Abstract: An actuator has a rotor with permanently magnetized North and South poles. A housing supports three pole members having respective coils and pole pieces. The first pole piece is always a North pole but the coils on the second and third pole pieces are arranged such that each can be of either polarity with the other simultaneously being of the opposite polarity. The rotor therefore has two stable positions, one when the second pole piece is a North pole and the third pole piece is a South pole and the other being 120.degree. from this position in the clockwise direction (when the second pole piece is a South pole and the third pole piece is a North pole). The rotor movement may however be stopped 15.degree. short of each such position by mechanical detents, so as to limit the maximum angular movement to 90.degree.. The actuator may be used to drive a switchable microwave coupling arrangement.