A perpetual-motion machine is often defined as one that never stops running, but a better definition is a machine that puts out more energy than it takes in. It is not hard to build devices that, aside from the inevitable wearing out of parts, will run virtually forever. Perpetual clocks, for instance, which go back to the eighteenth century, are perpetually rewound by changes in air pressure or temperature, but of course they are no more perpetual- motion devices than a watch that is rewound by the motions of the person who carries it.
Windmills, water wheels, and machines that run on tidal or solar energy are other examples of pseudo-perpetual motion because they require outside energy. The earth's rotation and revolution, the Brownian movements of particles in liquid suspension, the motions of molecules in a gas, of electrons in an atom, of electrical currents in supercooled substances--all such motions are of no help in making a device that will produce more energy than it extracts from its source.
Another way to define a perpetual-motion machine is to call it a device that violates the first law of thermodynamics. This is the law stating that energy (in relativity theory one must speak of mass-energy) is always conserved. It may alter in form, but the total energy output of a machine can never exceed its total input. Because of unavoidable heat loss from friction, air resistance, electrical resistance, and other retarding forces, all machines with no outside power source will eventually stop for the same reason that a spinning top soon falls over. Although the conservation of energy law is empirical, it is so firmly entrenched in modern physics that searching for a counterexample is as foolish as trying to lift yourself by your own bootstraps.
It is important to remember, however, that before the conservation of energy was well understood, the search for perpetual motion was thorough-____________________