Electron Emission from Tungsten, Molybdenum and Tantalum

Thremionic emission from tungsten, tantalum and molybdenum as a function of temperature.—The tubes used in these series of careful measurements, each contained a long V-shaped filament and an anode consisting of a calcium surface deposited on the wall, and were thoroughly heat treated so as to get rid of adsorbed gases. The data obtained were corrected for lead losses and for the Schottky effect. In the case of tungsten, specially prepared pure metal was used and the temperature scale of Worthing and Forsythe was adopted. The results for a number of filaments agree well with each other and with the equation $I=A{T}^2{e}^{-\frac{b_0}{T}}$ where if $I$ is in amp./${\mathrm{cm}}^2$, $A$ has the value 60.2 (${log}_{10}A=1.779\mathrm{}$), the value derived by ${\mathrm{Dushman}}^1$ from the "chemical" constant, assuming the Sackur-Tetrode theory, and $b_0=52,600\mathrm{}\pm 250$, which is in good agreement with previous results. With tantalum, the accuracy of the results is not as great because of uncertainty as to the temperature scale, but the indication is that the value of $A$ is the same as for tungsten (the most probable value of ${log}_{10}A=1.7\mathrm{}\pm .1$). Assuming the theoretical value, $b_0$ comes out 47,800±500. With molybdenum, the variation in the results for different runs indicates that the surfaces were not sufficiently cleaned and were contaminated with oxide which would tend to raise the value of $A$. (The mean value of ${log}_{10}A$ obtained is about 2.2.) However assuming ${log}_{10}A=1.779\mathrm{}$, $b_0$ comes out 50,000±500. Theoretical constancy of $A$. It is pointed out that theoretically $A$ can be expected to be a universal constant only for surfaces for which the surface heat due to positive charge is zero. Experiments with coated filaments show that electronegative atoms like oxygen increase $A$ and electropositive elements like Th and Cs decrease $A$.