Nicolas Léonard Sadi Carnot (01.06.1796-24.08.1832) was a French physicist and engineer, one of the founders of thermodynamics. He was born in Paris. He graduated from the École Polytechnique (1814). In 1814-19 and 1826-27 he was in military service as an engineer.
In 1824 he published the work “Reflections on the Motive Power of Fire and on Machines Fitted to Develop That Power”, in which, starting from the impossibility of a perpetual motion machine, he was the first to show that useful work can be obtained only when heat passes from a heated body to a colder one (the second law of thermodynamics). Only the difference between the temperatures of the heater and the cooler determines the output (the efficiency) of a heat engine, while the nature of the working substance plays no part at all (Carnot's theorem). Carnot's work became widely known in 1834 through B. Clapeyron, who gave his ideas an accessible mathematical form.
He introduced the concepts of a cyclic and of a reversible process and of the ideal cycle of heat engines, and so laid the foundations of their theory. He showed the advantage of using high-pressure steam and its repeated expansion in steam engines, and formulated the principle on which gas heat engines work. He arrived at the concept of the mechanical equivalent of heat.