138 



ME. G. W. C. KAYE ON THE 



Aluminium Screens. Turning to fig. 7 (aluminium screens), we notice a general 

 resemblance between the curves for platinum, copper, nickel and iron radiators. They 

 indicate the kind of absorption usual with Rontgen rays that is, the coefficient of 

 absorption steadily diminishes with increasing thickness of screen. The four curves 

 become practically parallel with the thickest screens. The proximity of the early 

 portions of the nickel and copper radiation curves will be noticed. 



01 CM. -OZ 



THICKNESS OF A L. SCREE N 



Fig. 7. Al scrcun, 120,000 volts. 



It is the aluminium radiation that presents interest. The curve consists of an 

 earlier steeper portion (X = 120) which merges, when the screen attains a thickness of 

 about O'Ol cm., into a straight line for the rest of its path (X = 40). Throughout this 

 latter region, then, the absorption is exponential, and the aluminium radiation behaves 

 as if it were homogeneous. 



Copper Screens. If we inspect fig. 8, which embodies the results obtained with 

 copper screens, we no longer see this indication of homogeneity on the part of the 

 aluminium radiation. Instead, the curve presents the gradual diminution in gradient 

 with increasing thickness of screen that is typical of Rontgen rays. It is now the 

 copper radiation that appears homogeneous its graph for screens thicker than about 

 G'0015 cm. is a straight line (\ = 470). For thinner screens the curve is a little 

 steeper (X = 620). The graph for nickel is very nearly straight over most of its path. 



Iron and platinum yield normal curves like that of aluminium, but have a different 

 range of X's. The nearness of the early parts of the curves for nickel and copper 

 radiators will again be noticed. It is noteworthy that the radiation from platinum is 





