Thermometric scales practical

Thermometer correction. The temperature which is read on the thermometric scale must be corrected because there are several errors in such determinations. One source of error arises from the construction and calibration of the thermometer. The bore of the capillary may not have the same diameter throughout further, the scale graduation and the calibration of low-priced thermometers are not very accurate. A second source of error is the method used in the common melting point apparatus. The common thermometer has been calibrated while totally immersed in a bath. In the melting-point apparatus described, only a part of the stem is immersed. The column of mercury above the oil bath has a lower temperature than that at which the thermometer was calibrated. Therefore either a thermometer calibrated by partial immersion should be used or a correction must be made for the unequal heating of the mercury in the stem of the thermometer. Although thermometers calibrated by partial immersion are available, the latter practice is the more common. 

Fahrenheit s and Amontons scales have a lot of common features with modern thermometric scales, which enabled the fundamental problems in scientific thermometry to be solved namely to assign a number 0, called the empirical temperature, to any given thermal state, to decide whether two bodies have the same temperature or not, and to determine which body has the higher temperature. Later Maxwell recognized that for thermometry to be a logically closed system it is necessary to add a concept of thermal equilibrium and another theorem, sometimes called the zero law of thermodynamics, according to which two bodies which are in thermal equilibrium with a third one are also in thermal equilibrium with each other. By establishing this theorem, which encompassed the form of Euclid s first axiom, the development of the concept of empirical temperature was practically completed. 

Practical difficulties arise in making very precise determinations of temperature on the thermodynamic scale the precision of the more refined thermometric techniques considerably exceeds the accuracy with which the experimental thermometer scale may be related to the thermodynamic scale. For this reason, a scale known as the International Temperature Scale has been devised, with several fixed points and with interpolation formulas based on practical thermometers (e.g., the platinum resistance thermometer between 13.803 K and 1234.93 K). This scale is intended to correspond as closely as possible to the thermodynamic scale but to permit more precision in the measurement of temperatures. Further details about this scale are given in Chapter XVII. 

The simphcity of the relationship between the thermodynamic scale and the gas thermometer scale is due principally to the simple properties of rarefied gases, and also to the fortunate choice of mercury as thermometric substance by Celsius and Reaumur before the discovery of the gas laws. The coefficient of expansion of mercury happens to be almost exactly proportional to the coefficient of expansion of rarefied gases. All our thermodynamical relationships would have been very much more comphcated had water or alcohol, for example, or the resistance of a metal, been used for the definition of the practical scale of temperature. Their strict validity, however, would not have been affected. 

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