Nikola Tesla Books
Books written by or about Nikola Tesla
stand and spark length tried in this way. With a ball of 4" diam. the spark from the oscillator end was 1/2" while from the free end of the exp. coil it was 4". A still smaller capacity was now used in the belief that perhaps the 4" ball was too large but the experiments showed a contrary result. It was thought that the wave length being estimated from that of the oscillator and extra coil must be longer than that of the wire on experimental coil. This led to consideration of certain advantages of long waves allowing a great length of wire to be wound up on the experimental coil, this in certain instances overbalancing the advantages of the larger magnifying factor which the short waves offer.
The connection was now made to the second and then again to the first turn of oscillator secondary and as even in this case the effects were inconveniently strong, connection was made to the water pipe to diminish impressed e.m.f. But even now the streamers would go over the spark gap. Several balls were now experimented with. Results were as follows:
| Turns exp. Coil | Spark from oscill. end | Spark from free end | Spark between terminals | Ball diam. |
|---|---|---|---|---|
| 516 | 3/8" | 1 1/2" | 3 3/8" | no ball |
| 516 | 1/8" | 3/8" | 5/8" | 18" |
| 516 | 3/16" | 9/16" | 1 1/2" | 8" |
| 516 | 5/16" | 15/16" | 1 7/8" | 5" |
It was now important to get an idea of the magnifying ratio and the spark was tried on the water pipe and on the free end of coil and the lengths compared. On the water pipe it was 1/64" and on the free end 1 1/2". This was fair but the coil was not yet quite well tuned. Completing the adjustment with more care the spark on the pipe was found 1/100" and on the free end of the coil 2". This was quite satisfactory but not the best by far.
Further efforts to tune still more closely resulted in producing a spark between rods 2 1/4" and with the capacity of the experimenter on free rod, the same being disconnected from everything, 3 3/4". The capacity in the primary oscillating circuit was now 5 tanks on each side and the self-ind. box 7 turns in. This capacity did not secure the best vibration of the sender which was a little slower but was suitable for the coil and no further attempt was made to tune still more advantageously by winding up more turns on the experimental coil. An important fact not to be forgotten is that the experimental coil responded without any spark passing between the oscillator balls. Obviously it was seen that, although the experimental coil during the tuning was placed so as to avoid induction of the primary system, the same still existed to some extent. To ascertain how much induced e.m.f. was set up spark was first tried between the terminals of the coil without ground connection and the spark obtained was about 1/64". Now the coil was reversed so that the induced e.m.f. was against the directly communicated e.m.f. through the water pipe and it was found still that a spark of 1" between the rods was obtained. The same would have been probably longer had it not been for the fact that the end of the coil was influenced by the metal of the sink which was near. As this could not be helped the effect could only be approximately estimated. All this showed that the induced e.m.f. from the primary system was not to any considerable degree responsible for the rise of pressure on free end of experimental coil.
178
September 5
After a number of experiments, including a few outside the laboratory, Tesla once more concludes that parasitic capacity is very harmful, so he decides to try winding a coil to have minimum capacitance. Unfortunately he does not describe how this was done. In his desire to get the maximum possible voltage from the coil he went as far as thinking that it was best to have no capacity at the free terminal. From one aspect he was right (theoretically a coil gives the highest Q-factor with the least capacity in the resonant circuit), but without the “elevated” metal sphere the received signal was much weaker because the free terminal of the coil no longer had a monopole antenna. In the circuit which he in fact used he did not, however, go to such extremes. He added the “experimental” coil but left the metal sphere (aerial capacity) connected to one end of the sensitive device.
