Method of Determining Electromagnetic Capacity. 71 



deflexions, either steady or momentary, of an instrument such 

 as a galvanometer or electrometer. 



Let a be the resistance in that part of the resistance-coils 

 between which the condenser circuit is applied, h the resistance 

 in that part of the coils between which the permanent current 

 is applied . The galvanometer itself forms a sufficient resistance 

 to insert in the permanent-circuit branch, but its resistance 

 must be known at the temperature at which it is used. 



txdixh 



F now = 



TT X G X (i, X a' 



The above expression is not strictly accurate, because damp- 

 ing occurs on an open circuit when the condenser is being 

 charged, and on a closed circuit when the permanent current 

 is flowing ; but in a galvanometer with any approach to bal- 

 listic properties the error may be considered as negligible for 

 ordinary purposes ; for example, the logarithmic decrement 

 of a galvanometer of 2000 ohms used in this operation on 

 closed circuit was '0428, on open circuit '0415, involving an 

 error of only one part in 2000, if the damping is entirely 

 neglected in the two cases. With a Thomson galvanometer, 

 in which the decrement reached the value of "56 on open 

 circuit, the error involved would, however, amount to one per 

 cent. 



The imperfect elasticity of the fibre may still exert a 

 small influence, because the needle is swinging for a quarter 

 period in the condenser charge and for a half period in the 

 permanent-current observation ; but if the error is not entirely 

 obviated, it is brought down to very small limits. 



