TIIERMO'METER (from the Greek words Otp4s, hot, and mirpov, a nusincre) is an instrument by which the temperatures of bodies are ascertained. It consists of a glass tube with a capillary bore containing, in general, alcohol or mercury, which expanding or contracting by variations in the temperature of the atmosphere, or on the instrument being immersed in the liquid or gas which is to be examined, tho state of the atmosphere, liquid, or gas, with respect to heat, is indicated by a scale which is either applied to the tube or engraven on its exterior surface.
The end proposed by a thermometer is the measurement of the temperature of any body with relation to the temperature of some other substance, as of water at the point of freezing; but the measure so obtained must not be understood to express the absolute quantity or density of hest in any body, it being well known that different sub stances, though exhibiting the same apparent temperature, contain very different quantities of heat according to their capacities for that element. [Smarm HEAT; LATMT HEAT.] The thermometer must have been in use in the beginning of the 17th century, but it is not known, precisely, to whom the honour of the invention is due. A physician of Padua, named Santorio, and Cornelius Drebbel, of Alkmaar in Holland, are the persons to one of whom that honour is, with most probability, ascribed ; and the former, in his Commentaries on Avicenna' (1628), actually claims it for him self. It may, however, have happened with this, as with other scien tific discoveries, that the idea of tho instrument occurred to two persons or more at or about the same time.
The first thermometers were intended to indicate variations in the temperature of the atmosphere merely ; and the most simple of them consisted of a hollow glass-ball at one extremity of a long tube which was open at the opposite extremity. The air within the ball and tube being rarefied by the heat of a lamp, and the tube being in a vertical position, the open end was plunged into a vessel containing a coloured spirit ; as the enclosed air cooled, the pressure of the atmosphere on the liquid caused it to ascend in the tube till the expansive force of the air in the ball and the upper part of the tube became equal to the pressure. In this state, an increase of the
temperature of the atmosphere caused the air In the ball to expand and press down the spirit in time tube ; on the other hand, a diminution of temperature, by causing that air to contract, allowed the external pressure to raise the spirit. A scale was adapted to the tube in order to express the degree of temperature by the number of the graduation at the upper extremity of the spirit.
An effort was made to render the instrument portable by bending the lower part of the tube upwards, and terminating this branch also with a ball; and a small aperture was made in the latter in order that the external air might have access to the lower surface of the spirit. Mr. Boyle subsequently modified the air-thermometer by snaking the tube quite straight and open at both ends : the lower end was immersed in a small glass vessel containing both air and coloured spirit, and the vessel being formed with a neck which closely encircled the tube, it was hermetically sealed to the latter. The variations in the temperature of the atmosphere caused the air in the vessel to expand or contract, and thus to press with more or less force on the surface of the spirit ; the latter was consequently made to ascend or descend in the tube.
The air-thermometer invented by Amontons (1702) consisted of a tube nearly four feet long, open at both ends, and curved upwards at bottom, where it terminated in a balL This tube carried a column of mercury about 294 English inches high, so that the air In the ball was coruprovied by the weight of two atmospheres. A light body, in which was inserted the lower end of a wire, floated on the upper extremity of the column of mercury In the tube ; and near the upper end of the wire was an index by which the number of the graduation on a scale was shown. The variations of the temperature of the air in the ball caused the mercurial column to ascend or descend in the tube ; and thus were produced corresponding movements in the index. By this instrument it was proposed to measure high temperatures on a scale whose length was only half of that which was required with the simple airthermoineter.