CHAP, iv.] ELECTRICITY AND MAGNETISM. 



223 



called "trial-plates." He even went so far as to com- 

 pare the capacities of these " trial-plates " with that of a 

 sphere of 12^ inches diameter hung up in the middle of 

 a room. 



269. Faraday's Experiments. 1111837 Faraday, 

 who did not know of th&<hen ua- i I Bft^Ss. 

 published researches of Caven- w rjjg wL \ N 

 dish, independently discover-ec^? ^15 

 specific inductive capacity, and w *t U 

 measured its value for several 

 substances, using for trnVsgUi^, ijj Jj J| 

 pose two condensers of the form 

 shown in Fig. 103. Each 

 consisted of a brass ball A, 

 enclosed inside a hollow sphere 

 of brass B, and insulated 

 by a long plug of shellac, up 

 which passed a wire terminating 

 in a ball a. The outer sphere 

 consisted of two* parts which 

 could be separated from each 

 other in order to fill the hollow 

 space with any desired material : 

 the experimental process then 

 was to compare their capacities 

 when one was filled with the 

 substance to be examined, the 

 other containing only dry air. 

 The method of experimenting 

 was simple. One of the condensers was charged with 

 electricity. It was then made to share its charge with the 

 other condenser, by putting the two inner coatings into 

 metallic communication with one another, the outer 

 coatings also being in communication with one another. 

 If their capacities were equal they would share the charge 

 equally, and the potential after contact would be just 

 half what it was in the charged condenser before con- 



Fig. 103. 



