This is a small induction coil mounted on a wooden base, consisting of primary and secondary circuits wrapped around a core made of a bundle of soft iron wire. The primary circuit is connected to two terminals for connection to a battery, located on the instrument’s base. The secondary circuit is connected to two other terminals from which the induced current may be drawn; this is generated in the secondary circuit by interruptions in the current supplied to the primary circuit. The coil, which could be used for both laboratory experiments and electrotherapy applications, had the unusual features of a rotary switch (now missing) and the system for determining current direction in the primary winding. The windings at the ends of the soft iron cylinder of the switch draw from the mercury-filled trays fixed to the base of the instrument and connected to the battery terminals, touching the mercury’s convex meniscus. At every half turn the polarity is reversed and the attraction and repulsion between the switch cylinder and the iron coil core cause the interruption and inversion of the current in the primary coil.

The coil was restored in 2017 by Paolo Brenni and Anna Giatti in the Fondazione Scienza e Tecnica laboratory in Florence.

Device on display.