Absorption of the Different Types of Beta Rays. 127 



is given of some experiments in which the secondary radia- 

 tion both from the back and front of metal plates was studied 

 when these were traversed by 7 rays alone, and also when 

 pencils of ft rays of different types were allowed to fall on 

 them. 



In differentiating the effects due to the various types of 

 rays, the action of each was ascertained by deflecting pencils 

 of each type into an ionization-chamber away from the others 

 by means of a magnetic field. 



II. Apparatus. 



The arrangement adopted is shown in fig. 1. The re- 

 ceptacle for holding the radium was a lead cylinder A with 

 walls 4 cms. thick. One end of the cylinder was covered by 



Fisr. 1. 



To E/ectrometer 



a brass plate 2'5 mms. 1 thick, in the centre of which was 

 a hole 4*5 mms. in diameter. The capsule holding the 

 radium was held close against this plate in such a position 

 that the rays from the radium on issuing, passed between the 

 poles of a powerful electromagnet. Immediately beneath 

 the pole-pieces of the electromagnet was the ionization- 

 chamber B. It also was made of lead in the form of a 

 cylinder 6*7 cms. long, with walls 4*6 cms. in thickness, the 

 ionization-chamber proper being 4*7 cms. in diameter. At 

 the top and bottom were brass rings to hold different 

 thicknesses of selected absorbing materials over the ends of 

 the chamber. 



A properly screened and insulated electrode was inserted 



