178 Trowbridge and Adams — Circular Magnetization 



In order to study the frequency of oscillation, the revolving 

 mirror method was used. A concave mirror of about 300 cen- 

 timeters focal distance was attached to the shaft of an elec- 

 tric motor. The image of the spark was reflected by the 

 revolving mirror upon a photographic plate 5 and when the 

 plate was developed the length of this image could be meas- 

 ured. Some typical photographs are shown in the accompany- 

 ing cuts, tig. 2. The speed of the motor was obtained by means 

 of a chronograph, the cylinder of which was directly attached 

 to the shaft of the motor. 



The formula connecting the periodic time of an oscillatory 

 discharge, T, with the self-induction, L, and the capacity, C, of 

 a circuit is : 



T = 



1 LC 4L 2 f 



or since K 2 /4L 2 is very small compared with 1/LC, 

 T = 27TVLC - 



In this formula T is known from the length of the spark on 

 the photographic plate, the distance of the plate from the 

 mirror, and the speed of rotation of the mirror. 



C was determined by placing at A a coil of copper wire 

 whose self-induction to steady currents could be accurately 

 calculated. It was then assumed that for the low frequency 

 used, the self-induction to oscillatory currents did not differ 

 appreciably from its calculated value. That this was a legitimate 

 assumption within the limits of experimental error, was shown 

 by the close agreement that existed between this calculated 

 value, and the value obtained by measuring the self-induction 

 by means of Rayleigh's bridge, employing a frequency of 

 about 700. Knowing thus L and T, C could be calculated 

 by the above formula. The value of the capacity obtained 

 for the lowest frequency used, viz : 1*85 micro-farads, agrees 

 very closely with its value measured by comparison with a 

 standard condenser. 



Two different coils of copper wire, corresponding to fre- 

 quencies, when alone in series with the condenser, of 633 and 

 1423 respectively, were used. For the higher of these two 

 frequencies the capacity was about 5 per cent less than for the 

 lower. Part of this change may have been due to the fact 

 that the change of self-induction of the copper coil was neg- 

 lected, but this latter was very small. 



The iron wire, whose permeability was under investigation, 

 was arranged in the form of a rectangle, around the four sides 



