Double sparking magnet on the Nobili pattern. Two permanent horseshoe magnets, held together by brass brackets, are placed on a large wooden base. The coil is oscillated rapidly between the magnet poles by a crank-operated mechanism. Whenever the circuit consisting of the magnet and the flexible contact strip is broken, an electric spark is produced by the induced current due to the opening and closing of the circuit. The alternating current generated in this way can be collected by the commutator attached to the base. Direct current can be produced by removing one of the contacts.

Nobili's commutator may have been inspired by the one developed by Hippolyte Pixii for his magneto-electric machine (inv. 552).

This instrument was made by Corrado Wolf, about whom Nobili wrote in his Memorie (Florence, 1834): "The compound magnets are very finely made; like most of my other apparatuses, they are produced in the workshop of a highly skilled machine-maker, Mr. Wolf, who works for me to my full satisfaction and to the genuine benefit of science." Provenance: Lorraine collections.