124 Prof. Thomson, On the absence, etc. 



On the absence of excited radio-activity due to temporary 

 exposure to <y rays. By J. J. Thomson, M.A., F.R.S., Cavendish 

 Professor of Experimental Physics. 



[Head 30 January 1905.] 



Experiments were made to see if the radiation given out by 

 metals could be temporarily increased by exposure to the radia- 

 tion from radium. The method used was to measure the saturation 

 current inside a closed metallic vessel, then to place a sealed glass 

 tube containing 30 mg. of radium bromide inside the vessel and 

 leave it there for times varying from one hour to ten days ; the 

 radium was then removed and as soon as possible afterwards the 

 saturation leak again measured ; experiments were made with 

 vessels made of lead, brass, tin, but no increase in the saturation 

 current attributable to exposure to the radium was ever detected. 

 The measurement of the saturation current took at least five 

 minutes after the removal of the radium, so that a very short-lived 

 increase might escape detection by this method. This point is 

 dealt with in the following paper by Professor Bumstead. 



