The instrument demonstrates the forces produced by interactions between an electric current and the field of a magnet.
The circuit is formed by corrugated copper sheets and horizontal rods with tips bent at 90°; connection with the terminals of a battery by means of mercury contained in trays at the base of the instrument. A cylindrical magnet is immersed in the tray filled with mercury, suitably weighted to ensure that it remains in equilibrium, just above the surface of the mercury.

If the circuit is closed so that current flows and the ends of the horizontal rods are immersed in the mercury, one in the centre of the tray and the other near the edge, the magnet turns around the central point, pushed by the interaction of forces due to the current circulating in the mercury and the field of the magnet.

Another way of using the instrument is to make sure (by adding a little mercury) that the end of the horizontal rod is in contact with the top of the magnet in the centre of the tray. In this case, when the circuit is closed the magnet rotates.

Device on display.