Ther-mōm'e-ter, n.

n. [Fr. thermomètre, Sp. & It. termometro, from Gr. θερμή, heat, θερμός, hot, and μέτρον, measure.] An instrument for measuring temperature, founded on the principle that changes of temperature in bodies are accompanied by proportional changes in their volume or dimensions.

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 position of the top of the liquid column on a graduated scale.

Air-thermometer. See AIR-THERMOMETER. — Centigrade thermometer. See CENTIGRADE. — Differential thermometer, a thermometer usually with a U-shaped tube terminating in two bulbs, and containing a colored liquid, used for indicating the difference between the temperatures to which the two bulbs are exposed, by the change of position of the colored fluid, in consequence of the different expansions of the air in the bulbs. [See Illust. of Air-thermometer.] — Fahrenheit's thermometer, a thermometer, so called from its inventor, the scale of which has its zero at 32° below the freezing point of water, and contains 180 such degrees between the freezing and boiling points, the latter being at 212°. — Reaumur's thermometer, a thermometer, now little used, having its zero at the freezing point, and 80° between that and the boiling point of water. — Register or self-registering thermometer, a thermometer that registers the maximum and minimum of temperature occurring in a given interval of time.

A common form (Rutherford's) consists of two thermometers placed horizontally, a mercurial, containing a bit of steel wire to be pushed before the column and left at the point of maximum, and an alcohol thermometer, containing a slide of enamel, which is drawn back by the liquid, and left within it at the point of minimum.