Reprinted in Classical Scientific Papers. Physics. Facsimile reproductions of famous scientific papers. Mills and Boon Limited, London (1964) 31.
Radium gives out three distinct types of radiation: (1) The rays, which are very easily absorbed by thin layers of matter, and which give rise to the greater portion of the ionization of the gas observed under the usual experimental conditions. (2) The rays, which consist of negatively charged particles projected with high velocity and which are similar in all respects to cathode rays produced in a vacuum-tube.
(3) The rays, which are non-deviable by a magnetic field, and which are of a very penetrating character. These rays differ very widely in their power of penetrating matter. The following approximate numbers, which show the thickness of aluminium traversed before the intensity is reduced to one-half, illustrate this difference. center tabularcc Radiation & Thickness of Aluminium rays & to 1cm.0005 cmhfill rays & to 1cm .05 cmhfill rays & to 1cm 8 cmhfill tabular center In this paper an account will be given of some experiments which show that the rays are deviable by a strong magnetic electric field. The deviation is in the opposite sense to that of the cathode rays, so that the radiations must consist of positively charged bodies projected with great velocity. In a previous
paper ( Phil. Mag. (Jan 1903), 113) I have given an account of the indirect experimental evidence in support of the view that the rays consist of projected charged particles. Preliminary experiments undertaken to settle this question during the past two years gave negative results. The magnetic deviation, even in a strong magnetic field, is so small that very special methods are necessary to detect and measure it. The smallness of the magnetic deviation
of the rays, compared with that of the cathode rays in a vacuum-tube, may be judged from the fact that the rays projected at right angles to a magnetic field of strength 10000 c.g.s. units, describe the arc of a circle of radius about 39 cm, while under the same conditions the cathode rays would describe a circle of radius about .01 cm. In the early experiments radium of activity 1000 was used, but this did
not give out strong enough rays to push the experiment to the necessary limit. The general method employed was to pass the rays through narrow slits and to observe whether the rate of discharge, due to the issuing rays, was altered by the application of a magnetic field. When, however, the rays were sent through sufficiently narrow slits to detect a small deviation of the rays, the rate of discharge of the issuing rays became too small to measure, even with a sensitive electrometer. I have
recently obtained a sample of radium of activity 19000, and using an electroscope instead of an electrometer, I have been able to extend the experiments, and to show that the rays are all deviated by a strong magnetic field.
Related references See also F. Soddy, Proc. Chem. Soc. (1902);
E. Rutherford and Grier, Phil. Mag. (Sept 1902);
Observation that rays are the flow of doubly positive charged particles.