Hadioactivit)/ of Metal'i aeaeralhi. ^45 



Ed'pe rim ents. — Metal Efect, 



In these experiments cylinders of aluminium, zinc, lead. 

 tin, and copper were used, the outer ones being 120 cms. in 

 length and 24 cms. in diameter, and the inner 110 cms. in 

 lenjith and U> cms. in diameter. The measurements vvere 

 made with a quadrant electrometer whose sensitiveness was 

 such as to give a deflexion of 1000 mms. on a scale situated 

 at 1 metre from the needle for a potential of 1 volt. 



s 



1 < 



^ 



,^^, r 



Yy- L 



n 



fM 



The apparatus and its connexions are shown in fig, 1. 

 where A and B represent the cylinders o£ the metal examined. 

 The wire which led from the inner cylinder to the electro- 

 meter was of brass, and was surrounded by a tube of the 

 same metal to screen off induction effects. Tliis tube, as well 

 as the outer cylinder B and the metallic screen over the 

 electrometer, was kept joined to earth during an experiment. 

 The inner cylinder and the free quadrants were earched by 

 means of the brass rod D, which carried a platinum tip and 

 made contact with a small platinum plate attached to the 

 connecting wire as shown in the figure. The tube was pro- 

 vided with a small opening so arranged that the withdrawal 

 of the rod D did not affect the capacity of the system. Plugs 

 and supports of paraffin-wax pro^'ided insulation for the 

 apparatus. 



i\\ making a measurement the connecting rod D was 

 withdrawn and the apparatus left intact. The inner cylinder 

 B and the free quadrants then slowly acquired a negative 

 charge which finally reached a limiting value. The value 

 varied with the metal used in the construction of the cylinders 

 A and B^ but did not vary with samples of the same metal 

 obtained from different sources. It was also the same whether 

 the needle of the electrometer was positively or negatively 

 charged. With lead, tin, and copper the limiting value was 

 reached in about one hour, but with aluminium and zinc not 

 before four or five hours had elapsed. 



Readings for pairs of cylinders of these metals at the 



