THERMOMETER. An instrument for measuring variations of heat or tempera ture.
The principle upon which thermome ters are constructed, is the change of vol ume which takes place in bodies when their temperature undergoes an altera tion. Generally speaking, all bodies ex pand when heated, and contract when cooled, and in such a manner that, under the same circumstances of temperature, they return to the same dimensions ; so that the change of volume becomes the exponent of the temperature which pro duces it. But as it is necessary not mere ly that expansion and contraction take place, but that they be capable of being conveniently observed and measured, only a small number of bodies are adapted for thermometrical purposes. Solid bodies, for example, undergo so small a change of volume with moderate varia tions of temperature, that they are in general only used for measuring very high temperatures, as the heat of fur naces, of melting metals, &e. Instru ments for such purposes are called py rometers. (See PYROMETER.) The gaseous fluids, on the other hand, are extremely susceptible of the impressions of heat and cold ; and as their changes of volume are great even with moderate accessions of heat, they are only adapted for indicating very minute variations, or for forming differential thermometers. (See DIETER zrriAL THERMOMETER.) Liquids hold an intermediate place ; and by reason of their moderate but sensible expansion through the ranges of temperature, within which observations have to be made for by far the greater number of purposes, are commonly used for the construction of thermometers. Various liquids have been proposed, as oils, ether, spirits of wine, and mercury ; but scarcely any other than the two last are now ever used, and mercury by far the most gen erally.
The ordinary mercurial thermometer consists of a glass tube, with a bulb blown at the lower end. Some mercury is boiled in the tube, and the whole of the atmospheric air driven away by im mersing the open end of the tube in a cup of quicksilver ; it rises in the glass as the latter cools. It is again heated, and closed at the open end. It is then immersed in melting ice and in boiling water. The heights at which the quick silver stands in these cases respectively, are marked 82° and 212°. The intervening space is divided into 180°, to make a Fahrenheit thermometer, and 100° to make a centigrade thermometer. Alco hol thermometers are used to indicate degrees of cold, as that liquid cannot be broken.
The differential thermometer consists of two legs connected, with a fluid working between them as either is made hotter than the other. It has a scale, but is not very accurate.
Thermometers are often slow in exhib iting the heat of new situations, and time should always be allowed, according to circumstances, for the progression of the atomic motion into or out of the mercury in the bulb.
The degrees of Celsius, or the cen tigrade scale, when desired, may be found by adding or subtracting for every degree 1.8 degree to or from the degree of Fahrenheit, and those of Beau mur, by adding or subtracting 2.25 de grees to or from Fahrenheit.