By placing the photographic plate at different distances from the slit it was found that the rays, after entering the magnetic field, described the arc of a circle of radius ρ equal to 42·0 cms. The strength of field H was 9470 C.G.S. units, so that the value of Hρ for the α particles expelled from radium C is 398,000. This is in good agreement with the maximum values of Hρ, previously found for radium rays (see section 92).
The electric deviation of the rays from radium C has not yet been accurately measured, but an approximate determination of e / m for the α particles can be obtained by assuming that the heating effect of radium C is a measure of the kinetic energy of the α particles expelled from it. We have seen in section 246 that the heating effect of the radium C present in one gram of radium in radio-active equilibrium is 31 gram calories per hour, which corresponds to an