On Measuring the Electrical Capacity of a Condenser. 99 



by the deflection due to the steady current, and the resultant 

 deflection is zero. When this is the case, there is a relation 

 between the capacity of the condenser, the number of times 

 the condenser is charged and discharged in a second, and the 

 resistances in the various arms of the bridge." 



The investigation given by Maxwell is only approximate ; 

 we shall quote therefore the result given in Thomson's paper. 



Let a be the resistance of A C, 

 b „ „ AB, 



c „ „ AD, 



d „ „ BC, 



9 » » DC - 



Let C be the capacity of the condenser, n the number of times 

 it is charged and discharged per second ; then 



a { l ~ (a±c + g)(a + b + d) \ 



nC = 



^^JttA^ 



ag 



} 



>(a + b-\-d)) l d(a + c+g) 



The commutator was the one used by J. J. Thomson, and 

 we will quote his description : — lk The current from some 

 Grove cells passes first through a tuning-fork interrupter and 

 then through the coils L M (fig. 2) of an electromagnet. 



Fig. 2. 



P N is a strip of brass with a piece of iron wire attached to it. 

 When there is no current passing through the electromagnet, 



H 2 



