 Nicolas Léonard Sadi Carnot

Sadi Carnot investigated what limits the work obtainable by transferring heat between hot and cold bodies. His reversible-engine argument compares machines without tying the ideal performance to a particular working substance. A reversible reference cycle can run backward, so a supposedly better engine could be combined with it to produce an impossible net result. This reasoning underlies Carnot's theorem and the modern bound $\eta \le 1-T_c/T_h$ for reservoirs at absolute temperatures $T_h>T_c$. The temperature notation is modern, rather than Carnot's original formulation.^[ref]

Nicolas Léonard Sadi Carnot (1796–1832) was a French physicist and engineer, often referred to as the "father of thermodynamics." He is best known for his seminal work on the principles of heat and work in mechanical systems, particularly through his formulation of the Carnot cycle. This idealized thermodynamic cycle helps to demonstrate the maximum possible efficiency of a heat engine operating between two temperatures.