• The traditional liquid-filled thermometer (usually with mercury or red alcohol) relies on thermal expansion of the liquid. As the temperature rises, the liquid becomes less dense and expands upward to show a higher reading. Others use an expanding piece of metal (often a coil) to move a pointer in response to changes in temperature. Electronic thermometers might use semiconductor devices whose electrical behavior (resistance or current) changes with temperature. Thermocouples rely on a changing voltage with temperature. Optical thermometers measure infrared light emission.

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