Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 398

June 6

Tesla spent some time in intensive research on X-rays, publishing his results in ten articles in the period 11th March 1896 to 11th August 1897⁽⁷⁾. On the 6th of April 1897 he also gave a lecture on his X-ray studies⁽¹⁷⁾ and presented designs of a number of devices for generating powerful rays. During this lecture he reported interesting data from his earlier experiments with Crooke's tubes in 1894. He had then observed that some tubes which produced only feeble visible light had more effect on photographic plates than tubes which were brighter to the eye. The goal of his research was to obtain true phosphorescence (“cold light”), so that he postponed further investigation of this phenomenon, and of the cause of various spots and hazing on photographic plates which had been kept in the laboratory for a time before use. When he finally did get around to it a fire broke out in the laboratory, destroying practically everything (13th March 1895). It was several months before he could resume his work, and in the meantime Rӧntgen made his discovery. When Tesla heard about it, it was immediately plain what had been happening in his laboratory. He repeated Rӧntgen's experiments, which were rather cryptically described, and realized that he had been mistaken in not following up certain chance observations during his work with Crooke's tubes.

During 1896 and 1897 Tesla carried out many experiments with X-rays, also speculating about their nature. He thought “that the effects on the sensitive plate are due to projected particles, or else to vibration far beyond any frequency which we are able to obtain by means of condenser discharges” (Lit.⁽¹⁾, p. A-30). He immediately realized the importance of high voltages for producing powerful rays and suggested using his single-terminal tubes connected to the secondary of the disruptive discharge coil. It is interesting to note that Rӧntgen too, in a lecture to the Physical Medical Society of Würzburg the same month as Tesla published his first article, also pointed out the great advantage of using Tesla's high-frequency oscillator in generating X-rays.⁽⁶⁶⁾

Tesla measured the reflection and transmission of X-rays for several metals, lead glass, mica and ebonite. It is not clear, however, whether what he measured was true reflected radiation or secondary radiation. He also tried to detect refraction but did not succeed, for reasons which are today obvious. In papers and in a lecture before the New York Academy of Science he described a number of tubes for producing powerful X-rays, most of them resembling Lenard tubes (which he often mentions) but without the anode terminal.

June 7

Descriptions of the high-frequency transformer are to be found in Tesla's publications and patents from 1891 onwards^{(15, 4)}, but he did not patent it until 1897⁽²⁶⁾. The invention protected by this patent is “A transformer for developing or converting currents of high potential, comprising a primary and secondary coil, one terminal of the secondary being electrically connected with the primary, and with earth when the transformer is in use, as set forth”. It in particular protects the spiral form of the secondary, and a conical form is also mentioned. For ordinary uses a cylindrical secondary divided into two parts is proposed. A new feature is the specification that the length of the secondary should be “approximately one quarter of the wavelength of the electrical disturbance in the secondary circuit, based on the velocity of propagation of the electrical disturbance through the coil itself”, or, in general, “so that at one terminal the potential would be zero and at the other maximum”.