Mercury relay

A mercury relay (mercury displacement relay, mercury contactor) is a relay that uses mercury as the switching element.

They are used as high-current switches or contactors, where contact erosion from constant cycling would be a problem for conventional relay contacts.

Owing to environmental considerations about the toxicity of mercury, mercury relays are mostly obsolete though modern encapsulated units still have applications. They are generally being replaced by solid state relays.

Mercury relays consist of a vertical (usually glass) tube containing liquid mercury. They have isolated contacts at the bottom of the tube and partway up, usually in a side arm of the glass. The relay works by displacement. A pool of mercury fills the lower portion of the tube, but is insufficient to bridge the contacts. A magneticslug of iron or steel is placed in the tube, where it sinks by gravity so as to displace the mercury. The displaced mercury rises in the tube, sufficiently to bridge the contacts and complete the circuit between them.

Around the top part of the tube is placed the coil. When energised, this coil attracts the slug, lifting it upwards and out of the mercury pool. The mercury is no longer displaced, so flows downwards, away from the upper contact and the circuit opens. This allows for normally closed operation.

For the more traditional normally open relay operation, the side contact is arranged somewhat higher up (or the volume of mercury is reduced) so that contact is not made when the iron slug is freely floating on the pool of mercury. The control coil is mounted below the rest level of the slug and when energised draws down the slug deeper into the pool displacing additional mercury thus raising the level to the previously uncovered side contact and closing the circuit.

The mercury relay thus allows for switching of higher currents with a small control current for a large number of cycles. They are often installed into automatic controllers that required extended periods of unattended continuous switching operation. The mercury surface is self-restoring after an arc, and the contact resistance is low and stable.

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