tin foil were placed over the radium. The second table gives the ratio I / I ₀ where I ₀ is the rate of discharge observed before the absorbing screen is introduced. The mean value of the absorption constant λ was deduced from the equation
where d is the thickness of matter traversed.
The values included in the brackets have not the same accuracy as the others. There is thus a wide difference in penetrating power of the β particles emitted from radium, and some of them are very readily absorbed.
When a lead screen 3 mms. thick was placed over the radium—a thickness sufficient to absorb all the readily deflectable β rays—a small negative charge was still given to the plate, corresponding to ·29 per cent. of the maximum. This is a very much smaller value than was observed by Paschen (see Fig. 30).
| Thickness of Tin in mms. | I / I ₀ | λ |
|---|---|---|
| 0·00834 | ·869 | 175 |
| 0·0166 | ·802 | 132·5 |
| 0·0421 | ·653 | 101·5 |
| 0·0818 | ·466 | 93·5 |
| 0·124 | ·359 | 82·5 |
| 0·166 | ·289 | 74·9 |
| 0·205 | ·230 | 71·5 |
| 0·270 | ·170 | 65·4 |
| 0·518 | ·065 } | 53} |
| 0·789 | ·031 } | 44} |
| 1·585 | ·0059} | 32} |
| 2·16 | ·0043} | 25} |
This difference may, in part, be due to the fact that, in Paschen’s experiments, a large proportion of the slow velocity electrons were absorbed in the glass tube of ·5 mm. thickness containing the radium.
Seitz also determined the relative thickness, compared with tin, of different substances which reduced the