﻿■11G2 



Prof. S. Rnss and Mr. L. H. Clark on a 



compensates this so that the time for a balance does not vary 

 appreciably over a considerable range of intensity. This is 



Fig. 3. 



200 



INTENSITY. 



300 Oiv/min. 



well shown by the curves of fig. 4, which were obtained with 

 the model of fig. 1. The position of the gold leaf was read 

 at 3-second intervals from the moment the excitation of the 

 Coolidge bulb was begun. The ultimate steady deflexions 

 were 2T2, 292, and 47\5 respectively, yet the time taken to 

 reach these values was approximately 30 seconds in each 

 case. 



Saturation of the ionized air in is of course essential if 

 the method is to be used for the quantitative measurement 

 of the intensity of the b, j am of X-rays. With 200 volts on 

 C, it was found that saturation was obtained provided that 

 the radium was in such .a position that the leaf balanced at a 

 deflexion corresponding to a voltage on the leaf of about 

 120 ; this left 80 volts for the ionization current. If the 

 radium is removed to a greater distance the leaf deflects 

 more, but the corresponding rise in voltage now ieaves 

 insufficient for saturation. One result of this is that a 

 spurious steadiness obtains in the balance ; when saturation 



