In a similar way it may be shown that the energy W2 of the β rays reaching the surface is given by
where E2 and λ2 are the values for the β rays corresponding to E1 and λ1 for the α rays. Thus it follows that
E1 λ1W1
---- = ------
E2 λ2W2
λ1 and λ2 are difficult to determine directly for the radio-active substance itself, but it is probable that the ratio λ1/λ2 is not very different from the ratio for the absorption coefficients for another substance like aluminium. This follows from the general result that the absorption of both α and β rays is proportional to the density of the substance; for it has already been shown in the case of the β rays from uranium that the absorption of the rays in the radio-active material is about the same as for non-radio-active matter of the same density.
With a thick layer of uranium oxide spread over an area of 22 sq. cms., it was found that the saturation current between parallel plates 6·1 cms. apart, due to the α rays, was 12·7 times as great as the current due to the β rays. Since the α rays were entirely absorbed between the plates and the total ionization produced by the β rays is 154 times the value at the surface of the plates,
W1 total number of ions due to α rays
---- = ------------------------------------