POTENTIAL AND CAPACITY 



97 



jar. Connect A to the inside coating of J, and B to the outside 

 coating of J. If the charges are equal these connections result in 

 complete discharge, and if A is now connected to a quadrant 

 electrometer no charge is shown. But if J has the larger capacity 

 some positive charge is left unneutralised and is indicated by the 

 electrometer. If J is the smaller, some negative charge is indicated. 

 The distance apart of the two plates A B must be varied till no 

 charge is indicated and the capacities are then equal. The 



A B 



FIG. 70. 



difficulty in such measurement arises from "residual charge," which 

 will be discussed in Chapter VIII. This method was used by 

 Cavendish (Electrical Researches, p. 144?). 



Capacity of a gold-leaf electroscope by comparison 

 "With a sphere. We shall describe the method used by C. T. R. 

 Wilson (P.R.S. Ixviii. p. 157). The electroscope was charged by 

 a battery of small accumulators to different potentials so that the 

 relative values of the deflections of the gold leaf were known. A 

 brass sphere, radius 2*13 cm., was suspended by a silk thread at a 

 distance great compared with its radius from all other conductors 

 except two fine wires, one earth-connected with which it rested in 

 contact, and another leading from the electroscope and with its free 

 end near the sphere. The electroscope was charged to some 

 potential V given by the deflection of the gold leaf. Then the 

 charge on it was CV, where C was its capacity. The sphere was 

 then drawn aside by a silk thread so that it momentarily broke 

 contact with the earth and came into contact with the wire from 

 the electroscope. Suppose the new value of the potential indicated 

 by the electroscope was V. The charge on it was now CV. But 

 the charge lost, viz. CV CV 7 , was given up to the sphere with 

 capacity 2*13, and raised its potential to V so that its amount was 

 2-13 V. Equating 



C(V - Y) = 2-13 V' 



G 



