French physicist (1796–1832)

Carnot came from a distinguished Parisian political family; his father, Lazare, was a leading politician under Napoleon Bonaparte. He studied at the Ecole Polytechnique, from which he graduated in 1814. For the next few years he worked as a military engineer, but the political climate had changed with the fall of Bonaparte and, in 1819, he transferred to Paris and concentrated on scientific research.

The fruits of this work ripened in 1824 in the form of a book called Réflexions sur la puissance motrice de feu (On the Motive Power of Fire). The main theme of this masterpiece was an analysis of the efficiency of engines in converting heat into work. He found a simple formula depending only on the temperature differences in the engine and not on intermediate stages through which the engine passed. He also introduced the concept of reversibility in the form of the ideal Carnot cycle. Using these ideas he derived an early form of the second law of thermodynamics, stating that heat always flows from hot to cold. It became an inspiration, many years later, for Rudolf Clausius's formulations of thermodynamics. Carnot died of cholera at the age of 36.

The French physicist Nicolas Léonard Sadi Carnot (1796-1832), with his analysis of the working of an ideal heat engine, opened the road to the science of thermodynamics.

Sadi Carnot was born on June 1, 1796, in Paris. He was given the name Sadi because of the admiration of his father, Lazare Carnot, for Sadi (Muslihal-Din), a medieval Persian poet and moralist. Sadi entered the famed Polytechnique at the age of 16, and he graduated first the next year in the artillery section. In 1814 he went to Antwerp, where his father fought the British, made a successful stand against them, and became the only undefeated French general, as other Napoleonic armies fell to the Allies.

Following the peace treaty of 1814, father and son returned to Paris and from there Sadi went to Metz, where in the military school he wrote an able analysis of the use of theodolites. After Waterloo the elder Carnot was exiled, and Sadi was sent from one military fortress to another to do routine engineering work. In 1818 he made use of a new royal decree and successfully took the exams for admission to the corps of general staff officers. After that his life was spent largely in studies and in the cultivation of the arts, especially music.

Birth of Thermodynamics

The only interruption of Carnot's stay in Paris came in 1821, when he visited his father in exile in Magdeburg; his father died 2 years later. Carnot's interest turned more and more toward fundamental questions concerning industry, economics, and social organization. He kept visiting museums, factories, and offices, trying to find the key to the most efficient utilization of power.

These concerns led to the writing of Carnot's masterpiece, Réflexions sur la puissance motrice du feu et sur les machines propres àdévelopper cette puissance (Reflections on the Motive Power of Fire and on Machines Fitted to Develop That Power), published in 1824. It stands as the beginning of the science of thermodynamics. This is not to say that Carnot wanted to write physics. His work had in view a wide audience and also contained many errors and half truths. Yet, as his analysis of the efficiency of heat engines aimed at fundamental elucidation of the problem, Carnot inevitably opened up new avenues in physics. He succeeded in making it clear that there was a theoretical limit to the efficiency of any heat engine: "The motive power of heat is independent of the agents employed to realize it; its quantity is fixed solely by the temperatures of the bodies between which is effected, finally, the transfer of the caloric."

The only favorable reaction to Carnot's work was a long review in the Revue encyclopédique. However, when E. Clapeyron returned in 1830 from Russia and began to work on his "Memoir on the Motive Power of Heat," he found that Carnot had anticipated him in several respects but that Carnot's ideas and experimental data needed considerable reworking. Clapeyron stated his indebtedness to Carnot at the beginning of his memoir, which contained the first diagrammatic representation of the so-called Carnot cycle. Still it was not until 1843, when Clapeyron's memoir appeared in German translation in the Annalen der Physik und Chemie, that the world of science began to take notice.

By then Carnot had been dead for 11 years. His last 8 years were spent in an intense search for an improved system of economics, of taxation, and of scientific education. In 1830, 2 years after his retirement from the army, he helped organize the Réunion Polytechnique Industrielle to promote collaboration among the alumni of the Polytechnique in support of the foregoing program. He was also an active member of the Association Polytechnique, devoted to the dissemination of useful knowledge among the wider segments of society.

Further Reading

The principal information on Carnot's life is the essay written in 1872 by his younger brother, Hippolyte. It was reprinted in 1878 in a new edition of Sadi Carnot's Réflexions (1824), together with some of his unpublished manuscripts. An English translation of Carnot's Reflections on the Motive Power of Fire (repr. 1960) contains a very informative introduction by the editor, E. Mendoza, on Carnot's life, on early thermodynamics, and on the significance of the Carnot cycle.

For more information on Nicolas-Léonard- Sadi Carnot, visit Britannica.com.

(nēkōlä' lāônär' sädē' kärnō') , 1796–1832, French physicist, a founder of modern thermodynamics; son of Lazare N. M. Carnot. His famous work on the motive power of heat (Réflexions sur la puissance motrice du feu, 1824) is concerned with the relation between heat and mechanical energy. Carnot devised an ideal engine in which a gas is allowed to expand to do work, absorbing heat in the process, and is expanded again without transfer of heat but with a temperature drop. The gas is then compressed, heat being given off, and finally it is returned to its original condition by another compression, accompanied by a rise in temperature. This series of operations, known as Carnot's cycle, shows that even under ideal conditions a heat engine cannot convert into mechanical energy all the heat energy supplied to it; some of the heat energy must be rejected. This is an illustration of the second law of thermodynamics. Carnot's work anticipated that of Joule, Kelvin, and others.