﻿Emission of Electrons by X-Rays, 803 



conditions of the X-ray tube. The voltage of the tube was 

 varied and the ratio of the electron emissions from the two 

 metals was measured. Results of such measurements are 

 given in Table II. The observations have been taken in 

 groups, each group corresponding to a certain range of 

 potential on the tube, the potential being measured by the 1 

 equivalent spark-gap between points. 



Table 



II. 





Range of Spark-Gap. 





Cu/Sn 



0- 5 cm. 





0-57 



5- 9 ., 





0-55 



9-12 „ 





0-55 



12->12„ 





0-56 



It is clear from this table that, under the experimental 

 conditions, the ratio of the electron emission from these 

 elements is practically independent of the potential used to 

 excite the tube. It was expected that there would be a dis- 

 continuity in the ratio at the point where the K-radiations 

 of tin became prominent. This should occur at a potential 

 of about 50,000 volts. No such discontinuity appeared. 



The reason probably lies in the fact that in these experi- 

 ments the tube was fitted with a tungsten anticathode. The 

 applied potential was never sufficient to excite the K-radiations 

 of tungsten to any extent, while the L-radiations would be 

 very largely absorbed in the walls of the tube and in the 

 window of the examination vessel. Under these conditions, 

 the radiation used was what is generally termed " white." 

 Its quality varies with the potential used to excite the tube, 

 but not to a very marked extent. Ulrey* lias shown that, 

 under conditions which are very similar to those obtaining 

 in these experiments, there is a maximum X-radiation at a 

 wave-length which obeys the approximate law — 



A. V 2 = constant 



(max.) 



where Y is the applied potential. Thus quite a large 

 difference in the potential produces a relatively small change 

 in the position of the wave-length to which corresponds the 

 maximum X-radiation. 



It would appear, therefore, that the electron emission 

 associated with the characteristic radiations of the elements 

 bombarded is not of sufficient intensity as materially to affect 



* C. T. Ulrey, Phys. Rev. xi. p. 401 (1918). 



