The concept of entropy arose from Rudolf Clausius 's study of the Carnot cycle that is a thermodynamic cycle performed by a Carnot heat engine as a reversible heat engine. In a Carnot cycle the heat Q H is transferred from a hot reservoir to a working gas at the constant temperature T H during isothermal expansion stage and the heat Q C is transferred from a working gas to a cold reservoir at the constant temperature T C during isothermal compression stage. According to Carnot's theorem, a heat engine with two thermal reservoirs can produce a work W if and only if there is a temperature difference between reservoirs. Originally, Carnot did not distinguish between heats Q H and Q C , as he assumed caloric theory to be valid and hence that the total heat in the system was conserved. But in fact, the magnitude of heat Q H is greater than the magnitude of heat Q C . Through the efforts of Clausius and Kelvin, the work W done by a reversible heat engine was found to be the product of the Carnot efficiency (i.e., the efficiency of all reversible heat engines with the same pair of thermal reservoirs) and the heat Q H absorbed by a working body of the engine during isothermal expansion: W = T H − − T C T H ⋅ ⋅ Q H = ( 1 − − T C T H ) Q H To derive the Carnot efficiency Kelvin had to evaluate the ratio of the work output to the heat absorbed during the isothermal expansion with the help of the Carnot–Clapeyron equation, which contained an unknown function called the Carnot function. The possibility that the Carnot function could be the temperature as measured from a zero point of temperature was suggested by Joule in a letter to Kelvin. This allowed Kelvin to establish his absolute temperature scale.