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Thermometer
device that measures temperature or a temperature gradient / From Wikipedia, the free encyclopedia
A thermometer is an instrument for measuring or showing temperature (how hot or cold something is). One type of thermometer is a narrow, concealed glass tube containing mercury or alcohol which extends along the tube as it expands. Another type is a digital thermometer, which uses electronics to measure temperature.
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Early thermometers from the time of Galileo measured the expansion and contraction of air. After the middle 17th century many used alcohol or mercury. In the 19th century a mechanical thermometer was in invented that used a bimetallic strip to move a pointer. This kind is still popular where people like to read temperature from a distance.