Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 15

the Nikola Tesla Museum in Belgrade. According to Tesla's caption these diagrams are “Illustrating various ways of using highfrequency 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⁽³⁵⁾. Only some of these diagrams have been published in^{(4, 6, 13)}, 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^{(13, 14)} on his apparatus and system for wireless transmission of power (these patents refer to Tesla's disruptive dis charge 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⁽⁶⁾, 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⁽¹²⁾.

Between 1893 and 1898 Tesla applied for and was granted seven American patents on his HF oscillator as a whole⁽²⁵⁾, one on his HF transformer⁽²⁶⁾, and eight on various types of electric circuit controller⁽²⁷⁾. In a later article⁽²⁸⁾ 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⁽¹⁷⁾. In a lecture before the American Electro-Therapeutic Association in Buffalo September 1898⁽¹⁸⁾ 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”⁽⁵⁹⁾, 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 of 20th March 1900⁽¹³⁾ and patent No. 649621 of 15th May 1900⁽¹⁴⁾. Unlike other radio experimenters of the time who worked either with damped oscillations at very high frequencies⁽⁴³⁾, 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

* 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 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 Stone⁽⁶⁵⁾.

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