On the Dark Side
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”:
In 1822, Thomas Seebeck demonstrated 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, which