To understand the physics behind a homopolar motor, first and foremost you have to understand that besides the electric current in a current-carrying wire there is also a magnetic current (which the contemporary science calls magnetic field). The two currents are inseparably bound to each other very similar to the water swirling current and the cavitation spiral current which an underwater propeller creates. The water swirling current corresponds to the electric current, whereas the cavitation spiral current corresponds to the magnetic current. Watch the cavitation spiral current in this short