170 



Mr Kleeman, The nature of the ionisation 



absorption of the rays as a whole is the same as the /3 rays of 

 uranium, or the penetrating y8 rays of radium. These calculations 

 are therefore not satisfactory, they may easily be very considerably 

 out. In fact McLennan* deduced that the ionisation in a chamber 

 not due to the secondary radiation from the walls is equal to 

 about one half of the total ionisation in the chamber, while Wilsonf 

 makes it equal to one sixth of the total ionisation. It does not 

 therefore appear quite certain that the total ionisation is not due 

 entirely to the secondary radiation from the walls. The writer 

 therefore made some direct experiments to test this point. 



Several slightly different arrangements were used, each of 

 which involved the deflection of the secondary radiation from 

 the apparatus by means of a magnetic field. A diagram of the 

 apparatus first used is shown in figure 1. ^ is an ionisation 



Fig. 1. 



chamber on a lead block B, 5 cm. thick, the chamber being placed 

 symmetrically with respect to an aperture a in the block. 30 

 mgrm. of radium were placed at C underneath the aperture. A 

 magnet whose poles measured 5 "5 cm. by 5 '5 cm., was placed so 

 that the lines of magnetic force traversed the chamber. Since 

 the electrons produced by y rays are ejected in the direction of 



* Phil. Mag. Dec. 1907. 



t Phil. Mag. p. 216, Jan. 1909. 



