Prof. Bumstead, On the absence, etc. 125 



On the absence of excited radio-activity due to temporary 

 exposure to 7 rays. By Professor H. A. Bumstead. (Communi- 

 cated by Professor J. J. Thomson.) 



[Bead 30 January 1905.] 



The following experiments form a continuation of the investi- 

 gation described in the preceding paper by Professor J. J. Thomson, 

 and were undertaken at his suggestion. Their object was to 

 ascertain whether certain metals, after removal from exposure to 

 the j3 and 7 rays of radium, continue to give out secondary rays 

 for a very short time — a time too short for the effect to be detected 

 in Professor Thomson's experiments. For this purpose a hori- 

 zontal copper disc 18 cm. in diameter was mounted so that it 

 could be rotated about a vertical axis at various speeds by a small 

 motor. Above the disc, at one side, was a cylindrical tin vessel, 

 6"5 cm. in diameter and 12'5 cm. high, provided with a central 

 rod, insulated by sulphur and protected by an earthed guard tube 

 in the usual manner. The vessel could be charged to various 

 potentials by means of a battery of small storage cells, and the 

 central rod was connected to one of C. T. R. Wilson's tilted elec- 

 troscopes which served to measure the ionization current in the 

 cylinder. In the bottom of the cylinder was a hole, 5 5 cm. in 

 diameter, which was covered with aluminium leaf O'0OOO5 cm. 

 thick ; this window was 0"7 cm. above the rotating disc. Above a 

 point on the disc whose angular distance from the centre of the 

 testing vessel was about 120° was supported a sealed tube of thin 

 glass containing 30 milligrams of pure radium bromide ; the 

 radium was 10'5 cm. above the disc, and was surrounded by a 

 hollow lead cylinder whose walls were 55 cm. thick. The bottom 

 of this lead cylinder was 2*5 cm. above the rotating disc ; its 

 internal diameter was 5 cm., so that the part of the disc exposed 

 to the radium rays had a greater area than the aluminium window 

 in the testing vessel. The total height of the lead cylinder was 

 1.9 cm. 



Notwithstanding the 5*5 cm. of lead between the radium and 

 the testing vessel the 7 rays which got through the lead and the 

 secondary rays due to them were sufficiently intense to make the 

 current in the testing vessel (with the disc at rest) about 60 times 

 that due to the " spontaneous " ionization of the air. In order 

 therefore to make the test one of any delicacy it became necessary 



