By the expression "source of
heat," we mean a body exterior to the system and capable of furnishing
or withdrawing heat from it.
Starting with this principle, we arrive, as does Clausius, at the
demonstration that the output of a reversible machine working between
two given temperatures is greater than that of any non-reversible
engine, and that it is the same for all reversible machines working
between these two temperatures.
This is the very proposition of Carnot; but the proposition thus
stated, while very useful for the theory of engines, does not yet
present any very general interest. Clausius, however, drew from it
much more important consequences. First, he showed that the principle
conduces to the definition of an absolute scale of temperature; and
then he was brought face to face with a new notion which allows a
strong light to be thrown on the questions of physical equilibrium. I
refer to entropy.
It is still rather difficult to strip entirely this very important
notion of all analytical adornment. Many physicists hesitate to
utilize it, and even look upon it with some distrust, because they see
in it a purely mathematical function without any definite physical
meaning.
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