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Nikola Tesla: Lecture Before 
The New York Academy of Sciences
April 6, 1897
Tesla Presents series, Part 2

Leland I. Anderson, Editor
xix, 123 pages
ISBN: 0-9636012-7-X
351-NTL ... $12.95

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Following Nikola Tesla On His Work With Alternating Currents, this book is the second in the Tesla Presents series offering the reader what has been, up until now, unavailable material on the pioneering work of Nikola Tesla in field of radio frequency electrical engineering. While first delivered under the title "On the Streams of Lenard and Roentgen with Novel Apparatus for Their Use" the information carried within the text of the lecture goes far beyond this topic. In addition to his opening remarks on X-ray discovery, a major portion of Tesla's commentary deals with the specially designed high frequency oscillators that were used in conjunction with his work, plus clear descriptions of stroboscopic instruments he designed for measurement of frequency and phase. Other topics addressed include wireless receiving methods and the genesis of Tesla's particle beam projector of 1937. During the talk Tesla had displayed approximately 120 drawings of specially constructed vacuum tubes, many being of the Lenard type and also the single-electrode type of his own design. Also among the drawings were renderings of various tubes used in his wireless communications experiments. Enhanced photographs of these images are among the 30 illustrations which fill out this fine volume.

At the close of 1894, realizing the necessity of recovery from a straining task, on which I have been laboring for a number of years and which still commands my energies, it occurred to me to investigate the actinic action of phosphorescent bodies. The subject did not appear to have been studied, and I began the work at once securing later, at the suggestion of some friends connected with the Century Magazine, the assistance of Messrs. Tonnel & Company, artists' photographers, of this city, then doing work for this magazine. In these experiments I employed an improved apparatus for the production of powerful electrical vibrations as well as one of my high frequency alternators of old design. A great variety of Crooks tubes, single electrode globes, and vacuum bulbs without external electrodes were experimented upon. A surprising fact was soon brought to light; namely that the actinic power of the Crooks bulbs varied greatly and that some, which emitted a comparatively strong luminosity, hardly showed an effect, while others, of much smaller light-giving power, produced strong impressions. I wish to state here, in order to be clear, that my efforts were directed toward investigating such actions of true phosphorescent light, as furnished from bulbs without appreciable emission of heat, and not so much as those of incandescent vacuum tubes, although some photographs were likewise taken with these. As both the artists and myself were busy on other matters, the plates in their ordinary holders were frequently put in some corner of the laboratory until a suitable opportunity for carrying on the experiments was found. During these investigations many plates gave a result, while many others failed, and on some of these both Mr. Alley, who then assisted me, and myself noted unaccountable marks and defects. Mr. Alley particularly found it extraordinary, that, in spite of his care, many plates proved defective and unsuccessful. The taking of these photographic impressions by means of Crooks bulbs brought freshly to my mind the experiments of Lenard, some features of which, particularly the action on a sensitive plate, had fascinated me from the start, and I resolved to go over the ground covered by him with assistance and improved appliances. Just as my attention was arrested by this feature, my laboratory with almost everything it contained was destroyed; and the few months following passed in intense activity which made me temporarily forget my projects. I had hardly finished the work of reconstruction and resumed the course of my ideas when the news of Roentgen's achievement reached me. Instantly the truth flashed upon my mind. I hurried to repeat his incompletely reported experiments, and there I beheld the wonder myself. Thentoo late
I realized that my guiding spirit had again prompted me and that I had failed to comprehend his mysterious signs. . . .

But while I have failed to see what others in my place might have perceived, it was always since my conviction, which is now firmer than ever, that I have not been forsaken by my kind spirit who then communed with me, but that, on the contrary, he has further guided me and guided me right in comprehension of the nature of these manifestations. Perhaps, in bringing to your attention some new facts which I have since discovered in addition to those already announced, I may induce, at least some of you, to interpret these phenomena as I do.  For fear, though, that I might miss my chief object this evening, I must ask your kind indulgence to dwell in a few works on the novel appliances which are exhibited here for your inspection.  When I trace their origin, I find it clearly in my early recognition of the fact that an economical method of producing electrical vibrations of very high frequency was the key for the solution of a number of most important problems in science and industry.  Insignificant as these machines may seem to you, they are nevertheless the result of labors extending through a number of years, and I can truthfully say that many times the difficulties which I have encountered in my endeavors to perfect them have appeared to me so great as to almost deprive me of the courage to continue the work.  When the experimenter has to spend several years of patient effort only to recognize that a mere microscopical cavity or air bubble in the essential parts of this apparatus is fatal to the attainment of the result sought for by him; when he has to find that his machine does not perform well because a wire he uses is a quarter of an inch too long or too short; when he notes that now a part of his apparatus when in action will grow colder in an apparently inexplicable way, and next that the same part will get overheated, though to all appearance the conditions are unchanged; when he makes puzzling observations at every step and ordinary instruments and methods of measurement are not available, then his progress is necessarily slow and his energies are severely taxed.  Finally, I am glad to say, I have triumphed over at least the chief obstacles, and nothing of any serious consequence stands now in the way of obtaining electrical oscillations of frequencies up to a few millions a second from ordinary supply circuits with simple and fairly economical appliances.  What this means I need not discuss.  It will be duly judged by those who have kept in touch with the development in this and allied fields.  These machines you see are only a few of the types I have developed, and as they stand here they are chiefly intended to replace the ordinary induction coil in its numerous uses.

Fig. 1.--Method of transformation of electrical energy by oscillatory condenser discharges. 

As to the broad principle, these transformers or electrical oscillators, as they might be most properly called, it is simple enough and has been advanced by me some five or six years ago.  A condenser is charged from a suitable source and is then in any convenient way discharged through a circuit containing, as it does here, the primary of the transformer.  The first diagram, Fig. 1, illustrates a generator G, a condenser C, and for charging and discharging the latter any kind of device b adapted to produce an intermittent break in the dielectric.  The circuit L containing the high or low tension devices d through which the condenser discharges being properly adjusted, extremely rapid electrical vibrations which, so far we know are unattainable by any other means, result; and these set up, by inductive action in any neighboring circuits, similar vibrations which give rise to many curious phenomena.  Having familiarized myself with these at the time when some laws governing them were not quite well understood, I have retained certain conceptions which I have then formed and which, though primitive, might stand even now in the light of our present advanced knowledge. 

Fig. 2.--Mechanical analogy of electrical oscillator.

I have likened a condenser to a reservoir R into which by means of a pump p an incompressible fluid as water W is supplied through a feed pipe p, as illustrated in the second diagram, Fig. 2, the fluid representing electricity, the pump the generator and the feed pipe the connecting wire.  The reservoir has a movable bottom, held up by a spring S, and opens the ports oo when the fluid in the vessel has reached a certain height and the pressure has become sufficient to overcome the elastic force of the spring.  To complete the model, adjustable weights w, a screw s for allowing the tension of the spring, and a valve v for regulating the flow of the fluid are provided.  With the giving away of the bottom, the fluid in the reservoir acquires velocity and consequently momentum, which results, in an increased pressure against the bottom causing the latter to open wider, and more of the fluid rushes out than the feed pipe can supply, whereupon the spring reasserts itself, closing again the ports, and the same process is repeated in more or less rapid succession.  This opening and closing of the bottom may be likened to the making and breaking of the conducting path, the frictional resistance in this mechanical system to the ohmic resistance and, obviously, the inertia of the moving masses to the self-induction of the electric circuit.  Now it is evident that, in order to keep in action the mechanism without the employment of auxiliary means, the average rate of supply through the pipe must be inferior to the average rate of discharge through the bottom; for, if it be otherwise, the ports will simply remain open and no vibration will take place.  The more nearly the average rate of supply equals the average rate of discharge, the quicker will the bottom open and close; and it is furthermore clear from a consideration of simple mechanical principles that, if the fluid be supplied so fast through the feed pipe that the bottom vibrates as it would of its own accord, then the amplitude of the vibration will be the largest, the pressure against the bottom the strongest, and the greatest amount of fluid will be passed through the ports.  All these considerations hold good for the electric circuit, and in experiments with high frequency machines, in which these effects were purposely magnified with the view of rendering their observation more easy, I have found that that condition is fulfilled when the capacity, self *induction, and frequency of vibration bear a certain relation, which observation I have since utilized in the adjustment of inductive circuits.  You will note that this condition governing the rate of supply and discharge, most important in practice, especially when no positively acting mechanical means are employed for effecting the rupture of the dielectric, is a distinct one and should not be confounded with the condition determining the oscillatory character of the discharge investigated long ago by Lord Kelvin.

Fig. 3.--System illustrated in figure 1 with self-induction coil.

Fig. 4.--Coil wound to secure greatly increased capacity.

Fig. 5.--Associating a secondary coil with a primary circuit coil.

Fig.6.--System adopted for existing municipal circuits.

Fig. 7.--Circuit controller allowing condensers to discharge alternately and successively.

The next step in the evolution of the principle and its adaptation to practical uses was to associate with the system illustrated in Fig. 1 a self-induction coil L, as shown in diagram Fig. 3, which modified the action in many now well understood ways.  In a simplified form of this arrangement the condenser, as a distinctive part of the system, was done away with, the necessary capacity being given to the coil itself, and for this purpose the turns of the latter were wound as illustrated in Fig. 4 so as to allow the storage of the proper and generally the largest possible amount of energy.  Then I associated a secondary coil S with the primary circuit P, as shown in Fig. 5, this enabling the obtaining of any tension required.  After this, the arrangement in diagram Fig. 6. was adopted as best suitable for the existing municipal circuits.  Again, the self-explanatory diagram Fig. 7. typically illustrates a further improved disposition as used in some of these machines with two or more circuits.  A modification of this plan with one continuous contact common to the two circuits, and independent interrupters for each of these, allows easy adjustment of the phase of the currents through the primary, which is of practical advantage in some uses of the apparatus.  Finally, in diagram Fig. 8 is shown the exact arrangement of the parts and circuits of one of these small oscillators with a break similar to that usually employed in connection with induction coils.  Although the majority of the preceding arrangements have been described by me before, I thought it necessary to dwell on them here in order to present clearly and comprehensively the subject.

A specific result of value in the operation of Roentgen bulbs is obtainable by the use of two circuits linked as shown in Fig. 7, or otherwise, or entirely independent with two separate primaries.  Namely, in the usual commercial bulbs the vacuum gets higher when the current is passed through the primary in a certain direction and is lowered when the direction of the current is reversed.  This is a direct consequence of some conditions which, as a rule, are present in the operation of the usual apparatus; that is, the asymmetry of the opposite current impulses, the unequal size, configuration or temperature of the two electrodes, or like causes which tend to render unequal the dissipation of the energy from both the electrodes.  It should be stated, though, that beyond a certain point, when the electrodes begin to act as entirely independent, the vacuum continues to increase no matter which way the current is passed through the primary. In the scheme illustrated in Fig. 7, or in its modifications referred to, the trouble attendant upon the operation of ordinary apparatus is practically done away with as the current though the primary is automatically reversed, and in this manner a tube which is first brought to the proper degree of exhaustion by means of one circuit can be worked for a long time without appreciable increase of vacuum or diminution of effectiveness. . . .

Editorial Remarks

     Skirmishes on non-publication of lecture
Lecture Commentary
     High frequency apparatus
     Lenard and Roentgen rays
     Harmful actions from Lenard and Roentgen tubes
The Lecture:
     Section I — Improved Apparatus for the Production of Powerful Electrical Vibrations; Novel Frequency Measurement Methods.
     Section I Addendum — Wireless Telegraphy Receiving Methods.
     Section II — The Hurtful Actions of Lenard and Roentgen Tubes.
     Section III — The Source of Roentgen Rays and the Practical Construction and Safe Operation of Lenard Tubes.
     Contemporary reviews of lecture

"Tesla's lecture of 1897 was never published in full. In this monograph..., Anderson has reconstructed the lecture from a partial typescript and from two articles by Tesla in the May 5 and August 11, 1997, issues of Electrical Review (N.Y.). Tesla begins by recounting his observations of emanations from dozens of differently designed Crooks tubes using a variety of powerful high frequency supplies of his own design... He provided evidence that Roentgen-rays were produced where the cathode rays first struck, e.g., the glass wall of the vacuum tube... In "The Hurtful Actions of Lenard and Roentgen Tubes," Tesla describes his own experiences with damage to the skin produced by both, and includes sensible advice for minimizing the damage. In addition to its historical interest, Tesla's presentation of experiments that revolutionized physical science provides a fascinating view of the analogies and metaphors guiding the thoughts of one important contributor to the revolution." (CHOICE, July/August 1995, Vol. 32, No. 11/12)

"Suddenly, without any preparation, Roentgen surprised the world..." So begins one of the most sought after of Tesla's treasures—his unpublished 1897 X-Ray lecture before the New York Academy of Sciences. Several puzzles become clear with this historical document. Tesla's independent discovery of X-Rays, unlike Roentgen's, was primarily based on sources which produced X-Rays by vacuum high field emission and the process now known as bremsstrahlung. While Roentgen employed a gaseous discharge tube utilizing electron avalanche, Tesla's cold cathode tubes worked best with high vacuum. Tesla's distinctive approach presaged the way for high energy particle accelerators, permitting the utilization of megavolt potentials, single electrode tubes, atmospheric bremsstrahlung, and a variety of intense beam techniques. As usual, he was years ahead with his inventions: Fowler and Nordheim's quantum mechanical considerations, necessary to understand Tesla's sources, were not available for another 32 years! (By then Tesla was developing a macroscopic charged particle beam.) This broad lecture provides a remarkable amount of collateral information. He not only discussed his approach to X-Ray experimentation, but also included a surprising amount of information on high frequency RF techniques, coupled oscillators, magnetic receiver technology, the development of fluorescent, lighting, and the creation of a stroboscopic measurement apparatus. The stroboscopy, tachometry, and chronography alone would have made the lecture a classic! The lecture demands careful study and balanced evaluation. Drawing on a lifetime of historical investigation and scholarship, Leland Anderson has once again contributed a splendid record of documentation to the scientific community. His unique insight into electrical history has not only furnished us with another unusual primary source in the history of science but, in this critical publication, Mr. Anderson has molded a singular instrument for the serious analysis of Tesla's professional activities.—Kenneth and James Corum

Nikola Tesla was born of Serbian parents at Smiljan, in the Austro-Hungarian border province of Lika, now part of Croatia, at midnight July 9-10, 1856. His father, Milutin, was a Serbian Orthodox priest, and his mother, nee Djouka Mandic, was of a family line whose sons were of the clergy and whose daughters were wives of the clergy. The Serbian Orthodox church then used the Julian calendar, and it continues to use this calendar today for days of observance. The American colonies had converted to the Gregorian calendar 132 years before Tesla arrived at New York in 1884. When Tesla crossed this "date line," 11 days dropped from his personal calendar. Most institutions observe Tesla's birth date as July 10, which date Tesla held for himself, but if the tolling church bells in Lika could have been heard in America when Tesla was born, the calendar date would have been July 21, 1856.

Establishing himself in the United States, Tesla became a citizen in 1891. He brought to the world great inventive gifts for which he became famousthe induction motor and the multi-phase alternating current power distribution system driving it (1888); the fundamental system of wireless telegraphy embodying the "Tesla coil" (1893); telemechanics (1898); the Tesla turbine (1913, 1920), which is attracting great revival interest; and, among other leading inventive achievements, VTOL aircraft (1928).

For 50 years following presentation of the principles of wireless telegraphy, now called radio, during his demonstration lecture in St. Louis, Tesla steadfastly asserted his inventive claim. It was not until five months following his death at the age of 86 in 1943 that the U.S. Supreme Court declared the basic radio patent of Marconi invalid, recognizing the prior art of Tesla for the system concept and apparatus, John Stone Stone for the method of selectivity, and Oliver Lodge for variable tuning.

In his lifetime, Tesla was granted over 30 honorary degrees and foundation medals from the world over. The unit of magnetic flux density in the MKS system was named "tesla" on the occasion of the centennial year of his birth. The only other American to share such recognition is Joseph Henry. Leland Anderson

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