He wrote in 1824 "Réflexions sur la puissance motrice du feu et sur les machines propes à développer cette puissance"
In this work, oriented to recognize in which way the maximum amount of work can be extracted by a given amount of heat, the concepts of cycle and of thermodyanmic transformation are introduced for the first time. Carnot carries out the analysis of a steam engine believing that the heat is a sort of fluid (the caloric) which flows from a hot body towards a colder.
The work of Carnot was neglected until it was reconsidered by Clapeyron who gave the mathematical formulation of the Carnot theoreme.
Clapeyron, as well Carnot, believed in the caloric theory that entered in crisis after the experiments on the transformation of mechanical work to heat
carried out by Joule..
He contributes to the abatement of the theory of caloric in favour of a theory based on the molecular mouvement. He formulates the first principle of Thermodynamic (Principle of conservation of energy). He provides a formulation of the second principle that sounds :
A process having as unique effect the extraction of heat from a colder body and its release to a warmer body it is not possible.
He introduces the concept of entropy.
The refrigerator of Einstein and Szilard
After having formulated the theory of the general relativity, Einstein works for many years, togheter with L. Szilard, on the absorption refrigeration cycles. In 1930 they got in the United States the registration of an absorptino refrigeration process with a single pressure level. The cycle adopts buthane as refrigerating fluid, ammonia as the fluid stabilizing the pressure and water as the fluid absorbing the ammonia that allows its separation from buthane. The cycle is quite different from the more known aamonia-water-hydrogen.