radium is, however, readily shown by the photographic method. We have already seen (section 93) that very low velocity electrons accompany the [Greek: alpha] rays from radium or radio-tellurium spread on a metal plate. These electrons are probably liberated when the [Greek: alpha] rays escape from or impinge upon matter, and the number emitted depends upon the kind of matter used as a screen. The differences shown in the above table when the screens were interchanged are explained simply in this way.
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| | Thickness | Current |
| Screens employed | in mms. | observed |
|-+-++
| Aluminium | 0·01 | |
| Cardboard | 0·005 | 17·9 |
| | | |
| Cardboard | 0·005 | |
| Aluminium | 0·01 | 6·7 |
| | | |
| Aluminium | 0·01 | |
| Tin | 0·005 | 150 |
| | | |
| Tin | 0·005 | |
| Aluminium | 0·01 | 126 |
| | | |
| Tin | 0·005 | |
| Cardboard | 0·005 | 13·9 |
| | | |
| Cardboard | 0·005 | |
| Tin | 0·005 | 4·4 |
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Fig. 45.
111. Secondary rays produced by β and γ rays. An
examination of the amount and character of the secondary
radiation emitted by various substances, when exposed to the
