572 Sir E. Rutherford on the Mass of the 



the total absorption in the path of the rajs was adjusted to- 

 9'4 cm. of air. The zinc-sniphide screen was placed close to 

 the aluminium absorbers, and the microscope so adjusted that,. 

 on exciting the magnetic field in one direction, the a particles 

 fell over the whole surface of the screen viewed by the- 

 microscope, and by reversing the field the scintillations were- 

 confined to the lower half of the field of view. The ratio of 

 the number of scintillations per minute, usually 4 or 5 to 1,. 

 was determined for the two fields, and this gave a measure 

 of the amount of deflexion of the rays. This ratio was com- 

 pared directly with that found for the thorium particles of 

 range of 8*6 cm. For this purpose a much weaker source, of 

 a rays was obtained by dipping a nickel plate for a few 

 seconds in a more dilute solution of radiothorium. The 

 experimental arrangement was identical with that described' 

 above, except that the absorption in the path of the a rays was 

 reduced to 5*4 cm. of air. This stopped the a particles of range 

 5*0 cm. from thorium C, while those of range 8*6 cm. gave- 

 bright scintillations on the screen. In all cases it was found 

 that the long-range particles were less deflected than those 

 of range 8'6 cm. By determining the value of the ratio for 

 different strengths of magnetic field, the relative magnetic 

 'deflexion of the two types of rays could be directly compared. 

 The different determinations made in this way varied between 

 1*08 and 1*12, with an average value of 1*10. 



In order to confirm these results, the deflexions were com- 

 pared by a more direct method, identical in principle witb 

 that employed previously to measure the deflexion of the 

 swift H atoms set in motion by impact with a particles (Phil. 

 Mag. xxxvii. p. 563 (1919)). The rays from the source placed 

 behind a horizontal slit of width 1 mm. passed through another 

 slit of equal width placed midway between the first slit and the 

 zinc-sulphide screen. The distance between the source and 

 screen was 8*3 cm. With the magnetic fields employed, the 

 band of scintillation observed on the screen, due to « particles 

 of range 8' 6 cm. from a strong source of thorium 0, was 

 displaced 5'7 mm. by reversal of the field. The amount of 

 deflexion of the long-range particles by reversing a current 

 through the electromagnet of 6 amps, was directly compared 

 with the deflexion due to the a rays of range 8' 6 cm. from 

 thorium C under similar conditions. For a current of 

 5 amp., the deflexion of the pencil of rays, range 8' 6 cm., 

 was "965 of the pencil of long-range particles with a current 

 of 6 amp., giving a field 1*12 that for 5 amp. For equal fields > 



