514 



FOOTE AND MOHLER: LOW VOLTAGE DISCHARGE 



heated to about 300°C. and evacuated to a pressure of about 

 10~^ cm. Hg as registered by a McLeod gauge. For observing 

 the spectral hnes a diffraction grating having 15,000 Hnes to 

 the inch was employed and the observations were made with an 

 eye piece of such magnification that the two D lines showed 

 distinctly with a rather wide slit opening. This was necessary 

 on account of the low intensity of the light at small voltages. 



/? 



JB 



3PECTR03C0I 



Fig. 1. Diagram of apparatus. 



A visible yellow glow could be detected when the accelerating 

 field across AC was as low as 0.8 volts. Since the resonance 

 potential is 2.1 volts, the initial velocity of a considerable num- 

 ber of the electrons emitted by the hot cathode must have been 

 about 1.3 volts. As the voltage increased the intensity of the 

 discharge rapidly increased. At an applied potential of 5.2 

 volts the character of the arc changed entirely, the second sub- 

 ordinate series of sodium appearing, thus indicating ionization; 

 and the arc became very brilliant. The applied potential of 5.2 



