The temperature in a contact between a carbon brush and a copper ring or commutator
One often meets the idea that very high temperatures are generated in the brush-copper contacts, and are in part accountable for the formation of the familiar dark collector film on the copper. Certainly, in case of poor commutation, flashes of high temperature appear in moments when the brush leaves the trailing edge of a segment, particularly if an arc ignites. Sometimes unevenness of the collector makes the brush jump, causing more or less complete current interruptions accompanied by temperature flashes on middle points of the segment. But the average supertemperature of the brush-copper contact above the bulk temperature of the copper usually is below the softening point of the copper, cf. the example at the end of this section. Such supertemperatures in contact a-spots of average size and in the hottest isotherm above them within the brush are the subject of this chapter. The bulk temperature of the copper is relatively easily determined by direct observation, whereas the supertemperature in the contact spots is confined to regions too small to be tested by any device. We here regard the bulk temperature of the copper as given; it is the basic temperature above which the supertemperatures are measured.
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