Nikola Tesla: Colorado Springs Notes, 1899-1900 Page 103

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 the secondary, and resonant frequency of the extra coil has to be equal to the resonant frequency of the coupled primary-secondary system. It seems that it did not occur to Tesla that when the coupling was tight the combined system produced different spectra during and after the spark. It would seem therefore, all the more significant that he was able to reach this correct conclusion, through a combination of empirical results, simple theory and intuition.

Tesla notes that during discharge in the secondary sparks went across the lightning arresters. Since the arresters were connected to the power line, Tesla thought that the HF voltage came from a wave propagating through the earth and getting into the line somewhere else. We have no evidence which would support this statement or establish whether it was not due to coupling between the oscillator and the mains via the power transformer.

The third part of this entry refers to measurement of the capacity to ground of the secondary coil as a whole. Tesla does not explain how he performed the comparison with a standard capacitor nor at what frequency.

** For the primary to oscillate at half the frequency the capacity would have to be quadrupled. It is possible that instead of connecting the banks of 8 - 9 jars in series, equivalent to the capacitance of 4 - 4 1/2 jars, Tesla connected the previously series connected jars in parallel, achieving an equivalent of 16 - 18 jars, i.e. four times the capacitance of the series configuration.

July 24

During that day the notes are divided in three parts (previous day in two parts), which is shown by designation of original handwritten pages. The first part pertains to experiments with oscillator secondary with 35 turns, and it is described on three pages; second part on five pages, pertains to this experiments continuation, and third part on three pages is devoted to the question of oscillator secondary capacitance. The oscillator secondary consists of 35 turns.

When adjusting the oscillator; Tesla most frequently changes the regulating coil in the primary, and so obtains the maximum voltage in secondary (which he determines according to the longest spark). In the secondary he connects "additional coil" to the open secondary terminal. He checks the operation with harmonics so that he doubles the primary capacitance*, and by small regulations of primary inductance he changes the primary operating frequency, in order to achieve maximum effect on the secondary which is supposed to oscillate at double frequency - primary harmonic. He doesn't obtain the expected results and continues the experiment with capacitive loading changes in secondary circuit, and looks for most suitable elements in the primary circuit. He considers that he obtained the best result when sparks in the secondary are most powerful.

In continuation of experiments with the transmitter, he looks for an explanation for the appearance of largest current streamers in the secondary when the inductance of regulating coil in the primary circuit is reduced to a minimum (and then it is practically disconnected from the circuit). Tesla was confused with the fact that maximum voltage at the open end of the additional coil (this coil is connected to the secondary similar as on Fig. 2 on July 11) doesn't occur when the excitation current frequency is equal to the coil's self resonant frequency. After an extensive analysis, Tesla came to the correct conclusion (opposite to the one of June 30 which was correct only in a special case) that in cases where the secondary free vibration is emphasized, parameters, of the primary circuit have to be weighed such as to achieve the maximum voltage in the secondary, and the additional coil self-resonant frequency has to be equal to the frequency of the combined system primary-secondary. It appears Tesla did not assume that when the link is good, the combined system produces a distinguished spectrum during and after spark interruption. It seems to us that much more important is the abovementioned conclusion which he established when combining the experimental results, simple theory and intuition.

When experimenting with his oscillator, Tesla observed that during a discharge in the secondary the sparking appears on lighting arresters as well. As these arresters are connected to the supply feeder, he assumed that the high frequency voltage originates from a wave which propagated through the ground, and at a certain location entered the feeder. The data is missing which would help to establish whether this high frequency voltage is actually a consequence of the oscillator link via supply transformer with the supply feeder.

In third part of notes on July 24 he gives measurement results of secondary coil capacitance as a whole with respect to the ground. He didn't mentioned how he performed the measurement by comparison with standard capacitor and then, by using several methods he calculates the wire capacitance of the same length as coil wire (C₁₎ according to the equation for capacitance of a single ellipsoide of which the longer axis is considerably longer than short one, and C₂ according to the equation for conductor capacitance above an ideally conductive plane under the condition when the distance from the plane is considerably larger than wire radius). Expanding on considerations for capacitances, he describes the methods for achieving the largest capacitances, and moves further and further away from his initial topic. He ends the notes with a discussion on the spheres filled with hydrogen which were supposed to have maximum capacitance for given dimensions, and which would therefore be used for telegraphy without wires.

* In order to get the primary to oscillate on two times lower frequency, the capacitance has to be four-fold. It is possible instead of series connection 8-9 jars which is equivalent to 4-4 1/2 jars he connected two previous series connections in parallel, and obtained the equivalent capacitance of 16-18 jars, that is four times larger than the equivalent capacitance of jars connected in series.