11] IONIZATION THEORY OF GASES 65 
dV 
«=0. By making the ionization very mtense, the value of Fi 
can be made extremely small. 

Putting A=0, 
Ve i 87rt,v 
da = NOUR, 
This gives the potential gradient between the plates for differ- 
ent values of «. 
Integrating between the limits 0 and d, 


‘ O Vite x3 
or t= a9 as WKGe 
If 7, is the value of the current when the electric field 1s 
reversed, and K, the velocity of the negative ion, 
: Ohya 

and , ies 
The current in the two directions is thus directly proportional 
to the velocities of the positive and negative ions. The current 
should vary directly as the square of the potential difference 
applied, and inversely as the cube of the distance between the 
plates. 
The theoretical condition of surface ionization cannot be fulfilled 
by the ionization due to active substances, as the ionization extends 
some centimetres from the active plate. If, however, the distance 
between the plates is large compared with the distance over which 
the ionization extends, the results will be in rough agreement with 
the theory. Using an active preparation of radium, the writer has 
made’ some experiments on the variation of current with voltage 
between parallel plates distant about 10 cms. from each other. 
1 Phil. Mag. Aug. 1901. 
g g 
