Definition
A heavy, silvery metallic element that is liquid at normal temperatures. Because of its high density and uniform expansion with temperature, mercury is used in barometers, manometers, and certain thermometers to measure pressure and temperature.
Plain English
A heavy liquid metal used inside instruments to measure pressure. It rises and falls in a sealed tube as the surrounding pressure changes, and the height of the column tells you the reading.
Context Anchor
Seen in weather, altimeter-setting, and barometric pressure discussions, especially in the unit inches of mercury.
Derivation
Named after the Roman god Mercury, the swift messenger, because the liquid metal moves quickly and slips around like a fast-running fluid. The chemical symbol Hg comes from the Latin hydrargyrum, meaning 'liquid silver.'
Why Pilots Care
Correct altimeter settings in inches of mercury prevent altitude errors during flight.
Grounding Statement
In a traditional barometer, changing air pressure pushes a column of mercury higher or lets it fall lower.
Intuition Check
Mercury does not mean the planet here. In this aviation context, it means the liquid metal used as the reference in pressure measurement.
Example Sentence 1
The current altimeter setting is 30.05 inches of mercury.
Example Sentence 2
Manifold pressure dropped to 18 inches of mercury during the climb.