640 Mr. R. D. Kleeman on the Secondary Cathode 



thick screen o£ lead. This is explained if the radiation from 

 aluminium is decreased in a greater proportion than that 

 from carbon, an element of smaller atomic weight, when the 

 thin screen is replaced by the thick screen (a similar assump- 

 tion we have seen fits the facts for Pb and Ai). For, the 

 numerical value of (k a — 1) of (C — Al) is then decreased, since 

 it is negative ; and this decrease may be of such a magnitude 

 that the value of (k a — 1)A1 becomes less than its previous 

 value when multiplied by a factor which brings [k x — 1) Al to 

 its previous value. It will be observed that it follows from the 

 investigation in the preceding paragraph that this factor 

 tends to increase the value of (0 — Al). Thus the greater 

 decrease of the radiation from aluminium than that from 

 carbon, with increase of thickness of lead screen, shows that 

 the rays that are most efficient in producing secondary 

 radiation from carbon are less absorbed by a screen of lead 

 than the rays that are most efficient in producing secondary 

 radiation from aluminium. 



Since the results obtained with a screen of mercury 

 or bismuth resemble those obtained with the lead screen, 

 the foregoing conclusions hold good for these screens 

 also. 



Next, let us investigate the results obtained with the 

 screens of iron, copper, zinc, and tin. We should expect 

 from the foregoing results, and the fact that the differences 

 of the radiating powers obtained with these screens resemble 

 one another, that each of these screens would absorb 

 approximately to the same extent the rays that are respectively 

 most efficient in producing secondary radiation from aluminium 

 and the substances N m or (S, Fe, Ni, Cu, Zn, Sn). Also, we 

 should expect these rays to be more easily absorbed by these 

 screens than the rays that are most efficient in producing 

 secondary radiation from lead. In this case the radiation 

 from aluminium should decrease in a greater proportion than 

 that from lead, when the thin lead screen is replaced by one 

 of the above-mentioned screens, and the radiation from each 

 of the substances N m and aluminium decrease in the same 

 proportion. Therefore the value of (k 1 — l) will be increased, 

 while each value of (Jc a — 1) of the differences (N m — Al) will 

 remain approximately the same. Therefore, when (k 1 — 1)A1 

 or (Pb-Al) and the values of (k a — 1)A1 or (N m — Al) are 

 each multiplied by a factor which reduces (Pb — Al) to its 

 previous value, the values of (N m — Al) will become less than 

 their previous value. Now, this result is approximately 

 obtained by experiment with the differences (N m — Al), when 

 the thin lead screen is replaced by a screen of iron, copper, 



