Mercury relay

The displaced mercury rises in the tube,[ii] sufficiently to bridge the contacts and complete the circuit between them.

When energised, this coil attracts the slug,[iii] lifting it upwards and out of the mercury pool.

The control coil is mounted below the rest level of the slug, and when energised draws down the slug deeper into the pool, thereby displacing additional mercury and thus raising the level to the previously uncovered side contact and closing the circuit.

They are often installed into automatic controllers that required extended periods of unattended continuous switching operation.

As sensitivity depends upon angle, they are unsuitable for use on mobile equipment or with conditions of high vibration.

For some applications, particularly inductive loads, this alone may be a reason for their use – the timing of contact closure is not rapid, but the avoidance of bounce is valuable.

This gives the low bounce advantage of mercury, although the current capacity is still limited to broadly that of the original reed relay.