160 Sir E. Rutherford on the Penetrating Power of 



in the experiments of Rutherford and Andrade * were '072 

 and *099 A.U., corresponding on the quantum relation to 

 waves excited by 174,000 and 125,000 volts. The values 

 of fjb/p for aluminium corresponding to X rays excited at 

 these voltages are about '09 and '12 respectively, while the 

 observed value of /uu/p for the penetrating gamma rays from 

 radium G is much less, viz. *026. Since undoubtedly for 

 such high frequencies, p/p varies very slowly with f requenc} 7 , 

 it is clear that the wave-length of the more penetrating 

 radiation is considerably smaller than that of the shortest 

 waves observed by Rutherford and Andrade. In other 

 words, the wave-length of the main gamma rays is much 

 shorter than was previously supposed. This conclusion is 

 still more strongly confirmed by the observations on the 

 absorption of the radiation by lead. For a voltage of 

 196,000 volts, corresponding to a still shorter wave-length 

 than the shortest observed by Rutherford and Andrade, the 

 observed value of fi/p in lead was 0'75, while the value of fi/p 

 found by Ishino for the penetrating gamma rays was "042 — 

 a ratio of nearly 20 times. Even allowing that the true 

 value of fi/p for waves generated at 196,000 volts is some- 

 what smaller than the value observed, the largeness of the 

 ratio shows that the gamma rays must be much shorter than 

 those generated at 200,000 volts, i. e. much shorter than 

 X = '062 A.U. 



In our present ignorance of the law of variation of p/p 

 with frequency in this region of the spectrum, it is only 

 possible to estimate the actual wave-length of the most 

 penetrating gamma rays. It is clear, however, that the 

 waves are at least three times and may be ten times shorter 

 than those which correspond to 200,000 volts, i. e. they 

 correspond to waves generated by voltages between 600,000 

 and 2,000,000 volts, and thus lie between '02 and '007 i.U. 

 It is thus clear that the gamma rays from radium C consist 

 mainly of waves of about ^ho ^ ne wave-length of the soft 

 gamma rays from radium B, and are of considerably shorter 

 wave-length than any so far observed in an X-ray tube, 

 with the highest voltages at our disposal. 



Another very interesting and important point arises from 

 this discussion. It is well known that the ft rays from 

 radium B and radium C when examined in a magnetic field 

 give a veritable spectrum of bright lines corresponding to 

 definite groups of ft rays, each group consisting of electrons 

 •expelled with a characteristic and definite velocity. The 



* Rutherford and Andrade, Phil. Mag. xxviii. p. 263 (1914). 



