Page:Radio-activity.djvu/169

 in the two directions, on developing the plate two fine diverging lines are found traced on the plate. The distance between these lines at any point is a measure of twice the average deviation at that point, corresponding to the value of the magnetic field. By measuring the distance between the trajectories at various points, Becquerel found that the radius of curvature of the path of the rays increased with the distance from the slit. The product H[Greek: rho] of the strength of the field and the radius of curvature of the path of the rays is shown in the following table.

Distance in mms. from the slit  H[Greek: rho] 1           2·91 × 10^5       3            2·99   "       5            3·06   "       7            3·15   "       8            3·27   "       9            3·41   "

The writer (loc. cit.) showed that the maximum value of H[Greek: rho] for complete deviation of the [Greek: alpha] rays was 390,000. The results are thus in good agreement. Since H[Greek: rho] = (m/e)V these results show that the values either of V or of e/m for the projected particles vary at different distances from the source. Becquerel considered that the rays were homogeneous, and, in order to explain the results, has suggested that the charge on the projected particles may gradually decrease with the distance traversed, so that the radius of curvature of the path steadily increases with the distance from the source. It, however, seems more probable that the rays consist of particles projected with different velocities, and that the slower particles are more quickly absorbed in the gas. In consequence of this, only the swifter particles are present some distance from the source.

This conclusion is borne out by some recent experiments of Bragg and Kleeman on the nature of the absorption of [Greek: alpha] particles by matter, which are discussed in more detail in sections 103 and 104. They found that the [Greek: alpha] particles from a thick layer of radium are complex, and have a wide range of penetrating power and presumably of velocity. This is due to the fact that the [Greek: alpha] particles