BerAiuerd and Rbntgen Rays in a Magnetic Field. 



75 



" On tlie Behaviour of the Becquerel and Rontgen Eays in a 

 Magnetic Field." By the Hon. R. J. Strutt, B.A., 

 Scholar of Trinity College, Cambridge. Communicated 

 by LOED Rayleigh, F.R.S. Received January 9, — Read 

 January 18, 1900. 



In the current number of Wiedeman's ' Annalen,' an experiment is 

 described by Giesel showing that the Becquerel rays are deflected in a 

 magnetic field. This result is of great interest, on account of the light 

 which it throws on the nature of the rays. Up to the present, the 

 ' evidence has tended to show that the Becquerel rays were of the same 

 nature as the Rontgen rays, both being capable of penetrating thin 

 metal sheets, of affecting a photographic plate, and of producing ionisa- 

 tion in the surrounding air. Neither could be refracted or reflected ; 

 and so far as has yet appeared, neither could be polarised. 



These facts seemed to form a fairly strong body of evidence that 

 the two kinds of radiation were essentially similar. But the announce- 

 ment of the magnetic deflectibility of the Becquerel rays seems to 

 throw doubt on this conclusion. The Rontgen rays, so far as is 

 known, are quite unafl'ected by magnetic force. Under these circum- 

 stances it seemed worth while to make a new attempt to discover such 

 an effect on the Rontgen rays. This attempt I have carried out. It 

 will be best to say at once that the result is negative. 



A focus tube was employed as the source of radiation. It was 

 placed at a distance of about 35 cm. from a powerful electro-magnet, 

 and in such a position that the cathode rays in the tube were parallel 

 to the magnetic force due to the magnet. The line joining the oblique 

 anti-cathode to the centre of the magnetic field lay in the plane of the 

 anti-cathode. 



A short distance in front of the magnet a wire was placed at right 

 angles to the direction of the rays, and in the plane of the anti-cathode. 

 It was thus at an angle of about 50° to the magnetic force — the same 

 angle as that between the axis of the cathode stream and the anti- 

 cathode. This wire was used to cast a shadow on a photographic plate 

 placed at a distance of 65 cm. on the other side of the magnet. 



An exposure was first made with the magnetic force in one direction. 

 The exposure was then stopped, the field reversed, and another ex- 

 posure given of course without shifting the plate. If then the rays 

 had been appreciably deflected, the photograph should have shown two 

 shadows, either overlapping, or altogether separated. 



The rays casting the shadow were those emitted at a grazing 

 angle from the anti-cathode. The reason for using these very 

 oblique rays was that owing to the foreshortening of the anti-cathode, 

 the source was virtually narrower than it would have been, had raj^s 



VOL. LXVI. H 



