This an exemplar of the most widespread version of the device used by Francois Arago (1786-1853) in 1824 in the experiments in which he showed that a copper disk placed horizontally and rotating around a vertical axis passing through its centre pulls with it a magnetic needle (or bar magnet) free to rotate around a vertical axis passing through its barycentre and coincident with the axis of rotation of the disk when it is placed at a distance of a few millimetres from the disk (spin magnetism).
The phenomenon was explained by Michael Faraday (1791-1867) in 1831 with the discovery of electromagnetic induction.
The device maintains its validity as a teaching aid in the study of induced currents in massive conductors (eddy currents) which find their simplest explanation in Lenz's law.
In fact, during the rotation of the disk, in all the closed paths that can be imagined as approaching or moving away from the magnet, there is the generation of induced currents which, by Lenz's law, on opposing the relative disk-magnet motion, pull the magnet into the rotation.
The device has a large rectangular wooden base (67.5 x 34 cm) and is composed of a copper disk (diameter 20.5 cm), mounted on a pin passing through its centre and of a large wooden pulley with a knob for making it rotate by hand.
The rotation of the pulley is transmitted to the copper disk by a belt connecting it to a smaller pulley, it too made of wood, attached to the disk's rotation axis.
Missing is the instrument's glass case which must have contained the magnetic needle (or bar magnet) suspended from a small vertical pin above the centre of the copper disk. In any case, the experiment can be performed by placing a plate of glass lying on the wooden supports which originally held the entire case as the supporting plane for the magnetic needle.
Daguin (1878), T. III, p. 747
Ganot (1864), p. 636
Privat Deschanel (1869), p. 788