460 



MEASUREMENT OF CAPACITY. DIELECTRICS. 



of equilibrium, and which consequently are applicable to capacities 

 of all kinds. 



The first requires the use of three condensers of comparison, one 

 of which at least has a variable capacity. 



Let C, C', G!, Ci be the four capacities to be compared (Fig. 

 218). Two of them C and C x are charged to the same potential V , 

 and then C is connected with C', and C : with Ci, all the outer 

 coatings being to earth. Let V and V 1 be the potentials on either 

 side ; we have 



(C + C')V = CV 



, 



If the two potentials V and Vj are equal, a galvanometer in- 

 serted between the two systems shows no current; we have then 



c+e c 



earth 



earth 



Fig. 218. 



It is advantageous to replace the galvanometer by an electro- 

 meter ; for, as each system retains its charge, there is every 

 opportunity of adjusting the capacity of the variable condenser so 

 that the condition is satisfied. 



In the case in which the galvanometer is used, we must wait 

 until the charge is in equilibrium before closing the current of the 

 galvanometer between D and D'. This time may amount to several 

 seconds when we are dealing with large capacities and high re- 

 sistances. 



The second method only requires one standard of comparison, 

 which, however, must be unchanged. The outer coatings of the 

 two capacities C and C' to be compared (Fig. 219) are insulated 

 and connected with each other by a wire D, and the internal arma- 

 tures are in relation with the ends of a considerable resistance AB 



