produced by the Action of Thorium Compounds. 167 



Conditions for the Production of Radioactivity in 

 Substances. 



In order to confine the induced radioactivity produced by 

 thorium compounds to any particular conductor, it is necessary 

 that it should be charged — and all other bodies in the field + . 

 In order to produce radioactivity in all bodies in the neigh- 

 bourhood, no electric field is required. If thorium oxide is 

 placed in a closed vessel connected to earth, the sides of the 

 vessel and any solid bodies near, whether conductors or 

 insulators, become radio-active. If, in addition, the surface of 

 the thorium oxide is covered with paper or thin aluminium- 

 foil, the side of the paper away from the oxide becomes 

 radio-active. When no electromotive forces are acting, the 

 amount of radioactivity in a given time per unit area is 

 greater the nearer the body to the thorium oxide. 



With electromotive forces acting, the substance to which 

 the radioactivity is due appears to travel along the lines 

 of force from the + to the — charged body. It is thus 

 possible to concentrate the radioactivity on small plates or 

 fine wires by placing them in a closed metal vessel connected 

 to earth and charging them — . 



If the bodies are all uncharged, the particles producing 

 radioactivity, by the process of diffusion through the gas, 

 are carried to the sides of the bodies and adhere to them. A 

 fine wire fixed in the centre of a vessel on the bottom of which 

 the active salt is placed becomes only slightly radio-active, since 

 only a few of the active particles reach its surface. The closer 

 a body is to the thorium, other conditions remaining unaltered, 

 the more active it becomes. 



Fig-. 3. 



Fig. 3 shows the general arrangement for concentrating the 

 activity on a small area of a conductor. A metal vessel V 



^ 



