Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
Sept. 7, 1899
A new experimental coil wound with 400 turns on same drum 10" diam. 66" long.
The coil when attached to a water pipe gave on free end spark 5/8". To test whether the wave length is greater 72 turns were added and sparks were decidedly stronger. But adding 50 more turns the effect was weaker. The self-induction was now calculated to get a better idea of the probable wave length and L was 2,000,000 cm. approx. As with this L the capacity would have to be extremely small, far less than the coil evidently had, it was safe to proceed in taking wire off. Gradually shortening the wire increased the spark length until at 405 turns and a capacity of 15 sq. inches tinfoil the longest spark was obtained about 1". Calculating from wire length λ/4 was 1010 feet approx. giving n=245,500 per sec. approx.
As there was a possibility of confounding the true vibration with a harmonic, wire in definite lengths was taken off. With 270 turns and small capacity on end the effect was still good. From that point on the diminution was steady.
The wire No. 20 was now taken off and wire No. 18 wound in place to study the effect of diminished resistance. New exp. coil wound on drum 10" diam. used before. It was estimated that for the vibrating system before described, comprising ball 38 cm. capacity and 120 feet cable 400,000 c. mills, about 400 - 420 turns would be needed. There was wire enough for 495 turns. The spark was taken to the body of the experimenter,
the length being at once read off by a simple arrangement comprising a small rule of insulating material and a metal strip, the position of which was adjustable relative to end of the insulating rule. The metal strip was held in hand and the end of the insulating rule was maintained almost in touch with the wire forming the free terminal of the coil which was carefully placed in the proper position such that there was no induced e.m.f. from the primary but only through the ground connection could the coil be excited. The connection of coil and manner of reading off spark-length is indicated in the above diagram.
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