On the “Kew” scale of temperature and its relation to the international hydrogen scale

Abstract In October, 1887, the International Committee of Weights and Measures adopted as the standard thermometric scale to which all temperature measurements were to be referred that of the constant-volume hydrogen thermometer. When, therefore, thermometric readings are expressed on any other scale the correction to be applied to these to bring them into accord with this standard becomes of importance. By far the majority of temperature measurements are made by means of mercury thermometers. The ideal mercury thermometer would be one which when subjected to any steady temperature would assume immediately a steady reading identical with that given by the hydrogen thermometer at the same temperature. This ideal is, as might be expected, not attained by any known mercury-in-glass thermometer, and the amount of the departure from the ideal at different temperatures depends on the particular kind of glass employed. So long ago as 1847 Regnault was aware of this fact, and showed that for several of the best thermometric glasses then in use the departure might attain as much as 10° C. above 300° C.