To see that every reversible engine operating between reservoirs formula_24 and formula_25 must have the same efficiency, assume that two reversible heat engines have different values of formula_2, and let the more efficient engine (M) drive the less efficient engine (L) as a heat pump. As the figure shows, this will cause heat to flow from the cold to the hot reservoir without any external work or energy, which violates the second law of thermodynamics. Therefore, both (reversible) heat engines have the same efficiency, and we conclude that: