146 



MR. G. W. C. KAYE ON THE 



hardness, but with harder primary rays such secondary radiation is replaced by one 

 of a softer type. The absorptive power of aluminium is small, and we should expect 

 the change from the steep part of the curve to the rest to be considerable, but 

 gradual. This will be seen to be the case in fig. 7, where X gradually changes from 

 120 to 40 in a thickness 0'012 cm. of aluminium. 



For copper and platinum the secondary radiations are much softer than the 

 primary, and as the absorptive powers of the metals are high, we should expect, as 

 figs. 8 and 9 show, a sharp alteration in the slope. With copper, X changes from 

 620 to 470 in a thickness 0'0015 cm., while X for platinum changes from 3700 to 2350 

 at a thickness 0'0004 cm. 



It is of distinct interest to note that MoOLELLAND,* working on the absorption of 

 the /3 rays from radium by metal screens, obtained a similar steepening of his 

 logarithmic curves of transmission in their early stages. His results for the ratios 

 of the initial to the final slopes in the case of aluminium, copper and platinum are 

 tabulated against the present values obtained for llontgen rays. 



The agreement with copper and platinum screens is certainly noteworthy. The 

 lack of it with aluminium may be ascribed to the abnormal character of its secondary 

 Rontgen radiation. 



Effect of the Potential Difference between the Electrodes. The next point investi- 

 gated was the effect of a change in the potential difference applied to the terminals of 

 the tube, on the apparent homogeneity indicated when screen and radiator are alike. 

 Measurements were made at three potentials, about 8,000, 20,000, and 43,000 volts. 

 The logarithmic curves of transmission with copper screens and copper and platinum 

 anticathodes are given in fig. 13, where the three full-line curves stand for the copper 

 radiation, and the dotted curves for the platinum radiation. For ease of comparison 

 the initial intensities are made the same at all potentials for each radiator, though 

 actually they were very different. It will be seen that the copper radiation is 

 transmitted exponentially at both 8,000 and 20,000 volts ; the only difference is that 

 a rather lower absorption coefficient accompanies the higher voltage. 



A departure from the exponential law is evident with thick screens in the curve for 

 copper radiation at43,000volts. The range of indicated homogeneity becomes restricted, 

 and the curve begins to display the features which characterise a normal transmission. 

 * MCCLELLAND, 'Sci. Trans. Roy. Dub. Soc.,' IX., p. 25, Feb., 1906. 



