299 



TKMPERATL'KE SCALE. 



The usual method of nieasurini: temperature is by the expansion of 

 some substance, such as mercury in the ordinary glass thermometer, or gas 

 in the more retined work. With such a method, however, the magnitude 

 of a degree will depend upon the nature of the substance employed, which 

 Is undesirable in scientific work. A theoretical thermometric scale, inde- 

 pendent of any substance used, has been worked out by Lord Kelvin and 

 is known as the "Thermodynamic Scale." Temperatures on this scale are 

 measured by the work done in carrying a substance around a Carnot's 

 cj'cle working between two sources at constant temperature. 



Without attempting any proof here, the theory gives the following re- 

 lation, 



Ti _ Qi 



T2 Q2 



where T is the absolute temperature and Q is the quantity of heat, which 

 can be measured in terms of energy since by the first law of thermody- 

 namics heat is proportional to work. Hence the ratio of any two tempera- 

 tures may be determined from purely mechanical considerations and will 

 furthermore be independent of the substance used in the conversion of work 

 into heat. Experiment has shown, however, that no known gas is perfect, 

 and that, furthermore, no gas is satisfactory throughout the entire range 

 of temperatures which arp used in gas thermometry.^ The practical stand- 

 ard is the international Normal Scale of the constant volume hydrogen 

 thermometer. Hydrogen can be used for very low temperatures, but above 

 300° C. it is unreliable. Nitrogen, on the other hand, can not be used for 

 low temperatures, but is suitable for high temperatures. In the absence 

 of a perfect gas we have practical standard gas thermometers, such as hy- 

 drogen and nitrogen, for which thermodynamic corrections have been de- 

 termined. In practice, however, the gas thermometer is never employed by 

 reason of the difficulties inherent in its use and. furthermore, because there 

 are numerous other thermometers more convenient which can be compared 

 with the gas thermometer. 



In exact work, it is necessary, therefore, to define temperature in the 

 terms of the lluM-modynaniir- scale I'atlw'r than the "Normal" or "Gas 

 Scale." Especially is (his true in the case of radiation pyrometvy, where 

 the laws und fornuilas developed have their foundation in the second law 



'Bnllotin. Bureau of Standards, Vol. 3, p. 237. 



