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+*** START OF THE PROJECT GUTENBERG EBOOK 13476 ***
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 13476-h.htm or 13476-h.zip:
+      (https://www.gutenberg.org/dirs/1/3/4/7/13476/13476-h/13476-h.htm)
+      or
+      (https://www.gutenberg.org/dirs/1/3/4/7/13476/13476-h.zip)
+
+
+
+
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY
+
+A Lecture Delivered before the Institution of Electrical Engineers, London
+
+by
+
+NIKOLA TESLA
+
+With a Portrait and Biographical Sketch of the Author
+
+NEW YORK
+
+1892
+
+
+
+
+
+
+
+Biographical Sketch of Nikola Tesla.
+
+
+While a large portion of the European family has been surging westward
+during the last three or four hundred years, settling the vast
+continents of America, another, but smaller, portion has been doing
+frontier work in the Old World, protecting the rear by beating back
+the "unspeakable Turk" and reclaiming gradually the fair lands that
+endure the curse of Mohammedan rule. For a long time the Slav
+people--who, after the battle of Kosovopjolje, in which the Turks
+defeated the Servians, retired to the confines of the present
+Montenegro, Dalmatia, Herzegovina and Bosnia, and "Borderland" of
+Austria--knew what it was to deal, as our Western pioneers did, with
+foes ceaselessly fretting against their frontier; and the races of
+these countries, through their strenuous struggle against the armies
+of the Crescent, have developed notable qualities of bravery and
+sagacity, while maintaining a patriotism and independence unsurpassed
+in any other nation.
+
+It was in this interesting border region, and from among these valiant
+Eastern folk, that Nikola Tesla was born in the year 1857, and the
+fact that he, to-day, finds himself in America and one of our foremost
+electricians, is striking evidence of the extraordinary attractiveness
+alike of electrical pursuits and of the country where electricity
+enjoys its widest application. Mr. Tesla's native place was Smiljan,
+Lika, where his father was an eloquent clergyman of the Greek Church,
+in which, by the way, his family is still prominently represented. His
+mother enjoyed great fame throughout the countryside for her skill and
+originality in needlework, and doubtless transmitted her ingenuity to
+Nikola; though it naturally took another and more masculine direction.
+
+The boy was early put to his books, and upon his father's removal to
+Gospic he spent four years in the public school, and later, three
+years in the Real School, as it is called. His escapades were such as
+most quick witted boys go through, although he varied the programme on
+one occasion by getting imprisoned in a remote mountain chapel rarely
+visited for service; and on another occasion by falling headlong into
+a huge kettle of boiling milk, just drawn from the paternal herds. A
+third curious episode was that connected with his efforts to fly when,
+attempting to navigate the air with the aid of an old umbrella, he
+had, as might be expected, a very bad fall, and was laid up for six
+weeks.
+
+About this period he began to take delight in arithmetic and physics.
+One queer notion he had was to work out everything by three or the
+power of three. He was now sent to an aunt at Cartstatt, Croatia, to
+finish his studies in what is known as the Higher Real School. It was
+there that, coming from the rural fastnesses, he saw a steam engine
+for the first time with a pleasure that he remembers to this day. At
+Cartstatt he was so diligent as to compress the four years' course
+into three, and graduated in 1873. Returning home during an epidemic
+of cholera, he was stricken down by the disease and suffered so
+seriously from the consequences that his studies were interrupted for
+fully two years. But the time was not wasted, for he had become
+passionately fond of experimenting, and as much as his means and
+leisure permitted devoted his energies to electrical study and
+investigation. Up to this period it had been his father's intention to
+make a priest of him, and the idea hung over the young physicist like
+a very sword of Damocles. Finally he prevailed upon his worthy but
+reluctant sire to send him to Gratz in Austria to finish his studies
+at the Polytechnic School, and to prepare for work as professor of
+mathematics and physics. At Gratz he saw and operated a Gramme machine
+for the first time, and was so struck with the objections to the use
+of commutators and brushes that he made up his mind there and then to
+remedy that defect in dynamo-electric machines. In the second year of
+his course he abandoned the intention of becoming a teacher and took
+up the engineering curriculum. After three years of absence he
+returned home, sadly, to see his father die; but, having resolved to
+settle down in Austria, and recognizing the value of linguistic
+acquirements, he went to Prague and then to Buda-Pesth with the view
+of mastering the languages he deemed necessary. Up to this time he had
+never realized the enormous sacrifices that his parents had made in
+promoting his education, but he now began to feel the pinch and to
+grow unfamiliar with the image of Francis Joseph I. There was
+considerable lag between his dispatches and the corresponding
+remittance from home; and when the mathematical expression for the
+value of the lag assumed the shape of an eight laid flat on its back,
+Mr. Tesla became a very fair example of high thinking and plain
+living, but he made up his mind to the struggle and determined to go
+through depending solely on his own resources. Not desiring the fame
+of a faster, he cast about for a livelihood, and through the help of
+friends he secured a berth as assistant in the engineering department
+of the government telegraphs. The salary was five dollars a week. This
+brought him into direct contact with practical electrical work and
+ideas, but it is needless to say that his means did not admit of much
+experimenting. By the time he had extracted several hundred thousand
+square and cube roots for the public benefit, the limitations,
+financial and otherwise, of the position had become painfully
+apparent, and he concluded that the best thing to do was to make a
+valuable invention. He proceeded at once to make inventions, but their
+value was visible only to the eye of faith, and they brought no grist
+to the mill. Just at this time the telephone made its appearance in
+Hungary, and the success of that great invention determined his
+career, hopeless as the profession had thus far seemed to him. He
+associated himself at once with telephonic work, and made various
+telephonic inventions, including an operative repeater; but it did not
+take him long to discover that, being so remote from the scenes of
+electrical activity, he was apt to spend time on aims and results
+already reached by others, and to lose touch. Longing for new
+opportunities and anxious for the development of which he felt himself
+possible, if once he could place himself within the genial and direct
+influences of the gulf streams of electrical thought, he broke away
+from the ties and traditions of the past, and in 1881 made his way to
+Paris. Arriving in that city, the ardent young Likan obtained
+employment as an electrical engineer with one of the largest electric
+lighting companies. The next year he went to Strasburg to install a
+plant, and on returning to Paris sought to carry out a number of ideas
+that had now ripened into inventions. About this time, however, the
+remarkable progress of America in electrical industry attracted his
+attention, and once again staking everything on a single throw, he
+crossed the Atlantic.
+
+Mr. Tesla buckled down to work as soon as he landed on these shores,
+put his best thought and skill into it, and soon saw openings for his
+talent. In a short while a proposition was made to him to start his
+own company, and, accepting the terms, he at once worked up a
+practical system of arc lighting, as well as a potential method of
+dynamo regulation, which in one form is now known as the "third brush
+regulation." He also devised a thermo-magnetic motor and other kindred
+devices, about which little was published, owing to legal
+complications. Early in 1887 the Tesla Electric Company of New York
+was formed, and not long after that Mr. Tesla produced his admirable
+and epoch-marking motors for multiphase alternating currents, in
+which, going back to his ideas of long ago, he evolved machines having
+neither commutator nor brushes. It will be remembered that about the
+time that Mr. Tesla brought out his motors, and read his thoughtful
+paper before the American Institute of Electrical Engineers, Professor
+Ferraris, in Europe, published his discovery of principles analogous
+to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
+Tesla was an independent inventor of this rotary field motor, for
+although anticipated in dates by Ferraris, he could not have known
+about Ferraris' work as it had not been published. Professor Ferraris
+stated himself, with becoming modesty, that he did not think Tesla
+could have known of his (Ferraris') experiments at that time, and adds
+that he thinks Tesla was an independent and original inventor of this
+principle. With such an acknowledgment from Ferraris there can be
+little doubt about Tesla's originality in this matter.
+
+Mr. Tesla's work in this field was wonderfully timely, and its worth
+was promptly appreciated in various quarters. The Tesla patents were
+acquired by the Westinghouse Electric Company, who undertook to
+develop his motor and to apply it to work of different kinds. Its use
+in mining, and its employment in printing, ventilation, etc., was
+described and illustrated in _The Electrical World_ some years ago.
+The immense stimulus that the announcement of Mr. Tesla's work gave to
+the study of alternating current motors would, in itself, be enough to
+stamp him as a leader.
+
+Mr. Tesla is only 35 years of age. He is tall and spare with a
+clean-cut, thin, refined face, and eyes that recall all the stories
+one has read of keenness of vision and phenomenal ability to see
+through things. He is an omnivorous reader, who never forgets; and he
+possesses the peculiar facility in languages that enables the least
+educated native of eastern Europe to talk and write in at least half a
+dozen tongues. A more congenial companion cannot be desired for the
+hours when one "pours out heart affluence in discursive talk," and
+when the conversation, dealing at first with things near at hand and
+next to us, reaches out and rises to the greater questions of life,
+duty and destiny.
+
+In the year 1890 he severed his connection with the Westinghouse
+Company, since which time he has devoted himself entirely to the study
+of alternating currents of high frequencies and very high potentials,
+with which study he is at present engaged. No comment is necessary on
+his interesting achievements in this field; the famous London lecture
+published in this volume is a proof in itself. His first lecture on
+his researches in this new branch of electricity, which he may be said
+to have created, was delivered before the American Institute of
+Electrical Engineers on May 20, 1891, and remains one of the most
+interesting papers read before that society. It will be found
+reprinted in full in _The Electrical World_, July 11, 1891. Its
+publication excited such interest abroad that he received numerous
+requests from English and French electrical engineers and scientists
+to repeat it in those countries, the result of which has been the
+interesting lecture published in this volume.
+
+The present lecture presupposes a knowledge of the former, but it may
+be read and understood by any one even though he has not read the
+earlier one. It forms a sort of continuation of the latter, and
+includes chiefly the results of his researches since that time.
+
+
+
+
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY
+
+
+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 time 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 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[A] 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.
+
+[Footnote A: For Mr. Tesla's American lecture on this subject see THE
+ELECTRICAL WORLD of July 11, 1891, and for a report of his French
+lecture see THE ELECTRICAL WORLD of March 26, 1892.]
+
+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 immediately the most 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 with 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 may 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 tests 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. 1.)
+
+[Illustration: FIG. 1.--DISCHARGE BETWEEN TWO WIRES WITH FREQUENCIES
+OF A FEW HUNDRED THOUSAND PER SECOND.]
+
+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. 2.
+
+[Illustration: FIG. 2.--IMITATING THE SPARK OF A HOLTZ MACHINE.]
+
+G is an ordinarily constructed alternator, supplying the primary P of
+an induction coil, the secondary S of which charges the condensers or
+jars CC. 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 pp of a second induction coil. This primary pp has a
+small air gap ab.
+
+The secondary s of this coil is provided with knobs or spheres KK of
+the proper size and set at a distance suitable for the experiment.
+
+A long arc is established between the terminals AB of the first
+induction coil. MM are the mica plates.
+
+Each time the arc is broken between A and B the jars are quickly
+charged and discharged through the primary pp, producing a snapping
+spark between the knobs KK. 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 ab until the arc is again broken by
+the draught.
+
+In this manner sudden impulses, at long intervals, are produced in the
+primary pp, which in the secondary s give a corresponding number of
+impulses of great intensity. If the secondary knobs or spheres, KK,
+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[A]
+before the American Institute of Electrical Engineers, May 20, 1891.
+
+[Footnote A: See THE ELECTRICAL WORLD, July 11, 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 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. 3.--DISRUPTIVE DISCHARGE COIL.]
+
+It is contained in a box B (Fig. 3) of thick boards of hard wood,
+covered on the outside with 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 RR, 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 FF, 24 centimetres square, the space between the flanges being
+about 3 centimetres. The secondary, SS, 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 PP 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 tt. 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.Fs. 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.
+
+[Illustration: FIG. 4.--ARRANGEMENT OF IMPROVED DISCHARGER AND
+MAGNET.]
+
+One of the changes is that the adjustable knobs A and B (Fig. 4),
+of the discharger are held in jaws of brass, JJ, by spring pressure,
+this allowing of turning them successively into different positions,
+and so doing away with the tedious process of frequent polishing up.
+
+The other change consists in the employment of a strong electromagnet
+NS, 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, MM, 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. LL are screws for fixing in position the rods RR, 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.
+
+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 AB, in Fig. 2 (the knobs ab 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. 5, 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.
+
+[Illustration: FIG. 5.--ARRANGEMENT WITH LOW-FREQUENCY ALTERNATOR AND
+IMPROVED DISCHARGER.]
+
+[Illustration: FIG. 6.--DISCHARGER WITH MULTIPLE GAPS.]
+
+The other form of discharger used in these and similar experiments is
+indicated in Figs. 6 and 7. It consists of a number of brass pieces cc
+(Fig. 6), 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 RR, with planed grooves gg (Fig. 7) to fit the
+middle portion of the pieces cc, serve to clamp the latter and hold
+them firmly in position by means of two bolts CC (of which only one is
+shown) passing through the ends of the strips.
+
+[Illustration: FIG. 7.--DISCHARGER WITH MULTIPLE GAPS.]
+
+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. 2,
+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 7 metres in length. They are supported on insulating cords
+at a distance of about 30 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 set at the beginning the condenser plates at maximum distance. If
+the streams for 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. 8.--EFFECT PRODUCED BY CONCENTRATING STREAMS.]
+
+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. 8), 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.
+
+Here, for instance, I have two plates, RR, of hard rubber (Fig. 9),
+upon which I have glued two very thin wires ww, 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 tt. The plates are placed in line
+at a sufficient distance to prevent a spark passing from one to the
+other wire. 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.
+
+[Illustration: FIG. 9.--WIRES RENDERED INTENSELY LUMINOUS.]
+
+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 then when produced with such a coil as the present. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.
+
+[Illustration: FIG. 10.--LUMINOUS DISCS.]
+
+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. 10), 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_ but _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 oil and placed parallel to each
+other at a distance of about 10 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 a merely 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 often occasion 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.
+
+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 (dd, Fig. 11) 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.
+
+[Illustration: FIG. 11.--PHANTOM STREAMS.]
+
+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 1 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
+immersion, 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 in commerce two kinds of gutta-percha wire: 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 hard wood 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, become, at least with higher frequencies, so easy that
+they could be hardly called engineering feats. With oil insulation
+and alternate current motors transmissions of power can be effected
+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 purposes 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
+20, 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; this
+allowing the use of a much bigger 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.
+
+In bulbs provided with a conducting terminal, though it be of
+aluminium, 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. 12 and 13.
+
+In Fig. 12 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 aluminium 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.
+
+[Illustration: FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.]
+
+The construction shown in Fig. 13 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 conducting 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. 14.--FORMS AND PHASES OF THE ROTATING BRUSH.]
+
+Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
+Fig. 14 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. 12 and 13) is exhausted to a very high
+degree, generally the bulb is not excited upon connecting the wire w
+(Fig. 12) or the tinfoil coating of the bulb (Fig. 13) 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. 15. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 16, 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. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING
+BRUSH.]
+
+When the brush assumes the form indicated in Fig. 16, it maybe 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
+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 in
+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 result. 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 aluminium 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 overlap partly 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. 17), comprising a coil and iron core, and a freely movable copper
+disc in proximity to the latter.
+
+[Illustration: FIG. 17.--SINGLE WIRE AND "NO-WIRE" MOTOR.]
+
+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 subtile 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
+alternate currents 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 inclosed 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 as 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 aluminium sheet, which fitted the stem
+and was held on it by spring pressure. The function of this aluminium
+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, time
+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 the coil, the largest bulb being placed at the end of the wire, at
+some distance from it the smallest bulb, and 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 it
+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 about in 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. In 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 otherwise
+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 the 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 aluminium, 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
+aluminium sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a piece of
+aluminium sheet of the 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 aluminium 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 aluminium
+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
+aluminium 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 degrees 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, by far, so 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 a means for
+concentrating more energy upon the same.
+
+To whatever extent the aluminium 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 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 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 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.
+
+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 clearness of some of the remarks before made, I must
+now refer to Figs. 18, 19 and 20, which illustrate various
+arrangements with a type of bulb most generally used.
+
+[Illustration: FIG. 18.--BULB WITH MICA TUBE AND ALUMINIUM SCREEN.]
+
+[Illustration: FIG. 19.--IMPROVED BULB WITH SOCKET AND SCREEN.]
+
+Fig. 18 is a section through a spherical bulb L, with the glass stem
+s, containing 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 aluminium tube.
+
+Fig. 19 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, as mica powder.
+
+[Illustration: FIG. 20.--BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.]
+
+Fig. 20 shows a bulb made for experimental purposes. In this bulb the
+aluminium 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. 21), 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, aluminium 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
+tried, 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 very high.
+
+Fig. 22 illustrates a similar arrangement, with a large tube T
+protruding in to the part of the bulb containing the refractors button
+m. In this case the wire leading from the outside into the bulb is
+omitted, the energy required being supplied through condenser coatings
+CC. 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 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.
+
+[Illustration: FIG. 21.--IMPROVED BULB WITH NON-CONDUCTING BUTTON.]
+
+[Illustration: FIG. 22.--TYPE OF BULB WITHOUT LEADING-IN WIRE.]
+
+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 currents. 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.
+
+[Illustration: FIG. 23.--EFFECT PRODUCED BY A RUBY DROP.]
+
+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. 21, 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.
+
+A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 23. 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 aluminium 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 aluminium 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. 24 illustrates one of the bulbs used. It consists
+of a spherical globe L, provided with a long neck n, on the 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. 24 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 a different kind were mounted
+in the bulb as, for instance, indicated in Fig. 23, 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.
+This 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 larger 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 others, 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 is 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 endeavored repeatedly
+to fuse zirconia, placing it in a cup or arc light carbon as indicated
+in Fig. 23. 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
+the 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.
+23 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 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 invisible 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 say
+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 found 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 phosphorescence, 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 phosphorescence. 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 phosphorescence, 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 could the
+observer call 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 aluminium 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 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. This 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 eye 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 a 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
+of 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
+reflecting one would think that in pushing so far the incandescence of
+the electrode it would be instantly volatilized. But after a careful
+consideration he would find that, theoretically, it should not occur,
+and in this fact--which, however, 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 is forced the incandescence 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 is 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 shatter quickly 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 the 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 most
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 22, 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 illuminating
+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 as 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 gentle 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. 19, 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. 24.--BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT
+OF PROJECTED MATTER.]
+
+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.
+
+[Illustration: FIG. 25.--IMPROVED EXPERIMENTAL BULB.]
+
+Again, in another of the early experiments, a bulb was used as
+illustrated in Fig. 12. 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 result 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.
+
+[Illustration: FIG. 26.--IMPROVED BULB WITH INTENSIFYING REFLECTOR.]
+
+In Fig. 24, 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. 25 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. 26. In this case the construction of the bulb is
+as shown and described before, when reference was made to Fig. 19. A
+zinc sheet Z, with a tubular extension T, is slipped 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.
+
+[Illustration: FIG. 27.--PHOSPHORESCENT TUBE WITH INTENSIFYING
+REFLECTOR.]
+
+A similar disposition with a phosphorescent tube is illustrated in
+Fig. 27. The tube T is prepared from two short tubes of a different
+diameter, which are sealed on the ends. On the lower end is placed an
+outside 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.
+
+Suppose a small helix with many well insulated turns, as in experiment
+Fig. 17, 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 the 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 the
+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
+more one or 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 the 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 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 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.
+
+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. 28.
+
+[Illustration: FIG. 28.--LAMP WITH AUXILIARY BULB FOR CONFINING THE
+ACTION TO THE CENTRE.]
+
+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. 18, for example. The small bulb is conveniently supported upon
+the stem s, carrying the refractory button m. It is separated from the
+aluminium tube a by several layers of mica M, in order to prevent the
+cracking of the neck by the rapid heating of the aluminium 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. 28 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. 29. 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 use a large bulb. It was found, in the course of
+experiences with bulbs such as illustrated in Fig. 29, 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. 29, namely, the
+vacuum in the bulb b would be impaired in a comparatively short time.
+
+[Illustration: FIG. 29.--LAMP WITH INDEPENDENT AUXILIARY BULB.]
+
+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. 28 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 the 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, and
+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 sufficiently 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 knocks 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 1 inch in
+diameter and 1 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 different
+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 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
+insulated 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 a 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 moving matter. In a gas the speed may be 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 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 presently 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 discarded 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. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF
+EXHAUSTION.]
+
+The apparatus is illustrated in a drawing shown in Fig. 30. 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 connection 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 mercury in 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 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 moisture off 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 almost in 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 the
+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 r 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 15 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 1-20, 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 act 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 claim 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 allowing
+barely the light to shine through. A metallic clasp, 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 aluminium 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 not by far
+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 a high frequency current,
+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 the 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 he
+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 to 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 as 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.
+
+[Illustration: FIG. 31.--BULB SHOWING RADIANT LIME STREAM AT LOW
+EXHAUSTION.]
+
+I have prepared a bulb to illustrate by an experiment the correctness
+of these assertions. In a globe L (Fig. 31) 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. 19, 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. 31) 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.
+
+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.
+
+A thought which naturally presents itself in connection with high
+frequency currents, is to make use of their powerful electro-dynamic
+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. 32 and Fig. 33.
+
+In Fig. 32 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 aluminium
+wire, the ends of which were provided with small spheres t and t_1 of
+aluminium, 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 currents 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.
+
+[Illustration: FIG. 32.--ELECTRO-DYNAMIC INDUCTION TUBE.]
+
+[Illustration: FIG. 33--ELECTRO-DYNAMIC INDUCTION LAMP.]
+
+In Fig. 33 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 just a trifle heated 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, was 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. 33 an
+aluminium 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 experiences 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, gets 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 ball 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, still, 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 far 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 he works
+with condenser charges--and they are the only means up to the present
+known for reaching these extreme frequencies--he gets to electromotive
+forces of several thousands of volts per turn of the primary. He
+cannot multiply the electro-dynamic inductive effect by taking more
+turns in the primary, for he arrives at the conclusion that the best
+way is to work with one single turn--though he must sometimes depart
+from this rule--and he must get along with whatever inductive effect
+he can obtain with one turn. But before he has long experimented with
+the extreme frequencies required to set up in a small bulb an
+electromotive force of several thousands of volts he 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 electro-dynamic
+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 condenser to a higher potential, establish electrostatic
+alternating fields which acted through the whole extent of a 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.
+
+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 the 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 the 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. 34 and Fig. 35 two such tubes are illustrated
+which are prepared for the occasion. In Fig. 34 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 aluminium 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.
+
+[Illustration: FIG. 34.--TUBE WITH FILAMENT RENDERED INCANDESCENT IN
+AN ELECTROSTATIC FIELD.]
+
+[Illustration: FIG. 35.--CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.]
+
+A more interesting piece of apparatus is illustrated in Fig. 35. 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 aluminium 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. From 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, their 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 this argument is not
+devoid of force-but because in a burner the higher vibrations can
+never be reached except by passing through all the low ones. For how
+is a flame produced 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 remain 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 very high frequencies, many
+advantages, such as a 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, 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 though 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 sent 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 distance 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.
+
+
+
+[Transcriber's note: Corrected the following typesetting errors:
+     1) 'preceived' to 'perceived', page 16.
+     2) 'disharging' to 'discharging', page 30.
+     3) 'park' to 'spark', page 33.
+     4) 'pssition' to 'position', page 50.
+     5) 'to th opposite side' to 'to the opposite side', page 56.
+     6) 's resses' to 'stresses', page 147.]
+
+*** END OF THE PROJECT GUTENBERG EBOOK 13476 ***
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+<div>*** START OF THE PROJECT GUTENBERG EBOOK 13476 ***</div>
+<h1>The Project Gutenberg eBook, Experiments with Alternate Currents of High
+Potential and High Frequency, by Nikola Tesla</h1>
+<br>
+<br>
+<br>
+<br>
+<hr class="full" noshade>
+<div align="center"><img src="images/title.gif" alt="Title Page"
+width="470" border="1"></div>
+<br>
+<br>
+<br>
+<h2>EXPERIMENTS</h2>
+<h3>WITH</h3>
+<h1>ALTERNATE CURRENTS</h1>
+<h3>OF</h3>
+<h2>HIGH POTENTIAL AND HIGH FREQUENCY.</h2>
+<h3>BY</h3>
+<h2>NIKOLA TESLA.</h2>
+
+<hr>
+
+<h2>A LECTURE</h2>
+<h3>DELIVERED BEFORE THE</h3>
+<h2>INSTITUTION OF ELECTRICAL ENGINEERS, LONDON.</h2>
+
+<hr>
+
+<div align="center"><i>With a Portrait and Biographical Sketch</i><br>
+<i>of the Author</i>.<br>
+
+<hr>
+
+NEW YORK:<br>
+1892</div>
+
+<p>&nbsp;</p><!-- Page 2 -->
+<p>&nbsp;</p><!-- Page 3 -->
+
+<!-- The following image was obtained from another source. -->
+<div align="center">
+<img src="images/tesla.gif" alt="Portrait of Nikola Tesla" width="280" height="459" border="0">
+</div>
+
+<h2>Biographical Sketch of Nikola Tesla.</h2>
+<hr>
+<p>&nbsp;</p>
+
+<p>While a large portion of the European family has been surging westward
+during the last three or four hundred years, settling the vast
+continents of America, another, but smaller, portion has been doing
+frontier work in the Old World, protecting the rear by beating back
+the &quot;unspeakable Turk&quot; and reclaiming gradually the fair lands that
+endure the curse of Mohammedan rule. For a long time the Slav
+people&mdash;who, after the battle of Kosovopjolje, in which the Turks
+defeated the Servians, retired to the confines of the present
+Montenegro, Dalmatia, Herzegovina and Bosnia, and &quot;Borderland&quot; of
+Austria&mdash;knew what it was to deal, as our Western pioneers did, with
+foes ceaselessly fretting against their frontier; and the races of
+these countries, through their strenuous struggle against the armies
+of the Crescent, have developed notable qualities of bravery and
+sagacity, while maintaining a patriotism and independence unsurpassed
+in any other nation.</p>
+
+<p>It was in this interesting border region, and from among these valiant
+Eastern folk, that Nikola Tesla was born in the year 1857, and the
+fact that he, to-day, finds himself in America and one of our foremost
+electricians, is striking evidence of the extraordinary attractiveness
+alike of electrical pursuits and of the country where electricity
+enjoys its widest application.
+<!-- Page 4 -->
+Mr. Tesla's native place was Smiljan,
+Lika, where his father was an eloquent clergyman of the Greek Church,
+in which, by the way, his family is still prominently represented. His
+mother enjoyed great fame throughout the countryside for her skill and
+originality in needlework, and doubtless transmitted her ingenuity to
+Nikola; though it naturally took another and more masculine direction.</p>
+
+<p>The boy was early put to his books, and upon his father's removal to
+Gospic he spent four years in the public school, and later, three
+years in the Real School, as it is called. His escapades were such as
+most quick witted boys go through, although he varied the programme on
+one occasion by getting imprisoned in a remote mountain chapel rarely
+visited for service; and on another occasion by falling headlong into
+a huge kettle of boiling milk, just drawn from the paternal herds. A
+third curious episode was that connected with his efforts to fly when,
+attempting to navigate the air with the aid of an old umbrella, he
+had, as might be expected, a very bad fall, and was laid up for six weeks.</p>
+
+<p>About this period he began to take delight in arithmetic and physics.
+One queer notion he had was to work out everything by three or the
+power of three. He was now sent to an aunt at Cartstatt, Croatia, to
+finish his studies in what is known as the Higher Real School. It was
+there that, coming from the rural fastnesses, he saw a steam engine
+for the first time with a pleasure that he remembers to this day. At
+Cartstatt he was so diligent as to compress the four years' course into three,
+and graduated in 1873. Returning home during an epidemic of cholera, he was
+<!-- Page 5 -->
+stricken down by the disease and suffered so
+seriously from the consequences that his studies were interrupted for
+fully two years. But the time was not wasted, for he had become
+passionately fond of experimenting, and as much as his means and
+leisure permitted devoted his energies to electrical study and
+investigation. Up to this period it had been his father's intention to
+make a priest of him, and the idea hung over the young physicist like
+a very sword of Damocles. Finally he prevailed upon his worthy but
+reluctant sire to send him to Gratz in Austria to finish his studies
+at the Polytechnic School, and to prepare for work as professor of
+mathematics and physics. At Gratz he saw and operated a Gramme machine
+for the first time, and was so struck with the objections to the use
+of commutators and brushes that he made up his mind there and then to
+remedy that defect in dynamo-electric machines. In the second year of
+his course he abandoned the intention of becoming a teacher and took
+up the engineering curriculum. After three years of absence he
+returned home, sadly, to see his father die; but, having resolved to
+settle down in Austria, and recognizing the value of linguistic
+acquirements, he went to Prague and then to Buda-Pesth with the view
+of mastering the languages he deemed necessary. Up to this time he had
+never realized the enormous sacrifices that his parents had made in
+promoting his education, but he now began to feel the pinch and to
+grow unfamiliar with the image of Francis Joseph I. There was
+considerable lag between his dispatches and the corresponding
+remittance from home; and when the mathematical expression for
+<!-- Page 6 -->
+the value of the lag assumed the shape of an eight laid flat on its back,
+Mr. Tesla became a very fair example of high thinking and plain
+living, but he made up his mind to the struggle and determined to go
+through depending solely on his own resources. Not desiring the fame
+of a faster, he cast about for a livelihood, and through the help of
+friends he secured a berth as assistant in the engineering department
+of the government telegraphs. The salary was five dollars a week. This
+brought him into direct contact with practical electrical work and
+ideas, but it is needless to say that his means did not admit of much
+experimenting. By the time he had extracted several hundred thousand
+square and cube roots for the public benefit, the limitations,
+financial and otherwise, of the position had become painfully
+apparent, and he concluded that the best thing to do was to make a
+valuable invention. He proceeded at once to make inventions, but their
+value was visible only to the eye of faith, and they brought no grist
+to the mill. Just at this time the telephone made its appearance in
+Hungary, and the success of that great invention determined his
+career, hopeless as the profession had thus far seemed to him. He
+associated himself at once with telephonic work, and made various
+telephonic inventions, including an operative repeater; but it did not
+take him long to discover that, being so remote from the scenes of
+electrical activity, he was apt to spend time on aims and results
+already reached by others, and to lose touch. Longing for new opportunities
+and anxious for the development of which he felt himself possible, if once
+he could place himself within the genial and direct influences of the gulf
+<!-- Page 7 -->
+streams of electrical thought, he broke away from the ties and traditions of the past,
+and in 1881 made his way to Paris. Arriving in that city, the ardent young Likan obtained
+employment as an electrical engineer with one of the largest electric
+lighting companies. The next year he went to Strasburg to install a
+plant, and on returning to Paris sought to carry out a number of ideas
+that had now ripened into inventions. About this time, however, the
+remarkable progress of America in electrical industry attracted his
+attention, and once again staking everything on a single throw, he
+crossed the Atlantic.</p>
+
+<p>Mr. Tesla buckled down to work as soon as he landed on these shores,
+put his best thought and skill into it, and soon saw openings for his
+talent. In a short while a proposition was made to him to start his
+own company, and, accepting the terms, he at once worked up a
+practical system of arc lighting, as well as a potential method of
+dynamo regulation, which in one form is now known as the &quot;third brush
+regulation.&quot; He also devised a thermo-magnetic motor and other kindred
+devices, about which little was published, owing to legal
+complications. Early in 1887 the Tesla Electric Company of New York
+was formed, and not long after that Mr. Tesla produced his admirable
+and epoch-marking motors for multiphase alternating currents, in
+which, going back to his ideas of long ago, he evolved machines having
+neither commutator nor brushes. It will be remembered that about the
+time that Mr. Tesla brought out his motors, and read his thoughtful
+paper before the American Institute of Electrical Engineers, Professor
+Ferraris, in Europe, published his discovery of principles
+<!-- Page 8 -->
+analogous to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
+Tesla was an independent inventor of this rotary field motor, for
+although anticipated in dates by Ferraris, he could not have known
+about Ferraris' work as it had not been published. Professor Ferraris
+stated himself, with becoming modesty, that he did not think Tesla
+could have known of his (Ferraris') experiments at that time, and adds
+that he thinks Tesla was an independent and original inventor of this
+principle. With such an acknowledgment from Ferraris there can be
+little doubt about Tesla's originality in this matter.</p>
+
+<p>Mr. Tesla's work in this field was wonderfully timely, and its worth
+was promptly appreciated in various quarters. The Tesla patents were
+acquired by the Westinghouse Electric Company, who undertook to
+develop his motor and to apply it to work of different kinds. Its use
+in mining, and its employment in printing, ventilation, etc., was
+described and illustrated in <i>The Electrical World</i> some years ago.
+The immense stimulus that the announcement of Mr. Tesla's work gave to
+the study of alternating current motors would, in itself, be enough to
+stamp him as a leader.</p>
+
+<p>Mr. Tesla is only 35 years of age. He is tall and spare with a
+clean-cut, thin, refined face, and eyes that recall all the stories
+one has read of keenness of vision and phenomenal ability to see
+through things. He is an omnivorous reader, who never forgets; and he
+possesses the peculiar facility in languages that enables the least
+educated native of eastern Europe to talk and write in at least half a
+dozen tongues. A more congenial companion cannot be desired for the
+hours when one &quot;pours out heart affluence in discursive
+<!-- Page 9 -->
+talk,&quot; and when the conversation, dealing at first with things near at hand and
+next to us, reaches out and rises to the greater questions of life, duty and destiny.</p>
+
+<p>In the year 1890 he severed his connection with the Westinghouse
+Company, since which time he has devoted himself entirely to the study
+of alternating currents of high frequencies and very high potentials,
+with which study he is at present engaged. No comment is necessary on
+his interesting achievements in this field; the famous London lecture
+published in this volume is a proof in itself. His first lecture on
+his researches in this new branch of electricity, which he may be said
+to have created, was delivered before the American Institute of
+Electrical Engineers on May 20, 1891, and remains one of the most
+interesting papers read before that society. It will be found
+reprinted in full in <i>The Electrical World</i>, July 11, 1891. Its
+publication excited such interest abroad that he received numerous
+requests from English and French electrical engineers and scientists
+to repeat it in those countries, the result of which has been the
+interesting lecture published in this volume.</p>
+
+<p>The present lecture presupposes a knowledge of the former, but it may
+be read and understood by any one even though he has not read the
+earlier one. It forms a sort of continuation of the latter, and
+includes chiefly the results of his researches since that time.</p>
+<!-- Page 10 -->
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+
+<h1>EXPERIMENTS</h1>
+<h3>WITH </h3>
+<h2>Alternate Currents of High Potential </h2>
+<h2>and High Frequency.</h2>
+
+<hr>
+
+<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 time 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
+<!-- Page 11 -->
+detail about that charming work I can remember 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 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<a name="FNanchor_A_1">
+</a><a href="#Footnote_A_1"><sup>[A]</sup></a> 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
+<!-- Page 12 -->
+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>
+
+<a name="Footnote_A_1"></a><a href="#FNanchor_A_1">[A]</a>
+<div class="fnote"><p> For Mr. Tesla's American lecture on this subject see THE
+ELECTRICAL WORLD of July 11, 1891, and for a report of his French
+lecture see THE ELECTRICAL WORLD of March 26, 1892.</p></div>
+
+<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 immediately the most 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 with 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
+<!-- Page 13 -->
+that; in all branches of industry alternating currents&mdash;electric wave
+motion&mdash;will have the sway.</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
+<!-- Page 14 -->
+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 may 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.</p>
+
+<p>Here still another, which by my fingers' touch casts a shadow&mdash;the
+Crookes shadow, 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
+<!-- Page 15 -->
+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>
+<!-- Page 16 -->
+<p>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 tests 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
+<!-- Page 17 -->
+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. 1.)</p>
+
+<div align="center"><img src="images/fig01.gif" width="492" height="599" border="0"
+alt="FIG. 1.&mdash;DISCHARGE BETWEEN TWO WIRES WITH FREQUENCIES OF A FEW HUNDRED THOUSAND PER SECOND.">
+</div>
+
+<p>Now compare this phenomenon which you have just witnessed 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
+<!-- Page 18 -->
+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 <br>
+succession of sparks of great intensity and small quantity, which possess
+<!-- Page 19 -->
+the same brilliancy, and are accompanied by the same sharp crackling sound,
+as those obtained from a friction or influence machine.</p>
+
+<img src="images/fig02.gif" width="178" height="663" border="0" align="left" hspace="10"
+alt="FIG. 2.&mdash;IMITATING THE SPARK OF A HOLTZ MACHINE.">
+
+<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. 2.</p>
+
+<p><i>G</i> is an ordinarily constructed alternator, supplying the primary <i>P</i>
+of an induction coil, the secondary <i>S</i> of which
+<!-- Page 20 -->
+charges the condensers or jars <i>C&nbsp;C</i>. 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&nbsp;p</i> of a second induction coil. This
+primary <i>p&nbsp;p</i> has a small air gap <i>a&nbsp;b</i>.</p>
+
+<p>The secondary <i>s</i> of this coil is provided with knobs or spheres <i>K&nbsp;K</i>
+of the proper size and set at a distance suitable for the experiment.</p>
+
+<p>A long arc is established between the terminals <i>A&nbsp;B</i> of the first
+induction coil. <i>M&nbsp;M</i> are the mica plates.</p>
+
+<p>Each time the arc is broken between <i>A</i> and <i>B</i> the jars are quickly
+charged and discharged through the primary <i>p&nbsp;p</i>, producing a snapping
+spark between the knobs <i>K&nbsp;K</i>. Upon the arc forming between <i>A</i> and <i>B</i>
+the potential falls, and the jars cannot be charged to such high
+potential as to break through the air gap <i>a&nbsp;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&nbsp;p</i>, which in the secondary <i>s</i> give a corresponding number
+of impulses of great intensity. If the secondary knobs or spheres,
+<i>K&nbsp;K</i>, 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 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.</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
+<!-- Page 21 -->
+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
+<a name="FNanchor_A_2"></a><a href="#Footnote_A_2"><sup>[A]</sup></a>
+before the American Institute of Electrical Engineers, May 20, 1891.</p>
+
+<a name="Footnote_A_2"></a><a href="#FNanchor_A_2">[A]</a><div class="fnote">
+<p> See THE ELECTRICAL WORLD, July 11, 1891.</p></div>
+
+<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
+<!-- Page 22 -->
+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&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
+<!-- Page 23 -->
+may vary them in many ways, and, if it were 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, 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,
+<!-- Page 24 -->
+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 align="center"><img src="images/fig03.gif" width="476" height="575" border="0"
+alt="FIG. 3.&mdash;DISRUPTIVE DISCHARGE COIL."></div>
+
+<p>It is contained in a box <i>B</i> (Fig. 3) of thick boards of hard wood,
+covered on the outside with zinc sheet <i>Z</i>, which is
+<!-- Page 25 -->
+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, 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 <i>R&nbsp;R</i>, held apart at a
+distance of 10 centimetres by bolts <i>c</i> and nuts <i>n</i>, likewise of hard
+rubber. Each spool comprises a tube <i>T</i> of approximately 8 centimetres
+inside diameter, and 3 millimetres thick, upon which are screwed two
+flanges <i>F&nbsp;F</i>, 24 centimetres square, the space between the flanges
+being about 3 centimetres. The secondary, <i>S&nbsp;S</i>, 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
+<!-- Page 26 -->
+its terminals <i>T</i><sub>1</sub>&nbsp;<i>T</i><sub>1</sub> 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 <i>P&nbsp;P</i> is wound in two parts, and oppositely, upon a wooden
+spool <i>W</i>, and the four ends are led out of the oil through hard
+rubber tubes <i>t&nbsp;t</i>. The ends of the secondary <i>T</i><sub>1</sub>&nbsp;<i>T</i><sub>1</sub>
+are also led out of the oil through rubber tubes <i>t</i><sub>1</sub>&nbsp;<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 E.M.Fs. 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 <i>B</i> surrounding the
+whole is used. </p>
+<!-- Page 27 -->
+<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>
+
+<div align="center"><img src="images/fig04.gif" width="692" height="367" border="0"
+alt="FIG. 4.&mdash;ARRANGEMENT OF IMPROVED DISCHARGER AND MAGNET.">
+</div>
+
+<p>One of the changes is that the adjustable knobs <i>A</i> and <i>B</i> (Fig. 4),
+of the discharger are held in jaws of brass, <i>J&nbsp;J</i>, 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>
+
+<p>The other change consists in the employment of a strong electromagnet
+<i>N&nbsp;S</i>, which is placed with its axis at right angles to the line
+joining the knobs <i>A</i> and <i>B</i>, and produces a strong magnetic field
+between them. The pole pieces of
+<!-- Page 28 -->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, <i>M&nbsp;M</i>, of sufficient thickness.
+<i>s</i><sub>1</sub>&nbsp;<i>s</i><sub>1</sub>
+and <i>s</i><sub>2</sub>&nbsp;<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. <i>L&nbsp;L</i> are screws for fixing in
+position the rods <i>R&nbsp;R</i>, which support the knobs.</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>
+
+<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 <i>A&nbsp;B</i>, in Fig. 2 (the knobs <i>a&nbsp;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>
+
+<div align="center"><img src="images/fig05.gif" width="588" height="210" border="0"
+alt="FIG. 5.&mdash;ARRANGEMENT WITH LOW-FREQUENCY ALTERNATOR AND IMPROVED DISCHARGER.">
+</div>
+
+<p>When a magnet is employed to break the arc, it is
+<!-- Page 29 -->
+better to choose the connection indicated diagrammatically in Fig. 5, 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>
+
+<div align="center"><img src="images/fig06.gif" width="564" height="226" border="0"
+alt="FIG. 6.&mdash;DISCHARGER WITH MULTIPLE GAPS."></div>
+
+<p>The other form of discharger used in these and similar experiments is
+indicated in Figs. 6 and 7. It consists of a number of brass pieces
+<i>c&nbsp;c</i> (Fig. 6), 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 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
+<!-- Page 30 -->
+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 <i>R&nbsp;R</i>, with planed grooves <i>g&nbsp;g</i> (Fig. 7)
+to fit the middle portion of the pieces <i>c&nbsp;c</i>, serve to clamp the latter
+and hold them firmly in position by means of two bolts <i>C&nbsp;C</i> (of which
+only one is shown) passing through the ends of the strips.</p>
+
+<div align="center"><img src="images/fig07.gif" width="557" height="373" border="0"
+alt="FIG. 7.&mdash;DISCHARGER WITH MULTIPLE GAPS."></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 apparatus affords some
+<!-- Page 31 -->
+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. 2,
+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
+<!-- Page 32 -->
+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 note-worthy of these
+discharge phenomena.</p>
+
+<p>I have stretched across the room two ordinary cotton covered wires,
+each about 7 metres in length. They are supported on insulating cords
+at a distance of about 30 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.</p>
+
+<p>A convenient way is to use an oil condenser of very small capacity,
+consisting of two small adjustable metal
+<!-- Page 33 -->
+plates, in connection with this and similar experiments. In such case I take wires
+rather short and set at the beginning the condenser plates at maximum distance.
+If the streams for 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 difficult. One
+<!-- Page 34 -->
+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>
+
+<p>The luminous intensity of the streams is, of course, considerably
+<!-- Page 35 -->
+increased when they are focused upon a small surface. This may be
+shown by the following experiment:</p>
+
+<div align="center">
+<img src="images/fig08.gif" width="554" height="514" border="0"
+alt="FIG. 8.&mdash;EFFECT PRODUCED BY CONCENTRATING STREAMS.">
+</div>
+
+<p>I attach to one of the terminals of the coil a wire <i>w</i> (Fig. 8), 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 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>
+
+<p>Here, for instance, I have two plates, <i>R&nbsp;R</i>, of hard rubber (Fig. 9),
+upon which I have glued two very thin wires <i>w&nbsp;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
+<!-- Page 36 -->
+<i>t&nbsp;t</i>. The plates are placed in line at a sufficient distance to prevent a
+spark passing from one to the other wire. The two tinfoil coatings I have joined by a
+conductor <i>C</i>, 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>
+
+<div align="center">
+<img src="images/fig09.gif" width="557" height="528" border="0"
+alt="FIG. 9.&mdash;WIRES RENDERED INTENSELY LUMINOUS.">
+</div>
+
+<p>It is perhaps preferable to perform this experiment with a coil
+operated from an alternator of high frequency, as
+<!-- Page 37 -->
+then, owing to the harmonic rise and fall, the streams are very uniform, though
+they are less abundant then when produced with such a coil as the present. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.</p>
+
+<div align="center">
+<img src="images/fig10.gif" width="325" height="559" border="0"
+alt="FIG. 10.&mdash;LUMINOUS DISCS.">
+</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, <i>C</i> and <i>c</i> (Fig. 10), of rather stout wire, one being about
+<!-- Page 38 -->
+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.</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> but <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
+<!-- Page 39 -->
+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 oil and placed parallel to each
+other at a distance of about 10 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 a
+merely incidental necessity. These experiments teach us that, in endeavoring
+to discover novel methods of producing light by the agitation of atoms, or
+<!-- Page 40 -->
+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 often occasion 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
+<!-- Page 41 -->
+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>
+
+<p>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&mdash;or, better still, two sharp-edged metal
+discs (<i>d&nbsp;d</i>, Fig. 11) 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
+<!-- Page 42 -->
+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. </p>
+
+<div align="center">
+<img src="images/fig11.gif" width="548" height="535" border="0"
+alt="FIG. 11.&mdash;PHANTOM STREAMS.">
+</div>
+
+<p>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
+<!-- Page 43 -->
+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 1 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
+<!-- Page 44 -->
+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 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
+<!-- Page 45 -->
+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
+immersion, 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 in commerce two kinds of gutta-percha wire: 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.</p>
+<!-- Page 46 -->
+<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 hard wood 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
+<!-- Page 47 -->
+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 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, become, at least with higher frequencies, so easy that
+they could be hardly called engineering
+<!-- Page 48 -->
+feats. With oil insulation and alternate current motors transmissions of power
+can be effected 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.</p>
+
+<p>The exclusion of gaseous matter from any apparatus
+<!-- Page 49 -->
+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 purposes 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
+20, 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
+<!-- Page 50 -->
+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; this
+allowing the use of a much bigger 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 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,
+<!-- Page 51 -->
+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 self-induction of both the latter. The condenser
+<!-- Page 52 -->
+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>
+
+<p>In bulbs provided with a conducting terminal, though it be of
+aluminium, 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. 12 and 13.</p>
+
+<div align="center">
+<img src="images/fig12_13.gif" width="518" height="578" border="0"
+alt="FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.">
+</div>
+
+<p>In Fig. 12 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 aluminium sheet, may be slipped in the barometer
+tube, but it is not important to employ it.</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>
+<!-- Page 53 -->
+<p>The construction shown in Fig. 13 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
+<!-- Page 54 -->
+inductively upon the moderately rarefied and highly conducting 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 align="center">
+<img src="images/fig14.gif" width="578" height="425" border="0"
+alt="FIG. 14.&mdash;FORMS AND PHASES OF THE ROTATING BRUSH.">
+</div>
+
+<p>Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
+Fig. 14 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. 12 and 13) is exhausted to a
+<!-- Page 55 -->
+very high degree, generally the bulb is not excited upon connecting the wire
+<i>w</i> (Fig. 12) or the tinfoil coating of the bulb (Fig. 13) 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
+<!-- Page 56 -->
+for some time the bulb appears as in Fig. 15. From this stage the
+phenomenon will gradually pass to that indicated in Fig. 16, 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 align="center">
+<img src="images/fig15_16.gif" width="518" height="552" border="0"
+alt="FIG. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING BRUSH.">
+</div>
+
+<p>When the brush assumes the form indicated in Fig. 16, it maybe 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.</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
+<!-- Page 57 -->
+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
+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
+<!-- Page 58 -->
+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 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
+<!-- Page 59 -->
+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 in
+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.</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,
+<!-- Page 60 -->
+the great importance of the relation of capacity, self-induction and frequency
+as regards the general result. 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 aluminium 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
+<!-- Page 61 -->
+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.</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 &quot;electric&quot;
+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.
+<!-- Page 62 -->
+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.</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
+<!-- Page 63 -->
+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 overlap partly 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. 17), comprising a coil and iron core, and a freely movable copper
+disc in proximity to the latter.</p>
+<!-- Page 64 -->
+<div align="center">
+<img src="images/fig17.gif" width="556" height="569" border="0"
+alt="FIG. 17.&mdash;SINGLE WIRE AND &quot;NO-WIRE&quot; MOTOR.">
+</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 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
+<!-- Page 65 -->
+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.</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,
+<!-- Page 66 -->
+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 &quot;no-wire&quot; 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
+<!-- Page 67 -->
+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 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 subtile 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
+alternate currents 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
+<!-- Page 68 -->
+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.</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
+<!-- Page 69 -->
+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 inclosed 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 as 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 &quot;radiant,&quot; 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
+<!-- Page 70 -->
+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 aluminium sheet, which fitted the stem
+and was held on it by spring pressure. The function of this aluminium
+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&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
+<!-- Page 71 -->
+one of the coatings of a condenser. Accordingly, time 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 the coil, the largest bulb being placed at the end of the wire, at
+some distance from it the smallest bulb, and 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 it
+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 about in 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 mounted in its centre is the best to employ. In 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 otherwise
+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.
+<!-- Page 72 -->
+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 the 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>
+<!-- Page 73 -->
+<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.</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 aluminium, 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
+aluminium sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a piece of aluminium
+<!-- Page 74 -->
+sheet of the 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 aluminium 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 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
+<!-- Page 75 -->
+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 aluminium
+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
+aluminium 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,
+<!-- Page 76 -->
+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 degrees of heat.</p>
+
+<p>The effectiveness of the metal tube as an electrostatic screen depends
+largely on the degree of exhaustion.</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
+&quot;non-striking&quot; 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 &quot;non-striking&quot; vacuum for a coil operated, as ordinarily, by
+impulses, or currents, of low-frequency, is not, by far, so 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
+<!-- Page 77 -->
+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 a means for
+concentrating more energy upon the same.</p>
+
+<p>To whatever extent the aluminium 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 as
+follows: Suppose a rhythmical bombardment to occur against the
+conducting tube by reason of its imperfect action as a screen,
+<!-- Page 78 -->
+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 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 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>
+
+<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 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
+<!-- Page 79 -->
+through the bulb, a much higher potential is needed if a conductor,
+especially of much surface, be present.</p>
+
+<p>For the sake of clearness of some of the remarks before made, I must
+now refer to Figs. 18, 19 and 20, which illustrate various
+arrangements with a type of bulb most generally used.</p>
+
+
+
+<img src="images/fig18.gif" width="315" height="560" border="0" hspace="10" align="left"
+alt="FIG. 18.&mdash;BULB WITH MICA TUBE AND ALUMINIUM SCREEN.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 18 is a section through a spherical bulb <i>L</i>, with the glass stem
+<i>s</i>, containing 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. <i>M</i> is a sheet of thin
+<!-- Page 80 -->
+mica wound in several layers around the stem <i>s</i>, and <i>a</i> is the aluminium tube.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig19.gif" width="263" height="563" border="0" align="left" hspace="10"
+alt="FIG. 19.&mdash;IMPROVED BULB WITH SOCKET AND SCREEN.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 19 illustrates such a bulb in a somewhat more advanced stage of
+perfection. A metallic tube <i>S</i> is fastened by means of some cement to
+the neck of the tube. In the tube is screwed a plug <i>P</i>, 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 <i>S</i>, therefore, if the cement used
+is conducting&mdash;and most generally it is sufficiently so&mdash;the space
+between the plug <i>P</i> and the neck of the bulb should be filled with
+some good insulating material, as mica powder.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig20.gif" width="274" height="564" border="0" align="left" hspace="10"
+alt="FIG. 20.&mdash;BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 20 shows a bulb made for experimental purposes. In this bulb the
+aluminium 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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig21.gif" width="278" height="562" border="0" align="left" hspace="10"
+alt="FIG. 21.&mdash;IMPROVED BULB WITH NON-CONDUCTING BUTTON.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+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
+<!-- Page 81 -->
+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 <i>T</i> (Fig. 21), 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, aluminium 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, for instance&mdash;and the space between
+ought to be filled out with some excellent insulator. Among many
+insulating powders I have tried, I have found that mica powder is the
+best to employ. If this precaution is not taken, the tube <i>T</i>,
+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 very high.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig22.gif" width="252" height="570" border="0" align="left" hspace="10"
+alt="FIG. 22.&mdash;TYPE OF BULB WITHOUT LEADING-IN WIRE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 22 illustrates a similar arrangement, with a large tube <i>T</i>
+protruding in to the part of the bulb containing the refractors 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
+<i>C&nbsp;C</i>. The insulating packing <i>P</i> 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>.
+<!-- Page 82 -->
+The molecular bombardment against the glass stem in the bulb is a source
+of great trouble. As 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.</p>
+<!-- Page 83 -->
+<br clear="all">&nbsp;<br>
+
+<p>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
+<!-- Page 84 -->
+conductor of low resistance is connected to a source of alternating currents.
+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>r&ocirc;le</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>
+<!-- Page 85 -->
+<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-conducting material, as,
+for instance, in the bulb described before in Fig. 21, 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 align="center">
+<img src="images/fig23.gif" width="452" height="566" border="0"
+alt="FIG. 23.&mdash;EFFECT PRODUCED BY A RUBY DROP.">
+</div>
+
+<p>A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 23. 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 aluminium 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 aluminium 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,
+<!-- Page 86 -->
+in which the radiant matter was projected against, or focused upon, the body
+to be rendered incandescent. Fig. 24 illustrates one of the bulbs used. It consists
+of a spherical globe <i>L</i>, provided with a long neck <i>n</i>, on the top,
+for increasing the action in some cases by the application of an
+external conducting coating. The globe <i>L</i> is blown out on the bottom
+into a very small bulb <i>b</i>, which serves to hold it firmly in a socket
+<i>S</i> 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 <i>L</i>. 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 <i>S</i> 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. 24 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 a different kind were
+mounted in the bulb as, for instance, indicated in Fig. 23, 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.
+This quality appeared to depend principally on the point of
+<!-- Page 87 -->
+fusion, and on the facility with which the body was &quot;evaporated,&quot; or,
+generally speaking, disintegrated&mdash;meaning by the latter term not only
+the throwing off of atoms, but likewise of larger 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 others, 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>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 is 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,
+<!-- Page 88 -->
+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>
+<!-- Page 89 -->
+<p>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 endeavored repeatedly
+to fuse zirconia, placing it in a cup or arc light carbon as indicated
+in Fig. 23. 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
+the 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
+<!-- Page 90 -->
+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.
+23 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 &quot;tired,&quot; 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,
+<!-- Page 91 -->
+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 evolution of <i>less</i> light?</p>
+
+<p>When the potential of a body is rapidly alternated it is
+<!-- Page 92 -->
+certain 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 invisible 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
+<!-- Page 93 -->
+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 say
+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.</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
+<!-- Page 94 -->
+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 &quot;carborundum&quot; 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 &quot;crystals&quot;
+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
+<!-- Page 95 -->
+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.</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 found 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
+<!-- Page 96 -->
+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 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
+<!-- Page 97 -->
+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 &quot;crystals,&quot;
+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 phosphorescence, unless it is that property which
+characterizes it as a
+<!-- Page 98 -->
+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 phosphorescence. 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 phosphorescence, 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
+<!-- Page 99 -->
+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 could the
+observer call 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.</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
+<!-- Page 100 -->
+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 aluminium 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
+<!-- Page 101 -->
+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.</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 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
+<!-- Page 102 -->
+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. This 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 eye 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 a 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.
+<!-- Page 103 -->
+The intensity of the light emitted depends principally on the frequency and
+potential of the impulses, and on the electric density of 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 reflecting one would think that in pushing
+so far the incandescence of the electrode it would be instantly volatilized.
+But after a careful consideration he would find that, theoretically, it should
+not occur, and in this fact&mdash;which, however, 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 is forced the incandescence 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,
+<!-- Page 104 -->
+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 is 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 shatter quickly 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,
+<!-- Page 105 -->
+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 the 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
+<!-- Page 106 -->
+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 most
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 22, 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 illuminating
+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
+<!-- Page 107 -->
+in the least as 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 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
+<!-- Page 108 -->
+by delivering frequent gentle 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. 19, 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
+<!-- Page 109 -->
+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>
+<!-- Page 110 -->
+<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 as
+illustrated in Fig. 12. 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 result as the application of an external negative electrode under
+ordinary circumstances.</p>
+
+<p>In all these experiments the action was intensified by
+<!-- Page 111 -->
+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>
+
+<img src="images/fig24.gif" width="489" height="648" border="0" align="left" hspace="10"
+alt="FIG. 24.&mdash;BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT OF PROJECTED MATTER." >
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 24, for example, an experimental bulb <i>L</i> 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.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig25.gif" width="269" height="662" border="0" align="left" hspace="10"
+alt="FIG. 25.&mdash;IMPROVED EXPERIMENTAL BULB.">
+
+<p>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Such a lamp as illustrated in Fig. 25 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>
+
+<br clear="all">&nbsp;<br>
+
+<div align="center">
+<img src="images/fig26.gif" width="586" height="389" border="0"
+alt="FIG. 26.&mdash;IMPROVED BULB WITH INTENSIFYING REFLECTOR.">
+</div>
+
+<p>A more perfected arrangement used in some of these bulbs is
+illustrated in Fig. 26. In this case the construction
+<!-- Page 112 -->
+of the bulb is as shown and described before, when reference was made to Fig. 19.
+A zinc sheet <i>Z</i>, with a tubular extension <i>T</i>, is slipped over the
+metallic socket <i>S</i>. The bulb hangs downward from the terminal <i>t</i>,
+the zinc sheet <i>Z</i>, 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 <i>P</i>.</p>
+
+<div align="center">
+<img src="images/fig27.gif" width="500" height="557" border="0"
+alt="FIG. 27.&mdash;PHOSPHORESCENT TUBE WITH INTENSIFYING REFLECTOR.">
+</div>
+
+<p>A similar disposition with a phosphorescent tube is illustrated
+<!-- Page 113 -->
+in Fig. 27. The tube <i>T</i> is prepared from two short tubes of a different
+diameter, which are sealed on the ends. On the lower end is placed an
+outside conducting coating <i>C</i>, 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
+<i>T</i> is another conducting coating <i>C</i><sub>1</sub> upon which is slipped a
+metallic reflector <i>Z</i>, 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, 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>
+
+<p>Suppose a small helix with many well insulated turns, as in experiment
+Fig. 17, 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 the 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 the
+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?
+<!-- Page 114 -->
+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 more one or 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 the 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 be very great. The
+total energy lost per unit of time is proportionate
+<!-- Page 115 -->
+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&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>
+
+<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>
+<!-- Page 116 -->
+<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 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
+<!-- Page 117 -->
+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
+<!-- Page 118 -->
+judiciously designed, would not present the slightest danger.</p>
+
+<p>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&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
+<!-- Page 119 -->
+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 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.</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
+<!-- Page 120 -->
+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. 28.</p>
+
+<img src="images/fig28.gif" width="490" height="565" border="0" align="left" hspace="10"
+alt="FIG. 28.&mdash;LAMP WITH AUXILIARY BULB FOR CONFINING THE ACTION TO THE CENTRE.">
+
+<p>The globe <i>L</i> 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. 18, for example. The small bulb is conveniently
+supported upon the stem <i>s</i>, carrying
+<!-- Page 121 -->
+the refractory button <i>m</i>. It is separated from the aluminium tube <i>a</i>
+by several layers of mica <i>M</i>, in order to prevent the cracking of the neck by the
+rapid heating of the aluminium 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. 28 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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig29.gif" width="503" height="563" border="0" align="left" hspace="10"
+alt="FIG. 29.&mdash;LAMP WITH INDEPENDENT AUXILIARY BULB.">
+
+<p>Many bulbs were constructed on the plan illustrated in Fig. 29. 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 <i>L</i>, 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 use a large bulb. It was found, in the course of experiences
+with bulbs such as illustrated in Fig. 29, 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 <i>L</i> was exhausted
+<!-- Page 122 -->
+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 <i>L</i> 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
+<!-- Page 123 -->
+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. 29, namely, the vacuum in the bulb <i>b</i> would be
+impaired in a comparatively short time.</p>
+
+<br clear="all">
+
+<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. 28 be
+chosen, in which both bulbs communicate.</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 the 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
+<!-- Page 124 -->
+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, and
+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 &quot;extinction&quot; 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 sufficiently 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 first. In a flame we set up light vibrations
+by causing molecules, or atoms, to collide.
+<!-- Page 125 -->
+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 knocks 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
+<!-- Page 126 -->
+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 1 inch in
+diameter and 1 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 different
+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 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
+<!-- Page 127 -->
+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
+insulated 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 a 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 with inconceivable
+<!-- Page 128 -->
+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 moving matter. In a gas the speed may be 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 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
+<!-- Page 129 -->
+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 &quot;non-striking&quot;
+vacuum being reached. But presently 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 discarded 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
+<!-- Page 130 -->
+will render this investigation more complete, might be permitted.</p>
+
+
+<div align="center">
+<img src="images/fig30.gif"  width="495" height="566" border="0"
+alt="FIG. 30.&mdash;APPARATUS USED FOR OBTAINING HIGH DEGREES OF EXHAUSTION.">
+</div>
+
+<p>The apparatus is illustrated in a drawing shown in Fig. 30. <i>S</i>
+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 <i>s</i> has been fitted in the neck
+<!-- Page 131 -->
+of the reservoir <i>R</i>. 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 connection of the latter with the fall tube.</p>
+
+<p>The pump is connected through a U-shaped tube <i>t</i> to a very large
+reservoir <i>R</i><sub>1</sub>. 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
+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 <i>C</i>, 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 <i>R</i><sub>1</sub> was connected by means of a rubber tube to a
+slightly larger reservoir <i>R</i><sub>2</sub>, each of the two reservoirs being
+provided with a stopcock <i>C</i><sub>1</sub> and <i>C</i><sub>2</sub>, respectively.
+The reservoir <i>R</i><sub>1</sub> 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 <i>C</i><sub>2</sub> closed, so as to form a Torricellian vacuum in
+it when raised, it could be lifted so high that the mercury in reservoir <i>R</i><sub>1</sub>
+would stand a little above stopcock <i>C</i><sub>1</sub>; and when this stopcock was
+closed and the reservoir <i>R</i><sub>2</sub> descended, so as to form a Torricellian vacuum in
+<!-- Page 132 -->
+reservoir <i>R</i><sub>1</sub>, it could be lowered so far as to
+completely empty the latter, the mercury filling the reservoir <i>R</i><sub>2</sub>
+up to a little above stopcock <i>C</i><sub>2</sub>.</p>
+
+<p>The capacity of the pump and of the connections was taken as small as
+possible relatively to the volume of reservoir <i>R</i><sub>1</sub>, 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 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 moisture off 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 <i>C</i> and <i>C</i><sub>1</sub> being open, and all other connections
+closed, the reservoir <i>R</i><sub>2</sub> was raised so far that the mercury filled the
+reservoir <i>R</i><sub>1</sub> 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 <i>R</i><sub>2</sub> was lowered, the experimenter keeping the mercury
+at about the same level.
+<!-- Page 133 -->
+The reservoir <i>R</i><sub>2</sub> was balanced by a long spring which facilitated
+the operation, and the friction of the parts was generally sufficient to keep it almost in any position.
+When the Sprengel pump had done its work, the reservoir <i>R</i><sub>2</sub> was
+further lowered and the mercury descended in <i>R</i><sub>1</sub> and filled <i>R</i><sub>2</sub>,
+whereupon stopcock <i>C</i><sub>2</sub> was closed. The air adhering to the walls of
+<i>R</i><sub>1</sub> and that absorbed by the mercury was carried off, and to free the
+mercury of all air the reservoir <i>R</i><sub>2</sub>  was for a long time worked up and
+down. During this process some air, which would gather below stopcock
+<i>C</i><sub>2</sub>, was expelled from <i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> when it was lowered, the caustic potash was resorted to.
+The reservoir <i>R</i><sub>2</sub> was now again raised until the mercury in
+<i>R</i><sub>1</sub> stood above stopcock <i>C</i><sub>1</sub>. The caustic potash
+was fused and boiled, and the 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
+<i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> was lowered, and
+<!-- Page 134 -->
+upon reservoir <i>R</i><sub>1</sub> being emptied the receiver <i>r</i> was
+quickly sealed up.</p>
+
+<p>When a new bulb was put on, the mercury was always raised above
+stopcock <i>C</i><sub>1</sub> which was closed, so as to always keep the mercury and
+both the reservoirs in fine condition, and the mercury was never
+withdrawn from <i>R</i><sub>1</sub> 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 15 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 <i>R</i> was about 1-20, 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
+<!-- Page 135 -->
+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 act 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&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
+<!-- Page 136 -->
+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 claim 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 allowing
+barely the light to shine through. A metallic clasp, 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 aluminium 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 not by far
+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
+<!-- Page 137 -->
+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.</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 a high frequency current,
+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 the heating is due to the
+rarefied gas. Here the gas might only act as a conductor of no impedance
+<!-- Page 138 -->
+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 he
+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 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 to 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 &quot;radiant state&quot; and the
+&quot;non-striking vacuum.&quot;</p>
+
+<p>Any one who has studied Crookes' work must have received the
+impression that the &quot;radiant state&quot; is a property
+<!-- Page 139 -->
+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
+<!-- Page 140 -->
+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 as 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>
+
+<div align="center">
+<img src="images/fig31.gif" width="492" height="526" border="0"
+alt="FIG. 31.&mdash;BULB SHOWING RADIANT LIME STREAM AT LOW EXHAUSTION.">
+</div>
+
+<p>I have prepared a bulb to illustrate by an experiment the correctness
+of these assertions. In a globe <i>L</i> (Fig. 31) 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. 19, 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 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
+<!-- Page 141 -->
+that point which is intensely heated, and a white stream of lime particles (Fig. 31)
+then breaks forth from that point. This stream is composed of &quot;radiant&quot;
+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>
+
+<p>As to the &quot;non-striking vacuum,&quot; 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
+<!-- Page 142 -->
+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 electro-dynamic
+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.</p>
+
+<p>Many bulbs were constructed as shown in Fig. 32 and Fig. 33.</p>
+
+<img src="images/fig32.gif" width="236" height="594" border="0" align="left" hspace="10"
+alt="FIG. 32.&mdash;ELECTRO-DYNAMIC INDUCTION TUBE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 32 a wide tube <i>T</i> was sealed to a smaller W-shaped tube <i>U</i>,
+of phosphorescent glass. In the tube <i>T</i> was placed a coil <i>C</i> of
+aluminium wire, the ends of which were provided with small spheres <i>t</i>
+and <i>t</i><sub>1</sub> of aluminium, and reached into the <i>U</i> tube.
+The tube <i>T</i> was slipped into a socket containing a primary coil
+through which usually the discharges of Leyden jars were directed, and
+<!-- Page 143 -->
+the rarefied gas in the small <i>U</i> tube was excited to strong luminosity
+by the high-tension currents induced in the coil <i>C</i>. When Leyden jar
+discharges were used to induce currents in the coil <i>C</i>, it was found
+necessary to pack the tube <i>T</i> tightly with insulating powder, as a
+discharge would occur frequently between the turns of the coil, especially
+<!-- Page 144 -->
+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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig33.gif" width="260" height="543" border="0" align="left" hspace="10"
+alt="FIG. 33&mdash;ELECTRO-DYNAMIC INDUCTION LAMP.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 33 is illustrated another form of the bulb constructed. In
+this case a tube <i>T</i> is sealed to a globe <i>L</i>. The tube contains a
+coil <i>C</i>, 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 <i>T</i>. 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 <i>L</i> communicated with the tube
+<i>T</i>. For this purpose the ends of the small tubes <i>t</i> and <i>t</i><sub>1</sub> were
+just a trifle heated in the burner, merely to hold the wires, but not
+to interfere with the communication. The tube <i>T</i>, with the small
+tubes, wires through the same, and the refractory buttons <i>m</i> and
+<i>m</i><sub>1</sub>, was first prepared, and then sealed to globe <i>L</i>, whereupon
+the coil <i>C</i> 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. 33 an aluminium 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 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 <i>C</i>. 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>
+<!-- Page 145 -->
+<br clear="all">
+
+<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 experiences 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, gets 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 ball 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, still, 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 far as desired, and the efficiency
+<!-- Page 146 -->
+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 he works
+with condenser charges&mdash;and they are the only means up to the present
+known for reaching these extreme frequencies&mdash;he gets to electromotive
+forces of several thousands of volts per turn of the primary. He
+cannot multiply the electro-dynamic inductive effect by taking more
+turns in the primary, for he arrives at the conclusion that the best
+way is to work with one single turn&mdash;though he must sometimes depart
+from this rule&mdash;and he must get along with whatever inductive effect
+he can obtain with one turn. But before he has long experimented with
+the extreme frequencies required to set up in a small bulb an
+electromotive force of several thousands of volts he 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 electro-dynamic
+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>
+<!-- Page 147 -->
+<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 condenser to a higher potential, establish electrostatic
+alternating fields which acted through the whole extent of a 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.</p>
+
+<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
+<!-- Page 148 -->
+best an unskilled lecturer can do is to begin and finish with the exhibition of these
+singular effects. I take a tube in the 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 the 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.</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. 34 and Fig. 35 two such tubes are illustrated
+which are prepared for the occasion.</p>
+
+<img src="images/fig34.gif" width="232" height="591" border="0" align="left" hspace="10"
+alt="FIG. 34.&mdash;TUBE WITH FILAMENT RENDERED INCANDESCENT IN AN ELECTROSTATIC FIELD.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 34 a short tube <i>T</i><sub>1</sub>, sealed to another long tube <i>T</i>,
+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
+<!-- Page 149 -->
+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, <i>C</i> and
+<i>C</i><sub>1</sub> 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
+<!-- Page 150 -->
+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 aluminium 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 <i>T</i><sub>1</sub> anywhere in the
+electrostatic field the filament is rendered incandescent.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig35.gif" width="259" height="592" border="0" align="left" hspace="10"
+alt="FIG. 35.&mdash;CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+A more interesting piece of apparatus is illustrated in Fig. 35. 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 <i>C</i>. 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 aluminium 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>
+
+<br clear="all">
+
+<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 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
+<!-- Page 151 -->
+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. From 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, their 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 this argument is not
+devoid of force-but because in a burner the higher vibrations can
+never be reached except by passing through all the low ones. For how
+is a flame produced 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;
+<!-- Page 152 -->
+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 remain 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 effectively 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 very high frequencies, many
+advantages, such as a 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
+<!-- Page 153 -->
+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, 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 though 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
+<!-- Page 154 -->
+the length of the wire, then corresponding short waves will be sent 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 distance 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
+<!-- Page 155 -->
+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 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>
+
+<p>&nbsp;</p>
+
+<center>
+<table border=0 bgcolor="ccccff" cellpadding=10>
+  <tr>
+    <td valign="top">
+      Transcriber's note:
+    </td>
+    <td>
+          Corrected the following typesetting errors:<br>
+		  1) 'preceived' to 'perceived', page 16. <br>
+		  2) 'disharging' to 'discharging', page 30.<br>
+		  3) 'park' to 'spark', page 33.<br>
+		  4) 'pssition' to 'position', page 50.<br>
+		  5) 'to th opposite side' to 'to the opposite side', page 56.<br>
+          6) 's resses' to 'stresses', page 147.
+	</td>
+  </tr>
+</table>
+</center>
+<br>
+<br>
+<div>*** END OF THE PROJECT GUTENBERG EBOOK 13476 ***</div>
+</body>
+</html>
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+Project Gutenberg (https://www.gutenberg.org) public repository for
+eBook #13476 (https://www.gutenberg.org/ebooks/13476)
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+<h1>The Project Gutenberg eBook, Experiments with Alternate Currents of High
+Potential and High Frequency, by Nikola Tesla</h1>
+<pre>
+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 <a href = "https://www.gutenberg.org">www.gutenberg.org</a></pre>
+<p>Title: Experiments with Alternate Currents of High Potential and High Frequency</p>
+<p>Author: Nikola Tesla</p>
+<p>Release Date: September 16, 2004  [eBook #13476]</p>
+<p>Language: English</p>
+<p>Character set encoding: ISO-8859-1</p>
+<p>***START OF THE PROJECT GUTENBERG EBOOK EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY***</p>
+<br>
+<br>
+<h3>E-text prepared by Robert Shimmin, Ronald Holder,<br>
+    and the Project Gutenberg Online Distributed Proofreading Team</h3>
+<br>
+<br>
+<hr class="full" noshade>
+<div align="center"><img src="images/title.gif" alt="Title Page"
+width="470" border="1"></div>
+<br>
+<br>
+<br>
+<h2>EXPERIMENTS</h2>
+<h3>WITH</h3>
+<h1>ALTERNATE CURRENTS</h1>
+<h3>OF</h3>
+<h2>HIGH POTENTIAL AND HIGH FREQUENCY.</h2>
+<h3>BY</h3>
+<h2>NIKOLA TESLA.</h2>
+
+<hr>
+
+<h2>A LECTURE</h2>
+<h3>DELIVERED BEFORE THE</h3>
+<h2>INSTITUTION OF ELECTRICAL ENGINEERS, LONDON.</h2>
+
+<hr>
+
+<div align="center"><i>With a Portrait and Biographical Sketch</i><br>
+<i>of the Author</i>.<br>
+
+<hr>
+
+NEW YORK:<br>
+1892</div>
+
+<p>&nbsp;</p><!-- Page 2 -->
+<p>&nbsp;</p><!-- Page 3 -->
+
+<!-- The following image was obtained from another source. -->
+<div align="center">
+<img src="images/tesla.gif" alt="Portrait of Nikola Tesla" width="280" height="459" border="0">
+</div>
+
+<h2>Biographical Sketch of Nikola Tesla.</h2>
+<hr>
+<p>&nbsp;</p>
+
+<p>While a large portion of the European family has been surging westward
+during the last three or four hundred years, settling the vast
+continents of America, another, but smaller, portion has been doing
+frontier work in the Old World, protecting the rear by beating back
+the &quot;unspeakable Turk&quot; and reclaiming gradually the fair lands that
+endure the curse of Mohammedan rule. For a long time the Slav
+people&mdash;who, after the battle of Kosovopjolje, in which the Turks
+defeated the Servians, retired to the confines of the present
+Montenegro, Dalmatia, Herzegovina and Bosnia, and &quot;Borderland&quot; of
+Austria&mdash;knew what it was to deal, as our Western pioneers did, with
+foes ceaselessly fretting against their frontier; and the races of
+these countries, through their strenuous struggle against the armies
+of the Crescent, have developed notable qualities of bravery and
+sagacity, while maintaining a patriotism and independence unsurpassed
+in any other nation.</p>
+
+<p>It was in this interesting border region, and from among these valiant
+Eastern folk, that Nikola Tesla was born in the year 1857, and the
+fact that he, to-day, finds himself in America and one of our foremost
+electricians, is striking evidence of the extraordinary attractiveness
+alike of electrical pursuits and of the country where electricity
+enjoys its widest application.
+<!-- Page 4 -->
+Mr. Tesla's native place was Smiljan,
+Lika, where his father was an eloquent clergyman of the Greek Church,
+in which, by the way, his family is still prominently represented. His
+mother enjoyed great fame throughout the countryside for her skill and
+originality in needlework, and doubtless transmitted her ingenuity to
+Nikola; though it naturally took another and more masculine direction.</p>
+
+<p>The boy was early put to his books, and upon his father's removal to
+Gospic he spent four years in the public school, and later, three
+years in the Real School, as it is called. His escapades were such as
+most quick witted boys go through, although he varied the programme on
+one occasion by getting imprisoned in a remote mountain chapel rarely
+visited for service; and on another occasion by falling headlong into
+a huge kettle of boiling milk, just drawn from the paternal herds. A
+third curious episode was that connected with his efforts to fly when,
+attempting to navigate the air with the aid of an old umbrella, he
+had, as might be expected, a very bad fall, and was laid up for six weeks.</p>
+
+<p>About this period he began to take delight in arithmetic and physics.
+One queer notion he had was to work out everything by three or the
+power of three. He was now sent to an aunt at Cartstatt, Croatia, to
+finish his studies in what is known as the Higher Real School. It was
+there that, coming from the rural fastnesses, he saw a steam engine
+for the first time with a pleasure that he remembers to this day. At
+Cartstatt he was so diligent as to compress the four years' course into three,
+and graduated in 1873. Returning home during an epidemic of cholera, he was
+<!-- Page 5 -->
+stricken down by the disease and suffered so
+seriously from the consequences that his studies were interrupted for
+fully two years. But the time was not wasted, for he had become
+passionately fond of experimenting, and as much as his means and
+leisure permitted devoted his energies to electrical study and
+investigation. Up to this period it had been his father's intention to
+make a priest of him, and the idea hung over the young physicist like
+a very sword of Damocles. Finally he prevailed upon his worthy but
+reluctant sire to send him to Gratz in Austria to finish his studies
+at the Polytechnic School, and to prepare for work as professor of
+mathematics and physics. At Gratz he saw and operated a Gramme machine
+for the first time, and was so struck with the objections to the use
+of commutators and brushes that he made up his mind there and then to
+remedy that defect in dynamo-electric machines. In the second year of
+his course he abandoned the intention of becoming a teacher and took
+up the engineering curriculum. After three years of absence he
+returned home, sadly, to see his father die; but, having resolved to
+settle down in Austria, and recognizing the value of linguistic
+acquirements, he went to Prague and then to Buda-Pesth with the view
+of mastering the languages he deemed necessary. Up to this time he had
+never realized the enormous sacrifices that his parents had made in
+promoting his education, but he now began to feel the pinch and to
+grow unfamiliar with the image of Francis Joseph I. There was
+considerable lag between his dispatches and the corresponding
+remittance from home; and when the mathematical expression for
+<!-- Page 6 -->
+the value of the lag assumed the shape of an eight laid flat on its back,
+Mr. Tesla became a very fair example of high thinking and plain
+living, but he made up his mind to the struggle and determined to go
+through depending solely on his own resources. Not desiring the fame
+of a faster, he cast about for a livelihood, and through the help of
+friends he secured a berth as assistant in the engineering department
+of the government telegraphs. The salary was five dollars a week. This
+brought him into direct contact with practical electrical work and
+ideas, but it is needless to say that his means did not admit of much
+experimenting. By the time he had extracted several hundred thousand
+square and cube roots for the public benefit, the limitations,
+financial and otherwise, of the position had become painfully
+apparent, and he concluded that the best thing to do was to make a
+valuable invention. He proceeded at once to make inventions, but their
+value was visible only to the eye of faith, and they brought no grist
+to the mill. Just at this time the telephone made its appearance in
+Hungary, and the success of that great invention determined his
+career, hopeless as the profession had thus far seemed to him. He
+associated himself at once with telephonic work, and made various
+telephonic inventions, including an operative repeater; but it did not
+take him long to discover that, being so remote from the scenes of
+electrical activity, he was apt to spend time on aims and results
+already reached by others, and to lose touch. Longing for new opportunities
+and anxious for the development of which he felt himself possible, if once
+he could place himself within the genial and direct influences of the gulf
+<!-- Page 7 -->
+streams of electrical thought, he broke away from the ties and traditions of the past,
+and in 1881 made his way to Paris. Arriving in that city, the ardent young Likan obtained
+employment as an electrical engineer with one of the largest electric
+lighting companies. The next year he went to Strasburg to install a
+plant, and on returning to Paris sought to carry out a number of ideas
+that had now ripened into inventions. About this time, however, the
+remarkable progress of America in electrical industry attracted his
+attention, and once again staking everything on a single throw, he
+crossed the Atlantic.</p>
+
+<p>Mr. Tesla buckled down to work as soon as he landed on these shores,
+put his best thought and skill into it, and soon saw openings for his
+talent. In a short while a proposition was made to him to start his
+own company, and, accepting the terms, he at once worked up a
+practical system of arc lighting, as well as a potential method of
+dynamo regulation, which in one form is now known as the &quot;third brush
+regulation.&quot; He also devised a thermo-magnetic motor and other kindred
+devices, about which little was published, owing to legal
+complications. Early in 1887 the Tesla Electric Company of New York
+was formed, and not long after that Mr. Tesla produced his admirable
+and epoch-marking motors for multiphase alternating currents, in
+which, going back to his ideas of long ago, he evolved machines having
+neither commutator nor brushes. It will be remembered that about the
+time that Mr. Tesla brought out his motors, and read his thoughtful
+paper before the American Institute of Electrical Engineers, Professor
+Ferraris, in Europe, published his discovery of principles
+<!-- Page 8 -->
+analogous to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
+Tesla was an independent inventor of this rotary field motor, for
+although anticipated in dates by Ferraris, he could not have known
+about Ferraris' work as it had not been published. Professor Ferraris
+stated himself, with becoming modesty, that he did not think Tesla
+could have known of his (Ferraris') experiments at that time, and adds
+that he thinks Tesla was an independent and original inventor of this
+principle. With such an acknowledgment from Ferraris there can be
+little doubt about Tesla's originality in this matter.</p>
+
+<p>Mr. Tesla's work in this field was wonderfully timely, and its worth
+was promptly appreciated in various quarters. The Tesla patents were
+acquired by the Westinghouse Electric Company, who undertook to
+develop his motor and to apply it to work of different kinds. Its use
+in mining, and its employment in printing, ventilation, etc., was
+described and illustrated in <i>The Electrical World</i> some years ago.
+The immense stimulus that the announcement of Mr. Tesla's work gave to
+the study of alternating current motors would, in itself, be enough to
+stamp him as a leader.</p>
+
+<p>Mr. Tesla is only 35 years of age. He is tall and spare with a
+clean-cut, thin, refined face, and eyes that recall all the stories
+one has read of keenness of vision and phenomenal ability to see
+through things. He is an omnivorous reader, who never forgets; and he
+possesses the peculiar facility in languages that enables the least
+educated native of eastern Europe to talk and write in at least half a
+dozen tongues. A more congenial companion cannot be desired for the
+hours when one &quot;pours out heart affluence in discursive
+<!-- Page 9 -->
+talk,&quot; and when the conversation, dealing at first with things near at hand and
+next to us, reaches out and rises to the greater questions of life, duty and destiny.</p>
+
+<p>In the year 1890 he severed his connection with the Westinghouse
+Company, since which time he has devoted himself entirely to the study
+of alternating currents of high frequencies and very high potentials,
+with which study he is at present engaged. No comment is necessary on
+his interesting achievements in this field; the famous London lecture
+published in this volume is a proof in itself. His first lecture on
+his researches in this new branch of electricity, which he may be said
+to have created, was delivered before the American Institute of
+Electrical Engineers on May 20, 1891, and remains one of the most
+interesting papers read before that society. It will be found
+reprinted in full in <i>The Electrical World</i>, July 11, 1891. Its
+publication excited such interest abroad that he received numerous
+requests from English and French electrical engineers and scientists
+to repeat it in those countries, the result of which has been the
+interesting lecture published in this volume.</p>
+
+<p>The present lecture presupposes a knowledge of the former, but it may
+be read and understood by any one even though he has not read the
+earlier one. It forms a sort of continuation of the latter, and
+includes chiefly the results of his researches since that time.</p>
+<!-- Page 10 -->
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+<p>&nbsp;</p>
+
+<h1>EXPERIMENTS</h1>
+<h3>WITH </h3>
+<h2>Alternate Currents of High Potential </h2>
+<h2>and High Frequency.</h2>
+
+<hr>
+
+<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 time 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
+<!-- Page 11 -->
+detail about that charming work I can remember 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 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<a name="FNanchor_A_1">
+</a><a href="#Footnote_A_1"><sup>[A]</sup></a> 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
+<!-- Page 12 -->
+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>
+
+<a name="Footnote_A_1"></a><a href="#FNanchor_A_1">[A]</a>
+<div class="fnote"><p> For Mr. Tesla's American lecture on this subject see THE
+ELECTRICAL WORLD of July 11, 1891, and for a report of his French
+lecture see THE ELECTRICAL WORLD of March 26, 1892.</p></div>
+
+<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 immediately the most 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 with 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
+<!-- Page 13 -->
+that; in all branches of industry alternating currents&mdash;electric wave
+motion&mdash;will have the sway.</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
+<!-- Page 14 -->
+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 may 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.</p>
+
+<p>Here still another, which by my fingers' touch casts a shadow&mdash;the
+Crookes shadow, 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
+<!-- Page 15 -->
+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>
+<!-- Page 16 -->
+<p>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 tests 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
+<!-- Page 17 -->
+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. 1.)</p>
+
+<div align="center"><img src="images/fig01.gif" width="492" height="599" border="0"
+alt="FIG. 1.&mdash;DISCHARGE BETWEEN TWO WIRES WITH FREQUENCIES OF A FEW HUNDRED THOUSAND PER SECOND.">
+</div>
+
+<p>Now compare this phenomenon which you have just witnessed 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
+<!-- Page 18 -->
+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 <br>
+succession of sparks of great intensity and small quantity, which possess
+<!-- Page 19 -->
+the same brilliancy, and are accompanied by the same sharp crackling sound,
+as those obtained from a friction or influence machine.</p>
+
+<img src="images/fig02.gif" width="178" height="663" border="0" align="left" hspace="10"
+alt="FIG. 2.&mdash;IMITATING THE SPARK OF A HOLTZ MACHINE.">
+
+<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. 2.</p>
+
+<p><i>G</i> is an ordinarily constructed alternator, supplying the primary <i>P</i>
+of an induction coil, the secondary <i>S</i> of which
+<!-- Page 20 -->
+charges the condensers or jars <i>C&nbsp;C</i>. 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&nbsp;p</i> of a second induction coil. This
+primary <i>p&nbsp;p</i> has a small air gap <i>a&nbsp;b</i>.</p>
+
+<p>The secondary <i>s</i> of this coil is provided with knobs or spheres <i>K&nbsp;K</i>
+of the proper size and set at a distance suitable for the experiment.</p>
+
+<p>A long arc is established between the terminals <i>A&nbsp;B</i> of the first
+induction coil. <i>M&nbsp;M</i> are the mica plates.</p>
+
+<p>Each time the arc is broken between <i>A</i> and <i>B</i> the jars are quickly
+charged and discharged through the primary <i>p&nbsp;p</i>, producing a snapping
+spark between the knobs <i>K&nbsp;K</i>. Upon the arc forming between <i>A</i> and <i>B</i>
+the potential falls, and the jars cannot be charged to such high
+potential as to break through the air gap <i>a&nbsp;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&nbsp;p</i>, which in the secondary <i>s</i> give a corresponding number
+of impulses of great intensity. If the secondary knobs or spheres,
+<i>K&nbsp;K</i>, 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 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.</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
+<!-- Page 21 -->
+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
+<a name="FNanchor_A_2"></a><a href="#Footnote_A_2"><sup>[A]</sup></a>
+before the American Institute of Electrical Engineers, May 20, 1891.</p>
+
+<a name="Footnote_A_2"></a><a href="#FNanchor_A_2">[A]</a><div class="fnote">
+<p> See THE ELECTRICAL WORLD, July 11, 1891.</p></div>
+
+<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
+<!-- Page 22 -->
+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&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
+<!-- Page 23 -->
+may vary them in many ways, and, if it were 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, 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,
+<!-- Page 24 -->
+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 align="center"><img src="images/fig03.gif" width="476" height="575" border="0"
+alt="FIG. 3.&mdash;DISRUPTIVE DISCHARGE COIL."></div>
+
+<p>It is contained in a box <i>B</i> (Fig. 3) of thick boards of hard wood,
+covered on the outside with zinc sheet <i>Z</i>, which is
+<!-- Page 25 -->
+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, 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 <i>R&nbsp;R</i>, held apart at a
+distance of 10 centimetres by bolts <i>c</i> and nuts <i>n</i>, likewise of hard
+rubber. Each spool comprises a tube <i>T</i> of approximately 8 centimetres
+inside diameter, and 3 millimetres thick, upon which are screwed two
+flanges <i>F&nbsp;F</i>, 24 centimetres square, the space between the flanges
+being about 3 centimetres. The secondary, <i>S&nbsp;S</i>, 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
+<!-- Page 26 -->
+its terminals <i>T</i><sub>1</sub>&nbsp;<i>T</i><sub>1</sub> 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 <i>P&nbsp;P</i> is wound in two parts, and oppositely, upon a wooden
+spool <i>W</i>, and the four ends are led out of the oil through hard
+rubber tubes <i>t&nbsp;t</i>. The ends of the secondary <i>T</i><sub>1</sub>&nbsp;<i>T</i><sub>1</sub>
+are also led out of the oil through rubber tubes <i>t</i><sub>1</sub>&nbsp;<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 E.M.Fs. 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 <i>B</i> surrounding the
+whole is used. </p>
+<!-- Page 27 -->
+<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>
+
+<div align="center"><img src="images/fig04.gif" width="692" height="367" border="0"
+alt="FIG. 4.&mdash;ARRANGEMENT OF IMPROVED DISCHARGER AND MAGNET.">
+</div>
+
+<p>One of the changes is that the adjustable knobs <i>A</i> and <i>B</i> (Fig. 4),
+of the discharger are held in jaws of brass, <i>J&nbsp;J</i>, 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>
+
+<p>The other change consists in the employment of a strong electromagnet
+<i>N&nbsp;S</i>, which is placed with its axis at right angles to the line
+joining the knobs <i>A</i> and <i>B</i>, and produces a strong magnetic field
+between them. The pole pieces of
+<!-- Page 28 -->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, <i>M&nbsp;M</i>, of sufficient thickness.
+<i>s</i><sub>1</sub>&nbsp;<i>s</i><sub>1</sub>
+and <i>s</i><sub>2</sub>&nbsp;<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. <i>L&nbsp;L</i> are screws for fixing in
+position the rods <i>R&nbsp;R</i>, which support the knobs.</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>
+
+<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 <i>A&nbsp;B</i>, in Fig. 2 (the knobs <i>a&nbsp;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>
+
+<div align="center"><img src="images/fig05.gif" width="588" height="210" border="0"
+alt="FIG. 5.&mdash;ARRANGEMENT WITH LOW-FREQUENCY ALTERNATOR AND IMPROVED DISCHARGER.">
+</div>
+
+<p>When a magnet is employed to break the arc, it is
+<!-- Page 29 -->
+better to choose the connection indicated diagrammatically in Fig. 5, 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>
+
+<div align="center"><img src="images/fig06.gif" width="564" height="226" border="0"
+alt="FIG. 6.&mdash;DISCHARGER WITH MULTIPLE GAPS."></div>
+
+<p>The other form of discharger used in these and similar experiments is
+indicated in Figs. 6 and 7. It consists of a number of brass pieces
+<i>c&nbsp;c</i> (Fig. 6), 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 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
+<!-- Page 30 -->
+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 <i>R&nbsp;R</i>, with planed grooves <i>g&nbsp;g</i> (Fig. 7)
+to fit the middle portion of the pieces <i>c&nbsp;c</i>, serve to clamp the latter
+and hold them firmly in position by means of two bolts <i>C&nbsp;C</i> (of which
+only one is shown) passing through the ends of the strips.</p>
+
+<div align="center"><img src="images/fig07.gif" width="557" height="373" border="0"
+alt="FIG. 7.&mdash;DISCHARGER WITH MULTIPLE GAPS."></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 apparatus affords some
+<!-- Page 31 -->
+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. 2,
+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
+<!-- Page 32 -->
+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 note-worthy of these
+discharge phenomena.</p>
+
+<p>I have stretched across the room two ordinary cotton covered wires,
+each about 7 metres in length. They are supported on insulating cords
+at a distance of about 30 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.</p>
+
+<p>A convenient way is to use an oil condenser of very small capacity,
+consisting of two small adjustable metal
+<!-- Page 33 -->
+plates, in connection with this and similar experiments. In such case I take wires
+rather short and set at the beginning the condenser plates at maximum distance.
+If the streams for 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 difficult. One
+<!-- Page 34 -->
+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>
+
+<p>The luminous intensity of the streams is, of course, considerably
+<!-- Page 35 -->
+increased when they are focused upon a small surface. This may be
+shown by the following experiment:</p>
+
+<div align="center">
+<img src="images/fig08.gif" width="554" height="514" border="0"
+alt="FIG. 8.&mdash;EFFECT PRODUCED BY CONCENTRATING STREAMS.">
+</div>
+
+<p>I attach to one of the terminals of the coil a wire <i>w</i> (Fig. 8), 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 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>
+
+<p>Here, for instance, I have two plates, <i>R&nbsp;R</i>, of hard rubber (Fig. 9),
+upon which I have glued two very thin wires <i>w&nbsp;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
+<!-- Page 36 -->
+<i>t&nbsp;t</i>. The plates are placed in line at a sufficient distance to prevent a
+spark passing from one to the other wire. The two tinfoil coatings I have joined by a
+conductor <i>C</i>, 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>
+
+<div align="center">
+<img src="images/fig09.gif" width="557" height="528" border="0"
+alt="FIG. 9.&mdash;WIRES RENDERED INTENSELY LUMINOUS.">
+</div>
+
+<p>It is perhaps preferable to perform this experiment with a coil
+operated from an alternator of high frequency, as
+<!-- Page 37 -->
+then, owing to the harmonic rise and fall, the streams are very uniform, though
+they are less abundant then when produced with such a coil as the present. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.</p>
+
+<div align="center">
+<img src="images/fig10.gif" width="325" height="559" border="0"
+alt="FIG. 10.&mdash;LUMINOUS DISCS.">
+</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, <i>C</i> and <i>c</i> (Fig. 10), of rather stout wire, one being about
+<!-- Page 38 -->
+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.</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> but <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
+<!-- Page 39 -->
+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 oil and placed parallel to each
+other at a distance of about 10 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 a
+merely incidental necessity. These experiments teach us that, in endeavoring
+to discover novel methods of producing light by the agitation of atoms, or
+<!-- Page 40 -->
+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 often occasion 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
+<!-- Page 41 -->
+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>
+
+<p>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&mdash;or, better still, two sharp-edged metal
+discs (<i>d&nbsp;d</i>, Fig. 11) 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
+<!-- Page 42 -->
+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. </p>
+
+<div align="center">
+<img src="images/fig11.gif" width="548" height="535" border="0"
+alt="FIG. 11.&mdash;PHANTOM STREAMS.">
+</div>
+
+<p>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
+<!-- Page 43 -->
+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 1 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
+<!-- Page 44 -->
+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 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
+<!-- Page 45 -->
+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
+immersion, 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 in commerce two kinds of gutta-percha wire: 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.</p>
+<!-- Page 46 -->
+<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 hard wood 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
+<!-- Page 47 -->
+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 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, become, at least with higher frequencies, so easy that
+they could be hardly called engineering
+<!-- Page 48 -->
+feats. With oil insulation and alternate current motors transmissions of power
+can be effected 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.</p>
+
+<p>The exclusion of gaseous matter from any apparatus
+<!-- Page 49 -->
+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 purposes 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
+20, 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
+<!-- Page 50 -->
+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; this
+allowing the use of a much bigger 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 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,
+<!-- Page 51 -->
+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 self-induction of both the latter. The condenser
+<!-- Page 52 -->
+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>
+
+<p>In bulbs provided with a conducting terminal, though it be of
+aluminium, 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. 12 and 13.</p>
+
+<div align="center">
+<img src="images/fig12_13.gif" width="518" height="578" border="0"
+alt="FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.">
+</div>
+
+<p>In Fig. 12 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 aluminium sheet, may be slipped in the barometer
+tube, but it is not important to employ it.</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>
+<!-- Page 53 -->
+<p>The construction shown in Fig. 13 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
+<!-- Page 54 -->
+inductively upon the moderately rarefied and highly conducting 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 align="center">
+<img src="images/fig14.gif" width="578" height="425" border="0"
+alt="FIG. 14.&mdash;FORMS AND PHASES OF THE ROTATING BRUSH.">
+</div>
+
+<p>Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
+Fig. 14 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. 12 and 13) is exhausted to a
+<!-- Page 55 -->
+very high degree, generally the bulb is not excited upon connecting the wire
+<i>w</i> (Fig. 12) or the tinfoil coating of the bulb (Fig. 13) 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
+<!-- Page 56 -->
+for some time the bulb appears as in Fig. 15. From this stage the
+phenomenon will gradually pass to that indicated in Fig. 16, 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 align="center">
+<img src="images/fig15_16.gif" width="518" height="552" border="0"
+alt="FIG. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING BRUSH.">
+</div>
+
+<p>When the brush assumes the form indicated in Fig. 16, it maybe 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.</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
+<!-- Page 57 -->
+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
+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
+<!-- Page 58 -->
+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 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
+<!-- Page 59 -->
+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 in
+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.</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,
+<!-- Page 60 -->
+the great importance of the relation of capacity, self-induction and frequency
+as regards the general result. 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 aluminium 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
+<!-- Page 61 -->
+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.</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 &quot;electric&quot;
+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.
+<!-- Page 62 -->
+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.</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
+<!-- Page 63 -->
+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 overlap partly 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. 17), comprising a coil and iron core, and a freely movable copper
+disc in proximity to the latter.</p>
+<!-- Page 64 -->
+<div align="center">
+<img src="images/fig17.gif" width="556" height="569" border="0"
+alt="FIG. 17.&mdash;SINGLE WIRE AND &quot;NO-WIRE&quot; MOTOR.">
+</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 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
+<!-- Page 65 -->
+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.</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,
+<!-- Page 66 -->
+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 &quot;no-wire&quot; 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
+<!-- Page 67 -->
+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 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 subtile 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
+alternate currents 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
+<!-- Page 68 -->
+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.</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
+<!-- Page 69 -->
+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 inclosed 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 as 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 &quot;radiant,&quot; 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
+<!-- Page 70 -->
+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 aluminium sheet, which fitted the stem
+and was held on it by spring pressure. The function of this aluminium
+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&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
+<!-- Page 71 -->
+one of the coatings of a condenser. Accordingly, time 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 the coil, the largest bulb being placed at the end of the wire, at
+some distance from it the smallest bulb, and 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 it
+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 about in 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 mounted in its centre is the best to employ. In 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 otherwise
+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.
+<!-- Page 72 -->
+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 the 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>
+<!-- Page 73 -->
+<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.</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 aluminium, 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
+aluminium sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a piece of aluminium
+<!-- Page 74 -->
+sheet of the 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 aluminium 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 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
+<!-- Page 75 -->
+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 aluminium
+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
+aluminium 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,
+<!-- Page 76 -->
+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 degrees of heat.</p>
+
+<p>The effectiveness of the metal tube as an electrostatic screen depends
+largely on the degree of exhaustion.</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
+&quot;non-striking&quot; 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 &quot;non-striking&quot; vacuum for a coil operated, as ordinarily, by
+impulses, or currents, of low-frequency, is not, by far, so 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
+<!-- Page 77 -->
+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 a means for
+concentrating more energy upon the same.</p>
+
+<p>To whatever extent the aluminium 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 as
+follows: Suppose a rhythmical bombardment to occur against the
+conducting tube by reason of its imperfect action as a screen,
+<!-- Page 78 -->
+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 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 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>
+
+<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 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
+<!-- Page 79 -->
+through the bulb, a much higher potential is needed if a conductor,
+especially of much surface, be present.</p>
+
+<p>For the sake of clearness of some of the remarks before made, I must
+now refer to Figs. 18, 19 and 20, which illustrate various
+arrangements with a type of bulb most generally used.</p>
+
+
+
+<img src="images/fig18.gif" width="315" height="560" border="0" hspace="10" align="left"
+alt="FIG. 18.&mdash;BULB WITH MICA TUBE AND ALUMINIUM SCREEN.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 18 is a section through a spherical bulb <i>L</i>, with the glass stem
+<i>s</i>, containing 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. <i>M</i> is a sheet of thin
+<!-- Page 80 -->
+mica wound in several layers around the stem <i>s</i>, and <i>a</i> is the aluminium tube.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig19.gif" width="263" height="563" border="0" align="left" hspace="10"
+alt="FIG. 19.&mdash;IMPROVED BULB WITH SOCKET AND SCREEN.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 19 illustrates such a bulb in a somewhat more advanced stage of
+perfection. A metallic tube <i>S</i> is fastened by means of some cement to
+the neck of the tube. In the tube is screwed a plug <i>P</i>, 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 <i>S</i>, therefore, if the cement used
+is conducting&mdash;and most generally it is sufficiently so&mdash;the space
+between the plug <i>P</i> and the neck of the bulb should be filled with
+some good insulating material, as mica powder.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig20.gif" width="274" height="564" border="0" align="left" hspace="10"
+alt="FIG. 20.&mdash;BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 20 shows a bulb made for experimental purposes. In this bulb the
+aluminium 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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig21.gif" width="278" height="562" border="0" align="left" hspace="10"
+alt="FIG. 21.&mdash;IMPROVED BULB WITH NON-CONDUCTING BUTTON.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+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
+<!-- Page 81 -->
+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 <i>T</i> (Fig. 21), 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, aluminium 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, for instance&mdash;and the space between
+ought to be filled out with some excellent insulator. Among many
+insulating powders I have tried, I have found that mica powder is the
+best to employ. If this precaution is not taken, the tube <i>T</i>,
+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 very high.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig22.gif" width="252" height="570" border="0" align="left" hspace="10"
+alt="FIG. 22.&mdash;TYPE OF BULB WITHOUT LEADING-IN WIRE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Fig. 22 illustrates a similar arrangement, with a large tube <i>T</i>
+protruding in to the part of the bulb containing the refractors 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
+<i>C&nbsp;C</i>. The insulating packing <i>P</i> 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>.
+<!-- Page 82 -->
+The molecular bombardment against the glass stem in the bulb is a source
+of great trouble. As 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.</p>
+<!-- Page 83 -->
+<br clear="all">&nbsp;<br>
+
+<p>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
+<!-- Page 84 -->
+conductor of low resistance is connected to a source of alternating currents.
+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>r&ocirc;le</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>
+<!-- Page 85 -->
+<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-conducting material, as,
+for instance, in the bulb described before in Fig. 21, 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 align="center">
+<img src="images/fig23.gif" width="452" height="566" border="0"
+alt="FIG. 23.&mdash;EFFECT PRODUCED BY A RUBY DROP.">
+</div>
+
+<p>A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 23. 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 aluminium 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 aluminium 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,
+<!-- Page 86 -->
+in which the radiant matter was projected against, or focused upon, the body
+to be rendered incandescent. Fig. 24 illustrates one of the bulbs used. It consists
+of a spherical globe <i>L</i>, provided with a long neck <i>n</i>, on the top,
+for increasing the action in some cases by the application of an
+external conducting coating. The globe <i>L</i> is blown out on the bottom
+into a very small bulb <i>b</i>, which serves to hold it firmly in a socket
+<i>S</i> 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 <i>L</i>. 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 <i>S</i> 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. 24 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 a different kind were
+mounted in the bulb as, for instance, indicated in Fig. 23, 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.
+This quality appeared to depend principally on the point of
+<!-- Page 87 -->
+fusion, and on the facility with which the body was &quot;evaporated,&quot; or,
+generally speaking, disintegrated&mdash;meaning by the latter term not only
+the throwing off of atoms, but likewise of larger 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 others, 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>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 is 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,
+<!-- Page 88 -->
+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>
+<!-- Page 89 -->
+<p>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 endeavored repeatedly
+to fuse zirconia, placing it in a cup or arc light carbon as indicated
+in Fig. 23. 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
+the 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
+<!-- Page 90 -->
+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.
+23 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 &quot;tired,&quot; 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,
+<!-- Page 91 -->
+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 evolution of <i>less</i> light?</p>
+
+<p>When the potential of a body is rapidly alternated it is
+<!-- Page 92 -->
+certain 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 invisible 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
+<!-- Page 93 -->
+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 say
+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.</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
+<!-- Page 94 -->
+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 &quot;carborundum&quot; 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 &quot;crystals&quot;
+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
+<!-- Page 95 -->
+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.</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 found 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
+<!-- Page 96 -->
+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 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
+<!-- Page 97 -->
+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 &quot;crystals,&quot;
+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 phosphorescence, unless it is that property which
+characterizes it as a
+<!-- Page 98 -->
+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 phosphorescence. 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 phosphorescence, 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
+<!-- Page 99 -->
+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 could the
+observer call 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.</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
+<!-- Page 100 -->
+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 aluminium 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
+<!-- Page 101 -->
+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.</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 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
+<!-- Page 102 -->
+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. This 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 eye 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 a 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.
+<!-- Page 103 -->
+The intensity of the light emitted depends principally on the frequency and
+potential of the impulses, and on the electric density of 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 reflecting one would think that in pushing
+so far the incandescence of the electrode it would be instantly volatilized.
+But after a careful consideration he would find that, theoretically, it should
+not occur, and in this fact&mdash;which, however, 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 is forced the incandescence 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,
+<!-- Page 104 -->
+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 is 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 shatter quickly 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,
+<!-- Page 105 -->
+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 the 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
+<!-- Page 106 -->
+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 most
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 22, 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 illuminating
+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
+<!-- Page 107 -->
+in the least as 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 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
+<!-- Page 108 -->
+by delivering frequent gentle 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. 19, 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
+<!-- Page 109 -->
+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>
+<!-- Page 110 -->
+<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 as
+illustrated in Fig. 12. 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 result as the application of an external negative electrode under
+ordinary circumstances.</p>
+
+<p>In all these experiments the action was intensified by
+<!-- Page 111 -->
+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>
+
+<img src="images/fig24.gif" width="489" height="648" border="0" align="left" hspace="10"
+alt="FIG. 24.&mdash;BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT OF PROJECTED MATTER." >
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 24, for example, an experimental bulb <i>L</i> 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.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig25.gif" width="269" height="662" border="0" align="left" hspace="10"
+alt="FIG. 25.&mdash;IMPROVED EXPERIMENTAL BULB.">
+
+<p>
+&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+Such a lamp as illustrated in Fig. 25 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>
+
+<br clear="all">&nbsp;<br>
+
+<div align="center">
+<img src="images/fig26.gif" width="586" height="389" border="0"
+alt="FIG. 26.&mdash;IMPROVED BULB WITH INTENSIFYING REFLECTOR.">
+</div>
+
+<p>A more perfected arrangement used in some of these bulbs is
+illustrated in Fig. 26. In this case the construction
+<!-- Page 112 -->
+of the bulb is as shown and described before, when reference was made to Fig. 19.
+A zinc sheet <i>Z</i>, with a tubular extension <i>T</i>, is slipped over the
+metallic socket <i>S</i>. The bulb hangs downward from the terminal <i>t</i>,
+the zinc sheet <i>Z</i>, 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 <i>P</i>.</p>
+
+<div align="center">
+<img src="images/fig27.gif" width="500" height="557" border="0"
+alt="FIG. 27.&mdash;PHOSPHORESCENT TUBE WITH INTENSIFYING REFLECTOR.">
+</div>
+
+<p>A similar disposition with a phosphorescent tube is illustrated
+<!-- Page 113 -->
+in Fig. 27. The tube <i>T</i> is prepared from two short tubes of a different
+diameter, which are sealed on the ends. On the lower end is placed an
+outside conducting coating <i>C</i>, 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
+<i>T</i> is another conducting coating <i>C</i><sub>1</sub> upon which is slipped a
+metallic reflector <i>Z</i>, 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, 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>
+
+<p>Suppose a small helix with many well insulated turns, as in experiment
+Fig. 17, 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 the 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 the
+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?
+<!-- Page 114 -->
+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 more one or 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 the 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 be very great. The
+total energy lost per unit of time is proportionate
+<!-- Page 115 -->
+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&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>
+
+<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>
+<!-- Page 116 -->
+<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 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
+<!-- Page 117 -->
+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
+<!-- Page 118 -->
+judiciously designed, would not present the slightest danger.</p>
+
+<p>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&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
+<!-- Page 119 -->
+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 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.</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
+<!-- Page 120 -->
+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. 28.</p>
+
+<img src="images/fig28.gif" width="490" height="565" border="0" align="left" hspace="10"
+alt="FIG. 28.&mdash;LAMP WITH AUXILIARY BULB FOR CONFINING THE ACTION TO THE CENTRE.">
+
+<p>The globe <i>L</i> 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. 18, for example. The small bulb is conveniently
+supported upon the stem <i>s</i>, carrying
+<!-- Page 121 -->
+the refractory button <i>m</i>. It is separated from the aluminium tube <i>a</i>
+by several layers of mica <i>M</i>, in order to prevent the cracking of the neck by the
+rapid heating of the aluminium 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. 28 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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig29.gif" width="503" height="563" border="0" align="left" hspace="10"
+alt="FIG. 29.&mdash;LAMP WITH INDEPENDENT AUXILIARY BULB.">
+
+<p>Many bulbs were constructed on the plan illustrated in Fig. 29. 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 <i>L</i>, 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 use a large bulb. It was found, in the course of experiences
+with bulbs such as illustrated in Fig. 29, 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 <i>L</i> was exhausted
+<!-- Page 122 -->
+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 <i>L</i> 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
+<!-- Page 123 -->
+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. 29, namely, the vacuum in the bulb <i>b</i> would be
+impaired in a comparatively short time.</p>
+
+<br clear="all">
+
+<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. 28 be
+chosen, in which both bulbs communicate.</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 the 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
+<!-- Page 124 -->
+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, and
+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 &quot;extinction&quot; 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 sufficiently 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 first. In a flame we set up light vibrations
+by causing molecules, or atoms, to collide.
+<!-- Page 125 -->
+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 knocks 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
+<!-- Page 126 -->
+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 1 inch in
+diameter and 1 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 different
+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 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
+<!-- Page 127 -->
+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
+insulated 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 a 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 with inconceivable
+<!-- Page 128 -->
+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 moving matter. In a gas the speed may be 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 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
+<!-- Page 129 -->
+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 &quot;non-striking&quot;
+vacuum being reached. But presently 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 discarded 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
+<!-- Page 130 -->
+will render this investigation more complete, might be permitted.</p>
+
+
+<div align="center">
+<img src="images/fig30.gif"  width="495" height="566" border="0"
+alt="FIG. 30.&mdash;APPARATUS USED FOR OBTAINING HIGH DEGREES OF EXHAUSTION.">
+</div>
+
+<p>The apparatus is illustrated in a drawing shown in Fig. 30. <i>S</i>
+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 <i>s</i> has been fitted in the neck
+<!-- Page 131 -->
+of the reservoir <i>R</i>. 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 connection of the latter with the fall tube.</p>
+
+<p>The pump is connected through a U-shaped tube <i>t</i> to a very large
+reservoir <i>R</i><sub>1</sub>. 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
+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 <i>C</i>, 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 <i>R</i><sub>1</sub> was connected by means of a rubber tube to a
+slightly larger reservoir <i>R</i><sub>2</sub>, each of the two reservoirs being
+provided with a stopcock <i>C</i><sub>1</sub> and <i>C</i><sub>2</sub>, respectively.
+The reservoir <i>R</i><sub>1</sub> 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 <i>C</i><sub>2</sub> closed, so as to form a Torricellian vacuum in
+it when raised, it could be lifted so high that the mercury in reservoir <i>R</i><sub>1</sub>
+would stand a little above stopcock <i>C</i><sub>1</sub>; and when this stopcock was
+closed and the reservoir <i>R</i><sub>2</sub> descended, so as to form a Torricellian vacuum in
+<!-- Page 132 -->
+reservoir <i>R</i><sub>1</sub>, it could be lowered so far as to
+completely empty the latter, the mercury filling the reservoir <i>R</i><sub>2</sub>
+up to a little above stopcock <i>C</i><sub>2</sub>.</p>
+
+<p>The capacity of the pump and of the connections was taken as small as
+possible relatively to the volume of reservoir <i>R</i><sub>1</sub>, 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 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 moisture off 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 <i>C</i> and <i>C</i><sub>1</sub> being open, and all other connections
+closed, the reservoir <i>R</i><sub>2</sub> was raised so far that the mercury filled the
+reservoir <i>R</i><sub>1</sub> 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 <i>R</i><sub>2</sub> was lowered, the experimenter keeping the mercury
+at about the same level.
+<!-- Page 133 -->
+The reservoir <i>R</i><sub>2</sub> was balanced by a long spring which facilitated
+the operation, and the friction of the parts was generally sufficient to keep it almost in any position.
+When the Sprengel pump had done its work, the reservoir <i>R</i><sub>2</sub> was
+further lowered and the mercury descended in <i>R</i><sub>1</sub> and filled <i>R</i><sub>2</sub>,
+whereupon stopcock <i>C</i><sub>2</sub> was closed. The air adhering to the walls of
+<i>R</i><sub>1</sub> and that absorbed by the mercury was carried off, and to free the
+mercury of all air the reservoir <i>R</i><sub>2</sub>  was for a long time worked up and
+down. During this process some air, which would gather below stopcock
+<i>C</i><sub>2</sub>, was expelled from <i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> when it was lowered, the caustic potash was resorted to.
+The reservoir <i>R</i><sub>2</sub> was now again raised until the mercury in
+<i>R</i><sub>1</sub> stood above stopcock <i>C</i><sub>1</sub>. The caustic potash
+was fused and boiled, and the 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
+<i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> 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 <i>R</i><sub>2</sub> was lowered, and
+<!-- Page 134 -->
+upon reservoir <i>R</i><sub>1</sub> being emptied the receiver <i>r</i> was
+quickly sealed up.</p>
+
+<p>When a new bulb was put on, the mercury was always raised above
+stopcock <i>C</i><sub>1</sub> which was closed, so as to always keep the mercury and
+both the reservoirs in fine condition, and the mercury was never
+withdrawn from <i>R</i><sub>1</sub> 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 15 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 <i>R</i> was about 1-20, 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
+<!-- Page 135 -->
+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 act 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&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
+<!-- Page 136 -->
+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 claim 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 allowing
+barely the light to shine through. A metallic clasp, 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 aluminium 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 not by far
+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
+<!-- Page 137 -->
+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.</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 a high frequency current,
+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 the heating is due to the
+rarefied gas. Here the gas might only act as a conductor of no impedance
+<!-- Page 138 -->
+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 he
+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 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 to 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 &quot;radiant state&quot; and the
+&quot;non-striking vacuum.&quot;</p>
+
+<p>Any one who has studied Crookes' work must have received the
+impression that the &quot;radiant state&quot; is a property
+<!-- Page 139 -->
+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
+<!-- Page 140 -->
+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 as 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>
+
+<div align="center">
+<img src="images/fig31.gif" width="492" height="526" border="0"
+alt="FIG. 31.&mdash;BULB SHOWING RADIANT LIME STREAM AT LOW EXHAUSTION.">
+</div>
+
+<p>I have prepared a bulb to illustrate by an experiment the correctness
+of these assertions. In a globe <i>L</i> (Fig. 31) 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. 19, 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 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
+<!-- Page 141 -->
+that point which is intensely heated, and a white stream of lime particles (Fig. 31)
+then breaks forth from that point. This stream is composed of &quot;radiant&quot;
+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>
+
+<p>As to the &quot;non-striking vacuum,&quot; 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
+<!-- Page 142 -->
+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 electro-dynamic
+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.</p>
+
+<p>Many bulbs were constructed as shown in Fig. 32 and Fig. 33.</p>
+
+<img src="images/fig32.gif" width="236" height="594" border="0" align="left" hspace="10"
+alt="FIG. 32.&mdash;ELECTRO-DYNAMIC INDUCTION TUBE.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 32 a wide tube <i>T</i> was sealed to a smaller W-shaped tube <i>U</i>,
+of phosphorescent glass. In the tube <i>T</i> was placed a coil <i>C</i> of
+aluminium wire, the ends of which were provided with small spheres <i>t</i>
+and <i>t</i><sub>1</sub> of aluminium, and reached into the <i>U</i> tube.
+The tube <i>T</i> was slipped into a socket containing a primary coil
+through which usually the discharges of Leyden jars were directed, and
+<!-- Page 143 -->
+the rarefied gas in the small <i>U</i> tube was excited to strong luminosity
+by the high-tension currents induced in the coil <i>C</i>. When Leyden jar
+discharges were used to induce currents in the coil <i>C</i>, it was found
+necessary to pack the tube <i>T</i> tightly with insulating powder, as a
+discharge would occur frequently between the turns of the coil, especially
+<!-- Page 144 -->
+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>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig33.gif" width="260" height="543" border="0" align="left" hspace="10"
+alt="FIG. 33&mdash;ELECTRO-DYNAMIC INDUCTION LAMP.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 33 is illustrated another form of the bulb constructed. In
+this case a tube <i>T</i> is sealed to a globe <i>L</i>. The tube contains a
+coil <i>C</i>, 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 <i>T</i>. 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 <i>L</i> communicated with the tube
+<i>T</i>. For this purpose the ends of the small tubes <i>t</i> and <i>t</i><sub>1</sub> were
+just a trifle heated in the burner, merely to hold the wires, but not
+to interfere with the communication. The tube <i>T</i>, with the small
+tubes, wires through the same, and the refractory buttons <i>m</i> and
+<i>m</i><sub>1</sub>, was first prepared, and then sealed to globe <i>L</i>, whereupon
+the coil <i>C</i> 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. 33 an aluminium 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 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 <i>C</i>. 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>
+<!-- Page 145 -->
+<br clear="all">
+
+<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 experiences 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, gets 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 ball 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, still, 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 far as desired, and the efficiency
+<!-- Page 146 -->
+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 he works
+with condenser charges&mdash;and they are the only means up to the present
+known for reaching these extreme frequencies&mdash;he gets to electromotive
+forces of several thousands of volts per turn of the primary. He
+cannot multiply the electro-dynamic inductive effect by taking more
+turns in the primary, for he arrives at the conclusion that the best
+way is to work with one single turn&mdash;though he must sometimes depart
+from this rule&mdash;and he must get along with whatever inductive effect
+he can obtain with one turn. But before he has long experimented with
+the extreme frequencies required to set up in a small bulb an
+electromotive force of several thousands of volts he 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 electro-dynamic
+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>
+<!-- Page 147 -->
+<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 condenser to a higher potential, establish electrostatic
+alternating fields which acted through the whole extent of a 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.</p>
+
+<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
+<!-- Page 148 -->
+best an unskilled lecturer can do is to begin and finish with the exhibition of these
+singular effects. I take a tube in the 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 the 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.</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. 34 and Fig. 35 two such tubes are illustrated
+which are prepared for the occasion.</p>
+
+<img src="images/fig34.gif" width="232" height="591" border="0" align="left" hspace="10"
+alt="FIG. 34.&mdash;TUBE WITH FILAMENT RENDERED INCANDESCENT IN AN ELECTROSTATIC FIELD.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+In Fig. 34 a short tube <i>T</i><sub>1</sub>, sealed to another long tube <i>T</i>,
+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
+<!-- Page 149 -->
+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, <i>C</i> and
+<i>C</i><sub>1</sub> 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
+<!-- Page 150 -->
+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 aluminium 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 <i>T</i><sub>1</sub> anywhere in the
+electrostatic field the filament is rendered incandescent.</p>
+
+<br clear="all">&nbsp;<br>
+
+<img src="images/fig35.gif" width="259" height="592" border="0" align="left" hspace="10"
+alt="FIG. 35.&mdash;CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.">
+
+<p>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>&nbsp;<br>
+A more interesting piece of apparatus is illustrated in Fig. 35. 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 <i>C</i>. 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 aluminium 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>
+
+<br clear="all">
+
+<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 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
+<!-- Page 151 -->
+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. From 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, their 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 this argument is not
+devoid of force-but because in a burner the higher vibrations can
+never be reached except by passing through all the low ones. For how
+is a flame produced 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;
+<!-- Page 152 -->
+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 remain 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 effectively 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 very high frequencies, many
+advantages, such as a 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
+<!-- Page 153 -->
+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, 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 though 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
+<!-- Page 154 -->
+the length of the wire, then corresponding short waves will be sent 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 distance 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
+<!-- Page 155 -->
+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 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>
+
+<p>&nbsp;</p>
+
+<center>
+<table border=0 bgcolor="ccccff" cellpadding=10>
+  <tr>
+    <td valign="top">
+      Transcriber's note:
+    </td>
+    <td>
+          Corrected the following typesetting errors:<br>
+		  1) 'preceived' to 'perceived', page 16. <br>
+		  2) 'disharging' to 'discharging', page 30.<br>
+		  3) 'park' to 'spark', page 33.<br>
+		  4) 'pssition' to 'position', page 50.<br>
+		  5) 'to th opposite side' to 'to the opposite side', page 56.<br>
+          6) 's resses' to 'stresses', page 147.
+	</td>
+  </tr>
+</table>
+</center>
+<br>
+<br>
+<hr class="full" noshade>
+<p>***END OF THE PROJECT GUTENBERG EBOOK EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY***</p>
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+The Project Gutenberg eBook, Experiments with Alternate Currents of High
+Potential and High Frequency, by Nikola Tesla
+
+
+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
+
+
+
+
+
+Title: Experiments with Alternate Currents of High Potential and High
+Frequency
+
+Author: Nikola Tesla
+
+Release Date: September 16, 2004  [eBook #13476]
+
+Language: English
+
+Character set encoding: ISO-646-US (US-ASCII)
+
+
+***START OF THE PROJECT GUTENBERG EBOOK EXPERIMENTS WITH ALTERNATE
+CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY***
+
+
+E-text prepared by Robert Shimmin, Ronald Holder, and the Project
+Gutenberg Online Distributed Proofreading Team
+
+
+
+Note: Project Gutenberg also has an HTML version of this
+      file which includes the original illustrations.
+      See 13476-h.htm or 13476-h.zip:
+      (https://www.gutenberg.org/dirs/1/3/4/7/13476/13476-h/13476-h.htm)
+      or
+      (https://www.gutenberg.org/dirs/1/3/4/7/13476/13476-h.zip)
+
+
+
+
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY
+
+A Lecture Delivered before the Institution of Electrical Engineers, London
+
+by
+
+NIKOLA TESLA
+
+With a Portrait and Biographical Sketch of the Author
+
+NEW YORK
+
+1892
+
+
+
+
+
+
+
+Biographical Sketch of Nikola Tesla.
+
+
+While a large portion of the European family has been surging westward
+during the last three or four hundred years, settling the vast
+continents of America, another, but smaller, portion has been doing
+frontier work in the Old World, protecting the rear by beating back
+the "unspeakable Turk" and reclaiming gradually the fair lands that
+endure the curse of Mohammedan rule. For a long time the Slav
+people--who, after the battle of Kosovopjolje, in which the Turks
+defeated the Servians, retired to the confines of the present
+Montenegro, Dalmatia, Herzegovina and Bosnia, and "Borderland" of
+Austria--knew what it was to deal, as our Western pioneers did, with
+foes ceaselessly fretting against their frontier; and the races of
+these countries, through their strenuous struggle against the armies
+of the Crescent, have developed notable qualities of bravery and
+sagacity, while maintaining a patriotism and independence unsurpassed
+in any other nation.
+
+It was in this interesting border region, and from among these valiant
+Eastern folk, that Nikola Tesla was born in the year 1857, and the
+fact that he, to-day, finds himself in America and one of our foremost
+electricians, is striking evidence of the extraordinary attractiveness
+alike of electrical pursuits and of the country where electricity
+enjoys its widest application. Mr. Tesla's native place was Smiljan,
+Lika, where his father was an eloquent clergyman of the Greek Church,
+in which, by the way, his family is still prominently represented. His
+mother enjoyed great fame throughout the countryside for her skill and
+originality in needlework, and doubtless transmitted her ingenuity to
+Nikola; though it naturally took another and more masculine direction.
+
+The boy was early put to his books, and upon his father's removal to
+Gospic he spent four years in the public school, and later, three
+years in the Real School, as it is called. His escapades were such as
+most quick witted boys go through, although he varied the programme on
+one occasion by getting imprisoned in a remote mountain chapel rarely
+visited for service; and on another occasion by falling headlong into
+a huge kettle of boiling milk, just drawn from the paternal herds. A
+third curious episode was that connected with his efforts to fly when,
+attempting to navigate the air with the aid of an old umbrella, he
+had, as might be expected, a very bad fall, and was laid up for six
+weeks.
+
+About this period he began to take delight in arithmetic and physics.
+One queer notion he had was to work out everything by three or the
+power of three. He was now sent to an aunt at Cartstatt, Croatia, to
+finish his studies in what is known as the Higher Real School. It was
+there that, coming from the rural fastnesses, he saw a steam engine
+for the first time with a pleasure that he remembers to this day. At
+Cartstatt he was so diligent as to compress the four years' course
+into three, and graduated in 1873. Returning home during an epidemic
+of cholera, he was stricken down by the disease and suffered so
+seriously from the consequences that his studies were interrupted for
+fully two years. But the time was not wasted, for he had become
+passionately fond of experimenting, and as much as his means and
+leisure permitted devoted his energies to electrical study and
+investigation. Up to this period it had been his father's intention to
+make a priest of him, and the idea hung over the young physicist like
+a very sword of Damocles. Finally he prevailed upon his worthy but
+reluctant sire to send him to Gratz in Austria to finish his studies
+at the Polytechnic School, and to prepare for work as professor of
+mathematics and physics. At Gratz he saw and operated a Gramme machine
+for the first time, and was so struck with the objections to the use
+of commutators and brushes that he made up his mind there and then to
+remedy that defect in dynamo-electric machines. In the second year of
+his course he abandoned the intention of becoming a teacher and took
+up the engineering curriculum. After three years of absence he
+returned home, sadly, to see his father die; but, having resolved to
+settle down in Austria, and recognizing the value of linguistic
+acquirements, he went to Prague and then to Buda-Pesth with the view
+of mastering the languages he deemed necessary. Up to this time he had
+never realized the enormous sacrifices that his parents had made in
+promoting his education, but he now began to feel the pinch and to
+grow unfamiliar with the image of Francis Joseph I. There was
+considerable lag between his dispatches and the corresponding
+remittance from home; and when the mathematical expression for the
+value of the lag assumed the shape of an eight laid flat on its back,
+Mr. Tesla became a very fair example of high thinking and plain
+living, but he made up his mind to the struggle and determined to go
+through depending solely on his own resources. Not desiring the fame
+of a faster, he cast about for a livelihood, and through the help of
+friends he secured a berth as assistant in the engineering department
+of the government telegraphs. The salary was five dollars a week. This
+brought him into direct contact with practical electrical work and
+ideas, but it is needless to say that his means did not admit of much
+experimenting. By the time he had extracted several hundred thousand
+square and cube roots for the public benefit, the limitations,
+financial and otherwise, of the position had become painfully
+apparent, and he concluded that the best thing to do was to make a
+valuable invention. He proceeded at once to make inventions, but their
+value was visible only to the eye of faith, and they brought no grist
+to the mill. Just at this time the telephone made its appearance in
+Hungary, and the success of that great invention determined his
+career, hopeless as the profession had thus far seemed to him. He
+associated himself at once with telephonic work, and made various
+telephonic inventions, including an operative repeater; but it did not
+take him long to discover that, being so remote from the scenes of
+electrical activity, he was apt to spend time on aims and results
+already reached by others, and to lose touch. Longing for new
+opportunities and anxious for the development of which he felt himself
+possible, if once he could place himself within the genial and direct
+influences of the gulf streams of electrical thought, he broke away
+from the ties and traditions of the past, and in 1881 made his way to
+Paris. Arriving in that city, the ardent young Likan obtained
+employment as an electrical engineer with one of the largest electric
+lighting companies. The next year he went to Strasburg to install a
+plant, and on returning to Paris sought to carry out a number of ideas
+that had now ripened into inventions. About this time, however, the
+remarkable progress of America in electrical industry attracted his
+attention, and once again staking everything on a single throw, he
+crossed the Atlantic.
+
+Mr. Tesla buckled down to work as soon as he landed on these shores,
+put his best thought and skill into it, and soon saw openings for his
+talent. In a short while a proposition was made to him to start his
+own company, and, accepting the terms, he at once worked up a
+practical system of arc lighting, as well as a potential method of
+dynamo regulation, which in one form is now known as the "third brush
+regulation." He also devised a thermo-magnetic motor and other kindred
+devices, about which little was published, owing to legal
+complications. Early in 1887 the Tesla Electric Company of New York
+was formed, and not long after that Mr. Tesla produced his admirable
+and epoch-marking motors for multiphase alternating currents, in
+which, going back to his ideas of long ago, he evolved machines having
+neither commutator nor brushes. It will be remembered that about the
+time that Mr. Tesla brought out his motors, and read his thoughtful
+paper before the American Institute of Electrical Engineers, Professor
+Ferraris, in Europe, published his discovery of principles analogous
+to those enunciated by Mr. Tesla. There is no doubt, however, that Mr.
+Tesla was an independent inventor of this rotary field motor, for
+although anticipated in dates by Ferraris, he could not have known
+about Ferraris' work as it had not been published. Professor Ferraris
+stated himself, with becoming modesty, that he did not think Tesla
+could have known of his (Ferraris') experiments at that time, and adds
+that he thinks Tesla was an independent and original inventor of this
+principle. With such an acknowledgment from Ferraris there can be
+little doubt about Tesla's originality in this matter.
+
+Mr. Tesla's work in this field was wonderfully timely, and its worth
+was promptly appreciated in various quarters. The Tesla patents were
+acquired by the Westinghouse Electric Company, who undertook to
+develop his motor and to apply it to work of different kinds. Its use
+in mining, and its employment in printing, ventilation, etc., was
+described and illustrated in _The Electrical World_ some years ago.
+The immense stimulus that the announcement of Mr. Tesla's work gave to
+the study of alternating current motors would, in itself, be enough to
+stamp him as a leader.
+
+Mr. Tesla is only 35 years of age. He is tall and spare with a
+clean-cut, thin, refined face, and eyes that recall all the stories
+one has read of keenness of vision and phenomenal ability to see
+through things. He is an omnivorous reader, who never forgets; and he
+possesses the peculiar facility in languages that enables the least
+educated native of eastern Europe to talk and write in at least half a
+dozen tongues. A more congenial companion cannot be desired for the
+hours when one "pours out heart affluence in discursive talk," and
+when the conversation, dealing at first with things near at hand and
+next to us, reaches out and rises to the greater questions of life,
+duty and destiny.
+
+In the year 1890 he severed his connection with the Westinghouse
+Company, since which time he has devoted himself entirely to the study
+of alternating currents of high frequencies and very high potentials,
+with which study he is at present engaged. No comment is necessary on
+his interesting achievements in this field; the famous London lecture
+published in this volume is a proof in itself. His first lecture on
+his researches in this new branch of electricity, which he may be said
+to have created, was delivered before the American Institute of
+Electrical Engineers on May 20, 1891, and remains one of the most
+interesting papers read before that society. It will be found
+reprinted in full in _The Electrical World_, July 11, 1891. Its
+publication excited such interest abroad that he received numerous
+requests from English and French electrical engineers and scientists
+to repeat it in those countries, the result of which has been the
+interesting lecture published in this volume.
+
+The present lecture presupposes a knowledge of the former, but it may
+be read and understood by any one even though he has not read the
+earlier one. It forms a sort of continuation of the latter, and
+includes chiefly the results of his researches since that time.
+
+
+
+
+
+EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY
+
+
+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 time 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 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[A] 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.
+
+[Footnote A: For Mr. Tesla's American lecture on this subject see THE
+ELECTRICAL WORLD of July 11, 1891, and for a report of his French
+lecture see THE ELECTRICAL WORLD of March 26, 1892.]
+
+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 immediately the most 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 with 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 may 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 tests 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. 1.)
+
+[Illustration: FIG. 1.--DISCHARGE BETWEEN TWO WIRES WITH FREQUENCIES
+OF A FEW HUNDRED THOUSAND PER SECOND.]
+
+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. 2.
+
+[Illustration: FIG. 2.--IMITATING THE SPARK OF A HOLTZ MACHINE.]
+
+G is an ordinarily constructed alternator, supplying the primary P of
+an induction coil, the secondary S of which charges the condensers or
+jars CC. 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 pp of a second induction coil. This primary pp has a
+small air gap ab.
+
+The secondary s of this coil is provided with knobs or spheres KK of
+the proper size and set at a distance suitable for the experiment.
+
+A long arc is established between the terminals AB of the first
+induction coil. MM are the mica plates.
+
+Each time the arc is broken between A and B the jars are quickly
+charged and discharged through the primary pp, producing a snapping
+spark between the knobs KK. 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 ab until the arc is again broken by
+the draught.
+
+In this manner sudden impulses, at long intervals, are produced in the
+primary pp, which in the secondary s give a corresponding number of
+impulses of great intensity. If the secondary knobs or spheres, KK,
+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[A]
+before the American Institute of Electrical Engineers, May 20, 1891.
+
+[Footnote A: See THE ELECTRICAL WORLD, July 11, 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 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. 3.--DISRUPTIVE DISCHARGE COIL.]
+
+It is contained in a box B (Fig. 3) of thick boards of hard wood,
+covered on the outside with 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 RR, 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 FF, 24 centimetres square, the space between the flanges being
+about 3 centimetres. The secondary, SS, 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 PP 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 tt. 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.Fs. 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.
+
+[Illustration: FIG. 4.--ARRANGEMENT OF IMPROVED DISCHARGER AND
+MAGNET.]
+
+One of the changes is that the adjustable knobs A and B (Fig. 4),
+of the discharger are held in jaws of brass, JJ, by spring pressure,
+this allowing of turning them successively into different positions,
+and so doing away with the tedious process of frequent polishing up.
+
+The other change consists in the employment of a strong electromagnet
+NS, 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, MM, 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. LL are screws for fixing in position the rods RR, 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.
+
+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 AB, in Fig. 2 (the knobs ab 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. 5, 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.
+
+[Illustration: FIG. 5.--ARRANGEMENT WITH LOW-FREQUENCY ALTERNATOR AND
+IMPROVED DISCHARGER.]
+
+[Illustration: FIG. 6.--DISCHARGER WITH MULTIPLE GAPS.]
+
+The other form of discharger used in these and similar experiments is
+indicated in Figs. 6 and 7. It consists of a number of brass pieces cc
+(Fig. 6), 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 RR, with planed grooves gg (Fig. 7) to fit the
+middle portion of the pieces cc, serve to clamp the latter and hold
+them firmly in position by means of two bolts CC (of which only one is
+shown) passing through the ends of the strips.
+
+[Illustration: FIG. 7.--DISCHARGER WITH MULTIPLE GAPS.]
+
+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. 2,
+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 7 metres in length. They are supported on insulating cords
+at a distance of about 30 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 set at the beginning the condenser plates at maximum distance. If
+the streams for 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. 8.--EFFECT PRODUCED BY CONCENTRATING STREAMS.]
+
+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. 8), 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.
+
+Here, for instance, I have two plates, RR, of hard rubber (Fig. 9),
+upon which I have glued two very thin wires ww, 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 tt. The plates are placed in line
+at a sufficient distance to prevent a spark passing from one to the
+other wire. 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.
+
+[Illustration: FIG. 9.--WIRES RENDERED INTENSELY LUMINOUS.]
+
+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 then when produced with such a coil as the present. This
+experiment, however, may be performed with low frequencies, but much
+less satisfactorily.
+
+[Illustration: FIG. 10.--LUMINOUS DISCS.]
+
+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. 10), 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_ but _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 oil and placed parallel to each
+other at a distance of about 10 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 a merely 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 often occasion 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.
+
+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 (dd, Fig. 11) 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.
+
+[Illustration: FIG. 11.--PHANTOM STREAMS.]
+
+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 1 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
+immersion, 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 in commerce two kinds of gutta-percha wire: 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 hard wood 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, become, at least with higher frequencies, so easy that
+they could be hardly called engineering feats. With oil insulation
+and alternate current motors transmissions of power can be effected
+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 purposes 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
+20, 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; this
+allowing the use of a much bigger 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.
+
+In bulbs provided with a conducting terminal, though it be of
+aluminium, 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. 12 and 13.
+
+In Fig. 12 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 aluminium 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.
+
+[Illustration: FIG. 12. FIG. 13. BULBS FOR PRODUCING ROTATING BRUSH.]
+
+The construction shown in Fig. 13 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 conducting 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. 14.--FORMS AND PHASES OF THE ROTATING BRUSH.]
+
+Figs. 14, 15 and 16 indicate different forms, or stages, of the brush.
+Fig. 14 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. 12 and 13) is exhausted to a very high
+degree, generally the bulb is not excited upon connecting the wire w
+(Fig. 12) or the tinfoil coating of the bulb (Fig. 13) 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. 15. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 16, 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. 15. FIG. 16. FORMS AND PHASES OF THE ROTATING
+BRUSH.]
+
+When the brush assumes the form indicated in Fig. 16, it maybe 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
+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 in
+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 result. 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 aluminium 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 overlap partly 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. 17), comprising a coil and iron core, and a freely movable copper
+disc in proximity to the latter.
+
+[Illustration: FIG. 17.--SINGLE WIRE AND "NO-WIRE" MOTOR.]
+
+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 subtile 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
+alternate currents 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 inclosed 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 as 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 aluminium sheet, which fitted the stem
+and was held on it by spring pressure. The function of this aluminium
+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, time
+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 the coil, the largest bulb being placed at the end of the wire, at
+some distance from it the smallest bulb, and 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 it
+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 about in 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. In 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 otherwise
+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 the 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 aluminium, 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
+aluminium sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a piece of
+aluminium sheet of the 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 aluminium 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 aluminium
+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
+aluminium 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 degrees 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, by far, so 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 a means for
+concentrating more energy upon the same.
+
+To whatever extent the aluminium 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 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 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 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.
+
+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 clearness of some of the remarks before made, I must
+now refer to Figs. 18, 19 and 20, which illustrate various
+arrangements with a type of bulb most generally used.
+
+[Illustration: FIG. 18.--BULB WITH MICA TUBE AND ALUMINIUM SCREEN.]
+
+[Illustration: FIG. 19.--IMPROVED BULB WITH SOCKET AND SCREEN.]
+
+Fig. 18 is a section through a spherical bulb L, with the glass stem
+s, containing 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 aluminium tube.
+
+Fig. 19 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, as mica powder.
+
+[Illustration: FIG. 20.--BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.]
+
+Fig. 20 shows a bulb made for experimental purposes. In this bulb the
+aluminium 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. 21), 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, aluminium 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
+tried, 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 very high.
+
+Fig. 22 illustrates a similar arrangement, with a large tube T
+protruding in to the part of the bulb containing the refractors button
+m. In this case the wire leading from the outside into the bulb is
+omitted, the energy required being supplied through condenser coatings
+CC. 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 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.
+
+[Illustration: FIG. 21.--IMPROVED BULB WITH NON-CONDUCTING BUTTON.]
+
+[Illustration: FIG. 22.--TYPE OF BULB WITHOUT LEADING-IN WIRE.]
+
+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 currents. 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.
+
+[Illustration: FIG. 23.--EFFECT PRODUCED BY A RUBY DROP.]
+
+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. 21, 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.
+
+A different arrangement used in some of the bulbs constructed is
+illustrated in Fig. 23. 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 aluminium 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 aluminium 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. 24 illustrates one of the bulbs used. It consists
+of a spherical globe L, provided with a long neck n, on the 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. 24 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 a different kind were mounted
+in the bulb as, for instance, indicated in Fig. 23, 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.
+This 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 larger 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 others, 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 is 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 endeavored repeatedly
+to fuse zirconia, placing it in a cup or arc light carbon as indicated
+in Fig. 23. 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
+the 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.
+23 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 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 invisible 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 say
+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 found 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 phosphorescence, 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 phosphorescence. 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 phosphorescence, 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 could the
+observer call 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 aluminium 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 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. This 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 eye 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 a 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
+of 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
+reflecting one would think that in pushing so far the incandescence of
+the electrode it would be instantly volatilized. But after a careful
+consideration he would find that, theoretically, it should not occur,
+and in this fact--which, however, 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 is forced the incandescence 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 is 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 shatter quickly 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 the 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 most
+generally used on account of convenience, as in employing condenser
+coatings in the manner indicated in Fig. 22, 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 illuminating
+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 as 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 gentle 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. 19, 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. 24.--BULB WITHOUT LEADING-IN WIRE, SHOWING EFFECT
+OF PROJECTED MATTER.]
+
+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.
+
+[Illustration: FIG. 25.--IMPROVED EXPERIMENTAL BULB.]
+
+Again, in another of the early experiments, a bulb was used as
+illustrated in Fig. 12. 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 result 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.
+
+[Illustration: FIG. 26.--IMPROVED BULB WITH INTENSIFYING REFLECTOR.]
+
+In Fig. 24, 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. 25 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. 26. In this case the construction of the bulb is
+as shown and described before, when reference was made to Fig. 19. A
+zinc sheet Z, with a tubular extension T, is slipped 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.
+
+[Illustration: FIG. 27.--PHOSPHORESCENT TUBE WITH INTENSIFYING
+REFLECTOR.]
+
+A similar disposition with a phosphorescent tube is illustrated in
+Fig. 27. The tube T is prepared from two short tubes of a different
+diameter, which are sealed on the ends. On the lower end is placed an
+outside 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.
+
+Suppose a small helix with many well insulated turns, as in experiment
+Fig. 17, 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 the 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 the
+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
+more one or 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 the 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 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 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.
+
+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. 28.
+
+[Illustration: FIG. 28.--LAMP WITH AUXILIARY BULB FOR CONFINING THE
+ACTION TO THE CENTRE.]
+
+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. 18, for example. The small bulb is conveniently supported upon
+the stem s, carrying the refractory button m. It is separated from the
+aluminium tube a by several layers of mica M, in order to prevent the
+cracking of the neck by the rapid heating of the aluminium 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. 28 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. 29. 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 use a large bulb. It was found, in the course of
+experiences with bulbs such as illustrated in Fig. 29, 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. 29, namely, the
+vacuum in the bulb b would be impaired in a comparatively short time.
+
+[Illustration: FIG. 29.--LAMP WITH INDEPENDENT AUXILIARY BULB.]
+
+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. 28 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 the 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, and
+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 sufficiently 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 knocks 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 1 inch in
+diameter and 1 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 different
+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 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
+insulated 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 a 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 moving matter. In a gas the speed may be 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 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 presently 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 discarded 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. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF
+EXHAUSTION.]
+
+The apparatus is illustrated in a drawing shown in Fig. 30. 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 connection 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 mercury in 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 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 moisture off 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 almost in 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 the
+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 r 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 15 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 1-20, 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 act 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 claim 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 allowing
+barely the light to shine through. A metallic clasp, 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 aluminium 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 not by far
+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 a high frequency current,
+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 the 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 he
+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 to 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 as 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.
+
+[Illustration: FIG. 31.--BULB SHOWING RADIANT LIME STREAM AT LOW
+EXHAUSTION.]
+
+I have prepared a bulb to illustrate by an experiment the correctness
+of these assertions. In a globe L (Fig. 31) 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. 19, 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. 31) 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.
+
+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.
+
+A thought which naturally presents itself in connection with high
+frequency currents, is to make use of their powerful electro-dynamic
+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. 32 and Fig. 33.
+
+In Fig. 32 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 aluminium
+wire, the ends of which were provided with small spheres t and t_1 of
+aluminium, 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 currents 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.
+
+[Illustration: FIG. 32.--ELECTRO-DYNAMIC INDUCTION TUBE.]
+
+[Illustration: FIG. 33--ELECTRO-DYNAMIC INDUCTION LAMP.]
+
+In Fig. 33 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 just a trifle heated 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, was 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. 33 an
+aluminium 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 experiences 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, gets 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 ball 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, still, 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 far 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 he works
+with condenser charges--and they are the only means up to the present
+known for reaching these extreme frequencies--he gets to electromotive
+forces of several thousands of volts per turn of the primary. He
+cannot multiply the electro-dynamic inductive effect by taking more
+turns in the primary, for he arrives at the conclusion that the best
+way is to work with one single turn--though he must sometimes depart
+from this rule--and he must get along with whatever inductive effect
+he can obtain with one turn. But before he has long experimented with
+the extreme frequencies required to set up in a small bulb an
+electromotive force of several thousands of volts he 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 electro-dynamic
+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 condenser to a higher potential, establish electrostatic
+alternating fields which acted through the whole extent of a 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.
+
+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 the 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 the 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. 34 and Fig. 35 two such tubes are illustrated
+which are prepared for the occasion. In Fig. 34 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 aluminium 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.
+
+[Illustration: FIG. 34.--TUBE WITH FILAMENT RENDERED INCANDESCENT IN
+AN ELECTROSTATIC FIELD.]
+
+[Illustration: FIG. 35.--CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.]
+
+A more interesting piece of apparatus is illustrated in Fig. 35. 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 aluminium 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. From 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, their 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 this argument is not
+devoid of force-but because in a burner the higher vibrations can
+never be reached except by passing through all the low ones. For how
+is a flame produced 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 remain 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 very high frequencies, many
+advantages, such as a 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, 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 though 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 sent 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 distance 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.
+
+
+
+[Transcriber's note: Corrected the following typesetting errors:
+     1) 'preceived' to 'perceived', page 16.
+     2) 'disharging' to 'discharging', page 30.
+     3) 'park' to 'spark', page 33.
+     4) 'pssition' to 'position', page 50.
+     5) 'to th opposite side' to 'to the opposite side', page 56.
+     6) 's resses' to 'stresses', page 147.]
+
+
+
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