620 Mr. R. D. Kleeman on the Secondary Cathode 



from the various substances, which would be independent of 

 the magnitude of these external quantities. It was also 

 necessary to elimininate the y secondary radiation. This 

 was obtained by the following process. After a reading had 

 been taken when the dish a contained the substance under 

 investigation, the dish with its contents was taken out of the 

 dish h and placed in the centre of the aluminium plate and 

 the dish b inverted and placed over it. A reading was then 

 taken with the bottom of the dish b as source of secondary 

 radiation. The bottom of the dish b consisted of sheet lead 

 3 mm. thick. A disk of aluminium of the same size as the 

 dish a was then placed into the dish b, and a reading taken 

 with this aluminium disk as source of secondary radiation. 

 A reading with the bottom of the dish b was then taken in 

 the same manner as before. The whole process was repeated 

 several times. It will now be shown that the difference 

 between the first and second readings is a measure of the 

 difference between the amounts of secondary cathode radia- 

 tion from the substance under investigation and from lead, 

 and the difference between the third and fourth readings a 

 measure of the difference between the amounts of secondary 

 radiation from lead and aluminium. 



Let nc, mc, uc 9 denote the secondary cathode radiations, 

 and ny, my, uy, the secondary 7 radiations, from the substance 

 under investigation and from lead and aluminium respec- 

 tively, as measured by their ionizing power. Let K denote 

 the leak that would be obtained if the substance in the dish a 

 and the lead bottom of the dish b did not emit either second- 

 ary y or secondary cathode radiation. Then, when the 

 substance n is in the dish a, the leak obtained is (K + nc 

 + ny-\-my). It is supposed that the y radiation from the 

 Lead is not appreciably diminished by passing through the 

 substance in the dish. When the dish a is covered by the 

 dish b the leak obtained is (K + mc + my + ny) . The difference 

 between these two leaks is (mc — nc), which is a measure of 

 the difference between the amount of cathode radiation from 

 lead and the substance n. In the same way it can be shown 

 that the difference between the leaks obtained with the 

 substances m and u, that is lead and aluminium, is (mc — uc). 



The mean values of (mc — nc) and (mc — uc), found by the 

 above process, were multiplied by a number which made 

 (mc — uc) equal to 800. The values of (mc — nc) for the 

 various substances were thus rendered comparable with 

 one another, being relatively independent of all external 

 quantities. 



To give an idea of the magnitude of the ionizing effect 



