Abstract: A slip ring assembly for an a.c. machine comprises a pair of copper rings mounted on an electrically insulating sleeve formed of two arcuate portions, each of which extends for less than the true semi-circle so that the sleeve is not a true cylinder of circular cross-section. Means internally of the sleeve urges the arcuate portions outwardly when the sleeve is mounted on a rotor shaft of the a.c. machine so as to retain the rings in position on the sleeve. The means may be in the form of a separate, split cylindrical spring or in the form of a taper on the inner surface of the sleeve which engages a corresponding taper on the rotor shaft.