The apparatus, invented by the Paris builders, the Breton Brothers, derives, with some simplifying changes and improvements, from one built and used by André Marie Ampère (1775-1836) to verify what he had deduced theoretically: that a magnetized bar, free to rotate around its own axis, begins to revolve when it is crossed by a current at a place not concentric to it. The rotation is caused by the action of the current on the part of the magnet that it crosses.
On portions of the magnet which are symmetrical to the centre, the action is contrary; for this reason, in Breton's configuration, in which the current crosses an entire half of the magnet, the effect is at its maximum.
On inverting the direction of the current, the direction of rotation of the magnet is also inverted.