﻿the a particles expelled from Radium and Actinium. 351 



of the apparatus. The vessel was connected to a mercury 

 pump and exhausted to a low vacuum. If necessary, the 

 exhaustion was completed by means of a side tube filled with 

 cocoanut charcoal and immersed in liquid air. 



Ffe. 1. 



The plates A and B were placed close together for several 

 reasons. In the first place, a strong electric field could be 

 produced between the plates for a comparatively small voltage. 

 The greatest P.D. necessary in the experiments was about 

 500 volts. Since the plates were about one-fifth of a milli- 

 metre apart, this voltage produced an electric field between 

 the plates corresponding to 25000 volts per cm. One ad- 

 vantage of the arrangement lies in the fact that, provided the 

 P.D. is below about 350 volts, there is no danger of a dis- 

 charge between the plates, even if there is not a good vacuum. 

 This is particularly convenient where it is found necessary to 

 expose the photographic plate to a weak source of radiation 



