In this case also, a diffuse impression is observed on the plate, giving, so to speak, a continuous spectrum of the rays and showing that the radiation is composed of rays of widely different curvatures. [Fig. 24] shows a photograph of this kind obtained by Becquerel, with strips of paper, aluminium, and platinum placed on the plate.
Fig. 24.
If screens of various thickness are placed on the plate, it is observed that the plate is not appreciably affected within a certain distance from the active matter, and that this distance increases with the thickness of the screen. This distance is obviously equal to twice the radius of curvature of the path of the rays, which are just able to produce an impression through the screen.
These experiments show very clearly that the most deviable rays are those most readily absorbed by matter. By observations of this kind Becquerel has determined approximately the inferior limit of the value of HR for rays which are transmitted through different thicknesses of matter.
The results are given in the table below:
| Substance | Thickness in mms. | Inferior limit of HR for transmitted rays |
|---|---|---|
| Black paper | 0·065 | 650 |
| Aluminium | 0·010 | 350 |
| “ | 0·100 | 1000 |
| ” | 0·200 | 1480 |
| Mica | 0·025 | 520 |
| Glass | 0·155 | 1130 |
| Platinum | 0·030 | 1310 |
| Copper | 0·085 | 1740 |
| Lead | 0·130 | 2610 |
If e/m is a constant for all the rays, the value of HR is proportional to the velocity of the rays, and it follows from the table that the velocity of the rays which just produce an effect on the plate through ·13 mms. of lead is about 7 times that of the rays which just produce an impression through ·01 mm. of aluminium. It will be shown, however, in [section 82], that e/m is not a constant for all speeds, but decreases with increase of velocity of the rays. The difference in velocity between the rays is in consequence not as great as this calculation would indicate. On examination of the rays from uranium, Becquerel found that the radiation is not as complex as that from radium, but consists wholly of rays for which the value of HR is about 2000.
78. Examination of the β rays by the electric method. The presence of easily deviable rays given off from an active substance can most readily be shown by the photographic method, but it is necessary, in addition, to show that the penetrating rays which produce the ionization in the gas are the same as those which cause the photographic action. This can be conveniently tested in an arrangement similar to that shown in [Fig. 25].
