2 ransf ovulations of Actinium C. 



819 



fact that when the a particles are near the end of their range, 

 they produce only very feeble scintillations which are difficult 

 to detect and count, especially when, as was actually the case, 

 a weak and decaying source was used. 



In the present experiments evidence of these long range 

 a particles was sought from observations of the ionization at 

 points beyond the range of the main pencil of a particles. 

 This method, although indirect, has the advantage that the 

 effects are mechanically recorded and do not depend on the 

 efficiency of the eye of the observer. 



The difficulty arises in practice owing to the small in- 

 tensity of the best actinium sources available, and the fact 

 that the effect to be measured is such a small fraction of the 

 whole ionization effect of the source. Further, the ioniz- 

 ation effect of the small portion of long range a particles 

 would ordinarily be masked by the relatively large ionization 



Fik 1. 



T 



J" 



M 



T> 



H 



~n 



E/1RTH 



r^TO BflTT£RV 



1 — 7 



TO ELECTROSCOPE 



I 

 I 

 I 



due to the /3 and 7 rays from the source. It was hoped that, 

 by placing the whole apparatus in a strong magnetic field, 

 the effect of the fi rays could be almost eliminated. 

 The arrangement is shown in fio-. 1. 



