page_title: Mercury relay
section_title: Operation
hierarchical_section_title: Mercury relay / Operation
caption_attribution_description: English: A mercury relay or contactor able to switch 60 Amp 115V AC power. This normally-open contactor is installed in a DEC Type 834 Power Control Panel mounted on the back side of a relay rack-mounted PDP-8 computer built in late 1965. The tubular devices connected across both the coil and the power contacts are thyrectors.
context_page_description: A mercury relay 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.
context_section_description: 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 magnetic slug 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, and thus flows downwards, away from the upper contact, and so 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, thereby displacing additional mercury and 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. 
  The glass tube of a mercury relay must be mounted near-vertically. The sensitivity of these relays can be altered by adjusting their angle relative to vertical. As sensitivity depends upon angle, they are unsuitable for use on mobile equipment or with conditions of high vibration.
caption: Normally open mercury relay, with coil around bottom of tube
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