Thermometer (Gr. θέρμη, heat, and μέτρονv, a moasure), an instrument to measure temperatures. It is formed of two or more different substances, the volumes of which expand and contract to different extents when they are simultaneously exposed to the same differences in intensity of heat. The first attempt at indicating to the eye differences of temperature seems to have been by the contrivance variously ascribed to Drebbel of Holland and Sanc-torius of Italy, about the beginning of the 17th century, and known as a weather glass. This was very rude and inaccurate, consisting of a glass bulb and tube inverted, opening below into a cup of colored liquid, which, the air of the bulb having been partly expelled by heat, rose in the tube, and stood at different heights according as the air remaining in the bull) was more or less expanded by heat. This, the origin of the common air thermometer, as improved by Boyle and by the Florentine academicians, became transformed to a smaller bulb with upright stem of somewhat fine bore, the contained liquid being colored spirits of wine; boiling this to expel air, the tube was hermetically sealed, and the whole then affixed to a case.

A scale of degrees was also introduced, its fixed points being the cold of snow or ice and the greatest heat known at Florence; it was of necessity very variable in its indications. At this stage in the progress of thermometry, much discussion in regard to the most suitable fixed points for the scale, the best substance for use in the instrument, etc, including that of the question whether water did not freeze at different temperatures in different latitudes, was carried on in England and on the continent, Hooke advocated as the lower fixed point the temperature of freezing water. Newton seems first to have discovered or taken advantage of the facts, that a thermometer placed in melting snow or ice always indicates the same temperature, and always very nearly one temperature in boiling water; but of oil, which.he suggested for the liquid in the bulb, the movements were found to be too sluggish and uncertain. • Romer, overcoming a prejudice that seems to have existed in regard to unequal expansion of mercury, first adopted that liquid; and he doubtless devised the instrument and scale usually attributed to Fahrenheit of Amsterdam (1720), the latter constructing and introducing the instrument, so that it became generally known throughout Europe in the first half of the 18th century.

Of this thermometer, the lower fixed point, or zero, was taken at 32° below freezing point of water; but whether as the cold obtained by its maker by mixing salt and snow, or as the greatest cold observed in Iceland, and in either case as the supposed point of absolute cold, is not now definitely known; and since Fahrenheit kept his graduation of thermometers a secret, the same must be said respecting the choice of a scale of 180° between the fixed points. Celsius of Sweden (1742) introduced a scale of 100° between the fixed points; this was adopted in France at the time of the revolution, and named the thermometre centigrade; and owing to its convenient decimal division, it has been wholly adopted in several countries of Europe, while it is coming into general use among scientific men throughout the world. For the general principles upon which the use of the thermometer depends, see Expansion, Heat, and Pyrometer. - An increase in the temperature of a body is generally accompanied by an increase in its volume, and a decrease in its temperature by a contraction in its volume.

Definite changes in the volume of a given substance may be used as indications of this substance having different definite temperatures, and this substance will have the temperature of the bodies by which it is surrounded, or of the medium in which it is immersed, and thus serve to measure their temperature. The substances generally used in the thermometer are glass and mercury, and the observed change of volume is the difference in the change of volume of the glass and of the mercury. The instrument which shows this difference in expansions is known as the mercurial thermometer. It consists of a tube of very small interior diameter, terminating in a bulb or reservoir. The bulb and a portion of the tube are tilled with mercury, and with an increase or a diminution of temperature the mercury will rise or fall in the tube; and the position of the mercury in the tube can be noted on a scale of equal parts either etched on the tube or marked on the surface or a plate to which the tube is attached. Mercury has several advantages as a thermometric substance. The successive increases in its volume for equal and successive additions of temperature, indicated by the air thermometer (see Pyrometer, vol. xiv., p.

Ill), are quite uniform; especially is this the case when we use the differential expansion of mercury and ordinary glass. The ordinary thermometer when constructed with care is trustworthy in the measure of temperatures up to 300° 0. Up to 100° C. mercurial thermometers made of any kind of glass indicate almost exactly the same temperatures as those given by the air thermometer. Another advantage of mercury is that it does not freeze above the low temperature of - 40° O., and does not boil below 360° C. But the mercury thermometer only gives accurate indications between - 35° and +300° C. For temperatures above 300° C. some form of pyrometer must be used. Mercury has a low specific heat, and this property combined with its high conductivity causes it rapidly to indicate the changes in the temperature of surrounding bodies or of the medium in which it is immersed. - Construction of the Mercurial Thermometer. The tube of the thermometer should be of uniform calibre throughout its whole interior.

To ascertain whether this is the case,, a short column of mercury is introduced into the tube; and if its length remains the same when it is moved throughout the length of the tube, we may be sure that the tube has a uniform bore, and hence that equal amounts of expansion of the mercury will cause equal additions in the length of the mercurial column in the tube. Since tubes of uniform bore are very rare, it is generally necessary to calibrate the tube before its graduation. This is done by etching on the tube a scale of equal parts, and then, from observations on the different lengths occupied by a column of mercury which is made to pass through the tube, forming a table which gives the temperatures corresponding to the arbitrary divisions on the tube. A bulb is now blown on the tube, and this bulb and a portion of the tube are filled with mercury as follows: The air in the bulb is heated while the open end of the tube clips into mercury. The heat having been withdrawn, the air in the bulb contracts and the mercury rises in the tube and partly fills the bulb. To the open end of the tube a funnel containing mercury is adapted, and the mercury in the bulb is boiled and thus expels all air and moisture from the. instrument, which on cooling necessarily tills completely with mercury.

The bulb is now placed in some fluid heated to a few degrees above the highest temperature which the thermometer is intended to measure, and when the mercury ceases to overflow the open end of the tube is sealed with a blowpipe flame. In order to graduate the instrument, the bulb and part of the tube are surrounded with melting ice, and. when the top of the mercury column has remained some time stationary, its position is marked by means of a line, or a note is made of this position, referred to the arbitrary scale etched on the tube. The point on the thermometer determined as above is designated as 0°, or zero degree, on the thermometers known as centigrade (Celsius) and Reaumur, and as 32° on the Fahrenheit system of graduation. To determine a higher point on the thermometer, the instrument is placed in the interior of a metallic vessel with double walls, between which circulates the steam from water boiling in the bottom of the vessel. "When the top of the mercury column in the thermometer has become stationary its position is marked on the tube.

The boiling point of water is constant at the same atmospheric pressure, and when the barometric column has a height of 29.922 inches or 760 millimetres, the boiling point of water is designated as 100° on the centigrade thermometer, 212° on the Fahrenheit, and 80° on the Reaumur. Hence, between the melting point of ice and the boiling point of water there are 100 equal degrees in the centigrade graduation, 180 in the Fahrenheit, and 80 in the Reaumur. To convert the indications of one of these thermometers into those of the other two, we have the following formula, in which F, C, and R denote equivalent temperatures expressed in degrees of Fahrenheit, centigrade, and Reaumur, respec-tivelv:

Thermometer 1500426

Fig. 2.

F= 9/5 C + 32= 9/4 R + 32.

C=5/4 R=4(F-32).

R= 4/5 C= 4/9 (F-32) Fig. 1 shows a thermometer graduated according to the three systems. A few weeks after a thermometer has been made and graduated it may be observed that the mercury will not quite descend to the melting point of ice when the instrument is immersed in pounded ice. It has been found that this "elevation of the zero point," as it is called, goes on gradually for about two years after the thermometer has been constructed, and at the expiration of that period the readings may all be too low by nearly a degree; hence it is necessary either to add the proper correction to the readings of the thermometer, or to slide down and refix the scale to which the thermometer is attached, so that it will read accurately. Alcohol, commonly used where temperatures much below 0° F. are to be observed, is liable at such range to much variation, although it does not freeze even at - 132° F.; and Parry, in his arctic voyages, observed differences of full 10° C. between alcohol thermometers by the best makers. - Self-recording Thermometers. Various instruments have been invented winch record the indications of the thermometer.

They may be divided into two classes, those which record only the maximum and the minimum of the temperatures occurring in any definite period, and those which produce continuous records. In the first class may be mentioned the two following instruments. An ordinary mercurial thermometer has its tube constricted to a thin passage at some point between its bulb and the beginning of its scale. This thermometer is placed in a horizontal position, and then as long as an increase of temperature takes place small portions of the mercury will go in a series of jumps across the constricted passage; but on a fall of temperature the mercury contracts into the portion of the thermometer below the constriction, leaving a column of mercury above it. The upper end of the hitter column marks the highest temperature reached during the time of exposure. To readjust this instrument, the mercury is sent into the vacant space below the constriction by swinging the instrument. Fig. 2 shows this "maximum thermometer," the invention of which has been claimed by several persons. The "minimum thermometer" of Rutherford, which is generally used, is made of alcohol contained in the ordinary glass bull and tube.

In the column of alcohol is a small index made of black glass and shown at n, tig. 3. This piece of glass is brought up to the end of the fluid column by inclining the instrument. The thermometer is then placed in a horizontal position, and as the temperature falls the top of the liquid column during its retraction carries the glass index with it, and leaves it at the point which indicates the minimum temperature reached during the exposure of the instrument. The thermometers of the second class give continuous records, either by causing a tracer attached to some simple or compound metallic bar to mark a continuous line on a cylinder which revolves once in 24 hours, or by the aid of photography a continuous impression of the image of the top of a thermometric column is obtained by illuminating a thermometer placed in front of the lens of a camera, while at the back of the camera is a sensitized plate on which the image is formed. The plate traverses athwart the beam issuing from the lens by a known distance each hour. - Differential Thermometer. This is a modification of the air thermometer, in which two large glass bulbs above are connected by a glass tube bent twice at right angles; the horizontal and parts of the upright tubes are filled in the common form with a colored liquid, which is depressed on either side as the corresponding bulb is more heated; thus the instrument indicates differences of the temperatures to which the two bulbs may be exposed.

It is very sensitive; and by a scale the results it affords are comparable with each other.

Thermometer with Fahrenheit, Reaumur, and Centigrade Scales.

Fig. 1. - Thermometer with Fahrenheit, Reaumur, and Centigrade Scales.

Maximum and Minimum Thermometers.

Fig. 2. - Maximum and Minimum Thermometers.

Index of Rutherford's Thermometer.

Fig. 3. - Index of Rutherford's Thermometer.