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Nicolas Léonard Sadi Carnot (French:; 1 June 1796 – 24 August 1832) was a French military engineer and physicist. A graduate of the École polytechnique, Carnot served as an officer in the Engineering Arm (le génie) of the French Army. He also pursued scientific studies and in June 1824 published an essay titled Reflections on the Motive Power of Fire. In that book, which would be his only publication, Carnot developed the first successful theory of the maximum efficiency of heat engines.

Carnot's scientific work attracted little attention during his lifetime, but in 1834 it became the object of a detailed commentary and explication by another French engineer, Émile Clapeyron. Clapeyron's commentary in turn attracted the attention of William Thomson (later known as Lord Kelvin) and Rudolf Clausius. Thomson used Carnot's analysis to develop an absolute thermodynamic temperature scale, while Clausius used it to define the concept of entropy, thus formalizing the second law of thermodynamics.

Sadi Carnot was the son of Lazare Carnot, an eminent mathematician, engineer, and commander of the French Revolutionary Army and later of the Napoleonic army. Some of the difficulties that Sadi faced in his own career might have been connected to the persecution of his family by the restored Bourbon monarchy after the fall of Napoleon in 1815. Sadi Carnot died in relative obscurity at the age of 36, but today he is often characterized as the "father of thermodynamics ".