Various Tesla book cover images

Nikola Tesla Books

Books written by or about Nikola Tesla

are 40 turns wire. Now A=surface of half wire: 131 feet, about 4000 cm. long; half circumference about 0.4 cm. gives A=1600 cm. sq. The distance of wires d was=5 cm. From this: Cs = 40 x $! {1600 \over {4 \pi \times 5}} $! = 1040 cm. approx. (probably accidentally close.)

Last night the 50,000 volts transformer brought from New York broke down. This happened upon connecting to the lower end of the secondary a condenser composed of two adjustable brass plates of 20" diam., one being connected to the ground, the other to the secondary. The plates were about 5" apart. The experiment was repeated with the transformer after repairing it and all was found in good order.

Experiments were now continued with the secondary of 40 turns wire No. 10 just one mile long. In connection with this secondary a coil was used wound on a drum 2 feet in diam. and 6 feet long, with wire No. 10 (cord), there being 260 turns. Approx. estimate of capacity after a previous similar estimate: 6 feet = 6 x 12 = 72" = 72 x 2.54 = 183 cm. Half circumference of wire 0.4 cm. Area = 183 x 0.4 = 74 sq. cm. (about) = a, distance of wires about 1 cm. =d. This would give roughly the capacity C1 = $! {a \over {4 \pi d}} $! = $! {74 \over {4 \pi}} $! for one pair of wires; there being 260 pairs, total capacity would be according to this $! {{260 \times 74} \over {4 \pi}} $! = 1532cm. = Ct (for coil). Now, the capacity in the secondary was previously found 1080 cm. Hence the total capacity of the system would be 1080 + 1532 = 2612 cm. = C’. This, of course, gives only an idea and the determination in this manner is far from being exact. Consider what the period would be with the capacity and secondary and coil together as a whole. Since the coil will have only about 12,000,000 cm. the inductance of the secondary will be the chief governing factor. Taking this at 5 x 107 cm. we would have a total inductance 5 x 107 x 12 x 106 or (50 + 12) x 106 = 62 x 106 cm. This would give

T = $! {{{2 \pi} \over 10^{3}} {\sqrt{{{62 \times 10^{6}} \over 10^{9}} \times {2612 \over {9 \times 10^{5}}}}}} $! = $! {{{2 \pi} \over {3 \times 10^{7}}} \sqrt{164,944}} $! =
= $! {{2 \pi \times 406} \over {3 \times 10^{7}}} $! = $! {2549.68 \over {3 \times 10^{7}}} $! = $! {849.9 \over 10^{7}} $! or $! {850 \over 10^{7}} $! = $! {85 \over 10^{6}} $!

and n = 11,800 roughly. To this the primary to be adjusted for first approximate trials. Now the primary has six turns. Since one turn is approx. 7 x 104 cm. we may put:

$! {85 \over 10^{6}} $! = T = $! {{2 \pi} \over 10^{3}} \sqrt{{{36 \times 7 \times 10^{4}} \over 10^{9}} C_{p}} $!

From this a rough idea of the capacity in primary may be gained. We get Cp = 0.0717mfd. roughly. Taking capacity of one jar = 0.003 mfd. we would want total $! {0.0717 \over 0.003} $! = 24 jars approx. or 48 jars on each side of primary. This vibration would be impracticable under the present conditions as the transformer could not charge this number of jars. Although for stationary waves in ground it would be desirable to use such a low frequency the vibration will have to be quickened. An octave would require only 12 jars on each side. This was tried and results were good although the octave vibration had only 1/4 of the energy as the fundamental would have had. To get the true vibration we shall want

60

July 3

The distributed capacitance of the secondary windings is difficult to determine. It depends on the coil diameter, the dimensions of the wire and the insulation and the winding pattern. In a single-layer coil it is due mostly to the capacity between neigh boring turns, and this is the way Tesla calculated it. He considers a greatly simplified model in which it is taken that the parasitic capacity per turn is equal to A/4πd, where A = rπl, half the surface area of the wire in one turn, and d is the distance between turns. The capacitance is calculated as that of a plate condenser of area A and gap d with air between the plates. This model is open to a good many criticisms, but it must not be forgotten that Tesla had to find some solution, whatever its shortcomings. It is also not correct that the total inductance and capacitance of the secondary circuit with the “additional coil” are additive, but Tesla was himself aware that this was guesswork, and often mentions the words “roughly”, “estimate”, etc.

In an earlier calculation (see June 20th) he had started from the primary circuit and worked out the values for the secondary, whereas here he attacks it from the other end: from the resonant frequency of the secondary circuit and the known primary inductance (one turn) he finds the required capacity of the primary circuit. He then checks whether this capacity can be used with an LF transformer of the given power. The formula is approximate, but gives a good rough guide for the power in the mains transformer. The peak power rating of the transformer must be even greater than the value found because the condenser is not charging all the time but only in short pulses.


July 3

Secondary distributed capacitance is one of the Tesla oscillator elements which cannot be determined easily. This magnitude depends on coil diameter, wire size and its insulation and coil winding method. At one layer coils it originates mainly due to influence of adjacent windings and Tesla calculates it so. He ultimately simplifies the model and takes that parasitic capacitance per turn is equal A/4πd, where A=rπl - ½ of one turn wire area, and 'd' is distance between turns. According to this model, capacitance is calculated as for a plate type capacitor of area A and distanced between plates with air insulation. There are a number of disadvantages of such a model, but it should not be forgotten that Tesla is looking for the solution. It is not correct also that he considers in the secondary circuit with "additional coil" all inductances and capacitances added together, but Tesla is aware that all this is just guessing and he frequently mentions words as "approximately", "estimate" and "similar".

Contrary to one previous calculation (please see June 20) when Tesla started with primary circuit and calculated the secondary, now he does the opposite; on the basis of secondary circuit resonant frequency and known primary inductance (one turn) he determines the primary circuit capacitance. After that he checks whether the found capacitance is allowed to be used with regards to the transformer power of low frequency. The applied equation is approximate, but it is good for orientational analysis of network transformer energy balance. Maximum transformer power has to be larger than the found one, because the capacitor in the primary is not charged all the time, and then only in short impulses.

Glossary

Lowercase tau - an irrational constant defined as the ratio of the circumference of a circle to its radius, equal to the radian measure of a full turn; approximately 6.283185307 (equal to 2π, or twice the value of π).
A natural rubber material obtained from Palaquium trees, native to South-east Asia. Gutta-percha made possible practical submarine telegraph cables because it was both waterproof and resistant to seawater as well as being thermoplastic. Gutta-percha's use as an electrical insulator was first suggested by Michael Faraday.
The Habirshaw Electric Cable Company, founded in 1886 by William M. Habirshaw in New York City, New York.
The Brown & Sharpe (B & S) Gauge, also known as the American Wire Gauge (AWG), is the American standard for making/ordering metal sheet and wire sizes.
A traditional general-purpose dry cell battery. Invented by the French engineer Georges Leclanché in 1866.
Refers to Manitou Springs, a small town just six miles west of Colorado Springs, and during Tesla's time there, producer of world-renown bottled water from its natural springs.
A French mineral water bottler.
Lowercase delta letter - used to denote: A change in the value of a variable in calculus. A functional derivative in functional calculus. An auxiliary function in calculus, used to rigorously define the limit or continuity of a given function.
America's oldest existing independent manufacturer of wire and cable, founded in 1878.
Lowercase lambda letter which, in physics and engineering, normally represents wavelength.
The lowercase omega letter, which represents angular velocity in physics.