Sadi Carnot

Major contributions

• Abstracted the steam engine into a general “heat engine” operating between a hot and a cold body, independent of the working substance
• Introduced the reversible cycle of isothermal and adiabatic steps, the Carnot cycle, and the concept of reversibility itself
• Argued that no engine can be more efficient than a reversible one between the same temperatures (Carnot's theorem), the seed of the second law
• Noted that efficiency depends on the temperature difference, suggesting high-temperature operation, a principle later used by Rudolf Diesel
• In unpublished notes (published 1878), moved towards treating heat as motion and estimated a mechanical equivalent of heat, anticipating the first law

Named concepts

Concept	Statement	Codex page
Carnot cycle	Reversible two-isothermal, two-adiabatic cycle	View entry
Carnot's theorem	No engine exceeds reversible-engine efficiency	View entry
Carnot efficiency	η = 1 − Tc/Th	View entry

Career

Carnot was the son of Lazare Carnot, a mathematician, engineer and leading political figure of the French Revolution. He entered the École Polytechnique in 1812 and then trained as a military engineer. Largely on half-pay or leave from the army, he studied physics, economics and industry in Paris, motivated by Britain's lead in steam power, which he saw as crucial to national strength.

He published Réflexions at his own expense in 1824 in a small edition, and it attracted little attention. He died of cholera in Paris in 1832, aged 36, and, as was customary for cholera victims, most of his belongings and papers were burned; some notes survived and were published by his brother Hippolyte in 1878.

Key publications

• Carnot, S. (1824). Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance. Paris: Bachelier.
• Carnot, S. (1878). Posthumous notes, appended to the 1878 edition of Réflexions, ed. H. Carnot. Paris: Gauthier-Villars.

Nobel Prize

Carnot died in 1832, long before the Nobel Prizes were established in 1901.

Historical context

Carnot reasoned within the caloric theory, in which heat was a conserved fluid, and compared an engine to a water wheel driven by falling caloric. His conclusions about efficiency nevertheless survived when the caloric theory was abandoned. Émile Clapeyron restated his work with pressure–volume diagrams in 1834, and William Thomson (Kelvin) and Rudolf Clausius reconciled it with energy conservation around 1850, creating classical thermodynamics. Historians regard Réflexions as one of the few cases in which a single short work founded a field, though its full significance was recognised only after his death (see Heat for the end of caloric theory).