The Rate of Vaporization of Mercury from an Anchored Cathode Spot

Two major difficulties which interfere with the measurement of the vaporization of mercury at a free cathode spot are the spray and the normal evaporation from the free surface. By anchoring the spot the spray has been eliminated, and by a specially designed, water-cooled cathode structure the free surface has been limited to small areas of controllable size. By means of a calibrated feed mechanism, mercury was fed to the cathode as fast as it vaporized. Measurements were made over a range of arc current, cooling water temperature, and exposed area. The rate of vaporization (g/coulomb) increased with arc current, temperature, and exposed area, all due to increased normal evaporation. The rate may also increase with arc current, because of cumulative effects within the spot itself. Extrapolated to zero arc current the rate was found to be approximately 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ g/coulomb. This lies within the limits set by von Issendorf. It means that eight elementary charges pass through the arc for every atom evaporated.