Effect of a Changing Magnetic Field. 471 



E x and E 2 are very nearly of the same phase the force is 



or the moment 



The two formulae must evidently yield the same result, but 

 the second has the manifest advantage of requiring merely 

 two voltmeter readings, together with a determination, once 

 for all, of the capacity. 



Method of Observation. 



The capacity of the condenser was determined by charging 

 and discharging hy means of a rated tuning-fork ; using a 

 storage-battery of known E.M.F. for charging, and measuring 

 the rate of discharge by means of a galvanometer of known 

 sensitiveness. The following is a specimen determination: — 



Sensitiveness of galvanometer. . . 525 X 10~ 10 amp./cm. 



E.M.F. applied 5*04 volts. 



Frequency of tuning-fork 89*1 vibr./sec. 



Deflexion produced 8*50 cm. 



/. Capacity 9' 9 x 10" 10 farad. 



The induced E.M.F. (E 2 ) was obtained by means of a coil 

 of 50 turns and of the same diameter as the condenser rings, 

 placed in the position occupied by the charged condenser 

 rings. A table was prepared giving the values of E 2 when 

 different E.M.F.'s were impressed on the magnetizing coils, 

 so that during the experiment it was only necessary to 

 observe the voltmeter which was left applied to the magne- 

 tizing circuit. 



The charging E.M.F. (E x ) was determined by observing 

 the E.M.F. impressed on the primary of the potential trans- 

 former, whose ratio of tranformation (20 : 1) was known. 



Both E x and E 2 were thus made to depend on the same 

 A.C voltmeter, and this in turn was compared with a Weston 

 laboratory standard voltmeter. 



In attempting to observe the electromagnetic deflexions 

 with this form or apparatus the only serious difficulty en- 

 countered was due to the electrostatic attraction between the 

 condenser parts. This caused a directive moment to be 

 exerted on the movable part, which was usually so large as 

 to completely overshadow the effect of the suspension wire, 

 although a strong suspension of manganin wire '13 mm. in 

 diameter and about 22 cm. long was used. Sometimes days 

 were spent in the attempt to so adjust the condenser as to 



