Page:Radio-activity.djvu/183

 The saturation current was measured between two parallel plates PP´ 3 cms. apart. The polonium A was placed in the metal box CC, and the rays from it, after passing through an opening in the lower plate P´, covered with a layer of thin foil T, ionized the gas between the plates. For a certain distance AT, of 4 cms. or more, no appreciable current was observed between P and P´. As the distance AT was diminished, the current increased in a very sudden manner, so that for a small variation of the distance AT there was a large increase of current. With still further decrease of distance the current increases in a more regular manner. The results are shown in the following table, where the screen T consisted of one and two layers of aluminium foil respectively. The current due to the rays, without the aluminium screen, is in each case taken as 100.

Fig. 34.

++—-+—-+—-++—-+ ++—-+—-+—-++—-+ ++—-+—-+—-++—-+
 * Distance AT in cms.        |3·5|2·5|1·9|1·45|0·5|
 * For 100 rays transmitted by one layer | 0 | 0 | 5 | 10 |25 |
 * For 100 rays transmitted by two layers| 0 | 0 | 0 | 0 |0·7|

The metallic screen thus cuts off a greater proportion of the rays the greater the distance of air which the radiations traverse. The effects are still more marked if the plates PP´ are close together. Results similar but not so marked are found if radium is substituted for the polonium.

It follows from these experiments that the ionization per unit volume, due to a large plate uniformly covered with the radio-active matter, falls off rapidly with the distance from the plate. At a distance of 10 cms. the [Greek: alpha] rays from uranium, thorium, or radium have been completely absorbed in the gas, and the small ionization then observed in the gas is due to the more penetrating [Greek: beta] and [Greek: gamma] rays. The relative amount of the ionization observed at