Colorado Spring Notes 1899-1900



The Nikola Tesla Museum is publishing the third book from the legacy of Nikola Tesla — his research diary written in Colorado Springs. The previous two books contain works that had already been published and had thus been accessible to the public. The most significant of Tesla's works had been selected and published in their integrity in the first book (Nikola Tesla: Lectures, Patents, Articles, Beograd, 1956) while the most important reviews of Tesla's works and the appreciation of their significance for world science are contained in the second book (Tribute to Tesla, Beograd, 1961).


Tesla's manuscript (written in English), that has up to now remained unpublished and unknown to the public, is appearing in this book for the first time. Nikola Tesla did not write his diary for the public, but exclusively for his personal use. Obviously, he was writing it to have an insight into the course of his research and due to the exceptional extent of the experiments of his research in the isolated laboratory he had erected in 1899 on the slopes of the mountain Pikes Peak, he was probably writing it with the wish to leave behind some evidence in case of fire or destruction of his laboratory.


Tesla evidently did not intend to publish this diary and left it among his other notes and writings. It was not until the whole legacy of Nikola Tesla had been systematically examined and put in order in the Nikola Tesla Museum that the manuscript of this diary was discovered. Like all testimonies of this kind, the diary of Nikola Tesla has the value and fascination of a most genuine testimony because it reveals Tesla's ideas in an important period of his research. It reveals the extraordinary enthusiasm and fervor of his inexhaustible and strikingly exploring imagination.


In fact, this diary brings to light all that made Tesla different from all other researchers: his creative spirit which often bewildered, amazed and infuriated many of his contemporaries and even some well-informed scientists, to whom it seemed that Tesla's ideas belonged to the sphere of illusion rather than to the acknowledged course of science. Tesla thus shared the fate of all exceptionally great and far-sighted explorers.


In fact, when one carefully studies the entire work of Tesla one can see that his principal aim was very clear: to search for the inexhaustible possibilities of dominating the forces of nature and subordinating them to human purposes thus increasing immensely the power of man and mankind in order to live more humanly. All that Tesla had done was subordinated to this principal aim. All his experiments in Colorado Springs, dealt with in this--diary, had also been dedicated to this basic aim.


Because of the extraordinary dimensions of his experiments, which would be unusual even for present-day experimental work in this field, this diary is not only a valuable historical testimony but also an inexhaustible inspiration for further research even when some mistakes are spotted. Tesla was so ingenious and devoted to his indefatigable search for new knowledge that he could permit his little errors to feed all kinds of small-minded people who learned how to calculate well but could never learn to seek for new ways of knowledge because they didn't have a creative gift.


Preliminary arrangements to prepare this diary for publication required a lot of time, effort and collaborators. The Nikola Tesla Museum thanks them all and especially the author of the commentaries, Prof. Aleksandar Marincik.


By publishing this diary the Nikola Tesla Museum wishes to mark the 120th anniversary of Tesla's birth, which has been celebrated all over the world, as well as to underline the deep devotion which Tesla felt towards this country where he was born.


Beograd, December 1977





In 1898 Tesla's creativity in the field of high frequencies was at its peak. From his initial ideas in 1890 and his first, pioneering steps, he had worked with such intensity that many of the inventions and discoveries which he had given the world by this time have remained unsurpassed to this day. Even the loss of his laboratory on Fifth Avenue in 1895, a severe blow for him, did not hold him back for long. He soon resumed his experiments in a new laboratory, on Houston Street, continuing to make new discoveries and inventions applying them with unflagging energy.


Tesla's polyphase system essentially solved the problem of generating, transmitting and utilization of electrical power. When he started working on high frequencies, he almost immediately began to perceive their vast possibilities for wireless transmission of "intelligible signals and perhaps power".


He worked on the practical development of his first ideas of 1891—1893 at such a rate that by 1897 he had already patented a system for wireless transmission of power and an apparatus utilizing this system. Shortly before this, during the ceremonial opening of the hydroelectric power plant on Niagara, at a time when the world was only just coming around to Tesla's polyphase system which for the first time in history enabled the transfer of electrical power over distance, he said:


"In fact, progress in this field has given me fresh hope that I shall see the fulfillment of one of my fondest dreams; namely, the transmission of power from station to station without the employment of any connecting wires."(16)


Always true to the principle that ideas must be experimentally verified, Tesla set about building powerful high-frequency generators and making experiments in wireless power transmission. The Nikola Tesla Museum in Belgrade possesses a Tesla's own slide which confirms that the experiment described in the patent "System of Transmission of Electrical Energy" was in fact carried out before the Examiner-in-Chief of the U.S. Patent Office.


For experimental verification of his method of wireless power transmission "by conduction through the intervening natural medium", on the global scale Tesla needed still higher voltages and more room (in the Houston Street laboratory he generated voltages of 2 to 4 MV using a high-frequency transformer with a coil diameter of 244 cm), he finally decided on Colorado Springs, a plateau about 2000 m above sea level, where he erected a shed large enough to house a high-frequency transformer with a coil diameter of 15 meters!




Tesla's arrival in Colorado Springs was reported in the press. According to the Philadelphia "Engineering Mechanics" Tesla arrived on the 18th of May 1899 (according to 68 he left to New York on 11th May 1899), with the intention of carrying out intensive research in wireless telegraphy and properties of the upper atmosphere. In his article "The transmission of electric energy without wires" (1904 1)


 Tesla writes that he came to Colorado Springs with the following goals:


  • To develop a transmitter of great power.


  • To perfect means for individualizing and isolating the energy transmitted.


  • To ascertain the laws of propagation of currents through the earth and the atmosphere.


Tesla had some ten years of experience with high frequency AC behind him by the time he moved to Colorado .Springs. In 1889, on his return from Pittsburg where he had been working as a consultant to Westinghouse on the development of his polyphase system, he began work on the construction of an alternator for generating currents at much higher frequencies than those used in ordinary power distribution. In 1890 he filed applications for two patents2 for alternators working at over 10 kHz.


One of these patents was in conjunction with a method for achieving quiet operation of arc lamps, but this was in fact a first step towards a new application of alternating currents, which soon became known as "Tesla currents". Tesla's alternators were an important milestone in electrical engineering and were the prototypes for alternators which were used some quarter-century later for driving high-power radio transmitters, and later on also for inductive heating. 24


Soon after he had started his research in high frequencies Tesla discovered there specific physiological action and suggested the possibility of medical applications. He did a lot of work on the utilization of high frequency AC for electric lighting by means of rarefied gas tubes of various shapes and types. During 1891 he publicized his results journals, patent applications and in his famous lecture to the AIEE at Columbia College4. This lecture, before a gathering of eminent electrical engineers, brought Tesla widespread recognition and soon made him world-famous.


This success was due in good measure to his convincing experiments too, which included a demonstration of rarefied gas luminescing in a tube not connected by wires to the source of power. This was the first experiment demonstrating wireless power transmission, and marked the birth of an idea to which Tesla was subsequently to devote a great part of his life. The necessary powerful electric field was created between the plates of a condenser connected across the secondary of a high-frequency transformer, whose was connected via a series condenser to a high-frequency alternator.


The system worked best when the primary and secondary circuits were in resonance. Tesla also made use of the resonant transformer with his spark oscillator, enabling easy and efficient generation of high-frequency AC from a DC or low frequency source. This oscillator was to play a key role in the development of HF engineering. Only a few years later it was to be found among the apparatus of practically every physics laboratory, under the name of the Tesla coil20.


The first record of Tesla's high-frequency coupled oscillatory circuit with an air-cored transformer is to be found in Patent No. 454622 of 23 June 1891 (application filed 25 April 1891) under the title " System of Electric Lighting". The oscillator converts low-frequency currents into "current of very high frequency and very high potential", which then supplies single-terminal lamps (see Fig. 2c).




Induction coil PS produces a high secondary voltage which charges condenser C until a spark occurs across air gap a. The discharge current flows through the air gap and the primary of the high-frequency induction coil P'. The discharge of the condenser in this case differs from the discharge through coil with ohmic resistance studied by Henry, already known by that time. In Tesla's oscillator the energy of the high-frequency oscillations in the primary circuit is gradually transferred circuit.


After energizing of the secondary circuit, the remaining energy is returned to the primary, then back to the secondary, and so on until losses reduce it sufficiently to interrupt spark across a in the primary circuit. Then condenser C begins to recharge from source G via induction coil (transformer) PS. Oberbeck published a theoretical analysis of Tesla's oscillator in 1895.


Tesla presented much new information about his discharge oscillators and his further research on high frequency currents in the lecture he gave to the IEE in London, February 1892 which he subsequently repeated in London and then in Paris*5'. He described at length the construction of a type of air-cored HF transformer and drew attention to the fact that the secondary voltage cannot even approximately be estimated from the primary/ /secondary turns ratio.


Tesla also did a lot of work on improvements of the spark gap and described several designs, some of which were subsequently attributed to other authors(24). In describing the apparatus with which he illustrated this lecture he explained several ways for interrupting arcs with the aid of a powerful magnetic field; using compressed air; multiple air gaps in series; single or multiple air gaps with rotating surfaces.


He describes how the capacity in the primary and secondary circuits of the HF transformer should be adjusted to get the maximum performance, stating that so far insufficient attention had been paid to this factor. He experimentally established that the secondary voltage could be increased by adding capacity to "compensate" the inductance of the secondary (resonant transformer).


He demonstrated several single-pole lamps which were connected to the secondary, describing the famous brush-discharge tube and expressing the opinion that it might find application in telegraphy. He noted that HF current readily passes through slightly rarefied gas and suggested that this might be used for driving motors and lamps at considerable distance from the source, the high-frequency resonant transformer being an important component of such a system.


The drawing shown in Fig. 3c dates from early on during Tesla's work with high frequencies, 1891—1892. It is taken from Tesla's original slide found in the archives of the Nikola Tesla Museum in Belgrade.




According to Tesla's caption these diagrams are "Illustrating various ways of using high-frequency alternator in the first experiment at Grand Street Laboratory 1891—1892". It seems that Tesla made these to prove his priority in a patent suit. Only some of these diagrams have been published in, so that this is an important document throwing new light on an exceptionally fertile but relatively little known period of Tesla's work. It is, for example, clear from these diagrams that he introduced an HF transformer in the open antenna circuit.


Circuits like that in Fig. 3c—4 are to be found later in two patents filed in 1897 on his apparatus and system for wireless transmission of power (these patents refer to Tesla's disruptive discharge oscillator as an alternative to the high-frequency alternator).


In February 1893 Tesla held a third lecture on high-frequency currents before the Franklin Institute in Philadelphia, and repeated it in March before the National Electric Light Association in St. Louis. The most significant part of this lecture is that which refers to a system for "transmitting intelligence or perhaps power, to any distance through the earth or environing medium".


What Tesla described here is often taken to be the foundation of radio engineering, since it embodies principles ideas of fundamental importance, viz.: the principle of adjusting for resonance to get maximum sensitivity and selective reception, inductive link between the driver and the tank circuit, an antenna circuit in which the antenna appears as a capacitive load(71). He also correctly noted the importance of the choice of the HF frequency and the advantages of a continuous carrier for transmitting signals over great distances.


Between 1893 and 1898 Tesla applied for and was granted seven American patents on his HF oscillator as a whole, one on his HF transformer, and eight on various types of electric circuit controller'27'. In a later article Tesla reviews his work on HF oscillators and reports that over a period of eight years from 1891 on he made no less than fifty types of oscillator powered either by DC or low-frequency AC.


Along with his work on the improvement of his HF oscillators Tesla was continuously exploring applications of the currents they produced. His work on the improvement of X-ray generating apparatus is well known — he reported it in a series of articles in 1896 and 1897'7' and in a lecture to the New York Academy of Sciences. In a lecture before the American Electro-Therapeutic Association in Buffalo September I898"8' he described applications of the HF oscillator for therapeutic and other purposes. The same year he took out his famous patent "Method of and Apparatus for Controlling Mechanism of Moving Vessels or Vehicles", which embodies the basic principles of telemechanics a field which only began to develop several decades after Tesla's invention.


On 2nd September 1897 Tesla filed patent application No. 650343, subsequently granted* as patent No. 645576 (System of Transmission of Electrical Energy) of 20th March 1900 and patent No. 649621 (Apparatus for Transmission of Electrical Energy) of 15th May 1900. Unlike other radio experimenters of the time who worked either with damped oscillations at very high frequencies, Tesla investigated undamped oscillations in the low HF range.


While others principally developed Hertz's apparatus with a spark-gap in the tank circuit (Lodge, Righi, Marconi, and others) and improved the receiver by introducing a sensitive coherer (Branly, Lodge, Popov, Marconi, and others), he set about implementing his ideas of 1892—1893. How far he had got in verifying his ideas for wireless power transmission before coming to Colorado Springs may be seen from patent No. 645576 (System of Transmission of Electrical Energy) and the diagram in Fig. 1c.


* The second of the two patents by which Tesla protected his apparatus for wireless power transmission, known as the "system of four tuned circuits", is particularly important in the history of radio. It was a subject of a long law suit between the Marconi Wireless Telegraph Company of America and the United States of America alleged to had used wireless devices that infringed on Marconi's patent No. 763772 (missing link!) of 28th June 1904. After 27 years the U.S. Supreme Court in 1943 invalidated the fundamental radio patent of Marconi as containing nothing which was not already contained in patents granted to Lodge, Tesla and Stoned.


Tesla based his hopes for wireless power transmission on the global scale on the principle that a gas at low pressure is an excellent conductor for high frequency currents. Since the limiting pressure at which the gas becomes a good conductor is higher the higher the voltage, he maintained that it would not be necessary to elevate a metal conductor to an altitude of some 15 miles above sea level, but that layers of the atmosphere which could be good conductors could be reached by a conductor (in fact an aerial) at much lower altitudes.


"Expressed briefly, (cit. patent 645576 - System of Transmission of Electrical Energy) my present invention, based upon these discoveries, consists then in producing at one point an electrical pressure of such character and magnitude as to cause thereby a current to traverse elevated strata of the air between the point of generation and a distant point at which the energy is to be received and utilized".


Figure 1c proves that Tesla did actually carry out an experimental demonstration of power transmission through rarefied gas before an official of the Patent Office. From the patent it may be seen that the pressure in the tube was between 120 and 150 mm Hg.


At this pressure, and with the circuits tuned to resonance, efficient power transfer was achieved with a voltage of 2—4 million volts on the transmitter aerial. In the application Tesla also claims patent rights to another, similar method of transmission, also using the Earth as one conductor, and rendered conductive high layers of the atmosphere as the other*.


Tesla spent about eight months in Colorado Springs. Something of his work and results from this period can be gleaned from articles in "American Inventor" and "Western Electrican". For instance, it is stated that Tesla intended to carry out wireless transmission of signals to Paris in 1900. An article of November 1899 reports that he was making rapid progress with his system for wireless transmission of signals and that there was no way of interfering with messages sent by it. Tesla returned to New York on the 11th of January 1900.


The diary which Tesla kept at that time gives a detailed day-by-day description of his research in the period from 1st June 1899 to 7th January 1900. Unlike many other records in the archives of the Nikola Tesla Museum in Belgrade, the Colorado Springs diary is continuous and orderly.


Since it was not intended for publication, Tesla probably kept it as a way of recording his research results. It could perhaps also have been a safety measure in case the laboratory should get destroyed, an eventuality by no means unlikely considering the dangerous experiments he was performing with powerful discharges. Some days he made no entries, but usually explained why at the beginning of the month.


* In the late eighties of the last century very little was known about the radiation and propagation of electromagnetic waves. Following the publication of Hertz's research in 1888, which provided confirmation of Maxwell's dynamic theory of the electromagnetic field published in 1865, scientists became more and more convinced that electromagnetic waves behaved like light waves, propagating in straight lines.


This led to pessimistic conclusions about the possible range of radio stations, which were soon refuted by experiments using the aerial-earth system designed by Tesla in 1893. Tesla did not go along with the general opinion that without wires "electrical vibrations" could only propagate in straight lines, being convinced that the globe was a good conductor through which electric power could be transmitted. He also suggested that the "upper strata of the air are conducting" (1893), and "that air strata at very moderate altitudes, which are easily accessible, offer, to all experimental evidence, a perfect conducting path" (1900).


It is interesting to note that this mode of propagation of radio waves was initially considered as something different from other modes then to be forgotten until recent years. In the 1950's Schumann, Bremmer, Budden, Wait, Galejs and other authors, working on the propagation of very low (3 to 30 kHz) and extremely low (1 to 3000 Hz) electromagnetic waves, founded their treatment on essentially the same principles as Tesla.


According to his notes, Tesla devoted the greatest proportion of his time (about 56%) to the transmitter, i.e. the high-power HF generator, about 21% to developing--receivers for small signals, about 16% to measuring the capacity of the vertical antenna, and about 6% to miscellaneous other research. He developed a large HF oscillator with three oscillatory circuits with which he generated voltages of the order of 10 million volts.


He tried out various modifications of the receiver with one or two coherers and special pre-excitation circuits. He made measurements of the electromagnetic radiations generated by natural electrical discharges, developed radio measurement methods, and worked on the design of modulators, shunt-fed antennas, etc.


The last few days covered by the diary Tesla devoted to photographing the laboratory inside and out. He describes 63 photographs in all, most of them showing the large oscillator in action with masses of streamers emerging from the outer windings of the secondary and the "extra coil". He probably derived special satisfaction from observing his artificial lightning, now a hundred times longer than the small sparks produced by his first oscillator in the Grand Street Laboratory in New York.


By then many leading scientists had been experimenting with "Tesla" currents but Tesla himself was still in the vanguard with new and unexpected results. When he finally finished his work in Colorado Springs he published some photographs of the oscillator in a blaze of streamers causing as much astonishment as had those from his famous lectures in the USA, England and France in 1891—1893. The famous German scientist Slaby wrote that the apparatuses of other radio experimenters were mere toys in comparison with Tesla's in Colorado Springs.


The descriptions of the photographs in. the diary also include detailed explanations of the circuitry and the operating conditions of the oscillator. The photographs themselves give an impressive picture of the scale of these experiments. Tesla maintains that bright patches on some of the photographs were a consequence of artificially generated fireballs. He also put forward a theory to explain this, still today somewhat enigmatic phenomenon.


Research on fireballs was not envisaged in his Colorado Springs work plan, but belonged to the special experiments which, in his own words, "were of an interest, purely scientific, at that time"*68*, which he carried out when he could spare the time.


Tesla used some parts of the diary in drawing up the patent applications which he filed between 1899 and 1902. Keeping such notes of his work was more a less a constant practice; they provided him with an aide-memoire when preparing to publish his discoveries.


The diary includes some descriptions of nature, mostly the surroundings of the laboratory and some meteorological phenomena, but only with the intention of bringing out certain facts of relevance to his current or planned research.


Immediately after he finished work at Colorado Springs Tesla wrote a long article entitled "The problem of increasing human energy" in which he often mentions his results from Colorado Springs. In 1902 he described how he worked on this article:


"The Century" began to press me very hard for completing the article which I have promised to them, and the text of this article required all my energies. I knew that the article would pass into history as I brought, for the first time, results before the world which were far beyond anything that was attempted before, either by myself or others".


The article really did create a sensation, and was reprinted and cited many times. The style he uses in describing Colorado Springs research differs greatly from that of the diary.


Tesla wrote about his Colorado Springs work again in 1904. Some interesting data is to be found in his replies before the United States Patent Office in 1902, in connection with a patent rights dispute between Tesla and Fessenden. This document includes statements by Tesla's assistant Fritz Lowenstein and secretary George Scherff. Tesla took Lowenstein on in New York in April 1899. At the end of May that year he summoned him to Colorado Springs, where Lowenstein remained until the end of September, when family matters obliged him to return to Germany. Tesla was satisfied with him as an assistant and asked him to return later, which he did, again becoming Tesla's assistant in February 1902.


Tesla did not break off his research in the field of radio after visiting Colorado Springs. Upon returning to New York on the 11th of January 1900 he took energetic steps to get backing for the implementation of a system of "World Telegraphy". He erected a building and an antenna on Long Island, and started fitting out a new laboratory. From his subsequent notes we learn that he intended to verify his ideas about resonance of the Earth's globe, referred to in a patent of 1900.


The experiments he wanted to perform were not in fact carried out until the sixties of this century, when it was found that the Earth resonates at 8, 14 and 20 Hz. Tesla predicted that the resonances would be at 6, 18 and 30 Hz. His preoccupation with this great idea slowed down the construction of his overseas radio station, and when radio transmission across the Atlantic was finally achieved with a simpler apparatus, he had to admit that his plans included not only the transmission of signals over large distances but also an attempt to transmit power without wires.


Commenting on Tesla's undertaking, one of the world's leading experts in this field, Wait, has written:


"From an historical standpoint, it is significant that the genius Nikola Tesla envisaged a world wide communication system using a huge spark gap transmitter located in Colorado Springs in 1899. A few years later he built a large facility in Long Island that he hoped would transmit signals to the Cornish coast of England. In addition, he proposed to use a modified version of the system to distribute power to all points of the globe.


Unfortunately, his sponsor, J. Pierpont Morgan, terminated his support at about this time. A factor here was Marconi's successful demonstration in 1901 of transatlantic signal transmission using much simpler and far cheaper instrumentation. Nevertheless, many of Tesla's early experiments have an intriguing similarity with later developments in ELF communications.


Tesla proposed that the earth itself could be set into a resonant mode at frequencies of the order of 10 Hz. He suggested that energy was reflected at the antipode of his Colorado Springs transmitter in such a manner that standing wave were set up."


In a letter to Morgan early in 1902 Tesla explained his research, in which he envisaged three "distinct steps to be made:


  1. the transmission of minute amounts of energy and the production of feeble effects, barely perceptible by sensitive devices
  2. the transmission of notable amounts of energy dispensing with the necessity of sensitive devices and enabling the positive operation of any kind of apparatus requiring a small amount of power
  3. the transmission of power in amounts of industrial significance


With the completion of my present undertaking the first step will be made". For the experiments with transmission of large power he envisaged the construction of a plant at Niagara to generate about 100 million volts.


However, Tesla did not succeed in getting the necessary financial backing, and after three years of abortive effort to finish his Long Island station he gave up his plans and turned to other fields of research. He wrote several times about his great idea for wireless transmission of power, and remained convinced to his death that it would one day become reality.


Today, when we have proof of the Earth's resonant modes (Schumann's resonances), and it is known that certain waves can propagate with very little attenuation, so little that standing waves can be set up in the Earth-ionosphere system, we can judge how right Tesla was when he said that the mechanism of electromagnetic wave propagation in "his system" was not the same as in Hertz's system with collimated radiation. Naturally, Tesla could not have known that the phenomena he was talking about would only become pronounced at very low frequencies, because it seems he was never able to carry out the experiments which he had so brilliantly planned, as early as 1893.


It is gratifying that after so many years Tesla's name is rightfully reappearing in papers dealing with the propagation of radio waves and the resonance of the Earth. In a recent book of a well known scientist (Jackson) it is stated that,


"this remarkable genius clearly outlines the idea of the earth as a resonating circuit (he did not know of the ionosphere), estimates the lowest resonant frequency as 6 Hz (close to 6.6 Hz for a perfectly conducting sphere), and describes generation and detection of these waves. I thank V.L. Fitch for this fascinating piece of history".


We believe that further studies of Tesla's writings will reveal some interesting details of his ideas in this field.


The publication of the Colorado Springs diary, a unique record of the work of a genius, means an enrichment of the scientific literature, not only in that throws light on a particularly interesting period of Tesla's creativity, but also as a source for the study of his work as a whole, and particularly of his part in the development of radio. It also facilitates the identification of many documents now at the Nikola Tesla Museum in Belgrade which lack date or description.


The preparation of this manuscript for publication required considerable time and labor in order to present its content in a form not deviating essentially from the original but more accessible to study. No alterations have been made even where the original contains certain minor errors, sometimes also in the use of power and energy units; some more important calculation errors which influence the conclusions drawn are also reproduced but are noted.


A section at the end of the book contains commentaries on the Diary with explanatory notes, and a survey of his earlier work and that of other researchers. For these commentaries reference was made to the large body of literature and documents in the archives of the Nikola Tesla Museum in Belgrade.


Aleksandar Marindic



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