Volume 5 — From Chambers's Journal. Thermometers.

An ordinary thermometer consists, as everybody knows, of a glass tube, fixed to a scale. This tube contains a fine bore, and has a bulb blown at one extremity. Some liquid, generally mercury or alcohol, is introduced into the tube, the air is driven out, and the tube is sealed. The quantity of fluid, say mercury, admitted into the tube is so regulated that at common temperatures the bulb and a portion of the bore are filled. The remainder of the bore, which is empty, affords space for the mercury to rise. This arrangement renders very perceptible the alterations in the volume of the mercury due to changes of temperature, a very slight increase or diminution of volume causing the mercury to rise or to fall appreciably in the fine bore. After sealing, the scale has to be adjusted to the tube, and the instrument is complete.

Thermometers of the most accurate make are called standard thermometers. In their manufacture, numerous precautions are necessary from the very outset. Even in so simple a matter as the choice of the tube of glass much care is requisite. The bore has to be tested, in order to ensure that it is of uniform capacity throughout. It is found that tubes, as they come from the glass-house, contain a bore wider at one extremity than the other. The bore is, in fact, a portion of a very elongated cone. In a hundredweight of tubes, not more than half a dozen or so can be picked out in which the bore is perfectly true. The bore is tested in a very ingenious though simple manner. A bulb is blown, and a very small quantity of mercury is admitted into the tube about as much as will fill an inch and a half of the bore. By alternately cooling and heating the bulb, this delicate thread of mercury is driven from one end of the tube to the other, and during this process its length is carefully measured in all parts of the tube. Should the length of the mercury alter in various situations, it is evident that the capacity of the bore is not uniform throughout, and the tube must be rejected. In blowing the bulb, an elastic ball, containing air, is used. The ordinary method of blowing glass bulbs by means of the breath is found to cause the introduction of moisture into the tube.

The size of the bulb has next to be considered. A large bulb renders the instrument slow in its indications of change, owing to the quantity of mercury that has to be acted on. On the other hand, if the bulb is too small, it will not contain sufficient mercury to register high temperatures, unless the bore is exceedingly fine.

The shape of the bulb is of importance. Spherical bulbs are best adapted to resist the varying pressure of the atmosphere; while cylindrical bulbs expose larger surfaces of mercury, and are therefore preferred for more delicate instruments. Various plans have been suggested in order to obtain thermometers of extreme sensitiveness for delicate experiments. Some have been made with very small thin bulbs, to contain a very small quantity of mercury; but in these the indicating column is generally so fine, that it can only be read by the aid of a powerful lens. Instruments have been contrived with spiral or coiled tubular bulbs; but the thickness of glass required to keep these in shape nullifies the effect sought to be obtained—namely, instantaneous action. Messrs. Negretti & Zambra, the well-known meteorological instrument-makers, have recently succeeded in constructing a thermometer which combines sensitiveness and quickness of action, and which presents a good visible column. The bulb of this thermometer is of a gridiron form. The reservoir is made of glass, so thin that it cannot be blown; it can only be formed by means of a spirit-lamp; yet its shape gives it such rigidity that its indications are not affected by altering its position or by standing it on its bulb. The reservoirs of the most delicate of these instruments contain about nine inches of excessively thin cylindrical glass, the outer diameter of which is not more than the twentieth of an inch, and, owing to the large surface thus presented to the air, the indications are positively instantaneous. This form of thermometer was constructed expressly to meet the requirements of scientific balloon ascents, to enable the observer to take thermometric readings at precise elevations. It was contemplated to procure a metallic thermometer; but, on the production of this perfect instrument, the idea was abandoned.

The shape and size of the bulb having been determined, the workman next proceeds to fill the tube. This is effected by heating the bulb while the open end of the tube is embedded in mercury. Upon allowing the bulb to cool, the atmospheric pressure drives some mercury into the tube. The process is continued until sufficient mercury has entered. The mercury used in filling should be quite pure, and should have been freed from moisture and air by recent boiling. It is again boiled in the tube after filling; and when the expulsion of air and moisture is deemed complete, and while the mercury fills the tube, the artist dexterously removes it from the source of heat, and at the same moment closes it with the flame of a blow-pipe. It sometimes happens that in spite of every care a little air still remains in the tube. Its presence is detected by inverting the tube, when, if the mercury falls to the extremity (or nearly so) of the bore, some air is present, which, of course, must be removed.

The thermometer, after being filled, has to be graduated. Common thermometers are fixed to a scale on which the degrees are marked; but the graduation of standards is engraved on the stem itself, in order to insure the greatest possible accuracy. The first steps in graduating are to ascertain the exact freezing-point and the exact and to mark on the tube the height of the mercury at these points. The freezing-point can be determined with comparative ease. Melting ice has always the same temperature in all places and under all circumstances, provided only that the water from which the ice is congealed is pure. The bulb and the lower portion of the tube are immersed in melting ice; the mercury descends; the point where it remains stationary is the freezing-point, and is marked on the tube.

The determination of the boiling-point is more difficult. The boiling-point varies with the pressure of the atmosphere. The normal boiling temperature of water is fixed at a barometric pressure of 29.922 inches of mercury having the temperature of melting ice, in the latitude of 45, and at the level of the sea. Of course, these conditions rarely if ever co-exist; and consequently the boiling-point has to be corrected for errors, and reduced for latitude. Tables of vapor tension, as they are called, computed from accurate experiments, are used for this purpose. Regnault's tables, the most recent, are considered the best.

An approximate boiling-point is first obtained by actual experiment. A copper boiler is used, which has at its top an open cylinder two or three inches in diameter, and of sufficient length to allow a thermometer to be introduced into it, without touching the water in the boiler. The cylinder is surrounded by a second one, fixed to the top of the boiler, but not entering it, the two being about an inch apart. The outer cylinder is intended to protect the inner one from contact with the cold external air. The thermometer to be graduated is placed in the inner cylinder, and held there by a thong of India-rubber. As the vapor of the boiling water rises from the boiler into the cylinder, it envelops the thermometer, and causes the mercury to ascend. As the mercury rises, the tube is gradually lowered, so as to keep the top of the mercury just visible above the cylinder. When the mercury becomes stationary, the position of the top of the column is marked on the tube; and the boiling-point, subject to corrections for errors, is obtained.

The freezing and boiling points being determined, the scale is applied by dividing the length between the two points into a certain number of equal degrees. This operation is performed by a machine called a dividing-engine, which engraves degrees of any required width with extreme accuracy.

The scale used in the United Kingdom, in the British colonies, and in North America, is that known as Fahrenheit's. Fahrenheit was a philosophical instrument maker of Amsterdam. About the year 1724, he invented the scale with which his name is associated. The freezing point of his scale is 32 degrees, the boiling-point 212 degrees, and the intermediate space is composed of 180 degrees. This peculiar division was thus derived. The lowest cold observed in Iceland was the zero of Fahrenheit. When the thermometer stood at zero, it was calculated to contain a volume of mercury represented by the figures 11,124. When plunged into melting snow, the mercury expanded to a volume represented by 11,156; hence the intermediate space was divided into thirty-two equal portions or degrees, and thirty-two was taken as the freezing-point of water. [Footnote 246] Similarly, at the boiling-point, the quick-silver expanded to 11,336. Fahrenheit's scale is convenient in some respects. The meteorological observer is seldom troubled with negative signs, the divisions of the scale are numerous, and tenths of degrees give all the minuteness usually requisite.

In 1742, Celsius, a Swede, proposed zero for the freezing-point, and 100 degrees for the boiling-point, all temperatures below freezing being distinguished by the negative sign (-). This scale is known as the Centigrade. It is in use in France, Sweden, and in the south of Europe; it has the advantage of decimal notation, with the disadvantage of the negative sign.

Reaumur's scale is in use in Spain, Switzerland, and Germany. It differs from the Centigrade in this, that the freezing and boiling points are separated by 80 degrees instead of 100 degrees.

It would not be difficult to construct a scale which should combine all the advantages of Fahrenheit's and of the Centigrade. Freezing-point should be fixed at 100 degrees; and boiling-point should be fixed at as many hundred divisions or degrees above 100 degrees as might be agreed on by practical men as most convenient. The advantages of decimal notation would thus remain as in the Centigrade scale, and the minus sign would be got rid of.

And now, having applied the scale, and having exercised every precaution, can we congratulate ourselves on possessing a perfect instrument? Disheartening as it may appear, the standard instrument of to-day may not be accurate to-morrow. It is more than probable that the freezing point will become displaced. This curious phenomenon has never been satisfactorily explained. Messrs. Negretti & Zambra, in their treatise on Meteorological Instruments, (a work which abounds with information of a most interesting nature,) say, in reference to displacement of the freezing point, that "either the prolonged effect of the atmospheric pressure upon the thin glass of the bulbs of thermometers, or the gradual restoration of the equilibrium of the particles of the glass after having been greatly disturbed by the operation of boiling the mercury, seems to be the cause of the freezing-points of standard thermometers reading from a few tenths to a degree higher in the course of some years." To obviate this small error, it is the practice of the makers in question "to place the tubes aside for about six months before fixing the freezing-point, in order to give time for the glass to regain its former state of aggregation. The making of accurate thermometers is a task attended with many difficulties, the principal one being the liability of the zero or freezing-point varying constantly; so much so, that a thermometer that is perfectly correct to-day, if immersed in boiling-water, will be no longer accurate; at least, it will take some time before it again settles into its normal state. Then, again, if a thermometer is recently blown, filled, and graduated immediately, or, at least, before some months have elapsed, though every care may have been taken with the production of the instrument, it will require some correction; so that the instrument, however carefully made, should from time to time be plunged into finely-pounded ice, in order to verify the freezing-point."