Mercury switch

A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs (tilt, displacement, radial, etc.) but they all share the common design strength of non-eroding switch contacts.
The most common is the mercury tilt switch. It is in one state (open or closed) when tilted one direction with respect to horizontal, and the other state when tilted the other direction. This is what older style thermostats used to turn a heater or air conditioner on or off.
The mercury displacement switch uses a 'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode. This design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers.

Uses

Roll sensing

Tilt switches provide a rollover or tip over warning for applications like construction equipment and lift vehicles that operate in rugged terrain. There are several non-mercury types, but few are implemented due to sensitivity to shock and vibration, causing false tripping. However, devices resistant to shock and vibration do exist.

Automotive uses

Automobile manufacturers once used mercury switches for lighting controls (for example, trunk lid lights), ride control, and anti-lock braking systems. Scrapped automobiles can leak mercury to the environment if these switches are not properly removed. Since 2003, new American-built cars no longer use mercury switches.

Fall alarms

Work performed in confined space (such as a welder inside a tank) raises special safety concerns. Tilt switches sound an alarm if a worker falls over.

Aircraft attitude indicators/artificial horizons

Electrically driven attitude indicators typically use mercury switches to keep the gyro axis vertical. When the gyro is off vertical, mercury switches trigger torque motors that move the gyro position back to the correct position. (Air driven attitude indicators use a different operating principle.)

Thermostats

Mercury switches were once common in bimetal thermostats. The weight of the movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position. The mercury also provided positive on-off switching, and could withstand millions of cycles without contact degradation.

Doorbells

Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters.

Pressure switches

Some pressure switches use a Bourdon tube and a mercury switch. The small force generated by the tube reliably operates the switch.

Vending

Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor. They are also commonly used in vending machines for tilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product.

Advantages

Mercury switches offer several advantages over other switch types:

• The contacts are enclosed, so oxidation of the contact points is unlikely.
• In hazardous locations, interrupting the circuit does not emit a spark that could ignite flammable gases.
• Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don't wear out.
• Even a small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size.
• Sensitivity of the drop to gravity provides a unique sensing function, and lends itself to simple, low-force mechanisms for manual or automatic operation.
• The switches are quiet, as no contacts abruptly snap together.
• The mass of the moving mercury drop provides an over center effect to avoid chattering as the switch tilts.
• The envelope can include contacts for two or more circuits.

Disadvantages

Mercury switches have several disadvantages:

• Their relatively slow operating rate (due to the inertia of the mercury drop) makes them unsuitable for applications that require many operating cycles per second.
• Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damage to the envelope.
• The mercury drop forms a common electrode, so circuits are not isolated from each other in a multi-pole switch.
• The sensitive to gravity may make them unsuitable in portable or mobile devices that can change orientation or that vibrate.
• Mercury compounds are highly toxic and accumulate in any food chain, so safety codes exclude mercury in many new designs.