A brief account is given here of some investigations made by the writer on the properties of the α rays from radium—investigations which were not completed in time for the results to be incorporated in the text.
The experiments were undertaken primarily with a view of determining accurately the value of e/m of the α particle from radium, in order to settle definitely whether or not it is an atom of helium. In the previous experiments of the writer, Becquerel, and Des Coudres, on this subject (sections 89, 90, and 91), a thick layer of radium in radio-active equilibrium has been used as a source of α rays. Bragg (section 103) has shown that the rays emitted from radium under such conditions are complex, and consist of particles projected over a considerable range of velocity. In order to obtain a homogeneous pencil of rays it is necessary to use a very thin layer of a simple radio-active substance as a source of rays. In the experiments that follow, this condition was fulfilled by using a fine wire which was made active by exposure for several hours in the presence of a large quantity of radium emanation. By charging the wire negatively the active deposit was concentrated upon the wire, which was made intensely active. The active deposit initially contains radium A, B, and C. The activity of radium A practically disappears in about fifteen minutes, and the α radiation is then due entirely to the single product radium C, since radium B is a rayless product. The activity of radium C decreases to about 15 per cent. of its initial value after two hours.
Magnetic deflection of the α rays. The photographic method was employed to determine the deviation of the pencil of rays in a magnetic field. The experimental arrangement is shown in Fig. 106. The rays from the active wire, which was placed in a slot, passed through a narrow slit and fell normally on a photographic plate, placed at a known distance above the slit. The apparatus was enclosed in a brass tube which could be exhausted rapidly to a low pressure by means of a Fleuss pump. The apparatus was placed in a strong uniform magnetic field parallel to the plane of the slit. The magnetic field was reversed every ten minutes, so that on developing the plate two narrow bands were observed, the distance between which represented twice the deviation from the normal of the pencil of rays by the magnetic field. The width of the band was found to be the same whether the magnetic field was applied or not, showing that the pencil of rays was homogeneous and consisted of α particles projected with the same velocity.