Rate of evaporation from tungsten filaments at 2825^∘{}K.---Using the Langmuir method, it was found that in vacuum the rate of evaporation per cm² 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 (b/a)=0.86; and this leads to the equation: ma log (b/a)=const., with which the experimental results agree. The constants for nitrogen and argon are inversely proportional to the molecular weights.
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Gorton R. Fonda (1923) studied this question.