Nikola Tesla Books
Books written by or about Nikola Tesla
Colorado Springs
Oct. 18, 1899
For special investigations particularly to prosecute further researches on the increase of capacity with elevation, a new coil is to be constructed as nearly as possible to exact dimensions which follow:
diam. of core 14" = 35.5 cm
length of core 8 x 12" = 243.84 cm.
The coil is to be wound with cord No. 20, before used in the coil of 404 turns on 25.25" diam. drum.
Allowance for thickness of insulation: 0.354 cm.
This makes total diameter d = 35.854 cm.
For approximate value of the surface of one loop we have then S = 1000 cm. sq.
There should go on this length 689 turns and this would make
$! {L = {4 \pi \over l} N^{2} S =} $! 24,490,000 cm. approx.
or
L = 0.02449 henry.
For the purposes contemplated this coil will be well adapted as the self-induction is large and owing to small diameter of the core and great thickness of insulation the capacity should be comparatively small. With this coil balls of 18" and 30" diam, are to be used:
Exact diam. of core should be 14.0485" = 35.6825 cm. for S = 1000 cm. sq.
Colorado Springs
Oct. 19, 1899
Experimental coil before used in investigating propagation of waves through ground. This coil was rewound and presently following particulars are good.
Outside diam. of core=10 3/8" = 10.375" = 26.3525 cm. Turns 550, No. 18 B. & S. wax covered, thickness of one insul $! {14" \over 1000} $!, length of core l = 40 7/8" = 40.875" = 103.8225 cm.
We may neglect insulation as the core is not perfectly round and any irregularity diminishes the area and we get a larger value as a rule. Now, from above data, surface of one loop S = $! {\pi \over 4} $! d2 = 0.7854 x 26.35252 = 545.241 cm. sq. This gives
$! {L = {4 \pi \over l} N^{2} S = {{12.5664 \times 302.500 \times 545.42} \over 103.8225}} $! = 19,970,000 cm, approx. or 0.01997 H
October 18
He calculates a new coil by means of which he wants to continue the tests on the sphere capacitance variation with relation to its elevated height. It is certain that he is still not satisfied with the achieved results, and he prepares the new elements for the more perfect experiments.
October 19
He rewinds the "experimental" coil by means of which he tested the wave propagation through the ground in the beginning of Sept. Now the rewound coil is to be used for sphere capacitance variation measurement with regard to elevated height variation. He calculates the rewound coil inductance (by means of equations for an infinitely long coil which he uses as a rule) and with the sphere of a certain diameter he calculates the system period. He neglects the coil distributed capacitances.
At the end he performs the experiment with the oscillator but it seems that he used only standard primary capacitors (jars) and he made the primary from several turns of thick wire. That is why the primary inductance appears as an unknown value.
