Definition of: thermometer

(thər·mom′ə·tər) noun
An instrument for measuring the temperature of a substance, body, or space. The ordinary thermometer consists of a graduated glass capillary tube or stem with a bulb containing mercury which expands or contracts as the temperature rises or falls. The differential thermometer has two air bulbs connected by a U–tube, containing colored liquid, so that when the bulbs are exposed to different temperatures a shifting of the liquid in the tube will be caused by the difference of expansion of air in the bulbs. A resistance thermometer indicates, by means of the change in electrical conductivity of wires with temperature, the temperature of any given wire or its environment.
—clinical thermometer A thermometer accurately calibrated for determining body temperature, especially of a person. [<THERMO- + METER]