Nikola Tesla Books
Books written by or about Nikola Tesla
upon the extra coil as the wires of the oscillator proper, wound on the wooden structure seen in the back behind the coil, are short-circuited. One of the terminals of the condensers is grounded so that when they are discharging through the circuit, chiefly composed of a number of turns of the regulating coil, there is a strong vibration propagated through the ground which through the ground wire w reaches the "extra coil". Now, generally, the energy which can thus be transmitted to the coil would be minute, but when the oscillations passing through the ground are exactly of the frequency of the "extra coil" system itself, a considerable current passes into the coil which then acts just as a hole would in a pipe through which a fluid is pumped by means of a pulsating piston. As the magnifying factor of the coil is very large the feeble impulses reaching the ground wire and lamps magnify the impressed e.m.f. and create considerable movement of electricity through the lamps which are thus brilliantly lighted, as shown in the photograph. In the experiment the capacity in the exciting oscillating circuit, impressing the vibrations upon the ground and wire w, was 3 tanks on each side or 1 1/2 tanks total, that is 54 bottles or 0.0009 x 54 = 0.0486 mfd, approx. The total inductance was 41,000 cm + ind. Of 6 1/4 turns of regulating coil = 41,000 + 19,368 = 60,000 cm, approx. or 0.00006 henry. From this the approximate period of the vibration impressed upon the ground would be
Tp = !$ {{2 \pi \over 10^{3}} \sqrt{0.0486 \times {6 \over 10^{5}}}} !$ = !$ {{2 \pi \over 10^{5}} \sqrt{0.02916}} !$ = !$ {{2 \pi \over 10^{5}} \times 0.1708} !$ = !$ {1.074 \over 10^{5}} !$
approx. and n = 93,110 and p = 585,000 approx. λ would be very nearly 2 miles and !$ {λ \over 4} !$ = 1/2 mile or about 2640 feet. In reality the length of the wire in the excited system - that is extra coil and ground wire, was found by measurement to be 2660 feet (98 turns, wire No. 6, 25' 11" each turn = 2540' + 3/4 turn of cable inside of secondary frame = 112' + continuation of ground cable outside of circle to ground plate = 28' + wire w = 20' + rubber covered wire on top of coil = 50' that is, total 2450' + 112' + 28' + 20' + 50' =2660 feet. From above data and taking resistance of extra coil at 1 ohm (in reality a little less) we get magnifying factor for coil alone !$ {p L \over R} !$ = !$ {{585,000 \times 0.018} \over 1} !$ = 10,530.
Taking, however, into consideration that the resistance of the lamps was about 1000 ohms roughly, when including the latter in the system the factor would be only about !$ {1 \over 1000} !$ of this, or approximately only 10.5. But I believe that the resistance of the lamps when operated by currents of such extreme frequencies is much smaller than the measured resistance according to the usual methods. The currents, namely, when produced in such ways as these here employed, have very high maximum values and the carbon is brought periodically to a much higher temperature than when operated with steady currents or currents of ordinary frequencies. I have observed this repeatedly. Furthermore when such currents as these here are used some part of the discharge also passes through
The explanation to Photograph XXII concerning the transmission of power from the excited primary circuit to the "extra coil" via the earth is similar to that he gave in 1893(6). The experiment to which the photograph refers was made with the aim of estimating the power of the oscillator from the thermal effect of the HF current. What Tesla calls the "total energy set in movement" would correspond to the total energy transferred to condenser in the secondary (i.e. the power) if an energy of !${1 \over 2}!$ CV2 is transferred in each half-cycle. It can be shown that the active power dissipated in the circuit is much less than this and is inversely proportional to the Q-factor of the oscillating circuit.
