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From The Collaborative International Dictionary of English v.0.48:
Thermometer \Ther*mom"e*ter\ (th[~e]r*m[o^]m"[-e]*t[~e]r), n. [Thermo- + -meter: cf. F. thermom[`e]tre. See Thermal.] (Physics) An instrument for measuring temperature, founded on the principle that changes of temperature in bodies are accompanied by proportional changes in their volumes or dimensions. [1913 Webster] Note: The thermometer usually consists of a glass tube of capillary bore, terminating in a bulb, and containing mercury or alcohol, which expanding or contracting according to the temperature to which it is exposed, indicates the degree of heat or cold by the amount of space occupied, as shown by the position of the top of the liquid column on a graduated scale. See Centigrade, Fahrenheit, and R['e]aumur. [1913 Webster] To reduce degrees Fahrenheit to degrees Centigrade, subtract 32[deg] and multiply by 5/9; to reduce degrees Centigrade to degrees Fahrenheit, multiply by 9/5 and add 32[deg]. [1913 Webster] Air thermometer, Balance thermometer, etc. See under Air, Balance, etc. Metallic thermometer, a form of thermometer indicating changes of temperature by the expansion or contraction of rods or strips of metal. Register thermometer, or Self-registering thermometer, a thermometer that registers the maximum and minimum of temperature occurring in the interval of time between two consecutive settings of the instrument. A common form contains a bit of steel wire to be pushed before the column and left at the point of maximum temperature, or a slide of enamel, which is drawn back by the liquid, and left within it at the point of minimum temperature. [1913 Webster] Thermometric