To demonstrate the presence of induction currents when there is relative motion between a conductor and a magnetic field.
- Arago's disc
The apparatus consists of a copper disc free to rotate about a vertical axis and a bar magnet pivoted above the centre of the disc. A thin sheet of glass separates the disc from the magnet and protects it from air currents.
Rotating the disc causes induction to be set up in the disc, these circulate in closed curves and are called eddy currents. The electromagnetic forces between the currents and the magnet tend to move the magnet in the same direction as the disc.