Nikola Tesla Articles
Induction-Coil Potentials Page 579
Philosophical Magazine - April 1st, 1914
Table VII.
| \(A\). | \(V_c\). | \(V_c / \sqrt{A}\). | |
|---|---|---|---|
| Case I. ……… | 4·4 | 71610 | 34130 |
| Case II. ……… | 6·9 | 87420 | 33310 |
| Case III. …… | 14·0 | 125700 | 33590 |
To within about 2·5 per cent., therefore, the calculated values of the maximum secondary potential are proportional to the values indicated by the electrometer.
With regard to the estimation of the absolute values of the potentials, no source of high potential suitable for standardizing the electrometer was available, and recourse was had to the method of spark potentials. It appears to be agreed that the potential difference required to produce a 1-centimetre spark, in air at atmospheric pressure, between ball electrodes 2 cm. in diameter, is not far from 31000 volts*.
The above theoretical expression for \(V_2\) gives in Case I. a maximum value of 31000 volts at \(i_0 = 1·298\) amperes, but this is calculated on the assumption that the inductances are constant and equal to their maximum values. At this current, however, \(L_{21}\) is (from Table I.) about ·916 of its maximum value, and we may assume that \(L_1\) and \(L_2\) are altered approximately in the same proportion. Hence from the expressions for \(n_1\), \(n_2\), \(A_1\), and \(A_2\) we find that \(V_2\) is approximately proportional, as far as small variations in the permeability of the core are concerned, to the square root of \(L_{21}\). Assuming this result, the calculated value of the current required to give a maximum voltage of 31000 is \(1·298 / \sqrt{·916}\), i.e. 1·356 amperes.
The least current observed to give a 1-cm. spark in Case I. was 1·36 amperes. This was repeated on many occasions and with both forms of interruptor, and found to be fairly definite. Sometimes the spark would not pass at less than 1·38 amperes, but on most occasions the result was as given above.
In Case II. the current giving 31000 volts, calculated
* Heydweiller (Wied. Ann. xlviii. 1893) gives 31290 volts, while according to C. Müller (Ann. d. Physik, xxviii. 1909) the voltage is 30240. An account of the results of different observers is given in Fleming’s ‘Principles of Electric Wave Telegraphy and Telephony,’ 2nd ed., pp. 203–8.
