Contribution to the Theory of the Contact between a Carbon Brush and a Copper Collector Ring

Abstract

The main subject is the interpretation of the fluctuations, ΔU, (0.3 to 0.6 v), in the contact voltage U between a carbon brush sliding smoothly on a copper ring. A special method was used and proved very informative. Within a fraction of a second, voltage or current was varied from an initial value (often zero) to a higher value and back again. Thereby voltage vs current or time were registered by means of an oscilloscope. It is shown that the sudden negative ΔU's are essentially due to electrical breakdowns—frittings at about 1 v—of the collector films, which are necessary in order to let the current through. The proof is given with stationary as well as sliding contacts by comparing the features involved with those of model diagrams, the design of which is based on the concept of fritting. Besides typical frittings of the type studied earlier, the concept of widening the area of conducting spots (generated through the collector films) by frittings as the current is increased, has been worked out. It is shown that every fritting of the collector films leads to a metallic junction by proving that consecutive records with decreasing voltage exhibit the character prescribed by the electrical and thermal constants of the electrode material used. Experimental proofs were obtained showing that ``frictional oxidation'' provided a noticeable contamination (of fritted spots) during one revolution between leaving the brush and meeting it again.