BALANCE OF CAPACITIES. 



459 



In the method used by De Sauty,* two condensers C and C' 

 (Fig. 217) take the place of the two branches b and b' of a Wheat- 

 stone's bridge, their external armatures being in connection with 

 the earth. The two resistances a and a' are adjusted so that, by 

 opening or closing the key K which sets up connection with the 

 battery, there is no current in the galvanometer. 



The condition of equilibrium requires evidently that the extremi- 

 ties B and B' of the bridge are at the same potential that is to 

 say, that at a given moment the charges Q and Q' are proportional 



Dearth 



^ earth 



Fig. 217. 



to the corresponding capacities C and C'. As the charges are pro- 

 portional to the currents which produce them, and these are inversely 

 as the corresponding resistances a and a, it follows that 



C'~ a 



It is clear that the galvanometer may be replaced by an electro- 

 meter. 



This method is good for capacities of mean value, and in which 

 electrical absorption has but little influence. It requires, in fact, 

 that the duration of the charge is the same for the two condensers. 

 Thus it cannot be applied to large capacities, such as submarine 

 cables. This is apparent from the fact that the adjustment of re- 

 sistances which is suitable for the charge is not so for the discharge. 



1049. Sir W. Thomson f has pointed out two methods which do 

 not depend on the time of the charge, but simply on the final state 



* L. CLARK and R. SABINE. Electrical Tables and Formula, p. 62. 1871. 

 t Sir W. THOMSON. Jottrnal of the Society of Telegraph Engineers. Vol. I., 

 5, p. 394. 1873. 



