Discussion
The patina on a healthy commutator surface consists of
Answer: D
Correct answer: a thin film of copper oxide and carbon
The patina is a thin oxide film with carbon from the brushes embedded in it, and it forms in service on a commutator that is running correctly. It lubricates the contact and provides the resistance that assists commutation, so it should be left alone - polishing a commutator back to bare copper does real harm.
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