In a good vacuum, the magnitude of the current observed is a measure of the number of β particles absorbed by the upper plate[137]. The following table shows the results obtained when different thicknesses of tin foil were placed over the radium. The second table gives the ratio _I_/_I₀_ where _I₀_ 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 $$ \frac {I} {I₀} = e^{–λ d} $$ 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 _I_/_I₀_ λ Tin in mms. 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 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. Substance Thickness Tin = 1 Lead ·745 Gold ·83 Platinum ·84 Silver 1 Steel 1·29 Aluminium 1·56 Water 1·66 Paraffin 1·69