thermometer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument for measuring temperature, especially one having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol, that expands and rises in the tube as the temperature increases.

Century Dictionary and Cyclopedia

1. n. An instrument by which the temperatures (see temperature and thermometry) of bodies are ascertained, founded on the common property belonging to all bodies, with very few exceptions, of expanding with heat, the rate or quantity of expansion being supposed to be proportional to the degree of heat applied, and hence indicating that degree. The expanding substance may be a liquid, as mercury or alcohol; a gas, as in the air-thermometer (which see); or a solid, as in the metallic thermometer (see below). The ordinary thermometer consists of a slender glass tube with a small bore, containing in general mercury or alcohol; this expands or contracts by variations in the temperature of the atmosphere, or on the instrument being brought into contact with any other body, or being immersed in a liquid or gas which is to be examined, and the state of the atmosphere, the body, liquid, or gas, with regard to heat, is indicated by a scale either applied to the tube or engraved on its exterior surface. The thermometer was invented by Galileo at some date prior to 1611, and was developed by his pupils through the first thirty years of the seventeenth century. In 1641 the Florentine philosophers were using a thermometer consisting of a bulb filled with alcohol, with sealed stem, and graduated on the stem according to an arbitrary scale, of which the divisions were, approximately, fiftieths of the volume of the bulb. Sagredo adopted a scale of 360 divisions, like the graduation of a circle, and fixed the application of the word degree to the thermometric spaces. No means of comparing observations made with thermometers containing different fluids and of different manufacture were possible until Fahrenheit adopted a graduation between two fixed temperatures. For the zero of his scale Fahrenheit adopted the lowest temperature observed by him in the winter of 1709, and for his upper fixed point he took the temperature of the body, and marked it 90°. By this system of numeration the temperature of melting ice became 32°, and the boiling-point of water 212°. This is the scale of the Fahrenheit thermometer commonly used by English-speaking peoples and in Holland. De l'Isle, about 1730, first used the melting-point of ice and the boiling-point of water as the fixed points of the thermometric scale, and they gradually came to be universally accepted. In Réaumur's thermometer (formerly largely used in Germany and Russia, but now being superseded) the space between the freezing-point and the boiling-point of water is divided into 80 equal parts, the zero being at freezing. In the centigrade thermometer, used widely throughout. Europe, and very extensively in scientific investigations everywhere, the space between the freezing-point and the boiling-point of water is divided into 100 equal parts or degrees, the freezing-point being zero and the boiling-point 100°. The absolute zero of temperature is the logical beginning of a thermometric scale, but since thermometric temperatures are primarily relative, the zero-point is arbitrary, and the Fahrenheit, Réaumur, and centigrade thermometers present the different systems of numeration that have come into use. The following formulæ give the conversion of these scales: Let F, R, and C represent any temperature as given by the three scales respectively, then F = R × + 32° = C × + 32°. The standard mercurial thermometer consists of a slender tube with capillary bore hermetically sealed at the top, and terminating at its lower end in a bulb filled with mercury. The melting-point of ice and the boiling-point of water at standard pressure are determined on the tube, and the intermediate space is subdivided into equal parts. The graduations are extended above and below the fiducial points, and finally the tube is calibrated, and outstanding errors of the graduation are determined. Ordinary thermometers covering any desired small range of temperature are graduated by comparison with a standard. For extreme degrees of cold, thermometers filled with spirit of wine must be employed, as no degree of cold known is capable of freezing that liquid, whereas mercury freezes at about 39° below zero on the Fahrenheit scale. On the other hand, spirit of wine is not adapted to high temperatures, as it is soon converted into vapor, whereas mercury does not boil till its temperature is raised to 660° F. Mercury thermometers designed for measuring temperatures up to 400° C. (752° F.) are made by filling the stem and an upper bulb above the stem with nitrogen. The mercury expands against the increasing pressure of the nitrogen, and its boiling-point is raised thereby. Temperatures higher than this limit are usually obtained with air- or steam-thermometers and other forms of pyrometer (which see). The air- (or gas-)thermometer consists of a quantity of pure dry air or gas contained in a reservoir such that its change of volume or of pressure with varying temperatures may be properly observed. Two forms have been used— the constant-pressure thermometer, in which the gas is maintained at constant pressure and its varying volume measured; the constant-volume thermometer, in which the increase of pressure under constant volume is measured. This is the ordinary form in which the instrument is used. For accuracy it is decidedly superior to the mercury thermometer, and has been adopted as the ultimate standard to which all other thermometers are referred. In the metallic thermometer, as generally constructed, temperature is measured by the change in form of composite metal bars, due to their differential expansion (hence more properly called bimetallic thermometer). One of the early forms was that of Bréguet, which consists of a fine spiral bar made of platinum, gold, and silver. One end of the spiral is fixed, the other end being connected with a simple mechanical device to convert the curving or torsion of the bar under changes of temperature into the movement of an index over a dial having a scale marked in a circle upon it. The same principle, with variations in the mechanical application, is now much used in the construction of thermographs. For indicating very slight variations of temperature a thermo-electric junction or the bolometer is employed.
2. n. Hence, figuratively, anything which (roughly) indicates temperature.
3. n. A thermometer whose action is based on the variation of electrical resistance produced by changes of temperature in a metallic conductor. The difference in the resistance between a current passing through a conductor of known and one of unknown temperature gives the difference of temperature between the two. Also called differential-resistance thermometer. The most delicate form in which the principle is applied is the bolometer.
4. n. The Rutherford maximum has a light movable steel index at the top of the mercurial column. The tube is placed horizontal, and as the temperature rises the mercury pushes the index before it. When the temperature falls, the index is left in situ to mark the position of the maximum.
5. n. In Phillips's maximum, a small bubble of air makes a break in the upper part of the mercurial column. When the temperature begins to fall, the detached portion of the column is left behind to register the highest temperature.
6. n. The Negretti maximum has the bore of the tube partly closed by a constriction just above the bulb. In rising temperatures mercury is forced from the bulb past the constriction, but when the temperature falls the mercury cannot readily return to the bulb, and the top of the mercurial column indicates the maximum temperature. In order to reset the thermometer to the current air-temperature, the mercury is forced back into the bulb by whirling the instrument on a swing-pin. This form of maximum is used at the stations of the United states Weather Bureau.

Wiktionary

1. n. An apparatus used to measure temperature.

GNU Webster's 1913

1. n. (Physics) An instrument for measuring temperature, founded on the principle that changes of temperature in bodies are accompanied by proportional changes in their volumes or dimensions.

WordNet 3.0

1. n. measuring instrument for measuring temperature