Induction-Coil Potentials Page 572

Table III.

It will be seen that there is no great difference between the results obtained by the two methods. Such differences as exist are to be accounted for by the difference in the circumstances of the experiments. In the sparking method the iron core is initially unmagnetized, and the oscillations are accompanied by reversals of the induced magnetism. In the short-circuiting method the core is initially strongly magnetized, and as the magnetization decays the oscillations produce small fluctuations in its intensity. The difference between the two cases may be illustrated by saying that, if \(N\) is the total flux of induction in the core and \(i\) the current in the secondary coil, then in the sparking method the frequency depends upon the coefficient \(N/i\), whereas in the short-circuiting method it depends upon \(dN/di\), and the value of this latter coefficient will be modified to some extent by the hysteresis of the core.

The circumstances of the sparking method are in closer accord with those of the actual case of an induction-coil, and the result obtained by that method, viz. \(k^2 = ·768\), is therefore adopted in the calculations of potential described below.

Determination of \(L_1 C_1\), \(L_2 C_2\), \(R_1/L_1\), \(R_2/L_2\).

The expression (1) for the secondary potential was worked out for three cases in which mica condensers, of capacities stated to be 3, 2, and 1 microfarad respectively, were connected across the primary coil. These values were, however, not assumed, but rather the value of the product \(L_1 C_1\) was determined for each case. This was done by coupling the primary (with the condenser across its terminals) loosely with a secondary of many windings to the terminals of which the electrometer was connected. The most sensitive electrometer