Nicolas Léonard Sadi Carnot

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Nicolas Léonard Sadi Carnot

Nicolas Léonard Sadi Carnot (born June 1, 1796 in Paris ; † August 24, 1832 there ) was a French physicist and engineer who founded a new branch of science, thermodynamics , with his theoretical consideration of the steam engine ( Carnot process ) .


Sadi Carnot was born as the second son of the politician and scientist Lazare Nicolas Marguerite Carnot . It was named after the Persian poet Sadi Shirazi . His father recognized his son's interest in mechanics and physics at an early age and prompted him to study technical sciences. Carnot began studying at the École polytechnique in Paris in 1812 , but left this college again in 1814 to become an engineer officer in the Geniekorps , a technical military force. Because of his Republican convictions, which he had in common with his father, he got into difficulties there, and the military service did not particularly appeal to him. In 1819 he asked for his temporary release so that he could devote himself entirely to science. He also attended lectures on chemistry, physics, mathematics, natural history and economics, visited industrial companies and studied models of machines in museums; but he also dealt with music and the works of Blaise Pascal , Molière and Jean de La Fontaine . He interrupted his activities only once to visit his father, who spent the last years of his life in exile in Magdeburg.


During his studies, Carnot recognized the possibilities for further developing the steam engine . Since these machines had so far only been improved on the basis of practical experience, he felt it was imperative to theoretically investigate the “phenomenon of the generation of movement through movement of heat”. The result appeared in 1824 in the 43-page book Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance (considerations on the moving power of fire and the machines suitable for developing this power). It is the only font by Carnot that was published during his lifetime. It was well received but was soon out of print and not reprinted. The English translation did not appear until 1890, and in 1892 Wilhelm Ostwald published a translation in German.

End of life

At the end of 1826 Carnot returned to the military and was promoted to captain as scheduled. In 1828 he took off his uniform for good in order to pursue a scientific career. He did not take part in the July Revolution in 1830 either , although he had great expectations of more democracy in it. When these were not fulfilled, he turned back to his experiments. In June 1832 Carnot fell ill with scarlet fever and "brain fever". He died a little later, at the age of only 36, during a cholera epidemic.

Late reactions to Carnot's writing

Émile Clapeyron (1799–1864) was the first to take up the ideas contained in Carnot's publication. He found the content to be “fruitful and free of throw-in”, but with his statements written in 1834, it was also hardly noticed. Only after a quarter of a century did the situation change: William Thomson, later Lord Kelvin , suggested Carnot's remarks in 1848 on his temperature scale. In an article on thermodynamics in Poggendorff's Annalen der Physik und Chemie of 1850, Rudolf Clausius explicitly referred to Carnot's scientific achievement, he spoke of the "... most important investigation that belongs here". It was surprising that neither Lord Kelvin nor Clausius had the original font and only knew the Clapeyron version. It was Wilhelm Ostwald who first translated Carnot's writing into German. He noted that “... it constitutes the essential content of what is called the second law of thermodynamics. As such, it has led to the most important results in the hands of Clausius and William Thomson. "

Meaning of the Carnot script

Carnot stated in his considerations that they refer not only to steam engines, "... but to every conceivable heat engine, whatever the material used, and what kind of action it is used". He noticed that the work performance was a periodic process and gave an ideally thought process for it, which is now called the Carnot process in his honor . With these two findings he also laid the foundation for the development of internal combustion engines; The Carnot process is still the basis for every design and calculation of periodically operating heat engines. Carnot also stated that the process must be reversible, from which the development of the heat pump resulted.

Carnot's fundamental theorem states that wherever there is a temperature difference, moving force can be generated, since heat always strives to change from a hot to a cold state.

Carnot proved that the work of steam engines is proportional to the amount of heat that is transferred from the boiler to the condenser, i.e. from the reservoir of high to the reservoir of low temperature. An “inflow” and an “outflow” of heat ( entropy ) are necessary . The drainage cannot normally take place at a temperature lower than the ambient temperature (298.15 K). As a result, the maximum efficiency cannot reach 1.

Carnot also demonstrated that the greater the temperature gradient, the greater the efficiency . The result is that no heat engine can have a higher degree of efficiency than that which is determined by the “maximum moving force resulting from the use of steam”. Every reversible machine is independent of the working substance, otherwise a suitable combination of machines with different degrees of efficiency could enable a perpetual motion machine of the second type.

Inadequacy of the Carnot script

Carnot bases his remarks on the theory that heat is a hypothetical, imponderable substance of a constant amount. This idea was generally held at the time, Antoine Laurent de Lavoisier spoke of "Calorique". Benjamin Thompson and Humphry Davy already saw a kind of movement due to their attempts at friction in heat. In 1850, Clausius then introduced the principle of equivalence in an essay, which abandons the idea of ​​an unchangeable amount of heat. Julius Robert von Mayer mentioned it as early as 1842, James Prescott Joule experimentally confirmed it the following year, and Hermann von Helmholtz generalized it - independently of Mayer - to all forms of energy. Clausius explicitly points out that it is not Carnot's basic principle that is objectionable, but only the addition that no heat is lost.

Sadi's brother Lazare Hippolyte Carnot (1801 to 1888) published an appendix with explanations from the estate in the second edition of the Carnot script in 1872. From these it emerges that Sadi later deviated again from his view, which was given in 1824, and now understood heat as a form of energy. He even determined the mechanical heat equivalent to be 370 kpm / kcal (= 2.7 cal / kpm) at least 10 years before Mayer, with which he came very close to a universal law of energy conservation. Revolutionary considerations on the authorship of the second law are still being discussed.


The Carnot battery is named after him.


  • Stephen S. Wilson: Sadi Carnot. Technology and theory of the steam engine. In: Wolfgang Neuser (Ed.): Newtons Universum. Materials on the history of the concept of force . Spectrum of Science, Heidelberg 1990, ISBN 3-89330-750-8 .
  • S. Carnot: Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance. In: Annales scientifiques de l'École Normale Supérieure Sér. 2, 1, 1872, pp. 393–457 (in French. Download link . )
  • S. Carnot: Considerations on the moving power of fire and the machines suitable for developing this power (1824). Transl. And ed. by W. Ostwald. Wilhelm Engelmann-Verlag, 1892. ( online at openlibrary)
  • Charles Gillispie , Raffaele Pisano: Lazare and Sadi Carnot. A scientific and filial relationship. 2nd Edition. Springer, 2014, ISBN 978-94-017-8010-0 .
  • James F. Challey: Carnot, Nicolas Léonard Sadi . In: Charles Coulston Gillispie (Ed.): Dictionary of Scientific Biography . tape 3 : Pierre Cabanis - Heinrich von Dechen . Charles Scribner's Sons, New York 1971, p. 79-84 .
  • Karl-Eugen Kurrer : The History of the Theory of Structures. Searching for Equilibrium , Berlin: Ernst & Sohn 2018, p. 158, p. 177, p. 438 and p. 442ff., ISBN 978-3-433-03229-9 .

See also

Web links

Individual evidence

  1. Sadi Carnot - a co-founder of technical thermodynamics (PDF; 15.8 MB) ( Memento from June 28, 2014 in the Internet Archive )