Réflexions sur la puissance motrice du feu et sur les machines propres a développer cette puissance

In the Réflexions, published in 1824, when he was 28, military engineer Nicolas Léonard Sadi Carnot (1796-1832) laid the foundations for the discipline that would later become known as “thermodynamics.” Carnot’s brief work is dedicated to the analysis of the performance of a steam engine, exemplified as a mechanical system that, to create “motive power,” exploits the passage of “caloric fluid” from a hot to a cold tank. The work also contains the first (of many) formulations of what would later become known as the second principle of thermodynamics. In particular, according to Carnot, the efficiency of any real motor can never equal 100%. That is to say, no perfectly reversible engine can be built, capable of transforming all the “caloric fluid” transferred from the hot tank to the cold tank into “motive power.” The plate below illustrates the elementary thermic machine used by Carnot to develop its demonstrations: a piston cylinder. It is a vessel filled with gas or steam whose variable volume depends on the position of the piston. Figure 1, in particular, shows the device in relation to the two tanks: the hottest, tank A, and coldest, tank B.