thermometry

• WordNet 3.6
  • n thermometry the measurement of temperature
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• • n Thermometry The estimation of temperature by the use of a thermometric apparatus.
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• • n thermometry The art of measuring temperature. A numerical unit of temperature difference is derived from the measurable physical effects produced in bodies by heat—for example, linear expansion, volumetric expansion, change of gaseous elastic pressure, and change in electric resistance. In the customary use of the thermometer, changes in temperature are assumed to be directly proportional to the observed changes in the thermometric material, and temperature units are defined in terms of the particular material and phenomenon adopted. The thermometric unit at present (1891) adopted by the International Bureau of Weights and Measures is one centigrade degree, or the hundredth part of the fractional increase of pressure of a volume of pure dry gas originally at a pressure of one standard atmosphere, and heated from the standard freezing-point to the standard boiling-point of water. With this unit, increments of temperature are closely proportional to increments of heat, and the air- (or gas-)thermometer of constant volume is the adopted instrumental standard. The air-thermometer, however, is not adapted to ordinary uses, and it is the object of thermometry to obtain comparable temperatures with convenient and portable instruments. The expansion of liquids is closely proportional to successive increments of heat, and is taken as the basis of the usual secondary thermometric standards. It should be observed, however, that in general the subject of measurement is not the simple expansion of the liquid, but the differential expansion of the liquid and the glass bulb in which it is contained; and from the standpoint of precise thermometry it is in this uncertain, irregular, and varying behavior of the glass that the principal residual discrepancies of normal mercurial thermometers lie. The most important of these sources of error in mercurial thermometers is a change in the zero-point with time and with the temperatures to which the thermometers are exposed. This change depends upon the nature of the glass. Glass of special composition is now used in the construction of thermometers, which will practically eliminate this source of error. The method of graduating thermometers between two fiducial points, instead of by volume, was an advance in construction adopted by Fahrenheit that first made possible the construction of comparable thermometers. The adoption later of the freezing-point and the boiling-point of water for these two standard temperatures brought different kinds of thermometers into substantial agreement. In the recent progress of precise thermometry, residual sources of error have been discovered, and outstanding discrepancies have been investigated, so as to render possible the reduction of all observed temperatures to the thermodynamic scale.
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