280 Spectrum of fi Rays excited in Metals by Soft X Rays. 



In accordance with anticipation, the /3-ray spectra did not 

 consist of fine lines, like those of the primary ft rays of 

 radium B and radium C, but of bands about 2 mm . broad, 

 due to the production of ft rays at different depths below the 

 surface of the radiator. The measurements were made to 

 the outside edges of the bands, corresponding to ft rays 

 escaping from the radiator without reduction in velocity by 

 absorption. 



The results already obtained are given in the following 

 table. The first column gives the values of H.p in gauss x cm., 

 and the second column the energy of the ft ray in terms of e, 

 the electronic charge (measured in absolute electromagnetic 

 units). For convenience in comparing experiment with 

 theory, the third column contains the wave-length of the 



X radiation corresponding to frequency v = y-, where E is 



the energy of the ft ray and h is Planck's constant. In 

 making the calculations we have taken the values of 

 c=l-591xl0- 20 e.m. units and /i = 6'62 x 10" 27 erg sec, 

 given recently by Millikan after a critical examination of 

 the data available *. 



Table of energies of ft rays expelled from iron 

 and lead by nickel X rays. 



Radiator. 



Hp of 

 expelled (3 ray. 



Energy 

 -rl0"e. 



Wave-length of 

 corresponding X. ray. 



Iron(l) 



(2) 



(3) 



267 

 334 

 364 



6-28 



9-79 



11-61 



l-985xl0" 8 cm. 



1-273 



1-074 



Lead(l) 



(2) 



1 



242 

 290 



5-16 

 7-43 



2-415 

 1-677 



Only those bands are included which are fairly strong on 

 the photographic plates : all have been obtained on two, 

 three, or lour different plates, and with different magnetic 

 fields. The bands vary in intensity, relatively to one another, 

 from plate to plate, according to the condition of the bulb 

 during the experiment. There is also evidence of a number 

 of much fainter bands, the existence of which requires 

 further confirmation : we have also observed faint but 



* Millikan, < Physical Review,' ii. p. 109 (1913). 



