produced by the Action of Ihorium Compounds. 18i 



diffuse through the gas and adhere to the surface on which 

 they impinge. A steady state will be reached when the rate 

 of supply of fresh radio-active particles due to diffusion is 

 balanced by the decay o£ the radiation from the surface. 

 The maximum intensity of the radiation on any surface in 

 the neighbourhood of a thorium compound is thus pro- 

 portional to the number of radio-active particles that reach it 

 by the processes of diffusion. 



Effect of E.M.F. on the Amount of Radioactivity. 



The amount of induced radioactivity in a given time increases 

 with the voltage for small voltages, but soon reaches a point 

 beyond which large increases in the E.M.F. have a very 

 small effect on it. In order to investigate the relation in 

 detail, the following arrangement was employed : — 



Two insulated concentric brass cylinders A and B (fig. 9) 

 were used, of diameters 5*5 and *7 centim. respectively. The 



Fig. 9. 



cf 



6- 



Hi 



ends were closed with paraffin stoppers C and D. The cylinder 

 A was connected with the + pole of a battery, the other pole 

 of which was to earth. The cylinder B was connected to the 

 electrometer in the usual manner. A layer of thorium oxide 

 in a paper envelope was placed along the bottom of the cylinder 

 A. The whole was exposed to the action of thorium oxide 

 for three days. The intensity of the radiation given out by 

 B had, after that interval, nearly reached its maximum value. 



