IONIZATION POTENTIALS IN DISSOCIATED GASES. 213 



particular critical potentials to atom or molecule has been justified by 

 the degree of consistency with probable processes of dissociation, 

 radiation, or ionization. The present investigations have yielded 

 definite and direct experimental evidence on the above points and 

 have thrown new light on the nature of low voltage arcs and on the 

 manner of excitation of the spectrum. 



Low Voltage Arc in Hydrogen. 



Dr. O, S. Duffendack has studied the relation of voltage to arc 

 currents and spectral excitation. The arcs were produced in gas at 

 pressures between 0.5 and 5.0 mm. between two electrodes, as shown 

 in Fig. I. AB was a tube of thin tungsten foil, which could be 



T 



IV 



B 



Fig. I. 



heated by a current through water-cooled leads. CD was a 20-miI 

 tungsten wire passing axially through the tube and also electrically 

 heated. The potential drops across the tube and the filament could 

 be adjusted to practical equality, so that they behaved as equipotential 

 electrodes to electrons emitted from the wire and drawn to the tube 

 by an applied difiference of potential V. The tube was 30 mm. long 

 and of 3.7 mm. radius. Ionization of gas within the tube was de- 

 tected by increase of current / between the electrodes, and the spec- 

 trum of the excited radiation was observed through an open end of 

 the tube. 



With the filament heated to a bright incandescence, but with the 

 tube relatively cool, the current-voltage variation is illustrated by 

 Fig. 2. From curve (a) it is seen that the current first is very small 

 and increases slowly with voltage until the ionization potential, about 



