IONISATION CAUSED BY PENETRATING y RAYS, 



141 



Suitable gearing, worked by a handle, was arranged so that 

 the contact piece could be moved along the wire at a rate 

 which was easily varied. One end of the wire was earthed. 



In measuring the ionisation current, the electrode was 

 disconnected from earth and the handle turned so as to keep 

 the needle of the electrometer at its zero position. The 

 time was taken by a stop-watch from the instant at which 

 the electrode was disconnected to the instant at which the 

 electrometer needle began to move after the sliding contact 

 had moved through a definite difference of potential. The 

 reciprocal of this time gave a measure of the ionisation 

 current in arbitrary units. 



To obtain values of kd relative to that of lead in the case 

 of any metal, the ionisation in the lead chamber is compared 

 with that in the chamber lined with the given metal. In 

 this latter case the y rays pass through a greater thick- 

 ness of material than with the unlined cylinder before 

 emerging into the chamber, so that the intensity of the 

 rays is not the same in the two chambers. To ensure the 



Fig. 1. 



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+ 1 1 -t 





T3 tc 

 lie 



> 







s* 





\<4^-t- 







+ ). 





























Thickness of Screen. 



