ROTATING A DIKU'XTKIC IN A M. \c\KTIC FIKI,D. 



131 



The following table gives the results obtained with various magnetic fields and 

 rates of revolution. 



The quantity 8///CS in the alx>ve table is approximately constant, which shows that 

 the deflection on reversing the current in the solenoid was proportional to the current 

 reversed, to the rate of revolution of the cylinder, and to the sensibility of the 

 electrometer for quantity. 



The numbers in the column headed " Electrometer sensibility " are the deflection 

 due to the charge induced on the small plate of the guard-ring condenser when 

 a difference of potential of 38'0 volts was maintained between the two plates of the 

 condenser. Denoting this charge by q, and the capacity of the condenser by a, we 

 have q = 38/300 electrostatic units. Now a = A./4nd, where A is the effective area 

 of the small plate and d the distance between the plates. Hence 



= 



2-00 



centim 



so that <y= - 8 = 0'0356 electrostatic unit. 



300 



The quantity of electricity (Q) required to produce a deflection equal to that 

 obtained by reversing C amperes with n revolutions per second is therefore 



Q = qS/S = 15-35 X 10~ 7 Cn electrostatic unit. 



s 2 



