Barlow's Wheel is the prototype of unipolar engine. Peter Barlow published its description in 1822 in the article "A curious electro-magnetic experiment".

The star-shaped copper wheel extends into a reservoir of mercury, which allows for higher currents (several Ampère) to pass through the wheel.

By applying a strong external magnetic field with appropriate orientation, a Lorentz force is exerted onto the wheel, which sets it in rapid motion.