Induction-Coil Potentials Page 570

0·61 cm. thick. The results are given in Table II., the capacities being in descending order of magnitude.

Table II.

Specimens of the photographs used in these measurements are shown in Plate VIII. figs. 4 & 5. In fig. 4 the first three waves correspond to the exciting spark, the remainder to the free oscillation. The lower wave is the trace of a 768 tuning-fork photographed simultaneously. The numbers in the fourth column show a gradual falling off of the coefficient of coupling as the capacity connected to the secondary coil diminishes, i.e. as the current changes from the uniform to the cosine distribution. The question, however, arises whether this diminution of \(k^2\) is due to the change in the distribution of the current, or whether it is not due to some other cause, e.g. to a falling off in the intensity of magnetization of the core as the frequency of the oscillation increases. For a given potential at the terminals of the secondary condenser the magnetizing current diminishes as the capacity of the condenser diminishes, being in fact inversely proportional to the frequency of the oscillations. Hence it might be expected that the permeability of the core, for oscillations of given amplitude, would fall off as the capacity of the condenser diminishes. The question whether this appreciably affects the value of \(k^2\) was tested by repeating the experiment with the largest capacity (condenser A) with a much smaller potential. The method was the same, but a much smaller discharge was employed for starting the oscillation, and the most sensitive form of electrometer⁹ was used for obtaining the curve. The potential and current in this experiment were not more than one-fifth of those employed in the former determination, and the value of \(k^2\) obtained was ·820. Hence the

⁹ Cf. Phil. Mag. xiv. p. 240 (1907).