326 Mr. W. P. Boynton on the 



The resistances of the two coils to steady currents are 

 small, of the order of '05 and *3 ohm respectively. R x and 

 R 2 will, however, contain not only these, increased perhaps 

 considerably on account of the peripheral distribution of the 

 current, but also the resistances of whatever measuring - 

 instruments are inserted, and of the spark-gaps, where such 

 exist. 



Period. 



If in equation (19) we insert these values, we find 



A,= -03V„K i; C^-gTVoK,. 



That is, the oscillation whose period is determined by the 

 value of 8 decidedly predominates in the primary circuit. 

 This is due simply to the choice of dimensions of the system. 

 The corresponding frequency hardly differs from the natural 

 frequency of the primary system. 



The experimental determinations of the period of oscillation 

 were made by photographing a spark by means of a rotating 

 mirror. The mirror itself was concave, silvered on the face, 

 of about 36-centim. focal length, and mounted on the end of 

 the shaft of an electric motor. The photographic plates were 

 set at a distance of 8L'5 centim. from the centre of the face of 

 the mirror, and the speed of the motor was determined by com- 

 parison with a standard tuning-fork by a stroboscopic method. 



The photographs of the most value were taken of the spark 

 in the primary circuit. Some were taken also of that in the 

 secondary circuit ; but these seem by the theory to represent 

 an oscillation superposed upon a current dying away loga- 

 rithmically, and the photographs are correspondingly hazy. 

 In each photograph there appear several distinct sparks, each 

 showing fine striations, which indicate the oscillations (see 



fig- 2)*. 



In Table I. are given (a) the number of revolutions per 

 second of the mirror, (b) the number of oscillations distinctly 

 visible in a given photograph, (c) the mean length of an 

 oscillation, (d) the double frequency of oscillation computed 

 from a and c. 



The dimensions of our apparatus would give, substituting 

 in equation (9), 



8 = 3,400,000; 



and the frequency would be 

 S 



= 542,000. 



7T 



2 _-—- , 



* In each photograph there appear several distinct sparks, each showing 

 Jine striations, which indicate the oscillations. The.se are unfortunately 

 hardly visible in the reproductions. 



