Thermometers measure temperature, usually by how far a liquid expands inside a narrow tube.

The longer version

Most ordinary thermometers work by expansion. A liquid sealed inside a narrow tube swells slightly as it warms and climbs higher up the scale, then contracts and falls as it cools. The narrow bore magnifies a tiny change in volume into a movement that is easy to read against a marked scale.

Alcohol and mercury have both been used because their expansion is steady and predictable across everyday temperatures. Electronic versions instead rely on the fact that the electrical resistance of a metal, or the voltage across a junction, changes in a repeatable way with temperature.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.