An electro-magnetic actuator includes a pair of magnetizable members, the one member being hollow and surrounding the other member. The other member defines a plurality of axially spaced circumferentially extending first ribs which are of reducing diameter towards one end of the member. The one member is generally of tapering construction and defines second ribs complementary to the first ribs. Recesses are defined between adjacent pairs of first ribs and accommodate windings. When electric current is applied to the windings the members move axially relative to each other to reduce the reluctance of the magnetic paths defined by the two members.