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 mms. of air, i.e. the value of λ is 1·6. The following table is an example of the variation of i with the distance between the plates.
| Distance | Saturation Current |
|---|---|
| 2·5 mms. | 32 |
| 5 „ | 55 |
| 7·5 „ | 72 |
| 10 „ | 85 |
| 12·5 „ | 96 |
| 15 „ | 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
where A, A1 are constants, and λ, λ1 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.