ELECTROSTATIC INDUCTION BALANCE. 477 



power. Spontaneous alteration of the surface produces the same 

 result. 



1066. Gibson and Barclay,* using concentric spheres, deter- 

 mined the specific inductive power of paraffine by comparison of the 

 capacities with a platymeter (1047) and a sliding condenser (1045). 



In his experiments on the inductive capacity of different kinds 

 of optical glass, Hopkinsonf used the guard-ring condenser, and 

 determined the thickness of the equivalent layer of air. The sliding 

 condenser was adjusted so as to give the same capacity as the 

 apparatus with the glass plate ; the glass plate being removed, the 

 disc was approached until equilibrium was restored; the sliding 

 condenser was then simply used as a tare. Only the acting surfaces 



Fig. 226. 



are compared those of the plate, that is to say all other con- 

 densers comprising the guard ring being connected with the earth. 

 1067. ELECTROSTATIC INDUCTION BALANCE. The part which 

 the dielectric plays has been made evident by Faraday, J by means 

 of an ingenious arrangement which he called the induction balance, 

 or differential inductometer. Between two plates A and B (Fig. 226), 

 which are respectively connected with two gold-leaves a and , an 

 insulated plate of the same dimensions is placed at equal distances. 

 The plates A and B are connected with the earth, C is positively 

 electrified, and connection broken ; the leaves a and b are then at 



* GIBSON and BARCLAY. Phil. Trans. Roy. Soc. Land, for 1871, p. 573. 



f HOPKINSON. Phil. Trans. Roy. Soc. Land, for 1878, p. 17. 



J FARADAY. Exper. Researches, Series XL, 1307, Vol. I., p. 413. 1838. 



