Definition
A temperature-measuring instrument that uses a column of mercury sealed inside a thin glass tube. As temperature rises, the mercury expands and rises up a calibrated scale; as temperature falls, the mercury contracts and the column drops. The reading on the scale beside the column gives the temperature.
Plain English
A glass tube with a silver liquid metal inside. When it gets warmer, the silver liquid rises higher in the tube. You read the temperature off the markings on the side.
Context Anchor
Seen in weather, aircraft maintenance, and instrument discussions where temperature must be measured or compared.
Derivation
Mercury is the liquid metal used inside the tube, named after the Roman messenger god because it moves quickly and freely. Thermometer comes from Greek thermos (heat) and metron (measure) -- literally a heat-measurer. Together: a heat-measurer that uses mercury.
Why Pilots Care
Pilots rarely use a mercury thermometer directly, but understanding how one works helps make sense of temperature, pressure, and altimeter discussions -- many of which trace back to mercury-based instruments.
Intuition Check
A mercury thermometer does not measure mercury. It uses mercury’s movement inside the glass to measure temperature.
Example Sentence 1
The old weather station still had a mercury thermometer mounted in the shaded enclosure for backup readings.
Example Sentence 2
Older aircraft sometimes carried a mercury thermometer to verify outside air temperature before digital gauges became standard.