Ions and Electrons through Gases. 37 



In fig. 1. When thrown to the left, connexion was made with 

 a subsidiary potentiometer system (6); in this position the 

 quadrants of the electrometer could be commutated in po- 

 tential between zero and any convenient potential read off 

 on the potentiometer. The use of this device in testing the 

 contact at the brushes, in estimating the value of /, the 

 fractional duration of contact, and in adjusting the position 

 of the electrometer-needle for observations, has been de- 

 scribed in the previous paper. 



For large current values readings were taken with the 

 capacities B and C added to the electrometer system ; the 

 capacity of the system was then increased 17| times. 



3. Experimental Procedure. 



For convenience in manipulation a table was prepared of 

 the potentials assumed by the gauze for different values of p, 

 V 1} V 2 , and It. This was effected by means of formula (1), 

 which for the purpose was put in the following form: — 



= V 1 + c(say) (2) 



v was always chosen equal to 40 volts and r was always 

 15,000 ohms. The calculated values of c for various values 

 of p and Vi + V 2 were then tabulated, and the value of x 

 under any desired conditions could be quickly obtained. 



Establishment of Potential. 



It was important to ascertain that the experimental con- 

 ditions admitted of an effectively instantaneous establishment 

 of the withdrawing potential — V 2 through the resistance R; 

 in other words, the ions must commence to retire as soon as 

 the commutator-brushes make contact with the fibre seg- 

 ments. This point was tested experimentally in the following 

 manner: the commutator and resistance R were put in series 

 with a battery V (fig. 3) of which one terminal was earthed ; 

 K represents a Kelvin multicellular electrostatic voltmeter 

 which was included in the manner shown in the diagram. 

 The commutator was set in motion at its highest speed, and 

 readings were taken on the voltmeter corresponding to 

 different values of R. If the values of R were excessively 

 large there would not be sufficient time during an alternation 

 to admit of the earth-connexion with the gauze being fully 



