﻿Emission of Electrons by X-Rays. 805 



varied with the voltage applied to the tube. Such experi- 

 ments are rendered difficult by the necessity of obtaining 

 some means of measuring the intensity of the X-ray beam. 

 As a rough measure of this, the ionization produced in the 

 standardizing vessel was used. The electron emission per 

 unit intensity was measured for various potentials ; Table III. 

 gives a typical set of such observations. 



Table III. 



Voltage. 



Electron Emission 

 Unit Intensity. 



per 



nVi 



28000 





85 







14200 



30000 





81 







15400 



45000 





71 







15000 



65000 





61 







15500 



78000 





53 







14800 



This Table shows that the electron emission per unit 

 intensity diminishes as the applied potential increases. 

 Column 3 shows that the product of the number of electrons 

 emitted per unit intensity and the square root of the voltage 

 is approximately constant. Owing to the method adopted 

 for the measurement of intensity, too much reliance must not 

 be attached to this result, but it gives some indication of the 

 nature of the variation. 



This suggests an intimate connexion between this result 

 and that obtained by Ulrey, to which reference has already 

 been made. As an approximation the X-iadiation used in 

 these experiments may be considered as a monochromatic 

 radiation of wave-length \ . Ulrev's result, combined 



D (max.) 



with that just given, leads to the result that the number of 

 electrons emitted per unit intensity is proportional to the 

 wave-length, or inversely proportional to the frequency of 

 the exciting radiation. As the size of the quantum is pro- 

 portional to the frequency, this may be interpreted as 

 meaning that the number of electrons emitted per unit 

 intensity is directly proportional to the number of quanta 

 involved. 



B. Low-Velocity Emission. 



(9) In studying the properties of these electrons, the 

 total number of electrons escaping from the radiator was 

 measured under the influence of varying, accelerating, and 

 retarding electric fields. For this purpose, the wide-meshed 

 wire cvlinder described above was charged to positive and 



