TEMPERATURE SCALE. 
The usual method of measuring temperature is by the expansion of 
some substance, such as mercury in the ordinary glass thermometer, or gas 
in the more refined 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 Seale.” Temperatures on this scale are 
measured by the work done in carrying a substance around a Carnot’s 
cycle working between two sources at constant temperature. 
Without attempting any proof here, the theory gives the following re- 
lation, 
dtr (an 
T2 Qe 
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 are 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 thermodynamic scale rather than the “Normal” or “Gas 
Seale.’ Especially is this true in the case of radiation pyrometry, where 
the laws uad formulas developed have their foundation in the second law 
+ Bulletin. Bureau of Standards, Vol. 3, p. 237, 
