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.
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