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+      The Project Gutenberg eBook of The Inventions Researches and Writings of Nikola Tesla, by Thomas Commerford Martin.
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+<pre>
+
+The Project Gutenberg EBook of The inventions, researches and writings of
+Nikola Tesla, by Thomas Commerford Martin
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org/license
+
+
+Title: The inventions, researches and writings of Nikola Tesla
+       With special reference to his work in polyphase currents
+              and high potential lighting
+
+Author: Thomas Commerford Martin
+
+Release Date: March 26, 2012 [EBook #39272]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE INVENTIONS, RESEARCHES ***
+
+
+
+
+Produced by Anna Hall, Albert L�szl� and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+</pre>
+
+
+<p>&nbsp;</p>
+
+
+
+<h1>THE INVENTIONS</h1>
+
+<h1>RESEARCHES AND WRITINGS</h1>
+
+<h4>OF</h4>
+
+<h1>NIKOLA TESLA</h1>
+
+<p>&nbsp;</p><p>&nbsp;</p>
+
+<h2>TO HIS COUNTRYMEN</h2>
+<h4>IN EASTERN EUROPE THIS RECORD OF<br />
+THE WORK ALREADY ACCOMPLISHED BY</h4>
+<h2>NIKOLA TESLA</h2>
+
+<h4>IS RESPECTFULLY DEDICATED</h4>
+<p>&nbsp;</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_004.jpg" width="640" height="1024" alt="" title="" />
+</div>
+<p>&nbsp;</p>
+
+<h2>THE INVENTIONS</h2>
+<h2>RESEARCHES AND WRITINGS</h2>
+<h5>OF</h5>
+<h1><span class="smcap">Nikola Tesla</span></h1>
+
+<h4>WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE<br />
+CURRENTS AND HIGH POTENTIAL LIGHTING</h4>
+
+<h5>BY</h5>
+
+<h2>THOMAS COMMERFORD MARTIN</h2>
+<h4>Editor <span class="smcap">The Electrical Engineer</span>; Past-President American Institute Electrical Engineers</h4>
+<p>&nbsp;</p>
+
+<h4>1894<br />
+THE ELECTRICAL ENGINEER<br />
+<small>NEW YORK</small><br />
+<b>D. VAN NOSTRAND COMPANY,<br />
+<small>NEW YORK.</small></b>
+</h4>
+
+<p>&nbsp;</p>
+
+<hr style="width: 10%;" />
+<p>&nbsp;</p>
+<h4>Entered according to Act of Congress in the year 1893 by<br />
+T. C. MARTIN<br />
+in the office of the Librarian of Congress at Washington</h4>
+
+<p>&nbsp;</p>
+
+<p class="center">Press of McIlroy &amp; Emmet, 36 Cortlandt St., N. Y.</p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_v" id="Page_v">[Pg v]</a></span></p>
+<h2><a name="PREFACE" id="PREFACE"></a>PREFACE.</h2>
+
+
+<p>The electrical problems of the present day lie largely in the
+economical transmission of power and in the radical improvement
+of the means and methods of illumination. To many
+workers and thinkers in the domain of electrical invention, the
+apparatus and devices that are familiar, appear cumbrous and
+wasteful, and subject to severe limitations. They believe that
+the principles of current generation must be changed, the area
+of current supply be enlarged, and the appliances used by the
+consumer be at once cheapened and simplified. The brilliant
+successes of the past justify them in every expectancy of still
+more generous fruition.</p>
+
+<p>The present volume is a simple record of the pioneer work
+done in such departments up to date, by Mr. Nikola Tesla, in
+whom the world has already recognized one of the foremost of
+modern electrical investigators and inventors. No attempt whatever
+has been made here to emphasize the importance of his
+researches and discoveries. Great ideas and real inventions win
+their own way, determining their own place by intrinsic merit.
+But with the conviction that Mr. Tesla is blazing a path that
+electrical development must follow for many years to come, the
+compiler has endeavored to bring together all that bears the impress
+of Mr. Tesla's genius, and is worthy of preservation. Aside
+from its value as showing the scope of his inventions, this
+volume may be of service as indicating the range of his thought.
+There is intellectual profit in studying the push and play of a
+vigorous and original mind.</p>
+
+<p>Although the lively interest of the public in Mr. Tesla's work
+is perhaps of recent growth, this volume covers the results of
+full ten years. It includes his lectures, miscellaneous articles<span class='pagenum'><a name="Page_vi" id="Page_vi">[Pg vi]</a></span>
+and discussions, and makes note of all his inventions thus far
+known, particularly those bearing on polyphase motors and the
+effects obtained with currents of high potential and high frequency.
+It will be seen that Mr. Tesla has ever pressed forward,
+barely pausing for an instant to work out in detail the utilizations
+that have at once been obvious to him of the new principles he
+has elucidated. Wherever possible his own language has been
+employed.</p>
+
+<p>It may be added that this volume is issued with Mr. Tesla's
+sanction and approval, and that permission has been obtained for
+the re-publication in it of such papers as have been read before
+various technical societies of this country and Europe. Mr.
+Tesla has kindly favored the author by looking over the proof
+sheets of the sections embodying his latest researches. The
+work has also enjoyed the careful revision of the author's
+friend and editorial associate, Mr. Joseph Wetzler, through
+whose hands all the proofs have passed.</p>
+
+<p><span class="smcap">December, 1893.</span></p>
+
+<p style='text-align: right'>T. C. M.</p>
+
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_vii" id="Page_vii">[Pg vii]</a></span></p>
+<h2>CONTENTS.</h2>
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="80%">
+<tr><td colspan='2' class='center2'><a href="#PART_I">PART I.</a><br />
+<small>POLYPHASE CURRENTS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER I.</td></tr>
+<tr><td align='left'><span class="smcap">Biographical and Introductory.</span></td><td align='right'><a href="#Page_3">3</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER II.</td></tr>
+<tr><td align='left'><span class="smcap">A New System of Alternating Current Motors and Transformers.</span></td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER III.</td></tr>
+<tr><td align='left'><span class="smcap">The Tesla Rotating Magnetic Field.&mdash;Motors with Closed Conductors.&mdash;Synchronizing Motors.&mdash;Rotating Field Transformers.</span></td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER IV.</td></tr>
+<tr><td align='left'><span class="smcap">Modifications and Expansions of the Tesla Polyphase Systems.</span></td><td align='right'><a href="#Page_26">26</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER V.</td></tr>
+<tr><td align='left'><span class="smcap">Utilizing Familiar Types of Generators of the Continuous Current Type.</span></td><td align='right'><a href="#Page_31">31</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VI.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Desired Speed of Motor Or Generator.</span></td><td align='right'><a href="#Page_36">36</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_viii" id="Page_viii">[Pg viii]</a></span><span class="smcap">Regulator for Rotary Current Motors.</span></td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER VIII.</td></tr>
+<tr><td align='left'><span class="smcap">Single Circuit, Self-starting Synchronizing Motors.</span></td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER IX.</td></tr>
+<tr><td align='left'><span class="smcap">Change from Double Current to Single Current Motors.</span></td><td align='right'><a href="#Page_56">56</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER X.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with "Current Lag" Artificially Secured.</span></td><td align='right'><a href="#Page_58">58</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XI.</td></tr>
+<tr><td align='left'><span class="smcap">Another Method of Transformation from a Torque to a Synchronizing Motor.</span></td><td align='right'><a href="#Page_62">62</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XII.</td></tr>
+<tr><td align='left'><span class="smcap">"Magnetic Lag" Motor.</span></td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIII.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Difference of Phase by Magnetic Shielding.</span></td><td align='right'><a href="#Page_71">71</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIV.</td></tr>
+<tr><td align='left'><span class="smcap">Type of Tesla Single-Phase Motor.</span></td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XV.</td></tr>
+<tr><td align='left'><span class="smcap">Motors With Circuits of Different Resistance.</span></td><td align='right'><a href="#Page_79">79</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVI.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with Equal Magnetic Energies in Field and Armature.</span></td><td align='right'><a href="#Page_81">81</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVII.</td></tr>
+<tr><td align='left'><span class="smcap">Motors with Coinciding Maxima of Magnetic Effect in Armature and Field.</span></td><td align='right'><a href="#Page_83">83</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XVIII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_ix" id="Page_ix">[Pg ix]</a></span><span class="smcap">Motor Based on the Difference of Phase in the Magnetization of the Inner and Outer Parts of an Iron Core.</span></td><td align='right'><a href="#Page_88">88</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XIX.</td></tr>
+<tr><td align='left'><span class="smcap">Another Type of Tesla Induction Motor.</span></td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XX.</td></tr>
+<tr><td align='left'><span class="smcap">Combinations of Synchronizing Motor and Torque Motor.</span></td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXI.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with a Condenser in the Armature Circuit.</span></td><td align='right'><a href="#Page_101">101</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXII.</td></tr>
+<tr><td align='left'><span class="smcap">Motor with Condenser in One of the Field Circuits.</span></td><td align='right'><a href="#Page_106">106</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIII.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Polyphase Transformer.</span></td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIV.</td></tr>
+<tr><td align='left'><span class="smcap">A Constant Current Transformer with Magnetic Shield Between Coils of Primary and Secondary.</span></td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_II">PART II.</a><br />
+<small>THE TESLA EFFECTS WITH HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXV.</td></tr>
+<tr><td align='left'><span class="smcap">Introductory.&mdash;The Scope of The Tesla Lectures.</span></td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVI.</td></tr>
+<tr><td align='left'><span class="smcap">The New York Lecture. Experiments with Alternate
+Currents of Very High Frequency, and Their
+Application to Methods of Artificial Illumination,
+May 20, 1891.</span></td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVII.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_x" id="Page_x">[Pg x]</a></span><span class="smcap">The London Lecture. Experiments with Alternate
+Currents of High Potential and High Frequency,
+February 3, 1892.</span></td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXVIII.</td></tr>
+<tr><td align='left'><span class="smcap">The Philadelphia and St. Louis Lecture. On Light
+and Other High Frequency Phenomena, February
+and March, 1893.</span></td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXIX.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Alternating Current Generators for High
+Frequency.</span></td><td align='right'><a href="#Page_374">374</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXX.</td></tr>
+<tr><td align='left'><span class="smcap">Alternate Current Electrostatic Induction Apparatus.</span></td><td align='right'><a href="#Page_392">392</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXI.</td></tr>
+<tr><td align='left'><span class="smcap">"Massage" with Currents of High Frequency.</span></td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXII.</td></tr>
+<tr><td align='left'><span class="smcap">Electric Discharge in Vacuum Tubes.</span></td><td align='right'><a href="#Page_396">396</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_III">PART III.</a><br />
+<small>MISCELLANEOUS INVENTIONS AND WRITINGS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIII.</td></tr>
+<tr><td align='left'><span class="smcap">Method of Obtaining Direct from Alternating Currents.</span></td><td align='right'><a href="#Page_409">409</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIV.</td></tr>
+<tr><td align='left'><span class="smcap">Condensers with Plates in Oil.</span></td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXV.</td></tr>
+<tr><td align='left'><span class="smcap">Electrolytic Registering Meter.</span></td><td align='right'><a href="#Page_420">420</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVI.</td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_xi" id="Page_xi">[Pg xi]</a></span><span class="smcap">Thermo-Magnetic Motors and Pyro-Magnetic Generators.</span></td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVII.</td></tr>
+<tr><td align='left'><span class="smcap">Anti-sparking Dynamo Brush and Commutator.</span></td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXVIII.</td></tr>
+<tr><td align='left'><span class="smcap">Auxiliary Brush Regulation of Direct Current Dynamos.</span></td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XXXIX.</td></tr>
+<tr><td align='left'><span class="smcap">Improvement in Dynamo and Motor Construction.</span></td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XL.</td></tr>
+<tr><td align='left'><span class="smcap">Tesla Direct Current Arc Lighting System.</span></td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLI.</td></tr>
+<tr><td align='left'><span class="smcap">Improvement in Unipolar Generators.</span></td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td colspan='2' class='center2'><a href="#PART_IV">PART IV.</a><br />
+<small>APPENDIX: EARLY PHASE MOTORS AND THE TESLA OSCILLATORS.</small></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLII.</td></tr>
+<tr><td align='left'><span class="smcap">Mr. Tesla's Personal Exhibit at the World's Fair.</span></td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td colspan='2' class='center1'>CHAPTER XLIII.</td></tr>
+<tr><td align='left'><span class="smcap">The Tesla Mechanical and Electrical Oscillators.</span></td><td align='right'><a href="#Page_486">486</a></td></tr>
+</table></div>
+
+<p><span class='pagenum'><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h1><small><a name="PART_I" id="PART_I"></a>PART I.</small><br /><br />
+
+POLYPHASE CURRENTS.</h1>
+<p><span class='pagenum'><a name="Page_2" id="Page_2">[Pg 2]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_3" id="Page_3">[Pg 3]</a></span></p>
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I.</h2>
+
+<h3><span class="smcap">Biographical and Introductory.</span></h3>
+
+
+<p>As an introduction to the record contained in this volume
+of Mr. Tesla's investigations and discoveries, a few words of a
+biographical nature will, it is deemed, not be out of place, nor
+other than welcome.</p>
+
+<p>Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland
+region of Austro-Hungary, of the Serbian race, which has maintained
+against Turkey and all comers so unceasing a struggle for
+freedom. His family is an old and representative one among
+these Switzers of Eastern Europe, and his father was an eloquent
+clergyman in the Greek Church. An uncle is to-day Metropolitan
+in Bosnia. His mother was a woman of inherited ingenuity,
+and delighted not only in skilful work of the ordinary household
+character, but in the construction of such mechanical appliances
+as looms and churns and other machinery required in a rural
+community. Nikola was educated at Gospich in the public
+school for four years, and then spent three years in the Real
+Schule. He was then sent to Carstatt, Croatia, where he continued
+his studies for three years in the Higher Real Schule.
+There for the first time he saw a steam locomotive. He graduated
+in 1873, and, surviving an attack of cholera, devoted himself
+to experimentation, especially in electricity and magnetism.
+His father would have had him maintain the family tradition by
+entering the Church, but native genius was too strong, and he
+was allowed to enter the Polytechnic School at Gratz, to finish
+his studies, and with the object of becoming a professor of mathematics
+and physics. One of the machines there experimented
+with was a Gramme dynamo, used as a motor. Despite his instructor's
+perfect demonstration of the fact that it was impossible
+to operate a dynamo without commutator or brushes, Mr. Tesla
+could not be convinced that such accessories were necessary or
+desirable. He had already seen with quick intuition that a way
+could be found to dispense with them; and from that time he may<span class='pagenum'><a name="Page_4" id="Page_4">[Pg 4]</a></span>
+be said to have begun work on the ideas that fructified ultimately
+in his rotating field motors.</p>
+
+<p>In the second year of his Gratz course, Mr. Tesla gave up the
+notion of becoming a teacher, and took up the engineering curriculum.
+His studies ended, he returned home in time to see his
+father die, and then went to Prague and Buda-Pesth to study
+languages, with the object of qualifying himself broadly for the
+practice of the engineering profession. For a short time he
+served as an assistant in the Government Telegraph Engineering
+Department, and then became associated with M. Puskas, a
+personal and family friend, and other exploiters of the telephone
+in Hungary. He made a number of telephonic inventions, but
+found his opportunities of benefiting by them limited in various
+ways. To gain a wider field of action, he pushed on to Paris
+and there secured employment as an electrical engineer with one
+of the large companies in the new industry of electric lighting.</p>
+
+<p>It was during this period, and as early as 1882, that he began
+serious and continued efforts to embody the rotating field principle
+in operative apparatus. He was enthusiastic about it; believed
+it to mark a new departure in the electrical arts, and could
+think of nothing else. In fact, but for the solicitations of a few
+friends in commercial circles who urged him to form a company
+to exploit the invention, Mr. Tesla, then a youth of little worldly
+experience, would have sought an immediate opportunity to publish
+his ideas, believing them to be worthy of note as a novel and
+radical advance in electrical theory as well as destined to have
+a profound influence on all dynamo electric machinery.</p>
+
+<p>At last he determined that it would be best to try his fortunes
+in America. In France he had met many Americans, and in
+contact with them learned the desirability of turning every new
+idea in electricity to practical use. He learned also of the ready
+encouragement given in the United States to any inventor who
+could attain some new and valuable result. The resolution was
+formed with characteristic quickness, and abandoning all his
+prospects in Europe, he at once set his face westward.</p>
+
+<p>Arrived in the United States, Mr. Tesla took off his coat the
+day he arrived, in the Edison Works. That place had been a
+goal of his ambition, and one can readily imagine the benefit and
+stimulus derived from association with Mr. Edison, for whom
+Mr. Tesla has always had the strongest admiration. It was impossible,
+however, that, with his own ideas to carry out, and his<span class='pagenum'><a name="Page_5" id="Page_5">[Pg 5]</a></span>
+own inventions to develop, Mr. Tesla could long remain in even
+the most delightful employ; and, his work now attracting attention,
+he left the Edison ranks to join a company intended to
+make and sell an arc lighting system based on some of his inventions
+in that branch of the art. With unceasing diligence he
+brought the system to perfection, and saw it placed on the market.
+But the thing which most occupied his time and thoughts, however,
+all through this period, was his old discovery of the rotating
+field principle for alternating current work, and the application
+of it in motors that have now become known the world over.</p>
+
+<p>Strong as his convictions on the subject then were, it is a fact
+that he stood very much alone, for the alternating current had
+no well recognized place. Few electrical engineers had ever
+used it, and the majority were entirely unfamiliar with its value,
+or even its essential features. Even Mr. Tesla himself did not,
+until after protracted effort and experimentation, learn how to
+construct alternating current apparatus of fair efficiency. But
+that he had accomplished his purpose was shown by the tests of
+Prof. Anthony, made in the of winter 1887-8, when Tesla motors
+in the hands of that distinguished expert gave an efficiency equal
+to that of direct current motors. Nothing now stood in the way
+of the commercial development and introduction of such motors,
+except that they had to be constructed with a view to operating
+on the circuits then existing, which in this country were all of
+high frequency.</p>
+
+<p>The first full publication of his work in this direction&mdash;outside
+his patents&mdash;was a paper read before the American Institute of
+Electrical Engineers in New York, in May, 1888 (read at the
+suggestion of Prof. Anthony and the present writer), when he
+exhibited motors that had been in operation long previous, and
+with which his belief that brushes and commutators could be
+dispensed with, was triumphantly proved to be correct. The
+section of this volume devoted to Mr. Tesla's inventions in the
+utilization of polyphase currents will show how thoroughly from
+the outset he had mastered the fundamental idea and applied it
+in the greatest variety of ways.</p>
+
+<p>Having noted for years the many advantages obtainable with
+alternating currents, Mr. Tesla was naturally led on to experiment
+with them at higher potentials and higher frequencies than
+were common or approved of. Ever pressing forward to determine
+in even the slightest degree the outlines of the unknown, he<span class='pagenum'><a name="Page_6" id="Page_6">[Pg 6]</a></span>
+was rewarded very quickly in this field with results of the most
+surprising nature. A slight acquaintance with some of these
+experiments led the compiler of this volume to urge Mr. Tesla
+to repeat them before the American Institute of Electrical Engineers.
+This was done in May, 1891, in a lecture that marked,
+beyond question, a distinct departure in electrical theory and
+practice, and all the results of which have not yet made themselves
+fully apparent. The New York lecture, and its successors,
+two in number, are also included in this volume, with a
+few supplementary notes.</p>
+
+<p>Mr. Tesla's work ranges far beyond the vast departments of
+polyphase currents and high potential lighting. The "Miscellaneous"
+section of this volume includes a great many other inventions
+in arc lighting, transformers, pyro-magnetic generators,
+thermo-magnetic motors, third-brush regulation, improvements
+in dynamos, new forms of incandescent lamps, electrical meters,
+condensers, unipolar dynamos, the conversion of alternating into
+direct currents, etc. It is needless to say that at this moment
+Mr. Tesla is engaged on a number of interesting ideas and inventions,
+to be made public in due course. The present volume
+deals simply with his work accomplished to date.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_7" id="Page_7">[Pg 7]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II.</h2>
+
+<h3><span class="smcap">A New System of Alternating Current Motors and
+Transformers.</span></h3>
+
+
+<p>The present section of this volume deals with polyphase currents,
+and the inventions by Mr. Tesla, made known thus far, in
+which he has embodied one feature or another of the broad
+principle of rotating field poles or <i>resultant attraction</i> exerted on
+the armature. It is needless to remind electricians of the great
+interest aroused by the first enunciation of the rotating field
+principle, or to dwell upon the importance of the advance from
+a single alternating current, to methods and apparatus which deal
+with more than one. Simply prefacing the consideration here
+attempted of the subject, with the remark that in nowise is the
+object of this volume of a polemic or controversial nature, it
+may be pointed out that Mr. Tesla's work has not at all been
+fully understood or realized up to date. To many readers, it is
+believed, the analysis of what he has done in this department
+will be a revelation, while it will at the same time illustrate the
+beautiful flexibility and range of the principles involved. It
+will be seen that, as just suggested, Mr. Tesla did not stop short
+at a mere rotating field, but dealt broadly with the shifting of
+the resultant attraction of the magnets. It will be seen that he
+went on to evolve the "multiphase" system with many ramifications
+and turns; that he showed the broad idea of motors employing
+currents of differing phase in the armature with direct
+currents in the field; that he first described and worked out the
+idea of an armature with a body of iron and coils closed upon
+themselves; that he worked out both synchronizing and torque
+motors; that he explained and illustrated how machines of ordinary
+construction might be adapted to his system; that he employed
+condensers in field and armature circuits, and went to the
+bottom of the fundamental principles, testing, approving or rejecting,
+it would appear, every detail that inventive ingenuity could
+hit upon.<span class='pagenum'><a name="Page_8" id="Page_8">[Pg 8]</a></span></p>
+
+<p>Now that opinion is turning so emphatically in favor of lower
+frequencies, it deserves special note that Mr. Tesla early recognized
+the importance of the low frequency feature in motor
+work. In fact his first motors exhibited publicly&mdash;and which, as
+Prof. Anthony showed in his tests in the winter of 1887-8, were
+the equal of direct current motors in efficiency, output and starting
+torque&mdash;were of the low frequency type. The necessity
+arising, however, to utilize these motors in connection with the
+existing high frequency circuits, our survey reveals in an interesting
+manner Mr. Tesla's fertility of resource in this direction.
+But that, after exhausting all the possibilities of this field, Mr.
+Tesla returns to low frequencies, and insists on the superiority of
+his polyphase system in alternating current distribution, need not
+at all surprise us, in view of the strength of his convictions, so
+often expressed, on this subject. This is, indeed, significant, and
+may be regarded as indicative of the probable development next
+to be witnessed.</p>
+
+<p>Incidental reference has been made to the efficiency of rotating
+field motors, a matter of much importance, though it is not the
+intention to dwell upon it here. Prof. Anthony in his remarks
+before the American Institute of Electrical Engineers, in May,
+1888, on the two small Tesla motors then shown, which he had
+tested, stated that one gave an efficiency of about 50 per cent.
+and the other a little over sixty per cent. In 1889, some tests
+were reported from Pittsburgh, made by Mr. Tesla and Mr.
+Albert Schmid, on motors up to 10 <span class="smcap">h.&nbsp;p.</span> and weighing about
+850 pounds. These machines showed an efficiency of nearly 90
+per cent. With some larger motors it was then found practicable
+to obtain an efficiency, with the three wire system, up to as
+high as 94 and 95 per cent. These interesting figures, which, of
+course, might be supplemented by others more elaborate and of
+later date, are cited to show that the efficiency of the system has
+not had to wait until the present late day for any demonstration
+of its commercial usefulness. An invention is none the less beautiful
+because it may lack utility, but it must be a pleasure to any
+inventor to know that the ideas he is advancing are fraught with
+substantial benefits to the public.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_9" id="Page_9">[Pg 9]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III.</h2>
+
+<h3><span class="smcap">The Tesla Rotating Magnetic Field.</span>&mdash;<span class="smcap">Motors With Closed
+Conductors.</span>&mdash;<span class="smcap">Synchronizing Motors.</span>&mdash;<span class="smcap">Rotating Field
+Transformers.</span></h3>
+
+
+<p>The best description that can be given of what he attempted,
+and succeeded in doing, with the rotating magnetic field, is to be
+found in Mr. Tesla's brief paper explanatory of his rotary current,
+polyphase system, read before the American Institute of
+Electrical Engineers, in New York, in May, 1888, under the
+title "A New System of Alternate Current Motors and Transformers."
+As a matter of fact, which a perusal of the paper
+will establish, Mr. Tesla made no attempt in that paper to describe
+all his work. It dealt in reality with the few topics enumerated
+in the caption of this chapter. Mr. Tesla's reticence
+was no doubt due largely to the fact that his action was governed
+by the wishes of others with whom he was associated, but
+it may be worth mention that the compiler of this volume&mdash;who
+had seen the motors running, and who was then chairman of the
+Institute Committee on Papers and Meetings&mdash;had great difficulty
+in inducing Mr. Tesla to give the Institute any paper at all.
+Mr. Tesla was overworked and ill, and manifested the greatest
+reluctance to an exhibition of his motors, but his objections were
+at last overcome. The paper was written the night previous to
+the meeting, in pencil, very hastily, and under the pressure
+just mentioned.</p>
+
+<p>In this paper casual reference was made to two special forms
+of motors not within the group to be considered. These two
+forms were: 1. A motor with one of its circuits in series with a
+transformer, and the other in the secondary of the transformer.
+2. A motor having its armature circuit connected to the generator,
+and the field coils closed upon themselves. The paper in
+its essence is as follows, dealing with a few leading features of
+the Tesla system, namely, the rotating magnetic field, motors<span class='pagenum'><a name="Page_10" id="Page_10">[Pg 10]</a></span>
+with closed conductors, synchronizing motors, and rotating field
+transformers:&mdash;</p>
+
+<p>The subject which I now have the pleasure of bringing to
+your notice is a novel system of electric distribution and transmission
+of power by means of alternate currents, affording peculiar
+advantages, particularly in the way of motors, which I am
+confident will at once establish the superior adaptability of these
+currents to the transmission of power and will show that many
+results heretofore unattainable can be reached by their use; results
+which are very much desired in the practical operation of
+such systems, and which cannot be accomplished by means of
+continuous currents.</p>
+
+<p>Before going into a detailed description of this system, I think
+it necessary to make a few remarks with reference to certain conditions
+existing in continuous current generators and motors,
+which, although generally known, are frequently disregarded.</p>
+
+<p>In our dynamo machines, it is well known, we generate alternate
+currents which we direct by means of a commutator, a complicated
+device and, it may be justly said, the source of most of
+the troubles experienced in the operation of the machines. Now,
+the currents so directed cannot be utilized in the motor, but
+they must&mdash;again by means of a similar unreliable device&mdash;be
+reconverted into their original state of alternate currents.
+The function of the commutator is entirely external, and in no
+way does it affect the internal working of the machines. In
+reality, therefore, all machines are alternate current machines,
+the currents appearing as continuous only in the external circuit
+during their transit from generator to motor. In view simply of
+this fact, alternate currents would commend themselves as a more
+direct application of electrical energy, and the employment of
+continuous currents would only be justified if we had dynamos
+which would primarily generate, and motors which would be
+directly actuated by, such currents.</p>
+
+<p>But the operation of the commutator on a motor is twofold;
+first, it reverses the currents through the motor, and secondly,
+it effects automatically, a progressive shifting of the poles of one
+of its magnetic constituents. Assuming, therefore, that both of
+the useless operations in the systems, that is to say, the directing
+of the alternate currents on the generator and reversing the direct
+currents on the motor, be eliminated, it would still be necessary,
+in order to cause a rotation of the motor, to produce a progressive<span class='pagenum'><a name="Page_11" id="Page_11">[Pg 11]</a></span>
+shifting of the poles of one of its elements, and the question
+presented itself&mdash;How to perform this operation by the direct
+action of alternate currents? I will now proceed to show how
+this result was accomplished.</p>
+
+<div class="figcenter" style="width: 640px;">
+<div class="figleft" style="width: 355px;">
+<img src="images/fig1.jpg" width="355" height="270" alt="Fig. 1." title="" />
+<span class="caption">Fig. 1.</span>
+</div>
+<div class="figright" style="width: 220px;">
+<img src="images/fig1a.jpg" width="220" height="270" alt="Fig. 1a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 1a.</span>
+</div>
+</div>
+
+
+<p>In the first experiment a drum-armature was provided with
+two coils at right angles to each other, and the ends of these coils
+were connected to two pairs of insulated contact-rings as usual.
+A ring was then made of thin insulated plates of sheet-iron and
+wound with four coils, each two opposite coils being connected
+together so as to produce free poles on diametrically opposite
+sides of the ring. The remaining free ends of the coils were then
+connected to the contact-rings of the generator armature so as
+to form two independent circuits, as indicated in Fig. 9. It
+may now be seen what results were secured in this combination,
+and with this view I would refer to the diagrams, Figs. 1 to 8<i>a</i>.
+The field of the generator being independently excited, the rotation
+of the armature sets up currents in the coils <small>C</small> <small>C<sub>1</sub></small>, varying in
+strength and direction in the well-known manner. In the position
+shown in Fig. 1, the current in coil <small>C</small> is nil, while coil <small>C<sub>1</sub></small> is
+traversed by its maximum current, and the connections may be
+such that the ring is magnetized by the coils <i>c</i><sub>1</sub> <i>c</i><sub>1</sub>, as indicated by
+the letters <small>N</small> <small>S</small> in Fig. 1<i>a</i>, the magnetizing effect of the coils
+<span class='pagenum'><a name="Page_12" id="Page_12">[Pg 12]</a></span><i>c</i> <i>c</i> being nil, since these coils are included in the circuit of
+coil <small>C</small>.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 430px;">
+<img src="images/fig2.jpg" width="430" height="280" alt="Fig. 2." title="" />
+<span class="caption">Fig. 2.</span>
+</div>
+<div class="figright" style="width: 315px;">
+<img src="images/fig2a.jpg" width="315" height="280" alt="Fig. 2a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 2a.</span>
+</div>
+</div>
+
+<p>In Fig. 2, the armature coils are shown in a more advanced
+position, one-eighth of one revolution being completed. Fig.
+2<i>a</i> illustrates the corresponding magnetic condition of the ring.
+At this moment the coil <small>C<sub>1</sub></small> generates a current of the same direction
+as previously, but weaker, producing the poles <i>n</i><sub>1</sub> <i>s</i><sub>1</sub> upon
+the ring; the coil <small>C</small> also generates a current of the same direction,
+and the connections may be such that the coils <i>c</i> <i>c</i> produce
+the poles <i>n</i> <i>s</i>, as shown in Fig. 2<i>a</i>. The resulting polarity is
+indicated by the letters <small>N</small> <small>S</small>, and it will be observed that the
+poles of the ring have been shifted one-eighth of the periphery
+of the same.</p>
+
+
+<div class="figcenter" style="width: 720px;">
+<div class="figleft" style="width: 376px;">
+<img src="images/fig3.jpg" width="376" height="250" alt="Fig. 3." title="" />
+<span class="caption">Fig. 3.</span>
+</div>
+<div class="figright" style="width: 310px;">
+<img src="images/fig3a.jpg" width="310" height="250" alt="Fig. 3a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 3a.</span>
+</div>
+</div>
+
+
+<p>In Fig. 3 the armature has completed one quarter of one
+revolution. In this phase the current in coil <small>C</small> is a maximum, and
+of such direction as to produce the poles <small>N</small> <small>S</small> in Fig. 3<i>a</i>, whereas
+the current in coil <small>C<sub>1</sub></small> is nil, this coil being at its neutral position.
+The poles <small>N</small> <small>S</small> in Fig. 3<i>a</i> are thus shifted one quarter of the
+circumference of the ring.</p>
+
+<div class="figcenter" style="width: 680px;">
+<div class="figleft" style="width: 355px;">
+<img src="images/fig4.jpg" width="355" height="260" alt="Fig. 4." title="" />
+<span class="caption">Fig. 4.</span>
+</div>
+<div class="figright" style="width: 265px;">
+<img src="images/fig4a.jpg" width="265" height="260" alt="Fig. 4a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 4a.</span>
+</div>
+</div>
+
+<p>Fig. 4 shows the coils <small>C</small> <small>C</small> in a still more advanced position,
+the armature having completed three-eighths of one revolution.
+At that moment the coil <small>C</small> still generates a current of the same
+direction as before, but of less strength, producing the compar<span class='pagenum'><a name="Page_13" id="Page_13">[Pg 13]</a></span>atively
+weaker poles <i>n s</i> in Fig. 4<i>a</i>. The current in the coil <small>C<sub>1</sub></small>
+is of the same strength, but opposite direction. Its effect is,
+therefore, to produce upon the ring the poles <i>n</i><sub>1</sub> <i>s</i><sub>1</sub>, as indicated,
+and a polarity, <small>N S</small>, results, the poles now being shifted three-eighths
+of the periphery of the ring.</p>
+
+<div class="figcenter" style="width: 640px;">
+<div class="figleft" style="width: 345px;">
+<img src="images/fig5.jpg" width="345" height="270" alt="Fig. 5." title="" />
+<span class="caption">Fig. 5.</span>
+</div>
+<div class="figright" style="width: 235px;">
+<img src="images/fig5a.jpg" width="235" height="270" alt="Fig. 5a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 5a.</span>
+</div>
+</div>
+
+<p>In Fig. 5 one half of one revolution of the armature is completed,
+and the resulting magnetic condition of the ring is indicated
+in Fig. 5<i>a</i>. Now the current in coil <small>C</small> is nil, while the coil
+<small>C<sub>1</sub></small> yields its maximum current, which is of the same direction as
+previously; the magnetizing effect is, therefore, due to the coils,
+<i>c</i><sub>1</sub> <i>c</i><sub>1</sub> alone, and, referring to Fig. 5<i>a</i>, it will be observed that
+the poles <small>N S</small> are shifted one half of the circumference of the
+ring. During the next half revolution the operations are repeated,
+as represented in the Figs. 6 to 8<i>a</i>.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 435px;">
+<img src="images/fig6.jpg" width="435" height="270" alt="Fig. 6." title="" />
+<span class="caption">Fig. 6.</span>
+</div>
+<div class="figright" style="width: 265px;">
+<img src="images/fig6a.jpg" width="265" height="270" alt="Fig. 6a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 6a.</span>
+</div>
+</div>
+
+<p>A reference to the diagrams will make it clear that during one
+revolution of the armature the poles of the ring are shifted once
+around its periphery, and, each revolution producing like effects,
+a rapid whirling of the poles in harmony with the rotation of the
+armature is the result. If the connections of either one of the
+circuits in the ring are reversed, the shifting of the poles is made
+to progress in the opposite direction, but the operation is identi<span class='pagenum'><a name="Page_14" id="Page_14">[Pg 14]</a></span>cally
+the same. Instead of using four wires, with like result,
+three wires may be used, one forming a common return for both
+circuits.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 410px;">
+<img src="images/fig7.jpg" width="410" height="270" alt="Fig. 7." title="" />
+<span class="caption">Fig. 7.</span>
+</div>
+<div class="figright" style="width: 330px;">
+<img src="images/fig7a.jpg" width="330" height="270" alt="Fig. 7a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 7a.</span>
+</div>
+</div>
+
+<p>This rotation or whirling of the poles manifests itself in a series
+of curious phenomena. If a delicately pivoted disc of steel or
+other magnetic metal is approached to the ring it is set in rapid
+rotation, the direction of rotation varying with the position of
+the disc. For instance, noting the direction outside of the ring
+it will be found that inside the ring it turns in an opposite direction,
+while it is unaffected if placed in a position symmetrical to
+the ring. This is easily explained. Each time that a pole approaches,
+it induces an opposite pole in the nearest point on the
+disc, and an attraction is produced upon that point; owing to this,
+as the pole is shifted further away from the disc a tangential pull
+is exerted upon the same, and the action being constantly repeated,
+a more or less rapid rotation of the disc is the result. As the
+pull is exerted mainly upon that part which is nearest to the
+ring, the rotation outside and inside, or right and left, respectively,
+is in opposite directions, Fig. 9. When placed symmetrically
+to the ring, the pull on the opposite sides of the disc being equal,
+no rotation results. The action is based on the magnetic inertia
+of iron; for this reason a disc of hard steel is much more affected
+than a disc of soft iron, the latter being capable of very
+rapid variations of magnetism. Such a disc has proved to be a
+very useful instrument in all these investigations, as it has enabled
+me to detect any irregularity in the action. A curious effect
+is also produced upon iron filings. By placing some upon a
+paper and holding them externally quite close to the ring, they
+are set in a vibrating motion, remaining in the same place, although
+the paper may be moved back and forth; but in lifting the paper
+to a certain height which seems to be dependent on the intensity
+of the poles and the speed of rotation, they are thrown away in<span class='pagenum'><a name="Page_15" id="Page_15">[Pg 15]</a></span>
+a direction always opposite to the supposed movement of the
+poles. If a paper with filings is put flat upon the ring and the
+current turned on suddenly, the existence of a magnetic whirl
+may easily be observed.</p>
+
+<p>To demonstrate the complete analogy between the ring and a
+revolving magnet, a strongly energized electro-magnet was rotated
+by mechanical power, and phenomena identical in every particular
+to those mentioned above were observed.</p>
+
+<p>Obviously, the rotation of the poles produces corresponding
+inductive effects and may be utilized to generate currents in a
+closed conductor placed within the influence of the poles. For
+this purpose it is convenient to wind a ring with two sets of
+superimposed coils forming respectively the primary and secondary
+circuits, as shown in Fig. 10. In order to secure the most
+economical results the magnetic circuit should be completely
+closed, and with this object in view the construction may be
+modified at will.</p>
+
+<div class="figcenter" style="width: 720px;">
+<div class="figleft" style="width: 385px;">
+<img src="images/fig8.jpg" width="385" height="270" alt="Fig. 8." title="" />
+<span class="caption">Fig. 8.</span>
+</div>
+<div class="figright" style="width: 270px;">
+<img src="images/fig8a.jpg" width="270" height="270" alt="Fig. 8a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 8a.</span>
+</div>
+</div>
+
+<p>The inductive effect exerted upon the secondary coils will be
+mainly due to the shifting or movement of the magnetic action;
+but there may also be currents set up in the circuits in consequence
+of the variations in the intensity of the poles. However,
+by properly designing the generator and determining the magnetizing
+effect of the primary coils, the latter element may be made
+to disappear. The intensity of the poles being maintained constant,
+the action of the apparatus will be perfect, and the same
+result will be secured as though the shifting were effected by
+means of a commutator with an infinite number of bars. In such
+case the theoretical relation between the energizing effect of each
+set of primary coils and their resultant magnetizing effect may
+be expressed by the equation of a circle having its centre coinciding
+with that of an orthogonal system of axes, and in which
+the radius represents the resultant and the co-ordinates both<span class='pagenum'><a name="Page_16" id="Page_16">[Pg 16]</a></span>
+of its components. These are then respectively the sine and
+cosine of the angle &#945; between the radius and one of the axes
+(<i>O&nbsp;X</i>). Referring to Fig. 11, we have <i>r</i><sup>2</sup> = <i>x</i><sup>2</sup> + <i>y</i><sup>2</sup>; where
+<i>x</i> = <i>r</i> cos &#945;, and <i>y</i> = <i>r</i> sin &#945;.</p>
+
+<p>Assuming the magnetizing effect of each set of coils in the
+transformer to be proportional to the current&mdash;which may be
+admitted for weak degrees of magnetization&mdash;then <i>x</i> = <i>Kc</i> and
+<i>y</i> = <i>Kc</i><sup>1</sup>, where <i>K</i> is a constant and <i>c</i> and <i>c</i><sup>1</sup> the current in both
+sets of coils respectively. Supposing, further, the field of the
+generator to be uniform, we have for constant speed <i>c</i><sup>1</sup> = <i>K</i><sup>1</sup> sin &#945;
+and <i>c</i> = <i>K</i><sup>1</sup> sin (90&deg; + &#945;) = <i>K</i><sup>1</sup> cos &#945;, where <i>K</i><sup>1</sup> is a constant.
+See Fig. 12.</p>
+
+<p>Therefore,</p>
+
+<p class="blockquot">
+<i>x</i> = <i>K c</i> = <i>K K</i><sup>1</sup> cos &#945;;<br />
+<i>y</i> = <i>K c</i><sup>1</sup> = <i>K K</i><sup>1</sup> sin &#945;; and<br />
+<i>K K</i><sup>1</sup> = <i>r</i>.
+</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_030.jpg" width="640" height="335" alt="Fig. 9." title="" />
+<span class="caption">Fig. 9.</span>
+</div>
+
+<p>That is, for a uniform field the disposition of the two coils at
+right angles will secure the theoretical result, and the intensity
+of the shifting poles will be constant. But from <i>r</i><sup>2</sup> = <i>x</i><sup>2</sup> + <i>y</i><sup>2</sup> it
+follows that for <i>y</i> = 0, <i>r</i> = <i>x</i>; it follows that the joint magnetizing
+effect of both sets of coils should be equal to the effect of
+one set when at its maximum action. In transformers and in a
+certain class of motors the fluctuation of the poles is not of great
+importance, but in another class of these motors it is desirable to
+obtain the theoretical result.</p>
+
+<p>In applying this principle to the construction of motors, two
+typical forms of motor have been developed. First, a form having
+a comparatively small rotary effort at the start but maintaining
+a perfectly uniform speed at all loads, which motor has been
+termed synchronous. Second, a form possessing a great rotary
+effort at the start, the speed being dependent on the load.<span class='pagenum'><a name="Page_17" id="Page_17">[Pg 17]</a></span></p>
+
+<p>These motors may be operated in three different ways: 1. By
+the alternate currents of the source only. 2. By a combined action
+of these and of induced currents. 3. By the joint action of
+alternate and continuous currents.</p>
+
+<div class="figcenter" style="width: 524px;">
+<img src="images/oi_031.jpg" width="524" height="480" alt="Fig. 10." title="" />
+<span class="caption">Fig. 10.</span>
+</div>
+
+<p>The simplest form of a synchronous motor is obtained by winding
+a laminated ring provided with pole projections with four
+coils, and connecting the same in the manner before indicated.
+An iron disc having a segment cut away on each side may be used
+as an armature. Such a motor is shown in Fig. 9. The disc
+being arranged to rotate freely within the ring in close proximity
+to the projections, it is evident that as the poles are shifted it
+will, owing to its tendency to place itself in such a position as to
+embrace the greatest number of the lines of force, closely follow
+the movement of the poles, and its motion will be synchronous
+with that of the armature of the generator; that is, in the peculiar
+disposition shown in Fig. 9, in which the armature produces by
+one revolution two current impulses in each of the circuits. It
+is evident that if, by one revolution of the armature, a greater
+number of impulses is produced, the speed of the motor will be
+correspondingly increased. Considering that the attraction exerted
+upon the disc is greatest when the same is in close proximity
+to the poles, it follows that such a motor will maintain exactly
+the same speed at all loads within the limits of its capacity.</p>
+
+<p>To facilitate the starting, the disc may be provided with a coil
+closed upon itself. The advantage secured by such a coil is evident.
+On the start the currents set up in the coil strongly ener<span class='pagenum'><a name="Page_18" id="Page_18">[Pg 18]</a></span>gize
+the disc and increase the attraction exerted upon the same by
+the ring, and currents being generated in the coil as long as the
+speed of the armature is inferior to that of the poles, considerable
+work may be performed by such a motor even if the speed
+be below normal. The intensity of the poles being constant, no
+currents will be generated in the coil when the motor is turning
+at its normal speed.</p>
+
+<p>Instead of closing the coil upon itself, its ends may be connected
+to two insulated sliding rings, and a continuous current supplied
+to these from a suitable generator. The proper way to start such
+a motor is to close the coil upon itself until the normal speed is
+reached, or nearly so, and then turn on the continuous current.
+If the disc be very strongly energized by a continuous
+current the motor may not be able to start, but if it be weakly
+energized, or generally so that the magnetizing effect of the ring
+is preponderating, it will start and reach the normal speed. Such
+a motor will maintain absolutely the same speed at all loads. It
+has also been found that if the motive power of the generator is
+not excessive, by checking the motor the speed of the generator is
+diminished in synchronism with that of the motor. It is characteristic
+of this form of motor that it cannot be reversed by reversing
+the continuous current through the coil.</p>
+
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 336px;">
+<img src="images/oi_032a.jpg" width="336" height="346" alt="Fig. 11." title="" />
+<span class="caption">Fig. 11.</span>
+</div>
+
+<div class="figright" style="width: 400px;">
+<img src="images/oi_032.jpg" width="400" height="346" alt="Fig. 12." title="" />
+<span class="caption">Fig. 12.</span>
+</div>
+</div>
+
+
+<p>The synchronism of these motors may be demonstrated experimentally
+in a variety of ways. For this purpose it is best to
+employ a motor consisting of a stationary field magnet and an
+armature arranged to rotate within the same, as indicated in
+Fig. 13. In this case the shifting of the poles of the armature
+produces a rotation of the latter in the opposite direction. It
+results therefrom that when the normal speed is reached, the
+poles of the armature assume fixed positions relatively to the<span class='pagenum'><a name="Page_19" id="Page_19">[Pg 19]</a></span>
+field magnet, and the same is magnetized by induction, exhibiting
+a distinct pole on each of the pole-pieces. If a piece of soft iron
+is approached to the field magnet, it will at the start be attracted
+with a rapid vibrating motion produced by the reversals of polarity
+of the magnet, but as the speed of the armature increases, the
+vibrations become less and less frequent and finally entirely cease.
+Then the iron is weakly but permanently attracted, showing that
+synchronism is reached and the field magnet energized by induction.</p>
+
+<p>The disc may also be used for the experiment. If held quite
+close to the armature it will turn as long as the speed of rotation
+of the poles exceeds that of the armature; but when the normal
+speed is reached, or very nearly so, it ceases to rotate and is permanently
+attracted.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_033.jpg" width="640" height="478" alt="Fig. 13." title="" />
+<span class="caption">Fig. 13.</span>
+</div>
+
+
+<p>A crude but illustrative experiment is made with an incandescent
+lamp. Placing the lamp in circuit with the continuous current
+generator and in series with the magnet coil, rapid fluctuations
+are observed in the light in consequence of the induced currents
+set up in the coil at the start; the speed increasing, the
+fluctuations occur at longer intervals, until they entirely disappear,
+showing that the motor has attained its normal speed. A
+telephone receiver affords a most sensitive instrument; when
+connected to any circuit in the motor the synchronism may be
+easily detected on the disappearance of the induced currents.</p>
+
+<p>In motors of the synchronous type it is desirable to maintain<span class='pagenum'><a name="Page_20" id="Page_20">[Pg 20]</a></span>
+the quantity of the shifting magnetism constant, especially if the
+magnets are not properly subdivided.</p>
+
+<p>To obtain a rotary effort in these motors was the subject of
+long thought. In order to secure this result it was necessary to
+make such a disposition that while the poles of one element of
+the motor are shifted by the alternate currents of the source, the
+poles produced upon the other elements should always be maintained
+in the proper relation to the former, irrespective of the
+speed of the motor. Such a condition exists in a continuous
+current motor; but in a synchronous motor, such as described,
+this condition is fulfilled only when the speed is normal.</p>
+
+<div class="figcenter" style="width: 335px;">
+<img src="images/oi_034.jpg" width="335" height="370" alt="Fig. 14." title="" />
+<span class="caption">Fig. 14.</span>
+</div>
+
+<p>The object has been attained by placing within the ring a properly
+subdivided cylindrical iron core wound with several independent
+coils closed upon themselves. Two coils at right angles as
+in Fig. 14, are sufficient, but a greater number may be advantageously
+employed. It results from this disposition that when
+the poles of the ring are shifted, currents are generated in the
+closed armature coils. These currents are the most intense at or
+near the points of the greatest density of the lines of force, and
+their effect is to produce poles upon the armature at right angles
+to those of the ring, at least theoretically so; and since this action
+is entirely independent of the speed&mdash;that is, as far as the location
+of the poles is concerned&mdash;a continuous pull is exerted upon the
+periphery of the armature. In many respects these motors are
+similar to the continuous current motors. If load is put on, the
+speed, and also the resistance of the motor, is diminished and
+more current is made to pass through the energizing coils, thus<span class='pagenum'><a name="Page_21" id="Page_21">[Pg 21]</a></span>
+increasing the effort. Upon the load being taken off, the
+counter-electromotive force increases and less current passes
+through the primary or energizing coils. Without any load the
+speed is very nearly equal to that of the shifting poles of the
+field magnet.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_035.jpg" width="640" height="199" alt="Fig. 15, 16, 17." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 15.</td><td class="caption">Fig. 16.</td><td class="caption">Fig. 17.</td></tr>
+</table>
+</div>
+
+<p>It will be found that the rotary effort in these motors fully
+equals that of the continuous current motors. The effort seems
+to be greatest when both armature and field magnet are without
+any projections; but as in such dispositions the field cannot be
+concentrated, probably the best results will be obtained by leaving
+pole projections on one of the elements only. Generally, it
+may be stated the projections diminish the torque and produce a
+tendency to synchronism.</p>
+
+<p>A characteristic feature of motors of this kind is their property
+of being very rapidly reversed. This follows from the peculiar
+action of the motor. Suppose the armature to be rotating and
+the direction of rotation of the poles to be reversed. The apparatus
+then represents a dynamo machine, the power to drive this
+machine being the momentum stored up in the armature and its
+speed being the sum of the speeds of the armature and the
+poles.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_035-1.jpg" width="640" height="132" alt="Fig. 18, 19, 20, 21." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 18.</td><td class="caption">Fig. 19.</td><td class="caption">Fig. 20.</td><td class="caption">Fig. 21.</td></tr>
+</table>
+</div>
+
+<p>If we now consider that the power to drive such a dynamo
+would be very nearly proportional to the third power of the
+speed, for that reason alone the armature should be quickly reversed.
+But simultaneously with the reversal another element is
+brought into action, namely, as the movement of the poles with
+respect to the armature is reversed, the motor acts like a transformer
+in which the resistance of the secondary circuit would be<span class='pagenum'><a name="Page_22" id="Page_22">[Pg 22]</a></span>
+abnormally diminished by producing in this circuit an additional
+electromotive force. Owing to these causes the reversal is instantaneous.</p>
+
+<p>If it is desirable to secure a constant speed, and at the same
+time a certain effort at the start, this result may be easily attained
+in a variety of ways. For instance, two armatures, one for torque
+and the other for synchronism, may be fastened on the same shaft
+and any desired preponderance may be given to either one, or an
+armature may be wound for rotary effort, but a more or less pronounced
+tendency to synchronism may be given to it by properly
+constructing the iron core; and in many other ways.</p>
+
+<p>As a means of obtaining the required phase of the currents in
+both the circuits, the disposition of the two coils at right angles
+is the simplest, securing the most uniform action; but the phase
+may be obtained in many other ways, varying with the machine
+employed. Any of the dynamos at present in use may be easily
+adapted for this purpose by making connections to proper points
+of the generating coils. In closed circuit armatures, such as used
+in the continuous current systems, it is best to make four derivations
+from equi-distant points or bars of the commutator, and to
+connect the same to four insulated sliding rings on the shaft. In
+this case each of the motor circuits is connected to two diametrically
+opposite bars of the commutator. In such a disposition the
+motor may also be operated at half the potential and on the three-wire
+plan, by connecting the motor circuits in the proper order to
+three of the contact rings.</p>
+
+<p>In multipolar dynamo machines, such as used in the converter
+systems, the phase is conveniently obtained by winding upon the
+armature two series of coils in such a manner that while the coils
+of one set or series are at their maximum production of current,
+the coils of the other will be at their neutral position, or nearly
+so, whereby both sets of coils may be subjected simultaneously
+or successively to the inducing action of the field magnets.</p>
+
+<p>Generally the circuits in the motor will be similarly disposed,
+and various arrangements may be made to fulfill the requirements;
+but the simplest and most practicable is to arrange primary circuits
+on stationary parts of the motor, thereby obviating, at least
+in certain forms, the employment of sliding contacts. In such a
+case the magnet coils are connected alternately in both the circuits;
+that is, 1, 3, 5 ... in one, and 2, 4, 6 ... in the other, and
+the coils of each set of series may be connected all in the same<span class='pagenum'><a name="Page_23" id="Page_23">[Pg 23]</a></span>
+manner, or alternately in opposition; in the latter case a motor
+with half the number of poles will result, and its action will be
+correspondingly modified. The Figs. 15, 16, and 17, show
+three different phases, the magnet coils in each circuit being connected
+alternately in opposition. In this case there will be always
+four poles, as in Figs. 15 and 17; four pole projections will be
+neutral; and in Fig. 16 two adjacent pole projections will have
+the same polarity. If the coils are connected in the same manner
+there will be eight alternating poles, as indicated by the letters
+<i>n'</i> <i>s'</i> in Fig. 15.</p>
+
+<p>The employment of multipolar motors secures in this system an
+advantage much desired and unattainable in the continuous current
+system, and that is, that a motor may be made to run exactly
+at a predetermined speed irrespective of imperfections in construction,
+of the load, and, within certain limits, of electromotive
+force and current strength.</p>
+
+<p>In a general distribution system of this kind the following plan
+should be adopted. At the central station of supply a generator
+should be provided having a considerable number of poles. The
+motors operated from this generator should be of the synchronous
+type, but possessing sufficient rotary effort to insure their starting.
+With the observance of proper rules of construction it may be
+admitted that the speed of each motor will be in some inverse
+proportion to its size, and the number of poles should be chosen
+accordingly. Still, exceptional demands may modify this rule.
+In view of this, it will be advantageous to provide each motor
+with a greater number of pole projections or coils, the number
+being preferably a multiple of two and three. By this means, by
+simply changing the connections of the coils, the motor may be
+adapted to any probable demands.</p>
+
+<p>If the number of the poles in the motor is even, the action will
+be harmonious and the proper result will be obtained; if this
+is not the case, the best plan to be followed is to make a
+motor with a double number of poles and connect the same in
+the manner before indicated, so that half the number of poles
+result. Suppose, for instance, that the generator has twelve poles,
+and it would be desired to obtain a speed equal to 12/7 of the speed
+of the generator. This would require a motor with seven pole
+projections or magnets, and such a motor could not be properly
+connected in the circuits unless fourteen armature coils would be
+provided, which would necessitate the employment of sliding<span class='pagenum'><a name="Page_24" id="Page_24">[Pg 24]</a></span>
+contacts. To avoid this, the motor should be provided with fourteen
+magnets and seven connected in each circuit, the magnets
+in each circuit alternating among themselves. The armature
+should have fourteen closed coils. The action of the motor will
+not be quite as perfect as in the case of an even number of poles,
+but the drawback will not be of a serious nature.</p>
+
+<p>However, the disadvantages resulting from this unsymmetrical
+form will be reduced in the same proportion as the number of
+the poles is augmented.</p>
+
+<p>If the generator has, say, <i>n</i>, and the motor <i>n</i><sub>1</sub> poles, the speed
+of the motor will be equal to that of the generator multiplied by
+<i>n</i>/<i>n</i><sub>1</sub>.</p>
+
+<p>The speed of the motor will generally be dependent on the
+number of the poles, but there may be exceptions to this rule.
+The speed may be modified by the phase of the currents in the
+circuit or by the character of the current impulses or by intervals
+between each or between groups of impulses. Some of the
+possible cases are indicated in the diagrams, Figs. 18, 19, 20 and
+21, which are self-explanatory. Fig. 18 represents the condition
+generally existing, and which secures the best result. In
+such a case, if the typical form of motor illustrated in Fig. 9
+is employed, one complete wave in each circuit will produce one
+revolution of the motor. In Fig. 19 the same result will be
+effected by one wave in each circuit, the impulses being successive;
+in Fig. 20 by four, and in Fig. 21 by eight waves.</p>
+
+<p>By such means any desired speed may be attained, that is, at
+least within the limits of practical demands. This system possesses
+this advantage, besides others, resulting from simplicity.
+At full loads the motors show an efficiency fully equal to that of
+the continuous current motors. The transformers present an
+additional advantage in their capability of operating motors.
+They are capable of similar modifications in construction, and will
+facilitate the introduction of motors and their adaptation to practical
+demands. Their efficiency should be higher than that of
+the present transformers, and I base my assertion on the following:</p>
+
+<p>In a transformer, as constructed at present, we produce the
+currents in the secondary circuit by varying the strength of the
+primary or exciting currents. If we admit proportionality with
+respect to the iron core the inductive effect exerted upon the<span class='pagenum'><a name="Page_25" id="Page_25">[Pg 25]</a></span>
+secondary coil will be proportional to the numerical sum of the
+variations in the strength of the exciting current per unit of time;
+whence it follows that for a given variation any prolongation of
+the primary current will result in a proportional loss. In order
+to obtain rapid variations in the strength of the current, essential
+to efficient induction, a great number of undulations are employed;
+from this practice various disadvantages result. These are:
+Increased cost and diminished efficiency of the generator; more
+waste of energy in heating the cores, and also diminished output
+of the transformer, since the core is not properly utilized, the
+reversals being too rapid. The inductive effect is also very small
+in certain phases, as will be apparent from a graphic representation,
+and there may be periods of inaction, if there are intervals
+between the succeeding current impulses or waves. In producing
+a shifting of the poles in a transformer, and thereby inducing
+currents, the induction is of the ideal character, being always
+maintained at its maximum action. It is also reasonable to assume
+that by a shifting of the poles less energy will be wasted
+than by reversals.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_26" id="Page_26">[Pg 26]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV.</h2>
+
+<h3><span class="smcap">Modifications and Expansions of the Tesla Polyphase
+Systems.</span></h3>
+
+
+<p>In his earlier papers and patents relative to polyphase currents,
+Mr. Tesla devoted himself chiefly to an enunciation of the broad
+lines and ideas lying at the basis of this new work; but he supplemented
+this immediately by a series of other striking inventions
+which may be regarded as modifications and expansions of
+certain features of the Tesla systems. These we shall now proceed
+to deal with.</p>
+
+<p>In the preceding chapters we have thus shown and described
+the Tesla electrical systems for the transmission of power and the
+conversion and distribution of electrical energy, in which the
+motors and the transformers contain two or more coils or sets of
+coils, which were connected up in independent circuits with
+corresponding coils of an alternating current generator, the operation
+of the system being brought about by the co-operation of
+the alternating currents in the independent circuits in progressively
+moving or shifting the poles or points of maximum magnetic
+effect of the motors or converters. In these systems two
+independent conductors are employed for each of the independent
+circuits connecting the generator with the devices for converting
+the transmitted currents into mechanical energy or into
+electric currents of another character. This, however, is not
+always necessary. The two or more circuits may have a single
+return path or wire in common, with a loss, if any, which is so
+extremely slight that it may be disregarded entirely. For the
+sake of illustration, if the generator have two independent coils
+and the motor two coils or two sets of coils in corresponding relations
+to its operative elements one terminal of each generator
+coil is connected to the corresponding terminals of the motor
+coils through two independent conductors, while the opposite
+terminals of the respective coils are both connected to one
+return wire. The following description deals with the modifica<span class='pagenum'><a name="Page_27" id="Page_27">[Pg 27]</a></span>tion.
+Fig. 22 is a diagrammatic illustration of a generator and
+single motor constructed and electrically connected in accordance
+with the invention. Fig. 23 is a diagram of the system
+as it is used in operating motors or converters, or both, in parallel,
+while Fig. 24 illustrates diagrammatically the manner of operating
+two or more motors or converters, or both, in series. Referring
+to Fig. 22, <small>A A</small> designate the poles of the field magnets of
+an alternating-current generator, the armature of which, being in
+this case cylindrical in form and mounted on a shaft, <small>C</small>, is wound
+longitudinally with coils <small>B B'</small>. The shaft <small>C</small> carries three insulated
+contact-rings, <i>a b c</i>, to two of which, as <i>b c</i>, one terminal of each
+coil, as <i>e d</i>, is connected. The remaining terminals, <i>f g</i>, are both
+connected to the third ring, <i>a</i>.</p>
+
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 378px;">
+<img src="images/fig22.jpg" width="378" height="640" alt="Fig. 22." title="" />
+<span class="caption">Fig. 22.</span>
+</div>
+<div class="figright" style="width: 341px;">
+<img src="images/fig24.jpg" width="341" height="640" alt="Fig. 24." title="" />
+<span class="caption">Fig. 24.</span>
+</div>
+</div>
+
+<p>A motor in this case is shown as composed of a ring, <small>H</small>, wound
+with four coils, <small>I I J J</small>, electrically connected, so as to co-operate
+in pairs, with a tendency to fix the poles of the ring at four points
+ninety degrees apart. Within the magnetic ring <small>H</small> is a disc or
+cylindrical core wound with two coils, <small>G G'</small>, which may be con<span class='pagenum'><a name="Page_28" id="Page_28">[Pg 28]</a></span>nected
+to form two closed circuits. The terminals <i>j k</i> of the two
+sets or pairs of coils are connected, respectively, to the binding-posts
+<small>E' F'</small>, and the other terminals, <i>h i</i>, are connected to a single
+binding-post, <small>D'</small>. To operate the motor, three line-wires are used
+to connect the terminals of the generator with those of the motor.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_042.jpg" width="600" height="675" alt="Fig. 23." title="" />
+<span class="caption">Fig. 23.</span>
+</div>
+
+
+<p>So far as the apparent action or mode of operation of this arrangement
+is concerned, the single wire <small>D</small>, which is, so to speak,
+a common return-wire for both circuits, may be regarded as two
+independent wires. In the illustration, with the order of connection
+shown, coil <small>B'</small> of the generator is producing its maximum
+current and coil <small>B</small> its minimum; hence the current which passes
+through wire <i>e</i>, ring <i>b</i>, brush <i>b'</i>, line-wire <small>E</small>, terminal <small>E'</small>, wire <i>j</i>,
+coils <small>I I</small>, wire or terminal <small>D'</small>, line-wire <small>D</small>, brush <i>a'</i>, ring <i>a</i>, and
+wire <i>f</i>, fixes the polar line of the motor midway between the<span class='pagenum'><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+two coils <small>I I</small>; but as the coil <small>B'</small> moves from the position indicated
+it generates less current, while coil <small>B</small>, moving into the field, generates
+more. The current from coil <small>B</small> passes through the devices
+and wires designated by the letters <i>d</i>, <i>c</i>, <i>c'</i> <small>F</small>, <small>F'</small> <i>k</i>, <small>J J</small>, <i>i</i>, <small>D'</small>, <small>D</small>, <i>a'</i>,
+<i>a</i>, and <i>g</i>, and the position of the poles of the motor will be due
+to the resultant effect of the currents in the two sets of coils&mdash;that
+is, it will be advanced in proportion to the advance or forward
+movement of the armature coils. The movement of the
+generator-armature through one-quarter of a revolution will obviously
+bring coil <small>B'</small> into its neutral position and coil <small>B</small> into its
+position of maximum effect, and this shifts the poles ninety degrees,
+as they are fixed solely by coils <small>B</small>. This action is repeated
+for each quarter of a complete revolution.</p>
+
+<p>When more than one motor or other device is employed, they
+may be run either in parallel or series. In Fig. 23 the former
+arrangement is shown. The electrical device is shown as a converter,
+<small>L</small>, of which the two sets of primary coils <i>p r</i> are connected,
+respectively, to the mains <small>F E</small>, which are electrically connected
+with the two coils of the generator. The cross-circuit
+wires <i>l m</i>, making these connections, are then connected to the
+common return-wire <small>D</small>. The secondary coils <i>p' p''</i> are in circuits
+<i>n o</i>, including, for example, incandescent lamps. Only one converter
+is shown entire in this figure, the others being illustrated
+diagrammatically.</p>
+
+<p>When motors or converters are to be run in series, the two
+wires <small>E F</small> are led from the generator to the coils of the first
+motor or converter, then continued on to the next, and so on
+through the whole series, and are then joined to the single wire
+<small>D</small>, which completes both circuits through the generator. This is
+shown in Fig. 24, in which <small>J I</small> represent the two coils or sets of
+coils of the motors.</p>
+
+<p>There are, of course, other conditions under which the same
+idea may be carried out. For example, in case the motor and
+generator each has three independent circuits, one terminal of
+each circuit is connected to a line-wire, and the other three terminals
+to a common return-conductor. This arrangement will
+secure similar results to those attained with a generator and motor
+having but two independent circuits, as above described.</p>
+
+<p>When applied to such machines and motors as have three or
+more induced circuits with a common electrical joint, the three
+or more terminals of the generator would be simply connected<span class='pagenum'><a name="Page_30" id="Page_30">[Pg 30]</a></span>
+to those of the motor. Mr. Tesla states, however, that the results
+obtained in this manner show a lower efficiency than do the
+forms dwelt upon more fully above.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_31" id="Page_31">[Pg 31]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V.</h2>
+
+<h3><span class="smcap">Utilizing Familiar Types of Generator of the Continuous
+Current Type.</span></h3>
+
+
+<p>The preceding descriptions have assumed the use of alternating
+current generators in which, in order to produce the progressive
+movement of the magnetic poles, or of the resultant attraction of
+independent field magnets, the current generating coils are independent
+or separate. The ordinary forms of continuous current
+dynamos may, however, be employed for the same work, in
+accordance with a method of adaptation devised by Mr. Tesla.
+As will be seen, the modification involves but slight changes in
+their construction, and presents other elements of economy.</p>
+
+<p>On the shaft of a given generator, either in place of or in addition
+to the regular commutator, are secured as many pairs of
+insulated collecting-rings as there are circuits to be operated.
+Now, it will be understood that in the operation of any dynamo
+electric generator the currents in the coils in their movement
+through the field of force undergo different phases&mdash;that is to
+say, at different positions of the coils the currents have certain
+directions and certain strengths&mdash;and that in the Tesla motors or
+transformers it is necessary that the currents in the energizing
+coils should undergo a certain order of variations in strength and
+direction. Hence, the further step&mdash;viz., the connection between
+the induced or generating coils of the machine and the contact-rings
+from which the currents are to be taken off&mdash;will be determined
+solely by what order of variations of strength and direction
+in the currents is desired for producing a given result in the
+electrical translating device. This may be accomplished in
+various ways; but in the drawings we give typical instances only
+of the best and most practicable ways of applying the invention
+to three of the leading types of machines in widespread use, in
+order to illustrate the principle.</p>
+
+<p>Fig. 25 is a diagram illustrative of the mode of applying the
+invention to the well-known type of "closed" or continuous cir<span class='pagenum'><a name="Page_32" id="Page_32">[Pg 32]</a></span>cuit
+machines. Fig. 26 is a similar diagram embodying an armature
+with separate coils connected diametrically, or what is generally
+called an "open-circuit" machine. Fig. 27 is a diagram
+showing the application of the invention to a machine the armature-coils
+of which have a common joint.</p>
+
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_046.jpg" width="600" height="628" alt="Fig. 25." title="" />
+<span class="caption">Fig. 25.</span>
+</div>
+
+
+<p>Referring to Fig. 25, let <small>A</small> represent a Tesla motor or transformer
+which, for convenience, we will designate as a "converter."
+It consists of an annular core, <small>B</small>, wound with four independent
+coils, <small>C</small> and <small>D</small>, those diametrically opposite being connected
+together so as to co-operate in pairs in establishing free
+poles in the ring, the tendency of each pair being to fix the poles
+at ninety degrees from the other. There may be an armature,
+<small>E</small>, within the ring, which is wound with coils closed upon themselves.
+The object is to pass through coils <small>C D</small> currents of such
+relative strength and direction as to produce a progressive shifting
+or movement of the points of maximum magnetic effect
+around the ring, and to thereby maintain a rotary movement of
+the armature. There are therefore secured to the shaft <small>F</small> of the
+generator, four insulated contact-rings, <i>a b c d</i>, upon which bear<span class='pagenum'><a name="Page_33" id="Page_33">[Pg 33]</a></span>
+the collecting-brushes <i>a' b' c' d'</i>, connected by wires <small>G G H H</small>, respectively,
+with the terminals of coils <small>C</small> and <small>D</small>.</p>
+
+<p>Assume, for sake of illustration, that the coils <small>D D</small> are to receive
+the maximum and coils <small>C C</small> at the same instant the minimum
+current, so that the polar line may be midway between the
+coils <small>D D</small>. The rings <i>a b</i> would therefore be connected to the
+continuous armature-coil at its neutral points with respect to the
+field, or the point corresponding with that of the ordinary commutator
+brushes, and between which exists the greatest difference
+of potential; while rings <i>c d</i> would be connected to two
+points in the coil, between which exists no difference of potential.
+The best results will be obtained by making these connections at
+points equidistant from one another, as shown. These connections
+are easiest made by using wires <small>L</small> between the rings and the
+loops or wires <small>J</small>, connecting the coil <small>I</small> to the segments of the
+commutator <small>K</small>. When the converters are made in this manner,
+it is evident that the phases of the currents in the sections of the
+generator coil will be reproduced in the converter coils. For
+example, after turning through an arc of ninety degrees the conductors
+<small>L L</small>, which before conveyed the maximum current, will
+receive the minimum current by reason of the change in the
+position of their coils, and it is evident that for the same reason
+the current in these coils has gradually fallen from the maximum
+to the minimum in passing through the arc of ninety degrees.
+In this special plan of connections, the rotation of the magnetic
+poles of the converter will be synchronous with that of the
+armature coils of the generator, and the result will be the same,
+whether the energizing circuits are derivations from a continuous
+armature coil or from independent coils, as in Mr. Tesla's
+other devices.</p>
+
+<p>In Fig. 25, the brushes <small>M M</small> are shown in dotted lines in their
+proper normal position. In practice these brushes may be removed
+from the commutator and the field of the generator
+excited by an external source of current; or the brushes may be
+allowed to remain on the commutator and to take off a converted
+current to excite the field, or to be used for other purposes.</p>
+
+<p>In a certain well-known class of machines known as the "open
+circuit," the armature contains a number of coils the terminals of
+which connect to commutator segments, the coils being connected
+across the armature in pairs. This type of machine is represented
+in Fig. 26. In this machine each pair of coils goes<span class='pagenum'><a name="Page_34" id="Page_34">[Pg 34]</a></span>
+through the same phases as the coils in some of the generators
+already shown, and it is obviously only necessary to utilize them
+in pairs or sets to operate a Tesla converter by extending the
+segments of the commutators belonging to each pair of coils and
+causing a collecting brush to bear on the continuous portion of
+each segment. In this way two or more circuits may be taken
+off from the generator, each including one or more pairs or sets
+of coils as may be desired.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_048.jpg" width="638" height="480" alt="Fig. 26, 27." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 26.</td><td class="caption">Fig. 27.</td></tr>
+</table>
+</div>
+
+
+<p>In Fig. 26 <small>I I</small> represent the armature coils, <small>T T</small> the poles of the
+field magnet, and <small>F</small> the shaft carrying the commutators, which
+are extended to form continuous portions <i>a b c d</i>. The brushes
+bearing on the continuous portions for taking off the alternating
+currents are represented by <i>a' b' c' d'</i>. The collecting brushes,
+or those which may be used to take off the direct current, are
+designated by <small>M M</small>. Two pairs of the armature coils and their
+commutators are shown in the figure as being utilized; but all
+may be utilized in a similar manner.</p>
+
+<p>There is another well-known type of machine in which three
+or more coils, <small>A' B' C'</small>, on the armature have a common joint,
+the free ends being connected to the segments of a commutator.
+This form of generator is illustrated in Fig. 27. In this case each
+terminal of the generator is connected directly or in derivation
+to a continuous ring, <i>a b c</i>, and collecting brushes, <i>a' b' c'</i>, bearing<span class='pagenum'><a name="Page_35" id="Page_35">[Pg 35]</a></span>
+thereon, take off the alternating currents that operate the motor.
+It is preferable in this case to employ a motor or transformer
+with three energizing coils, <small>A'' B'' C''</small>, placed symmetrically with
+those of the generator, and the circuits from the latter are connected
+to the terminals of such coils either directly&mdash;as when
+they are stationary&mdash;or by means of brushes <i>e'</i> and contact rings
+<i>e</i>. In this, as in the other cases, the ordinary commutator may
+be used on the generator, and the current taken from it utilized
+for exciting the generator field-magnets or for other purposes.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_36" id="Page_36">[Pg 36]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI.</h2>
+
+<h3><span class="smcap">Method of Obtaining Desired Speed of Motor or
+Generator.</span></h3>
+
+
+<p>With the object of obtaining the desired speed in motors
+operated by means of alternating currents of differing phase,
+Mr. Tesla has devised various plans intended to meet the practical
+requirements of the case, in adapting his system to types of
+multipolar alternating current machines yielding a large number
+of current reversals for each revolution.</p>
+
+<p>For example, Mr. Tesla has pointed out that to adapt a given
+type of alternating current generator, you may couple rigidly
+two complete machines, securing them together in such a way
+that the requisite difference in phase will be produced; or you
+may fasten two armatures to the same shaft within the influence
+of the same field and with the requisite angular displacement to
+yield the proper difference in phase between the two currents;
+or two armatures may be attached to the same shaft with their
+coils symmetrically disposed, but subject to the influence of two
+sets of field magnets duly displaced; or the two sets of coils
+may be wound on the same armature alternately or in such manner
+that they will develop currents the phases of which differ in
+time sufficiently to produce the rotation of the motor.</p>
+
+<p>Another method included in the scope of the same idea, whereby
+a single generator may run a number of motors either at its
+own rate of speed or all at different speeds, is to construct the
+motors with fewer poles than the generator, in which case their
+speed will be greater than that of the generator, the rate of speed
+being higher as the number of their poles is relatively less. This
+may be understood from an example, taking a generator that has
+two independent generating coils which revolve between two
+pole pieces oppositely magnetized; and a motor with energizing
+coils that produce at any given time two magnetic poles in one
+element that tend to set up a rotation of the motor. A generator
+thus constructed yields four reversals, or impulses, in each<span class='pagenum'><a name="Page_37" id="Page_37">[Pg 37]</a></span>
+revolution, two in each of its independent circuits; and the effect
+upon the motor is to shift the magnetic poles through three hundred
+and sixty degrees. It is obvious that if the four reversals
+in the same order could be produced by each half-revolution of
+the generator the motor would make two revolutions to the generator's
+one. This would be readily accomplished by adding two
+intermediate poles to the generator or altering it in any of the
+other equivalent ways above indicated. The same rule applies
+to generators and motors with multiple poles. For instance, if a
+generator be constructed with two circuits, each of which produces
+twelve reversals of current to a revolution, and these currents
+be directed through the independent energizing-coils of a
+motor, the coils of which are so applied as to produce twelve
+magnetic poles at all times, the rotation of the two will be synchronous;
+but if the motor-coils produce but six poles, the movable
+element will be rotated twice while the generator rotates once; or
+if the motor have four poles, its rotation will be three times as
+fast as that of the generator.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_051.jpg" width="640" height="405" alt="Fig. 28, 29." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 28.</td><td class="caption">Fig. 29.</td></tr>
+</table>
+</div>
+
+<p>These features, so far as necessary to an understanding of the
+principle, are here illustrated. Fig. 28 is a diagrammatic illustration
+of a generator constructed in accordance with the invention.
+Fig. 29 is a similar view of a correspondingly constructed
+motor. Fig. 30 is a diagram of a generator of modified construction.
+Fig. 31 is a diagram of a motor of corresponding
+character. Fig. 32 is a diagram of a system containing a generator
+and several motors adapted to run at various speeds.<span class='pagenum'><a name="Page_38" id="Page_38">[Pg 38]</a></span></p>
+
+<p>In Fig. 28, let <small>C</small> represent a cylindrical armature core wound
+longitudinally with insulated coils <small>A A</small>, which are connected up
+in series, the terminals of the series being connected to collecting-rings
+<i>a a</i> on the shaft <small>G</small>. By means of this shaft the armature
+is mounted to rotate between the poles of an annular field-magnet
+<small>D</small>, formed with polar projections wound with coils <small>E</small>, that
+magnetize the said projections. The coils <small>E</small> are included in the
+circuit of a generator <small>F</small>, by means of which the field-magnet is
+energized. If thus constructed, the machine is a well-known
+form of alternating-current generator. To adapt it to his system,
+however, Mr. Tesla winds on armature <small>C</small> a second set of
+coils <small>B B</small> intermediate to the first, or, in other words, in such positions
+that while the coils of one set are in the relative positions
+to the poles of the field-magnet to produce the maximum current,
+those of the other set will be in the position in which they produce
+the minimum current. The coils <small>B</small> are connected, also, in
+series and to two connecting-rings, secured generally to the
+shaft at the opposite end of the armature.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 350px;">
+<img src="images/fig30.jpg" width="350" height="225" alt="Fig. 30." title="" />
+<span class="caption">Fig. 30.</span>
+</div>
+<div class="figright" style="width: 350px;">
+<img src="images/fig31.jpg" width="350" height="225" alt="Fig. 31." title="" />
+<span class="caption">Fig. 31.</span>
+</div>
+</div>
+
+<p>The motor shown in Fig. 29 has an annular field-magnet <small>H</small>,
+with four pole-pieces wound with coils <small>I</small>. The armature is constructed
+similarly to the generator, but with two sets of two
+coils in closed circuits to correspond with the reduced number of
+magnetic poles in the field. From the foregoing it is evident that
+one revolution of the armature of the generator producing eight
+current impulses in each circuit will produce two revolutions of
+the motor-armature.</p>
+
+<p>The application of the principle of this invention is not, however,
+confined to any particular form of machine. In Figs. 30
+and 31 a generator and motor of another well-known type are
+shown. In Fig. 30, <small>J J</small> are magnets disposed in a circle and
+wound with coils <small>K</small>, which are in circuit with a generator which<span class='pagenum'><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+supplies the current that maintains the field of force. In the
+usual construction of these machines the armature-conductor <small>L</small> is
+carried by a suitable frame, so as to be rotated in face of the
+magnets <small>J J</small>, or between these magnets and another similar set
+in front of them. The magnets are energized so as to be of alternately
+opposite polarity throughout the series, so that as the
+conductor <small>C</small> is rotated the current impulses combine or are
+added to one another, those produced by the conductor in any
+given position being all in the same direction. To adapt such
+a machine to his system, Mr. Tesla adds a second set of induced
+conductors <small>M</small>, in all respects similar to the first, but so placed
+in reference to it that the currents produced in each will differ
+by a quarter-phase. With such relations it is evident that as the
+current decreases in conductor <small>L</small> it increases in conductor <small>M</small>, and
+conversely, and that any of the forms of Tesla motor invented
+for use in this system may be operated by such a generator.</p>
+
+<p>Fig. 31 is intended to show a motor corresponding to the machine
+in Fig. 30. The construction of the motor is identical with
+that of the generator, and if coupled thereto it will run synchronously
+therewith. <small>J' J'</small> are the field-magnets, and <small>K'</small> the
+coils thereon. <small>L'</small> is one of the armature-conductors and <small>M'</small> the
+other.</p>
+
+<p>Fig. 32 shows in diagram other forms of machine. The generator
+<small>N</small> in this case is shown as consisting of a stationary ring <small>O</small>,
+wound with twenty-four coils <small>P P'</small>, alternate coils being connected
+in series in two circuits. Within this ring is a disc or drum <small>Q</small>,
+with projections <small>Q'</small> wound with energizing-coils included in circuit
+with a generator <small>R</small>. By driving this disc or cylinder alternating
+currents are produced in the coils <small>P</small> and <small>P'</small>, which are
+carried off to run the several motors.</p>
+
+<p>The motors are composed of a ring or annular field-magnet <small>S</small>,
+wound with two sets of energizing-coils <small>T T'</small>, and armatures <small>U</small>,
+having projections <small>U'</small> wound with coils <small>V</small>, all connected in series
+in a closed circuit or each closed independently on itself.</p>
+
+<p>Suppose the twelve generator-coils <small>P</small> are wound alternately in
+opposite directions, so that any two adjacent coils of the same set
+tend to produce a free pole in the ring <small>O</small> between them and the
+twelve coils <small>P'</small> to be similarly wound. A single revolution of
+the disc or cylinder <small>Q</small>, the twelve polar projections of which are
+of opposite polarity, will therefore produce twelve current impulses
+in each of the circuits <small>W W'</small>. Hence the motor <small>X</small>, which<span class='pagenum'><a name="Page_40" id="Page_40">[Pg 40]</a></span>
+has sixteen coils or eight free poles, will make one and a half turns
+to the generator's one. The motor <small>Y</small>, with twelve coils or six
+poles, will rotate with twice the speed of the generator, and the
+motor <small>Z</small>, with eight coils or four poles, will revolve three times
+as fast as the generator. These multipolar motors have a peculiarity
+which may be often utilized to great advantage. For example,
+in the motor <small>X</small>, Fig. 32, the eight poles may be either
+alternately opposite or there may be at any given time alternately
+two like and two opposite poles. This is readily attained by
+making the proper electrical connections. The effect of such a
+change, however, would be the same as reducing the number of
+<span class='pagenum'><a name="Page_41" id="Page_41">[Pg 41]</a></span>poles one-half, and thereby doubling the speed of any given
+motor.</p>
+
+<div class="figcenter" style="width: 464px;">
+<img src="images/oi_054.jpg" width="464" height="640" alt="Fig. 32." title="" />
+<span class="caption">Fig. 32.</span>
+</div>
+
+
+<p>It is obvious that the Tesla electrical transformers which have
+independent primary currents may be used with the generators
+described. It may also be stated with respect to the devices
+we now describe that the most perfect and harmonious action
+of the generators and motors is obtained when the numbers of the
+poles of each are even and not odd. If this is not the case, there
+will be a certain unevenness of action which is the less appreciable
+as the number of poles is greater; although this may be in a
+measure corrected by special provisions which it is not here
+necessary to explain. It also follows, as a matter of course, that
+if the number of the poles of the motor be greater than that of
+the generator the motor will revolve at a slower speed than the
+generator.</p>
+
+<p>In this chapter, we may include a method devised by Mr.
+Tesla for avoiding the very high speeds which would be necessary
+with large generators. In lieu of revolving the generator
+armature at a high rate of speed, he secures the desired result by
+a rotation of the magnetic poles of one element of the generator,
+while driving the other at a different speed. The effect is the
+same as that yielded by a very high rate of rotation.</p>
+
+<p>In this instance, the generator which supplies the current for
+operating the motors or transformers consists of a subdivided
+ring or annular core wound with four diametrically-opposite
+coils, <small>E E'</small>, Fig. 33. Within the ring is mounted a cylindrical
+armature-core wound longitudinally with two independent coils,
+<small>F F'</small>, the ends of which lead, respectively, to two pairs of insulated
+contact or collecting rings, <small>D D' G G'</small>, on the armature shaft.
+Collecting brushes <i>d d' g g'</i> bear upon these rings, respectively,
+and convey the currents through the two independent line-circuits
+<small>M M'</small>. In the main line there may be included one or more
+motors or transformers, or both. If motors be used, they are of
+the usual form of Tesla construction with independent coils or
+sets of coils <small>J J'</small>, included, respectively, in the circuits <small>M M'</small>.
+These energizing-coils are wound on a ring or annular field or on
+pole pieces thereon, and produce by the action of the alternating
+currents passing through them a progressive shifting of the magnetism
+from pole to pole. The cylindrical armature <small>H</small> of the
+motor is wound with two coils at right angles, which form independent
+closed circuits.<span class='pagenum'><a name="Page_42" id="Page_42">[Pg 42]</a></span></p>
+
+<p>If transformers be employed, one set of the primary coils, as
+<small>N N</small>, wound on a ring or annular core is connected to one circuit,
+as <small>M'</small>, and the other primary coils, <small>N N'</small>, to the circuit <small>M</small>. The
+secondary coils <small>K K'</small> may then be utilized for running groups of
+incandescent lamps <small>P P'</small>.</p>
+
+<div class="figcenter" style="width: 557px;">
+<img src="images/oi_056.jpg" width="557" height="800" alt="Fig. 33." title="" />
+<span class="caption">Fig. 33.</span>
+</div>
+
+<p>With this generator an exciter is employed. This consists of
+two poles, <small>A A</small>, of steel permanently magnetized, or of iron excited
+by a battery or other generator of continuous currents, and
+a cylindrical armature core mounted on a shaft, <small>B</small>, and wound
+with two longitudinal coils, <small>C C'</small>. One end of each of these coils
+is connected to the collecting-rings <i>b c</i>, respectively, while the<span class='pagenum'><a name="Page_43" id="Page_43">[Pg 43]</a></span>
+other ends are both connected to a ring, <i>a</i>. Collecting-brushes
+<i>b' c'</i> bear on the rings <i>b c</i>, respectively, and conductors <small>L L</small> convey
+the currents therefrom through the coils <small>E</small> and <small>E</small> of the generator.
+<small>L'</small> is a common return-wire to brush <i>a'</i>. Two independent
+circuits are thus formed, one including coils <small>C</small> of the exciter
+and <small>E E</small> of the generator, the other coils <small>C'</small> of the exciter and <small>E'</small>
+<small>E'</small> of the generator. It results from this that the operation of
+the exciter produces a progressive movement of the magnetic
+poles of the annular field-core of the generator, the shifting or
+rotary movement of the poles being synchronous with the rotation
+of the exciter armature. Considering the operative conditions
+of a system thus established, it will be found that when
+the exciter is driven so as to energize the field of the generator,
+the armature of the latter, if left free to turn, would rotate at a
+speed practically the same as that of the exciter. If under such
+conditions the coils <small>F F'</small> of the generator armature be closed
+upon themselves or short-circuited, no currents, at least theoretically,
+will be generated in these armature coils. In practice
+the presence of slight currents is observed, the existence of which
+is attributable to more or less pronounced fluctuations in the intensity
+of the magnetic poles of the generator ring. So, if the
+armature-coils <small>F F'</small> be closed through the motor, the latter will
+not be turned as long as the movement of the generator armature
+is synchronous with that of the exciter or of the magnetic poles
+of its field. If, on the contrary, the speed of the generator armature
+be in any way checked, so that the shifting or rotation of
+the poles of the field becomes relatively more rapid, currents will
+be induced in the armature coils. This obviously follows from
+the passing of the lines of force across the armature conductors.
+The greater the speed of rotation of the magnetic poles relatively
+to that of the armature the more rapidly the currents developed
+in the coils of the latter will follow one another, and the more
+rapidly the motor will revolve in response thereto, and this continues
+until the armature generator is stopped entirely, as by a
+brake, when the motor, if properly constructed, runs at the speed
+with which the magnetic poles of the generator rotate.</p>
+
+<p>The effective strength of the currents developed in the armature
+coils of the generator is dependent upon the strength of the
+currents energizing the generator and upon the number of rotations
+per unit of time of the magnetic poles of the generator;
+hence the speed of the motor armature will depend in all cases<span class='pagenum'><a name="Page_44" id="Page_44">[Pg 44]</a></span>
+upon the relative speeds of the armature of the generator and of
+its magnetic poles. For example, if the poles are turned two
+thousand times per unit of time and the armature is turned eight
+hundred, the motor will turn twelve hundred times, or nearly so.
+Very slight differences of speed may be indicated by a delicately
+balanced motor.</p>
+
+<p>Let it now be assumed that power is applied to the generator
+armature to turn it in a direction opposite to that in which its
+magnetic poles rotate. In such case the result would be similar
+to that produced by a generator the armature and field magnets
+of which are rotated in opposite directions, and by reason of these
+conditions the motor armature will turn at a rate of speed equal
+to the sum of the speeds of the armature and magnetic poles of
+the generator, so that a comparatively low speed of the generator
+armature will produce a high speed in the motor.</p>
+
+<p>It will be observed in connection with this system that on
+diminishing the resistance of the external circuit of the generator
+armature by checking the speed of the motor or by adding
+translating devices in multiple arc in the secondary circuit or circuits
+of the transformer the strength of the current in the armature
+circuit is greatly increased. This is due to two causes: first,
+to the great differences in the speeds of the motor and generator,
+and, secondly, to the fact that the apparatus follows the analogy
+of a transformer, for, in proportion as the resistance of the armature
+or secondary circuits is reduced, the strength of the currents
+in the field or primary circuits of the generator is increased and
+the currents in the armature are augmented correspondingly.
+For similar reasons the currents in the armature-coils of the
+generator increase very rapidly when the speed of the armature
+is reduced when running in the same direction as the magnetic
+poles or conversely.</p>
+
+<p>It will be understood from the above description that the
+generator-armature may be run in the direction of the shifting of
+the magnetic poles, but more rapidly, and that in such case the
+speed of the motor will be equal to the difference between the
+two rates.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII.</h2>
+
+<h3><span class="smcap">Regulator for Rotary Current Motors.</span></h3>
+
+
+<p>An interesting device for regulating and reversing has been
+devised by Mr. Tesla for the purpose of varying the speed of
+polyphase motors. It consists of a form of converter or transformer
+with one element capable of movement with respect to
+the other, whereby the inductive relations may be altered, either
+manually or automatically, for the purpose of varying the
+strength of the induced current. Mr. Tesla prefers to construct
+this device in such manner that the induced or secondary element
+may be movable with respect to the other; and the invention,
+so far as relates merely to the construction of the device itself,
+consists, essentially, in the combination, with two opposite
+magnetic poles, of an armature wound with an insulated coil and
+mounted on a shaft, whereby it may be turned to the desired
+extent within the field produced by the poles. The normal position
+of the core of the secondary element is that in which it
+most completely closes the magnetic circuit between the poles
+of the primary element, and in this position its coil is in its
+most effective position for the inductive action upon it of the
+primary coils; but by turning the movable core to either side,
+the induced currents delivered by its coil become weaker until,
+by a movement of the said core and coil through 90&deg;, there will
+be no current delivered.</p>
+
+<p>Fig. 34 is a view in side elevation of the regulator. Fig. 35 is
+a broken section on line <i>x x</i> of Fig. 34. Fig. 36 is a diagram
+illustrating the most convenient manner of applying the regulator
+to ordinary forms of motors, and Fig. 37 is a similar diagram illustrating
+the application of the device to the Tesla alternating-current
+motors. The regulator may be constructed in many
+ways to secure the desired result; but that which is, perhaps, its
+best form is shown in Figs. 34 and 35.</p>
+
+<p><small>A</small> represents a frame of iron. <small>B B</small> are the cores of the induc<span class='pagenum'><a name="Page_46" id="Page_46">[Pg 46]</a></span>ing
+or primary coils <small>C C</small>. <small>D</small> is a shaft mounted on the side bars,
+<small>D'</small>, and on which is secured a sectional iron core, <small>E</small>, wound with
+an induced or secondary coil, <small>F</small>, the convolutions of which are
+parallel with the axis of the shaft. The ends of the core are
+rounded off so as to fit closely in the space between the two poles
+and permit the core <small>E</small> to be turned to and held at any desired
+point. A handle, <small>G</small>, secured to the projecting end of the shaft
+<small>D</small>, is provided for this purpose.</p>
+
+<div class="figcenter" style="width: 740px;">
+<div class="figleft" style="width: 310px;">
+<img src="images/fig34.jpg" width="310" height="427" alt="Fig. 34." title="" />
+<span class="caption">Fig. 34.</span>
+</div>
+<div class="figright" style="width: 380px;">
+<img src="images/fig35.jpg" width="380" height="427" alt="Fig. 35." title="" />
+<span class="caption">Fig. 35.</span>
+</div>
+</div>
+
+<p>In Fig. 36 let <small>H</small> represent an ordinary alternating current generator,
+the field-magnets of which are excited by a suitable
+source of current, <small>I</small>. Let <small>J</small> designate an ordinary form of electromagnetic
+motor provided with an armature, <small>K</small>, commutator <small>L</small>,
+and field-magnets <small>M</small>. It is well known that such a motor, if its
+field-magnet cores be divided up into insulated sections, may be
+practically operated by an alternating current; but in using this
+regulator with such a motor, Mr. Tesla includes one element of
+the motor only&mdash;say the armature-coils&mdash;in the main circuit of
+the generator, making the connections through the brushes and
+the commutator in the usual way. He also includes one of the
+elements of the regulator&mdash;say the stationary coils&mdash;in the same
+circuit, and in the circuit with the secondary or movable coil of
+the regulator he connects up the field-coils of the motor. He
+also prefers to use flexible conductors to make the connections
+from the secondary coil of the regulator, as he thereby avoids
+the use of sliding contacts or rings without interfering with the
+requisite movement of the core <small>E</small>.<span class='pagenum'><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+
+<p>If the regulator be in its normal position, or that in which its
+magnetic circuit is most nearly closed, it delivers its maximum
+induced current, the phases of which so correspond with those of
+the primary current that the motor will run as though both field
+and armature were excited by the main current.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_061.jpg" width="640" height="229" alt="Fig. 36." title="" />
+<span class="caption">Fig. 36.</span>
+</div>
+
+<p>To vary the speed of the motor to any rate between the minimum
+and maximum rates, the core <small>E</small> and coils <small>F</small> are turned in
+either direction to an extent which produces the desired result,
+for in its normal position the convolutions of coil <small>F</small> embrace the
+maximum number of lines of force, all of which act with the
+same effect upon the coil; hence it will deliver its maximum
+current; but by turning the coil <small>F</small> out of its position of maximum
+effect the number of lines of force embraced by it is diminished.
+The inductive effect is therefore impaired, and the current delivered
+by coil <small>F</small> will continue to diminish in proportion to the
+angle at which the coil <small>F</small> is turned until, after passing through
+an angle of ninety degrees, the convolutions of the coil will be
+at right angles to those of coils <small>C C</small>, and the inductive effect reduced
+to a minimum.</p>
+
+<p>Incidentally to certain constructions, other causes may influence
+the variation in the strength of the induced currents. For
+example, in the present case it will be observed that by the first
+movement of coil <small>F</small> a certain portion of its convolutions are carried
+beyond the line of the direct influence of the lines of force, and
+that the magnetic path or circuit for the lines is impaired; hence
+the inductive effect would be reduced. Next, that after moving
+through a certain angle, which is obviously determined by the
+relative dimensions of the bobbin or coil F, diagonally opposite
+portions of the coil will be simultaneously included in the field,
+but in such positions that the lines which produce a current-impulse
+in one portion of the coil in a certain direction will pro<span class='pagenum'><a name="Page_48" id="Page_48">[Pg 48]</a></span>duce
+in the diagonally opposite portion a corresponding impulse
+in the opposite direction; hence portions of the current will
+neutralize one another.</p>
+
+<p>As before stated, the mechanical construction of the device
+may be greatly varied; but the essential conditions of the principle
+will be fulfilled in any apparatus in which the movement of
+the elements with respect to one another effects the same results
+by varying the inductive relations of the two elements in a manner
+similar to that described.</p>
+
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_062.jpg" width="640" height="462" alt="Fig. 37." title="" />
+<span class="caption">Fig. 37.</span>
+</div>
+
+
+<p>It may also be stated that the core <small>E</small> is not indispensable to the
+operation of the regulator; but its presence is obviously beneficial.
+This regulator, however, has another valuable property
+in its capability of reversing the motor, for if the coil <small>F</small> be turned
+through a half-revolution, the position of its convolutions relatively
+to the two coils <small>C C</small> and to the lines of force is reversed, and
+consequently the phases of the current will be reversed. This
+will produce a rotation of the motor in an opposite direction.
+This form of regulator is also applied with great advantage to
+Mr. Tesla's system of utilizing alternating currents, in which the
+magnetic poles of the field of a motor are progressively shifted
+by means of the combined effects upon the field of magnetizing
+coils included in independent circuits, through which pass alternating
+currents in proper order and relations to each other.</p>
+
+<p>In Fig. 37, let <small>P</small> represent a Tesla generator having two independent
+coils, <small>P'</small> and <small>P''</small>, on the armature, and <small>T</small> a diagram of a<span class='pagenum'><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+motor having two independent energizing coils or sets of coils,
+<small>R R'</small>. One of the circuits from the generator, as <small>S' S'</small>, includes
+one set, <small>R' R'</small>, of the energizing coils of the motor, while the
+other circuit, as <small>S S</small>, includes the primary coils of the regulator.
+The secondary coil of the regulator includes the other coils, <small>R R</small>,
+of the motor.</p>
+
+<p>While the secondary coil of the regulator is in its normal position,
+it produces its maximum current, and the maximum rotary
+effect is imparted to the motor; but this effect will be diminished
+in proportion to the angle at which the coil <small>F</small> of the regulator is
+turned. The motor will also be reversed by reversing the position
+of the coil with reference to the coils <small>C C</small>, and thereby reversing
+the phases of the current produced by the generator. This
+changes the direction of the movement of the shifting poles which
+the armature follows.</p>
+
+<p>One of the main advantages of this plan of regulation is its
+economy of power. When the induced coil is generating its
+maximum current, the maximum amount of energy in the primary
+coils is absorbed; but as the induced coil is turned from its
+normal position the self-induction of the primary-coils reduces
+the expenditure of energy and saves power.</p>
+
+<p>It is obvious that in practice either coils <small>C C</small> or coil <small>F</small> may be
+used as primary or secondary, and it is well understood that their
+relative proportions may be varied to produce any desired difference
+or similarity in the inducing and induced currents.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_50" id="Page_50">[Pg 50]</a></span></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII.</h2>
+
+<h3><span class="smcap">Single Circuit, Self-Starting Synchronizing Motors.</span></h3>
+
+
+<p>In the first chapters of this section we have, bearing in mind
+the broad underlying principle, considered a distinct class of motors,
+namely, such as require for their operation a special generator
+capable of yielding currents of differing phase. As a matter
+of course, Mr. Tesla recognizing the desirability of utilizing his
+motors in connection with ordinary systems of distribution, addressed
+himself to the task of inventing various methods and
+ways of achieving this object. In the succeeding chapters,
+therefore, we witness the evolution of a number of ideas bearing
+upon this important branch of work. It must be obvious to
+a careful reader, from a number of hints encountered here and
+there, that even the inventions described in these chapters to follow
+do not represent the full scope of the work done in these
+lines. They might, indeed, be regarded as exemplifications.</p>
+
+<p>We will present these various inventions in the order which
+to us appears the most helpful to an understanding of the subject
+by the majority of readers. It will be naturally perceived that
+in offering a series of ideas of this nature, wherein some of the
+steps or links are missing, the descriptions are not altogether sequential;
+but any one who follows carefully the main drift of
+the thoughts now brought together will find that a satisfactory
+comprehension of the principles can be gained.</p>
+
+<p>As is well known, certain forms of alternating-current machines
+have the property, when connected in circuit with an alternating
+current generator, of running as a motor in synchronism therewith;
+but, while the alternating current will run the motor after
+it has attained a rate of speed synchronous with that of the generator,
+it will not start it. Hence, in all instances heretofore
+where these "synchronizing motors," as they are termed, have
+been run, some means have been adopted to bring the motors up
+to synchronism with the generator, or approximately so, before
+the alternating current of the generator is applied to drive them.<span class='pagenum'><a name="Page_51" id="Page_51">[Pg 51]</a></span>
+In some instances mechanical appliances have been utilized for
+this purpose. In others special and complicated forms of motor
+have been constructed. Mr. Tesla has discovered a much more
+simple method or plan of operating synchronizing motors, which
+requires practically no other apparatus than the motor itself. In
+other words, by a certain change in the circuit connections of the
+motor he converts it at will from a double circuit motor, or such
+as have been already described, and which will start under the
+action of an alternating current, into a synchronizing motor, or
+one which will be run by the generator only when it has reached
+a certain speed of rotation synchronous with that of the generator.
+In this manner he is enabled to extend very greatly the applications
+of his system and to secure all the advantages of both
+forms of alternating current motor.</p>
+
+<p>The expression "synchronous with that of the generator," is
+used here in its ordinary acceptation&mdash;that is to say, a motor is
+said to synchronize with the generator when it preserves a certain
+relative speed determined by its number of poles and the number
+of alternations produced per revolution of the generator. Its
+actual speed, therefore, may be faster or slower than that of the
+generator; but it is said to be synchronous so long as it preserves
+the same relative speed.</p>
+
+<p>In carrying out this invention Mr. Tesla constructs a motor
+which has a strong tendency to synchronism with the generator.
+The construction preferred is that in which the armature is provided
+with polar projections. The field-magnets are wound with
+two sets of coils, the terminals of which are connected to a switch
+mechanism, by means of which the line-current may be carried
+directly through these coils or indirectly through paths by
+which its phases are modified. To start such a motor, the switch
+is turned on to a set of contacts which includes in one motor
+circuit a dead resistance, in the other an inductive resistance, and,
+the two circuits being in derivation, it is obvious that the difference
+in phase of the current in such circuits will set up a rotation
+of the motor. When the speed of the motor has thus been
+brought to the desired rate the switch is shifted to throw the
+main current directly through the motor-circuits, and although
+the currents in both circuits will now be of the same phase the
+motor will continue to revolve, becoming a true synchronous
+motor. To secure greater efficiency, the armature or its polar
+projections are wound with coils closed on themselves.<span class='pagenum'><a name="Page_52" id="Page_52">[Pg 52]</a></span></p>
+
+<p>In the accompanying diagrams, Fig. 38 illustrates the details
+of the plan above set forth, and Figs. 39 and 40 modifications
+of the same.</p>
+
+<div class="figcenter" style="width: 466px;">
+<img src="images/oi_066.jpg" width="466" height="640" alt="Fig. 38, 39 and 40." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 38, 39 and 40.</span>
+</div>
+
+<p>Referring to Fig. 38, let <small>A</small> designate the field-magnets of a
+motor, the polar projections of which are wound with coils <small>B C</small>
+included in independent circuits, and <small>D</small> the armature with polar
+projections wound with coils <small>E</small> closed upon themselves, the
+motor in these respects being similar in construction to those<span class='pagenum'><a name="Page_53" id="Page_53">[Pg 53]</a></span>
+described already, but having on account of the polar projections
+on the armature core, or other similar and well-known features,
+the properties of a synchronizing-motor. <small>L L'</small> represents the
+conductors of a line from an alternating current generator <small>G</small>.</p>
+
+<p>Near the motor is placed a switch the action of which is that
+of the one shown in the diagrams, which is constructed as follows:
+<small>F F'</small> are two conducting plates or arms, pivoted at their
+ends and connected by an insulating cross-bar, <small>H</small>, so as to be
+shifted in parallelism. In the path of the bars <small>F F'</small> is the contact
+2, which forms one terminal of the circuit through coils <small>C</small>, and
+the contact 4, which is one terminal of the circuit through coils
+<small>B</small>. The opposite end of the wire of coils <small>C</small> is connected to the
+wire <small>L</small> or bar <small>F'</small>, and the corresponding end of coils <small>B</small> is connected
+to wire <small>L'</small> and bar <small>F</small>; hence if the bars be shifted so as to bear on
+contacts 2 and 4 both sets of coils <small>B C</small> will be included in the circuit
+<small>L L'</small> in multiple arc or derivation. In the path of the levers
+<small>F F'</small> are two other contact terminals, 1 and 3. The contact 1 is
+connected to contact 2 through an artificial resistance, <small>I</small>, and contact
+3 with contact 4 through a self-induction coil, <small>J</small>, so that when
+the switch levers are shifted upon the points 1 and 3 the circuits
+of coils <small>B</small> and <small>C</small> will be connected in multiple arc or derivation to
+the circuit <small>L L'</small>, and will include the resistance and self-induction
+coil respectively. A third position of the switch is that in which
+the levers <small>F</small> and <small>F'</small> are shifted out of contact with both sets of
+points. In this case the motor is entirely out of circuit.</p>
+
+<p>The purpose and manner of operating the motor by these devices
+are as follows: The normal position of the switch, the
+motor being out of circuit, is off the contact points. Assuming
+the generator to be running, and that it is desired to start the
+motor, the switch is shifted until its levers rest upon points 1 and
+3. The two motor-circuits are thus connected with the generator
+circuit; but by reason of the presence of the resistance <small>I</small> in one
+and the self-induction coil <small>J</small> in the other the coincidence of the
+phases of the current is disturbed sufficiently to produce a progression
+of the poles, which starts the motor in rotation. When
+the speed of the motor has run up to synchronism with the
+generator, or approximately so, the switch is shifted over upon
+the points 2 and 4, thus cutting out the coils <small>I</small> and <small>J</small>, so that the
+currents in both circuits have the same phase; but the motor
+now runs as a synchronous motor.</p>
+
+<p>It will be understood that when brought up to speed the mo<span class='pagenum'><a name="Page_54" id="Page_54">[Pg 54]</a></span>tor
+will run with only one of the circuits <small>B</small> or <small>C</small> connected with
+the main or generator circuit, or the two circuits may be connected
+in series. This latter plan is preferable when a current
+having a high number of alternations per unit of time is employed
+to drive the motor. In such case the starting of the
+motor is more difficult, and the dead and inductive resistances
+must take up a considerable proportion of the electromotive
+force of the circuits. Generally the conditions are so adjusted
+that the electromotive force used in each of the motor circuits is
+that which is required to operate the motor when its circuits are
+in series. The plan followed in this case is illustrated in Fig.
+39. In this instance the motor has twelve poles and the armature
+has polar projections <small>D</small> wound with closed coils <small>E</small>. The
+switch used is of substantially the same construction as that
+shown in the previous figure. There are, however, five contacts,
+designated as 5, 6, 7, 8, and 9. The motor-circuits <small>B C</small>, which include
+alternate field-coils, are connected to the terminals in the
+following order: One end of circuit <small>C</small> is connected to contact 9
+and to contact 5 through a dead resistance, <small>I</small>. One terminal of
+circuit <small>B</small> is connected to contact 7 and to contact 6 through a
+self-induction coil, <small>J</small>. The opposite terminals of both circuits are
+connected to contact 8.</p>
+
+<p>One of the levers, as <small>F</small>, of the switch is made with an extension,
+<i>f</i>, or otherwise, so as to cover both contacts 5 and 6 when
+shifted into the position to start the motor. It will be observed
+that when in this position and with lever <small>F'</small> on contact 8 the current
+divides between the two circuits <small>B C</small>, which from their difference
+in electrical character produce a progression of the poles
+that starts the motor in rotation. When the motor has attained
+the proper speed, the switch is shifted so that the levers cover
+the contacts 7 and 9, thereby connecting circuits <small>B</small> and <small>C</small> in series.
+It is found that by this disposition the motor is maintained
+in rotation in synchronism with the generator. This principle
+of operation, which consists in converting by a change of connections
+or otherwise a double-circuit motor, or one operating by
+a progressive shifting of the poles, into an ordinary synchronizing
+motor may be carried out in many other ways. For instance,
+instead of using the switch shown in the previous figures, we
+may use a temporary ground circuit between the generator and
+motor, in order to start the motor, in substantially the manner
+indicated in Fig. 40. Let <small>G</small> in this figure represent an ordinary<span class='pagenum'><a name="Page_55" id="Page_55">[Pg 55]</a></span>
+alternating-current generator with, say, two poles, <small>M M'</small>, and an
+armature wound with two coils, <small>N N'</small>, at right angles and connected
+in series. The motor has, for example, four poles wound
+with coils <small>B C</small>, which are connected in series, and an armature
+with polar projections <small>D</small> wound with closed coils <small>E E</small>. From the
+common joint or union between the two circuits of both the generator
+and the motor an earth connection is established, while
+the terminals or ends of these circuits are connected to the
+line. Assuming that the motor is a synchronizing motor or one
+that has the capability of running in synchronism with the generator,
+but not of starting, it may be started by the above-described
+apparatus by closing the ground connection from both
+generator and motor. The system thus becomes one with a two-circuit
+generator and motor, the ground forming a common return
+for the currents in the two circuits <small>L</small> and <small>L'</small>. When by
+this arrangement of circuits the motor is brought to speed, the
+ground connection is broken between the motor or generator, or
+both, ground-switches <small>P P'</small> being employed for this purpose.
+The motor then runs as a synchronizing motor.</p>
+
+<p>In describing the main features which constitute this invention
+illustrations have necessarily been omitted of the appliances used
+in conjunction with the electrical devices of similar systems&mdash;such,
+for instance, as driving-belts, fixed and loose pulleys for the
+motor, and the like; but these are matters well understood.</p>
+
+<p>Mr. Tesla believes he is the first to operate electro-magnetic
+motors by alternating currents in any of the ways herein described&mdash;that
+is to say, by producing a progressive movement or rotation
+of their poles or points of greatest magnetic attraction by
+the alternating currents until they have reached a given speed,
+and then by the same currents producing a simple alternation of
+their poles, or, in other words, by a change in the order or character
+of the circuit connections to convert a motor operating on
+one principle to one operating on another.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_56" id="Page_56">[Pg 56]</a></span></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX.</h2>
+
+<h3><span class="smcap">Change From Double Current to Single Current Motor.</span></h3>
+
+
+<p>A description is given elsewhere of a method of operating alternating
+current motors by first rotating their magnetic poles
+until they have attained synchronous speed, and then alternating
+the poles. The motor is thus transformed, by a simple change
+of circuit connections from one operated by the action of two or
+more independent energizing currents to one operated either by
+a single current or by several currents acting as one. Another
+way of doing this will now be described.</p>
+
+<p>At the start the magnetic poles of one element or field of the
+motor are progressively shifted by alternating currents differing
+in phase and passed through independent energizing circuits, and
+short circuit the coils of the other element. When the motor
+thus started reaches or passes the limit of speed synchronous with
+the generator, Mr. Tesla connects up the coils previously short-circuited
+with a source of direct current and by a change of the circuit
+connections produces a simple alternation of the poles. The
+motor then continues to run in synchronism with the generator.
+The motor here shown in Fig. 41 is one of the ordinary forms, with
+field-cores either laminated or solid and with a cylindrical laminated
+armature wound, for example, with the coils <small>A B</small> at right angles.
+The shaft of the armature carries three collecting or contact rings
+<small>C D E</small>. (Shown, for better illustration, as of different diameters.)</p>
+
+<p>One end of coil <small>A</small> connects to one ring, as <small>C</small>, and one end of
+coil <small>B</small> connects with ring <small>D</small>. The remaining ends are connected
+to ring <small>E</small>. Collecting springs or brushes <small>F G H</small> bear upon the
+rings and lead to the contacts of a switch, to be presently described.
+The field-coils have their terminals in binding-posts <small>K
+K</small>, and may be either closed upon themselves or connected with
+a source of direct current <small>L</small>, by means of a switch <small>M</small>. The main
+or controlling switch has five contacts <i>a b c d e</i> and two levers <i>f
+g</i>, pivoted and connected by an insulating cross-bar <i>h</i>, so as to
+move in parallelism. These levers are connected to the line<span class='pagenum'><a name="Page_57" id="Page_57">[Pg 57]</a></span>
+wires from a source of alternating currents <small>N</small>. Contact <i>a</i> is connected
+to brush <small>G</small> and coil <small>B</small> through a dead resistance <small>R</small> and
+wire <small>P</small>. Contact <i>b</i> is connected with brush <small>F</small> and coil <small>A</small> through
+a self-induction coil <small>S</small> and wire <small>O</small>. Contacts <i>c</i> and <i>e</i> are connected
+to brushes <small>G F</small>, respectively, through the wires <small>P O</small>, and contact
+<i>d</i> is directly connected with brush <small>H</small>. The lever <i>f</i> has a widened
+end, which may span the contacts <i>a b</i>. When in such position
+and with lever <i>g</i> on contact <i>d</i>, the alternating currents divide between
+the two motor-coils, and by reason of their different self-induction
+a difference of current-phase is obtained that starts the
+motor in rotation. In starting, the field-coils are short circuited.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_071.jpg" width="600" height="625" alt="Fig. 41." title="" />
+<span class="caption">Fig. 41.</span>
+</div>
+
+
+<p>When the motor has attained the desired speed, the switch is
+shifted to the position shown in dotted lines&mdash;that is to say, with
+the levers <i>f g</i> resting on points <i>c e</i>. This connects up the two
+armature coils in series, and the motor will then run as a synchronous
+motor. The field-coils are thrown into circuit with the
+direct current source when the main switch is shifted.</p>
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_58" id="Page_58">[Pg 58]</a></span></p>
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X.</h2>
+
+<h3><span class="smcap">Motor With "Current Lag" Artificially Secured.</span></h3>
+
+
+<p>One of the general ways followed by Mr. Tesla in developing
+his rotary phase motors is to produce practically independent
+currents differing primarily in phase and to pass these through the
+motor-circuits. Another way is to produce a single alternating
+current, to divide it between the motor-circuits, and to effect
+artificially a lag in one of these circuits or branches, as by
+giving to the circuits different self-inductive capacity, and in
+other ways. In the former case, in which the necessary difference
+of phase is primarily effected in the generation of currents,
+in some instances, the currents are passed through the energizing
+coils of both elements of the motor&mdash;the field and armature; but
+a further result or modification may be obtained by doing this
+under the conditions hereinafter specified in the case of motors
+in which the lag, as above stated, is artificially secured.</p>
+
+<p>Figs. 42 to 47, inclusive, are diagrams of different ways in which
+the invention is carried out; and Fig. 48, a side view of a form
+of motor used by Mr. Tesla for this purpose.</p>
+
+<div class="figcenter" style="width: 431px;">
+<img src="images/oi_073.jpg" width="431" height="640" alt="Figs. 42, 43 and 44." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 42, 43 and 44.</span>
+</div>
+
+<p><small>A B</small> in Fig. 42 indicate the two energizing circuits of a motor,
+and <small>C D</small> two circuits on the armature. Circuit or coil <small>A</small> is connected
+in series with circuit or coil <small>C</small>, and the two circuits <small>B D</small> are
+similarly connected. Between coils <small>A</small> and <small>C</small> is a contact-ring <i>e</i>,
+forming one terminal of the latter, and a brush <i>a</i>, forming one
+terminal of the former. A ring <i>d</i> and brush <i>c</i> similarly connect
+coils <small>B</small> and <small>D</small>. The opposite terminals of the field-coils connect
+to one binding post <i>h</i> of the motor, and those of the armature
+coils are similarly connected to the opposite binding post <i>i</i> through
+a contact-ring <i>f</i> and brush <i>g</i>. Thus each motor-circuit while in
+derivation to the other includes one armature and one field coil.
+These circuits are of different self-induction, and may be made
+so in various ways. For the sake of clearness, an artificial resistance
+<small>R</small> is shown in one of these circuits, and in the other a
+self-induction coil <small>S</small>. When an alternating current is passed
+<span class='pagenum'><a name="Page_59" id="Page_59">[Pg 59]</a></span>through this motor it divides between its two energizing-circuits.
+The higher self-induction of one circuit produces a greater retardation
+or lag in the current therein than in the other. The
+difference of phase between the two currents effects the rotation
+or shifting of the points of maximum magnetic effect that secures
+the rotation of the armature. In certain respects this plan of including
+both armature and field coils in circuit is a marked improvement.
+Such a motor has a good torque at starting; yet it
+has also considerable tendency to synchronism, owing to the fact<span class='pagenum'><a name="Page_60" id="Page_60">[Pg 60]</a></span>
+that when properly constructed the maximum magnetic effects in
+both armature and field coincide&mdash;a condition which in the usual
+construction of these motors with closed armature coils is not
+readily attained. The motor thus constructed exhibits too, a
+better regulation of current from no load to load, and there is
+less difference between the apparent and real energy expended
+in running it. The true synchronous speed of this form of motor
+is that of the generator when both are alike&mdash;that is to say, if
+the number of the coils on the armature and on the field is <i>x</i>, the
+motor will run normally at the same speed as a generator driving
+it if the number of field magnets or poles of the same be also <i>x</i>.</p>
+
+<div class="figcenter" style="width: 655px;">
+<img src="images/oi_074.jpg" width="655" height="600" alt="Figs. 45, 46 and 47." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 45, 46 and 47.</span>
+</div>
+
+<p>Fig. 43 shows a somewhat modified arrangement of circuits.
+There is in this case but one armature coil <small>E</small>, the winding of
+which maintains effects corresponding to the resultant poles produced
+by the two field-circuits.</p>
+
+<p>Fig. 44 represents a disposition in which both armature and
+field are wound with two sets of coils, all in multiple arc to the
+line or main circuit. The armature coils are wound to correspond
+with the field-coils with respect to their self-induction. A
+modification of this plan is shown in Fig. 45&mdash;that is to say, the<span class='pagenum'><a name="Page_61" id="Page_61">[Pg 61]</a></span>
+two field coils and two armature coils are in derivation to themselves
+and in series with one another. The armature coils in
+this case, as in the previous figure, are wound for different self-induction
+to correspond with the field coils.</p>
+
+<p>Another modification is shown in Fig. 46. In this case only
+one armature-coil, as <small>D</small>, is included in the line-circuit, while the
+other, as <small>C</small>, is short-circuited.</p>
+
+<p>In such a disposition as that shown in Fig. 43, or where only
+one armature-coil is employed, the torque on the start is somewhat
+reduced, while the tendency to synchronism is somewhat
+increased. In such a disposition as shown in Fig. 46, the opposite
+conditions would exist. In both instances, however, there
+is the advantage of dispensing with one contact-ring.</p>
+
+
+<div class="figcenter" style="width: 585px;">
+<img src="images/oi_075.jpg" width="585" height="480" alt="Fig. 48." title="" />
+<span class="caption">Fig. 48.</span>
+</div>
+
+
+<p>In Fig. 46 the two field-coils and the armature-coil <small>D</small> are in
+multiple arc. In Fig. 47 this disposition is modified, coil <small>D</small> being
+shown in series with the two field-coils.</p>
+
+<p>Fig. 48 is an outline of the general form of motor in which
+this invention is embodied. The circuit connections between
+the armature and field coils are made, as indicated in the previous
+figures, through brushes and rings, which are not shown.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_62" id="Page_62">[Pg 62]</a></span></p>
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI.</h2>
+
+<h3><span class="smcap">Another Method of Transformation from a Torque to a
+Synchronizing Motor.</span></h3>
+
+
+<p>In a preceding chapter we have described a method by which
+Mr. Tesla accomplishes the change in his type of rotating field
+motor from a torque to a synchronizing motor. As will be observed,
+the desired end is there reached by a change in the circuit
+connections at the proper moment. We will now proceed
+to describe another way of bringing about the same result. The
+principle involved in this method is as follows:&mdash;</p>
+
+<p>If an alternating current be passed through the field coils only
+of a motor having two energizing circuits of different self-induction
+and the armature coils be short-circuited, the motor will have
+a strong torque, but little or no tendency to synchronism with
+the generator; but if the same current which energizes the field
+be passed also through the armature coils the tendency to remain
+in synchronism is very considerably increased. This is due to
+the fact that the maximum magnetic effects produced in the field
+and armature more nearly coincide. On this principle Mr.
+Tesla constructs a motor having independent field circuits of
+different self-induction, which are joined in derivation to a
+source of alternating currents. The armature is wound with one
+or more coils, which are connected with the field coils through
+contact rings and brushes, and around the armature coils a shunt
+is arranged with means for opening or closing the same. In starting
+this motor the shunt is closed around the armature coils,
+which will therefore be in closed circuit. When the current is
+directed through the motor, it divides between the two circuits,
+(it is not necessary to consider any case where there are more
+than two circuits used), which, by reason of their different self-induction,
+secure a difference of phase between the two currents
+in the two branches, that produces a shifting or rotation of the
+poles. By the alternations of current, other currents are
+induced in the closed&mdash;or short-circuited&mdash;armature coils and the<span class='pagenum'><a name="Page_63" id="Page_63">[Pg 63]</a></span>
+motor has a strong torque. When the desired speed is reached,
+the shunt around the armature-coils is opened and the current
+directed through both armature and field coils. Under these
+conditions the motor has a strong tendency to synchronism.</p>
+
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_077.jpg" width="600" height="667" alt="Figs. 49, 50 and 51." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 49, 50 and 51.</span>
+</div>
+
+<p>In Fig. 49, <small>A</small> and <small>B</small> designate the field coils of the motor. As
+the circuits including these coils are of different self-induction,
+this is represented by a resistance coil <small>R</small> in circuit with <small>A</small>, and a
+self-induction coil <small>S</small> in circuit with <small>B</small>. The same result may of
+course be secured by the winding of the coils. <small>C</small> is the armature
+circuit, the terminals of which are rings <i>a b</i>. Brushes <i>c d</i> bear
+on these rings and connect with the line and field circuits. <small>D</small> is
+the shunt or short circuit around the armature. <small>E</small> is the switch
+in the shunt.</p>
+
+<p>It will be observed that in such a disposition as is illustrated in<span class='pagenum'><a name="Page_64" id="Page_64">[Pg 64]</a></span>
+Fig. 49, the field circuits <small>A</small> and <small>B</small> being of different self-induction,
+there will always be a greater lag of the current in one than the
+other, and that, generally, the armature phases will not correspond
+with either, but with the resultant of both. It is therefore
+important to observe the proper rule in winding the armature.
+For instance, if the motor have eight poles&mdash;four in each circuit&mdash;there
+will be four resultant poles, and hence the armature
+winding should be such as to produce four poles, in order to constitute
+a true synchronizing motor.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_078.jpg" width="640" height="307" alt="Fig. 52." title="" />
+<span class="caption">Fig. 52.</span>
+</div>
+
+
+<p>The diagram, Fig. 50, differs from the previous one only in
+respect to the order of connections. In the present case the armature-coil,
+instead of being in series with the field-coils, is in multiple
+arc therewith. The armature-winding may be similar to
+that of the field&mdash;that is to say, the armature may have two or
+more coils wound or adapted for different self-induction and
+adapted, preferably, to produce the same difference of
+phase as the field-coils. On starting the motor the shunt
+is closed around both coils. This is shown in Fig. 51, in
+which the armature coils are <small>F G</small>. To indicate their different
+electrical character, there are shown in circuit with them, respectively,
+the resistance <small>R'</small> and the self-induction coil <small>S'</small>. The two
+armature coils are in series with the field-coils and the same disposition
+of the shunt or short-circuit <small>D</small> is used. It is of advantage
+in the operation of motors of this kind to construct or wind
+the armature in such manner that when short-circuited on the
+start it will have a tendency to reach a higher speed than that
+which synchronizes with the generator. For example, a given
+motor having, say, eight poles should run, with the armature coil
+short-circuited, at two thousand revolutions per minute to bring
+it up to synchronism. It will generally happen, however, that<span class='pagenum'><a name="Page_65" id="Page_65">[Pg 65]</a></span>
+this speed is not reached, owing to the fact that the armature
+and field currents do not properly correspond, so that when the
+current is passed through the armature (the motor not being
+quite up to synchronism) there is a liability that it will not "hold
+on," as it is termed. It is preferable, therefore, to so wind or
+construct the motor that on the start, when the armature coils
+are short-circuited, the motor will tend to reach a speed higher
+than the synchronous&mdash;as for instance, double the latter. In
+such case the difficulty above alluded to is not felt, for the motor
+will always hold up to synchronism if the synchronous speed&mdash;in
+the case supposed of two thousand revolutions&mdash;is reached or
+passed. This may be accomplished in various ways; but for all
+practical purposes the following will suffice: On the armature
+are wound two sets of coils. At the start only one of these is
+short-circuited, thereby producing a number of poles on the armature,
+which will tend to run the speed up above the synchronous
+limit. When such limit is reached or passed, the current is
+directed through the other coil, which, by increasing the number
+of armature poles, tends to maintain synchronism.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_079.jpg" width="640" height="332" alt="Fig. 53." title="" />
+<span class="caption">Fig. 53.</span>
+</div>
+
+<p>In Fig. 52, such a disposition is shown. The motor having,
+say, eight poles contains two field-circuits <small>A</small> and <small>B</small>, of different
+self-induction. The armature has two coils <small>F</small> and <small>G</small>. The former
+is closed upon itself, the latter connected with the field and line
+through contact-rings <i>a b</i>, brushes <i>c d</i>, and a switch <small>E</small>. On the
+start the coil <small>F</small> alone is active and the motor tends to run at a
+speed above the synchronous; but when the coil <small>G</small> is connected
+to the circuit the number of armature poles is increased, while
+the motor is made a true synchronous motor. This disposition<span class='pagenum'><a name="Page_66" id="Page_66">[Pg 66]</a></span>
+has the advantage that the closed armature-circuit imparts to the
+motor torque when the speed falls off, but at the same time the
+conditions are such that the motor comes out of synchronism
+more readily. To increase the tendency to synchronism, two
+circuits may be used on the armature, one of which is short-circuited
+on the start and both connected with the external circuit
+after the synchronous speed is reached or passed. This disposition
+is shown in Fig. 53. There are three contact-rings <i>a b e</i>
+and three brushes <i>c d f</i>, which connect the armature circuits
+with the external circuit. On starting, the switch <small>H</small> is turned to
+complete the connection between one binding-post <small>P</small> and the field-coils.
+This short-circuits one of the armature-coils, as <small>G</small>. The
+other coil <small>F</small> is out of circuit and open. When the motor is up
+to speed, the switch <small>H</small> is turned back, so that the connection
+from binding-post <small>P</small> to the field coils is through the coil <small>G</small>, and
+switch <small>K</small> is closed, thereby including coil <small>F</small> in multiple arc with
+the field coils. Both armature coils are thus active.</p>
+
+<p>From the above-described instances it is evident that many
+other dispositions for carrying out the invention are possible.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_67" id="Page_67">[Pg 67]</a></span></p>
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII.</h2>
+
+<h3><span class="smcap">"Magnetic Lag" Motor.</span></h3>
+
+
+<p>The following description deals with another form of motor,
+namely, depending on "magnetic lag" or hysteresis, its peculiarity
+being that in it the attractive effects or phases while lagging
+behind the phases of current which produce them, are manifested
+simultaneously and not successively. The phenomenon
+utilized thus at an early stage by Mr. Tesla, was not generally
+believed in by scientific men, and Prof. Ayrton was probably
+first to advocate it or to elucidate the reason of its supposed existence.</p>
+
+<p>Fig. 54 is a side view of the motor, in elevation. Fig. 55 is
+a part-sectional view at right angles to Fig. 54. Fig. 56 is an
+end view in elevation and part section of a modification, and
+Fig. 57 is a similar view of another modification.</p>
+
+<p>In Figs. 54 and 55, <small>A</small> designates a base or stand, and B B
+the supporting-frame of the motor. Bolted to the supporting-frame
+are two magnetic cores or pole-pieces <small>C C'</small>, of iron or
+soft steel. These may be subdivided or laminated, in which
+case hard iron or steel plates or bars should be used, or they
+should be wound with closed coils. <small>D</small> is a circular disc armature,
+built up of sections or plates of iron and mounted in the
+frame between the pole-pieces <small>C C'</small>, curved to conform to the
+circular shape thereof. This disc may be wound with a number
+of closed coils <small>E</small>. <small>F F</small> are the main energizing coils, supported
+by the supporting-frame, so as to include within their magnetizing
+influence both the pole-pieces <small>C C'</small> and the armature <small>D</small>.
+The pole-pieces <small>C C'</small> project out beyond the coils <small>F F</small> on opposite
+sides, as indicated in the drawings. If an alternating
+current be passed through the coils <small>F F</small>, rotation of the armature
+will be produced, and this rotation is explained by the
+following apparent action, or mode of operation: An impulse
+of current in the coils <small>F F</small> establishes two polarities in the motor.
+The protruding end of pole-piece <small>C</small>, for instance, will be<span class='pagenum'><a name="Page_68" id="Page_68">[Pg 68]</a></span>
+of one sign, and the corresponding end of pole-piece <small>C'</small> will be
+of the opposite sign. The armature also exhibits two poles at
+right angles to the coils <small>F F</small>, like poles to those in the pole-pieces
+being on the same side of the coils. While the current
+is flowing there is no appreciable tendency to rotation developed;
+but after each current impulse ceases or begins to fall,
+the magnetism in the armature and in the ends of the pole-pieces
+<small>C C'</small> lags or continues to manifest itself, which produces a
+rotation of the armature by the repellent force between the
+more closely approximating points of maximum magnetic effect.
+This effect is continued by the reversal of current, the polarities
+of field and armature being simply reversed. One or both
+of the elements&mdash;the armature or field&mdash;may be wound with
+closed induced coils to intensify this effect. Although in the
+illustrations but one of the fields is shown, each element of the
+motor really constitutes a field, wound with the closed coils,
+the currents being induced mainly in those convolutions or coils
+which are parallel to the coils <small>F F</small>.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_082.jpg" width="640" height="400" alt="Fig. 54, 55." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 54.</td><td class="caption">Fig. 55.</td></tr>
+</table>
+</div>
+
+<p>A modified form of this motor is shown in Fig. 56. In this
+form <small>G</small> is one of two standards that support the bearings for
+the armature-shaft. <small>H H</small> are uprights or sides of a frame, preferably
+magnetic, the ends <small>C C'</small> of which are bent in the manner
+indicated, to conform to the shape of the armature <small>D</small> and form
+field-magnet poles. The construction of the armature may be
+the same as in the previous figure, or it may be simply a magnetic
+disc or cylinder, as shown, and a coil or coils <small>F F</small> are se<span class='pagenum'><a name="Page_69" id="Page_69">[Pg 69]</a></span>cured
+in position to surround both the armature and the poles
+<small>C C'</small>. The armature is detachable from its shaft, the latter being
+passed through the armature after it has been inserted in position.
+The operation of this form of motor is the same in principle
+as that previously described and needs no further explanation.</p>
+
+<div class="figcenter" style="width: 780px;">
+<div class="figleft" style="width: 350px;">
+<img src="images/fig56.jpg" width="350" height="336" alt="Fig. 56." title="" />
+<span class="caption">Fig. 56.</span>
+</div>
+<div class="figright" style="width: 360px;">
+<img src="images/oi_083.jpg" width="360" height="360" alt="Fig. 57." title="" />
+<span class="caption">Fig. 57.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>One of the most important features in alternating current
+motors is, however, that they should be adapted to and capable
+of running efficiently on the alternating circuits in present use,
+in which almost without exception the generators yield a very
+high number of alternations. Such a motor, of the type under
+consideration, Mr. Tesla has designed by a development of the
+principle of the motor shown in Fig. 56, making a multipolar
+motor, which is illustrated in Fig. 57. In the construction of
+this motor he employs an annular magnetic frame <small>J</small>, with inwardly-extending
+ribs or projections <small>K</small>, the ends of which all
+bend or turn in one direction and are generally shaped to conform
+to the curved surface of the armature. Coils <small>F F</small> are wound
+from one part <small>K</small> to the one next adjacent, the ends or loops of
+each coil or group of wires being carried over toward the shaft,
+so as to form <big><b>U</b></big>-shaped groups of convolutions at each end of the
+armature. The pole-pieces <small>C C'</small>, being substantially concentric
+with the armature, form ledges, along which the coils are laid
+and should project to some extent beyond the the coils, as shown.
+The cylindrical or drum armature <small>D</small> is of the same construction
+as in the other motors described, and is mounted to rotate within
+the annular frame J and between the <big><b>U</b></big>-shaped ends or bends of<span class='pagenum'><a name="Page_70" id="Page_70">[Pg 70]</a></span>
+the coils <small>F</small>. The coils <small>F</small> are connected in multiple or in series
+with a source of alternating currents, and are so wound that
+with a current or current impulse of given direction they will
+make the alternate pole-pieces <small>C</small> of one polarity and the other
+pole-pieces <small>C'</small> of the opposite polarity. The principle of the
+operation of this motor is the same as the other above described,
+for, considering any two pole-pieces <small>C C'</small>, a current
+impulse passing in the coil which bridges them or is wound
+over both tends to establish polarities in their ends of opposite
+sign and to set up in the armature core between them a polarity
+of the same sign as that of the nearest pole-piece <small>C</small>. Upon the
+fall or cessation of the current impulse that established these
+polarities the magnetism which lags behind the current phase,
+and which continues to manifest itself in the polar projections
+<small>C C'</small> and the armature, produces by repulsion a rotation of the
+armature. The effect is continued by each reversal of the current.
+What occurs in the case of one pair of pole-pieces occurs
+simultaneously in all, so that the tendency to rotation of the
+armature is measured by the sum of all the forces exerted by the
+pole-pieces, as above described. In this motor also the magnetic
+lag or effect is intensified by winding one or both cores
+with closed induced coils. The armature core is shown as thus
+wound. When closed coils are used, the cores should be laminated.</p>
+
+<p>It is evident that a pulsatory as well as an alternating current
+might be used to drive or operate the motors above described.</p>
+
+<p>It will be understood that the degree of subdivision, the mass
+of the iron in the cores, their size and the number of alternations
+in the current employed to run the motor, must be taken into
+consideration in order to properly construct this motor. In other
+words, in all such motors the proper relations between the number
+of alternations and the mass, size, or quality of the iron must
+be preserved in order to secure the best results.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_71" id="Page_71">[Pg 71]</a></span></p>
+<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII.</h2>
+
+<h3><span class="smcap">Method of Obtaining Difference of Phase by Magnetic
+Shielding.</span></h3>
+
+
+<p>In that class of motors in which two or more sets of energizing
+magnets are employed, and in which by artificial means a certain
+interval of time is made to elapse between the respective maximum
+or minimum periods or phases of their magnetic attraction
+or effect, the interval or difference in phase between the two sets
+of magnets is limited in extent. It is desirable, however, for the
+economical working of such motors that the strength or attraction
+of one set of magnets should be maximum, at the time when that
+of the other set is minimum, and conversely; but these conditions
+have not heretofore been realized except in cases where the two
+currents have been obtained from independent sources in the
+same or different machines. Mr. Tesla has therefore devised a
+motor embodying conditions that approach more nearly the theoretical
+requirements of perfect working, or in other words, he
+produces artificially a difference of magnetic phase by means of
+a current from a single primary source sufficient in extent to
+meet the requirements of practical and economical working. He
+employs a motor with two sets of energizing or field magnets,
+each wound with coils connected with a source of alternating or
+rapidly-varying currents, but forming two separate paths or
+circuits. The magnets of one set are protected to a certain extent
+from the energizing action of the current by means of a
+magnetic shield or screen interposed between the magnet and its
+energizing coil. This shield is properly adapted to the conditions
+of particular cases, so as to shield or protect the main core from
+magnetization until it has become itself saturated and no longer
+capable of containing all the lines of force produced by the current.
+It will be seen that by this means the energizing action
+begins in the protected set of magnets a certain arbitrarily-determined
+period of time later than in the other, and that by
+this means alone or in conjunction with other means or devices<span class='pagenum'><a name="Page_72" id="Page_72">[Pg 72]</a></span>
+heretofore employed a practical difference of magnetic phase
+may readily be secured.</p>
+
+<p>Fig. 58 is a view of a motor, partly in section, with a diagram
+illustrating the invention. Fig. 59 is a similar view of a
+modification of the same.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_086.jpg" width="800" height="382" alt="Fig. 58, 59." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 58.</td><td class="caption">Fig. 59.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 58, which exhibits the simplest form of the invention,
+<small>A A</small> is the field-magnet of a motor, having, say, eight poles or
+inwardly-projecting cores <small>B</small> and <small>C</small>. The cores <small>B</small> form one set of
+magnets and are energized by coils <small>D</small>. The cores <small>C</small>, forming
+the other set are energized by coils <small>E</small>, and the coils are
+connected, preferably, in series with one another, in two derived
+or branched circuits, <small>F G</small>, respectively, from a suitable
+source of current. Each coil <small>E</small> is surrounded by a magnetic
+shield <small>H</small>, which is preferably composed of an annealed, insulated,
+or oxidized iron wire wrapped or wound on the coils in the manner
+indicated so as to form a closed magnetic circuit around the
+coils and between the same and the magnetic cores <small>C</small>. Between
+the pole pieces or cores <small>B C</small> is mounted the armature <small>K</small>,
+which, as is usual in this type of machines, is wound with coils
+<small>L</small> closed upon themselves. The operation resulting from this
+disposition is as follows: If a current impulse be directed
+through the two circuits of the motor, it will quickly energize
+the cores <small>B</small>, but not so the cores <small>C</small>, for the reason that in
+passing through the coils <small>E</small> there is encountered the influence
+of the closed magnetic circuits formed by the shields <small>H</small>. The
+first effect is to retard effectively the current impulse in circuit
+<small>G</small>, while at the same time the proportion of current which does
+pass does not magnetize the cores <small>C</small>, which are shielded or<span class='pagenum'><a name="Page_73" id="Page_73">[Pg 73]</a></span>
+screened by the shields <small>H</small>. As the increasing electromotive
+force then urges more current through the coils <small>E</small>, the iron wire
+<small>H</small> becomes magnetically saturated and incapable of carrying all
+the lines of force, and hence ceases to protect the cores <small>C</small>, which
+becomes magnetized, developing their maximum effect after an
+interval of time subsequent to the similar manifestation of strength
+in the other set of magnets, the extent of which is arbitrarily
+determined by the thickness of the shield <small>H</small>, and other well-understood
+conditions.</p>
+
+<p>From the above it will be seen that the apparatus or device
+acts in two ways. First, by retarding the current, and, second,
+by retarding the magnetization of one set of the cores, from
+which its effectiveness will readily appear.</p>
+
+<p>Many modifications of the principle of this invention are possible.
+One useful and efficient application of the invention is
+shown in Fig. 59. In this figure a motor is shown similar in all
+respects to that above described, except that the iron wire <small>H</small>, which
+is wrapped around the coils <small>E</small>, is in this case connected in series
+with the coils <small>D</small>. The iron-wire coils <small>H</small>, are connected and wound,
+so as to have little or no self-induction, and being added to the
+resistance of the circuit <small>F</small>, the action of the current in that circuit
+will be accelerated, while in the other circuit <small>G</small> it will be
+retarded. The shield <small>H</small> may be made in many forms, as will be
+understood, and used in different ways, as appears from the
+foregoing description.</p>
+
+<p>As a modification of his type of motor with "shielded" fields,
+Mr. Tesla has constructed a motor with a field-magnet having
+two sets of poles or inwardly-projecting cores and placed side
+by side, so as practically to form two fields of force and alternately
+disposed&mdash;that is to say, with the poles of one set or field
+opposite the spaces between the other. He then connects the free
+ends of one set of poles by means of laminated iron bands or
+bridge-pieces of considerably smaller cross-section than the cores
+themselves, whereby the cores will all form parts of complete
+magnetic circuits. When the coils on each set of magnets are
+connected in multiple circuits or branches from a source of alternating
+currents, electromotive forces are set up in or impressed
+upon each circuit simultaneously; but the coils on the
+magnetically bridged or shunted cores will have, by reason of
+the closed magnetic circuits, a high self-induction, which retards
+the current, permitting at the beginning of each impulse but lit<span class='pagenum'><a name="Page_74" id="Page_74">[Pg 74]</a></span>tle
+current to pass. On the other hand, no such opposition being
+encountered in the other set of coils, the current passes freely
+through them, magnetizing the poles on which they are wound.
+As soon, however, as the laminated bridges become saturated
+and incapable of carrying all the lines of force which the rising
+electromotive force, and consequently increased current, produce,
+free poles are developed at the ends of the cores, which,
+acting in conjunction with the others, produce rotation of the
+armature.</p>
+
+<p>The construction in detail by which this invention is illustrated
+is shown in the accompanying drawings.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_088.jpg" width="800" height="340" alt="Fig. 60, 61." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 60.</td><td class="caption">Fig. 61.</td></tr>
+</table>
+</div>
+
+<p>Fig. 60 is a view in side elevation of a motor embodying the
+principle. Fig. 61 is a vertical cross-section of the motor. <small>A</small> is
+the frame of the motor, which should be built up of sheets of
+iron punched out to the desired shape and bolted together with
+insulation between the sheets. When complete, the frame makes
+a field-magnet with inwardly projecting pole-pieces <small>B</small> and <small>C</small>. To
+adapt them to the requirements of this particular case these pole-pieces
+are out of line with one another, those marked <small>B</small> surrounding
+one end of the armature and the others, as <small>C</small>, the opposite
+end, and they are disposed alternately&mdash;that is to say, the pole-pieces
+of one set occur in line with the spaces between those of the
+other sets.</p>
+
+<p>The armature <small>D</small> is of cylindrical form, and is also laminated in
+the usual way and is wound longitudinally with coils closed upon
+themselves. The pole-pieces <small>C</small> are connected or shunted by
+bridge-pieces <small>E</small>. These may be made independently and attached
+to the pole-pieces, or they may be parts of the forms or blanks
+stamped or punched out of sheet-iron. Their size or mass is
+de<span class='pagenum'><a name="Page_75" id="Page_75">[Pg 75]</a></span>termined by various conditions, such as the strength of the current
+to be employed, the mass or size of the cores to which they
+are applied, and other familiar conditions.</p>
+
+<p>Coils <small>F</small> surround the pole-pieces <small>B</small>, and other coils <small>G</small> are wound
+on the pole-pieces <small>C</small>. These coils are connected in series in two
+circuits, which are branches of a circuit from a generator of alternating
+currents, and they may be so wound, or the respective
+circuits in which they are included may be so arranged, that the
+circuit of coils <small>G</small> will have, independently of the particular construction
+described, a higher self-induction than the other circuit
+or branch.</p>
+
+<p>The function of the shunts or bridges <small>E</small> is that they shall form
+with the cores <small>C</small> a closed magnetic circuit for a current up to a
+predetermined strength, so that when saturated by such current
+and unable to carry more lines of force than such a current produces
+they will to no further appreciable extent interfere with
+the development, by a stronger current, of free magnetic poles at
+the ends of the cores <small>C</small>.</p>
+
+<p>In such a motor the current is so retarded in the coils <small>G</small>, and
+the manifestation of the free magnetism in the poles <small>C</small> is so delayed
+beyond the period of maximum magnetic effect in poles <small>B</small>, that a
+strong torque is produced and the motor operates with approximately
+the power developed in a motor of this kind energized
+by independently generated currents differing by a full quarter
+phase.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_76" id="Page_76">[Pg 76]</a></span></p>
+<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV.</h2>
+
+<h3><span class="smcap">Type of Tesla Single-Phase Motor.</span></h3>
+
+
+<p>Up to this point, two principal types of Tesla motors have
+been described: First, those containing two or more energizing
+circuits through which are caused to pass alternating currents
+differing from one another in phase to an extent sufficient to
+produce a continuous progression or shifting of the poles or
+points of greatest magnetic effect, in obedience to which the
+movable element of the motor is maintained in rotation; second,
+those containing poles, or parts of different magnetic susceptibility,
+which under the energizing influence of the same current
+or two currents coinciding in phase will exhibit differences in
+their magnetic periods or phases. In the first class of motors
+the torque is due to the magnetism established in different portions
+of the motor by currents from the same or from independent
+sources, and exhibiting time differences in phase. In
+the second class the torque results from the energizing effects of
+a current upon different parts of the motor which differ in magnetic
+susceptibility&mdash;in other words, parts which respond in the
+same relative degree to the action of a current, not simultaneously,
+but after different intervals of time.</p>
+
+<p>In another Tesla motor, however, the torque, instead of being
+solely the result of a time difference in the magnetic periods or
+phases of the poles or attractive parts to whatever cause due, is
+produced by an angular displacement of the parts which, though
+movable with respect to one another, are magnetized simultaneously,
+or approximately so, by the same currents. This principle
+of operation has been embodied practically in a motor in which
+the necessary angular displacement between the points of greatest
+magnetic attraction in the two elements of the motor&mdash;the armature
+and field&mdash;is obtained by the direction of the lamination of
+the magnetic cores of the elements.</p>
+
+<p>Fig. 62 is a side view of such a motor with a portion of its
+armature core exposed. Fig. 63 is an end or edge view of the<span class='pagenum'><a name="Page_77" id="Page_77">[Pg 77]</a></span>
+same. Fig. 64 is a central cross-section of the same, the armature
+being shown mainly in elevation.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_091.jpg" width="800" height="367" alt="Fig. 62, 63, 64." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 62.</td><td class="caption">Fig. 63.</td><td class="caption">Fig. 64.</td></tr>
+</table>
+</div>
+
+<p>Let <small>A A</small> designate two plates built up of thin sections or
+lamin&aelig; of soft iron insulated more or less from one another and
+held together by bolts <i>a</i> and secured to a base <small>B</small>. The inner
+faces of these plates contain recesses or grooves in which a coil
+or coils <small>D</small> are secured obliquely to the direction of the laminations.
+Within the coils <small>D</small> is a disc <small>E</small>, preferably composed of
+a spirally-wound iron wire or ribbon or a series of concentric
+rings and mounted on a shaft <small>F</small>, having bearings in the plates
+<small>A A</small>. Such a device when acted upon by an alternating current
+is capable of rotation and constitutes a motor, the operation of
+which may be explained in the following manner: A current or
+current-impulse traversing the coils <small>D</small> tends to magnetize the
+cores <small>A A</small> and <small>E</small>, all of which are within the influence of the
+field of the coils. The poles thus established would naturally
+lie in the same line at right angles to the coils <small>D</small>, but in the
+plates <small>A</small> they are deflected by reason of the direction of the
+laminations, and appear at or near the extremities of these plates.
+In the disc, however, where these conditions are not present, the
+poles or points of greatest attraction are on a line at right
+angles to the plane of the coils; hence there will be a torque established
+by this angular displacement of the poles or magnetic
+lines, which starts the disc in rotation, the magnetic lines of the
+armature and field tending toward a position of parallelism.
+This rotation is continued and maintained by the reversals of
+the current in coils <small>D D</small>, which change alternately the polarity of
+the field-cores <small>A A</small>. This rotary tendency or effect will be greatly<span class='pagenum'><a name="Page_78" id="Page_78">[Pg 78]</a></span>
+increased by winding the disc with conductors <small>G</small>, closed upon
+themselves and having a radial direction, whereby the magnetic
+intensity of the poles of the disc will be greatly increased by
+the energizing effect of the currents induced in the coils <small>G</small> by the
+alternating currents in coils <small>D</small>.</p>
+
+<p>The cores of the disc and field may or may not be of different
+magnetic susceptibility&mdash;that is to say, they may both be of the
+same kind of iron, so as to be magnetized at approximately the
+same instant by the coils <small>D</small>; or one may be of soft iron and the
+other of hard, in order that a certain time may elapse between
+the periods of their magnetization. In either case rotation will
+be produced; but unless the disc is provided with the closed energizing
+coils it is desirable that the above-described difference of
+magnetic susceptibility be utilized to assist in its rotation.</p>
+
+<p>The cores of the field and armature may be made in various
+ways, as will be well understood, it being only requisite that the
+laminations in each be in such direction as to secure the necessary
+angular displacement of the points of greatest attraction.
+Moreover, since the disc may be considered as made up of an
+infinite number of radial arms, it is obvious that what is true of
+a disc holds for many other forms of armature.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_79" id="Page_79">[Pg 79]</a></span></p>
+<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV.</h2>
+
+<h3><span class="smcap">Motors with Circuits of Different Resistance.</span></h3>
+
+
+<p>As has been pointed out elsewhere, the lag or retardation of
+the phases of an alternating current is directly proportional to
+the self-induction and inversely proportional to the resistance of
+the circuit through which the current flows. Hence, in order
+to secure the proper differences of phase between the two motor-circuits,
+it is desirable to make the self-induction in one much
+higher and the resistance much lower than the self-induction and
+resistance, respectively, in the other. At the same time the
+magnetic quantities of the two poles or sets of poles which the
+two circuits produce should be approximately equal. These
+requirements have led Mr. Tesla to the invention of a motor
+having the following general characteristics: The coils which
+are included in that energizing circuit which is to have the
+higher self-induction are made of coarse wire, or a conductor of
+relatively low resistance, and with the greatest possible length
+or number of turns. In the other set of coils a comparatively
+few turns of finer wire are used, or a wire of higher resistance.
+Furthermore, in order to approximate the magnetic quantities of
+the poles excited by these coils, Mr. Tesla employs in the self-induction
+circuit cores much longer than those in the other or
+resistance circuit.</p>
+
+<p>Fig. 65 is a part sectional view of the motor at right angles to
+the shaft. Fig. 66 is a diagram of the field circuits.</p>
+
+<p>In Fig. 66, let <small>A</small> represent the coils in one motor circuit, and B
+those in the other. The circuit <small>A</small> is to have the higher self-induction.
+There are, therefore, used a long length or a large
+number of turns of coarse wire in forming the coils of this circuit.
+For the circuit <small>B</small>, a smaller conductor is employed, or a
+conductor of a higher resistance than copper, such as German
+silver or iron, and the coils are wound with fewer turns. In applying
+these coils to a motor, Mr. Tesla builds up a field-magnet of
+plates <small>C</small>, of iron and steel, secured together in the usual manner<span class='pagenum'><a name="Page_80" id="Page_80">[Pg 80]</a></span>
+by bolts <small>D</small>. Each plate is formed with four (more or less) long
+cores <small>E</small>, around which is a space to receive the coil and an equal
+number of short projections <small>F</small> to receive the coils of the resistance-circuit.
+The plates are generally annular in shape, having an
+open space in the centre for receiving the armature <small>G</small>, which Mr.
+Tesla prefers to wind with closed coils. An alternating current
+divided between the two circuits is retarded as to its phases in
+the circuit <small>A</small> to a much greater extent than in the circuit <small>B</small>. By
+reason of the relative sizes and disposition of the cores and coils
+the magnetic effect of the poles <small>E</small> and <small>F</small> upon the armature closely
+approximate.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_094.jpg" width="800" height="334" alt="Fig. 65, 66." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 65.</td><td class="caption">Fig. 66.</td></tr>
+</table>
+</div>
+
+<p>An important result secured by the construction shown here
+is that these coils which are designed to have the higher self-induction
+are almost completely surrounded by iron, and that the
+retardation is thus very materially increased.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_81" id="Page_81">[Pg 81]</a></span></p>
+<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI.</h2>
+
+<h3><span class="smcap">Motor With Equal Magnetic Energies in Field and
+Armature.</span></h3>
+
+
+<p>Let it be assumed that the energy as represented in the magnetism
+in the field of a given rotating field motor is ninety and
+that of the armature ten. The sum of these quantities, which
+represents the total energy expended in driving the motor, is
+one hundred; but, assuming that the motor be so constructed
+that the energy in the field is represented by fifty, and that in
+the armature by fifty, the sum is still one hundred; but while in
+the first instance the product is nine hundred, in the second it is
+two thousand five hundred, and as the energy developed is in
+proportion to these products it is clear that those motors are the
+most efficient&mdash;other things being equal&mdash;in which the magnetic
+energies developed in the armature and field are equal. These
+results Mr. Tesla obtains by using the same amount of copper or
+ampere turns in both elements when the cores of both are equal,
+or approximately so, and the same current energizes both; or in
+cases where the currents in one element are induced to those of
+the other he uses in the induced coils an excess of copper over
+that in the primary element or conductor.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_095.jpg" width="640" height="430" alt="Fig. 67." title="" />
+<span class="caption">Fig. 67.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_82" id="Page_82">[Pg 82]</a></span></p>
+
+<p>The conventional figure of a motor here introduced, Fig. 67,
+will give an idea of the solution furnished by Mr. Tesla for the
+specific problem. Referring to the drawing, <small>A</small> is the field-magnet,
+<small>B</small> the armature, <small>C</small> the field coils, and <small>D</small> the armature-coils of
+the motor.</p>
+
+<p>Generally speaking, if the mass of the cores of armature and
+field be equal, the amount of copper or ampere turns of the
+energizing coils on both should also be equal; but these conditions
+will be modified in different forms of machine. It will be
+understood that these results are most advantageous when existing
+under the conditions presented where the motor is running
+with its normal load, a point to be well borne in mind.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_83" id="Page_83">[Pg 83]</a></span></p>
+<h2><a name="CHAPTER_XVII" id="CHAPTER_XVII"></a>CHAPTER XVII.</h2>
+
+<h3><span class="smcap">Motors With Coinciding Maxima of Magnetic Effect in
+Armature and Field.</span></h3>
+
+
+<p>In this form of motor, Mr. Tesla's object is to design and
+build machines wherein the maxima of the magnetic effects of
+the armature and field will more nearly coincide than in some of
+the types previously under consideration. These types are: First,
+motors having two or more energizing circuits of the same electrical
+character, and in the operation of which the currents used
+differ primarily in phase; second, motors with a plurality of
+energizing circuits of different electrical character, in or by
+means of which the difference of phase is produced artificially,
+and, third, motors with a plurality of energizing circuits, the
+currents in one being induced from currents in another. Considering
+the structural and operative conditions of any one of
+them&mdash;as, for example, that first named&mdash;the armature which is
+mounted to rotate in obedience to the co-operative influence or
+action of the energizing circuits has coils wound upon it which
+are closed upon themselves and in which currents are induced by
+the energizing-currents with the object and result of energizing
+the armature-core; but under any such conditions as must exist
+in these motors, it is obvious that a certain time must elapse
+between the manifestations of an energizing current impulse in
+the field coils, and the corresponding magnetic state or phase in
+the armature established by the current induced thereby; consequently
+a given magnetic influence or effect in the field which is
+the direct result of a primary current impulse will have become
+more or less weakened or lost before the corresponding effect in
+the armature indirectly produced has reached its maximum. This
+is a condition unfavorable to efficient working in certain cases&mdash;as,
+for instance, when the progress of the resultant poles or points
+of maximum attraction is very great, or when a very high number
+of alternations is employed&mdash;for it is apparent that a stronger<span class='pagenum'><a name="Page_84" id="Page_84">[Pg 84]</a></span>
+tendency to rotation will be maintained if the maximum magnetic
+attractions or conditions in both armature and field coincide,
+the energy developed by a motor being measured by the product
+of the magnetic quantities of the armature and field.</p>
+
+<p>To secure this coincidence of maximum magnetic effects, Mr.
+Tesla has devised various means, as explained below. Fig. 68 is
+a diagrammatic illustration of a Tesla motor system in which the
+alternating currents proceed from independent sources and differ
+primarily in phase.</p>
+
+<div class="figcenter" style="width: 700px;">
+<img src="images/oi_098.jpg" width="700" height="600" alt="Fig. 68, 69." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 68.</td><td class="caption">Fig. 69.</td></tr>
+</table>
+</div>
+
+<p><small>A</small> designates the field-magnet or magnetic frame of the motor;
+<small>B B</small>, oppositely located pole-pieces adapted to receive the coils of
+one energizing circuit; and <small>C C</small>, similar pole-pieces for the coils
+of the other energizing circuit. These circuits are designated,
+respectively, by <small>D E</small>, the conductor <small>D''</small> forming a common return
+to the generator <small>G</small>. Between these poles is mounted an armature&mdash;for
+example, a ring or annular armature, wound with a series
+of coils <small>F</small>, forming a closed circuit or circuits. The action or
+operation of a motor thus constructed is now well understood.
+It will be observed, however, that the magnetism of poles <small>B</small>, for<span class='pagenum'><a name="Page_85" id="Page_85">[Pg 85]</a></span>
+example, established by a current impulse in the coils thereon,
+precedes the magnetic effect set up in the armature by the induced
+current in coils <small>F</small>. Consequently the mutual attraction
+between the armature and field-poles is considerably reduced.
+The same conditions will be found to exist if, instead of assuming
+the poles <small>B</small> or <small>C</small> as acting independently, we regard the ideal resultant
+of both acting together, which is the real condition. To
+remedy this, the motor field is constructed with secondary poles
+<small>B' C'</small>, which are situated between the others. These pole-pieces
+are wound with coils <small>D' E'</small>, the former in derivation to the coils
+<small>D</small>, the latter to coils <small>E</small>. The main or primary coils <small>D</small> and <small>E</small> are
+wound for a different self-induction from that of the coils <small>D'</small> and
+<small>E'</small>, the relations being so fixed that if the currents in <small>D</small> and <small>E</small>
+differ, for example, by a quarter-phase, the currents in each
+secondary coil, as <small>D' E'</small>, will differ from those in its appropriate
+primary <small>D</small> or <small>E</small> by, say, forty-five degrees, or one-eighth of a
+period.</p>
+
+<p>Now, assuming that an impulse or alternation in circuit or
+branch <small>E</small> is just beginning, while in the branch <small>D</small> it is just falling
+from maximum, the conditions are those of a quarter-phase
+difference. The ideal resultant of the attractive forces of the two
+sets of poles <small>B C</small> therefore may be considered as progressing from
+poles <small>B</small> to poles <small>C</small>, while the impulse in <small>E</small> is rising to maximum,
+and that in <small>D</small> is falling to zero or minimum. The polarity set up
+in the armature, however, lags behind the manifestations of field
+magnetism, and hence the maximum points of attraction in armature
+and field, instead of coinciding, are angularly displaced.
+This effect is counteracted by the supplemental poles <small>B' C'</small>. The
+magnetic phases of these poles succeed those of poles <small>B C</small> by the
+same, or nearly the same, period of time as elapses between the
+effect of the poles <small>B C</small> and the corresponding induced effect in the
+armature; hence the magnetic conditions of poles <small>B' C'</small> and of
+the armature more nearly coincide and a better result is obtained.
+As poles <small>B' C'</small> act in conjunction with the poles in the armature
+established by poles <small>B C</small>, so in turn poles <small>C B</small> act similarly with
+the poles set up by <small>B' C'</small>, respectively. Under such conditions
+the retardation of the magnetic effect of the armature and that
+of the secondary poles will bring the maximum of the two more
+nearly into coincidence and a correspondingly stronger torque or
+magnetic attraction secured.</p>
+
+<p>In such a disposition as is shown in Fig. 68 it will be observed<span class='pagenum'><a name="Page_86" id="Page_86">[Pg 86]</a></span>
+that as the adjacent pole-pieces of either circuit are of like polarity
+they will have a certain weakening effect upon one another.
+Mr. Tesla therefore prefers to remove the secondary poles from
+the direct influence of the others. This may be done by constructing
+a motor with two independent sets of fields, and with
+either one or two armatures electrically connected, or by using
+two armatures and one field. These modifications are illustrated
+further on.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_100.jpg" width="800" height="564" alt="Fig. 70, 71." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 70.</td><td class="caption">Fig. 71.</td></tr>
+</table>
+</div>
+
+<p>Fig. 69 is a diagrammatic illustration of a motor and system in
+which the difference of phase is artificially produced. There are
+two coils <small>D D</small> in one branch and two coils <small>E E</small> in another branch
+of the main circuit from the generator <small>G</small>. These two circuits or
+branches are of different self-induction, one, as <small>D</small>, being higher
+than the other. This is graphically indicated by making coils <small>D</small>
+much larger than coils <small>E</small>. By reason of the difference in the
+electrical character of the two circuits, the phases of current in
+one are retarded to a greater extent than the other. Let this
+difference be thirty degrees. A motor thus constructed will
+rotate under the action of an alternating current; but as happens
+in the case previously described the corresponding magnetic effects
+of the armature and field do not coincide owing to the time
+that elapses between a given magnetic effect in the armature and<span class='pagenum'><a name="Page_87" id="Page_87">[Pg 87]</a></span>
+the condition of the field that produces it. The secondary or
+supplemental poles <small>B' C'</small> are therefore availed of. There being
+thirty degrees difference of phase between the currents in coils
+<small>D E</small>, the magnetic effect of poles <small>B' C'</small> should correspond to that
+produced by a current differing from the current in coils <small>D</small> or <small>E</small>
+by fifteen degrees. This we can attain by winding each supplemental
+pole <small>B' C'</small> with two coils <small>H H'</small>. The coils <small>H</small> are included
+in a derived circuit having the same self-induction as circuit <small>D</small>,
+and coils <small>H'</small> in a circuit having the same self-induction as circuit
+<small>E</small>, so that if these circuits differ by thirty degrees the magnetism
+of poles <small>B' C'</small> will correspond to that produced by a current differing
+from that in either <small>D</small> or <small>E</small> by fifteen degrees. This is true
+in all other cases. For example, if in Fig. 68 the coils <small>D' E'</small> be
+replaced by the coils <small>H H'</small> included in the derived circuits, the
+magnetism of the poles <small>B' C'</small> will correspond in effect or phase,
+if it may be so termed, to that produced by a current differing
+from that in either circuit <small>D</small> or <small>E</small> by forty-five degrees, or one-eighth
+of a period.</p>
+
+<p>This invention as applied to a derived circuit motor is illustrated
+in Figs. 70 and 71. The former is an end view of the motor
+with the armature in section and a diagram of connections, and
+Fig. 71 a vertical section through the field. These figures are
+also drawn to show one of the dispositions of two fields that may
+be adopted in carrying out the principle. The poles <small>B B C C</small> are
+in one field, the remaining poles in the other. The former are
+wound with primary coils <small>I J</small> and secondary coils <small>I' J'</small>, the latter
+with coils <small>K L</small>. The primary coils <small>I J</small> are in derived circuits, between
+which, by reason of their different self-induction, there is
+a difference of phase, say, of thirty degrees. The coils <small>I' K</small> are
+in circuit with one another, as also are coils <small>J' L</small>, and there should
+be a difference of phase between the currents in coils <small>K</small> and <small>L</small> and
+their corresponding primaries of, say, fifteen degrees. If the
+poles <small>B C</small> are at right angles, the armature-coils should be connected
+directly across, or a single armature core wound from end
+to end may be used; but if the poles <small>B C</small> be in line there should
+be an angular displacement of the armature coils, as will be well
+understood.</p>
+
+<p>The operation will be understood from the foregoing. The
+maximum magnetic condition of a pair of poles, as <small>B' B'</small>, coincides
+closely with the maximum effect in the armature, which lags behind
+the corresponding condition in poles <small>B B</small>.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_88" id="Page_88">[Pg 88]</a></span></p>
+<h2><a name="CHAPTER_XVIII" id="CHAPTER_XVIII"></a>CHAPTER XVIII.</h2>
+
+<h3><span class="smcap">Motor Based on the Difference of Phase in the Magnetization
+of the Inner and Outer Parts of an Iron Core.</span></h3>
+
+
+<p>It is well known that if a magnetic core, even if laminated or
+subdivided, be wound with an insulated coil and a current of
+electricity be directed through the coil, the magnetization of the
+entire core does not immediately ensue, the magnetizing effect
+not being exhibited in all parts simultaneously. This may be attributed
+to the fact that the action of the current is to energize
+first those lamin&aelig; or parts of the core nearest the surface and
+adjacent to the exciting-coil, and from thence the action progresses
+toward the interior. A certain interval of time therefore
+elapses between the manifestation of magnetism in the external
+and the internal sections or layers of the core. If the core be
+thin or of small mass, this effect may be inappreciable; but in
+the case of a thick core, or even of a comparatively thin one, if
+the number of alternations or rate of change of the current
+strength be very great, the time interval occurring between the
+manifestations of magnetism in the interior of the core and in
+those parts adjacent to the coil is more marked. In the construction
+of such apparatus as motors which are designed to be
+run by alternating or equivalent currents&mdash;such as pulsating or
+undulating currents generally&mdash;Mr. Tesla found it desirable and
+even necessary to give due consideration to this phenomenon and
+to make special provisions in order to obviate its consequences.
+With the specific object of taking advantage of this action or
+effect, and to render it more pronounced, he constructs a field
+magnet in which the parts of the core or cores that exhibit at
+different intervals of time the magnetic effect imparted to them
+by alternating or equivalent currents in an energizing coil or coils,
+are so placed with relation to a rotating armature as to exert
+thereon their attractive effect successively in the order of their
+magnetization. By this means he secures a result similar to that
+which he had previously attained in other forms or types of mo<span class='pagenum'><a name="Page_89" id="Page_89">[Pg 89]</a></span>tor
+in which by means of one or more alternating currents he
+has produced the rotation or progression of the magnetic poles.</p>
+
+<p>This new mode of operation will now be described. Fig. 72
+is a side elevation of such motor. Fig. 73 is a side elevation of
+a more practicable and efficient embodiment of the invention.
+Fig. 74 is a central vertical section of the same in the plane of
+the axis of rotation.</p>
+
+<div class="figcenter" style="width: 668px;">
+<img src="images/oi_103.jpg" width="668" height="600" alt="Fig. 72 and 73." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 72 and 73.</span>
+</div>
+
+<p>Referring to Fig. 72, let <small>X</small> represent a large iron core, which
+may be composed of a number of sheets or lamin&aelig; of soft iron
+or steel. Surrounding this core is a coil <small>Y</small>, which is connected
+with a source <small>E</small> of rapidly varying currents. Let us consider now
+the magnetic conditions existing in this core at any point, as <i>b</i>,
+at or near the centre, and any other point, as <i>a</i>, nearer the surface.
+When a current impulse is started in the magnetizing coil
+<small>Y</small>, the section or part at <i>a</i>, being close to the coil, is immediately
+energized, while the section or part at <i>b</i>, which, to use a convenient
+expression, is "protected" by the intervening sections or
+layers between <i>a</i> and <i>b</i>, does not at once exhibit its magnetism.
+However, as the magnetization of <i>a</i> increases, <i>b</i> becomes also
+affected, reaching finally its maximum strength some time later
+than <i>a</i>. Upon the weakening of the current the magnetization
+of <i>a</i> first diminishes, while <i>b</i> still exhibits its maximum strength;<span class='pagenum'><a name="Page_90" id="Page_90">[Pg 90]</a></span>
+but the continued weakening of <i>a</i> is attended by a subsequent
+weakening of <i>b</i>. Assuming the current to be an alternating one,
+<i>a</i> will now be reversed, while <i>b</i> still continues of the first imparted
+polarity. This action continues the magnetic condition of <i>b</i>, following
+that of <i>a</i> in the manner above described. If an armature&mdash;for
+instance, a simple disc <small>F</small>, mounted to rotate freely on an
+axis&mdash;be brought into proximity to the core, a movement of rotation
+will be imparted to the disc, the direction depending upon
+its position relatively to the core, the tendency being to turn the
+portion of the disc nearest to the core from <i>a</i> to <i>b</i>, as indicated
+in Fig. 72.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_104.jpg" width="336" height="443" alt="Fig. 74." title="" />
+<span class="caption">Fig. 74.</span>
+</div>
+
+<p>This action or principle of operation has been embodied in a
+practicable form of motor, which is illustrated in Fig. 73. Let <small>A</small>
+in that figure represent a circular frame of iron, from diametrically
+opposite points of the interior of which the cores project.
+Each core is composed of three main parts <small>B</small>, <small>B</small> and <small>C</small>, and they
+are similarly formed with a straight portion or body <i>e</i>, around
+which the energizing coil is wound, a curved arm or extension <i>c</i>,
+and an inwardly projecting pole or end <i>d</i>. Each core is made up
+of two parts <small>B B</small>, with their polar extensions reaching in one
+direction, and a part <small>C</small> between the other two, and with its polar
+extension reaching in the opposite direction. In order to lessen
+in the cores the circulation of currents induced therein, the several
+sections are insulated from one another in the manner usually<span class='pagenum'><a name="Page_91" id="Page_91">[Pg 91]</a></span>
+followed in such cases. These cores are wound with coils <small>D</small>, which
+are connected in the same circuit, either in parallel or series, and
+supplied with an alternating or a pulsating current, preferably
+the former, by a generator <small>E</small>, represented diagrammatically. Between
+the cores or their polar extensions is mounted a cylindrical
+or similar armature <small>F</small>, wound with magnetizing coils <small>G</small>, closed
+upon themselves.</p>
+
+<p>The operation of this motor is as follows: When a current
+impulse or alternation is directed through the coils <small>D</small>, the sections
+<small>B B</small> of the cores, being on the surface and in close proximity to
+the coils, are immediately energized. The sections <small>C</small>, on the other
+hand, are protected from the magnetizing influence of the coil
+by the interposed layers of iron <small>B B</small>. As the magnetism of <small>B B</small>
+increases, however, the sections <small>C</small> are also energized; but they
+do not attain their maximum strength until a certain time subsequent
+to the exhibition by the sections <small>B B</small> of their maximum.
+Upon the weakening of the current the magnetic strength of <small>B B</small>
+first diminishes, while the sections <small>C</small> have still their maximum
+strength; but as <small>B B</small> continue to weaken the interior sections are
+similarly weakened. <small>B B</small> may then begin to exhibit an opposite
+polarity, which is followed later by a similar change on <small>C</small>, and
+this action continues. <small>B B</small> and <small>C</small> may therefore be considered as
+separate field-magnets, being extended so as to act on the armature
+in the most efficient positions, and the effect is similar to
+that in the other forms of Tesla motor&mdash;viz., a rotation or progression
+of the maximum points of the field of force. Any
+armature&mdash;such, for instance, as a disc&mdash;mounted in this field
+would rotate from the pole first to exhibit its magnetism to that
+which exhibits it later.</p>
+
+<p>It is evident that the principle here described may be carried
+out in conjunction with other means for securing a more favorable
+or efficient action of the motor. For example, the polar
+extensions of the sections <small>C</small> may be wound or surrounded by
+closed coils. The effect of these coils will be to still more
+effectively retard the magnetization of the polar extensions of <small>C</small>.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_92" id="Page_92">[Pg 92]</a></span></p>
+<h2><a name="CHAPTER_XIX" id="CHAPTER_XIX"></a>CHAPTER XIX.</h2>
+
+<h3><span class="smcap">Another Type of Tesla Induction Motor.</span></h3>
+
+
+<p>It will have been gathered by all who are interested in the
+advance of the electrical arts, and who follow carefully, step by
+step, the work of pioneers, that Mr. Tesla has been foremost to
+utilize inductive effects in permanently closed circuits, in the
+operation of alternating motors. In this chapter one simple type
+of such a motor is described and illustrated, which will serve as
+an exemplification of the principle.</p>
+
+<p>Let it be assumed that an ordinary alternating current generator
+is connected up in a circuit of practically no self-induction,
+such, for example, as a circuit containing incandescent lamps
+only. On the operation of the machine, alternating currents will
+be developed in the circuit, and the phases of these currents will
+theoretically coincide with the phases of the impressed electromotive
+force. Such currents may be regarded and designated as
+the "unretarded currents."</p>
+
+<p>It will be understood, of course, that in practice there is always
+more or less self-induction in the circuit, which modifies to
+a corresponding extent these conditions; but for convenience
+this may be disregarded in the consideration of the principle of
+operation, since the same laws apply. Assume next that a path
+of currents be formed across any two points of the above circuit,
+consisting, for example, of the primary of an induction device.
+The phases of the currents passing through the primary,
+owing to the self-induction of the same, will not coincide with
+the phases of the impressed electromotive force, but will lag
+behind, such lag being directly proportional to the self-induction
+and inversely proportional to the resistance of the said coil.
+The insertion of this coil will also cause a lagging or retardation
+of the currents traversing and delivered by the generator behind
+the impressed electromotive force, such lag being the mean or
+resultant of the lag of the current through the primary alone and
+of the "unretarded current" in the entire working circuit. Next<span class='pagenum'><a name="Page_93" id="Page_93">[Pg 93]</a></span>
+consider the conditions imposed by the association in inductive
+relation with the primary coil, of a secondary coil. The current
+generated in the secondary coil will react upon the primary current,
+modifying the retardation of the same, according to the
+amount of self-induction and resistance in the secondary circuit.
+If the secondary circuit has but little self-induction&mdash;as, for instance,
+when it contains incandescent lamps only&mdash;it will increase
+the actual difference of phase between its own and the
+primary current, first, by diminishing the lag between the primary
+current and the impressed electromotive force, and, second,
+by its own lag or retardation behind the impressed electromotive
+force. On the other hand, if the secondary circuit have
+a high self-induction, its lag behind the current in the primary is
+directly increased, while it will be still further increased if the
+primary have a very low self-induction. The better results are
+obtained when the primary has a low self-induction.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_107.jpg" width="800" height="429" alt="Fig. 75, 76." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 75.</td><td class="caption">Fig. 76.</td></tr>
+</table>
+</div>
+
+<p>Fig. 75 is a diagram of a Tesla motor embodying this principle.
+Fig. 76 is a similar diagram of a modification of the same.
+In Fig. 75 let <small>A</small> designate the field-magnet of a motor which, as
+in all these motors, is built up of sections or plates. <small>B C</small> are polar
+projections upon which the coils are wound. Upon one pair
+of these poles, as <small>C</small>, are wound primary coils <small>D</small>, which are directly
+connected to the circuit of an alternating current generator
+<small>G</small>. On the same poles are also wound secondary coils <small>F</small>,
+either side by side or over or under the primary coils, and these
+are connected with other coils <small>E</small>, which surround the poles <small>B B</small>.<span class='pagenum'><a name="Page_94" id="Page_94">[Pg 94]</a></span>
+The currents in both primary and secondary coils in such a motor
+will be retarded or will lag behind the impressed electromotive
+force; but to secure a proper difference in phase between
+the primary and secondary currents themselves, Mr. Tesla increases
+the resistance of the circuit of the secondary and reduces
+as much as practicable its self-induction. This is done by using
+for the secondary circuit, particularly in the coils <small>E</small>, wire of comparatively
+small diameter and having but few turns around the
+cores; or by using some conductor of higher specific resistance,
+such as German silver; or by introducing at some point in the
+secondary circuit an artificial resistance <small>R</small>. Thus the self-induction
+of the secondary is kept down and its resistance increased,
+with the result of decreasing the lag between the impressed
+electro-motive force and the current in the primary coils and increasing
+the difference of phase between the primary and secondary
+currents.</p>
+
+<p>In the disposition shown in Fig. 76, the lag in the secondary
+is increased by increasing the self-induction of that circuit, while
+the increasing tendency of the primary to lag is counteracted by
+inserting therein a dead resistance. The primary coils <small>D</small> in this
+case have a low self-induction and high resistance, while the coils
+<small>E F</small>, included in the secondary circuit, have a high self-induction
+and low resistance. This may be done by the proper winding of
+the coils; or in the circuit including the secondary coils <small>E F</small>, we
+may introduce a self-induction coil <small>S</small>, while in the primary circuit
+from the generator <small>G</small> and including coils <small>D</small>, there may be inserted
+a dead resistance <small>R</small>. By this means the difference of
+phase between the primary and secondary is increased. It is evident
+that both means of increasing the difference of phase&mdash;namely,
+by the special winding as well as by the supplemental or
+external inductive and dead resistance&mdash;may be employed conjointly.</p>
+
+<p>In the operation of this motor the current impulses in the primary
+coils induce currents in the secondary coils, and by the conjoint
+action of the two the points of greatest magnetic attraction
+are shifted or rotated.</p>
+
+<p>In practice it is found desirable to wind the armature with
+closed coils in which currents are induced by the action thereon
+of the primaries.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_95" id="Page_95">[Pg 95]</a></span></p>
+<h2><a name="CHAPTER_XX" id="CHAPTER_XX"></a>CHAPTER XX.</h2>
+
+<h3><span class="smcap">Combinations of Synchronizing Motor and Torque Motor.</span></h3>
+
+
+<p>In the preceding descriptions relative to synchronizing motors
+and methods of operating them, reference has been made to the
+plan adopted by Mr. Tesla, which consists broadly in winding or
+arranging the motor in such manner that by means of suitable
+switches it could be started as a multiple-circuit motor, or one
+operating by a progression of its magnetic poles, and then, when
+up to speed, or nearly so, converted into an ordinary synchronizing
+motor, or one in which the magnetic poles were simply alternated.
+In some cases, as when a large motor is used and when
+the number of alternations is very high, there is more or less
+difficulty in bringing the motor to speed as a double or multiple-circuit
+motor, for the plan of construction which renders the
+motor best adapted to run as a synchronizing motor impairs its
+efficiency as a torque or double-circuit motor under the assumed
+conditions on the start. This will be readily understood, for in a
+large synchronizing motor the length of the magnetic circuit of
+the polar projections, and their mass, are so great that apparently
+considerable time is required for magnetization and demagnetization.
+Hence with a current of a very high number of alternations
+the motor may not respond properly. To avoid this objection
+and to start up a synchronizing motor in which these conditions
+obtain, Mr. Tesla has combined two motors, one a synchronizing
+motor, the other a multiple-circuit or torque motor, and by the
+latter he brings the first-named up to speed, and then either
+throws the whole current into the synchronizing motor or operates
+jointly both of the motors.</p>
+
+<p>This invention involves several novel and useful features. It
+will be observed, in the first place, that both motors are run,
+without commutators of any kind, and, secondly, that the speed
+of the torque motor may be higher than that of the synchronizing
+motor, as will be the case when it contains a fewer number of
+poles or sets of poles, so that the motor will be more readily and<span class='pagenum'><a name="Page_96" id="Page_96">[Pg 96]</a></span>
+easily brought up to speed. Thirdly, the synchronizing motor
+may be constructed so as to have a much more pronounced tendency
+to synchronism without lessening the facility with which
+it is started.</p>
+
+<p>Fig. 77 is a part sectional view of the two motors; Fig. 78 an
+end view of the synchronizing motor; Fig. 79 an end view and
+part section of the torque or double-circuit motor; Fig. 80 a
+diagram of the circuit connections employed; and Figs. 81, 82,
+83, 84 and 85 are diagrams of modified dispositions of the two
+motors.</p>
+
+
+<div class="figcenter" style="width: 629px;">
+<img src="images/oi_110.jpg" width="629" height="480" alt="Fig. 77." title="" />
+<span class="caption">Fig. 77.</span>
+</div>
+
+<p>Inasmuch as neither motor is doing any work while the current
+is acting upon the other, the two armatures are rigidly connected,
+both being mounted upon the same shaft <small>A</small>, the field-magnets <small>B</small>
+of the synchronizing and <small>C</small> of the torque motor being secured to
+the same base <small>D</small>. The preferably larger synchronizing motor has
+polar projections on its armature, which rotate in very close proximity
+to the poles of the field, and in other respects it conforms
+to the conditions that are necessary to secure synchronous action.
+The pole-pieces of the armature are, however, wound with closed
+coils <small>E</small>, as this obviates the employment of sliding contacts. The
+smaller or torque motor, on the other hand, has, preferably, a
+cylindrical armature <small>F</small>, without polar projections and wound with
+closed coils <small>G</small>. The field-coils of the torque motor are connected
+up in two series <small>H</small> and <small>I</small>, and the alternating current from the
+generator is directed through or divided between these two circuits
+in any manner to produce a progression of the poles or
+points of maximum magnetic effect. This result is secured by
+connecting the two motor-circuits in derivation with the circuit<span class='pagenum'><a name="Page_97" id="Page_97">[Pg 97]</a></span>
+from the generator, inserting in one motor circuit a dead resistance
+and in the other a self-induction coil, by which means a
+difference in phase between the two divisions of the current is
+secured. If both motors have the same number of field poles,
+the torque motor for a given number of alternations will tend to
+run at double the speed of the other, for, assuming the connections
+to be such as to give the best results, its poles are divided
+into two series and the number of poles is virtually reduced one-half,
+which being acted upon by the same number of alternations
+tend to rotate the armature at twice the speed. By this means
+the main armature is more easily brought to or above the required
+speed. When the speed necessary for synchronism is imparted
+to the main motor, the current is shifted from the torque motor
+into the other.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_111.jpg" width="800" height="411" alt="Fig. 78, 79." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 78.</td><td class="caption">Fig. 79.</td></tr>
+</table>
+</div>
+
+<p>A convenient arrangement for carrying out this invention is
+shown in Fig. 80, in which <small>J J</small> are the field coils of the synchronizing,
+and <small>H I</small> the field coils of the torque motor. <small>L L'</small> are
+the conductors of the main line. One end of, say, coils <small>H</small> is connected
+to wire <small>L</small> through a self-induction coil <small>M</small>. One end of the
+other set of coils <small>I</small> is connected to the same wire through a dead
+resistance <small>N</small>. The opposite ends of these two circuits are connected
+to the contact <i>m</i> of a switch, the handle or lever of which
+is in connection with the line-wire <small>L'</small>. One end of the field circuit
+of the synchronizing motor is connected to the wire <small>L</small>. The
+other terminates in the switch-contact <i>n</i>. From the diagram it
+will be readily seen that if the lever <small>P</small> be turned upon contact <i>m</i>,
+the torque motor will start by reason of the difference of phase
+between the currents in its two energizing circuits. Then when
+the desired speed is attained, if the lever <small>P</small> be shifted upon con<span class='pagenum'><a name="Page_98" id="Page_98">[Pg 98]</a></span>tact
+<i>n</i> the entire current will pass through the field coils of the
+synchronizing motor and the other will be doing no work.</p>
+
+<p>The torque motor may be constructed and operated in various
+ways, many of which have already been touched upon. It is not
+necessary that one motor be cut out of circuit while the other is
+in, for both may be acted upon by current at the same time, and
+Mr. Tesla has devised various dispositions or arrangements of the
+two motors for accomplishing this. Some of these arrangements
+are illustrated in Figs. 81 to 85.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_112.jpg" width="640" height="428" alt="Fig. 80." title="" />
+<span class="caption">Fig. 80.</span>
+</div>
+
+
+<p>Referring to Fig. 81, let <small>T</small> designate the torque or multiple
+circuit motor and <small>S</small> the synchronizing motor, <small>L L'</small> being the line-wires
+from a source of alternating current. The two circuits of
+the torque motor of different degrees of self-induction, and designated
+by <small>N M</small>, are connected in derivation to the wire L. They
+are then joined and connected to the energizing circuit of the
+synchronizing motor, the opposite terminal of which is connected
+to wire <small>L'</small>. The two motors are thus in series. To start them
+Mr. Tesla short-circuits the synchronizing motor by a switch <small>P'</small>,
+throwing the whole current through the torque motor. Then
+when the desired speed is reached the switch <small>P'</small> is opened, so
+that the current passes through both motors. In such an arrangement
+as this it is obviously desirable for economical and other
+reasons that a proper relation between the speeds of the two
+motors should be observed.</p>
+
+<p>In Fig. 82 another disposition is illustrated. <small>S</small> is the synchronizing
+motor and <small>T</small> the torque motor, the circuits of both being in
+parallel. <small>W</small> is a circuit also in derivation to the motor circuits
+and containing a switch <small>P''</small>. <small>S'</small> is a switch in the synchronizing
+motor circuit. On the start the switch <small>S'</small> is opened, cutting out
+the motor <small>S</small>. Then <small>P''</small> is opened, throwing the entire current<span class='pagenum'><a name="Page_99" id="Page_99">[Pg 99]</a></span>
+through the motor <small>T</small>, giving it a very strong torque. When the
+desired speed is reached, switch <small>S'</small> is closed and the current divides
+between both motors. By means of switch <small>P''</small> both motors may
+be cut out.</p>
+
+<div class="figcenter" style="width: 500px;">
+<img src="images/oi_113.jpg" width="500" height="800" alt="Fig. 81, 82, 83, 84 and 85." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 81, 82, 83, 84 and 85.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_100" id="Page_100">[Pg 100]</a></span></p>
+
+<p>In Fig. 83 the arrangement is substantially the same, except
+that a switch <small>T'</small> is placed in the circuit which includes the two circuits
+of the torque motor. Fig. 84 shows the two motors in
+series, with a shunt around both containing a switch <small>S T</small>. There
+is also a shunt around the synchronizing motor <small>S</small>, with a switch
+<small>P'</small>. In Fig. 85 the same disposition is shown; but each motor is
+provided with a shunt, in which are switches <small>P'</small> and <small>T''</small>, as shown.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_101" id="Page_101">[Pg 101]</a></span></p>
+
+<h2><a name="CHAPTER_XXI" id="CHAPTER_XXI"></a>CHAPTER XXI.</h2>
+
+<h3><span class="smcap">Motor with a Condenser in the Armature Circuit.</span></h3>
+
+
+<p>We now come to a new class of motors in which resort is had
+to condensers for the purpose of developing the required difference
+of phase and neutralizing the effects of self-induction. Mr.
+Tesla early began to apply the condenser to alternating apparatus,
+in just how many ways can only be learned from a perusal
+of other portions of this volume, especially those dealing with
+his high frequency work.</p>
+
+<p>Certain laws govern the action or effects produced by a condenser
+when connected to an electric circuit through which an
+alternating or in general an undulating current is made to pass.
+Some of the most important of such effects are as follows: First,
+if the terminals or plates of a condenser be connected with two
+points of a circuit, the potentials of which are made to rise and
+fall in rapid succession, the condenser allows the passage, or more
+strictly speaking, the transference of a current, although its
+plates or armatures may be so carefully insulated as to prevent
+almost completely the passage of a current of unvarying strength
+or direction and of moderate electromotive force. Second, if a
+circuit, the terminals of which are connected with the plates of
+the condenser, possess a certain self-induction, the condenser will
+overcome or counteract to a greater or less degree, dependent
+upon well-understood conditions, the effects of such self-induction.
+Third, if two points of a closed or complete circuit
+through which a rapidly rising and falling current flows be
+shunted or bridged by a condenser, a variation in the strength of
+the currents in the branches and also a difference of phase of the
+currents therein is produced. These effects Mr. Tesla has utilized
+and applied in a variety of ways in the construction and operation
+of his motors, such as by producing a difference in phase in the
+two energizing circuits of an alternating current motor by connecting
+the two circuits in derivation and connecting up a condenser
+in series in one of the circuits. A further development,
+<span class='pagenum'><a name="Page_102" id="Page_102">[Pg 102]</a></span>however, possesses certain novel features of practical value and involves
+a knowledge of facts less generally understood. It comprises
+the use of a condenser or condensers in connection with the induced
+or armature circuit of a motor and certain details of the construction
+of such motors. In an alternating current motor of the
+type particularly referred to above, or in any other which has
+an armature coil or circuit closed upon itself, the latter represents
+not only an inductive resistance, but one which is period<span class='pagenum'><a name="Page_103" id="Page_103">[Pg 103]</a></span>ically
+varying in value, both of which facts complicate and render
+difficult the attainment of the conditions best suited to the most
+efficient working conditions; in other words, they require, first,
+that for a given inductive effect upon the armature there should
+be the greatest possible current through the armature or induced
+coils, and, second, that there should always exist between the
+currents in the energizing and the induced circuits a given relation
+of phase. Hence whatever tends to decrease the self-induction
+and increase the current in the induced circuits will, other
+things being equal, increase the output and efficiency of the motor,
+and the same will be true of causes that operate to maintain
+the mutual attractive effect between the field magnets and armature
+at its maximum. Mr. Tesla secures these results by connecting
+with the induced circuit or circuits a condenser, in the
+manner described below, and he also, with this purpose in view,
+constructs the motor in a special manner.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_116.jpg" width="800" height="534" alt="Fig. 86." title="" />
+<span class="caption">Fig. 86.</span>
+</div>
+
+
+<div class="figcenter" style="width: 780px;">
+<img src="images/fig88-89.jpg" width="640" height="278" alt="Fig. 88, 89." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 88.</td><td class="caption">Fig. 89.</td></tr>
+</table>
+
+<div class="figleft" style="width: 276px;">
+<img src="images/fig87.jpg" width="276" height="448" alt="Fig. 87." title="" />
+<span class="caption">Fig. 87.</span>
+</div>
+
+<div class="figright" style="width: 448px;">
+<img src="images/fig90.jpg" width="448" height="295" alt="Fig. 90." title="" />
+<span class="caption">Fig. 90.</span>
+</div>
+</div>
+<div style="clear: both;"></div>
+
+<p>Referring to the drawings, Fig. 86, is a view, mainly diagrammatic,
+of an alternating current motor, in which the present
+principle is applied. Fig. 87 is a central section, in line with
+the shaft, of a special form of armature core. Fig. 88 is a similar
+section of a modification of the same. Fig. 89 is one of the
+sections of the core detached. Fig. 90 is a diagram showing a
+modified disposition of the armature or induced circuits.</p>
+
+<p>The general plan of the invention is illustrated in Fig. 86.
+<small>A A</small> in this figure represent the the frame and field magnets of
+an alternating current motor, the poles or projections of which
+are wound with coils <small>B</small> and <small>C</small>, forming independent energizing
+circuits connected either to the same or to independent sources
+of alternating currents, so that the currents flowing through the
+circuits, respectively, will have a difference of phase. Within
+the influence of this field is an armature core <small>D</small>, wound with coils
+<small>E</small>. In motors of this description heretofore these coils have been
+closed upon themselves, or connected in a closed series; but in
+the present case each coil or the connected series of coils terminates
+in the opposite plates of a condenser <small>F</small>. For this purpose
+the ends of the series of coils are brought out through the shaft
+to collecting rings <small>G</small>, which are connected to the condenser by
+contact brushes <small>H</small> and suitable conductors, the condenser being
+independent of the machine. The armature coils are wound or
+connected in such manner that adjacent coils produce opposite
+poles.<span class='pagenum'><a name="Page_104" id="Page_104">[Pg 104]</a></span></p>
+
+<p>The action of this motor and the effect of the plan followed
+in its construction are as follows: The motor being started in
+operation and the coils of the field magnets being traversed by
+alternating currents, currents are induced in the armature coils
+by one set of field coils, as <small>B</small>, and the poles thus established are
+acted upon by the other set, as <small>C</small>. The armature coils, however,
+have necessarily a high self-induction, which opposes the flow of
+the currents thus set up. The condenser <small>F</small> not only permits the
+passage or transference of these currents, but also counteracts
+the effects of self-induction, and by a proper adjustment of the
+capacity of the condenser, the self-induction of the coils, and the
+periods of the currents, the condenser may be made to overcome
+entirely the effect of self-induction.</p>
+
+<p>It is preferable on account of the undesirability of using sliding
+contacts of any kind, to associate the condenser with the armature
+directly, or make it a part of the armature. In some cases Mr.
+Tesla builds up the armature of annular plates <small>K K</small>, held by bolts
+<small>L</small> between heads <small>M</small>, which are secured to the driving shaft, and
+in the hollow space thus formed he places a condenser <small>F</small>, generally
+by winding the two insulated plates spirally around the
+shaft. In other cases he utilizes the plates of the core itself
+as the plates of the condenser. For example, in Figs. 88 and 89,
+<small>N</small> is the driving shaft, <small>M M</small> are the heads of the armature-core,
+and <small>K K'</small> the iron plates of which the core is built up. These
+plates are insulated from the shaft and from one another, and are
+held together by rods or bolts <small>L</small>. The bolts pass through a large
+hole in one plate and a small hole in the one next adjacent, and
+so on, connecting electrically all of plates <small>K</small>, as one armature of a
+condenser, and all of plates <small>K'</small> as the other.</p>
+
+<p>To either of the condensers above described the armature coils
+may be connected, as explained by reference to Fig. 86.</p>
+
+<p>In motors in which the armature coils are closed upon themselves&mdash;as,
+for example, in any form of alternating current motor
+in which one armature coil or set of coils is in the position of
+maximum induction with respect to the field coils or poles, while
+the other is in the position of minimum induction&mdash;the coils are
+best connected in one series, and two points of the circuit
+thus formed are bridged by a condenser. This is illustrated in
+Fig. 90, in which <small>E</small> represents one set of armature coils and <small>E'</small>
+the other. Their points of union are joined through a condenser
+<small>F</small>. It will be observed that in this disposition the self<span class='pagenum'><a name="Page_105" id="Page_105">[Pg 105]</a></span>-induction
+of the two branches <small>E</small> and <small>E'</small> varies with their position
+relatively to the field magnet, and that each branch is alternately
+the predominating source of the induced current. Hence the
+effect of the condenser <small>F</small> is twofold. First, it increases the current
+in each of the branches alternately, and, secondly, it alters
+the phase of the currents in the branches, this being the well-known
+effect which results from such a disposition of a condenser
+with a circuit, as above described. This effect is favorable
+to the proper working of the motor, because it increases the flow
+of current in the armature circuits due to a given inductive
+effect, and also because it brings more nearly into coincidence
+the maximum magnetic effects of the coacting field and armature
+poles.</p>
+
+<p>It will be understood, of course, that the causes that contribute
+to the efficiency of condensers when applied to such uses as
+the above must be given due consideration in determining the
+practicability and efficiency of the motors. Chief among these
+is, as is well known, the periodicity of the current, and hence the
+improvements described are more particularly adapted to systems
+in which a very high rate of alternation or change is maintained.</p>
+
+<p>Although this invention has been illustrated in connection
+with a special form of motor, it will be understood that it is
+equally applicable to any other alternating current motor in
+which there is a closed armature coil wherein the currents are
+induced by the action of the field, and the feature of utilizing
+the plates or sections of a magnetic core for forming the condenser
+is applicable, generally, to other kinds of alternating current
+apparatus.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_106" id="Page_106">[Pg 106]</a></span></p>
+<h2><a name="CHAPTER_XXII" id="CHAPTER_XXII"></a>CHAPTER XXII.</h2>
+
+<h3><span class="smcap">Motor with Condenser in one of the Field Circuits.</span></h3>
+
+
+<p>If the field or energizing circuits of a rotary phase motor be
+both derived from the same source of alternating currents and a
+condenser of proper capacity be included in one of the same, approximately,
+the desired difference of phase may be obtained between
+the currents flowing directly from the source and those
+flowing through the condenser; but the great size and expense
+of condensers for this purpose that would meet the requirements
+of the ordinary systems of comparatively low potential are particularly
+prohibitory to their employment.</p>
+
+<p>Another, now well-known, method or plan of securing a difference
+of phase between the energizing currents of motors of this
+kind is to induce by the currents in one circuit those in the other
+circuit or circuits; but as no means had been proposed that
+would secure in this way between the phases of the primary or
+inducing and the secondary or induced currents that difference&mdash;theoretically
+ninety degrees&mdash;that is best adapted for practical
+and economical working, Mr. Tesla devised a means which renders
+practicable both the above described plans or methods, and
+by which he is enabled to obtain an economical and efficient alternating
+current motor. His invention consists in placing a
+condenser in the secondary or induced circuit of the motor above
+described and raising the potential of the secondary currents to
+such a degree that the capacity of the condenser, which is in
+part dependent on the potential, need be quite small. The value
+of this condenser is determined in a well-understood manner with
+reference to the self-induction and other conditions of the circuit,
+so as to cause the currents which pass through it to differ from
+the primary currents by a quarter phase.</p>
+
+<p>Fig. 91 illustrates the invention as embodied in a motor
+in which the inductive relation of the primary and secondary
+circuits is secured by winding them inside the motor partly
+upon the same cores; but the invention applies, generally, to<span class='pagenum'><a name="Page_107" id="Page_107">[Pg 107]</a></span>
+other forms of motor in which one of the energizing currents is
+induced in any way from the other.</p>
+
+<p>Let <small>A B</small> represent the poles of an alternating current motor, of
+which <small>C</small> is the armature wound with coils <small>D</small>, closed upon themselves,
+as is now the general practice in motors of this kind. The
+poles <small>A</small>, which alternate with poles <small>B</small>, are wound with coils of
+ordinary or coarse wire <small>E</small> in such direction as to make them of
+alternate north and south polarity, as indicated in the diagram
+by the characters <small>N S</small>. Over these coils, or in other inductive relation
+to the same, are wound long fine-wire coils <small>F F</small>, and in the
+same direction throughout as the coils <small>E</small>. These coils are secondaries,
+in which currents of very high potential are induced. All
+the coils <small>E</small> in one series are connected, and all the secondaries <small>F</small>
+in another.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_121.jpg" width="600" height="609" alt="Fig. 91." title="" />
+<span class="caption">Fig. 91.</span>
+</div>
+
+<p>On the intermediate poles <small>B</small> are wound fine-wire energizing
+coils <small>G</small>, which are connected in series with one another, and also
+with the series of secondary coils <small>F</small>, the direction of winding being
+such that a current-impulse induced from the primary coils
+<small>E</small> imparts the same magnetism to the poles <small>B</small> as that produced<span class='pagenum'><a name="Page_108" id="Page_108">[Pg 108]</a></span>
+in poles <small>A</small> by the primary impulse. This condition is indicated
+by the characters <small>N' S'</small>.</p>
+
+<p>In the circuit formed by the two sets of coils <small>F</small> and <small>G</small> is introduced
+a condenser <small>H</small>; otherwise this circuit is closed upon
+itself, while the free ends of the circuit of coils <small>E</small> are connected
+to a source of alternating currents. As the condenser capacity
+which is needed in any particular motor of this kind is dependent
+upon the rate of alternation or the potential, or both, its size
+or cost, as before explained, may be brought within economical
+limits for use with the ordinary circuits if the potential of the
+secondary circuit in the motor be sufficiently high. By giving
+to the condenser proper values, any desired difference of phase
+between the primary and secondary energizing circuits may be
+obtained.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_109" id="Page_109">[Pg 109]</a></span></p>
+<h2><a name="CHAPTER_XXIII" id="CHAPTER_XXIII"></a>CHAPTER XXIII.</h2>
+
+<h3><span class="smcap">Tesla Polyphase Transformer.</span></h3>
+
+
+<p>Applying the polyphase principle to the construction of transformers
+as well to the motors already noticed, Mr. Tesla has invented
+some very interesting forms, which he considers free
+from the defects of earlier and, at present, more familiar forms.
+In these transformers he provides a series of inducing coils and
+corresponding induced coils, which are generally wound upon a
+core closed upon itself, usually a ring of laminated iron.</p>
+
+<p>The two sets of coils are wound side by side or superposed or
+otherwise placed in well-known ways to bring them into the most
+effective relations to one another and to the core. The inducing
+or primary coils wound on the core are divided into pairs or sets
+by the proper electrical connections, so that while the coils of
+one pair or set co-operate in fixing the magnetic poles of the
+core at two given diametrically opposite points, the coils of the
+other pair or set&mdash;assuming, for sake of illustration, that there
+are but two&mdash;tend to fix the poles ninety degrees from such
+points. With this induction device is used an alternating current
+generator with coils or sets of coils to correspond with those of
+the converter, and the corresponding coils of the generator and
+converter are then connected up in independent circuits. It results
+from this that the different electrical phases in the generator
+are attended by corresponding magnetic changes in the converter;
+or, in other words, that as the generator coils revolve,
+the points of greatest magnetic intensity in the converter will be
+progressively shifted or whirled around.</p>
+
+<p>Fig. 92 is a diagrammatic illustration of the converter and the
+electrical connections of the same. Fig. 93 is a horizontal central
+cross-section of Fig. 92. Fig. 94 is a diagram of the circuits
+of the entire system, the generator being shown in section.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_124.jpg" width="600" height="663" alt="Fig. 92 and 93." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 92 and 93.</span>
+</div>
+
+<p>Mr. Tesla uses a core, <small>A</small>, which is closed upon itself&mdash;that is to
+say, of an annular cylindrical or equivalent form&mdash;and as the
+efficiency of the apparatus is largely increased by the subdivision<span class='pagenum'><a name="Page_110" id="Page_110">[Pg 110]</a></span>
+of this core, he makes it of thin strips, plates, or wires of soft
+iron electrically insulated as far as practicable. Upon this core
+are wound, say, four coils, <small>B B B' B'</small>, used as primary coils, and for
+which long lengths of comparatively fine wire are employed.
+Over these coils are then wound shorter coils of coarser wire, <small>C C
+C' C'</small>, to constitute the induced or secondary coils. The construction
+of this or any equivalent form of converter may be carried
+further, as above pointed out, by inclosing these coils with iron&mdash;as,
+for example, by winding over the coils layers of insulated
+iron wire.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_125.jpg" width="800" height="469" alt="Fig. 94." title="" />
+<span class="caption">Fig. 94.</span>
+</div>
+
+<p>The device is provided with suitable binding posts, to which
+the ends of the coils are led. The diametrically opposite coils
+<small>B B</small> and <small>B' B'</small> are connected, respectively, in series, and the four
+terminals are connected to the binding posts. The induced
+coils are connected together in any desired manner. For example,
+as shown in Fig. 94, <small>C C</small> may be connected in multiple
+arc when a quantity current is desired&mdash;as for running a group
+of incandescent lamps&mdash;while <small>C' C'</small> may be independently connected
+in series in a circuit including arc lamps or the like. The
+generator in this system will be adapted to the converter in the
+<span class='pagenum'><a name="Page_111" id="Page_111">[Pg 111]</a></span>manner illustrated. For example, in the present case there are
+employed a pair of ordinary permanent or electro-magnets, <small>E E</small>,
+between which is mounted a cylindrical armature on a shaft, <small>F</small>,
+and wound with two coils, <small>G G'</small>. The terminals of these coils are
+connected, respectively, to four insulated contact or collecting
+rings, <small>H H H' H'</small>, and the four line circuit wires <small>L</small> connect the
+brushes <small>K</small>, bearing on these rings, to the converter in the order
+shown. Noting the results of this combination, it will be observed
+that at a given point of time the coil <small>G</small> is in its neutral
+position and is generating little or no current, while the other
+coil, <small>G'</small>, is in a position where it exerts its maximum effect.
+Assuming coil <small>G</small> to be connected in circuit with coils <small>B B</small> of the
+converter, and coil <small>G'</small> with coils <small>B' B'</small>, it is evident that the poles
+of the ring <small>A</small> will be determined by coils <small>B' B'</small> alone; but as the
+armature of the generator revolves, coil <small>G</small> develops more current
+and coil <small>G'</small> less, until <small>G</small> reaches its maximum and <small>G'</small> its neutral
+position. The obvious result will be to shift the poles of the
+ring <small>A</small> through one-quarter of its periphery. The movement of
+the coils through the next quarter of a turn&mdash;during which coil
+<small>G'</small> enters a field of opposite polarity and generates a current of
+opposite direction and increasing strength, while coil <small>G</small>, in passing
+from its maximum to its neutral position generates a current of
+decreasing strength and same direction as before&mdash;causes a further
+shifting of the poles through the second quarter of the ring.
+The second half-revolution will obviously be a repetition of the
+same action. By the shifting of the poles of the ring <small>A</small>, a power<span class='pagenum'><a name="Page_112" id="Page_112">[Pg 112]</a></span>ful
+dynamic inductive effect on the coils <small>C C'</small> is produced. Besides
+the currents generated in the secondary coils by dynamo-magnetic
+induction, other currents will be set up in the same
+coils in consequence of many variations in the intensity of the
+poles in the ring <small>A</small>. This should be avoided by maintaining the
+intensity of the poles constant, to accomplish which care should
+be taken in designing and proportioning the generator and in
+distributing the coils in the ring <small>A</small>, and balancing their effect.
+When this is done, the currents are produced by dynamo-magnetic
+induction only, the same result being obtained as though
+the poles were shifted by a commutator with an infinite number
+of segments.</p>
+
+<p>The modifications which are applicable to other forms of converter
+are in many respects applicable to this, such as those pertaining
+more particularly to the form of the core, the relative
+lengths and resistances of the primary and secondary coils, and
+the arrangements for running or operating the same.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_113" id="Page_113">[Pg 113]</a></span></p>
+<h2><a name="CHAPTER_XXIV" id="CHAPTER_XXIV"></a>CHAPTER XXIV.</h2>
+
+<h3><span class="smcap">A Constant Current Transformer with Magnetic Shield
+Between Coils of Primary and Secondary.</span></h3>
+
+
+<p>Mr. Tesla has applied his principle of magnetic shielding of
+parts to the construction also of transformers, the shield being
+interposed between the primary and secondary coils. In transformers
+of the ordinary type it will be found that the wave of
+electromotive force of the secondary very nearly coincides with
+that of the primary, being, however, in opposite sign. At the same
+time the currents, both primary and secondary, lag behind their
+respective electromotive forces; but as this lag is practically or
+nearly the same in the case of each it follows that the maximum
+and minimum of the primary and secondary currents will nearly
+coincide, but differ in sign or direction, provided the secondary
+be not loaded or if it contain devices having the property of
+self-induction. On the other hand, the lag of the primary
+behind the impressed electromotive force may be diminished by
+loading the secondary with a non-inductive or dead resistance&mdash;such
+as incandescent lamps&mdash;whereby the time interval between
+the maximum or minimum periods of the primary and secondary
+currents is increased. This time interval, however, is limited,
+and the results obtained by phase difference in the operation of
+such devices as the Tesla alternating current motors can only be
+approximately realized by such means of producing or securing
+this difference, as above indicated, for it is desirable in such cases
+that there should exist between the primary and secondary currents,
+or those which, however produced, pass through the two
+circuits of the motor, a difference of phase of ninety degrees;
+or, in other words, the current in one circuit should be a maximum
+when that in the other circuit is a minimum. To attain
+to this condition more perfectly, an increased retardation of the
+secondary current is secured in the following manner: Instead
+of bringing the primary and secondary coils or circuits of a
+transformer into the closest possible relations, as has hitherto<span class='pagenum'><a name="Page_114" id="Page_114">[Pg 114]</a></span>
+been done, Mr. Tesla protects in a measure the secondary from
+the inductive action or effect of the primary by surrounding
+either the primary or the secondary with a comparatively thin
+magnetic shield or screen. Under these modified conditions,
+as long as the primary current has a small value, the shield
+protects the secondary; but as soon as the primary current
+has reached a certain strength, which is arbitrarily determined,
+the protecting magnetic shield becomes saturated and the inductive
+action upon the secondary begins. It results, therefore, that
+the secondary current begins to flow at a certain fraction of a
+period later than it would without the interposed shield, and
+since this retardation may be obtained without necessarily retarding
+the primary current also, an additional lag is secured, and
+the time interval between the maximum or minimum periods of
+the primary and secondary currents is increased. Such a transformer
+may, by properly proportioning its several elements and
+determining the proper relations between the primary and
+secondary windings, the thickness of the magnetic shield, and
+other conditions, be constructed to yield a constant current at all
+loads.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_128.jpg" width="640" height="387" alt="Fig. 95." title="" />
+<span class="caption">Fig. 95.</span>
+</div>
+
+
+<p>Fig. 95 is a cross-section of a transformer embodying this improvement.
+Fig. 96 is a similar view of a modified form of
+transformer, showing diagrammatically the manner of using the
+same.</p>
+
+<p><small>A A</small> is the main core of the transformer, composed of a ring
+of soft annealed and insulated or oxidized iron wire. Upon this
+core is wound the secondary circuit or coil <small>B B</small>. This latter is
+then covered with a layer or layers of annealed and insulated
+iron wires <small>C C</small>, wound in a direction at right angles to the secondary<span class='pagenum'><a name="Page_115" id="Page_115">[Pg 115]</a></span>
+coil. Over the whole is then wound the primary coil or wire <small>D D</small>.
+From the nature of this construction it will be obvious that
+as long as the shield formed by the wires <small>C</small> is below magnetic
+saturation the secondary coil or circuit is effectually protected or
+shielded from the inductive influence of the primary, although
+on open circuit it may exhibit some electromotive force. When
+the strength of the primary reaches a certain value, the shield <small>C</small>,
+becoming saturated, ceases to protect the secondary from inductive
+action, and current is in consequence developed therein.
+For similar reasons, when the primary current weakens, the
+weakening of the secondary is retarded to the same or approximately
+the same extent.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_129.jpg" width="800" height="445" alt="Fig. 96." title="" />
+<span class="caption">Fig. 96.</span>
+</div>
+
+<p>The specific construction of the transformer is largely immaterial.
+In Fig. 96, for example, the core <small>A</small> is built up of thin
+insulated iron plates or discs. The primary circuit <small>D</small> is wound
+next the core <small>A</small>. Over this is applied the shield <small>C</small>, which in this
+case is made up of thin strips or plates of iron properly insulated
+and surrounding the primary, forming a closed magnetic circuit.
+The secondary <small>B</small> is wound over the shield <small>C</small>. In Fig. 96, also,
+<small>E</small> is a source of alternating or rapidly changing currents.
+The primary of the transformer is connected with the circuit of
+the generator. <small>F</small> is a two-circuit alternating current motor, one
+of the circuits being connected with the main circuit from the
+source <small>E</small>, and the other being supplied with currents from the
+secondary of the transformer.</p>
+<p><span class='pagenum'><a name="Page_116" id="Page_116">[Pg 116]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_117" id="Page_117">[Pg 117]</a></span></p>
+<h1><small><a name="PART_II" id="PART_II"></a>PART II.</small><br /><br />
+
+THE TESLA EFFECTS WITH HIGH FREQUENCY<br />
+AND HIGH POTENTIAL CURRENTS.</h1>
+<p><span class='pagenum'><a name="Page_118" id="Page_118">[Pg 118]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_119" id="Page_119">[Pg 119]</a></span></p>
+<h2><a name="CHAPTER_XXV" id="CHAPTER_XXV"></a>CHAPTER XXV.</h2>
+
+<h3><span class="smcap">Introduction.&mdash;The Scope of the Tesla Lectures.</span></h3>
+
+
+<p>Before proceeding to study the three Tesla lectures here
+presented, the reader may find it of some assistance to have his
+attention directed to the main points of interest and significance
+therein. The first of these lectures was delivered in New York,
+at Columbia College, before the American Institute of Electrical
+Engineers, May 20, 1891. The urgent desire expressed immediately
+from all parts of Europe for an opportunity to witness the
+brilliant and unusual experiments with which the lecture was
+accompanied, induced Mr. Tesla to go to England early in 1892,
+when he appeared before the Institution of Electrical Engineers,
+and a day later, by special request, before the Royal Institution.
+His reception was of the most enthusiastic and flattering nature on
+both occasions. He then went, by invitation, to France, and repeated
+his novel demonstrations before the Soci&eacute;t&eacute; Internationale
+des Electriciens, and the Soci&eacute;t&eacute; Fran&ccedil;aise de Physique. Mr. Tesla
+returned to America in the fall of 1892, and in February, 1893, delivered
+his third lecture before the Franklin Institute of Philadelphia,
+in fulfilment of a long standing promise to Prof. Houston.
+The following week, at the request of President James I. Ayer,
+of the National Electric Light Association, the same lecture was
+re-delivered in St. Louis. It had been intended to limit the invitations
+to members, but the appeals from residents in the city
+were so numerous and pressing that it became necessary to secure
+a very large hall. Hence it came about that the lecture was
+listened to by an audience of over 5,000 people, and was in some
+parts of a more popular nature than either of its predecessors.
+Despite this concession to the need of the hour and occasion, Mr.
+Tesla did not hesitate to show many new and brilliant experiments,
+and to advance the frontier of discovery far beyond any
+point he had theretofore marked publicly.</p>
+
+<p>We may now proceed to a running review of the lectures themselves.
+The ground covered by them is so vast that only the<span class='pagenum'><a name="Page_120" id="Page_120">[Pg 120]</a></span>
+leading ideas and experiments can here be touched upon; besides,
+it is preferable that the lectures should be carefully gone over for
+their own sake, it being more than likely that each student will
+discover a new beauty or stimulus in them. Taking up the
+course of reasoning followed by Mr. Tesla in his first lecture, it
+will be noted that he started out with the recognition of the fact,
+which he has now experimentally demonstrated, that for the production
+of light waves, primarily, electrostatic effects must be
+brought into play, and continued study has led him to the opinion
+that all electrical and magnetic effects may be referred to electrostatic
+molecular forces. This opinion finds a singular confirmation
+in one of the most striking experiments which he
+describes, namely, the production of a veritable flame by the
+agitation of electrostatically charged molecules. It is of the
+highest interest to observe that this result points out a way of
+obtaining a flame which consumes no material and in which no
+chemical action whatever takes place. It also throws a light on
+the nature of the ordinary flame, which Mr. Tesla believes to be
+due to electrostatic molecular actions, which, if true, would lead
+directly to the idea that even chemical affinities might be electrostatic
+in their nature and that, as has already been suggested,
+molecular forces in general may be referable to one and the same
+cause. This singular phenomenon accounts in a plausible manner
+for the unexplained fact that buildings are frequently set on
+fire during thunder storms without having been at all struck by
+lightning. It may also explain the total disappearance of ships
+at sea.</p>
+
+<p>One of the striking proofs of the correctness of the ideas advanced
+by Mr. Tesla is the fact that, notwithstanding the employment
+of the most powerful electromagnetic inductive effects, but
+feeble luminosity is obtainable, and this only in close proximity
+to the source of disturbance; whereas, when the electrostatic
+effects are intensified, the same initial energy suffices to excite
+luminosity at considerable distances from the source. That there
+are only electrostatic effects active seems to be clearly proved by
+Mr. Tesla's experiments with an induction coil operated with
+alternating currents of very high frequency. He shows how
+tubes may be made to glow brilliantly at considerable distances
+from any object when placed in a powerful, rapidly alternating,
+electrostatic field, and he describes many interesting phenomena
+observed in such a field. His experiments open up the possibility<span class='pagenum'><a name="Page_121" id="Page_121">[Pg 121]</a></span>
+of lighting an apartment by simply creating in it such an electrostatic
+field, and this, in a certain way, would appear to be the
+ideal method of lighting a room, as it would allow the illuminating
+device to be freely moved about. The power with which
+these exhausted tubes, devoid of any electrodes, light up is certainly
+remarkable.</p>
+
+<p>That the principle propounded by Mr. Tesla is a broad one is
+evident from the many ways in which it may be practically applied.
+We need only refer to the variety of the devices shown
+or described, all of which are novel in character and will, without
+doubt, lead to further important results at the hands of Mr.
+Tesla and other investigators. The experiment, for instance, of
+lighting up a single filament or block of refractory material with
+a single wire, is in itself sufficient to give Mr. Tesla's work the
+stamp of originality, and the numerous other experiments and
+effects which may be varied at will, are equally new and interesting.
+Thus, the incandescent filament spinning in an unexhausted
+globe, the well-known Crookes experiment on open circuit,
+and the many others suggested, will not fail to interest the
+reader. Mr. Tesla has made an exhaustive study of the various
+forms of the discharge presented by an induction coil when operated
+with these rapidly alternating currents, starting from the
+thread-like discharge and passing through various stages to the
+true electric flame.</p>
+
+<p>A point of great importance in the introduction of high tension
+alternating current which Mr. Tesla brings out is the necessity
+of carefully avoiding all gaseous matter in the high tension
+apparatus. He shows that, at least with very rapidly alternating
+currents of high potential, the discharge may work through almost
+any practicable thickness of the best insulators, if air is
+present. In such cases the air included within the apparatus is
+violently agitated and by molecular bombardment the parts may
+be so greatly heated as to cause a rupture of the insulation.
+The practical outcome of this is, that, whereas with steady currents,
+any kind of insulation may be used, with rapidly alternating
+currents oils will probably be the best to employ, a fact
+which has been observed, but not until now satisfactorily explained.
+The recognition of the above fact is of special importance
+in the construction of the costly commercial induction coils
+which are often rendered useless in an unaccountable manner.
+The truth of these views of Mr. Tesla is made evident by the in<span class='pagenum'><a name="Page_122" id="Page_122">[Pg 122]</a></span>teresting
+experiments illustrative of the behavior of the air between
+charged surfaces, the luminous streams formed by the
+charged molecules appearing even when great thicknesses of the
+best insulators are interposed between the charged surfaces.
+These luminous streams afford in themselves a very interesting
+study for the experimenter. With these rapidly alternating currents
+they become far more powerful and produce beautiful light
+effects when they issue from a wire, pinwheel or other object attached
+to a terminal of the coil; and it is interesting to note that
+they issue from a ball almost as freely as from a point, when the
+frequency is very high.</p>
+
+<p>From these experiments we also obtain a better idea of the
+importance of taking into account the capacity and self-induction
+in the apparatus employed and the possibilities offered by the
+use of condensers in conjunction with alternate currents, the employment
+of currents of high frequency, among other things,
+making it possible to reduce the condenser to practicable dimensions.
+Another point of interest and practical bearing is the
+fact, proved by Mr. Tesla, that for alternate currents, especially
+those of high frequency, insulators are required possessing a
+small specific inductive capacity, which at the same time have a
+high insulating power.</p>
+
+<p>Mr. Tesla also makes interesting and valuable suggestion in regard
+to the economical utilization of iron in machines and transformers.
+He shows how, by maintaining by continuous magnetization
+a flow of lines through the iron, the latter may be kept
+near its maximum permeability and a higher output and economy
+may be secured in such apparatus. This principle may prove of
+considerable commercial importance in the development of alternating
+systems. Mr. Tesla's suggestion that the same result can
+be secured by heating the iron by hysteresis and eddy currents,
+and increasing the permeability in this manner, while it may appear
+less practical, nevertheless opens another direction for investigation
+and improvement.</p>
+
+<p>The demonstration of the fact that with alternating currents
+of high frequency, sufficient energy may be transmitted under
+practicable conditions through the glass of an incandescent lamp
+by electrostatic or electromagnetic induction may lead to a departure
+in the construction of such devices. Another important
+experimental result achieved is the operation of lamps, and even
+motors, with the discharges of condensers, this method affording<span class='pagenum'><a name="Page_123" id="Page_123">[Pg 123]</a></span>
+a means of converting direct or alternating currents. In this
+connection Mr. Tesla advocates the perfecting of apparatus capable
+of generating electricity of high tension from heat energy,
+believing this to be a better way of obtaining electrical energy
+for practical purposes, particularly for the production of light.</p>
+
+<p>While many were probably prepared to encounter curious
+phenomena of impedance in the use of a condenser discharged
+disruptively, the experiments shown were extremely interesting
+on account of their paradoxical character. The burning of an
+incandescent lamp at any candle power when connected across a
+heavy metal bar, the existence of nodes on the bar and the possibility
+of exploring the bar by means of an ordinary Cardew
+voltmeter, are all peculiar developments, but perhaps the most
+interesting observation is the phenomenon of impedance observed
+in the lamp with a straight filament, which remains dark while
+the bulb glows.</p>
+
+<p>Mr. Tesla's manner of operating an induction coil by means of
+the disruptive discharge, and thus obtaining enormous differences
+of potential from comparatively small and inexpensive coils, will
+be appreciated by experimenters and will find valuable application
+in laboratories. Indeed, his many suggestions and hints in
+regard to the construction and use of apparatus in these investigations
+will be highly valued and will aid materially in future
+research.</p>
+
+<p>The London lecture was delivered twice. In its first form,
+before the Institution of Electrical Engineers, it was in some
+respects an amplification of several points not specially enlarged
+upon in the New York lecture, but brought forward many additional
+discoveries and new investigations. Its repetition, in
+another form, at the Royal Institution, was due to Prof. Dewar,
+who with Lord Rayleigh, manifested a most lively interest in Mr.
+Tesla's work, and whose kindness illustrated once more the strong
+English love of scientific truth and appreciation of its votaries.
+As an indefatigable experimenter, Mr. Tesla was certainly nowhere
+more at home than in the haunts of Faraday, and as the
+guest of Faraday's successor. This Royal Institution lecture
+summed up the leading points of Mr. Tesla's work, in the high
+potential, high frequency field, and we may here avail ourselves
+of so valuable a summarization, in a simple form, of a subject by
+no means easy of comprehension until it has been thoroughly
+studied.<span class='pagenum'><a name="Page_124" id="Page_124">[Pg 124]</a></span></p>
+
+<p>In these London lectures, among the many notable points made
+was first, the difficulty of constructing the alternators to obtain
+the very high frequencies needed. To obtain the high frequencies
+it was necessary to provide several hundred polar projections,
+which were necessarily small and offered many drawbacks,
+and this the more as exceedingly high peripheral speeds
+had to be resorted to. In some of the first machines both armature
+and field had polar projections. These machines produced
+a curious noise, especially when the armature was started from
+the state of rest, the field being charged. The most efficient
+machine was found to be one with a drum armature, the iron
+body of which consisted of very thin wire annealed with special
+care. It was, of course, desirable to avoid the employment of
+iron in the armature, and several machines of this kind, with
+moving or stationary conductors were constructed, but the results
+obtained were not quite satisfactory, on account of the
+great mechanical and other difficulties encountered.</p>
+
+<p>The study of the properties of the high frequency currents
+obtained from these machines is very interesting, as nearly every
+experiment discloses something new. Two coils traversed by
+such a current attract or repel each other with a force which,
+owing to the imperfection of our sense of touch, seems continuous.
+An interesting observation, already noted under another
+form, is that a piece of iron, surrounded by a coil through which
+the current is passing appears to be continuously magnetized.
+This apparent continuity might be ascribed to the deficiency of
+the sense of touch, but there is evidence that in currents of such
+high frequencies one of the impulses preponderates over the
+other.</p>
+
+<p>As might be expected, conductors traversed by such currents
+are rapidly heated, owing to the increase of the resistance, and
+the heating effects are relatively much greater in the iron.
+The hysteresis losses in iron are so great that an iron core,
+even if finely subdivided, is heated in an incredibly short time.
+To give an idea of this, an ordinary iron wire 1/16 inch in
+diameter inserted within a coil having 250 turns, with a current
+estimated to be five amperes passing through the coil, becomes
+within two seconds' time so hot as to scorch wood. Beyond a
+certain frequency, an iron core, no matter how finely subdivided,
+exercises a dampening effect, and it was easy to find a point at<span class='pagenum'><a name="Page_125" id="Page_125">[Pg 125]</a></span>
+which the impedance of a coil was not affected by the presence
+of a core consisting of a bundle of very thin well annealed and
+varnished iron wires.</p>
+
+<p>Experiments with a telephone, a conductor in a strong magnetic
+field, or with a condenser or arc, seem to afford certain
+proof that sounds far above the usually accepted limit of hearing
+would be perceived if produced with sufficient power. The arc
+produced by these currents possesses several interesting features.
+Usually it emits a note the pitch of which corresponds to twice
+the frequency of the current, but if the frequency be sufficiently
+high it becomes noiseless, the limit of audition being determined
+principally by the linear dimensions of the arc. A curious feature
+of the arc is its persistency, which is due partly to the inability
+of the gaseous column to cool and increase considerably
+in resistance, as is the case with low frequencies, and partly to
+the tendency of such a high frequency machine to maintain a
+constant current.</p>
+
+<p>In connection with these machines the condenser affords a particularly
+interesting study. Striking effects are produced by
+proper adjustments of capacity and self-induction. It is easy to
+raise the electromotive force of the machine to many times the
+original value by simply adjusting the capacity of a condenser
+connected in the induced circuit. If the condenser be at some
+distance from the machine, the difference of potential on the
+terminals of the latter may be only a small fraction of that on
+the condenser.</p>
+
+<p>But the most interesting experiences are gained when the tension
+of the currents from the machine is raised by means of an
+induction coil. In consequence of the enormous rate of change
+obtainable in the primary current, much higher potential differences
+are obtained than with coils operated in the usual ways,
+and, owing to the high frequency, the secondary discharge possesses
+many striking peculiarities. Both the electrodes behave
+generally alike, though it appears from some observations that
+one current impulse preponderates over the other, as before
+mentioned.</p>
+
+<p>The physiological effects of the high tension discharge are
+found to be so small that the shock of the coil can be supported
+without any inconvenience, except perhaps a small burn produced
+by the discharge upon approaching the hand to one of the terminals.
+The decidedly smaller physiological effects of these cur<span class='pagenum'><a name="Page_126" id="Page_126">[Pg 126]</a></span>rents
+are thought to be due either to a different distribution
+through the body or to the tissues acting as condensers. But in
+the case of an induction coil with a great many turns the harmlessness
+is principally due to the fact that but little energy is available
+in the external circuit when the same is closed through the
+experimenter's body, on account of the great impedance of the
+coil.</p>
+
+<p>In varying the frequency and strength of the currents through
+the primary of the coil, the character of the secondary discharge
+is greatly varied, and no less than five distinct forms are observed:&mdash;A
+weak, sensitive thread discharge, a powerful flaming
+discharge, and three forms of brush or streaming discharges.
+Each of these possesses certain noteworthy features, but the most
+interesting to study are the latter.</p>
+
+<p>Under certain conditions the streams, which are presumably
+due to the violent agitation of the air molecules, issue freely
+from all points of the coil, even through a thick insulation. If
+there is the smallest air space between the primary and secondary,
+they will form there and surely injure the coil by slowly warming
+the insulation. As they form even with ordinary frequencies
+when the potential is excessive, the air-space must be most carefully
+avoided. These high frequency streamers differ in aspect
+and properties from those produced by a static machine. The
+wind produced by them is small and should altogether cease if
+still considerably higher frequencies could be obtained. A peculiarity
+is that they issue as freely from surfaces as from points.
+Owing to this, a metallic vane, mounted in one of the terminals of
+the coil so as to rotate freely, and having one of its sides covered
+with insulation, is spun rapidly around. Such a vane would not
+rotate with a steady potential, but with a high frequency coil it
+will spin, even if it be entirely covered with insulation, provided
+the insulation on one side be either thicker or of a higher specific
+inductive capacity. A Crookes electric radiometer is also spun
+around when connected to one of the terminals of the coil, but
+only at very high exhaustion or at ordinary pressures.</p>
+
+<p>There is still another and more striking peculiarity of such a
+high frequency streamer, namely, it is hot. The heat is easily
+perceptible with frequencies of about 10,000, even if the potential
+is not excessively high. The heating effect is, of course, due
+to the molecular impacts and collisions. Could the frequency
+and potential be pushed far enough, then a brush could be pro<span class='pagenum'><a name="Page_127" id="Page_127">[Pg 127]</a></span>duced
+resembling in every particular a flame and giving light
+and heat, yet without a chemical process taking place.</p>
+
+<p>The hot brush, when properly produced, resembles a jet of
+burning gas escaping under great pressure, and it emits an extraordinary
+strong smell of ozone. The great ozonizing action is
+ascribed to the fact that the agitation of the molecules of the air
+is more violent in such a brush than in the ordinary streamer of
+a static machine. But the most powerful brush discharges were
+produced by employing currents of much higher frequencies than
+it was possible to obtain by means of the alternators. These
+currents were obtained by disruptively discharging a condenser
+and setting up oscillations. In this manner currents of a frequency
+of several hundred thousand were obtained.</p>
+
+<p>Currents of this kind, Mr. Tesla pointed out, produce striking
+effects. At these frequencies, the impedance of a copper bar is
+so great that a potential difference of several hundred volts can
+be maintained between two points of a short and thick bar, and
+it is possible to keep an ordinary incandescent lamp burning at
+full candle power by attaching the terminals of the lamp to two
+points of the bar no more than a few inches apart. When the
+frequency is extremely high, nodes are found to exist on such a
+bar, and it is easy to locate them by means of a lamp.</p>
+
+<p>By converting the high tension discharges of a low frequency
+coil in this manner, it was found practicable to keep a few lamps
+burning on the ordinary circuit in the laboratory, and by bringing
+the undulation to a low pitch, it was possible to operate small
+motors.</p>
+
+<p>This plan likewise allows of converting high tension discharges
+of one direction into low tension unidirectional currents, by adjusting
+the circuit so that there are no oscillations. In passing
+the oscillating discharges through the primary of a specially
+constructed coil, it is easy to obtain enormous potential differences
+with only few turns of the secondary.</p>
+
+<p>Great difficulties were at first experienced in producing a successful
+coil on this plan. It was found necessary to keep all air,
+or gaseous matter in general, away from the charged surfaces,
+and oil immersion was resorted to. The wires used were heavily
+covered with gutta-percha and wound in oil, or the air was pumped
+out by means of a Sprengel pump. The general arrangement
+was the following:&mdash;An ordinary induction coil, operated from
+a low frequency alternator, was used to charge Leyden jars. The<span class='pagenum'><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+jars were made to discharge over a single or multiple gap through
+the primary of the second coil. To insure the action of the gap,
+the arc was blown out by a magnet or air blast. To adjust the
+potential in the secondary a small oil condenser was used, or
+polished brass spheres of different sizes were screwed on the
+terminals and their distance adjusted.</p>
+
+<p>When the conditions were carefully determined to suit each
+experiment, magnificent effects were obtained. Two wires,
+stretched through the room, each being connected to one of the
+terminals of the coil, emitted streams so powerful that the light
+from them allowed distinguishing the objects in the room; the
+wires became luminous even though covered with thick and
+most excellent insulation. When two straight wires, or two concentric
+circles of wire, are connected to the terminals, and set at
+the proper distance, a uniform luminous sheet is produced between
+them. It was possible in this way to cover an area of
+more than one meter square completely with the streams. By
+attaching to one terminal a large circle of wire and to the other
+terminal a small sphere, the streams are focused upon the sphere,
+produce a strongly lighted spot upon the same, and present the
+appearance of a luminous cone. A very thin wire glued upon a
+plate of hard rubber of great thickness, on the opposite side of
+which is fastened a tinfoil coating, is rendered intensely luminous
+when the coating is connected to the other terminal of the coil.
+Such an experiment can be performed also with low frequency
+currents, but much less satisfactorily.</p>
+
+<p>When the terminals of such a coil, even of a very small one,
+are separated by a rubber or glass plate, the discharge spreads
+over the plate in the form of streams, threads or brilliant sparks,
+and affords a magnificent display, which cannot be equaled by
+the largest coil operated in the usual ways. By a simple adjustment
+it is possible to produce with the coil a succession of brilliant
+sparks, exactly as with a Holtz machine.</p>
+
+<p>Under certain conditions, when the frequency of the oscillation
+is very great, white, phantom-like streams are seen to break forth
+from the terminals of the coil. The chief interesting feature
+about them is, that they stream freely against the outstretched
+hand or other conducting object without producing any sensation,
+and the hand may be approached very near to the terminal
+without a spark being induced to jump. This is due presumably
+to the fact that a considerable portion of the energy is carried<span class='pagenum'><a name="Page_129" id="Page_129">[Pg 129]</a></span>
+away or dissipated in the streamers, and the difference of potential
+between the terminal and the hand is diminished.</p>
+
+<p>It is found in such experiments that the frequency of the
+vibration and the quickness of succession of the sparks between
+the knobs affect to a marked degree the appearance of the
+streams. When the frequency is very low, the air gives way in
+more or less the same manner as by a steady difference of potential,
+and the streams consist of distinct threads, generally mingled
+with thin sparks, which probably correspond to the successive
+discharges occurring between the knobs. But when the frequency
+is very high, and the arc of the discharge produces a
+sound which is loud and smooth (which indicates both that oscillation
+takes place and that the sparks succeed each other with
+great rapidity), then the luminous streams formed are perfectly
+uniform. They are generally of a purplish hue, but when the
+molecular vibration is increased by raising the potential, they assume
+a white color.</p>
+
+<p>The luminous intensity of the streams increases rapidly when
+the potential is increased; and with frequencies of only a few
+hundred thousand, could the coil be made to withstand a sufficiently
+high potential difference, there is no doubt that the
+space around a wire could be made to emit a strong light,
+merely by the agitation of the molecules of the air at ordinary
+pressure.</p>
+
+<p>Such discharges of very high frequency which render luminous
+the air at ordinary pressure we have very likely occasion to
+witness in the aurora borealis. From many of these experiments
+it seems reasonable to infer that sudden cosmic disturbances,
+such as eruptions on the sun, set the electrostatic charge
+of the earth in an extremely rapid vibration, and produce the
+glow by the violent agitation of the air in the upper and even in
+the lower strata. It is thought that if the frequency were low,
+or even more so if the charge were not at all vibrating, the
+lower dense strata would break down as in a lightning discharge.
+Indications of such breaking down have been repeatedly observed,
+but they can be attributed to the fundamental disturbances,
+which are few in number, for the superimposed vibration
+would be so rapid as not to allow a disruptive break.</p>
+
+<p>The study of these discharge phenomena has led Mr. Tesla to
+the recognition of some important facts. It was found, as already
+stated, that gaseous matter must be most carefully excluded from<span class='pagenum'><a name="Page_130" id="Page_130">[Pg 130]</a></span>
+any dielectric which is subjected to great, rapidly changing electrostatic
+stresses. Since it is difficult to exclude the gas perfectly
+when solid insulators are used, it is necessary to resort to liquid
+dielectrics. When a solid dielectric is used, it matters little how
+thick and how good it is; if air be present, streamers form,
+which gradually heat the dielectric and impair its insulating
+power, and the discharge finally breaks through. Under ordinary
+conditions the best insulators are those which possess the
+highest specific inductive capacity, but such insulators are not
+the best to employ when working with these high frequency
+currents, for in most cases the higher specific inductive capacity
+is rather a disadvantage. The prime quality of the insulating
+medium for these currents is continuity. For this reason principally
+it is necessary to employ liquid insulators, such as oils.
+If two metal plates, connected to the terminals of the coil, are
+immersed in oil and set a distance apart, the coil may be kept
+working for any length of time without a break occurring, or
+without the oil being warmed, but if air bubbles are introduced,
+they become luminous; the air molecules, by their impact
+against the oil, heat it, and after some time cause the insulation
+to give way. If, instead of the oil, a solid plate of the best
+dielectric, even several times thicker than the oil intervening
+between the metal plates, is inserted between the latter, the air
+having free access to the charged surfaces, the dielectric invariably
+is warmed and breaks down.</p>
+
+<p>The employment of oil is advisable or necessary even with low
+frequencies, if the potentials are such that streamers form, but
+only in such cases, as is evident from the theory of the action.
+If the potentials are so low that streamers do not form, then it
+is even disadvantageous to employ oil, for it may, principally by
+confining the heat, be the cause of the breaking down of the insulation.</p>
+
+<p>The exclusion of gaseous matter is not only desirable on account
+of the safety of the apparatus, but also on account of
+economy, especially in a condenser, in which considerable waste
+of power may occur merely owing to the presence of air, if the
+electric density on the charged surfaces is great.</p>
+
+<p>In the course of these investigations a phenomenon of special
+scientific interest was observed. It may be ranked among the
+brush phenomena, in fact it is a kind of brush which forms at, or
+near, a single terminal in high vacuum. In a bulb with a con<span class='pagenum'><a name="Page_131" id="Page_131">[Pg 131]</a></span>ducting
+electrode, even if the latter be of aluminum, the brush
+has only a very short existence, but it can be preserved for a considerable
+length of time in a bulb devoid of any conducting electrode.
+To observe the phenomenon it is found best to employ a
+large spherical bulb having in its centre a small bulb supported
+on a tube sealed to the neck of the former. The large bulb being
+exhausted to a high degree, and the inside of the small bulb
+being connected to one of the terminals of the coil, under certain
+conditions there appears a misty haze around the small bulb,
+which, after passing through some stages, assumes the form of a
+brush, generally at right angles to the tube supporting the small
+bulb. When the brush assumes this form it may be brought to
+a state of extreme sensitiveness to electrostatic and magnetic influence.
+The bulb hanging straight down, and all objects being
+remote from it, the approach of the observer within a few paces
+will cause the brush to fly to the opposite side, and if he walks
+around the bulb it will always keep on the opposite side. It may
+begin to spin around the terminal long before it reaches that sensitive
+stage. When it begins to turn around, principally, but
+also before, it is affected by a magnet, and at a certain stage it is
+susceptible to magnetic influence to an astonishing degree. A
+small permanent magnet, with its poles at a distance of no more
+than two centimetres will affect it visibly at a distance of two metres,
+slowing down or accelerating the rotation according to how
+it is held relatively to the brush.</p>
+
+<p>When the bulb hangs with the globe down, the rotation is always
+clockwise. In the southern hemisphere it would occur in
+the opposite direction, and on the (magnetic) equator the brush
+should not turn at all. The rotation may be reversed by a magnet
+kept at some distance. The brush rotates best, seemingly,
+when it is at right angles to the lines of force of the earth. It
+very likely rotates, when at its maximum speed, in synchronism
+with the alternations, say, 10,000 times a second. The rotation
+can be slowed down or accelerated by the approach or recession
+of the observer, or any conducting body, but it cannot be reversed
+by putting the bulb in any position. Very curious experiments
+may be performed with the brush when in its most sensitive
+state. For instance, the brush resting in one position, the
+experimenter may, by selecting a proper position, approach the
+hand at a certain considerable distance to the bulb, and he may
+cause the brush to pass off by merely stiffening the muscles of<span class='pagenum'><a name="Page_132" id="Page_132">[Pg 132]</a></span>
+the arm, the mere change of configuration of the arm and the
+consequent imperceptible displacement being sufficient to disturb
+the delicate balance. When it begins to rotate slowly, and the
+hands are held at a proper distance, it is impossible to make even
+the slightest motion without producing a visible effect upon the
+brush. A metal plate connected to the other terminal of the coil
+affects it at a great distance, slowing down the rotation often to
+one turn a second.</p>
+
+<p>Mr. Tesla hopes that this phenomenon will prove a valuable
+aid in the investigation of the nature of the forces acting in an
+electrostatic or magnetic field. If there is any motion which is
+measurable going on in the space, such a brush would be apt to
+reveal it. It is, so to speak, a beam of light, frictionless, devoid
+of inertia. On account of its marvellous sensitiveness to electrostatic
+or magnetic disturbances it may be the means of sending
+signals through submarine cables with any speed, and even of
+transmitting intelligence to a distance without wires.</p>
+
+<p>In operating an induction coil with these rapidly alternating
+currents, it is astonishing to note, for the first time, the great
+importance of the relation of capacity, self-induction, and frequency
+as bearing upon the general result. The combined effect
+of these elements produces many curious effects. For instance,
+two metal plates are connected to the terminals and set at a small
+distance, so that an arc is formed between them. This arc <i>prevents</i>
+a strong current from flowing through the coil. If the arc
+be interrupted by the interposition of a glass plate, the capacity
+of the condenser obtained counteracts the self-induction, and a
+stronger current is made to pass. The effects of capacity are the
+most striking, for in these experiments, since the self-induction
+and frequency both are high, the critical capacity is very small,
+and need be but slightly varied to produce a very considerable
+change. The experimenter brings his body in contact with the
+terminals of the secondary of the coil, or attaches to one or both
+terminals insulated bodies of very small bulk, such as exhausted
+bulbs, and he produces a considerable rise or fall of potential on
+the secondary, and greatly affects the flow of the current through
+the primary coil.</p>
+
+<p>In many of the phenomena observed, the presence of the air,
+or, generally speaking, of a medium of a gaseous nature (using
+this term not to imply specific properties, but in contradistinction
+to homogeneity or perfect continuity) plays an important part,<span class='pagenum'><a name="Page_133" id="Page_133">[Pg 133]</a></span>
+as it allows energy to be dissipated by molecular impact or bombardment.
+The action is thus explained:&mdash;When an insulated
+body connected to a terminal of the coil is suddenly charged to
+high potential, it acts inductively upon the surrounding air, or
+whatever gaseous medium there might be. The molecules or
+atoms which are near it are, of course, more attracted, and move
+through a greater distance than the further ones. When the
+nearest molecules strike the body they are repelled, and collisions
+occur at all distances within the inductive distance. It is now
+clear that, if the potential be steady, but little loss of energy can
+be caused in this way, for the molecules which are nearest to
+the body having had an additional charge imparted to them by
+contact, are not attracted until they have parted, if not with all,
+at least with most of the additional charge, which can be accomplished
+only after a great many collisions. This is inferred from
+the fact that with a steady potential there is but little loss in dry
+air. When the potential, instead of being steady, is alternating,
+the conditions are entirely different. In this case a rhythmical
+bombardment occurs, no matter whether the molecules after
+coming in contact with the body lose the imparted charge or
+not, and, what is more, if the charge is not lost, the impacts are
+all the more violent. Still, if the frequency of the impulses
+be very small, the loss caused by the impacts and collisions would
+not be serious unless the potential was excessive. But when
+extremely high frequencies and more or less high potentials are
+used, the loss may be very great. The total energy lost per unit
+of time is proportionate to the product of the number of impacts
+per second, or the frequency and the energy lost in each impact.
+But the energy of an impact must be proportionate to the square
+of the electric density of the body, on the assumption that the
+charge imparted to the molecule is proportionate to that density.
+It is concluded from this that the total energy lost must be proportionate
+to the product of the frequency and the square of the
+electric density; but this law needs experimental confirmation.
+Assuming the preceding considerations to be true, then, by rapidly
+alternating the potential of a body immersed in an insulating
+gaseous medium, any amount of energy may be dissipated
+into space. Most of that energy, then, is not dissipated in the
+form of long ether waves, propagated to considerable distance,
+as is thought most generally, but is consumed in impact and
+collisional losses&mdash;that is, heat vibrations&mdash;on the surface and in<span class='pagenum'><a name="Page_134" id="Page_134">[Pg 134]</a></span>
+the vicinity of the body. To reduce the dissipation it is necessary
+to work with a small electric density&mdash;the smaller, the
+higher the frequency.</p>
+
+<p>The behavior of a gaseous medium to such rapid alternations
+of potential makes it appear plausible that electrostatic disturbances
+of the earth, produced by cosmic events, may have
+great influence upon the meteorological conditions. When such
+disturbances occur both the frequency of the vibrations of the
+charge and the potential are in all probability excessive, and the
+energy converted into heat may be considerable. Since the
+density must be unevenly distributed, either in consequence of
+the irregularity of the earth's surface, or on account of the
+condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable
+variations in the temperature and pressure of the atmosphere
+may in this manner be caused at any point of the surface of the
+earth. The variations may be gradual or very sudden, according
+to the nature of the original disturbance, and may produce rain
+and storms, or locally modify the weather in any way.</p>
+
+<p>From many experiences gathered in the course of these investigations
+it appears certain that in lightning discharges the air is
+an element of importance. For instance, during a storm a
+stream may form on a nail or pointed projection of a building.
+If lightning strikes somewhere in the neighborhood, the harmless
+static discharge may, in consequence of the oscillations set
+up, assume the character of a high-frequency streamer, and the
+nail or projection may be brought to a high temperature by the
+violent impact of the air molecules. Thus, it is thought, a
+building may be set on fire without the lightning striking it. In
+like manner small metallic objects may be fused and volatilized&mdash;as
+frequently occurs in lightning discharges&mdash;merely because
+they are surrounded by air. Were they immersed in a practically
+continuous medium, such as oil, they would probably be
+safe, as the energy would have to spend itself elsewhere.</p>
+
+<p>An instructive experience having a bearing on this subject is
+the following:&mdash;A glass tube of an inch or so in diameter and
+several inches long is taken, and a platinum wire sealed into it,
+the wire running through the center of the tube from end to
+end. The tube is exhausted to a moderate degree. If a steady
+current is passed through the wire it is heated uniformly in all
+parts and the gas in the tube is of no consequence. But if high<span class='pagenum'><a name="Page_135" id="Page_135">[Pg 135]</a></span>
+frequency discharges are directed through the wire, it is heated
+more on the ends than in the middle portion, and if the frequency,
+or rate of charge, is high enough, the wire might as
+well be cut in the middle as not, for most of the heating on the
+ends is due to the rarefied gas. Here the gas might only act as
+a conductor of no impedance, diverting the current from the
+wire as the impedance of the latter is enormously increased, and
+merely heating the ends of the wire by reason of their resistance
+to the passage of the discharge. But it is not at all necessary that
+the gas in the tube should be conducting; it might be at an extremely
+low pressure, still the ends of the wire would be heated;
+however, as is ascertained by experience, only the two ends
+would in such case not be electrically connected through the
+gaseous medium. Now, what with these frequencies and potentials
+occurs in an exhausted tube, occurs in the lightning discharge
+at ordinary pressure.</p>
+
+<p>From the facility with which any amount of energy may be
+carried off through a gas, Mr. Tesla infers that the best way to
+render harmless a lightning discharge is to afford it in some way
+a passage through a volume of gas.</p>
+
+<p>The recognition of some of the above facts has a bearing upon
+far-reaching scientific investigations in which extremely high
+frequencies and potentials are used. In such cases the air is an
+important factor to be considered. So, for instance, if two wires
+are attached to the terminals of the coil, and the streamers issue
+from them, there is dissipation of energy in the form of heat
+and light, and the wires behave like a condenser of larger capacity.
+If the wires be immersed in oil, the dissipation of energy
+is prevented, or at least reduced, and the apparent capacity is
+diminished. The action of the air would seem to make it very
+difficult to tell, from the measured or computed capacity of a
+condenser in which the air is acted upon, its actual capacity or
+vibration period, especially if the condenser is of very small surface
+and is charged to a very high potential. As many important
+results are dependant upon the correctness of the estimation
+of the vibration period, this subject demands the most careful
+scrutiny of investigators.</p>
+
+<p>In Leyden jars the loss due to the presence of air is comparatively
+small, principally on account of the great surface of the
+coatings and the small external action, but if there are streamers
+on the top, the loss may be considerable, and the period of vibra<span class='pagenum'><a name="Page_136" id="Page_136">[Pg 136]</a></span>tion
+is affected. In a resonator, the density is small, but the
+frequency is extreme, and may introduce a considerable error.
+It appears certain, at any rate, that the periods of vibration of a
+charged body in a gaseous and in a continuous medium, such
+as oil, are different, on account of the action of the former, as
+explained.</p>
+
+<p>Another fact recognized, which is of some consequence, is,
+that in similar investigations the general considerations of static
+screening are not applicable when a gaseous medium is present.
+This is evident from the following experiment:&mdash;A short and
+wide glass tube is taken and covered with a substantial coating of
+bronze powder, barely allowing the light to shine a little through.
+The tube is highly exhausted and suspended on a metallic clasp
+from the end of a wire. When the wire is connected with one
+of the terminals of the coil, the gas inside of the tube is lighted
+in spite of the metal coating. Here the metal evidently does
+not screen the gas inside as it ought to, even if it be very thin
+and poorly conducting. Yet, in a condition of rest the metal
+coating, however thin, screens the inside perfectly.</p>
+
+<p>One of the most interesting results arrived at in pursuing these
+experiments, is the demonstration of the fact that a gaseous medium,
+upon which vibration is impressed by rapid changes of
+electrostatic potential, is rigid. In illustration of this result an
+experiment made by Mr. Tesla may by cited:&mdash;A glass tube about
+one inch in diameter and three feet long, with outside condenser
+coatings on the ends, was exhausted to a certain point, when, the
+tube being suspended freely from a wire connecting the upper coating
+to one of the terminals of the coil, the discharge appeared in
+the form of a luminous thread passing through the axis of the tube.
+Usually the thread was sharply defined in the upper part of the
+tube and lost itself in the lower part. When a magnet or the
+finger was quickly passed near the upper part of the luminous
+thread, it was brought out of position by magnetic or electrostatic
+influence, and a transversal vibration like that of a suspended
+cord, with one or more distinct nodes, was set up, which
+lasted for a few minutes and gradually died out. By suspending
+from the lower condenser coating metal plates of different sizes,
+the speed of the vibration was varied. This vibration would
+seem to show beyond doubt that the thread possessed rigidity,
+at least to transversal displacements.</p>
+
+<p>Many experiments were tried to demonstrate this property in<span class='pagenum'><a name="Page_137" id="Page_137">[Pg 137]</a></span>
+air at ordinary pressure. Though no positive evidence has been
+obtained, it is thought, nevertheless, that a high frequency brush
+or streamer, if the frequency could be pushed far enough, would
+be decidedly rigid. A small sphere might then be moved within
+it quite freely, but if thrown against it the sphere would rebound.
+An ordinary flame cannot possess rigidity to a marked degree
+because the vibration is directionless; but an electric arc, it is
+believed, must possess that property more or less. A luminous
+band excited in a bulb by repeated discharges of a Leyden jar
+must also possess rigidity, and if deformed and suddenly released
+should vibrate.</p>
+
+<p>From like considerations other conclusions of interest are
+reached. The most probable medium filling the space is one
+consisting of independent carriers immersed in an insulating
+fluid. If through this medium enormous electrostatic stresses
+are assumed to act, which vary rapidly in intensity, it would
+allow the motion of a body through it, yet it would be rigid and
+elastic, although the fluid itself might be devoid of these properties.
+Furthermore, on the assumption that the independent
+carriers are of any configuration such that the fluid resistance to
+motion in one direction is greater than in another, a stress of
+that nature would cause the carriers to arrange themselves in
+groups, since they would turn to each other their sides of the
+greatest electric density, in which position the fluid resistance to
+approach would be smaller than to receding. If in a medium of
+the above characteristics a brush would be formed by a steady
+potential, an exchange of the carriers would go on continually,
+and there would be less carriers per unit of volume in the brush
+than in the space at some distance from the electrode, this corresponding
+to rarefaction. If the potential were rapidly changing,
+the result would be very different; the higher the frequency
+of the pulses, the slower would be the exchange of the carriers;
+finally, the motion of translation through measurable space would
+cease, and, with a sufficiently high frequency and intensity of the
+stress, the carriers would be drawn towards the electrode, and
+compression would result.</p>
+
+<p>An interesting feature of these high frequency currents is that
+they allow of operating all kinds of devices by connecting the device
+with only one leading wire to the electric source. In fact,
+under certain conditions it may be more economical to supply the
+electrical energy with one lead than with two.<span class='pagenum'><a name="Page_138" id="Page_138">[Pg 138]</a></span></p>
+
+<p>An experiment of special interest shown by Mr. Tesla, is the
+running, by the use of only one insulated line, of a motor operating
+on the principle of the rotating magnetic field enunciated
+by Mr. Tesla. A simple form of such a motor is obtained by
+winding upon a laminated iron core a primary and close to it a
+secondary coil, closing the ends of the latter and placing a freely
+movable metal disc within the influence of the moving field.
+The secondary coil may, however, be omitted. When one of the
+ends of the primary coil of the motor is connected to one of the
+terminals of the high frequency coil and the other end to an
+insulated metal plate, which, it should be stated, is not absolutely
+necessary for the success of the experiment, the disc is set in
+rotation.</p>
+
+<p>Experiments of this kind seem to bring it within possibility to
+operate a motor at any point of the earth's surface from a central
+source, without any connection to the same except through
+the earth. If, by means of powerful machinery, rapid variations
+of the earth's potential were produced, a grounded wire reaching
+up to some height would be traversed by a current which could
+be increased by connecting the free end of the wire to a body of
+some size. The current might be converted to low tension and
+used to operate a motor or other device. The experiment, which
+would be one of great scientific interest, would probably best
+succeed on a ship at sea. In this manner, even if it were not
+possible to operate machinery, intelligence might be transmitted
+quite certainly.</p>
+
+<p>In the course of this experimental study special attention was
+devoted to the heating effects produced by these currents, which
+are not only striking, but open up the possibility of producing a
+more efficient illuminant. It is sufficient to attach to the coil
+terminal a thin wire or filament, to have the temperature of the
+latter perceptibly raised. If the wire or filament be enclosed in
+a bulb, the heating effect is increased by preventing the circulation
+of the air. If the air in the bulb be strongly compressed,
+the displacements are smaller, the impacts less violent, and the
+heating effect is diminished. On the contrary, if the air in the
+bulb be exhausted, an inclosed lamp filament is brought to incandescence,
+and any amount of light may thus be produced.</p>
+
+<p>The heating of the inclosed lamp filament depends on so
+many things of a different nature, that it is difficult to give a
+generally applicable rule under which the maximum heating<span class='pagenum'><a name="Page_139" id="Page_139">[Pg 139]</a></span>
+occurs. As regards the size of the bulb, it is ascertained that at
+ordinary or only slightly differing atmospheric pressures, when
+air is a good insulator, the filament is heated more in a small
+bulb, because of the better confinement of heat in this case. At
+lower pressures, when air becomes conducting, the heating effect
+is greater in a large bulb, but at excessively high degrees of
+exhaustion there seems to be, beyond a certain and rather small
+size of the vessel, no perceptible difference in the heating.</p>
+
+<p>The shape of the vessel is also of some importance, and it has
+been found of advantage for reasons of economy to employ a
+spherical bulb with the electrode mounted in its centre, where
+the rebounding molecules collide.</p>
+
+<p>It is desirable on account of economy that all the energy supplied
+to the bulb from the source should reach without loss the
+body to be heated. The loss in conveying the energy from the
+source to the body may be reduced by employing thin wires
+heavily coated with insulation, and by the use of electrostatic
+screens. It is to be remarked, that the screen cannot be connected
+to the ground as under ordinary conditions.</p>
+
+<p>In the bulb itself a large portion of the energy supplied may
+be lost by molecular bombardment against the wire connecting
+the body to be heated with the source. Considerable improvement
+was effected by covering the glass stem containing the wire
+with a closely fitting conducting tube. This tube is made to
+project a little above the glass, and prevents the cracking of the
+latter near the heated body. The effectiveness of the conducting
+tube is limited to very high degrees of exhaustion. It diminishes
+the energy lost in bombardment for two reasons; first, the
+charge given up by the atoms spreads over a greater area, and
+hence the electric density at any point is small, and the atoms
+are repelled with less energy than if they would strike against a
+good insulator; secondly, as the tube is electrified by the atoms
+which first come in contact with it, the progress of the following
+atoms against the tube is more or less checked by the repulsion
+which the electrified tube must exert upon the similarly electrified
+atoms. This, it is thought, explains why the discharge through
+a bulb is established with much greater facility when an insulator,
+than when a conductor, is present.</p>
+
+<p>During the investigations a great many bulbs of different construction,
+with electrodes of different material, were experimented
+upon, and a number of observations of interest were made. Mr.<span class='pagenum'><a name="Page_140" id="Page_140">[Pg 140]</a></span>
+Tesla has found that the deterioration of the electrode is the less,
+the higher the frequency. This was to be expected, as then the
+heating is effected by many small impacts, instead by fewer and
+more violent ones, which quickly shatter the structure. The deterioration
+is also smaller when the vibration is harmonic. Thus
+an electrode, maintained at a certain degree of heat, lasts much
+longer with currents obtained from an alternator, than with
+those obtained by means of a disruptive discharge. One of the
+most durable electrodes was obtained from strongly compressed
+carborundum, which is a kind of carbon recently produced by
+Mr. E. G. Acheson, of Monongahela City, Pa. From experience,
+it is inferred, that to be most durable, the electrode should
+be in the form of a sphere with a highly polished surface.</p>
+
+<p>In some bulbs refractory bodies were mounted in a carbon cup
+and put under the molecular impact. It was observed in
+such experiments that the carbon cup was heated at first, until a
+higher temperature was reached; then most of the bombardment
+was directed against the refractory body, and the carbon
+was relieved. In general, when different bodies were mounted
+in the bulb, the hardest fusible would be relieved, and would
+remain at a considerably lower temperature. This was necessitated
+by the fact that most of the energy supplied would find
+its way through the body which was more easily fused or "evaporated."</p>
+
+<p>Curiously enough it appeared in some of the experiments
+made, that a body was fused in a bulb under the molecular impact
+by evolution of less light than when fused by the application
+of heat in ordinary ways. This may be ascribed to a
+loosening of the structure of the body under the violent impacts
+and changing stresses.</p>
+
+<p>Some experiments seem to indicate that under certain conditions
+a body, conducting or nonconducting, may, when bombarded,
+emit light, which to all appearances is due to phosphorescence,
+but may in reality be caused by the incandescence of an
+infinitesimal layer, the mean temperature of the body being
+comparatively small. Such might be the case if each single
+rhythmical impact were capable of instantaneously exciting the
+retina, and the rhythm were just high enough to cause a continuous
+impression in the eye. According to this view, a coil operated
+by disruptive discharge would be eminently adapted to produce
+such a result, and it is found by experience that its power of<span class='pagenum'><a name="Page_141" id="Page_141">[Pg 141]</a></span>
+exciting phosphorescence is extraordinarily great. It is capable
+of exciting phosphorescence at comparatively low degrees of
+exhaustion, and also projects shadows at pressures far greater
+than those at which the mean free path is comparable to the
+dimensions of the vessel. The latter observation is of some importance,
+inasmuch as it may modify the generally accepted views
+in regard to the "radiant state" phenomena.</p>
+
+<p>A thought which early and naturally suggested itself to Mr.
+Tesla, was to utilize the great inductive effects of high frequency
+currents to produce light in a sealed glass vessel without the use
+of leading in wires. Accordingly, many bulbs were constructed
+in which the energy necessary to maintain a button or filament
+at high incandescence, was supplied through the glass by either
+electrostatic or electrodynamic induction. It was easy to regulate
+the intensity of the light emitted by means of an externally
+applied condenser coating connected to an insulated plate, or
+simply by means of a plate attached to the bulb which at the
+same time performed the function of a shade.</p>
+
+<p>A subject of experiment, which has been exhaustively treated
+in England by Prof. J. J. Thomson, has been followed up independently
+by Mr. Tesla from the beginning of this study, namely,
+to excite by electrodynamic induction a luminous band in a closed
+tube or bulb. In observing the behavior of gases, and the
+luminous phenomena obtained, the importance of the electrostatic
+effects was noted and it appeared desirable to produce
+enormous potential differences, alternating with extreme rapidity.
+Experiments in this direction led to some of the most interesting
+results arrived at in the course of these investigations. It
+was found that by rapid alternations of a high electrostatic potential,
+exhausted tubes could be lighted at considerable distances
+from a conductor connected to a properly constructed coil, and
+that it was practicable to establish with the coil an alternating
+electrostatic field, acting through the whole room and lighting a
+tube wherever it was placed within the four walls. Phosphorescent
+bulbs may be excited in such a field, and it is easy to regulate
+the effect by connecting to the bulb a small insulated metal
+plate. It was likewise possible to maintain a filament or button
+mounted in a tube at bright incandescence, and, in one experiment,
+a mica vane was spun by the incandescence of a platinum
+wire.</p>
+
+<p>Coming now to the lecture delivered in Philadelphia and St.<span class='pagenum'><a name="Page_142" id="Page_142">[Pg 142]</a></span>
+Louis, it may be remarked that to the superficial reader, Mr.
+Tesla's introduction, dealing with the importance of the eye, might
+appear as a digression, but the thoughtful reader will find therein
+much food for meditation and speculation. Throughout his discourse
+one can trace Mr. Tesla's effort to present in a popular
+way thoughts and views on the electrical phenomena which have
+in recent years captivated the scientific world, but of which the
+general public has even yet merely received an inkling. Mr.
+Tesla also dwells rather extensively on his well-known method of
+high-frequency conversion; and the large amount of detail information
+will be gratefully received by students and experimenters
+in this virgin field. The employment of apt analogies
+in explaining the fundamental principles involved makes it easy
+for all to gain a clear idea of their nature. Again, the ease with
+which, thanks to Mr. Tesla's efforts, these high-frequency currents
+may now be obtained from circuits carrying almost any
+kind of current, cannot fail to result in an extensive broadening
+of this field of research, which offers so many possibilities. Mr.
+Tesla, true philosopher as he is, does not hesitate to point out
+defects in some of his methods, and indicates the lines which to
+him seem the most promising. Particular stress is laid by him
+upon the employment of a medium in which the discharge
+electrodes should be immersed in order that this method of conversion
+may be brought to the highest perfection. He has evidently
+taken pains to give as much useful information as possible
+to those who wish to follow in his path, as he shows in detail the
+circuit arrangements to be adopted in all ordinary cases met with
+in practice, and although some of these methods were described
+by him two years before, the additional information is still timely
+and welcome.</p>
+
+<p>In his experiments he dwells first on some phenomena produced
+by electrostatic force, which he considers in the light of
+modern theories to be the most important force in nature for us
+to investigate. At the very outset he shows a strikingly novel
+experiment illustrating the effect of a rapidly varying electrostatic
+force in a gaseous medium, by touching with one hand one of
+the terminals of a 200,000 volt transformer and bringing the
+other hand to the opposite terminal. The powerful streamers
+which issued from his hand and astonished his audiences formed
+a capital illustration of some of the views advanced, and afforded
+Mr. Tesla an opportunity of pointing out the true reasons why,<span class='pagenum'><a name="Page_143" id="Page_143">[Pg 143]</a></span>
+with these currents, such an amount of energy can be passed
+through the body with impunity. He then showed by experiment
+the difference between a steady and a rapidly varying force
+upon the dielectric. This difference is most strikingly illustrated
+in the experiment in which a bulb attached to the end of a wire
+in connection with one of the terminals of the transformer is
+ruptured, although all extraneous bodies are remote from the
+bulb. He next illustrates how mechanical motions are produced
+by a varying electrostatic force acting through a gaseous medium.
+The importance of the action of the air is particularly illustrated
+by an interesting experiment.</p>
+
+<p>Taking up another class of phenomena, namely, those of dynamic
+electricity, Mr. Tesla produced in a number of experiments
+a variety of effects by the employment of only a single wire
+with the evident intent of impressing upon his audience the idea
+that electric vibration or current can be transmitted with ease,
+without any return circuit; also how currents so transmitted can
+be converted and used for many practical purposes. A number
+of experiments are then shown, illustrating the effects of frequency,
+self-induction and capacity; then a number of ways of
+operating motive and other devices by the use of a single lead.
+A number of novel impedance phenomena are also shown which
+cannot fail to arouse interest.</p>
+
+<p>Mr. Tesla next dwelt upon a subject which he thinks of great
+importance, that is, electrical resonance, which he explained in a
+popular way. He expressed his firm conviction that by observing
+proper conditions, intelligence, and possibly even power, can
+be transmitted through the medium or through the earth; and
+he considers this problem worthy of serious and immediate consideration.</p>
+
+<p>Coming now to the light phenomena in particular, he illustrated
+the four distinct kinds of these phenomena in an original way,
+which to many must have been a revelation. Mr. Tesla attributes
+these light effects to molecular or atomic impacts produced by a
+varying electrostatic stress in a gaseous medium. He illustrated
+in a series of novel experiments the effect of the gas surrounding
+the conductor and shows beyond a doubt that with high frequency
+and high potential currents, the surrounding gas is of
+paramount importance in the heating of the conductor. He
+attributes the heating partially to a conduction current and partially
+to bombardment, and demonstrates that in many cases the<span class='pagenum'><a name="Page_144" id="Page_144">[Pg 144]</a></span>
+heating may be practically due to the bombardment alone. He
+pointed out also that the skin effect is largely modified by the
+presence of the gas or of an atomic medium in general. He
+showed also some interesting experiments in which the effect of
+convection is illustrated. Probably one of the most curious experiments
+in this connection is that in which a thin platinum wire
+stretched along the axis of an exhausted tube is brought to incandescence
+at certain points corresponding to the position of
+the stri&aelig;, while at others it remains dark. This experiment
+throws an interesting light upon the nature of the stri&aelig; and may
+lead to important revelations.</p>
+
+<p>Mr. Tesla also demonstrated the dissipation of energy through
+an atomic medium and dwelt upon the behavior of vacuous
+space in conveying heat, and in this connection showed the curious
+behavior of an electrode stream, from which he concludes that
+the molecules of a gas probably cannot be acted upon directly
+at measurable distances.</p>
+
+<p>Mr. Tesla summarized the chief results arrived at in pursuing
+his investigations in a manner which will serve as a valuable
+guide to all who may engage in this work. Perhaps most interest
+will centre on his general statements regarding the phenomena
+of phosphorescence, the most important fact revealed in this direction
+being that when exciting a phosphorescent bulb a certain
+definite potential gives the most economical result.</p>
+
+<p>The lectures will now be presented in the order of their date
+of delivery.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_145" id="Page_145">[Pg 145]</a></span></p>
+<h2><a name="CHAPTER_XXVI" id="CHAPTER_XXVI"></a>CHAPTER XXVI.</h2>
+
+<h3><span class="smcap">Experiments With Alternate Currents of Very High Frequency
+and Their Application to Methods of Artificial
+Illumination.</span><a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a></h3>
+
+
+<p>There is no subject more captivating, more worthy of study,
+than nature. To understand this great mechanism, to discover
+the forces which are active, and the laws which govern them, is
+the highest aim of the intellect of man.</p>
+
+<p>Nature has stored up in the universe infinite energy. The
+eternal recipient and transmitter of this infinite energy is the
+ether. The recognition of the existence of ether, and of the
+functions it performs, is one of the most important results of
+modern scientific research. The mere abandoning of the idea of
+action at a distance, the assumption of a medium pervading all
+space and connecting all gross matter, has freed the minds of
+thinkers of an ever present doubt, and, by opening a new horizon&mdash;new
+and unforeseen possibilities&mdash;has given fresh interest to
+phenomena with which we are familiar of old. It has been a
+great step towards the understanding of the forces of nature and
+their multifold manifestations to our senses. It has been for
+the enlightened student of physics what the understanding of
+the mechanism of the firearm or of the steam engine is for the
+barbarian. Phenomena upon which we used to look as wonders
+baffling explanation, we now see in a different light. The spark
+of an induction coil, the glow of an incandescent lamp, the manifestations
+of the mechanical forces of currents and magnets are
+no longer beyond our grasp; instead of the incomprehensible, as
+before, their observation suggests now in our minds a simple
+mechanism, and although as to its precise nature all is still conjecture,
+yet we know that the truth cannot be much longer hidden,
+and instinctively we feel that the understanding is dawning
+upon us. We still admire these beautiful phenomena, these<span class='pagenum'><a name="Page_146" id="Page_146">[Pg 146]</a></span>
+strange forces, but we are helpless no longer; we can in a certain
+measure explain them, account for them, and we are hopeful of
+finally succeeding in unraveling the mystery which surrounds
+them.</p>
+
+<p>In how far we can understand the world around us is the ultimate
+thought of every student of nature. The coarseness of our
+senses prevents us from recognizing the ulterior construction of
+matter, and astronomy, this grandest and most positive of natural
+sciences, can only teach us something that happens, as it were, in
+our immediate neighborhood: of the remoter portions of the
+boundless universe, with its numberless stars and suns, we know
+nothing. But far beyond the limit of perception of our senses
+the spirit still can guide us, and so we may hope that even these
+unknown worlds&mdash;infinitely small and great&mdash;may in a measure
+become known to us. Still, even if this knowledge should reach
+us, the searching mind will find a barrier, perhaps forever unsurpassable,
+to the <i>true</i> recognition of that which <i>seems</i> to be, the
+mere <i>appearance</i> of which is the only and slender basis of all
+our philosophy.</p>
+
+<p>Of all the forms of nature's immeasurable, all-pervading
+energy, which ever and ever changing and moving, like a soul
+animates the inert universe, electricity and magnetism are perhaps
+the most fascinating. The effects of gravitation, of heat
+and light we observe daily, and soon we get accustomed to
+them, and soon they lose for us the character of the marvelous
+and wonderful; but electricity and magnetism, with their singular
+relationship, with their seemingly dual character, unique among
+the forces in nature, with their phenomena of attractions, repulsions
+and rotations, strange manifestations of mysterious agents,
+stimulate and excite the mind to thought and research. What is
+electricity, and what is magnetism? These questions have been
+asked again and again. The most able intellects have ceaselessly
+wrestled with the problem; still the question has not as yet been
+fully answered. But while we cannot even to-day state what
+these singular forces are, we have made good headway towards
+the solution of the problem. We are now confident that
+electric and magnetic phenomena are attributable to ether, and
+we are perhaps justified in saying that the effects of static electricity
+are effects of ether under strain, and those of dynamic
+electricity and electro-magnetism effects of ether in motion. But
+this still leaves the question, as to what electricity and magnetism
+are, unanswered.<span class='pagenum'><a name="Page_147" id="Page_147">[Pg 147]</a></span></p>
+
+<p>First, we naturally inquire, What is electricity, and is there
+such a thing as electricity? In interpreting electric phenomena,
+we may speak of electricity or of an electric condition, state or
+effect. If we speak of electric effects we must distinguish two
+such effects, opposite in character and neutralizing each other, as
+observation shows that two such opposite effects exist. This is
+unavoidable, for in a medium of the properties of ether, we cannot
+possibly exert a strain, or produce a displacement or motion
+of any kind, without causing in the surrounding medium an
+equivalent and opposite effect. But if we speak of electricity,
+meaning a <i>thing</i>, we must, I think, abandon the idea of two
+electricities, as the existence of two such things is highly improbable.
+For how can we imagine that there should be two things,
+equivalent in amount, alike in their properties, but of opposite
+character, both clinging to matter, both attracting and completely
+neutralizing each other? Such an assumption, though suggested
+by many phenomena, though most convenient for explaining
+them, has little to commend it. If there <i>is</i> such a thing as electricity,
+there can be only <i>one</i> such thing, and excess and want
+of that one thing, possibly; but more probably its condition determines
+the positive and negative character. The old theory of
+Franklin, though falling short in some respects, is, from a certain
+point of view, after all, the most plausible one. Still, in spite
+of this, the theory of the two electricities is generally accepted,
+as it apparently explains electric phenomena in a more satisfactory
+manner. But a theory which better explains the facts is not
+necessarily true. Ingenious minds will invent theories to suit
+observation, and almost every independent thinker has his own
+views on the subject.</p>
+
+<p>It is not with the object of advancing an opinion, but with
+the desire of acquainting you better with some of the results,
+which I will describe, to show you the reasoning I have followed,
+the departures I have made&mdash;that I venture to express,
+in a few words, the views and convictions which have led me to
+these results.</p>
+
+<p>I adhere to the idea that there is a thing which we have been
+in the habit of calling electricity. The question is, What is that
+thing? or, What, of all things, the existence of which we know,
+have we the best reason to call electricity? We know that it acts
+like an incompressible fluid; that there must be a constant quantity
+of it in nature; that it can be neither produced nor destroyed;<span class='pagenum'><a name="Page_148" id="Page_148">[Pg 148]</a></span>
+and, what is more important, the electro-magnetic theory of light
+and all facts observed teach us that electric and ether phenomena
+are identical. The idea at once suggests itself, therefore, that
+electricity might be called ether. In fact, this view has in a certain
+sense been advanced by Dr. Lodge. His interesting work
+has been read by everyone and many have been convinced by
+his arguments. His great ability and the interesting nature of
+the subject, keep the reader spellbound; but when the impressions
+fade, one realizes that he has to deal only with ingenious
+explanations. I must confess, that I cannot believe in two electricities,
+much less in a doubly-constituted ether. The puzzling
+behavior of the ether as a solid to waves of light and heat, and
+as a fluid to the motion of bodies through it, is certainly explained
+in the most natural and satisfactory manner by assuming
+it to be in motion, as Sir William Thomson has suggested; but
+regardless of this, there is nothing which would enable us to
+conclude with certainty that, while a fluid is not capable of transmitting
+transverse vibrations of a few hundred or thousand per
+second, it might not be capable of transmitting such vibrations
+when they range into hundreds of million millions per second.
+Nor can anyone prove that there are transverse ether waves
+emitted from an alternate current machine, giving a small number
+of alternations per second; to such slow disturbances, the ether,
+if at rest, may behave as a true fluid.</p>
+
+<p>Returning to the subject, and bearing in mind that the existence
+of two electricities is, to say the least, highly improbable,
+we must remember, that we have no evidence of electricity, nor
+can we hope to get it, unless gross matter is present. Electricity,
+therefore, cannot be called ether in the broad sense of the term;
+but nothing would seem to stand in the way of calling electricity
+ether associated with matter, or bound ether; or, in other words,
+that the so-called static charge of the molecule is ether associated
+in some way with the molecule. Looking at it in that light, we
+would be justified in saying, that electricity is concerned in all
+molecular actions.</p>
+
+<p>Now, precisely what the ether surrounding the molecules is,
+wherein it differs from ether in general, can only be conjectured.
+It cannot differ in density, ether being incompressible:
+it must, therefore, be under some strain or in motion, and the
+latter is the most probable. To understand its functions, it
+would be necessary to have an exact idea of the physical con<span class='pagenum'><a name="Page_149" id="Page_149">[Pg 149]</a></span>struction
+of matter, of which, of course, we can only form a
+mental picture.</p>
+
+<p>But of all the views on nature, the one which assumes one
+matter and one force, and a perfect uniformity throughout, is
+the most scientific and most likely to be true. An infinitesimal
+world, with the molecules and their atoms spinning and moving
+in orbits, in much the same manner as celestial bodies, carrying
+with them and probably spinning with them ether, or in other
+words, carrying with them static charges, seems to my mind the
+most probable view, and one which, in a plausible manner, accounts
+for most of the phenomena observed. The spinning of
+the molecules and their ether sets up the ether tensions or electrostatic
+strains; the equalization of ether tensions sets up ether
+motions or electric currents, and the orbital movements produce
+the effects of electro and permanent magnetism.</p>
+
+<p>About fifteen years ago, Prof. Rowland demonstrated a most
+interesting and important fact, namely, that a static charge carried
+around produces the effects of an electric current. Leaving
+out of consideration the precise nature of the mechanism, which
+produces the attraction and repulsion of currents, and conceiving
+the electrostatically charged molecules in motion, this experimental
+fact gives us a fair idea of magnetism. We can conceive lines
+or tubes of force which physically exist, being formed of rows
+of directed moving molecules; we can see that these lines must be
+closed, that they must tend to shorten and expand, etc. It likewise
+explains in a reasonable way, the most puzzling phenomenon
+of all, permanent magnetism, and, in general, has all the beauties
+of the Ampere theory without possessing the vital defect of the
+same, namely, the assumption of molecular currents. Without
+enlarging further upon the subject, I would say, that I look upon
+all electrostatic, current and magnetic phenomena as being due
+to electrostatic molecular forces.</p>
+
+<p>The preceding remarks I have deemed necessary to a full
+understanding of the subject as it presents itself to my mind.</p>
+
+<p>Of all these phenomena the most important to study are the
+current phenomena, on account of the already extensive and ever-growing
+use of currents for industrial purposes. It is now a century
+since the first practical source of current was produced,
+and, ever since, the phenomena which accompany the flow of
+currents have been diligently studied, and through the untiring
+efforts of scientific men the simple laws which govern them have<span class='pagenum'><a name="Page_150" id="Page_150">[Pg 150]</a></span>
+been discovered. But these laws are found to hold good only
+when the currents are of a steady character. When the currents
+are rapidly varying in strength, quite different phenomena, often
+unexpected, present themselves, and quite different laws hold
+good, which even now have not been determined as fully as is
+desirable, though through the work, principally, of English scientists,
+enough knowledge has been gained on the subject to enable
+us to treat simple cases which now present themselves in daily
+practice.</p>
+
+<p>The phenomena which are peculiar to the changing character
+of the currents are greatly exalted when the rate of change is
+increased, hence the study of these currents is considerably facilitated
+by the employment of properly constructed apparatus.
+It was with this and other objects in view that I constructed
+alternate current machines capable of giving more than two
+million reversals of current per minute, and to this circumstance
+it is principally due, that I am able to bring to your attention
+some of the results thus far reached, which I hope will prove to
+be a step in advance on account of their direct bearing upon one
+of the most important problems, namely, the production of a
+practical and efficient source of light.</p>
+
+<p>The study of such rapidly alternating currents is very interesting.
+Nearly every experiment discloses something new. Many
+results may, of course, be predicted, but many more are unforeseen.
+The experimenter makes many interesting observations.
+For instance, we take a piece of iron and hold it against a magnet.
+Starting from low alternations and running up higher and higher
+we feel the impulses succeed each other faster and faster, get
+weaker and weaker, and finally disappear. We then observe a
+continuous pull; the pull, of course, is not continuous; it only
+appears so to us; our sense of touch is imperfect.</p>
+
+<p>We may next establish an arc between the electrodes and
+observe, as the alternations rise, that the note which accompanies
+alternating arcs gets shriller and shriller, gradually weakens, and
+finally ceases. The air vibrations, of course, continue, but they
+are too weak to be perceived; our sense of hearing fails us.</p>
+
+<p>We observe the small physiological effects, the rapid heating of
+the iron cores and conductors, curious inductive effects, interesting
+condenser phenomena, and still more interesting light phenomena
+with a high tension induction coil. All these experiments
+and observations would be of the greatest interest to the<span class='pagenum'><a name="Page_151" id="Page_151">[Pg 151]</a></span>
+student, but their description would lead me too far from the
+principal subject. Partly for this reason, and partly on account
+of their vastly greater importance, I will confine myself to the
+description of the light effects produced by these currents.</p>
+
+<p>In the experiments to this end a high tension induction coil or
+equivalent apparatus for converting currents of comparatively
+low into currents of high tension is used.</p>
+
+<p>If you will be sufficiently interested in the results I shall describe
+as to enter into an experimental study of this subject; if you
+will be convinced of the truth of the arguments I shall advance&mdash;your
+aim will be to produce high frequencies and high potentials;
+in other words, powerful electrostatic effects. You will then encounter
+many difficulties, which, if completely overcome, would
+allow us to produce truly wonderful results.</p>
+
+<p>First will be met the difficulty of obtaining the required frequencies
+by means of mechanical apparatus, and, if they be obtained
+otherwise, obstacles of a different nature will present
+themselves. Next it will be found difficult to provide the requisite
+insulation without considerably increasing the size of the
+apparatus, for the potentials required are high, and, owing to the
+rapidity of the alternations, the insulation presents peculiar difficulties.
+So, for instance, when a gas is present, the discharge
+may work, by the molecular bombardment of the gas and consequent
+heating, through as much as an inch of the best solid
+insulating material, such as glass, hard rubber, porcelain, sealing
+wax, etc.; in fact, through any known insulating substance. The
+chief requisite in the insulation of the apparatus is, therefore, the
+exclusion of any gaseous matter.</p>
+
+<p>In general my experience tends to show that bodies which
+possess the highest specific inductive capacity, such as glass,
+afford a rather inferior insulation to others, which, while they are
+good insulators, have a much smaller specific inductive capacity,
+such as oils, for instance, the dielectric losses being no doubt
+greater in the former. The difficulty of insulating, of course,
+only exists when the potentials are excessively high, for with
+potentials such as a few thousand volts there is no particular difficulty
+encountered in conveying currents from a machine giving,
+say, 20,000 alternations per second, to quite a distance. This
+number of alternations, however, is by far too small for many
+purposes, though quite sufficient for some practical applications.
+This difficulty of insulating is fortunately not a vital drawback;<span class='pagenum'><a name="Page_152" id="Page_152">[Pg 152]</a></span>
+it affects mostly the size of the apparatus, for, when excessively
+high potentials would be used, the light-giving devices would be
+located not far from the apparatus, and often they would be quite
+close to it. As the air-bombardment of the insulated wire is dependent
+on condenser action, the loss may be reduced to a trifle
+by using excessively thin wires heavily insulated.</p>
+
+<p>Another difficulty will be encountered in the capacity and self-induction
+necessarily possessed by the coil. If the coil be large,
+that is, if it contain a great length of wire, it will be generally
+unsuited for excessively high frequencies; if it be small, it may
+be well adapted for such frequencies, but the potential might
+then not be as high as desired. A good insulator, and preferably
+one possessing a small specific inductive capacity, would
+afford a two-fold advantage. First, it would enable us to construct
+a very small coil capable of withstanding enormous differences
+of potential; and secondly, such a small coil, by reason of
+its smaller capacity and self-induction, would be capable of a
+quicker and more vigorous vibration. The problem then of constructing
+a coil or induction apparatus of any kind possessing
+the requisite qualities I regard as one of no small importance,
+and it has occupied me for a considerable time.</p>
+
+<p>The investigator who desires to repeat the experiments which
+I will describe, with an alternate current machine, capable of
+supplying currents of the desired frequency, and an induction
+coil, will do well to take the primary coil out and mount the secondary
+in such a manner as to be able to look through the tube
+upon which the secondary is wound. He will then be able to
+observe the streams which pass from the primary to the insulating
+tube, and from their intensity he will know how far he can
+strain the coil. Without this precaution he is sure to injure
+the insulation. This arrangement permits, however, an easy
+exchange of the primaries, which is desirable in these experiments.</p>
+
+<p>The selection of the type of machine best suited for the purpose
+must be left to the judgment of the experimenter. There
+are here illustrated three distinct types of machines, which,
+besides others, I have used in my experiments.</p>
+
+<p>Fig. 97 represents the machine used in my experiments before
+this Institute. The field magnet consists of a ring of wrought
+iron with 384 pole projections. The armature comprises a steel
+disc to which is fastened a thin, carefully welded rim of wrought<span class='pagenum'><a name="Page_153" id="Page_153">[Pg 153]</a></span>
+iron. Upon the rim are wound several layers of fine, well
+annealed iron wire, which, when wound, is passed through
+shellac. The armature wires are wound around brass pins,
+wrapped with silk thread. The diameter of the armature wire
+in this type of machine should not be more than 1/6 of the thickness
+of the pole projections, else the local action will be considerable.</p>
+
+<div class="figcenter" style="width: 503px;">
+<img src="images/oi_167.jpg" width="503" height="480" alt="Fig. 97." title="" />
+<span class="caption">Fig. 97.</span>
+</div>
+
+
+<p>Fig. 98 represents a larger machine of a different type. The
+field magnet of this machine consists of two like parts which
+either enclose an exciting coil, or else are independently wound.
+Each part has 480 pole projections, the projections of one facing
+those of the other. The armature consists of a wheel of hard
+bronze, carrying the conductors which revolve between the projections
+of the field magnet. To wind the armature conductors,
+I have found it most convenient to proceed in the following
+manner. I construct a ring of hard bronze of the required size.
+This ring and the rim of the wheel are provided with the
+proper number of pins, and both fastened upon a plate. The
+armature conductors being wound, the pins are cut off and the
+ends of the conductors fastened by two rings which screw to the<span class='pagenum'><a name="Page_154" id="Page_154">[Pg 154]</a></span>
+bronze ring and the rim of the wheel, respectively. The whole
+may then be taken off and forms a solid structure. The conductors
+in such a type of machine should consist of sheet copper,
+the thickness of which, of course, depends on the thickness of
+the pole projections; or else twisted thin wires should be employed.</p>
+
+<p>Fig. 99 is a smaller machine, in many respects similar to the
+former, only here the armature conductors and the exciting coil
+are kept stationary, while only a block of wrought iron is revolved.</p>
+
+<div class="figcenter" style="width: 567px;">
+<img src="images/oi_168.jpg" width="567" height="480" alt="Fig. 98." title="" />
+<span class="caption">Fig. 98.</span>
+</div>
+
+<p>It would be uselessly lengthening this description were I to
+dwell more on the details of construction of these machines.
+Besides, they have been described somewhat more elaborately in
+<i>The Electrical Engineer</i>, of March 18, 1891. I deem it well,
+however, to call the attention of the investigator to two things,
+the importance of which, though self evident, he is nevertheless
+apt to underestimate; namely, to the local action in the conductors
+which must be carefully avoided, and to the clearance,
+which must be small. I may add, that since it is desirable to use
+very high peripheral speeds, the armature should be of very
+large diameter in order to avoid impracticable belt speeds. Of<span class='pagenum'><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+the several types of these machines which have been constructed
+by me, I have found that the type illustrated in Fig. 97 caused
+me the least trouble in construction, as well as in maintenance,
+and on the whole, it has been a good experimental machine.</p>
+
+<p>In operating an induction coil with very rapidly alternating
+currents, among the first luminous phenomena noticed are naturally
+those presented by the high-tension discharge. As the number
+of alternations per second is increased, or as&mdash;the number
+being high&mdash;the current through the primary is varied, the discharge
+gradually changes in appearance. It would be difficult to
+describe the minor changes which occur, and the conditions which
+bring them about, but one may note five distinct forms of the
+discharge.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_169.jpg" width="640" height="437" alt="Fig. 99." title="" />
+<span class="caption">Fig. 99.</span>
+</div>
+
+<p>First, one may observe a weak, sensitive discharge in the form
+of a thin, feeble-colored thread. (Fig. 100a.) It always occurs
+when, the number of alternations per second being high, the current
+through the primary is very small. In spite of the excessively
+small current, the rate of change is great, and the difference
+of potential at the terminals of the secondary is therefore
+considerable, so that the arc is established at great distances; but
+the quantity of "electricity" set in motion is insignificant, barely
+sufficient to maintain a thin, threadlike arc. It is excessively
+sensitive and may be made so to such a degree that the mere act
+of breathing near the coil will affect it, and unless it is perfectly<span class='pagenum'><a name="Page_156" id="Page_156">[Pg 156]</a></span>
+well protected from currents of air, it wriggles around constantly.
+Nevertheless, it is in this form excessively persistent, and when
+the terminals are approached to, say, one-third of the striking
+distance, it can be blown out only with difficulty. This exceptional
+persistency, when short, is largely due to the arc being
+excessively thin; presenting, therefore, a very small surface
+to the blast. Its great sensitiveness, when very long, is probably
+due to the motion of the particles of dust suspended in the air.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_170.jpg" width="800" height="269" alt="Fig. 100a, 100b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 100a.</td><td class="caption1"><span class="smcap">Fig.</span> 100b.</td></tr>
+</table>
+</div>
+
+<p>When the current through the primary is increased, the discharge
+gets broader and stronger, and the effect of the capacity
+of the coil becomes visible until, finally, under proper conditions,
+a white flaming arc, Fig. 100<small>B</small>, often as thick as one's finger, and
+striking across the whole coil, is produced. It develops remarkable
+heat, and may be further characterized by the absence of
+the high note which accompanies the less powerful discharges.
+To take a shock from the coil under these conditions would not
+be advisable, although under different conditions, the potential
+being much higher, a shock from the coil may be taken with
+impunity. To produce this kind of discharge the number of
+alternations per second must not be too great for the coil used;
+and, generally speaking, certain relations between capacity, self-induction
+and frequency must be observed.</p>
+
+<p>The importance of these elements in an alternate current circuit
+is now well-known, and under ordinary conditions, the general
+rules are applicable. But in an induction coil exceptional
+conditions prevail. First, the self-induction is of little importance
+before the arc is established, when it asserts itself, but perhaps
+never as prominently as in ordinary alternate current circuits,
+because the capacity is distributed all along the coil, and by reason
+of the fact that the coil usually discharges through very great
+resistances; hence the currents are exceptionally small. Secondly,<span class='pagenum'><a name="Page_157" id="Page_157">[Pg 157]</a></span>
+the capacity goes on increasing continually as the potential rises,
+in consequence of absorption which takes place to a considerable
+extent. Owing to this there exists no critical relationship between
+these quantities, and ordinary rules would not seem to be applicable.
+As the potential is increased either in consequence of the
+increased frequency or of the increased current through the
+primary, the amount of the energy stored becomes greater and
+greater, and the capacity gains more and more in importance.
+Up to a certain point the capacity is beneficial, but after that it
+begins to be an enormous drawback. It follows from this that
+each coil gives the best result with a given frequency and primary
+current. A very large coil, when operated with currents of very
+high frequency, may not give as much as 1/8 inch spark. By adding
+capacity to the terminals, the condition may be improved, but
+what the coil really wants is a lower frequency.</p>
+
+<p>When the flaming discharge occurs, the conditions are evidently
+such that the greatest current is made to flow through the
+circuit. These conditions may be attained by varying the frequency
+within wide limits, but the highest frequency at which
+the flaming arc can still be produced, determines, for a given
+primary current, the maximum striking distance of the coil. In
+the flaming discharge the <i>eclat</i> effect of the capacity is not perceptible;
+the rate at which the energy is being stored then just
+equals the rate at which it can be disposed of through the circuit.
+This kind of discharge is the severest test for a coil; the break,
+when it occurs, is of the nature of that in an overcharged Leyden
+jar. To give a rough approximation I would state that, with an
+ordinary coil of, say 10,000 ohms resistance, the most powerful
+arc would be produced with about 12,000 alternations per second.</p>
+
+<p>When the frequency is increased beyond that rate, the potential,
+of course, rises, but the striking distance may, nevertheless,
+diminish, paradoxical as it may seem. As the potential rises the
+coil attains more and more the properties of a static machine
+until, finally, one may observe the beautiful phenomenon of the
+streaming discharge, Fig. 101, which may be produced across the
+whole length of the coil. At that stage streams begin to issue
+freely from all points and projections. These streams will also be
+seen to pass in abundance in the space between the primary and
+the insulating tube. When the potential is excessively high they
+will always appear, even if the frequency be low, and even if the
+primary be surrounded by as much as an inch of wax, hard rub<span class='pagenum'><a name="Page_158" id="Page_158">[Pg 158]</a></span>ber,
+glass, or any other insulating substance. This limits greatly
+the output of the coil, but I will later show how I have been able
+to overcome to a considerable extent this disadvantage in the
+ordinary coil.</p>
+
+<p>Besides the potential, the intensity of the streams depends on
+the frequency; but if the coil be very large they show themselves,
+no matter how low the frequencies used. For instance,
+in a very large coil of a resistance of 67,000 ohms, constructed
+by me some time ago, they appear with as low as 100 alternations
+per second and less, the insulation of the secondary being 3/4 inch
+of ebonite. When very intense they produce a noise similar to
+that produced by the charging of a Holtz machine, but much
+more powerful, and they emit a strong smell of ozone. The
+lower the frequency, the more apt they are to suddenly injure
+the coil. With excessively high frequencies they may pass freely
+without producing any other effect than to heat the insulation
+slowly and uniformly.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_172.jpg" width="800" height="252" alt="Fig. 101, 102." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 101.</td><td class="caption">Fig. 102.</td></tr>
+</table>
+</div>
+
+<p>The existence of these streams shows the importance of constructing
+an expensive coil so as to permit of one's seeing
+through the tube surrounding the primary, and the latter should
+be easily exchangeable; or else the space between the primary
+and secondary should be completely filled up with insulating
+material so as to exclude all air. The non-observance of this
+simple rule in the construction of commercial coils is responsible
+for the destruction of many an expensive coil.</p>
+
+<p>At the stage when the streaming discharge occurs, or with
+somewhat higher frequencies, one may, by approaching the terminals
+quite nearly, and regulating properly the effect of capacity,
+produce a veritable spray of small silver-white sparks, or a
+bunch of excessively thin silvery threads (Fig. 102) amidst a
+powerful brush&mdash;each spark or thread possibly corresponding<span class='pagenum'><a name="Page_159" id="Page_159">[Pg 159]</a></span>
+to one alternation. This, when produced under proper conditions,
+is probably the most beautiful discharge, and when an air
+blast is directed against it, it presents a singular appearance.
+The spray of sparks, when received through the body, causes
+some inconvenience, whereas, when the discharge simply
+streams, nothing at all is likely to be felt if large conducting
+objects are held in the hands to protect them from receiving
+small burns.</p>
+
+<p>If the frequency is still more increased, then the coil refuses
+to give any spark unless at comparatively small distances, and the
+fifth typical form of discharge may be observed (Fig. 103). The
+tendency to stream out and dissipate is then so great that when
+the brush is produced at one terminal no sparking occurs, even
+if, as I have repeatedly tried, the hand, or any conducting object,
+is held within the stream; and, what is more singular, the luminous
+stream is not at all easily deflected by the approach of a
+conducting body.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_173.jpg" width="800" height="351" alt="Fig. 103, 104." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 103.</td><td class="caption">Fig. 104.</td></tr>
+</table>
+</div>
+
+<p>At this stage the streams seemingly pass with the greatest
+freedom through considerable thicknesses of insulators, and it is
+particularly interesting to study their behavior. For this purpose
+it is convenient to connect to the terminals of the coil two
+metallic spheres which may be placed at any desired distance,
+Fig. 104. Spheres are preferable to plates, as the discharge can
+be better observed. By inserting dielectric bodies between the
+spheres, beautiful discharge phenomena may be observed. If
+the spheres be quite close and a spark be playing between them, by
+interposing a thin plate of ebonite between the spheres the spark
+instantly ceases and the discharge spreads into an intensely luminous
+circle several inches in diameter, provided the spheres are<span class='pagenum'><a name="Page_160" id="Page_160">[Pg 160]</a></span>
+sufficiently large. The passage of the streams heats, and, after
+a while, softens, the rubber so much that two plates may be
+made to stick together in this manner. If the spheres are so far
+apart that no spark occurs, even if they are far beyond the striking
+distance, by inserting a thick plate of glass the discharge is
+instantly induced to pass from the spheres to the glass in the
+form of luminous streams. It appears almost as though these
+streams pass <i>through</i> the dielectric. In reality this is not the
+case, as the streams are due to the molecules of the air which
+are violently agitated in the space between the oppositely charged
+surfaces of the spheres. When no dielectric other than air is
+present, the bombardment goes on, but is too weak to be visible;
+by inserting a dielectric the inductive effect is much increased,
+and besides, the projected air molecules find an obstacle and the
+bombardment becomes so intense that the streams become luminous.
+If by any mechanical means we could effect such a violent
+agitation of the molecules we could produce the same phenomenon.
+A jet of air escaping through a small hole under
+enormous pressure and striking against an insulating substance,
+such as glass, may be luminous in the dark, and it might be possible
+to produce a phosphorescence of the glass or other insulators
+in this manner.</p>
+
+<p>The greater the specific inductive capacity of the interposed
+dielectric, the more powerful the effect produced. Owing to
+this, the streams show themselves with excessively high potentials
+even if the glass be as much as one and one-half to two
+inches thick. But besides the heating due to bombardment,
+some heating goes on undoubtedly in the dielectric, being apparently
+greater in glass than in ebonite. I attribute this to the
+greater specific inductive capacity of the glass, in consequence of
+which, with the same potential difference, a greater amount of
+energy is taken up in it than in rubber. It is like connecting to
+a battery a copper and a brass wire of the same dimensions. The
+copper wire, though a more perfect conductor, would heat more
+by reason of its taking more current. Thus what is otherwise
+considered a virtue of the glass is here a defect. Glass usually
+gives way much quicker than ebonite; when it is heated to a certain
+degree, the discharge suddenly breaks through at one point,
+assuming then the ordinary form of an arc.</p>
+
+<p>The heating effect produced by molecular bombardment of
+the dielectric would, of course, diminish as the pressure of the<span class='pagenum'><a name="Page_161" id="Page_161">[Pg 161]</a></span>
+air is increased, and at enormous pressure it would be negligible,
+unless the frequency would increase correspondingly.</p>
+
+<p>It will be often observed in these experiments that when the
+spheres are beyond the striking distance, the approach of a glass
+plate, for instance, may induce the spark to jump between the
+spheres. This occurs when the capacity of the spheres is somewhat
+below the critical value which gives the greatest difference
+of potential at the terminals of the coil. By approaching a dielectric,
+the specific inductive capacity of the space between the
+spheres is increased, producing the same effect as if the capacity
+of the spheres were increased. The potential at the terminals
+may then rise so high that the air space is cracked. The experiment
+is best performed with dense glass or mica.</p>
+
+<p>Another interesting observation is that a plate of insulating
+material, when the discharge is passing through it, is strongly
+attracted by either of the spheres, that is by the nearer one, this
+being obviously due to the smaller mechanical effect of the bombardment
+on that side, and perhaps also to the greater electrification.</p>
+
+<p>From the behavior of the dielectrics in these experiments, we
+may conclude that the best insulator for these rapidly alternating
+currents would be the one possessing the smallest specific inductive
+capacity and at the same time one capable of withstanding
+the greatest differences of potential; and thus two diametrically
+opposite ways of securing the required insulation are indicated,
+namely, to use either a perfect vacuum or a gas under great pressure;
+but the former would be preferable. Unfortunately neither
+of these two ways is easily carried out in practice.</p>
+
+<p>It is especially interesting to note the behavior of an excessively
+high vacuum in these experiments. If a test tube, provided
+with external electrodes and exhausted to the highest possible
+degree, be connected to the terminals of the coil, Fig. 105, the
+electrodes of the tube are instantly brought to a high temperature
+and the glass at each end of the tube is rendered intensely phosphorescent,
+but the middle appears comparatively dark, and for a
+while remains cool.</p>
+
+<p>When the frequency is so high that the discharge shown in
+Fig. 103 is observed, considerable dissipation no doubt occurs in
+the coil. Nevertheless the coil may be worked for a long time,
+as the heating is gradual.</p>
+
+<p>In spite of the fact that the difference of potential may be<span class='pagenum'><a name="Page_162" id="Page_162">[Pg 162]</a></span>
+enormous, little is felt when the discharge is passed through the
+body, provided the hands are armed. This is to some extent due
+to the higher frequency, but principally to the fact that less energy
+is available externally, when the difference of potential
+reaches an enormous value, owing to the circumstance that, with
+the rise of potential, the energy absorbed in the coil increases as
+the square of the potential. Up to a certain point the energy
+available externally increases with the rise of potential, then it
+begins to fall off rapidly. Thus, with the ordinary high tension
+induction coil, the curious paradox exists, that, while with a given
+current through the primary the shock might be fatal, with many
+times that current it might be perfectly harmless, even if the
+frequency be the same. With high frequencies and excessively
+high potentials when the terminals are not connected to bodies
+of some size, practically all the energy supplied to the primary is
+taken up by the coil. There is no breaking through, no local injury,
+but all the material, insulating and conducting, is uniformly
+heated.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_176.jpg" width="800" height="336" alt="Fig. 105, 106." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 105.</td><td class="caption">Fig. 106.</td></tr>
+</table>
+</div>
+
+<p>To avoid misunderstanding in regard to the physiological
+effect of alternating currents of very high frequency, I think it
+necessary to state that, while it is an undeniable fact that they are
+incomparably less dangerous than currents of low frequencies,
+it should not be thought that they are altogether harmless.
+What has just been said refers only to currents from an ordinary
+high tension induction coil, which currents are necessarily very
+small; if received directly from a machine or from a secondary
+of low resistance, they produce more or less powerful effects, and
+may cause serious injury, especially when used in conjunction
+with condensers.<span class='pagenum'><a name="Page_163" id="Page_163">[Pg 163]</a></span></p>
+
+<p>The streaming discharge of a high tension induction coil differs
+in many respects from that of a powerful static machine. In
+color it has neither the violet of the positive, nor the brightness
+of the negative, static discharge, but lies somewhere between,
+being, of course, alternatively positive and negative. But since
+the streaming is more powerful when the point or terminal is
+electrified positively, than when electrified negatively, it follows
+that the point of the brush is more like the positive, and the root
+more like the negative, static discharge. In the dark, when the
+brush is very powerful, the root may appear almost white. The
+wind produced by the escaping streams, though it may be very
+strong&mdash;often indeed to such a degree that it may be felt quite a
+distance from the coil&mdash;is, nevertheless, considering the quantity
+of the discharge, smaller than that produced by the positive
+brush of a static machine, and it affects the flame much less
+powerfully. From the nature of the phenomenon we can conclude
+that the higher the frequency, the smaller must, of course,
+be the wind produced by the streams, and with sufficiently high
+frequencies no wind at all would be produced at the ordinary
+atmospheric pressures. With frequencies obtainable by means
+of a machine, the mechanical effect is sufficiently great to revolve,
+with considerable speed, large pin-wheels, which in the dark
+present a beautiful appearance owing to the abundance of the
+streams (Fig. 106).</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_177.jpg" width="800" height="375" alt="Fig. 107, 108." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 107.</td><td class="caption">Fig. 108.</td></tr>
+</table>
+</div>
+
+<p>In general, most of the experiments usually performed with a
+static machine can be performed with an induction coil when
+operated with very rapidly alternating currents. The effects produced,
+however, are much more striking, being of incomparably<span class='pagenum'><a name="Page_164" id="Page_164">[Pg 164]</a></span>
+greater power. When a small length of ordinary cotton covered
+wire, Fig. 107, is attached to one terminal of the coil, the streams
+issuing from all points of the wire may be so intense as to produce
+a considerable light effect. When the potentials and frequencies
+are very high, a wire insulated with gutta percha or rubber and
+attached to one of the terminals, appears to be covered with a
+luminous film. A very thin bare wire when attached to a terminal
+emits powerful streams and vibrates continually to and fro
+or spins in a circle, producing a singular effect (Fig. 108). Some
+of these experiments have been described by me in <i>The Electrical
+World</i>, of February 21, 1891.</p>
+
+<p>Another peculiarity of the rapidly alternating discharge of the
+induction coil is its radically different behavior with respect to
+points and rounded surfaces.</p>
+
+<p>If a thick wire, provided with a ball at one end and with a
+point at the other, be attached to the positive terminal of a static
+machine, practically all the charge will be lost through the point,
+on account of the enormously greater tension, dependent on the
+radius of curvature. But if such a wire is attached to one of the
+terminals of the induction coil, it will be observed that with very
+high frequencies streams issue from the ball almost as copiously
+as from the point (Fig. 109).</p>
+
+<p>It is hardly conceivable that we could produce such a condition
+to an equal degree in a static machine, for the simple reason,
+that the tension increases as the square of the density, which in
+turn is proportional to the radius of curvature; hence, with a
+steady potential an enormous charge would be required to make
+streams issue from a polished ball while it is connected with a
+point. But with an induction coil the discharge of which alternates
+with great rapidity it is different. Here we have to deal
+with two distinct tendencies. First, there is the tendency to
+escape which exists in a condition of rest, and which depends on
+the radius of curvature; second, there is the tendency to dissipate
+into the surrounding air by condenser action, which depends
+on the surface. When one of these tendencies is a maximum,
+the other is at a minimum. At the point the luminous
+stream is principally due to the air molecules coming bodily in
+contact with the point; they are attracted and repelled, charged
+and discharged, and, their atomic charges being thus disturbed,
+vibrate and emit light waves. At the ball, on the contrary, there
+is no doubt that the effect is to a great extent produced induc<span class='pagenum'><a name="Page_165" id="Page_165">[Pg 165]</a></span>tively,
+the air molecules not <i>necessarily</i> coming in contact with
+the ball, though they undoubtedly do so. To convince ourselves
+of this we only need to exalt the condenser action, for instance,
+by enveloping the ball, at some distance, by a better conductor
+than the surrounding medium, the conductor being, of course,
+insulated; or else by surrounding it with a better dielectric and
+approaching an insulated conductor; in both cases the streams
+will break forth more copiously. Also, the larger the ball with
+a given frequency, or the higher the frequency, the more will
+the ball have the advantage over the point. But, since a certain
+intensity of action is required to render the streams visible, it is
+obvious that in the experiment described the ball should not be
+taken too large.</p>
+
+<p>In consequence of this two-fold tendency, it is possible to produce
+by means of points, effects identical to those produced by
+capacity. Thus, for instance, by attaching to one terminal of
+the coil a small length of soiled wire, presenting many points
+and offering great facility to escape, the potential of the coil
+may be raised to the same value as by attaching to the terminal
+a polished ball of a surface many times greater than that of the
+wire.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_179.jpg" width="800" height="329" alt="Fig. 109, 110." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 109.</td><td class="caption">Fig. 110.</td></tr>
+</table>
+</div>
+
+<p>An interesting experiment, showing the effect of the points,
+may be performed in the following manner: Attach to one of
+the terminals of the coil a cotton covered wire about two feet in
+length, and adjust the conditions so that streams issue from the
+wire. In this experiment the primary coil should be preferably
+placed so that it extends only about half way into the secondary
+coil. Now touch the free terminal of the secondary with a conducting
+object held in the hand, or else connect it to an insulated<span class='pagenum'><a name="Page_166" id="Page_166">[Pg 166]</a></span>
+body of some size. In this manner the potential on the wire
+may be enormously raised. The effect of this will be either to
+increase, or to diminish, the streams. If they increase, the wire
+is too short; if they diminish, it is too long. By adjusting the
+length of the wire, a point is found where the touching of the
+other terminal does not at all affect the streams. In this case
+the rise of potential is exactly counteracted by the drop through
+the coil. It will be observed that small lengths of wire produce
+considerable difference in the magnitude and luminosity of the
+streams. The primary coil is placed sidewise for two reasons:
+First, to increase the potential at the wire; and, second, to increase
+the drop through the coil. The sensitiveness is thus augmented.</p>
+
+<p>There is still another and far more striking peculiarity of the
+brush discharge produced by very rapidly alternating currents.
+To observe this it is best to replace the usual terminals of the
+coil by two metal columns insulated with a good thickness of
+ebonite. It is also well to close all fissures and cracks with wax
+so that the brushes cannot form anywhere except at the tops of
+the columns. If the conditions are carefully adjusted&mdash;which,
+of course, must be left to the skill of the experimenter&mdash;so that
+the potential rises to an enormous value, one may produce two
+powerful brushes several inches long, nearly white at their roots,
+which in the dark bear a striking resemblance to two flames of
+a gas escaping under pressure (Fig. 110). But they do not only
+<i>resemble</i>, they <i>are</i> veritable flames, for they are hot. Certainly
+they are not as hot as a gas burner, <i>but they would be so if the
+frequency and the potential would be sufficiently high</i>. Produced
+with, say, twenty thousand alternations per second, the heat is
+easily perceptible even if the potential is not excessively high.
+The heat developed is, of course, due to the impact of the air
+molecules against the terminals and against each other. As, at
+the ordinary pressures, the mean free path is excessively small,
+it is possible that in spite of the enormous initial speed imparted
+to each molecule upon coming in contact with the terminal, its
+progress&mdash;by collision with other molecules&mdash;is retarded to such
+an extent, that it does not get away far from the terminal, but
+may strike the same many times in succession. The higher the
+frequency, the less the molecule is able to get away, and this the
+more so, as for a given effect the potential required is smaller;
+and a frequency is conceivable&mdash;perhaps even obtainable&mdash;at<span class='pagenum'><a name="Page_167" id="Page_167">[Pg 167]</a></span>
+which practically the same molecules would strike the terminal.
+Under such conditions the exchange of the molecules would be
+very slow, and the heat produced at, and very near, the terminal
+would be excessive. But if the frequency would go on increasing
+constantly, the heat produced would begin to diminish for obvious
+reasons. In the positive brush of a static machine the exchange
+of the molecules is very rapid, the stream is constantly
+of one direction, and there are fewer collisions; hence the heating
+effect must be very small. Anything that impairs the facility
+of exchange tends to increase the local heat produced. Thus, if
+a bulb be held over the terminal of the coil so as to enclose the
+brush, the air contained in the bulb is very quickly brought to
+a high temperature. If a glass tube be held over the brush so
+as to allow the draught to carry the brush upwards, scorching hot
+air escapes at the top of the tube. Anything held within the
+brush is, of course, rapidly heated, and the possibility of using
+such heating effects for some purpose or other suggests itself.</p>
+
+<p>When contemplating this singular phenomenon of the hot
+brush, we cannot help being convinced that a similar process
+must take place in the ordinary flame, and it seems strange that
+after all these centuries past of familiarity with the flame, now,
+in this era of electric lighting and heating, we are finally led to
+recognize, that since time immemorial we have, after all, always
+had "electric light and heat" at our disposal. It is also of no
+little interest to contemplate, that we have a possible way of
+producing&mdash;by other than chemical means&mdash;a veritable flame,
+which would give light and heat without any material being
+consumed, without any chemical process taking place, and to
+accomplish this, we only need to perfect methods of producing
+enormous frequencies and potentials. I have no doubt that if
+the potential could be made to alternate with sufficient rapidity
+and power, the brush formed at the end of a wire would lose its
+electrical characteristics and would become flamelike. The flame
+must be due to electrostatic molecular action.</p>
+
+<p>This phenomenon now explains in a manner which can hardly
+be doubted the frequent accidents occurring in storms. It is well
+known that objects are often set on fire without the lightning
+striking them. We shall presently see how this can happen.
+On a nail in a roof, for instance, or on a projection of any kind,
+more or less conducting, or rendered so by dampness, a powerful
+brush may appear. If the lightning strikes somewhere in the<span class='pagenum'><a name="Page_168" id="Page_168">[Pg 168]</a></span>
+neighborhood the enormous potential may be made to alternate
+or fluctuate perhaps many million times a second. The air
+molecules are violently attracted and repelled, and by their impact
+produce such a powerful heating effect that a fire is started.
+It is conceivable that a ship at sea may, in this manner, catch fire
+at many points at once. When we consider, that even with the
+comparatively low frequencies obtained from a dynamo machine,
+and with potentials of no more than one or two hundred thousand
+volts, the heating effects are considerable, we may imagine
+how much more powerful they must be with frequencies and potentials
+many times greater; and the above explanation seems, to
+say the least, very probable. Similar explanations may have been
+suggested, but I am not aware that, up to the present, the heating
+effects of a brush produced by a rapidly alternating potential
+have been experimentally demonstrated, at least not to such a
+remarkable degree.</p>
+
+<div class="figcenter" style="width: 568px;">
+<img src="images/oi_182.jpg" width="568" height="480" alt="Fig. 111." title="" />
+<span class="caption">Fig. 111.</span>
+</div>
+
+
+<p>By preventing completely the exchange of the air molecules,
+the local heating effect may be so exalted as to bring a body to
+incandescence. Thus, for instance, if a small button, or preferably
+a very thin wire or filament be enclosed in an unexhausted
+globe and connected with the terminal of the coil, it may be
+rendered incandescent. The phenomenon is made much more
+interesting by the rapid spinning round in a circle of the top of
+the filament, thus presenting the appearance of a luminous funnel,
+Fig. 111, which widens when the potential is increased.
+When the potential is small the end of the filament may perform
+irregular motions, suddenly changing from one to the other, or
+it may describe an ellipse; but when the potential is very
+high it always spins in a circle; and so does generally a thin<span class='pagenum'><a name="Page_169" id="Page_169">[Pg 169]</a></span>
+straight wire attached freely to the terminal of the coil. These
+motions are, of course, due to the impact of the molecules, and
+the irregularity in the distribution of the potential, owing to the
+roughness and dissymmetry of the wire or filament. With a
+perfectly symmetrical and polished wire such motions would
+probably not occur. That the motion is not likely to be due to
+others causes is evident from the fact that it is not of a definite
+direction, and that in a very highly exhausted globe it ceases
+altogether. The possibility of bringing a body to incandescence
+in an exhausted globe, or even when not at all enclosed, would
+seem to afford a possible way of obtaining light effects, which,
+in perfecting methods of producing rapidly alternating potentials,
+might be rendered available for useful purposes.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_183.jpg" width="800" height="346" alt="Fig. 112a." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 112a.</span>
+</div>
+
+
+<p>In employing a commercial coil, the production of very powerful
+brush effects is attended with considerable difficulties, for
+when these high frequencies and enormous potentials are used,
+the best insulation is apt to give way. Usually the coil is insulated
+well enough to stand the strain from convolution to convolution,
+since two double silk covered paraffined wires will withstand
+a pressure of several thousand volts; the difficulty lies
+principally in preventing the breaking through from the secondary
+to the primary, which is greatly facilitated by the streams
+issuing from the latter. In the coil, of course, the strain is greatest
+from section to section, but usually in a larger coil there are
+so many sections that the danger of a sudden giving way is not
+very great. No difficulty will generally be encountered in that
+direction, and besides, the liability of injuring the coil internally
+is very much reduced by the fact that the effect most likely to
+be produced is simply a gradual heating, which, when far enough<span class='pagenum'><a name="Page_170" id="Page_170">[Pg 170]</a></span>
+advanced, could not fail to be observed. The principal necessity
+is then to prevent the streams between the primary and the tube,
+not only on account of the heating and possible injury, but also
+because the streams may diminish very considerably the potential
+difference available at the terminals. A few hints as to how
+this may be accomplished will probably be found useful in most
+of these experiments with the ordinary induction coil.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_184.jpg" width="640" height="369" alt="Fig. 112b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 112b.</span>
+</div>
+
+<p>One of the ways is to wind a short primary, Fig. 112a, so that
+the difference of potential is not at that length great enough to
+cause the breaking forth of the streams through the insulating
+tube. The length of the primary should be determined by experiment.
+Both the ends of the coil should be brought out on one
+end through a plug of insulating material fitting in the tube as
+illustrated. In such a disposition one terminal of the secondary
+is attached to a body, the surface of which is determined with the
+greatest care so as to produce the greatest rise in the potential.
+At the other terminal a powerful brush appears, which may be
+experimented upon.</p>
+
+<p>The above plan necessitates the employment of a primary of
+comparatively small size, and it is apt to heat when powerful effects
+are desirable for a certain length of time. In such a case it
+is better to employ a larger coil, Fig. 112b, and introduce it
+from one side of the tube, until the streams begin to appear. In
+this case the nearest terminal of the secondary may be connected
+to the primary or to the ground, which is practically the same
+thing, if the primary is connected directly to the machine. In the
+case of ground connections it is well to determine experimentally
+the frequency which is best suited under the conditions of the
+test. Another way of obviating the streams, more or less, is to<span class='pagenum'><a name="Page_171" id="Page_171">[Pg 171]</a></span>
+make the primary in sections and supply it from separate, well
+insulated sources.</p>
+
+<p>In many of these experiments, when powerful effects are
+wanted for a short time, it is advantageous to use iron cores with
+the primaries. In such case a very large primary coil may be
+wound and placed side by side with the secondary, and, the nearest
+terminal of the latter being connected to the primary, a laminated
+iron core is introduced through the primary into the secondary
+as far as the streams will permit. Under these conditions
+an excessively powerful brush, several inches long, which may
+be appropriately called "St. Elmo's hot fire," may be caused to
+appear at the other terminal of the secondary, producing striking
+effects. It is a most powerful ozonizer, so powerful indeed, that
+only a few minutes are sufficient to fill the whole room with the
+smell of ozone, and it undoubtedly possesses the quality of exciting
+chemical affinities.</p>
+
+<p>For the production of ozone, alternating currents of very
+high frequency are eminently suited, not only on account of the
+advantages they offer in the way of conversion but also because
+of the fact, that the ozonizing action of a discharge is dependent
+on the frequency as well as on the potential, this being undoubtedly
+confirmed by observation.</p>
+
+<p>In these experiments if an iron core is used it should be carefully
+watched, as it is apt to get excessively hot in an incredibly
+short time. To give an idea of the rapidity of the heating, I
+will state, that by passing a powerful current through a coil with
+many turns, the inserting within the same of a thin iron wire for
+no more than one second's time is sufficient to heat the wire to
+something like 100&deg; C.</p>
+
+<p>But this rapid heating need not discourage us in the use
+of iron cores in connection with rapidly alternating currents.
+I have for a long time been convinced that in the industrial distribution
+by means of transformers, some such plan as the following
+might be practicable. We may use a comparatively small iron
+core, subdivided, or perhaps not even subdivided. We may surround
+this core with a considerable thickness of material which
+is fire-proof and conducts the heat poorly, and on top of that we
+may place the primary and secondary windings. By using either
+higher frequencies or greater magnetizing forces, we may by
+hysteresis and eddy currents heat the iron core so far as to bring
+it nearly to its maximum permeability, which, as Hopkinson has<span class='pagenum'><a name="Page_172" id="Page_172">[Pg 172]</a></span>
+shown, may be as much as sixteen times greater than that at ordinary
+temperatures. If the iron core were perfectly enclosed,
+it would not be deteriorated by the heat, and, if the enclosure of
+fire-proof material would be sufficiently thick, only a limited
+amount of energy could be radiated in spite of the high temperature.
+Transformers have been constructed by me on that
+plan, but for lack of time, no thorough tests have as yet been
+made.</p>
+
+<p>Another way of adapting the iron core to rapid alternations,
+or, generally speaking, reducing the frictional losses, is to produce
+by continuous magnetization a flow of something like seven
+thousand or eight thousand lines per square centimetre through
+the core, and then work with weak magnetizing forces and preferably
+high frequencies around the point of greatest permeability.
+A higher efficiency of conversion and greater output are
+obtainable in this manner. I have also employed this principle
+in connection with machines in which there is no reversal of
+polarity. In these types of machines, as long as there are only
+few pole projections, there is no great gain, as the maxima and
+minima of magnetization are far from the point of maximum
+permeability; but when the number of the pole projections is
+very great, the required rate of change may be obtained, without
+the magnetization varying so far as to depart greatly from the
+point of maximum permeability, and the gain is considerable.</p>
+
+<p>The above described arrangements refer only to the use of
+commercial coils as ordinarily constructed. If it is desired to
+construct a coil for the express purpose of performing with it
+such experiments as I have described, or, generally, rendering it
+capable of withstanding the greatest possible difference of potential,
+then a construction as indicated in Fig. 113 will be found of
+advantage. The coil in this case is formed of two independent
+parts which are wound oppositely, the connection between both
+being made near the primary. The potential in the middle being
+zero, there is not much tendency to jump to the primary and not
+much insulation is required. In some cases the middle point
+may, however, be connected to the primary or to the ground. In
+such a coil the places of greatest difference of potential are far
+apart and the coil is capable of withstanding an enormous strain.
+The two parts may be movable so as to allow a slight adjustment
+of the capacity effect.</p>
+
+<p>As to the manner of insulating the coil, it will be found con<span class='pagenum'><a name="Page_173" id="Page_173">[Pg 173]</a></span>venient
+to proceed in the following way: First, the wire should
+be boiled in paraffine until all the air is out; then the coil is
+wound by running the wire through melted paraffine, merely for
+the purpose of fixing the wire. The coil is then taken off from
+the spool, immersed in a cylindrical vessel filled with pure melted
+wax and boiled for a long time until the bubbles cease to appear.
+The whole is then left to cool down thoroughly, and then the
+mass is taken out of the vessel and turned up in a lathe. A coil
+made in this manner and with care is capable of withstanding
+enormous potential differences.</p>
+
+<div class="figcenter" style="width: 525px;">
+<img src="images/oi_187.jpg" width="525" height="480" alt="Fig. 113." title="" />
+<span class="caption">Fig. 113.</span>
+</div>
+
+
+<p>It may be found convenient to immerse the coil in paraffine oil
+or some other kind of oil; it is a most effective way of insulating,
+principally on account of the perfect exclusion of air, but it may
+be found that, after all, a vessel filled with oil is not a very convenient
+thing to handle in a laboratory.</p>
+
+<p>If an ordinary coil can be dismounted, the primary may be
+taken out of the tube and the latter plugged up at one end, filled
+with oil, and the primary reinserted. This affords an excellent
+insulation and prevents the formation of the streams.</p>
+
+<p>Of all the experiments which may be performed with rapidly
+alternating currents the most interesting are those which concern
+the production of a practical illuminant. It cannot be denied
+that the present methods, though they were brilliant advances,
+are very wasteful. Some better methods must be invented, some
+more perfect apparatus devised. Modern research has opened
+new possibilities for the production of an efficient source of light,
+and the attention of all has been turned in the direction indicated<span class='pagenum'><a name="Page_174" id="Page_174">[Pg 174]</a></span>
+by able pioneers. Many have been carried away by the enthusiasm
+and passion to discover, but in their zeal to reach results, some
+have been misled. Starting with the idea of producing electro-magnetic
+waves, they turned their attention, perhaps, too much
+to the study of electro-magnetic effects, and neglected the study
+of electrostatic phenomena. Naturally, nearly every investigator
+availed himself of an apparatus similar to that used in earlier
+experiments. But in those forms of apparatus, while the electro-magnetic
+inductive effects are enormous, the electrostatic effects
+are excessively small.</p>
+
+<p>In the Hertz experiments, for instance, a high tension induction
+coil is short circuited by an arc, the resistance of which is
+very small, the smaller, the more capacity is attached to the terminals;
+and the difference of potential at these is enormously
+diminished. On the other hand, when the discharge is not passing
+between the terminals, the static effects may be considerable,
+but only qualitatively so, not quantitatively, since their rise and
+fall is very sudden, and since their frequency is small. In neither
+case, therefore, are powerful electrostatic effects perceivable.
+Similar conditions exist when, as in some interesting experiments
+of Dr. Lodge, Leyden jars are discharged disruptively. It has
+been thought&mdash;and I believe asserted&mdash;that in such cases
+most of the energy is radiated into space. In the light of the
+experiments which I have described above, it will now not be
+thought so. I feel safe in asserting that in such cases most of
+the energy is partly taken up and converted into heat in the arc
+of the discharge and in the conducting and insulating material of
+the jar, some energy being, of course, given off by electrification
+of the air; but the amount of the directly radiated energy is very
+small.</p>
+
+<p>When a high tension induction coil, operated by currents alternating
+only 20,000 times a second, has its terminals closed through
+even a very small jar, practically all the energy passes through
+the dielectric of the jar, which is heated, and the electrostatic
+effects manifest themselves outwardly only to a very weak degree.
+Now the external circuit of a Leyden jar, that is, the arc and the
+connections of the coatings, may be looked upon as a circuit generating
+alternating currents of excessively high frequency and
+fairly high potential, which is closed through the coatings and
+the dielectric between them, and from the above it is evident
+that the external electrostatic effects must be very small, even if a<span class='pagenum'><a name="Page_175" id="Page_175">[Pg 175]</a></span>
+recoil circuit be used. These conditions make it appear that with
+the apparatus usually at hand, the observation of powerful electrostatic
+effects was impossible, and what experience has been
+gained in that direction is only due to the great ability of the
+investigators.</p>
+
+<p>But powerful electrostatic effects are a <i>sine qua non</i> of light
+production on the lines indicated by theory. Electro-magnetic
+effects are primarily unavailable, for the reason that to produce
+the required effects we would have to pass current impulses
+through a conductor, which, long before the required frequency
+of the impulses could be reached, would cease to transmit them.
+On the other hand, electro-magnetic waves many times longer
+than those of light, and producible by sudden discharge of a condenser,
+could not be utilized, it would seem, except we avail ourselves
+of their effect upon conductors as in the present methods,
+which are wasteful. We could not affect by means of such waves
+the static molecular or atomic charges of a gas, cause them to vibrate
+and to emit light. Long transverse waves cannot, apparently,
+produce such effects, since excessively small electro-magnetic
+disturbances may pass readily through miles of air. Such dark
+waves, unless they are of the length of true light waves, cannot,
+it would seem, excite luminous radiation in a Geissler tube, and
+the luminous effects, which are producible by induction in a tube
+devoid of electrodes, I am inclined to consider as being of an electrostatic
+nature.</p>
+
+<p>To produce such luminous effects, straight electrostatic thrusts
+are required; these, whatever be their frequency, may disturb
+the molecular charges and produce light. Since current impulses
+of the required frequency cannot pass through a conductor of
+measurable dimensions, we must work with a gas, and then the
+production of powerful electrostatic effects becomes an imperative
+necessity.</p>
+
+<p>It has occurred to me, however, that electrostatic effects are in
+many ways available for the production of light. For instance,
+we may place a body of some refractory material in a closed, and
+preferably more or less exhausted, globe, connect it to a source of
+high, rapidly alternating potential, causing the molecules of the
+gas to strike it many times a second at enormous speeds, and in
+this manner, with trillions of invisible hammers, pound it until it
+gets incandescent; or we may place a body in a very highly exhausted
+globe, in a non-striking vacuum, and, by employing very<span class='pagenum'><a name="Page_176" id="Page_176">[Pg 176]</a></span>
+high frequencies and potentials, transfer sufficient energy from it
+to other bodies in the vicinity, or in general to the surroundings,
+to maintain it at any degree of incandescence; or we may, by
+means of such rapidly alternating high potentials, disturb the
+ether carried by the molecules of a gas or their static charges,
+causing them to vibrate and to emit light.</p>
+
+<p>But, electrostatic effects being dependent upon the potential
+and frequency, to produce the most powerful action it is desirable
+to increase both as far as practicable. It may be possible to
+obtain quite fair results by keeping either of these factors small,
+provided the other is sufficiently great; but we are limited in
+both directions. My experience demonstrates that we cannot go
+below a certain frequency, for, first, the potential then becomes
+so great that it is dangerous; and, secondly, the light production
+is less efficient.</p>
+
+<p>I have found that, by using the ordinary low frequencies, the
+physiological effect of the current required to maintain at a certain
+degree of brightness a tube four feet long, provided at the
+ends with outside and inside condenser coatings, is so powerful
+that, I think, it might produce serious injury to those not accustomed
+to such shocks; whereas, with twenty thousand alternations
+per second, the tube may be maintained at the same degree
+of brightness without any effect being felt. This is due principally
+to the fact that a much smaller potential is required to produce
+the same light effect, and also to the higher efficiency in the
+light production. It is evident that the efficiency in such cases
+is the greater, the higher the frequency, for the quicker the process
+of charging and discharging the molecules, the less energy
+will be lost in the form of dark radiation. But, unfortunately,
+we cannot go beyond a certain frequency on account of the difficulty
+of producing and conveying the effects.</p>
+
+<p>I have stated above that a body inclosed in an unexhausted
+bulb may be intensely heated by simply connecting it with a
+source of rapidly alternating potential. The heating in such a
+case is, in all probability, due mostly to the bombardment of the
+molecules of the gas contained in the bulb. When the bulb is
+exhausted, the heating of the body is much more rapid, and there
+is no difficulty whatever in bringing a wire or filament to any
+degree of incandescence by simply connecting it to one terminal
+of a coil of the proper dimensions. Thus, if the well-known apparatus
+of Prof. Crookes, consisting of a bent platinum wire with<span class='pagenum'><a name="Page_177" id="Page_177">[Pg 177]</a></span>
+vanes mounted over it (Fig. 114), be connected to one terminal of
+the coil&mdash;either one or both ends of the platinum wire being connected&mdash;the
+wire is rendered almost instantly incandescent, and
+the mica vanes are rotated as though a current from a battery
+were used. A thin carbon filament, or, preferably, a button of
+some refractory material (Fig. 115), even if it be a comparatively
+poor conductor, inclosed in an exhausted globe, may be rendered
+highly incandescent; and in this manner a simple lamp capable
+of giving any desired candle power is provided.</p>
+
+<p>The success of lamps of this kind would depend largely on the
+selection of the light-giving bodies contained within the bulb.
+Since, under the conditions described, refractory bodies&mdash;which
+are very poor conductors and capable of withstanding for a long
+time excessively high degrees of temperature&mdash;may be used,
+such illuminating devices may be rendered successful.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_191.jpg" width="800" height="321" alt="Fig. 114, 115." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 114.</td><td class="caption">Fig. 115.</td></tr>
+</table>
+</div>
+
+
+<p>It might be thought at first that if the bulb, containing the
+filament or button of refractory material, be perfectly well exhausted&mdash;that
+is, as far as it can be done by the use of the best
+apparatus&mdash;the heating would be much less intense, and that in
+a perfect vacuum it could not occur at all. This is not confirmed
+by my experience; quite the contrary, the better the vacuum
+the more easily the bodies are brought to incandescence. This
+result is interesting for many reasons.</p>
+
+<p>At the outset of this work the idea presented itself to me,
+whether two bodies of refractory material enclosed in a bulb exhausted
+to such a degree that the discharge of a large induction
+coil, operated in the usual manner, cannot pass through, could be
+rendered incandescent by mere condenser action. Obviously, to
+reach this result enormous potential differences and very high
+frequencies are required, as is evident from a simple calculation.<span class='pagenum'><a name="Page_178" id="Page_178">[Pg 178]</a></span></p>
+
+<p>But such a lamp would possess a vast advantage over an ordinary
+incandescent lamp in regard to efficiency. It is well-known
+that the efficiency of a lamp is to some extent a function of the
+degree of incandescence, and that, could we but work a filament
+at many times higher degrees of incandescence, the efficiency
+would be much greater. In an ordinary lamp this is impracticable
+on account of the destruction of the filament, and it has been
+determined by experience how far it is advisable to push the incandescence.
+It is impossible to tell how much higher efficiency
+could be obtained if the filament could withstand indefinitely,
+as the investigation to this end obviously cannot be carried beyond
+a certain stage; but there are reasons for believing that it
+would be very considerably higher. An improvement might be
+made in the ordinary lamp by employing a short and thick carbon;
+but then the leading-in wires would have to be thick, and,
+besides, there are many other considerations which render such a
+modification entirely impracticable. But in a lamp as above described,
+the leading in wires may be very small, the incandescent
+refractory material may be in the shape of blocks offering a very
+small radiating surface, so that less energy would be required to
+keep them at the desired incandescence; and in addition to this,
+the refractory material need not be carbon, but may be manufactured
+from mixtures of oxides, for instance, with carbon or other
+material, or may be selected from bodies which are practically
+non-conductors, and capable of withstanding enormous degrees of
+temperature.</p>
+
+<p>All this would point to the possibility of obtaining a much
+higher efficiency with such a lamp than is obtainable in ordinary
+lamps. In my experience it has been demonstrated that the
+blocks are brought to high degrees of incandescence with much
+lower potentials than those determined by calculation, and the
+blocks may be set at greater distances from each other. We may
+freely assume, and it is probable, that the molecular bombardment
+is an important element in the heating, even if the globe
+be exhausted with the utmost care, as I have done; for although
+the number of the molecules is, comparatively speaking, insignificant,
+yet on account of the mean free path being very great,
+there are fewer collisions, and the molecules may reach much
+higher speeds, so that the heating effect due to this cause may
+be considerable, as in the Crookes experiments with radiant
+matter.<span class='pagenum'><a name="Page_179" id="Page_179">[Pg 179]</a></span></p>
+
+<p>But it is likewise possible that we have to deal here with an
+increased facility of losing the charge in very high vacuum, when
+the potential is rapidly alternating, in which case most of the
+heating would be directly due to the surging of the charges in
+the heated bodies. Or else the observed fact may be largely
+attributable to the effect of the points which I have mentioned
+above, in consequence of which the blocks or filaments contained
+in the vacuum are equivalent to condensers of many times
+greater surface than that calculated from their geometrical dimensions.
+Scientific men still differ in opinion as to whether a
+charge should, or should not, be lost in a perfect vacuum, or in
+other words, whether ether is, or is not, a conductor. If the
+former were the case, then a thin filament enclosed in a perfectly
+exhausted globe, and connected to a source of enormous, steady
+potential, would be brought to incandescence.</p>
+
+<div class="figcenter" style="width: 619px;">
+<img src="images/oi_193.jpg" width="619" height="480" alt="Fig. 116, 117." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 116.</td><td class="caption">Fig. 117.</td></tr>
+</table>
+</div>
+
+<p>Various forms of lamps on the above described principle, with
+the refractory bodies in the form of filaments, Fig. 116, or blocks,
+Fig. 117, have been constructed and operated by me, and investigations
+are being carried on in this line. There is no difficulty in
+reaching such high degrees of incandescence that ordinary carbon
+is to all appearance melted and volatilized. If the vacuum
+could be made absolutely perfect, such a lamp, although inoperative
+with apparatus ordinarily used, would, if operated with cur<span class='pagenum'><a name="Page_180" id="Page_180">[Pg 180]</a></span>rents
+of the required character, afford an illuminant which would
+never be destroyed, and which would be far more efficient than
+an ordinary incandescent lamp. This perfection can, of course,
+never be reached, and a very slow destruction and gradual diminution
+in size always occurs, as in incandescent filaments; but there
+is no possibility of a sudden and premature disabling which occurs
+in the latter by the breaking of the filament, especially
+when the incandescent bodies are in the shape of blocks.</p>
+
+<p>With these rapidly alternating potentials there is, however, no
+necessity of enclosing two blocks in a globe, but a single block,
+as in Fig. 115, or filament, Fig. 118, may be used. The potential
+in this case must of course be higher, but is easily obtainable,
+and besides it is not necessarily dangerous.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_194.jpg" width="336" height="378" alt="Fig. 118." title="" />
+<span class="caption">Fig. 118.</span>
+</div>
+
+
+<p>The facility with which the button or filament in such a lamp
+is brought to incandescence, other things being equal, depends
+on the size of the globe. If a perfect vacuum could be obtained,
+the size of the globe would not be of importance, for then the
+heating would be wholly due to the surging of the charges, and
+all the energy would be given off to the surroundings by radiation.
+But this can never occur in practice. There is always
+some gas left in the globe, and although the exhaustion may be
+carried to the highest degree, still the space inside of the bulb
+must be considered as conducting when such high potentials are
+used, and I assume that, in estimating the energy that may be
+given off from the filament to the surroundings, we may consider<span class='pagenum'><a name="Page_181" id="Page_181">[Pg 181]</a></span>
+the inside surface of the bulb as one coating of a condenser, the
+air and other objects surrounding the bulb forming the other
+coating. When the alternations are very low there is no doubt
+that a considerable portion of the energy is given off by the electrification
+of the surrounding air.</p>
+
+<p>In order to study this subject better, I carried on some experiments
+with excessively high potentials and low frequencies. I
+then observed that when the hand is approached to the bulb,&mdash;the
+filament being connected with one terminal of the coil,&mdash;a
+powerful vibration is felt, being due to the attraction and repulsion
+of the molecules of the air which are electrified by induction
+through the glass. In some cases when the action is very
+intense I have been able to hear a sound, which must be due to
+the same cause.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_195.jpg" width="800" height="375" alt="Fig. 119, 120." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 119.</td><td class="caption">Fig. 120.</td></tr>
+</table>
+</div>
+
+<p>When the alternations are low, one is apt to get an excessively
+powerful shock from the bulb. In general, when one attaches
+bulbs or objects of some size to the terminals of the coil, one
+should look out for the rise of potential, for it may happen that
+by merely connecting a bulb or plate to the terminal, the potential
+may rise to many times its original value. When lamps are
+attached to the terminals, as illustrated in Fig. 119, then the
+capacity of the bulbs should be such as to give the maximum
+rise of potential under the existing conditions. In this manner
+one may obtain the required potential with fewer turns of
+wire.</p>
+
+<p>The life of such lamps as described above depends, of course,
+largely on the degree of exhaustion, but to some extent also on
+the shape of the block of refractory material. Theoretically it<span class='pagenum'><a name="Page_182" id="Page_182">[Pg 182]</a></span>
+would seem that a small sphere of carbon enclosed in a sphere of
+glass would not suffer deterioration from molecular bombardment,
+for, the matter in the globe being radiant, the molecules
+would move in straight lines, and would seldom strike the sphere
+obliquely. An interesting thought in connection with such a
+lamp is, that in it "electricity" and electrical energy apparently
+must move in the same lines.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_196.jpg" width="480" height="536" alt="Fig. 121a, 121b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 121a.</td><td class="caption1"><span class="smcap">Fig.</span> 121b.</td></tr>
+</table>
+</div>
+
+<p>The use of alternating currents of very high frequency makes
+it possible to transfer, by electrostatic or electromagnetic induction
+through the glass of a lamp, sufficient energy to keep a filament
+at incandescence and so do away with the leading-in wires.
+Such lamps have been proposed, but for want of proper apparatus
+they have not been successfully operated. Many forms of
+lamps on this principle with continuous and broken filaments
+have been constructed by me and experimented upon. When
+using a secondary enclosed within the lamp, a condenser is advantageously
+combined with the secondary. When the transference
+is effected by electrostatic induction, the potentials used are,
+of course, very high with frequencies obtainable from a machine.
+For instance, with a condenser surface of forty square centimetres,<span class='pagenum'><a name="Page_183" id="Page_183">[Pg 183]</a></span>
+which is not impracticably large, and with glass of good quality
+1 mm. thick, using currents alternating twenty thousand times
+a second, the potential required is approximately 9,000 volts.
+This may seem large, but since each lamp may be included
+in the secondary of a transformer of very small dimensions, it
+would not be inconvenient, and, moreover, it would not produce
+fatal injury. The transformers would all be preferably in series.
+The regulation would offer no difficulties, as with currents of such
+frequencies it is very easy to maintain a constant current.</p>
+
+<p>In the accompanying engravings some of the types of lamps of
+this kind are shown. Fig. 120 is such a lamp with a broken filament,
+and Figs. 121 <small>A</small> and 121 <small>B</small> one with a single outside and
+inside coating and a single filament. I have also made lamps
+with two outside and inside coatings and a continuous loop connecting
+the latter. Such lamps have been operated by me with
+current impulses of the enormous frequencies obtainable by the
+disruptive discharge of condensers.</p>
+
+<p>The disruptive discharge of a condenser is especially suited for
+operating such lamps&mdash;with no outward electrical connections&mdash;by
+means of electromagnetic induction, the electromagnetic inductive
+effects being excessively high; and I have been able to
+produce the desired incandescence with only a few short turns of
+wire. Incandescence may also be produced in this manner in a
+simple closed filament.</p>
+
+<p>Leaving now out of consideration the practicability of such
+lamps, I would only say that they possess a beautiful and desirable
+feature, namely, that they can be rendered, at will, more or
+less brilliant simply by altering the relative position of the outside
+and inside condenser coatings, or inducing and induced circuits.</p>
+
+<p>When a lamp is lighted by connecting it to one terminal only
+of the source, this may be facilitated by providing the globe with
+an outside condenser coating, which serves at the same time as a
+reflector, and connecting this to an insulated body of some size.
+Lamps of this kind are illustrated in Fig. 122 and Fig. 123.
+Fig. 124 shows the plan of connection. The brilliancy of the
+lamp may, in this case, be regulated within wide limits by varying
+the size of the insulated metal plate to which the coating is
+connected.</p>
+
+<p>It is likewise practicable to light with one leading wire lamps
+such as illustrated in Fig. 116 and Fig. 117, by connecting one<span class='pagenum'><a name="Page_184" id="Page_184">[Pg 184]</a></span>
+terminal of the lamp to one terminal of the source, and the
+other to an insulated body of the required size. In all cases
+the insulated body serves to give off the energy into the surrounding
+space, and is equivalent to a return wire. Obviously,
+in the two last-named cases, instead of connecting the wires to
+an insulated body, connections may be made to the ground.</p>
+
+<p>The experiments which will prove most suggestive and of
+most interest to the investigator are probably those performed
+with exhausted tubes. As might be anticipated, a source of such
+rapidly alternating potentials is capable of exciting the tubes at
+a considerable distance, and the light effects produced are remarkable.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_198.jpg" width="800" height="503" alt="Fig. 122, 123." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 122.</td><td class="caption">Fig. 123.</td></tr>
+</table>
+</div>
+
+<p>During my investigations in this line I endeavored to excite
+tubes, devoid of any electrodes, by electromagnetic induction,
+making the tube the secondary of the induction device, and
+passing through the primary the discharges of a Leyden jar.
+These tubes were made of many shapes, and I was able to
+obtain luminous effects which I then thought were due wholly
+to electromagnetic induction. But on carefully investigating
+the phenomena I found that the effects produced were more
+of an electrostatic nature. It may be attributed to this circumstance
+that this mode of exciting tubes is very wasteful,
+namely, the primary circuit being closed, the potential, and
+consequently the electrostatic inductive effect, is much diminished.<span class='pagenum'><a name="Page_185" id="Page_185">[Pg 185]</a></span></p>
+
+<p>When an induction coil, operated as above described, is used,
+there is no doubt that the tubes are excited by electrostatic induction,
+and that electromagnetic induction has little, if anything,
+to do with the phenomena.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_199.jpg" width="800" height="293" alt="Fig. 124." title="" />
+<span class="caption">Fig. 124.</span>
+</div>
+
+
+<p>This is evident from many experiments. For instance, if a
+tube be taken in one hand, the observer being near the coil, it is
+brilliantly lighted and remains so no matter in what position it is
+held relatively to the observer's body. Were the action electromagnetic,
+the tube could not be lighted when the observer's
+body is interposed between it and the coil, or at least its luminosity
+should be considerably diminished. When the tube is
+held exactly over the centre of the coil&mdash;the latter being wound
+in sections and the primary placed symmetrically to the secondary&mdash;it
+may remain completely dark, whereas it is rendered
+intensely luminous by moving it slightly to the right or left
+from the centre of the coil. It does not light because in the
+middle both halves of the coil neutralize each other, and the
+electric potential is zero. If the action were electromagnetic,
+the tube should light best in the plane through the centre of the
+coil, since the electromagnetic effect there should be a maximum.
+When an arc is established between the terminals, the tubes and
+lamps in the vicinity of the coil go out, but light up again
+when the arc is broken, on account of the rise of potential. Yet
+the electromagnetic effect should be practically the same in both
+cases.</p>
+
+<p>By placing a tube at some distance from the coil, and nearer to
+one terminal&mdash;preferably at a point on the axis of the coil&mdash;one
+may light it by touching the remote terminal with an insulated
+body of some size or with the hand, thereby raising the potential
+at that terminal nearer to the tube. If the tube is shifted nearer
+to the coil so that it is lighted by the action of the nearer termi<span class='pagenum'><a name="Page_186" id="Page_186">[Pg 186]</a></span>nal,
+it may be made to go out by holding, on an insulated support,
+the end of a wire connected to the remote terminal, in the
+vicinity of the nearer terminal, by this means counteracting the
+action of the latter upon the tube. These effects are evidently
+electrostatic. Likewise, when a tube is placed at a considerable
+distance from the coil, the observer may, standing upon an insulated
+support between coil and tube, light the latter by approaching
+the hand to it; or he may even render it luminous by simply
+stepping between it and the coil. This would be impossible with
+electro-magnetic induction, for the body of the observer would
+act as a screen.</p>
+
+<p>When the coil is energized by excessively weak currents, the
+experimenter may, by touching one terminal of the coil with the
+tube, extinguish the latter, and may again light it by bringing it
+out of contact with the terminal and allowing a small arc to form.
+This is clearly due to the respective lowering and raising of the
+potential at that terminal. In the above experiment, when the
+tube is lighted through a small arc, it may go out when the arc is
+broken, because the electrostatic inductive effect alone is too
+weak, though the potential may be much higher; but when the
+arc is established, the electrification of the end of the tube is
+much greater, and it consequently lights.</p>
+
+<p>If a tube is lighted by holding it near to the coil, and in the
+hand which is remote, by grasping the tube anywhere with the
+other hand, the part between the hands is rendered dark, and the
+singular effect of wiping out the light of the tube may be produced
+by passing the hand quickly along the tube and at the
+same time withdrawing it gently from the coil, judging properly
+the distance so that the tube remains dark afterwards.</p>
+
+<p>If the primary coil is placed sidewise, as in Fig. 112 <small>B</small> for instance,
+and an exhausted tube be introduced from the other side
+in the hollow space, the tube is lighted most intensely because of
+the increased condenser action, and in this position the stri&aelig; are
+most sharply defined. In all these experiments described, and in
+many others, the action is clearly electrostatic.</p>
+
+<p>The effects of screening also indicate the electrostatic nature
+of the phenomena and show something of the nature of electrification
+through the air. For instance, if a tube is placed in the
+direction of the axis of the coil, and an insulated metal plate be
+interposed, the tube will generally increase in brilliancy, or if it
+be too far from the coil to light, it may even be rendered lumin<span class='pagenum'><a name="Page_187" id="Page_187">[Pg 187]</a></span>ous
+by interposing an insulated metal plate. The magnitude of
+the effects depends to some extent on the size of the plate. But if
+the metal plate be connected by a wire to the ground, its interposition
+will always make the tube go out even if it be very near the
+coil. In general, the interposition of a body between the coil and
+tube, increases or diminishes the brilliancy of the tube, or its
+facility to light up, according to whether it increases or diminishes
+the electrification. When experimenting with an insulated
+plate, the plate should not be taken too large, else it will generally
+produce a weakening effect by reason of its great facility for giving
+off energy to the surroundings.</p>
+
+<p>If a tube be lighted at some distance from the coil, and a plate
+of hard rubber or other insulating substance be interposed, the
+tube may be made to go out. The interposition of the dielectric
+in this case only slightly increases the inductive effect, but diminishes
+considerably the electrification through the air.</p>
+
+<p>In all cases, then, when we excite luminosity in exhausted
+tubes by means of such a coil, the effect is due to the rapidly
+alternating electrostatic potential; and, furthermore, it must be
+attributed to the harmonic alternation produced directly by the
+machine, and not to any superimposed vibration which might be
+thought to exist. Such superimposed vibrations are impossible
+when we work with an alternate current machine. If a spring be
+gradually tightened and released, it does not perform independent
+vibrations; for this a sudden release is necessary. So with
+the alternate currents from a dynamo machine; the medium is
+harmonically strained and released, this giving rise to only one
+kind of waves; a sudden contact or break, or a sudden giving
+way of the dielectric, as in the disruptive discharge of a Leyden
+jar, are essential for the production of superimposed waves.</p>
+
+<p>In all the last described experiments, tubes devoid of any electrodes
+may be used, and there is no difficulty in producing by
+their means sufficient light to read by. The light effect is, however,
+considerably increased by the use of phosphorescent bodies
+such as yttria, uranium glass, etc. A difficulty will be found
+when the phosphorescent material is used, for with these powerful
+effects, it is carried gradually away, and it is preferable to use
+material in the form of a solid.</p>
+
+<p>Instead of depending on induction at a distance to light the
+tube, the same may be provided with an external&mdash;and, if desired,
+also with an internal&mdash;condenser coating, and it may then<span class='pagenum'><a name="Page_188" id="Page_188">[Pg 188]</a></span>
+be suspended anywhere in the room from a conductor connected
+to one terminal of the coil, and in this manner a soft illumination
+may be provided.</p>
+
+<div class="figcenter" style="width: 286px;">
+<img src="images/oi_202.jpg" width="286" height="640" alt="Fig. 125." title="" />
+<span class="caption">Fig. 125.</span>
+</div>
+
+
+<p>The ideal way of lighting a hall or room would, however, be
+to produce such a condition in it that an illuminating device
+could be moved and put anywhere, and that it is lighted, no matter
+where it is put and without being electrically connected to<span class='pagenum'><a name="Page_189" id="Page_189">[Pg 189]</a></span>
+anything. I have been able to produce such a condition by creating
+in the room a powerful, rapidly alternating electrostatic
+field. For this purpose I suspend a sheet of metal a distance
+from the ceiling on insulating cords and connect it to one terminal
+of the induction coil, the other terminal being preferably connected
+to the ground. Or else I suspend two sheets as illustrated
+in Fig. 125, each sheet being connected with one of the terminals
+of the coil, and their size being carefully determined. An exhausted
+tube may then be carried in the hand anywhere between
+the sheets or placed anywhere, even a certain distance
+beyond them; it remains always luminous.</p>
+
+<p>In such an electrostatic field interesting phenomena may be
+observed, especially if the alternations are kept low and the potentials
+excessively high. In addition to the luminous phenomena
+mentioned, one may observe that any insulated conductor gives
+sparks when the hand or another object is approached to it, and
+the sparks may often be powerful. When a large conducting
+object is fastened on an insulating support, and the hand approached
+to it, a vibration, due to the rythmical motion of the
+air molecules is felt, and luminous streams may be perceived
+when the hand is held near a pointed projection. When a telephone
+receiver is made to touch with one or both of its terminals
+an insulated conductor of some size, the telephone emits a loud
+sound; it also emits a sound when a length of wire is attached to
+one or both terminals, and with very powerful fields a sound may
+be perceived even without any wire.</p>
+
+<p>How far this principle is capable of practical application, the
+future will tell. It might be thought that electrostatic effects
+are unsuited for such action at a distance. Electromagnetic inductive
+effects, if available for the production of light, might be
+thought better suited. It is true the electrostatic effects diminish
+nearly with the cube of the distance from the coil, whereas
+the electromagnetic inductive effects diminish simply with the
+distance. But when we establish an electrostatic field of force,
+the condition is very different, for then, instead of the differential
+effect of both the terminals, we get their conjoint effect.
+Besides, I would call attention to the effect, that in an alternating
+electrostatic field, a conductor, such as an exhausted tube,
+for instance, tends to take up most of the energy, whereas in an
+electromagnetic alternating field the conductor tends to take up
+the least energy, the waves being reflected with but little loss.<span class='pagenum'><a name="Page_190" id="Page_190">[Pg 190]</a></span>
+This is one reason why it is difficult to excite an exhausted tube,
+at a distance, by electromagnetic induction. I have wound coils
+of very large diameter and of many turns of wire, and connected
+a Geissler tube to the ends of the coil with the object of exciting
+the tube at a distance; but even with the powerful inductive
+effects producible by Leyden jar discharges, the tube could not
+be excited unless at a very small distance, although some judgment
+was used as to the dimensions of the coil. I have also
+found that even the most powerful Leyden jar discharges are
+capable of exciting only feeble luminous effects in a closed exhausted
+tube, and even these effects upon thorough examination
+I have been forced to consider of an electrostatic nature.</p>
+
+<p>How then can we hope to produce the required effects at a
+distance by means of electromagnetic action, when even in the
+closest proximity to the source of disturbance, under the most
+advantageous conditions, we can excite but faint luminosity? It
+is true that when acting at a distance we have the resonance to
+help us out. We can connect an exhausted tube, or whatever
+the illuminating device may be, with an insulated system of the
+proper capacity, and so it may be possible to increase the effect
+qualitatively, and only qualitatively, for we would not get <i>more</i>
+energy through the device. So we may, by resonance effect,
+obtain the required electromotive force in an exhausted tube, and
+excite faint luminous effects, but we cannot get enough energy to
+render the light practically available, and a simple calculation,
+based on experimental results, shows that even if all the energy
+which a tube would receive at a certain distance from the source
+should be wholly converted into light, it would hardly satisfy the
+practical requirements. Hence the necessity of directing, by
+means of a conducting circuit, the energy to the place of transformation.
+But in so doing we cannot very sensibly depart from
+present methods, and all we could do would be to improve the
+apparatus.</p>
+
+<p>From these considerations it would seem that if this ideal way
+of lighting is to be rendered practicable it will be only by the use
+of electrostatic effects. In such a case the most powerful electrostatic
+inductive effects are needed; the apparatus employed must,
+therefore, be capable of producing high electrostatic potentials
+changing in value with extreme rapidity. High frequencies are
+especially wanted, for practical considerations make it desirable
+to keep down the potential. By the employment of machines,<span class='pagenum'><a name="Page_191" id="Page_191">[Pg 191]</a></span>
+or, generally speaking, of any mechanical apparatus, but low
+frequencies can be reached; recourse must, therefore, be had to
+some other means. The discharge of a condenser affords us a
+means of obtaining frequencies by far higher than are obtainable
+mechanically, and I have accordingly employed condensers in the
+experiments to the above end.</p>
+
+<p>When the terminals of a high tension induction coil, Fig. 126,
+are connected to a Leyden jar, and the latter is discharging disruptively
+into a circuit, we may look upon the arc playing between
+the knobs as being a source of alternating, or generally
+speaking, undulating currents, and then we have to deal with
+the familiar system of a generator of such currents, a circuit connected
+to it, and a condenser bridging the circuit. The condenser
+in such case is a veritable transformer, and since the frequency is
+excessive, almost any ratio in the strength of the currents in both
+the branches may be obtained. In reality the analogy is not quite
+complete, for in the disruptive discharge we have most generally
+a fundamental instantaneous variation of comparatively low frequency,
+and a superimposed harmonic vibration, and the laws
+governing the flow of currents are not the same for both.</p>
+
+<p>In converting in this manner, the ratio of conversion should
+not be too great, for the loss in the arc between the knobs increases
+with the square of the current, and if the jar be discharged
+through very thick and short conductors, with the view of obtaining
+a very rapid oscillation, a very considerable portion of the
+energy stored is lost. On the other hand, too small ratios are not
+practicable for many obvious reasons.</p>
+
+<p>As the converted currents flow in a practically closed circuit,
+the electrostatic effects are necessarily small, and I therefore convert
+them into currents or effects of the required character. I
+have effected such conversions in several ways. The preferred
+plan of connections is illustrated in Fig. 127. The manner of operating
+renders it easy to obtain by means of a small and inexpensive
+apparatus enormous differences of potential which have been
+usually obtained by means of large and expensive coils. For this
+it is only necessary to take an ordinary small coil, adjust to it a
+condenser and discharging circuit, forming the primary of an
+auxiliary small coil, and convert upward. As the inductive effect
+of the primary currents is excessively great, the second coil need
+have comparatively but very few turns. By properly adjusting
+the elements, remarkable results may be secured.<span class='pagenum'><a name="Page_192" id="Page_192">[Pg 192]</a></span></p>
+
+<p>In endeavoring to obtain the required electrostatic effects in
+this manner, I have, as might be expected, encountered many
+difficulties which I have been gradually overcoming, but I am not
+as yet prepared to dwell upon my experiences in this direction.</p>
+
+<p>I believe that the disruptive discharge of a condenser will play
+an important part in the future, for it offers vast possibilities,
+not only in the way of producing light in a more efficient manner
+and in the line indicated by theory, but also in many other respects.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_206.jpg" width="640" height="230" alt="Fig. 126." title="" />
+<span class="caption">Fig. 126.</span>
+</div>
+
+<p>For years the efforts of inventors have been directed towards
+obtaining electrical energy from heat by means of the thermopile.
+It might seem invidious to remark that but few know
+what is the real trouble with the thermopile. It is not the inefficiency
+or small output&mdash;though these are great drawbacks&mdash;but
+the fact that the thermopile has its phylloxera, that is, that
+by constant use it is deteriorated, which has thus far prevented its
+introduction on an industrial scale. Now that all modern research
+seems to point with certainty to the use of electricity of excessively
+high tension, the question must present itself to many
+whether it is not possible to obtain in a practicable manner this
+form of energy from heat. We have been used to look upon
+an electrostatic machine as a plaything, and somehow we couple
+with it the idea of the inefficient and impractical. But now we
+must think differently, for now we know that everywhere we
+have to deal with the same forces, and that it is a mere question
+of inventing proper methods or apparatus for rendering them
+available.</p>
+
+<p>In the present systems of electrical distribution, the employment
+of the iron with its wonderful magnetic properties allows
+us to reduce considerably the size of the apparatus; but, in spite
+of this, it is still very cumbersome. The more we progress in
+the study of electric and magnetic phenomena, the more we be<span class='pagenum'><a name="Page_193" id="Page_193">[Pg 193]</a></span>come
+convinced that the present methods will be short-lived. For
+the production of light, at least, such heavy machinery would
+seem to be unnecessary. The energy required is very small, and
+if light can be obtained as efficiently as, theoretically, it appears
+possible, the apparatus need have but a very small output.
+There being a strong probability that the illuminating methods
+of the future will involve the use of very high potentials, it seems
+very desirable to perfect a contrivance capable of converting the
+energy of heat into energy of the requisite form. Nothing to
+speak of has been done towards this end, for the thought that
+electricity of some 50,000 or 100,000 volts pressure or more, even
+if obtained, would be unavailable for practical purposes, has deterred
+inventors from working in this direction.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_207.jpg" width="640" height="229" alt="Fig. 127." title="" />
+<span class="caption">Fig. 127.</span>
+</div>
+
+
+<p>In Fig. 126 a plan of connections is shown for converting
+currents of high, into currents of low, tension by means of the
+disruptive discharge of a condenser. This plan has been used by
+me frequently for operating a few incandescent lamps required
+in the laboratory. Some difficulties have been encountered in the
+arc of the discharge which I have been able to overcome to a great
+extent; besides this, and the adjustment necessary for the proper
+working, no other difficulties have been met with, and it was easy
+to operate ordinary lamps, and even motors, in this manner.
+The line being connected to the ground, all the wires could be
+handled with perfect impunity, no matter how high the potential
+at the terminals of the condenser. In these experiments a high
+tension induction coil, operated from a battery or from an alternate
+current machine, was employed to charge the condenser; but
+the induction coil might be replaced by an apparatus of a different
+kind, capable of giving electricity of such high tension. In
+this manner, direct or alternating currents may be converted, and
+in both cases the current-impulses may be of any desired frequency.
+When the currents charging the condenser are of the<span class='pagenum'><a name="Page_194" id="Page_194">[Pg 194]</a></span>
+same direction, and it is desired that the converted currents
+should also be of one direction, the resistance of the discharging
+circuit should, of course, be so chosen that there are no
+oscillations.</p>
+
+<div class="figcenter" style="width: 324px;">
+<img src="images/oi_208.jpg" width="324" height="640" alt="Fig. 128." title="" />
+<span class="caption">Fig. 128.</span>
+</div>
+
+
+<p>In operating devices on the above plan I have observed curious
+phenomena of impedance which are of interest. For instance
+if a thick copper bar be bent, as indicated in Fig. 128, and shunted
+by ordinary incandescent lamps, then, by passing the discharge
+between the knobs, the lamps may be brought to incandescence
+although they are short-circuited. When a large induction coil
+is employed it is easy to obtain nodes on the bar, which are
+rendered evident by the different degree of brilliancy of the
+lamps, as shown roughly in Fig. 128. The nodes are never clearly
+defined, but they are simply maxima and minima of potentials
+along the bar. This is probably due to the irregularity of the arc
+between the knobs. In general when the above-described plan
+of conversion from high to low tension is used, the behavior of
+the disruptive discharge may be closely studied. The nodes may
+also be investigated by means of an ordinary Cardew voltmeter<span class='pagenum'><a name="Page_195" id="Page_195">[Pg 195]</a></span>
+which should be well insulated. Geissler tubes may also be
+lighted across the points of the bent bar; in this case, of course,
+it is better to employ smaller capacities. I have found it practicable
+to light up in this manner a lamp, and even a Geissler
+tube, shunted by a short, heavy block of metal, and this result
+seems at first very curious. In fact, the thicker the copper bar
+in Fig. 128, the better it is for the success of the experiments, as
+they appear more striking. When lamps with long slender filaments
+are used it will be often noted that the filaments are from
+time to time violently vibrated, the vibration being smallest at
+the nodal points. This vibration seems to be due to an electrostatic
+action between the filament and the glass of the bulb.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_209.jpg" width="640" height="437" alt="Fig. 129." title="" />
+<span class="caption">Fig. 129.</span>
+</div>
+
+
+<p>In some of the above experiments it is preferable to use special
+lamps having a straight filament as shown in Fig. 129. When
+such a lamp is used a still more curious phenomenon than those
+described may be observed. The lamp may be placed across the
+copper bar and lighted, and by using somewhat larger capacities,
+or, in other words, smaller frequencies or smaller impulsive impedances,
+the filament may be brought to any desired degree of
+incandescence. But when the impedance is increased, a point is
+reached when comparatively little current passes through the
+carbon, and most of it through the rarefied gas; or perhaps it
+may be more correct to state that the current divides nearly
+evenly through both, in spite of the enormous difference in the
+resistance, and this would be true unless the gas and the filament
+behave differently. It is then noted that the whole bulb is brilliantly
+illuminated, and the ends of the leading-in wires become
+incandescent and often throw off sparks in consequence of the
+violent bombardment, but the carbon filament remains dark.
+This is illustrated in Fig. 129. Instead of the filament a single<span class='pagenum'><a name="Page_196" id="Page_196">[Pg 196]</a></span>
+wire extending through the whole bulb may be used, and in this
+case the phenomenon would seem to be still more interesting.</p>
+
+<p>From the above experiment it will be evident, that when ordinary
+lamps are operated by the converted currents, those should
+be preferably taken in which the platinum wires are far apart,
+and the frequencies used should not be too great, else the discharge
+will occur at the ends of the filament or in the base of the
+lamp between the leading-in wires, and the lamp might then be
+damaged.</p>
+
+<p>In presenting to you these results of my investigation on the
+subject under consideration, I have paid only a passing notice to
+facts upon which I could have dwelt at length, and among many
+observations I have selected only those which I thought most
+likely to interest you. The field is wide and completely unexplored,
+and at every step a new truth is gleaned, a novel fact
+observed.</p>
+
+<p>How far the results here borne out are capable of practical
+applications will be decided in the future. As regards the production
+of light, some results already reached are encouraging
+and make me confident in asserting that the practical solution of
+the problem lies in the direction I have endeavored to indicate.
+Still, whatever may be the immediate outcome of these experiments
+I am hopeful that they will only prove a step in further
+development towards the ideal and final perfection. The possibilities
+which are opened by modern research are so vast that
+even the most reserved must feel sanguine of the future. Eminent
+scientists consider the problem of utilizing one kind of
+radiation without the others a rational one. In an apparatus designed
+for the production of light by conversion from any form
+of energy into that of light, such a result can never be reached,
+for no matter what the process of producing the required vibrations,
+be it electrical, chemical or any other, it will not be possible
+to obtain the higher light vibrations without going through
+the lower heat vibrations. It is the problem of imparting to a
+body a certain velocity without passing through all lower velocities.
+But there is a possibility of obtaining energy not only in
+the form of light, but motive power, and energy of any other
+form, in some more direct way from the medium. The time will
+be when this will be accomplished, and the time has come when
+one may utter such words before an enlightened audience without
+being considered a visionary. We are whirling through<span class='pagenum'><a name="Page_197" id="Page_197">[Pg 197]</a></span>
+endless space with an inconceivable speed, all around us everything
+is spinning, everything is moving, everywhere is energy.
+There <i>must</i> be some way of availing ourselves of this energy
+more directly. Then, with the light obtained from the medium,
+with the power derived from it, with every form of energy
+obtained without effort, from the store forever inexhaustible,
+humanity will advance with giant strides. The mere contemplation
+of these magnificent possibilities expands our minds, strengthens
+our hopes and fills our hearts with supreme delight.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_198" id="Page_198">[Pg 198]</a></span></p>
+<h2><a name="CHAPTER_XXVII" id="CHAPTER_XXVII"></a>CHAPTER XXVII.</h2>
+
+<h3><span class="smcap">Experiments with Alternate Currents of High Potential
+and High Frequency.</span><a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a></h3>
+
+
+<p>I cannot find words to express how deeply I feel the honor of
+addressing some of the foremost thinkers of the present time,
+and so many able scientific men, engineers and electricians, of
+the country greatest in scientific achievements.</p>
+
+<p>The results which I have the honor to present before such a
+gathering I cannot call my own. There are among you not a
+few who can lay better claim than myself on any feature of
+merit which this work may contain. I need not mention many
+names which are world-known&mdash;names of those among you who
+are recognized as the leaders in this enchanting science; but one,
+at least, I must mention&mdash;a name which could not be omitted in
+a demonstration of this kind. It is a name associated with the
+most beautiful invention ever made: it is Crookes!</p>
+
+<p>When I was at college, a good while ago, I read, in a translation
+(for then I was not familiar with your magnificent language), the
+description of his experiments on radiant matter. I read it only
+once in my life&mdash;that time&mdash;yet every detail about that charming
+work I can remember to this day. Few are the books, let me
+say, which can make such an impression upon the mind of a
+student.</p>
+
+<p>But if, on the present occasion, I mention this name as one of
+many your Institution can boast of, it is because I have more
+than one reason to do so. For what I have to tell you and to
+show you this evening concerns, in a large measure, that same
+vague world which Professor Crookes has so ably explored; and,
+more than this, when I trace back the mental process which led
+me to these advances&mdash;which even by myself cannot be considered
+trifling, since they are so appreciated by you&mdash;I believe
+that their real origin, that which started me to work in this
+<span class='pagenum'><a name="Page_199" id="Page_199">[Pg 199]</a></span>direction, and brought me to them, after a long period of constant
+thought, was that fascinating little book which I read many
+years ago.</p>
+
+<p>And now that I have made a feeble effort to express my
+homage and acknowledge my indebtedness to him and others
+among you, I will make a second effort, which I hope you will
+not find so feeble as the first, to entertain you.</p>
+
+<p>Give me leave to introduce the subject in a few words.</p>
+
+<p>A short time ago I had the honor to bring before our American
+Institute of Electrical Engineers some results then arrived
+at by me in a novel line of work. I need not assure you that
+the many evidences which I have received that English scientific
+men and engineers were interested in this work have been for
+me a great reward and encouragement. I will not dwell upon
+the experiments already described, except with the view of completing,
+or more clearly expressing, some ideas advanced by me
+before, and also with the view of rendering the study here presented
+self-contained, and my remarks on the subject of this
+evening's lecture consistent.</p>
+
+<p>This investigation, then, it goes without saying, deals with
+alternating currents, and to be more precise, with alternating
+currents of high potential and high frequency. Just in how
+much a very high frequency is essential for the production of
+the results presented is a question which, even with my present
+experience, would embarrass me to answer. Some of the experiments
+may be performed with low frequencies; but very high
+frequencies are desirable, not only on account of the many effects
+secured by their use, but also as a convenient means of obtaining,
+in the induction apparatus employed, the high potentials, which in
+their turn are necessary to the demonstration of most of the experiments
+here contemplated.</p>
+
+<p>Of the various branches of electrical investigation, perhaps the
+most interesting and the most immediately promising is that
+dealing with alternating currents. The progress in this branch
+of applied science has been so great in recent years that it justifies
+the most sanguine hopes. Hardly have we become familiar
+with one fact, when novel experiences are met and new avenues
+of research are opened. Even at this hour possibilities not
+dreamed of before are, by the use of these currents, partly realized.
+As in nature all is ebb and tide, all is wave motion, so it
+seems that in all branches of industry alternating currents&mdash;electric
+wave motion&mdash;will have the sway.<span class='pagenum'><a name="Page_200" id="Page_200">[Pg 200]</a></span></p>
+
+<p>One reason, perhaps, why this branch of science is being so
+rapidly developed is to be found in the interest which is attached
+to its experimental study. We wind a simple ring of iron with
+coils; we establish the connections to the generator, and with
+wonder and delight we note the effects of strange forces which
+we bring into play, which allow us to transform, to transmit and
+direct energy at will. We arrange the circuits properly, and we
+see the mass of iron and wires behave as though it were endowed
+with life, spinning a heavy armature, through invisible connections,
+with great speed and power&mdash;with the energy possibly conveyed
+from a great distance. We observe how the energy of an
+alternating current traversing the wire manifests itself&mdash;not so
+much in the wire as in the surrounding space&mdash;in the most surprising
+manner, taking the forms of heat, light, mechanical
+energy, and, most surprising of all, even chemical affinity. All
+these observations fascinate us, and fill us with an intense desire
+to know more about the nature of these phenomena. Each day
+we go to our work in the hope of discovering,&mdash;in the hope that
+some one, no matter who, may find a solution of one of the pending
+great problems,&mdash;and each succeeding day we return to our
+task with renewed ardor; and even if we <i>are</i> unsuccessful, our
+work has not been in vain, for in these strivings, in these efforts,
+we have found hours of untold pleasure, and we have directed
+our energies to the benefit of mankind.</p>
+
+<p>We may take&mdash;at random, if you choose&mdash;any of the many experiments
+which may be performed with alternating currents;
+a few of which only, and by no means the most striking, form
+the subject of this evening's demonstration; they are all equally
+interesting, equally inciting to thought.</p>
+
+<p>Here is a simple glass tube from which the air has been partially
+exhausted. I take hold of it; I bring my body in contact
+with a wire conveying alternating currents of high potential, and
+the tube in my hand is brilliantly lighted. In whatever position
+I may put it, wherever I move it in space, as far as I can reach,
+its soft, pleasing light persists with undiminished brightness.</p>
+
+<p>Here is an exhausted bulb suspended from a single wire.
+Standing on an insulated support, I grasp it, and a platinum button
+mounted in it is brought to vivid incandescence.</p>
+
+<p>Here, attached to a leading wire, is another bulb, which, as I
+touch its metallic socket, is filled with magnificent colors of phosphorescent
+light.<span class='pagenum'><a name="Page_201" id="Page_201">[Pg 201]</a></span></p>
+
+<p>Here still another, which by my fingers' touch casts a shadow&mdash;the
+Crookes shadow&mdash;of the stem inside of it.</p>
+
+<p>Here, again, insulated as I stand on this platform, I bring my
+body in contact with one of the terminals of the secondary of
+this induction coil&mdash;with the end of a wire many miles long&mdash;and
+you see streams of light break forth from its distant end, which
+is set in violent vibration.</p>
+
+<p>Here, once more, I attach these two plates of wire gauze to the
+terminals of the coil; I set them a distance apart, and I set the
+coil to work. You may see a small spark pass between the
+plates. I insert a thick plate of one of the best dielectrics between
+them, and instead of rendering altogether impossible, as
+we are used to expect, I <i>aid</i> the passage of the discharge, which,
+as I insert the plate, merely changes in appearance and assumes
+the form of luminous streams.</p>
+
+<p>Is there, I ask, can there be, a more interesting study than that
+of alternating currents?</p>
+
+<p>In all these investigations, in all these experiments, which are
+so very, very interesting, for many years past&mdash;ever since the
+greatest experimenter who lectured in this hall discovered its
+principle&mdash;we have had a steady companion, an appliance familiar
+to every one, a plaything once, a thing of momentous importance
+now&mdash;the induction coil. There is no dearer appliance to the
+electrician. From the ablest among you, I dare say, down to the
+inexperienced student, to your lecturer, we all have passed many
+delightful hours in experimenting with the induction coil. We
+have watched its play, and thought and pondered over the beautiful
+phenomena which it disclosed to our ravished eyes. So
+well known is this apparatus, so familiar are these phenomena to
+every one, that my courage nearly fails me when I think that I
+have ventured to address so able an audience, that I have ventured
+to entertain you with that same old subject. Here in reality
+is the same apparatus, and here are the same phenomena, only
+the apparatus is operated somewhat differently, the phenomena
+are presented in a different aspect. Some of the results we find
+as expected, others surprise us, but all captivate our attention, for
+in scientific investigation each novel result achieved may be the
+centre of a new departure, each novel fact learned may lead to
+important developments.</p>
+
+<p>Usually in operating an induction coil we have set up a vibration
+of moderate frequency in the primary, either by means of an<span class='pagenum'><a name="Page_202" id="Page_202">[Pg 202]</a></span>
+interrupter or break, or by the use of an alternator. Earlier
+English investigators, to mention only Spottiswoode and J. E. H.
+Gordon, have used a rapid break in connection with the coil.
+Our knowledge and experience of to-day enables us to see clearly
+why these coils under the conditions of the test did not disclose
+any remarkable phenomena, and why able experimenters failed
+to perceive many of the curious effects which have since been
+observed.</p>
+
+<p>In the experiments such as performed this evening, we operate
+the coil either from a specially constructed alternator capable of
+giving many thousands of reversals of current per second, or, by
+disruptively discharging a condenser through the primary, we set
+up a vibration in the secondary circuit of a frequency of many
+hundred thousand or millions per second, if we so desire; and in
+using either of these means we enter a field as yet unexplored.</p>
+
+<p>It is impossible to pursue an investigation in any novel line
+without finally making some interesting observation or learning
+some useful fact. That this statement is applicable to the subject
+of this lecture the many curious and unexpected phenomena
+which we observe afford a convincing proof. By way of illustration,
+take for instance the most obvious phenomena, those of the
+discharge of the induction coil.</p>
+
+<p>Here is a coil which is operated by currents vibrating with
+extreme rapidity, obtained by disruptively discharging a Leyden
+jar. It would not surprise a student were the lecturer to say
+that the secondary of this coil consists of a small length of comparatively
+stout wire; it would not surprise him were the lecturer
+to state that, in spite of this, the coil is capable of giving any
+potential which the best insulation of the turns is able to withstand;
+but although he may be prepared, and even be indifferent
+as to the anticipated result, yet the aspect of the discharge of the
+coil will surprise and interest him. Every one is familiar with
+the discharge of an ordinary coil; it need not be reproduced
+here. But, by way of contrast, here is a form of discharge of a
+coil, the primary current of which is vibrating several hundred
+thousand times per second. The discharge of an ordinary coil
+appears as a simple line or band of light. The discharge of this
+coil appears in the form of powerful brushes and luminous
+streams issuing from all points of the two straight wires attached
+to the terminals of the secondary. (Fig. 130.)</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_217.jpg" width="600" height="628" alt="Fig. 130, 131." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 130.</td><td class="caption">Fig. 131.</td></tr>
+</table>
+</div>
+
+<p>Now compare this phenomenon which you have just witnessed
+<span class='pagenum'><a name="Page_203" id="Page_203">[Pg 203]</a></span>with the discharge of a Holtz or Wimshurst machine&mdash;that other
+interesting appliance so dear to the experimenter. What a difference
+there is between these phenomena! And yet, had I made
+the necessary arrangements&mdash;which could have been made easily,
+were it not that they would interfere with other experiments&mdash;I
+could have produced with this coil sparks which, had I the coil
+hidden from your view and only two knobs exposed, even the
+keenest observer among you would find it difficult, if not impossible,
+to distinguish from those of an influence or friction machine.
+This may be done in many ways&mdash;for instance, by operating
+the induction coil which charges the condenser from an
+alternating-current machine of very low frequency, and preferably
+adjusting the discharge circuit so that there are no oscillations
+set up in it. We then obtain in the secondary circuit, if the
+knobs are of the required size and properly set, a more or less<span class='pagenum'><a name="Page_204" id="Page_204">[Pg 204]</a></span>
+rapid succession of sparks of great intensity and small quantity,
+which possess the same brilliancy, and are accompanied by the
+same sharp crackling sound, as those obtained from a friction or
+influence machine.</p>
+
+<p>Another way is to pass through two primary circuits, having a
+common secondary, two currents of a slightly different period,
+which produce in the secondary circuit sparks occurring at comparatively
+long intervals. But, even with the means at hand
+this evening, I may succeed in imitating the spark of a Holtz
+machine. For this purpose I establish between the terminals of
+the coil which charges the condenser a long, unsteady arc, which
+is periodically interrupted by the upward current of air produced
+by it. To increase the current of air I place on each side of the
+arc, and close to it, a large plate of mica. The condenser charged
+from this coil discharges into the primary circuit of a second
+coil through a small air gap, which is necessary to produce a
+sudden rush of current through the primary. The scheme of
+connections in the present experiment is indicated in Fig. 131.</p>
+
+<p><small>G</small> is an ordinarily constructed alternator, supplying the primary
+<small>P</small> of an induction coil, the secondary <small>S</small> of which charges
+the condensers or jars <small>C C</small>. The terminals of the secondary are
+connected to the inside coatings of the jars, the outer coatings
+being connected to the ends of the primary <i>p p</i> of a second induction
+coil. This primary <i>p p</i> has a small air gap <i>a b</i>.</p>
+
+<p>The secondary <i>s</i> of this coil is provided with knobs or spheres
+<small>K K</small> of the proper size and set at a distance suitable for the experiment.</p>
+
+<p>A long arc is established between the terminals <small>A B</small> of the first
+induction coil. <small>M M</small> are the mica plates.</p>
+
+<p>Each time the arc is broken between <small>A</small> and <small>B</small> the jars are
+quickly charged and discharged through the primary <i>p p</i>, producing
+a snapping spark between the knobs <small>K K</small>. Upon the arc
+forming between <small>A</small> and <small>B</small> the potential falls, and the jars cannot
+be charged to such high potential as to break through the air
+gap <i>a b</i> until the arc is again broken by the draught.</p>
+
+<p>In this manner sudden impulses, at long intervals, are produced
+in the primary <i>p p</i>, which in the secondary <i>s</i> give a corresponding
+number of impulses of great intensity. If the secondary
+knobs or spheres, <small>K K</small>, are of the proper size, the sparks
+show much resemblance to those of a Holtz machine.</p>
+
+<p>But these two effects, which to the eye appear so very differ<span class='pagenum'><a name="Page_205" id="Page_205">[Pg 205]</a></span>ent,
+are only two of the many discharge phenomena. We only
+need to change the conditions of the test, and again we make
+other observations of interest.</p>
+
+<p>When, instead of operating the induction coil as in the last
+two experiments, we operate it from a high frequency alternator,
+as in the next experiment, a systematic study of the phenomena
+is rendered much more easy. In such case, in varying the
+strength and frequency of the currents through the primary, we
+may observe five distinct forms of discharge, which I have described
+in my former paper on the subject before the American
+Institute of Electrical Engineers, May 20, 1891.</p>
+
+<p>It would take too much time, and it would lead us too far
+from the subject presented this evening, to reproduce all these
+forms, but it seems to me desirable to show you one of them. It
+is a brush discharge, which is interesting in more than one respect.
+Viewed from a near position it resembles much a jet of
+gas escaping under great pressure. We know that the phenomenon
+is due to the agitation of the molecules near the terminal,
+and we anticipate that some heat must be developed by the impact
+of the molecules against the terminal or against each other.
+Indeed, we find that the brush is hot, and only a little thought
+leads us to the conclusion that, could we but reach sufficiently
+high frequencies, we could produce a brush which would give
+intense light and heat, and which would resemble in every particular
+an ordinary flame, save, perhaps, that both phenomena
+might not be due to the same agent&mdash;save, perhaps, that chemical
+affinity might not be <i>electrical</i> in its nature.</p>
+
+<p>As the production of heat and light is here due to the impact
+of the molecules, or atoms of air, or something else besides,
+and, as we can augment the energy simply by raising the
+potential, we might, even with frequencies obtained from
+a dynamo machine, intensify the action to such a degree as to
+bring the terminal to melting heat. But with such low frequencies
+we would have to deal always with something of the nature
+of an electric current. If I approach a conducting object to the
+brush, a thin little spark passes, yet, even with the frequencies
+used this evening, the tendency to spark is not very great. So,
+for instance, if I hold a metallic sphere at some distance above
+the terminal, you may see the whole space between the terminal
+and sphere illuminated by the streams without the spark passing;
+and with the much higher frequencies obtainable by the disrup<span class='pagenum'><a name="Page_206" id="Page_206">[Pg 206]</a></span>tive
+discharge of a condenser, were it not for the sudden impulses,
+which are comparatively few in number, sparking would not
+occur even at very small distances. However, with incomparably
+higher frequencies, which we may yet find means to produce
+efficiently, and provided that electric impulses of such high
+frequencies could be transmitted through a conductor, the electrical
+characteristics of the brush discharge would completely
+vanish&mdash;no spark would pass, no shock would be felt&mdash;yet we
+would still have to deal with an <i>electric</i> phenomenon, but in the
+broad, modern interpretation of the word. In my first paper, before
+referred to, I have pointed out the curious properties of the
+brush, and described the best manner of producing it, but I have
+thought it worth while to endeavor to express myself more clearly
+in regard to this phenomenon, because of its absorbing interest.</p>
+
+<p>When a coil is operated with currents of very high frequency,
+beautiful brush effects may be produced, even if the coil be of
+comparatively small dimensions. The experimenter may vary
+them in many ways, and, if it were for nothing else, they afford a
+pleasing sight. What adds to their interest is that they may be
+produced with one single terminal as well as with two&mdash;in fact,
+often better with one than with two.</p>
+
+<p>But of all the discharge phenomena observed, the most pleasing
+to the eye, and the most instructive, are those observed with
+a coil which is operated by means of the disruptive discharge of
+a condenser. The power of the brushes, the abundance of the
+sparks, when the conditions are patiently adjusted, is often amazing.
+With even a very small coil, if it be so well insulated as to
+stand a difference of potential of several thousand volts per turn,
+the sparks may be so abundant that the whole coil may appear
+a complete mass of fire.</p>
+
+<p>Curiously enough the sparks, when the terminals of the coil
+are set at a considerable distance, seem to dart in every possible
+direction as though the terminals were perfectly independent of
+each other. As the sparks would soon destroy the insulation, it
+is necessary to prevent them. This is best done by immersing
+the coil in a good liquid insulator, such as boiled-out oil. Immersion
+in a liquid may be considered almost an absolute necessity
+for the continued and successful working of such a coil.</p>
+
+<p>It is, of course, out of the question, in an experimental lecture,
+with only a few minutes at disposal for the performance of each
+experiment, to show these discharge phenomena to advantage,<span class='pagenum'><a name="Page_207" id="Page_207">[Pg 207]</a></span>
+as, to produce each phenomenon at its best, a very careful adjustment
+is required. But even if imperfectly produced, as they are
+likely to be this evening, they are sufficiently striking to interest
+an intelligent audience.</p>
+
+<p>Before showing some of these curious effects I must, for the
+sake of completeness, give a short description of the coil and
+other apparatus used in the experiments with the disruptive discharge
+this evening.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_221.jpg" width="600" height="674" alt="Fig. 132." title="" />
+<span class="caption">Fig. 132.</span>
+</div>
+
+
+<p>It is contained in a box <small>B</small> (Fig. 132) of thick boards of hard
+wood, covered on the outside with a zinc sheet <small>Z</small>, which is carefully
+soldered all around. It might be advisable, in a strictly scientific
+investigation, when accuracy is of great importance, to do away
+with the metal cover, as it might introduce many errors, principally
+on account of its complex action upon the coil, as a condenser
+of very small capacity and as an electrostatic and electromagnetic
+screen. When the coil is used for such experiments as
+are here contemplated, the employment of the metal cover offers
+some practical advantages, but these are not of sufficient importance
+to be dwelt upon.</p>
+
+<p>The coil should be placed symmetrically to the metal cover,<span class='pagenum'><a name="Page_208" id="Page_208">[Pg 208]</a></span>
+and the space between should, of course, not be too small, certainly
+not less than, say, five centimetres, but much more if possible;
+especially the two sides of the zinc box, which are at right
+angles to the axis of the coil, should be sufficiently remote from
+the latter, as otherwise they might impair its action and be a
+source of loss.</p>
+
+<p>The coil consists of two spools of hard rubber <small>R R</small>, held apart
+at a distance of 10 centimetres by bolts <small>C</small> and nuts <i>n</i>, likewise of
+hard rubber. Each spool comprises a tube <small>T</small> of approximately 8
+centimetres inside diameter, and 3 millimetres thick, upon which
+are screwed two flanges <small>F F</small>, 24 centimetres square, the space between
+the flanges being about 3 centimetres. The secondary, <small>S S</small>,
+of the best gutta percha-covered wire, has 26 layers, 10 turns in
+each, giving for each half a total of 260 turns. The two halves
+are wound oppositely and connected in series, the connection between
+both being made over the primary. This disposition, besides
+being convenient, has the advantage that when the coil is
+well balanced&mdash;that is, when both of its terminals <small>T<sub>1</sub></small>, <small>T<sub>1</sub></small>, are connected
+to bodies or devices of equal capacity&mdash;there is not much
+danger of breaking through to the primary, and the insulation
+between the primary and the secondary need not be thick. In
+using the coil it is advisable to attach to <i>both</i> terminals devices of
+nearly equal capacity, as, when the capacity of the terminals is
+not equal, sparks will be apt to pass to the primary. To avoid
+this, the middle point of the secondary may be connected to the
+primary, but this is not always practicable.</p>
+
+<p>The primary <small>P P</small> is wound in two parts, and oppositely, upon
+a wooden spool w, and the four ends are led out of the oil through
+hard rubber tubes <i>t t</i>. The ends of the secondary <small>T<sub>1</sub> T<sub>1</sub></small>, are also
+led out of the oil through rubber tubes <i>t</i><sub>1</sub> <i>t</i><sub>1</sub> of great thickness.
+The primary and secondary layers are insulated by cotton cloth,
+the thickness of the insulation, of course, bearing some proportion
+to the difference of potential between the turns of the different
+layers. Each half of the primary has four layers, 24 turns
+in each, this giving a total of 96 turns. When both the parts
+are connected in series, this gives a ratio of conversion of about
+1:2.7, and with the primaries in multiple, 1:5.4; but in operating
+with very rapidly alternating currents this ratio does not convey
+even an approximate idea of the ratio of the <span class="smcap">e. m. f</span>'s. in the
+primary and secondary circuits. The coil is held in position in
+the oil on wooden supports, there being about 5 centimetres<span class='pagenum'><a name="Page_209" id="Page_209">[Pg 209]</a></span>
+thickness of oil all round. Where the oil is not specially needed,
+the space is filled with pieces of wood, and for this purpose
+principally the wooden box B surrounding the whole is used.</p>
+
+<p>The construction here shown is, of course, not the best on
+general principles, but I believe it is a good and convenient one
+for the production of effects in which an excessive potential and
+a very small current are needed.</p>
+
+<p>In connection with the coil I use either the ordinary form of
+discharger or a modified form. In the former I have introduced
+two changes which secure some advantages, and which are obvious.
+If they are mentioned, it is only in the hope that some
+experimenter may find them of use.</p>
+
+<p>One of the changes is that the adjustable knobs <small>A</small> and <small>B</small> (Fig.
+133), of the discharger are held in jaws of brass, <small>J J</small>, by spring
+pressure, this allowing of turning them successively into different
+positions, and so doing away with the tedious process of frequent
+polishing up.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_223.jpg" width="800" height="461" alt="Fig. 133." title="" />
+<span class="caption">Fig. 133.</span>
+</div>
+
+<p>The other change consists in the employment of a strong electromagnet
+<small>N S</small>, which is placed with its axis at right angles to
+the line joining the knobs <small>A</small> and <small>B</small>, and produces a strong magnetic
+field between them. The pole pieces of the magnet are
+movable and properly formed so as to protrude between the brass
+knobs, in order to make the field as intense as possible; but to
+prevent the discharge from jumping to the magnet the pole
+pieces are protected by a layer of mica, <small>M M</small>, of sufficient thickness;
+<i>s</i><sub>1</sub> <i>s</i><sub>1</sub> and <i>s</i><sub>2</sub> <i>s</i><sub>2</sub> are screws for fastening the wires. On each
+side one of the screws is for large and the other for small wires.
+<small>L L</small> are screws for fixing in position the rods <small>R R</small>, which support
+the knobs.<span class='pagenum'><a name="Page_210" id="Page_210">[Pg 210]</a></span></p>
+
+<p>In another arrangement with the magnet I take the discharge
+between the rounded pole pieces themselves, which in such
+case are insulated and preferably provided with polished brass
+caps.</p>
+
+<p>The employment of an intense magnetic field is of advantage
+principally when the induction coil or transformer which charges
+the condenser is operated by currents of very low frequency. In
+such a case the number of the fundamental discharges between
+the knobs may be so small as to render the currents produced in
+the secondary unsuitable for many experiments. The intense
+magnetic field then serves to blow out the arc between the knobs
+as soon as it is formed, and the fundamental discharges occur in
+quicker succession.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_224.jpg" width="800" height="331" alt="Fig. 134." title="" />
+<span class="caption">Fig. 134.</span>
+</div>
+
+<p>Instead of the magnet, a draught or blast of air may be employed
+with some advantage. In this case the arc is preferably
+established between the knobs <small>A B</small>, in Fig. 131 (the knobs <i>a b</i>
+being generally joined, or entirely done away with), as in this
+disposition the arc is long and unsteady, and is easily affected by
+the draught.</p>
+
+<p>When a magnet is employed to break the arc, it is better to
+choose the connection indicated diagrammatically in Fig. 134,
+as in this case the currents forming the arc are much more powerful,
+and the magnetic field exercises a greater influence. The
+use of the magnet permits, however, of the arc being replaced by
+a vacuum tube, but I have encountered great difficulties in working
+with an exhausted tube.</p>
+
+<p>The other form of discharger used in these and similar experiments
+is indicated in Figs. 135 and 136. It consists of a number
+of brass pieces <i>c c</i> (Fig. 135), each of which comprises a spherical
+middle portion <i>m</i> with an extension <i>e</i> below&mdash;which is merely used
+to fasten the piece in a lathe when polishing up the discharging<span class='pagenum'><a name="Page_211" id="Page_211">[Pg 211]</a></span>
+surface&mdash;and a column above, which consists of a knurled flange
+<i>f</i> surmounted by a threaded stem <i>l</i> carrying a nut <i>n</i>, by means
+of which a wire is fastened to the column. The flange <i>f</i> conveniently
+serves for holding the brass piece when fastening the
+wire, and also for turning it in any position when it becomes
+necessary to present a fresh discharging surface. Two stout
+strips of hard rubber <small>R R</small>, with planed grooves <i>g g</i> (Fig. 136) to fit
+the middle portion of the pieces <i>c c</i>, serve to clamp the latter
+and hold them firmly in position by means of two bolts <small>C C</small>
+(of which only one is shown) passing through the ends of the
+strips.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_225.jpg" width="800" height="312" alt="Fig. 135." title="" />
+<span class="caption">Fig. 135.</span>
+</div>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_225-1.jpg" width="800" height="506" alt="Fig. 136." title="" />
+<span class="caption">Fig. 136.</span>
+</div>
+
+<p>In the use of this kind of discharger I have found three principal
+advantages over the ordinary form. First, the dielectric
+strength of a given total width of air space is greater when a
+great many small air gaps are used instead of one, which permits
+of working with a smaller length of air gap, and that means
+smaller loss and less deterioration of the metal; secondly, by
+reason of splitting the arc up into smaller arcs, the polished
+surfaces are made to last much longer; and, thirdly, the appa<span class='pagenum'><a name="Page_212" id="Page_212">[Pg 212]</a></span>ratus
+affords some gauge in the experiments. I usually set the
+pieces by putting between them sheets of uniform thickness at a
+certain very small distance which is known from the experiments
+of Sir William Thomson to require a certain electromotive force
+to be bridged by the spark.</p>
+
+<p>It should, of course, be remembered that the sparking distance
+is much diminished as the frequency is increased. By taking
+any number of spaces the experimenter has a rough idea of the
+electromotive force, and he finds it easier to repeat an experiment,
+as he has not the trouble of setting the knobs again and
+again. With this kind of discharger I have been able to maintain
+an oscillating motion without any spark being visible with
+the naked eye between the knobs, and they would not show a
+very appreciable rise in temperature. This form of discharge
+also lends itself to many arrangements of condensers and circuits
+which are often very convenient and time-saving. I have used
+it preferably in a disposition similar to that indicated in Fig. 131,
+when the currents forming the arc are small.</p>
+
+<p>I may here mention that I have also used dischargers with
+single or multiple air gaps, in which the discharge surfaces were
+rotated with great speed. No particular advantage was, however,
+gained by this method, except in cases where the currents
+from the condenser were large and the keeping cool of the surfaces
+was necessary, and in cases when, the discharge not being
+oscillating of itself, the arc as soon as established was broken by
+the air current, thus starting the vibration at intervals in rapid
+succession. I have also used mechanical interrupters in many
+ways. To avoid the difficulties with frictional contacts, the preferred
+plan adopted was to establish the arc and rotate through
+it at great speed a rim of mica provided with many holes and
+fastened to a steel plate. It is understood, of course, that the
+employment of a magnet, air current, or other interrupter, produces
+no effect worth noticing, unless the self-induction, capacity
+and resistance are so related that there are oscillations set up
+upon each interruption.</p>
+
+<p>I will now endeavor to show you some of the most noteworthy
+of these discharge phenomena.</p>
+
+<p>I have stretched across the room two ordinary cotton covered
+wires, each about seven metres in length. They are supported
+on insulating cords at a distance of about thirty centimetres. I
+attach now to each of the terminals of the coil one of the wires,<span class='pagenum'><a name="Page_213" id="Page_213">[Pg 213]</a></span>
+and set the coil in action. Upon turning the lights off in the
+room you see the wires strongly illuminated by the streams issuing
+abundantly from their whole surface in spite of the cotton
+covering, which may even be very thick. When the experiment
+is performed under good conditions, the light from the wires is
+sufficiently intense to allow distinguishing the objects in a room.
+To produce the best result it is, of course, necessary to adjust
+carefully the capacity of the jars, the arc between the knobs and
+the length of the wires. My experience is that calculation of the
+length of the wires leads, in such case, to no result whatever. The
+experimenter will do best to take the wires at the start very long,
+and then adjust by cutting off first long pieces, and then smaller
+and smaller ones as he approaches the right length.</p>
+
+<p>A convenient way is to use an oil condenser of very small
+capacity, consisting of two small adjustable metal plates, in connection
+with this and similar experiments. In such case I take
+wires rather short and at the beginning set the condenser plates
+at maximum distance. If the streams from the wires increase by
+approach of the plates, the length of the wires is about right; if
+they diminish, the wires are too long for that frequency and potential.
+When a condenser is used in connection with experiments
+with such a coil, it should be an oil condenser by all means,
+as in using an air condenser considerable energy might be wasted.
+The wires leading to the plates in the oil should be very thin,
+heavily coated with some insulating compound, and provided
+with a conducting covering&mdash;this preferably extending under the
+surface of the oil. The conducting cover should not be too near
+the terminals, or ends, of the wire, as a spark would be apt to
+jump from the wire to it. The conducting coating is used to
+diminish the air losses, in virtue of its action as an electrostatic
+screen. As to the size of the vessel containing the oil, and the
+size of the plates, the experimenter gains at once an idea from a
+rough trial. The size of the plates <i>in oil</i> is, however, calculable,
+as the dielectric losses are very small.</p>
+
+<p>In the preceding experiment it is of considerable interest to
+know what relation the quantity of the light emitted bears to
+the frequency and potential of the electric impulses. My opinion
+is that the heat as well as light effects produced should be proportionate,
+under otherwise equal conditions of test, to the product
+of frequency and square of potential, but the experimental verification
+of the law, whatever it may be, would be exceedingly<span class='pagenum'><a name="Page_214" id="Page_214">[Pg 214]</a></span>
+difficult. One thing is certain, at any rate, and that is, that in
+augmenting the potential and frequency we rapidly intensify the
+streams; and, though it may be very sanguine, it is surely not
+altogether hopeless to expect that we may succeed in producing
+a practical illuminant on these lines. We would then be simply
+using burners or flames, in which there would be no chemical
+process, no consumption of material, but merely a transfer of
+energy, and which would, in all probability, emit more light and
+less heat than ordinary flames.</p>
+
+<div class="figcenter" style="width: 471px;">
+<img src="images/oi_228.jpg" width="471" height="640" alt="Fig. 137." title="" />
+<span class="caption">Fig. 137.</span>
+</div>
+
+<p>The luminous intensity of the streams is, of course, considerably
+increased when they are focused upon a small surface. This may
+be shown by the following experiment:</p>
+
+<p>I attach to one of the terminals of the coil a wire <i>w</i> (Fig. 137),
+bent in a circle of about 30 centimetres in diameter, and to the
+other terminal I fasten a small brass sphere <i>s</i>, the surface of the
+wire being preferably equal to the surface of the sphere, and the
+centre of the latter being in a line at right angles to the plane of
+the wire circle and passing through its centre. When the discharge
+is established under proper conditions, a luminous hollow
+cone is formed, and in the dark one-half of the brass sphere is
+strongly illuminated, as shown in the cut.</p>
+
+<p>By some artifice or other it is easy to concentrate the streams<span class='pagenum'><a name="Page_215" id="Page_215">[Pg 215]</a></span>
+upon small surfaces and to produce very strong light effects.
+Two thin wires may thus be rendered intensely luminous.</p>
+
+<p>In order to intensify the streams the wires should be very thin
+and short; but as in this case their capacity would be generally
+too small for the coil&mdash;at least for such a one as the present&mdash;it
+is necessary to augment the capacity to the required value, while,
+at the same time, the surface of the wires remains very small.
+This may be done in many ways.</p>
+
+<div class="figcenter" style="width: 550px;">
+<img src="images/oi_229.jpg" width="550" height="480" alt="Fig. 138." title="" />
+<span class="caption">Fig. 138.</span>
+</div>
+
+
+<p>Here, for instance, I have two plates, <small>R R</small>, of hard rubber (Fig.
+138), upon which I have glued two very thin wires <i>w w</i>, so as to
+form a name. The wires may be bare or covered with the best
+insulation&mdash;it is immaterial for the success of the experiment.
+Well insulated wires, if anything, are preferable. On the back
+of each plate, indicated by the shaded portion, is a tinfoil coating
+<i>t t</i>. The plates are placed in line at a sufficient distance to prevent
+a spark passing from one wire to the other. The two tinfoil
+coatings I have joined by a conductor <small>C</small>, and the two wires I
+presently connect to the terminals of the coil. It is now easy, by
+varying the strength and frequency of the currents through the
+primary, to find a point at which the capacity of the system is
+best suited to the conditions, and the wires become so strongly
+luminous that, when the light in the room is turned off the name
+formed by them appears in brilliant letters.</p>
+
+<p>It is perhaps preferable to perform this experiment with a
+coil operated from an alternator of high frequency, as then,<span class='pagenum'><a name="Page_216" id="Page_216">[Pg 216]</a></span>
+owing to the harmonic rise and fall, the streams are very uniform,
+though they are less abundant than when produced with such a
+coil as the present one. This experiment, however, may be performed
+with low frequencies, but much less satisfactorily.</p>
+
+<div class="figcenter" style="width: 406px;">
+<img src="images/oi_230.jpg" width="406" height="640" alt="Fig. 139." title="" />
+<span class="caption">Fig. 139.</span>
+</div>
+
+
+<p>When two wires, attached to the terminals of the coil, are set
+at the proper distance, the streams between them may be so intense
+as to produce a continuous luminous sheet. To show this
+phenomenon I have here two circles, <small>C</small> and <i>c</i> (Fig. 139), of rather
+stout wire, one being about 80 centimetres and the other 30 centimetres
+in diameter. To each of the terminals of the coil I
+attach one of the circles. The supporting wires are so bent that
+the circles may be placed in the same plane, coinciding as nearly
+as possible. When the light in the room is turned off and the
+coil set to work, you see the whole space between the wires uniformly
+filled with streams, forming a luminous disc, which could
+be seen from a considerable distance, such is the intensity of the
+streams. The outer circle could have been much larger than the
+present one; in fact, with this coil I have used much larger
+circles, and I have been able to produce a strongly luminous
+sheet, covering an area of more than one square metre, which is
+a remarkable effect with this very small coil. To avoid uncer<span class='pagenum'><a name="Page_217" id="Page_217">[Pg 217]</a></span>tainty,
+the circle has been taken smaller, and the area is now
+about 0.43 square metre.</p>
+
+<p>The frequency of the vibration, and the quickness of succession
+of the sparks between the knobs, affect to a marked degree
+the appearance of the streams. When the frequency is very
+low, the air gives way in more or less the same manner, as by a
+steady difference of potential, and the streams consist of distinct
+threads, generally mingled with thin sparks, which probably correspond
+to the successive discharges occurring between the
+knobs. But when the frequency is extremely high, and the arc
+of the discharge produces a very <i>loud</i> and <i>smooth</i> sound&mdash;showing
+both that oscillation takes place and that the sparks succeed
+each other with great rapidity&mdash;then the luminous streams
+formed are perfectly uniform. To reach this result very small
+coils and jars of small capacity should be used. I take two
+tubes of thick Bohemian glass, about 5 centimetres in diameter
+and 20 centimetres long. In each of the tubes I slip a primary
+of very thick copper wire. On the top of each tube I wind a
+secondary of much thinner gutta-percha covered wire. The two
+secondaries I connect in series, the primaries preferably in multiple
+arc. The tubes are then placed in a large glass vessel, at a distance
+of 10 to 15 centimetres from each other, on insulating supports,
+and the vessel is filled with boiled-out oil, the oil reaching
+about an inch above the tubes. The free ends of the secondary
+are lifted out of the coil and placed parallel to each other at a
+distance of about ten centimetres. The ends which are scraped
+should be dipped in the oil. Two four-pint jars joined in series
+may be used to discharge through the primary. When the necessary
+adjustments in the length and distance of the wires above
+the oil and in the arc of discharge are made, a luminous sheet is
+produced between the wires which is perfectly smooth and textureless,
+like the ordinary discharge through a moderately exhausted
+tube.</p>
+
+<p>I have purposely dwelt upon this apparently insignificant experiment.
+In trials of this kind the experimenter arrives at the
+startling conclusion that, to pass ordinary luminous discharges
+through gases, no particular degree of exhaustion is needed, but
+that the gas may be at ordinary or even greater pressure. To
+accomplish this, a very high frequency is essential; a high potential
+is likewise required, but this is merely an incidental necessity.
+These experiments teach us that, in endeavoring to dis<span class='pagenum'><a name="Page_218" id="Page_218">[Pg 218]</a></span>cover
+novel methods of producing light by the agitation of atoms,
+or molecules, of a gas, we need not limit our research to the
+vacuum tube, but may look forward quite seriously to the possibility
+of obtaining the light effects without the use of any vessel
+whatever, with air at ordinary pressure.</p>
+
+<p>Such discharges of very high frequency, which render luminous
+the air at ordinary pressures, we have probably occasion often to
+witness in Nature. I have no doubt that if, as many believe, the
+aurora borealis is produced by sudden cosmic disturbances, such
+as eruptions at the sun's surface, which set the electrostatic charge
+of the earth in an extremely rapid vibration, the red glow observed
+is not confined to the upper rarefied strata of the air, but
+the discharge traverses, by reason of its very high frequency,
+also the dense atmosphere in the form of a <i>glow</i>, such as we ordinarily
+produce in a slightly exhausted tube. If the frequency
+were very low, or even more so, if the charge were not at all
+vibrating, the dense air would break down as in a lightning discharge.
+Indications of such breaking down of the lower dense
+strata of the air have been repeatedly observed at the occurrence
+of this marvelous phenomenon; but if it does occur, it can only
+be attributed to the fundamental disturbances, which are few in
+number, for the vibration produced by them would be far too
+rapid to allow a disruptive break. It is the original and irregular
+impulses which affect the instruments; the superimposed vibrations
+probably pass unnoticed.</p>
+
+<p>When an ordinary low frequency discharge is passed through
+moderately rarefied air, the air assumes a purplish hue. If by
+some means or other we increase the intensity of the molecular,
+or atomic, vibration, the gas changes to a white color. A similar
+change occurs at ordinary pressures with electric impulses of very
+high frequency. If the molecules of the air around a wire are
+moderately agitated, the brush formed is reddish or violet; if
+the vibration is rendered sufficiently intense, the streams become
+white. We may accomplish this in various ways. In the experiment
+before shown with the two wires across the room, I have
+endeavored to secure the result by pushing to a high value both
+the frequency and potential; in the experiment with the thin
+wires glued on the rubber plate I have concentrated the action
+upon a very small surface&mdash;in other words, I have worked with
+a great electric density.</p>
+
+<div class="figcenter" style="width: 619px;">
+<img src="images/oi_233.jpg" width="619" height="600" alt="Fig. 140." title="" />
+<span class="caption">Fig. 140.</span>
+</div>
+
+
+<p>A most curious form of discharge is observed with such a coil
+<span class='pagenum'><a name="Page_219" id="Page_219">[Pg 219]</a></span>when the frequency and potential are pushed to the extreme
+limit. To perform the experiment, every part of the coil should
+be heavily insulated, and only two small spheres&mdash;or, better still,
+two sharp-edged metal discs (<i>d d</i>, Fig. 140) of no more than
+a few centimetres in diameter&mdash;should be exposed to the air.
+The coil here used is immersed in oil, and the ends of the
+secondary reaching out of the oil are covered with an air-tight
+cover of hard rubber of great thickness. All cracks, if there
+are any, should be carefully stopped up, so that the brush discharge
+cannot form anywhere except on the small spheres or
+plates which are exposed to the air. In this case, since there
+are no large plates or other bodies of capacity attached to the
+terminals, the coil is capable of an extremely rapid vibration.
+The potential may be raised by increasing, as far as the experimenter
+judges proper, the rate of change of the primary current.
+With a coil not widely differing from the present, it is
+best to connect the two primaries in multiple arc; but if the
+secondary should have a much greater number of turns the
+primaries should preferably be used in series, as otherwise the
+vibration might be too fast for the secondary. It occurs under
+these conditions that misty white streams break forth from the
+edges of the discs and spread out phantom-like into space.
+With this coil, when fairly well produced, they are about 25 to
+30 centimetres long. When the hand is held against them no
+sensation is produced, and a spark, causing a shock, jumps from<span class='pagenum'><a name="Page_220" id="Page_220">[Pg 220]</a></span>
+the terminal only upon the hand being brought much nearer.
+If the oscillation of the primary current is rendered intermittent
+by some means or other, there is a corresponding throbbing of
+the streams, and now the hand or other conducting object may
+be brought in still greater proximity to the terminal without a
+spark being caused to jump.</p>
+
+<p>Among the many beautiful phenomena which may be produced
+with such a coil, I have here selected only those which appear
+to possess some features of novelty, and lead us to some
+conclusions of interest. One will not find it at all difficult to
+produce in the laboratory, by means of it, many other phenomena
+which appeal to the eye even more than these here shown, but
+present no particular feature of novelty.</p>
+
+<p>Early experimenters describe the display of sparks produced by
+an ordinary large induction coil upon an insulating plate separating
+the terminals. Quite recently Siemens performed some experiments
+in which fine effects were obtained, which were seen
+by many with interest. No doubt large coils, even if operated
+with currents of low frequencies, are capable of producing
+beautiful effects. But the largest coil ever made could not, by
+far, equal the magnificent display of streams and sparks obtained
+from such a disruptive discharge coil when properly adjusted.
+To give an idea, a coil such as the present one will cover easily
+a plate of one metre in diameter completely with the streams.
+The best way to perform such experiments is to take a very thin
+rubber or a glass plate and glue on one side of it a narrow ring
+of tinfoil of very large diameter, and on the other a circular
+washer, the centre of the latter coinciding with that of the ring,
+and the surfaces of both being preferably equal, so as to keep
+the coil well balanced. The washer and ring should be connected
+to the terminals by heavily insulated thin wires. It is easy in
+observing the effect of the capacity to produce a sheet of uniform
+streams, or a fine network of thin silvery threads, or a
+mass of loud brilliant sparks, which completely cover the plate.</p>
+
+<p>Since I have advanced the idea of the conversion by means of
+the disruptive discharge, in my paper before the American Institute
+of Electrical Engineers at the beginning of the past year,
+the interest excited in it has been considerable. It affords us a
+means for producing any potentials by the aid of inexpensive
+coils operated from ordinary systems of distribution, and&mdash;what
+is perhaps more appreciated&mdash;it enables us to convert currents of<span class='pagenum'><a name="Page_221" id="Page_221">[Pg 221]</a></span>
+any frequency into currents of any other lower or higher frequency.
+But its chief value will perhaps be found in the help
+which it will afford us in the investigations of the phenomena
+of phosphorescence, which a disruptive discharge coil is capable
+of exciting in innumerable cases where ordinary coils, even the
+largest, would utterly fail.</p>
+
+<p>Considering its probable uses for many practical purposes, and
+its possible introduction into laboratories for scientific research,
+a few additional remarks as to the construction of such a coil
+will perhaps not be found superfluous.</p>
+
+<p>It is, of course, absolutely necessary to employ in such a coil
+wires provided with the best insulation.</p>
+
+<p>Good coils may be produced by employing wires covered with
+several layers of cotton, boiling the coil a long time in pure wax,
+and cooling under moderate pressure. The advantage of such a
+coil is that it can be easily handled, but it cannot probably give
+as satisfactory results as a coil immersed in pure oil. Besides, it
+seems that the presence of a large body of wax affects the coil
+disadvantageously, whereas this does not seem to be the case with
+oil. Perhaps it is because the dielectric losses in the liquid are
+smaller.</p>
+
+<p>I have tried at first silk and cotton covered wires with oil immersions,
+but I have been gradually led to use gutta-percha
+covered wires, which proved most satisfactory. Gutta-percha
+insulation adds, of course, to the capacity of the coil, and this,
+especially if the coil be large, is a great disadvantage when extreme
+frequencies are desired; but, on the other hand, gutta-percha
+will withstand much more than an equal thickness of oil,
+and this advantage should be secured at any price. Once the
+coil has been immersed, it should never be taken out of the oil
+for more than a few hours, else the gutta-percha will crack up
+and the coil will not be worth half as much as before. Gutta-percha
+is probably slowly attacked by the oil, but after an immersion
+of eight to nine months I have found no ill effects.</p>
+
+<p>I have obtained two kinds of gutta-percha wire known in commerce:
+in one the insulation sticks tightly to the metal, in the
+other it does not. Unless a special method is followed to expel all
+air, it is much safer to use the first kind. I wind the coil within
+an oil tank so that all interstices are filled up with the oil. Between
+the layers I use cloth boiled out thoroughly in oil,
+calculating the thickness according to the difference of potential<span class='pagenum'><a name="Page_222" id="Page_222">[Pg 222]</a></span>
+between the turns. There seems not to be a very great difference
+whatever kind of oil is used; I use paraffine or linseed oil.</p>
+
+<p>To exclude more perfectly the air, an excellent way to proceed,
+and easily practicable with small coils, is the following:
+Construct a box of hardwood of very thick boards which have
+been for a long time boiled in oil. The boards should be so
+joined as to safely withstand the external air pressure. The coil
+being placed and fastened in position within the box, the latter
+is closed with a strong lid, and covered with closely fitting metal
+sheets, the joints of which are soldered very carefully. On the
+top two small holes are drilled, passing through the metal sheet
+and the wood, and in these holes two small glass tubes are inserted
+and the joints made air-tight. One of the tubes is connected
+to a vacuum pump, and the other with a vessel containing a
+sufficient quantity of boiled-out oil. The latter tube has a very
+small hole at the bottom, and is provided with a stopcock.
+When a fairly good vacuum has been obtained, the stopcock is
+opened and the oil slowly fed in. Proceeding in this manner,
+it is impossible that any big bubbles, which are the principal
+danger, should remain between the turns. The air is most completely
+excluded, probably better than by boiling out, which,
+however, when gutta-percha coated wires are used, is not practicable.</p>
+
+<p>For the primaries I use ordinary line wire with a thick cotton
+coating. Strands of very thin insulated wires properly interlaced
+would, of course, be the best to employ for the primaries,
+but they are not to be had.</p>
+
+<p>In an experimental coil the size of the wires is not of great
+importance. In the coil here used the primary is No. 12 and the
+secondary No. 24 Brown &amp; Sharpe gauge wire; but the sections
+may be varied considerably. It would only imply different adjustments;
+the results aimed at would not be materially affected.</p>
+
+<p>I have dwelt at some length upon the various forms of brush
+discharge because, in studying them, we not only observe phenomena
+which please our eye, but also afford us food for thought,
+and lead us to conclusions of practical importance. In the use
+of alternating currents of very high tension, too much precaution
+cannot be taken to prevent the brush discharge. In a main conveying
+such currents, in an induction coil or transformer, or in a
+condenser, the brush discharge is a source of great danger to the
+insulation. In a condenser, especially, the gaseous matter must<span class='pagenum'><a name="Page_223" id="Page_223">[Pg 223]</a></span>
+be most carefully expelled, for in it the charged surfaces are near
+each other, and if the potentials are high, just as sure as a weight
+will fall if let go, so the insulation will give way if a single
+gaseous bubble of some size be present, whereas, if all gaseous
+matter were carefully excluded, the condenser would safely
+withstand a much higher difference of potential. A main conveying
+alternating currents of very high tension may be injured
+merely by a blow hole or small crack in the insulation, the more
+so as a blowhole is apt to contain gas at low pressure; and as it
+appears almost impossible to completely obviate such little imperfections,
+I am led to believe that in our future distribution of
+electrical energy by currents of very high tension, liquid insulation
+will be used. The cost is a great drawback, but if we employ
+an oil as an insulator the distribution of electrical energy
+with something like 100,000 volts, and even more, becomes, at
+least with higher frequencies, so easy that it could be hardly
+called an engineering feat. With oil insulation and alternate current
+motors, transmissions of power can be affected with safety
+and upon an industrial basis at distances of as much as a thousand
+miles.</p>
+
+<p>A peculiar property of oils, and liquid insulation in general,
+when subjected to rapidly changing electric stresses, is to disperse
+any gaseous bubbles which may be present, and diffuse them
+through its mass, generally long before any injurious break can
+occur. This feature may be easily observed with an ordinary induction
+coil by taking the primary out, plugging up the end of
+the tube upon which the secondary is wound, and filling it with
+some fairly transparent insulator, such as paraffine oil. A primary
+of a diameter something like six millimetres smaller than the
+inside of the tube may be inserted in the oil. When the coil is
+set to work one may see, looking from the top through the oil,
+many luminous points&mdash;air bubbles which are caught by inserting
+the primary, and which are rendered luminous in consequence
+of the violent bombardment. The occluded air, by its impact
+against the oil, heats it; the oil begins to circulate, carrying some
+of the air along with it, until the bubbles are dispersed and the
+luminous points disappear. In this manner, unless large bubbles
+are occluded in such way that circulation is rendered impossible,
+a damaging break is averted, the only effect being a moderate
+warming up of the oil. If, instead of the liquid, a solid insulation,
+no matter how thick, were used, a breaking through and injury
+of the apparatus would be inevitable.<span class='pagenum'><a name="Page_224" id="Page_224">[Pg 224]</a></span></p>
+
+<p>The exclusion of gaseous matter from any apparatus in which
+the dielectric is subjected to more or less rapidly changing electric
+forces is, however, not only desirable in order to avoid a
+possible injury of the apparatus, but also on account of economy.
+In a condenser, for instance, as long as only a solid or only a
+liquid dielectric is used, the loss is small; but if a gas under ordinary
+or small pressure be present the loss may be very great.
+Whatever the nature of the force acting in the dielectric may be,
+it seems that in a solid or liquid the molecular displacement produced
+by the force is small: hence the product of force and
+displacement is insignificant, unless the force be very great; but
+in a gas the displacement, and therefore this product, is considerable;
+the molecules are free to move, they reach high speeds, and
+the energy of their impact is lost in heat or otherwise. If the
+gas be strongly compressed, the displacement due to the force is
+made smaller, and the losses are reduced.</p>
+
+<p>In most of the succeeding experiments I prefer, chiefly on
+account of the regular and positive action, to employ the alternator
+before referred to. This is one of the several machines
+constructed by me for the purpose of these investigations. It has
+384 pole projections, and is capable of giving currents of a frequency
+of about 10,000 per second. This machine has been illustrated
+and briefly described in my first paper before the American
+Institute of Electrical Engineers, May 20th, 1891, to which I have
+already referred. A more detailed description, sufficient to enable
+any engineer to build a similar machine, will be found in
+several electrical journals of that period.</p>
+
+<p>The induction coils operated from the machine are rather small,
+containing from 5,000 to 15,000 turns in the secondary. They
+are immersed in boiled-out linseed oil, contained in wooden boxes
+covered with zinc sheet.</p>
+
+<p>I have found it advantageous to reverse the usual position of
+the wires, and to wind, in these coils, the primaries on the top;
+thus allowing the use of a much larger primary, which, of course,
+reduces the danger of overheating and increases the output of
+the coil. I make the primary on each side at least one centimetre
+shorter than the secondary, to prevent the breaking through on the
+ends, which would surely occur unless the insulation on the top
+of the secondary be very thick, and this, of course, would be disadvantageous.</p>
+
+<p>When the primary is made movable, which is necessary in<span class='pagenum'><a name="Page_225" id="Page_225">[Pg 225]</a></span>
+some experiments, and many times convenient for the purposes
+of adjustment, I cover the secondary with wax, and turn it off
+in a lathe to a diameter slightly smaller than the inside of the
+primary coil. The latter I provide with a handle reaching out
+of the oil, which serves to shift it in any position along the
+secondary.</p>
+
+<p>I will now venture to make, in regard to the general manipulation
+of induction coils, a few observations bearing upon points
+which have not been fully appreciated in earlier experiments
+with such coils, and are even now often overlooked.</p>
+
+<p>The secondary of the coil possesses usually such a high self-induction
+that the current through the wire is inappreciable, and
+may be so even when the terminals are joined by a conductor of
+small resistance. If capacity is added to the terminals, the self-induction
+is counteracted, and a stronger current is made to flow
+through the secondary, though its terminals are insulated from
+each other. To one entirely unacquainted with the properties of
+alternating currents nothing will look more puzzling. This feature
+was illustrated in the experiment performed at the beginning
+with the top plates of wire gauze attached to the terminals and
+the rubber plate. When the plates of wire gauze were close together,
+and a small arc passed between them, the arc <i>prevented</i> a
+strong current from passing through the secondary, because it
+did away with the capacity on the terminals; when the rubber
+plate was inserted between, the capacity of the condenser formed
+counteracted the self-induction of the secondary, a stronger current
+passed now, the coil performed more work, and the discharge
+was by far more powerful.</p>
+
+<p>The first thing, then, in operating the induction coil is to combine
+capacity with the secondary to overcome the self-induction.
+If the frequencies and potentials are very high, gaseous matter
+should be carefully kept away from the charged surfaces. If
+Leyden jars are used, they should be immersed in oil, as otherwise
+considerable dissipation may occur if the jars are greatly
+strained. When high frequencies are used, it is of equal importance
+to combine a condenser with the primary. One may
+use a condenser connected to the ends of the primary or to the
+terminals of the alternator, but the latter is not to be recommended,
+as the machine might be injured. The best way is
+undoubtedly to use the condenser in series with the primary and
+with the alternator, and to adjust its capacity so as to annul the<span class='pagenum'><a name="Page_226" id="Page_226">[Pg 226]</a></span>
+self-induction of both the latter. The condenser should be adjustable
+by very small steps, and for a finer adjustment a small
+oil condenser with movable plates may be used conveniently.</p>
+
+<p>I think it best at this juncture to bring before you a phenomenon,
+observed by me some time ago, which to the purely
+scientific investigator may perhaps appear more interesting than
+any of the results which I have the privilege to present to you
+this evening.</p>
+
+<p>It may be quite properly ranked among the brush phenomena&mdash;in
+fact, it is a brush, formed at, or near, a single terminal
+in high vacuum.</p>
+
+<div class="figcenter" style="width: 748px;">
+<img src="images/oi_240.jpg" width="748" height="600" alt="Fig. 141, 142." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 141.</td><td class="caption">Fig. 142.</td></tr>
+</table>
+</div>
+
+<p>In bulbs provided with a conducting terminal, though it be of
+aluminum, the brush has but an ephemeral existence, and cannot,
+unfortunately, be indefinitely preserved in its most sensitive
+state, even in a bulb devoid of any conducting electrode.
+In studying the phenomenon, by all means a bulb having no
+leading-in wire should be used. I have found it best to use
+bulbs constructed as indicated in Figs. 141 and 142.</p>
+
+<p>In Fig. 141 the bulb comprises an incandescent lamp globe <i>L</i>,
+in the neck of which is sealed a barometer tube <i>b</i>, the end of which
+is blown out to form a small sphere <i>s</i>. This sphere should be
+sealed as closely as possible in the centre of the large globe.
+Before sealing, a thin tube <i>t</i>, of aluminum sheet, may be slipped
+in the barometer tube, but it is not important to employ it.<span class='pagenum'><a name="Page_227" id="Page_227">[Pg 227]</a></span></p>
+
+<p>The small hollow sphere <i>s</i> is filled with some conducting
+powder, and a wire <i>w</i> is cemented in the neck for the purpose of
+connecting the conducting powder with the generator.</p>
+
+<p>The construction shown in Fig. 142 was chosen in order to
+remove from the brush any conducting body which might possibly
+affect it. The bulb consists in this case of a lamp globe <i>L</i>,
+which has a neck <i>n</i>, provided with a tube <i>b</i> and small sphere <i>s</i>,
+sealed to it, so that two entirely independent compartments are
+formed, as indicated in the drawing. When the bulb is in use
+the neck <i>n</i> is provided with a tinfoil coating, which is connected
+to the generator and acts inductively upon the moderately rarefied
+and highly conducted gas inclosed in the neck. From there
+the current passes through the tube <i>b</i> into the small sphere <i>s</i>, to
+act by induction upon the gas contained in the globe <i>L</i>.</p>
+
+<p>It is of advantage to make the tube <i>t</i> very thick, the hole
+through it very small, and to blow the sphere <i>s</i> very thin. It is
+of the greatest importance that the sphere <i>s</i> be placed in the
+centre of the globe <i>L</i>.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_241.jpg" width="480" height="548" alt="Fig. 143." title="" />
+<span class="caption">Fig. 143.</span>
+</div>
+
+
+<p>Figs. 143, 144 and 145 indicate different forms, or stages, of
+the brush. Fig. 143 shows the brush as it first appears in a bulb
+provided with a conducting terminal; but, as in such a bulb it
+very soon disappears&mdash;often after a few minutes&mdash;I will confine
+myself to the description of the phenomenon as seen in a bulb
+without conducting electrode. It is observed under the following
+conditions:</p>
+
+<p>When the globe <i>L</i> (Figs. 141 and 142) is exhausted to a very
+high degree, generally the bulb is not excited upon connecting
+the wire <i>w</i> (Fig. 141) or the tinfoil coating of the bulb (Fig.<span class='pagenum'><a name="Page_228" id="Page_228">[Pg 228]</a></span>
+142) to the terminal of the induction coil. To excite it, it is
+usually sufficient to grasp the globe <i>L</i> with the hand. An intense
+phosphorescence then spreads at first over the globe, but
+soon gives place to a white, misty light. Shortly afterward one
+may notice that the luminosity is unevenly distributed in the
+globe, and after passing the current for some time the bulb appears
+as in Fig. 144. From this stage the phenomenon will
+gradually pass to that indicated in Fig. 145, after some minutes,
+hours, days or weeks, according as the bulb is worked. Warming
+the bulb or increasing the potential hastens the transit.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_242.jpg" width="800" height="530" alt="Fig. 144, 145." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 144.</td><td class="caption">Fig. 145.</td></tr>
+</table>
+</div>
+
+<p>When the brush assumes the form indicated in Fig. 145, it may
+be brought to a state of extreme sensitiveness to electrostatic
+and magnetic influence. The bulb hanging straight down from
+a wire, and all objects being remote from it, the approach of the
+observer at a few paces from the bulb will cause the brush to fly
+to the opposite side, and if he walks around the bulb it will
+always keep on the opposite side. It may begin to spin around
+the terminal long before it reaches that sensitive stage. When
+it begins to turn around, principally, but also before, it is affected
+by a magnet, and at a certain stage it is susceptible to magnetic
+influence to an astonishing degree. A small permanent magnet,
+with its poles at a distance of no more than two centimetres, will
+affect it visibly at a distance of two metres, slowing down or accelerating
+the rotation according to how it is held relatively to<span class='pagenum'><a name="Page_229" id="Page_229">[Pg 229]</a></span>
+the brush. I think I have observed that at the stage when it is
+most sensitive to magnetic, it is not most sensitive to electrostatic,
+influence. My explanation is, that the electrostatic attraction
+between the brush and the glass of the bulb, which retards the
+rotation, grows much quicker than the magnetic influence when
+the intensity of the stream is increased.</p>
+
+<p>When the bulb hangs with the globe <i>L</i> down, the rotation is
+always clockwise. In the southern hemisphere it would occur
+in the opposite direction and on the equator the brush should
+not turn at all. The rotation may be reversed by a magnet kept
+at some distance. The brush rotates best, seemingly, when it is
+at right angles to the lines of force of the earth. It very likely
+rotates, when at its maximum speed, in synchronism with the
+alternations, say, 10,000 times a second. The rotation can be
+slowed down or accelerated by the approach or receding of the
+observer, or any conducting body, but it cannot be reversed by
+putting the bulb in any position. When it is in the state of the
+highest sensitiveness and the potential or frequency be varied,
+the sensitiveness is rapidly diminished. Changing either of
+these but little will generally stop the rotation. The sensitiveness
+is likewise affected by the variations of temperature. To
+attain great sensitiveness it is necessary to have the small sphere
+<i>s</i> in the centre of the globe <i>L</i>, as otherwise the electrostatic
+action of the glass of the globe will tend to stop the rotation.
+The sphere <i>s</i> should be small and of uniform thickness; any dissymmetry
+of course has the effect to diminish the sensitiveness.</p>
+
+<p>The fact that the brush rotates in a definite direction in a permanent
+magnetic field seems to show that in alternating currents
+of very high frequency the positive and negative impulses are
+not equal, but that one always preponderates over the other.</p>
+
+<p>Of course, this rotation in one direction may be due to the
+action of the two elements of the same current upon each other,
+or to the action of the field produced by one of the elements
+upon the other, as in a series motor, without necessarily one impulse
+being stronger than the other. The fact that the brush
+turns, as far as I could observe, in any position, would speak for
+this view. In such case it would turn at any point of the earth's
+surface. But, on the other hand, it is then hard to explain why
+a permanent magnet should reverse the rotation, and one must
+assume the preponderance of impulses of one kind.</p>
+
+<p>As to the causes of the formation of the brush or stream, I<span class='pagenum'><a name="Page_230" id="Page_230">[Pg 230]</a></span>
+think it is due to the electrostatic action of the globe and the
+dissymmetry of the parts. If the small bulb <i>s</i> and the globe <i>L</i>
+were perfect concentric spheres, and the glass throughout of the
+same thickness and quality, I think the brush would not form,
+as the tendency to pass would be equal on all sides. That the
+formation of the stream is due to an irregularity is apparent from
+the fact that it has the tendency to remain in one position, and
+rotation occurs most generally only when it is brought out of
+this position by electrostatic or magnetic influence. When in an
+extremely sensitive state it rests in one position, most curious experiments
+may be performed with it. For instance, the experimenter
+may, by selecting a proper position, approach the hand
+at a certain considerable distance to the bulb, and he may cause
+the brush to pass off by merely stiffening the muscles of the arm.
+When it begins to rotate slowly, and the hands are held at a
+proper distance, it is impossible to make even the slightest motion
+without producing a visible effect upon the brush. A metal
+plate connected to the other terminal of the coil affects it at a
+great distance, slowing down the rotation often to one turn a
+second.</p>
+
+<p>I am firmly convinced that such a brush, when we learn how
+to produce it properly, will prove a valuable aid in the investigation
+of the nature of the forces acting in an electrostatic or
+magnetic field. If there is any motion which is measurable going
+on in the space, such a brush ought to reveal it. It is, so to
+speak, a beam of light, frictionless, devoid of inertia.</p>
+
+<p>I think that it may find practical applications in telegraphy.
+With such a brush it would be possible to send dispatches across
+the Atlantic, for instance, with any speed, since its sensitiveness
+may be so great that the slightest changes will affect it. If it
+were possible to make the stream more intense and very narrow,
+its deflections could be easily photographed.</p>
+
+<p>I have been interested to find whether there is a rotation of
+the stream itself, or whether there is simply a stress traveling
+around the bulb. For this purpose I mounted a light mica fan
+so that its vanes were in the path of the brush. If the stream
+itself was rotating the fan would be spun around. I could produce
+no distinct rotation of the fan, although I tried the experiment
+repeatedly; but as the fan exerted a noticeable influence
+on the stream, and the apparent rotation of the latter was, in this
+case, never quite satisfactory, the experiment did not appear to
+be conclusive.<span class='pagenum'><a name="Page_231" id="Page_231">[Pg 231]</a></span></p>
+
+<p>I have been unable to produce the phenomenon with the disruptive
+discharge coil, although every other of these phenomena
+can be well produced by it&mdash;many, in fact, much better than
+with coils operated from an alternator.</p>
+
+<p>It may be possible to produce the brush by impulses of one
+direction, or even by a steady potential, in which case it would
+be still more sensitive to magnetic influence.</p>
+
+<p>In operating an induction coil with rapidly alternating currents,
+we realize with astonishment, for the first time, the great importance
+of the relation of capacity, self-induction and frequency as
+regards the general results. The effects of capacity are the most
+striking, for in these experiments, since the self-induction and
+frequency both are high, the critical capacity is very small, and
+need be but slightly varied to produce a very considerable change.
+The experimenter may bring his body in contact with the terminals
+of the secondary of the coil, or attach to one or both terminals
+insulated bodies of very small bulk, such as bulbs, and he
+may produce a considerable rise or fall of potential, and greatly
+affect the flow of the current through the primary. In the experiment
+before shown, in which a brush appears at a wire
+attached to one terminal, and the wire is vibrated when the experimenter
+brings his insulated body in contact with the other
+terminal of the coil, the sudden rise of potential was made evident.</p>
+
+<p>I may show you the behavior of the coil in another manner
+which possesses a feature of some interest. I have here a little light
+fan of aluminum sheet, fastened to a needle and arranged to
+rotate freely in a metal piece screwed to one of the terminals of
+the coil. When the coil is set to work, the molecules of the air
+are rhythmically attracted and repelled. As the force with
+which they are repelled is greater than that with which they are
+attracted, it results that there is a repulsion exerted on the surfaces
+of the fan. If the fan were made simply of a metal sheet,
+the repulsion would be equal on the opposite sides, and would
+produce no effect. But if one of the opposing surfaces is screened,
+or if, generally speaking, the bombardment on this side is
+weakened in some way or other, there remains the repulsion exerted
+upon the other, and the fan is set in rotation. The screening
+is best effected by fastening upon one of the opposing sides
+of the fan insulated conducting coatings, or, if the fan is made
+in the shape of an ordinary propeller screw, by fastening on one<span class='pagenum'><a name="Page_232" id="Page_232">[Pg 232]</a></span>
+side, and close to it, an insulated metal plate. The static screen
+may, however, be omitted, and simply a thickness of insulating
+material fastened to one of the sides of the fan.</p>
+
+<p>To show the behavior of the coil, the fan may be placed upon
+the terminal and it will readily rotate when the coil is operated
+by currents of very high frequency. With a steady potential,
+of course, and even with alternating currents of very low frequency,
+it would not turn, because of the very slow exchange of
+air and, consequently, smaller bombardment; but in the latter
+case it might turn if the potential were excessive. With a pin
+wheel, quite the opposite rule holds good; it rotates best with
+a steady potential, and the effort is the smaller the higher the
+frequency. Now, it is very easy to adjust the conditions so that
+the potential is normally not sufficient to turn the fan, but that
+by connecting the other terminal of the coil with an insulated
+body it rises to a much greater value, so as to rotate the fan, and
+it is likewise possible to stop the rotation by connecting to the
+terminal a body of different size, thereby diminishing the potential.</p>
+
+<p>Instead of using the fan in this experiment, we may use the
+"electric" radiometer with similar effect. But in this case it will
+be found that the vanes will rotate only at high exhaustion or at
+ordinary pressures; they will not rotate at moderate pressures,
+when the air is highly conducting. This curious observation was
+made conjointly by Professor Crookes and myself. I attribute
+the result to the high conductivity of the air, the molecules of
+which then do not act as independent carriers of electric charges,
+but act all together as a single conducting body. In such case,
+of course, if there is any repulsion at all of the molecules from
+the vanes, it must be very small. It is possible, however, that
+the result is in part due to the fact that the greater part of the
+discharge passes from the leading-in wire through the highly conducting
+gas, instead of passing off from the conducting vanes.</p>
+
+<p>In trying the preceding experiment with the electric radiometer
+the potential should not exceed a certain limit, as then the electrostatic
+attraction between the vanes and the glass of the bulb
+may be so great as to stop the rotation.</p>
+
+<p>A most curious feature of alternate currents of high frequencies
+and potentials is that they enable us to perform many experiments
+by the use of one wire only. In many respects this feature
+is of great interest.<span class='pagenum'><a name="Page_233" id="Page_233">[Pg 233]</a></span></p>
+
+<p>In a type of alternate current motor invented by me some years
+ago I produced rotation by inducing, by means of a single alternating
+current passed through a motor circuit, in the mass or other
+circuits of the motor, secondary currents, which, jointly with the
+primary or inducing current, created a moving field of force. A
+simple but crude form of such a motor is obtained by winding
+upon an iron core a primary, and close to it a secondary coil, joining
+the ends of the latter and placing a freely movable metal disc
+within the influence of the field produced by both. The iron core
+is employed for obvious reasons, but it is not essential to the
+operation. To improve the motor, the iron core is made to encircle
+the armature. Again to improve, the secondary coil is
+made to partly overlap the primary, so that it cannot free itself
+from a strong inductive action of the latter, repel its lines as it
+may. Once more to improve, the proper difference of phase is
+obtained between the primary and secondary currents by a condenser,
+self-induction, resistance or equivalent windings.</p>
+
+<p>I had discovered, however, that rotation is produced by means
+of a single coil and core; my explanation of the phenomenon, and
+leading thought in trying the experiment, being that there must
+be a true time lag in the magnetization of the core. I remember
+the pleasure I had when, in the writings of Professor Ayrton,
+which came later to my hand, I found the idea of the time lag
+advocated. Whether there is a true time lag, or whether the retardation
+is due to eddy currents circulating in minute paths, must
+remain an open question, but the fact is that a coil wound upon
+an iron core and traversed by an alternating current creates a
+moving field of force, capable of setting an armature in rotation.
+It is of some interest, in conjunction with the historical Arago
+experiment, to mention that in lag or phase motors I have produced
+rotation in the opposite direction to the moving field, which
+means that in that experiment the magnet may not rotate, or may
+even rotate in the opposite direction to the moving disc. Here,
+then, is a motor (diagrammatically illustrated in Fig. 146), comprising
+a coil and iron core, and a freely movable copper disc in
+proximity to the latter.</p>
+
+<div class="figcenter" style="width: 613px;">
+<img src="images/oi_248.jpg" width="613" height="600" alt="Fig. 146." title="" />
+<span class="caption">Fig. 146.</span>
+</div>
+
+
+<p>To demonstrate a novel and interesting feature, I have, for a
+reason which I will explain, selected this type of motor. When
+the ends of the coil are connected to the terminals of an alternator
+the disc is set in rotation. But it is not this experiment,
+now well known, which I desire to perform. What I wish to
+<span class='pagenum'><a name="Page_234" id="Page_234">[Pg 234]</a></span>show you is that this motor rotates with <i>one single</i> connection between
+it and the generator; that is to say, one terminal of the
+motor is connected to one terminal of the generator&mdash;in this case
+the secondary of a high-tension induction coil&mdash;the other terminals
+of motor and generator being insulated in space. To produce
+rotation it is generally (but not absolutely) necessary to
+connect the free end of the motor coil to an insulated body of
+some size. The experimenter's body is more than sufficient. If
+he touches the free terminal with an object held in the hand, a
+current passes through the coil and the copper disc is set in rotation.
+If an exhausted tube is put in series with the coil, the tube
+lights brilliantly, showing the passage of a strong current. Instead
+of the experimenter's body, a small metal sheet suspended
+on a cord may be used with the same result. In this case the
+plate acts as a condenser in series with the coil. It counteracts
+the self-induction of the latter and allows a strong current to
+pass. In such a combination, the greater the self-induction of
+the coil the smaller need be the plate, and this means that a lower
+frequency, or eventually a lower potential, is required to operate
+the motor. A single coil wound upon a core has a high self-induction;
+for this reason, principally, this type of motor was
+chosen to perform the experiment. Were a secondary closed
+coil wound upon the core, it would tend to diminish the
+self-<span class='pagenum'><a name="Page_235" id="Page_235">[Pg 235]</a></span>induction, and then it would be necessary to employ a much
+higher frequency and potential. Neither would be advisable, for
+a higher potential would endanger the insulation of the small
+primary coil, and a higher frequency would result in a materially
+diminished torque.</p>
+
+<p>It should be remarked that when such a motor with a
+closed secondary is used, it is not at all easy to obtain rotation
+with excessive frequencies, as the secondary cuts off
+almost completely the lines of the primary&mdash;and this, of
+course, the more, the higher the frequency&mdash;and allows the passage
+of but a minute current. In such a case, unless the secondary
+is closed through a condenser, it is almost essential, in order
+to produce rotation, to make the primary and secondary coils
+overlap each other more or less.</p>
+
+<p>But there is an additional feature of interest about this motor,
+namely, it is not necessary to have even a single connection between
+the motor and generator, except, perhaps, through the
+ground; for not only is an insulated plate capable of giving off
+energy into space, but it is likewise capable of deriving it from
+an alternating electrostatic field, though in the latter case the
+available energy is much smaller. In this instance one of the
+motor terminals is connected to the insulated plate or body
+located within the alternating electrostatic field, and the other
+terminal preferably to the ground.</p>
+
+<p>It is quite possible, however, that such "no wire" motors, as
+they might be called, could be operated by conduction through
+the rarefied air at considerable distances. Alternate currents,
+especially of high frequencies, pass with astonishing freedom
+through even slightly rarefied gases. The upper strata of the air
+are rarefied. To reach a number of miles out into space requires
+the overcoming of difficulties of a merely mechanical nature.
+There is no doubt that with the enormous potentials obtainable by
+the use of high frequencies and oil insulation, luminous discharges
+might be passed through many miles of rarefied air, and that, by
+thus directing the energy of many hundreds or thousands of horse-power,
+motors or lamps might be operated at considerable
+distances from stationary sources. But such schemes are mentioned
+merely as possibilities. We shall have no need to transmit
+power in this way. We shall have no need to <i>transmit</i> power
+at all. Ere many generations pass, our machinery will be driven
+by a power obtainable at any point of the universe. This idea is<span class='pagenum'><a name="Page_236" id="Page_236">[Pg 236]</a></span>
+not novel. Men have been led to it long ago by instinct or reason.
+It has been expressed in many ways, and in many places, in the
+history of old and new. We find it in the delightful myth of
+Antheus, who derives power from the earth; we find it among
+the subtle speculations of one of your splendid mathematicians,
+and in many hints and statements of thinkers of the present time.
+Throughout space there is energy. Is this energy static or kinetic?
+If static our hopes are in vain; if kinetic&mdash;and this we know it
+is, for certain&mdash;then it is a mere question of time when men will
+succeed in attaching their machinery to the very wheelwork of
+nature. Of all, living or dead, Crookes came nearest to doing it.
+His radiometer will turn in the light of day and in the darkness
+of the night; it will turn everywhere where there is heat, and
+heat is everywhere. But, unfortunately, this beautiful little
+machine, while it goes down to posterity as the most interesting,
+must likewise be put on record as the most inefficient machine
+ever invented!</p>
+
+<p>The preceding experiment is only one of many equally interesting
+experiments which may be performed by the use of only
+one wire with alternations of high potential and frequency. We
+may connect an insulated line to a source of such currents, we
+may pass an inappreciable current over the line, and on any
+point of the same we are able to obtain a heavy current, capable
+of fusing a thick copper wire. Or we may, by the help of some
+artifice, decompose a solution in any electrolytic cell by connecting
+only one pole of the cell to the line or source of energy.
+Or we may, by attaching to the line, or only bringing into its
+vicinity, light up an incandescent lamp, an exhausted tube, or a
+phosphorescent bulb.</p>
+
+<p>However impracticable this plan of working may appear in
+many cases, it certainly seems practicable, and even recommendable,
+in the production of light. A perfected lamp would require
+but little energy, and if wires were used at all we ought to be able
+to supply that energy without a return wire.</p>
+
+<p>It is now a fact that a body may be rendered incandescent or
+phosphorescent by bringing it either in single contact or merely
+in the vicinity of a source of electric impulses of the proper
+character, and that in this manner a quantity of light sufficient
+to afford a practical illuminant may be produced. It is, therefore,
+to say the least, worth while to attempt to determine the
+best conditions and to invent the best appliances for attaining
+this object.<span class='pagenum'><a name="Page_237" id="Page_237">[Pg 237]</a></span></p>
+
+<p>Some experiences have already been gained in this direction,
+and I will dwell on them briefly, in the hope that they might
+prove useful.</p>
+
+<p>The heating of a conducting body inclosed in a bulb, and connected
+to a source of rapidly alternating electric impulses, is
+dependent on so many things of a different nature, that it would
+be difficult to give a generally applicable rule under which the
+maximum heating occurs. As regards the size of the vessel, I
+have lately found that at ordinary or only slightly differing
+atmospheric pressures, when air is a good insulator, and hence
+practically the same amount of energy by a certain potential and
+frequency is given off from the body, whether the bulb be small
+or large, the body is brought to a higher temperature if enclosed
+in a small bulb, because of the better confinement of heat in this
+case.</p>
+
+<p>At lower pressures, when air becomes more or less conducting,
+or if the air be sufficiently warmed to become conducting, the
+body is rendered more intensely incandescent in a large bulb,
+obviously because, under otherwise equal conditions of test, more
+energy may be given off from the body when the bulb is large.</p>
+
+<p>At very high degrees of exhaustion, when the matter in the
+bulb becomes "radiant," a large bulb has still an advantage, but
+a comparatively slight one, over the small bulb.</p>
+
+<p>Finally, at excessively high degrees of exhaustion, which cannot
+be reached except by the employment of special means, there
+seems to be, beyond a certain and rather small size of vessel, no
+perceptible difference in the heating.</p>
+
+<p>These observations were the result of a number of experiments,
+of which one, showing the effect of the size of the bulb at a high
+degree of exhaustion, may be described and shown here, as it
+presents a feature of interest. Three spherical bulbs of 2 inches,
+3 inches and 4 inches diameter were taken, and in the centre of
+each was mounted an equal length of an ordinary incandescent
+lamp filament of uniform thickness. In each bulb the piece of
+filament was fastened to the leading-in wire of platinum, contained
+in a glass stem sealed in the bulb; care being taken, of
+course, to make everything as nearly alike as possible. On each
+glass stem in the inside of the bulb was slipped a highly polished
+tube made of aluminum sheet, which fitted the stem and was held
+on it by spring pressure. The function of this aluminum tube will
+be explained subsequently. In each bulb an equal length of fila<span class='pagenum'><a name="Page_238" id="Page_238">[Pg 238]</a></span>ment
+protruded above the metal tube. It is sufficient to say now
+that under these conditions equal lengths of filament of the same
+thickness&mdash;in other words, bodies of equal bulk&mdash;were brought
+to incandescence. The three bulbs were sealed to a glass tube,
+which was connected to a Sprengel pump. When a high vacuum
+had been reached, the glass tube carrying the bulbs was sealed
+off. A current was then turned on successively on each bulb,
+and it was found that the filaments came to about the same
+brightness, and, if anything, the smallest bulb, which was placed
+midway between the two larger ones, may have been slightly
+brighter. This result was expected, for when either of the bulbs
+was connected to the coil the luminosity spread through the
+other two, hence the three bulbs constituted really one vessel.
+When all the three bulbs were connected in multiple arc to the
+coil, in the largest of them the filament glowed brightest, in the
+next smaller it was a little less bright, and in the smallest it only
+came to redness. The bulbs were then sealed off and separately
+tried. The brightness of the filaments was now such as would
+have been expected on the supposition that the energy given off
+was proportionate to the surface of the bulb, this surface in each
+case representing one of the coatings of a condenser. Accordingly,
+there was less difference between the largest and the
+middle sized than between the latter and the smallest bulb.</p>
+
+<p>An interesting observation was made in this experiment. The
+three bulbs were suspended from a straight bare wire connected
+to a terminal of a coil, the largest bulb being placed at the end
+of the wire, at some distance from it the smallest bulb, and at an
+equal distance from the latter the middle-sized one. The carbons
+glowed then in both the larger bulbs about as expected, but the
+smallest did not get its share by far. This observation led me to
+exchange the position of the bulbs, and I then observed that
+whichever of the bulbs was in the middle was by far less bright
+than it was in any other position. This mystifying result was,
+of course, found to be due to the electrostatic action between the
+bulbs. When they were placed at a considerable distance, or
+when they were attached to the corners of an equilateral triangle
+of copper wire, they glowed in about the order determined by
+their surfaces.</p>
+
+<p>As to the shape of the vessel, it is also of some importance, especially
+at high degrees of exhaustion. Of all the possible constructions,
+it seems that a spherical globe with the refractory body<span class='pagenum'><a name="Page_239" id="Page_239">[Pg 239]</a></span>
+mounted in its centre is the best to employ. By experience it
+has been demonstrated that in such a globe a refractory body of
+a given bulk is more easily brought to incandescence than when
+differently shaped bulbs are used. There is also an advantage in
+giving to the incandescent body the shape of a sphere, for self-evident
+reasons. In any case the body should be mounted in the
+centre, where the atoms rebounding from the glass collide. This
+object is best attained in the spherical bulb; but it is also attained
+in a cylindrical vessel with one or two straight filaments
+coinciding with its axis, and possibly also in parabolical or spherical
+bulbs with refractory body or bodies placed in the focus or
+foci of the same; though the latter is not probable, as the electrified
+atoms should in all cases rebound normally from the
+surface they strike, unless the speed were excessive, in which
+case they <i>would</i> probably follow the general law of reflection.
+No matter what shape the vessel may have, if the exhaustion be
+low, a filament mounted in the globe is brought to the same
+degree of incandescence in all parts; but if the exhaustion be
+high and the bulb be spherical or pear-shaped, as usual, focal
+points form and the filament is heated to a higher degree at or
+near such points.</p>
+
+<p>To illustrate the effect, I have here two small bulbs which are
+alike, only one is exhausted to a low and the other to a very high
+degree. When connected to the coil, the filament in the former
+glows uniformly throughout all its length; whereas in the latter,
+that portion of the filament which is in the centre of the bulb
+glows far more intensely than the rest. A curious point is that
+the phenomenon occurs even if two filaments are mounted in a
+bulb, each being connected to one terminal of the coil, and, what
+is still more curious, if they be very near together, provided the
+vacuum be very high. I noted in experiments with such bulbs
+that the filaments would give way usually at a certain point, and
+in the first trials I attributed it to a defect in the carbon. But
+when the phenomenon occurred many times in succession I
+recognized its real cause.</p>
+
+<p>In order to bring a refractory body inclosed in a bulb to incandescence,
+it is desirable, on account of economy, that all the
+energy supplied to the bulb from the source should reach without
+loss the body to be heated; from there, and from nowhere else,
+it should be radiated. It is, of course, out of the question to
+reach this theoretical result, but it is possible by a proper construction
+of the illuminating device to approximate it more or less.<span class='pagenum'><a name="Page_240" id="Page_240">[Pg 240]</a></span></p>
+
+<p>For many reasons, the refractory body is placed in the centre
+of the bulb, and it is usually supported on a glass stem containing
+the leading-in wire. As the potential of this wire is alternated,
+the rarefied gas surrounding the stem is acted upon inductively,
+and the glass stem is violently bombarded and heated. In this
+manner by far the greater portion of the energy supplied to the
+bulb&mdash;especially when exceedingly high frequencies are used&mdash;may
+be lost for the purpose contemplated. To obviate this loss,
+or at least to reduce it to a minimum, I usually screen the rarefied
+gas surrounding the stem from the inductive action of the leading-in
+wire by providing the stem with a tube or coating of conducting
+material. It seems beyond doubt that the best among metals to
+employ for this purpose is aluminum, on account of its many remarkable
+properties. Its only fault is that it is easily fusible,
+and, therefore, its distance from the incandescing body should be
+properly estimated. Usually, a thin tube, of a diameter somewhat
+smaller than that of the glass stem, is made of the finest
+aluminum sheet, and slipped on the stem. The tube is conveniently
+prepared by wrapping around a rod fastened in a lathe a
+piece of aluminum sheet of proper size, grasping the sheet firmly
+with clean chamois leather or blotting paper, and spinning the
+rod very fast. The sheet is wound tightly around the rod, and a
+highly polished tube of one or three layers of the sheet is obtained.
+When slipped on the stem, the pressure is generally sufficient to
+prevent it from slipping off, but, for safety, the lower edge of
+the sheet may be turned inside. The upper inside corner of the
+sheet&mdash;that is, the one which is nearest to the refractory incandescent
+body&mdash;should be cut out diagonally, as it often happens
+that, in consequence of the intense heat, this corner turns toward
+the inside and comes very near to, or in contact with, the wire, or
+filament, supporting the refractory body. The greater part of
+the energy supplied to the bulb is then used up in heating the
+metal tube, and the bulb is rendered useless for the purpose.
+The aluminum sheet should project above the glass stem more or
+less&mdash;one inch or so&mdash;or else, if the glass be too close to the incandescing
+body, it may be strongly heated and become more or
+less conducting, whereupon it may be ruptured, or may, by its
+conductivity, establish a good electrical connection between the
+metal tube and the leading-in wire, in which case, again, most of
+the energy will be lost in heating the former. Perhaps the best
+way is to make the top of the glass tube, for about an inch, of a<span class='pagenum'><a name="Page_241" id="Page_241">[Pg 241]</a></span>
+much smaller diameter. To still further reduce the danger
+arising from the heating of the glass stem, and also with the view
+of preventing an electrical connection between the metal tube
+and the electrode, I preferably wrap the stem with several layers
+of thin mica, which extends at least as far as the metal tube. In
+some bulbs I have also used an outside insulating cover.</p>
+
+<p>The preceding remarks are only made to aid the experimenter
+in the first trials, for the difficulties which he encounters he may
+soon find means to overcome in his own way.</p>
+
+<p>To illustrate the effect of the screen, and the advantage of
+using it, I have here two bulbs of the same size, with their stems,
+leading-in wires and incandescent lamp filaments tied to the latter,
+as nearly alike as possible. The stem of one bulb is provided
+with an aluminum tube, the stem of the other has none. Originally
+the two bulbs were joined by a tube which was connected
+to a Sprengel pump. When a high vacuum had been reached,
+first the connecting tube, and then the bulbs, were sealed off;
+they are therefore of the same degree of exhaustion. When they
+are separately connected to the coil giving a certain potential, the
+carbon filament in the bulb provided with the aluminum screen
+is rendered highly incandescent, while the filament in the other
+bulb may, with the same potential, not even come to redness,
+although in reality the latter bulb takes generally more energy
+than the former. When they are both connected together to the
+terminal, the difference is even more apparent, showing the importance
+of the screening. The metal tube placed on the stem containing
+the leading-in wire performs really two distinct functions: First,
+it acts more or less as an electrostatic screen, thus economizing
+the energy supplied to the bulb; and, second, to whatever extent
+it may fail to act electrostatically, it acts mechanically, preventing
+the bombardment, and consequently intense heating and
+possible deterioration of the slender support of the refractory incandescent
+body, or of the glass stem containing the leading-in
+wire. I say <i>slender</i> support, for it is evident that in order to
+confine the heat more completely to the incandescing body its support
+should be very thin, so as to carry away the smallest possible
+amount of heat by conduction. Of all the supports used I have
+found an ordinary incandescent lamp filament to be the best,
+principally because among conductors it can withstand the highest
+degree of heat.</p>
+
+<p>The effectiveness of the metal tube as an electrostatic screen
+depends largely on the degree of exhaustion.<span class='pagenum'><a name="Page_242" id="Page_242">[Pg 242]</a></span></p>
+
+<p>At excessively high degrees of exhaustion&mdash;which are reached
+by using great care and special means in connection with the
+Sprengel pump&mdash;when the matter in the globe is in the ultra-radiant
+state, it acts most perfectly. The shadow of the upper
+edge of the tube is then sharply defined upon the bulb.</p>
+
+<p>At a somewhat lower degree of exhaustion, which is about the
+ordinary "non-striking" vacuum, and generally as long as the
+matter moves predominantly in straight lines, the screen still
+does well. In elucidation of the preceding remark it is necessary
+to state that what is a "non-striking" vacuum for a coil operated
+as ordinarily, by impulses, or currents, of low frequency, is not
+so, by far, when the coil is operated by currents of very high frequency.
+In such case the discharge may pass with great freedom
+through the rarefied gas through which a low frequency discharge
+may not pass, even though the potential be much higher.
+At ordinary atmospheric pressures just the reverse rule holds
+good: the higher the frequency, the less the spark discharge is
+able to jump between the terminals, especially if they are knobs
+or spheres of some size.</p>
+
+<p>Finally, at very low degrees of exhaustion, when the gas is well
+conducting, the metal tube not only does not act as an electrostatic
+screen, but even is a drawback, aiding to a considerable
+extent the dissipation of the energy laterally from the leading-in
+wire. This, of course, is to be expected. In this case, namely,
+the metal tube is in good electrical connection with the leading-in
+wire, and most of the bombardment is directed upon the tube.
+As long as the electrical connection is not good, the conducting
+tube is always of some advantage, for although it may not greatly
+economize energy, still it protects the support of the refractory
+button, and is the means of concentrating more energy upon the
+same.</p>
+
+<p>To whatever extent the aluminum tube performs the function
+of a screen, its usefulness is therefore limited to very high degrees
+of exhaustion when it is insulated from the electrode&mdash;that
+is, when the gas as a whole is non-conducting, and the molecules,
+or atoms, act as independent carriers of electric charges.</p>
+
+<p>In addition to acting as a more or less effective screen, in the
+true meaning of the word, the conducting tube or coating may
+also act, by reason of its conductivity, as a sort of equalizer or
+dampener of the bombardment against the stem. To be explicit,
+I assume the action to be as follows: Suppose a rhythmical bom<span class='pagenum'><a name="Page_243" id="Page_243">[Pg 243]</a></span>bardment
+to occur against the conducting tube by reason of its
+imperfect action as a screen, it certainly must happen that some
+molecules, or atoms, strike the tube sooner than others. Those
+which come first in contact with it give up their superfluous
+charge, and the tube is electrified, the electrification instantly
+spreading over its surface. But this must diminish the energy
+lost in the bombardment, for two reasons: first, the charge given
+up by the atoms spreads over a great area, and hence the electric
+density at any point is small, and the atoms are repelled with less
+energy than they would be if they struck against a good insulator;
+secondly, as the tube is electrified by the atoms which first
+come in contact with it, the progress of the following atoms
+against the tube is more or less checked by the repulsion which
+the electrified tube must exert upon the similarly electrified
+atoms. This repulsion may perhaps be sufficient to prevent a
+large portion of the atoms from striking the tube, but at any rate
+it must diminish the energy of their impact. It is clear that
+when the exhaustion is very low, and the rarefied gas well conducting,
+neither of the above effects can occur, and, on the other
+hand, the fewer the atoms, with the greater freedom they move;
+in other words, the higher the degree of exhaustion, up to a
+limit, the more telling will be both the effects.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_257.jpg" width="800" height="475" alt="Fig. 147, 148." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 147.</td><td class="caption">Fig. 148.</td></tr>
+</table>
+</div>
+
+<p>What I have just said may afford an explanation of the phenomenon
+observed by Prof. Crookes, namely, that a discharge
+through a bulb is established with much greater facility when an<span class='pagenum'><a name="Page_244" id="Page_244">[Pg 244]</a></span>
+insulator than when a conductor is present in the same. In my
+opinion, the conductor acts as a dampener of the motion of the
+atoms in the two ways pointed out; hence, to cause a visible discharge
+to pass through the bulb, a much higher potential is
+needed if a conductor, especially of much surface, be present.</p>
+
+<p>For the sake of elucidating of some of the remarks before made,
+I must now refer to Figs. 147, 148 and 149, which illustrate
+various arrangements with a type of bulb most generally used.</p>
+
+<p>Fig. 147 is a section through a spherical bulb <small>L</small>, with the glass
+stem <i>s</i>, contains the leading-in wire <i>w</i>, which has a lamp filament
+<i>l</i> fastened to it, serving to support the refractory button <i>m</i> in the
+centre. <small>M</small> is a sheet of thin mica wound in several layers around
+the stem <i>s</i>, and <i>a</i> is the aluminum tube.</p>
+
+<p>Fig. 148 illustrates such a bulb in a somewhat more advanced
+stage of perfection. A metallic tube <small>S</small> is fastened by means of
+some cement to the neck of the tube. In the tube is screwed a
+plug <small>P</small>, of insulating material, in the centre of which is fastened
+a metallic terminal <i>t</i>, for the connection to the leading-in wire <i>w</i>.
+This terminal must be well insulated from the metal tube <small>S</small>;
+therefore, if the cement used is conducting&mdash;and most generally
+it is sufficiently so&mdash;the space between the plug <small>P</small> and the neck
+of the bulb should be filled with some good insulating material,
+such as mica powder.</p>
+
+
+<p>Fig. 149 shows a bulb made for experimental purposes. In this
+bulb the aluminum tube is provided with an external connection,
+which serves to investigate the effect of the tube under various
+conditions. It is referred to chiefly to suggest a line of experiment
+followed.</p>
+
+<p>Since the bombardment against the stem containing the leading-in
+wire is due to the inductive action of the latter upon the
+rarefied gas, it is of advantage to reduce this action as far as
+practicable by employing a very thin wire, surrounded by a very
+thick insulation of glass or other material, and by making the
+wire passing through the rarefied gas as short as practicable. To
+combine these features I employ a large tube <small>T</small> (Fig. 150), which
+protrudes into the bulb to some distance, and carries on the top a
+very short glass stem <i>s</i>, into which is sealed the leading-in wire
+<i>w</i>, and I protect the top of the glass stem against the heat by a
+small aluminum tube <i>a</i> and a layer of mica underneath the same,
+as usual. The wire <i>w</i>, passing through the large tube to the
+outside of the bulb, should be well insulated&mdash;with a glass tube,<span class='pagenum'><a name="Page_245" id="Page_245">[Pg 245]</a></span>
+for instance&mdash;and the space between ought to be filled out with
+some excellent insulator. Among many insulating powders I
+have found that mica powder is the best to employ. If this precaution
+is not taken, the tube <small>T</small>, protruding into the bulb, will
+surely be cracked in consequence of the heating by the brushes
+which are apt to form in the upper part of the tube, near the exhausted
+globe, especially if the vacuum be excellent, and therefore
+the potential necessary to operate the lamp be very high.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_259.jpg" width="800" height="500" alt="Fig. 149, 150." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 149.</td><td class="caption">Fig. 150.</td></tr>
+</table>
+</div>
+
+<p>Fig. 151 illustrates a similar arrangement, with a large tube <small>T</small>
+protruding into the part of the bulb containing the refractory
+button <i>m</i>. In this case the wire leading from the outside into
+the bulb is omitted, the energy required being supplied through
+condenser coatings <small>C C</small>. The insulating packing <small>P</small> should in
+this construction be tightly fitting to the glass, and rather wide,
+or otherwise the discharge might avoid passing through the wire
+<i>w</i>, which connects the inside condenser coating to the incandescent
+button <i>m</i>.</p>
+
+<p>The molecular bombardment against the glass stem in the bulb
+is a source of great trouble. As an illustration I will cite a phenomenon
+only too frequently and unwillingly observed. A bulb,
+preferably a large one, may be taken, and a good conducting
+body, such as a piece of carbon, may be mounted in it upon a platinum
+wire sealed in the glass stem. The bulb may be exhausted
+to a fairly high degree, nearly to the point when phosphorescence<span class='pagenum'><a name="Page_246" id="Page_246">[Pg 246]</a></span>
+begins to appear. When the bulb is connected with the coil, the
+piece of carbon, if small, may become highly incandescent at
+first, but its brightness immediately diminishes, and then the discharge
+may break through the glass somewhere in the middle of
+the stem, in the form of bright sparks, in spite of the fact that
+the platinum wire is in good electrical connection with the rarefied
+gas through the piece of carbon or metal at the top. The
+first sparks are singularly bright, recalling those drawn from a
+clear surface of mercury. But, as they heat the glass rapidly,
+they, of course, lose their brightness, and cease when the glass at
+the ruptured place becomes incandescent, or generally sufficiently
+hot to conduct. When observed for the first time the phenomenon
+must appear very curious, and shows in a striking manner
+how radically different alternate currents, or impulses, of high
+frequency behave, as compared with steady currents, or currents
+of low frequency. With such currents&mdash;namely, the latter&mdash;the
+phenomenon would of course not occur. When frequencies such
+as are obtained by mechanical means are used, I think that the rupture
+of the glass is more or less the consequence of the bombardment,
+which warms it up and impairs its insulating power; but
+with frequencies obtainable with condensers I have no doubt
+that the glass may give way without previous heating. Although
+this appears most singular at first, it is in reality what we might
+expect to occur. The energy supplied to the wire leading into
+the bulb is given off partly by direct action through the carbon
+button, and partly by inductive action through the glass surrounding
+the wire. The case is thus analogous to that in which a condenser
+shunted by a conductor of low resistance is connected to
+a source of alternating current. As long as the frequencies are
+low, the conductor gets the most and the condenser is perfectly
+safe; but when the frequency becomes excessive, the <i>role</i> of the
+conductor may become quite insignificant. In the latter case the
+difference of potential at the terminals of the condenser may become
+so great as to rupture the dielectric, notwithstanding the
+fact that the terminals are joined by a conductor of low resistance.</p>
+
+<p>It is, of course, not necessary, when it is desired to produce
+the incandescence of a body inclosed in a bulb by means of these
+currents, that the body should be a conductor, for even a perfect
+non-conductor may be quite as readily heated. For this purpose
+it is sufficient to surround a conducting electrode with a non-con<span class='pagenum'><a name="Page_247" id="Page_247">[Pg 247]</a></span>ducting
+material, as, for instance, in the bulb described before in
+Fig. 150, in which a thin incandescent lamp filament is coated
+with a non-conductor, and supports a button of the same material
+on the top. At the start the bombardment goes on by inductive
+action through the non-conductor, until the same is sufficiently
+heated to become conducting, when the bombardment continues
+in the ordinary way.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_261.jpg" width="800" height="556" alt="Fig. 151, 152." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 151.</td><td class="caption">Fig. 152.</td></tr>
+</table>
+</div>
+
+<p>A different arrangement used in some of the bulbs constructed
+is illustrated in Fig. 152. In this instance a non-conductor <i>m</i> is
+mounted in a piece of common arc light carbon so as to project
+some small distance above the latter. The carbon piece is connected
+to the leading-in wire passing through a glass stem, which
+is wrapped with several layers of mica. An aluminum tube <i>a</i> is
+employed as usual for screening. It is so arranged that it reaches
+very nearly as high as the carbon and only the non-conductor <i>m</i>
+projects a little above it. The bombardment goes at first against
+the upper surface of carbon, the lower parts being protected by
+the aluminum tube. As soon, however, as the non-conductor <i>m</i>
+is heated it is rendered good conducting, and then it becomes the
+centre of the bombardment, being most exposed to the same.</p>
+
+<p>I have also constructed during these experiments many such
+single-wire bulbs with or without internal electrode, in which the
+radiant matter was projected against, or focused upon, the body<span class='pagenum'><a name="Page_248" id="Page_248">[Pg 248]</a></span>
+to be rendered incandescent. Fig. 153 (page 263) illustrates one
+of the bulbs used. It consists of a spherical globe <small>L</small>, provided
+with a long neck <i>n</i>, on top, for increasing the action in some cases
+by the application of an external conducting coating. The globe <small>L</small>
+is blown out on the bottom into a very small bulb <i>b</i>, which serves
+to hold it firmly in a socket <small>S</small> of insulating material into which it
+is cemented. A fine lamp filament <i>f</i>, supported on a wire <i>w</i>,
+passes through the centre of the globe <small>L</small>. The filament is rendered
+incandescent in the middle portion, where the bombardment
+proceeding from the lower inside surface of the globe is
+most intense. The lower portion of the globe, as far as the
+socket <small>S</small> reaches, is rendered conducting, either by a tinfoil coating
+or otherwise, and the external electrode is connected to a
+terminal of the coil.</p>
+
+<p>The arrangement diagrammatically indicated in Fig. 153 was
+found to be an inferior one when it was desired to render incandescent
+a filament or button supported in the centre of the globe,
+but it was convenient when the object was to excite phosphorescence.</p>
+
+<p>In many experiments in which bodies of different kind were
+mounted in the bulb as, for instance, indicated in Fig. 152, some
+observations of interest were made.</p>
+
+<p>It was found, among other things, that in such cases, no matter
+where the bombardment began, just as soon as a high temperature
+was reached there was generally one of the bodies
+which seemed to take most of the bombardment upon itself, the
+other, or others, being thereby relieved. The quality appeared
+to depend principally on the point of fusion, and on the facility
+with which the body was "evaporated," or, generally speaking,
+disintegrated&mdash;meaning by the latter term not only the throwing
+off of atoms, but likewise of large lumps. The observation made
+was in accordance with generally accepted notions. In a highly
+exhausted bulb, electricity is carried off from the electrode by
+independent carriers, which are partly the atoms, or molecules,
+of the residual atmosphere, and partly the atoms, molecules, or
+lumps thrown off from the electrode. If the electrode is composed
+of bodies of different character, and if one of these is more
+easily disintegrated than the other, most of the electricity supplied
+is carried off from that body, which is then brought to a
+higher temperature than the others, and this the more, as upon
+an increase of the temperature the body is still more easily disintegrated.</p>
+<p><span class='pagenum'><a name="Page_249" id="Page_249">[Pg 249]</a></span></p>
+<p>It seems to me quite probable that a similar process takes place
+in the bulb even with a homogeneous electrode, and I think it
+to be the principal cause of the disintegration. There is bound
+to be some irregularity, even if the surface is highly polished,
+which, of course, is impossible with most of the refractory bodies
+employed as electrodes. Assume that a point of the electrode
+gets hotter; instantly most of the discharge passes through that
+point, and a minute patch it probably fused and evaporated. It
+is now possible that in consequence of the violent disintegration
+the spot attacked sinks in temperature, or that a counter force is
+created, as in an arc; at any rate, the local tearing off meets with
+the limitations incident to the experiment, whereupon the same
+process occurs on another place. To the eye the electrode appears
+uniformly brilliant, but there are upon it points constantly
+shifting and wandering around, of a temperature far above the
+mean, and this materially hastens the process of deterioration.
+That some such thing occurs, at least when the electrode is at a lower
+temperature, sufficient experimental evidence can be obtained in
+the following manner: Exhaust a bulb to a very high degree, so
+that with a fairly high potential the discharge cannot pass&mdash;that
+is, not a <i>luminous</i> one, for a weak invisible discharge occurs
+always, in all probability. Now raise slowly and carefully the
+potential, leaving the primary current on no more than for an
+instant. At a certain point, two, three, or half a dozen phosphorescent
+spots will appear on the globe. These places of the
+glass are evidently more violently bombarded than others, this
+being due to the unevenly distributed electric density, necessitated,
+of course, by sharp projections, or, generally speaking, irregularities
+of the electrode. But the luminous patches are
+constantly changing in position, which is especially well observable
+if one manages to produce very few, and this indicates that
+the configuration of the electrode is rapidly changing.</p>
+
+<p>From experiences of this kind I am led to infer that, in order
+to be most durable, the refractory button in the bulb should be
+in the form of a sphere with a highly polished surface. Such a
+small sphere could be manufactured from a diamond or some
+other crystal, but a better way would be to fuse, by the employment
+of extreme degrees of temperature, some oxide&mdash;as, for
+instance, zirconia&mdash;into a small drop, and then keep it in the
+bulb at a temperature somewhat below its point of fusion.</p>
+
+<p>Interesting and useful results can, no doubt, be reached in the<span class='pagenum'><a name="Page_250" id="Page_250">[Pg 250]</a></span>
+direction of extreme degrees of heat. How can such high temperatures
+be arrived at? How are the highest degrees of heat
+reached in nature? By the impact of stars, by high speeds and
+collisions. In a collision any rate of heat generation may be
+attained. In a chemical process we are limited. When oxygen
+and hydrogen combine, they fall, metaphorically speaking, from
+a definite height. We cannot go very far with a blast, nor by
+confining heat in a furnace, but in an exhausted bulb we can
+concentrate any amount of energy upon a minute button. Leaving
+practicability out of consideration, this, then, would be the
+means which, in my opinion, would enable us to reach the highest
+temperature. But a great difficulty when proceeding in this way
+is encountered, namely, in most cases the body is carried off before
+it can fuse and form a drop. This difficulty exists principally
+with an oxide, such as zirconia, because it cannot be compressed
+in so hard a cake that it would not be carried off quickly.
+I have endeavored repeatedly to fuse zirconia, placing it in a cup of
+arc light carbon, as indicated in Fig. 152. It glowed with a most
+intense light, and the stream of the particles projected out of the
+carbon cup was of a vivid white; but whether it was compressed
+in a cake or made into a paste with carbon, it was carried off
+before it could be fused. The carbon cup, containing zirconia,
+had to be mounted very low in the neck of a large bulb, as the
+heating of the glass by the projected particles of the oxide was
+so rapid that in the first trial the bulb was cracked almost in an
+instant, when the current was turned on. The heating of the
+glass by the projected particles was found to be always greater
+when the carbon cup contained a body which was rapidly carried
+off&mdash;I presume, because in such cases, with the same potential,
+higher speeds were reached, and also because, per unit of time,
+more matter was projected&mdash;that is, more particles would strike
+the glass.</p>
+
+<p>The before-mentioned difficulty did not exist, however, when
+the body mounted in the carbon cup offered great resistance to
+deterioration. For instance, when an oxide was first fused in
+an oxygen blast, and then mounted in the bulb, it melted very
+readily into a drop.</p>
+
+<p>Generally, during the process of fusion, magnificent light
+effects were noted, of which it would be difficult to give an adequate
+idea. Fig. 152 is intended to illustrate the effect observed
+with a ruby drop. At first one may see a narrow funnel of<span class='pagenum'><a name="Page_251" id="Page_251">[Pg 251]</a></span>
+white light projected against the top of the globe, where it
+produces an irregularly outlined phosphorescent patch. When the
+point of the ruby fuses, the phosphorescence becomes very powerful;
+but as the atoms are projected with much greater speed
+from the surface of the drop, soon the glass gets hot and "tired,"
+and now only the outer edge of the patch glows. In this manner
+an intensely phosphorescent, sharply defined line, <i>l</i>, corresponding
+to the outline of the drop, is produced, which spreads slowly
+over the globe as the drop gets larger. When the mass begins
+to boil, small bubbles and cavities are formed, which cause dark
+colored spots to sweep across the globe. The bulb may be
+turned downward without fear of the drop falling off, as the
+mass possesses considerable viscosity.</p>
+
+<p>I may mention here another feature of some interest, which
+I believe to have noted in the course of these experiments,
+though the observations do not amount to a certitude. It <i>appeared</i>
+that under the molecular impact caused by the rapidly
+alternating potential, the body was fused and maintained in that
+state at a lower temperature in a highly exhausted bulb than
+was the case at normal pressure and application of heat in the
+ordinary way&mdash;that is, at least, judging from the quantity of the
+light emitted. One of the experiments performed may be mentioned
+here by way of illustration. A small piece of pumice
+stone was stuck on a platinum wire, and first melted to it in a
+gas burner. The wire was next placed between two pieces of
+charcoal, and a burner applied, so as to produce an intense heat,
+sufficient to melt down the pumice stone into a small glass-like
+button. The platinum wire had to be taken of sufficient thickness,
+to prevent its melting in the fire. While in the charcoal
+fire, or when held in a burner to get a better idea of the degree
+of heat, the button glowed with great brilliancy. The wire with
+the button was then mounted in a bulb, and upon exhausting the
+same to a high degree, the current was turned on slowly, so as to
+prevent the cracking of the button. The button was heated to
+the point of fusion, and when it melted, it did not, apparently,
+glow with the same brilliancy as before, and this would indicate
+a lower temperature. Leaving out of consideration the observer's
+possible, and even probable, error, the question is, can a body
+under these conditions be brought from a solid to a liquid state
+with the evolution of <i>less</i> light?</p>
+
+<p>When the potential of a body is rapidly alternated, it is certain<span class='pagenum'><a name="Page_252" id="Page_252">[Pg 252]</a></span>
+that the structure is jarred. When the potential is very high,
+although the vibrations may be few&mdash;say 20,000 per second&mdash;the
+effect upon the structure may be considerable. Suppose, for example,
+that a ruby is melted into a drop by a steady application
+of energy. When it forms a drop, it will emit visible and invisible
+waves, which will be in a definite ratio, and to the eye the
+drop will appear to be of a certain brilliancy. Next, suppose we
+diminish to any degree we choose the energy steadily supplied,
+and, instead, supply energy which rises and falls according to a
+certain law. Now, when the drop is formed, there will be emitted
+from it three different kinds of vibrations&mdash;the ordinary
+visible, and two kinds of invisible waves: that is, the ordinary
+dark waves of all lengths, and, in addition, waves of a well defined
+character. The latter would not exist by a steady supply
+of the energy; still they help to jar and loosen the structure. If
+this really be the case, then the ruby drop will emit relatively
+less visible and more invisible waves than before. Thus it would
+seem that when a platinum wire, for instance, is fused by currents
+alternating with extreme rapidity, it emits at the point of fusion
+less light and more visible radiation than it does when melted by
+a steady current, though the total energy used up in the process
+of fusion is the same in both cases. Or, to cite another example,
+a lamp filament is not capable of withstanding as long with currents
+of extreme frequency as it does with steady currents,
+assuming that it be worked at the same luminous intensity. This
+means that for rapidly alternating currents the filament should
+be shorter and thicker. The higher the frequency&mdash;that is, the
+greater the departure from the steady flow&mdash;the worse it would
+be for the filament. But if the truth of this remark were demonstrated,
+it would be erroneous to conclude that such a refractory
+button as used in these bulbs would be deteriorated quicker
+by currents of extremely high frequency than by steady or low
+frequency currents. From experience I may say that just the
+opposite holds good: the button withstands the bombardment
+better with currents of very high frequency. But this is due to
+the fact that a high frequency discharge passes through a rarefied
+gas with much greater freedom than a steady or low frequency
+discharge, and this will mean that with the former we can work
+with a lower potential or with a less violent impact. As long,
+then, as the gas is of no consequence, a steady or low frequency
+current is better; but as soon as the action of the gas is desired
+and important, high frequencies are preferable.<span class='pagenum'><a name="Page_253" id="Page_253">[Pg 253]</a></span></p>
+
+<p>In the course of these experiments a great many trials were
+made with all kinds of carbon buttons. Electrodes made of ordinary
+carbon buttons were decidedly more durable when the
+buttons were obtained by the application of enormous pressure.
+Electrodes prepared by depositing carbon in well known ways
+did not show up well; they blackened the globe very quickly.
+From many experiences I conclude that lamp filaments obtained
+in this manner can be advantageously used only with low potentials
+and low frequency currents. Some kinds of carbon withstand
+so well that, in order to bring them to the point of fusion, it is
+necessary to employ very small buttons. In this case the observation
+is rendered very difficult on account of the intense heat
+produced. Nevertheless there can be no doubt that all kinds of
+carbon are fused under the molecular bombardment, but the
+liquid state must be one of great instability. Of all the bodies
+tried there were two which withstood best&mdash;diamond and carborundum.
+These two showed up about equally, but the latter
+was preferable for many reasons. As it is more than likely that
+this body is not yet generally known, I will venture to call your
+attention to it.</p>
+
+<p>It has been recently produced by Mr. E. G. Acheson, of
+Monongahela City, Pa., U. S. A. It is intended to replace ordinary
+diamond powder for polishing precious stones, etc., and I
+have been informed that it accomplishes this object quite successfully.
+I do not know why the name "carborundum" has
+been given to it, unless there is something in the process of its
+manufacture which justifies this selection. Through the kindness
+of the inventor, I obtained a short while ago some samples which
+I desired to test in regard to their qualities of phosphorescence
+and capability of withstanding high degrees of heat.</p>
+
+<p>Carborundum can be obtained in two forms&mdash;in the form of
+"crystals" and of powder. The former appear to the naked eye
+dark colored, but are very brilliant; the latter is of nearly the
+same color as ordinary diamond powder, but very much finer.
+When viewed under a microscope the samples of crystals given
+to me did not appear to have any definite form, but rather resembled
+pieces of broken up egg coal of fine quality. The
+majority were opaque, but there were some which were transparent
+and colored. The crystals are a kind of carbon containing
+some impurities; they are extremely hard, and withstand for a
+long time even an oxygen blast. When the blast is directed<span class='pagenum'><a name="Page_254" id="Page_254">[Pg 254]</a></span>
+against them they at first form a cake of some compactness, probably
+in consequence of the fusion of impurities they contain. The
+mass withstands for a very long time the blast without further
+fusion; but a slow carrying off, or burning, occurs, and, finally,
+a small quantity of a glass-like residue is left, which, I suppose,
+is melted alumina. When compressed strongly they conduct very
+well, but not as well as ordinary carbon. The powder, which is
+obtained from the crystals in some way, is practically non-conducting.
+It affords a magnificent polishing material for stones.</p>
+
+<p>The time has been too short to make a satisfactory study of
+the properties of this product, but enough experience has been
+gained in a few weeks I have experimented upon it to say that
+it does possess some remarkable properties in many respects. It
+withstands excessively high degrees of heat, it is little deteriorated
+by molecular bombardment, and it does not blacken the globe as
+ordinary carbon does. The only difficulty which I have experienced
+in its use in connection with these experiments was to find some
+binding material which would resist the heat and the effect of the
+bombardment as successfully as carborundum itself does.</p>
+
+<p>I have here a number of bulbs which I have provided with
+buttons of carborundum. To make such a button of carborundum
+crystals I proceed in the following manner: I take an ordinary
+lamp filament and dip its point in tar, or some other
+thick substance or paint which may be readily carbonized. I
+next pass the point of the filament through the crystals, and then
+hold it vertically over a hot plate. The tar softens and forms a
+drop on the point of the filament, the crystals adhering to the
+surface of the drop. By regulating the distance from the plate
+the tar is slowly dried out and the button becomes solid. I then
+once more dip the button in tar and hold it again over a plate
+until the tar is evaporated, leaving only a hard mass which firmly
+binds the crystals. When a larger button is required I repeat
+the process several times, and I generally also cover the filament
+a certain distance below the button with crystals. The button
+being mounted in a bulb, when a good vacuum has been reached,
+first a weak and then a strong discharge is passed through the
+bulb to carbonize the tar and expel all gases, and later it is brought
+to a very intense incandescence.</p>
+
+<p>When the powder is used I have found it best to proceed as
+follows: I make a thick paint of carborundum and tar, and pass
+a lamp filament through the paint. Taking then most of the<span class='pagenum'><a name="Page_255" id="Page_255">[Pg 255]</a></span>
+paint off by rubbing the filament against a piece of chamois
+leather, I hold it over a hot plate until the tar evaporates and the
+coating becomes firm. I repeat this process as many times as it
+is necessary to obtain a certain thickness of coating. On the
+point of the coated filament I form a button in the same
+manner.</p>
+
+<p>There is no doubt that such a button&mdash;properly prepared under
+great pressure&mdash;of carborundum, especially of powder of the best
+quality, will withstand the effect of the bombardment fully as
+well as anything we know. The difficulty is that the binding
+material gives way, and the carborundum is slowly thrown off
+after some time. As it does not seem to blacken the globe in the
+least, it might be found useful for coating the filaments of ordinary
+incandescent lamps, and I think that it is even possible to produce
+thin threads or sticks of carborundum which will replace the ordinary
+filaments in an incandescent lamp. A carborundum coating
+seems to be more durable than other coatings, not only
+because the carborundum can withstand high degrees of heat, but
+also because it seems to unite with the carbon better than any
+other material I have tried. A coating of zirconia or any other
+oxide, for instance, is far more quickly destroyed. I prepared
+buttons of diamond dust in the same manner as of carborundum,
+and these came in durability nearest to those prepared of carborundum,
+but the binding paste gave way much more quickly
+in the diamond buttons; this, however, I attributed to the size
+and irregularity of the grains of the diamond.</p>
+
+<p>It was of interest to find whether carborundum possesses the
+quality of phosphorescence. One is, of course, prepared to encounter
+two difficulties: first, as regards the rough product, the
+"crystals," they are good conducting, and it is a fact that conductors
+do not phosphoresce; second, the powder, being exceedingly
+fine, would not be apt to exhibit very prominently this
+quality, since we know that when crystals, even such as diamond
+or ruby, are finely powdered, they lose the property of phosphorescence
+to a considerable degree.</p>
+
+<p>The question presents itself here, can a conductor phosphoresce?
+What is there in such a body as a metal, for instance, that
+would deprive it of the quality of phosphoresence, unless it is
+that property which characterizes it as a conductor? For it is a
+fact that most of the phosphorescent bodies lose that quality when
+they are sufficiently heated to become more or less conducting.<span class='pagenum'><a name="Page_256" id="Page_256">[Pg 256]</a></span>
+Then, if a metal be in a large measure, or perhaps entirely, deprived
+of that property, it should be capable of phosphoresence.
+Therefore it is quite possible that at some extremely high frequency,
+when behaving practically as a non-conductor, a metal
+or any other conductor might exhibit the quality of phosphoresence,
+even though it be entirely incapable of phosphorescing
+under the impact of a low-frequency discharge. There is, however,
+another possible way how a conductor might at least <i>appear</i>
+to phosphoresce.</p>
+
+<p>Considerable doubt still exists as to what really is phosphorescence,
+and as to whether the various phenomena comprised
+under this head are due to the same causes. Suppose that in an
+exhausted bulb, under the molecular impact, the surface of a
+piece of metal or other conductor is rendered strongly luminous,
+but at the same time it is found that it remains comparatively
+cool, would not this luminosity be called phosphorescence? Now
+such a result, theoretically at least, is possible, for it is a mere
+question of potential or speed. Assume the potential of the
+electrode, and consequently the speed of the projected atoms, to
+be sufficiently high, the surface of the metal piece, against which
+the atoms are projected, would be rendered highly incandescent,
+since the process of heat generation would be incomparably faster
+than that of radiating or conducting away from the surface of
+the collision. In the eye of the observer a single impact of the
+atoms would cause an instantaneous flash, but if the impacts were
+repeated with sufficient rapidity, they would produce a continuous
+impression upon his retina. To him then the surface of the
+metal would appear continuously incandescent and of constant
+luminous intensity, while in reality the light would be either
+intermittent, or at least changing periodically in intensity. The
+metal piece would rise in temperature until equilibrium was
+attained&mdash;that is, until the energy continuously radiated would
+equal that intermittently supplied. But the supplied energy
+might under such conditions not be sufficient to bring the body
+to any more than a very moderate mean temperature, especially
+if the frequency of the atomic impacts be very low&mdash;just enough
+that the fluctuation of the intensity of the light emitted could
+not be detected by the eye. The body would now, owing to the
+manner in which the energy is supplied, emit a strong light, and
+yet be at a comparatively very low mean temperature. How
+should the observer name the luminosity thus produced? Even if<span class='pagenum'><a name="Page_257" id="Page_257">[Pg 257]</a></span>
+the analysis of the light would teach him something definite, still
+he would probably rank it under the phenomena of phosphorescence.
+It is conceivable that in such a way both conducting
+and non-conducting bodies may be maintained at a certain luminous
+intensity, but the energy required would very greatly vary
+with the nature and properties of the bodies.</p>
+
+<p>These and some foregoing remarks of a speculative nature
+were made merely to bring out curious features of alternate
+currents or electric impulses. By their help we may cause a body
+to emit <i>more</i> light, while at a certain mean temperature, than it
+would emit if brought to that temperature by a steady supply;
+and, again, we may bring a body to the point of fusion, and cause
+it to emit <i>less</i> light than when fused by the application of energy
+in ordinary ways. It all depends on how we supply the energy,
+and what kind of vibrations we set up; in one case the vibrations
+are more, in the other less, adapted to affect our sense of vision.</p>
+
+<p>Some effects, which I had not observed before, obtained with
+carborundum in the first trials, I attributed to phosphorescence,
+but in subsequent experiments it appeared that it was devoid of
+that quality. The crystals possess a noteworthy feature. In a
+bulb provided with a single electrode in the shape of a small
+circular metal disc, for instance, at a certain degree of exhaustion
+the electrode is covered with a milky film, which is separated by
+a dark space from the glow filling the bulb. When the metal
+disc is covered with carborundum crystals, the film is far more
+intense, and snow-white. This I found later to be merely an
+effect of the bright surface of the crystals, for when an aluminum
+electrode was highly polished, it exhibited more or less the same
+phenomenon. I made a number of experiments with the samples
+of crystals obtained, principally because it would have been of
+special interest to find that they are capable of phosphorescence,
+on account of their being conducting. I could not produce phosphorescence
+distinctly, but I must remark that a decisive opinion
+cannot be formed until other experimenters have gone over the
+same ground.</p>
+
+<p>The powder behaved in some experiments as though it contained
+alumina, but it did not exhibit with sufficient distinctness
+the red of the latter. Its dead color brightens considerably under
+the molecular impact, but I am now convinced it does not
+phosphoresce. Still, the tests with the powder are not conclusive,
+because powdered carborundum probably does not behave like a<span class='pagenum'><a name="Page_258" id="Page_258">[Pg 258]</a></span>
+phosphorescent sulphide, for example, which could be finely
+powdered without impairing the phosphorescence, but rather like
+powdered ruby or diamond, and therefore it would be necessary,
+in order to make a decisive test, to obtain it in a large lump and
+polish up the surface.</p>
+
+<p>If the carborundum proves useful in connection with these
+and similar experiments, its chief value will be found in the
+production of coatings, thin conductors, buttons, or other electrodes
+capable of withstanding extremely high degrees of heat.</p>
+
+<p>The production of a small electrode, capable of withstanding
+enormous temperatures, I regard as of the greatest importance
+in the manufacture of light. It would enable us to obtain, by
+means of currents of very high frequencies, certainly 20 times, if
+not more, the quantity of light which is obtained in the present
+incandescent lamp by the same expenditure of energy. This
+estimate may appear to many exaggerated, but in reality I think
+it is far from being so. As this statement might be misunderstood,
+I think it is necessary to expose clearly the problem with
+which, in this line of work, we are confronted, and the manner
+in which, in my opinion, a solution will be arrived at.</p>
+
+<p>Any one who begins a study of the problem will be apt to
+think that what is wanted in a lamp with an electrode is a very
+high degree of incandescence of the electrode. There he will be
+mistaken. The high incandescence of the button is a necessary
+evil, but what is really wanted is the high incandescence of the
+gas surrounding the button. In other words, the problem in
+such a lamp is to bring a mass of gas to the highest possible incandescence.
+The higher the incandescence, the quicker the
+mean vibration, the greater is the economy of the light production.
+But to maintain a mass of gas at a high degree of incandescence
+in a glass vessel, it will always be necessary to keep the incandescent
+mass away from the glass; that is, to confine it as much as
+possible to the central portion of the globe.</p>
+
+<p>In one of the experiments this evening a brush was produced
+at the end of a wire. The brush was a flame, a source of heat
+and light. It did not emit much perceptible heat, nor did it
+glow with an intense light; but is it the less a flame because it
+does not scorch my hand? Is it the less a flame because it does
+not hurt my eyes by its brilliancy? The problem is precisely to
+produce in the bulb such a flame, much smaller in size, but incomparably
+more powerful. Were there means at hand for<span class='pagenum'><a name="Page_259" id="Page_259">[Pg 259]</a></span>
+producing electric impulses of a sufficiently high frequency, and
+for transmitting them, the bulb could be done away with, unless
+it were used to protect the electrode, or to economize the energy
+by confining the heat. But as such means are not at disposal, it
+becomes necessary to place the terminal in the bulb and rarefy
+the air in the same. This is done merely to enable the apparatus
+to perform the work which it is not capable of performing at ordinary
+air pressure. In the bulb we are able to intensify the
+action to any degree&mdash;so far that the brush emits a powerful
+light.</p>
+
+<p>The intensity of the light emitted depends principally on the
+frequency and potential of the impulses, and on the electric density
+on the surface of the electrode. It is of the greatest importance
+to employ the smallest possible button, in order to push
+the density very far. Under the violent impact of the molecules
+of the gas surrounding it, the small electrode is of course brought
+to an extremely high temperature, but around it is a mass of
+highly incandescent gas, a flame photosphere, many hundred
+times the volume of the electrode. With a diamond, carborundum
+or zirconia button the photosphere can be as much as one
+thousand times the volume of the button. Without much reflection
+one would think that in pushing so far the incandescence
+of the electrode it would be instantly volatilized. But after a
+careful consideration one would find that, theoretically, it should
+not occur, and in this fact&mdash;which, moreover, is experimentally
+demonstrated&mdash;lies principally the future value of such a lamp.</p>
+
+<p>At first, when the bombardment begins, most of the work is
+performed on the surface of the button, but when a highly conducting
+photosphere is formed the button is comparatively relieved.
+The higher the incandescence of the photosphere, the
+more it approaches in conductivity to that of the electrode, and
+the more, therefore, the solid and the gas form one conducting
+body. The consequence is that the further the incandescence is
+forced the more work, comparatively, is performed on the gas,
+and the less on the electrode. The formation of a powerful
+photosphere is consequently the very means for protecting the
+electrode. This protection, of course, is a relative one, and it
+should not be thought that by pushing the incandescence higher
+the electrode is actually less deteriorated. Still, theoretically,
+with extreme frequencies, this result must be reached, but probably
+at a temperature too high for most of the refractory bodies<span class='pagenum'><a name="Page_260" id="Page_260">[Pg 260]</a></span>
+known. Given, then, an electrode which can withstand to a very
+high limit the effect of the bombardment and outward strain, it
+would be safe, no matter how much it was forced beyond that
+limit. In an incandescent lamp quite different considerations
+apply. There the gas is not at all concerned; the whole of the
+work is performed on the filament; and the life of the lamp
+diminishes so rapidly with the increase of the degree of incandescence
+that economical reasons compel us to work it at a low
+incandescence. But if an incandescent lamp is operated with
+currents of very high frequency, the action of the gas cannot be
+neglected, and the rules for the most economical working must
+be considerably modified.</p>
+
+<p>In order to bring such a lamp with one or two electrodes to a
+great perfection, it is necessary to employ impulses of very high
+frequency. The high frequency secures, among others, two chief
+advantages, which have a most important bearing upon the
+economy of the light production. First, the deterioration of the
+electrode is reduced by reason of the fact that we employ a great
+many small impacts, instead of a few violent ones, which quickly
+shatter the structure; secondly, the formation of a large photosphere
+is facilitated.</p>
+
+<p>In order to reduce the deterioration of the electrode to the
+minimum, it is desirable that the vibration be harmonic, for any
+suddenness hastens the process of destruction. An electrode lasts
+much longer when kept at incandescence by currents, or impulses,
+obtained from a high frequency alternator, which rise and fall
+more or less harmonically, than by impulses obtained from a disruptive
+discharge coil. In the latter case there is no doubt that
+most of the damage is done by the fundamental sudden discharges.</p>
+
+<p>One of the elements of loss in such a lamp is the bombardment
+of the globe. As the potential is very high, the molecules
+are projected with great speed; they strike the glass, and usually excite
+a strong phosphorescence. The effect produced is very pretty,
+but for economical reasons it would be perhaps preferable to prevent,
+or at least reduce to a minimum, the bombardment against
+the globe, as in such case it is, as a rule, not the object to excite
+phosphorescence, and as some loss of energy results from the
+bombardment. This loss in the bulb is principally dependent
+on the potential of the impulses and on the electric density on
+the surface of the electrode. In employing very high frequen<span class='pagenum'><a name="Page_261" id="Page_261">[Pg 261]</a></span>cies
+the loss of energy by the bombardment is greatly reduced,
+for, first, the potential needed to perform a given amount of work
+is much smaller; and, secondly, by producing a highly conducting
+photosphere around the electrode, the same result is obtained
+as though the electrode were much larger, which is equivalent to
+a smaller electric density. But be it by the diminution of the
+maximum potential or of the density, the gain is effected in the
+same manner, namely, by avoiding violent shocks, which strain
+the glass much beyond its limit of elasticity. If the frequency
+could be brought high enough, the loss due to the imperfect
+elasticity of the glass would be entirely negligible. The loss due
+to bombardment of the globe may, however, be reduced by using
+two electrodes instead of one. In such case each of the electrodes
+may be connected to one of the terminals; or else, if it is
+preferable to use only one wire, one electrode may be connected
+to one terminal and the other to the ground or to an insulated
+body of some surface, as, for instance, a shade on the lamp. In
+the latter case, unless some judgment is used, one of the electrodes
+might glow more intensely than the other.</p>
+
+<p>But on the whole I find it preferable, when using such high
+frequencies, to employ only one electrode and one connecting
+wire. I am convinced that the illuminating device of the near
+future will not require for its operation more than one lead, and,
+at any rate, it will have no leading-in wire, since the energy required
+can be as well transmitted through the glass. In experimental
+bulbs the leading-in wire is not generally used on account
+of convenience, as in employing condenser coatings in the manner
+indicated in Fig. 151, for example, there is some difficulty in
+fitting the parts, but these difficulties would not exist if a great
+many bulbs were manufactured; otherwise the energy can be
+conveyed through the glass as well as through a wire, and with
+these high frequencies the losses are very small. Such illustrating
+devices will necessarily involve the use of very high
+potentials, and this, in the eyes of practical men, might be an objectionable
+feature. Yet, in reality, high potentials are not
+objectionable&mdash;certainly not in the least so far as the safety of
+the devices is concerned.</p>
+
+<p>There are two ways of rendering an electric appliance safe.
+One is to use low potentials, the other is to determine the dimensions
+of the apparatus so that it is safe, no matter how high a
+potential is used. Of the two, the latter seems to me the better<span class='pagenum'><a name="Page_262" id="Page_262">[Pg 262]</a></span>
+way, for then the safety is absolute, unaffected by any possible
+combination of circumstances which might render even a low-potential
+appliance dangerous to life and property. But the practical
+conditions require not only the judicious determination of the
+dimensions of the apparatus; they likewise necessitate the employment
+of energy of the proper kind. It is easy, for instance,
+to construct a transformer capable of giving, when operated from
+an ordinary alternate current machine of low tension, say 50,000
+volts, which might be required to light a highly exhausted phosphorescent
+tube, so that, in spite of the high potential, it is
+perfectly safe, the shock from it producing no inconvenience.
+Still such a transformer would be expensive, and in itself inefficient;
+and, besides, what energy was obtained from it would not
+be economically used for the production of light. The economy
+demands the employment of energy in the form of extremely rapid
+vibrations. The problem of producing light has been likened to
+that of maintaining a certain high-pitch note by means of a bell.
+It should be said a <i>barely audible</i> note; and even these words
+would not express it, so wonderful is the sensitiveness of the eye.
+We may deliver powerful blows at long intervals, waste a good
+deal of energy, and still not get what we want; or we may keep
+up the note by delivering frequent taps, and get nearer to the
+object sought by the expenditure of much less energy. In the
+production of light, as far as the illuminating device is concerned,
+there can be only one rule&mdash;that is, to use as high frequencies as
+can be obtained; but the means for the production and conveyance
+of impulses of such character impose, at present at least,
+great limitations. Once it is decided to use very high frequencies,
+the return wire becomes unnecessary, and all the appliances
+are simplified. By the use of obvious means the same result is
+obtained as though the return wire were used. It is sufficient for
+this purpose to bring in contact with the bulb, or merely in the
+vicinity of the same, an insulated body of some surface. The
+surface need, of course, be the smaller, the higher the frequency
+and potential used, and necessarily, also, the higher the economy
+of the lamp or other device.</p>
+
+<p>This plan of working has been resorted to on several occasions
+this evening. So, for instance, when the incandescence of a
+button was produced by grasping the bulb with the hand, the
+body of the experimenter merely served to intensify the action.
+The bulb used was similar to that illustrated in Fig. 148, and<span class='pagenum'><a name="Page_263" id="Page_263">[Pg 263]</a></span>
+the coil was excited to a small potential, not sufficient to bring
+the button to incandescence when the bulb was hanging from
+the wire; and incidentally, in order to perform the experiment
+in a more suitable manner, the button was taken so large that a
+perceptible time had to elapse before, upon grasping the bulb, it
+could be rendered incandescent. The contact with the bulb was,
+of course, quite unnecessary. It is easy, by using a rather large
+bulb with an exceedingly small electrode, to adjust the conditions
+so that the latter is brought to bright incandescence by the mere
+approach of the experimenter within a few feet of the bulb, and
+that the incandescence subsides upon his receding.</p>
+
+<div class="figcenter" style="width: 710px;">
+<img src="images/oi_277.jpg" width="710" height="600" alt="Fig. 153, 154." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 153.</td><td class="caption">Fig. 154.</td></tr>
+</table>
+</div>
+
+<p>In another experiment, when phosphorescence was excited, a
+similar bulb was used. Here again, originally, the potential was
+not sufficient to excite phosphorescence until the action was intensified&mdash;in
+this case, however, to present a different feature, by
+touching the socket with a metallic object held in the hand. The
+electrode in the bulb was a carbon button so large that it could
+not be brought to incandescence, and thereby spoil the effect
+produced by phosphorescence.</p>
+
+<p>Again, in another of the early experiments, a bulb was used,<span class='pagenum'><a name="Page_264" id="Page_264">[Pg 264]</a></span>
+as illustrated in Fig. 141. In this instance, by touching the bulb
+with one or two fingers, one or two shadows of the stem inside
+were projected against the glass, the touch of the finger producing
+the same results as the application of an external negative electrode
+under ordinary circumstances.</p>
+
+<p>In all these experiments the action was intensified by augmenting
+the capacity at the end of the lead connected to the terminal.
+As a rule, it is not necessary to resort to such means, and would
+be quite unnecessary with still higher frequencies; but when it
+<i>is</i> desired, the bulb, or tube, can be easily adapted to the purpose.</p>
+
+<p>In Fig. 153, for example, an experimental bulb, <small>L</small>, is shown,
+which is provided with a neck, <i>n</i>, on the top, for the application
+of an external tinfoil coating, which may be connected to a body
+of larger surface. Such a lamp as illustrated in Fig. 154 may
+also be lighted by connecting the tinfoil coating on the neck <i>n</i>
+to the terminal, and the leading-in wire, <i>w</i>, to an insulated plate.
+If the bulb stands in a socket upright, as shown in the cut, a
+shade of conducting material may be slipped in the neck, <i>n</i>, and
+the action thus magnified.</p>
+
+<p>A more perfected arrangement used in some of these bulbs is
+illustrated in Fig. 155. In this case the construction of the bulb
+is as shown and described before, when reference was made to
+Fig. 148. A zinc sheet, <small>Z</small>, with a tubular extension, <small>T</small>, is applied
+over the metallic socket, <small>S</small>. The bulb hangs downward from the
+terminal, <i>t</i>, the zinc sheet, <small>Z</small>, performing the double office of intensifier
+and reflector. The reflector is separated from the terminal,
+<i>t</i>, by an extension of the insulating plug, <small>P</small>.</p>
+
+<p>A similar disposition with a phosphorescent tube is illustrated
+in Fig. 156. The tube, <small>T</small>, is prepared from two short tubes of
+different diameter, which are sealed on the ends. On the lower
+end is placed an inside conducting coating, <small>C</small>, which connects to
+the wire <i>w</i>. The wire has a hook on the upper end for suspension,
+and passes through the centre of the inside tube, which is
+filled with some good and tightly packed insulator. On the outside
+of the upper end of the tube, <small>T</small>, is another conducting coating,
+<small>C<sub>1</sub></small>, upon which is slipped a metallic reflector <small>Z</small>, which should
+be separated by a thick insulation from the end of wire <i>w</i>.</p>
+
+<p>The economical use of such a reflector or intensifier would require
+that all energy supplied to an air condenser should be recoverable,
+or, in other words, that there should not be any losses,<span class='pagenum'><a name="Page_265" id="Page_265">[Pg 265]</a></span>
+neither in the gaseous medium nor through its action elsewhere.
+This is far from being so, but, fortunately, the losses may be reduced
+to anything desired. A few remarks are necessary on
+this subject, in order to make the experiences gathered in the
+course of these investigations perfectly clear.</p>
+
+<div class="figcenter" style="width: 670px;">
+<img src="images/oi_279.jpg" width="670" height="600" alt="Fig. 155." title="" />
+<span class="caption">Fig. 155.</span>
+</div>
+
+
+<p>Suppose a small helix with many well insulated turns, as in
+experiment Fig. 146, has one of its ends connected to one of the
+terminals of the induction coil, and the other to a metal plate,
+or, for the sake of simplicity, a sphere, insulated in space. When
+the coil is set to work, the potential of the sphere is alternated,
+and a small helix now behaves as though its free end were connected
+to the other terminal of the induction coil. If an iron
+rod be held within a small helix, it is quickly brought to a high
+temperature, indicating the passage of a strong current through
+the helix. How does the insulated sphere act in this case? It
+can be a condenser, storing and returning the energy supplied to
+it, or it can be a mere sink of energy, and the conditions of the
+experiment determine whether it is rather one than the other.
+The sphere being charged to a high potential, it acts inductively
+upon the surrounding air, or whatever gaseous medium there might
+be. The molecules, or atoms, which are near the sphere, are of
+course more attracted, and move through a greater distance than
+the farther ones. When the nearest molecules strike the sphere,
+they are repelled, and collisions occur at all distances within the
+inductive action of the sphere. It is now clear that, if the poten<span class='pagenum'><a name="Page_266" id="Page_266">[Pg 266]</a></span>tial
+be steady, but little loss of energy can be caused in this way,
+for the molecules which are nearest to the sphere, having had an
+additional charge imparted to them by contact, are not attracted
+until they have parted, if not with all, at least with most of the
+additional charge, which can be accomplished only after a great
+many collisions. From the fact, that with a steady potential
+there is but little loss in dry air, one must come to such a conclusion.
+When the potential of a sphere, instead of being steady,
+is alternating, the conditions are entirely different. In this case
+a rhythmical bombardment occurs, no matter whether the molecules,
+after coming in contact with the sphere, lose the imparted
+charge or not; what is more, if the charge is not lost, the impacts
+are only the more violent. Still, if the frequency of the impulses
+be very small, the loss caused by the impacts and collisions
+would not be serious, unless the potential were excessive. But
+when extremely high frequencies and more or less high potentials
+are used, the loss may very great. The total energy lost per unit
+of time is proportionate to the product of the number of impacts
+per second, or the frequency and the energy lost in each impact.
+But the energy of an impact must be proportionate to the square
+of the electric density of the sphere, since the charge imparted
+<span class='pagenum'><a name="Page_267" id="Page_267">[Pg 267]</a></span>to the molecule is proportionate to that density. I conclude from
+this that the total energy lost must be proportionate to the product
+of the frequency and the square of the electric density; but
+this law needs experimental confirmation. Assuming the preceding
+considerations to be true, then, by rapidly alternating the
+potential of a body immersed in an insulating gaseous medium,
+any amount of energy may be dissipated into space. Most of
+that energy then, I believe, is not dissipated in the form of long
+ether waves, propagated to considerable distance, as is thought
+most generally, but is consumed&mdash;in the case of an insulated
+sphere, for example&mdash;in impact and collisional losses&mdash;that is,
+heat vibrations&mdash;on the surface and in the vicinity of the sphere.
+To reduce the dissipation, it is necessary to work with a small
+electric density&mdash;the smaller, the higher the frequency.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_280.jpg" width="600" height="669" alt="Fig. 156." title="" />
+<span class="caption">Fig. 156.</span>
+</div>
+
+<p>But since, on the assumption before made, the loss is diminished
+with the square of the density, and since currents of very
+high frequencies involve considerable waste when transmitted
+through conductors, it follows that, on the whole, it is better to
+employ one wire than two. Therefore, if motors, lamps, or devices
+of any kind are perfected, capable of being advantageously
+operated by currents of extremely high frequency, economical
+reasons will make it advisable to use only one wire, especially if
+the distances are great.</p>
+
+<p>When energy is absorbed in a condenser, the same behaves as
+though its capacity were increased. Absorption always exists
+more or less, but generally it is small and of no consequence as
+long as the frequencies are not very great. In using extremely
+high frequencies, and, necessarily in such case, also high potentials,
+the absorption&mdash;or, what is here meant more particularly
+by this term, the loss of energy due to the presence of a gaseous
+medium&mdash;is an important factor to be considered, as the energy
+absorbed in the air condenser may be any fraction of the supplied
+energy. This would seem to make it very difficult to tell from
+the measured or computed capacity of an air condenser its actual
+capacity or vibration period, especially if the condenser is of very
+small surface and is charged to a very high potential. As many
+important results are dependent upon the correctness of the
+estimation of the vibration period, this subject demands the most
+careful scrutiny of other investigators. To reduce the probable
+error as much as possible in experiments of the kind alluded to,
+it is advisable to use spheres or plates of large surface, so as to<span class='pagenum'><a name="Page_268" id="Page_268">[Pg 268]</a></span>
+make the density exceedingly small. Otherwise, when it is
+practicable, an oil condenser should be used in preference. In
+oil or other liquid dielectrics there are seemingly no such losses
+as in gaseous media. It being impossible to exclude entirely the
+gas in condensers with solid dielectrics, such condensers should
+be immersed in oil, for economical reasons, if nothing else; they
+can then be strained to the utmost, and will remain cool. In
+Leyden jars the loss due to air is comparatively small, as the tinfoil
+coatings are large, close together, and the charged surfaces
+not directly exposed; but when the potentials are very high, the
+loss may be more or less considerable at, or near, the upper edge
+of the foil, where the air is principally acted upon. If the jar
+be immersed in boiled-out oil, it will be capable of performing
+four times the amount of work which it can for any length of
+time when used in the ordinary way, and the loss will be inappreciable.</p>
+
+<p>It should not be thought that the loss in heat in an air condenser
+is necessarily associated with the formation of <i>visible</i>
+streams or brushes. If a small electrode, inclosed in an unexhausted
+bulb, is connected to one of the terminals of the coil,
+streams can be seen to issue from the electrode, and the air in the
+bulb is heated; if instead of a small electrode a large sphere is
+inclosed in the bulb, no streams are observed, still the air is
+heated.</p>
+
+<p>Nor should it be thought that the temperature of an air condenser
+would give even an approximate idea of the loss in heat
+incurred, as in such case heat must be given off much more
+quickly, since there is, in addition to the ordinary radiation, a
+very active carrying away of heat by independent carriers going
+on, and since not only the apparatus, but the air at some distance
+from it is heated in consequence of the collisions which must
+occur.</p>
+
+<p>Owing to this, in experiments with such a coil, a rise of temperature
+can be distinctly observed only when the body connected
+to the coil is very small. But with apparatus on a larger scale,
+even a body of considerable bulk would be heated, as, for instance,
+the body of a person; and I think that skilled physicians might
+make observations of utility in such experiments, which, if the
+apparatus were judiciously designed, would not present the slightest
+danger.</p>
+
+<p>A question of some interest, principally to meteorologists,<span class='pagenum'><a name="Page_269" id="Page_269">[Pg 269]</a></span>
+presents itself here. How does the earth behave? The earth is
+an air condenser, but is it a perfect or a very imperfect one&mdash;a
+mere sink of energy? There can be little doubt that to such
+small disturbance as might be caused in an experiment, the earth
+behaves as an almost perfect condenser. But it might be different
+when its charge is set in vibration by some sudden disturbance
+occurring in the heavens. In such case, as before stated,
+probably only little of the energy of the vibrations set up would
+be lost into space in the form of long ether radiations, but most
+of the energy, I think, would spend itself in molecular impacts
+and collisions, and pass off into space in the form of short heat,
+and possibly light, waves. As both the frequency of the vibrations
+of the charge and the potential are in all probability excessive,
+the energy converted into heat may be considerable. Since
+the density must be unevenly distributed, either in consequence
+of the irregularity of the earth's surface, or on account of the
+condition of the atmosphere in various places, the effect produced
+would accordingly vary from place to place. Considerable variations
+in the temperature and pressure of the atmosphere may in
+this manner be caused at any point of the surface of the earth.
+The variations may be gradual or very sudden, according to the
+nature of the general disturbance, and may produce rain and
+storms, or locally modify the weather in any way.</p>
+
+<p>From the remarks before made, one may see what an important
+factor of loss the air in the neighborhood of a charged surface
+becomes when the electric density is great and the frequency of
+the impulses excessive. But the action, as explained, implies
+that the air is insulating&mdash;that is, that it is composed of independent
+carriers immersed in an insulating medium. This is the case
+only when the air is at something like ordinary or greater, or at
+extremely small, pressure. When the air is slightly rarefied and
+conducting, then true conduction losses occur also. In such case,
+of course, considerable energy may be dissipated into space even
+with a steady potential, or with impulses of low frequency, if the
+density is very great.</p>
+
+<p>When the gas is at very low pressure, an electrode is heated
+more because higher speeds can be reached. If the gas around
+the electrode is strongly compressed, the displacements, and
+consequently the speeds, are very small, and the heating is insignificant.
+But if in such case the frequency could be sufficiently
+increased, the electrode would be brought to a high tem<span class='pagenum'><a name="Page_270" id="Page_270">[Pg 270]</a></span>perature
+as well as if the gas were at very low pressure; in fact,
+exhausting the bulb is only necessary because we cannot produce,
+(and possibly not convey) currents of the required frequency.</p>
+
+<p>Returning to the subject of electrode lamps, it is obviously of
+advantage in such a lamp to confine as much as possible the heat
+to the electrode by preventing the circulation of the gas in the
+bulb. If a very small bulb be taken, it would confine the heat
+better than a large one, but it might not be of sufficient capacity
+to be operated from the coil, or, if so, the glass might get too
+hot. A simple way to improve in this direction is to employ a
+globe of the required size, but to place a small bulb, the diameter
+of which is properly estimated, over the refractory button contained
+in the globe. This arrangement is illustrated in Fig. 157.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_284.jpg" width="800" height="554" alt="Fig. 157, 158." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 157.</td><td class="caption">Fig. 158.</td></tr>
+</table>
+</div>
+
+<p>The globe <small>L</small> has in this case a large neck <i>n</i>, allowing the small
+bulb <i>b</i> to slip through. Otherwise the construction is the same
+as shown in Fig. 147, for example. The small bulb is conveniently
+supported upon the stem <i>s</i>, carrying the refractory button
+<i>m</i>. It is separated from the aluminum tube <i>a</i> by several layers
+of mica <small>M</small>, in order to prevent the cracking of the neck by the
+rapid heating of the aluminum tube upon a sudden turning on
+of the current. The inside bulb should be as small as possible
+when it is desired to obtain light only by incandescence of the
+electrode. If it is desired to produce phosphorescence, the bulb<span class='pagenum'><a name="Page_271" id="Page_271">[Pg 271]</a></span>
+should be larger, else it would be apt to get too hot, and the
+phosphorescence would cease. In this arrangement usually only
+the small bulb shows phosphorescence, as there is practically no
+bombardment against the outer globe. In some of these bulbs
+constructed as illustrated in Fig. 157, the small tube was coated
+with phosphorescent paint, and beautiful effects were obtained.
+Instead of making the inside bulb large, in order to avoid undue
+heating, it answers the purpose to make the electrode <i>m</i> larger.
+In this case the bombardment is weakened by reason of the
+smaller electric density.</p>
+
+<p>Many bulbs were constructed on the plan illustrated in Fig.
+158. Here a small bulb <i>b</i>, containing the refractory button <i>m</i>,
+upon being exhausted to a very high degree was sealed in a large
+globe <small>L</small>, which was then moderately exhausted and sealed off.
+The principal advantage of this construction was that it allowed
+of reaching extremely high vacua, and, at the same time of using a
+large bulb. It was found, in the course of experiments with
+bulbs such as illustrated in Fig. 158, that it was well to make
+the stem <i>s</i>, near the seal at <i>e</i>, very thick, and the leading-in wire
+<i>w</i> thin, as it occurred sometimes that the stem at <i>e</i> was heated
+and the bulb was cracked. Often the outer globe <small>L</small> was exhausted
+only just enough to allow the discharge to pass through, and the
+space between the bulbs appeared crimson, producing a curious
+effect. In some cases, when the exhaustion in globe <small>L</small> was very
+low, and the air good conducting, it was found necessary, in order
+to bring the button <i>m</i> to high incandescence, to place, preferably
+on the upper part of the neck of the globe, a tinfoil coating which
+was connected to an insulated body, to the ground, or to the
+other terminal of the coil, as the highly conducting air weakened
+the effect somewhat, probably by being acted upon inductively
+from the wire <i>w</i>, where it entered the bulb at <i>e</i>. Another difficulty&mdash;which,
+however, is always present when the refractory
+button is mounted in a very small bulb&mdash;existed in the construction
+illustrated in Fig. 158, namely, the vacuum in the bulb <i>b</i>
+would be impaired in a comparatively short time.</p>
+
+<p>The chief idea in the two last described constructions was to
+confine the heat to the central portion of the globe by preventing
+the exchange of air. An advantage is secured, but owing to the
+heating of the inside bulb and slow evaporation of the glass, the
+vacuum is hard to maintain, even if the construction illustrated
+in Fig. 157 be chosen, in which both bulbs communicate.<span class='pagenum'><a name="Page_272" id="Page_272">[Pg 272]</a></span></p>
+
+<p>But by far the better way&mdash;the ideal way&mdash;would be to reach
+sufficiently high frequencies. The higher the frequency, the
+slower would be the exchange of the air, and I think that a frequency
+may be reached, at which there would be no exchange
+whatever of the air molecules around the terminal. We would
+then produce a flame in which there would be no carrying away
+of material, and a queer flame it would be, for it would be rigid!
+With such high frequencies the inertia of the particles would come
+into play. As the brush, or flame, would gain rigidity in virtue
+of the inertia of the particles, the exchange of the latter would
+be prevented. This would necessarily occur, for, the number of
+impulses being augmented, the potential energy of each would
+diminish, so that finally only atomic vibrations could be set up,
+and the motion of translation through measurable space would
+cease. Thus an ordinary gas burner connected to a source of
+rapidly alternating potential might have its efficiency augmented
+to a certain limit, and this for two reasons&mdash;because of the additional
+vibration imparted, and because of a slowing down of the
+process of carrying off. But the renewal being rendered difficult,
+a renewal being necessary to maintain the <i>burner</i>, a continued
+increase of the frequency of the impulses, assuming they could
+be transmitted to and impressed upon the flame, would result in
+the "extinction" of the latter, meaning by this term only the
+cessation of the chemical process.</p>
+
+<p>I think, however, that in the case of an electrode immersed in
+a fluid insulating medium, and surrounded by independent carriers
+of electric charges, which can be acted upon inductively, a
+sufficient high frequency of the impulses would probably result
+in a gravitation of the gas all around toward the electrode. For
+this it would be only necessary to assume that the independent
+bodies are irregularly shaped; they would then turn toward the
+electrode their side of the greatest electric density, and this
+would be a position in which the fluid resistance to approach
+would be smaller than that offered to the receding.</p>
+
+<p>The general opinion, I do not doubt, is that it is out of the
+question to reach any such frequencies as might&mdash;assuming some
+of the views before expressed to be true&mdash;produce any of the results
+which I have pointed out as mere possibilities. This may be
+so, but in the course of these investigations, from the observation
+of many phenomena, I have gained the conviction that these frequencies
+would be much lower than one is apt to estimate at<span class='pagenum'><a name="Page_273" id="Page_273">[Pg 273]</a></span>
+first. In a flame we set up light vibrations by causing molecules,
+or atoms, to collide. But what is the ratio of the frequency of
+the collisions and that of the vibrations set up? Certainly it
+must be incomparably smaller than that of the strokes of the bell
+and the sound vibrations, or that of the discharges and the oscillations
+of the condenser. We may cause the molecules of the
+gas to collide by the use of alternate electric impulses of high
+frequency, and so we may imitate the process in a flame; and
+from experiments with frequencies which we are now able to
+obtain, I think that the result is producible with impulses which
+are transmissible through a conductor.</p>
+
+<p>In connection with thoughts of a similar nature, it appeared to
+me of great interest to demonstrate the rigidity of a vibrating gaseous
+column. Although with such low frequencies as, say 10,000
+per second, which I was able to obtain without difficulty from a
+specially constructed alternator, the task looked discouraging at
+first, I made a series of experiments. The trials with air at ordinary
+pressure led to no result, but with air moderately rarefied I
+obtain what I think to be an unmistakable experimental evidence
+of the property sought for. As a result of this kind might lead
+able investigators to conclusions of importance, I will describe
+one of the experiments performed.</p>
+
+<p>It is well known that when a tube is slightly exhausted, the
+discharge may be passed through it in the form of a thin luminous
+thread. When produced with currents of low frequency,
+obtained from a coil operated as usual, this thread is inert. If a
+magnet be approached to it, the part near the same is attracted
+or repelled, according to the direction of the lines of force of the
+magnet. It occurred to me that if such a thread would be produced
+with currents of very high frequency, it should be more
+or less rigid, and as it was visible it could be easily studied.
+Accordingly I prepared a tube about one inch in diameter and
+one metre long, with outside coating at each end. The tube was
+exhausted to a point at which, by a little working, the thread discharge
+could be obtained. It must be remarked here that the
+general aspect of the tube, and the degree of exhaustion, are
+quite other than when ordinary low frequency currents are
+used. As it was found preferable to work with one terminal,
+the tube prepared was suspended from the end of a wire connected
+to the terminal, the tinfoil coating being connected to the
+wire, and to the lower coating sometimes a small insulated plate<span class='pagenum'><a name="Page_274" id="Page_274">[Pg 274]</a></span>
+was attached. When the thread was formed, it extended through
+the upper part of the tube and lost itself in the lower end. If it
+possessed rigidity it resembled, not exactly an elastic cord
+stretched tight between two supports, but a cord suspended from
+a height with a small weight attached at the end. When the
+finger or a small magnet was approached to the upper end of the
+luminous thread, it could be brought locally out of position by
+electrostatic or magnetic action; and when the disturbing object
+was very quickly removed, an analogous result was produced, as
+though a suspended cord would be displaced and quickly released
+near the point of suspension. In doing this the luminous thread
+was set in vibration, and two very sharply marked nodes, and a
+third indistinct one, were formed. The vibration, once set up,
+continued for fully eight minutes, dying gradually out. The
+speed of the vibration often varied perceptibly, and it could be
+observed that the electrostatic attraction of the glass affected the
+vibrating thread; but it was clear that the electrostatic action
+was not the cause of the vibration, for the thread was most generally
+stationary, and could always be set in vibration by passing
+the finger quickly near the upper part of the tube. With a
+magnet the thread could be split in two and both parts vibrated.
+By approaching the hand to the lower coating of the tube, or
+insulation plate if attached, the vibration was quickened; also, as
+far as I could see, by raising the potential or frequency. Thus,
+either increasing the frequency or passing a stronger discharge
+of the same frequency corresponded to a tightening of the cord.
+I did not obtain any experimental evidence with condenser discharges.
+A luminous band excited in the bulb by repeated discharges
+of a Leyden jar must possess rigidity, and if deformed
+and suddenly released, should vibrate. But probably the amount
+of vibrating matter is so small that in spite of the extreme speed,
+the inertia cannot prominently assert itself. Besides, the observation
+in such a case is rendered extremely difficult on account
+of the fundamental vibration.</p>
+
+<p>The demonstration of the fact&mdash;which still needs better experimental
+confirmation&mdash;that a vibrating gaseous column possesses
+rigidity, might greatly modify the views of thinkers.
+When with low frequencies and insignificant potentials indications
+of that property may be noted, how must a gaseous medium behave
+under the influence of enormous electrostatic stresses which
+may be active in the interstellar space, and which may alternate<span class='pagenum'><a name="Page_275" id="Page_275">[Pg 275]</a></span>
+with inconceivable rapidity? The existence of such an electrostatic,
+rhythmically throbbing force&mdash;of a vibrating electrostatic
+field&mdash;would show a possible way how solids might have formed
+from the ultra-gaseous uterus, and how transverse and all kinds
+of vibrations may be transmitted through a gaseous medium filling
+all space. Then, ether might be a true fluid, devoid of
+rigidity, and at rest, it being merely necessary as a connecting
+link to enable interaction. What determines the rigidity of a
+body? It must be the speed and the amount of motive matter.
+In a gas the speed maybe considerable, but the density is exceedingly
+small; in a liquid the speed would be likely to be small,
+though the density may be considerable; and in both cases the
+inertia resistance offered to displacement is practically <i>nil</i>. But
+place a gaseous (or liquid) column in an intense, rapidly alternating
+electrostatic field, set the particles vibrating with enormous
+speeds, then the inertia resistance asserts itself. A body might
+move with more or less freedom through the vibrating mass, but
+as a whole it would be rigid.</p>
+
+<p>There is a subject which I must mention in connection with
+these experiments: it is that of high vacua. This is a subject,
+the study of which is not only interesting, but useful, for it may
+lead to results of great practical importance. In commercial apparatus,
+such as incandescent lamps, operated from ordinary
+systems of distribution, a much higher vacuum than is obtained at
+present would not secure a very great advantage. In such a case
+the work is performed on the filament, and the gas is little concerned;
+the improvement, therefore, would be but trifling. But
+when we begin to use very high frequencies and potentials, the
+action of the gas becomes all important, and the degree of exhaustion
+materially modifies the results. As long as ordinary
+coils, even very large ones, were used, the study of the subject
+was limited, because just at a point when it became most interesting
+it had to be interrupted on account of the "non-striking"
+vacuum being reached. But at present we are able to obtain
+from a small disruptive discharge coil potentials much higher
+than even the largest coil was capable of giving, and, what is
+more, we can make the potential alternate with great rapidity.
+Both of these results enable us now to pass a luminous discharge
+through almost any vacua obtainable, and the field of our investigations
+is greatly extended. Think we as we may, of all the
+possible directions to develop a practical illuminant, the line of<span class='pagenum'><a name="Page_276" id="Page_276">[Pg 276]</a></span>
+high vacua seems to be the most promising at present. But to
+reach extreme vacua the appliances must be much more improved,
+and ultimate perfection will not be attained until we shall have
+discharged the mechanical and perfected an <i>electrical</i> vacuum
+pump. Molecules and atoms can be thrown out of a bulb under
+the action of an enormous potential: <i>this</i> will be the principle
+of the vacuum pump of the future. For the present, we must
+secure the best results we can with mechanical appliances. In
+this respect, it might not be out of the way to say a few words
+about the method of, and apparatus for, producing excessively
+high degrees of exhaustion of which I have availed myself in the
+course of these investigations. It is very probable that other experimenters
+have used similar arrangements; but as it is possible
+that there may be an item of interest in their description, a few
+remarks, which will render this investigation more complete,
+might be permitted.</p>
+
+<div class="figcenter" style="width: 480px;">
+<img src="images/oi_290.jpg" width="480" height="611" alt="Fig. 159." title="" />
+<span class="caption">Fig. 159.</span>
+</div>
+
+
+<p>The apparatus is illustrated in a drawing shown in Fig. 159.
+<small>S</small> represents a Sprengel pump, which has been specially constructed
+to better suit the work required. The stop-cock which<span class='pagenum'><a name="Page_277" id="Page_277">[Pg 277]</a></span>
+is usually employed has been omitted, and instead of it a hollow
+stopper s has been fitted in the neck of the reservoir <small>R</small>. This
+stopper has a small hole <i>h</i>, through which the mercury descends;
+the size of the outlet <i>o</i> being properly determined with respect
+to the section of the fall tube <i>t</i>, which is sealed to the reservoir
+instead of being connected to it in the usual manner. This
+arrangement overcomes the imperfections and troubles which
+often arise from the use of the stopcock on the reservoir and the
+connections of the latter with the fall tube.</p>
+
+<p>The pump is connected through a <big><b>U</b></big>-shaped tube <i>t</i> to a very
+large reservoir <small>R<sub>1</sub></small>. Especial care was taken in fitting the grinding
+surfaces of the stoppers <i>p</i> and <i>p</i><sub>1</sub>, and both of these and the
+mercury caps above them were made exceptionally long. After
+the <big><b>U</b></big>-shaped tube was fitted and put in place, it was heated, so
+as to soften and take off the strain resulting from imperfect
+fitting. The <big><b>U</b></big>-shaped tube was provided with a stopcock <small>C</small>,
+and two ground connections <i>g</i> and <i>g</i><sub>1</sub>,&mdash;one for a small bulb <i>b</i>,
+usually containing caustic potash, and the other for the receiver
+<i>r</i>, to be exhausted.</p>
+
+<p>The reservoir <small>R<sub>1</sub></small>, was connected by means of a rubber tube to
+a slightly larger reservoir <small>R<sub>2</sub></small>, each of the two reservoirs being
+provided with a stopcock <small>C<sub>1</sub></small> and <small>C<sub>2</sub></small>, respectively. The reservoir
+<small>R<sub>2</sub></small> could be raised and lowered by a wheel and rack, and the
+range of its motion was so determined that when it was filled
+with mercury and the stopcock <small>C<sub>2</sub></small> closed, so as to form a Torricellian
+vacuum in it when raised, it could be lifted so high that
+the reservoir <small>R<sub>1</sub></small> would stand a little above stopcock <small>C<sub>1</sub></small>; and when
+this stopcock was closed and the reservoir <small>R<sub>2</sub></small> descended, so as to
+form a Torricellian vacuum in reservoir <small>R<sub>1</sub></small>, it could be lowered
+so far as to completely empty the latter, the mercury filling the
+reservoir <small>R<sub>2</sub></small> up to a little above stopcock <small>C<sub>2</sub></small>.</p>
+
+<p>The capacity of the pump and of the connections was taken
+as small as possible relatively to the volume of reservoir, <small>R<sub>1</sub></small>,
+since, of course, the degree of exhaustion depended upon the
+ratio of these quantities.</p>
+
+<p>With this apparatus I combined the usual means indicated by
+former experiments for the production of very high vacua. In
+most of the experiments it was most convenient to use caustic
+potash. I may venture to say, in regard to its use, that much
+time is saved and a more perfect action of the pump insured by
+fusing and boiling the potash as soon as, or even before, the<span class='pagenum'><a name="Page_278" id="Page_278">[Pg 278]</a></span>
+pump settles down. If this course is not followed, the sticks, as
+ordinarily employed, may give off moisture at a certain very
+slow rate, and the pump may work for many hours without
+reaching a very high vacuum. The potash was heated either by
+a spirit lamp or by passing a discharge through it, or by passing
+a current through a wire contained in it. The advantage in the
+latter case was that the heating could be more rapidly repeated.</p>
+
+<p>Generally the process of exhaustion was the following:&mdash;At
+the start, the stop-cocks <small>C</small> and <small>C<sub>1</sub></small> being open, and all other connections
+closed, the reservoir <small>R<sub>2</sub></small> was raised so far that the mercury
+filled the reservoir <small>R<sub>1</sub></small> and a part of the narrow connecting
+<big><b>U</b></big>-shaped tube. When the pump was set to work, the mercury
+would, of course, quickly rise in the tube, and reservoir <small>R<sub>2</sub></small> was
+lowered, the experimenter keeping the mercury at about the
+same level. The reservoir <small>R<sub>2</sub></small> was balanced by a long spring
+which facilitated the operation, and the friction of the parts was
+generally sufficient to keep it in almost any position. When the
+Sprengel pump had done its work, the reservoir <small>R<sub>2</sub></small> was further lowered
+and the mercury descended in <small>R</small><sub>1</sub> and filled <small>R<sub>2</sub></small>, whereupon stopcock
+<small>C<sub>2</sub></small> was closed. The air adhering to the walls of <small>R<sub>1</sub></small> and that
+absorbed by the mercury was carried off, and to free the mercury
+of all air the reservoir <small>R<sub>2</sub></small> was for a long time worked up and
+down. During this process some air, which would gather below
+stopcock <small>C<sub>2</sub></small>, was expelled from <small>R<sub>2</sub></small> by lowering it far enough and
+opening the stopcock, closing the latter again before raising the
+reservoir. When all the air had been expelled from the mercury,
+and no air would gather in <small>R<sub>2</sub></small> when it was lowered, the caustic
+potash was resorted to. The reservoir <small>R<sub>2</sub></small> was now again raised
+until the mercury in <small>R<sub>1</sub></small>, stood above stopcock <small>C<sub>1</sub></small>. The caustic
+potash was fused and boiled, and moisture partly carried off by
+the pump and partly re-absorbed; and this process of heating
+and cooling was repeated many times, and each time, upon the
+moisture being absorbed or carried off, the reservoir <small>R<sub>2</sub></small> was for
+a long time raised and lowered. In this manner all the moisture
+was carried off from the mercury, and both the reservoirs were
+in proper condition to be used. The reservoir <small>R<sub>2</sub></small> was then again
+raised to the top, and the pump was kept working for a long
+time. When the highest vacuum obtainable with the pump had
+been reached, the potash bulb was usually wrapped with cotton
+which was sprinkled with ether so as to keep the potash at a
+very low temperature, then the reservoir <small>R<sub>2</sub></small> was lowered, and upon
+<span class='pagenum'><a name="Page_279" id="Page_279">[Pg 279]</a></span>reservoir
+<small>R<sub>1</sub></small> being emptied the receiver was quickly sealed up.</p>
+
+<p>When a new bulb was put on, the mercury was always raised
+above stopcock <small>C<sub>1</sub></small>, which was closed, so as to always keep the
+mercury and both the reservoirs in fine condition, and the mercury
+was never withdrawn from <small>R<sub>1</sub></small> except when the pump had
+reached the highest degree of exhaustion. It is necessary to observe
+this rule if it is desired to use the apparatus to advantage.</p>
+
+<p>By means of this arrangement I was able to proceed very
+quickly, and when the apparatus was in perfect order it was possible
+to reach the phosphorescent stage in a small bulb in less
+than fifteen minutes, which is certainly very quick work for a
+small laboratory arrangement requiring all in all about 100 pounds
+of mercury. With ordinary small bulbs the ratio of the capacity
+of the pump, receiver, and connections, and that of reservoir <small>R</small>
+was about 1 to 20, and the degrees of exhaustion reached were
+necessarily very high, though I am unable to make a precise and
+reliable statement how far the exhaustion was carried.</p>
+
+<p>What impresses the investigator most in the course of these
+experiences is the behavior of gases when subjected to great rapidly
+alternating electrostatic stresses. But he must remain in
+doubt as to whether the effects observed are due wholly to the
+molecules, or atoms, of the gas which chemical analysis discloses
+to us, or whether there enters into play another medium of a
+gaseous nature, comprising atoms, or molecules, immersed in a
+fluid pervading the space. Such a medium surely must exist,
+and I am convinced that, for instance, even if air were absent,
+the surface and neighborhood of a body in space would be heated
+by rapidly alternating the potential of the body; but no such
+heating of the surface or neighborhood could occur if all free
+atoms were removed and only a homogeneous, incompressible, and
+elastic fluid&mdash;such as ether is supposed to be&mdash;would remain, for
+then there would be no impacts, no collisions. In such a case,
+as far as the body itself is concerned, only frictional losses in the
+inside could occur.</p>
+
+<p>It is a striking fact that the discharge through a gas is established
+with ever-increasing freedom as the frequency of the
+impulses is augmented. It behaves in this respect quite contrarily
+to a metallic conductor. In the latter the impedance enters
+prominently into play as the frequency is increased, but the gas
+acts much as a series of condensers would; the facility with
+which the discharge passes through, seems to depend on the rate
+of change of potential. If it acts so, then in a vacuum tube even<span class='pagenum'><a name="Page_280" id="Page_280">[Pg 280]</a></span>
+of great length, and no matter how strong the current, self-induction
+could not assert itself to any appreciable degree. We
+have, then, as far as we can now see, in the gas a conductor
+which is capable of transmitting electric impulses of any frequency
+which we may be able to produce. Could the frequency be
+brought high enough, then a queer system of electric distribution,
+which would be likely to interest gas companies, might be realized:
+metal pipes filled with gas&mdash;the metal being the insulator,
+the gas the conductor&mdash;supplying phosphorescent bulbs, or perhaps
+devices as yet uninvented. It is certainly possible to take
+a hollow core of copper, rarefy the gas in the same, and by passing
+impulses of sufficiently high frequency through a circuit
+around it, bring the gas inside to a high degree of incandescence;
+but as to the nature of the forces there would be considerable
+uncertainty, for it would be doubtful whether with such impulses
+the copper core would act as a static screen. Such paradoxes and
+apparent impossibilities we encounter at every step in this line of
+work, and therein lies, to a great extent, the charm of the study.</p>
+
+<p>I have here a short and wide tube which is exhausted to a
+high degree and covered with a substantial coating of bronze, the
+coating barely allowing the light to shine through. A metallic
+cap, with a hook for suspending the tube, is fastened around the
+middle portion of the latter, the clasp being in contact with the
+bronze coating. I now want to light the gas inside by suspending
+the tube on a wire connected to the coil. Any one who
+would try the experiment for the first time, not having any previous
+experience, would probably take care to be quite alone
+when making the trial, for fear that he might become the joke of
+his assistants. Still, the bulb lights in spite of the metal coating,
+and the light can be distinctly perceived through the latter. A
+long tube covered with aluminum bronze lights when held in
+one hand&mdash;the other touching the terminal of the coil&mdash;quite
+powerfully. It might be objected that the coatings are not
+sufficiently conducting; still, even if they were highly resistant,
+they ought to screen the gas. They certainly screen it perfectly
+in a condition of rest, but far from perfectly when the charge
+is surging in the coating. But the loss of energy which occurs
+within the tube, notwithstanding the screen, is occasioned principally
+by the presence of the gas. Were we to take a large
+hollow metallic sphere and fill it with a perfect, incompressible,
+fluid dielectric, there would be no loss inside of the sphere, and<span class='pagenum'><a name="Page_281" id="Page_281">[Pg 281]</a></span>
+consequently the inside might be considered as perfectly screened,
+though the potential be very rapidly alternating. Even were
+the sphere filled with oil, the loss would be incomparably smaller
+than when the fluid is replaced by a gas, for in the latter case the
+force produces displacements; that means impact and collisions
+in the inside.</p>
+
+<p>No matter what the pressure of the gas may be, it becomes an
+important factor in the heating of a conductor when the electric
+density is great and the frequency very high. That in the heating
+of conductors by lightning discharges, air is an element of
+great importance, is almost as certain as an experimental fact. I
+may illustrate the action of the air by the following experiment:
+I take a short tube which is exhausted to a moderate degree and
+has a platinum wire running through the middle from one end
+to the other. I pass a steady or low frequency current through
+the wire, and it is heated uniformly in all parts. The heating
+here is due to conduction, or frictional losses, and the gas around
+the wire has&mdash;as far as we can see&mdash;no function to perform.
+But now let me pass sudden discharges, or high frequency currents,
+through the wire. Again the wire is heated, this time
+principally on the ends and least in the middle portion; and if
+the frequency of the impulses, or the rate of change, is high
+enough, the wire might as well be cut in the middle as not, for
+practically all heating is due to the rarefied gas. Here the gas
+might only act as a conductor of no impedance diverting the current
+from the wire as the impedance of the latter is enormously
+increased, and merely heating the ends of the wire by reason of
+their resistance to the passage of the discharge. But it is not
+at all necessary that the gas in the tube should be conducting; it
+might be at an extremely low pressure, still the ends of the wire
+would be heated&mdash;as, however, is ascertained by experience&mdash;only
+the two ends would in such case not be electrically connected
+through the gaseous medium. Now what with these frequencies
+and potentials occurs in an exhausted tube, occurs in the
+lightning discharges at ordinary pressure. We only need remember
+one of the facts arrived at in the course of these investigations,
+namely, that to impulses of very high frequency the gas
+at ordinary pressure behaves much in the same manner as though
+it were at moderately low pressure. I think that in lightning
+discharges frequently wires or conducting objects are volatilized
+merely because air is present, and that, were the conductor im<span class='pagenum'><a name="Page_282" id="Page_282">[Pg 282]</a></span>mersed
+in an insulating liquid, it would be safe, for then the
+energy would have to spend itself somewhere else. From the
+behavior of gases under sudden impulses of high potential, I am
+led to conclude that there can be no surer way of diverting a
+lightning discharge than by affording it a passage through a
+volume of gas, if such a thing can be done in a practical manner.</p>
+
+<p>There are two more features upon which I think it necessary
+to dwell in connection with these experiments&mdash;the "radiant
+state" and the "non-striking vacuum."</p>
+
+<p>Any one who has studied Crookes' work must have received
+the impression that the "radiant state" is a property of the gas
+inseparably connected with an extremely high degree of exhaustion.
+But it should be remembered that the phenomena
+observed in an exhausted vessel are limited to the character and
+capacity of the apparatus which is made use of. I think that in
+a bulb a molecule, or atom, does not precisely move in a straight
+line because it meets no obstacle, but because the velocity imparted
+to it is sufficient to propel it in a sensibly straight line.
+The mean free path is one thing, but the velocity&mdash;the energy
+associated with the moving body&mdash;is another, and under ordinary
+circumstances I believe that it is a mere question of potential or
+speed. A disruptive discharge coil, when the potential is pushed
+very far, excites phosphorescence and projects shadows, at comparatively
+low degrees of exhaustion. In a lightning discharge,
+matter moves in straight lines at ordinary pressure when the
+mean free path is exceedingly small, and frequently images of
+wires or other metallic objects have been produced by the particles
+thrown off in straight lines.</p>
+
+<p>I have prepared a bulb to illustrate by an experiment the
+correctness of these assertions. In a globe <small>L</small>, Fig. 160, I have
+mounted upon a lamp filament <i>f</i> a piece of lime <i>l</i>. The lamp
+filament is connected with a wire which leads into the bulb, and
+the general construction of the latter is as indicated in Fig. 148,
+before described. The bulb being suspended from a wire
+connected to the terminal of the coil, and the latter being set to
+work, the lime piece <i>l</i> and the projecting parts of the filament <i>f</i>
+are bombarded. The degree of exhaustion is just such that with
+the potential the coil is capable of giving, phosphorescence of the
+glass is produced, but disappears as soon as the vacuum is impaired.
+The lime containing moisture, and moisture being given
+off as soon as heating occurs, the phosphorescence lasts only for<span class='pagenum'><a name="Page_283" id="Page_283">[Pg 283]</a></span>
+a few moments. When the lime has been sufficiently heated,
+enough moisture has been given off to impair materially the
+vacuum of the bulb. As the bombardment goes on, one point
+of the lime piece is more heated than other points, and the result
+is that finally practically all the discharge passes through that
+point which is intensely heated, and a white stream of lime particles
+(Fig. 160) then breaks forth from that point. This stream
+is composed of "radiant" matter, yet the degree of exhaustion
+is low. But the particles move in straight lines because the
+velocity imparted to them is great, and this is due to three
+causes&mdash;to the great electric density, the high temperature of the
+small point, and the fact that the particles of the lime are easily
+torn and thrown off&mdash;far more easily than those of carbon. With
+frequencies such as we are able to obtain, the particles are bodily
+thrown off and projected to a considerable distance; but with
+sufficiently high frequencies no such thing would occur; in such
+case only a stress would spread or a vibration would be propagated
+through the bulb. It would be out of the question to
+reach any such frequency on the assumption that the atoms move
+with the speed of light; but I believe that such a thing is impossible;
+for this an enormous potential would be required.
+With potentials which we are able to obtain, even with a disruptive
+discharge coil, the speed must be quite insignificant.</p>
+
+<div class="figcenter" style="width: 467px;">
+<img src="images/oi_297.jpg" width="467" height="640" alt="Fig. 160." title="" />
+<span class="caption">Fig. 160.</span>
+</div>
+
+
+<p>As to the "non-striking vacuum," the point to be noted is,
+that it can occur only with low frequency impulses, and it is<span class='pagenum'><a name="Page_284" id="Page_284">[Pg 284]</a></span>
+necessitated by the impossibility of carrying off enough energy
+with such impulses in high vacuum, since the few atoms which
+are around the terminal upon coming in contact with the same,
+are repelled and kept at a distance for a comparatively long
+period of time, and not enough work can be performed to render
+the effect perceptible to the eye. If the difference of potential
+between the terminals is raised, the dielectric breaks down. But
+with very high frequency impulses there is no necessity for such
+breaking down, since any amount of work can be performed by
+continually agitating the atoms in the exhausted vessel, provided
+the frequency is high enough. It is easy to reach&mdash;even with
+frequencies obtained from an alternator as here used&mdash;a stage at
+which the discharge does not pass between two electrodes in a
+narrow tube, each of these being connected to one of the terminals
+of the coil, but it is difficult to reach a point at which a
+luminous discharge would not occur around each electrode.</p>
+
+<p>A thought which naturally presents itself in connection with
+high frequency currents, is to make use of their powerful electrodynamic
+inductive action to produce light effects in a sealed glass
+globe. The leading-in wire is one of the defects of the present
+incandescent lamp, and if no other improvement were made,
+that imperfection at least should be done away with. Following<span class='pagenum'><a name="Page_285" id="Page_285">[Pg 285]</a></span>
+this thought, I have carried on experiments in various directions,
+of which some were indicated in my former paper. I may here
+mention one or two more lines of experiment which have been
+followed up.</p>
+
+<p>Many bulbs were constructed as shown in Fig. 161 and Fig.
+162.</p>
+
+<div class="figcenter" style="width: 693px;">
+<img src="images/oi_298.jpg" width="693" height="600" alt="Fig. 161, 162." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 161.</td><td class="caption">Fig. 162.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 161, a wide tube, <small>T</small>, was sealed to a smaller <big><b>W</b></big> shaped
+tube <small>U</small>, of phosphorescent glass. In the tube <small>T</small>, was placed a coil
+<small>C</small>, of aluminum wire, the ends of which were provided with
+small spheres, <i>t</i> and <i>t</i><sub>1</sub>, of aluminum, and reached into the <small>U</small> tube.
+The tube <small>T</small> was slipped into a socket containing a primary coil,
+through which usually the discharges of Leyden jars were directed,
+and the rarefied gas in the small <small>U</small> tube was excited to
+strong luminosity by the high-tension current induced in the coil <small>C</small>.
+When Leyden jar discharges were used to induce currents in the
+coil <small>C</small>, it was found necessary to pack the tube <small>T</small> tightly with insulating
+powder, as a discharge would occur frequently between
+the turns of the coil, especially when the primary was thick and
+the air gap, through which the jars discharged, large, and no
+little trouble was experienced in this way.</p>
+
+<p>In Fig. 162 is illustrated another form of the bulb constructed.
+In this case a tube <small>T</small> is sealed to a globe <small>L</small>. The tube contains a
+coil <small>C</small>, the ends of which pass through two small glass tubes <i>t</i>
+and <i>t</i><sub>1</sub>, which are sealed to the tube <small>T</small>. Two refractory buttons
+<i>m</i> and <i>m</i><sub>1</sub>, are mounted on lamp filaments which are fastened to
+the ends of the wires passing through the glass tubes <i>t</i> and <i>t</i><sub>1</sub>.
+Generally in bulbs made on this plan the globe <small>L</small> communicated
+with the tube <small>T</small>. For this purpose the ends of the small tubes <i>t</i>
+and <i>t</i><sub>1</sub> were heated just a trifle in the burner, merely to hold the
+wires, but not to interfere with the communication. The tube <small>T</small>,
+with the small tubes, wires through the same, and the refractory
+buttons <i>m</i> and <i>m</i><sub>1</sub>, were first prepared, and then sealed to globe <small>L</small>,
+whereupon the coil <small>C</small> was slipped in and the connections made to
+its ends. The tube was then packed with insulating powder,
+jamming the latter as tight as possible up to very nearly the end;
+then it was closed and only a small hole left through which the
+remainder of the powder was introduced, and finally the end of
+the tube was closed. Usually in bulbs constructed as shown in
+Fig. 162 an aluminum tube <i>a</i> was fastened to the upper end <i>s</i>
+of each of the tubes <i>t</i> and <i>t</i><sub>1</sub> in order to protect that end against
+<span class='pagenum'><a name="Page_286" id="Page_286">[Pg 286]</a></span>the heat. The buttons <i>m</i> and <i>m</i><sub>1</sub> could be brought to any degree
+of incandescence by passing the discharges of Leyden jars
+around the coil <small>C</small>. In such bulbs with two buttons a very curious
+effect is produced by the formation of the shadows of each
+of the two buttons.</p>
+
+<p>Another line of experiment, which has been assiduously followed,
+was to induce by electro-dynamic induction a current or
+luminous discharge in an exhausted tube or bulb. This matter
+has received such able treatment at the hands of Prof. J. J.
+Thomson, that I could add but little to what he has made known,
+even had I made it the special subject of this lecture. Still,
+since experiments in this line have gradually led me to the present
+views and results, a few words must be devoted here to this
+subject.</p>
+
+<p>It has occurred, no doubt, to many that as a vacuum tube is
+made longer, the electromotive force per unit length of the tube,
+necessary to pass a luminous discharge through the latter, becomes
+continually smaller; therefore, if the exhausted tube be made
+long enough, even with low frequencies a luminous discharge
+could be induced in such a tube closed upon itself. Such a tube
+might be placed around a hall or on a ceiling, and at once a simple
+appliance capable of giving considerable light would be obtained.
+But this would be an appliance hard to manufacture
+and extremely unmanageable. It would not do to make the
+tube up of small lengths, because there would be with ordinary
+frequencies considerable loss in the coatings, and besides, if coatings
+were used, it would be better to supply the current directly
+to the tube by connecting the coatings to a transformer. But
+even if all objections of such nature were removed, with
+low frequencies the light conversion itself would be inefficient,
+as I have before stated. In using extremely high frequencies
+the length of the secondary&mdash;in other words, the size of the vessel&mdash;can
+be reduced as much as desired, and the efficiency of the
+light conversion is increased, provided that means are invented
+for efficiently obtaining such high frequencies. Thus one is led,
+from theoretical and practical considerations, to the use of high
+frequencies, and this means high electromotive forces and small
+currents in the primary. When one works with condenser
+charges&mdash;and they are the only means up to the present known
+for reaching these extreme frequencies&mdash;one gets to electromotive
+forces of several thousands of volts per turn of the primary. We
+cannot multiply the electro-dynamic inductive effect by taking<span class='pagenum'><a name="Page_287" id="Page_287">[Pg 287]</a></span>
+more turns in the primary, for we arrive at the conclusion that
+the best way is to work with one single turn&mdash;though we must
+sometimes depart from this rule&mdash;and we must get along with
+whatever inductive effect we can obtain with one turn. But before
+one has long experimented with the extreme frequencies required
+to set up in a small bulb an electromotive force of several
+thousands of volts, one realizes the great importance of electrostatic
+effects, and these effects grow relatively to the electro-dynamic
+in significance as the frequency is increased.</p>
+
+<p>Now, if anything is desirable in this case, it is to increase the
+frequency, and this would make it still worse for the electrodynamic
+effects. On the other hand, it is easy to exalt the electrostatic
+action as far as one likes by taking more turns on the
+secondary, or combining self-induction and capacity to raise the
+potential. It should also be remembered that, in reducing the
+current to the smallest value and increasing the potential,
+the electric impulses of high frequency can be more easily transmitted
+through a conductor.</p>
+
+<p>These and similar thoughts determined me to devote more attention
+to the electrostatic phenomena, and to endeavor to produce
+potentials as high as possible, and alternating as fast as
+they could be made to alternate. I then found that I could excite
+vacuum tubes at considerable distance from a conductor
+connected to a properly constructed coil, and that I could, by
+converting the oscillatory current of a conductor to a higher potential,
+establish electrostatic alternating fields which acted
+through the whole extent of the room, lighting up a tube no
+matter where it was held in space. I thought I recognized that
+I had made a step in advance, and I have persevered in this line;
+but I wish to say that I share with all lovers of science and progress
+the one and only desire&mdash;to reach a result of utility to men
+in any direction to which thought or experiment may lead me.
+I think that this departure is the right one, for I cannot see,
+from the observation of the phenomena which manifest themselves
+as the frequency is increased, what there would remain to
+act between two circuits conveying, for instance, impulses of
+several hundred millions per second, except electrostatic forces.
+Even with such trifling frequencies the energy would be practically
+all potential, and my conviction has grown strong that, to whatever
+kind of motion light may be due, it is produced by tremendous
+electrostatic stresses vibrating with extreme rapidity.<span class='pagenum'><a name="Page_288" id="Page_288">[Pg 288]</a></span></p>
+
+<div class="figcenter" style="width: 517px;">
+<img src="images/oi_302.jpg" width="517" height="800" alt="Fig. 163, 164." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 163.</td><td class="caption">Fig. 164.</td></tr>
+</table>
+</div>
+
+<p>Of all these phenomena observed with currents, or electric
+impulses, of high frequency, the most fascinating for an audience
+are certainly those which are noted in an electrostatic field
+acting through considerable distance; and the best an unskilled
+lecturer can do is to begin and finish with the exhibition of these
+singular effects. I take a tube in my hand and move it about,
+and it is lighted wherever I may hold it; throughout space the
+invisible forces act. But I may take another tube and it might
+not light, the vacuum being very high. I excite it by means of a
+disruptive discharge coil, and now it will light in the electrostatic
+field. I may put it away for a few weeks or months, still it retains
+the faculty of being excited. What change have I produced in the
+tube in the act of exciting it? If a motion imparted to atoms, it
+is difficult to perceive how it can persist so long without being
+arrested by frictional losses; and if a strain exerted in the dielectric,
+such as a simple electrification would produce, it is easy to
+see how it may persist indefinitely, but very difficult to understand
+why such a condition should aid the excitation when we
+have to deal with potentials which are rapidly alternating.<span class='pagenum'><a name="Page_289" id="Page_289">[Pg 289]</a></span></p>
+
+<p>Since I have exhibited these phenomena for the first time, I
+have obtained some other interesting effects. For instance, I
+have produced the incandescence of a button, filament, or wire
+enclosed in a tube. To get to this result it was necessary to
+economize the energy which is obtained from the field, and direct
+most of it on the small body to be rendered incandescent. At
+the beginning the task appeared difficult, but the experiences
+gathered permitted me to reach the result easily. In Fig. 163
+and Fig. 164, two such tubes are illustrated, which are prepared for
+the occasion. In Fig. 163 a short tube <small>T<sub>1</sub></small>, sealed to another long
+tube <small>T</small>, is provided with a stem <i>s</i>, with a platinum wire sealed in
+the latter. A very thin lamp filament <i>l</i>, is fastened to this wire
+and connection to the outside is made through a thin copper wire
+<i>w</i>. The tube is provided with outside and inside coatings, <small>C</small> and
+<small>C<sub>1</sub></small>, respectively, and is filled as far as the coatings reach with conducting,
+and the space above with insulating, powder. These
+coatings are merely used to enable me to perform two experiments
+with the tube&mdash;namely, to produce the effect desired either
+by direct connection of the body of the experimenter or of another
+body to the wire <i>w</i>, or by acting inductively through the
+glass. The stem <i>s</i> is provided with an aluminum tube <i>a</i>, for
+purposes before explained, and only a small part of the filament
+reaches out of this tube. By holding the tube <small>T<sub>1</sub></small> anywhere in
+the electrostatic field, the filament is rendered incandescent.</p>
+
+<p>A more interesting piece of apparatus is illustrated in Fig. 164.
+The construction is the same as before, only instead of the lamp
+filament a small platinum wire <i>p</i>, sealed in a stem <i>s</i>, and bent
+above it in a circle, is connected to the copper wire <i>w</i>, which is
+joined to an inside coating <small>C</small>. A small stem <i>s</i><sub>1</sub> is provided with
+a needle, on the point of which is arranged, to rotate very freely,
+a very light fan of mica <i>v</i>. To prevent the fan from falling out,
+a thin stem of glass <i>g</i>, is bent properly and fastened to the aluminum
+tube. When the glass tube is held anywhere in the electrostatic
+field the platinum wire becomes incandescent, and the
+mica vanes are rotated very fast.</p>
+
+<p>Intense phosphorescence may be excited in a bulb by merely
+connecting it to a plate within the field, and the plate need not
+be any larger than an ordinary lamp shade. The phosphorescence
+excited with these currents is incomparably more powerful
+than with ordinary apparatus. A small phosphorescent bulb,
+when attached to a wire connected to a coil, emits sufficient light<span class='pagenum'><a name="Page_290" id="Page_290">[Pg 290]</a></span>
+to allow reading ordinary print at a distance of five to six paces.
+It was of interest to see how some of the phosphorescent bulbs
+of Professor Crookes would behave with these currents, and he
+has had the kindness to lend me a few for the occasion. The
+effects produced are magnificent, especially by the sulphide of
+calcium and sulphide of zinc. With the disruptive discharge
+coil they glow intensely merely by holding them in the hand and
+connecting the body to the terminal of the coil.</p>
+
+<p>To whatever results investigations of this kind may lead, the
+chief interest lies, for the present, in the possibilities they offer
+for the production of an efficient illuminating device. In no
+branch of electric industry is an advance more desired than in
+the manufacture of light. Every thinker, when considering the
+barbarous methods employed, the deplorable losses incurred in
+our best systems of light production, must have asked himself,
+What is likely to be the light of the future? Is it to be an incandescent
+solid, as in the present lamp, or an incandescent gas,
+or a phosphorescent body, or something like a burner, but incomparably
+more efficient?</p>
+
+<p>There is little chance to perfect a gas burner; not, perhaps,
+because human ingenuity has been bent upon that problem for
+centuries without a radical departure having been made&mdash;though
+the argument is not devoid of force&mdash;but because in a
+burner the highest vibrations can never be reached, except by
+passing through all the low ones. For how is a flame to proceed
+unless by a fall of lifted weights? Such process cannot be maintained
+without renewal, and renewal is repeated passing from low
+to high vibrations. One way only seems to be open to improve
+a burner, and that is by trying to reach higher degrees of incandescence.
+Higher incandescence is equivalent to a quicker vibration:
+that means more light from the same material, and that
+again, means more economy. In this direction some improvements
+have been made, but the progress is hampered by many
+limitations. Discarding, then, the burner, there remains the
+three ways first mentioned, which are essentially electrical.</p>
+
+<p>Suppose the light of the immediate future to be a solid, rendered
+incandescent by electricity. Would it not seem that it is
+better to employ a small button than a frail filament? From
+many considerations it certainly must be concluded that a button
+is capable of a higher economy, assuming, of course, the difficulties
+connected with the operation of such a lamp to be effec<span class='pagenum'><a name="Page_291" id="Page_291">[Pg 291]</a></span>tively
+overcome. But to light such a lamp we require a high
+potential; and to get this economically, we must use high frequencies.</p>
+
+<p>Such considerations apply even more to the production of light
+by the incandescence of a gas, or by phosphorescence. In all
+cases we require high frequencies and high potentials. These
+thoughts occurred to me a long time ago.</p>
+
+<p>Incidentally we gain, by the use of high frequencies, many advantages,
+such as higher economy in the light production, the
+possibility of working with one lead, the possibility of doing away
+with the leading-in wire, etc.</p>
+
+<p>The question is, how far can we go with frequencies? Ordinary
+conductors rapidly lose the facility of transmitting electric
+impulses when the frequency is greatly increased. Assume the
+means for the production of impulses of very great frequency
+brought to the utmost perfection, every one will naturally ask
+how to transmit them when the necessity arises. In transmitting
+such impulses through conductors we must remember that we
+have to deal with <i>pressure</i> and <i>flow</i>, in the ordinary interpretation
+of these terms. Let the pressure increase to an enormous value,
+and let the flow correspondingly diminish, then such impulses&mdash;variations
+merely of pressure, as it were&mdash;can no doubt be
+transmitted through a wire even if their frequency be many
+hundreds of millions per second. It would, of course, be out of
+question to transmit such impulses through a wire immersed in a
+gaseous medium, even if the wire were provided with a thick
+and excellent insulation, for most of the energy would be lost in
+molecular bombardment and consequent heating. The end of
+the wire connected to the source would be heated, and the remote
+end would receive but a trifling part of the energy supplied.
+The prime necessity, then, if such electric impulses are
+to be used, is to find means to reduce as much as possible the
+dissipation.</p>
+
+<p>The first thought is, to employ the thinnest possible wire surrounded
+by the thickest practicable insulation. The next thought
+is to employ electrostatic screens. The insulation of the wire
+may be covered with a thin conducting coating and the latter
+connected to the ground. But this would not do, as then all the
+energy would pass through the conducting coating to the ground
+and nothing would get to the end of the wire. If a ground connection
+is made it can only be made through a conductor offer<span class='pagenum'><a name="Page_292" id="Page_292">[Pg 292]</a></span>ing
+an enormous impedance, or through a condenser of extremely
+small capacity. This, however, does not do away with
+other difficulties.</p>
+
+<p>If the wave length of the impulses is much smaller than the
+length of the wire, then corresponding short waves will be set
+up in the conducting coating, and it will be more or less the
+same as though the coating were directly connected to earth. It
+is therefore necessary to cut up the coating in sections much
+shorter than the wave length. Such an arrangement does not
+still afford a perfect screen, but it is ten thousand times better
+than none. I think it preferable to cut up the conducting coating
+in small sections, even if the current waves be much longer
+than the coating.</p>
+
+<p>If a wire were provided with a perfect electrostatic screen, it
+would be the same as though all objects were removed from it at
+infinite distance. The capacity would then be reduced to the
+capacity of the wire itself, which would be very small. It
+would then be possible to send over the wire current vibrations
+of very high frequencies at enormous distances, without affecting
+greatly the character of the vibrations. A perfect screen is of
+course out of the question, but I believe that with a screen such
+as I have just described telephony could be rendered practicable
+across the Atlantic. According to my ideas, the gutta-percha
+covered wire should be provided with a third conducting coating
+subdivided in sections. On the top of this should be again
+placed a layer of gutta-percha and other insulation, and on the
+top of the whole the armor. But such cables will not be constructed,
+for ere long intelligence&mdash;transmitted without wires&mdash;will
+throb through the earth like a pulse through a living organism.
+The wonder is that, with the present state of knowledge
+and the experiences gained, no attempt is being made to disturb
+the electrostatic or magnetic condition of the earth, and
+transmit, if nothing else, intelligence.</p>
+
+<p>It has been my chief aim in presenting these results to point
+out phenomena or features of novelty, and to advance ideas
+which I am hopeful will serve as starting points of new departures.
+It has been my chief desire this evening to entertain you
+with some novel experiments. Your applause, so frequently
+and generously accorded, has told me that I have succeeded.</p>
+
+<p>In conclusion, let me thank you most heartily for your kindness
+and attention, and assure you that the honor I have had in<span class='pagenum'><a name="Page_293" id="Page_293">[Pg 293]</a></span>
+addressing such a distinguished audience, the pleasure I have had
+in presenting these results to a gathering of so many able men&mdash;and
+among them also some of those in whose work for many
+years past I have found enlightenment and constant pleasure&mdash;I
+shall never forget.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_294" id="Page_294">[Pg 294]</a></span></p>
+<h2><a name="CHAPTER_XXVIII" id="CHAPTER_XXVIII"></a>CHAPTER XXVIII.</h2>
+
+<h3><span class="smcap">On Light and Other High Frequency Phenomena.</span><a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a></h3>
+
+<h5>INTRODUCTORY.&mdash;SOME THOUGHTS ON THE EYE.</h5>
+
+
+<p>When we look at the world around us, on Nature, we are impressed
+with its beauty and grandeur. Each thing we perceive,
+though it may be vanishingly small, is in itself a world, that is,
+like the whole of the universe, matter and force governed by
+law,&mdash;a world, the contemplation of which fills us with feelings
+of wonder and irresistibly urges us to ceaseless thought and inquiry.
+But in all this vast world, of all objects our senses reveal
+to us, the most marvellous, the most appealing to our
+imagination, appears no doubt a highly developed organism, a
+thinking being. If there is anything fitted to make us admire
+Nature's handiwork, it is certainly this inconceivable structure,
+which performs its innumerable motions of obedience to external
+influence. To understand its workings, to get a deeper insight
+into this Nature's masterpiece, has ever been for thinkers a fascinating
+aim, and after many centuries of arduous research men have
+arrived at a fair understanding of the functions of its organs and
+senses. Again, in all the perfect harmony of its parts, of the
+parts which constitute the material or tangible of our being, of all
+its organs and senses, the eye is the most wonderful. It is the
+most precious, the most indispensable of our perceptive or directive
+organs, it is the great gateway through which all knowledge
+enters the mind. Of all our organs, it is the one, which is in the
+<span class='pagenum'><a name="Page_295" id="Page_295">[Pg 295]</a></span>most intimate relation with that which we call intellect. So intimate
+is this relation, that it is often said, the very soul shows
+itself in the eye.</p>
+
+<p>It can be taken as a fact, which the theory of the action of the
+eye implies, that for each external impression, that is, for each
+image produced upon the retina, the ends of the visual nerves,
+concerned in the conveyance of the impression to the mind, must
+be under a peculiar stress or in a vibratory state. It now does
+not seem improbable that, when by the power of thought an image
+is evoked, a distinct reflex action, no matter how weak, is
+exerted upon certain ends of the visual nerves, and therefore
+upon the retina. Will it ever be within human power to analyze
+the condition of the retina when disturbed by thought or reflex
+action, by the help of some optical or other means of such sensitiveness,
+that a clear idea of its state might be gained at any
+time? If this were possible, then the problem of reading one's
+thoughts with precision, like the characters of an open book,
+might be much easier to solve than many problems belonging to
+the domain of positive physical science, in the solution of which
+many, if not the majority, of scientific men implicitly believe.
+Helmholtz, has shown that the fundi of the eye are themselves,
+luminous, and he was able to <i>see</i>, in total darkness, the movement
+of his arm by the light of his own eyes. This is one of the
+most remarkable experiments recorded in the history of science,
+and probably only a few men could satisfactorily repeat it, for it
+is very likely, that the luminosity of the eyes is associated with
+uncommon activity of the brain and great imaginative power. It
+is fluorescence of brain action, as it were.</p>
+
+<p>Another fact having a bearing on this subject which has probably
+been noted by many, since it is stated in popular expressions,
+but which I cannot recollect to have found chronicled as a positive
+result of observation is, that at times, when a sudden idea or
+image presents itself to the intellect, there is a distinct and sometimes
+painful sensation of luminosity produced in the eye, observable
+even in broad daylight.</p>
+
+<p>The saying then, that the soul shows itself in the eye, is deeply
+founded, and we feel that it expresses a great truth. It has a
+profound meaning even for one who, like a poet or artist, only
+following his inborn instinct or love for Nature, finds delight in
+aimless thoughts and in the mere contemplation of natural phenomena,
+but a still more profound meaning for one who, in the
+<span class='pagenum'><a name="Page_296" id="Page_296">[Pg 296]</a></span>spirit of positive scientific investigation, seeks to ascertain the
+causes of the effects. It is principally the natural philosopher,
+the physicist, for whom the eye is the subject of the most intense
+admiration.</p>
+
+<p>Two facts about the eye must forcibly impress the mind of the
+physicist, notwithstanding he may think or say that it is an
+imperfect optical instrument, forgetting, that the very conception
+of that which is perfect or seems so to him, has been gained
+through this same instrument. First, the eye is, as far as our
+positive knowledge goes, the only organ which is <i>directly</i> affected
+by that subtile medium, which as science teaches us, must fill all
+space; secondly, it is the most sensitive of our organs, incomparably
+more sensitive to external impressions than any other.</p>
+
+<p>The organ of hearing implies the impact of ponderable bodies,
+the organ of smell the transference of detached material particles,
+and the organs of taste, and of touch or force, the direct contact,
+or at least some interference of ponderable matter, and this is
+true even in those instances of animal organisms, in which some
+of these organs are developed to a degree of truly marvelous
+perfection. This being so, it seems wonderful that the organ of
+sight solely should be capable of being stirred by that, which all
+our other organs are powerless to detect, yet which plays an essential
+part in all natural phenomena, which transmits all energy
+and sustains all motion and, that most intricate of all, life, but
+which has properties such that even a scientifically trained mind
+cannot help drawing a distinction between it and all that is called
+matter. Considering merely this, and the fact that the eye, by
+its marvelous power, widens our otherwise very narrow range of
+perception far beyond the limits of the small world which is our
+own, to embrace myriads of other worlds, suns and stars in the
+infinite depths of the universe, would make it justifiable to assert,
+that it is an organ of a higher order. Its performances are beyond
+comprehension. Nature as far as we know never produced anything
+more wonderful. We can get barely a faint idea of its
+prodigious power by analyzing what it does and by comparing.
+When ether waves impinge upon the human body, they produce
+the sensations of warmth or cold, pleasure or pain, or perhaps other
+sensations of which we are not aware, and any degree or intensity
+of these sensations, which degrees are infinite in number, hence an
+infinite number of distinct sensations. But our sense of touch, or
+our sense of force, cannot reveal to us these differences in degree<span class='pagenum'><a name="Page_297" id="Page_297">[Pg 297]</a></span>
+or intensity, unless they are very great. Now we can readily conceive
+how an organism, such as the human, in the eternal process
+of evolution, or more philosophically speaking, adaptation to
+Nature, being constrained to the use of only the sense of touch or
+force, for instance, might develop this sense to such a degree of
+sensitiveness or perfection, that it would be capable of distinguishing
+the minutest differences in the temperature of a body even
+at some distance, to a hundredth, or thousandth, or millionth part
+of a degree. Yet, even this apparently impossible performance
+would not begin to compare with that of the eye, which is capable
+of distinguishing and conveying to the mind in a single
+instant innumerable peculiarities of the body, be it in form,
+or color, or other respects. This power of the eye rests upon
+two things, namely, the rectilinear propagation of the disturbance
+by which it is effected, and upon its sensitiveness.
+To say that the eye is sensitive is not saying anything. Compared
+with it, all other organs are monstrously crude. The organ of
+smell which guides a dog on the trail of a deer, the organ of touch
+or force which guides an insect in its wanderings, the organ of
+hearing, which is affected by the slightest disturbances of the air,
+are sensitive organs, to be sure, but what are they compared with
+the human eye! No doubt it responds to the faintest echoes or
+reverberations of the medium; no doubt, it brings us tidings from
+other worlds, infinitely remote, but in a language we cannot as
+yet always understand. And why not? Because we live in a
+medium filled with air and other gases, vapors and a dense mass
+of solid particles flying about. These play an important part in
+many phenomena; they fritter away the energy of the vibrations
+before they can reach the eye; they too, are the carriers of germs
+of destruction, they get into our lungs and other organs, clog up
+the channels and imperceptibly, yet inevitably, arrest the stream
+of life. Could we but do away with all ponderable matter in the
+line of sight of the telescope, it would reveal to us undreamt of
+marvels. Even the unaided eye, I think, would be capable of distinguishing
+in the pure medium, small objects at distances measured
+probably by hundreds or perhaps thousands of miles.</p>
+
+<p>But there is something else about the eye which impresses us
+still more than these wonderful features which we observed, viewing
+it from the standpoint of a physicist, merely as an optical
+instrument,&mdash;something which appeals to us more than its marvelous
+faculty of being directly affected by the vibrations of the<span class='pagenum'><a name="Page_298" id="Page_298">[Pg 298]</a></span>
+medium, without interference of gross matter, and more than its
+inconceivable sensitiveness and discerning power. It is its significance
+in the processes of life. No matter what one's views on
+nature and life may be, he must stand amazed when, for the first
+time in his thoughts, he realizes the importance of the eye in the
+physical processes and mental performances of the human organism.
+And how could it be otherwise, when he realizes, that the
+eye is the means through which the human race has acquired
+the entire knowledge it possesses, that it controls all our motions,
+more still, all our actions.</p>
+
+<p>There is no way of acquiring knowledge except through the eye.
+What is the foundation of all philosophical systems of ancient
+and modern times, in fact, of all the philosophy of man? <i>I am,
+I think; I think, therefore I am.</i> But how could I think and how
+would I know that I exist, if I had not the eye? For knowledge
+involves consciousness; consciousness involves ideas, conceptions;
+conceptions involve pictures or images, and images the sense of
+vision, and therefore the organ of sight. But how about blind
+men, will be asked? Yes, a blind man may depict in magnificent
+poems, forms and scenes from real life, from a world he physically
+does not see. A blind man may touch the keys of an instrument
+with unerring precision, may model the fastest boat, may discover
+and invent, calculate and construct, may do still greater wonders&mdash;but
+all the blind men who have done such things have descended
+from those who had seeing eyes. Nature may reach the same result
+in many ways. Like a wave in the physical world, in the infinite
+ocean of the medium which pervades all, so in the world of
+organisms, in life, an impulse started proceeds onward, at times,
+may be, with the speed of light, at times, again, so slowly that
+for ages and ages it seems to stay, passing through processes of a
+complexity inconceivable to men, but in all its forms, in all its
+stages, its energy ever and ever integrally present. A single ray
+of light from a distant star falling upon the eye of a tyrant in bygone
+times, may have altered the course of his life, may have
+changed the destiny of nations, may have transformed the surface
+of the globe, so intricate, so inconceivably complex are the
+processes in Nature. In no way can we get such an overwhelming
+idea of the grandeur of Nature, as when we consider, that in
+accordance with the law of the conservation of energy, throughout
+the infinite, the forces are in a perfect balance, and hence the
+energy of a single thought may determine the motion of a Uni<span class='pagenum'><a name="Page_299" id="Page_299">[Pg 299]</a></span>verse.
+It is not necessary that every individual, not even that
+every generation or many generations, should have the physical
+instrument of sight, in order to be able to form images and to
+think, that is, form ideas or conceptions; but sometime or other,
+during the process of evolution, the eye certainly must have existed,
+else thought, as we understand it, would be impossible;
+else conceptions, like spirit, intellect, mind, call it as you may,
+could not exist. It is conceivable, that in some other world, in
+some other beings, the eye is replaced by a different organ, equally
+or more perfect, but these beings cannot be men.</p>
+
+<p>Now what prompts us all to voluntary motions and actions of
+any kind? Again the eye. If I am conscious of the motion, I
+must have an idea or conception, that is, an image, therefore the
+eye. If I am not precisely conscious of the motion, it is, because
+the images are vague or indistinct, being blurred by the superimposition
+of many. But when I perform the motion, does the
+impulse which prompts me to the action come from within or from
+without? The greatest physicists have not disdained to endeavor
+to answer this and similar questions and have at times
+abandoned themselves to the delights of pure and unrestrained
+thought. Such questions are generally considered not to belong
+to the realm of positive physical science, but will before long be
+annexed to its domain. Helmholtz has probably thought more
+on life than any modern scientist. Lord Kelvin expressed his
+belief that life's process is electrical and that there is a force inherent
+to the organism and determining its motions. Just as
+much as I am convinced of any physical truth I am convinced
+that the motive impulse must come from the outside. For, consider
+the lowest organism we know&mdash;and there are probably
+many lower ones&mdash;an aggregation of a few cells only. If it is
+capable of voluntary motion it can perform an infinite number
+of motions, all definite and precise. But now a mechanism consisting
+of a finite number of parts and few at that, cannot perform
+an infinite number of definite motions, hence the impulses
+which govern its movements must come from the environment.
+So, the atom, the ulterior element of the Universe's structure, is
+tossed about in space, eternally, a play to external influences, like
+a boat in a troubled sea. Were it to stop its motion <i>it would die</i>.
+Matter at rest, if such a thing could exist, would be matter dead.
+Death of matter! Never has a sentence of deeper philosophical
+meaning been uttered. This is the way in which Prof. Dewar<span class='pagenum'><a name="Page_300" id="Page_300">[Pg 300]</a></span>
+forcibly expresses it in the description of his admirable experiments,
+in which liquid oxygen is handled as one handles water,
+and air at ordinary pressure is made to condense and even to
+solidify by the intense cold. Experiments, which serve to illustrate,
+in his language, the last feeble manifestations of life, the
+last quiverings of matter about to die. But human eyes shall
+not witness such death. There is no death of matter, for
+throughout the infinite universe, all has to move, to vibrate, that
+is, to live.</p>
+
+<p>I have made the preceding statements at the peril of treading
+upon metaphysical ground, in my desire to introduce the subject
+of this lecture in a manner not altogether uninteresting, I may
+hope, to an audience such as I have the honor to address. But
+now, then, returning to the subject, this divine organ of sight,
+this indispensable instrument for thought and all intellectual enjoyment,
+which lays open to us the marvels of this universe,
+through which we have acquired what knowledge we possess, and
+which prompts us to, and controls, all our physical and mental
+activity. By what is it affected? By light! What is light?</p>
+
+<p>We have witnessed the great strides which have been made in
+all departments of science in recent years. So great have been
+the advances that we cannot refrain from asking ourselves, Is
+this all true, or is it but a dream? Centuries ago men have
+lived, have thought, discovered, invented, and have believed that
+they were soaring, while they were merely proceeding at a snail's
+pace. So we too may be mistaken. But taking the truth of the
+observed events as one of the implied facts of science, we must
+rejoice in the immense progress already made and still more in the
+anticipation of what must come, judging from the possibilities
+opened up by modern research. There is, however, an advance
+which we have been witnessing, which must be particularly
+gratifying to every lover of progress. It is not a discovery, or
+an invention, or an achievement in any particular direction. It
+is an advance in all directions of scientific thought and experiment.
+I mean the generalization of the natural forces and phenomena,
+the looming up of a certain broad idea on the scientific
+horizon. It is this idea which has, however, long ago taken possession
+of the most advanced minds, to which I desire to call your
+attention, and which I intend to illustrate in a general way, in
+these experiments, as the first step in answering the question
+"What is light?" and to realize the modern meaning of this
+word.<span class='pagenum'><a name="Page_301" id="Page_301">[Pg 301]</a></span></p>
+
+<p>It is beyond the scope of my lecture to dwell upon the subject
+of light in general, my object being merely to bring presently to
+your notice a certain class of light effects and a number of phenomena
+observed in pursuing the study of these effects. But to
+be consistent in my remarks it is necessary to state that, according
+to that idea, now accepted by the majority of scientific men as a
+positive result of theoretical and experimental investigation, the
+various forms or manifestations of energy which were generally
+designated as "electric" or more precisely "electromagnetic" are
+energy manifestations of the same nature as those of radiant
+heat and light. Therefore the phenomena of light and heat and
+others besides these, may be called electrical phenomena. Thus
+electrical science has become the mother science of all and its
+study has become all important. The day when we shall know
+exactly what "electricity" is, will chronicle an event probably
+greater, more important than any other recorded in the history
+of the human race. The time will come when the comfort, the
+very existence, perhaps, of man will depend upon that wonderful
+agent. For our existence and comfort we require heat, light
+and mechanical power. How do we now get all these? We get
+them from fuel, we get them by consuming material. What
+will man do when the forests disappear, when the coal fields are
+exhausted? Only one thing, according to our present knowledge
+will remain; that is, to transmit power at great distances. Men
+will go to the waterfalls, to the tides, which are the stores of an
+infinitesimal part of Nature's immeasurable energy. There will
+they harness the energy and transmit the same to their settlements,
+to warm their homes by, to give them light, and to keep
+their obedient slaves, the machines, toiling. But how will they
+transmit this energy if not by electricity? Judge then, if the
+comfort, nay, the very existence, of man will not depend on electricity.
+I am aware that this view is not that of a practical
+engineer, but neither is it that of an illusionist, for it is certain,
+that power transmission, which at present is merely a stimulus to
+enterprise, will some day be a dire necessity.</p>
+
+<p>It is more important for the student, who takes up the study
+of light phenomena, to make himself thoroughly acquainted with
+certain modern views, than to peruse entire books on the subject
+of light itself, as disconnected from these views. Were I therefore
+to make these demonstrations before students seeking
+information&mdash;and for the sake of the few of those who may be<span class='pagenum'><a name="Page_302" id="Page_302">[Pg 302]</a></span>
+present, give me leave to so assume&mdash;it would be my principal
+endeavor to impress these views upon their minds in this series of
+experiments.</p>
+
+<p>It might be sufficient for this purpose to perform a simple and
+well-known experiment. I might take a familiar appliance, a
+Leyden jar, charge it from a frictional machine, and then discharge
+it. In explaining to you its permanent state when charged,
+and its transitory condition when discharging, calling your attention
+to the forces which enter into play and to the various phenomena
+they produce, and pointing out the relation of the forces
+and phenomena, I might fully succeed in illustrating that modern
+idea. No doubt, to the thinker, this simple experiment would
+appeal as much as the most magnificent display. But this is to
+be an experimental demonstration, and one which should possess,
+besides instructive, also entertaining features and as such, a simple
+experiment, such as the one cited, would not go very far towards
+the attainment of the lecturer's aim. I must therefore choose
+another way of illustrating, more spectacular certainly, but perhaps
+also more instructive. Instead of the frictional machine and
+Leyden jar, I shall avail myself in these experiments, of an induction
+coil of peculiar properties, which was described in detail by me
+in a lecture before the London Institution of Electrical Engineers,
+in Feb., 1892. This induction coil is capable of yielding currents of
+enormous potential differences, alternating with extreme rapidity.
+With this apparatus I shall endeavor to show you three distinct
+classes of effects, or phenomena, and it is my desire that each
+experiment, while serving for the purposes of illustration, should
+at the same time teach us some novel truth, or show us some
+novel aspect of this fascinating science. But before doing this, it
+seems proper and useful to dwell upon the apparatus employed,
+and method of obtaining the high potentials and high-frequency
+currents which are made use of in these experiments.</p>
+
+
+<p><span class='pagenum'><a name="Page_303" id="Page_303">[Pg 303]</a></span></p>
+<h5>ON THE APPARATUS AND METHOD OF CONVERSION.</h5>
+
+<p>These high-frequency currents are obtained in a peculiar manner.
+The method employed was advanced by me about two
+years ago in an experimental lecture before the American Institute
+of Electrical Engineers. A number of ways, as practiced in
+the laboratory, of obtaining these currents either from continuous
+or low frequency alternating currents, is diagramatically indicated
+in Fig. 165, which will be later described in detail. The general
+<span class='pagenum'><a name="Page_304" id="Page_304">[Pg 304]</a></span>
+plan is to charge condensers, from a direct or alternate-current
+source, preferably of high-tension, and to discharge them
+disruptively while observing well-known conditions necessary
+to maintain the oscillations of the current. In view of the
+general interest taken in high-frequency currents and effects producible
+by them, it seems to me advisable to dwell at some length
+upon this method of conversion. In order to give you a clear
+idea of the action, I will suppose that a continuous-current generator
+is employed, which is often very convenient. It is desirable
+that the generator should possess such high tension as to be able
+to break through a small air space. If this is not the case, then
+auxiliary means have to be resorted to, some of which will be indicated
+subsequently. When the condensers are charged to a
+certain potential, the air, or insulating space, gives way and a disruptive
+discharge occurs. There is then a sudden rush of current
+and generally a large portion of accumulated electrical energy
+spends itself. The condensers are thereupon quickly charged and
+the same process is repeated in more or less rapid succession.
+To produce such sudden rushes of current it is necessary to observe
+certain conditions. If the rate at which the condensers are
+discharged is the same as that at which they are charged, then,
+clearly, in the assumed case the condensers do not come into
+play. If the rate of discharge be smaller than the rate of charging,
+then, again, the condensers cannot play an important part.
+But if, on the contrary, the rate of discharging is greater than
+that of charging, then a succession of rushes of current is obtained.
+It is evident that, if the rate at which the energy is
+dissipated by the discharge is very much greater than the rate of
+supply to the condensers, the sudden rushes will be comparatively
+few, with long-time intervals between. This always occurs
+when a condenser of considerable capacity is charged by means
+of a comparatively small machine. If the rates of supply and
+dissipation are not widely different, then the rushes of current
+will be in quicker succession, and this the more, the more nearly
+equal both the rates are, until limitations incident to each case
+and depending upon a number of causes are reached. Thus we
+are able to obtain from a continuous-current generator as rapid a
+succession of discharges as we like. Of course, the higher the
+tension of the generator, the smaller need be the capacity of the
+condensers, and for this reason, principally, it is of advantage to
+employ a generator of very high tension. Besides, such a generator
+permits the attaining of greater rates of vibration.<span class='pagenum'><a name="Page_305" id="Page_305">[Pg 305]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_317.jpg" width="1024" height="686" alt="Fig. 165." title="" />
+<span class="caption">Fig. 165.</span>
+</div>
+
+<p>The rushes of current may be of the same direction under the
+conditions before assumed, but most generally there is an oscillation
+superimposed upon the fundamental vibration of the current.
+When the conditions are so determined that there are no oscillations,
+the current impulses are unidirectional and thus a means is
+provided of transforming a continuous current of high tension,
+into a direct current of lower tension, which I think may find
+employment in the arts.</p>
+
+<p>This method of conversion is exceedingly interesting and I
+was much impressed by its beauty when I first conceived it. It is
+ideal in certain respects. It involves the employment of no mechanical
+devices of any kind, and it allows of obtaining currents
+of any desired frequency from an ordinary circuit, direct or alternating.
+The frequency of the fundamental discharges depending
+on the relative rates of supply and dissipation can be readily
+varied within wide limits, by simple adjustments of these quantities,
+and the frequency of the superimposed vibration by the
+determination of the capacity, self-induction and resistance of the
+circuit. The potential of the currents, again, may be raised as
+high as any insulation is capable of withstanding safely by combining
+capacity and self-induction or by induction in a secondary,
+which need have but comparatively few turns.</p>
+
+<p>As the conditions are often such that the intermittence or oscillation
+does not readily establish itself, especially when a direct
+current source is employed, it is of advantage to associate an interrupter
+with the arc, as I have, some time ago, indicated the
+use of an air-blast or magnet, or other such device readily at
+hand. The magnet is employed with special advantage in the
+conversion of direct currents, as it is then very effective. If the
+primary source is an alternate current generator, it is desirable,
+as I have stated on another occasion, that the frequency should
+be low, and that the current forming the arc be large, in order
+to render the magnet more effective.</p>
+
+<p>A form of such discharger with a magnet which has been
+found convenient, and adopted after some trials, in the conversion
+of direct currents particularly, is illustrated in Fig. 166. <small>N S</small> are
+the pole pieces of a very strong magnet which is excited by a coil
+C. The pole pieces are slotted for adjustment and can be fastened
+in any position by screws <i>s s</i><sub>1</sub>. The discharge rods <i>d d</i><sub>1</sub>, thinned
+down on the ends in order to allow a closer approach of the magnetic
+pole pieces, pass through the columns of brass <i>b b</i><sub>1</sub> and are
+<span class='pagenum'><a name="Page_306" id="Page_306">[Pg 306]</a></span>fastened in position by screws <i>s</i><sub>2</sub> <i>s</i><sub>2</sub>. Springs <i>r r</i><sub>1</sub> and collars <i>c c</i><sub>1</sub>
+are slipped on the rods, the latter serving to set the points of the
+rods at a certain suitable distance by means of screws <i>s</i><sub>3</sub> <i>s</i><sub>3</sub>, and
+the former to draw the points apart. When it is desired to start
+the arc, one of the large rubber handles <i>h h</i><sub>1</sub> is tapped quickly
+with the hand, whereby the points of the rods are brought in
+contact but are instantly separated by the springs <i>r r</i><sub>1</sub>. Such an
+arrangement has been found to be often necessary, namely in
+cases when the <span class="smcap">e. m. f.</span> was not large enough to cause the discharge
+to break through the gap, and also when it was desirable to avoid
+short circuiting of the generator by the metallic contact of the
+rods. The rapidity of the interruptions of the current with a
+magnet depends on the intensity of the magnetic field and on the
+potential difference at the end of the arc. The interruptions are
+generally in such quick succession as to produce a musical sound.
+Years ago it was observed that when a powerful induction coil
+is discharged between the poles of a strong magnet, the discharge
+produces a loud noise, not unlike a small pistol shot. It was
+vaguely stated that the spark was intensified by the presence of
+the magnetic field. It is now clear that the discharge current,
+flowing for some time, was interrupted a great number of times
+by the magnet, thus producing the sound. The phenomenon is
+especially marked when the field circuit of a large magnet or
+dynamo is broken in a powerful magnetic field.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_320.jpg" width="800" height="535" alt="Fig. 166." title="" />
+<span class="caption">Fig. 166.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_307" id="Page_307">[Pg 307]</a></span></p>
+
+
+<p>When the current through the gap is comparatively large, it is
+of advantage to slip on the points of the discharge rods pieces of
+very hard carbon and let the arc play between the carbon pieces.
+This preserves the rods, and besides has the advantage of keeping
+the air space hotter, as the heat is not conducted away as
+quickly through the carbons, and the result is that a smaller
+<span class="smcap">e. m. f.</span> in the arc gap is required to maintain a succession of
+discharges.</p>
+
+<div class="figcenter" style="width: 695px;">
+<img src="images/oi_321.jpg" width="695" height="600" alt="Fig. 167." title="" />
+<span class="caption">Fig. 167.</span>
+</div>
+
+<p>Another form of discharger, which may be employed with advantage
+in some cases, is illustrated in Fig. 167. In this form
+the discharge rods <i>d d</i><sub>1</sub> pass through perforations in a wooden
+box <small>B</small>, which is thickly coated with mica on the inside, as indicated
+by the heavy lines. The perforations are provided with
+mica tubes <i>m m</i><sub>1</sub> of some thickness, which are preferably not in
+contact with the rods <i>d d</i><sub>1</sub>. The box has a cover <small>C</small> which is a
+little larger and descends on the outside of the box. The spark
+gap is warmed by a small lamp <i>l</i> contained in the box. A plate
+<i>p</i> above the lamp allows the draught to pass only through the
+chimney <i>e</i> of the lamp, the air entering through holes <i>o o</i> in or
+near the bottom of the box and following the path indicated by
+the arrows. When the discharger is in operation, the door of the
+box is closed so that the light of the arc is not visible outside.<span class='pagenum'><a name="Page_308" id="Page_308">[Pg 308]</a></span>
+It is desirable to exclude the light as perfectly as possible, as it
+interferes with some experiments. This form of discharger is simple
+and very effective when properly manipulated. The air
+being warmed to a certain temperature, has its insulating power
+impaired; it becomes dielectrically weak, as it were, and the consequence
+is that the arc can be established at much greater distance.
+The arc should, of course, be sufficiently insulating to
+allow the discharge to pass through the gap <i>disruptively</i>. The
+arc formed under such conditions, when long, may be made extremely
+sensitive, and the weak draught through the lamp
+chimney <i>c</i> is quite sufficient to produce rapid interruptions. The
+adjustment is made by regulating the temperature and velocity
+of the draught. Instead of using the lamp, it answers the purpose
+to provide for a draught of warm air in other ways. A
+very simple way which has been practiced is to enclose the arc
+in a long vertical tube, with plates on the top and bottom for
+regulating the temperature and velocity of the air current.
+Some provision had to be made for deadening the sound.</p>
+
+<p>The air may be rendered dielectrically weak also by rarefaction.
+Dischargers of this kind have likewise been used by me
+in connection with a magnet. A large tube is for this purpose
+provided with heavy electrodes of carbon or metal, between
+which the discharge is made to pass, the tube being placed in a
+powerful magnetic field. The exhaustion of the tube is carried
+to a point at which the discharge breaks through easily, but the
+pressure should be more than 75 millimetres, at which the ordinary
+thread discharge occurs. In another form of discharger,
+combining the features before mentioned, the discharge was
+made to pass between two adjustable magnetic pole pieces, the
+space between them being kept at an elevated temperature.</p>
+
+<p>It should be remarked here that when such, or interrupting
+devices of any kind, are used and the currents are passed through
+the primary of a disruptive discharge coil, it is not, as a rule, of
+advantage to produce a number of interruptions of the current
+per second greater than the natural frequency of vibration of the
+dynamo supply circuit, which is ordinarily small. It should also
+be pointed out here, that while the devices mentioned in connection
+with the disruptive discharge are advantageous under certain
+conditions, they may be sometimes a source of trouble, as
+they produce intermittences and other irregularities in the vibration
+which it would be very desirable to overcome.<span class='pagenum'><a name="Page_309" id="Page_309">[Pg 309]</a></span></p>
+
+<p>There is, I regret to say, in this beautiful method of conversion
+a defect, which fortunately is not vital, and which I have been
+gradually overcoming. I will best call attention to this defect
+and indicate a fruitful line of work, by comparing the electrical
+process with its mechanical analogue. The process may be illustrated
+in this manner. Imagine a tank with a wide opening at
+the bottom, which is kept closed by spring pressure, but so that
+it snaps off <i>suddenly</i> when the liquid in the tank has reached a
+certain height. Let the fluid be supplied to the tank by means
+of a pipe feeding at a certain rate. When the critical height of
+the liquid is reached, the spring gives way and the bottom of the
+tank drops out. Instantly the liquid falls through the wide opening,
+and the spring, reasserting itself, closes the bottom again.
+The tank is now filled, and after a certain time interval the same
+process is repeated. It is clear, that if the pipe feeds the fluid
+quicker than the bottom outlet is capable of letting it pass
+through, the bottom will remain off and the tank will still overflow.
+If the rates of supply are exactly equal, then the bottom lid will
+remain partially open and no vibration of the same and of the
+liquid column will generally occur, though it might, if started by
+some means. But if the inlet pipe does not feed the fluid fast
+enough for the outlet, then there will be always vibration.
+Again, in such case, each time the bottom flaps up or down, the
+spring and the liquid column, if the pliability of the spring and
+the inertia of the moving parts are properly chosen, will perform
+independent vibrations. In this analogue the fluid may be likened
+to electricity or electrical energy, the tank to the condenser,
+the spring to the dielectric, and the pipe to the conductor through
+which electricity is supplied to the condenser. To make this
+analogy quite complete it is necessary to make the assumption,
+that the bottom, each time it gives way, is knocked violently
+against a non-elastic stop, this impact involving some loss of energy;
+and that, besides, some dissipation of energy results due to
+frictional losses. In the preceding analogue the liquid is supposed
+to be under a steady pressure. If the presence of the fluid
+be assumed to vary rhythmically, this may be taken as corresponding
+to the case of an alternating current. The process is
+then not quite as simple to consider, but the action is the same in
+principle.</p>
+
+<p>It is desirable, in order to maintain the vibration economically,
+to reduce the impact and frictional losses as much as possible.<span class='pagenum'><a name="Page_310" id="Page_310">[Pg 310]</a></span>
+As regards the latter, which in the electrical analogue correspond
+to the losses due to the resistance of the circuits, it is impossible
+to obviate them entirely, but they can be reduced to a minimum
+by a proper selection of the dimensions of the circuits and by the
+employment of thin conductors in the form of strands. But
+the loss of energy caused by the first breaking through of the
+dielectric&mdash;which in the above example corresponds to the violent
+knock of the bottom against the inelastic stop&mdash;would be more important
+to overcome. At the moment of the breaking through,
+the air space has a very high resistance, which is probably reduced
+to a very small value when the current has reached some
+strength, and the space is brought to a high temperature. It
+would materially diminish the loss of energy if the space were
+always kept at an extremely high temperature, but then there
+would be no disruptive break. By warming the space moderately
+by means of a lamp or otherwise, the economy as far as the
+arc is concerned is sensibly increased. But the magnet or other
+interrupting device does not diminish the loss in the arc. Likewise,
+a jet of air only facilitates the carrying off of the energy.
+Air, or a gas in general, behaves curiously in this respect. When
+two bodies charged to a very high potential, discharge disruptively
+through an air space, any amount of energy may be carried
+off by the air. This energy is evidently dissipated by bodily
+carriers, in impact and collisional losses of the molecules. The
+exchange of the molecules in the space occurs with inconceivable
+rapidity. A powerful discharge taking place between two electrodes,
+they may remain entirely cool, and yet the loss in the
+air may represent any amount of energy. It is perfectly practicable,
+with very great potential differences in the gap, to dissipate
+several horse-power in the arc of the discharge without even
+noticing a small increase in the temperature of the electrodes.
+All the frictional losses occur then practically in the air. If the
+exchange of the air molecules is prevented, as by enclosing the air
+hermetically, the gas inside of the vessel is brought quickly to a
+high temperature, even with a very small discharge. It is difficult
+to estimate how much of the energy is lost in sound waves,
+audible or not, in a powerful discharge. When the currents
+through the gap are large, the electrodes may become rapidly
+heated, but this is not a reliable measure of the energy wasted in
+the arc, as the loss through the gap itself may be comparatively
+small. The air or a gas in general is, at ordinary pressure at least,<span class='pagenum'><a name="Page_311" id="Page_311">[Pg 311]</a></span>
+clearly not the best medium through which a disruptive discharge
+should occur. Air or other gas under great pressure is of
+course a much more suitable medium for the discharge gap. I
+have carried on long-continued experiments in this direction, unfortunately
+less practicable on account of the difficulties and expense
+in getting air under great pressure. But even if the
+medium in the discharge space is solid or liquid, still the same
+losses take place, though they are generally smaller, for just as
+soon as the arc is established, the solid or liquid is volatilized.
+Indeed, there is no body known which would not be disintegrated
+by the arc, and it is an open question among scientific men,
+whether an arc discharge could occur at all in the air itself without
+the particles of the electrodes being torn off. When the
+current through the gap is very small and the arc very long, I
+believe that a relatively considerable amount of heat is taken up
+in the disintegration of the electrodes, which partially on this account
+may remain quite cold.</p>
+
+<p>The ideal medium for a discharge gap should only <i>crack</i>, and
+the ideal electrode should be of some material which cannot be
+disintegrated. With small currents through the gap it is best to
+employ aluminum, but not when the currents are large. The disruptive
+break in the air, or more or less in any ordinary medium,
+is not of the nature of a crack, but it is rather comparable to the
+piercing of innumerable bullets through a mass offering great
+frictional resistances to the motion of the bullets, this involving
+considerable loss of energy. A medium which would merely
+crack when strained electrostatically&mdash;and this possibly might be
+the case with a perfect vacuum, that is, pure ether&mdash;would involve
+a very small loss in the gap, so small as to be entirely negligible,
+at least theoretically, because a crack may be produced by an
+infinitely small displacement. In exhausting an oblong bulb
+provided with two aluminum terminals, with the greatest care, I
+have succeeded in producing such a vacuum that the secondary
+discharge of a disruptive discharge coil would break disruptively
+through the bulb in the form of fine spark streams. The
+curious point was that the discharge would completely ignore the
+terminals and start far behind the two aluminum plates which
+served as electrodes. This extraordinary high vacuum could only
+be maintained for a very short while. To return to the ideal
+medium, think, for the sake of illustration, of a piece of glass or
+similar body clamped in a vice, and the latter tightened more and<span class='pagenum'><a name="Page_312" id="Page_312">[Pg 312]</a></span>
+more. At a certain point a minute increase of the pressure will
+cause the glass to crack. The loss of energy involved in splitting
+the glass may be practically nothing, for though the force is great,
+the displacement need be but extremely small. Now imagine
+that the glass would possess the property of closing again perfectly
+the crack upon a minute diminution of the pressure.
+This is the way the dielectric in the discharge space should
+behave. But inasmuch as there would be always some loss in the
+gap, the medium, which should be continuous, should exchange
+through the gap at a rapid rate. In the preceding example, the
+glass being perfectly closed, it would mean that the dielectric in
+the discharge space possesses a great insulating power; the glass
+being cracked, it would signify that the medium in the space is
+a good conductor. The dielectric should vary enormously in
+resistance by minute variations of the <span class="smcap">e. m. f.</span> across the
+discharge space. This condition is attained, but in an extremely
+imperfect manner, by warming the air space to a certain
+critical temperature, dependent on the <span class="smcap">e. m. f.</span> across the gap,
+or by otherwise impairing the insulating power of the air. But
+as a matter of fact the air does never break down <i>disruptively</i>,
+if this term be rigorously interpreted, for before the sudden
+rush of the current occurs, there is always a weak current
+preceding it, which rises first gradually and then with comparative
+suddenness. That is the reason why the rate of change is
+very much greater when glass, for instance, is broken through,
+than when the break takes place through an air space of equivalent
+dielectric strength. As a medium for the discharge space, a
+solid, or even a liquid, would be preferable therefor. It is somewhat
+difficult to conceive of a solid body which would possess the
+property of closing instantly after it has been cracked. But a
+liquid, especially under great pressure, behaves practically like a
+solid, while it possesses the property of closing the crack. Hence
+it was thought that a liquid insulator might be more suitable as a
+dielectric than air. Following out this idea, a number of different
+forms of dischargers in which a variety of such insulators, sometimes
+under great pressure, were employed, have been experimented
+upon. It is thought sufficient to dwell in a few words
+upon one of the forms experimented upon. One of these dischargers
+is illustrated in Figs. 168<i>a</i> and 168<i>b</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_327.jpg" width="800" height="327" alt="Fig. 168a, 168b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 168a.</td><td class="caption1"><span class="smcap">Fig.</span> 168b.</td></tr>
+</table>
+</div>
+
+<p>A hollow metal pulley <small>P</small> (Fig. 168<i>a</i>), was fastened upon an arbor
+<i>a</i>, which by suitable means was rotated at a considerable
+<span class='pagenum'><a name="Page_313" id="Page_313">[Pg 313]</a></span>speed. On the inside of the pulley, but disconnected from the
+same, was supported a thin disc <i>h</i> (which is shown thick for the
+sake of clearness), of hard rubber in which there were embedded
+two metal segments <i>s s</i> with metallic extensions <i>e e</i> into which
+were screwed conducting terminals <i>t t</i> covered with thick tubes
+of hard rubber <i>t t</i>. The rubber disc <i>h</i> with its metallic segments
+<i>s s</i>, was finished in a lathe, and its entire surface highly polished
+so as to offer the smallest possible frictional resistance to the motion
+through a fluid. In the hollow of the pulley an insulating
+liquid such as a thin oil was poured so as to reach very nearly to
+the opening left in the flange <i>f</i>, which was screwed tightly on the
+front side of the pulley. The terminals <i>t t</i>, were connected to the
+opposite coatings of a battery of condensers so that the discharge
+occurred through the liquid. When the pulley was rotated, the
+liquid was forced against the rim of the pulley and considerable
+fluid pressure resulted. In this simple way the discharge gap
+was filled with a medium which behaved practically like a solid,
+which possessed the quality of closing instantly upon the occurrence
+of the break, and which moreover was circulating through
+the gap at a rapid rate. Very powerful effects were produced by
+discharges of this kind with liquid interrupters, of which a number
+of different forms were made. It was found that, as expected,
+a longer spark for a given length of wire was obtainable
+in this way than by using air as an interrupting device. Generally
+the speed, and therefore also the fluid pressure, was limited
+by reason of the fluid friction, in the form of discharger described,
+but the practically obtainable speed was more than sufficient to
+produce a number of breaks suitable for the circuits ordinarily
+used. In such instances the metal pulley <small>P</small> was provided with a
+few projections inwardly, and a definite number of breaks was
+then produced which could be computed from the speed of<span class='pagenum'><a name="Page_314" id="Page_314">[Pg 314]</a></span>
+rotation of the pulley. Experiments were also carried on with
+liquids of different insulating power with the view of reducing
+the loss in the arc. When an insulating liquid is moderately
+warmed, the loss in the arc is diminished.</p>
+
+<p>A point of some importance was noted in experiments with
+various discharges of this kind. It was found, for instance, that
+whereas the conditions maintained in these forms were favorable
+for the production of a great spark length, the current so obtained
+was not best suited to the production of light effects. Experience
+undoubtedly has shown, that for such purposes a harmonic
+rise and fall of the potential is preferable. Be it that a
+solid is rendered incandescent, or phosphorescent, or be it that energy
+is transmitted by condenser coating through the glass, it is
+quite certain that a harmonically rising and falling potential produces
+less destructive action, and that the vacuum is more permanently
+maintained. This would be easily explained if it were
+ascertained that the process going on in an exhausted vessel is of
+an electrolytic nature.</p>
+
+<p>In the diagrammatical sketch, Fig. 165, which has been already
+referred to, the cases which are most likely to be met with in
+practice are illustrated. One has at his disposal either direct or
+alternating currents from a supply station. It is convenient for
+an experimenter in an isolated laboratory to employ a machine <small>G</small>,
+such as illustrated, capable of giving both kinds of currents. In
+such case it is also preferable to use a machine with multiple
+circuits, as in many experiments it is useful and convenient to
+have at one's disposal currents of different phases. In the
+sketch, <small>D</small> represents the direct and <small>A</small> the alternating circuit. In
+each of these, three branch circuits are shown, all of which are
+provided with double line switches <i>s s s s s s</i>. Consider first the
+direct current conversion; <small>I</small><i>a</i> represents the simplest case. If
+the <span class="smcap">e. m. f.</span> of the generator is sufficient to break through a small
+air space, at least when the latter is warmed or otherwise rendered
+poorly insulating, there is no difficulty in maintaining a
+vibration with fair economy by judicious adjustment of the
+capacity, self-induction and resistance of the circuit <small>L</small> containing
+the devices <i>l l m</i>. The magnet <small>N</small>, <small>S</small>, can be in this case advantageously
+combined with the air space. The discharger <i>d d</i> with
+the magnet may be placed either way, as indicated by the full or
+by the dotted lines. The circuit <small>I</small><i>a</i> with the connections and devices
+is supposed to possess dimensions such as are suitable for<span class='pagenum'><a name="Page_315" id="Page_315">[Pg 315]</a></span>
+the maintenance of a vibration. But usually the <span class="smcap">e. m. f.</span> on the
+circuit or branch <small>I</small><i>a</i> will be something like a 100 volts or so, and
+in this case it is not sufficient to break through the gap. Many
+different means may be used to remedy this by raising the <span class="smcap">e. m. f.</span>
+across the gap. The simplest is probably to insert a large self-induction
+coil in series with the circuit <small>L</small>. When the arc is
+established, as by the discharger illustrated in Fig. 166, the magnet
+blows the arc out the instant it is formed. Now the extra
+current of the break, being of high <span class="smcap">e. m. f.</span>, breaks through the
+gap, and a path of low resistance for the dynamo current being
+again provided, there is a sudden rush of current from the
+dynamo upon the weakening or subsidence of the extra current.
+This process is repeated in rapid succession, and in this manner I
+have maintained oscillation with as low as 50 volts, or even less,
+across the gap. But conversion on this plan is not to be recommended
+on account of the too heavy currents through the gap
+and consequent heating of the electrodes; besides, the frequencies
+obtained in this way are low, owing to the high self-induction
+necessarily associated with the circuit. It is very desirable
+to have the <span class="smcap">e. m. f.</span> as high as possible, first, in order to increase
+the economy of the conversion, and, secondly, to obtain high
+frequencies. The difference of potential in this electric oscillation
+is, of course, the equivalent of the stretching force in the
+mechanical vibration of the spring. To obtain very rapid vibration
+in a circuit of some inertia, a great stretching force or difference
+of potential is necessary. Incidentally, when the <span class="smcap">e. m. f.</span> is
+very great, the condenser which is usually employed in connection
+with the circuit need but have a small capacity, and many
+other advantages are gained. With a view of raising the <span class="smcap">e. m. f.</span>
+to a many times greater value than obtainable from ordinary
+distribution circuits, a rotating transformer <i>g</i> is used, as indicated
+at <small>II</small><i>a</i>, Fig. 165, or else a separate high potential machine
+is driven by means of a motor operated from the generator <small>G</small>.
+The latter plan is in fact preferable, as changes are easier made.
+The connections from the high tension winding are quite similar
+to those in branch <small>I</small><i>a</i> with the exception that a condenser <small>C</small>,
+which should be adjustable, is connected to the high tension
+circuit. Usually, also, an adjustable self-induction coil in series
+with the circuit has been employed in these experiments. When
+the tension of the currents is very high, the magnet ordinarily
+used in connection with the discharger is of comparatively small<span class='pagenum'><a name="Page_316" id="Page_316">[Pg 316]</a></span>
+value, as it is quite easy to adjust the dimensions of the circuit
+so that oscillation is maintained. The employment of a steady
+<span class="smcap">e. m. f.</span> in the high frequency conversion affords some advantages
+over the employment of alternating <span class="smcap">e. m. f.</span>, as the adjustments
+are much simpler and the action can be easier controlled.
+But unfortunately one is limited by the obtainable potential difference.
+The winding also breaks down easily in consequence
+of the sparks which form between the sections of the armature
+or commutator when a vigorous oscillation takes place. Besides,
+these transformers are expensive to build. It has been found by
+experience that it is best to follow the plan illustrated at <small>III</small><i>a</i>.
+In this arrangement a rotating transformer <i>g</i>, is employed to
+convert the low tension direct currents into low frequency alternating
+currents, preferably also of small tension. The tension
+of the currents is then raised in a stationary transformer <small>T</small>. The
+secondary <small>S</small> of this transformer is connected to an adjustable condenser
+<small>C</small> which discharges through the gap or discharger <i>d d</i>, placed
+in either of the ways indicated, through the primary <small>P</small> of a disruptive
+discharge coil, the high frequency current being obtained
+from the secondary <small>S</small> of this coil, as described on previous occasions.
+This will undoubtedly be found the cheapest and most convenient
+way of converting direct currents.</p>
+
+<p>The three branches of the circuit <small>A</small> represent the usual cases
+met in practice when alternating currents are converted. In
+Fig. 1<i>b</i> a condenser <small>C</small>, generally of large capacity, is connected to the
+circuit <small>L</small> containing the devices <i>l l</i>, <i>m m</i>. The devices <i>m m</i> are supposed
+to be of high self-induction so as to bring the frequency of
+the circuit more or less to that of the dynamo. In this instance
+the discharger <i>d d</i> should best have a number of makes and breaks
+per second equal to twice the frequency of the dynamo. If not
+so, then it should have at least a number equal to a multiple or
+even fraction of the dynamo frequency. It should be observed,
+referring to <small>I</small><i>b</i>, that the conversion to a high potential is also
+effected when the discharger <i>d d</i>, which is shown in the sketch, is
+omitted. But the effects which are produced by currents which
+rise instantly to high values, as in a disruptive discharge, are
+entirely different from those produced by dynamo currents which
+rise and fall harmonically. So, for instance, there might be in a
+given case a number of makes and breaks at <i>d d</i> equal to just
+twice the frequency of the dynamo, or in other words, there may
+be the same number of fundamental oscillations as would be pro<span class='pagenum'><a name="Page_317" id="Page_317">[Pg 317]</a></span>duced
+without the discharge gap, and there might even not be any
+quicker superimposed vibration; yet the differences of potential at
+the various points of the circuit, the impedance and other phenomena,
+dependent upon the rate of change, will bear no similarity in
+the two cases. Thus, when working with currents discharging disruptively,
+the element chiefly to be considered is not the frequency,
+as a student might be apt to believe, but the rate of change per
+unit of time. With low frequencies in a certain measure the same
+effects may be obtained as with high frequencies, provided the rate
+of change is sufficiently great. So if a low frequency current is
+raised to a potential of, say, 75,000 volts, and the high tension current
+passed through a series of high resistance lamp filaments, the
+importance of the rarefied gas surrounding the filament is clearly
+noted, as will be seen later; or, if a low frequency current of several
+thousand amperes is passed through a metal bar, striking phenomena
+of impedance are observed, just as with currents of high
+frequencies. But it is, of course, evident that with low frequency
+currents it is impossible to obtain such rates of change per unit of
+time as with high frequencies, hence the effects produced by the
+latter are much more prominent. It is deemed advisable to
+make the preceding remarks, inasmuch as many more recently
+described effects have been unwittingly identified with high
+frequencies. Frequency alone in reality does not mean anything,
+except when an undisturbed harmonic oscillation is considered.</p>
+
+<p>In the branch <small>III</small><i>b</i> a similar disposition to that in <small>I</small><i>b</i> is illustrated,
+with the difference that the currents discharging through the gap
+<i>d d</i> are used to induce currents in the secondary <small>S</small> of a transformer
+<small>T</small>. In such case the secondary should be provided with an
+adjustable condenser for the purpose of tuning it to the primary.</p>
+
+<p><small>II</small><i>b</i> illustrates a plan of alternate current high frequency
+conversion which is most frequently used and which is found to
+be most convenient. This plan has been dwelt upon in detail on
+previous occasions and need not be described here.</p>
+
+<p>Some of these results were obtained by the use of a high
+frequency alternator. A description of such machines will be
+found in my original paper before the American Institute of
+Electrical Engineers, and in periodicals of that period, notably
+in <span class="smcap">The Electrical Engineer</span> of March 18, 1891.</p>
+
+<p>I will now proceed with the experiments.<span class='pagenum'><a name="Page_318" id="Page_318">[Pg 318]</a></span></p>
+
+<h5>ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE.</h5>
+
+<p>The first class of effects I intend to show you are effects produced
+by electrostatic force. It is the force which governs the
+the motion of the atoms, which causes them to collide and develop
+the life-sustaining energy of heat and light, and which
+causes them to aggregate in an infinite variety of ways, according
+to Nature's fanciful designs, and to form all these wondrous
+structures we perceive around us; it is, in fact, if our present
+views be true, the most important force for us to consider in Nature.
+As the term <i>electrostatic</i> might imply a steady electric
+condition, it should be remarked, that in these experiments the
+force is not constant, but varies at a rate which may be considered
+moderate, about one million times a second, or thereabouts.
+This enables me to produce many effects which are not producible
+with an unvarying force.</p>
+
+<p>When two conducting bodies are insulated and electrified,
+we say that an electrostatic force is acting between them. This
+force manifests itself in attractions, repulsions and stresses in the
+bodies and space or medium without. So great may be the strain
+exerted in the air, or whatever separates the two conducting
+bodies, that it may break down, and we observe sparks or bundles
+of light or streamers, as they are called. These streamers form
+abundantly when the force through the air is rapidly varying. I
+will illustrate this action of electrostatic force in a novel experiment
+in which I will employ the induction coil before referred
+to. The coil is contained in a trough filled with oil, and placed
+under the table. The two ends of the secondary wire pass
+through the two thick columns of hard rubber which protrude
+to some height above the table. It is necessary to insulate the
+ends or terminals of the secondary heavily with hard rubber, because
+even dry wood is by far too poor an insulator for these currents
+of enormous potential differences. On one of the terminals
+of the coil, I have placed a large sphere of sheet brass, which
+is connected to a larger insulated brass plate, in order to enable
+me to perform the experiments under conditions, which, as you
+will see, are more suitable for this experiment. I now set the
+coil to work and approach the free terminal with a metallic object
+held in my hand, this simply to avoid burns. As I approach the
+metallic object to a distance of eight or ten inches, a torrent of furious
+sparks breaks forth from the end of the secondary wire, which<span class='pagenum'><a name="Page_319" id="Page_319">[Pg 319]</a></span>
+passes through the rubber column. The sparks cease when the
+metal in my hand touches the wire. My arm is now traversed
+by a powerful electric current, vibrating at about the rate of one
+million times a second. All around me the electrostatic force
+makes itself felt, and the air molecules and particles of dust flying
+about are acted upon and are hammering violently against my
+body. So great is this agitation of the particles, that when the
+lights are turned out you may see streams of feeble light appear
+on some parts of my body. When such a streamer breaks out on
+any part of the body, it produces a sensation like the pricking of
+a needle. Were the potentials sufficiently high and the frequency
+of the vibration rather low, the skin would probably be ruptured
+under the tremendous strain, and the blood would rush out
+with great force in the form of fine spray or jet so thin as to be
+invisible, just as oil will when placed on the positive terminal of
+a Holtz machine. The breaking through of the skin though it
+may seem impossible at first, would perhaps occur, by reason of
+the tissues under the skin being incomparably better conducting.
+This, at least, appears plausible, judging from some observations.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_333.jpg" width="640" height="408" alt="Fig. 169." title="" />
+<span class="caption">Fig. 169.</span>
+</div>
+
+
+<p>I can make these streams of light visible to all, by touching
+with the metallic object one of the terminals as before, and
+approaching my free hand to the brass sphere, which is connected
+to the second terminal of the coil. As the hand is
+approached, the air between it and the sphere, or in the immediate
+neighborhood, is more violently agitated, and you see
+streams of light now break forth from my finger tips and
+from the whole hand (Fig. 169). Were I to approach the hand
+closer, powerful sparks would jump from the brass sphere to
+my hand, which might be injurious. The streamers offer no
+particular inconvenience, except that in the ends of the finger<span class='pagenum'><a name="Page_320" id="Page_320">[Pg 320]</a></span>
+tips a burning sensation is felt. They should not be confounded
+with those produced by an influence machine, because in many
+respects they behave differently. I have attached the brass sphere
+and plate to one of the terminals in order to prevent the formation
+of visible streamers on that terminal, also in order to prevent
+sparks from jumping at a considerable distance. Besides, the
+attachment is favorable for the working of the coil.</p>
+
+<p>The streams of light which you have observed issuing from my
+hand are due to a potential of about 200,000 volts, alternating in
+rather irregular intervals, sometimes like a million times a second.
+A vibration of the same amplitude, but four times as fast, to maintain
+which over 3,000,000 volts would be required, would be
+more than sufficient to envelop my body in a complete sheet of
+flame. But this flame would not burn me up; quite contrarily,
+the probability is that I would not be injured in the least. Yet a
+hundredth part of that energy, otherwise directed, would be amply
+sufficient to kill a person.</p>
+
+<p>The amount of energy which may thus be passed into the body
+of a person depends on the frequency and potential of the currents,
+and by making both of these very great, a vast amount of
+energy may be passed into the body without causing any discomfort,
+except perhaps, in the arm, which is traversed by a true
+conduction current. The reason why no pain in the body is felt,
+and no injurious effect noted, is that everywhere, if a current be
+imagined to flow through the body, the direction of its flow
+would be at right angles to the surface; hence the body of the
+experimenter offers an enormous section to the current, and the
+density is very small, with the exception of the arm, perhaps,
+where the density may be considerable. But if only a small
+fraction of that energy would be applied in such a way that a current
+would traverse the body in the same manner as a low frequency
+current, a shock would be received which might be fatal.
+A direct or low frequency alternating current is fatal, I think,
+principally because its distribution through the body is not
+uniform, as it must divide itself in minute streamlets of great
+density, whereby some organs are vitally injured. That such a
+process occurs I have not the least doubt, though no evidence
+might apparently exist, or be found upon examination. The
+surest to injure and destroy life, is a continuous current, but the
+most painful is an alternating current of very low frequency.
+The expression of these views, which are the result of long con<span class='pagenum'><a name="Page_321" id="Page_321">[Pg 321]</a></span>tinued
+experiment and observation, both with steady and varying
+currents, is elicited by the interest which is at present taken in
+this subject, and by the manifestly erroneous ideas which are
+daily propounded in journals on this subject.</p>
+
+<p>I may illustrate an effect of the electrostatic force by another
+striking experiment, but before, I must call your attention to one
+or two facts. I have said before, that when the medium between
+two oppositely electrified bodies is strained beyond a certain
+limit it gives way and, stated in popular language, the
+opposite electric charges unite and neutralize each other. This
+breaking down of the medium occurs principally when the force
+acting between the bodies is steady, or varies at a moderate rate.
+Were the variation sufficiently rapid, such a destructive break
+would not occur, no matter how great the force, for all the energy
+would be spent in radiation, convection and mechanical and
+chemical action. Thus the <i>spark</i> length, or greatest distance
+which a <i>spark</i> will jump between the electrified bodies is the
+smaller, the greater the variation or time rate of change. But
+this rule may be taken to be true only in a general way, when
+comparing rates which are widely different.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_335.jpg" width="800" height="346" alt="Fig. 170a, 170b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 170a.</td><td class="caption1"><span class="smcap">Fig.</span> 170b.</td></tr>
+</table>
+</div>
+
+<p>I will show you by an experiment the difference in the effect
+produced by a rapidly varying and a steady or moderately varying
+force. I have here two large circular brass plates <i>p p</i> (Fig.
+170<i>a</i> and Fig. 170<i>b</i>), supported on movable insulating stands on
+the table, connected to the ends of the secondary of a coil similar
+to the one used before. I place the plates ten or twelve inches
+apart and set the coil to work. You see the whole space between
+the plates, nearly two cubic feet, filled with uniform light, Fig.
+170<i>a</i>. This light is due to the streamers you have seen in the first
+experiment, which are now much more intense. I have already
+pointed out the importance of these streamers in commercial apparatus
+and their still greater importance in some purely scientific
+investigations. Often they are too weak to be visible, but<span class='pagenum'><a name="Page_322" id="Page_322">[Pg 322]</a></span>
+they always exist, consuming energy and modifying the action
+of the apparatus. When intense, as they are at present, they
+produce ozone in great quantity, and also, as Professor Crookes
+has pointed out, nitrous acid. So quick is the chemical action that
+if a coil, such as this one, is worked for a very long time it will
+make the atmosphere of a small room unbearable, for the eyes
+and throat are attacked. But when moderately produced, the
+streamers refresh the atmosphere wonderfully, like a thunder-storm,
+and exercises unquestionably a beneficial effect.</p>
+
+<p>In this experiment the force acting between the plates changes
+in intensity and direction at a very rapid rate. I will now make
+the rate of change per unit time much smaller. This I effect by
+rendering the discharges through the primary of the induction
+coil less frequent, and also by diminishing the rapidity of the vibration
+in the secondary. The former result is conveniently secured
+by lowering the <span class="smcap">e. m. f.</span> over the air gap in the primary
+circuit, the latter by approaching the two brass plates to a distance
+of about three or four inches. When the coil is set to work,
+you see no streamers or light between the plates, yet the medium
+between them is under a tremendous strain. I still further augment
+the strain by raising the <span class="smcap">e. m. f.</span> in the primary circuit, and
+soon you see the air give way and the hall is illuminated by a
+shower of brilliant and noisy sparks, Fig. 170<i>b</i>. These sparks could
+be produced also with unvarying force; they have been for many
+years a familiar phenomenon, though they were usually obtained
+from an entirely different apparatus. In describing these two
+phenomena so radically different in appearance, I have advisedly
+spoken of a "force" acting between the plates. It would be in
+accordance with accepted views to say, that there was an "alternating
+<span class="smcap">e. m. f,</span>" acting between the plates. This term is quite
+proper and applicable in all cases where there is evidence of at
+least a possibility of an essential inter-dependence of the electric
+state of the plates, or electric action in their neighborhood. But
+if the plates were removed to an infinite distance, or if at a finite
+distance, there is no probability or necessity whatever for such
+dependence. I prefer to use the term "electrostatic force," and
+to say that such a force is acting around each plate or electrified insulated
+body in general. There is an inconvenience in using this
+expression as the term incidentally means a steady electric condition;
+but a proper nomenclature will eventually settle this difficulty.<span class='pagenum'><a name="Page_323" id="Page_323">[Pg 323]</a></span></p>
+
+<p>I now return to the experiment to which I have already alluded,
+and with which I desire to illustrate a striking effect produced
+by a rapidly varying electrostatic force. I attach to the end
+of the wire, <i>l</i> (Fig. 171), which is in connection with one of the
+terminals of the secondary of the induction coil, an exhausted
+bulb <i>b</i>. This bulb contains a thin carbon filament <i>f</i>, which is
+fastened to a platinum wire <i>w</i>, sealed in the glass and leading
+outside of the bulb, where it connects to the wire <i>l</i>. The
+bulb may be exhausted to any degree attainable with ordinary
+apparatus. Just a moment before, you have witnessed the breaking
+down of the air between the charged brass plates. You know
+that a plate of glass, or any other insulating material, would break
+down in like manner. Had I therefore a metallic coating attached
+to the outside of the bulb, or placed near the same, and
+were this coating connected to the other terminal of the coil, you
+would be prepared to see the glass give way if the strain were
+sufficiently increased. Even were the coating not connected to
+the other terminal, but to an insulated plate, still, if you have
+followed recent developments, you would naturally expect a rupture
+of the glass.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_337.jpg" width="800" height="474" alt="Fig. 171, 172a, 172b." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 171.</td><td class="caption1"><span class="smcap">Fig.</span> 172a.</td><td class="caption1"><span class="smcap">Fig.</span> 172b.</td></tr>
+</table>
+</div>
+
+
+<p>But it will certainly surprise you to note that under the action
+of the varying electrostatic force, the glass gives way when all
+other bodies are removed from the bulb. In fact, all the surrounding
+bodies we perceive might be removed to an infinite distance
+without affecting the result in the slightest. When the coil
+is set to work, the glass is invariably broken through at the seal,
+or other narrow channel, and the vacuum is quickly impaired.<span class='pagenum'><a name="Page_324" id="Page_324">[Pg 324]</a></span>
+Such a damaging break would not occur with a steady force, even
+if the same were many times greater. The break is due to the
+agitation of the molecules of the gas within the bulb, and outside
+of the same. This agitation, which is generally most violent in
+the narrow pointed channel near the seal, causes a heating and
+rupture of the glass. This rupture, would, however, not occur,
+not even with a varying force, if the medium filling the inside of
+the bulb, and that surrounding it, were perfectly homogeneous.
+The break occurs much quicker if the top of the bulb is drawn
+out into a fine fibre. In bulbs used with these coils such narrow,
+pointed channels must therefore be avoided.</p>
+
+<p>When a conducting body is immersed in air, or similar insulating
+medium, consisting of, or containing, small freely movable
+particles capable of being electrified, and when the electrification
+of the body is made to undergo a very rapid change&mdash;which is
+equivalent to saying that the electrostatic force acting around
+the body is varying in intensity,&mdash;the small particles are attracted
+and repelled, and their violent impacts against the body may
+cause a mechanical motion of the latter. Phenomena of this
+kind are noteworthy, inasmuch as they have not been observed
+before with apparatus such as has been commonly in use. If a
+very light conducting sphere be suspended on an exceedingly fine
+wire, and charged to a steady potential, however high, the sphere
+will remain at rest. Even if the potential would be rapidly
+varying, provided that the small particles of matter, molecules or
+atoms, are evenly distributed, no motion of the sphere should result.
+But if one side of the conducting sphere is covered with a
+thick insulating layer, the impacts of the particles will cause the
+sphere to move about, generally in irregular curves, Fig. 172<i>a</i>.
+In like manner, as I have shown on a previous occasion, a fan of
+sheet metal, Fig. 172<i>b</i>, covered partially with insulating material
+as indicated, and placed upon the terminal of the coil so as to turn
+freely on it, is spun around.</p>
+
+<p>All these phenomena you have witnessed and others which
+will be shown later, are due to the presence of a medium like
+air, and would not occur in a continuous medium. The action
+of the air may be illustrated still better by the following experiment.
+I take a glass tube <i>t</i>, Fig. 173, of about an inch in diameter,
+which has a platinum wire <i>w</i> sealed in the lower end,
+and to which is attached a thin lamp filament <i>f</i>. I connect the
+wire with the terminal of the coil and set the coil to work. The<span class='pagenum'><a name="Page_325" id="Page_325">[Pg 325]</a></span>
+platinum wire is now electrified positively and negatively
+in rapid succession and the wire and air inside of the tube
+is rapidly heated by the impacts of the particles, which may be
+so violent as to render the filament incandescent. But if I pour
+oil in the tube, just as soon as the wire is covered with the oil,
+all action apparently ceases and there is no marked evidence of
+heating. The reason of this is that the oil is a practically continuous
+medium. The displacements in such a continuous medium
+are, with these frequencies, to all appearance incomparably
+smaller than in air, hence the work performed in such a medium
+is insignificant. But oil would behave very differently with frequencies
+many times as great, for even though the displacements
+be small, if the frequency were much greater, considerable work
+might be performed in the oil.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_339.jpg" width="800" height="567" alt="Fig. 173, 174." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 173.</td><td class="caption">Fig. 174.</td></tr>
+</table>
+</div>
+
+<p>The electrostatic attractions and repulsions between bodies of
+measurable dimensions are, of all the manifestations of this force,
+the first so-called <i>electrical</i> phenomena noted. But though they
+have been known to us for many centuries, the precise nature of
+the mechanism concerned in these actions is still unknown to us,
+and has not been even quite satisfactorily explained. What kind
+of mechanism must that be? We cannot help wondering when
+we observe two magnets attracting and repelling each other with
+a force of hundreds of pounds with apparently nothing between
+them. We have in our commercial dynamos magnets capable of
+sustaining in mid-air tons of weight. But what are even these<span class='pagenum'><a name="Page_326" id="Page_326">[Pg 326]</a></span>
+forces acting between magnets when compared with the tremendous
+attractions and repulsions produced by electrostatic force, to
+which there is apparently no limit as to intensity. In lightning
+discharges bodies are often charged to so high a potential that
+they are thrown away with inconceivable force and torn asunder
+or shattered into fragments. Still even such effects cannot compare
+with the attractions and repulsions which exist between
+charged molecules or atoms, and which are sufficient to project
+them with speeds of many kilometres a second, so that under their
+violent impact bodies are rendered highly incandescent and are
+volatilized. It is of special interest for the thinker who inquires
+into the nature of these forces to note that whereas the actions
+between individual molecules or atoms occur seemingly under any
+conditions, the attractions and repulsions of bodies of measurable
+dimensions imply a medium possessing insulating properties. So,
+if air, either by being rarefied or heated, is rendered more or less
+conducting, these actions between two electrified bodies practically
+cease, while the actions between the individual atoms continue to
+manifest themselves.</p>
+
+<p>An experiment may serve as an illustration and as a means of
+bringing out other features of interest. Some time ago I showed
+that a lamp filament or wire mounted in a bulb and connected to
+one of the terminals of a high tension secondary coil is set spinning,
+the top of the filament generally describing a circle. This
+vibration was very energetic when the air in the bulb was at
+ordinary pressure and became less energetic when the air in the
+bulb was strongly compressed. It ceased altogether when the air
+was exhausted so as to become comparatively good conducting. I
+found at that time that no vibration took place when the bulb
+was very highly exhausted. But I conjectured that the vibration
+which I ascribed to the electrostatic action between the walls of
+the bulb and the filament should take place also in a highly
+exhausted bulb. To test this under conditions which were more
+favorable, a bulb like the one in Fig. 174, was constructed. It
+comprised a globe <i>b</i>, in the neck of which was sealed a platinum
+wire <i>w</i> carrying a thin lamp filament <i>f</i>. In the lower part of
+the globe a tube <i>t</i> was sealed so as to surround the filament. The
+exhaustion was carried as far as it was practicable with the apparatus
+employed.</p>
+
+<p>This bulb verified my expectation, for the filament was set
+spinning when the current was turned on, and became incandes<span class='pagenum'><a name="Page_327" id="Page_327">[Pg 327]</a></span>cent.
+It also showed another interesting feature, bearing upon
+the preceding remarks, namely, when the filament had been
+kept incandescent some time, the narrow tube and the space inside
+were brought to an elevated temperature, and as the gas in
+the tube then became conducting, the electrostatic attraction between
+the glass and the filament became very weak or ceased, and
+the filament came to rest. When it came to rest it would glow
+far more intensely. This was probably due to its assuming the
+position in the centre of the tube where the molecular bombardment
+was most intense, and also partly to the fact that the individual
+impacts were more violent and that no part of the supplied
+energy was converted into mechanical movement. Since, in accordance
+with accepted views, in this experiment the incandescence
+must be attributed to the impacts of the particles, molecules or
+atoms in the heated space, these particles must therefore, in order
+to explain such action, be assumed to behave as independent carriers
+of electric charges immersed in an insulating medium; yet
+there is no attractive force between the glass tube and the filament
+because the space in the tube is, as a whole, conducting.</p>
+
+<p>It is of some interest to observe in this connection that whereas
+the attraction between two electrified bodies may cease owing to
+the impairing of the insulating power of the medium in which
+they are immersed, the repulsion between the bodies may still be
+observed. This may be explained in a plausible way. When the
+bodies are placed at some distance in a poorly conducting medium,
+such as slightly warmed or rarefied air, and are suddenly electrified,
+opposite electric charges being imparted to them, these
+charges equalize more or less by leakage through the air. But if
+the bodies are similarly electrified, there is less opportunity afforded
+for such dissipation, hence the repulsion observed in such
+case is greater than the attraction. Repulsive actions in a gaseous
+medium are however, as Prof. Crookes has shown, enhanced
+by molecular bombardment.</p>
+
+
+<h5>ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA.</h5>
+
+<p>So far, I have considered principally effects produced by a
+varying electrostatic force in an insulating medium, such as air.
+When such a force is acting upon a conducting body of measurable
+dimensions, it causes within the same, or on its surface,
+displacements of the electricity and gives rise to electric currents,
+and these produce another kind of phenomena, some of which I<span class='pagenum'><a name="Page_328" id="Page_328">[Pg 328]</a></span>
+shall presently endeavor to illustrate. In presenting this second
+class of electrical effects, I will avail myself principally of such
+as are producible without any return circuit, hoping to interest
+you the more by presenting these phenomena in a more or less
+novel aspect.</p>
+
+<p>It has been a long time customary, owing to the limited
+experience with vibratory currents, to consider an electric current
+as something circulating in a closed conducting path. It
+was astonishing at first to realize that a current may flow through
+the conducting path even if the latter be interrupted, and it
+was still more surprising to learn, that sometimes it may be
+even easier to make a current flow under such conditions
+than through a closed path. But that old idea is gradually disappearing,
+even among practical men, and will soon be entirely
+forgotten.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_342.jpg" width="800" height="332" alt="Fig. 175." title="" />
+<span class="caption">Fig. 175.</span>
+</div>
+
+
+<p>If I connect an insulated metal plate <small>P</small>, Fig. 175, to one of the
+terminals <small>T</small> of the induction coil by means of a wire, though this
+plate be very well insulated, a current passes through the
+wire when the coil is set to work. First I wish to give you
+evidence that there <i>is</i> a current passing through the connecting
+wire. An obvious way of demonstrating this is to insert between
+the terminal of the coil and the insulated plate a very thin platinum
+or german silver wire <i>w</i> and bring the latter to incandescence
+or fusion by the current. This requires a rather large plate
+or else current impulses of very high potential and frequency.
+Another way is to take a coil <small>C</small>, Fig. 175, containing many turns of
+thin insulated wire and to insert the same in the path of the current
+to the plate. When I connect one of the ends of the coil to the
+wire leading to another insulated plate <small>P<sub>1</sub></small>, and its other end to the
+terminal <small>T<sub>1</sub></small> of the induction coil, and set the latter to work, a current
+passes through the inserted coil <small>C</small> and the existence of the
+current may be made manifest in various ways. For instance, I<span class='pagenum'><a name="Page_329" id="Page_329">[Pg 329]</a></span>
+insert an iron core <i>i</i> within the coil. The current being one of
+very high frequency, will, if it be of some strength, soon bring the
+iron core to a noticeably higher temperature, as the hysteresis and
+current losses are great with such high frequencies. One might
+take a core of some size, laminated or not, it would matter little;
+but ordinary iron wire 1/16th or 1/8th of an inch thick is suitable
+for the purpose. While the induction coil is working, a current
+traverses the inserted coil and only a few moments are sufficient
+to bring the iron wire <i>i</i> to an elevated temperature sufficient to
+soften the sealing-wax <i>s</i>, and cause a paper washer <i>p</i> fastened by
+it to the iron wire to fall off. But with the apparatus such as I
+have here, other, much more interesting, demonstrations of this
+kind can be made. I have a secondary <small>S</small>, Fig 176, of coarse wire,
+wound upon a coil similar to the first. In the preceding experiment
+the current through the coil <small>C</small>, Fig. 175, was very small, but
+there being many turns a strong heating effect was, nevertheless,
+produced in the iron wire. Had I passed that current through a
+conductor in order to show the heating of the latter, the current
+might have been too small to produce the effect desired. But with
+this coil provided with a secondary winding, I can now transform
+the feeble current of high tension which passes through the primary
+<small>P</small> into a strong secondary current of low tension, and this
+current will quite certainly do what I expect. In a small glass
+tube (<i>t</i>, Fig. 176), I have enclosed a coiled platinum wire, <i>w</i>, this
+merely in order to protect the wire. On each end of the glass
+tube is sealed a terminal of stout wire to which one of the ends of
+the platinum wire <i>w</i>, is connected. I join the terminals of the
+secondary coil to these terminals and insert the primary <i>p</i>,
+between the insulated plate <small>P<sub>1</sub></small>, and the terminal <small>T<sub>1</sub></small>, of the induction
+coil as before. The latter being set to work, instantly the
+platinum wire <i>w</i> is rendered incandescent and can be fused, even
+if it be very thick.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_343.jpg" width="800" height="306" alt="Fig. 176." title="" />
+<span class="caption">Fig. 176.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_330" id="Page_330">[Pg 330]</a></span></p>
+
+<p>Instead of the platinum wire I now take an ordinary 50-volt
+16 <span class="smcap">c. p.</span> lamp. When I set the induction coil in operation the
+lamp filament is brought to high incandescence. It is, however,
+not necessary to use the insulated plate, for the lamp (<i>l</i>, Fig. 177)
+is rendered incandescent even if the plate <small>P<sub>1</sub></small> be disconnected.
+The secondary may also be connected to the primary as indicated
+by the dotted line in Fig. 177, to do away more or less with the
+electrostatic induction or to modify the action otherwise.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_344.jpg" width="800" height="463" alt="Fig. 177." title="" />
+<span class="caption">Fig. 177.</span>
+</div>
+
+<p>I may here call attention to a number of interesting observations
+with the lamp. First, I disconnect one of the terminals of
+the lamp from the secondary <small>S</small>. When the induction coil plays,
+a glow is noted which fills the whole bulb. This glow is due to
+electrostatic induction. It increases when the bulb is grasped
+with the hand, and the capacity of the experimenter's body thus
+added to the secondary circuit. The secondary, in effect, is equivalent
+to a metallic coating, which would be placed near the primary.
+If the secondary, or its equivalent, the coating, were placed
+symmetrically to the primary, the electrostatic induction would
+be nil under ordinary conditions, that is, when a primary return
+circuit is used, as both halves would neutralize each other. The
+secondary <i>is</i> in fact placed symmetrically to the primary, but the
+action of both halves of the latter, when only one of its ends is
+connected to the induction coil, is not exactly equal; hence electrostatic
+induction takes place, and hence the glow in the bulb. I
+can nearly equalize the action of both halves of the primary by
+connecting the other, free end of the same to the insulated plate,
+as in the preceding experiment. When the plate is connected,
+the glow disappears. With a smaller plate it would not entirely
+disappear and then it would contribute to the brightness of the
+filament when the secondary is closed, by warming the air in the
+bulb.<span class='pagenum'><a name="Page_331" id="Page_331">[Pg 331]</a></span></p>
+
+<div class="figcenter" style="width: 563px;">
+<img src="images/oi_345.jpg" width="563" height="800" alt="Fig. 178a, 178b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 178a. &nbsp; &nbsp; <span class="smcap">Fig.</span> 178b.</span>
+</div>
+
+<div class="figcenter" style="width: 519px;">
+<img src="images/oi_346.jpg" width="519" height="800" alt="Fig. 179a, 179b." title="" />
+<span class="caption1"><span class="smcap">Fig.</span> 179a. &nbsp; &nbsp; <span class="smcap">Fig.</span> 179b.</span>
+</div>
+
+
+<p>To demonstrate another interesting feature, I have adjusted
+the coils used in a certain way. I first connect both the terminals
+of the lamp to the secondary, one end of the primary being connected
+to the terminal <small>T<sub>1</sub></small> of the induction coil and the other to
+the insulated plate <small>P<sub>1</sub></small> as before. When the current is turned on,
+the lamp glows brightly, as shown in Fig. 178<i>b</i>, in which <small>C</small> is a fine
+wire coil and <small>S</small> a coarse wire secondary wound upon it. If the
+insulated plate <small>P<sub>1</sub></small> is disconnected, leaving one of the ends <i>a</i> of the
+primary insulated, the filament becomes dark or generally it diminishes
+in brightness (Fig. 178<i>a</i>). Connecting again the plate <small>P<sub>1</sub></small>
+and raising the frequency of the current, I make the filament
+quite dark or barely red (Fig. 179<i>b</i>). Once more I will disconnect
+the plate. One will of course infer that when the plate is
+disconnected, the current through the primary will be weakened,
+that therefore the <span class="smcap">e. m. f.</span> will fall in the secondary <small>S</small>, and that
+the brightness of the lamp will diminish. This might be the
+case and the result can be secured by an easy adjustment of the
+<span class='pagenum'><a name="Page_332" id="Page_332">[Pg 332]</a></span>coils; also by varying the frequency and potential of the currents.
+But it is perhaps of greater interest to note, that the lamp
+increases in brightness when the plate is disconnected (Fig. 179<i>a</i>).
+In this case all the energy the primary receives is now sunk into
+it, like the charge of a battery in an ocean cable, but most of that
+energy is recovered through the secondary and used to light the
+lamp. The current traversing the primary is strongest at the end
+<i>b</i> which is connected to the terminal <small>T<sub>1</sub></small> of the induction coil, and
+diminishes in strength towards the remote end <i>a</i>. But the dynamic
+inductive effect exerted upon the secondary <small>S</small> is now greater
+than before, when the suspended plate was connected to the
+primary. These results might have been produced by a number
+of causes. For instance, the plate <small>P<sub>1</sub></small> being connected, the reaction
+from the coil <small>C</small> may be such as to diminish the potential at
+the terminal <small>T<sub>1</sub></small> of the induction coil, and therefore weaken the
+current through the primary of the coil <small>C</small>. Or the disconnecting<span class='pagenum'><a name="Page_333" id="Page_333">[Pg 333]</a></span>
+of the plate may diminish the capacity effect with relation to the
+primary of the latter coil to such an extent that the current
+through it is diminished, though the potential at the terminal <small>T<sub>1</sub></small>
+of the induction coil may be the same or even higher. Or the
+result might have been produced by the change of phase of the
+primary and secondary currents and consequent reaction. But
+the chief determining factor is the relation of the self-induction
+and capacity of coil <small>C</small> and plate <small>P<sub>1</sub></small> and the frequency of the currents.
+The greater brightness of the filament in Fig. 179<i>a</i>, is,
+however, in part due to the heating of the rarefied gas in the
+lamp by electrostatic induction, which, as before remarked, is
+greater when the suspended plate is disconnected.</p>
+
+<p>Still another feature of some interest I may here bring to your
+attention. When the insulated plate is disconnected and the secondary
+of the coil opened, by approaching a small object to the
+secondary, but very small sparks can be drawn from it, showing
+that the electrostatic induction is small in this case. But upon
+the secondary being closed upon itself or through the lamp, the
+filament glowing brightly, strong sparks are obtained from the
+secondary. The electrostatic induction is now much greater,
+because the closed secondary determines a greater flow of current
+through the primary and principally through that half of it which
+is connected to the induction coil. If now the bulb be grasped
+with the hand, the capacity of the secondary with reference to the
+primary is augmented by the experimenter's body and the luminosity
+of the filament is increased, the incandescence now being
+due partly to the flow of current through the filament and
+partly to the molecular bombardment of the rarefied gas in the
+bulb.</p>
+
+<p>The preceding experiments will have prepared one for the next
+following results of interest, obtained in the course of these investigations.
+Since I can pass a current through an insulated
+wire merely by connecting one of its ends to the source of electrical
+energy, since I can induce by it another current, magnetize
+an iron core, and, in short, perform all operations as though a return
+circuit were used, clearly I can also drive a motor by the aid
+of only one wire. On a former occasion I have described a simple
+form of motor comprising a single exciting coil, an iron core
+and disc. Fig. 180 illustrates a modified way of operating such
+an alternate current motor by currents induced in a transformer
+connected to one lead, and several other arrangements of circuits<span class='pagenum'><a name="Page_334" id="Page_334">[Pg 334]</a></span>
+for operating a certain class of alternating motors founded on the
+action of currents of differing phase. In view of the present
+state of the art it is thought sufficient to describe these arrangements
+in a few words only. The diagram, Fig. 180 II., shows
+a primary coil <small>P</small>, connected with one of its ends to the line <small>L</small> leading
+from a high tension transformer terminal <small>T<sub>1</sub></small>. In inductive
+relation to this primary <small>P</small> is a secondary <small>S</small> of coarse wire in the
+circuit of which is a coil <i>c</i>. The currents induced in the secondary
+energize the iron core <i>i</i>, which is preferably, but not necessarily,
+subdivided, and set the metal disc <i>d</i> in rotation. Such a
+motor <small>M<sub>2</sub></small> as diagramatically shown in Fig. 180 II., has been
+called a "magnetic lag motor," but this expression may be objected
+to by those who attribute the rotation of the disc to eddy
+currents circulating in minute paths when the core <i>i</i> is finally
+subdivided. In order to operate such a motor effectively on the
+plan indicated, the frequencies should not be too high, not more
+than four or five thousand, though the rotation is produced even
+with ten thousand per second, or more.</p>
+
+<p>In Fig. 180 I., a motor <small>M<sub>1</sub></small> having two energizing circuits, <small>A</small> and
+<small>B</small>, is diagrammatically indicated. The circuit <small>A</small> is connected to
+the line <small>L</small> and in series with it is a primary <small>P</small>, which may have its
+free end connected to an insulated plate <small>P<sub>1</sub></small>, such connection
+being indicated by the dotted lines. The other motor circuit <small>B</small>
+is connected to the secondary <small>S</small> which is in inductive relation to
+the primary <small>P</small>. When the transformer terminal <small>T<sub>1</sub></small> is alternately
+electrified, currents traverse the open line <small>L</small> and also circuit <small>A</small> and
+primary <small>P</small>. The currents through the latter induce secondary
+currents in the circuit <small>S</small>, which pass through the energizing coil
+<small>B</small> of the motor. The currents through the secondary <small>S</small> and those
+through the primary <small>P</small> differ in phase 90 degrees, or nearly so, and
+are capable of rotating an armature placed in inductive relation
+to the circuits <small>A</small> and <small>B</small>.</p>
+
+<p>In Fig. 180 III., a similar motor <small>M<sub>3</sub></small> with two energizing circuits
+<small>A<sub>1</sub></small> and <small>B<sub>1</sub></small> is illustrated. A primary <small>P</small>, connected with one
+of its ends to the line <small>L</small> has a secondary <small>S</small>, which is preferably
+wound for a tolerably high <span class="smcap">e. m. f.</span>, and to which the two energizing
+circuits of the motor are connected, one directly to the
+ends of the secondary and the other through a condenser <small>C</small>, by the
+action of which the currents traversing the circuit <small>A<sub>1</sub></small> and <small>B<sub>1</sub></small> are
+made to differ in phase.<span class='pagenum'><a name="Page_335" id="Page_335">[Pg 335]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_349-1.jpg" width="1024" height="281" alt="Fig. 180." title="" />
+<span class="caption">Fig. 180.</span>
+
+<img src="images/oi_349.jpg" width="1024" height="221" alt="Fig. 181, 182." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 181.</td><td class="caption">Fig. 182.</td></tr>
+</table>
+
+</div>
+
+<p>In Fig. 180 IV., still another arrangement is shown. In this
+case two primaries <small>P<sub>1</sub></small> and <small>P<sub>2</sub></small> are connected to the line <small>L</small>, one
+<span class='pagenum'><a name="Page_336" id="Page_336">[Pg 336]</a></span>
+through a condenser <small>C</small> of small capacity, and the other directly.
+The primaries are provided with secondaries <small>S<sub>1</sub></small> and <small>S<sub>2</sub></small> which are
+in series with the energizing circuits, <small>A<sub>2</sub></small> and <small>B<sub>2</sub></small> and a motor <small>M<sub>3</sub></small>,
+the condenser <small>C</small> again serving to produce the requisite difference
+in the phase of the currents traversing the motor circuits. As
+such phase motors with two or more circuits are now well known
+in the art, they have been here illustrated diagrammatically. No
+difficulty whatever is found in operating a motor in the manner
+indicated, or in similar ways; and although such experiments up
+to this day present only scientific interest, they may at a period
+not far distant, be carried out with practical objects in view.</p>
+
+<p>It is thought useful to devote here a few remarks to the subject
+of operating devices of all kinds by means of only one leading
+wire. It is quite obvious, that when high-frequency currents are
+made use of, ground connections are&mdash;at least when the <span class="smcap">e. m. f.</span>
+of the currents is great&mdash;better than a return wire. Such ground
+connections are objectionable with steady or low frequency currents
+on account of destructive chemical actions of the former
+and disturbing influences exerted by both on the neighboring circuits;
+but with high frequencies these actions practically do not
+exist. Still, even ground connections become superfluous when
+the <span class="smcap">e. m. f.</span> is very high, for soon a condition is reached, when the
+current may be passed more economically through open, than
+through closed, conductors. Remote as might seem an industrial
+application of such single wire transmission of energy to one not
+experienced in such lines of experiment, it will not seem so to
+anyone who for some time has carried on investigations of such
+nature. Indeed I cannot see why such a plan should not be
+practicable. Nor should it be thought that for carrying out such
+a plan currents of very high frequency are expressly required,
+for just as soon as potentials of say 30,000 volts are used, the
+single wire transmission may be effected with low frequencies,
+and experiments have been made by me from which these inferences
+are made.</p>
+
+<p>When the frequencies are very high it has been found in laboratory
+practice quite easy to regulate the effects in the manner
+shown in diagram Fig. 181. Here two primaries <small>P</small> and <small>P<sub>1</sub></small> are shown,
+each connected with one of its ends to the line <small>L</small> and with the
+other end to the condenser plates <small>C</small> and <small>C</small>, respectively. Near
+these are placed other condenser plates <small>C<sub>1</sub></small> and <small>C<sub>1</sub></small>, the former being
+connected to the line <small>L</small> and the latter to an insulated larger<span class='pagenum'><a name="Page_337" id="Page_337">[Pg 337]</a></span>
+plate <small>P<sub>2</sub></small>. On the primaries are wound secondaries <small>S</small> and <small>S<sub>1</sub></small>, of
+coarse wire, connected to the devices <i>d</i> and <i>l</i> respectively. By
+varying the distances of the condenser plates <small>C</small> and <small>C<sub>1</sub></small>, and <small>C</small> and
+<small>C<sub>1</sub></small> the currents through the secondaries <small>S</small> and <small>S<sub>1</sub></small> are varied in
+intensity. The curious feature is the great sensitiveness, the
+slightest change in the distance of the plates producing considerable
+variations in the intensity or strength of the currents. The
+sensitiveness may be rendered extreme by making the frequency
+such, that the primary itself, without any plate attached to its
+free end, satisfies, in conjunction with the closed secondary, the
+condition of resonance. In such condition an extremely small
+change in the capacity of the free terminal produces great variations.
+For instance, I have been able to adjust the conditions so
+that the mere approach of a person to the coil produces a considerable
+change in the brightness of the lamps attached to the
+secondary. Such observations and experiments possess, of course,
+at present, chiefly scientific interest, but they may soon become
+of practical importance.</p>
+
+<p>Very high frequencies are of course not practicable with
+motors on account of the necessity of employing iron cores. But
+one may use sudden discharges of low frequency and thus obtain
+certain advantages of high-frequency currents without rendering
+the iron core entirely incapable of following the changes and
+without entailing a very great expenditure of energy in the core.
+I have found it quite practicable to operate with such low frequency
+disruptive discharges of condensers, alternating-current
+motors. A certain class of such motors which I advanced a few
+years ago, which contain closed secondary circuits, will rotate
+quite vigorously when the discharges are directed through the
+exciting coils. One reason that such a motor operates so well
+with these discharges is that the difference of phase between the
+primary and secondary currents is 90 degrees, which is generally
+not the case with harmonically rising and falling currents of low
+frequency. It might not be without interest to show an experiment
+with a simple motor of this kind, inasmuch as it is commonly
+thought that disruptive discharges are unsuitable for such
+purposes. The motor is illustrated in Fig. 182. It comprises a
+rather large iron core <i>i</i> with slots on the top into which are embedded
+thick copper washers <i>c c</i>. In proximity to the core is
+a freely-movable metal disc <small>D</small>. The core is provided with a primary
+<span class='pagenum'><a name="Page_338" id="Page_338">[Pg 338]</a></span>exciting coil <small>C<sub>1</sub></small> the ends <i>a</i> and <i>b</i> of which are connected to
+the terminals of the secondary <small>S</small> of an ordinary transformer, the
+primary <small>P</small> of the latter being connected to an alternating distribution
+circuit or generator <small>G</small> of low or moderate frequency.
+The terminals of the secondary <small>S</small> are attached to a condenser <small>C</small>
+which discharges through an air gap <i>d d</i> which may be placed
+in series or shunt to the coil <small>C<sub>1</sub></small>. When the conditions are
+properly chosen the disc <small>D</small> rotates with considerable effort and the
+iron core <i>i</i> does not get very perceptibly hot. With currents from
+a high-frequency alternator, on the contrary, the core gets rapidly
+hot and the disc rotates with a much smaller effort. To perform
+the experiment properly it should be first ascertained that the
+disc <small>D</small> is not set in rotation when the discharge is not occurring
+at <i>d d</i>. It is preferable to use a large iron core and a condenser
+of large capacity so as to bring the superimposed quicker oscillation
+to a very low pitch or to do away with it entirely. By
+observing certain elementary rules I have also found it practicable
+to operate ordinary series or shunt direct-current motors
+with such disruptive discharges, and this can be done with or
+without a return wire.</p>
+
+
+<h5>IMPEDANCE PHENOMENA.</h5>
+
+<p>Among the various current phenomena observed, perhaps the
+most interesting are those of impedance presented by conductors
+to currents varying at a rapid rate. In my first paper before the
+American Institute of Electrical Engineers, I have described a
+few striking observations of this kind. Thus I showed that when
+such currents or sudden discharges are passed through a thick
+metal bar there may be points on the bar only a few inches apart,
+which have a sufficient potential difference between them to
+maintain at bright incandescence an ordinary filament lamp. I
+have also described the curious behavior of rarefied gas surrounding
+a conductor, due to such sudden rushes of current. These
+phenomena have since been more carefully studied and one or
+two novel experiments of this kind are deemed of sufficient interest
+to be described here.</p>
+
+<p>Referring to Fig. 183<i>a</i>, <small>B</small> and <small>B<sub>1</sub></small> are very stout copper bars
+connected at their lower ends to plates <small>C</small> and <small>C<sub>1</sub></small>, respectively, of a
+condenser, the opposite plates of the latter being connected to the
+terminals of the secondary <small>S</small> of a high-tension transformer, the
+primary <small>P</small> of which is supplied with alternating currents from an
+ordinary low-frequency dynamo <small>G</small> or distribution circuit. The<span class='pagenum'><a name="Page_339" id="Page_339">[Pg 339]</a></span>
+condenser discharges through an adjustable gap <i>d d</i> as usual. By
+establishing a rapid vibration it was found quite easy to perform
+the following curious experiment. The bars <small>B</small> and <small>B<sub>1</sub></small> were joined
+at the top by a low-voltage lamp <i>l</i><sub>3</sub>; a little lower was placed by
+means of clamps <i>c c</i>, a 50-volt lamp <i>l</i><sub>2</sub>; and still lower another 100-volt
+lamp <i>l</i><sub>1</sub>; and finally, at a certain distance below the latter
+lamp, an exhausted tube <small>T</small>. By carefully determining the positions
+of these devices it was found practicable to maintain them
+all at their proper illuminating power. Yet they were all connected
+in multiple arc to the two stout copper bars and required
+widely different pressures. This experiment requires of course
+some time for adjustment but is quite easily performed.</p>
+
+<div class="figcenter" style="width: 600px;">
+<img src="images/oi_353.jpg" width="600" height="767" alt="Fig. 183a, 183b and 183c." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 183a, 183b and 183c.</span>
+</div>
+
+<p>In Figs. 183<i>b</i> and 183<i>c</i>, two other experiments are illustrated
+which, unlike the previous experiment, do not require very careful
+<span class='pagenum'><a name="Page_340" id="Page_340">[Pg 340]</a></span>adjustments. In Fig. 183<i>b</i>, two lamps, <i>l</i><sub>1</sub> and <i>l</i><sub>2</sub>, the former a
+100-volt and the latter a 50-volt are placed in certain positions as
+indicated, the 100-volt lamp being below the 50-volt lamp. When
+the arc is playing at <i>d d</i> and the sudden discharges are passed
+through the bars <small>B B<sub>1</sub></small>, the 50-volt lamp will, as a rule, burn brightly,
+or at least this result is easily secured, while the 100-volt lamp
+will burn very low or remain quite dark, Fig. 183<i>b</i>. Now the
+bars <small>B B<sub>1</sub></small> may be joined at the top by a thick cross bar <small>B<sub>2</sub></small> and it
+is quite easy to maintain the 100-volt lamp at full candle-power
+while the 50-volt lamp remains dark, Fig. 183<i>c</i>. These results,
+as I have pointed out previously, should not be considered to be
+due exactly to frequency but rather to the time rate of change
+which may be great, even with low frequencies. A great many
+other results of the same kind, equally interesting, especially to
+those who are only used to manipulate steady currents, may be
+obtained and they afford precious clues in investigating the nature
+of electric currents.</p>
+
+<p>In the preceding experiments I have already had occasion to
+show some light phenomena and it would now be proper to study
+these in particular; but to make this investigation more complete
+I think it necessary to make first a few remarks on the
+subject of electrical resonance which has to be always observed
+in carrying out these experiments.</p>
+
+
+<h5>ON ELECTRICAL RESONANCE.</h5>
+
+<p>The effects of resonance are being more and more noted by engineers
+and are becoming of great importance in the practical operation
+of apparatus of all kinds with alternating currents. A few
+general remarks may therefore be made concerning these effects.
+It is clear, that if we succeed in employing the effects of resonance
+practically in the operation of electric devices the return wire will,
+as a matter of course, become unnecessary, for the electric vibration
+may be conveyed with one wire just as well as, and sometimes
+even better than, with two. The question first to answer is, then,
+whether pure resonance effects are producible. Theory and experiment
+both show that such is impossible in Nature, for as the
+oscillation becomes more and more vigorous, the losses in the vibrating
+bodies and environing media rapidly increase and necessarily
+check the vibration which otherwise would go on increasing
+forever. It is a fortunate circumstance that pure resonance is
+not producible, for if it were there is no telling what dangers
+might not lie in wait for the innocent experimenter. But to a<span class='pagenum'><a name="Page_341" id="Page_341">[Pg 341]</a></span>
+certain degree resonance is producible, the magnitude of the
+effects being limited by the imperfect conductivity and imperfect
+elasticity of the media or, generally stated, by frictional losses. The
+smaller these losses, the more striking are the effects. The same
+is the case in mechanical vibration. A stout steel bar may be set
+in vibration by drops of water falling upon it at proper intervals;
+and with glass, which is more perfectly elastic, the resonance
+effect is still more remarkable, for a goblet may be burst by
+singing into it a note of the proper pitch. The electrical resonance
+is the more perfectly attained, the smaller the resistance or the
+impedance of the conducting path and the more perfect the dielectric.
+In a Leyden jar discharging through a short stranded cable
+of thin wires these requirements are probably best fulfilled, and
+the resonance effects are therefore very prominent. Such is not
+the case with dynamo machines, transformers and their circuits,
+or with commercial apparatus in general in which the presence
+of iron cores complicates the action or renders it impossible.
+In regard to Leyden jars with which resonance effects are
+frequently demonstrated, I would say that the effects observed
+are often <i>attributed</i> but are seldom <i>due</i> to true resonance, for
+an error is quite easily made in this respect. This may be
+undoubtedly demonstrated by the following experiment. Take,
+for instance, two large insulated metallic plates or spheres which
+I shall designate <small>A</small> and <small>B</small>; place them at a certain small distance
+apart and charge them from a frictional or influence
+machine to a potential so high that just a slight increase of the
+difference of potential between them will cause the small air or
+insulating space to break down. This is easily reached by making
+a few preliminary trials. If now another plate&mdash;fastened on
+an insulating handle and connected by a wire to one of the terminals
+of a high tension secondary of an induction coil, which
+is maintained in action by an alternator (preferably high frequency)&mdash;is
+approached to one of the charged bodies <small>A</small> or <small>B</small>, so as
+to be nearer to either one of them, the discharge will invariably
+occur between them; at least it will, if the potential of the coil
+in connection with the plate is sufficiently high. But the explanation
+of this will soon be found in the fact that the approached
+plate acts inductively upon the bodies <small>A</small> and <small>B</small> and causes a spark
+to pass between them. When this spark occurs, the charges which
+were previously imparted to these bodies from the influence machine,
+must needs be lost, since the bodies are brought in electri<span class='pagenum'><a name="Page_342" id="Page_342">[Pg 342]</a></span>cal
+connection through the arc formed. Now this arc is formed
+whether there be resonance or not. But even if the spark would
+not be produced, still there is an alternating <span class="smcap">e. m. f.</span> set up between
+the bodies when the plate is brought near one of them; therefore
+the approach of the plate, if it <i>does</i> not always actually, will, at any
+rate, <i>tend</i> to break down the air space by inductive action. Instead
+of the spheres or plates <small>A</small> and <small>B</small> we may take the coatings of a Leyden
+jar with the same result, and in place of the machine,&mdash;which
+is a high frequency alternator preferably, because it is more suitable
+for the experiment and also for the argument,&mdash;we may take
+another Leyden jar or battery of jars. When such jars are discharging
+through a circuit of low resistance the same is traversed
+by currents of very high frequency. The plate may now be connected
+to one of the coatings of the second jar, and when it is
+brought near to the first jar just previously charged to a high
+potential from an influence machine, the result is the same as before,
+and the first jar will discharge through a small air space
+upon the second being caused to discharge. But both jars and
+their circuits need not be tuned any closer than a basso profundo
+is to the note produced by a mosquito, as small sparks will be produced
+through the air space, or at least the latter will be considerably
+more strained owing to the setting up of an alternating
+<span class="smcap">e. m. f.</span> by induction, which takes place when one of the jars begins
+to discharge. Again another error of a similar nature is quite
+easily made. If the circuits of the two jars are run parallel and
+close together, and the experiment has been performed of discharging
+one by the other, and now a coil of wire be added to one
+of the circuits whereupon the experiment does not succeed, the
+conclusion that this is due to the fact that the circuits are now
+not tuned, would be far from being safe. For the two circuits
+act as condenser coatings and the addition of the coil to one of
+them is equivalent to bridging them, at the point where the coil
+is placed, by a small condenser, and the effect of the latter might
+be to prevent the spark from jumping through the discharge space
+by diminishing the alternating <span class="smcap">e. m. f.</span> acting across the same.
+All these remarks, and many more which might be added but for
+fear of wandering too far from the subject, are made with the
+pardonable intention of cautioning the unsuspecting student, who
+might gain an entirely unwarranted opinion of his skill at seeing
+every experiment succeed; but they are in no way thrust upon
+the experienced as novel observations.<span class='pagenum'><a name="Page_343" id="Page_343">[Pg 343]</a></span></p>
+
+<p>In order to make reliable observations of electric resonance
+effects it is very desirable, if not necessary, to employ an alternator
+giving currents which rise and fall harmonically, as in
+working with make and break currents the observations are not
+always trustworthy, since many phenomena, which depend on
+the rate of change, may be produced with widely different frequencies.
+Even when making such observations with an alternator
+one is apt to be mistaken. When a circuit is connected to an
+alternator there are an indefinite number of values for capacity and
+self-induction which, in conjunction, will satisfy the condition of
+resonance. So there are in mechanics an infinite number of tuning
+forks which will respond to a note of a certain pitch, or loaded
+springs which have a definite period of vibration. But the resonance
+will be most perfectly attained in that case in which the motion
+is effected with the greatest freedom. Now in mechanics,
+considering the vibration in the common medium&mdash;that is, air&mdash;it
+is of comparatively little importance whether one tuning fork be
+somewhat larger than another, because the losses in the air are
+not very considerable. One may, of course, enclose a tuning fork
+in an exhausted vessel and by thus reducing the air resistance to
+a minimum obtain better resonant action. Still the difference
+would not be very great. But it would make a great difference if
+the tuning fork were immersed in mercury. In the electrical
+vibration it is of enormous importance to arrange the conditions
+so that the vibration is effected with the greatest freedom. The
+magnitude of the resonance effect depends, under otherwise equal
+conditions, on the quantity of electricity set in motion or on the
+strength of the current driven through the circuit. But the circuit
+opposes the passage of the currents by reason of its impedance
+and therefore, to secure the best action it is necessary to reduce
+the impedance to a minimum. It is impossible to overcome
+it entirely, but merely in part, for the ohmic resistance cannot be
+overcome. But when the frequency of the impulses is very great,
+the flow of the current is practically determined by self-induction.
+Now self-induction can be overcome by combining it with capacity.
+If the relation between these is such, that at the frequency
+used they annul each other, that is, have such values as to
+satisfy the condition of resonance, and the greatest quantity of
+electricity is made to flow through the external circuit, then the
+best result is obtained. It is simpler and safer to join the condenser
+in series with the self-induction. It is clear that in such<span class='pagenum'><a name="Page_344" id="Page_344">[Pg 344]</a></span>
+combinations there will be, for a given frequency, and considering
+only the fundamental vibration, values which will give the best
+result, with the condenser in shunt to the self-induction coil; of
+course more such values than with the condenser in series. But
+practical conditions determine the selection. In the latter case
+in performing the experiments one may take a small self-induction
+and a large capacity or a small capacity and a large self-induction,
+but the latter is preferable, because it is inconvenient to adjust
+a large capacity by small steps. By taking a coil with a very
+large self-induction the critical capacity is reduced to a very small
+value, and the capacity of the coil itself may be sufficient. It is
+easy, especially by observing certain artifices, to wind a coil
+through which the impedance will be reduced to the value of the
+ohmic resistance only; and for any coil there is, of course, a frequency
+at which the maximum current will be made to pass
+through the coil. The observation of the relation between self-induction,
+capacity and frequency is becoming important in the
+operation of alternate current apparatus, such as transformers or
+motors, because by a judicious determination of the elements the
+employment of an expensive condenser becomes unnecessary.
+Thus it is possible to pass through the coils of an alternating
+current motor under the normal working conditions the required
+current with a low <span class="smcap">e. m. f.</span> and do away entirely with the false
+current, and the larger the motor, the easier such a plan becomes
+practicable; but it is necessary for this to employ currents of very
+high potential or high frequency.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_358.jpg" width="800" height="267" alt="Fig. 184." title="" />
+<span class="caption">Fig. 184.</span>
+</div>
+
+
+<p>In Fig. 184 I. is shown a plan which has been followed in the
+study of the resonance effects by means of a high frequency alternator.
+<small>C<sub>1</sub></small> is a coil of many turns, which is divided into small
+separate sections for the purpose of adjustment. The final adjustment
+was made sometimes with a few thin iron wires (though
+this is not always advisable) or with a closed secondary. The coil<span class='pagenum'><a name="Page_345" id="Page_345">[Pg 345]</a></span>
+<small>C<sub>1</sub></small> is connected with one of its ends to the line <small>L</small> from the alternator
+<small>G</small> and with the other end to one of the plates <i>c</i> of a condenser
+<i>c c</i><sub>1</sub>, the plate (<i>c</i><sub>1</sub>) of the latter being connected to a much
+larger plate <small>P<sub>1</sub></small>. In this manner both capacity and self-induction
+were adjusted to suit the dynamo frequency.</p>
+
+<p>As regards the rise of potential through resonant action, of
+course, theoretically, it may amount to anything since it depends
+on self-induction and resistance and since these may have any
+value. But in practice one is limited in the selection of these
+values and besides these, there are other limiting causes. One
+may start with, say, 1,000 volts and raise the <span class="smcap">e. m. f.</span> to 50 times
+that value, but one cannot start with 100,000 and raise it to ten
+times that value because of the losses in the media which are
+great, especially if the frequency is high. It should be possible
+to start with, for instance, two volts from a high or low frequency
+circuit of a dynamo and raise the <span class="smcap">e. m. f.</span> to many hundred
+times that value. Thus coils of the proper dimensions
+might be connected each with only one of its ends to the
+mains from a machine of low <span class="smcap">e. m. f.</span>, and though the circuit of
+the machine would not be closed in the ordinary acceptance of the
+term, yet the machine might be burned out if a proper resonance
+effect would be obtained. I have not been able to produce, nor
+have I observed with currents from a dynamo machine, such
+great rises of potential. It is possible, if not probable, that with
+currents obtained from apparatus containing iron the disturbing
+influence of the latter is the cause that these theoretical possibilities
+cannot be realized. But if such is the case I attribute
+it solely to the hysteresis and Foucault current losses in the core.
+Generally it was necessary to transform upward, when the <span class="smcap">e. m. f.</span>
+was very low, and usually an ordinary form of induction coil
+was employed, but sometimes the arrangement illustrated in Fig.
+184 II., has been found to be convenient. In this case a coil <small>C</small> is
+made in a great many sections, a few of these being used as a
+primary. In this manner both primary and secondary are adjustable.
+One end of the coil is connected to the line <small>L<sub>1</sub></small> from
+the alternator, and the other line <small>L</small> is connected to the intermediate
+point of the coil. Such a coil with adjustable primary and
+secondary will be found also convenient in experiments with the
+disruptive discharge. When true resonance is obtained the top
+of the wave must of course be on the free end of the coil as, for
+instance, at the terminal of the phosphorescence bulb <small>B</small>. This is<span class='pagenum'><a name="Page_346" id="Page_346">[Pg 346]</a></span>
+easily recognized by observing the potential of a point on the
+wire <i>w</i> near to the coil.</p>
+
+<p>In connection with resonance effects and the problem of transmission
+of energy over a single conductor which was previously
+considered, I would say a few words on a subject which constantly
+fills my thoughts and which concerns the welfare of all. I mean
+the transmission of intelligible signals or perhaps even power to
+any distance without the use of wires. I am becoming daily
+more convinced of the practicability of the scheme; and though
+I know full well that the great majority of scientific men will
+not believe that such results can be practically and immediately
+realized, yet I think that all consider the developments in recent
+years by a number of workers to have been such as to encourage
+thought and experiment in this direction. My conviction has
+grown so strong, that I no longer look upon this plan of energy
+or intelligence transmission as a mere theoretical possibility, but as
+a serious problem in electrical engineering, which must be carried
+out some day. The idea of transmitting intelligence without
+wires is the natural outcome of the most recent results of electrical
+investigations. Some enthusiasts have expressed their belief
+that telephony to any distance by induction through the air
+is possible. I cannot stretch my imagination so far, but I do
+firmly believe that it is practicable to disturb by means of powerful
+machines the electrostatic condition of the earth and thus
+transmit intelligible signals and perhaps power. In fact, what is
+there against the carrying out of such a scheme? We now know
+that electric vibration may be transmitted through a single conductor.
+Why then not try to avail ourselves of the earth for
+this purpose? We need not be frightened by the idea of distance.
+To the weary wanderer counting the mile-posts the earth
+may appear very large, but to that happiest of all men, the astronomer,
+who gazes at the heavens and by their standard judges
+the magnitude of our globe, it appears very small. And so I
+think it must seem to the electrician, for when he considers the
+speed with which an electric disturbance is propagated through
+the earth all his ideas of distance must completely vanish.</p>
+
+<p>A point of great importance would be first to know what is the
+capacity of the earth? and what charge does it contain if electrified?
+Though we have no positive evidence of a charged body
+existing in space without other oppositely electrified bodies being
+near, there is a fair probability that the earth is such a body, for<span class='pagenum'><a name="Page_347" id="Page_347">[Pg 347]</a></span>
+by whatever process it was separated from other bodies&mdash;and this
+is the accepted view of its origin&mdash;it must have retained a charge,
+as occurs in all processes of mechanical separation. If it be a
+charged body insulated in space its capacity should be extremely
+small, less than one-thousandth of a farad. But the upper strata
+of the air are conducting, and so, perhaps, is the medium in free
+space beyond the atmosphere, and these may contain an opposite
+charge. Then the capacity might be incomparably greater. In
+any case it is of the greatest importance to get an idea of what
+quantity of electricity the earth contains. It is difficult to say
+whether we shall ever acquire this necessary knowledge, but there
+is hope that we may, and that is, by means of electrical resonance.
+If ever we can ascertain at what period the earth's charge, when
+disturbed, oscillates with respect to an oppositely electrified system
+or known circuit, we shall know a fact possibly of the greatest
+importance to the welfare of the human race. I propose to seek
+for the period by means of an electrical oscillator, or a source of
+alternating electric currents. One of the terminals of the source
+would be connected to earth as, for instance, to the city water
+mains, the other to an insulated body of large surface. It is possible
+that the outer conducting air strata, or free space, contain
+an opposite charge and that, together with the earth, they form a
+condenser of very large capacity. In such case the period of
+vibration may be very low and an alternating dynamo machine
+might serve for the purpose of the experiment. I would then
+transform the current to a potential as high as it would be found
+possible and connect the ends of the high tension secondary to the
+ground and to the insulated body. By varying the frequency of the
+currents and carefully observing the potential of the insulated body
+and watching for the disturbance at various neighboring points of
+the earth's surface resonance might be detected. Should, as the
+majority of scientific men in all probability believe, the period be
+extremely small, then a dynamo machine would not do and a
+proper electrical oscillator would have to be produced and perhaps
+it might not be possible to obtain such rapid vibrations. But
+whether this be possible or not, and whether the earth contains a
+charge or not, and whatever may be its period of vibration, it certainly
+is possible&mdash;for of this we have daily evidence&mdash;to produce
+some electrical disturbance sufficiently powerful to be perceptible
+by suitable instruments at any point of the earth's
+surface.<span class='pagenum'><a name="Page_348" id="Page_348">[Pg 348]</a></span></p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_362.jpg" width="800" height="94" alt="Fig. 185." title="" />
+<span class="caption">Fig. 185.</span>
+</div>
+
+
+<p>Assume that a source of alternating current <small>S</small> be connected, as
+in Fig. 185, with one of its terminals to earth (conveniently to the
+water mains) and with the other to a body of large surface <small>P</small>.
+When the electric oscillation is set up there will be
+a movement of electricity in and out of <small>P</small>, and alternating
+currents will pass through the earth, converging
+to, or diverging from, the point <small>C</small> where
+the ground connection is made. In this manner
+neighboring points on the earth's surface within a
+certain radius will be disturbed. But the disturbance
+will diminish with the distance, and the distance
+at which the effect will still be perceptible
+will depend on the quantity of electricity set in
+motion. Since the body <small>P</small> is insulated, in order to
+displace a considerable quantity, the potential of
+the source must be excessive, since there would be
+limitations as to the surface of <small>P</small>. The conditions
+might be adjusted so that the generator or source
+<small>S</small> will set up the same electrical movement as
+though its circuit were closed. Thus it is certainly
+practicable to impress an electric vibration at least
+of a certain low period upon the earth by means of
+proper machinery. At what distance such a vibration
+might be made perceptible can only be conjectured.
+I have on another occasion considered the
+question how the earth might behave to electric
+disturbances. There is no doubt that, since in such
+an experiment the electrical density at the surface
+could be but extremely small considering the size
+of the earth, the air would not act as a very disturbing
+factor, and there would be not much energy
+lost through the action of the air, which would be
+the case if the density were great. Theoretically,
+then, it could not require a great amount of energy
+to produce a disturbance perceptible at great distance,
+or even all over the surface of the globe.
+Now, it is quite certain that at any point within a
+certain radius of the source <small>S</small> a properly adjusted
+self-induction and capacity device can be set in action
+by resonance. But not only can this be done, but another source
+<span class='pagenum'><a name="Page_349" id="Page_349">[Pg 349]</a></span>
+<small>S<sub>1</sub></small>, Fig. 185, similar to <small>S</small>, or any number of such sources, can be set
+to work in synchronism with the latter, and the vibration thus
+intensified and spread over a large area, or a flow of electricity
+produced to or from the source <small>S<sub>1</sub></small> if the same be of
+opposite phase to the source <small>S</small>. I think that beyond doubt
+it is possible to operate electrical devices in a city through
+the ground or pipe system by resonance from an electrical
+oscillator located at a central point. But the practical solution
+of this problem would be of incomparably smaller benefit to man
+than the realization of the scheme of transmitting intelligence, or
+perhaps power, to any distance through the earth or environing
+medium. If this is at all possible, distance does not mean anything.
+Proper apparatus must first be produced by means of
+which the problem can be attacked and I have devoted much
+thought to this subject. I am firmly convinced that it can be
+done and hope that we shall live to see it done.</p>
+
+
+<h5>ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CURRENTS
+OF HIGH POTENTIAL AND GENERAL REMARKS RELATING
+TO THE SUBJECT.</h5>
+
+<p>Returning now to the light effects which it has been the chief
+object to investigate, it is thought proper to divide these effects
+into four classes: 1. Incandescence of a solid. 2. Phosphorescence.
+3. Incandescence or phosphorescence of a rarefied gas; and
+4. Luminosity produced in a gas at ordinary pressure. The first
+question is: How are these luminous effects produced? In order
+to answer this question as satisfactorily as I am able to do in the
+light of accepted views and with the experience acquired, and to
+add some interest to this demonstration, I shall dwell here upon
+a feature which I consider of great importance, inasmuch as it
+promises, besides, to throw a better light upon the nature of most
+of the phenomena produced by high-frequency electric currents.
+I have on other occasions pointed out the great importance of the
+presence of the rarefied gas, or atomic medium in general, around
+the conductor through which alternate currents of high frequency
+are passed, as regards the heating of the conductor by the currents.
+My experiments, described some time ago, have shown
+that, the higher the frequency and potential difference of the currents,
+the more important becomes the rarefied gas in which the
+conductor is immersed, as a factor of the heating. The potential
+difference, however, is, as I then pointed out, a more im<span class='pagenum'><a name="Page_350" id="Page_350">[Pg 350]</a></span>portant
+element than the frequency. When both of these are
+sufficiently high, the heating may be almost entirely due to the
+presence of the rarefied gas. The experiments to follow will
+show the importance of the rarefied gas, or, generally, of gas at ordinary
+or other pressure as regards the incandescence or other
+luminous effects produced by currents of this kind.</p>
+
+<p>I take two ordinary 50-volt 16 <span class="smcap">c. p.</span> lamps which are in every
+respect alike, with the exception, that one has been opened at the
+top and the air has filled the bulb, while the other is at the ordinary
+degree of exhaustion of commercial lamps. When I attach
+the lamp which is exhausted to the terminal of the secondary of
+the coil, which I have already used, as in experiments illustrated
+in Fig. 179<i>a</i> for instance, and turn on the current, the filament, as
+you have before seen, comes to high incandescence. When I
+attach the second lamp, which is filled with air, instead of the
+former, the filament still glows, but much less brightly. This
+experiment illustrates only in part the truth of the statements
+before made. The importance of the filament's being immersed
+in rarefied gas is plainly noticeable but not to such a degree as
+might be desirable. The reason is that the secondary of this coil is
+wound for low tension, having only 150 turns, and the potential
+difference at the terminals of the lamp is therefore small. Were
+I to take another coil with many more turns in the secondary,
+the effect would be increased, since it depends partially on the
+potential difference, as before remarked. But since the effect
+likewise depends on the frequency, it maybe properly stated that
+it depends on the time rate of the variation of the potential difference.
+The greater this variation, the more important becomes
+the gas as an element of heating. I can produce a much greater
+rate of variation in another way, which, besides, has the advantage
+of doing away with the objections, which might be made in
+the experiment just shown, even if both the lamps were connected
+in series or multiple arc to the coil, namely, that in consequence
+of the reactions existing between the primary and
+secondary coil the conclusions are rendered uncertain. This result
+I secure by charging, from an ordinary transformer which is
+fed from the alternating current supply station, a battery of condensers,
+and discharging the latter directly through a circuit of
+small self-induction, as before illustrated in Figs. 183<i>a</i>, 183<i>b</i>,
+and 183<i>c</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_365.jpg" width="800" height="308" alt="Fig. 186a, 186b, 186c." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption1"><span class="smcap">Fig.</span> 186a.</td><td class="caption1"><span class="smcap">Fig.</span> 186b.</td><td class="caption1"><span class="smcap">Fig.</span> 186c.</td></tr>
+</table>
+</div>
+
+<p>In Figs. 186<i>a</i>, 186<i>b</i> and 186<i>c</i>, the heavy copper bars <small>B B<sub>1</sub></small>, are
+<span class='pagenum'><a name="Page_351" id="Page_351">[Pg 351]</a></span>connected to the opposite coatings of a battery of condensers,
+or generally in such way, that the high frequency or sudden
+discharges are made to traverse them. I connect first an
+ordinary 50-volt incandescent lamp to the bars by means of
+the clamps <i>c c</i>. The discharges being passed through the lamp,
+the filament is rendered incandescent, though the current
+through it is very small, and would not be nearly sufficient to
+produce a visible effect under the conditions of ordinary use of
+the lamp. Instead of this I now attach to the bars another
+lamp exactly like the first, but with the seal broken off, the bulb
+being therefore filled with air at ordinary pressure. When the
+discharges are directed through the filament, as before, it does
+not become incandescent. But the result might still be attributed
+to one of the many possible reactions. I therefore connect
+both the lamps in multiple arc as illustrated in Fig. 186<i>a</i>. Passing
+the discharges through both the lamps, again the filament in the
+exhausted lamp <i>l</i> glows very brightly while that in the non-exhausted
+lamp <i>l</i><sub>1</sub> remains dark, as previously. But it should not
+be thought that the latter lamp is taking only a small fraction of
+the energy supplied to both the lamps; on the contrary, it may
+consume a considerable portion of the energy and it may become
+even hotter than the one which burns brightly. In this experiment
+the potential difference at the terminals of the lamps varies
+in sign theoretically three to four million times a second. The
+ends of the filaments are correspondingly electrified, and the gas
+in the bulbs is violently agitated and a large portion of the supplied
+energy is thus converted into heat. In the non-exhausted
+bulb, there being a few million times more gas molecules than in
+the exhausted one, the bombardment, which is most violent at
+the ends of the filament, in the neck of the bulb, consumes a<span class='pagenum'><a name="Page_352" id="Page_352">[Pg 352]</a></span>
+large portion of the energy without producing any visible effect.
+The reason is that, there being many molecules, the bombardment
+is quantitatively considerable, but the individual impacts are
+not very violent, as the speeds of the molecules are comparatively
+small owing to the small free path. In the exhausted bulb, on
+the contrary, the speeds are very great, and the individual impacts
+are violent and therefore better adapted to produce a visible
+effect. Besides, the convection of heat is greater in the former
+bulb. In both the bulbs the current traversing the filaments is
+very small, incomparably smaller than that which they require on
+an ordinary low-frequency circuit. The potential difference,
+however, at the ends of the filaments is very great and might be
+possibly 20,000 volts or more, if the filaments were straight and
+their ends far apart. In the ordinary lamp a spark generally occurs
+between the ends of the filament or between the platinum
+wires outside, before such a difference of potential can be
+reached.</p>
+
+<p>It might be objected that in the experiment before shown the
+lamps, being in multiple arc, the exhausted lamp might take a
+much larger current and that the effect observed might not be
+exactly attributable to the action of the gas in the bulbs. Such
+objections will lose much weight if I connect the lamps in series,
+with the same result. When this is done and the discharges are
+directed through the filaments, it is again noted that the filament
+in the non-exhausted bulb <i>l</i><sub>1</sub>, remains dark, while that in the
+exhausted one (<i>l</i>) glows even more intensely than under its
+normal conditions of working, Fig. 186<i>b</i>. According to general
+ideas the current through the filaments should now be the same,
+were it not modified by the presence of the gas around the
+filaments.</p>
+
+<p>At this juncture I may point out another interesting feature,
+which illustrates the effect of the rate of change of potential
+of the currents. I will leave the two lamps connected in series
+to the bars <small>B B<sub>1</sub></small>, as in the previous experiment, Fig. 186<i>b</i>, but will
+presently reduce considerably the frequency of the currents,
+which was excessive in the experiment just before shown. This
+I may do by inserting a self-induction coil in the path of the discharges,
+or by augmenting the capacity of the condensers. When
+I now pass these low-frequency discharges through the lamps,
+the exhausted lamp <i>l</i> again is as bright as before, but it is noted
+<span class='pagenum'><a name="Page_353" id="Page_353">[Pg 353]</a></span>also that the non-exhausted lamp <i>l</i><sub>1</sub> glows, though not quite
+as intensely as the other. Reducing the current through the
+lamps, I may bring the filament in the latter lamp to redness, and,
+though the filament in the exhausted lamp <i>l</i> is bright, Fig. 186<i>c</i>,
+the degree of its incandescence is much smaller than in Fig. 186<i>b</i>,
+when the currents were of a much higher frequency.</p>
+
+<p>In these experiments the gas acts in two opposite ways in determining
+the degree of the incandescence of the filaments, that
+is, by convection and bombardment. The higher the frequency and
+potential of the currents, the more important becomes the bombardment.
+The convection on the contrary should be the smaller,
+the higher the frequency. When the currents are steady there is
+practically no bombardment, and convection may therefore with
+such currents also considerably modify the degree of incandescence
+and produce results similar to those just before shown. Thus, if
+two lamps exactly alike, one exhausted and one not exhausted,
+are connected in multiple arc or series to a direct-current machine,
+the filament in the non-exhausted lamp will require a considerably
+greater current to be rendered incandescent. This result is
+entirely due to convection, and the effect is the more prominent
+the thinner the filament. Professor Ayrton and Mr. Kilgour
+some time ago published quantitative results concerning the
+thermal emissivity by radiation and convection in which the effect
+with thin wires was clearly shown. This effect may be strikingly
+illustrated by preparing a number of small, short, glass tubes,
+each containing through its axis the thinnest obtainable platinum
+wire. If these tubes be highly exhausted, a number of them
+may be connected in multiple arc to a direct-current machine and
+all of the wires may be kept at incandescence with a smaller current
+than that required to render incandescent a single one of the
+wires if the tube be not exhausted. Could the tubes be so highly
+exhausted that convection would be nil, then the relative amounts
+of heat given off by convection and radiation could be determined
+without the difficulties attending thermal quantitative
+measurements. If a source of electric impulses of high frequency
+and very high potential is employed, a still greater number of
+the tubes may be taken and the wires rendered incandescent by a
+current not capable of warming perceptibly a wire of the same
+size immersed in air at ordinary pressure, and conveying the
+energy to all of them.</p>
+
+<p>I may here describe a result which is still more interesting,
+and to which I have been led by the observation of these phe<span class='pagenum'><a name="Page_354" id="Page_354">[Pg 354]</a></span>nomena.
+I noted that small differences in the density of the air
+produced a considerable difference in the degree of incandescence
+of the wires, and I thought that, since in a tube, through which
+a luminous discharge is passed, the gas is generally not of uniform
+density, a very thin wire contained in the tube might be
+rendered incandescent at certain places of smaller density of the
+gas, while it would remain dark at the places of greater density,
+where the convection would be greater and the bombardment less
+intense. Accordingly a tube <i>t</i> was prepared, as illustrated in Fig.
+187, which contained through the middle a very fine platinum wire
+<i>w</i>. The tube was exhausted to a moderate degree and it was found
+that when it was attached to the terminal of a high-frequency coil
+the platinum wire <i>w</i> would indeed, become incandescent in patches,
+as illustrated in Fig. 187. Later a number of these tubes with one
+or more wires were prepared, each showing this result. The effect
+was best noted when the striated discharge occurred in the
+tube, but was also produced when the stri&aelig; were not visible,
+showing that, even then, the gas in the tube was not of uniform
+density. The position of the stri&aelig; was generally such, that the
+rarefactions corresponded to the places of incandescence or greater
+brightness on the wire <i>w</i>. But in a few instances it was noted, that
+the bright spots on the wire were covered by the dense parts of
+the striated discharge as indicated by <i>l</i> in Fig. 187, though the effect
+was barely perceptible. This was explained in a plausible way
+by assuming that the convection was not widely different in the
+dense and rarefied places, and that the bombardment was greater
+on the dense places of the striated discharge. It is, in fact, often
+observed in bulbs, that under certain conditions a thin wire is
+brought to higher incandescence when the air is not too highly
+rarefied. This is the case when the potential of the coil is not
+high enough for the vacuum, but the result may be attributed to
+many different causes. In all cases this curious phenomenon of
+incandescence disappears when the tube, or rather the wire,
+acquires throughout a uniform temperature.</p>
+
+<div class="figcenter" style="width: 713px;">
+<img src="images/oi_369.jpg" width="713" height="600" alt="Fig. 187, 188." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 187.</td><td class="caption">Fig. 188.</td></tr>
+</table>
+</div>
+
+<p>Disregarding now the modifying effect of convection there are
+then two distinct causes which determine the incandescence of a
+wire or filament with varying currents, that is, conduction current
+and bombardment. With steady currents we have to deal
+only with the former of these two causes, and the heating effect
+is a minimum, since the resistance is least to steady flow. When
+the current is a varying one the resistance is greater, and hence
+<span class='pagenum'><a name="Page_355" id="Page_355">[Pg 355]</a></span>the heating effect is increased. Thus if the rate of change of
+the current is very great, the resistance may increase to such
+an extent that the filament is brought to incandescence with inappreciable
+currents, and we are able to take a short and thick
+block of carbon or other material and bring it to bright incandescence
+with a current incomparably smaller than that required
+to bring to the same degree of incandescence an ordinary thin
+lamp filament with a steady or low frequency current. This result
+is important, and illustrates how rapidly our views on these subjects
+are changing, and how quickly our field of knowledge is extending.
+In the art of incandescent lighting, to view this result
+in one aspect only, it has been commonly considered as an essential
+requirement for practical success, that the lamp filament
+should be thin and of high resistance. But now we know that
+the resistance of the filament to the steady flow does not mean
+anything; the filament might as well be short and thick; for if it
+be immersed in rarefied gas it will become incandescent by the
+passage of a small current. It all depends on the frequency and
+potential of the currents. We may conclude from this, that it
+<span class='pagenum'><a name="Page_356" id="Page_356">[Pg 356]</a></span>would be of advantage, so far as the lamp is considered, to employ
+high frequencies for lighting, as they allow the use of short
+and thick filaments and smaller currents.</p>
+
+<p>If a wire or filament be immersed in a homogeneous medium, all
+the heating is due to true conduction current, but if it be enclosed
+in an exhausted vessel the conditions are entirely different. Here
+the gas begins to act and the heating effect of the conduction current,
+as is shown in many experiments, may be very small compared
+with that of the bombardment. This is especially the case if
+the circuit is not closed and the potentials are of course very high.
+Suppose that a fine filament enclosed in an exhausted vessel be
+connected with one of its ends to the terminal of a high tension
+coil and with its other end to a large insulated plate. Though
+the circuit is not closed, the filament, as I have before shown, is
+brought to incandescence. If the frequency and potential be
+comparatively low, the filament is heated by the current passing
+<i>through it</i>. If the frequency and potential, and principally the
+latter, be increased, the insulated plate need be but very small, or
+may be done away with entirely; still the filament will become
+incandescent, practically all the heating being then due to the bombardment.
+A practical way of combining both the effects of
+conduction currents and bombardment is illustrated in Fig. 188,
+in which an ordinary lamp is shown provided with a very thin
+filament which has one of the ends of the latter connected to a
+shade serving the purpose of the insulated plate, and the other
+end to the terminal of a high tension source. It should not be
+thought that only rarefied gas is an important factor in the heating
+of a conductor by varying currents, but gas at ordinary pressure
+may become important, if the potential difference and frequency
+of the currents is excessive. On this subject I have already
+stated, that when a conductor is fused by a stroke of
+lightning, the current through it may be exceedingly small, not
+even sufficient to heat the conductor perceptibly, were the latter
+immersed in a homogeneous medium.</p>
+
+<p>From the preceding it is clear that when a conductor of high
+resistance is connected to the terminals of a source of high frequency
+currents of high potential, there may occur considerable
+dissipation of energy, principally at the ends of the conductor, in
+consequence of the action of the gas surrounding the conductor.
+Owing to this, the current through a section of the conductor at
+a point midway between its ends may be much smaller than
+<span class='pagenum'><a name="Page_357" id="Page_357">[Pg 357]</a></span>through a section near the ends. Furthermore, the current passes
+principally through the outer portions of the conductor, but this
+effect is to be distinguished from the skin effect as ordinarily interpreted,
+for the latter would, or should, occur also in a continuous
+incompressible medium. If a great many incandescent lamps
+are connected in series to a source of such currents, the lamps at
+the ends may burn brightly, whereas those in the middle may remain
+entirely dark. This is due principally to bombardment, as
+before stated. But even if the currents be steady, provided the
+difference of potential is very great, the lamps at the end will
+burn more brightly than those in the middle. In such case there
+is no rhythmical bombardment, and the result is produced entirely
+by leakage. This leakage or dissipation into space when
+the tension is high, is considerable when incandescent lamps are
+used, and still more considerable with arcs, for the latter act like
+flames. Generally, of course, the dissipation is much smaller
+with steady, than with varying, currents.</p>
+
+<p>I have contrived an experiment which illustrates in an interesting
+manner the effect of lateral diffusion. If a very long tube
+is attached to the terminal of a high frequency coil, the luminosity
+is greatest near the terminal and falls off gradually towards
+the remote end. This is more marked if the tube is narrow.</p>
+
+<p>A small tube about one-half inch in diameter and twelve
+inches long (Fig. 189), has one of its ends drawn out into a fine
+fibre <i>f</i> nearly three feet long. The tube is placed in a brass socket
+<small>T</small> which can be screwed on the terminal <small>T<sub>1</sub></small> of the induction coil.
+The discharge passing through the tube first illuminates the bottom
+of the same, which is of comparatively large section; but
+through the long glass fibre the discharge cannot pass. But
+gradually the rarefied gas inside becomes warmed and more conducting
+and the discharge spreads into the glass fibre. This spreading
+is so slow, that it may take half a minute or more until the
+discharge has worked through up to the top of the glass fibre,
+then presenting the appearance of a strongly luminous thin
+thread. By adjusting the potential at the terminal the light may
+be made to travel upwards at any speed. Once, however, the
+glass fibre is heated, the discharge breaks through its entire
+length instantly. The interesting point to be noted is that, the
+higher the frequency of the currents, or in other words, the
+greater relatively the lateral dissipation, at a slower rate may the
+light be made to propagate through the fibre. This experiment
+<span class='pagenum'><a name="Page_358" id="Page_358">[Pg 358]</a></span>is best performed with a highly exhausted and freshly made tube.
+When the tube has been used for some time the experiment
+often fails. It is possible that the gradual and slow impairment
+of the vacuum is the cause. This slow propagation of the discharge
+through a very narrow glass tube corresponds exactly to
+the propagation of heat through a bar warmed at one end. The
+quicker the heat is carried away laterally the longer time it will
+take for the heat to warm the remote end. When the current
+of a low frequency coil is passed through the fibre from end to
+end, then the lateral dissipation is small and the discharge instantly
+breaks through almost without exception.</p>
+
+<div class="figcenter" style="width: 636px;">
+<img src="images/oi_372.jpg" width="636" height="600" alt="Fig. 189, 190." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 189.</td><td class="caption">Fig. 190.</td></tr>
+</table>
+</div>
+
+<p>After these experiments and observations which have shown
+the importance of the discontinuity or atomic structure of the
+medium and which will serve to explain, in a measure at least,
+the nature of the four kinds of light effects producible with
+these currents, I may now give you an illustration of these
+effects. For the sake of interest I may do this in a manner
+which to many of you might be novel. You have seen before
+that we may now convey the electric vibration to a body by
+means of a single wire or conductor of any kind. Since the
+<span class='pagenum'><a name="Page_359" id="Page_359">[Pg 359]</a></span>human frame is conducting I may convey the vibration through
+my body.</p>
+
+<p>First, as in some previous experiments, I connect my body with
+one of the terminals of a high-tension transformer and take in my
+hand an exhausted bulb which contains a small carbon button
+mounted upon a platinum wire leading to the outside of the bulb,
+and the button is rendered incandescent as soon as the transformer
+is set to work (Fig. 190). I may place a conducting shade on the
+bulb which serves to intensify the action, but is not necessary.
+Nor is it required that the button should be in conducting connection
+with the hand through a wire leading through the glass,
+for sufficient energy may be transmitted through the glass itself
+by inductive action to render the button incandescent.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_373.jpg" width="800" height="508" alt="Fig. 191, 192." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 191.</td><td class="caption">Fig. 192.</td></tr>
+</table>
+</div>
+
+<p>Next I take a highly exhausted bulb containing a strongly
+phosphorescent body, above which is mounted a small plate of
+aluminum on a platinum wire leading to the outside, and the currents
+flowing through my body excite intense phosphorescence
+in the bulb (Fig. 191). Next again I take in my hand a simple
+exhausted tube, and in the same manner the gas inside the tube
+is rendered highly incandescent or phosphorescent (Fig. 192).
+Finally, I may take in my hand a wire, bare or covered with thick
+insulation, it is quite immaterial; the electrical vibration is so
+intense as to cover the wire with a luminous film (Fig. 193).<span class='pagenum'><a name="Page_360" id="Page_360">[Pg 360]</a></span></p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_375.jpg" width="800" height="453" alt="Fig. 193, 194, 195." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 193.</td><td class="caption">Fig. 194.</td><td class="caption">Fig. 195.</td></tr>
+</table>
+</div>
+
+<p>A few words must now be devoted to each of these phenomena.
+In the first place, I will consider the incandescence of a button or of
+a solid in general, and dwell upon some facts which apply equally
+to all these phenomena. It was pointed out before that when a
+thin conductor, such as a lamp filament, for instance, is connected
+with one of its ends to the terminal of a transformer of high
+tension the filament is brought to incandescence partly by a
+conduction current and partly by bombardment. The shorter
+and thicker the filament the more important becomes the latter,
+and finally, reducing the filament to a mere button, all the heating
+must practically be attributed to the bombardment. So in
+the experiment before shown, the button is rendered incandescent
+by the rhythmical impact of freely movable small bodies in the
+bulb. These bodies may be the molecules of the residual gas,
+particles of dust or lumps torn from the electrode; whatever they
+are, it is certain that the heating of the button is essentially connected
+with the pressure of such freely movable particles, or of
+atomic matter in general in the bulb. The heating is the more
+intense the greater the number of impacts per second and the
+greater the energy of each impact. Yet the button would
+be heated also if it were connected to a source of a steady potential.
+In such a case electricity would be carried away from
+the button by the freely movable carriers or particles flying
+about, and the quantity of electricity thus carried away might be
+sufficient to bring the button to incandescence by its passage
+through the latter. But the bombardment could not be of great
+importance in such case. For this reason it would require a comparatively
+very great supply of energy to the button to maintain
+it at incandescence with a steady potential. The higher the frequency
+of the electric impulses the more economically can the
+button be maintained at incandescence. One of the chief reasons
+why this is so, is, I believe, that with impulses of very high
+frequency there is less exchange of the freely movable carriers
+around the electrode and this means, that in the bulb the heated
+matter is better confined to the neighborhood of the button. If
+a double bulb, as illustrated in Fig. 194 be made, comprising a
+large globe <small>B</small> and a small one <i>b</i>, each containing as usual a filament
+<i>f</i> mounted on a platinum wire <i>w</i> and <i>w</i><sub>1</sub>, it is found, that if
+the filaments <i>f f</i> be exactly alike, it requires less energy to keep
+the filament in the globe <i>b</i> at a certain degree of incandescence,
+than that in the globe <small>B</small>. This is due to the confinement of the
+<span class='pagenum'><a name="Page_361" id="Page_361">[Pg 361]</a></span>movable particles around the button. In this case it is also ascertained,
+that the filament in the small globe <i>b</i> is less deteriorated
+when maintained a certain length of time at incandescence. This
+is a necessary consequence of the fact that the gas in the small
+bulb becomes strongly heated and therefore a very good conductor,
+and less work is then performed on the button, since the
+bombardment becomes less intense as the conductivity of the gas
+increases. In this construction, of course, the small bulb becomes
+very hot and when it reaches an elevated temperature the convection
+and radiation on the outside increase. On another occasion
+I have shown bulbs in which this drawback was largely
+avoided. In these instances a very small bulb, containing a refractory
+button, was mounted in a large globe and the space between
+the walls of both was highly exhausted. The outer large
+globe remained comparatively cool in such constructions. When
+the large globe was on the pump and the vacuum between the
+walls maintained permanent by the continuous action of the
+pump, the outer globe would remain quite cold, while the button
+in the small bulb was kept at incandescence. But when the seal
+was made, and the button in the small bulb maintained incandescent
+some length of time, the large globe too would become
+warmed. From this I conjecture that if vacuous space (as Prof.
+Dewar finds) cannot convey heat, it is so merely in virtue of our
+rapid motion through space or, generally speaking, by the motion
+of the medium relatively to us, for a permanent condition could
+<span class='pagenum'><a name="Page_362" id="Page_362">[Pg 362]</a></span>not be maintained without the medium being constantly renewed.
+A vacuum cannot, according to all evidence, be permanently
+maintained around a hot body.</p>
+
+<p>In these constructions, before mentioned, the small bulb inside
+would, at least in the first stages, prevent all bombardment
+against the outer large globe. It occurred to me then to ascertain
+how a metal sieve would behave in this respect, and several
+bulbs, as illustrated in Fig. 195, were prepared for this purpose.
+In a globe <i>b</i>, was mounted a thin filament <i>f</i> (or button) upon a
+platinum wire <i>w</i> passing through a glass stem and leading to the
+outside of the globe. The filament <i>f</i> was surrounded by a metal
+sieve <i>s</i>. It was found in experiments with such bulbs that a sieve
+with wide meshes apparently did not in the slightest affect the
+bombardment against the globe <i>b</i>. When the vacuum was high,
+the shadow of the sieve was clearly projected against the globe
+and the latter would get hot in a short while. In some bulbs the
+sieve <i>s</i> was connected to a platinum wire sealed in the glass.
+When this wire was connected to the other terminal of the induction
+coil (the <span class="smcap">e. m. f.</span> being kept low in this case), or to an insulated
+plate, the bombardment against the outer globe <i>b</i> was
+diminished. By taking a sieve with fine meshes the bombardment
+against the globe <i>b</i> was always diminished, but even then
+if the exhaustion was carried very far, and when the potential of
+the transformer was very high, the globe <i>b</i> would be bombarded
+and heated quickly, though no shadow of the sieve was visible,
+owing to the smallness of the meshes. But a glass tube or other
+continuous body mounted so as to surround the filament, did entirely
+cut off the bombardment and for a while the outer globe <i>b</i>
+would remain perfectly cold. Of course when the glass tube
+was sufficiently heated the bombardment against the outer globe
+could be noted at once. The experiments with these bulbs
+seemed to show that the speeds of the projected molecules or
+particles must be considerable (though quite insignificant when
+compared with that of light), otherwise it would be difficult to
+understand how they could traverse a fine metal sieve without
+being affected, unless it were found that such small particles or
+atoms cannot be acted upon directly at measurable distances.
+In regard to the speed of the projected atoms, Lord Kelvin has
+recently estimated it at about one kilometre a second or thereabouts
+in an ordinary Crookes bulb. As the potentials obtainable
+with a disruptive discharge coil are much higher than with or<span class='pagenum'><a name="Page_363" id="Page_363">[Pg 363]</a></span>dinary
+coils, the speeds must, of course, be much greater when
+the bulbs are lighted from such a coil. Assuming the speed to
+be as high as five kilometres and uniform through the whole
+trajectory, as it should be in a very highly exhausted vessel, then
+if the alternate electrifications of the electrode would be of a
+frequency of five million, the greatest distance a particle could
+get away from the electrode would be one millimetre, and if it
+could be acted upon directly at that distance, the exchange of
+electrode matter or of the atoms would be very slow and there
+would be practically no bombardment against the bulb. This at
+least should be so, if the action of an electrode upon the atoms
+of the residual gas would be such as upon electrified bodies which
+we can perceive. A hot body enclosed in an exhausted bulb
+produces always atomic bombardment, but a hot body has no
+definite rhythm, for its molecules perform vibrations of all kinds.</p>
+
+<p>If a bulb containing a button or filament be exhausted as high
+as is possible with the greatest care and by the use of the best artifices,
+it is often observed that the discharge cannot, at first,
+break through, but after some time, probably in consequence of
+some changes within the bulb, the discharge finally passes through
+and the button is rendered incandescent. In fact, it appears that
+the higher the degree of exhaustion the easier is the incandescence
+produced. There seem to be no other causes to which the incandescence
+might be attributed in such case except to the bombardment
+or similar action of the residual gas, or of particles of
+matter in general. But if the bulb be exhausted with the greatest
+care can these play an important part? Assume the vacuum
+in the bulb to be tolerably perfect, the great interest then centres
+in the question: Is the medium which pervades all space continuous
+or atomic? If atomic, then the heating of a conducting
+button or filament in an exhausted vessel might be due largely
+to ether bombardment, and then the heating of a conductor in
+general through which currents of high frequency or high potential
+are passed must be modified by the behavior of such medium;
+then also the skin effect, the apparent increase of the ohmic resistance,
+etc., admit, partially at least, of a different explanation.</p>
+
+<p>It is certainly more in accordance with many phenomena observed
+with high-frequency currents to hold that all space is pervaded
+with free atoms, rather than to assume that it is devoid of
+these, and dark and cold, for so it must be, if filled with a continuous
+medium, since in such there can be neither heat nor light.<span class='pagenum'><a name="Page_364" id="Page_364">[Pg 364]</a></span>
+Is then energy transmitted by independent carriers or by the
+vibration of a continuous medium? This important question is
+by no means as yet positively answered. But most of the effects
+which are here considered, especially the light effects, incandescence,
+or phosphorescence, involve the presence of free atoms and
+would be impossible without these.</p>
+
+<p>In regard to the incandescence of a refractory button (or filament)
+in an exhausted receiver, which has been one of the subjects
+of this investigation, the chief experiences, which may serve
+as a guide in constructing such bulbs, may be summed up as follows:
+1. The button should be as small as possible, spherical,
+of a smooth or polished surface, and of refractory material which
+withstands evaporation best. 2. The support of the button
+should be very thin and screened by an aluminum and mica sheet,
+as I have described on another occasion. 3. The exhaustion of
+the bulb should be as high as possible. 4. The frequency of the
+currents should be as high as practicable. 5. The currents should
+be of a harmonic rise and fall, without sudden interruptions. 6.
+The heat should be confined to the button by inclosing the same
+in a small bulb or otherwise. 7. The space between the walls of
+the small bulb and the outer globe should be highly exhausted.</p>
+
+<p>Most of the considerations which apply to the incandescence
+of a solid just considered may likewise be applied to phosphorescence.
+Indeed, in an exhausted vessel the phosphorescence is,
+as a rule, primarily excited by the powerful beating of the electrode
+stream of atoms against the phosphorescent body. Even in
+many cases, where there is no evidence of such a bombardment,
+I think that phosphorescence is excited by violent impacts of
+atoms, which are not necessarily thrown off from the electrode
+but are acted upon from the same inductively through the
+medium or through chains of other atoms. That mechanical
+shocks play an important part in exciting phosphorescence in a
+bulb may be seen from the following experiment. If a bulb,
+constructed as that illustrated in Fig. 174, be taken and exhausted
+with the greatest care so that the discharge cannot pass, the filament
+<i>f</i> acts by electrostatic induction upon the tube <i>t</i> and the
+latter is set in vibration. If the tube <i>o</i> be rather wide, about an
+inch or so, the filament may be so powerfully vibrated that whenever
+it hits the glass tube it excites phosphorescence. But the
+phosphorescence ceases when the filament comes to rest. The
+vibration can be arrested and again started by varying the<span class='pagenum'><a name="Page_365" id="Page_365">[Pg 365]</a></span>
+frequency of the currents. Now the filament has its own
+period of vibration, and if the frequency of the currents is such
+that there is resonance, it is easily set vibrating, though the potential
+of the currents be small. I have often observed that the
+filament in the bulb is destroyed by such mechanical resonance.
+The filament vibrates as a rule so rapidly that it cannot be seen
+and the experimenter may at first be mystified. When such an
+experiment as the one described is carefully performed, the potential
+of the currents need be extremely small, and for this
+reason I infer that the phosphorescence is then due to the
+mechanical shock of the filament against the glass, just as it is
+produced by striking a loaf of sugar with a knife. The mechanical
+shock produced by the projected atoms is easily noted when
+a bulb containing a button is grasped in the hand and the current
+turned on suddenly. I believe that a bulb could be shattered
+by observing the conditions of resonance.</p>
+
+<p>In the experiment before cited it is, of course, open to say,
+that the glass tube, upon coming in contact with the filament, retains
+a charge of a certain sign upon the point of contact. If
+now the filament again touches the glass at the same point while
+it is oppositely charged, the charges equalize under evolution of
+light. But nothing of importance would be gained by such an
+explanation. It is unquestionable that the initial charges given
+to the atoms or to the glass play some part in exciting phosphorescence.
+So, for instance, if a phosphorescent bulb be first excited
+by a high frequency coil by connecting it to one of the terminals
+of the latter and the degree of luminosity be noted, and then
+the bulb be highly charged from a Holtz machine by attaching
+it preferably to the positive terminal of the machine, it is found
+that when the bulb is again connected to the terminal of the high
+frequency coil, the phosphorescence is far more intense. On
+another occasion I have considered the possibility of some phosphorescent
+phenomena in bulbs being produced by the incandescence
+of an infinitesimal layer on the surface of the phosphorescent
+body. Certainly the impact of the atoms is powerful enough
+to produce intense incandescence by the collisions, since they bring
+quickly to a high temperature a body of considerable bulk. If any
+such effect exists, then the best appliance for producing phosphorescence
+in a bulb, which we know so far, is a disruptive discharge
+coil giving an enormous potential with but few fundamental discharges,
+say 25-30 per second, just enough to produce a continu<span class='pagenum'><a name="Page_366" id="Page_366">[Pg 366]</a></span>ous
+impression upon the eye. It is a fact that such a coil excites
+phosphorescence under almost any condition and at all degrees
+of exhaustion, and I have observed effects which appear to be due
+to phosphorescence even at ordinary pressures of the atmosphere,
+when the potentials are extremely high. But if phosphorescent
+light is produced by the equalization of charges of electrified
+atoms (whatever this may mean ultimately), then the higher the
+frequency of the impulses or alternate electrifications, the
+more economical will be the light production. It is a long
+known and noteworthy fact that all the phosphorescent bodies
+are poor conductors of electricity and heat, and that all bodies
+cease to emit phosphorescent light when they are brought to a
+certain temperature. Conductors on the contrary do not possess
+this quality. There are but few exceptions to the rule. Carbon
+is one of them. Becquerel noted that carbon phosphoresces at
+a certain elevated temperature preceding the dark red. This
+phenomenon may be easily observed in bulbs provided with a
+rather large carbon electrode (say, a sphere of six millimetres diameter).
+If the current is turned on after a few seconds, a snow
+white film covers the electrode, just before it gets dark red.
+Similar effects are noted with other conducting bodies, but many
+scientific men will probably not attribute them to true phosphorescence.
+Whether true incandescence has anything to do with
+phosphorescence excited by atomic impact or mechanical shocks
+still remains to be decided, but it is a fact that all conditions,
+which tend to localize and increase the heating effect at the point
+of impact, are almost invariably the most favorable for the production
+of phosphorescence. So, if the electrode be very small,
+which is equivalent to saying in general, that the electric density
+is great; if the potential be high, and if the gas be highly rarefied,
+all of which things imply high speed of the projected atoms,
+or matter, and consequently violent impacts&mdash;the phosphorescence
+is very intense. If a bulb provided with a large and small
+electrode be attached to the terminal of an induction coil, the
+small electrode excites phosphorescence while the large one may
+not do so, because of the smaller electric density and hence
+smaller speed of the atoms. A bulb provided with a large electrode
+may be grasped with the hand while the electrode is connected
+to the terminal of the coil and it may not phosphoresce;
+but if instead of grasping the bulb with the hand, the same be
+touched with a pointed wire, the phosphorescence at once spreads<span class='pagenum'><a name="Page_367" id="Page_367">[Pg 367]</a></span>
+through the bulb, because of the great density at the point of
+contact. With low frequencies it seems that gases of great
+atomic weight excite more intense phosphorescence than those
+of smaller weight, as for instance, hydrogen. With high frequencies
+the observations are not sufficiently reliable to draw a
+conclusion. Oxygen, as is well-known, produces exceptionally
+strong effects, which may be in part due to chemical action. A
+bulb with hydrogen residue seems to be most easily excited.
+Electrodes which are most easily deteriorated produce more
+intense phosphorescence in bulbs, but the condition is not permanent
+because of the impairment of the vacuum and the deposition
+of the electrode matter upon the phosphorescent surfaces.
+Some liquids, as oils, for instance, produce magnificent effects of
+phosphorescence (or fluorescence?), but they last only a few
+seconds. So if a bulb has a trace of oil on the walls and the
+current is turned on, the phosphorescence only persists for a few
+moments until the oil is carried away. Of all bodies so far tried,
+sulphide of zinc seems to be the most susceptible to phosphorescence.
+Some samples, obtained through the kindness of Prof.
+Henry in Paris, were employed in many of these bulbs. One of
+the defects of this sulphide is, that it loses its quality of emitting
+light when brought to a temperature which is by no means high.
+It can therefore, be used only for feeble intensities. An observation
+which might deserve notice is, that when violently bombarded
+from an aluminum electrode it assumes a black color, but
+singularly enough, it returns to the original condition when it
+cools down.</p>
+
+<p>The most important fact arrived at in pursuing investigations
+in this direction is, that in all cases it is necessary, in order to excite
+phosphorescence with a minimum amount of energy, to observe
+certain conditions. Namely, there is always, no matter what
+the frequency of the currents, degree of exhaustion and character
+of the bodies in the bulb, a certain potential (assuming the bulb
+excited from one terminal) or potential difference (assuming the
+bulb to be excited with both terminals) which produces the most
+economical result. If the potential be increased, considerable
+energy may be wasted without producing any more light, and if
+it be diminished, then again the light production is not as economical.
+The exact condition under which the best result is obtained
+seems to depend on many things of a different nature, and it is to
+be yet investigated by other experimenters, but it will certainly<span class='pagenum'><a name="Page_368" id="Page_368">[Pg 368]</a></span>
+have to be observed when such phosphorescent bulbs are operated,
+if the best results are to be obtained.</p>
+
+<p>Coming now to the most interesting of these phenomena, the
+incandescence or phosphorescence of gases, at low pressures or at
+the ordinary pressure of the atmosphere, we must seek the explanation
+of these phenomena in the same primary causes, that is,
+in shocks or impacts of the atoms. Just as molecules or atoms
+beating upon a solid body excite phosphorescence in the same or
+render it incandescent, so when colliding among themselves they
+produce similar phenomena. But this is a very insufficient explanation
+and concerns only the crude mechanism. Light is produced
+by vibrations which go on at a rate almost inconceivable.
+If we compute, from the energy contained in the form of known
+radiations in a definite space the force which is necessary to set
+up such rapid vibrations, we find, that though the density of the
+ether be incomparably smaller than that of any body we know,
+even hydrogen, the force is something surpassing comprehension.
+What is this force, which in mechanical measure may amount to
+thousands of tons per square inch? It is electrostatic force in the
+light of modern views. It is impossible to conceive how a body
+of measurable dimensions could be charged to so high a potential
+that the force would be sufficient to produce these vibrations.
+Long before any such charge could be imparted to the body it
+would be shattered into atoms. The sun emits light and heat, and
+so does an ordinary flame or incandescent filament, but in neither
+of these can the force be accounted for if it be assumed that it is
+associated with the body as a whole. Only in one way may we
+account for it, namely, by identifying it with the atom. An
+atom is so small, that if it be charged by coming in contact with
+an electrified body and the charge be assumed to follow the same
+law as in the case of bodies of measurable dimensions, it must
+retain a quantity of electricity which is fully capable of accounting
+for these forces and tremendous rates of vibration. But the
+atom behaves singularly in this respect&mdash;it always takes the same
+"charge."</p>
+
+<p>It is very likely that resonant vibration plays a most important
+part in all manifestations of energy in nature. Throughout space
+all matter is vibrating, and all rates of vibration are represented,
+from the lowest musical note to the highest pitch of the chemical
+rays, hence an atom, or complex of atoms, no matter what its
+period, must find a vibration with which it is in resonance.<span class='pagenum'><a name="Page_369" id="Page_369">[Pg 369]</a></span>
+When we consider the enormous rapidity of the light vibrations,
+we realize the impossibility of producing such vibrations directly
+with any apparatus of measurable dimensions, and we are driven
+to the only possible means of attaining the object of setting up
+waves of light by electrical means and economically, that is, to
+affect the molecules or atoms of a gas, to cause them to collide and
+vibrate. We then must ask ourselves&mdash;How can free molecules
+or atoms be affected?</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_384.jpg" width="800" height="338" alt="Fig. 196, 197." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 196.</td><td class="caption">Fig. 197.</td></tr>
+</table>
+</div>
+
+<p>It is a fact that they can be affected by electrostatic force, as is
+apparent in many of these experiments. By varying the electrostatic
+force we can agitate the atoms, and cause them to collide
+accompanied by evolution of heat and light. It is not demonstrated
+beyond doubt that we can affect them otherwise. If a luminous
+discharge is produced in a closed exhausted tube, do the atoms
+arrange themselves in obedience to any other but to electrostatic
+force acting in straight lines from atom to atom? Only recently
+I investigated the mutual action between two circuits with extreme
+rates of vibration. When a battery of a few jars (<i>c c c c</i>, Fig.
+196) is discharged through a primary <small>P</small> of low resistance (the connections
+being as illustrated in Figs. 183<i>a</i>, 183<i>b</i> and 183<i>c</i>), and the
+frequency of vibration is many millions there are great differences
+of potential between points on the primary not more than
+a few inches apart. These differences may be 10,000 volts per
+inch, if not more, taking the maximum value of the <span class="smcap">e. m. f.</span> The
+secondary <i>s</i> is therefore acted upon by electrostatic induction,
+which is in such extreme cases of much greater importance than
+the electro-dynamic. To such sudden impulses the primary as
+well as the secondary are poor conductors, and therefore great
+differences of potential may be produced by electrostatic induction
+between adjacent points on the secondary. Then sparks may
+jump between the wires and streamers become visible in the dark
+if the light of the discharge through the spark gap <i>d d</i> be carefully
+excluded. If now we substitute a closed vacuum tube for the
+metallic secondary <i>s</i>, the differences of potential produced in the
+tube by electrostatic induction from the primary are fully sufficient
+to excite portions of it; but as the points of certain differences
+of potential on the primary are not fixed, but are generally
+constantly changing in position, a luminous band is produced in
+the tube, apparently not touching the glass, as it should, if the
+points of maximum and minimum differences of potential were
+fixed on the primary. I do not exclude the possibility of such a
+<span class='pagenum'><a name="Page_370" id="Page_370">[Pg 370]</a></span>tube being excited only by electro-dynamic induction, for very
+able physicists hold this view; but in my opinion, there is as yet
+no positive proof given that atoms of a gas in a closed tube may
+arrange themselves in chains under the action of an electromotive
+impulse produced by electro-dynamic induction in the tube. I
+have been unable so far to produce stri&aelig; in a tube, however long,
+and at whatever degree of exhaustion, that is, stri&aelig; at right
+angles to the supposed direction of the discharge or the axis of
+the tube; but I have distinctly observed in a large bulb, in which
+a wide luminous band was produced by passing a discharge of a
+battery through a wire surrounding the bulb, a circle of feeble
+luminosity between two luminous bands, one of which was more
+intense than the other. Furthermore, with my present experience
+I do not think that such a gas discharge in a closed tube
+can vibrate, that is, vibrate as a whole. I am convinced that no
+discharge through a gas can vibrate. The atoms of a gas behave
+very curiously in respect to sudden electric impulses. The
+gas does not seem to possess any appreciable inertia to such
+impulses, for it is a fact, that the higher the frequency of
+the impulses, with the greater freedom does the discharge
+pass through the gas. If the gas possesses no inertia then
+it cannot vibrate, for some inertia is necessary for the free vibration.
+I conclude from this that if a lightning discharge occurs
+between two clouds, there can be no oscillation, such as would
+be expected, considering the capacity of the clouds. But if
+the lightning discharge strike the earth, there is always vibration&mdash;in
+the earth, but not in the cloud. In a gas discharge each
+atom vibrates at its own rate, but there is no vibration of the
+conducting gaseous mass as a whole. This is an important
+consideration in the great problem of producing light economi<span class='pagenum'><a name="Page_371" id="Page_371">[Pg 371]</a></span>cally,
+for it teaches us that to reach this result we must use
+impulses of very high frequency and necessarily also of high
+potential. It is a fact that oxygen produces a more intense
+light in a tube. Is it because oxygen atoms possess some inertia
+and the vibration does not die out instantly? But then nitrogen
+should be as good, and chlorine and vapors of many other bodies
+much better than oxygen, unless the magnetic properties of the
+latter enter prominently into play. Or, is the process in the tube
+of an electrolytic nature? Many observations certainly speak for
+it, the most important being that matter is always carried away
+from the electrodes and the vacuum in a bulb cannot be permanently
+maintained. If such process takes place in reality, then
+again must we take refuge in high frequencies, for, with such,
+electrolytic action should be reduced to a minimum, if not rendered
+entirely impossible. It is an undeniable fact that with very
+high frequencies, provided the impulses be of harmonic nature,
+like those obtained from an alternator, there is less deterioration
+and the vacua are more permanent. With disruptive discharge
+coils there are sudden rises of potential and the vacua are
+more quickly impaired, for the electrodes are deteriorated in a
+very short time. It was observed in some large tubes, which
+were provided with heavy carbon blocks <small>B B<sub>1</sub></small>, connected to platinum
+wires <i>w w</i><sub>1</sub> (as illustrated in Fig. 197), and which were employed
+in experiments with the disruptive discharge instead of the
+ordinary air gap, that the carbon particles under the action of the
+powerful magnetic field in which the tube was placed, were deposited
+in regular fine lines in the middle of the tube, as illustrated.
+These lines were attributed to the deflection or distortion
+of the discharge by the magnetic field, but why the deposit
+occurred principally where the field was most intense did not
+appear quite clear. A fact of interest, likewise noted, was
+that the presence of a strong magnetic field increases the deterioration
+of the electrodes, probably by reason of the rapid interruptions
+it produces, whereby there is actually a higher <span class="smcap">e. m. f.</span>
+maintained between the electrodes.</p>
+
+<p>Much would remain to be said about the luminous effects produced
+in gases at low or ordinary pressures. With the present
+experiences before us we cannot say that the essential nature of
+these charming phenomena is sufficiently known. But investigations
+in this direction are being pushed with exceptional ardor.
+Every line of scientific pursuit has its fascinations, but electrical<span class='pagenum'><a name="Page_372" id="Page_372">[Pg 372]</a></span>
+investigation appears to possess a peculiar attraction, for there is
+no experiment or observation of any kind in the domain of this
+wonderful science which would not forcibly appeal to us. Yet
+to me it seems, that of all the many marvelous things we observe,
+a vacuum tube, excited by an electric impulse from a distant
+source, bursting forth out of the darkness and illuminating the
+room with its beautiful light, is as lovely a phenomenon as can
+greet our eyes. More interesting still it appears when, reducing
+the fundamental discharges across the gap to a very small number
+and waving the tube about we produce all kinds of designs
+in luminous lines. So by way of amusement I take a straight
+long tube, or a square one, or a square attached to a straight tube,
+and by whirling them about in the hand, I imitate the spokes of
+a wheel, a Gramme winding, a drum winding, an alternate current
+motor winding, etc. (Fig. 198). Viewed from a distance the
+effect is weak and much of its beauty is lost, but being near or
+holding the tube in the hand, one cannot resist its charm.</p>
+
+<div class="figcenter" style="width: 617px;">
+<img src="images/oi_386.jpg" width="617" height="600" alt="Fig. 198." title="" />
+<span class="caption">Fig. 198.</span>
+</div>
+
+<p><span class='pagenum'><a name="Page_373" id="Page_373">[Pg 373]</a></span></p>
+
+<p>In presenting these insignificant results I have not attempted
+to arrange and co-ordinate them, as would be proper in a strictly
+scientific investigation, in which every succeeding result should
+be a logical sequence of the preceding, so that it might be guessed
+in advance by the careful reader or attentive listener. I have
+preferred to concentrate my energies chiefly upon advancing
+novel facts or ideas which might serve as suggestions to others,
+and this may serve as an excuse for the lack of harmony. The
+explanations of the phenomena have been given in good faith
+and in the spirit of a student prepared to find that they admit of
+a better interpretation. There can be no great harm in a student
+taking an erroneous view, but when great minds err, the world
+must dearly pay for their mistakes.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_374" id="Page_374">[Pg 374]</a></span></p>
+<h2><a name="CHAPTER_XXIX" id="CHAPTER_XXIX"></a>CHAPTER XXIX.</h2>
+
+<h3><span class="smcap">Tesla Alternating Current Generators for High Frequency,
+in Detail.</span></h3>
+
+
+<p>It has become a common practice to operate arc lamps by alternating
+or pulsating, as distinguished from continuous, currents;
+but an objection which has been raised to such systems exists in
+the fact that the arcs emit a pronounced sound, varying with the
+rate of the alternations or pulsations of current. This noise is
+due to the rapidly alternating heating and cooling, and consequent
+expansion and contraction, of the gaseous matter forming
+the arc, which corresponds with the periods or impulses of the
+current. Another disadvantageous feature is found in the difficulty
+of maintaining an alternating current arc in consequence of
+the periodical increase in resistance corresponding to the periodical
+working of the current. This feature entails a further disadvantage,
+namely, that small arcs are impracticable.</p>
+
+<p>Theoretical considerations have led Mr. Tesla to the belief
+that these disadvantageous features could be obviated by employing
+currents of a sufficiently high number of alternations, and his
+anticipations have been confirmed in practice. These rapidly
+alternating currents render it possible to maintain small arcs
+which, besides, possess the advantages of silence and persistency.
+The latter quality is due to the necessarily rapid alternations, in
+consequence of which the arc has no time to cool, and is always
+maintained at a high temperature and low resistance.</p>
+
+<p>At the outset of his experiments Mr. Tesla encountered great
+difficulties in the construction of high frequency machines. A
+generator of this kind is described here, which, though constructed
+quite some time ago, is well worthy of a detailed description.
+It may be mentioned, in passing, that dynamos of
+this type have been used by Mr. Tesla in his lighting researches and
+experiments with currents of high potential and high frequency,
+and reference to them will be found in his lectures
+elsewhere printed in this volume.<a name="FNanchor_4_4" id="FNanchor_4_4"></a><a href="#Footnote_4_4" class="fnanchor">[4]</a></p>
+<p><span class='pagenum'><a name="Page_375" id="Page_375">[Pg 375]</a></span></p>
+<p>In the accompanying engravings, Figs. 199 and 200 show the
+machine, respectively, in side elevation and vertical cross-section;
+Figs. 201, 202 and 203 showing enlarged details of construction.
+As will be seen, <small>A</small> is an annular magnetic frame, the interior of
+which is provided with a large number of pole-pieces <small>D</small>.</p>
+
+<p>Owing to the very large number and small size of the poles
+and the spaces between them, the field coils are applied by winding
+an insulated conductor <small>F</small> zigzag through the grooves, as shown
+in Fig. 203, carrying the wire around the annulus to form as
+many layers as is desired. In this way the pole-pieces <small>D</small> will be
+energized with alternately opposite polarity around the entire
+ring.</p>
+
+<p>For the armature, Mr. Tesla employs a spider carrying a ring
+<small>J</small>, turned down, except at its edges, to form a trough-like receptacle
+for a mass of fine annealed iron wires <small>K</small>, which are wound
+in the groove to form the core proper for the armature-coils.
+Pins <small>L</small> are set in the sides of the ring <small>J</small> and the coils <small>M</small> are wound
+over the periphery of the armature-structure and around the pins.
+The coils <small>M</small> are connected together in series, and these terminals
+<small>N</small> carried through the hollow shaft <small>H</small> to contact-rings <small>P P</small>, from
+which the currents are taken off by brushes <small>O</small>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_389.jpg" width="800" height="569" alt="Fig. 199." title="" />
+<span class="caption">Fig. 199.</span>
+</div>
+
+<p>In this way a machine with a very large number of poles may
+be constructed. It is easy, for instance, to obtain in this manner
+three hundred and seventy-five to four hundred poles in a machine
+that may be safely driven at a speed of fifteen hundred or sixteen
+hundred revolutions per minute, which will produce ten<span class='pagenum'><a name="Page_376" id="Page_376">[Pg 376]</a></span>
+thousand or eleven thousand alternations of current per second.
+Arc lamps <small>R R</small> are shown in the diagram as connected up in series
+with the machine in Fig. 200. If such a current be applied to
+running arc lamps, the sound produced by or in the arc becomes
+practically inaudible, for, by increasing the rate of change in the
+current, and consequently the number of vibrations per unit of
+time of the gaseous material of the arc up to, or beyond, ten
+thousand or eleven thousand per second, or to what is regarded
+as the limit of audition, the sound due to such vibrations will not
+be audible. The exact number of changes or undulations necessary
+to produce this result will vary somewhat according to the
+size of the arc&mdash;that is to say, the smaller the arc, the greater the
+number of changes that will be required to render it inaudible
+within certain limits. It should also be stated that the arc should
+not exceed a certain length.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_390.jpg" width="800" height="560" alt="Fig. 200, 201, 202 and 203." title="" />
+<span class="caption1"><span class="smcap">Figs.</span> 200, 201, 202 and 203.</span>
+</div>
+
+<p>The difficulties encountered in the construction of these
+machines are of a mechanical as well as an electrical nature.
+The machines may be designed on two plans: the field may be
+formed either of alternating poles, or of polar projections of the
+same polarity. Up to about 15,000 alternations per second in an
+experimental machine, the former plan may be followed, but a
+more efficient machine is obtained on the second plan.</p>
+
+<p>In the machine above described, which was capable of running
+two arcs of normal candle power, the field was composed of a<span class='pagenum'><a name="Page_377" id="Page_377">[Pg 377]</a></span>
+ring of wrought iron 32 inches outside diameter, and about 1
+inch thick. The inside diameter was 30 inches. There were 384
+polar projections. The wire was wound in zigzag form, but two
+wires were wound so as to completely envelop the projections.
+The distance between the projections is about 3/16 inch, and they
+are a little over 1/16 inch thick. The field magnet was made relatively
+small so as to adapt the machine for a constant current.
+There are 384 coils connected in two series. It was found impracticable
+to use any wire much thicker than No. 26 B. and S.
+gauge on account of the local effects. In such a machine the
+clearance should be as small as possible; for this reason the
+machine was made only 1&frac14; inch wide, so that the binding wires
+might be obviated. The armature wires must be wound with
+great care, as they are apt to fly off in consequence of the great
+peripheral speed. In various experiments this machine has been
+run as high as 3,000 revolutions per minute. Owing to the great
+speed it was possible to obtain as high as 10 amperes out of the
+machine. The electromotive force was regulated by means of
+an adjustable condenser within very wide limits, the limits
+being the greater, the greater the speed. This machine was
+frequently used to run Mr. Tesla's laboratory lights.</p>
+
+<div class="figcenter" style="width: 649px;">
+<img src="images/oi_391.jpg" width="649" height="600" alt="Fig. 204." title="" />
+<span class="caption">Fig. 204.</span>
+</div>
+
+<p>The machine above described was only one of many such
+types constructed. It serves well for an experimental machine,
+but if still higher alternations are required and higher efficiency
+is necessary, then a machine on a plan shown in Figs. 204 to<span class='pagenum'><a name="Page_378" id="Page_378">[Pg 378]</a></span>
+207, is preferable. The principal advantage of this type of
+machine is that there is not much magnetic leakage, and that a
+field may be produced, varying greatly in intensity in places not
+much distant from each other.</p>
+
+<p>In these engravings, Figs. 204 and 205 illustrate a machine in
+which the armature conductor and field coils are stationary, while
+the field magnet core revolves. Fig. 206 shows a machine
+embodying the same plan of construction, but having a stationary
+field magnet and rotary armature.</p>
+
+<p>The conductor in which the currents are induced may be
+arranged in various ways; but Mr. Tesla prefers the following
+method: He employs an annular plate of copper <small>D</small>, and by
+means of a saw cuts in it radial slots from one edge nearly
+through to the other, beginning alternately from opposite edges.
+In this way a continuous zigzag conductor is formed. When the
+polar projections are 1/8 inch wide, the width of the conductor
+should not, under any circumstances, be more than 1/32 inch wide;
+even then the eddy effect is considerable.</p>
+
+
+<div class="figcenter" style="width: 771px;">
+<img src="images/oi_392.jpg" width="771" height="600" alt="Fig. 205." title="" />
+<span class="caption">Fig. 205.</span>
+</div>
+
+<p>To the inner edge of this plate are secured two rings of non-magnetic
+metal <small>E</small>, which are insulated from the copper conductor,
+but held firmly thereto by means of the bolts <small>F</small>. Within the
+rings <small>E</small> is then placed an annular coil <small>G</small>, which is the energizing
+coil for the field magnet. The conductor <small>D</small> and the parts attached
+thereto are supported by means of the cylindrical shell or<span class='pagenum'><a name="Page_379" id="Page_379">[Pg 379]</a></span>
+casting <small>A A</small>, the two parts of which are brought together and
+clamped to the outer edge of the conductor <small>D</small>.</p>
+
+<div class="figcenter" style="width: 654px;">
+<img src="images/oi_393.jpg" width="654" height="600" alt="Fig. 206." title="" />
+<span class="caption">Fig. 206.</span>
+</div>
+
+
+<p>The core for the field magnet is built up of two circular parts
+<small>H H</small>, formed with annular grooves <small>I</small>, which, when the two parts
+are brought together, form a space for the reception of the energizing
+coil <small>G</small>. The hubs of the cores are trued off, so as to fit
+closely against one another, while the outer portions or flanges
+which form the polar faces <small>J J</small>, are reduced somewhat in thickness
+to make room for the conductor <small>D</small>, and are serrated on their
+faces. The number of serrations in the polar faces is arbitrary;
+but there must exist between them and the radial portions of
+the conductor <small>D</small> certain relation, which will be understood by
+reference to Fig. 207 in which <small>N N</small> represent the projections or
+points on one face of the core of the field, and <small>S S</small> the points of
+the other face. The conductor <small>D</small> is shown in this figure in section
+<i>a a'</i> designating the radial portions of the conductor, and <i>b</i> the
+insulating divisions between them. The relative width of the
+parts <i>a a'</i> and the space between any two adjacent points <small>N N</small> or
+<small>S S</small> is such that when the radial portions <i>a</i> of the conductor are
+passing between the opposite points <small>N S</small> where the field is strongest,
+the intermediate radial portions <i>a'</i> are passing through the<span class='pagenum'><a name="Page_380" id="Page_380">[Pg 380]</a></span>
+widest spaces midway between such points and where the field is
+weakest. Since the core on one side is of opposite polarity to
+the part facing it, all the projections of one polar face will be of
+opposite polarity to those of the other face. Hence, although
+the space between any two adjacent points on the same face may
+be extremely small, there will be no leakage of the magnetic
+lines between any two points of the same name, but the lines of
+force will pass across from one set of points to the other. The
+construction followed obviates to a great degree the distortion of
+the magnetic lines by the action of the current in the conductor
+<small>D</small>, in which it will be observed the current is flowing at any given
+time from the centre toward the periphery in one set of radial
+parts <i>a</i> and in the opposite direction in the adjacent parts <i>a'</i>.</p>
+
+<p>In order to connect the energizing coil <small>G</small>, Fig. 204, with a source
+of continuous current, Mr. Tesla utilizes two adjacent radial portions
+of the conductor <small>D</small> for connecting the terminals of the coil
+<small>G</small> with two binding posts <small>M</small>. For this purpose the plate <small>D</small> is cut
+entirely through, as shown, and the break thus made is bridged
+over by a short conductor <small>C</small>. The plate <small>D</small> is cut through to form
+two terminals <i>d</i>, which are connected to binding posts <small>N</small>. The
+core <small>H H</small>, when rotated by the driving pulley, generates in the conductors
+<small>D</small> an alternating current, which is taken off from the
+binding posts <small>N</small>.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_394.jpg" width="640" height="275" alt="Fig. 207." title="" />
+<span class="caption">Fig. 207.</span>
+</div>
+
+
+
+<p>When it is desired to rotate the conductor between the faces
+of a stationary field magnet, the construction shown in Fig.
+206, is adopted. The conductor <small>D</small> in this case is or may be
+made in substantially the same manner as above described by
+slotting an annular conducting-plate and supporting it between
+two heads <small>O</small>, held together by bolts <i>o</i> and fixed to the driving-shaft
+<small>K</small>. The inner edge of the plate or conductor <small>D</small> is preferably
+flanged to secure a firmer union between it and the heads <small>O</small>. It
+is insulated from the head. The field-magnet in this case consists
+of two annular parts <small>H H</small>, provided with annular grooves <small>I</small>
+for the reception of the coils. The flanges or faces surrounding<span class='pagenum'><a name="Page_381" id="Page_381">[Pg 381]</a></span>
+the annular groove are brought together, while the inner flanges
+are serrated, as in the previous case, and form the polar faces.
+The two parts <small>H H</small> are formed with a base <small>R</small>, upon which the
+machine rests. <small>S S</small> are non-magnetic bushings secured or set in
+the central opening of the cores. The conductor <small>D</small> is cut entirely
+through at one point to form terminals, from which insulated
+conductors <small>T</small> are led through the shaft to collecting-rings <small>V</small>.</p>
+
+<p>In one type of machine of this kind constructed by Mr. Tesla,
+the field had 480 polar projections on each side, and from this
+machine it was possible to obtain 30,000 alternations per second.
+As the polar projections must necessarily be very narrow, very
+thin wires or sheets must be used to avoid the eddy current
+effects. Mr. Tesla has thus constructed machines with a stationary
+armature and rotating field, in which case also the field-coil
+was supported so that the revolving part consisted only of a
+wrought iron body devoid of any wire and also machines with a
+rotating armature and stationary field. The machines may be
+either drum or disc, but Mr. Tesla's experience shows the latter
+to be preferable.</p>
+
+<hr style='width: 15%;' />
+
+<p>In the course of a very interesting article contributed to the
+<i>Electrical World</i> in February, 1891, Mr. Tesla makes some suggestive
+remarks on these high frequency machines and his experiences
+with them, as well as with other parts of the high
+frequency apparatus. Part of it is quoted here and is as
+follows:&mdash;</p>
+
+<p>The writer will incidentally mention that any one who attempts
+for the first time to construct such a machine will have a
+tale of woe to tell. He will first start out, as a matter of course,
+by making an armature with the required number of polar projections.
+He will then get the satisfaction of having produced
+an apparatus which is fit to accompany a thoroughly Wagnerian
+opera. It may besides possess the virtue of converting mechanical
+energy into heat in a nearly perfect manner. If there is a
+reversal in the polarity of the projections, he will get heat out of
+the machine; if there is no reversal, the heating will be less, but
+the output will be next to nothing. He will then abandon the
+iron in the armature, and he will get from the Scylla to the
+Charybdis. He will look for one difficulty and will find another,
+but, after a few trials, he may get nearly what he wanted.<span class='pagenum'><a name="Page_382" id="Page_382">[Pg 382]</a></span></p>
+
+<p>Among the many experiments which may be performed with
+such a machine, of not the least interest are those performed
+with a high-tension induction coil. The character of the discharge
+is completely changed. The arc is established at much
+greater distances, and it is so easily affected by the slightest current
+of air that it often wriggles around in the most singular
+manner. It usually emits the rhythmical sound peculiar to the
+alternate current arcs, but the curious point is that the sound
+may be heard with a number of alternations far above ten thousand
+per second, which by many is considered to be about the
+limit of audition. In many respects the coil behaves like a static
+machine. Points impair considerably the sparking interval, electricity
+escaping from them freely, and from a wire attached to
+one of the terminals streams of light issue, as though it were
+connected to a pole of a powerful Toepler machine. All these
+phenomena are, of course, mostly due to the enormous differences
+of potential obtained. As a consequence of the self-induction
+of the coil and the high frequency, the current is minute
+while there is a corresponding rise of pressure. A current impulse
+of some strength started in such a coil should persist to
+flow no less than four ten-thousandths of a second. As this time
+is greater than half the period, it occurs that an opposing electromotive
+force begins to act while the current is still flowing. As
+a consequence, the pressure rises as in a tube filled with liquid
+and vibrated rapidly around its axis. The current is so small
+that, in the opinion and involuntary experience of the writer, the
+discharge of even a very large coil cannot produce seriously injurious
+effects, whereas, if the same coil were operated with a
+current of lower frequency, though the electromotive force would
+be much smaller, the discharge would be most certainly injurious.
+This result, however, is due in part to the high frequency.
+The writer's experiences tend to show that the higher the frequency
+the greater the amount of electrical energy which may
+be passed through the body without serious discomfort; whence
+it seems certain that human tissues act as condensers.</p>
+
+<p>One is not quite prepared for the behavior of the coil when
+connected to a Leyden jar. One, of course, anticipates that since
+the frequency is high the capacity of the jar should be small. He
+therefore takes a very small jar, about the size of a small wine
+glass, but he finds that even with this jar the coil is practically
+short-circuited. He then reduces the capacity until he comes to<span class='pagenum'><a name="Page_383" id="Page_383">[Pg 383]</a></span>
+about the capacity of two spheres, say, ten centimetres in diameter
+and two to four centimetres apart. The discharge then assumes
+the form of a serrated band exactly like a succession of
+sparks viewed in a rapidly revolving mirror; the serrations, of
+course, corresponding to the condenser discharges. In this case
+one may observe a queer phenomenon. The discharge starts at
+the nearest points, works gradually up, breaks somewhere near
+the top of the spheres, begins again at the bottom, and so on.
+This goes on so fast that several serrated bands are seen at once.
+One may be puzzled for a few minutes, but the explanation is
+simple enough. The discharge begins at the nearest points, the air
+is heated and carries the arc upward until it breaks, when it is re-established
+at the nearest points, etc. Since the current passes
+easily through a condenser of even small capacity, it will be found
+quite natural that connecting only one terminal to a body of the
+same size, no matter how well insulated, impairs considerably the
+striking distance of the arc.</p>
+
+<p>Experiments with Geissler tubes are of special interest. An
+exhausted tube, devoid of electrodes of any kind, will light up at
+some distance from the coil. If a tube from a vacuum pump is
+near the coil the whole of the pump is brilliantly lighted. An
+incandescent lamp approached to the coil lights up and gets perceptibly
+hot. If a lamp have the terminals connected to one of
+the binding posts of the coil and the hand is approached to the
+bulb, a very curious and rather unpleasant discharge from the
+glass to the hand takes place, and the filament may become incandescent.
+The discharge resembles to some extent the stream
+issuing from the plates of a powerful Toepler machine, but is of
+incomparably greater quantity. The lamp in this case acts as a
+condenser, the rarefied gas being one coating, the operator's hand
+the other. By taking the globe of a lamp in the hand, and by
+bringing the metallic terminals near to or in contact with a conductor
+connected to the coil, the carbon is brought to bright incandescence
+and the glass is rapidly heated. With a 100-volt 10 <span class="smcap">c.
+p.</span> lamp one may without great discomfort stand as much current
+as will bring the lamp to a considerable brilliancy; but it can be
+held in the hand only for a few minutes, as the glass is heated in
+an incredibly short time. When a tube is lighted by bringing it
+near to the coil it may be made to go out by interposing a metal
+plate on the hand between the coil and tube; but if the metal
+plate be fastened to a glass rod or otherwise insulated, the tube<span class='pagenum'><a name="Page_384" id="Page_384">[Pg 384]</a></span>
+may remain lighted if the plate be interposed, or may even increase
+in luminosity. The effect depends on the position of the
+plate and tube relatively to the coil, and may be always easily
+foretold by <i>assuming</i> that conduction takes place from one terminal
+of the coil to the other. According to the position of the
+plate, it may either divert from or direct the current to the tube.</p>
+
+<p>In another line of work the writer has in frequent experiments
+maintained incandescent lamps of 50 or 100 volts burning at any
+desired candle power with both the terminals of each lamp connected
+to a stout copper wire of no more than a few feet in
+length. These experiments seem interesting enough, but they
+are not more so than the queer experiment of Faraday, which
+has been revived and made much of by recent investigators, and
+in which a discharge is made to jump between two points of a
+bent copper wire. An experiment may be cited here which may
+seem equally interesting. If a Geissler tube, the terminals of
+which are joined by a copper wire, be approached to the coil, certainly
+no one would be prepared to see the tube light up.
+Curiously enough, it does light up, and, what is more, the
+wire does not seem to make much difference. Now one is
+apt to think in the first moment that the impedance of the
+wire might have something to do with the phenomenon. But
+this is of course immediately rejected, as for this an enormous
+frequency would be required. This result, however, seems
+puzzling only at first; for upon reflection it is quite clear that
+the wire can make but little difference. It may be explained in
+more than one way, but it agrees perhaps best with observation
+to assume that conduction takes place from the terminals of the
+coil through the space. On this assumption, if the tube with the
+wire be held in any position, the wire can divert little more than
+the current which passes through the space occupied by the wire
+and the metallic terminals of the tube; through the adjacent
+space the current passes practically undisturbed. For this reason,
+if the tube be held in any position at right angles to the line
+joining the binding posts of the coil, the wire makes hardly any
+difference, but in a position more or less parallel with that line
+it impairs to a certain extent the brilliancy of the tube and its
+facility to light up. Numerous other phenomena may be explained
+on the same assumption. For instance, if the ends of the
+tube be provided with washers of sufficient size and held in the
+line joining the terminals of the coil, it will not light up, and
+then nearly the whole of the current, which would otherwise<span class='pagenum'><a name="Page_385" id="Page_385">[Pg 385]</a></span>
+pass uniformly through the space between the washers, is diverted
+through the wire. But if the tube be inclined sufficiently
+to that line, it will light up in spite of the washers. Also, if a
+metal plate be fastened upon a glass rod and held at right angles
+to the line joining the binding posts, and nearer to one of them,
+a tube held more or less parallel with the line will light up instantly
+when one of the terminals touches the plate, and will go
+out when separated from the plate. The greater the surface of
+the plate, up to a certain limit, the easier the tube will light up.
+When a tube is placed at right angles to the straight line joining
+the binding posts, and then rotated, its luminosity steadily increases
+until it is parallel with that line. The writer must state,
+however, that he does not favor the idea of a leakage or current
+through the space any more than as a suitable explanation, for he
+is convinced that all these experiments could not be performed with
+a static machine yielding a constant difference of potential, and
+that condenser action is largely concerned in these phenomena.</p>
+
+<p>It is well to take certain precautions when operating a Ruhmkorff
+coil with very rapidly alternating currents. The primary
+current should not be turned on too long, else the core may get
+so hot as to melt the gutta-percha or paraffin, or otherwise injure
+the insulation, and this may occur in a surprisingly short time,
+considering the current's strength. The primary current being
+turned on, the fine wire terminals may be joined without great
+risk, the impedance being so great that it is difficult to force
+enough current through the fine wire so as to injure it, and in
+fact the coil may be on the whole much safer when the terminals
+of the fine wire are connected than when they are insulated;
+but special care should be taken when the terminals are connected
+to the coatings of a Leyden jar, for with anywhere near
+the critical capacity, which just counteracts the self-induction at
+the existing frequency, the coil might meet the fate of St. Polycarpus.
+If an expensive vacuum pump is lighted up by being
+near to the coil or touched with a wire connected to one of the
+terminals, the current should be left on no more than a few
+moments, else the glass will be cracked by the heating of the
+rarefied gas in one of the narrow passages&mdash;in the writer's own
+experience <i>quod erat demonstrandum</i>.<a name="FNanchor_5_5" id="FNanchor_5_5"></a><a href="#Footnote_5_5" class="fnanchor">[5]</a></p>
+
+<p><span class='pagenum'><a name="Page_386" id="Page_386">[Pg 386]</a></span></p>
+<p>There are a good many other points of interest which may be
+observed in connection with such a machine. Experiments with
+the telephone, a conductor in a strong field or with a condenser
+or arc, seem to afford certain proof that sounds far above the
+usual accepted limit of hearing would be perceived. A telephone
+will emit notes of twelve to thirteen thousand vibrations per
+second; then the inability of the core to follow such rapid alternations
+begins to tell. If, however, the magnet and core be
+replaced by a condenser and the terminals connected to the high-tension
+secondary of a transformer, higher notes may still be
+heard. If the current be sent around a finely laminated core
+and a small piece of thin sheet iron be held gently against the
+core, a sound may be still heard with thirteen to fourteen thousand
+alternations per second, provided the current is sufficiently
+strong. A small coil, however, tightly packed between the poles
+of a powerful magnet, will emit a sound with the above number
+of alternations, and arcs may be audible with a still higher frequency.
+The limit of audition is variously estimated. In Sir
+William Thomson's writings it is stated somewhere that ten
+thousand per second, or nearly so, is the limit. Other, but less
+reliable, sources give it as high as twenty-four thousand per
+second. The above experiments have convinced the writer that
+notes of an incomparably higher number of vibrations per second
+would be perceived provided they could be produced with sufficient
+power. There is no reason why it should not be so. The
+condensations and rarefactions of the air would necessarily set
+the diaphragm in a corresponding vibration and some sensation
+would be produced, whatever&mdash;within certain limits&mdash;the velocity
+of transmission to their nerve centres, though it is probable that
+for want of exercise the ear would not be able to distinguish any
+such high note. With the eye it is different; if the sense of
+vision is based upon some resonance effect, as many believe, no
+amount of increase in the intensity of the ethereal vibration
+could extend our range of vision on either side of the visible
+spectrum.</p>
+
+<p>The limit of audition of an arc depends on its size. The
+greater the surface by a given heating effect in the arc, the higher
+the limit of audition. The highest notes are emitted by the
+high-tension discharges of an induction coil in which the arc is,
+so to speak, all surface. If <i>R</i> be the resistance of an arc, and <i>C</i>
+the current, and the linear dimensions be <i>n</i> times increased, then<span class='pagenum'><a name="Page_387" id="Page_387">[Pg 387]</a></span>
+the resistance is <i>R</i>/<i>n</i>, and with the same current density the current
+would be <i>n</i><sup>2</sup><i>C</i>; hence the heating effect is <i>n</i><sup>3</sup> times greater,
+while the surface is only <i>n</i><sup>2</sup> times as great. For this reason very
+large arcs would not emit any rhythmical sound even with a very
+low frequency. It must be observed, however, that the sound
+emitted depends to some extent also on the composition of the
+carbon. If the carbon contain highly refractory material, this,
+when heated, tends to maintain the temperature of the arc uniform
+and the sound is lessened; for this reason it would seem
+that an alternating arc requires such carbons.</p>
+
+<p>With currents of such high frequencies it is possible to obtain
+noiseless arcs, but the regulation of the lamp is rendered extremely
+difficult on account of the excessively small attractions
+or repulsions between conductors conveying these currents.</p>
+
+<p>An interesting feature of the arc produced by these rapidly
+alternating currents is its persistency. There are two causes for
+it, one of which is always present, the other sometimes only.
+One is due to the character of the current and the other to a
+property of the machine. The first cause is the more important
+one, and is due directly to the rapidity of the alternations.
+When an arc is formed by a periodically undulating current,
+there is a corresponding undulation in the temperature of the
+gaseous column, and, therefore, a corresponding undulation in
+the resistance of the arc. But the resistance of the arc varies
+enormously with the temperature of the gaseous column, being
+practically infinite when the gas between the electrodes is cold.
+The persistence of the arc, therefore, depends on the inability of
+the column to cool. It is for this reason impossible to maintain
+an arc with the current alternating only a few times a second.
+On the other hand, with a practically continuous current, the arc
+is easily maintained, the column being constantly kept at a high
+temperature and low resistance. The higher the frequency the
+smaller the time interval during which the arc may cool and increase
+considerably in resistance. With a frequency of 10,000
+per second or more in an arc of equal size excessively small variations
+of temperature are superimposed upon a steady temperature,
+like ripples on the surface of a deep sea. The heating effect is
+practically continuous and the arc behaves like one produced by
+a continuous current, with the exception, however, that it may
+not be quite as easily started, and that the electrodes are equally<span class='pagenum'><a name="Page_388" id="Page_388">[Pg 388]</a></span>
+consumed; though the writer has observed some irregularities in
+this respect.</p>
+
+<p>The second cause alluded to, which possibly may not be present,
+is due to the tendency of a machine of such high frequency
+to maintain a practically constant current. When the arc is
+lengthened, the electromotive force rises in proportion and the
+arc appears to be more persistent.</p>
+
+<p>Such a machine is eminently adapted to maintain a constant
+current, but it is very unfit for a constant potential. As a matter
+of fact, in certain types of such machines a nearly constant current
+is an almost unavoidable result. As the number of poles or
+polar projections is greatly increased, the clearance becomes of
+great importance. One has really to do with a great number of
+very small machines. Then there is the impedance in the armature,
+enormously augmented by the high frequency. Then,
+again, the magnetic leakage is facilitated. If there are three or
+four hundred alternate poles, the leakage is so great that it is
+virtually the same as connecting, in a two-pole machine, the poles
+by a piece of iron. This disadvantage, it is true, may be obviated
+more or less by using a field throughout of the same polarity,
+but then one encounters difficulties of a different nature. All
+these things tend to maintain a constant current in the armature
+circuit.</p>
+
+<p>In this connection it is interesting to notice that even to-day
+engineers are astonished at the performance of a constant current
+machine, just as, some years ago, they used to consider it an extraordinary
+performance if a machine was capable of maintaining
+a constant potential difference between the terminals. Yet one
+result is just as easily secured as the other. It must only be
+remembered that in an inductive apparatus of any kind, if constant
+potential is required, the inductive relation between the
+primary or exciting and secondary or armature circuit must be
+the closest possible; whereas, in an apparatus for constant current
+just the opposite is required. Furthermore, the opposition
+to the current's flow in the induced circuit must be as small as
+possible in the former and as great as possible in the latter case.
+But opposition to a current's flow may be caused in more than
+one way. It may be caused by ohmic resistance or self-induction.
+One may make the induced circuit of a dynamo machine
+or transformer of such high resistance that when operating devices
+of considerably smaller resistance within very wide limits a<span class='pagenum'><a name="Page_389" id="Page_389">[Pg 389]</a></span>
+nearly constant current is maintained. But such high resistance
+involves a great loss in power, hence it is not practicable. Not
+so self-induction. Self-induction does not necessarily mean loss
+of power. The moral is, use self-induction instead of resistance.
+There is, however, a circumstance which favors the adoption of
+this plan, and this is, that a very high self-induction may be
+obtained cheaply by surrounding a comparatively small length
+of wire more or less completely with iron, and, furthermore, the
+effect may be exalted at will by causing a rapid undulation of the
+current. To sum up, the requirements for constant current
+are: Weak magnetic connection between the induced and
+inducing circuits, greatest possible self-induction with the
+least resistance, greatest practicable rate of change of the
+current. Constant potential, on the other hand, requires: Closest
+magnetic connection between the circuits, steady induced
+current, and, if possible, no reaction. If the latter conditions
+could be fully satisfied in a constant potential machine, its output
+would surpass many times that of a machine primarily designed
+to give constant current. Unfortunately, the type of machine
+in which these conditions may be satisfied is of little practical
+value, owing to the small electromotive force obtainable and the
+difficulties in taking off the current.</p>
+
+<p>With their keen inventor's instinct, the now successful arc-light
+men have early recognized the desiderata of a constant
+current machine. Their arc light machines have weak fields,
+large armatures, with a great length of copper wire and few
+commutator segments to produce great variations in the current's
+strength and to bring self-induction into play. Such machines
+may maintain within considerable limits of variation in the resistance
+of the circuit a practically constant current. Their output
+is of course correspondingly diminished, and, perhaps with
+the object in view not to cut down the output too much, a simple
+device compensating exceptional variations is employed.
+The undulation of the current is almost essential to the commercial
+success of an arc-light system. It introduces in the circuit a
+steadying element taking the place of a large ohmic resistance,
+without involving a great loss in power, and, what is more important,
+it allows the use of simple clutch lamps, which with a
+current of a certain number of impulses per second, best suitable
+for each particular lamp, will, if properly attended to, regulate
+even better than the finest clock-work lamps. This discovery
+has been made by the writer&mdash;several years too late.<span class='pagenum'><a name="Page_390" id="Page_390">[Pg 390]</a></span></p>
+
+<p>It has been asserted by competent English electricians that in a
+constant-current machine or transformer the regulation is effected
+by varying the phase of the secondary current. That this view
+is erroneous may be easily proved by using, instead of lamps, devices
+each possessing self-induction and capacity or self-induction
+and resistance&mdash;that is, retarding and accelerating components&mdash;in
+such proportions as to not affect materially the phase of the
+secondary current. Any number of such devices may be inserted
+or cut out, still it will be found that the regulation occurs, a constant
+current being maintained, while the electromotive force is
+varied with the number of the devices. The change of phase of
+the secondary current is simply a result following from the
+changes in resistance, and, though secondary reaction is always
+of more or less importance, yet the real cause of the regulation
+lies in the existence of the conditions above enumerated. It
+should be stated, however, that in the case of a machine the above
+remarks are to be restricted to the cases in which the machine is
+independently excited. If the excitation be effected by commutating
+the armature current, then the fixed position of the brushes
+makes any shifting of the neutral line of the utmost importance,
+and it may not be thought immodest of the writer to mention
+that, as far as records go, he seems to have been the first who has
+successfully regulated machines by providing a bridge connection
+between a point of the external circuit and the commutator by
+means of a third brush. The armature and field being properly
+proportioned and the brushes placed in their determined positions,
+a constant current or constant potential resulted from the
+shifting of the diameter of commutation by the varying loads.</p>
+
+<p>In connection with machines of such high frequencies, the
+condenser affords an especially interesting study. It is easy to
+raise the electromotive force of such a machine to four or five
+times the value by simply connecting the condenser to the circuit,
+and the writer has continually used the condenser for the
+the purposes of regulation, as suggested by Blakesley in his book
+on alternate currents, in which he has treated the most frequently
+occurring condenser problems with exquisite simplicity and clearness.
+The high frequency allows the use of small capacities and
+renders investigation easy. But, although in most of the experiments
+the result may be foretold, some phenomena observed seem
+at first curious. One experiment performed three or four months
+ago with such a machine and a condenser may serve as an il<span class='pagenum'><a name="Page_391" id="Page_391">[Pg 391]</a></span>lustration.
+A machine was used giving about 20,000 alternations
+per second. Two bare wires about twenty feet long and two
+millimetres in diameter, in close proximity to each other, were
+connected to the terminals of the machine at the one end, and
+to a condenser at the other. A small transformer without an
+iron core, of course, was used to bring the reading within range
+of a Cardew voltmeter by connecting the voltmeter to the
+secondary. On the terminals of the condenser the electromotive
+force was about 120 volts, and from there inch by inch it gradually
+fell until at the terminals of the machine it was about 65
+volts. It was virtually as though the condenser were a generator,
+and the line and armature circuit simply a resistance connected
+to it. The writer looked for a case of resonance, but he
+was unable to augment the effect by varying the capacity very
+carefully and gradually or by changing the speed of the machine.
+A case of pure resonance he was unable to obtain.
+When a condenser was connected to the terminals of the machine&mdash;the
+self-induction of the armature being first determined
+in the maximum and minimum position and the mean value taken&mdash;the
+capacity which gave the highest electromotive force corresponded
+most nearly to that which just counteracted the self-induction
+with the existing frequency. If the capacity was increased
+or diminished, the electromotive force fell as expected.</p>
+
+<p>With frequencies as high as the above mentioned, the condenser
+effects are of enormous importance. The condenser
+becomes a highly efficient apparatus capable of transferring
+considerable energy.</p>
+
+<hr style='width: 15%;' />
+
+<p>In an appendix to this book will be found a description of the
+Tesla oscillator, which its inventor believes will among other great
+advantages give him the necessary high frequency conditions,
+while relieving him of the inconveniences that attach to generators
+of the type described at the beginning of this chapter.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_392" id="Page_392">[Pg 392]</a></span></p>
+<h2><a name="CHAPTER_XXX" id="CHAPTER_XXX"></a>CHAPTER XXX.</h2>
+
+<h3><span class="smcap">Alternate Current Electrostatic Induction Apparatus.<a name="FNanchor_6_6" id="FNanchor_6_6"></a><a href="#Footnote_6_6" class="fnanchor">[6]</a></span></h3>
+
+
+<p>About a year and a half ago while engaged in the study of
+alternate currents of short period, it occurred to me that such
+currents could be obtained by rotating charged surfaces in close
+proximity to conductors. Accordingly I devised various forms
+of experimental apparatus of which two are illustrated in the
+accompanying engravings.</p>
+
+<div class="figcenter" style="width: 639px;">
+<img src="images/oi_406.jpg" width="639" height="600" alt="Fig. 208." title="" />
+<span class="caption">Fig. 208.</span>
+</div>
+
+<p>In the apparatus shown in Fig. 208, <small>A</small> is a ring of dry shellacked
+hard wood provided on its inside with two sets of tin-foil
+coatings, <i>a</i> and <i>b</i>, all the <i>a</i> coatings and all the <i>b</i> coatings being
+connected together, respectively, but independent from each
+other. These two sets of coatings are connected to two termi<span class='pagenum'><a name="Page_393" id="Page_393">[Pg 393]</a></span>nals,
+<small>T</small>. For the sake of clearness only a few coatings are shown.
+Inside of the ring <small>A</small>, and in close proximity to it there is arranged
+to rotate a cylinder <small>B</small>, likewise of dry, shellacked hard wood, and
+provided with two similar sets of coatings, <i>a</i><sup>1</sup> and <i>b</i><sup>1</sup>, all the coatings
+<i>a</i><sup>1</sup> being connected to one ring and all the others, <i>b</i><sup>1</sup>, to
+another marked &#43; and &minus;. These two sets, <i>a</i><sup>1</sup> and <i>b</i><sup>1</sup> are charged
+to a high potential by a Holtz or Wimshurst machine, and may
+be connected to a jar of some capacity. The inside of ring <small>A</small> is
+coated with mica in order to increase the induction and also to
+allow higher potentials to be used.</p>
+
+<div class="figcenter" style="width: 627px;">
+<img src="images/oi_407.jpg" width="627" height="600" alt="Fig. 209." title="" />
+<span class="caption">Fig. 209.</span>
+</div>
+
+
+<p>When the cylinder <small>B</small> with the charged coatings is rotated, a
+circuit connected to the terminals <small>T</small> is traversed by alternating
+currents. Another form of apparatus is illustrated in Fig. 209.
+In this apparatus the two sets of tin-foil coatings are glued on a
+plate of ebonite, and a similar plate which is rotated, and the
+coatings of which are charged as in Fig. 208, is provided.</p>
+
+<p>The output of such an apparatus is very small, but some of
+the effects peculiar to alternating currents of short periods may
+be observed. The effects, however, cannot be compared with
+those obtainable with an induction coil which is operated by an
+alternate current machine of high frequency, some of which
+were described by me a short while ago.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_394" id="Page_394">[Pg 394]</a></span></p>
+<h2><a name="CHAPTER_XXXI" id="CHAPTER_XXXI"></a>CHAPTER XXXI.</h2>
+
+<h3><span class="smcap">"Massage" With Currents of High Frequency.<a name="FNanchor_7_7" id="FNanchor_7_7"></a><a href="#Footnote_7_7" class="fnanchor">[7]</a></span></h3>
+
+
+<p>I trust that the present brief communication will not be interpreted
+as an effort on my part to put myself on record as a
+"patent medicine" man, for a serious worker cannot despise
+anything more than the misuse and abuse of electricity which we
+have frequent occasion to witness. My remarks are elicited by
+the lively interest which prominent medical practitioners evince
+at every real advance in electrical investigation. The progress
+in recent years has been so great that every electrician and electrical
+engineer is confident that electricity will become the means
+of accomplishing many things that have been heretofore, with
+our existing knowledge, deemed impossible. No wonder then
+that progressive physicians also should expect to find in it a
+powerful tool and help in new curative processes. Since I had
+the honor to bring before the American Institute of Electrical
+Engineers some results in utilizing alternating currents of high
+tension, I have received many letters from noted physicians inquiring
+as to the physical effects of such currents of high frequency.
+It may be remembered that I then demonstrated that
+a body perfectly well insulated in air can be heated by simply
+connecting it with a source of rapidly alternating high potential.
+The heating in this case is due in all probability to the bombardment
+of the body by air, or possibly by some other medium,
+which is molecular or atomic in construction, and the presence
+of which has so far escaped our analysis&mdash;for according to my
+ideas, the true ether radiation with such frequencies as even a
+few millions per second must be very small. This body may be
+a good conductor or it may be a very poor conductor of electricity
+with little change in the result. The human body is, in
+such a case, a fine conductor, and if a person insulated in a room,
+or no matter where, is brought into contact with such a source of
+<span class='pagenum'><a name="Page_395" id="Page_395">[Pg 395]</a></span>rapidly alternating high potential, the skin is heated by bombardment.
+It is a mere question of the dimensions and character
+of the apparatus to produce any degree of heating desired.</p>
+
+<p>It has occurred to me whether, with such apparatus properly
+prepared, it would not be possible for a skilled physician to find
+in it a means for the effective treatment of various types of disease.
+The heating will, of course, be superficial, that is, on the
+skin, and would result, whether the person operated on were in
+bed or walking around a room, whether dressed in thick clothes or
+whether reduced to nakedness. In fact, to put it broadly, it is
+conceivable that a person entirely nude at the North Pole might
+keep himself comfortably warm in this manner.</p>
+
+<p>Without vouching for all the results, which must, of course, be
+determined by experience and observation, I can at least warrant
+the fact that heating would occur by the use of this method of
+subjecting the human body to bombardment by alternating currents
+of high potential and frequency such I have long worked
+with. It is only reasonable to expect that some of the novel effects
+will be wholly different from those obtainable with the old
+familiar therapeutic methods generally used. Whether they
+would all be beneficial or not remains to be proved.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_396" id="Page_396">[Pg 396]</a></span></p>
+<h2><a name="CHAPTER_XXXII" id="CHAPTER_XXXII"></a>CHAPTER XXXII.</h2>
+
+<h3><span class="smcap">Electric Discharge in Vacuum Tubes.<a name="FNanchor_8_8" id="FNanchor_8_8"></a><a href="#Footnote_8_8" class="fnanchor">[8]</a></span></h3>
+
+
+<p>In <i>The Electrical Engineer</i> of June 10 I have noted the description
+of some experiments of Prof. J. J. Thomson, on the
+"Electric Discharge in Vacuum Tubes," and in your issue of June
+24 Prof. Elihu Thomson describes an experiment of the same
+kind. The fundamental idea in these experiments is to set up
+an electromotive force in a vacuum tube&mdash;-preferably devoid of
+any electrodes&mdash;by means of electro-magnetic induction, and to
+excite the tube in this manner.</p>
+
+<p>As I view the subject I should, think that to any experimenter
+who had carefully studied the problem confronting us and who
+attempted to find a solution of it, this idea must present itself as
+naturally as, for instance, the idea of replacing the tinfoil coatings
+of a Leyden jar by rarefied gas and exciting luminosity in
+the condenser thus obtained by repeatedly charging and discharging
+it. The idea being obvious, whatever merit there is in this
+line of investigation must depend upon the completeness of the
+study of the subject and the correctness of the observations. The
+following lines are not penned with any desire on my part to put
+myself on record as one who has performed similar experiments,
+but with a desire to assist other experimenters by pointing out
+certain peculiarities of the phenomena observed, which, to all appearances,
+have not been noted by Prof. J. J. Thomson, who,
+however, seems to have gone about systematically in his investigations,
+and who has been the first to make his results known.
+These peculiarities noted by me would seem to be at variance
+with the views of Prof. J. J. Thomson, and present the phenomena
+in a different light.</p>
+
+<p>My investigations in this line occupied me principally during
+the winter and spring of the past year. During this time many different
+experiments were performed, and in my exchanges of ideas
+<span class='pagenum'><a name="Page_397" id="Page_397">[Pg 397]</a></span>on this subject with Mr. Alfred S. Brown, of the Western Union
+Telegraph Company, various different dispositions were suggested
+which were carried out by me in practice. Fig. 210 may serve
+as an example of one of the many forms of apparatus used. This
+consisted of a large glass tube sealed at one end and projecting
+into an ordinary incandescent lamp bulb. The primary, usually
+consisting of a few turns of thick, well-insulated copper sheet was
+inserted within the tube, the inside space of the bulb furnishing
+the secondary. This form of apparatus was arrived at after some
+experimenting, and was used principally with the view of enabling
+me to place a polished reflecting surface on the inside of
+the tube, and for this purpose the last turn of the primary was
+covered with a thin silver sheet. In all forms of apparatus used
+there was no special difficulty in exciting a luminous circle or
+cylinder in proximity to the primary.</p>
+
+<div class="figcenter" style="width: 336px;">
+<img src="images/oi_411.jpg" width="336" height="448" alt="Fig. 210." title="" />
+<span class="caption">Fig. 210.</span>
+</div>
+
+<p>As to the number of turns, I cannot quite understand why
+Prof. J. J. Thomson should think that a few turns were "quite
+sufficient," but lest I should impute to him an opinion he may
+not have, I will add that I have gained this impression from the
+reading of the published abstracts of his lecture. Clearly, the
+number of turns which gives the best result in any case, is dependent
+on the dimensions of the apparatus, and, were it not for
+various considerations, one turn would always give the best
+result.</p>
+
+<p>I have found that it is preferable to use in these experiments
+an alternate current machine giving a moderate number of alter<span class='pagenum'><a name="Page_398" id="Page_398">[Pg 398]</a></span>nations
+per second to excite the induction coil for charging the
+Leyden jar which discharges through the primary&mdash;shown diagrammatically
+in Fig. 211,&mdash;as in such case, before the disruptive
+discharge takes place, the tube or bulb is slightly excited and
+the formation of the luminous circle is decidedly facilitated.
+But I have also used a Wimshurst machine in some experiments.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_412.jpg" width="640" height="180" alt="Fig. 211." title="" />
+<span class="caption">Fig. 211.</span>
+</div>
+
+<p>Prof. J. J. Thomson's view of the phenomena under consideration
+seems to be that they are wholly due to electro-magnetic
+action. I was, at one time, of the same opinion, but upon carefully
+investigating the subject I was led to the conviction that
+they are more of an electrostatic nature. It must be remembered
+that in these experiments we have to deal with primary
+currents of an enormous frequency or rate of change and of high
+potential, and that the secondary conductor consists of a rarefied
+gas, and that under such conditions electrostatic effects must play
+an important part.</p>
+
+<div class="figcenter" style="width: 588px;">
+<img src="images/oi_412-1.jpg" width="588" height="480" alt="Fig. 212." title="" />
+<span class="caption">Fig. 212.</span>
+</div>
+
+
+<p>In support of my view I will describe a few experiments made
+by me. To excite luminosity in the tube it is not absolutely
+necessary that the conductor should be closed. For instance, if<span class='pagenum'><a name="Page_399" id="Page_399">[Pg 399]</a></span>
+an ordinary exhausted tube (preferably of large diameter) be
+surrounded by a spiral of thick copper wire serving as the primary,
+a feebly luminous spiral may be induced in the tube, roughly
+shown in Fig. 212. In one of these experiments a curious phenomenon
+was observed; namely, two intensely luminous circles,
+each of them close to a turn of the primary spiral, were formed
+inside of the tube, and I attributed this phenomenon to the existence
+of nodes on the primary. The circles were connected by
+a faint luminous spiral parallel to the primary and in close proximity
+to it. To produce this effect I have found it necessary to
+strain the jar to the utmost. The turns of the spiral tend to
+close and form circles, but this, of course, would be expected,
+and does not necessarily indicate an electro-magnetic effect;
+Whereas the fact that a glow can be produced along the primary
+in the form of an open spiral argues for an electrostatic effect.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_413.jpg" width="640" height="334" alt="Fig. 213." title="" />
+<span class="caption">Fig. 213.</span>
+</div>
+
+<p>In using Dr. Lodge's recoil circuit, the electrostatic action is
+likewise apparent. The arrangement is illustrated in Fig. 213.
+In his experiment two hollow exhausted tubes <small>H H</small> were slipped
+over the wires of the recoil circuit and upon discharging the jar
+in the usual manner luminosity was excited in the tubes.</p>
+
+<p>Another experiment performed is illustrated in Fig. 214. In
+this case an ordinary lamp-bulb was surrounded by one or two
+turns of thick copper wire <small>P</small> and the luminous circle <small>L</small> excited
+in the bulb by discharging the jar through the primary. The
+lamp-bulb was provided with a tinfoil coating on the side opposite
+to the primary and each time the tinfoil coating was connected
+to the ground or to a large object the luminosity of the
+circle was considerably increased. This was evidently due to
+electrostatic action.</p>
+
+<p>In other experiments I have noted that when the primary
+touches the glass the luminous circle is easier produced and is<span class='pagenum'><a name="Page_400" id="Page_400">[Pg 400]</a></span>
+more sharply defined; but I have not noted that, generally speaking,
+the circles induced were very sharply defined, as Prof. J. J.
+Thomson has observed; on the contrary, in my experiments they
+were broad and often the whole of the bulb or tube was illuminated;
+and in one case I have observed an intensely purplish
+glow, to which Prof. J. J. Thomson refers. But the circles were
+always in close proximity to the primary and were considerably
+easier produced when the latter was very close to the glass, much
+more so than would be expected assuming the action to be electromagnetic
+and considering the distance; and these facts speak
+for an electrostatic effect.</p>
+
+<div class="figcenter" style="width: 534px;">
+<img src="images/oi_414.jpg" width="534" height="480" alt="Fig. 214." title="" />
+<span class="caption">Fig. 214.</span>
+</div>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_414-1.jpg" width="640" height="449" alt="Fig. 215." title="" />
+<span class="caption">Fig. 215.</span>
+</div>
+
+
+<p>Furthermore I have observed that there is a molecular bombardment
+in the plane of the luminous circle at right angles to
+the glass&mdash;supposing the circle to be in the plane of the primary<span class='pagenum'><a name="Page_401" id="Page_401">[Pg 401]</a></span>&mdash;this
+bombardment being evident from the rapid heating of the
+glass near the primary. Were the bombardment not at right
+angles to the glass the heating could not be so rapid. If there
+is a circumferential movement of the molecules constituting the
+luminous circle, I have thought that it might be rendered manifest
+by placing within the tube or bulb, radially to the circle, a
+thin plate of mica coated with some phosphorescent material and
+another such plate tangentially to the circle. If the molecules
+would move circumferentially, the former plate would be rendered
+more intensely phosphorescent. For want of time I have,
+however, not been able to perform the experiment.</p>
+
+<p>Another observation made by me was that when the specific
+inductive capacity of the medium between the primary and
+secondary is increased, the inductive effect is augmented. This
+is roughly illustrated in Fig. 215. In this case luminosity was
+excited in an exhausted tube or bulb <small>B</small> and a glass tube <small>T</small> slipped
+between the primary and the bulb, when the effect pointed out
+was noted. Were the action wholly electromagnetic no change
+could possibly have been observed.</p>
+
+<p>I have likewise noted that when a bulb is surrounded by a
+wire closed upon itself and in the plane of the primary, the formation
+of the luminous circle within the bulb is not prevented.
+But if instead of the wire a broad strip of tinfoil is glued upon
+the bulb, the formation of the luminous band was prevented, because
+then the action was distributed over a greater surface. The
+effect of the closed tinfoil was no doubt of an electrostatic nature,
+for it presented a much greater resistance than the closed wire
+and produced therefore a much smaller electromagnetic effect.</p>
+
+<p>Some of the experiments of Prof. J. J. Thomson also would
+seem to show some electrostatic action. For instance, in the experiment
+with the bulb enclosed in a bell jar, I should think
+that when the latter is exhausted so far that the gas enclosed
+reaches the maximum conductivity, the formation of the circle
+in the bulb and jar is prevented because of the space surrounding
+the primary being highly conducting; when the jar is further
+exhausted, the conductivity of the space around the primary
+diminishes and the circles appear necessarily first in the bell jar,
+as the rarefied gas is nearer to the primary. But were the inductive
+effect very powerful, they would probably appear in the
+bulb also. If, however, the bell jar were exhausted to the highest
+degree they would very likely show themselves in the bulb<span class='pagenum'><a name="Page_402" id="Page_402">[Pg 402]</a></span>
+only, that is, supposing the vacuous space to be non-conducting.
+On the assumption that in these phenomena electrostatic actions
+are concerned we find it easily explicable why the introduction
+of mercury or the heating of the bulb prevents the formation of
+the luminous band or shortens the after-glow; and also why in
+some cases a platinum wire may prevent the excitation of the
+tube. Nevertheless some of the experiments of Prof. J. J.
+Thomson would seem to indicate an electromagnetic effect. I
+may add that in one of my experiments in which a vacuum was
+produced by the Torricellian method, I was unable to produce
+the luminous band, but this may have been due to the weak exciting
+current employed.</p>
+
+<p>My principal argument is the following: I have experimentally
+proved that if the same discharge which is barely sufficient
+to excite a luminous band in the bulb when passed through the
+primary circuit be so directed as to exalt the electrostatic inductive
+effect&mdash;namely, by converting upwards&mdash;an exhausted tube,
+devoid of electrodes, may be excited at a distance of several feet.</p>
+
+<hr style='width: 15%;' />
+
+<h5>SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES.<a name="FNanchor_9_9" id="FNanchor_9_9"></a><a href="#Footnote_9_9" class="fnanchor">[9]</a></h5>
+
+<h5>BY PROF. J. J. THOMSON, M.A., F.R.S.</h5>
+
+
+
+<div  class="blockquot">
+<p>The phenomena of vacuum discharges were, Prof. Thomson said, greatly
+simplified when their path was wholly gaseous, the complication of the dark
+space surrounding the negative electrode, and the stratifications so commonly
+observed in ordinary vacuum tubes, being absent. To produce discharges in
+tubes devoid of electrodes was, however, not easy to accomplish, for the only
+available means of producing an electromotive force in the discharge circuit
+was by electro-magnetic induction. Ordinary methods of producing variable
+induction were valueless, and recourse was had to the oscillatory discharge of a
+<span class='pagenum'><a name="Page_403" id="Page_403">[Pg 403]</a></span>Leyden jar, which combines the two essentials of a current whose maximum
+value is enormous, and whose rapidity of alternation is immensely great. The
+discharge circuits, which may take the shape of bulbs, or of tubes bent in the
+form of coils, were placed in close proximity to glass tubes filled with mercury,
+which formed the path of the oscillatory discharge. The parts thus corresponded
+to the windings of an induction coil, the vacuum tubes being the secondary,
+and the tubes filled with mercury the primary. In such an apparatus
+the Leyden jar need not be large, and neither primary nor secondary need have
+many turns, for this would increase the self-induction of the former, and
+lengthen the discharge path in the latter. Increasing the self-induction of the
+primary reduces the <span class="smcap">e. m. f.</span> induced in the secondary, whilst lengthening the
+secondary does not increase the <span class="smcap">e. m. f.</span> per unit length. The two or three
+turns, as shown in Fig. 216, in each, were found to be quite sufficient, and, on
+discharging the Leyden jar between two highly polished knobs in the primary
+circuit, a plain uniform band of light was seen to pass round the secondary.
+An exhausted bulb, Fig. 217, containing traces of oxygen was placed within a
+primary spiral of three turns, and, on passing the jar discharge, a circle of light
+was seen within the bulb in close proximity to the primary circuit, accompanied
+by a purplish glow, which lasted for a second or more. On heating the
+bulb, the duration of the glow was greatly diminished, and it could be instantly
+extinguished by the presence of an electro-magnet. Another exhausted
+bulb, Fig. 218, surrounded by a primary spiral, was contained in a bell-jar,
+and when the pressure of air in the jar was about that of the atmosphere, the
+secondary discharge occurred in the bulb, as is ordinarily the case. On exhausting
+the jar, however, the luminous discharge grew fainter, and a point
+was reached at which no secondary discharge was visible. Further exhaustion
+of the jar caused the secondary discharge to appear outside of the bulb. The
+fact of obtaining no luminous discharge, either in the bulb or jar, the author<span class='pagenum'><a name="Page_404" id="Page_404">[Pg 404]</a></span>
+could only explain on two suppositions, viz.: that under the conditions then existing
+the specific inductive capacity of the gas was very great, or that a discharge
+could pass without being luminous. The author had also observed
+that the conductivity of a vacuum tube without electrodes increased as the pressure
+diminished, until a certain point was reached, and afterwards diminished
+again, thus showing that the high resistance of a nearly perfect vacuum is in
+no way due to the presence of the electrodes. One peculiarity of the discharges
+was their local nature, the rings of light being much more sharply defined than
+was to be expected. They were also found to be most easily produced when
+the chain of molecules in the discharge were all of the same kind. For example,
+a discharge could be easily sent through a tube many feet long, but the
+introduction of a small pellet of mercury in the tube stopped the discharge,
+although the conductivity of the mercury was much greater than that of the
+vacuum. In some cases he had noticed that a very fine wire placed within a
+tube, on the side remote from the primary circuit, would prevent a luminous
+discharge in that tube.</p>
+
+<p>Fig. 219 shows an exhausted secondary coil of one loop containing bulbs;
+the discharge passed along the inner side of the bulbs, the primary coils being
+placed within the secondary.</p>
+</div>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_416.jpg" width="800" height="456" alt="Fig. 216, 217." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 216.</td><td class="caption">Fig. 217.</td></tr>
+</table>
+</div>
+
+<div class="figcenter" style="width: 766px;">
+<img src="images/oi_417.jpg" width="766" height="600" alt="Fig. 218, 219." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 218.</td><td class="caption">Fig. 219.</td></tr>
+</table>
+</div>
+
+<hr style='width: 15%;' />
+
+<p><a name="FNanchor_9_10" id="FNanchor_9_10"></a><a href="#Footnote_9_10" class="fnanchor">[9]</a>In <i>The Electrical Engineer</i> of August 12, I find some remarks
+of Prof. J. J. Thomson, which appeared originally in the
+London <i>Electrician</i> and which have a bearing upon some experiments
+described by me in your issue of July 1.</p>
+
+<p>I did not, as Prof. J. J. Thomson seems to believe, misunderstand
+his position in regard to the cause of the phenomena
+considered, but I thought that in his experiments, as well as in
+my own, electrostatic effects were of great importance. It did
+not appear, from the meagre description of his experiments, that
+all possible precautions had been taken to exclude these effects.
+I did not doubt that luminosity could be excited in a closed tube
+when electrostatic action is completely excluded. In fact, at the
+outset, I myself looked for a purely electrodynamic effect and
+believed that I had obtained it. But many experiments performed
+at that time proved to me that the electrostatic effects
+were generally of far greater importance, and admitted of a more
+satisfactory explanation of most of the phenomena observed.</p>
+
+<p>In using the term <i>electrostatic</i> I had reference rather to the
+nature of the action than to a stationary condition, which is the
+usual acceptance of the term. To express myself more clearly,
+I will suppose that near a closed exhausted tube be placed a small
+sphere charged to a very high potential. The sphere would act
+inductively upon the tube, and by distributing electricity over
+<span class='pagenum'><a name="Page_405" id="Page_405">[Pg 405]</a></span>the same would undoubtedly produce luminosity (if the potential
+be sufficiently high), until a permanent condition would be
+reached. Assuming the tube to be perfectly well insulated,
+there would be only one instantaneous flash during the act of
+distribution. This would be due to the electrostatic action
+simply.</p>
+
+<p>But now, suppose the charged sphere to be moved at short intervals
+with great speed along the exhausted tube. The tube
+would now be permanently excited, as the moving sphere would
+cause a constant redistribution of electricity and collisions of the
+molecules of the rarefied gas. We would still have to deal with
+an electrostatic effect, and in addition an electrodynamic effect
+would be observed. But if it were found that, for instance, the
+effect produced depended more on the specific inductive capacity
+than on the magnetic permeability of the medium&mdash;which
+would certainly be the case for speeds incomparably lower than
+that of light&mdash;then I believe I would be justified in saying that
+the effect produced was more of an electrostatic nature. I do
+not mean to say, however, that any similar condition prevails in
+the case of the discharge of a Leyden jar through the primary,
+but I think that such an action would be desirable.</p>
+
+<p>It is in the spirit of the above example that I used the terms
+"more of an electrostatic nature," and have investigated the influence
+of bodies of high specific inductive capacity, and observed,
+for instance, the importance of the quality of glass of which the
+tube is made. I also endeavored to ascertain the influence of a
+medium of high permeability by using oxygen. It appeared
+from rough estimation that an oxygen tube when excited under
+similar conditions&mdash;that is, as far as could be determined&mdash;gives
+more light; but this, of course, may be due to many causes.</p>
+
+<p>Without doubting in the least that, with the care and precautions
+taken by Prof. J. J. Thomson, the luminosity excited was
+due solely to electrodynamic action, I would say that in many
+experiments I have observed curious instances of the ineffectiveness
+of the screening, and I have also found that the electrification
+through the air is often of very great importance, and may,
+in some cases, determine the excitation of the tube.</p>
+
+<p>In his original communication to the <i>Electrician</i>, Prof. J. J.
+Thomson refers to the fact that the luminosity in a tube near a
+wire through which a Leyden jar was discharged was noted by
+Hittorf. I think that the feeble luminous effect referred to has<span class='pagenum'><a name="Page_406" id="Page_406">[Pg 406]</a></span>
+been noted by many experimenters, but in my experiments the
+effects were much more powerful than those usually noted.</p>
+
+<p>The following is the communication<a name="FNanchor_10_11" id="FNanchor_10_11"></a><a href="#Footnote_10_11" class="fnanchor">[10]</a> referred to:&mdash;</p>
+
+<hr style='width: 15%;' />
+
+<div  class="blockquot">
+<p>"Mr. Tesla seems to ascribe the effects he observed to electrostatic action,
+and I have no doubt, from the description he gives of his method of conducting
+his experiments, that in them electrostatic action plays a very important
+part. He seems, however, to have misunderstood my position with respect to
+the cause of these discharges, which is not, as he implies, that luminosity in
+tubes without electrodes cannot be produced by electrostatic action, but that it
+can also be produced when this action is excluded. As a matter of fact, it is
+very much easier to get the luminosity when these electrostatic effects are
+operative than when they are not. As an illustration of this I may mention
+that the first experiment I tried with the discharge of a Leyden jar produced
+luminosity in the tube, but it was not until after six weeks' continuous experimenting
+that I was able to get a discharge in the exhausted tube which I was
+satisfied was due to what is ordinarily called electrodynamic action. It is advisable
+to have a clear idea of what we mean by electrostatic action. If,
+previous to the discharge of the jar, the primary coil is raised to a high potential,
+it will induce over the glass of the tube a distribution of electricity.
+When the potential of the primary suddenly falls, this electrification will redistribute
+itself, and may pass through the rarefied gas and produce luminosity
+in doing so. Whilst the discharge of the jar is going on, it is difficult, and,
+from a theoretical point of view, undesirable, to separate the effect into parts,
+one of which is called electrostatic, the other electromagnetic; what we can
+prove is that in this case the discharge is not such as would be produced by
+electromotive forces derived from a potential function. In my experiments the
+primary coil was connected to earth, and, as a further precaution, the primary
+was separated from the discharge tube by a screen of blotting paper, moistened
+with dilute sulphuric acid, and connected to earth. Wet blotting paper is a
+sufficiently good conductor to screen off a stationary electrostatic effect, though
+it is not a good enough one to stop waves of alternating electromotive intensity.
+When showing the experiments to the Physical Society I could not, of course,
+keep the tubes covered up, but, unless my memory deceives me, I stated the
+precautions which had been taken against the electrostatic effect. To correct
+misapprehension I may state that I did not read a formal paper to the Society,
+my object being to exhibit a few of the most typical experiments. The account
+of the experiments in the <i>Electrician</i> was from a reporter's note, and was
+not written, or even read, by me. I have now almost finished writing out, and
+hope very shortly to publish, an account of these and a large number of allied
+experiments, including some analogous to those mentioned by Mr. Tesla on the
+effect of conductors placed near the discharge tube, which I find, in some
+cases, to produce a diminution, in others an increase, in the brightness of the
+discharge, as well as some on the effect of the presence of substances of large
+specific inductive capacity. These seem to me to admit of a satisfactory explanation,
+for which, however, I must refer to my paper."</p>
+</div>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_407" id="Page_407">[Pg 407]</a></span></p>
+<h1><small><a name="PART_III" id="PART_III"></a>PART III.</small><br /><br />
+
+MISCELLANEOUS INVENTIONS AND<br />
+WRITINGS.</h1>
+<p><span class='pagenum'><a name="Page_408" id="Page_408">[Pg 408]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_409" id="Page_409">[Pg 409]</a></span></p>
+<h2><a name="CHAPTER_XXXIII" id="CHAPTER_XXXIII"></a>CHAPTER XXXIII.</h2>
+
+<h3><span class="smcap">Method of Obtaining Driect From Alternating Currents.</span></h3>
+
+
+<p>This method consists in obtaining direct from alternating
+currents, or in directing the waves of an alternating current so as
+to produce direct or substantially direct currents by developing
+or producing in the branches of a circuit including a source of alternating
+currents, either permanently or periodically, and by
+electric, electro-magnetic, or magnetic agencies, manifestations of
+energy, or what may be termed active resistances of opposite
+electrical character, whereby the currents or current waves of opposite
+sign will be diverted through different circuits, those of
+one sign passing over one branch and those of opposite sign over
+the other.</p>
+
+<p>We may consider herein only the case of a circuit divided into
+two paths, inasmuch as any further subdivision involves merely
+an extension of the general principle. Selecting, then, any circuit
+through which is flowing an alternating current, Mr. Tesla
+divides such circuit at any desired point into two branches or
+paths. In one of these paths he inserts some device to create
+an electromotive force counter to the waves or impulses of current
+of one sign and a similar device in the other branch which
+opposes the waves of opposite sign. Assume, for example, that
+these devices are batteries, primary or secondary, or continuous
+current dynamo machines. The waves or impulses of opposite
+direction composing the main current have a natural tendency to
+divide between the two branches; but by reason of the opposite
+electrical character or effect of the two branches, one will offer
+an easy passage to a current of a certain direction, while the other
+will offer a relatively high resistance to the passage of the same
+current. The result of this disposition is, that the waves of current
+of one sign will, partly or wholly, pass over one of the paths
+or branches, while those of the opposite sign pass over the other.
+There may thus be obtained from an alternating current two or
+more direct currents without the employment of any commutator<span class='pagenum'><a name="Page_410" id="Page_410">[Pg 410]</a></span>
+such as it has been heretofore regarded as necessary to use. The
+current in either branch may be used in the same way and for
+the same purposes as any other direct current&mdash;that is, it may be
+made to charge secondary batteries, energize electro-magnets, or
+for any other analogous purpose.</p>
+
+<p>Fig. 220 represents a plan of directing the alternating currents
+by means of devices purely electrical in character. Figs. 221,
+222, 223, 224, 225, and 226 are diagrams illustrative of other
+ways of carrying out the invention.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_424.jpg" width="640" height="300" alt="Fig. 220." title="" />
+<span class="caption">Fig. 220.</span>
+</div>
+
+
+<p>In Fig. 220, <small>A</small> designates a generator of alternating currents,
+and <small>B B</small> the main or line circuit therefrom. At any given point
+in this circuit at or near which it is desired to obtain direct currents,
+the circuit <b>B</b> is divided into two paths or branches <small>C D</small>. In
+each of these branches is placed an electrical generator, which
+for the present we will assume produces direct or continuous currents.
+The direction of the current thus produced is opposite in
+one branch to that of the current in the other branch, or, considering
+the two branches as forming a closed circuit, the generators
+<small>E F</small> are connected up in series therein, one generator in
+each part or half of the circuit. The electromotive force of the
+current sources <small>E</small> and <small>F</small> may be equal to or higher or lower than
+the electromotive forces in the branches <small>C D</small>, or between the points
+<small>X</small> and <small>Y</small> of the circuit <small>B B</small>. If equal, it is evident that current
+waves of one sign will be opposed in one branch and assisted in
+the other to such an extent that all the waves of one sign will
+pass over one branch and those of opposite sign over the other.
+If, on the other hand, the electromotive force of the sources <small>E F</small>
+be lower than that between <small>X</small> and <small>Y</small>, the currents in both
+branches will be alternating, but the waves of one sign will preponderate.
+One of the generators or sources of current <small>E</small> or <small>F</small>
+may be dispensed with; but it is preferable to employ both, if<span class='pagenum'><a name="Page_411" id="Page_411">[Pg 411]</a></span>
+they offer an appreciable resistance, as the two branches will be
+thereby better balanced. The translating or other devices to be
+acted upon by the current are designated by the letters <small>G</small>, and
+they are inserted in the branches <small>C D</small> in any desired manner; but
+in order to better preserve an even balance between the branches
+due regard should, of course, be had to the number and character
+of the devices.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_425.jpg" width="640" height="284" alt="Fig. 221." title="" />
+<span class="caption">Fig. 221.</span>
+</div>
+
+<p>Figs. 221, 222, 223, and 224 illustrate what may termed "electro-magnetic"
+devices for accomplishing a similar result&mdash;that is
+to say, instead of producing directly by a generator an electromotive
+force in each branch of the circuit, Mr. Tesla establishes
+a field or fields of force and leads the branches through the same
+in such manner that an active opposition of opposite effect or direction
+will be developed therein by the passage, or tendency to
+pass, of the alternations of current. In Fig. 221, for example, <small>A</small> is
+the generator of alternating currents, <small>B B</small> the line circuit, and <small>C D</small>
+the branches over which the alternating currents are directed. In
+each branch is included the secondary of a transformer or induction
+coil, which, since they correspond in their functions to the
+batteries of the previous figure, are designated by the letters <small>E F</small>.
+The primaries <small>H H'</small> of the induction coils or transformers are
+connected either in parallel or series with a source of direct or
+continuous currents <small>I</small>, and the number of convolutions is so calculated
+for the strength of the current from <small>I</small> that the cores <small>J J'</small> will be saturated. The connections are such that the conditions
+in the two transformers are of opposite character&mdash;that is to say,
+the arrangement is such that a current wave or impulse corresponding
+in direction with that of the direct current in one primary,
+as <small>H</small>, is of opposite direction to that in the other primary <small>H'</small>.
+It thus results that while one secondary offers a resistance or op<span class='pagenum'><a name="Page_412" id="Page_412">[Pg 412]</a></span>position
+to the passage through it of a wave of one sign, the other
+secondary similarly opposes a wave of opposite sign. In consequence,
+the waves of one sign will, to a greater or less extent, pass
+by way of one branch, while those of opposite sign in like manner
+pass over the other branch.</p>
+
+<p>In lieu of saturating the primaries by a source of continuous
+current, we may include the primaries in the branches <small>C D</small>, respectively,
+and periodically short-circuit by any suitable mechanical
+devices&mdash;such as an ordinary revolving commutator&mdash;their
+secondaries. It will be understood, of course, that the rotation
+and action of the commutator must be in synchronism or in
+proper accord with the periods of the alternations in order to
+secure the desired results. Such a disposition is represented
+diagrammatically in Fig. 222. Corresponding to the previous
+figures, <small>A</small> is the generator of alternating currents, <small>B B</small> the line,
+and <small>C D</small> the two branches for the direct currents. In branch <small>C</small>
+are included two primary coils <small>E E'</small>, and in branch <small>D</small> are two
+similar primaries <small>F F'</small> The corresponding secondaries for these
+coils and which are on the same subdivided cores <small>J</small> or <small>J'</small>, are in
+circuits the terminals of which connect to opposite segments
+<small>K K'</small>, and <small>L L'</small>, respectively, of a commutator. Brushes <i>b b</i> bear
+upon the commutator and alternately short-circuit the plates <small>K</small>
+and <small>K'</small>, and <small>L</small> and <small>L'</small>, through a connection <i>c</i>. It is obvious that
+either the magnets and commutator, or the brushes, may revolve.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_426.jpg" width="640" height="347" alt="Fig. 222." title="" />
+<span class="caption">Fig. 222.</span>
+</div>
+
+
+<p>The operation will be understood from a consideration of the
+effects of closing or short-circuiting the secondaries. For example,
+if at the instant when a given wave of current passes, one<span class='pagenum'><a name="Page_413" id="Page_413">[Pg 413]</a></span>
+set of secondaries be short-circuited, nearly all the current flows
+through the corresponding primaries; but the secondaries of the
+other branch being open-circuited, the self-induction in the
+primaries is highest, and hence little or no current will pass
+through that branch. If, as the current alternates, the secondaries
+of the two branches are alternately short-circuited, the
+result will be that the currents of one sign pass over one branch
+and those of the opposite sign over the other. The disadvantages
+of this arrangement, which would seem to result from the
+employment of sliding contacts, are in reality very slight, inasmuch
+as the electromotive force of the secondaries may be made
+exceedingly low, so that sparking at the brushes is avoided.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_427.jpg" width="640" height="269" alt="Fig. 223." title="" />
+<span class="caption">Fig. 223.</span>
+</div>
+
+
+<p>Fig. 223 is a diagram, partly in section, of another plan of
+carrying out the invention. The circuit <small>B</small> in this case is divided,
+as before, and each branch includes the coils of both the fields
+and revolving armatures of two induction devices. The armatures
+<small>O P</small> are preferably mounted on the same shaft, and are adjusted
+relatively to one another in such manner that when the
+self-induction in one branch, as <small>C</small>, is maximum, in the other branch
+<small>D</small> it is minimum. The armatures are rotated in synchronism with
+the alternations from the source <small>A</small>. The winding or position
+of the armature coils is such that a current in a given direction
+passed through both armatures would establish in one, poles similar
+to those in the adjacent poles of the field, and in the other,
+poles unlike the adjacent field poles, as indicated by <i>n n s s</i> in
+the diagram. If the like poles are presented, as shown in circuit
+<small>D</small>, the condition is that of a closed secondary upon a primary,
+or the position of least inductive resistance; hence a given alternation
+of current will pass mainly through <small>D</small>. A half revolution
+of the armatures produces an opposite effect and the succeeding<span class='pagenum'><a name="Page_414" id="Page_414">[Pg 414]</a></span>
+current impulse passes through <small>C</small>. Using this figure as an illustration,
+it is evident that the fields <small>N M</small> may be permanent magnets
+or independently excited and the armatures <small>O P</small> driven, as in
+the present case, so as to produce alternate currents, which will
+set up alternately impulses of opposite direction in the two
+branches <small>D C</small>, which in such case would include the armature circuits
+and translating devices only.</p>
+
+<p>In Fig. 224 a plan alternative with that shown in Fig. 222 is
+illustrated. In the previous case illustrated, each branch <small>C</small> and <small>D</small>
+contained one or more primary coils, the secondaries of which
+were periodically short circuited in synchronism with the alternations
+of current from the main source <small>A</small>, and for this purpose
+a commutator was employed. The latter may, however, be dispensed
+with and an armature with a closed coil substituted.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_428.jpg" width="640" height="261" alt="Fig. 224." title="" />
+<span class="caption">Fig. 224.</span>
+</div>
+
+
+<p>Referring to Fig. 224 in one of the branches, as <small>C</small>, are two coils
+<small>M'</small>, wound on laminated cores, and in the other branches <small>D</small> are
+similar coils <small>N'</small>. A subdivided or laminated armature <small>O'</small>, carrying
+a closed coil <small>R'</small>, is rotatably supported between the coils <small>M' N'</small>,
+as shown. In the position shown&mdash;that is, with the coil <small>R'</small> parallel
+with the convolutions of the primaries <small>N' M'</small>&mdash;practically the
+whole current will pass through branch <small>D</small>, because the self-induction
+in coils <small>M' M'</small> is maximum. If, therefore, the armature
+and coil be rotated at a proper speed relatively to the periods or
+alternations of the source <small>A</small>, the same results are obtained as in
+the case of Fig. 222.</p>
+
+<p>Fig. 225 is an instance of what may be called, in distinction to
+the others, a "magnetic" means of securing the result. <small>V</small> and
+<small>W</small> are two strong permanent magnets provided with armatures
+<small>V' W'</small>, respectively. The armatures are made of thin lamin&aelig; of
+soft iron or steel, and the amount of magnetic metal which they<span class='pagenum'><a name="Page_415" id="Page_415">[Pg 415]</a></span>
+contain is so calculated that they will be fully or nearly saturated
+by the magnets. Around the armatures are coils <small>E F</small>, contained,
+respectively, in the circuits <small>C</small> and <small>D</small>. The connections and electrical
+conditions in this case are similar to those in Fig. 221,
+except that the current source of <small>I</small>, Fig. 221, is dispensed with
+and the saturation of the core of coils <small>E F</small> obtained from the permanent
+magnets.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_429.jpg" width="640" height="286" alt="Fig. 225." title="" />
+<span class="caption">Fig. 225.</span>
+</div>
+
+<p>The previous illustrations have all shown the two branches or
+paths containing the translating or induction devices as in derivation
+one to the other; but this is not always necessary. For
+example, in Fig. 226, <small>A</small> is an alternating-current generator; <small>B B</small>,
+the line wires or circuit. At any given point in the circuit let
+us form two paths, as <small>D D'</small>, and at another point two paths, as <small>C</small>
+<small>C'</small>. Either pair or group of paths is similar to the previous dispositions
+with the electrical source or induction device in one
+branch only, while the two groups taken together form the
+obvious equivalent of the cases in which an induction device or
+generator is included in both branches. In one of the paths, as
+<small>D</small>, are included the devices to be operated by the current. In
+the other branch, as <small>D'</small>, is an induction device that opposes the
+current impulses of one direction and directs them through the
+branch <small>D</small>. So, also, in branch <small>C</small> are translating devices <small>G</small>, and in
+branch <small>C'</small> an induction device or its equivalent that diverts
+through <small>C</small> impulses of opposite direction to those diverted by the
+device in branch <small>D'</small>. The diagram shows a special form of induction
+device for this purpose. <small>J J'</small> are the cores, formed with
+pole-pieces, upon which are wound the coils <small>M N</small>. Between these
+pole-pieces are mounted at right angles to one another the magnetic
+armatures <small>O P</small>, preferably mounted on the same shaft and<span class='pagenum'><a name="Page_416" id="Page_416">[Pg 416]</a></span>
+designed to be rotated in synchronism with the alternations of
+current. When one of the armatures is in line with the poles or
+in the position occupied by armature <small>P</small>, the magnetic circuit of
+the induction device is practically closed; hence there will be
+the greatest opposition to the passage of a current through coils
+<small>N N</small>. The alternation will therefore pass by way of branch <small>D</small>.
+At the same time, the magnetic circuit of the other induction
+device being broken by the position of the armature <small>O</small>, there will
+be less opposition to the current in coils <small>M</small>, which will shunt the
+current from branch <small>C</small>. A reversal of the current being attended
+by a shifting of the armatures, the opposite effect is produced.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_430.jpg" width="640" height="370" alt="Fig. 226." title="" />
+<span class="caption">Fig. 226.</span>
+</div>
+
+
+<p>Other modifications of these methods are possible, but need
+not be pointed out. In all these plans, it will be observed, there
+is developed in one or all of these branches of a circuit from a
+source of alternating currents, an active (as distinguished from a
+dead) resistance or opposition to the currents of one sign, for the
+purpose of diverting the currents of that sign through the other
+or another path, but permitting the currents of opposite sign to
+pass without substantial opposition.</p>
+
+<p>Whether the division of the currents or waves of current of
+opposite sign be effected with absolute precision or not is immaterial,
+since it will be sufficient if the waves are only partially
+diverted or directed, for in such case the preponderating influence
+in each branch of the circuit of the waves of one sign secures
+the same practical results in many if not all respects as though
+the current were direct and continuous.<span class='pagenum'><a name="Page_417" id="Page_417">[Pg 417]</a></span></p>
+
+<p>An alternating and a direct current have been combined so that
+the waves of one direction or sign were partially or wholly overcome
+by the direct current; but by this plan only one set of alternations
+are utilized, whereas by the system just described the
+entire current is rendered available. By obvious applications of
+this discovery Mr. Tesla is enabled to produce a self-exciting alternating
+dynamo, or to operate direct current meters on alternating-current
+circuits or to run various devices&mdash;such as arc lamps&mdash;by
+direct currents in the same circuit with incandescent lamps
+or other devices operated by alternating currents.</p>
+
+<p>It will be observed that if an intermittent counter or opposing
+force be developed in the branches of the circuit and of higher
+electromotive force than that of the generator, an alternating
+current will result in each branch, with the waves of one sign
+preponderating, while a constantly or uniformly acting opposition
+in the branches of higher electromotive force than the
+generator would produce a pulsating current, which conditions
+would be, under some circumstances, the equivalent of those described.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_418" id="Page_418">[Pg 418]</a></span></p>
+<h2><a name="CHAPTER_XXXIV" id="CHAPTER_XXXIV"></a>CHAPTER XXXIV.</h2>
+
+<h3><span class="smcap">Condensers with Plates in Oil.</span></h3>
+
+<p>In experimenting with currents of high frequency and high
+potential, Mr. Tesla has found that insulating materials such as
+glass, mica, and in general those bodies which possess the highest
+specific inductive capacity, are inferior as insulators in such devices
+when currents of the kind described are employed compared
+with those possessing high insulating power, together with a smaller
+specific inductive capacity; and he has also found that it is very desirable
+to exclude all gaseous matter from the apparatus, or any access
+of the same to the electrified surfaces, in order to prevent heating
+by molecular bombardment and the loss or injury consequent
+thereon. He has therefore devised a method to accomplish these
+results and produce highly efficient and reliable condensers, by
+using oil as the dielectric<a name="FNanchor_11_12" id="FNanchor_11_12"></a><a href="#Footnote_11_12" class="fnanchor">[11]</a>. The plan admits of a particular
+con<span class='pagenum'><a name="Page_419" id="Page_419">[Pg 419]</a></span>struction of condenser, in which the distance between the plates
+is adjustable, and of which he takes advantage.</p>
+
+<div class="figcenter" style="width: 700px;">
+<div class="figleft" style="width: 407px;">
+<img src="images/fig227.jpg" width="407" height="220" alt="Fig. 227." title="" />
+<span class="caption">Fig. 227.</span>
+</div>
+<div class="figright" style="width: 231px;">
+<img src="images/oi_432.jpg" width="231" height="220" alt="Fig. 228." title="" />
+<span class="caption">Fig. 228.</span>
+</div>
+</div>
+
+<p>In the accompanying illustrations, Fig. 227 is a section of a
+condenser constructed in accordance with this principle and having
+stationary plates; and Fig. 228 is a similar view of a condenser
+with adjustable plates.</p>
+
+<p>Any suitable box or receptacle <small>A</small> may be used to contain the
+plates or armatures. These latter are designated by <small>B</small> and <small>C</small> and
+are connected, respectively, to terminals <small>D</small> and <small>E</small>, which pass out
+through the sides of the case. The plates ordinarily are separated
+by strips of porous insulating material <small>F</small>, which are used merely
+for the purpose of maintaining them in position. The space
+within the can is filled with oil <small>G</small>. Such a condenser will prove
+highly efficient and will not become heated or permanently injured.</p>
+
+<p>In many cases it is desirable to vary or adjust the capacity of
+a condenser, and this is provided for by securing the plates to adjustable
+supports&mdash;as, for example, to rods <small>H</small>&mdash;passing through
+stuffing boxes <small>K</small> in the sides of case <small>A</small> and furnished with nuts <small>L</small>,
+the ends of the rods being threaded for engagement with the
+nuts.</p>
+
+<p>It is well known that oils possess insulating properties, and it
+has been a common practice to interpose a body of oil between
+two conductors for purposes of insulation; but Mr. Tesla believes
+he has discovered peculiar properties in oils which render
+them very valuable in this particular form of device.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_420" id="Page_420">[Pg 420]</a></span></p>
+<h2><a name="CHAPTER_XXXV" id="CHAPTER_XXXV"></a>CHAPTER XXXV.</h2>
+
+<h3><span class="smcap">Electrolytic Registering Meter.</span></h3>
+
+
+<p>An ingenious form of electrolytic meter attributable to Mr.
+Tesla is one in which a conductor is immersed in a solution, so
+arranged that metal may be deposited from the solution or taken
+away in such a manner that the electrical resistance of the conductor
+is varied in a definite proportion to the strength of the
+current the energy of which is to be computed, whereby this
+variation in resistance serves as a measure of the energy and also
+may actuate registering mechanism, whenever the resistance
+rises above or falls below certain limits.</p>
+
+<p>In carrying out this idea Mr. Tesla employs an electrolytic
+cell, through which extend two conductors parallel and
+in close proximity to each other. These conductors he connects
+in series through a resistance, but in such manner that there is
+an equal difference of potential between them throughout their
+entire extent. The free ends or terminals of the conductors are
+connected either in series in the circuit supplying the current to
+the lamps or other devices, or in parallel to a resistance in the
+circuit and in series with the current consuming devices. Under
+such circumstances a current passing through the conductors
+establishes a difference of potential between them which is proportional
+to the strength of the current, in consequence of which
+there is a leakage of current from one conductor to the other
+across the solution. The strength of this leakage current is proportional
+to the difference of potential, and, therefore, in proportion
+to the strength of the current passing through the conductors.
+Moreover, as there is a constant difference of potential between
+the two conductors throughout the entire extent that is exposed
+to the solution, the current density through such solution is the
+same at all corresponding points, and hence the deposit is uniform
+along the whole of one of the conductors, while the metal
+is taken away uniformly from the other. The resistance of one
+conductor is by this means diminished, while that of the other is<span class='pagenum'><a name="Page_421" id="Page_421">[Pg 421]</a></span>
+increased, both in proportion to the strength of the current passing
+through the conductors. From such variation in the resistance
+of either or both of the conductors forming the positive
+and negative electrodes of the cell, the current energy expended
+may be readily computed. Figs. 229 and 230 illustrate two
+forms of such a meter.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_435.jpg" width="640" height="471" alt="Fig. 229." title="" />
+<span class="caption">Fig. 229.</span>
+</div>
+
+
+<p>In Fig. 229 <small>G</small> designates a direct-current generator. <small>L L</small> are
+the conductors of the circuit extending therefrom. <small>A</small> is a tube
+of glass, the ends of which are sealed, as by means of insulating
+plugs or caps <small>B B</small>. <small>C C'</small> are two conductors extending
+through the tube <small>A</small>, their ends passing out through the plugs <small>B</small> to
+terminals thereon. These conductors may be corrugated or
+formed in other proper ways to offer the desired electrical resistance.
+<small>R</small> is a resistance connected in series with the two conductors
+<small>C C'</small>, which by their free terminals are connected up in
+circuit with one of the conductors <small>L</small>.</p>
+
+<p>The method of using this device and computing by means
+thereof the energy of the current will be readily understood.
+First, the resistances of the two conductors <small>C C'</small>, respectively, are
+accurately measured and noted. Then a known current is passed
+through the instrument for a given time, and by a second measurement
+the increase and diminution of the resistances of the two
+conductors are respectively taken. From these data the constant is<span class='pagenum'><a name="Page_422" id="Page_422">[Pg 422]</a></span>
+obtained&mdash;that is to say, for example, the increase of resistance of
+one conductor or the diminution of the resistance of the other per
+lamp hour. These two measurements evidently serve as a check,
+since the gain of one conductor should equal the loss of the other.
+A further check is afforded by measuring both wires in series with
+the resistance, in which case the resistance of the whole should
+remain constant.</p>
+
+<div class="figcenter" style="width: 631px;">
+<img src="images/oi_436.jpg" width="631" height="480" alt="Fig. 230." title="" />
+<span class="caption">Fig. 230.</span>
+</div>
+
+<p>In Fig. 230 the conductors <small>C C'</small> are connected in parallel, the
+current device at <small>X</small> passing in one branch first through a resistance
+<small>R'</small> and then through conductor <small>C</small>, while on the other branch
+it passes first through conductor <small>C'</small>, and then through resistance
+<small>R''</small>. The resistances <small>R' R''</small> are equal, as also are the resistances of
+the conductors <small>C C'</small>. It is, moreover, preferable that the respective
+resistances of the conductors <small>C C'</small> should be a known and convenient
+fraction of the coils or resistances <small>R' R''</small>. It will be observed
+that in the arrangement shown in Fig. 230 there is a constant
+potential difference between the two conductors <small>C C'</small> throughout
+their entire length.</p>
+
+<p>It will be seen that in both cases illustrated, the proportionality
+of the increase or decrease of resistance to the current strength
+will always be preserved, for what one conductor gains the other
+loses, and the resistances of the conductors <small>C C'</small> being small as<span class='pagenum'><a name="Page_423" id="Page_423">[Pg 423]</a></span>
+compared with the resistances in series with them. It will be
+understood that after each measurement or registration of a given
+variation of resistance in one or both conductors, the direction of
+the current should be changed or the instrument reversed, so that
+the deposit will be taken from the conductor which has gained
+and added to that which has lost. This principle is capable of
+many modifications. For instance, since there is a section of the
+circuit&mdash;to wit, the conductor <small>C</small> or <small>C'</small>&mdash;that varies in resistance in
+proportion to the current strength, such variation may be utilized,
+as is done in many analogous cases, to effect the operation of
+various automatic devices, such as registers. It is better, however,
+for the sake of simplicity to compute the energy by measurements
+of resistance.</p>
+
+<p>The chief advantages of this arrangement are, first, that it is
+possible to read off directly the amount of the energy expended
+by means of a properly constructed ohm-meter and without resorting
+to weighing the deposit; secondly it is not necessary to
+employ shunts, for the whole of the current to be measured may
+be passed through the instrument; third, the accuracy of the instrument
+and correctness of the indications are but slightly affected
+by changes in temperature. It is also said that such meters
+have the merit of superior economy and compactness, as well as
+of cheapness in construction. Electrolytic meters seem to need
+every auxiliary advantage to make them permanently popular and
+successful, no matter how much ingenuity may be shown in their
+design.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_424" id="Page_424">[Pg 424]</a></span></p>
+<h2><a name="CHAPTER_XXXVI" id="CHAPTER_XXXVI"></a>CHAPTER XXXVI.</h2>
+
+<h3><span class="smcap">Thermo-Magnetic Motors and Pyro-Magnetic Generators.</span></h3>
+
+
+<p>No electrical inventor of the present day dealing with the
+problems of light and power considers that he has done himself
+or his opportunities justice until he has attacked the subject of
+thermo-magnetism. As far back as the beginning of the seventeenth
+century it was shown by Dr. William Gilbert, the father
+of modern electricity, that a loadstone or iron bar when heated
+to redness loses its magnetism; and since that time the influence
+of heat on the magnetic metals has been investigated frequently,
+though not with any material or practical result.</p>
+
+<p>For a man of Mr. Tesla's inventive ability, the problems in
+this field have naturally had no small fascination, and though he
+has but glanced at them, it is to be hoped he may find time to
+pursue the study deeper and further. For such as he, the investigation
+must undoubtedly bear fruit. Meanwhile he has
+worked out one or two operative devices worthy of note.<a name="FNanchor_12_13" id="FNanchor_12_13"></a><a href="#Footnote_12_13" class="fnanchor">[12]</a> He
+obtains mechanical power by a reciprocating action resulting
+from the joint operations of heat, magnetism, and a spring or
+weight or other force&mdash;that is to say he subjects a body magnetized
+by induction or otherwise to the action of heat until the
+magnetism is sufficiently neutralized to allow a weight or spring
+to give motion to the body and lessen the action of the heat, so
+that the magnetism may be sufficiently restored to move the
+<span class='pagenum'><a name="Page_425" id="Page_425">[Pg 425]</a></span>body in the opposite direction, and again subject the same to the
+demagnetizing power of the heat.</p>
+
+<p>Use is made of either an electro-magnet or a permanent magnet,
+and the heat is directed against a body that is magnetized
+by induction, rather than directly against a permanent magnet,
+thereby avoiding the loss of magnetism that might result in the
+permanent magnet by the action of heat. Mr. Tesla also provides
+for lessening the volume of the heat or for intercepting the same
+during that portion of the reciprocation in which the cooling
+action takes place.</p>
+
+<p>In the diagrams are shown some of the numerous arrangements
+that may be made use of in carrying out this idea. In all
+of these figures the magnet-poles are marked <small>N S</small>, the armature
+<small>A</small>, the Bunsen burner or other source of heat <small>H</small>, the axis of motion
+<small>M</small>, and the spring or the equivalent thereof&mdash;namely, a
+weight&mdash;is marked <small>W</small>.</p>
+
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_439.jpg" width="800" height="370" alt="Fig. 232, 231, 233." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 232.</td><td class="caption">Fig. 231.</td><td class="caption">Fig. 233.</td></tr>
+</table>
+</div>
+
+
+<p>In Fig. 231 the permanent magnet <small>N</small> is connected with a frame,
+<small>F</small>, supporting the axis <small>M</small>, from which the arm <small>P</small> hangs, and at the
+lower end of which the armature <small>A</small> is supported. The stops 2
+and 3 limit the extent of motion, and the spring <small>W</small> tends to draw
+the armature <small>A</small> away from the magnet <small>N</small>. It will now be understood
+that the magnetism of <small>N</small> is sufficient to overcome the
+spring <small>W</small> and draw the armature <small>A</small> toward the magnet <small>N</small>. The
+heat acting upon the armature <small>A</small> neutralizes its induced magnetism
+sufficiently for the spring <small>W</small> to draw the armature A away
+from the magnet <small>N</small> and also from the heat at <small>H</small>. The armature
+now cools, and the attraction of the magnet <small>N</small> overcomes the
+spring <small>W</small> and draws the armature <small>A</small> back again above the burner<span class='pagenum'><a name="Page_426" id="Page_426">[Pg 426]</a></span>
+<small>H</small>, so that the same is again heated and the operations are repeated.
+The reciprocating movements thus obtained are employed
+as a source of mechanical power in any desired manner.
+Usually a connecting-rod to a crank upon a fly-wheel shaft would
+be made use of, as indicated in Fig. 240.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_440.jpg" width="800" height="369" alt="Fig. 234, 236, 235." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 234.</td><td class="caption">Fig. 236.</td><td class="caption">Fig. 235.</td></tr>
+</table>
+</div>
+
+<p>Fig. 232 represents the same parts as before described; but an
+electro-magnet is illustrated in place of a permanent magnet.
+The operations, however, are the same.</p>
+
+<p>In Fig. 233 are shown the same parts as in Figs. 231 and 232,
+but they are differently arranged. The armature <small>A</small>, instead of
+swinging, is stationary and held by arm <small>P'</small>, and the core <small>N S</small> of
+the electro-magnet is made to swing within the helix <small>Q</small>, the
+core being suspended by the arm <small>P</small> from the pivot <small>M</small>. A shield,
+<small>R</small>, is connected with the magnet-core and swings with it, so
+that after the heat has demagnetized the armature <small>A</small> to such an
+extent that the spring <small>W</small> draws the core <small>N S</small> away from the armature
+<small>A</small>, the shield <small>R</small> comes between the flame <small>H</small> and armature <small>A</small>,
+thereby intercepting the action of the heat and allowing the armature
+to cool, so that the magnetism, again preponderating,
+causes the movement of the core <small>N S</small> toward the armature <small>A</small> and
+the removal of the shield <small>R</small> from above the flame, so that the heat
+again acts to lessen or neutralize the magnetism. A rotary or
+other movement may be obtained from this reciprocation.</p>
+
+<p>Fig. 234 corresponds in every respect with Fig. 233, except
+that a permanent horseshoe-magnet, <small>N S</small> is represented as taking
+the place of the electro-magnet in Fig. 233.</p>
+
+<p>In Fig. 235 is shown a helix, <small>Q</small>, with an armature adapted to
+swing toward or from the helix. In this case there may be a soft<span class='pagenum'><a name="Page_427" id="Page_427">[Pg 427]</a></span>-iron
+core in the helix, or the armature may assume the form of a
+solenoid core, there being no permanent core within the helix.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_441.jpg" width="800" height="323" alt="Fig. 237, 238, 239." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 237.</td><td class="caption">Fig. 238.</td><td class="caption">Fig. 239.</td></tr>
+</table>
+</div>
+
+<p>Fig. 236 is an end view, and Fig. 237 a plan view, illustrating
+the method as applied to a swinging armature, <small>A</small>, and a stationary
+permanent magnet, <small>N S</small>. In this instance Mr. Tesla applies the
+heat to an auxiliary armature or keeper, <small>T</small>, which is adjacent to
+and preferably in direct contact with the magnet. This armature
+<small>T</small>, in the form of a plate of sheet-iron, extends across from
+one pole to the other and is of sufficient section to practically
+form a keeper for the magnet, so that when the armature <small>T</small> is
+cool nearly all the lines of force pass over the same and very little
+free magnetism is exhibited. Then the armature <small>A</small>, which swings
+freely on the pivots <small>M</small> in front of the poles <small>N S</small>, is very little attracted
+and the spring <small>W</small> pulls the same way from the poles into
+the position indicated in the diagram. The heat is directed upon
+the iron plate <small>T</small> at some distance from the magnet, so as to allow
+the magnet to keep comparatively cool. This heat is applied beneath
+the plate by means of the burners <small>H</small>, and there is a connection
+from the armature <small>A</small> or its pivot to the gas-cock 6, or
+other device for regulating the heat. The heat acting upon the
+middle portion of the plate <small>T</small>, the magnetic conductivity of the
+heated portion is diminished or destroyed, and a great number of
+the lines of force are deflected over the armature <small>A</small>, which is now
+powerfully attracted and drawn into line, or nearly so, with the
+poles <small>N S</small>. In so doing the cock 6 is nearly closed and the plate
+<small>T</small> cools, the lines of force are again deflected over the same, the
+attraction exerted upon the armature <small>A</small> is diminished, and the
+spring <small>W</small> pulls the same away from the magnet into the position
+shown by full lines, and the operations are repeated. The ar<span class='pagenum'><a name="Page_428" id="Page_428">[Pg 428]</a></span>rangement
+shown in Fig. 236 has the advantages that the magnet
+and armature are kept cool and the strength of the permanent
+magnet is better preserved, as the magnetic circuit is
+constantly closed.</p>
+
+<p>In the plan view, Fig. 238, is shown a permanent magnet and
+keeper plate, <small>T</small>, similar to those in Figs. 236 and 237, with the
+burners <small>H</small> for the gas beneath the same; but the armature is
+pivoted at one end to one pole of the magnet and the other end
+swings toward and from the other pole of the magnet. The spring
+<small>W</small> acts against a lever arm that projects from the armature, and
+the supply of heat has to be partly cut off by a connection to the
+swinging armature, so as to lessen the heat acting upon the keeper
+plate when the armature <small>A</small> has been attracted.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_442.jpg" width="800" height="469" alt="Fig. 240, 241." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 240.</td><td class="caption">Fig. 241.</td></tr>
+</table>
+</div>
+
+<p>Fig. 239 is similar to Fig. 238, except that the keeper <small>T</small> is not
+made use of and the armature itself swings into and out of the
+range of the intense action of the heat from the burner <small>H</small>. Fig.
+240 is a diagram similar to Fig. 231, except that in place of using a
+spring and stops, the armature is shown as connected by a link,
+to the crank of a fly-wheel, so that the fly-wheel will be revolved
+as rapidly as the armature can be heated and cooled to the
+necessary extent. A spring may be used in addition, as in Fig.
+231. In Fig. 241 the armatures <small>A A</small> are connected by a link, so
+that one will be heating while the other is cooling, and the attraction
+exerted to move the cooled armature is availed of to draw
+away the heated armature instead of using a spring.<span class='pagenum'><a name="Page_429" id="Page_429">[Pg 429]</a></span></p>
+
+<p>Mr. Tesla has also devoted his attention to the development of
+a pyromagnetic generator of electricity<a name="FNanchor_13_14" id="FNanchor_13_14"></a><a href="#Footnote_13_14" class="fnanchor">[13]</a> based upon the following
+laws: First, that electricity or electrical energy is developed in
+any conducting body by subjecting such body to a varying magnetic
+influence; and second, that the magnetic properties of iron
+or other magnetic substance may be partially or entirely destroyed
+or caused to disappear by raising it to a certain temperature, but
+restored and caused to reappear by again lowering its temperature
+to a certain degree. These laws may be applied in the production
+of electrical currents in many ways, the principle of
+which is in all cases the same, viz., to subject a conductor to a
+varying magnetic influence, producing such variations by the application
+of heat, or, more strictly speaking, by the application or
+action of a varying temperature upon the source of the magnetism.
+This principle of operation may be illustrated by a simple
+experiment: Place end to end, and preferably in actual contact,
+a permanently magnetized steel bar and a strip or bar of soft iron.
+Around the end of the iron bar or plate wind a coil of insulated wire.
+Then apply to the iron between the coil and the steel bar a flame
+or other source of heat which will be capable of raising that portion
+of the iron to an orange red, or a temperature of about 600&deg;
+centigrade. When this condition is reached, the iron somewhat
+suddenly loses its magnetic properties, if it be very thin, and the
+same effect is produced as though the iron had been moved away
+from the magnet or the heated section had been removed. This
+change of position, however, is accompanied by a shifting of the
+magnetic lines, or, in other words, by a variation in the magnetic
+influence to which the coil is exposed, and a current in the coil
+is the result. Then remove the flame or in any other way reduce
+the temperature of the iron. The lowering of its temperature is
+accompanied by a return of its magnetic properties, and another
+change of magnetic conditions occurs, accompanied by a current
+in an opposite direction in the coil. The same operation may be
+<span class='pagenum'><a name="Page_430" id="Page_430">[Pg 430]</a></span>repeated indefinitely, the effect upon the coil being similar to
+that which would follow from moving the magnetized bar to and
+from the end of the iron bar or plate.</p>
+
+<p>The device illustrated below is a means of obtaining this
+result, the features of novelty in the invention being, first, the
+employment of an artificial cooling device, and, second, inclosing
+the source of heat and that portion of the magnetic circuit exposed
+to the heat and artificially cooling the heated part.</p>
+
+<p>These improvements are applicable generally to the generators
+constructed on the plan above described&mdash;that is to say, we may
+use an artificial cooling device in conjunction with a variable or
+varied or uniform source of heat.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_444.jpg" width="800" height="393" alt="Fig. 242, 243." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 242.</td><td class="caption">Fig. 243.</td></tr>
+</table>
+</div>
+
+<p>Fig. 242 is a central vertical longitudinal section of the complete
+apparatus and Fig. 243 is a cross-section of the magnetic
+armature-core of the generator.</p>
+
+<p>Let <small>A</small> represent a magnetized core or permanent magnet the
+poles of which are bridged by an armature-core composed of a
+casing or shell <small>B</small> inclosing a number of hollow iron tubes <small>C</small>.
+Around this core are wound the conductors <small>E E'</small>, to form the
+coils in which the currents are developed. In the circuits of
+these coils are current-consuming devices, as <small>F F'</small>.</p>
+
+<p><small>D</small> is a furnace or closed fire-box, through which the central
+portion of the core <small>B</small> extends. Above the fire is a boiler <small>K</small>, containing
+water. The flue <small>L</small> from the fire-box may extend up
+through the boiler.</p>
+
+<p><small>G</small> is a water-supply pipe, and <small>H</small> is the steam-exhaust pipe,
+which communicates with all the tubes <small>C</small> in the armature <small>B</small>, so
+that steam escaping from the boiler will pass through the tubes.<span class='pagenum'><a name="Page_431" id="Page_431">[Pg 431]</a></span></p>
+
+<p>In the steam-exhaust pipe <small>H</small> is a valve <small>V</small>, to which is connected
+the lever <small>I</small>, by the movement of which the valve is opened
+or closed. In such a case as this the heat of the fire may be
+utilized for other purposes after as much of it as may be needed
+has been applied to heating the core <small>B</small>. There are special advantages
+in the employment of a cooling device, in that the
+metal of the core <small>B</small> is not so quickly oxidized. Moreover, the
+difference between the temperature of the applied heat and of
+the steam, air, or whatever gas or fluid be applied as the cooling
+medium, may be increased or decreased at will, whereby the
+rapidity of the magnetic changes or fluctuations may be regulated.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_432" id="Page_432">[Pg 432]</a></span></p>
+<h2><a name="CHAPTER_XXXVII" id="CHAPTER_XXXVII"></a>CHAPTER XXXVII.</h2>
+
+<h3><span class="smcap">Anti-Sparking Dynamo Brush and Commutator.</span></h3>
+
+
+<p>In direct current dynamos of great electromotive force&mdash;such,
+for instance, as those used for arc lighting&mdash;when one commutator
+bar or plate comes out of contact with the collecting-brush a
+spark is apt to appear on the commutator. This spark may be
+due to the break of the complete circuit, or to a shunt of low
+resistance formed by the brush between two or more commutator-bars.
+In the first case the spark is more apparent, as there is
+at the moment when the circuit is broken a discharge of the
+magnets through the field helices, producing a great spark or
+flash which causes an unsteady current, rapid wear of the commutator
+bars and brushes, and waste of power. The sparking
+may be reduced by various devices, such as providing a path for
+the current at the moment when the commutator segment or bar
+leaves the brush, by short-circuiting the field-helices, by increasing
+the number of the commutator-bars, or by other similar
+means; but all these devices are expensive or not fully available,
+and seldom attain the object desired.</p>
+
+<p>To prevent this sparking in a simple manner, Mr. Tesla some
+years ago employed with the commutator-bars and intervening
+insulating material, mica, asbestos paper or other insulating and
+incombustible material, arranged to bear on the surface of the
+commutator, near to and behind the brush.</p>
+
+<p>In the drawings, Fig. 244 is a section of a commutator with
+an asbestos insulating device; and Fig. 245 is a similar view, representing
+two plates of mica upon the back of the brush.</p>
+
+<p>In 244, <small>C</small> represents the commutator and intervening
+insulating material; <small>B B</small>, the brushes. <i>d d</i> are sheets of asbestos
+paper or other suitable non-conducting material. <i>f f</i> are springs,
+the pressure of which may be adjusted by means of the screws
+<i>g g</i>.</p>
+
+<p>In Fig. 245 a simple arrangement is shown with two plates of
+mica or other material. It will be seen that whenever one com<span class='pagenum'><a name="Page_433" id="Page_433">[Pg 433]</a></span>mutator
+segment passes out of contact with the brush, the formation
+of the arc will be prevented by the intervening insulating
+material coming in contact with the insulating material on the
+brush.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_447.jpg" width="800" height="242" alt="Fig. 244, 245." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 244.</td><td class="caption">Fig. 245.</td></tr>
+</table>
+</div>
+
+<p>Asbestos paper or cloth impregnated with zinc-oxide, magnesia,
+zirconia, or other suitable material, may be used, as the
+paper and cloth are soft, and serve at the same time to wipe and
+polish the commutator; but mica or any other suitable material
+can be employed, provided the material be an insulator or a bad
+conductor of electricity.</p>
+
+<p>A few years later Mr. Tesla turned his attention again to the
+same subject, as, perhaps, was very natural in view of the fact
+that the commutator had always been prominent in his thoughts,
+and that so much of his work was even aimed at dispensing with
+it entirely as an objectionable and unnecessary part of dynamos
+and motors. In these later efforts to remedy commutator troubles,
+Mr. Tesla constructs a commutator and the collectors therefor in
+two parts mutually adapted to one another, and, so far as the essential
+features are concerned, alike in mechanical structure. Selecting
+as an illustration a commutator of two segments adapted
+for use with an armature the coils or coil of which have but two
+free ends, connected respectively to the segments, the bearing-surface
+is the face of a disc, and is formed of two metallic quadrant
+segments and two insulating segments of the same dimensions,
+and the face of the disc is smoothed off, so that the metal
+and insulating segments are flush. The part which takes the
+place of the usual brushes, or the "collector," is a disc of the
+same character as the commutator and has a surface similarly
+formed with two insulating and two metallic segments. These
+two parts are mounted with their faces in contact and in such
+manner that the rotation of the armature causes the commutator
+to turn upon the collector, whereby the currents induced in the<span class='pagenum'><a name="Page_434" id="Page_434">[Pg 434]</a></span>
+coils are taken off by the collector segments and thence conveyed
+off by suitable conductors leading from the collector segments.
+This is the general plan of the construction adopted. Aside from
+certain adjuncts, the nature and functions of which are set forth
+later, this means of commutation will be seen to possess many important
+advantages. In the first place the short-circuiting and the
+breaking of the armature coil connected to the commutator-segments
+occur at the same instant, and from the nature of the construction
+this will be done with the greatest precision; secondly, the
+duration of both the break and of the short circuit will be reduced
+to a minimum. The first results in a reduction which amounts
+practically to a suppression of the spark, since the break and
+the short circuit produce opposite effects in the armature-coil.
+The second has the effect of diminishing the destructive effect
+of a spark, since this would be in a measure proportional to the
+duration of the spark; while lessening the duration of the short
+circuit obviously increases the efficiency of the machine.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_448.jpg" width="800" height="510" alt="Fig. 246, 247." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 246.</td><td class="caption">Fig. 247.</td></tr>
+</table>
+</div>
+
+<p>The mechanical advantages will be better understood by referring
+to the accompanying diagrams, in which Fig. 246 is a
+central longitudinal section of the end of a shaft with the improved
+commutator carried thereon. Fig. 247 is a view of the
+inner or bearing face of the collector. Fig. 248 is an end view
+from the armature side of a modified form of commutator. Figs.<span class='pagenum'><a name="Page_435" id="Page_435">[Pg 435]</a></span>
+249 and 250 are views of details of Fig. 248. Fig. 251 is a longitudinal
+central section of another modification, and Fig. 252 is a
+sectional view of the same. <small>A</small> is the end of the armature-shaft
+of a dynamo-electric machine or motor. <small>A'</small> is a sleeve of insulating
+material around the shaft, secured in place by a screw, <i>a'</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_449.jpg" width="800" height="295" alt="Fig. 248, 249, 250." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 248.</td><td class="caption">Fig. 249. &nbsp; &nbsp; Fig. 250.</td></tr>
+</table>
+</div>
+
+
+<p>The commutator proper is in the form of a disc which is made
+up of four segments <small>D D'</small> <small>G G'</small>, similar to those shown in Fig. 248.
+Two of these segments, as <small>D D'</small>, are of metal and are in electrical
+connection with the ends of the coils on the armature. The
+other two segments are of insulating material. The segments are
+held in place by a band, <small>B</small>, of insulating material. The disc is
+held in place by friction or by screws, <i>g' g'</i>, Fig. 248, which
+secure the disc firmly to the sleeve <small>A'</small>.</p>
+
+<p>The collector is made in the same form as the commutator. It
+is composed of the two metallic segments <small>E E'</small> and the two insulating
+segments <small>F F'</small>, bound together by a band, <small>C</small>. The metallic
+segments <small>E E'</small> are of the same or practically the same width or
+extent as the insulating segments or spaces of the commutator.
+The collector is secured to a sleeve, <small>B'</small>, by screws <i>g g</i>, and the sleeve
+is arranged to turn freely on the shaft <small>A</small>. The end of the sleeve
+<small>B'</small> is closed by a plate, <i>f</i>, upon which presses a pivot-pointed
+screw, <i>h</i>, adjustable in a spring, <small>H</small>, which acts to maintain the
+collector in close contact with the commutator and to compensate
+for the play of the shaft. The collector is so fixed that it cannot
+turn with the shaft. For example, the diagram shows a slotted
+plate, <small>K</small>, which is designed to be attached to a stationary support,
+and an arm extending from the collector and carrying a clamping
+screw, <small>L</small>, by which the collector may be adjusted and set to the
+desired position.</p>
+
+<p>Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit<span class='pagenum'><a name="Page_436" id="Page_436">[Pg 436]</a></span>
+the insulating segments of both commutator and collector loosely
+and to provide some means&mdash;as, for example, light springs, <i>e e</i>,
+secured to the bands <small>A' B'</small>, respectively, and bearing against the
+segments&mdash;to exert a light pressure upon them and keep them in
+close contact and to compensate for wear. The metal segments
+of the commutator may be moved forward by loosening the
+screw <i>a'</i>.</p>
+
+<p>The line wires are fed from the metal segments of the collector,
+being secured thereto in any convenient manner, the plan of connections
+being shown as applied to a modified form of the commutator
+in Fig. 251. The commutator and the collector in thus
+presenting two flat and smooth bearing surfaces prevent most effectually
+by mechanical action the occurrence of sparks.</p>
+
+<p>The insulating segments are made of some hard material capable
+of being polished and formed with sharp edges. Such materials
+as glass, marble, or soapstone may be advantageously used.
+The metal segments are preferably of copper or brass; but they
+may have a facing or edge of durable material&mdash;such as platinum
+or the like&mdash;where the sparks are liable to occur.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_450.jpg" width="800" height="409" alt="Fig. 251, 252." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 251.</td><td class="caption">Fig. 252.</td></tr>
+</table>
+</div>
+
+<p>In Fig. 248 a somewhat modified form of the invention is
+shown, a form designed to facilitate the construction and replacing
+of the parts. In this modification the commutator and collector
+are made in substantially the same manner as previously
+described, except that the bands <small>B C</small> are omitted. The four segments
+of each part, however, are secured to their respective sleeves
+by screws <i>g' g'</i>, and one edge of each segment is cut away, so that
+small plates <i>a b</i> may be slipped into the spaces thus formed. Of<span class='pagenum'><a name="Page_437" id="Page_437">[Pg 437]</a></span>
+these plates <i>a a</i> are of metal, and are in contact with the metal segments
+<small>D D'</small>, respectively. The other two, <i>b b</i>, are of glass or marble,
+and they are all better square, as shown in Figs. 249 and 250,
+so that they may be turned to present new edges should any edge
+become worn by use. Light springs <i>d</i> bear upon these plates
+and press those in the commutator toward those in the collector,
+and insulating strips <i>c c</i> are secured to the periphery of the discs
+to prevent the blocks from being thrown out by centrifugal action.
+These plates are, of course, useful at those edges of the segments
+only where sparks are liable to occur, and, as they are easily replaced,
+they are of great advantage. It is considered best to coat
+them with platinum or silver.</p>
+
+<p>In Figs. 251 and 252 is shown a construction where, instead of
+solid segments, a fluid is employed. In this case the commutator
+and collector are made of two insulating discs, <small>S T</small>, and in
+lieu of the metal segments a space is cut out of each part, as at
+<small>R R'</small>, corresponding in shape and size to a metal segment. The
+two parts are fitted smoothly and the collector <small>T</small> held by the
+screw <i>h</i> and spring <small>H</small> against the commutator <small>S</small>. As in the other
+cases, the commutator revolves while the collector remains stationary.
+The ends of the coils are connected to binding-posts
+<i>s s</i>, which are in electrical connection with metal plates <i>t t</i> within
+the recesses in the two parts <small>S T</small>. These chambers or recesses
+are filled with mercury, and in the collector part are tubes <small>W W</small>,
+with screws <i>w w</i>, carrying springs <small>X</small> and pistons <small>X'</small>, which compensate
+for the expansion and contraction of the mercury under
+varying temperatures, but which are sufficiently strong not to
+yield to the pressure of the fluid due to centrifugal action, and
+which serve as binding-posts.</p>
+
+<p>In all the above cases the commutators are adapted for a single
+coil, and the device is particularly suited to such purposes. The
+number of segments may be increased, however, or more than
+one commutator used with a single armature. Although the
+bearing-surfaces are shown as planes at right angles to the shaft
+or axis, it is evident that in this particular the construction may
+be very greatly modified.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_438" id="Page_438">[Pg 438]</a></span></p>
+<h2><a name="CHAPTER_XXXVIII" id="CHAPTER_XXXVIII"></a>CHAPTER XXXVIII.</h2>
+
+<h3><span class="smcap">Auxiliary Brush Regulation of Direct Current Dynamos.</span></h3>
+
+
+<p>An interesting method devised by Mr. Tesla for the regulation
+of direct current dynamos, is that which has come to be
+known as the "third brush" method. In machines of this type,
+devised by him as far back as 1885, he makes use of two main
+brushes to which the ends of the field magnet coils are connected,
+an auxiliary brush, and a branch or shunt connection from an intermediate
+point of the field wire to the auxiliary brush.<a name="FNanchor_14_15" id="FNanchor_14_15"></a><a href="#Footnote_14_15" class="fnanchor">[14]</a></p>
+
+<p>The relative positions of the respective brushes are varied,
+either automatically or by hand, so that the shunt becomes inoperative
+when the auxiliary brush has a certain position upon
+the commutator; but when the auxiliary brush is moved in its
+relation to the main brushes, or the latter are moved in their
+relation to the auxiliary brush, the electric condition is disturbed
+and more or less of the current through the field-helices is
+diverted through the shunt or a current is passed over the shunt
+to the field-helices. By varying the relative position upon the
+commutator of the respective brushes automatically in proportion
+to the varying electrical conditions of the working-circuit,
+the current developed can be regulated in proportion to the demands
+in the working-circuit.</p>
+
+<p>Fig. 253 is a diagram illustrating the invention, showing one
+core of the field-magnets with one helix wound in the same direction
+throughout. Figs. 254 and 255 are diagrams showing one
+core of the field-magnets with a portion of the helices wound in
+opposite directions. Figs. 256 and 257 are diagrams illustrating
+<span class='pagenum'><a name="Page_439" id="Page_439">[Pg 439]</a></span>the electric devices that may be employed for automatically
+adjusting the brushes, and Fig. 258 is a diagram illustrating the
+positions of the brushes when the machine is being energized at
+the start.</p>
+
+<p><i>a</i> and <i>b</i> are the positive and negative brushes of the main or
+working-circuit, and <i>c</i> the auxiliary brush. The working-circuit
+<small>D</small> extends from the brushes <i>a</i> and <i>b</i>, as usual, and contains electric
+lamps or other devices, <small>D'</small>, either in series or in multiple
+arc.</p>
+
+<p><small>M M'</small> represent the field-helices, the ends of which are connected
+to the main brushes <i>a</i> and <i>b</i>. The branch or shunt wire
+<i>c'</i> extends from the auxiliary brush <i>c</i> to the circuit of the field-helices,
+and is connected to the same at an intermediate point, <i>x</i>.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_453.jpg" width="800" height="480" alt="Fig. 253." title="" />
+<span class="caption">Fig. 253.</span>
+</div>
+
+
+<p><small>H</small> represents the commutator, with the plates of ordinary construction.
+When the auxiliary brush <i>c</i> occupies such a position
+upon the commutator that the electro-motive force between the
+brushes <i>a</i> and <i>c</i> is to the electro-motive force between the brushes
+<i>c</i> and <i>b</i> as the resistance of the circuit <i>a</i> <small>M</small> <i>c' c</i> <small>A</small> is to the resistance
+of the circuit <i>b</i> <small>M'</small> <i>c' c</i> <small>B</small>, the potentials of the points <i>x</i> and <small>Y</small> will
+be equal, and no current will flow over the auxiliary brush; but
+when the brush <i>c</i> occupies a different position the potentials of
+the points <i>x</i> and <small>Y</small> will be different, and a current will flow over
+the auxiliary brush to and from the commutator, according to the
+relative position of the brushes. If, for instance, the commutator-space
+between the brushes <i>a</i> and <i>c</i>, when the latter is at the
+neutral point, is diminished, a current will flow from the point <small>Y</small>
+over the shunt <i>c</i> to the brush <i>b</i>, thus strengthening the current
+in the part <small>M'</small>, and partly neutralizing the current in part <small>M</small>; but
+if the space between the brushes <i>a</i> and <i>c</i> is increased, the cur<span class='pagenum'><a name="Page_440" id="Page_440">[Pg 440]</a></span>rent
+will flow over the auxiliary brush in an opposite direction,
+and the current in <small>M</small> will be strengthened, and in <small>M'</small>, partly neutralized.</p>
+
+<p>By combining with the brushes <i>a</i>, <i>b</i>, and <i>c</i> any usual automatic
+regulating mechanism, the current developed can be regulated in
+proportion to the demands in the working circuit. The parts <small>M</small>
+and <small>M'</small> of the field wire may be wound in the same direction.
+In this case they are arranged as shown in Fig. 253; or the part
+<small>M</small> may be wound in the opposite direction, as shown in Figs.
+254 and 255.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_454.jpg" width="800" height="366" alt="Fig. 254." title="" />
+<span class="caption">Fig. 254.</span>
+</div>
+
+<p>It will be apparent that the respective cores of the field-magnets
+are subjected to neutralizing or intensifying effects of the
+current in the shunt through <i>c'</i>, and the magnetism of the cores
+will be partially neutralized, or the points of greatest magnetism
+shifted, so that it will be more or less remote from or approaching
+to the armature, and hence the aggregate energizing actions
+of the field magnets on the armature will be correspondingly
+varied.</p>
+
+<p>In the form indicated in Fig. 253 the regulation is effected by
+shifting the point of greatest magnetism, and in Figs. 254 and
+255 the same effect is produced by the action of the current in
+the shunt passing through the neutralizing helix.</p>
+
+<p>The relative positions of the respective brushes may be varied
+by moving the auxiliary brush, or the brush <i>c</i> may remain stationary
+and the core <small>P</small> be connected to the main-brush holder <small>A</small>,
+so as to adjust the brushes <i>a b</i> in their relation to the brush <i>c</i>.
+If, however, an adjustment is applied to all the brushes, as seen
+in Fig. 257, the solenoid should be connected to both <i>a</i> and <i>c</i>, so
+as to move them toward or away from each other.</p>
+
+<p>There are several known devices for giving motion in propor<span class='pagenum'><a name="Page_441" id="Page_441">[Pg 441]</a></span>tion
+to an electric current. In Figs. 256 and 257 the moving
+cores are shown as convenient devices for obtaining the required
+extent of motion with very slight changes in the current passing
+through the helices. It is understood that the adjustment of
+the main brushes causes variations in the strength of the current
+independently of the relative position of those brushes to the
+auxiliary brush. In all cases the adjustment should be such that
+no current flows over the auxiliary brush when the dynamo is
+running with its normal load.</p>
+
+<p>In Figs. 256 and 257 <small>A A</small> indicate the main-brush holder,
+carrying the main brushes, and <small>C</small> the auxiliary-brush holder,
+carrying the auxiliary brush. These brush-holders are movable
+in arcs concentric with the centre of the commutator-shaft. An
+iron piston, <small>P</small>, of the solenoid <small>S</small>, Fig. 256, is attached to the auxiliary-brush
+holder <small>C</small>. The adjustment is effected by means of a
+spring and screw or tightener.</p>
+
+<p>In Fig. 257 instead of a solenoid, an iron tube inclosing a coil
+is shown. The piston of the coil is attached to both brush-holders
+<small>A A</small> and <small>C</small>. When the brushes are moved directly by
+electrical devices, as shown in Figs. 256 and 257, these are so
+constructed that the force exerted for adjusting is practically
+uniform through the whole length of motion.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_455.jpg" width="800" height="366" alt="Fig. 255." title="" />
+<span class="caption">Fig. 255.</span>
+</div>
+
+<p>It is true that auxiliary brushes have been used in connection
+with the helices of the field-wire; but in these instances the
+helices receive the entire current through the auxiliary brush or
+brushes, and these brushes could not be taken off without breaking
+the circuit through the field. These brushes cause, moreover,
+heavy sparking at the commutator. In the present
+case the auxiliary brush causes very little or no sparking, and
+can be taken off without breaking the circuit through the field<span class='pagenum'><a name="Page_442" id="Page_442">[Pg 442]</a></span>-helices.
+The arrangement has, besides, the advantage of facilitating
+the self-excitation of the machine in all cases where the resistance
+of the field-wire is very great comparatively to the resistance
+of the main circuit at the start&mdash;for instance, on arc-light
+machines. In this case the auxiliary brush <i>c</i> is placed near to, or
+better still in contact with, the brush <i>b</i>, as shown in Fig. 258.
+In this manner the part <small>M'</small> is completely cut out, and as the part
+<small>M</small> has a considerably smaller resistance than the whole length of
+the field-wire the machine excites itself, whereupon the auxiliary
+brush is shifted automatically to its normal position.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_456.jpg" width="800" height="281" alt="Fig. 256, 257." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 256.</td><td class="caption">Fig. 257.</td></tr>
+</table>
+</div>
+
+<p>In a further method devised by Mr. Tesla, one or more auxiliary
+brushes are employed, by means of which a portion or the
+whole of the field coils is shunted. According to the relative position
+upon the commutator of the respective brushes more or
+less current is caused to pass through the helices of the field, and
+the current developed by the machine can be varied at will by
+varying the relative positions of the brushes.</p>
+
+<div class="figcenter" style="width: 361px;">
+<img src="images/oi_456-1.jpg" width="361" height="336" alt="Fig. 258." title="" />
+<span class="caption">Fig. 258.</span>
+</div>
+
+<p>In Fig. 259, <i>a</i> and <i>b</i> are the positive and negative brushes of
+the main circuit, and <i>c</i> an auxiliary brush. The main circuit <small>D</small>
+extends from the brushes <i>a</i> and <i>b</i>, as usual, and contains the
+helices <small>M</small> of the field wire and the electric lamps or other working
+devices. The auxiliary brush <i>c</i> is connected to the point <i>x</i>
+of the main circuit by means of the wire <i>c'</i>. <small>H</small> is a commutator<span class='pagenum'><a name="Page_443" id="Page_443">[Pg 443]</a></span>
+of ordinary construction. It will have been seen from what was
+said already that when the electro-motive force between the brushes
+<i>a</i> and <i>c</i> is to the electromotive force between the brushes <i>c</i>
+and <i>b</i> as the resistance of the circuit <i>a</i> <small>M</small> <i>c' c</i> <small>A</small> is to the resistance
+of the circuit <i>b</i> <small>C B</small> <i>c c'</i> <small>D</small>, the potentials of the points <i>x</i> and <i>y</i>
+will be equal, and no current will pass over the auxiliary brush
+<i>c</i>; but if that brush occupies a different position relatively to the
+main brushes the electric condition is disturbed, and current
+will flow either from <i>y</i> to <i>x</i> or from <i>x</i> to <i>y</i>, according to the relative
+position of the brushes. In the first case the current through
+the field-helices will be partly neutralized and the magnetism of
+the field magnets will be diminished. In the second case the
+current will be increased and the magnets gain strength. By
+combining with the brushes at <i>a b c</i> any automatic regulating
+mechanism, the current developed can be regulated automatically
+in proportion to the demands of the working circuit.</p>
+
+<p>In Figs. 264 and 265 some of the automatic means are represented
+that maybe used for moving the brushes. The core <small>P</small>,
+Fig. 264, of the solenoid-helix <small>S</small> is connected with the brush <i>a</i> to
+move the same, and in Fig. 265 the core <small>P</small> is shown as within the
+helix <small>S</small>, and connected with brushes <i>a</i> and <i>c</i>, so as to move the
+same toward or from each other, according to the strength of the
+current in the helix, the helix being within an iron tube, <small>S'</small>, that
+becomes magnetized and increases the action of the solenoid.</p>
+
+<p>In practice it is sufficient to move only the auxiliary brush, as
+shown in Fig. 264, as the regulation is very sensitive to the
+slightest changes; but the relative position of the auxiliary brush
+to the main brushes may be varied by moving the main brushes,
+or both main and auxiliary brushes may be moved, as illustrated
+in Fig. 265. In the latter two cases, it will be understood, the
+motion of the main brushes relatively to the neutral line of the
+machine causes variations in the strength of the current independently
+of their relative position to the auxiliary brush. In
+all cases the adjustment may be such that when the machine is
+running with the ordinary load, no current flows over the auxiliary
+brush.</p>
+
+<p>The field helices may be connected, as shown in Fig. 259, or a
+part of the field helices may be in the outgoing and the other part
+in the return circuit, and two auxiliary brushes may be employed
+as shown in Figs. 261 and 262. Instead of shunting the whole
+of the field helices, a portion only of such helices may be shunted,
+as shown in Figs. 260 and 262.<span class='pagenum'><a name="Page_444" id="Page_444">[Pg 444]</a></span></p>
+
+<p>The arrangement shown in Fig. 262 is advantageous, as it diminishes
+the sparking upon the commutator, the main circuit being
+closed through the auxiliary brushes at the moment of the break
+of the circuit at the main brushes.</p>
+
+<div class="figcenter" style="width: 800px;">
+
+<img src="images/oi_458-1.jpg" width="800" height="321" alt="Fig. 259." title="" />
+<span class="caption">Fig. 259.</span>
+
+<img src="images/oi_458-2.jpg" width="800" height="270" alt="Fig. 260." title="" />
+<span class="caption">Fig. 260.</span>
+
+<img src="images/oi_458-3.jpg" width="800" height="283" alt="Fig. 261." title="" />
+<span class="caption">Fig. 261.</span>
+
+<img src="images/oi_458.jpg" width="800" height="277" alt="Fig. 262, 263." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 262.</td><td class="caption">Fig. 263.</td></tr>
+</table>
+
+</div>
+
+<p>The field helices may be wound in the same direction, or a part
+may be wound in opposite directions.</p>
+
+<p>The connection between the helices and the auxiliary brush or
+brushes may be made by a wire of small resistance, or a resistance
+may be interposed (<small>R</small>, Fig. 263,) between the point <i>x</i> and the<span class='pagenum'><a name="Page_445" id="Page_445">[Pg 445]</a></span>
+auxiliary brush or brushes to divide the sensitiveness when the
+brushes are adjusted.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_459.jpg" width="800" height="281" alt="Fig. 264, 265." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 264.</td><td class="caption">Fig. 265.</td></tr>
+</table>
+</div>
+
+<p>The accompanying sketches also illustrate improvements made
+by Mr. Tesla in the mechanical devices used to effect the shifting
+of the brushes, in the use of an auxiliary brush. Fig. 266 is
+an elevation of the regulator with the frame partly in section;
+and Fig. 267 is a section at the line <i>x x</i>, Fig. 266. <small>C</small> is the commutator;
+<small>B</small> and <small>B'</small>, the brush-holders, <small>B</small> carrying the main
+brushes <i>a a'</i>, and <small>B'</small> the auxiliary or shunt brushes <i>b b</i>. The
+axis of the brush-holder <small>B</small> is supported by two pivot-screws, <i>p p</i>.
+The other brush-holder, <small>B'</small>, has a sleeve, <i>d</i>, and is movable
+around the axis of the brush-holder <small>B</small>. In this way both brush-holders
+can turn very freely, the friction of the parts being
+reduced to a minimum. Over the brush-holders is mounted the
+solenoid <small>S</small>, which rests upon a forked column, <i>c</i>. This column
+also affords a support for the pivots <i>p p</i>, and is fastened upon a
+solid bracket or projection, <small>P</small>, which extends from the base of
+the machine, and is cast in one piece with the same. The
+brush-holders <small>B B'</small> are connected by means of the links <i>e e</i>
+and the cross-piece <small>F</small> to the iron core <small>I</small>, which slides freely in the
+tube <small>T</small> of the solenoid. The iron core <small>I</small> has a screw, <i>s</i>, by means
+of which it can be raised and adjusted in its position relatively
+to the solenoid, so that the pull exerted upon it by the solenoid
+is practically uniform through the whole length of motion which
+is required to effect the regulation. In order to effect the
+adjustment with greater precision, the core <small>I</small> is provided with a
+small iron screw, <i>s'</i>. The core being first brought very nearly
+in the required position relatively to the solenoid by means of
+the screw <i>s</i>, the small screw <i>s'</i> is then adjusted until the magnetic
+attraction upon the core is the same when the core is in any position.
+A convenient stop, <i>t</i>, serves to limit the upward movement
+of the iron core.<span class='pagenum'><a name="Page_446" id="Page_446">[Pg 446]</a></span></p>
+
+<p>To check somewhat the movement of the core <small>I</small>, a dash-pot, <small>K</small>,
+is used. The piston <small>L</small> of the dash-pot is provided with a valve,
+<small>V</small>, which opens by a downward pressure and allows an easy
+downward movement of the iron core <small>I</small>, but closes and checks
+the movement of the core when it is pulled up by the action
+of the solenoid.</p>
+
+<p>To balance the opposing forces, the weight of the moving
+parts, and the pull exerted by the solenoid upon the iron core,
+the weights <small>W W</small> may be used. The adjustment is such that
+when the solenoid is traversed by the normal current it is just
+strong enough to balance the downward pull of the parts.</p>
+
+<div class="figcenter" style="width: 733px;">
+<img src="images/oi_460.jpg" width="733" height="600" alt="Fig. 266, 267." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 266.</td><td class="caption">Fig. 267.</td></tr>
+</table>
+</div>
+
+<p>The electrical circuit-connections are substantially the same as
+indicated in the previous diagrams, the solenoid being in series
+with the circuit when the translating devices are in series, and in
+shunt when the devices are in multiple arc. The operation of
+the device is as follows: When upon a decrease of the resistance
+of the circuit or for some other reason, the current is
+increased, the solenoid <small>S</small> gains in strength and pulls up the iron
+core <small>I</small>, thus shifting the main brushes in the direction of rotation
+and the auxiliary brushes in the opposite way. This diminishes
+the strength of the current until the opposing forces are balanced
+and the solenoid is traversed by the normal current; but if from
+any cause the current in the circuit is diminished, then the weight
+of the moving parts overcomes the pull of the solenoid, the iron<span class='pagenum'><a name="Page_447" id="Page_447">[Pg 447]</a></span>
+core <small>I</small> descends, thus shifting the brushes the opposite way and
+increasing the current to the normal strength. The dash-pot
+connected to the iron core <small>I</small> may be of ordinary construction;
+but it is better, especially in machines for arc lights, to provide
+the piston of the dash-pot with a valve, as indicated in the diagrams.
+This valve permits a comparatively easy downward movement
+of the iron core, but checks its movement when it is drawn
+up by the solenoid. Such an arrangement has the advantage
+that a great number of lights may be put on without diminishing
+the light-power of the lamps in the circuit, as the brushes assume
+at once the proper position. When lights are cut out, the dash-pot
+acts to retard the movement; but if the current is considerably
+increased the solenoid gets abnormally strong and the brushes
+are shifted instantly. The regulator being properly adjusted,
+lights or other devices may be put on or out with scarcely any
+perceptible difference. It is obvious that instead of the dash-pot
+any other retarding device may be used.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_448" id="Page_448">[Pg 448]</a></span></p>
+<h2><a name="CHAPTER_XXXIX" id="CHAPTER_XXXIX"></a>CHAPTER XXXIX.</h2>
+
+<h3><span class="smcap">Improvement in the Construction of Dynamos and Motors.</span></h3>
+
+
+<p>This invention of Mr. Tesla is an improvement in the construction
+of dynamo or magneto electric machines or motors,
+consisting in a novel form of frame and field magnet which renders
+the machine more solid and compact as a structure, which
+requires fewer parts, and which involves less trouble and expense
+in its manufacture. It is applicable to generators and motors
+generally, not only to those which have independent circuits
+adapted for use in the Tesla alternating current system, but to
+other continuous or alternating current machines of the ordinary
+type generally used.</p>
+
+<p>Fig. 268 shows the machine in side elevation. Fig. 269 is a
+vertical sectional view of the field magnets and frame and an end
+view of the armature; and Fig. 270 is a plan view of one of
+the parts of the frame and the armature, a portion of the latter
+being cut away.</p>
+
+<p>The field magnets and frame are cast in two parts. These
+parts are identical in size and shape, and each consists of the solid
+plates or ends <small>A B</small>, from which project inwardly the cores <small>C D</small> and
+the side bars or bridge pieces, <small>E F</small>. The precise shape of these
+parts is largely a matter of choice&mdash;that is to say, each casting,
+as shown, forms an approximately rectangular frame; but it might
+obviously be more or less oval, round, or square, without departure
+from the invention. It is also desirable to reduce the
+width of the side bars, <small>E F</small>, at the center and to so proportion the
+parts that when the frame is put together the spaces between the
+pole pieces will be practically equal to the arcs which the surfaces
+of the poles occupy.</p>
+
+<p>The bearings <small>G</small> for the armature shaft are cast in the side bars
+<small>E F</small>. The field coils are either wound on the pole pieces or on a
+form and then slipped on over the ends of the pole pieces.
+The lower part or casting is secured to the base after being
+finished off. The armature <small>K</small> on its shaft is then mounted in<span class='pagenum'><a name="Page_449" id="Page_449">[Pg 449]</a></span>
+the bearings of the lower casting and the other part of the frame
+placed in position, dowel pins <small>L</small> or any other means being used to
+secure the two parts in proper position.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_463-1.jpg" width="800" height="407" alt="Fig. 268." title="" />
+<span class="caption">Fig. 268.</span>
+
+<img src="images/oi_463-2.jpg" width="800" height="426" alt="Fig. 269." title="" />
+<span class="caption">Fig. 269.</span>
+
+<img src="images/oi_463.jpg" width="800" height="513" alt="Fig. 270." title="" />
+<span class="caption">Fig. 270.</span>
+
+</div>
+
+<p>In order to secure an easier fit, the side bars <small>E F</small>, and end pieces,
+<small>A B</small>, are so cast that slots <small>M</small> are formed when the two parts are
+put together.<span class='pagenum'><a name="Page_450" id="Page_450">[Pg 450]</a></span></p>
+
+<p>This machine possesses several advantages. For example, if we
+magnetize the cores alternately, as indicated by the characters <small>N</small>
+<small>S</small>, it will be seen that the magnetic circuit between the poles of
+each part of a casting is completed through the solid iron side
+bars. The bearings for the shaft are located at the neutral points
+of the field, so that the armature core is not affected by the magnetic
+condition of the field.</p>
+
+<p>The improvement is not restricted to the use of four pole pieces,
+as it is evident that each pole piece could be divided or more than
+four formed by the shape of the casting.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_451" id="Page_451">[Pg 451]</a></span></p>
+<h2><a name="CHAPTER_XL" id="CHAPTER_XL"></a>CHAPTER XL.</h2>
+
+<h3><span class="smcap">Tesla Direct Current Arc Lighting System.</span></h3>
+
+
+<p>At one time, soon after his arrival in America, Mr. Tesla was
+greatly interested in the subject of arc lighting, which then occupied
+public attention and readily enlisted the support of capital.
+He therefore worked out a system which was confided to a company
+formed for its exploitation, and then proceeded to devote
+his energies to the perfection of the details of his more celebrated
+"rotary field" motor system. The Tesla arc lighting apparatus
+appeared at a time when a great many other lamps and machines
+were in the market, but it commanded notice by its ingenuity.
+Its chief purpose was to lessen the manufacturing cost and simplify
+the processes of operation.</p>
+
+<p>We will take up the dynamo first. Fig. 271 is a longitudinal
+section, and Fig. 272 a cross section of the machine. Fig. 273 is
+a top view, and Fig. 274 a side view of the magnetic frame. Fig.
+275 is an end view of the commutator bars, and Fig. 276 is a
+section of the shaft and commutator bars. Fig. 277 is a diagram
+illustrating the coils of the armature and the connections to the
+commutator plates.</p>
+
+<p>The cores <i>c c c c</i> of the field-magnets are tapering in both
+directions, as shown, for the purposes of concentrating the magnetism
+upon the middle of the pole-pieces.</p>
+
+<p>The connecting-frame <small>F F</small> of the field-magnets is in the form
+indicated in the side view, Fig. 274, the lower part being provided
+with the spreading curved cast legs <i>e e</i>, so that the machine
+will rest firmly upon two base-bars, <i>r r</i>.</p>
+
+<p>To the lower pole, <small>S</small>, of the field-magnet <small>M</small> is fastened, by
+means of babbitt or other fusible diamagnetic material, the base
+<small>B</small>, which is provided with bearings <i>b</i> for the armature-shaft <small>H</small>.
+The base <small>B</small> has a projection, <small>P</small>, which supports the brush-holders
+and the regulating devices, which are of a special character devised
+by Mr. Tesla.</p>
+
+<p>The armature is constructed with the view to reduce to a min<span class='pagenum'><a name="Page_452" id="Page_452">[Pg 452]</a></span>imum
+the loss of power due to Foucault currents and to the
+change of polarity, and also to shorten as much as possible the
+length of the inactive wire wound upon the armature core.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_466.jpg" width="640" height="301" alt="Fig. 271." title="" />
+<span class="caption">Fig. 271.</span>
+</div>
+
+<p>It is well known that when the armature is revolved between
+the poles of the field-magnets, currents are generated in the iron
+body of the armature which develop heat, and consequently cause
+a waste of power. Owing to the mutual action of the lines of
+force, the magnetic properties of iron, and the speed of the different
+portions of the armature core, these currents are generated
+principally on and near the surface of the armature core, diminishing
+in strength gradually toward the centre of the core.
+Their quantity is under some conditions proportional to the
+length of the iron body in the direction in which these currents
+are generated. By subdividing the iron core electrically in this
+direction, the generation of these currents can be reduced to a
+great extent. For instance, if the length of the armature-core is
+twelve inches, and by a suitable construction it is subdivided
+electrically, so that there are in the generating direction six inches
+of iron and six inches of intervening air-spaces or insulating material,
+the waste currents will be reduced to fifty per cent.</p>
+
+<p>As shown in the diagrams, the armature is constructed of thin
+iron discs <small>D D D</small>, of various diameters, fastened upon the armature-shaft
+in a suitable manner and arranged according to their
+sizes, so that a series of iron bodies, <i>i i i</i>, is formed, each of which
+diminishes in thickness from the centre toward the periphery.
+At both ends of the armature the inwardly curved discs <i>d d</i>, of
+cast iron, are fastened to the armature shaft.</p>
+
+<p>The armature core being constructed as shown, it will be easily
+seen that on those portions of the armature that are the most
+remote from the axis, and where the currents are principally developed,
+the length of iron in the generating direction is only a<span class='pagenum'><a name="Page_453" id="Page_453">[Pg 453]</a></span>
+small fraction of the total length of the armature core, and besides
+this the iron body is subdivided in the generating direction,
+and therefore the Foucault currents are greatly reduced. Another
+cause of heating is the shifting of the poles of the armature core.
+In consequence of the subdivision of the iron in the armature
+and the increased surface for radiation, the risk of heating is
+lessened.</p>
+
+<p>The iron discs <small>D D D</small> are insulated or coated with some insulating-paint,
+a very careful insulation being unnecessary, as an
+electrical contact between several discs can only occur at places
+where the generated currents are comparatively weak. An
+armature core constructed in the manner described may be revolved
+between the poles of the field magnets without showing
+the slightest increase of temperature.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_467.jpg" width="800" height="404" alt="Fig. 272, 273." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 272.</td><td class="caption">Fig. 273.</td></tr>
+</table>
+</div>
+
+<p>The end discs, <i>d d</i>, which are of sufficient thickness and, for
+the sake of cheapness, of cast-iron, are curved inwardly, as indicated
+in the drawings. The extent of the curve is dependent
+on the amount of wire to be wound upon the armatures. In this
+machine the wire is wound upon the armature in two superimposed
+parts, and the curve of the end discs, <i>d d</i>, is so calculated
+that the first part&mdash;that is, practically half of the wire&mdash;just fills
+up the hollow space to the line <i>x x</i>; or, if the wire is wound in
+any other manner, the curve is such that when the whole of the
+wire is wound, the outside mass of wires, <i>w</i>, and the inside mass
+of wires, <i>w'</i>, are equal at each side of the plane <i>x x</i>. In this case
+the passive or electrically-inactive wires are of the smallest
+length practicable. The arrangement has further the advantage<span class='pagenum'><a name="Page_454" id="Page_454">[Pg 454]</a></span>
+that the total lengths of the crossing wires at the two sides of
+the plane <i>x x</i> are practically equal.</p>
+
+<div class="figcenter" style="width: 617px;">
+<img src="images/oi_468.jpg" width="617" height="480" alt="Fig. 274." title="" />
+<span class="caption">Fig. 274.</span>
+</div>
+
+<p>To equalize further the armature coils at both sides of the
+plates that are in contact with the brushes, the winding and connecting
+up is effected in the following manner: The whole wire
+is wound upon the armature-core in two superimposed parts,
+which are thoroughly insulated from each other. Each of these
+two parts is composed of three separated groups of coils. The
+first group of coils of the first part of wire being wound and
+connected to the commutator-bars in the usual manner, this group
+is insulated and the second group wound; but the coils of this
+second group, instead of being connected to the next following
+commutator bars, are connected to the directly opposite bars of
+the commutator. The second group is then insulated and the
+third group wound, the coils of this group being connected to
+those bars to which they would be connected in the usual way.
+The wires are then thoroughly insulated and the second part of
+wire is wound and connected in the same manner.</p>
+
+<p>Suppose, for instance, that there are twenty-four coils&mdash;that is,
+twelve in each part&mdash;and consequently twenty-four commutator
+plates. There will be in each part three groups, each containing
+four coils, and the coils will be connected as follows:</p>
+
+<div class='center'>
+<table border="0" cellpadding="1" cellspacing="0" summary="">
+<tr><td align='left'></td><td align='center'><i>Groups.</i> &nbsp; &nbsp;</td><td align='center'><i>Commutator Bars.</i></td></tr>
+<tr><td colspan='3'>&nbsp;</td></tr>
+<tr><td align='left'></td><td align='left'>First</td><td align='center'>1&mdash;5</td></tr>
+<tr><td align='left'>First part of wire</td><td align='left'>Second</td><td align='center'>17&mdash;21</td></tr>
+<tr><td align='left'></td><td align='left'>Third</td><td align='center'>9&mdash;13</td></tr>
+<tr><td colspan='3'>&nbsp;</td></tr>
+<tr><td align='left'></td><td align='left'>First</td><td align='center'>13&mdash;17</td></tr>
+<tr><td align='left'>Second part of wire &nbsp; &nbsp; </td><td align='left'>Second &nbsp; </td><td align='left'>5&mdash;9</td></tr>
+<tr><td align='left'></td><td align='left'>Third</td><td align='center'>21&mdash;1</td></tr>
+</table></div>
+
+<p>In constructing the armature core and winding and connecting
+the coils in the manner indicated, the passive or electrically in<span class='pagenum'><a name="Page_455" id="Page_455">[Pg 455]</a></span>active
+wire is reduced to a minimum, and the coils at each
+side of the plates that are in contact with the brushes are practically
+equal. In this way the electrical efficiency of the machine
+is increased.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_469.jpg" width="800" height="302" alt="Fig. 275, 276." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 275.</td><td class="caption">Fig. 276.</td></tr>
+</table>
+</div>
+
+<p>The commutator plates <i>t</i> are shown as outside the bearing <i>b</i> of
+the armature shaft. The shaft <small>H</small> is tubular and split at the end
+portion, and the wires are carried through the same in the usual
+manner and connected to the respective commutator plates. The
+commutator plates are upon a cylinder, <i>u</i>, and insulated, and this
+cylinder is properly placed and then secured by expanding the
+split end of the shaft by a tapering screw plug, <i>v</i>.</p>
+
+<div class="figcenter" style="width: 645px;">
+<img src="images/oi_469-1.jpg" width="645" height="600" alt="Fig. 277." title="" />
+<span class="caption">Fig. 277.</span>
+</div>
+
+
+<p>The arc lamps invented by Mr. Tesla for use on the circuits
+from the above described dynamo are those in which the separation
+and feed of the carbon electrodes or their equivalents is accomplished
+by means of electro-magnets or solenoids in connection
+with suitable clutch mechanism, and were designed for the purpose<span class='pagenum'><a name="Page_456" id="Page_456">[Pg 456]</a></span>
+of remedying certain faults common to arc lamps.</p>
+
+<p>He proposed to prevent the frequent vibrations of the movable
+carbon "point" and flickering of the light arising therefrom; to
+prevent the falling into contact of the carbons; to dispense with
+the dash pot, clock work, or gearing and similar devices; to render
+the lamp extremely sensitive, and to feed the carbon almost
+imperceptibly, and thereby obtain a very steady and uniform
+light.</p>
+
+<p>In that class of lamps where the regulation of the arc is effected
+by forces acting in opposition on a free, movable rod or lever directly
+connected with the electrode, all or some of the forces
+being dependent on the strength of the current, any change in
+the electrical condition of the circuit causes a vibration and a corresponding
+flicker in the light. This difficulty is most apparent
+when there are only a few lamps in circuit. To lessen this difficulty
+lamps have been constructed in which the lever or armature,
+after the establishing of the arc, is kept in a fixed position and
+cannot vibrate during the feed operation, the feed mechanism
+acting independently; but in these lamps, when a clamp is employed,
+it frequently occurs that the carbons come into contact
+and the light is momentarily extinguished, and frequently parts
+of the circuit are injured. In both these classes of lamps it has
+been customary to use dash pot, clock work, or equivalent retarding
+devices; but these are often unreliable and objectionable, and
+increase the cost of construction.</p>
+
+<p>Mr. Tesla combines two electro-magnets&mdash;one of low resistance
+in the main or lamp circuit, and the other of comparatively
+high resistance in a shunt around the arc&mdash;a movable armature
+lever, and a special feed mechanism, the parts being arranged so
+that in the normal working position of the armature lever the
+same is kept almost rigidly in one position, and is not affected
+even by considerable changes in the electric circuit; but if the
+carbons fall into contact the armature will be actuated by the
+magnets so as to move the lever and start the arc, and hold the
+carbons until the arc lengthens and the armature lever returns to
+the normal position. After this the carbon rod holder is released
+by the action of the feed mechanism, so as to feed the carbon and
+restore the arc to its normal length.</p>
+
+<p>Fig. 278 is an elevation of the mechanism made use of in
+this arc lamp. Fig. 279 is a plan view. Fig. 280 is an elevation
+of the balancing lever and spring; Fig. 281 is a de<span class='pagenum'><a name="Page_457" id="Page_457">[Pg 457]</a></span>tached
+plan view of the pole pieces and armatures upon the
+friction clamp, and Fig. 282 is a section of the clamping tube.</p>
+
+<p><small>M</small> is a helix of coarse wire in a circuit from the lower carbon
+holder to the negative binding screw &minus;. <small>N</small> is a helix of fine wire
+in a shunt between the positive binding screw &#43; and the
+negative binding screw &minus;. The upper carbon holder <small>S</small> is a parallel
+rod sliding through the plates <small>S' S<sup>2</sup></small> of the frame of the lamp,
+and hence the electric current passes from the positive binding
+post &#43; through the plate <small>S<sup>2</sup></small>, carbon holder <small>S</small>, and upper carbon
+to the lower carbon, and thence by the holder and a metallic
+connection to the helix <small>M</small>.</p>
+
+
+<div class="figcenter" style="width: 1024px;">
+<div class="figleft" style="width: 448px;">
+<img src="images/oi_471-1.jpg" width="414" height="640" alt="Fig. 278, 282." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 278.</td><td class="caption">Fig. 282.</td></tr>
+</table>
+</div>
+
+<div class="figright" style="width: 520px;">
+<img src="images/oi_471-2.jpg" width="520" height="480" alt="Fig. 279." title="" /><br />
+<span class="caption">Fig. 279.</span>
+</div>
+
+<div class="figright" style="width: 520px;">
+<img src="images/oi_471-4.jpg" width="520" height="480" alt="Fig. 280." title="" /><br />
+<span class="caption">Fig. 280.</span>
+</div>
+
+<div class="figleft" style="width: 448px;">
+<img src="images/oi_471-3.jpg" width="448" height="133" alt="Fig. 281." title="" /><br />
+<span class="caption">Fig. 281.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>The carbon holders are of the usual character, and to insure
+electric connections the springs <i>l</i> are made use of to grasp the
+upper carbon holding rod <small>S</small>, but to allow the rod to slide freely
+through the same. These springs <i>l</i> may be adjusted in their
+pressure by the screw <i>m</i>, and the spring <i>l</i> maybe sustained upon<span class='pagenum'><a name="Page_458" id="Page_458">[Pg 458]</a></span>
+any suitable support. They are shown as connected with the
+upper end of the core of the magnet <small>N</small>.</p>
+
+<p>Around the carbon-holding rod <small>S</small>, between the plates <small>S' S<sup>2</sup></small>,
+there is a tube, <small>R</small>, which forms a clamp. This tube is counter-bored,
+as seen in the section Fig. 282, so that it bears upon the
+rod <small>S</small> at its upper end and near the middle, and at the lower end of
+this tubular clamp <small>R</small> there are armature segments <i>r</i> of soft iron.
+A frame or arm, <i>n</i>, extending, preferably, from the core <small>N<sup>2</sup></small>, supports
+the lever <small>A</small> by a fulcrum-pin, <i>o</i>. This lever <small>A</small> has a hole,
+through which the upper end of the tubular clamp <small>R</small> passes
+freely, and from the lever <small>A</small> is a link, <i>q</i>, to the lever <i>t</i>, which
+lever is pivoted at <i>y</i> to a ring upon one of the columns <small>S<sup>3</sup></small>. This
+lever <i>t</i> has an opening or bow surrounding the tubular clamp
+<small>R</small>, and there are pins or pivotal connections <i>w</i> between the lever
+<i>t</i> and this clamp <small>R</small>, and a spring, <i>r</i><sup>2</sup>, serves to support or suspend
+the weight of the parts and balance them, or nearly so. This
+spring is adjustable.</p>
+
+<p>At one end of the lever <small>A</small> is a soft-iron armature block, <i>a</i>, over
+the core <small>M'</small> of the helix <small>M</small>, and there is a limiting screw, <i>c</i>, passing
+through this armature block <i>a</i>, and at the other end of the
+lever <small>A</small> is a soft iron armature block, <i>b</i>, with the end tapering or
+wedge shaped, and the same comes close to and in line with the
+lateral projection <i>e</i> on the core <small>N<sup>2</sup></small>. The lower ends of the cores
+<small>M' N<sup>2</sup></small> are made with laterally projecting pole-pieces <small>M<sup>3</sup> N<sup>3</sup></small>, respectively,
+and these pole-pieces are concave at their outer ends, and
+are at opposite sides of the armature segments <i>r</i> at the lower end
+of the tubular clamp <small>R</small>.</p>
+
+<p>The operation of these devices is as follows: In the condition
+of inaction, the upper carbon rests upon the lower one, and when
+the electric current is turned on it passes freely, by the frame
+and spring <i>l</i>, through the rods and carbons to the coarse wire and
+helix <small>M</small>, and to the negative binding post <small>V</small> and the core <small>M'</small> thereby
+is energized. The pole piece <small>M<sup>3</sup></small> attracts the armature <i>r</i>, and by
+the lateral pressure causes the clamp <small>R</small> to grasp the rod <small>S'</small>, and
+the lever <small>A</small> is simultaneously moved from the position shown by
+dotted lines, Fig. 278, to the normal position shown in full lines,
+and in so doing the link <i>q</i> and lever <i>t</i> are raised, lifting the clamp
+<small>R</small> and <small>S</small>, separating the carbons and forming the arc. The magnetism
+of the pole piece <i>e</i> tends to hold the lever <small>A</small> level, or
+nearly so, the core <small>N<sup>2</sup></small> being energized by the current in the shunt
+which contains the helix <small>N</small>. In this position the lever <small>A</small> is not<span class='pagenum'><a name="Page_459" id="Page_459">[Pg 459]</a></span>
+moved by any ordinary variation in the current, because the armature
+<i>b</i> is strongly attracted by the magnetism of <i>e</i>, and these
+parts are close to each other, and the magnetism of <i>e</i> acts at right
+angles to the magnetism of the core <small>M'</small>. If, now, the arc becomes
+too long, the current through the helix <small>M</small> is lessened, and the magnetism
+of the core <small>N<sup>3</sup></small> is increased by the greater current passing
+through the shunt, and this core <small>N<sup>3</sup></small>, attracting the segmental armature
+<i>r</i>, lessens the hold of the clamp <small>R</small> upon the rod <small>S</small>, allowing
+the latter to slide and lessen the length of the arc, which instantly
+restores the magnetic equilibrium and causes the clamp <small>R</small> to hold
+the rod <small>S</small>. If it happens that the carbons fall into contact, then
+the magnetism of <small>N<sup>2</sup></small> is lessened so much that the attraction of
+the magnet <small>M</small> will be sufficient to move the armature <i>a</i> and lever
+<small>A</small> so that the armature <i>b</i> passes above the normal position, so as
+to separate the carbons instantly; but when the carbons burn
+away, a greater amount of current will pass through the shunt
+until the attraction of the core <small>N<sup>2</sup></small> will overcome the attraction of
+the core <small>M'</small> and bring the armature lever <small>A</small> again into the normal
+horizontal position, and this occurs before the feed can take place.
+The segmental armature pieces <i>r</i> are shown as nearly semicircular.
+They are square or of any other desired shape, the ends of the
+pole pieces <small>M<sup>3</sup></small>, <small>N<sup>3</sup></small> being made to correspond in shape.</p>
+
+<p>In a modification of this lamp, Mr. Tesla provided means for
+automatically withdrawing a lamp from the circuit, or cutting
+it out when, from a failure of the feed, the arc reached an
+abnormal length; and also means for automatically reinserting
+such lamp in the circuit when the rod drops and the carbons
+come into contact.</p>
+
+<p>Fig. 283 is an elevation of the lamp with the case in section.
+Fig. 284 is a sectional plan at the line <i>x x</i>. Fig. 285 is an elevation,
+partly in section, of the lamp at right angles to Fig. 283.
+Fig. 286 is a sectional plan at the line <i>y y</i> of Fig. 283. Fig. 287
+is a section of the clamp in about full size. Fig. 288 is a detached
+section illustrating the connection of the spring to the
+lever that carries the pivots of the clamp, and Fig. 289 is a
+diagram showing the circuit-connections of the lamp.</p>
+
+<p>In Fig. 283, <small>M</small> represents the main and <small>N</small> the shunt magnet, both
+securely fastened to the base <small>A</small>, which with its side columns, <small>S S</small>,
+are cast in one piece of brass or other diamagnetic material. To
+the magnets are soldered or otherwise fastened the brass washers
+or discs <i>a a a a</i>. Similar washers, <i>b b</i>, of fibre or other insu<span class='pagenum'><a name="Page_460" id="Page_460">[Pg 460]</a></span>lating
+material, serve to insulate the wires from the brass washers.</p>
+
+<p>The magnets <small>M</small> and <small>N</small> are made very flat, so that their width
+exceeds three times their thickness, or even more. In this way
+a comparatively small number of convolutions is sufficient to produce
+the required magnetism, while a greater surface is offered
+for cooling off the wires.</p>
+
+<div class="figcenter" style="width: 1024px;">
+<div class="figleft" style="width: 480px;">
+<img src="images/oi_474-1.jpg" width="480" height="533" alt="Fig. 286, 283." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 286.</td><td class="caption">Fig. 283.</td></tr>
+</table>
+</div>
+
+<div class="figright" style="width: 480px;">
+<img src="images/oi_474-2.jpg" width="480" height="631" alt="Fig. 285." title="" /><br />
+<span class="caption">Fig. 285.</span>
+</div>
+
+<div class="figright" style="width: 336px;">
+<img src="images/oi_474-4.jpg" width="336" height="414" alt="Fig. 287, 288." title="" /><br />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 287.</td><td class="caption">Fig. 288.</td></tr>
+</table>
+</div>
+
+<div class="figleft" style="width: 480px;">
+<img src="images/oi_474-3.jpg" width="480" height="480" alt="Fig. 284." title="" /><br />
+<span class="caption">Fig. 284.</span>
+</div>
+</div>
+
+<div style="clear: both;"></div>
+
+<p>The upper pole pieces, <i>m n</i>, of the magnets are curved, as indicated
+in the drawings, Fig. 283. The lower pole pieces <i>m' n'</i>,
+are brought near together, tapering toward the armature <i>g</i>, as
+shown in Figs. 284 and 286. The object of this taper is to concentrate
+the greatest amount of the developed magnetism upon
+the armature, and also to allow the pull to be exerted always upon
+the middle of the armature <i>g</i>. This armature <i>g</i> is a piece of iron<span class='pagenum'><a name="Page_461" id="Page_461">[Pg 461]</a></span>
+in the shape of a hollow cylinder, having on each side a segment
+cut away, the width of which is equal to the width of the pole
+pieces <i>m' n'</i>.</p>
+
+<p>The armature is soldered or otherwise fastened to the clamp <i>r</i>,
+which is formed of a brass tube, provided with gripping-jaws <i>e e</i>,
+Fig. 287. These jaws are arcs of a circle of the diameter of the
+rod <small>R</small>, and are made of hardened German silver. The guides
+<i>f f</i>, through which the carbon-holding rod <small>R</small> slides, are made of
+the same material. This has the advantage of reducing greatly the
+wear and corrosion of the parts coming in frictional contact with
+the rod, which frequently causes trouble. The jaws <i>e e</i> are
+fastened to the inside of the tube <i>r</i>, so that one is a little lower
+than the other. The object of this is to provide a greater opening
+for the passage of the rod when the same is released by the
+clamp. The clamp <i>r</i> is supported on bearings <i>w w</i>, Figs. 283,
+285 and 287, which are just in the middle between the jaws <i>e e</i>.
+The bearings <i>w w</i> are carried by a lever, <i>t</i>, one end of which
+rests upon an adjustable support, <i>q</i>, of the side columns, <small>S</small>, the
+other end being connected by means of the link <i>e'</i> to the armature-lever
+<small>L</small>. The armature-lever <small>L</small> is a flat piece of iron in <big><b>N</b></big>
+shape, having its ends curved so as to correspond to the form of
+the upper pole-pieces of the magnets <small>M</small> and <small>N</small>. It is hung upon
+the pivots <i>v v</i>, Fig. 284, which are in the jaw <i>x</i> of the
+top plate <small>B</small>. This plate <small>B</small>, with the jaw, is cast in one piece
+and screwed to the side columns, <small>S S</small>, that extend up from the
+base <small>A</small>. To partly balance the overweight of the moving parts,
+a spring, <i>s'</i>, Figs. 284 and 288, is fastened to the top plate, <small>B</small>,
+and hooked to the lever <i>t</i>. The hook <i>o</i> is toward one side of the
+lever or bent a little sidewise, as seen in Fig. 288. By this means
+a slight tendency is given to swing the armature toward the
+pole-piece <i>m'</i> of the main magnet.</p>
+
+<p>The binding-posts <small>K K'</small> are screwed to the base <small>A</small>. A manual
+switch, for short-circuiting the lamp when the carbons are renewed,
+is also fastened to the base. This switch is of ordinary
+character, and is not shown in the drawings.</p>
+
+<p>The rod <small>R</small> is electrically connected to the lamp-frame by means
+of a flexible conductor or otherwise. The lamp-case receives a
+removable cover, <i>s</i><sup>2</sup>, to inclose the parts.</p>
+
+<p>The electrical connections are as indicated diagrammatically in
+Fig. 289. The wire in the main magnet consists of two parts,
+<i>x'</i> and <i>p'</i>. These two parts may be in two separated coils or in<span class='pagenum'><a name="Page_462" id="Page_462">[Pg 462]</a></span>
+one single helix, as shown in the drawings. The part <i>x'</i> being
+normally in circuit, is, with the fine wire upon the shunt-magnet,
+wound and traversed by the current in the same direction, so as
+to tend to produce similar poles, <small>N N</small> or <small>S S</small>, on the corresponding
+pole-pieces of the magnets <small>M</small> and <small>N</small>. The part <i>p'</i> is only in circuit
+when the lamp is cut out, and then the current being in the
+opposite direction produces in the main magnet, magnetism of
+the opposite polarity.</p>
+
+<p>The operation is as follows: At the start the carbons are to
+be in contact, and the current passes from the positive binding-post
+<small>K</small> to the lamp-frame, carbon-holder, upper and lower carbon,
+insulated return-wire in one of the side rods, and from there
+through the part <i>x'</i> of the wire on the main magnet to the negative
+binding-post. Upon the passage of the current the main
+magnet is energized and attracts the clamping-armature <i>g</i>, swinging
+the clamp and gripping the rod by means of the gripping
+jaws <i>e e</i>. At the same time the armature lever <small>L</small> is pulled down
+and the carbons are separated. In pulling down the armature lever
+<small>L</small> the main magnet is assisted by the shunt-magnet <small>N</small>, the latter
+being magnetized by magnetic induction from the magnet <small>M</small>.</p>
+
+<div class="figcenter" style="width: 616px;">
+<img src="images/oi_476.jpg" width="616" height="480" alt="Fig. 289." title="" />
+<span class="caption">Fig. 289.</span>
+</div>
+
+
+<p>It will be seen that the armatures <small>L</small> and <i>g</i> are practically the
+keepers for the magnets <small>M</small> and <small>N</small>, and owing to this fact both
+magnets with either one of the armatures <small>L</small> and <i>g</i> may be considered
+as one horseshoe magnet, which we might term a "compound
+magnet." The whole of the soft-iron parts <small>M</small>, <i>m'</i>, <i>g</i>, <i>n'</i>,
+<small>N</small> and <small>L</small> form a compound magnet.<span class='pagenum'><a name="Page_463" id="Page_463">[Pg 463]</a></span></p>
+
+<p>The carbons being separated, the fine wire receives a portion
+of the current. Now, the magnetic induction from the magnet
+<small>M</small> is such as to produce opposite poles on the corresponding ends
+of the magnet <small>N</small>; but the current traversing the helices tends to
+produce similar poles on the corresponding ends of both magnets,
+and therefore as soon as the fine wire is traversed by sufficient
+current the magnetism of the whole compound magnet is diminished.</p>
+
+<p>With regard to the armature <i>g</i> and the operation of the lamp,
+the pole <i>m'</i> may be considered as the "clamping" and the pole <i>n'</i>
+as the "releasing" pole.</p>
+
+<p>As the carbons burn away, the fine wire receives more current
+and the magnetism diminishes in proportion. This causes the
+armature lever <small>L</small> to swing and the armature <i>g</i> to descend gradually
+under the weight of the moving parts until the end <i>p</i>, Fig.
+283, strikes a stop on the top plate, <small>B</small>. The adjustment is such
+that when this takes place the rod <small>R</small> is yet gripped securely by
+the jaws <i>e e</i>. The further downward movement of the armature
+lever being prevented, the arc becomes longer as the carbons are
+consumed, and the compound magnet is weakened more and
+more until the clamping armature <i>g</i> releases the hold of the
+gripping-jaws <i>e e</i> upon the rod <small>R</small>, and the rod is allowed to drop
+a little, thus shortening the arc. The fine wire now receiving
+less current, the magnetism increases, and the rod is clamped
+again and slightly raised, if necessary. This clamping and releasing
+of the rod continues until the carbons are consumed. In
+practice the feed is so sensitive that for the greatest part of the
+time the movement of the rod cannot be detected without some
+actual measurement. During the normal operation of the lamp
+the armature lever <small>L</small> remains practically stationary, in the position
+shown in Fig. 283.</p>
+
+<p>Should it happen that, owing to an imperfection in it, the rod
+and the carbons drop too far, so as to make the arc too short, or
+even bring the carbons in contact, a very small amount of current
+passes through the fine wire, and the compound magnet
+becomes sufficiently strong to act as at the start in pulling the
+armature lever <small>L</small> down and separating the carbons to a greater
+distance.</p>
+
+<p>It occurs often in practical work that the rod sticks in the
+guides. In this case the are reaches a great length, until it finally
+breaks. Then the light goes out, and frequently the fine wire is<span class='pagenum'><a name="Page_464" id="Page_464">[Pg 464]</a></span>
+injured. To prevent such an accident Mr. Tesla provides this
+lamp with an automatic cut-out which operates as follows: When,
+upon a failure of the feed, the arc reaches a certain predetermined
+length, such an amount of current is diverted through
+the fine wire that the polarity of the compound magnet is reversed.
+The clamping armature <i>g</i> is now moved against the
+shunt magnet <small>N</small> until it strikes the releasing pole <i>n'</i>. As soon
+as the contact is established, the current passes from the positive
+binding post over the clamp <i>r</i>, armature <i>g</i>, insulated shunt magnet,
+and the helix <i>p'</i> upon the main magnet <small>M</small> to the negative
+binding post. In this case the current passes in the opposite direction
+and changes the polarity of the magnet <small>M</small>, at the same
+time maintaining by magnetic induction in the core of the shunt
+magnet the required magnetism without reversal of polarity, and
+the armature <i>g</i> remains against the shunt magnet pole <i>n'</i>. The
+lamp is thus cut out as long as the carbons are separated. The
+cut out may be used in this form without any further improvement;
+but Mr. Tesla arranges it so that if the rod drops and the
+carbons come in contact the arc is started again. For this purpose
+he proportions the resistance of part <i>p'</i> and the number of
+the convolutions of the wire upon the main magnet so that when
+the carbons come in contact a sufficient amount of current is diverted
+through the carbons and the part <i>x'</i> to destroy or neutralize
+the magnetism of the compound magnet. Then the armature
+<i>g</i>, having a slight tendency to approach to the clamping pole
+<i>m'</i>, comes out of contact with the releasing pole <i>n'</i>. As soon as
+this happens, the current through the part <i>p'</i> is interrupted, and
+the whole current passes through the part <i>x</i>. The magnet <small>M</small> is
+now strongly magnetized, the armature <i>g</i> is attracted, and the
+rod clamped. At the same time the armature lever <small>L</small> is pulled
+down out of its normal position and the arc started. In this way
+the lamp cuts itself out automatically when the arc gets too long,
+and reinserts itself automatically in the circuit if the carbons drop
+together.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_465" id="Page_465">[Pg 465]</a></span></p>
+<h2><a name="CHAPTER_XLI" id="CHAPTER_XLI"></a>CHAPTER XLI.</h2>
+
+<h3><span class="smcap">Improvement in "Unipolar" Generators.</span></h3>
+
+
+<p>Another interesting class of apparatus to which Mr. Tesla has
+directed his attention, is that of "unipolar" generators, in which a
+disc or a cylindrical conductor is mounted between magnetic
+poles adapted to produce an approximately uniform field. In
+the disc armature machines the currents induced in the rotating
+conductor flow from the centre to the periphery, or conversely,
+according to the direction of rotation or the lines of force as determined
+by the signs of the magnetic poles, and these currents
+are taken off usually by connections or brushes applied to the
+disc at points on its periphery and near its centre. In the case
+of the cylindrical armature machine, the currents developed in
+the cylinder are taken off by brushes applied to the sides of the
+cylinder at its ends.</p>
+
+<p>In order to develop economically an electromotive force available
+for practicable purposes, it is necessary either to rotate the
+conductor at a very high rate of speed or to use a disc of large
+diameter or a cylinder of great length; but in either case it becomes
+difficult to secure and maintain a good electrical connection
+between the collecting brushes and the conductor, owing to the
+high peripheral speed.</p>
+
+<p>It has been proposed to couple two or more discs together in
+series, with the object of obtaining a higher electro-motive force;
+but with the connections heretofore used and using other conditions
+of speed and dimension of disc necessary to securing good
+practicable results, this difficulty is still felt to be a serious
+obstacle to the use of this kind of generator. These objections
+Mr. Tesla has sought to avoid by constructing a machine with
+two fields, each having a rotary conductor mounted between its
+poles. The same principle is involved in the case of both forms
+of machine above described, but the description now given is
+confined to the disc type, which Mr. Tesla is inclined to favor for
+that machine. The discs are formed with flanges, after the<span class='pagenum'><a name="Page_466" id="Page_466">[Pg 466]</a></span>
+manner of pulleys, and are connected together by flexible conducting
+bands or belts.</p>
+
+<p>The machine is built in such manner that the direction of
+magnetism or order of the poles in one field of force is opposite
+to that in the other, so that rotation of the discs in the same direction
+develops a current in one from centre to circumference
+and in the other from circumference to centre. Contacts applied
+therefore to the shafts upon which the discs are mounted form
+the terminals of a circuit the electro-motive force in which is the
+sum of the electro-motive forces of the two discs.</p>
+
+<p>It will be obvious that if the direction of magnetism in both
+fields be the same, the same result as above will be obtained by
+driving the discs in opposite directions and crossing the connecting
+belts. In this way the difficulty of securing and maintaining
+good contact with the peripheries of the discs is avoided and a
+cheap and durable machine made which is useful for many purposes&mdash;such
+as for an exciter for alternating current generators,
+for a motor, and for any other purpose for which dynamo machines
+are used.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_480.jpg" width="640" height="600" alt="Fig. 290, 291." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 290.</td><td class="caption">Fig. 291.</td></tr>
+</table>
+</div>
+
+<p>Fig. 290 is a side view, partly in section, of this machine.
+Fig. 291 is a vertical section of the same at right angles to the
+shafts.<span class='pagenum'><a name="Page_467" id="Page_467">[Pg 467]</a></span></p>
+
+<p>In order to form a frame with two fields of force, a support,
+<small>A</small>, is cast with two pole pieces <small>B B'</small> integral with it. To this are
+joined by bolts <small>E</small> a casting <small>D</small>, with two similar and corresponding
+pole pieces <small>C C'</small>. The pole pieces <small>B B'</small> are wound and connected
+to produce a field of force of given polarity, and the pole
+pieces <small>C C'</small> are wound so as to produce a field of opposite polarity.
+The driving shafts <small>F G</small> pass through the poles and are
+journaled in insulating bearings in the casting <small>A D</small>, as shown.</p>
+
+<p><small>H K</small> are the discs or generating conductors. They are composed
+of copper, brass, or iron and are keyed or secured to their respective
+shafts. They are provided with broad peripheral flanges
+<small>J</small>. It is of course obvious that the discs may be insulated from their
+shafts, if so desired. A flexible metallic belt <small>L</small> is passed over the
+flanges of the two discs, and, if desired, may be used to drive one
+of the discs. It is better, however, to use this belt merely as a
+conductor, and for this purpose sheet steel, copper, or other suitable
+metal is used. Each shaft is provided with a driving pulley
+<small>M</small>, by which power is imparted from a driving shaft.</p>
+
+<p><small>N N</small> are the terminals. For the sake of clearness they are shown
+as provided with springs <small>P</small>, that bear upon the ends of the shafts.
+This machine, if self-exciting, would have copper bands around
+its poles; or conductors of any kind&mdash;such as wires shown in
+the drawings&mdash;may be used.</p>
+
+<hr style='width: 15%;' />
+
+<p>It is thought appropriate by the compiler to append here some
+notes on unipolar dynamos, written by Mr. Tesla, on a recent occasion.</p>
+
+
+
+<h5>NOTES ON A UNIPOLAR DYNAMO.<a name="FNanchor_15_16" id="FNanchor_15_16"></a><a href="#Footnote_15_16" class="fnanchor">[15]</a></h5>
+
+<p>It is characteristic of fundamental discoveries, of great achievements
+of intellect, that they retain an undiminished power upon
+the imagination of the thinker. The memorable experiment of
+Faraday with a disc rotating between the two poles of a magnet,
+which has borne such magnificent fruit, has long passed into
+every-day experience; yet there are certain features about this
+embryo of the present dynamos and motors which even to-day
+appear to us striking, and are worthy of the most careful study.</p>
+
+<p>Consider, for instance, the case of a disc of iron or other metal
+<span class='pagenum'><a name="Page_468" id="Page_468">[Pg 468]</a></span>revolving between the two opposite poles of a magnet, and the
+polar surfaces completely covering both sides of the disc, and
+assume the current to be taken off or conveyed to the same by
+contacts uniformly from all points of the periphery of the disc.
+Take first the case of a motor. In all ordinary motors the operation
+is dependent upon some shifting or change of the resultant
+of the magnetic attraction exerted upon the armature, this process
+being effected either by some mechanical contrivance on the
+motor or by the action of currents of the proper character. We
+may explain the operation of such a motor just as we can that of
+a water-wheel. But in the above example of the disc surrounded
+completely by the polar surfaces, there is no shifting of the magnetic
+action, no change whatever, as far as we know, and yet
+rotation ensues. Here, then, ordinary considerations do not
+apply; we cannot even give a superficial explanation, as in ordinary
+motors, and the operation will be clear to us only when we
+shall have recognized the very nature of the forces concerned,
+and fathomed the mystery of the invisible connecting mechanism.</p>
+
+<p>Considered as a dynamo machine, the disc is an equally interesting
+object of study. In addition to its peculiarity of giving
+currents of one direction without the employment of commutating
+devices, such a machine differs from ordinary dynamos in
+that there is no reaction between armature and field. The armature
+current tends to set up a magnetization at right angles to
+that of the field current, but since the current is taken off uniformly
+from all points of the periphery, and since, to be exact,
+the external circuit may also be arranged perfectly symmetrical
+to the field magnet, no reaction can occur. This, however, is
+true only as long as the magnets are weakly energized, for when
+the magnets are more or less saturated, both magnetizations at
+right angles seemingly interfere with each other.</p>
+
+<p>For the above reason alone it would appear that the output of
+such a machine should, for the same weight, be much greater
+than that of any other machine in which the armature current
+tends to demagnetize the field. The extraordinary output of the
+Forbes unipolar dynamo and the experience of the writer confirm
+this view.</p>
+
+<p>Again, the facility with which such a machine may be made to
+excite itself is striking, but this may be due&mdash;besides to the absence
+of armature reaction&mdash;to the perfect smoothness of the current
+and non-existence of self-induction.<span class='pagenum'><a name="Page_469" id="Page_469">[Pg 469]</a></span></p>
+
+<p>If the poles do not cover the disc completely on both sides,
+then, of course, unless the disc be properly subdivided, the
+machine will be very inefficient. Again, in this case there are
+points worthy of notice. If the disc be rotated and the field
+current interrupted, the current through the armature will continue
+to flow and the field magnets will lose their strength comparatively
+slowly. The reason for this will at once appear when
+we consider the direction of the currents set up in the disc.</p>
+
+<div class="figcenter" style="width: 613px;">
+<img src="images/oi_483.jpg" width="613" height="480" alt="Fig. 292." title="" />
+<span class="caption">Fig. 292.</span>
+</div>
+
+
+<p>Referring to the diagram Fig. 292, <i>d</i> represents the disc with
+the sliding contacts <small>B B'</small> on the shaft and periphery. <small>N</small> and <small>S</small>
+represent the two poles of a magnet. If the pole <small>N</small> be above, as
+indicated in the diagram, the disc being supposed to be in the
+plane of the paper, and rotating in the direction of the arrow <small>D</small>,
+the current set up in the disc will flow from the centre to the
+periphery, as indicated by the arrow <small>A</small>. Since the magnetic action
+is more or less confined to the space between the poles <small>N S</small>,
+the other portions of the disc may be considered inactive. The
+current set up will therefore not wholly pass through the external
+circuit <small>F</small>, but will close through the disc itself, and generally, if
+the disposition be in any way similar to the one illustrated, by far
+the greater portion of the current generated will not appear externally,
+as the circuit <small>F</small> is practically short-circuited by the inactive
+portions of the disc. The direction of the resulting currents
+in the latter may be assumed to be as indicated by the dotted<span class='pagenum'><a name="Page_470" id="Page_470">[Pg 470]</a></span>
+lines and arrows <i>m</i> and <i>n</i>; and the direction of the energizing
+field current being indicated by the arrows <i>a b c d</i>, an inspection of
+the figure shows that one of the two branches of the eddy current,
+that is, <small>A B'</small> <i>m</i> <small>B</small>, will tend to demagnetize the field, while the
+other branch, that is, <small>A B'</small> <i>n</i> <small>B</small>, will have the opposite effect.
+Therefore, the branch <small>A B'</small> <i>m</i> <small>B</small>, that is, the one which is <i>approaching</i>
+the field, will repel the lines of the same, while branch <small>A B'</small>
+<i>n</i> <small>B</small>, that is, the one <i>leaving</i> the field, will gather the lines of
+force upon itself.</p>
+
+<p>In consequence of this there will be a constant tendency to
+reduce the current flow in the path <small>A B'</small> <i>m</i> <small>B</small>, while on the other
+hand no such opposition will exist in path <small>A B'</small> <i>n</i> <small>B</small>, and the effect
+of the latter branch or path will be more or less preponderating
+over that of the former. The joint effect of both the assumed
+branch currents might be represented by that of one single current
+of the same direction as that energizing the field. In other
+words, the eddy currents circulating in the disc will energize the
+field magnet. This is a result quite contrary to what we might
+be led to suppose at first, for we would naturally expect that the
+resulting effect of the armature currents would be such as to
+oppose the field current, as generally occurs when a primary and
+secondary conductor are placed in inductive relations to each
+other. But it must be remembered that this results from the
+peculiar disposition in this case, namely, two paths being afforded
+to the current, and the latter selecting that path which offers the
+least opposition to its flow. From this we see that the eddy
+currents flowing in the disc partly energize the field, and for this
+reason when the field current is interrupted the currents in the
+disc will continue to flow, and the field magnet will lose its
+strength with comparative slowness and may even retain a certain
+strength as long as the rotation of the disc is continued.</p>
+
+<p>The result will, of course, largely depend on the resistance
+and geometrical dimensions of the path of the resulting eddy
+current and on the speed of rotation; these elements, namely,
+determine the retardation of this current and its position relative
+to the field. For a certain speed there would be a maximum
+energizing action; then at higher speeds, it would gradually fall
+off to zero and finally reverse, that is, the resultant eddy current
+effect would be to weaken the field. The reaction would be
+best demonstrated experimentally by arranging the fields <small>N S</small>,
+<small>N' S'</small>, freely movable on an axis concentric with the shaft of the<span class='pagenum'><a name="Page_471" id="Page_471">[Pg 471]</a></span>
+disc. If the latter were rotated as before in the direction of the
+arrow <small>D</small>, the field would be dragged in the same direction with a
+torque, which, up to a certain point, would go on increasing with
+the speed of rotation, then fall off, and, passing through zero,
+finally become negative; that is, the field would begin to rotate
+in opposite direction to the disc. In experiments with alternate
+current motors in which the field was shifted by currents of
+differing phase, this interesting result was observed. For very
+low speeds of rotation of the field the motor would show a
+torque of 900 lbs. or more, measured on a pulley 12 inches
+in diameter. When the speed of rotation of the poles was
+increased, the torque would diminish, would finally go down to
+zero, become negative, and then the armature would begin to
+rotate in opposite direction to the field.</p>
+
+<p>To return to the principal subject; assume the conditions to be
+such that the eddy currents generated by the rotation of the disc
+strengthen the field, and suppose the latter gradually removed
+while the disc is kept rotating at an increased rate. The current,
+once started, may then be sufficient to maintain itself and even
+increase in strength, and then we have the case of Sir William
+Thomson's "current accumulator." But from the above considerations
+it would seem that for the success of the experiment
+the employment of a disc <i>not subdivided</i><a name="FNanchor_16_17" id="FNanchor_16_17"></a><a href="#Footnote_16_17" class="fnanchor">[16]</a> would be essential,
+for if there should be a radial subdivision, the eddy currents
+could not form and the self-exciting action would cease. If
+such a radially subdivided disc were used it would be necessary
+to connect the spokes by a conducting rim or in any proper
+manner so as to form a symmetrical system of closed circuits.</p>
+
+<p>The action of the eddy currents may be utilized to excite a machine
+of any construction. For instance, in Figs. 293 and 294 an
+arrangement is shown by which a machine with a disc armature
+might be excited. Here a number of magnets, <small>N S</small>, <small>N S</small>, are
+placed radially on each side of a metal disc <small>D</small> carrying on its rim
+a set of insulated coils, <small>C C</small>. The magnets form two separate
+fields, an internal and an external one, the solid disc rotating in the
+<span class='pagenum'><a name="Page_472" id="Page_472">[Pg 472]</a></span>field nearest the axis, and the coils in the field further from it.
+Assume the magnets slightly energized at the start; they could be
+strengthened by the action of the eddy currents in the solid disc
+so as to afford a stronger field for the peripheral coils. Although
+there is no doubt that under proper conditions a machine might
+be excited in this or a similar manner, there being sufficient experimental
+evidence to warrant such an assertion, such a mode of
+excitation would be wasteful.</p>
+
+<p>But a unipolar dynamo or motor, such as shown in Fig. 292,
+may be excited in an efficient manner by simply properly subdividing
+the disc or cylinder in which the currents are set up, and
+it is practicable to do away with the field coils which are usually
+employed. Such a plan is illustrated in Fig. 295. The disc or
+cylinder <small>D</small> is supposed to be arranged to rotate between the two
+poles <small>N</small> and <small>S</small> of a magnet, which completely cover it on both
+sides, the contours of the disc and poles being represented by the
+circles <i>d</i> and <i>d</i><sup>1</sup> respectively, the upper pole being omitted for
+the sake of clearness. The cores of the magnet are supposed to
+be hollow, the shaft <small>C</small> of the disc passing through them. If the
+unmarked pole be below, and the disc be rotated screw fashion,
+the current will be, as before, from the centre to the periphery,
+and may be taken off by suitable sliding contacts, <small>B B'</small>, on the
+shaft and periphery respectively. In this arrangement the current
+flowing through the disc and external circuit will have no
+appreciable effect on the field magnet.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_486.jpg" width="800" height="557" alt="Fig. 293, 294." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 293.</td><td class="caption">Fig. 294.</td></tr>
+</table>
+</div>
+
+<p>But let us now suppose the disc to be subdivided spirally, as
+<span class='pagenum'><a name="Page_473" id="Page_473">[Pg 473]</a></span>indicated by the full or dotted lines, Fig. 295. The difference of
+potential between a point on the shaft and a point on the periphery
+will remain unchanged, in sign as well as in amount. The
+only difference will be that the resistance of the disc will be augmented
+and that there will be a greater fall of potential from a
+point on the shaft to a point on the periphery when the same current
+is traversing the external circuit. But since the current is
+forced to follow the lines of subdivision, we see that it will tend
+either to energize or de-energize the field, and this will depend,
+other things being equal, upon the direction of the lines of subdivision.
+If the subdivision be as indicated by the full lines in
+Fig. 295, it is evident that if the current is of the same direction
+as before, that is, from centre to periphery, its effect will be to
+strengthen the field magnet; Whereas, if the subdivision be as indicated
+by the dotted lines, the current generated will tend to
+weaken the magnet. In the former case the machine will be
+capable of exciting itself when the disc is rotated in the direction
+of arrow <small>D</small>; in the latter case the direction of rotation must be
+reversed. Two such discs may be combined, however, as indicated,
+the two discs rotating in opposite fields, and in the same
+or opposite direction.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_487.jpg" width="800" height="382" alt="Fig. 295, 296." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 295.</td><td class="caption">Fig. 296.</td></tr>
+</table>
+</div>
+
+<p>Similar disposition may, of course, be made in a type of
+machine in which, instead of a disc, a cylinder is rotated. In
+such unipolar machines, in the manner indicated, the usual field
+coils and poles may be omitted and the machine may be made to
+consist only of a cylinder or of two discs enveloped by a metal
+casting.</p>
+
+<p>Instead of subdividing the disc or cylinder spirally, as indicated
+in Fig. 295, it is more convenient to interpose one or more turns<span class='pagenum'><a name="Page_474" id="Page_474">[Pg 474]</a></span>
+between the disc and the contact ring on the periphery, as illustrated
+in Fig. 296.</p>
+
+<p>A Forbes dynamo may, for instance, be excited in such a manner.
+In the experience of the writer it has been found that instead
+of taking the current from two such discs by sliding
+contacts, as usual, a flexible conducting belt may be employed
+to advantage. The discs are in such case provided with large
+flanges, affording a very great contact surface. The belt should
+be made to bear on the flanges with spring pressure to take up
+the expansion. Several machines with belt contact were constructed
+by the writer two years ago, and worked satisfactorily;
+but for want of time the work in that direction has been temporarily
+suspended. A number of features pointed out above have
+also been used by the writer in connection with some types of
+alternating current motors.</p>
+
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_475" id="Page_475">[Pg 475]</a></span></p>
+<h1><small><a name="PART_IV" id="PART_IV"></a>PART IV.</small><br /><br />
+
+APPENDIX.&mdash;EARLY PHASE MOTORS AND THE<br />
+TESLA MECHANICAL AND ELECTRICAL<br />
+OSCILLATOR.</h1>
+<p><span class='pagenum'><a name="Page_476" id="Page_476">[Pg 476]</a></span></p>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_477" id="Page_477">[Pg 477]</a></span></p>
+<h2><a name="CHAPTER_XLII" id="CHAPTER_XLII"></a>CHAPTER XLII.</h2>
+
+<h3><span class="smcap">Mr. Tesla's Personal Exhibit at the World's Fair.</span></h3>
+
+<p>While the exhibits of firms engaged in the manufacture of
+electrical apparatus of every description at the Chicago World's
+Fair, afforded the visitor ample opportunity for gaining an excellent
+knowledge of the state of the art, there were also numbers
+of exhibits which brought out in strong relief the work of the
+individual inventor, which lies at the foundation of much, if not
+all, industrial or mechanical achievement. Prominent among
+such personal exhibits was that of Mr. Tesla, whose apparatus
+occupied part of the space of the Westinghouse Company, in
+Electricity Building.</p>
+
+<p>This apparatus represented the results of work and thought
+covering a period of ten years. It embraced a large number of
+different alternating motors and Mr. Tesla's earlier high frequency
+apparatus. The motor exhibit consisted of a variety of
+fields and armatures for two, three and multiphase circuits, and
+gave a fair idea of the gradual evolution of the fundamental idea
+of the rotating magnetic field. The high frequency exhibit included
+Mr. Tesla's earlier machines and disruptive discharge coils
+and high frequency transformers, which he used in his investigations
+and some of which are referred to in his papers printed
+in this volume.</p>
+
+<p>Fig. 297 shows a view of part of the exhibits containing the
+motor apparatus. Among these is shown at A a large ring intended
+to exhibit the phenomena of the rotating magnetic field.
+The field produced was very powerful and exhibited striking
+effects, revolving copper balls and eggs and bodies of various
+shapes at considerable distances and at great speeds. This ring
+was wound for two-phase circuits, and the winding was so distributed
+that a practically uniform field was obtained. This ring
+was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician
+of the Westinghouse Electric and Manufacturing Company.</p>
+<p><span class='pagenum'><a name="Page_478" id="Page_478">[Pg 478]</a></span></p>
+
+<div class="figcenter" style="width: 1024px;">
+<img src="images/oi_492.jpg" width="1024" height="487" alt="Fig. 297." title="" />
+<span class="caption">Fig. 297.</span>
+</div>
+
+
+<p><span class='pagenum'><a name="Page_479" id="Page_479">[Pg 479]</a></span></p>
+<p>A smaller ring, shown at <small>B</small>, was arranged like the one exhibited
+at <small>A</small> but designed especially to exhibit the rotation of an
+armature in a rotating field. In connection with these two
+rings there was an interesting exhibit shown by Mr. Tesla which
+consisted of a magnet with a coil, the magnet being arranged to
+rotate in bearings. With this magnet he first demonstrated the
+identity between a rotating field and a rotating magnet; the latter,
+when rotating, exhibited the same phenomena as the rings when
+they were energized by currents of differing phase. Another
+prominent exhibit was a model illustrated at <small>C</small> which is a two-phase
+motor, as well as an induction motor and transformer. It
+consists of a large outer ring of laminated iron wound with
+two superimposed, separated windings which can be connected
+in a variety of ways. This is one of the first models used by
+Mr. Tesla as an induction motor and rotating transformer. The
+armature was either a steel or wrought iron disc with a closed
+coil. When the motor was operated from a two phase generator
+the windings were connected in two groups, as usual. When
+used as an induction motor, the current induced in one of the
+windings of the ring was passed through the other winding on
+the ring and so the motor operated with only two wires. When
+used as a transformer the outer winding served, for instance, as
+a secondary and the inner as a primary. The model shown at
+D is one of the earliest rotating field motors, consisting of a thin
+iron ring wound with two sets of coils and an armature consisting
+of a series of steel discs partly cut away and arranged on a small
+arbor.</p>
+
+<p>At <small>E</small> is shown one of the first rotating field or induction motors
+used for the regulation of an arc lamp and for other purposes. It
+comprises a ring of discs with two sets of coils having different
+self-inductions, one set being of German silver and the other of
+copper wire. The armature is wound with two closed-circuited
+coils at right angles to each other. To the armature shaft are
+fastened levers and other devices to effect the regulation. At <small>F</small>
+is shown a model of a magnetic lag motor; this embodies a casting
+with pole projections protruding from two coils between
+which is arranged to rotate a smooth iron body. When an alternating
+current is sent through the two coils the pole projections
+of the field and armature within it are similarly magnetized, and
+upon the cessation or reversal of the current the armature and
+field repel each other and rotation is produced in this way.<span class='pagenum'><a name="Page_480" id="Page_480">[Pg 480]</a></span>
+Another interesting exhibit, shown at <small>G</small>, is an early model of a
+two field motor energized by currents of different phase. There
+are two independent fields of laminated iron joined by brass
+bolts; in each field is mounted an armature, both armatures being
+on the same shaft. The armatures were originally so arranged
+as to be placed in any position relatively to each other,
+and the fields also were arranged to be connected in a number
+of ways. The motor has served for the exhibition of a number
+of features; among other things, it has been used as a dynamo
+for the production of currents of any frequency between wide
+limits. In this case the field, instead of being energized by direct
+current, was energized by currents differing in phase, which
+produced a rotation of the field; the armature was then rotated
+in the same or in opposite direction to the movement of the field;
+and so any number of alternations of the currents induced in the
+armature, from a small to a high number, determined by the
+frequency of the energizing field coils and the speed of the armature,
+was obtained.</p>
+
+<div class="figcenter" style="width: 632px;">
+<img src="images/oi_494.jpg" width="632" height="480" alt="Fig. 298." title="" />
+<span class="caption">Fig. 298.</span>
+</div>
+
+<p>The models <small>H</small>, <small>I</small>, <small>J</small>, represent a variety of rotating field, synchronous
+motors which are of special value in long distance transmission
+work. The principle embodied in these motors was enunciated
+by Mr. Tesla in his lecture before the American Institute of
+Electrical Engineers, in May, 1888<a name="FNanchor_17_18" id="FNanchor_17_18"></a><a href="#Footnote_17_18" class="fnanchor">[17]</a>. It involves the production
+<span class='pagenum'><a name="Page_481" id="Page_481">[Pg 481]</a></span>of the rotating field in one of the elements of the motor by currents
+differing in phase and energizing the other element by
+direct currents. The armatures are of the two and three phase
+type. <small>K</small> is a model of a motor shown in an enlarged view in Fig.
+298. This machine, together with that shown in Fig. 299, was
+exhibited at the same lecture, in May, 1888. They were
+the first rotating field motors which were independently tested,
+having for that purpose been placed in the hands of Prof. Anthony
+in the winter of 1887-88. From these tests it was shown
+that the efficiency and output of these motors was quite satisfactory
+in every respect.</p>
+
+<div class="figcenter" style="width: 640px;">
+<img src="images/oi_495.jpg" width="640" height="410" alt="Fig. 299." title="" />
+<span class="caption">Fig. 299.</span>
+</div>
+
+
+<p>It was intended to exhibit the model shown in Fig. 299, but it
+was unavailable for that purpose owing to the fact that it was
+some time ago handed over to the care of Prof. Ayrton in England.
+This model was originally provided with twelve independent
+coils; this number, as Mr. Tesla pointed out in his first lecture,
+being divisible by two and three, was selected in order to make
+various connections for two and three-phase operations, and during
+Mr. Tesla's experiments was used in many ways with from two to
+six phases. The model, Fig. 298, consists of a magnetic frame of
+laminated iron with four polar projections between which an armature
+is supported on brass bolts passing through the frame. A
+great variety of armatures was used in connection with these two
+and other fields. Some of the armatures are shown in front on
+the table, Fig. 297, and several are also shown enlarged in Figs.
+300 to 310. An interesting exhibit is that shown at <small>L</small>, Fig. 297.
+This is an armature of hardened steel which was used in a demon<span class='pagenum'><a name="Page_482" id="Page_482">[Pg 482]</a></span>stration
+before the Society of Arts in Boston, by Prof. Anthony.
+Another curious exhibit is shown enlarged in Fig. 301. This
+consists of thick discs of wrought iron placed lengthwise, with a
+mass of copper cast around them. The discs were arranged
+longitudinally to afford an easier starting by reason of the induced
+current formed in the iron discs, which differed in phase from
+those in the copper. This armature would start with a single circuit
+and run in synchronism, and represents one of the earliest
+types of such an armature. Fig. 305 is another striking exhibit.
+This is one of the earliest types of an armature with holes beneath
+the periphery, in which copper conductors are imbedded. The
+armature has eight closed circuits and was used in many different
+ways. Fig. 304 is a type of synchronous armature consisting of
+a block of soft steel wound with a coil closed upon itself. This
+armature was used in connection with the field shown in Fig. 298
+and gave excellent results.</p>
+
+<div class="figcenter" style="width: 800px;">
+<img src="images/oi_496-1.jpg" width="800" height="139" alt="Fig. 300, 301, 302." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 300.</td><td class="caption">Fig. 301.</td><td class="caption">Fig. 302.</td></tr>
+</table>
+
+<img src="images/oi_496-2.jpg" width="800" height="172" alt="Fig. 303, 304, 305." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 303.</td><td class="caption">Fig. 304.</td><td class="caption">Fig. 305.</td></tr>
+</table>
+
+<img src="images/oi_496-3.jpg" width="800" height="150" alt="Fig. 306, 307, 308." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 306.</td><td class="caption">Fig. 307.</td><td class="caption">Fig. 308.</td></tr>
+</table>
+
+<img src="images/oi_496.jpg" width="800" height="139" alt="Fig. 309, 310." title="" />
+<table border="0" cellpadding="4" cellspacing="0" summary="" width="100%">
+<tr><td class="caption">Fig. 309.</td><td class="caption">Fig. 310.</td></tr>
+</table>
+</div>
+
+
+<p>Fig. 302 represents a synchronous armature with a large coil
+around a body of iron. There is another very small coil at right
+angles to the first. This small coil was used for the purpose of<span class='pagenum'><a name="Page_483" id="Page_483">[Pg 483]</a></span>
+increasing the starting torque and was found very effective in
+this connection. Figs. 306 and 308 show a favorite construction
+of armature; the iron body is made up of two sets of discs cut
+away and placed at right angles to each other, the interstices being
+wound with coils. The one shown in Fig. 308 is provided
+with an additional groove on each of the projections formed by
+the discs, for the purpose of increasing the starting torque by a
+wire wound in these projections. Fig. 307 is a form of armature
+similarly constructed, but with four independent coils wound upon
+the four projections. This armature was used to reduce the
+speed of the motor with reference to that of the generator. Fig.
+300 is still another armature with a great number of independent
+circuits closed upon themselves, so that all the dead points on
+the armature are done away with, and the armature has a large
+starting torque. Fig. 303 is another type of armature for a four-pole
+motor but with coils wound upon a smooth surface. A
+number of these armatures have hollow shafts, as they have been
+used in many ways. Figs. 309 and 310 represent armatures to
+which either alternating or direct current was conveyed by
+means of sliding rings. Fig. 309 consists of a soft iron body
+with a single coil wound around it, the ends of the coil being
+connected to two sliding rings to which, usually, direct current
+was conveyed. The armature shown in Fig. 310 has three insulated
+rings on a shaft and was used in connection with two or
+three phase circuits.</p>
+
+<p>All these models shown represent early work, and the enlarged
+engravings are made from photographs taken early in
+1888. There is a great number of other models which were exhibited,
+but which are not brought out sharply in the engraving,
+Fig. 297. For example at <small>M</small> is a model of a motor comprising
+an armature with a hollow shaft wound with two or three coils for
+two or three-phase circuits; the armature was arranged to be stationary
+and the generating circuits were connected directly to
+the generator. Around the armature is arranged to rotate on
+its shaft a casting forming six closed circuits. On the outside
+this casting was turned smooth and the belt was placed on it for
+driving with any desired appliance. This also is a very early
+model.</p>
+
+<p>On the left side of the table there are seen a large variety of
+models, <small>N</small>, <small>O</small>, <small>P</small>, etc., with fields of various shapes. Each of these
+models involves some distinct idea and they all represent gradual<span class='pagenum'><a name="Page_484" id="Page_484">[Pg 484]</a></span>
+development chiefly interesting as showing Mr. Tesla's efforts to
+adapt his system to the existing high frequencies.</p>
+
+<p>On the right side of the table, at <small>S</small>, <small>T</small>, are shown, on separate
+supports, larger and more perfected armatures of commercial
+motors, and in the space around the table a variety of motors and
+generators supplying currents to them was exhibited.</p>
+
+<p>The high frequency exhibit embraced Mr. Tesla's first original
+apparatus used in his investigations. There was exhibited a
+glass tube with one layer of silk-covered wire wound at the top
+and a copper ribbon on the inside. This was the first disruptive
+discharge coil constructed by him. At <small>U</small> is shown the disruptive
+discharge coil exhibited by him in his lecture before the American
+Institute of Electrical Engineers, in May, 1891.<a name="FNanchor_18_19" id="FNanchor_18_19"></a><a href="#Footnote_18_19" class="fnanchor">[18]</a> At <small>V</small> and <small>W</small>
+are shown some of the first high frequency transformers. A
+number of various fields and armatures of small models of high
+frequency apparatus as shown at <small>X</small> and <small>Y</small>, and others not visible
+in the picture, were exhibited. In the annexed space the dynamo
+then used by Mr. Tesla at Columbia College was exhibited; also
+another form of high frequency dynamo used.</p>
+
+<div class="figcenter" style="width: 611px;">
+<img src="images/oi_498.jpg" width="611" height="480" alt="Fig. 311." title="" />
+<span class="caption">Fig. 311.</span>
+</div>
+
+<p>In this space also was arranged a battery of Leyden jars and
+his large disruptive discharge coil which was used for exhibiting
+<span class='pagenum'><a name="Page_485" id="Page_485">[Pg 485]</a></span>the light phenomena in the adjoining dark room. The coil was
+operated at only a small fraction of its capacity, as the necessary
+condensers and transformers could not be had and as Mr. Tesla's
+stay was limited to one week; notwithstanding, the phenomena
+were of a striking character. In the room were arranged two
+large plates placed at a distance of about eighteen feet from each
+other. Between them were placed two long tables with all sorts
+of phosphorescent bulbs and tubes; many of these were prepared
+with great care and marked legibly with the names which would
+shine with phosphorescent glow. Among them were some with
+the names of Helmholtz, Faraday, Maxwell, Henry, Franklin,
+etc. Mr. Tesla had also not forgotten the greatest living poet of
+his own country, Zmaj Jovan; two or three were prepared with
+inscriptions, like "Welcome, Electricians," and produced a beautiful
+effect. Each represented some phase of this work and stood
+for some individual experiment of importance. Outside the room
+was the small battery seen in Fig. 311, for the exhibition of some
+of the impedance and other phenomena of interest. Thus, for
+instance, a thick copper bar bent in arched form was provided
+with clamps for the attachment of lamps, and a number of lamps
+were kept at incandescence on the bar; there was also a little motor
+shown on the table operated by the disruptive discharge.</p>
+
+<p>As will be remembered by those who visited the Exposition,
+the Westinghouse Company made a line exhibit of the various
+commercial motors of the Tesla system, while the twelve generators
+in Machinery Hall were of the two-phase type constructed
+for distributing light and power. Mr. Tesla, also exhibited
+some models of his oscillators.</p>
+
+
+
+<hr style="width: 65%;" />
+<p><span class='pagenum'><a name="Page_486" id="Page_486">[Pg 486]</a></span></p>
+<h2><a name="CHAPTER_XLIII" id="CHAPTER_XLIII"></a>CHAPTER XLIII.</h2>
+
+<h3><span class="smcap">The Tesla Mechanical and Electrical Oscillators.</span></h3>
+
+
+<p>On the evening of Friday, August 25, 1893, Mr. Tesla delivered
+a lecture on his mechanical and electrical oscillators, before
+the members of the Electrical Congress, in the hall adjoining
+the Agricultural Building, at the World's Fair, Chicago. Besides
+the apparatus in the room, he employed an air compressor,
+which was driven by an electric motor.</p>
+
+<p>Mr. Tesla was introduced by Dr. Elisha Gray, and began by
+stating that the problem he had set out to solve was to construct,
+first, a mechanism which would produce oscillations of a perfectly
+constant period independent of the pressure of steam or
+air applied, within the widest limits, and also independent of
+frictional losses and load. Secondly, to produce electric currents
+of a perfectly constant period independently of the working
+conditions, and to produce these currents with mechanism
+which should be reliable and positive in its action without resorting
+to spark gaps and breaks. This he successfully accomplished
+in his apparatus, and with this apparatus, now, scientific men will
+be provided with the necessaries for carrying on investigations
+with alternating currents with great precision. These two inventions
+Mr. Tesla called, quite appropriately, a mechanical and
+an electrical oscillator, respectively.</p>
+
+<p>The former is substantially constructed in the following way.
+There is a piston in a cylinder made to reciprocate automatically
+by proper dispositions of parts, similar to a reciprocating tool.
+Mr. Tesla pointed out that he had done a great deal of work in
+perfecting his apparatus so that it would work efficiently at such
+high frequency of reciprocation as he contemplated, but he did not
+dwell on the many difficulties encountered. He exhibited, however,
+the pieces of a steel arbor which had been actually torn
+apart while vibrating against a minute air cushion.</p>
+
+<p>With the piston above referred to there is associated in one of
+his models in an independent chamber an air spring, or dash pot,<span class='pagenum'><a name="Page_487" id="Page_487">[Pg 487]</a></span>
+or else he obtains the spring within the chambers of the oscillator
+itself. To appreciate the beauty of this it is only necessary to say
+that in that disposition, as he showed it, no matter what the
+rigidity of the spring and no matter what the weight of the moving
+parts, in other words, no matter what the period of vibrations,
+the vibrations of the spring are always isochronous with the applied
+pressure. Owing to this, the results obtained with these
+vibrations are truly wonderful. Mr. Tesla provides for an air
+spring of tremendous rigidity, and he is enabled to vibrate big
+weights at an enormous rate, considering the inertia, owing to the
+recoil of the spring. Thus, for instance, in one of these experiments,
+he vibrates a weight of approximately 20 pounds at the
+rate of about 80 per second and with a stroke of about 7/8 inch, but
+by shortening the stroke the weight could be vibrated many hundred
+times, and has been, in other experiments.</p>
+
+<p>To start the vibrations, a powerful blow is struck, but the adjustment
+can be so made that only a minute effort is required to
+start, and, even without any special provision it will start by
+merely turning on the pressure suddenly. The vibration being,
+of course, isochronous, any change of pressure merely produces a
+shortening or lengthening of the stroke. Mr. Tesla showed a
+number of very clear drawings, illustrating the construction of
+the apparatus from which its working was plainly discernible.
+Special provisions are made so as to equalize the pressure
+within the dash pot and the outer atmosphere. For this purpose
+the inside chambers of the dash pot are arranged to communicate
+with the outer atmosphere so that no matter how the temperature
+of the enclosed air might vary, it still retains the same mean
+density as the outer atmosphere, and by this means a spring of
+constant rigidity is obtained. Now, of course, the pressure of
+the atmosphere may vary, and this would vary the rigidity of the
+spring, and consequently the period of vibration, and this feature
+constitutes one of the great beauties of the apparatus; for, as Mr.
+Tesla pointed out, this mechanical system acts exactly like a
+string tightly stretched between two points, and with fixed nodes,
+so that slight changes of the tension do not in the least alter the
+period of oscillation.</p>
+
+<p>The applications of such an apparatus are, of course, numerous
+and obvious. The first is, of course, to produce electric
+currents, and by a number of models and apparatus on the lecture
+platform, Mr. Tesla showed how this could be carried out in<span class='pagenum'><a name="Page_488" id="Page_488">[Pg 488]</a></span>
+practice by combining an electric generator with his oscillator.
+He pointed out what conditions must be observed in order that
+the period of vibration of the electrical system might not disturb
+the mechanical oscillation in such a way as to alter the periodicity,
+but merely to shorten the stroke. He combines a condenser
+with a self-induction, and gives to the electrical system the same
+period as that at which the machine itself oscillates, so that both
+together then fall in step and electrical and mechanical resonance
+is obtained, and maintained absolutely unvaried.</p>
+
+<p>Next he showed a model of a motor with delicate wheelwork,
+which was driven by these currents at a constant speed, no matter
+what the air pressure applied was, so that this motor could
+be employed as a clock. He also showed a clock so constructed
+that it could be attached to one of the oscillators, and would
+keep absolutely correct time. Another curious and interesting
+feature which Mr. Tesla pointed out was that, instead of controlling
+the motion of the reciprocating piston by means of a
+spring, so as to obtain isochronous vibration, he was actually able
+to control the mechanical motion by the natural vibration of the
+electro-magnetic system, and he said that the case was a very
+simple one, and was quite analogous to that of a pendulum.
+Thus, supposing we had a pendulum of great weight, preferably,
+which would be maintained in vibration by force, periodically
+applied; now that force, no matter how it might vary, although
+it would oscillate the pendulum, would have no control over its
+period.</p>
+
+<p>Mr. Tesla also described a very interesting phenomenon which
+he illustrated by an experiment. By means of this new apparatus,
+he is able to produce an alternating current in which the
+<span class="smcap">e. m. f.</span> of the impulses in one direction preponderates over that
+of those in the other, so that there is produced the effect of a
+direct current. In fact he expressed the hope that these currents
+would be capable of application in many instances, serving
+as direct currents. The principle involved in this preponderating
+<span class="smcap">e. m. f.</span> he explains in this way: Suppose a conductor is
+moved into the magnetic field and then suddenly withdrawn. If
+the current is not retarded, then the work performed will be a
+mere fractional one; but if the current is retarded, then the
+magnetic field acts as a spring. Imagine that the motion of the
+conductor is arrested by the current generated, and that at the
+instant when it stops to move into the field, there is still the<span class='pagenum'><a name="Page_489" id="Page_489">[Pg 489]</a></span>
+maximum current flowing in the conductor; then this current
+will, according to Lenz's law, drive the conductor out of the field
+again, and if the conductor has no resistance, then it would leave
+the field with the velocity it entered it. Now it is clear that if,
+instead of simply depending on the current to drive the conductor
+out of the field, the mechanically applied force is so timed
+that it helps the conductor to get out of the field, then it might
+leave the field with higher velocity than it entered it, and
+thus one impulse is made to preponderate in <span class="smcap">e. m. f.</span> over the
+other.</p>
+
+<p>With a current of this nature, Mr. Tesla energized magnets
+strongly, and performed many interesting experiments bearing
+out the fact that one of the current impulses preponderates.
+Among them was one in which he attached to his oscillator a ring
+magnet with a small air gap between the poles. This magnet was
+oscillated up and down 80 times a second. A copper disc, when
+inserted within the air gap of the ring magnet, was brought into
+rapid rotation. Mr. Tesla remarked that this experiment also
+seemed to demonstrate that the lines of flow of current through
+a metallic mass are disturbed by the presence of a magnet in a
+manner quite independently of the so-called Hall effect. He
+showed also a very interesting method of making a connection
+with the oscillating magnet. This was accomplished by attaching
+to the magnet small insulated steel rods, and connecting to these
+rods the ends of the energizing coil. As the magnet was vibrated,
+stationary nodes were produced in the steel rods, and at these
+points the terminals of a direct current source were attached.
+Mr. Tesla also pointed out that one of the uses of currents, such
+as those produced in his apparatus, would be to select any given
+one of a number of devices connected to the same circuit by picking
+out the vibration by resonance. There is indeed little doubt
+that with Mr. Tesla's devices, harmonic and synchronous telegraphy
+will receive a fresh impetus, and vast possibilities are
+again opened up.</p>
+
+<p>Mr. Tesla was very much elated over his latest achievements,
+and said that he hoped that in the hands of practical, as well as
+scientific men, the devices described by him would yield important
+results. He laid special stress on the facility now afforded for
+investigating the effect of mechanical vibration in all directions,
+and also showed that he had observed a number of facts in connection
+with iron cores.<span class='pagenum'><a name="Page_490" id="Page_490">[Pg 490]</a></span></p>
+
+
+<div class="figcenter" style="width: 558px;">
+<img src="images/oi_504.jpg" width="558" height="480" alt="Fig. 312." title="" />
+<span class="caption">Fig. 312.</span>
+</div>
+
+
+<p>The engraving, Fig. 312, shows, in perspective, one of the
+forms of apparatus used by Mr. Tesla in his earlier investigations
+in this field of work, and its interior construction is made plain
+by the sectional view shown in Fig. 313. It will be noted that the
+piston <small>P</small> is fitted into the hollow of a cylinder <small>C</small> which is provided
+with channel ports <small>O O</small>, and <i>I</i>, extending all around the inside
+surface. In this particular apparatus there are two channels <small>O O</small>
+for the outlet of the working fluid and one, <i>I</i>, for the inlet.
+The piston <small>P</small> is provided with two slots <small>S S'</small> at a carefully determined
+distance, one from the other. The tubes <small>T T</small> which are
+screwed into the holes drilled into the piston, establish communication
+between the slots <small>S S'</small> and chambers on each side of the
+piston, each of these chambers connecting with the slot which is
+remote from it. The piston <small>P</small> is screwed tightly on a shaft <small>A</small>
+<span class='pagenum'><a name="Page_491" id="Page_491">[Pg 491]</a></span>
+which passes through fitting boxes at the end of the cylinder <small>C</small>.
+The boxes project to a carefully determined distance into the hollow
+of the cylinder <small>C</small>, thus determining the length of the stroke.</p>
+
+<p>Surrounding the whole is a jacket <small>J</small>. This jacket acts chiefly to
+diminish the sound produced by the oscillator and as a jacket when
+the oscillator is driven by steam, in which case a somewhat different
+arrangement of the magnets is employed. The apparatus here
+illustrated was intended for demonstration purposes, air being
+used as most convenient for this purpose.</p>
+
+<p>A magnetic frame <small>M M</small> is fastened so as to closely surround the
+oscillator and is provided with energizing coils which establish
+two strong magnetic fields on opposite sides. The magnetic frame
+is made up of thin sheet iron. In the intensely concentrated
+field thus produced, there are arranged two pairs of coils <small>H H</small> supported
+in metallic frames which are screwed on the shaft <b>A</b> of
+the piston and have additional bearings in the boxes <small>B B</small> on each
+side. The whole is mounted on a metallic base resting on two
+wooden blocks.</p>
+
+<div class="figcenter" style="width: 576px;">
+<img src="images/oi_505.jpg" width="576" height="480" alt="Fig. 313." title="" />
+<span class="caption">Fig. 313.</span>
+</div>
+
+<p>The operation of the device is as follows: The working fluid
+being admitted through an inlet pipe to the slot <small>I</small> and the piston
+being supposed to be in the position indicated, it is sufficient,
+though not necessary, to give a gentle tap on one of the shaft<span class='pagenum'><a name="Page_492" id="Page_492">[Pg 492]</a></span>
+ends protruding from the boxes <small>B</small>. Assume that the motion imparted
+be such as to move the piston to the left (when looking at
+the diagram) then the air rushes through the slot <small>S'</small> and tube <small>T</small>
+into the chamber to the left. The pressure now drives the piston
+towards the right and, owing to its inertia, it overshoots the
+position of equilibrium and allows the air to rush through the
+slot <small>S</small> and tube <small>T</small> into the chamber to the right, while the communication
+to the left hand chamber is cut off, the air of the
+latter chamber escaping through the outlet <small>O</small> on the left. On
+the return stroke a similar operation takes place on the right
+hand side. This oscillation is maintained continuously and the
+apparatus performs vibrations from a scarcely perceptible quiver
+amounting to no more than <small><sup>1</sup></small> of an inch, up to vibrations of a little
+over 3/8 of an inch, according to the air pressure and load. It is
+indeed interesting to see how an incandescent lamp is kept burning
+with the apparatus showing a scarcely perceptible quiver.</p>
+
+<p>To perfect the mechanical part of the apparatus so that oscillations
+are maintained economically was one thing, and Mr. Tesla
+hinted in his lecture at the great difficulties he had first encountered
+to accomplish this. But to produce oscillations which would
+be of constant period was another task of no mean proportions.
+As already pointed out, Mr. Tesla obtains the constancy of period
+in three distinct ways. Thus, he provides properly calculated
+chambers, as in the case illustrated, in the oscillator itself; or he associates
+with the oscillator an air spring of constant resilience. But
+the most interesting of all, perhaps, is the maintenance of the constancy
+of oscillation by the reaction of the electromagnetic part of
+the combination. Mr. Tesla winds his coils, by preference, for high
+tension and associates with them a condenser, making the natural
+period of the combination fairly approximating to the average period
+at which the piston would oscillate without any particular provision
+being made for the constancy of period under varying pressure
+and load. As the piston with the coils is perfectly free to move,
+it is extremely susceptible to the influence of the natural vibration
+set up in the circuits of the coils <small>H H</small>. The mechanical efficiency
+of the apparatus is very high owing to the fact that friction
+is reduced to a minimum and the weights which are moved are
+small; the output of the oscillator is therefore a very large one.</p>
+
+<p>Theoretically considered, when the various advantages which
+Mr. Tesla holds out are examined, it is surprising, considering
+the simplicity of the arrangement, that nothing was done in this<span class='pagenum'><a name="Page_493" id="Page_493">[Pg 493]</a></span>
+direction before. No doubt many inventors, at one time or
+other, have entertained the idea of generating currents by attaching
+a coil or a magnetic core to the piston of a steam engine,
+or generating currents by the vibrations of a tuning fork, or
+similar devices, but the disadvantages of such arrangements from
+an engineering standpoint must be obvious. Mr. Tesla, however,
+in the introductory remarks of his lecture, pointed out how by a
+series of conclusions he was driven to take up this new line of
+work by the necessity of producing currents of constant period
+and as a result of his endeavors to maintain electrical oscillation
+in the most simple and economical manner.</p>
+
+<hr style="width: 100%;" />
+<h3>FOOTNOTES</h3>
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> A lecture delivered before the American Institute of Electrical Engineers,
+at Columbia College, N. Y., May 20, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> Lecture delivered before the Institution of Electrical Engineers, London,
+February, 1892.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a>  A lecture delivered before the Franklin Institute, Philadelphia, February,
+1893, and before the National Electric Light Association, St. Louis, March,
+1893.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> See pages 153-4 5.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> It is thought necessary to remark that, although the induction coil may
+give quite a good result when operated with such rapidly alternating currents,
+yet its construction, quite irrespective of the iron core, makes it very unfit for
+such high frequencies, and to obtain the best results the construction should be
+greatly modified.</p></div>
+
+<div class="footnote"><p><a name="Footnote_6_6" id="Footnote_6_6"></a><a href="#FNanchor_6_6"><span class="label">[6]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>, N. Y., May 6, 1891.</p></div>
+
+
+<div class="footnote"><p><a name="Footnote_7_7" id="Footnote_7_7"></a><a href="#FNanchor_7_7"><span class="label">[7]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i> of Dec. 23d, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_8_8" id="Footnote_8_8"></a><a href="#FNanchor_8_8"><span class="label">[8]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>. N. Y., July 1, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_9" id="Footnote_9_9"></a><a href="#FNanchor_9_9"><span class="label">[9]</span></a> Abstract of a paper read before Physical Society of London.</p></div>
+
+<div class="footnote"><p><a name="Footnote_9_10" id="Footnote_9_10"></a><a href="#FNanchor_9_10"><span class="label">[9]</span></a> Article by Mr. Tesla in <i>The Electrical Engineer</i>, N. Y., August 26, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_10_11" id="Footnote_10_11"></a><a href="#FNanchor_10_11"><span class="label">[10]</span></a> Note by Prof. J. J. Thomson in the London <i>Electrician</i>, July 24, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_11_12" id="Footnote_11_12"></a><a href="#FNanchor_11_12"><span class="label">[11]</span></a> Mr. Tesla's experiments, as the careful reader of his three lectures will
+perceive, have revealed a very important fact which is taken advantage of in
+this invention. Namely, he has shown that in a condenser a considerable
+amount of energy may be wasted, and the condenser may break down merely
+because gaseous matter is present between the surfaces. A number of experiments
+are described in the lectures, which bring out this fact forcibly and serve
+as a guide in the operation of high tension apparatus. But besides bearing
+upon this point, these experiments also throw a light upon investigations of a
+purely scientific nature and explain now the lack of harmony among the observations
+of various investigators. Mr. Tesla shows that in a fluid such as oil
+the losses are very small as compared with those incurred in a gas.</p></div>
+
+<div class="footnote"><p><a name="Footnote_12_13" id="Footnote_12_13"></a><a href="#FNanchor_12_13"><span class="label">[12]</span></a> It will, of course, be inferred from the nature of these devices that the
+vibration obtained in this manner is very slow owing to the inability of the
+iron to follow rapid changes in temperature. In an interview with Mr. Tesla
+on this subject, the compiler learned of an experiment which will interest
+students. A simple horseshoe magnet is taken and a piece of sheet iron bent in
+the form of an L is brought in contact with one of the poles and placed in
+such a position that it is kept in the attraction of the opposite pole delicately
+suspended. A spirit lamp is placed under the sheet iron piece and when the
+iron is heated to a certain temperature it is easily set in vibration oscillating as
+rapidly as 400 to 500 times a minute. The experiment is very easily performed
+and is interesting principally on account of the very rapid rate of
+vibration.</p></div>
+
+<div class="footnote"><p><a name="Footnote_13_14" id="Footnote_13_14"></a><a href="#FNanchor_13_14"><span class="label">[13]</span></a> The chief point to be noted is that Mr. Tesla attacked this problem in a
+way which was, from the standpoint of theory, and that of an engineer, far
+better than that from which some earlier trials in this direction started. The
+enlargement of these ideas will be found in Mr. Tesla's work on the pyromagnetic
+generator, treated in this chapter. The chief effort of the inventor was
+to economize the heat, which was accomplished by inclosing the iron in a source
+of heat well insulated, and by cooling the iron by means of steam, utilizing the
+steam over again. The construction also permits of more rapid magnetic
+changes per unit of time, meaning larger output.</p></div>
+
+<div class="footnote"><p><a name="Footnote_14_15" id="Footnote_14_15"></a><a href="#FNanchor_14_15"><span class="label">[14]</span></a> The compiler has learned partially from statements made on several
+occasions in journals and partially by personal inquiry of Mr. Tesla, that a
+great deal of work in this interesting line is unpublished. In these inventions
+as will be seen, the brushes are automatically shifted, but in the broad method
+barely suggested here the regulation is effected without any change in the
+position of the brushes. This auxiliary brush invention, it will be remembered,
+was very much discussed a few years ago, and it may be of interest that
+this work of Mr. Tesla, then unknown in this field, is now brought to light.</p></div>
+
+<div class="footnote"><p><a name="Footnote_15_16" id="Footnote_15_16"></a><a href="#FNanchor_15_16"><span class="label">[15]</span></a> Article by Mr. Tesla, contributed to <i>The Electrical Engineer</i>, N. Y.,
+Sept. 2, 1891.</p></div>
+
+<div class="footnote"><p><a name="Footnote_16_17" id="Footnote_16_17"></a><a href="#FNanchor_16_17"><span class="label">[16]</span></a> Mr. Tesla here refers to an interesting article which appeared in July,
+1865, in the <i>Phil. Magazine</i>, by Sir W. Thomson, in which Sir William,
+speaking of his "uniform electric current accumulator," assumes that for
+self-excitation it is desirable to subdivide the disc into an infinite number of infinitely
+thin spokes, in order to prevent diffusion of the current. Mr. Tesla
+shows that diffusion is absolutely necessary for the excitation and that when
+the disc is subdivided no excitation can occur.</p></div>
+
+<div class="footnote"><p><a name="Footnote_17_18" id="Footnote_17_18"></a><a href="#FNanchor_17_18"><span class="label">[17]</span></a> See Part I, Chap. III, page 9.</p></div>
+
+<div class="footnote"><p><a name="Footnote_18_19" id="Footnote_18_19"></a><a href="#FNanchor_18_19"><span class="label">[18]</span></a> See Part II, Chap. XXVI., page 145.</p></div>
+
+
+<hr style="width: 100%;" />
+<p><span class='pagenum'><a name="Page_494" id="Page_494">[Pg 494]</a></span></p>
+<h2><a name="INDEX" id="INDEX"></a>INDEX.</h2>
+
+
+
+<div class='center'>
+<table border="0" cellpadding="4" cellspacing="0" summary="">
+<tr><td align='left'>Alternate Current Electrostatic Apparatus</td><td align='right'><a href="#Page_392">392</a></td></tr>
+<tr><td align='left'>Alternating Current Generators for High Frequency</td><td align='right'><a href="#Page_152">152</a>, <a href="#Page_374">374</a>, <a href="#Page_224">224</a></td></tr>
+<tr><td align='left'>Alternating Motors and Transformers</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'>American Institute Electrical Engineers Lecture</td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td align='left'>Anthony, W. A., Tests of Tesla Motors</td><td align='right'><a href="#Page_8">8</a></td></tr>
+<tr><td align='left'>Apparatus for Producing High Vacua</td><td align='right'><a href="#Page_276">276</a></td></tr>
+<tr><td align='left'>Arc Lighting, Tesla Direct, System</td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td align='left'>Auxiliary Brush Regulation</td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td align='left'>Biography, Tesla</td><td align='right'><a href="#Page_4">4</a></td></tr>
+<tr><td align='left'>Brush, Anti-Sparking</td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td align='left'>Brush, Third, Regulation</td><td align='right'><a href="#Page_438">438</a></td></tr>
+<tr><td align='left'>Brush, Phenomena in High Vacuum</td><td align='right'><a href="#Page_226">226</a></td></tr>
+<tr><td align='left'>Carborundum Button for Tesla Lamps</td><td align='right'><a href="#Page_140">140</a>, <a href="#Page_253">253</a></td></tr>
+<tr><td align='left'>Commutator, Anti-Sparking</td><td align='right'><a href="#Page_432">432</a></td></tr>
+<tr><td align='left'>Combination of Synchronizing and Torque Motor</td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td align='left'>Condensers with Plates in Oil</td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td align='left'>Conversion with Disruptive Discharge</td><td align='right'><a href="#Page_193">193</a>, <a href="#Page_204">204</a>, <a href="#Page_303">303</a></td></tr>
+<tr><td align='left'>Current or Dynamic Electricity Phenomena</td><td align='right'><a href="#Page_327">327</a></td></tr>
+<tr><td align='left'>Direct Current Arc Lighting</td><td align='right'><a href="#Page_451">451</a></td></tr>
+<tr><td align='left'>Dischargers, Forms of</td><td align='right'><a href="#Page_305">305</a></td></tr>
+<tr><td align='left'>Disruptive Discharge Coil</td><td align='right'><a href="#Page_207">207</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'>Disruptive Discharge Phenomena</td><td align='right'><a href="#Page_212">212</a></td></tr>
+<tr><td align='left'>Dynamos, Improved Direct Current</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'>Early Phase Motors</td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td align='left'>Effects with High Frequency and High Potential Currents</td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td align='left'>Electrical Congress Lecture, Chicago</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Electric Resonance</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'>Electric Discharges in Vacuum Tubes</td><td align='right'><a href="#Page_396">396</a></td></tr>
+<tr><td align='left'>Electrolytic Registering Meter</td><td align='right'><a href="#Page_420">420</a></td></tr>
+<tr><td align='left'>Eye, Observations on the</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Flames, Electrostatic, Non-Consuming</td><td align='right'><a href="#Page_166">166</a>, <a href="#Page_272">272</a></td></tr>
+<tr><td align='left'>Forbes Unipolar Generator</td><td align='right'><a href="#Page_468">468</a>, <a href="#Page_474">474</a></td></tr>
+<tr><td align='left'>Franklin Institute Lecture</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Generators, Pyromagnetic</td><td align='right'><a href="#Page_429">429</a></td></tr>
+<tr><td align='left'>High Potential, High Frequency:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Brush Phenomena in High Vacuum</td><td align='right'><a href="#Page_226">226</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Carborundum Buttons</td><td align='right'><a href="#Page_140">140</a>, <a href="#Page_253">253</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Disruptive Discharge Phenomena</td><td align='right'><a href="#Page_212">212</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Flames, Electrostatic, Non-Consuming</td><td align='right'><a href="#Page_166">166</a>, <a href="#Page_272">272</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Impedance, Novel Phenomena</td><td align='right'><a href="#Page_194">194</a>, <a href="#Page_338">338</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Lighting Lamps Through Body</td><td align='right'><a href="#Page_359">359</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Luminous Effects with Gases</td><td align='right'><a href="#Page_368">368</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "Massage" with Currents</td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Motor with Single Wire</td><td align='right'><a href="#Page_234">234</a>, <a href="#Page_330">330</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "No Wire" Motors</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Oil Insulation of Induction Coils</td><td align='right'><a href="#Page_173">173</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; <span class='pagenum'><a name="Page_495" id="Page_495">[Pg 495]</a></span>Ozone, Production of</td><td align='right'><a href="#Page_171">171</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Phosphorescence</td><td align='right'><a href="#Page_367">367</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Physiological Effects</td><td align='right'><a href="#Page_162">162</a>, <a href="#Page_394">394</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Resonance</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Spinning Filament</td><td align='right'><a href="#Page_168">168</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Streaming Discharges of High Tension Coil</td><td align='right'><a href="#Page_155">155</a>, <a href="#Page_163">163</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Telegraphy without Wires</td><td align='right'><a href="#Page_346">346</a></td></tr>
+<tr><td align='left'>Impedance, Novel Phenomena</td><td align='right'><a href="#Page_194">194</a>, <a href="#Page_338">338</a></td></tr>
+<tr><td align='left'>Improvements in Unipolar Generators</td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td align='left'>Improved Direct Current Dynamos and Motors</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'>Induction Motors</td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td align='left'>Institution Electrical Engineers Lecture</td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td align='left'>Lamps and Motor operated on a Single Wire</td><td align='right'><a href="#Page_330">330</a></td></tr>
+<tr><td align='left'>Lamps with Single Straight Fiber</td><td align='right'><a href="#Page_183">183</a></td></tr>
+<tr><td align='left'>Lamps containing only a Gas</td><td align='right'><a href="#Page_188">188</a></td></tr>
+<tr><td align='left'>Lamps with Refractory Button</td><td align='right'><a href="#Page_177">177</a>, <a href="#Page_239">239</a>, <a href="#Page_360">360</a></td></tr>
+<tr><td align='left'>Lamps for Simple Phosphorescence</td><td align='right'><a href="#Page_187">187</a>, <a href="#Page_282">282</a>, <a href="#Page_364">364</a></td></tr>
+<tr><td align='left'>Lecture, Tesla before:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; American Institute Electrical Engineers</td><td align='right'><a href="#Page_145">145</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Royal Institution</td><td align='right'><a href="#Page_124">124</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Institution Electrical Engineers</td><td align='right'><a href="#Page_198">198</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Franklin Institute and National Electric Light Association</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Electrical Congress, Chicago</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Lighting Lamps Through the Body</td><td align='right'><a href="#Page_359">359</a></td></tr>
+<tr><td align='left'>Light Phenomena with High Frequencies</td><td align='right'><a href="#Page_349">349</a></td></tr>
+<tr><td align='left'>Luminous Effects with Gases at Low-Pressure</td><td align='right'><a href="#Page_368">368</a></td></tr>
+<tr><td align='left'>"Magnetic Lag" Motor</td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td align='left'>"Massage" with Currents of High Frequency</td><td align='right'><a href="#Page_394">394</a></td></tr>
+<tr><td align='left'>Mechanical and Electrical Oscillators</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'>Method of obtaining Direct from Alternating currents</td><td align='right'><a href="#Page_409">409</a></td></tr>
+<tr><td align='left'>Method of obtaining Difference of Phase by Magnetic Shielding</td><td align='right'><a href="#Page_71">71</a></td></tr>
+<tr><td align='left'>Motors:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Circuits of Different Resistance</td><td align='right'><a href="#Page_79">79</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Closed Conductors</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Combination of Synchronizing and Torque</td><td align='right'><a href="#Page_95">95</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Condenser in Armature Circuit</td><td align='right'><a href="#Page_101">101</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Condenser in one of the Field Circuits</td><td align='right'><a href="#Page_106">106</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Coinciding Maxima of Magnetic Effect in Armature and Field</td><td align='right'><a href="#Page_83">83</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With "Current Lag" Artificially Secured</td><td align='right'><a href="#Page_58">58</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Early Phase</td><td align='right'><a href="#Page_477">477</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Equal Magnetic Energies in Field and Armature</td><td align='right'><a href="#Page_81">81</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Or Generator, obtaining Desired Speed of</td><td align='right'><a href="#Page_36">36</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Improved Direct Current</td><td align='right'><a href="#Page_448">448</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Induction</td><td align='right'><a href="#Page_92">92</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "Magnetic Lag"</td><td align='right'><a href="#Page_67">67</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; "No Wire"</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Phase Difference in Magnetization of Inner and Outer Parts of Core</td><td align='right'><a href="#Page_88">88</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Regulator for Rotary Current</td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Single Circuit, Self-starting Synchronizing</td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Single Phase</td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; With Single Wire to Generator</td><td align='right'><a href="#Page_234">234</a>, <a href="#Page_330">330</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Synchronizing</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Thermo-Magnetic</td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Utilizing Continuous Current Generators</td><td align='right'><a href="#Page_31">31</a></td></tr>
+<tr><td align='left'>National Electric Light Association Lecture</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>"No Wire" Motor</td><td align='right'><a href="#Page_235">235</a></td></tr>
+<tr><td align='left'>Observations on the Eye</td><td align='right'><a href="#Page_294">294</a></td></tr>
+<tr><td align='left'>Oil, Condensers with Plates in</td><td align='right'><a href="#Page_418">418</a></td></tr>
+<tr><td align='left'>Oil Insulation of Induction Coils</td><td align='right'><a href="#Page_173">173</a>, <a href="#Page_221">221</a></td></tr>
+<tr><td align='left'>Oscillators, Mechanical and Electrical</td><td align='right'><a href="#Page_486">486</a></td></tr>
+<tr><td align='left'><span class='pagenum'><a name="Page_496" id="Page_496">[Pg 496]</a></span>Ozone, Production of</td><td align='right'><a href="#Page_171">171</a></td></tr>
+<tr><td align='left'>Phenomena Produced by Electrostatic Force</td><td align='right'><a href="#Page_318">318</a></td></tr>
+<tr><td align='left'>Phosphorescence and Sulphide of Zinc</td><td align='right'><a href="#Page_367">367</a></td></tr>
+<tr><td align='left'>Physiological Effects of High Frequency</td><td align='right'><a href="#Page_162">162</a>, <a href="#Page_394">394</a></td></tr>
+<tr><td align='left'>Polyphase Systems</td><td align='right'><a href="#Page_26">26</a></td></tr>
+<tr><td align='left'>Polyphase Transformer</td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td align='left'>Pyromagnetic Generators</td><td align='right'><a href="#Page_429">429</a></td></tr>
+<tr><td align='left'>Regulator for Rotary Current Motors</td><td align='right'><a href="#Page_45">45</a></td></tr>
+<tr><td align='left'>Resonance, Electric, Phenomena of</td><td align='right'><a href="#Page_340">340</a></td></tr>
+<tr><td align='left'>"Resultant Attraction"</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'>Rotating Field Transformers</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Rotating Magnetic Field</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Royal Institution Lecture</td><td align='right'><a href="#Page_124">124</a></td></tr>
+<tr><td align='left'>Scope of Lectures</td><td align='right'><a href="#Page_119">119</a></td></tr>
+<tr><td align='left'>Single Phase Motor</td><td align='right'><a href="#Page_76">76</a></td></tr>
+<tr><td align='left'>Single Circuit, Self-Starting Synchronizing Motors</td><td align='right'><a href="#Page_50">50</a></td></tr>
+<tr><td align='left'>Spinning Filament Effects</td><td align='right'><a href="#Page_168">168</a></td></tr>
+<tr><td align='left'>Streaming Discharges of High Tension Coil</td><td align='right'><a href="#Page_155">155</a>, <a href="#Page_163">163</a></td></tr>
+<tr><td align='left'>Synchronizing Motors</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Telegraphy without Wires</td><td align='right'><a href="#Page_346">346</a></td></tr>
+<tr><td align='left'>Transformer with Shield between Primary and Secondary</td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td align='left'>Thermo-Magnetic Motors</td><td align='right'><a href="#Page_424">424</a></td></tr>
+<tr><td align='left'>Thomson, J. J., on Vacuum Tubes</td><td align='right'><a href="#Page_397">397</a>, <a href="#Page_402">402</a>, <a href="#Page_406">406</a></td></tr>
+<tr><td align='left'>Thomson, Sir W., Current Accumulator</td><td align='right'><a href="#Page_471">471</a></td></tr>
+<tr><td align='left'>Transformers:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Alternating</td><td align='right'><a href="#Page_7">7</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Magnetic Shield</td><td align='right'><a href="#Page_113">113</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Polyphase</td><td align='right'><a href="#Page_109">109</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Rotating Field</td><td align='right'><a href="#Page_9">9</a></td></tr>
+<tr><td align='left'>Tubes:</td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Coated with Yttria, etc.</td><td align='right'><a href="#Page_187">187</a></td></tr>
+<tr><td align='left'> &nbsp; &nbsp; Coated with Sulphide of Zinc, etc.</td><td align='right'><a href="#Page_290">290</a>, <a href="#Page_367">367</a></td></tr>
+<tr><td align='left'>Unipolar Generators</td><td align='right'><a href="#Page_465">465</a></td></tr>
+<tr><td align='left'>Unipolar Generator, Forbes</td><td align='right'><a href="#Page_468">468</a>, <a href="#Page_474">474</a></td></tr>
+<tr><td align='left'>Yttria, Coated Tubes</td><td align='right'><a href="#Page_187">187</a></td></tr>
+<tr><td align='left'>Zinc, Tubes Coated with Sulphide of</td><td align='right'><a href="#Page_367">367</a></td></tr>
+</table></div>
+
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
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+</body>
+</html>
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