11] IONIZATION THEORY OF GASES 59 
the ionization is not uniform, on account of the absorption of the 
radiation in the gas, the current does not decrease directly as the 
pressure until the pressure is reduced so far that the ionization 
is sensibly uniform. Consider the variation with pressure of the 
saturation current 7 between two large parallel plates, one of which 
is covered with a uniform layer of active matter. 
Let X, =absorption constant of the radiation in the gas for 
unit pressure. 
For a pressure p, the intensity J at any point « is given by 
ili 
5 a e Put The saturation current 7 1s thus proportional to 
0 
d rd T 
| pldx = | (OGL, Ce seh) (1 — ep), 
0 10 Ay 
If r be the ratio of the saturation currents for the pressures p, 
and pz 
i e pid 
—— 
1 — e-PAvd 
The ratio is thus dependent on the distance d between the 
plates and the absorption of the radiation by the gas. 
The difference in the shape of the pressure-current curves? is 
well illustrated in Fig. 8, where curves are given for hydrogen, air, 
and carbonic acid for plates 3°5 cms. apart. 
For the purpose of comparison, the current at atmospheric 
pressure and temperature in each case is taken as unity. The 
actual value of the current was greatest in carbonic acid and 
least in hydrogen. In hydrogen, where the absorption is small, 
the current over the whole range is nearly proportional to the 
pressure. In carbonic acid, where the absorption is large, the 
current diminishes at first slowly with the pressure, but is nearly 
proportional to it below the pressure of 235 mms. of mercury. 
The curve for air occupies an intermediate position. 
In cases where the distance between the plates is large, the 
saturation current will remain constant with diminution of pres- 
sure until the absorption is so reduced that the radiation reaches 
the other plate. 
1 Rutherford, Phil. Mag. Jan. 1899. 
