Consider a Carnot engine using a perfect gas as a working fluid. Let the high temperature reservoir be at and low temperature reservoir be at

(20.23)

or,

But the absolute temperature scale is defined by the relation

(20.24)

Therefore,

Since the numerical value assigned to the triple point of water can be chosen to be same for both the perfect gas scale and the absolute temperature scale, . Hereafter, the common symbol may be used to represent the temperature.