proper winding of the coils; or in the circuit
+including the secondary coils E F, we may introduce a self-induction
+coil S, while in the primary circuit from the generator G and including
+coils D, there may be inserted a dead resistance R. By this means the
+difference of phase between the primary and secondary is increased. It
+is evident that both means of increasing the difference of
+phase--namely, by the special winding as well as by the supplemental or
+external inductive and dead resistance--may be employed conjointly.
+
+In the operation of this motor the current impulses in the primary coils
+induce currents in the secondary coils, and by the conjoint action of
+the two the points of greatest magnetic attraction are shifted or
+rotated.
+
+In practice it is found desirable to wind the armature with closed coils
+in which currents are induced by the action thereon of the primaries.
+
+
+
+
+CHAPTER XX.
+
+COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR.
+
+
+In the preceding descriptions relative to synchronizing motors and
+methods of operating them, reference has been made to the plan adopted
+by Mr. Tesla, which consists broadly in winding or arranging the motor
+in such manner that by means of suitable switches it could be started as
+a multiple-circuit motor, or one operating by a progression of its
+magnetic poles, and then, when up to speed, or nearly so, converted into
+an ordinary synchronizing motor, or one in which the magnetic poles were
+simply alternated. In some cases, as when a large motor is used and when
+the number of alternations is very high, there is more or less
+difficulty in bringing the motor to speed as a double or
+multiple-circuit motor, for the plan of construction which renders the
+motor best adapted to run as a synchronizing motor impairs its
+efficiency as a torque or double-circuit motor under the assumed
+conditions on the start. This will be readily understood, for in a large
+synchronizing motor the length of the magnetic circuit of the polar
+projections, and their mass, are so great that apparently considerable
+time is required for magnetization and demagnetization. Hence with a
+current of a very high number of alternations the motor may not respond
+properly. To avoid this objection and to start up a synchronizing motor
+in which these conditions obtain, Mr. Tesla has combined two motors, one
+a synchronizing motor, the other a multiple-circuit or torque motor, and
+by the latter he brings the first-named up to speed, and then either
+throws the whole current into the synchronizing motor or operates
+jointly both of the motors.
+
+This invention involves several novel and useful features. It will be
+observed, in the first place, that both motors are run, without
+commutators of any kind, and, secondly, that the speed of the torque
+motor may be higher than that of the synchronizing motor, as will be the
+case when it contains a fewer number of poles or sets of poles, so that
+the motor will be more readily and easily brought up to speed. Thirdly,
+the synchronizing motor may be constructed so as to have a much more
+pronounced tendency to synchronism without lessening the facility with
+which it is started.
+
+Fig. 77 is a part sectional view of the two motors; Fig. 78 an end view
+of the synchronizing motor; Fig. 79 an end view and part section of the
+torque or double-circuit motor; Fig. 80 a diagram of the circuit
+connections employed; and Figs. 81, 82, 83, 84 and 85 are diagrams of
+modified dispositions of the two motors.
+
+[Illustration: FIG. 77.]
+
+Inasmuch as neither motor is doing any work while the current is acting
+upon the other, the two armatures are rigidly connected, both being
+mounted upon the same shaft A, the field-magnets B of the synchronizing
+and C of the torque motor being secured to the same base D. The
+preferably larger synchronizing motor has polar projections on its
+armature, which rotate in very close proximity to the poles of the
+field, and in other respects it conforms to the conditions that are
+necessary to secure synchronous action. The pole-pieces of the armature
+are, however, wound with closed coils E, as this obviates the employment
+of sliding contacts. The smaller or torque motor, on the other hand,
+has, preferably, a cylindrical armature F, without polar projections and
+wound with closed coils G. The field-coils of the torque motor are
+connected up in two series H and I, and the alternating current from the
+generator is directed through or divided between these two circuits in
+any manner to produce a progression of the poles or points of maximum
+magnetic effect. This result is secured by connecting the two
+motor-circuits in derivation with the circuit from the generator,
+inserting in one motor circuit a dead resistance and in the other a
+self-induction coil, by which means a difference in phase between the
+two divisions of the current is secured. If both motors have the same
+number of field poles, the torque motor for a given number of
+alternations will tend to run at double the speed of the other, for,
+assuming the connections to be such as to give the best results, its
+poles are divided into two series and the number of poles is virtually
+reduced one-half, which being acted upon by the same number of
+alternations tend to rotate the armature at twice the speed. By this
+means the main armature is more easily brought to or above the required
+speed. When the speed necessary for synchronism is imparted to the main
+motor, the current is shifted from the torque motor into the other.
+
+[Illustration: FIG. 78.]
+
+[Illustration: FIG. 79.]
+
+A convenient arrangement for carrying out this invention is shown in
+Fig. 80, in which J J are the field coils of the synchronizing, and H I
+the field coils of the torque motor. L L' are the conductors of the main
+line. One end of, say, coils H is connected to wire L through a
+self-induction coil M. One end of the other set of coils I is connected
+to the same wire through a dead resistance N. The opposite ends of these
+two circuits are connected to the contact _m_ of a switch, the handle or
+lever of which is in connection with the line-wire L'. One end of the
+field circuit of the synchronizing motor is connected to the wire L. The
+other terminates in the switch-contact _n_. From the diagram it will be
+readily seen that if the lever P be turned upon contact _m_, the torque
+motor will start by reason of the difference of phase between the
+currents in its two energizing circuits. Then when the desired speed is
+attained, if the lever P be shifted upon contact _n_ the entire current
+will pass through the field coils of the synchronizing motor and the
+other will be doing no work.
+
+The torque motor may be constructed and operated in various ways, many
+of which have already been touched upon. It is not necessary that one
+motor be cut out of circuit while the other is in, for both may be acted
+upon by current at the same time, and Mr. Tesla has devised various
+dispositions or arrangements of the two motors for accomplishing this.
+Some of these arrangements are illustrated in Figs. 81 to 85.
+
+[Illustration: FIG. 80.]
+
+Referring to Fig. 81, let T designate the torque or multiple circuit
+motor and S the synchronizing motor, L L' being the line-wires from a
+source of alternating current. The two circuits of the torque motor of
+different degrees of self-induction, and designated by N M, are
+connected in derivation to the wire L. They are then joined and
+connected to the energizing circuit of the synchronizing motor, the
+opposite terminal of which is connected to wire L'. The two motors are
+thus in series. To start them Mr. Tesla short-circuits the synchronizing
+motor by a switch P', throwing the whole current through the torque
+motor. Then when the desired speed is reached the switch P' is opened,
+so that the current passes through both motors. In such an arrangement
+as this it is obviously desirable for economical and other reasons that
+a proper relation between the speeds of the two motors should be
+observed.
+
+In Fig. 82 another disposition is illustrated. S is the synchronizing
+motor and T the torque motor, the circuits of both being in parallel. W
+is a circuit also in derivation to the motor circuits and containing a
+switch P''. S' is a switch in the synchronizing motor circuit. On the
+start the switch S' is opened, cutting out the motor S. Then P'' is
+opened, throwing the entire current through the motor T, giving it a
+very strong torque. When the desired speed is reached, switch S' is
+closed and the current divides between both motors. By means of switch
+P'' both motors may be cut out.
+
+[Illustration: FIGS. 81, 82, 83, 84 and 85.]
+
+In Fig. 83 the arrangement is substantially the same, except that a
+switch T' is placed in the circuit which includes the two circuits of
+the torque motor. Fig. 84 shows the two motors in series, with a shunt
+around both containing a switch S T. There is also a shunt around the
+synchronizing motor S, with a switch P'. In Fig. 85 the same disposition
+is shown; but each motor is provided with a shunt, in which are switches
+P' and T'', as shown.
+
+
+
+
+CHAPTER XXI.
+
+MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT.
+
+
+We now come to a new class of motors in which resort is had to
+condensers for the purpose of developing the required difference of
+phase and neutralizing the effects of self-induction. Mr. Tesla early
+began to apply the condenser to alternating apparatus, in just how many
+ways can only be learned from a perusal of other portions of this
+volume, especially those dealing with his high frequency work.
+
+Certain laws govern the action or effects produced by a condenser when
+connected to an electric circuit through which an alternating or in
+general an undulating current is made to pass. Some of the most
+important of such effects are as follows: First, if the terminals or
+plates of a condenser be connected with two points of a circuit, the
+potentials of which are made to rise and fall in rapid succession, the
+condenser allows the passage, or more strictly speaking, the
+transference of a current, although its plates or armatures may be so
+carefully insulated as to prevent almost completely the passage of a
+current of unvarying strength or direction and of moderate electromotive
+force. Second, if a circuit, the terminals of which are connected with
+the plates of the condenser, possess a certain self-induction, the
+condenser will overcome or counteract to a greater or less degree,
+dependent upon well-understood conditions, the effects of such
+self-induction. Third, if two points of a closed or complete circuit
+through which a rapidly rising and falling current flows be shunted or
+bridged by a condenser, a variation in the strength of the currents in
+the branches and also a difference of phase of the currents therein is
+produced. These effects Mr. Tesla has utilized and applied in a variety
+of ways in the construction and operation of his motors, such as by
+producing a difference in phase in the two energizing circuits of an
+alternating current motor by connecting the two circuits in derivation
+and connecting up a condenser in series in one of the circuits. A
+further development, however, possesses certain novel features of
+practical value and involves a knowledge of facts less generally
+understood. It comprises the use of a condenser or condensers in
+connection with the induced or armature circuit of a motor and certain
+details of the construction of such motors. In an alternating current
+motor of the type particularly referred to above, or in any other which
+has an armature coil or circuit closed upon itself, the latter
+represents not only an inductive resistance, but one which is
+periodically varying in value, both of which facts complicate and
+render difficult the attainment of the conditions best suited to the
+most efficient working conditions; in other words, they require, first,
+that for a given inductive effect upon the armature there should be the
+greatest possible current through the armature or induced coils, and,
+second, that there should always exist between the currents in the
+energizing and the induced circuits a given relation of phase. Hence
+whatever tends to decrease the self-induction and increase the current
+in the induced circuits will, other things being equal, increase the
+output and efficiency of the motor, and the same will be true of causes
+that operate to maintain the mutual attractive effect between the field
+magnets and armature at its maximum. Mr. Tesla secures these results by
+connecting with the induced circuit or circuits a condenser, in the
+manner described below, and he also, with this purpose in view,
+constructs the motor in a special manner.
+
+[Illustration: FIG. 86.]
+
+[Illustration: FIG. 88.]
+
+[Illustration: FIG. 89.]
+
+[Illustration: FIG. 87.]
+
+[Illustration: FIG. 90.]
+
+Referring to the drawings, Fig. 86, is a view, mainly diagrammatic, of
+an alternating current motor, in which the present principle is applied.
+Fig. 87 is a central section, in line with the shaft, of a special form
+of armature core. Fig. 88 is a similar section of a modification of the
+same. Fig. 89 is one of the sections of the core detached. Fig. 90 is a
+diagram showing a modified disposition of the armature or induced
+circuits.
+
+The general plan of the invention is illustrated in Fig. 86. A A in this
+figure represent the the frame and field magnets of an alternating
+current motor, the poles or projections of which are wound with coils B
+and C, forming independent energizing circuits connected either to the
+same or to independent sources of alternating currents, so that the
+currents flowing through the circuits, respectively, will have a
+difference of phase. Within the influence of this field is an armature
+core D, wound with coils E. In motors of this description heretofore
+these coils have been closed upon themselves, or connected in a closed
+series; but in the present case each coil or the connected series of
+coils terminates in the opposite plates of a condenser F. For this
+purpose the ends of the series of coils are brought out through the
+shaft to collecting rings G, which are connected to the condenser by
+contact brushes H and suitable conductors, the condenser being
+independent of the machine. The armature coils are wound or connected in
+such manner that adjacent coils produce opposite poles.
+
+The action of this motor and the effect of the plan followed in its
+construction are as follows: The motor being started in operation and
+the coils of the field magnets being traversed by alternating currents,
+currents are induced in the armature coils by one set of field coils, as
+B, and the poles thus established are acted upon by the other set, as C.
+The armature coils, however, have necessarily a high self-induction,
+which opposes the flow of the currents thus set up. The condenser F not
+only permits the passage or transference of these currents, but also
+counteracts the effects of self-induction, and by a proper adjustment of
+the capacity of the condenser, the self-induction of the coils, and the
+periods of the currents, the condenser may be made to overcome entirely
+the effect of self-induction.
+
+It is preferable on account of the undesirability of using sliding
+contacts of any kind, to associate the condenser with the armature
+directly, or make it a part of the armature. In some cases Mr. Tesla
+builds up the armature of annular plates K K, held by bolts L between
+heads M, which are secured to the driving shaft, and in the hollow space
+thus formed he places a condenser F, generally by winding the two
+insulated plates spirally around the shaft. In other cases he utilizes
+the plates of the core itself as the plates of the condenser. For
+example, in Figs. 88 and 89, N is the driving shaft, M M are the heads
+of the armature-core, and K K' the iron plates of which the core is
+built up. These plates are insulated from the shaft and from one
+another, and are held together by rods or bolts L. The bolts pass
+through a large hole in one plate and a small hole in the one next
+adjacent, and so on, connecting electrically all of plates K, as one
+armature of a condenser, and all of plates K' as the other.
+
+To either of the condensers above described the armature coils may be
+connected, as explained by reference to Fig. 86.
+
+In motors in which the armature coils are closed upon themselves--as,
+for example, in any form of alternating current motor in which one
+armature coil or set of coils is in the position of maximum induction
+with respect to the field coils or poles, while the other is in the
+position of minimum induction--the coils are best connected in one
+series, and two points of the circuit thus formed are bridged by a
+condenser. This is illustrated in Fig. 90, in which E represents one set
+of armature coils and E' the other. Their points of union are joined
+through a condenser F. It will be observed that in this disposition the
+self-induction of the two branches E and E' varies with their position
+relatively to the field magnet, and that each branch is alternately the
+predominating source of the induced current. Hence the effect of the
+condenser F is twofold. First, it increases the current in each of the
+branches alternately, and, secondly, it alters the phase of the currents
+in the branches, this being the well-known effect which results from
+such a disposition of a condenser with a circuit, as above described.
+This effect is favorable to the proper working of the motor, because it
+increases the flow of current in the armature circuits due to a given
+inductive effect, and also because it brings more nearly into
+coincidence the maximum magnetic effects of the coacting field and
+armature poles.
+
+It will be understood, of course, that the causes that contribute to the
+efficiency of condensers when applied to such uses as the above must be
+given due consideration in determining the practicability and efficiency
+of the motors. Chief among these is, as is well known, the periodicity
+of the current, and hence the improvements described are more
+particularly adapted to systems in which a very high rate of alternation
+or change is maintained.
+
+Although this invention has been illustrated in connection with a
+special form of motor, it will be understood that it is equally
+applicable to any other alternating current motor in which there is a
+closed armature coil wherein the currents are induced by the action of
+the field, and the feature of utilizing the plates or sections of a
+magnetic core for forming the condenser is applicable, generally, to
+other kinds of alternating current apparatus.
+
+
+
+
+CHAPTER XXII.
+
+MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS.
+
+
+If the field or energizing circuits of a rotary phase motor be both
+derived from the same source of alternating currents and a condenser of
+proper capacity be included in one of the same, approximately, the
+desired difference of phase may be obtained between the currents flowing
+directly from the source and those flowing through the condenser; but
+the great size and expense of condensers for this purpose that would
+meet the requirements of the ordinary systems of comparatively low
+potential are particularly prohibitory to their employment.
+
+Another, now well-known, method or plan of securing a difference of
+phase between the energizing currents of motors of this kind is to
+induce by the currents in one circuit those in the other circuit or
+circuits; but as no means had been proposed that would secure in this
+way between the phases of the primary or inducing and the secondary or
+induced currents that difference--theoretically ninety degrees--that is
+best adapted for practical and economical working, Mr. Tesla devised a
+means which renders practicable both the above described plans or
+methods, and by which he is enabled to obtain an economical and
+efficient alternating current motor. His invention consists in placing a
+condenser in the secondary or induced circuit of the motor above
+described and raising the potential of the secondary currents to such a
+degree that the capacity of the condenser, which is in part dependent on
+the potential, need be quite small. The value of this condenser is
+determined in a well-understood manner with reference to the
+self-induction and other conditions of the circuit, so as to cause the
+currents which pass through it to differ from the primary currents by a
+quarter phase.
+
+Fig. 91 illustrates the invention as embodied in a motor in which the
+inductive relation of the primary and secondary circuits is secured by
+winding them inside the motor partly upon the same cores; but the
+invention applies, generally, to other forms of motor in which one of
+the energizing currents is induced in any way from the other.
+
+Let A B represent the poles of an alternating current motor, of which C
+is the armature wound with coils D, closed upon themselves, as is now
+the general practice in motors of this kind. The poles A, which
+alternate with poles B, are wound with coils of ordinary or coarse wire
+E in such direction as to make them of alternate north and south
+polarity, as indicated in the diagram by the characters N S. Over these
+coils, or in other inductive relation to the same, are wound long
+fine-wire coils F F, and in the same direction throughout as the coils
+E. These coils are secondaries, in which currents of very high potential
+are induced. All the coils E in one series are connected, and all the
+secondaries F in another.
+
+[Illustration: FIG. 91.]
+
+On the intermediate poles B are wound fine-wire energizing coils G,
+which are connected in series with one another, and also with the series
+of secondary coils F, the direction of winding being such that a
+current-impulse induced from the primary coils E imparts the same
+magnetism to the poles B as that produced in poles A by the primary
+impulse. This condition is indicated by the characters N' S'.
+
+In the circuit formed by the two sets of coils F and G is introduced a
+condenser H; otherwise this circuit is closed upon itself, while the
+free ends of the circuit of coils E are connected to a source of
+alternating currents. As the condenser capacity which is needed in any
+particular motor of this kind is dependent upon the rate of alternation
+or the potential, or both, its size or cost, as before explained, may be
+brought within economical limits for use with the ordinary circuits if
+the potential of the secondary circuit in the motor be sufficiently
+high. By giving to the condenser proper values, any desired difference
+of phase between the primary and secondary energizing circuits may be
+obtained.
+
+
+
+
+CHAPTER XXIII.
+
+TESLA POLYPHASE TRANSFORMER.
+
+
+Applying the polyphase principle to the construction of transformers as
+well to the motors already noticed, Mr. Tesla has invented some very
+interesting forms, which he considers free from the defects of earlier
+and, at present, more familiar forms. In these transformers he provides
+a series of inducing coils and corresponding induced coils, which are
+generally wound upon a core closed upon itself, usually a ring of
+laminated iron.
+
+The two sets of coils are wound side by side or superposed or otherwise
+placed in well-known ways to bring them into the most effective
+relations to one another and to the core. The inducing or primary coils
+wound on the core are divided into pairs or sets by the proper
+electrical connections, so that while the coils of one pair or set
+co-operate in fixing the magnetic poles of the core at two given
+diametrically opposite points, the coils of the other pair or
+set--assuming, for sake of illustration, that there are but two--tend to
+fix the poles ninety degrees from such points. With this induction
+device is used an alternating current generator with coils or sets of
+coils to correspond with those of the converter, and the corresponding
+coils of the generator and converter are then connected up in
+independent circuits. It results from this that the different electrical
+phases in the generator are attended by corresponding magnetic changes
+in the converter; or, in other words, that as the generator coils
+revolve, the points of greatest magnetic intensity in the converter will
+be progressively shifted or whirled around.
+
+Fig. 92 is a diagrammatic illustration of the converter and the
+electrical connections of the same. Fig. 93 is a horizontal central
+cross-section of Fig. 92. Fig. 94 is a diagram of the circuits of the
+entire system, the generator being shown in section.
+
+Mr. Tesla uses a core, A, which is closed upon itself--that is to say,
+of an annular cylindrical or equivalent form--and as the efficiency of
+the apparatus is largely increased by the subdivision of this core, he
+makes it of thin strips, plates, or wires of soft iron electrically
+insulated as far as practicable. Upon this core are wound, say, four
+coils, B B B' B', used as primary coils, and for which long lengths of
+comparatively fine wire are employed. Over these coils are then wound
+shorter coils of coarser wire, C C C' C', to constitute the induced or
+secondary coils. The construction of this or any equivalent form of
+converter may be carried further, as above pointed out, by inclosing
+these coils with iron--as, for example, by winding over the coils layers
+of insulated iron wire.
+
+[Illustration: FIGS. 92 and 93.]
+
+[Illustration: FIG. 94.]
+
+The device is provided with suitable binding posts, to which the ends of
+the coils are led. The diametrically opposite coils B B and B' B' are
+connected, respectively, in series, and the four terminals are connected
+to the binding posts. The induced coils are connected together in any
+desired manner. For example, as shown in Fig. 94, C C may be connected
+in multiple arc when a quantity current is desired--as for running a
+group of incandescent lamps--while C' C' may be independently connected
+in series in a circuit including arc lamps or the like. The generator in
+this system will be adapted to the converter in the manner illustrated.
+For example, in the present case there are employed a pair of ordinary
+permanent or electro-magnets, E E, between which is mounted a
+cylindrical armature on a shaft, F, and wound with two coils, G G'. The
+terminals of these coils are connected, respectively, to four insulated
+contact or collecting rings, H H H' H', and the four line circuit wires
+L connect the brushes K, bearing on these rings, to the converter in the
+order shown. Noting the results of this combination, it will be observed
+that at a given point of time the coil G is in its neutral position and
+is generating little or no current, while the other coil, G', is in a
+position where it exerts its maximum effect. Assuming coil G to be
+connected in circuit with coils B B of the converter, and coil G' with
+coils B' B', it is evident that the poles of the ring A will be
+determined by coils B' B' alone; but as the armature of the generator
+revolves, coil G develops more current and coil G' less, until G reaches
+its maximum and G' its neutral position. The obvious result will be to
+shift the poles of the ring A through one-quarter of its periphery. The
+movement of the coils through the next quarter of a turn--during which
+coil G' enters a field of opposite polarity and generates a current of
+opposite direction and increasing strength, while coil G, in passing
+from its maximum to its neutral position generates a current of
+decreasing strength and same direction as before--causes a further
+shifting of the poles through the second quarter of the ring. The second
+half-revolution will obviously be a repetition of the same action. By
+the shifting of the poles of the ring A, a powerful dynamic inductive
+effect on the coils C C' is produced. Besides the currents generated in
+the secondary coils by dynamo-magnetic induction, other currents will be
+set up in the same coils in consequence of many variations in the
+intensity of the poles in the ring A. This should be avoided by
+maintaining the intensity of the poles constant, to accomplish which
+care should be taken in designing and proportioning the generator and in
+distributing the coils in the ring A, and balancing their effect. When
+this is done, the currents are produced by dynamo-magnetic induction
+only, the same result being obtained as though the poles were shifted by
+a commutator with an infinite number of segments.
+
+The modifications which are applicable to other forms of converter are
+in many respects applicable to this, such as those pertaining more
+particularly to the form of the core, the relative lengths and
+resistances of the primary and secondary coils, and the arrangements for
+running or operating the same.
+
+
+
+
+CHAPTER XXIV.
+
+A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD BETWEEN COILS OF
+PRIMARY AND SECONDARY.
+
+
+Mr. Tesla has applied his principle of magnetic shielding of parts to
+the construction also of transformers, the shield being interposed
+between the primary and secondary coils. In transformers of the ordinary
+type it will be found that the wave of electromotive force of the
+secondary very nearly coincides with that of the primary, being,
+however, in opposite sign. At the same time the currents, both primary
+and secondary, lag behind their respective electromotive forces; but as
+this lag is practically or nearly the same in the case of each it
+follows that the maximum and minimum of the primary and secondary
+currents will nearly coincide, but differ in sign or direction, provided
+the secondary be not loaded or if it contain devices having the property
+of self-induction. On the other hand, the lag of the primary behind the
+impressed electromotive force may be diminished by loading the secondary
+with a non-inductive or dead resistance--such as incandescent
+lamps--whereby the time interval between the maximum or minimum periods
+of the primary and secondary currents is increased. This time interval,
+however, is limited, and the results obtained by phase difference in the
+operation of such devices as the Tesla alternating current motors can
+only be approximately realized by such means of producing or securing
+this difference, as above indicated, for it is desirable in such cases
+that there should exist between the primary and secondary currents, or
+those which, however produced, pass through the two circuits of the
+motor, a difference of phase of ninety degrees; or, in other words, the
+current in one circuit should be a maximum when that in the other
+circuit is a minimum. To attain to this condition more perfectly, an
+increased retardation of the secondary current is secured in the
+following manner: Instead of bringing the primary and secondary coils or
+circuits of a transformer into the closest possible relations, as has
+hitherto been done, Mr. Tesla protects in a measure the secondary from
+the inductive action or effect of the primary by surrounding either the
+primary or the secondary with a comparatively thin magnetic shield or
+screen. Under these modified conditions, as long as the primary current
+has a small value, the shield protects the secondary; but as soon as the
+primary current has reached a certain strength, which is arbitrarily
+determined, the protecting magnetic shield becomes saturated and the
+inductive action upon the secondary begins. It results, therefore, that
+the secondary current begins to flow at a certain fraction of a period
+later than it would without the interposed shield, and since this
+retardation may be obtained without necessarily retarding the primary
+current also, an additional lag is secured, and the time interval
+between the maximum or minimum periods of the primary and secondary
+currents is increased. Such a transformer may, by properly proportioning
+its several elements and determining the proper relations between the
+primary and secondary windings, the thickness of the magnetic shield,
+and other conditions, be constructed to yield a constant current at all
+loads.
+
+[Illustration: FIG. 95.]
+
+Fig. 95 is a cross-section of a transformer embodying this improvement.
+Fig. 96 is a similar view of a modified form of transformer, showing
+diagrammatically the manner of using the same.
+
+A A is the main core of the transformer, composed of a ring of soft
+annealed and insulated or oxidized iron wire. Upon this core is wound
+the secondary circuit or coil B B. This latter is then covered with a
+layer or layers of annealed and insulated iron wires C C, wound in a
+direction at right angles to the secondary coil. Over the whole is then
+wound the primary coil or wire D D. From the nature of this construction
+it will be obvious that as long as the shield formed by the wires C is
+below magnetic saturation the secondary coil or circuit is effectually
+protected or shielded from the inductive influence of the primary,
+although on open circuit it may exhibit some electromotive force. When
+the strength of the primary reaches a certain value, the shield C,
+becoming saturated, ceases to protect the secondary from inductive
+action, and current is in consequence developed therein. For similar
+reasons, when the primary current weakens, the weakening of the
+secondary is retarded to the same or approximately the same extent.
+
+[Illustration: FIG. 96.]
+
+The specific construction of the transformer is largely immaterial. In
+Fig. 96, for example, the core A is built up of thin insulated iron
+plates or discs. The primary circuit D is wound next the core A. Over
+this is applied the shield C, which in this case is made up of thin
+strips or plates of iron properly insulated and surrounding the primary,
+forming a closed magnetic circuit. The secondary B is wound over the
+shield C. In Fig. 96, also, E is a source of alternating or rapidly
+changing currents. The primary of the transformer is connected with the
+circuit of the generator. F is a two-circuit alternating current motor,
+one of the circuits being connected with the main circuit from the
+source E, and the other being supplied with currents from the secondary
+of the transformer.
+
+
+
+
+PART II.
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.
+
+
+
+
+CHAPTER XXV.
+
+INTRODUCTION.--THE SCOPE OF THE TESLA LECTURES.
+
+
+Before proceeding to study the three Tesla lectures here presented, the
+reader may find it of some assistance to have his attention directed to
+the main points of interest and significance therein. The first of these
+lectures was delivered in New York, at Columbia College, before the
+American Institute of Electrical Engineers, May 20, 1891. The urgent
+desire expressed immediately from all parts of Europe for an opportunity
+to witness the brilliant and unusual experiments with which the lecture
+was accompanied, induced Mr. Tesla to go to England early in 1892, when
+he appeared before the Institution of Electrical Engineers, and a day
+later, by special request, before the Royal Institution. His reception
+was of the most enthusiastic and flattering nature on both occasions. He
+then went, by invitation, to France, and repeated his novel
+demonstrations before the Societe Internationale des Electriciens, and
+the Societe Francaise de Physique. Mr. Tesla returned to America in the
+fall of 1892, and in February, 1893, delivered his third lecture before
+the Franklin Institute of Philadelphia, in fulfilment of a long standing
+promise to Prof. Houston. The following week, at the request of
+President James I. Ayer, of the National Electric Light Association, the
+same lecture was re-delivered in St. Louis. It had been intended to
+limit the invitations to members, but the appeals from residents in the
+city were so numerous and pressing that it became necessary to secure a
+very large hall. Hence it came about that the lecture was listened to by
+an audience of over 5,000 people, and was in some parts of a more
+popular nature than either of its predecessors. Despite this concession
+to the need of the hour and occasion, Mr. Tesla did not hesitate to show
+many new and brilliant experiments, and to advance the frontier of
+discovery far beyond any point he had theretofore marked publicly.
+
+We may now proceed to a running review of the lectures themselves. The
+ground covered by them is so vast that only the leading ideas and
+experiments can here be touched upon; besides, it is preferable that the
+lectures should be carefully gone over for their own sake, it being more
+than likely that each student will discover a new beauty or stimulus in
+them. Taking up the course of reasoning followed by Mr. Tesla in his
+first lecture, it will be noted that he started out with the recognition
+of the fact, which he has now experimentally demonstrated, that for the
+production of light waves, primarily, electrostatic effects must be
+brought into play, and continued study has led him to the opinion that
+all electrical and magnetic effects may be referred to electrostatic
+molecular forces. This opinion finds a singular confirmation in one of
+the most striking experiments which he describes, namely, the production
+of a veritable flame by the agitation of electrostatically charged
+molecules. It is of the highest interest to observe that this result
+points out a way of obtaining a flame which consumes no material and in
+which no chemical action whatever takes place. It also throws a light on
+the nature of the ordinary flame, which Mr. Tesla believes to be due to
+electrostatic molecular actions, which, if true, would lead directly to
+the idea that even chemical affinities might be electrostatic in their
+nature and that, as has already been suggested, molecular forces in
+general may be referable to one and the same cause. This singular
+phenomenon accounts in a plausible manner for the unexplained fact that
+buildings are frequently set on fire during thunder storms without
+having been at all struck by lightning. It may also explain the total
+disappearance of ships at sea.
+
+One of the striking proofs of the correctness of the ideas advanced by
+Mr. Tesla is the fact that, notwithstanding the employment of the most
+powerful electromagnetic inductive effects, but feeble luminosity is
+obtainable, and this only in close proximity to the source of
+disturbance; whereas, when the electrostatic effects are intensified,
+the same initial energy suffices to excite luminosity at considerable
+distances from the source. That there are only electrostatic effects
+active seems to be clearly proved by Mr. Tesla's experiments with an
+induction coil operated with alternating currents of very high
+frequency. He shows how tubes may be made to glow brilliantly at
+considerable distances from any object when placed in a powerful,
+rapidly alternating, electrostatic field, and he describes many
+interesting phenomena observed in such a field. His experiments open up
+the possibility of lighting an apartment by simply creating in it such
+an electrostatic field, and this, in a certain way, would appear to be
+the ideal method of lighting a room, as it would allow the illuminating
+device to be freely moved about. The power with which these exhausted
+tubes, devoid of any electrodes, light up is certainly remarkable.
+
+That the principle propounded by Mr. Tesla is a broad one is evident
+from the many ways in which it may be practically applied. We need only
+refer to the variety of the devices shown or described, all of which are
+novel in character and will, without doubt, lead to further important
+results at the hands of Mr. Tesla and other investigators. The
+experiment, for instance, of lighting up a single filament or block of
+refractory material with a single wire, is in itself sufficient to give
+Mr. Tesla's work the stamp of originality, and the numerous other
+experiments and effects which may be varied at will, are equally new and
+interesting. Thus, the incandescent filament spinning in an unexhausted
+globe, the well-known Crookes experiment on open circuit, and the many
+others suggested, will not fail to interest the reader. Mr. Tesla has
+made an exhaustive study of the various forms of the discharge presented
+by an induction coil when operated with these rapidly alternating
+currents, starting from the thread-like discharge and passing through
+various stages to the true electric flame.
+
+A point of great importance in the introduction of high tension
+alternating current which Mr. Tesla brings out is the necessity of
+carefully avoiding all gaseous matter in the high tension apparatus. He
+shows that, at least with very rapidly alternating currents of high
+potential, the discharge may work through almost any practicable
+thickness of the best insulators, if air is present. In such cases the
+air included within the apparatus is violently agitated and by molecular
+bombardment the parts may be so greatly heated as to cause a rupture of
+the insulation. The practical outcome of this is, that, whereas with
+steady currents, any kind of insulation may be used, with rapidly
+alternating currents oils will probably be the best to employ, a fact
+which has been observed, but not until now satisfactorily explained. The
+recognition of the above fact is of special importance in the
+construction of the costly commercial induction coils which are often
+rendered useless in an unaccountable manner. The truth of these views of
+Mr. Tesla is made evident by the interesting experiments illustrative
+of the behavior of the air between charged surfaces, the luminous
+streams formed by the charged molecules appearing even when great
+thicknesses of the best insulators are interposed between the charged
+surfaces. These luminous streams afford in themselves a very interesting
+study for the experimenter. With these rapidly alternating currents they
+become far more powerful and produce beautiful light effects when they
+issue from a wire, pinwheel or other object attached to a terminal of
+the coil; and it is interesting to note that they issue from a ball
+almost as freely as from a point, when the frequency is very high.
+
+From these experiments we also obtain a better idea of the importance of
+taking into account the capacity and self-induction in the apparatus
+employed and the possibilities offered by the use of condensers in
+conjunction with alternate currents, the employment of currents of high
+frequency, among other things, making it possible to reduce the
+condenser to practicable dimensions. Another point of interest and
+practical bearing is the fact, proved by Mr. Tesla, that for alternate
+currents, especially those of high frequency, insulators are required
+possessing a small specific inductive capacity, which at the same time
+have a high insulating power.
+
+Mr. Tesla also makes interesting and valuable suggestion in regard to
+the economical utilization of iron in machines and transformers. He
+shows how, by maintaining by continuous magnetization a flow of lines
+through the iron, the latter may be kept near its maximum permeability
+and a higher output and economy may be secured in such apparatus. This
+principle may prove of considerable commercial importance in the
+development of alternating systems. Mr. Tesla's suggestion that the same
+result can be secured by heating the iron by hysteresis and eddy
+currents, and increasing the permeability in this manner, while it may
+appear less practical, nevertheless opens another direction for
+investigation and improvement.
+
+The demonstration of the fact that with alternating currents of high
+frequency, sufficient energy may be transmitted under practicable
+conditions through the glass of an incandescent lamp by electrostatic or
+electromagnetic induction may lead to a departure in the construction of
+such devices. Another important experimental result achieved is the
+operation of lamps, and even motors, with the discharges of condensers,
+this method affording a means of converting direct or alternating
+currents. In this connection Mr. Tesla advocates the perfecting of
+apparatus capable of generating electricity of high tension from heat
+energy, believing this to be a better way of obtaining electrical energy
+for practical purposes, particularly for the production of light.
+
+While many were probably prepared to encounter curious phenomena of
+impedance in the use of a condenser discharged disruptively, the
+experiments shown were extremely interesting on account of their
+paradoxical character. The burning of an incandescent lamp at any candle
+power when connected across a heavy metal bar, the existence of nodes on
+the bar and the possibility of exploring the bar by means of an ordinary
+Cardew voltmeter, are all peculiar developments, but perhaps the most
+interesting observation is the phenomenon of impedance observed in the
+lamp with a straight filament, which remains dark while the bulb glows.
+
+Mr. Tesla's manner of operating an induction coil by means of the
+disruptive discharge, and thus obtaining enormous differences of
+potential from comparatively small and inexpensive coils, will be
+appreciated by experimenters and will find valuable application in
+laboratories. Indeed, his many suggestions and hints in regard to the
+construction and use of apparatus in these investigations will be highly
+valued and will aid materially in future research.
+
+The London lecture was delivered twice. In its first form, before the
+Institution of Electrical Engineers, it was in some respects an
+amplification of several points not specially enlarged upon in the New
+York lecture, but brought forward many additional discoveries and new
+investigations. Its repetition, in another form, at the Royal
+Institution, was due to Prof. Dewar, who with Lord Rayleigh, manifested
+a most lively interest in Mr. Tesla's work, and whose kindness
+illustrated once more the strong English love of scientific truth and
+appreciation of its votaries. As an indefatigable experimenter, Mr.
+Tesla was certainly nowhere more at home than in the haunts of Faraday,
+and as the guest of Faraday's successor. This Royal Institution lecture
+summed up the leading points of Mr. Tesla's work, in the high potential,
+high frequency field, and we may here avail ourselves of so valuable a
+summarization, in a simple form, of a subject by no means easy of
+comprehension until it has been thoroughly studied.
+
+In these London lectures, among the many notable points made was first,
+the difficulty of constructing the alternators to obtain the very high
+frequencies needed. To obtain the high frequencies it was necessary to
+provide several hundred polar projections, which were necessarily small
+and offered many drawbacks, and this the more as exceedingly high
+peripheral speeds had to be resorted to. In some of the first machines
+both armature and field had polar projections. These machines produced a
+curious noise, especially when the armature was started from the state
+of rest, the field being charged. The most efficient machine was found
+to be one with a drum armature, the iron body of which consisted of very
+thin wire annealed with special care. It was, of course, desirable to
+avoid the employment of iron in the armature, and several machines of
+this kind, with moving or stationary conductors were constructed, but
+the results obtained were not quite satisfactory, on account of the
+great mechanical and other difficulties encountered.
+
+The study of the properties of the high frequency currents obtained from
+these machines is very interesting, as nearly every experiment discloses
+something new. Two coils traversed by such a current attract or repel
+each other with a force which, owing to the imperfection of our sense of
+touch, seems continuous. An interesting observation, already noted under
+another form, is that a piece of iron, surrounded by a coil through
+which the current is passing appears to be continuously magnetized. This
+apparent continuity might be ascribed to the deficiency of the sense of
+touch, but there is evidence that in currents of such high frequencies
+one of the impulses preponderates over the other.
+
+As might be expected, conductors traversed by such currents are rapidly
+heated, owing to the increase of the resistance, and the heating effects
+are relatively much greater in the iron. The hysteresis losses in iron
+are so great that an iron core, even if finely subdivided, is heated in
+an incredibly short time. To give an idea of this, an ordinary iron wire
+1/16 inch in diameter inserted within a coil having 250 turns, with a
+current estimated to be five amperes passing through the coil, becomes
+within two seconds' time so hot as to scorch wood. Beyond a certain
+frequency, an iron core, no matter how finely subdivided, exercises a
+dampening effect, and it was easy to find a point at which the
+impedance of a coil was not affected by the presence of a core
+consisting of a bundle of very thin well annealed and varnished iron
+wires.
+
+Experiments with a telephone, a conductor in a strong magnetic field, or
+with a condenser or arc, seem to afford certain proof that sounds far
+above the usually accepted limit of hearing would be perceived if
+produced with sufficient power. The arc produced by these currents
+possesses several interesting features. Usually it emits a note the
+pitch of which corresponds to twice the frequency of the current, but if
+the frequency be sufficiently high it becomes noiseless, the limit of
+audition being determined principally by the linear dimensions of the
+arc. A curious feature of the arc is its persistency, which is due
+partly to the inability of the gaseous column to cool and increase
+considerably in resistance, as is the case with low frequencies, and
+partly to the tendency of such a high frequency machine to maintain a
+constant current.
+
+In connection with these machines the condenser affords a particularly
+interesting study. Striking effects are produced by proper adjustments
+of capacity and self-induction. It is easy to raise the electromotive
+force of the machine to many times the original value by simply
+adjusting the capacity of a condenser connected in the induced circuit.
+If the condenser be at some distance from the machine, the difference of
+potential on the terminals of the latter may be only a small fraction of
+that on the condenser.
+
+But the most interesting experiences are gained when the tension of the
+currents from the machine is raised by means of an induction coil. In
+consequence of the enormous rate of change obtainable in the primary
+current, much higher potential differences are obtained than with coils
+operated in the usual ways, and, owing to the high frequency, the
+secondary discharge possesses many striking peculiarities. Both the
+electrodes behave generally alike, though it appears from some
+observations that one current impulse preponderates over the other, as
+before mentioned.
+
+The physiological effects of the high tension discharge are found to be
+so small that the shock of the coil can be supported without any
+inconvenience, except perhaps a small burn produced by the discharge
+upon approaching the hand to one of the terminals. The decidedly smaller
+physiological effects of these currents are thought to be due either to
+a different distribution through the body or to the tissues acting as
+condensers. But in the case of an induction coil with a great many turns
+the harmlessness is principally due to the fact that but little energy
+is available in the external circuit when the same is closed through the
+experimenter's body, on account of the great impedance of the coil.
+
+In varying the frequency and strength of the currents through the
+primary of the coil, the character of the secondary discharge is greatly
+varied, and no less than five distinct forms are observed:--A weak,
+sensitive thread discharge, a powerful flaming discharge, and three
+forms of brush or streaming discharges. Each of these possesses certain
+noteworthy features, but the most interesting to study are the latter.
+
+Under certain conditions the streams, which are presumably due to the
+violent agitation of the air molecules, issue freely from all points of
+the coil, even through a thick insulation. If there is the smallest air
+space between the primary and secondary, they will form there and surely
+injure the coil by slowly warming the insulation. As they form even with
+ordinary frequencies when the potential is excessive, the air-space must
+be most carefully avoided. These high frequency streamers differ in
+aspect and properties from those produced by a static machine. The wind
+produced by them is small and should altogether cease if still
+considerably higher frequencies could be obtained. A peculiarity is that
+they issue as freely from surfaces as from points. Owing to this, a
+metallic vane, mounted in one of the terminals of the coil so as to
+rotate freely, and having one of its sides covered with insulation, is
+spun rapidly around. Such a vane would not rotate with a steady
+potential, but with a high frequency coil it will spin, even if it be
+entirely covered with insulation, provided the insulation on one side be
+either thicker or of a higher specific inductive capacity. A Crookes
+electric radiometer is also spun around when connected to one of the
+terminals of the coil, but only at very high exhaustion or at ordinary
+pressures.
+
+There is still another and more striking peculiarity of such a high
+frequency streamer, namely, it is hot. The heat is easily perceptible
+with frequencies of about 10,000, even if the potential is not
+excessively high. The heating effect is, of course, due to the molecular
+impacts and collisions. Could the frequency and potential be pushed far
+enough, then a brush could be produced resembling in every particular a
+flame and giving light and heat, yet without a chemical process taking
+place.
+
+The hot brush, when properly produced, resembles a jet of burning gas
+escaping under great pressure, and it emits an extraordinary strong
+smell of ozone. The great ozonizing action is ascribed to the fact that
+the agitation of the molecules of the air is more violent in such a
+brush than in the ordinary streamer of a static machine. But the most
+powerful brush discharges were produced by employing currents of much
+higher frequencies than it was possible to obtain by means of the
+alternators. These currents were obtained by disruptively discharging a
+condenser and setting up oscillations. In this manner currents of a
+frequency of several hundred thousand were obtained.
+
+Currents of this kind, Mr. Tesla pointed out, produce striking effects.
+At these frequencies, the impedance of a copper bar is so great that a
+potential difference of several hundred volts can be maintained between
+two points of a short and thick bar, and it is possible to keep an
+ordinary incandescent lamp burning at full candle power by attaching the
+terminals of the lamp to two points of the bar no more than a few inches
+apart. When the frequency is extremely high, nodes are found to exist on
+such a bar, and it is easy to locate them by means of a lamp.
+
+By converting the high tension discharges of a low frequency coil in
+this manner, it was found practicable to keep a few lamps burning on the
+ordinary circuit in the laboratory, and by bringing the undulation to a
+low pitch, it was possible to operate small motors.
+
+This plan likewise allows of converting high tension discharges of one
+direction into low tension unidirectional currents, by adjusting the
+circuit so that there are no oscillations. In passing the oscillating
+discharges through the primary of a specially constructed coil, it is
+easy to obtain enormous potential differences with only few turns of the
+secondary.
+
+Great difficulties were at first experienced in producing a successful
+coil on this plan. It was found necessary to keep all air, or gaseous
+matter in general, away from the charged surfaces, and oil immersion was
+resorted to. The wires used were heavily covered with gutta-percha and
+wound in oil, or the air was pumped out by means of a Sprengel pump. The
+general arrangement was the following:--An ordinary induction coil,
+operated from a low frequency alternator, was used to charge Leyden
+jars. The jars were made to discharge over a single or multiple gap
+through the primary of the second coil. To insure the action of the gap,
+the arc was blown out by a magnet or air blast. To adjust the potential
+in the secondary a small oil condenser was used, or polished brass
+spheres of different sizes were screwed on the terminals and their
+distance adjusted.
+
+When the conditions were carefully determined to suit each experiment,
+magnificent effects were obtained. Two wires, stretched through the
+room, each being connected to one of the terminals of the coil, emitted
+streams so powerful that the light from them allowed distinguishing the
+objects in the room; the wires became luminous even though covered with
+thick and most excellent insulation. When two straight wires, or two
+concentric circles of wire, are connected to the terminals, and set at
+the proper distance, a uniform luminous sheet is produced between them.
+It was possible in this way to cover an area of more than one meter
+square completely with the streams. By attaching to one terminal a large
+circle of wire and to the other terminal a small sphere, the streams are
+focused upon the sphere, produce a strongly lighted spot upon the same,
+and present the appearance of a luminous cone. A very thin wire glued
+upon a plate of hard rubber of great thickness, on the opposite side of
+which is fastened a tinfoil coating, is rendered intensely luminous when
+the coating is connected to the other terminal of the coil. Such an
+experiment can be performed also with low frequency currents, but much
+less satisfactorily.
+
+When the terminals of such a coil, even of a very small one, are
+separated by a rubber or glass plate, the discharge spreads over the
+plate in the form of streams, threads or brilliant sparks, and affords a
+magnificent display, which cannot be equaled by the largest coil
+operated in the usual ways. By a simple adjustment it is possible to
+produce with the coil a succession of brilliant sparks, exactly as with
+a Holtz machine.
+
+Under certain conditions, when the frequency of the oscillation is very
+great, white, phantom-like streams are seen to break forth from the
+terminals of the coil. The chief interesting feature about them is, that
+they stream freely against the outstretched hand or other conducting
+object without producing any sensation, and the hand may be approached
+very near to the terminal without a spark being induced to jump. This is
+due presumably to the fact that a considerable portion of the energy is
+carried away or dissipated in the streamers, and the difference of
+potential between the terminal and the hand is diminished.
+
+It is found in such experiments that the frequency of the vibration and
+the quickness of succession of the sparks between the knobs affect to a
+marked degree the appearance of the streams. When the frequency is very
+low, the air gives way in more or less the same manner as by a steady
+difference of potential, and the streams consist of distinct threads,
+generally mingled with thin sparks, which probably correspond to the
+successive discharges occurring between the knobs. But when the
+frequency is very high, and the arc of the discharge produces a sound
+which is loud and smooth (which indicates both that oscillation takes
+place and that the sparks succeed each other with great rapidity), then
+the luminous streams formed are perfectly uniform. They are generally of
+a purplish hue, but when the molecular vibration is increased by raising
+the potential, they assume a white color.
+
+The luminous intensity of the streams increases rapidly when the
+potential is increased; and with frequencies of only a few hundred
+thousand, could the coil be made to withstand a sufficiently high
+potential difference, there is no doubt that the space around a wire
+could be made to emit a strong light, merely by the agitation of the
+molecules of the air at ordinary pressure.
+
+Such discharges of very high frequency which render luminous the air at
+ordinary pressure we have very likely occasion to witness in the aurora
+borealis. From many of these experiments it seems reasonable to infer
+that sudden cosmic disturbances, such as eruptions on the sun, set the
+electrostatic charge of the earth in an extremely rapid vibration, and
+produce the glow by the violent agitation of the air in the upper and
+even in the lower strata. It is thought that if the frequency were low,
+or even more so if the charge were not at all vibrating, the lower dense
+strata would break down as in a lightning discharge. Indications of such
+breaking down have been repeatedly observed, but they can be attributed
+to the fundamental disturbances, which are few in number, for the
+superimposed vibration would be so rapid as not to allow a disruptive
+break.
+
+The study of these discharge phenomena has led Mr. Tesla to the
+recognition of some important facts. It was found, as already stated,
+that gaseous matter must be most carefully excluded from any dielectric
+which is subjected to great, rapidly changing electrostatic stresses.
+Since it is difficult to exclude the gas perfectly when solid insulators
+are used, it is necessary to resort to liquid dielectrics. When a solid
+dielectric is used, it matters little how thick and how good it is; if
+air be present, streamers form, which gradually heat the dielectric and
+impair its insulating power, and the discharge finally breaks through.
+Under ordinary conditions the best insulators are those which possess
+the highest specific inductive capacity, but such insulators are not the
+best to employ when working with these high frequency currents, for in
+most cases the higher specific inductive capacity is rather a
+disadvantage. The prime quality of the insulating medium for these
+currents is continuity. For this reason principally it is necessary to
+employ liquid insulators, such as oils. If two metal plates, connected
+to the terminals of the coil, are immersed in oil and set a distance
+apart, the coil may be kept working for any length of time without a
+break occurring, or without the oil being warmed, but if air bubbles are
+introduced, they become luminous; the air molecules, by their impact
+against the oil, heat it, and after some time cause the insulation to
+give way. If, instead of the oil, a solid plate of the best dielectric,
+even several times thicker than the oil intervening between the metal
+plates, is inserted between the latter, the air having free access to
+the charged surfaces, the dielectric invariably is warmed and breaks
+down.
+
+The employment of oil is advisable or necessary even with low
+frequencies, if the potentials are such that streamers form, but only in
+such cases, as is evident from the theory of the action. If the
+potentials are so low that streamers do not form, then it is even
+disadvantageous to employ oil, for it may, principally by confining the
+heat, be the cause of the breaking down of the insulation.
+
+The exclusion of gaseous matter is not only desirable on account of the
+safety of the apparatus, but also on account of economy, especially in a
+condenser, in which considerable waste of power may occur merely owing
+to the presence of air, if the electric density on the charged surfaces
+is great.
+
+In the course of these investigations a phenomenon of special scientific
+interest was observed. It may be ranked among the brush phenomena, in
+fact it is a kind of brush which forms at, or near, a single terminal in
+high vacuum. In a bulb with a conducting electrode, even if the latter
+be of aluminum, the brush has only a very short existence, but it can be
+preserved for a considerable length of time in a bulb devoid of any
+conducting electrode. To observe the phenomenon it is found best to
+employ a large spherical bulb having in its centre a small bulb
+supported on a tube sealed to the neck of the former. The large bulb
+being exhausted to a high degree, and the inside of the small bulb being
+connected to one of the terminals of the coil, under certain conditions
+there appears a misty haze around the small bulb, which, after passing
+through some stages, assumes the form of a brush, generally at right
+angles to the tube supporting the small bulb. When the brush assumes
+this form it may be brought to a state of extreme sensitiveness to
+electrostatic and magnetic influence. The bulb hanging straight down,
+and all objects being remote from it, the approach of the observer
+within a few paces will cause the brush to fly to the opposite side, and
+if he walks around the bulb it will always keep on the opposite side. It
+may begin to spin around the terminal long before it reaches that
+sensitive stage. When it begins to turn around, principally, but also
+before, it is affected by a magnet, and at a certain stage it is
+susceptible to magnetic influence to an astonishing degree. A small
+permanent magnet, with its poles at a distance of no more than two
+centimetres will affect it visibly at a distance of two metres, slowing
+down or accelerating the rotation according to how it is held relatively
+to the brush.
+
+When the bulb hangs with the globe down, the rotation is always
+clockwise. In the southern hemisphere it would occur in the opposite
+direction, and on the (magnetic) equator the brush should not turn at
+all. The rotation may be reversed by a magnet kept at some distance. The
+brush rotates best, seemingly, when it is at right angles to the lines
+of force of the earth. It very likely rotates, when at its maximum
+speed, in synchronism with the alternations, say, 10,000 times a second.
+The rotation can be slowed down or accelerated by the approach or
+recession of the observer, or any conducting body, but it cannot be
+reversed by putting the bulb in any position. Very curious experiments
+may be performed with the brush when in its most sensitive state. For
+instance, the brush resting in one position, the experimenter may, by
+selecting a proper position, approach the hand at a certain considerable
+distance to the bulb, and he may cause the brush to pass off by merely
+stiffening the muscles of the arm, the mere change of configuration of
+the arm and the consequent imperceptible displacement being sufficient
+to disturb the delicate balance. When it begins to rotate slowly, and
+the hands are held at a proper distance, it is impossible to make even
+the slightest motion without producing a visible effect upon the brush.
+A metal plate connected to the other terminal of the coil affects it at
+a great distance, slowing down the rotation often to one turn a second.
+
+Mr. Tesla hopes that this phenomenon will prove a valuable aid in the
+investigation of the nature of the forces acting in an electrostatic or
+magnetic field. If there is any motion which is measurable going on in
+the space, such a brush would be apt to reveal it. It is, so to speak, a
+beam of light, frictionless, devoid of inertia. On account of its
+marvellous sensitiveness to electrostatic or magnetic disturbances it
+may be the means of sending signals through submarine cables with any
+speed, and even of transmitting intelligence to a distance without
+wires.
+
+In operating an induction coil with these rapidly alternating currents,
+it is astonishing to note, for the first time, the great importance of
+the relation of capacity, self-induction, and frequency as bearing upon
+the general result. The combined effect of these elements produces many
+curious effects. For instance, two metal plates are connected to the
+terminals and set at a small distance, so that an arc is formed between
+them. This arc _prevents_ a strong current from flowing through the
+coil. If the arc be interrupted by the interposition of a glass plate,
+the capacity of the condenser obtained counteracts the self-induction,
+and a stronger current is made to pass. The effects of capacity are the
+most striking, for in these experiments, since the self-induction and
+frequency both are high, the critical capacity is very small, and need
+be but slightly varied to produce a very considerable change. The
+experimenter brings his body in contact with the terminals of the
+secondary of the coil, or attaches to one or both terminals insulated
+bodies of very small bulk, such as exhausted bulbs, and he produces a
+considerable rise or fall of potential on the secondary, and greatly
+affects the flow of the current through the primary coil.
+
+In many of the phenomena observed, the presence of the air, or,
+generally speaking, of a medium of a gaseous nature (using this term not
+to imply specific properties, but in contradistinction to homogeneity or
+perfect continuity) plays an important part, as it allows energy to be
+dissipated by molecular impact or bombardment. The action is thus
+explained:--When an insulated body connected to a terminal of the coil
+is suddenly charged to high potential, it acts inductively upon the
+surrounding air, or whatever gaseous medium there might be. The
+molecules or atoms which are near it are, of course, more attracted, and
+move through a greater distance than the further ones. When the nearest
+molecules strike the body they are repelled, and collisions occur at all
+distances within the inductive distance. It is now clear that, if the
+potential be steady, but little loss of energy can be caused in this
+way, for the molecules which are nearest to the body having had an
+additional charge imparted to them by contact, are not attracted until
+they have parted, if not with all, at least with most of the additional
+charge, which can be accomplished only after a great many collisions.
+This is inferred from the fact that with a steady potential there is but
+little loss in dry air. When the potential, instead of being steady, is
+alternating, the conditions are entirely different. In this case a
+rhythmical bombardment occurs, no matter whether the molecules after
+coming in contact with the body lose the imparted charge or not, and,
+what is more, if the charge is not lost, the impacts are all the more
+violent. Still, if the frequency of the impulses be very small, the loss
+caused by the impacts and collisions would not be serious unless the
+potential was excessive. But when extremely high frequencies and more or
+less high potentials are used, the loss may be very great. The total
+energy lost per unit of time is proportionate to the product of the
+number of impacts per second, or the frequency and the energy lost in
+each impact. But the energy of an impact must be proportionate to the
+square of the electric density of the body, on the assumption that the
+charge imparted to the molecule is proportionate to that density. It is
+concluded from this that the total energy lost must be proportionate to
+the product of the frequency and the square of the electric density; but
+this law needs experimental confirmation. Assuming the preceding
+considerations to be true, then, by rapidly alternating the potential of
+a body immersed in an insulating gaseous medium, any amount of energy
+may be dissipated into space. Most of that energy, then, is not
+dissipated in the form of long ether waves, propagated to considerable
+distance, as is thought most generally, but is consumed in impact and
+collisional losses--that is, heat vibrations--on the surface and in the
+vicinity of the body. To reduce the dissipation it is necessary to work
+with a small electric density--the smaller, the higher the frequency.
+
+The behavior of a gaseous medium to such rapid alternations of potential
+makes it appear plausible that electrostatic disturbances of the earth,
+produced by cosmic events, may have great influence upon the
+meteorological conditions. When such disturbances occur both the
+frequency of the vibrations of the charge and the potential are in all
+probability excessive, and the energy converted into heat may be
+considerable. Since the density must be unevenly distributed, either in
+consequence of the irregularity of the earth's surface, or on account of
+the condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable variations in
+the temperature and pressure of the atmosphere may in this manner be
+caused at any point of the surface of the earth. The variations may be
+gradual or very sudden, according to the nature of the original
+disturbance, and may produce rain and storms, or locally modify the
+weather in any way.
+
+From many experiences gathered in the course of these investigations it
+appears certain that in lightning discharges the air is an element of
+importance. For instance, during a storm a stream may form on a nail or
+pointed projection of a building. If lightning strikes somewhere in the
+neighborhood, the harmless static discharge may, in consequence of the
+oscillations set up, assume the character of a high-frequency streamer,
+and the nail or projection may be brought to a high temperature by the
+violent impact of the air molecules. Thus, it is thought, a building may
+be set on fire without the lightning striking it. In like manner small
+metallic objects may be fused and volatilized--as frequently occurs in
+lightning discharges--merely because they are surrounded by air. Were
+they immersed in a practically continuous medium, such as oil, they
+would probably be safe, as the energy would have to spend itself
+elsewhere.
+
+An instructive experience having a bearing on this subject is the
+following:--A glass tube of an inch or so in diameter and several inches
+long is taken, and a platinum wire sealed into it, the wire running
+through the center of the tube from end to end. The tube is exhausted to
+a moderate degree. If a steady current is passed through the wire it is
+heated uniformly in all parts and the gas in the tube is of no
+consequence. But if high frequency discharges are directed through the
+wire, it is heated more on the ends than in the middle portion, and if
+the frequency, or rate of charge, is high enough, the wire might as well
+be cut in the middle as not, for most of the heating on the ends is due
+to the rarefied gas. Here the gas might only act as a conductor of no
+impedance, diverting the current from the wire as the impedance of the
+latter is enormously increased, and merely heating the ends of the wire
+by reason of their resistance to the passage of the discharge. But it is
+not at all necessary that the gas in the tube should be conducting; it
+might be at an extremely low pressure, still the ends of the wire would
+be heated; however, as is ascertained by experience, only the two ends
+would in such case not be electrically connected through the gaseous
+medium. Now, what with these frequencies and potentials occurs in an
+exhausted tube, occurs in the lightning discharge at ordinary pressure.
+
+From the facility with which any amount of energy may be carried off
+through a gas, Mr. Tesla infers that the best way to render harmless a
+lightning discharge is to afford it in some way a passage through a
+volume of gas.
+
+The recognition of some of the above facts has a bearing upon
+far-reaching scientific investigations in which extremely high
+frequencies and potentials are used. In such cases the air is an
+important factor to be considered. So, for instance, if two wires are
+attached to the terminals of the coil, and the streamers issue from
+them, there is dissipation of energy in the form of heat and light, and
+the wires behave like a condenser of larger capacity. If the wires be
+immersed in oil, the dissipation of energy is prevented, or at least
+reduced, and the apparent capacity is diminished. The action of the air
+would seem to make it very difficult to tell, from the measured or
+computed capacity of a condenser in which the air is acted upon, its
+actual capacity or vibration period, especially if the condenser is of
+very small surface and is charged to a very high potential. As many
+important results are dependant upon the correctness of the estimation
+of the vibration period, this subject demands the most careful scrutiny
+of investigators.
+
+In Leyden jars the loss due to the presence of air is comparatively
+small, principally on account of the great surface of the coatings and
+the small external action, but if there are streamers on the top, the
+loss may be considerable, and the period of vibration is affected. In a
+resonator, the density is small, but the frequency is extreme, and may
+introduce a considerable error. It appears certain, at any rate, that
+the periods of vibration of a charged body in a gaseous and in a
+continuous medium, such as oil, are different, on account of the action
+of the former, as explained.
+
+Another fact recognized, which is of some consequence, is, that in
+similar investigations the general considerations of static screening
+are not applicable when a gaseous medium is present. This is evident
+from the following experiment:--A short and wide glass tube is taken and
+covered with a substantial coating of bronze powder, barely allowing the
+light to shine a little through. The tube is highly exhausted and
+suspended on a metallic clasp from the end of a wire. When the wire is
+connected with one of the terminals of the coil, the gas inside of the
+tube is lighted in spite of the metal coating. Here the metal evidently
+does not screen the gas inside as it ought to, even if it be very thin
+and poorly conducting. Yet, in a condition of rest the metal coating,
+however thin, screens the inside perfectly.
+
+One of the most interesting results arrived at in pursuing these
+experiments, is the demonstration of the fact that a gaseous medium,
+upon which vibration is impressed by rapid changes of electrostatic
+potential, is rigid. In illustration of this result an experiment made
+by Mr. Tesla may by cited:--A glass tube about one inch in diameter and
+three feet long, with outside condenser coatings on the ends, was
+exhausted to a certain point, when, the tube being suspended freely from
+a wire connecting the upper coating to one of the terminals of the coil,
+the discharge appeared in the form of a luminous thread passing through
+the axis of the tube. Usually the thread was sharply defined in the
+upper part of the tube and lost itself in the lower part. When a magnet
+or the finger was quickly passed near the upper part of the luminous
+thread, it was brought out of position by magnetic or electrostatic
+influence, and a transversal vibration like that of a suspended cord,
+with one or more distinct nodes, was set up, which lasted for a few
+minutes and gradually died out. By suspending from the lower condenser
+coating metal plates of different sizes, the speed of the vibration was
+varied. This vibration would seem to show beyond doubt that the thread
+possessed rigidity, at least to transversal displacements.
+
+Many experiments were tried to demonstrate this property in air at
+ordinary pressure. Though no positive evidence has been obtained, it is
+thought, nevertheless, that a high frequency brush or streamer, if the
+frequency could be pushed far enough, would be decidedly rigid. A small
+sphere might then be moved within it quite freely, but if thrown against
+it the sphere would rebound. An ordinary flame cannot possess rigidity
+to a marked degree because the vibration is directionless; but an
+electric arc, it is believed, must possess that property more or less. A
+luminous band excited in a bulb by repeated discharges of a Leyden jar
+must also possess rigidity, and if deformed and suddenly released should
+vibrate.
+
+From like considerations other conclusions of interest are reached. The
+most probable medium filling the space is one consisting of independent
+carriers immersed in an insulating fluid. If through this medium
+enormous electrostatic stresses are assumed to act, which vary rapidly
+in intensity, it would allow the motion of a body through it, yet it
+would be rigid and elastic, although the fluid itself might be devoid of
+these properties. Furthermore, on the assumption that the independent
+carriers are of any configuration such that the fluid resistance to
+motion in one direction is greater than in another, a stress of that
+nature would cause the carriers to arrange themselves in groups, since
+they would turn to each other their sides of the greatest electric
+density, in which position the fluid resistance to approach would be
+smaller than to receding. If in a medium of the above characteristics a
+brush would be formed by a steady potential, an exchange of the carriers
+would go on continually, and there would be less carriers per unit of
+volume in the brush than in the space at some distance from the
+electrode, this corresponding to rarefaction. If the potential were
+rapidly changing, the result would be very different; the higher the
+frequency of the pulses, the slower would be the exchange of the
+carriers; finally, the motion of translation through measurable space
+would cease, and, with a sufficiently high frequency and intensity of
+the stress, the carriers would be drawn towards the electrode, and
+compression would result.
+
+An interesting feature of these high frequency currents is that they
+allow of operating all kinds of devices by connecting the device with
+only one leading wire to the electric source. In fact, under certain
+conditions it may be more economical to supply the electrical energy
+with one lead than with two.
+
+An experiment of special interest shown by Mr. Tesla, is the running, by
+the use of only one insulated line, of a motor operating on the
+principle of the rotating magnetic field enunciated by Mr. Tesla. A
+simple form of such a motor is obtained by winding upon a laminated iron
+core a primary and close to it a secondary coil, closing the ends of the
+latter and placing a freely movable metal disc within the influence of
+the moving field. The secondary coil may, however, be omitted. When one
+of the ends of the primary coil of the motor is connected to one of the
+terminals of the high frequency coil and the other end to an insulated
+metal plate, which, it should be stated, is not absolutely necessary for
+the success of the experiment, the disc is set in rotation.
+
+Experiments of this kind seem to bring it within possibility to operate
+a motor at any point of the earth's surface from a central source,
+without any connection to the same except through the earth. If, by
+means of powerful machinery, rapid variations of the earth's potential
+were produced, a grounded wire reaching up to some height would be
+traversed by a current which could be increased by connecting the free
+end of the wire to a body of some size. The current might be converted
+to low tension and used to operate a motor or other device. The
+experiment, which would be one of great scientific interest, would
+probably best succeed on a ship at sea. In this manner, even if it were
+not possible to operate machinery, intelligence might be transmitted
+quite certainly.
+
+In the course of this experimental study special attention was devoted
+to the heating effects produced by these currents, which are not only
+striking, but open up the possibility of producing a more efficient
+illuminant. It is sufficient to attach to the coil terminal a thin wire
+or filament, to have the temperature of the latter perceptibly raised.
+If the wire or filament be enclosed in a bulb, the heating effect is
+increased by preventing the circulation of the air. If the air in the
+bulb be strongly compressed, the displacements are smaller, the impacts
+less violent, and the heating effect is diminished. On the contrary, if
+the air in the bulb be exhausted, an inclosed lamp filament is brought
+to incandescence, and any amount of light may thus be produced.
+
+The heating of the inclosed lamp filament depends on so many things of a
+different nature, that it is difficult to give a generally applicable
+rule under which the maximum heating occurs. As regards the size of the
+bulb, it is ascertained that at ordinary or only slightly differing
+atmospheric pressures, when air is a good insulator, the filament is
+heated more in a small bulb, because of the better confinement of heat
+in this case. At lower pressures, when air becomes conducting, the
+heating effect is greater in a large bulb, but at excessively high
+degrees of exhaustion there seems to be, beyond a certain and rather
+small size of the vessel, no perceptible difference in the heating.
+
+The shape of the vessel is also of some importance, and it has been
+found of advantage for reasons of economy to employ a spherical bulb
+with the electrode mounted in its centre, where the rebounding molecules
+collide.
+
+It is desirable on account of economy that all the energy supplied to
+the bulb from the source should reach without loss the body to be
+heated. The loss in conveying the energy from the source to the body may
+be reduced by employing thin wires heavily coated with insulation, and
+by the use of electrostatic screens. It is to be remarked, that the
+screen cannot be connected to the ground as under ordinary conditions.
+
+In the bulb itself a large portion of the energy supplied may be lost by
+molecular bombardment against the wire connecting the body to be heated
+with the source. Considerable improvement was effected by covering the
+glass stem containing the wire with a closely fitting conducting tube.
+This tube is made to project a little above the glass, and prevents the
+cracking of the latter near the heated body. The effectiveness of the
+conducting tube is limited to very high degrees of exhaustion. It
+diminishes the energy lost in bombardment for two reasons; first, the
+charge given up by the atoms spreads over a greater area, and hence the
+electric density at any point is small, and the atoms are repelled with
+less energy than if they would strike against a good insulator;
+secondly, as the tube is electrified by the atoms which first come in
+contact with it, the progress of the following atoms against the tube is
+more or less checked by the repulsion which the electrified tube must
+exert upon the similarly electrified atoms. This, it is thought,
+explains why the discharge through a bulb is established with much
+greater facility when an insulator, than when a conductor, is present.
+
+During the investigations a great many bulbs of different construction,
+with electrodes of different material, were experimented upon, and a
+number of observations of interest were made. Mr. Tesla has found that
+the deterioration of the electrode is the less, the higher the
+frequency. This was to be expected, as then the heating is effected by
+many small impacts, instead by fewer and more violent ones, which
+quickly shatter the structure. The deterioration is also smaller when
+the vibration is harmonic. Thus an electrode, maintained at a certain
+degree of heat, lasts much longer with currents obtained from an
+alternator, than with those obtained by means of a disruptive discharge.
+One of the most durable electrodes was obtained from strongly compressed
+carborundum, which is a kind of carbon recently produced by Mr. E. G.
+Acheson, of Monongahela City, Pa. From experience, it is inferred, that
+to be most durable, the electrode should be in the form of a sphere with
+a highly polished surface.
+
+In some bulbs refractory bodies were mounted in a carbon cup and put
+under the molecular impact. It was observed in such experiments that the
+carbon cup was heated at first, until a higher temperature was reached;
+then most of the bombardment was directed against the refractory body,
+and the carbon was relieved. In general, when different bodies were
+mounted in the bulb, the hardest fusible would be relieved, and would
+remain at a considerably lower temperature. This was necessitated by the
+fact that most of the energy supplied would find its way through the
+body which was more easily fused or "evaporated."
+
+Curiously enough it appeared in some of the experiments made, that a
+body was fused in a bulb under the molecular impact by evolution of less
+light than when fused by the application of heat in ordinary ways. This
+may be ascribed to a loosening of the structure of the body under the
+violent impacts and changing stresses.
+
+Some experiments seem to indicate that under certain conditions a body,
+conducting or nonconducting, may, when bombarded, emit light, which to
+all appearances is due to phosphorescence, but may in reality be caused
+by the incandescence of an infinitesimal layer, the mean temperature of
+the body being comparatively small. Such might be the case if each
+single rhythmical impact were capable of instantaneously exciting the
+retina, and the rhythm were just high enough to cause a continuous
+impression in the eye. According to this view, a coil operated by
+disruptive discharge would be eminently adapted to produce such a
+result, and it is found by experience that its power of exciting
+phosphorescence is extraordinarily great. It is capable of exciting
+phosphorescence at comparatively low degrees of exhaustion, and also
+projects shadows at pressures far greater than those at which the mean
+free path is comparable to the dimensions of the vessel. The latter
+observation is of some importance, inasmuch as it may modify the
+generally accepted views in regard to the "radiant state" phenomena.
+
+A thought which early and naturally suggested itself to Mr. Tesla, was
+to utilize the great inductive effects of high frequency currents to
+produce light in a sealed glass vessel without the use of leading in
+wires. Accordingly, many bulbs were constructed in which the energy
+necessary to maintain a button or filament at high incandescence, was
+supplied through the glass by either electrostatic or electrodynamic
+induction. It was easy to regulate the intensity of the light emitted by
+means of an externally applied condenser coating connected to an
+insulated plate, or simply by means of a plate attached to the bulb
+which at the same time performed the function of a shade.
+
+A subject of experiment, which has been exhaustively treated in England
+by Prof. J. J. Thomson, has been followed up independently by Mr. Tesla
+from the beginning of this study, namely, to excite by electrodynamic
+induction a luminous band in a closed tube or bulb. In observing the
+behavior of gases, and the luminous phenomena obtained, the importance
+of the electrostatic effects was noted and it appeared desirable to
+produce enormous potential differences, alternating with extreme
+rapidity. Experiments in this direction led to some of the most
+interesting results arrived at in the course of these investigations. It
+was found that by rapid alternations of a high electrostatic potential,
+exhausted tubes could be lighted at considerable distances from a
+conductor connected to a properly constructed coil, and that it was
+practicable to establish with the coil an alternating electrostatic
+field, acting through the whole room and lighting a tube wherever it was
+placed within the four walls. Phosphorescent bulbs may be excited in
+such a field, and it is easy to regulate the effect by connecting to the
+bulb a small insulated metal plate. It was likewise possible to maintain
+a filament or button mounted in a tube at bright incandescence, and, in
+one experiment, a mica vane was spun by the incandescence of a platinum
+wire.
+
+Coming now to the lecture delivered in Philadelphia and St. Louis, it
+may be remarked that to the superficial reader, Mr. Tesla's
+introduction, dealing with the importance of the eye, might appear as a
+digression, but the thoughtful reader will find therein much food for
+meditation and speculation. Throughout his discourse one can trace Mr.
+Tesla's effort to present in a popular way thoughts and views on the
+electrical phenomena which have in recent years captivated the
+scientific world, but of which the general public has even yet merely
+received an inkling. Mr. Tesla also dwells rather extensively on his
+well-known method of high-frequency conversion; and the large amount of
+detail information will be gratefully received by students and
+experimenters in this virgin field. The employment of apt analogies in
+explaining the fundamental principles involved makes it easy for all to
+gain a clear idea of their nature. Again, the ease with which, thanks to
+Mr. Tesla's efforts, these high-frequency currents may now be obtained
+from circuits carrying almost any kind of current, cannot fail to result
+in an extensive broadening of this field of research, which offers so
+many possibilities. Mr. Tesla, true philosopher as he is, does not
+hesitate to point out defects in some of his methods, and indicates the
+lines which to him seem the most promising. Particular stress is laid by
+him upon the employment of a medium in which the discharge electrodes
+should be immersed in order that this method of conversion may be
+brought to the highest perfection. He has evidently taken pains to give
+as much useful information as possible to those who wish to follow in
+his path, as he shows in detail the circuit arrangements to be adopted
+in all ordinary cases met with in practice, and although some of these
+methods were described by him two years before, the additional
+information is still timely and welcome.
+
+In his experiments he dwells first on some phenomena produced by
+electrostatic force, which he considers in the light of modern theories
+to be the most important force in nature for us to investigate. At the
+very outset he shows a strikingly novel experiment illustrating the
+effect of a rapidly varying electrostatic force in a gaseous medium, by
+touching with one hand one of the terminals of a 200,000 volt
+transformer and bringing the other hand to the opposite terminal. The
+powerful streamers which issued from his hand and astonished his
+audiences formed a capital illustration of some of the views advanced,
+and afforded Mr. Tesla an opportunity of pointing out the true reasons
+why, with these currents, such an amount of energy can be passed
+through the body with impunity. He then showed by experiment the
+difference between a steady and a rapidly varying force upon the
+dielectric. This difference is most strikingly illustrated in the
+experiment in which a bulb attached to the end of a wire in connection
+with one of the terminals of the transformer is ruptured, although all
+extraneous bodies are remote from the bulb. He next illustrates how
+mechanical motions are produced by a varying electrostatic force acting
+through a gaseous medium. The importance of the action of the air is
+particularly illustrated by an interesting experiment.
+
+Taking up another class of phenomena, namely, those of dynamic
+electricity, Mr. Tesla produced in a number of experiments a variety of
+effects by the employment of only a single wire with the evident intent
+of impressing upon his audience the idea that electric vibration or
+current can be transmitted with ease, without any return circuit; also
+how currents so transmitted can be converted and used for many practical
+purposes. A number of experiments are then shown, illustrating the
+effects of frequency, self-induction and capacity; then a number of ways
+of operating motive and other devices by the use of a single lead. A
+number of novel impedance phenomena are also shown which cannot fail to
+arouse interest.
+
+Mr. Tesla next dwelt upon a subject which he thinks of great importance,
+that is, electrical resonance, which he explained in a popular way. He
+expressed his firm conviction that by observing proper conditions,
+intelligence, and possibly even power, can be transmitted through the
+medium or through the earth; and he considers this problem worthy of
+serious and immediate consideration.
+
+Coming now to the light phenomena in particular, he illustrated the four
+distinct kinds of these phenomena in an original way, which to many must
+have been a revelation. Mr. Tesla attributes these light effects to
+molecular or atomic impacts produced by a varying electrostatic stress
+in a gaseous medium. He illustrated in a series of novel experiments the
+effect of the gas surrounding the conductor and shows beyond a doubt
+that with high frequency and high potential currents, the surrounding
+gas is of paramount importance in the heating of the conductor. He
+attributes the heating partially to a conduction current and partially
+to bombardment, and demonstrates that in many cases the heating may be
+practically due to the bombardment alone. He pointed out also that the
+skin effect is largely modified by the presence of the gas or of an
+atomic medium in general. He showed also some interesting experiments in
+which the effect of convection is illustrated. Probably one of the most
+curious experiments in this connection is that in which a thin platinum
+wire stretched along the axis of an exhausted tube is brought to
+incandescence at certain points corresponding to the position of the
+striae, while at others it remains dark. This experiment throws an
+interesting light upon the nature of the striae and may lead to important
+revelations.
+
+Mr. Tesla also demonstrated the dissipation of energy through an atomic
+medium and dwelt upon the behavior of vacuous space in conveying heat,
+and in this connection showed the curious behavior of an electrode
+stream, from which he concludes that the molecules of a gas probably
+cannot be acted upon directly at measurable distances.
+
+Mr. Tesla summarized the chief results arrived at in pursuing his
+investigations in a manner which will serve as a valuable guide to all
+who may engage in this work. Perhaps most interest will centre on his
+general statements regarding the phenomena of phosphorescence, the most
+important fact revealed in this direction being that when exciting a
+phosphorescent bulb a certain definite potential gives the most
+economical result.
+
+The lectures will now be presented in the order of their date of
+delivery.
+
+
+
+
+CHAPTER XXVI.
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY HIGH FREQUENCY AND THEIR
+APPLICATION TO METHODS OF ARTIFICIAL ILLUMINATION.[1]
+
+  [1] A lecture delivered before the American Institute of
+      Electrical Engineers, at Columbia College, N. Y.,
+      May 20, 1891.
+
+
+There is no subject more captivating, more worthy of study, than nature.
+To understand this great mechanism, to discover the forces which are
+active, and the laws which govern them, is the highest aim of the
+intellect of man.
+
+Nature has stored up in the universe infinite energy. The eternal
+recipient and transmitter of this infinite energy is the ether. The
+recognition of the existence of ether, and of the functions it performs,
+is one of the most important results of modern scientific research. The
+mere abandoning of the idea of action at a distance, the assumption of a
+medium pervading all space and connecting all gross matter, has freed
+the minds of thinkers of an ever present doubt, and, by opening a new
+horizon--new and unforeseen possibilities--has given fresh interest to
+phenomena with which we are familiar of old. It has been a great step
+towards the understanding of the forces of nature and their multifold
+manifestations to our senses. It has been for the enlightened student of
+physics what the understanding of the mechanism of the firearm or of the
+steam engine is for the barbarian. Phenomena upon which we used to look
+as wonders baffling explanation, we now see in a different light. The
+spark of an induction coil, the glow of an incandescent lamp, the
+manifestations of the mechanical forces of currents and magnets are no
+longer beyond our grasp; instead of the incomprehensible, as before,
+their observation suggests now in our minds a simple mechanism, and
+although as to its precise nature all is still conjecture, yet we know
+that the truth cannot be much longer hidden, and instinctively we feel
+that the understanding is dawning upon us. We still admire these
+beautiful phenomena, these strange forces, but we are helpless no
+longer; we can in a certain measure explain them, account for them, and
+we are hopeful of finally succeeding in unraveling the mystery which
+surrounds them.
+
+In how far we can understand the world around us is the ultimate thought
+of every student of nature. The coarseness of our senses prevents us
+from recognizing the ulterior construction of matter, and astronomy,
+this grandest and most positive of natural sciences, can only teach us
+something that happens, as it were, in our immediate neighborhood: of
+the remoter portions of the boundless universe, with its numberless
+stars and suns, we know nothing. But far beyond the limit of perception
+of our senses the spirit still can guide us, and so we may hope that
+even these unknown worlds--infinitely small and great--may in a measure
+become known to us. Still, even if this knowledge should reach us, the
+searching mind will find a barrier, perhaps forever unsurpassable, to
+the _true_ recognition of that which _seems_ to be, the mere
+_appearance_ of which is the only and slender basis of all our
+philosophy.
+
+Of all the forms of nature's immeasurable, all-pervading energy, which
+ever and ever changing and moving, like a soul animates the inert
+universe, electricity and magnetism are perhaps the most fascinating.
+The effects of gravitation, of heat and light we observe daily, and soon
+we get accustomed to them, and soon they lose for us the character of
+the marvelous and wonderful; but electricity and magnetism, with their
+singular relationship, with their seemingly dual character, unique among
+the forces in nature, with their phenomena of attractions, repulsions
+and rotations, strange manifestations of mysterious agents, stimulate
+and excite the mind to thought and research. What is electricity, and
+what is magnetism? These questions have been asked again and again. The
+most able intellects have ceaselessly wrestled with the problem; still
+the question has not as yet been fully answered. But while we cannot
+even to-day state what these singular forces are, we have made good
+headway towards the solution of the problem. We are now confident that
+electric and magnetic phenomena are attributable to ether, and we are
+perhaps justified in saying that the effects of static electricity are
+effects of ether under strain, and those of dynamic electricity and
+electro-magnetism effects of ether in motion. But this still leaves the
+question, as to what electricity and magnetism are, unanswered.
+
+First, we naturally inquire, What is electricity, and is there such a
+thing as electricity? In interpreting electric phenomena, we may speak
+of electricity or of an electric condition, state or effect. If we speak
+of electric effects we must distinguish two such effects, opposite in
+character and neutralizing each other, as observation shows that two
+such opposite effects exist. This is unavoidable, for in a medium of the
+properties of ether, we cannot possibly exert a strain, or produce a
+displacement or motion of any kind, without causing in the surrounding
+medium an equivalent and opposite effect. But if we speak of
+electricity, meaning a _thing_, we must, I think, abandon the idea of
+two electricities, as the existence of two such things is highly
+improbable. For how can we imagine that there should be two things,
+equivalent in amount, alike in their properties, but of opposite
+character, both clinging to matter, both attracting and completely
+neutralizing each other? Such an assumption, though suggested by many
+phenomena, though most convenient for explaining them, has little to
+commend it. If there _is_ such a thing as electricity, there can be only
+_one_ such thing, and excess and want of that one thing, possibly; but
+more probably its condition determines the positive and negative
+character. The old theory of Franklin, though falling short in some
+respects, is, from a certain point of view, after all, the most
+plausible one. Still, in spite of this, the theory of the two
+electricities is generally accepted, as it apparently explains electric
+phenomena in a more satisfactory manner. But a theory which better
+explains the facts is not necessarily true. Ingenious minds will invent
+theories to suit observation, and almost every independent thinker has
+his own views on the subject.
+
+It is not with the object of advancing an opinion, but with the desire
+of acquainting you better with some of the results, which I will
+describe, to show you the reasoning I have followed, the departures I
+have made--that I venture to express, in a few words, the views and
+convictions which have led me to these results.
+
+I adhere to the idea that there is a thing which we have been in the
+habit of calling electricity. The question is, What is that thing? or,
+What, of all things, the existence of which we know, have we the best
+reason to call electricity? We know that it acts like an incompressible
+fluid; that there must be a constant quantity of it in nature; that it
+can be neither produced nor destroyed; and, what is more important, the
+electro-magnetic theory of light and all facts observed teach us that
+electric and ether phenomena are identical. The idea at once suggests
+itself, therefore, that electricity might be called ether. In fact, this
+view has in a certain sense been advanced by Dr. Lodge. His interesting
+work has been read by everyone and many have been convinced by his
+arguments. His great ability and the interesting nature of the subject,
+keep the reader spellbound; but when the impressions fade, one realizes
+that he has to deal only with ingenious explanations. I must confess,
+that I cannot believe in two electricities, much less in a
+doubly-constituted ether. The puzzling behavior of the ether as a solid
+to waves of light and heat, and as a fluid to the motion of bodies
+through it, is certainly explained in the most natural and satisfactory
+manner by assuming it to be in motion, as Sir William Thomson has
+suggested; but regardless of this, there is nothing which would enable
+us to conclude with certainty that, while a fluid is not capable of
+transmitting transverse vibrations of a few hundred or thousand per
+second, it might not be capable of transmitting such vibrations when
+they range into hundreds of million millions per second. Nor can anyone
+prove that there are transverse ether waves emitted from an alternate
+current machine, giving a small number of alternations per second; to
+such slow disturbances, the ether, if at rest, may behave as a true
+fluid.
+
+Returning to the subject, and bearing in mind that the existence of two
+electricities is, to say the least, highly improbable, we must remember,
+that we have no evidence of electricity, nor can we hope to get it,
+unless gross matter is present. Electricity, therefore, cannot be called
+ether in the broad sense of the term; but nothing would seem to stand in
+the way of calling electricity ether associated with matter, or bound
+ether; or, in other words, that the so-called static charge of the
+molecule is ether associated in some way with the molecule. Looking at
+it in that light, we would be justified in saying, that electricity is
+concerned in all molecular actions.
+
+Now, precisely what the ether surrounding the molecules is, wherein it
+differs from ether in general, can only be conjectured. It cannot differ
+in density, ether being incompressible: it must, therefore, be under
+some strain or in motion, and the latter is the most probable. To
+understand its functions, it would be necessary to have an exact idea of
+the physical construction of matter, of which, of course, we can only
+form a mental picture.
+
+But of all the views on nature, the one which assumes one matter and one
+force, and a perfect uniformity throughout, is the most scientific and
+most likely to be true. An infinitesimal world, with the molecules and
+their atoms spinning and moving in orbits, in much the same manner as
+celestial bodies, carrying with them and probably spinning with them
+ether, or in other words, carrying with them static charges, seems to my
+mind the most probable view, and one which, in a plausible manner,
+accounts for most of the phenomena observed. The spinning of the
+molecules and their ether sets up the ether tensions or electrostatic
+strains; the equalization of ether tensions sets up ether motions or
+electric currents, and the orbital movements produce the effects of
+electro and permanent magnetism.
+
+About fifteen years ago, Prof. Rowland demonstrated a most interesting
+and important fact, namely, that a static charge carried around produces
+the effects of an electric current. Leaving out of consideration the
+precise nature of the mechanism, which produces the attraction and
+repulsion of currents, and conceiving the electrostatically charged
+molecules in motion, this experimental fact gives us a fair idea of
+magnetism. We can conceive lines or tubes of force which physically
+exist, being formed of rows of directed moving molecules; we can see
+that these lines must be closed, that they must tend to shorten and
+expand, etc. It likewise explains in a reasonable way, the most puzzling
+phenomenon of all, permanent magnetism, and, in general, has all the
+beauties of the Ampere theory without possessing the vital defect of the
+same, namely, the assumption of molecular currents. Without enlarging
+further upon the subject, I would say, that I look upon all
+electrostatic, current and magnetic phenomena as being due to
+electrostatic molecular forces.
+
+The preceding remarks I have deemed necessary to a full understanding of
+the subject as it presents itself to my mind.
+
+Of all these phenomena the most important to study are the current
+phenomena, on account of the already extensive and ever-growing use of
+currents for industrial purposes. It is now a century since the first
+practical source of current was produced, and, ever since, the phenomena
+which accompany the flow of currents have been diligently studied, and
+through the untiring efforts of scientific men the simple laws which
+govern them have been discovered. But these laws are found to hold good
+only when the currents are of a steady character. When the currents are
+rapidly varying in strength, quite different phenomena, often
+unexpected, present themselves, and quite different laws hold good,
+which even now have not been determined as fully as is desirable, though
+through the work, principally, of English scientists, enough knowledge
+has been gained on the subject to enable us to treat simple cases which
+now present themselves in daily practice.
+
+The phenomena which are peculiar to the changing character of the
+currents are greatly exalted when the rate of change is increased, hence
+the study of these currents is considerably facilitated by the
+employment of properly constructed apparatus. It was with this and other
+objects in view that I constructed alternate current machines capable of
+giving more than two million reversals of current per minute, and to
+this circumstance it is principally due, that I am able to bring to your
+attention some of the results thus far reached, which I hope will prove
+to be a step in advance on account of their direct bearing upon one of
+the most important problems, namely, the production of a practical and
+efficient source of light.
+
+The study of such rapidly alternating currents is very interesting.
+Nearly every experiment discloses something new. Many results may, of
+course, be predicted, but many more are unforeseen. The experimenter
+makes many interesting observations. For instance, we take a piece of
+iron and hold it against a magnet. Starting from low alternations and
+running up higher and higher we feel the impulses succeed each other
+faster and faster, get weaker and weaker, and finally disappear. We then
+observe a continuous pull; the pull, of course, is not continuous; it
+only appears so to us; our sense of touch is imperfect.
+
+We may next establish an arc between the electrodes and observe, as the
+alternations rise, that the note which accompanies alternating arcs gets
+shriller and shriller, gradually weakens, and finally ceases. The air
+vibrations, of course, continue, but they are too weak to be perceived;
+our sense of hearing fails us.
+
+We observe the small physiological effects, the rapid heating of the
+iron cores and conductors, curious inductive effects, interesting
+condenser phenomena, and still more interesting light phenomena with a
+high tension induction coil. All these experiments and observations
+would be of the greatest interest to the student, but their description
+would lead me too far from the principal subject. Partly for this
+reason, and partly on account of their vastly greater importance, I will
+confine myself to the description of the light effects produced by these
+currents.
+
+In the experiments to this end a high tension induction coil or
+equivalent apparatus for converting currents of comparatively low into
+currents of high tension is used.
+
+If you will be sufficiently interested in the results I shall describe
+as to enter into an experimental study of this subject; if you will be
+convinced of the truth of the arguments I shall advance--your aim will
+be to produce high frequencies and high potentials; in other words,
+powerful electrostatic effects. You will then encounter many
+difficulties, which, if completely overcome, would allow us to produce
+truly wonderful results.
+
+First will be met the difficulty of obtaining the required frequencies
+by means of mechanical apparatus, and, if they be obtained otherwise,
+obstacles of a different nature will present themselves. Next it will be
+found difficult to provide the requisite insulation without considerably
+increasing the size of the apparatus, for the potentials required are
+high, and, owing to the rapidity of the alternations, the insulation
+presents peculiar difficulties. So, for instance, when a gas is present,
+the discharge may work, by the molecular bombardment of the gas and
+consequent heating, through as much as an inch of the best solid
+insulating material, such as glass, hard rubber, porcelain, sealing wax,
+etc.; in fact, through any known insulating substance. The chief
+requisite in the insulation of the apparatus is, therefore, the
+exclusion of any gaseous matter.
+
+In general my experience tends to show that bodies which possess the
+highest specific inductive capacity, such as glass, afford a rather
+inferior insulation to others, which, while they are good insulators,
+have a much smaller specific inductive capacity, such as oils, for
+instance, the dielectric losses being no doubt greater in the former.
+The difficulty of insulating, of course, only exists when the potentials
+are excessively high, for with potentials such as a few thousand volts
+there is no particular difficulty encountered in conveying currents from
+a machine giving, say, 20,000 alternations per second, to quite a
+distance. This number of alternations, however, is by far too small for
+many purposes, though quite sufficient for some practical applications.
+This difficulty of insulating is fortunately not a vital drawback; it
+affects mostly the size of the apparatus, for, when excessively high
+potentials would be used, the light-giving devices would be located not
+far from the apparatus, and often they would be quite close to it. As
+the air-bombardment of the insulated wire is dependent on condenser
+action, the loss may be reduced to a trifle by using excessively thin
+wires heavily insulated.
+
+Another difficulty will be encountered in the capacity and
+self-induction necessarily possessed by the coil. If the coil be large,
+that is, if it contain a great length of wire, it will be generally
+unsuited for excessively high frequencies; if it be small, it may be
+well adapted for such frequencies, but the potential might then not be
+as high as desired. A good insulator, and preferably one possessing a
+small specific inductive capacity, would afford a two-fold advantage.
+First, it would enable us to construct a very small coil capable of
+withstanding enormous differences of potential; and secondly, such a
+small coil, by reason of its smaller capacity and self-induction, would
+be capable of a quicker and more vigorous vibration. The problem then of
+constructing a coil or induction apparatus of any kind possessing the
+requisite qualities I regard as one of no small importance, and it has
+occupied me for a considerable time.
+
+The investigator who desires to repeat the experiments which I will
+describe, with an alternate current machine, capable of supplying
+currents of the desired frequency, and an induction coil, will do well
+to take the primary coil out and mount the secondary in such a manner as
+to be able to look through the tube upon which the secondary is wound.
+He will then be able to observe the streams which pass from the primary
+to the insulating tube, and from their intensity he will know how far he
+can strain the coil. Without this precaution he is sure to injure the
+insulation. This arrangement permits, however, an easy exchange of the
+primaries, which is desirable in these experiments.
+
+The selection of the type of machine best suited for the purpose must be
+left to the judgment of the experimenter. There are here illustrated
+three distinct types of machines, which, besides others, I have used in
+my experiments.
+
+Fig. 97 represents the machine used in my experiments before this
+Institute. The field magnet consists of a ring of wrought iron with 384
+pole projections. The armature comprises a steel disc to which is
+fastened a thin, carefully welded rim of wrought iron. Upon the rim are
+wound several layers of fine, well annealed iron wire, which, when
+wound, is passed through shellac. The armature wires are wound around
+brass pins, wrapped with silk thread. The diameter of the armature wire
+in this type of machine should not be more than 1/6 of the thickness of
+the pole projections, else the local action will be considerable.
+
+[Illustration: FIG. 97.]
+
+Fig. 98 represents a larger machine of a different type. The field
+magnet of this machine consists of two like parts which either enclose
+an exciting coil, or else are independently wound. Each part has 480
+pole projections, the projections of one facing those of the other. The
+armature consists of a wheel of hard bronze, carrying the conductors
+which revolve between the projections of the field magnet. To wind the
+armature conductors, I have found it most convenient to proceed in the
+following manner. I construct a ring of hard bronze of the required
+size. This ring and the rim of the wheel are provided with the proper
+number of pins, and both fastened upon a plate. The armature conductors
+being wound, the pins are cut off and the ends of the conductors
+fastened by two rings which screw to the bronze ring and the rim of the
+wheel, respectively. The whole may then be taken off and forms a solid
+structure. The conductors in such a type of machine should consist of
+sheet copper, the thickness of which, of course, depends on the
+thickness of the pole projections; or else twisted thin wires should be
+employed.
+
+Fig. 99 is a smaller machine, in many respects similar to the former,
+only here the armature conductors and the exciting coil are kept
+stationary, while only a block of wrought iron is revolved.
+
+[Illustration: FIG. 98.]
+
+It would be uselessly lengthening this description were I to dwell more
+on the details of construction of these machines. Besides, they have
+been described somewhat more elaborately in _The Electrical Engineer_,
+of March 18, 1891. I deem it well, however, to call the attention of the
+investigator to two things, the importance of which, though self
+evident, he is nevertheless apt to underestimate; namely, to the local
+action in the conductors which must be carefully avoided, and to the
+clearance, which must be small. I may add, that since it is desirable to
+use very high peripheral speeds, the armature should be of very large
+diameter in order to avoid impracticable belt speeds. Of the several
+types of these machines which have been constructed by me, I have found
+that the type illustrated in Fig. 97 caused me the least trouble in
+construction, as well as in maintenance, and on the whole, it has been a
+good experimental machine.
+
+In operating an induction coil with very rapidly alternating currents,
+among the first luminous phenomena noticed are naturally those presented
+by the high-tension discharge. As the number of alternations per second
+is increased, or as--the number being high--the current through the
+primary is varied, the discharge gradually changes in appearance. It
+would be difficult to describe the minor changes which occur, and the
+conditions which bring them about, but one may note five distinct forms
+of the discharge.
+
+[Illustration: FIG. 99.]
+
+First, one may observe a weak, sensitive discharge in the form of a
+thin, feeble-colored thread. (Fig. 100a.) It always occurs when, the
+number of alternations per second being high, the current through the
+primary is very small. In spite of the excessively small current, the
+rate of change is great, and the difference of potential at the
+terminals of the secondary is therefore considerable, so that the arc is
+established at great distances; but the quantity of "electricity" set in
+motion is insignificant, barely sufficient to maintain a thin,
+threadlike arc. It is excessively sensitive and may be made so to such a
+degree that the mere act of breathing near the coil will affect it, and
+unless it is perfectly well protected from currents of air, it wriggles
+around constantly. Nevertheless, it is in this form excessively
+persistent, and when the terminals are approached to, say, one-third of
+the striking distance, it can be blown out only with difficulty. This
+exceptional persistency, when short, is largely due to the arc being
+excessively thin; presenting, therefore, a very small surface to the
+blast. Its great sensitiveness, when very long, is probably due to the
+motion of the particles of dust suspended in the air.
+
+[Illustration: FIG. 100a.]
+
+[Illustration: FIG. 100b.]
+
+When the current through the primary is increased, the discharge gets
+broader and stronger, and the effect of the capacity of the coil becomes
+visible until, finally, under proper conditions, a white flaming arc,
+Fig. 100 B, often as thick as one's finger, and striking across the
+whole coil, is produced. It develops remarkable heat, and may be further
+characterized by the absence of the high note which accompanies the
+less powerful discharges. To take a shock from the coil under these
+conditions would not be advisable, although under different conditions,
+the potential being much higher, a shock from the coil may be taken with
+impunity. To produce this kind of discharge the number of alternations
+per second must not be too great for the coil used; and, generally
+speaking, certain relations between capacity, self-induction and
+frequency must be observed.
+
+The importance of these elements in an alternate current circuit is now
+well-known, and under ordinary conditions, the general rules are
+applicable. But in an induction coil exceptional conditions prevail.
+First, the self-induction is of little importance before the arc is
+established, when it asserts itself, but perhaps never as prominently as
+in ordinary alternate current circuits, because the capacity is
+distributed all along the coil, and by reason of the fact that the coil
+usually discharges through very great resistances; hence the currents
+are exceptionally small. Secondly, the capacity goes on increasing
+continually as the potential rises, in consequence of absorption which
+takes place to a considerable extent. Owing to this there exists no
+critical relationship between these quantities, and ordinary rules would
+not seem to be applicable. As the potential is increased either in
+consequence of the increased frequency or of the increased current
+through the primary, the amount of the energy stored becomes greater and
+greater, and the capacity gains more and more in importance. Up to a
+certain point the capacity is beneficial, but after that it begins to be
+an enormous drawback. It follows from this that each coil gives the best
+result with a given frequency and primary current. A very large coil,
+when operated with currents of very high frequency, may not give as much
+as 1/8 inch spark. By adding capacity to the terminals, the condition
+may be improved, but what the coil really wants is a lower frequency.
+
+When the flaming discharge occurs, the conditions are evidently such
+that the greatest current is made to flow through the circuit. These
+conditions may be attained by varying the frequency within wide limits,
+but the highest frequency at which the flaming arc can still be
+produced, determines, for a given primary current, the maximum striking
+distance of the coil. In the flaming discharge the _eclat_ effect of the
+capacity is not perceptible; the rate at which the energy is being
+stored then just equals the rate at which it can be disposed of through
+the circuit. This kind of discharge is the severest test for a coil; the
+break, when it occurs, is of the nature of that in an overcharged Leyden
+jar. To give a rough approximation I would state that, with an ordinary
+coil of, say 10,000 ohms resistance, the most powerful arc would be
+produced with about 12,000 alternations per second.
+
+When the frequency is increased beyond that rate, the potential, of
+course, rises, but the striking distance may, nevertheless, diminish,
+paradoxical as it may seem. As the potential rises the coil attains more
+and more the properties of a static machine until, finally, one may
+observe the beautiful phenomenon of the streaming discharge, Fig. 101,
+which may be produced across the whole length of the coil. At that stage
+streams begin to issue freely from all points and projections. These
+streams will also be seen to pass in abundance in the space between the
+primary and the insulating tube. When the potential is excessively high
+they will always appear, even if the frequency be low, and even if the
+primary be surrounded by as much as an inch of wax, hard rubber, glass,
+or any other insulating substance. This limits greatly the output of the
+coil, but I will later show how I have been able to overcome to a
+considerable extent this disadvantage in the ordinary coil.
+
+Besides the potential, the intensity of the streams depends on the
+frequency; but if the coil be very large they show themselves, no matter
+how low the frequencies used. For instance, in a very large coil of a
+resistance of 67,000 ohms, constructed by me some time ago, they appear
+with as low as 100 alternations per second and less, the insulation of
+the secondary being 3/4 inch of ebonite. When very intense they produce
+a noise similar to that produced by the charging of a Holtz machine, but
+much more powerful, and they emit a strong smell of ozone. The lower the
+frequency, the more apt they are to suddenly injure the coil. With
+excessively high frequencies they may pass freely without producing any
+other effect than to heat the insulation slowly and uniformly.
+
+[Illustration: FIG. 101.]
+
+[Illustration: FIG. 102.]
+
+The existence of these streams shows the importance of constructing an
+expensive coil so as to permit of one's seeing through the tube
+surrounding the primary, and the latter should be easily exchangeable;
+or else the space between the primary and secondary should be completely
+filled up with insulating material so as to exclude all air. The
+non-observance of this simple rule in the construction of commercial
+coils is responsible for the destruction of many an expensive coil.
+
+At the stage when the streaming discharge occurs, or with somewhat
+higher frequencies, one may, by approaching the terminals quite nearly,
+and regulating properly the effect of capacity, produce a veritable
+spray of small silver-white sparks, or a bunch of excessively thin
+silvery threads (Fig. 102) amidst a powerful brush--each spark or thread
+possibly corresponding to one alternation. This, when produced under
+proper conditions, is probably the most beautiful discharge, and when an
+air blast is directed against it, it presents a singular appearance. The
+spray of sparks, when received through the body, causes some
+inconvenience, whereas, when the discharge simply streams, nothing at
+all is likely to be felt if large conducting objects are held in the
+hands to protect them from receiving small burns.
+
+If the frequency is still more increased, then the coil refuses to give
+any spark unless at comparatively small distances, and the fifth typical
+form of discharge may be observed (Fig. 103). The tendency to stream out
+and dissipate is then so great that when the brush is produced at one
+terminal no sparking occurs, even if, as I have repeatedly tried, the
+hand, or any conducting object, is held within the stream; and, what is
+more singular, the luminous stream is not at all easily deflected by the
+approach of a conducting body.
+
+[Illustration: FIG. 103.]
+
+[Illustration: FIG. 104.]
+
+At this stage the streams seemingly pass with the greatest freedom
+through considerable thicknesses of insulators, and it is particularly
+interesting to study their behavior. For this purpose it is convenient
+to connect to the terminals of the coil two metallic spheres which may
+be placed at any desired distance, Fig. 104. Spheres are preferable to
+plates, as the discharge can be better observed. By inserting dielectric
+bodies between the spheres, beautiful discharge phenomena may be
+observed. If the spheres be quite close and a spark be playing between
+them, by interposing a thin plate of ebonite between the spheres the
+spark instantly ceases and the discharge spreads into an intensely
+luminous circle several inches in diameter, provided the spheres are
+sufficiently large. The passage of the streams heats, and, after a
+while, softens, the rubber so much that two plates may be made to stick
+together in this manner. If the spheres are so far apart that no spark
+occurs, even if they are far beyond the striking distance, by inserting
+a thick plate of glass the discharge is instantly induced to pass from
+the spheres to the glass in the form of luminous streams. It appears
+almost as though these streams pass _through_ the dielectric. In reality
+this is not the case, as the streams are due to the molecules of the air
+which are violently agitated in the space between the oppositely charged
+surfaces of the spheres. When no dielectric other than air is present,
+the bombardment goes on, but is too weak to be visible; by inserting a
+dielectric the inductive effect is much increased, and besides, the
+projected air molecules find an obstacle and the bombardment becomes so
+intense that the streams become luminous. If by any mechanical means we
+could effect such a violent agitation of the molecules we could produce
+the same phenomenon. A jet of air escaping through a small hole under
+enormous pressure and striking against an insulating substance, such as
+glass, may be luminous in the dark, and it might be possible to produce
+a phosphorescence of the glass or other insulators in this manner.
+
+The greater the specific inductive capacity of the interposed
+dielectric, the more powerful the effect produced. Owing to this, the
+streams show themselves with excessively high potentials even if the
+glass be as much as one and one-half to two inches thick. But besides
+the heating due to bombardment, some heating goes on undoubtedly in the
+dielectric, being apparently greater in glass than in ebonite. I
+attribute this to the greater specific inductive capacity of the glass,
+in consequence of which, with the same potential difference, a greater
+amount of energy is taken up in it than in rubber. It is like connecting
+to a battery a copper and a brass wire of the same dimensions. The
+copper wire, though a more perfect conductor, would heat more by reason
+of its taking more current. Thus what is otherwise considered a virtue
+of the glass is here a defect. Glass usually gives way much quicker than
+ebonite; when it is heated to a certain degree, the discharge suddenly
+breaks through at one point, assuming then the ordinary form of an arc.
+
+The heating effect produced by molecular bombardment of the dielectric
+would, of course, diminish as the pressure of the air is increased, and
+at enormous pressure it would be negligible, unless the frequency would
+increase correspondingly.
+
+It will be often observed in these experiments that when the spheres are
+beyond the striking distance, the approach of a glass plate, for
+instance, may induce the spark to jump between the spheres. This occurs
+when the capacity of the spheres is somewhat below the critical value
+which gives the greatest difference of potential at the terminals of the
+coil. By approaching a dielectric, the specific inductive capacity of
+the space between the spheres is increased, producing the same effect as
+if the capacity of the spheres were increased. The potential at the
+terminals may then rise so high that the air space is cracked. The
+experiment is best performed with dense glass or mica.
+
+Another interesting observation is that a plate of insulating material,
+when the discharge is passing through it, is strongly attracted by
+either of the spheres, that is by the nearer one, this being obviously
+due to the smaller mechanical effect of the bombardment on that side,
+and perhaps also to the greater electrification.
+
+From the behavior of the dielectrics in these experiments, we may
+conclude that the best insulator for these rapidly alternating currents
+would be the one possessing the smallest specific inductive capacity and
+at the same time one capable of withstanding the greatest differences of
+potential; and thus two diametrically opposite ways of securing the
+required insulation are indicated, namely, to use either a perfect
+vacuum or a gas under great pressure; but the former would be
+preferable. Unfortunately neither of these two ways is easily carried
+out in practice.
+
+It is especially interesting to note the behavior of an excessively high
+vacuum in these experiments. If a test tube, provided with external
+electrodes and exhausted to the highest possible degree, be connected to
+the terminals of the coil, Fig. 105, the electrodes of the tube are
+instantly brought to a high temperature and the glass at each end of the
+tube is rendered intensely phosphorescent, but the middle appears
+comparatively dark, and for a while remains cool.
+
+When the frequency is so high that the discharge shown in Fig. 103 is
+observed, considerable dissipation no doubt occurs in the coil.
+Nevertheless the coil may be worked for a long time, as the heating is
+gradual.
+
+In spite of the fact that the difference of potential may be enormous,
+little is felt when the discharge is passed through the body, provided
+the hands are armed. This is to some extent due to the higher frequency,
+but principally to the fact that less energy is available externally,
+when the difference of potential reaches an enormous value, owing to the
+circumstance that, with the rise of potential, the energy absorbed in
+the coil increases as the square of the potential. Up to a certain point
+the energy available externally increases with the rise of potential,
+then it begins to fall off rapidly. Thus, with the ordinary high tension
+induction coil, the curious paradox exists, that, while with a given
+current through the primary the shock might be fatal, with many times
+that current it might be perfectly harmless, even if the frequency be
+the same. With high frequencies and excessively high potentials when the
+terminals are not connected to bodies of some size, practically all the
+energy supplied to the primary is taken up by the coil. There is no
+breaking through, no local injury, but all the material, insulating and
+conducting, is uniformly heated.
+
+[Illustration: FIG. 105.]
+
+[Illustration: FIG. 106.]
+
+To avoid misunderstanding in regard to the physiological effect of
+alternating currents of very high frequency, I think it necessary to
+state that, while it is an undeniable fact that they are incomparably
+less dangerous than currents of low frequencies, it should not be
+thought that they are altogether harmless. What has just been said
+refers only to currents from an ordinary high tension induction coil,
+which currents are necessarily very small; if received directly from a
+machine or from a secondary of low resistance, they produce more or less
+powerful effects, and may cause serious injury, especially when used in
+conjunction with condensers.
+
+The streaming discharge of a high tension induction coil differs in many
+respects from that of a powerful static machine. In color it has neither
+the violet of the positive, nor the brightness of the negative, static
+discharge, but lies somewhere between, being, of course, alternatively
+positive and negative. But since the streaming is more powerful when the
+point or terminal is electrified positively, than when electrified
+negatively, it follows that the point of the brush is more like the
+positive, and the root more like the negative, static discharge. In the
+dark, when the brush is very powerful, the root may appear almost white.
+The wind produced by the escaping streams, though it may be very
+strong--often indeed to such a degree that it may be felt quite a
+distance from the coil--is, nevertheless, considering the quantity of
+the discharge, smaller than that produced by the positive brush of a
+static machine, and it affects the flame much less powerfully. From the
+nature of the phenomenon we can conclude that the higher the frequency,
+the smaller must, of course, be the wind produced by the streams, and
+with sufficiently high frequencies no wind at all would be produced at
+the ordinary atmospheric pressures. With frequencies obtainable by means
+of a machine, the mechanical effect is sufficiently great to revolve,
+with considerable speed, large pin-wheels, which in the dark present a
+beautiful appearance owing to the abundance of the streams (Fig. 106).
+
+[Illustration: FIG. 107.]
+
+[Illustration: FIG. 108.]
+
+In general, most of the experiments usually performed with a static
+machine can be performed with an induction coil when operated with very
+rapidly alternating currents. The effects produced, however, are much
+more striking, being of incomparably greater power. When a small length
+of ordinary cotton covered wire, Fig. 107, is attached to one terminal
+of the coil, the streams issuing from all points of the wire may be so
+intense as to produce a considerable light effect. When the potentials
+and frequencies are very high, a wire insulated with gutta percha or
+rubber and attached to one of the terminals, appears to be covered with
+a luminous film. A very thin bare wire when attached to a terminal emits
+powerful streams and vibrates continually to and fro or spins in a
+circle, producing a singular effect (Fig. 108). Some of these
+experiments have been described by me in _The Electrical World_, of
+February 21, 1891.
+
+Another peculiarity of the rapidly alternating discharge of the
+induction coil is its radically different behavior with respect to
+points and rounded surfaces.
+
+If a thick wire, provided with a ball at one end and with a point at the
+other, be attached to the positive terminal of a static machine,
+practically all the charge will be lost through the point, on account of
+the enormously greater tension, dependent on the radius of curvature.
+But if such a wire is attached to one of the terminals of the induction
+coil, it will be observed that with very high frequencies streams issue
+from the ball almost as copiously as from the point (Fig. 109).
+
+It is hardly conceivable that we could produce such a condition to an
+equal degree in a static machine, for the simple reason, that the
+tension increases as the square of the density, which in turn is
+proportional to the radius of curvature; hence, with a steady potential
+an enormous charge would be required to make streams issue from a
+polished ball while it is connected with a point. But with an induction
+coil the discharge of which alternates with great rapidity it is
+different. Here we have to deal with two distinct tendencies. First,
+there is the tendency to escape which exists in a condition of rest, and
+which depends on the radius of curvature; second, there is the tendency
+to dissipate into the surrounding air by condenser action, which depends
+on the surface. When one of these tendencies is a maximum, the other is
+at a minimum. At the point the luminous stream is principally due to the
+air molecules coming bodily in contact with the point; they are
+attracted and repelled, charged and discharged, and, their atomic
+charges being thus disturbed, vibrate and emit light waves. At the ball,
+on the contrary, there is no doubt that the effect is to a great extent
+produced inductively, the air molecules not _necessarily_ coming in
+contact with the ball, though they undoubtedly do so. To convince
+ourselves of this we only need to exalt the condenser action, for
+instance, by enveloping the ball, at some distance, by a better
+conductor than the surrounding medium, the conductor being, of course,
+insulated; or else by surrounding it with a better dielectric and
+approaching an insulated conductor; in both cases the streams will break
+forth more copiously. Also, the larger the ball with a given frequency,
+or the higher the frequency, the more will the ball have the advantage
+over the point. But, since a certain intensity of action is required to
+render the streams visible, it is obvious that in the experiment
+described the ball should not be taken too large.
+
+In consequence of this two-fold tendency, it is possible to produce by
+means of points, effects identical to those produced by capacity. Thus,
+for instance, by attaching to one terminal of the coil a small length of
+soiled wire, presenting many points and offering great facility to
+escape, the potential of the coil may be raised to the same value as by
+attaching to the terminal a polished ball of a surface many times
+greater than that of the wire.
+
+[Illustration: FIG. 109.]
+
+[Illustration: FIG. 110.]
+
+An interesting experiment, showing the effect of the points, may be
+performed in the following manner: Attach to one of the terminals of the
+coil a cotton covered wire about two feet in length, and adjust the
+conditions so that streams issue from the wire. In this experiment the
+primary coil should be preferably placed so that it extends only about
+half way into the secondary coil. Now touch the free terminal of the
+secondary with a conducting object held in the hand, or else connect it
+to an insulated body of some size. In this manner the potential on the
+wire may be enormously raised. The effect of this will be either to
+increase, or to diminish, the streams. If they increase, the wire is too
+short; if they diminish, it is too long. By adjusting the length of the
+wire, a point is found where the touching of the other terminal does not
+at all affect the streams. In this case the rise of potential is exactly
+counteracted by the drop through the coil. It will be observed that
+small lengths of wire produce considerable difference in the magnitude
+and luminosity of the streams. The primary coil is placed sidewise for
+two reasons: First, to increase the potential at the wire; and, second,
+to increase the drop through the coil. The sensitiveness is thus
+augmented.
+
+There is still another and far more striking peculiarity of the brush
+discharge produced by very rapidly alternating currents. To observe this
+it is best to replace the usual terminals of the coil by two metal
+columns insulated with a good thickness of ebonite. It is also well to
+close all fissures and cracks with wax so that the brushes cannot form
+anywhere except at the tops of the columns. If the conditions are
+carefully adjusted--which, of course, must be left to the skill of the
+experimenter--so that the potential rises to an enormous value, one may
+produce two powerful brushes several inches long, nearly white at their
+roots, which in the dark bear a striking resemblance to two flames of a
+gas escaping under pressure (Fig. 110). But they do not only _resemble_,
+they _are_ veritable flames, for they are hot. Certainly they are not as
+hot as a gas burner, _but they would be so if the frequency and the
+potential would be sufficiently high_. Produced with, say, twenty
+thousand alternations per second, the heat is easily perceptible even if
+the potential is not excessively high. The heat developed is, of course,
+due to the impact of the air molecules against the terminals and against
+each other. As, at the ordinary pressures, the mean free path is
+excessively small, it is possible that in spite of the enormous initial
+speed imparted to each molecule upon coming in contact with the
+terminal, its progress--by collision with other molecules--is retarded
+to such an extent, that it does not get away far from the terminal, but
+may strike the same many times in succession. The higher the frequency,
+the less the molecule is able to get away, and this the more so, as for
+a given effect the potential required is smaller; and a frequency is
+conceivable--perhaps even obtainable--at which practically the same
+molecules would strike the terminal. Under such conditions the exchange
+of the molecules would be very slow, and the heat produced at, and very
+near, the terminal would be excessive. But if the frequency would go on
+increasing constantly, the heat produced would begin to diminish for
+obvious reasons. In the positive brush of a static machine the exchange
+of the molecules is very rapid, the stream is constantly of one
+direction, and there are fewer collisions; hence the heating effect must
+be very small. Anything that impairs the facility of exchange tends to
+increase the local heat produced. Thus, if a bulb be held over the
+terminal of the coil so as to enclose the brush, the air contained in
+the bulb is very quickly brought to a high temperature. If a glass tube
+be held over the brush so as to allow the draught to carry the brush
+upwards, scorching hot air escapes at the top of the tube. Anything held
+within the brush is, of course, rapidly heated, and the possibility of
+using such heating effects for some purpose or other suggests itself.
+
+When contemplating this singular phenomenon of the hot brush, we cannot
+help being convinced that a similar process must take place in the
+ordinary flame, and it seems strange that after all these centuries past
+of familiarity with the flame, now, in this era of electric lighting and
+heating, we are finally led to recognize, that since time immemorial we
+have, after all, always had "electric light and heat" at our disposal.
+It is also of no little interest to contemplate, that we have a possible
+way of producing--by other than chemical means--a veritable flame, which
+would give light and heat without any material being consumed, without
+any chemical process taking place, and to accomplish this, we only need
+to perfect methods of producing enormous frequencies and potentials. I
+have no doubt that if the potential could be made to alternate with
+sufficient rapidity and power, the brush formed at the end of a wire
+would lose its electrical characteristics and would become flamelike.
+The flame must be due to electrostatic molecular action.
+
+This phenomenon now explains in a manner which can hardly be doubted the
+frequent accidents occurring in storms. It is well known that objects
+are often set on fire without the lightning striking them. We shall
+presently see how this can happen. On a nail in a roof, for instance, or
+on a projection of any kind, more or less conducting, or rendered so by
+dampness, a powerful brush may appear. If the lightning strikes
+somewhere in the neighborhood the enormous potential may be made to
+alternate or fluctuate perhaps many million times a second. The air
+molecules are violently attracted and repelled, and by their impact
+produce such a powerful heating effect that a fire is started. It is
+conceivable that a ship at sea may, in this manner, catch fire at many
+points at once. When we consider, that even with the comparatively low
+frequencies obtained from a dynamo machine, and with potentials of no
+more than one or two hundred thousand volts, the heating effects are
+considerable, we may imagine how much more powerful they must be with
+frequencies and potentials many times greater; and the above explanation
+seems, to say the least, very probable. Similar explanations may have
+been suggested, but I am not aware that, up to the present, the heating
+effects of a brush produced by a rapidly alternating potential have been
+experimentally demonstrated, at least not to such a remarkable degree.
+
+[Illustration: FIG. 111.]
+
+By preventing completely the exchange of the air molecules, the local
+heating effect may be so exalted as to bring a body to incandescence.
+Thus, for instance, if a small button, or preferably a very thin wire or
+filament be enclosed in an unexhausted globe and connected with the
+terminal of the coil, it may be rendered incandescent. The phenomenon is
+made much more interesting by the rapid spinning round in a circle of
+the top of the filament, thus presenting the appearance of a luminous
+funnel, Fig. 111, which widens when the potential is increased. When the
+potential is small the end of the filament may perform irregular
+motions, suddenly changing from one to the other, or it may describe an
+ellipse; but when the potential is very high it always spins in a
+circle; and so does generally a thin straight wire attached freely to
+the terminal of the coil. These motions are, of course, due to the
+impact of the molecules, and the irregularity in the distribution of the
+potential, owing to the roughness and dissymmetry of the wire or
+filament. With a perfectly symmetrical and polished wire such motions
+would probably not occur. That the motion is not likely to be due to
+others causes is evident from the fact that it is not of a definite
+direction, and that in a very highly exhausted globe it ceases
+altogether. The possibility of bringing a body to incandescence in an
+exhausted globe, or even when not at all enclosed, would seem to afford
+a possible way of obtaining light effects, which, in perfecting methods
+of producing rapidly alternating potentials, might be rendered available
+for useful purposes.
+
+[Illustration: FIG. 112a.]
+
+In employing a commercial coil, the production of very powerful brush
+effects is attended with considerable difficulties, for when these high
+frequencies and enormous potentials are used, the best insulation is apt
+to give way. Usually the coil is insulated well enough to stand the
+strain from convolution to convolution, since two double silk covered
+paraffined wires will withstand a pressure of several thousand volts;
+the difficulty lies principally in preventing the breaking through from
+the secondary to the primary, which is greatly facilitated by the
+streams issuing from the latter. In the coil, of course, the strain is
+greatest from section to section, but usually in a larger coil there are
+so many sections that the danger of a sudden giving way is not very
+great. No difficulty will generally be encountered in that direction,
+and besides, the liability of injuring the coil internally is very much
+reduced by the fact that the effect most likely to be produced is simply
+a gradual heating, which, when far enough advanced, could not fail to
+be observed. The principal necessity is then to prevent the streams
+between the primary and the tube, not only on account of the heating and
+possible injury, but also because the streams may diminish very
+considerably the potential difference available at the terminals. A few
+hints as to how this may be accomplished will probably be found useful
+in most of these experiments with the ordinary induction coil.
+
+[Illustration: FIG. 112b.]
+
+One of the ways is to wind a short primary, Fig. 112a, so that the
+difference of potential is not at that length great enough to cause the
+breaking forth of the streams through the insulating tube. The length of
+the primary should be determined by experiment. Both the ends of the
+coil should be brought out on one end through a plug of insulating
+material fitting in the tube as illustrated. In such a disposition one
+terminal of the secondary is attached to a body, the surface of which is
+determined with the greatest care so as to produce the greatest rise in
+the potential. At the other terminal a powerful brush appears, which may
+be experimented upon.
+
+The above plan necessitates the employment of a primary of comparatively
+small size, and it is apt to heat when powerful effects are desirable
+for a certain length of time. In such a case it is better to employ a
+larger coil, Fig. 112b, and introduce it from one side of the tube,
+until the streams begin to appear. In this case the nearest terminal of
+the secondary may be connected to the primary or to the ground, which is
+practically the same thing, if the primary is connected directly to the
+machine. In the case of ground connections it is well to determine
+experimentally the frequency which is best suited under the conditions
+of the test. Another way of obviating the streams, more or less, is to
+make the primary in sections and supply it from separate, well insulated
+sources.
+
+In many of these experiments, when powerful effects are wanted for a
+short time, it is advantageous to use iron cores with the primaries. In
+such case a very large primary coil may be wound and placed side by side
+with the secondary, and, the nearest terminal of the latter being
+connected to the primary, a laminated iron core is introduced through
+the primary into the secondary as far as the streams will permit. Under
+these conditions an excessively powerful brush, several inches long,
+which may be appropriately called "St. Elmo's hot fire," may be caused
+to appear at the other terminal of the secondary, producing striking
+effects. It is a most powerful ozonizer, so powerful indeed, that only a
+few minutes are sufficient to fill the whole room with the smell of
+ozone, and it undoubtedly possesses the quality of exciting chemical
+affinities.
+
+For the production of ozone, alternating currents of very high frequency
+are eminently suited, not only on account of the advantages they offer
+in the way of conversion but also because of the fact, that the
+ozonizing action of a discharge is dependent on the frequency as well as
+on the potential, this being undoubtedly confirmed by observation.
+
+In these experiments if an iron core is used it should be carefully
+watched, as it is apt to get excessively hot in an incredibly short
+time. To give an idea of the rapidity of the heating, I will state, that
+by passing a powerful current through a coil with many turns, the
+inserting within the same of a thin iron wire for no more than one
+second's time is sufficient to heat the wire to something like 100 deg. C.
+
+But this rapid heating need not discourage us in the use of iron cores
+in connection with rapidly alternating currents. I have for a long time
+been convinced that in the industrial distribution by means of
+transformers, some such plan as the following might be practicable. We
+may use a comparatively small iron core, subdivided, or perhaps not even
+subdivided. We may surround this core with a considerable thickness of
+material which is fire-proof and conducts the heat poorly, and on top of
+that we may place the primary and secondary windings. By using either
+higher frequencies or greater magnetizing forces, we may by hysteresis
+and eddy currents heat the iron core so far as to bring it nearly to its
+maximum permeability, which, as Hopkinson has shown, may be as much as
+sixteen times greater than that at ordinary temperatures. If the iron
+core were perfectly enclosed, it would not be deteriorated by the heat,
+and, if the enclosure of fire-proof material would be sufficiently
+thick, only a limited amount of energy could be radiated in spite of the
+high temperature. Transformers have been constructed by me on that plan,
+but for lack of time, no thorough tests have as yet been made.
+
+Another way of adapting the iron core to rapid alternations, or,
+generally speaking, reducing the frictional losses, is to produce by
+continuous magnetization a flow of something like seven thousand or
+eight thousand lines per square centimetre through the core, and then
+work with weak magnetizing forces and preferably high frequencies around
+the point of greatest permeability. A higher efficiency of conversion
+and greater output are obtainable in this manner. I have also employed
+this principle in connection with machines in which there is no reversal
+of polarity. In these types of machines, as long as there are only few
+pole projections, there is no great gain, as the maxima and minima of
+magnetization are far from the point of maximum permeability; but when
+the number of the pole projections is very great, the required rate of
+change may be obtained, without the magnetization varying so far as to
+depart greatly from the point of maximum permeability, and the gain is
+considerable.
+
+The above described arrangements refer only to the use of commercial
+coils as ordinarily constructed. If it is desired to construct a coil
+for the express purpose of performing with it such experiments as I have
+described, or, generally, rendering it capable of withstanding the
+greatest possible difference of potential, then a construction as
+indicated in Fig. 113 will be found of advantage. The coil in this case
+is formed of two independent parts which are wound oppositely, the
+connection between both being made near the primary. The potential in
+the middle being zero, there is not much tendency to jump to the primary
+and not much insulation is required. In some cases the middle point may,
+however, be connected to the primary or to the ground. In such a coil
+the places of greatest difference of potential are far apart and the
+coil is capable of withstanding an enormous strain. The two parts may be
+movable so as to allow a slight adjustment of the capacity effect.
+
+As to the manner of insulating the coil, it will be found convenient to
+proceed in the following way: First, the wire should be boiled in
+paraffine until all the air is out; then the coil is wound by running
+the wire through melted paraffine, merely for the purpose of fixing the
+wire. The coil is then taken off from the spool, immersed in a
+cylindrical vessel filled with pure melted wax and boiled for a long
+time until the bubbles cease to appear. The whole is then left to cool
+down thoroughly, and then the mass is taken out of the vessel and turned
+up in a lathe. A coil made in this manner and with care is capable of
+withstanding enormous potential differences.
+
+[Illustration: FIG. 113.]
+
+It may be found convenient to immerse the coil in paraffine oil or some
+other kind of oil; it is a most effective way of insulating, principally
+on account of the perfect exclusion of air, but it may be found that,
+after all, a vessel filled with oil is not a very convenient thing to
+handle in a laboratory.
+
+If an ordinary coil can be dismounted, the primary may be taken out of
+the tube and the latter plugged up at one end, filled with oil, and the
+primary reinserted. This affords an excellent insulation and prevents
+the formation of the streams.
+
+Of all the experiments which may be performed with rapidly alternating
+currents the most interesting are those which concern the production of
+a practical illuminant. It cannot be denied that the present methods,
+though they were brilliant advances, are very wasteful. Some better
+methods must be invented, some more perfect apparatus devised. Modern
+research has opened new possibilities for the production of an efficient
+source of light, and the attention of all has been turned in the
+direction indicated by able pioneers. Many have been carried away by
+the enthusiasm and passion to discover, but in their zeal to reach
+results, some have been misled. Starting with the idea of producing
+electro-magnetic waves, they turned their attention, perhaps, too much
+to the study of electro-magnetic effects, and neglected the study of
+electrostatic phenomena. Naturally, nearly every investigator availed
+himself of an apparatus similar to that used in earlier experiments. But
+in those forms of apparatus, while the electro-magnetic inductive
+effects are enormous, the electrostatic effects are excessively small.
+
+In the Hertz experiments, for instance, a high tension induction coil is
+short circuited by an arc, the resistance of which is very small, the
+smaller, the more capacity is attached to the terminals; and the
+difference of potential at these is enormously diminished. On the other
+hand, when the discharge is not passing between the terminals, the
+static effects may be considerable, but only qualitatively so, not
+quantitatively, since their rise and fall is very sudden, and since
+their frequency is small. In neither case, therefore, are powerful
+electrostatic effects perceivable. Similar conditions exist when, as in
+some interesting experiments of Dr. Lodge, Leyden jars are discharged
+disruptively. It has been thought--and I believe asserted--that in such
+cases most of the energy is radiated into space. In the light of the
+experiments which I have described above, it will now not be thought so.
+I feel safe in asserting that in such cases most of the energy is partly
+taken up and converted into heat in the arc of the discharge and in the
+conducting and insulating material of the jar, some energy being, of
+course, given off by electrification of the air; but the amount of the
+directly radiated energy is very small.
+
+When a high tension induction coil, operated by currents alternating
+only 20,000 times a second, has its terminals closed through even a very
+small jar, practically all the energy passes through the dielectric of
+the jar, which is heated, and the electrostatic effects manifest
+themselves outwardly only to a very weak degree. Now the external
+circuit of a Leyden jar, that is, the arc and the connections of the
+coatings, may be looked upon as a circuit generating alternating
+currents of excessively high frequency and fairly high potential, which
+is closed through the coatings and the dielectric between them, and from
+the above it is evident that the external electrostatic effects must be
+very small, even if a recoil circuit be used. These conditions make it
+appear that with the apparatus usually at hand, the observation of
+powerful electrostatic effects was impossible, and what experience has
+been gained in that direction is only due to the great ability of the
+investigators.
+
+But powerful electrostatic effects are a _sine qua non_ of light
+production on the lines indicated by theory. Electro-magnetic effects
+are primarily unavailable, for the reason that to produce the required
+effects we would have to pass current impulses through a conductor,
+which, long before the required frequency of the impulses could be
+reached, would cease to transmit them. On the other hand,
+electro-magnetic waves many times longer than those of light, and
+producible by sudden discharge of a condenser, could not be utilized, it
+would seem, except we avail ourselves of their effect upon conductors as
+in the present methods, which are wasteful. We could not affect by means
+of such waves the static molecular or atomic charges of a gas, cause
+them to vibrate and to emit light. Long transverse waves cannot,
+apparently, produce such effects, since excessively small
+electro-magnetic disturbances may pass readily through miles of air.
+Such dark waves, unless they are of the length of true light waves,
+cannot, it would seem, excite luminous radiation in a Geissler tube, and
+the luminous effects, which are producible by induction in a tube devoid
+of electrodes, I am inclined to consider as being of an electrostatic
+nature.
+
+To produce such luminous effects, straight electrostatic thrusts are
+required; these, whatever be their frequency, may disturb the molecular
+charges and produce light. Since current impulses of the required
+frequency cannot pass through a conductor of measurable dimensions, we
+must work with a gas, and then the production of powerful electrostatic
+effects becomes an imperative necessity.
+
+It has occurred to me, however, that electrostatic effects are in many
+ways available for the production of light. For instance, we may place a
+body of some refractory material in a closed, and preferably more or
+less exhausted, globe, connect it to a source of high, rapidly
+alternating potential, causing the molecules of the gas to strike it
+many times a second at enormous speeds, and in this manner, with
+trillions of invisible hammers, pound it until it gets incandescent; or
+we may place a body in a very highly exhausted globe, in a non-striking
+vacuum, and, by employing very high frequencies and potentials,
+transfer sufficient energy from it to other bodies in the vicinity, or
+in general to the surroundings, to maintain it at any degree of
+incandescence; or we may, by means of such rapidly alternating high
+potentials, disturb the ether carried by the molecules of a gas or their
+static charges, causing them to vibrate and to emit light.
+
+But, electrostatic effects being dependent upon the potential and
+frequency, to produce the most powerful action it is desirable to
+increase both as far as practicable. It may be possible to obtain quite
+fair results by keeping either of these factors small, provided the
+other is sufficiently great; but we are limited in both directions. My
+experience demonstrates that we cannot go below a certain frequency,
+for, first, the potential then becomes so great that it is dangerous;
+and, secondly, the light production is less efficient.
+
+I have found that, by using the ordinary low frequencies, the
+physiological effect of the current required to maintain at a certain
+degree of brightness a tube four feet long, provided at the ends with
+outside and inside condenser coatings, is so powerful that, I think, it
+might produce serious injury to those not accustomed to such shocks;
+whereas, with twenty thousand alternations per second, the tube may be
+maintained at the same degree of brightness without any effect being
+felt. This is due principally to the fact that a much smaller potential
+is required to produce the same light effect, and also to the higher
+efficiency in the light production. It is evident that the efficiency in
+such cases is the greater, the higher the frequency, for the quicker the
+process of charging and discharging the molecules, the less energy will
+be lost in the form of dark radiation. But, unfortunately, we cannot go
+beyond a certain frequency on account of the difficulty of producing and
+conveying the effects.
+
+I have stated above that a body inclosed in an unexhausted bulb may be
+intensely heated by simply connecting it with a source of rapidly
+alternating potential. The heating in such a case is, in all
+probability, due mostly to the bombardment of the molecules of the gas
+contained in the bulb. When the bulb is exhausted, the heating of the
+body is much more rapid, and there is no difficulty whatever in bringing
+a wire or filament to any degree of incandescence by simply connecting
+it to one terminal of a coil of the proper dimensions. Thus, if the
+well-known apparatus of Prof. Crookes, consisting of a bent platinum
+wire with vanes mounted over it (Fig. 114), be connected to one
+terminal of the coil--either one or both ends of the platinum wire being
+connected--the wire is rendered almost instantly incandescent, and the
+mica vanes are rotated as though a current from a battery were used. A
+thin carbon filament, or, preferably, a button of some refractory
+material (Fig. 115), even if it be a comparatively poor conductor,
+inclosed in an exhausted globe, may be rendered highly incandescent; and
+in this manner a simple lamp capable of giving any desired candle power
+is provided.
+
+The success of lamps of this kind would depend largely on the selection
+of the light-giving bodies contained within the bulb. Since, under the
+conditions described, refractory bodies--which are very poor conductors
+and capable of withstanding for a long time excessively high degrees of
+temperature--may be used, such illuminating devices may be rendered
+successful.
+
+[Illustration: FIG. 114.]
+
+[Illustration: FIG. 115.]
+
+It might be thought at first that if the bulb, containing the filament
+or button of refractory material, be perfectly well exhausted--that is,
+as far as it can be done by the use of the best apparatus--the heating
+would be much less intense, and that in a perfect vacuum it could not
+occur at all. This is not confirmed by my experience; quite the
+contrary, the better the vacuum the more easily the bodies are brought
+to incandescence. This result is interesting for many reasons.
+
+At the outset of this work the idea presented itself to me, whether two
+bodies of refractory material enclosed in a bulb exhausted to such a
+degree that the discharge of a large induction coil, operated in the
+usual manner, cannot pass through, could be rendered incandescent by
+mere condenser action. Obviously, to reach this result enormous
+potential differences and very high frequencies are required, as is
+evident from a simple calculation.
+
+But such a lamp would possess a vast advantage over an ordinary
+incandescent lamp in regard to efficiency. It is well-known that the
+efficiency of a lamp is to some extent a function of the degree of
+incandescence, and that, could we but work a filament at many times
+higher degrees of incandescence, the efficiency would be much greater.
+In an ordinary lamp this is impracticable on account of the destruction
+of the filament, and it has been determined by experience how far it is
+advisable to push the incandescence. It is impossible to tell how much
+higher efficiency could be obtained if the filament could withstand
+indefinitely, as the investigation to this end obviously cannot be
+carried beyond a certain stage; but there are reasons for believing that
+it would be very considerably higher. An improvement might be made in
+the ordinary lamp by employing a short and thick carbon; but then the
+leading-in wires would have to be thick, and, besides, there are many
+other considerations which render such a modification entirely
+impracticable. But in a lamp as above described, the leading in wires
+may be very small, the incandescent refractory material may be in the
+shape of blocks offering a very small radiating surface, so that less
+energy would be required to keep them at the desired incandescence; and
+in addition to this, the refractory material need not be carbon, but may
+be manufactured from mixtures of oxides, for instance, with carbon or
+other material, or may be selected from bodies which are practically
+non-conductors, and capable of withstanding enormous degrees of
+temperature.
+
+All this would point to the possibility of obtaining a much higher
+efficiency with such a lamp than is obtainable in ordinary lamps. In my
+experience it has been demonstrated that the blocks are brought to high
+degrees of incandescence with much lower potentials than those
+determined by calculation, and the blocks may be set at greater
+distances from each other. We may freely assume, and it is probable,
+that the molecular bombardment is an important element in the heating,
+even if the globe be exhausted with the utmost care, as I have done; for
+although the number of the molecules is, comparatively speaking,
+insignificant, yet on account of the mean free path being very great,
+there are fewer collisions, and the molecules may reach much higher
+speeds, so that the heating effect due to this cause may be
+considerable, as in the Crookes experiments with radiant matter.
+
+But it is likewise possible that we have to deal here with an increased
+facility of losing the charge in very high vacuum, when the potential is
+rapidly alternating, in which case most of the heating would be directly
+due to the surging of the charges in the heated bodies. Or else the
+observed fact may be largely attributable to the effect of the points
+which I have mentioned above, in consequence of which the blocks or
+filaments contained in the vacuum are equivalent to condensers of many
+times greater surface than that calculated from their geometrical
+dimensions. Scientific men still differ in opinion as to whether a
+charge should, or should not, be lost in a perfect vacuum, or in other
+words, whether ether is, or is not, a conductor. If the former were the
+case, then a thin filament enclosed in a perfectly exhausted globe, and
+connected to a source of enormous, steady potential, would be brought to
+incandescence.
+
+[Illustration: FIG. 116.]
+
+[Illustration: FIG. 117.]
+
+Various forms of lamps on the above described principle, with the
+refractory bodies in the form of filaments, Fig. 116, or blocks, Fig.
+117, have been constructed and operated by me, and investigations are
+being carried on in this line. There is no difficulty in reaching such
+high degrees of incandescence that ordinary carbon is to all appearance
+melted and volatilized. If the vacuum could be made absolutely perfect,
+such a lamp, although inoperative with apparatus ordinarily used, would,
+if operated with currents of the required character, afford an
+illuminant which would never be destroyed, and which would be far more
+efficient than an ordinary incandescent lamp. This perfection can, of
+course, never be reached, and a very slow destruction and gradual
+diminution in size always occurs, as in incandescent filaments; but
+there is no possibility of a sudden and premature disabling which occurs
+in the latter by the breaking of the filament, especially when the
+incandescent bodies are in the shape of blocks.
+
+With these rapidly alternating potentials there is, however, no
+necessity of enclosing two blocks in a globe, but a single block, as in
+Fig. 115, or filament, Fig. 118, may be used. The potential in this case
+must of course be higher, but is easily obtainable, and besides it is
+not necessarily dangerous.
+
+[Illustration: FIG. 118.]
+
+The facility with which the button or filament in such a lamp is brought
+to incandescence, other things being equal, depends on the size of the
+globe. If a perfect vacuum could be obtained, the size of the globe
+would not be of importance, for then the heating would be wholly due to
+the surging of the charges, and all the energy would be given off to the
+surroundings by radiation. But this can never occur in practice. There
+is always some gas left in the globe, and although the exhaustion may be
+carried to the highest degree, still the space inside of the bulb must
+be considered as conducting when such high potentials are used, and I
+assume that, in estimating the energy that may be given off from the
+filament to the surroundings, we may consider the inside surface of the
+bulb as one coating of a condenser, the air and other objects
+surrounding the bulb forming the other coating. When the alternations
+are very low there is no doubt that a considerable portion of the energy
+is given off by the electrification of the surrounding air.
+
+In order to study this subject better, I carried on some experiments
+with excessively high potentials and low frequencies. I then observed
+that when the hand is approached to the bulb,--the filament being
+connected with one terminal of the coil,--a powerful vibration is felt,
+being due to the attraction and repulsion of the molecules of the air
+which are electrified by induction through the glass. In some cases when
+the action is very intense I have been able to hear a sound, which must
+be due to the same cause.
+
+[Illustration: FIG. 119.]
+
+[Illustration: FIG. 120.]
+
+When the alternations are low, one is apt to get an excessively powerful
+shock from the bulb. In general, when one attaches bulbs or objects of
+some size to the terminals of the coil, one should look out for the rise
+of potential, for it may happen that by merely connecting a bulb or
+plate to the terminal, the potential may rise to many times its original
+value. When lamps are attached to the terminals, as illustrated in Fig.
+119, then the capacity of the bulbs should be such as to give the
+maximum rise of potential under the existing conditions. In this manner
+one may obtain the required potential with fewer turns of wire.
+
+The life of such lamps as described above depends, of course, largely on
+the degree of exhaustion, but to some extent also on the shape of the
+block of refractory material. Theoretically it would seem that a small
+sphere of carbon enclosed in a sphere of glass would not suffer
+deterioration from molecular bombardment, for, the matter in the globe
+being radiant, the molecules would move in straight lines, and would
+seldom strike the sphere obliquely. An interesting thought in connection
+with such a lamp is, that in it "electricity" and electrical energy
+apparently must move in the same lines.
+
+[Illustration: FIG. 121a.]
+
+[Illustration: FIG. 121b.]
+
+The use of alternating currents of very high frequency makes it possible
+to transfer, by electrostatic or electromagnetic induction through the
+glass of a lamp, sufficient energy to keep a filament at incandescence
+and so do away with the leading-in wires. Such lamps have been proposed,
+but for want of proper apparatus they have not been successfully
+operated. Many forms of lamps on this principle with continuous and
+broken filaments have been constructed by me and experimented upon. When
+using a secondary enclosed within the lamp, a condenser is
+advantageously combined with the secondary. When the transference is
+effected by electrostatic induction, the potentials used are, of course,
+very high with frequencies obtainable from a machine. For instance, with
+a condenser surface of forty square centimetres, which is not
+impracticably large, and with glass of good quality 1 mm. thick, using
+currents alternating twenty thousand times a second, the potential
+required is approximately 9,000 volts. This may seem large, but since
+each lamp may be included in the secondary of a transformer of very
+small dimensions, it would not be inconvenient, and, moreover, it would
+not produce fatal injury. The transformers would all be preferably in
+series. The regulation would offer no difficulties, as with currents of
+such frequencies it is very easy to maintain a constant current.
+
+In the accompanying engravings some of the types of lamps of this kind
+are shown. Fig. 120 is such a lamp with a broken filament, and Figs. 121
+A and 121 B one with a single outside and inside coating and a single
+filament. I have also made lamps with two outside and inside coatings
+and a continuous loop connecting the latter. Such lamps have been
+operated by me with current impulses of the enormous frequencies
+obtainable by the disruptive discharge of condensers.
+
+The disruptive discharge of a condenser is especially suited for
+operating such lamps--with no outward electrical connections--by means
+of electromagnetic induction, the electromagnetic inductive effects
+being excessively high; and I have been able to produce the desired
+incandescence with only a few short turns of wire. Incandescence may
+also be produced in this manner in a simple closed filament.
+
+Leaving now out of consideration the practicability of such lamps, I
+would only say that they possess a beautiful and desirable feature,
+namely, that they can be rendered, at will, more or less brilliant
+simply by altering the relative position of the outside and inside
+condenser coatings, or inducing and induced circuits.
+
+When a lamp is lighted by connecting it to one terminal only of the
+source, this may be facilitated by providing the globe with an outside
+condenser coating, which serves at the same time as a reflector, and
+connecting this to an insulated body of some size. Lamps of this kind
+are illustrated in Fig. 122 and Fig. 123. Fig. 124 shows the plan of
+connection. The brilliancy of the lamp may, in this case, be regulated
+within wide limits by varying the size of the insulated metal plate to
+which the coating is connected.
+
+It is likewise practicable to light with one leading wire lamps such as
+illustrated in Fig. 116 and Fig. 117, by connecting one terminal of the
+lamp to one terminal of the source, and the other to an insulated body
+of the required size. In all cases the insulated body serves to give off
+the energy into the surrounding space, and is equivalent to a return
+wire. Obviously, in the two last-named cases, instead of connecting the
+wires to an insulated body, connections may be made to the ground.
+
+The experiments which will prove most suggestive and of most interest to
+the investigator are probably those performed with exhausted tubes. As
+might be anticipated, a source of such rapidly alternating potentials is
+capable of exciting the tubes at a considerable distance, and the light
+effects produced are remarkable.
+
+[Illustration: FIG. 122.]
+
+[Illustration: FIG. 123.]
+
+During my investigations in this line I endeavored to excite tubes,
+devoid of any electrodes, by electromagnetic induction, making the tube
+the secondary of the induction device, and passing through the primary
+the discharges of a Leyden jar. These tubes were made of many shapes,
+and I was able to obtain luminous effects which I then thought were due
+wholly to electromagnetic induction. But on carefully investigating the
+phenomena I found that the effects produced were more of an
+electrostatic nature. It may be attributed to this circumstance that
+this mode of exciting tubes is very wasteful, namely, the primary
+circuit being closed, the potential, and consequently the electrostatic
+inductive effect, is much diminished.
+
+When an induction coil, operated as above described, is used, there is
+no doubt that the tubes are excited by electrostatic induction, and that
+electromagnetic induction has little, if anything, to do with the
+phenomena.
+
+[Illustration: FIG. 124.]
+
+This is evident from many experiments. For instance, if a tube be taken
+in one hand, the observer being near the coil, it is brilliantly lighted
+and remains so no matter in what position it is held relatively to the
+observer's body. Were the action electromagnetic, the tube could not be
+lighted when the observer's body is interposed between it and the coil,
+or at least its luminosity should be considerably diminished. When the
+tube is held exactly over the centre of the coil--the latter being wound
+in sections and the primary placed symmetrically to the secondary--it
+may remain completely dark, whereas it is rendered intensely luminous by
+moving it slightly to the right or left from the centre of the coil. It
+does not light because in the middle both halves of the coil neutralize
+each other, and the electric potential is zero. If the action were
+electromagnetic, the tube should light best in the plane through the
+centre of the coil, since the electromagnetic effect there should be a
+maximum. When an arc is established between the terminals, the tubes and
+lamps in the vicinity of the coil go out, but light up again when the
+arc is broken, on account of the rise of potential. Yet the
+electromagnetic effect should be practically the same in both cases.
+
+By placing a tube at some distance from the coil, and nearer to one
+terminal--preferably at a point on the axis of the coil--one may light
+it by touching the remote terminal with an insulated body of some size
+or with the hand, thereby raising the potential at that terminal nearer
+to the tube. If the tube is shifted nearer to the coil so that it is
+lighted by the action of the nearer terminal, it may be made to go out
+by holding, on an insulated support, the end of a wire connected to the
+remote terminal, in the vicinity of the nearer terminal, by this means
+counteracting the action of the latter upon the tube. These effects are
+evidently electrostatic. Likewise, when a tube is placed at a
+considerable distance from the coil, the observer may, standing upon an
+insulated support between coil and tube, light the latter by approaching
+the hand to it; or he may even render it luminous by simply stepping
+between it and the coil. This would be impossible with electro-magnetic
+induction, for the body of the observer would act as a screen.
+
+When the coil is energized by excessively weak currents, the
+experimenter may, by touching one terminal of the coil with the tube,
+extinguish the latter, and may again light it by bringing it out of
+contact with the terminal and allowing a small arc to form. This is
+clearly due to the respective lowering and raising of the potential at
+that terminal. In the above experiment, when the tube is lighted through
+a small arc, it may go out when the arc is broken, because the
+electrostatic inductive effect alone is too weak, though the potential
+may be much higher; but when the arc is established, the electrification
+of the end of the tube is much greater, and it consequently lights.
+
+If a tube is lighted by holding it near to the coil, and in the hand
+which is remote, by grasping the tube anywhere with the other hand, the
+part between the hands is rendered dark, and the singular effect of
+wiping out the light of the tube may be produced by passing the hand
+quickly along the tube and at the same time withdrawing it gently from
+the coil, judging properly the distance so that the tube remains dark
+afterwards.
+
+If the primary coil is placed sidewise, as in Fig. 112 B for instance,
+and an exhausted tube be introduced from the other side in the hollow
+space, the tube is lighted most intensely because of the increased
+condenser action, and in this position the striae are most sharply
+defined. In all these experiments described, and in many others, the
+action is clearly electrostatic.
+
+The effects of screening also indicate the electrostatic nature of the
+phenomena and show something of the nature of electrification through
+the air. For instance, if a tube is placed in the direction of the axis
+of the coil, and an insulated metal plate be interposed, the tube will
+generally increase in brilliancy, or if it be too far from the coil to
+light, it may even be rendered luminous by interposing an insulated
+metal plate. The magnitude of the effects depends to some extent on the
+size of the plate. But if the metal plate be connected by a wire to the
+ground, its interposition will always make the tube go out even if it be
+very near the coil. In general, the interposition of a body between the
+coil and tube, increases or diminishes the brilliancy of the tube, or
+its facility to light up, according to whether it increases or
+diminishes the electrification. When experimenting with an insulated
+plate, the plate should not be taken too large, else it will generally
+produce a weakening effect by reason of its great facility for giving
+off energy to the surroundings.
+
+If a tube be lighted at some distance from the coil, and a plate of hard
+rubber or other insulating substance be interposed, the tube may be made
+to go out. The interposition of the dielectric in this case only
+slightly increases the inductive effect, but diminishes considerably the
+electrification through the air.
+
+In all cases, then, when we excite luminosity in exhausted tubes by
+means of such a coil, the effect is due to the rapidly alternating
+electrostatic potential; and, furthermore, it must be attributed to the
+harmonic alternation produced directly by the machine, and not to any
+superimposed vibration which might be thought to exist. Such
+superimposed vibrations are impossible when we work with an alternate
+current machine. If a spring be gradually tightened and released, it
+does not perform independent vibrations; for this a sudden release is
+necessary. So with the alternate currents from a dynamo machine; the
+medium is harmonically strained and released, this giving rise to only
+one kind of waves; a sudden contact or break, or a sudden giving way of
+the dielectric, as in the disruptive discharge of a Leyden jar, are
+essential for the production of superimposed waves.
+
+In all the last described experiments, tubes devoid of any electrodes
+may be used, and there is no difficulty in producing by their means
+sufficient light to read by. The light effect is, however, considerably
+increased by the use of phosphorescent bodies such as yttria, uranium
+glass, etc. A difficulty will be found when the phosphorescent material
+is used, for with these powerful effects, it is carried gradually away,
+and it is preferable to use material in the form of a solid.
+
+Instead of depending on induction at a distance to light the tube, the
+same may be provided with an external--and, if desired, also with an
+internal--condenser coating, and it may then be suspended anywhere in
+the room from a conductor connected to one terminal of the coil, and in
+this manner a soft illumination may be provided.
+
+[Illustration: FIG. 125.]
+
+The ideal way of lighting a hall or room would, however, be to produce
+such a condition in it that an illuminating device could be moved and
+put anywhere, and that it is lighted, no matter where it is put and
+without being electrically connected to anything. I have been able to
+produce such a condition by creating in the room a powerful, rapidly
+alternating electrostatic field. For this purpose I suspend a sheet of
+metal a distance from the ceiling on insulating cords and connect it to
+one terminal of the induction coil, the other terminal being preferably
+connected to the ground. Or else I suspend two sheets as illustrated in
+Fig. 125, each sheet being connected with one of the terminals of the
+coil, and their size being carefully determined. An exhausted tube may
+then be carried in the hand anywhere between the sheets or placed
+anywhere, even a certain distance beyond them; it remains always
+luminous.
+
+In such an electrostatic field interesting phenomena may be observed,
+especially if the alternations are kept low and the potentials
+excessively high. In addition to the luminous phenomena mentioned, one
+may observe that any insulated conductor gives sparks when the hand or
+another object is approached to it, and the sparks may often be
+powerful. When a large conducting object is fastened on an insulating
+support, and the hand approached to it, a vibration, due to the
+rythmical motion of the air molecules is felt, and luminous streams may
+be perceived when the hand is held near a pointed projection. When a
+telephone receiver is made to touch with one or both of its terminals an
+insulated conductor of some size, the telephone emits a loud sound; it
+also emits a sound when a length of wire is attached to one or both
+terminals, and with very powerful fields a sound may be perceived even
+without any wire.
+
+How far this principle is capable of practical application, the future
+will tell. It might be thought that electrostatic effects are unsuited
+for such action at a distance. Electromagnetic inductive effects, if
+available for the production of light, might be thought better suited.
+It is true the electrostatic effects diminish nearly with the cube of
+the distance from the coil, whereas the electromagnetic inductive
+effects diminish simply with the distance. But when we establish an
+electrostatic field of force, the condition is very different, for then,
+instead of the differential effect of both the terminals, we get their
+conjoint effect. Besides, I would call attention to the effect, that in
+an alternating electrostatic field, a conductor, such as an exhausted
+tube, for instance, tends to take up most of the energy, whereas in an
+electromagnetic alternating field the conductor tends to take up the
+least energy, the waves being reflected with but little loss. This is
+one reason why it is difficult to excite an exhausted tube, at a
+distance, by electromagnetic induction. I have wound coils of very large
+diameter and of many turns of wire, and connected a Geissler tube to the
+ends of the coil with the object of exciting the tube at a distance; but
+even with the powerful inductive effects producible by Leyden jar
+discharges, the tube could not be excited unless at a very small
+distance, although some judgment was used as to the dimensions of the
+coil. I have also found that even the most powerful Leyden jar
+discharges are capable of exciting only feeble luminous effects in a
+closed exhausted tube, and even these effects upon thorough examination
+I have been forced to consider of an electrostatic nature.
+
+How then can we hope to produce the required effects at a distance by
+means of electromagnetic action, when even in the closest proximity to
+the source of disturbance, under the most advantageous conditions, we
+can excite but faint luminosity? It is true that when acting at a
+distance we have the resonance to help us out. We can connect an
+exhausted tube, or whatever the illuminating device may be, with an
+insulated system of the proper capacity, and so it may be possible to
+increase the effect qualitatively, and only qualitatively, for we would
+not get _more_ energy through the device. So we may, by resonance
+effect, obtain the required electromotive force in an exhausted tube,
+and excite faint luminous effects, but we cannot get enough energy to
+render the light practically available, and a simple calculation, based
+on experimental results, shows that even if all the energy which a tube
+would receive at a certain distance from the source should be wholly
+converted into light, it would hardly satisfy the practical
+requirements. Hence the necessity of directing, by means of a conducting
+circuit, the energy to the place of transformation. But in so doing we
+cannot very sensibly depart from present methods, and all we could do
+would be to improve the apparatus.
+
+From these considerations it would seem that if this ideal way of
+lighting is to be rendered practicable it will be only by the use of
+electrostatic effects. In such a case the most powerful electrostatic
+inductive effects are needed; the apparatus employed must, therefore, be
+capable of producing high electrostatic potentials changing in value
+with extreme rapidity. High frequencies are especially wanted, for
+practical considerations make it desirable to keep down the potential.
+By the employment of machines, or, generally speaking, of any
+mechanical apparatus, but low frequencies can be reached; recourse must,
+therefore, be had to some other means. The discharge of a condenser
+affords us a means of obtaining frequencies by far higher than are
+obtainable mechanically, and I have accordingly employed condensers in
+the experiments to the above end.
+
+When the terminals of a high tension induction coil, Fig. 126, are
+connected to a Leyden jar, and the latter is discharging disruptively
+into a circuit, we may look upon the arc playing between the knobs as
+being a source of alternating, or generally speaking, undulating
+currents, and then we have to deal with the familiar system of a
+generator of such currents, a circuit connected to it, and a condenser
+bridging the circuit. The condenser in such case is a veritable
+transformer, and since the frequency is excessive, almost any ratio in
+the strength of the currents in both the branches may be obtained. In
+reality the analogy is not quite complete, for in the disruptive
+discharge we have most generally a fundamental instantaneous variation
+of comparatively low frequency, and a superimposed harmonic vibration,
+and the laws governing the flow of currents are not the same for both.
+
+In converting in this manner, the ratio of conversion should not be too
+great, for the loss in the arc between the knobs increases with the
+square of the current, and if the jar be discharged through very thick
+and short conductors, with the view of obtaining a very rapid
+oscillation, a very considerable portion of the energy stored is lost.
+On the other hand, too small ratios are not practicable for many obvious
+reasons.
+
+As the converted currents flow in a practically closed circuit, the
+electrostatic effects are necessarily small, and I therefore convert
+them into currents or effects of the required character. I have effected
+such conversions in several ways. The preferred plan of connections is
+illustrated in Fig. 127. The manner of operating renders it easy to
+obtain by means of a small and inexpensive apparatus enormous
+differences of potential which have been usually obtained by means of
+large and expensive coils. For this it is only necessary to take an
+ordinary small coil, adjust to it a condenser and discharging circuit,
+forming the primary of an auxiliary small coil, and convert upward. As
+the inductive effect of the primary currents is excessively great, the
+second coil need have comparatively but very few turns. By properly
+adjusting the elements, remarkable results may be secured.
+
+In endeavoring to obtain the required electrostatic effects in this
+manner, I have, as might be expected, encountered many difficulties
+which I have been gradually overcoming, but I am not as yet prepared to
+dwell upon my experiences in this direction.
+
+I believe that the disruptive discharge of a condenser will play an
+important part in the future, for it offers vast possibilities, not only
+in the way of producing light in a more efficient manner and in the line
+indicated by theory, but also in many other respects.
+
+[Illustration: FIG. 126.]
+
+For years the efforts of inventors have been directed towards obtaining
+electrical energy from heat by means of the thermopile. It might seem
+invidious to remark that but few know what is the real trouble with the
+thermopile. It is not the inefficiency or small output--though these are
+great drawbacks--but the fact that the thermopile has its phylloxera,
+that is, that by constant use it is deteriorated, which has thus far
+prevented its introduction on an industrial scale. Now that all modern
+research seems to point with certainty to the use of electricity of
+excessively high tension, the question must present itself to many
+whether it is not possible to obtain in a practicable manner this form
+of energy from heat. We have been used to look upon an electrostatic
+machine as a plaything, and somehow we couple with it the idea of the
+inefficient and impractical. But now we must think differently, for now
+we know that everywhere we have to deal with the same forces, and that
+it is a mere question of inventing proper methods or apparatus for
+rendering them available.
+
+In the present systems of electrical distribution, the employment of the
+iron with its wonderful magnetic properties allows us to reduce
+considerably the size of the apparatus; but, in spite of this, it is
+still very cumbersome. The more we progress in the study of electric and
+magnetic phenomena, the more we become convinced that the present
+methods will be short-lived. For the production of light, at least, such
+heavy machinery would seem to be unnecessary. The energy required is
+very small, and if light can be obtained as efficiently as,
+theoretically, it appears possible, the apparatus need have but a very
+small output. There being a strong probability that the illuminating
+methods of the future will involve the use of very high potentials, it
+seems very desirable to perfect a contrivance capable of converting the
+energy of heat into energy of the requisite form. Nothing to speak of
+has been done towards this end, for the thought that electricity of some
+50,000 or 100,000 volts pressure or more, even if obtained, would be
+unavailable for practical purposes, has deterred inventors from working
+in this direction.
+
+[Illustration: FIG. 127.]
+
+In Fig. 126 a plan of connections is shown for converting currents of
+high, into currents of low, tension by means of the disruptive discharge
+of a condenser. This plan has been used by me frequently for operating a
+few incandescent lamps required in the laboratory. Some difficulties
+have been encountered in the arc of the discharge which I have been able
+to overcome to a great extent; besides this, and the adjustment
+necessary for the proper working, no other difficulties have been met
+with, and it was easy to operate ordinary lamps, and even motors, in
+this manner. The line being connected to the ground, all the wires could
+be handled with perfect impunity, no matter how high the potential at
+the terminals of the condenser. In these experiments a high tension
+induction coil, operated from a battery or from an alternate current
+machine, was employed to charge the condenser; but the induction coil
+might be replaced by an apparatus of a different kind, capable of giving
+electricity of such high tension. In this manner, direct or alternating
+currents may be converted, and in both cases the current-impulses may be
+of any desired frequency. When the currents charging the condenser are
+of the same direction, and it is desired that the converted currents
+should also be of one direction, the resistance of the discharging
+circuit should, of course, be so chosen that there are no oscillations.
+
+[Illustration: FIG. 128.]
+
+In operating devices on the above plan I have observed curious phenomena
+of impedance which are of interest. For instance if a thick copper bar
+be bent, as indicated in Fig. 128, and shunted by ordinary incandescent
+lamps, then, by passing the discharge between the knobs, the lamps may
+be brought to incandescence although they are short-circuited. When a
+large induction coil is employed it is easy to obtain nodes on the bar,
+which are rendered evident by the different degree of brilliancy of the
+lamps, as shown roughly in Fig. 128. The nodes are never clearly
+defined, but they are simply maxima and minima of potentials along the
+bar. This is probably due to the irregularity of the arc between the
+knobs. In general when the above-described plan of conversion from high
+to low tension is used, the behavior of the disruptive discharge may be
+closely studied. The nodes may also be investigated by means of an
+ordinary Cardew voltmeter which should be well insulated. Geissler
+tubes may also be lighted across the points of the bent bar; in this
+case, of course, it is better to employ smaller capacities. I have found
+it practicable to light up in this manner a lamp, and even a Geissler
+tube, shunted by a short, heavy block of metal, and this result seems at
+first very curious. In fact, the thicker the copper bar in Fig. 128, the
+better it is for the success of the experiments, as they appear more
+striking. When lamps with long slender filaments are used it will be
+often noted that the filaments are from time to time violently vibrated,
+the vibration being smallest at the nodal points. This vibration seems
+to be due to an electrostatic action between the filament and the glass
+of the bulb.
+
+[Illustration: FIG. 129.]
+
+In some of the above experiments it is preferable to use special lamps
+having a straight filament as shown in Fig. 129. When such a lamp is
+used a still more curious phenomenon than those described may be
+observed. The lamp may be placed across the copper bar and lighted, and
+by using somewhat larger capacities, or, in other words, smaller
+frequencies or smaller impulsive impedances, the filament may be brought
+to any desired degree of incandescence. But when the impedance is
+increased, a point is reached when comparatively little current passes
+through the carbon, and most of it through the rarefied gas; or perhaps
+it may be more correct to state that the current divides nearly evenly
+through both, in spite of the enormous difference in the resistance, and
+this would be true unless the gas and the filament behave differently.
+It is then noted that the whole bulb is brilliantly illuminated, and the
+ends of the leading-in wires become incandescent and often throw off
+sparks in consequence of the violent bombardment, but the carbon
+filament remains dark. This is illustrated in Fig. 129. Instead of the
+filament a single wire extending through the whole bulb may be used,
+and in this case the phenomenon would seem to be still more interesting.
+
+From the above experiment it will be evident, that when ordinary lamps
+are operated by the converted currents, those should be preferably taken
+in which the platinum wires are far apart, and the frequencies used
+should not be too great, else the discharge will occur at the ends of
+the filament or in the base of the lamp between the leading-in wires,
+and the lamp might then be damaged.
+
+In presenting to you these results of my investigation on the subject
+under consideration, I have paid only a passing notice to facts upon
+which I could have dwelt at length, and among many observations I have
+selected only those which I thought most likely to interest you. The
+field is wide and completely unexplored, and at every step a new truth
+is gleaned, a novel fact observed.
+
+How far the results here borne out are capable of practical applications
+will be decided in the future. As regards the production of light, some
+results already reached are encouraging and make me confident in
+asserting that the practical solution of the problem lies in the
+direction I have endeavored to indicate. Still, whatever may be the
+immediate outcome of these experiments I am hopeful that they will only
+prove a step in further development towards the ideal and final
+perfection. The possibilities which are opened by modern research are so
+vast that even the most reserved must feel sanguine of the future.
+Eminent scientists consider the problem of utilizing one kind of
+radiation without the others a rational one. In an apparatus designed
+for the production of light by conversion from any form of energy into
+that of light, such a result can never be reached, for no matter what
+the process of producing the required vibrations, be it electrical,
+chemical or any other, it will not be possible to obtain the higher
+light vibrations without going through the lower heat vibrations. It is
+the problem of imparting to a body a certain velocity without passing
+through all lower velocities. But there is a possibility of obtaining
+energy not only in the form of light, but motive power, and energy of
+any other form, in some more direct way from the medium. The time will
+be when this will be accomplished, and the time has come when one may
+utter such words before an enlightened audience without being considered
+a visionary. We are whirling through endless space with an
+inconceivable speed, all around us everything is spinning, everything is
+moving, everywhere is energy. There _must_ be some way of availing
+ourselves of this energy more directly. Then, with the light obtained
+from the medium, with the power derived from it, with every form of
+energy obtained without effort, from the store forever inexhaustible,
+humanity will advance with giant strides. The mere contemplation of
+these magnificent possibilities expands our minds, strengthens our hopes
+and fills our hearts with supreme delight.
+
+
+
+
+CHAPTER XXVII.
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH
+FREQUENCY.[2]
+
+  [2] Lecture delivered before the Institution of Electrical
+      Engineers, London, February, 1892.
+
+
+I cannot find words to express how deeply I feel the honor of addressing
+some of the foremost thinkers of the present time, and so many able
+scientific men, engineers and electricians, of the country greatest in
+scientific achievements.
+
+The results which I have the honor to present before such a gathering I
+cannot call my own. There are among you not a few who can lay better
+claim than myself on any feature of merit which this work may contain. I
+need not mention many names which are world-known--names of those among
+you who are recognized as the leaders in this enchanting science; but
+one, at least, I must mention--a name which could not be omitted in a
+demonstration of this kind. It is a name associated with the most
+beautiful invention ever made: it is Crookes!
+
+When I was at college, a good while ago, I read, in a translation (for
+then I was not familiar with your magnificent language), the description
+of his experiments on radiant matter. I read it only once in my
+life--that time--yet every detail about that charming work I can
+remember to this day. Few are the books, let me say, which can make such
+an impression upon the mind of a student.
+
+But if, on the present occasion, I mention this name as one of many your
+Institution can boast of, it is because I have more than one reason to
+do so. For what I have to tell you and to show you this evening
+concerns, in a large measure, that same vague world which Professor
+Crookes has so ably explored; and, more than this, when I trace back the
+mental process which led me to these advances--which even by myself
+cannot be considered trifling, since they are so appreciated by you--I
+believe that their real origin, that which started me to work in this
+direction, and brought me to them, after a long period of constant
+thought, was that fascinating little book which I read many years ago.
+
+And now that I have made a feeble effort to express my homage and
+acknowledge my indebtedness to him and others among you, I will make a
+second effort, which I hope you will not find so feeble as the first, to
+entertain you.
+
+Give me leave to introduce the subject in a few words.
+
+A short time ago I had the honor to bring before our American Institute
+of Electrical Engineers some results then arrived at by me in a novel
+line of work. I need not assure you that the many evidences which I have
+received that English scientific men and engineers were interested in
+this work have been for me a great reward and encouragement. I will not
+dwell upon the experiments already described, except with the view of
+completing, or more clearly expressing, some ideas advanced by me
+before, and also with the view of rendering the study here presented
+self-contained, and my remarks on the subject of this evening's lecture
+consistent.
+
+This investigation, then, it goes without saying, deals with alternating
+currents, and to be more precise, with alternating currents of high
+potential and high frequency. Just in how much a very high frequency is
+essential for the production of the results presented is a question
+which, even with my present experience, would embarrass me to answer.
+Some of the experiments may be performed with low frequencies; but very
+high frequencies are desirable, not only on account of the many effects
+secured by their use, but also as a convenient means of obtaining, in
+the induction apparatus employed, the high potentials, which in their
+turn are necessary to the demonstration of most of the experiments here
+contemplated.
+
+Of the various branches of electrical investigation, perhaps the most
+interesting and the most immediately promising is that dealing with
+alternating currents. The progress in this branch of applied science has
+been so great in recent years that it justifies the most sanguine hopes.
+Hardly have we become familiar with one fact, when novel experiences are
+met and new avenues of research are opened. Even at this hour
+possibilities not dreamed of before are, by the use of these currents,
+partly realized. As in nature all is ebb and tide, all is wave motion,
+so it seems that in all branches of industry alternating
+currents--electric wave motion--will have the sway.
+
+One reason, perhaps, why this branch of science is being so rapidly
+developed is to be found in the interest which is attached to its
+experimental study. We wind a simple ring of iron with coils; we
+establish the connections to the generator, and with wonder and delight
+we note the effects of strange forces which we bring into play, which
+allow us to transform, to transmit and direct energy at will. We arrange
+the circuits properly, and we see the mass of iron and wires behave as
+though it were endowed with life, spinning a heavy armature, through
+invisible connections, with great speed and power--with the energy
+possibly conveyed from a great distance. We observe how the energy of an
+alternating current traversing the wire manifests itself--not so much in
+the wire as in the surrounding space--in the most surprising manner,
+taking the forms of heat, light, mechanical energy, and, most surprising
+of all, even chemical affinity. All these observations fascinate us, and
+fill us with an intense desire to know more about the nature of these
+phenomena. Each day we go to our work in the hope of discovering,--in
+the hope that some one, no matter who, may find a solution of one of the
+pending great problems,--and each succeeding day we return to our task
+with renewed ardor; and even if we _are_ unsuccessful, our work has not
+been in vain, for in these strivings, in these efforts, we have found
+hours of untold pleasure, and we have directed our energies to the
+benefit of mankind.
+
+We may take--at random, if you choose--any of the many experiments which
+may be performed with alternating currents; a few of which only, and by
+no means the most striking, form the subject of this evening's
+demonstration; they are all equally interesting, equally inciting to
+thought.
+
+Here is a simple glass tube from which the air has been partially
+exhausted. I take hold of it; I bring my body in contact with a wire
+conveying alternating currents of high potential, and the tube in my
+hand is brilliantly lighted. In whatever position I may put it, wherever
+I move it in space, as far as I can reach, its soft, pleasing light
+persists with undiminished brightness.
+
+Here is an exhausted bulb suspended from a single wire. Standing on an
+insulated support, I grasp it, and a platinum button mounted in it is
+brought to vivid incandescence.
+
+Here, attached to a leading wire, is another bulb, which, as I touch its
+metallic socket, is filled with magnificent colors of phosphorescent
+light.
+
+Here still another, which by my fingers' touch casts a shadow--the
+Crookes shadow--of the stem inside of it.
+
+Here, again, insulated as I stand on this platform, I bring my body in
+contact with one of the terminals of the secondary of this induction
+coil--with the end of a wire many miles long--and you see streams of
+light break forth from its distant end, which is set in violent
+vibration.
+
+Here, once more, I attach these two plates of wire gauze to the
+terminals of the coil; I set them a distance apart, and I set the coil
+to work. You may see a small spark pass between the plates. I insert a
+thick plate of one of the best dielectrics between them, and instead of
+rendering altogether impossible, as we are used to expect, I _aid_ the
+passage of the discharge, which, as I insert the plate, merely changes
+in appearance and assumes the form of luminous streams.
+
+Is there, I ask, can there be, a more interesting study than that of
+alternating currents?
+
+In all these investigations, in all these experiments, which are so
+very, very interesting, for many years past--ever since the greatest
+experimenter who lectured in this hall discovered its principle--we have
+had a steady companion, an appliance familiar to every one, a plaything
+once, a thing of momentous importance now--the induction coil. There is
+no dearer appliance to the electrician. From the ablest among you, I
+dare say, down to the inexperienced student, to your lecturer, we all
+have passed many delightful hours in experimenting with the induction
+coil. We have watched its play, and thought and pondered over the
+beautiful phenomena which it disclosed to our ravished eyes. So well
+known is this apparatus, so familiar are these phenomena to every one,
+that my courage nearly fails me when I think that I have ventured to
+address so able an audience, that I have ventured to entertain you with
+that same old subject. Here in reality is the same apparatus, and here
+are the same phenomena, only the apparatus is operated somewhat
+differently, the phenomena are presented in a different aspect. Some of
+the results we find as expected, others surprise us, but all captivate
+our attention, for in scientific investigation each novel result
+achieved may be the centre of a new departure, each novel fact learned
+may lead to important developments.
+
+Usually in operating an induction coil we have set up a vibration of
+moderate frequency in the primary, either by means of an interrupter or
+break, or by the use of an alternator. Earlier English investigators, to
+mention only Spottiswoode and J. E. H. Gordon, have used a rapid break
+in connection with the coil. Our knowledge and experience of to-day
+enables us to see clearly why these coils under the conditions of the
+test did not disclose any remarkable phenomena, and why able
+experimenters failed to perceive many of the curious effects which have
+since been observed.
+
+In the experiments such as performed this evening, we operate the coil
+either from a specially constructed alternator capable of giving many
+thousands of reversals of current per second, or, by disruptively
+discharging a condenser through the primary, we set up a vibration in
+the secondary circuit of a frequency of many hundred thousand or
+millions per second, if we so desire; and in using either of these means
+we enter a field as yet unexplored.
+
+It is impossible to pursue an investigation in any novel line without
+finally making some interesting observation or learning some useful
+fact. That this statement is applicable to the subject of this lecture
+the many curious and unexpected phenomena which we observe afford a
+convincing proof. By way of illustration, take for instance the most
+obvious phenomena, those of the discharge of the induction coil.
+
+Here is a coil which is operated by currents vibrating with extreme
+rapidity, obtained by disruptively discharging a Leyden jar. It would
+not surprise a student were the lecturer to say that the secondary of
+this coil consists of a small length of comparatively stout wire; it
+would not surprise him were the lecturer to state that, in spite of
+this, the coil is capable of giving any potential which the best
+insulation of the turns is able to withstand; but although he may be
+prepared, and even be indifferent as to the anticipated result, yet the
+aspect of the discharge of the coil will surprise and interest him.
+Every one is familiar with the discharge of an ordinary coil; it need
+not be reproduced here. But, by way of contrast, here is a form of
+discharge of a coil, the primary current of which is vibrating several
+hundred thousand times per second. The discharge of an ordinary coil
+appears as a simple line or band of light. The discharge of this coil
+appears in the form of powerful brushes and luminous streams issuing
+from all points of the two straight wires attached to the terminals of
+the secondary. (Fig. 130.)
+
+[Illustration: FIG. 130.]
+
+[Illustration: FIG. 131.]
+
+Now compare this phenomenon which you have just witnessed with the
+discharge of a Holtz or Wimshurst machine--that other interesting
+appliance so dear to the experimenter. What a difference there is
+between these phenomena! And yet, had I made the necessary
+arrangements--which could have been made easily, were it not that they
+would interfere with other experiments--I could have produced with this
+coil sparks which, had I the coil hidden from your view and only two
+knobs exposed, even the keenest observer among you would find it
+difficult, if not impossible, to distinguish from those of an influence
+or friction machine. This may be done in many ways--for instance, by
+operating the induction coil which charges the condenser from an
+alternating-current machine of very low frequency, and preferably
+adjusting the discharge circuit so that there are no oscillations set up
+in it. We then obtain in the secondary circuit, if the knobs are of the
+required size and properly set, a more or less rapid succession of
+sparks of great intensity and small quantity, which possess the same
+brilliancy, and are accompanied by the same sharp crackling sound, as
+those obtained from a friction or influence machine.
+
+Another way is to pass through two primary circuits, having a common
+secondary, two currents of a slightly different period, which produce in
+the secondary circuit sparks occurring at comparatively long intervals.
+But, even with the means at hand this evening, I may succeed in
+imitating the spark of a Holtz machine. For this purpose I establish
+between the terminals of the coil which charges the condenser a long,
+unsteady arc, which is periodically interrupted by the upward current of
+air produced by it. To increase the current of air I place on each side
+of the arc, and close to it, a large plate of mica. The condenser
+charged from this coil discharges into the primary circuit of a second
+coil through a small air gap, which is necessary to produce a sudden
+rush of current through the primary. The scheme of connections in the
+present experiment is indicated in Fig. 131.
+
+G is an ordinarily constructed alternator, supplying the primary P of an
+induction coil, the secondary S of which charges the condensers or jars
+C C. The terminals of the secondary are connected to the inside coatings
+of the jars, the outer coatings being connected to the ends of the
+primary _p p_ of a second induction coil. This primary _p p_ has a small
+air gap _a b_.
+
+The secondary _s_ of this coil is provided with knobs or spheres K K of
+the proper size and set at a distance suitable for the experiment.
+
+A long arc is established between the terminals A B of the first
+induction coil. M M are the mica plates.
+
+Each time the arc is broken between A and B the jars are quickly charged
+and discharged through the primary _p p_, producing a snapping spark
+between the knobs K K. Upon the arc forming between A and B the
+potential falls, and the jars cannot be charged to such high potential
+as to break through the air gap _a b_ until the arc is again broken by
+the draught.
+
+In this manner sudden impulses, at long intervals, are produced in the
+primary _p p_, which in the secondary _s_ give a corresponding number of
+impulses of great intensity. If the secondary knobs or spheres, K K, are
+of the proper size, the sparks show much resemblance to those of a Holtz
+machine.
+
+But these two effects, which to the eye appear so very different, are
+only two of the many discharge phenomena. We only need to change the
+conditions of the test, and again we make other observations of
+interest.
+
+When, instead of operating the induction coil as in the last two
+experiments, we operate it from a high frequency alternator, as in the
+next experiment, a systematic study of the phenomena is rendered much
+more easy. In such case, in varying the strength and frequency of the
+currents through the primary, we may observe five distinct forms of
+discharge, which I have described in my former paper on the subject
+before the American Institute of Electrical Engineers, May 20, 1891.
+
+It would take too much time, and it would lead us too far from the
+subject presented this evening, to reproduce all these forms, but it
+seems to me desirable to show you one of them. It is a brush discharge,
+which is interesting in more than one respect. Viewed from a near
+position it resembles much a jet of gas escaping under great pressure.
+We know that the phenomenon is due to the agitation of the molecules
+near the terminal, and we anticipate that some heat must be developed by
+the impact of the molecules against the terminal or against each other.
+Indeed, we find that the brush is hot, and only a little thought leads
+us to the conclusion that, could we but reach sufficiently high
+frequencies, we could produce a brush which would give intense light and
+heat, and which would resemble in every particular an ordinary flame,
+save, perhaps, that both phenomena might not be due to the same
+agent--save, perhaps, that chemical affinity might not be _electrical_
+in its nature.
+
+As the production of heat and light is here due to the impact of the
+molecules, or atoms of air, or something else besides, and, as we can
+augment the energy simply by raising the potential, we might, even with
+frequencies obtained from a dynamo machine, intensify the action to such
+a degree as to bring the terminal to melting heat. But with such low
+frequencies we would have to deal always with something of the nature of
+an electric current. If I approach a conducting object to the brush, a
+thin little spark passes, yet, even with the frequencies used this
+evening, the tendency to spark is not very great. So, for instance, if I
+hold a metallic sphere at some distance above the terminal, you may see
+the whole space between the terminal and sphere illuminated by the
+streams without the spark passing; and with the much higher frequencies
+obtainable by the disruptive discharge of a condenser, were it not for
+the sudden impulses, which are comparatively few in number, sparking
+would not occur even at very small distances. However, with incomparably
+higher frequencies, which we may yet find means to produce efficiently,
+and provided that electric impulses of such high frequencies could be
+transmitted through a conductor, the electrical characteristics of the
+brush discharge would completely vanish--no spark would pass, no shock
+would be felt--yet we would still have to deal with an _electric_
+phenomenon, but in the broad, modern interpretation of the word. In my
+first paper, before referred to, I have pointed out the curious
+properties of the brush, and described the best manner of producing it,
+but I have thought it worth while to endeavor to express myself more
+clearly in regard to this phenomenon, because of its absorbing interest.
+
+When a coil is operated with currents of very high frequency, beautiful
+brush effects may be produced, even if the coil be of comparatively
+small dimensions. The experimenter may vary them in many ways, and, if
+it were for nothing else, they afford a pleasing sight. What adds to
+their interest is that they may be produced with one single terminal as
+well as with two--in fact, often better with one than with two.
+
+But of all the discharge phenomena observed, the most pleasing to the
+eye, and the most instructive, are those observed with a coil which is
+operated by means of the disruptive discharge of a condenser. The power
+of the brushes, the abundance of the sparks, when the conditions are
+patiently adjusted, is often amazing. With even a very small coil, if it
+be so well insulated as to stand a difference of potential of several
+thousand volts per turn, the sparks may be so abundant that the whole
+coil may appear a complete mass of fire.
+
+Curiously enough the sparks, when the terminals of the coil are set at a
+considerable distance, seem to dart in every possible direction as
+though the terminals were perfectly independent of each other. As the
+sparks would soon destroy the insulation, it is necessary to prevent
+them. This is best done by immersing the coil in a good liquid
+insulator, such as boiled-out oil. Immersion in a liquid may be
+considered almost an absolute necessity for the continued and successful
+working of such a coil.
+
+It is, of course, out of the question, in an experimental lecture, with
+only a few minutes at disposal for the performance of each experiment,
+to show these discharge phenomena to advantage, as, to produce each
+phenomenon at its best, a very careful adjustment is required. But even
+if imperfectly produced, as they are likely to be this evening, they are
+sufficiently striking to interest an intelligent audience.
+
+Before showing some of these curious effects I must, for the sake of
+completeness, give a short description of the coil and other apparatus
+used in the experiments with the disruptive discharge this evening.
+
+[Illustration: FIG. 132.]
+
+It is contained in a box B (Fig. 132) of thick boards of hard wood,
+covered on the outside with a zinc sheet Z, which is carefully soldered
+all around. It might be advisable, in a strictly scientific
+investigation, when accuracy is of great importance, to do away with the
+metal cover, as it might introduce many errors, principally on account
+of its complex action upon the coil, as a condenser of very small
+capacity and as an electrostatic and electromagnetic screen. When the
+coil is used for such experiments as are here contemplated, the
+employment of the metal cover offers some practical advantages, but
+these are not of sufficient importance to be dwelt upon.
+
+The coil should be placed symmetrically to the metal cover, and the
+space between should, of course, not be too small, certainly not less
+than, say, five centimetres, but much more if possible; especially the
+two sides of the zinc box, which are at right angles to the axis of the
+coil, should be sufficiently remote from the latter, as otherwise they
+might impair its action and be a source of loss.
+
+The coil consists of two spools of hard rubber R R, held apart at a
+distance of 10 centimetres by bolts C and nuts _n_, likewise of hard
+rubber. Each spool comprises a tube T of approximately 8 centimetres
+inside diameter, and 3 millimetres thick, upon which are screwed two
+flanges F F, 24 centimetres square, the space between the flanges being
+about 3 centimetres. The secondary, S S, of the best gutta
+percha-covered wire, has 26 layers, 10 turns in each, giving for each
+half a total of 260 turns. The two halves are wound oppositely and
+connected in series, the connection between both being made over the
+primary. This disposition, besides being convenient, has the advantage
+that when the coil is well balanced--that is, when both of its
+terminals T_{1}, T_{1}, are connected to bodies or devices of equal
+capacity--there is not much danger of breaking through to the primary,
+and the insulation between the primary and the secondary need not be
+thick. In using the coil it is advisable to attach to _both_ terminals
+devices of nearly equal capacity, as, when the capacity of the terminals
+is not equal, sparks will be apt to pass to the primary. To avoid this,
+the middle point of the secondary may be connected to the primary, but
+this is not always practicable.
+
+The primary P P is wound in two parts, and oppositely, upon a wooden
+spool w, and the four ends are led out of the oil through hard rubber
+tubes _t t_. The ends of the secondary T_{1} T_{1}, are also led out of
+the oil through rubber tubes t_{1} t_{1} of great thickness. The
+primary and secondary layers are insulated by cotton cloth, the
+thickness of the insulation, of course, bearing some proportion to the
+difference of potential between the turns of the different layers. Each
+half of the primary has four layers, 24 turns in each, this giving a
+total of 96 turns. When both the parts are connected in series, this
+gives a ratio of conversion of about 1:2.7, and with the primaries in
+multiple, 1:5.4; but in operating with very rapidly alternating currents
+this ratio does not convey even an approximate idea of the ratio of the
+E. M. F's. in the primary and secondary circuits. The coil is held in
+position in the oil on wooden supports, there being about 5 centimetres
+thickness of oil all round. Where the oil is not specially needed, the
+space is filled with pieces of wood, and for this purpose principally
+the wooden box B surrounding the whole is used.
+
+The construction here shown is, of course, not the best on general
+principles, but I believe it is a good and convenient one for the
+production of effects in which an excessive potential and a very small
+current are needed.
+
+In connection with the coil I use either the ordinary form of discharger
+or a modified form. In the former I have introduced two changes which
+secure some advantages, and which are obvious. If they are mentioned, it
+is only in the hope that some experimenter may find them of use.
+
+One of the changes is that the adjustable knobs A and B (Fig. 133), of
+the discharger are held in jaws of brass, J J, by spring pressure, this
+allowing of turning them successively into different positions, and so
+doing away with the tedious process of frequent polishing up.
+
+[Illustration: FIG. 133.]
+
+The other change consists in the employment of a strong electromagnet
+N S, which is placed with its axis at right angles to the line joining
+the knobs A and B, and produces a strong magnetic field between them.
+The pole pieces of the magnet are movable and properly formed so as to
+protrude between the brass knobs, in order to make the field as intense
+as possible; but to prevent the discharge from jumping to the magnet the
+pole pieces are protected by a layer of mica, M M, of sufficient
+thickness; s_{1} s_{1} and s_{2} s_{2} are screws for fastening the
+wires. On each side one of the screws is for large and the other for
+small wires. L L are screws for fixing in position the rods R R, which
+support the knobs.
+
+In another arrangement with the magnet I take the discharge between the
+rounded pole pieces themselves, which in such case are insulated and
+preferably provided with polished brass caps.
+
+The employment of an intense magnetic field is of advantage principally
+when the induction coil or transformer which charges the condenser is
+operated by currents of very low frequency. In such a case the number of
+the fundamental discharges between the knobs may be so small as to
+render the currents produced in the secondary unsuitable for many
+experiments. The intense magnetic field then serves to blow out the arc
+between the knobs as soon as it is formed, and the fundamental
+discharges occur in quicker succession.
+
+[Illustration: FIG. 134.]
+
+Instead of the magnet, a draught or blast of air may be employed with
+some advantage. In this case the arc is preferably established between
+the knobs A B, in Fig. 131 (the knobs _a b_ being generally joined, or
+entirely done away with), as in this disposition the arc is long and
+unsteady, and is easily affected by the draught.
+
+When a magnet is employed to break the arc, it is better to choose the
+connection indicated diagrammatically in Fig. 134, as in this case the
+currents forming the arc are much more powerful, and the magnetic field
+exercises a greater influence. The use of the magnet permits, however,
+of the arc being replaced by a vacuum tube, but I have encountered great
+difficulties in working with an exhausted tube.
+
+The other form of discharger used in these and similar experiments is
+indicated in Figs. 135 and 136. It consists of a number of brass pieces
+_c c_ (Fig. 135), each of which comprises a spherical middle portion _m_
+with an extension _e_ below--which is merely used to fasten the piece in
+a lathe when polishing up the discharging surface--and a column above,
+which consists of a knurled flange _f_ surmounted by a threaded stem _l_
+carrying a nut _n_, by means of which a wire is fastened to the column.
+The flange _f_ conveniently serves for holding the brass piece when
+fastening the wire, and also for turning it in any position when it
+becomes necessary to present a fresh discharging surface. Two stout
+strips of hard rubber R R, with planed grooves _g g_ (Fig. 136) to fit
+the middle portion of the pieces _c c_, serve to clamp the latter and
+hold them firmly in position by means of two bolts C C (of which only
+one is shown) passing through the ends of the strips.
+
+[Illustration: FIG. 135.]
+
+[Illustration: FIG. 136.]
+
+In the use of this kind of discharger I have found three principal
+advantages over the ordinary form. First, the dielectric strength of a
+given total width of air space is greater when a great many small air
+gaps are used instead of one, which permits of working with a smaller
+length of air gap, and that means smaller loss and less deterioration of
+the metal; secondly, by reason of splitting the arc up into smaller
+arcs, the polished surfaces are made to last much longer; and, thirdly,
+the apparatus affords some gauge in the experiments. I usually set the
+pieces by putting between them sheets of uniform thickness at a certain
+very small distance which is known from the experiments of Sir William
+Thomson to require a certain electromotive force to be bridged by the
+spark.
+
+It should, of course, be remembered that the sparking distance is much
+diminished as the frequency is increased. By taking any number of spaces
+the experimenter has a rough idea of the electromotive force, and he
+finds it easier to repeat an experiment, as he has not the trouble of
+setting the knobs again and again. With this kind of discharger I have
+been able to maintain an oscillating motion without any spark being
+visible with the naked eye between the knobs, and they would not show a
+very appreciable rise in temperature. This form of discharge also lends
+itself to many arrangements of condensers and circuits which are often
+very convenient and time-saving. I have used it preferably in a
+disposition similar to that indicated in Fig. 131, when the currents
+forming the arc are small.
+
+I may here mention that I have also used dischargers with single or
+multiple air gaps, in which the discharge surfaces were rotated with
+great speed. No particular advantage was, however, gained by this
+method, except in cases where the currents from the condenser were large
+and the keeping cool of the surfaces was necessary, and in cases when,
+the discharge not being oscillating of itself, the arc as soon as
+established was broken by the air current, thus starting the vibration
+at intervals in rapid succession. I have also used mechanical
+interrupters in many ways. To avoid the difficulties with frictional
+contacts, the preferred plan adopted was to establish the arc and rotate
+through it at great speed a rim of mica provided with many holes and
+fastened to a steel plate. It is understood, of course, that the
+employment of a magnet, air current, or other interrupter, produces no
+effect worth noticing, unless the self-induction, capacity and
+resistance are so related that there are oscillations set up upon each
+interruption.
+
+I will now endeavor to show you some of the most noteworthy of these
+discharge phenomena.
+
+I have stretched across the room two ordinary cotton covered wires, each
+about seven metres in length. They are supported on insulating cords at
+a distance of about thirty centimetres. I attach now to each of the
+terminals of the coil one of the wires, and set the coil in action.
+Upon turning the lights off in the room you see the wires strongly
+illuminated by the streams issuing abundantly from their whole surface
+in spite of the cotton covering, which may even be very thick. When the
+experiment is performed under good conditions, the light from the wires
+is sufficiently intense to allow distinguishing the objects in a room.
+To produce the best result it is, of course, necessary to adjust
+carefully the capacity of the jars, the arc between the knobs and the
+length of the wires. My experience is that calculation of the length of
+the wires leads, in such case, to no result whatever. The experimenter
+will do best to take the wires at the start very long, and then adjust
+by cutting off first long pieces, and then smaller and smaller ones as
+he approaches the right length.
+
+A convenient way is to use an oil condenser of very small capacity,
+consisting of two small adjustable metal plates, in connection with this
+and similar experiments. In such case I take wires rather short and at
+the beginning set the condenser plates at maximum distance. If the
+streams from the wires increase by approach of the plates, the length of
+the wires is about right; if they diminish, the wires are too long for
+that frequency and potential. When a condenser is used in connection
+with experiments with such a coil, it should be an oil condenser by all
+means, as in using an air condenser considerable energy might be wasted.
+The wires leading to the plates in the oil should be very thin, heavily
+coated with some insulating compound, and provided with a conducting
+covering--this preferably extending under the surface of the oil. The
+conducting cover should not be too near the terminals, or ends, of the
+wire, as a spark would be apt to jump from the wire to it. The
+conducting coating is used to diminish the air losses, in virtue of its
+action as an electrostatic screen. As to the size of the vessel
+containing the oil, and the size of the plates, the experimenter gains
+at once an idea from a rough trial. The size of the plates _in oil_ is,
+however, calculable, as the dielectric losses are very small.
+
+In the preceding experiment it is of considerable interest to know what
+relation the quantity of the light emitted bears to the frequency and
+potential of the electric impulses. My opinion is that the heat as well
+as light effects produced should be proportionate, under otherwise equal
+conditions of test, to the product of frequency and square of potential,
+but the experimental verification of the law, whatever it may be, would
+be exceedingly difficult. One thing is certain, at any rate, and that
+is, that in augmenting the potential and frequency we rapidly intensify
+the streams; and, though it may be very sanguine, it is surely not
+altogether hopeless to expect that we may succeed in producing a
+practical illuminant on these lines. We would then be simply using
+burners or flames, in which there would be no chemical process, no
+consumption of material, but merely a transfer of energy, and which
+would, in all probability, emit more light and less heat than ordinary
+flames.
+
+[Illustration: FIG. 137.]
+
+The luminous intensity of the streams is, of course, considerably
+increased when they are focused upon a small surface. This may be shown
+by the following experiment:
+
+I attach to one of the terminals of the coil a wire _w_ (Fig. 137), bent
+in a circle of about 30 centimetres in diameter, and to the other
+terminal I fasten a small brass sphere _s_, the surface of the wire
+being preferably equal to the surface of the sphere, and the centre of
+the latter being in a line at right angles to the plane of the wire
+circle and passing through its centre. When the discharge is established
+under proper conditions, a luminous hollow cone is formed, and in the
+dark one-half of the brass sphere is strongly illuminated, as shown in
+the cut.
+
+By some artifice or other it is easy to concentrate the streams upon
+small surfaces and to produce very strong light effects. Two thin wires
+may thus be rendered intensely luminous.
+
+In order to intensify the streams the wires should be very thin and
+short; but as in this case their capacity would be generally too small
+for the coil--at least for such a one as the present--it is necessary to
+augment the capacity to the required value, while, at the same time, the
+surface of the wires remains very small. This may be done in many ways.
+
+[Illustration: FIG. 138.]
+
+Here, for instance, I have two plates, R R, of hard rubber (Fig. 138),
+upon which I have glued two very thin wires _w w_, so as to form a name.
+The wires may be bare or covered with the best insulation--it is
+immaterial for the success of the experiment. Well insulated wires, if
+anything, are preferable. On the back of each plate, indicated by the
+shaded portion, is a tinfoil coating _t t_. The plates are placed in
+line at a sufficient distance to prevent a spark passing from one wire
+to the other. The two tinfoil coatings I have joined by a conductor C,
+and the two wires I presently connect to the terminals of the coil. It
+is now easy, by varying the strength and frequency of the currents
+through the primary, to find a point at which the capacity of the system
+is best suited to the conditions, and the wires become so strongly
+luminous that, when the light in the room is turned off the name formed
+by them appears in brilliant letters.
+
+It is perhaps preferable to perform this experiment with a coil operated
+from an alternator of high frequency, as then, owing to the harmonic
+rise and fall, the streams are very uniform, though they are less
+abundant than when produced with such a coil as the present one. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.
+
+[Illustration: FIG. 139.]
+
+When two wires, attached to the terminals of the coil, are set at the
+proper distance, the streams between them may be so intense as to
+produce a continuous luminous sheet. To show this phenomenon I have here
+two circles, C and _c_ (Fig. 139), of rather stout wire, one being about
+80 centimetres and the other 30 centimetres in diameter. To each of the
+terminals of the coil I attach one of the circles. The supporting wires
+are so bent that the circles may be placed in the same plane, coinciding
+as nearly as possible. When the light in the room is turned off and the
+coil set to work, you see the whole space between the wires uniformly
+filled with streams, forming a luminous disc, which could be seen from a
+considerable distance, such is the intensity of the streams. The outer
+circle could have been much larger than the present one; in fact, with
+this coil I have used much larger circles, and I have been able to
+produce a strongly luminous sheet, covering an area of more than one
+square metre, which is a remarkable effect with this very small coil. To
+avoid uncertainty, the circle has been taken smaller, and the area is
+now about 0.43 square metre.
+
+The frequency of the vibration, and the quickness of succession of the
+sparks between the knobs, affect to a marked degree the appearance of
+the streams. When the frequency is very low, the air gives way in more
+or less the same manner, as by a steady difference of potential, and the
+streams consist of distinct threads, generally mingled with thin sparks,
+which probably correspond to the successive discharges occurring between
+the knobs. But when the frequency is extremely high, and the arc of the
+discharge produces a very _loud_ and _smooth_ sound--showing both that
+oscillation takes place and that the sparks succeed each other with
+great rapidity--then the luminous streams formed are perfectly uniform.
+To reach this result very small coils and jars of small capacity should
+be used. I take two tubes of thick Bohemian glass, about 5 centimetres
+in diameter and 20 centimetres long. In each of the tubes I slip a
+primary of very thick copper wire. On the top of each tube I wind a
+secondary of much thinner gutta-percha covered wire. The two secondaries
+I connect in series, the primaries preferably in multiple arc. The tubes
+are then placed in a large glass vessel, at a distance of 10 to 15
+centimetres from each other, on insulating supports, and the vessel is
+filled with boiled-out oil, the oil reaching about an inch above the
+tubes. The free ends of the secondary are lifted out of the coil and
+placed parallel to each other at a distance of about ten centimetres.
+The ends which are scraped should be dipped in the oil. Two four-pint
+jars joined in series may be used to discharge through the primary. When
+the necessary adjustments in the length and distance of the wires above
+the oil and in the arc of discharge are made, a luminous sheet is
+produced between the wires which is perfectly smooth and textureless,
+like the ordinary discharge through a moderately exhausted tube.
+
+I have purposely dwelt upon this apparently insignificant experiment. In
+trials of this kind the experimenter arrives at the startling conclusion
+that, to pass ordinary luminous discharges through gases, no particular
+degree of exhaustion is needed, but that the gas may be at ordinary or
+even greater pressure. To accomplish this, a very high frequency is
+essential; a high potential is likewise required, but this is merely an
+incidental necessity. These experiments teach us that, in endeavoring to
+discover novel methods of producing light by the agitation of atoms, or
+molecules, of a gas, we need not limit our research to the vacuum tube,
+but may look forward quite seriously to the possibility of obtaining the
+light effects without the use of any vessel whatever, with air at
+ordinary pressure.
+
+Such discharges of very high frequency, which render luminous the air at
+ordinary pressures, we have probably occasion often to witness in
+Nature. I have no doubt that if, as many believe, the aurora borealis is
+produced by sudden cosmic disturbances, such as eruptions at the sun's
+surface, which set the electrostatic charge of the earth in an extremely
+rapid vibration, the red glow observed is not confined to the upper
+rarefied strata of the air, but the discharge traverses, by reason of
+its very high frequency, also the dense atmosphere in the form of a
+_glow_, such as we ordinarily produce in a slightly exhausted tube. If
+the frequency were very low, or even more so, if the charge were not at
+all vibrating, the dense air would break down as in a lightning
+discharge. Indications of such breaking down of the lower dense strata
+of the air have been repeatedly observed at the occurrence of this
+marvelous phenomenon; but if it does occur, it can only be attributed to
+the fundamental disturbances, which are few in number, for the vibration
+produced by them would be far too rapid to allow a disruptive break. It
+is the original and irregular impulses which affect the instruments; the
+superimposed vibrations probably pass unnoticed.
+
+When an ordinary low frequency discharge is passed through moderately
+rarefied air, the air assumes a purplish hue. If by some means or other
+we increase the intensity of the molecular, or atomic, vibration, the
+gas changes to a white color. A similar change occurs at ordinary
+pressures with electric impulses of very high frequency. If the
+molecules of the air around a wire are moderately agitated, the brush
+formed is reddish or violet; if the vibration is rendered sufficiently
+intense, the streams become white. We may accomplish this in various
+ways. In the experiment before shown with the two wires across the room,
+I have endeavored to secure the result by pushing to a high value both
+the frequency and potential; in the experiment with the thin wires glued
+on the rubber plate I have concentrated the action upon a very small
+surface--in other words, I have worked with a great electric density.
+
+[Illustration: FIG. 140.]
+
+A most curious form of discharge is observed with such a coil when the
+frequency and potential are pushed to the extreme limit. To perform the
+experiment, every part of the coil should be heavily insulated, and only
+two small spheres--or, better still, two sharp-edged metal discs (_d d_,
+Fig. 140) of no more than a few centimetres in diameter--should be
+exposed to the air. The coil here used is immersed in oil, and the ends
+of the secondary reaching out of the oil are covered with an air-tight
+cover of hard rubber of great thickness. All cracks, if there are any,
+should be carefully stopped up, so that the brush discharge cannot form
+anywhere except on the small spheres or plates which are exposed to the
+air. In this case, since there are no large plates or other bodies of
+capacity attached to the terminals, the coil is capable of an extremely
+rapid vibration. The potential may be raised by increasing, as far as
+the experimenter judges proper, the rate of change of the primary
+current. With a coil not widely differing from the present, it is best
+to connect the two primaries in multiple arc; but if the secondary
+should have a much greater number of turns the primaries should
+preferably be used in series, as otherwise the vibration might be too
+fast for the secondary. It occurs under these conditions that misty
+white streams break forth from the edges of the discs and spread out
+phantom-like into space. With this coil, when fairly well produced, they
+are about 25 to 30 centimetres long. When the hand is held against them
+no sensation is produced, and a spark, causing a shock, jumps from the
+terminal only upon the hand being brought much nearer. If the
+oscillation of the primary current is rendered intermittent by some
+means or other, there is a corresponding throbbing of the streams, and
+now the hand or other conducting object may be brought in still greater
+proximity to the terminal without a spark being caused to jump.
+
+Among the many beautiful phenomena which may be produced with such a
+coil, I have here selected only those which appear to possess some
+features of novelty, and lead us to some conclusions of interest. One
+will not find it at all difficult to produce in the laboratory, by means
+of it, many other phenomena which appeal to the eye even more than these
+here shown, but present no particular feature of novelty.
+
+Early experimenters describe the display of sparks produced by an
+ordinary large induction coil upon an insulating plate separating the
+terminals. Quite recently Siemens performed some experiments in which
+fine effects were obtained, which were seen by many with interest. No
+doubt large coils, even if operated with currents of low frequencies,
+are capable of producing beautiful effects. But the largest coil ever
+made could not, by far, equal the magnificent display of streams and
+sparks obtained from such a disruptive discharge coil when properly
+adjusted. To give an idea, a coil such as the present one will cover
+easily a plate of one metre in diameter completely with the streams. The
+best way to perform such experiments is to take a very thin rubber or a
+glass plate and glue on one side of it a narrow ring of tinfoil of very
+large diameter, and on the other a circular washer, the centre of the
+latter coinciding with that of the ring, and the surfaces of both being
+preferably equal, so as to keep the coil well balanced. The washer and
+ring should be connected to the terminals by heavily insulated thin
+wires. It is easy in observing the effect of the capacity to produce a
+sheet of uniform streams, or a fine network of thin silvery threads, or
+a mass of loud brilliant sparks, which completely cover the plate.
+
+Since I have advanced the idea of the conversion by means of the
+disruptive discharge, in my paper before the American Institute of
+Electrical Engineers at the beginning of the past year, the interest
+excited in it has been considerable. It affords us a means for producing
+any potentials by the aid of inexpensive coils operated from ordinary
+systems of distribution, and--what is perhaps more appreciated--it
+enables us to convert currents of any frequency into currents of any
+other lower or higher frequency. But its chief value will perhaps be
+found in the help which it will afford us in the investigations of the
+phenomena of phosphorescence, which a disruptive discharge coil is
+capable of exciting in innumerable cases where ordinary coils, even the
+largest, would utterly fail.
+
+Considering its probable uses for many practical purposes, and its
+possible introduction into laboratories for scientific research, a few
+additional remarks as to the construction of such a coil will perhaps
+not be found superfluous.
+
+It is, of course, absolutely necessary to employ in such a coil wires
+provided with the best insulation.
+
+Good coils may be produced by employing wires covered with several
+layers of cotton, boiling the coil a long time in pure wax, and cooling
+under moderate pressure. The advantage of such a coil is that it can be
+easily handled, but it cannot probably give as satisfactory results as a
+coil immersed in pure oil. Besides, it seems that the presence of a
+large body of wax affects the coil disadvantageously, whereas this does
+not seem to be the case with oil. Perhaps it is because the dielectric
+losses in the liquid are smaller.
+
+I have tried at first silk and cotton covered wires with oil immersions,
+but I have been gradually led to use gutta-percha covered wires, which
+proved most satisfactory. Gutta-percha insulation adds, of course, to
+the capacity of the coil, and this, especially if the coil be large, is
+a great disadvantage when extreme frequencies are desired; but, on the
+other hand, gutta-percha will withstand much more than an equal
+thickness of oil, and this advantage should be secured at any price.
+Once the coil has been immersed, it should never be taken out of the oil
+for more than a few hours, else the gutta-percha will crack up and the
+coil will not be worth half as much as before. Gutta-percha is probably
+slowly attacked by the oil, but after an immersion of eight to nine
+months I have found no ill effects.
+
+I have obtained two kinds of gutta-percha wire known in commerce: in one
+the insulation sticks tightly to the metal, in the other it does not.
+Unless a special method is followed to expel all air, it is much safer
+to use the first kind. I wind the coil within an oil tank so that all
+interstices are filled up with the oil. Between the layers I use cloth
+boiled out thoroughly in oil, calculating the thickness according to the
+difference of potential between the turns. There seems not to be a very
+great difference whatever kind of oil is used; I use paraffine or
+linseed oil.
+
+To exclude more perfectly the air, an excellent way to proceed, and
+easily practicable with small coils, is the following: Construct a box
+of hardwood of very thick boards which have been for a long time boiled
+in oil. The boards should be so joined as to safely withstand the
+external air pressure. The coil being placed and fastened in position
+within the box, the latter is closed with a strong lid, and covered with
+closely fitting metal sheets, the joints of which are soldered very
+carefully. On the top two small holes are drilled, passing through the
+metal sheet and the wood, and in these holes two small glass tubes are
+inserted and the joints made air-tight. One of the tubes is connected to
+a vacuum pump, and the other with a vessel containing a sufficient
+quantity of boiled-out oil. The latter tube has a very small hole at the
+bottom, and is provided with a stopcock. When a fairly good vacuum has
+been obtained, the stopcock is opened and the oil slowly fed in.
+Proceeding in this manner, it is impossible that any big bubbles, which
+are the principal danger, should remain between the turns. The air is
+most completely excluded, probably better than by boiling out, which,
+however, when gutta-percha coated wires are used, is not practicable.
+
+For the primaries I use ordinary line wire with a thick cotton coating.
+Strands of very thin insulated wires properly interlaced would, of
+course, be the best to employ for the primaries, but they are not to be
+had.
+
+In an experimental coil the size of the wires is not of great
+importance. In the coil here used the primary is No. 12 and the
+secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
+varied considerably. It would only imply different adjustments; the
+results aimed at would not be materially affected.
+
+I have dwelt at some length upon the various forms of brush discharge
+because, in studying them, we not only observe phenomena which please
+our eye, but also afford us food for thought, and lead us to conclusions
+of practical importance. In the use of alternating currents of very high
+tension, too much precaution cannot be taken to prevent the brush
+discharge. In a main conveying such currents, in an induction coil or
+transformer, or in a condenser, the brush discharge is a source of great
+danger to the insulation. In a condenser, especially, the gaseous matter
+must be most carefully expelled, for in it the charged surfaces are
+near each other, and if the potentials are high, just as sure as a
+weight will fall if let go, so the insulation will give way if a single
+gaseous bubble of some size be present, whereas, if all gaseous matter
+were carefully excluded, the condenser would safely withstand a much
+higher difference of potential. A main conveying alternating currents of
+very high tension may be injured merely by a blow hole or small crack in
+the insulation, the more so as a blowhole is apt to contain gas at low
+pressure; and as it appears almost impossible to completely obviate such
+little imperfections, I am led to believe that in our future
+distribution of electrical energy by currents of very high tension,
+liquid insulation will be used. The cost is a great drawback, but if we
+employ an oil as an insulator the distribution of electrical energy with
+something like 100,000 volts, and even more, becomes, at least with
+higher frequencies, so easy that it could be hardly called an
+engineering feat. With oil insulation and alternate current motors,
+transmissions of power can be affected with safety and upon an
+industrial basis at distances of as much as a thousand miles.
+
+A peculiar property of oils, and liquid insulation in general, when
+subjected to rapidly changing electric stresses, is to disperse any
+gaseous bubbles which may be present, and diffuse them through its mass,
+generally long before any injurious break can occur. This feature may be
+easily observed with an ordinary induction coil by taking the primary
+out, plugging up the end of the tube upon which the secondary is wound,
+and filling it with some fairly transparent insulator, such as paraffine
+oil. A primary of a diameter something like six millimetres smaller than
+the inside of the tube may be inserted in the oil. When the coil is set
+to work one may see, looking from the top through the oil, many luminous
+points--air bubbles which are caught by inserting the primary, and which
+are rendered luminous in consequence of the violent bombardment. The
+occluded air, by its impact against the oil, heats it; the oil begins to
+circulate, carrying some of the air along with it, until the bubbles are
+dispersed and the luminous points disappear. In this manner, unless
+large bubbles are occluded in such way that circulation is rendered
+impossible, a damaging break is averted, the only effect being a
+moderate warming up of the oil. If, instead of the liquid, a solid
+insulation, no matter how thick, were used, a breaking through and
+injury of the apparatus would be inevitable.
+
+The exclusion of gaseous matter from any apparatus in which the
+dielectric is subjected to more or less rapidly changing electric forces
+is, however, not only desirable in order to avoid a possible injury of
+the apparatus, but also on account of economy. In a condenser, for
+instance, as long as only a solid or only a liquid dielectric is used,
+the loss is small; but if a gas under ordinary or small pressure be
+present the loss may be very great. Whatever the nature of the force
+acting in the dielectric may be, it seems that in a solid or liquid the
+molecular displacement produced by the force is small: hence the product
+of force and displacement is insignificant, unless the force be very
+great; but in a gas the displacement, and therefore this product, is
+considerable; the molecules are free to move, they reach high speeds,
+and the energy of their impact is lost in heat or otherwise. If the gas
+be strongly compressed, the displacement due to the force is made
+smaller, and the losses are reduced.
+
+In most of the succeeding experiments I prefer, chiefly on account of
+the regular and positive action, to employ the alternator before
+referred to. This is one of the several machines constructed by me for
+the purpose of these investigations. It has 384 pole projections, and is
+capable of giving currents of a frequency of about 10,000 per second.
+This machine has been illustrated and briefly described in my first
+paper before the American Institute of Electrical Engineers, May 20th,
+1891, to which I have already referred. A more detailed description,
+sufficient to enable any engineer to build a similar machine, will be
+found in several electrical journals of that period.
+
+The induction coils operated from the machine are rather small,
+containing from 5,000 to 15,000 turns in the secondary. They are
+immersed in boiled-out linseed oil, contained in wooden boxes covered
+with zinc sheet.
+
+I have found it advantageous to reverse the usual position of the wires,
+and to wind, in these coils, the primaries on the top; thus allowing the
+use of a much larger primary, which, of course, reduces the danger of
+overheating and increases the output of the coil. I make the primary on
+each side at least one centimetre shorter than the secondary, to prevent
+the breaking through on the ends, which would surely occur unless the
+insulation on the top of the secondary be very thick, and this, of
+course, would be disadvantageous.
+
+When the primary is made movable, which is necessary in some
+experiments, and many times convenient for the purposes of adjustment, I
+cover the secondary with wax, and turn it off in a lathe to a diameter
+slightly smaller than the inside of the primary coil. The latter I
+provide with a handle reaching out of the oil, which serves to shift it
+in any position along the secondary.
+
+I will now venture to make, in regard to the general manipulation of
+induction coils, a few observations bearing upon points which have not
+been fully appreciated in earlier experiments with such coils, and are
+even now often overlooked.
+
+The secondary of the coil possesses usually such a high self-induction
+that the current through the wire is inappreciable, and may be so even
+when the terminals are joined by a conductor of small resistance. If
+capacity is added to the terminals, the self-induction is counteracted,
+and a stronger current is made to flow through the secondary, though its
+terminals are insulated from each other. To one entirely unacquainted
+with the properties of alternating currents nothing will look more
+puzzling. This feature was illustrated in the experiment performed at
+the beginning with the top plates of wire gauze attached to the
+terminals and the rubber plate. When the plates of wire gauze were close
+together, and a small arc passed between them, the arc _prevented_ a
+strong current from passing through the secondary, because it did away
+with the capacity on the terminals; when the rubber plate was inserted
+between, the capacity of the condenser formed counteracted the
+self-induction of the secondary, a stronger current passed now, the coil
+performed more work, and the discharge was by far more powerful.
+
+The first thing, then, in operating the induction coil is to combine
+capacity with the secondary to overcome the self-induction. If the
+frequencies and potentials are very high, gaseous matter should be
+carefully kept away from the charged surfaces. If Leyden jars are used,
+they should be immersed in oil, as otherwise considerable dissipation
+may occur if the jars are greatly strained. When high frequencies are
+used, it is of equal importance to combine a condenser with the primary.
+One may use a condenser connected to the ends of the primary or to the
+terminals of the alternator, but the latter is not to be recommended, as
+the machine might be injured. The best way is undoubtedly to use the
+condenser in series with the primary and with the alternator, and to
+adjust its capacity so as to annul the self-induction of both the
+latter. The condenser should be adjustable by very small steps, and for
+a finer adjustment a small oil condenser with movable plates may be used
+conveniently.
+
+I think it best at this juncture to bring before you a phenomenon,
+observed by me some time ago, which to the purely scientific
+investigator may perhaps appear more interesting than any of the results
+which I have the privilege to present to you this evening.
+
+It may be quite properly ranked among the brush phenomena--in fact, it
+is a brush, formed at, or near, a single terminal in high vacuum.
+
+[Illustration: FIG. 141.]
+
+[Illustration: FIG. 142.]
+
+In bulbs provided with a conducting terminal, though it be of aluminum,
+the brush has but an ephemeral existence, and cannot, unfortunately, be
+indefinitely preserved in its most sensitive state, even in a bulb
+devoid of any conducting electrode. In studying the phenomenon, by all
+means a bulb having no leading-in wire should be used. I have found it
+best to use bulbs constructed as indicated in Figs. 141 and 142.
+
+In Fig. 141 the bulb comprises an incandescent lamp globe _L_, in the
+neck of which is sealed a barometer tube _b_, the end of which is blown
+out to form a small sphere _s_. This sphere should be sealed as closely
+as possible in the centre of the large globe. Before sealing, a thin
+tube _t_, of aluminum sheet, may be slipped in the barometer tube, but
+it is not important to employ it.
+
+The small hollow sphere _s_ is filled with some conducting powder, and a
+wire _w_ is cemented in the neck for the purpose of connecting the
+conducting powder with the generator.
+
+The construction shown in Fig. 142 was chosen in order to remove from
+the brush any conducting body which might possibly affect it. The bulb
+consists in this case of a lamp globe _L_, which has a neck _n_,
+provided with a tube _b_ and small sphere _s_, sealed to it, so that two
+entirely independent compartments are formed, as indicated in the
+drawing. When the bulb is in use the neck _n_ is provided with a tinfoil
+coating, which is connected to the generator and acts inductively upon
+the moderately rarefied and highly conducted gas inclosed in the neck.
+From there the current passes through the tube _b_ into the small sphere
+_s_, to act by induction upon the gas contained in the globe _L_.
+
+It is of advantage to make the tube _t_ very thick, the hole through it
+very small, and to blow the sphere _s_ very thin. It is of the greatest
+importance that the sphere _s_ be placed in the centre of the globe _L_.
+
+[Illustration: FIG. 143.]
+
+Figs. 143, 144 and 145 indicate different forms, or stages, of the
+brush. Fig. 143 shows the brush as it first appears in a bulb provided
+with a conducting terminal; but, as in such a bulb it very soon
+disappears--often after a few minutes--I will confine myself to the
+description of the phenomenon as seen in a bulb without conducting
+electrode. It is observed under the following conditions:
+
+When the globe _L_ (Figs. 141 and 142) is exhausted to a very high
+degree, generally the bulb is not excited upon connecting the wire _w_
+(Fig. 141) or the tinfoil coating of the bulb (Fig. 142) to the
+terminal of the induction coil. To excite it, it is usually sufficient
+to grasp the globe _L_ with the hand. An intense phosphorescence then
+spreads at first over the globe, but soon gives place to a white, misty
+light. Shortly afterward one may notice that the luminosity is unevenly
+distributed in the globe, and after passing the current for some time
+the bulb appears as in Fig. 144. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 145, after some minutes, hours,
+days or weeks, according as the bulb is worked. Warming the bulb or
+increasing the potential hastens the transit.
+
+[Illustration: FIG. 144.]
+
+[Illustration: FIG. 145.]
+
+When the brush assumes the form indicated in Fig. 145, it may be brought
+to a state of extreme sensitiveness to electrostatic and magnetic
+influence. The bulb hanging straight down from a wire, and all objects
+being remote from it, the approach of the observer at a few paces from
+the bulb will cause the brush to fly to the opposite side, and if he
+walks around the bulb it will always keep on the opposite side. It may
+begin to spin around the terminal long before it reaches that sensitive
+stage. When it begins to turn around, principally, but also before, it
+is affected by a magnet, and at a certain stage it is susceptible to
+magnetic influence to an astonishing degree. A small permanent magnet,
+with its poles at a distance of no more than two centimetres, will
+affect it visibly at a distance of two metres, slowing down or
+accelerating the rotation according to how it is held relatively to the
+brush. I think I have observed that at the stage when it is most
+sensitive to magnetic, it is not most sensitive to electrostatic,
+influence. My explanation is, that the electrostatic attraction between
+the brush and the glass of the bulb, which retards the rotation, grows
+much quicker than the magnetic influence when the intensity of the
+stream is increased.
+
+When the bulb hangs with the globe _L_ down, the rotation is always
+clockwise. In the southern hemisphere it would occur in the opposite
+direction and on the equator the brush should not turn at all. The
+rotation may be reversed by a magnet kept at some distance. The brush
+rotates best, seemingly, when it is at right angles to the lines of
+force of the earth. It very likely rotates, when at its maximum speed,
+in synchronism with the alternations, say, 10,000 times a second. The
+rotation can be slowed down or accelerated by the approach or receding
+of the observer, or any conducting body, but it cannot be reversed by
+putting the bulb in any position. When it is in the state of the highest
+sensitiveness and the potential or frequency be varied, the
+sensitiveness is rapidly diminished. Changing either of these but little
+will generally stop the rotation. The sensitiveness is likewise affected
+by the variations of temperature. To attain great sensitiveness it is
+necessary to have the small sphere _s_ in the centre of the globe _L_,
+as otherwise the electrostatic action of the glass of the globe will
+tend to stop the rotation. The sphere _s_ should be small and of uniform
+thickness; any dissymmetry of course has the effect to diminish the
+sensitiveness.
+
+The fact that the brush rotates in a definite direction in a permanent
+magnetic field seems to show that in alternating currents of very high
+frequency the positive and negative impulses are not equal, but that one
+always preponderates over the other.
+
+Of course, this rotation in one direction may be due to the action of
+the two elements of the same current upon each other, or to the action
+of the field produced by one of the elements upon the other, as in a
+series motor, without necessarily one impulse being stronger than the
+other. The fact that the brush turns, as far as I could observe, in any
+position, would speak for this view. In such case it would turn at any
+point of the earth's surface. But, on the other hand, it is then hard to
+explain why a permanent magnet should reverse the rotation, and one must
+assume the preponderance of impulses of one kind.
+
+As to the causes of the formation of the brush or stream, I think it is
+due to the electrostatic action of the globe and the dissymmetry of the
+parts. If the small bulb _s_ and the globe _L_ were perfect concentric
+spheres, and the glass throughout of the same thickness and quality, I
+think the brush would not form, as the tendency to pass would be equal
+on all sides. That the formation of the stream is due to an irregularity
+is apparent from the fact that it has the tendency to remain in one
+position, and rotation occurs most generally only when it is brought out
+of this position by electrostatic or magnetic influence. When in an
+extremely sensitive state it rests in one position, most curious
+experiments may be performed with it. For instance, the experimenter
+may, by selecting a proper position, approach the hand at a certain
+considerable distance to the bulb, and he may cause the brush to pass
+off by merely stiffening the muscles of the arm. When it begins to
+rotate slowly, and the hands are held at a proper distance, it is
+impossible to make even the slightest motion without producing a visible
+effect upon the brush. A metal plate connected to the other terminal of
+the coil affects it at a great distance, slowing down the rotation often
+to one turn a second.
+
+I am firmly convinced that such a brush, when we learn how to produce it
+properly, will prove a valuable aid in the investigation of the nature
+of the forces acting in an electrostatic or magnetic field. If there is
+any motion which is measurable going on in the space, such a brush ought
+to reveal it. It is, so to speak, a beam of light, frictionless, devoid
+of inertia.
+
+I think that it may find practical applications in telegraphy. With such
+a brush it would be possible to send dispatches across the Atlantic, for
+instance, with any speed, since its sensitiveness may be so great that
+the slightest changes will affect it. If it were possible to make the
+stream more intense and very narrow, its deflections could be easily
+photographed.
+
+I have been interested to find whether there is a rotation of the stream
+itself, or whether there is simply a stress traveling around the bulb.
+For this purpose I mounted a light mica fan so that its vanes were in
+the path of the brush. If the stream itself was rotating the fan would
+be spun around. I could produce no distinct rotation of the fan,
+although I tried the experiment repeatedly; but as the fan exerted a
+noticeable influence on the stream, and the apparent rotation of the
+latter was, in this case, never quite satisfactory, the experiment did
+not appear to be conclusive.
+
+I have been unable to produce the phenomenon with the disruptive
+discharge coil, although every other of these phenomena can be well
+produced by it--many, in fact, much better than with coils operated from
+an alternator.
+
+It may be possible to produce the brush by impulses of one direction, or
+even by a steady potential, in which case it would be still more
+sensitive to magnetic influence.
+
+In operating an induction coil with rapidly alternating currents, we
+realize with astonishment, for the first time, the great importance of
+the relation of capacity, self-induction and frequency as regards the
+general results. The effects of capacity are the most striking, for in
+these experiments, since the self-induction and frequency both are high,
+the critical capacity is very small, and need be but slightly varied to
+produce a very considerable change. The experimenter may bring his body
+in contact with the terminals of the secondary of the coil, or attach to
+one or both terminals insulated bodies of very small bulk, such as
+bulbs, and he may produce a considerable rise or fall of potential, and
+greatly affect the flow of the current through the primary. In the
+experiment before shown, in which a brush appears at a wire attached to
+one terminal, and the wire is vibrated when the experimenter brings his
+insulated body in contact with the other terminal of the coil, the
+sudden rise of potential was made evident.
+
+I may show you the behavior of the coil in another manner which
+possesses a feature of some interest. I have here a little light fan of
+aluminum sheet, fastened to a needle and arranged to rotate freely in a
+metal piece screwed to one of the terminals of the coil. When the coil
+is set to work, the molecules of the air are rhythmically attracted and
+repelled. As the force with which they are repelled is greater than that
+with which they are attracted, it results that there is a repulsion
+exerted on the surfaces of the fan. If the fan were made simply of a
+metal sheet, the repulsion would be equal on the opposite sides, and
+would produce no effect. But if one of the opposing surfaces is
+screened, or if, generally speaking, the bombardment on this side is
+weakened in some way or other, there remains the repulsion exerted upon
+the other, and the fan is set in rotation. The screening is best
+effected by fastening upon one of the opposing sides of the fan
+insulated conducting coatings, or, if the fan is made in the shape of an
+ordinary propeller screw, by fastening on one side, and close to it, an
+insulated metal plate. The static screen may, however, be omitted, and
+simply a thickness of insulating material fastened to one of the sides
+of the fan.
+
+To show the behavior of the coil, the fan may be placed upon the
+terminal and it will readily rotate when the coil is operated by
+currents of very high frequency. With a steady potential, of course, and
+even with alternating currents of very low frequency, it would not turn,
+because of the very slow exchange of air and, consequently, smaller
+bombardment; but in the latter case it might turn if the potential were
+excessive. With a pin wheel, quite the opposite rule holds good; it
+rotates best with a steady potential, and the effort is the smaller the
+higher the frequency. Now, it is very easy to adjust the conditions so
+that the potential is normally not sufficient to turn the fan, but that
+by connecting the other terminal of the coil with an insulated body it
+rises to a much greater value, so as to rotate the fan, and it is
+likewise possible to stop the rotation by connecting to the terminal a
+body of different size, thereby diminishing the potential.
+
+Instead of using the fan in this experiment, we may use the "electric"
+radiometer with similar effect. But in this case it will be found that
+the vanes will rotate only at high exhaustion or at ordinary pressures;
+they will not rotate at moderate pressures, when the air is highly
+conducting. This curious observation was made conjointly by Professor
+Crookes and myself. I attribute the result to the high conductivity of
+the air, the molecules of which then do not act as independent carriers
+of electric charges, but act all together as a single conducting body.
+In such case, of course, if there is any repulsion at all of the
+molecules from the vanes, it must be very small. It is possible,
+however, that the result is in part due to the fact that the greater
+part of the discharge passes from the leading-in wire through the highly
+conducting gas, instead of passing off from the conducting vanes.
+
+In trying the preceding experiment with the electric radiometer the
+potential should not exceed a certain limit, as then the electrostatic
+attraction between the vanes and the glass of the bulb may be so great
+as to stop the rotation.
+
+A most curious feature of alternate currents of high frequencies and
+potentials is that they enable us to perform many experiments by the use
+of one wire only. In many respects this feature is of great interest.
+
+In a type of alternate current motor invented by me some years ago I
+produced rotation by inducing, by means of a single alternating current
+passed through a motor circuit, in the mass or other circuits of the
+motor, secondary currents, which, jointly with the primary or inducing
+current, created a moving field of force. A simple but crude form of
+such a motor is obtained by winding upon an iron core a primary, and
+close to it a secondary coil, joining the ends of the latter and placing
+a freely movable metal disc within the influence of the field produced
+by both. The iron core is employed for obvious reasons, but it is not
+essential to the operation. To improve the motor, the iron core is made
+to encircle the armature. Again to improve, the secondary coil is made
+to partly overlap the primary, so that it cannot free itself from a
+strong inductive action of the latter, repel its lines as it may. Once
+more to improve, the proper difference of phase is obtained between the
+primary and secondary currents by a condenser, self-induction,
+resistance or equivalent windings.
+
+I had discovered, however, that rotation is produced by means of a
+single coil and core; my explanation of the phenomenon, and leading
+thought in trying the experiment, being that there must be a true time
+lag in the magnetization of the core. I remember the pleasure I had
+when, in the writings of Professor Ayrton, which came later to my hand,
+I found the idea of the time lag advocated. Whether there is a true time
+lag, or whether the retardation is due to eddy currents circulating in
+minute paths, must remain an open question, but the fact is that a coil
+wound upon an iron core and traversed by an alternating current creates
+a moving field of force, capable of setting an armature in rotation. It
+is of some interest, in conjunction with the historical Arago
+experiment, to mention that in lag or phase motors I have produced
+rotation in the opposite direction to the moving field, which means that
+in that experiment the magnet may not rotate, or may even rotate in the
+opposite direction to the moving disc. Here, then, is a motor
+(diagrammatically illustrated in Fig. 146), comprising a coil and iron
+core, and a freely movable copper disc in proximity to the latter.
+
+[Illustration: FIG. 146.]
+
+To demonstrate a novel and interesting feature, I have, for a reason
+which I will explain, selected this type of motor. When the ends of the
+coil are connected to the terminals of an alternator the disc is set in
+rotation. But it is not this experiment, now well known, which I desire
+to perform. What I wish to show you is that this motor rotates with
+_one single_ connection between it and the generator; that is to say,
+one terminal of the motor is connected to one terminal of the
+generator--in this case the secondary of a high-tension induction
+coil--the other terminals of motor and generator being insulated in
+space. To produce rotation it is generally (but not absolutely)
+necessary to connect the free end of the motor coil to an insulated body
+of some size. The experimenter's body is more than sufficient. If he
+touches the free terminal with an object held in the hand, a current
+passes through the coil and the copper disc is set in rotation. If an
+exhausted tube is put in series with the coil, the tube lights
+brilliantly, showing the passage of a strong current. Instead of the
+experimenter's body, a small metal sheet suspended on a cord may be used
+with the same result. In this case the plate acts as a condenser in
+series with the coil. It counteracts the self-induction of the latter
+and allows a strong current to pass. In such a combination, the greater
+the self-induction of the coil the smaller need be the plate, and this
+means that a lower frequency, or eventually a lower potential, is
+required to operate the motor. A single coil wound upon a core has a
+high self-induction; for this reason, principally, this type of motor
+was chosen to perform the experiment. Were a secondary closed coil wound
+upon the core, it would tend to diminish the self-induction, and then
+it would be necessary to employ a much higher frequency and potential.
+Neither would be advisable, for a higher potential would endanger the
+insulation of the small primary coil, and a higher frequency would
+result in a materially diminished torque.
+
+It should be remarked that when such a motor with a closed secondary is
+used, it is not at all easy to obtain rotation with excessive
+frequencies, as the secondary cuts off almost completely the lines of
+the primary--and this, of course, the more, the higher the
+frequency--and allows the passage of but a minute current. In such a
+case, unless the secondary is closed through a condenser, it is almost
+essential, in order to produce rotation, to make the primary and
+secondary coils overlap each other more or less.
+
+But there is an additional feature of interest about this motor, namely,
+it is not necessary to have even a single connection between the motor
+and generator, except, perhaps, through the ground; for not only is an
+insulated plate capable of giving off energy into space, but it is
+likewise capable of deriving it from an alternating electrostatic field,
+though in the latter case the available energy is much smaller. In this
+instance one of the motor terminals is connected to the insulated plate
+or body located within the alternating electrostatic field, and the
+other terminal preferably to the ground.
+
+It is quite possible, however, that such "no wire" motors, as they might
+be called, could be operated by conduction through the rarefied air at
+considerable distances. Alternate currents, especially of high
+frequencies, pass with astonishing freedom through even slightly
+rarefied gases. The upper strata of the air are rarefied. To reach a
+number of miles out into space requires the overcoming of difficulties
+of a merely mechanical nature. There is no doubt that with the enormous
+potentials obtainable by the use of high frequencies and oil insulation,
+luminous discharges might be passed through many miles of rarefied air,
+and that, by thus directing the energy of many hundreds or thousands of
+horse-power, motors or lamps might be operated at considerable distances
+from stationary sources. But such schemes are mentioned merely as
+possibilities. We shall have no need to transmit power in this way. We
+shall have no need to _transmit_ power at all. Ere many generations
+pass, our machinery will be driven by a power obtainable at any point of
+the universe. This idea is not novel. Men have been led to it long ago
+by instinct or reason. It has been expressed in many ways, and in many
+places, in the history of old and new. We find it in the delightful myth
+of Antheus, who derives power from the earth; we find it among the
+subtle speculations of one of your splendid mathematicians, and in many
+hints and statements of thinkers of the present time. Throughout space
+there is energy. Is this energy static or kinetic? If static our hopes
+are in vain; if kinetic--and this we know it is, for certain--then it is
+a mere question of time when men will succeed in attaching their
+machinery to the very wheelwork of nature. Of all, living or dead,
+Crookes came nearest to doing it. His radiometer will turn in the light
+of day and in the darkness of the night; it will turn everywhere where
+there is heat, and heat is everywhere. But, unfortunately, this
+beautiful little machine, while it goes down to posterity as the most
+interesting, must likewise be put on record as the most inefficient
+machine ever invented!
+
+The preceding experiment is only one of many equally interesting
+experiments which may be performed by the use of only one wire with
+alternations of high potential and frequency. We may connect an
+insulated line to a source of such currents, we may pass an
+inappreciable current over the line, and on any point of the same we are
+able to obtain a heavy current, capable of fusing a thick copper wire.
+Or we may, by the help of some artifice, decompose a solution in any
+electrolytic cell by connecting only one pole of the cell to the line or
+source of energy. Or we may, by attaching to the line, or only bringing
+into its vicinity, light up an incandescent lamp, an exhausted tube, or
+a phosphorescent bulb.
+
+However impracticable this plan of working may appear in many cases, it
+certainly seems practicable, and even recommendable, in the production
+of light. A perfected lamp would require but little energy, and if wires
+were used at all we ought to be able to supply that energy without a
+return wire.
+
+It is now a fact that a body may be rendered incandescent or
+phosphorescent by bringing it either in single contact or merely in the
+vicinity of a source of electric impulses of the proper character, and
+that in this manner a quantity of light sufficient to afford a practical
+illuminant may be produced. It is, therefore, to say the least, worth
+while to attempt to determine the best conditions and to invent the best
+appliances for attaining this object.
+
+Some experiences have already been gained in this direction, and I will
+dwell on them briefly, in the hope that they might prove useful.
+
+The heating of a conducting body inclosed in a bulb, and connected to a
+source of rapidly alternating electric impulses, is dependent on so many
+things of a different nature, that it would be difficult to give a
+generally applicable rule under which the maximum heating occurs. As
+regards the size of the vessel, I have lately found that at ordinary or
+only slightly differing atmospheric pressures, when air is a good
+insulator, and hence practically the same amount of energy by a certain
+potential and frequency is given off from the body, whether the bulb be
+small or large, the body is brought to a higher temperature if enclosed
+in a small bulb, because of the better confinement of heat in this case.
+
+At lower pressures, when air becomes more or less conducting, or if the
+air be sufficiently warmed to become conducting, the body is rendered
+more intensely incandescent in a large bulb, obviously because, under
+otherwise equal conditions of test, more energy may be given off from
+the body when the bulb is large.
+
+At very high degrees of exhaustion, when the matter in the bulb becomes
+"radiant," a large bulb has still an advantage, but a comparatively
+slight one, over the small bulb.
+
+Finally, at excessively high degrees of exhaustion, which cannot be
+reached except by the employment of special means, there seems to be,
+beyond a certain and rather small size of vessel, no perceptible
+difference in the heating.
+
+These observations were the result of a number of experiments, of which
+one, showing the effect of the size of the bulb at a high degree of
+exhaustion, may be described and shown here, as it presents a feature of
+interest. Three spherical bulbs of 2 inches, 3 inches and 4 inches
+diameter were taken, and in the centre of each was mounted an equal
+length of an ordinary incandescent lamp filament of uniform thickness.
+In each bulb the piece of filament was fastened to the leading-in wire
+of platinum, contained in a glass stem sealed in the bulb; care being
+taken, of course, to make everything as nearly alike as possible. On
+each glass stem in the inside of the bulb was slipped a highly polished
+tube made of aluminum sheet, which fitted the stem and was held on it by
+spring pressure. The function of this aluminum tube will be explained
+subsequently. In each bulb an equal length of filament protruded above
+the metal tube. It is sufficient to say now that under these conditions
+equal lengths of filament of the same thickness--in other words, bodies
+of equal bulk--were brought to incandescence. The three bulbs were
+sealed to a glass tube, which was connected to a Sprengel pump. When a
+high vacuum had been reached, the glass tube carrying the bulbs was
+sealed off. A current was then turned on successively on each bulb, and
+it was found that the filaments came to about the same brightness, and,
+if anything, the smallest bulb, which was placed midway between the two
+larger ones, may have been slightly brighter. This result was expected,
+for when either of the bulbs was connected to the coil the luminosity
+spread through the other two, hence the three bulbs constituted really
+one vessel. When all the three bulbs were connected in multiple arc to
+the coil, in the largest of them the filament glowed brightest, in the
+next smaller it was a little less bright, and in the smallest it only
+came to redness. The bulbs were then sealed off and separately tried.
+The brightness of the filaments was now such as would have been expected
+on the supposition that the energy given off was proportionate to the
+surface of the bulb, this surface in each case representing one of the
+coatings of a condenser. Accordingly, there was less difference between
+the largest and the middle sized than between the latter and the
+smallest bulb.
+
+An interesting observation was made in this experiment. The three bulbs
+were suspended from a straight bare wire connected to a terminal of a
+coil, the largest bulb being placed at the end of the wire, at some
+distance from it the smallest bulb, and at an equal distance from the
+latter the middle-sized one. The carbons glowed then in both the larger
+bulbs about as expected, but the smallest did not get its share by far.
+This observation led me to exchange the position of the bulbs, and I
+then observed that whichever of the bulbs was in the middle was by far
+less bright than it was in any other position. This mystifying result
+was, of course, found to be due to the electrostatic action between the
+bulbs. When they were placed at a considerable distance, or when they
+were attached to the corners of an equilateral triangle of copper wire,
+they glowed in about the order determined by their surfaces.
+
+As to the shape of the vessel, it is also of some importance, especially
+at high degrees of exhaustion. Of all the possible constructions, it
+seems that a spherical globe with the refractory body mounted in its
+centre is the best to employ. By experience it has been demonstrated
+that in such a globe a refractory body of a given bulk is more easily
+brought to incandescence than when differently shaped bulbs are used.
+There is also an advantage in giving to the incandescent body the shape
+of a sphere, for self-evident reasons. In any case the body should be
+mounted in the centre, where the atoms rebounding from the glass
+collide. This object is best attained in the spherical bulb; but it is
+also attained in a cylindrical vessel with one or two straight filaments
+coinciding with its axis, and possibly also in parabolical or spherical
+bulbs with refractory body or bodies placed in the focus or foci of the
+same; though the latter is not probable, as the electrified atoms should
+in all cases rebound normally from the surface they strike, unless the
+speed were excessive, in which case they _would_ probably follow the
+general law of reflection. No matter what shape the vessel may have, if
+the exhaustion be low, a filament mounted in the globe is brought to the
+same degree of incandescence in all parts; but if the exhaustion be high
+and the bulb be spherical or pear-shaped, as usual, focal points form
+and the filament is heated to a higher degree at or near such points.
+
+To illustrate the effect, I have here two small bulbs which are alike,
+only one is exhausted to a low and the other to a very high degree. When
+connected to the coil, the filament in the former glows uniformly
+throughout all its length; whereas in the latter, that portion of the
+filament which is in the centre of the bulb glows far more intensely
+than the rest. A curious point is that the phenomenon occurs even if two
+filaments are mounted in a bulb, each being connected to one terminal of
+the coil, and, what is still more curious, if they be very near
+together, provided the vacuum be very high. I noted in experiments with
+such bulbs that the filaments would give way usually at a certain point,
+and in the first trials I attributed it to a defect in the carbon. But
+when the phenomenon occurred many times in succession I recognized its
+real cause.
+
+In order to bring a refractory body inclosed in a bulb to incandescence,
+it is desirable, on account of economy, that all the energy supplied to
+the bulb from the source should reach without loss the body to be
+heated; from there, and from nowhere else, it should be radiated. It is,
+of course, out of the question to reach this theoretical result, but it
+is possible by a proper construction of the illuminating device to
+approximate it more or less.
+
+For many reasons, the refractory body is placed in the centre of the
+bulb, and it is usually supported on a glass stem containing the
+leading-in wire. As the potential of this wire is alternated, the
+rarefied gas surrounding the stem is acted upon inductively, and the
+glass stem is violently bombarded and heated. In this manner by far the
+greater portion of the energy supplied to the bulb--especially when
+exceedingly high frequencies are used--may be lost for the purpose
+contemplated. To obviate this loss, or at least to reduce it to a
+minimum, I usually screen the rarefied gas surrounding the stem from the
+inductive action of the leading-in wire by providing the stem with a
+tube or coating of conducting material. It seems beyond doubt that the
+best among metals to employ for this purpose is aluminum, on account of
+its many remarkable properties. Its only fault is that it is easily
+fusible, and, therefore, its distance from the incandescing body should
+be properly estimated. Usually, a thin tube, of a diameter somewhat
+smaller than that of the glass stem, is made of the finest aluminum
+sheet, and slipped on the stem. The tube is conveniently prepared by
+wrapping around a rod fastened in a lathe a piece of aluminum sheet of
+proper size, grasping the sheet firmly with clean chamois leather or
+blotting paper, and spinning the rod very fast. The sheet is wound
+tightly around the rod, and a highly polished tube of one or three
+layers of the sheet is obtained. When slipped on the stem, the pressure
+is generally sufficient to prevent it from slipping off, but, for
+safety, the lower edge of the sheet may be turned inside. The upper
+inside corner of the sheet--that is, the one which is nearest to the
+refractory incandescent body--should be cut out diagonally, as it often
+happens that, in consequence of the intense heat, this corner turns
+toward the inside and comes very near to, or in contact with, the wire,
+or filament, supporting the refractory body. The greater part of the
+energy supplied to the bulb is then used up in heating the metal tube,
+and the bulb is rendered useless for the purpose. The aluminum sheet
+should project above the glass stem more or less--one inch or so--or
+else, if the glass be too close to the incandescing body, it may be
+strongly heated and become more or less conducting, whereupon it may be
+ruptured, or may, by its conductivity, establish a good electrical
+connection between the metal tube and the leading-in wire, in which
+case, again, most of the energy will be lost in heating the former.
+Perhaps the best way is to make the top of the glass tube, for about an
+inch, of a much smaller diameter. To still further reduce the danger
+arising from the heating of the glass stem, and also with the view of
+preventing an electrical connection between the metal tube and the
+electrode, I preferably wrap the stem with several layers of thin mica,
+which extends at least as far as the metal tube. In some bulbs I have
+also used an outside insulating cover.
+
+The preceding remarks are only made to aid the experimenter in the first
+trials, for the difficulties which he encounters he may soon find means
+to overcome in his own way.
+
+To illustrate the effect of the screen, and the advantage of using it, I
+have here two bulbs of the same size, with their stems, leading-in wires
+and incandescent lamp filaments tied to the latter, as nearly alike as
+possible. The stem of one bulb is provided with an aluminum tube, the
+stem of the other has none. Originally the two bulbs were joined by a
+tube which was connected to a Sprengel pump. When a high vacuum had been
+reached, first the connecting tube, and then the bulbs, were sealed off;
+they are therefore of the same degree of exhaustion. When they are
+separately connected to the coil giving a certain potential, the carbon
+filament in the bulb provided with the aluminum screen is rendered
+highly incandescent, while the filament in the other bulb may, with the
+same potential, not even come to redness, although in reality the latter
+bulb takes generally more energy than the former. When they are both
+connected together to the terminal, the difference is even more
+apparent, showing the importance of the screening. The metal tube placed
+on the stem containing the leading-in wire performs really two distinct
+functions: First, it acts more or less as an electrostatic screen, thus
+economizing the energy supplied to the bulb; and, second, to whatever
+extent it may fail to act electrostatically, it acts mechanically,
+preventing the bombardment, and consequently intense heating and
+possible deterioration of the slender support of the refractory
+incandescent body, or of the glass stem containing the leading-in wire.
+I say _slender_ support, for it is evident that in order to confine the
+heat more completely to the incandescing body its support should be very
+thin, so as to carry away the smallest possible amount of heat by
+conduction. Of all the supports used I have found an ordinary
+incandescent lamp filament to be the best, principally because among
+conductors it can withstand the highest degree of heat.
+
+The effectiveness of the metal tube as an electrostatic screen depends
+largely on the degree of exhaustion.
+
+At excessively high degrees of exhaustion--which are reached by using
+great care and special means in connection with the Sprengel pump--when
+the matter in the globe is in the ultra-radiant state, it acts most
+perfectly. The shadow of the upper edge of the tube is then sharply
+defined upon the bulb.
+
+At a somewhat lower degree of exhaustion, which is about the ordinary
+"non-striking" vacuum, and generally as long as the matter moves
+predominantly in straight lines, the screen still does well. In
+elucidation of the preceding remark it is necessary to state that what
+is a "non-striking" vacuum for a coil operated as ordinarily, by
+impulses, or currents, of low frequency, is not so, by far, when the
+coil is operated by currents of very high frequency. In such case the
+discharge may pass with great freedom through the rarefied gas through
+which a low frequency discharge may not pass, even though the potential
+be much higher. At ordinary atmospheric pressures just the reverse rule
+holds good: the higher the frequency, the less the spark discharge is
+able to jump between the terminals, especially if they are knobs or
+spheres of some size.
+
+Finally, at very low degrees of exhaustion, when the gas is well
+conducting, the metal tube not only does not act as an electrostatic
+screen, but even is a drawback, aiding to a considerable extent the
+dissipation of the energy laterally from the leading-in wire. This, of
+course, is to be expected. In this case, namely, the metal tube is in
+good electrical connection with the leading-in wire, and most of the
+bombardment is directed upon the tube. As long as the electrical
+connection is not good, the conducting tube is always of some advantage,
+for although it may not greatly economize energy, still it protects the
+support of the refractory button, and is the means of concentrating more
+energy upon the same.
+
+To whatever extent the aluminum tube performs the function of a screen,
+its usefulness is therefore limited to very high degrees of exhaustion
+when it is insulated from the electrode--that is, when the gas as a
+whole is non-conducting, and the molecules, or atoms, act as independent
+carriers of electric charges.
+
+In addition to acting as a more or less effective screen, in the true
+meaning of the word, the conducting tube or coating may also act, by
+reason of its conductivity, as a sort of equalizer or dampener of the
+bombardment against the stem. To be explicit, I assume the action to be
+as follows: Suppose a rhythmical bombardment to occur against the
+conducting tube by reason of its imperfect action as a screen, it
+certainly must happen that some molecules, or atoms, strike the tube
+sooner than others. Those which come first in contact with it give up
+their superfluous charge, and the tube is electrified, the
+electrification instantly spreading over its surface. But this must
+diminish the energy lost in the bombardment, for two reasons: first, the
+charge given up by the atoms spreads over a great area, and hence the
+electric density at any point is small, and the atoms are repelled with
+less energy than they would be if they struck against a good insulator;
+secondly, as the tube is electrified by the atoms which first come in
+contact with it, the progress of the following atoms against the tube is
+more or less checked by the repulsion which the electrified tube must
+exert upon the similarly electrified atoms. This repulsion may perhaps
+be sufficient to prevent a large portion of the atoms from striking the
+tube, but at any rate it must diminish the energy of their impact. It is
+clear that when the exhaustion is very low, and the rarefied gas well
+conducting, neither of the above effects can occur, and, on the other
+hand, the fewer the atoms, with the greater freedom they move; in other
+words, the higher the degree of exhaustion, up to a limit, the more
+telling will be both the effects.
+
+[Illustration: FIG. 147.]
+
+[Illustration: FIG. 148.]
+
+What I have just said may afford an explanation of the phenomenon
+observed by Prof. Crookes, namely, that a discharge through a bulb is
+established with much greater facility when an insulator than when a
+conductor is present in the same. In my opinion, the conductor acts as a
+dampener of the motion of the atoms in the two ways pointed out; hence,
+to cause a visible discharge to pass through the bulb, a much higher
+potential is needed if a conductor, especially of much surface, be
+present.
+
+For the sake of elucidating of some of the remarks before made, I must
+now refer to Figs. 147, 148 and 149, which illustrate various
+arrangements with a type of bulb most generally used.
+
+Fig. 147 is a section through a spherical bulb L, with the glass stem
+_s_, contains the leading-in wire _w_, which has a lamp filament _l_
+fastened to it, serving to support the refractory button _m_ in the
+centre. M is a sheet of thin mica wound in several layers around the
+stem _s_, and _a_ is the aluminum tube.
+
+Fig. 148 illustrates such a bulb in a somewhat more advanced stage of
+perfection. A metallic tube S is fastened by means of some cement to the
+neck of the tube. In the tube is screwed a plug P, of insulating
+material, in the centre of which is fastened a metallic terminal _t_,
+for the connection to the leading-in wire _w_. This terminal must be
+well insulated from the metal tube S; therefore, if the cement used is
+conducting--and most generally it is sufficiently so--the space between
+the plug P and the neck of the bulb should be filled with some good
+insulating material, such as mica powder.
+
+
+Fig. 149 shows a bulb made for experimental purposes. In this bulb the
+aluminum tube is provided with an external connection, which serves to
+investigate the effect of the tube under various conditions. It is
+referred to chiefly to suggest a line of experiment followed.
+
+Since the bombardment against the stem containing the leading-in wire is
+due to the inductive action of the latter upon the rarefied gas, it is
+of advantage to reduce this action as far as practicable by employing a
+very thin wire, surrounded by a very thick insulation of glass or other
+material, and by making the wire passing through the rarefied gas as
+short as practicable. To combine these features I employ a large tube T
+(Fig. 150), which protrudes into the bulb to some distance, and carries
+on the top a very short glass stem _s_, into which is sealed the
+leading-in wire _w_, and I protect the top of the glass stem against the
+heat by a small aluminum tube _a_ and a layer of mica underneath the
+same, as usual. The wire _w_, passing through the large tube to the
+outside of the bulb, should be well insulated--with a glass tube, for
+instance--and the space between ought to be filled out with some
+excellent insulator. Among many insulating powders I have found that
+mica powder is the best to employ. If this precaution is not taken, the
+tube T, protruding into the bulb, will surely be cracked in consequence
+of the heating by the brushes which are apt to form in the upper part of
+the tube, near the exhausted globe, especially if the vacuum be
+excellent, and therefore the potential necessary to operate the lamp be
+very high.
+
+[Illustration: FIG. 149.]
+
+[Illustration: FIG. 150.]
+
+Fig. 151 illustrates a similar arrangement, with a large tube T
+protruding into the part of the bulb containing the refractory button
+_m_. In this case the wire leading from the outside into the bulb is
+omitted, the energy required being supplied through condenser coatings C
+C. The insulating packing P should in this construction be tightly
+fitting to the glass, and rather wide, or otherwise the discharge might
+avoid passing through the wire _w_, which connects the inside condenser
+coating to the incandescent button _m_.
+
+The molecular bombardment against the glass stem in the bulb is a source
+of great trouble. As an illustration I will cite a phenomenon only too
+frequently and unwillingly observed. A bulb, preferably a large one, may
+be taken, and a good conducting body, such as a piece of carbon, may be
+mounted in it upon a platinum wire sealed in the glass stem. The bulb
+may be exhausted to a fairly high degree, nearly to the point when
+phosphorescence begins to appear. When the bulb is connected with the
+coil, the piece of carbon, if small, may become highly incandescent at
+first, but its brightness immediately diminishes, and then the discharge
+may break through the glass somewhere in the middle of the stem, in the
+form of bright sparks, in spite of the fact that the platinum wire is in
+good electrical connection with the rarefied gas through the piece of
+carbon or metal at the top. The first sparks are singularly bright,
+recalling those drawn from a clear surface of mercury. But, as they heat
+the glass rapidly, they, of course, lose their brightness, and cease
+when the glass at the ruptured place becomes incandescent, or generally
+sufficiently hot to conduct. When observed for the first time the
+phenomenon must appear very curious, and shows in a striking manner how
+radically different alternate currents, or impulses, of high frequency
+behave, as compared with steady currents, or currents of low frequency.
+With such currents--namely, the latter--the phenomenon would of course
+not occur. When frequencies such as are obtained by mechanical means are
+used, I think that the rupture of the glass is more or less the
+consequence of the bombardment, which warms it up and impairs its
+insulating power; but with frequencies obtainable with condensers I have
+no doubt that the glass may give way without previous heating. Although
+this appears most singular at first, it is in reality what we might
+expect to occur. The energy supplied to the wire leading into the bulb
+is given off partly by direct action through the carbon button, and
+partly by inductive action through the glass surrounding the wire. The
+case is thus analogous to that in which a condenser shunted by a
+conductor of low resistance is connected to a source of alternating
+current. As long as the frequencies are low, the conductor gets the most
+and the condenser is perfectly safe; but when the frequency becomes
+excessive, the _role_ of the conductor may become quite insignificant.
+In the latter case the difference of potential at the terminals of the
+condenser may become so great as to rupture the dielectric,
+notwithstanding the fact that the terminals are joined by a conductor of
+low resistance.
+
+It is, of course, not necessary, when it is desired to produce the
+incandescence of a body inclosed in a bulb by means of these currents,
+that the body should be a conductor, for even a perfect non-conductor
+may be quite as readily heated. For this purpose it is sufficient to
+surround a conducting electrode with a non-conducting material, as, for
+instance, in the bulb described before in Fig. 150, in which a thin
+incandescent lamp filament is coated with a non-conductor, and supports
+a button of the same material on the top. At the start the bombardment
+goes on by inductive action through the non-conductor, until the same is
+sufficiently heated to become conducting, when the bombardment continues
+in the ordinary way.
+
+[Illustration: FIG. 151.]
+
+[Illustration: FIG. 152.]
+
+A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 152. In this instance a non-conductor _m_ is mounted
+in a piece of common arc light carbon so as to project some small
+distance above the latter. The carbon piece is connected to the
+leading-in wire passing through a glass stem, which is wrapped with
+several layers of mica. An aluminum tube _a_ is employed as usual for
+screening. It is so arranged that it reaches very nearly as high as the
+carbon and only the non-conductor _m_ projects a little above it. The
+bombardment goes at first against the upper surface of carbon, the lower
+parts being protected by the aluminum tube. As soon, however, as the
+non-conductor _m_ is heated it is rendered good conducting, and then it
+becomes the centre of the bombardment, being most exposed to the same.
+
+I have also constructed during these experiments many such single-wire
+bulbs with or without internal electrode, in which the radiant matter
+was projected against, or focused upon, the body to be rendered
+incandescent. Fig. 153 (page 263) illustrates one of the bulbs used. It
+consists of a spherical globe L, provided with a long neck _n_, on top,
+for increasing the action in some cases by the application of an
+external conducting coating. The globe L is blown out on the bottom into
+a very small bulb _b_, which serves to hold it firmly in a socket S of
+insulating material into which it is cemented. A fine lamp filament _f_,
+supported on a wire _w_, passes through the centre of the globe L. The
+filament is rendered incandescent in the middle portion, where the
+bombardment proceeding from the lower inside surface of the globe is
+most intense. The lower portion of the globe, as far as the socket S
+reaches, is rendered conducting, either by a tinfoil coating or
+otherwise, and the external electrode is connected to a terminal of the
+coil.
+
+The arrangement diagrammatically indicated in Fig. 153 was found to be
+an inferior one when it was desired to render incandescent a filament or
+button supported in the centre of the globe, but it was convenient when
+the object was to excite phosphorescence.
+
+In many experiments in which bodies of different kind were mounted in
+the bulb as, for instance, indicated in Fig. 152, some observations of
+interest were made.
+
+It was found, among other things, that in such cases, no matter where
+the bombardment began, just as soon as a high temperature was reached
+there was generally one of the bodies which seemed to take most of the
+bombardment upon itself, the other, or others, being thereby relieved.
+The quality appeared to depend principally on the point of fusion, and
+on the facility with which the body was "evaporated," or, generally
+speaking, disintegrated--meaning by the latter term not only the
+throwing off of atoms, but likewise of large lumps. The observation made
+was in accordance with generally accepted notions. In a highly exhausted
+bulb, electricity is carried off from the electrode by independent
+carriers, which are partly the atoms, or molecules, of the residual
+atmosphere, and partly the atoms, molecules, or lumps thrown off from
+the electrode. If the electrode is composed of bodies of different
+character, and if one of these is more easily disintegrated than the
+other, most of the electricity supplied is carried off from that body,
+which is then brought to a higher temperature than the others, and this
+the more, as upon an increase of the temperature the body is still more
+easily disintegrated.
+
+It seems to me quite probable that a similar process takes place in the
+bulb even with a homogeneous electrode, and I think it to be the
+principal cause of the disintegration. There is bound to be some
+irregularity, even if the surface is highly polished, which, of course,
+is impossible with most of the refractory bodies employed as electrodes.
+Assume that a point of the electrode gets hotter; instantly most of the
+discharge passes through that point, and a minute patch it probably
+fused and evaporated. It is now possible that in consequence of the
+violent disintegration the spot attacked sinks in temperature, or that a
+counter force is created, as in an arc; at any rate, the local tearing
+off meets with the limitations incident to the experiment, whereupon the
+same process occurs on another place. To the eye the electrode appears
+uniformly brilliant, but there are upon it points constantly shifting
+and wandering around, of a temperature far above the mean, and this
+materially hastens the process of deterioration. That some such thing
+occurs, at least when the electrode is at a lower temperature,
+sufficient experimental evidence can be obtained in the following
+manner: Exhaust a bulb to a very high degree, so that with a fairly high
+potential the discharge cannot pass--that is, not a _luminous_ one, for
+a weak invisible discharge occurs always, in all probability. Now raise
+slowly and carefully the potential, leaving the primary current on no
+more than for an instant. At a certain point, two, three, or half a
+dozen phosphorescent spots will appear on the globe. These places of the
+glass are evidently more violently bombarded than others, this being due
+to the unevenly distributed electric density, necessitated, of course,
+by sharp projections, or, generally speaking, irregularities of the
+electrode. But the luminous patches are constantly changing in position,
+which is especially well observable if one manages to produce very few,
+and this indicates that the configuration of the electrode is rapidly
+changing.
+
+From experiences of this kind I am led to infer that, in order to be
+most durable, the refractory button in the bulb should be in the form of
+a sphere with a highly polished surface. Such a small sphere could be
+manufactured from a diamond or some other crystal, but a better way
+would be to fuse, by the employment of extreme degrees of temperature,
+some oxide--as, for instance, zirconia--into a small drop, and then keep
+it in the bulb at a temperature somewhat below its point of fusion.
+
+Interesting and useful results can, no doubt, be reached in the
+direction of extreme degrees of heat. How can such high temperatures be
+arrived at? How are the highest degrees of heat reached in nature? By
+the impact of stars, by high speeds and collisions. In a collision any
+rate of heat generation may be attained. In a chemical process we are
+limited. When oxygen and hydrogen combine, they fall, metaphorically
+speaking, from a definite height. We cannot go very far with a blast,
+nor by confining heat in a furnace, but in an exhausted bulb we can
+concentrate any amount of energy upon a minute button. Leaving
+practicability out of consideration, this, then, would be the means
+which, in my opinion, would enable us to reach the highest temperature.
+But a great difficulty when proceeding in this way is encountered,
+namely, in most cases the body is carried off before it can fuse and
+form a drop. This difficulty exists principally with an oxide, such as
+zirconia, because it cannot be compressed in so hard a cake that it
+would not be carried off quickly. I have endeavored repeatedly to fuse
+zirconia, placing it in a cup of arc light carbon, as indicated in Fig.
+152. It glowed with a most intense light, and the stream of the
+particles projected out of the carbon cup was of a vivid white; but
+whether it was compressed in a cake or made into a paste with carbon, it
+was carried off before it could be fused. The carbon cup, containing
+zirconia, had to be mounted very low in the neck of a large bulb, as the
+heating of the glass by the projected particles of the oxide was so
+rapid that in the first trial the bulb was cracked almost in an instant,
+when the current was turned on. The heating of the glass by the
+projected particles was found to be always greater when the carbon cup
+contained a body which was rapidly carried off--I presume, because in
+such cases, with the same potential, higher speeds were reached, and
+also because, per unit of time, more matter was projected--that is, more
+particles would strike the glass.
+
+The before-mentioned difficulty did not exist, however, when the body
+mounted in the carbon cup offered great resistance to deterioration. For
+instance, when an oxide was first fused in an oxygen blast, and then
+mounted in the bulb, it melted very readily into a drop.
+
+Generally, during the process of fusion, magnificent light effects were
+noted, of which it would be difficult to give an adequate idea. Fig. 152
+is intended to illustrate the effect observed with a ruby drop. At first
+one may see a narrow funnel of white light projected against the top of
+the globe, where it produces an irregularly outlined phosphorescent
+patch. When the point of the ruby fuses, the phosphorescence becomes
+very powerful; but as the atoms are projected with much greater speed
+from the surface of the drop, soon the glass gets hot and "tired," and
+now only the outer edge of the patch glows. In this manner an intensely
+phosphorescent, sharply defined line, _l_, corresponding to the outline
+of the drop, is produced, which spreads slowly over the globe as the
+drop gets larger. When the mass begins to boil, small bubbles and
+cavities are formed, which cause dark colored spots to sweep across the
+globe. The bulb may be turned downward without fear of the drop falling
+off, as the mass possesses considerable viscosity.
+
+I may mention here another feature of some interest, which I believe to
+have noted in the course of these experiments, though the observations
+do not amount to a certitude. It _appeared_ that under the molecular
+impact caused by the rapidly alternating potential, the body was fused
+and maintained in that state at a lower temperature in a highly
+exhausted bulb than was the case at normal pressure and application of
+heat in the ordinary way--that is, at least, judging from the quantity
+of the light emitted. One of the experiments performed may be mentioned
+here by way of illustration. A small piece of pumice stone was stuck on
+a platinum wire, and first melted to it in a gas burner. The wire was
+next placed between two pieces of charcoal, and a burner applied, so as
+to produce an intense heat, sufficient to melt down the pumice stone
+into a small glass-like button. The platinum wire had to be taken of
+sufficient thickness, to prevent its melting in the fire. While in the
+charcoal fire, or when held in a burner to get a better idea of the
+degree of heat, the button glowed with great brilliancy. The wire with
+the button was then mounted in a bulb, and upon exhausting the same to a
+high degree, the current was turned on slowly, so as to prevent the
+cracking of the button. The button was heated to the point of fusion,
+and when it melted, it did not, apparently, glow with the same
+brilliancy as before, and this would indicate a lower temperature.
+Leaving out of consideration the observer's possible, and even probable,
+error, the question is, can a body under these conditions be brought
+from a solid to a liquid state with the evolution of _less_ light?
+
+When the potential of a body is rapidly alternated, it is certain that
+the structure is jarred. When the potential is very high, although the
+vibrations may be few--say 20,000 per second--the effect upon the
+structure may be considerable. Suppose, for example, that a ruby is
+melted into a drop by a steady application of energy. When it forms a
+drop, it will emit visible and invisible waves, which will be in a
+definite ratio, and to the eye the drop will appear to be of a certain
+brilliancy. Next, suppose we diminish to any degree we choose the energy
+steadily supplied, and, instead, supply energy which rises and falls
+according to a certain law. Now, when the drop is formed, there will be
+emitted from it three different kinds of vibrations--the ordinary
+visible, and two kinds of invisible waves: that is, the ordinary dark
+waves of all lengths, and, in addition, waves of a well defined
+character. The latter would not exist by a steady supply of the energy;
+still they help to jar and loosen the structure. If this really be the
+case, then the ruby drop will emit relatively less visible and more
+invisible waves than before. Thus it would seem that when a platinum
+wire, for instance, is fused by currents alternating with extreme
+rapidity, it emits at the point of fusion less light and more visible
+radiation than it does when melted by a steady current, though the total
+energy used up in the process of fusion is the same in both cases. Or,
+to cite another example, a lamp filament is not capable of withstanding
+as long with currents of extreme frequency as it does with steady
+currents, assuming that it be worked at the same luminous intensity.
+This means that for rapidly alternating currents the filament should be
+shorter and thicker. The higher the frequency--that is, the greater the
+departure from the steady flow--the worse it would be for the filament.
+But if the truth of this remark were demonstrated, it would be erroneous
+to conclude that such a refractory button as used in these bulbs would
+be deteriorated quicker by currents of extremely high frequency than by
+steady or low frequency currents. From experience I may say that just
+the opposite holds good: the button withstands the bombardment better
+with currents of very high frequency. But this is due to the fact that a
+high frequency discharge passes through a rarefied gas with much greater
+freedom than a steady or low frequency discharge, and this will mean
+that with the former we can work with a lower potential or with a less
+violent impact. As long, then, as the gas is of no consequence, a steady
+or low frequency current is better; but as soon as the action of the gas
+is desired and important, high frequencies are preferable.
+
+In the course of these experiments a great many trials were made with
+all kinds of carbon buttons. Electrodes made of ordinary carbon buttons
+were decidedly more durable when the buttons were obtained by the
+application of enormous pressure. Electrodes prepared by depositing
+carbon in well known ways did not show up well; they blackened the globe
+very quickly. From many experiences I conclude that lamp filaments
+obtained in this manner can be advantageously used only with low
+potentials and low frequency currents. Some kinds of carbon withstand so
+well that, in order to bring them to the point of fusion, it is
+necessary to employ very small buttons. In this case the observation is
+rendered very difficult on account of the intense heat produced.
+Nevertheless there can be no doubt that all kinds of carbon are fused
+under the molecular bombardment, but the liquid state must be one of
+great instability. Of all the bodies tried there were two which
+withstood best--diamond and carborundum. These two showed up about
+equally, but the latter was preferable for many reasons. As it is more
+than likely that this body is not yet generally known, I will venture to
+call your attention to it.
+
+It has been recently produced by Mr. E. G. Acheson, of Monongahela City,
+Pa., U. S. A. It is intended to replace ordinary diamond powder for
+polishing precious stones, etc., and I have been informed that it
+accomplishes this object quite successfully. I do not know why the name
+"carborundum" has been given to it, unless there is something in the
+process of its manufacture which justifies this selection. Through the
+kindness of the inventor, I obtained a short while ago some samples
+which I desired to test in regard to their qualities of phosphorescence
+and capability of withstanding high degrees of heat.
+
+Carborundum can be obtained in two forms--in the form of "crystals" and
+of powder. The former appear to the naked eye dark colored, but are very
+brilliant; the latter is of nearly the same color as ordinary diamond
+powder, but very much finer. When viewed under a microscope the samples
+of crystals given to me did not appear to have any definite form, but
+rather resembled pieces of broken up egg coal of fine quality. The
+majority were opaque, but there were some which were transparent and
+colored. The crystals are a kind of carbon containing some impurities;
+they are extremely hard, and withstand for a long time even an oxygen
+blast. When the blast is directed against them they at first form a
+cake of some compactness, probably in consequence of the fusion of
+impurities they contain. The mass withstands for a very long time the
+blast without further fusion; but a slow carrying off, or burning,
+occurs, and, finally, a small quantity of a glass-like residue is left,
+which, I suppose, is melted alumina. When compressed strongly they
+conduct very well, but not as well as ordinary carbon. The powder, which
+is obtained from the crystals in some way, is practically
+non-conducting. It affords a magnificent polishing material for stones.
+
+The time has been too short to make a satisfactory study of the
+properties of this product, but enough experience has been gained in a
+few weeks I have experimented upon it to say that it does possess some
+remarkable properties in many respects. It withstands excessively high
+degrees of heat, it is little deteriorated by molecular bombardment, and
+it does not blacken the globe as ordinary carbon does. The only
+difficulty which I have experienced in its use in connection with these
+experiments was to find some binding material which would resist the
+heat and the effect of the bombardment as successfully as carborundum
+itself does.
+
+I have here a number of bulbs which I have provided with buttons of
+carborundum. To make such a button of carborundum crystals I proceed in
+the following manner: I take an ordinary lamp filament and dip its point
+in tar, or some other thick substance or paint which may be readily
+carbonized. I next pass the point of the filament through the crystals,
+and then hold it vertically over a hot plate. The tar softens and forms
+a drop on the point of the filament, the crystals adhering to the
+surface of the drop. By regulating the distance from the plate the tar
+is slowly dried out and the button becomes solid. I then once more dip
+the button in tar and hold it again over a plate until the tar is
+evaporated, leaving only a hard mass which firmly binds the crystals.
+When a larger button is required I repeat the process several times, and
+I generally also cover the filament a certain distance below the button
+with crystals. The button being mounted in a bulb, when a good vacuum
+has been reached, first a weak and then a strong discharge is passed
+through the bulb to carbonize the tar and expel all gases, and later it
+is brought to a very intense incandescence.
+
+When the powder is used I have found it best to proceed as follows: I
+make a thick paint of carborundum and tar, and pass a lamp filament
+through the paint. Taking then most of the paint off by rubbing the
+filament against a piece of chamois leather, I hold it over a hot plate
+until the tar evaporates and the coating becomes firm. I repeat this
+process as many times as it is necessary to obtain a certain thickness
+of coating. On the point of the coated filament I form a button in the
+same manner.
+
+There is no doubt that such a button--properly prepared under great
+pressure--of carborundum, especially of powder of the best quality, will
+withstand the effect of the bombardment fully as well as anything we
+know. The difficulty is that the binding material gives way, and the
+carborundum is slowly thrown off after some time. As it does not seem to
+blacken the globe in the least, it might be found useful for coating the
+filaments of ordinary incandescent lamps, and I think that it is even
+possible to produce thin threads or sticks of carborundum which will
+replace the ordinary filaments in an incandescent lamp. A carborundum
+coating seems to be more durable than other coatings, not only because
+the carborundum can withstand high degrees of heat, but also because it
+seems to unite with the carbon better than any other material I have
+tried. A coating of zirconia or any other oxide, for instance, is far
+more quickly destroyed. I prepared buttons of diamond dust in the same
+manner as of carborundum, and these came in durability nearest to those
+prepared of carborundum, but the binding paste gave way much more
+quickly in the diamond buttons; this, however, I attributed to the size
+and irregularity of the grains of the diamond.
+
+It was of interest to find whether carborundum possesses the quality of
+phosphorescence. One is, of course, prepared to encounter two
+difficulties: first, as regards the rough product, the "crystals," they
+are good conducting, and it is a fact that conductors do not
+phosphoresce; second, the powder, being exceedingly fine, would not be
+apt to exhibit very prominently this quality, since we know that when
+crystals, even such as diamond or ruby, are finely powdered, they lose
+the property of phosphorescence to a considerable degree.
+
+The question presents itself here, can a conductor phosphoresce? What is
+there in such a body as a metal, for instance, that would deprive it of
+the quality of phosphoresence, unless it is that property which
+characterizes it as a conductor? For it is a fact that most of the
+phosphorescent bodies lose that quality when they are sufficiently
+heated to become more or less conducting. Then, if a metal be in a
+large measure, or perhaps entirely, deprived of that property, it should
+be capable of phosphoresence. Therefore it is quite possible that at
+some extremely high frequency, when behaving practically as a
+non-conductor, a metal or any other conductor might exhibit the quality
+of phosphoresence, even though it be entirely incapable of
+phosphorescing under the impact of a low-frequency discharge. There is,
+however, another possible way how a conductor might at least _appear_ to
+phosphoresce.
+
+Considerable doubt still exists as to what really is phosphorescence,
+and as to whether the various phenomena comprised under this head are
+due to the same causes. Suppose that in an exhausted bulb, under the
+molecular impact, the surface of a piece of metal or other conductor is
+rendered strongly luminous, but at the same time it is found that it
+remains comparatively cool, would not this luminosity be called
+phosphorescence? Now such a result, theoretically at least, is possible,
+for it is a mere question of potential or speed. Assume the potential of
+the electrode, and consequently the speed of the projected atoms, to be
+sufficiently high, the surface of the metal piece, against which the
+atoms are projected, would be rendered highly incandescent, since the
+process of heat generation would be incomparably faster than that of
+radiating or conducting away from the surface of the collision. In the
+eye of the observer a single impact of the atoms would cause an
+instantaneous flash, but if the impacts were repeated with sufficient
+rapidity, they would produce a continuous impression upon his retina. To
+him then the surface of the metal would appear continuously incandescent
+and of constant luminous intensity, while in reality the light would be
+either intermittent, or at least changing periodically in intensity. The
+metal piece would rise in temperature until equilibrium was
+attained--that is, until the energy continuously radiated would equal
+that intermittently supplied. But the supplied energy might under such
+conditions not be sufficient to bring the body to any more than a very
+moderate mean temperature, especially if the frequency of the atomic
+impacts be very low--just enough that the fluctuation of the intensity
+of the light emitted could not be detected by the eye. The body would
+now, owing to the manner in which the energy is supplied, emit a strong
+light, and yet be at a comparatively very low mean temperature. How
+should the observer name the luminosity thus produced? Even if the
+analysis of the light would teach him something definite, still he would
+probably rank it under the phenomena of phosphorescence. It is
+conceivable that in such a way both conducting and non-conducting bodies
+may be maintained at a certain luminous intensity, but the energy
+required would very greatly vary with the nature and properties of the
+bodies.
+
+These and some foregoing remarks of a speculative nature were made
+merely to bring out curious features of alternate currents or electric
+impulses. By their help we may cause a body to emit _more_ light, while
+at a certain mean temperature, than it would emit if brought to that
+temperature by a steady supply; and, again, we may bring a body to the
+point of fusion, and cause it to emit _less_ light than when fused by
+the application of energy in ordinary ways. It all depends on how we
+supply the energy, and what kind of vibrations we set up; in one case
+the vibrations are more, in the other less, adapted to affect our sense
+of vision.
+
+Some effects, which I had not observed before, obtained with carborundum
+in the first trials, I attributed to phosphorescence, but in subsequent
+experiments it appeared that it was devoid of that quality. The crystals
+possess a noteworthy feature. In a bulb provided with a single electrode
+in the shape of a small circular metal disc, for instance, at a certain
+degree of exhaustion the electrode is covered with a milky film, which
+is separated by a dark space from the glow filling the bulb. When the
+metal disc is covered with carborundum crystals, the film is far more
+intense, and snow-white. This I found later to be merely an effect of
+the bright surface of the crystals, for when an aluminum electrode was
+highly polished, it exhibited more or less the same phenomenon. I made a
+number of experiments with the samples of crystals obtained, principally
+because it would have been of special interest to find that they are
+capable of phosphorescence, on account of their being conducting. I
+could not produce phosphorescence distinctly, but I must remark that a
+decisive opinion cannot be formed until other experimenters have gone
+over the same ground.
+
+The powder behaved in some experiments as though it contained alumina,
+but it did not exhibit with sufficient distinctness the red of the
+latter. Its dead color brightens considerably under the molecular
+impact, but I am now convinced it does not phosphoresce. Still, the
+tests with the powder are not conclusive, because powdered carborundum
+probably does not behave like a phosphorescent sulphide, for example,
+which could be finely powdered without impairing the phosphorescence,
+but rather like powdered ruby or diamond, and therefore it would be
+necessary, in order to make a decisive test, to obtain it in a large
+lump and polish up the surface.
+
+If the carborundum proves useful in connection with these and similar
+experiments, its chief value will be found in the production of
+coatings, thin conductors, buttons, or other electrodes capable of
+withstanding extremely high degrees of heat.
+
+The production of a small electrode, capable of withstanding enormous
+temperatures, I regard as of the greatest importance in the manufacture
+of light. It would enable us to obtain, by means of currents of very
+high frequencies, certainly 20 times, if not more, the quantity of light
+which is obtained in the present incandescent lamp by the same
+expenditure of energy. This estimate may appear to many exaggerated, but
+in reality I think it is far from being so. As this statement might be
+misunderstood, I think it is necessary to expose clearly the problem
+with which, in this line of work, we are confronted, and the manner in
+which, in my opinion, a solution will be arrived at.
+
+Any one who begins a study of the problem will be apt to think that what
+is wanted in a lamp with an electrode is a very high degree of
+incandescence of the electrode. There he will be mistaken. The high
+incandescence of the button is a necessary evil, but what is really
+wanted is the high incandescence of the gas surrounding the button. In
+other words, the problem in such a lamp is to bring a mass of gas to the
+highest possible incandescence. The higher the incandescence, the
+quicker the mean vibration, the greater is the economy of the light
+production. But to maintain a mass of gas at a high degree of
+incandescence in a glass vessel, it will always be necessary to keep the
+incandescent mass away from the glass; that is, to confine it as much as
+possible to the central portion of the globe.
+
+In one of the experiments this evening a brush was produced at the end
+of a wire. The brush was a flame, a source of heat and light. It did not
+emit much perceptible heat, nor did it glow with an intense light; but
+is it the less a flame because it does not scorch my hand? Is it the
+less a flame because it does not hurt my eyes by its brilliancy? The
+problem is precisely to produce in the bulb such a flame, much smaller
+in size, but incomparably more powerful. Were there means at hand for
+producing electric impulses of a sufficiently high frequency, and for
+transmitting them, the bulb could be done away with, unless it were used
+to protect the electrode, or to economize the energy by confining the
+heat. But as such means are not at disposal, it becomes necessary to
+place the terminal in the bulb and rarefy the air in the same. This is
+done merely to enable the apparatus to perform the work which it is not
+capable of performing at ordinary air pressure. In the bulb we are able
+to intensify the action to any degree--so far that the brush emits a
+powerful light.
+
+The intensity of the light emitted depends principally on the frequency
+and potential of the impulses, and on the electric density on the
+surface of the electrode. It is of the greatest importance to employ the
+smallest possible button, in order to push the density very far. Under
+the violent impact of the molecules of the gas surrounding it, the small
+electrode is of course brought to an extremely high temperature, but
+around it is a mass of highly incandescent gas, a flame photosphere,
+many hundred times the volume of the electrode. With a diamond,
+carborundum or zirconia button the photosphere can be as much as one
+thousand times the volume of the button. Without much reflection one
+would think that in pushing so far the incandescence of the electrode it
+would be instantly volatilized. But after a careful consideration one
+would find that, theoretically, it should not occur, and in this
+fact--which, moreover, is experimentally demonstrated--lies principally
+the future value of such a lamp.
+
+At first, when the bombardment begins, most of the work is performed on
+the surface of the button, but when a highly conducting photosphere is
+formed the button is comparatively relieved. The higher the
+incandescence of the photosphere, the more it approaches in conductivity
+to that of the electrode, and the more, therefore, the solid and the gas
+form one conducting body. The consequence is that the further the
+incandescence is forced the more work, comparatively, is performed on
+the gas, and the less on the electrode. The formation of a powerful
+photosphere is consequently the very means for protecting the electrode.
+This protection, of course, is a relative one, and it should not be
+thought that by pushing the incandescence higher the electrode is
+actually less deteriorated. Still, theoretically, with extreme
+frequencies, this result must be reached, but probably at a temperature
+too high for most of the refractory bodies known. Given, then, an
+electrode which can withstand to a very high limit the effect of the
+bombardment and outward strain, it would be safe, no matter how much it
+was forced beyond that limit. In an incandescent lamp quite different
+considerations apply. There the gas is not at all concerned; the whole
+of the work is performed on the filament; and the life of the lamp
+diminishes so rapidly with the increase of the degree of incandescence
+that economical reasons compel us to work it at a low incandescence. But
+if an incandescent lamp is operated with currents of very high
+frequency, the action of the gas cannot be neglected, and the rules for
+the most economical working must be considerably modified.
+
+In order to bring such a lamp with one or two electrodes to a great
+perfection, it is necessary to employ impulses of very high frequency.
+The high frequency secures, among others, two chief advantages, which
+have a most important bearing upon the economy of the light production.
+First, the deterioration of the electrode is reduced by reason of the
+fact that we employ a great many small impacts, instead of a few violent
+ones, which quickly shatter the structure; secondly, the formation of a
+large photosphere is facilitated.
+
+In order to reduce the deterioration of the electrode to the minimum, it
+is desirable that the vibration be harmonic, for any suddenness hastens
+the process of destruction. An electrode lasts much longer when kept at
+incandescence by currents, or impulses, obtained from a high frequency
+alternator, which rise and fall more or less harmonically, than by
+impulses obtained from a disruptive discharge coil. In the latter case
+there is no doubt that most of the damage is done by the fundamental
+sudden discharges.
+
+One of the elements of loss in such a lamp is the bombardment of the
+globe. As the potential is very high, the molecules are projected with
+great speed; they strike the glass, and usually excite a strong
+phosphorescence. The effect produced is very pretty, but for economical
+reasons it would be perhaps preferable to prevent, or at least reduce to
+a minimum, the bombardment against the globe, as in such case it is, as
+a rule, not the object to excite phosphorescence, and as some loss of
+energy results from the bombardment. This loss in the bulb is
+principally dependent on the potential of the impulses and on the
+electric density on the surface of the electrode. In employing very high
+frequencies the loss of energy by the bombardment is greatly reduced,
+for, first, the potential needed to perform a given amount of work is
+much smaller; and, secondly, by producing a highly conducting
+photosphere around the electrode, the same result is obtained as though
+the electrode were much larger, which is equivalent to a smaller
+electric density. But be it by the diminution of the maximum potential
+or of the density, the gain is effected in the same manner, namely, by
+avoiding violent shocks, which strain the glass much beyond its limit of
+elasticity. If the frequency could be brought high enough, the loss due
+to the imperfect elasticity of the glass would be entirely negligible.
+The loss due to bombardment of the globe may, however, be reduced by
+using two electrodes instead of one. In such case each of the electrodes
+may be connected to one of the terminals; or else, if it is preferable
+to use only one wire, one electrode may be connected to one terminal and
+the other to the ground or to an insulated body of some surface, as, for
+instance, a shade on the lamp. In the latter case, unless some judgment
+is used, one of the electrodes might glow more intensely than the other.
+
+But on the whole I find it preferable, when using such high frequencies,
+to employ only one electrode and one connecting wire. I am convinced
+that the illuminating device of the near future will not require for its
+operation more than one lead, and, at any rate, it will have no
+leading-in wire, since the energy required can be as well transmitted
+through the glass. In experimental bulbs the leading-in wire is not
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 151, for example, there is some
+difficulty in fitting the parts, but these difficulties would not exist
+if a great many bulbs were manufactured; otherwise the energy can be
+conveyed through the glass as well as through a wire, and with these
+high frequencies the losses are very small. Such illustrating devices
+will necessarily involve the use of very high potentials, and this, in
+the eyes of practical men, might be an objectionable feature. Yet, in
+reality, high potentials are not objectionable--certainly not in the
+least so far as the safety of the devices is concerned.
+
+There are two ways of rendering an electric appliance safe. One is to
+use low potentials, the other is to determine the dimensions of the
+apparatus so that it is safe, no matter how high a potential is used. Of
+the two, the latter seems to me the better way, for then the safety is
+absolute, unaffected by any possible combination of circumstances which
+might render even a low-potential appliance dangerous to life and
+property. But the practical conditions require not only the judicious
+determination of the dimensions of the apparatus; they likewise
+necessitate the employment of energy of the proper kind. It is easy, for
+instance, to construct a transformer capable of giving, when operated
+from an ordinary alternate current machine of low tension, say 50,000
+volts, which might be required to light a highly exhausted
+phosphorescent tube, so that, in spite of the high potential, it is
+perfectly safe, the shock from it producing no inconvenience. Still such
+a transformer would be expensive, and in itself inefficient; and,
+besides, what energy was obtained from it would not be economically used
+for the production of light. The economy demands the employment of
+energy in the form of extremely rapid vibrations. The problem of
+producing light has been likened to that of maintaining a certain
+high-pitch note by means of a bell. It should be said a _barely audible_
+note; and even these words would not express it, so wonderful is the
+sensitiveness of the eye. We may deliver powerful blows at long
+intervals, waste a good deal of energy, and still not get what we want;
+or we may keep up the note by delivering frequent taps, and get nearer
+to the object sought by the expenditure of much less energy. In the
+production of light, as far as the illuminating device is concerned,
+there can be only one rule--that is, to use as high frequencies as can
+be obtained; but the means for the production and conveyance of impulses
+of such character impose, at present at least, great limitations. Once
+it is decided to use very high frequencies, the return wire becomes
+unnecessary, and all the appliances are simplified. By the use of
+obvious means the same result is obtained as though the return wire were
+used. It is sufficient for this purpose to bring in contact with the
+bulb, or merely in the vicinity of the same, an insulated body of some
+surface. The surface need, of course, be the smaller, the higher the
+frequency and potential used, and necessarily, also, the higher the
+economy of the lamp or other device.
+
+This plan of working has been resorted to on several occasions this
+evening. So, for instance, when the incandescence of a button was
+produced by grasping the bulb with the hand, the body of the
+experimenter merely served to intensify the action. The bulb used was
+similar to that illustrated in Fig. 148, and the coil was excited to a
+small potential, not sufficient to bring the button to incandescence
+when the bulb was hanging from the wire; and incidentally, in order to
+perform the experiment in a more suitable manner, the button was taken
+so large that a perceptible time had to elapse before, upon grasping the
+bulb, it could be rendered incandescent. The contact with the bulb was,
+of course, quite unnecessary. It is easy, by using a rather large bulb
+with an exceedingly small electrode, to adjust the conditions so that
+the latter is brought to bright incandescence by the mere approach of
+the experimenter within a few feet of the bulb, and that the
+incandescence subsides upon his receding.
+
+[Illustration: FIG. 153.]
+
+[Illustration: FIG. 154.]
+
+In another experiment, when phosphorescence was excited, a similar bulb
+was used. Here again, originally, the potential was not sufficient to
+excite phosphorescence until the action was intensified--in this case,
+however, to present a different feature, by touching the socket with a
+metallic object held in the hand. The electrode in the bulb was a carbon
+button so large that it could not be brought to incandescence, and
+thereby spoil the effect produced by phosphorescence.
+
+Again, in another of the early experiments, a bulb was used, as
+illustrated in Fig. 141. In this instance, by touching the bulb with one
+or two fingers, one or two shadows of the stem inside were projected
+against the glass, the touch of the finger producing the same results as
+the application of an external negative electrode under ordinary
+circumstances.
+
+In all these experiments the action was intensified by augmenting the
+capacity at the end of the lead connected to the terminal. As a rule, it
+is not necessary to resort to such means, and would be quite unnecessary
+with still higher frequencies; but when it _is_ desired, the bulb, or
+tube, can be easily adapted to the purpose.
+
+In Fig. 153, for example, an experimental bulb, L, is shown, which
+is provided with a neck, _n_, on the top, for the application of an
+external tinfoil coating, which may be connected to a body of larger
+surface. Such a lamp as illustrated in Fig. 154 may also be lighted by
+connecting the tinfoil coating on the neck _n_ to the terminal, and the
+leading-in wire, _w_, to an insulated plate. If the bulb stands in a
+socket upright, as shown in the cut, a shade of conducting material may
+be slipped in the neck, _n_, and the action thus magnified.
+
+A more perfected arrangement used in some of these bulbs is illustrated
+in Fig. 155. In this case the construction of the bulb is as shown and
+described before, when reference was made to Fig. 148. A zinc sheet, Z,
+with a tubular extension, T, is applied over the metallic socket, S.
+The bulb hangs downward from the terminal, _t_, the zinc sheet, Z,
+performing the double office of intensifier and reflector. The reflector
+is separated from the terminal, _t_, by an extension of the insulating
+plug, P.
+
+A similar disposition with a phosphorescent tube is illustrated in
+Fig. 156. The tube, T, is prepared from two short tubes of different
+diameter, which are sealed on the ends. On the lower end is placed an
+inside conducting coating, C, which connects to the wire _w_. The wire
+has a hook on the upper end for suspension, and passes through the
+centre of the inside tube, which is filled with some good and tightly
+packed insulator. On the outside of the upper end of the tube, T, is
+another conducting coating, C_{1}, upon which is slipped a metallic
+reflector Z, which should be separated by a thick insulation from the
+end of wire _w_.
+
+The economical use of such a reflector or intensifier would require that
+all energy supplied to an air condenser should be recoverable, or, in
+other words, that there should not be any losses, neither in the
+gaseous medium nor through its action elsewhere. This is far from being
+so, but, fortunately, the losses may be reduced to anything desired. A
+few remarks are necessary on this subject, in order to make the
+experiences gathered in the course of these investigations perfectly
+clear.
+
+[Illustration: FIG. 155.]
+
+Suppose a small helix with many well insulated turns, as in experiment
+Fig. 146, has one of its ends connected to one of the terminals of the
+induction coil, and the other to a metal plate, or, for the sake of
+simplicity, a sphere, insulated in space. When the coil is set to work,
+the potential of the sphere is alternated, and a small helix now behaves
+as though its free end were connected to the other terminal of the
+induction coil. If an iron rod be held within a small helix, it is
+quickly brought to a high temperature, indicating the passage of a
+strong current through the helix. How does the insulated sphere act in
+this case? It can be a condenser, storing and returning the energy
+supplied to it, or it can be a mere sink of energy, and the conditions
+of the experiment determine whether it is rather one than the other. The
+sphere being charged to a high potential, it acts inductively upon the
+surrounding air, or whatever gaseous medium there might be. The
+molecules, or atoms, which are near the sphere, are of course more
+attracted, and move through a greater distance than the farther ones.
+When the nearest molecules strike the sphere, they are repelled, and
+collisions occur at all distances within the inductive action of the
+sphere. It is now clear that, if the potential be steady, but little
+loss of energy can be caused in this way, for the molecules which are
+nearest to the sphere, having had an additional charge imparted to them
+by contact, are not attracted until they have parted, if not with all,
+at least with most of the additional charge, which can be accomplished
+only after a great many collisions. From the fact, that with a steady
+potential there is but little loss in dry air, one must come to such a
+conclusion. When the potential of a sphere, instead of being steady, is
+alternating, the conditions are entirely different. In this case a
+rhythmical bombardment occurs, no matter whether the molecules, after
+coming in contact with the sphere, lose the imparted charge or not; what
+is more, if the charge is not lost, the impacts are only the more
+violent. Still, if the frequency of the impulses be very small, the loss
+caused by the impacts and collisions would not be serious, unless the
+potential were excessive. But when extremely high frequencies and more
+or less high potentials are used, the loss may very great. The total
+energy lost per unit of time is proportionate to the product of the
+number of impacts per second, or the frequency and the energy lost in
+each impact. But the energy of an impact must be proportionate to the
+square of the electric density of the sphere, since the charge imparted
+to the molecule is proportionate to that density. I conclude from this
+that the total energy lost must be proportionate to the product of the
+frequency and the square of the electric density; but this law needs
+experimental confirmation. Assuming the preceding considerations to be
+true, then, by rapidly alternating the potential of a body immersed in
+an insulating gaseous medium, any amount of energy may be dissipated
+into space. Most of that energy then, I believe, is not dissipated in
+the form of long ether waves, propagated to considerable distance, as is
+thought most generally, but is consumed--in the case of an insulated
+sphere, for example--in impact and collisional losses--that is, heat
+vibrations--on the surface and in the vicinity of the sphere. To reduce
+the dissipation, it is necessary to work with a small electric
+density--the smaller, the higher the frequency.
+
+[Illustration: FIG. 156.]
+
+But since, on the assumption before made, the loss is diminished with
+the square of the density, and since currents of very high frequencies
+involve considerable waste when transmitted through conductors, it
+follows that, on the whole, it is better to employ one wire than two.
+Therefore, if motors, lamps, or devices of any kind are perfected,
+capable of being advantageously operated by currents of extremely high
+frequency, economical reasons will make it advisable to use only one
+wire, especially if the distances are great.
+
+When energy is absorbed in a condenser, the same behaves as though its
+capacity were increased. Absorption always exists more or less, but
+generally it is small and of no consequence as long as the frequencies
+are not very great. In using extremely high frequencies, and,
+necessarily in such case, also high potentials, the absorption--or, what
+is here meant more particularly by this term, the loss of energy due to
+the presence of a gaseous medium--is an important factor to be
+considered, as the energy absorbed in the air condenser may be any
+fraction of the supplied energy. This would seem to make it very
+difficult to tell from the measured or computed capacity of an air
+condenser its actual capacity or vibration period, especially if the
+condenser is of very small surface and is charged to a very high
+potential. As many important results are dependent upon the correctness
+of the estimation of the vibration period, this subject demands the most
+careful scrutiny of other investigators. To reduce the probable error as
+much as possible in experiments of the kind alluded to, it is advisable
+to use spheres or plates of large surface, so as to make the density
+exceedingly small. Otherwise, when it is practicable, an oil condenser
+should be used in preference. In oil or other liquid dielectrics there
+are seemingly no such losses as in gaseous media. It being impossible to
+exclude entirely the gas in condensers with solid dielectrics, such
+condensers should be immersed in oil, for economical reasons, if nothing
+else; they can then be strained to the utmost, and will remain cool. In
+Leyden jars the loss due to air is comparatively small, as the tinfoil
+coatings are large, close together, and the charged surfaces not
+directly exposed; but when the potentials are very high, the loss may be
+more or less considerable at, or near, the upper edge of the foil, where
+the air is principally acted upon. If the jar be immersed in boiled-out
+oil, it will be capable of performing four times the amount of work
+which it can for any length of time when used in the ordinary way, and
+the loss will be inappreciable.
+
+It should not be thought that the loss in heat in an air condenser is
+necessarily associated with the formation of _visible_ streams or
+brushes. If a small electrode, inclosed in an unexhausted bulb, is
+connected to one of the terminals of the coil, streams can be seen to
+issue from the electrode, and the air in the bulb is heated; if instead
+of a small electrode a large sphere is inclosed in the bulb, no streams
+are observed, still the air is heated.
+
+Nor should it be thought that the temperature of an air condenser would
+give even an approximate idea of the loss in heat incurred, as in such
+case heat must be given off much more quickly, since there is, in
+addition to the ordinary radiation, a very active carrying away of heat
+by independent carriers going on, and since not only the apparatus, but
+the air at some distance from it is heated in consequence of the
+collisions which must occur.
+
+Owing to this, in experiments with such a coil, a rise of temperature
+can be distinctly observed only when the body connected to the coil is
+very small. But with apparatus on a larger scale, even a body of
+considerable bulk would be heated, as, for instance, the body of a
+person; and I think that skilled physicians might make observations of
+utility in such experiments, which, if the apparatus were judiciously
+designed, would not present the slightest danger.
+
+A question of some interest, principally to meteorologists, presents
+itself here. How does the earth behave? The earth is an air condenser,
+but is it a perfect or a very imperfect one--a mere sink of energy?
+There can be little doubt that to such small disturbance as might be
+caused in an experiment, the earth behaves as an almost perfect
+condenser. But it might be different when its charge is set in vibration
+by some sudden disturbance occurring in the heavens. In such case, as
+before stated, probably only little of the energy of the vibrations set
+up would be lost into space in the form of long ether radiations, but
+most of the energy, I think, would spend itself in molecular impacts and
+collisions, and pass off into space in the form of short heat, and
+possibly light, waves. As both the frequency of the vibrations of the
+charge and the potential are in all probability excessive, the energy
+converted into heat may be considerable. Since the density must be
+unevenly distributed, either in consequence of the irregularity of the
+earth's surface, or on account of the condition of the atmosphere in
+various places, the effect produced would accordingly vary from place to
+place. Considerable variations in the temperature and pressure of the
+atmosphere may in this manner be caused at any point of the surface of
+the earth. The variations may be gradual or very sudden, according to
+the nature of the general disturbance, and may produce rain and storms,
+or locally modify the weather in any way.
+
+From the remarks before made, one may see what an important factor of
+loss the air in the neighborhood of a charged surface becomes when the
+electric density is great and the frequency of the impulses excessive.
+But the action, as explained, implies that the air is insulating--that
+is, that it is composed of independent carriers immersed in an
+insulating medium. This is the case only when the air is at something
+like ordinary or greater, or at extremely small, pressure. When the air
+is slightly rarefied and conducting, then true conduction losses occur
+also. In such case, of course, considerable energy may be dissipated
+into space even with a steady potential, or with impulses of low
+frequency, if the density is very great.
+
+When the gas is at very low pressure, an electrode is heated more
+because higher speeds can be reached. If the gas around the electrode is
+strongly compressed, the displacements, and consequently the speeds, are
+very small, and the heating is insignificant. But if in such case the
+frequency could be sufficiently increased, the electrode would be
+brought to a high temperature as well as if the gas were at very low
+pressure; in fact, exhausting the bulb is only necessary because we
+cannot produce, (and possibly not convey) currents of the required
+frequency.
+
+Returning to the subject of electrode lamps, it is obviously of
+advantage in such a lamp to confine as much as possible the heat to the
+electrode by preventing the circulation of the gas in the bulb. If a
+very small bulb be taken, it would confine the heat better than a large
+one, but it might not be of sufficient capacity to be operated from the
+coil, or, if so, the glass might get too hot. A simple way to improve in
+this direction is to employ a globe of the required size, but to place a
+small bulb, the diameter of which is properly estimated, over the
+refractory button contained in the globe. This arrangement is
+illustrated in Fig. 157.
+
+[Illustration: FIG. 157.]
+
+[Illustration: FIG. 158.]
+
+The globe L has in this case a large neck _n_, allowing the small bulb
+_b_ to slip through. Otherwise the construction is the same as shown in
+Fig. 147, for example. The small bulb is conveniently supported upon the
+stem _s_, carrying the refractory button _m_. It is separated from the
+aluminum tube _a_ by several layers of mica M, in order to prevent the
+cracking of the neck by the rapid heating of the aluminum tube upon a
+sudden turning on of the current. The inside bulb should be as small as
+possible when it is desired to obtain light only by incandescence of the
+electrode. If it is desired to produce phosphorescence, the bulb should
+be larger, else it would be apt to get too hot, and the phosphorescence
+would cease. In this arrangement usually only the small bulb shows
+phosphorescence, as there is practically no bombardment against the
+outer globe. In some of these bulbs constructed as illustrated in Fig.
+157, the small tube was coated with phosphorescent paint, and beautiful
+effects were obtained. Instead of making the inside bulb large, in order
+to avoid undue heating, it answers the purpose to make the electrode _m_
+larger. In this case the bombardment is weakened by reason of the
+smaller electric density.
+
+Many bulbs were constructed on the plan illustrated in Fig. 158. Here a
+small bulb _b_, containing the refractory button _m_, upon being
+exhausted to a very high degree was sealed in a large globe L, which was
+then moderately exhausted and sealed off. The principal advantage of
+this construction was that it allowed of reaching extremely high vacua,
+and, at the same time of using a large bulb. It was found, in the course
+of experiments with bulbs such as illustrated in Fig. 158, that it was
+well to make the stem _s_, near the seal at _e_, very thick, and the
+leading-in wire _w_ thin, as it occurred sometimes that the stem at _e_
+was heated and the bulb was cracked. Often the outer globe L was
+exhausted only just enough to allow the discharge to pass through, and
+the space between the bulbs appeared crimson, producing a curious
+effect. In some cases, when the exhaustion in globe L was very low, and
+the air good conducting, it was found necessary, in order to bring the
+button _m_ to high incandescence, to place, preferably on the upper part
+of the neck of the globe, a tinfoil coating which was connected to an
+insulated body, to the ground, or to the other terminal of the coil, as
+the highly conducting air weakened the effect somewhat, probably by
+being acted upon inductively from the wire _w_, where it entered the
+bulb at _e_. Another difficulty--which, however, is always present when
+the refractory button is mounted in a very small bulb--existed in the
+construction illustrated in Fig. 158, namely, the vacuum in the bulb _b_
+would be impaired in a comparatively short time.
+
+The chief idea in the two last described constructions was to confine
+the heat to the central portion of the globe by preventing the exchange
+of air. An advantage is secured, but owing to the heating of the inside
+bulb and slow evaporation of the glass, the vacuum is hard to maintain,
+even if the construction illustrated in Fig. 157 be chosen, in which
+both bulbs communicate.
+
+But by far the better way--the ideal way--would be to reach sufficiently
+high frequencies. The higher the frequency, the slower would be the
+exchange of the air, and I think that a frequency may be reached, at
+which there would be no exchange whatever of the air molecules around
+the terminal. We would then produce a flame in which there would be no
+carrying away of material, and a queer flame it would be, for it would
+be rigid! With such high frequencies the inertia of the particles would
+come into play. As the brush, or flame, would gain rigidity in virtue of
+the inertia of the particles, the exchange of the latter would be
+prevented. This would necessarily occur, for, the number of impulses
+being augmented, the potential energy of each would diminish, so that
+finally only atomic vibrations could be set up, and the motion of
+translation through measurable space would cease. Thus an ordinary gas
+burner connected to a source of rapidly alternating potential might have
+its efficiency augmented to a certain limit, and this for two
+reasons--because of the additional vibration imparted, and because of a
+slowing down of the process of carrying off. But the renewal being
+rendered difficult, a renewal being necessary to maintain the _burner_,
+a continued increase of the frequency of the impulses, assuming they
+could be transmitted to and impressed upon the flame, would result in
+the "extinction" of the latter, meaning by this term only the cessation
+of the chemical process.
+
+I think, however, that in the case of an electrode immersed in a fluid
+insulating medium, and surrounded by independent carriers of electric
+charges, which can be acted upon inductively, a sufficient high
+frequency of the impulses would probably result in a gravitation of the
+gas all around toward the electrode. For this it would be only necessary
+to assume that the independent bodies are irregularly shaped; they would
+then turn toward the electrode their side of the greatest electric
+density, and this would be a position in which the fluid resistance to
+approach would be smaller than that offered to the receding.
+
+The general opinion, I do not doubt, is that it is out of the question
+to reach any such frequencies as might--assuming some of the views
+before expressed to be true--produce any of the results which I have
+pointed out as mere possibilities. This may be so, but in the course of
+these investigations, from the observation of many phenomena, I have
+gained the conviction that these frequencies would be much lower than
+one is apt to estimate at first. In a flame we set up light vibrations
+by causing molecules, or atoms, to collide. But what is the ratio of the
+frequency of the collisions and that of the vibrations set up? Certainly
+it must be incomparably smaller than that of the strokes of the bell and
+the sound vibrations, or that of the discharges and the oscillations of
+the condenser. We may cause the molecules of the gas to collide by the
+use of alternate electric impulses of high frequency, and so we may
+imitate the process in a flame; and from experiments with frequencies
+which we are now able to obtain, I think that the result is producible
+with impulses which are transmissible through a conductor.
+
+In connection with thoughts of a similar nature, it appeared to me of
+great interest to demonstrate the rigidity of a vibrating gaseous
+column. Although with such low frequencies as, say 10,000 per second,
+which I was able to obtain without difficulty from a specially
+constructed alternator, the task looked discouraging at first, I made a
+series of experiments. The trials with air at ordinary pressure led to
+no result, but with air moderately rarefied I obtain what I think to be
+an unmistakable experimental evidence of the property sought for. As a
+result of this kind might lead able investigators to conclusions of
+importance, I will describe one of the experiments performed.
+
+It is well known that when a tube is slightly exhausted, the discharge
+may be passed through it in the form of a thin luminous thread. When
+produced with currents of low frequency, obtained from a coil operated
+as usual, this thread is inert. If a magnet be approached to it, the
+part near the same is attracted or repelled, according to the direction
+of the lines of force of the magnet. It occurred to me that if such a
+thread would be produced with currents of very high frequency, it should
+be more or less rigid, and as it was visible it could be easily studied.
+Accordingly I prepared a tube about one inch in diameter and one metre
+long, with outside coating at each end. The tube was exhausted to a
+point at which, by a little working, the thread discharge could be
+obtained. It must be remarked here that the general aspect of the tube,
+and the degree of exhaustion, are quite other than when ordinary low
+frequency currents are used. As it was found preferable to work with one
+terminal, the tube prepared was suspended from the end of a wire
+connected to the terminal, the tinfoil coating being connected to the
+wire, and to the lower coating sometimes a small insulated plate was
+attached. When the thread was formed, it extended through the upper part
+of the tube and lost itself in the lower end. If it possessed rigidity
+it resembled, not exactly an elastic cord stretched tight between two
+supports, but a cord suspended from a height with a small weight
+attached at the end. When the finger or a small magnet was approached to
+the upper end of the luminous thread, it could be brought locally out of
+position by electrostatic or magnetic action; and when the disturbing
+object was very quickly removed, an analogous result was produced, as
+though a suspended cord would be displaced and quickly released near the
+point of suspension. In doing this the luminous thread was set in
+vibration, and two very sharply marked nodes, and a third indistinct
+one, were formed. The vibration, once set up, continued for fully eight
+minutes, dying gradually out. The speed of the vibration often varied
+perceptibly, and it could be observed that the electrostatic attraction
+of the glass affected the vibrating thread; but it was clear that the
+electrostatic action was not the cause of the vibration, for the thread
+was most generally stationary, and could always be set in vibration by
+passing the finger quickly near the upper part of the tube. With a
+magnet the thread could be split in two and both parts vibrated. By
+approaching the hand to the lower coating of the tube, or insulation
+plate if attached, the vibration was quickened; also, as far as I could
+see, by raising the potential or frequency. Thus, either increasing the
+frequency or passing a stronger discharge of the same frequency
+corresponded to a tightening of the cord. I did not obtain any
+experimental evidence with condenser discharges. A luminous band excited
+in the bulb by repeated discharges of a Leyden jar must possess
+rigidity, and if deformed and suddenly released, should vibrate. But
+probably the amount of vibrating matter is so small that in spite of the
+extreme speed, the inertia cannot prominently assert itself. Besides,
+the observation in such a case is rendered extremely difficult on
+account of the fundamental vibration.
+
+The demonstration of the fact--which still needs better experimental
+confirmation--that a vibrating gaseous column possesses rigidity, might
+greatly modify the views of thinkers. When with low frequencies and
+insignificant potentials indications of that property may be noted, how
+must a gaseous medium behave under the influence of enormous
+electrostatic stresses which may be active in the interstellar space,
+and which may alternate with inconceivable rapidity? The existence of
+such an electrostatic, rhythmically throbbing force--of a vibrating
+electrostatic field--would show a possible way how solids might have
+formed from the ultra-gaseous uterus, and how transverse and all kinds
+of vibrations may be transmitted through a gaseous medium filling all
+space. Then, ether might be a true fluid, devoid of rigidity, and at
+rest, it being merely necessary as a connecting link to enable
+interaction. What determines the rigidity of a body? It must be the
+speed and the amount of motive matter. In a gas the speed maybe
+considerable, but the density is exceedingly small; in a liquid the
+speed would be likely to be small, though the density may be
+considerable; and in both cases the inertia resistance offered to
+displacement is practically _nil_. But place a gaseous (or liquid)
+column in an intense, rapidly alternating electrostatic field, set the
+particles vibrating with enormous speeds, then the inertia resistance
+asserts itself. A body might move with more or less freedom through the
+vibrating mass, but as a whole it would be rigid.
+
+There is a subject which I must mention in connection with these
+experiments: it is that of high vacua. This is a subject, the study of
+which is not only interesting, but useful, for it may lead to results of
+great practical importance. In commercial apparatus, such as
+incandescent lamps, operated from ordinary systems of distribution, a
+much higher vacuum than is obtained at present would not secure a very
+great advantage. In such a case the work is performed on the filament,
+and the gas is little concerned; the improvement, therefore, would be
+but trifling. But when we begin to use very high frequencies and
+potentials, the action of the gas becomes all important, and the degree
+of exhaustion materially modifies the results. As long as ordinary
+coils, even very large ones, were used, the study of the subject was
+limited, because just at a point when it became most interesting it had
+to be interrupted on account of the "non-striking" vacuum being reached.
+But at present we are able to obtain from a small disruptive discharge
+coil potentials much higher than even the largest coil was capable of
+giving, and, what is more, we can make the potential alternate with
+great rapidity. Both of these results enable us now to pass a luminous
+discharge through almost any vacua obtainable, and the field of our
+investigations is greatly extended. Think we as we may, of all the
+possible directions to develop a practical illuminant, the line of high
+vacua seems to be the most promising at present. But to reach extreme
+vacua the appliances must be much more improved, and ultimate perfection
+will not be attained until we shall have discharged the mechanical and
+perfected an _electrical_ vacuum pump. Molecules and atoms can be thrown
+out of a bulb under the action of an enormous potential: _this_ will be
+the principle of the vacuum pump of the future. For the present, we must
+secure the best results we can with mechanical appliances. In this
+respect, it might not be out of the way to say a few words about the
+method of, and apparatus for, producing excessively high degrees of
+exhaustion of which I have availed myself in the course of these
+investigations. It is very probable that other experimenters have used
+similar arrangements; but as it is possible that there may be an item of
+interest in their description, a few remarks, which will render this
+investigation more complete, might be permitted.
+
+[Illustration: FIG. 159.]
+
+The apparatus is illustrated in a drawing shown in Fig. 159. S
+represents a Sprengel pump, which has been specially constructed to
+better suit the work required. The stop-cock which is usually employed
+has been omitted, and instead of it a hollow stopper _s_ has been fitted
+in the neck of the reservoir R. This stopper has a small hole _h_,
+through which the mercury descends; the size of the outlet _o_ being
+properly determined with respect to the section of the fall tube _t_,
+which is sealed to the reservoir instead of being connected to it in the
+usual manner. This arrangement overcomes the imperfections and troubles
+which often arise from the use of the stopcock on the reservoir and the
+connections of the latter with the fall tube.
+
+The pump is connected through a U-shaped tube _t_ to a very large
+reservoir R_{1}. Especial care was taken in fitting the grinding
+surfaces of the stoppers p and p_{1}, and both of these and the mercury
+caps above them were made exceptionally long. After the U-shaped tube
+was fitted and put in place, it was heated, so as to soften and take
+off the strain resulting from imperfect fitting. The U-shaped tube was
+provided with a stopcock C, and two ground connections g and g_{1},--one
+for a small bulb _b_, usually containing caustic potash, and the other
+for the receiver _r_, to be exhausted.
+
+The reservoir R_{1}, was connected by means of a rubber tube to a
+slightly larger reservoir R_{2}, each of the two reservoirs being
+provided with a stopcock C_{1} and C_{2}, respectively. The reservoir
+R_{2} could be raised and lowered by a wheel and rack, and the range of
+its motion was so determined that when it was filled with mercury and
+the stopcock C_{2} closed, so as to form a Torricellian vacuum in it
+when raised, it could be lifted so high that the reservoir R_{1} would
+stand a little above stopcock C_{1}; and when this stopcock was closed
+and the reservoir R_{2} descended, so as to form a Torricellian vacuum
+in reservoir R_{1}, it could be lowered so far as to completely empty
+the latter, the mercury filling the reservoir R_{2} up to a little above
+stopcock C_{2}.
+
+The capacity of the pump and of the connections was taken as small as
+possible relatively to the volume of reservoir, R_{1}, since, of course,
+the degree of exhaustion depended upon the ratio of these quantities.
+
+With this apparatus I combined the usual means indicated by former
+experiments for the production of very high vacua. In most of the
+experiments it was most convenient to use caustic potash. I may venture
+to say, in regard to its use, that much time is saved and a more perfect
+action of the pump insured by fusing and boiling the potash as soon as,
+or even before, the pump settles down. If this course is not followed,
+the sticks, as ordinarily employed, may give off moisture at a certain
+very slow rate, and the pump may work for many hours without reaching a
+very high vacuum. The potash was heated either by a spirit lamp or by
+passing a discharge through it, or by passing a current through a wire
+contained in it. The advantage in the latter case was that the heating
+could be more rapidly repeated.
+
+Generally the process of exhaustion was the following:--At the start,
+the stop-cocks C and C_{1} being open, and all other connections closed,
+the reservoir R_{2} was raised so far that the mercury filled the
+reservoir R_{1} and a part of the narrow connecting U-shaped tube.
+When the pump was set to work, the mercury would, of course, quickly
+rise in the tube, and reservoir R_{2} was lowered, the experimenter
+keeping the mercury at about the same level. The reservoir R_{2} was
+balanced by a long spring which facilitated the operation, and the
+friction of the parts was generally sufficient to keep it in almost any
+position. When the Sprengel pump had done its work, the reservoir R_{2}
+was further lowered and the mercury descended in R_{1} and filled R_{2},
+whereupon stopcock C_{2} was closed. The air adhering to the walls of
+R_{1} and that absorbed by the mercury was carried off, and to free the
+mercury of all air the reservoir R_{2} was for a long time worked up and
+down. During this process some air, which would gather below stopcock
+C_{2}, was expelled from R_{2} by lowering it far enough and opening the
+stopcock, closing the latter again before raising the reservoir. When
+all the air had been expelled from the mercury, and no air would gather
+in R_{2} when it was lowered, the caustic potash was resorted to. The
+reservoir R_{2} was now again raised until the mercury in R_{1}, stood
+above stopcock C_{1}. The caustic potash was fused and boiled, and
+moisture partly carried off by the pump and partly re-absorbed; and this
+process of heating and cooling was repeated many times, and each time,
+upon the moisture being absorbed or carried off, the reservoir R_{2} was
+for a long time raised and lowered. In this manner all the moisture was
+carried off from the mercury, and both the reservoirs were in proper
+condition to be used. The reservoir R_{2} was then again raised to the
+top, and the pump was kept working for a long time. When the highest
+vacuum obtainable with the pump had been reached, the potash bulb was
+usually wrapped with cotton which was sprinkled with ether so as to keep
+the potash at a very low temperature, then the reservoir R_{2} was
+lowered, and upon reservoir R_{1} being emptied the receiver was quickly
+sealed up.
+
+When a new bulb was put on, the mercury was always raised above stopcock
+C_{1}, which was closed, so as to always keep the mercury and both the
+reservoirs in fine condition, and the mercury was never withdrawn from
+R_{1} except when the pump had reached the highest degree of exhaustion.
+It is necessary to observe this rule if it is desired to use the
+apparatus to advantage.
+
+By means of this arrangement I was able to proceed very quickly, and
+when the apparatus was in perfect order it was possible to reach the
+phosphorescent stage in a small bulb in less than fifteen minutes, which
+is certainly very quick work for a small laboratory arrangement
+requiring all in all about 100 pounds of mercury. With ordinary small
+bulbs the ratio of the capacity of the pump, receiver, and connections,
+and that of reservoir R was about 1 to 20, and the degrees of exhaustion
+reached were necessarily very high, though I am unable to make a precise
+and reliable statement how far the exhaustion was carried.
+
+What impresses the investigator most in the course of these experiences
+is the behavior of gases when subjected to great rapidly alternating
+electrostatic stresses. But he must remain in doubt as to whether the
+effects observed are due wholly to the molecules, or atoms, of the gas
+which chemical analysis discloses to us, or whether there enters into
+play another medium of a gaseous nature, comprising atoms, or molecules,
+immersed in a fluid pervading the space. Such a medium surely must
+exist, and I am convinced that, for instance, even if air were absent,
+the surface and neighborhood of a body in space would be heated by
+rapidly alternating the potential of the body; but no such heating of
+the surface or neighborhood could occur if all free atoms were removed
+and only a homogeneous, incompressible, and elastic fluid--such as ether
+is supposed to be--would remain, for then there would be no impacts, no
+collisions. In such a case, as far as the body itself is concerned, only
+frictional losses in the inside could occur.
+
+It is a striking fact that the discharge through a gas is established
+with ever-increasing freedom as the frequency of the impulses is
+augmented. It behaves in this respect quite contrarily to a metallic
+conductor. In the latter the impedance enters prominently into play as
+the frequency is increased, but the gas acts much as a series of
+condensers would; the facility with which the discharge passes through,
+seems to depend on the rate of change of potential. If it acts so, then
+in a vacuum tube even of great length, and no matter how strong the
+current, self-induction could not assert itself to any appreciable
+degree. We have, then, as far as we can now see, in the gas a conductor
+which is capable of transmitting electric impulses of any frequency
+which we may be able to produce. Could the frequency be brought high
+enough, then a queer system of electric distribution, which would be
+likely to interest gas companies, might be realized: metal pipes
+filled with gas--the metal being the insulator, the gas the
+conductor--supplying phosphorescent bulbs, or perhaps devices as yet
+uninvented. It is certainly possible to take a hollow core of copper,
+rarefy the gas in the same, and by passing impulses of sufficiently high
+frequency through a circuit around it, bring the gas inside to a high
+degree of incandescence; but as to the nature of the forces there would
+be considerable uncertainty, for it would be doubtful whether with such
+impulses the copper core would act as a static screen. Such paradoxes
+and apparent impossibilities we encounter at every step in this line of
+work, and therein lies, to a great extent, the charm of the study.
+
+I have here a short and wide tube which is exhausted to a high degree
+and covered with a substantial coating of bronze, the coating barely
+allowing the light to shine through. A metallic cap, with a hook for
+suspending the tube, is fastened around the middle portion of the
+latter, the clasp being in contact with the bronze coating. I now want
+to light the gas inside by suspending the tube on a wire connected to
+the coil. Any one who would try the experiment for the first time, not
+having any previous experience, would probably take care to be quite
+alone when making the trial, for fear that he might become the joke of
+his assistants. Still, the bulb lights in spite of the metal coating,
+and the light can be distinctly perceived through the latter. A long
+tube covered with aluminum bronze lights when held in one hand--the
+other touching the terminal of the coil--quite powerfully. It might be
+objected that the coatings are not sufficiently conducting; still, even
+if they were highly resistant, they ought to screen the gas. They
+certainly screen it perfectly in a condition of rest, but far from
+perfectly when the charge is surging in the coating. But the loss of
+energy which occurs within the tube, notwithstanding the screen, is
+occasioned principally by the presence of the gas. Were we to take a
+large hollow metallic sphere and fill it with a perfect, incompressible,
+fluid dielectric, there would be no loss inside of the sphere, and
+consequently the inside might be considered as perfectly screened,
+though the potential be very rapidly alternating. Even were the sphere
+filled with oil, the loss would be incomparably smaller than when the
+fluid is replaced by a gas, for in the latter case the force produces
+displacements; that means impact and collisions in the inside.
+
+No matter what the pressure of the gas may be, it becomes an important
+factor in the heating of a conductor when the electric density is great
+and the frequency very high. That in the heating of conductors by
+lightning discharges, air is an element of great importance, is almost
+as certain as an experimental fact. I may illustrate the action of the
+air by the following experiment: I take a short tube which is exhausted
+to a moderate degree and has a platinum wire running through the middle
+from one end to the other. I pass a steady or low frequency current
+through the wire, and it is heated uniformly in all parts. The heating
+here is due to conduction, or frictional losses, and the gas around the
+wire has--as far as we can see--no function to perform. But now let me
+pass sudden discharges, or high frequency currents, through the wire.
+Again the wire is heated, this time principally on the ends and least in
+the middle portion; and if the frequency of the impulses, or the rate of
+change, is high enough, the wire might as well be cut in the middle as
+not, for practically all heating is due to the rarefied gas. Here the
+gas might only act as a conductor of no impedance diverting the current
+from the wire as the impedance of the latter is enormously increased,
+and merely heating the ends of the wire by reason of their resistance to
+the passage of the discharge. But it is not at all necessary that the
+gas in the tube should be conducting; it might be at an extremely low
+pressure, still the ends of the wire would be heated--as, however, is
+ascertained by experience--only the two ends would in such case not be
+electrically connected through the gaseous medium. Now what with these
+frequencies and potentials occurs in an exhausted tube, occurs in the
+lightning discharges at ordinary pressure. We only need remember one of
+the facts arrived at in the course of these investigations, namely, that
+to impulses of very high frequency the gas at ordinary pressure behaves
+much in the same manner as though it were at moderately low pressure. I
+think that in lightning discharges frequently wires or conducting
+objects are volatilized merely because air is present, and that, were
+the conductor immersed in an insulating liquid, it would be safe, for
+then the energy would have to spend itself somewhere else. From the
+behavior of gases under sudden impulses of high potential, I am led to
+conclude that there can be no surer way of diverting a lightning
+discharge than by affording it a passage through a volume of gas, if
+such a thing can be done in a practical manner.
+
+There are two more features upon which I think it necessary to dwell in
+connection with these experiments--the "radiant state" and the
+"non-striking vacuum."
+
+Any one who has studied Crookes' work must have received the impression
+that the "radiant state" is a property of the gas inseparably connected
+with an extremely high degree of exhaustion. But it should be remembered
+that the phenomena observed in an exhausted vessel are limited to the
+character and capacity of the apparatus which is made use of. I think
+that in a bulb a molecule, or atom, does not precisely move in a
+straight line because it meets no obstacle, but because the velocity
+imparted to it is sufficient to propel it in a sensibly straight line.
+The mean free path is one thing, but the velocity--the energy associated
+with the moving body--is another, and under ordinary circumstances I
+believe that it is a mere question of potential or speed. A disruptive
+discharge coil, when the potential is pushed very far, excites
+phosphorescence and projects shadows, at comparatively low degrees of
+exhaustion. In a lightning discharge, matter moves in straight lines at
+ordinary pressure when the mean free path is exceedingly small, and
+frequently images of wires or other metallic objects have been produced
+by the particles thrown off in straight lines.
+
+I have prepared a bulb to illustrate by an experiment the correctness of
+these assertions. In a globe L, Fig. 160, I have mounted upon a lamp
+filament _f_ a piece of lime _l_. The lamp filament is connected with a
+wire which leads into the bulb, and the general construction of the
+latter is as indicated in Fig. 148, before described. The bulb being
+suspended from a wire connected to the terminal of the coil, and the
+latter being set to work, the lime piece _l_ and the projecting parts of
+the filament _f_ are bombarded. The degree of exhaustion is just such
+that with the potential the coil is capable of giving, phosphorescence
+of the glass is produced, but disappears as soon as the vacuum is
+impaired. The lime containing moisture, and moisture being given off as
+soon as heating occurs, the phosphorescence lasts only for a few
+moments. When the lime has been sufficiently heated, enough moisture has
+been given off to impair materially the vacuum of the bulb. As the
+bombardment goes on, one point of the lime piece is more heated than
+other points, and the result is that finally practically all the
+discharge passes through that point which is intensely heated, and a
+white stream of lime particles (Fig. 160) then breaks forth from that
+point. This stream is composed of "radiant" matter, yet the degree of
+exhaustion is low. But the particles move in straight lines because the
+velocity imparted to them is great, and this is due to three causes--to
+the great electric density, the high temperature of the small point, and
+the fact that the particles of the lime are easily torn and thrown
+off--far more easily than those of carbon. With frequencies such as we
+are able to obtain, the particles are bodily thrown off and projected to
+a considerable distance; but with sufficiently high frequencies no such
+thing would occur; in such case only a stress would spread or a
+vibration would be propagated through the bulb. It would be out of the
+question to reach any such frequency on the assumption that the atoms
+move with the speed of light; but I believe that such a thing is
+impossible; for this an enormous potential would be required. With
+potentials which we are able to obtain, even with a disruptive discharge
+coil, the speed must be quite insignificant.
+
+[Illustration: FIG. 160.]
+
+As to the "non-striking vacuum," the point to be noted is, that it can
+occur only with low frequency impulses, and it is necessitated by the
+impossibility of carrying off enough energy with such impulses in high
+vacuum, since the few atoms which are around the terminal upon coming in
+contact with the same, are repelled and kept at a distance for a
+comparatively long period of time, and not enough work can be performed
+to render the effect perceptible to the eye. If the difference of
+potential between the terminals is raised, the dielectric breaks down.
+But with very high frequency impulses there is no necessity for such
+breaking down, since any amount of work can be performed by continually
+agitating the atoms in the exhausted vessel, provided the frequency is
+high enough. It is easy to reach--even with frequencies obtained from an
+alternator as here used--a stage at which the discharge does not pass
+between two electrodes in a narrow tube, each of these being connected
+to one of the terminals of the coil, but it is difficult to reach a
+point at which a luminous discharge would not occur around each
+electrode.
+
+[Illustration: FIG. 161.]
+
+[Illustration: FIG. 162.]
+
+A thought which naturally presents itself in connection with high
+frequency currents, is to make use of their powerful electrodynamic
+inductive action to produce light effects in a sealed glass globe. The
+leading-in wire is one of the defects of the present incandescent lamp,
+and if no other improvement were made, that imperfection at least should
+be done away with. Following this thought, I have carried on
+experiments in various directions, of which some were indicated in my
+former paper. I may here mention one or two more lines of experiment
+which have been followed up.
+
+Many bulbs were constructed as shown in Fig. 161 and Fig. 162.
+
+In Fig. 161, a wide tube, T, was sealed to a smaller W
+shaped tube U, of phosphorescent glass. In the tube T, was placed a coil
+C, of aluminum wire, the ends of which were provided with small spheres,
+t and t_{1}, of aluminum, and reached into the U tube. The tube T
+was slipped into a socket containing a primary coil, through which
+usually the discharges of Leyden jars were directed, and the rarefied
+gas in the small U tube was excited to strong luminosity by the
+high-tension current induced in the coil C. When Leyden jar discharges
+were used to induce currents in the coil C, it was found necessary to
+pack the tube T tightly with insulating powder, as a discharge would
+occur frequently between the turns of the coil, especially when the
+primary was thick and the air gap, through which the jars discharged,
+large, and no little trouble was experienced in this way.
+
+In Fig. 162 is illustrated another form of the bulb constructed. In this
+case a tube T is sealed to a globe L. The tube contains a coil C, the
+ends of which pass through two small glass tubes t and t_{1}, which
+are sealed to the tube T. Two refractory buttons m and m_{1}, are
+mounted on lamp filaments which are fastened to the ends of the wires
+passing through the glass tubes t and t_{1}. Generally in bulbs made
+on this plan the globe L communicated with the tube T. For this purpose
+the ends of the small tubes t and t_{1} were heated just a trifle in
+the burner, merely to hold the wires, but not to interfere with the
+communication. The tube T, with the small tubes, wires through the same,
+and the refractory buttons m and m_{1}, were first prepared, and
+then sealed to globe L, whereupon the coil C was slipped in and the
+connections made to its ends. The tube was then packed with insulating
+powder, jamming the latter as tight as possible up to very nearly the
+end; then it was closed and only a small hole left through which the
+remainder of the powder was introduced, and finally the end of the tube
+was closed. Usually in bulbs constructed as shown in Fig. 162 an
+aluminum tube _a_ was fastened to the upper end _s_ of each of the tubes
+t and t_{1} in order to protect that end against the heat. The
+buttons m and m_{1} could be brought to any degree of incandescence
+by passing the discharges of Leyden jars around the coil C. In such
+bulbs with two buttons a very curious effect is produced by the
+formation of the shadows of each of the two buttons.
+
+Another line of experiment, which has been assiduously followed, was to
+induce by electro-dynamic induction a current or luminous discharge in
+an exhausted tube or bulb. This matter has received such able treatment
+at the hands of Prof. J. J. Thomson, that I could add but little to what
+he has made known, even had I made it the special subject of this
+lecture. Still, since experiments in this line have gradually led me to
+the present views and results, a few words must be devoted here to this
+subject.
+
+It has occurred, no doubt, to many that as a vacuum tube is made longer,
+the electromotive force per unit length of the tube, necessary to pass a
+luminous discharge through the latter, becomes continually smaller;
+therefore, if the exhausted tube be made long enough, even with low
+frequencies a luminous discharge could be induced in such a tube closed
+upon itself. Such a tube might be placed around a hall or on a ceiling,
+and at once a simple appliance capable of giving considerable light
+would be obtained. But this would be an appliance hard to manufacture
+and extremely unmanageable. It would not do to make the tube up of small
+lengths, because there would be with ordinary frequencies considerable
+loss in the coatings, and besides, if coatings were used, it would be
+better to supply the current directly to the tube by connecting the
+coatings to a transformer. But even if all objections of such nature
+were removed, with low frequencies the light conversion itself would be
+inefficient, as I have before stated. In using extremely high
+frequencies the length of the secondary--in other words, the size of the
+vessel--can be reduced as much as desired, and the efficiency of the
+light conversion is increased, provided that means are invented for
+efficiently obtaining such high frequencies. Thus one is led, from
+theoretical and practical considerations, to the use of high
+frequencies, and this means high electromotive forces and small currents
+in the primary. When one works with condenser charges--and they are the
+only means up to the present known for reaching these extreme
+frequencies--one gets to electromotive forces of several thousands of
+volts per turn of the primary. We cannot multiply the electro-dynamic
+inductive effect by taking more turns in the primary, for we arrive at
+the conclusion that the best way is to work with one single turn--though
+we must sometimes depart from this rule--and we must get along with
+whatever inductive effect we can obtain with one turn. But before one
+has long experimented with the extreme frequencies required to set up in
+a small bulb an electromotive force of several thousands of volts, one
+realizes the great importance of electrostatic effects, and these
+effects grow relatively to the electro-dynamic in significance as the
+frequency is increased.
+
+Now, if anything is desirable in this case, it is to increase the
+frequency, and this would make it still worse for the electrodynamic
+effects. On the other hand, it is easy to exalt the electrostatic action
+as far as one likes by taking more turns on the secondary, or combining
+self-induction and capacity to raise the potential. It should also be
+remembered that, in reducing the current to the smallest value and
+increasing the potential, the electric impulses of high frequency can be
+more easily transmitted through a conductor.
+
+These and similar thoughts determined me to devote more attention to the
+electrostatic phenomena, and to endeavor to produce potentials as high
+as possible, and alternating as fast as they could be made to alternate.
+I then found that I could excite vacuum tubes at considerable distance
+from a conductor connected to a properly constructed coil, and that I
+could, by converting the oscillatory current of a conductor to a higher
+potential, establish electrostatic alternating fields which acted
+through the whole extent of the room, lighting up a tube no matter where
+it was held in space. I thought I recognized that I had made a step in
+advance, and I have persevered in this line; but I wish to say that I
+share with all lovers of science and progress the one and only
+desire--to reach a result of utility to men in any direction to which
+thought or experiment may lead me. I think that this departure is the
+right one, for I cannot see, from the observation of the phenomena which
+manifest themselves as the frequency is increased, what there would
+remain to act between two circuits conveying, for instance, impulses of
+several hundred millions per second, except electrostatic forces. Even
+with such trifling frequencies the energy would be practically all
+potential, and my conviction has grown strong that, to whatever kind of
+motion light may be due, it is produced by tremendous electrostatic
+stresses vibrating with extreme rapidity.
+
+[Illustration: FIG. 163.]
+
+[Illustration: FIG. 164.]
+
+Of all these phenomena observed with currents, or electric impulses, of
+high frequency, the most fascinating for an audience are certainly those
+which are noted in an electrostatic field acting through considerable
+distance; and the best an unskilled lecturer can do is to begin and
+finish with the exhibition of these singular effects. I take a tube in
+my hand and move it about, and it is lighted wherever I may hold it;
+throughout space the invisible forces act. But I may take another tube
+and it might not light, the vacuum being very high. I excite it by means
+of a disruptive discharge coil, and now it will light in the
+electrostatic field. I may put it away for a few weeks or months, still
+it retains the faculty of being excited. What change have I produced in
+the tube in the act of exciting it? If a motion imparted to atoms, it is
+difficult to perceive how it can persist so long without being arrested
+by frictional losses; and if a strain exerted in the dielectric, such as
+a simple electrification would produce, it is easy to see how it may
+persist indefinitely, but very difficult to understand why such a
+condition should aid the excitation when we have to deal with potentials
+which are rapidly alternating.
+
+Since I have exhibited these phenomena for the first time, I have
+obtained some other interesting effects. For instance, I have produced
+the incandescence of a button, filament, or wire enclosed in a tube. To
+get to this result it was necessary to economize the energy which is
+obtained from the field, and direct most of it on the small body to be
+rendered incandescent. At the beginning the task appeared difficult, but
+the experiences gathered permitted me to reach the result easily. In
+Fig. 163 and Fig. 164, two such tubes are illustrated, which are
+prepared for the occasion. In Fig. 163 a short tube T_{1}, sealed to
+another long tube T, is provided with a stem _s_, with a platinum wire
+sealed in the latter. A very thin lamp filament _l_, is fastened to this
+wire and connection to the outside is made through a thin copper wire
+_w_. The tube is provided with outside and inside coatings, C and C_{1},
+respectively, and is filled as far as the coatings reach with
+conducting, and the space above with insulating, powder. These coatings
+are merely used to enable me to perform two experiments with the
+tube--namely, to produce the effect desired either by direct connection
+of the body of the experimenter or of another body to the wire _w_, or
+by acting inductively through the glass. The stem _s_ is provided with
+an aluminum tube _a_, for purposes before explained, and only a small
+part of the filament reaches out of this tube. By holding the tube T_{1}
+anywhere in the electrostatic field, the filament is rendered
+incandescent.
+
+A more interesting piece of apparatus is illustrated in Fig. 164. The
+construction is the same as before, only instead of the lamp filament a
+small platinum wire _p_, sealed in a stem _s_, and bent above it in a
+circle, is connected to the copper wire _w_, which is joined to an
+inside coating C. A small stem s_{1} is provided with a needle, on the
+point of which is arranged, to rotate very freely, a very light fan of
+mica _v_. To prevent the fan from falling out, a thin stem of glass _g_,
+is bent properly and fastened to the aluminum tube. When the glass tube
+is held anywhere in the electrostatic field the platinum wire becomes
+incandescent, and the mica vanes are rotated very fast.
+
+Intense phosphorescence may be excited in a bulb by merely connecting it
+to a plate within the field, and the plate need not be any larger than
+an ordinary lamp shade. The phosphorescence excited with these currents
+is incomparably more powerful than with ordinary apparatus. A small
+phosphorescent bulb, when attached to a wire connected to a coil, emits
+sufficient light to allow reading ordinary print at a distance of five
+to six paces. It was of interest to see how some of the phosphorescent
+bulbs of Professor Crookes would behave with these currents, and he has
+had the kindness to lend me a few for the occasion. The effects produced
+are magnificent, especially by the sulphide of calcium and sulphide of
+zinc. With the disruptive discharge coil they glow intensely merely by
+holding them in the hand and connecting the body to the terminal of the
+coil.
+
+To whatever results investigations of this kind may lead, the chief
+interest lies, for the present, in the possibilities they offer for the
+production of an efficient illuminating device. In no branch of electric
+industry is an advance more desired than in the manufacture of light.
+Every thinker, when considering the barbarous methods employed, the
+deplorable losses incurred in our best systems of light production, must
+have asked himself, What is likely to be the light of the future? Is it
+to be an incandescent solid, as in the present lamp, or an incandescent
+gas, or a phosphorescent body, or something like a burner, but
+incomparably more efficient?
+
+There is little chance to perfect a gas burner; not, perhaps, because
+human ingenuity has been bent upon that problem for centuries without a
+radical departure having been made--though the argument is not devoid of
+force--but because in a burner the highest vibrations can never be
+reached, except by passing through all the low ones. For how is a flame
+to proceed unless by a fall of lifted weights? Such process cannot be
+maintained without renewal, and renewal is repeated passing from low to
+high vibrations. One way only seems to be open to improve a burner, and
+that is by trying to reach higher degrees of incandescence. Higher
+incandescence is equivalent to a quicker vibration: that means more
+light from the same material, and that again, means more economy. In
+this direction some improvements have been made, but the progress is
+hampered by many limitations. Discarding, then, the burner, there
+remains the three ways first mentioned, which are essentially
+electrical.
+
+Suppose the light of the immediate future to be a solid, rendered
+incandescent by electricity. Would it not seem that it is better to
+employ a small button than a frail filament? From many considerations it
+certainly must be concluded that a button is capable of a higher
+economy, assuming, of course, the difficulties connected with the
+operation of such a lamp to be effectively overcome. But to light such
+a lamp we require a high potential; and to get this economically, we
+must use high frequencies.
+
+Such considerations apply even more to the production of light by the
+incandescence of a gas, or by phosphorescence. In all cases we require
+high frequencies and high potentials. These thoughts occurred to me a
+long time ago.
+
+Incidentally we gain, by the use of high frequencies, many advantages,
+such as higher economy in the light production, the possibility of
+working with one lead, the possibility of doing away with the leading-in
+wire, etc.
+
+The question is, how far can we go with frequencies? Ordinary conductors
+rapidly lose the facility of transmitting electric impulses when the
+frequency is greatly increased. Assume the means for the production of
+impulses of very great frequency brought to the utmost perfection, every
+one will naturally ask how to transmit them when the necessity arises.
+In transmitting such impulses through conductors we must remember that
+we have to deal with _pressure_ and _flow_, in the ordinary
+interpretation of these terms. Let the pressure increase to an enormous
+value, and let the flow correspondingly diminish, then such
+impulses--variations merely of pressure, as it were--can no doubt be
+transmitted through a wire even if their frequency be many hundreds of
+millions per second. It would, of course, be out of question to transmit
+such impulses through a wire immersed in a gaseous medium, even if the
+wire were provided with a thick and excellent insulation, for most of
+the energy would be lost in molecular bombardment and consequent
+heating. The end of the wire connected to the source would be heated,
+and the remote end would receive but a trifling part of the energy
+supplied. The prime necessity, then, if such electric impulses are to be
+used, is to find means to reduce as much as possible the dissipation.
+
+The first thought is, to employ the thinnest possible wire surrounded by
+the thickest practicable insulation. The next thought is to employ
+electrostatic screens. The insulation of the wire may be covered with a
+thin conducting coating and the latter connected to the ground. But this
+would not do, as then all the energy would pass through the conducting
+coating to the ground and nothing would get to the end of the wire. If a
+ground connection is made it can only be made through a conductor
+offering an enormous impedance, or through a condenser of extremely
+small capacity. This, however, does not do away with other difficulties.
+
+If the wave length of the impulses is much smaller than the length of
+the wire, then corresponding short waves will be set up in the
+conducting coating, and it will be more or less the same as though the
+coating were directly connected to earth. It is therefore necessary to
+cut up the coating in sections much shorter than the wave length. Such
+an arrangement does not still afford a perfect screen, but it is ten
+thousand times better than none. I think it preferable to cut up the
+conducting coating in small sections, even if the current waves be much
+longer than the coating.
+
+If a wire were provided with a perfect electrostatic screen, it would be
+the same as though all objects were removed from it at infinite
+distance. The capacity would then be reduced to the capacity of the wire
+itself, which would be very small. It would then be possible to send
+over the wire current vibrations of very high frequencies at enormous
+distances, without affecting greatly the character of the vibrations. A
+perfect screen is of course out of the question, but I believe that with
+a screen such as I have just described telephony could be rendered
+practicable across the Atlantic. According to my ideas, the gutta-percha
+covered wire should be provided with a third conducting coating
+subdivided in sections. On the top of this should be again placed a
+layer of gutta-percha and other insulation, and on the top of the whole
+the armor. But such cables will not be constructed, for ere long
+intelligence--transmitted without wires--will throb through the earth
+like a pulse through a living organism. The wonder is that, with the
+present state of knowledge and the experiences gained, no attempt is
+being made to disturb the electrostatic or magnetic condition of the
+earth, and transmit, if nothing else, intelligence.
+
+It has been my chief aim in presenting these results to point out
+phenomena or features of novelty, and to advance ideas which I am
+hopeful will serve as starting points of new departures. It has been my
+chief desire this evening to entertain you with some novel experiments.
+Your applause, so frequently and generously accorded, has told me that I
+have succeeded.
+
+In conclusion, let me thank you most heartily for your kindness and
+attention, and assure you that the honor I have had in addressing such
+a distinguished audience, the pleasure I have had in presenting these
+results to a gathering of so many able men--and among them also some of
+those in whose work for many years past I have found enlightenment and
+constant pleasure--I shall never forget.
+
+
+
+
+CHAPTER XXVIII.
+
+ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA.[3]
+
+  [3] A lecture delivered before the Franklin Institute, Philadelphia,
+      February, 1893, and before the National Electric Light
+      Association, St. Louis, March, 1893.
+
+
+INTRODUCTORY.--SOME THOUGHTS ON THE EYE.
+
+When we look at the world around us, on Nature, we are impressed with
+its beauty and grandeur. Each thing we perceive, though it may be
+vanishingly small, is in itself a world, that is, like the whole of the
+universe, matter and force governed by law,--a world, the contemplation
+of which fills us with feelings of wonder and irresistibly urges us to
+ceaseless thought and inquiry. But in all this vast world, of all
+objects our senses reveal to us, the most marvellous, the most appealing
+to our imagination, appears no doubt a highly developed organism, a
+thinking being. If there is anything fitted to make us admire Nature's
+handiwork, it is certainly this inconceivable structure, which performs
+its innumerable motions of obedience to external influence. To
+understand its workings, to get a deeper insight into this Nature's
+masterpiece, has ever been for thinkers a fascinating aim, and after
+many centuries of arduous research men have arrived at a fair
+understanding of the functions of its organs and senses. Again, in all
+the perfect harmony of its parts, of the parts which constitute the
+material or tangible of our being, of all its organs and senses, the eye
+is the most wonderful. It is the most precious, the most indispensable
+of our perceptive or directive organs, it is the great gateway through
+which all knowledge enters the mind. Of all our organs, it is the one,
+which is in the most intimate relation with that which we call
+intellect. So intimate is this relation, that it is often said, the very
+soul shows itself in the eye.
+
+It can be taken as a fact, which the theory of the action of the eye
+implies, that for each external impression, that is, for each image
+produced upon the retina, the ends of the visual nerves, concerned in
+the conveyance of the impression to the mind, must be under a peculiar
+stress or in a vibratory state. It now does not seem improbable that,
+when by the power of thought an image is evoked, a distinct reflex
+action, no matter how weak, is exerted upon certain ends of the visual
+nerves, and therefore upon the retina. Will it ever be within human
+power to analyze the condition of the retina when disturbed by thought
+or reflex action, by the help of some optical or other means of such
+sensitiveness, that a clear idea of its state might be gained at any
+time? If this were possible, then the problem of reading one's thoughts
+with precision, like the characters of an open book, might be much
+easier to solve than many problems belonging to the domain of positive
+physical science, in the solution of which many, if not the majority, of
+scientific men implicitly believe. Helmholtz, has shown that the fundi
+of the eye are themselves, luminous, and he was able to _see_, in total
+darkness, the movement of his arm by the light of his own eyes. This is
+one of the most remarkable experiments recorded in the history of
+science, and probably only a few men could satisfactorily repeat it, for
+it is very likely, that the luminosity of the eyes is associated with
+uncommon activity of the brain and great imaginative power. It is
+fluorescence of brain action, as it were.
+
+Another fact having a bearing on this subject which has probably been
+noted by many, since it is stated in popular expressions, but which I
+cannot recollect to have found chronicled as a positive result of
+observation is, that at times, when a sudden idea or image presents
+itself to the intellect, there is a distinct and sometimes painful
+sensation of luminosity produced in the eye, observable even in broad
+daylight.
+
+The saying then, that the soul shows itself in the eye, is deeply
+founded, and we feel that it expresses a great truth. It has a profound
+meaning even for one who, like a poet or artist, only following his
+inborn instinct or love for Nature, finds delight in aimless thoughts
+and in the mere contemplation of natural phenomena, but a still more
+profound meaning for one who, in the spirit of positive scientific
+investigation, seeks to ascertain the causes of the effects. It is
+principally the natural philosopher, the physicist, for whom the eye is
+the subject of the most intense admiration.
+
+Two facts about the eye must forcibly impress the mind of the physicist,
+notwithstanding he may think or say that it is an imperfect optical
+instrument, forgetting, that the very conception of that which is
+perfect or seems so to him, has been gained through this same
+instrument. First, the eye is, as far as our positive knowledge goes,
+the only organ which is _directly_ affected by that subtile medium,
+which as science teaches us, must fill all space; secondly, it is the
+most sensitive of our organs, incomparably more sensitive to external
+impressions than any other.
+
+The organ of hearing implies the impact of ponderable bodies, the organ
+of smell the transference of detached material particles, and the organs
+of taste, and of touch or force, the direct contact, or at least some
+interference of ponderable matter, and this is true even in those
+instances of animal organisms, in which some of these organs are
+developed to a degree of truly marvelous perfection. This being so, it
+seems wonderful that the organ of sight solely should be capable of
+being stirred by that, which all our other organs are powerless to
+detect, yet which plays an essential part in all natural phenomena,
+which transmits all energy and sustains all motion and, that most
+intricate of all, life, but which has properties such that even a
+scientifically trained mind cannot help drawing a distinction between it
+and all that is called matter. Considering merely this, and the fact
+that the eye, by its marvelous power, widens our otherwise very narrow
+range of perception far beyond the limits of the small world which is
+our own, to embrace myriads of other worlds, suns and stars in the
+infinite depths of the universe, would make it justifiable to assert,
+that it is an organ of a higher order. Its performances are beyond
+comprehension. Nature as far as we know never produced anything more
+wonderful. We can get barely a faint idea of its prodigious power by
+analyzing what it does and by comparing. When ether waves impinge upon
+the human body, they produce the sensations of warmth or cold, pleasure
+or pain, or perhaps other sensations of which we are not aware, and any
+degree or intensity of these sensations, which degrees are infinite in
+number, hence an infinite number of distinct sensations. But our sense
+of touch, or our sense of force, cannot reveal to us these differences
+in degree or intensity, unless they are very great. Now we can readily
+conceive how an organism, such as the human, in the eternal process of
+evolution, or more philosophically speaking, adaptation to Nature, being
+constrained to the use of only the sense of touch or force, for
+instance, might develop this sense to such a degree of sensitiveness or
+perfection, that it would be capable of distinguishing the minutest
+differences in the temperature of a body even at some distance, to a
+hundredth, or thousandth, or millionth part of a degree. Yet, even this
+apparently impossible performance would not begin to compare with that
+of the eye, which is capable of distinguishing and conveying to the mind
+in a single instant innumerable peculiarities of the body, be it in
+form, or color, or other respects. This power of the eye rests upon two
+things, namely, the rectilinear propagation of the disturbance by which
+it is effected, and upon its sensitiveness. To say that the eye is
+sensitive is not saying anything. Compared with it, all other organs are
+monstrously crude. The organ of smell which guides a dog on the trail of
+a deer, the organ of touch or force which guides an insect in its
+wanderings, the organ of hearing, which is affected by the slightest
+disturbances of the air, are sensitive organs, to be sure, but what are
+they compared with the human eye! No doubt it responds to the faintest
+echoes or reverberations of the medium; no doubt, it brings us tidings
+from other worlds, infinitely remote, but in a language we cannot as yet
+always understand. And why not? Because we live in a medium filled with
+air and other gases, vapors and a dense mass of solid particles flying
+about. These play an important part in many phenomena; they fritter away
+the energy of the vibrations before they can reach the eye; they too,
+are the carriers of germs of destruction, they get into our lungs and
+other organs, clog up the channels and imperceptibly, yet inevitably,
+arrest the stream of life. Could we but do away with all ponderable
+matter in the line of sight of the telescope, it would reveal to us
+undreamt of marvels. Even the unaided eye, I think, would be capable of
+distinguishing in the pure medium, small objects at distances measured
+probably by hundreds or perhaps thousands of miles.
+
+But there is something else about the eye which impresses us still more
+than these wonderful features which we observed, viewing it from the
+standpoint of a physicist, merely as an optical instrument,--something
+which appeals to us more than its marvelous faculty of being directly
+affected by the vibrations of the medium, without interference of gross
+matter, and more than its inconceivable sensitiveness and discerning
+power. It is its significance in the processes of life. No matter what
+one's views on nature and life may be, he must stand amazed when, for
+the first time in his thoughts, he realizes the importance of the eye in
+the physical processes and mental performances of the human organism.
+And how could it be otherwise, when he realizes, that the eye is the
+means through which the human race has acquired the entire knowledge it
+possesses, that it controls all our motions, more still, all our
+actions.
+
+There is no way of acquiring knowledge except through the eye. What is
+the foundation of all philosophical systems of ancient and modern times,
+in fact, of all the philosophy of man? _I am, I think; I think,
+therefore I am._ But how could I think and how would I know that I
+exist, if I had not the eye? For knowledge involves consciousness;
+consciousness involves ideas, conceptions; conceptions involve pictures
+or images, and images the sense of vision, and therefore the organ of
+sight. But how about blind men, will be asked? Yes, a blind man may
+depict in magnificent poems, forms and scenes from real life, from a
+world he physically does not see. A blind man may touch the keys of an
+instrument with unerring precision, may model the fastest boat, may
+discover and invent, calculate and construct, may do still greater
+wonders--but all the blind men who have done such things have descended
+from those who had seeing eyes. Nature may reach the same result in many
+ways. Like a wave in the physical world, in the infinite ocean of the
+medium which pervades all, so in the world of organisms, in life, an
+impulse started proceeds onward, at times, may be, with the speed of
+light, at times, again, so slowly that for ages and ages it seems to
+stay, passing through processes of a complexity inconceivable to men,
+but in all its forms, in all its stages, its energy ever and ever
+integrally present. A single ray of light from a distant star falling
+upon the eye of a tyrant in bygone times, may have altered the course of
+his life, may have changed the destiny of nations, may have transformed
+the surface of the globe, so intricate, so inconceivably complex are the
+processes in Nature. In no way can we get such an overwhelming idea of
+the grandeur of Nature, as when we consider, that in accordance with the
+law of the conservation of energy, throughout the infinite, the forces
+are in a perfect balance, and hence the energy of a single thought may
+determine the motion of a Universe. It is not necessary that every
+individual, not even that every generation or many generations, should
+have the physical instrument of sight, in order to be able to form
+images and to think, that is, form ideas or conceptions; but sometime or
+other, during the process of evolution, the eye certainly must have
+existed, else thought, as we understand it, would be impossible; else
+conceptions, like spirit, intellect, mind, call it as you may, could not
+exist. It is conceivable, that in some other world, in some other
+beings, the eye is replaced by a different organ, equally or more
+perfect, but these beings cannot be men.
+
+Now what prompts us all to voluntary motions and actions of any kind?
+Again the eye. If I am conscious of the motion, I must have an idea or
+conception, that is, an image, therefore the eye. If I am not precisely
+conscious of the motion, it is, because the images are vague or
+indistinct, being blurred by the superimposition of many. But when I
+perform the motion, does the impulse which prompts me to the action come
+from within or from without? The greatest physicists have not disdained
+to endeavor to answer this and similar questions and have at times
+abandoned themselves to the delights of pure and unrestrained thought.
+Such questions are generally considered not to belong to the realm of
+positive physical science, but will before long be annexed to its
+domain. Helmholtz has probably thought more on life than any modern
+scientist. Lord Kelvin expressed his belief that life's process is
+electrical and that there is a force inherent to the organism and
+determining its motions. Just as much as I am convinced of any physical
+truth I am convinced that the motive impulse must come from the outside.
+For, consider the lowest organism we know--and there are probably many
+lower ones--an aggregation of a few cells only. If it is capable of
+voluntary motion it can perform an infinite number of motions, all
+definite and precise. But now a mechanism consisting of a finite number
+of parts and few at that, cannot perform an infinite number of definite
+motions, hence the impulses which govern its movements must come from
+the environment. So, the atom, the ulterior element of the Universe's
+structure, is tossed about in space, eternally, a play to external
+influences, like a boat in a troubled sea. Were it to stop its motion
+_it would die_. Matter at rest, if such a thing could exist, would be
+matter dead. Death of matter! Never has a sentence of deeper
+philosophical meaning been uttered. This is the way in which Prof.
+Dewar forcibly expresses it in the description of his admirable
+experiments, in which liquid oxygen is handled as one handles water, and
+air at ordinary pressure is made to condense and even to solidify by the
+intense cold. Experiments, which serve to illustrate, in his language,
+the last feeble manifestations of life, the last quiverings of matter
+about to die. But human eyes shall not witness such death. There is no
+death of matter, for throughout the infinite universe, all has to move,
+to vibrate, that is, to live.
+
+I have made the preceding statements at the peril of treading upon
+metaphysical ground, in my desire to introduce the subject of this
+lecture in a manner not altogether uninteresting, I may hope, to an
+audience such as I have the honor to address. But now, then, returning
+to the subject, this divine organ of sight, this indispensable
+instrument for thought and all intellectual enjoyment, which lays open
+to us the marvels of this universe, through which we have acquired what
+knowledge we possess, and which prompts us to, and controls, all our
+physical and mental activity. By what is it affected? By light! What is
+light?
+
+We have witnessed the great strides which have been made in all
+departments of science in recent years. So great have been the advances
+that we cannot refrain from asking ourselves, Is this all true, or is it
+but a dream? Centuries ago men have lived, have thought, discovered,
+invented, and have believed that they were soaring, while they were
+merely proceeding at a snail's pace. So we too may be mistaken. But
+taking the truth of the observed events as one of the implied facts of
+science, we must rejoice in the immense progress already made and still
+more in the anticipation of what must come, judging from the
+possibilities opened up by modern research. There is, however, an
+advance which we have been witnessing, which must be particularly
+gratifying to every lover of progress. It is not a discovery, or an
+invention, or an achievement in any particular direction. It is an
+advance in all directions of scientific thought and experiment. I mean
+the generalization of the natural forces and phenomena, the looming up
+of a certain broad idea on the scientific horizon. It is this idea which
+has, however, long ago taken possession of the most advanced minds, to
+which I desire to call your attention, and which I intend to illustrate
+in a general way, in these experiments, as the first step in answering
+the question "What is light?" and to realize the modern meaning of this
+word.
+
+It is beyond the scope of my lecture to dwell upon the subject of light
+in general, my object being merely to bring presently to your notice a
+certain class of light effects and a number of phenomena observed in
+pursuing the study of these effects. But to be consistent in my remarks
+it is necessary to state that, according to that idea, now accepted by
+the majority of scientific men as a positive result of theoretical and
+experimental investigation, the various forms or manifestations of
+energy which were generally designated as "electric" or more precisely
+"electromagnetic" are energy manifestations of the same nature as those
+of radiant heat and light. Therefore the phenomena of light and heat and
+others besides these, may be called electrical phenomena. Thus
+electrical science has become the mother science of all and its study
+has become all important. The day when we shall know exactly what
+"electricity" is, will chronicle an event probably greater, more
+important than any other recorded in the history of the human race. The
+time will come when the comfort, the very existence, perhaps, of man
+will depend upon that wonderful agent. For our existence and comfort we
+require heat, light and mechanical power. How do we now get all these?
+We get them from fuel, we get them by consuming material. What will man
+do when the forests disappear, when the coal fields are exhausted? Only
+one thing, according to our present knowledge will remain; that is, to
+transmit power at great distances. Men will go to the waterfalls, to the
+tides, which are the stores of an infinitesimal part of Nature's
+immeasurable energy. There will they harness the energy and transmit the
+same to their settlements, to warm their homes by, to give them light,
+and to keep their obedient slaves, the machines, toiling. But how will
+they transmit this energy if not by electricity? Judge then, if the
+comfort, nay, the very existence, of man will not depend on electricity.
+I am aware that this view is not that of a practical engineer, but
+neither is it that of an illusionist, for it is certain, that power
+transmission, which at present is merely a stimulus to enterprise, will
+some day be a dire necessity.
+
+It is more important for the student, who takes up the study of light
+phenomena, to make himself thoroughly acquainted with certain modern
+views, than to peruse entire books on the subject of light itself, as
+disconnected from these views. Were I therefore to make these
+demonstrations before students seeking information--and for the sake of
+the few of those who may be present, give me leave to so assume--it
+would be my principal endeavor to impress these views upon their minds
+in this series of experiments.
+
+It might be sufficient for this purpose to perform a simple and
+well-known experiment. I might take a familiar appliance, a Leyden jar,
+charge it from a frictional machine, and then discharge it. In
+explaining to you its permanent state when charged, and its transitory
+condition when discharging, calling your attention to the forces which
+enter into play and to the various phenomena they produce, and pointing
+out the relation of the forces and phenomena, I might fully succeed in
+illustrating that modern idea. No doubt, to the thinker, this simple
+experiment would appeal as much as the most magnificent display. But
+this is to be an experimental demonstration, and one which should
+possess, besides instructive, also entertaining features and as such, a
+simple experiment, such as the one cited, would not go very far towards
+the attainment of the lecturer's aim. I must therefore choose another
+way of illustrating, more spectacular certainly, but perhaps also more
+instructive. Instead of the frictional machine and Leyden jar, I shall
+avail myself in these experiments, of an induction coil of peculiar
+properties, which was described in detail by me in a lecture before the
+London Institution of Electrical Engineers, in Feb., 1892. This
+induction coil is capable of yielding currents of enormous potential
+differences, alternating with extreme rapidity. With this apparatus I
+shall endeavor to show you three distinct classes of effects, or
+phenomena, and it is my desire that each experiment, while serving for
+the purposes of illustration, should at the same time teach us some
+novel truth, or show us some novel aspect of this fascinating science.
+But before doing this, it seems proper and useful to dwell upon the
+apparatus employed, and method of obtaining the high potentials and
+high-frequency currents which are made use of in these experiments.
+
+
+[Illustration: FIG. 165.]
+
+ON THE APPARATUS AND METHOD OF CONVERSION.
+
+These high-frequency currents are obtained in a peculiar manner. The
+method employed was advanced by me about two years ago in an
+experimental lecture before the American Institute of Electrical
+Engineers. A number of ways, as practiced in the laboratory, of
+obtaining these currents either from continuous or low frequency
+alternating currents, is diagramatically indicated in Fig. 165, which
+will be later described in detail. The general plan is to charge
+condensers, from a direct or alternate-current source, preferably of
+high-tension, and to discharge them disruptively while observing
+well-known conditions necessary to maintain the oscillations of the
+current. In view of the general interest taken in high-frequency
+currents and effects producible by them, it seems to me advisable to
+dwell at some length upon this method of conversion. In order to give
+you a clear idea of the action, I will suppose that a continuous-current
+generator is employed, which is often very convenient. It is desirable
+that the generator should possess such high tension as to be able to
+break through a small air space. If this is not the case, then auxiliary
+means have to be resorted to, some of which will be indicated
+subsequently. When the condensers are charged to a certain potential,
+the air, or insulating space, gives way and a disruptive discharge
+occurs. There is then a sudden rush of current and generally a large
+portion of accumulated electrical energy spends itself. The condensers
+are thereupon quickly charged and the same process is repeated in more
+or less rapid succession. To produce such sudden rushes of current it is
+necessary to observe certain conditions. If the rate at which the
+condensers are discharged is the same as that at which they are charged,
+then, clearly, in the assumed case the condensers do not come into play.
+If the rate of discharge be smaller than the rate of charging, then,
+again, the condensers cannot play an important part. But if, on the
+contrary, the rate of discharging is greater than that of charging, then
+a succession of rushes of current is obtained. It is evident that, if
+the rate at which the energy is dissipated by the discharge is very much
+greater than the rate of supply to the condensers, the sudden rushes
+will be comparatively few, with long-time intervals between. This always
+occurs when a condenser of considerable capacity is charged by means of
+a comparatively small machine. If the rates of supply and dissipation
+are not widely different, then the rushes of current will be in quicker
+succession, and this the more, the more nearly equal both the rates are,
+until limitations incident to each case and depending upon a number of
+causes are reached. Thus we are able to obtain from a continuous-current
+generator as rapid a succession of discharges as we like. Of course, the
+higher the tension of the generator, the smaller need be the capacity of
+the condensers, and for this reason, principally, it is of advantage to
+employ a generator of very high tension. Besides, such a generator
+permits the attaining of greater rates of vibration.
+
+The rushes of current may be of the same direction under the conditions
+before assumed, but most generally there is an oscillation superimposed
+upon the fundamental vibration of the current. When the conditions are
+so determined that there are no oscillations, the current impulses are
+unidirectional and thus a means is provided of transforming a continuous
+current of high tension, into a direct current of lower tension, which I
+think may find employment in the arts.
+
+This method of conversion is exceedingly interesting and I was much
+impressed by its beauty when I first conceived it. It is ideal in
+certain respects. It involves the employment of no mechanical devices of
+any kind, and it allows of obtaining currents of any desired frequency
+from an ordinary circuit, direct or alternating. The frequency of the
+fundamental discharges depending on the relative rates of supply and
+dissipation can be readily varied within wide limits, by simple
+adjustments of these quantities, and the frequency of the superimposed
+vibration by the determination of the capacity, self-induction and
+resistance of the circuit. The potential of the currents, again, may be
+raised as high as any insulation is capable of withstanding safely by
+combining capacity and self-induction or by induction in a secondary,
+which need have but comparatively few turns.
+
+As the conditions are often such that the intermittence or oscillation
+does not readily establish itself, especially when a direct current
+source is employed, it is of advantage to associate an interrupter with
+the arc, as I have, some time ago, indicated the use of an air-blast or
+magnet, or other such device readily at hand. The magnet is employed
+with special advantage in the conversion of direct currents, as it is
+then very effective. If the primary source is an alternate current
+generator, it is desirable, as I have stated on another occasion, that
+the frequency should be low, and that the current forming the arc be
+large, in order to render the magnet more effective.
+
+A form of such discharger with a magnet which has been found convenient,
+and adopted after some trials, in the conversion of direct currents
+particularly, is illustrated in Fig. 166. N S are the pole pieces of a
+very strong magnet which is excited by a coil C. The pole pieces are
+slotted for adjustment and can be fastened in any position by screws s
+s_{1}. The discharge rods d d_{1}, thinned down on the ends in order
+to allow a closer approach of the magnetic pole pieces, pass through the
+columns of brass b b_{1} and are fastened in position by screws s_{2}
+s_{2}. Springs r r_{1} and collars c c_{1} are slipped on the
+rods, the latter serving to set the points of the rods at a certain
+suitable distance by means of screws s_{3} s_{3}, and the former to
+draw the points apart. When it is desired to start the arc, one of the
+large rubber handles h h_{1} is tapped quickly with the hand, whereby
+the points of the rods are brought in contact but are instantly
+separated by the springs r r_{1}. Such an arrangement has been found
+to be often necessary, namely in cases when the E. M. F. was not large
+enough to cause the discharge to break through the gap, and also when it
+was desirable to avoid short circuiting of the generator by the metallic
+contact of the rods. The rapidity of the interruptions of the current
+with a magnet depends on the intensity of the magnetic field and on the
+potential difference at the end of the arc. The interruptions are
+generally in such quick succession as to produce a musical sound. Years
+ago it was observed that when a powerful induction coil is discharged
+between the poles of a strong magnet, the discharge produces a loud
+noise, not unlike a small pistol shot. It was vaguely stated that the
+spark was intensified by the presence of the magnetic field. It is now
+clear that the discharge current, flowing for some time, was interrupted
+a great number of times by the magnet, thus producing the sound. The
+phenomenon is especially marked when the field circuit of a large magnet
+or dynamo is broken in a powerful magnetic field.
+
+[Illustration: FIG. 166.]
+
+When the current through the gap is comparatively large, it is of
+advantage to slip on the points of the discharge rods pieces of very
+hard carbon and let the arc play between the carbon pieces. This
+preserves the rods, and besides has the advantage of keeping the air
+space hotter, as the heat is not conducted away as quickly through the
+carbons, and the result is that a smaller E. M. F. in the arc gap is
+required to maintain a succession of discharges.
+
+[Illustration: FIG. 167.]
+
+Another form of discharger, which may be employed with advantage in
+some cases, is illustrated in Fig. 167. In this form the discharge rods
+d d_{1} pass through perforations in a wooden box B, which is thickly
+coated with mica on the inside, as indicated by the heavy lines. The
+perforations are provided with mica tubes m m_{1} of some thickness,
+which are preferably not in contact with the rods d d_{1}. The box has
+a cover C which is a little larger and descends on the outside of the
+box. The spark gap is warmed by a small lamp _l_ contained in the box. A
+plate _p_ above the lamp allows the draught to pass only through the
+chimney _e_ of the lamp, the air entering through holes _o o_ in or near
+the bottom of the box and following the path indicated by the arrows.
+When the discharger is in operation, the door of the box is closed so
+that the light of the arc is not visible outside. It is desirable to
+exclude the light as perfectly as possible, as it interferes with some
+experiments. This form of discharger is simple and very effective when
+properly manipulated. The air being warmed to a certain temperature, has
+its insulating power impaired; it becomes dielectrically weak, as it
+were, and the consequence is that the arc can be established at much
+greater distance. The arc should, of course, be sufficiently insulating
+to allow the discharge to pass through the gap _disruptively_. The arc
+formed under such conditions, when long, may be made extremely
+sensitive, and the weak draught through the lamp chimney _c_ is quite
+sufficient to produce rapid interruptions. The adjustment is made by
+regulating the temperature and velocity of the draught. Instead of using
+the lamp, it answers the purpose to provide for a draught of warm air in
+other ways. A very simple way which has been practiced is to enclose the
+arc in a long vertical tube, with plates on the top and bottom for
+regulating the temperature and velocity of the air current. Some
+provision had to be made for deadening the sound.
+
+The air may be rendered dielectrically weak also by rarefaction.
+Dischargers of this kind have likewise been used by me in connection
+with a magnet. A large tube is for this purpose provided with heavy
+electrodes of carbon or metal, between which the discharge is made to
+pass, the tube being placed in a powerful magnetic field. The exhaustion
+of the tube is carried to a point at which the discharge breaks through
+easily, but the pressure should be more than 75 millimetres, at which
+the ordinary thread discharge occurs. In another form of discharger,
+combining the features before mentioned, the discharge was made to pass
+between two adjustable magnetic pole pieces, the space between them
+being kept at an elevated temperature.
+
+It should be remarked here that when such, or interrupting devices of
+any kind, are used and the currents are passed through the primary of a
+disruptive discharge coil, it is not, as a rule, of advantage to produce
+a number of interruptions of the current per second greater than the
+natural frequency of vibration of the dynamo supply circuit, which is
+ordinarily small. It should also be pointed out here, that while the
+devices mentioned in connection with the disruptive discharge are
+advantageous under certain conditions, they may be sometimes a source of
+trouble, as they produce intermittences and other irregularities in the
+vibration which it would be very desirable to overcome.
+
+There is, I regret to say, in this beautiful method of conversion a
+defect, which fortunately is not vital, and which I have been gradually
+overcoming. I will best call attention to this defect and indicate a
+fruitful line of work, by comparing the electrical process with its
+mechanical analogue. The process may be illustrated in this manner.
+Imagine a tank with a wide opening at the bottom, which is kept closed
+by spring pressure, but so that it snaps off _suddenly_ when the liquid
+in the tank has reached a certain height. Let the fluid be supplied to
+the tank by means of a pipe feeding at a certain rate. When the critical
+height of the liquid is reached, the spring gives way and the bottom of
+the tank drops out. Instantly the liquid falls through the wide opening,
+and the spring, reasserting itself, closes the bottom again. The tank is
+now filled, and after a certain time interval the same process is
+repeated. It is clear, that if the pipe feeds the fluid quicker than the
+bottom outlet is capable of letting it pass through, the bottom will
+remain off and the tank will still overflow. If the rates of supply are
+exactly equal, then the bottom lid will remain partially open and no
+vibration of the same and of the liquid column will generally occur,
+though it might, if started by some means. But if the inlet pipe does
+not feed the fluid fast enough for the outlet, then there will be always
+vibration. Again, in such case, each time the bottom flaps up or down,
+the spring and the liquid column, if the pliability of the spring and
+the inertia of the moving parts are properly chosen, will perform
+independent vibrations. In this analogue the fluid may be likened to
+electricity or electrical energy, the tank to the condenser, the spring
+to the dielectric, and the pipe to the conductor through which
+electricity is supplied to the condenser. To make this analogy quite
+complete it is necessary to make the assumption, that the bottom, each
+time it gives way, is knocked violently against a non-elastic stop, this
+impact involving some loss of energy; and that, besides, some
+dissipation of energy results due to frictional losses. In the preceding
+analogue the liquid is supposed to be under a steady pressure. If the
+presence of the fluid be assumed to vary rhythmically, this may be taken
+as corresponding to the case of an alternating current. The process is
+then not quite as simple to consider, but the action is the same in
+principle.
+
+It is desirable, in order to maintain the vibration economically, to
+reduce the impact and frictional losses as much as possible. As regards
+the latter, which in the electrical analogue correspond to the losses
+due to the resistance of the circuits, it is impossible to obviate them
+entirely, but they can be reduced to a minimum by a proper selection of
+the dimensions of the circuits and by the employment of thin conductors
+in the form of strands. But the loss of energy caused by the first
+breaking through of the dielectric--which in the above example
+corresponds to the violent knock of the bottom against the inelastic
+stop--would be more important to overcome. At the moment of the breaking
+through, the air space has a very high resistance, which is probably
+reduced to a very small value when the current has reached some
+strength, and the space is brought to a high temperature. It would
+materially diminish the loss of energy if the space were always kept at
+an extremely high temperature, but then there would be no disruptive
+break. By warming the space moderately by means of a lamp or otherwise,
+the economy as far as the arc is concerned is sensibly increased. But
+the magnet or other interrupting device does not diminish the loss in
+the arc. Likewise, a jet of air only facilitates the carrying off of the
+energy. Air, or a gas in general, behaves curiously in this respect.
+When two bodies charged to a very high potential, discharge disruptively
+through an air space, any amount of energy may be carried off by the
+air. This energy is evidently dissipated by bodily carriers, in impact
+and collisional losses of the molecules. The exchange of the molecules
+in the space occurs with inconceivable rapidity. A powerful discharge
+taking place between two electrodes, they may remain entirely cool, and
+yet the loss in the air may represent any amount of energy. It is
+perfectly practicable, with very great potential differences in the gap,
+to dissipate several horse-power in the arc of the discharge without
+even noticing a small increase in the temperature of the electrodes. All
+the frictional losses occur then practically in the air. If the exchange
+of the air molecules is prevented, as by enclosing the air hermetically,
+the gas inside of the vessel is brought quickly to a high temperature,
+even with a very small discharge. It is difficult to estimate how much
+of the energy is lost in sound waves, audible or not, in a powerful
+discharge. When the currents through the gap are large, the electrodes
+may become rapidly heated, but this is not a reliable measure of the
+energy wasted in the arc, as the loss through the gap itself may be
+comparatively small. The air or a gas in general is, at ordinary
+pressure at least, clearly not the best medium through which a
+disruptive discharge should occur. Air or other gas under great pressure
+is of course a much more suitable medium for the discharge gap. I have
+carried on long-continued experiments in this direction, unfortunately
+less practicable on account of the difficulties and expense in getting
+air under great pressure. But even if the medium in the discharge space
+is solid or liquid, still the same losses take place, though they are
+generally smaller, for just as soon as the arc is established, the solid
+or liquid is volatilized. Indeed, there is no body known which would not
+be disintegrated by the arc, and it is an open question among scientific
+men, whether an arc discharge could occur at all in the air itself
+without the particles of the electrodes being torn off. When the current
+through the gap is very small and the arc very long, I believe that a
+relatively considerable amount of heat is taken up in the disintegration
+of the electrodes, which partially on this account may remain quite
+cold.
+
+The ideal medium for a discharge gap should only _crack_, and the ideal
+electrode should be of some material which cannot be disintegrated. With
+small currents through the gap it is best to employ aluminum, but not
+when the currents are large. The disruptive break in the air, or more or
+less in any ordinary medium, is not of the nature of a crack, but it is
+rather comparable to the piercing of innumerable bullets through a mass
+offering great frictional resistances to the motion of the bullets, this
+involving considerable loss of energy. A medium which would merely crack
+when strained electrostatically--and this possibly might be the case
+with a perfect vacuum, that is, pure ether--would involve a very small
+loss in the gap, so small as to be entirely negligible, at least
+theoretically, because a crack may be produced by an infinitely small
+displacement. In exhausting an oblong bulb provided with two aluminum
+terminals, with the greatest care, I have succeeded in producing such a
+vacuum that the secondary discharge of a disruptive discharge coil would
+break disruptively through the bulb in the form of fine spark streams.
+The curious point was that the discharge would completely ignore the
+terminals and start far behind the two aluminum plates which served as
+electrodes. This extraordinary high vacuum could only be maintained for
+a very short while. To return to the ideal medium, think, for the sake
+of illustration, of a piece of glass or similar body clamped in a vice,
+and the latter tightened more and more. At a certain point a minute
+increase of the pressure will cause the glass to crack. The loss of
+energy involved in splitting the glass may be practically nothing, for
+though the force is great, the displacement need be but extremely small.
+Now imagine that the glass would possess the property of closing again
+perfectly the crack upon a minute diminution of the pressure. This is
+the way the dielectric in the discharge space should behave. But
+inasmuch as there would be always some loss in the gap, the medium,
+which should be continuous, should exchange through the gap at a rapid
+rate. In the preceding example, the glass being perfectly closed, it
+would mean that the dielectric in the discharge space possesses a great
+insulating power; the glass being cracked, it would signify that the
+medium in the space is a good conductor. The dielectric should vary
+enormously in resistance by minute variations of the E. M. F. across the
+discharge space. This condition is attained, but in an extremely
+imperfect manner, by warming the air space to a certain critical
+temperature, dependent on the E. M. F. across the gap, or by otherwise
+impairing the insulating power of the air. But as a matter of fact the
+air does never break down _disruptively_, if this term be rigorously
+interpreted, for before the sudden rush of the current occurs, there is
+always a weak current preceding it, which rises first gradually and then
+with comparative suddenness. That is the reason why the rate of change
+is very much greater when glass, for instance, is broken through, than
+when the break takes place through an air space of equivalent dielectric
+strength. As a medium for the discharge space, a solid, or even a
+liquid, would be preferable therefor. It is somewhat difficult to
+conceive of a solid body which would possess the property of closing
+instantly after it has been cracked. But a liquid, especially under
+great pressure, behaves practically like a solid, while it possesses the
+property of closing the crack. Hence it was thought that a liquid
+insulator might be more suitable as a dielectric than air. Following out
+this idea, a number of different forms of dischargers in which a variety
+of such insulators, sometimes under great pressure, were employed, have
+been experimented upon. It is thought sufficient to dwell in a few words
+upon one of the forms experimented upon. One of these dischargers is
+illustrated in Figs. 168_a_ and 168_b_.
+
+[Illustration: FIG. 168a.]
+
+[Illustration: FIG. 168b.]
+
+A hollow metal pulley P (Fig. 168_a_), was fastened upon an arbor _a_,
+which by suitable means was rotated at a considerable speed. On the
+inside of the pulley, but disconnected from the same, was supported a
+thin disc _h_ (which is shown thick for the sake of clearness), of hard
+rubber in which there were embedded two metal segments _s s_ with
+metallic extensions _e e_ into which were screwed conducting terminals
+_t t_ covered with thick tubes of hard rubber _t t_. The rubber disc _h_
+with its metallic segments _s s_, was finished in a lathe, and its
+entire surface highly polished so as to offer the smallest possible
+frictional resistance to the motion through a fluid. In the hollow of
+the pulley an insulating liquid such as a thin oil was poured so as to
+reach very nearly to the opening left in the flange _f_, which was
+screwed tightly on the front side of the pulley. The terminals _t t_,
+were connected to the opposite coatings of a battery of condensers so
+that the discharge occurred through the liquid. When the pulley was
+rotated, the liquid was forced against the rim of the pulley and
+considerable fluid pressure resulted. In this simple way the discharge
+gap was filled with a medium which behaved practically like a solid,
+which possessed the quality of closing instantly upon the occurrence of
+the break, and which moreover was circulating through the gap at a rapid
+rate. Very powerful effects were produced by discharges of this kind
+with liquid interrupters, of which a number of different forms were
+made. It was found that, as expected, a longer spark for a given length
+of wire was obtainable in this way than by using air as an interrupting
+device. Generally the speed, and therefore also the fluid pressure, was
+limited by reason of the fluid friction, in the form of discharger
+described, but the practically obtainable speed was more than sufficient
+to produce a number of breaks suitable for the circuits ordinarily used.
+In such instances the metal pulley P was provided with a few projections
+inwardly, and a definite number of breaks was then produced which could
+be computed from the speed of rotation of the pulley. Experiments were
+also carried on with liquids of different insulating power with the view
+of reducing the loss in the arc. When an insulating liquid is moderately
+warmed, the loss in the arc is diminished.
+
+A point of some importance was noted in experiments with various
+discharges of this kind. It was found, for instance, that whereas the
+conditions maintained in these forms were favorable for the production
+of a great spark length, the current so obtained was not best suited to
+the production of light effects. Experience undoubtedly has shown, that
+for such purposes a harmonic rise and fall of the potential is
+preferable. Be it that a solid is rendered incandescent, or
+phosphorescent, or be it that energy is transmitted by condenser coating
+through the glass, it is quite certain that a harmonically rising and
+falling potential produces less destructive action, and that the vacuum
+is more permanently maintained. This would be easily explained if it
+were ascertained that the process going on in an exhausted vessel is of
+an electrolytic nature.
+
+In the diagrammatical sketch, Fig. 165, which has been already referred
+to, the cases which are most likely to be met with in practice are
+illustrated. One has at his disposal either direct or alternating
+currents from a supply station. It is convenient for an experimenter in
+an isolated laboratory to employ a machine G, such as illustrated,
+capable of giving both kinds of currents. In such case it is also
+preferable to use a machine with multiple circuits, as in many
+experiments it is useful and convenient to have at one's disposal
+currents of different phases. In the sketch, D represents the direct and
+A the alternating circuit. In each of these, three branch circuits are
+shown, all of which are provided with double line switches _s s s s s
+s_. Consider first the direct current conversion; I_a_ represents the
+simplest case. If the E. M. F. of the generator is sufficient to break
+through a small air space, at least when the latter is warmed or
+otherwise rendered poorly insulating, there is no difficulty in
+maintaining a vibration with fair economy by judicious adjustment of the
+capacity, self-induction and resistance of the circuit L containing the
+devices _l l m_. The magnet N, S, can be in this case advantageously
+combined with the air space. The discharger _d d_ with the magnet may be
+placed either way, as indicated by the full or by the dotted lines. The
+circuit I_a_ with the connections and devices is supposed to possess
+dimensions such as are suitable for the maintenance of a vibration. But
+usually the E. M. F. on the circuit or branch I_a_ will be something
+like a 100 volts or so, and in this case it is not sufficient to break
+through the gap. Many different means may be used to remedy this by
+raising the E. M. F. across the gap. The simplest is probably to insert
+a large self-induction coil in series with the circuit L. When the arc
+is established, as by the discharger illustrated in Fig. 166, the magnet
+blows the arc out the instant it is formed. Now the extra current of the
+break, being of high E. M. F., breaks through the gap, and a path of low
+resistance for the dynamo current being again provided, there is a
+sudden rush of current from the dynamo upon the weakening or subsidence
+of the extra current. This process is repeated in rapid succession, and
+in this manner I have maintained oscillation with as low as 50 volts, or
+even less, across the gap. But conversion on this plan is not to be
+recommended on account of the too heavy currents through the gap and
+consequent heating of the electrodes; besides, the frequencies obtained
+in this way are low, owing to the high self-induction necessarily
+associated with the circuit. It is very desirable to have the E. M. F.
+as high as possible, first, in order to increase the economy of the
+conversion, and, secondly, to obtain high frequencies. The difference of
+potential in this electric oscillation is, of course, the equivalent of
+the stretching force in the mechanical vibration of the spring. To
+obtain very rapid vibration in a circuit of some inertia, a great
+stretching force or difference of potential is necessary. Incidentally,
+when the E. M. F. is very great, the condenser which is usually employed
+in connection with the circuit need but have a small capacity, and many
+other advantages are gained. With a view of raising the E. M. F. to a
+many times greater value than obtainable from ordinary distribution
+circuits, a rotating transformer _g_ is used, as indicated at II_a_,
+Fig. 165, or else a separate high potential machine is driven by means
+of a motor operated from the generator G. The latter plan is in fact
+preferable, as changes are easier made. The connections from the high
+tension winding are quite similar to those in branch I_a_ with the
+exception that a condenser C, which should be adjustable, is connected
+to the high tension circuit. Usually, also, an adjustable self-induction
+coil in series with the circuit has been employed in these experiments.
+When the tension of the currents is very high, the magnet ordinarily
+used in connection with the discharger is of comparatively small value,
+as it is quite easy to adjust the dimensions of the circuit so that
+oscillation is maintained. The employment of a steady E. M. F. in the
+high frequency conversion affords some advantages over the employment of
+alternating E. M. F., as the adjustments are much simpler and the action
+can be easier controlled. But unfortunately one is limited by the
+obtainable potential difference. The winding also breaks down easily in
+consequence of the sparks which form between the sections of the
+armature or commutator when a vigorous oscillation takes place. Besides,
+these transformers are expensive to build. It has been found by
+experience that it is best to follow the plan illustrated at III_a_. In
+this arrangement a rotating transformer _g_, is employed to convert the
+low tension direct currents into low frequency alternating currents,
+preferably also of small tension. The tension of the currents is then
+raised in a stationary transformer T. The secondary S of this
+transformer is connected to an adjustable condenser C which discharges
+through the gap or discharger _d d_, placed in either of the ways
+indicated, through the primary P of a disruptive discharge coil, the
+high frequency current being obtained from the secondary S of this coil,
+as described on previous occasions. This will undoubtedly be found the
+cheapest and most convenient way of converting direct currents.
+
+The three branches of the circuit A represent the usual cases met in
+practice when alternating currents are converted. In Fig. 1_b_ a
+condenser C, generally of large capacity, is connected to the circuit L
+containing the devices _l l_, _m m_. The devices _m m_ are supposed to
+be of high self-induction so as to bring the frequency of the circuit
+more or less to that of the dynamo. In this instance the discharger _d
+d_ should best have a number of makes and breaks per second equal to
+twice the frequency of the dynamo. If not so, then it should have at
+least a number equal to a multiple or even fraction of the dynamo
+frequency. It should be observed, referring to I_b_, that the conversion
+to a high potential is also effected when the discharger _d d_, which is
+shown in the sketch, is omitted. But the effects which are produced by
+currents which rise instantly to high values, as in a disruptive
+discharge, are entirely different from those produced by dynamo currents
+which rise and fall harmonically. So, for instance, there might be in a
+given case a number of makes and breaks at _d d_ equal to just twice the
+frequency of the dynamo, or in other words, there may be the same number
+of fundamental oscillations as would be produced without the discharge
+gap, and there might even not be any quicker superimposed vibration; yet
+the differences of potential at the various points of the circuit, the
+impedance and other phenomena, dependent upon the rate of change, will
+bear no similarity in the two cases. Thus, when working with currents
+discharging disruptively, the element chiefly to be considered is not
+the frequency, as a student might be apt to believe, but the rate of
+change per unit of time. With low frequencies in a certain measure the
+same effects may be obtained as with high frequencies, provided the rate
+of change is sufficiently great. So if a low frequency current is raised
+to a potential of, say, 75,000 volts, and the high tension current
+passed through a series of high resistance lamp filaments, the
+importance of the rarefied gas surrounding the filament is clearly
+noted, as will be seen later; or, if a low frequency current of several
+thousand amperes is passed through a metal bar, striking phenomena of
+impedance are observed, just as with currents of high frequencies. But
+it is, of course, evident that with low frequency currents it is
+impossible to obtain such rates of change per unit of time as with high
+frequencies, hence the effects produced by the latter are much more
+prominent. It is deemed advisable to make the preceding remarks,
+inasmuch as many more recently described effects have been unwittingly
+identified with high frequencies. Frequency alone in reality does not
+mean anything, except when an undisturbed harmonic oscillation is
+considered.
+
+In the branch III_b_ a similar disposition to that in I_b_ is
+illustrated, with the difference that the currents discharging through
+the gap _d d_ are used to induce currents in the secondary S of a
+transformer T. In such case the secondary should be provided with an
+adjustable condenser for the purpose of tuning it to the primary.
+
+II_b_ illustrates a plan of alternate current high frequency conversion
+which is most frequently used and which is found to be most convenient.
+This plan has been dwelt upon in detail on previous occasions and need
+not be described here.
+
+Some of these results were obtained by the use of a high frequency
+alternator. A description of such machines will be found in my original
+paper before the American Institute of Electrical Engineers, and in
+periodicals of that period, notably in THE ELECTRICAL ENGINEER of March
+18, 1891.
+
+I will now proceed with the experiments.
+
+
+ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.
+
+The first class of effects I intend to show you are effects produced by
+electrostatic force. It is the force which governs the the motion of the
+atoms, which causes them to collide and develop the life-sustaining
+energy of heat and light, and which causes them to aggregate in an
+infinite variety of ways, according to Nature's fanciful designs, and to
+form all these wondrous structures we perceive around us; it is, in
+fact, if our present views be true, the most important force for us to
+consider in Nature. As the term _electrostatic_ might imply a steady
+electric condition, it should be remarked, that in these experiments the
+force is not constant, but varies at a rate which may be considered
+moderate, about one million times a second, or thereabouts. This enables
+me to produce many effects which are not producible with an unvarying
+force.
+
+When two conducting bodies are insulated and electrified, we say that an
+electrostatic force is acting between them. This force manifests itself
+in attractions, repulsions and stresses in the bodies and space or
+medium without. So great may be the strain exerted in the air, or
+whatever separates the two conducting bodies, that it may break down,
+and we observe sparks or bundles of light or streamers, as they are
+called. These streamers form abundantly when the force through the air
+is rapidly varying. I will illustrate this action of electrostatic force
+in a novel experiment in which I will employ the induction coil before
+referred to. The coil is contained in a trough filled with oil, and
+placed under the table. The two ends of the secondary wire pass through
+the two thick columns of hard rubber which protrude to some height above
+the table. It is necessary to insulate the ends or terminals of the
+secondary heavily with hard rubber, because even dry wood is by far too
+poor an insulator for these currents of enormous potential differences.
+On one of the terminals of the coil, I have placed a large sphere of
+sheet brass, which is connected to a larger insulated brass plate, in
+order to enable me to perform the experiments under conditions, which,
+as you will see, are more suitable for this experiment. I now set the
+coil to work and approach the free terminal with a metallic object held
+in my hand, this simply to avoid burns. As I approach the metallic
+object to a distance of eight or ten inches, a torrent of furious sparks
+breaks forth from the end of the secondary wire, which passes through
+the rubber column. The sparks cease when the metal in my hand touches
+the wire. My arm is now traversed by a powerful electric current,
+vibrating at about the rate of one million times a second. All around me
+the electrostatic force makes itself felt, and the air molecules and
+particles of dust flying about are acted upon and are hammering
+violently against my body. So great is this agitation of the particles,
+that when the lights are turned out you may see streams of feeble light
+appear on some parts of my body. When such a streamer breaks out on any
+part of the body, it produces a sensation like the pricking of a needle.
+Were the potentials sufficiently high and the frequency of the vibration
+rather low, the skin would probably be ruptured under the tremendous
+strain, and the blood would rush out with great force in the form of
+fine spray or jet so thin as to be invisible, just as oil will when
+placed on the positive terminal of a Holtz machine. The breaking through
+of the skin though it may seem impossible at first, would perhaps occur,
+by reason of the tissues under the skin being incomparably better
+conducting. This, at least, appears plausible, judging from some
+observations.
+
+[Illustration: FIG. 169.]
+
+I can make these streams of light visible to all, by touching with the
+metallic object one of the terminals as before, and approaching my free
+hand to the brass sphere, which is connected to the second terminal of
+the coil. As the hand is approached, the air between it and the sphere,
+or in the immediate neighborhood, is more violently agitated, and you
+see streams of light now break forth from my finger tips and from the
+whole hand (Fig. 169). Were I to approach the hand closer, powerful
+sparks would jump from the brass sphere to my hand, which might be
+injurious. The streamers offer no particular inconvenience, except that
+in the ends of the finger tips a burning sensation is felt. They should
+not be confounded with those produced by an influence machine, because
+in many respects they behave differently. I have attached the brass
+sphere and plate to one of the terminals in order to prevent the
+formation of visible streamers on that terminal, also in order to
+prevent sparks from jumping at a considerable distance. Besides, the
+attachment is favorable for the working of the coil.
+
+The streams of light which you have observed issuing from my hand are
+due to a potential of about 200,000 volts, alternating in rather
+irregular intervals, sometimes like a million times a second. A
+vibration of the same amplitude, but four times as fast, to maintain
+which over 3,000,000 volts would be required, would be more than
+sufficient to envelop my body in a complete sheet of flame. But this
+flame would not burn me up; quite contrarily, the probability is that I
+would not be injured in the least. Yet a hundredth part of that energy,
+otherwise directed, would be amply sufficient to kill a person.
+
+The amount of energy which may thus be passed into the body of a person
+depends on the frequency and potential of the currents, and by making
+both of these very great, a vast amount of energy may be passed into the
+body without causing any discomfort, except perhaps, in the arm, which
+is traversed by a true conduction current. The reason why no pain in the
+body is felt, and no injurious effect noted, is that everywhere, if a
+current be imagined to flow through the body, the direction of its flow
+would be at right angles to the surface; hence the body of the
+experimenter offers an enormous section to the current, and the density
+is very small, with the exception of the arm, perhaps, where the density
+may be considerable. But if only a small fraction of that energy would
+be applied in such a way that a current would traverse the body in the
+same manner as a low frequency current, a shock would be received which
+might be fatal. A direct or low frequency alternating current is fatal,
+I think, principally because its distribution through the body is not
+uniform, as it must divide itself in minute streamlets of great density,
+whereby some organs are vitally injured. That such a process occurs I
+have not the least doubt, though no evidence might apparently exist, or
+be found upon examination. The surest to injure and destroy life, is a
+continuous current, but the most painful is an alternating current of
+very low frequency. The expression of these views, which are the result
+of long continued experiment and observation, both with steady and
+varying currents, is elicited by the interest which is at present taken
+in this subject, and by the manifestly erroneous ideas which are daily
+propounded in journals on this subject.
+
+I may illustrate an effect of the electrostatic force by another
+striking experiment, but before, I must call your attention to one or
+two facts. I have said before, that when the medium between two
+oppositely electrified bodies is strained beyond a certain limit it
+gives way and, stated in popular language, the opposite electric charges
+unite and neutralize each other. This breaking down of the medium occurs
+principally when the force acting between the bodies is steady, or
+varies at a moderate rate. Were the variation sufficiently rapid, such a
+destructive break would not occur, no matter how great the force, for
+all the energy would be spent in radiation, convection and mechanical
+and chemical action. Thus the _spark_ length, or greatest distance which
+a _spark_ will jump between the electrified bodies is the smaller, the
+greater the variation or time rate of change. But this rule may be taken
+to be true only in a general way, when comparing rates which are widely
+different.
+
+[Illustration: FIG. 170a.]
+
+[Illustration: FIG. 170b.]
+
+I will show you by an experiment the difference in the effect produced
+by a rapidly varying and a steady or moderately varying force. I have
+here two large circular brass plates _p p_ (Fig. 170_a_ and Fig.
+170_b_), supported on movable insulating stands on the table, connected
+to the ends of the secondary of a coil similar to the one used before. I
+place the plates ten or twelve inches apart and set the coil to work.
+You see the whole space between the plates, nearly two cubic feet,
+filled with uniform light, Fig. 170_a_. This light is due to the
+streamers you have seen in the first experiment, which are now much more
+intense. I have already pointed out the importance of these streamers in
+commercial apparatus and their still greater importance in some purely
+scientific investigations. Often they are too weak to be visible, but
+they always exist, consuming energy and modifying the action of the
+apparatus. When intense, as they are at present, they produce ozone in
+great quantity, and also, as Professor Crookes has pointed out, nitrous
+acid. So quick is the chemical action that if a coil, such as this one,
+is worked for a very long time it will make the atmosphere of a small
+room unbearable, for the eyes and throat are attacked. But when
+moderately produced, the streamers refresh the atmosphere wonderfully,
+like a thunder-storm, and exercises unquestionably a beneficial effect.
+
+In this experiment the force acting between the plates changes in
+intensity and direction at a very rapid rate. I will now make the rate
+of change per unit time much smaller. This I effect by rendering the
+discharges through the primary of the induction coil less frequent, and
+also by diminishing the rapidity of the vibration in the secondary. The
+former result is conveniently secured by lowering the E. M. F. over the
+air gap in the primary circuit, the latter by approaching the two brass
+plates to a distance of about three or four inches. When the coil is set
+to work, you see no streamers or light between the plates, yet the
+medium between them is under a tremendous strain. I still further
+augment the strain by raising the E. M. F. in the primary circuit, and
+soon you see the air give way and the hall is illuminated by a shower of
+brilliant and noisy sparks, Fig. 170_b_. These sparks could be produced
+also with unvarying force; they have been for many years a familiar
+phenomenon, though they were usually obtained from an entirely different
+apparatus. In describing these two phenomena so radically different in
+appearance, I have advisedly spoken of a "force" acting between the
+plates. It would be in accordance with accepted views to say, that there
+was an "alternating E. M. F," acting between the plates. This term is
+quite proper and applicable in all cases where there is evidence of at
+least a possibility of an essential inter-dependence of the electric
+state of the plates, or electric action in their neighborhood. But if
+the plates were removed to an infinite distance, or if at a finite
+distance, there is no probability or necessity whatever for such
+dependence. I prefer to use the term "electrostatic force," and to say
+that such a force is acting around each plate or electrified insulated
+body in general. There is an inconvenience in using this expression as
+the term incidentally means a steady electric condition; but a proper
+nomenclature will eventually settle this difficulty.
+
+I now return to the experiment to which I have already alluded, and with
+which I desire to illustrate a striking effect produced by a rapidly
+varying electrostatic force. I attach to the end of the wire, _l_ (Fig.
+171), which is in connection with one of the terminals of the secondary
+of the induction coil, an exhausted bulb _b_. This bulb contains a thin
+carbon filament _f_, which is fastened to a platinum wire _w_, sealed in
+the glass and leading outside of the bulb, where it connects to the wire
+_l_. The bulb may be exhausted to any degree attainable with ordinary
+apparatus. Just a moment before, you have witnessed the breaking down of
+the air between the charged brass plates. You know that a plate of
+glass, or any other insulating material, would break down in like
+manner. Had I therefore a metallic coating attached to the outside of
+the bulb, or placed near the same, and were this coating connected to
+the other terminal of the coil, you would be prepared to see the glass
+give way if the strain were sufficiently increased. Even were the
+coating not connected to the other terminal, but to an insulated plate,
+still, if you have followed recent developments, you would naturally
+expect a rupture of the glass.
+
+[Illustration: FIG. 171.]
+
+[Illustration: FIG. 172a.]
+
+[Illustration: FIG. 172b.]
+
+But it will certainly surprise you to note that under the action of the
+varying electrostatic force, the glass gives way when all other bodies
+are removed from the bulb. In fact, all the surrounding bodies we
+perceive might be removed to an infinite distance without affecting the
+result in the slightest. When the coil is set to work, the glass is
+invariably broken through at the seal, or other narrow channel, and the
+vacuum is quickly impaired. Such a damaging break would not occur with
+a steady force, even if the same were many times greater. The break is
+due to the agitation of the molecules of the gas within the bulb, and
+outside of the same. This agitation, which is generally most violent in
+the narrow pointed channel near the seal, causes a heating and rupture
+of the glass. This rupture, would, however, not occur, not even with a
+varying force, if the medium filling the inside of the bulb, and that
+surrounding it, were perfectly homogeneous. The break occurs much
+quicker if the top of the bulb is drawn out into a fine fibre. In bulbs
+used with these coils such narrow, pointed channels must therefore be
+avoided.
+
+When a conducting body is immersed in air, or similar insulating medium,
+consisting of, or containing, small freely movable particles capable of
+being electrified, and when the electrification of the body is made to
+undergo a very rapid change--which is equivalent to saying that the
+electrostatic force acting around the body is varying in intensity,--the
+small particles are attracted and repelled, and their violent impacts
+against the body may cause a mechanical motion of the latter. Phenomena
+of this kind are noteworthy, inasmuch as they have not been observed
+before with apparatus such as has been commonly in use. If a very light
+conducting sphere be suspended on an exceedingly fine wire, and charged
+to a steady potential, however high, the sphere will remain at rest.
+Even if the potential would be rapidly varying, provided that the small
+particles of matter, molecules or atoms, are evenly distributed, no
+motion of the sphere should result. But if one side of the conducting
+sphere is covered with a thick insulating layer, the impacts of the
+particles will cause the sphere to move about, generally in irregular
+curves, Fig. 172_a_. In like manner, as I have shown on a previous
+occasion, a fan of sheet metal, Fig. 172_b_, covered partially with
+insulating material as indicated, and placed upon the terminal of the
+coil so as to turn freely on it, is spun around.
+
+All these phenomena you have witnessed and others which will be shown
+later, are due to the presence of a medium like air, and would not occur
+in a continuous medium. The action of the air may be illustrated still
+better by the following experiment. I take a glass tube _t_, Fig. 173,
+of about an inch in diameter, which has a platinum wire _w_ sealed in
+the lower end, and to which is attached a thin lamp filament _f_. I
+connect the wire with the terminal of the coil and set the coil to work.
+The platinum wire is now electrified positively and negatively in rapid
+succession and the wire and air inside of the tube is rapidly heated by
+the impacts of the particles, which may be so violent as to render the
+filament incandescent. But if I pour oil in the tube, just as soon as
+the wire is covered with the oil, all action apparently ceases and there
+is no marked evidence of heating. The reason of this is that the oil is
+a practically continuous medium. The displacements in such a continuous
+medium are, with these frequencies, to all appearance incomparably
+smaller than in air, hence the work performed in such a medium is
+insignificant. But oil would behave very differently with frequencies
+many times as great, for even though the displacements be small, if the
+frequency were much greater, considerable work might be performed in the
+oil.
+
+[Illustration: FIG. 173.]
+
+[Illustration: FIG. 174.]
+
+The electrostatic attractions and repulsions between bodies of
+measurable dimensions are, of all the manifestations of this force, the
+first so-called _electrical_ phenomena noted. But though they have been
+known to us for many centuries, the precise nature of the mechanism
+concerned in these actions is still unknown to us, and has not been even
+quite satisfactorily explained. What kind of mechanism must that be? We
+cannot help wondering when we observe two magnets attracting and
+repelling each other with a force of hundreds of pounds with apparently
+nothing between them. We have in our commercial dynamos magnets capable
+of sustaining in mid-air tons of weight. But what are even these forces
+acting between magnets when compared with the tremendous attractions and
+repulsions produced by electrostatic force, to which there is apparently
+no limit as to intensity. In lightning discharges bodies are often
+charged to so high a potential that they are thrown away with
+inconceivable force and torn asunder or shattered into fragments. Still
+even such effects cannot compare with the attractions and repulsions
+which exist between charged molecules or atoms, and which are sufficient
+to project them with speeds of many kilometres a second, so that under
+their violent impact bodies are rendered highly incandescent and are
+volatilized. It is of special interest for the thinker who inquires into
+the nature of these forces to note that whereas the actions between
+individual molecules or atoms occur seemingly under any conditions, the
+attractions and repulsions of bodies of measurable dimensions imply a
+medium possessing insulating properties. So, if air, either by being
+rarefied or heated, is rendered more or less conducting, these actions
+between two electrified bodies practically cease, while the actions
+between the individual atoms continue to manifest themselves.
+
+An experiment may serve as an illustration and as a means of bringing
+out other features of interest. Some time ago I showed that a lamp
+filament or wire mounted in a bulb and connected to one of the terminals
+of a high tension secondary coil is set spinning, the top of the
+filament generally describing a circle. This vibration was very
+energetic when the air in the bulb was at ordinary pressure and became
+less energetic when the air in the bulb was strongly compressed. It
+ceased altogether when the air was exhausted so as to become
+comparatively good conducting. I found at that time that no vibration
+took place when the bulb was very highly exhausted. But I conjectured
+that the vibration which I ascribed to the electrostatic action between
+the walls of the bulb and the filament should take place also in a
+highly exhausted bulb. To test this under conditions which were more
+favorable, a bulb like the one in Fig. 174, was constructed. It
+comprised a globe _b_, in the neck of which was sealed a platinum wire
+_w_ carrying a thin lamp filament _f_. In the lower part of the globe a
+tube _t_ was sealed so as to surround the filament. The exhaustion was
+carried as far as it was practicable with the apparatus employed.
+
+This bulb verified my expectation, for the filament was set spinning
+when the current was turned on, and became incandescent. It also showed
+another interesting feature, bearing upon the preceding remarks, namely,
+when the filament had been kept incandescent some time, the narrow tube
+and the space inside were brought to an elevated temperature, and as the
+gas in the tube then became conducting, the electrostatic attraction
+between the glass and the filament became very weak or ceased, and the
+filament came to rest. When it came to rest it would glow far more
+intensely. This was probably due to its assuming the position in the
+centre of the tube where the molecular bombardment was most intense, and
+also partly to the fact that the individual impacts were more violent
+and that no part of the supplied energy was converted into mechanical
+movement. Since, in accordance with accepted views, in this experiment
+the incandescence must be attributed to the impacts of the particles,
+molecules or atoms in the heated space, these particles must therefore,
+in order to explain such action, be assumed to behave as independent
+carriers of electric charges immersed in an insulating medium; yet there
+is no attractive force between the glass tube and the filament because
+the space in the tube is, as a whole, conducting.
+
+It is of some interest to observe in this connection that whereas the
+attraction between two electrified bodies may cease owing to the
+impairing of the insulating power of the medium in which they are
+immersed, the repulsion between the bodies may still be observed. This
+may be explained in a plausible way. When the bodies are placed at some
+distance in a poorly conducting medium, such as slightly warmed or
+rarefied air, and are suddenly electrified, opposite electric charges
+being imparted to them, these charges equalize more or less by leakage
+through the air. But if the bodies are similarly electrified, there is
+less opportunity afforded for such dissipation, hence the repulsion
+observed in such case is greater than the attraction. Repulsive actions
+in a gaseous medium are however, as Prof. Crookes has shown, enhanced by
+molecular bombardment.
+
+
+ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.
+
+So far, I have considered principally effects produced by a varying
+electrostatic force in an insulating medium, such as air. When such a
+force is acting upon a conducting body of measurable dimensions, it
+causes within the same, or on its surface, displacements of the
+electricity and gives rise to electric currents, and these produce
+another kind of phenomena, some of which I shall presently endeavor to
+illustrate. In presenting this second class of electrical effects, I
+will avail myself principally of such as are producible without any
+return circuit, hoping to interest you the more by presenting these
+phenomena in a more or less novel aspect.
+
+It has been a long time customary, owing to the limited experience with
+vibratory currents, to consider an electric current as something
+circulating in a closed conducting path. It was astonishing at first to
+realize that a current may flow through the conducting path even if the
+latter be interrupted, and it was still more surprising to learn, that
+sometimes it may be even easier to make a current flow under such
+conditions than through a closed path. But that old idea is gradually
+disappearing, even among practical men, and will soon be entirely
+forgotten.
+
+[Illustration: FIG. 175.]
+
+If I connect an insulated metal plate P, Fig. 175, to one of the
+terminals T of the induction coil by means of a wire, though this plate
+be very well insulated, a current passes through the wire when the coil
+is set to work. First I wish to give you evidence that there _is_ a
+current passing through the connecting wire. An obvious way of
+demonstrating this is to insert between the terminal of the coil and the
+insulated plate a very thin platinum or german silver wire _w_ and bring
+the latter to incandescence or fusion by the current. This requires a
+rather large plate or else current impulses of very high potential and
+frequency. Another way is to take a coil C, Fig. 175, containing many
+turns of thin insulated wire and to insert the same in the path of the
+current to the plate. When I connect one of the ends of the coil to the
+wire leading to another insulated plate P_{1}, and its other end to the
+terminal T_{1} of the induction coil, and set the latter to work, a
+current passes through the inserted coil C and the existence of the
+current may be made manifest in various ways. For instance, I insert an
+iron core _i_ within the coil. The current being one of very high
+frequency, will, if it be of some strength, soon bring the iron core to
+a noticeably higher temperature, as the hysteresis and current losses
+are great with such high frequencies. One might take a core of some
+size, laminated or not, it would matter little; but ordinary iron wire
+1/16th or 1/8th of an inch thick is suitable for the purpose. While the
+induction coil is working, a current traverses the inserted coil and
+only a few moments are sufficient to bring the iron wire _i_ to an
+elevated temperature sufficient to soften the sealing-wax _s_, and cause
+a paper washer _p_ fastened by it to the iron wire to fall off. But with
+the apparatus such as I have here, other, much more interesting,
+demonstrations of this kind can be made. I have a secondary S, Fig 176,
+of coarse wire, wound upon a coil similar to the first. In the preceding
+experiment the current through the coil C, Fig. 175, was very small, but
+there being many turns a strong heating effect was, nevertheless,
+produced in the iron wire. Had I passed that current through a conductor
+in order to show the heating of the latter, the current might have been
+too small to produce the effect desired. But with this coil provided
+with a secondary winding, I can now transform the feeble current of high
+tension which passes through the primary P into a strong secondary
+current of low tension, and this current will quite certainly do what I
+expect. In a small glass tube (_t_, Fig. 176), I have enclosed a coiled
+platinum wire, _w_, this merely in order to protect the wire. On each
+end of the glass tube is sealed a terminal of stout wire to which one of
+the ends of the platinum wire _w_, is connected. I join the terminals of
+the secondary coil to these terminals and insert the primary _p_,
+between the insulated plate P_{1}, and the terminal T_{1}, of the
+induction coil as before. The latter being set to work, instantly the
+platinum wire _w_ is rendered incandescent and can be fused, even if it
+be very thick.
+
+[Illustration: FIG. 176.]
+
+Instead of the platinum wire I now take an ordinary 50-volt 16 C. P.
+lamp. When I set the induction coil in operation the lamp filament is
+brought to high incandescence. It is, however, not necessary to use the
+insulated plate, for the lamp (_l_, Fig. 177) is rendered incandescent
+even if the plate P_{1} be disconnected. The secondary may also be
+connected to the primary as indicated by the dotted line in Fig. 177, to
+do away more or less with the electrostatic induction or to modify the
+action otherwise.
+
+[Illustration: FIG. 177.]
+
+I may here call attention to a number of interesting observations with
+the lamp. First, I disconnect one of the terminals of the lamp from the
+secondary S. When the induction coil plays, a glow is noted which fills
+the whole bulb. This glow is due to electrostatic induction. It
+increases when the bulb is grasped with the hand, and the capacity of
+the experimenter's body thus added to the secondary circuit. The
+secondary, in effect, is equivalent to a metallic coating, which would
+be placed near the primary. If the secondary, or its equivalent, the
+coating, were placed symmetrically to the primary, the electrostatic
+induction would be nil under ordinary conditions, that is, when a
+primary return circuit is used, as both halves would neutralize each
+other. The secondary _is_ in fact placed symmetrically to the primary,
+but the action of both halves of the latter, when only one of its ends
+is connected to the induction coil, is not exactly equal; hence
+electrostatic induction takes place, and hence the glow in the bulb. I
+can nearly equalize the action of both halves of the primary by
+connecting the other, free end of the same to the insulated plate, as in
+the preceding experiment. When the plate is connected, the glow
+disappears. With a smaller plate it would not entirely disappear and
+then it would contribute to the brightness of the filament when the
+secondary is closed, by warming the air in the bulb.
+
+[Illustration: FIG. 178a.]
+
+[Illustration: FIG. 178b.]
+
+[Illustration: FIG. 179a.]
+
+[Illustration: FIG. 179b.]
+
+To demonstrate another interesting feature, I have adjusted the coils
+used in a certain way. I first connect both the terminals of the lamp to
+the secondary, one end of the primary being connected to the terminal
+T_{1} of the induction coil and the other to the insulated plate P_{1}
+as before. When the current is turned on, the lamp glows brightly, as
+shown in Fig. 178_b_, in which C is a fine wire coil and S a coarse wire
+secondary wound upon it. If the insulated plate P_{1} is disconnected,
+leaving one of the ends _a_ of the primary insulated, the filament
+becomes dark or generally it diminishes in brightness (Fig. 178_a_).
+Connecting again the plate P_{1} and raising the frequency of the
+current, I make the filament quite dark or barely red (Fig. 179_b_).
+Once more I will disconnect the plate. One will of course infer that
+when the plate is disconnected, the current through the primary will be
+weakened, that therefore the E. M. F. will fall in the secondary S, and
+that the brightness of the lamp will diminish. This might be the case
+and the result can be secured by an easy adjustment of the coils; also
+by varying the frequency and potential of the currents. But it is
+perhaps of greater interest to note, that the lamp increases in
+brightness when the plate is disconnected (Fig. 179_a_). In this case
+all the energy the primary receives is now sunk into it, like the charge
+of a battery in an ocean cable, but most of that energy is recovered
+through the secondary and used to light the lamp. The current traversing
+the primary is strongest at the end _b_ which is connected to the
+terminal T_{1} of the induction coil, and diminishes in strength towards
+the remote end _a_. But the dynamic inductive effect exerted upon the
+secondary S is now greater than before, when the suspended plate was
+connected to the primary. These results might have been produced by a
+number of causes. For instance, the plate P_{1} being connected, the
+reaction from the coil C may be such as to diminish the potential at the
+terminal T_{1} of the induction coil, and therefore weaken the current
+through the primary of the coil C. Or the disconnecting of the plate
+may diminish the capacity effect with relation to the primary of the
+latter coil to such an extent that the current through it is diminished,
+though the potential at the terminal T_{1} of the induction coil may be
+the same or even higher. Or the result might have been produced by the
+change of phase of the primary and secondary currents and consequent
+reaction. But the chief determining factor is the relation of the
+self-induction and capacity of coil C and plate P_{1} and the frequency
+of the currents. The greater brightness of the filament in Fig. 179_a_,
+is, however, in part due to the heating of the rarefied gas in the lamp
+by electrostatic induction, which, as before remarked, is greater when
+the suspended plate is disconnected.
+
+Still another feature of some interest I may here bring to your
+attention. When the insulated plate is disconnected and the secondary of
+the coil opened, by approaching a small object to the secondary, but
+very small sparks can be drawn from it, showing that the electrostatic
+induction is small in this case. But upon the secondary being closed
+upon itself or through the lamp, the filament glowing brightly, strong
+sparks are obtained from the secondary. The electrostatic induction is
+now much greater, because the closed secondary determines a greater flow
+of current through the primary and principally through that half of it
+which is connected to the induction coil. If now the bulb be grasped
+with the hand, the capacity of the secondary with reference to the
+primary is augmented by the experimenter's body and the luminosity of
+the filament is increased, the incandescence now being due partly to the
+flow of current through the filament and partly to the molecular
+bombardment of the rarefied gas in the bulb.
+
+The preceding experiments will have prepared one for the next following
+results of interest, obtained in the course of these investigations.
+Since I can pass a current through an insulated wire merely by
+connecting one of its ends to the source of electrical energy, since I
+can induce by it another current, magnetize an iron core, and, in short,
+perform all operations as though a return circuit were used, clearly I
+can also drive a motor by the aid of only one wire. On a former occasion
+I have described a simple form of motor comprising a single exciting
+coil, an iron core and disc. Fig. 180 illustrates a modified way of
+operating such an alternate current motor by currents induced in a
+transformer connected to one lead, and several other arrangements of
+circuits for operating a certain class of alternating motors founded on
+the action of currents of differing phase. In view of the present state
+of the art it is thought sufficient to describe these arrangements in a
+few words only. The diagram, Fig. 180 II., shows a primary coil P,
+connected with one of its ends to the line L leading from a high tension
+transformer terminal T_{1}. In inductive relation to this primary P is a
+secondary S of coarse wire in the circuit of which is a coil _c_. The
+currents induced in the secondary energize the iron core _i_, which is
+preferably, but not necessarily, subdivided, and set the metal disc _d_
+in rotation. Such a motor M_{2} as diagramatically shown in Fig. 180
+II., has been called a "magnetic lag motor," but this expression may be
+objected to by those who attribute the rotation of the disc to eddy
+currents circulating in minute paths when the core _i_ is finally
+subdivided. In order to operate such a motor effectively on the plan
+indicated, the frequencies should not be too high, not more than four or
+five thousand, though the rotation is produced even with ten thousand
+per second, or more.
+
+In Fig. 180 I., a motor M_{1} having two energizing circuits, A and B,
+is diagrammatically indicated. The circuit A is connected to the line L
+and in series with it is a primary P, which may have its free end
+connected to an insulated plate P_{1}, such connection being indicated
+by the dotted lines. The other motor circuit B is connected to the
+secondary S which is in inductive relation to the primary P. When the
+transformer terminal T_{1} is alternately electrified, currents traverse
+the open line L and also circuit A and primary P. The currents through
+the latter induce secondary currents in the circuit S, which pass
+through the energizing coil B of the motor. The currents through the
+secondary S and those through the primary P differ in phase 90 degrees,
+or nearly so, and are capable of rotating an armature placed in
+inductive relation to the circuits A and B.
+
+In Fig. 180 III., a similar motor M_{3} with two energizing circuits
+A_{1} and B_{1} is illustrated. A primary P, connected with one of its
+ends to the line L has a secondary S, which is preferably wound for a
+tolerably high E. M. F., and to which the two energizing circuits of the
+motor are connected, one directly to the ends of the secondary and the
+other through a condenser C, by the action of which the currents
+traversing the circuit A_{1} and B_{1} are made to differ in phase.
+
+[Illustration: FIG. 180.]
+
+[Illustration: FIG. 181.]
+
+[Illustration: FIG. 182.]
+
+In Fig. 180 IV., still another arrangement is shown. In this case two
+primaries P_{1} and P_{2} are connected to the line L, one through a
+condenser C of small capacity, and the other directly. The primaries are
+provided with secondaries S_{1} and S_{2} which are in series with the
+energizing circuits, A_{2} and B_{2} and a motor M_{3}, the condenser C
+again serving to produce the requisite difference in the phase of the
+currents traversing the motor circuits. As such phase motors with two or
+more circuits are now well known in the art, they have been here
+illustrated diagrammatically. No difficulty whatever is found in
+operating a motor in the manner indicated, or in similar ways; and
+although such experiments up to this day present only scientific
+interest, they may at a period not far distant, be carried out with
+practical objects in view.
+
+It is thought useful to devote here a few remarks to the subject of
+operating devices of all kinds by means of only one leading wire. It is
+quite obvious, that when high-frequency currents are made use of, ground
+connections are--at least when the E. M. F. of the currents is
+great--better than a return wire. Such ground connections are
+objectionable with steady or low frequency currents on account of
+destructive chemical actions of the former and disturbing influences
+exerted by both on the neighboring circuits; but with high frequencies
+these actions practically do not exist. Still, even ground connections
+become superfluous when the E. M. F. is very high, for soon a condition
+is reached, when the current may be passed more economically through
+open, than through closed, conductors. Remote as might seem an
+industrial application of such single wire transmission of energy to one
+not experienced in such lines of experiment, it will not seem so to
+anyone who for some time has carried on investigations of such nature.
+Indeed I cannot see why such a plan should not be practicable. Nor
+should it be thought that for carrying out such a plan currents of very
+high frequency are expressly required, for just as soon as potentials of
+say 30,000 volts are used, the single wire transmission may be effected
+with low frequencies, and experiments have been made by me from which
+these inferences are made.
+
+When the frequencies are very high it has been found in laboratory
+practice quite easy to regulate the effects in the manner shown in
+diagram Fig. 181. Here two primaries P and P_{1} are shown, each
+connected with one of its ends to the line L and with the other end to
+the condenser plates C and C, respectively. Near these are placed other
+condenser plates C_{1} and C_{1}, the former being connected to the line
+L and the latter to an insulated larger plate P_{2}. On the primaries
+are wound secondaries S and S_{1}, of coarse wire, connected to the
+devices _d_ and _l_ respectively. By varying the distances of the
+condenser plates C and C_{1}, and C and C_{1} the currents through the
+secondaries S and S_{1} are varied in intensity. The curious feature is
+the great sensitiveness, the slightest change in the distance of the
+plates producing considerable variations in the intensity or strength of
+the currents. The sensitiveness may be rendered extreme by making the
+frequency such, that the primary itself, without any plate attached to
+its free end, satisfies, in conjunction with the closed secondary, the
+condition of resonance. In such condition an extremely small change in
+the capacity of the free terminal produces great variations. For
+instance, I have been able to adjust the conditions so that the mere
+approach of a person to the coil produces a considerable change in the
+brightness of the lamps attached to the secondary. Such observations and
+experiments possess, of course, at present, chiefly scientific interest,
+but they may soon become of practical importance.
+
+Very high frequencies are of course not practicable with motors on
+account of the necessity of employing iron cores. But one may use sudden
+discharges of low frequency and thus obtain certain advantages of
+high-frequency currents without rendering the iron core entirely
+incapable of following the changes and without entailing a very great
+expenditure of energy in the core. I have found it quite practicable to
+operate with such low frequency disruptive discharges of condensers,
+alternating-current motors. A certain class of such motors which I
+advanced a few years ago, which contain closed secondary circuits, will
+rotate quite vigorously when the discharges are directed through the
+exciting coils. One reason that such a motor operates so well with these
+discharges is that the difference of phase between the primary and
+secondary currents is 90 degrees, which is generally not the case with
+harmonically rising and falling currents of low frequency. It might not
+be without interest to show an experiment with a simple motor of this
+kind, inasmuch as it is commonly thought that disruptive discharges are
+unsuitable for such purposes. The motor is illustrated in Fig. 182. It
+comprises a rather large iron core _i_ with slots on the top into which
+are embedded thick copper washers _c c_. In proximity to the core is a
+freely-movable metal disc D. The core is provided with a primary
+exciting coil C_{1} the ends _a_ and _b_ of which are connected to the
+terminals of the secondary S of an ordinary transformer, the primary P
+of the latter being connected to an alternating distribution circuit or
+generator G of low or moderate frequency. The terminals of the secondary
+S are attached to a condenser C which discharges through an air gap _d
+d_ which may be placed in series or shunt to the coil C_{1}. When the
+conditions are properly chosen the disc D rotates with considerable
+effort and the iron core _i_ does not get very perceptibly hot. With
+currents from a high-frequency alternator, on the contrary, the core
+gets rapidly hot and the disc rotates with a much smaller effort. To
+perform the experiment properly it should be first ascertained that the
+disc D is not set in rotation when the discharge is not occurring at _d
+d_. It is preferable to use a large iron core and a condenser of large
+capacity so as to bring the superimposed quicker oscillation to a very
+low pitch or to do away with it entirely. By observing certain
+elementary rules I have also found it practicable to operate ordinary
+series or shunt direct-current motors with such disruptive discharges,
+and this can be done with or without a return wire.
+
+
+IMPEDANCE PHENOMENA.
+
+Among the various current phenomena observed, perhaps the most
+interesting are those of impedance presented by conductors to currents
+varying at a rapid rate. In my first paper before the American Institute
+of Electrical Engineers, I have described a few striking observations of
+this kind. Thus I showed that when such currents or sudden discharges
+are passed through a thick metal bar there may be points on the bar only
+a few inches apart, which have a sufficient potential difference between
+them to maintain at bright incandescence an ordinary filament lamp. I
+have also described the curious behavior of rarefied gas surrounding a
+conductor, due to such sudden rushes of current. These phenomena have
+since been more carefully studied and one or two novel experiments of
+this kind are deemed of sufficient interest to be described here.
+
+Referring to Fig. 183_a_, B and B_{1} are very stout copper bars
+connected at their lower ends to plates C and C_{1}, respectively, of a
+condenser, the opposite plates of the latter being connected to the
+terminals of the secondary S of a high-tension transformer, the primary
+P of which is supplied with alternating currents from an ordinary
+low-frequency dynamo G or distribution circuit. The condenser
+discharges through an adjustable gap _d d_ as usual. By establishing a
+rapid vibration it was found quite easy to perform the following curious
+experiment. The bars B and B_{1} were joined at the top by a low-voltage
+lamp l_{3}; a little lower was placed by means of clamps _c c_, a
+50-volt lamp l_{2}; and still lower another 100-volt lamp l_{1}; and
+finally, at a certain distance below the latter lamp, an exhausted tube
+T. By carefully determining the positions of these devices it was found
+practicable to maintain them all at their proper illuminating power. Yet
+they were all connected in multiple arc to the two stout copper bars and
+required widely different pressures. This experiment requires of course
+some time for adjustment but is quite easily performed.
+
+[Illustration: FIGS. 183a, 183b and 183c.]
+
+In Figs. 183_b_ and 183_c_, two other experiments are illustrated which,
+unlike the previous experiment, do not require very careful adjustments.
+In Fig. 183_b_, two lamps, l_{1} and l_{2}, the former a 100-volt
+and the latter a 50-volt are placed in certain positions as indicated,
+the 100-volt lamp being below the 50-volt lamp. When the arc is playing
+at _d d_ and the sudden discharges are passed through the bars B B_{1},
+the 50-volt lamp will, as a rule, burn brightly, or at least this result
+is easily secured, while the 100-volt lamp will burn very low or remain
+quite dark, Fig. 183_b_. Now the bars B B_{1} may be joined at the top
+by a thick cross bar B_{2} and it is quite easy to maintain the 100-volt
+lamp at full candle-power while the 50-volt lamp remains dark, Fig.
+183_c_. These results, as I have pointed out previously, should not be
+considered to be due exactly to frequency but rather to the time rate of
+change which may be great, even with low frequencies. A great many other
+results of the same kind, equally interesting, especially to those who
+are only used to manipulate steady currents, may be obtained and they
+afford precious clues in investigating the nature of electric currents.
+
+In the preceding experiments I have already had occasion to show some
+light phenomena and it would now be proper to study these in particular;
+but to make this investigation more complete I think it necessary to
+make first a few remarks on the subject of electrical resonance which
+has to be always observed in carrying out these experiments.
+
+
+ON ELECTRICAL RESONANCE.
+
+The effects of resonance are being more and more noted by engineers and
+are becoming of great importance in the practical operation of apparatus
+of all kinds with alternating currents. A few general remarks may
+therefore be made concerning these effects. It is clear, that if we
+succeed in employing the effects of resonance practically in the
+operation of electric devices the return wire will, as a matter of
+course, become unnecessary, for the electric vibration may be conveyed
+with one wire just as well as, and sometimes even better than, with two.
+The question first to answer is, then, whether pure resonance effects
+are producible. Theory and experiment both show that such is impossible
+in Nature, for as the oscillation becomes more and more vigorous, the
+losses in the vibrating bodies and environing media rapidly increase and
+necessarily check the vibration which otherwise would go on increasing
+forever. It is a fortunate circumstance that pure resonance is not
+producible, for if it were there is no telling what dangers might not
+lie in wait for the innocent experimenter. But to a certain degree
+resonance is producible, the magnitude of the effects being limited by
+the imperfect conductivity and imperfect elasticity of the media or,
+generally stated, by frictional losses. The smaller these losses, the
+more striking are the effects. The same is the case in mechanical
+vibration. A stout steel bar may be set in vibration by drops of water
+falling upon it at proper intervals; and with glass, which is more
+perfectly elastic, the resonance effect is still more remarkable, for a
+goblet may be burst by singing into it a note of the proper pitch. The
+electrical resonance is the more perfectly attained, the smaller the
+resistance or the impedance of the conducting path and the more perfect
+the dielectric. In a Leyden jar discharging through a short stranded
+cable of thin wires these requirements are probably best fulfilled, and
+the resonance effects are therefore very prominent. Such is not the case
+with dynamo machines, transformers and their circuits, or with
+commercial apparatus in general in which the presence of iron cores
+complicates the action or renders it impossible. In regard to Leyden
+jars with which resonance effects are frequently demonstrated, I would
+say that the effects observed are often _attributed_ but are seldom
+_due_ to true resonance, for an error is quite easily made in this
+respect. This may be undoubtedly demonstrated by the following
+experiment. Take, for instance, two large insulated metallic plates or
+spheres which I shall designate A and B; place them at a certain small
+distance apart and charge them from a frictional or influence machine to
+a potential so high that just a slight increase of the difference of
+potential between them will cause the small air or insulating space to
+break down. This is easily reached by making a few preliminary trials.
+If now another plate--fastened on an insulating handle and connected by
+a wire to one of the terminals of a high tension secondary of an
+induction coil, which is maintained in action by an alternator
+(preferably high frequency)--is approached to one of the charged bodies
+A or B, so as to be nearer to either one of them, the discharge will
+invariably occur between them; at least it will, if the potential of the
+coil in connection with the plate is sufficiently high. But the
+explanation of this will soon be found in the fact that the approached
+plate acts inductively upon the bodies A and B and causes a spark to
+pass between them. When this spark occurs, the charges which were
+previously imparted to these bodies from the influence machine, must
+needs be lost, since the bodies are brought in electrical connection
+through the arc formed. Now this arc is formed whether there be
+resonance or not. But even if the spark would not be produced, still
+there is an alternating E. M. F. set up between the bodies when the
+plate is brought near one of them; therefore the approach of the plate,
+if it _does_ not always actually, will, at any rate, _tend_ to break
+down the air space by inductive action. Instead of the spheres or plates
+A and B we may take the coatings of a Leyden jar with the same result,
+and in place of the machine,--which is a high frequency alternator
+preferably, because it is more suitable for the experiment and also for
+the argument,--we may take another Leyden jar or battery of jars. When
+such jars are discharging through a circuit of low resistance the same
+is traversed by currents of very high frequency. The plate may now be
+connected to one of the coatings of the second jar, and when it is
+brought near to the first jar just previously charged to a high
+potential from an influence machine, the result is the same as before,
+and the first jar will discharge through a small air space upon the
+second being caused to discharge. But both jars and their circuits need
+not be tuned any closer than a basso profundo is to the note produced by
+a mosquito, as small sparks will be produced through the air space, or
+at least the latter will be considerably more strained owing to the
+setting up of an alternating E. M. F. by induction, which takes place
+when one of the jars begins to discharge. Again another error of a
+similar nature is quite easily made. If the circuits of the two jars are
+run parallel and close together, and the experiment has been performed
+of discharging one by the other, and now a coil of wire be added to one
+of the circuits whereupon the experiment does not succeed, the
+conclusion that this is due to the fact that the circuits are now not
+tuned, would be far from being safe. For the two circuits act as
+condenser coatings and the addition of the coil to one of them is
+equivalent to bridging them, at the point where the coil is placed, by a
+small condenser, and the effect of the latter might be to prevent the
+spark from jumping through the discharge space by diminishing the
+alternating E. M. F. acting across the same. All these remarks, and many
+more which might be added but for fear of wandering too far from the
+subject, are made with the pardonable intention of cautioning the
+unsuspecting student, who might gain an entirely unwarranted opinion of
+his skill at seeing every experiment succeed; but they are in no way
+thrust upon the experienced as novel observations.
+
+In order to make reliable observations of electric resonance effects it
+is very desirable, if not necessary, to employ an alternator giving
+currents which rise and fall harmonically, as in working with make and
+break currents the observations are not always trustworthy, since many
+phenomena, which depend on the rate of change, may be produced with
+widely different frequencies. Even when making such observations with an
+alternator one is apt to be mistaken. When a circuit is connected to an
+alternator there are an indefinite number of values for capacity and
+self-induction which, in conjunction, will satisfy the condition of
+resonance. So there are in mechanics an infinite number of tuning forks
+which will respond to a note of a certain pitch, or loaded springs which
+have a definite period of vibration. But the resonance will be most
+perfectly attained in that case in which the motion is effected with the
+greatest freedom. Now in mechanics, considering the vibration in the
+common medium--that is, air--it is of comparatively little importance
+whether one tuning fork be somewhat larger than another, because the
+losses in the air are not very considerable. One may, of course, enclose
+a tuning fork in an exhausted vessel and by thus reducing the air
+resistance to a minimum obtain better resonant action. Still the
+difference would not be very great. But it would make a great difference
+if the tuning fork were immersed in mercury. In the electrical vibration
+it is of enormous importance to arrange the conditions so that the
+vibration is effected with the greatest freedom. The magnitude of the
+resonance effect depends, under otherwise equal conditions, on the
+quantity of electricity set in motion or on the strength of the current
+driven through the circuit. But the circuit opposes the passage of the
+currents by reason of its impedance and therefore, to secure the best
+action it is necessary to reduce the impedance to a minimum. It is
+impossible to overcome it entirely, but merely in part, for the ohmic
+resistance cannot be overcome. But when the frequency of the impulses is
+very great, the flow of the current is practically determined by
+self-induction. Now self-induction can be overcome by combining it with
+capacity. If the relation between these is such, that at the frequency
+used they annul each other, that is, have such values as to satisfy the
+condition of resonance, and the greatest quantity of electricity is made
+to flow through the external circuit, then the best result is obtained.
+It is simpler and safer to join the condenser in series with the
+self-induction. It is clear that in such combinations there will be,
+for a given frequency, and considering only the fundamental vibration,
+values which will give the best result, with the condenser in shunt to
+the self-induction coil; of course more such values than with the
+condenser in series. But practical conditions determine the selection.
+In the latter case in performing the experiments one may take a small
+self-induction and a large capacity or a small capacity and a large
+self-induction, but the latter is preferable, because it is inconvenient
+to adjust a large capacity by small steps. By taking a coil with a very
+large self-induction the critical capacity is reduced to a very small
+value, and the capacity of the coil itself may be sufficient. It is
+easy, especially by observing certain artifices, to wind a coil through
+which the impedance will be reduced to the value of the ohmic resistance
+only; and for any coil there is, of course, a frequency at which the
+maximum current will be made to pass through the coil. The observation
+of the relation between self-induction, capacity and frequency is
+becoming important in the operation of alternate current apparatus, such
+as transformers or motors, because by a judicious determination of the
+elements the employment of an expensive condenser becomes unnecessary.
+Thus it is possible to pass through the coils of an alternating current
+motor under the normal working conditions the required current with a
+low E. M. F. and do away entirely with the false current, and the larger
+the motor, the easier such a plan becomes practicable; but it is
+necessary for this to employ currents of very high potential or high
+frequency.
+
+[Illustration: FIG. 184.]
+
+In Fig. 184 I. is shown a plan which has been followed in the study of
+the resonance effects by means of a high frequency alternator. C_{1} is
+a coil of many turns, which is divided into small separate sections for
+the purpose of adjustment. The final adjustment was made sometimes with
+a few thin iron wires (though this is not always advisable) or with a
+closed secondary. The coil C_{1} is connected with one of its ends to
+the line L from the alternator G and with the other end to one of the
+plates _c_ of a condenser c c_{1}, the plate (c_{1}) of the latter
+being connected to a much larger plate P_{1}. In this manner both
+capacity and self-induction were adjusted to suit the dynamo frequency.
+
+As regards the rise of potential through resonant action, of course,
+theoretically, it may amount to anything since it depends on
+self-induction and resistance and since these may have any value. But in
+practice one is limited in the selection of these values and besides
+these, there are other limiting causes. One may start with, say, 1,000
+volts and raise the E. M. F. to 50 times that value, but one cannot
+start with 100,000 and raise it to ten times that value because of the
+losses in the media which are great, especially if the frequency is
+high. It should be possible to start with, for instance, two volts from
+a high or low frequency circuit of a dynamo and raise the E. M. F. to
+many hundred times that value. Thus coils of the proper dimensions might
+be connected each with only one of its ends to the mains from a machine
+of low E. M. F., and though the circuit of the machine would not be
+closed in the ordinary acceptance of the term, yet the machine might be
+burned out if a proper resonance effect would be obtained. I have not
+been able to produce, nor have I observed with currents from a dynamo
+machine, such great rises of potential. It is possible, if not probable,
+that with currents obtained from apparatus containing iron the
+disturbing influence of the latter is the cause that these theoretical
+possibilities cannot be realized. But if such is the case I attribute it
+solely to the hysteresis and Foucault current losses in the core.
+Generally it was necessary to transform upward, when the E. M. F. was
+very low, and usually an ordinary form of induction coil was employed,
+but sometimes the arrangement illustrated in Fig. 184 II., has been
+found to be convenient. In this case a coil C is made in a great many
+sections, a few of these being used as a primary. In this manner both
+primary and secondary are adjustable. One end of the coil is connected
+to the line L_{1} from the alternator, and the other line L is connected
+to the intermediate point of the coil. Such a coil with adjustable
+primary and secondary will be found also convenient in experiments with
+the disruptive discharge. When true resonance is obtained the top of the
+wave must of course be on the free end of the coil as, for instance, at
+the terminal of the phosphorescence bulb B. This is easily recognized
+by observing the potential of a point on the wire _w_ near to the coil.
+
+In connection with resonance effects and the problem of transmission of
+energy over a single conductor which was previously considered, I would
+say a few words on a subject which constantly fills my thoughts and
+which concerns the welfare of all. I mean the transmission of
+intelligible signals or perhaps even power to any distance without the
+use of wires. I am becoming daily more convinced of the practicability
+of the scheme; and though I know full well that the great majority of
+scientific men will not believe that such results can be practically and
+immediately realized, yet I think that all consider the developments in
+recent years by a number of workers to have been such as to encourage
+thought and experiment in this direction. My conviction has grown so
+strong, that I no longer look upon this plan of energy or intelligence
+transmission as a mere theoretical possibility, but as a serious problem
+in electrical engineering, which must be carried out some day. The idea
+of transmitting intelligence without wires is the natural outcome of the
+most recent results of electrical investigations. Some enthusiasts have
+expressed their belief that telephony to any distance by induction
+through the air is possible. I cannot stretch my imagination so far, but
+I do firmly believe that it is practicable to disturb by means of
+powerful machines the electrostatic condition of the earth and thus
+transmit intelligible signals and perhaps power. In fact, what is there
+against the carrying out of such a scheme? We now know that electric
+vibration may be transmitted through a single conductor. Why then not
+try to avail ourselves of the earth for this purpose? We need not be
+frightened by the idea of distance. To the weary wanderer counting the
+mile-posts the earth may appear very large, but to that happiest of all
+men, the astronomer, who gazes at the heavens and by their standard
+judges the magnitude of our globe, it appears very small. And so I think
+it must seem to the electrician, for when he considers the speed with
+which an electric disturbance is propagated through the earth all his
+ideas of distance must completely vanish.
+
+A point of great importance would be first to know what is the capacity
+of the earth? and what charge does it contain if electrified? Though we
+have no positive evidence of a charged body existing in space without
+other oppositely electrified bodies being near, there is a fair
+probability that the earth is such a body, for by whatever process it
+was separated from other bodies--and this is the accepted view of its
+origin--it must have retained a charge, as occurs in all processes of
+mechanical separation. If it be a charged body insulated in space its
+capacity should be extremely small, less than one-thousandth of a farad.
+But the upper strata of the air are conducting, and so, perhaps, is the
+medium in free space beyond the atmosphere, and these may contain an
+opposite charge. Then the capacity might be incomparably greater. In any
+case it is of the greatest importance to get an idea of what quantity of
+electricity the earth contains. It is difficult to say whether we shall
+ever acquire this necessary knowledge, but there is hope that we may,
+and that is, by means of electrical resonance. If ever we can ascertain
+at what period the earth's charge, when disturbed, oscillates with
+respect to an oppositely electrified system or known circuit, we shall
+know a fact possibly of the greatest importance to the welfare of the
+human race. I propose to seek for the period by means of an electrical
+oscillator, or a source of alternating electric currents. One of the
+terminals of the source would be connected to earth as, for instance, to
+the city water mains, the other to an insulated body of large surface.
+It is possible that the outer conducting air strata, or free space,
+contain an opposite charge and that, together with the earth, they form
+a condenser of very large capacity. In such case the period of vibration
+may be very low and an alternating dynamo machine might serve for the
+purpose of the experiment. I would then transform the current to a
+potential as high as it would be found possible and connect the ends of
+the high tension secondary to the ground and to the insulated body. By
+varying the frequency of the currents and carefully observing the
+potential of the insulated body and watching for the disturbance at
+various neighboring points of the earth's surface resonance might be
+detected. Should, as the majority of scientific men in all probability
+believe, the period be extremely small, then a dynamo machine would not
+do and a proper electrical oscillator would have to be produced and
+perhaps it might not be possible to obtain such rapid vibrations. But
+whether this be possible or not, and whether the earth contains a charge
+or not, and whatever may be its period of vibration, it certainly is
+possible--for of this we have daily evidence--to produce some electrical
+disturbance sufficiently powerful to be perceptible by suitable
+instruments at any point of the earth's surface.
+
+[Illustration: FIG. 185.]
+
+Assume that a source of alternating current S be connected, as in Fig.
+185, with one of its terminals to earth (conveniently to the water
+mains) and with the other to a body of large surface P. When the
+electric oscillation is set up there will be a movement of electricity
+in and out of P, and alternating currents will pass through the earth,
+converging to, or diverging from, the point C where the ground
+connection is made. In this manner neighboring points on the earth's
+surface within a certain radius will be disturbed. But the disturbance
+will diminish with the distance, and the distance at which the effect
+will still be perceptible will depend on the quantity of electricity set
+in motion. Since the body P is insulated, in order to displace a
+considerable quantity, the potential of the source must be excessive,
+since there would be limitations as to the surface of P. The conditions
+might be adjusted so that the generator or source S will set up the same
+electrical movement as though its circuit were closed. Thus it is
+certainly practicable to impress an electric vibration at least of a
+certain low period upon the earth by means of proper machinery. At what
+distance such a vibration might be made perceptible can only be
+conjectured. I have on another occasion considered the question how the
+earth might behave to electric disturbances. There is no doubt that,
+since in such an experiment the electrical density at the surface could
+be but extremely small considering the size of the earth, the air would
+not act as a very disturbing factor, and there would be not much energy
+lost through the action of the air, which would be the case if the
+density were great. Theoretically, then, it could not require a great
+amount of energy to produce a disturbance perceptible at great distance,
+or even all over the surface of the globe. Now, it is quite certain that
+at any point within a certain radius of the source S a properly adjusted
+self-induction and capacity device can be set in action by resonance.
+But not only can this be done, but another source S_{1}, Fig. 185,
+similar to S, or any number of such sources, can be set to work in
+synchronism with the latter, and the vibration thus intensified and
+spread over a large area, or a flow of electricity produced to or from
+the source S_{1} if the same be of opposite phase to the source S. I
+think that beyond doubt it is possible to operate electrical devices in
+a city through the ground or pipe system by resonance from an electrical
+oscillator located at a central point. But the practical solution of
+this problem would be of incomparably smaller benefit to man than the
+realization of the scheme of transmitting intelligence, or perhaps
+power, to any distance through the earth or environing medium. If this
+is at all possible, distance does not mean anything. Proper apparatus
+must first be produced by means of which the problem can be attacked and
+I have devoted much thought to this subject. I am firmly convinced that
+it can be done and hope that we shall live to see it done.
+
+
+ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS OF HIGH
+POTENTIAL AND GENERAL REMARKS RELATING TO THE SUBJECT.
+
+Returning now to the light effects which it has been the chief object to
+investigate, it is thought proper to divide these effects into four
+classes: 1. Incandescence of a solid. 2. Phosphorescence. 3.
+Incandescence or phosphorescence of a rarefied gas; and 4. Luminosity
+produced in a gas at ordinary pressure. The first question is: How are
+these luminous effects produced? In order to answer this question as
+satisfactorily as I am able to do in the light of accepted views and
+with the experience acquired, and to add some interest to this
+demonstration, I shall dwell here upon a feature which I consider of
+great importance, inasmuch as it promises, besides, to throw a better
+light upon the nature of most of the phenomena produced by
+high-frequency electric currents. I have on other occasions pointed out
+the great importance of the presence of the rarefied gas, or atomic
+medium in general, around the conductor through which alternate currents
+of high frequency are passed, as regards the heating of the conductor by
+the currents. My experiments, described some time ago, have shown that,
+the higher the frequency and potential difference of the currents, the
+more important becomes the rarefied gas in which the conductor is
+immersed, as a factor of the heating. The potential difference, however,
+is, as I then pointed out, a more important element than the frequency.
+When both of these are sufficiently high, the heating may be almost
+entirely due to the presence of the rarefied gas. The experiments to
+follow will show the importance of the rarefied gas, or, generally, of
+gas at ordinary or other pressure as regards the incandescence or other
+luminous effects produced by currents of this kind.
+
+I take two ordinary 50-volt 16 C. P. lamps which are in every respect
+alike, with the exception, that one has been opened at the top and the
+air has filled the bulb, while the other is at the ordinary degree of
+exhaustion of commercial lamps. When I attach the lamp which is
+exhausted to the terminal of the secondary of the coil, which I have
+already used, as in experiments illustrated in Fig. 179_a_ for instance,
+and turn on the current, the filament, as you have before seen, comes to
+high incandescence. When I attach the second lamp, which is filled with
+air, instead of the former, the filament still glows, but much less
+brightly. This experiment illustrates only in part the truth of the
+statements before made. The importance of the filament's being immersed
+in rarefied gas is plainly noticeable but not to such a degree as might
+be desirable. The reason is that the secondary of this coil is wound for
+low tension, having only 150 turns, and the potential difference at the
+terminals of the lamp is therefore small. Were I to take another coil
+with many more turns in the secondary, the effect would be increased,
+since it depends partially on the potential difference, as before
+remarked. But since the effect likewise depends on the frequency, it
+maybe properly stated that it depends on the time rate of the variation
+of the potential difference. The greater this variation, the more
+important becomes the gas as an element of heating. I can produce a much
+greater rate of variation in another way, which, besides, has the
+advantage of doing away with the objections, which might be made in the
+experiment just shown, even if both the lamps were connected in series
+or multiple arc to the coil, namely, that in consequence of the
+reactions existing between the primary and secondary coil the
+conclusions are rendered uncertain. This result I secure by charging,
+from an ordinary transformer which is fed from the alternating current
+supply station, a battery of condensers, and discharging the latter
+directly through a circuit of small self-induction, as before
+illustrated in Figs. 183_a_, 183_b_, and 183_c_.
+
+[Illustration: FIG. 186a.]
+
+[Illustration: FIG. 186b.]
+
+[Illustration: FIG. 186c.]
+
+In Figs. 186_a_, 186_b_ and 186_c_, the heavy copper bars B B_{1}, are
+connected to the opposite coatings of a battery of condensers, or
+generally in such way, that the high frequency or sudden discharges are
+made to traverse them. I connect first an ordinary 50-volt incandescent
+lamp to the bars by means of the clamps _c c_. The discharges being
+passed through the lamp, the filament is rendered incandescent, though
+the current through it is very small, and would not be nearly sufficient
+to produce a visible effect under the conditions of ordinary use of the
+lamp. Instead of this I now attach to the bars another lamp exactly like
+the first, but with the seal broken off, the bulb being therefore filled
+with air at ordinary pressure. When the discharges are directed through
+the filament, as before, it does not become incandescent. But the result
+might still be attributed to one of the many possible reactions. I
+therefore connect both the lamps in multiple arc as illustrated in Fig.
+186_a_. Passing the discharges through both the lamps, again the
+filament in the exhausted lamp _l_ glows very brightly while that in the
+non-exhausted lamp l_{1} remains dark, as previously. But it should
+not be thought that the latter lamp is taking only a small fraction of
+the energy supplied to both the lamps; on the contrary, it may consume a
+considerable portion of the energy and it may become even hotter than
+the one which burns brightly. In this experiment the potential
+difference at the terminals of the lamps varies in sign theoretically
+three to four million times a second. The ends of the filaments are
+correspondingly electrified, and the gas in the bulbs is violently
+agitated and a large portion of the supplied energy is thus converted
+into heat. In the non-exhausted bulb, there being a few million times
+more gas molecules than in the exhausted one, the bombardment, which is
+most violent at the ends of the filament, in the neck of the bulb,
+consumes a large portion of the energy without producing any visible
+effect. The reason is that, there being many molecules, the bombardment
+is quantitatively considerable, but the individual impacts are not very
+violent, as the speeds of the molecules are comparatively small owing to
+the small free path. In the exhausted bulb, on the contrary, the speeds
+are very great, and the individual impacts are violent and therefore
+better adapted to produce a visible effect. Besides, the convection of
+heat is greater in the former bulb. In both the bulbs the current
+traversing the filaments is very small, incomparably smaller than that
+which they require on an ordinary low-frequency circuit. The potential
+difference, however, at the ends of the filaments is very great and
+might be possibly 20,000 volts or more, if the filaments were straight
+and their ends far apart. In the ordinary lamp a spark generally occurs
+between the ends of the filament or between the platinum wires outside,
+before such a difference of potential can be reached.
+
+It might be objected that in the experiment before shown the lamps,
+being in multiple arc, the exhausted lamp might take a much larger
+current and that the effect observed might not be exactly attributable
+to the action of the gas in the bulbs. Such objections will lose much
+weight if I connect the lamps in series, with the same result. When this
+is done and the discharges are directed through the filaments, it is
+again noted that the filament in the non-exhausted bulb l_{1}, remains
+dark, while that in the exhausted one (_l_) glows even more intensely
+than under its normal conditions of working, Fig. 186_b_. According to
+general ideas the current through the filaments should now be the same,
+were it not modified by the presence of the gas around the filaments.
+
+At this juncture I may point out another interesting feature, which
+illustrates the effect of the rate of change of potential of the
+currents. I will leave the two lamps connected in series to the bars
+B B_{1}, as in the previous experiment, Fig. 186_b_, but will presently
+reduce considerably the frequency of the currents, which was excessive
+in the experiment just before shown. This I may do by inserting a
+self-induction coil in the path of the discharges, or by augmenting the
+capacity of the condensers. When I now pass these low-frequency
+discharges through the lamps, the exhausted lamp _l_ again is as bright
+as before, but it is noted also that the non-exhausted lamp l_{1}
+glows, though not quite as intensely as the other. Reducing the current
+through the lamps, I may bring the filament in the latter lamp to
+redness, and, though the filament in the exhausted lamp _l_ is bright,
+Fig. 186_c_, the degree of its incandescence is much smaller than in
+Fig. 186_b_, when the currents were of a much higher frequency.
+
+In these experiments the gas acts in two opposite ways in determining
+the degree of the incandescence of the filaments, that is, by convection
+and bombardment. The higher the frequency and potential of the currents,
+the more important becomes the bombardment. The convection on the
+contrary should be the smaller, the higher the frequency. When the
+currents are steady there is practically no bombardment, and convection
+may therefore with such currents also considerably modify the degree of
+incandescence and produce results similar to those just before shown.
+Thus, if two lamps exactly alike, one exhausted and one not exhausted,
+are connected in multiple arc or series to a direct-current machine, the
+filament in the non-exhausted lamp will require a considerably greater
+current to be rendered incandescent. This result is entirely due to
+convection, and the effect is the more prominent the thinner the
+filament. Professor Ayrton and Mr. Kilgour some time ago published
+quantitative results concerning the thermal emissivity by radiation and
+convection in which the effect with thin wires was clearly shown. This
+effect may be strikingly illustrated by preparing a number of small,
+short, glass tubes, each containing through its axis the thinnest
+obtainable platinum wire. If these tubes be highly exhausted, a number
+of them may be connected in multiple arc to a direct-current machine and
+all of the wires may be kept at incandescence with a smaller current
+than that required to render incandescent a single one of the wires if
+the tube be not exhausted. Could the tubes be so highly exhausted that
+convection would be nil, then the relative amounts of heat given off by
+convection and radiation could be determined without the difficulties
+attending thermal quantitative measurements. If a source of electric
+impulses of high frequency and very high potential is employed, a still
+greater number of the tubes may be taken and the wires rendered
+incandescent by a current not capable of warming perceptibly a wire of
+the same size immersed in air at ordinary pressure, and conveying the
+energy to all of them.
+
+I may here describe a result which is still more interesting, and to
+which I have been led by the observation of these phenomena. I noted
+that small differences in the density of the air produced a considerable
+difference in the degree of incandescence of the wires, and I thought
+that, since in a tube, through which a luminous discharge is passed, the
+gas is generally not of uniform density, a very thin wire contained in
+the tube might be rendered incandescent at certain places of smaller
+density of the gas, while it would remain dark at the places of greater
+density, where the convection would be greater and the bombardment less
+intense. Accordingly a tube _t_ was prepared, as illustrated in Fig.
+187, which contained through the middle a very fine platinum wire _w_.
+The tube was exhausted to a moderate degree and it was found that when
+it was attached to the terminal of a high-frequency coil the platinum
+wire _w_ would indeed, become incandescent in patches, as illustrated in
+Fig. 187. Later a number of these tubes with one or more wires were
+prepared, each showing this result. The effect was best noted when the
+striated discharge occurred in the tube, but was also produced when the
+striae were not visible, showing that, even then, the gas in the tube was
+not of uniform density. The position of the striae was generally such,
+that the rarefactions corresponded to the places of incandescence or
+greater brightness on the wire _w_. But in a few instances it was noted,
+that the bright spots on the wire were covered by the dense parts of the
+striated discharge as indicated by _l_ in Fig. 187, though the effect
+was barely perceptible. This was explained in a plausible way by
+assuming that the convection was not widely different in the dense and
+rarefied places, and that the bombardment was greater on the dense
+places of the striated discharge. It is, in fact, often observed in
+bulbs, that under certain conditions a thin wire is brought to higher
+incandescence when the air is not too highly rarefied. This is the case
+when the potential of the coil is not high enough for the vacuum, but
+the result may be attributed to many different causes. In all cases this
+curious phenomenon of incandescence disappears when the tube, or rather
+the wire, acquires throughout a uniform temperature.
+
+[Illustration: FIG. 187.]
+
+[Illustration: FIG. 188.]
+
+Disregarding now the modifying effect of convection there are then two
+distinct causes which determine the incandescence of a wire or filament
+with varying currents, that is, conduction current and bombardment. With
+steady currents we have to deal only with the former of these two
+causes, and the heating effect is a minimum, since the resistance is
+least to steady flow. When the current is a varying one the resistance
+is greater, and hence the heating effect is increased. Thus if the rate
+of change of the current is very great, the resistance may increase to
+such an extent that the filament is brought to incandescence with
+inappreciable currents, and we are able to take a short and thick block
+of carbon or other material and bring it to bright incandescence with a
+current incomparably smaller than that required to bring to the same
+degree of incandescence an ordinary thin lamp filament with a steady or
+low frequency current. This result is important, and illustrates how
+rapidly our views on these subjects are changing, and how quickly our
+field of knowledge is extending. In the art of incandescent lighting, to
+view this result in one aspect only, it has been commonly considered as
+an essential requirement for practical success, that the lamp filament
+should be thin and of high resistance. But now we know that the
+resistance of the filament to the steady flow does not mean anything;
+the filament might as well be short and thick; for if it be immersed in
+rarefied gas it will become incandescent by the passage of a small
+current. It all depends on the frequency and potential of the currents.
+We may conclude from this, that it would be of advantage, so far as the
+lamp is considered, to employ high frequencies for lighting, as they
+allow the use of short and thick filaments and smaller currents.
+
+If a wire or filament be immersed in a homogeneous medium, all the
+heating is due to true conduction current, but if it be enclosed in an
+exhausted vessel the conditions are entirely different. Here the gas
+begins to act and the heating effect of the conduction current, as is
+shown in many experiments, may be very small compared with that of the
+bombardment. This is especially the case if the circuit is not closed
+and the potentials are of course very high. Suppose that a fine filament
+enclosed in an exhausted vessel be connected with one of its ends to the
+terminal of a high tension coil and with its other end to a large
+insulated plate. Though the circuit is not closed, the filament, as I
+have before shown, is brought to incandescence. If the frequency and
+potential be comparatively low, the filament is heated by the current
+passing _through it_. If the frequency and potential, and principally
+the latter, be increased, the insulated plate need be but very small, or
+may be done away with entirely; still the filament will become
+incandescent, practically all the heating being then due to the
+bombardment. A practical way of combining both the effects of conduction
+currents and bombardment is illustrated in Fig. 188, in which an
+ordinary lamp is shown provided with a very thin filament which has one
+of the ends of the latter connected to a shade serving the purpose of
+the insulated plate, and the other end to the terminal of a high tension
+source. It should not be thought that only rarefied gas is an important
+factor in the heating of a conductor by varying currents, but gas at
+ordinary pressure may become important, if the potential difference and
+frequency of the currents is excessive. On this subject I have already
+stated, that when a conductor is fused by a stroke of lightning, the
+current through it may be exceedingly small, not even sufficient to heat
+the conductor perceptibly, were the latter immersed in a homogeneous
+medium.
+
+From the preceding it is clear that when a conductor of high resistance
+is connected to the terminals of a source of high frequency currents of
+high potential, there may occur considerable dissipation of energy,
+principally at the ends of the conductor, in consequence of the action
+of the gas surrounding the conductor. Owing to this, the current through
+a section of the conductor at a point midway between its ends may be
+much smaller than through a section near the ends. Furthermore, the
+current passes principally through the outer portions of the conductor,
+but this effect is to be distinguished from the skin effect as
+ordinarily interpreted, for the latter would, or should, occur also in a
+continuous incompressible medium. If a great many incandescent lamps are
+connected in series to a source of such currents, the lamps at the ends
+may burn brightly, whereas those in the middle may remain entirely dark.
+This is due principally to bombardment, as before stated. But even if
+the currents be steady, provided the difference of potential is very
+great, the lamps at the end will burn more brightly than those in the
+middle. In such case there is no rhythmical bombardment, and the result
+is produced entirely by leakage. This leakage or dissipation into space
+when the tension is high, is considerable when incandescent lamps are
+used, and still more considerable with arcs, for the latter act like
+flames. Generally, of course, the dissipation is much smaller with
+steady, than with varying, currents.
+
+I have contrived an experiment which illustrates in an interesting
+manner the effect of lateral diffusion. If a very long tube is attached
+to the terminal of a high frequency coil, the luminosity is greatest
+near the terminal and falls off gradually towards the remote end. This
+is more marked if the tube is narrow.
+
+A small tube about one-half inch in diameter and twelve inches long
+(Fig. 189), has one of its ends drawn out into a fine fibre _f_ nearly
+three feet long. The tube is placed in a brass socket T which can be
+screwed on the terminal T_{1} of the induction coil. The discharge
+passing through the tube first illuminates the bottom of the same, which
+is of comparatively large section; but through the long glass fibre the
+discharge cannot pass. But gradually the rarefied gas inside becomes
+warmed and more conducting and the discharge spreads into the glass
+fibre. This spreading is so slow, that it may take half a minute or more
+until the discharge has worked through up to the top of the glass fibre,
+then presenting the appearance of a strongly luminous thin thread. By
+adjusting the potential at the terminal the light may be made to travel
+upwards at any speed. Once, however, the glass fibre is heated, the
+discharge breaks through its entire length instantly. The interesting
+point to be noted is that, the higher the frequency of the currents, or
+in other words, the greater relatively the lateral dissipation, at a
+slower rate may the light be made to propagate through the fibre. This
+experiment is best performed with a highly exhausted and freshly made
+tube. When the tube has been used for some time the experiment often
+fails. It is possible that the gradual and slow impairment of the vacuum
+is the cause. This slow propagation of the discharge through a very
+narrow glass tube corresponds exactly to the propagation of heat through
+a bar warmed at one end. The quicker the heat is carried away laterally
+the longer time it will take for the heat to warm the remote end. When
+the current of a low frequency coil is passed through the fibre from end
+to end, then the lateral dissipation is small and the discharge
+instantly breaks through almost without exception.
+
+[Illustration: FIG. 189.]
+
+[Illustration: FIG. 190.]
+
+After these experiments and observations which have shown the importance
+of the discontinuity or atomic structure of the medium and which will
+serve to explain, in a measure at least, the nature of the four kinds of
+light effects producible with these currents, I may now give you an
+illustration of these effects. For the sake of interest I may do this in
+a manner which to many of you might be novel. You have seen before that
+we may now convey the electric vibration to a body by means of a single
+wire or conductor of any kind. Since the human frame is conducting I
+may convey the vibration through my body.
+
+First, as in some previous experiments, I connect my body with one of
+the terminals of a high-tension transformer and take in my hand an
+exhausted bulb which contains a small carbon button mounted upon a
+platinum wire leading to the outside of the bulb, and the button is
+rendered incandescent as soon as the transformer is set to work (Fig.
+190). I may place a conducting shade on the bulb which serves to
+intensify the action, but is not necessary. Nor is it required that the
+button should be in conducting connection with the hand through a wire
+leading through the glass, for sufficient energy may be transmitted
+through the glass itself by inductive action to render the button
+incandescent.
+
+[Illustration: FIG. 191.]
+
+[Illustration: FIG. 192.]
+
+Next I take a highly exhausted bulb containing a strongly phosphorescent
+body, above which is mounted a small plate of aluminum on a platinum
+wire leading to the outside, and the currents flowing through my body
+excite intense phosphorescence in the bulb (Fig. 191). Next again I take
+in my hand a simple exhausted tube, and in the same manner the gas
+inside the tube is rendered highly incandescent or phosphorescent (Fig.
+192). Finally, I may take in my hand a wire, bare or covered with thick
+insulation, it is quite immaterial; the electrical vibration is so
+intense as to cover the wire with a luminous film (Fig. 193).
+
+[Illustration: FIG. 193.]
+
+[Illustration: FIG. 194.]
+
+[Illustration: FIG. 195.]
+
+A few words must now be devoted to each of these phenomena. In the first
+place, I will consider the incandescence of a button or of a solid in
+general, and dwell upon some facts which apply equally to all these
+phenomena. It was pointed out before that when a thin conductor, such as
+a lamp filament, for instance, is connected with one of its ends to the
+terminal of a transformer of high tension the filament is brought to
+incandescence partly by a conduction current and partly by bombardment.
+The shorter and thicker the filament the more important becomes the
+latter, and finally, reducing the filament to a mere button, all the
+heating must practically be attributed to the bombardment. So in the
+experiment before shown, the button is rendered incandescent by the
+rhythmical impact of freely movable small bodies in the bulb. These
+bodies may be the molecules of the residual gas, particles of dust or
+lumps torn from the electrode; whatever they are, it is certain that the
+heating of the button is essentially connected with the pressure of such
+freely movable particles, or of atomic matter in general in the bulb.
+The heating is the more intense the greater the number of impacts per
+second and the greater the energy of each impact. Yet the button would
+be heated also if it were connected to a source of a steady potential.
+In such a case electricity would be carried away from the button by the
+freely movable carriers or particles flying about, and the quantity of
+electricity thus carried away might be sufficient to bring the button to
+incandescence by its passage through the latter. But the bombardment
+could not be of great importance in such case. For this reason it would
+require a comparatively very great supply of energy to the button to
+maintain it at incandescence with a steady potential. The higher the
+frequency of the electric impulses the more economically can the button
+be maintained at incandescence. One of the chief reasons why this is so,
+is, I believe, that with impulses of very high frequency there is less
+exchange of the freely movable carriers around the electrode and this
+means, that in the bulb the heated matter is better confined to the
+neighborhood of the button. If a double bulb, as illustrated in Fig. 194
+be made, comprising a large globe B and a small one _b_, each containing
+as usual a filament _f_ mounted on a platinum wire w and w_{1}, it
+is found, that if the filaments _f f_ be exactly alike, it requires less
+energy to keep the filament in the globe _b_ at a certain degree of
+incandescence, than that in the globe B. This is due to the confinement
+of the movable particles around the button. In this case it is also
+ascertained, that the filament in the small globe _b_ is less
+deteriorated when maintained a certain length of time at incandescence.
+This is a necessary consequence of the fact that the gas in the small
+bulb becomes strongly heated and therefore a very good conductor, and
+less work is then performed on the button, since the bombardment becomes
+less intense as the conductivity of the gas increases. In this
+construction, of course, the small bulb becomes very hot and when it
+reaches an elevated temperature the convection and radiation on the
+outside increase. On another occasion I have shown bulbs in which this
+drawback was largely avoided. In these instances a very small bulb,
+containing a refractory button, was mounted in a large globe and the
+space between the walls of both was highly exhausted. The outer large
+globe remained comparatively cool in such constructions. When the large
+globe was on the pump and the vacuum between the walls maintained
+permanent by the continuous action of the pump, the outer globe would
+remain quite cold, while the button in the small bulb was kept at
+incandescence. But when the seal was made, and the button in the small
+bulb maintained incandescent some length of time, the large globe too
+would become warmed. From this I conjecture that if vacuous space (as
+Prof. Dewar finds) cannot convey heat, it is so merely in virtue of our
+rapid motion through space or, generally speaking, by the motion of the
+medium relatively to us, for a permanent condition could not be
+maintained without the medium being constantly renewed. A vacuum cannot,
+according to all evidence, be permanently maintained around a hot body.
+
+In these constructions, before mentioned, the small bulb inside would,
+at least in the first stages, prevent all bombardment against the outer
+large globe. It occurred to me then to ascertain how a metal sieve would
+behave in this respect, and several bulbs, as illustrated in Fig. 195,
+were prepared for this purpose. In a globe _b_, was mounted a thin
+filament _f_ (or button) upon a platinum wire _w_ passing through a
+glass stem and leading to the outside of the globe. The filament _f_ was
+surrounded by a metal sieve _s_. It was found in experiments with such
+bulbs that a sieve with wide meshes apparently did not in the slightest
+affect the bombardment against the globe _b_. When the vacuum was high,
+the shadow of the sieve was clearly projected against the globe and the
+latter would get hot in a short while. In some bulbs the sieve _s_ was
+connected to a platinum wire sealed in the glass. When this wire was
+connected to the other terminal of the induction coil (the E. M. F.
+being kept low in this case), or to an insulated plate, the bombardment
+against the outer globe _b_ was diminished. By taking a sieve with fine
+meshes the bombardment against the globe _b_ was always diminished, but
+even then if the exhaustion was carried very far, and when the potential
+of the transformer was very high, the globe _b_ would be bombarded and
+heated quickly, though no shadow of the sieve was visible, owing to the
+smallness of the meshes. But a glass tube or other continuous body
+mounted so as to surround the filament, did entirely cut off the
+bombardment and for a while the outer globe _b_ would remain perfectly
+cold. Of course when the glass tube was sufficiently heated the
+bombardment against the outer globe could be noted at once. The
+experiments with these bulbs seemed to show that the speeds of the
+projected molecules or particles must be considerable (though quite
+insignificant when compared with that of light), otherwise it would be
+difficult to understand how they could traverse a fine metal sieve
+without being affected, unless it were found that such small particles
+or atoms cannot be acted upon directly at measurable distances. In
+regard to the speed of the projected atoms, Lord Kelvin has recently
+estimated it at about one kilometre a second or thereabouts in an
+ordinary Crookes bulb. As the potentials obtainable with a disruptive
+discharge coil are much higher than with ordinary coils, the speeds
+must, of course, be much greater when the bulbs are lighted from such a
+coil. Assuming the speed to be as high as five kilometres and uniform
+through the whole trajectory, as it should be in a very highly exhausted
+vessel, then if the alternate electrifications of the electrode would be
+of a frequency of five million, the greatest distance a particle could
+get away from the electrode would be one millimetre, and if it could be
+acted upon directly at that distance, the exchange of electrode matter
+or of the atoms would be very slow and there would be practically no
+bombardment against the bulb. This at least should be so, if the action
+of an electrode upon the atoms of the residual gas would be such as upon
+electrified bodies which we can perceive. A hot body enclosed in an
+exhausted bulb produces always atomic bombardment, but a hot body has no
+definite rhythm, for its molecules perform vibrations of all kinds.
+
+If a bulb containing a button or filament be exhausted as high as is
+possible with the greatest care and by the use of the best artifices, it
+is often observed that the discharge cannot, at first, break through,
+but after some time, probably in consequence of some changes within the
+bulb, the discharge finally passes through and the button is rendered
+incandescent. In fact, it appears that the higher the degree of
+exhaustion the easier is the incandescence produced. There seem to be no
+other causes to which the incandescence might be attributed in such case
+except to the bombardment or similar action of the residual gas, or of
+particles of matter in general. But if the bulb be exhausted with the
+greatest care can these play an important part? Assume the vacuum in the
+bulb to be tolerably perfect, the great interest then centres in the
+question: Is the medium which pervades all space continuous or atomic?
+If atomic, then the heating of a conducting button or filament in an
+exhausted vessel might be due largely to ether bombardment, and then the
+heating of a conductor in general through which currents of high
+frequency or high potential are passed must be modified by the behavior
+of such medium; then also the skin effect, the apparent increase of the
+ohmic resistance, etc., admit, partially at least, of a different
+explanation.
+
+It is certainly more in accordance with many phenomena observed with
+high-frequency currents to hold that all space is pervaded with free
+atoms, rather than to assume that it is devoid of these, and dark and
+cold, for so it must be, if filled with a continuous medium, since in
+such there can be neither heat nor light. Is then energy transmitted by
+independent carriers or by the vibration of a continuous medium? This
+important question is by no means as yet positively answered. But most
+of the effects which are here considered, especially the light effects,
+incandescence, or phosphorescence, involve the presence of free atoms
+and would be impossible without these.
+
+In regard to the incandescence of a refractory button (or filament) in
+an exhausted receiver, which has been one of the subjects of this
+investigation, the chief experiences, which may serve as a guide in
+constructing such bulbs, may be summed up as follows: 1. The button
+should be as small as possible, spherical, of a smooth or polished
+surface, and of refractory material which withstands evaporation best.
+2. The support of the button should be very thin and screened by an
+aluminum and mica sheet, as I have described on another occasion. 3. The
+exhaustion of the bulb should be as high as possible. 4. The frequency
+of the currents should be as high as practicable. 5. The currents should
+be of a harmonic rise and fall, without sudden interruptions. 6. The
+heat should be confined to the button by inclosing the same in a small
+bulb or otherwise. 7. The space between the walls of the small bulb and
+the outer globe should be highly exhausted.
+
+Most of the considerations which apply to the incandescence of a solid
+just considered may likewise be applied to phosphorescence. Indeed, in
+an exhausted vessel the phosphorescence is, as a rule, primarily excited
+by the powerful beating of the electrode stream of atoms against the
+phosphorescent body. Even in many cases, where there is no evidence of
+such a bombardment, I think that phosphorescence is excited by violent
+impacts of atoms, which are not necessarily thrown off from the
+electrode but are acted upon from the same inductively through the
+medium or through chains of other atoms. That mechanical shocks play an
+important part in exciting phosphorescence in a bulb may be seen from
+the following experiment. If a bulb, constructed as that illustrated in
+Fig. 174, be taken and exhausted with the greatest care so that the
+discharge cannot pass, the filament _f_ acts by electrostatic induction
+upon the tube _t_ and the latter is set in vibration. If the tube _o_ be
+rather wide, about an inch or so, the filament may be so powerfully
+vibrated that whenever it hits the glass tube it excites
+phosphorescence. But the phosphorescence ceases when the filament comes
+to rest. The vibration can be arrested and again started by varying the
+frequency of the currents. Now the filament has its own period of
+vibration, and if the frequency of the currents is such that there is
+resonance, it is easily set vibrating, though the potential of the
+currents be small. I have often observed that the filament in the bulb
+is destroyed by such mechanical resonance. The filament vibrates as a
+rule so rapidly that it cannot be seen and the experimenter may at first
+be mystified. When such an experiment as the one described is carefully
+performed, the potential of the currents need be extremely small, and
+for this reason I infer that the phosphorescence is then due to the
+mechanical shock of the filament against the glass, just as it is
+produced by striking a loaf of sugar with a knife. The mechanical shock
+produced by the projected atoms is easily noted when a bulb containing a
+button is grasped in the hand and the current turned on suddenly. I
+believe that a bulb could be shattered by observing the conditions of
+resonance.
+
+In the experiment before cited it is, of course, open to say, that the
+glass tube, upon coming in contact with the filament, retains a charge
+of a certain sign upon the point of contact. If now the filament again
+touches the glass at the same point while it is oppositely charged, the
+charges equalize under evolution of light. But nothing of importance
+would be gained by such an explanation. It is unquestionable that the
+initial charges given to the atoms or to the glass play some part in
+exciting phosphorescence. So, for instance, if a phosphorescent bulb be
+first excited by a high frequency coil by connecting it to one of the
+terminals of the latter and the degree of luminosity be noted, and then
+the bulb be highly charged from a Holtz machine by attaching it
+preferably to the positive terminal of the machine, it is found that
+when the bulb is again connected to the terminal of the high frequency
+coil, the phosphorescence is far more intense. On another occasion I
+have considered the possibility of some phosphorescent phenomena in
+bulbs being produced by the incandescence of an infinitesimal layer on
+the surface of the phosphorescent body. Certainly the impact of the
+atoms is powerful enough to produce intense incandescence by the
+collisions, since they bring quickly to a high temperature a body of
+considerable bulk. If any such effect exists, then the best appliance
+for producing phosphorescence in a bulb, which we know so far, is a
+disruptive discharge coil giving an enormous potential with but few
+fundamental discharges, say 25-30 per second, just enough to produce a
+continuous impression upon the eye. It is a fact that such a coil
+excites phosphorescence under almost any condition and at all degrees of
+exhaustion, and I have observed effects which appear to be due to
+phosphorescence even at ordinary pressures of the atmosphere, when the
+potentials are extremely high. But if phosphorescent light is produced
+by the equalization of charges of electrified atoms (whatever this may
+mean ultimately), then the higher the frequency of the impulses or
+alternate electrifications, the more economical will be the light
+production. It is a long known and noteworthy fact that all the
+phosphorescent bodies are poor conductors of electricity and heat, and
+that all bodies cease to emit phosphorescent light when they are brought
+to a certain temperature. Conductors on the contrary do not possess this
+quality. There are but few exceptions to the rule. Carbon is one of
+them. Becquerel noted that carbon phosphoresces at a certain elevated
+temperature preceding the dark red. This phenomenon may be easily
+observed in bulbs provided with a rather large carbon electrode (say, a
+sphere of six millimetres diameter). If the current is turned on after a
+few seconds, a snow white film covers the electrode, just before it gets
+dark red. Similar effects are noted with other conducting bodies, but
+many scientific men will probably not attribute them to true
+phosphorescence. Whether true incandescence has anything to do with
+phosphorescence excited by atomic impact or mechanical shocks still
+remains to be decided, but it is a fact that all conditions, which tend
+to localize and increase the heating effect at the point of impact, are
+almost invariably the most favorable for the production of
+phosphorescence. So, if the electrode be very small, which is equivalent
+to saying in general, that the electric density is great; if the
+potential be high, and if the gas be highly rarefied, all of which
+things imply high speed of the projected atoms, or matter, and
+consequently violent impacts--the phosphorescence is very intense. If a
+bulb provided with a large and small electrode be attached to the
+terminal of an induction coil, the small electrode excites
+phosphorescence while the large one may not do so, because of the
+smaller electric density and hence smaller speed of the atoms. A bulb
+provided with a large electrode may be grasped with the hand while the
+electrode is connected to the terminal of the coil and it may not
+phosphoresce; but if instead of grasping the bulb with the hand, the
+same be touched with a pointed wire, the phosphorescence at once
+spreads through the bulb, because of the great density at the point of
+contact. With low frequencies it seems that gases of great atomic weight
+excite more intense phosphorescence than those of smaller weight, as for
+instance, hydrogen. With high frequencies the observations are not
+sufficiently reliable to draw a conclusion. Oxygen, as is well-known,
+produces exceptionally strong effects, which may be in part due to
+chemical action. A bulb with hydrogen residue seems to be most easily
+excited. Electrodes which are most easily deteriorated produce more
+intense phosphorescence in bulbs, but the condition is not permanent
+because of the impairment of the vacuum and the deposition of the
+electrode matter upon the phosphorescent surfaces. Some liquids, as
+oils, for instance, produce magnificent effects of phosphorescence (or
+fluorescence?), but they last only a few seconds. So if a bulb has a
+trace of oil on the walls and the current is turned on, the
+phosphorescence only persists for a few moments until the oil is carried
+away. Of all bodies so far tried, sulphide of zinc seems to be the most
+susceptible to phosphorescence. Some samples, obtained through the
+kindness of Prof. Henry in Paris, were employed in many of these bulbs.
+One of the defects of this sulphide is, that it loses its quality of
+emitting light when brought to a temperature which is by no means high.
+It can therefore, be used only for feeble intensities. An observation
+which might deserve notice is, that when violently bombarded from an
+aluminum electrode it assumes a black color, but singularly enough, it
+returns to the original condition when it cools down.
+
+The most important fact arrived at in pursuing investigations in this
+direction is, that in all cases it is necessary, in order to excite
+phosphorescence with a minimum amount of energy, to observe certain
+conditions. Namely, there is always, no matter what the frequency of the
+currents, degree of exhaustion and character of the bodies in the bulb,
+a certain potential (assuming the bulb excited from one terminal) or
+potential difference (assuming the bulb to be excited with both
+terminals) which produces the most economical result. If the potential
+be increased, considerable energy may be wasted without producing any
+more light, and if it be diminished, then again the light production is
+not as economical. The exact condition under which the best result is
+obtained seems to depend on many things of a different nature, and it is
+to be yet investigated by other experimenters, but it will certainly
+have to be observed when such phosphorescent bulbs are operated, if the
+best results are to be obtained.
+
+Coming now to the most interesting of these phenomena, the incandescence
+or phosphorescence of gases, at low pressures or at the ordinary
+pressure of the atmosphere, we must seek the explanation of these
+phenomena in the same primary causes, that is, in shocks or impacts of
+the atoms. Just as molecules or atoms beating upon a solid body excite
+phosphorescence in the same or render it incandescent, so when colliding
+among themselves they produce similar phenomena. But this is a very
+insufficient explanation and concerns only the crude mechanism. Light is
+produced by vibrations which go on at a rate almost inconceivable. If we
+compute, from the energy contained in the form of known radiations in a
+definite space the force which is necessary to set up such rapid
+vibrations, we find, that though the density of the ether be
+incomparably smaller than that of any body we know, even hydrogen, the
+force is something surpassing comprehension. What is this force, which
+in mechanical measure may amount to thousands of tons per square inch?
+It is electrostatic force in the light of modern views. It is impossible
+to conceive how a body of measurable dimensions could be charged to so
+high a potential that the force would be sufficient to produce these
+vibrations. Long before any such charge could be imparted to the body it
+would be shattered into atoms. The sun emits light and heat, and so does
+an ordinary flame or incandescent filament, but in neither of these can
+the force be accounted for if it be assumed that it is associated with
+the body as a whole. Only in one way may we account for it, namely, by
+identifying it with the atom. An atom is so small, that if it be charged
+by coming in contact with an electrified body and the charge be assumed
+to follow the same law as in the case of bodies of measurable
+dimensions, it must retain a quantity of electricity which is fully
+capable of accounting for these forces and tremendous rates of
+vibration. But the atom behaves singularly in this respect--it always
+takes the same "charge."
+
+It is very likely that resonant vibration plays a most important part in
+all manifestations of energy in nature. Throughout space all matter is
+vibrating, and all rates of vibration are represented, from the lowest
+musical note to the highest pitch of the chemical rays, hence an atom,
+or complex of atoms, no matter what its period, must find a vibration
+with which it is in resonance. When we consider the enormous rapidity
+of the light vibrations, we realize the impossibility of producing such
+vibrations directly with any apparatus of measurable dimensions, and we
+are driven to the only possible means of attaining the object of setting
+up waves of light by electrical means and economically, that is, to
+affect the molecules or atoms of a gas, to cause them to collide and
+vibrate. We then must ask ourselves--How can free molecules or atoms be
+affected?
+
+[Illustration: FIG. 196.]
+
+[Illustration: FIG. 197.]
+
+It is a fact that they can be affected by electrostatic force, as is
+apparent in many of these experiments. By varying the electrostatic
+force we can agitate the atoms, and cause them to collide accompanied by
+evolution of heat and light. It is not demonstrated beyond doubt that we
+can affect them otherwise. If a luminous discharge is produced in a
+closed exhausted tube, do the atoms arrange themselves in obedience to
+any other but to electrostatic force acting in straight lines from atom
+to atom? Only recently I investigated the mutual action between two
+circuits with extreme rates of vibration. When a battery of a few jars
+(_c c c c_, Fig. 196) is discharged through a primary P of low
+resistance (the connections being as illustrated in Figs. 183_a_, 183_b_
+and 183_c_), and the frequency of vibration is many millions there are
+great differences of potential between points on the primary not more
+than a few inches apart. These differences may be 10,000 volts per inch,
+if not more, taking the maximum value of the E. M. F. The secondary _s_
+is therefore acted upon by electrostatic induction, which is in such
+extreme cases of much greater importance than the electro-dynamic. To
+such sudden impulses the primary as well as the secondary are poor
+conductors, and therefore great differences of potential may be produced
+by electrostatic induction between adjacent points on the secondary.
+Then sparks may jump between the wires and streamers become visible in
+the dark if the light of the discharge through the spark gap _d d_ be
+carefully excluded. If now we substitute a closed vacuum tube for the
+metallic secondary _s_, the differences of potential produced in the
+tube by electrostatic induction from the primary are fully sufficient to
+excite portions of it; but as the points of certain differences of
+potential on the primary are not fixed, but are generally constantly
+changing in position, a luminous band is produced in the tube,
+apparently not touching the glass, as it should, if the points of
+maximum and minimum differences of potential were fixed on the primary.
+I do not exclude the possibility of such a tube being excited only by
+electro-dynamic induction, for very able physicists hold this view; but
+in my opinion, there is as yet no positive proof given that atoms of a
+gas in a closed tube may arrange themselves in chains under the action
+of an electromotive impulse produced by electro-dynamic induction in the
+tube. I have been unable so far to produce striae in a tube, however
+long, and at whatever degree of exhaustion, that is, striae at right
+angles to the supposed direction of the discharge or the axis of the
+tube; but I have distinctly observed in a large bulb, in which a wide
+luminous band was produced by passing a discharge of a battery through a
+wire surrounding the bulb, a circle of feeble luminosity between two
+luminous bands, one of which was more intense than the other.
+Furthermore, with my present experience I do not think that such a gas
+discharge in a closed tube can vibrate, that is, vibrate as a whole. I
+am convinced that no discharge through a gas can vibrate. The atoms of a
+gas behave very curiously in respect to sudden electric impulses. The
+gas does not seem to possess any appreciable inertia to such impulses,
+for it is a fact, that the higher the frequency of the impulses, with
+the greater freedom does the discharge pass through the gas. If the gas
+possesses no inertia then it cannot vibrate, for some inertia is
+necessary for the free vibration. I conclude from this that if a
+lightning discharge occurs between two clouds, there can be no
+oscillation, such as would be expected, considering the capacity of the
+clouds. But if the lightning discharge strike the earth, there is always
+vibration--in the earth, but not in the cloud. In a gas discharge each
+atom vibrates at its own rate, but there is no vibration of the
+conducting gaseous mass as a whole. This is an important consideration
+in the great problem of producing light economically, for it teaches us
+that to reach this result we must use impulses of very high frequency
+and necessarily also of high potential. It is a fact that oxygen
+produces a more intense light in a tube. Is it because oxygen atoms
+possess some inertia and the vibration does not die out instantly? But
+then nitrogen should be as good, and chlorine and vapors of many other
+bodies much better than oxygen, unless the magnetic properties of the
+latter enter prominently into play. Or, is the process in the tube of an
+electrolytic nature? Many observations certainly speak for it, the most
+important being that matter is always carried away from the electrodes
+and the vacuum in a bulb cannot be permanently maintained. If such
+process takes place in reality, then again must we take refuge in high
+frequencies, for, with such, electrolytic action should be reduced to a
+minimum, if not rendered entirely impossible. It is an undeniable fact
+that with very high frequencies, provided the impulses be of harmonic
+nature, like those obtained from an alternator, there is less
+deterioration and the vacua are more permanent. With disruptive
+discharge coils there are sudden rises of potential and the vacua are
+more quickly impaired, for the electrodes are deteriorated in a very
+short time. It was observed in some large tubes, which were provided
+with heavy carbon blocks B B_{1}, connected to platinum wires w w_{1}
+(as illustrated in Fig. 197), and which were employed in experiments
+with the disruptive discharge instead of the ordinary air gap, that the
+carbon particles under the action of the powerful magnetic field in
+which the tube was placed, were deposited in regular fine lines in the
+middle of the tube, as illustrated. These lines were attributed to the
+deflection or distortion of the discharge by the magnetic field, but why
+the deposit occurred principally where the field was most intense did
+not appear quite clear. A fact of interest, likewise noted, was that the
+presence of a strong magnetic field increases the deterioration of the
+electrodes, probably by reason of the rapid interruptions it produces,
+whereby there is actually a higher E. M. F. maintained between the
+electrodes.
+
+Much would remain to be said about the luminous effects produced in
+gases at low or ordinary pressures. With the present experiences before
+us we cannot say that the essential nature of these charming phenomena
+is sufficiently known. But investigations in this direction are being
+pushed with exceptional ardor. Every line of scientific pursuit has its
+fascinations, but electrical investigation appears to possess a
+peculiar attraction, for there is no experiment or observation of any
+kind in the domain of this wonderful science which would not forcibly
+appeal to us. Yet to me it seems, that of all the many marvelous things
+we observe, a vacuum tube, excited by an electric impulse from a distant
+source, bursting forth out of the darkness and illuminating the room
+with its beautiful light, is as lovely a phenomenon as can greet our
+eyes. More interesting still it appears when, reducing the fundamental
+discharges across the gap to a very small number and waving the tube
+about we produce all kinds of designs in luminous lines. So by way of
+amusement I take a straight long tube, or a square one, or a square
+attached to a straight tube, and by whirling them about in the hand, I
+imitate the spokes of a wheel, a Gramme winding, a drum winding, an
+alternate current motor winding, etc. (Fig. 198). Viewed from a distance
+the effect is weak and much of its beauty is lost, but being near or
+holding the tube in the hand, one cannot resist its charm.
+
+[Illustration: FIG. 198.]
+
+In presenting these insignificant results I have not attempted to
+arrange and co-ordinate them, as would be proper in a strictly
+scientific investigation, in which every succeeding result should be a
+logical sequence of the preceding, so that it might be guessed in
+advance by the careful reader or attentive listener. I have preferred to
+concentrate my energies chiefly upon advancing novel facts or ideas
+which might serve as suggestions to others, and this may serve as an
+excuse for the lack of harmony. The explanations of the phenomena have
+been given in good faith and in the spirit of a student prepared to find
+that they admit of a better interpretation. There can be no great harm
+in a student taking an erroneous view, but when great minds err, the
+world must dearly pay for their mistakes.
+
+
+
+
+CHAPTER XXIX.
+
+TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FREQUENCY, IN DETAIL.
+
+
+It has become a common practice to operate arc lamps by alternating or
+pulsating, as distinguished from continuous, currents; but an objection
+which has been raised to such systems exists in the fact that the arcs
+emit a pronounced sound, varying with the rate of the alternations or
+pulsations of current. This noise is due to the rapidly alternating
+heating and cooling, and consequent expansion and contraction, of the
+gaseous matter forming the arc, which corresponds with the periods or
+impulses of the current. Another disadvantageous feature is found in the
+difficulty of maintaining an alternating current arc in consequence of
+the periodical increase in resistance corresponding to the periodical
+working of the current. This feature entails a further disadvantage,
+namely, that small arcs are impracticable.
+
+Theoretical considerations have led Mr. Tesla to the belief that these
+disadvantageous features could be obviated by employing currents of a
+sufficiently high number of alternations, and his anticipations have
+been confirmed in practice. These rapidly alternating currents render it
+possible to maintain small arcs which, besides, possess the advantages
+of silence and persistency. The latter quality is due to the necessarily
+rapid alternations, in consequence of which the arc has no time to cool,
+and is always maintained at a high temperature and low resistance.
+
+At the outset of his experiments Mr. Tesla encountered great
+difficulties in the construction of high frequency machines. A generator
+of this kind is described here, which, though constructed quite some
+time ago, is well worthy of a detailed description. It may be mentioned,
+in passing, that dynamos of this type have been used by Mr. Tesla in his
+lighting researches and experiments with currents of high potential and
+high frequency, and reference to them will be found in his lectures
+elsewhere printed in this volume.[4]
+
+  [4] See pages 153-4 5.
+
+In the accompanying engravings, Figs. 199 and 200 show the machine,
+respectively, in side elevation and vertical cross-section; Figs. 201,
+202 and 203 showing enlarged details of construction. As will be seen, A
+is an annular magnetic frame, the interior of which is provided with a
+large number of pole-pieces D.
+
+Owing to the very large number and small size of the poles and the
+spaces between them, the field coils are applied by winding an insulated
+conductor F zigzag through the grooves, as shown in Fig. 203, carrying
+the wire around the annulus to form as many layers as is desired. In
+this way the pole-pieces D will be energized with alternately opposite
+polarity around the entire ring.
+
+For the armature, Mr. Tesla employs a spider carrying a ring J, turned
+down, except at its edges, to form a trough-like receptacle for a mass
+of fine annealed iron wires K, which are wound in the groove to form the
+core proper for the armature-coils. Pins L are set in the sides of the
+ring J and the coils M are wound over the periphery of the
+armature-structure and around the pins. The coils M are connected
+together in series, and these terminals N carried through the hollow
+shaft H to contact-rings P P, from which the currents are taken off by
+brushes O.
+
+[Illustration: FIG. 199.]
+
+In this way a machine with a very large number of poles may be
+constructed. It is easy, for instance, to obtain in this manner three
+hundred and seventy-five to four hundred poles in a machine that may be
+safely driven at a speed of fifteen hundred or sixteen hundred
+revolutions per minute, which will produce ten thousand or eleven
+thousand alternations of current per second. Arc lamps R R are shown in
+the diagram as connected up in series with the machine in Fig. 200. If
+such a current be applied to running arc lamps, the sound produced by or
+in the arc becomes practically inaudible, for, by increasing the rate of
+change in the current, and consequently the number of vibrations per
+unit of time of the gaseous material of the arc up to, or beyond, ten
+thousand or eleven thousand per second, or to what is regarded as the
+limit of audition, the sound due to such vibrations will not be audible.
+The exact number of changes or undulations necessary to produce this
+result will vary somewhat according to the size of the arc--that is to
+say, the smaller the arc, the greater the number of changes that will be
+required to render it inaudible within certain limits. It should also be
+stated that the arc should not exceed a certain length.
+
+[Illustration: FIGS. 200, 201, 202 and 203.]
+
+The difficulties encountered in the construction of these machines are
+of a mechanical as well as an electrical nature. The machines may be
+designed on two plans: the field may be formed either of alternating
+poles, or of polar projections of the same polarity. Up to about 15,000
+alternations per second in an experimental machine, the former plan may
+be followed, but a more efficient machine is obtained on the second
+plan.
+
+In the machine above described, which was capable of running two arcs of
+normal candle power, the field was composed of a ring of wrought iron
+32 inches outside diameter, and about 1 inch thick. The inside diameter
+was 30 inches. There were 384 polar projections. The wire was wound in
+zigzag form, but two wires were wound so as to completely envelop the
+projections. The distance between the projections is about 3/16 inch,
+and they are a little over 1/16 inch thick. The field magnet was made
+relatively small so as to adapt the machine for a constant current.
+There are 384 coils connected in two series. It was found impracticable
+to use any wire much thicker than No. 26 B. and S. gauge on account of
+the local effects. In such a machine the clearance should be as small as
+possible; for this reason the machine was made only 1-1/4 inch wide, so
+that the binding wires might be obviated. The armature wires must be
+wound with great care, as they are apt to fly off in consequence of the
+great peripheral speed. In various experiments this machine has been run
+as high as 3,000 revolutions per minute. Owing to the great speed it was
+possible to obtain as high as 10 amperes out of the machine. The
+electromotive force was regulated by means of an adjustable condenser
+within very wide limits, the limits being the greater, the greater the
+speed. This machine was frequently used to run Mr. Tesla's laboratory
+lights.
+
+[Illustration: FIG. 204.]
+
+The machine above described was only one of many such types constructed.
+It serves well for an experimental machine, but if still higher
+alternations are required and higher efficiency is necessary, then a
+machine on a plan shown in Figs. 204 to 207, is preferable. The
+principal advantage of this type of machine is that there is not much
+magnetic leakage, and that a field may be produced, varying greatly in
+intensity in places not much distant from each other.
+
+In these engravings, Figs. 204 and 205 illustrate a machine in which the
+armature conductor and field coils are stationary, while the field
+magnet core revolves. Fig. 206 shows a machine embodying the same plan
+of construction, but having a stationary field magnet and rotary
+armature.
+
+The conductor in which the currents are induced may be arranged in
+various ways; but Mr. Tesla prefers the following method: He employs an
+annular plate of copper D, and by means of a saw cuts in it radial slots
+from one edge nearly through to the other, beginning alternately from
+opposite edges. In this way a continuous zigzag conductor is formed.
+When the polar projections are 1/8 inch wide, the width of the conductor
+should not, under any circumstances, be more than 1/32 inch wide; even
+then the eddy effect is considerable.
+
+[Illustration: FIG. 205.]
+
+To the inner edge of this plate are secured two rings of non-magnetic
+metal E, which are insulated from the copper conductor, but held firmly
+thereto by means of the bolts F. Within the rings E is then placed an
+annular coil G, which is the energizing coil for the field magnet. The
+conductor D and the parts attached thereto are supported by means of the
+cylindrical shell or casting A A, the two parts of which are brought
+together and clamped to the outer edge of the conductor D.
+
+[Illustration: FIG. 206.]
+
+The core for the field magnet is built up of two circular parts H H,
+formed with annular grooves I, which, when the two parts are brought
+together, form a space for the reception of the energizing coil G. The
+hubs of the cores are trued off, so as to fit closely against one
+another, while the outer portions or flanges which form the polar faces
+J J, are reduced somewhat in thickness to make room for the conductor D,
+and are serrated on their faces. The number of serrations in the polar
+faces is arbitrary; but there must exist between them and the radial
+portions of the conductor D certain relation, which will be understood
+by reference to Fig. 207 in which N N represent the projections or
+points on one face of the core of the field, and S S the points of the
+other face. The conductor D is shown in this figure in section _a a'_
+designating the radial portions of the conductor, and _b_ the insulating
+divisions between them. The relative width of the parts _a a'_ and the
+space between any two adjacent points N N or S S is such that when the
+radial portions _a_ of the conductor are passing between the opposite
+points N S where the field is strongest, the intermediate radial
+portions _a'_ are passing through the widest spaces midway between such
+points and where the field is weakest. Since the core on one side is of
+opposite polarity to the part facing it, all the projections of one
+polar face will be of opposite polarity to those of the other face.
+Hence, although the space between any two adjacent points on the same
+face may be extremely small, there will be no leakage of the magnetic
+lines between any two points of the same name, but the lines of force
+will pass across from one set of points to the other. The construction
+followed obviates to a great degree the distortion of the magnetic lines
+by the action of the current in the conductor D, in which it will be
+observed the current is flowing at any given time from the centre toward
+the periphery in one set of radial parts _a_ and in the opposite
+direction in the adjacent parts _a'_.
+
+In order to connect the energizing coil G, Fig. 204, with a source of
+continuous current, Mr. Tesla utilizes two adjacent radial portions of
+the conductor D for connecting the terminals of the coil G with two
+binding posts M. For this purpose the plate D is cut entirely through,
+as shown, and the break thus made is bridged over by a short conductor
+C. The plate D is cut through to form two terminals _d_, which are
+connected to binding posts N. The core H H, when rotated by the driving
+pulley, generates in the conductors D an alternating current, which is
+taken off from the binding posts N.
+
+[Illustration: FIG. 207.]
+
+When it is desired to rotate the conductor between the faces of a
+stationary field magnet, the construction shown in Fig. 206, is adopted.
+The conductor D in this case is or may be made in substantially the same
+manner as above described by slotting an annular conducting-plate and
+supporting it between two heads O, held together by bolts _o_ and fixed
+to the driving-shaft K. The inner edge of the plate or conductor D is
+preferably flanged to secure a firmer union between it and the heads O.
+It is insulated from the head. The field-magnet in this case consists of
+two annular parts H H, provided with annular grooves I for the reception
+of the coils. The flanges or faces surrounding the annular groove are
+brought together, while the inner flanges are serrated, as in the
+previous case, and form the polar faces. The two parts H H are formed
+with a base R, upon which the machine rests. S S are non-magnetic
+bushings secured or set in the central opening of the cores. The
+conductor D is cut entirely through at one point to form terminals, from
+which insulated conductors T are led through the shaft to
+collecting-rings V.
+
+In one type of machine of this kind constructed by Mr. Tesla, the field
+had 480 polar projections on each side, and from this machine it was
+possible to obtain 30,000 alternations per second. As the polar
+projections must necessarily be very narrow, very thin wires or sheets
+must be used to avoid the eddy current effects. Mr. Tesla has thus
+constructed machines with a stationary armature and rotating field, in
+which case also the field-coil was supported so that the revolving part
+consisted only of a wrought iron body devoid of any wire and also
+machines with a rotating armature and stationary field. The machines may
+be either drum or disc, but Mr. Tesla's experience shows the latter to
+be preferable.
+
+       *       *       *       *       *
+
+In the course of a very interesting article contributed to the
+_Electrical World_ in February, 1891, Mr. Tesla makes some suggestive
+remarks on these high frequency machines and his experiences with them,
+as well as with other parts of the high frequency apparatus. Part of it
+is quoted here and is as follows:--
+
+The writer will incidentally mention that any one who attempts for the
+first time to construct such a machine will have a tale of woe to tell.
+He will first start out, as a matter of course, by making an armature
+with the required number of polar projections. He will then get the
+satisfaction of having produced an apparatus which is fit to accompany a
+thoroughly Wagnerian opera. It may besides possess the virtue of
+converting mechanical energy into heat in a nearly perfect manner. If
+there is a reversal in the polarity of the projections, he will get heat
+out of the machine; if there is no reversal, the heating will be less,
+but the output will be next to nothing. He will then abandon the iron in
+the armature, and he will get from the Scylla to the Charybdis. He will
+look for one difficulty and will find another, but, after a few trials,
+he may get nearly what he wanted.
+
+Among the many experiments which may be performed with such a machine,
+of not the least interest are those performed with a high-tension
+induction coil. The character of the discharge is completely changed.
+The arc is established at much greater distances, and it is so easily
+affected by the slightest current of air that it often wriggles around
+in the most singular manner. It usually emits the rhythmical sound
+peculiar to the alternate current arcs, but the curious point is that
+the sound may be heard with a number of alternations far above ten
+thousand per second, which by many is considered to be about the limit
+of audition. In many respects the coil behaves like a static machine.
+Points impair considerably the sparking interval, electricity escaping
+from them freely, and from a wire attached to one of the terminals
+streams of light issue, as though it were connected to a pole of a
+powerful Toepler machine. All these phenomena are, of course, mostly due
+to the enormous differences of potential obtained. As a consequence of
+the self-induction of the coil and the high frequency, the current is
+minute while there is a corresponding rise of pressure. A current
+impulse of some strength started in such a coil should persist to flow
+no less than four ten-thousandths of a second. As this time is greater
+than half the period, it occurs that an opposing electromotive force
+begins to act while the current is still flowing. As a consequence, the
+pressure rises as in a tube filled with liquid and vibrated rapidly
+around its axis. The current is so small that, in the opinion and
+involuntary experience of the writer, the discharge of even a very large
+coil cannot produce seriously injurious effects, whereas, if the same
+coil were operated with a current of lower frequency, though the
+electromotive force would be much smaller, the discharge would be most
+certainly injurious. This result, however, is due in part to the high
+frequency. The writer's experiences tend to show that the higher the
+frequency the greater the amount of electrical energy which may be
+passed through the body without serious discomfort; whence it seems
+certain that human tissues act as condensers.
+
+One is not quite prepared for the behavior of the coil when connected to
+a Leyden jar. One, of course, anticipates that since the frequency is
+high the capacity of the jar should be small. He therefore takes a very
+small jar, about the size of a small wine glass, but he finds that even
+with this jar the coil is practically short-circuited. He then reduces
+the capacity until he comes to about the capacity of two spheres, say,
+ten centimetres in diameter and two to four centimetres apart. The
+discharge then assumes the form of a serrated band exactly like a
+succession of sparks viewed in a rapidly revolving mirror; the
+serrations, of course, corresponding to the condenser discharges. In
+this case one may observe a queer phenomenon. The discharge starts at
+the nearest points, works gradually up, breaks somewhere near the top of
+the spheres, begins again at the bottom, and so on. This goes on so fast
+that several serrated bands are seen at once. One may be puzzled for a
+few minutes, but the explanation is simple enough. The discharge begins
+at the nearest points, the air is heated and carries the arc upward
+until it breaks, when it is re-established at the nearest points, etc.
+Since the current passes easily through a condenser of even small
+capacity, it will be found quite natural that connecting only one
+terminal to a body of the same size, no matter how well insulated,
+impairs considerably the striking distance of the arc.
+
+Experiments with Geissler tubes are of special interest. An exhausted
+tube, devoid of electrodes of any kind, will light up at some distance
+from the coil. If a tube from a vacuum pump is near the coil the whole
+of the pump is brilliantly lighted. An incandescent lamp approached to
+the coil lights up and gets perceptibly hot. If a lamp have the
+terminals connected to one of the binding posts of the coil and the hand
+is approached to the bulb, a very curious and rather unpleasant
+discharge from the glass to the hand takes place, and the filament may
+become incandescent. The discharge resembles to some extent the stream
+issuing from the plates of a powerful Toepler machine, but is of
+incomparably greater quantity. The lamp in this case acts as a
+condenser, the rarefied gas being one coating, the operator's hand the
+other. By taking the globe of a lamp in the hand, and by bringing the
+metallic terminals near to or in contact with a conductor connected to
+the coil, the carbon is brought to bright incandescence and the glass is
+rapidly heated. With a 100-volt 10 C. P. lamp one may without great
+discomfort stand as much current as will bring the lamp to a
+considerable brilliancy; but it can be held in the hand only for a few
+minutes, as the glass is heated in an incredibly short time. When a tube
+is lighted by bringing it near to the coil it may be made to go out by
+interposing a metal plate on the hand between the coil and tube; but if
+the metal plate be fastened to a glass rod or otherwise insulated, the
+tube may remain lighted if the plate be interposed, or may even
+increase in luminosity. The effect depends on the position of the plate
+and tube relatively to the coil, and may be always easily foretold by
+_assuming_ that conduction takes place from one terminal of the coil to
+the other. According to the position of the plate, it may either divert
+from or direct the current to the tube.
+
+In another line of work the writer has in frequent experiments
+maintained incandescent lamps of 50 or 100 volts burning at any desired
+candle power with both the terminals of each lamp connected to a stout
+copper wire of no more than a few feet in length. These experiments seem
+interesting enough, but they are not more so than the queer experiment
+of Faraday, which has been revived and made much of by recent
+investigators, and in which a discharge is made to jump between two
+points of a bent copper wire. An experiment may be cited here which may
+seem equally interesting. If a Geissler tube, the terminals of which are
+joined by a copper wire, be approached to the coil, certainly no one
+would be prepared to see the tube light up. Curiously enough, it does
+light up, and, what is more, the wire does not seem to make much
+difference. Now one is apt to think in the first moment that the
+impedance of the wire might have something to do with the phenomenon.
+But this is of course immediately rejected, as for this an enormous
+frequency would be required. This result, however, seems puzzling only
+at first; for upon reflection it is quite clear that the wire can make
+but little difference. It may be explained in more than one way, but it
+agrees perhaps best with observation to assume that conduction takes
+place from the terminals of the coil through the space. On this
+assumption, if the tube with the wire be held in any position, the wire
+can divert little more than the current which passes through the space
+occupied by the wire and the metallic terminals of the tube; through the
+adjacent space the current passes practically undisturbed. For this
+reason, if the tube be held in any position at right angles to the line
+joining the binding posts of the coil, the wire makes hardly any
+difference, but in a position more or less parallel with that line it
+impairs to a certain extent the brilliancy of the tube and its facility
+to light up. Numerous other phenomena may be explained on the same
+assumption. For instance, if the ends of the tube be provided with
+washers of sufficient size and held in the line joining the terminals of
+the coil, it will not light up, and then nearly the whole of the
+current, which would otherwise pass uniformly through the space between
+the washers, is diverted through the wire. But if the tube be inclined
+sufficiently to that line, it will light up in spite of the washers.
+Also, if a metal plate be fastened upon a glass rod and held at right
+angles to the line joining the binding posts, and nearer to one of them,
+a tube held more or less parallel with the line will light up instantly
+when one of the terminals touches the plate, and will go out when
+separated from the plate. The greater the surface of the plate, up to a
+certain limit, the easier the tube will light up. When a tube is placed
+at right angles to the straight line joining the binding posts, and then
+rotated, its luminosity steadily increases until it is parallel with
+that line. The writer must state, however, that he does not favor the
+idea of a leakage or current through the space any more than as a
+suitable explanation, for he is convinced that all these experiments
+could not be performed with a static machine yielding a constant
+difference of potential, and that condenser action is largely concerned
+in these phenomena.
+
+It is well to take certain precautions when operating a Ruhmkorff coil
+with very rapidly alternating currents. The primary current should not
+be turned on too long, else the core may get so hot as to melt the
+gutta-percha or paraffin, or otherwise injure the insulation, and this
+may occur in a surprisingly short time, considering the current's
+strength. The primary current being turned on, the fine wire terminals
+may be joined without great risk, the impedance being so great that it
+is difficult to force enough current through the fine wire so as to
+injure it, and in fact the coil may be on the whole much safer when the
+terminals of the fine wire are connected than when they are insulated;
+but special care should be taken when the terminals are connected to the
+coatings of a Leyden jar, for with anywhere near the critical capacity,
+which just counteracts the self-induction at the existing frequency, the
+coil might meet the fate of St. Polycarpus. If an expensive vacuum pump
+is lighted up by being near to the coil or touched with a wire connected
+to one of the terminals, the current should be left on no more than a
+few moments, else the glass will be cracked by the heating of the
+rarefied gas in one of the narrow passages--in the writer's own
+experience _quod erat demonstrandum_.[5]
+
+  [5] It is thought necessary to remark that, although the induction
+      coil may give quite a good result when operated with such
+      rapidly alternating currents, yet its construction, quite
+      irrespective of the iron core, makes it very unfit for such
+      high frequencies, and to obtain the best results the
+      construction should be greatly modified.
+
+There are a good many other points of interest which may be observed in
+connection with such a machine. Experiments with the telephone, a
+conductor in a strong field or with a condenser or arc, seem to afford
+certain proof that sounds far above the usual accepted limit of hearing
+would be perceived. A telephone will emit notes of twelve to thirteen
+thousand vibrations per second; then the inability of the core to follow
+such rapid alternations begins to tell. If, however, the magnet and core
+be replaced by a condenser and the terminals connected to the
+high-tension secondary of a transformer, higher notes may still be
+heard. If the current be sent around a finely laminated core and a small
+piece of thin sheet iron be held gently against the core, a sound may be
+still heard with thirteen to fourteen thousand alternations per second,
+provided the current is sufficiently strong. A small coil, however,
+tightly packed between the poles of a powerful magnet, will emit a sound
+with the above number of alternations, and arcs may be audible with a
+still higher frequency. The limit of audition is variously estimated. In
+Sir William Thomson's writings it is stated somewhere that ten thousand
+per second, or nearly so, is the limit. Other, but less reliable,
+sources give it as high as twenty-four thousand per second. The above
+experiments have convinced the writer that notes of an incomparably
+higher number of vibrations per second would be perceived provided they
+could be produced with sufficient power. There is no reason why it
+should not be so. The condensations and rarefactions of the air would
+necessarily set the diaphragm in a corresponding vibration and some
+sensation would be produced, whatever--within certain limits--the
+velocity of transmission to their nerve centres, though it is probable
+that for want of exercise the ear would not be able to distinguish any
+such high note. With the eye it is different; if the sense of vision is
+based upon some resonance effect, as many believe, no amount of increase
+in the intensity of the ethereal vibration could extend our range of
+vision on either side of the visible spectrum.
+
+The limit of audition of an arc depends on its size. The greater the
+surface by a given heating effect in the arc, the higher the limit of
+audition. The highest notes are emitted by the high-tension discharges
+of an induction coil in which the arc is, so to speak, all surface. If
+_R_ be the resistance of an arc, and _C_ the current, and the linear
+dimensions be _n_ times increased, then the resistance is _R_/_n_, and
+with the same current density the current would be _n_^2_C_; hence the
+heating effect is _n_^3 times greater, while the surface is only _n_^2
+times as great. For this reason very large arcs would not emit any
+rhythmical sound even with a very low frequency. It must be observed,
+however, that the sound emitted depends to some extent also on the
+composition of the carbon. If the carbon contain highly refractory
+material, this, when heated, tends to maintain the temperature of the
+arc uniform and the sound is lessened; for this reason it would seem
+that an alternating arc requires such carbons.
+
+With currents of such high frequencies it is possible to obtain
+noiseless arcs, but the regulation of the lamp is rendered extremely
+difficult on account of the excessively small attractions or repulsions
+between conductors conveying these currents.
+
+An interesting feature of the arc produced by these rapidly alternating
+currents is its persistency. There are two causes for it, one of which
+is always present, the other sometimes only. One is due to the character
+of the current and the other to a property of the machine. The first
+cause is the more important one, and is due directly to the rapidity of
+the alternations. When an arc is formed by a periodically undulating
+current, there is a corresponding undulation in the temperature of the
+gaseous column, and, therefore, a corresponding undulation in the
+resistance of the arc. But the resistance of the arc varies enormously
+with the temperature of the gaseous column, being practically infinite
+when the gas between the electrodes is cold. The persistence of the arc,
+therefore, depends on the inability of the column to cool. It is for
+this reason impossible to maintain an arc with the current alternating
+only a few times a second. On the other hand, with a practically
+continuous current, the arc is easily maintained, the column being
+constantly kept at a high temperature and low resistance. The higher the
+frequency the smaller the time interval during which the arc may cool
+and increase considerably in resistance. With a frequency of 10,000 per
+second or more in an arc of equal size excessively small variations of
+temperature are superimposed upon a steady temperature, like ripples on
+the surface of a deep sea. The heating effect is practically continuous
+and the arc behaves like one produced by a continuous current, with the
+exception, however, that it may not be quite as easily started, and that
+the electrodes are equally consumed; though the writer has observed
+some irregularities in this respect.
+
+The second cause alluded to, which possibly may not be present, is due
+to the tendency of a machine of such high frequency to maintain a
+practically constant current. When the arc is lengthened, the
+electromotive force rises in proportion and the arc appears to be more
+persistent.
+
+Such a machine is eminently adapted to maintain a constant current, but
+it is very unfit for a constant potential. As a matter of fact, in
+certain types of such machines a nearly constant current is an almost
+unavoidable result. As the number of poles or polar projections is
+greatly increased, the clearance becomes of great importance. One has
+really to do with a great number of very small machines. Then there is
+the impedance in the armature, enormously augmented by the high
+frequency. Then, again, the magnetic leakage is facilitated. If there
+are three or four hundred alternate poles, the leakage is so great that
+it is virtually the same as connecting, in a two-pole machine, the poles
+by a piece of iron. This disadvantage, it is true, may be obviated more
+or less by using a field throughout of the same polarity, but then one
+encounters difficulties of a different nature. All these things tend to
+maintain a constant current in the armature circuit.
+
+In this connection it is interesting to notice that even to-day
+engineers are astonished at the performance of a constant current
+machine, just as, some years ago, they used to consider it an
+extraordinary performance if a machine was capable of maintaining a
+constant potential difference between the terminals. Yet one result is
+just as easily secured as the other. It must only be remembered that in
+an inductive apparatus of any kind, if constant potential is required,
+the inductive relation between the primary or exciting and secondary or
+armature circuit must be the closest possible; whereas, in an apparatus
+for constant current just the opposite is required. Furthermore, the
+opposition to the current's flow in the induced circuit must be as small
+as possible in the former and as great as possible in the latter case.
+But opposition to a current's flow may be caused in more than one way.
+It may be caused by ohmic resistance or self-induction. One may make the
+induced circuit of a dynamo machine or transformer of such high
+resistance that when operating devices of considerably smaller
+resistance within very wide limits a nearly constant current is
+maintained. But such high resistance involves a great loss in power,
+hence it is not practicable. Not so self-induction. Self-induction does
+not necessarily mean loss of power. The moral is, use self-induction
+instead of resistance. There is, however, a circumstance which favors
+the adoption of this plan, and this is, that a very high self-induction
+may be obtained cheaply by surrounding a comparatively small length of
+wire more or less completely with iron, and, furthermore, the effect may
+be exalted at will by causing a rapid undulation of the current. To sum
+up, the requirements for constant current are: Weak magnetic connection
+between the induced and inducing circuits, greatest possible
+self-induction with the least resistance, greatest practicable rate of
+change of the current. Constant potential, on the other hand, requires:
+Closest magnetic connection between the circuits, steady induced
+current, and, if possible, no reaction. If the latter conditions could
+be fully satisfied in a constant potential machine, its output would
+surpass many times that of a machine primarily designed to give constant
+current. Unfortunately, the type of machine in which these conditions
+may be satisfied is of little practical value, owing to the small
+electromotive force obtainable and the difficulties in taking off the
+current.
+
+With their keen inventor's instinct, the now successful arc-light men
+have early recognized the desiderata of a constant current machine.
+Their arc light machines have weak fields, large armatures, with a great
+length of copper wire and few commutator segments to produce great
+variations in the current's strength and to bring self-induction into
+play. Such machines may maintain within considerable limits of variation
+in the resistance of the circuit a practically constant current. Their
+output is of course correspondingly diminished, and, perhaps with the
+object in view not to cut down the output too much, a simple device
+compensating exceptional variations is employed. The undulation of the
+current is almost essential to the commercial success of an arc-light
+system. It introduces in the circuit a steadying element taking the
+place of a large ohmic resistance, without involving a great loss in
+power, and, what is more important, it allows the use of simple clutch
+lamps, which with a current of a certain number of impulses per second,
+best suitable for each particular lamp, will, if properly attended to,
+regulate even better than the finest clock-work lamps. This discovery
+has been made by the writer--several years too late.
+
+It has been asserted by competent English electricians that in a
+constant-current machine or transformer the regulation is effected by
+varying the phase of the secondary current. That this view is erroneous
+may be easily proved by using, instead of lamps, devices each possessing
+self-induction and capacity or self-induction and resistance--that is,
+retarding and accelerating components--in such proportions as to not
+affect materially the phase of the secondary current. Any number of such
+devices may be inserted or cut out, still it will be found that the
+regulation occurs, a constant current being maintained, while the
+electromotive force is varied with the number of the devices. The change
+of phase of the secondary current is simply a result following from the
+changes in resistance, and, though secondary reaction is always of more
+or less importance, yet the real cause of the regulation lies in the
+existence of the conditions above enumerated. It should be stated,
+however, that in the case of a machine the above remarks are to be
+restricted to the cases in which the machine is independently excited.
+If the excitation be effected by commutating the armature current, then
+the fixed position of the brushes makes any shifting of the neutral line
+of the utmost importance, and it may not be thought immodest of the
+writer to mention that, as far as records go, he seems to have been the
+first who has successfully regulated machines by providing a bridge
+connection between a point of the external circuit and the commutator by
+means of a third brush. The armature and field being properly
+proportioned and the brushes placed in their determined positions, a
+constant current or constant potential resulted from the shifting of the
+diameter of commutation by the varying loads.
+
+In connection with machines of such high frequencies, the condenser
+affords an especially interesting study. It is easy to raise the
+electromotive force of such a machine to four or five times the value by
+simply connecting the condenser to the circuit, and the writer has
+continually used the condenser for the the purposes of regulation, as
+suggested by Blakesley in his book on alternate currents, in which he
+has treated the most frequently occurring condenser problems with
+exquisite simplicity and clearness. The high frequency allows the use of
+small capacities and renders investigation easy. But, although in most
+of the experiments the result may be foretold, some phenomena observed
+seem at first curious. One experiment performed three or four months ago
+with such a machine and a condenser may serve as an illustration. A
+machine was used giving about 20,000 alternations per second. Two bare
+wires about twenty feet long and two millimetres in diameter, in close
+proximity to each other, were connected to the terminals of the machine
+at the one end, and to a condenser at the other. A small transformer
+without an iron core, of course, was used to bring the reading within
+range of a Cardew voltmeter by connecting the voltmeter to the
+secondary. On the terminals of the condenser the electromotive force was
+about 120 volts, and from there inch by inch it gradually fell until at
+the terminals of the machine it was about 65 volts. It was virtually as
+though the condenser were a generator, and the line and armature circuit
+simply a resistance connected to it. The writer looked for a case of
+resonance, but he was unable to augment the effect by varying the
+capacity very carefully and gradually or by changing the speed of the
+machine. A case of pure resonance he was unable to obtain. When a
+condenser was connected to the terminals of the machine--the
+self-induction of the armature being first determined in the maximum and
+minimum position and the mean value taken--the capacity which gave the
+highest electromotive force corresponded most nearly to that which just
+counteracted the self-induction with the existing frequency. If the
+capacity was increased or diminished, the electromotive force fell as
+expected.
+
+With frequencies as high as the above mentioned, the condenser effects
+are of enormous importance. The condenser becomes a highly efficient
+apparatus capable of transferring considerable energy.
+
+       *       *       *       *       *
+
+In an appendix to this book will be found a description of the Tesla
+oscillator, which its inventor believes will among other great
+advantages give him the necessary high frequency conditions, while
+relieving him of the inconveniences that attach to generators of the
+type described at the beginning of this chapter.
+
+
+
+
+CHAPTER XXX.
+
+ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.[6]
+
+
+  [6] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
+      May 6, 1891.
+
+About a year and a half ago while engaged in the study of alternate
+currents of short period, it occurred to me that such currents could be
+obtained by rotating charged surfaces in close proximity to conductors.
+Accordingly I devised various forms of experimental apparatus of which
+two are illustrated in the accompanying engravings.
+
+[Illustration: FIG. 208.]
+
+In the apparatus shown in Fig. 208, A is a ring of dry shellacked hard
+wood provided on its inside with two sets of tin-foil coatings, _a_ and
+_b_, all the _a_ coatings and all the _b_ coatings being connected
+together, respectively, but independent from each other. These two sets
+of coatings are connected to two terminals, T. For the sake of
+clearness only a few coatings are shown. Inside of the ring A, and in
+close proximity to it there is arranged to rotate a cylinder B, likewise
+of dry, shellacked hard wood, and provided with two similar sets of
+coatings, _a^1_ and _b^1_, all the coatings _a^1_ being connected to one
+ring and all the others, _b^1_, to another marked + and -. These two
+sets, _a^1_ and _b^1_ are charged to a high potential by a Holtz or
+Wimshurst machine, and may be connected to a jar of some capacity. The
+inside of ring A is coated with mica in order to increase the induction
+and also to allow higher potentials to be used.
+
+[Illustration: FIG. 209.]
+
+When the cylinder B with the charged coatings is rotated, a circuit
+connected to the terminals T is traversed by alternating currents.
+Another form of apparatus is illustrated in Fig. 209. In this apparatus
+the two sets of tin-foil coatings are glued on a plate of ebonite, and a
+similar plate which is rotated, and the coatings of which are charged as
+in Fig. 208, is provided.
+
+The output of such an apparatus is very small, but some of the effects
+peculiar to alternating currents of short periods may be observed. The
+effects, however, cannot be compared with those obtainable with an
+induction coil which is operated by an alternate current machine of high
+frequency, some of which were described by me a short while ago.
+
+
+
+
+CHAPTER XXXI.
+
+"MASSAGE" WITH CURRENTS OF HIGH FREQUENCY.[7]
+
+  [7] Article by Mr. Tesla in _The Electrical Engineer_ of Dec. 23d,
+      1891.
+
+I trust that the present brief communication will not be interpreted as
+an effort on my part to put myself on record as a "patent medicine" man,
+for a serious worker cannot despise anything more than the misuse and
+abuse of electricity which we have frequent occasion to witness. My
+remarks are elicited by the lively interest which prominent medical
+practitioners evince at every real advance in electrical investigation.
+The progress in recent years has been so great that every electrician
+and electrical engineer is confident that electricity will become the
+means of accomplishing many things that have been heretofore, with our
+existing knowledge, deemed impossible. No wonder then that progressive
+physicians also should expect to find in it a powerful tool and help in
+new curative processes. Since I had the honor to bring before the
+American Institute of Electrical Engineers some results in utilizing
+alternating currents of high tension, I have received many letters from
+noted physicians inquiring as to the physical effects of such currents
+of high frequency. It may be remembered that I then demonstrated that a
+body perfectly well insulated in air can be heated by simply connecting
+it with a source of rapidly alternating high potential. The heating in
+this case is due in all probability to the bombardment of the body by
+air, or possibly by some other medium, which is molecular or atomic in
+construction, and the presence of which has so far escaped our
+analysis--for according to my ideas, the true ether radiation with such
+frequencies as even a few millions per second must be very small. This
+body may be a good conductor or it may be a very poor conductor of
+electricity with little change in the result. The human body is, in such
+a case, a fine conductor, and if a person insulated in a room, or no
+matter where, is brought into contact with such a source of rapidly
+alternating high potential, the skin is heated by bombardment. It is a
+mere question of the dimensions and character of the apparatus to
+produce any degree of heating desired.
+
+It has occurred to me whether, with such apparatus properly prepared, it
+would not be possible for a skilled physician to find in it a means for
+the effective treatment of various types of disease. The heating will,
+of course, be superficial, that is, on the skin, and would result,
+whether the person operated on were in bed or walking around a room,
+whether dressed in thick clothes or whether reduced to nakedness. In
+fact, to put it broadly, it is conceivable that a person entirely nude
+at the North Pole might keep himself comfortably warm in this manner.
+
+Without vouching for all the results, which must, of course, be
+determined by experience and observation, I can at least warrant the
+fact that heating would occur by the use of this method of subjecting
+the human body to bombardment by alternating currents of high potential
+and frequency such I have long worked with. It is only reasonable to
+expect that some of the novel effects will be wholly different from
+those obtainable with the old familiar therapeutic methods generally
+used. Whether they would all be beneficial or not remains to be proved.
+
+
+
+
+CHAPTER XXXII.
+
+ELECTRIC DISCHARGE IN VACUUM TUBES.[8]
+
+  [8] Article by Mr. Tesla in _The Electrical Engineer_. N. Y.,
+      July 1, 1891.
+
+
+In _The Electrical Engineer_ of June 10 I have noted the description of
+some experiments of Prof. J. J. Thomson, on the "Electric Discharge in
+Vacuum Tubes," and in your issue of June 24 Prof. Elihu Thomson
+describes an experiment of the same kind. The fundamental idea in these
+experiments is to set up an electromotive force in a vacuum
+tube---preferably devoid of any electrodes--by means of electro-magnetic
+induction, and to excite the tube in this manner.
+
+As I view the subject I should, think that to any experimenter who had
+carefully studied the problem confronting us and who attempted to find a
+solution of it, this idea must present itself as naturally as, for
+instance, the idea of replacing the tinfoil coatings of a Leyden jar by
+rarefied gas and exciting luminosity in the condenser thus obtained by
+repeatedly charging and discharging it. The idea being obvious, whatever
+merit there is in this line of investigation must depend upon the
+completeness of the study of the subject and the correctness of the
+observations. The following lines are not penned with any desire on my
+part to put myself on record as one who has performed similar
+experiments, but with a desire to assist other experimenters by pointing
+out certain peculiarities of the phenomena observed, which, to all
+appearances, have not been noted by Prof. J. J. Thomson, who, however,
+seems to have gone about systematically in his investigations, and who
+has been the first to make his results known. These peculiarities noted
+by me would seem to be at variance with the views of Prof. J. J.
+Thomson, and present the phenomena in a different light.
+
+My investigations in this line occupied me principally during the winter
+and spring of the past year. During this time many different experiments
+were performed, and in my exchanges of ideas on this subject with Mr.
+Alfred S. Brown, of the Western Union Telegraph Company, various
+different dispositions were suggested which were carried out by me in
+practice. Fig. 210 may serve as an example of one of the many forms of
+apparatus used. This consisted of a large glass tube sealed at one end
+and projecting into an ordinary incandescent lamp bulb. The primary,
+usually consisting of a few turns of thick, well-insulated copper sheet
+was inserted within the tube, the inside space of the bulb furnishing
+the secondary. This form of apparatus was arrived at after some
+experimenting, and was used principally with the view of enabling me to
+place a polished reflecting surface on the inside of the tube, and for
+this purpose the last turn of the primary was covered with a thin silver
+sheet. In all forms of apparatus used there was no special difficulty in
+exciting a luminous circle or cylinder in proximity to the primary.
+
+[Illustration: FIG. 210.]
+
+As to the number of turns, I cannot quite understand why Prof. J. J.
+Thomson should think that a few turns were "quite sufficient," but lest
+I should impute to him an opinion he may not have, I will add that I
+have gained this impression from the reading of the published abstracts
+of his lecture. Clearly, the number of turns which gives the best result
+in any case, is dependent on the dimensions of the apparatus, and, were
+it not for various considerations, one turn would always give the best
+result.
+
+I have found that it is preferable to use in these experiments an
+alternate current machine giving a moderate number of alternations per
+second to excite the induction coil for charging the Leyden jar which
+discharges through the primary--shown diagrammatically in Fig. 211,--as
+in such case, before the disruptive discharge takes place, the tube or
+bulb is slightly excited and the formation of the luminous circle is
+decidedly facilitated. But I have also used a Wimshurst machine in some
+experiments.
+
+[Illustration: FIG. 211.]
+
+Prof. J. J. Thomson's view of the phenomena under consideration seems to
+be that they are wholly due to electro-magnetic action. I was, at one
+time, of the same opinion, but upon carefully investigating the subject
+I was led to the conviction that they are more of an electrostatic
+nature. It must be remembered that in these experiments we have to deal
+with primary currents of an enormous frequency or rate of change and of
+high potential, and that the secondary conductor consists of a rarefied
+gas, and that under such conditions electrostatic effects must play an
+important part.
+
+[Illustration: FIG. 212.]
+
+In support of my view I will describe a few experiments made by me. To
+excite luminosity in the tube it is not absolutely necessary that the
+conductor should be closed. For instance, if an ordinary exhausted tube
+(preferably of large diameter) be surrounded by a spiral of thick copper
+wire serving as the primary, a feebly luminous spiral may be induced in
+the tube, roughly shown in Fig. 212. In one of these experiments a
+curious phenomenon was observed; namely, two intensely luminous circles,
+each of them close to a turn of the primary spiral, were formed inside
+of the tube, and I attributed this phenomenon to the existence of nodes
+on the primary. The circles were connected by a faint luminous spiral
+parallel to the primary and in close proximity to it. To produce this
+effect I have found it necessary to strain the jar to the utmost. The
+turns of the spiral tend to close and form circles, but this, of course,
+would be expected, and does not necessarily indicate an electro-magnetic
+effect; Whereas the fact that a glow can be produced along the primary
+in the form of an open spiral argues for an electrostatic effect.
+
+[Illustration: FIG. 213.]
+
+In using Dr. Lodge's recoil circuit, the electrostatic action is
+likewise apparent. The arrangement is illustrated in Fig. 213. In his
+experiment two hollow exhausted tubes H H were slipped over the wires of
+the recoil circuit and upon discharging the jar in the usual manner
+luminosity was excited in the tubes.
+
+Another experiment performed is illustrated in Fig. 214. In this case an
+ordinary lamp-bulb was surrounded by one or two turns of thick copper
+wire P and the luminous circle L excited in the bulb by discharging the
+jar through the primary. The lamp-bulb was provided with a tinfoil
+coating on the side opposite to the primary and each time the tinfoil
+coating was connected to the ground or to a large object the luminosity
+of the circle was considerably increased. This was evidently due to
+electrostatic action.
+
+In other experiments I have noted that when the primary touches the
+glass the luminous circle is easier produced and is more sharply
+defined; but I have not noted that, generally speaking, the circles
+induced were very sharply defined, as Prof. J. J. Thomson has observed;
+on the contrary, in my experiments they were broad and often the whole
+of the bulb or tube was illuminated; and in one case I have observed an
+intensely purplish glow, to which Prof. J. J. Thomson refers. But the
+circles were always in close proximity to the primary and were
+considerably easier produced when the latter was very close to the
+glass, much more so than would be expected assuming the action to be
+electromagnetic and considering the distance; and these facts speak for
+an electrostatic effect.
+
+[Illustration: FIG. 214.]
+
+[Illustration: FIG. 215.]
+
+Furthermore I have observed that there is a molecular bombardment in the
+plane of the luminous circle at right angles to the glass--supposing the
+circle to be in the plane of the primary--this bombardment being
+evident from the rapid heating of the glass near the primary. Were the
+bombardment not at right angles to the glass the heating could not be so
+rapid. If there is a circumferential movement of the molecules
+constituting the luminous circle, I have thought that it might be
+rendered manifest by placing within the tube or bulb, radially to the
+circle, a thin plate of mica coated with some phosphorescent material
+and another such plate tangentially to the circle. If the molecules
+would move circumferentially, the former plate would be rendered more
+intensely phosphorescent. For want of time I have, however, not been
+able to perform the experiment.
+
+Another observation made by me was that when the specific inductive
+capacity of the medium between the primary and secondary is increased,
+the inductive effect is augmented. This is roughly illustrated in Fig.
+215. In this case luminosity was excited in an exhausted tube or bulb B
+and a glass tube T slipped between the primary and the bulb, when the
+effect pointed out was noted. Were the action wholly electromagnetic no
+change could possibly have been observed.
+
+I have likewise noted that when a bulb is surrounded by a wire closed
+upon itself and in the plane of the primary, the formation of the
+luminous circle within the bulb is not prevented. But if instead of the
+wire a broad strip of tinfoil is glued upon the bulb, the formation of
+the luminous band was prevented, because then the action was distributed
+over a greater surface. The effect of the closed tinfoil was no doubt of
+an electrostatic nature, for it presented a much greater resistance than
+the closed wire and produced therefore a much smaller electromagnetic
+effect.
+
+Some of the experiments of Prof. J. J. Thomson also would seem to show
+some electrostatic action. For instance, in the experiment with the bulb
+enclosed in a bell jar, I should think that when the latter is exhausted
+so far that the gas enclosed reaches the maximum conductivity, the
+formation of the circle in the bulb and jar is prevented because of the
+space surrounding the primary being highly conducting; when the jar is
+further exhausted, the conductivity of the space around the primary
+diminishes and the circles appear necessarily first in the bell jar, as
+the rarefied gas is nearer to the primary. But were the inductive effect
+very powerful, they would probably appear in the bulb also. If, however,
+the bell jar were exhausted to the highest degree they would very likely
+show themselves in the bulb only, that is, supposing the vacuous space
+to be non-conducting. On the assumption that in these phenomena
+electrostatic actions are concerned we find it easily explicable why the
+introduction of mercury or the heating of the bulb prevents the
+formation of the luminous band or shortens the after-glow; and also why
+in some cases a platinum wire may prevent the excitation of the tube.
+Nevertheless some of the experiments of Prof. J. J. Thomson would seem
+to indicate an electromagnetic effect. I may add that in one of my
+experiments in which a vacuum was produced by the Torricellian method, I
+was unable to produce the luminous band, but this may have been due to
+the weak exciting current employed.
+
+My principal argument is the following: I have experimentally proved
+that if the same discharge which is barely sufficient to excite a
+luminous band in the bulb when passed through the primary circuit be so
+directed as to exalt the electrostatic inductive effect--namely, by
+converting upwards--an exhausted tube, devoid of electrodes, may be
+excited at a distance of several feet.
+
+
+SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.[9]
+
+BY PROF. J. J. THOMSON, M.A., F.R.S.
+
+  [9] Abstract of a paper read before Physical Society of London.
+
+    [Illustration: FIG. 216.]
+
+    [Illustration: FIG. 217.]
+
+    [Illustration: FIG. 218.]
+
+    [Illustration: FIG. 219.]
+
+    The phenomena of vacuum discharges were, Prof. Thomson said,
+    greatly simplified when their path was wholly gaseous, the
+    complication of the dark space surrounding the negative electrode,
+    and the stratifications so commonly observed in ordinary vacuum
+    tubes, being absent. To produce discharges in tubes devoid of
+    electrodes was, however, not easy to accomplish, for the only
+    available means of producing an electromotive force in the
+    discharge circuit was by electro-magnetic induction. Ordinary
+    methods of producing variable induction were valueless, and
+    recourse was had to the oscillatory discharge of a Leyden jar,
+    which combines the two essentials of a current whose maximum value
+    is enormous, and whose rapidity of alternation is immensely great.
+    The discharge circuits, which may take the shape of bulbs, or of
+    tubes bent in the form of coils, were placed in close proximity to
+    glass tubes filled with mercury, which formed the path of the
+    oscillatory discharge. The parts thus corresponded to the windings
+    of an induction coil, the vacuum tubes being the secondary, and the
+    tubes filled with mercury the primary. In such an apparatus the
+    Leyden jar need not be large, and neither primary nor secondary
+    need have many turns, for this would increase the self-induction of
+    the former, and lengthen the discharge path in the latter.
+    Increasing the self-induction of the primary reduces the E. M. F.
+    induced in the secondary, whilst lengthening the secondary does not
+    increase the E. M. F. per unit length. The two or three turns, as
+    shown in Fig. 216, in each, were found to be quite sufficient, and,
+    on discharging the Leyden jar between two highly polished knobs in
+    the primary circuit, a plain uniform band of light was seen to pass
+    round the secondary. An exhausted bulb, Fig. 217, containing traces
+    of oxygen was placed within a primary spiral of three turns, and,
+    on passing the jar discharge, a circle of light was seen within the
+    bulb in close proximity to the primary circuit, accompanied by a
+    purplish glow, which lasted for a second or more. On heating the
+    bulb, the duration of the glow was greatly diminished, and it could
+    be instantly extinguished by the presence of an electro-magnet.
+    Another exhausted bulb, Fig. 218, surrounded by a primary spiral,
+    was contained in a bell-jar, and when the pressure of air in the
+    jar was about that of the atmosphere, the secondary discharge
+    occurred in the bulb, as is ordinarily the case. On exhausting the
+    jar, however, the luminous discharge grew fainter, and a point was
+    reached at which no secondary discharge was visible. Further
+    exhaustion of the jar caused the secondary discharge to appear
+    outside of the bulb. The fact of obtaining no luminous discharge,
+    either in the bulb or jar, the author could only explain on two
+    suppositions, viz.: that under the conditions then existing the
+    specific inductive capacity of the gas was very great, or that a
+    discharge could pass without being luminous. The author had also
+    observed that the conductivity of a vacuum tube without electrodes
+    increased as the pressure diminished, until a certain point was
+    reached, and afterwards diminished again, thus showing that the
+    high resistance of a nearly perfect vacuum is in no way due to the
+    presence of the electrodes. One peculiarity of the discharges was
+    their local nature, the rings of light being much more sharply
+    defined than was to be expected. They were also found to be most
+    easily produced when the chain of molecules in the discharge were
+    all of the same kind. For example, a discharge could be easily sent
+    through a tube many feet long, but the introduction of a small
+    pellet of mercury in the tube stopped the discharge, although the
+    conductivity of the mercury was much greater than that of the
+    vacuum. In some cases he had noticed that a very fine wire placed
+    within a tube, on the side remote from the primary circuit, would
+    prevent a luminous discharge in that tube.
+
+    Fig. 219 shows an exhausted secondary coil of one loop containing
+    bulbs; the discharge passed along the inner side of the bulbs, the
+    primary coils being placed within the secondary.
+
+
+[9]In _The Electrical Engineer_ of August 12, I find some remarks of
+Prof. J. J. Thomson, which appeared originally in the London
+_Electrician_ and which have a bearing upon some experiments described
+by me in your issue of July 1.
+
+  [9] Article by Mr. Tesla in _The Electrical Engineer_, N. Y.,
+      August 26, 1891.
+
+I did not, as Prof. J. J. Thomson seems to believe, misunderstand his
+position in regard to the cause of the phenomena considered, but I
+thought that in his experiments, as well as in my own, electrostatic
+effects were of great importance. It did not appear, from the meagre
+description of his experiments, that all possible precautions had been
+taken to exclude these effects. I did not doubt that luminosity could be
+excited in a closed tube when electrostatic action is completely
+excluded. In fact, at the outset, I myself looked for a purely
+electrodynamic effect and believed that I had obtained it. But many
+experiments performed at that time proved to me that the electrostatic
+effects were generally of far greater importance, and admitted of a more
+satisfactory explanation of most of the phenomena observed.
+
+In using the term _electrostatic_ I had reference rather to the nature
+of the action than to a stationary condition, which is the usual
+acceptance of the term. To express myself more clearly, I will suppose
+that near a closed exhausted tube be placed a small sphere charged to a
+very high potential. The sphere would act inductively upon the tube, and
+by distributing electricity over the same would undoubtedly produce
+luminosity (if the potential be sufficiently high), until a permanent
+condition would be reached. Assuming the tube to be perfectly well
+insulated, there would be only one instantaneous flash during the act of
+distribution. This would be due to the electrostatic action simply.
+
+But now, suppose the charged sphere to be moved at short intervals with
+great speed along the exhausted tube. The tube would now be permanently
+excited, as the moving sphere would cause a constant redistribution of
+electricity and collisions of the molecules of the rarefied gas. We
+would still have to deal with an electrostatic effect, and in addition
+an electrodynamic effect would be observed. But if it were found that,
+for instance, the effect produced depended more on the specific
+inductive capacity than on the magnetic permeability of the
+medium--which would certainly be the case for speeds incomparably lower
+than that of light--then I believe I would be justified in saying that
+the effect produced was more of an electrostatic nature. I do not mean
+to say, however, that any similar condition prevails in the case of the
+discharge of a Leyden jar through the primary, but I think that such an
+action would be desirable.
+
+It is in the spirit of the above example that I used the terms "more of
+an electrostatic nature," and have investigated the influence of bodies
+of high specific inductive capacity, and observed, for instance, the
+importance of the quality of glass of which the tube is made. I also
+endeavored to ascertain the influence of a medium of high permeability
+by using oxygen. It appeared from rough estimation that an oxygen tube
+when excited under similar conditions--that is, as far as could be
+determined--gives more light; but this, of course, may be due to many
+causes.
+
+Without doubting in the least that, with the care and precautions taken
+by Prof. J. J. Thomson, the luminosity excited was due solely to
+electrodynamic action, I would say that in many experiments I have
+observed curious instances of the ineffectiveness of the screening, and
+I have also found that the electrification through the air is often of
+very great importance, and may, in some cases, determine the excitation
+of the tube.
+
+In his original communication to the _Electrician_, Prof. J. J. Thomson
+refers to the fact that the luminosity in a tube near a wire through
+which a Leyden jar was discharged was noted by Hittorf. I think that the
+feeble luminous effect referred to has been noted by many
+experimenters, but in my experiments the effects were much more powerful
+than those usually noted.
+
+The following is the communication[10] referred to:--
+
+  [10] Note by Prof. J. J. Thomson in the London _Electrician_,
+       July 24, 1891.
+
+    "Mr. Tesla seems to ascribe the effects he observed to
+    electrostatic action, and I have no doubt, from the description he
+    gives of his method of conducting his experiments, that in them
+    electrostatic action plays a very important part. He seems,
+    however, to have misunderstood my position with respect to the
+    cause of these discharges, which is not, as he implies, that
+    luminosity in tubes without electrodes cannot be produced by
+    electrostatic action, but that it can also be produced when this
+    action is excluded. As a matter of fact, it is very much easier to
+    get the luminosity when these electrostatic effects are operative
+    than when they are not. As an illustration of this I may mention
+    that the first experiment I tried with the discharge of a Leyden
+    jar produced luminosity in the tube, but it was not until after six
+    weeks' continuous experimenting that I was able to get a discharge
+    in the exhausted tube which I was satisfied was due to what is
+    ordinarily called electrodynamic action. It is advisable to have a
+    clear idea of what we mean by electrostatic action. If, previous to
+    the discharge of the jar, the primary coil is raised to a high
+    potential, it will induce over the glass of the tube a distribution
+    of electricity. When the potential of the primary suddenly falls,
+    this electrification will redistribute itself, and may pass through
+    the rarefied gas and produce luminosity in doing so. Whilst the
+    discharge of the jar is going on, it is difficult, and, from a
+    theoretical point of view, undesirable, to separate the effect into
+    parts, one of which is called electrostatic, the other
+    electromagnetic; what we can prove is that in this case the
+    discharge is not such as would be produced by electromotive forces
+    derived from a potential function. In my experiments the primary
+    coil was connected to earth, and, as a further precaution, the
+    primary was separated from the discharge tube by a screen of
+    blotting paper, moistened with dilute sulphuric acid, and connected
+    to earth. Wet blotting paper is a sufficiently good conductor to
+    screen off a stationary electrostatic effect, though it is not a
+    good enough one to stop waves of alternating electromotive
+    intensity. When showing the experiments to the Physical Society I
+    could not, of course, keep the tubes covered up, but, unless my
+    memory deceives me, I stated the precautions which had been taken
+    against the electrostatic effect. To correct misapprehension I may
+    state that I did not read a formal paper to the Society, my object
+    being to exhibit a few of the most typical experiments. The account
+    of the experiments in the _Electrician_ was from a reporter's note,
+    and was not written, or even read, by me. I have now almost
+    finished writing out, and hope very shortly to publish, an account
+    of these and a large number of allied experiments, including some
+    analogous to those mentioned by Mr. Tesla on the effect of
+    conductors placed near the discharge tube, which I find, in some
+    cases, to produce a diminution, in others an increase, in the
+    brightness of the discharge, as well as some on the effect of the
+    presence of substances of large specific inductive capacity. These
+    seem to me to admit of a satisfactory explanation, for which,
+    however, I must refer to my paper."
+
+
+
+
+PART III.
+
+MISCELLANEOUS INVENTIONS AND WRITINGS.
+
+
+
+
+CHAPTER XXXIII.
+
+METHOD OF OBTAINING DRIECT FROM ALTERNATING CURRENTS.
+
+
+This method consists in obtaining direct from alternating currents, or
+in directing the waves of an alternating current so as to produce direct
+or substantially direct currents by developing or producing in the
+branches of a circuit including a source of alternating currents, either
+permanently or periodically, and by electric, electro-magnetic, or
+magnetic agencies, manifestations of energy, or what may be termed
+active resistances of opposite electrical character, whereby the
+currents or current waves of opposite sign will be diverted through
+different circuits, those of one sign passing over one branch and those
+of opposite sign over the other.
+
+We may consider herein only the case of a circuit divided into two
+paths, inasmuch as any further subdivision involves merely an extension
+of the general principle. Selecting, then, any circuit through which is
+flowing an alternating current, Mr. Tesla divides such circuit at any
+desired point into two branches or paths. In one of these paths he
+inserts some device to create an electromotive force counter to the
+waves or impulses of current of one sign and a similar device in the
+other branch which opposes the waves of opposite sign. Assume, for
+example, that these devices are batteries, primary or secondary, or
+continuous current dynamo machines. The waves or impulses of opposite
+direction composing the main current have a natural tendency to divide
+between the two branches; but by reason of the opposite electrical
+character or effect of the two branches, one will offer an easy passage
+to a current of a certain direction, while the other will offer a
+relatively high resistance to the passage of the same current. The
+result of this disposition is, that the waves of current of one sign
+will, partly or wholly, pass over one of the paths or branches, while
+those of the opposite sign pass over the other. There may thus be
+obtained from an alternating current two or more direct currents without
+the employment of any commutator such as it has been heretofore
+regarded as necessary to use. The current in either branch may be
+used in the same way and for the same purposes as any other direct
+current--that is, it may be made to charge secondary batteries, energize
+electro-magnets, or for any other analogous purpose.
+
+Fig. 220 represents a plan of directing the alternating currents by
+means of devices purely electrical in character. Figs. 221, 222, 223,
+224, 225, and 226 are diagrams illustrative of other ways of carrying
+out the invention.
+
+[Illustration: FIG. 220.]
+
+In Fig. 220, A designates a generator of alternating currents, and B B
+the main or line circuit therefrom. At any given point in this circuit
+at or near which it is desired to obtain direct currents, the circuit B
+is divided into two paths or branches C D. In each of these branches is
+placed an electrical generator, which for the present we will assume
+produces direct or continuous currents. The direction of the current
+thus produced is opposite in one branch to that of the current in the
+other branch, or, considering the two branches as forming a closed
+circuit, the generators E F are connected up in series therein, one
+generator in each part or half of the circuit. The electromotive force
+of the current sources E and F may be equal to or higher or lower than
+the electromotive forces in the branches C D, or between the points X
+and Y of the circuit B B. If equal, it is evident that current waves of
+one sign will be opposed in one branch and assisted in the other to such
+an extent that all the waves of one sign will pass over one branch and
+those of opposite sign over the other. If, on the other hand, the
+electromotive force of the sources E F be lower than that between X and
+Y, the currents in both branches will be alternating, but the waves of
+one sign will preponderate. One of the generators or sources of current
+E or F may be dispensed with; but it is preferable to employ both, if
+they offer an appreciable resistance, as the two branches will be
+thereby better balanced. The translating or other devices to be acted
+upon by the current are designated by the letters G, and they are
+inserted in the branches C D in any desired manner; but in order to
+better preserve an even balance between the branches due regard should,
+of course, be had to the number and character of the devices.
+
+[Illustration: FIG. 221.]
+
+Figs. 221, 222, 223, and 224 illustrate what may termed
+"electro-magnetic" devices for accomplishing a similar result--that is
+to say, instead of producing directly by a generator an electromotive
+force in each branch of the circuit, Mr. Tesla establishes a field or
+fields of force and leads the branches through the same in such manner
+that an active opposition of opposite effect or direction will be
+developed therein by the passage, or tendency to pass, of the
+alternations of current. In Fig. 221, for example, A is the generator of
+alternating currents, B B the line circuit, and C D the branches over
+which the alternating currents are directed. In each branch is included
+the secondary of a transformer or induction coil, which, since they
+correspond in their functions to the batteries of the previous figure,
+are designated by the letters E F. The primaries H H' of the induction
+coils or transformers are connected either in parallel or series with a
+source of direct or continuous currents I, and the number of
+convolutions is so calculated for the strength of the current from I
+that the cores J J' will be saturated. The connections are such that the
+conditions in the two transformers are of opposite character--that is to
+say, the arrangement is such that a current wave or impulse
+corresponding in direction with that of the direct current in one
+primary, as H, is of opposite direction to that in the other primary H'.
+It thus results that while one secondary offers a resistance or
+opposition to the passage through it of a wave of one sign, the other
+secondary similarly opposes a wave of opposite sign. In consequence, the
+waves of one sign will, to a greater or less extent, pass by way of one
+branch, while those of opposite sign in like manner pass over the other
+branch.
+
+In lieu of saturating the primaries by a source of continuous current,
+we may include the primaries in the branches C D, respectively, and
+periodically short-circuit by any suitable mechanical devices--such as
+an ordinary revolving commutator--their secondaries. It will be
+understood, of course, that the rotation and action of the commutator
+must be in synchronism or in proper accord with the periods of the
+alternations in order to secure the desired results. Such a disposition
+is represented diagrammatically in Fig. 222. Corresponding to the
+previous figures, A is the generator of alternating currents, B B the
+line, and C D the two branches for the direct currents. In branch C are
+included two primary coils E E', and in branch D are two similar
+primaries F F' The corresponding secondaries for these coils and which
+are on the same subdivided cores J or J', are in circuits the terminals
+of which connect to opposite segments K K', and L L', respectively, of a
+commutator. Brushes _b b_ bear upon the commutator and alternately
+short-circuit the plates K and K', and L and L', through a connection
+_c_. It is obvious that either the magnets and commutator, or the
+brushes, may revolve.
+
+[Illustration: FIG. 222.]
+
+The operation will be understood from a consideration of the effects of
+closing or short-circuiting the secondaries. For example, if at the
+instant when a given wave of current passes, one set of secondaries be
+short-circuited, nearly all the current flows through the corresponding
+primaries; but the secondaries of the other branch being open-circuited,
+the self-induction in the primaries is highest, and hence little or no
+current will pass through that branch. If, as the current alternates,
+the secondaries of the two branches are alternately short-circuited, the
+result will be that the currents of one sign pass over one branch and
+those of the opposite sign over the other. The disadvantages of this
+arrangement, which would seem to result from the employment of sliding
+contacts, are in reality very slight, inasmuch as the electromotive
+force of the secondaries may be made exceedingly low, so that sparking
+at the brushes is avoided.
+
+[Illustration: FIG. 223.]
+
+Fig. 223 is a diagram, partly in section, of another plan of carrying
+out the invention. The circuit B in this case is divided, as before, and
+each branch includes the coils of both the fields and revolving
+armatures of two induction devices. The armatures O P are preferably
+mounted on the same shaft, and are adjusted relatively to one another in
+such manner that when the self-induction in one branch, as C, is
+maximum, in the other branch D it is minimum. The armatures are rotated
+in synchronism with the alternations from the source A. The winding or
+position of the armature coils is such that a current in a given
+direction passed through both armatures would establish in one, poles
+similar to those in the adjacent poles of the field, and in the other,
+poles unlike the adjacent field poles, as indicated by _n n s s_ in the
+diagram. If the like poles are presented, as shown in circuit D, the
+condition is that of a closed secondary upon a primary, or the position
+of least inductive resistance; hence a given alternation of current will
+pass mainly through D. A half revolution of the armatures produces an
+opposite effect and the succeeding current impulse passes through C.
+Using this figure as an illustration, it is evident that the fields N M
+may be permanent magnets or independently excited and the armatures O P
+driven, as in the present case, so as to produce alternate currents,
+which will set up alternately impulses of opposite direction in the two
+branches D C, which in such case would include the armature circuits and
+translating devices only.
+
+In Fig. 224 a plan alternative with that shown in Fig. 222 is
+illustrated. In the previous case illustrated, each branch C and D
+contained one or more primary coils, the secondaries of which were
+periodically short circuited in synchronism with the alternations of
+current from the main source A, and for this purpose a commutator was
+employed. The latter may, however, be dispensed with and an armature
+with a closed coil substituted.
+
+[Illustration: FIG. 224.]
+
+Referring to Fig. 224 in one of the branches, as C, are two coils M',
+wound on laminated cores, and in the other branches D are similar coils
+N'. A subdivided or laminated armature O', carrying a closed coil R', is
+rotatably supported between the coils M' N', as shown. In the position
+shown--that is, with the coil R' parallel with the convolutions of the
+primaries N' M'--practically the whole current will pass through branch
+D, because the self-induction in coils M' M' is maximum. If, therefore,
+the armature and coil be rotated at a proper speed relatively to the
+periods or alternations of the source A, the same results are obtained
+as in the case of Fig. 222.
+
+Fig. 225 is an instance of what may be called, in distinction to the
+others, a "magnetic" means of securing the result. V and W are two
+strong permanent magnets provided with armatures V' W', respectively.
+The armatures are made of thin laminae of soft iron or steel, and the
+amount of magnetic metal which they contain is so calculated that they
+will be fully or nearly saturated by the magnets. Around the armatures
+are coils E F, contained, respectively, in the circuits C and D. The
+connections and electrical conditions in this case are similar to those
+in Fig. 221, except that the current source of I, Fig. 221, is dispensed
+with and the saturation of the core of coils E F obtained from the
+permanent magnets.
+
+[Illustration: FIG. 225.]
+
+The previous illustrations have all shown the two branches or paths
+containing the translating or induction devices as in derivation one to
+the other; but this is not always necessary. For example, in Fig. 226, A
+is an alternating-current generator; B B, the line wires or circuit. At
+any given point in the circuit let us form two paths, as D D', and at
+another point two paths, as C C'. Either pair or group of paths is
+similar to the previous dispositions with the electrical source or
+induction device in one branch only, while the two groups taken together
+form the obvious equivalent of the cases in which an induction device or
+generator is included in both branches. In one of the paths, as D, are
+included the devices to be operated by the current. In the other branch,
+as D', is an induction device that opposes the current impulses of one
+direction and directs them through the branch D. So, also, in branch C
+are translating devices G, and in branch C' an induction device or its
+equivalent that diverts through C impulses of opposite direction to
+those diverted by the device in branch D'. The diagram shows a special
+form of induction device for this purpose. J J' are the cores, formed
+with pole-pieces, upon which are wound the coils M N. Between these
+pole-pieces are mounted at right angles to one another the magnetic
+armatures O P, preferably mounted on the same shaft and designed to be
+rotated in synchronism with the alternations of current. When one of the
+armatures is in line with the poles or in the position occupied by
+armature P, the magnetic circuit of the induction device is practically
+closed; hence there will be the greatest opposition to the passage of a
+current through coils N N. The alternation will therefore pass by way of
+branch D. At the same time, the magnetic circuit of the other induction
+device being broken by the position of the armature O, there will be
+less opposition to the current in coils M, which will shunt the current
+from branch C. A reversal of the current being attended by a shifting of
+the armatures, the opposite effect is produced.
+
+[Illustration: FIG. 226.]
+
+Other modifications of these methods are possible, but need not be
+pointed out. In all these plans, it will be observed, there is developed
+in one or all of these branches of a circuit from a source of
+alternating currents, an active (as distinguished from a dead)
+resistance or opposition to the currents of one sign, for the purpose of
+diverting the currents of that sign through the other or another path,
+but permitting the currents of opposite sign to pass without substantial
+opposition.
+
+Whether the division of the currents or waves of current of opposite
+sign be effected with absolute precision or not is immaterial, since it
+will be sufficient if the waves are only partially diverted or directed,
+for in such case the preponderating influence in each branch of the
+circuit of the waves of one sign secures the same practical results in
+many if not all respects as though the current were direct and
+continuous.
+
+An alternating and a direct current have been combined so that the waves
+of one direction or sign were partially or wholly overcome by the direct
+current; but by this plan only one set of alternations are utilized,
+whereas by the system just described the entire current is rendered
+available. By obvious applications of this discovery Mr. Tesla is
+enabled to produce a self-exciting alternating dynamo, or to operate
+direct current meters on alternating-current circuits or to run various
+devices--such as arc lamps--by direct currents in the same circuit with
+incandescent lamps or other devices operated by alternating currents.
+
+It will be observed that if an intermittent counter or opposing force be
+developed in the branches of the circuit and of higher electromotive
+force than that of the generator, an alternating current will result in
+each branch, with the waves of one sign preponderating, while a
+constantly or uniformly acting opposition in the branches of higher
+electromotive force than the generator would produce a pulsating
+current, which conditions would be, under some circumstances, the
+equivalent of those described.
+
+
+
+
+CHAPTER XXXIV.
+
+CONDENSERS WITH PLATES IN OIL.
+
+
+[Illustration: FIG. 227.]
+
+[Illustration: FIG. 228.]
+
+In experimenting with currents of high frequency and high potential, Mr.
+Tesla has found that insulating materials such as glass, mica, and in
+general those bodies which possess the highest specific inductive
+capacity, are inferior as insulators in such devices when currents of
+the kind described are employed compared with those possessing high
+insulating power, together with a smaller specific inductive capacity;
+and he has also found that it is very desirable to exclude all gaseous
+matter from the apparatus, or any access of the same to the electrified
+surfaces, in order to prevent heating by molecular bombardment and the
+loss or injury consequent thereon. He has therefore devised a method to
+accomplish these results and produce highly efficient and reliable
+condensers, by using oil as the dielectric[11]. The plan admits of a
+particular construction of condenser, in which the distance between the
+plates is adjustable, and of which he takes advantage.
+
+  [11] Mr. Tesla's experiments, as the careful reader of his three
+    lectures will perceive, have revealed a very important fact which
+    is taken advantage of in this invention. Namely, he has shown that
+    in a condenser a considerable amount of energy may be wasted, and
+    the condenser may break down merely because gaseous matter is
+    present between the surfaces. A number of experiments are described
+    in the lectures, which bring out this fact forcibly and serve as a
+    guide in the operation of high tension apparatus. But besides
+    bearing upon this point, these experiments also throw a light upon
+    investigations of a purely scientific nature and explain now the
+    lack of harmony among the observations of various investigators.
+    Mr. Tesla shows that in a fluid such as oil the losses are very
+    small as compared with those incurred in a gas.
+
+In the accompanying illustrations, Fig. 227 is a section of a condenser
+constructed in accordance with this principle and having stationary
+plates; and Fig. 228 is a similar view of a condenser with adjustable
+plates.
+
+Any suitable box or receptacle A may be used to contain the plates or
+armatures. These latter are designated by B and C and are connected,
+respectively, to terminals D and E, which pass out through the sides of
+the case. The plates ordinarily are separated by strips of porous
+insulating material F, which are used merely for the purpose of
+maintaining them in position. The space within the can is filled with
+oil G. Such a condenser will prove highly efficient and will not become
+heated or permanently injured.
+
+In many cases it is desirable to vary or adjust the capacity of a
+condenser, and this is provided for by securing the plates to adjustable
+supports--as, for example, to rods H--passing through stuffing boxes K
+in the sides of case A and furnished with nuts L, the ends of the rods
+being threaded for engagement with the nuts.
+
+It is well known that oils possess insulating properties, and it has
+been a common practice to interpose a body of oil between two conductors
+for purposes of insulation; but Mr. Tesla believes he has discovered
+peculiar properties in oils which render them very valuable in this
+particular form of device.
+
+
+
+
+CHAPTER XXXV.
+
+ELECTROLYTIC REGISTERING METER.
+
+
+An ingenious form of electrolytic meter attributable to Mr. Tesla is one
+in which a conductor is immersed in a solution, so arranged that metal
+may be deposited from the solution or taken away in such a manner that
+the electrical resistance of the conductor is varied in a definite
+proportion to the strength of the current the energy of which is to be
+computed, whereby this variation in resistance serves as a measure of
+the energy and also may actuate registering mechanism, whenever the
+resistance rises above or falls below certain limits.
+
+In carrying out this idea Mr. Tesla employs an electrolytic cell,
+through which extend two conductors parallel and in close proximity to
+each other. These conductors he connects in series through a resistance,
+but in such manner that there is an equal difference of potential
+between them throughout their entire extent. The free ends or terminals
+of the conductors are connected either in series in the circuit
+supplying the current to the lamps or other devices, or in parallel to a
+resistance in the circuit and in series with the current consuming
+devices. Under such circumstances a current passing through the
+conductors establishes a difference of potential between them which is
+proportional to the strength of the current, in consequence of which
+there is a leakage of current from one conductor to the other across the
+solution. The strength of this leakage current is proportional to the
+difference of potential, and, therefore, in proportion to the strength
+of the current passing through the conductors. Moreover, as there is a
+constant difference of potential between the two conductors throughout
+the entire extent that is exposed to the solution, the current density
+through such solution is the same at all corresponding points, and hence
+the deposit is uniform along the whole of one of the conductors, while
+the metal is taken away uniformly from the other. The resistance of one
+conductor is by this means diminished, while that of the other is
+increased, both in proportion to the strength of the current passing
+through the conductors. From such variation in the resistance of either
+or both of the conductors forming the positive and negative electrodes
+of the cell, the current energy expended may be readily computed. Figs.
+229 and 230 illustrate two forms of such a meter.
+
+[Illustration: FIG. 229.]
+
+In Fig. 229 G designates a direct-current generator. L L are the
+conductors of the circuit extending therefrom. A is a tube of glass, the
+ends of which are sealed, as by means of insulating plugs or caps B B. C
+C' are two conductors extending through the tube A, their ends passing
+out through the plugs B to terminals thereon. These conductors may be
+corrugated or formed in other proper ways to offer the desired
+electrical resistance. R is a resistance connected in series with the
+two conductors C C', which by their free terminals are connected up in
+circuit with one of the conductors L.
+
+The method of using this device and computing by means thereof the
+energy of the current will be readily understood. First, the resistances
+of the two conductors C C', respectively, are accurately measured and
+noted. Then a known current is passed through the instrument for a given
+time, and by a second measurement the increase and diminution of the
+resistances of the two conductors are respectively taken. From these
+data the constant is obtained--that is to say, for example, the
+increase of resistance of one conductor or the diminution of the
+resistance of the other per lamp hour. These two measurements evidently
+serve as a check, since the gain of one conductor should equal the loss
+of the other. A further check is afforded by measuring both wires in
+series with the resistance, in which case the resistance of the whole
+should remain constant.
+
+[Illustration: FIG. 230.]
+
+In Fig. 230 the conductors C C' are connected in parallel, the current
+device at X passing in one branch first through a resistance R' and then
+through conductor C, while on the other branch it passes first through
+conductor C', and then through resistance R''. The resistances R' R''
+are equal, as also are the resistances of the conductors C C'. It is,
+moreover, preferable that the respective resistances of the conductors C
+C' should be a known and convenient fraction of the coils or resistances
+R' R''. It will be observed that in the arrangement shown in Fig. 230
+there is a constant potential difference between the two conductors C C'
+throughout their entire length.
+
+It will be seen that in both cases illustrated, the proportionality of
+the increase or decrease of resistance to the current strength will
+always be preserved, for what one conductor gains the other loses, and
+the resistances of the conductors C C' being small as compared with the
+resistances in series with them. It will be understood that after each
+measurement or registration of a given variation of resistance in one or
+both conductors, the direction of the current should be changed or the
+instrument reversed, so that the deposit will be taken from the
+conductor which has gained and added to that which has lost. This
+principle is capable of many modifications. For instance, since there is
+a section of the circuit--to wit, the conductor C or C'--that varies in
+resistance in proportion to the current strength, such variation may be
+utilized, as is done in many analogous cases, to effect the operation of
+various automatic devices, such as registers. It is better, however, for
+the sake of simplicity to compute the energy by measurements of
+resistance.
+
+The chief advantages of this arrangement are, first, that it is possible
+to read off directly the amount of the energy expended by means of a
+properly constructed ohm-meter and without resorting to weighing the
+deposit; secondly it is not necessary to employ shunts, for the whole of
+the current to be measured may be passed through the instrument; third,
+the accuracy of the instrument and correctness of the indications are
+but slightly affected by changes in temperature. It is also said that
+such meters have the merit of superior economy and compactness, as well
+as of cheapness in construction. Electrolytic meters seem to need every
+auxiliary advantage to make them permanently popular and successful, no
+matter how much ingenuity may be shown in their design.
+
+
+
+
+CHAPTER XXXVI.
+
+THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS.
+
+
+No electrical inventor of the present day dealing with the problems of
+light and power considers that he has done himself or his opportunities
+justice until he has attacked the subject of thermo-magnetism. As far
+back as the beginning of the seventeenth century it was shown by Dr.
+William Gilbert, the father of modern electricity, that a loadstone or
+iron bar when heated to redness loses its magnetism; and since that time
+the influence of heat on the magnetic metals has been investigated
+frequently, though not with any material or practical result.
+
+For a man of Mr. Tesla's inventive ability, the problems in this field
+have naturally had no small fascination, and though he has but glanced
+at them, it is to be hoped he may find time to pursue the study deeper
+and further. For such as he, the investigation must undoubtedly bear
+fruit. Meanwhile he has worked out one or two operative devices worthy
+of note.[12] He obtains mechanical power by a reciprocating action
+resulting from the joint operations of heat, magnetism, and a spring or
+weight or other force--that is to say he subjects a body magnetized by
+induction or otherwise to the action of heat until the magnetism is
+sufficiently neutralized to allow a weight or spring to give motion to
+the body and lessen the action of the heat, so that the magnetism may be
+sufficiently restored to move the body in the opposite direction, and
+again subject the same to the demagnetizing power of the heat.
+
+  [12] It will, of course, be inferred from the nature of these devices
+       that the vibration obtained in this manner is very slow owing to
+       the inability of the iron to follow rapid changes in temperature.
+       In an interview with Mr. Tesla on this subject, the compiler
+       learned of an experiment which will interest students. A simple
+       horseshoe magnet is taken and a piece of sheet iron bent in the
+       form of an L is brought in contact with one of the poles and
+       placed in such a position that it is kept in the attraction of
+       the opposite pole delicately suspended. A spirit lamp is placed
+       under the sheet iron piece and when the iron is heated to a
+       certain temperature it is easily set in vibration oscillating
+       as rapidly as 400 to 500 times a minute. The experiment is very
+       easily performed and is interesting principally on account of the
+       very rapid rate of vibration.
+
+Use is made of either an electro-magnet or a permanent magnet, and the
+heat is directed against a body that is magnetized by induction, rather
+than directly against a permanent magnet, thereby avoiding the loss of
+magnetism that might result in the permanent magnet by the action of
+heat. Mr. Tesla also provides for lessening the volume of the heat or
+for intercepting the same during that portion of the reciprocation in
+which the cooling action takes place.
+
+In the diagrams are shown some of the numerous arrangements that may be
+made use of in carrying out this idea. In all of these figures the
+magnet-poles are marked N S, the armature A, the Bunsen burner or other
+source of heat H, the axis of motion M, and the spring or the equivalent
+thereof--namely, a weight--is marked W.
+
+[Illustration: FIG. 232.]
+
+[Illustration: FIG. 231.]
+
+[Illustration: FIG. 233.]
+
+
+In Fig. 231 the permanent magnet N is connected with a frame, F,
+supporting the axis M, from which the arm P hangs, and at the lower end
+of which the armature A is supported. The stops 2 and 3 limit the extent
+of motion, and the spring W tends to draw the armature A away from the
+magnet N. It will now be understood that the magnetism of N is
+sufficient to overcome the spring W and draw the armature A toward the
+magnet N. The heat acting upon the armature A neutralizes its induced
+magnetism sufficiently for the spring W to draw the armature A away from
+the magnet N and also from the heat at H. The armature now cools, and
+the attraction of the magnet N overcomes the spring W and draws the
+armature A back again above the burner H, so that the same is again
+heated and the operations are repeated. The reciprocating movements thus
+obtained are employed as a source of mechanical power in any desired
+manner. Usually a connecting-rod to a crank upon a fly-wheel shaft would
+be made use of, as indicated in Fig. 240.
+
+[Illustration: FIG. 234.]
+
+[Illustration: FIG. 236.]
+
+[Illustration: FIG. 235.]
+
+Fig. 232 represents the same parts as before described; but an
+electro-magnet is illustrated in place of a permanent magnet. The
+operations, however, are the same.
+
+In Fig. 233 are shown the same parts as in Figs. 231 and 232, but they
+are differently arranged. The armature A, instead of swinging, is
+stationary and held by arm P', and the core N S of the electro-magnet is
+made to swing within the helix Q, the core being suspended by the arm P
+from the pivot M. A shield, R, is connected with the magnet-core and
+swings with it, so that after the heat has demagnetized the armature A
+to such an extent that the spring W draws the core N S away from the
+armature A, the shield R comes between the flame H and armature A,
+thereby intercepting the action of the heat and allowing the armature to
+cool, so that the magnetism, again preponderating, causes the movement
+of the core N S toward the armature A and the removal of the shield R
+from above the flame, so that the heat again acts to lessen or
+neutralize the magnetism. A rotary or other movement may be obtained
+from this reciprocation.
+
+Fig. 234 corresponds in every respect with Fig. 233, except that a
+permanent horseshoe-magnet, N S is represented as taking the place of
+the electro-magnet in Fig. 233.
+
+In Fig. 235 is shown a helix, Q, with an armature adapted to swing
+toward or from the helix. In this case there may be a soft-iron core in
+the helix, or the armature may assume the form of a solenoid core, there
+being no permanent core within the helix.
+
+[Illustration: FIG. 237.]
+
+[Illustration: FIG. 238.]
+
+[Illustration: FIG. 239.]
+
+
+Fig. 236 is an end view, and Fig. 237 a plan view, illustrating the
+method as applied to a swinging armature, A, and a stationary permanent
+magnet, N S. In this instance Mr. Tesla applies the heat to an auxiliary
+armature or keeper, T, which is adjacent to and preferably in direct
+contact with the magnet. This armature T, in the form of a plate of
+sheet-iron, extends across from one pole to the other and is of
+sufficient section to practically form a keeper for the magnet, so that
+when the armature T is cool nearly all the lines of force pass over the
+same and very little free magnetism is exhibited. Then the armature A,
+which swings freely on the pivots M in front of the poles N S, is very
+little attracted and the spring W pulls the same way from the poles into
+the position indicated in the diagram. The heat is directed upon the
+iron plate T at some distance from the magnet, so as to allow the magnet
+to keep comparatively cool. This heat is applied beneath the plate by
+means of the burners H, and there is a connection from the armature A or
+its pivot to the gas-cock 6, or other device for regulating the heat.
+The heat acting upon the middle portion of the plate T, the magnetic
+conductivity of the heated portion is diminished or destroyed, and a
+great number of the lines of force are deflected over the armature A,
+which is now powerfully attracted and drawn into line, or nearly so,
+with the poles N S. In so doing the cock 6 is nearly closed and the
+plate T cools, the lines of force are again deflected over the same, the
+attraction exerted upon the armature A is diminished, and the spring W
+pulls the same away from the magnet into the position shown by full
+lines, and the operations are repeated. The arrangement shown in Fig.
+236 has the advantages that the magnet and armature are kept cool and
+the strength of the permanent magnet is better preserved, as the
+magnetic circuit is constantly closed.
+
+In the plan view, Fig. 238, is shown a permanent magnet and keeper
+plate, T, similar to those in Figs. 236 and 237, with the burners H for
+the gas beneath the same; but the armature is pivoted at one end to one
+pole of the magnet and the other end swings toward and from the other
+pole of the magnet. The spring W acts against a lever arm that projects
+from the armature, and the supply of heat has to be partly cut off by a
+connection to the swinging armature, so as to lessen the heat acting
+upon the keeper plate when the armature A has been attracted.
+
+[Illustration: FIG. 240.]
+
+[Illustration: FIG. 241.]
+
+Fig. 239 is similar to Fig. 238, except that the keeper T is not made
+use of and the armature itself swings into and out of the range of the
+intense action of the heat from the burner H. Fig. 240 is a diagram
+similar to Fig. 231, except that in place of using a spring and stops,
+the armature is shown as connected by a link, to the crank of a
+fly-wheel, so that the fly-wheel will be revolved as rapidly as the
+armature can be heated and cooled to the necessary extent. A spring may
+be used in addition, as in Fig. 231. In Fig. 241 the armatures A A are
+connected by a link, so that one will be heating while the other is
+cooling, and the attraction exerted to move the cooled armature is
+availed of to draw away the heated armature instead of using a spring.
+
+Mr. Tesla has also devoted his attention to the development of a
+pyromagnetic generator of electricity[13] based upon the following laws:
+First, that electricity or electrical energy is developed in any
+conducting body by subjecting such body to a varying magnetic influence;
+and second, that the magnetic properties of iron or other magnetic
+substance may be partially or entirely destroyed or caused to disappear
+by raising it to a certain temperature, but restored and caused to
+reappear by again lowering its temperature to a certain degree. These
+laws may be applied in the production of electrical currents in many
+ways, the principle of which is in all cases the same, viz., to subject
+a conductor to a varying magnetic influence, producing such variations
+by the application of heat, or, more strictly speaking, by the
+application or action of a varying temperature upon the source of the
+magnetism. This principle of operation may be illustrated by a simple
+experiment: Place end to end, and preferably in actual contact, a
+permanently magnetized steel bar and a strip or bar of soft iron. Around
+the end of the iron bar or plate wind a coil of insulated wire. Then
+apply to the iron between the coil and the steel bar a flame or other
+source of heat which will be capable of raising that portion of the iron
+to an orange red, or a temperature of about 600 deg. centigrade. When this
+condition is reached, the iron somewhat suddenly loses its magnetic
+properties, if it be very thin, and the same effect is produced as
+though the iron had been moved away from the magnet or the heated
+section had been removed. This change of position, however, is
+accompanied by a shifting of the magnetic lines, or, in other words, by
+a variation in the magnetic influence to which the coil is exposed, and
+a current in the coil is the result. Then remove the flame or in any
+other way reduce the temperature of the iron. The lowering of its
+temperature is accompanied by a return of its magnetic properties, and
+another change of magnetic conditions occurs, accompanied by a current
+in an opposite direction in the coil. The same operation may be
+repeated indefinitely, the effect upon the coil being similar to that
+which would follow from moving the magnetized bar to and from the end of
+the iron bar or plate.
+
+  [13] The chief point to be noted is that Mr. Tesla attacked this
+       problem in a way which was, from the standpoint of theory, and
+       that of an engineer, far better than that from which some
+       earlier trials in this direction started. The enlargement of
+       these ideas will be found in Mr. Tesla's work on the pyromagnetic
+       generator, treated in this chapter. The chief effort of the
+       inventor was to economize the heat, which was accomplished by
+       inclosing the iron in a source of heat well insulated, and by
+       cooling the iron by means of steam, utilizing the steam over
+       again. The construction also permits of more rapid magnetic
+       changes per unit of time, meaning larger output.
+
+The device illustrated below is a means of obtaining this result, the
+features of novelty in the invention being, first, the employment of an
+artificial cooling device, and, second, inclosing the source of heat and
+that portion of the magnetic circuit exposed to the heat and
+artificially cooling the heated part.
+
+These improvements are applicable generally to the generators
+constructed on the plan above described--that is to say, we may use an
+artificial cooling device in conjunction with a variable or varied or
+uniform source of heat.
+
+[Illustration: FIG. 242.]
+
+[Illustration: FIG. 243.]
+
+Fig. 242 is a central vertical longitudinal section of the complete
+apparatus and Fig. 243 is a cross-section of the magnetic armature-core
+of the generator.
+
+Let A represent a magnetized core or permanent magnet the poles of which
+are bridged by an armature-core composed of a casing or shell B
+inclosing a number of hollow iron tubes C. Around this core are wound
+the conductors E E', to form the coils in which the currents are
+developed. In the circuits of these coils are current-consuming devices,
+as F F'.
+
+D is a furnace or closed fire-box, through which the central portion of
+the core B extends. Above the fire is a boiler K, containing water. The
+flue L from the fire-box may extend up through the boiler.
+
+G is a water-supply pipe, and H is the steam-exhaust pipe, which
+communicates with all the tubes C in the armature B, so that steam
+escaping from the boiler will pass through the tubes.
+
+In the steam-exhaust pipe H is a valve V, to which is connected the
+lever I, by the movement of which the valve is opened or closed. In such
+a case as this the heat of the fire may be utilized for other purposes
+after as much of it as may be needed has been applied to heating the
+core B. There are special advantages in the employment of a cooling
+device, in that the metal of the core B is not so quickly oxidized.
+Moreover, the difference between the temperature of the applied heat and
+of the steam, air, or whatever gas or fluid be applied as the cooling
+medium, may be increased or decreased at will, whereby the rapidity of
+the magnetic changes or fluctuations may be regulated.
+
+
+
+
+CHAPTER XXXVII.
+
+ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR.
+
+
+In direct current dynamos of great electromotive force--such, for
+instance, as those used for arc lighting--when one commutator bar or
+plate comes out of contact with the collecting-brush a spark is apt to
+appear on the commutator. This spark may be due to the break of the
+complete circuit, or to a shunt of low resistance formed by the brush
+between two or more commutator-bars. In the first case the spark is more
+apparent, as there is at the moment when the circuit is broken a
+discharge of the magnets through the field helices, producing a great
+spark or flash which causes an unsteady current, rapid wear of the
+commutator bars and brushes, and waste of power. The sparking may be
+reduced by various devices, such as providing a path for the current at
+the moment when the commutator segment or bar leaves the brush, by
+short-circuiting the field-helices, by increasing the number of the
+commutator-bars, or by other similar means; but all these devices are
+expensive or not fully available, and seldom attain the object desired.
+
+To prevent this sparking in a simple manner, Mr. Tesla some years ago
+employed with the commutator-bars and intervening insulating material,
+mica, asbestos paper or other insulating and incombustible material,
+arranged to bear on the surface of the commutator, near to and behind
+the brush.
+
+In the drawings, Fig. 244 is a section of a commutator with an asbestos
+insulating device; and Fig. 245 is a similar view, representing two
+plates of mica upon the back of the brush.
+
+In 244, C represents the commutator and intervening insulating material;
+B B, the brushes. _d d_ are sheets of asbestos paper or other suitable
+non-conducting material. _f f_ are springs, the pressure of which may be
+adjusted by means of the screws _g g_.
+
+In Fig. 245 a simple arrangement is shown with two plates of mica or
+other material. It will be seen that whenever one commutator segment
+passes out of contact with the brush, the formation of the arc will be
+prevented by the intervening insulating material coming in contact with
+the insulating material on the brush.
+
+[Illustration: FIG. 244.]
+
+[Illustration: FIG. 245.]
+
+Asbestos paper or cloth impregnated with zinc-oxide, magnesia, zirconia,
+or other suitable material, may be used, as the paper and cloth are
+soft, and serve at the same time to wipe and polish the commutator; but
+mica or any other suitable material can be employed, provided the
+material be an insulator or a bad conductor of electricity.
+
+A few years later Mr. Tesla turned his attention again to the same
+subject, as, perhaps, was very natural in view of the fact that the
+commutator had always been prominent in his thoughts, and that so much
+of his work was even aimed at dispensing with it entirely as an
+objectionable and unnecessary part of dynamos and motors. In these later
+efforts to remedy commutator troubles, Mr. Tesla constructs a commutator
+and the collectors therefor in two parts mutually adapted to one
+another, and, so far as the essential features are concerned, alike in
+mechanical structure. Selecting as an illustration a commutator of two
+segments adapted for use with an armature the coils or coil of which
+have but two free ends, connected respectively to the segments, the
+bearing-surface is the face of a disc, and is formed of two metallic
+quadrant segments and two insulating segments of the same dimensions,
+and the face of the disc is smoothed off, so that the metal and
+insulating segments are flush. The part which takes the place of the
+usual brushes, or the "collector," is a disc of the same character as
+the commutator and has a surface similarly formed with two insulating
+and two metallic segments. These two parts are mounted with their faces
+in contact and in such manner that the rotation of the armature causes
+the commutator to turn upon the collector, whereby the currents induced
+in the coils are taken off by the collector segments and thence
+conveyed off by suitable conductors leading from the collector segments.
+This is the general plan of the construction adopted. Aside from certain
+adjuncts, the nature and functions of which are set forth later, this
+means of commutation will be seen to possess many important advantages.
+In the first place the short-circuiting and the breaking of the armature
+coil connected to the commutator-segments occur at the same instant, and
+from the nature of the construction this will be done with the greatest
+precision; secondly, the duration of both the break and of the short
+circuit will be reduced to a minimum. The first results in a reduction
+which amounts practically to a suppression of the spark, since the break
+and the short circuit produce opposite effects in the armature-coil. The
+second has the effect of diminishing the destructive effect of a spark,
+since this would be in a measure proportional to the duration of the
+spark; while lessening the duration of the short circuit obviously
+increases the efficiency of the machine.
+
+[Illustration: FIG. 246.]
+
+[Illustration: FIG. 247.]
+
+The mechanical advantages will be better understood by referring to the
+accompanying diagrams, in which Fig. 246 is a central longitudinal
+section of the end of a shaft with the improved commutator carried
+thereon. Fig. 247 is a view of the inner or bearing face of the
+collector. Fig. 248 is an end view from the armature side of a modified
+form of commutator. Figs. 249 and 250 are views of details of Fig. 248.
+Fig. 251 is a longitudinal central section of another modification, and
+Fig. 252 is a sectional view of the same. A is the end of the
+armature-shaft of a dynamo-electric machine or motor. A' is a sleeve of
+insulating material around the shaft, secured in place by a screw, _a'_.
+
+[Illustration: FIG. 248.]
+
+[Illustration: FIG. 249.]
+
+[Illustration: FIG. 250.]
+
+The commutator proper is in the form of a disc which is made up of four
+segments D D' G G', similar to those shown in Fig. 248. Two of these
+segments, as D D', are of metal and are in electrical connection with
+the ends of the coils on the armature. The other two segments are of
+insulating material. The segments are held in place by a band, B, of
+insulating material. The disc is held in place by friction or by screws,
+_g' g'_, Fig. 248, which secure the disc firmly to the sleeve A'.
+
+The collector is made in the same form as the commutator. It is composed
+of the two metallic segments E E' and the two insulating segments F F',
+bound together by a band, C. The metallic segments E E' are of the same
+or practically the same width or extent as the insulating segments or
+spaces of the commutator. The collector is secured to a sleeve, B', by
+screws _g g_, and the sleeve is arranged to turn freely on the shaft A.
+The end of the sleeve B' is closed by a plate, _f_, upon which presses a
+pivot-pointed screw, _h_, adjustable in a spring, H, which acts to
+maintain the collector in close contact with the commutator and to
+compensate for the play of the shaft. The collector is so fixed that it
+cannot turn with the shaft. For example, the diagram shows a slotted
+plate, K, which is designed to be attached to a stationary support, and
+an arm extending from the collector and carrying a clamping screw, L, by
+which the collector may be adjusted and set to the desired position.
+
+Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit the
+insulating segments of both commutator and collector loosely and to
+provide some means--as, for example, light springs, _e e_, secured to
+the bands A' B', respectively, and bearing against the segments--to
+exert a light pressure upon them and keep them in close contact and to
+compensate for wear. The metal segments of the commutator may be moved
+forward by loosening the screw _a'_.
+
+The line wires are fed from the metal segments of the collector, being
+secured thereto in any convenient manner, the plan of connections being
+shown as applied to a modified form of the commutator in Fig. 251. The
+commutator and the collector in thus presenting two flat and smooth
+bearing surfaces prevent most effectually by mechanical action the
+occurrence of sparks.
+
+The insulating segments are made of some hard material capable of being
+polished and formed with sharp edges. Such materials as glass, marble,
+or soapstone may be advantageously used. The metal segments are
+preferably of copper or brass; but they may have a facing or edge of
+durable material--such as platinum or the like--where the sparks are
+liable to occur.
+
+[Illustration: FIG. 251.]
+
+[Illustration: FIG. 252.]
+
+In Fig. 248 a somewhat modified form of the invention is shown, a form
+designed to facilitate the construction and replacing of the parts. In
+this modification the commutator and collector are made in substantially
+the same manner as previously described, except that the bands B C are
+omitted. The four segments of each part, however, are secured to their
+respective sleeves by screws _g' g'_, and one edge of each segment is
+cut away, so that small plates _a b_ may be slipped into the spaces thus
+formed. Of these plates _a a_ are of metal, and are in contact with the
+metal segments D D', respectively. The other two, _b b_, are of glass or
+marble, and they are all better square, as shown in Figs. 249 and 250,
+so that they may be turned to present new edges should any edge become
+worn by use. Light springs _d_ bear upon these plates and press those in
+the commutator toward those in the collector, and insulating strips _c
+c_ are secured to the periphery of the discs to prevent the blocks from
+being thrown out by centrifugal action. These plates are, of course,
+useful at those edges of the segments only where sparks are liable to
+occur, and, as they are easily replaced, they are of great advantage. It
+is considered best to coat them with platinum or silver.
+
+In Figs. 251 and 252 is shown a construction where, instead of solid
+segments, a fluid is employed. In this case the commutator and collector
+are made of two insulating discs, S T, and in lieu of the metal segments
+a space is cut out of each part, as at R R', corresponding in shape and
+size to a metal segment. The two parts are fitted smoothly and the
+collector T held by the screw _h_ and spring H against the commutator S.
+As in the other cases, the commutator revolves while the collector
+remains stationary. The ends of the coils are connected to binding-posts
+_s s_, which are in electrical connection with metal plates _t t_ within
+the recesses in the two parts S T. These chambers or recesses are filled
+with mercury, and in the collector part are tubes W W, with screws _w
+w_, carrying springs X and pistons X', which compensate for the
+expansion and contraction of the mercury under varying temperatures, but
+which are sufficiently strong not to yield to the pressure of the fluid
+due to centrifugal action, and which serve as binding-posts.
+
+In all the above cases the commutators are adapted for a single coil,
+and the device is particularly suited to such purposes. The number of
+segments may be increased, however, or more than one commutator used
+with a single armature. Although the bearing-surfaces are shown as
+planes at right angles to the shaft or axis, it is evident that in this
+particular the construction may be very greatly modified.
+
+
+
+
+CHAPTER XXXVIII.
+
+AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DYNAMOS.
+
+
+An interesting method devised by Mr. Tesla for the regulation of direct
+current dynamos, is that which has come to be known as the "third brush"
+method. In machines of this type, devised by him as far back as 1885, he
+makes use of two main brushes to which the ends of the field magnet
+coils are connected, an auxiliary brush, and a branch or shunt
+connection from an intermediate point of the field wire to the auxiliary
+brush.[14]
+
+  [14] The compiler has learned partially from statements made on
+       several occasions in journals and partially by personal inquiry
+       of Mr. Tesla, that a great deal of work in this interesting line
+       is unpublished. In these inventions as will be seen, the brushes
+       are automatically shifted, but in the broad method barely
+       suggested here the regulation is effected without any change in
+       the position of the brushes. This auxiliary brush invention, it
+       will be remembered, was very much discussed a few years ago, and
+       it may be of interest that this work of Mr. Tesla, then unknown
+       in this field, is now brought to light.
+
+The relative positions of the respective brushes are varied, either
+automatically or by hand, so that the shunt becomes inoperative when the
+auxiliary brush has a certain position upon the commutator; but when the
+auxiliary brush is moved in its relation to the main brushes, or the
+latter are moved in their relation to the auxiliary brush, the electric
+condition is disturbed and more or less of the current through the
+field-helices is diverted through the shunt or a current is passed over
+the shunt to the field-helices. By varying the relative position upon
+the commutator of the respective brushes automatically in proportion to
+the varying electrical conditions of the working-circuit, the current
+developed can be regulated in proportion to the demands in the
+working-circuit.
+
+Fig. 253 is a diagram illustrating the invention, showing one core of
+the field-magnets with one helix wound in the same direction throughout.
+Figs. 254 and 255 are diagrams showing one core of the field-magnets
+with a portion of the helices wound in opposite directions. Figs. 256
+and 257 are diagrams illustrating the electric devices that may be
+employed for automatically adjusting the brushes, and Fig. 258 is a
+diagram illustrating the positions of the brushes when the machine is
+being energized at the start.
+
+_a_ and _b_ are the positive and negative brushes of the main or
+working-circuit, and _c_ the auxiliary brush. The working-circuit D
+extends from the brushes _a_ and _b_, as usual, and contains electric
+lamps or other devices, D', either in series or in multiple arc.
+
+M M' represent the field-helices, the ends of which are connected to the
+main brushes _a_ and _b_. The branch or shunt wire _c'_ extends from the
+auxiliary brush _c_ to the circuit of the field-helices, and is
+connected to the same at an intermediate point, _x_.
+
+[Illustration: FIG. 253.]
+
+H represents the commutator, with the plates of ordinary construction.
+When the auxiliary brush _c_ occupies such a position upon the
+commutator that the electro-motive force between the brushes _a_ and _c_
+is to the electro-motive force between the brushes _c_ and _b_ as the
+resistance of the circuit _a_ M _c' c_ A is to the resistance of the
+circuit _b_ M' _c' c_ B, the potentials of the points _x_ and Y will be
+equal, and no current will flow over the auxiliary brush; but when the
+brush _c_ occupies a different position the potentials of the points _x_
+and Y will be different, and a current will flow over the auxiliary
+brush to and from the commutator, according to the relative position of
+the brushes. If, for instance, the commutator-space between the brushes
+_a_ and _c_, when the latter is at the neutral point, is diminished, a
+current will flow from the point Y over the shunt _c_ to the brush _b_,
+thus strengthening the current in the part M', and partly neutralizing
+the current in part M; but if the space between the brushes _a_ and _c_
+is increased, the current will flow over the auxiliary brush in an
+opposite direction, and the current in M will be strengthened, and in
+M', partly neutralized.
+
+By combining with the brushes _a_, _b_, and _c_ any usual automatic
+regulating mechanism, the current developed can be regulated in
+proportion to the demands in the working circuit. The parts M and M' of
+the field wire may be wound in the same direction. In this case they are
+arranged as shown in Fig. 253; or the part M may be wound in the
+opposite direction, as shown in Figs. 254 and 255.
+
+[Illustration: FIG. 254.]
+
+It will be apparent that the respective cores of the field-magnets are
+subjected to neutralizing or intensifying effects of the current in the
+shunt through _c'_, and the magnetism of the cores will be partially
+neutralized, or the points of greatest magnetism shifted, so that it
+will be more or less remote from or approaching to the armature, and
+hence the aggregate energizing actions of the field magnets on the
+armature will be correspondingly varied.
+
+In the form indicated in Fig. 253 the regulation is effected by shifting
+the point of greatest magnetism, and in Figs. 254 and 255 the same
+effect is produced by the action of the current in the shunt passing
+through the neutralizing helix.
+
+The relative positions of the respective brushes may be varied by moving
+the auxiliary brush, or the brush _c_ may remain stationary and the core
+P be connected to the main-brush holder A, so as to adjust the brushes
+_a b_ in their relation to the brush _c_. If, however, an adjustment is
+applied to all the brushes, as seen in Fig. 257, the solenoid should be
+connected to both _a_ and _c_, so as to move them toward or away from
+each other.
+
+There are several known devices for giving motion in proportion to an
+electric current. In Figs. 256 and 257 the moving cores are shown as
+convenient devices for obtaining the required extent of motion with very
+slight changes in the current passing through the helices. It is
+understood that the adjustment of the main brushes causes variations in
+the strength of the current independently of the relative position of
+those brushes to the auxiliary brush. In all cases the adjustment should
+be such that no current flows over the auxiliary brush when the dynamo
+is running with its normal load.
+
+In Figs. 256 and 257 A A indicate the main-brush holder, carrying the
+main brushes, and C the auxiliary-brush holder, carrying the auxiliary
+brush. These brush-holders are movable in arcs concentric with the
+centre of the commutator-shaft. An iron piston, P, of the solenoid S,
+Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is
+effected by means of a spring and screw or tightener.
+
+In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is
+shown. The piston of the coil is attached to both brush-holders A A and
+C. When the brushes are moved directly by electrical devices, as shown
+in Figs. 256 and 257, these are so constructed that the force exerted
+for adjusting is practically uniform through the whole length of motion.
+
+[Illustration: FIG. 255.]
+
+It is true that auxiliary brushes have been used in connection with the
+helices of the field-wire; but in these instances the helices receive
+the entire current through the auxiliary brush or brushes, and these
+brushes could not be taken off without breaking the circuit through the
+field. These brushes cause, moreover, heavy sparking at the commutator.
+In the present case the auxiliary brush causes very little or no
+sparking, and can be taken off without breaking the circuit through the
+field-helices. The arrangement has, besides, the advantage of
+facilitating the self-excitation of the machine in all cases where the
+resistance of the field-wire is very great comparatively to the
+resistance of the main circuit at the start--for instance, on arc-light
+machines. In this case the auxiliary brush _c_ is placed near to, or
+better still in contact with, the brush _b_, as shown in Fig. 258. In
+this manner the part M' is completely cut out, and as the part M has a
+considerably smaller resistance than the whole length of the field-wire
+the machine excites itself, whereupon the auxiliary brush is shifted
+automatically to its normal position.
+
+[Illustration: FIG. 256.]
+
+[Illustration: FIG. 257.]
+
+In a further method devised by Mr. Tesla, one or more auxiliary brushes
+are employed, by means of which a portion or the whole of the field
+coils is shunted. According to the relative position upon the commutator
+of the respective brushes more or less current is caused to pass through
+the helices of the field, and the current developed by the machine can
+be varied at will by varying the relative positions of the brushes.
+
+[Illustration: FIG. 258.]
+
+In Fig. 259, _a_ and _b_ are the positive and negative brushes of the
+main circuit, and _c_ an auxiliary brush. The main circuit D extends
+from the brushes _a_ and _b_, as usual, and contains the helices M of
+the field wire and the electric lamps or other working devices. The
+auxiliary brush _c_ is connected to the point _x_ of the main circuit by
+means of the wire _c'_. H is a commutator of ordinary construction. It
+will have been seen from what was said already that when the
+electro-motive force between the brushes _a_ and _c_ is to the
+electromotive force between the brushes _c_ and _b_ as the resistance of
+the circuit _a_ M _c' c_ A is to the resistance of the circuit _b_ C B
+_c c'_ D, the potentials of the points _x_ and _y_ will be equal, and no
+current will pass over the auxiliary brush _c_; but if that brush
+occupies a different position relatively to the main brushes the
+electric condition is disturbed, and current will flow either from _y_
+to _x_ or from _x_ to _y_, according to the relative position of the
+brushes. In the first case the current through the field-helices will be
+partly neutralized and the magnetism of the field magnets will be
+diminished. In the second case the current will be increased and the
+magnets gain strength. By combining with the brushes at _a b c_ any
+automatic regulating mechanism, the current developed can be regulated
+automatically in proportion to the demands of the working circuit.
+
+In Figs. 264 and 265 some of the automatic means are represented that
+maybe used for moving the brushes. The core P, Fig. 264, of the
+solenoid-helix S is connected with the brush _a_ to move the same, and
+in Fig. 265 the core P is shown as within the helix S, and connected
+with brushes _a_ and _c_, so as to move the same toward or from each
+other, according to the strength of the current in the helix, the helix
+being within an iron tube, S', that becomes magnetized and increases the
+action of the solenoid.
+
+In practice it is sufficient to move only the auxiliary brush, as shown
+in Fig. 264, as the regulation is very sensitive to the slightest
+changes; but the relative position of the auxiliary brush to the main
+brushes may be varied by moving the main brushes, or both main and
+auxiliary brushes may be moved, as illustrated in Fig. 265. In the
+latter two cases, it will be understood, the motion of the main brushes
+relatively to the neutral line of the machine causes variations in the
+strength of the current independently of their relative position to the
+auxiliary brush. In all cases the adjustment may be such that when the
+machine is running with the ordinary load, no current flows over the
+auxiliary brush.
+
+The field helices may be connected, as shown in Fig. 259, or a part of
+the field helices may be in the outgoing and the other part in the
+return circuit, and two auxiliary brushes may be employed as shown in
+Figs. 261 and 262. Instead of shunting the whole of the field helices, a
+portion only of such helices may be shunted, as shown in Figs. 260 and
+262.
+
+The arrangement shown in Fig. 262 is advantageous, as it diminishes the
+sparking upon the commutator, the main circuit being closed through the
+auxiliary brushes at the moment of the break of the circuit at the main
+brushes.
+
+[Illustration: FIG. 259.]
+
+[Illustration: FIG. 260.]
+
+[Illustration: FIG. 261.]
+
+[Illustration: FIG. 262.]
+
+[Illustration: FIG. 263.]
+
+The field helices may be wound in the same direction, or a part may be
+wound in opposite directions.
+
+The connection between the helices and the auxiliary brush or brushes
+may be made by a wire of small resistance, or a resistance may be
+interposed (R, Fig. 263,) between the point _x_ and the auxiliary brush
+or brushes to divide the sensitiveness when the brushes are adjusted.
+
+[Illustration: FIG. 264.]
+
+[Illustration: FIG. 265.]
+
+The accompanying sketches also illustrate improvements made by Mr. Tesla
+in the mechanical devices used to effect the shifting of the brushes, in
+the use of an auxiliary brush. Fig. 266 is an elevation of the regulator
+with the frame partly in section; and Fig. 267 is a section at the line
+_x x_, Fig. 266. C is the commutator; B and B', the brush-holders, B
+carrying the main brushes _a a'_, and B' the auxiliary or shunt brushes
+_b b_. The axis of the brush-holder B is supported by two pivot-screws,
+_p p_. The other brush-holder, B', has a sleeve, _d_, and is movable
+around the axis of the brush-holder B. In this way both brush-holders
+can turn very freely, the friction of the parts being reduced to a
+minimum. Over the brush-holders is mounted the solenoid S, which rests
+upon a forked column, _c_. This column also affords a support for the
+pivots _p p_, and is fastened upon a solid bracket or projection, P,
+which extends from the base of the machine, and is cast in one piece
+with the same. The brush-holders B B' are connected by means of the
+links _e e_ and the cross-piece F to the iron core I, which slides
+freely in the tube T of the solenoid. The iron core I has a screw, _s_,
+by means of which it can be raised and adjusted in its position
+relatively to the solenoid, so that the pull exerted upon it by the
+solenoid is practically uniform through the whole length of motion which
+is required to effect the regulation. In order to effect the adjustment
+with greater precision, the core I is provided with a small iron screw,
+_s'_. The core being first brought very nearly in the required position
+relatively to the solenoid by means of the screw _s_, the small screw
+_s'_ is then adjusted until the magnetic attraction upon the core is the
+same when the core is in any position. A convenient stop, _t_, serves to
+limit the upward movement of the iron core.
+
+To check somewhat the movement of the core I, a dash-pot, K, is used.
+The piston L of the dash-pot is provided with a valve, V, which opens by
+a downward pressure and allows an easy downward movement of the iron
+core I, but closes and checks the movement of the core when it is pulled
+up by the action of the solenoid.
+
+To balance the opposing forces, the weight of the moving parts, and the
+pull exerted by the solenoid upon the iron core, the weights W W may be
+used. The adjustment is such that when the solenoid is traversed by the
+normal current it is just strong enough to balance the downward pull of
+the parts.
+
+[Illustration: FIG. 266.]
+
+[Illustration: FIG. 267.]
+
+The electrical circuit-connections are substantially the same as
+indicated in the previous diagrams, the solenoid being in series with
+the circuit when the translating devices are in series, and in shunt
+when the devices are in multiple arc. The operation of the device is as
+follows: When upon a decrease of the resistance of the circuit or for
+some other reason, the current is increased, the solenoid S gains in
+strength and pulls up the iron core I, thus shifting the main brushes in
+the direction of rotation and the auxiliary brushes in the opposite way.
+This diminishes the strength of the current until the opposing forces
+are balanced and the solenoid is traversed by the normal current; but if
+from any cause the current in the circuit is diminished, then the weight
+of the moving parts overcomes the pull of the solenoid, the iron core I
+descends, thus shifting the brushes the opposite way and increasing the
+current to the normal strength. The dash-pot connected to the iron core
+I may be of ordinary construction; but it is better, especially in
+machines for arc lights, to provide the piston of the dash-pot with a
+valve, as indicated in the diagrams. This valve permits a comparatively
+easy downward movement of the iron core, but checks its movement when it
+is drawn up by the solenoid. Such an arrangement has the advantage that
+a great number of lights may be put on without diminishing the
+light-power of the lamps in the circuit, as the brushes assume at once
+the proper position. When lights are cut out, the dash-pot acts to
+retard the movement; but if the current is considerably increased the
+solenoid gets abnormally strong and the brushes are shifted instantly.
+The regulator being properly adjusted, lights or other devices may be
+put on or out with scarcely any perceptible difference. It is obvious
+that instead of the dash-pot any other retarding device may be used.
+
+
+
+
+CHAPTER XXXIX.
+
+IMPROVEMENT IN THE CONSTRUCTION OF DYNAMOS AND MOTORS.
+
+
+This invention of Mr. Tesla is an improvement in the construction of
+dynamo or magneto electric machines or motors, consisting in a novel
+form of frame and field magnet which renders the machine more solid and
+compact as a structure, which requires fewer parts, and which involves
+less trouble and expense in its manufacture. It is applicable to
+generators and motors generally, not only to those which have
+independent circuits adapted for use in the Tesla alternating current
+system, but to other continuous or alternating current machines of the
+ordinary type generally used.
+
+Fig. 268 shows the machine in side elevation. Fig. 269 is a vertical
+sectional view of the field magnets and frame and an end view of the
+armature; and Fig. 270 is a plan view of one of the parts of the frame
+and the armature, a portion of the latter being cut away.
+
+The field magnets and frame are cast in two parts. These parts are
+identical in size and shape, and each consists of the solid plates or
+ends A B, from which project inwardly the cores C D and the side bars or
+bridge pieces, E F. The precise shape of these parts is largely a matter
+of choice--that is to say, each casting, as shown, forms an
+approximately rectangular frame; but it might obviously be more or less
+oval, round, or square, without departure from the invention. It is also
+desirable to reduce the width of the side bars, E F, at the center and
+to so proportion the parts that when the frame is put together the
+spaces between the pole pieces will be practically equal to the arcs
+which the surfaces of the poles occupy.
+
+The bearings G for the armature shaft are cast in the side bars E F. The
+field coils are either wound on the pole pieces or on a form and then
+slipped on over the ends of the pole pieces. The lower part or casting
+is secured to the base after being finished off. The armature K on its
+shaft is then mounted in the bearings of the lower casting and the
+other part of the frame placed in position, dowel pins L or any other
+means being used to secure the two parts in proper position.
+
+[Illustration: FIG. 268.]
+
+[Illustration: FIG. 269.]
+
+[Illustration: FIG. 270.]
+
+In order to secure an easier fit, the side bars E F, and end pieces, A
+B, are so cast that slots M are formed when the two parts are put
+together.
+
+This machine possesses several advantages. For example, if we magnetize
+the cores alternately, as indicated by the characters N S, it will be
+seen that the magnetic circuit between the poles of each part of a
+casting is completed through the solid iron side bars. The bearings for
+the shaft are located at the neutral points of the field, so that the
+armature core is not affected by the magnetic condition of the field.
+
+The improvement is not restricted to the use of four pole pieces, as it
+is evident that each pole piece could be divided or more than four
+formed by the shape of the casting.
+
+
+
+
+CHAPTER XL.
+
+TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.
+
+
+At one time, soon after his arrival in America, Mr. Tesla was greatly
+interested in the subject of arc lighting, which then occupied public
+attention and readily enlisted the support of capital. He therefore
+worked out a system which was confided to a company formed for its
+exploitation, and then proceeded to devote his energies to the
+perfection of the details of his more celebrated "rotary field" motor
+system. The Tesla arc lighting apparatus appeared at a time when a great
+many other lamps and machines were in the market, but it commanded
+notice by its ingenuity. Its chief purpose was to lessen the
+manufacturing cost and simplify the processes of operation.
+
+We will take up the dynamo first. Fig. 271 is a longitudinal section,
+and Fig. 272 a cross section of the machine. Fig. 273 is a top view, and
+Fig. 274 a side view of the magnetic frame. Fig. 275 is an end view of
+the commutator bars, and Fig. 276 is a section of the shaft and
+commutator bars. Fig. 277 is a diagram illustrating the coils of the
+armature and the connections to the commutator plates.
+
+The cores _c c c c_ of the field-magnets are tapering in both
+directions, as shown, for the purposes of concentrating the magnetism
+upon the middle of the pole-pieces.
+
+The connecting-frame F F of the field-magnets is in the form indicated
+in the side view, Fig. 274, the lower part being provided with the
+spreading curved cast legs _e e_, so that the machine will rest firmly
+upon two base-bars, _r r_.
+
+To the lower pole, S, of the field-magnet M is fastened, by means of
+babbitt or other fusible diamagnetic material, the base B, which is
+provided with bearings _b_ for the armature-shaft H. The base B has a
+projection, P, which supports the brush-holders and the regulating
+devices, which are of a special character devised by Mr. Tesla.
+
+The armature is constructed with the view to reduce to a minimum the
+loss of power due to Foucault currents and to the change of polarity,
+and also to shorten as much as possible the length of the inactive wire
+wound upon the armature core.
+
+[Illustration: FIG. 271.]
+
+It is well known that when the armature is revolved between the poles of
+the field-magnets, currents are generated in the iron body of the
+armature which develop heat, and consequently cause a waste of power.
+Owing to the mutual action of the lines of force, the magnetic
+properties of iron, and the speed of the different portions of the
+armature core, these currents are generated principally on and near the
+surface of the armature core, diminishing in strength gradually toward
+the centre of the core. Their quantity is under some conditions
+proportional to the length of the iron body in the direction in which
+these currents are generated. By subdividing the iron core electrically
+in this direction, the generation of these currents can be reduced to a
+great extent. For instance, if the length of the armature-core is twelve
+inches, and by a suitable construction it is subdivided electrically, so
+that there are in the generating direction six inches of iron and six
+inches of intervening air-spaces or insulating material, the waste
+currents will be reduced to fifty per cent.
+
+As shown in the diagrams, the armature is constructed of thin iron discs
+D D D, of various diameters, fastened upon the armature-shaft in a
+suitable manner and arranged according to their sizes, so that a series
+of iron bodies, _i i i_, is formed, each of which diminishes in
+thickness from the centre toward the periphery. At both ends of the
+armature the inwardly curved discs _d d_, of cast iron, are fastened to
+the armature shaft.
+
+The armature core being constructed as shown, it will be easily seen
+that on those portions of the armature that are the most remote from the
+axis, and where the currents are principally developed, the length of
+iron in the generating direction is only a small fraction of the total
+length of the armature core, and besides this the iron body is
+subdivided in the generating direction, and therefore the Foucault
+currents are greatly reduced. Another cause of heating is the shifting
+of the poles of the armature core. In consequence of the subdivision of
+the iron in the armature and the increased surface for radiation, the
+risk of heating is lessened.
+
+The iron discs D D D are insulated or coated with some insulating-paint,
+a very careful insulation being unnecessary, as an electrical contact
+between several discs can only occur at places where the generated
+currents are comparatively weak. An armature core constructed in the
+manner described may be revolved between the poles of the field magnets
+without showing the slightest increase of temperature.
+
+[Illustration: FIG. 272.]
+
+[Illustration: FIG. 273.]
+
+The end discs, _d d_, which are of sufficient thickness and, for the
+sake of cheapness, of cast-iron, are curved inwardly, as indicated in
+the drawings. The extent of the curve is dependent on the amount of wire
+to be wound upon the armatures. In this machine the wire is wound upon
+the armature in two superimposed parts, and the curve of the end discs,
+_d d_, is so calculated that the first part--that is, practically half
+of the wire--just fills up the hollow space to the line _x x_; or, if
+the wire is wound in any other manner, the curve is such that when the
+whole of the wire is wound, the outside mass of wires, _w_, and the
+inside mass of wires, _w'_, are equal at each side of the plane _x x_.
+In this case the passive or electrically-inactive wires are of the
+smallest length practicable. The arrangement has further the advantage
+that the total lengths of the crossing wires at the two sides of the
+plane _x x_ are practically equal.
+
+[Illustration: FIG. 274.]
+
+To equalize further the armature coils at both sides of the plates that
+are in contact with the brushes, the winding and connecting up is
+effected in the following manner: The whole wire is wound upon the
+armature-core in two superimposed parts, which are thoroughly insulated
+from each other. Each of these two parts is composed of three separated
+groups of coils. The first group of coils of the first part of wire
+being wound and connected to the commutator-bars in the usual manner,
+this group is insulated and the second group wound; but the coils of
+this second group, instead of being connected to the next following
+commutator bars, are connected to the directly opposite bars of the
+commutator. The second group is then insulated and the third group
+wound, the coils of this group being connected to those bars to which
+they would be connected in the usual way. The wires are then thoroughly
+insulated and the second part of wire is wound and connected in the same
+manner.
+
+Suppose, for instance, that there are twenty-four coils--that is, twelve
+in each part--and consequently twenty-four commutator plates. There will
+be in each part three groups, each containing four coils, and the coils
+will be connected as follows:
+
+                        _Groups._   _Commutator Bars._
+                      { First            1--5
+  First part of wire  { Second          17--21
+                      { Third            9--13
+
+                      { First           13--17
+  Second part of wire { Second           5--9
+                      { Third           21--1
+
+In constructing the armature core and winding and connecting the coils
+in the manner indicated, the passive or electrically inactive wire is
+reduced to a minimum, and the coils at each side of the plates that are
+in contact with the brushes are practically equal. In this way the
+electrical efficiency of the machine is increased.
+
+[Illustration: FIG. 275.]
+
+[Illustration: FIG. 276.]
+
+The commutator plates _t_ are shown as outside the bearing _b_ of the
+armature shaft. The shaft H is tubular and split at the end portion, and
+the wires are carried through the same in the usual manner and connected
+to the respective commutator plates. The commutator plates are upon a
+cylinder, _u_, and insulated, and this cylinder is properly placed and
+then secured by expanding the split end of the shaft by a tapering screw
+plug, _v_.
+
+[Illustration: FIG. 277.]
+
+The arc lamps invented by Mr. Tesla for use on the circuits from the
+above described dynamo are those in which the separation and feed of the
+carbon electrodes or their equivalents is accomplished by means of
+electro-magnets or solenoids in connection with suitable clutch
+mechanism, and were designed for the purpose of remedying certain
+faults common to arc lamps.
+
+He proposed to prevent the frequent vibrations of the movable carbon
+"point" and flickering of the light arising therefrom; to prevent the
+falling into contact of the carbons; to dispense with the dash pot,
+clock work, or gearing and similar devices; to render the lamp extremely
+sensitive, and to feed the carbon almost imperceptibly, and thereby
+obtain a very steady and uniform light.
+
+In that class of lamps where the regulation of the arc is effected by
+forces acting in opposition on a free, movable rod or lever directly
+connected with the electrode, all or some of the forces being dependent
+on the strength of the current, any change in the electrical condition
+of the circuit causes a vibration and a corresponding flicker in the
+light. This difficulty is most apparent when there are only a few lamps
+in circuit. To lessen this difficulty lamps have been constructed in
+which the lever or armature, after the establishing of the arc, is kept
+in a fixed position and cannot vibrate during the feed operation, the
+feed mechanism acting independently; but in these lamps, when a clamp is
+employed, it frequently occurs that the carbons come into contact and
+the light is momentarily extinguished, and frequently parts of the
+circuit are injured. In both these classes of lamps it has been
+customary to use dash pot, clock work, or equivalent retarding devices;
+but these are often unreliable and objectionable, and increase the cost
+of construction.
+
+Mr. Tesla combines two electro-magnets--one of low resistance in the
+main or lamp circuit, and the other of comparatively high resistance in
+a shunt around the arc--a movable armature lever, and a special feed
+mechanism, the parts being arranged so that in the normal working
+position of the armature lever the same is kept almost rigidly in one
+position, and is not affected even by considerable changes in the
+electric circuit; but if the carbons fall into contact the armature will
+be actuated by the magnets so as to move the lever and start the arc,
+and hold the carbons until the arc lengthens and the armature lever
+returns to the normal position. After this the carbon rod holder is
+released by the action of the feed mechanism, so as to feed the carbon
+and restore the arc to its normal length.
+
+Fig. 278 is an elevation of the mechanism made use of in this arc lamp.
+Fig. 279 is a plan view. Fig. 280 is an elevation of the balancing lever
+and spring; Fig. 281 is a detached plan view of the pole pieces and
+armatures upon the friction clamp, and Fig. 282 is a section of the
+clamping tube.
+
+M is a helix of coarse wire in a circuit from the lower carbon holder to
+the negative binding screw -. N is a helix of fine wire in a shunt
+between the positive binding screw + and the negative binding screw -.
+The upper carbon holder S is a parallel rod sliding through the plates
+S' S^{2} of the frame of the lamp, and hence the electric current passes
+from the positive binding post + through the plate S^{2}, carbon holder
+S, and upper carbon to the lower carbon, and thence by the holder and a
+metallic connection to the helix M.
+
+[Illustration: FIG. 278.]
+
+[Illustration: FIG. 279.]
+
+[Illustration: FIG. 280.]
+
+[Illustration: FIG. 281.]
+
+[Illustration: FIG. 282.]
+
+The carbon holders are of the usual character, and to insure electric
+connections the springs _l_ are made use of to grasp the upper carbon
+holding rod S, but to allow the rod to slide freely through the same.
+These springs _l_ may be adjusted in their pressure by the screw _m_,
+and the spring _l_ maybe sustained upon any suitable support. They are
+shown as connected with the upper end of the core of the magnet N.
+
+Around the carbon-holding rod S, between the plates S' S^{2}, there is a
+tube, R, which forms a clamp. This tube is counter-bored, as seen in the
+section Fig. 282, so that it bears upon the rod S at its upper end and
+near the middle, and at the lower end of this tubular clamp R there are
+armature segments _r_ of soft iron. A frame or arm, _n_, extending,
+preferably, from the core N^{2}, supports the lever A by a fulcrum-pin,
+_o_. This lever A has a hole, through which the upper end of the tubular
+clamp R passes freely, and from the lever A is a link, _q_, to the lever
+_t_, which lever is pivoted at _y_ to a ring upon one of the columns
+S^{3}. This lever _t_ has an opening or bow surrounding the tubular
+clamp R, and there are pins or pivotal connections _w_ between the lever
+_t_ and this clamp R, and a spring, _r^{2}_, serves to support or
+suspend the weight of the parts and balance them, or nearly so. This
+spring is adjustable.
+
+At one end of the lever A is a soft-iron armature block, _a_, over the
+core M' of the helix M, and there is a limiting screw, _c_, passing
+through this armature block _a_, and at the other end of the lever A is
+a soft iron armature block, _b_, with the end tapering or wedge shaped,
+and the same comes close to and in line with the lateral projection _e_
+on the core N^{2}. The lower ends of the cores M' N^{2} are made with
+laterally projecting pole-pieces M^{3} N^{3}, respectively, and these
+pole-pieces are concave at their outer ends, and are at opposite sides
+of the armature segments _r_ at the lower end of the tubular clamp R.
+
+The operation of these devices is as follows: In the condition of
+inaction, the upper carbon rests upon the lower one, and when the
+electric current is turned on it passes freely, by the frame and spring
+_l_, through the rods and carbons to the coarse wire and helix M, and to
+the negative binding post V and the core M' thereby is energized. The
+pole piece M^{3} attracts the armature _r_, and by the lateral pressure
+causes the clamp R to grasp the rod S', and the lever A is
+simultaneously moved from the position shown by dotted lines, Fig. 278,
+to the normal position shown in full lines, and in so doing the link _q_
+and lever _t_ are raised, lifting the clamp R and S, separating the
+carbons and forming the arc. The magnetism of the pole piece _e_ tends
+to hold the lever A level, or nearly so, the core N^{2} being energized
+by the current in the shunt which contains the helix N. In this position
+the lever A is not moved by any ordinary variation in the current,
+because the armature _b_ is strongly attracted by the magnetism of _e_,
+and these parts are close to each other, and the magnetism of _e_ acts
+at right angles to the magnetism of the core M'. If, now, the arc
+becomes too long, the current through the helix M is lessened, and the
+magnetism of the core N^{3} is increased by the greater current passing
+through the shunt, and this core N^{3}, attracting the segmental
+armature _r_, lessens the hold of the clamp R upon the rod S, allowing
+the latter to slide and lessen the length of the arc, which instantly
+restores the magnetic equilibrium and causes the clamp R to hold the rod
+S. If it happens that the carbons fall into contact, then the magnetism
+of N^{2} is lessened so much that the attraction of the magnet M will be
+sufficient to move the armature _a_ and lever A so that the armature _b_
+passes above the normal position, so as to separate the carbons
+instantly; but when the carbons burn away, a greater amount of current
+will pass through the shunt until the attraction of the core N^{2} will
+overcome the attraction of the core M' and bring the armature lever A
+again into the normal horizontal position, and this occurs before the
+feed can take place. The segmental armature pieces _r_ are shown as
+nearly semicircular. They are square or of any other desired shape, the
+ends of the pole pieces M^{3}, N^{3} being made to correspond in shape.
+
+In a modification of this lamp, Mr. Tesla provided means for
+automatically withdrawing a lamp from the circuit, or cutting it out
+when, from a failure of the feed, the arc reached an abnormal length;
+and also means for automatically reinserting such lamp in the circuit
+when the rod drops and the carbons come into contact.
+
+Fig. 283 is an elevation of the lamp with the case in section. Fig. 284
+is a sectional plan at the line _x x_. Fig. 285 is an elevation, partly
+in section, of the lamp at right angles to Fig. 283. Fig. 286 is a
+sectional plan at the line _y y_ of Fig. 283. Fig. 287 is a section of
+the clamp in about full size. Fig. 288 is a detached section
+illustrating the connection of the spring to the lever that carries the
+pivots of the clamp, and Fig. 289 is a diagram showing the
+circuit-connections of the lamp.
+
+In Fig. 283, M represents the main and N the shunt magnet, both securely
+fastened to the base A, which with its side columns, S S, are cast in
+one piece of brass or other diamagnetic material. To the magnets are
+soldered or otherwise fastened the brass washers or discs _a a a a_.
+Similar washers, _b b_, of fibre or other insulating material, serve to
+insulate the wires from the brass washers.
+
+The magnets M and N are made very flat, so that their width exceeds
+three times their thickness, or even more. In this way a comparatively
+small number of convolutions is sufficient to produce the required
+magnetism, while a greater surface is offered for cooling off the wires.
+
+[Illustration: FIG. 286.]
+
+[Illustration: FIG. 283.]
+
+[Illustration: FIG. 285.]
+
+[Illustration: FIG. 284.]
+
+[Illustration: FIG. 287.]
+
+[Illustration: FIG. 288.]
+
+The upper pole pieces, _m n_, of the magnets are curved, as indicated in
+the drawings, Fig. 283. The lower pole pieces _m' n'_, are brought near
+together, tapering toward the armature _g_, as shown in Figs. 284 and
+286. The object of this taper is to concentrate the greatest amount of
+the developed magnetism upon the armature, and also to allow the pull to
+be exerted always upon the middle of the armature _g_. This armature _g_
+is a piece of iron in the shape of a hollow cylinder, having on each
+side a segment cut away, the width of which is equal to the width of the
+pole pieces _m' n'_.
+
+The armature is soldered or otherwise fastened to the clamp _r_, which
+is formed of a brass tube, provided with gripping-jaws _e e_, Fig. 287.
+These jaws are arcs of a circle of the diameter of the rod R, and are
+made of hardened German silver. The guides _f f_, through which the
+carbon-holding rod R slides, are made of the same material. This has the
+advantage of reducing greatly the wear and corrosion of the parts coming
+in frictional contact with the rod, which frequently causes trouble. The
+jaws _e e_ are fastened to the inside of the tube _r_, so that one is a
+little lower than the other. The object of this is to provide a greater
+opening for the passage of the rod when the same is released by the
+clamp. The clamp _r_ is supported on bearings _w w_, Figs. 283, 285 and
+287, which are just in the middle between the jaws _e e_. The bearings
+_w w_ are carried by a lever, _t_, one end of which rests upon an
+adjustable support, _q_, of the side columns, S, the other end being
+connected by means of the link _e'_ to the armature-lever L. The
+armature-lever L is a flat piece of iron in N shape, having its ends
+curved so as to correspond to the form of the upper pole-pieces of the
+magnets M and N. It is hung upon the pivots _v v_, Fig. 284, which are
+in the jaw _x_ of the top plate B. This plate B, with the jaw, is cast
+in one piece and screwed to the side columns, S S, that extend up from
+the base A. To partly balance the overweight of the moving parts, a
+spring, _s'_, Figs. 284 and 288, is fastened to the top plate, B, and
+hooked to the lever _t_. The hook _o_ is toward one side of the lever or
+bent a little sidewise, as seen in Fig. 288. By this means a slight
+tendency is given to swing the armature toward the pole-piece _m'_ of
+the main magnet.
+
+The binding-posts K K' are screwed to the base A. A manual switch, for
+short-circuiting the lamp when the carbons are renewed, is also fastened
+to the base. This switch is of ordinary character, and is not shown in
+the drawings.
+
+The rod R is electrically connected to the lamp-frame by means of a
+flexible conductor or otherwise. The lamp-case receives a removable
+cover, _s^{2}_, to inclose the parts.
+
+The electrical connections are as indicated diagrammatically in Fig.
+289. The wire in the main magnet consists of two parts, _x'_ and _p'_.
+These two parts may be in two separated coils or in one single helix,
+as shown in the drawings. The part _x'_ being normally in circuit, is,
+with the fine wire upon the shunt-magnet, wound and traversed by the
+current in the same direction, so as to tend to produce similar poles, N
+N or S S, on the corresponding pole-pieces of the magnets M and N. The
+part _p'_ is only in circuit when the lamp is cut out, and then the
+current being in the opposite direction produces in the main magnet,
+magnetism of the opposite polarity.
+
+The operation is as follows: At the start the carbons are to be in
+contact, and the current passes from the positive binding-post K to the
+lamp-frame, carbon-holder, upper and lower carbon, insulated return-wire
+in one of the side rods, and from there through the part _x'_ of the
+wire on the main magnet to the negative binding-post. Upon the passage
+of the current the main magnet is energized and attracts the
+clamping-armature _g_, swinging the clamp and gripping the rod by means
+of the gripping jaws _e e_. At the same time the armature lever L is
+pulled down and the carbons are separated. In pulling down the armature
+lever L the main magnet is assisted by the shunt-magnet N, the latter
+being magnetized by magnetic induction from the magnet M.
+
+[Illustration: FIG. 289.]
+
+It will be seen that the armatures L and _g_ are practically the keepers
+for the magnets M and N, and owing to this fact both magnets with either
+one of the armatures L and _g_ may be considered as one horseshoe
+magnet, which we might term a "compound magnet." The whole of the
+soft-iron parts M, _m'_, _g_, _n'_, N and L form a compound magnet.
+
+The carbons being separated, the fine wire receives a portion of the
+current. Now, the magnetic induction from the magnet M is such as to
+produce opposite poles on the corresponding ends of the magnet N; but
+the current traversing the helices tends to produce similar poles on the
+corresponding ends of both magnets, and therefore as soon as the fine
+wire is traversed by sufficient current the magnetism of the whole
+compound magnet is diminished.
+
+With regard to the armature _g_ and the operation of the lamp, the pole
+_m'_ may be considered as the "clamping" and the pole _n'_ as the
+"releasing" pole.
+
+As the carbons burn away, the fine wire receives more current and the
+magnetism diminishes in proportion. This causes the armature lever L to
+swing and the armature _g_ to descend gradually under the weight of the
+moving parts until the end _p_, Fig. 283, strikes a stop on the top
+plate, B. The adjustment is such that when this takes place the rod R is
+yet gripped securely by the jaws _e e_. The further downward movement of
+the armature lever being prevented, the arc becomes longer as the
+carbons are consumed, and the compound magnet is weakened more and more
+until the clamping armature _g_ releases the hold of the gripping-jaws
+_e e_ upon the rod R, and the rod is allowed to drop a little, thus
+shortening the arc. The fine wire now receiving less current, the
+magnetism increases, and the rod is clamped again and slightly raised,
+if necessary. This clamping and releasing of the rod continues until the
+carbons are consumed. In practice the feed is so sensitive that for the
+greatest part of the time the movement of the rod cannot be detected
+without some actual measurement. During the normal operation of the lamp
+the armature lever L remains practically stationary, in the position
+shown in Fig. 283.
+
+Should it happen that, owing to an imperfection in it, the rod and the
+carbons drop too far, so as to make the arc too short, or even bring the
+carbons in contact, a very small amount of current passes through the
+fine wire, and the compound magnet becomes sufficiently strong to act as
+at the start in pulling the armature lever L down and separating the
+carbons to a greater distance.
+
+It occurs often in practical work that the rod sticks in the guides. In
+this case the are reaches a great length, until it finally breaks. Then
+the light goes out, and frequently the fine wire is injured. To prevent
+such an accident Mr. Tesla provides this lamp with an automatic cut-out
+which operates as follows: When, upon a failure of the feed, the arc
+reaches a certain predetermined length, such an amount of current is
+diverted through the fine wire that the polarity of the compound magnet
+is reversed. The clamping armature _g_ is now moved against the shunt
+magnet N until it strikes the releasing pole _n'_. As soon as the
+contact is established, the current passes from the positive binding
+post over the clamp _r_, armature _g_, insulated shunt magnet, and the
+helix _p'_ upon the main magnet M to the negative binding post. In this
+case the current passes in the opposite direction and changes the
+polarity of the magnet M, at the same time maintaining by magnetic
+induction in the core of the shunt magnet the required magnetism without
+reversal of polarity, and the armature _g_ remains against the shunt
+magnet pole _n'_. The lamp is thus cut out as long as the carbons are
+separated. The cut out may be used in this form without any further
+improvement; but Mr. Tesla arranges it so that if the rod drops and the
+carbons come in contact the arc is started again. For this purpose he
+proportions the resistance of part _p'_ and the number of the
+convolutions of the wire upon the main magnet so that when the carbons
+come in contact a sufficient amount of current is diverted through the
+carbons and the part _x'_ to destroy or neutralize the magnetism of the
+compound magnet. Then the armature _g_, having a slight tendency to
+approach to the clamping pole _m'_, comes out of contact with the
+releasing pole _n'_. As soon as this happens, the current through the
+part _p'_ is interrupted, and the whole current passes through the part
+_x_. The magnet M is now strongly magnetized, the armature _g_ is
+attracted, and the rod clamped. At the same time the armature lever L is
+pulled down out of its normal position and the arc started. In this way
+the lamp cuts itself out automatically when the arc gets too long, and
+reinserts itself automatically in the circuit if the carbons drop
+together.
+
+
+
+
+CHAPTER XLI.
+
+IMPROVEMENT IN "UNIPOLAR" GENERATORS.
+
+
+Another interesting class of apparatus to which Mr. Tesla has directed
+his attention, is that of "unipolar" generators, in which a disc or a
+cylindrical conductor is mounted between magnetic poles adapted to
+produce an approximately uniform field. In the disc armature machines
+the currents induced in the rotating conductor flow from the centre to
+the periphery, or conversely, according to the direction of rotation or
+the lines of force as determined by the signs of the magnetic poles, and
+these currents are taken off usually by connections or brushes applied
+to the disc at points on its periphery and near its centre. In the case
+of the cylindrical armature machine, the currents developed in the
+cylinder are taken off by brushes applied to the sides of the cylinder
+at its ends.
+
+In order to develop economically an electromotive force available for
+practicable purposes, it is necessary either to rotate the conductor at
+a very high rate of speed or to use a disc of large diameter or a
+cylinder of great length; but in either case it becomes difficult to
+secure and maintain a good electrical connection between the collecting
+brushes and the conductor, owing to the high peripheral speed.
+
+It has been proposed to couple two or more discs together in series,
+with the object of obtaining a higher electro-motive force; but with the
+connections heretofore used and using other conditions of speed and
+dimension of disc necessary to securing good practicable results, this
+difficulty is still felt to be a serious obstacle to the use of this
+kind of generator. These objections Mr. Tesla has sought to avoid by
+constructing a machine with two fields, each having a rotary conductor
+mounted between its poles. The same principle is involved in the case of
+both forms of machine above described, but the description now given is
+confined to the disc type, which Mr. Tesla is inclined to favor for that
+machine. The discs are formed with flanges, after the manner of
+pulleys, and are connected together by flexible conducting bands or
+belts.
+
+The machine is built in such manner that the direction of magnetism or
+order of the poles in one field of force is opposite to that in the
+other, so that rotation of the discs in the same direction develops a
+current in one from centre to circumference and in the other from
+circumference to centre. Contacts applied therefore to the shafts upon
+which the discs are mounted form the terminals of a circuit the
+electro-motive force in which is the sum of the electro-motive forces of
+the two discs.
+
+It will be obvious that if the direction of magnetism in both fields be
+the same, the same result as above will be obtained by driving the discs
+in opposite directions and crossing the connecting belts. In this way
+the difficulty of securing and maintaining good contact with the
+peripheries of the discs is avoided and a cheap and durable machine made
+which is useful for many purposes--such as for an exciter for
+alternating current generators, for a motor, and for any other purpose
+for which dynamo machines are used.
+
+[Illustration: FIG. 290.]
+
+[Illustration: FIG. 291.]
+
+Fig. 290 is a side view, partly in section, of this machine. Fig. 291 is
+a vertical section of the same at right angles to the shafts.
+
+In order to form a frame with two fields of force, a support, A, is cast
+with two pole pieces B B' integral with it. To this are joined by bolts
+E a casting D, with two similar and corresponding pole pieces C C'. The
+pole pieces B B' are wound and connected to produce a field of force of
+given polarity, and the pole pieces C C' are wound so as to produce a
+field of opposite polarity. The driving shafts F G pass through the
+poles and are journaled in insulating bearings in the casting A D, as
+shown.
+
+H K are the discs or generating conductors. They are composed of copper,
+brass, or iron and are keyed or secured to their respective shafts. They
+are provided with broad peripheral flanges J. It is of course obvious
+that the discs may be insulated from their shafts, if so desired. A
+flexible metallic belt L is passed over the flanges of the two discs,
+and, if desired, may be used to drive one of the discs. It is better,
+however, to use this belt merely as a conductor, and for this purpose
+sheet steel, copper, or other suitable metal is used. Each shaft is
+provided with a driving pulley M, by which power is imparted from a
+driving shaft.
+
+N N are the terminals. For the sake of clearness they are shown as
+provided with springs P, that bear upon the ends of the shafts. This
+machine, if self-exciting, would have copper bands around its poles; or
+conductors of any kind--such as wires shown in the drawings--may be
+used.
+
+       *       *       *       *       *
+
+It is thought appropriate by the compiler to append here some notes on
+unipolar dynamos, written by Mr. Tesla, on a recent occasion.
+
+
+NOTES ON A UNIPOLAR DYNAMO.[15]
+
+  [15] Article by Mr. Tesla, contributed to _The Electrical Engineer_,
+       N. Y., Sept. 2, 1891.
+
+It is characteristic of fundamental discoveries, of great achievements
+of intellect, that they retain an undiminished power upon the
+imagination of the thinker. The memorable experiment of Faraday with a
+disc rotating between the two poles of a magnet, which has borne such
+magnificent fruit, has long passed into every-day experience; yet there
+are certain features about this embryo of the present dynamos and motors
+which even to-day appear to us striking, and are worthy of the most
+careful study.
+
+Consider, for instance, the case of a disc of iron or other metal
+revolving between the two opposite poles of a magnet, and the polar
+surfaces completely covering both sides of the disc, and assume the
+current to be taken off or conveyed to the same by contacts uniformly
+from all points of the periphery of the disc. Take first the case of a
+motor. In all ordinary motors the operation is dependent upon some
+shifting or change of the resultant of the magnetic attraction exerted
+upon the armature, this process being effected either by some mechanical
+contrivance on the motor or by the action of currents of the proper
+character. We may explain the operation of such a motor just as we can
+that of a water-wheel. But in the above example of the disc surrounded
+completely by the polar surfaces, there is no shifting of the magnetic
+action, no change whatever, as far as we know, and yet rotation ensues.
+Here, then, ordinary considerations do not apply; we cannot even give a
+superficial explanation, as in ordinary motors, and the operation will
+be clear to us only when we shall have recognized the very nature of the
+forces concerned, and fathomed the mystery of the invisible connecting
+mechanism.
+
+Considered as a dynamo machine, the disc is an equally interesting
+object of study. In addition to its peculiarity of giving currents of
+one direction without the employment of commutating devices, such a
+machine differs from ordinary dynamos in that there is no reaction
+between armature and field. The armature current tends to set up a
+magnetization at right angles to that of the field current, but since
+the current is taken off uniformly from all points of the periphery, and
+since, to be exact, the external circuit may also be arranged perfectly
+symmetrical to the field magnet, no reaction can occur. This, however,
+is true only as long as the magnets are weakly energized, for when the
+magnets are more or less saturated, both magnetizations at right angles
+seemingly interfere with each other.
+
+For the above reason alone it would appear that the output of such a
+machine should, for the same weight, be much greater than that of any
+other machine in which the armature current tends to demagnetize the
+field. The extraordinary output of the Forbes unipolar dynamo and the
+experience of the writer confirm this view.
+
+Again, the facility with which such a machine may be made to excite
+itself is striking, but this may be due--besides to the absence of
+armature reaction--to the perfect smoothness of the current and
+non-existence of self-induction.
+
+If the poles do not cover the disc completely on both sides, then, of
+course, unless the disc be properly subdivided, the machine will be very
+inefficient. Again, in this case there are points worthy of notice. If
+the disc be rotated and the field current interrupted, the current
+through the armature will continue to flow and the field magnets will
+lose their strength comparatively slowly. The reason for this will at
+once appear when we consider the direction of the currents set up in the
+disc.
+
+[Illustration: FIG. 292.]
+
+Referring to the diagram Fig. 292, _d_ represents the disc with the
+sliding contacts B B' on the shaft and periphery. N and S represent the
+two poles of a magnet. If the pole N be above, as indicated in the
+diagram, the disc being supposed to be in the plane of the paper, and
+rotating in the direction of the arrow D, the current set up in the disc
+will flow from the centre to the periphery, as indicated by the arrow A.
+Since the magnetic action is more or less confined to the space between
+the poles N S, the other portions of the disc may be considered
+inactive. The current set up will therefore not wholly pass through the
+external circuit F, but will close through the disc itself, and
+generally, if the disposition be in any way similar to the one
+illustrated, by far the greater portion of the current generated will
+not appear externally, as the circuit F is practically short-circuited
+by the inactive portions of the disc. The direction of the resulting
+currents in the latter may be assumed to be as indicated by the dotted
+lines and arrows _m_ and _n_; and the direction of the energizing field
+current being indicated by the arrows _a b c d_, an inspection of the
+figure shows that one of the two branches of the eddy current, that is,
+A B' _m_ B, will tend to demagnetize the field, while the other branch,
+that is, A B' _n_ B, will have the opposite effect. Therefore, the
+branch A B' _m_ B, that is, the one which is _approaching_ the field,
+will repel the lines of the same, while branch A B' _n_ B, that is, the
+one _leaving_ the field, will gather the lines of force upon itself.
+
+In consequence of this there will be a constant tendency to reduce the
+current flow in the path A B' _m_ B, while on the other hand no such
+opposition will exist in path A B' _n_ B, and the effect of the latter
+branch or path will be more or less preponderating over that of the
+former. The joint effect of both the assumed branch currents might be
+represented by that of one single current of the same direction as that
+energizing the field. In other words, the eddy currents circulating in
+the disc will energize the field magnet. This is a result quite contrary
+to what we might be led to suppose at first, for we would naturally
+expect that the resulting effect of the armature currents would be such
+as to oppose the field current, as generally occurs when a primary and
+secondary conductor are placed in inductive relations to each other. But
+it must be remembered that this results from the peculiar disposition in
+this case, namely, two paths being afforded to the current, and the
+latter selecting that path which offers the least opposition to its
+flow. From this we see that the eddy currents flowing in the disc partly
+energize the field, and for this reason when the field current is
+interrupted the currents in the disc will continue to flow, and the
+field magnet will lose its strength with comparative slowness and may
+even retain a certain strength as long as the rotation of the disc is
+continued.
+
+The result will, of course, largely depend on the resistance and
+geometrical dimensions of the path of the resulting eddy current and on
+the speed of rotation; these elements, namely, determine the retardation
+of this current and its position relative to the field. For a certain
+speed there would be a maximum energizing action; then at higher speeds,
+it would gradually fall off to zero and finally reverse, that is, the
+resultant eddy current effect would be to weaken the field. The reaction
+would be best demonstrated experimentally by arranging the fields N S,
+N' S', freely movable on an axis concentric with the shaft of the disc.
+If the latter were rotated as before in the direction of the arrow D,
+the field would be dragged in the same direction with a torque, which,
+up to a certain point, would go on increasing with the speed of
+rotation, then fall off, and, passing through zero, finally become
+negative; that is, the field would begin to rotate in opposite direction
+to the disc. In experiments with alternate current motors in which the
+field was shifted by currents of differing phase, this interesting
+result was observed. For very low speeds of rotation of the field the
+motor would show a torque of 900 lbs. or more, measured on a pulley 12
+inches in diameter. When the speed of rotation of the poles was
+increased, the torque would diminish, would finally go down to zero,
+become negative, and then the armature would begin to rotate in opposite
+direction to the field.
+
+To return to the principal subject; assume the conditions to be such
+that the eddy currents generated by the rotation of the disc strengthen
+the field, and suppose the latter gradually removed while the disc is
+kept rotating at an increased rate. The current, once started, may then
+be sufficient to maintain itself and even increase in strength, and then
+we have the case of Sir William Thomson's "current accumulator." But
+from the above considerations it would seem that for the success of the
+experiment the employment of a disc _not subdivided_[16] would be
+essential, for if there should be a radial subdivision, the eddy
+currents could not form and the self-exciting action would cease. If
+such a radially subdivided disc were used it would be necessary to
+connect the spokes by a conducting rim or in any proper manner so as to
+form a symmetrical system of closed circuits.
+
+  [16] Mr. Tesla here refers to an interesting article which appeared
+       in July, 1865, in the _Phil. Magazine_, by Sir W. Thomson, in
+       which Sir William, speaking of his "uniform electric current
+       accumulator," assumes that for self-excitation it is desirable
+       to subdivide the disc into an infinite number of infinitely thin
+       spokes, in order to prevent diffusion of the current. Mr. Tesla
+       shows that diffusion is absolutely necessary for the excitation
+       and that when the disc is subdivided no excitation can occur.
+
+The action of the eddy currents may be utilized to excite a machine of
+any construction. For instance, in Figs. 293 and 294 an arrangement is
+shown by which a machine with a disc armature might be excited. Here a
+number of magnets, N S, N S, are placed radially on each side of a metal
+disc D carrying on its rim a set of insulated coils, C C. The magnets
+form two separate fields, an internal and an external one, the solid
+disc rotating in the field nearest the axis, and the coils in the field
+further from it. Assume the magnets slightly energized at the start;
+they could be strengthened by the action of the eddy currents in the
+solid disc so as to afford a stronger field for the peripheral coils.
+Although there is no doubt that under proper conditions a machine might
+be excited in this or a similar manner, there being sufficient
+experimental evidence to warrant such an assertion, such a mode of
+excitation would be wasteful.
+
+But a unipolar dynamo or motor, such as shown in Fig. 292, may be
+excited in an efficient manner by simply properly subdividing the disc
+or cylinder in which the currents are set up, and it is practicable to
+do away with the field coils which are usually employed. Such a plan is
+illustrated in Fig. 295. The disc or cylinder D is supposed to be
+arranged to rotate between the two poles N and S of a magnet, which
+completely cover it on both sides, the contours of the disc and poles
+being represented by the circles _d_ and _d^{1}_ respectively, the upper
+pole being omitted for the sake of clearness. The cores of the magnet
+are supposed to be hollow, the shaft C of the disc passing through them.
+If the unmarked pole be below, and the disc be rotated screw fashion,
+the current will be, as before, from the centre to the periphery, and
+may be taken off by suitable sliding contacts, B B', on the shaft and
+periphery respectively. In this arrangement the current flowing through
+the disc and external circuit will have no appreciable effect on the
+field magnet.
+
+[Illustration: FIG. 293.]
+
+[Illustration: FIG. 294.]
+
+But let us now suppose the disc to be subdivided spirally, as indicated
+by the full or dotted lines, Fig. 295. The difference of potential
+between a point on the shaft and a point on the periphery will remain
+unchanged, in sign as well as in amount. The only difference will be
+that the resistance of the disc will be augmented and that there will be
+a greater fall of potential from a point on the shaft to a point on the
+periphery when the same current is traversing the external circuit. But
+since the current is forced to follow the lines of subdivision, we see
+that it will tend either to energize or de-energize the field, and this
+will depend, other things being equal, upon the direction of the lines
+of subdivision. If the subdivision be as indicated by the full lines in
+Fig. 295, it is evident that if the current is of the same direction as
+before, that is, from centre to periphery, its effect will be to
+strengthen the field magnet; Whereas, if the subdivision be as indicated
+by the dotted lines, the current generated will tend to weaken the
+magnet. In the former case the machine will be capable of exciting
+itself when the disc is rotated in the direction of arrow D; in the
+latter case the direction of rotation must be reversed. Two such discs
+may be combined, however, as indicated, the two discs rotating in
+opposite fields, and in the same or opposite direction.
+
+[Illustration: FIG. 295.]
+
+[Illustration: FIG. 296.]
+
+Similar disposition may, of course, be made in a type of machine in
+which, instead of a disc, a cylinder is rotated. In such unipolar
+machines, in the manner indicated, the usual field coils and poles may
+be omitted and the machine may be made to consist only of a cylinder or
+of two discs enveloped by a metal casting.
+
+Instead of subdividing the disc or cylinder spirally, as indicated in
+Fig. 295, it is more convenient to interpose one or more turns between
+the disc and the contact ring on the periphery, as illustrated in Fig.
+296.
+
+A Forbes dynamo may, for instance, be excited in such a manner. In the
+experience of the writer it has been found that instead of taking the
+current from two such discs by sliding contacts, as usual, a flexible
+conducting belt may be employed to advantage. The discs are in such case
+provided with large flanges, affording a very great contact surface. The
+belt should be made to bear on the flanges with spring pressure to take
+up the expansion. Several machines with belt contact were constructed by
+the writer two years ago, and worked satisfactorily; but for want of
+time the work in that direction has been temporarily suspended. A number
+of features pointed out above have also been used by the writer in
+connection with some types of alternating current motors.
+
+
+
+
+PART IV.
+
+APPENDIX.--EARLY PHASE MOTORS AND THE TESLA MECHANICAL AND ELECTRICAL
+OSCILLATOR.
+
+
+
+
+CHAPTER XLII.
+
+MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.
+
+While the exhibits of firms engaged in the manufacture of electrical
+apparatus of every description at the Chicago World's Fair, afforded the
+visitor ample opportunity for gaining an excellent knowledge of the
+state of the art, there were also numbers of exhibits which brought out
+in strong relief the work of the individual inventor, which lies at the
+foundation of much, if not all, industrial or mechanical achievement.
+Prominent among such personal exhibits was that of Mr. Tesla, whose
+apparatus occupied part of the space of the Westinghouse Company, in
+Electricity Building.
+
+This apparatus represented the results of work and thought covering a
+period of ten years. It embraced a large number of different alternating
+motors and Mr. Tesla's earlier high frequency apparatus. The motor
+exhibit consisted of a variety of fields and armatures for two, three
+and multiphase circuits, and gave a fair idea of the gradual evolution
+of the fundamental idea of the rotating magnetic field. The high
+frequency exhibit included Mr. Tesla's earlier machines and disruptive
+discharge coils and high frequency transformers, which he used in his
+investigations and some of which are referred to in his papers printed
+in this volume.
+
+Fig. 297 shows a view of part of the exhibits containing the motor
+apparatus. Among these is shown at A a large ring intended to exhibit
+the phenomena of the rotating magnetic field. The field produced was
+very powerful and exhibited striking effects, revolving copper balls and
+eggs and bodies of various shapes at considerable distances and at great
+speeds. This ring was wound for two-phase circuits, and the winding was
+so distributed that a practically uniform field was obtained. This ring
+was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician of
+the Westinghouse Electric and Manufacturing Company.
+
+[Illustration: FIG. 297.]
+
+A smaller ring, shown at B, was arranged like the one exhibited at A but
+designed especially to exhibit the rotation of an armature in a rotating
+field. In connection with these two rings there was an interesting
+exhibit shown by Mr. Tesla which consisted of a magnet with a coil, the
+magnet being arranged to rotate in bearings. With this magnet he first
+demonstrated the identity between a rotating field and a rotating
+magnet; the latter, when rotating, exhibited the same phenomena as the
+rings when they were energized by currents of differing phase. Another
+prominent exhibit was a model illustrated at C which is a two-phase
+motor, as well as an induction motor and transformer. It consists of a
+large outer ring of laminated iron wound with two superimposed,
+separated windings which can be connected in a variety of ways. This is
+one of the first models used by Mr. Tesla as an induction motor and
+rotating transformer. The armature was either a steel or wrought iron
+disc with a closed coil. When the motor was operated from a two phase
+generator the windings were connected in two groups, as usual. When used
+as an induction motor, the current induced in one of the windings of the
+ring was passed through the other winding on the ring and so the motor
+operated with only two wires. When used as a transformer the outer
+winding served, for instance, as a secondary and the inner as a primary.
+The model shown at D is one of the earliest rotating field motors,
+consisting of a thin iron ring wound with two sets of coils and an
+armature consisting of a series of steel discs partly cut away and
+arranged on a small arbor.
+
+At E is shown one of the first rotating field or induction motors used
+for the regulation of an arc lamp and for other purposes. It comprises a
+ring of discs with two sets of coils having different self-inductions,
+one set being of German silver and the other of copper wire. The
+armature is wound with two closed-circuited coils at right angles to
+each other. To the armature shaft are fastened levers and other devices
+to effect the regulation. At F is shown a model of a magnetic lag motor;
+this embodies a casting with pole projections protruding from two coils
+between which is arranged to rotate a smooth iron body. When an
+alternating current is sent through the two coils the pole projections
+of the field and armature within it are similarly magnetized, and upon
+the cessation or reversal of the current the armature and field repel
+each other and rotation is produced in this way. Another interesting
+exhibit, shown at G, is an early model of a two field motor energized by
+currents of different phase. There are two independent fields of
+laminated iron joined by brass bolts; in each field is mounted an
+armature, both armatures being on the same shaft. The armatures were
+originally so arranged as to be placed in any position relatively to
+each other, and the fields also were arranged to be connected in a
+number of ways. The motor has served for the exhibition of a number of
+features; among other things, it has been used as a dynamo for the
+production of currents of any frequency between wide limits. In this
+case the field, instead of being energized by direct current, was
+energized by currents differing in phase, which produced a rotation of
+the field; the armature was then rotated in the same or in opposite
+direction to the movement of the field; and so any number of
+alternations of the currents induced in the armature, from a small to a
+high number, determined by the frequency of the energizing field coils
+and the speed of the armature, was obtained.
+
+[Illustration: FIG. 298.]
+
+The models H, I, J, represent a variety of rotating field, synchronous
+motors which are of special value in long distance transmission work.
+The principle embodied in these motors was enunciated by Mr. Tesla in
+his lecture before the American Institute of Electrical Engineers, in
+May, 1888[17]. It involves the production of the rotating field in one
+of the elements of the motor by currents differing in phase and
+energizing the other element by direct currents. The armatures are of
+the two and three phase type. K is a model of a motor shown in an
+enlarged view in Fig. 298. This machine, together with that shown in
+Fig. 299, was exhibited at the same lecture, in May, 1888. They were the
+first rotating field motors which were independently tested, having for
+that purpose been placed in the hands of Prof. Anthony in the winter of
+1887-88. From these tests it was shown that the efficiency and output of
+these motors was quite satisfactory in every respect.
+
+  [17] See Part I, Chap. III, page 9.
+
+[Illustration: FIG. 299.]
+
+It was intended to exhibit the model shown in Fig. 299, but it was
+unavailable for that purpose owing to the fact that it was some time ago
+handed over to the care of Prof. Ayrton in England. This model was
+originally provided with twelve independent coils; this number, as Mr.
+Tesla pointed out in his first lecture, being divisible by two and
+three, was selected in order to make various connections for two and
+three-phase operations, and during Mr. Tesla's experiments was used in
+many ways with from two to six phases. The model, Fig. 298, consists of
+a magnetic frame of laminated iron with four polar projections between
+which an armature is supported on brass bolts passing through the frame.
+A great variety of armatures was used in connection with these two and
+other fields. Some of the armatures are shown in front on the table,
+Fig. 297, and several are also shown enlarged in Figs. 300 to 310. An
+interesting exhibit is that shown at L, Fig. 297. This is an armature of
+hardened steel which was used in a demonstration before the Society of
+Arts in Boston, by Prof. Anthony. Another curious exhibit is shown
+enlarged in Fig. 301. This consists of thick discs of wrought iron
+placed lengthwise, with a mass of copper cast around them. The discs
+were arranged longitudinally to afford an easier starting by reason of
+the induced current formed in the iron discs, which differed in phase
+from those in the copper. This armature would start with a single
+circuit and run in synchronism, and represents one of the earliest types
+of such an armature. Fig. 305 is another striking exhibit. This is one
+of the earliest types of an armature with holes beneath the periphery,
+in which copper conductors are imbedded. The armature has eight closed
+circuits and was used in many different ways. Fig. 304 is a type of
+synchronous armature consisting of a block of soft steel wound with a
+coil closed upon itself. This armature was used in connection with the
+field shown in Fig. 298 and gave excellent results.
+
+[Illustration: FIG. 300.]
+
+[Illustration: FIG. 301.]
+
+[Illustration: FIG. 302.]
+
+[Illustration: FIG. 303.]
+
+[Illustration: FIG. 304.]
+
+[Illustration: FIG. 305.]
+
+[Illustration: FIG. 306.]
+
+[Illustration: FIG. 307.]
+
+[Illustration: FIG. 308.]
+
+[Illustration: FIG. 309.]
+
+[Illustration: FIG. 310.]
+
+Fig. 302 represents a synchronous armature with a large coil around a
+body of iron. There is another very small coil at right angles to the
+first. This small coil was used for the purpose of increasing the
+starting torque and was found very effective in this connection. Figs.
+306 and 308 show a favorite construction of armature; the iron body is
+made up of two sets of discs cut away and placed at right angles to each
+other, the interstices being wound with coils. The one shown in Fig. 308
+is provided with an additional groove on each of the projections formed
+by the discs, for the purpose of increasing the starting torque by a
+wire wound in these projections. Fig. 307 is a form of armature
+similarly constructed, but with four independent coils wound upon the
+four projections. This armature was used to reduce the speed of the
+motor with reference to that of the generator. Fig. 300 is still another
+armature with a great number of independent circuits closed upon
+themselves, so that all the dead points on the armature are done away
+with, and the armature has a large starting torque. Fig. 303 is another
+type of armature for a four-pole motor but with coils wound upon a
+smooth surface. A number of these armatures have hollow shafts, as they
+have been used in many ways. Figs. 309 and 310 represent armatures to
+which either alternating or direct current was conveyed by means of
+sliding rings. Fig. 309 consists of a soft iron body with a single coil
+wound around it, the ends of the coil being connected to two sliding
+rings to which, usually, direct current was conveyed. The armature shown
+in Fig. 310 has three insulated rings on a shaft and was used in
+connection with two or three phase circuits.
+
+All these models shown represent early work, and the enlarged engravings
+are made from photographs taken early in 1888. There is a great number
+of other models which were exhibited, but which are not brought out
+sharply in the engraving, Fig. 297. For example at M is a model of a
+motor comprising an armature with a hollow shaft wound with two or three
+coils for two or three-phase circuits; the armature was arranged to be
+stationary and the generating circuits were connected directly to the
+generator. Around the armature is arranged to rotate on its shaft a
+casting forming six closed circuits. On the outside this casting was
+turned smooth and the belt was placed on it for driving with any desired
+appliance. This also is a very early model.
+
+On the left side of the table there are seen a large variety of models,
+N, O, P, etc., with fields of various shapes. Each of these models
+involves some distinct idea and they all represent gradual development
+chiefly interesting as showing Mr. Tesla's efforts to adapt his system
+to the existing high frequencies.
+
+On the right side of the table, at S, T, are shown, on separate
+supports, larger and more perfected armatures of commercial motors, and
+in the space around the table a variety of motors and generators
+supplying currents to them was exhibited.
+
+The high frequency exhibit embraced Mr. Tesla's first original apparatus
+used in his investigations. There was exhibited a glass tube with one
+layer of silk-covered wire wound at the top and a copper ribbon on the
+inside. This was the first disruptive discharge coil constructed by him.
+At U is shown the disruptive discharge coil exhibited by him in his
+lecture before the American Institute of Electrical Engineers, in May,
+1891.[18] At V and W are shown some of the first high frequency
+transformers. A number of various fields and armatures of small models
+of high frequency apparatus as shown at X and Y, and others not visible
+in the picture, were exhibited. In the annexed space the dynamo then
+used by Mr. Tesla at Columbia College was exhibited; also another form
+of high frequency dynamo used.
+
+  [18] See Part II, Chap. XXVI., page 145.
+
+[Illustration: FIG. 311.]
+
+In this space also was arranged a battery of Leyden jars and his large
+disruptive discharge coil which was used for exhibiting the light
+phenomena in the adjoining dark room. The coil was operated at only a
+small fraction of its capacity, as the necessary condensers and
+transformers could not be had and as Mr. Tesla's stay was limited to one
+week; notwithstanding, the phenomena were of a striking character. In
+the room were arranged two large plates placed at a distance of about
+eighteen feet from each other. Between them were placed two long tables
+with all sorts of phosphorescent bulbs and tubes; many of these were
+prepared with great care and marked legibly with the names which would
+shine with phosphorescent glow. Among them were some with the names of
+Helmholtz, Faraday, Maxwell, Henry, Franklin, etc. Mr. Tesla had also
+not forgotten the greatest living poet of his own country, Zmaj Jovan;
+two or three were prepared with inscriptions, like "Welcome,
+Electricians," and produced a beautiful effect. Each represented some
+phase of this work and stood for some individual experiment of
+importance. Outside the room was the small battery seen in Fig. 311, for
+the exhibition of some of the impedance and other phenomena of interest.
+Thus, for instance, a thick copper bar bent in arched form was provided
+with clamps for the attachment of lamps, and a number of lamps were kept
+at incandescence on the bar; there was also a little motor shown on the
+table operated by the disruptive discharge.
+
+As will be remembered by those who visited the Exposition, the
+Westinghouse Company made a line exhibit of the various commercial
+motors of the Tesla system, while the twelve generators in Machinery
+Hall were of the two-phase type constructed for distributing light and
+power. Mr. Tesla, also exhibited some models of his oscillators.
+
+
+
+
+CHAPTER XLIII.
+
+THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.
+
+
+On the evening of Friday, August 25, 1893, Mr. Tesla delivered a lecture
+on his mechanical and electrical oscillators, before the members of the
+Electrical Congress, in the hall adjoining the Agricultural Building, at
+the World's Fair, Chicago. Besides the apparatus in the room, he
+employed an air compressor, which was driven by an electric motor.
+
+Mr. Tesla was introduced by Dr. Elisha Gray, and began by stating that
+the problem he had set out to solve was to construct, first, a mechanism
+which would produce oscillations of a perfectly constant period
+independent of the pressure of steam or air applied, within the widest
+limits, and also independent of frictional losses and load. Secondly, to
+produce electric currents of a perfectly constant period independently
+of the working conditions, and to produce these currents with mechanism
+which should be reliable and positive in its action without resorting to
+spark gaps and breaks. This he successfully accomplished in his
+apparatus, and with this apparatus, now, scientific men will be provided
+with the necessaries for carrying on investigations with alternating
+currents with great precision. These two inventions Mr. Tesla called,
+quite appropriately, a mechanical and an electrical oscillator,
+respectively.
+
+The former is substantially constructed in the following way. There is a
+piston in a cylinder made to reciprocate automatically by proper
+dispositions of parts, similar to a reciprocating tool. Mr. Tesla
+pointed out that he had done a great deal of work in perfecting his
+apparatus so that it would work efficiently at such high frequency of
+reciprocation as he contemplated, but he did not dwell on the many
+difficulties encountered. He exhibited, however, the pieces of a steel
+arbor which had been actually torn apart while vibrating against a
+minute air cushion.
+
+With the piston above referred to there is associated in one of his
+models in an independent chamber an air spring, or dash pot, or else he
+obtains the spring within the chambers of the oscillator itself. To
+appreciate the beauty of this it is only necessary to say that in that
+disposition, as he showed it, no matter what the rigidity of the spring
+and no matter what the weight of the moving parts, in other words, no
+matter what the period of vibrations, the vibrations of the spring are
+always isochronous with the applied pressure. Owing to this, the results
+obtained with these vibrations are truly wonderful. Mr. Tesla provides
+for an air spring of tremendous rigidity, and he is enabled to vibrate
+big weights at an enormous rate, considering the inertia, owing to the
+recoil of the spring. Thus, for instance, in one of these experiments,
+he vibrates a weight of approximately 20 pounds at the rate of about 80
+per second and with a stroke of about 7/8 inch, but by shortening the
+stroke the weight could be vibrated many hundred times, and has been, in
+other experiments.
+
+To start the vibrations, a powerful blow is struck, but the adjustment
+can be so made that only a minute effort is required to start, and, even
+without any special provision it will start by merely turning on the
+pressure suddenly. The vibration being, of course, isochronous, any
+change of pressure merely produces a shortening or lengthening of the
+stroke. Mr. Tesla showed a number of very clear drawings, illustrating
+the construction of the apparatus from which its working was plainly
+discernible. Special provisions are made so as to equalize the pressure
+within the dash pot and the outer atmosphere. For this purpose the
+inside chambers of the dash pot are arranged to communicate with the
+outer atmosphere so that no matter how the temperature of the enclosed
+air might vary, it still retains the same mean density as the outer
+atmosphere, and by this means a spring of constant rigidity is obtained.
+Now, of course, the pressure of the atmosphere may vary, and this would
+vary the rigidity of the spring, and consequently the period of
+vibration, and this feature constitutes one of the great beauties of the
+apparatus; for, as Mr. Tesla pointed out, this mechanical system acts
+exactly like a string tightly stretched between two points, and with
+fixed nodes, so that slight changes of the tension do not in the least
+alter the period of oscillation.
+
+The applications of such an apparatus are, of course, numerous and
+obvious. The first is, of course, to produce electric currents, and by a
+number of models and apparatus on the lecture platform, Mr. Tesla showed
+how this could be carried out in practice by combining an electric
+generator with his oscillator. He pointed out what conditions must be
+observed in order that the period of vibration of the electrical system
+might not disturb the mechanical oscillation in such a way as to alter
+the periodicity, but merely to shorten the stroke. He combines a
+condenser with a self-induction, and gives to the electrical system the
+same period as that at which the machine itself oscillates, so that both
+together then fall in step and electrical and mechanical resonance is
+obtained, and maintained absolutely unvaried.
+
+Next he showed a model of a motor with delicate wheelwork, which was
+driven by these currents at a constant speed, no matter what the air
+pressure applied was, so that this motor could be employed as a clock.
+He also showed a clock so constructed that it could be attached to one
+of the oscillators, and would keep absolutely correct time. Another
+curious and interesting feature which Mr. Tesla pointed out was that,
+instead of controlling the motion of the reciprocating piston by means
+of a spring, so as to obtain isochronous vibration, he was actually able
+to control the mechanical motion by the natural vibration of the
+electro-magnetic system, and he said that the case was a very simple
+one, and was quite analogous to that of a pendulum. Thus, supposing we
+had a pendulum of great weight, preferably, which would be maintained in
+vibration by force, periodically applied; now that force, no matter how
+it might vary, although it would oscillate the pendulum, would have no
+control over its period.
+
+Mr. Tesla also described a very interesting phenomenon which he
+illustrated by an experiment. By means of this new apparatus, he is able
+to produce an alternating current in which the E. M. F. of the impulses
+in one direction preponderates over that of those in the other, so that
+there is produced the effect of a direct current. In fact he expressed
+the hope that these currents would be capable of application in many
+instances, serving as direct currents. The principle involved in this
+preponderating E. M. F. he explains in this way: Suppose a conductor is
+moved into the magnetic field and then suddenly withdrawn. If the
+current is not retarded, then the work performed will be a mere
+fractional one; but if the current is retarded, then the magnetic field
+acts as a spring. Imagine that the motion of the conductor is arrested
+by the current generated, and that at the instant when it stops to move
+into the field, there is still the maximum current flowing in the
+conductor; then this current will, according to Lenz's law, drive the
+conductor out of the field again, and if the conductor has no
+resistance, then it would leave the field with the velocity it entered
+it. Now it is clear that if, instead of simply depending on the current
+to drive the conductor out of the field, the mechanically applied force
+is so timed that it helps the conductor to get out of the field, then it
+might leave the field with higher velocity than it entered it, and thus
+one impulse is made to preponderate in E. M. F. over the other.
+
+With a current of this nature, Mr. Tesla energized magnets strongly, and
+performed many interesting experiments bearing out the fact that one of
+the current impulses preponderates. Among them was one in which he
+attached to his oscillator a ring magnet with a small air gap between
+the poles. This magnet was oscillated up and down 80 times a second. A
+copper disc, when inserted within the air gap of the ring magnet, was
+brought into rapid rotation. Mr. Tesla remarked that this experiment
+also seemed to demonstrate that the lines of flow of current through a
+metallic mass are disturbed by the presence of a magnet in a manner
+quite independently of the so-called Hall effect. He showed also a very
+interesting method of making a connection with the oscillating magnet.
+This was accomplished by attaching to the magnet small insulated steel
+rods, and connecting to these rods the ends of the energizing coil. As
+the magnet was vibrated, stationary nodes were produced in the steel
+rods, and at these points the terminals of a direct current source were
+attached. Mr. Tesla also pointed out that one of the uses of currents,
+such as those produced in his apparatus, would be to select any given
+one of a number of devices connected to the same circuit by picking out
+the vibration by resonance. There is indeed little doubt that with Mr.
+Tesla's devices, harmonic and synchronous telegraphy will receive a
+fresh impetus, and vast possibilities are again opened up.
+
+Mr. Tesla was very much elated over his latest achievements, and said
+that he hoped that in the hands of practical, as well as scientific men,
+the devices described by him would yield important results. He laid
+special stress on the facility now afforded for investigating the effect
+of mechanical vibration in all directions, and also showed that he had
+observed a number of facts in connection with iron cores.
+
+[Illustration: FIG. 312.]
+
+The engraving, Fig. 312, shows, in perspective, one of the forms of
+apparatus used by Mr. Tesla in his earlier investigations in this field
+of work, and its interior construction is made plain by the sectional
+view shown in Fig. 313. It will be noted that the piston P is fitted
+into the hollow of a cylinder C which is provided with channel ports
+O O, and _I_, extending all around the inside surface. In this
+particular apparatus there are two channels O O for the outlet of the
+working fluid and one, _I_, for the inlet. The piston P is provided with
+two slots S S' at a carefully determined distance, one from the other.
+The tubes T T which are screwed into the holes drilled into the piston,
+establish communication between the slots S S' and chambers on each side
+of the piston, each of these chambers connecting with the slot which is
+remote from it. The piston P is screwed tightly on a shaft A which
+passes through fitting boxes at the end of the cylinder C. The boxes
+project to a carefully determined distance into the hollow of the
+cylinder C, thus determining the length of the stroke.
+
+Surrounding the whole is a jacket J. This jacket acts chiefly to
+diminish the sound produced by the oscillator and as a jacket when the
+oscillator is driven by steam, in which case a somewhat different
+arrangement of the magnets is employed. The apparatus here illustrated
+was intended for demonstration purposes, air being used as most
+convenient for this purpose.
+
+A magnetic frame M M is fastened so as to closely surround the
+oscillator and is provided with energizing coils which establish two
+strong magnetic fields on opposite sides. The magnetic frame is made up
+of thin sheet iron. In the intensely concentrated field thus produced,
+there are arranged two pairs of coils H H supported in metallic frames
+which are screwed on the shaft A of the piston and have additional
+bearings in the boxes B B on each side. The whole is mounted on a
+metallic base resting on two wooden blocks.
+
+[Illustration: FIG. 313.]
+
+The operation of the device is as follows: The working fluid being
+admitted through an inlet pipe to the slot I and the piston being
+supposed to be in the position indicated, it is sufficient, though not
+necessary, to give a gentle tap on one of the shaft ends protruding
+from the boxes B. Assume that the motion imparted be such as to move the
+piston to the left (when looking at the diagram) then the air rushes
+through the slot S' and tube T into the chamber to the left. The
+pressure now drives the piston towards the right and, owing to its
+inertia, it overshoots the position of equilibrium and allows the air to
+rush through the slot S and tube T into the chamber to the right, while
+the communication to the left hand chamber is cut off, the air of the
+latter chamber escaping through the outlet O on the left. On the return
+stroke a similar operation takes place on the right hand side. This
+oscillation is maintained continuously and the apparatus performs
+vibrations from a scarcely perceptible quiver amounting to no more than
+1 of an inch, up to vibrations of a little over 3/8 of an inch,
+according to the air pressure and load. It is indeed interesting to see
+how an incandescent lamp is kept burning with the apparatus showing a
+scarcely perceptible quiver.
+
+To perfect the mechanical part of the apparatus so that oscillations are
+maintained economically was one thing, and Mr. Tesla hinted in his
+lecture at the great difficulties he had first encountered to accomplish
+this. But to produce oscillations which would be of constant period was
+another task of no mean proportions. As already pointed out, Mr. Tesla
+obtains the constancy of period in three distinct ways. Thus, he
+provides properly calculated chambers, as in the case illustrated, in
+the oscillator itself; or he associates with the oscillator an air
+spring of constant resilience. But the most interesting of all, perhaps,
+is the maintenance of the constancy of oscillation by the reaction of
+the electromagnetic part of the combination. Mr. Tesla winds his coils,
+by preference, for high tension and associates with them a condenser,
+making the natural period of the combination fairly approximating to the
+average period at which the piston would oscillate without any
+particular provision being made for the constancy of period under
+varying pressure and load. As the piston with the coils is perfectly
+free to move, it is extremely susceptible to the influence of the
+natural vibration set up in the circuits of the coils H H. The
+mechanical efficiency of the apparatus is very high owing to the fact
+that friction is reduced to a minimum and the weights which are moved
+are small; the output of the oscillator is therefore a very large one.
+
+Theoretically considered, when the various advantages which Mr. Tesla
+holds out are examined, it is surprising, considering the simplicity of
+the arrangement, that nothing was done in this direction before. No
+doubt many inventors, at one time or other, have entertained the idea of
+generating currents by attaching a coil or a magnetic core to the piston
+of a steam engine, or generating currents by the vibrations of a tuning
+fork, or similar devices, but the disadvantages of such arrangements
+from an engineering standpoint must be obvious. Mr. Tesla, however, in
+the introductory remarks of his lecture, pointed out how by a series of
+conclusions he was driven to take up this new line of work by the
+necessity of producing currents of constant period and as a result of
+his endeavors to maintain electrical oscillation in the most simple and
+economical manner.
+
+
+
+
+INDEX.
+
+
+Alternate Current Electrostatic Apparatus 392
+
+Alternating Current Generators for High Frequency 152, 374, 224
+
+Alternating Motors and Transformers 7
+
+American Institute Electrical Engineers Lecture 145
+
+Anthony, W. A., Tests of Tesla Motors 8
+
+Apparatus for Producing High Vacua 276
+
+Arc Lighting, Tesla Direct, System 451
+
+Auxiliary Brush Regulation 438
+
+
+Biography, Tesla 4
+
+Brush, Anti-Sparking 432
+
+Brush, Third, Regulation 438
+
+Brush, Phenomena in High Vacuum 226
+
+
+Carborundum Button for Tesla Lamps 140, 253
+
+Commutator, Anti-Sparking 432
+
+Combination of Synchronizing and Torque Motor 95
+
+Condensers with Plates in Oil 418
+
+Conversion with Disruptive Discharge 193, 204, 303
+
+Current or Dynamic Electricity Phenomena 327
+
+
+Direct Current Arc Lighting 451
+
+Dischargers, Forms of 305
+
+Disruptive Discharge Coil 207, 221
+
+Disruptive Discharge Phenomena 212
+
+Dynamos, Improved Direct Current 448
+
+
+Early Phase Motors 477
+
+Effects with High Frequency and High Potential Currents 119
+
+Electrical Congress Lecture, Chicago. 486
+
+Electric Resonance 340
+
+Electric Discharges in Vacuum Tubes 396
+
+Electrolytic Registering Meter 420
+
+Eye, Observations on the 294
+
+
+Flames, Electrostatic, Non-Consuming 166, 272
+
+Forbes Unipolar Generator 468, 474
+
+Franklin Institute Lecture 294
+
+
+Generators, Pyromagnetic 429
+
+
+High Potential, High Frequency:
+
+  Brush Phenomena in High Vacuum 226
+  Carborundum Buttons 140, 253
+  Disruptive Discharge Phenomena 212
+  Flames, Electrostatic, Non-Consuming 166, 272
+  Impedance, Novel Phenomena 194, 338
+  Lighting Lamps Through Body 359
+  Luminous Effects with Gases 368
+  "Massage" with Currents 394
+  Motor with Single Wire 234, 330
+  "No Wire" Motors 235
+  Oil Insulation of Induction Coils 173, 221
+  Ozone, Production of 171
+  Phosphorescence 367
+  Physiological Effects 162, 394
+  Resonance 340
+  Spinning Filament 168
+  Streaming Discharges of High Tension Coil 155, 163
+  Telegraphy without Wires 346
+
+
+Impedance, Novel Phenomena 194, 338
+
+Improvements in Unipolar Generators 465
+
+Improved Direct Current Dynamos and Motors 448
+
+Induction Motors 92
+
+Institution Electrical Engineers Lecture 198
+
+
+Lamps and Motor operated on a Single Wire 330
+
+Lamps with Single Straight Fiber 183
+
+Lamps containing only a Gas 188
+
+Lamps with Refractory Button 177, 239, 360
+
+Lamps for Simple Phosphorescence 187, 282, 364
+
+Lecture, Tesla before:
+
+ American Institute Electrical Engineers 145
+ Royal Institution 124
+ Institution Electrical Engineers 198
+ Franklin Institute and National Electric Light Association 294
+ Electrical Congress, Chicago 486
+
+Lighting Lamps Through the Body 359
+
+Light Phenomena with High Frequencies 349
+
+Luminous Effects with Gases at Low-Pressure 368
+
+
+"Magnetic Lag" Motor 67
+
+"Massage" with Currents of High Frequency 394
+
+Mechanical and Electrical Oscillators 486
+
+Method of obtaining Direct from Alternating currents 409
+
+Method of obtaining Difference of Phase by Magnetic Shielding 71
+
+Motors:
+
+  With Circuits of Different Resistance 79
+  With Closed Conductors 9
+  Combination of Synchronizing and Torque 95
+  With Condenser in Armature Circuit 101
+  With Condenser in one of the Field Circuits 106
+  With Coinciding Maxima of Magnetic Effect in Armature and Field 83
+  With "Current Lag" Artificially Secured 58
+  Early Phase 477
+  With Equal Magnetic Energies in Field and Armature 81
+  Or Generator, obtaining Desired Speed of 36
+  Improved Direct Current 448
+  Induction 92
+  "Magnetic Lag" 67
+  "No Wire" 235
+  With Phase Difference in Magnetization of Inner and Outer Parts
+          of Core 88
+  Regulator for Rotary Current 45
+  Single Circuit, Self-starting Synchronizing 50
+  Single Phase 76
+  With Single Wire to Generator 234, 330
+  Synchronizing 9
+  Thermo-Magnetic 424
+  Utilizing Continuous Current Generators 31
+
+
+National Electric Light Association Lecture 294
+
+"No Wire" Motor 235
+
+
+Observations on the Eye 294
+
+Oil, Condensers with Plates in 418
+
+Oil Insulation of Induction Coils 173, 221
+
+Oscillators, Mechanical and Electrical 486
+
+Ozone, Production of 171
+
+Phenomena Produced by Electrostatic Force 318
+
+Phosphorescence and Sulphide of Zinc 367
+
+Physiological Effects of High Frequency 162, 394
+
+Polyphase Systems 26
+
+Polyphase Transformer 109
+
+Pyromagnetic Generators 429
+
+Regulator for Rotary Current Motors 45
+
+Resonance, Electric, Phenomena of 340
+
+"Resultant Attraction" 7
+
+Rotating Field Transformers 9
+
+Rotating Magnetic Field 9
+
+Royal Institution Lecture 124
+
+Scope of Lectures 119
+
+Single Phase Motor 76
+
+Single Circuit, Self-Starting Synchronizing Motors 50
+
+Spinning Filament Effects 168
+
+Streaming Discharges of High Tension Coil 155, 163
+
+Synchronizing Motors 9
+
+Telegraphy without Wires 246
+
+Transformer with Shield between Primary and Secondary 113
+
+Thermo-Magnetic Motors 424
+
+Thomson, J. J., on Vacuum Tubes 397, 402, 406
+
+Thomson, Sir W., Current Accumulator 471
+
+Transformers:
+
+  Alternating 7
+  Magnetic Shield 113
+  Polyphase 109
+  Rotating Field 9
+
+Tubes:
+
+  Coated with Yttria, etc. 187
+  Coated with Sulphide of Zinc, etc. 290, 367
+
+Unipolar Generators 465
+
+Unipolar Generator, Forbes 468, 474
+
+Yttria, Coated Tubes 187
+
+Zinc, Tubes Coated with Sulphide of 367
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The inventions, researches and
+writings of Nikola Tesla, by Thomas Commerford Martin
+
+*** END OF THIS PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES ***
+
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