A rotor for an electric motor or generator, the rotor comprising a housing having a first surface on which a first set of magnets is mounted; and an annular clamping ring for retaining a second set of magnets to a second surface of the housing, wherein the position of the second set of magnets on the second surface allows the position of the first set of magnets to be determined, wherein the annular clamping ring includes a mounting point for allowing the annular clamping ring to be mounted to the housing for retaining the second set of magnets to the second surface, wherein the cross sectional area of the annular clamping ring is reduced in a region adjacent to the mounting point and wherein the annular clamping ring is mounted over the second set of magnets.