Something Is Conserved
To the modern student, the term energy has a meaning that is almost self-evident.
This meaning was far from clear, however, to scientists of the early nineteenth
century. The many effects that would finally be unified by the concept of energy
were still seen mostly as diverse phenomena. It was suspected that mechanical,
thermal, chemical, electrical, and magnetic effects had something in common,
but the connections were incomplete and confused.
What was most obvious by the 1820s and 1830s was that strikingly diverse
effects were interconvertible. Alessandro Volta’s electric cell, invented in 1800,
produced electrical effects from chemical effects. In 1820, Hans Christian Oersted
observed magnetic effects produced by electrical effects. Magnetism produces
motion (mechanical effects), and for many years it had been known that motion
can produce electrical effects through friction. This sequence is a chain of “conversions”:
Chemical effect electrical effect magnetic effect mechanical effect
electrical effect.
In 1822, Thomas Seebeckdemonstrated that a bimetallic junction produces an
electrical effect when heated, and twelve years later Jean Peltier reported the
reverse conversion: cooling produced by an electrical effect. Heat engines perform
as conversion devices, converting a thermal effect (heat) into a mechanical
effect (work).
Most of the major theories of science have been discovered by one scientist,
or at most by a few. The search for broad theoretical unities tends to be difficult,
solitary work, and important scientific discoveries are usually subtle enough that
special kinds of genius are needed to recognize and develop them. But, as Thomas
Kuhn points out, there is at least one prominent exception to this rule. The
theoretical studies inspired by the discoveries of conversion processes, whichenergy concept, were far from a singular effort. Kuhn lists
twelve scientists who contributed importantly during the early stages of this “simultaneous
discovery.”
The idea that occurred to all twelve—not quite simultaneously, but independently—
was that conversion was somehow linked with conservation. When one
effect was converted to another, some measure of the first effect was quantitatively
replaced by the same kind of measure of the second. This measure, applicable
to all the various interconvertible effects, was conserved: throughout a conversion
process its total amount, whether it assessed one effect, the other effect,
or both, was precisely constant.
The twelve simultaneous discoverers were not the first to make important use
of a conservation principle. In one form or another, conservation principles had
been popular, almost intuitive it seems, with scientists for many years. Theorists
had counted among their most impressive achievements discoveries of quantities
that were both indestructible and uncreatable. Adherents of the caloric theory of
heat had postulated conservation of heat. In the late eighteenth century, Antoine-
Laurent Lavoisier and others had established that mass is conserved in chemical
reactions; when a chemical reaction proceeds in a closed container, there is no
change in total mass.
So it was natural for theorists who studied conversion processes to attempt to
build their theories from a conservation law. But, as always in the formulation
of a conservation principle, a difficult question had to be asked at the outset:
what is the quantity conserved? As it turned out, a workable answer to this
question was practically impossible without some knowledge of the conservation
law itself, because the most obvious property of the conserved quantity, ultimately
identified as energy, was that it was conserved. No direct measurement
like that of mass could be made for verification of the conservation property. This
was a search for something that could not be fully defined until it was actually
found.
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