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The current density permitted at a carbon brush contact is of the order of

  • A.100 amperes per square centimetre
  • B.1 ampere per square centimetre
  • C.1,000 amperes per square centimetre
  • D.10 amperes per square centimetre

Answer: D

Grades vary between about eight and twelve for ordinary carbon and rather more for metal-graphite, which is what fixes the brush area for a given current. Exceeding it overheats the contact and burns the commutator surface.

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