Working body

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A depiction of the Papin engine (Papin, 1690), Carnot engine (Carnot, 1824), and Clausius engine (Clausius, 1865), wherein the "hot body" (e.g. fire or hot boiler), i.e. a highertemperature body, "cold body" (e.g. spray of cold water or air), i.e. a lower ↓ temperature body, and the "working body" (Clausius, 1865), aka "working substance" (Carnot, 1824), which is made to expand or contract, in so-called work cycles, defined originally via the "reversible" Carnot cycle (1824), then modified, by changing "caloric" to "entropy", into the "irreversible" Clausius cycle (1865).

In thermodynamics, work body (TR:147) (LH:8) (TL:155), aka "working substance" (Carnot, 1824), as compared to the "hot body" or "cold body", refers to any body in the universe at temperature in between that of the temperature of a hotter body and a colder body; generally confined to a volume, e.g. that inside of piston and cylinder, which is made to increase or decrease in volume, via alternating contact with the hot body and cold body, respectively, in the so-called standard heat engine or steam engine model.


In 1824, Sadi Carnot, developed a abstract “heat engine” model, wherein he conceptualizing all steam engines, as being Papin engines in basic principle, and where the boiler was the “hot body”, the spraying cold water was the “cold body”, and the matter inside of the piston and cylinder was the “working substance” or “working body” (Clausius, 1865).

In 1834, Emile Clapeyron extended Carnot’s “hot body” and “cold body” terminology and logic, by presenting it in graphical form, using the “indicator diagram” of Watt.

In 1848, William Thomson began working on the heat engine ideas of Carnot.

In 1850, Rudolf Clausius, via Thomson, began to expand on these models to form the science of thermodynamics.


The following are quotes:

“In the phenomenon of the production of motion by heat, it is necessary to establish principles applicable not only to steam engines, but to all imaginable heat engines, whatever the working substance [working body] and whatever the method by which it is operated.”
Sadi Carnot (1824), On the Motive Power of Fire (pg. 6)[1]

End matter


  1. Carnot, Sadi. (1824). Reflections on the Motive Power of Fire: and on Machines Fitted to Develop that Power (editor: Eric Mendoza) (pg. 6). Dover, 1960.

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