Manchester Memoirs, Vol. xlviii. (1904), No. 11. 3 



current flowing through the galvanometer (viz., j 



must equal «£(r. Hence nC= p , ^ . 



A: + Cr 



Knowing the frequency of the vibrating key, and the 

 resistance of the galvanometer, we thus obtain the value 

 of the capacity in electro-magnetic units. 



In Thomson and Searle's application of this method, 

 a rotating commutator was employed to work the condenser 

 key, and with n equal to 64, the direct deflection method 

 described above was found to give unsteady deflections, 

 and a zero method using a form of Wheatstone's Bridge 

 was substituted. 



Such a rotating commutator is, however, not always 

 available in a general physical laboratory, and it was 

 decided to use an electrically driven tuning fork to work 

 the key. 



Two small rigid riders of aluminium were fastened to 



Fiff. 2, 



