Nicolas Léonard Sadi Carnot (English: kar-NOH, French: [nikɔla leɔnaʁ sadi kaʁno]; 1 June 1796 – 24 August 1832), known as Sadi Carnot, was a French military engineer and physicist. A graduate of the École polytechnique, Carnot served as an officer in the Engineering Arm (le génie) of the French Army. He also pursued scientific studies, and in June 1824 published an essay titled Reflections on the Motive Power of Fire. In that book, which would be his only publication, Carnot developed the first successful theory of the maximum efficiency of heat engines.
Carnot's scientific work attracted little attention during his lifetime, but in 1834 it became the object of a detailed commentary and explanation by another French engineer, Émile Clapeyron. Clapeyron's commentary in turn attracted the attention of William Thomson (later Lord Kelvin) and Rudolf Clausius. Thomson used Carnot's analysis to develop an absolute thermodynamic temperature scale, while Clausius used it to define the concept of entropy, thus formalizing the second law of thermodynamics.
Carnot was the son of Lazare Carnot, an eminent mathematician, engineer, and commander of the French Revolutionary Army and later of the Napoleonic army. Some of the difficulties that Carnot faced in his own career might have been connected to the persecution of his family by the restored Bourbon monarchy after the fall of Napoleon in 1815. Carnot died in relative obscurity at the age of 36, but today he is often characterized as the "father of thermodynamics".
Sadi Carnot was born in Paris on 1 June 1796, at the Petit Luxembourg palace, where his father Lazare resided as one of the five members of the Directory, the highest governing body of the French First Republic in the immediate aftermath of the Thermidorian Reaction. His mother, Sophie née Dupont (1764-1813), came from a wealthy family based in Saint-Omer.
Carnot's father Lazare named him after the 13th-century Persian poet Sadi of Shiraz. An elder brother, also named Sadi, had been born in 1794 but died in infancy the following year. "Sadi" is the only given name that appears in the second-born's civil birth certificate, dated 14 prairial, year IV in the French Republican calendar. On 11 July 1796, the child was baptized in the Catholic church of Saint-Louis-d'Antin as "Nicolas-Léonard Dupont". The principal witness at that baptism was his maternal grandfather, Jacques-Antoine-Léonard Dupont. The father is wrongly identified in the baptismal record as Jacques-Léonard-Joseph-Auguste Dupont (who was, in fact, the child's maternal uncle). Following the biographical notice published long after his death by his brother Hippolyte, most sources now give his full name as "Nicolas Léonard Sadi", but there is no evidence that he ever used any name other than "Sadi".
Carnot had a younger brother, Hippolyte Carnot, who was born in 1801 in Saint-Omer and who would later become a prominent politician. Hippolyte's eldest son Sadi Carnot served as President of France from 1887 to 1894. Another of Hippolyte's sons was the chemist, mining engineer and politician Adolphe Carnot. Carnot himself would remain a bachelor and left no descendants.
The young Carnot was educated first at home by his father and later at the Lycée Charlemagne, in Paris, where he prepared for the examinations required to enter the École polytechnique, which his father had helped to establish. In 1811, at the age of 16 (the minimum allowed), Carnot became a cadet of the École polytechnique, where his classmates included the future mathematician Michel Chasles. Among his professors were André-Marie Ampère, Siméon Denis Poisson, François Arago, and Gaspard-Gustave Coriolis. Thus, the school had become renowned for its instruction in mathematics and physics.
During the Battle of Paris in March of 1814, Carnot, Chasles, and other cadets of the École polytechnique participated in the defense of Vincennes. This appears to have been Carnot's only experience of battle. Carnot graduated in 1814 and was admitted at the École d'application de l'artillerie et du génie ("School of Applied Artillery and Military Engineering") in Metz, where he completed a two-year course. Carnot then became an officer in the French army's corps of engineers.
Carnot's father Lazare served as Napoleon's minister of the interior during the "Hundred Days", and, after Napoleon's final defeat in 1815, Lazare was forced into exile in the German city of Magdeburg. Carnot's position in the army, under the restored Bourbon monarchy of King Louis XVIII, became increasingly difficult. Lazare never returned to France, dying in Magdeburg in 1823.
Carnot became a captain in the Génie and was posted to various locations, where he inspected fortifications, tracked plans, and wrote many reports. However, it appeared that his recommendations were ignored and that his career was stagnating. On 15 September 1818, at the age of 22, he took a six-month leave to prepare for the entrance examination to the newly formed General Staff in Paris. Carnot passed the exam and joined the General Staff in January 1819, with the lower rank of lieutenant. He remained on call for military duty, but from then on he dedicated most of his attention to private intellectual pursuits and received only two-thirds pay.
In Paris, Carnot befriended Nicolas Clément and Charles-Bernard Desormes and attended lectures on physics and chemistry at the Sorbonne and the Collège de France. He also attended the Conservatoire national des arts et métiers, where he followed the lectures on chemistry by Clément and those on economics by Jean-Baptiste Say. Carnot became interested in understanding the limits to improving the performance of steam engines, which led him to the investigations that became his Reflections on the Motive Power of Fire, published at his own expense in June 1824.
Carnot was finally promoted to his former rank of captain in September of 1827, but the following April he quit the army, having completed only fifteen months of active service and without right to a pension. In a directory of alumni of the École polytechnique published by Ambroise Fourcy in 1828, Carnot is listed as "maker of steam engines". This and some other indications suggest that Carnot may have been involved in a practical scheme for the improvement of steam engines, but no patents or other concrete evidences of that work have emerged.
Carnot was interested in political economy. His political orientation was liberal, but he seems to have preferred the more interventionist doctrines of Jean de Sismondi to the laissez-faire policies advocated by classical liberal economists such as Say and David Ricardo. Out of Carnot's private writings on economics, only some fragmentary notes survive.
Carnot initially welcomed the July Revolution of 1830, which ended the Bourbonic regime under Charles X and established a new constitutional monarchy under "Citizen King" Louis Philippe. According to his brother Hippolyte, there was some discussion among leaders of the new regime of incorporating Sadi to the Chamber of Peers, as he could be considered to have inherited the Imperial title of "Count Carnot" that Napoleon had bestowed on his father Lazare in 1815. Nothing came of this, however, perhaps because Sadi's republican convictions prevented him from accepting a hereditary distinction.
According to recollections published long after Carnot's death by his brother Hippolyte, Carnot was an avid reader of Blaise Pascal, Molière and Jean de La Fontaine. Hippolyte recalled that Carnot was a philosophical theist who believed in divine causality but not in divine punishment. Carnot wrote in his private papers that "what to an ignorant man is chance, cannot be chance to one better instructed". He was critical of established religion, but spoke in favor of "the belief in an all-powerful Being, who loves us and watches over us".
Hippolyte also described his brother as a talented violin player, interested principally in the music of Jean-Baptiste Lully and Giovanni Battista Viotti, who also cultivated gymnastics, fencing, swimming, dancing, and skating. According to historian of science James F. Challey, "although sensitive and perceptive", Carnot "appeared extremely introverted, even aloof, to all but a few close friends". This may help explain why Carnot's work failed to make any significant impression within either the scientific or the engineering community during his lifetime.