The coil comprises a primary and secondary circuit, isolated from each other and consisting of wire wound around the iron core. These coils are enclosed in a brass casing; each is connected to a pair of clamps placed at the base of the instrument.
At the top of the instrument there is a switch connected to the primary circuit, consisting of a brass sheet fixed to a pin on one side and free on the other side, just above the core of the coil. The current flowing in the primary circuit magnetizes the core which attracts the metal sheet; when this is pulled down it breaks the circuit, the core is demagnetized and the sheet goes back up, closing the circuit once more; the process is repeated.

Device on display.