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A thermometer is a device for measuring temperature. The two simplest ways to do this both rely on the principle that hot things expand and cold things contract. A liquid such as colored alcohol or mercury can be placed in an evacuated glass tube. The expansion and contraction of the liquid indicates the temperature, which can be read from a scale next to the expanded liquid. A helical spring can also be used, as in the common refrigerator thermometer. In such a refrigerator, the thermometer points at a scale.

The first useful thermometer was invented in 1714 by Gabriel Fahrenheit,[1] who used mercury in a glass tube graduated with 180 degrees between the freezing and boiling points of water (32° F and 212° F). Modern scientists use the Celsius scale, graduated with 100 degrees between freezing and boiling (0° C to 100° C).