THERMOMETER (Gr. literally, heat measurer), a term which, in spite of its derivation, is usually restricted to instruments which measure temperature (see HEAT) by the expan n'on of bodies. Like that of the telescope and microscope, and many other valuable pieces of philosophical apparatus, its early history is very obscure. There are various clitimants who seek to share at least a part in the credit of its invention; and they agree pretty well in referring it to somewhere in the beginning of the 17th century. 1%e shall not waste space in endeavoring to settle such matters of history, but proceed at once to s description of the forms of the instrument now most commonly used; after which we shall say a few words about the actual value of their indications, and finish by a rapid sketch of a few other instruments also adapted for the measurement of temperature, but not usually known by the name of thermometer.
Let us commence with the ordinary spirit-thermometer, as it is called; where the indications are given by the expansion of a quantity of alcohol which fills entirely a glass bulb, and partially a narrow tube attached to it.
To construct such an instrument, a capillary tube is selected, of as uniform a bore as possible. The easiest method of testing its uniformity is to introduce a column of mer cury, about an inch long, into the tube, and gradually move it along by inclining the tube, carefully measuring the length of the column in each of its successive positions. It is obvious that the column will be longer the smaller is the mean section of the por tion of the tube occupied at any time by the drop of mercury. If considerable differ ences of length are found, the tube is rejected at once. The best tubes are those which, if showing any change, taper very slowly but nearly uniformly from one extremity to the other; a defect which can easily be allowed for in the subsequent graduation of the instrument. A bulb is blown on one end of the selected tube; large, if the instrument is meant to he very delicate; small, if a common instrument is to be made, or oue which will work through a great range of temperature. The bulb is heated to expand the con
tained air, and then the open end of the tube is plunged into alcohol, usually tinged with coloring matter, for greater visibility. As the bulb cools, the atmospheric pressure on the alcohol in the vessel forces some of it into the stem, and perhaps a little into the bulb. The tube being then inverted, a few dextrous taps suffice to shake the greater part of the alcohol into the bulb. The lamp is again applied, with caution, until the alcohol boils, and the rapidly escaping vapor drives the air almost entirely from the tube, whose open end is immediately plunged again into the colored spirit. Unless the stem be nearly 40 ft. in length—and thermometers have been made by Forbes (q.v.) of a length approaching to this for the measurement of underground temperature—the alco hol fills the whole of the ball and stem as soon as the glass has cooled. The bulb is again cautiously heated, so that, by the expansion of the spirit, such a portion may be expelled, that, when the whole has again cooled, the level of the liquid iu the tube may stand near-some point previously determined on with reference to the particular employ ment for which the instrument is destined. Finally, the lamp being again applied to the tube, near the upper surface of the liquid, that portion of the spirit is again made to boil; and while the vapor keeps the free end of the tube clear of air, that end is her metically sealed; and the glass-blower's part of the work is done. A somewhat similar, but more difficult process has to be gone through, if other liquids, such as ether, sul phuric acid, mercury, etc., are employed to fill the bulb; each of these liquids having its own special use in certain philosophical inquiries, as we shall presently see. It only remains that the instrument be graduated, so that some definite information may be given by its indications.