94: Dr. P. D. Foote and Dr. W. F. Meggers on Atomic 



series lines, however, converge at 2p, and hence do not affect 

 the intensity of: l'5s — 2p , which may still be excited at the 

 same collision. 



In fig. 6 we have plotted the logarithm of the intensity 

 for X8521, X6973, and X4555 per unit number of electrons 

 reaching the outer cylinder as the accelerating voltage is 



Fur. P. 





z 3 — ' 



J— 







—7 





?» 8521 





















* 6 9 74 





^— - — " 

















?l 4555 





(^ 









1 Z 3 4 5 



Volts flf>(>l'ed accelerating jaotential 



Logarithm of intensity per unit number of electrons reaching anode 

 as a function of the applied accelerating voltage. 



increased. At higher voltages this ratio becomes a constant 

 for each line in agreement with the work of Jolly*, who 

 observed that in discharge through hydrogen "the radiation 

 is proportional to the current density both for the whole 

 spectrum and for any portion of it." This follows (approxi- 

 mately) directly from the quantum theory. Above a certain 

 minimum voltage the number of electronic atomic collisions, 

 and hence the number of quanta of any particular frequency 

 produced, is proportional to the number of electrons present. 

 The existence of l'5s — 2p± when the intensity of all other 

 lines has dropped to zero is well illustrated in this plot. The 

 above law is not rigorously applicable in the present case, 

 since some of the electrons reaching the anode have been 

 produced by ionization, and, in general, would not be able to 

 accumulate sufficient velocity to cause radiation upon further 

 collision. 



An interesting phenomenon was observed in the operation 

 of the caesium arc at 120 volts. It was found that, after once 



* Phil. Mag. xxvi. p. 801 (1913). 



