462 MEASUREMENT OF CAPACITY. DIELECTRICS. 



1050. MEASUREMENT OF POTENTIALS. In order to compare 

 two capacities C and C', we may compare by an electrometer the 

 potential V which the first acquires for a definite charge M of 

 electricity, with the potential V of the system when the two 

 capacities C and C' are connected; we have, in fact (83), 



from which follows 



This, for instance, is the method which Faraday* used to deter- 

 mine the ratio of two spherical condensers of the same dimensions, 

 one containing air and the other a solid dielectric. By means of 

 the torsion balance (801) Faraday measured the quantities of elec- 

 tricity taken by an insulated ball from the same point of the first 

 condenser before and after contact with the second. The ratio 

 of the charges was equal to the ratio of the potentials V and V. 



V 



If the ratio - be determined directly by a graduated electro- 

 meter, and the capacity y of the instrument cannot be neglected 

 in comparison with that which is to be estimated, the former must 

 be eliminated. For this the experiment must be repeated by first 

 charging the electrometer alone to a potential U and then con- 

 necting it with a capacity C, which lowers the potential to a value U ; 

 we have then 



7'u-'- 



As the comparison of the capacities C and C' gives 



C' V 



it follows that 



C 



"G 



* FARADAY. Experimental Researches, Vol. I., p. 371. 



