Thermometry

International Temperature Scale.—The experimental diffi culties incident to the realization of the thermodynamic scale have made it expedient to adopt a practical scale designated as the International Temperature Scale. This scale has been pro posed as the result of discussion extending over a considerable period between the national laboratories of Germany, Great Britain, and the United States, and has been adopted pro visionally by the Seventh General Conference of Weights and Measures, meeting in Paris in 1927. It is intended to represent the thermodynamic scale as closely as is possible with present knowledge for scientific and industrial purposes, and may be subject to revision as more accurate methods of measurement are evolved. It is based on a number of fixed and reproducible equilibrium temperatures to which numerical values are assigned, and upon the indications of interpolation instruments calibrated according to a specified procedure at the fixed temperatures. A summary of the basic fixed points, and of the procedure to be adopted in realizing the practical scale, will be given at the end of this article.

Mercurial thermometers will doubtless continue to be employed for the majority of measurements for which they are suited, especially in cases where facility of observation is more im portant than the highest attainable degree of precision. It is to be presumed, however, that the indications of such instruments will be reduced when necessary to the international scale by comparison with a suitable standard instead of being expressed in terms of the mercurial scale as previously defined. The some what exacting process of the calibration of the bore of a stand ard mercurial thermometer, which was essential to the reproduc tion of the mercurial scale, becomes of secondary importance, since the calibration correction of each instrument will be auto matically included in the table of corrections supplied by the testing laboratories. Other corrections depend on the conditions under which the instrument is used.

Correction for Changes of Zero.

The changes of zero are of two kinds : (a) Secular rise of zero due to gradual recovery from changes or strains acquired by the bulb during the process of manufacture. This process may be hastened and subsequent changes practically eliminated by annealing the bulb after manu facture, and before final adjustment, at a high temperature, such as that of boiling sulphur (about 45o° C). A thermometer which has not been so treated may show a rise of zero amounting to as much as 2o° or 3o° when exposed for some time to a tempera ture of 35o° C. (b) Temporary depression of zero after each exposure to a high temperature, followed by a slow recovery which may last for days or weeks. The best thermometers of hard glass show a depression of zero amounting to about one-tenth of I° C after exposure to ioo° C. In softer glass the depression

is usually greater and more persistent, and may amount to half a degree after ioo° C. At higher temperatures the depression generally increases roughly as the square of the temperature above o° C. It may amount to 2° or 3° at 3oo° C. The effect cannot be calculated or predicted in any series of observations because it depends in so complicated a manner on the past history and on the time. It is a most serious difficulty in accurate mercurial thermometry, especially at high temperatures. The most satis factory method of correction appears to be to observe the zero immediately after each reading, and to reckon the temperature from the variable zero thus observed. The rationale of this pro cedure is that the depression is produced at the high temperature much more rapidly than the subsequent recovery at the low tem perature. The thermometer is taken from the bath and allowed to cool rapidly by free exposure to the air. As soon as it reaches or 50° C, it is plunged in the melting ice, and the lowest point reached is taken as the temporary zero.

The following formulae have been proposed by various ob servers to represent the depression of zero for different kinds of glass :— Pernet, French cristal, dz = . . . . Guillaume, Verre dur, o—ioo° C, d'z= Bottcher, Crista/ dur, 0— z90° C, dz = 7 • „ Jena, 16, dz= (7zoot . . .

The symbol dz in these formulae stands for the depression of . zero produced by an exposure to a temperature t. The depression is about three times as large in French crystal as in English flint glass, and varies roughly as the square of t. Verre dur and Jena, 16, iii., are varieties of hard glass chosen as standards in France and Germany respectively, on account of the comparatively small depression of zero to which they are liable. At low temperatures, up to 50° C, the depression is very nearly proportional to t, but at temperatures above oo° C it is necessary to adopt another formula in which the term depending upon is more important. These formulae are useful as giving an idea of the probable size of the correction in any case, but they cannot be employed in practice except in the simplest cases and at low temperatures. On account of these temporary changes of zero, a mercury thermom eter intended for the most accurate work at ordinary tempera tures (as in calorimetry) should preferably never be heated above or 50° C, and certainly never above Ioo° C. Above oo° C the changes of zero become more irregular and more variable, depending on the rate of cooling and on the sequence of previous observations, so that even if the method of observing the zero after each reading is adopted, the order of precision attainable rapidly diminishes.