32 



THE THEOKY OF IONIZATION OF GASES 



in millimetres of mercury, for which the current is 

 a maximum. 



TABLE X. 



The agreement between the figures in the last column 

 shows that P is proportional to the force E/. 



When E is expressed in volts and I in centimetres, the 



value of the pressure for which the current is a maximum 



v 

 is P=^r^. In these experiments the potentials and the 



Ota 



distances between the plates were small, so that there 

 can be no appreciable effect due to ionization by the action 

 of positive ions. There may be a small variation of the 

 number of ions set free from the plate due to the varia- 

 tion in the pressure, but the principal variations must 

 be due to the ionization produced by the motion of the 

 negative ions. The pressure for which the current is a 

 maximum should therefore be practically the same as 

 that at which a becomes a maximum. This pressure can 



be deduced by differentiating the formula, a=pf(} 



