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Brushes are used everywhere for transmitting electrical current between two parts that have an unlimited relative motion.

Brushes are typically made of graphite mixed with some binder. The graphite conducts the electrical current, but it also acts as a lubricant.

The metallic part that is in contact with the brush is called a slip ring, if it is continuous, like in synchronous motors, or a collector ring if it is segmented, like in DC motors or single-phase motors with brushes.

I've probably taken apart 10 automotive alternators. Every single one had brushes.

yeah I misspoke, I meant to say that it's a brush riding on a slip-ring (continuous contact, no arcing, lasts long) rather than a bunch of contacts in a cylinder (commutator, arcing, wears out).

Slip rings have brushes.

Yes but they wear less than DC brushed motors exactly because it's a slip ring and not a commutator

Because it's the discontinuities in the commutator where the sparks fly (with much help from self-induction of the motor's coils) and erode the ring and brushes.