ITS Prof. E. Rutherford cm the Magnetic and 



that of the cathode rajs, so that the radiations must consist of 

 positively charged hodies projected with great velocity. In a 

 previous paper* I have given an account of the indirect experi- 

 mental evidence in support of the view that the a 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 a rays, compared with that of 

 the cathode rays in a vacuum-tube, may be judged from the 

 fact that the a rays, projected at right angles to a magnetic 

 field of strength 10,000 c.G.s, units, describe the arc of a 

 circle of radius about 39 cms., 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 expe- 

 riment 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 f of activity 

 19,000, and using an electroscope instead of an electrometer, 

 I have been able to extend the experiments, and to show that 

 the a rays are all deviated bv a strong- magnetic field. 



Magnetic Deviation of the Rays. 



Fig. 1 A shows the general arrangement of the experiment. 

 The rays from a thin layer of radium passed upwards through 

 a number of narrow slits, Gr, in parallel, and then through a 

 thin layer of aluminium foil "00034 cm. thick into the 

 testing vessel V. The ionization produced by the rays in 

 the testing vessel was measured by the rate of movement of 

 the leaves of a gold-leaf electroscope B. This was arranged 

 after the manner of C. T. R. Wilson in his experiments on 



* Phil. Magr. Jan. 1903, p. 113. It was long ago suggested by Strutt 

 (Phil. Trans. Roy. Soc. 1900) that the a rays consist of positively charged 

 particles projected from the active substance. The same idea has lately 

 been advanced by Sir Wm. Crookes (Proc. Roy. Soc. 1900). 



f The sample of radium of greater activity than that usually sold was 

 obtained from the Societe Cent rale de Produits Chimiques, through the 

 kinduess of M. P. Curie. 



