Positive and Negative Thermionic Emission from Tungsten

The positive and negative thermionic emission and the rate of evaporation of neutral atoms have been studied for well outgassed tungsten. To fix a reliable temperature scale the spectral emissivity for $\lambda =0.669\mathrm{}$μ has been determined for a cylinder heated in a vacuum. The emissivity has been found to be a constant and equal to 0.46 from 1200°K-2200°K. A determination of the heat of evaporation of neutral atoms gave a value of 8.44 ev. The electron work function was found to be 4.55 ev with an A of 75 amp./${\mathrm{cm}}^2$/${\mathrm{degree}}^2$ until the filament was heated to a temperature of 3100°K, after which the work function was 4.63 ev and the value of $A$ was 212. The average positive ion work function was 11.93 ev. This value closes the thermionic cycle if Russell's value of 8.1 for the ionization potential is used.