Abstract: An efficient and light weight energy storage and retrieval flywheel is described. Two monopolar motor-generators are assembled in a single rotating magnetic cylinder. The ends of the cylinder are enclosed by two magnetic disks with bearings mounted on a fixed cylinder. The two end disks and the flywheel cylinder rotate as an energy storage flywheel. Two rotors are mounted within the magnetic cylinder on two output shafts which drive gear trains. Two fixed pole pieces support two field coils and provide magnetic paths for flux which crosses the rotor armatures at right angles. Two nonmagnetic contact rings and two rotating mercury contact rings provide a current path between the rotor armatures and the outer cylinder. A nonmagnetic contact disk between the two rotors closes a low resistance current loop which interacts with the magnetic flux to provide equal and opposite torques on the rotors and flywheel cylinder. The rotors rotate in the opposite direction to the flywheel.