Dependence of the Conduction Mechanism on Polarity in Stationary and Sliding Contacts When High-Resistivity Film Is Present in the Contact

High-resistivity films thicker than 50 angstroms, which are present in a sliding contact between a graphite brush and a copper ring, are fritted at very small currents. Two kinds of fritting are observed: 1) when the copper is anodic, metallic bridges through the film between members are formed (the minimum field strength is approximately 4X104 volts per cm); and 2) when the copper is cathodic, the film is pulled away by electric forces to make place for a smaller spot which conducts more current. Bridge formation during sliding requires overvoltages because the time is too short for equilibrium states. The consequence on anodic copper is oxidation (limited by wear) of the clean cuts of broken bridges and fritting of overvoltages 1-1.5 volts above a basic level Uo. However, the contact spots on cathodic copper are protected against oxidation by a tunnel conducting film, and U remains constant at Uo. The metallic bridges are responsible for the great wear on the cathodic brush.