Evaporation Characteristics of Tungsten

Rate of evaporation from tungsten filaments at 2825°K.—Using the Langmuir method, it was found that in vacuum the rate of evaporation per ${\mathrm{cm}}^2$ $m$ is independent of the diameter $a$, 1/20 to 1/4 mm, but is over forty per cent greater for fine-grained than for very coarse-grained wires, probably because the evaporation at the boundaries of the crystals is abnormal. In nitrogen the rate varied from 2 to 5 per cent of that for the same wire in vacuum, and in argon from 1.3 to 3 per cent, increasing as the diameter decreased from 1/4 to 1/15 mm. The variation with size when gas was present is in agreement with the Langmuir theory of heat conduction for hot filaments which supposes that there is around the wire a stationary film of gas through which the evaporated atoms must diffuse. The diameter of the outer surface of this film $b$ varies with $a$ according to the equation: $b log \left(\frac{b}{a}\right)=0.86\mathrm{}$; and this leads to the equation: $\mathrm{ma} log \left(\frac{b}{a}\right)=\mathrm{const}.\mathrm{}$, with which the experimental results agree. The constants for nitrogen and argon are inversely proportional to the molecular weights.