M‘Ciutvanp— The Energy of Secondary Radiation. 13 
Apparatus AND Meruop or EXPERIMENT. 
The radium & was placed as shown in fig. 2. P is the lead plate which 
was the source of the secondary rays; and it could be placed in position as 
shown, or removed when desired. When it was removed, the primary § rays 
passed into an ionisation-vessel consisting of a brass tube A, 20 cm. long and 
7-5 cm. diameter, with an insulated brass rod B fixed along its axis and connected 
Cells 
Electrometer 
Electrometer. 
to an electrometer. ‘lhe end of the tube through which the radiation passes 
was covered with gold leaf. The tube was kept at a high potential, and the 
saturation-current to 6 measured in the usual way with the electrometer. The 
ionisation-vessel was then moved into the position A’ such that CC’ was equal 
to CR, and @ was 45°; and the lead plate P was placed as shown in the position 
