the Ionizing Potential of Sodium Vapour. 183 



the potential difference results from the drop across the 

 spiral. The glow appears around the tungsten loop W and 

 its copper supporting wires, which are at a temperature suffi- 

 ciently high to prevent any condensation of sodium. W 

 remains below a red heat, while W is at incandescence. 



Fig. 4. 



The potential difference can be varied somewhat by changing 

 the heating current, and consequently the temperature and 

 resistance of the spiral. 



We found in this case that the D-line glow appeared and 

 disappeared at the same instant on the tungsten loop W 

 and its two supporting wires of copper : this appears to 

 indicate that contact difference of potential plays no part 

 in the production of the D-line glow. 



The conclusion that we have reached, as the result of all 

 of our experiments, is that the D-line emission results from 

 the application of a potential of 0*5 volt or more, and that the 

 subordinate series appears at 2*3 V. 



The two points are determined, however, by the visibility 

 of the lines in a Schmidt & Haensch pocket spectroscope, 

 which is a most efficient instrument, but the lines fade 

 gradually in each case, and there seems to be no point at 

 which there is a discontinuity. This is perhaps to be 

 expected, as a result of the wide variation in velocity of the 

 electrons expelled from the hot cathode. 



The average velocity of these, in the case of an incan- 

 descent tungsten wire, is the equivalent of a potential drop of 

 about 0'4 volt. 



Assuming the Maxwell distribution, there must be a con- 

 siderable number moving with a sufficient velocity to excite 

 the D-line emission without the application of any electro- 

 motive force. 



It seems desirable to arrange an experiment with sodium 

 vapour in which we can deal with a stream of electrons in 

 which all are moving at very nearly the same velocity. This 

 could be accomplished perhaps by magnetic separation. 



Johns Hopkins University. 

 . May 1917. 



