710 



Prof. L. T. More on the Fatigue of 



of secondary radiation is variable ; the changes, however, 

 differ in many respects from the photo-electric effect. 



Until we can devise a method of obtaining a constant 

 source of Rontgen radiation, we are limited to a choice of 

 comparative measurements. Another complication which 

 makes accuracy difficult, is the impossibility of making an 

 induction-coil work steadily for the length of time employed 

 in photo-electric work. So the attempt was made to measure 

 such small effects as might take place during a few hours of 

 exposure. The method adopted to eliminate irregularities 

 of coil and bulb was a comparative one. The secondary 

 radiation excited by the Rontgen rays in two similar plates 

 was measured. One of them, which I shall call the standard 

 plate, was then screened from the rays that continued to fall 

 upon the other, or trial plate. At intervals afterwards the 

 secondary radiation from both plates was observed. Varia- 

 tions in the ratios of the radiation from the fresh plate and 

 the other exposed continuously to the rays, are a relative 

 measure of the effect of the Rontgen rays on the trial plate. 

 To have values accurate as well as characteristic, a coil was 

 chosen which worked with steadiness, and the bulb was pro- 

 vided with a regulator which, with gentle heating, kept the 

 rays soft and as uniform as possible. 



Fiar. 1. 



^H# 



The apparatus adopted is shown in fig. 1. Two exactly 

 similar brass boxes, 12 cms. in each direction, served as 



