m1] IONIZATION THEORY OF GASES 57 
energy dJ absorbed in producing ions in a layer of unit area and 
thickness dw at a distance # from the plane is given by 
dI =qw.dx 
@ 
Integrating, lose a = a+A, 
where A is a constant. 
Since J = J, when 2=0, A =log, /,, and 
w 
os 5 tn 
il = @ do Ge, 
I, 
0) 
where = aa a constant. 
» will be called the absorption constant of the gas for the 
particular kind of radiation considered. 
If q is the rate of production of ions at the surface of the 
plate, Tg, 
0 
Consider two parallel plates placed as in Fig. 1, one of which 1s 
covered with a uniform layer of radio-active matter. If the distance 
d between the plates is small compared with the dimensions of the 
plates, the ionization near the centre of the plates will be sensibly 
uniform over any plane parallel to the plates and lying between 
them. ‘The saturation current 7 per unit area is given by 
d 
1= | ge’ dx, where e’ is the charge on an ion, 
0 
d qe’ 
= qe [ede = 1 es); 
0 Xr 
when Ad is small, 7.e. when the ionization between the plates is 
nearly constant, 
t= qed. 
The current is thus proportional to the distance between the 
plates. When Ad is large, the saturation current 2, 1s equal to ae 
and is independent of further increase in the value of d. In such 
