Sadi Carnot

Nicolas Léonard Sadi Carnot was born in Paris in 1796, the eldest son of Lazare Carnot, the mathematician and 'Organiser of Victory' who marshalled the armies of revolutionary France. Trained at the École Polytechnique as a military engineer, the younger Carnot turned his attention to a question of national importance: why were British steam engines so much more efficient than French ones, and what set the ultimate limit on the work that heat could produce?

His answer appeared in 1824 in a slim volume, Réflexions sur la puissance motrice du feu (Reflections on the Motive Power of Fire). Reasoning almost entirely in prose, with scarcely a formula, Carnot imagined an idealised, reversible engine and showed that its efficiency depends only on the temperatures between which it works, not on the working substance — the seed of the second law of thermodynamics and of the concept of absolute temperature.

The book sank almost without trace, read by few and understood by fewer. It was rescued a decade later by Émile Clapeyron, who recast Carnot's reasoning into the graphical language of the pressure–volume diagram, and only then, through Clapeyron, did Carnot's ideas reach Kelvin and Clausius, who built the second law upon them.

Carnot died in the Paris cholera epidemic of 1832, aged 36, his contribution still largely unrecognised. He went to his grave believing his work had failed. His private notebooks, published long after his death, revealed that he had by then abandoned the caloric theory and grasped that heat and work are interconvertible — anticipating the first law of thermodynamics as well as the second.