Electrical Resistance of Carbon Brushes on Copper Rings [includes discussion]

By using an accurate machine and specially shaped brushes, a method has been developed to analyze the resistance of carbon to copper sliding contacts. It is found that the conducting areas on the copper ring are long narrow strips. Although they act in somewhat different ways, the current density in the conducting areas is a constant of the order of 2X105 amperes per centimeter (cm)2 for both anode and cathode brushes. The brush rides on a film of water which is dispersed when the ring stops, or at high current, or in dry air. This film causes an increase in voltage on starting which is remarkably independent of the operating conditions and this increase has a value of about 0.35 volt. Not much higher voltage is required for the electrical breakdown (fritting) of the thin oxide film around the conducting spots. This leads to a theory that the size of the conducting spots is determined by an equilibrium between oxidation of the copper and fritting or breakdown of the oxide layer, which is then substantiated by some further experiments. In brief, it is found that the mechanism of conduction in sliding contacts can be explained by well-known physical effects. These include constriction resistance, water film, tunnel effect, oxidation, and film breakdown.