456 MEASUREMENT OF CAPACITY. DIELECTRICS. 



If R and R x are the radii of the internal and external cylinders, 

 the value of the coefficient a in electrostatic units is* 



The capacity C , which corresponds to the standard or zero 

 of the scale, is determined by comparison. 



The capacity of the sliding condenser is somewhat small ; but a 

 suitable capacity may always be added, and the first only used to 

 effect the adjustment. 



In order, finally, to avoid any perturbation arising from adjacent 

 conductors, the insulated part AC is enclosed in a conducting cylin- 

 der in connection with the earth. 



Fig. 215 represents a sliding condenser used*by Sir W. Thomson, 



._ 



/* 



Fig. 215, 



which, by a double adjustment, enables us to obtain a given capacity 

 with great approximation. The cylinder A is insulated, the cylinders 

 B and C communicate with each other and with the earth, as well as 

 with the cylinders D and D'; the cylinders D and C are movable. 

 The capacity of A increases when the cylinder D or the cylinder C 

 is pushed in, but much more in the first case than in the second for 

 the same displacement. The first motion will enable us to obtain 

 an approximate equilibrium in a comparative experiment, and the 

 adjustment is effected by moving the cylinder C in either direction. 



1046. COMPARISON OF Two CAPACITIES. METHODS OF OP- 

 POSITION. A method of opposition employed so long ago as last 

 century by Volta and by Cavendish enables us to verify the equality 

 of two condensers. They are charged to equal and opposite 



* In the condenser employed by Gibson and Barclay the central cylinder was 

 moved by a screw sliding in a groove of the envelope A, in front of a divided 

 scale. Here Rj =2-4807 cm., R= 1-2515, which gives = 0-6514. The en- 

 velopes A and C were 30 cm. in length, and the cylinder 36 cm. 



