The charge and nature of the α-particle

Abstract

In the previous paper, we have determined the number of α-particles expelled per second per gramme of radium by a direct counting method. Knowing this number, the charge carried by each particle can be determined by measuring the total charge carried by the α-particles expelled per second from a known quantity of radium. Since radium C was used as a source of radiation in the counting experiments, it was thought desirable to determine directly the charge carried by the α-particles expelled from this substance. In a paper some years ago,* one of us has investigated the experimental conditions necessary for an accurate determination of the total charge carried by the α-rays, and has measured the charge carried by the α-particles expelled from a thin film of radium itself. In the present experiments the same general method has been used, with certain modifications, rendered necessary by the choice of radium C as a source of α-rays. The experimental arrangement is clearly seen in fig. 1. A cylindrical glass tube HH of diameter 4 cm. is closed at the ends by ground-glass stoppers D, E. The source of radiation R is attached to the lower stopper E. The radiation from this passes into the testing chamber, which is rigidly attached to the stopper D by means of an ebonite tube F. The testing chamber consists of two parallel plates A and B about 2 mm. apart. A circular opening, 1.92 cm. in diameter, cut in the brass plate B, is covered by a sheet of thin aluminium foil. The upper chamber AC consists of a shallow brass vessel of aperture 2.5 cm., the lower surface of which is covered also with a sheet of aluminium foil.† The plate B is connected through a side glass tube to one terminal of a battery, the other pole of which is earthed. The chamber AC, which is insulated from the plate B, is connected with one pair of quadrants of a Dolezalek electrometer, the other of which is earthed. The whole apparatus is placed between the poles NS of a large electromagnet marked by the dotted lines in the figure, so that the α-rays in their passage from the source R to the testing chamber pass through a strong magnetic field.