Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 417

August 28

Tesla's idea of the Earth as a perfectly conducting sphere lead him to a mistaken hypothesis about the general behaviour of the electromagnetic field around the grounding of the transmitter. What he expected at frequencies of the order of 10 kHz in fact occurs at much lower frequencies⁽⁷²⁾, at which, as far as can be seen from his notes, he did not work in Colorado Springs. He correctly observed that the decisive factor determining whether predominantly waves of the “Hertzian type” or the waves which he thought to be propagated through the earth (in fact waves in the spherical condenser constituted by the Earth and the ionosphere) would be excited was the excitation of the “Earth”. Tesla was also certainly in error when he tried to make generalizations concerning the wave frequency, and in his conviction that he needed extremely high voltages to “create” the second conductor for a system of wireless power transmission. He could not know that this conductor already existed permitting transmission at very low loss of very low frequency waves, and that it would not matter whether the energy transmitted was high or low.

August 29

Although the circuit looks simple enough, an analysis of Tesla’s receiver with a “magnifying effect” is rather complicated, because transient phenomena have to be taken into account and the resistance law of the sensitive devices as a function of voltage has to be known. It was not easy to adjust a receiver like this to work properly.

Apparently there was an earphone T in the secondary circuit of the transformer, but it is not mentioned in the notes. The sensitivity of an earphone would normally be much greater than that of a relay, so it would be interesting to find out how this apparatus performed. Unfortunately, earphones are practically not mentioned anywhere in the diary.

Tesla here at last makes a few remarks about how the sensitivity of receivers was estimated. To test its response he put a “small capacity” across sensitive device a, but of what value, and whether it was charged or not he does not say.

September 3-4

The aim of these experiments is not explained, but it was probably associated with the “experimental” coil with which he examined currents in the water pipe. This was a resonant coil which in the receiver played a part analogous to that of the “extra” coil in the transmitter. Its purpose was to maximize the received signal. Since Tesla connected one terminal to ground, it appears that he wanted to pick up electrical vibrations from the earth. In this case too he found that it was not sufficient just to increase the Q-factor $! {pL \over R} $!, but also that it was necessary to keep the coil's distributed capacity as low as possible. This conclusion was consistent with what he had earlier found about the influence of distributed capacity of the coil on the length of wire needed to achieve resonance. Conclusion (5) is interesting in that it shows Tesla was aware that the secondary and the extra coil, although excited by the same primary, would each oscillate at its own resonant frequency, and if these were not the same, they would beat.