(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "The inventions, researches and writings of Nikola Tesla, with special reference to his work in polyphase currents and high potential lighting"






I 1 





SOl^ 



1 1 







I i 







AM-UBRARY 




A\\E-UNIVERS/A 








s 5 

jn V 

5 





^\\EUNIVER% 




II 















UIBRARYQr 



% T s ""' 5 
'rtitnM'WfF 




.5JWVER%. 




















I |^| *f; 

I 





^ 

S S 

s I 





% I 








* 
1 i 







s i 



i * 








^Aavaan-3^ 




THE INVENTIONS 
RESEARCHES AND WRITINGS 

OF 

NIKOLA TESLA 



TO HIS COUNTRYMEN 



IN EASTERN EUROPE THIS RECORD OF 
THE WORK ALREADY ACCOMPLISHED BY 

NIKOLA TESLA 

IS RESPECTFULLY DEDICATED 



THE INVENTIONS 
RESEARCHES AND WRITINGS 

OF 

NIKOLA TESLA 



WITH SPECIAL REFERENCE TO HIS WORK IN POLYPHASE 
CURRENTS AND HIGH POTENTIAL LIGHTING 



HY 



THOMAS COMMERFORD MARTIN 

Editor THE ELKCTHICAL ENGiNEtR ; Past-President American Institute Electrical Engineers 



1894 

THE ELECTRICAL ENGINEER 

NEW YORK 

D. VAN NOSTRAND COMPANY, 
NEW YORK. 



Entered according to Act of Congress in the year 1893 by 

T. C. MARTIN 
in the office of the Librarian of Congress at Washington 



Press of Mcllroy & Emmet, 36 Cortlandt St., N. Y. 



PREFACE. 



rpHE electrical problems of the present day lie largely in the 
** economical transmission of power and in the radical im- 
provement of the means and methods of illumination. To many 
workers and thinkers in the domain of electrical invention, the 
apparatus and devices that are familiar, appear cumbrous and 
wasteful, and subject to severe limitations. They believe that 
the principles of current generation must be changed, the area 
of current supply be enlarged, and the appliances used by the 
consumer be at once cheapened and simplified. The brilliant 
successes of the past justify them in every expectancy of still 
more generous fruition. 

The present volume is a simple record of the pioneer work 
done in such departments up to date, by Mr. Nikola Tesla, in 
whom the world has already recognized one of the foremost of 
modern electrical investigators and inventors. No attempt what- 
ever 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 patli that 
electrical development must follow for many years to come, the 
compiler has endeavored to bring together all that bears the im- 
press 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. 

Althqugh 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 



vi PREFACE. 

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 fre- 
quency. It will be seen that Mr. Tesla has ever pressed forward, 
barely pausing for an instant to work out in detail the utilizations 
that have at once been obvious to him of the new principles he 
has elucidated. Wherever possible his own language has been 
employed. 

It may be added that this volume is issued with Mr. Tesla's 
sanction and approval, and that permission has been obtained for 
the re-publication in it of such papers as have been read before 
various technical societies of this country and Europe. Mr. 
Tesla has kindly favored the author by looking over the proof 
sheets of the sections embodying his latest researches. The 
Work has also enjoyed the careful revision of the author's 
friend and editorial associate, Mr. Joseph Wetzler, through 
whose hands all the proofs have passed. 

DECEMBER, 1893. T. C. M. 



CONTENTS. 



PART I, 

POLYPHASE CUERENTS. 

CHAPTEK I. 

BIOGRAPHICAL AND INTRODUCTORY.. 



CHAPTER II. 

A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND 

TRANSFORMERS Y 

CHAPTER III. 

THE TESLA ROTATING MAGNETIC FIELD. MOTORS WITH 
CLOSED CONDUCTORS. SYNCHRONIZING MOTORS. ROTA- 
TING FIELD TRANSFORMERS 9 

CHAPTER IV. 

MODIFICATIONS AND EXPANSIONS OF THE TESLA POLYPHASE 

SYSTEMS 26 

CHAPTER V. 

UTILIZING FAMILIAR TYPES OF GENERATORS OF THE CON- 
TINUOUS CURRENT TYPE 31 

CHAPTER VI. 

METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR 

GENERATOR 36 



v iii 

CHAPTER VII. 

REGULATOR FOR ROTARY CURRENT MOTORS 45 

CHAPTER VIII. 
SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS. . . 50 

CHAPTER IX. 

CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT 

MO.TORS 56 

CHAPTER X. 

MOTOR WITH " CURRENT LAG " ARTIFICIALLY SECURED 58 

CHAPTER XL 

ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO 

A SYNCHRONIZING MOTOR ... 62 

CHAPTER XII. 
" MAGNETIC LAG " MOTOR 67 

CHAPTER XIII. 

METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAG- 
NETIC SHIELDING 71 

CHAPTER XIV. 
TYPE OF TESLA SINGLE-PHASE MOTOR 76 

CHAPTER XV. 

MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE 79 

CHAPTER XVI. 

MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND 

ARMATURE g^ 

CHAPTER XVII. 

MOTORS WITH COINCIDING MAXIMA OK MAGNETIC EFFECT 

IN ARMATURE AND FIELD 83 



CONTENTS. ix 

CHAPTER XVIII. 

MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAG- 
NETIZATION OF THE INNER AND OUTER PARTS OF AN 
IRON CORE 88 

CHAPTER XIX. 
ANOTHER TYPE OF TESLA INDUCTION MOTOR 92 

CHAPTER XX. 

COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE 

MOTOR 95 

CHAPTER XXI. 

MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT . . . 101 

CHAPTER XXII. 
MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS. 106 

CHAPTER XXIII. 

TESLA POLYPHASE TRANSFORMER 1 J9 

CHAPTER XXIV. 

A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC 

SHIELD BETWEEN COILS OF PRIMARY AND SECONDARY 113 



PART II. 

THE TESLA EFFECTS WITH HIGH FREQUENCY AND 
HIGH POTENTIAL CURRENTS. 

CHAPTER XXV. 

INTRODUCTORY. THE SCOPE OF THE TESLA LECTURES 119 

CHAPTER XXVI. 

THE NEW YORK LECTURE. EXPERIMENTS WITH ALTERNATE 
CURRENTS OF VERY HIGH FREQUENCY, AND THEIR 
APPLICATION TO METHODS OF ARTIFICIAL ILLUMINA- 
TION, MAY 20, 1S91 145 



x CONTENTS. 

CHAPTER XXVII. 

THE LONDON LECTURE. EXPERIMENTS WITH ALTERNATE 
CURRENTS OF HIGH POTENTIAL AND HIGH FREQUENCY, 
FEBRUARY 3, 1892 198 

CHAPTER XXVIII. 

THE PHILADELPHIA AND ST. Louis LECTURE. ON LIGHT 
AND OTHER HIGH FREQUENCY PHENOMENA, FEBRUARY 
AND MARCH, 1893. 294 

CHAPTER XXIX. 

TESLA ALTERNATING CURRENT GENERATORS FOR HIGH 

FREQUENCY 374 

CHAPTER XXX. 
ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS 392 

CHAPTER XXXI. 

" MASSAGE " WITH CURRENTS OF HIGH FREQUENCY 394 

CHAPTER XXXII. 

ELECTRIC DISCHARGE IN VACUUM TUBES 396 



PART III. 

MISCELLANEOUS INVENTIONS AND WEITINGS. 

CHAPTER XXXIII. 

METHOD OF OBTAINING DIRECT FROM ALTERNATING CUR- 
RENTS 409 

CHAPTER XXXIV. 
CONDENSERS WITH PLATES IN OIL 418 

CHAPTER XXXV. 

ELECTROLYTIC REGISTERING ]!UETER . 420 



CONTENTS. xi 

CHAPTER XXXVI. 

THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENE- 
RATORS , 424 

CHAPTER XXXVII. 
ANTI-SPARKING DYNAMO BRUSH AND COMMUTATOR 432 

CHAPTER XXXVIII. 

AUXILIARY BRUSH REGULATION OF DIRECT CURRENT DY- 
NAMOS 438 

CHAPTER XXXIX. 

IMPROVEMENT IN DYNAMO AND MOTOR CONSTRUCTION 448 

CHAPTER XL. 
TESLA DIRECT CURRENT ARC LIGHTING SYSTEM 451 

CHAPTER XLI. 

IMPROVEMENT IN UNIPOLAR GENERATORS . . . 465 



PART IV. 

APPENDIX : EARLY PHASE MOTORS AND THE TESLA 
OSCILLATORS. 

CHAPTER XLIL 

MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR . . . 4.77 

CHAPTER XLIII. 

THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS... 486 



PART I. 



POLYPHASE CURRENTS. 



CHAPTER I. 

BIOGRAPHICAL AND INTRODUCTORY. 

As AN introduction to the record contained in this volume 
of Mr. Tesla' s investigations and discoveries, a few words of "a 
biographical nature will, it is deemed, not be out of place, nor 
other than welcome. 

Nikola Tesla was born in 1857 at Smiljan, Lika, a borderland 
region of Austro-Hungary, of the Serbian race, w r hich has main- 
tained 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 Metropoli- 
tan 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 
Scliule. He was then sent to Carstatt, Croatia, where he con- 
tinued his studies for three years in the Higher Real Scliule. 
There for the first time he saw a steam locomotive. He gradu- 
ated in 1873, and, surviving an attack of cholera, devoted him- 
self to experimentation, especially in electricity and magnetism. 
His father would have had him maintain the family tradition by 
Altering 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 math- 
ematics and physics. One of the machines there experimented 
with was a Gramme dynamo, used as a motor. Despite his in- 
structor's perfect demonstration of the fact that it \vas impossible 
to operate a dynamo without commutator or brushes, Mr. Tesla 
could not be convinced that such accessories \\eiv K-cessary 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 



4 INVENTIONS OF NIKOLA TESLA. 

be said to have begun work on the ideas that fructified ultimately 
in his rotating field motors. 

In the second year of his Gratz course, Mr. Tesla gave up the 
notion of becoming a teacher, and took up the engineering cur- 
riculum. 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 Engineer- 
ing 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 inyentions, but 
found his opportunities of benefiting by them limited in various 
ways. To gain a wider field* of action, he pushed on to Paris 
and there secured employment as an electrical engineer with one 
of the large companies in the new industry of electric lighting. 

It was during this period, and as early as 1882, that he began 
serious and continued efforts to embody the rotating field prin- 
ciple' in operative apparatus. He was enthusiastic about it ; be- 
lieved 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 pub- 
lish his ideas, believing them to be worthy of note as a novel and 
radical advance in electrical theory as well as destined to have 
a profound influence on all dynamo electric machinery. 

At last he determined that it would be best to try his fortunes 
in America. In France he had met many Americans, and in 
contact with them learned the desirability of turning every new 
idea in electricity to practical use. He learned also of the ready 
encouragement given in the United States to any inventor who 
could attain some new and valuable result. The resolution \v. .., 
formed with characteristic quickness, and abandoning all his 
prospects in Europe, he at once set his face westward. 

Arrived in the United States, Mr. Tesla took off his coat tin- 
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 'aas always had the strongest admiration. It was im- 
possible, however, that, with -his own ideas to carry out, and his 



POLYPHASE CURRENTS. 5 

own inventions to develop, Mr. Tesla could long remain in even 
the most delightful employ ; and, his work now attracting atten- 
tion, he left the Edison ranks to join a company intended to 
make and sell an arc lighting system based on some of his inven- 
tions 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, how- 
ever, all through this period, was his old discovery of the rotating 
field principle for alternating current work, and the application 
of it in motors that have now become known the world over. 

Strong as his convictions on the subject then were, it is a fact , 
that he stood very much alone, for the alternating current had 
no well recognized place. Few electrical engineers had ever 
used it, and the majority were entirely unfamiliar with its value, 
or even its essential features. Even Mr. Tesla himself did not, 
until after protracted effort and experimentation, learn how to 
construct alternating current apparatus of fair efficiency. But 
that he had accomplished his purpose was shown by the tests of 
Prof. Anthony, made in the of winter 188T-8, when Tesla motors 
in the hands of that distinguished expert gave an efficiency equal 
to that of direct current motors. Nothing now stood in the way 
of the commercial development and introduction of such motors, 
except that they had to be constructed with a view to operating 
on the circuits then existing, which in this country were all of 
high frequency. 

The first full publication of his work in this direction outside 
his patents was a paper read before the American Institute of 
Electrical Engineers in New York, in May, 1888 (read at the 
suggestion of Prof. Anthony and the present writer), when he 
exhibited motors that had been in operation long previous, and 
with which his belief that brushes and commutators could be 
dispensed with, was triumphantly proved to be correct. The 
section of this volume devoted to Mr. Tesla's inventions in the 
utilization of polyphase currents will show how thoroughly from 
the outset he had mastered the fundamental idea and applied it 
in the greatest variety of ways. 

Having noted for years the many advantages obtainable with 
alternating currents, Mr. Tesla was naturally led on to experi- 
ment with them at higher potentials and higher frequencies than 
were common or approved of. Ever pressing forward to deter- 
mine in even the slightest degree the outlines of the unknown, he 



6 INVENTIONS OF NIKOLA TESLA. 

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 En- 
gineers. 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 them- 
selves fully apparent. The New York lecture, and its suc- 
cessors, two in number, are also included in this volume, with a 
few supplementary notes. 

Mr. Tesla's work ranges far beyond the vast departments of 
polyphase currents and high potential lighting. The " Miscella- 
neous " section of this volume includes a great many other in- 
ventions 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 inven- 
tions, to be made public in due course. The present volume 
deals simply with his work accomplished to date. 



CHAPTER II. 

A NEW SYSTEM OF ALTERNATING CURRENT MOTORS AND 
TRANSFORMERS. 

THE present section of this volume deals with polyphase cur- 
rents, and the inventions by Mr. Tesla, made known thus far, in 
which he has embodied one feature or another of the broad 
principle of rotating field poles or resultant attraction exerted on 
the armature. It is needless to remind electricians of the great 
interest aroused by the first enunciation of the rotating field 
principle, or to dwell upon the importance of the advance from 
a single alternating current, to methods and apparatus which deal 
with more than one. Simply prefacing the consideration here 
attempted of the subject, with the remark that in nowise is the 
object of this volume of a polemic or controversial nature, it 
may be pointed out that Mr. Tesla's work has not at all been 
fully understood or realized up to date. To many readers, it is 
believed, the analysis of what he has done in this department 
will be a revelation, while it will at the same time illustrate the 
beautiful flexibility and range of the principles involved. It 
will be seen that, as just suggested, Mr. Tesla did not stop short 
at a mere rotating field, but dealt broadly with the shifting of 
the resultant attraction of the magnets. It will be seen that he 
went on to evolve the " multiphase " system with many ramifica- 
tions and turns; that he showed the broad idea of motors em- 
ploying 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 ordi- 
nary construction might be adapted to his system ; that he em- 
ployed condensers in field and armature circuits, and went to the 
bottom of the fundamental principles, testing, approving or reject- 
ing, it would appear, every detail that inventive ingenuity could 
hit upon. 



8 INVENTIONS OF NIKOLA TESLA. 

Now that opinion is turning so emphatically in favor of lower 
frequencies, it deserves special note that Mr. Tesla early re- 
cognized the importance of the low frequency feature in motor 
work. In fact his first motors exhibited publicly and which, as 
Prof. Anthony showed in his tests in the winter of 1887-8, were 
the equal of direct current motors in efficiency, output and start- 
ing torque were of the low frequency type. The necessity 
arising, however, to utilize these motors in connection with the 
existing high frequency circuits, our survey reveals in an inter- 
esting manner Mr. Tesla's fertility of resource in this direction. 
But that, after exhausting all the possibilities of this field, Mr. 
Tesla returns to low frequencies, and insists on the superiority of 
his polyphase system in alternating current distribution, need not 
at all surprise us, in view of the strength of his convictions, so 
often expressed, on this subject. This is, indeed, significant, and 
may be regarded as indicative of the probable development next 
to be witnessed. 

Incidental reference has been made to the efficiency of rotating 
field motors, a matter of much importance, though it is not the 
intention to dwell upon it here. Prof. Anthony in his remarks 
before the American Institute of Electrical Engineers, in May, 
1888, on the two small Tesla motors then shown, which he had 
tested, stated that one gave an efficiency of about 50 per cent, 
and the other a little over sixty per cent. In 1889, some tests 
were reported from Pittsburgh, made by Mr. Tesla and Mr. 
Albert Schmid, on motors up to 10 H. p. and weighing about 
850 pounds. These machines showed an efficiency of nearly 90 
per cent. With some larger motors it was then found practic- 
able 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 beauti- 
ful 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. 



CHAPTEE III. 

THE TESLA KOTATING MAGNETIC FIELD. MOTORS WITH CLOSED 
CONDUCTORS. SYNCHRONIZING MOTORS. KOTATING FIELD 
TRANSFORMERS. 

THE best description that can be given of what he attempted, 
and succeeded in doing, with the rotating magnetic field, is to be 
found in Mr. Tesla's brief paper explanatory of his rotary cur- 
rent, 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 Trans- 
formers." As a matter of fact, which a perusal of the paper 
will establish, Mr. Tesla made no attempt in that paper to de- 
scribe all his work. It dealt in reality with the few topics enu- 
merated in the caption of this chapter. Mr. Tesla's reticence 
was no doubt due largely to the fact that his action was gov- 
erned by the wishes of others with whom lie was associated, but 
it may be worth mention that the compiler of this volume who 
had seen the motors running, and who was then chairman of the 
Institute Committee on Papers and Meetings had great diffi- 
culty in inducing Mr. Tesla to give the Institute any paper at all. 
Mr. Tesla was overworked and ill, and manifested the greatest 
reluctance to an exhibition of his motors, but his objections were 
at last overcome. The paper was written the night previous to 
the meeting, in pencil, very hastily, and under the pressure 
just mentioned. 

In this paper casual reference was made to two special forms 
of motors not within the group to be considered. These two 
forms were : 1. A motor with one of its circuits in series with a 
transformer, and the other in the secondary of the transformer. 
2. A motor having its armature circuit connected to the gener- 
ator, and the field coils closed upon themselves. The paper in 
its essence is as follows, dealing witli a few leading features of 
the Tesla system, namely, the rotating magnetic field, motors 



10 INVENTIONS OF NIKOLA TESLA. 

with closed conductors, synchronizing motors, and rotating field 
transformers : 

The subject which I now have the pleasure of bringing to 
your notice is a novel system of electric distribution and trans- 
mission of power by means of alternate currents, affording pecu- 
liar 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 ; re- 
sults which are very much desired in the practical operation of 
such systems, and which cannot be accomplished by means of 
continuous currents. 

Before going into a detailed description of this system, I think 
it necessary to make a few remarks with reference to certain con- 
ditions existing in continuous current generators and motors, 
which, although generally known, are frequently disregarded. 

In our dynamo machines, it is well known, we generate alter- 
nate currents which we direct by means of a commutator, a com- 
plicated device and, it may be justly said, the source of most of 
the troubles experienced in the operation of the machines. Now, 
the currents so directed cannot be utilized in the motor, but 
they must again by means of a similar unreliable device 
be reconverted into their original state of alternate currents.^ 
The function of the commutator is entirely external, and in no 
way does it affect the internal working of the machines. In 
reality, therefore, all machines are alternate current machines, 
the currents appearing as continuous only in the external circuit 
during their transit from generator to motor. In view simply of 
this fact, alternate currents would commend themselves as a more 
direct application of electrical energy, and the employment of 
continuous currents would only be justified if we had dynamos 
which would primarily generate, and motors which would be 
directly actuated by, such currents. 

But the operation of the commutator on a motor is twofold ; 
first, it reverses the currents through the motor, and secondly, 
it effects automatically, a progressive shifting of the poles of one 
of its magnetic constituents. Assuming, therefore, that both of 
the useless operations in the systems, that is to say, the directing 
of the alternate currents on the generator and reversing the direct 
currents on the motor, be eliminated, it would still be necessary, 
in order to cause a rotation of the motor, to produce a progressive 



POLYPHASE CURRENTS. 



11 



shifting of the poles of one of its elements, and the question 
presented itself How to perform this operation by the direct 
action of alternate currents ? I will now proceed to show how 
this result was accomplished. 

In the first experiment a drum-armature was provided with 






Fie. l. 



FIG. la. 



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 witli this view I would refer to the diagrams, Figs. 1 to 8#. 
The field of the generator being independently excited, the rota- 
tion of the armature sets up currents in the coils c c l5 varying in 




FIG 



FIG. 2a. 



strength and direction in the well-known manner. In the posi- 
tion shown in Fig. 1, the current in coil c is nil, while coil c { is 
traversed by its maximum current, and the connections may be 
such that the ring is magnetized by the coils c t <?j, as indicated by 
the letters N s in Fig. 1#, the magnetizing effect of the coils 



12 INVENTIONS OF NIKOLA TE8L A. 

c c being nil, since these coils are included in the circuit of 
coil c. 

In Fig. 2, the armature coils are shown in a more advanced 
position, one-eighth of one revolution being completed. Fig. 
la illustrates the corresponding magnetic condition of the ring. 
At this moment the coil c, generates a current of the same di- 




FIG. 3. 



FIG. 3a. 



rection as previously, but weaker, producing the poles w t .Vj upon 
the ring ; the coil c also generates a current of the same direc- 
tion, and the connections may be such that the coils c c produce 
the poles n *, as shown in Fig. 'la. The resulting polarity is 
indicated by the letters x s, and it will be observed that the 
poles of the ring have been shifted one-eighth of the periphery 
of the same. 

In Fig. 3 the armature has completed one quarter of one 
revolution. In this phase the current in coil c is a maximum, and 
of such direction as to produce the poles N s in Fig. 3a, whereas 
the current in coil c v is nil, this coil being at its neutral position. 




FIG. 4. 



FIG. 4a. 



The poles N s in Fig. 3a are thus shifted one quarter of the 
circumference of the ring. 

Fig. 4 shows the coils c c in a still more advanced position, 
the armature having completed three-eighths of one revolution. 
At that moment the coil c still generates a current of the same 
direction as before, but of less strength, producing the compar- 



POLYPHASE CURRENTS. 13 

atively weaker poles n .y in Fig. 4. The current in the coil Cj 
is of the same strength, but opposite direction. Its effect is, 
therefore, to produce upon the ring the poles n -^ as indicated, 
and a polarity, N s, results, the poles now being shifted three- 
eighths of the periphery of the ring. 

In Fig. 5 one half of one revolution of the armature is com- 





pleted, and the resulting magnetic condition of the ring is indi- 
cated in Fig. 5. Now the current in coil c is nil, while the coil 
c t yields its maximum current, which is of the same direction as 
previously ; the magnetizing effect is, therefore, due to the coils, 
6 ( ! e n alone, and, referring to Fig. 5, it will be observed that 
the poles N s are shifted one half of the circumference of the 
ring. During the next half revolution the operations are repeated, 
as represented in the Figs, f> to 8a. 

A reference to the diagrams will make it clear that during one 




FIG. Q. 



FIG. 6a. 



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- 



14 INVENTIONS OF NIKOLA TESLA. 

cally the same. Instead of using four wires, with like result) 
three wires may be used, one forming a common return for both 
circuits. 

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 




FIG. 7. FIG. Ta. 

the disc. For instance, noting the direction outside of the ring 
it will he found that inside the ring it turns in an opposite direc- 
tion, while it is unaffected if placed in a position symmetrical to 
the ring. This is easily explained. Each time that a pole ap- 
proaches, 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 repeat- 
ed, 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 af- 
fected 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 en- 
abled me to detect any irregularity in the action. A curious ef- 
fect is also produced upon iron tilings. 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 



POLYPHASE CURRENTS. 15 

a direction always opposite to the supposed movement of the 
poles. If a paper with filings is put flat upon the ring and the 
current turned on suddenly, the existence of a magnetic whirl 
may easily be observed. 

To demonstrate the complete analogy between the ring and a 
revolving magnet, a strongly energized electro-magnet was rota- 
ted by mechanical power, and phenomena identical in every par- 
ticular to those mentioned above were observed. 

Obviously, the rotation of the poles produces corresponding 
inductive effects and may be utilized to generate currents in a 
closed conductor placed within the influence of the poles. For 
this purpose it is convenient to wind a ring with two sets of 
superimposed coils forming respectively the primary and second- 
ary 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. 

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 conse- 
quence of the variations in the intensity of the poles. However, 
by properly designing the generator and determining the magneti- 
zing effect of the primary coils, the latter element may be made 
to disappear. The intensity of the poles being maintained con- 




FIG. 



FIG. 8a. 



stant, 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 coin- 
ciding with that of an orthogonal system of axes, and in which 
the radius represents the resultant and the co-ordinates both 



16 INVENTIONS OF NIKOLA TESLA. 

of its components. These are then respectively the sine and 
cosine of the angle a between* the radius and one of the axes 
(O X\ Referring to Fig. 11, we have ,* = x? + f ; where 
./ = r cos a, and y = r sin a. 

Assuming the magnetizing effect of each set of coils in the 
transformer to be proportional to the current which may be 
admitted for weak degrees of magnetization then x = KG and 
y _ K C ^ w here ^is a constant and c and c 1 the current in both 
sets of coils respectively. Supposing, further, the field of the 
generator to be uniform, we have for constant speed c 1 = A" 1 sin a 
and c = K l sin (90 + a) = K l cos a, where K l is a constant. 
See Fig. 12. 

Therefore, a? = K c K K^ cos a; 

y = Kc l = K K l sin a; and 




FIG. 9. 

That is, for a uniform field the disposition of the two coils at 
right angles will secure the theoretical result, and the intensity 
of the shifting poles will be constant. But from ^ = x? -J- >f it 
follows that for y = 0, r = x; it follows that the joint magnet- 
izing effect of both sets of coils should be equal to the effect of 
one set when at its maximum action. In transformers and in a 
certain class of motors the fluctuation of the poles is not of great 
importance, but in another class of these motors it is desirable to 
obtain the theoretical result. 

In applying this principle to the construction of motors, two 
typical forms of motor have been developed. First, a form hav- 
ing 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. 



I'OL YMIAKK 



17 



These motors may be operated in three different ways : 1. By 
the alternate currents of the source only. 2. By a combined ac- 
tion of these and of induced currents. 3. By the joint action of 
alternate and continuous currents. 

The simplest form of a synchronous motor is obtained by wind- 
ing 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 




Fit* 10. 

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 ex- 
erted upon the disc is greatest when the same is in close proximity 
to the poles, it follows that such a motor will maintain exactly 
the same speed at all loads within the limits of its capacity. 

To facilitate the starting, the disc may be provided with a coil 
closed upon itself. The advantage secured by such a coil is evi- 
dent. On the start the currents set up in the coil strongly ener- 



IS 



INVENTIONS OF NIKOLA TKKLA. 



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, consider- 
able work may be performed by such a motor even if the speed 
be below normal. The intensity of the poles being constant, no 
currents will be generated in the coil when the motor is turning 
at its normal speed. 

Instead of closing the coil upon itself, its ends may be connected 
to two insulated sliding rings, and a continuous current supplied 
to these from a suitable generator. The proper way to start such 
a motor is to close the coil upon itself until the normal speed is 
reached, or nearly so, and then turn on the continuous cur- 
rent. 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 eifect 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 charac- 
teristic of this form of motor that it cannot be reversed by revers- 
ing the continuous current through the coil. 

The synchronism of these motors may be demonstrated experi- 
mentally 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 readied, the 
poles of the armature assume fixed positions relatively to the 



POLYPHASE CURRENTS. 



lit 



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 polar- 
ity 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 in- 
duction. 

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 




FIG. 13. 

speed is reached, or very nearly so, it ceases to rotate and is per- 
manently attracted. 

A crude but illustrative experiment is made with an incandes- 
cent lamp. Placing the lamp in circuit with the continuous cur- 
rent generator and in series with the magnet coil, rapid fluctua- 
tions are observed in the light in consequence of the induced cur- 
rents set up in the coil at the start ; the speed increasing, the 
fluctuations occur at longer intervals, until they entirely disap- 
pear, showing that the motor has attained its normal speed. A 
telephone receiver affords a most sensitive instrument ; when 
connected to any circuit in the motor the synchronism may be 
easily detected on the disappearance of the induced currents. 

In motors of the synchronous type it is desirable to maintain 



20 



INVENTIONS OF NIKOLA TKSLA. 



the quantity of the shifting magnetism constant, especially if the 
magnets are not properly subdivided. 

To obtain a rotary effort in these motors was the subject of 
long thought. In order to secure this result it was necessary to 
make such a disposition that while the poles of one element of 
the motor are shifted by the alternate currents of the source, the 
poles produced upon the other elements should always be main- 
tained 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. 

The object has been attained by placing within the ring a prop- 
erly subdivided cylindrical iron core wound with several indepen- 
dent coils closed upon themselves. Two coils at right angles as 




6 



FIG. 14. 



in Fig. 14, are sufficient, but a greater number may be advan- 
tageously employed. It results from this disposition that when 
the poles of the ring are shifted, currents are generated in the 
closed armature coils. These currents are the most intense at or 
near the points of the greatest density of the lines of force, and 
their effect is to produce poles upon the armature at right angles 
to those of the ring, at least theoretically so ; and since this action 
is entirely independent of the speed that is, as far as the location 
of the poles is concerned a continuous pull is exerted upon the 
periphery of the armature. In many respects these motors are 
similar to the continuous current motors. If load is put on, the 
speed, and also the resistance of the motor, is diminished and 
more current is made to pass through the energizing coils, thus 



POLYPHASE CURRENTS. 21 

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 
iield magnet. 

It will be found that the rotary effort in these motors fully 




FIG. 15. FIG. 16. FIG. 17. 

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 leav- 
ing pole projections on one of the elements only. Generally, it 
may be stated the projections diminish the torque and produce a 
tendency to synchronism. 

A characteristic feature of motors of this kind is their property 
of being very rapidly reversed. This follows from the peculiar 
action of the motor. Suppose the armature to be rotating and 
the direction of rotation of the poles to be reversed. The appa- 
ratus 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. 

If we now consider that the power to drive such a dynamo 



'\AAA/ 




FIG. 18. FIG. 19. FIG. 20. FIG. 21. 

would be very nearly proportional to the third power of the 
speed, for that reason alone the armature should be quickly re- 
versed. 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 trans- 
former in which the resistance of the secondarv circuit would be 



gg INVENTIONS OF NIKOLA TE8LA. 

abnormally diminished by producing in this circuit an additional 
electromotive force. Owing to these causes the reversal is in- 
stantaneous. 

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 pro- 
nounced tendency to synchronism may be given to it by properly 
constructing the iron core ; and in many other ways. 

As a means of obtaining the required phase of the currents in 
both the circuits, the disposition of the two coils at right angles 
is the simplest, securing the most uniform action ; but the phase 
may be obtained in many other ways, varying with the machine 
employed. Any of the dynamos at present in use may be easily 
adapted for this purpose by making connections to proper points 
of the generating coils. In closed circuit armatures, such as used 
in the continuous current systems, it is best to make four deriva- 
tions 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 diametri- 
cally opposite bars of the commutator. In such a disposition the 
motor may also be operated at half the potential and on the three- 
wire plan, by connecting the motor circuits in the proper order to 
three of the contact rings. 

In multipolar dynamo machines, such as used in the converter 
systems, the phase is conveniently obtained by winding upon the 
armature two series of coils in such a manner that while the coils 
of one set or series are at their maximum production of current, 
the coils of the other will be at their neutral position, or nearly 
so, whereby both sets of coils may be subjected simultaneously 
or successively to the inducing action of the field magnets. 

Generally the circuits in the motor will be similarly disposed, 
and various arrangements may be made to fulfill the requirements; 
but the simplest and most practicable is to arrange primary cir- 
cuits 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 cir- 
cuits ; 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 



POLYPHASE CURRENTS. 23 

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 con- 
nected alternately in opposition. In this case there will be always 
four poles, as in Figs. 15 and 17 ; four pole projections will be 
neutral ; and in Fig. 16 two adjacent pole projections will have 
the same polarity. If the coils are connected in the same manner 
there will be eight alternating poles, as indicated by the letters 
n' s' in Fig. 15. 

The employment of multipolar motors secures in this system an 
advantage much desired and unattainable in the continuous cur- 
rent system, and that is, that a motor may be made to run exactly 
at a predetermined speed irrespective of imperfections in con- 
struction, of the load, and, within certain limits, of electromotive 
force and current strength. 

In a general distribution system of this kind the following plan 
should be adopted. At the central station of supply a generator 
should be provided having a considerable number of poles. The 
motors operated from this generator should be of the synchronous 
type, but possessing sufficient rotary effort to insure their starting. 
With the observance of proper rules of construction it may be 
admitted that the speed of each motor will be in some inverse 
proportion to its size, and the number of poles should be chosen 
accordingly. Still, exceptional demands may modify this rule. 
In view of this, it will be advantageous to provide each motor 
with a greater number of pole projections or coils, the number 
being preferably a multiple of two and three. By this means, by 
simply changing the connections of the coils, the motor may be 
adapted to any probable demands. 

If the number of the poles in the motor is even, the action will 
be harmonious and the proper result will be obtained ; if this 
is not the case, the best plan to be followed is to make a 
motor with a double number of poles and connect the same in 
the manner before indicated, so that half the number of poles 
result. Suppose, for instance, that the generator has twelve poles, 
and it would be desired to obtain a speed equal to ^ 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 



4 INVENTIONS OF NIKOLA TESLA. 

contacts. To avoid this, the motor should be provided with four- 
teen magnets and seven connected in each circuit, the magnets 
in each circuit alternating among themselves. The armature 
should have fourteen closed coils. The action of the motor will 
not be quite as perfect as in the case of an even number of poles, 
but the drawback will not be of a serious nature. 

However, the disadvantages resulting from this unsymmetrical 
form will be reduced in the same proportion as the number of 
the poles is augmented. 

If the generator has, say, n, and the motor % poles, the speed 
of the motor will be equal to that of the generator multiplied by 



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 inter- 
vals 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 condi- 
tion 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 resiilt will be 
effected by one wave in each circuit, the impulses being succes- 
sive; in Fig. 20 by four, and in Fig. 21 by eight waves. 

By such means any desired speed may be attained, that is, at 
least within the limits of practical demands. This system pos- 
sesses 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 prac- 
tical demands. Their efficiency should be higher than that of 
the present transformers, and I base my assertion on the fol- 
lowing : 

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 



POLYPHASE CURRENTS. 25 

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 employ- 
ed ; 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 representa- 
tion, 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 as- 
sume that by a shifting of the poles less energy will be wasted 
than by reversals. 



CHAPTER IV. 

MODIFICATIONS AND EXPANSIONS or THE TESLA POLYPHASE 

SYSTEMS. 

IN his earlier papers and patents relative to polyphase currents, 
Mr. Tesla devoted himself chiefly to an enunciation of the broad 
lines and ideas lying at the basis of this new work ; but he sup- 
plemented this immediately by a series of other striking inven- 
tions which may be regarded as modifications and expansions of 
certain features of the Tesla systems. These we shall now pro- 
ceed to deal with. 

In the preceding chapters we have thus shown and described 
the Tesla electrical systems for the transmission of power and the 
conversion and distribution of electrical energy, in which the 
motors and the transformers contain two or more coils or sets of 
coils, which were connected up in independent circuits with 
corresponding coils of an alternating current generator, the opera- 
tion of the system being brought about by the co-operation of 
the alternating currents in the independent circuits in progres- 
sively moving or shifting the poles or points of maximum mag- 
netic effect of the motors or converters. In these systems two 
independent conductors are employed for each of the independ- 
ent circuits connecting the generator with the devices for con- 
verting 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 Vela- 
tions 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- 



POLYPHASE CURRENTS. 



27 



tion. Fig. 22 is a diagrammatic illustration of a generator and 
single motor constructed and electrically connected in accord- 
ance with the invention. Fig. 23 is a diagram of the system 
as it is nsed in operating motors or converters, or both, in parallel, 
while Fig. 24 illustrates diagrammatically the manner of operat- 
ing two or more motors or converters, or both, in series. Refer- 
ring to Fig. 22, A A designate the poles of the field magnets of 
an alternating-current generator, the armature of which, being in 
this case cylindrical in form and mounted on a shaft, c, is wound 




FIG. 24. 



longitudinally with coils B B'. The shaft c carries three insulated 
contact-rings, a b c, to two of which, as 5 c, one terminal of each 
coil, as e d, is connected. The remaining terminals, f g, are both 
connected to the third ring, a. 

A motor in this case is shown as composed of a ring, H, wound 
with four coils, i i j j, electrically connected, so as to co-operate 
in pairs, with a tendency to fix the poles of the ring at four points 
ninety degrees apart. Within the magnetic ring H is a disc or 
cylindrical core wound with two coils, G a', which may be con- 



28 



INVENTIONS OF NIKOLA TESLA. 



nected to form two closed circuits. The terminals j k of the two 
sets or pairs of coils are connected, respectively, to the binding- 
posts E' F', and the other terminals, h i, are connected to a single 
binding-post, D'. To operate the motor, three line-wires are used 
to connect the terminals of the generator with those of the mo- 
tor. 

So far as the apparent action or mode of operation of this ar- 
rangement is concerned, the single wire D, which is, so to speak, 





FIG. 23. 

a common return-wire for both circuits, may be regarded as two 
independent wires. In the illustration, with the order of con- 
nection shown, coil B' of the generator is producing its maximum 
current and coil B its minimum ; hence the current which passes 
through wire e, ring 5, brush b' ', line-wire E, terminal E', wire,;', 
coils i i, wire or terminal D', line-wire D, brush a', ring a, and 
wire/, fixes the polar line of the motor midway between the 



POLYPHASE VURRKNT8. 25) 

two coils i i ; but as the coil B' moves from the position indicated 
it generates less current, while coil B, moving into the field, gen- 
erates more. The current from coil B passes through the devices 
and wires designated by the letters d, c, c' F, F' &, j j, i, D', D, #', 
, and g, and the position of the poles of the motor will be due 
to the resultant effect of the currents in the two sets of coils 
that is, it will be advanced in proportion to the advance or for- 
ward movement of the armature coils. The movement of the 
generator-armature through one-quarter of a revolution will ob- 
viously bring coil B' into its neutral position and coil B into its 
position of maximum effect, and this shifts the poles ninety de- 
grees, as they are fixed solely by coils B. This action is repeated 
for each quarter of a complete revolution. 

When more than one motor or other device is employed, they 
may be run either in parallel or series. In Fig. 23 the former 
arrangement is shown. The electrical device is shown as a con- 
verter, L, of which the two sets of primary coils p r are con- 
nected, respectively, to the mains F E, which are electrically con- 
nected with the two coils of the generator. The cross-circuit 
wires I m, making these connections, are then connected to the 
common return-wire D. The secondary coils p' p" are in circuits 
n <>, including, for example, incandescent lamps. Only one con- 
verter is shown entire in this figure, the others being illustrated 
diagrammatically. 

When motors or converters are to be run in series, the two 
wires E F are led from the generator to the coils of the first 
motor or converter, then continued on to the next, and so on 
through the whole series, and are then joined to the single wire 
D, which completes both circuits through the generator. This is 
shown in Fig. 24, in which j i represent the two coils or sets of 
coils of the motors. 

There are, of course, other conditions under which the same 
idea may be carried out. For example, in case the motor and 
generator each has three independent circuits, one terminal of 
each circuit is connected to a line-wire, and the other three ter- 
minals to a common return-conductor. This arrangement will 
secure similar results to those attained with a generator and motor 
having but two independent circuits, as above described.- 

When applied to such machines and motors as have three or 
more induced circuits with a common electrical joint, the three 
or more terminals of the generator would be simply connected 



30 INVENTIONS OF NIKOLA TESLA. 

to those of the motor. Mr. Tesla states, however, that the re- 
sults obtained in this manner show a lower efficiency than do the 
forms dwelt upon more fully above. 



CHAPTER V. 

UTILIZING FAMILIAR TYPES OF GENERATOR OF THE CONTINUOUS 
CURRENT TYPE. 

THE preceding descriptions have assumed the use of alternating 
current generators in which, in order to produce the progressive 
movement of the magnetic poles, or of the resultant attraction of 
independent field magnets, the current generating coils are inde- 
pendent or separate. The ordinary forms of continuous current 
dynamos may, however, be employed for the same work, in 
accordance with a method of adaptation devised by Mr. Tesla. 
As will be seen, the modification involves but slight changes in 
their construction, and presents other elements of economy. 

On the shaft of a given generator, either in place of or in ad- 
dition to the regular commutator, are secured as many pairs of 
insulated collecting-rings as there are circuits to be operated. 
Now, it will be understood that in the operation of any dynamo 
electric generator the currents in the coils in their movement 
through the field of force undergo different phases that is to 
say, at different positions of the coils the currents have certain 
directions and certain strengths and that in the Tesla motors or 
transformers it is necessary that the currents in the energizing 
coils should undergo a certain order of variations in strength and 
direction. Hence, the further step viz., the connection between 
the induced or generating coils of the machine and the contact- 
rings from which the currents are to be taken off will be deter- 
mined solely by what order of variations of strength and direction 
in the currents is desired for producing a given result in the 
electrical translating device. This may be accomplished in 
various ways ; but in the drawings we give typical instances only 
of the best and most practicable ways of applying the invention 
to three of the leading types of machines in widespread use, in 
order to illustrate the principle. 

Fig. 25 is a diagram illustrative of the mode of applying the 
invention to the well-known type of " closed " or continuous cir- 



32 INVENTIONS OF NIKOLA TESLA. 

cuit machines. Fig. 26 is a similar diagram embodying an arma- 
ture with separate coils connected diametrically, or what is gener- 
ally called an "open-circuit" machine. Fig. 27 is a diagram 
showing the application of the invention to a machine the arm- 
ature-coils of which have -a common joint. 

Keferring to Fig. 25, let A represent a Tesla motor or trans- 
former which, for convenience, we will designate as a "con- 
verter." It consists of an annular core, B, wound with four inde- 
pendent coils, c and D, those diametrically opposite being con- 




FIG. 25. 

nected together so as to co-operate in pairs in establishing free 
poles in the ring, the tendency of each pair being to fix the poles 
at ninety degrees from the other. There may be an armature, 
E, within the ring, which is wound with coils closed upon them- 
selves. The object is to pass through coils c D currents of such 
relative strength and direction as to produce a progressive shift- 
ing or movement of the points of maximum magnetic effect 
around the ring, and to thereby maintain a rotary movement of 
the armature. There are therefore secured to the shaft F of the 
generator, four insulated contact-rings, abed, upon which bear 



POLYPHASE CURRENTS. 33 

the collecting-brushes a' b' c' d', connected by wires G G H H, re- 
spectively, with the terminals of coils c and D. 

Assume, for sake of illustration, that the coils D D are to re- 
ceive the maximum and coils c c at the same instant the mini- 
mum current, so that the polar line may be midway between the 
coils D D. The rings a 5 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 com- 
mutator brushes, and between which exists the greatest differ- 
ence of potential ; while rings c d would be connected to two 
points in the coil, between which exists no difference of potential. 
The best results will be obtained by making these connections at 
points equidistant from one another, as shown. These connec- 
tions are easiest made by using wires L between the rings and the 
loops or wires j, connecting the coil i to the segments of the 
commutator K. When the converters are made in this manner, 
it is evident that the phases of the currents in the sections of the 
generator coil will be reproduced in the converter coils. For 
example, after turning through an arc of ninety degrees the con- 
ductors L L, which before conveyed the maximum current, will 
receive the minimum current by reason of the change in the 
position of their coils, and it is evident that for the same reason 
the current in these coils lias gradually fallen from the maximum 
to the minimum in passing through the arc of ninety degrees. 
In this special plan of connections, the rotation of the magnetic 
poles of the converter will be synchronous with that of the 
armature coils of the generator, and the result will be the same, 
whether the energizing circuits are derivations from a continuous 
armature coil or from independent coils, as in Mr. Tesla's 
other devices. 

In Fig. 25, the brushes M M are shown in dotted lines in their 
proper normal position. In practice these brushes may be re- 
moved from the commutator and the field of the generator 
excited by an external source of current; or the brushes may be 
allowed to remain on the commutator and to take off a converted 
current to excite the field, or to be used for other purposes. 

In a certain well-known class of machines known as the "open 
circuit," the armature contains a number of coils the terminals of 
which connect to commutator segments, the coils being connected 
across the armature in pairs. This type of machine is repre- 
sented in Fig. 2fi. In this machine each pair of coils goes 



34 INVENTIONS OF NIKOLA TESLA. 

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. 

In Fig. 2H i i represent the armature coils, T T the poles of the 
field magnet, and F the shaft carrying the commutators, which 
are extended to form continuous portions a I c d. The brushes 




FIG. 26. 



FIG. 27. 



bearing on the continuous portions for taking off the alternating 
currents are represented by a' V c' d'. The collecting brushes, 
or those which may be used to take off the direct current, are 
designated by M M. Two pairs of the armature coils and their 
commutators are shown in the figure as being utilized; but all 
may be utilized in a similar manner. 

There is another well-known type of machine in which three 
or more coils, A' ' c', on the armature have a common joint, 
the free ends being connected to the segments of a commutator. 
This form of generator is illustrated in Fig. 27. In this case each 
terminal of the generator is connected directly or in derivation 
to a continuous ring, a 1) <?, and collecting brushes, a' V c', bearing 



POLYPHASE CURRENT*. 33 

thereon, take oft' the alternating currents that operate the motor. 
It is preferable in this case to employ a motor or transformer 
with three energizing coils, A" B" c", placed symmetrically with 
those of the generator, and the circuits from the latter are con- 
nected to the terminals of such coils either directly as when 
they are stationary or by means of brushes e' and contact rings 
e. In this, as in the other cases, the ordinary commutator may 
be used on the generator, and the current taken from it utilized 
for exciting the generator iielcl-magnets or for other purposes. 



CHAPTER VI. 

METHOD OF OBTAINING DESIRED SPEED OF MOTOR OR 
GENERATOR. 

WITH the object of obtaining the desired speed in motors 
operated by means of alternating currents of differing phase, 
Mr. Tesla has devised various plans intended to meet the prac- 
tical requirements of the case, in adapting his system to types of 
multipolar alternating current machines yielding a large number 
of current reversals for each revolution. 

For example, Mr. Tesla has pointed out that to adapt a given 
type of alternating current generator, you may couple rigidly 
two complete machines, securing them together in such a way 
that the requisite difference in phase will be produced ; or you 
may fasten two armatures to the same shaft within the influence 
of the same field and with the requisite angular displacement to 
yield the proper difference in phase between the two currents; 
or two armatures may be attached to the same shaft with their 
coils symmetrically disposed, but subject to the influence of two 
sets of field magnets duly displaced ; or the two sets of coils 
may be wound on the same armature alternately or in such man- 
ner that they will develop currents the phases of which differ in 
time sufficiently to produce the rotation of the motor. 

Another method included in the scope of the same idea, where- 
by 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 genera- 
tor thus constructed yields four reversals, or impulses, in each 



POLYPHASE CURRENTS. 37 

revolution, two in each of its independent circuits ; and the effect 
upon the motor is to shift the magnetic poles through three hun- 
dred 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 gen- 
erator'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 pro- 
duces twelve reversals of current to a revolution, and these cur- 
rents be directed through the independent energizing-coils of a 
motor, the coils of which are so applied as to produce twelve 




FIG. 28, FIG. 29. 

magnetic poles at all times, the rotation of the two will be syn- 
chronous ; 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. 

These features, so far as necessary to an understanding of the 
principle, are here illustrated. Fig. 28 is a diagrammatic illus- 
tration of a generator constructed in accordance with the inven- 
tion. Fig. 29 is a similar view of a correspondingly constructed 
motor. Fig. 30 is a diagram of a generator of modified con- 
struction. Fig. 31 is a diagram of a motor of corresponding 
character. Fig. 32 is a diagram of a system containing a gener- 
ator and several motors adapted to run at various speeds. 



38 INVENTIONS OF NIKOLA TESLA. 

In Fig. 28, let c represent a cylindrical armature core wound 
longitudinally with insulated coils A A, which are connected up 
in series, the terminals of the series being connected to collecting- 
rings a a on the shaft G. By means of this shaft the armature 
is mounted to rotate between the poles of an annular field-mag- 
net D, formed with polar projections wound with coils E, that 
magnetize the said projections. The coils E are included in the 
circuit of a generator F, by means of which the field-magnet is 
energized. If thus constucted, the machine is a well-known 
form of alternating-current generator. To adapt it to his sys- 
tem, however, Mr. Tesla winds on armature c a second set of 
coils B B intermediate to the first, or, in other words, in such po- 
sitions 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 pro- 
duce the minimum current. The coils B are connected, also, in 




FIG. 30. 



FIG. 81. 



series and to two connecting-rings, secured generally to the 
shaft at the opposite end of the armature. 

The motor shown in Fig. 29 has an annular field-magnet H, 
with four pole-pieces wound with coils i. The armature is con- 
structed similarly to the generator, but with two sets of two 
coils in closed circuits to correspond with the reduced number of 
magnetic poles in the field. From the foregoing it is evident that 
one revolution of the armature of the generator producing eight 
current impulses in each circuit will produce two revolutions of 
the motor-armature. 

The application of the principle of this invention is not, how- 
ever, confined to any particular form of machine. In Figs. 30 
and 31 a generator and motor of another well-known type are 
shown. In Fig. 30, j j are magnets disposed in a circle and 
wound with coils K, which are in circuit with a generator which 



POLYPHASE CURRENTS. 39 

supplies the current that maintains the field of force. In the 
usual construction of these machines the armature-conductor L is 
carried by a suitable frame, so as to be rotated in face of the 
magnets j .1, or between these magnets and another similar set 
in front of them. The magnets are energized so as to be of al- 
ternately opposite polarity throughout the series, so that as the 
conductor c is rotated the current impulses combine or are 
added to one another, those produced by the conductor in any 
given position being all in the same direction. To adapt such 
a machine to his system, Mr. Tesla adds a second set of induced 
conductors M, in all respects similar to the first, but so placed 
in reference to it that the currents produced in each will differ 
by a quarter-phase. With such relations it is evident that as the 
current decreases in conductor L it increases in conductor M, and 
conversely, and that any of the forms of Tesla motor invented 
for use in this system may be operated by such a generator. 

Fig. 31 is intended to show a motor corresponding to the ma- 
chine in Fig. 30. The construction of the motor is identical with 
that of the generator, and if coupled thereto it will run syn- 
chronously therewith, j' j' are the field-magnets, and K' the 
coils thereon, i/ is one of the armature-conductors and M' the 
other. 

Fig. 32 shows in diagram other forms of machine. The gene- 
rator N in this case is shown as consisting of a stationary ring o, 
wound with twenty-four coils p p', alternate coils being connected 
in series in two circuits. Within this ring is a disc or drum Q, 
with projections Q' wound with energizing-coils included in cir- 
cuit with a generator K. By driving this disc or cylinder alter- 
nating currents are produced in the coils p and p', which are 
carried off to run the several motors. 

The motors are composed of a ring or annular field-magnet s, 
wound with two sets of energizing-coils T T', and armatures u, 
having projections L T/ wound with coils v, all connected in series 
in a closed circuit or each closed independently on itself. 

Suppose the twelve generator-coils p are wound alternately in 
opposite directions, so that any two adjacent coils of the same set 
tend to produce a free pole in the ring o between them and the 
twelve coils p' to be similarly wound. A single revolution of 
the disc or cylinder Q, the twelve polar projections of which are 
of opposite polarity, will therefore produce twelve current im- 
pulses in each of the circuits w w'. Hence the motor x, which 



40 INVENTIONS OF NIKOLA TE8LA. 

has sixteen coils or eight free poles, will make one and a half turns 
to the generator's one. The motor Y, with twelve coils or six 
poles, will rotate with twice the speed of the generator, and the 
motor z, with eight coils or four poles, will revolve three times 
as fast as the generator. These multipolar motors have a peculi- 
arity which may be often utilized to great advantage. For ex- 




FTG. 32. 

ample, in the motor x, Fig. 32, the eight poles may be either 
alternately opposite or there may be at any given time alternately 
two like and two opposite poles. This is readily attained by 
making the proper electrical connections. The effect of such a 
change, however, would be the same as reducing the number of 



POLYPHASE CURRENTS. 41 

poles one-half, and thereby doubling the speed of any given 
motor. 

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 appreci- 
able as the number of poles is greater; although this may be in a 
measure corrected by special provisions which it is not here 
necessary to explain. It also follows, as a matter of course, that 
if the number of the poles of the motor be greater than that of 
the generator the motor will revolve at a slower speed than the 
generator. 

In this chapter, we may include a method devised by Mr. 
Tesla for avoiding the very high speeds which would be neces- 
sary with large generators. In lieu of revolving the generator 
armature at a high rate of speed, he secures the desired result by 
a rotation of the magnetic poles of one element of the generator, 
while driving the other at a different speed. The effect is the 
same as that yielded by a very high rate of rotation. 

In this instance, the generator which supplies the current for 
operating the motors or transformers consists of a subdivided 
ring or annular core wound with four diametrically-opposite 
coils, E F/, Fig. 33. Within the ring is mounted a cylindrical 
armature-core wound longitudinally with two independent coils, 
F F', the ends of which lead, respectively, to two pairs of insu- 
lated contact or collecting rings, D D' G G', on the armature shaft. 
Collecting brushes d d' g g' bear upon these rings, respectively, 
and convey the currents through the two independent line-cir- 
cuits M M'. In the main line there may be included one or more 
motors or transformers, or both. If motors be used, they are of 
the usual form of Tesla construction with independent coils or 
sets of coils j j', included, respectively, in the circuits M M'. 
These energizing-coils are wound on a ring or annular field or on 
pole pieces thereon, and produce by the action of the alternating 
currents passing through them a progressive shifting of the mag- 
netism from pole to pole. The cylindrical armature H of the 
motor is wound with two coils at right angles, which form inde- 
pendent closed circuits. 



42 INVENTIONS OF NIKOLA TESLA. 

If transformers be employed, one set of the primary coils, as 
N N, wound on a ring or annular core is connected to one circuit, 
as M', and the other primary coils, N N', to the circuit M. The 
secondary coils K K' may then be utilized for running groups of 
incandescent lamps p p'. 

With this generator an exciter is employed. This consists of 




FIG. 33. 

two poles, A A, of steel permanently magnetized, or of iron ex- 
cited by a battery or other generator of continuous currents, and 
a cylindrical armature core mounted on a shaft, B, and wound 
with two longitudinal coils, c c'. One end of each of these coils 
is connected to the collecting-rings I c, respectively, while the 



POLYPHASE CURRENTS. 43 

other ends are both connected to a ring, a. Collecting-brushes 
b' e' bear on the rings b c, respectively, and conductors L L con- 
vey tlie currents therefrom through the coils E and E of the gen- 
erator, i/ is a common return-wire to brush a'. Two indepen- 
dent circuits are thus formed, one including coils c of the exciter 
and E E of the generator, the other coils c' of the exciter and E' 
E' of the generator. It results from this that the operation of 
the exciter produces a progressive movement of the magnetic 
poles of the annular field-core of the generator, the shifting or 
rotary movement of the poles being synchronous with the rota- 
tion of the exciter armature. Considering the operative con- 
ditions of a system thus established, it will be found that when 
the exciter is driven so as to energize the field of the generator, 
the armature of the latter, if left free to turn, would rotate at a 
speed practically the same as that of the exciter. If under such 
conditions the coils F F' of the generator armature be closed 
upon themselves or short-circuited, no currents, at least theoreti- 
cally, 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 in- 
tensity of the magnetic poles of the generator ring. So, if the 
armature-coils F F' be closed through the motor, the latter will 
not be turned as long as the movement of the generator armature 
is synchronous with that of the exciter or of the magnetic poles 
of its lield. If, on the contrary, the speed of the generator arm- 
ature 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 con- 
tinues until the armature generator is stopped entirely, as by a 
brake, when the motor, if properly constructed, runs at the speed 
with which the magnetic poles of the generator rotate. 

The effective strength of the currents developed in the arma- 
ture coils of the generator is dependent upon the strength of the 
currents energizing the generator and upon the number of rota- 
tions per unit of time of the magnetic poles of the generator; 
hence the speed of the motor armature will depend in all cases 



44 INVENTIONS OF NIKOLA TESLA. 

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 diiferences of speed may be indicated by a delicately 
balanced motor. 

Let it now be assumed that power is applied to the generator 
armature to turn it in a direction opposite to that in which its 
magnetic poles rotate. In such case the result would be similar 
to that produced by a generator the armature and field magnets 
of which are rotated in opposite directions, and by reason of these 
conditions the motor armature will turn at a rate of speed equal 
to the sum of the speeds of the armature and magnetic poles of 
the generator, so that a comparatively low speed of the generator 
armature will produce a high speed in the motor. 

It will be observed in connection with this system that on 
diminishing the resistance of the external circuit of the generator 
armature by checking the speed of the motor or by adding 
translating devices in multiple arc in the secondary circuit or cir- 
cuits of the transformer the strength of the current in the arma- 
ture 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 arma- 
ture or secondary circuits is reduced, the strength of the currents 
in the field or primary circuits of the generator is increased and 
the currents in the armature are augmented correspondingly. 
For similar reasons the currents in the armature-coils of the 
generator increase very rapidly when the speed of the armature 
is reduced when running in the same direction as the magnetic 
poles or conversely. 

It will be understood from the above description that the 
generator-armature may be run in the direction of the shifting of 
the magnetic poles, but more rapidly, and that in such case the 
speed of the motor will be equal to the difference between the 
two rates. 



CHAPTER VII. 

FOR RoTARY CURRENT MoTORS. 

AN interesting device for regulating and reversing has been 
devised by Mr. Tesla for the purpose of varying the speed of 
polyphase motors. It consists of a form of converter or trans- 
former 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 ele- 
ment may be movable with respect to the other ; and the inven- 
tion, so far as relates merely to the construction of the device it- 
self, 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 po- 
sition of the core of the secondary element is that in which it 
most completely closes the magnetic circuit between the poles 
of the primary element, and in this position its coil is in its 
most effective position for the inductive action upon it of the 
primary coils ; but by turning the movable core to either side, 
the induced currents delivered by its coil become weaker until, 
by a movement of the said core and coil through 90, there will 
be no current delivered. 

Fig. 34 is a view in side elevation of the regulator. Fig. 35 is 
a broken section on line a 1 a? 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 illus- 
trating the application of the device to the Tesla alternating- 
current motors. The regulator may be constructed in many 
ways to secure the desired result ; but that which is, perhaps, its 
best form is shown in Figs. 34 and 35. 

A represents a frame of iron. B B are the cores of the indue- 



Iti 



INVENTIONS OF NIKOLA TEKLA. 



ing or primary coils c c. i> is a shaft mounted on the side bars, 
D', and on which is secured a sectional iron core, E, wound with 
an' induced or secondary coil, F, the convolutions of which are 
parallel with the axis of the shaft. The ends of the core are 
rounded off so as to fit closely in the space between the two poles 
and permit the core E to be turned to and held at any desiivd 
point. A handle, G, secured to the projecting end of the shaft 
D, is provided for this purpose. 

In Fig. 36 let n represent an ordinary alternating current gen- 
erator, the field-magnets of which are excited by a suitable 
source of current, i. Let j designate an ordinary form of electro- 
magnetic motor provided with an armature, K, commutator L, 
and field-magnets M. It is well known that such a motor, if its 




FIG. 34. 



field-magnet cores be divided up into insulated sections, may be 
practically operated by an alternating current ; but in using this 
regulator with such a motor, Mr. Tesla includes one element of 
the motor only say the armature-coils in the main circuit of 
the generator, making the connections through the brushes and 
the commutator in the usual way. He also includes one of the 
elements of the regulator say the stationary coils in the same 
circuit, and in the circuit with the secondary or movable coil of 
the regulator he connects up the field-coils of the motor. He 
also prefers to use flexible conductors to make the connections 
from the secondary coil of the regulator, as he thereby avoids 
the use of sliding contacts or rings without interfering with the 
requisite movement of the core E. 



POLYPHASE CURRENTS. 



47 



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 lield 
and armature were excited by the main current. 

To vary the speed of the motor to any rate between the mini- 
mum and maximum rates, the core E and coils F are turned in 
either direction to an extent which produces the desired result, 
for in its normal position the convolutions of coil F embrace the 
maximum number of lines of force, all of which act with the 
same effect upon the coil ; hence it will deliver its maximum 
current ; but by turning the coil F out of its position of maximum 
effect the number of lines of force embraced by it is diminished. 
The inductive effect is therefore impaired, and the current de- 
livered by coil F will continue to diminish in proportion to the 
angle at which the coil F is turned until, after passing through 




FIG. 36. 

an angle of ninety degrees, the convolutions of the coil will be 
at right angles to those of coils c c, and the inductive effect re- 
duced to a minimum. 

Incidentally to certain constructions, other causes may influ- 
ence the variation in the strength of the induced currents. For 
example, in the present case it will be observed that by the first 
movement of coil F a certain portion of its convolutions are carried 
beyond the line of the direct influence of the lines of force, and 
that the magnetic path or circuit for the lines is impaired ; hence 
the inductive effect would be reduced. Next, that after moving 
through a certain angle, which is obviously determined by the 
relative dimensions of the bobbin or coil F, diagonally opposite 
portions of the coil will be simultaneously included in the field, 
but in such positions that the lines which produce a current- 
impulse in one portion of the coil in a certain direction will pro- 



INVENTIONS OF NIKOLA TESLA. 



duce in the diagonally opposite portion a corresponding impulse 
in the opposite direction; hence portions of the current will 
neutralize one another. 

As before stated, the mechanical construction of the device 
may be greatly varied ; but the essential conditions of the princi- 
ple 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 man- 
ner similar to that described. 

It may also be stated that the core E is not indispensable to the 
operation of the regulator ; but its presence is obviously bene- 
ficial. This regulator, however, has another valuable property 
in its capability of reversing the motor, for if the coil F be turned 




through a half-revolution, the position of its convolutions rela- 
tively to the two coils c c and to the lines of force is reversed, and 
consequently the phases of the current will be reversed. This 
will produce a rotation of the motor in an opposite direction. 
This form of regulator is also applied with great advantage to 
Mr. Tesla's system of utilizing alternating currents, in which the 
magnetic poles of the field of a motor are progressively shifted 
by means of the combined effects upon the field of magnetizing 
coils included in independent circuits, through which pass alter- 
nating currents in proper order and relations to each other. 

In Fig. 37, let P represent a Tesla generator having two inde- 
pendent coils, P' and P", on the armature, and T a diagram of a 



POL7PHAHE CURRENTS. 49 

motor having two independent energizing coils or sets of coils, 
R R'. One of the circuits from the generator, as s' s', includes 
one set, R' R', of the energizing coils of the motor, while the 
other circuit, as s s, includes the primary coils of the regulator. 
The secondary coil of the regulator includes the other coils, R R, 
of the motor. 

While the secondary coil of the regulator is in its normal posi- 
tion, it produces its maximum current, and the maximum rotary 
effect is imparted to the motor; but this effect will be diminished 
in proportion to the angle at which the coil F of the regulator is 
turned. The motor will also be reversed by reversing the posi- 
tion of the coil with reference to the coils c c, and thereby re- 
versing the phases of the current produced by the generator. This 
changes the direction of the movement of the shifting poles which 
the armature follows. 

One of the main advantages of this plan of regulation is its 
economy of power. When the induced coil is generating its 
maximum current, the maximum amount of energy in the prim- 
ary coils is absorbed ; but as the induced coil is turned from its 
normal position the self-induction of the primary-coils reduces 
the expenditure of energy and saves power. 

It is obvious that in practice either coils c <: or coil v may be 
used as primary or secondary, and it is well understood that their 
relative proportions may be varied to produce any desired differ- 
ence or similarity in the inducing and induced currents. 



CHAPTER VIII. 

SINGLE CIRCUIT, SELF-STARTING SYNCHRONIZING MOTORS. 

In the first chapters of this section we have, bearing in mind 
the broad underlying principle, considered a distinct class of mo- 
tors, namely, such as require for their operation a special genera- 
tor 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, ad- 
dressed 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 fol- 
low do not represent the full scope of the work done in these 
lines. They might, indeed, be regarded as exemplifications. 

We will present these various inventions in the order which 
to us appears the most helpful to an understanding of the subject 
by the majority of readers. It will be naturally perceived that 
in offering a series of ideas of this nature, wherein some of the 
steps or links are missing, the descriptions are not altogether se- 
quential; but any one who follows carefully the main drift of 
the thoughts now brought together will find that a satisfactory 
comprehension of the principles can be gained. 

As is well known, certain forms of alternating-current machines 
have the property, when connected in circuit with an alternating 
current generator, of running as a motor in synchronism there- 
with ; but, while the alternating current will run the motor after 
it has attained a rate of speed synchronous with that of the gen- 
erator, 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. 



POLYPHASE CURRENTS. 51 

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 genera- 
tor. In this manner he is enabled to extend very greatly the ap- 
plications of his system and to secure all the advantages of both 
forms of alternating current motor. 

The expression " synchronous with that of the generator," is 
used here in its ordinary acceptation that is to say, a motor is 
said to synchronize with the generator when it preserves a certain 
relative speed determined by its number of poles and the number 
of alternations produced per revolution of the generator. Its 
actual speed, therefore, may be faster or slower than that of the 
generator; but it is said to be synchronous so long as it preserves 
the same relative speed. 

In carrying out this invention Mr. Tesla constructs a motor 
which has a strong tendency to synchronism with the generator. 
The construction preferred is that in which the armature is pro- 
vided 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 differ- 
ence 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. 



53 INVENTIONS OF NIKOLA TESLA. 

In the accompanying diagrams, Fig. 38 illustrates the details 
of the plan above set forth, and Figs. 39 and 40 modifications 
of the same. 

Referring to Fig. 38, let A designate the neld-magnets of a 




FK;S. : 



motor, the polar projections of which are wound with coils is c 
included in independent circuits, and D the armature with polar 
projections wound with coils E closed upon themselves, the 
motor in these respects being similar in construction to those 



POLYPHASE CURRENTS. 53 

described already, but having OH account of the polar projections 
on the armature core, or other similar and well-known features, 
the properties of a synch ronizing-motor. L i/ represents the 
conductors of a line from an alternating current generator <j. 

Near the motor is placed a switch the action of which is that 
of the one shown in the diagrams, which is constructed as fol- 
lows : F F' are two conducting plates or arms, pivoted at their 
ends and connected by an insulating cross-bar, H, so as to be 
shifted in parallelism. In the path of the bars F F 7 is the contact 
2, which forms one terminal of the circuit through coils c, and 
the contact 4, which is one terminal of the circuit through coils 
B. The opposite end of the wire of coils c is connected to the 
wire L or bar F' , and the corresponding end of coils B is connected 
to wire i/ and bar F; hence if the bars be shifted so as to bear on 
contacts 2 and 4 both sets of coils B c: will be included in the cir- 
cuit L i/ in multiple arc or derivation. In the path of the levers 
F F' are two other contact terminals, L and 3. The contact 1 is 
connected to contact 2 through an artificial resistance, i, and con- 
tact 3 with contact 4 through a self-induction coil, j, so that when 
the switch levers are shifted upon the points ] and 3 the circuits 
of coils B and c will be connected in multiple arc or derivation to 
the circuit L i/, and will include the resistance and self-induction 
coil respectively. A third position of the switch is that in which 
the levers F and F' are shifted out of contact with both sets of 
points. In this case the motor is entirely out of circuit. 

The purpose and manner of operating the motor by these de- 
vices are as follows : The normal position of the switch, the 
motor being out of circuit, is off the contact points. Assuming 
the generator to be running, and that it is desired to start the 
motor, the switch is shifted until its levers rest upon points 1 and 
3. The two motor-circuits are thus connected with the generator 
circuit ; but by reason of the presence of the resistance i in one 
and the self-induction coil j in the other the coincidence of the 
phases of the current is disturbed sufficiently to produce a pro- 
gression of the poles, which starts the motor in rotation. When 
tl.'e speed of the motor has run up to synchronism with the 
generator, or approximately so, the switch is shifted over upon 
the points 2 and 4, thus cutting out the coils i and j, so that the 
currents in both circuits have the same phase; but the motor 
now runs as a synchronous motor. 

It will be understood that when brought up to speed the mo 



r>4 INVENTIONS OF NIKOLA TESLA. 

tor will run with only one of the circuits B or c connected with 
the main or generator circuit, or the two circuits may be con- 
nected in series. This latter plan is preferable when a current 
having a high number of alternations per unit of time is em- 
ployed 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 arma- 
ture has polar projections D wound with closed coils E. The 
switch used is of substantially the same construction as that 
shown in the previous figure. There are, however, five contacts, 
designated as 5, 6, 7, 8, and 9. The motor-circuits B c, which in- 
clude alternate field-coils, are connected to the terminals in the 
following order : One end of circuit c is connected to contact 9 
and to contact 5 through a dead resistance, i. One terminal of 
circuit B is connected to contact 7 and to contact 6 through a 
self-induction coil, J. The opposite terminals of both circuits are 
connected to contact 8. 

One of the levers, as F, of the switch is made with an exten- 
sion, /, or otherwise, so as to cover both contacts 5 and 6 when 
shifted into the position to start the motor. It will be observed 
that when in this position and with lever F' on contact 8 the cur- 
rent divides between the two circuits B c, which from their dif- 
ference in electrical character produce a progression of the poles 
that starts the motor in rotation. When the motor has attained 
the proper speed, the switch is shifted so that the levers cover 
the contacts 7 and 9, thereby connecting circuits B and c in se- 
ries. 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 con- 
nections or otherwise a double-circuit motor, or one operating by 
a progressive shifting of the poles, into an ordinary synchroniz- 
ing motor may be carried out in many other ways. For instance, 
instead of using the switch shown in the previous figures, we 
may use a temporary ground circuit between the generator and 
motor, in order to start the motor, in substantially the manner 
indicated in Fig. 40. Let G in this figure represent an ordinary 



POLYPHASE CURRENTS. 55 

alternating-current generator with, say, two poles, M M', and an 
armature wound with two coils, N N', at right angles and con- 
nected in series. The motor has, for example, four poles wound 
with coils B c, which are connected in series, and an armature 
with polar projections D wound with closed coils E E. From the 
common joint or union between the two circuits of both the gen- 
erator 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 gen- 
erator, 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 re- 
turn for the currents in the two circuits L and i/. 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 PP' being employed for this purpose. 
The motor then runs as a synchronizing motor. 

In describing the main features which constitute this invention 
illustrations have necessarily been omitted of the appliances used 
in conjunction with the electrical devices of similar systems 
such, for instance, as driving-belts, fixed and loose pulleys for the 
motor, and the like ; but these are matters well understood. 

Mr. Tesla believes he is the first to operate electro-magnetic 
motors by alternating currents in any of the ways herein described 
that is to say, by producing a progressive movement or rota- 
tion 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 char- 
acter of the circuit connections to convert a motor operating on 
one principle to one operating on another. 



CHAPTER IX. 

CHANGE FROM DOUBLE CURRENT TO SINGLE CURRENT MOTOR. 

A DESCRIPTION is given elsewhere of a method of operating al- 
ternating current motors by first rotating their magnetic poles 
until they have attained synchronous speed, and then alternating 
the poles. The motor is thus transformed, by a simple change 
of circuit connections from one operated by the action of two or 
more independent energizing currents to one operated either by 
a single current or by several currents acting as one. Another 
way of doing this will now be described. 

At the start the magnetic poles of one element or field of the 
motor are progressively shifted by alternating currents differing 
in phase and passed through independent energizing circuits, and 
short circuit the coils of the other element. When the motor 
thus started reaches or passes the limit of speed synchronous with 
the generator, Mr. Tesla connects up the coils previously short-cir- 
cuited with a source of direct current and by a change of the cir- 
cuit 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 lamin- 
ated armature wound, for example, with the coils A B at right angles. 
The shaft of the armature carries three collecting or contact rings 
c D E. (Shown, for better illustration, as of different diameters.) 

One end of coil A connects to one ring, as c, and one end of 
coil B connects with ring D. The remaining ends are connected 
to ring E. Collecting springs or brushes F G H bear upon the 
rings and lead to the contacts of a switch, to be presently de- 
scribed. The field-coils have their terminals in binding-posts K 
K, and may be either closed upon themselves or connected w r ith 
a source of direct current L, by means of a switch M. The main 
or controlling switch has five contacts a b c d e and two levers/ 
g, pivoted and connected by an insulating cross-bar A, so as to 
move in parallelism. These levers are connected to the line 



POLYPHASE CURRENTS. 



5? 



wires from a source of alternating currents N. Contact a is con- 
nected to brush o and coil B through a dead resistance R and 
wire P. Contact b is connected with brush F and coil A through 
a self-induction coil s and wire o. Contacts c and e are connected 
to brushes <; F, respectively, through the wires P o, and contact 
<l is directly connected with brush H. The lever /has a widened 
end, which may span the contacts a 1>. When in such position 
and with lever g on contact d, the alternating currents divide be- 
tween 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 cir 
cuited. 

When the motor has attained the desired speed, the switch is 
shifted to the position shown in dotted lines that is to say, with 
the levers fg resting on points c e. This connects up the two 
armature coils in series, and the motor will then run as a syn- 
chronous motor. The field-coils are thrown into circuit with the 
direct current source when the main switch is shifted. 



CHAPTER X. 

MOTOR WITH " CURRENT LAG" ARTIFICIALLY SECURED. 

ONE of the general ways followed by Mr. Tesla in developing 
his rotary phase motors is to produce practically independent 
currents differing primarily in phase and to pass these through the 
motor-circuits. Another way is to produce a single alternating 
current, to divide it between the motor-circuits, and to effect 
artificially a lag in one of these circuits or branches, as by 
giving to the circuits different self-inductive capacity, and in 
other ways. In the former case, in which the necessary differ- 
ence of phase is primarily effected in the generation of currents, 
in some instances, the currents are passed through the energizing 
coils of both elements of the motor the field and armature ; but 
a further result or modification may be obtained by doing this 
under the conditions hereinafter specified in the case of motors 
in which the lag, as above stated, is artificially secured. 

Figs. 42 to 4T, inclusive, are diagrams of different ways in which 
the invention is carried out ; and Fig. 48, a side view of a foam 
of motor used by Mr. Tesla for this purpose. 

A B in Fig. 42 indicate the two energizing circuits of a motor, 
and c D two circuits on the armature. Circuit or coil A is con- 
nected in series with circuit or coil c, and the two circuits B D are 
similarly connected. Between coils A and c is a contact-ring , 
forming one terminal of the latter, and a brush , forming one 
terminal of the former. A ring d and brush c similarly connect 
coils B and D. The opposite terminals of the field-coils connect 
to one binding post h of the motor, and those of the armature 
coils are similarly connected to the opposite binding post i through 
a contact-ring f and brush g. Thus each motor-circuit while in 
derivation to the other includes one armature and one field coil. 
These circuits are of different self-induction, and may be made 
so in various ways. For the sake of clearness, an artificial re- 
sistance R is shown in one of these circuits, and in the other a 
self-induction coil s. When an alternating current is passed 



POLYPHASE CURRENTS. 



through this motor it divides between its two energizing-circuits. 
The higher self-induction of one circuit produces a greater re- 
tardation 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 



t www HM5&RJT *& nffiMT | 

^ l&t-*--* 




FIGS. 42, 43 and 44. 

the rotation of the armature. In certain respects this plan of in- 
cluding both armature and field coils in circuit is a marked im- 
provement. Such a motor has a good torque at starting ; yet it 
has also considerable tendency to synchronism, owing to the fact 



60 INVENTIONS OF NIKOLA TE8LA. 

that when properly constructed the maximum magnetic effects in 
both armature and field coincide a condition which in the usual 
construction of these motors with closed armature coils is not 
readily attained. The motor thus constructed exhibits too, a 
better regulation of current from no load to load, and there is 
less difference between the apparent and real energy expended 
in running it. The true synchronous speed of this form of motor 
is that of the generator when both are alike that is to say, if 
the number of the coils on the armature and on the field is a?, the 
motor will run normally at the same speed as a generator driving 




Uv^-^Mfa^ Lum' 



Fms. 45, 46 and 47. 

it if the number of field magnets or poles of the same be also or. 

Fig. 43 shows a somewhat modified arrangement of circuits. 
There is in this case but one armature coil E, the winding of 
which maintains effects corresponding to the resultant poles pro- 
duced by the two field-circuits. 

Fig. 44 represents a disposition in which both armature and 
field are wound with two sets of coils, all in multiple arc to the 
line or main circuit. The armature coils are wound to corre- 
spond with the field-coils with respect to their self-induction. A 
modification of this plan is shown in Fig. 45 that is to say, the 



POLYPHASE CURRENTS 



61 



two field coils and two armature coils are in derivation to them- 
selves and in series with one another. The armature coils in 
this case, as in the previous figure, are wound for different self- 
induction to correspond with the field coils. 

Another modification is shown in Fig. 46. In this case only 
one armature-coil, as D, is included in the line-circuit, while the 
other, as c, is short-circuited. 

In such a disposition as that shown in Fig. 43, or where only 
one armature-coil is employed, the torque on the start is some- 
what reduced, while the tendency to synchronism is somewhat 




FIG. 48. 

increased. In such a disposition as shown in Fig. 4H, the oppo- 
site conditions would exist. In both instances, however, there 
is the advantage of dispensing with one contact-ring. 

In Fig. 4(5 the two field-coils and the armature-coil D are in 
multiple arc. In Fig. 47 this disposition is modified, coil D be- 
ing shown in series with the two field-coils. 

Fig. 48 is an outline of the general form of motor in which 
this invention is embodied. The circuit connections between 
the armature and field coils are made, as indicated in the previ- 
ous figures, through brushes and rings, which are not shown. 



CHAPTER XI. 

ANOTHER METHOD OF TRANSFORMATION FROM A TORQUE TO A 
SYNCHRONIZING MOTOR. 

IN a preceding chapter we have described a method by which 
Mr. Tesla accomplishes the change in his type of rotating field 
motor from a torque to a synchronizing motor. As will be ob- 
served, the desired end is there reached by a change in the cir- 
cuit connections at the proper moment. We will now proceed 
to describe another way of bringing about the same result. The 
principle involved in this method is as follows : 

If an alternating current be passed through the field coils only 
of a motor having two energizing circuits of different self-induc- 
tion 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 start- 
ing 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 
of the poles. By the alternations of current, other currents are 
induced in the closed or short-circuited armature coils and the 



POLYPHASE CURRENTS. 



63 



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. 

In Fig. 49, A and B designate the field coils of the motor. As 
the circuits including these coils are of different self-induction, 
this is represented by a resistance coil R in circuit with A, and a 





FKJS. 49 ( 50 and 51. 



self-induction coil s in circuit with B. The same result may of 
course be secured by the winding of the coils, c is the armature 
circuit, the terminals of which are rings a J. Brushes c d bear 
on these rings and connect with the line and field circuits. D is 
the shunt or short circuit around the armature. E is the switch 
in the shunt. 

It will be observed that in such a disposition as is illustrated in 



vi 



INVENTIONS OF NIKOLA TESLA. 



Fig. 49, the field circuits A and B being of different self-induction, 
there will always be a greater lag of the current in one than the 
other, and that, generally, the armature phases will not corre- 
spond with either, but with the resultant of both. It is therefore 
important to observe the proper rule in winding the armature. 
For instance, if the motor have eight poles four in each circuit 
there will be four resultant poles, and hence the armature 
winding should be such as to produce four poles, in order to con- 
stitute a true synchronizing motor. 

The diagram, Fig. 50, differs from the previous one only in 
respect to the order of connections. In the present case the arm- 
ature-coil, instead of being in series with the field-coils, is in mul- 
tiple arc therewith. The armature- winding may be similar to 
that of the field that is to say, the armature may have two or 
more coils wound or adapted for different self-induction and 




FIG. 52. 

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 K <;. To indicate their different 
electrical, character, there are shown in circuit with them, respect- 
ively, the resistance R' and the self-induction coil s'. The two 
armature coils are in series with the field-coils and the same dis- 
position of the shunt or short-circuit u is used. It is of advan- 
tage 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, tha't 



POLYPHASE CURRENTS. 65 

this speed is not reached, owing to the fact that the armature 
and field currents do not properly correspond, so that when the 
current is passed through the armature (the motor not being 
quite up to synchronism) there is a liability that it will not "hold 
on," as it is termed. It is preferable, therefore, to so wind or 
construct the motor that on the start, when the armature coils 
are short-circuited, the motor will tend to reach a speed higher 
than the synchronous as for instance, double the latter. In 
such case the difficulty above alluded to is not felt, for the mo- 
tor will always hold up to synchronism if the synchronous speed 
in the case supposed of two thousand revolutions is reached or 
passed. This may be accomplished in various ways ; but for all 
practical purposes the following will suffice : On the armature 
are wound two sets of coils. At the start only one of these is 




Fm. 53. 

short-circuited, thereby producing a number of poles on the ar- 
mature, which will tend to run the speed up above the synchron- 
ous limit. When such limit is reached or passed, the current is 
directed through the other coil, which, by increasing the number 
<>f armature poles, tends to maintain synchronism. 

In Fig. 52, such a disposition is shown. The motor having, 
say, eight poles contains two field-circuits A and B, of different 
self-induction. The armature has two coils F and G. The former 
is closed upon itself, the latter connected with the field and line 
through contact-rings a 5, brushes G d, and a switch K. On the 
start the coil F alone is active and the motor tends to run at a 
speed above the synchronous; but when the coil G is connected 
to the circuit the number of armature poles is increased, while 
the motor is made a true synchronous motor. This disposition 



66 INVENTIONS OF NIKOLA TESLA. 

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-cir- 
cuited on the start and both connected with the external circuit 
after the synchronous speed is reached or passed. This disposi- 
tion is shown in Fig. 53. There are three contact-rings a b e 
and three brushes c d f, which connect the armature circuits 
with the external circuit. ( )n starting, the switch H is turned to 
complete the connection between one binding-post p and the field- 
coils. This short-circuits one of the armature-coils, as G. The 
other coil F is out of circuit and open. When the motor is up 
to speed, the switch H is turned back, so that the connection 
from binding-post p to the field coils is through the coil G, and 
switch K is closed, thereby including coil F in multiple arc with 
the field coils. Both armature coils arethus active. 

From the above-described instances it is evident that many 
other dispositions for carrying out the invention are possible. 



CHAPTER XII. 

" MAGNETIC LAG " MOTOK. 

THE following description deals with another form of motor, 
namely, depending on " magnetic lag " or hysteresis, its peculiar- 
ity being that in it the attractive effects or phases while lagging 
behind the phases of current which produce them, are mani- 
fested 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 
iirst to advocate it or to elucidate the reason of its supposed ex- 
istence. 

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 T in elevation and part section of a modification, and 
Fig. 57 is a similar view of another modification. 

In Figs. 54 and 55, A designates a base or stand, and B B 
the supporting-frame of the motor. Bolted to the supporting- 
frame are two magnetic cores or pole-pieces c c', of iron or 
soft steel. These may be subdivided or laminated, in which 
case hard iron or steel plates or bars should be used, or they 
should be wound with closed coils. D is a circular disc arma- 
ture, built up of sections or plates of iron and mounted in the 
frame between the pole-pieces c c', curved to conform to the 
circular shape thereof. This disc may be wound with a number 
of closed coils E. v F are the main energizing coils, supported 
by the supporting-frame, so as to include within their magnet- 
izing influence both the pole-pieces c c' and the armature i>. 
The pole-pieces c c' project out beyond the coils F F on op- 
posite sides, as indicated in the drawings. If an alternating 
current be passed through the coils F F, rotation of the arma- 
ture will be produced, and this rotation is explained by the 
following apparent action, or mode of operation : An impulse 
of current in the coils F F establishes two polarities in the mo- 
tor. The protruding end of pole-piece c, for instance, will be 



68 INVENTIONS OF NIKOLA TE8LA. 

of one sign, and the corresponding end of pole-piece c will be 
of the opposite sign. The armature also exhibits two poles. at 
right angles to the coils r F, like poles to those in the pole- 
pieces being 011 the same side of the coils. While the current 
is flowing there is no appreciable tendency to rotation devel- 
oped ; but after each current impulse ceases or begins to fall, 
the magnetism in the armature and in the ends of the pole- 
pieces c c' lags or continues to manifest itself, which produces a 
rotation of the armature by the repellent force between the 
more closely approximating points of maximum magnetic effect. 
This effect is continued by the reversal of current, the polari- 
ties of field and armature being simply reversed. One or both 
of the elements the armature or field may be wound with 




FIG. 54 



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 r F. 

A modified form of this motor is shown in Fig. 5(5. In this 
form G is one of two standards that support the bearings for 
the armature-shaft. H H are uprights or sides of a frame, prefer- 
ably magnetic, the ends c c' of which are bent in the manner 
indicated, to conform to the shape of the armature D and form 
field-magnet poles. The construction of the armature may be 
the same as in the previous figure, or it may be simply a mag- 
netic disc or cylinder, as shown, and a coil or coils F F are se- 



POLYPHASE CURRENT*. 



69 



cured in position to surround both the armature and the poles 
c c'. The armature is detachable from its shaft, the latter being 
passed through the armature after it has been inserted in posi- 
tion. The operation of this form of motor is the same in prin- 
ciple as that previously described and needs no further explana- 
tion. 

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 




FIG. 56. 



FIG. 57. 



this motor he employs an annular magnetic frame j, with in- 
wardly-extending ribs or projections K, the ends of which all 
bend or turn in one direction and are generally shaped to con- 
form to the curved surface of the armature. Coils F F are wound 
from one part K to the one next adjacent, the ends or loops of 
each coil or group of wires being carried over toward the shaft, 
so as to form y -shaped groups of convolutions at each end of the 
armature. The pole-pieces C C', being substantially concentric 
with the armature, form ledges, along which the coils are laid 
and should project to some extent beyond the the coils, as shown. 
The cylindrical or drum armature D is of the same construction 
as in the other motors described, and is mounted to rotate within 
the annular frame j and 1 Jet ween the U-shaped ends or bends of 



70 INVENTIONS OF NIKOLA TE8LA. 

the coils F. The coils F are connected in multiple or in series 
with a source of alternating currents, and are so wound that 
with a current or current impulse of given direction they will 
make the alternate pole-pieces c of one polarity and the other 
pole-pieces c' of the opposite polarity. The principle of the 
operation of this motor is the same as the other above de- 
scribed, for, considering any two pole-pieces c c', a current 
impulse passing in the coil which bridges them or is wound 
over both tends to establish polarities in their ends of opposite 
sign and to set up in the armature core between them a polarity 
of the same sign as that of the nearest pole-piece c. Upon the 
fall or cessation of the current impulse that established these 
polarities the magnetism which lags behind the current phase, 
and which continues to manifest itself in the polar projections 
c c' and the armature, produces by repulsion a rotation of the 
armature. The effect is continued by each reversal of the cur- 
rent. 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 mag- 
netic 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 lamin- 
ated. 

It is evident that a pulsatory as well as an alternating current 
might be used to drive or operate the motors above described. 

It will be understood that the degree of subdivision, the mass 
of the iron in the cores, their size and the number of alternations 
in the current employed to run the motor, must be taken into 
consideration in order to properly construct this motor. In other 
words, in all such motors the proper relations between the num- 
ber of alternations and the mass, size, or quality of the iron must 
be preserved in order to secure the best results. 



CHAPTEE XIII. 

METHOD OF OBTAINING DIFFERENCE OF PHASE BY MAGNETIC 
SHIELDING. 

IN that class of motors in which two or more sets of energizing 
magnets are employed, and in which by artificial means a certain 
interval of time is made to elapse between the respective max- 
imum 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 theo- 
retical 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 ex- 
tent 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 cur- 
rent. 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 



72 INVENTIONS OF NIKOLA TESLA. 

heretofore employed a practical difference of magnetic phase 
may readily be secured. 

Fig. 58 is a view of a motor, partly in section, with a dia- 
gram illustrating the invention. Fig. 59 is a similar view of a 
modification of the same. 

In Fig. 58, which exhibits the simplest form of the invention, 
A A is the field-magnet of a motor, having, say, eight poles or 
inwardly-projecting cores B and c. The cores B form one set of 
magnets and are energized by coils D. The cores c, forming 
the other set are energized by coils E, and the coils are 
connected, preferablv, in series with one another, in two de- 
rived or branched circuits, r o, respectively, from a suitable 
source of current. Each coil E is surrounded by a magnetic 
shield n, which is preferably composed of an annealed, insulated, 




FIG. 58. 



FIG. 59. 



or oxidized iron wire wrapped or wound on the coils in the man- 
ner indicated so as to form a closed magnetic circuit around the 
coils and between the same and the magnetic cores c. Be- 
tween the pole pieces or cores B c is mounted the- armature K, 
which, as is usual in this type of machines, is wound with coils 
L closed upon themselves. The operation resulting from this 
disposition is as follows: If a current impulse be directed 
through the two circuits of the motor, it will quickly energize 
the cores B, but not so the cores c, for the reason that in 
passing through the coils E there is encountered the influence 
of the closed magnetic circuits formed by the shields H. The 
first effect is to retard effectively the current impulse in circuit 
G, while at the same time the proportion of current which does 
pass does not magnetize the cores c, which are shielded or 



POLYPHASE CURRENTS. 73 

screened by the shields H. As the increasing electromotive 
force then urges more current through the coils E, the iron wire 
H becomes magnetically saturated and incapable of carrying all 
the lines of force, and hence ceases to protect the cores c, which 
becomes magnetized, developing their maximum effect after an 
interval of time subsequent to the similar manifestation of strength 
in the other set of magnets, the extent of which is arbitrarily 
determined by the thickness of the shield H, and other well-un- 
derstood conditions. 

From the above it will be seen that the apparatus or device 
acts in two ways. First, by retarding the current, and, second, 
by retarding the magnetization of one set of the cores, from 
which its effectiveness will readily appear. 

Many modifications of the principle of this invention are pos- 
sible. One useful and efficient application of the invention is 
shown in Fig. 59. In this figure a motor is shown similar in all 
respects to that above described, except that the iron wire H, which 
is wrapped around the coils E, is in this case connected in series 
with the coils D. The iron-wire coils H, are connected and wound, 
so as to have little or no self-induction, and being added to the 
resistance of the circuit F, the action of the current in that cir- 
cuit will be accelerated, while in the other circuit G it will be 
retarded. The shield H may be made in many forms, as will be 
understood, and used in different ways, as appears from the 
foregoing description. 

As a modification of his type of motor with " shielded " fields^ 
Mr. Tesla has constructed a motor with a field-magnet having 
two sets of poles or inwardly-projecting cores and placed side 
Uy side, so as practically to form two fields of force and alter- 
nately disposed that is to say, with the poles of one set or field 
opposite the spaces between the other. He then connects the free 
ends of one set of poles by means of laminated iron bands or 
bridge-pieces of considerably smaller cross-section than the cores 
themselves, whereby the cores will all form parts of complete 
magnetic circuits. When the coils on each set of magnets are 
connected in multiple circuits or branches from a source of al- 
ternating currents, electromotive forces are set up in or im- 
pressed upon each circuit simultaneously ; but the coils on the 
magnetically bri'dged 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- 



7-1 



INVENTIONS OF NIKOLA TESLA. 



tie 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, pro- 
duce, free poles are developed at the ends of the cores, which, 
acting in conjunction with the others, produce rotation of the 
armature. 

The construction in detail by which this invention is illustrated 
is shown in the accompanying drawings. 

Fig. 60 is a view in side elevation of a motor embodying the 
principle. Fig. 61 is a vertical cross-section of the motor. A is 
the frame of the motor, which should be built up of sheets of 
iron punched out to the desired shape and bolted together witli 





FIG. 60. 



FIG. 61. 



insulation between the sheets. When complete, the frame makes 
a field-magnet with inwardly projecting pole-pieces B and c. To 
adapt them to the requirements of this particular case these pole- 
pieces are out of line with one another, those marked B surround- 
ing one end of the armature and the others, as c, the opposite 
end, and they are disposed alternately that is to say, the pole- 
pieces of one set occur in line with the spaces between those of the 
other sets. 

The armature D is of cylindrical form, and is also laminated in 
the 'usual way and is wound longitudinally with coils closed upon 
themselves. The pole-pieces c are connected or shunted by 
bridge-pieces E. These may be made independently and attached 
to the pole-pieces, or they may be parts of the forms or blanks 
stamped or punched out of sheet-iron. Their size or mass is de- 



POLYPHASE CURRENTS. 75 

termined by various conditions, such as the strength of the cur- 
rent to be employed, the mass or size of the cores to which they 
are applied, and other familiar conditions. 

Coils F surround the pole-pieces B, and other coils G are wound 
on the pole-pieces c. These coils are connected in series in two 
circuits, which are branches of a circuit from a generator of alter- 
nating currents, and they may be so wound, or the respective 
circuits in which they are included may be so arranged, that the 
circuit of coils G will have, independently of the particular con- 
struction described, a higher self-induction than the other circuit 
or branch. 

The function of the shunts or bridges E is that they shall form 
with the cores c a closed magnetic circuit for a current up to a 
predetermined strength, so that when saturated by such current 
and unable to carry more lines of force than such a current pro- 
duces they will to no further appreciable extent interfere with 
the development, by a stronger current, of free magnetic poles at 
the ends of the cores c. 

In such a motor the current is so retarded in the coils G, and 
the manifestation of the free magnetism in the poles c is so delayed 
beyond the period of maximum magnetic effect in poles B, that a 
strong torque is produced and the motor operates with approx- 
imately the power developed in a motor of this kind energized 
by independently generated currents differing by a full quarter 
phase. 



CHAPTEK XIV. 
TYPE OF TESLA SINGLE-PHASE MOTOR. 

UP TO this point, two principal types of Tesla motors have 
been described : First, those containing two or more energizing 
circuits through which are caused to pass alternating currents 
differing from one another in phase to an extent sufficient to 
produce a continuous progression or shifting of the poles or 
points of greatest magnetic eifect, in obedience to which the 
movable element of the motor is maintained in rotation ; second, 
those containing poles, or parts of different magnetic suscepti- 
bility, 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 por- 
tions of the motor by currents from the same or from inde- 
pendent 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 mag- 
netic susceptibility in other words, parts which respond in the 
same relative degree to the action of a current, not simultaneously, 
but after different intervals of time. 

In another Tesla motor, however, the torque, instead of being 
solely the result of a tjme 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 simultane- 
ously, or approximately so, by the same currents. This principle 
of operation has been embodied practically in a motor in which 
the necessary angular displacement between the points of greatest 
magnetic attraction in the two elements of the motor the arma- 
ture and field is obtained by the direction of the lamination of 
the magnetic cores of the elements. 

Fig. 62 is a side view of such a motor with a portion of its 
armature core exposed. Fig. 63 is an end or edge view of the 



POLYPHASE CURRENTS. 



77 



same. Fig. 64 is a central cross-section of the same, the arma- 
ture being shown mainly in elevation. 

Let A A designate two plates built up of thin sections or 
laminae of soft iron insulated more or less from one another and 
held together by bolts a and secured to a base B. The inner 
faces of these plates contain recesses or grooves in which a coil 
or coils D are secured obliquely to the direction of the lamina- 
tions. Within the coils D is a disc E, preferably composed of 
a spirally- wound iron wire or ribbon or a series of concentric- 
rings and mounted on a shaft r, having bearings in the plates 
A A. Such a device when acted upon by an alternating current 
is capable of rotation and constitutes a motor, the operation of 
which may be explained in the following manner : A current or 
current-impulse traversing the coils n tends to magnetize the 




FIG. 62. 



FIG. 63. 



FIG. 64. 



cores A A and E, all of which are within the influence of the 
lield of the coils. The poles thus established would naturally 
lie in the same line at right angles to the coils D, but in the 
plates A they are deflected by reason of the direction of the 
laminations, and appear at or near the extremities of these plates. 
In the disc, however, where these conditions are not present, the 
poles or points of greatest attraction are on a line at right 
angles to the plane of the coils; hence there will be a torque es- 
tablished by this angular displacement of the poles or magnetic 
lines, which starts the disc in rotation, the magnetic lines of the 
armature and field tending toward a position of parallelism. 
This rotation is continued and maintained by the reversals of 
the current in coils D D, which change alternately the polarity of 
the field-cores A A. This rotary tendency or effect will be greatly 



78 INVENTIONS OF NIKOLA TESLA. 

increased by winding the disc with conductors G, closed upon 
themselves and having a radial direction, whereby the magnetic 
intensity of the poles of the disc will be greatly increased by 
the energizing effect of the currents induced in the coils G by the 
alternating currents in coils D. 

The cores of the disc and field may or may not be of different 
magnetic susceptibility that is to say, they may both be of the 
same kind of iron, so as to be magnetized at approximately the 
same instant by the coils D; or one may be of soft iron and the 
other of hard, in order that a certain time may elapse between 
the periods of their magnetization. In either case rotation will 
be produced ; but unless the disc is provided with the closed en- 
ergizing coils it is desirable that the above-described difference of 
magnetic susceptibility be utilized to assist in its rotation. 

The cores of the field and armature may be made in various 
ways, as will be well understood, it being only requisite that the 
laminations in each be in such direction as to secure the neces- 
sary angular displacement of the points of greatest attraction. 
Moreover, since the disc may be considered as made up of an 
infinite number of radial arms, it is obvious that what is true of 
a disc holds for many other forms of armature. 



CHAPTER XV. 

MOTORS WITH CIRCUITS OF DIFFERENT RESISTANCE. 

As lias 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 liner wire are used, or a wire of higher resistance. 
Furthermore, in order to approximate the magnetic quantities of 
the poles excited by these coils, Mr. Tesla employs in the self- 
induction circuit cores much longer than those in the other or 
resistance circuit. 

Fig. 65 is a part sectional view of the motor at right angles to 
the shaft. Fig. 66 is a diagram of the tield circuits. 

In Fig. 66, let A represent the coils in one motor circuit, and H 
those in the other. The circuit A is to have the higher self- 
induction. There are, therefore, used a long length or a large 
number of turns of coarse wire in forming the coils of this cir- 
cuit. For the circuit B, a smaller conductor is employed, or a 
conductor of a higher resistance than copper, such as German 
silver or iron, and the coils are wound with fewer turns. In apply- 
ing these coils to a motor, Mr. Tesla builds up a field-magnet of 
plates c, of iron and steel, secured together in the usual manner 



80 INVENTIONS OF NIKOLA TESLA. 

by bolts D. Each plate is formed with four (more or less) long 
cores E, around which is a space to receive the coil and an equal 
number of short projections F to receive the coils of the resistance- 
circuit. The plates are generally annular in shape, having an 
open space in the centre for receiving the armature G, which Mr. 
Tesla prefers to wind with closed coils. An alternating current 
divided between the two circuits is retarded as to its phases in 
the circuit A to a mucli greater extent than in the circuit B. By 




FIG. 65. 



FIG. 



reason of the relative sizes and disposition of the cores and coils 
the magnetic effect of the poles E and F upon the armature closely 
approximate. 

An important result secured by the construction shown here 
is that these coils which are designed to have the higher self- 
induction are almost completely surrounded by iron, and that the 
retardation is thus very materially increased. 



CHAPTER XVI. 

MOTOR WITH EQUAL MAGNETIC ENERGIES IN FIELD AND 
ARMATURE. 

LET it be assumed that the energy as represented in the magnet- 
ism in the field of a given rotating field motor is ninety and 
thafe 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 




FIG. 67. 

two thousand five hundred, and as the energy developed is in 
proportion to these products it is clear that those motors are the 
most efficient other things being equal in which the magnetic 
energies developed in the armature and field are equal. These 
results Mr. Tesla obtains by using the same amount of copper or 
ampere turns in both elements when the cores of both are equal, 
or approximately so, and the same current energizes both ; or in 
cases where the currents in one element are induced to those of 
the other he uses in the induced coils an excess of copper over 
that in the primary element or conductor. 



S3 INVENTIONS OF NIKOLA TESLA. 

The conventional figure of a motor here introduced, Fig. H7, 
will give an idea of the solution furnished by Mr. Tesla for the 
specific problem. Referring to the drawing, A is the field-mag- 
net, B the armature, c the field coils, and D the armature-coils of 
the motor. 

Generally speaking, if the mass of the cores of armature and 
field be equal, the amount of copper or ampere turns of the 
energizing coils on both should also be equal ; but these condi- 
tions will be modified in different forms of machine. It will be 
understood that these results are most advantageous when exist- 
ing under the conditions presented where the motor is running 
with its normal load, a point to be well borne in mind. 



CHAPTER XVII. 

MOTORS WITH COINCIDING MAXIMA OF MAGNETIC EFFECT IN 
ARMATURE AND FIELD. 

IN THIS forin 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 elec- 
trical 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. Con- 
sidering the structural and operative conditions of any one of 
them as, for example, that first named the armature which is 
mounted to rotate in obedience to the co-operative influence or 
action of the energizing circuits has coils wound upon it which 
are closed upon themselves and in which currents are induced by 
the energizing-currents with the object and result of energizing 
the armature-core ; but under any such conditions as must exist 
in these motors, it is obvious that a certain time must elapse 
between the manifestations of an energizing current impulse in 
the field coils, and the corresponding magnetic state or phase in 
the armature established by the current induced thereby; conse- 
quently a given magnetic influence or effect in the field which is 
the direct result of a primary current impulse will have become 
more or less weakened or lost before the corresponding effect in 
the armature indirectly produced has reached its maximum. This 
is a condition unfavorable to efficient working in certain cases as, 
for instance, when the progress of the resultant poles or points 
of maximum attraction is verj* great, or when a very high num- 
ber of alternations is employed for it is apparent that a stronger 



84 



INVENTIONS OF NIKOLA TESLA. 



tendency to rotation will be maintained if the maximum mag- 
netic attractions or conditions in both armature and field coincide, 
the energy developed by a motor being measured by the product 
of the magnetic quantities of the armature and field. 

To secure this coincidence of maximum magnetic effects, Mr. 
Tesla has devised various means, as explained below. Fig. 68 is 
a diagrammatic illustration of a Tesla motor system in which the 
alternating currents proceed from independent sources and differ 
primarily in phase. 

A designates the field-magnet or magnetic frame of the motor; 




FIG. 68. 



FIG. 69. 



B B, oppositely located pole-pieces adapted to receive the coils of 
one energizing circuit ; and c c, similar pole-pieces for the coils 
of the other energizing circuit. These circuits are designated, 
respectively, by D E, the conductor B" forming a common return 
to the generator G. Between these poles is mounted an armature 
for example, a ring or annular armature, wound with a series 
of coils F, forming a closed circuit or circuits. The action or 
operation of a motor thus constructed is now well understood. 
It will be observed, however, that the magnetism of poles B, for 



POLYPHASK CURRENTS. 85 

example, established by a current impulse in the coils thereon, 
precedes the magnetic effect set up in the armature by the in- 
duced current in coils F. Consequently the mutual attraction 
between the armature and field-poles is considerably reduced. 
The same conditions will be found to exist if, instead of assuming 
the poles B or c as acting independently, we regard the ideal re- 
sultant of both acting together, which is the real condition. To 
remedy this, the motor field is constructed with secondary poles 
B' c', which are situated between the others. These pole-pieces 
are wound with coils D' E', the former in derivation to the coils 
D, the latter to coils E. The main or primary coils D and E are 
wound for a different self-induction from that of the coils*D' and 
E', the relations being so fixed that if the currents in D and E 
differ, for example, by a quarter-phase, the currents in each 
secondary coil, as D' E', will differ from those in its appropriate 
primary D or E by, say, forty-five degrees, or one-eighth of a 
period. 

Now, assuming that an impulse or alternation in circuit or 
branch E is just beginning, while in the branch u it is just falling 
from maximum, the conditions are those of a quarter-phase 
difference. The ideal resultant of the attractive forces of the two 
sets of poles B c therefore may be considered as progressing from 
poles B to poles c, while the impulse in E is rising to maximum, 
and that in D is falling to zero or minimum. The polarity set up 
in the armature, however, lags behind the manifestations of field 
magnetism, and hence the maximum points of attraction in arma- 
ture and field, instead of coinciding, are angularly displaced. 
This effect is counteracted by the supplemental poles B' c'. The 
magnetic phases of these poles succeed those of poles B c by the 
same, or nearly the same, period of time as elapses between the 
effect of the poles B c and the corresponding induced effect in the 
armature ; hence the magnetic conditions of poles B' c' and of 
the armature more nearly coincide and a better result is obtained. 
As poles B' c' act in conjunction with the poles in the armature 
established by poles B c, so in turn poles c B act similarly with 
the poles set up by B' c', respectively. Under such conditions 
the retardation of the magnetic effect of the armature and that 
of the secondary poles will bring the maximum of the two more 
nearly into coincidence and a correspondingly stronger torque or 
magnetic attraction secured. 

In such a disposition as is shown in Fig. fiS it will be observed 



86 



INVENTIONS OF NIKOLA TESLA. 



that as the adjacent pole-pieces of either circuit are of like polar- 
ity 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 con- 
structing 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. 

Fig. 69 is a diagrammatic illustration of a motor and system in 
which the difference of phase is artificially produced. There are 
two coils D i) in one branch and two coils E E in another branch 




FIG. 71. 



of the main circuit from the generator o. These two circuits or 
branches are of different self-induction, one, as D, being higher 
than the other. This is graphically indicated by making coils D 
much larger than coils E. By reason of the difference in the 
electrical character of the two circuits, the phases of current in 
one are retarded to a greater extent than the other. Let this 
difference be thirty degrees. A motor thus constructed will 
rotate under the action of an alternating current ; but as happens 
in the case previously described the corresponding magnetic ef- 
fects of the armature and field do not coincide owing to the time 
that elapses between a given magnetic effect in the armature and 



POLYPHASE CURRENTS 87 

the condition of the field that produces it. The secondary or 
supplemental poles B' c' are therefore availed of. There being 
thirty degrees difference of phase between the currents in coils 
D E, the magnetic effect of poles B' c' should correspond to that 
produced by a current differing from the current in coils D or K 
by fifteen degrees. This we can attain by winding each supple- 
mental pole B' c' with two coils H H'. The coils H are included 
in a derived circuit having the same self-induction as circuit D, 
and coils H' in a circuit having the same self-induction as circuit 
E, so that if these circuits differ by thirty degrees the magnetism 
of poles B' c' will correspond to that produced by a current dif- 
fering from that in either D or E by fifteen degrees. This is true 
in all other cases. For example, if in Fig. 68 the coils D' E' be 
replaced by the coils H H' included in the derived circuits, the 
magnetism of the poles B' c' will correspond in effect or phase, 
if it may be so termed, to that produced by a current differing 
from that in either circuit D or E by forty-five degrees, or one- 
eighth of a period. 

This invention as applied to a derived circuit motor is illustra- 
ted in Figs. 70 and 71. The former is an end view of the motor 
with the armature in section and a diagram of connections, and 
Fig. 71 a vertical section through the field. These figures are 
also drawn to show one of the dispositions of two fields that may 
be adopted in carrying out the principle. The poles B B c c are 
in one field, the remaining poles in the other. The former are 
wound with primary coils i j and secondary coils i' j', the latter 
with coils K L. The primary coils i j are in derived circuits, be- 
tween which, by reason of their different self-induction, there is 
a difference of phase, say, of thirty degrees. The coils i' K are 
in circuit with one another, as also are coils j' L, and there should 
be a difference of phase between the currents in coils K and L and 
their corresponding primaries of, say, fifteen degrees. If the 
poles B c are at right angles, the armature-coils should be con- 
.nected directly across, or a single armature core wound from end 
to end may be used ; but if the poles B c be in line there should 
be an angular displacement of the armature coils, as will be well 
understood. 

The operation will be understood from the foregoing. The 
maximum magnetic condition of a pair of poles, as B' B', coincides 
closely with the maximum effect in the armature, which lags be- 
hind the corresponding condition in poles H n. 



CHAPTER XVIII. 

MOTOR BASED ON THE DIFFERENCE OF PHASE IN THE MAGNETIZA- 
TION OF THE INNER AND OUTER PARTS OF AN IRON CORE. 

IT is well known that if a magnetic core, even if laminated or 
subdivided, be wound with an insulated coil and a current of 
electricity be directed through the coil, the magnetization of the 
entire core does not immediately ensue, the magnetizing effect 
not being exhibited in all parts simultaneously. This may be at- 
tributed to the fact that the action of the current is to energize 
first those laminae or parts of the core nearest the surface and 
adjacent to the exciting-coil, and from thence the action pro- 
gresses 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 con- 
struction of such apparatus as motors which are designed to be 
run by alternating or equivalent currents such as pulsating or 
undulating currents generally Mr. Tesla found it desirable and 
even necessary to give due consideration to this phenomenon and 
to make special provisions in order to obviate its consequences. 
With the specific object of taking advantage of this action or 
effect, and to render it more pronounced, he constructs a field 
magnet in which the parts of the core or cores that exhibit at 
different intervals of time the magnetic effect imparted to them 
by alternating or equivalent currents in an energizing coil or coils, 
are so placed with relation to a rotating armature as to exert 
thereon their attractive effect successively in the order of their 
magnetization. By this means he secures a result similar to that 
which he had previously attained in other forms or types of mo- 



POLYPHASE CURRENTS. 89 

tor in which by means of one or more alternating currents he 
lias produced the rotation or progression of the magnetic poles. 

This new mode of operation will now be described. Fig. 72 
is a side elevation of such motor. Fig. 73 is a side elevation of 
a more practicable and efficient embodiment of the invention. 
Fig. 74 is a central vertical section of the same in the plane of 
the axis of rotation. 

Referring to Fig. 72, let x represent a large iron core, which 
may be composed of a number of sheets or laminae of soft iron 
or steel. Surrounding this core is a coil Y, which is connected 
with a source E of rapidly varying currents. Let us consider now 





FIGS. 72 and 73. 

the magnetic conditions existing in this core at any point, as 5, 
at or near the centre, and any other point, as #, nearer the sur- 
face. When a current impulse is started in the magnetizing coil 
Y, the section or part at <z, being close to the coil, is immediately 
energized, while the section or part at J, which, to use a conveni- 
ent expression, is " protected " by the intervening sections or 
layers between a and J, does not at once exhibit its magnetism. 
However, as the magnetization of a increases, 5 becomes also 
affected, reaching finally its maximum strength some time later 
than a. Upon the weakening of the current the magnetization 
of a first diminishes, while J still exhibits its maximum strength ; 



90 INVENTIONS OF NIKOLA TESLA. 

but the continued weakening of a is attended by a subsequent 
weakening of b. Assuming the current to be an alternating one, 
a will now be reversed, while b still continues of the first imparted 
polarity. This action continues the magnetic condition of &, fol- 
lowing that of a in the manner above described. If an armature 
for instance, a simple disc F, mounted to rotate freely on an 
axis be brought into proximity to the core, a movement of rota- 
tion will be imparted to the disc, the .direction depending upon 
its position relatively to the core, the tendency being to turn the 
portion of the disc nearest to the core from a to >, as indicated 
in Fig. 72. 

This action or principle of operation has been embodied in a 
practicable form of motor, which is illustrated in Fig. 73. Let A 




FIG. 74. 

in that figure represent a circular frame of iron, from diametric- 
ally opposite points of the interior of which the cores project. 
Each core is composed of three main parts B, B and c, and they 
are similarly formed with a straight portion or body <?, around 
which the energizing coil is wound, a curved arm or extension e, 
and an inwardly projecting pole or end d. Each core is made up 
of two parts B B, with their polar extensions reaching in one 
direction, and a part c between the other two, and with its polar 
extension reaching in the opposite direction. In order to lessen 
in the cores the circulation of currents induced therein, the several 
sections are insulated from one another in the manner usually 



POLYPHASE CURRENTS. 91 

followed in such cases. These cores are wound with coils D, which 
are connected in the same circuit, either in parallel or series, arid 
supplied with an alternating or a pulsating current, preferably 
the former, by a generator K, represented diagrammatically. Be- 
tween the cores or their polar extensions is mounted a cylindrical 
or similar armature F, wound with magnetizing coils G, closed 
upon themselves. 

The operation of this motor is as follows : When a current 
impulse or alternation is directed through the coils D, the sections 
B B of the cores, being on the surface and in close proximity to 
the coils, are immediately energized. The sections c, on the other 
hand, are protected from the magnetizing influence of the coil 
by the interposed layers of iron B B. As the magnetism of B B 
increases, however, the sections c are also energized ; but they 
do not attain their maximum strength until a certain time subse- 
quent to the exhibition by the sections B B of their maximum. 
Upon the weakening of the current the magnetic strength of B B 
first diminishes, while the sections c have still their maximum 
strength ; but as B B continue to weaken the interior sections are 
similarly weakened. B B may then begin to exhibit an opposite 
polarity, which is followed later by a similar change on c, and 
this action continues. B B and c may therefore be considered as 
separate field-magnets, being extended so as to act on the arma- 
ture in the most efficient positions, and the effect is similar to 
that in the other forms of Tesla motor viz., a rotation or pro- 
gression of the maximum points of the field of force. Any 
armature such, for instance, as a disc mounted in this field 
would rotate from the pole first to exhibit its magnetism to that 
which exhibits it later. 

It is evident that the principle here described may be carried 
out in conjunction with other means for securing a more favor- 
able or efficient action of the motor. For example, the polar 
extensions of the sections c may be wound or surrounded by 
closed coils. The effect of these coils will be to still more 
effectively retard the magnetization of the polar extensions of c. 



CHAPTER XIX. 

ANOTHER TYPE OF TESLA INDUCTION MOTOR. 

IT WILL have been gathered by all who are interested in the 
advance of the electrical arts, and who follow carefully, step by 
step, the work of pioneers, that Mr. Tesla ha been foremost to 
utilize inductive effects in permanently closed circuits, in the 
operation of alternating motors. In this chapter one simple type 
of such a motor is described and illustrated, which will serve as 
an exemplification of the principle. 

Let it be assumed that an ordinary alternating current genera- 
tor 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 electro- 
motive force. Such currents may be regarded and designated as 
the "unretarded currents." 

It will be understood, of course, that in practice there is al- 
ways 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 cir- 
cuit, consisting, for example, of the primary of an induction de- 
vice. 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 



POL YPHAXE CURRENTS. 



88 



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 cur- 
rent, modifying the retardation of the same, according to the 
amount of self-induction and resistance in the secondary circuit. 
If the secondary circuit has but little self-induction as, for in- 
stance, when it contains incandescent lamps only it will in- 
crease the actual difference of phase between its own and the 
primary current, first, by diminishing the lag between the pri- 
mary current and the impressed electromotive force, and, sec- 
ond, by its own lag or retardation behind the impressed electro- 
motive force. On the other hand, if the secondary circuit have 
a high self-induction, its lag behind the current in the primary is 





FIG. 7.-). 

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. 

Fig. 75 is a diagram of a Tesla motor embodying this princi- 
ple. Fig. 76 is a similar diagram of a modification of the same. 
In Fig. 75 let A designate the field-magnet of a motor which, as 
in all these motors, is built up of sections or plates. B c are po- 
lar projections upon which the coils are wound. Upon one pair 
of these poles, as c, are wound primary coils i>, which are di- 
rectly connected to the circuit of an alternating current genera- 
tor a. On the same poles are also wound secondary coils r, 
either side by side or over or under the primary coils, and these 
are connected with other coils E, which surround the poles B B. 



94 INVENTIONS OF NIKOLA TESLA. 

The currents in both primary and secondary coils in such a mo- 
tor will be retarded or will lag behind the impressed electro- 
motive force ; but to secure a proper difference in phase between 
the primary and secondary currents themselves, Mr. Tesla in- 
creases the resistance of the circuit of the secondary and reduces 
as much as practicable its self-induction. This is done by using 
for the secondary circuit, particularly in the coils E, wire of com- 
paratively 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 K. Thus the self-induc- 
tion 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 in- 
creasing the difference of phase between the primary and secon- 
dary currents. 

In the disposition shown in Fig. 76, the lag in the secondary 
is increased by increasing the self-induction of that circuit, while 
the increasing tendency of the primary to lag is counteracted by 
inserting therein a dead resistance. The primary coils D in this 
case have a low self-induction and high resistance, while the coils 
E F, included in the secondary circuit, have a high self-induction 
and low resistance. This may be done by the proper winding of 
the coils ; or in the circuit including the secondary coils E F, we 
may introducb a self-induction coil s, while in the primary cir- 
cuit from the generator o and including coils D, there may be in 
serted a dead resistance R. By this means the difference of 
phase between the primary and secondary is increased. It is evi- 
dent that both means of increasing the difference of phase 
namely, by the special winding as well as by the supplemental or 
external inductive and dead resistance may be employed con- 
jointly. 

In the operation of this motor the current impulses in the pri- 
mary coils induce currents in the secondary coils, and by the con- 
joint action of the two the points of greatest magnetic attraction 
are shifted or rotated. 

In practice it is found desirable to wind the armature with 
closed coils in which currents are induced by the action thereon 
of the primaries. 



CHAPTER XX. 

COMBINATIONS OF SYNCHRONIZING MOTOR AND TORQUE MOTOR. 

IN THE preceding descriptions relative to synchronizing motors 
and methods of operating them, reference has been made to the 
plan adopted by Mr. Tesla, which consists broadly in winding or 
arranging the motor in such manner that by means of suitable 
switches it could be started as a multiple-circuit motor, or one 
operating by a progression of its magnetic poles, and then, when 
up to speed, or nearly so, converted into an ordinary synchroniz- 
ing motor, or one in which the magnetic poles were simply alter- 
nated. 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 demagnetiza- 
tion. Hence with a current of a very high number of alternations 
the motor may not respond properly. To avoid this objection 
and to start up a synchronizing motor in which these conditions 
obtain, Mr. Tesla has combined two motors, one a synchronizing 
motor, the other a multiple-circuit or torque motor, and by the 
latter he brings the first-named up to speed, and then either 
throws the whole current into the synchronizing motor or operates 
jointly both of the motors. 

This invention involves several novel and useful features. It 
will be observed, in the first place, that both motors are run, 
without commutators of any kind, and, secondly, that the speed 
of the torque motor may be higher than that of the synchroniz- 
ing 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 



96 INVENTIONS OF NIKOLA TESLA. 

easily brought up to speed. Thirdly, the synchronizing motor 
may be constructed so as to have a much more pronounced ten- 
dency to synchronism without lessening the facility with which 
it is started. 

Fig. 77 is a part sectional view of the two motors ; Fig. 78 an 
end view of the synchronizing motor ; Fig. 79 an end view and 
part section of the torque or double-circuit motor; Fig. 80 a 
diagram of the circuit connections employed ; and Figs. 81, 82, 
83, 84 and 85 are diagrams of modified dispositions of the two 
motors. 

Inasmuch as neither motor is doing any work while the current 
is acting upon the other, the two armatures are rigidly connected, 
both being mounted upon the same shaft A, the field-magnets B 
of the synchronizing and c of the torque motor being secured to 




the same base D. The preferably larger synchronizing motor has 
polar projections on its armature, which rotate in very close prox- 
imity to the poles of the field, and in other respects it conforms 
to the conditions that are necessary to secure synchronous action. 
The pole-pieces of the armature are, however, wound with closed 
coils E, as this obviates the employment of sliding contacts. The 
smaller or torque motor, on the other hand, has, preferably, a 
cylindrical armature F, without polar projections and wound with 
closed coils G. The field-coils of the torque motor are connected 
up in two series H and i, and the alternating current from the 
generator is directed through or divided between these two cir- 
cuits 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 witli the circuit 



POLYPHASE CURRENTS. 



from the generator, inserting in one motor circuit a dead resist- 
ance 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 connec- 
tions 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. 

A convenient arrangement for carrying out this invention is 





FIG. 78. 



FIG. 79. 



shown in Fig. 80, in which j .1 are the field coils of the syn- 
chronizing, and H i the field coils of the torque motor. L L' are 
the conductors of the main line. One end of, say, coils H is con- 
nected to wire L through a self-induction coil M. One end of the 
other set of coils i is connected to the same wire through a dead 
resistance N. The opposite ends of these two circuits are con- 
nected to the contact m of a switch, the handle or lever of which 
is in connection with the line-wire L', One end of the field cir- 
cuit of the synchronizing motor is connected to the wire L. The 
other terminates in the switch-contact n. From the diagram it 
will be readily seen that if the lever p be turned upon contact m, 
the torque motor will start by reason of the difference of phase 
between the currents in its two energizing circuits. Then when 
the desired speed is attained, if the lever p be shifted upon con- 



98 INVENTIONS OF NIKOLA TESLA. 

tact ;/ the entire current will pass through the field coils of the 
synchronizing motor and the other will be doing no work. 

The torque motor may be constructed and operated in various 
ways, many of which have already been touched upon. It is not 
necessary that one motor be cut out of circuit while the other is 
in, for both may be acted upon by current at the same time, and 
Mr. Tesla has devised various dispositions or arrangements of the 
two motors for accomplishing this. Some of these arrangements 
are illustrated in Figs. 81 to 85. 

Referring to Fig. 81, let T designate the torque or multiple 
circuit motor and s the synchronizing motor, L i,' being the line- 
wires from a source of alternating current. The two circuits of 
the torque motor of different degrees of self-induction, and de- 
signated by N M, are connected in derivation to the wire L. They 
are then joined and connected to the energizing circuit of the 




FIG. 



synchronizing motor, the opposite terminal of which is connected 
to wire L'. The two motors are thus in series. To start them 
Mr. Tesla short-circuits the synchronizing motor by a switch P', 
throwing the whole current through the torque motor. Then 
when the desired speed is reached the switch p' is opened, so 
that the current passes through both motors. In such an arrange- 
ment as this it is obviously desirable for economical and other 
reasons that a proper relation between the speeds of the two 
motors should be observed. 

In Fig. 82 another disposition is illustrated, s is the synchron- 
izing motor and T the torque motor, the circuits of both being in 
parallel, w is a circuit also in derivation to the motor circuits 
and containing a switch P". s' is a switch in the synchronizing 
motor circuit. On the start, the switch s' is opened, cutting out 
the motor s. Then P" is opened, throwing the entire current 



POLYPHASE CURRENTS. 



through the motor T, giving it a very strong torque. When the 
desired speed is reached, switch s' is closed and the current divides 





FIGS. 81, 82, 83, 84 and 85. 

between both motors. By means of switch p" both motors may 
be cut out. 



100 INVENTIONS OF NIKOLA TE8LA. 

In Fig. 83 the arrangement is substantially the same, except 
that a switch T' is placed in the circuit which includes the two cir- 
cuits of the torque motor. Fig. 84 shows the two motors in 
series, with a shunt around both containing a switch s T. There 
is also a shunt around the synchronizing motor s, with a switch 
p'. In Fig. 85 the same disposition is shown ; but each motor is 
provided witli a shunt, in which are switches P' and T*, as shown. 



CHAPTER XXL 

MOTOR WITH A CONDENSER IN THE ARMATURE CIRCUIT. 

WE NOW come to a new class of motors in which resort is had 
to condensers for the purpose of developing the required differ- 
ence of phase and neutralizing the effects of self-induction. Mr. 
Tesla early began to apply the condenser to alternating appara- 
tus, in just how r many ways can only be learned from a perusal 
of other portions of this volume, especially those dealing with 
his high frequency work. 

Certain laws govern the action or effects produced by a con- 
denser 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-induc- 
tion. Third, if two points of a closed or complete circuit 
through w T hich 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 con- 
necting the two circuits in derivation and connecting up a con- 
denser in series in one of the circuits. A further development, 



102 



INVENTIONS OF NIKOLA TESLA. 



however, possesses certain novel features of practical value and in- 
volves 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 con- 




. 87. 



FIG. 90. 



struction 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 repre- 
sents not only an inductive resistance, but one which is period- 



POLYPHASE VUKRENT8. 103 

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 rela- 
tion of phase. Hence whatever tends to decrease the self-induc- 
tion and increase the current in the induced circuits will, other 
things being equal, increase the output arid efficiency of the mo- 
tor, and the same will be true of causes that operate to maintain 
the mutual attractive effect between the field magnets and arma- 
ture at its maximum. Mr. Tesla secures these results by con- 
necting 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. 

Referring to the drawings, Fig. 86, is a view, mainly dia- 
grammatic, 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 simi- 
lar section of a modification of the same. Fig. 89 is one of the 
sections of the core detached. Fig. 90 is a diagram showing a 
modified disposition of the armature or induced circuits. 

The general plan of the invention is illustrated iji Fig. 86. 
A A in this figure represent the the frame and field magnets of 
an alternating current motor, the poles or projections of which 
are wound with coils B and c, forming independent energizing 
circuits connected either to the same or to independent sources 
of alternating currents, so that the currents flowing through the 
circuits, respectively, will have a difference of phase. Within 
the influence of this field is an armature core D, wound with coils 
E. In motors of this description heretofore these coils have been 
closed upon themselves, or connected in a closed series; but in 
the present case each coil or the connected series of coils termi- 
nates in the opposite plates of a condenser F. For this purpose 
the ends of the series of coils are brought out through the shaft 
to collecting rings G, which are connected to the condenser by 
contact brushes H and suitable conductors, the condenser being 
independent of the machine. The armature coils are wound or 
connected in such manner that adjacent coils produce opposite 
poles. 



104 INVENTIONS OF NIKOLA TK8LA. 

The action, of this motor and the effect of the plan followed 
in its construction are as follows : The motor being started in 
operation and the coils of the field magnets being traversed by 
alternating currents, currents are induced in the armature coils 
by one set of field coils, as B, and the poles thus established are 
acted upon by the other set, as c. The armature coils, however, 
have necessarily a high self-induction, which opposes the flow of 
the currents thus set up. The condenser F not only permits the 
passage or transference of these currents, but also counteracts 
the effects of self-induction, and by a proper adjustment of the 
capacity of the condenser, the self-induction of the coils, and the 
periods of the currents, the condenser may be made to overcome 
entirely the effect of self-induction. 

It is preferable on account of the undesirability of using sliding 
contacts of any kind, to associate the condenser with the armature 
directly, or make it a part of the armature. In some cases Mr. 
Tesla builds up the armature of annular plates K K, held by bolts 
L between heads M, which are secured to the driving shaft, and 
in the hollow space thus formed he places a condenser F, gener- 
ally by winding the two insulated plates spirally around the 
shaft. In other cases he utilizes the plates of the core itself 
as the plates of the condenser. For example, in Figs. 88 and 89, 
N is the driving shaft, M M are the heads of the armature-core, 
and K K' the iron plates of which the core is built up. These 
plates are insulated from the shaft and from one another, and are 
held together by rods or bolts L. The bolts pass through a large 
hole in one plate and a small hole in the one next adjacent, and 
so on, connecting electrically all of plates K, as one armature of a 
condenser, and all of plates K' as the other. 

To either of the condensers above described the armature coils 
may be connected, as explained by reference to Fig. 86. 

In motors in which the armature coils are closed upon them- 
selves as, for example, in any form of alternating current motor 
in which one armature coil or set of coils is in the position of 
maximum induction with respect to the field coils or poles, while 
the other is in the position of minimum induction the coils are 
best connected in one series, and two points of the circuit 
thus formed are bridged by a condenser. This is illustrated in 
Fig. 90, in which E represents one set of armature coils and E' 
the other. Their points of uniou are joined through a con- 
denser F. It will be observed that in this disposition the self- 



POLYPHASE CURRENTS. 105 

induction of the two branches E and E' varies with their position 
relatively to the field magnet, and that each branch is alternately 
the predominating source of the induced current. Hence the 
effect of the condenser F is twofold. First, it increases the cur- 
rent 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 con- 
denser with a circuit, as above described. This effect is favorable 
to the proper working of the motor, because it increases the flow 
of current in the armature circuits due to a given inductive 
effect, and also because it brings more nearly into coincidence 
the maximum magnetic effects of the coacting field and armature 
poles. 

It will be understood, of course, that the causes that contri- 
bute 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 mgre particularly adapted to systems 
in which a very high rate of alternation or change is main- 
tained. 

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 con- 
denser is applicable, generally, to other kinds of alternating cur- 
rent apparatus. 



CHAPTEK XXII. 

MOTOR WITH CONDENSER IN ONE OF THE FIELD CIRCUITS. 

IF THE field or energizing circuits of a rotary phase motor be 
both derived from the same source of alternating currents and a 
condenser of proper capacity be included in one of the same, ap- 
proximately, the desired difference of phase may be obtained be- 
tween 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 par- 
ticularly prohibitory to their employment. 

Another, now well-known, method or plan of securing a differ- 
ence of phase between the energizing currents of motors of this 
kind is to induce by the currents in one circuit those in the other 
circuit or circuits ; but as no means had been proposed that 
would secure in this way between the phases of the primary or 
inducing and the secondary or induced currents that difference 
theoretically ninety degrees that is best adapted for practical 
and economical working, Mr. Tesla devised a means which ren- 
ders practicable both the above described plans or methods, and 
by which he is enabled to obtain an economical and efficient al- 
ternating current motor. His invention consists in placing a 
condenser in the secondary or induced circuit of the motor above 
described and raising the potential of the secondary currents to 
such a degree that the capacity of the condenser, which is in 
part dependent on the potential, need be quite small. The value 
of this condenser is determined in a well-understood manner with 
reference to the self-induction and other conditions of the circuit, 
so as to cause the currents which pass through it to differ from 
the primary currents by a quarter phase. 

Fig. 91 illustrates the invention as embodied in a motor 
in which the inductive relation of the primary and secondary 
circuits is secured by winding them inside the motor partly 
upon the same cores ; but the invention applies, generally, to 



POL YPHA&K d 



107 



other forms of motor in which one of the energizing currents is 
induced in any way from the other. 

Let A B represent the poles of an alternating current motor, of 
which c is the armature wound with coils D, closed upon them- 
selves, as is now the general practice in motors of this kind. The 
poles A, which alternate with poles B, are wound with coils of 
ordinary or coarse wire E in such direction as to make them of 
alternate north and south polarity, as indicated in the diagram 
by the characters N s. Over these coils, or in other inductive re- 
lation to the same, are wound long fine-wire coils F F, and in the 




FIG. 91. 

same direction throughout as the coils E. These coils are secon- 
daries, in which currents of very high potential are induced. All 
the coils E in one series are connected, and all the secondaries F 
in another. 

On the intermediate poles B are wound line-wire energizing 
coils G, which are connected in series with one another, and also 
with the series of secondary coils F, the direction of winding be- 
.ing such that a current-impulse induced from the primary coils 
K imparts the same magnetism to the poles B as that produced 



108 INVENTIONS OF NIKOLA TESLA. 

in poles A by the primary impulse. Tins condition is indicated 
by the characters N' s'. 

In the circuit formed by the two sets of coils F and G is intro- 
duced a condenser H ; otherwise this circuit is closed upon 
itself, while the free ends of the circuit of coils E are connected 
to a source of alternating currents. As the condenser capacity 
which is needed in any particular motor of this kind is depend- 
ent upon the rate of alternation or the potential, or both, its size 
or cost, as before explained, may be brought within economical 
limits for use with the ordinary circuits if the potential of the 
secondary circuit in the motor be sufficiently high. By giving 
to the condenser proper values, any desired difference of phase 
between the primary and secondary energizing circuits may be 
obtained. 



CHAPTER XXIII. 

TESLA POLYPHASE TRANSFORMER. 

APPLYING the polyphase principle to the construction of trans- 
formers as well to the motors already noticed, Mr. Tesla has in- 
vented some very interesting forms, which he considers free 
from the defects of earlier and, at present, more familiar forms. 
In these transformers he provides a series of inducing coils and 
corresponding induced coils, which are generally wound upon a 
core closed upon itself, usually a ring of laminated iron. 

The two sets of coils are wound side by side or superposed or 
otherwise placed in well-known ways to bring them into the most 
effective relations to one another and to the core. The inducing 
or primary coils wound on the core are divided into pairs or sets 
by the proper electrical connections, so that while the coils of 
one pair or set co-operate in fixing the magnetic poles of the 
core at two given diametrically opposite points, the coils of the 
other pair or set assuming, for sake of illustration, that there 
are but two tend to fix the poles ninety degrees from such 
points. With this induction device is used an alternating current 
generator with coils or sets of coils to correspond with those of 
the converter, and the corresponding coils of the generator and 
converter are then connected up in independent circuits. It re- 
sults from this that the different electrical phases in the genera- 
tor are attended by corresponding magnetic changes in the con- 
verter; or, in other words, that as the generator coils revolve, 
the points of greatest magnetic intensity in the converter will be 
progressively shifted or whirled around. 

Fig. 92 is a diagrammatic illustration of the converter and the 
electrical connections of the same. Fig. 93 is a horizontal cen- 
tral cross-section of Fig. 92. Fig. 94 is a diagram of the circuits 
of the entire system, the generator being shown in section. 

Mr. Tesla uses a core, A, which is closed upon itself that is to 
say, of an annular cylindrical or equivalent form and as the 
efficiency of the apparatus is largely increased by the subdivision 



no 



INVENTIONS 0V NIKOLA TKKLA. 



of tliis 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, BBS' B', used as primary coils, and 'for 
which long lengths of comparatively fine wire are employed. 
Over these coils are then wound shorter coils of coarser wire, c c 
c' c', to constitute the induced or secondary coils. The construc- 
tion of this or any equivalent form of converter may be carried 
further, as above pointed out, by inclosing these coils with iron 
as, for example, by winding over the coils layers of insulated 
iron wire. 

The device is provided with suitable binding posts, to which 




FIGS. 92 and 93. 

the ends of the coils are led. The diametrically opposite coils 
B R and B' B' are connected, respectively, in series, and the four 
terminals are connected to the binding posts. The induced 
coils are connected together in any desired manner. For ex- 
ample, as shown in Fig. 94, c c may be connected in multiple 
arc when a quantity current is desired as for running a group 
of incandescent lamps while c' c' may be independently con- 
nected in series in a circuit including arc lamps or the like. The 
generator in this system will be adapted to the converter in the 



POLYPHASE CURRENTS. 



Ill 



manner illustrated. For example, in the present case there are 
employed a pair of ordinary permanent or electro-magnets, E E, 
between which is mounted a cylindrical armature on a shaft, F, 
and wound with two coils, G G'. The terminals of these coils are 
connected, respectively, to four insulated contact or collecting 
rings, H H H' H', and the four line circuit wires L connect the 
brushes K, bearing on these rings, to the converter in the order 
shown. Noting the results of this combination, it will be ob- 
served that at a given point of time the coil G is in its neutral 
position and is generating little or no current, while the other 
coil, G', is in a position where it exerts its maximum effect. 
Assuming coil G to be connected in circuit with coils B B of the 
converter, and coil G' with coils B' B', it is evident that the poles 




FIG. 94. 



of the ring A will be determined by coils B' B' alone ; but as the 
armature of the generator revolves, coil G develops more current 
and coil G' less, until G reaches its maximum and G' its neutral 
position. The obvious result will be to shift the poles of the 
ring A through one-quarter of its periphery. The movement of 
the coils through the next quarter of a turn during which coil 
(/ enters a tield of opposite polarity and generates a current of 
opposite direction and increasing strength, while coil G, in passing 
from its maximum to its neutral position generates a current of 
decreasing strength and same direction as before causes a further 
shifting of the poles through the second quarter of the ring. 
The second half -re volution will obviously be a repetition of the 
same action. By the shifting of the poles of the ring A, a power- 



112 INVENTIONS OF NIKOLA TE8LA. 

ful dynamic inductive effect on the coils c c' is produced. Be- 
sides the currents generated in the secondary coils by dynamo- 
magnetic induction, other currents will be set up in the same 
coils in consequence of many variations in the intensity of the 
poles in the ring A. This should be avoided by maintaining the 
intensity of the poles constant, to accomplish which care should 
be taken in designing and proportioning the generator and in 
distributing the coils in the ring A, and balancing their effect. 
When this is done, the currents are produced by dynamo-mag- 
netic induction only, the same result being obtained as though 
the poles were shifted by a commutator with an infinite number 
of segments. 

The modifications which are applicable to other forms of con- 
verter are in many respects applicable to this, such as those per- 
taining 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. 



CHAPTEK XXIV. 

A CONSTANT CURRENT TRANSFORMER WITH MAGNETIC SHIELD 
BETWEEN COILS OF PRIMARY AND SECONDARY. 

MR. TESLA has applied his principle of magnetic shielding of 
parts to the construction also of transformers, the shield being 
interposed between the primary and secondary coils. In trans- 
formers of the ordinary type it will be found that the wave of 
electromotive force of the secondary very nearly coincides with 
that of the primary, being, however, in opposite sign. At the same 
time the currents, both primary and secondary, lag behind their 
respective electromotive forces ; but as this lag is practically or 
nearly the same in the case of each it follows that the maximum 
and minimum of the primary and secondary currents will nearly 
coincide, but differ in sign or direction, provided the secondary 
be not loaded or if it contain devices having the property of 
self-induction. On the other hand, the lag of the primary 
behind the impressed electromotive force may be diminished by 
loading the secondary with a non-inductive or dead resistance 
such as incandescent lamps whereby the time interval between 
the maximum or minimum periods of the primary and secondary 
currents is increased. This time interval, however, is limited, 
and the results obtained by phase difference in the operation of 
such devices as the Tesla alternating current motors can only be 
approximately realized by such means of producing or securing 
this difference, as above indicated, for it is desirable in such cases 
that there should exist between the primary and secondary cur- 
rents, 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 maxi- 
mum 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 



114 



INVENTIONS OF NIKOLA TESLA. 



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 induc- 
tive action upon the secondary begins. It results, therefore, that 
the secondary current begins to How at a certain fraction of a 
period later than it would without the interposed shield, and 
since this retardation may be obtained without necessarily retard- 
ing 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 trans- 




FIG. 95. 

former 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. 

Fig. 95 is a cross-section of a transformer embodying this im- 
provement. Fig. 96 is a similar view of a modified form of 
transformer, showing diagrammatically the manner of using the 
same. 

A A is the main core of the transformer, composed of a ring 
of soft annealed and insulated or oxidized iron wire. Upon this 
core is wound the secondary circuit or coil B B. This latter is 
then covered with a layer or layers of annealed and insulated 
iron wires c c, wound in a direction at right angles to the secondary 



POLYPHASE CURRENTS. 



115 



coil. Over the whole is then wound the primary coil or wire D D. 
From the nature of this construction it will be obvious that 
as long as the shield formed by the wires c is below magnetic 
saturation the secondary coil or circuit is effectually protected or 
shielded from the inductive influence of the primary, although 
on open circuit it may exhibit some electromotive force. When 
the strength of the primary reaches a certain value, the shield c, 
becoming saturated, ceases to protect the secondary from induc- 
tive 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 approxi- 
mately the same extent. 

The specific construction of the transformer is largely imma- 




FIG. 90. 

terial. In Fig. 90, for example, the core A is built up of thin 
insulated iron plates or discs. The primary circuit D is wound 
next the core A. Over this is applied the shield c, which in this 
case is made up of thin strips or plates of iron properly insulated 
and surrounding the primary, forming a closed magnetic circuit. 
The secondary B is wound over the shield c. In Fig. 06, also, 
K is a source of alternating or rapidly changing currents. 
The primary of the transformer is connected with the circuit of 
the generator. F is a two-circuit alternating current motor, one 
of the circuits being connected with the main circuit from the 
source E, and the other being supplied with currents from the 
secondarv of the transformer. 



PART II. 



THE TESLA EFFECTS WITH HIGH FREQUENCY 
AND HIGH POTENTIAL CURRENTS. 



CHAPTER XXV. 

INTRODUCTION. THE SCOPE OF THE TESLA LECTURES. 

BEFORE proceeding to study the three Tesla lectures here 
presented, the reader may find it of some assistance to have his 
attention directed to the main points of interest and significance 
therein. The lirst 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 immedi- 
ately 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 re- 
peated his novel demonstrations before the Societe Internationale 
des Electriciens, and the Societe Frangaise de Physique. Mr. Tesla 
returned to America in the fall of 1892, and in February, 1893, de- 
livered his third lecture before the Franklin Institute of Philadel- 
phia, 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 in- 
vitations 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 experi- 
ments, and to advance the frontier of discovery far beyond any 
point he had theretofore marked publicly. 

We may now proceed to a running review of the lectures them- 
selves. The ground covered by them is so vast that only the 



120 INVENTIONS OF NIKOLA TKSLA. 

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 pro- 
duction of light waves, primarily, electrostatic effects must be 
_^Jjrought into play, and continuedjrtudy has led him tpjtheLOpinion 
that_all electrical and magnetic effects may be referred to ek-c- 
trostajtic _irLolecular _ forces. This opinion finds a singular con- 
firmation 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 electro- 
static 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 man- 
ner for the unexplained fact that buildings are frequently set on 
fire during thunder storms with'out having been at all struck by 
-\v lightning. It may also explain the total disappearance of ships 
at sea. 

One of the striking proofs of the correctness of the ideas ad- 
vanced by Mr. Tesla is the fact that, notwithstanding the employ- 
ment 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 



man FHKQUENVY AND HIGH POTENTIAL CVKRKNT*. 121 

of lighting an apartment by simply creating in it sucli an electro- 
static field, and this, in a certain way, would appear to be the 
ideal method of lighting a room, as it would allow the illuminat- 
ing device to be freely moved about. The power with which 
these exhausted tubes, devoid of any electrodes, light up is cer- 
tainly remarkable. 

That the principle propounded by Mr. Tesla is a broad one is 
evident from the many ways in which it may be practically ap- 
plied. We need only refer to the variety of the devices shown 
or described, all of which are novel in character and will, with- 
out 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 interest- 
ing. Thus, the incandescent filament spinning in an unex- 
hausted globe, the well-known Crookes experiment on open cir- 
cuit, 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 op- 
erated with these rapidly alternating currents, starting from the 
thread-like discharge and passing through various stages to the 
true electric flame. 

A point of great importance in the introduction of high ten- 
sion alternating current which Mr. Tesla brings out is the neces- 
sity 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 al- 
most 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 cur- 
rents, any kind of insulation may be used, with rapidly alternat- 
ing currents oils will probably be the best to employ, a fact 
which has been observed, but not until now satisfactorily ex- 
plained. The recognition of the above fact is of special impor- 
tance 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- 



122 INVENTIONS OF NIKOLA TESLA. 

. teresting experiments illustrative of the behavior of the air be- 
tween charged surfaces, the luminous streams formed by the 
charged molecules appearing even when great thicknesses of tin- 
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 cur- 
rents they become far more powerful and produce beautiful light 
effects when they issue from a wire, pinwheel or other object at- 
tached to a terminal of the coil ; and it is interesting to note that 
they issue from a ball almost as freely as from a point, when the 
frequency is very high. 

From these experiments we also obtain a better idea of the 
importance of taking into account the capacity and self-induction 
in the apparatus employed and the possibilities offered by the 
use of condensers in conjunction with alternate currents, the em- 
ployment of currents of high frequency, among other things, 
making it possible to reduce the condenser to practicable dirnen- 
(sions. Another point of interest and practical bearing is the 
fact, proved by Mr. Tesla, that for alternate currents, especially 
those of high frequency, insulators are required possessing a 
small specific inductive capacity, which at the same time have a 
high insulating power. 

Mr. Tesla also makes interesting and valuable suggestion in re- 
gard to the economical utilization of iron in machines and trans- 
formers. He shows how, by maintaining by continuous magnet- 
ization 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 alter- 
nating 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 ap- 
pear less practical, nevertheless opens another direction for inves- 
tigation and improvement. 

The demonstration of the fact that with alternating currents 

of high frequency, sufficient energy may be transmitted under 

practicable conditions through the glass of an incandescent lamp 

by electrostatic or electromagnetic induction may lead to a de- 

^-parture in the construction of such devices. Another important 

i I experimental result achieved is the operation of lamps, and even 

\ 1 .motors, with the discharges of condensers, this method affording 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 123 

a means of converting direct or alternating currents. In this 
connection Mr. Tesla advocates the perfecting of apparatus capa- 
ble of generating electricity of high tension from heat energy, 
believing this to be a better way of obtaining electrical energy 
for practical purposes, particularly for the production of light. 

While many were probably prepared to encounter curious 
phenomena of impedance in the use of a condenser discharged 
disruptively, the experiments shown were extremely interesting 
on account of their paradoxical character. The burning of an 
incandescent lamp at any candle power when connected across a 
heavy metal bar, the existence of nodes on the bar and the possi- 
bility of exploring the bar by means of an ordinary Garde w 
voltmeter, are all peculiar developments, but perhaps the most 
interesting observation is the phenomenon of impedance observed 
in the lamp with a straight filament, which remains dark while 
the bulb glows. 

Mr. Tesla's manner of operating an induction coil by means of 
the disruptive discharge, and thus obtaining enormous differences 
of potential from comparatively small and inexpensive coils, will 
be appreciated by experimenters and will find valuable applica- 
tion in laboratories. Indeed, his many suggestions and hints in 
regard to the construction and use of apparatus in these investi- 
gations will be highly valued and will aid materially in future- 
research. 

The London lecture was delivered twice. In its first form, 
before the Institution of Electrical Engineers, it was in some 
respects an amplification of several j>oints not specially enlarged 
upon in the JS T ew York lecture, but brought forward many addi- 
tional discoveries and new investigations. Its repetition, in-.""-] 
another form, at the Royal Institution, was due to Prof. Dewar, 
who with Lord Ray leigh, manifested a most lively interest in Mr. 'j 
Tesla's work, and whose kindness illustrated once more the strong V } 
English love of scientific truth and appreciation of its votaries. } 
As an indefatigable experimenter, Mr. Tesla was certainly no-^ 
where more at home than in the haunts of Faraday, and as the / 
guest of Faraday's successor. This Royal Institution lecture W 
summed up the leading points of Mr. Tesla's work, in the high / 
potential, high frequency field, and we may here avail ourselves J 
of so valuable a summarization, in a simple form, of a subject by 
no means easv of comprehension until it has been thoroughly 
studied. 



124 INVENTIONS OF NIKOLA TKSLA. 

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 fre- 
quencies it was necessary to provide several hundred polar pro- 
jections, which were necessarily small and offered many draw- 
backs, and this the more as exceedingly high peripheral speeds 
had to be resorted to. In some of the first machines both arma- 
ture 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 re- 
sults obtained were not quite satisfactory, on account of the 
great mechanical and other difficulties encountered. 

The study of the properties of the high frequency currents 
obtained from these machines is very interesting, as nearly every 
experiment discloses something new. Two coils traversed by 
such a current attract or repel each other with a force which, 
owing to the imperfection of our sense of touch, seems contin- 
uous. An interesting observation, already noted under another 
form, is that a piece of iron, surrounded by a coil through which 
the current is passing appears to be continuously magnetized. 
This apparent continuity might be ascribed to the deficiency of 
the sense of touch, but there is evidence that in currents of such 
high frequencies one of the impulses preponderates over the 
other. 

As might be expected, conductors traversed by such currents 
are rapidly heated, owing to the increase of the resistance, and 
the heating effects are relatively much greater in the iron. 
The hysteresis losses in iron are so great that an iron core, 
even if finely subdivided, is heated in an incredibly short time. 
To give an idea of this, an ordinary iron wire -^g- 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 



HIGH FRJSQVKNCy AND HIGH POTENTIAL CURRENTS. 125 

whicli tlie impedance <>f a coil was not affected by the presence 
of a core consisting of a bundle of very thin well annealed and 
varnished iron wires. 

Experiments with a telephone, a conductor in a strong mag- 
netic 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 fea- 
ture of the arc is its persistency, which is due partly to the in- 
ability of the gaseous column to cool and increase considerably 
in resistance, as is the case with low frequencies, and partly to 
the tendency of such a high frequency machine to maintain a 
constant current. 

In connection with these machines the condenser affords a par- 
ticularly interesting study. Striking effects are produced by 
proper adjustments of capacity and self-induction. It is easy to 
raise the electromotive force of the machine to many times the 
original value by simply adjusting the capacity of a condenser 
connected in the induced circuit. If the condenser be at some 
distance from the machine, the difference of potential on the 
terminals of the latter may be only a small fraction of that on 
the condenser. 

But the most interesting experiences are gained when the ten- 
sion 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 differ- 
ences are obtained than with coils operated in the usual ways, 
and, owing to the high frequency, the secondary discharge pos- 
sesses many striking peculiarities. Both the electrodes behave 
generally alike, though it appears from some observations that 
one current impulse preponderates over the other, as before 
mentioned. 

The physiological effects of the high tension discharge are 
found to be so small that the shock of the coil can be supported 
without any inconvenience, except perhaps a small burn produced 
by the discharge upon approaching the hand to one of the ter- 
minals. The decidedly smaller physiological effects of these cur- 



126 INVENTIONS OF NIKOLA TESLA. 

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 harmless- 
ness is principally due to the fact that but little energy is avail- 
able in the external circuit when the same is closed through the 
experimenter's body, on account of the great impedance of the 
coil. 

In varying the frequency and strenth of the currents through 
the primary of the coil, the character of the secondary discharge 
is greatly varied, and no less than five distincts forms are ob- 
served : A weak, sensitive thread discharge, a powerful naming 
discharge, and three forms of brush or streaming discharges. 
Each of these possesses certain noteworthy features, but the most 
interesting to study are the latter. 

Under certain conditions the streams, which are presumably 
due to the violent agitation of the air molecules, issue freely 
from all points of the coil, even through a thick insulation. If 
there is the smallest air space between the primary and secondary, 
they will form there and surely injure the coil by slowly warm- 
ing the insulation. As they form even with ordinary frequencies 
when the potential is excessive, the air-space must be most care- 
fully 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 pe- 
culiarity is that they issue as freely from surfaces as from points. 
( hving to this, a metallic vane, mounted in one of the terminals of 
the coil so as to rotate freely, and having one of its sides covered 
with insulation, is spun rapidly around. Such a vane would not 
rotate with a steady potential, but with a high frequency coil it 
will spin, even if it be entirely covered with insulation, provided 
the insulation on one side be either thicker or of a higher specific 
inductive capacity. A Crookes electric radiometer is also spun 
around when connected to one of the terminals of the coil, but 
only at very high exhaustion or at ordinary pressures. 

There is still another and more striking peculiarity of such a 
high frequency streamer, namely, it is hot. The heat is easily 
perceptible with frequencies of about 10,000, even if the poten- 
tial 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 pr- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 127 

duced resembling in every particular a flame and giving light 
and heat, jet without a chemical process taking place. 

The hot brush, when properly produced, resembles a jet of 
burning gas escaping under great pressure, and it emits an extra- 
ordinary 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 fre- 
quency of several hundred thousand were obtained. 

Currents of this kind, Mr. Tesla pointed out, produce striking 
effects. At these frequencies, the impedance of a copper bar is 
so great that a potential difference of several hundred volts can 
be maintained between two points of a short and thick bar, and 
it is possible to keep an ordinary incandescent lamp burning at 
full candle power by attaching the terminals of the lamp to two 
points of the bar no more than a few inches apart, When the 
frequency is extremely high, nodes are found to exist on such a 
bar, and it is easy to locate them by means of a lamp. 

By converting the high tension discharges of a low frequency 
coil in this manner, it was found practicable to keep a few lamps 
burning on the ordinary circuit in the laboratory, and by bring- 
ing the undulation to a low pitch, it was possible to operate small 
motors. 

This plan likewise allows of converting high tension discharges 
of one direction into low tension unidirectional currents, by ad- 
justing the circuit so that there are no oscillations. In passing 
the oscillating discharges through the primary of a specially 
constructed coil, it is easy to obtain enormous potential differences 
with only few turns of the secondary. 

Great difficulties were at first experienced in producing a suc- 
cessful coil on this plan. It was found necessary to keep all air, 
or gaseous matter in general, away from the charged surfaces, 
and oil immersion was resorted to. The wires used were heavily 
covered with gutta-percha and wound in oil, or the air was pumped 
out by means of a Sprengel pump. The general arrangement 
was the following: An ordinary induction coil, operated from 
a low frequency alternator, was used to charge Leyden jars. The 



128 INVENTIONS OF NIKOLA TESLA. 

jars were made to discharge over a single or multiple gap through 
the primary of the second coil. To insure the action of the gap, 
the arc was blown out by a magnet or air blast. To adjust the 
potential in the secondary a small oil condenser was used, or 
polished brass spheres of different sizes were screwed on the 
terminals and their distance adjusted. 

When the conditions were carefully determined to suit each 
experiment, magnificent effects were obtained. Two wires, 
stretched through the room, each being connected to one of the 
terminals of the coil, emitted streams so powerful that the light 
from them allowed distinguishing the objects in the room ; the 
wires became luminous even though covered with thick and 
most excellent insulation. When two straight wires, or two con- 
centric circles of wire, are connected to the terminals, and set at 
the proper distance, a uniform luminous sheet is produced be- 
tween them. It was possible in this way to cover an ana of 
more than one meter square completely with the streams. By 
attaching to one terminal a large circle of wire and to the other 
terminal a small sphere, the streams are focused upon the sphere, 
produce a strongly lighted spot upon the same, and present the 
appearance of a luminous cone. A very thin wire glued upon a 
plate of hard rubber of great thickness, on the opposite side of 
which is fastened a tinfoil coating, is rendered intensely luminous 
when the coating is connected to the other terminal of the coil. 
Such an experiment can be performed also with low frequency 
currents, but much less satisfactorily. 

When the terminals of such a coil, even of a very small one, 
are separated by a rubber or glass plate, the discharge spreads 
over the plate in the form of streams, threads or brilliant sparks, 
and affords a magnificent display, which cannot be equaled by 
the largest coil operated in the usual ways. By a simple adjust- 
ment it is possible to produce with the coil a succession of bril- 
liant sparks, exactly as with a Holtz machine. 

Under certain conditions, when the frequency of the oscillation 
is very great, white, phantom-like streams are seen to break forth 
from the terminals of the coil. The chief interesting feature 
about them is, that they stream freely against the outstretched 
hand or other conducting object without producing any sensa- 
tion, 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 129 

away or dissipated in the streamers, and the difference of poten- 
tial between the terminal and the hand is diminished. 

It is found in such experiments that the frequency of the 
vibration and the quickness of succession of the sparks between 
the knobs affect to a marked degree the appearance of the 
streams. When the frequency is very low, the air gives way in 
more or less the same manner as by a steady difference of poten- 
tial, 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 fre- 
quency is very high, and the arc of the discharge produces a 
sound which is loud and smooth (which indicates both that oscil- 
lation 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 as- 
sume a white color. 

The luminous intensity of the streams increases rapidly when 
the potential is increased; and with frequencies of only a few 
hundred thousand, could the coil be made to withstand a suffi- 
ciently high potential difference, there is no doubt that the 
space around a wire could be made to emit a strong light, 
merely by the agitation of the molecules of the air at ordinary 
pressure. 

Such discharges of very high frequency which render lumi- 
nous the air at ordinary pressure we have very likely occasion to 
witness in the aurora borealis. From many of these experi- 
ments it seems reasonable to infer that sudden cosmic disturb- 
ances, 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 ob- 
served, but they can be attributed to the fundamental disturb- 
ances, which are few in number, for the superimposed vibration 
would be so rapid as not to allow a disruptive break. 

The study of these discharge phenomena has led Mr. Tesla to 
the recognition of some important facts. It was found, as already 
stated, that uascous matter must be most carefully excluded from 



130 INVENTIONS OP' NIKOLA TESLA. 

any dielectric which is subjected to great, rapidly changing elec- 
trostatic 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 ordi- 
nary 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 prin- 
cipally 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 i vari- 
ably is warmed and breaks down. 

The employment of oil is advisable or necessary even with low 
frequencies, if the potentials are such that streamers form, but 
only in such cases, as is evident from the theory of the action. 
If the potentials are so low that streamers do not form, then it 
is even disadvantageous to employ oil, for it may, principally by 
confining the heat, be the cause of the breaking down of the in- 
sulation. 

The exclusion of gaseous matter is not only desirable on ac- 
count of the safety of the apparatus, but also on account of 
economy, especially in a condenser, in which considerable waste 
of power may occur merely owing to the presence of air, if the 
electric density on the charged surfaces is great. 

In the course of these investigations a phenomenon of special 
scientific interest was observed. It may be ranked among the 
brush phenomena, in fact it is a kind of brush which forms at, or 
near, a single terminal in high vacuum. In a bulb with a con- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 131 

ducting electrode, even if the latter be of aluminum, the brush 
has only a very short existence, but it can be preserved for a con- 
siderable length of time in a bulb devoid of any conducting elec- 
trode. 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 be- 
ing 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 in- 
fluence. 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 sen- 
sitive 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 me- 
tres, slowing down or accelerating the rotation according to how 
it is held relatively to the brush. 

When the bulb hangs with the globe down, the rotation is al- 
ways 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 mag- 
net 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 re- 
versed by putting the bulb in any position. Very curious experi- 
ments may be performed with the brush when in its most sensi- 
tive 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 
cjuisi' the brush to pass oft bv merely stiffening the muscles of 



132 INVENTIONS OF NIKOLA TESLA. 

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 tin- 
hands are held at a proper distance, it is impossible to make even 
the slightest motion without producing a visible effect upon the 
brush. A metal plate connected to the other terminal of the coil 
affects it at a great distance, slowing down the rotation often to 
one turn a second. 

Mr. Tesla hopes that this phenomenon will prove a valuable 
aid in the investigation of the nature of the forces acting in an 
electrostatic or magnetic field. If there is any motion which is 
measurable going on in the space, such a brush would be apt to 
reveal it. It is, so to speak, a beam of light, frictionless, devoid 
of inertia. On account of its marvellous sensitiveness to electro- 
static or magnetic disturbances it may be the means of sending 
signals through submarine cables with any speed, and even of 
transmitting intelligence to a .distance without wires. 

In operating an induction coil with these rapidly alternating 
currents, it is astonishing to note, for the first time, the great 
importance of the relation of capacity, self-induction, and fre- 
quency 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 />/v- 
vents a strong current from flowing through the coil. If the art- 
be interrupted by the interposition of a glass plate, the capacity 
of the condenser obtained counteracts the self-induction, and a 
stronger current is made to pass. The effects of capacity are the 
most striking, for in these experiments, since the self-induction 
and frequency both are high, the critical capacity is very small, 
and need be but slightly varied to produce a very considerable 
change. The experimenter brings his body in contact with the 
terminals of the secondary of the coil, or attaches to one or both 
terminals insulated bodies of very small bulk, such as exhausted 
bulbs, and he produces a considerable rise or fall of potential on 
the secondary, and greatly affects the flow of the current through 
the primary coil. 

In many of the phenomena observed, the presence of the air, 
or, generally speaking, of a medium of a gaseous nature (using 
this term not to imply specific properties, but in contradistinction 
to homogeneity or perfect continuity) plays an important part. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 133 

as it allows energy to be dissipated by molecular impact or bom- 
bardment. The action is thus explained: When an insulated 
body connected to a terminal of the coil is suddenly charged to 
high potential, it acts inductively upon the surrounding air, or 
whatever gaseous medium there might be. The molecules or 
atoms which are near it are, of course, more attracted, and move 
through a greater distance than the further ones. When the 
nearest molecules strike the body they are repelled, and collisions 
occur at all distances within the inductive distance. It is now 
clear that, if the potential be steady, bat 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 accom- 
plished 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 pro- 
portionate 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 ra- 
pidly alternating the potential of a body immersed in an insulat- 
ing gaseous medium, any amount of energy may be dissipated 
into space. Most of that energy, then, is not dissipated in the 
form of long ether waves, propagated to considerable distance, 
as is thought most generally, but is consumed in impact and 
collisional losses that is, heat vibrations on the surface and in 



134 INVENTIONS OF NIKOLA TESLA. 

the vicinity of the body. To reduce the dissipation it is neces- 
sary to work with a small electric density the smaller, the 
higher the frequency. 

The behavior of a gaseous medium to such rapid alternations 
of potential makes it appear plausible that electrostatic dis- 
turbances of the earth, produced by cosmic events, may have 
great influence upon the meteorological condition^. 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 pro- 
duced would accordingly vary from place to place. Considerable 
variations in the temperature and pressure of the atmosphere 
may in this manner be caused at any point of the surface of the 
earth. The variations may be gradual or very sudden, according 
to the nature of the original disturbance, and may produce rain 
and storms, or locally modify the weather in any way. 

From many experiences gathered in the course of these inves- 
tigations 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 harm- 
less static discharge may, in consequence of the oscillations set 
up, assume the character of a high-frequency streamer, and the 
nail or projection may be brought to a high temperature by the 
violent impact of the air molecules. Thus, it is thought, a 
building may be set on fire without the lightning striking it. In 
like manner small metallic objects may be fused and volatilized 
as frequently occurs in lightning discharges merely because 
they are surrounded by air. Were they immersed in a practi- 
cally continuous medium, such as oil, they would probably be 
safe, as the energy would have to spend itself elsewhere. 

An instructive experience having a bearing on this subject is 
the following: A glass tube of an inch or so in diameter and 
several inches long is taken, and a platnium 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENT*. 135 

frequency discharges are directed through the wire, it is heated 
more on the ends than in the middle portion, and if the fre- 
quency, 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 he conducting ; it might be at an ex- 
tremely 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 poten- 
tials occurs in an exhausted tube, occurs in the lightning discharge 
at ordinary pressure. 

From the facility with which any amount of energy may be 
carried off through a gas, Mr. Tesla infers that the best w T ay to 
render harmless a lightning discharge is to afford it in some way 
a passage through a volume of gas. 

The recognition of some of the above facts has a bearing upon 
far-reaching scientific investigations in which extremely high 
frequencies and potentials are used. In such cases the air is an 
important factor to be considered. So, for instance, if two wires 
are attached to the terminals of the coil, and the streamers issue 
from' them, there is dissipation of energy in the form of heat 
and light, and the wires behave like a condenser of larger capac- 
ity. 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 sur- 
face and is charged to a very high potential. As many import- 
ant results are dependant upon the correctness of the estimation 
of the vibration period, this subject demands the most careful 
scrutiny of investigators. 

In Leyden jars the loss due to the presence of air is compara- 
tively 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- 



136 INVENTIONS OF NIKOLA TESLA. 

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. 

Another fact recognized, which is of some consequence, is, 
that in similar investigations the general considerations of static 
screening are not applicable when a gaseous medium is present. 
This is evident from the following experiment : A short and 
wide glass tube is taken and covered with a substantial coating of 
bronze powder, barely allowing the light to shine a little through. 
The tube is highly exhausted and suspended on a metallic clasp 
from the end of a wire. When the wire is connected with one 
of the terminals of the coil, the gas inside of the tube is lighted 
in spite of the metal coating. Here the metal evidently does 
not screen the gas inside as it ought to, even if it be very thin 
and poorly conducting. Yet, in a condition of rest the metal 
coating, however thin, screens the inside perfectly. 

One of the most interesting results arrived at in pursuing these 
experiments, is the demonstration of the fact that a gaseous me- 
dium, upon which vibration is impressed by rapid changes of 
electrostatic potential, is rigid. In illustration of this result an 
experiment made by Mr. Tesla may by cited : A glass tube about 
one inch in diameter and three feet long, with outside condenser 
coatings on the ends, was exhausted to a certain point, when, the 
tube being suspended freely from a wire connecting the upper coat- 
ing 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 electro- 
static influence, and a transversal vibration like that of a sus- 
pended cord, with one or more distinct nodes, was set up, which 
lasted for a few minutes and gradually died out. By suspending 
from the lower condenser coating metal plates of different sizes, 
the speed of the vibration was varied. This vibration would 
seem to show beyond doubt that the thread possessed rigidity, 
at least to transversal displacements. 

Many experiments were tried to demonstrate this property in 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 137 

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 tin-own against it the sphere would rebound. 
An ordinary flame cannot possess rigidity to a marked degree 
because the vibration is directionless ; but an electric arc, it is 
believed, must possess that property more or less. A luminous 
band excited in a bulb by repeated discharges of a Leyden jar 
must also possess rigidity, and if deformed and suddenly released 
should vibrate. 

From like considerations other conclusions of interest are 
readied. 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 pro- 
perties. 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 cor- 
responding to rarefaction. If the potential were rapidly chang- 
ing, the result would be very different ; the higher the freqency 
of the pulses, the slower would be the exchange of the carriers ; 
finally, the motion of translation through measurable space would 
cease, and, with a sufficiently high frequency and intensity of the 
stress, the carriers would be drawn towards the electrode, and 
compression would result. 

An interesting feature of these high frequency currents is that 
they allow of operating all kinds of devices by connecting the de- 
vice with only one leading wire to the electric source. In fact, 
under certain conditions it may be more economical to supply the 
electrical energy witli one lead than with two. 



138 INVENTIONS OF NIKOLA TESLA. 

An experiment of special interest shown by Mr. Tesla, is the 
running, by the use of only one insulated line, of a motor oper- 
ating 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 arid the other end to an 
insulated metal plate, which, it should be stated, is not absolutely 
necessary for the success of the experiment, the disc is set in 
rotation. 

Experiments of this kind seem to bring it within possibility to 
operate a motor at any point of the earth's surface from a cen- 
tral source, without any connection to the same except through 
the earth. If, by means of powerful machinery, rapid variations 
of the earth's potential were produced, a grounded wire reaching 
up to some height would be traversed by a current which could 
be increased by connecting the free end of the wire to a body of 
some size. The current might be converted to low tension and 
used to operate a motor or other device. The experiment, which 
would be one of great scientific interest, would probably best 
succeed on a ship at sea. In this manner, even if it were not 
possible to operate machinery, intelligence might be transmitted 
quite certainly. 

In the course of this experimental study special attention was 
devoted to the heating effects produced by these currents, which 
are not only striking, but open up the possibility of producing a 
more efficient illumiuant. 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 circula- 
tion 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 in- 
candescence, and any amount of light may thus be produced. 

The heating of the inclosed lamp filament depends on so 
many things of a different nature, that it is difficult to give a 
generally applicable rule under which the maximum heating 



ITIGH FREQUENCY AND HTGH POTENTIAL CURRENTS. 139 

occurs. As regards the size of the bull), 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 ef- 
fect is greater in a large bull), but at excessively high degrees of 
exhaustion there seems to be, beyond a certain and rather small 
size of the vessel, no perceptible difference in the heating. 

The shape of the vessel is also of some importance, and it has 
been found of advantage for reasons of economy to employ a 
spherical bulb with the electrode mounted in its centre, where 
the rebounding molecules collide. 

It is desirable on account of economy that all the energy sup- 
plied 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 con- 
nected to the ground as under ordinary conditions. 

In the bulb itself a large portion of the energy' supplied may 
be lost by molecular bombardment against the wire connecting 
the body to be heated with the source. Considerable improve- 
ment was effected by covering the glass stem containing the wire 
with a closely fitting conducting tube. This tube is made to 
project a little above the glass, and prevents the cracking of the 
latter near the heated body. The effectiveness of the conducting 
tube is limited to very high degrees of exhaustion. It diminishes 
the energy lost in bombardment for two reasons; first, the 
charge given up by the atoms spreads over a greater area, and 
hence the electric density at any point is small, and the atoms 
are repelled with less energy than if they would strike against a 
good insulator; secondly, as the tube is electrified by the atoms 
which first come in contact with it, the progress of the following- 
atoms against the tube is more or less checked by the repulsion 
which the electrified tube must exert upon the similarly electrified 
atoms. This, it is thought, explains why the discharge through 
a bulb is established with much greater facility when an insulator, 
than when a conductor, is present. 

During the investigations a great many bulbs of different con- 
struction, with electrodes of different material, were experimented 
upon, and a number of observations of interest were made. Mr. 



140 INVENTIONS OF NIKOLA TESLA. 

Tesla has found tlmt 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 de- 
terioration 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 experi- 
ence, it is inferred, that to be most durable, the electrode should 
be in the form of a sphere with a highly polished surface. 

In some bulbs refractory bodies were mounted in a carbon cup 
and put under the molecular impact. It was observed in 
such experiments that the carbon cup was heated at first, until a 
higher temperature was reached; then most of the bombard- 
ment 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 necessi- 
tated by the fact that most of the energy supplied would find 
its way through the body \vhioh was more easily fused or "evap- 
orated." 

Curiously enough it appeared in some of the experiments 
made, that a body was fused in a bulb under the molecular im- 
pact by evolution" of less light than when fused by the applica- 
tion of heat in ordinary ways. This may be ascribed to a 
loosening of the structure of the body under the violent impacts 
and changing stresses. 

Some experiments seem to indicate that under certain condi- 
tions a body, conducting or nonconducting, may, when bom- 
barded, emit light, which to all appearances is due to phosphor- 
escence, 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 141 

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 im- 
portance, inasmuch as it may modify the generally accepted views 
in regard to the "radiant state" phenomena. 

A thought which early and naturally suggested itself to JVI r. 
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 regu- 
late the intensity of the light emitted by means of an externally 
applied condenser coating connected to an insulated plate, or 
simply by means of a plate attached to the bulb which at the 
same time performed the function of a shade. 

A subject of experiment, which has been exhaustively treated 
in England by Prof. J. J. Thomson, has been followed up inde- 
pendently 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 electro- 
static 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 interest- 
ing results arrived at in the course of these investigations. It 
was found that by rapid alternations of a high electrostatic po- 
tential, 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. Phosphores- 
cent bulbs may be excited in such a field, and it is easy to regu- 
late 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 experi- 
ment, a mica vane was spun by the incandescence of a platinum 
wire. 

Coming now to the lecture delivered in Philadelphia and St. 



142 INVENTIONS OF NIKOLA TESLA. 

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 dis- 
course 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 in- 
formation will be gratefully received by students and experi- 
menters 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 cur- 
rents 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. M r. 
Tesla, true philosopher as he is, does not hesitate to point out 
defects in some of his methods, and indicates the lines which t<> 
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 con- 
version may be brought to the highest perfection. He has evi- 
dently taken pains to give as much useful information as possible 
to those who wish to follow in his path, as he shows in detail the 
circuit arrangements to be adopted in all ordinary cases met with 
in practice, and although some of these methods were described 
by him two years before, the additional information is still timely 
and welcome. 

In his experiments he dwells first on some phenomena pro- 
duced 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 electrosta- 
tic force in a gaseous medium, by touching with one hand one of 
the terminals of a 200,000 volt transformer and bringing tin- 
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. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 143 

with these currents, such an amount of energy can be passed 
through the body with impunity. He then showed by experi- 
ment the difference between a steady and a rapidly varying force 
upon the dielectric. This difference is most strikingly illustrated 
in the experiment in which a bulb attached to the end of a wire 
in connection with one of the terminals of the transformer is 
ruptured, although all extraneous bodies are remote from the 
bulb. He next illustrates how mechanical motions are produced 
by a varying electrostatic force acting through a gaseous medium. 
The importance of the action of the air is particularly illustrated 
by an interesting experiment. 

Taking up another class of phenomena, namely, those of dyna- 
mic 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 witli 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 fre- 
quency, self-induction and capacity; then a number of ways of 
operating motive and other devices by the use of a single lead. 
A number of novel impedance phenomena are also shown which 
cannot fail to arouse interest. 

Mr. Tesla next dwelt upon a subject which he thinks of great 
importance, that is, electrical resonance, which he explained in a 
popular way. He expressed his firm conviction that by observ- 
ing 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 con- 
sideration. 

Coming now to the light phenomena in particular, lie 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. Fie illustrated 
in a series of novel experiments the effect of the gas surround- 
ing the conductor and shows beyond a doubt that with high fre- 
quency 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 par- 
tially to bombardment, and demonstrates that in manv cases the 



144 INVENTIONS OF NIKOLA TESLA. 

heating may be practically due to the bombardment alone. He 
pointed out also that the skin effect is largely modi lied 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 ex- 
periments in this connection is that in which a thin platinum wire 
stretched along the axis of an exhausted tube is brought to in- 
candescence at certain points corresponding to the position of 
the striae, while at others it remains dark. This experiment 
throws an interesting light upon the nature of the strife and may 
lead to important revelations. 

Mr. Tesla also demonstrated the dissipation of energy through 
an atomic medium and dwelt upon the behavior of vacuous 
space in conveying heat, and in this connection showed the curious 
behavior of an electrode stream, from which he concludes that 
the molecules of a gas probably cannot be acted upon directly 
at measurable distances. 

Mr. Tesla summarized the chief results arrived at in pursuing 
his investigations in a manner which will serve as a valuable 
guide to all who may engage in this work. Perhaps most inter- 
est will centre on his general statements regarding the phenomena 
of phosphorescence, the most important fact revealed in this di- 
rection being that when exciting a phosphorescent bulb a certain 
definite potential gives the most economical result. 

The lectures will now be presented in the order of their date 
of delivery. 



CHAPTER XXVI. 

EXPEEIMENTS WlTH ALTERNATE CURRENTS OF VERY HlGH FRE- 
QUENCY AND THEIR APPLICATION TO METHODS OF ARTIFICIAL 
ILLUMINATION. J 

THERE is no subject more captivating, more worthy of study, 
than nature. To understand this great mechanism, to discover 
the forces which are active, and the laws which govern them, is 
the highest aim of the intellect of man. 

Nature has stored up in the universe infinite energy. The 
eternal recipient and transmitter of this infinite energy is the 
ether. The recognition of the existence of ether, and of the 
functions it performs, is one of the most important results of 
modern scientific research. The mere abandoning of the idea of 
action at a distance, the assumption of a medium pervading all 
space and connecting all gross matter, has freed the minds of 
thinkers of an ever present doubt, and, by opening a new horizon 
new and unforeseen possibilities has given fresh interest to 
phenomena witli 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 mani- 
festations 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 con- 
jecture, yet we know that the truth cannot be much longer hid- 
den, and instinctively we feel that the understanding is dawning 
upon us. We still admire these beautiful phenomena, these 

1. A lecture delivered before the American Institute of Electrical Engineers, 
at Columbia College, N. Y., May 20, 1891. 



146 INVENTIONS OF NIKOLA TESLA. 

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. 

Iri how far we can understand the world around us is the ulti- 
mate thought of every student of nature. The coarseness of our 
senses prevents us from recognizing the ulterior construction of 
matter, and astronomy, this grandest and most positive of natural 
sciences, can only teach us something that happens, as it were, in 
our immediate neighborhood ; of the remoter portions of the 
boundless universe, with its numberless stars and suns, we know 
nothing. But far beyond the limit of perception of our senses 
the spirit still can guide us, and so we may hope that even these 
unknown worlds infinitely small and great may in a measure 
become known to us. Still, even if this knowledge should reacli 
us, the searching mind will find a barrier, perhaps forever unsur- 
passable, to the true recognition of that which seems to be, the 
mere appearcmce of which is the only and slender basis of all 
our philosophy. 

Of all the forms of nature's immeasurable, all-pervading 
energy, which ever and ever changing and moving, like a soul 
animates the inert universe, electricity and magnetism are per- 
haps 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, repul- 
sions 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 to- 
wards 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 elec- 
tricity 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 
arc, unanswered. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 147 

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 can- 
not possibly exert a strain, or produce a displacement or motion 
of any kind, without causing in the surrounding medium an 
equivalent and opposite effect. But if we speak of electricity, 
meaning a thing, we must, I think, abandon the idea of two 
electricities, as the existence of two such things is highly improb- 
able. For how can we imagine that there should be two things, 
equivalent in amount, alike in their properties, but of opposite 
character, botli clinging to matter, both attracting and completely 
neutralizing each other? Such an assumption, though suggested 
by many phenomena, though most convenient for explaining 
them, has little to commend it. If there is such a thing as elec- 
tricity, there can be only one such thing, and, excess and want 
of that one thing, possibly; but more probably its condition de- 
termines 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 satisfac- 
tor manner. But a theory which better explains the facts is not 
necessarily true. Ingenious minds will invent theories to suit 
observation, and almost every independent thinker has his own 
views on the subject. 

It is not with the object of advancing an opinion, but with 
the desire of acquainting you better with some of the results, 
which I will describe, to show you the reasoning I have fol- 
lowed, the departures I have made that I venture to express, 
in a few words, the views and convictions which have led me to 
these results. 

I adhere to the idea that there is a thing which we have been 
in the habit of calling electricity. The question is, What is that 
thing? or, What, of all tilings, 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 quan- 
tity of it in nature ; that it can be neither produced nor destroyed ; 



148 INVENTIONS OF NIKOLA TESLA. 

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 cer- 
tain 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 impres- 
sions fade, one realizes that he has to deal only with ingenious 
explanations. I must confess, that I cannot believe in two elec- 
tricities, 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 ex- 
plained 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 trans- 
mitting 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 num- 
ber of alternations per second ; to such slow disturbances, the ether, 
if at rest, may behave as a true fluid. 

Returning to the subject, and bearing in mind that the exist- 
ence of two electricities is, to say the least, highly improbable, 
we must remember, that we have no evidence of electricity, nor 
can we hope to get it, unless gross matter is present. Electricity, 
therefore, cannot be called ether in the broad sense of the term ; 
but nothing would seem to stand in the way of calling electricity 
ether associated with matter, or bound ether; or, in other words, 
that the so-called static charge of the molecule is ether associated 
in some way with the molecule. Looking at it in that light, we 
would be justified in saying, that electricity is concerned in all 
molecular actions. 

Now, precisely what the ether surrounding the molecules is, 
wherein it differs from ether in general, can only be conject- 
ured. 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- 

v ' 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 149 

struction of matter, of which, of course, we can only form a 
mental picture. 

But of all the views on nature, the one which assumes one 
matter and one force, and a perfect uniformity throughout, is 
the most scientific and most likely to be true. An infinitesimal 
world, with the molecules and their atoms spinning and moving 
in orbits, in much the same manner as celestial bodies, carrying 
with them and probably spinning with them ether, or in other 
words, carrying with them static charges, seems to my mind the 
most probable view, and one which, in a plausible manner, ac- 
counts for most of the phenomena observed. The spinning of 
the molecules and their ether sets up the ether tensions or elec- 
trostatic strains ; the equalization of ether tensions sets up ether 
motions or electric currents, and the orbital movements produce 
the effects of electro and permanent magnetism. 

About fifteen years ago, Prof. Rowland demonstrated a most 
interesting and important fact, namely, that a static charge car- 
ried 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 experimen- 
tal 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 like- 
wise explains in a reasonable way, the most puzzling phenomenon 
of all, permanent magnetism, and, in general, lias 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, 1 would say, that I look upon 
all electrostatic, current and magnetic phenomena as being due 
to electrostatic molecular forces. 

The preceding remarks I have deemed necessary to a full 
understanding of the subject as it presents itself to my mind. 

Of all these phenomena the most important to study are the 
current phenomena, on account of the already extensive and ever- 
growing use of currents for industrial purposes. It is now a cen- 
tury 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 



150 INVENTIONS OF NIKOLA TESLA. 

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 scien- 
tists, enough knowledge has been gained on the subject to enable 
us to treat simple cases which now present themselves in daily 
practice. 

The phenomena which are peculiar to the changing character 
of the currents are greatly exalted when the rate of change is 
increased, hence the study of these currents is considerably facil- 
itated by the employment of properly constructed apparatus. 
It was with this and other objects in view that I constructed 
alternate current machines capable of giving more than two 
million reversals of current per minute, and to this circumstance 
it is principally due, that I am able to bring to your attention 
some of the results thus far reached, which I hope will prove to 
be a step in advance on account of their direct bearing upon one 
of the most important problems, namely, the production of a 
practical and efficient source of light. 

The study of such rapidly alternating currents is very interest- 
ing. Nearly every experiment discloses something new. Many 
results may, of course, be predicted, but many more are unfore- 
seen. The experimenter makes many interesting observations. 
For instance, we take a piece of iron and hold it against a magnet. 
Starting from low alternations and running up higher and higher 
we feel the impulses succeed each other faster and faster, get 
weaker and weaker, and finally disappear. We then observe a 
continuous pull ; the pull, of course, is not continuous ; it only 
appears so to us ; our sense of touch is imperfect. 

We may next establish an arc between the electrodes and 
observe, as the alternations rise, that the note which accompanies 
alternating arcs gets shriller and shriller, gradually weakens, and 
finally ceases. The air vibrations, of course, continue, but they 
are too weak to be perceived ; our sense of hearing fails us. 

We observe the small physiological effects, the rapid heating of 
the iron cores and conductors, curious inductive effects, interest- 
ing condenser phenomena, and still more interesting light phe- 
nomena with a high tension induction coil. All these experi- 
ments and observations would be of the greatest interest to the 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 151 

student, but their description would lead me too far from the 
principal subject. Partly for this reason, and partly on account 
of their vastly greater importance, I will confine myself to the 
description of the light effects produced by these currents. 

In the experiments to this end a high tension induction coil or 
equivalent apparatus for converting currents of comparatively 
low into currents of high tension is used. 

If you will be sufficiently interested in the results I shall de- 
scribe as to enter into an experimental study of this subject ; if you 
will be convinced of the truth of the arguments I shall advance 
your aim will be to produce high frequencies and high potentials j 
in other words, powerful electrostatic effects. You will then en- 
counter many difficulties, which, if completely overcome, would 
allow us to produce truly wonderful results. 

First will be met the difficulty of obtaining the required fre- 
quencies by means of mechanical apparatus, and, if they be ob. 
tained otherwise, obstacles of a different nature will present 
themselves. Next it will be found difficult to provide the requi- 
site 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 diffi- 
culties. So, for instance, when a gas is present, the discharge 
may work, by the molecular bombardment of the gas and con- 
sequent heating, through as much as an inch of the best solid 
insulating material, such as glass, hard rubber, porcelain, sealing 
wax, etc. ; in fact, through any known insulating substance. The 
chief requisite in the insulation of the apparatus is, therefore, the 
exclusion of any gaseous matter. 

In general my experience tends to show that bodies which 
possess the highest specific inductive capacity, such as glass, 
afford a rather inferior insulation to others, which, while they are 
good insulators, have a much smaller specific inductive capacity, 
such as oils, for instance, the dielectric losses being no doubt 
greater in the former. The difficulty of insulating, of course, 
only exists when the potentials are excessively high, for with 
potentials such as a few thousand volts there is no particular diffi- 
culty 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 ; 



153 INVENTIONS OP NIKOLA TESLA. 

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 de- 
pendent on condenser action, the loss may be reduced to a trifle 
by using excessively thin wires heavily insulated. 

Another difficulty will be encountered in the capacity and self- 
induction necessarily possessed by the coil. If the coil be large, 
that is, if it contain a great length of wire, it will be generally 
un suited 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 prefera- 
bly one possessing a small specific inductive capacity, would 
afford a two-fold advantage. First, it would enable us to con- 
struct a very small coil capable of withstanding enormous differ- 
ences 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 con- 
structing a coil or induction apparatus of any kind possessing 
the requisite qualities I regard as one of no small importance, 
and it has occupied me for a considerable time. 

The investigator who desires to repeat the experiments which 
I will describe, with an alternate current machine, capable of 
supplying currents of the desired frequency, and an induction 
coil, will do well to take the primary coil out and mount the sec- 
ondary 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 insulat- 
ing tube, and from their intensity he will know jiow far he can 
strain the coil. Without this precaution he is sure to injure 
the insulation. This arrangment permito, however, an easy 
exchange of the primaries, which is desirable in these experi- 
ments. 

The selection of the type of machine best suited for the pur- 
pose must be left to the judgment of the experimenter. There 
are here illustrated three distinct types of machines, which, 
besides others, I have used in my experiments. 

Fig. 97 represents the machine used in my experiments before 
this Institute. The field magnet consists of a ring of wrought 
iron with 384 pole projections. The armature comprises a steel 
disc to which is fastened a thin, carefully welded rim of wrought 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 153 

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 of the thick- 
ness of the pole projections, else the local action will be con- 
siderable. 

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. 




FIG. 97. 

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 pro- 
jections 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 



154 



INVENTIONS OF NIKOLA TE8LA. 



bronze ring and the rim of the wheel, respectively. The whole 
may then be taken off and forms a solid structure. The con- 
ductors 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 em- 
ployed. 

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 re- 
volved. 

It would be uselessly lengthening this description were I to 




dwell more 011 the details of construction of these machines. 
Besides, they have been described somewhat more elaborately in 
The Electrical Engineer, of March 18, 1891. I deem it well, 
however, to call the attention of the investigator to two things, 
the importance of which, though self evident, he is nevertheless 
apt to underestimate ; namely, to the local action in the con- 
ductors 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 



SIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 155 

the several types of these machines which have been constructed 
by me, I have found that the type illustrated in Fig. 97 caused 
me the least trouble in construction, as well as in maintenance, 
and on the whole, it has been a good experimental machine. 

In operating an induction coil with very rapidly alternating 
currents, among the first luminous phenomena noticed are natur- 
ally those presented by the high-tension discharge. As the num- 
ber of alternations per second is increased, or as the number 
being high the current through the primary is varied, the dis- 
charge gradually changes in appearance. It would be difficult to 
describe the minor changes which occur, and the conditions which 



i 




FIG. 99. 

bring them about, but one may note five distinct forms of the 
discharge. 

First, one may observe a weak, sensitive discharge in the form 
of a thin, feeble-colored thread. (Fig. lOOa.) It always occurs 
when, the number of alternations per second being high, the cur- 
rent through the primary is very small. In spite of the exces- 
sively small current, the rate of change is great, and the differ- 
ence 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 



156 INVENTIONS OF NIKOLA TESLA. 

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 excep- 
tional 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. 

When the current through the primary is increased, the dis- 
charge gets broader and stronger, and the effect of the capacity 
of the coil becomes visible until, finally, under proper conditions, 
a white naming arc, Fig. 100 B, often as thick as one's finger, and 
striking across the whole coil, is produced. It develops remark- 
able 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 





FIG. lOOa. FIG. lOOh. 

be advisable, although under different conditions, the potential 
being much higher, a shock from the coil may be taken with 
impunity. To produce this kind of discharge the number of 
alternations per second must not be too great for the coil used ; 
and, generally speaking, certain relations between capacity, self- 
induction and frequency must be observed. 

The importance of these elements in an alternate current cir- 
cuit is now well-known, and under ordinary conditions, the gen- 
eral 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, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 157 

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 appli- 
cable. 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 incli spark. By adding 
capacity to the terminals, the condition may be improved, but 
what the coil really wants is a lower frequency. 

When the flaming discharge occurs, the conditions are evi- 
dently such that the greatest current is made to flow through the 
circuit. These conditions may be attained by varying the fre- 
quency within wide limits, but the highest frequency at which 
the flaming arc can still be produced, determines, for a given 
primary current, the maximum striking distance of the coil. In 
the flaming discharge the eclat effect of the capacity is not per- 
ceptible ; 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 Ley den 
jar. To give a rough approximation I would state that, with an 
ordinary coil of, say 10,000 ohms resistance, the most powerful 
arc would be produced with about 12,000 alternations per second. 

When the frequency is increased beyond that rate, the poten- 
tial, 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 nib- 



15 g INVENTIONS OF NIKOLA TE8LA. 

her, glass, or any other insulating substance. This limits greatly 
the' output of the coil, but I will later show how I have been able 
to overcome to a considerable extent this disadvantage in the 
ordinary coil. 

Besides the potential, the intensity of the streams depends on 
the frequency ; but if the coil be very large they show them- 
selves, 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 f 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 




FIG. 101. 



without producing any other effect than to heat the insulation 
slowly and uniformly. 

The existence of these streams shows the importance of con- 
structing an expensive coil so as to permit of one's seeing 
through the tube surrounding the primary, and the latter should 
be easily exchangeable ; or else the space between the primary 
and secondary should be completely filled up with insulating 
material so as to exclude all air. The non-observance of this 
simple rule in the construction of commercial coils is responsible 
for the destruction of many an expensive coil. 

At the stage when the streaming discharge occurs, or with 
somewhat higher frequencies, one may, by approaching the ter- 
minals quite nearly, and regulating properly the effect of capac- 
ity, produce a veritable spray of small silver-white sparks, or a 
bunch of excessively thin silvery threads (Fig. 102) amidst a 
powerful brush each spark or thread possibly corresponding 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 159 

to one alternation. This, when produced under proper condi- 
tions, 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, no tiling at all is likely to be felt if large conducting 
objects are held in the hands to protect them from receiving 
small burns. 

If the frequency is still more increased, then the coil refuses 
to give any spark unless at comparatively small distances, and the 
fifth typical form of discharge may be observed (Fig. 103). The 
tendency to stream out and dissipate is then so great that when 
the brush is produced at one terminal no sparking occurs, even 
if, as I have repeatedly tried, the hand, or any conducting object, 
is held within the stream ; and, what is more singular, the lumi- 





FIG. 103. FIG. 104. 

nous stream is not at all easily deflected by the approach of a 
conducting body. 

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 pur- 
pose 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 lumi- 
nous circle several inches in diameter, provided the spheres are 



160 INVENTIONS OF NIKOLA TE8LA. 

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 strik- 
ing distance, by inserting a thick plate of glass the discharge is 
instantly induced to pass from the spheres to the glass in the 
form of luminous streams. It appears almost as though these 
streams pass through the dielectric. In reality this is not the 
case, as the streams are due to the molecules of the air which 
are violently agitated in the space between the oppositely charged 
.surfaces of the spheres. When no dielectric other than air is 
present, the bombardment goes on, but is too weak to be visible ; 
by inserting a dielectric the inductive effect is much increased, 
and besides, the projected air molecules find an obstacle and the 
bombardment becomes so intense that the streams become lumi- 
nous. If by any mechanical means we could effect such a vio- 
lent agitation of the molecules we could produce the same phe- 
nomenon. 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 pos- 
sible to produce a phosphorescence of the glass or other insulators 
in this manner. 

The greater the specific inductive capacity of the interposed 
dielectric, the more powerful the effect produced. Owing to 
this, the streams show themselves with excessively high poten- 
tials 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 th^ dielectric, being ap- 
parently 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 cer- 
tain degree, the discharge suddenly breaks through at one point, 
assuming then the ordinary form of an arc. 

The heating effect produced by molecular bombardment of 
the dielectric would, of course, diminish as the pressure of the 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 161 

air is increased, and at enormous pressure it would be negligible, 
unless the frequency would increase correspondingly. 

It will be often observed in these experiments that when the 
spheres are beyond the striking distance, the approach of a glass 
plate, for instance, may induce the spark to jump between the 
spheres. This occurs when the capacity of the spheres is some- 
what below the critical value which gives the greatest difference 
of potential at the terminals of the coil. By approaching a di- 
electric, 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 experi- 
ment is best performed with dense glass or mica. 

Another interesting observation is that a plate of insulating 
material, when the discharge is passing through it, is strongly 
attracted by either of the spheres, that is by the nearer one, this 
being obviously due to the smaller mechanical effect of the bom- 
bardment on that side, and perhaps also to the greater electrifica- 
tion. 

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 induc- 
tive 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 press- 
ure ; but the former would be preferable. Unfortunately neither 
of these two ways is easily carried out in practice. 

It is especially interesting to note the behavior of an exces- 
sively 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 phos- 
phorescent, but the middle appears comparatively dark, and for a 
while remains cool. 

When the frequency is so high that the discharge shown in 
Fig. 103 is observed, considerable dissipation no doubt occurs in 
the coil. Nevertheless the coil may be worked for a long time, 
as the heating is gradual. 

In spite of the fact that the difference of potential may be 



182 



INVENTIONS OF NIKOLA TE8LA. 



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 en- 
ergy 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 




FIG. 10"). 



FIG. l<m. 



taken up by the coil. There is no breaking through, no local in- 
jury, but all the material, insulating and conducting, is uniformly 
heated. 

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. 



HIGH FfiEQ UENOY AND HIGH POTENTIAL CURRENTS. 163 

The streaming discharge of a high tension induction coil differs 
in many respects from that of a powerful static machine. In 
color it has neither the violet of the positive, nor the brightness 
of the negative, static discharge, but lies somewhere between, 
being, of course, alternatively positive and negative. But since 
the streaming is more powerful when the point or terminal is 
electrified positively, than when electrified negatively, it follows 
that the point of the brush is more like the positive, and the root 
more like the negative, static discharge. In the dark, when the 
brush is very powerful, the root may appear almost white. The 
wind produced by the escaping streams, though it may be very 
strong often indeed to such a degree that it may be felt quite a 
distance from the coil is, nevertheless, considering the quantity 
of the discharge, smaller than that produced by the positive 




FIG. 107. FIG. 108. 

brush of a static machine, and it affects the flame much less 
powerfully. From the nature of the phenomenon we can con- 
clude 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). 

In general, most of the experiments usually performed with a 
static machine can be performed with an induction coil when 
operated witli very rapidly alternating currents. The effects pro- 
duced, however,' are much more striking, being of incomparably 



164 INVENTIONS OF NIKOLA TESLA. 

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 ter- 
minal emits powerful streams and vibrates continually to and fro 
or spins in a circle, producing a singular effect (Fig. 108). Some 
of these experiments have been described by me in The Electrical 
W(M, of February 21, 1891. 

Another peculiarity of the rapidly alternating discharge of the 
induction coil is its radically different behavior with respect to 
points and rounded surfaces. 

If a thick wire, provided with a ball at one end and with a 
point at the other, be attached to the positive terminal of a static 
machine, practically all the charge will be lost through the point, 
on account of the enormously greater tension, dependent on the 
radius of curvature. But if such a wire is attached to one of the 
terminals of the induction coil, it will be observed that with very 
high frequencies streams issue from the ball almost as copiously 
as from the point (Fig. 109). 

It is hardly conceivable that we could produce such a condi- 
tion 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 alter- 
nates 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 OH 
the radius of curvature; second, there is the tendency to dissi- 
pate into the surrounding air by condenser action, which de- 
pends on the surface. When one of these tendencies is a maxi- 
mum, 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 indue- 



&IGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 166 

tively, the air molecules not necessarily coining in contact with 
the ball, though they undoubtedly do so. To convince ourselves 
of this we only need to exalt the condenser action, for instance, 
by enveloping the ball, at some distance, by a better conductor 
than the surrounding medium, the conductor being, of course, 
insulated ; or else by surrounding it with a better dielectric and 
approaching an insulated conductor; in both cases the streams 
will break forth more copiously. Also, the larger the ball with 
a given frequency, or the higher the frequency, the more will 
the ball have the advantage over the point. But, since a certain 
intensity of action is required to render the streams visible, it is 
obvious that in the experiment described the ball should not be 
taken too large. 

In consequence of this two-fold tendency, it is possible to pro- 
duce by means of points, effects identical to those produced by 




FIG. 109. FIG. 110. 

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. 

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 con- 
ducting object held in the hand, or else connect it to an insulated 



166 INVENTIONS OF NIKOLA TESLA. 

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 side wise for two reasons: 
First, to increase the potential at the wire ; and, second, to in- 
crease the drop through the coil. The sensitiveness is thus aug- 
mented. 

There is still another and far more striking peculiarity of the 
brush discharge produced by very rapidly alternating currents. 
To observe this it is best to replace the usual terminals of the 
coil by two metal columns insulated with a good thickness of 
ebonite. It is also well to close all fissures and cracks with wax 
so that the brushes cannot form anywhere except at the tops of 
the columns. If the conditions are carefully adjusted which, 
of course, must be left to the skill of the experimenter so that 
the potential rises to an enormous value, one may produce two 
powerful brushes several inches long, nearly white at their roots, 
which in the dark bear a striking resemblance to two flames of 
a gas escaping under pressure (Fig. 110). But they do not only 
resemble, they are veritable flames, for they are hot. Certainly 
they are not as hot as a gas burner, but they would be so if the 
frequency cmd the potential would be sufficiently high. Produced 
with, say, twenty thousand alternations per second, the heat is 
easily perceptible even if the potential is not excessively high. 
The heat developed is, of course, due to the impact of the air 
molecules against the terminals and against each other. As, at 
the ordinary pressures, the mean free path is excessively small, 
it is possible that in spite of the enormous initial speed imparted 
to each molecule upon coming in contact with the terminal, its 
progress by collision with other molecules is retarded to such 
an extent, that it does not get away far from the terminal, but 
may strike the same many times in succession. The higher the 
frequency, the less the molecule is able to get away, and this the 
more so, as for a given effect the potential required is smaller ; 
and a frequency is conceivable perhaps even obtainable at 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 167 

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 ob- 
vious reasons. In the positive brush of a static machine the ex- 
change of the molecules is very rapid, the stream is constantly 
of one direction, and there are fewer collisions ; hence the heating 
effect must be very small. Anything that impairs the facility 
of exchange tends to increase the local heat produced. Thus, if 
a bulb be held over the terminal of the coil so as to enclose the 
brush, the air contained in the bulb is very quickly brought to 
a high temperature. If a glass tube be held over the brush so 
as to allow the draught to carry the brush upwards, scorching hot 
air escapes at the top of the tube. Anything held within the 
brush is, of course, rapidly heated, and the possibility of using 
such heating effects for some purpose or other suggests itself. 

When contemplating this singular phenomenon of the hot 
brush, we cannot help being convinced that a similar process 
must take place in the ordinary flame, and it seems strange that 
after all these centuries past of familiarity with the flame, now, 
in this era of electric lighting and heating, we are finally led to 
recognize, that since time immemorial we have, after all, always 
had " electric light and heat " at our disposal. It is also of no 
little interest to contemplate, that we have a possible way of 
producing by other than chemical means a veritable flame, 
which would give light and heat without any material being 
consumed, without any chemical process taking place, and to 
accomplish this, we only need to perfect methods of producing 
enormous frequencies and potentials. I have no doubt that if 
the potential could be made to alternate with sufficient rapidity 
and power, the brush formed at the end of a wire would lose its 
electrical characteristics and would become flamelike. The flame 
must be due to electrostatic molecular action. 

This phenomenon now explains in a manner which can hardly 
be doubted the frequent accidents occurring in storms. It is well 
known that objects are often set on fire without the lightning 
striking them. We shall presently see how this can happen. 
On a nail in a roof, for instance, or on a projection of any kind, 
more or less conducting, or rendered so by dampness, a powerful 
brush may appear. If the lightning strikes somewhere in the 



168 INVENTIONS OF NIKOLA TESLA. 

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 im- 
pact produce such a powerful heating effect that a lire 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 thous- 
and volts, the heating effects are considerable, we may imagine 
how much more powerful they must be with frequencies and po- 
tentials 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 heat- 
ing effects of a brush produced by a rapidly alternating potential 




FIG. ill. 

have been experimentally demonstrated, at least not to such a 
remarkable degree. 

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 prefer- 
ably 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 fun- 
nel, Fig. Ill, which widens when the potential is increased. 
When the potential is small the end of the filament may perf orm 
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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 169 

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 eifects, which, 
in perfecting methods of producing rapidly alternating potentials, 
might be rendered available for useful purposes. 

In employing a commercial coil, the production of very power- 
ful brush effects is attended with considerable difficulties, for 




FIG. 112*. 

when these high frequencies and enormous potentials are used, 
the best insulation is apt to give way. Usually the coil is insu- 
lated well enough to stand the strain from convolution to convo- 
lution, since two double silk covered paraffined wires will with- 
stand a pressure of several thousand volts; the difficulty lies 
principally in preventing the breaking through from the secon- 
dary to the primary, which is greatly facilitated by the streams 
issuing from the latter. In the coil, of course, the strain is great- 
est 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 



1?0 INVENTIONS OF NIKOLA TE8LA. 

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. 

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 expe- 
riment. 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 




FIG. 112b. 

greatest care so as to produce the greatest rise in the potential. 
At the other terminal a powerful brush appears, which may be 
experimented upon. 

The above plan necessitates the employment of a primary of 
comparatively small size, and it is apt to heat when powerful ef- 
fects 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 



Iimil FRKQURNOY AND HIGH POTENTIAL CURRENTS. Ill 

make the primary in sections and supply it from separate, well 
insulated sources. 

In many of these experiments, when powerful effects are 
wanted for a short time, it is advantageous to use iron cores with 
the primaries. In such case a very large primary coil may be 
wound and placed side by side witli the secondary, and, the near- 
est terminal of the latter being connected to the primary, a lami- 
nated iron core is introduced through the primary into the sec- 
ondary 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 excit- 
ing chemical affinities. 

For the production of ozone, alternating currents of very 
high frequency are eminently suited, not only on account of the 
advantages they offer in the way of conversion but also because 
of the fact, that the ozonizing action of a discharge is dependent 
on the frequency as well as on the potential, this being undoubt- 
edly confirmed by observation. 

In these experiments if an iron core is used it should be care- 
fully watched, as it is apt to get excessively hot in an incredibly 
short time. To give an idea of the rapidity of the heating, I 
will state, that by passing a powerful current through a coil with 
many turns, the inserting within the same of a thin iron wire for 
no more than one second's time is sufficient to heat the wire to 
something like 100 C. 

But this rapid heating need not discourage us in the use 
of iron cores in connection with rapidly alternating currents. 
I have for a long time been convinced that in the industrial distri- 
bution 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 sur- 
round 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 



172 INVENTION'S OF NIKOLA TE8LA. 

shown, may be as much as sixteen times greater than that at or- 
dinary temperatures. If the iron core were perfectly enclosed, 
it would not be deteriorated by the heat, and, if the enclosure of 
tire-proof material would be sufficiently thick, only a limited 
amount of energy could be radiated in spite of the high tem- 
perature. Transformers have been constructed by me on that 
plan, but for lack of time, no thorough tests have as yet been 
made. 

Another way of adapting the iron core to rapid alternations, 
or, generally speaking, reducing the frictional losses, is to pro- 
duce 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 pre- 
ferably high frequencies around the point of greatest permeabil- 
ity. A higher efficiency of conversion and greater output are 
obtainable in this manner. I have also employed this principle 
in connection with machines in which there is no reversal of 
polarity. In these types of machines, as long as there are only 
few pole projections, there is no great gain, as the maxima and 
minima of magnetization are far from the point of maximum 
permeability ; but when the number of the pole projections is 
very great, the required rate of change may be obtained, without 
the magnetization varying so far as to depart greatly from the 
point of maximum permeability, and the gain is considerable. 

The above described arrangements refer only to the use of 
commercial coils as ordinarily constructed. If it is desired to 
construct a coil for the express purpose of performing with it 
such experiments as I have described, or, generally, rendering it 
capable of withstanding the greatest possible difference of poten- 
tial, then a construction as indicated in Fig. 113 will be found of 
advantage. The coil in this case is formed of two independent 
parts which are wound oppositely, the connection between both 
being made near the primary. The potential in the middle being 
zero, there is not much tendency to jump to the primary and not 
much insulation is required. In some cases the middle point 
may, however, be connected to the primary or to the ground. In 
such a coil the places of greatest difference of potential are far 
apart and the coil is capable of withstanding an enormous strain. 
The two parts may be movable so as to allow a slight adjustment 
of the capacity effect. 

As to the manner of insulating the coil, it will be found con- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 173 

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. 

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 




FIG. 113. 



be found that, after all, a vessel filled with oil is not a very con- 
venient thing to handle in a laboratory. 

If an ordinary coil can be dismounted, the primary may be 
taken out of the tube and the latter plugged up at one end, filled 
with oil, and the primary reinserted. This affords an excellent 
insulation and prevents the formation of the streams. 

Of all the experiments which may be performed with rapidly 
alternating currents the most interesting are those which concern 
the production of a practical illuminant. It cannot be denied 
that the present methods, though they were brilliant advances, 
are very wasteful. Some better methods must be invented, some 
more perfect apparatus devised. Modern research has opened 
new possibilities for the production of an efficient source of light, 
and the attention of all has been turned in the direction indicated 



174 INVENTIONS OF NIKOLA TESLA. 

by able pioneers. Many have been carried away by the enthusiasm 
and passion to discover, but in their zeal to reach results, some 
have been misled. Starting with the idea of producing electro- 
magnetic waves, they turned their attention, perhaps, too much 
to the study of electro-magnetic effects, and neglected the study 
of electrostatic phenomena. Naturally, nearly every investigator 
availed himself of an apparatus similar to that used in earlier 
experiments. But in those forms of apparatus, while the electro- 
magnetic inductive effects are enormous, the electrostatic effects 
are excessively small. 

In the Hertz experiments, for instance, a high tension induc- 
tion coil is short circuited by an arc, the resistance of which is 
very small, the smaller, the more capacity is attached to the ter- 
minals ; and the difference of potential at these is enormously 
diminished. On the other hand, when the discharge is not pass- 
ing 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, Ley den jars are discharged disruptively. It has 
been thought and I believe asserted that in such cases 
most of the energy is radiated into space. In the light of the 
experiments which I have described above, it will now not be 
thought so. I feel safe in asserting that in such cases 'most of 
the energy is partly taken up and converted into heat in the arc 
of the discharge and in the conducting and insulating material of 
the jar, some energy being, of course, given off by electrification 
of the air ; but the amount of the directly radiated energy is very 
small. 

When a high tension induction coil, operated by currents alter- 
nating 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 gen- 
erating 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 175 

recoil circuit be used. These conditions make it appear that with 
the apparatus usually at hand, the observation of powerful elec- 
trostatic effects was impossible, and what experience has been 
gained in that direction is only due to the great ability of the 
investigators. 

But powerful electrostatic eifects are a sitw qua -non of light 
production on the lines indicated by theory. Electro-magnetic 
eifects 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 con- 
denser, could not be utilized, it would seem, except we avail our- 
selves 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 vi- 
brate 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 elec- 
trostatic nature. 

To produce such luminous effects, straight electrostatic thrusts 
are required; these, whatever be their frequency, may disturb 
the molecular charges and produce light. Since current impulses 
of the required frequency cannot pass through a conductor of 
measurable dimensions, we must work with a gas, and then the 
production of powerful electrostatic effects becomes an imperative 
necessity. 

It has occurred to me, however, that electrostatic effects are in 
many ways available for the production of light. For instance, 
we may place a body of some refractory material in a closed, and 
preferably more or less exhausted, globe, connect it to a source of 
high, rapidly alternating potential, causing the molecules of the 
gas to strike it many times a second at enormous speeds, and in 
this manner, with trillions of invisible hammers, pound it until it 
gets incandescent ; or we may place a body in a very highly ex- 
hausted globe, in a non-striking vacuum, and, by employing very 



176 INVENTIONS OF NIKOLA TE8LA. 

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 viorate and to emit light. 

But, electrostatic eifects being dependent upon the potential 
and frequency, to produce the most powerful action it is desira- 
ble to increase both as far as practicable. It may be possible to 
obtain quite fair results by keeping either of these factors small, 
provided the other is sufficiently great ; but we are limited in 
both directions. My experience demonstrates that we cannot go 
below a certain frequency, for, first, the potential then becomes 
so great that it is dangerous ; and, secondly, the light production 
is less efficient. 

I have found that, by using the ordinary low frequencies, the 
physiological effect of the current required to maintain at a cer- 
tain 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 accus- 
tomed to such shocks ; whereas, with twenty thousand alterna- 
tions per second, the tube may be maintained at the same degree 
of brightness without any effect being felt. This is due princi- 
pally to the fact that a much smaller potential is required to pro- 
duce the same light effect, and also to the higher efficiency in the 
light production. It is evident that the efficiencv in such cases 
is the greater, the higher the frequency, for the quicker the pro- 
cess 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 diffi- 
culty of producing and conveying the effects. 

I have stated above that a body inclosed in an unexhausted 
bulb may be intensely heated by simply connecting it with a 
source of rapidly alternating potential. The heating in such a 
case is, in all probability, due mostly to the bombardment of the 
molecules of the gas contained in the bulb. When the bulb is 
exhausted, the heating of the body is much more rapid, and there 
is no difficulty whatever in bringing a wire or filament to any 
degree of incandescence by simply connecting it to one terminal 
of a coil of the proper dimensions. Thus, if the well-known ap- 
paratus of Prof. Crookes, consisting of a bent platinum wire with 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 177 

vanes mounted over it (Fig. 114), be connected to one terminal of 
the coil either one or both ends of the platinum wire being con- 
nected the wire is rendered almost instantly incandescent, and 
the mica vanes are rotated as though a current from a battery 
were used. A thin carbon filament, or, preferably, a button of 
some refractory material (Fig. 115), even if it be a comparatively 
poor conductor, inclosed in an exhausted globe, may be rendered 
highly incandescent ; and in this manner a simple lamp capable 
of giving any desired candle power is provided. 

The success of lamps of this kind would depend largely on the 
selection of the light-giving bodies contained within the bulb. 
Since, under the conditions described, refractory bodies which 
are very poor conductors and capable of withstanding for a long 
time excessively high degrees of temperature may be used, 
such illuminating devices may be rendered successful. 

It might be thought at first that if the bulb, containing the 





FIG. 114. FIG. 115. 

filament or button of refractory material, be perfectly well ex- 
hausted that is, as far as it can be done by the use of the best 
apparatus the heating would be much less intense, and that in 
a perfect vacuum it could not occur at all. This is not confirmed 
by my experience; quite the contrary, the better the vacuum 
the more easily the bodies are brought to incandescence. This 
result is interesting for many reasons. 

At the outset of this work the idea presented itself to me, 
whether two bodies of refractory material enclosed in a bulb ex- 
hausted 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 calcula- 
tion. 



178 INVENTIONS OF NIKOLA TESLA. 

But such a lamp would possess a vast advantage over an ordi- 
nary 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 impractic- 
able on account of the destruction of the filament, and it has been 
determined by experience how far it is advisable to push the in- 
candescence. 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 be- 
yond 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 car- 
bon ; 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 de- 
scribed, 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 manufac- 
tured from mixtures of oxides, for instance, with carbon or other 
material, or may be selected from bodies which are practically 
non-conductors, and capable of withstanding enormous degrees of 
temperature. 

All this would point to the possibility of obtaining a much 
higher efficiency with such a lamp than is obtainable in ordinary 
lamps. In my experience it has been demonstrated that the 
blocks are brought to high degrees of incandescence with much 
lower potentials than those determined by calculation, and the 
blocks may be set at greater distances from each other. We may 
freely assume, and it is probable, that the molecular bombard- 
ment 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, insign- 
ificant, 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. 



HIGH FREQUENCY AND HIGH POTENTIAL VUlillENTt*. 179 



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 dimen- 
sion^. 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 





FIG. 116. 



FIG. 117. 



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. 

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 investi- 
gations are being carried on in this line. There is no difficulty in 
reaching such high degrees of incandescence that ordinary car- 
bon is to all appearance melted and volatilized. If the vacuum 
could be made absolutely perfect, such a lamp, although inopera- 
tive with apparatus ordinarily used, would, if operated with cur- 



180 INVENTIONS OF NIKOLA TESLA. 

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 diminu- 
tion in size always occurs, as in incandescent filaments ; but there 
is no possibility of a sudden and premature disabling which oc- 
curs in the latter by the breaking of the filament, especially 
when the incandescent bodies are in the shape of blocks. 

With these rapidly alternating potentials there is, however, no 
necessity of enclosing two blocks in a globe, but a single block, 
as in Fig. 115, or filament, Fig. 118, may be used. The poten- 
tial in this case must of course be higher, but is easily obtainable, 
and besides it is not necessarily dangerous. 

The facility with which the button or filament in such a lamp 




FIG. 118. 

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 radia- 
tion. 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURREN1S. 181 

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 elec- 
trification, of the surrounding air. 

In order to study this subject better, I carried on some experi- 
ments with excessively high potentials and low frequencies. I 
then observed that when the hand is approached to the bulb, 
the filament being connected with one terminal of the coil, a 
powerful vibration is felt, being due to the attraction and repul- 
sion of the molecules of the air which are electrified by induc- 
tion 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. 

When the alternations are low, one is agt to get an excessively 





FIG. 119. FIG. 120. 

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 poten- 
tial 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 man- 
ner one may obtain the required potential with fewer turns of 
wire. 

The life of such lamps as described above depends, of course, 
largely on the degree of exhaustion, but to some extent also on 
the shape of the block of refractory material. Theoretically it 



INVENTIONS OF NIKOLA TESLA. 



would seem that a small sphere of carbon enclosed in a sphere of 
glass would not suffer deterioration from molecular bombard- 
ment, 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. 

The use of alternating currents of very high frequency makes 
it possible to transfer, by electrostatic or electromagnetic induc- 
tion through the glass of a lamp, sufficient energy to keep a fila- 





FIG. 121a. 



FIG. 121b. 



ment at incandescence and so do away with the leading-in wires. 
Such lamps have been proposed, but for want of proper appara- 
tus 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 ad- 
vantageously combined with the secondary. When the transfer- 
ence 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, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 183 

which is not impracticably large, and with glass of good quality 
1 ram. thick, using currents alternating twenty thousand times 
a second, the potential required is approximately 9,000 volts. 
This may seem large, but since each lamp may be included 
in- the secondary of a transformer of very small dimensions, it 
would not be inconvenient, and, moreover, it would not produce 
fatal injury. The transformers would all be preferably in series. 
The regulation would offer no difficulties, as with currents of such 
frequencies it is very easy to maintain a constant current. 

In the accompanying engravings some of the types of lamps of 
this kind are shown. Fig. 120 is such a lamp with a broken fila- 
ment, and Figs. 121 A and 121 B one with a single outside and 
inside coating and a single filament. I have also made lamps 
with two outside and inside coatings and a continuous loop con- 
necting the latter. Such lamps have been operated by me with 
current impulses of the enormous frequencies obtainable by the 
disruptive discharge of condensers. 

The disruptive discharge of a condenser is especially suited for 
operating such lamps with no outward electrical connections 
by means of electromagnetic induction, the electromagnetic in- 
ductive effects being excessively high ; and I have been able to 
produce the desired incandescence with only a few short turns of 
wire. Incandescence may also be produced in this manner in a 
simple closed filament. 

Leaving now out of consideration the practicability of such 
lamps, I would only say that they possess a beautiful and desir- 
able feature, namely, that they can be rendered, at will, more or 
less brilliant simply by altering the relative position of the out- 
side and inside condenser coatings, or inducing and induced cir- 
cuits. 

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 vary- 
ing the size of the insulated metal plate to which the coating is 
connected. 

It is likewise practicable to light with one leading wire lamps 
such as illustrated in Fig. 116 and Fig. 117, by connecting one 



184 INVENTIONS OF NIKOLA TKSLA. 

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 sur- 
rounding space, and is equivalent to a return wire. Obviously, 
in the two last-named cases, instead of connecting the wires to 
an insulated body, connections may be made to the ground. 

The experiments which will prove most suggestive and of 
most interest to the investigator are probably those performed 
with exhausted tubes. As might be anticipated, a source of such 
rapidly alternating potentials is capable of exciting the tubes at 
a considerable distance, and the light effects produced are re- 
markable. 

During my investigations in this line I endeavored to excite 





FIG. 122. FIG. 123. 

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 cir- 
cumstance 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 dimin- 
ished. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 185 

When an induction coil, operated as above described, is used, 
there is no doubt that the tubes are excited by electrostatic in- 
duction, and that electromagnetic induction has little, if any- 
thing, to do with the phenomena. 

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 electro- 
magnetic, the tube could not be lighted when the observer's 
body is interposed between it and the coil, or at least its lumi- 
nosity should be considerably diminished. When the tube is 
held exactly over the centre of the coil the latter being wound 
in sections and the primary placed symmetrically to the sec- 
ondary 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 




FIG. 124. 

middle both halves of the coil neutralize each other, and the 
electric potential is zero. If the action were electromagnetic, 
the tube should light best in the plane through the centre of the 
coil, since the electromagnetic effect there should be a maximum. 
When an arc is established between the terminals, the tubes and 
lamps in the vicinity of the coil go out, but light up again 
when the arc is broken, on account of the rise of potential. Yet 
the electromagnetic effect should be practically the same in both 
cases. 

By placing a tube at some distance from the coil, and nearer to 
one terminal preferably at a point on the axis of the coil one 
may light it by touching the remote terminal with an insulated 
body of some size or with the hand, thereby raising the potential 
at that terminal nearer to the tube. If the tube is shifted nearer 
to the coil so that it is lighted by the action of the nearer termi- 



186 INVENTIONS OF NIKOLA TEBLA. 

nal, it may be made to go out by holding, on an insulated sup- 
port, 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 insu- 
lated support between coil and tube, light the latter by approach- 
ing the hand to it ; or he may even render it luminous by simply 
stepping between it and the coil. This would be impossible with 
electro-magnetic induction, for the body of the observer would 
act as a screen. 

When the coil is energized by excessively weak currents, the 
experimenter may, by touching one terminal of the coil with the 
tube, extinguish the latter, and may again light it by bringing it 
out of contact with the terminal and allowing a small arc to form. 
This is clearly due to the respective lowering and raising of the 
potential at that terminal. In the above experiment, when the 
tube is lighted through a small arc, it may go out when the arc is 
broken, because the electrostatic inductive effect alone is too 
weak, though the potential may be much higher ; but when the 
arc is established, the electrification of the end of the tube is 
much greater, and it consequently lights. 

If a tube is lighted by holding it near to the coil, and in the 
hand which is remote, by grasping the tube anywhere with the 
other hand, the part between the hands is rendered dark, and the 
singular effect of wiping out the light of the tube may be pro- 
duced by passing the hand quickly along the tube and at the 
same time withdrawing it gently from the coil, judging prop- 
erly the distance so that the tube remains dark afterwards. 

If the primary coil is placed sidewise, as in Fig. 112 B for in- 
stance, and an exhausted tube be introduced from the other side 
in the hollow space, the tube is lighted most intensely because of 
the increased condenser action, and in this position the striae are 
most sharply defined. In all these experiments described, and in 
many others, the action is clearly electrostatic. 

The effects of screening also indicate the electrostatic nature 
of the phenomena and show something of the nature of electri- 
fication 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 
l>e too far from the coil to light, it may even be rendered lumin- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 187 

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 interpo- 
sition 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 dimin- 
ishes 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 giv- 
ing off energy to the surroundings. 

If a tube be lighted at some distance from the coil, and a plate 
of hard rubber or other insulating substance be interposed, the 
tube may be made to go out. The interposition of the dielectric 
in this case only slightly increases the inductive effect, but dimin- 
ishes considerably the electrification through the air. 

In all cases, then, when we excite luminosity in exhausted 
tubes by means of such a coil, the effect is due to the rapidly 
alternating electrostatic potential ; and, furthermore, it must be 
attributed to the harmonic alternation produced directly by the 
machine, and not to any superimposed vibration which might be 
thought to exist. Such superimposed vibrations are impossible 
when we work with an alternate current machine. If a spring be 
gradually tightened and released, it does not perform independ- 
ent vibrations ; for this a sudden release is necessary. So with 
the alternate currents from a dynamo machine ; the medium is 
harmonically strained and released, this giving rise to only one 
kind of waves ; a sudden contact or break, or a sudden giving 
way of the dielectric, as in the disruptive discharge of a Leyden 
jar, are essential for the production of superimposed waves. 

In all the last described experiments, tubes devoid of any elec- 
trodes may be used, and there is no difficulty in producing by 
their means sufficient light to read by. The light effect is, how- 
ever, 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 power- 
ful effects, it is carried gradually away, and it is preferable to use 
material in the form of a solid. 

Instead of depending on induction at a distance to light the 
tube, the same may be provided with an external and, if de- 
sired, also with an internal condenser coating, and it may then 



188 



INVENTIONS OF NIKOLA TESLA. 



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. 

The ideal way of lighting a hall or room would, however, be 




FIG. 125. 



to produce such a condition in it that an illuminating device 
could be moved and put anywhere, and that it is lighted, no mat- 
ter where it is put and without being electrically connected to 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 189 

anything. I have been able to produce such a condition by creat- 
ing 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 termi- 
nal of the induction coil, the other terminal being preferably con- 
nected 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 ex- 
hausted tube may then be carried in the hand anywhere be- 
tween the sheets or placed anywhere, even a certain distance 
beyond them ; it remains always luminous. 

In such an electrostatic field interesting phenomena may be 
observed, especially if the alternations are kept low and the po- 
tentials 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 ap- 
proached 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 tele- 
phone receiver is made to touch with one or both of its terminals 
an insulated conductor of some size, the telephone emits a loud 
sound ; it also emits a sound when a length of wire is attached to 
one or both terminals, and with very powerful fields a sound may 
be perceived even without any wire. 

How far this principle is capable of practical application, the 
future will tell. It might be thought that electrostatic effects 
are unsuited for such action at a distance. Electromagnetic in- 
ductive effects, if available for the production of light, might be 
thought better suited. It is true the electrostatic effects dimin- 
ish 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 differen- 
tial effect of both the terminals, we get their conjoint effect. 
Besides, I would call attention to the effect, that in an alternat- 
ing 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 
tlie least energy, the waves being reflected with but little- loss. 



190 INVENTIONS OF NIKOLA TESLA. 

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 Ley den jar discharges, the tube could not 
be excited unless at a very small distance, although some judg- 
ment 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 ex- 
hausted tube, and even these effects upon thorough examination 
I have been forced to consider of an electrostatic nature. 

How then can we hope to produce the required effects at a 
distance by means of electromagnetic action, when even in the 
closest proximity to the source of disturbance, under the most 
advantageous conditions, we can excite but faint luminosity '* It 
is true that when acting at a distance we have the resonance to 
help us out. We can connect an exhausted tube, or whatever 
the illuminating device may be, with an insulated system of the 
proper capacity, and so it may be possible to increase the effect 
qualitatively, and only qualitatively, for we would not get more 
energy through the device. So we may, by resonance effect, 
obtain the required electromotive force in an exhausted tube, and 
excite faint luminous effects, but we cannot get enough energy to 
render the light practically available, and a simple calculation, 
based on experimental results, shows that even if all the energy 
which a tube would receive at a certain distance from the source 
should be wholly converted into light, it would hardly satisfy the 
practical requirements. Hence the necessity of directing, by 
means of a conducting circuit, the energy to the place of trans- 
formation. But in so doing we cannot very sensibly depart from 
present methods, and all we could do would be to improve the 1 
apparatus. 

From these considerations it would seem that if this ideal way 
of lighting is to be rendered practicable it will be only by the use 
of electrostatic effects. In such a case the most powerful electro- 
static 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, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 191 

or, generally speaking, of any mechanical apparatus, but low 
frequencies can be reached ; recourse must, therefore, be had to 
some other means. The discharge of a condenser affords us a 
means of obtaining frequencies by far higher than are obtainable 
mechanically, and I have accordingly employed condensers in the 
experiments to the above end. 

When the terminals of a high tension induction coil, Fig. 120, 
are connected to a Leyden jar, and the latter is discharging dis- 
ruptively into a circuit, we may look upon the arc playing be- 
tween 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 con- 
nected 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 fre- 
quency, and a superimposed harmonic vibration, and the laws 
governing the flow of currents are not the same for both. 

In converting in this manner, the ratio of conversion should 
not be too great, for the loss in the arc between the knobs in- 
creases with the square of the current, and if the jar be discharged 
through very thick and short conductors, with the view of ob- 
taining a very rapid oscillation, a very considerable portion of the 
energy stored is lost. On the other hand, too small ratios are not 
practicable for many obvious reasons. 

As the converted currents flow in a practically closed circuit, 
the electrostatic effects are necessarily small, and I therefore con- 
vert 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 oper- 
ating renders it easy to obtain by means of a small and inexpen- 
sive 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. 



192 INVENTIONS OF NIKOLA TESLA. 

In endeavoring to obtain tlie required electrostatic effects in 
this manner, I have, as might be expected, encountered many 
difficulties which I have been gradually overcoming, but I am not 
as yet prepared to dwell upon my experiences in this direction. 

I believe that the disruptive discharge of a condenser will play 
an important part in the future, for it offers vast possibilities, 
not only in the way of producing light in a more efficient manner 
and in the line indicated by theory, but also in many other re- 
spects. 

For years the efforts of inventors have been directed towards 
obtaining electrical energy from heat by means of the thermo- 
pile. It might seem invidious to remark that but few know 
what is the real trouble with the thermopile. It is not the in- 
efficiency or small output though these are great drawbacks 
but the fact that the thermopile has its phylloxera, that is, that 
by constant use it is deteriorated, which has thus far prevented its 




FIG. 126. 

introduction on an industrial scale. Now that all modern re- 
search seems to point with certainty to the use of electricity of ex- 
cessively 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 w r e know that everywhere we 
have to deal with the same forces, and that it is a mere question 
of inventing proper methods or apparatus for rendering them 
available. 

In the present systems oij electrical distribution, the employ- 
ment 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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 193 

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 de- 
terred inventors from working in this direction. 

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 




FIG. 127. 

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 alter- 
nate current machine, was employed to charge the condenser ; but 
the induction coil might be replaced by an apparatus of a differ- 
ent 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 fre- 
quency. "When the currents charging the condenser are of the 



194 INVENTIONS OF NIKOLA TE8LA. 

same direction, and it is desired that the converted currents 
should also he of one direction, the resistance of the discharg- 
ing circuit should, of course, be so chosen that there are no 
oscillations. 

In operating devices on the above plan I have observed curi- 
ous 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 




FIG. 128. 

is employed it is easy to obtain 
rendered evident by the different degree of brilliancy of the 
lamps, as shown roughly in Fig. 12S. The nodes are never clearly 
delined, 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 ordinarv Cardew voltmeter 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 195 

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 prac- 
ticable 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 fila- 
ments are used it w T ill 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 electro- 
static action between the filament and the glass of the bulb. 

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 




Fro. 129. 

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 im- 
pedances, the filament may be brought to any desired degree of 
incandescence. But Avhen 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 bril- 
liantly 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 



196 INVENTIONS OF NIKOLA TESLA. 

wire extending through the whole bulb may be used, and in this 
case the phenomenon would seem to be still more interesting. 

From the above experiment it will be evident, that when ordi- 
nary 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 dis- 
charge will occur at the ends of the filament or in the base of the 
lamp between the leading-in wires, and the lamp might then be 
damaged. 

In presenting to you these results of my investigation on the 
subject under consideration, I have paid only a passing notice to 
facts upon which I could have dwelt at length, and among many 
observations I have selected only those which I thought most 
likely to interest you. The field is wide and completely unex- 
plored, and at every step a new truth is gleaned, a novel fact 
observed. 

How far the results here borne out are capable of practical 
applications will be decided in the future. As regards the pro- 
duction 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 experi- 
ments I am hopeful that they will only prove a step in further 
development towards the ideal and final perfection. The possi- 
bilities which are opened by modern research are so vast that 
even the most reserved must feel sanguine of the future. Emi- 
nent scientists consider the problem of utilizing one kind of 
radiation without the others a rational one. In an apparatus de- 
signed 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 vibra- 
tions, be it electrical, chemical or any other, it will not be possi- 
ble 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 veloci- 
ties. 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 with- 
out being considered a visionary. AVe are whirling through 



moil FREQUENCY AND HIGH POTENTIAL CURRENTS. 197 

endless space with an inconceivable speed, all around us every- 
thing is spinning, everything is moving, everywhere is energy. 
There must be some way of availing ourselves of this energy 
more directly. Then, with the light obtained from the medium, 
with the power derived from it, with every form of energy 
obtained without effort, from the store forever inexhaustible, 
humanity will advance with giant strides. The mere contempla- 
tion of these magnificent possibilities expands our minds, strength- 
ens our hopes and fills our hearts with supreme delight. 



CHAPTEE XXVII. 

EXPERIMENTS WITH ALTERNATE CURRENTS OF HIGH POTENTIAL 

AND HlGH FREQUENCY. 1 

I CANNOT find words to express how deeply I feel the honor of 
addressing some of the foremost thinkers of the present time, 
and so many able scientific men, engineers and electricians, of 
the country greatest in scientific achievements. 

The results which I have the honor to present before such a 
gathering I cannot call my own. There are among you not a 
few who can lay better claim than myself on any feature of 
merit which this work may contain. I need not mention many 
names which are world-known names of those among you who 
are recognized as the leaders in this enchanting science ; but one, 
at least, I must mention a name which could not be omitted in 
a demonstration of this kind. It is a name associated with the 
most beautiful invention ever made : it is Crookes ! 

When I was at college, a good while ago, I read, in a translation 
(for then I was not familiar with your magnificent language), the 
description of his experiments on radiant matter. I read it only 
once in my life that time yet every detail about that charm- 
ing work I can remember to this day. Few are the books, let me 
say, which can make such an impression upon the mind of a 
student. 

But if, on the present occasion, I mention this name as one of 
many your Institution can boast of, it is because I have more 
than one reason to do so. For what I have to tell you and to 
show you this evening concerns, in a large measure, that same 
vague world which Professor Crookes has so ably explored ; and, 
more than this, when I trace back the mental process which led 
me to these advances which even by myself cannot be consid- 
ered trifling, since they are so appreciated by you I believe 
that their real origin, that which started me to work in this 

1. Lecture delivered before the Institution of Electrical Engineers, London, 
February, 1892. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 199 

direction, and brought me to them, after a long period of con- 
stant thought, was that fascinating little book which I read many 
years ago. 

And now that I have made a feeble effort to express my 
homage and acknowledge my indebedness to him and others 
among you, I will make a second effort, which I hope you will 
not find so feeble as the first, to entertain you. 

Give me leave to introduce the subject in a few words. 

A short time ago I had the honor to bring before our Ameri- 
can 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 com- 
pleting, or more clearly expressing, some ideas^ advanced by me 
before, and also with the view of rendering the study here pre- 
sented self-contained, and my remarks on the subject of this 
evening's lecture consistent. 

This investigation, then, it goes without saying, deals with 
alternating currents, and to be more precise, with alternating 
currents of high potential and high frequency. Just in how 
much a very high frequency is essential for the production of 
the results presented is a question which, even with my present 
experience, would embarrass me to answer. Some of the experi- 
ments 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 ex- 
periments here contemplated. 

Of the various branches of electrical investigation, perhaps the 
most interesting and the most immediately promising is that 
dealing with alternating currents. The progress in this branch 
of applied science has been so great in recent years that it justi- 
fies 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 re- 
alized. As in nature all is ebb and tide, all is wave motion, so it 
seems that in all branches of industry alternating currents elec- 
tric wave motion will have the sway. 



soo INVENTIONS OF NIKOLA TESLA. 

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 connec- 
tions, with great speed and power with the energy possibly con- 
veyed from a great distance. We observe how the energy of an 
alternating current traversing the wire manifests itself not so 
much in the wire as in the surrounding space in the most sur- 
prising manner, taking the forms of heat, light, mechanical 
energy, and, most surprising of all, even chemical affinity. All 
these observations fascinate us, and fill us with an intense desire 
to know more about the nature of these phenomena. Each day 
we go to our work in the hope of discovering, in the hope that 
some one, no matter who, may find a solution of one of the pend- 
ing great problems, and each succeeding day we return to our 
task with renewed ardor ; and even if we are unsuccessful, our 
work has not been in vain, for in these strivings, in these efforts, 
we have found hours of untold pleasure, and we have directed 
our energies to the benefit of mankind. 

We may take at random, if you choose any of the many ex- 
periments which may be performed with alternating currents ; 
a few of which only, and by no means the most striking, form 
the subject of this evening's demonstration ; they are all equally 
interesting, equally inciting to thought. 

Here is a simple glass tube from which the air has been par- 
tially exhausted. I take hold of it ; I bring my body in contact 
with a wire conveying alternating currents of high potential, and 
the tube in my hand is brilliantly lighted. In whatever position 
I may put it, wherever I move it in space, as far as I can reach, 
its soft, pleasing light persists with undiminished brightness. 

Here is an exhausted bulb suspended from a single wire. 
Standing on an insulated support, I grasp it, and a platinum but- 
ton mounted in it is brought to vivid incandescence. 

Here, attached to a leading wire, is another bulb, which, as I 
touch its metallic socket, is filled with magnificent colors of phos- 
phorescent light. 



I 

HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 201 

Here still another, which by my fingers' touch casts a shadow 
the Crookes shadow of the stem inside of it. 

Here, again, insulated as I stand on this platform, I bring my 
body in contact with one of the terminals of the secondary of 
this induction coil with the end of a wire many miles long and 
you see streams of light break forth from its distant end, which 
is set in violent vibration. 

Here, once more, I attach these two plates of wire gauze to the 
terminals of the coil ; I set them a distance apart, and I set the 
coil to work. You may see a small spark pass between the 
plates. I insert a thick plate of one of the best dielectrics be- 
tween them, and instead of rendering altogether impossible, as 
we are used to expect, I aid the passage of the discharge, which, 
as I insert the plate, merely changes in appearance and assumes 
the form of luminous streams. 

Is there, I ask, can there be, a more interesting study than that 
of alternating currents ? 

In all these investigations, in all these experiments, which are 
so very, very interesting, for many years past ever since the 
greatest experimenter who lectured in this hall discovered its 
principle we have had a steady companion, an appliance familiar 
to every one, a plaything once, a thing of momentous importance 
now the induction coil. There is no dearer appliance to the 
electrician. From the ablest among you, I dare say, down to the 
inexperienced student, to your lecturer, we all have passed many 
delightful hours in experimenting with the induction coil. We 
have watched its play, and thought and pondered over the beau- 
tiful 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 ven- 
tured to entertain you with that same old subject. Here in reality 
is the same apparatus, and here are the same phenomena, only 
the apparatus is operated somewhat differently, the phenomena 
are presented in a different aspect. Some of the results we find 
as expected, others surprise us, but all captivate our attention, for 
in scientific investigation each novel result achieved may be the 
centre of a new departure, each novel fact learned may lead to 
important developments. 

Usually in operating an induction coil we have set up a vibra- 
tion of moderate frequency in the primary, either by means of an 



203 INVENTIONS OF NIKOLA TE8LA. 

interrupter or break, or by the use of an alternator. Earlier 
English investigators, to mention only Spottiswoode and J. E. H. 
Gordon, have used a rapid break in connection with the coil. 
Our knowledge and experience of to-day enables us to see clearly 
why these coils under the conditions of the test did not disclose 
any remarkable phenomena, and why able experimenters failed 
to perceive many of the curious effects which have since been 
observed. 

In the experiments such as performed this evening, we operate 
the coil either from a specially constructed alternator capable of 
giving many thousands of reversals of current per second, or, by 
disrupt! vely discharging a condenser through the primary, we set 
up a vibration in the secondary circuit of a frequency of many 
hundred thousand or millions per second, if we so desire ; and in 
using either of these means we enter a field as yet unexplored. 

It is impossible to pursue an investigation in any novel line 
without finally making some interesting observation or learning 
some useful fact. That this statement is applicable to the sub- 
ject of this lecture the many curious and unexpected phenomena 
which we observe afford a convincing proof. By way of illustra- 
tion, take for instance the most obvious phenomena, those of the 
discharge of the induction coil. 

Here is a coil which is operated by currents vibrating with 
extreme rapidity, obtained by disruptively discharging a Leyden 
jar. It would not surprise a student were the lecturer to say 
that the secondary of this coil consists of a small length of com- 
paratively 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 with- 
stand ; 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.) 

compare this phenomenon which you have just witnessed 



HIGH FKEQ UENCY AND HIGH POTENTIAL CURRENTS. 203 

with the discharge of a Holtz or Wimshurst machine that other 
interesting appliance so dear to the experimenter. What a differ- 
ence there is between these phenomena ! And yet, had I made 
the necessary arrangements which could have been made easily, 
were it not that they would interfere with other experiments I 
could have produced with this coil sparks which, had I the coil 





FIG. 131. 

hidden from your view and only two knobs exposed, even the 
keenest observer among you would find it difficult, if not impos- 
sible, to distinguish from those of an influence or friction ma- 
chine. This may be done in many ways for instance, by oper- 
ating the induction coil which charges the condenser from an 
alternating-current machine of very low frequency, and prefer- 
ably 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 



204 INVENTIONS OF NIKOLA TESLA. 

rapid succession of sparks of great intensity and small quantity, 
which possess the same brilliancy, and are accompanied by the 
same sharp crackling sound, as those obtained from a friction or 
influence machine. 

Another way is to pass through two primary circuits, having a 
common secondary, two currents of a slightly different period, 
which produce in the secondary circuit sparks occurring at com- 
paratively long intervals. But, even with the means at hand 
this evening, I may succeed in imitating the spark of a Holtz 
machine. For this purpose I establish between the terminals of 
the coil which charges the condenser a long, unsteady arc, which 
is periodically interrupted by the upward current of air produced 
by it. To increase the current of air I place on each side of the 
arc, and close to it, a large plate of mica. The condenser charged 
from this coil discharges into the primary circuit of a second 
coil through a small air gap, which is necessary to produce a 
sudden rush of current through the primary. The scheme of 
connections in the present experiment is indicated in Fig. 131. 

G is an ordinarily constructed alternator, supplying the pri- 
mary P of an induction coil, the secondary s of which charges 
the condensers or jars c c. The terminals of the secondary are 
connected to the inside coatings of the jars, the outer coatings 
being connected to the ends of the primary p p of a second in- 
duction coil. This primary p p has a small air gap a b. 

The secondary s of this coil is provided with knobs or spheres 
K K of the proper size and set at a distance suitable for the ex- 
periment. 

A long arc is established between the terminals A B of the first 
induction coil. M M are the mica plates. 

Each time the arc is broken between A and B the jars are 
quickly charged and discharged through the primary p p, pro- 
ducing a snapping spark between the knobs K K. Upon the arc 
forming between A and B the potential falls, and the jars cannot 
be charged to such high potential as to break through the air 
gap a ~b until the arc is again broken by the draught. 

In this manner sudden impulses, at long intervals, are pro- 
duced in the primary p p, which in the secondary s give a cor- 
responding number of impulses of great intensity. If the sec- 
ondary knobs or spheres, K K, are of the proper size, the sparks 
show much resemblance to those of a Holtz machine. 

But these two effects, which to the e,ye appear so very differ- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 205 

eut, are only two of the many discharge phenomena. We only 
need to change the conditions of the test, and again we make 
other observations of interest. 

When, instead of operating the induction coil as in the last 
two experiments, we operate it from a high frequency alternator, 
as in the next experiment, a systematic study of the phenomena 
is rendered much more easy. In such case, in varying the 
strength and frequency of the currents through the primary, we 
may observe live distinct forms of discharge, which I have de- 
scribed in my former paper on the subject before the American 
Institute of Electrical Engineers, May 20, 1891. 

It would take too much time, and it would lead us too far 
from the subject presented this evening, to reproduce all these 
forms, but it seems to me desirable to show you one of them. It 
is a brush discharge, which is interesting in more than one re- 
spect. Viewed from a near position it resembles much a jet of 
gas escaping under great pressure. We know that the phenom- 
enon is due to the agitation of the molecules near the terminal, 
and we anticipate that some heat must be developed by the im- 
pact 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 par- 
ticular an ordinary flame, save, perhaps, that both phenomena 
might not be due to the same agent save, perhaps, that chemical 
affinity might not be electrical in its nature. 

As the production 'of heat and light is here due to the impact 
of the molecules, or atoms of air, or something else besides, 
and, as we can augment the energy simply by raising the 
potential, we might, even with frequencies obtained from 
a dynamo machine, intensify the action to such a degree as to 
bring the terminal to melting heat. But with such low' frequen- 
cies 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- 



206 INVENTIONS OF NIKOLA TESLA. 

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 incompar- 
ably higher frequencies, which we may yet lind means to pro- 
duce efficiently, and provided that electric impulses of such high 
frequencies could be transmitted through a conductor, the elec- 
trical characteristics of the brush discharge would completely 
vanish no spark would pass, no shock would he felt yet we 
would still have to deal with an electric phenomenon, but in the 
broad, modern interpretation of the word. In my first paper, be- 
fore referred to, I have pointed out the curious properties of the 
brush, and described the best manner of producing it, but I have 
thought it worth while to endeavor to express myself more clearly 
in regard to this phenomenon, because of its absorbing interest. 

When a coil is operated with currents of very high freqency, 
beautiful brush effects may be produced, even if the coil be of 
comparatively small dimensions. The experimenter may vary 
them in many ways, and, if it were for nothing else, they afford a 
pleasing sight. What adds to their interest is that they may be 
produced with one single terminal as well as with two in fact, 
often better with one than with two. 

But of all the discharge phenomena observed, the most pleas- 
ing 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 amaz- 
ing. With even a very small coil, if it be so well insulated as to 
stand a difference of potential of several thousand volts per turn, 
the sparks may be so abundant that the whole coil may appear 
a complete mass of fire. 

Curiously enough the sparks, when the terminals of the coil 
are set at a considerable distance, seem to dart in every possible 
direction as though the terminals were perfectly independent of 
each other. As the sparks would soon destroy the insulation, it 
is necessary to prevent them. This is best done by immersing 
the coil in a good liquid insulator, such as boiled-out oil. Immer- 
sion in a liquid may be considered almost an absolute necessity 
for the continued and successful working of such a coil. 

It is, of course, out of the question, in an experimental lecture, 
with only a few minutes at disposal for the performance of each 
experiment, to show these discharge phenomena to advantage, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 207 

as, to produce each phenomenon at its best, a very careful adjust- 
ment is required. But even if imperfectly produced, as they are 
likely to be this evening, they are sufficiently striking to interest 
an intelligent audience. 

Before showing some of these curious effects I must, for the 
sake of completeness, give a short description of the coil and 
other apparatus used in the experiments with the disruptive dis- 
charge this evening. 

It is contained in a box u (Fig. 13:2) of thick boards of hard 




wood, covered on the outside with a zinc sheet z, which is carefully 
soldered all around. It might be advisable, in a strictly scientific 
investigation, when accuracy is of great importance, to do away 
with the metal cover, as it might introduce many errors, princi- 
pally on account of its complex action upon the coil, as a con- 
denser of very small capacity and as an electrostatic and electro- 
magnetic 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 import- 
ance to be dwelt upon. 

The coil should be placed symmetrically to the metal cover, 



308 INVENTIONS OF NIKOLA TESLA. 

and the space between should, of course, not be too small, cer- 
tainly not less than, say, five centimetres, but much more if pos- 
sible ; especially the two sides of the zinc box, which are at right 
angles to the axis of the coil, should be sufficiently remote from 
the latter, as otherwise they might impair its action and be a 
source of loss. 

The coil consists of two spools of hard rubber R K, held apart 
at a distance of 10 centimetres by bolts c and nuts w, likewise of 
hard rubber. Each spool comprises a tube T of approximately 8 
centimetres inside diameter, and 3 millimetres thick, upon which 
are screwed two flanges F F, 24 centimetres square, the space be- 
tween the flanges being about 3 centimetres. The secondary, s s, 
of the best gutta percha-covered wire, has 26 layers, 10 turns in 
each, giving for each half a total of 260 turns. The two halves 
are wound oppositely and connected in series, the connection be- 
tween both being made over the primary. This disposition, be- 
sides being convenient, has the advantage that when the coil is 
well balanced that is, when both of its terminals TJ, T,, are con- 
nected to bodies or devices of equal capacity there is not much 
danger of breaking through to the primary, and the insulation 
between the primary and the secondary need not be thick. In 
using the coil it is advisable to attach to both terminals devices of 
nearly equal capacity, as, when the capacity of the terminals is 
not equal, sparks will be apt to pass to the primary. To avoid 
this, the middle point of the secondary may be connected to the 
primary, but this is not always practicable. 

The primary p p is wound in two parts, and oppositely, upon 
a wooden spool w, and.the four ends are led out of the oil through 
hard rubber tubes t t. The ends of the secondary T t T t are also 
led out of the oil through rubber tubes t t v of great thickness. 
The primary and secondary layers are insulated by cotton cloth, 
the thickness of the .insulation, of course, bearing some propor- 
tion to the difference of potential between the turns of the differ" 
ent layers. Each half of the primary has four layers, 24 turns 
in each, this giving a total of 96 turns. When both the parts 
are connected in series, this gives a ratio of conversion of about 
1 : 2.7, and with the primaries in multiple, 1 : 5.4 ; but in operating 
with very rapidly alternating currents this ratio does not convey 
even an approximate idea of the ratio of the E. M. F'S. in the 
primary and secondary circuits. The coil is held in position in 
the oil on wooden supports, there being about 5 centimetres 



HIGH FRKQUENCY AND HIGH POTENTIAL CURRKNTti. 309 

thickness of oil all round. Where the oil is not specially needed, 
the space is filled with pieces of wood, and for this purpose 
principally the wooden box B surrounding the whole is used. 

The construction here shown is, of course, not the best on 
general principles, but I believe it is a good and convenient one 
for the production of effects in which an excessive potential and 
a very small current are needed. 

In connection with the coil I use either the ordinary form of* 
discharger or a modified form. In the former I have introduced 
two changes which secure some advantages, and which are ob- 
vious. If they are mentioned, it is only in the hope that some 
experimenter may find them of use. 

One of the changes is that the adjustable knobs A and B (Fig. 
183), of the discharger are held in jaws of brass, .1 ,T, by spring 
pressure, this allowing of turning them successively into different 




FIG. 133. 

positions, and so doing away with the tedious process of frequent 
polishing up. 

The other change consists in the employment of a strong elec- 
tromagnet N s, which is placed with its axis at right angles to 
the line joining the knobs A and B, and produces a strong mag- 
netic field between them. The pole pieces of the magnet are 
movable and properly formed so as to protrude between the brass 
knobs, in order to make the field as intense as possible; but to 
prevent the discharge from jumping to the magnet the pole 
pieces are protected by a layer of mica, M M, of sufficient thick- 
ness; s t s l and ,9 2 .? 2 are screws for fastening the wires. On each 
side one of the screws is for large and the other for small wires. 
L L are screws for fixing in position the rods R K, which support 
the knobs. 



210 INVENTIONS OF NIKOLA TESLA. 

In another arrangement with the magnet I take the discharge 
between the rounded pole pieces themselves, which in such 
case are insulated and preferably provided with polished brass 
caps. 

The employment of an intense magnetic field is of advantage 
principally when the induction coil or transformer which charges 
the condenser is operated by currents of very low frequency. In 
such a case the number of the fundamental discharges between 
the knobs may be so small as to render the currents produced in 
the secondary unsuitable for many experiments. The intense 
magnetic field then serves to blow out the arc between the knobs 
as soon as it is formed, and the fundamental discharges occur in 
quicker succession. 

Instead of the magnet, a draught or blast of air may be em- 
ployed with some advantage. In this case the arc is preferably 




FIG. 134. 

established between the knobs A B, in Fig. 181 (the knob- " l> 
being generally joined, or entirely done away with), as in this 
disposition the arc is long and unsteady, and is easily affected by 
the draught. 

When a magnet is employed to break the arc, it is better to 
choose the connection indicated diagrammatically in Fig. 134, 
as in this case the currents forming the arc are much more pow- 
erful, 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 work- 
ing with an exhausted tube. 

The other form of discharger used in these and similar experi- 
ments is indicated in Figs. 135 and 13H. It consists of a number 
of brass pieces e c (Fig. 135), each of which comprises a spherical 
middle portion /// with an extension e below which is merely used 
to fasten the piece in a lathe when polishing up the discharging 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 211 

surface and a column above, which consists of a knurled flange 
f surmounted by a threaded stem I carrying a nut w, by means 
of which a wire is fastened to the column. The flange/ con- 
veniently serves for holding the brass piece when fastening the 




FIG. 135. 

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 K K, with planed grooves g g (Fig. 136) to fit 
the middle portion of the pieces c c, serve to clamp the latter 
and hold them firmly in position by means of two bolts c c 
(of which only one is shown) passing through the ends of the 
strips. 

In the use of this kind of discharger I have found three prin- 
cipal advantages over the ordinary form. First, the dielectric 
strength of a given total widtli of air space is greater when a 
great many small air gaps are used instead of one, which permits 




FIG. 136. 

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- 



212 INVENTIONS OF NIKOLA TEftLA. 

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. 

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 experi- 
ment, as he has not the trouble of setting the knobs again and 
again. With this kind of discharger I have been able to main- 
tain an oscillating motion without any spark being visible with 
the naked eye between the knobs, and they would not show a 
very appeciable rise in temperature. This form of discharge 
also lends itself to many arrangements of condensers and circuits 
which are often very convenient and time-saving. I have used 
it preferably in a disposition similar to that indicated in Fig. 131, 
when the currents forming the arc are small. 

I may here mention that I have also used dischargers with 
single or multiple air gaps, in which the discharge surfaces were 
rotated with great speed. No particular advantage was, how- 
ever, gained by this method, except in cases where the currents 
from the condenser were large and the keeping cool of the sur- 
faces was necessary, and in cases Avhen, 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 pre- 
ferred 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, pro- 
duces no effect worth noticing, unless the self-induction, capacity 
and resistance are so related that there are oscillations set up 
upon each interruption. 

I will now endeavor to show you some of the most noteworthy 
of these discharge phenomena. 

I have stretched across the room two ordinary cotton covered 
wires, each about seven metres in length. They are supported 
011 insulating cords at a distance of about thirty centimetres. I 
attach now to each of the terminals of the coil one of the wires. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 213 

and set the coil in action. Upon turning the lights off in the 
room yon see the wires strongly illuminated by the streams issu- 
ing abundantly from their whole surface in spite of the cotton 
covering, which may even be very thick. When the experiment 
is performed under good conditions, the light from the wires is 
sufficiently intense to allow distinguishing the objects in a room. 
To produce the best result it is, of course, necessary to adjust 
carefully the capacity of the jars, the arc between the knobs and 
the length of the wires. My experience is that calculation of the 
length of the wires leads, in such case, to no result whatever. The 
experimenter will do best to take the wires at the start very long, 
and then adjust by cutting off first long pieces, and then smaller 
and smaller ones as he approaches the right length. 

A convenient way is to use an oil condenser of very small 
capacity, consisting of two small adjustable metal plates, in con- 
nection 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 po- 
tential. When a condenser is used in connection with experi- 
ments with such a coil, it should be an oil condenser by all means, 
as in using an air condenser considerable energy might be wasted. 
The wires leading to the plates in the oil should be very thin, 
heavily coated with some insulating compound, and provided 
with a conducting covering this preferably extending under the 
surface of the oil. The conducting cover should not be too near 
the terminals, or ends, of the wire, as a spark would be apt to 
jump from the wire to it. The conducting coating is used to 
diminish the air losses, in virtue of its action as an electrostatic 
screen. As to the size of the vessel containing the oil, and the 
size of the plates, the experimenter gains at once an idea from a 
rough trial. The size of the plates in oil is, however, calculable, 
as the dielectric losses are very small. 

In the preceding experiment it is of considerable interest to 
know what relation the quantity of the light emitted bears to 
the frequency and potential of the electric impulses. My opinion 
is that the heat as well as light effects produced should be pro- 
portionate, under otherwise equal conditions of test, to the product 
of frequency and square of potential, but the experimental veri- 
fication of the law, whatever it may be, would be exceedingly 



214 INVENTIONS OF NIKOLA TESLA. 

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. 

The luminous intensity of the streams is, of course, considerably 




FIG. 137. 

increased when they are focused upon a small surface. This may 
be shown by the following experiment : 

I attach to one of the terminals of the coil a wire w (Fig. 137), 
bent in a circle of about 30 centimetres in diameter, and to the 
other terminal I fasten a small brass sphere s, the surface of the 
wire being preferably equal to the surface of the sphere, and the 
centre of the latter being in a line at right angles to the plane of 
the wire circle and passing through its centre. When the dis- 
charge is established under proper conditions, a luminous hollow 
cone is formed, and in the dark one-half of the brass sphere is 
strongly illuminated, as shown in the cut. 

By some artifice or other it is easy to concentrate the streams 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 215 

upon small surfaces and to produce very strong light effects. 
Two thin wires may thus be rendered intensely luminous. 

In order to intensify the streams the wires should be very thin 
and short ; but as in this case their capacity would be generally 
too small for the coil at least for such a one as the present it 
is necessary to augment the capacity to the required value, while, 
at the same time, the surface of the wires remains very small. 
This may be done in many ways. 

Here, for instance, I have two plates, K K, of hard rubber (Fig. 
188), upon which I have glued two very thin wires w w, so as to 
form a name. The wires may be bare or covered with the best 
insulation it is immaterial for the success of the experiment. 
Well insulated wires, if anything, are preferable. On the back 




FIG. 138. 

of each plate, indicated by the shaded portion, is a tinfoil coating 
t t. The plates are placed in line at a sufficient distance to pre- 
vent a spark passing from one wire to the other. The two tin- 
foil coatings I have joined by a conductor c, and the two wires I 
presently connect to the terminals of the coil. It is now easy, by 
varying the strength and frequency of the currents through the 
primary, to find a point at which the capacity of the system is 
best suited to the conditions, and the wires become so strongly 
luminous that, when the light in the room is turned off the name 
formed by them appears in brilliant letters. 

It is perhaps preferable to perform this experiment with a 
coil operated from an alternator of high frequency, as then, 



216 INVENTIONS OF NIKOLA TESLA. 

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 per- 
formed with low frequencies, but much less satisfactorily. 

When two wires, attached to the terminals of the coil, are set 
at the proper distance, the streams between them may be so in- 
tense as to produce a continuous luminous sheet. To show this 
phenomenon I have here two circles, c andc (Fig. 139), of rather 
stout wire, one being about 80 centimetres and the other 30 cen- 
timetres in diameter. To each of the terminals of the coil I 
attach one of the circles. The supporting wires are so bent that 




FIG. 139. 

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 uni- 
formly 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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 217 

tainty, the circle has been taken smaller, and the area is now 
about 0.43 square metre. 

The frequency of the vibration, and the quickness of succes- 
sion 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 cor- 
respond to the successive discharges occurring between the 
knobs. But when the frequency is extremely high, and the arc 
of the discharge produces a very loud and smooth sound show- 
ing both that oscillation takes place and that the sparks succeed 
each other with great rapidity then the luminous streams 
formed are perfectly uniform. To reach this result very small 
coils and jars of small capacity should be used. I take two 
tubes of thick Bohemian glass, about 5 centimetres in diameter 
and 20 centimetres long. In each of the tubes I slip a primary 
of very thick copper wire. On the top of each tube I wind a 
secondary of much thinner gutta-percha covered wire. The two 
secondaries I connect in series, the primaries preferably in multiple 
arc. The tubes are then placed in a large glass vessel, at a dis- 
tance of 10 to 15 centimetres from each other, on insulating sup- 
ports, and the vessel is filled witli 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 ne- 
cessary 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 tex- 
tureless, like the ordinary discharge through a moderately ex- 
hausted tube. 

I have purposely dwelt upon this apparently insignificant ex- 
periment. 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 po- 
tential is likewise required, but this is merely an incidental neces- 
sity. These experiments teach us that, in endeavoring to dis- 



218 INVENTIONS OF NIKOLA TE8LA. 

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 possi- 
bility of obtaining the light effects without the use of any vessel 
whatever, with air at ordinary pressure. 

Such discharges of very high frequency, which render luminous 
the air at ordinary pressures, we have probably occasion often to 
witness in Nature. I have no doubt that if, as many believe, the 
aurora borealis is produced by sudden cosmic disturbances, such 
as eruptions at the sun's surface, which set the electrostatic charge 
of the earth in an extremely rapid vibration, the red glow ob- 
served is not confined to the upper rarefied strata of the air, but 
the discharge traverses, by reason of its very high frequency, 
also the dense atmosphere in the form of a glow, such as we or- 
dinarily 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 dis- 
charge. Indications of such breaking down of the lower dense 
strata of the air have been repeatedly observed at the occurence 
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 vibra- 
tions probably pass unnoticed. 

When an ordinary low frequency discharge is passed through 
moderately rarefied air, the air assumes a purplish hue. If by 
some means or other we increase the intensity of the molecular, 
or atomic, vibration, the gas changes to a white color. A similar 
change occurs at ordinary pressures with electric impulses of very 
high frequency. If the molecules of the air around a wire are 
moderately agitated, the brush formed is reddish or violet ; if 
the vibration is rendered sufficiently intense, the streams become 
white. We may accomplish this in various ways. In the experi- 
ment before shown with the two wires across the room, I have 
endeavored to secure the result by pushing to a high value both 
the frequency and potential ; in the experiment with the thin 
wires glued on the rubber plate I have concentrated the action 
upon a very small surface in other words, I have worked with 
a great electric density. 

A most curious form of discharge is observed with such a coil 



man FREQUENCY AND man POTENTIAL CURRENTS. 219 

when the frequency and potential are pushed to the extreme 
limit. To perform the experiment, every part of the coil should 
be heavily insulated, and only two small spheres or, better still, 
two sharp-edged metal discs (d d, Fig. 140) of no more than 
a few centimetres in diameter should be exposed to the air. 
The coil here used is immersed in oil, and the ends of the 
secondary reaching out of the oil are covered with an air-tight 
cover of hard rubber of great thickness. All cracks, if there 
are any, should be carefully stopped up, so that the brush dis- 
charge 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. 




FIG. 140. 

The potential may be raised by increasing, as far as the experi- 
menter judges proper, the rate of change of the primary cur- 
rent. 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 



220 INVENTIONS OF NIKOLA TESLA. 

the terminal only upon the hand being brought much nearer. 
If the oscillation of the primary current is rendered intermittent 
by some means or other, there is a corresponding throbbing of 
the streams, and now the hand or other conducting object may 
be brought in still greater proximity to the terminal without a 
spark being caused to jump. 

Among the many beautiful phenomena which may be pro- 
duced with such a coil, I have here selected only those which ap- 
pear to possess some features of novelty, and lead us to some 
conclusions of interest. One will not tind it at all difficult to 
produce in the laboratory, by means of it, many other phenomena 
which appeal to the eye even more than these here shown, but 
present no particular feature of novelty. 

Early experimenters describe the display of sparks produced by 
an ordinary large induction coil upon an insulating plate separat- 
ing the terminals. Quite recently Siemens performed some ex- 
periments 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 uni- 
form streams, or a fine network of thin silvery threads, or a 
mass of loud brilliant sparks, which completely cover the plate. 

Since I have advanced the idea of the conversion by means of 
the disruptive discharge, in my paper before the American In- 
stitute of Electrical Engineers at the beginning of the past year, 
the interest excited in it has been considerable. It affords us a 
means for producing any potentials by the aid of inexpensive 
coils operated from ordinary systems of distribution, and what 
is perhaps more appreciated it enables us to convert cunvnt* <>!' 



HIGH FliKQ.UENCY AND HIGH POTENTIAL CURRENTS. 231 

any frequency into currents of any other lower or higher fre- 
quency. But its chief value will perhaps be found in the help 
which it will afford us in the investigations of the phenomena 
of phosphorescence, which a disruptive discharge coil is capable 
of exciting in innumerable cases where ordinary coils, even the 
largest, would utterly fail. 

Considering its probable uses for many practical purposes, and 
its possible introduction into laboratories for scientific research, 
a few additional remarks as to the construction of such a coil 
will perhaps not be found superfluous. 

It is, of course, absolutely necessary to employ in such a coil 
wires provided with the best insulation. 

Good coils may be produced by employing wires covered with 
several layers of cotton, boiling the coil a long time in pure wax, 
and cooling under moderate pressure. The advantage of such a 
coil is that it can be easily handled, but it cannot probably give 
as satisfactory results as a coil immersed in pure oil. Besides, it 
seems that the presence of a large body of wax affects the coil 
disadvantageously, whereas this does not seem to be the case with 
oil. Perhaps it is because the dielectric losses in the liquid are 
smaller. 

I have tried at iirst silk and cotton covered wires with oil im- 
mersions, 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 ex- 
treme 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 im- 
mersion of eight to nine months I have found no ill effects. 

I have obtained two kinds of gutta-percha wire known in com- 
merce : 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 iirst kind. I wind the coil within 
an oil tank so that all interstices are filled up with the oil. Be- 
tween the layers I use cloth boiled out thoroughly in oil, 
calculating the thickness according to the difference of potential 



222 INVENTIONS OF NIKOLA TESLA. 

between the turns. There seems not to be a very great differ- 
ence whatever kind of oil is used ; I use paraffine or linseed oil. 

To exclude more perfectly the air, an excellent way to pro- 
ceed, 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 insert- 
ed 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 com- 
pletely excluded, probably better than by boiling out, which, 
however, when gutta-percha coated wires are used, is not prac- 
ticable. 

For the primaries I use ordinary line wire with a thick cotton 
coating. Strands of very thin insulated wires properly inter- 
laced would, of course, be the best to employ for the primaries, 
but they are not to be had. 

In an experimental coil the size of the wires is not of great 
importance. In the coil here used the primary is No. 12 and the 
secondary No. 24 Brown & Sharpe gauge wire ; but the sections 
may be varied considerably. It would only imply different ad- 
justments ; the results aimed at would not be materially affected. 

I have dwelt at some length upon the various forms of brush 
discharge because, in studying them, we not only observe pheno- 
mena 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 con- 
veying 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 223 

be most carefully expelled, for in it the charged surfaces are near 
each other, and if the potentials are high, just assure 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 con- 
veying 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 im- 
perfections, I am led to believe that in our future distribution of 
electrical energy by currents of very high tension, liquid insula- 
tion will be used. The cost is a great drawback, but if we em- 
ploy 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 cur- 
rent motors, transmissions of power can be affected with safety 
and upon an industrial basis at distances of as much as a thousand 
miles. 

A peculiar property of oils, and liquid insulation in general, 
when subjected to rapidly changing electric stresses, is to disperse 
any gaseous bubbles which may be present, and diffuse them 
through its mass, generally long before any injurious break can 
occur. This feature may be easily observed with an ordinary in- 
duction 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 paraffme oil. A prim- 
ary of a diameter something like six millimetres smaller than the 
inside of the tube may be inserted in the oil. When the coil is 
set to work one may see, looking from the top through the oil, 
many luminous points air bubbles which are caught by insert- 
ing 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 insula- 
tion, no matter how thick, were used, a breaking through and in- 
jury of the apparatus would be inevitable. 



234 INVENTIONS OF NIKOLA TKSLA. 

The exclusion of gaseous matter from any apparatus in which 
the dielectric is subjected to more or less rapidly changing elec- 
tric 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 or- 
dinary 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 pro- 
duced 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 consider- 
able ; the molecules are free to move, they reach high speeds, and 
the energy of their impact is lost in heat or otherwise. If the 
gas be strongly compressed, the displacement due to the force is 
made smaller, and the losses are reduced. 

In most of the succeeding experiments I prefer, chiefly on 
account of the regular and positive action, to employ the alter- 
nator 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 fre- 
quency of about 10,000 per second. This machine has been illus- 
trated 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 en- 
able any engineer to build a similar machine, will be found in 
several electrical journals of that period. 

The induction coils operated from the machine are rather small, 
containing from 5,000 to 15,000 turns in the secondary. They 
are immersed in boiled-out linseed oil, contained in wooden boxes 
covered with zinc sheet. 

I have found it advantageous to reverse the usual position of 
the wires, and to wind, in these coils, the primaries on the top ; 
thus allowing the use of a much larger primary, which, of course, 
reduces the danger of overheating and increases the output of 
the coil. I make the primary on each side at least one centimetre 
shorter than the secondary, to prevent the breaking through on the 
ends, which would surely occur unless the insulation on the top 
of the secondary be very thick, and this, of course, would be dis- 
advantageous. 

When the primary is made movable, which is necessary in 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 225 

some experiments, and many times convenient for the purposes 
of adjustment, I cover the secondary with wax, and turn it off 
in a lathe to a diameter slightly smaller than the inside of the 
primary coil. The latter I provide with a handle reaching out 
of the oil, which serves to shift it in any position along the 
secondary. 

I will now venture to make, in regard to the general mani- 
pulation of induction coils, a few observations bearing upon points 
which have not been fully appreciated in earlier experiments 
with such coils, and are even now often overlooked. 

The secondary of the coil possesses usually such a high self- 
induction that the current through the wire is inappreciable, and 
may be so even when the terminals are joined by a conductor of 
small resistance. If capacity is added to the terminals, the self- 
induction is counteracted, and a stronger current is made to flow 
through the secondary, though its terminals are insulated from 
each other. To one entirely unacquainted with the properties of 
alternating currents nothing will look more puzzling. This fea- 
ture 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 to- 
gether, and a small arc passed between them, the arc prevented a 
strong current from passing through the secondary, because it 
did away with the capacity on the terminals ; when the rubber 
plate was inserted between, the capacity of the condenser formed 
counteracted the self-induction of the secondary, a stronger cur- 
rent passed now, the coil performed more work, and the discharge 
was by far more powerful. 

The first thing, then, in operating the induction coil is to com- 
bine 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 other- 
wise considerable dissipation may occur if the jars are greatly 
strained. When high frequencies are used, it is of equal im- 
portance 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 recom- 
mended, 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 



326 



INVENTIONS OF NIKOLA TE8LA. 



self-induction of both the latter. The condenser should be ad- 
justable by very small steps, and for a finer adjustment a small 
oil condenser with movable plates may be used conveniently. 

I think it best at this juncture to bring before you a phe- 
nomenon, observed by me some time ago, which to the purely 
scientific investigator may perhaps appear more interesting than 
any of the results which I have the privilege to present to you 
this evening. 

It may be quite properly ranked among the brush phenom- 
ena in fact, it is a brush, formed at, or near, a single terminal 
in high vacuum. 

In bulbs provided with a conducting terminal, though it be of 





FIG. 141. 



FIG. 142. 



aluminum, the brush -has but an ephemeral existence, and can- 
not, unfortunately, be indefinitely preserved in its most sensi- 
tive state, even in a bulb devoid of any conducting electrode. 
In studying the phenomenon, by all means a bulb having no 
leading-in wire should be used. I have found it best to use 
bulbs constructed as indicated in Figs. 141 and 142. 

In Fig. 141 the bulb comprises an incandescent lamp globe Z, 
in the neck of which is sealed a barometer tube &, the end of which 
is blown out to form a small sphere s. This sphere should be 
sealed as closely as possible in the centre of the large globe. 
Before sealing, a thin tube t, of aluminum sheet, may be slipped 
in the barometer tube, but it is not important to employ it. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 227 

The small hollow sphere s is filled with some conducting 
powder, and a wire w is cemented in the neck for the purpose of 
connecting the conducting powder with the generator. 

The construction shown in Fig. 142 was chosen in order to 
remove from the brush any conducting body which might possi- 
bly affect it. The bulb consists in this case of a lamp globe Z, 
which has a neck n, provided with a tube b and small sphere s, 
sealed to it, so that two entirely independent compartments are 
formed, as indicated in the drawing. When the bulb is in use 
the neck n is provided with a tinfoil coating, which is connected 
to the generator and acts inductively upon the moderately rare- 
fied and highly conducted gas inclosed in the neck. From there 
the current passes through the tube b into the small sphere *, to 
act by induction upon the gas contained in the globe L. 

It is of advantage to make the tube -very thick, the hole 




FIG. 143. 



through it very small, and to blow the sphere * very thin. It is 
of the greatest importance that the sphere * be placed in the 
centre of the globe L. 

Figs. 143, 144 and 145 indicate different forms, or stages, of 
the brush. Fig. 143 shows the brush as it first appears in a bulb 
provided with a conducting terminal ; but, as in such a bulb it 
very soon disappears often after a few minutes I will confine 
myself to the description of the phenomenon as seen in a bulb 
without conducting electrode. It is observed under the follow- 
ing conditions : 

When the globe L (Figs. 141 and 142) is exhausted to a very 
high degree, generally the bulb is not excited upon connecting 
the wire w (Fig. 141) or the tinfoil coating of the bulb (Fig. 



INVENTIONS OF NIKOLA TESLA. 



142) to the terminal of the induction coil. To excite it, it is 
usually sufficient to grasp the globe L with the hand. An in- 
tense phosphorescence then spreads at tirst 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 ap- 
pears 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. Warm- 
ing the bulb or increasing the potential hastens the transit. 

When the brush assumes the form indicated in Fig. 145, it may 
be brought to a state of extreme sensitiveness to electrostatic 





FIG. 144. 



FIG. 145. 



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 
aft'ect it visibly at a distance of two metres, slowing down or ac- 
elerating the rotation according to how it is held relatively to 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 229 

the brush. I think I have observed that at the stage when it is 
most sensitive to magnetic, it is not most sensitive to electrostatic, 
influence. My explanation is, that the electrostatic attraction 
between the brush and the glass of the bulb, which retards the 
rotation, grows much quicker than the magnetic influence when 
the intensity of the stream is increased. 

When the bulb hangs with the globe L down, the rotation is 
always clockwise. In the southern hemisphere it would occur 
in the opposite direction and on the equator the brush should 
not turn at all. The rotation may be reversed by a magnet kept 
at some distance. The brush rotates best, seemingly, when it is 
at right angles to the lines of force of the earth. It very likely 
rotates, when at its maximum speed, in synchronism with the 
alternations, say, 10,000 times a second, The rotation can be 
slowed down or accelerated by the approach or receding of the 
observer, or any conducting body, but it cannot be reversed by 
putting the bulb in any position. When it is in the state of the 
highest sensitiveness and the potential or frequency be varied, 
the sensitiveness is rapidly diminished. Changing either of 
these but little will generally stop the rotation. The sensitive- 
ness is likewise affected by the variations of temperature. To 
attain great sensitiveness it is necessary to have the small sphere 
s in the centre of the globe Z, as otherwise the electrostatic 
action of the glass of the globe will tend to stop the rotation. 
The sphere s should be small and of uniform thickness ; any dis- 
symmetry of course has the effect to diminish the sensitiveness. 

The fact that the brush rotates in a delinite direction in a per- 
manent magnetic tield seems to show that in alternating currents 
of very high frequency the positive and negative impulses are 
not equal, but that one always preponderates over the other. 

Of course, this rotation in one direction may be due to the 
action of the two elements of the same current upon each other, 
or to the action of the field produced by one of the elements 
upon the other, as in a series motor, without necessarily one im- 
pulse being stronger than the other. The fact that the brush 
turns, as far as I could observe, in any position, would speak for 
this view. In such case it would turn at any point of the earth's 
surface. But, on the other hand, it is then hard to explain why 
a permanent magnet should reverse the rotation, and one must 
assume the preponderance of impulses of one kind. 

As to the causes of the formation of the brush or stream, i 



230 INVENTIONS OF NIKOLA TESLA. 

think it is due to the electrostatic action of the globe and the 
dissymmetry of the parts. If the small bulb * and the globe Z 
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 ex- 
periments may be performed with it. For instance, the experi- 
menter 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 oif by merely stiffening the muscles of the arm. 
When it begins to rotate slowly, and the hands are held at a 
proper distance, it is impossible to make even the slightest motion 
without producing a visible effect upon the brush. A metal 
plate connected to the other terminal of the coil affects it at a 
great distance, slowing down the rotation often to one turn a 
second. 

I am firmly convinced that such a brush, when we learn how 
to produce it properly, will prove a valuable aid in the investi- 
gation of the nature of the forces acting in an electrostatic or 
magnetic field. If there is any motion which is measurable going 
on in the space, such a brush ought to reveal it. It is, so to 
speak, a beam of light, frictionless, devoid of inertia. 

I think that it may find practical applications in telegraphy. 
With such a brush it would be possible to send dispatches across 
the Atlantic, for instance, with any speed, since its sensitiveness 
may be so great that the slightest changes will affect it. If it 
were possible to make the stream more intense and very narrow, 
its deflections could be easily photographed. 

I have been interested to find whether there is a rotation of 
the stream itself, or whether there is simply a stress traveling 
around the bulb. For this purpose I mounted a light mica fan 
so that its vanes were in the path of the brush. If the stream 
itself was rotating the fan would be spun around. I could pro- 
duce no distinct rotation of the fan, although I tried the experi- 
ment 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. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 231 

I have been unable to produce the phenomenon with the dis- 
ruptive discharge coil, although every other of these phenomena 
can be well produced by it many, in fact, much better than 
with coils operated from an alternator. 

It may be possible to produce the brush by impulses of one 
direction, or even by a steady potential, in which case it would 
be still more sensitive to magnetic influence. 

In operating an induction coil with rapidly alternating currents, 
we realize with astonishment, for the first time, the great import- 
ance 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 ter- 
minals of the secondary of the coil, or attach to one or both ter- 
minals insulated bodies of very small bulk, such as bulbs, and lie 
may produce a considerable rise or fall of potential, and greatly 
affect the now of the current through the primary. In the ex- 
periment before shown, in which a brush appears at a wire 
attached to one terminal, and the wire is vibrated when the ex- 
perimenter brings his insulated body in contact with the other 
terminal of the coil, the sudden rise of potential was made evi- 
dent, 

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 sur- 
faces 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 screen- 
ed, or if, generally speaking, the bombardment on this side is 
weakened in some way or other, there remains the repulsion ex- 
erted upon the other, and the fan is set in rotation. The screen- 
ing 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 



232 INVENTIONS OF NIKOLA TESLA. 

side, and close to it, an insulated metal plate. The static screen 
may, however, be omitted, and simply a thickness of insulating 
material fastened to one of the sides of the fan. 

To show the behavior of the coil, the fan may be placed upon 
the terminal and it will readily rotate when the coil is operated 
by currents of very high frequency. With a steady potential, 
of course, and even with alternating currents of very low fre- 
quency, 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 eifort 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 potent- 
ial. 

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 Crcokes 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 con- 
ducting gas, instead of passing off from the conducting vanes. 

In trying the preceding experiment with the electric radiometer 
the potential should not exceed a certain limit, as then the elec- 
trostatic attraction between the vanes and the glass of the bulb 
may be so great as to stop the rotation. 

A most curious feature of alternate currents of high frequen- 
cies and potentials is that they enable us to perform many experi- 
ments by the use of one wire only. In many respects this feat, 
ure is of great interest. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 233 

In a type of alternate current motor invented by me some years 
ago I produced rotation by inducing, by means of a single alter- 
nating 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, join- 
ing 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 en- 
circle 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 con- 
denser, self-induction, resistance or equivalent windings. 

I had discovered, however, that rotation is produced by means 
of a single coil and core; my explanation of the phenomenon, and 
leading thought in trying the experiment, being that there must 
be a true time lag in the magnetization of the core. I remember 
the pleasure I had when, in the writings of Professor Ayrton, 
which came later to my hand, I found the idea of the time lag 
advocated. Whether there is a true time lag, or whether the re- 
tardation 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 pro- 
duced 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), com- 
prising a coil and iron core, and a freely movable copper disc in 
proximity to the latter. 

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 alter- 
nator the disc is set in rotation. But it is not this experiment, 
now well known, which I desire to perform. What I wish to 



234 INVENTIONS OF NIKOLA TESLA. 

show you is that this motor rotates with one single connection be- 
tween it and the generator; that is to say, one terminal of the 
motor is connected to one terminal of the generator in this case 
the secondary of a high-tension induction coil the other term- 
inals of motor and generator being insulated in space. To pro- 
duce 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 rota- 
tion. If an exhausted tube is put in series with the coil, the tube 
lights brilliantly, showing the passage of a strong current. In- 




PIG. 146. 

stead 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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 285 

induction, and then it would be necessary to employ a much 
higher frequency and potential. Neither would be advisable, for 
a higher potential would endanger the insulation of the small 
primary coil, and a higher frequency would result in a materially 
diminished torque. 

It should be remarked that when such a motor with a 
closed secondary is used, it is not at all easy to obtain rota- 
tion with excessive frequencies, as the secondary cuts off 
almost completely the lines of the primary and this, of 
course, the more, the higher the frequency and allows the pass- 
age of but a minute current. In such a case, unless the second- 
ary is closed through a condenser, it is almost essential, in order 
to produce rotation, to make the primary and secondary coils 
overlap each other more or less. 

But there is an additional feature of interest about this motor, 
namely, it is not necessary to have even a single connection be- 
tween the motor and generator, except, perhaps, through the 
ground; for not only is an insulated plate capable of giving off 
energy into space, but it is likewise capable of deriving it from 
an alternating electrostatic field, though in the latter case the 
available energy is much smaller. In this instance one of the 
motor terminals is connected to the insulated plate or body 
located within the alternating electrostatic field, and the other 
terminal preferably to the ground. 

It is quite possible, however, that such " no wire " motors, as 
they might be called, could be operated by conduction through 
the rarefied air at considerable distances. Alternate currents, 
especially of high frequencies, pass with astonishing freedom 
through even slightly rarefied gases. The upper strata of the air 
are rarefied. To reach a number of miles out into space requires 
the overcoming of difficulties of a merely mechanical nature. 
There is no doubt that with the enormous potentials obtainable by 
the use of high frequencies and oil insulation, luminous discharges 
might be passed through many miles of rarefied air, and that, by 
thus directing the energy of many hundreds or thousands of horse- 
power, motors or lamps might be operated at considerable 
distances from stationary sources. But such schemes are men- 
tioned merely as possibilities. We shall have no need to transmit 
power in this way. We shall have no need to transmit power 
at all. Ere many generations pass, our machinery will be driven 
by a power obtainable at any point of the universe. This idea is 



236 INVENTIONS OF NIKOLA TESLA. 

not novel. Men have been led to it long ago by instinct or reason. 
It has been expressed in many ways, and in many places, in the 
history of old and new. We find it in the delightful myth of 
Antheus, who derives power from the earth ; we find it among 
the subtle speculations of one of your splendid mathematicians, 
and in many hints and statements of thinkers of the present time. 
Throughout space there is energy. Is this energy static or kinetic ? 
If static our hopes are in vain; if kinetic and this we know it 
is, for certain then it is a mere question of time when men will 
succeed in attaching their machinery to the very wheelwork of 
nature. Of all, living or dead, Crookes came nearest to doing it. 
His radiometer will turn in the light of day and in the darkness 
of the night; it will turn everywhere where there is heat, and 
heat is everywhere. But, unfortunately, this beautiful little 
machine, while it goes down to posterity as the most interesting, 
must likewise be put on record as the most inefficient machine 
ever invented ! 

The preceding experiment is only one of many equally inter- 
esting 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 con- 
necting only one pole of the cell to the line or source of energy. 
Or we may, by attaching to the line, or only bringing into its 
vicinity, light up an incandescent lamp, an exhausted tube, or a 
phosphorescent bulb. 

However impracticable this plan of working may appear in 
many cases, it certainly seems practicable, and even recommend- 
able, in the production of light. A perfected lamp would require 
but little energy, and if wires were used at all we ought to be able 
to supply that energy without a return wire. 

It is now a fact that a body may be rendered incandescent or 
phosphorescent by bringing it either in single contact or merely 
in the vicinity of a source of electric impulses of the proper 
character, and that in this manner a quantity of light sufficient 
to afford a practical illuminant may be produced. It is, there- 
fore, to say the least, worth while to attempt to determine the 
best conditions and to invent the best appliances for attaining 
this object. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 237 

Some experiences have already been gained in this direction, 
and I will dwell on them briefly, in the hope that they might 
prove useful. 

The heating of a conducting body inclosed in a bulb, and con- 
nected to a source of rapidly alternating electric impulses, is 
dependent on so many things of a different nature, that it would 
be difficult to give a generally applicable rule under which the 
maximum heating occurs. As regards the size of the vessel, I 
have lately found that at ordinary or only slightly differing 
atmospheric pressures, when air is a good insulator, and hence 
practically the same amount of energy by a certain potential and 
frequency is given off from the body, whether the bulb be small 
or large, the body is brought to a higher temperature if enclosed 
in a small bulb, because of the better confinement of heat in this 
case. 

At lower pressures, when air becomes more or less conducting, 
or if the air be sufficiently warmed to become conducting, the 
body is rendered more intensely incandescent in a large bulb, 
obviously because, under otherwise equal conditions of test, more 
energy may be given off from the body when the bulb is large. 

At very high degrees of exhaustion, when the matter in the 
bulb becomes " radiant," a large bulb has still an advantage, but 
a comparatively slight one, over the small bulb. 

Finally, at excessively high degrees of exhaustion, which can- 
not be reached except by the employment of special means, there 
seems to be, beyond a certain and rather small size of vessel, no 
perceptible difference in the heating-. 

These observations were the result of a number of experiments, 
of which one, showing the effect of the size of the bulb at a high 
degree of exhaustion, may be described and shown here, as it 
presents a feature of interest. Three spherical bulbs of 2 inches, 
3 inches and 4 inches diameter were taken, and in the centre of 
each was mounted an equal length of an ordinary incandescent 
lamp filament of uniform thickness. In each bulb the piece of 
filament was fastened to the leading-in wire of platinum, con- 
tained 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 
bo explained subsequently. In each bulb an equal length of fila- 



238 INVENTIONS OF NIKOLA TESLA. 

ment protruded above the metal tube. It is sufficient to say now 
that under these conditions equal lengths of filament of the same 
thickness in other words, bodies of equal bulk were brought 
to incandescence. The three bulbs were sealed to a glass tube, 
which was connected to a Sprengel pump. When a high vacuum 
had been reached, the glass tube carrying the bulbs was sealed 
off. A current was then turned on successively on each bulb, 
and it was found that the filaments came to about the same 
brightness, and, if anything, the smallest bulb, which was placed 
midway between the two larger ones, may have been slightly 
brighter. This result was expected, for when either of the bulbs 
was connected to the coil the luminosity spread through the 
other two, hence the three bulbs constituted really one vessel. 
When all the three bulbs were connected in multiple arc to the 
coil, in the largest of them the filament glowed brightest, in the 
next smaller it was a little less bright, and in the smallest it only 
came to redness. The bulbs were then sealed off and separately 
tried. The brightness of the filaments was now such as would 
have been expected on the supposition that the energy given off 
was proportionate to the surface of the bulb, this surface in each 
case representing one of the cpatings of a condenser. Accord- 
ingly, there was less difference between the largest and the 
middle sized than between the latter and the smallest bulb. 

An interesting observation was made in this experiment. The 
three bulbs were suspended from a straight bare wire connected 
to a terminal of a coil, the largest bulb being placed at the end 
of the wire, at some distance from it the smallest bulb, and at an 
equal distance from the latter the middle-sized one. The carbons 
glowed then in both the larger bulbs about as expected, but the 
smallest did not get its share by far. This observation led me to 
exchange the position of the bulbs, and I then observed that 
whichever of the bulbs was in the middle was by far less bright 
than it was in any other position. This mystifying result was, 
of course, found to be due to the electrostatic action between the 
bulbs. When they were placed at a considerable distance, or 
when they were attached to the corners of an equilateral triangle 
of copper wire, they glowed in about the order determined by 
their surfaces. 

As to the shape of the vessel, it is also of some importance, especi- 
ally at high degrees of exhaustion. Of all the possible construc- 
tions, it seems that a spherical globe with the refractory body 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 239 

mounted in its centre is the best to employ. By experience it 
lias 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 at- 
tained in a cylindrical vessel with one or two straight filaments 
coinciding with its axis, and possibly also in parabolical or spheri- 
cal bulbs with refractory body or bodies placed in the focus or 
foci of the same; though the latter is not probable, as the elec- 
trified atoms should in all cases rebound normally from the 
surface they strike, unless the speed were excessive, in which 
case they would probably follow the general law of reflection. 
]S r o matter what shape the vessel may have, if the exhaustion be 
low, a filament mounted in the globe is brought to the same 
degree of incandescence in all parts ; but if the exhaustion be 
high and the bulb be spherical or pear-shaped, as usual, focal 
points form and the filament is heated to a higher degree at or 
near such points. 

To illustrate the effect, I have here two small bulbs which are 
alike, only one is exhausted to a low and the other to a very high 
degree. When connected to the coil, the filament in the former 
glows uniformly throughout all its length ; whereas in the latter, 
that portion of the filament which is in the centre of the bulb 
glows far more intensely than the rest. A curious point is that 
the phenomenon occurs even if two filaments are mounted in a 
bulb, each being connected to one terminal of the coil, and, what 
is still more curious, if they be very near together, provided the 
vacuum be very high. I noted in experiments with such bulbs 
that the filaments would give way usually at a certain point, and 
in the first trials I attributed it to a defect in the carbon. But 
when the phenomenon occurred many times in succession I 
recognized its real cause. 

In order to bring a refractory body inclosed in a bulb to in- 
candescence, 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 construc- 
tion of the illuminating device to approximate it more or less. 



240 INVENTIONS OF NIKOLA TESLA. 

For many reasons, the refractory body is placed in the centre 
of the bulb, and it is usually supported on a glass stem containing 
the leading-in wire. As the potential of this wire is alternated, 
the rarefied gas surrounding the stem is acted upon inductively, 
and the glass stem is violently bombarded and heated. In this 
manner by far the greater portion of the energy supplied to the 
bulb especially when exceedingly high frequencies are used 
may be lost for the purpose contemplated. To obviate this loss, 
or at least to reduce it to a minimum, I usually screen the rarefied 
gas surrounding the stem from the inductive action of the leading-in 
wire by providing the stem with a tube or coating of conducting 
material. It seems beyond doubt that the best among metals to 
employ for this purpose is aluminum, on account of its many re- 
markable 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 some- 
what smaller than that of the glass stem, is made of the finest 
aluminum sheet, and slipped on the stem. The tube is conveni- 
ently prepared by wrapping around a rod fastened in a lathe a 
piece of aluminum sheet of proper size, grasping the sheet firmly 
with clean chamois leather or blotting paper, and spinning the 
rod very fast. The sheet is wound tightly around the rod, and a 
highly polished tube of one or three layers of the sheet is obtained. 
When slipped on the stem, the pressure is generally sufficient to 
prevent it from slipping off, but, for safety, the lower edge of 
the sheet may be turned inside. The upper inside corner of the 
sheet that is, the one which is nearest to the refractory incan- 
descent body should be cut out diagonally, as it often happens 
that, in consequence of the intense heat, this corner turns toward 
the inside and comes very near to, or in contact with, the wire, or 
filament, supporting the refractory body. The greater part of 
the energy supplied to the bulb is then used up in heating the 
metal tube, and the bulb is rendered useless for the purpose. 
The aluminum sheet should project above the glass stem more or 
less one inch or so or else, if the glass be too close to the in- 
candescing 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 241 

much smaller diameter. To still further reduce the danger 
arising from the heating of the glass stem, and also with the view 
of preventing an electrical connection between the metal tube 
and the electrode, I preferably wrap the stem with several layers 
of thin mica, which extends at least as far as the metal tube. In 
some bulbs I have also used an outside insulating cover. 

The preceding remarks are only made to aid the experimenter 
in the first trials, for the difficulties which he encounters he may 
soon find means to overcome in his own way. 

To illustrate the effect of the screen, and the advantage of 
using it, I have here two bulbs of the same size, with their stems, 
leading-in wires and incandescent lamp filaments tied to the latter, 
as nearly alike as possible. The stem of one bulb is provided 
with an aluminum tube, the stem of the other has none. Origi- 
nally 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 impor- 
tance of the screening. The metal tube placed on the stem contain- 
ing 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, prevent- 
ing the bombardment, and consequently intense heating and 
possible deterioration of the slender support of the refractory in- 
candescent body, or of the glass stem containing the leading-in 
wire. I say slender support, for it is evident that in order to 
confine the heat more completely to the incandescing body its sup- 
port 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 high- 
est degree of heat. 

The effectiveness of the metal tube as an electrostatic screen 
depend? largely on the degree of exhaustion. 



242, INVENTIONS OF NIKOLA TESLA. 

At excessively high degrees of exhaustion which are reached 
by using great care and special means in connection with the 
Sprengel pump when the matter in the globe is in the ultra- 
radiant state, it acts most perfectly. The shadow of the upper 
edge of the tube is then sharply defined upon the bulb. 

At a somewhat lower degree of exhaustion, which is about the 
ordinary "non-striking" vacuum, and generally as long as the 
matter moves predominantly in straight Hues, 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 fre- 
quency. In such case the discharge may pass with great freedom 
through the rarefied gas through which a low frequency dis- 
charge may not pass, even though the potential be much higher. 
At ordinary atmospheric pressures just the reverse rule holds 
good : the higher the frequency, the less the spark discharge is 
able to jump between the terminals, especially if they are knobs 
or spheres of some size. 

Finally, at very low degrees of exhaustion, when the gas is well 
conducting, the metal tube not only does not act as an electro- 
static screen, but even is a drawback, aiding to a considerable 
extent the dissipation of the energy laterally from the leading-in 
wire. This, of course, is to be expected. In this case, namely, 
the metal tube is in good electrical connection with the leading- 
in wire, and most of the bombardment is directed upon the tube. 
As long as the electrical connection is not good, the conducting 
tube is always of some advantage, for although it may not greatly 
economize energy, still it protects the support of the refractory 
button, and is the means of concentrating more energy upon the 
same. 

To whatever extent the aluminum tube performs the function 
of a screen, its usefulness is therefore limited to very high de- 
grees of exhaustion when it is insulated from the electrode that 
is, when the gas as a whole is non-conducting, and the molecu- 
les, or atoms, act as independent carriers of electric charges. 

In addition to acting as a more or less effective screen, in the 
true meaning of the word, the conducting tube or coating may 
also act, by reason of its conductivity, as a sort of equalizer or 
dampener of the bombardment against the stem. To be explicit, 
I assume the action to be as follows: Suppose a rhythmical bom- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 243 

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 insu- 
lator ; 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 





FIG. 147. FIG. 148. 

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 con- 
ducting, neither of the above effects can occur, and, on the other 
hand, the fewer the atoms, with the greater freedom they move ; 
in other words, the higher the degree of exhaustion, up to a 
limit, the more telling will be both the effects. 

What I have just said may afford an explanation of the phe- 
nomenon observed by Prof. Crookes, namely, that a discharge 
through a bulb is established \vith much greater facility when an 



344 INVENTIONS OF NIKOLA TE8LA. 

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 dis- 
charge to pass through the bulb, a much higher potential is 
needed if a conductor, especially of much surface, be present. 

For the sake of elucidating of some of the remarks before made, 
I must now refer to Figs. 147, 148 and 149, which illustrate 
various arrangements with a type of bulb most generally used. 

Fig. 147 is a section through a spherical bulb L, with the glass 
stem *, contains the leading-in wire ?r, which has a lamp filament 
I fastened to it, serving to support the refractory button m in the 
centre. M is a sheet of thin mica wound in several layer* around 
the stem s, and a is the aluminum tube. 

Fig. 148 illustrates such a bulb in a somewhat more advanced 
stage of perfection. A metallic tube s is fastened by means of 
some cement to the neck of the tube. In the tube is screwed a 
plug P, of insulating material, in the centre of which is fastened 
a metallic terminal t, for the connection to the leading-in wire w. 
This terminal must be well insulated from the metal tube s ; 
therefore, if the cement used is conducting and most generally 
it is sufficiently so the space between the plug P and the neck 
of the bulb should be filled with some good insulating material, 
such as mica powder. 

Fig. 149 shows a bulb made for experimental purposes. In this 
bulb the aluminum tube is provided with an external connection, 
which serves to investigate the eifect of the tube under various 
conditions. It is referred to chiefly to suggest a line of e.xprri- 
ment followed. 

Since the bombardment against the stem containing the lead- 
ing-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 verv 
thick insulation of glass or other material, and by making the 
wire passing through the rarefied gas as short as practicable. To 
combine these features I employ a large tube T (Fig. 150), which 
protrudes into the bulb to some distance, and carries on the top a 
very short glass stem , into which is sealed the leading-in wire 
w, and I protect the top of the glass stem against the heat by a 
small aluminum tube a and a layer of mica underneath the same, 
as usual. The wire u\ passing through the large tube to the 
outside of the bulb, should be well insulated with a s;lass tube, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 245 

for instance and the space between ought to be filled out with 
some excellent insulator. Among many insulating powders I 
have found that mica powder is the best to employ. If this pre- 
caution is not taken, the tube T, protruding into the bulb, will 
surely be cracked in consequence of the heating by the brushes 
which are apt to form in the upper part of the tube, near the ex- 
hausted globe, especially if the vacuum be excellent, and therefore 
the potential necessary to operate the lamp be very high. 

Fig. 151 illustrates a similar arrangement, with a large tube T 
protruding into the part of the bulb containing the refractory 
button -ni. In this case the wire leading from the outside into 
the bulb is omitted, the energy required being supplied through 





FIG. 149. 



FIG. 150. 



condenser coatings c o. The insulating packing p should in 
this construction be tightly litting to the glass, and rather wide, 
or otherwise the discharge might avoid passing through the wire 
ie, which connects the inside condenser coating to the incandes- 
cent button ///. 

The molecular bombardment against the glass stem in the bulb 
is a source of great trouble. As an illustration I will cite a phe- 
nomenon 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 plati- 
num wire sealed in the glass stem. The bulb may be exhausted 
to a fairly high degree, nearly to the point when phosphorescence 



046 INVENTIONS OF NIKOLA TE8LA. 

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 dis- 
charge 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 rare- 
fied 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 phenome- 
non must appear very curious, and shows in a striking manner 
how radically different alternate currents, or impulses, of high 
frequency behave, as compared with steady currents, or currents 
of low frequency. With such currents namely, the latter the 
phenomenon would of course not occur. When frequencies such 
as are obtained by mechanical means are used, I think that the rup- 
ture of the glass is more or less the consequence of the bombard, 
ment, 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 surround- 
ing the wire. The case is thus analogous to that in which a con- 
denser shunted by a conductor of low resistance is connected to 
a source of alternating current. As long as the frequencies are 
low, the conductor gets the most and the condenser is perfectly 
safe ; but when the frequency becomes excessive, the role of the 
conductor may become quite insignificant. In the latter case the 
difference of potential at the terminals of the condenser may be- 
come so great as to rupture the dielectric, notwithstanding the 
fact that the terminals are joined by a conductor of low resis 
tance. 

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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 247 



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. 

A different arrangement used in some of the bulbs constructed 
is illustrated in Fig. 152. In this instance a non-conductor ra is 
mounted in a piece of common arc light carbon so as to project 
some small distance above the latter. The carbon piece is con- 
nected to the leading-ill wire passing through a glass stem, which 





FIG. 151. 



FIG. 152. 



is wrapped with several layers of mica. An aluminum tube a is 
employed as usual for screening. It is so arranged that it reaches 
very nearly as high as the carbon and only the non-conductor m 
projects a little above it. The bombardment goes at first against 
the upper surface of carbon, the lower parts being protected by 
the aluminum tube. As soon, however, as the non-conductor m 
is heated it is rendered good conducting, and then it becomes the 
centre of the bombardment, being most exposed to the same. 

I have also constructed during these experiments many such 
single-wire bulbs with or without internal electrode, in which the 
radiant matter was projected against, or focused upon, the body 



248 INVENTIONS OF NIKOLA TE8LA. 

to be rendered incandescent. Fig. 153 (page 263) illustrates one 
of the bulbs used. It consists of a spherical globe L, provided 
with a long neck n, on top, for increasing the action in some cases 
by the application of an external conducting coating. The globe L 
is blown out on the bottom into a very small bulb Z>, which serves 
to hold it firmly in a socket s of insulating material into which it 
is cemented. A fine lamp filament f, supported on a wire w, 
passes through the centre of the globe L. The filament is ren- 
dered incandescent in the middle portion, where the bombard- 
ment proceeding from the lower inside surface of the globe is 
most intense. The lower portion of the globe, as far as the 
socket s reaches, is rendered conducting, either by a tinfoil coat- 
ing or otherwise, and the external electrode is connected to a 
terminal of the coil. 

The arrangement diagrammatically indicated in Fig. 153 was 
found to be an inferior one when it was desired to render incan- 
descent a filament or button supported in the centre of the globe, 
but it was convenient when the object was to excite phosphor- 
escence. 

In many experiments in which bodies of different kind were 
mounted in the bulb as, for instance, indicated in Fig. 152, some 
observations of interest were made. 

It was found, among other things, that in such cases, no mat- 
ter where the bombardment began, just as soon as a high tem- 
perature was reached there was generally one of the bodies 
which seemed to take most of the bombardment upon itself, the 
other, or others, being thereby relieved. The quality appeared 
to depend principally on the point of fusion, and on the facility 
with which the body was " evaporated," or, generally speaking, 
disintegrated meaning by the latter term not only the throwing 
off of atoms, but likewise of large lumps. The observation made 
was in accordance with generally accepted notions. In a highly 
exhausted bulb, electricity is carried off from the electrode by 
independent carriers, which are partly the atoms, or molecules, 
of the residual atmosphere, and partly the atoms, molecules, or 
lumps thrown off from the electrode. If the electrode is com- 
posed of bodies of different character, and if one of these is more 
easily disentegrated than the other, most of the electricity sup- 
plied 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 dis- 
intregrated. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 249 

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 ap- 
pears uniformly brilliant, but there are upon it points constantly 
shifting and wandering around, of a temperature far above the 
mean, and this materially hastens the process of deterioration. 
That some such thing occurs, at least when the electrode is at a lower 
temperature, sufficient experimental evidence can be obtained in 
the following manner : Exhaust a bulb to a very high degree, so 
that with a fairly high potential the discharge cannot pass that 
is, not a luminous one, for a weak invisible discharge occurs 
always, in all probability. Now raise slowly and carefully the 
potential, leaving the primary current on no more than for an 
instant. At a certain point, two, three, or half a dozen phos- 
phorescent 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, necessi- 
tated, of course, by sharp projections, or, generally speaking, ir- 
regularities of the electrode. But the luminous patches are 
constantly changing in position, which is especially well observ- 
able if one manages to produce very few, and this indicates that 
the configuration of the electrode is rapidly changing. 

From experiences of this kind I am led to infer that, in order 
to be most durable, the refractory button in the bulb should be 
in the form of a sphere with a highly polished surface. Such a 
small sphere could be manufactured from a diamond or some 
other crystal, but a better way would be to fuse, by the employ- 
ment of extreme degrees of temperature, some oxide as, fo 
instance, zirconia into a small drop, and then keep it in the 
bulb at a temperature somewhat below its point of fusion. 

Interesting and useful results can, no doubt, be reached in the 



250 INVENTIONS OF NIKOLA TESLA. 

direction of extreme degrees of heat. How can such high tem- 
peratures he arrived at ? How are the highest degrees of heat 
readied 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. Leav- 
ing 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 be- 
fore it can fuse and form a drop. This difficulty exists princip- 
ally with an oxide, such as zirconia, because it cannot be com- 
pressed in so hard a cake that it would not be carried off quickly. 
I have endeavored repeatedly to fuse zirconia, placing it in a cup of 
arc light carbon, as indicated in Fig. 152. It glowed with a most 
intense light, and the stream of the particles projected out of the 
carbon cup was of a vivid white ; but whether it was compressed 
in a cake or made into a paste with carbon, it was carried off 
before it could be fused. The carbon cup, containing zirconia, 
had to be mounted very low in the neck of a large bulb, as the 
heating of the glass by the projected particles of the oxide was 
so rapid that in the first trial the bulb was cracked almost in an 
instant, when the current was turned on. The heating of the 
glass by the projected particles was found to be always greater 
when the carbon cup contained a body which was rapidly carried 
off I presume, because in such cases, with the same potential, 
higher speeds were reached, and also because, per unit of time, 
more matter was projected that is, more particles would strike 
the glass. 

The before-mentioned difficulty did not exist, however, when 
the body mounted in the carbon cup offered great resistance to 
deterioration. For instance, when an oxide was first fused in 
an oxygen blast, and then mounted in the bulb, it melted very 
readily into a drop. 

Generally, during the process of fusion, magnificent light 
effects were noted, of which it would be difficult to give an ade- 
quate idea. Fig. 152 is intended to illustrate the effect observed 
with a ruby drop. At first one may see a narrow funnel of 



man FREQUENCY AND HTGH POTENTIAL CURRENTS 251 

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 power- 
ful ; 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, , correspond- 
ing to the outline of the drop, is produced, which spreads slowly 
over the globe as the drop gets larger. When the mass begins 
to boil, small bubbles and cavities are formed, which cause dark 
colored spots to sweep across the globe. The bulb may be 
turned downward without fear of the drop falling off, as the 
mass possesses considerable viscosity. 

I may mention here another feature of some interest, which 
I believe to have noted in the course of these experiments, 
though the observations do not amount to a certitude. It ap- 
peared that under the molecular impact caused by the rapidly 
alternating potential, the body was fused and maintained in that 
state at a lower temperature in a highly exhausted bulb than 
was the case at normal pressure and application of heat in the 
ordinary way that is, at least, judging from the quantity of the 
light emitted. One of the experiments performed may be men- 
tioned 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 thick- 
ness, 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 observ- 
er's possible, and even probable, error, the question is, can a body 
under these conditions be brought from a solid to a liquid state 
with the evolution of less light ? 

When the potential of a body is rapidly alternated, it is certain 



252 . INVENTIONS OF NIKOLA TESLA. 

that the structure is jarred. When the potential is very high, 
although the vibrations may be few say 20,000 per second the 
effect upon the structure may be considerable. Suppose, for ex- 
ample, that a ruby is melted into a drop by a steady application" 
of energy. When it forms a drop, it will emit visible and in- 
visible 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 emit- 
ted from it three different kinds of vibrations the ordinary 
visible, and two kinds of invisible waves : that is, the ordinary 
dark waves of all lengths, and, in addition, waves of a well de- 
fined 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 cur- 
rents of extreme frequency as it does with steady currents, 
assuming that it be worked at the same luminous intensity. This 
means that for rapidly alternating currents the filament should 
be shorter and thicker. The higher the frequency that is, the 
greater the departure from the steady flow the worse it would 
be for the filament. But if the truth of this remark were de- 
monstrated, it would be erroneous to conclude that such a refrac- 
tory 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. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 253 

In the course of these experiments a great many trials were 
made with all kinds of carbon buttons. Electrodes made of or- 
dinary 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 poten- 
tials 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 obser- 
vation is rendered very difficult on account of the intense heat 
produced. Nevertheless there can be no doubt that all kinds of 
carbon are fused under the molecular bombardment, but the 
liquid state must be one of great instability. Of all the bodies 
tried there were two which withstood best diamond and car- 
borundum. 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 Avill venture to call your 
attention to it. 

It has been recently produced by Mr. E. G. Acheson, of 
Monongahela City, Pa., II. S. A. It is intended to replace ordi- 
nary diamond powder for polishing precious stones, etc., and I 
have been informed that it accomplishes this object quite suc- 
cessfully. I do not know why the name " carborundum " has 
been given to it, unless there is something in the process of its 
manufacture which justifies this selection. Through the kindness 
of the inventor, I obtained a short while ago some samples which 
I desired to test in regard to their qualities of phosphorescence 
and capability of withstanding high degrees of heat. 

Carborundum can be obtained in two forms in the form of 
"crystals" and of powder. The former appear to the naked eye 
dark colored, but are very brilliant ; the latter is of nearly the 
same color as ordinary diamond powder, but very much finer. 
When viewed under a microscope the samples of crystals given 
to me did not appear to have any definite form, but rather re- 
sembled pieces of broken up egg coal of fine quality. The 
majority were opaque, but there were some which were trans- 
parent 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 



254 INVENTIONS OF NIKOLA TESLA. 

against them they at first form a cake of some compactness, prob- 
ably 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, w r hich, 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-con- 
ducting. It affords a magnificent polishing material for stones. 

The time has been too short to make a satisfactory study of 
the properties of this product, but enough experience has been 
gained in a few weeks I have experimented upon it to say that 
it does possess some remarkable properties in many respects. It 
withstands excessively high degrees of heat, it is little deteriorated 
by molecular bombardment, and it does not blacken the globe as 
ordinary carbon does. The only difficulty which I have experienced 
in its use in connection with these experiments was to find some 
binding material which would resist the heat and the effect of the 
bombardment as successfully as carborundum itself does. 

I have here a number of bulbs which I have provided with 
buttons of carborundum. To make such a button of carborun- 
dum crystals I proceed in the following manner: I take an or- 
dinary lamp filament and dip its point in tar, or some other 
thick substance or paint which may be readily carbonized. I 
next pass the point of the filament through the crystals, and then 
hold it vertically over a hot plate. The tar softens and forms a 
drop on the point of the filament, the crystals adhering to the 
surface of the drop. By regulating the distance from the plate 
the tar is slowly dried out and the button becomes solid. I then 
once more dip the button in tar and hold it again over a plate 
until the tar is evaporated, leaving only a hard mass which firmly 
binds the crystals. When a larger button is required I repeat 
the process several times, and I generally also cover the filament 
a certain distance below the button with crystals. The button 
being mounted in a bulb, when a good vacuum has been reached, 
first a weak and then a strong discharge is passed through the 
bulb to carbonize the tar and expel all gases, and later it is brought 
to a very intense incandescence. 

When the powder is used I have found it best to proceed as 
follows : I make a thick paint of carborundum and tar, and pass 
a lamp filament through the paint. Taking then most of the 



UIGU FREQUENCY AND HIGH POTENTIAL CURRENTS. 255 

paint off by rubbing the filament against a piece of chamois 
leather, I hold it over a hot plate until the tar evaporates and the 
coating becomes firm. I repeat this process as many times as it 
is necessary to obtain a certain thickness of coating. On the 
point of the coated filament I form a button in the same 
manner. 

There is no doubt that such a button properly prepared under 
great pressure of carborundum, especially of powder of the best 
quality, will withstand the effect of the bombardment fully as 
well as anything we know. The difficulty is that the binding 
material gives way, and the carborundum is slowly thrown off 
after some time. As it does not seem to blacken the globe in the 
least, it might be found useful for coating the filaments of ordinary 
incandescent lamps, and I think that it is even possible to produce 
thin threads or sticks of carborundum which will replace the or- 
dinary filaments in an incandescent lamp. A carborundum coat- 
ing 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 car- 
borundum, but the binding paste gave way much more quickly 
in the diamond buttons ; this, however, I attributed to the size 
and irregularity of the grains of the diamond. 

It was of interest to find whether carborundum possesses the 
quality of phosphorescence. One is, of course, prepared to en- 
counter two difficulties : first, as regards the rough product, the 
"crystals," they are good conducting, and it is a fact that con- 
ductors do not phosphoresce; second, the powder, being exceed- 
ingly 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 phos- 
phorescence to a considerable degree. 

The question presents itself here, can a conductor phosphor- 
esce ? 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. 



256 INVENTIONS OF NIKOLA TESLA. 

Then, if a metal be in a large measure, or perhaps entirely, de- 
prived of that property, it should be capable of phosphoresence. 
Therefore it is quite possible that at some extremely high fre- 
quency, when behaving practically as a non-conductor, a metal 
or any other conductor might exhibit the quality of phosphores- 
ence, even though it be entirely incapable of phosphorescing 
under the impact of a low-frequency discharge. There is, how- 
ever, another possible way how a conductor might at least appear 
to phosphoresce. 

Considerable doubt still exists as to what really is phosphor- 
escence, 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 continu- 
ous impression upon his retina. To him then the surface of the 
metal would appear continuously incandescent and of constant 
luminous intensity, while in reality the light would be either 
intermittent, or at least changing periodically in intensity. The 
metal piece would rise in temperature until equilibrium was 
attained that is, until the energy continuously radiated would 
equal that intermittently supplied. But the supplied energy 
might under such conditions not be sufficient to bring the body 
to any more than a very moderate mean temperature, especially 
if the frequency of the atomic impacts be very low just enough 
that the fluctuation of the intensity of the light emitted could 
not be detected by the eye. The body would now, owing to the 
manner in which the energy is supplied, emit a strong light, and 
yet be at a comparatively very low mean temperature. How 
should the observer name the luminosity thus produced ? Even if 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 257 

the analysis of the light would teach him something definite, still 
he would probably rank it under the phenomena of phosphor- 
escence. It is conceivable that in such a way both conducting 
and non-conducting bodies may be maintained at a certain lumin- 
ous intensity, but the energy required would very greatly vary 
with the nature and properties of the bodies. 

These and some foregoing remarks of a speculative nature 
were made merely to bring out curious features of alternate 
currents or electric impulses. By their help we may cause a body 
to emit more light, while at a certain mean temperature, than it 
would emit if brought to that temperature by a steady supply ; 
and, again, we may bring a body to the point of fusion, and cause 
it to emit less light than when fused by the application of energy 
in ordinary ways. It all depends on how we supply the energy, 
and what kind of vibrations we set up ; in one case the vibrations 
are more, in the other less, adapted to affect our sense of vision. 

Some effects, which I had not observed before, obtained with 
carborundum in the first trials, I attributed to phosphorescence, 
but in subsequent experiments it appeared that it was devoid of 
that quality. The crystals possess a noteworthy feature. In a 
bulb provided with a single electrode in the shape of a small 
circular metal disc, for instance, at a certain degree of exhaustion 
the electrode is covered with a milky, film, which is separated by 
a dark space from the glow filling the bulb. When the metal 
disc is covered with carborundum crystals, the film is far more 
intense, and snow-white. This I found later to be merely an 
effect of the bright surface of the crystals, for when an aluminum 
electrode was highly polished, it exhibited more or less the same 
phenomenon. I made a number of experiments with the samples 
of crystals obtained, principally because it would have been of 
special interest to find that they are capable of phosphorescence, 
on account of their being conducting. I could not produce phos- 
phorescence distinctly, but I must remark that a decisive opinion 
cannot be formed until other experimenters have gone over the 
same ground. 

The powder behaved in some experiments as though it con- 
tained alumina, but it did not exhibit with sufficient distinctness 
the red of the latter. Its dead color brightens considerably un- 
der 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 



258 INVENTIONS OF NIKOLA TESLA. 

phosphorescent sulphide, for example, which could be finely 
powdered without impairing the phosphorescence, but rather like 
powdered ruby or diamond, and therefore it would be necessary, 
in order to make a decisive test, to obtain it in a large lump and 
polish up the surface. 

If the carborundum proves useful in connection with these 
and similar experiments, its chief value will be found in the 
production of coatings, thin conductors, buttons, or other elec- 
trodes capable of withstanding extremely high degrees of heat. 

The production of a sniall electrode, capable of wit hstan< lino- 
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 misunder- 
stood, I think it is necessary to expose clearly the problem with 
which, in this line of work, we are confronted, and the manner 
in which, in my opinion, a solution will be arrived at. 

Any one who begins a study of the problem will be apt to 
think that what is wanted in a lamp with an electrode is a very 
high degree of incandescence of the electrode. There he will be 
mistaken. The high incandescence of the button is a necessary 
evil, but what is really wanted is the high incandescence of the 
gas surrounding the button. In other words, the problem in 
such a lamp is to bring a mass of gas to the highest, possible in- 
candescence. 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 incande- 
scent mass away from the glass ; that is, to confine it as much as 
possible to the central portion of the globe. 

In one of the experiments this evening a brush was produced 
at the end of a wire. The brush was a flame, a source of heat 
and light. It did not emit much perceptible heat, nor did it 
glow with an intense light ; but is it the less a flame because it 
does not scorch my hand { Is it the less a flame because it does 
not hurt my eyes by its brilliancy ? The problem is precisely to 
produce in the bulb such a flame, much smaller in size, but in- 
comparably more powerful. Were there means at hand for 



HIGH FREQVENCF AND HIGH POTENTIAL CURRENTS. 259 

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 or- 
dinary air pressure. In the bulb we are able to intensify the 
action to any degree so far that the brush emits a powerful 
light. 

The intensity of the light emitted depends principally on the 
frequency and potential of the impulses, and on the electric den- 
sity on the surface of the electrode. It is of the greatest impor- 
tance 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, carborun- 
dum or zirconia button the photosphere can be as much as one 
thousand times the volume of the button. Without much re- 
flection one would tl link that in pushing so far the incandescence 
of the electrode it would be instantly volatilized. But after a 
careful consideration one would find that, theoretically, it should 
not occur, and in this fact which, moreover, is experimentally 
demonstrated lies principally the future value of such a lamp. 

At first, when the bombardment begins, most of the work is 
performed on the surface of the button, but when a highly con- 
ducting photosphere is formed the button is comparatively re- 
lieved. 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 prob- 
ably at a temperature too high for most of the refractory bodies 



360 INVENTIONS OF NIKOLA TEFL A. 

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 the life of the lamp 
diminishes so rapidly with the increase of the degree of incan- 
descence that economical reasons compel us to work it at a low 
incandescence. But if an incandescent lamp is operated with 
currents of very high frequency, the action of the gas cannot be 
neglected, and the rules for the most economical working must 
be considerably modified. 

In order to bring such a lamp with one or two electrodes to a 
great perfection, it is necessary to employ impulses of very high 
frequency. The high frequency secures, among others, two chief 
advantages, which have a most important bearing upon the 
economy of the light production. First, the deterioration of the 
electrode is reduced by reason of the fact that we employ a great 
many small impacts, instead of a few violent ones, which quickly 
shatter the structure ; secondly, the formation of a large photo- 
shere is facilitated. 

In order to reduce the deterioration of the electrode to the 
minimum, it is desirable that the vibration be harmonic, for any 
suddenness hastens the process of destruction. An electrode lasts 
much longer when kept at incandescence by currents, or impulses, 
obtained from a high frequency alternator, which rise and fall 
more or less harmonically, than by impulses obtained from a dis- 
ruptive discharge coil. In the latter case there is no doubt that 
most of the damage is done by the fundamental sudden dis- 
charges. 

One of the elements of loss in such a lamp is the bombard- 
ment of the globe. As the potential is very high, the molecules 
are pro jected with great speed ; they strike the glass, and usually ex- 
cite a strong phosphorescence. The effect produced is very pretty , 
but for economical reasons it would be perhaps preferable to pre- 
vent, 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 \ cry high frecjuen- 






HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 261 

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 conduct- 
ting 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 elec- 
trodes 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 elec- 
trodes might glow more intensely than the other. 

But on the whole I find it preferable, when using such high 
frequencies, to employ only one electrode and one connecting 
wire. I am convinced that the illuminating device of the near 
future will not require for its operation more than one lead, and, 
at any rate, it will have no leading-in wire, since the energy re- 
quired can be as well transmitted through the glass. In experi- 
mental 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 
titting 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 illustrat- 
ing devices will necessarilly involve the use of very high 
potentials, and this, in the eyes of practical men, might be an ob- 
jectionable feature. Yet, in reality, high potentials are not 
objectionable certainly not in the least so far as the safety of 
the devices is concerned. 

There are two ways of rendering an electric appliance safe. 
One is to use low potentials, the other is to determine the dimen- 
sions 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 



362 INVENTIONS OF NIKOLA TESLA. 

way, for then the safety is absolute, unaffected by any possible 
combination of circumstances which might render even alow-poten- 
tial 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 em- 
ployment 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 phos- 
phorescent 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 ineffi- 
cient; 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-pitcli note by means of a bell. 
It should be said a barely audible note ; and even these words 
xvould not express it, so wonderful is the sensitiveness of the eye. 
We may deliver powerful blows at long intervals, waste a good 
deal of energy, and still not get what we want ; or we may keep 
up the note by delivering frequent taps, and get nearer to the 
object sought by the expenditure of much less energy. In the 
production of light, as far as the illuminating device is concerned, 
there can be only one rule that is, to use as high frequencies as 
can be obtained ; but the means for the production and convey- 
ance of impulses of such character impose, at present at least, 
great limitations. Once it is decided to use very high frequen- 
cies, the return wire becomes unnecessary, and all the appliances 
are simplified. By the use of obvious means the same result is 
obtained as though the return wire were used. It is sufficient for 
this purpose to bring in contact with the bulb, or merely in the 
vicinity of the same, an insulated body of some surface. The 
surface need, of course, be the smaller, the higher the frequency 
and potential used, and necessarily, also, the higher the economy 
of the lamp or other device. 

This plan of working has been resorted to on several occasions 
this evening. So, for instance, when the incandescence of a 
button was produced by grasping the bulb with the hand, the 
body of the experimenter merely served to intensify the action. 
The bulb used was similar to that illustrated in Fig. 148, and 



HIGH FREQ UENCY AND HIGH POTENTIAL CURRENTS. 263 



tlie coil was excited to a small potential, not sufficient to bring 
the button to incandescence when the bull) was hanging from 
the Avire ; 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. 





FIG. 153. 



FIG. 154. 



In another experiment, when phosphorescence was excited, a 
similar bulb was used. Here again, originally, the potential was 
not sufficient to excite phosphorescence until the action was in- 
tensified in this case, however, to present a different feature, by 
touching the socket with a metallic object held in the hand. The 
electrode in the bulb was a carbon button so large that it could 
not be brought to incandescence, and thereby spoil the effect 
produced by phosphorescence. 

Again, in another of the early experiments, a bulb was used, 



264 INVENTIONS OF NIKOLA TESLA. 

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 elec- 
trode under ordinary circumstances. 

In all these experiments the action was intensified by augment- 
ing the capacity at the end of the lead connected to the terminal. 
As a rule, it is not necessary to resort to such means, and would 
be quite unnecessary with still higher frequencies ; but when it 
Is desired, the bulb, or tube, can be easily adapted to the pur- 
pose. 

In Fig. 153, for example, an experimental bull), i,, is shown, 
which is provided with a neck, n, on the top, for the application 
of an external tinfoil coating, which may be connected to a body 
of larger surface. Such a lamp as illustrated in Fig. 154 may 
also be lighted by connecting the tinfoil coating on the neck n, 
to the terminal, and the leading-in wire, w, to an insulated plate. 
If the bulb stands in a socket upright, as shown in the cut, a 
shade of conducting material may be slipped in the neck, n, and 
the action thus magnified. 

A more perfected arrangement used in some of these bulbs is 
illustrated in Fig. 155. In this case the construction of the bulb 
is as shown and described before, when reference was made to 
Fig. 148. A zinc sheet, z, with a tubular extension, T, is applied 
over the metallic socket, s. The bulb hangs downward from the 
terminal, t, the zinc sheet, z, performing the double office of in- 
tensifier and reflector. The reflector is separated from the ter- 
minal, t, by an extension of the insulating plug, P. 

A similar disposition with a phosphorescent tube is illustrated 
in Fig. 156. The tube, T, is prepared from two short tubes of 
different diameter, which are sealed on the ends. Oil the lower 
end is placed an inside conducting coating, c, which connects to 
the wire w. The wire has a hook on the upper end for suspen- 
sion, and passes through the centre of the inside tube, which is 
filled witli some good and tightly packed insulator. On the out- 
side of the upper end of the tube, T, is another conducting coat- 
ing, o l} upon which is slipped a metallic reflector z, which should 
be separated by a thick insulation from the end of wire u\ 

The economical use of such a reflector or intensifier would re- 
quire that all energy supplied to an air condenser should be re- 
coverable, or, in other words, that there should not be any losses, 



HIGH FREq UENGY AND HIGIt POTENTIAL CURRENTS. 265 

neither in the gaseous medium nor through its action elsewhere. 
This is far from being so, but, fortunately, the losses may be re- 
duced to anything desired. A few remarks are necessary on 
this subject, in order to make the experiences gathered in the 
course of these investigations perfectly clear. 

Suppose a small helix with many well insulated turns, as in 
experiment Fig. 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 con- 
nected 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 




FIG. 155. 

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- 



266 INVENTIONS OF NIKOLA TE8LA. 

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 con- 
clusion. 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 mole- 
cules, after coming in contact with the sphere, lose the imparted 




FIG. 156. 

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 im- 
pulses be very small, the loss caused by the impacts and collisions 
would not be serious, unless the potential \vere 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 267 

to the molecule is proportionate to that density. I conclude from 
this that the total energy lost must be proportionate to the pro- 
duct of the frequency and the square of the electric density ; but 
this law needs experimental confirmation. Assuming the pre- 
ceding- considerations to be true, then, by rapidly alternating the 
potential of a body immersed in an insulating gaseous medium, 
any amount of energy may be dissipated into space. Most of 
that energy then, I believe, is not dissipated in the form of long 
ether waves, propagated to considerable distance, as is thought 
most generally, but is consumed in the case of an insulated 
sphere, for example in impact and collisional losses that is, 
heat vibrations on the surface and in the vicinity of the sphere. 
To reduce the dissipation, it is necessary to work with a small 
electric density the smaller, the higher the frequency. 

But since, on the assumption before made, the loss is dimin- 
ished 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 de- 
vices of any kind are perfected, capable of being advantageously 
operated by currents of extremely high frequency, economical 
reasons will make it advisable to use only one wire, especially if 
the distances are great. 

When energy is absorbed in a condenser, the same behaves as 
though its capacity were increased. Absorption always exists 
more or less, but generally it is small and of no consequence us 
long as the frequencies are not very great, In using extremely 
high frequencies, and, necessarily in such case, also high poten- 
tials, the absorption or, what is here meant more particularly 
by this term, the loss of energy due to the presence of "a gaseous 
medium is an important factor to be considered, as the energy 
absorbed in the air condenser may be any fraction of the supplied 
energy. This would seem to make it very difficult to tell from 
the measured or computed capacity of an air condenser its actual 
capacity or vibration period, especially if the condenser is of very 
small surface and is charged to a very high potential. As many 
important results are dependent upon the correctness of the 
estimation of the vibration period, this subject demands the most 
careful scrutiny of other investigators. To reduce the probable 
error as much as possible in experiments of the kind alluded to, 
it is advisable to use spheres or plates of large surface, so as to 



268 INVENTIONS OF NIKOLA TEFL A. 

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 tin- 
foil 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 inappre- 
ciable. 

It should not be thought that the loss in heat in an air con- 
denser is necessarily associated with the formation of /-/VJ/r 
streams or brushes. If a small electrode, inclosed in an un- 
exhausted 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. 

j^or should it be thought that the temperature of an air con- 
denser would give even an approximate idea of the loss in heat 
incurred, as in such case heat must be given off much more 
quickly, since there is, in addition to the ordinary radiation, a 
very active carrying away of heat by independent carriers going 
on, and since not only the apparatus, but the air at some distance 
from it is heated in consequence of the collisions which must 
occur. 

Owing to this, in experiments with such a coil, a rise of tem- 
perature 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 slight- 
est danger. 

A question of some interest, principally to meteorologists, 



HIGH FEEQUENCY AND HIGH POTENTIAL CURRENTS. 269 

presents itself here. How does the earth behave ? The earth is 
an air condenser, but is it a perfect or a very imperfect one a 
mere sink of energy ? There can be little doubt that to such 
small disturbance as might be caused in an experiment, the earth 
behaves as an almost perfect condenser. But it might be differ- 
ent when its charge is set in yibration by some sudden disturb- 
ance 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 vibra- 
tions of the charge and the potential are in all probability exces- 
sive, 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 varia- 
tions in the temperature and pressure of the atmosphere may in 
this manner be caused at any point of the surface of the earth. 
The variations may be gradual or very sudden, according to the 
nature of the general disturbance, and may produce rain and 
storms, or locally modify the weather in any way. 

From the remarks before made, one may see what an import- 
ant factor of loss the air in the neighborhood of a charged surface 
becomes when the electric density is great and the frequency of 
the impulses excessive. But the action, as explained, implies 
that the air is insulating that is, that it is composed of independ- 
ent 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 rareiied and 
conducting, then true conduction losses occur also. In such case, 
of course, considerable energy may be dissipated into space even 
with a steady potential, or with impulses of low frequency, if the 
density is very great. 

When the gas is -at very low pressure, an electrode is heated 
more because higher speeds can be reached. If the gas around 
the electrode is strongly compressed, the displacements, and 
consequently the speeds, are very small, and the heating is in- 
signincant. But if in such case the frequency could be suffici- 
ently increased, the electrode would be brought to a high tern- 



270 



INVENTIONS OF NIKOLA TESLA. 



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. 

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 con- 





FIG. 157. 

tained in the globe. This arrangement is illustrated in Fig. 157. 
The globe L has in this case a large neck n, allowing the small 
bulb b to slip through. Otherwise the construction is the same 
as shown in Fig. 147, for example. The small bulb is conveni- 
ently supported upon the stem s, carrying the refractory button 
in. It is separated from the aluminum tube a by several layers 
of mica M, in order to prevent the cracking of the neck by the 
rapid heating of the aluminum tube upon a sudden turning on 
of the current. The inside bulb should be as small as possible 
when it is desired to obtain light only by incandescence of the 
electrode. If it is desired to produce phosphorescence, the bulb 



man FEEQUENCT AND HIGH POTENTIAL CUERENTS. 271 

should be larger, else it would be apt to get too hot, and the 
phosphorescence would cease. In this arrangement usually only 
the small bulb shows phosphorescence, as there is practically no 
bombardment against the outer globe. In some of these bulbs 
constructed as illustrated in Fig. 157, the small tube was coated 
with phosphorescent paint, and beautiful effects were obtained. 
Instead of making the inside bulb large, in order to avoid undue 
heating, it answers the purpose to make the electrode m larger. 
In this case the bombardment is weakened by reason of the 
smaller electric density. 

Many bulbs were constructed on the plan illustrated in Fig. 
158. Here a small bulb Z>, containing the refractory button >//, 
upon being exhausted to a very high degree Avas sealed in a large 
globe L, which w T as 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 *, near the seal at <*, very thick, and the leading-in wire 
// thin, as it occurred sometimes that the stem at e was heated 
and the bulb was cracked. Often the outer globe L was exhausted 
only just enough to allow the discharge to pass through, and the 
space between the bulbs appeared crimson, producing a curious 
effect. In some cases, when the exhaustion in globe L was very 
low, and the air good conducting, it was found necessary, in order 
to bring the button in to high incandescence, to place, preferably 
on the upper part of the neck of the globe, a tinfoil coating which 
was connected to an insulated body, to the ground, or to the 
other terminal of the coil, as the highly conducting air weakened 
the effect somewhat, probably by being acted upon inductively 
from the wire w, where it entered the bulb at e. Another diffi- 
culty which, however, is always present when the refractory 
button is mounted in a very small bulb existed in the construc- 
tion illustrated in Fig. 158, namely, the vacuum in, the bulb 1> 
would be impaired in a comparatively short time. 

The chief idea in the two last described constructions was to 
confine the heat to the central portion of the globe by preventing 
the exchange of air. An advantage is secured, but owing to the 
heating of the inside bulb and slow evaporation of the glass, the 
vacuum is hard to maintain, even if the construction illustrated 
in Fig. 157 be chosen, in which both bulbs communicate. 



272 INVENTIONS OF NIKOLA TE8LA, 

But by far the better way the ideal way would be to reach 
sufficiently high frequencies. The higher the frequency, the 
slower would be the exchange of the air, and I think that a fre- 
quency may be reached, at which there would be no exchange 
whatever of the air molecules around the terminal. We would 
then produce a flame in which there would be no carrying away 
of material, and a queer flame it would be, for it would be rigid ! 
With such high frequencies the inertia of the particles would come 
into play. As the brush, or flame, would gain rigidity in virtue 
of the inertia of the particles, the exchange of the latter would 
be prevented. This would necessarily occur, for, the number of 
impulses being augmented, the potential energy of each would 
diminish, so that finally only atomic vibrations could be set up, 
and the motion of translation through measurable space would 
cease. Thus an ordinary gas burner connected to a source of 
rapidly alternating potential might have its efficiency augmented 
to a certain limit, and this for two reasons because of the addi- 
tional vibration imparted, and because of a slowing down of the 
process of carrying off. But the renewal being rendered difficult, 
a renewal being necessary to maintain the burner, a continued 
increase of the frequency of the impulses, assuming they could 
be transmitted to and impressed upon the flame, would result in 
the " extinction " of the latter, meaning by this term only the 
cessation of the chemical process. 

I think, however, that in the case of an electrode immersed in 
a fluid insulating medium, and surrounded by independent car- 
riers of electric charges, which can be acted upon inductively, a 
sufficient high frequency of the impulses would probably result 
in a gravitation of the gas all around toward the electrode. For 
this it would be only necessary to assume that the independent 
bodies are irregularly shaped ; they would then turn toward the 
electrode their side of the greatest electric density, and this 
would be a position in which the fluid resistance to approach 
.would be smaller than that offered to the receding. 

The general opinion, I do not doubt, is that it is out of the 
question to reach any such frequencies as might assuming some 
of the views before expressed to be true produce any of the re. 
suits 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 fre- 
quencies would be much lower than one is apt to estimate at 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 273 

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 oscil- 
lations of the condenser. We may cause the molecules of the 
gas to collide by the use of alternate electric impulses of high 
frequency, and so we may imitate the process in a flame ; and 
from experiments with frequencies which we are now able to 
obtain, I think that the result is producible with impulses which 
are transmissible through a conductor. 

In connection with thoughts of a similar nature, it appeared to 
me of great interest to demonstrate the rigidity of a vibrating gas- 
eous 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 ordi- 
nary pressure led to no result, but with air moderately rarefied I 
obtain what I think to be an unmistakable experimental evidence 
of the property sought for. As a result of this kind might lead 
able investigators to conclusions of importance, I will describe 
one of the experiments performed. 

It is well known that when a tube is slightly exhausted, the 
discharge may be passed through it in the form of a thin lumin- 
ous 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 pro- 
duced 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 dis- 
charge 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 con- 
nected to the terminal, the tinfoil coating being connected to the 
wire, and to the lower coating sometimes a small insulated plate 



274 INVENTIONS OF NIKOLA TESLA. 

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 gen- 
erally 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 dis- 
charges. A luminous band excited in the bulb by repeated dis- 
charges 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 obser- 
vation in such a case is rendered extremely difficult on account 
of the fundamental vibration. 

The demonstration of the fact which still needs better ex- 
perimental confirmation that a vibrating gaseous column pos- 
sesses 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 be- 
liave under the influence of enormous electrostatic stresses which 
may be active in the interstellar space, and which may alternate 



HTGH FREQUKNCY AND HIGH POTENTIAL CURRENTS. 275 

with inconceivable rapidity ? The existence of such an electro- 
static, rhythmically throbbing force of a vibrating electrostatic 
field would show a possible way how solids might have formed 
from the ultra-gaseous uterus, and how transverse and all kinds 
of vibrations may be transmitted through a gaseous medium fill- 
ing 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 may be considerable, but the density is exceed- 
ingly small ; in a liquid the speed would be likely to be small, 
though the density may be considerable ; and in both cases the 
inertia resistance offered to displacement is practically nil. But 
place a gaseous (or liquid) column in an intense,rapidly alternating 
electrostatic field, set the particles vibrating with enormous 
speeds, then the inertia resistance asserts itself. A body might 
move with more or less freedom through the vibrating mass, but 
as a whole it would be rigid. 

There is a subject which I must mention in connection with 
these experiments : it is that of high vacua. This is a subject, 
the study of which is not only interesting, but useful, for it may 
lead to results of great practical importance. In commercial ap-' 
paratus, 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 con- 
cerned ; 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 ex- 
haustion 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 inter- 
esting 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 inves- 
tigations is greatly extended. Think we as we may, of all the 
possible directions to develop a practical illnminant, the line of 



276 INVENTIONS OF NIKOLA TESLA. 

high vacua seems to be the most promising at present. But to 
reach extreme vacua the appliances must be much more improved, 
and ultimate perfection will not be attained until we shall have 
discharged the mechanical and perfected an electrical vacuum 
pump. Molecules and atoms can be thrown out of a bulb under 
the action of an enormous potential : this will be the principle 
of the vacuum pump of the future. For the present, we must 
secure the best results we can with mechanical appliances. In 
this respect, it might not be out of the way to say a few words 
about the method of, and apparatus for, producing excessively 




FIG. 159. 

high degrees of exhaustion of which I have availed myself in the 
course of these investigations. It is very probable that other ex- 
perimenters 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. 

The apparatus is illustrated in a drawing shown in Fig. 159. 
s represents a Sprengel pump, which has been specially con- 
structed to better suit the work required. The stop-cock which 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 277 

is usually employed has been omitted, and instead of it a hollow 
stopper s has been fitted in the neck of the reservoir K. This 
stopper has a small hole A, through which the mercury descends ; 
the size of the outlet o being properly determined with respect 
to the section of the fall tube , which is sealed to the reservoir 
instead of being connected to it in the usual manner. This 
arrangement overcomes the imperfections and troubles which 
often arise from the use of the stopcock on the reservoir and the 
connections of the latter with the fall tube. 

The pump is connected through a (J- sna ped tube t to a very 
large reservoir & lm Especial care was taken in fitting the grind- 
ing surfaces of the stoppers p and p lt and both of these and the 
mercury caps above them were made exceptionally long. After 
the U-shaped tube was fitted and put in place, it was heated, so 
as to soften and take off the strain resulting from imperfect 
fitting. The (J -shaped tube was provided with a stopcock c. 
and two ground connections y and y l one for a small bulb b, 
usually containing caustic potash, and the other for the receiver 
/, to be exhausted. 

The reservoir E b was connected by means of a rubber tube to 
a slightly larger reservoir R^, each of the two reservoirs being 
provided with a stopcock c t and c 2 , respectively. The reservoir 
RJ could be raised and lowered by a wheel and rack, and the 
range of its motion was so determined that when it was filled 
with mercury and the stopcock c 2 closed, so as to form a Torri- 
cellian vacuum in it when raised, it could be lifted so high that 
the reservoir EJ would stand a little above stopcock c^ ; and when 
this stopcock was closed and the reservoir Eg descended, so as to 
form a Torricellian vacuum in reservoir R,, it could be lowered 
so far as to completely empty the latter, the mercury filling the 
reservoir RJ up to a little above stopcock c 2 . 

The capacity of the pump and of the connections was taken 
as small as possible relatively to the volume of reservoir, E l5 
since, of course, the degree of exhaustion depended upon the 
ratio of these quantities. 

With this apparatus I combined the usual means indicated by 
former experiments for the production of very high vacua. In 
most of the experiments it was most convenient to use caustic 
potash. I may venture to say, in regard to its use, that much 
time is saved and a more perfect action of the pump insured by 
fusing and boiling the potash as soon as, or even before, the 



278 INVENTIONS OF NIKOLA TESLA. 

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 hy 
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 couldjbe more rapidly repeated. 
Generally the process of exhaustion was the following : At 
the start, the stop-cocks c and c t being open, and all other con- 
nections closed, the reservoir n> was raised so far that the mer- 
cury filled the reservoir R t and a part of the narrow connecting 
U-shaped tube. When the pump^was set to work, the mercury 
would, of course, quickly rise in the tube, and reservoir RJ was 
lowered, the experimenter keeping ^the mercury at about the 
same level. The reservoir RJ wasj balanced by a long spring 
which facilitated the operation, and the friction of the parts was 
generally sufficient to keep it in almost any position. When the 
Sprengel pump had done its work, the'reservoir R% was further low- 
ered and the mercury descended in R t and tilled R>, whereupon stop- 
cock 02 was closed. The air adhering to the walls of R t and that 
absorbed by the mercury was carried off, and to free the mercury 
of all air the reservoir E 2 was for a long time worked up and 
down. During this process some air, which would gather below 
stopcock c 2 , was expelled from R 2 by lowering it far enough and 
opening the stopcock, closing the latter again before raising the 
reservoir. When all the air had been expelled from the mercury, 
and no air would gather in Rg when it was lowered, the caustic 
potash was resorted to. The reservoir RJ was now again raised 
until the mercury in RJ stood above stopcock Ci. 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 B^ was for 
a long time raised and lowered. In this manner all the moisture 
was carried off from the mercury, and both the reservoirs were 
in proper condition to be used. The reservoir R 2 was then again 
raised to the top, and the pump was kept working for a long 
time. When the highest vacuum obtainable with the pump had 
been reached, the potash bulb was usually wrapped with cotton 
which was sprinkled with ether so as to keep the potash at a 
very low temperature, then the reservoir R 2 was lowered, and upon 
reservoir R t being emptied the receiver was]quickly sealed up. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 279 

When a new bulb was put on, the mercury was always raised 
above stopcock c,, which was closed, so as to always keep the 
mercury and both the reservoirs in fine condition, and the mer- 
cury was never withdrawn from R t except when the pump had 
reached the highest degree of exhaustion. It is necessary to ob- 
serve this rule if it is desired to use the apparatus to advantage. 

By means of this arrangement I was able to proceed very 
quickly, and when the apparatus was in perfect order it was pos- 
sible to reach the phosphorescent stage in a small bulb in less 
than fifteen minutes, which is certainly very quick work for a 
small laboratory arrangement requiring all in all about 100 pounds 
of mercury. With ordinary small bulbs the ratio of the capacity 
of the pump, receiver, and connections, and that of reservoir R 
was about 1 to 20, and the degrees of exhaustion reached were 
necessarily very high, though I am unable to make a precise and 
reliable statement how far the exhaustion was carried. 

What impresses the investigator most in the course of these 
experiences is the behavior of gases when subjected to great^rap- 
idly alternating^ electrostatic stresses. But he must remain in 
doubt as to whether the effects observed are due wholly to the 
molecules, or atoms, of the gas which chemical analysis discloses 
to us, or whether there enters into play another medium of a 
gaseous nature, comprising atoms, or molecules, immersed in a 
fluid pervading the space. Such a medium surely must exist, 
and I am convinced that, for instance, even if air were absent, 
the surface and neighborhood of a body in space would be heated 
by rapidly alternating the potential of the body; but no such 
heating of the surface or neighborhood could occur if all free 
atoms were removed and only a homogeneous, incompressible, and 
elastic fluid such as ether is supposed to be would remain, for 
then there would be no impacts, no collisions. In such a case, 
as far as the body itself is concerned, only f rictional losses in the 
inside could occur. 

It is a striking fact that the discharge through a gas is es- 
tablished 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 



280 INVENTIONS OF NIKOLA TESLA. 

of great length, and no matter how strong the current, self-in- 
duction 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 fre- 
quency 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 real- 
ized : metal pipes filled with gas the metal being the insulator, 
the gas the conductor supplying phosphorescent bulbs, or per- 
haps devices as yet uninvented. It is certainly possible to take 
a hollow core of copper, rarefy the gas in the same, and by pas- 
sing impulses of sufficiently high frequency through a circuit 
around it, bring the gas inside to a high degree of incandescence ; 
but as to the nature of the forces there would be considerable 
uncertainty, for it would be doubtful whether with such impulses 
the copper core would act as a static screen. Such paradoxes and 
apparent impossibilities we encounter at every step in this line of 
work, and therein lies, to a great extent, the charm of the study. 
I have here a short and wide tube which is exhausted to a 
high degree and covered with a substantial coating of bronze, the 
coating barely allowing the light to shine through. A metallic 
cap, with a hook for suspending the tube, is fastened around the 
middle portion of the latter, the clasp being in contact with the 
bronze coating. I now want to light the gas inside by suspend- 
ing the tube on a wire connected to the coil. Any one who 
would try the experiment for the first time, not having any pre- 
vious experience, would probably take care to be quite alone 
when making the trial, for fear that he might become the joke of 
his assistants. Still, the bulb lights in spite of the metal coating, 
and the light can be distinctly perceived through the latter. A 
long tube covered with aluminum bronze lights when held in 
one hand the other touching the terminal of the coil quite 
powerfully. It might be objected that the coatings are not 
sufficiently conducting ; still, even if they were highly resistant, 
they ought to screen the gas. They certainly screen it perfectly 
in a condition of rest, but far from perfectly when the charge 
is surging in the coating. But the loss of energy which occurs 
within the tube, notwithstanding the screen, is occasioned prin- 
cipally 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 281 

consequently the inside might be considered as perfectly screened, 
though the potential be very rapidly alternating. Even were 
the sphere filled with oil, the loss would be incomparably smaller 
than when the fluid is replaced by a gas, for in the latter case the 
force produces displacements ; that means impact and collisions 
in the inside. 

No matter what the pressure of the gas may be, it becomes an 
important factor in the heating of a conductor when the electric 
density is great and the frequency very high. That in the heat- 
ing of conductors by lightning discharges, air is an element of 
great importance, is almost as certain as an experimental fact. I 
may illustrate the action of the air by the following experiment: 
I take a short tube which is exhausted to a moderate degree and 
has a platinum wire running through the middle from one end 
to the other. I pass a steady or low frequency current through 
the wire, and it is heated uniformly in all parts. The heating 
here is due to conduction, or frictional losses, and the gas around 
the wire has as far as we can see no function to perform. 
But now let me pass sudden discharges, or high frequency cur- 
rents, 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 cur- 
rent from the wire as the impedance of the latter is enormously 
increased, and merely heating the ends of the wire by reason of 
their resistance to the passage of the discharge. But it is not 
at all necessary that the gas in the tube should be conducting ; it 
might be at an extremely low pressure, still the ends of the wire 
would be heated as, however, is ascertained by experience 
only the two ends would in such case not be electrically con- 
nected through the gaseous medium. Now what with these fre- 
quencies and potentials occurs in an exhausted tube, occurs in the 
lightning discharges at ordinary pressure. We only need re- 
member one of the facts arrived at in the course of these investi- 
gations, 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- 



882 INVENTIONS OF NIKOLA TESLA. 

merged in an insulating liquid, it would be safe, for then the 
energy would have to spend itself somewhere else. From the 
behavior of gases under sudden impulses of high potential, I am 
led to conclude that there can be no surer way of diverting a 
lightning discharge than by affording it a passage through a 
volume of gas, if such a thing can be done in a practical manner. 

There are two more features upon which I think it necessary 
to dwell in connection with these experiments the " radiant 
state " and the " non-striking vacuum." 

Any one who has studied Crookes' work must have received 
the impression that the " radiant state " is a property of the gas 
inseparably connected with an extremely high degree of ex- 
haustion. 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. 1 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 im- 
parted to it is sufficient to propel it in a sensibly straight line. 
The mean free path is one thing, but the velocity the energy 
associated with the moving body is another, and under ordinary 
circumstances I believe that it is a mere question of potential or 
speed. A disruptive discharge coil, when the potential is pushed 
very far, excites phosphorescence and projects shadows, at com- 
paratively 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 par- 
ticles thrown off in straight lines. 

I have prepared a bulb to illustrate by an experiment the 
correctness of these assertions. In a globe L, Fig. 160, I have 
mounted upon a lamp filament f a piece of lime /. 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 and the projecting parts of the filament f 
are bombarded. The degree of exhaustion is just such that with 
the potential the coil is capable of giving, phosphorescence of the 
glass is produced, but disappears as soon as the vacuum is im- 
paired. The lime containing moisture, and moisture being given 
off as soon as heating occurs, the phosphorescence lasts only for 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 283 

a few moments. When the lime has been sufficiently heated, 
enough moisture has been given oft' 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 par- 
ticles (Fig. 160) then breaks forth from that point. This stream 
is composed of " radiant " matter, yet the degree of exhaustion 
is low. But the particles move in straight lines because the 
velocity imparted to them is great, and this is due to three 
causes to the great electric density, the high temperature of the 
small point, and the fact that the particles of the lime are easily 




FIG. 160. 



torn and thrown off far more easily than those of carbon. With 
frequencies such as we are able to obtain, the particles are bodily 
thrown off and projected to a considerable distance ; but with 
sufficiently high frequencies no such thing would occur ; in such 
case only a stress would spread or a vibration would be propa- 
gated through the bulb. It would be out of the question to 
reacli any such frequency on the assumption that the atoms move 
with the speed of light ; but I believe that such a thing is impos- 
sible ; for this an enormous potential would be required. 
With potentials which we are able to obtain, even with a disrup- 
tive discharge coil, the speed must be quite insignificant. 

As to the " non-striking vacuum," the point to be noted is, 
that it can occur only with low frequency impulses, and it is 



284 



INVENTIONS OF NIKOLA TKSLA. 



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 coining 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 higli enough. It is easy to reach even with 





FIG. 161. 



FIG. 162. 



frequencies obtained from an alternator as here used a stage at 
which the discharge does not pass between two electrodes in a 
narrow tube, each of these being connected to one of the termi- 
nals of the coil, but it is difficult to reach a point at which a 
luminous discharge would not occur around each electrode. 

A thought which naturally presents itself in connection with 
high frequency currents, is to make use of their powerful electro- 
dynamic inductive action to produce light effects in a sealed glass 
globe. The leading-in wire is one of the defects of the present 
incandescent lamp, and if no other improvement were made, 
that imperfection at least should be done away with. Following 



HIGH FREQUENCY AND 1110 U POTENTIAL CURRENTS. 285 

this thought, I have carried on experiments in various directions, 
of which some were indicated in my former paper. I may here 
mention one or two more lines of experiment which have been 
followed up. 

Many bulbs were constructed as shown in Fig. 161 and Fig. 
162. 

In Fig. 161, a wide tube, T, was sealed to a smaller W shaped 
tube u, of phosphorescent glass. In the tube T, was placed a coil 
c, of aluminum wire, the ends of which were provided with 
small spheres, t and ^, of aluminum, and reached into the u tube. 
The tube T was slipped into a socket containing a primary coil, 
through which usually the discharges of Leyden jars were di- 
rected, and the rarefied gas in the small u tube was excited to 
strong luminosity by the high-tension current induced in the coil c. 
When Leyden jar discharges were used to induce currents in the 
coil c, it was found necessary to pack the tube T tightly with in- 
sulating powder, as a discharge would occur frequently between 
the turns of the coil, especially when the primary was thick and 
the air gap, through which the jars discharged, large, and no 
little trouble was experienced in this way. 

In Fig. 162 is illustrated another form of the bulb constructed. 
In this case a tube T is sealed to a globe L. The tube contains a 
coil c, the ends of which pass through two small glass tubes t 
and ti, which are sealed to the tube T. Two refractory buttons 
m and m t are mounted on lamp filaments which are fastened to 
the ends of the wires passing through the glass tubes t and ,. 
Generally in bulbs made on this plan the globe L communicated 
with the tube T. For this purpose the ends of the small tubes t 
and ti were heated just a trifle in the burner, merely to hold the 
wires, but not to interfere with the communication. The tube T, 
with the small tubes, wires through the same, and the refractory 
buttons in and m l9 were first prepared, and then sealed to globe L, 
whereupon the coil c was slipped in and the connections made to 
its ends. The tube was then packed with insulating powder, 
jamming the latter as tight as possible up to very nearly the end ; 
then it was closed and only a small hole left through which the 
remainder of the powder was introduced, and finally the end of 
the tube was closed. Usually in bulbs constructed as shown in 
Fig. 162 an aluminum tube a was fastened to the upper end s 
of each of the tubes t and b in order to protect that end against 
the heat. The buttons m and m^ could be brought to any degree 



286 INVENTIONS OF NIKOLA TESLA. 

of incandescence by passing the discharges of Leyden jars 
around the coil c. In such bulbs with two buttons a very curi- 
ous effect is produced by the formation of the shadows of each 
of the two buttons. 

Another line of experiment, which has been assiduously fol- 
lowed, 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 pres- 
ent views and results, a few words must be devoted here to this 
subject. 

It has occured, 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 sim- 
ple appliance capable of giving considerable light would be ob- 
tained. 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 coat- 
ings were used, it would be better to supply the current directly 
to the tube by connecting the coatings to a transformer. But 
even if all objections of such nature were removed, with 
low frequencies the light conversion itself would be inefficient, 
as I have before stated. In using extremely high frequencies 
the length of the secondary in other words, the size of the ves- 
sel can be reduced as much as desired, and the efficiency of the 
light conversion is increased, provided that means are invented 
for efficiently obtaining such high frequencies. Thus one is led, 
from theoretical and practical considerations, to the use of high 
frequencies, and this means high electromotive forces and small 
currents in the primary. When one works with condenser 
charges and they are the only means up to the present known 
for reaching these extreme frequencies one gets to electromotive 
forces of several thousands of volts per turn of the primary. "We 
cannot multiply the electro-dynamic inductive effect by taking 



man FREQUENCY AND HIGH POTENTIAL CURRENTS. 287 

more turns in the primary, for we arrive at the conclusion that 
the best way is to work with one single turn though we must 
sometimes depart from this rule and we must get along with 
whatever inductive effect we can obtain with one turn. But be- 
fore one has long experimented with the extreme frequencies re- 
quired to set up in a small bulb an electromotive force of several 
thousands of volts, one realizes the great importance of electrosta- 
tic effects, and these effects grow relatively to the electro-dyna- 
mic in significance as the frequency is increased. 

Kow, if anything is desirable in this case, it is to increase the 
frequency, and this would make it still worse for the electro- 
dynamic effects. On the other hand, it is easy to exalt the elec- 
trostatic 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 
the current to the smallest value and increasing the potential, 
the electric impulses of high frequency can be more easily trans- 
mitted through a conductor. 

These and similar thoughts determined me to devote more at 
tention to the electrostatic phenomena, and to endeavor to pro- 
duce potentials as high as possible, and alternating as fast as 
they could be made to alternate. I then found that I could ex- 
cite 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 po- 
tential, 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 pro- 
gress the one and only desire to reach a result of utility to men 
in any direction to which thought or experiment may lead me. 
I think that this departure is the right one, for I cannot see, 
from the observation of the phenomena which manifest them- 
selves 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. 



288 INVENTIONS OF NIKOLA TESLA. 

Of all these phenomena observed with currents, or electric 
impulses, of high frequency, the most fascinating for an aud- 
ience 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 



FIG. 163. FIG. 164. 

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 f rictional losses ; and if a strain exerted in the dielec- 
tric, such as a simple electrification would produce, it is easy to 
see how it may persist indefinitely, but very difficult to under- 
stand why such a condition should aid the excitation when we 
have to deal with potentials which are rapidly alternating. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 289 

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 TJ, sealed to another long 
tube T, is provided with a stem .$, with a platinum wire sealed in 
the latter. A very thin lamp filament I, is fastened to this wire 
and connection to the outside is made through a thin copper wire 
w. The tube is provided with outside and inside coatings, c and 
GJ, respectively, and is filled as far as the coatings reach with con- 
ducting, and the space above with insulating, powder. These 
coatings are merely used to enable me to perform two experi- 
ments with the tube namely, to produce the effect desired either 
by direct connection of the body of the experimenter or of an- 
other body to the wire w, or by acting inductively through the 
glass. The stem s is provided with an aluminum tube , for 
purposes before explained, and only a small part of the filament 
reaches out of this tube. By holding the tube T : anywhere in 
the electrostatic field, the filament is rendered incandescent. 

A more interesting piece of apparatus is illustrated in Fig. 164. 
The construction is the same as before, only instead of the lamp 
filament a small platinum wire ^>, sealed in a stem s, and bent 
above it in a circle, is connected to the copper wire w, which is 
joined to an inside coating c. A small stem * M is provided with 
a needle, on the point of which is arranged, to rotate very freely, 
a very light fan of mica v. To prevent the fan from falling out, 
a thin stem of glass </, is bent properly and fastened to the alu- 
minum tube. When the glass tube is held anywhere in the elec- 
trostatic field the platinum wire becomes incandescent, and the 
mica vanes are rotated very fast. 

Intense phosphorescence may be excited in a bulb by merely 
connecting it to a plate within the field, and the plate need not 
be any larger than an ordinary lamp shade. The phosphores- 
cence 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 



290 INVENTIONS OF NIKOLA TESLA. 

to allow reading ordinary print at a distance of five to six paces. 
It was of interest to see how some of the phosphorescent bulbs 
of Professor Crookes would behave with these currents, and he 
has had the kindness to lend me a few for the occasion. The 
effects produced are magnificent, especially by the sulphide of 
calcium and sulphide of zinc. With the disruptive discharge 
coil they glow intensely merely by holding them in the hand and 
connecting the body to the terminal of the coil. 

To whatever results investigations of this kind may lead, the 
chief interest lies, for the present, in the possibilities they offer 
for the production of an efficient illuminating device. In no 
branch of electric industry is an advance more desired than in 
the manufacture of light. Every thinker, when considering the 
barbarous methods employed, the deplorable losses incurred in 
our best systems of light production, must have asked himself, 
What is likely to be the light of the future ? Is it to be an in- 
candescent solid, as in the present lamp, or an incandescent gas, 
or a phosphorescent body, or something like a burner, but in- 
comparably more efficient ? 

There is little chance to perfect a gas burner ; not, perhaps, 
because human ingenuity has been bent upon that problem for 
centuries without a radical departure having been made 
though the argument is not devoid of force but because in a 
burner the highest vibrations can never be reached, except by 
passing through all the low ones. For how is a flame to proceed 
unless by a fall of lifted weights ? Such process cannot be main- 
tained 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 incan- 
descence. Higher incandescence is equivalent to a quicker vi- 
bration : that means more light from the same material, and that 
again, means niore economy. In this direction some improve- 
ments have been made, but the progress is hampered by many 
limitations. Discarding, then, the burner, there remains the 
three ways first mentioned, which are essentially electrical. 

Suppose the light of the immediate future to be a solid, ren- 
dered 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 diffi- 
culties connected with the operation of such a lamp to be effec- 



HIGH FRKQUENCY AND HIGH POTENTIAL CURRENTS. 291 

lively overcome. But to light such a lamp we require a high 
potential ; and to get this economically, we must use high fre- 
quencies. 

Such considerations apply even more to the production of light 
by the incandescence of a gas, or by phosphorescence. In all 
cases we require high frequencies and high potentials. These 
thoughts occurred to me a long time ago. 

Incidentally we gain, by the use of high frequencies, many ad- 
vantages, such as higher economy in the light production, the 
possibility of working with one lead, the possibility of doing away 
with the leading-in wire, etc. 

The question is, how far can we go with frequencies ? Ordi- 
nary conductors rapidly lose the facility of transmitting electric 
impulses when the frequency is greatly increased. Assume the 
means for the production of impulses of very great frequency 
brought to the utmost perfection, every one will naturally ask 
how to transmit them when the necessity arises. In transmitting 
such impulses through conductors we must remember that we 
have to deal with pressure and flow, in the ordinary interpretation 
of these terms. Let the pressure increase to an enormous value, 
and let the flow correspondingly diminish, then such impulses 
variations merely of pressure, as it were can no doubt be 
transmitted through a wire even if their frequency be many 
hundreds of millions per second. It would, of course, be out of 
question to transmit such impulses through a wire immersed in a 
gaseous medium, even if the wire were provided with a thick 
and excellent insulation, for most of the energy would be lost in 
molecular bombardment and consequent heating. The end of 
the wire connected to the source would be heated, and the re- 
mote end would receive but a trifling part of the energy sup- 
plied. The prime necessity, then, if such electric impulses are 
to be used, is to find means to reduce as much as possible the 
dissipation. 

The first thought is, to employ the thinnest possible wire sur- 
rounded 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 con- 
nection is made it can only be made through a conductor offer- 



292 INVENTIONS OF NIKOLA TESLA. 

ing an enormous impedance, or through a condenser of ex- 
tremely small capacity. This, however, does not do away with 
other difficulties. 

If the wave length of the impulses is much smaller than the 
length of the wire, then corresponding short waves will be set 
up in the conducting coating, and it will be more or less the 
same as though the coating were directly connected to earth. It 
is therefore necessary to cut up the coating in sections much 
shorter than the wave length. Such an arrangement does not 
still afford a perfect screen, but it is ten thousand times better 
than none. I think it preferable to cut up the conducting coat- 
ing in small sections, even if the current waves be much longer 
than the coating. 

If a wire were provided with a perfect electrostatic screen, it 
would be the same as though all objects were removed from it at 
infinite distance. The capacity would then be reduced to the 
capacity of the wire itself, which would be very small. It 
would then be possible to send over the wire current vibrations 
of very high frequencies at enormous distances, without affecting 
greatly the character of the vibrations. A perfect screen is of 
course out of the question, but I believe that with a screen such 
as I have just described telephony could be rendered practicable 
across the Atlantic. According to my ideas, the gutta-percha 
covered wire should be provided with a third conducting coating 
subdivided in sections. On the top of this should be again 
placed a layer of gutta-percha and other insulation, and on the 
top of the whole the armor. But such cables will not be con- 
structed, for ere long intelligence transmitted without wires 
will throb through the earth like a pulse through a living organ- 
ism. The wonder is that, with the present state of knowledge 
and the experiences gained, no attempt is being made to dis- 
turb the electrostatic or magnetic condition of the earth, and 
transmit, if nothing else, intelligence. 

It has been, my chief aim in presenting these results to point 
out phenomena or features of novelty, and to advance ideas 
which I am hopeful will serve as starting points of new depart- 
ures. It has been my chief desire this evening to entertain you 
with some novel experiments. Your applause, so frequently 
and generously accorded, has told me that I have succeeded. 

In conclusion, let me thank you most heartily for your kind- 
ness and attention, and assure you that the honor I have had in 



HJGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 293 

addressing such a distinguished audience, the pleasure I have had 
in presenting these results to a gathering of so many able men 
and among them also some of those in whose work for many 
years past I have found enlightenment and constant pleasure 
I shall never forget. 



CHAPTER XXVIII. 
ON LIGHT AND OTHER HIGH FREQUENCY PHENOMENA. 1 

INTRODUCTORY. SOME THOUGHTS ON THE EYE. 

WHEN we look at the world around us, on Nature, we are im- 
pressed with its beauty and grandeur. Each thing we perceive, 
though it may be vanishingly small, is in itself a world, that is, 
like the whole of the universe, matter and force governed by 
law, a world, the contemplation of which fills us with feelings 
of wonder and irresistibly urges us to ceaseless thought and in- 
quiry. But in all this vast world, of all objects our senses re- 
veal 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 fascin- 
ating 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 direct- 
ive 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 

1. A lecture delivered before the Franklin Institute, Philadelphia, February* 
1893, and before the National Electric Light Association, St. Louis, March, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 295 

most intimate relation with that which we call intellect. So inti- 
mate is this relation, that it is often said, the very soul shows 
itself in the eye. 

It can he 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 im- 
age 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 sensi- 
tiveness, that a clear idea of its state might be gained at any 
time 2 If this were possible, then the problem of reading cne's 
thoughts with precision, like the characters of an open book, 
might be much easier to solve than many problems belonging to 
the domain of positive physical science, in the solution of which 
many, if not the majority, of scientific men implicitly believe. 
Helmholtz, has shown that the fundi of the eye are themselves, 
luminous, and he was able to see, in total darkness, the move- 
ment of his arm by the light of his own eyes. This is one of the 
most remarkable experiments recorded in the history of science, 
and probably only a few men could satisfactorily repeat it, for it 
is very likely, that the luminosity of the eyes is associated with 
uncommon activity of the brain and great imaginative power. It 
is fluorescence of brain action, as it were. 

Another fact having a bearing on this subject which has prob- 
ably been noted by many, since it is stated in popular expressions, 
but which I cannot recollect to have found chronicled as a posi- 
tive result of observation is, that at times, when a sudden idea or 
image presents itself to the intellect, there is a distinct and some- 
times painful sensation of luminosity produced in the eye, ob- 
servable even in broad daylight. 

The saying then, that the soul shows itself in the eye, is deep- 
ly 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 phe- 
nomena, but a still more profound meaning for one who, in the 



290 INVENTIONS OF NIKOLA TESLA. 

spirit of positive scientific investigation, seeks to ascertain the 
causes of the effects. It is principally the natural philospher, 
the physicist, for whom the eye is the subject of the most intense 
admiration. 

Two facts about the eye must forcibly impress the mind of the 
physicist, notwithstanding he may think or say that it is an 
imperfect optical instrument, forgetting, that the very conception 
of that which is perfect or seems so to him, has been gained 
through this same instrument. First, the eye is, as far as our 
positive knowledge goes, the only organ which is directly affected 
by that subtile medium, which as science teaches us, must fill all 
space ; secondly, it is the most sensitive of our organs, incompar- 
ably more sensitive to external impressions than any other. 

The organ of hearing implies the impact of ponderable bodies, 
the organ of smell the transference of detached material particles, 
and the organs of taste, and of touch or force, the direct contact, 
or at least some interference of ponderable matter, and this is 
true even in those instances of animal organisms, in which some 
of these organs are developed to a degree of truly marvelous 
perfection. This being so, it seems wonderful that the organ of 
, ^' c sight solely should be capable of being stirred by that, which all 
> our other organs are powerless to detect, yet which plays an es- 
sential 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 
L 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 
v * 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 any- 
t *t thing 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 
#v~a sensations of which we are not aware, and any degree or intensity 
/^cWof 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 

' 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 297 

or intensity, unless they are very great. Now we can readily con- 
ceive 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 
senstiveness or perfection, that it would be capable of distinguish- 
ing 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 cap- 
able 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 disturb- 
ance 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 
r 3 ve liberations 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 dis- 
tinguishing in the pure medium, small objects at distances meas- 
ured probably by hundreds or perhaps thousands of miles. 

But there is something else about the eye which impresses us 
still more than these wonderful features which we observed, view- 
ing it from the standpoint of a physicist, merely as an optical 
instrument, something which appeals to us more than its marvel- 
ous faculty of being directly affected by the vibrations of the 



298 INVENTIONS OF NIKOLA TESLA. 

medium, without interference of gross matter, and more than its 
inconceivable sensitiveness and discerning power. It is its sig- 
nificance in the processes of life. No matter what one's views oh 
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 organ- 
ism. And how could it be otherwise, when he realizes, that the 
eye is the means through which the human race has acquired 
the entire knowledge it possesses, that it controls all our motions, 
more still, all our actions. 

There is no way of acquiring knowledge except through the eye. 
What is the foundation of all philosophical systems of ancient 
and modern times, in fact, of all the philosophy of man ? / am, 
I think I think, therefore Iain. But how could I think and how 
would I know that I exist, if I had not the eye ? For knowledge 
involves consciousness ; consciousness involves ideas, conceptions ; 
conceptions involve pictures or images, and images the sense of 
vision, and therefore the organ of sight. But how about blind 
men, will be asked ? Yes, a blind man may depict in magnificent 
poems, forms and scenes from real life, from a world he physically 
does not see. A blind man may touch the keys of an instrument 
with unerring precision, may model the fastest boat, may discover 
and invent, calculate and construct, may do still greater wonders 
but all the blind men who have done such things have descended 
from those who had seeing eyes. Nature may reach the same re- 
sult in many ways. Like a wave in the physical world, in the in- 
finite 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 by- 
gone times, may have altered the course of his life, may have 
changed the destiny of nations, may have transformed the sur- 
face of the globe, so intricate, so inconceivably complex are the 
processes in Nature. In no way can we get such an overwhelm- 
ing 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- 

*7"&#3i tVVH/t. 

^^ ^ \***^^svt*4&f -^/-' 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 299 

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 ex- 
isted, else thought, as we understand it, would be impossible ; 
else conceptions, like spirit, intellect, mind, call it as you may, 
could not exist. It is conceivable, that in some other world, in 
some other beings, the eye is replaced by a different organ, equally 
or more perfect, but these beings cannot be men. 

Now r 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 superim- 
position 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 en- 
deavor 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 in- 
herent 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, con- 
sider the lowest organism we know and there are probably 
many lower ones an aggregation of a few cells only.' If it is 
capable of voluntary motion it can perform an infinite number 
of motions, all definite and precise. But now a mechanism con- 
sisting of a finite number of parts and few at that, cannot per- 
form an infinite number of definite motions, hence the impulses 
which govern its movements must come from the environment. 
So, the atom, the ulterior element of the Universe's structure, is 
tossed about in space, eternally, a play to external influences, like 
a boat in a troubled sea. Were it to stop its motion it would die. 
Matter at rest, if such a thing could exist, would be matter dead. 
Death of matter ! Never has a sentence of deeper philosophical 
meaning been uttered. This is the way in which Prof. Dewar 



800 INVENTIONS OF NIKOLA TESLA. 

forcibly expresses it in the description of his admirable experi- 
ments, 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 illus- 
trate, 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 t3 move, to vibrate, that 
is, to live. 

I have made the preceding statements at the peril of treading 
upon metaphysical ground, in my desire to introduce the subject 
of this lecture in a manner not altogether uninteresting, I may 
hope, to an audience such as I have the honor to address. But 
now, then, returning to the subject, this divine organ of sight, 
this indispensable instrument for thought and all intellectual en- 
joyment, which lays open to us the marvels of this universe, 
through which we have acquired what knowledge we possess, and 
which prompts us to, and controls, all our physical and mental 
activity. By what is it affected? By light ! What is light ? 

We have witnessed the great strides which have been made in 
all departments of science in recent years. So great have been 
the advances that we cannot refrain from asking ourselves, Is 
this all true, or is it but a dream ? Centuries ago men have 
lived, have thought, discovered, invented, and have believed that 
they were soaring, while they were merely proceeding at a snail's 
pace. So we too may be mistaken. But taking the truth of the 
observed events as one of the implied facts of science, we must 
rejoice in the immense progress already made and still more in the 
anticipation of what must come, judging from the possibilities 
opened up by modern research. There is, however, an advance 
which we have been witnessing, which must be particularly 
gratifying to every lover of progress. It is not a discovery, or 
an invention, or an achievement in any particular direction. It 
is an advance in all directions of scientific thought and experi- 
ment. I mean the generalization of the natural forces and phe- 
nomena, the looming up of a certain broad idea on the scientific 
horizon. It is this idea which has, however, long ago taken pos- 
session 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. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 301 

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 phe- 
nomena 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 settle- 
ments, to warm their homes by, to give them light, and to keep 
their ooedient 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 elec- 
tricity. I am aware that this view is not that of a practical 
engineer, but neither is it that of an illusionist, for it is certain, 
that power transmission, which at present is merely a stimulus to 
enterprise, will some day be a dire necessity. 

It is more important for the student, who takes up the study 
of light phenomena, to make himself thoroughly acquainted with 
certain modern views, than to peruse entire books on the subject 
of light itself, as disconnected from these views. Were I there- 
fore to make these demonstrations before students seeking 
information and for the sake of the few of those who may be 



802 INVENTIONS OF NIKOLA TESLA. 

present, give me leave to so assume it would be my principal 
endeavor to impress these views upon their minds in this series of 
experiments. 

It might be sufficient for tins purpose to perform a simple and 
well-known experiment. I might take a familiar appliance, a 
L3yden jar, charge it from a frictional machine, and then dis- 
charge it. In explaining to you its permanent state when charged, 
and its transitory condition when discharging, calling your atten- 
tion to the forces which enter into play and to the various phen- 
omena they produce, and pointing out the relation of the forces 
and phenomena, I might fully succeed in illustrating that modern 
idea. Xo 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 per- 
haps also more instructive. Instead of the frictional machine and 
Leyden jar, I shall avail myself in these experiments, of an induc- 
tion 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. 

ON THE APPARATUS AND METHOD OF CONVERSION. 

These high-frequency currents are obtained in a peculiar man- 
ner. The method employed was advanced by me about two 
years ago in an experimental lecture before the American Insti- 
tute 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 




304 INVENTIONS OF NIKOLA TESLA. 

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 neces- 
sary to maintain the oscillations of the current. In view of the 
general interest taken in high-frequency currents and effects pro- 
ducible 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 gen- 
erator 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 in- 
dicated subsequently. When the condensers are charged to a 
certain potential, the air, or insulating space, gives way and a dis- 
ruptive 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 sadden rushes of current it is necessary to ob- 
serve certain conditions. If the rate at which the condensers are 
disci mrged 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 charg- 
ing, 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 ob- 
tained. 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 compara- 
tively few, with long-time intervals between. This alwavs 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 eacli 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 gener- 
ator permits the attaining of greater rates of vibration. 



1UG11 FREQUENCY AND HIGH POTENTIAL CURRENTS. 305 

The rushes of current may be of the same direction under the 
conditions before assumed, but most generally there is an oscilla- 
tion superimposed upon the fundamental vibration of the current. 
When the conditions are so determined that there are no oscilla- 
tions, the current impulses are unidirectional and thus a means is 
provided of transforming a continuous current of high tension, 
into a direct current of lower tension, which I think may find 
employment in the arts. 

This method of conversion is exceedingly interesting and I 
was much impressed by its beauty when I first conceived it. It is 
ideal in certain respects. It involves the employment of no me- 
chanical devices of any kind, and it allows of obtaining currents 
of any desired frequency from an ordinary circuit, direct or al- 
ternating. The frequency of the fundamental discharges depend- 
ing on the relative rates of supply and dissipation can be readily 
varied within wide limits, by simple adjustments of these quanti- 
ties, 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 com- 
bining capacity and self-induction or by induction in a secondary, 
which need have but comparatively few turns. 

As the conditions are often such that the intermittence or os- 
cillation does not readily establish itself, especially when a direct 
current source is employed, it is of advantage to associate an in- 
terrupter with the arc, as I have, some time ago, indicated the 
use of an air-blast or magnet, or other such device readily at 
hand. The magnet is employed with special advantage in the 
conversion of direct currents, as it is then very effective. If the 
primary source is an alternate current generator, it is desirable, 
as I have stated on another occasion, that the frequency should 
be low, and that the current forming the arc be large, in order 
to render the magnet more effective. 

A form of such discharger with a magnet which has been 
found convenient, and adopted after some trials, in the conversion 
of direct currents particularly, is illustrated in Fig. 166. N s are 
the pole pieces of a very strong magnet which is excited by a coil 
c. The pole pieces are slotted for adjustment and can be fastened 
in any position by screws s s^ The discharge rods d d t1 thinned 
down on the ends in order to allow a closer approach of the mag- 
netic pole pieces, pass through the columns of brass b ^ and are 
fastened in position by screws # 2 $2- Springs r r t and collars c c 



306 INVENTIONS OF NIKOLA TESLA. 

are slipped on the rods, the latter serving to set the points of the 
rods at [a certain suitable distance by means of screws # 3 s s , and 
the former to draw the points apart. When it is desired to start 
the arc, one of the large rubber handles h Ji is tapped quickly 
with the [hand, whereby the points of the rods are brought in 
contact but are instantly separated by the springs r r^ Such an 
arrangements-has been found to be often necessary, namely in 
cases when the E. M. r. 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 




FIG. ICG. 

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. 



HIGH FREQUENCE: AND HIGH POTENTIAL CURRENTS. 307 

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 keep- 
ing the air space hotter, as the heat is not conducted away as 
quickly through the carbons, and the result is that a smaller 
E. M. F. in the arc gap is required to maintain a succession of 
discharges. 

Another form of discharger, which may be employed with ad- 
vantage in some cases, is illustrated in Fig. 167. In this form 
the discharge rods d d^ pass through perforations in a wooden 




FIG. 107. 



box B, which is thickly coated with mica on the inside, as indi- 
cated by the heavy lines. The perforations are provided with 
mica tubes m m^ of some thickness, which are preferably not in 
contact with the rods d d { . The box has a cover c which is a 
little larger and descends on the outside of the box. The spark 
gap is warmed by a small lamp I contained in the box. A plate 
p above the lamp allows the draught to pass only through the 
chimney <? of the lamp, the air entering through holes o o in or 
near the bottom of the box and following the path indicated by 
the arrows. When the discharger is in operation, the door of the 
box is closed so that the light of the arc is not visible outside. 



308 INVENTIONS OF NIKOLA TESLA. 

It is desirable to exclude the light as perfectly as possible, as it 
interferes with some experiments. This form of discharger is sim- 
ple 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 con- 
sequence is that the arc can be established at much greater dis- 
tance. The arc should, of course, be sufficiently insulating to 
allow the discharge to pass through the gap disruptively. The 
arc formed under such conditions, when long, may be made ex- 
tremely sensitive, and the weak draught through the lamp 
chimney c is quite sufficient to produce rapid interruptions. The 
adjustment is made by regulating the temperature and velocity 
of the draught. Instead of using the lamp, it answers the pur- 
pose to provide for a draught of warm air in other ways. A 
very simple way which has been practiced is to enclose the arc 
in a long vertical tube, with plates on the top and bottom for 
regulating the temperature and velocity of the air current. 
Some provision had to be made for deadening the sound. 

The air may be rendered dielectrically weak also by rarefac- 
tion. 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 Y5 millimetres, at which the ordi. 
nary thread discharge occurs. In another form of discharger, 
combining the features before mentioned, the discharge was 
made to pass between two adjustable magnetic pole pieces, the 
space between them being kept at an elevated temperature. 

It should be remarked here that when such, or interrupting 
devices of any kind, are used and the currents are passed through 
the primary of a disruptive discharge coil, it is not, as a rule, of 
advantage to produce a number of interruptions of the current 
per second greater than the natural frequency of vibration of the 
dynamo supply circuit, which is ordinarily small. It should also 
ba pointed out here, that while the devices mentioned in connec- 
tion with the disruptive discharge are advantageous under cer- 
tain conditions, they may be sometimes a source of trouble, as 
they produce intermittences and other irregularities in the vibra- 
tion which it would be very desirable to overcome. 



IIIGII FREQUENCY AND IIIGI1 POTENTIAL CURRENTS. 309 

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 illus- 
trated in this manner. Imagine a tank with a wide opening at 
the bottom, which is kept closed by spring pressure, but so that 
it snaps off suddenly when the liquid in the tank has reached a 
certain height. Let the fluid be supplied to the tank by means 
of a pipe feeding at a certain rate. When the critical height of 
the liquid is reached, the spring gives way and the bottom of the 
tank drops out. Instantly the liquid falls through the wide open- 
ing, 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 lik- 
ened 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 en- 
ergy ; and that, besides, some dissipation of energy results due to 
frictional losses. In the preceding analogue the liquid is sup- 
posed to be under a steady pressure. If the presence of the fluid 
be assumed to vary rhythmically, this may be taken as corres- 
ponding to the case of an alternating current. The process is 
then not quite as simple to consider, but the action is the same in 
principle. 

It is desirable, in order to maintain the vibration economically, 
to reduce the impact and frictional losses as much as possible. 



310 INVENTIONS OF NIKOLA TKSLA. 

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 
the employment of thin conductors in the form of strands. But 
the loss of energy caused by the first breaking through of the 
dielectric which in the above example corresponds to the violent 
knock of the bottom against the inelastic stop would be more im- 
portant to overcome. At the moment of the breaking through, 
the air space has a very high resistance, which is probably re- 
duced 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 moder- 
ately 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. Like- 
wise, 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 disrupt- 
ively 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 elec- 
trodes, they may remain entirely cool, and yet the loss in the 
air may represent any amount of energy. It is perfectly prac- 
ticable, with very great potential differences in the gap, to dissi- 
pate 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 diffi- 
cult 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, 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 311 

clearly not the best medium through which a disruptive dis- 
charge 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, un- 
fortunately less practicable on account of the difficulties and ex- 
pense 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 jusfr 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 with- 
out 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 ac- 
count may remain quite cold. 

The ideal medium for a discharge gap should only crack, and 
the ideal electrode should be of some material which cannot be 
disintegrated. With small currents through the gap it is best to 
employ aluminum, but not when the currents are large. The dis- 
ruptive break in the air, or more or less in any ordinary medium, 
is not of the nature of a crack, but it is rather comparable to the 
piercing of innumerable bullets through a mass offering great 
frictional resistances to the motion of the bullets, this involving 
considerable loss of energy. A medium which would merely 
crack when strained electrostatically and this possibly might be 
the case with a perfect vacuum, that is, pure ether would involve 
a very small loss in the gap, so small as to be entirely negligible, 
at least theoretically, because a crack may be produced by an 
infinitely small displacement. In exhausting an oblong bulb 
provided with two aluminum terminals, with the greatest care, I 
have succeeded in producing such a vacuum that the secondary 
discharge of a disruptive discharge coil would break disrup- 
tively 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 



312 INVENTIONS OF NIKOLA TESLA, 

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 per- 
fectly the crack upon a minute diminution of the pressure. 
This is the way the dielectric in the discharge space should 
behave. But inasmuch as there would be always some loss in the 
gap, the medium, which should be continuous, should exchange 
through the gap at a rapid rate. In the preceding example, the 
glass being perfectly closed, it would mean that the dielectric in 
the discharge space possesses a great insulating power ; the glass 
being cracked, it would signify that the medium in the space is 
a good conductor. The dielectric should vary enormously in 
resistance by minute variations of the E. M. F. across the 
discharge space. This condition is attained, but in an extremely 
imperfect manner, by warming the air space to a certain 
critical temperature, dependent on the E. M. F. across the gap, 
or by otherwise impairing the insulating power of the air. But 
as a matter of fact the air does never break down disruptively, 
if this term be rigorously interpreted, for before the sudden 
rush of the current occurs, there is always a weak current 
preceding it, which rises first gradually and then with compara- 
tive 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 equiva- 
lent dielectric strength. As a medium for the discharge space, a 
solid, or even a liquid, would be preferable therefor. It is some- 
what 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, some- 
times under great pressure, were employed, have been experi- 
mented upon. It is thought sufficient to dwell in a few words 
upon one of the forms experimented upon. One of these dis- 
chargers is illustrated in Figs. 168 and 168 b. 

A hollow metal pulley P (Fig. 16 8), was fastened upon an ar- 
bor #, which by suitable means was rotated at a considerable 



HIQII FREQUENCY AND HIOII POTENTIAL CURRENTS. 313 

speed. On the inside of the pulley, but disconnected from the 
same, was supported a thin disc h (which is shown thick for the 
sake of clearness), of hard rubber in which there were embedded 
two metal segments s s with metallic extensions e e into which 
were screwed conducting terminals t t covered with thick tubes 
of hard rubber 1 1. The rubber disc h with its metallic segments 
s ,9, was finished in a lathe, and its entire surface highly polished 
so as to offer the smallest possible frictional resistance to the mo- 
tion 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/, which was screwed tightly on the 
front side of the pulley. The terminals t t, were connected to the 
opposite coatings of a battery of condensers so that the discharge 
occurred through the liquid. When the pulley was rotated, the 
liquid was forced against the rim of the pulley and considerable 
fluid pressure resulted. In this simple way the discharge gap 





FIG. 168a. 



FIG. 168b. 



was filled with a medium which behaved practically like a solid, 
which possessed the quality of closing instantly upon the occur- 
rence 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 num- 
ber of different forms were made. It was found that, as ex- 
pected, a longer spark for a given length of wire was obtainable 
in this way than by using air as an interrupting device. Gener- 
ally the speed, and therefore also the fluid pressure, was limited 
by reason of the fluid friction, in the form of discharger described, 
but the practically obtainable speed was more than sufficient to 
produce a number of breaks suitable for the circuits ordinarily 
used. In such instances the metal pulley P was provided with a 
few projections inwardly, and a definite number of breaks was 
then produced which could be computed from the speed of 



314 1A' VJHNTION8 OF NIKOLA TESLA. 

rotation of the pulley. Experiments were also carried on with 
liquids of different insulating power with the view of reducing 
the loss in the arc. When an insulating liquid is moderately 
warmed, the loss in the arc is diminished. 

A point of some importance was noted in experiments with 
various discharges of this kind. It was found, for instance, that 
whereas the conditions maintained in these forms were favorable 
for the production of a great spark length, the current so ob- 
tained was not best suited to the production of light effects. Ex- 
perience undoubtedly has shown, that for such purposes a har- 
monic rise and fall of the potential is preferable. Be it that a 
solid is rendered incandescent, or phosphorescent, or be it that en- 
ergy is transmitted by condenser coating through the glass, it is 
quite certain that a harmonically rising and falling potential pro- 
duces less destructive action, and that the vacuum is more per- 
manently maintained. This would be easily explained if it were 
ascertained that the process going on in an exhausted vessel is of 
an electrolytic nature. 

In the diagrammatical sketch, Fig. 165, which has been already 
referred to, the cases which are most likely to be met with in 
practice are illustrated. One has at his disposal either direct or 
alternating currents from a supply station. It is convenient for 
an experimenter in an isolated laboratory to employ a machine G, 
such as illustrated, capable of giving both kinds of currents. In 
such case it is also preferable to use a machine with multiple 
circuits, as in many experiments it is useful and convenient to 
have at one's disposal currents of different phases. In the 
sketch, D represents the direct and A the alternating circuit. In 
each of these, three branch circuits are shown, all of which are 
provided with double line switches s s s s s s. Consider first the 
direct current conversion ; m represents the simplest case. If 
the E. M. F. of the generator is sufficient to break through a small 
air space, at least when the latter is warmed or otherwise rend- 
ered poorly insulating, there is no difficulty in maintaining a 
vibration with fair economy by judicious adjustment of the 
capacity, self-induction and resistance of the circuit L containing 
the devices II m. The magnet N, s, can be in this case advan- 
tageously combined with the air space. The discharger d d with 
the magnet may be placed either way, as indicated by the full or 
by the dotted lines. The circuit la with the connections and de- 
vices is supposed to possess dimensions such as are suitable for 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 315 

the maintenance of a vibration. But usually the E. M. r. on the 
circuit or branch \a will be something like a 1 00 volts or so, and 
in this case it is not sufficient to break through the gap. Many 
different means may be used to remedy this by raising the E. M. F. 
across the gap. The simplest is probably to insert a large self- 
induction coil in series with the circuit L. When the arc is 
established, as by the discharger illustrated in B'ig. 166, the mag- 
net blows the arc out the instant it is formed. Now the extra 
current of the break, being of high E. M. F., breaks through the 
gap, and a path of low resistance for the dynamo current being 
again provided, there is a sudden rush of current from the 
dynamo upon the weakening or subsidence of the extra current. 
This process is repeated in rapid succession, and in this manner I 
have maintained oscillation with as low as 50 volts, or even less, 
across the gap. But conversion on this plan is not to be recom- 
mended on account of the too heavy currents through the gap 
and consequent heating of the electrodes ; besides, the frequen- 
cies obtained in this way are low, owing to the high self-induc- 
tion necessarily associated with the circuit. It is very desirable 
to have the E. M. F. as high as possible, first, in order to increase 
the economy of the conversion, and, secondly, to obtain high 
frequencies. The difference of potential in this electric oscilla- 
tion is, of course, the equivalent of the stretching force in the 
mechanical vibration of the spring. To obtain very rapid vibra- 
tion in a circuit of some inertia, a great stretching force or differ- 
ence of potential is necessary. Incidentally, when the E. M. F. is 
very great, the condenser which is usually employed in connec- 
tion with the circuit need but have a small capacity, and many 
other advantages are gained. With a view of raising the E. M. F. 
to a many times greater value than obtainable from ordinary 
distribution circuits, a rotating transformer g is used, as indi- 
cated at i la, Fig. 165, or else a separate high potential machine 
is driven by means of a motor operated from the generator G. 
The latter plan is in fact preferable, as changes are easier made. 
The connections from the high tension winding are quite similar 
to those in branch la with the exception that a condenser c, 
which should be adjustable, is connected to the high tension 
circuit. Usually, also, an adjustable self-induction coil in series 
with the circuit has been employed in these experiments. When 
the tension of the currents is very high, the magnet ordinarily 
used in connection with the discharger is of comparatively small 



316 INVENTIONS OF NIKOLA TKSLA. 

value, as it is quite easy to adjust the dimensions of the circuit 
so that oscillation is maintained. The employment of a steady 
E. M. F. in the high frequency conversion affords some advan- 
tages over the employment of alternating E. M. F., as the adjust- 
ments are much simpler and the action can be easier controlled. 
But unfortunately one is limited by the obtainable potential dif- 
ference. 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 ma. 
In this arrangement a rotating transformer g, is employed to 
convert the low tension direct currents into low frequency alter- 
nating currents, preferably also of small tension. The tension 
of the currents is then raised in a stationary transformer T. The 
secondary s of this transformer is connected to an adjustable con- 
denser c which discharges through the gap or discharger dd, placed 
in either of the ways indicated, through the primary p of a dis- 
ruptive discharge coil, the high frequency current being obtained 
from the secondary s of this coil, as described on previous occa- 
sions. This will undoubtedly be found the cheapest and most con- 
venient way of converting direct currents. 

The three branches of the circuit A represent the usual cases 
met in practice when alternating currents are converted. In 
Fig. 15 a condenser c., generally of large capacity, is connected to the 
circuit L containing the devices Z Z, m m. The devices mm are sup- 
posed to be of high self-induction so as to bring the frequency of 
the circuit more or less to that of the dynamo. In this instance 
the discharger d d should best have a number of makes and breaks 
per second equal to twice the frequency of the dynamo. If not 
so, then it should have at least a number equal to a multiple or 
even fraction of the dynamo frequency. It should be observed, 
referring to iJ, that the conversion to a high potential is also 
effected when the discharger d d, which is shown in the sketch, is 
omitted. But the effects which are produced by currents which 
rise instantly to high values, as in a disruptive discharge, are 
entirely different from those produced by dynamo currents which 
rise and fall harmonically. So, for instance, there might be in a 
given case a number of makes and breaks at d d equal to just 
twice the frequency of the dynamo,"or in other words, there may 
be the same number of fundamental oscillations as would be pro- 



HIGH FREQUENCY AND HIGH POTENTIAL CURBENT8. 317 

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 pheno- 
mena, dependent upon the rate of change, will bear no similarity in 
the two cases. Thus, when working with currents discharging dis- 
ruptively, 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 cur- 
rent 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 phe- 
nomena of impedance are observed, just as with currents of high 
frequencies. But it is, of course, evident that with low frequency 
currents it is impossible to obtain such rates of change per unit of 
time as with high frequencies, hence the effects produced by the 
latter are much more prominent. It is deemed advisable to 
make the preceding remarks, inasmuch as many more recently 
described effects have been unwittingly identified with high 
frequencies. Frequency alone in reality does not mean anything, 
except when an undisturbed harmonic oscillation is considered. 

In the branch uib a similar disposition to that in ib is illustrated, 
with the difference that the currents discharging through the gap 
d d are used to induce currents in the secondary s of a trans- 
former T. In such case the secondary should be provided with an 
adjustable condenser for the purpose of tuning it to the primary. 

lib illustrates a plan of alternate current high frequency 
conversion which is most frequently used and which is found to 
be most convenient. This plan has been dwelt upon in detail on 
previous occasions and need not be described here. 

Some of these results were obtained by the use of a high 
frequency alternator. A description of such machines will be 
found in my original paper before the American Institute of 
Electrical Engineers, and in periodicals of that period, notably 
in THE ELECTRICAL ENGINEER of March 18, 1891. 

I will now proceed with the experiments. 



318 INVENTIONS OF NIKOLA TESLA. 

ON PHENOMENA PRODUCED BY ELECTROSTATIC FORCE. 

The first class of effects I intend to show you are effects pro- 
duced by electrostatic force. It is the force which governs the 
the motion of the atoms, which causes them to collide and de- 
velop the life-sustaining energy of heat and light, and which 
causes them to aggregate in an in finite 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 Na- 
ture. As the term electrostatic might imply a steady electric 
condition, it should be remarked, that in these experiments the 
force is not constant, but varies at a rate which may be consid- 
ered moderate, about one million times a second, or thereabouts. 
This enables me to produce many effects which are not produ- 
cible with an unvarying force. 

When two conducting bodies are insulated and electrified, 
we say that an electrostatic force is acting between them. This 
force manifests itself in attractions, repulsions and stresses in the 
bodies and space or medium without. So great may be the strain 
exerted in the air, or whatever separates the two conducting 
bodies, that it may break down, and we observe sparks or bundles 
of light or streamers, as they are called. These streamers form 
abundantly when the force through the air is rapidly varying. I 
will illustrate this action of electrostatic force in a novel experi- 
ment 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, be- 
cause even dry wood is by far too poor an insulator for these cur- 
rents of enormous potential differences. On one of the termi- 
nals 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 ob- 
ject 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 furi- 
ous sparks breaks forth from the end of the secondary wire, which 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 319 

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 rup- 
tured 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 




FIG. 169. 

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. 
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 con- 
nected to the second terminal of the coil. As the hand is 
approached, the air between it and the sphere, or in the imme- 
diate 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 



820 INVENTIONS OF NIKOLA TE8LA. 

tips a burning sensation is felt. They should not be confounded 
with those produced by an influence machine, because in many 
respects they behave differently. I have attached the brass sphere 
and plate to one of the terminals in order to prevent the formation 
of visible streamers on that terminal, also in order to prevent 
sparks from jumping at a considerable distance. Besides, the 
attachment is favorable for the working of the coil. 

The streams of light which you have observed issuing from my 
hand are due to a potential of about 200,000 volts, alternating in 
rather irregular intervals, sometimes like a million times a second. 
A vibration of the same amplitude, but four times as fast, to main- 
tain which over 3,000,000 volts would be required, would be 
more than sufficient to envelop my body in a complete sheet of 
flame. But this flame would not burn me up ; quite contrarily, 
the probability is that I would not be injured in the least. Yet a 
hundredth part of that energy, otherwise directed, would be amply 
sufficient to kill a person. 

The amount of energy which may thus be passed into the body 
of a person depends on the frequency and potential of the cur- 
rents, and by making both of these very great, a vast amount of 
energy may be passed into the body without causing any discom- 
fort, 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 cur- 
rent would traverse the body in the same manner as a low fre- 
quency 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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 321 

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. 

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 be- 
tween two oppositely electrified bodies is strained beyond a cer- 
tain 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 en- 
ergy would be spent in radiation, convection and mechanical and 
chemical action. Thus the spark length, or greatest distance 
which a spark will jump between the electrified bodies is the 




FIG. 170a. FIG. 170b. 

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. 

I will show you by an experiment the difference in the effect 
produced by a rapidly varying and a steady or moderately vary- 
ing force. I have here two large circular brass plates p p (Fig. 
170# and Fig. 1706), 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. 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 ap- 
paratus and their still greater importance in some purely scien- 
tific investigations. Often they are too weak to be visible, but 



322 INVENTIONS OF NIKOLA TESLA. 

they always exist, consuming energy and modifying the action 
of the apparatus. When intense, as they are at present, they 
produce ozone in great quantity, and also, as Professor Crookes 
has pointed out, nitrous acid. So quick is the chemical action that 
if a coil, such as this one, is worked for a very long time it will 
make the atmosphere of a small room unbearable, for the eyes 
and throat are attacked. But when moderately produced, the 
streamers refresh the atmosphere wonderfully, like a thunder- 
storm, and exercises unquestionably a beneficial effect. 

In this experiment the force acting between the plates changes 
in intensity and direction at a very rapid rate. I will now make 
the rate of change per unit time much smaller. This I effect by 
rendering the discharges through the primary of the induction 
coil less frequent, and also by diminishing the rapidity of the vi- 
bration in the secondary. The former result is conveniently se- 
cured by lowering the E. M. r. over the air gap in the primary 
circuit, the latter by approaching the two brass plates to a dis- 
tance 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 aug- 
ment the strain by raising the E. M. F. in the primary circuit, and 
soon you see the air give way and the hall is illuminated by a 
shower of brilliant and noisy sparks, Fig. 1TO&. 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 " alter- 
nating E. M. F," acting between the plates. This term is quite 
proper and applicable in all cases where there is evidence of at 
least a possibility of an essential inter-dependence of the electric 
state of the plates, or electric action in their neighborhood. But 
if the plates were removed to an infinite distance, or if at a finite 
distance, there is no probability or necessity whatever for such 
dependence. I prefer to use the term " electrostatic force," and 
to say that such a force is acting around each plate or electrified in- 
sulated body in general. There is an inconvenience in using this 
express/on as the term incidentally means a steady electric con- 
dition ; but a proper nomenclature will eventually settle this dif- 
ficulty. 



HIGH FItEQ UENCY AND HIQII POTENTIAL CUKRENTS. 323 

I now return to the experiment to which I have already al- 
luded, and with which I desire to illustrate a striking effect pro- 
duced by a rapidly varying electrostatic force. I attach to the end 
of the wire, I (Fig. 171), which is in connection with one of the 
terminals of the secondary of the induction coil, an exhausted 
bulb I. This bulb contains a thin carbon filament/, which is 
fastened to a platinum wire w, sealed in the glass and leading 
outside of the bulb, where it connects to the wire I. The 
bulb may be exhausted to any degree attainable with ordinary 
apparatus. Just a moment before, you have witnessed the break- 
ing 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 at- 
tached to the outside of the bulb, or placed near the same, and 





FIG. 171. 



FIG. 172a 



FIG. 172b. 



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 rup- 
ture of the glass. 

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 sur- 
rounding bodies we perceive might be removed to an infinite dis- 
tance without affecting the result in the slightest. Wher^he coil 
is set to work, the glass is invariably broken through at the seal, 
or other narrow channel, and the vacuum is quickly impaired. 



324 INVENTIONS OF NIKOLA TKSLA. 

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 h'ne fibre. In bulbs used with these coils such nar- 
row, pointed channels must therefore be avoided. 

When a conducting body is immersed in air, or similar insulat- 
ing medium, consisting of, or containing, small freely movable 
particles capable of being electrified, and when the electrification 
of the body is made to undergo a very rapid change which is 
equivalent to saying that the electrostatic force acting around 
the body is varying in intensity, the small particles are attracted 
and repelled, and their violent impacts against the body may 
cause a mechanical motion of the latter. Phenomena of this 
kind are noteworthy, inasmuch as they have not been observed 
before with apparatus such as has been commonly in use. If a 
very light conducting sphere be suspended on an exceedingly fine 
wire, and charged to a steady potential, however high, the sphere 
will remain at rest. Even if the potential would be rapidly 
varying, provided that the small particles of matter, molecules or 
atoms, are evenly distributed, no motion of the sphere should re- 
sult. 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&. 
In like manner, as I have shown on a previous occasion, a fan of 
sheet metal, Fig. 1 72&, covered partially with insulating material 
as indicated, and placed upon the terminal of the coil so as to turn 
freely, on it, is spun around. 

All these phenomena you have witnessed and others which 
will be shown later, are due to the presence of a medium like 
air, and would not occur in a continuous medium. The action 
of the air may be illustrated still better by the following experi- 
ment. I take a glass tube t, Fig. 173, of about an inch in di- 
ameter, which has a platinum wire w sealed in the lower end, 
and to which is attached a thin lamp filament f. I connect the 
wire with the terminal of the coil and set the coil to work. The 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 325 

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 con- 
tinuous 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 fre- 
quencies many times as great, for even though the displacements 





FIG. 178. 



FIG. 174. 



be small, if the frequency were much greater, considerable work 
might be performed in the oil. 

The electrostatic attractions and repulsions between bodies of 
measurable dimensions are, of all the manifestations of this force, 
the first so-called electrical phenomena noted. But though they 
have been known to us for many centuries, the precise nature of 
the mechanism concerned in these actions is still unknown to us, 
and has not been even quite satisfactorily explained. What kind 
of mechanism must that be ? We cannot help wondering when 
we observe two magnets attracting and repelling each other with 
a force of hundreds of pounds with apparently nothing between 
them. We have in our commercial dynamos magnets capable of 
sustaining in mid-air tons of weight. But what are even these 



326 INVENTIONS OF NIKOLA TESLA. 

forces acting between magnets when compared with the tremen- 
dous 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 com- 
pare 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 mtev- ,t for the thinker who inquires 
into the nature of these forces I./ note that whereas the actions 
between individual molecules ( atoms occur seemingly under any 
conditions, the attractions and repulsions of bodies of measurable 
dimensions imply a medium possessing insulating properties. So, 
if air, either by being rarefied or heated, is rendered more or less 
conducting, these actions between two electrified bodies practically 
cease, while the actions between the individual atoms continue to 
manifest themselves. 

An experiment may serve as an illustration and as a means of 
bringing out other features of interest. Some time ago I showed 
that a lamp filament or wire mounted in a bulb and connected to 
one of the terminals of a high tension secondary coil is set spin- 
ning, 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 inore 
favorable, a bulb like the one in Fig. 174, was constructed. It 
comprised a globe 5, in the neck of which was sealed a platinum 
wire w carrying a thin lamp filament/. In the lower part of 
the globe a tube t was sealed so as to surround the filament. The 
exhaustion was carried as far as it was practicable with .the appa- 
ratus employed. 

This bulb verified my expectation, for the filament was set 
spinning when the current was turned on, and became incandes- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 327 

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 in- 
side were brought to an elevated temperature, and as the gas in 
the tube then became conducting, the electrostatic attraction be- 
tween 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 bombard- 
ment was most intense, and also partly to the fact that the indi- 
vidual impacts were more violent and that no part of the supplied 
energy was converted into mechanical movement. Since, in ac- 
cordance 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 car- 
riers of electric charges immersed in an insulating medium ; yet 
there is no attractive force between the glass tube and the fila- 
ment because the space in the tube is, as a whole, conducting. 

It is of some interest to observe in this connection that whereas 
the attraction between two electrified bodies may cease owing to 
the impairing of the insulating power of the medium in which 
they are immersed, the repulsion between the bodies may still be 
observed. This may be explained in a plausible way. When the 
bodies are placed at some distance in a poorly conducting medium, 
such as slightly warmed or rarefied air, and are suddenly electri- 
fied, 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 af- 
forded for such dissipation, hence the repulsion observed in such 
case is greater than the attraction. Repulsive actions in a gas- 
eous medium are however, as Prof. Crookes has shown, enhanced 
by molecular bombardment. 

ON CURRENT OR DYNAMIC ELECTRICITY PHENOMENA. 

So far, I have considered principally effects produced by a 
varying electrostatic force in an insulating medium, such as air. 
When such a force is acting upon a conducting body of measur- 
able 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 



328 INVENTIONS OF NIKOLA TKSLA. 

shall presently endeavor to illustrate. In presenting this second 
class of electrical effects, I will avail myself principally of such 
as are producible without any return circuit, hoping to interest 
you the more by presenting these phenomena in a more or less 
novel aspect. 

It has been a long time customary, owing to the limited 
experience with vibratory currents, to consider an electric cur- 
rent 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 dis 
appearing, even among practical men, and will soon be entirely 
forgotten. 

If I connect an insulated metal plate P, Fig. 175, to one of the 
terminals T of the induction coil by means of a wire, though this 




FIG. 175. 

plate be very well insulated, a current passes through the 
wire when the coil is set to work. First I wish to give you 
evidence that there is a current passing through the connecting 
wire. An obvious way of demonstrating this is to insert between 
the terminal of the coil and the insulated plate a very thin plati- 
num or german silver wire w and bring the latter to incandes- 
cence or fusion by the current. This requires a rather large plate 
or else current impulses of very high potential and frequency. 
Another way is to take a coil c, Fig. 175, containing many turns of 
thin insulated wire and to insert the same in the path of the cur- 
rent to the plate. When I connect one of the ends of the coil to the 
wire leading to another insulated plate p l5 and its other end to the 
terminal TJ of the induction coil, and set the latter to work, a cur- 
rent passes through the inserted coil c and the existence of the 
current may be made manifest in various ways. For instance, I 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 329 

insert an iron core * 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 witli such high frequencies. One might 
take a core of some size, laminated or not, it would matter little ; 
but ordinary iron wire -^th or th of an inch thick is suitable 
for the purpose. While the induction coil is working, a current 
traverses the inserted coil and only a few moments are sufficient 
to bring the iron wire i to an elevated temperature sufficient to 
soften the sealing-wax s, and cause a paper washer p fastened by 
it to the iron wire to fall off. But with the apparatus such as I 
have here, other, much more interesting, demonstrations of this 
kind can be made. I have a secondary s, Fig 176, of coarse wire, 
wound upon a coil similar to the first. In the preceding experi- 
ment the current through the coil c, Fig. 175, was very small, but 
there being many turns a strong heating effect was, nevertheless, 




FIG. 176. 



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 prim- 
ary P into a strong secondary current of low tension, and this 
current will quite certainly do what I expect. In a small glass 
tube (t, Fig. 176), I have enclosed a coiled platinum wire, w, this 
merely in order to protect the wire. On each end of the glass 
tube is sealed a terminal of stout wire to which one of the ends of 
the platinum wire w, is connected. I join the terminals of the 
secondary coil to these terminals and insert the primary p, 
between the insulated plate r l5 and the terminal TJ, of the induc- 
tion coil as before. The latter being set to work, instantly the 
platinum wire w is rendered incandescent and can be fused, even 
if it be verv thick. 



330 INVENTIONS OF NIKOLA TESLA. 

Instead of the platinum wire I now take an ordinary 50-volt 
Ifi c. p. lamp. When I set the induction coil in operation the 
lamp filament is brought to high incandescence. It is, however, 
not necessary to use the insulated plate, for the lamp (7, Fig. 177) 
is rendered incandescent even if the plate p t 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. 

I may here call attention to a number of interesting observa- 
tions with the lamp. First, I disconnect one of the terminals of 
the lamp from the secondary s. When the induction coil plays, 
a glow is noted which tills 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 equi- 
valent to a metallic coating, which would be placed near the pri- 




FIG. 177. 

mary . If the secondary, or its equivalent, the coating, were placed 
symmetrically to the primary, the electrostatic induction would 
be nil under ordinary conditions, that is, when a primary return 
circuit is used, as both halves would neutralize each other. The 
secondary is in fact placed symmetrically to the primary, but the 
action of both halves of the latter, when only one of its ends is 
connected to the induction coil, is not exactly equal ; hence elec- 
trostatic 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. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 331 

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 con- 
nected to the terminal TJ of the induction coil and the other to 
the insulated plate p t as before. When the current is turned on, 
the lamp glows brightly, as shown in Fig. 17S&, in which c is a fine 
wire coil and s a coarse wire secondary wound upon it. If the 
insulated plate p t is disconnected, leaving one of the ends a of the 




FIG. 178b. 

primary insulated, the filament becomes dark or generally it dim- 
inishes in brightness (Fig. 1780). Connecting again the plate p t 
and raising the frequency of the current, I make the filament 
quite dark or barely red (Fig. 179J). Once more I will discon- 
nect the plate. One will of course infer that when the plate is 
disconnected, the current through the primary will be weakened, 
that therefore the E. M. F. will fall in the secondary s, and that 
the brightness of the lamp will diminish. This might be the 
case and the result can be secured by an easy adjustment of the 



332 



INVENTIONS OF NIKOLA TESLA. 



coils ; also by varying the frequency and potential of the cur- 
rents. But it is perhaps of greater interest to note, that the lamp 
increases in brightness when the plate is disconnected (Fig. 179#). 
In this case all the energy the primary receives is now sunk into 
it, like the charge of a battery in an ocean cable, but most of that 
energy is recovered through the secondary and used to light the 
lamp. The current traversing the primary is strongest at the end 
b which is connected to the terminal T X of the induction coil, and 




FIG 179b. 

diminishes in strength towards the remote end a. But the dyna- 
mic inductive effect exerted upon the secondary s is now greater 
than before, when the suspended plate was connected to the 
primary. These results might have been produced by a number 
of causes. For instance, the plate P! being connected, the reac- 
tion from the coil c may be such as to diminish the potential at 
the terminal T t of the induction coil, and therefore weaken the 
current through the primary of the coil c. Or the disconnecting 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 333 

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 TJ 
of the induction coil may be the same or even higher. Or the 
result might have been produced by the change of phase of the 
primary and secondary currents and consequent reaction. But 
the chief determining factor is the relation of the self-induction 
and capacity of coil c and plate p t and the frequency of the cur- 
rents. The greater brightness of the filament in Fig. 179&, is, 
however, in part due to the heating of the rarefied gas in the 
lamp by electrostatic induction, which, as before remarked, is 
greater when the suspended plate is disconnected. 

Still another feature of some interest I may here bring to your 
attention. When the insulated plate is disconnected and the sec- 
ondary 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 lumi- 
nosity of the filament is increased, the incandescence now being 
due partly to the flow of current through the filament and 
partly to the molecular bombardment of the rarefied gas in the 
bulb. 

The preceding experiments will have prepared one for the next 
following results of interest, obtained in the course of these in- 
vestigations. Since I can pass a current through an insulated 
wire merely by connecting one of its ends to the source of elec- 
trical energy, since I can induce by it another current, magnetize 
an iron core, and, in short, perform all operations as though a re- 
turn circuit were used, clearly I can also drive a motor by the aid 
of only one wire. On a former occasion 1 have described a sim- 
ple 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 



334 INVENTIONS OF NIKOLA TE8LA. 

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 arrange- 
ments in a few words only. The diagram, Fig. 180 II., shows 
a primary coil P, connected with one of its ends to the line L lead- 
ing from a high tension transformer terminal TJ. In inductive 
relation to this primary P is a secondary s of coarse wire in the 
circuit of which is a coil c. The currents induced in the second- 
ary energize the iron core ?', which is preferably, but not neces- 
sarily, subdivided, and set the metal disc d in rotation. Such a 
motor M 2 as diagramatically shown in Fig. 180 II., has been 
called a " magnetic lag motor," but this expression may be ob- 
jected to by those who attribute the rotation of the disc to eddy 
currents circulating in minute paths when the core i is finally 
subdivided. In order to operate such a motor effectively on the 
plan indicated, the frequencies should not be too high, not more 
than four or five thousand, though the rotation is produced even 
with ten thousand per second, or more. 

In Fig. 180 I., a motor M t having two energizing circuits, A and 
B, is diagrammatical ly indicated. The circuit A is connected to 
the line L and in series with it is a primary p, which may have its 
free end connected to an insulated plate p l5 such connection 
being indicated by the dotted lines. The other motor circuit B 
is connected to the secondary s which is in inductive relation to 
the primary p. When the transformer terminal T t is alternately 
electrified, currents traverse the open line L and also circuit A and 
primary p. The currents through the latter induce secondary 
currents in the circuit s, which pass through the energizing coil 
B of the motor. The currents through the secondary s and those 
through the primary p differ in phase 90 degrees, or nearly so, and 
are capable of rotating an armature placed in inductive relation 
to the circuits A and B. 

In Fig. 180 III., a similar motor M 3 with two energizing cir- 
cuits A! and B! is illustrated. A primary p, connected with one 
of its ends to the line L has a secondary s, which is preferably 
wound for a tolerably high E. M. r., and to which the two ener- 
gizing circuits of the motor are connected, one directly to the 
ends of the secondary and the other through a condenser c, by the 
action of which the currents traversing the circuit A t and B t are 
made to differ in phase. 

In Fig. 180 IV., still another arrangement is shown. In this 
case two primaries p t and P 2 are connected to the line L, one 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 335 









-e 





336 INVENTIONS OF NIKOLA TESLA. 

through a condenser c of small capacity, and the other directly. 
The primaries are provided witli secondaries s t and s 2 which are 
in series with the energizing circuits, A 2 and B 2 and a motor M 3 , 
the condenser c again serving to produce the requisite difference 
in the phase of the currents traversing the motor circuits. As 
such phase motors with two or more circuits are now well known 
in the art, they have been here illustrated diagrammatically. No 
difficulty whatever is found in operating a motor in the manner 
indicated, or in similar ways ; and although such experiments up 
to this day present only scientific interest, they may at a period 
not far distant, be carried out with practical objects in view. 

It is thought useful to devote here a few remarks to the sub- 
ject of operating devices of all kinds by means of only one leading 
wire. It is quite obvious, that when high-frequency currents are 
made use of, ground connections are at least when the E. M. F. 
of the currents is great better than a return wire. Such ground 
connections are objectionable witli steady or low frequency cur- 
rents on account of destructive chemical actions of the former 
and disturbing influences exerted by both on the neighboring cir- 
cuits; but with high frequencies these actions practically do not 
exist. Still, even ground connections become superfluous when 
the E. M. F. is very high, for soon a condition is reached, when the 
current may be passed more economically through open, than 
through closed, conductors. Remote as might seem an industrial 
application of such single wire transmission of energy to one not 
experienced in such lines of experiment, it will not seem so to 
anyone who for some time has carried on investigations of such 
nature. Indeed I cannot see why such a plan should not be 
practicable. Nor should it be thought that for carrying out such 
a plan currents of very high frequency are expressly required, 
for just as soon as potentials of say 30,000 volts are used, the 
single wire transmission may be effected with low frequencies, 
and experiments have been made by me from which these infer- 
ences are made. 

When the frequencies are very high it has been found in lab- 
oratory practice quite easy to regulate the effects in the manner 
shown in diagram Fig. 181. Here two primaries p and p l are shown, 
each connected with one of its ends to the line L and with the 
other end to the condenser plates c and c, respectively. Near 
these are placed other condenser plates c x and c,, the former be- 
ing connected to the line L and the latter to an insulated larger 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 337 

plate P 2 . On the primaries are wound secondaries s and s t , of 
coarse wire, connected to the devices d and I respectively. By- 
varying the distances of the condenser plates c and c l5 and c and 
c t the currents through the secondaries s and s t are varied in 
intensity. The curious feature is the great sensitiveness, the 
slightest change in the distance of the plates producing consid- 
erable 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 varia- 
tions. For instance, I have been able to adjust the conditions so 
that the mere approach of a person to the coil produces a con- 
siderable 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. 

Yery 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 s.uch low fre- 
quency 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 experi- 
ment with a simple motor of this kind, inasmuch as it is com- 
monly thought that disruptive discharges are unsuitable for such 
purposes. The motor is illustrated in Fig. 182. It comprises a 
rather large iron core * with slots on the top into which are em- 
bedded thick copper washers c c. In proximity to the core is 
a freely -movable metal disc D. The core is provided with a pri- 
mary exciting coil c a the ends a and b of which are connected to 



338 INVENTIONS OF NIKOLA TKSLA. 

the terminals of the secondary s of an ordinary transformer, the 
primary p of the latter being connected to an alternating distri- 
bution circuit or generator o of low or moderate frequency. 
The terminals of the secondary s are attached to a condenser c 
which discharges through an air gap d d which may be placed 
in series or shunt to the coil c x . When the conditions are 
properly chosen the disc D rotates with considerable effort and the 
iron core i does not get very perceptibly hot. With currents from 
a high-frequency alternator, on the contrary, the core gets rapidly 
hot and the disc rotates with a much smaller effort. To perform 
the experiment properly it should be first ascertained that the 
disc D is not set in rotation when the discharge is not occurring 
at d d. It is preferable to use a large iron core and a condenser 
of large capacity so as to bring the superimposed quicker oscil- 
lation to a very low pitch or to do away \vith it entirely. By 
observing certain elementary rules I have also found it practi- 
cable to operate ordinary series or shunt direct-current motors 
with such disruptive discharges, and this can be done with or 
without a return wire. 

IMPEDANCE PHENOMENA. 

Among the various current phenomena observed, perhaps the 
most interesting are those of impedance presented by conductors 
to currents varying at a rapid rate. In my first paper before the 
American Institute of Electrical Engineers, I have described a 
few striking observations of this kind. Thus I showed that when 
such currents or sudden dischaiges 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 surround- 
ing 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 in- 
terest to be described here. 

Referring to Fig. 1830, B and BJ are very stout copper bars 
connected at their lower ends to plates c and c 1? respectively, of a 
condenser, the opposite plates of the latter being connected to the 
terminals of the secondary s of a high-tension transformer, the 
primary p of which is supplied with alternating currents from an 
ordinary low-frequency dynamo & or distribution circuit. The 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 339 

condenser discharges through an adjustable gap dd&& usual. By 
establishing a rapid vibration it was found quite easy to perform 
the following curious experiment. The bars B and B t were joined 
at the top by a low-voltage lamp Z 3 ; a little lower was placed by 
means of clamps c c, a 50-volt lamp 4 ; and still lower another 100- 
volt lamp /! ; and finally, at a certain distance below the latter 
lamp, an exhausted tube T. By carefully determining the po- 
sitions of these devices it was found practicable to maintain them 




FIQB. 183a, 183b and 183c. 

all at their proper illuminating power. Yet they were all con- 
nected 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. 

In Figs. 1835 and 1836', two other experiments are illustrated 
which, unlike the previous experiment, do not require very care- 
ful adjustments. In Fig. 1835, two lamps, ^ and 4, the former a 



340 INVENTIONS OF NIKOLA TESLA. 

100-volt and the latter a 50-volt are placed in certain positions as 
indicated, the 100-volt lamp being below the 50-volt lamp. When 
the arc is playing at d ' d and the sudden discharges are passed 
through the bars B B,, 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. 1835. Now the 
bars B B! may be joined at the top by a thick cross bar -^ and it 
is quite easy to maintain the 100-volt lamp at full candle-power 
while the 50-volt lamp remains dark, Fig. 183c. 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 na- 
ture of electric currents. 

In the preceding experiments I have already had occasion to 
show some light phenomena and it would now be proper to study 
these in particular ; but to make this investigation more com- 
plete I think it necessary to make first a few remarks on the 
subject of electrical resonance which has to be always observed 
in carrying out these experiments. 

ON ELECTRICAL RESONANCE. 

The effects of resonance are being more and more noted by engi- 
neers and are becoming of great importance in the practical opera- 
tion 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 vibra- 
tion 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 ex- 
periment both show that such is impossible in Nature, for as the 
oscillation becomes more and more vigorous, the losses in the vi- 
brating bodies and environing media rapidly increase and necessa- 
rily 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 



HIGH FREQ UENCT AND HIGH POTENTIAL CURRENTS. 341 

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 f rictional losses. The 
smaller these tosses, 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 dielec- 
tric. In a Leyden jar discharging through a short stranded cable 
of thin wires these requirements are probably best fulfilled, and 
the resonance effects are therefore very prominent. Such is not 
the case with dynamo machines, transformers and their circuits, 
or with commercial apparatus in general in which the presence 
of iron cores complicates the action or renders it impossible. 
In regard to Leyden jars . with which resonance effects are 
frequently demonstrated, I would say that the effects observed 
are often attributed but are seldom due to true resonance, for 
an error is quite easily made in this respect. This may be 
undoubtedly demonstrated by the following experiment. Take, 
for instance, two large insulated metallic plates or spheres which 
I shall designate A and B; place them at a certain small dis- 
tance 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 mak- 
ing a few preliminary trials. If now another plate fastened on 
an insulating handle and connected by a wire to one of the ter- 
minals of a high tension secondary of an induction coil, which 
is maintained in action by an alternator (preferably high fre- 
quency) is approached to one of the charged bodies A or B, so as 
to be nearer to either one of them, the discharge will invariably 
occur between them ; at least it will, if the potential of the coil 
in connection with the plate is sufficiently high. But the expla- 
nation of this will soon be found in the fact that the approached 
plate acts inductively upon the bodies A and B and causes a spark 
to pass between them. When this spark occurs, the charges which 
were previously imparted to these bodies from the influence ma- 
chine, must needs be lost, since the bodies are brought in electri- 



342 INVENTIONS OF NIKOLA TESLA. 

cal connection through the arc formed. Xow this arc is formed 
whether there be resonance or not. But even if the spark would 
not be produced, still there is an alternating E. M. F. set up between 
the bodies when the plate is brought near one of thfem ; therefore 
the approach of the plate, if it does not always actually, will, at any 
rate, tend to break down the air space by inductive action. Instead 
of the spheres or plates A and B we may take the coatings of a Ley- 
den jar with the same result, and in place of the machine, which 
is a high frequency alternator preferably, because it is more suit- 
able for the experiment and also for the argument, we may take 
another Leyden jar or battery of jars. When such jars are dis- 
charging through a circuit of low resistance the same is traversed 
by currents of very high frequency. The plate may now be con- 
nected 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 be- 
fore, 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 pro- 
duced through the air space, or at least the latter will be consider- 
ably more strained owing to the setting up of an alternating 
K. M. F. by induction, which takes place when one of the jars be- 
gins 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 dis- 
charging one by the other, and now a coil of wire be added to one 
of the circuits whereupon the experiment does not succeed, the 
conclusion that this is due to the fact that the circuits are now 
not tuned, would be far from being safe. For the two circuits 
act as condenser coatings and the addition of the coil to one of 
them is equivalent to bridging them, at the point where the coil 
is placed, by a small condenser, and the effect of the latter might 
be to prevent the spark from jumping through the discharge space 
by diminishing the alternating E. M. F. acting across the same. 
All these remarks, and many more which might be added but for 
fear of wandering too far from the subject, are made with the 
pardonable intention of cautioning the unsuspecting student, who 
might gain an entirely unwarranted opinion of his skill at see- 
ing every experiment succeed ; but they are in no way thrust upon 
the experienced as novel observations. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 843 

In order to make reliable observations of electric resonance 
effects it is very desirable, if not necessary, to employ an alter- 
nator 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 fre- 
quencies. 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 tun- 
ing forks which will respond to a note of a certain pitch, or loaded 
springs which have a definite period of vibration. But the reson- 
ance will be most perfectly attained in that case in which the mo- 
tion is effected with the greatest freedom. Now in mechanics, 
considering the vibration in the common medium that is, air it 
is of comparatively little importance whether one tuning fork be 
somewhat larger than another, because the losses in the air are 
not very considerable. One may, of course, enclose a tuning fork 
in an exhausted vessel and by thus reducing the air resistance to 
a minimum obtain better resonant action. Still the difference 
would not be very great. But it would make a great difference if 
the tuning fork were immersed in mercury. In the electrical 
vibration it is of enormous importance to arrange the conditions 
so that the vibration is effected with the greatest freedom. The 
magnitude of the resonance effect depends, under otherwise equal 
conditions, on the quantity of electricity set in motion or on the 
strength of the current driven through the circuit. But the cir- 
cuit opposes the passage of the currents by reason of its imped- 
ance and therefore, to secure the best action it is necessary to re- 
duce the impedance to a minimum. It is impossible to overcome 
it entirely, but merely in part, for the ohmic resistance cannot be 
overcame. 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 capac- 
ity. 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 con- 
denser in series with the self-induction. It is clear that in such 



844 INVENTIONS OF NIKOLA TESLA. 

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-induc- 
tion, but the latter is preferable, because it is inconvenient to ad- 
just 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 fre- 
quency at which the maximum current will be made to pass 
through the coil. The observation of the relation between self- 




FIG. 184. 

induction, capacity and frequency is becoming important in the 
operation of alternate current apparatus, such as transformers or 
motors, because by a judicious determination of the elements the 
employment of an expensive condenser becomes unnecessary. 
Thus it is possible to pass through the coils of an alternating 
current motor under the normal working conditions the required 
current with a low E. M. F. and do away entirely with the false 
current, and the larger the motor, the easier such a plan becomes 
practicable ; but it is necessary for this to employ currents of very 
high potential or high frequency. 

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 al- 
ternator. G! is a coil of many turns, which is divided into small 
separate sections for the purpose of adjustment. The final ad- 
justment was made sometimes with a few thin iron wires (though 
this is not always advisable) or with a closed secondary. The coil 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 345 

Cj is connected with one of its ends to the line L from the alter- 
nator G and with the other end to one of the plates c of a con- 
denser c GI, the plate (c^ of the latter being connected to a much 
larger plate PJ. In this manner both capacity and self-induction 
were adjusted to suit the dynamo frequency. 

As regards the rise of potential through resonant action, of 
course, theoretically, it may amount to anything since it depends 
on self-induction and resistance and since these may have any 
value. But in practice one is limited in the selection of these 
values and besides these, there are other limiting causes. One 
may start with, say, 1,000 volts and raise the E. M. F. to 50 times 
that value, but one cannot start with 100,000 and raise it to ten 
times that value because of the losses in the media which are 
great, especially if the frequency is high. It should be possible 
to start with, for instance, two volts from a high or low fre- 
quency circuit of a dynamo and raise the E. M. r. to many hun- 
dred times that value. Thus coils of the proper dimensions 
might be connected each with only one of its ends to the 
mains from a machine of low E. M. F., and though the circuit of 
the machine would not be closed in the ordinary acceptance of the 
term, yet the machine might be burned out if a proper resonance 
effect would be obtained. I have not been able to produce, nor 
have I observed with currents from a dynamo machine, such 
great rises of potential. It is possible, if not probable, that with 
currents obtained from apparatus containing iron the disturbing 
influence of the latter is the cause that these theoretical pos- 
sibilities cannot be realized. But if such is the case I attribute 
it solely to the hysteresis and Foucault current losses in the core. 
Generally it was necessary to transform upward, when the E. M. 
F. was very low, and usually an ordinary form of induction coil 
was employed, but sometimes the arrangement illustrated in Fig. 
184 II., has been found to be convenient. In this case a coil cis 
made in a great many sections, a few of these being used as a 
primary. In this manner both primary and secondary are ad- 
justable. One end of the coil is connected to the line i^ from 
the alternator, and the other line L is connected to the intermedi- 
ate point of the coil. Such a coil with adjustable primary and 
secondary will be found also convenient in experiments with the 
disruptive discharge. When true resonance is obtained the top 
of the wave must of course be on the free end of the coil as, for 
instance, at the terminal of the phosphorescence bulb B. This is 



346 INVENTIONS OF NIKOLA TESLA. 

easily recognized by observing the potential of a point on tl it- 
wire w near to the coil. 

In connection with resonance effects and the problem of trans- 
mission 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 elec- 
trical investigations. Some enthusiasts have expressed their be- 
lief 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 power- 
ful 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 con- 
ductor. Why then not try to avail ourselves of the earth for 
this purpose ? We need not be frightened by the idea of dis- 
tance. To the weary wanderer counting the mile-posts the earth 
may appear very large, but to that happiest of all men, the as- 
tronomer, who gazes at the heavens and by their standard judges 
the magnitude of our globe, it appears very small. And so I 
think it must seem to the electrician, for when he considers the 
speed with which an electric disturbance is propagated through 
the earth all his ideas of distance must completely vanish. 

A point of great importance would be first to know what is the 
capacity of the earth ? and what charge does it contain if electri- 
fied ? 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 347 

by whatever process it was separated from other bodies and this 
is the accepted view of its origin it must have retained a charge, 
as occurs in all processes of mechanical separation. If it be a 
charged body insulated in space its capacity should be extremely 
small, less than one-thousandth of a farad. But the upper strata 
of the air are conducting, and so, perhaps, is the medium in free 
space beyond the atmosphere, and these may contain an opposite 
charge. Then the capacity might be incomparably greater. In 
any case it is of the greatest importance to get an idea of what 
quantity of electricity the earth contains. It is difficult to say 
whether we shall ever acquire this necessary knowledge, but there 
is hope that we may, and that is, by means of electrical resonance. 
If ever we can ascertain at what period the earth's charge, when 
disturbed, oscillates with respect to an oppositely electrified system 
or known circuit, we shall know a fact possibly of the greatest 
importance to the welfare of the human race. I propose to seek 
for the period by means of an electrical oscillator, or a source of 
alternating electric currents. One of the terminals of the source 
would be connected to earth as, for instance, to the city water 
mains* the other to an insulated body of large surface. It is pos- 
sible 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 cer- 
tainly is possible for of this we have daily evidence to pro- 
duce some electrical disturbance sufficiently powerful to be per- 
ceptible by suitable instruments at any point of the earth's 
surface. 



348 INVENTIONS OF NIKOLA TESLA. 

Assume that a source of alternating currentss be connected, as 
in Fig. 185, with one of its terminals to earth (conveniently to the 
water mains) and with the other to a body of large surface p. 
When the electric oscillation is set up there will be 
a movement of electricity in and out of p, and alter- 
nating currents will pass through the earth, con- 
verging to, or diverging from, the point c where 
the ground connection is made. In this manner 
neighboring points on the earth's surface within a 
certain radius will be disturbed. But the distur- 
bance will diminish with the distance, and the dis- 
tance at which the effect will still be perceptible 
will depend on the quantity of electricity set in 
motion. Since the body p is insulated, in order to 
displace a considerable quantity, the potential of 
the source must be excessive, since there would be 
limitations as to the surface of p. The conditions 
might be adjusted so that the generator or source 
s will set up the same electrical movement as 
though its circuit were closed. Thus it is certainly 
$2 practicable to impress an electric vibration at least 
g of a certain low period upon the earth by means of 
proper machinery. At what distance such a vibra- 
tion might be made perceptible can only be conjec- 
tured. 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 dis- 
turbing 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 dis- 
tance, or even all over the surface of the globe. 
. tq Now, it is quite certain that at any point within a 

certain radius of the source s a properly adjusted 
self-induction and capacity device can be set in action 
by resonance. But not only can this be done, but another source 
s,, Fig. 185, similar to s, or any number of such sources, can be set 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 349 

to work in synchronism with the latter, and the vibration thus 
intensified and spread over a large area, or a flow of elec- 
tricity produced to or from the source s t if the same be of 
opposite phase to the source s. I think that beyond doubt 
it is possible to operate electrical devices in a city through 
the ground or pipe system by resonance from an electrical 
oscillator located at a central point. But the practical solution 
of this problem would be of incomparably smaller benefit to man 
than the realization of the scheme of transmitting intelligence, or 
perhaps power, to any distance through the earth or environing 
medium. If this is at all possible, distance does not mean any- 
thing. Proper apparatus must first be produced by means of 
which the problem can be attacked and I have devoted much 
thought to this subject. I am firmly convinced that it can be 
done and hope that we shall live to see it done. 



ON THE LIGHT PHENOMENA PRODUCED BY HIGH-FREQUENCY CUR- 
RENTS OF HIGH POTENTIAL AND GENERAL REMARKS RELATING 
TO THE SUBJECT. 

Returning now to the light effects which it has been the chief 
object to investigate, it is thought proper to divide these effects 
into four classes : 1. Incandescence of a solid. 2. Phosphorescence. 

3. Incandescence or phosphorescence of a rarefied gas ; and 

4. Luminosity produced in a gas at ordinary pressure. The first 
question is : How are these luminous effects produced ? In order 
to answer this question as satisfactorily as I am able to do in the 
light of accepted views and with the experience acquired, and to 
add some interest to this demonstration, I shall dwell here upon 
a feature which I consider of great importance, inasmuch as it 
promises, besides, to throw a better light upon the nature of most 
of the phenomena produced by high-frequency electric currents. 
I have on other occasions pointed out the great importance of the 
presence of the rarefied gas, or atomic medium in general, around 
the conductor through which alternate currents of high frequency 
are passed, as regards the heating of the conductor by the cur- 
rents. My experiments, described some time ago, have shown 
that, the higher the frequency and potential difference of the cur- 
rents, the more important becomes the rarefied gas in which the 
conductor is immersed, as a factor of the heating. The poten- 
tial difference, however, is, as I then pointed out, a more im- 



350 INVENTIONS OF NIKOLA TESLA. 

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 or- 
dinary or other pressure as regards the incandescence or other 
luminous effects produced by currents of this kind. 

I take two ordinary 50-volt 16 c. p. lamps which are in every 
respect alike, with the exception, that one has been opened at the 
top and the air has filled the bulb, while the other is at the ordi- 
nary 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 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 may be properly stated that 
it depends on the time rate of the variation of the potential dif- 
ference. 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 advan- 
tage of doing away with the objections, which might be made in 
the experiment just shown, even if both the lamps were con- 
nected in series or multiple arc to the coil, namely, that in con- 
sequence of the reactions existing between the primary and 
secondary coil the conclusions are rendered uncertain. This re- 
sult I secure by charging, from an ordinary transformer which is 
fed from the alternating current supply station, a battery of con- 
densers, and discharging the latter directly through a circuit of 
small self-induction, as before illustrated in Figs. 183*, 183&, 
and 1836-. 

In Figs. 186, 1865 and 186c, the heavy copper bars BB^ are 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 351 

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 o c. The discharges being passed through the lamp, 
the filament is rendered incandescent, though the current 
through it is very small, and would not be nearly sufficient to 
produce a visible effect under the conditions of ordinary use of 
the lamp. Instead of this I now attach to the bars another 
lamp exactly like the first, but with the seal broken off, the bulb 
being therefore filled with air at ordinary pressure. When the 
discharges are directed through the filament, as before, it does 
not become incandescent. But the result might still be attri- 
buted to one of the many possible reactions. I therefore connect 
both the lamps in multiple arc as illustrated in Fig. 186. Passing 






FIG. 186a. 



FIG. 186b. 



FIG. 186c. 



the discharges through both the lamps, again the filament in the 
exhausted lamp I glows very brightly while that in the non-ex- 
hausted lamp Zi 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 experi- 
ment 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 sup- 
plied 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 



352 INVENTIONS OF NIKOLA TESLA. 

large portion of the energy without producing any visible effect. 
The reason is that, there being many molecules, the bombard- 
ment 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 im- 
pacts are violent and therefore better adapted to produce a visi- 
ble 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 oc- 
curs between the ends of the filament or between the platinum 
wires outside, before such a difference of potential can be 
reached. 

It might be objected that in the experiment before shown the 
lamps, being in multiple arc, the exhausted lamp might take a 
much larger current and that the effect observed might not be 
exactly attributable to the action of the gas in the bulbs. Such 
objections will lose much weight if I connect the lamps in series, 
with the same result. When this is done and the discharges are 
directed through the filaments, it is again noted that the filament 
in the non-exhausted bulb l^ remains dark, while that in the 
exhausted one (7) glows even more intensely than under its 
normal conditions of working, Fig. 1865. According to general 
ideas the current through the filaments should now be the same, 
were it not modified by the presence of the gas around the 
filaments. 

At this juncture I may point out another interesting feature, 
which illustrates the effect of the rate of change of potential 
of the currents. I will leave the two lamps connected in series 
to the bars BB,, as in the previous experiment, Fig. 186&, 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 dis- 
charges, or by augmenting the capacity of the condensers. When 
I now pass these low-frequency discharges through the lamps, 
the exhausted lamp I again is as bright as before, but it is noted 
also that the non-exhausted lamp l glows, though not quite 



HIGH FJiKQ UENCY AND HIGH POTENTIAL CURRENTS. 353 

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 is bright, Fig. I860, 
the degree of its incandescence is much smaller than in Fig. 1865, 
when the currents were of a much higher frequency. 

In these experiments the gas acts in two opposite ways in de- 
termining 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 bom- 
bardment. 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 considera- 
bly 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 ef- 
fect with thin wires was clearly shown. This effect may be strik- 
ingly 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 cur- 
rent 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 deter- 
mined 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 df them. 

I may here describe a result which is still more interesting, 
and to which I have been led by the observation of these phe- 



354 INVENTIONS OF NIKOLA TKSLA. 

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 uni- 
form density, a very thin wire contained in the tube might be 
rendered incandescent at certain places of smaller density of the 
gas, while it would remain dark at the places of greater density, 
where the convection would be greater and the bombardment less 
intense. Accordingly a tube t was prepared, as illustrated in Fig. 
187, which contained through the middle a very line platinum wire 
w. The tube was exhausted to a moderate degree and it was found 
that when it was attached to the terminal of a high-frequency coil 
the platinum wire w would indeed, become incandescent in patches, 
as illustrated in Fig. 187. Later a number of these tubes with one 
or more wires were prepared, each showing this result. The ef- 
fect was best noted when the striated discharge occurred in the 
tube, but was also produced when the stride were not vi-ible, 
showing that, even then, the gas in the tube was not of uniform 
density. The position of the strirp was generally such, that the 
rarefactions corresponded to the places of incandescence or greater 
brightness on the wire w. But in a few instances it was noted, that 
the bright spots on the wire were covered by the dense parts of 
the striated discharge as indicated by / in Fig. 187, though the effect 
Avas 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. 

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 cur- 
rent 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 fiow. When 
the current is a varying one the resistance is greater, and hence 



HIGH FREQ UWCY AND HIGH POTENTIA L CURRENTS. 355 



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 in- 
appreciable currents, and we are able to take a short and thick 
block of carbon or other material and bring it to bright incan- 
descence 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 sub- 
jects are changing, and how quickly our field of knowledge is ex- 





FIG. 187. 



FIG. 188. 



tending. In the art of incandescent lighting, to view this result 
in one aspect only, it has been commonly considered as an essen- 
tial 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 rareiied 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 



356 INVENTIONS OF NIKOLA TESLA. 

would be of advantage, so far as the lamp is considered, to em- 
ploy high frequencies for lighting, as they allow the use of short 
and thick filaments and smaller currents. 

If a wire or filament be immersed in a homogeneous medium, all 
the heating is due to true conduction current, but if it be enclosed 
in an exhausted vessel the conditions are entirely different. Here 
the gas begins to act and the heating effect of the conduction cur- 
rent, as is shown in many experiments, may be very small com- 
pared with that of the bombardment. This is especially the case if 
the circuit is not closed and the potentials are of course very high. 
Suppose that a fine filament enclosed in an exhausted vessel be 
connected with one of its ends to the terminal of a high tension 
coil and with its other end to a large insulated plate. Though 
the circuit is not closed, the filament, as I have before shown, is 
brought to incandescence. If the frequency and potential be 
comparatively low, the filament is heated by the current passing 
through it. If the frequency and potential, and principally the 
latter, be increased, the insulated plate need 'be but very small, or 
may be done away with entirely ; still the filament will become 
incandescent, practically all the heating being then due to the bom- 
bardment. 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 heat- 
ing of a conductor by varying currents, but gas at ordinary pres- 
sure may become important, if the potential difference and fre- 
quency of the currents is excessive. On this subject I have al- 
ready stated, that when a conductor is fused by a stroke of 
lightning, the current through it may be exceedingly small, not 
even sufficient to heat the conductor perceptibly, were the latter 
immersed in a homogeneous medium. 

From the preceding it is clear that when a conductor of high 
resistance is connected to the terminals of a source of high fre- 
quency 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 



HIGH FRE^JENOY AND HIGH POTENTIAL CURRENTS. 357 

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 in- 
terpreted, for the latter would, or should, occur also in a continu- 
ous 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 re- 
main 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 en- 
tirely by leakage. This leakage or dissipation into space when 
the tension is high, is considerable when incandescent lamps are 
used, and still more considerable with arcs, for the latter act like 
flames. Generally, of course, the dissipation is much smaller 
with steady, than with varying, currents. 

I have contrived an experiment which illustrates in an inter- 
esting manner the effect of lateral diffusion. If a very long tube 
is attached to the terminal of a high frequency coil, the luminos- 
ity is greatest near the terminal and falls off gradually towards 
the remote end. This is more marked if the tube is narrow. 

A small tube about one-half inch in diameter and twelve 
inches long (Fig. 189), has one of its ends drawn out into a fine 
fibre/ nearly three feet long. The tube is placed in a brass socket 
T which can be screwed on the terminal T X of the induction coil. 
The discharge passing through the tube first illuminates the bot- 
tom 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 con- 
ducting and the discharge spreads into the glass fibre. This spread- 
ing 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 



358 INVENTIONS OF NIKOLA TE8L*. ~ 

\ 

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 dis- 
charge 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 in- 
stantly breaks through almost without exception. 




FIG. 189. 



FIG. l0. 



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 



HIGH FREQ UENCY AND HIGH FOTENTIA L C URRENTS. 359 



human frame is conducting I may convey the vibration through 
my body. 

First, as in some previous experiments, I connect my body with 
one of the terminals of a high-tension transformer and take in my 
hand an exhausted bulb which contains a small carbon button 
mounted upon a platinum wire leading to the outside of the bulb, 
and the button is rendered incandescent as soon as the transformer 
is set to work (Fig. 190). I may place a conducting shade on the 
bulb which serves to intensify the action, but is not necessary. 
Nor is it required that the button should be in conducting con- 
nection witli the hand through a wire leading through the glass, 





FIG. 192. 

for sufficient energy may be transmitted through the glass itself 
by inductive action to render the button incandescent. 

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 cur- 
rents 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 su 
intense as to cover the wire with a luminous film (Fig. 193). 



360 INVENTIONS OF NIKOLA TESLA. 

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 heat- 
ing 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 con- 
nected 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 po- 
tential. 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 com- 
paratively very great supply of energy to the button to maintain 
it at incandescence with a steady potential. The higher the fre- 
quency of the electric impulses the more economically can the 
button be maintained at incandescence. One of the chief rea- 
sons why this is so, is, I believe, that with impulses of very high 
frequency there is less exchange of the freely movable carriers 
around the electrode and this means, that in the bulb the heated 
matter is better confined to the neighborhood of the button. If 
a double bulb, as illustrated in Fig. 194 be made, comprising a 
large globe B and a small one 5, each containing as usual a fila- 
ment/" mounted on a platinum wire w and w t , it is found, that if 
the filaments ff be exactly alike, it requires less energy to keep 
the filament in the globe b at a certain degree of incandescence, 
than that in the globe B. This is due to the confinement of the 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS 361 

movable particles around the button. In this case it is also ascer- 
tained, that the filament in the small globe 5 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 con- 
ductor, 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 con- 
vection and radiation on the outside increase. On another oc- 
casion I have shown bulbs in which this drawback was largely 
avoided. In these instances a very small bulb, containing a re- 
fractory button, was mounted in a large globe and the space be- 




FIG. 193. 



FIG. 194. 




tween 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 incan- 
descent 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 moti n 
of the medium relatively to us, for a permanent condition could 



362 INVENTIONS OF NIKOLA TESLA. 

not be maintained without the medium being constantly renewed. 
A vacuum cannot, according to all evidence, be permanently 
maintained around a hot body. 

In these constructions, before mentioned, the small bulb inside 
would, at least in the first stages, prevent all bombardment 
a* - against the outer large globe. It occurred to me then to ascer- 
tain how a metal sieve would behave in this respect, and several 
bulbs, as illustrated in Fig. 195, were prepared for this purpose. 
r . In a globe &, was mounted a thin filament f (or button) upon a 
platinum wire w passing through a glass stem and leading to the 
outside of the globe. The filament /"was surrounded by a metal 
sieve s. It was found in experiments with such bulbs that a sieve 
with wide meshes apparently did not in the slightest affect the 
bombardment against the globe b. When the vacuum was high, 
the shadow of the sieve was clearly projected against the globe 
and the latter would get hot in a short while. In some bulbs the 
sieve .s was connected to a platinum wire sealed in the glass. 
When this w r ire was connected to the other terminal of the induc- 
tion coil (the E. M. F. being kept low in this case), or to an insu- 
lated plate, the bombardment against the outer globe 1) was 
diminished. By taking a sieve with fine meshes the bombard- 
ment against the globe b was always diminished, but even then 
if the exhaustion was carried very far, and when the potential of 
the transformer was very high, the globe 1} would be bombarded 
and heated quickly, though no shadow pf 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 en- 
tirely cut off the bombardment and for a while the outer globe b 
would remain perfectly cold. Of course when the glass tube 
was sufficiently heated the bombardment against the outer globe 
could be noted at once. The experiments with these bulbs 
seemed to show that the speeds of the projected molecules or 
particles must be considerable (though quite insignificant when 
compared with that of light), otherwise it would be difficult to 
understand how they could traverse a fine metal sieve without 
being affected, unless it were found that such small particles or 
atoms cannot be acted upon directly at measurable distances. 
In regard to the speed of the projected atoms, Lord Kelvin has 
recently estimated it at about one kilometre a second or there- 
abouts in an ordinary Crookes bulb. As the potentials obtainable 
with a disruptive discharge coil are much higher than with or- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. :',(>:; 

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. 

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 ar- 
tifices, 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 in- 
candescence might be attributed in such case except to the bom- 
bardment or similar action of the residual gas, or of particles of 
matter in general. But if the bulb be exhausted with the great- 
est 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 con- 
tinuous 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 poten- 
tial are passed must be modified by the behavior of such medium ; 
then also the skin effect, the apparent increase of the ohmic re- 
sistance, etc., admit, partially at least, of a different explanation. 

It is certainly more in accordance with many phenomena ob- 
served with high-frequency currents to hold that all space is per- 
vaded 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 con- 
tinuous medium, since in such there can be neither heat nor light. 



364 INVENTIONS OF NIKOLA TE8LA. 

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, incandes- 
cence, or phosphorescence, involve the presence of free atoms and 
would be impossible without these. 

In regard to the incandescence of a refractory button (or fila- 
ment) in an exhausted receiver, which has been one of the sub- 
jects of this investigation, the chief experiences, which may serve 
as a guide in constructing such bulbs, may be summed up as fol- 
lows : 1. The button should be as small as possible, spherical, 
of a smooth or polished surface, and of refractory material which 
withstands evaporation best. 2. The support of the button 
should be very thin and screened by an aluminum and mica sheet, 
as I have described on another occasion. 3. The exhaustion of 
the bulb should be as high as possible. 4. The frequency of the 
currents should be as high as practicable. 5. The currents should 
be of a harmonic rise and fall, without sudden interruptions. 6. 
The heat should be confined to the button by inclosing the same 
in a small bulb or otherwise. 7. The space between the walls of 
the small bulb and the outer globe should be highly exhausted. 
Most of the considerations which apply to the incandescence 
of a solid just considered may likewise be applied to phosphor- 
escence. Indeed, in an exhausted vessel the phosphorescence is, 
as a rule, primarily excited by the powerful beating of the elec- 
trode 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. 1Y4, be taken and exhausted 
with the greatest care so that the discharge cannot pass, the fila- 
ment f acts by electrostatic induction upon the tube t and the 
latter is set in vibration. If the tube o be rather wide, about an 
inch or so, the filament may be so powerfully vibrated that when- 
ever 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 



HIGH FEEQ UENCT AND HIGH POTENTIAL CURRENTS. 365 

frequency of the currents. Now the filament lias 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 po- 
tential 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 po- 
tential 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 mechani- 
cal shock produced by the projected atoms is easily noted when 
a bulb containing a button is grasped in the hand and the cur- 
rent turned on suddenly. I believe that a bulb could be shat- 
tered by observing the conditions of resonance. 

In tlie experiment before cited it is, of course, open to say, 
that the glass tube, upon coming in contact with the filament, re- 
tains 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 phospho- 
rescence. So, for instance, if a phosphorescent bulb be first ex- 
cited by a high frequency coil by connecting it to one of the ter- 
minals 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 phos- 
phorescent phenomena in bulbs being produced by the incandes- 
cence of an infinitesimal layer on the surface of the phosphores- 
cent 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 phospho- 
rescence in a bulb, which we know so far, is a disruptive discharge 
coil giving an enormous potential with but few fundamental dis- 
charges, say 25-30 per second, just enough to produce a continu- 



366 INVENTIONS OF NIKOLA TES1.A. 

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 
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 di- 
ameter). 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 phosphor- 
escence. 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, 
w T hich tend to localize and increase the heating effect at the point 
of impact, are almost invariably the most favorable for the pro- 
duction 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, arid if the gas be highly rare- 
fied, all of which things imply high speed of the projected atoms, 
or matter, and consequently violent impacts the phosphores- 
cence 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 elec- 
trode may be grasped with the hand Avhile the electrode is con- 
nected 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 



11IG1I VllI&Q UENCY A ND HIGH POTENTIAL CURRKltTS. 307 

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 fre- 
quencies 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 per- 
manent because of the impairment of the vacuum and the deposi- 
tion 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 phosphores- 
cence. 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 obser- 
vation which might deserve notice is, that when violently bom- 
barded from an aluminum electrode it assumes a black color, but 
singularly enough, it returns to the original condition when it 
cools down. 

The most important fact arrived at in pursuing investigations 
in this direction is, that in all cases it is necessary, in order to ex- 
cite phosphorescence with a minimum amount of energy, to ob- 
serve 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 econom- 
ical. 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 



368 INVENTIONS OF NIKOLA TESLA. 

have to be observed when such phosphorescent bulbs are oper- 
ated, if the best results are to be obtained. 

Coming now to the most interesting of these phenomena, the 
incandescence or phosphorescence of gases, at low pressures or at 
the ordinary pressure of the atmosphere, we must seek the ex- 
planation 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 ex- 
planation and concerns only the crude mechanism. Light is pro- 
duced 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 account- 
ing for these forces and tremendous rates of vibration. But the 
atom behaves singularly in this respect it always takes the same 
" charge." 

It is very likely that resonant vibration plays a most important 
part in all manifestations of energy in nature. Throughout space 
all matter is vibrating, and all rates of vibration are represented, 
from the lowest musical note to the highest pitch of the chemical 
rays, hence an atom, or complex of atoms, no matter what its 
period, must find a vibration with which it is in resonance. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 869 

"When we consider the enormous rapidity of the light vibrations, 
we realize the impossibility of producing such vibrations directly 
with any apparatus of measurable dimensions, and we are driven 
to the only possible means of attaining the object of setting up 
waves of light by electrical means and economically, that is, to 
affect the molecules or atoms of a gas, to cause them to collide and 
vibrate. We then must ask ourselves How can free molecules 
or atoms .be affected ? 

It is a fact that they can be affected by electrostatic force, as is 
apparent in many of these experiments. By varying the electro- 
static force we can agitate the atoms, and cause them to collide 
accompanied by evolution of heat and light. It is not demonstrated 
beyond doubt that we can affect them otherwise. If a luminous 
discharge is produced in a closed exhausted tube, do the atoms 
arrange themselves in obedience to any other but to electrostatic 
force acting in straight lines from atom to atom ? Only recently 
I investigated the mutual action between two circuits with extreme 
rates of vibration. When a battery of a few jars (c c c c, Fig. 
196) is discharged through a primary p of low resistance (the con- 
nections being as illustrated in Figs. 183, 183&andl83c), and the 
frequency of vibration is many millions there are great differ- 
ences of potential between points on the primary not more than 
a few inches apart. These differences may be 10,000 volts per 
inch, if not more, taking the maximum value of the E. M. F. The 
secondary s is therefore acted upon by electrostatic induction, 
which is in such extreme cases of much greater importance than 
the electro-dynamic. To such sudden impulses the primary as 
well as the secondary are poor conductors, and therefore great 
differences of potential may be produced by electrostatic induc- 
tion 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 c? c? be carefully 
excluded. If now we substitute a closed vacuum tube for the 
metallic secondary s, the differences of potential produced in the 
tube by electrostatic induction from the primary are fully suffi- 
cient to excite portions of it ; but as the points of certain differ- 
ences 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 



370 INVENTIONS OF NIKOLA TESLA. 

tube being excited only by electro-dynamic induction, for very 
able physicists hold this view ; but in my opinion, there is as yet 
no positive proof given that atoms of a gas in a closed tube may 
arrange themselves in chains under the action of an electromotive 
impulse produced by electro-dynamic induction in the tube. I 
have been unable so far to produce striae in a tube, however long, 
and at whatever degree of exhaustion, that is, striae at right 
angles to the supposed direction of the discharge or the axis of 
the tube ; but I have distinctly observed in a large bulb, in which 
a wide luminous band was produced by passing a discharge of a 
battery through a wire surrounding the bulb, a circle of feeble 
luminosity between two luminous bands, one of which was more 
intense than the other. Furthermore, with my present experi- 
ence 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 




V 




FIG. 196. FIG. 197. 

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 thqui 
it cannot vibrate, for some inertia is necessary for the free vibra- 
tion. 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 vibra- 
tion 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- 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 371 

callj, 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 perma- 
nently 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 ren_ 
dered 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 deteri- 
oration and the vacua are more permanent. With disruptive dis- 
charge coils there are sudden rises of potential and the vacua are 
more quickly impaired, for the electrodes are deteriorated in a 
very short time. It was observed in some large tubes, which 
were provided with heavy carbon blocks B B l5 connected to plati- 
num wires w w^ (as illustrated in Fig. 197), and which were em- 
ployed 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 de- 
posited in regular fine lines in the middle of the tube, as illus- 
trated. These lines were attributed to the deflection or distortion 
of the discharge by the magnetic field, but why the deposit 
occiirred 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 deteri- 
oration of the electrodes, probably by reason of the rapid inter- 
ruptions it produces, whereby there is actually a higher E. M. F. 
maintained between the electrodes. 

Much would remain to be said about the luminous effects pro- 
duced 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 investiga- 
tions in this direction are being pushed with exceptional ardor. 
Every line of scientific pursuit has its fascinations, but electrical 



372 



INVENTIONS OF NIKOLA TESLA. 



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 Avould 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 nuiu- 






FIG. 198. 

ber 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 cur- 
rent 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. 



HIGH FREQUENCY AND 1IIQR POTENTIAL CURRENTS. 373 

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. 



CHAPTEK XXIX. 

TESLA ALTERNATING CURRENT GENERATORS FOR HIGH FRE- 
QUENCY, IN DETAIL. 

It lias become a common practice to operate arc lamps by alter- 
nating 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 conse- 
quent 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 diffi- 
culty of maintaining an alternating current arc in consequence of 
the periodical increase in resistance corresponding to the periodi- 
cal working of the current. This feature entails a further dis- 
advantage, namely, that small arcs are impracticable. 

Theoretical considerations have led Mr. Tesla to the belief 
that these disadvantageous features could be obviated by employ- 
ing 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 alternation^ in 
consequence of which the arc has no time to cool, and is always 
maintained at a high temperature and low resistance. 

At the outset of his experiments Mr. Tesla encountered great 
difficulties in the construction of high frequency machines. A 
generator of this kind is described here, which, though con- 
structed quite some time ago, is well worthy of a detailed de- 
scription. 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 fre- 
quency, and reference to them will be found in his lectures 
elsewhere printed in this volume. 1 

1. See pages 153-4 5. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 375 

In the aecompaning engravings, Figs. 199 and 200 show the 
machine, respectively, in side elevation and vertical cross-section ; 
Figs. 201, 202 and 203 showing enlarged details of construction. 
As will be seen, A is an annular magnetic frame, the interior of 
which is provided with a large number of pole-pieces D. 

Owing to the very large number and small size of the poles 
and the spaces between them, the field coils are applied by wind- 
ing an insulated conductor F zigzag through the grooves, as shown 
in Fig. 203, carrying the wire around the annulus to form as 
many layers as is desired. In this way the pole-pieces D will be 
energized with alternately opposite polarity around the entire 
ring. 

For the armature, Mr. Tesla employs a spider carrying a ring 




j, turned down, except at its edges, to form a trough-like recep- 
tacle for a mass of fine annealed iron wires K, which are wound 
in the groove to form the core proper for the armature-coils. 
Pins L are set in the sides of the ring j and the coils M are wound 
over the periphery of the armature-structure and around the pins. 
The coils M are connected together in series, and these terminals 
N carried through the hollow shaft H to contact-rings P P, from 
which the currents are taken off by brushes o. 

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 six- 
teen hundred revolutions per minute, which will produce ten 



376 



INVENTIONS OF NIKOLA TESLA, 



thousand or eleven thousand alternations of current per second. 
Arc lamps K R are shown in the diagram as connected up in series 
with the machine in Fig. 200. If such a current be applied to 
running arc lamps, the sound produced by or in the arc becomes 
practically inaudible, for, by increasing the rate of change in the 
current, and consequently the number of vibrations per unit of 
time of the gaseous material of the arc up to, or beyond, ten 
thousand or eleven thousand per second, or to what is regarded 
as the limit of audition, the sound due to such vibrations will not 
be audible. The exact number of changes or undulations neces- 
sary to produce this result will vary somewhat according to the 
size of the arc that is to say, the smaller the arc, the greater the 




FIGS. 200, 201, 202 and 203. 

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. 

The difficulties encountered in the construction of these 
machines are of a mechanical as well as an electrical nature. 
The machines may be designed on two plans : the field may be 
formed either of alternating poles, or of polar projections of the 
same polarity. Up to about 15,000 alternations per second in an 
experimental machine, the former plan may be followed, but a 
more efficient machine is obtained on the second plan. 

In the machine above described, which was capable of running 
two arcs of normal candle power, the field was composed of a 



HIGH FREQUENCY AND HLOU POTENTIAL CURRENTS. 37? 

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 pro jections is about T 3 ^ inch, and they 
are a little over j\ inch thick. The field magnet was made rela- 
tively small so as to adapt the machine for a constant current. 
There are 384 coils connected in two series. It was found im- 
practicable 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 inch wide, so that the binding wires 
might be obviated. The armature wires must be wound with 




FIG. 204. 

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. 

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 



378 



INVENTIONS OF NIKOLA TE8LA. 



207, is preferable. The principal advantage of this type of 
machine is that there is not much magnetic leakage, and that a 
field may be produced, varying greatly in intensity in places not 
much distant from each other. 

In these engravings, Figs. 204 and 205 illustrate a machine in 
which the armature conductor and field coils are stationary, while 
the field magnet core revolves. Fig. 206 shows a machine 
embodying the same plan of construction, but having a stationary 
field magnet and rotary armature. 

The conductor in which the currents are induced may be 
arranged in various ways ; but Mr. Tesla prefers the following 
method : He employs an annular plate of copper D, and by 




FIG. 205. 

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 inch wide, the width of the conductor 
should not, under any circumstances, be more than ^ inch wide ; 
even then the eddy effect is considerable. 

To the inner edge of this plate are secured two rings of non- 
magnetic metal E, which are insulated from the copper conductor, 
but held firmly thereto by means of the bolts F. Within the 
rings E is then placed an annular coil G, which is the energizing 
coil for the field magnet. The conductor D and the parts at- 
tached thereto are supported by means of the cylindrical shell or 



HIGH FREQ UENCr AND HIGH POTENTIA L CURRENTS. 379 

casting A A, the two parts of which are brought together and 
clamped to the outer edge of the conductor D. 

The core for the field magnet is built up of two circular parts 
H H, formed with annular grooves i, which, when the two parts 
are brought together, form a space for the reception of the ener- 
gizing coil G. The hubs of the cores are trued off, so as to fit 
closely against one another, while the outer portions or flanges 
which form the polar faces j j, are reduced somewhat in thick- 
ness to make room for the conductor D, and are serrated on their 
faces. The number of serrations in the polar faces is arbitrary ; 




FIG. 206. 

but there must exist between them and the radial portions of 
the conductor D certain relation, which will be understood by 
reference to Fig. 207 in which N N represent the projections or 
points on one face of the core of the field, and s s the points of 
the other face. The conductor D is shown in this figure in section 
a a' designating the radial portions of the conductor, and 5 the 
insulating divisions between them. The relative width of the 
parts a a' and the space between any two adjacent points N N or 
s s is such that when the radial portions a of the conductor are 
passing between the opposite points N s where the field is strong- 
est, the intermediate radial portions a' are passing through the 



380 INVENTIONS OF NIKOLA TESLA. 

widest spaces midway between such points and where the field is 
weakest. Since the core on one side is of opposite polarity to 
the part facing it, all the projections of one polar face will be of 
opposite polarity to those of the other face. Hence, although 
the space between any two adjacent points on the same face may 
be extremely small, there will be no leakage of the magnetic 
lines between any two points of the same name, but the lines of 
force will pass across from one set of points to the other. The 
construction followed obviates to a great degree the distortion of 
the magnetic lines by the action of the current in the conductor 
D, in which it will be observed the current is flowing at any given 
time from the centre toward the periphery in one set of radial 
parts a and in the opposite direction in the adjacent parts a'. 

In order to connect the energizing coil G, Fig. 204, with a source 
of continuous current, Mr. Tesla utilizes two adjacent radial por- 
tions of the conductor D for connecting the terminals of the coil 
G with two binding posts M. For this purpose the plate D is cut 



vwwyy/ 

tr m.mmmm ...'> 



FIG. 207. 

entirely through, as shown, and the break thus made is bridged 
over by a short conductor c. The plate D is cut through to form 
two terminals d, which are connected to binding posts N. The 
core H H, when rotated by the driving pulley, generates in the con- 
ductors D an alternating current, which is taken off from the 
binding posts ST. 

When it is desired to rotate the conductor between the faces 
of a stationary field magnet, the construction shown in Fig. 
206, is adopted. The conductor D in this case is or may be 
made in substantially the same manner as above described by 
slotting an annular conducting-plate and supporting it between 
two heads o, held together by bolts o and fixed to the driving-shaft 
K. The inner edge of the plate or conductor D is preferably 
flanged to secure a firmer union between it and the heads o. It 
is insulated from the head. The field-magnet in this case con- 
sists of two annular parts H H, provided with annular grooves i 
for the reception of the coils. The flanges or faces surrounding 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 381 

the annular groove are brought together, while the inner flanges 
are serrated, as in the previous case, and form the polar faces. 
The two parts H H are formed with a base E, upon which the 
machine rests, s s are non-magnetic bushings secured or set in 
the central opening of the cores. The conductor D is cut entirely 
through at one point to form terminals, from which insulated 
conductors T are led through the shaft to collecting-rings v. 

In one type of machine of this kind constructed by Mr. Tesla, 
the field had 480 polar projections on each side, and from this 
machine it was possible to obtain 30,000 alternations per second. 
As the polar projections must necessarily be very narrow, very 
thin wires or sheets must be used to avoid the eddy current 
effects. Mr. Tesla has thus constructed machines with a station- 
ary 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 he 
either drum or disc, but Mr. Tesla's experience shows the latter 
to be preferable. 



In the course of a very interesting article contributed to the 
Electrical World in February, 1891, Mr. Tesla makes some sug- 
gestive remarks on these high frequency machines and his ex- 
periences with them, as well as with other parts of the high 
frequency apparatus. Part of it is quoted here and is as 
follows : 

The writer will incidentally mention that any one who at- 
tempts 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 pro- 
jections. 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 mechani- 
cal 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. 



3*2 INVENTIONS OF NIKOLA TK8LA. 

Among the many experiments winch 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 dis- 
charge is completely changed. The arc is established at much 
greater distances, and it is so easily affected by the slightest cur- 
rent 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 thou- 
sand 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, elec- 
tricity 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 differ- 
ences of potential obtained. As a consequence of the self-induc- 
tion of the coil and the high frequency, the current is minute 
while there is a corresponding rise of pressure. A current im- 
pulse 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 electro- 
motive 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 in- 
jurious 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 injuri- 
ous. This result, however, is due in part to the high frequency. 
The writer's experiences tend to show that the higher the fre- 
quency the greater the amount of electrical energy which may 
be passed through the body without serious discomfort ; whence 
it seems certain that human tissues act as condensers. 

One is not quite prepared for the behavior of the coil when 
connected to a Leyden jar. One, of course, anticipates that since 
the frequency is high the capacity of the jar should be small. He 
therefore takes a very small jar, about the size of a small wine 
glass, but he finds that even with this jar the coil is practically 
short-circuited. He then reduces the capacity until he comes to 



HIGH FItEQ UENCT AND HIG1I POTENTIAL CURliENTS, 383 

about the capacity of two spheres, say, ten centimetres in diam- 
eter and two to four centimetres apart. The discharge then as- 
sumes 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 
(juite natural that connecting only one terminal to a body of the 
same size, no matter how well insulated, impairs considerably the 
striking distance of the arc. 

Experiments with Greissler 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 per- 
ceptibly 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 in- 
candescent. The diSc"Iiarge 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 con- 
ductor connected to the coil, the carbon is brought to bright in- 
candescence and the glass is rapidly heated. With a 100- volt 10 c. 
p. lanro 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 



384 INVENTIONS OF NIKOLA TE8LA. 

may remain lighted if the plate be interposed, or may even in- 
crease in luminosity. The effect depends on the position of the 
plate and tube relatively to the coil, and may be always easily 
foretold by assuming that conduction takes place from one ter- 
minal of the coil to the other. According to the position of the 
plate, it may either divert from or direct the current to the tube. 
In another line of work the writer has in frequent experiments 
maintained incandescent lamps of 50 or 100 volts burning at any 
desired candle power with both the terminals of each lamp con- 
nected 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, cer- 
tainly 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 h'rst ; 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 ex- 
plained 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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 385 

pass uniformly through the space between the washers, is di- 
verted 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 in- 
stantly 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 in- 
creases until it is parallel with that line. The writer must state, 
however, that he does nat favor the idea of a leakage or current 
through the space any more than as a suitable explanation, for he 
is convinced that all these experiments could not be performed with 
a static machine yielding a constant difference of potential, and 
that condenser action is largely concerned in these phenomena. 

It is well to take certain precautions when operating a Ruhm- 
korff 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 tine 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 con- 
nected 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. Poly- 
carpus. If an expensive vacuum pump is lighted up by being 
near to the coil or touched with a wire connected to one of the 
terminals, the current should be left on no more than a few 
moments, else the glass will be cracked by the heating of the 
rarefied gas in one of the narrow passages in the writer's own 
experience quod erat demonstrandum. 1 

1. 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. 



386 INVENTIONS OF NIKOLA TESLA. 

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 alter- 
nations 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 thou- 
sand 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 fre- 
quency. 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 suffi- 
cient power. There is no reason why it should not be so. The 
condensations and rarefactions of the air would necessarily set 
the diaphragm in a corresponding vibration and some sensation 
would be produced, whatever within certain limits the velocity 
of transmission to their nerve centres, though it is probable that 
for want of exercise the ear would not be able to distinguish any 
such high note. With the eye it is different ; if the sense of 
vision is based upon some resonance effect, as many believe, no 
amount of increase in the intensity of the ethereal vibration 
could extend our range of vision on either side of the visible 
spectrum. 

The limit of audition of an arc depends on its size. The 
greater the surface by a given heating effect in the arc, the higher 
the limit of audition. The highest notes are emitted by the 
high-tension discharges of an induction coil in which the arc is, 
so to speak, all surface. If R be the resistance of an arc, and C 
the current, and the linear dimensions be n times increased, then 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 387 

the resistance is , and with the same current density the cur- 
rent would be v?C; hence the heating effect is n* times greater, 
while the surface is only n* 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 uni- 
form and the sound is lessened ; for this reason it would seem 
that an alternating arc requires such carbons. 

With currents of such high frequencies it is possible to obtain 
noiseless arcs, but the regulation of the lamp is rendered ex- 
tremely difficult on account of the excessively small attractions 
or repulsions between conductors conveying these currents. 

An interesting feature of the arc produced by these rapidly 
alternating currents is its persistency. There are two causes for 
it, one of which is always present, the other sometimes only. 
One' is due to the character of the current and the other to a 
property of the machine. The first cause is the more important 
one, and is due directly to the rapidity of the alternations. 
When an arc is formed by a periodically undulating current, 
there is a corresponding undulation in the temperature of the 
gaseous column, and, therefore, a corresponding undulation in 
the resistance of the arc. But the resistance of the arc varies 
enormously with the temperature of the gaseous column, being 
practically infinite when the gas between the electrodes is cold. 
The persistence of the arc, therefore, depends on the inability of 
the column to cool. It is for this reason impossible to maintain 
an arc with the current alternating only a few times a second. 
On the other hand, with a practically continuous current, the arc 
is easily maintained, the column being constantly kept at a high 
temperature and low resistance. The higher the frequency the 
smaller the time interval during which the arc may cool and in- 
crease considerably in resistance. With a frequency of 10,000 
per second or more in an arc of equal size excessively small varia- 
tions of temperature are superimposed upon a steady temperature, 
like ripples 011 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 



888 INVENTIONS OF NIKOLA TESLA. 

consumed ; though the writer has observed some irregularities in 
this respect. 

The second cause alluded to, which possibly may not be pre- 
sent, is due to the tendency of a machine of such high frequency 
to maintain a practically constant current. When the arc is 
lengthened, the electromotive force rises in proportion and the 
arc appears to be more persistent. 

Such a machine is eminently adapted to maintain a constant 
current, but it is very unfit for a constant potential. As a matter 
of fact, in certain types of such machines a nearly constant cur- 
rent 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 arma- 
ture, enormously augmented by the high frequency. Then, 
again, the magnetic leakage is facilitated. If there are three or 
four hundred alternate poles, the leakage is so great that it is 
virtually the same as connecting, in a two-pole machine, the poles 
by a piece of iron. This disadvantage, it is true, may be obviated 
more or less by using a field throughout of the same polarity, 
but then one encounters difficulties of a different nature. All 
these things tend to maintain a constant current in the armature 
circuit. 

In this connection it is interesting to notice that even to-day 
engineers are astonished at the performance of a constant current 
machine, just as, some years ago, they used to consider it an ex- 
traordinary 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 con- 
stant 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 cur- 
rent 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-induc- 
tion. One may make the induced circuit of a dynamo machine 
or transformer of such high resistance that when operating de- 
vices of considerably smaller resistance within very wide limits a 



HIGH FREQ UENCY AND HIGH POTENTIAL CURRENTS. 389 

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 : Clos- 
est 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 iii taking off the current. 

With their keen inventor's instinct, the now successful arc- 
light men have early recognized the desiderata of a constant 
current machine. Their arc light machines have weak fields, 
large armatures, with a great length of copper wire and few 
commutator segments to produce great variations in the current's 
strength and to bring self-induction into play. Such machines 
may maintain within considerable limits of variation in the re- 
sistance of the circuit a practically constant current. Their out- 
put is of course correspondingly diminished, and, perhaps with 
the object in view not to cut down the output too much, a sim- 
ple device compensating exceptional variations is employed. 
The undulation of the current is almost essential to the commer- 
cial 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 im- 
portant, it allows the use of simple clutch lamps, which with a 
current of a certain number of impulses per second, best suitable 
for each particular lamp, will, if properly attended to, regulate 
even better than the finest clock-work lamps. This discovery 
has been made by the writer several years too late. 



:-MK) INVENTIONS OF NIKOLA TESLA. 

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, de- 
vices each possessing self-induction and capacity or self-induction 
and resistance that is, retarding and accelerating components 
in such proportions as to not affect materially the phase of the 
secondary current. Any number of such devices may be inserted 
or cut out, still it will be found that the regulation occurs, a con- 
stant 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 commu- 
tating the armature current, then the iixed 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 th eir determined posi- 
tions, a constant current or constant potential resulted from the 
shifting of the diameter of commutation by the varying loads. 

In connection with machines of such high frequencies, the 
condenser affords an especially interesting study. It is easy to 
raise the electromotive force of such a machine to four or five 
times the value by simply connecting the condenser to the cir- 
cuit, 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 clear- 
ness. The high frequency allow r s the use of small capacities and 
renders investigation easy. But, although in most of the experi- 
ments 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- 



HI&H FREQUENCY AND HIGH POTENTIAL CURRENTS. 391 

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 gradu- 
ally fell until at the terminals of the machine it was about 65 
volts. It was virtually as though the condenser were a gene- 
rator, and the line and armature circuit simply a resistance con- 
nected 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 ma- 
chine. A case of pure resonance he was unable to obtain. 
When a condenser was connected to the terminals of the ma- 
chine the self-induction of the armature being first determined 
in the maximum and minimum position and the mean value taken 
the capacity which gave the highest electromotive force corre- 
sponded most nearly to that which just counteracted the self-in- 
duction with the existing frequency. If the capacity was in- 
creased or diminished, the electromotive force fell as expected. 

With frequencies as high as the above mentioned, the con- 
denser effects are of enormous importance. The condenser 
becomes a highly efficient apparatus capable of transferring 
considerable energy. 



In an appendix to this book will be found a description of the 
Tesla oscillator, which its inventor believes will among other great 
advantages give him the necessary high frequency conditions, 
while relieving him of the inconveniences that attach to genera- 
tors of the type described at the beginning of this chapter. 



CHAPTEK XXX. 

ALTERNATE CURRENT ELECTROSTATIC INDUCTION APPARATUS.* 

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 




FIG. 208. 

of experimental apparatus of which two are illustrated in the 
accompanying engravings. 

In the apparatus shown in Fig. 208, A is a ring of dry shel- 
lacked hard wood provided on its inside with two sets of tin-foil 
coatings, a and J, all the a coatings and all the I coatings being 
connected together, respectively, but independent from each 
other. These two sets of coatings are connected to two termi- 

1. Article by Mr. Tesla in The Electrical Engineer, N. Y., May 6, 1891. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 393 

nals, T. For the sake of clearness only a few coatings are shown. 
Inside of the ring A, and in close proximity to it there is arranged 
to rotate a cylinder B, likewise of dry, shellacked hard wood, and 
provided with two similar sets of coatings, a 1 and J 1 , all the coat- 
ings a} being connected to one ring and all the others, S l , to 
another marked -f- and . These two sets, a 1 and J 1 are charged 
to a high potential by a Holtz or Wimshurst machine, and may 
be connected to a jar of some capacity. The inside of ring A is 
coated with mica in order to increase the induction and also to 
allow higher potentials to be used. 

When the cylinder B with the charged coatings is rotated, a 




FIG. 20J. 



circuit connected to the terminals T is traversed by alternating 
currents. Another form of apparatus is illustrated in Fig. 209. 
In this apparatus the two sets of tin-foil coatings are glued on a 
plate of ebonite, and a similar plate which is rotated, and the 
coatings of which are charged as in Fig. 208, is provided. 

The output of such an apparatus is very small, but some of 
the effects peculiar to alternating currents of short periods may 
l>e observed. The effects, however, cannot be compared with 
those obtainable with an induction coil which is operated by an 
alternate current machine of high frequency, some of which 
were described by me a short while ago. 



CHAPTER XXXI. 

" MASSAGE " WITH CURRENTS OF HIGH FREQUENCY. 1 

I TRUST that the present brief communication will not be inter- 
preted 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 elec- 
trical engineer is confident that electricity will become the means 
of accomplishing many things that have been heretofore, with 
our existing knowledge, deemed impossible. ]S r o 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 in- 
quiring as to the physical effects of such currents of high fre- 
quency. 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 bombard- 
ment of the body by air, or possibly by some other medium, 
which is molecular or atomic in construction, and the presence 
of which has so far escaped our analysis for according to my 
ideas, the true ether radiation with such frequencies as even a 
few millions per second must be very small. This body may be 
a good conductor or it may be a very poor conductor of elec- 
tricity 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 

1. Article by Mr. Tesla in Tlie EUctrical Engineer of Deo. 23d, 1891. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 395 

rapidly alternating high potential, the skin is heated by bom- 
bardment. It is a mere question of the dimensions and character 
of the apparatus to produce any degree of heating desired. 

It has occurred to me whether, with such apparatus properly 
prepared, it would not be possible for a skilled physician to find 
in it a means for the effective treatment of various types of dis- 
ease. The heating will, of course, be superficial, that is, on the 
skin, and would result, whether the person operated on were in 
bed or walking around a room, whether dressed in thick clothes or 
whether reduced to nakedness. In fact, to put it broadly, it is 
conceivable that a person entirely nude at the North Pole might 
keep himself comfortably warm in this manner. 

Without vouching for all the results, which must, of course, be 
determined by experience and observation, I can at least warrant 
the fact that heating would occur by the use of this method of 
subjecting the human body to bombardment by alternating cur- 
rents of high potential and frequency such as I have long worked 
with. It is only reasonable to expect that some of the novel ef- 
fects 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. 



CHAPTEE XXXII. 

ELECTRIC DISCHARGE IN VACUUM TUBES.* 

IN The Electrical Engineer of June 10 I have noted the de- 
scription of some experiments of Prof. J. J. Thomson, on the 
" Electric Discharge in Vacuum Tubes," and in your issue of June 
24 Prof. Elihu Thomson describes an experiment of the same 
kind. The fundamental idea in these experiments is to set up 
an electromotive force in a vacuum tube preferably devoid of 
any electrodes by means of electro-magnetic induction, and to 
excite the tube in this manner. 

As I view the subject I should, think that to any experimenter 
who had carefully studied the problem confronting us and who 
attempted to find a solution of it, this idea must present itself as 
naturally as, for instance, the idea of replacing the tinfoil coat- 
ings of a Leyden jar by rarefied gas and exciting luminosity in 
the condenser thus obtained by repeatedly charging and discharg- 
ing 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 ap- 
pearances, have not been noted by Prof. J. J. Thomson, who, 
however, seems to have gone about systematically in his investi- 
gations, 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 pheno- 
mena in a different light. 

My investigations in this line occupied me principally during 
the winter and spring of the past year. During this time many dif- 
ferent experiments were performed, and in my exchanges of ideas 

1. Article by Mr. Tesla in The Electrical Engineer. N. Y., July 1, 1891. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 397 

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 en- 
abling 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 




FIG. 210. 

there was no special difficulty in exciting a luminous circle or 
cylinder in proximity to the primary. 

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 de- 
pendent on the dimensions of the apparatus, and, were it not for 
various considerations, one turn would always give the best 
result. 

I have found that it is preferable to use in these experiments 
an alternate current machine giving a moderate number of alter- 



398 . NVENTION8 OF NIKOLA TESLA. 

nations per second to excite the induction coil for charging the 
Leyden jar which discharges through the primary shown dia- 
grammatically in Fig. 211, as in such case, before the disrup- 
tive discharge takes place, the tube or bulb is slightly excited and 
the formation of the luminous circle is decidedly facilitated. 




FIG. 211. 

But I have also used a Wimshurst machine in some experi- 
ments. 

Prof. J. J. Thomson's view of the phenomena under consid- 
eration seems to be that they are wholly due to electro-magnetic 
action. I was, at one time, of the same opinion, but upon care- 
fully investigating the subject I was led to the conviction that 
they are more of an electrostatic nature. It must be remem- 
bered 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 




FIG. 212. 



gas, and that under such conditions electrostatic effects must play 
an important part. 

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 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 399 

an ordinary exhausted tube (preferably of large diameter) be 
surrounded by a spiral of thick copper wire serving as the prim- 
ary^ a feebly luminous spiral may be induced in the tube, roughly 
shown in Fig. 212. In one of these experiments a curious phe- 
nomenon 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 ex- 
istence of nodes on the primary. The circles were connected by 
a faint luminous spiral parallel to the primary and in close prox- 
imity 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. 




FIG. 213. 

In using Dr. Lodge's recoil circuit, the electrostatic action is 
likewise apparent. The arrangement is illustrated in Fig. 213. 
In his experiment two hollow exhausted tubes H H were slipped 
over the wires of the recoil circuit and upon discharging the jar 
in the usual manner luminosity was excited in the tubes. 

Another experiment performed is illustrated in Fig. 214. In 
this case an ordinary lamp-bulb was surrounded by one or two 
turns of thick copper wire P and the luminous circle L excited 
in the bulb by discharging the jar through the primary. The 
lamp-bulb was provided with a tinfoil coating on the side oppo- 
site to the primary and each time the tinfoil coating was con- 
nected to the ground or to a large object the luminosity of the 
circle was considerably increased. This was evidently due to 
electrostatic action. 

In other experiments I have noted that when the primary 
touches the glass the luminous circle is easier produced and is 



400 



INVENTIONS OF NIKOLA TESLA. 



more sharply defined ; but I have not noted that, generally speak- 
ing, 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 illumi- 
nated ; and in one case I have observed an intensely purplish 




FIG. 214. 



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 elec- 




FIG. 215. 

tromagnetic and considering the distance ; and these facts speak 
for an electrostatic effect. 

Furthermore I have observed that there is a molecular bom- 
bardment in the plane of the luminous circle at right angles to 
the glass supposing the circle to be in the plane of the primary 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 401 

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 mani- 
fest 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 ren- 
dered more intensely phosphorescent. For want of time I have, 
however, not been able to perform the experiment. 

Another observation made by me was that when the specific 
inductive capacity of the medium between the primary and 
secondary is increased, the inductive effect is augmented. This 
is roughly illustrated in Fig. 215. In this case luminosity was 
excited in an exhausted tube or bulb B and a glass tube T slipped 
between the primary and the bulb, when the effect pointed out 
was noted. Were the action wholly electromagnetic no change 
could possibly have been observed. 

I have likewise noted that when a bulb is surrounded by a 
wire closed upon itself and in the plane of the primary, the for- 
mation 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, be- 
cause then the action was distributed over a greater surface. The 
effect of the closed tinfoil was no doubt of an electrostatic nature, 
for it presented a much greater resistance than the closed wire 
and produced therefore a much smaller electromagnetic effect. 

Some of the experiments of Prof. J. J. Thomson also would 
seem to show some electrostatic action. For instance, in the ex- 
periment 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 in- 
ductive effect very powerful, they would probably appear in the 
bulb also. If, however, the bell jar were exhausted to the high- 
est degree they would very likely show themselves in the bulb 



403 



INVENTIONS OF NIKOLA TE8LA. 



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 ex- 
citing current employed. 

My principal argument is the following : I have experiment- 
ally 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 induc- 
tive effect namely, by converting upwards an exhausted tube, 
devoid of electrodes, may be excited at a distance of several feet. 



SOME EXPERIMENTS ON THE ELECTRIC DISCHARGE IN VACUUM TUBES. 
BY PROF. J. J. THOMSON, M.A., F.R.S. 

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 vaciium tubes, being absent. To produce discharges in 





FIG. 216. 



FIG. 2V, 



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 

1. Abstract of a paper read before Physical Society of London. 



HIGH FREQUENCY AND HTGII POTENTIAL CURRENTS. 403 

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 corres- 
ponded to the windings of an induction coil, the vacuum tubes being the sec- 
ondary, and the tubes filled with mercury the primary. In such an apparatus 
the Leyden jar need not be large, and neither primary nor secondary need have 
many turns, for this would increase the self-induction of the former, and 
lengthen the discharge path in the latter. Increasing the self-induction of the 
primary reduces the E. M. F. induced in the secondary, whilst lengthening the 
secondary does not increase the E. M. F. per unit length. The two or three 
turns, as shown in Fig. 216, in each, were found to be quite sufficient, and, on 
discharging the Leyden jar between two highly polished knobs in the primary 





FIG. 218. 



FIG. 219. 



circuit, a plain uniform band of light was seen to pass round the secondary. 
An exhausted bulb, Fig. 217, containing traces of oxygen was plaeed within a 
primary spiral of three turns, and, on passing the jar discharge, a circle of light 
was seen within the bulb inclose proximity to the primary circuit, accom- 
panied 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 in- 
stantly 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 ex- 
hausting the jar, however, the luminous discharge grew fainter, and a point 
was reached at which no secondary discharge was visible. J urther 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 



404 INVENTIONS OF NIKOLA TESLA. 

could only explain on two suppositions, viz.: that under the conditions then ex- 
isting the specific inductive capacity of the gas was very great, or that a dis- 
charge could pass without being luminous. '1 he author had also .observed 
that the conductivity of a vacuum tube without electrodes increased as the pres- 
sure 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 denned 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 ex- 
ample, 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. 

Pig. 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. 



1 In The Electrical Engineer of August 12, I find some re- 
marks of Prof. J. J. Thomson, which appeared originally in the 
London Electrician and which have a bearing upon some experi- 
ments described by me in your issue of July 1. 

I did not, as Prof. J. J. Thomson seems to believe, misunder- 
stand 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 per- 
formed at that time proved to me that the electrostatic effects 
were generally of far greater importance, and admitted of a more 
satisfactory explanation of most of the phenomena observed. 

In using the term electrostatic I had reference rather to the 
nature of the action than to a stationary condition, which is the 
usual acceptance of the term. To express myself more clearly, 
I will suppose that near a closed exhausted tube be placed a small 
sphere charged to a very high potential. The sphere would act 
inductively upon the tube, and by distributing electricity over 
1. Article by Mr. Tesla in The Electrical Engineer, N. Y., August 26, 1891. 



HIGH FREQUENCY AND HIGH POTENTIAL CURRENTS. 405 

the same would undoubtedly produce luminosity (if the potential 
be sufficiently high), until a permanent condition would be 
reached. Assuming the tube to be perfectly well insulated, 
there would be only one instantaneous flash during the act of 
distribution. This would be due to the electrostatic action 
simply. 

But now, suppose the charged sphere to be moved at short in- 
tervals 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 eifect, 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 capa- 
city than on the magnetic permeability of the medium which 
would certainly be the case for speeds incomparably lower than 
that of light then I believe I would be justified in saying that 
the effect produced was more of an electrostatic nature. I do 
not mean to say, however, that any similar condition prevails in 
the case of the discharge of a Leyden jar through the primary, 
but I think that such an action would be desirable. 

It is in the spirit of the above example that I used the terms 
" more of an electrostatic nature," and have investigated the in- 
fluence of bodies of high specific inductive capacity, and observed, 
for instance, the importance of the quality of glass of which the 
tube is made. I also endeavored to ascertain the influence of a 
medium of high permeability by using oxygen. It appeared 
from rough estimation that an oxygen tube when excited under 
similar conditions that is, as far as could be determined gives 
more light ; but this, of course, may be due to many causes. 

Without doubting in the least that, with the care and precau- 
tions 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 ineffective- 
ness of the screening, and I have also found that the electritica. 
tion through the air is often of very great importance, and may, 
in some cases, determine the excitation of the tube. 

In his original communication to the Electrician, Prof. J. J. 
Thomson refers to the fact that the luminosity in a tube near a 
wire through which a Leyden jar was discharged was noted by 
Hittorf. I think that the feeble luminous effect referred to has 



406 INVENTIONS OF NIKOLA TESLA. 

been noted by many experimenters, but in my experiments the 
effects were much more powerful than those usually noted. 
The following is the communication 1 referred to : 



"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 conduct- 
ing 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 experi- 
menting 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 ad- 
visable 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 po- 
tential, it will induce over the glass of the tube a distribution of electricity. 
When the potential of the primary suddenly falls, this electrification will re- 
distribute 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 ben 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 ac- 
count of the experiments in the Electrician was from a reporter's note, and was 
not written, or even read, by me. I have now almost finished writing out, and 
hope very shortly to publish, an account of these and a large number of allied 
experiments, including some analogous to those mentioned by Mr. Tesla on the 
effect of conductors placed near the discharge tube, which I find, in some 
cases, to produce a diminution, in others an increase, in the brightness of the 
discharge, as well as some on the effect of the presence of substances of large 
specific inductive capacity. These seem to me to admit of a satisfactory ex- 
planation, for which, however, I must refer to my paper." 

1. Note by Prof. J. J. Thomson in the London Electrician, July 24, 1891. 



PART III. 



MISCELLANEOUS INVENTIONS AND 
WRITINGS. 



CHAPTER XXXIII. 

METHOD OF OBTAINING DIRECT FROM ALTERNATING CURRENTS. 

THIS method consists in obtaining direct from alternating 
currents, or in directing the waves of an alternating current so as 
to produce direct or substantially direct currents by developing 
or producing in the branches of a circuit including a source of al- 
ternating 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 op- 
posite sign will be diverted through different circuits, those of 
one sign passing over one branch and those of opposite sign over 
the other. 

We may consider herein only the case of a circuit divided into 
two paths, inasmuch as any further subdivision involves merely 
an extension of the general principle. Selecting, then, any cir- 
cuit 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 cur- 
rent 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 cur- 
rent 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 



410 INVENTIONS OF NIKOLA TESLA. 

such as it has been heretofore regarded as necessary to use. The 
current in either branch may be used in the same way and for 
the same purposes as any other direct current that is, it may be 
made to charge secondary batteries, energize electro-magnets, or 
for any other analogous purpose. 

Fig. 220 represents a plan of directing the alternating currents 
by means of devices purely electrical in character. Figs. 221, 
222, 223, 224, 225, and 226 are diagrams illustrative of other 
ways of carrying out the invention. 

In Fig. 220, A designates a generator of alternating currents, 
and B B the main or line circuit therefrom. At any given point 
in this circuit at or near which it is desired to obtain direct cur- 
rents, the circuit B is divided into two paths or branches c D. In 
each of these branches is placed an electrical generator, which 
for the present we will assume produces direct or continuous cur- 



FIG. 220. 

rents. The direction of the current thus produced is opposite in 
one branch to that of the current in the other branch, or, con- 
sidering the two branches as forming a closed circuit, the gene- 
rators E F are connected up in series therein, one generator in 
each part or half of the circuit. The electromotive force of the 
current sources E and F may be equal to or higher or lower than 
the electromotive forces in the branches c D, or between the points 
x and Y of the circuit B B. If equal, it is evident that current 
waves of one sign will be opposed in one branch and assisted in 
the other to such an extent that all the waves of one sign will 
pass over one branch and those of opposite sign over the other. 
If, on the other hand, the electromotive force of the sources E F 
be lower than that between x and Y, the currents in both 
branches will be alternating, but the waves of one sign will pre- 
ponderate. One of the generators or sources of current E or F 
may be dispensed with ; but it is preferable to employ both, if 



OBTAINING DIRECT FROM ALTERNATING CURRENTS. 411 

they offer an appreciable resistance, as the two brandies will be 
thereby better balanced. The translating or other devices to be 
acted upon by the current are designated by the letters G, and 
they are inserted in the branches c D in any desired manner ; but 
in order to better preserve an even balance between the branches 
due regard should, of course, be had to the number and character 
of the devices. 

Figs. 221, 222, 223, and 224 illustrate what may termed "elec- 
tro-magnetic" devices for accomplishing a similar result that is 
to say, instead of producing directly by a generator an electro- 
motive 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 di- 
rection will be developed therein by the passage, or tendency to 
pass, of the alternations of current. In Fig. 221, for example, A is 




FIG. 221. 

the generator of alternating currents, B B the line circuit, and c D 
the branches over which the alternating currents are directed. In 
each branch is included the secondary of a transformer or induc- 
tion coil, which, since they correspond in their functions to the 
batteries of the previous figure, are designated by the letters E F. 
The primaries H H' of the induction coils or transformers are 
connected either in parallel or series with a source of direct or 
continuous currents i, and the number of convolutions is so cal- 
culated for the strength of the current from i that the cores J j' 
will be saturated. The connections are such that the conditions 
in the two transformers are of opposite character that is to say, 
the arrangement is such that a current wave or impulse corres- 
ponding in direction with that of the direct current in one pri- 
mary, as H, is of opposite direction to that in the other primary H'. 
It thus results that while one secondary offers a resistance or op- 



412 



INVENTIONS OF NIKOLA TESLA. 



position to the passage through it of a wave of one sign, the other 
secondary similarly opposes a wave of opposite sign. In conse- 
quence, 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 man- 
ner pass over the other branch. 

In lieu of saturating the primaries by a source of continuous 
current, we may include the primaries in the branches c D, re- 
spectively, and periodically short-circuit by any suitable mechani- 
cal devices such as an ordinary revolving commutator their 
secondaries. It will be understood, of course, that the rotation 
and action of the commutator must be in synchronism or in 
proper accord with the periods of the alternations in order to 
secure the desired results. Such a disposition is represented 




FIG. 222. 

diagrammatically in Fig. 222. Corresponding to the previous 
figures, A is the generator of alternating currents, B B the line, 
and c D the two branches for the direct currents. In branch c 
are included two primary coils E E', and in branch D are two 
similar primaries F F' The corresponding secondaries for these 
coils and which are on the same subdivided cores j or j', are in 
circuits the terminals of which connect to opposite segments K 
K', and L i/, respectively, of a commutator. Brushes b I bear 
upon the commutator and alternately short-circuit the plates K 
and K', and L and L', through a connection c. It is obvious that 
either the magnets and commutator, or the brushes, may revolve. 
The operation will be understood from a consideration of the 
effects of closing or short-circuiting the secondaries. For ex- 
ample, if at the instant when a given wave of current passes, one 



OBTAINING DIRECT FROM ALTERNATING CURRENTS. 418 

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 second- 
aries 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 disadvan- 
tages of this arrangement, which would seem to result from the 
employment of sliding contacts, are in reality very slight, inas- 
much as the electromotive force of the secondaries may be made 
exceedingly low, so that sparking at the brushes is avoided. 

Fig. 223 is a diagram, partly in section, of another plan of 
carrying out the invention. The circuit B in this case is divided, 
as before, and each branch includes the coils of both the fields 




FIG. 223. 

and revolving armatures of two induction devices. The arma- 
tures o P are preferably mounted on the same shaft, and are ad- 
justed relatively to one another in such manner that when the 
self-induction in one branch, as c. is maximum, in the other branch 
D it is minimum. The armatures are rotated in synchronism with 
the alternations from the source A. The winding or position 
of the armature coils is such that a current in a given direction 
passed through both armatures would establish in one, poles simi- 
lar to those in the adjacent poles of the field, and in the other, 
poles unlike the adjacent field poles, as indicated by n n s s in 
the diagram. If the like poles are presented, as shown in cir- 
cuit D, the condition is that of a closed secondary upon a primary, 
or the position of least inductive resistance ; hence a given alter- 
nation of current will pass mainly through D. A half revolution 
of the armatures produces an opposite effect and the succeeding 



414 



INVENTIONS OF NIKOLA TE8LA. 



current impulse passes through c. Using this figure as an illus- 
tration, it is evident that the fields N M may be permanent mag- 
nets or independently excited and the armatures o P driven, as in 
the present case, so as to produce alternate currents, which will 
set up alternately impulses of opposite direction in the two 
branches D c, which in such case would include the armature cir- 
cuits and translating devices only. 

In Fig. 224 a plan alternative with that shown in Fig. 222 is 
illustrated. In the previous case illustrated, each branch c and D 
contained one or more primary coils, the secondaries of which 
were periodically short circuited in synchronism with the alter- 
nations of current from the main source A, and' for this purpose 
a commutator was employed. The latter may, however, be dis- 
pensed with and an armature with a closed coil substituted. 

Referring to Fig. 224 in one of the branches, as c, are two coils 




FIG. 224. 

M', wound on laminated cores, and in the other branches D are 
similar coils N'. A subdivided or laminated armature o 7 , carry- 
ing a closed coil R', is rotatably supported between the coils M' N', 
as shown. In the position shown that is, with the coil K' paral- 
lel with the convolutions of the primaries N' M' practically the 
whole current will pass through branch D, because the self-in- 
duction in coils M' M' is maximum. If, therefore, the armature 
and coil be rotated at a proper speed relatively to the periods or 
alternations of the source A, the same results are obtained as in 
the case of Fig. 222. 

Fig. 225 is an instance of what may be called, in distinction to 
the others, a " magnetic " means of securing the result, v and 
w are two strong permanent magnets provided with armatures 
v' w', respectively. The armatures are made of thin laminae of 
soft iron or steel, and the amount of magnetic metal which they 



OBTAINING DIRECT FROM ALTERNATING CURRENTS. 4ir> 

contain is so calculated that they will be fully or nearly saturated 
by the magnets. Around the armatures are coils E F, contained, 
respectively, in the circuits c and D. The connections and elec- 
trical conditions in this case are similar to those in Fig. 221, 
except that the current source of i, Fig. 221, is dispensed with 
and the saturation of the core of coils E F obtained froth the per- 
manent magnets. 

The previous illustrations have all shown the two branches or 
paths containing the translating or induction devices as in deriva- 
tion one to the other ; but this is not always necessary. For 
example, in Fig. 226, A is an alternating-current generator; B B, 
the line wires or circuit. At any given point in the circuit let 
us form two paths, as D D', and at another point two paths, as c 
c' '. Either pair or group of paths is similar to the previous dis- 




FIG. 225. 

positions with the electrical source or induction device in one 
branch only, while the two groups taken together form the 
obvious equivalent of the cases in which an induction device or 
generator is included in both branches. In one of the paths, as 
D, are included the devices to be operated by the current. In 
the other branch, as D', is an induction device that opposes the 
current impulses of one direction and directs them through the 
branch D. So, also, in branch c are translating devices o, and in 
branch c' an induction device or its equivalent that diverts 
through c impulses of opposite direction to those diverted by the 
device in branch D'. The diagram shows a special form of in- 
duction device for this purpose, .r / are the cores, formed with 
pole-pieces, upon which are wound the coils M N. Between these 
pole-pieces are mounted at right angles to one another the mag- 
netic armatures o P, preferably mounted on the same shaft and 



416 



INVENTIONS OF NIKOLA TESLA. 



designed to be rotated in synchronism with the alternations of 
current. When one of the armatures is in line with the poles or 
in the position occupied by armature p, the magnetic circuit of 
the induction device is practically closed ; hence there will be 
the greatest opposition to the passage of a current through coils 
N N. The alternation will therefore pass by way of branch D. 
At the same time, the magnetic circuit of the other induction 
device being broken by the position of the armature o, there will 
be less opposition to the current in coils M, which will shunt the 
current from branch c. A reversal of the current being attended 
by a shifting of the armatures, the opposite effect is produced. 

Other modifications of these methods are possible, but need 
not be pointed out. In all these plans, it will be observed, there 




PIG 



is developed in one or all of these branches of a circuit from a 
source of alternating currents, an active (as distinguished from a 
dead) resistance or opposition to the currents of one sign, for the 
purpose of diverting the currents of that sign through the other 
or another path, but permitting the currents of opposite sign to 
pass without substantial opposition. 

Whether the division of the currents or waves of current of 
opposite sign be effected with absolute precision or not is imma- 
terial, 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. 



OBTAINING DIRECT FROM ALTERNATING CURRENTS. 417 

An alternating and a direct current have been combined so that 
the waves of one direction or sign were partially or wholly over- 
come by the direct current ; but by this plan only one set of al- 
ternations 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 al- 
ternating dynamo, or to operate direct current meters on alter- 
nating-current circuits or to run various devices such as arc lamps 
by direct currents in the same circuit with incandescent lamps 
or other devices operated by alternating currents. 

It will be observed that if an intermittent counter or opposing 
force be developed in the branches of the circuit and of higher 
electromotive force than that of the generator, an alternating 
current will result in each branch, with the waves of one sign 
preponderating, while a constantly or uniformly acting oppo- 
sition 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 de- 
scribed. 



CHAPTER XXXIY. 

CONDENSERS WITH PLATES IN OIL. 

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 de- 
vices 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 de- 
sirable to exclude all gaseous matter from the apparatus, or any ac- 





FIG. 227. 



FIG. 228. 



cess of the same to the electrified surfaces, in order to prevent heat- 
ing 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 1 . The plan admits of a particular con- 

1. 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 experi- 
ments 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 ob- 
servations 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. 



. 



CONDENSERS WITH PL A TEN IN OIL. 419 

struction of condenser, in whicli the distance between the plates 
is adjustable, and of which he takes advantage. 

In the accompanying illustrations, Fig. 227 is a section of a 
condenser constructed in accordance with this principle and hav- 
ing stationary plates ; and Fig. 228 is a similar view of a condenser 
with adjustable plates. 

Any suitable box or receptacle A may be used to contain the 
plates or armatures. These latter are designated by B and c and 
are connected, respectively, to terminals i> and E, which pass out 
through the sides of the case. The plates ordinarily are separated 
by strips of porous insulating material F, which are used merely 
for the purpose of maintaining them in position. The space 
within the can is filled with oil G. Such a condenser will prove 
highly efficient and will not become heated or permanently in- 
jured. 

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 ad- 
justable supports as, for example, to rods n passing through 
stuffing boxes K in the sides of case A and furnished with nuts L, 
the ends of the rods being threaded for engagement with the 
nuts. 

It is well known that oils possess insulating properties, and it 
it has been a common practice to interpose a body of oil between 
two conductors for purposes of insulation ; but Mr. Tesla be- 
lieves he has discovered peculiar properties in oils which ren- 
der them very valuable in this particular form of device. 



CHAPTER XXXY. 

ELECTROLYTIC REGISTERING METER. 

AN ingenious form of electrolytic meter attributable to Mr. 
Tesla is one in which a conductor is immersed in a solution, so 
arranged that metal may be deposited from the solution or taken 
away in such a manner that the electrical resistance of the con- 
ductor is varied in a definite proportion to the strength of the 
current the energy of which is to be computed, whereby this 
variation in resistance serves as a measure of the energy and also 
may actuate registering mechanism, whenever the resistance 
rises above or falls below certain limits. 

In carrying out this idea Mr. Tesla employs an electroly- 
tic 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 pro- 
portional 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 pro- 
portional to the difference of potential, and, therefore, in propor- 
tion 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 uni- 
form 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 



ELECTROLYTIC REGISTERING METER. 



421 



increased, both in proportion to the strength of the current pass- 
ing through the conductors. From sucli variation in the resis- 
tance 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. 

In Fig. 229 G designates a direct-current generator. L L are 
the conductors of the circuit extending therefrom. A is a tube 
of glass, the ends of which are scaled, as by means of in- 
sulating plugs or caps B B. c c' are two conductors extending 
through the tube A, their ends passing out through the plugs B to 




FIG. 229. 



terminals thereon. These conductors may be corrugated or 
formed in other proper ways to offer the desired electrical resis- 
tance. K is a resistance connected in series witli the two con- 
ductors c c', which by their free terminals are connected up in 
circuit with one of the conductors L. 

The method of using this device and computing by means 
thereof the energy of the current will be readily understood. 
First, the resistances of the two conductors c c', respectively, are 
accurately measured and noted. Then a known current is passed 
through the instrument for a -given time, and by a second meas- 
urement the increase and diminution of the resistances of the two 
conductors are respectively taken. From these data the constant is 



422 



INVENTIONS OF NIKOLA TKSLA. 



obtained that is to say, for example, the increase of resistance of 
one conductor or the diminution of the resistance of the other per 
lamp hour. These two measurements evidently serve as a check, 
since the gain of one conductor should equal the loss of the other. 
A further check is afforded by measuring both wires in series with 
the resistance, in which case the resistance of the whole should 
remain constant. 

In Fig. 230 the conductors c c' are connected in parallel, the 
current device at x passing in one branch iirst through a resis- 
tance R' and then through conductor c, while on the other branch 
it passes iirst through conductor c', and then through resistance 




FIG. 280. 



R". The resistances R' R" are equal, as also are the resistances of 
the conductors c c'. It is, moreover, preferable that the respective 
resistances of the conductors c c' should be a known and con- 
venient fraction of the coils or resistances R' R". It will be ob- 
served that in the arrangement shown in Fig. 230 there is a constant 
potential difference between the two conductors c c' throughout 
their entire length. 

It will be seen that in both cases illustrated, the proportionality 
of the increase or decrease of resistance to the current strength 
will always be preserved, for what one conductor gains the other 
loses, and the resistances of the conductors c c' being small as 



ELECTROLYTIC REGISTERING METER. 423 

compared with the resistances in series with them. It will be 
understood that after. each measurement or registration of a given 
variation of resistance in one or both conductors, the direction of 
the current should -be changed or the instrument reversed, so that 
the deposit will be taken from the conductor which has gained 
and added to that which has lost. This principle is capable of 
many modifications. For instance, since there is a section of the 
circuit to wit, the conductor c or c' that varies in resistance in 
proportion to the current strength,. such variation maybe utilized, 
as is done in many analogous cases, to effect the operation of 
various automatic devices, such as registers. It is better, how- 
ever, for the sake of simplicity to compute the energy by meas- 
urements of resistance. 

The chief advantages of this arrangement are, first, that it is 
possible to read off directly the amount of the energy expended 
by means of a properly constructed ohm-meter and without re- 
sorting 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 in- 
strument and correctness of the indications are but slightly af- 
fected 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. 



CHAPTEK XXXVI. 

THERMO-MAGNETIC MOTORS AND PYRO-MAGNETIC GENERATORS. 

No electrical inventor of the present day dealing with the 
problems of light and power considers that he has done himself 
or his opportunities justice until he has attacked the subject of 
thermo-magiietism. As far back as the beginning of the seven- 
teenth century it was shown by Dr. William Gilbert, the father 
of modern electricity, that a loadstone or iron bar when heated 
to redness loses its magnetism ; and since that time the influence 
of heat on the magnetic metals has been investigated frequently, 
though not with any material or practical result. 

For a man of Mr. Tesla's inventive ability, the problems in 
this field have naturally had no small fascination, and though he 
has but glanced at them, it is to be hoped he may find time to 
pursue the study deeper and further. For such as he, the in- 
vestigation must undoubtedly bear fruit. Meanwhile he has 
worked out one or two operative devices worthy of note. 1 He 
obtains mechanical power by a reciprocating action resulting 
from the joint operations of heat, magnetism, and a spring or 
weight or other force that is to say he subjects a body magnet- 
ized 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 

1. 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 per- 
formed and is interesting principally on account of the very rapid rate of 
vibration. 



TlIK II MO- MAGNETISM AND PYRO MAGNKTIHM. 



4-25 



body in tlie opposite direction, and again subject the same to the 
demagnetizing power of the heat. 

Use is made of either an electro-magnet or a permanent mag- 
net, and the heat is directed against a body that is magnetized 
by induction, rather than directly against a permanent magnet, 
thereby avoiding the loss of magnetism that might result in the 
permanent magnet by the action of heat. Mr. Tesla also provides 
for lessening the volume of the heat or for intercepting the same 
during that portion of the reciprocation in which the cooling 
action takes place. 

In the diagrams are shown some of the numerous arrangements 
that may be made use of in carrying out this idea. In all 
of these figures the magnet-poles are marked N s, the armature 
A, the Bunsen burner or other source of heat H, the axis of mo- 




FIG. 232. 



FIG. 231. 



FIG. 233. 



tion M, and the spring or the equivalent thereof namely, a 
weight is marked w. 

In Fig. 281 the permanent magnet N is connected with a frame, 
F, supporting the axis M, from which the arm P hangs, and at the 
lower end of which the armature A is supported. The stops 2 
and :-J limit the extent of motion, and the spring w tends to draw 
the armature A away from the magnet N. It will now be under- 
stood that the magnetism of > is sufficient to overcome the 
spring w and draw the armature A toward the magnet N. The 
heat acting upon the armature A neutralizes its induced magnet- 
ism sufficiently for the spring w to draw the armature A away 
from the magnet M and also from the heat at ir. The armature 
now cools, and the attraction of the magnet N overcomes the 
spring w and draws the armature A back again above the burm-i- 



42(5 



INVENTIONS OF NIKOLA TESLA. 



ii, so that the same is again heated and the operations are re- 
peated. The reciprocating movements thus obtained are em- 
ployed 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. 

Fig. 232 represents the same parts as before described ; but an 




Fie. 234. 





FIG. 235. 



electro-magnet is illustrated in place of a permanent magnet. 
The operations, however, are the same. 

In Fig. 233 are shown the same parts as in Figs. 231 and 232, 
but they are differently arranged. The armature A, instead of 
swinging, is stationary and held by arm p', and the core N s of 
the electro-magnet is made to swing within the helix Q, the 
core being suspended by the arm p from the pivot M. A shield, 
R, is connected with the magnet-core and swings with it, so 
that after the heat has demagnetized the armature A to such an 
extent that the spring w draws the core N s away from the arma- 
ture A, the shield K comes between the flame H and armature A, 
thereby intercepting the action of the heat and allowing the ar- 
mature to cool, so that the magnetism, again preponderating, 
causes the movement of the core N s toward the armature A and 
the removal of the shield R from above the flame, so that the heat 
again acts to lessen or neutralize the magnetism. A rotary or 
other movement may be obtained from this reciprocation. 

Fig. 234 corresponds in every respect with Fig. 233, except 
that a permanent horseshoe-magnet, N s is represented as taking 
the place of the electro-magnet in Fig. 233. 

In Fig. 235 is shown a helix, Q, with an armature adapted to 
swing toward or from the helix. In this case there may be a soft- 



THERMO-MAGNETI8M AND PJEO-MAGNETI8M. 



4-21 



iron core in the helix, or the armature may assume the form of a 
solenoid core, there being no permanent core within the helix. 

Fig. 23tf is an end view, and Fig. 237 a plan view, illustrating 
the method as applied to a swinging armature, A, and a stationary 
permanent magnet, N s. In this instance Mr. Tesla applies the 
heat to an auxiliary armature or keeper, T, which is adjacent to 
and preferably in direct contact with the magnet. This arma- 
ture T, in the form of a plate of sheet-iron, extends across from 
one pole to the other and is of sufficient section to practically 
form a keeper for the magnet, so that when the armature T is 
cool nearly all the lines of force pass over the same and very little 
free magnetism is exhibited. Then the armature A, which swings 
freely on the pivots M in front of the poles N s, is very little at- 
tracted and the spring w pulls the same way from the poles into 
the position indicated in the diagram. The heat is directed upon 
the iron plate T at some distance from the magnet, so as to allow 
the magnet to keep comparatively cool. This heat is applied be- 
neath the plate by means of the burners H, and there is a con- 
nection from the armature A or its pivot to the gas-cock 6, or 
other device for regulating the heat. The heat acting upon the 
middle portion of the plate T, the magnetic conductivity of the 
heated portion is diminished or destroyed, and a great number of 
the lines of force are deflected over the armature A, which is now 




FIG. 287. 



FIG. 238. 




FIG. 



powerfully attracted and drawn into line, or nearly so, with the 
poles N s. In so doing the cock 6 is nearly closed and the plate 
T cools, the lines of force are again deflected over the same, the 
attraction exerted upon the armature A is diminished, and the 
spring w pulls the same away from the magnet into the position 
shown by full lines, and the operations are repeated. 



The ar- 



438 



INVENTIONS OF NIKOLA TE8LA 



rangement shown in Fig. 236 has the advantages that the mag- 
net and armature are kept cool and the strength of the per- 
manent magnet is better preserved, as the magnetic circuit is 
constantly closed. 

In the plan view, Fig. 238, is shown a permanent magnet and 
keeper plate, T, similar to those in Figs. 236 and 237, with the 
burners H for the gas beneath the same ; but the armature is 
pivoted at one end to one pole of the magnet and the other end 
swings toward and from the other pole of the magnet. The spring 
w acts against a lever arm that projects from the armature, and 
the supply of heat has to be partly cut oif by a connection to the 
swinging armature, so as to lessen the heat acting upon the keeper 
plate when the armature A has been attracted. 





0.;N 



FIG. 240. 



FIG. 241. 



Fig. 239 is similar to Fig. 238, except that the keeper T is not 
made use of and the armature itself swings into and out of the 
range of the intense action of the heat from the burner H. Fig. 
240 is a diagram similar to Fig. 231, except that in place of using a 
spring and stops, the armature is shown as connected by a link, 
to the crank of a fly-wheel, so that the fly-wheel will be revolved 
as rapidly as the armature can be heated and cooled to the 
necessary extent. A spring may be used in addition, as in Fig. 
231. In Fig. 241 the armatures A A are connected by a link, so 
that one will be heating while the other is cooling, and the attrac- 
tion exerted to move the cooled armature is availed of to draw 
away the heated armature instead of using a spring. 



THEKMO-MAGNETISM AND PT110-MAGNETISM. 429 

Mr. Tesla has also devoted his attention to the development of 
a pyromagnetic generator of electricity 1 based upon the following 
laws : First, that electricity or electrical energy is developed in 
any conducting body by subjecting such body to a varying mag- 
netic 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 tempera- 
ture to a certain degree. These laws may be applied in the pro- 
duction 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 ap- 
plication of heat, or, more strictly speaking, by the application or 
action of a varying temperature upon the source of the magnet- 
ism. 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 por- 
tion of the iron to an orange red, or a temperature of about 600 
centigrade. "When this condition is reached, the iron somewhat 
suddenly loses its magnetic properties, if it be very thin, and the 
same effect is produced as though the iron had been moved away 
from the magnet or the heated section had been removed. This 
change of position, however, is accompanied by a shifting of the 
magnetic lines, or, in other words, by a variation in the magnetic 
influence to which the coil is exposed, and a current in the coil 
is the result. Then remove the flame or in any other way reduce 
the temperature of the iron. The lowering of its temperature is 
accompanied by a return of its magnetic properties, and another 
change of magnetic conditions occurs, accompanied by a current 
in an opposite direction in the coil. The same operation may be 

1. 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 pyromag- 
netic 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. 



430 



INVENTIONS OF NIKOLA TESLA. 



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. 

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 ex- 
posed to the heat and artificially cooling the heated part. 

These improvements are applicable generally to the generators 
constructed on the plan above described that is to say, we may 
use an artificial cooling device in conjunction with a variable or 
varied or uniform source of heat. 

Fig. 242 is a central vertical longitudinal section of the com- 




FIG. 242. 



FIG. 243. 



plete apparatus and Fig. 243 is a cross-section of the magnetic 
armature-core of the generator. 

Let A represent a magnetized core or permanent magnet the 
poles of which are bridged by an armature-core composed of a 
casing or shell B inclosing a number of hollow iron tubes c. 
Around this core are wound the conductors E E', to form the 
coils in which the currents are developed. In the circuits of 
these coils are current-consuming devices, as F F'. 

n is a furnace or closed fire-box, through which the central 
portion of the core B extends. Above the fire is a boiler K, con- 
taining water. The flue L from the fire-box may extend up 
through the boiler. 

G is a water-supply pipe, and H is the steam-exhaust pipe, 
which communicates with all the tubes c in the armature B, so 
that steam escaping from the boiler will pass through the tubes. 



THKRMO-MA ONETISM AND P7RO-MA QNETISM. 431 

In tlie steam-exhaust pipe H is a valve v, to which is connected 
the lever i, by the movement of which the valve is opened 
or closed. In such a case as this the heat of the fire may he 
utilized for other purposes after as much of it as may be needed 
has been applied to heating the core u. There are special ad- 
vantages in the employment of a cooling device, in that the 
metal of the core B is not so quickly oxidized. Moreover, the 
difference between the temperature of the applied heat and of 
the steam, air, or whatever gas or fluid be applied as the cooling 
medium, may be increased or decreased at will, whereby the 
rapidity of the magnetic changes or fluctuations may be regulated. 



CHAPTEK XXXVII. 
ANTI-SPAKKING DYNAMO BRUSH AND COMMUTATOR. 

IN direct current dynamos of great electromotive force such, 
for instance, as those used for arc lighting when one commuta- 
tor 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 commuta- 
tor-bars. In the lirst 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 
Hash which causes an unsteady current, rapid wear of the com- 
mutator 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 increas- 
ing the number of the commutator-bars, or by other similar 
means ; but all these devices are expensive or not fully available, 
and seldom attain the object desired. 

To prevent this sparking in a simple manner, Mr. Tesla some 
years ago employed with the commutator-bars and intervening 
insulating material, mica, asbestos paper or other insulating and 
incombustible material, arranged to bear on the surface of the 
commutator, near to and behind the brush. 

In the drawings, Fig. 244 is a section of a commutator with 
an asbestos insulating device ; and Fig. 245 is a similar view, re- 
presenting two plates of mica upon the back of the brush. 

In Fig. 244, c represents the commutator and intervening 
insulating material ; B B, the brushes, d d are sheets of asbestos 
paper or other suitable non-conducting material. // are springs, 
the pressure of which may be adjusted by means of the screws 

V 9- 

In Fig. 245 a simple arrangement is shown with two plates of 
mica or other material. It will be seen that whenever one com- 



ANTL SPARKING BRUSHES AND COMMUTATORS. 433 

imitator segment passes out of contact with the brush, the forma- 
tion of the arc will be prevented by the intervening insulating 
material coming in contact with the insulating material on the 
brush. 

Asbestos paper or cloth impregnated with zinc-oxide, mag- 
nesia, zirconia, or other suitable material, may be used, as the 




FIG. 244. 



FIG. 245. 



paper and cloth are soft, and serve at the same time to wipe and 
polish the commutator ; but mica or any other suitable material 
can be employed, provided the material be an insulator or a bad 
conductor of electricity. 

A few years later Mr. Tesla turned his attention again to the 
same subject, as, perhaps, was very natural in view of the fact 
that the commutator had always been prominent in his thoughts, 
and that so much of his work was even aimed at dispensing with 
it entirely as an objectionable and unnecessary part of dynamos 
and motors. In these later efforts to remedy commutator troubles, 
Mr. Tesla constructs a commutator and the collectors therefor in 
two parts mutually adapted to one another, and, so far as the es- 
sential features are concerned, alike in mechanical structure. Se- 
lecting 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 quad- 
rant 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 



434 



INVENTIONS OF NIKOLA TESLA. 



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 im- 
portant advantages. In the first place the short-circuiting and the 
breaking of the armature coil connected to the commutator-seg- 
ments occur at the same instant, and from the nature of the con- 
struction 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. 





FIG. 246. 



FIG. 247. 



The mechanical advantages will be better understood by re- 
ferring to the accompanying diagrams, in which Fig. 246 is a 
central longitudinal section of the end of a shaft with the im- 
proved 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. 



ANTI-SPARKING BRUSHES AND COMMUTATORS. 435 

249 and 250 are views of details of Fig. 248. Fig. 251 is a longi- 
tudinal central section of another modification, and Fig. 252 is a 
sectional view of the same. A is the end of the armature-shaft 
of a dynamo-electric machine or motor. A' is a sleeve of insu- 
lating material around the shaft, secured in place by a screw a'. 




FIG. 248 FIG. 249. FIG. 250. 

The commutator proper is in the form of a disc which is made 
up of four segments n D' G G', similar to those shown in Fig. 248. 
Two of these segments, as D D', are of metal and are in electrical 
connection with the ends of the coils on the armature. The 
other two segments are of insulating material. The segments are 
held in place by a band, B, of insulating material. The disc is 
held in place by friction or by screws, y' g' , Fig. 248, which 
secure the disc firmly to the sleeve A'. 

The collector is made in the same form as the commutator. It 
is composed of the two metallic segments E E' and the two insu- 
lating segments r F', bound together by a band, c. The metallic 
segments E E' are of the same or practically the same width or 
extent as the insulating segments or spaces of the commutator. 
The collector is secured to a sleeve, B', by screws g g, and the sleeve 
is arranged to turn freely on the shaft A. The end of the sleeve 
B' is closed by a plate, /, upon which presses a pivot-pointed 
screw, /i, adjustable in a spring, H, which acts to maintain the 
collector in close contact with the commutator and to compensate 
for the play of the shaft. The collector is so fixed that it cannot 
turn with the shaft. For example, the diagram shows a slotted 
plate, K, which is designed to be attached to a stationary support, 
and an arm extending from the collector and carrying a clamping 
screw, L, by which the collector may be adjusted and set to the 
desired position. 

Mr. Tesla prefers the form shown in Figs. 246 and 247 to fit 



436 



INVENTIONS OF NIKOLA TESLA. 



the insulating segments of both commutator and collector loosely 
and to provide some means as, for example, light springs, e e, 
secured to the bands A' B', respectively, and bearing against the 
segments to exert a light pressure upon them and keep them in 
close contact and to compensate for wear. The metal segments 
of the commutator may be moved forward by loosening the 
screw a'. 

The line wires are fed from the metal segments of the collector, 
being secured thereto in any convenient manner, the plan of con- 
nections being shown as applied to a modified form of the com- 
mutator in Fig. 251. The commutator and the collector in thus 
presenting two flat and smooth bearing surfaces prevent most ef- 
fectually by mechanical action the occurrence of sparks. 

The insulating segments are made of some hard material capa- 
ble of being polished and formed with sharp edges. Such mater- 
ials as glass, marble, or soapstone may be advantageously used. 
The metal segments are preferably of copper or brass ; but they 
may have a facing or edge of durable material such as platinum 
or the like where the sparks are liable to occur. 

In Fig. 248 a somewhat modified form of the invention is 
shown, a form designed to facilitate the construction and replac- 





FIG. 251. 



FIG. 252. 



ing of the parts. In this modification the commutator and col- 
lector are made in substantially the same manner as previously 
described, except that the bands B o are omitted. The four seg- 
ments of each part, however, are secured to their respective sleeves 
by screws g' g' , and one edge of each segment is cut away, so that 
small plates a b may be slipped into the spaces thus formed. Of 



ANTI-SPARKINO BRUSHKS AND COMMUTATORS. 437 

these plates a a are of metal, and are in contact with the metal seg- 
ments D D', respectively. The other two, b &, are of glass or mar- 
ble, and they are all better square, as shown in Figs. 249 and 250, 
so that they may be turned to present new edges should any edge 
become worn by use. Light springs d bear upon these plates 
and press those in the commutator toward those in the collector, 
and insulating strips c c are secured to the periphery of the discs 
to prevent the blocks from being thrown out by centrifugal action. 
These plates are, of course, useful at those edges of the segments 
only where sparks are liable to occur, and, as they are easily re- 
placed, they are of great advantage. It is considered best to coat 
them with platinum or silver. 

In Figs. 251 and 252 is shown a construction where, instead of 
solid segments, a fluid is employed. In this case the commuta- 
tor and collector are made of two insulating discs, s T, and in 
lieu of the metal segments a space is cut out of each part, as at 
K K', corresponding in shape and size to a metal segment. The 
two parts are iitted smoothly and the collector T held by the 
screw h and spring n against the commutator s. As in the other 
cases, the commutator revolves while the collector remains sta- 
tionary. The ends of the coils are connected to binding-posts 
$ -v, which are in electrical connection with metal plates t 2 within 
the recesses in the two parts s T. These chambers or recesses 
are filled with mercury, and in the collector part are tubes w w, 
with screws w w, carrying springs x and pistons x', which com- 
pensate for the expansion and contraction of the mercury under 
varying temperatures, but which are sufficiently strong not to 
yield to the pressure of the fluid due to centrifugal action, and 
which serve as binding-posts. 

In all the above cases the commutators are adapted fora single 
coil, and the device is particularly suited to such purposes. The 
number of segments may be increased, however, or more than 
one commutator used with a single armature. Although the 
bearing-surfaces are shown as planes at right angles to the shaft 
or axis, it is evident that in this particular the construction 'may 
be very greatly modified. 



CHAPTER XXXVIII. 
AUXILIARY BKUSH REGULATION OF DIRECT CURRENT DYNAMOS. 

AN interesting method devised by Mr. Tesla for the regula- 
tion of direct current dynamos, is that which lias 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 in- 
termediate point of the iield wire to the auxiliary brush. 1 

The relative positions of the respective brushes are varied, 
either automatically or by hand, so that the shunt becomes in- 
operative when the auxiliary brash 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 propor- 
tion to the varying electrical conditions of the working-circuit, 
the current developed can be regulated in proportion to the de- 
mands in the working-circuit. 

Fig. 253 is a diagram illustrating the invention, showing one 
core of the field-magnets with one helix wound in the same direc- 
tion 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 

1. 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 remem- 
bered, 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 



A UXILIA R Y BR U8U REG ULA TION. 439 

the electric devices that may be employed for automatically 
adjusting the brushes, and Fig. 258 is a diagram illustrating the 
positions of the brushes when the machine is being energized at 
the start. 

a and 5 are the positive and negative brushes of the main or 
working-circuit, and c the auxiliary brush. The working-circuit 
i) extends from the brushes a and b, as usual, and contains elec- 
tric lamps or other devices, D', either in series or in multiple 
arc. 

M M' represent the field-helices, the ends of which are con- 
nected to the main brushes a and 5. The branch or shunt wire 
c' extends from the auxiliary brush c to the circuit of the field- 
helices, and is connected to the same at an intermediate point, v. 

H represents the commutator, with the plates of ordinary con- 




FIG. 253. 

struction. When the auxiliary brush c occupies such a position 
upon the commutator that the electro-motive force between the 
brushes a and c is to the electro-motive force between the brushes 
c and b as the resistance of the circuit a M c' c A is to the resistance 
of the circuit b M' c' c B, the potentials of the points x and Y will 
be equal, and no current will flow over the auxiliary brush ; but 
when the brush c occupies a different position the potentials of 
the points x and Y will be different, and a current will flow over 
the auxiliary brush to and from the commutator, according to the 
relative position of the brushes. If, for instance, the commu- 
tator-space between the brushes a and c, when the latter is at the 
neutral point, is diminished, a current will flow from the point Y 
over the shunt c to the brush J, thus strengthening the current 
in the part M', and partly neutralizing the current in part M ; but 
if the space between the brushes a and c is increased, the cur- 



440 



INVENTIONS OF NIKOLA TESLA. 



rent will flow over the auxiliary brush in an opposite direction, 
and the current in M will be strengthened, and in M' partly neu- 
tralized. 

By combining with the brushes a, I, and c any usual automatic 
regulating mechanism, the current developed can be regulated in 
proportion to the demands in the working circuit. The parts M 




FIG. 254. 

and M' of the Held wire may be wound in the same direction. 
In this case they are arranged as shown in Fig. 253 ; or the part 
M may be wound in the opposite direction, as shown in Figs. 

254 and 255. 

It will be apparent that the respective cores of the tield-rnag- 
nets are subjected to neutralizing or intensifying effects of the 
current in the shunt through c', and the magnetism of the cores 
will be partially neutralized, or the points of greatest magnetism 
shifted, so that it will be more or less remote from or approach- 
ing to the armature, and hence the aggregate energizing actions 
of the field magnets on the armature will be correspondingly 
varied. 

In the form indicated in Fig. 253 the regulation is effected by 
shifting the point of greatest magnetism, and in Figs. 254 and 

255 the same effect is produced by the action of the current in 
the shunt passing through the neutralizing helix. 

The relative positions of the respective brushes may be varied 
by moving the auxiliary brush, or the brush c may remain station- 
ary and the core P be connected to the main-brush holder A, 
so as to adjust the brushes a b in their relation to the brush c. 
If, however, an adjustment is applied to all the brushes, as seen 
in Fig. 257, the solenoid should be connected to both a and c, so 
as to move them toward or away from each other. 

There are several known devices for giving motion in propor- 



A UXfLIARY BRUSH It KG ULAT1ON. 



441 



tion to an electric current. In Figs. 25i and 257 the moving 
cores are shown as convenient devices for obtaining the required 
extent of motion with very slight changes in the current passing 
through the helices. It is understood that the adjustment of 
the main brushes causes variations in the strength of the current 
independently of the relative position of those brushes to the 
auxiliary brush. In all cases the adjustment should be such that 
no current flows over the auxiliary brush when the dynamo is 
running with its normal load. 

In Figs. 256 and 25 7 A A indicate the main-brush holder, 
carrying the main brushes, and c the auxiliary-brush holder, 
carrying the auxiliary brush. These brush-holders are movable 
in arcs concentric with the centre of the commutator-shaft. An 
iron piston, p, of the solenoid s, Fig. 25(5, is attached to the aux- 
iliary-brush holder c;. The adjustment is effected by means of a 
spring and screw or tightener. 

In Fig. 257 instead of a solenoid, an iron tube inclosing a coil 
is shown. The piston of the coil is attached to both brush- 
holders A A and c. When the brushes are moved directly by 
electrical devices, as shown in Figs. 25(5 and 257, these are so 
constructed that the force exerted for adjusting is practically 
uniform through the whole length of motion. 

It is true that auxiliary brushes have been used in connection 
with the helices of the field-wire; but in these instances the 




FIG. 255. 

helices receive the entire current through the auxiliary brush or 
brushes, and these brushes could not be taken off without break- 
ing the circuit through the field. These brushes cause, move- 
over, 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- 



442 



INVENTIONS OF NIKOLA TESLA. 



helices. The arrangement lias, besides, the ad vantage of facilitating 
the self-excitation of the machine in all cases where the resis- 
tance of the field- wire is very great comparatively to the resis-' 
tance of the main circuit at the start for instance, on arc-light 




FIG. 256. 



machines. In this case the auxiliary brush e is placed near to, or 
better still in contact with, the brush , as shown in Fig. 258. 
In this manner the part M' is completely cut out, and as the part 
M has a considerably smaller resistance than the whole length of 
the field-wire the machine excites itself, whereupon the auxiliary 
brush is shifted automatically to its normal position. 

In a further method devised by Mr. Tesla, one or more auxili- 
ary brushes are employed, by means of which a portion or the 
whole of the field coils is shunted. According to the relative po- 
sition 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. 

In Fig. 259, a and 1) are the positive and negative brushes of 
the main circuit, and c an auxiliary brush. The main circuit D 




FIG. 258. 

extends from the brushes a and b, as usual, and contains the 
helices M of the field wire and the electric lamps or other work- 
ing devices. The auxiliary brush c is connected to the point x 
of the main circuit by means of the wire c' . H is a commutator 



A UXILIAR T Bit USE REG ULA TION. 443 

of ordinary construction. It will have been seen from what was 
said already that when the electro-motive force between the brushes 
a and c is to the electromotive force between the brushes c 
and b as the resistance of the circuit a M c' c A is to the resistance 
of the circuit b c B c c' D, the potentials of the points a? and y 
will be equal, and no current will pass over the auxiliary brush 
c; but if that brush occupies a different position relatnely to the 
main brushes the electric condition is disturbed, and current 
will flow either from y to x or from a? to y, according to the rela- 
tive 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 a 1) c any automatic regulating 
mechanism, the current developed can be regulated automatically 
in proportion to the demands of the working circuit. 

In Figs. 264 and 265 some of the automatic means are repre- 
sented that may be used for moving the brushes. The core P, 
Fig. 264, of the solenoid-helix s is connected with the brush c to 
move the same, and in Fig. 265 the core P is shown as within the 
helix s, and connected with brushes a and c, so as to move the 
same toward or from each other, according to the strength of the 
current in the helix, the helix being within an iron tube, s', that 
becomes magnetized and increases the action of the solenoid. 

In practice it is sufficient to move only the auxiliary brush, as 
shown in Fig. 264, as the regulation is very sensitive to the 
slightest changes ; but the relative position of the auxiliary brush 
to the main brushes may be varied by moving the main brushes, 
or both main and auxiliary brushes may be moved, as illustrated 
in Fig. 265. In the latter two cases, it will be understood, the 
motion of the main brushes relatively to the neutral line of the 
machine causes variations in the strength of the current inde- 
pendently 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 fiows over the auxil- 
iary brush. 

The field helices may be connected, as shown in Fig. 25!>, 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 maybe shunted, 
as shown in Figs. 260 and 262. 



444 



INVENTIONS OF NIKOLA TESLA. 



The arrangement shown in Fig. '2ti*2 is advantageous, as it dim- 
inishes 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. 




FIG. 259. 




FIG. 261. 




FIG. 262. 



FIG. 263. 



The field helices may be wound in the same direction, or a part 
may be wound in opposite directions. 

The connection between the helices and the auxiliary brush or 
brushes may be made by a wire of small resistance, or a resistance 
may be interposed (R, Fig. 263,) between the point ./ and the 



A UXILIARY BRUSH REG ULATWN. 44~> 

auxiliary brush or brushes to divide the sensitiveness when the 
'brushes are adjusted. 

The accompanying sketches also illustrate improvements made 
by Mr. Tesla in the mechanical devices used to effect the shift- 
ing 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 a? a?, Fig. 266. c is the com- 
mutator; B and B', the brush-holders, B carrying the main 
brushes a a' , and B' the auxiliary or shunt brushes b b. The 
axis of the brush-holder B is supported by two pivot-screws, JP />. 
The other brush-holder, B', has a sleeve, d, and is movable 
around the axis of the brush-holder B. In this way both brush- 
holders can turn very freely, the friction of the parts being 
reduced to a minimum. Over the brush-holders is mounted the 
solenoid s, which rests upon a forked column, c. This column 




FIG. 264. Fro. 265. 

also affords a support for the pivots p p, and is fastened upon a 
solid bracket or projection, p, which extends from the base of 
the machine, and is cast in one piece with the same. The 
brush-holders B B' are connected by means of the links e e 
and the cross-piece F to the iron core i, which slides freely in the 
tube T of the solenoid. The iron core i has a screw, s, by means 
of which it can be raised and adjusted in its position relatively 
to the solenoid, so that the pull exerted upon it by the solenoid 
is practically uniform through the whole length of motion which 
is required to effect the regulation. In order to effect the 
adjustment with greater precision, the core i is provided with a 
small iron screw, s'. The core being first brought very nearly 
in the required position relatively to the solenoid by means of 
the screw s, the small screw s' is then adjusted until the magnetic 
attraction upon the core is the same when the core is in any posi. 
tion. A convenient stop, , serves to limit the upward move- 
ment of the iron core. 



44(5 



INVENTIONS OF NIKOLA TESLA. 



To check somewhat the movement of the core i, a dash-pot, K, 
is used. The piston L of the dash-pot is provided with a vah^e, 
v, which opens by a downward pressure and allows an easy- 
downward movement of the iron core i, but closes and checks 
the movement of the core when it is pulled up by the action 
of the solenoid. 

To balance the opposing forces, the weight of the moving 
parts, and the pull exerted by the solenoid upon the iron core, 
the weights w w may be used. The adjustment is such that 
when the solenoid is traversed by the normal current it is just 
strong enough to balance the downward pull of the parts. 

The electrical circuit-connections are substantially the same as 




FIG. 266 



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 resis- 
tance of the circuit or for some other reason, the current is 
increased, the solenoid s gains in strength and pulls up the iron 
core i, thus shifting the main brushes in the direction of rotation 
and the auxiliary brushes in the opposite way. This diminishes 
the strength of the current until the opposing forces are balanced 
and the solenoid is traversed by the normal current ; but if from 
any cause the current in the circuit is diminished, then the weight 
of the moving parts overcomes the pull of the solenoid, the iron 



A UXILTAR Y BR U8II RKO ULA TION. 447 

core i descends, thus shifting the brashes the opposite way and 
increasing the current to the normal strength. The dash-pot 
connected to the iron core i may he 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 dia- 
grams. This valve permits a comparatively easy downward move- 
ment of the iron core, but checks its movement when it is drawn 
up by the solenoid. Such an arrangement has the advantage 
that a great number of lights may be put on without diminishing 
the light- power of the lamps in the circuit, as the brushes assume 
at once the proper position. When lights are cut out, the dash- 
pot acts to retard the movement ; but if the current is considerably 
increased the solenoid gets abnormally strong and the brushes 
are shifted instantly. The regulator being properly adjusted, 
lights or other devices may be put on or out with scarcely any 
perceptible difference. It is obvious that instead of the dash-pot 
any other retarding device may be used. 



CHAPTER XXXIX. 

IMPROVEMENT IN THE CONSTRUCTION OF DYNAMOS AND MOTORS. 

THIS invention of Mr. Tesla is an improvement in the con- 
struction of dynamo or magneto electric machines or motors, 
consisting in a novel form of frame and field magnet which ren- 
ders the machine more solid and compact as a structure, which 
requires fewer parts, and which involves less trouble and expense 
in its manufacture. It is applicable to generators and motors 
generally, not only to those which have independent circuits 
adapted for use in the Tesla alternating current system, but to 
other continuous or alternating current machines of the ordinary 
type generally used. 

Fig. 268 shows the machine in side elevation. Fig. 269 is a 
vertical sectional view of the field magnets and frame and an end 
view of the armature ; and Fig. 270 is a plan view of one of 
the parts of the frame and the armature, a portion of the latter 
being cut away. 

The field magnets and frame are cast in two parts. These 
parts are identical in size and shape, and each consists of the solid 
plates or ends A B, from which project inwardly the cores c D and 
the side bars or bridge pieces, E F. The precise shape of these 
parts is largely a matter of choice that is to say, each casting, 
as shown, forms an approximately rectangular frame ; but it might 
obviously be more or less oval, round, or square, without de- 
parture from the invention. It is also desirable to reduce the 
width of the side bars, E F, at the center and to so proportion the 
parts that when the frame is put together the spaces between the 
pole pieces will be practically equal to the arcs which the sur-. 
faces of the poles occupy. 

The bearings G for the armature shaft are cast in the side bars 
E F. The field coils are either wound on the pole pieces or on a 
form and then slipped on over the ends of the pole pieces. 
The lower part or casting is secured to the base after being 
finished off. The armature K on its shaft is then mounted in 



IMPROVEMENTS IN DYNAMOS AND MOTORS. 



449 



the bearings of the lower casting and the other part of the frame 
placed in position, dowel pins L or any other means being used to 
secure the two parts in proper position. 




FIG. 268. 




FIG. 270. 



In order to secure an easier fit, the side bars E F, and end pieces, 
A B, are so cast that slots M are formed when the two parts are 
put together. 



450 INVENTIONS OF NIKOLA TESLA. 

This machine possesses several advantages. For example, if we 
magnetize the cores alternately, as indicated by the characters y 
s, it will be seen that the magnetic circuit between the poles of 
each part of a casting is completed through the solid iron side 
bars. The bearings for the shaft are located at the neutral points 
of the field, so that the armature core is not affected by the mag- 
netic condition of the field. 

The improvement is not restricted to the use of four pole pieces, 
as it is evident that each pole piece could be divided or more than 
four formed by the shape of the casting. 



CHAPTER XI, 

TKSLA DIRECT CURRENT ARC LIGHTING SYSTKM. 

AT one time, soon after his arrival in America, Mr. Tesla was 
greatly interested in the subject of arc lighting, which then occu- 
pied public attention and readily enlisted the support of capital. 
He therefore worked out a system which was confided to a com- 
pany 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 sim- 
plify the processes of operation. 

We will take up the dynamo first. Fig. 271 is a longitudinal 
section, and Fig. 272 a cross section of the machine. Fig. 273 is 
a top view, and Fig. 274 a side view of the magnetic frame. Fig. 
275 is an end view of the commutator bars, and Fig. 276 is a 
section of the shaft and commutator bars. Fig. 277 is a diagram 
illustrating the coils of the armature and the connections to the 
commutator plates. 

The cores c c c c of the field-magnets are tapering in both 
directions, as shown, for the purposes of concentrating the mag- 
netism upon the middle of the pole-pieces. 

The connecting-frame F F of the field-magnets is in the form 
indicated in the side view, Fig. 274, the lower part being pro- 
vided with the spreading curved cast legs e e, so that the machine 
will rest firmly upon two base-bars, r r. 

To the lower pole, s, of the field-magnet M is fastened, by 
means of babbitt or other fusible diamagnetic material, the base 
B, which is provided with bearings b for the armature-shaft H. 
The base B has a projection, p, which supports the brush-holders 
and the regulating devices, which are of a special character de- 
vised by Mr. Tesla. 

The armature is constructed with the view to reduce to a min- 



453 



INVENTIONS OF NIKOLA TEHLA. 



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. 

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 




FIG. 271. 

a waste of power. Owing to the mutual action of the lines of 
force, the magnetic properties of iron, and the speed of the dif- 
ferent portions of the armature core, these currents are generated 
principally on and near the surface of the armature core, dimin- 
ishing 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 ma- 
terial, the waste currents will be reduced to fifty per cent. 

As shown in the diagrams, the armature is constructed of thin 
iron discs n D D, of various diameters, fastened upon the arma- 
ture-shaft in a suitable manner and arranged according to their 
sizes, so that a series of iron bodies, i i i, is formed, each of which 
diminishes in thickness from the centre toward the periphery. 
At both ends of the armature the inwardly curved discs d d, of 
cast iron, are fastened to the armature shaft. 

The armature core being constructed as shown, it will be easily 
seen that on those portions of the armature that are the most 
remote from the axis, and where the currents are principally de- 
veloped, the length of iron in the generating direction is only a 



DIRECT CURRKNT ARC LIGHTING SYSTEM. 



453 



small fraction of the total length of the armature core, and be- 
sides this the iron body is subdivided in the generating direction, 
and therefore the Foueault currents are greatly reduced. Another 
cause of heating is the shifting of the poles of the armature core. 
In consequence of the subdivision of the iron in the armature 
and the increased surface for radiation, the risk of heating is 
lessened. 

The iron discs D D D are insulated or coated with some insulat- 
ing-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 re- 
volved between the poles of the field magnets without showing 
the slightest increase of temperature. 

The end discs, d d, which are of sufficient thickness and, for 
the sake of cheapness, of cast-iron, are curved inwardly, as in- 
dicated 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 super- 
imposed parts, and the curve of the end discs, dd, is so calculated 
that the first part that is, practically half of the wire just fills 





Fro. 273. 

up the hollow space to the line xx; 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, M>, and the inside mass 
of wires, w', are equal at each side of the plane x x. In this case 
the passive or electrically-inactive wires are of the smallest 
length practicable. The arrangement has further the advantage 



454 



INVENTIONS OF NIKOLA TESLA. 



that the total lengths of the crossing wires at the two sides of 
the plane x x are practically equal. 

To equalize further the armature coils at both sides of the 
plates that are in contact with the brushes, the winding and con- 
necting 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 < -f this 
second group, instead of being connected to the next following 
commutator bars, are connected to the directly opposite bars of 
the commutator. The second group is then insulated and the 
third group wound, the coils of this group being connected to 
those bars to which they would be connected in the usual way. 
The wires are then thoroughly insulated and the second part of 
wire is wound and connected in the same manner. 

Suppose, for instance, that there are twenty-four coils that is, 
twelve in each part and consequently twenty-four commutator 
plates. There will be in each part three groups, each containing 
four coils, and the coils will be connected as follows: 

Groups. Commutator J><ir*\ 

( First 15 

First part of wire I Second 17 21 

( Third 913 

( First 1317 

Second part of wire < Second 5 9 

( Third 21 1 

In constructing the armature core and winding and connecting 
the coils in the manner indicated, the passive or electrically in- 



DIRECT CURRENT ARC LIGHTING SYSTEM. 



455 



active wire is reduced to a minimum, and the coils at each 
side of the plates that are in contact with the brushes are prac- 
tically equal. In this way the electrical efficiency of the ma- 
chine is increased. 

The commutator plates t are shown as outside the bearing b of 




FIG. 275. 



FIG. 276. 



the armature shaft. The shaft H is tubular and split at the end 
portion, and the wires are carried through the same in the usual 
manner and connected to the respective commutator plates. The 
commutator plates are upon a cylinder, w, and insulated, and this 
cylinder is properly placed and then secured by expanding* the 
split end of the shaft by a tapering screw plug, v. 




FIG. 277. 

The arc lamps invented by Mr. Tesla for use on the circuits 
from the above described dynamo are those in which the separa- 
tion and feed of the carbon electrodes or their equivalents is ac- 
complished by means of electro-magnets or solenoids in connection 
with suitable clutch mechanism, and were designed for the purpose 



4o6 INVENTIONS OF NIKOLA TESLA. 

of remedying certain faults common to arc lamps. 

He proposed to prevent the frequent vibrations of the movable 
carbon "point" and flickering of the light arising therefrom; to 
prevent the falling into contact of the carbons ; to dispense with 
the dash pot, clock work, or gearing and similar devices; to ren- 
der the lamp extremely sensitive, and to feed the carbon almost 
imperceptibly, and thereby obtain a very steady and uniform 
light. 

In that class of lamps where the regulation of the arc is effected 
by forces acting in opposition on a free, movable rod or lever di- 
rectly 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 cor- 
responding flicker in the light. This difficulty is most apparent 
when there are only a few lamps in circuit. To lessen this diffi- 
culty 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 em- 
ployed, 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 retard- 
ing devices ; but these are often unreliable and objectionable, and 
increase the cost of construction. 

Mr. Tesla combines two electro-magnets one of low resis- 
tance in the main or lamp circuit, and the other of comparatively 
high resistance in a shunt around the arc a movable armature 
lever, and a special feed mechanism, the parts being arranged so 
that in the normal working position of the armature lever the 
same is kept almost rigidly in one position, and is not affected 
even by considerable changes in the electric circuit ; but if the 
carbons fall into contact the armature will be actuated by the 
magnets so as to move the lever and start the arc, and hold the 
carbons until the arc lengthens and the armature lever returns to 
the normal position. After this the carbon rod holder is released 
by the action of the feed mechanism, so as to feed the carbon and 
restore the arc to its normal length. 

Fig. 278 is an elevation of the mechanism made use of in 
this arc lamp. Fig. 279 is a plan view. Fig. 280 is an ele- 
vation of the balancing lever and spring; Fig. 281 is a de- 



DIRECT CURRENT ARC LIGHTING 8YSTKM. 



4f>7 



taclied plan view of the pole pieces and armatures upon the 
friction clamp, and Fig. 282 is a section of the clamping tube. 

M is a helix of coarse wire in a circuit from the lower carbon 
holder to the negative binding screw . N is a helix of fine wire 
in a shunt between the positive binding screw -\- and the 
negative binding screw . The upper carbon holder s is a paral- 
lel rod sliding through the plates s' s 2 of the frame of the lamp, 
and hence the electric current passes from the positive binding 








FIG. 279, 




FIG. 281. 



FIG. 280. 



post _j_ through the plate s 2 , carbon holder s, and upper carbon 
to the lower carbon, and thence by the holder and a metallic 
connection to the helix M. 

The carbon holders are of the usual character, and to insure 
electric connections the springs I are made use of to grasp the 
upper carbon holding rod s, but to allow the rod to slide freely 
through the same. These springs / may be adjusted in their 
pressure by the screw m, and the spring / may be sustained upon 



4.->s INVENTIONS OF NIKOLA TE8LA. 

any suitable support. They are shown as connected with the 
upper end of the core of the magnet N. 

Around the carbon-holding rod s, between the plates s' s 2 ? 
there is a tube, R, which forms a clamp. This tube is counter- 
bored, as seen in the section Fig. 282, so that it bears upon the 
rod s at its upper end and near the middle, and at the lower end of 
this tubular clamp K there are armature segments r of soft iron. 
A frame or arm, n, extending, preferably, from the core N 2 , sup- 
ports the lever A by a fulcrum-pin, o. This lever A has a hole, 
through which the upper end of the tubular clamp E passes 
freely, and from the lever A is a link, q, to the lever 2, which 
lever is pivoted at y to a ring upon one of the columns s 8 . This 
lever t has an opening or bow surrounding the tubular clamp 
K, and there are pins or pivotal connections w between the lever 
t and this clamp R, and a spring, r, serves to support or suspend 
the weight of the parts and balance them, or nearly so. This 
spring is adjustable. 

At one end of the lever A is a soft-iron armature block, , over 
the core M' of the helix M, and there is a limiting screw, c, pass- 
ing through this armature block , and at the other end of the 
lever A is a soft iron armature block, 5, with the end tapering or 
wedge shaped, and the same comes close to and in line with the 
lateral projection e on the core N 2 . The lower ends of the cores 
M' N 2 are made with laterally projecting pole-pieces M 3 N 3 , respect- 
ively, and these pole-pieces are concave at their outer ends, and 
are at opposite sides of the armature segments ; at the lower end 
of the tubular clamp R. 

The operation of these devices is as follows : In the condition 
of inaction, the upper carbon rests upon the lower one, and when 
the electric current is turned on it passes freely, by the frame 
and spring /, through the rods and carbons to the coarse wire and 
helix M, and to the negative binding post v and the core M' thereby 
is energized. The pole piece M 3 attracts the armature r, and by 
the lateral pressure causes the clamp R to grasp the rod s', and 
the lever A is simultaneously moved from the position shown by 
dotted lines, Fig. 278, to the normal position shown in full lines, 
and in so doing the link q and lever t are raised, lifting the clamp 
R and s, separating the carbons and forming the arc. The mag- 
netism of the pole piece e tends to hold the lever A level, or 
nearly so, the core N 2 being energized by the current in the shunt 
which contains the helix N. In this position the lever A is not 



DIRECT CUKRKNT ABC LIGHTING SYSTEM. 4.V.) 

moved by any ordinary variation in the current, because the arm- 
ature b is strongly attracted by the magnetism of <?, and these 
parts are close to each other, and the magnetism of e acts at right 
angles to the magnetism of the core M'. If, now, the arc becomes 
too long, the current through the helix M is lessened, and the mag- 
netism of the core N 3 is increased by the greater current passing 
through the shunt, and this core N 3 , attracting the segmental arm- 
ature /, lessens the hold of the clamp R upon the rod s, allowing 
the latter to slide and lessen the length of the arc, which instantly 
restores the magnetic equilibrium and causes the clamp R to hold 
the rod s. If it happens that the carbons fall into contact, then 
the magnetism of N 2 is lessened so much that the attraction of 
the magnet M will be sufficient to move the armature a and lever 
A so that the armature b passes above the normal position, so as 
to separate the carbons instantly; but when the carbons burn 
away, a greater amount of current will pass through the shunt 
until the attraction of the core N 2 will overcome the attraction of 
the core M' and bring the armature lever A again into the normal 
horizontal position, and this occurs before the feed can take place. 
The segmental armature pieces '/ are shown as nearly semicircular. 
They are square or of any other desired shape, the ends of the 
pole pieces M 3 , N 3 being made to correspond in shape. 

In a modification of this lamp, Mr. Tesla provided means for 
automatically withdrawing a lamp from the circuit, or cutting 
it out when, from a failure of the feed, the arc reached an 
abnormal length ; and also means for automatically reinserting 
such lamp in the circuit when the rod drops and the carbons 
come into contact. 

Fig. 283 is an elevation of the lamp with the case in section. 
Fig. 284 is a sectional plan at the line x .r. Fig. 285 is an ele- 
vation, partly in section, of the lamp at right angles to Fig. 283. 
Fig. 286 is a sectional plan at the line y y of Fig. 283. Fig. 287 
is a section of the clamp in about full size. Fig. 288 is a de- 
tached section illustrating the connection of the spring to the 
lever that carries the pivots of the clamp, and Fig. 289 is a 
diagram showing the circuit-connections of the lamp. 

In Fig. 283, M represents the main and N the shunt magnet, both 
securely fastened to the base A, which with its side columns, s s, 
are cast in one piece of brass or other diamagnetic material. To 
the magnets are soldered or otherwise fastened the brass washers 
or discs a a a a. Similar washers, b &, of fibre or other insii- 



460 



IKVKNTIOXS OF NIKOLA TKSLA. 



lating material, serve to insulate the wires from the brass washers. 
The magnets M and N are made very flat, so that their width 
exceeds three times their thickness, or even more. In this way 
a comparatively small number of convolutions is sufficient to pro- 
duce the required magnetism, while a greater surface is offered 
for cooling off the wires. 




FIG. 284. 



FIG. 287. FIG. 288. 



The upper pole pieces, /// , of the magnets are curved, as in- 
dicated in the drawings, Fig. 283. The lower pole pieces/// /i', 
are brought near together, tapering toward the armature g, as 
shown in Figs. 284 and 286. The object of this taper is to con- 
centrate 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 y. This armature yisa piece of iron 



DIRECT CURRKNT ARC LIGHTING SYSTEM. 461 

in the shape of a hollow cylinder, having on each side a segment 
cut away, the width of which is equal to the width of the pole 
pieces m' n'. 

The armature is soldered or otherwise fastened to the clamp /-, 
which is formed of a brass tube, provided with gripping-jaws e 
Fig. 287. These jaws are arcs of a circle of the diameter of the 
rod R, and are made of hardened German silver. The guides 
/"/', through which the carbon-holding rod E slides, are made of 
the same material. This has the advantage of reducing greatly the 
wear and corrosion of the parts coming in frictional contact with 
the rod, which frequently causes trouble. The jaws e e are 
fastened to the inside of the tube r, so that one is a little lower 
than the other. The object of this is to provide a greater open- 
ing for the passage of the rod when the same is released by the 
clamp. The clamp r is supported on bearings w w, Figs. 283, 
285 and 287, which are just in the middle between the jaws e e. 
The bearings w w are carried by a lever, t, one end of which 
rests upon an adjustable support, -, of the side columns, s, the 
other end being connected by means of the link e' to the arma- 
ture-lever L. The armature-lever L is a flat piece of iron in |sj 
shape, having its ends curved so as to correspond to the form of 
the upper pole-pieces of the magnets M and N. It is hung upon 
the pivots v v, Fig. 284, which are in the jaw x of the 
top plate B. This plate B, with the jaw, is cast in one piece 
and screwed to the side columns, s s, that extend up from the 
base A. To partly balance the overweight of the moving parts, 
a spring, ', Figs. 284 and 288, is fastened to the top plate, B, 
and hooked to the lever t. The hook o is toward one side of the 
lever or bent a little sidewise, as seen in Fig. 288. By this means 
a slight tendency is given to swing the armature toward the 
pole-piece m' of the main magnet. 

The binding-posts K K' are screwed to the base A. A manual 
switch, for short-circuiting the lamp when the carbons are re- 
newed, is also fastened to the base. This switch is of ordinary 
character, and is not shown in the drawings. 

The rod E is electrically connected to the lamp-frame by means 
of a flexible conductor or otherwise. The lamp-case receives a 
removable cover, s 2 , to inclose the parts. 

The electrical connections are as indicated diagrammatically in 
Fig. 289. The wire in the main magnet consists of two parts, 
a?' and p'. These two parts may be in two separated coils or in 



4(52 



INVENTIONS OF NIKOLA TKSLA. 



one single helix, as shown in the drawings. The part ,// being 
normally in circuit, is, with the fine wire upon the shunt-magnet, 
wound and traversed by the current in the same direction, so as 
to tend to produce similar poles, N N or s s, on the corresponding 
pole-pieces of the magnets M and N. The part p' is only in cir- 
cuit when the lamp is cut out, and then the current being in the 
opposite direction produces in the main magnet, magnetism of 
the opposite polarity. 

The operation is as follows : At the start the carbons are to 
be in contact, and the current passes from the positive binding- 
post K to the lamp-frame, carbon-holder, upper and lower carbon, 
insulated return- wire in one of the side rods, and from there 
through the part x' of the wire on the main magnet to the nega- 




Fro. 289. 

tive binding-post. Upon the passage of the current the main 
magnet is energized and attracts the clamping-armature g, swing- 
ing the clamp and gripping the rod by means of the gripping 
jaws e e. At the same time the armature lever L is pulled down 
and the carbons are separated. In pulling down the armature lever 
L the main magnet is assisted by the shunt-magnet N, the latter 
being magnetized by magnetic induction from the magnet M. 

Tt will be seen that the armatures L and g are practically the 
keepers for the magnets M and N, and owing to this fact both 
magnets with either one of the armatures L and g may be con- 
sidered as one horseshoe magnet, which we might term a " com- 
pound magnet." The whole of the soft-iron parts M, m' t g, n' y 
N and i, form a compound magnet. 



DIRECT CURRENT ARC LIGHTING SYXTKM. 4fc] 

The carbons being separated, the fine wire receives a portion 
of the current. Now, the magnetic induction from the magnet 
M is such as to produce opposite poles on the corresponding ends 
of the magnet N ; but the current traversing the helices tends to 
produce similar poles on the corresponding ends of both magnets, 
and therefore as soon as the fine wire is traversed by sufficient 
current the magnetism of the whole compound magnet is dimin- 
ished. 

With regard to the armature g and the operation of the lamp, 
the pole 77i ' may be considered as the " clamping " and the pole /// 
as the " releasing " pole. 

As the carbons burn away, the fine wire receives more current 
and the magnetism diminishes in proportion. This causes the 
armature lever L to swing and the armature g to descend grad- 
ually under the weight of the moving parts until the end/>, Fig. 
283, strikes a stop on the top plate, B. The adjustment is such 
that when this takes place the rod K is yet gripped securely by 
the jaws ee. The further downward movement of the armature 
lever being prevented, the arc becomes longer as the carbons are 
consumed, and the compound magnet is weakened more and 
more until the clamping armature g releases the hold of the 
gripping-jaws e e upon the rod R, and the rod is allowed to drop 
a little, thus shortening the arc. The fine wire now receiving 
less current, the magnetism increases, and the rod is clamped 
again and slightly raised, if necessary. This clamping and re- 
leasing of the rod continues until the carbons are consumed. In 
practice the feed is so sensitive that for the greatest part of the 
time the movement of the rod cannot be detected without some 
actual measurement. During the normal operation of the lamp 
the armature lever L remains practically stationary, in the posi- 
tion show T n in Fig. 283. 

Should it happen that, owing to an imperfection in it, the rod 
and the carbons drop too far, so as to make the arc too short, or 
even bring the carbons in contact, a very small amount of cur- 
rent passes through the fine wire, and the compound magnet 
becomes sufficiently strong to act as at the start in pulling the 
armature lever L down and separating the c'arbons to a greater 
distance. 

It occurs often in practical work that the rod sticks in the 
guides. In this case the arc reaches a great length, until it finally 
breaks. Then the light goes out, and frequently the fine wire is 



464 INVENTIONS OF NIKOLA TESLA, 

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 predeter- 
mined length, such an amount of current is diverted through 
the fine wire that the polarity of the compound magnet is re- 
versed. The clamping armature g is now moved against the 
shunt magnet N until it strikes the releasing pole n'. As soon 
as the contact is established, the current passes from the positive 
binding post over the clamp />, armature g, insulated shunt mag- 
net, and the helix p' upon the main magnet M to the negative 
binding post. In this case the current passes in the opposite di- 
rection and changes the polarity of the magnet M, at the same 
time maintaining by magnetic induction in the core of the shunt 
magnet the required magnetism without reversal of polarity, and 
the armature g remains against the shunt magnet pole n'. The 
lamp is thus cut out as long as the carbons are separated. The 
cut out may be used in this form without any further improve- 
ment ; but Mr. Tesla arranges it so that if the rod drops and the 
carbons come in contact the arc is started again. For this pur- 
pose he proportions the resistance of part j!/ 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 di- 
verted through the carbons and the part x' to destroy or neutral- 
ize the magnetism of the compound magnet, Then the arma- 
ture g, having a slight tendency to approach to the clamping pole 
m' t comes out of contact with the releasing pole n'. As soon as 
this happens, the current through the part j?' is interrupted, and 
the whole current passes through the part x. The magnet M is 
now strongly magnetized, the armature g is attracted, and the 
rod clamped. At the same time the armature lever L is pulled 
down out of its normal position and the arc started. In this way 
the lamp cuts itself out automatically when the arc gets too long, 
and reinserts itself automatically in the circuit if the carbons drop 
together. 



CHAPTER XLI. 

IMPROVEMENT IN "UNIPOLAR" GENERATORS. 

ANOTHER interesting class of apparatus to which Mr. Tesla has 
directed his attention, is that of " unipolar " generators, in which a 
disc or a cylindrical conductor is mounted between magnetic 
poles adapted to produce an approximately uniform field. In 
the disc armature machines the currents induced in the rotating 
conductor flow from the centre to the periphery, or conversely, 
according to the direction of rotation or the lines of force as de- 
termined by the signs of the magnetic poles, and these currents 
are taken off usually by connections or brushes applied to the 
disc at points on its periphery and near its centre. In the case 
of the cylindrical armature machine, the currents developed in 
the cylinder are taken off .by brushes applied to the sides of the 
cylinder at its ends. 

In order to develop economically an electromotive force avail- 
able 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 be- 
comes difficult to secure and maintain a good electrical connection 
between the collecting brushes and the conductor, owing to the 
high peripheral speed. 

It has been proposed to couple two or more discs together in 
series, with the object of obtaining a higher electro-motive force ; 
but with the connections heretofore used and using other condi- 
tions 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 tho 



466 



INVENTIONS OF NIKOLA TESLA. 



manner of pulleys, and are connected together by flexible con- 
ducting bands or belts. 

The machine is built in such manner that the direction of 
magnetism or order of the poles in one tield of force is opposite 
to that in the other, so that rotation of the discs in the same di- 
rection 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 dises. 

It will be obvious that if the direction of magnetism in both 




Fro. 290. 



FIG. 291. 



fields be the same, the same result as above will be obtained by 
driving the discs in opposite directions and crossing the connect- 
ing 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 pur- 
poses such as for an exciter for alternating current generators, 
for a motor, and for any other purpose for which dynamo ma- 
chines are used. 

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. 



UNIPOLAR GENERATORS. 467 

In order to form a frame with two fields of force, a support, 
A, is cast with two pole pieces u B' integral with it. To this are 
joined by bolts E a casting D, with two similar and corresponding 
pole pieces c c'. The pole pieces B B' are wound and connected 
to produce a field of force of given polarity, and the pole 
pieces c c' are wound so as to produce a field of opposite po- 
larity. The driving shafts F G pass through the poles and are 
journaled in insulating bearings in the casting A u, as shown. 

H K are the discs or generating conductors. They are com- 
posed of copper, brass, or iron and are keyed or secured to their re- 
spective shafts. They are provided with broad peripheral flanges 
j. It is of course obvious that the discs may be insulated from their 
shafts, if so desired. A flexible metallic belt L is passed over the 
flanges of the two discs, and, if desired, maybe 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 suit- 
able metal is used. Each shaft is provided with a driving pulley 
M, by which power is imparted from a driving shaft. 

N N are the terminals. For the sake of clearness they are shown 
as provided with springs p, that bear upon the ends of the shafts. 
This machine, if self-exciting, would have copper bands around 
its poles ; or conductors of any kind such as wires shown in 
thexlrawings may be used. 



It is thought appropriate by the compiler to append here some 
notes on unipolar dynamos, written by Mr. Tesla, on a recent oc- 
casion. 



It is characteristic of fundamental discoveries, of great achieve- 
ments of intellect, that they retain an undiminished power upon 
the imagination of the thinker. The memorable experiment of 
Faraday with a disc rotating between the two poles of a magnet, 
which has borne such magnificent fruit, has long passed into 
every-day experience ; yet there are certain features about this 
embryo of the present dynamos and motors which even to-day 
appear to us striking, and are worthy of the most careful study. 

Consider, for instance, the case of a disc of iron or other metal 

1. Article by Mr. Tesla, contributed to The Electrical Engineer, N. Y., 
Sept. 2, 1891. 



468 INVENTIONS OF NIKOLA TESLA. 

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 opera- 
tion is dependent upon some shifting or change of the resultant 
of the magnetic attraction exerted upon the armature, this pro- 
cess 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 mag- 
netic 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 ordi- 
nary 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 mechan- 
ism. 

Considered as a dynamo machine, the disc is an equally inter- 
esting object of study. In addition to its peculiarity of giving 
currents of one direction without the employment of commutat- 
ing devices, such a machine differs from ordinary dynamos in 
that there is no reaction between armature and field. The arma- 
ture current tends to set up a magnetization at right angles to 
that of the field current, but since the current is taken off uni- 
formly from all points of the periphery, and since, to be exact, 
the external circuit may also be arranged perfectly symmetrical 
to the field magnet, no reaction can occur. This, however, is 
true only as long as the magnets are weakly energized, for when 
the magnets are more or less saturated, both magnetizations at 
right angles seemingly interfere with each other. 

For the above reason alone it would appear that the output of 
such a machine should, for the same weight, be much greater 
than that of any other machine in which the armature current 
tends to demagnetize the field. The extraordinary output of the 
Forbes unipolar dynamo and the experience of the writer con- 
firm this view. 

Again, the facility with which such a machine may be made to 
excite itself is striking, but this may be due besides to the ab- 
sence of armature reaction to the perfect smoothness of the cur- 
rent and non-existence of self-induction. 



UNIPOLAR GENERATORS. 



469 



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 con- 
tinue to flow and the field magnets will lose their strength com- 
paratively slowly. The reason for this will at once appear when 
we consider the direction of the currents set up in the disc. 

Referring to the diagram Fig. 292, d represents the disc with 
the sliding contacts B B' on the shaft and periphery. N and s 
represent the two poles of a magnet. If the pole N be above, as 
indicated in the diagram, the disc being supposed to be in the 




Fio. 292. 

plane of the paper, and rotating in the direction of the arrow D, 
the current set up in the disc will flow from the centre to the 
periphery, as indicated by the arrow A. Since the magnetic ac- 
tion is more or less confined to the space between the poles N s, 
the other portions of the disc may be considered inactive. The 
current set up will therefore not wholly pass through the external 
circuit F, but will close through the disc itself, and generally, if 
the disposition be in any way similar to the one illustrated, by far 
the greater portion of the current generated will not appear ex- 
ternally, as the circuit F is practically short-circuited by the inac- 
tive portions of the disc. The direction of the resulting currents 
in the latter may be assumed to be as indicated by the dotted 



470 INVENTIONS OF NIKOLA TESLA. 

lines and arrows HI and n / and tlie direction of the energizing 
field current being indicated by the arrows a b c d, an inspection of 
the figure shows that one of the two branches of the eddy current-, 
that is, A B' m B, will tend to demagnetize the field, while the 
other branch, that is, A B' n B, will have the opposite effect. 
Therefore, the branch A B' m B, that is, the one which is approach- 
ing the field, will repel the lines of the same, while branch A B' 
n B, that is, the one leaving the field, will gather the lines of 
force upon itself. 

In consequence of this there will be a constant tendency to 
reduce the current flow in the path A B' m B, while on the other 
hand no such opposition will exist in path A B' n B, and the effect 
of the latter branch or path will be more or less preponderating 
over that of the former. The joint effect of both the assumed 
branch currents might be represented by that of one single cur- 
rent 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 cer- 
tain strength as long as the rotation of the disc is continued. 

The result will, of course, largely depend on the resistance 
and geometrical dimensions of the path of the resulting eddy 
current and on the speed of rotation ; these elements, namely, 
determine the retardation of this current and its position relative 
to the field. For a certain speed there would be a maximum 
energizing action ; then at higher speeds, it would gradually fall 
off to zero and finally reverse, that is, the resultant eddy current 
effect would be to weaken the field. The reaction would be 
best demonstrated experimentally by arranging the fields N s, 
N' s', freely movable on an axis concentric with the shaft of the 



UNIPOLAR GENERATORS. 471 

disc. If the latter were rotated as before in the direction of the 
arrow D, the field would be dragged in the same direction with a 
torque, which, up to a certain point, would go on increasing with 
the speed of rotation, then fall off, and, passing through zero, 
finally become negative ; that is, the field would begin to rotate 
in opposite direction to the disc. In experiments with alternate 
current motors in which the field was shifted by currents of 
differing phase, this interesting result was observed. For very 
low speeds of rotation of the field the motor would show a 
torque of 900 Ibs. or more, measured on a pulley 12 inches 
in diameter. When the speed of rotation of the poles was 
increased, the torque would diminish, would finally go down to 
zero, become negative, and then the armature would begin to 
rotate in opposite direction to the field. 

To return to the principal subject ; assume the conditions to be 
such that the eddy currents generated by the rotation of the disc 
strengthen the field, and suppose the latter gradually removed 
while the disc is kept rotating at an increased rate. The current, 
once started, may then be sufficient to maintain itself and even 
increase in strength, and then we have the case of Sir William 
Thomson's "current accumulator." But from the above con- 
siderations it would seem that for the success of the experi- 
ment the employment of a disc not subdivided 1 would be es- 
sential, for if there should be a radial subdivision, the eddy cur- 
rents 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. 

The action of the eddy currents may be utilized to excite a ma- 
chine of any construction. For instance, in Figs. 293 and 294 an 
arrangement is shown by which a machine with a disc armature 
might be excited. Here a number of magnets, N s, N s, are 
placed radially on each side of a metal disc D carrying on its rim 
a set of insulated coils, c c. The magnets form two separate 
fields, an internal and an external one, the solid disc rotating in the 

1. Mr. Tesla here refers to an interesting article which appeared in July, 
1865, in the Phil. Magazine, by Sir W. Thomson, in which Sir William, 
speaking of his " uniform electric current accumulator," assumes that for 
self-excitation it is desirable to subdivide the disc into an infinite number of in- 
finitely 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. 



472 INVENTIONS OF NIKOLA TESLA. 

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 ex- 
perimental evidence to warrant such an assertion, such a mode of 
excitation would be wasteful. 

But a unipolar dynamo or motor, such as shown in Fig. 292, 
may be excited in an efficient manner by simply properly subdi- 
viding 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 




FIG. 293. FIG. 294. 

cylinder D is supposed to be arranged to rotate between the two 
poles N and s of a' magnet, which completely cover it on both 
sides, the contours of the disc and poles being represented by the 
circles d and d 1 respectively, the upper pole being omitted for 
the sake of clearness. The cores of the magnet are supposed to 
be hollow, the shaft c of the disc passing through them. If the 
unmarked pole be below, and the disc be rotated screw fashion, 
the current will be, as before, from the centre to the periphery, 
and may be taken off by suitable sliding contacts, B B', on the 
shaft and periphery respectively. In this arrangement the cur- 
rent flowing through the disc and external circuit will have no 
appreciable effect on the field magnet. 

But let us now suppose the disc to be subdivided spirally, as 



UNIPOLAR GENERATORS. 



478 



indicated by the full or dotted lines, Fig. 295. The difference of 
potential between a point on the shaft and a point on the peri- 
phery will remain unchanged, in sign as well as in amount. The 
only difference will be that the resistance of the disc will be aug- 
mented 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 cur- 
rent 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 sub- 
division. 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 in- 




FIG. 295. 



FIG. 296. 



dicated by the dotted lines, the current generated will tend to 
weaken the magnet. In the former case the machine will be 
capable of exciting itself when the disc is rotated in the direction 
of arrow D ; in the latter case the direction of rotation must be 
reversed. Two such discs may be combined, however, as indi- 
cated, the two discs rotating in opposite fields, and in the same 
or opposite direction. 

Similar disposition may, of course, be made in a type of 
machine in which, instead of a disc, a cylinder is rotated. In 
such unipolar machines, in the manner indicated, the usual field 
coils and poles may be omitted and the machine may be made to 
consist only of a cylinder or of two discs enveloped by a metal 
casting. 

Instead of subdividing the disc or cylinder spirally, as indicated 
in Fig. 295, it is more convenient to interpose one or more turns 



474 INVENTIONS OF NIKOLA TESLA. 

between the disc and the contact ring on the periphery, as illus- 
trated in Fig. 296. 

A Forbes dynamo may, for instance, be excited in such a man- 
ner. In the experience of the writer it has been found that in- 
stead 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 con- 
structed by the writer two years ago, and worked satisfactorily ; 
but for want of time the work in that direction has been tempor- 
arily suspended. A number of features pointed out above have 
also been used by the writer in connection with some types of 
alternating current motors. 



PART IV. 



APPENDIX.-EARLY PHASE MOTORS AND THE 
TESLA MECHANICAL AND ELEC- 
TRICAL OSCILLATOR. 



CHAPTER XLII. 

MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR. 

WHILE the exhibits of firms engaged in the manufacture of 
electrical apparatus of every description at the Chicago World's 
Fair, afforded the visitor ample opportunity for gaining an ex- 
cellent knowledge of the state of the art, there were also numbers 
of exhibits which brought out in strong relief the work of the 
individual inventor, which lies at the foundation of much, if not 
all, industrial or mechanical achievement. Prominent among 
such personal exhibits was that of Mr. Tesla, whose apparatus 
occupied part of the space of the Westinghouse Company, in 
Electricity Building. 

This apparatus represented the results of work and thought 
covering a period of ten years. It embraced a large number of 
different alternating motors and Mr. Tesla's earlier high fre- 
quency 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 in- 
cluded Mr. Tesla's earlier machines and disruptive discharge coils 
and high frequency transformers, which he used in his investi- 
gations and some of which are referred to in his papers printed 
in this volume. 

Fig. 297 shows a view of part of the exhibits containing the 
motor apparatus. Among these is shown at A a large ring in- 
tended 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 dis- 
tributed that a practically uniform field was obtained. This ring 
was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, elec- 
trician of the Westinghouse Electric and Manufacturing Com- 
pany. 



478 



INVENTION* OF NIKOLA 7/:s/..|. 




PERSONAL EXHIBIT AT THE WORLD? 8 FAIR. 479 

A smaller ring, shown at B, was arranged like the one exhibited 
at A but designed especially to exhibit the rotation of an 
armature in a rotating field. In connection with these two 
rings there was an interesting exhibit shown by Mr. Tesla which 
consisted of a magnet with a coil, the magnet being arranged to 
rotate in bearings. With this magnet he first demonstrated the 
identity between a rotating field and a rotating magnet ; the latter, 
when rotating, exhibited the same phenomena as the rings when 
they were energized by currents of differing phase. Another 
prominent exhibit was a model illustrated at c which is a two- 
phase motor, as well as an induction motor and transformer. It 
consists of a large outer ring of laminated iron wound with 
two superimposed, separated windings which can be connected 
in a variety of ways. This is one of the first models used by 
Mr. Tesla as an induction motor and rotating transformer. The 
armature was either a steel or wrought iron disc with a closed 
coil. When the motor was operated from a two phase generator 
the windings were connected in two groups, as usual. When 
used as an induction motor, the current induced in one of the 
windings of the ring was passed through the other winding on 
the ring and so the motor operated with only two wires. When 
iised as a transformer the outer winding served, for instance, as 
a secondary and the inner as a primary. The model shown at 
D is one of the earliest rotating field motors, consisting of a thin 
iron ring wound with two sets of coils and an armature consisting 
of a series of steel discs partly cut away and arranged on a small 
arbor. 

At E is shown one of the first rotating field or induction motors 
used for the regulation of an arc lamp and for other purposes. It 
comprises a ring of discs with two sets of coils having different 
self-inductions, one set being of German silver and the other of 
copper wire. The armature is wound with two closed-circuited 
coils at right angles to each other. To the armature shaft are 
fastened levers and other devices to effect the regulation. At F 
is shown a model of a magnetic lag motor ; this embodies a cast- 
ing with pole projections protruding from two coils between 
which is arranged to rotate a smooth iron body. When an alter- 
nating 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. 



480 INVENTIONS OF NIKOLA TE SLA. 

Another interesting exhibit, shown at G, is an early model of a 
two field motor energized by currents of different phase. There 
are two independent fields of laminated iron joined by brass' 
bolts ; in each field is mounted an armature, both armatures be- 
ing on the same shaft. The armatures were originally so ar- 
ranged 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 di- 
rect current, was energized by currents differing in phase, which 




FIG. 298. 

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 arma- 
ture, was obtained. 

The models H, i, j, represent a variety of rotating field, synchron- 
ous motors which are of special value in long distance transmis- 
sion work. The principle embodied in these motors was enunci- 
ated by Mr. Tesla in his lecture before the American Institute of 
Electrical Engineers, in May, 1888 1 . It involves the production 

1. See Part I, Chap. Ill, page 9. 



PERSONAL EXHIBIT AT TllK WORLD'S FAIR. 481 

of the rotating field in one of the elements of the motor by cur 
rents differing in phase and energizing the other element by 
direct currents. The armatures are of the two and three phase 
type. K is a model of a motor shown in an enlarged view in Fig. 
298. This machine, together with that shown in Fig. 299, was 
exhibited at the same lecture, in May, 1888. They were 
the first rotating field motors which were independently tested, 
having for that purpose been placed in the hands of Prof. An- 
thony in the winter of 188T-88. From these tests it was shown 
that the efficiency and output of these motors was quite satisfac- 
tory in every respect. 

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 




FIG. 299. 

some time ago handed over to the care of Prof. Ayrton in Eng- 
land. This model was originally provided with twelve independ- 
ent coils ; this number, as Mr. Tesla pointed out in his first lec- 
ture, 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 arm- 
ature is supported on brass bolts passing through the frame. A 
great variety of armatures was used in connection with these two 
and other fields. Some of the armatures are shown in front on 
the table, Fig. 297, and several are also shown enlarged in Figs. 
300 to 310. An interesting exhibit is that shown at L, Fig. 297.. 
This is an armature of hardened steel which was used in a demon- 



482 



INVENTIONS OF NIKOLA TESLA. 



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 cir- 
cuit and run in synchronism, and represents one of the earliest 
types of such an armature. Fig. 305 is another striking exhibit. 




FIG. 303. 



FIG. 304. 



FIG. 305. 




FIG. 306 



FIG. 307 



FIG. 308. 




FIG. 309. 



FIG. 310. 



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. 

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 



PERSONAL EXHIBIT AT THE WORLD'S FAIR. 483 

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 be- 
ing 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 insu- 
lated rings on a shaft and was used in connection with two or 
three phase circuits. 

All these models shown represent early work, and the en- 
larged engravings are made from photographs taken early in 
1888. There is a great number of other models which were ex- 
hibited, but which are not brought out sharply in the engraving, 
Fig. 297. For example at M is a model of a motor comprising 
an armature with a hollow shaft wound with two or three coils for 
two or three-phase circuits ; the armature was arranged to be sta- 
tionary and the generating circuits were connected directly to 
the generator. Around the armature is arranged to rotate on 
its shaft a casting forming six closed circuits. On the outside 
this casting was turned smooth and the belt was placed on it for 
driving with any desired appliance. This also is a very early 
model. 

On the left side of the table there are seen a large variety of 
models, N, o, P, etc., with fields of various shapes. Each of these 
models involves some distinct idea and they all represent gradual 



484 INVENTIONS OF NIKOLA TESLA. 

development chiefly interesting as showing Mr. Tesla's efforts to 
adapt his system to the existing high frequencies. 

On the right side of the table, at s, T, are shown, on separate 
supports, larger and more perfected armatures of commercial 
motors, and in the space around the table a variety of motors and 
generators supplying currents to them was exhibited. 

The high frequency exhibit embraced Mr. Tesla's first original 
apparatus used in his investigations. There was exhibited a 
glass tube with one layer of silk-covered wire wound at the top 
and a copper ribbon on the inside. This was the first disruptive 
discharge coil constructed by him. At u is shown the disruptive 




FIG. 811. 

discharge coil exhibited by him in his lecture before the Ameri- 
can Institute of Electrical Engineers, in May, 1891. 1 At v and w 
are shown some of the first high frequency transformers. A 
number of various fields and armatures of small models of high 
frequency apparatus as shown at x and Y, and others not visible 
in the picture, were exhibited. In the annexed space the dynamo 
then used by Mr. Tesla at Columbia College was exhibited ; also 
another form of high frequency dynamo used. 

In this space also was arranged a battery of Leyden jars and 
his large disruptive discharge coil which was used for exhibiting 

1. See Part II, Chap. XXVI., page 145. 



PERSONAL EXHIBIT AT THE WORLD' 8 FAIR. 485 

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 beau- 
tiful 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 mo- 
tor shown on the table operated by the disruptive discharge. 

As will be remembered by those who visited the Exposition, 
the Westinghouse Company made a fine exhibit of the various 
commercial motors of the Tesla system, while the twelve genera- 
tors in Machinery Hall were of the two-phase type constructed 
for distributing light and power. Mr. Tesla, also exhibited 
some models of his oscillators. 



CHAPTER XLIII. 

THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS. 

ON the evening of Friday, August 25, 1893, Mr. Tesla de- 
livered a lecture on his mechanical and electrical oscillators, be- 
fore the members of the Electrical Congress, in the hall adjoin- 
ing the Agricultural Building, at the World's Fair, Chicago. Be r 
sides the apparatus in the room, he employed an air compressor, 
which was driven by an electric motor. 

Mr. Tesla was introduced by Dr. Elisha Gray, and began by 
stating that the problem he had set out to solve was to construct, 
first, a mechanism which would produce oscillations of a per- 
fectly 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 cur- 
rents of a perfectly constant period independently of the work- 
ing conditions, and to produce these currents with mechanism 
which should be reliable and positive in its action without resort- 
ing 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 in- 
ventions Mr. Tesla called, quite appropriately, a mechanical and 
an electrical oscillator, respectively. 

The former is substantially constructed in the following way. 
There is a piston in a cylinder made to reciprocate automatically 
by proper dispositions of parts, similar to a reciprocating tool. 
Mr. Tesla pointed out that he had done a great deal of work in 
perfecting his apparatus so that it would work efficiently at such 
high frequency of reciprocation as he contemplated, but he did not 
dwell on the many difficulties encountered. He exhibited, how- 
ever, the pieces of a steel arbor which had been actually torn 
apart while vibrating against a minute air cushion. 

With the piston above referred to there is associated in one of 
his models in an independent chamber an air spring, or dash pot, 



MECUANICAL AND ELECTRICAL OSCILLATORS. 487 

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 mov- 
ing parts, in other words, no matter what the period of vibrations, 
the vibrations of the spring are always isochronous with the ap- 
plied 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 experi- 
ments, he vibrates a weight of approximately 20 pounds at the 
rate of about 80 per second and with a stroke of about f- inch, but 
by shortening the stroke the weight could be vibrated many hun- 
dred times, and has been, in other experiments. 

To start the vibrations, a powerful blow is struck, but the ad- 
justment 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 communi- 
cate with the outer atmosphere so that no matter how the tempera- 
ture of the enclosed air might vary, it still retains the same mean 
density as the outer atmosphere, and by this means a spring of 
constant rigidity is obtained. Now, of course, the pressure of 
the atmosphere may vary, and this would vary the rigidity of the 
spring, and consequently the period of vibration, and this feature 
constitutes one of the great beauties of the apparatus ; for, as Mr. 
Tesla pointed out, this mechanical system acts exactly like a 
string tightly stretched between two points, and with fixed nodes, 
so that slight changes of the tension do not in the least alter the 
period of oscillation. 

The applications of such an apparatus are, of course, numer- 
ous 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 



488 INVENTIONS OF NIKOLA TESLA. 

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. 

IsText he showed a model of a motor with delicate wheelwork, 
which was driven by these currents at a constant speed, no mat- 
ter 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 con- 
trolling the motion of the reciprocating piston by means of a 
spring, so as to obtain isochronous vibration, he was actually able 
to control the mechanical motion by the natural vibration of the 
electro-magnetic system, and he said that the case was a very 
simple one, and was quite analogous to that of a pendulum. 
Thus, supposing we had a pendulum of great weight, preferably, 
which would be .maintained in vibration by force, periodically 
applied ; now that force, no matter how it might vary, although 
it would oscillate the pendulum, would have no control over its 
period. 

Mr. Tesla also described a very interesting phenomenon which 
he illustrated by an experiment. By means of this new appara- 
tus, he is able to produce an alternating current in which the 
E. M. F. of the impulses in one direction preponderates over that 
of those in the other, so that there is produced the effect of a 
direct current. In fact he expressed the hope that these cur- 
rents would be capable of application in many instances, serving 
as direct currents. The principle involved in this preponderat- 
ing E. M. F. he explains in this way : Suppose a conductor is 
moved into the magnetic field and then suddenly withdrawn. If 
the current is not retarded, then the work performed will be a 
mere fractional one ; but if the current is retarded, then the 
magnetic field acts as a spring. Imagine that the motion of the 
conductor is arrested by the current generated, and that at the 
instant when it stops to move into the field, there is still the 



MECHANICAL AND ELECTRICAL OSCILLATORS. 489 

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 conduc- 
tor out of the field, the mechanically applied force is so timed 
that it helps the conductor to get out of the field, then it might 
leave the field with higher velocity than it entered it, and 
thus one impulse is made to preponderate in E. M. r. over the 
other. 

With a current of this nature, Mr. Tesla energized magnets 
strongly, and performed many interesting experiments bearing 
out the fact that one of the current impulses preponderates. 
Among them was one in which he attached to his oscillator a ring 
magnet with a small air gap between the poles. This magnet was 
oscillated up and down 80 times a second. A copper disc, when 
inserted within the air gap of the ring magnet, was brought into 
rapid rotation. Mr. Tesla remarked that this experiment also 
seemed to demonstrate that the lines of flow of current through 
a metallic mass are disturbed by the presence of a magnet in a 
manner quite independently of the so-called Hall effect. He 
showed also a very interesting method of making a connection 
with the oscillating magnet. This was accomplished by attaching 
to the magnet small insulated steel rods, and connecting to these 
rods the ends of the energizing coil. As the magnet was vibrated, 
stationary nodes were produced in the steel rods, and at these 
points the terminals of a direct current source were attached. 
Mr. Tesla also pointed out that one of the uses of currents, such 
as those produced in his apparatus, would be to select any given 
one of a number of devices connected to the same circuit by pick- 
ing out the vibration by resonance. There is indeed little doubt 
that with Mr. Tesla's devices, harmonic and synchronous tele- 
graphy will receive a fresh impetus, and vast possibilities are 
again opened up. 

Mr. Tesla was very much elated over his latest achievements, 
and said that he hoped that in the hands of practical, as well as 
scientific men, the devices described by him would yield important 
results. He laid special stress on the facility now afforded for 
investigating the effect of mechanical vibration in all directions, 
and also showed that he had observed a number of facts in con- 
nection with iron cores. 



490 INVENTIONS OF NIKOLA TE8LA. 

The engraving, Fig. 312, shows, in perspective, one of the 
forms of apparatus used by Mr. Tesla in his earlier investigations 
in this field of work, and its interior construction is made plain, 
by the sectional view shown in Fig. 313. It will be noted that the 
piston P is fitted into the hollow of a cylinder c which is provided 
with channel ports o o, and i, extending all around the inside 
surface. In this particular apparatus there are two channels o o 




for the outlet of the working fluid and one, i, for the inlet. 
The piston P is provided with two slots s s' at a carefully deter- 
mined distance, one from the other. The tubes T T which are 
sere wed into the holes drilled into the piston, establish communi- 
cation between the slots s s' and chambers on each side of the 
piston, each of these chambers connecting with the slot which is 
remote from it. The piston P is screwed tightly' on a shaft A 



MECHANICAL AND ELECTRICAL OSCILLATORS. 



491 



which passes through fitting boxes at the end of the cylinder c. 
The boxes project to a carefully determined distance into the hol- 
low of the cylinder c, thus determining the length of the stroke. 

Surrounding the whole is a jacket j. This jacket acts chiefly to 
diminish the sound produced by the oscillator and as a jacket when 
the oscillator is driven by steam, in which case a somewhat differ- 
ent arrangement of the magnets is employed. The apparatus here 
illustrated was intended for demonstration purposes, air being 
used as most convenient for this purpose. 

A magnetic frame .M M is fastened so as to closely surround the 
oscillator and is provided with energizing coils which establish 




FIG. 313. 

two strong magnetic fields on opposite sides. The magnetic frame 
is made up of thin sheet iron. In the intensely concentrated 
field thus produced, there are arranged two pairs of coils H H sup- 
ported in metallic frames which are screwed on the shaft A of 
the piston and have additional bearings in the boxes B B on each 
side. The whole is mounted on a metallic base resting on two 
wooden blocks. 

The operation of the device is as follows : The working fluid 
being admitted through an inlet pipe to the slot i and the piston 
being supposed to be in the position indicated, it is sufficient, 
though not necessary, to give a gentle tap on one of the shaft 



492 INVENTIONS OF NIKOLA TE8LA. 

ends protruding from the boxes B. Assume that the motion im- 
parted be such as to move the piston to the left (when looking at 
the diagram) then the air rushes through the slot s' and tube T 
into the chamber to the left. The pressure now drives the pie- 
ton towards the right and, owing to its inertia, it overshoots the 
position of equilibrium and allows the air to rush through the 
slot s and tube T into the chamber to the right, while the com- 
munication to the left hand chamber is cut off, the air of the 
latter chamber escaping through the outlet o on the left. On 
the return stroke a similar operation takes place on the right 
hand side. This oscillation is maintained continuously and the 
apparatus performs vibrations from a scarcely perceptible quiver 
amounting to no more than l of an inch, up to vibrations of a little 
over | of an inch, according to the air pressure and load. It is 
indeed interesting to see how an incandescent lamp is kept burn- 
ing with the apparatus showing a scarcely perceptible quiver. 

To perfect the mechanical part of the apparatus so that oscil- 
lations are maintained economically was one thing, and Mr. Tesla 
hinted in his lecture at the great difficulties he had first encoun- 
tered 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 as- 
sociates with the oscillator an air spring of constant resilience. But 
the most interesting of all, perhaps, is the maintenance of the con- 
stancy 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 vibra- 
tion set up in the circuits of the coils H H. The mechanical effici- 
ency of the apparatus is very high owing to the fact that friction 
is reduced to a minimum and the weights which are moved are 
small ; the output of the oscillator is therefore a very large one. 

Theoretically considered, when the various advantages which 
Mr. Tesla holds out are examined, it is surprising, considering 
the simplicity of the arrangement, that nothing was done in this 



MECHANICAL AND ELECTRICAL OSCILLATORS. 493 

direction before. No doubt many inventors, at one time or 
other, have entertained the idea of generating currents by at- 
taching a coil or a magnetic core to the piston of a steam engine, 
or generating currents by the vibrations of a tuning fork, or 
similar devices, but the disadvantages of such arrangements from 
an engineering standpoint must be obvious. Mr. Tesla, however, 
in the introductory remarks of his lecture, pointed out how by a 
series of conclusions he was driven to take up this new line of 
work by the necessity of producing currents of constant period 
and as a result of his endeavors to maintain electrical oscillation 
in the most simple and economical manner. 



INDEX. 



Alternate Current Electrostatic 

Apparatus 392 

Alternating Current Generators 

for High Frequency.. 152, 374, 224 
Alternating Motors and Trans- 
formers 7 

American Institute Electrical 

Engineers Lecture 145 

Anthony, W. A., Tests of Tesla 

Motors 8 

Apparatus for Producing High 

Vacua 276 

Arc Lighting, Tesla Direct, Sys- 
tem 451 

Auxiliary Brush Regulation ... 438 

Biography, Tesla 4 

Brush. Anti-Sparking 432 

" Third, Regulation 438 

Phenomena in High 

Vacuum 226 

Carborundum Button for Tesla 

Lamps 140, 253 

Commutator, Anti-Sparking. . . . 432 
Combination of Synchronizing 

and Torque Motor 94 

Condensers with Plates in Oil. . 418 
Conversion with Disruptive Dis- 
charge 193,204, 303 

Current or Dynamic Electricity 

Phenomena , . . . 327 

Direct Current Arc Lighting 451 

Dischargers, Forms of 305 

Disruptive Discharge Coil . . 207, 221 
Disruptive Discharge Phenom- 
ena... .. 212 



Dynamos, Improved Direct Cur- 
rent 448 

Early Phase Motors 477 

Effects with High Frequency 

and High Potential Currents. 119 
Electrical Congress Lecture, 

Chicago 486 

Electric Resonance 340 

Electric Discharges in Vacuum 

Tubes 396 

Electrolytic Registering Meter. 420 

Eye, Observations on the 294 

Flames, Electrostatic, Non-Con- 
suming 166,272 

Forbes Unipolar Generator. 468, 474 

Franklin Institute Lecture 294 

Generators, Pyromagnetic 429 

High Potential, High Frequency : 
Brush Phenomena in High 

Vacuum 226 

Carborundum Buttons.. 140, 253 
Disruptive Discharge Pheno- 
mena 212 

Flames, Electrostatic, Non- 
Consuming 166, 272 

Impedance, Novel Phenom- 
ena 194, 338 

Lighting Lamps Through 

Body 359 

Luminous Effects with 

Gases 368 

' ' Massage " with Currents 394 
Motor with Single Wire. 234, 330 

"No Wire " Motors 235 

Oil Insulation of Induction 
Coils 173, 221 



INDEX. 



495 



High Potential. Continued. 

Ozone, Production of 171 

Phosphorescence 367 

Physiological Effects... 162, 394 

Resonance 340 

Spinning Filament . 168 
Streaming Discharges of 

High Tension Coil. . ..155, 163 
Telegraphy without Wires 346 
Impedance, Novel Phenomena. 

194, 338 

Improvements in Unipolar Gen- 
erators 465 

Improved Direct Current Dyna- 
mos and Motors 448 

Induction Motors 92 

Institution Electrical Engineers 

Lecture 189 

Lamps and Motor operated on 

a Single Wire 330 

Lamps with Single Straight 

Fiber 183 

Lamps containing only a Gas. . 188 
Lamps with Refractory But- 
ton 177, 239, 360 

Lamps for Simple Phosphore- 
scence... 187, 282, 364 

Lecture, Tesla before : 

American Institute Elec- 
trical Engineers 145 

Royal Institution 124 

Institution Electrical Eng- 
ineers 189 

Franklin Institute and Nat- 
ional Electric Light Asso- 
ciation 294 

Electrical Congress, Chicago 486 
Lighting Lamps Through the 

Body 359 

Light Phenomena with High 

Frequencies. 349 

Luminous Effects with Gases at 

Low Pressure 368 

" Magnetic Lag " Motor 67 

"Massage" with Currents of 

High Frequency 394 

Mechanical and Electrical Oscil- 
lators 486 

Method of obtaining Direct from 
Alternating currents 409 



Method of obtaining Difference 
of Phase by Magnetic Shield- 
ing 71 

Motors : 

With Circuits of Different 

Resistance 79 

With Closed Conductors. . . 9 
Combination of Synchroniz- 
ing and Torque 94 

With Condenser in Arma- 
ture Circuit 100 

With Condenser in one of 

the Field Circuits 106 

With Coinciding Maxima of 
Magnetic Effect in Arma- 
ture and Field 83 

With "Current Lag " Arti- 
ficially Secured 58 

Early Phase 477 

With Equal Magnetic Ener- 
gies in Field and Arma- 
ture 81 

Or Generator, obtaining De- 
sired Speed of 36 

Improved Direct Current. . . 448 

Induction 92 

' ' Magnetic Lag " 67 

"No Wire" 235 

With Phase Difference in 
Magnetization of Inner 
and Outer Parts of Core.. 88 
Regulator for Rotary Cur- 
rent 45 

Single Circuit, Self-starting 

Synchronizing .... 50 

Single Phase 76 

With Single Wire to Genera- 
tor 234, 330 

Synchronizing 9 

Thermo-Magnetic 424 

Utilizing Continuous Cur- 
rent Generators . . 31 

National Electric Light Asso- 
ciation Lecture 294 

" No Wire " Motor 285 

Observations on the Eye. 294 

Oil, Condensers with Plates in . 418 
Oil Insulation of Induction Coils 

173, 221 

Oscillators, Mechanical and 
Electrical... .. 486 



INDEX. 



Ozone, Production of 171 

Phenomena Produced by Elec- 
trostatic Force 319 

Phosphorescence and Sulphide 

of Zinc 367 

Physiological Effects of High 

Frequency 162, 394 

Polyphase Systems 26 

Polyphase Transformer 109 

Pyromagnetic Generators 429 

Regulator for Rotary Current 

Motors 45 

Resonance, Electric, Phenom- 
ena of 340 

' ' Resultant Attraction " 7 

Rotating Field Transformers. . . 9 

Rotating Magnetic Field .9 

Royal Institution Lecture 124 

Scope of Lectures 119 

Single Phase Motor 76 

Single Circuit, Self-Starting 

Synchronizing Motors 50 

Spinning Filament Effects 168 

Streaming Discharges of High 
Tension Coil 155, 163 



Synchronizing Motors 9 

Telegraphy without Wires. . . . 346 
Transformer with Shield be- 
tween Primary and Second- 
ary 113 

Thermo-Magnetic Motors 424 

Thomson, J. J., on Vacuum 

Tubes 397,402, 406 

j Thomson, Sir W., Current Ac- 



cumulator 471 

Transformers : 

Alternating 7' 

Magnetic Shield 113 

Polyphase, 109 

Rotating Field 9 

Tubes : 

Coated with Yttria, etc 187 

Coated with Sulphide of 

Zinc, etc 290, 367 

Unipolar Generators 465 

Unipolar Generator, Forbes,468, 474 

Yttria, Coated Tubes 187 

Zinc, Tubes Coated with Sul- 
phide of 367 










hteh 




L 






A 000 



I 

361 176 1 b J 

* (9 ^^^X: 




lOFCAlIF' 



i 


















1? 



^ 

















i ufy. ^P LU/llrul ' v f- ^v"- v