an instrument that measures temperature. Thermometers provide a way to estimate temperature from its effect on a substance with known characteristics (as a gas that expands when heated). Various types of thermometers measure temperature by measuring the change in pressure of a gas kept at a constant volume, the change in electrical resistance of metals, or the galvanic effect of dissimilar metals in contact. The most common thermometer is the mercury-filled glass tube that indicates temperature by the expansion of the liquid mercury.
