Thus, any reversible heat engine operating between temperatures ''T''<sub>1</sub> and ''T''<sub>2</sub> must have the same efficiency, that is to say, the efficiency is the function of temperatures only: | Thus, any reversible heat engine operating between temperatures ''T''<sub>1</sub> and ''T''<sub>2</sub> must have the same efficiency, that is to say, the efficiency is the function of temperatures only: |