Encyclopædia Britannica, Ninth Edition/Thermometer

by Hugh Robert Mill

THERMOMETER, an instrument for detecting and measuring differences in temperature. The name is usually restricted to instruments adapted for use at moderate temperatures; those for measuring high temperatures are termed pyrometers (see Pyrometer). Thermometry has been treated theoretically under Heat (see vol. xi. p. 558 sq.). It here remains to trace the history of thermometers, and to describe the principal forms in use.

Fig. 1.

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Fig. 2.

History.—The honour of inventing the thermometer has been given to several natural philosophers of the 16th century; the claims of Robert Fludd are more tangible than those of Drebbel and Santorio, but the instrument invented by Galileo before 1597 seems best entitled to be considered the precursor of accurate thermometers. All the early instruments were air thermoscopes, and, until the variations of atmospheric pressure were discovered, their use was only deceptive. Galileo’s thermometer (fig. 1) consisted of a glass bulb containing air, terminating below in a long glass tube which dipped into a vessel containing a coloured fluid. The variations of volume of the enclosed air caused the fluid to fall or rise in the tube, to which an arbitrary scale was attached. The great step in advance of inventing the alcohol thermometer is also due to Galileo, but the date (probably 1611 or 1612) is not precisely known. Rinieri certainly had alcohol thermometers made before 1647, and they are referred to as familiarly known in the oldest memoirs of the Accademia del Cimento (1667). In form they resembled those now in use; they had large spherical (or, occasionally, cylindrical or helical) bulbs, and the degrees intended to represent thousandths of the volume of the reservoir were marked with beads of enamel fused on to the stem (fig. 2). All the Florentine instruments were graduated in the same way, but the scale was arbitrary, and the recorded readings were accordingly supposed for a long time to be useless. In 1829 the fortunate discovery by Antinori of a number of those early Florentine thermometers enabled their scale to be ascertained and translated into known degrees. The temperature of melting ice was marked by them as 135, while 50 corresponded with 55° C. No means of comparing observations made by thermometers of different manufacture existed until certain fixed points of universal accessibility were discovered. The thermal conditions of freezing water were studied with great care, but natural congelation was generally supposed to take place at variable temperatures, until Fahrenheit proved that, however much water could be cooled down without freezing, the temperature when ice began to form was always the same. Hooke, in 1665 (Micrographia, p. 38), describes the manufacture and graduation of comparable spirit thermometers with the freezing point of water as the zero of their scales, and he evidently recognized it as fixed. Halley in 1693 stated that the temperature of boiling water is constant, and this was again proved by Amontons in 1702. In 1694 Renaldeni of Padua proposed to graduate thermometers by taking as standards of temperature mixtures of definite volumes of ice-cold and boiling water. This method, although theoretically admirable (see Heat, vol. xi. p. 559), is defective in practice. Seven years later Newton proposed anonymously (Phil. Trans., 1701, vol. xxii. p. 824) a thermometer scale on which the temperature of freezing water was 0°, and that of the blood of a healthy man 12°. Continuing the graduation of a linseed-oil thermometer above this point, he found that water boiled at 34°. Fahrenheit in 1714 took as fixed points the temperature of the human body and that of a mixture of ice and sal ammoniac or common salt. In 1721 he made a mercury thermometer according to Halley’s suggestion of 1693, and by means of it he proved the dependence of the boiling point on pressure. It was not until after Fahrenheit’s death that the freezing and boiling points of water were universally accepted as fixed points on the thermometric scale. The thermometer has remained unchanged in its main features since the middle of the 18th century. Mercury has been found the most convenient fluid for ordinary use, in spite of the advantages (Heat, vol. xi.. p. 561 sq.) presented by lighter and more volatile liquids. Graduation of thermometers, by marking off volumes of the stem equal to a given fraction of the capacity of the bulb, although reintroduced by Réaumur in 1730, has now been entirely discontinued.

The idea of a self-registering thermometer early presented itself. Many forms were devised by natural philosophers and instrument-makers. That of Sixe, in 1782, a precursor of which, dating from the 17th century, is preserved amongst the instruments of the Florentine Academy, was the most successful.

Scales.—The absolute zero of temperature is the logical beginning of a thermometric scale, but some point easy of reference is desirable, and this is found in the temperature at which ice melts and water freezes. The second accepted fixed point is that at which distilled water boils under the pressure of 760 millimetres (29·92 in.) of mercury. For the division of the space between the two fixed points into degrees of convenient length only three of the innumerable methods proposed have survived, and one of these, the centigrade, is rapidly becoming universal. The oldest system, that of Fahrenheit, dates from 1724. It is used for meteorological purposes, and popularly, in Great Britain, the British colonies, and the United States. The freezing point is marked 32° and the boiling point of water 212°. At first Fahrenheit employed a scale of 180 degrees; the zero was placed at "temperate" (9° C.); 90° at "blood-heat," the point to which the alcohol rose when the thermometer was placed under the arm of a healthy man; and -90° at the temperature of a mixture of ice and salt, then believed to be the greatest possible cold. In 1714 Fahrenheit changed his scale at the suggestion of the Danish astronomer Roemer, placed 0° at his "absolute zero," and divided the space between that and the warmth of the human body into 24 degrees. The freezing point of water thus became 8°. For convenience, these long degrees were divided into quarters, which were afterwards termed degrees; thus the freezing point became 32° and blood heat 96°. A mercury thermometer graduated in this way, with divisions of equal length continued above blood heat, registered 212° in boiling water. Thus the Fahrenheit scale came from a duodecimal reckoning.

De Lisle, in 1724, introduced a scale in which the boiling point of water was marked 0° and the temperature of the cellars of the Paris Observatory 100°. He afterwards adopted the freezing point of water as his upper fixed point, and called it 150°. This scale was used for many years in Russia, but is now obsolete.

In 1730 Réaumur made alcohol thermometers with their zero at the freezing point of water, and degrees of one-thousandth of the volume of the bulb. On some of these the boiling point of water was 80°; but the instruments were defective in principle and very unequal in their indications. Deluc introduced mercury thermometers graduated from 0° in melting ice to 80° in boiling water, and these, with Réaumur’s name attached, are in use for popular purposes in Germany, Holland, and other parts of the Continent.

Celsius adopted a centesimal scale in 1742. The boiling point was marked 0° and the freezing point of water 100°. Linnæus introduced the mode of reckoning from 0° in melting ice to 100° in boiling water, which is now known as the centigrade, and is used universally in laboratories, and in all except English-speaking countries for every scientific purpose.

Fahrenheit’s scale is convenient for meteorological work on account of its short degrees, admitting of great accuracy in reading and compactness in recording, and on account of its low zero, which makes it possible in temperate climates to dispense with negative quantities. On the other hand, the centigrade scale is on the whole so convenient, its use is so nearly universal, and the advantage of a uniform system is so great that it must ultimately be adopted for all purposes.^[1]

Air Thermometer.—Under constant pressure gases expand equally for equal increments of heat. Hence, when an air thermometer is graduated between two fixed points the graduation may be continued above and below these points in degrees of the same length; and any number of air thermometers so made will agree amongst themselves at every temperature. The principle of air thermometers is treated of in Heat (ut sup), and examples of special forms are described in that article and in Pyrometer. The air thermometer is the ultimate standard of reference to which all other thermometers are referred.}}

Alcohol Thermometer.—Alcohol, the first liquid used for thermometric purposes, possesses numerous advantages, and on account of its low freezing point it is always used for observations in polar regions. Alcohol thermometers are graduated by fixing the freezing point in melting ice and by comparison with a mercury or air thermometer at several higher and lower temperatures. Recently low-temperature thermometers have been verified at Kew in melting mercury at the temperature of -40. The law of expansion of alcohol in glass at low temperatures is not known with such precision as to make the minimum indications of Arctic expeditions entirely trustworthy. The graduation of ordinary minimum alcohol thermometers used for meteorological purposes is effected by comparison with mercury standards, and their indications, so far as this source of uncertainty is concerned, may consequently be relied on.

Mercury in Glass Thermometer.—The simplest form is the Weight Thermometer, a large glass bulb terminating in a capillary tube, and filled with a known weight of mercury at 0° C. The weight of mercury that escapes when the apparatus is heated to 100° is determined, and the temperature of any enclosure is then ascertained by placing in it the thermometer filled at zero, and weighing the liquid that runs out. Thermometers on this principle were used by Regnault in his celebrated researches on steam.

Standard Thermometers.—The tube is sometimes made with elliptical bore to ensure visibility of the mercury column, but it is usually circular in section. The internal diameter must be as nearly as possible uniform. This is tested by a preliminary calibration in which a short thread of mercury is measured in different parts of the tube. The length of stem and the range of the thermometer having been decide upon, the size of the bulb is calculated from the known expansibility of mercury and the section of the bore. The bulb is made as nearly as possible the required size, either by blowing it from a tube or preferably by forming it of a glass cylinder, and attached to the stem. The bulb is usually cylindrical in form and it must be uniform in thickness. The utmost care requires to be exercised to keep the bulb and stem dry and clean and to fill them with pure mercury recently distilled. The mercury is boiled in the thermometer for some time to drive out all traces of air and moisture, and the point of the stem is sealed off. If the thermometer is not intended to measure temperatures up to the boiling point of mercury, an expansion should be made at the top of the tube to prevent bursting from accidental overheating. Under Heat (vol. xi. p. 561) the changes of volume which thermometer bulbs undergo in cooling and for a long time afterwards are discussed. The process of annealing by heating to a temperature exceeding 400° C. for some hours as originally proposed by Person,^[2] or in vapour of mercury for several days as recently practised at Kew, renders the thermometer much less liable to suffer change of zero by the lapse of time or by heating to any lower temperature. All instruments of precision should be treated in this -way, or kept for several years after they have been filled and sealed before they are graduated.

The first fixed point on the scale is marked at the place where the mercury stands when the thermometer is buried in melting ice from which the water is allowed to drain away, the second at the place where the mercury stands when the thermometer is immersed in steam of water boiling freely under the pressure of 760 mm. (29·92 inches) of mercury corrected to 0° C. The space between these may he graduated either in arbitrary equidistant divisions, as it is best to do in delicate instruments, or in degrees of any scale. Each degree centigrade is 1/100 of the volume of the tube between the freezing and boiling points; if the tube is quite uniform in bore the degrees will be of equal length and may be marked off correctly by a dividing engine. If the preliminary calibration showed the tube to vary in diameter, the degree marks are often adjusted to correspond to intervals of equal volume. It is better in all cases, whether degrees or arbitrary divisions are adopted, to have them of equal length and correct the readings by the calibration curve. The scale may be continued above and beneath the fixed points in degrees or divisions of the same length.

Calibration consists in measuring the internal volume of the thermometer tube by means of a thread of mercury detached from the main column. There are several ways of doing this, for particulars of which reference may be made to the British Association Report on the subject (1882, pp. 145-204), where references to original memoirs are given. The best and simplest is Gay Lussac's "step by step" method.

The most recent and approved processes of manufacturing, testing, and using standard thermometers of great delicacy and high precision are described by Guillaume in his “Etudes Thermométriques” (Travaux et Mémoires du Bureau International des Poids et Mesures, v., 1886);^[3] for additional information the work of Pickering cited below may also be consulted.

Comparison of Thermometers.—As the apparent expansion of mercury in glass from —39° to 100° C.^[4] is very nearly proportional to the amount of heat imparted to it, a thermometer made and divided as indicated above is a natural standard. But the apparent expansion with different kinds of glass differs (see Heat, vol. xi. pp. 563–4),^[5] and, except at the fixed points or near them, mercury thermometers of different construction will only fortuitously agree absolutely among themselves or with the air thermometer. Bosscha^[6] states that at 50° C. the mercury thermometer shows an error of 0°·5, other experimenters place it as high as 1°, but Mascart found it to amount only to 0°·06.^[7] For purposes of ordinary experiment thermometers are compared at several temperatures with some standard instrument of known value—that of the Kew observatory for Great Britain,—and all results are stated in terms of the standard. The methods of comparison at Kew are described by Welsh (Proc. R. S., vi. 181) and Whipple (Phil. Mag., [5], xxi., 1886, p. 27).

The reading of thermometers is greatly facilitated .by the process of enamelling the back, and still more by that of entirely surrounding the instrument with enamel except over a narrow strip through which the mercury is seen.^[8] The enamel must not be allowed to encroach on the bulb, for that would endanger the homogeneity and strength of the glass. Thermometers Employed for Special Purposes.—Physical and Chemical Work.—For all purposes of minute accuracy where thermometers are applicable standard instruments must be employed. They must be used in one position only. The stem is usually engraved with an arbitrary scale of equal divisions, the total range not exceeding 15°·0., and readings are made by a cathetometer at some distance. The use of an intermediate bulb, first recommended by Person, enables the fixed points to be observed on instruments of very short rangs. Results of great accuracy, certainly to 0°·005 C., may be obtained in this way for comparative purposes if sufficient care be taken; but the greater the sensitiveness of a thermometer the more difficult is it to obtain a series of concordant readings (Heat, vol. xi. p. 562). Pickering^[9] uses thermometers of extreme sensitiveness, in which, by conveying the excess of mercury into an expansion at the top of the stem, he secures that the same part of the short arbitrary scale is used for every temperature that has to be measured. In physical researches thermoelectric junctions are more often used than thermometers for measuring very small differences of temperature.

For ordinary work in a chemical or physical laboratory thermometers are used which can be read easily to one-tenth of a degree centigrade, and have a range from 0° to 100°, or in some cases to 350°·0. They are always either engraved on the stem or graduated on an included scale (see Heat, figs. 4, 5), and are not mounted onframes of any kind. It is not necessary to calibrate such thermometers; but they should be compared with a standard at several temperatures and frequently verified in melting ice and steam of boiling water.

Zincke's chemical thermometer for high temperature has a scale commencing at 100° C. In Geissler’s nitrogen thermometer the range is extended by raising the boiling point of the included mercury, the upper part of the tube being filled with rarefied nitrogen.

Meteorological.—The thermometer was early applied to the study of differences of climate, and this is still one of its most important uses. The wet and dry bulb thermometers placed in the shade give the temperature and humidity (see Hygrometry) of the surrounding air, but “shade” and “surrounding air” require to be defined. Shade is intended to exclude rain and prevent all radiation; and the surrounding air is that of the atmosphere in the neighbourhood of the thermometer outside any shelter that may be used. The simplest way of observing is to hang up a thermometer in the shadow of some rather distant object and leave it until it acquires a steady temperature; but this method has been found impracticable and does not give very exact results.

In different countries different patterns of thermometer shelter are employed and exposure takes place at a different height above the ground. Results so obtained cannot be critically compared, and the relative mean temperatures of the atmosphere in different countries are only known to within one or two degrees. The Stevenson double-louvred screen (see vol. xvi. p. 115), a box open below, provided with a solid roof, is used at all meteorological stations in Great Britain. It is placed 4 feet from the ground, and painted white outside and inside. The results derived from its use are comparable, because the conditions in which it is employed are the same, but the general introduction of a double roof would greatly add to its efficiency. Exposure outside windows or in wall boxes is the rule in Austria. In France the Renou screen is largely used; it is a flat roof one square metre in extent, and double; the thermometers are hung under it two metres from the ground. A similar roof, but of much larger size, is employed in Australia, in combination with a metal thermometer-box. A metallic box, constructed of double louvres with an air-space between, finds favour in Spain. In Russia and Switzerland Wild’s shelter is extensively employed. The thermometers are enclosed in a case composed of two or three concentric zinc cylinders perforated to admit air, and placed 11 feet above the ground. They are protected by a large shelter of wood, the south wall and roof of which are double and made of solid boards, between which air circulates; the east and west sides are louvred, and the north side entirely open. A similar shelter is used in Canada, to cover a box of single sheet-iron louvres in which the thermometers are placed 41/2 feet from the ground. Various systems of exposure were authorized in the United States until 1885. It was then decided, as the result of experiments^[10] carried on for nearly two years, that a uniform pattern of shelter be adopted by the Signal Service. It is a single-louvred wooden box, 3 feet 6 inches long, 3 feet wide and high, with a movable bottom and a double roof. The louvres are provided with an upright flange on their inner side, designed to keep rain from the thermometers. The bottom of the shelter is to be fied either 9 feet above a roof or 16 feet above grass.

All these screens are confessedly imperfect, although most of them are well adapted for the climates in which they are used. Numerous comparisons of different screens with each other have been made,^[11] but in some cases sufficient precautions in the way of using instruments precisely similar and only dissimilarly situated have not been observed, and the results are uncertain. A critical comparison of the leading forms of thermometer shelter in use is still a desideratum.

Fig. 3.—Aitken's Thermometer Screen for Maximum Thermometer.

The sling thermometer^[12] (thermometre fronde), a small thermometer whirled in the air at the end of a string, is often used as a standard, and gives more correct readings than most closed screens. All open screens are untrustworthy. Aitken^[13] has devised a series of thermometer boxes on a new principle, radiation being taken advantage of to produce a constant draught over the thermometer bulbs by the use of a long blackened chimney. These give admirable results. Very small and bright objects are little affected by radiation: hence thermometers with bulbs of small diameter and coated with a bright deposit of gold or silver have been used without screens. The air temperature has also been calculated by means of a formula from the readings of two similar thermometers, the bulbs of which are unequally affected by radiation. Some form of sling thermometer should always be used for observations at sea; the Board of Trade screen generally employed is thoroughly objectionable, and can only give moderately good results by the exercise of great precautions on the part of the observer.^[14]

As a rule, thermometers for meteorological purposes are made with spherical bulbs, although cylindrical reservoirs present certain advantages. To ensure perfect uniformity in registration, the bulbs should all be as nearly as possible of one size, constructed of one kind of glass, and the mounting perfectly uniform. Better-class instruments have the bulb clear of the frame, and the stem attached to a slab of metal, of porcelain, or of glass backed by wood; but sometimes they are simply fixed to a boxwood scale. In all cases they should he graduated on the stem, and compared with a standard, but in view of the uncertainty of the methods of thermometer exposure great delicacy is undesirable.

The influence of height on thermometers for ascertaining the temperature of the air as been investigated with somewhat conflicting results;^[15] the disparity is at least partly due to the use of dissimilar instruments.

Registering Thermometers.—Rutherford's maximum, invented before 1790,^[16] was an ordinary mercury thermometer placed horizontally; the column pushed before it a small steel index, which was left at the highest point reached. It is little used now. The maximum thermometers in common use for meteorological purposes are Negretti & Zambra’s and Phillips’s. The former is a modified outflow thermometer. It is made with a constriction in the tube near the bulb, past which the mercury easily expands, but cannot return when the temperature falls, as the column breaks at the narrowed point when the fluid in the bulb begins to contract. The thermometer acts horizontally, but Everett devised a modification which is hung bulb uppermost, and the mercury, as it passes the constriction, falls down and stands as a column in the inverted tube. The thermometer is set by swinging it. Phillips’s maximum, claimed also by Walferdin, has a portion of the mercury thread separated from the rest by a minute bubble of air. It is placed horizontally, and, as temperature increases, the detached portion of mercury is pushed forward and is not withdrawn when the main column retreats toward the bulb on cooling. It is set for a new observation by bringing it into a vertical position and tapping it slightly. By reducing the length of the index and the bore of the stem this thermometer may be made suitable for use in any position without altering its register. Walferdin’s outflow maximum thermometer is a modification of that of Lord Charles Cavendish^[17] and the type of a number of similar instruments. It is set by filling the stem entirely with mercury from a lateral chamber at the top (fig. 4). The instrument is placed vertically, and as temperature rises mercury overflows into the reservoir. To read, the thermometer is brought back to its original temperature, then the number of degree spaces left vacant at the top of the tube shows the excess of maximum temperature above that at the time of setting.

Fig. 4.

The minimum thermometer in most frequent use is that of John Rutherford, invented in 1790. It is a spirit thermometer, preferably filled with amyl alcohol to reduce risk of distillation, in the column of which a small porcelain index is included. The instrument is hung horizontally, and, as temperature falls, the index is drawn back by the surface tension of the fluid. When temperature rises, the liquid flows past the index easily, leaving it at the lowest point attained. Baudin invented a modification called the thermomètre à marteau in 1862; it acts vertically, the index being fixed by a spring, as in Sixe’s thermometer, and set by a long glass needle included in the stem, which, when the instrument is inverted, falls on the index and drives it to the surface of the alcohol. The mercurial minimum of Casella is an instrument of great delicacy and beauty, extremely difficult to make, and requiring careful handling in its use. A side tube of wide bore ac (fig. 5) is joined to the stem of an ordinary mercurial thermometer near the bulb. This tube terminates in a small chamber ab, cut off by a perpendicular glass diaphragm which is perforated by a hole of greater diameter than the thermometer stem. When set, the mercury in the stem indicates the actual temperature, and the chamber is empty. On the principle of Balfour Stewart’s fluctuation thermometer,^[18] when the instrument is heated the mercury remains stationary in the stem but expands into the chamber ab. When cooled, the mercury passes out of the chamber; when this is empty, the temperature has returned to that at which the instrument was set, the surface attraction of glass and mercury prevents the fluid leaving the diaphragm b, and all subsequent contraction takes place from the stem. The position of the mercury column in the stem marks the minimum temperature since last setting. The instrument is set by raising the bulb end and allowing all the mercury to flow from the chamber.

Fig. 5.

Thermometers which record the actual temperature at any required time, by a change of position produced by a clock, were employed by Blackadder^[19] in 1826. His process was complicated and uncertain. Negretti 8r Zambra have a simpler arrangement that works well. Several of their reversing thermometers (see under Deep-Sea Thermometers) are pivoted on a frame, and held upright by catches which are withdrawn in turn at definite intervals by an electrical arrangement regulated by a clock. Each instrument, when it reverses, preserves the record of temperature at that moment until it is set again.

No thoroughly satisfactory self-registering maximum or minimum thermometer has yet been produced. In all existing forms the indications are liable to be disturbed by shaking. Where alcohol is the fluid used, it is apt to volatilize and accumulate at the top of the tube, so registering a much lower temperature than actually occurs. It is extremely difficult also to free alcohol thermometers from air, which gradually escapes from solution in the fluid and renders the instrument untrustworthy or even useless.

Radiation Thermometers.—The intensity of solar radiation is measured by the pyrheliometer, which usually consists of a body heated by the sun’s rays and a thermometer to measure the rise of temperature. In meteorology radiation is measured by thermometers simply exposed with blackened bulbs. Results of the utmost diversity are given by different methods. As there is no means of determining the true measure of radiation, all that can be done is to have the instruments whose indications are to be compared constructed and exposed in the same way. The usual form, as suggested by Herschel, is a maximum thermometer with a spherical bulb half an inch in diameter coated with lamp-black and placed in the centre of a spherical vessel of clear glass, 21/2 inches in diameter, and exhausted of air. The state of the vacuum may be shown by including a small mercurial manometer, or a radiometer, or by soldering in platinum electrodes through which a discharge can be made in the interior. It is not essential that the vacuum be very perfect; some observers prefer to employ a globe filled with dry air. For separate instruments to be comparable, Whipple^[20] and Ferrel^[21] have shown that the bulbs must be truly spherical, of equal thickness and size (a difference of 8 per cent. in diameter produces variations of several degrees), blackened sufficiently to absorb all radiation falling on them, and placed accurately in the centre of perfectly spherical enclosures, which must also be of equal diameter. The stem should be as small as possible in proportion to the bulb; and before being used for comparative purposes all radiation thermometers should be compared with an arbitrary standard by daily exposure for several weeks to sunshine.

Minimum radiation thermometers, intended to measure radiation from the earth at night, are usually filled with alcohol, and much ingenuity has been expended on increasing their delicacy. The bulbs are made very large relatively to the bore, and constructed so as to expose a great surface, the reservoir being often helical, lenticular, annular, spoon-shaped, forked, or even like a gridiron.

Earth Thermometers.—Saussure introduced the use of sluggish thermometers packed in non-conducting material for taking the temperature of the soil at different depths. Symon’s earth thermometer on this principle is a slow-action instrument cased in felt, and is lowered by a chain into an iron tube which has previously been sunk to the required depth. It may be withdrawn and read without changing its record. The underground temperature committee of the British Association have used both slow-action and self-registering thermometers for their observations in mines and shafts.^[22]

Thermometers with very long stems, which can be read above ground, fitted in deep borings in the rock, are used at the observatories of Greenwich and Edinburgh for investigating earth temperature. Those at present established at the Royal Observatory, Edinburgh,^[23] are the successors of a set fixed in the rock in 1837, and broken accidentally in 1876. They are placed with their bulbs at depths of 25, 12, 6, 3 feet beneath the surface respectively, and one has its bulb just covered. The readings of the intermediate thermometers supply data for correcting the long columns of alcohol in the deeper ones for the different temperatures of their different parts. Allowance may be made for this effect without calculation by utilizing the principle applied by Sainte-Claire Deville to pyrometers. A second stem, similar in every way to that of the thermometer, nearly filled with the same fluid, but hermetically sealed at the lower end, is fixed beside the thermometer stem. The fluctuations it shows are due solely to causes affecting the stem and not the bulb of the thermometer, and they are eliminated from the readings of the latter by taking account only of the difference of level of the fluid in the two tubes.

Deep-Sea Thermometers.—The earliest observations of warmth beneath the surface were made by raising samples of water in a valved box and noting the temperature when it was brought on board. Saussure, in addition to this, used sluggish thermometers, which he left immersed for several hours before reading. His latest thermometer for sea-work was filled with alcohol, and had a bulb more than an inch in diameter, which was imbedded in a mass of wax and enclosed in a stout wooden case. It attained the temperature of its surroundings very slowly, preserved it for a long time, and gave, in his hands, thoroughly trustworthy results. On the introduction of registering thermometers these were used, but the unsuspected magnitude of the effect of pressure at great depths made the earlier records entirely misleading.

Fig. 6.—Miller–Casella Thermometer.

A modification of Sixe’s thermometer, protected from pressure by the addition of an outer bulb partially filled with a liquid, is now usually employed on deep-sea expeditions. Those used on the “Challenger,” under the name of Miller-Casella thermometers, were of the form shown in fig. 6. The tube is U-shaped, the bend and part of each limb filled with mercury, the rest of the tube, the bulb, and part of the expansion on the other side with alcohol. A steel index, held in its place by the pressure of a hair, is immersed in the spirit in each limb above the mercury, which pushes one or other before it as the temperature is rising or falling, and leaves them at points denoting the highest and lowest temperatures passed through. The indexes are set by a magnet. The “Challenger” thermometers, which were not graduated on the stems, were secured side by side with porcelain temperature scales to vulcanite frames and placed in copper cases perforated to allow a circulation of water. Tait investigated the whole subject of pressure corrections after the return of the expedition, and found that the high result obtained by a previous experimenter was due mainly to heat developed by compression of the vulcanite, which affected the thermometer in the press, but would not do so at sea. The correction which had to be applied was rather less than 1/7 of a degree Fahr. per mile of depth.^[24] These thermometers require to be immersed from twenty minutes to half an hour before they acquire the temperature of the water, they can only be read to quarter degrees Fahr., and they simply indicate the extreme temperatures through which they have passed. Buchanan has greatly Improved the instrument by reducing the bore of the tube on the minimum side, which is that most frequently used, thus giving long degrees. An arbitrary scale is engraved on the stem.^[25] His mercury piezometer is affected by temperature and by pressure, and enables the actual temperature at any known depth to be found.

Fig. 7.—Negretti and Zambra's Deep-Sea Thermometer (inverted).

Aimé in 1845^[26] invented a very ingenious arrangement of outflow thermometers. which were inverted by a weight slipping down the line, and registered as they were being drawn up. His instruments were accurate, but very delicate and troublesome to manage. Within the last few years Negretti and Zambra have patented several forms of modified outflow thermometers. The first instrument of the kind was complicated and unmanageable, but that now before the public is both simple and convenient. It consists of a mercury thermometer with a cylindrical bulb and a stem AC (fig. 7) of wide bore terminating in a small pyriform aneurism. The stem is contracted and contorted just above the bulb, and when the instrument is turned upside down the mercury column breaks at this point and flows down into the tube, which is graduated in the inverted position. To protect it from pressure the thermometer is hermetically sealed in a strong glass tube, the portion of which surrounding the bulb contains a quantity of mercury secured by a ring of india-rubber cement. When the thermometer is made to turn over at any depth in water of any temperature, the record remains nearly unaltered, and, until set for a new observation, enables the actual temperature at the instant of reversal to be ascertained at any subsequent time and in any other place. The detached column standing in the tube changes its length slightly by change of temperature. A series of experiments with twelve instruments has shown that for 60° F. change of temperature there is a difference of one degree in the reading of the inverted thermometer. Hence a correction must be applied in all cases where the temperature at which the thermometer is read differs more than a few degrees from that at which it was inverted, contrary to the opinion of the German observers.^[27] If a thermometer is inverted in water and read while wet, the temperature by which it should be corrected is obviously that given by the wet-bulb in air. In view of the great range of temperature experienced in deep-sea work in the tropics, the size of the little overflow cell B, which prevents mercury from the bulb from entering the tube must be considerably increased before the thermometer can be used with safety for such purposes. The Negretti and Zambra thermometer acquires the temperature of its surroundings very rapidly (two or three minutes are usually sufficient); it can be read easily to tenths of a degree Fahr.; and, above all, it ascertains temperature at exact points of depth, and has thus revealed layers of remarkably varying temperature^[28] which could not have been detected by the other instruments in use.

The loaded wooden frame originally employed for reversing the thermometer is unsatisfactory, and Magnaghi’s reversing gear actuated by the revolution of a small propeller set in motion by the water when the thermometer is drawn up briskly, is not to be trusted in shallow water or where there are rapid currents. When the pin is withdrawn the thermometer case turns over and is clamped by a side-spring on the frame. Rung^[29] adopted a simpler and better though somewhat clumsy frame, in which the thermometer was made to turn by slipping a weight down the line. The United States Fish Commission^[30] employ the thermometer in a frame adapted for use on a wire sounding line, and also actuated by a messenger, but the thermometer is not clamped on turning over. The Scottish marine station produced^[31] a modification of Magnaghi’s frame, the propeller being replaced by a forked lever held down by a spiral spring and raised when the thermometer is to be reversed by the Impact of a Rung’s messenger (fig. 8). A messenger placed on the line below, and hung by a loop to the upper groove of the thermometer, is let go when the thermometer turns and reverses another instrument lower down. Instead of being lashed to the sounding line, the frame is retained by a ram’s horn spiral below and clamped by a small vice at the upper end. Buchanan has modified and simplified the frame, combining its mode of attachment to the line with the American method of reversing.

Fig. 8.—Scottish Frame for Deep-Sea Thermometer. Messenger descending to reverse the instrument.

Neumayer^[32] has attempted to use a photographic thermograph for deep-sea work, the light being supplied by a Geissler tube excited by a small battery. Siemens’s electrical thermometer has also been experimented with,^[33] but has hardly been brought to a practicable state, and the same may be said for the use of thermoelectric junctions.

Hypsometer.—The boiling-point thermometer or hypsometer may be used to obtain an independent measure of the pressure of the atmosphere, and so to determine an altitude or verify an aneroid barometer. It consists of a very delicate mercury thermometer graduated only for 20 or 25 degrees Fahr. in the neighbourhood of the boiling point of water and divided on the stem into tenths. A large aneurism on the tube a little above the bulb should allow the freezing point to be verified from time to time on the portion of stem beneath it. The thermometer is hung in a cylindrical tin vessel in which water is boiled by a spirit lamp placed underneath. The bulb must be raised considerably above the level of the water, and the whole stem to the top of the mercury column immersed in the steam. After steam has been escaping freely for some time the temperature is read, and by reference to a table the barometric pressure, and consequently the altitude, is obtained.

Clinical Thermometers.—The first use to which thermometers were applied was the study of the temperature of the blood in fevers; and the constancy of the temperature of the healthy human body was for a century considered sufficient to entitle it to the position of a fixed point in graduating thermometers. The increased importance now attached to temperature in disease has led to the production of many forms of clinical thermometer. The large instruments intended to be read in situ are now entirely superseded by small maximum self-registering thermometers. Graduation is carried to one-fifth of a degree, and the usual range is about 25 degrees Fahr., —from 85° or 90° to 110° or 115°. Olive-shaped bulbs have been used, but a cylindrical form is most common. There should be an arrangement like that suggested for hypsometers to enable the freezing point to be verified. Casella's thermometer on Phillips’s system has a small expansion on the stem, followed by a contraction, to prevent the index following the rest of the mercury into the bulb when the instrument is not in use. The “half-minute thermometer” is quick in action; it has a bulb of very small diameter and an extremely fine bore, the mercury thread being rendered visible by Hicks’s arrangement of a lens-fronted stem. Immisch’s avitreous thermometer is recommended for clinical use on account of its small size, convenient shape, and non-liability to get out of order.

Thermometers for Technical Purposes.—These are made in an infinite variety of forms, adapted to the various processes of manufacture and industry. The scale is often dispensed with in these instruments, a movable pointer being fixed at the point at which the mercury is to be kept. Air or steam thermometers (see Pyrometer) are rapidly superseding mercury instruments for all temperatures above the boiling point of water. The cheap German paper-scale thermometers are largely used, fitted in wooden cases, as dairy thermometers, and a larger size for brewing purposes. Alarm thermometers are often employed, in which electric contact is made and a bell rung when the temperature exceeds or falls short of a certain limit. Thermostats of various forms are made use of, in which a thermometer, by the position of the mercury in the stem, regulates the gas-supply of a burner and thus the heat of an enclosure.

Metallic Thermometers.—Thermometers depending on change in length or form of composite metal bars, such as Crighton’s zinc-iron bar and Bréguet’s silver-gold-platinum spiral (see Pyrometer), are converted into registering instruments by the addition of two light pointers pushed forward by the index needle as it travels round the graduated arc to either side and left at their extreme points. Jürgensen in 1841 constructed a chronometer, the balance wheel of which was arranged so as to exaggerate the effects of change of temperature and thus to affect the rate. It furnished a very close approximation to the mean temperature between the intervals of rating, and was approved by Arago for use in observations. Hermann and Pfister's metallic thermometer^[34] is probably the best adapted for meteorological purposes, and has given satisfactory results at the Zurich observatory. It is a flat spiral of brass and steel, which unrolls and coils up according to changes of temperature, moving an index on a divided horizontal circle and marking the maximum and minimum by light pointers. In order to secure regular results, the instrument must be annealed by heating for some time in boiling linseed oil.

Fig. 9.—Immisch's Avitreous Thermometer.

Several instruments known popularly as metallic thermometers depend on a different principle, that of the change of form in a thin metallic enclosure containing liquid. Immisch’s avitreous thermometer (fig. 9) is an example. A minute Bourdon’s tube is fixed at one end, and the other bears on the short arm of a lever, the long arm of which acts by a rack on the pinion forming the axis of the pointer. It is only one inch in diameter and extremely accurate.

Thermographs.—The first form of thermograph, due to Wheatstone, was an electrical apparatus. It has recently been improved by Van Rysselberghe, in whose hands it has assumed the following form. The thermometer is of rather wide bore and open above. At intervals of quarter of an hour a wire is moved gradually down the tube by a clock until it touches the mercury; an electric circuit is thus completed, and causes an indentation by a diamond point which moves in the same way as the wire down a rotating cylinder covered with thin sheet copper or zinc. The metal sheet is renewed at each revolution of the cylinder, and it is sufficient to join the indented points with a graver to have a plate from which any number of copies of the record may be printed. Cripp’s thermograph records hourly on a revolving cylinder. It consists essentially of a mercury thermometer coiled into a flat spiral and suspended on a horizontal axis. Any change of temperature displaces the centre of gravity of the system, and the instrument rotates through an arc, moving a pencil as it does so. A perfectly continuous record is produced by the photographic thermograph. Wet and dry bulb thermometers are so arranged that a beam of light passes through an air-speck, which separates part of the mercury thread, or through the vacant part of the tube, and falls on a rotating cylinder covered with photographic paper on which it traces the curve of temperature fluctuation. This apparatus is probably the most perfect of its kind. In Bowkett's thermograph the change of form of a curved tube containing oil moves a pencil radially over a card turned horizontally by a clock. The resulting curve is referable to polar instead of rectangular coordinates; the radius measures temperature, the angle time. Richard's thermograph is also actuated by means of a sealed metallic capsule containing fluid. It draws a continuous curve in ink on a revolving drum on which one sheet lasts for seven days. This instrument is largely employed in observatories to check eye-observations, and is peculiarly adapted for use in positions to which access can only be had occasionally. It is made in many forms, one of which is specially adapted for marine work, the sealed capsule being rolled into the form of a cylinder and exposed to the water on both surfaces. (H. R. M.)

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1. The process of converting readings of any one of the three existing scales into those of any other is a simple matter of proportion. They stand in the ratio of 80 : 100 : 180 (32 being subtracted from Fahrenheit temperatures before the calculation is made, and added to the result when converting from Réaumur or centigrade into Fahrenheit). An easy rule for changing centigrade readings into Fahrenheit mentally is—multiply the centigrade temperature by 2, subtract one-tenth of the product, and add 32: e.g., 10° C.=20-2+32=50° F. These rules are only to be applied to thermometers made with all modern precautions. When the boiling point was determined by immersing the bulb of the thermometer in boiling water or in steam at any pressure other than 760 mm. appropriate corrections have to be applied. For a detailed historical account, see Renou, “Histoire du Thermomètre,” Annuaire Soc. Mét. de France, 1876.
2. Comptes Rendus, xix., 1844, p. 1314.
3. Abstract by Guillaume in the Séances de la Soc. Française de Physique, 1886, p. 219.
4. Ayrton and Perry, Phil. Mag. [5], xxii. 1886, p. 325.
5. See also Kraffts, Comptes Rendus, xcv. 836.
6. Comptes Rendus, lxix. 875. See Note by Regnault, ibid., 879.
7. Berthelot, Mécanique Chémique, i. 158
8. Whipple, Brit. Assoc. Reports, 1885, p. 937.
9. Phil. Mag., [5], xxi., 1886, p. 331; xxiii., 1887, pp. 401, 406.
10. H. A. Hazen, “Thermometer Exposure,“ Prof. Papers of Signal Service, No. xviii., 1885.
11. Gaster, Quart. Weather Report for 1879 (1882), Appendix ii.; Wild, Mittheil. der naturforsch. Gesellsch. in Bern, 1860, 108; Marriott, Quart. J. Roy. Met. Soc, 1879, v. 217; Stow, ib., 1882, viii. 228; Gill, ib., 1882, viii. 238; Mawley, ib., 1884. x. l; Aitken, Proc. R. S. E., 1884, xii. 681; Dickson, ib., 1885, xiii. 199; Hazen, loc. cit.
12. The first use of this instrument is usually stated to have been by Arago (Euvres, 1858, viii. p. 500), but Saussure employed it for wet-bulb observations, and doubtless invented it (see Voyages dans les Alpes, 1796, iv. p. 267).
13. Proc. R. S. E., 1884, xii. 660; 1885, xiii. 199; 1886, xiii. 632.
14. Osborne, Quart. J. Roy. Met. Soc., 1881, vii. 10.
15. Hazen, lcc. cit.; Wild and Cantoni in Report of Vienna Meteorological Conference, 1874; Symons, Proc. R. S., 1883. xxxv. 310; Omond, Proc. R. S. E., 1886-87.
16. Trans. R. S. E., iii., 1794, p. 247.
17. Phil. Trans., i., 1757, p. 300. Henry Cavendish’s register thermometer is on another principle and a much less practical instrument (see Wilson's Life of Cavendish, p. 477).
18. Proc. R. S., viii. 195.
19. Trans. R. S. E., 1826, x. 381, 440.
20. Quart. J. R. Met. Soc., 1879, v. 142; 1884. x. 45.
21. Signal Service Prof. Papers, No. xlii., 1884, p. 34.
22. For a résumé of the methods and work of the committee, see Brit. Assoc. Reports, 1882, p. 74.
23. Trans. R. S. E., 1880, xxix. p. 637.
24. “Challenger" Narrative, ii., App. 1., 1882.
25. For a general account of deep-sea thermometers, see Buchanan, Proc. R. S. E., x. 1878, 77; and “Chal." Reports, Narrative, vol. i., 1884, p. 84.
26. Ann. Chim. Phys., [3], 1845, xv. 1.
27. Ergebnisse der Untersuchungensfahrt der Drache. Berlin, 1886, p. 2.
28. Mill, Jour. Scot. Met. Soc. [3], 1886, No. iii. p. 289.
29. Den Tekniske Forenings Tidskrift, 1883.
30. Report, 1882.
31. Mill, Proc. R. S. E., xii., 1884, 928.
32. Nature, viii. 195.
33. “Challenger Reports", Narrative. 1884. l. p. 95.
34. Report. für Meteorologie, i. pt. 1. p. 7.