142 



S. E. PIERCE. 



same intensity of rays in both cases, when the ionisation 

 in the unlined cylinder is being measured, the bottom plate 

 of the lining to be afterwards used is placed as a screen 

 between the radium bromide and the lead cylinder. The 

 ratio of the two ionisation currents, taken in this way, 

 corrected for the differences in volume of the air and for 

 the natural leak in the chambers, gives the value of the fed 

 of the material of the lining relative to that of lead. 



Values of the product fed have been obtained by this 

 method for the metals tin, copper, zinc, iron, and aluminium 

 and also for a card cylinder. The source of y rays was 5 

 mg. of radium bromide placed on the axis of the lead 

 cylinder 20 cm. below its base. Experiments were first 

 made to find out whether varying the distance of the 

 radium from the cylinder along its axis had any effect. No 

 difference was found except when the radium was screened 

 with thick screens and was put close to the base of the 

 cylinder. In this case an increase in the observed relative 

 kd of the metal was found. Now when the radium is close 



Fig. 2. 

















R) 



















2^ 







Y — 1 1 h 



— ± — h ' 





- + -r- -)- \ 







^ 















1 













Z 3 4 



Thickness of Screen. 



