of the α rays for different thicknesses of matter, an apparatus similar to that shown in Fig. 17, p. 98, was employed48. A thin layer of the active material was spread uniformly over an area of about 30 sq. cms., and the saturation current observed between two plates 3·5 cms. apart. With a thin layer49 of active material, the ionization between the plates is due almost entirely to the α rays. The ionization due to the β and γ rays is generally less than 1% of the total.
The following table shows the variation of the saturation current between the plates due to the α rays from radium and polonium, with successive layers of aluminium foil interposed, each ·00034 cm. in thickness. In order to get rid of the ionization due to the β rays from radium, the radium chloride employed was dissolved in water and evaporated. This renders the active compound, for the time, nearly free from β rays.
The initial current with 1 layer of aluminium over the active material is taken as 100. It will be observed that the current due
| Polonium. | Radium. | ||||
|---|---|---|---|---|---|
| Layers of aluminium | Current | Ratio of decrease for each layer | Layers of aluminium | Current | Ratio of decrease for each layer |
| 0 | 100 | 0 | 100 | ||
| ·41 | ·48 | ||||
| 1 | 41 | 1 | 48 | ||
| ·31 | ·48 | ||||
| 2 | 12·6 | 2 | 23 | ||
| ·17 | ·60 | ||||
| 3 | 2·1 | 3 | 13·6 | ||
| ·067 | ·47 | ||||
| 4 | ·14 | 4 | 6·4 | ||
| ·39 | |||||
| 5 | 0 | 5 | 2·5 | ||
| ·36 | |||||
| 6 | ·9 | ||||
| 7 | 0 |
to the radium rays decreases very nearly