Thoriated tungsten filaments

The rate of evaporation of electrons, ν e , from a thoriated tungsten filament depends on the temperature T and on θ, the fraction of the surface covered by thorium atoms. The relation of ν e to θ and T has been given by Brattain and Becker. From ν e the change in contact potential V of the filament surface produced by the adsorbed thorium can be calculated by the Boltzmann equation. Knowing σ the number of thorium atoms per cm. 3 , the dipole moment M of each is given by V = 2 τσM . By an equation already used for Cs films on tungsten, the 2-dimensional equation of state of the adsorbed film can be calculated from M . Then by Gibbs' adsorption equation the relation of the atom evaporation rate ν a to θ and T can be determined. It is thus calculated that in the range from θ = 0.2 to θ = 0.6, ν a varies in proportion to ϵ Hθ where H = 8.1 in good agreement with the value H = 7.8 given by Brattain and Becker's measurements. A recalculation of the data of numerous experiments in 1921–1923, using the new relations of ν e to θ 1 gives data on ν a as a function of T and θ in good agreement with the values of ν a calculated from ν e . The diffusion coefficients of Th through tungsten crystals, along grain boundaries and over the free filament surface, are calculated. A theory is given for the cause of the variation in the surface diffusion coefficient with σ. The probable mechanism of the production of the metallic thorium within the filament is discussed. At 2400° the thorium which arrives at the surface along grain boundaries, for some unknown reason, does not spread out over the surface as it does at lower temperatures (1900–2100°).