Relative velocities of the alpha-particles emitted by certain radioactive elements

The ordinary method of determining the velocity of alpha-particles depends on the measurement of the deflection of a beam of the rays in a magnetic field of known strength. The difficult measurement of the strength of the magnetic field gives rise to the largest source of error in the method. This source of error can be avoided in a direct comparison of the velocity of the alpha-particles with the velocity of the particles emitted by radium C′, which has been measured accurately by Rutherford and Robinson, and by Briggs, with excellent agreement. The method depends on the use of a combined source, consisting of the two radioactive elements to be compared. The radius of curvature of the path of the alpha-particles from each of the elements is measured in the usual way. Then the ratio of the velocities of the alpha-particles is equal to the ratio of the radii of curvature of their paths (after applying a small correction for the increase in mass at high velocities). In this manner the velocities of the rays from radium C′, thorium C and C′, and radium F, have been compared. Using Briggs’ value of the velocity of the particles emitted by radium C′, the velocities of the others are calculated from the ratios thus obtained. II.Apparatus The apparatus is similar to that used by Briggs and described in his paper. Briefly, it consists of a rigid frame, supporting the wire source at one end, a partition with three slits in it parallel to the wire source in the centre, and a photographic plate at the other end. The distances from source to slits and from slits to plate were both about 10 cm., but varied somewhat with different measurements. This frame fits in a brass box between the poles of an electric magnet. The box was closed by a ground glass plate, which, when greased, permitted evacuation to 10-4 mm. A windlass raised a shutter which covered the slits while the chamber was being pumped out. The magnet and box were the ones used by Briggs.