300 Danysz and Duane — Electric Charges of a- and j3- Rays. 



tions : The effect produced by the magnetic field H on the 

 secondary rays is to make them move along the surfaces of 

 circular cylinders, whose axes lie in the direction of the 

 magnetic force, and whose radii r are given by the well 

 known formula 



TT mv 

 tir = 



e 



Now none of the electrons starting from one sheet of aluminium 

 can reach the other, if the maximum diameter of these cylin- 

 ders corresponding to the maximum value of v is less than 

 the smallest distance between the sheets. To allow for the 



Fig. 3. 



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bulging of the sheets this distance may be taken to be about 

 •8 mm . The maximum vertical velocity is 37'4xl0 8 , and there 

 is no reason for supposing that the horizontal velocity is any 

 greater. Hence substituting r = '^ m and v = 3*74 XlO 8 , we 

 have 



H = 53 gauss 



This is not far from the value of the magnetic field at the end 

 of the sharp fall in the currents represented by curve 1 (iig. 3), 

 and we may conclude that the currents beyond the minimum 

 valve of this curve are due to the a- and /3-rays alone, the 

 secondary rays having been suppressed. 



Curve 2 (figure 3) represents currents measured with a sheet 

 of aluminium -04:8 mni thick covering the hole C. This sheet 

 stops the a-rays but lets most of the /3-rays through, and it is 

 interesting to note that there is no initial drop in the curve, 



