Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 416

August 22

In this entry he returns once again to the receivers. He tried out two receiver circuits using one battery and one sensitive device. He changed the capacity in the primary circuit over a wide range, but it is not clear why 1 µF proved best. It remains unexplained what was the relationship between the frequency of the incoming signal and that generated by the receiver itself. Could it perhaps be, if the rotating coherer behaved as a nonlinear element, that the signal was amplified as in a heterodyne receiver⁽⁵⁵⁾?

August 23

He now put the extra coil in the center of the primary, retaining this configuration from then on. After the usual adjustment of the oscillator he got sparks 2 m, and later 4 m long, indicating a voltage of around 2 million volts.

August 26

Tesla experimented with twice the interruption rate. The oscillator worked better and there was heavy sparking across the lightning arresters (Fig. 4). Investigating the cause of this sparking he inserted a coil in the lead of the metal sphere (Fig. 1) to reject high frequencies. In an earlier experiment (see July 30th) inserting such a choke coil in the ground line had stopped sparking across the arresters. This time it did not, so Tesla tried the circuit in Fig. 2. Still there was no marked change, the sparking across the arresters was only slightly reduced. After this experiment he began to wonder whether the grounding point of the secondary was not perhaps a peak rather than a node of the standing wave. It must be understood that Tesla thought that standing waves were set up around the transmitter (like waves on an open transmission line. With shorter waves the rate of change of amplitude with distance would be faster (i.e. maxima and minima would occur at shorter distance intervals), so he thought that a large potential difference could be obtained with a short distance between the grounding of the secondary and that of the lightning arrester.

In order to explain what happened when the sphere was not grounded (which would mean that there were no short waves) but the sparking across the arrester did not stop, Tesla found it necessary to formulate a new hypothesis: “ Could the sparks be produced by static induction upon wire through the air and not chiefly by conduction through earth?” The experiment with which he tried to verify this hypothesis did not yield any definite answer.

August 27

Although he has noted several times already that good results were obtained with various decoherence techniques (rotation, interruption of the excitation current), this re-examination of his old ideas shows that Tesla is still seeking a more reliable solution. One of the ideas he was gathering together for further investigation is illustrated by the diagram in Fig. 4, in which a rotary interrupter, condenser, choke and battery provide bias for the sensitive device. When interrupter d breaks, the voltage on C can be higher than the battery voltage. With proper choice of the values the coherer can be biased to threshold, making it very sensitive.