An electro-magnetic device comprises a first annular member and a second member having a cylindrical surface, the two members being interengaged. The presented surfaces of the two members are each provided with a two start or a multiple of a two start, thread formation thereby defining helical ribs on the two members. A winding is wound on the second member the winding arrangement being such that in the grooves or adjacent grooves the direction of current flow will be in the opposite direction when the winding is energized whereby the two members will move relative to each other to align the ribs on the members.