The Contact Potential Between the Solid and Liquid Phases of Bismuth

Abstract

Contact potential between the solid and liquid phases of bismuth.---In order to obtain as gas-free metal as possible, the bismuth was distilled into the highly exhausted apparatus. The contact potential was measured, by the standard Kelvin method, against a copper plate whose absolute potential was maintained constant by protecting it from the temperature changes incident to melting and solidifying the bismuth. The results were remarkably free from disturbing factors which arise from residual gas effects. Solid bismuth was found to be of the order of -0.020 volts with respect to the molten metal. Small variations from this value were caused, presumably, by the chance crystalline state of the solid surface.Relation between thermo-electric power and temperature change at contact potential.---The equation $\frac{\mathrm{dV}}{\mathrm{dt}}=\frac{\mathrm{dE}}{\mathrm{dt}}$, which expresses the equality of the thermoelectric power to the temperature rate of change of contact potential, has so far failed of experimental verification. The present experiment, though not adapted to a real test of the equation, yielded evidence that the discrepancy can be greatly reduced by improvement of experimental conditions.The effect of surface charges on melting conditions.---From the results on the contact potential between the two phases of bismuth, it can be shown thermodynamically, following Bridgman's treatment of the matter, that, when one coulomb per ${\mathrm{cm}}^{2}$ is added to the surface of bismuth at constant volume, pressure, and capacity the melting point of the metal increases by $(\frac{1}{\mathrm{v}})\ifmmode\times\else\texttimes\fi{}0.02\ifmmode^\circ\else\textdegree\fi{}$C where v is the volume per unit area of the solid bismuth at its melting point.