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Thermometer. An instrument for measuring the temperature (q.v.) of a body or space. In its simplest form, it consists essentially of a glass tube sealed under vacuum that contains a column of liquid which expands and contracts, i.e., rises and falls, with changes in temperature. The temperature is indicated by the top of the liquid column on a calibrated scale marked either on the tube or on its frame. Common types of thermometers include metallic thermometers, pyrometers (q.v.), resistance thermometers, and others, all of which operate according to different principles.