the Ionizing Potential of Sodium Vapt 



181 



as. in this case the sodium glow could be viewed against an 

 absolutely black background and the light of the filameut 

 cut off by means of a suitably placed screen. 



Ficr. 2. 



The second method was very similar except that we 

 arranged the circuits as shown in fig. 2, the rotatiDg wheel 

 applying the potential between the hot cathode and the 

 anode wire only for the time intervals during which the 

 heating current was shut off. The condenser C was 

 inserted to lessen the spark at the break. In both cases 

 the wheel was turned at a speed sufficient to maintain 

 the filament at a constant intensity without any visible 

 flicker. 



observations were confirmed by both o£ these 

 The D lines appeared w r hen the potential 

 hot cathode and the wire was 0'5 V. or 

 As this value is so much below that required by 

 the quantum relation (2'1 V.) it was necessary to make sure 

 lhat the emission of the D lines was not due to some 

 secondary action, the light from the incandescent filament 

 for example. 



To test this point we mounted thetungsien filament at the 

 centre of a steel tube, as shown in fig. 3. Immediately 



Our earlier 

 experiments, 

 between the 



greater. 



