38 Mr. E. M. Wellisch on the Motion of 



established, and in consequence the steady reading of the 

 voltmeter would be too large. It was found that when the 

 small-frequency commutator was employed this steady 

 reading remained constant for values of R up to 1,000,000 

 ohms ; for the high - frequency commutator the value 

 R = 200,000 afforded a reading greater than the normal 

 by less than 2 per cent. Inasmuch as the potential (V 2 ) 



Ma:. 3. 



— ^vWvVWWVV f 



TO GAUZE 



was always chosen considerably greater numerically than 

 the advancing potential (a), the value R = 200,000 was 

 sufficient to ensure the realization of the desired conditions. 

 In the present series of experiments this value of R was 

 chosen in preference to a smaller value because in the deter- 

 mination of electron velocities Y L is often small, and it is 

 advisable to have c in formula (2) small compared with Y v 



Manipulation of Switches. 



In general, when the gauze is raised to any potential, the 

 electrode e is raised by induction to a potential which bas 

 to be taken into consideration when the electric field is 

 estimated. It was found possible, however, by a suitable 

 manipulation of the switches S,/, and g to arrange that the 

 electrode e was practically at zero potential when the potential 

 (x) had been established on the gauze, so that no correction 

 for induction was necessary. The series of operations 

 involved in taking a single reading was as follows : — 



(i.) Potentiometer (b) fixed at a convenient value so that 

 the electrometer-needle should have a suitable range of 



