In symbols, S is the symbol for entropy, assigned by German physicist Rudolf Clausius.

In 1824, French physicist Sadi Carnot used small "s" for heat.

In 1865, Clausius, possibly on the earlier usage of small "s" for heat by Carnot, assigned the capital "S" symbol for entropy as follows: [1]

“I propose to call S the entropy of the body, from the Greek word τροπή, transformation.”

It remains to be pinned down why exactly Clausius used the symbol S here in place of his early 1856 symbol N (or -N) depending (see: entropy etymology).

In 1872, Austrian physicist Ludwig Boltzmann used the symbol E to represent Clausius' entropy. [2]

In 1873, American engineer Willard Gibbs used the Greek symbol eta η to represent Clausius' entropy. [3]

See also
S = k ln W

1. (a) Clausius, Rufolf. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies (terms: “entropy” and “S”, pgs. 357, 363, 367) (URL). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
(b) Read at the Philosophical Society of Zurich on the 24th of April, 1865, published in the Vierteljahrsschrift of this society, Bd. x. S. 1.; Pogg. Ann. July, 1865, Bd. cxxv. S. 353; Journ. de Liouville, 2e ser. t. x. p. 361.
2. Boltzmann, Ludwig. (1872). "Further Studies on the Thermal Equilibrium of Gas Molecules" (“Weitere Studien über das Wärmegleichgewicht unter Gasmolekülen”), in Sitzungsberichte der Akademie der Wissenschaften, Mathematische-Naturwissenschaftliche Klasse (pgs. 275-370; term: "entropy" symbol "E", pgs. 263, 308, 346, etc), Bd. 66, Dritte Heft, Zweite Abteilung, Vienna: Gerold.
3. Gibbs, J. Willard. (1873). "Graphical Methods in the Thermodynamics of Fluids" (symbol table, pg. 1), Transactions of the Connecticut Academy, I. pp. 309-342, April-May.

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