Page:Radio-activity.djvu/159

 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). This

++-+        | ++-+-+
 * 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}   |

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 negative charge communicated to P by a definite amount. A few of the numbers are given below, and expressed in terms of tin as unity.

+-+-+ +-+-+ +-+-+
 * | Thickness |
 * Substance | Tin = 1  |
 * Lead     |    ·745   |
 * Gold     |    ·83    |
 * Platinum |    ·84    |
 * Silver   |   1       |
 * Steel    |   1·29    |
 * Aluminium |  1·56    |
 * Water    |   1·66    |
 * Paraffin |   1·69    |

The thickness required to stop a given proportion of the β rays thus decreases with the density, but not nearly so fast as the