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THE ROYAL SOCIETY OF CANADA 



portion of the air between GH and CD was confined to a very thin 

 layer close to the latter plate. 



The experiment consisted in measuring the current between CD 

 and GH with various positive and negative voltages applied to AB 

 at the different pressures. The formula for determining the mobilities 

 which is applicable to the present case is that given by Rutherford^ 

 and Child. 2 



Expressed in electrostatic units the mobility of an ion is given by : 

 32.7i.dK[ 



K = 



9V2 



cm. a second per 300 volts a cm 



(1) 



where i is the current between CD and GH in e.s.u. per square cm. 

 cross section, d the distance in cm. between GH and CD and V the 

 potential difference between them in electrostatic units. 



Expressed in practical electromagnetic units: 

 3200.7r.dli 



^ ^ r\T2 ^"^- ^ second per volt a cm (2) 



where d is in cm., V is in volts, and i is in electrostatic units and is 

 the current per square cm. cross-section between CD and GH. 



As d was 1 cm. in the apparatus used by us the relation (2j re- 

 duces to: 



3200. TT.i 



K = ^V^ (3) 



From equation (3) it will be seen that for a selected pressure the 

 current i should be proportional to V". 



Table I. 

 Air 

 Pressure = 145 -35 atmospheres. 



^Rutherford, Phys. Rev., Vol. XIII (6), p. 321, 1901. 

 ^Child, Phys. Rev., Vol. XII (3) p. 137, 1901. 



