Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
July 24, 1899
Experiments with the secondary 35 turns were continued today. In connection with the secondary an âextra coilâ was used. The same was before described having 260 turns on a drum 2 feet in diam. of cord No. 10. The inductance of the coil as before found was approximately 13,900,000 cm. or $! {139 \over 10^{4}} $! henry. The coil was connected with the lower end to the free terminal of the secondary while the upper end was left free, a few feet of wire extending into the air. Resonance, as evidenced by streamers (maximum) on free end of coil, was obtained exactly as in a previously recorded experiment with 9 jars in each set of the primary condensers, or slightly less, at any rate 8 - 9 jars caused the largest display of the streamers on the free end. The streamers were only three feet long as the energy from the supply circuit was limited, the intention being to first study the peculiarities and behaviour of the transformers before taking them to their full output. A simple computation showed that the resonant rise on the free end was chiefly, if not wholly, due to the rise in the coil itself, the free vibration of the secondary being comparatively of small moment. To study the harmonics the capacity in the primary circuit was doubled but the effect, as expected, was very small. Now the capacity in primary was again doubled, it being expected that the streamers would be of considerable power under these conditions. This was the first undertone and it should have been fairly strong. But singularly nothing to speak of was noted although the adjustments were carefully gone through again and again. It was thought that owing to a very small arc in the primary the oscillation did not readily establish itself but this was not highly probable though such has taken place in experiments which I made before. The arc was necessarily small as the capacity was very large in the primary. The tuning was very sharp with twice the capacity in primary so that a little variation in the self-induction regulating coil made the streamers change very considerably. I expect that it was sharper still with four times the primary capacity so that, after all, the resonant condition may have been missed. This might have been the case easily as all the variation from no streamers to their maximum would have taken place by going through only one quarter of one turn, and possibly less, of the regulating coil. This sharpness of tuning noted here and in previous instances in some arrangements again impresses me with the value of such dispositions in telegraphy, when it is of great importance to isolate messages. It seems possible to secure in such or similar ways an almost absolute privacy. The experiments with only two circuits show this sufficiently. Continuing the experiments one of the balls of 30" diameter was connected to the free end of the coil and now the donating condition was secured with 23 jars on each side of the primary. Summing up the results the vibration with coil alone, without ball, with 9 jars on each side of primary was, approximately, taking the primary capacity equal to $! {{{9 \over 2} \times 0.003} = {0.027 \over 2} = 0.0135} $! mfd.
$! {{T_{1}} = {{2 \over 10^{3}} \sqrt{0.0135 \times {7 \over 10^{5}}}} = {{2 \pi \over 10^{3}} \sqrt{945 \over 10^{9}}} =} $!
$! {= {{2 \pi \over 10^{7}} \sqrt{94.5}} = {{2 \pi \over 10^{7}}9.72} = {61.0416 \over 10^{7}}} $! or approx. = $! {61 \over 10^{7}} $!
and n = 164,000 per second.
101
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.
