Environmental Engineering Reference
In-Depth Information
1.3
Left, James Prescott Joule (1818-1889); right, William
Thomson, Lord Kelvin (1824-1907).
took from published sources. This error had no effect
on the validity of Mayer's law or on his conclusion
that muscles are the equivalents of heat engines and that
mammals convert food or feed energy to work (mechan-
ical energy) with an efficiency of about 20%. The law
of conservation of energy is now commonly known
as the first law of thermodynamics. All of its many
restatements, ranging from precise formulations (heat
flowing into a system is equal to the increase of the sys-
tem's internal energy and the work done by the system)
to catchy summations (you can't get something for noth-
ing) make it clear that energy can be neither created nor
destroyed.
The correct value for the equivalence of heat and
mechanical energy was found, contemporaneously with
Mayer's work but entirely independently, by the English
physicist James Prescott Joule (fig. 1.3) after he con-
ducted a large number of carefully executed churning
experiments. Joule used very sensitive thermometers
to measure the temperature of water that was being
churned by an assembly of revolving vanes driven by
descending weights; this arrangement made it possible
to measure fairly accurately the mechanical energy
invested in the churning process. Joule reported his first
results to the British Association meeting at Oxford in
1847, and the full paper finally appeared three years later
(Joule 1850). Joule's later revision set the equivalence
with an error of less than 1%. The third independent for-
mulation of the law was the work of a German physiolo-
gist, Hermann von Helmholtz (1821-1894), who, like
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