C. D. Cooksey — Secondary Cathode Hays. 51 



nig upon whether the ionization in A or B was more intense. 

 By a suitable adjustment of the screen, F, the ionization in the 

 two chambers could be made equal, when there would be no 

 motion of the gold leaf. After this adjustment was made the 

 tube was stopped, and the front holder, E, reversed so that the 

 gold side faced into A. The leaf in the electroscope, E„ was 

 then charged to a potential of about 200 volts, and the tube 

 again excited. When the leaf reached a certain division on 

 the scale the wire, C, was again insulated, and after the leaf in 

 E 2 had passed over a convenient number of scale divisions the 

 tube was stopped. The position of the leaf in E a was then 

 observed, and C connected to earth through a potentiometer, 

 and a sufficient potential applied to keep the leaf at its 

 observed position. Calling this potential a, we can take it as 

 a measure of the excess of ionization in A over that in B. If 

 e denotes the ionization produced by the emergence secondary 

 rays from the gold, and e' by the emergence secondary rays 

 from aluminium, we shall have 



e — e' = ka 

 where k is a constant. 



After this measurement was made the holder, E, was turned 

 back with the aluminium face in, and the other holder, I, 

 reversed so that the gold faced in. The tube was again 

 excited, and the potential, h, acquired by the leaf in E, while 

 the leaf in E 2 passed over the same interval as before, was 

 measured. Denoting the ionization produced by the incidence 

 secondary rays from the gold by i, and that by the incidence 

 secondary rays from the aluminium by i', we can write as 

 before 



i — i' = kb 



Letting e'/e = to, and i'/i = n we shall have 



e/i = a/b (1 — n)/(l — m) 



where e/i is the ratio of the emergence secondary radiations 

 produced in the gold by a certain beam of X-rays from B, to 

 the incidence secondary radiations produced by the same beam 

 after traversing the ionization chamber. In the light of the 

 results obtained in a previous paper,* and owing to the fact 

 that the ionization chambers used in the present work were 

 only two-fifths as long as those used before, we may assume 

 that the secondary X-rays from the metal produced a negli- 

 gible amount of ionization in the chamber compared to that 

 produced by the cathode rays from the metal. 



No data are readily obtainable in regard to to, but it is 

 known that the amount of cathode rays given out by aluminium 



* This Journal, xxiv, p. 285, 1907. 



