Definition
A sealed glass tube containing a small pool of liquid mercury and two or more electrical contacts. When the tube is tilted to a specified angle, the mercury flows and bridges the contacts, completing the electrical circuit. When tilted the other way, the mercury rolls clear of the contacts and breaks the circuit.
Plain English
A small switch that turns a circuit on or off depending on the angle it is tilted to. A blob of liquid metal inside the switch slides one way to make contact, and the other way to break it.
Context Anchor
Seen in older aircraft electrical systems and maintenance manuals, especially where a circuit needs to respond to position or tilt.
Derivation
Named after mercury, the only metal that is liquid at room temperature. Its ability to flow while still conducting electricity is what makes the switch work.
Why Pilots Care
Mercury switches react to aircraft attitude or movement, so they can trigger warnings or activate equipment based on pitch, bank, or impact. They are also a hazardous-materials concern during maintenance and disposal because mercury is toxic.
Analogy
It works somewhat like a tiny sealed level: when it tilts, the liquid inside runs to one side. In a mercury switch, that movement also controls an electrical connection.
Intuition Check
Do not read “mercury” here as the planet or as a general temperature word. In a mercury switch, mercury means the liquid metal inside the switch.
Example Sentence 1
The old crash-activated emergency locator beacon used a mercury switch to detect a sudden change in attitude.
Example Sentence 2
Maintenance crews replaced the mercury switch in the attitude indicator after it failed to sense a wings-level condition.