COMPARISON OF TWO CAPACITIES. 



457 



potentials, and then discharged on each other. If their capacities 

 are equal, they will both be brought to the neutral state. 



Let us consider, for instance, two Leyden jars the internal 

 armatures of which are A and A' and the external armatures 

 B and B'. A is connected with B' and B with A'; and one of 

 these systems being put to earth, the other is electrified by a 

 source at high potential. The connections with the source and 

 with the earth are then broken, and the internal and external 

 armatures are connected respectively with each other. The two 

 capacities are equal provided the jars are entirely discharged. In 

 this form the method is only exact provided the capacity of the 

 condensers is independent of the choice of the armatures that is, 

 if they could be considered as quite closed. In condensers with 

 the guard ring the acting surfaces alone should thus be compared 

 that is, the surface of the plate comprised within the guard ring. 



A simple method of charging the two surfaces to equal and 

 opposite potentials consists in putting them in communication re- 



Fig. 216. 



spectively with the two poles of a battery the centre of which is 

 to earth. After having broken connection with the battery, the 

 two capacities are connected. The charge is reduced to zero if 

 they are equal. 



1047. PLATYMETER. Sir W. Thomson* has given this name 

 to a double condenser formed of two cylindrical rings A and A' 

 (Fig. 216) of the same length and radius, perfectly insulated, and 

 placed about a cylinder BB' of the same axis which is connected 

 with an electrometer or an electroscope. Keeping at first the 

 cylinder B in connection with the earth, one of the rings A is 

 charged to a potential V; connection with the source and with 

 the earth is then broken. If the two rings are then joined, 



V 



the common potential becomes by the division of the charges, 



* Sir W. THOMSON. British Association Report, 1855. Glasgow. See also 

 GIBSON and BARCLAY, Transactions of the Royal Society, 1871, p. 570. 



