
58 IONIZATION THEORY OF GASES 
a case the radiation is completely absorbed in producing ions be- 
tween the plates, and - —]—¢ 4, 
0 
For example, in the case of a thin layer of uranium oxide spread 
over a large plate, the ionization is mostly produced by rays the 
intensity of which is reduced to half value in passing through 
4-3 mums. of air;2.e. the value of X is 16. The following table is an 
example of the variation of ¢ with the distance between the plates. 
Distance Saturation Current 
2°5 mms. 32 
5 x 55 
Coe 72 
10 53 85 
NAGY 96 
15 55 100 
Thus the increase of current for equal increments of distance 
between the plates decreases rapidly with the distance traversed by 
the radiation. 
The distance of 15 mms. was not sufficient to completely absorb 
all the radiation, so that the current had not reached its limiting 
value. 
When more than one type of radiation is present, the saturation 
current between parallel plates is given by 
1=A (1 —-&%)4+ A, (1 —e) + &e. 
where A, A, are constants and 2X, A, the absorption constants of 
the radiations in the gas. 
Since the radiations are unequally absorbed in different gases, 
the variation of current with distance depends on the nature of the 
gas between the plates. 
44. Variation of the current with pressure. The rate 
of production of ions by the radiations from active substances is 
directly proportional to the pressure of the gas. The absorption of 
the radiation in the gas also varies directly as the pressure. The 
latter result necessarily follows if the energy required to produce 
an ion is independent of the pressure. 
In cases where the ionization is uniform between two parallel 
plates, the current will vary directly as the pressure; when however 
