Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 411

electrical discharges. A major advance was Branly's observation in 1890 that a spark changed the conductivity of a metal powder at a distance. He carried out many experiments with various metal powders, determining their change in resistance by connecting them in series with a galvanometer and battery. In 1894 Lodge^* showed that the conductivity of a metal powder could be altered by an electromagnetic wave; this was the final step which preceded the widespread introduction of coherers for the detection of radio waves. From the period 1895 - 1896 the coherers used by Popov and Marconi are well known.^{(43, 47)}

Once activated, a coherer remains in the conducting state. To re-establish the high-resistance state it has to be shaken. The strength and timing of the shaking have to be properly adjusted. A novel method of decoherence of powders was invented by Popov, and used by him in his receiver and later, by others⁽⁴³⁾. In 1898 Rupp⁽⁴⁸⁾ found that constant slow rotation of the coherer keeps it sensitive. The decohering effect of rotation had been discovered earlier, in 1884, by Calzecchi-Onesti⁽⁴⁹⁾.

Tesla mentions that he had worked with a rotating coherer in the New York laboratory, so it is possible that he used decoherence by rotation before Rupp. He finds it superior to other methods of decoherence, because then the sensitive device behaves like a selenium sell, conducting only when radiation acts upon it. Also its sensitivity can be controlled by changing the rate of rotation.

July 24

From the pagination of the manuscript it may be seen that the entry for this day was divided into three parts (the previous day two parts). The first part, three pages, refers to experiments with a 35-turn secondary on the oscillator, the second part, five pages, to a resumption of these experiments, and the third, three pages, to the determination of the capacity of the 35-turn secondary.

Tesla adjusted the regulating coil in the primary to obtain the maximum secondary voltage, judged by the size of streamers. He connected an “extra coil” to the free terminal of the secondary. He investigated the operation of the transformer at harmonic frequencies by doubling the primary capacity^** and making fine adjustments of the primary frequency by varying the inductance in order to get maximum response of the secondary to the harmonic of the primary.

On resuming the experiments Tesla sought an explanation for the occurrence of the largest streamers from the secondary when the regulating inductance was practically cut out. He found it confusing that the highest voltage at the free terminal of the extra coil (connected to the secondary like in Fig. 2 of July 11th) was not obtained when the frequency of the excitation was equal to the natural resonant frequency of the coil. After an extensive analysis he came to the correct conclusion (unlike that of June 30th, which was valid only for a special case), that when free oscillation of the secondary becomes influential, the parameters of the primary have to be adjusted to get maximum voltage across