66 



APPLIED RADIOACTIVITY 



H is varied by the current exciting the pole pieces of the electromagnet. 

 The result is that electrons of the same velocity, though shot up at 

 different angles, will describe circles of radius R, and converge to a line 

 focus on the photographic plate. Groups of low-velocity electrons 



iffiN 



Vacuum pump 



' ' l> 



I j /' 

 y rays | I ( 



1 .',' 

 ll" 



>l" 

 I II 





//, 





^Z-Z^. /3rays 





^\ 





Photographic plate 



•::::,\\ 



TJ 



Fig. II-5. A form of beta-ray spectrograph. This shows how the magnetic field, 

 perpendicular to the plane of the paper, acts as a lens to focus the electrons. First 

 used by Chadwick in 1914. 



converge nearer to the source than the high-velocity groups. For low- 

 speed electrons the relative curvature of path is determined by Hev 

 = m v 2 /R. For the very high-speed electrons the relativity mass of 

 the electron must be used, so that 



HR = 



?)IqC 



v/c 



e Vl - v 2 /& 



where ra is the rest mass of the electron of charge e and velocity v, and c 

 is the velocity of light. 



The photographic images of the radiative source are lines lying 

 parallel to the linear source of the beta rays. The line images due to the 

 higher-velocity electrons are found farther from the slit. If radon had 

 been used as the beta-ray source, the developed photographic plate 

 would have shown a composite effect, a general background darkening 

 with superimposed sharp linear images. A photometric analysis of 

 such a plate is shown diagrammatically in Fig. II-6. The line images 

 appear as humps on the curve. These are homogeneous velocity groups 

 of emission. This curve indicates that theirs is a relatively small effect 

 as compared to the numbers producing the continuous electron spec- 

 trum. The latter produce the general background darkening of the 

 photographic plate and are represented by the area under the broken 

 curve. 



