Abstract: A cylindrical outer surface of a commutator is formed with circumferencially extending striations separated by sharp-edged ridges which bite into the abutting surfaces of the brushes engaging the commutator to improve electrical conductivity and reduce static mechanical friction. These striations and the ridges may extend helically so that, on relative movement between the commutator and the brushes, the abutting surfaces of the brushes are scoured. There is also a tendency for the brushes to be moved axially of the commutator, against the restoring force provided by the brush leaf, until this restoring force rises sufficiently to cause the brush to return towards its original axial position so that the surfaces of the brushes and the commutator scrape against each other so as to present fresh surfaces for abutment.