A brass axle with plastic discs is free to roll along the rails and completes the electrical contact between them. When the axle is placed on the rails between the poles of the magnet, and a power supply unit is connected, the axle rolls along the rails, away from the centre of the magnetic field.

Apparatus To Show Force On Conductor In Magnetic Field - Comprising a strong U-shaped magnet and a pair of brass rails with 4mm socket terminals.