It is a mercury switch designed by Jean Bernard Léon Foucault (1819-1868), which speeds up the opening of the primary circuit in Ruhmkorff coils. It replaced the hammer switch with which the contacts, even when made of platinum, were deteriorated by the spark and, in the larger models, there was even the risk of their melting.
The exemplar presented here is a classic model which is mounted on a rectangular wooden platform (30 x 20 cm). Essentially, it consists of two identical mercury switches: one, which we may call the main switch, for powering the primary of Ruhmkorff's coils, and the other, which we may call the auxiliary switch for powering the circuit of an electromagnet composed of two vertical coils wound around a U-shaped soft iron core.
Parallel to the longer sides of the rectangular wooden platform, there are four vertical wire clamps on one side and four on the other.
The poles of the generator are connected to two of these on one side. The generator, through a commutator, powers the primary coil; the pile that powers the electromagnet through another commutator is connected to the others. Of those on the other side, two are for connection of the primary coil and the other two are for a possible connection of the condenser.
All the connections of the different parts of the device with the commutators and the outlet terminals are by means of wide strips of bare copper on the wooden platform.
Each of the two switches consists of a glass filled with mercury covered by a thin layer of alcohol, into which is placed a thin brass rod with a platinum point which is wet by the alcohol and penetrates slightly into the mercury. The glass is attached to a copper coaster to which at the centre is soldered a small platinum electrode which, on penetrating into the glass from the bottom, is in contact with the mercury. Each of the two switches is attached to the wooden platform by means of a brass support which is connected to the commutator and thus to one of the poles of the corresponding generator.
The thin rod with the platinum point of the main switch is attached to the end of the longer arm of a brass lever having arms of different lengths; this, instead of lying free, is attached transversally to a flexible, vertical, copper reed held by a sturdy brass bracket by means of a rack shaft that allows regulation of its height.
The brass bracket communicates with one of the two terminals for connection to the primary Ruhmkorff coil; the other terminal is connected to the commutator and then to the other pole of the generator. In this way, the primary Ruhmkorff coil has one of its ends connected directly to one of the poles of the generator and the other end to the other pole, through the mercury switch.
Also the rod with the platinum tip of the auxiliary switch is attached to the longer arm of the brass lever, but centrally. It, through the same arm, the copper reed and its support, is connected to one of the terminals of the electromagnet. The other terminal of the electromagnet is connected directly to the commutator and thus to the other pole of the pile. Thus, the electromagnet has one terminal connected directly to one of the poles of the pile and the other to the other pole through the mercury switch.
Above the flexible reed there is a steel rod with a spherical brass counterweight that can be set at different heights; when it is removed from its position of equilibrium and then let go, it oscillates at a frequency depending on the height at which the counterweight was set. At the end of the shorter arm of the lever there is a soft-iron bar (plate) which falls near the expansions of the electromagnet.

Daguin (1878), T. III, p. 765
Du Moncel (1873), p. 251
Jamin (1889), p. 252

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