﻿Emission of Electrons by X-Mays. SO 7 



Table IV. 



Voltage. Ou. Sn. Pb. 



100 100 100 



+400 136 140 139 



-400 81 79 80 



The results of the following experiment throw some light 

 on the source of these slow electrons. The wire cylinder 

 was removed, and measurements were made on the effect of 

 retarding fields. It was found that, under these conditions, 

 the diminution was much more marked, and in some cases 

 the brass cube even acquired a negative charge with a 

 sufficiently large retarding field. This implied that more 

 electrons were being driven from the walls into the cube 

 than were coming from the cube owing to the direct action 

 of the X-rays. As precautions had been taken to prevent 

 the X-rays from falling directly on the walls of the vessel, 

 the only sources of electrons from the walls were the high- 

 speed electrons and the scattered and characteristic radiations 

 from the cube. These X-radiations would be of small 

 intensity compared with the direct X-ray beam, and their 

 effect would be still further reduced by the paper lining. 

 On the other hand, the walls were subject to direct bombard- 

 ment by all the fast electrons, and it seems certain that it was 

 this electronic bombardment which liberated the low-speed 

 electrons. As the energy of the slow electrons is so small, it 

 is reasonable to assume that one high-speed electron can 

 produce a large number of low-speed electrons ; and this 

 would account for the observed fact that the number of 

 electrons driven back by the action of the retarding field 

 was greater than the number of electrons liberated by the 

 X-ray beam. With the cylinder in position, the field 

 between the walls and the cylinder prevented the electrons 

 from the walls from reaching the central insulated system, 

 while, owing to the wide mesh of the cylinder, the number 

 liberated from it would be small. 



The properties of these slow electrons are very similar to 

 those of the delta rays produced by bombardment by alpha 

 rays. 



These results indicate that the low-speed emission is of a 

 secondary nature, and owes its origin not to the direct action 

 of the X-rays, but to the high-speed electrons ejected hj the 

 X-rays. The process appears to be exactly analogous to the 

 phenomenon of ionization in a gas. The photographs obtained 

 by O. T. R. Wilson show that each high-speed electron is 



