Sadi Carnot was the French engineer whose work on heat engines laid the foundation for the second law of thermodynamics.
In 1824, Carnot published Reflections on the Motive Power of Fire, studying how steam engines convert heat into useful work. He argued that engine efficiency depends on the temperatures of the hot and cold reservoirs, not on the particular working substance. His ideal reversible engine later became known as the Carnot engine.
Carnot still used the then-popular caloric theory, which treated heat as a fluid. Later scientists reinterpreted his results using energy conservation and entropy. Rudolf Clausius formulated a modern version of the second law, while William Thomson—Lord Kelvin—developed the absolute temperature scale from Carnot’s ideas.
A common mix-up is crediting Clausius alone. Clausius gave the law its formal nineteenth-century formulation, but Carnot’s 1824 heat-engine analysis supplied its historical foundation.