Abstract: An improved homopolar motor-generator having a hollow rotor, or a plurality of nested hollow rotors connected in series, with one magnetic pole of the stator located inside the rotor. This enables both sliding electrical contacts to be located close to the axis of rotation and thus decreases the linear velocity at the sliding contact. An alternative design of this invention comprises a hollow magnetized stator with an annular slot through which one or several ?-shaped current-carrying rods are free to rotate. The rods can also be embedded in a single cylindrical rotor. This design also enables the sliding electrical contacts to be located near the ends of the rods, and therefore close to the axis of rotation, and similarly decreases the linear velocity at the sliding contact.