﻿Irans formation of Radium A, B, and C. 



75 



necessity of determining this ratio for every /3-rav decay 

 curve, and the difficulties involved in obtaining it with suf- 

 ficient accuracy, seemed to make the /3-ray decay curve 

 untrustworthy for the purposes of analysis. It seemed worth 

 while, however, to determine approximately the maximum 

 value for this ratio, and at the same time to get an approximate 

 value for the ratio of the ionization produced by the /3 rays 

 to that produced by the a rays under certain definite conditions. 

 The conditions of the problem were such that the method 

 of measurement and the testing-vessel employed had to be 

 different from those which were found to give the best results 

 in the measurement of decay curves. The arrangement of 

 the apparatus is seen in fig. 1. The insulated plate M was 



Fig. 1. 



J 3i? 



To E fee fro meter 



K 



AL 



Active Wi 



ire 



CD 



EJMSMME^ 



To Earth 



connected to an electrometer. The plate N was of aluminium- 

 foil about 0-00029 cm. thick. The active wire was placed 

 beneath N and arranged so that it could be easily covered 

 with an aluminium cap P, about 0*005 cm. in thickness. This 

 was just sufficient to cut off all the a. rays. The ionization- 

 chamber between M and N was so small that the a. particles, 

 after passing through the foil N, would practically penetrate 

 to the side-walls in every direction. The difference between 

 the ionization produced by the a rays and that produced by 

 the /3 rays was so large that the constant-deflexion method 

 was not adapted for the measurements. The ionization 

 currents were therefore measured in the ordinary way by 

 means of an electrometer with suitable capacities in parallel. 

 Measurements were taken alternately with and without the 



