Carnot, (Nicolas Léonard) Sadi (1796-1832)

French scientist and military engineer who founded the science of thermodynamics. His pioneering work was Reflexions sur la puissance motrice du feu/On the Motive Power of Fire, which considered the changes that would take place in an idealized, frictionless steam engine.

Carnot's theorem showed that the amount of work that an engine can produce depends only on the temperature difference that occurs in the engine. He described the maximum amount of heat convertible into work by the formula (T₁ − T₂)/T₂, where T₁ is the temperature of the hottest part of the machine and T₂ is the coldest part.

Carnot was born in Paris and educated there and at the Ecole Genie in Metz. He became an army engineer, at first inspecting and reporting on fortifications and from 1819 based in Paris. In formulating his theorem, Carnot considered the case of an ideal heat engine following a reversible sequence known as the Carnot cycle. This cycle consists of the isothermal expansion and adiabatic expansion of a quantity of gas, producing work and consuming heat, followed by isothermal compression and adiabatic compression, consuming work and producing heat to restore the gas to its original state of pressure, volume, and temperature. Carnot's law states that no engine is more efficient than a reversible engine working between the same temperatures. The Carnot cycle differs from that of any practical engine in that heat is consumed at a constant temperature and produced at another constant temperature; no work is done in overcoming friction at any stage; and no heat is lost to the surroundings. At the time he wrote Reflexions, Carnot believed that heat was a form of fluid. But notes discovered 1878 indicate that he later arrived at the idea that heat is essentially work, or rather work that has changed its form. He had calculated a conversion constant for heat and work and showed he believed that the total quantity of work in the universe is constant - the first law of thermodynamics.