Alpha Rays from Polonium in Mercury Vapour. 297 



was slipped into the small tube that extended above the 

 main tube and through which the rod supporting A ex- 

 tended. A similar earthed guard-ring was put around the 



Fig. 1. 



TO ELECTROMETER 



outside of the same tube. These rings were split so as to 

 allow for the expansion when heated. They prevented any 

 leakage over the surface of the glass to the plate A. 



