of Helium in the Extreme Ultra-Violet. 817 



in helium examined with the large spectroscope yields strong 

 impurity lines of shorter wave-length than \ 585. 



It may appear curious that this resonance line was not 

 discovered in previous researches when a disruptive dis- 

 charge was employed. This may be explained in two ways : 

 first, on the ground that the impurities known to be liberated 

 by the disruptive discharge carry the current to the exclu- 

 sion of the helium ; second, by reference to the work of 

 Compton, Lilly, and Olmstead* on the minimum arcing 

 voltages, in which they have shown that when current density 

 is high it is possible to obtain an arc at about 8 volts with 

 the emission of the ordinary helium spectrum, while the 

 resonance potential line requires upward of twenty volts. 

 It would seem, therefore, that with high current density it 

 might be quite reasonable to expect a bright helium spectrum 

 in the visible spectrum without the resonance line in the 

 extreme ultra-violet. 



In conclusion, it is interesting to note that, if we follow 

 the speculations of Bohr, Sommerfeld t, and others J, two 

 resonance potential lines in helium might be expected corre- 

 sponding to the two principal series of this element. The 

 wave numbers of the limits of these two principal series are 

 respectively 32031 and 38453, these figures corresponding 

 to potentials of 4*0 and 4*8 volts. Taking the ionization 

 potential of helium as 25*2 volts and subtracting from it the 

 values 4*0 and 4'8, we obtain the values 21*2 and 20'4 as the 

 two values of the resonance potentials in helium. The line 

 585 corresponds to the value 21'2 : that we have only this 

 line in the spectrum and not the line corresponding to 20*4 

 volts appears to agree with the work of Franck and 

 Knipping. 



Jefferson Physical Laboratory, 

 Harvard University, 

 December 1920." 



* K. T. Compton, E. T. Lilly, & P. S. Olmstead, Physical Review, 

 xvi. p. 282 (1920). Compare T. C. Hebb, Phys. Eev. xvi. p. 375 

 (1920). 



t Sommerfeld, Atombau u. Spektrallinien, 2nd Ed. p. 287. 



\ J. Franck & P. Knippiug, loc. cit. 



