Mr Townsend, Secondary Rontgen Rays. 



219 



the object at D with a sheet of aluminium \ mm. thick placed in 

 the position H. 



Table I. Giving electrometer deflections obtained with 

 secondary rays, in 10 seconds. 



A rough estimate of the intensity of the primary rays was 

 made by placing W at D, so that the direct rays should pass 

 through the gauze cylinder. In \ a second the deflection ex- 

 ceeded 500 divisions. In order to compare the secondary radiation 

 with Uranium radiation, a large disc of uranium, 7 centimetres in 

 diameter, which Lord Kelvin kindly lent me, was placed at D. 

 The uranium rays gave a deflection of 1"5 divisions in 15 seconds. 

 When rays from the Crookes tube fell on the uranium, a deflection 

 of 26 divisions was obtained in the same time. Under similar 

 conditions a piece of copper at D gave a deflection of 96 divisions. 



These results show the ionization which is produced by rays 

 from various bodies at a distance of about 6 centimetres from the 

 radiating body. 



We see that there are different kinds of radiation given out by 

 different substances. The table shows that brass, zinc and copper 

 give out a radiation of which only a small fraction passes through 

 a sheet of aluminium, whereas the rays from the other substances 

 traverse aluminium comparatively well. Sagnac obtained a result 

 similar to this. 



These secondary rays, which produce an effect at a distance 

 from the radiating body, are not much affected by the state of the 

 surface of the body. It was found that the strength of the rays 

 from polished brass was only two or three per cent, greater than 

 the radiation from brass coated with oxide. 



When the brass was covered with moistened filter paper, the 

 deflections were cut down from 66 to 46. 



