474 MEASUREMENT OF CAPACITY. DIELECTRICS. 



of the charging current is CE, and if equilibrium of the bridge 

 is produced in the two experiments, we have 



The plate L is put in motion by an electrical trembler, and the 

 contacts take place on platinised surfaces ; the number of vibrations 

 is given by the height of the note. The values obtained for the 

 capacity are very concordant, with a less divergence than 0*002 when 

 the number of oscillations is varied from 16 to 128 per second, 

 and when the relative duration of the contacts is modified within 

 wide limits. The phenomenon, therefore, is well defined, and the 

 condenser had time to take at each contact what is called an in- 

 stantaneous discharge. 



The expression for R 2 may be simplified if we suitably choose 

 the different resistances, as Mr. Glazebrook* has done. Writing 

 this expression in the form 



_ 



we see that, if the resistances R and a' are small in comparison 

 with b' and r^ we have sensibly 



2nC a' 'a + r 



\ 1063. COMPARISON OF A CAPACITY WITH A COEFFICIENT OF 

 MUTUAL INDUCTION. The discharge of a condenser presents the 

 same characteristics as induced discharges. 



If a capacity C, communicates with two points of a circuit 

 traversed by a permanent current I, and separated by a resistance 

 R, the charge it takes, 



* R. T. GLAZEBROOK. Phil. Mag. [5], Vol. xvin., p. 98. 1884. In 

 Mr. Glazebrook's experiments, a'= 10, R = 5 to 6, 01 = 240 to 1800, '=1000, 

 and 2=11,000 ohms. 



