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Thermometer

stem, mercury, glass, bulb, boiling, instrument, air, thermometers, water and tube

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THERMOMETER (Greek "heat-meas urer)), an instrument for determining the tem peratures of bodies. The general problem of thermometry is considered, in this encyclopedia, under the heading THERMOMETRY ; and the present article will he devoted mainly to the consideration of the common mercury-in-glass form of the instrument, and to certain of its modifications. The mercury-in-glass thermom eter depends for its action upon the fact that mercury expands about seven times as much as glass, for a given rise in temperature; so that when mercury is enclosed in a glass vessel, its apparent expansion is quite considerable. The mercury-in-glass thermometer consists essen tially of four parts, these being (1) the mer cury, (2) the spherical or cylindrical bulb of glass which contains it, (3) the fine tube which is attached to the bulb and which serves to make ,the expansion of the mercury evident, and (4) the graduated scale which is affixed to the capil lary tube and from which the indications of the instrument are read. In the manufacture of thermometers which are intended for accurate work, the mercury is carefully purified by fil tration through leather under pressure, and by subsequent distillation, and, in many cases, by chemical treatment also; and immediately before the mercury is placed in the thermometer it should be boiled so that it may be rendered free from moisture and from air. In the man ufacture of the capillary stems of thermometers, some kind of glass which experience indicates to be adapted to this end is melted and the op erator takes up a ball of it on the end of his blowpipe, blowing it out gradually and adding more glass to the mass from time to time. When the ball of molten glass has thus been brought to a convenient size, a second workman at es his blowpipe to it also, and the two, still blowing, walk apart, so that the sphere of glass is drawn out into a very long and fine tube, which, when it has cooled, is cut into lengths and annealed. The calibre of each of these lengths is subsequently measured under the microscope, so that the instrument maker may know how large a bulb must be attached to each piece, in order that the degree-spaces on the finished thermometer may be of approxi mately the desired size. The bulb of the ther mometer is usually made of a different kind of glass from the stem; and the process of mak ing it consists simply in fusing to one end of the open stem a knob of special glass and then blowing it to size through the stem. In ther mometers that are to be used for accurate sci entific work, the bulbs should always be made of one. of the three kinds of glass that are re spectively known as "verre dur,) Jena low and Jena The first of these is a French glass, which has been demonstrated to be peculiarly adapted for use in thermometer bulbs, by the elaborate experiments made at the International Bureau of Weights and Meas ures, at Paris. The two are made at Jena, Germany, and have been similarly proved to be adapted for use in accurate thermometers, by the experiments made at the Reichsanstalt, in Berlin. Since 1917 American glasses are manu factured that are well suited to the construction of accurate thermometers; but the problem to be solved is a difficult one and it will require much study and experiment. The stem and bulb of the thermometer being thus completed, the next step consists in cleaning them thor oughly on the inside. For this purpose they are washed out with hot nitric acid, with distilled water and with ether. They are then thoroughly dried, preferably by repeatedly exhausting them, while hot, with an air pump, and then filling them again with air that has been carefully dried. The next operation consists in filling the thermometer with pure mercury. In order

to do this the bulb is heated until the air that it contains is partially expelled and the open end of the stem is then dipped beneath the mercury. As the bulb cools, the air remaining within it contracts and mercury rises through the stem until the bulb has become partially filled; this operation being repeated until the bulb is full. The instrument is next heated to a temperature considerably higher than the highest temper ature to which it is to be exposed in use, the mercury that it contains becoming thereby so much expanded that it fills the entire stem and runs over at the top; and while the stem is still filled in this manner it is sealed off at the end by means of a blowpipe. In the higher grades of thermometers, a tiny pear-shaped bulb is left at the top of the stem, partly as a precaution against the destruction of the thermometer in case it is accidentally exposed to too high a temperature in its subsecjuent service and partly as an aid in the calibration of the stem. When such a pear-shaped bulb is provided, the stem may be sealed off at the end while the in ternal space is exhausted by means of an air pump, instead of while it is filled with mercury; or the tube may be filled, above the mercury column, with dry nitrogen or some other inert gas. The glass part of the instnunent having been completed, it remains to affix the scale to the stem. In high grade thermometers, the scale is engraved upon the stem directly; but in the cheaper forms it is usually engraved or stamped upon a piece of metal or of wood, to which the thermometer is finally secured. Let us consider the high grade instruments first and 'the cheaper ones afterward. Instruments of the fortner class are graduated by finding, experimentally, two definite points upon the stem, corresponding to two known tempera tures; the two known temperatures which are selected for this purpose being the boiling point and freezing point of water. When these two points are found, the space between them is divided into a certain number of equal parts, which are called degrees. In determining the position of the boiling point upon a thermom eter, the instrument is placed in steam that is rising from water that is boiling freely under a barometric pressure equal to that which would be produced by a column of pure, ice-cold mercury, 760 millimeters high, at sea-level in latitude 45°. When the mercury column in the thermometer ceases rising and becomes station ary, the point opposite which it stands is marked upon the stem and is called the "boiling point." If the barometric pressure under which the ex periment is performed is not identically equal to the value assumed above, allowance must be made for that fact by the aid of the experiments of Regnault (or others) upon the variation of the boiling point of water per millimeter of change of barometric pressure. The boiling point having been marked upon the thermom eter as here indicated, the instrument is then placed in a mixture of water and finely pulver ized ice, as quickly as this can .be safely done; and the point to which the mercury sinks is marlced and called the "freezing pomt." The distance, on the stem, between the boiling and freezing points, is then marked off, by means of a dividing engine, into as many equal spaces as there are degrees between the freezing and boiling points of water and (save for the affix ing of numbers to the degree-marks) the thermometer is complete. It may be, of course, that the thermometer is to be divided into half degrees, or into tenths; but the operation is precisely.the same, in this case, as it is when the division is to be made to negrees only.

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