hence, when λd is small, i.e. when the ionization between the plates is nearly constant,
The current is thus proportional to the distance between the plates. When λd is large, the saturation current i₀ is equal to q₀e´/λ, and is independent of further increase in the value of d. In such a case the radiation is completely absorbed in producing ions between the plates, and
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