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THERMOMETRY (Gr. Bepµos, warm; µerpov...

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Originally appearing in Volume V26, Page 822 of the 1911 Encyclopedia Britannica.

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THERMOMETRY (Gr. Bepµos, warm; µerpov, a measure)

THERMOMETRY (Gr. Bepµos, warm; µerpov, a measure) , the See also:

art of measuring temperature or degree of See also:

HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat. The See also:

INSTRUMENTS

instruments used for this purpose are known as thermometers, or sometimes, when the temperatures to be measured are high, as pyrometers. 1. A brief See also:

SKETCH (directly adapted from Dutch schets, which was taken from Ital. schizzo, a rough draft, Lat. schedium, something hastily made, Gr. oxE&os, sudden, off-hand, axeb6v, near by; Ger. Skizze and Fr. esquisse are from the same source)

sketch of the See also:

EVOLUTION

evolution of the thermometer is included in the See also:

ARTICLE (from Lat. articulus, a joint)

article HEAT, §§ 2 and 3. The See also:

OBJECT

object of thepresent article is to discuss the See also:

general principles on which the accurate measurement of temperature depends, and to describe the application of these principles to the construction and use of the most important types of thermometer. See also:

SPECIAL

Special See also:

ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention will be devoted to more See also:

RECENT

recent advances in scientific methods of testing thermometers and to the application of See also:

electrical and See also:

OPTICAL

optical methods to the difficult problem of measuring high temperatures. In the article See also:

PYROMETER (Gr. iri p, fire, µErpov, a measure)

PYROMETER an See also:

account will be found of some of the thermoscopic methods employed in the arts for determining high temperatures. 2. Zero: Fundamental See also:

INTERVAL

Interval.—In all systems of measuring temperature it is necessary (I) to choose a zero or starting-point from which to reckon, (2) to determine the See also:

SIZE

size of the degree by subdividing the interval between two selected fixed points of the See also:

scale (called the " fundamental interval ") into a given number of equal parts. The fundamental interval selected is that between the temperature of melting See also:

ICE (a word common to Teutonic languages; cf. Ger. Eis)

ice and the temperature of condensing See also:

steam, under See also:

standard atmospheric pressure. On the Centigrade See also:

SYSTEM

system the fundamental interval is divided into See also:

LOO (formerly called " Lanterloo," Fr. lanturlu, the refrain of a popular 17th-century song)

loo parts, and the melting-point of ice is taken as the zero of the scale.

We shall denote temperature reckoned on this system by the See also:

LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

letter t, or by affixing the letter C. It is often convenient to reckon temperature, not from the melting-point of ice, but from a theoretical or See also:

ABSOLUTE (Lat. absolvere, to loose, set free)

absolute zero representing the lowest conceivable temperature. We shall denote temperature reckoned in this manner by the letter T, or 0, or by affixing the letters Abs. In practice, since the absolute zero is unattainable, the absolute temperature is deduced from the Centigrade temperature by adding a See also:

CONSTANT, BENJAMIN (1845-1902)

constant quantity, To, representing the interval between the absolute zero and the melting-point of ice; thus T=t+To. 3. Arbitrary Scales.—An arbitrary scale can be constructed by selecting any See also:

PHYSICAL

physical See also:

PROPERTY

property of a substance which varies regularly with the temperature, such as the See also:

VOLUME

volume of a liquid, or the pressure or See also:

DENSITY (Lat. densus, thick)

density of a See also:

gas, or the electrical resistance of a See also:

metal. Thus if V denote the volume of a given See also:

mass at the temperature t, and if Vo, VI represent the volumes of the same mass at the temperatures o° and See also:

roe C., the size of re C. on the scale of this arbitrary thermometer is one hundredth See also:

PART

part of the fundamental interval, namely (V,—Vo)/loo, and the temperature t at volume V is the number of these degrees contained in the expansion V—Vo between o° and t° C. We thus arrive at the See also:

formula t= too (V—Vo)/(Vi—Vo) . . (I), which is the general expression for the temperature Centigrade on any such arbitrary scale, provided that we substitute for V the particular physical property selected as the basis of the scale. If we prefer to reckon temperature from an arbitrary zero defined by the vanishing of V, which may conveniently be called the fundamental zero of the scale considered, we have, putting V=o in See also:

EQUATION (from Lat. aequatio, aequare, to equalize)

equation (I), the numerical values of the fundamental zero To, and of the temperature T reckoned from this zero To=looVo/(V,—Vo), and T=.ToV/Vo=t+To . . (2). It is frequently convenient to measure temperature in this manner when dealing with gases, or electrical resistance thermometers.

4. Absolute Scale.—It is necessary for theoretical purposes to reduce all experimental results as far as possible to the absolute scale, defined as explained in HEAT, § 21, on the basis of See also:

Carnot's principle, which is See also:

INDEPENDENT

independent of the properties of any particular substance. Temperature on this scale measured from the absolute zero will be denoted by the letter B. This scale can be most nearly realized in practice by observing the temperature T on the scale of a gas-thermometer, and making special experiments on the gas to determine how far its scale deviates from that of the thermodynamical See also:

engine. In the See also:

case of the gases See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen and See also:

HELIUM (from Gr. i)Xcor, the sun)

helium, which can exist in the liquid See also:

state only at very See also:

low temperatures, the deviations from the absolute scale at See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary temperatures are so small that they cannot be certainly determined. Thermometers containing these gases are generally taken as the ultimate See also:

STANDARDS

standards of reference in See also:

PRACTICAL

practical thermometry.

End of Article: THERMOMETRY (Gr. Bepµos, warm; µerpov, a measure)

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