Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 31

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".

June 7

Already in 1891 a description of a high frequency transformer is mentioned in Tesla's works and patents^(4,15), but he patented it as late as 1897⁽¹⁶⁾. In this patent the discovery of ".... transformer for production or conversion of high voltage currents which consists of primary and secondary coil with one secondary terminal electrically connected to primary, and with grounding when the transformer is used as mentioned...". The secondary in the shape of a spiral is particularly protected, and a cone shape is mentioned as well. For the usual applications he suggest the cylindrical secondary divided into two parts. New information is that the secondary has to have the length of approximately ¼ wavelength determined on the basis of phenomenon propagation speed through the secondary or generally such length that the voltage at the open secondary terminal is maximum.

The primary coil of Tesla'a high frequency transformer almost always has one to two turns. Even with such a small number of turns the primary coil inductances are sufficiently large regarding the dimensions of these coils. For example, in his patent - "Electrical Energy Transmission System"⁽¹³⁾ the mentioned diameter of primary coil amounts to approximately 244 cm, and its inductance is approximately 8000 cms. He decided to use an even larger coil, which can be seen on the basis of data (radius 762 cm!).

In his note, Tesla does not talk about work on the laboratory construction and what he notes are the ideas and the data necessary for the experimentation. So, for example, now he describes the measurement of inductance by means of a balanced bridge, but he doesn't give the equation of the balance, and consequently some claims (and schematic) could not be proven with assurance.