EMISSION AND TRANSMISSION OF RONTGEN BAYS. 



133 



Platinum Screens. If we now turn to the transmission curves secured under the 

 same conditions with platinum screens (fig. 5), we see in the main a resemblance to 

 those obtained with aluminium screens, but with some differences. The radiation 

 value for lead is now throughout about 10 per cent, less than that of platinum, but with 

 the thickest screens the two curves show some signs of approaching. It is d propos 

 to notice here that if screens of lead are used, then the radiation value of lead is 

 about 5 per cent, larger than that of platinum for screens of all thicknesses except the 

 very thinnest. 



Thus platinum and lead show the phenomenon of selective transmission very nicely. 



With platinum screens the radiation curves for copper, nickel, and iron show very 

 flat minima a result consequent on the radiation value of platinum being kept 

 constant. Platinum thus shows a not very marked selective transmission over this 

 range. Speaking generally, the radiation values with a platinum screen are lower 

 than the corresponding ones with an aluminium screen. 



The comparative effect of the three screens -aluminium, copper, and platinum on 

 the different radiations is exemplified in the following table. For ease of comparison 

 the screens have been chosen of thicknesses which cut down the platinum radiation 

 by the same amount in each case. 



Iron, Nickel, and Zinc Screens. Still using the same conditions of experiment, 

 measurements of the relative radiations from the different anticathodes were made 

 with screens of iron, nickel, and zinc. 



It will be sufficient to say that in each case the curves of transmission are similar 

 to those for screens of copper. 



When screen and anticathode are alike or have adjoining atomic weights, the 

 radiation is, over a certain region, largely augmented relative to that from any other 

 anticathode. The more remote the atomic weights of the radiator and screen, the 

 more limited is this region, and the sooner does the anticathode assume a normal 

 radiation value, judged on an atomic weight basis. Generally speaking, the lower the 

 atomic weight of the radiator in a group, the thinner is the range of screens of like 

 metal for which its radiation shows abnormal penetrability. It would seem, therefore, 



