216 COMPTON— ARC SPECTRA AND 



Incidentally the character of the current-voltage curves yields 

 knowledge of the processes which produce the arc. At voltages so 

 low as to avoid ionization the current is due entirely to the electronic 

 emission from the filament, and this is limited to a very small value, 

 independent of the temperature of the filament, provided it is high, by 

 the negative space charge of the electrons immediately surrounding 

 it.^ When ionization occurs each positive ion, drawn toward the 

 filament, neutralizes the space charge of about 243 electrons and thus 

 permits the escape of that additional number. This large number of 

 electrons liberated by each positive ion is due to the relatively small 

 speed with which the heavy positive ion moves through the region of 

 space charge. The current increase is therefore due not so much to 

 the addition of ions by ionization as to the effect of the positive ions 

 in permitting the escape of many more electrons from the filament. 

 Thus the current increases with increasing voltage until it approaches 

 the thermionic saturation current characteristic of the size and tem- 

 perament of the filament. Then there are no longer sufficient elec- 

 trons to neutralize the effect of the positive ions near the filament so 

 that the space charge changes from negative to positive, creating an 

 accelerating field for the emitted electrons and giving the saturation 

 thermionic current plus the ionization current. This is the arc. It 

 is characterized by its sudden appearance and by the concentration of 

 luminosity in the region very near the filament, where the principal 

 portion of the potential drop occurs, with positive space charge. 



Excitation of the Hydrogen Spectrum. 



In molecular hydrogen there was no visible spectrum until the arc 



flashed in at or above 16.3 volts. Then there appeared the Balmer 



series lines and part of the secondary, or band, spectrum. The part 



appearing was Group I., of Fulcher's Classification,- which includes 



those lines which show little intensification with increasing voltage. 



These lines are also those which show no Zeeman effect and are 



apparently the ones which Merton found not to be enhanced by the 



admixture of helium with the hydrogen. =^ 



1 Langmuir, Phys. Rev., 2, p. 543, 1913. 

 - Astrophys. Jour., 37, p. 65, 1913. 

 ^Roy. Soc. Proc, A., 96, p. 382, 1919. 



