on Ionisation in Closed Vessels. 



479 



reduction of the ionisation amounting to about 12 °/ of the normal. 

 The iron screen which was about 5 cm. thick gave a reduction of 

 about 20 °/ . The effect of the screens of wood and paper was 

 much smaller, as might have been expected. In the case of the 

 paper screen, which consisted of books, it was a reduction which 

 was too small to be measured and could only just be detected. 

 The wooden screen was about 15 cm. thick and gave a slight 

 increase. This was no doubt due to some radiation from the 

 wood similar to that observed by Cooke in the case of bricks, as 

 in both cases it was found that the increase was annulled by the 

 interposition of a lead screen 1 mm. thick. In the subsequent 

 experiments the vessel in which the ionisation was measured was 

 varied, vessels of lead, tin, iron, aluminium, and zinc being used. 

 All the vessels were of the same dimensions. It was of course 

 found that the ionisation varied very much with the material of 

 the vessel. The following table gives the average values obtained 

 for the various metals, the scale being arbitrary. For purposes of 

 comparison the corresponding values obtained by Strutt* are. 

 placed alongside. His numbers are doubled to render comparison 

 easier. 



The ionisation was also measured in each of those vessels 

 while it was surrounded with a lead screen. The screen was about 

 1'3 cm. thick, and the reduction of the ionisation effected by it in 

 the different cases is shown below. 



* Loc. cit. 



