1] IONIZATION THEORY OF GASES 35 
Now the greater the value of V required to obtain a given 
value of p (supposed small compared with unity), the greater the 
‘potential required to produce saturation. 
It thus follows from the equation that : 
(1) For a given intensity of radiation, the saturation P.D. 
increases with the distance between the plates. In the equation, 
for small values of p, V varies as /?. This is found to be the case 
for uniform ionization, but it only holds approximately for non- 
uniform ionization. 
(2) Fora given distance between the plates, the saturation 
P.D.1s greater, the greater the intensity of ionization between the 
plates. This is found to be the case for the ionization produced 
by radio-active substances. With a very active substance like 
radium, the ionization produced is so intense that very large 
voltages are required to produce approximate saturation. On the 
other hand, only a fraction of a volt per cm. 1s necessary to produce 
saturation in a gas where the ionization is very slight, for example, 
in the case of the natural ionization observed in a closed vessel, 
where no radio-active substances are present. 
For a given intensity of radiation, the saturation P.D. decreases 
rapidly with the lowering of the pressure of the gas. This is due 
to two causes operating in the same direction, viz. a decrease in 
the intensity of the ionization and an increase in the velocity of 
the ions. The ionization varies directly as the pressure, while the 
velocity varies inversely as the pressure. This will obviously have 
the effect of causing more rapid saturation, since the rate of 
recombination is slower and the time taken for the ions to travel 
between the electrodes is less. 
The saturation curves observed for the gases hydrogen and 
carbon dioxide! are very similar in shape to those obtained for air. 
For a given intensity of radiation, saturation is more readily 
obtained in hydrogen than in air, since the ionization is less than 
in air while the velocity of the ions is greater. Carbon dioxide on 
the other hand requires a greater P.D. to produce saturation than 
does air, since the ionization 1s more intense and the velocity of 
the ions less than in air. 
1 Rutherford, Phil. Mag. Jan. 1899. 
