Barlow's wheel of 1822 is a kind of electric motor that shows the interplay between electricity and magnetism. It combines a circuit made from a battery (originally a voltaic pile), a metallic wheel rotating through a mercury pool, and a horseshoe magnet. The metallic wheel rapidly reaches a maximum angular velocity. The vectors representing the force, the magnetic field, and the direction of the current are shown. The angular velocity is proportional to the intensity of current and the magnetic field.