High temperature measurement 497 



solid and liquid, used in dilatation pyrometers expand fairly 

 uniformly as the temperature rises, and between the same 

 fixed points all give practically the same scale. When, how- 

 ever, observations are made at much higher temperatures it 

 is found that different kinds of glass expand differently, hence 

 two thermometers made of different glasses and both correct 

 at 0° and ioo° might differ very considerably at 300" C. 



For this reason, therefore, it is desirable to refer all tempera- 

 ture measurements to some scale independent of the properties 

 of the substance used to indicate it. The methods for the practi- 

 cal realisation of such an " absolute " scale have been shown by 

 Lord Kelvin, but it is not necessary to enter upon them here. 

 Many years ago it was found that nearly all gases, when 

 sufficiently far from their liquefying points, expand nearly 

 equally for equal rises of temperature. Thus for the mercury 

 in the mercury thermometer we may substitute air, hydrogen 

 or nitrogen, and measure temperature by measuring the 

 changes of either pressure or volume of the gas thus con- 

 fined. Owing to the larger coefficient of expansion of 

 the gas as compared with the mercury, much greater sensi- 

 tiveness may be obtained if required, and what is of more 

 importance the expansion of the containing envelope, if its 

 material be suitably chosen, may be made very small in com- 

 parison with that of the measuring gas. Hence uncertainties in 

 the knowledge of this expansion become of much less account. 

 A gas-thermometer has also the advantage of being applicable 

 over a very wide range both in the downward and upward 

 direction. 



In 1887 the International Committee of Weights and Measures, 

 meeting at Sevres, fixed provisionally the Fundamental Inter- 

 national Scale of Temperature, as defined in the following 

 resolution : 



"That the International Committee of Weights and Measures 

 adopt as the Normal Thermometric Scale . . . the Centigrade 

 Scale of the Hydrogen Thermometer, having as fixed points the 

 temperature of melting ice (q°) and that of the vapour of distilled 

 water in ebullition (100°) under the normal atmospheric pressure, 

 the hydrogen being taken under the manometric initial pressure 

 of one metre of mercury. . . ." 



This International Scale was founded on the classic work of 

 Chappuis done at the Sevres laboratory with the large gas- 



