Metals subjected to Rontgen Radiation. 



713 



current was determined by noting -svith a stop-watch the 

 time which the gold-leaf of the electroscope required to move 

 over a given number of divisions of the microscope-micro- 

 meter. The reciprocal of this time is a measure of the 

 current produced by the secondary radiation of the metal. 

 Alternative readings were made with the standard and trial 

 plates and an average taken of the reciprocals for each plate. 

 The ratios of these averages are the values given in the first 

 column for each metal. A decrease in the number therefore 

 means less radiation, and an increase, a greater radiation from 

 the trial plate. 



If the coil or tube worked irregularly while any one set 

 of readings was taken, the values for the standard plate did 

 not agree, and such a set could then be discarded. Fortu- 

 nately this happened very seldom. Variations in the tube 

 between two sets of readings did not affect the character of 

 the change taking place in the trial plate, although they 

 would alter the absolute value of the per-cent. change. 



Table I. 

 Unpolished Metal Plates in Air. 





Iron. 



Lead. 



yiCKEL. 



Zinc. 



Copper. 



Aluminium. 



Exposure. 



Eatio to 



Staudard 



Plate. 



Ratio. 



Ratio. 



Ratio. 



Ratio. 



Ratio. 



hours 



100 



100 



100 



100 



100 



100 



1 „ 



91 





97 



97 



100 



99 



2 „ 





93 



95 



94 



99 



96 



3 » 



90 



94 



92 



91 



98 



98 



; 4 » 





95 











5 ., 















6 „ 





94 











Rest 



100 



... 





94 



98 





30 rain 



45 „ 





100 











18 hours ... 







97 



99 







The examples chosen are characteristic of the different 

 metals when the surface has been allowed to age in air for 

 some months, and they are not at all exceptional in uniformity 

 of the readings. They show, with the possible exception of 

 copper and aluminium, a decided falling-off in the power of 

 giving off secondary radiation. This fatigue is, however, a 



