Perpetual Motion Machine

perpetuum mobile, an imaginary machine which once set in motion would perform work for an infinitely long time without borrowing energy from outside itself. The perpetual motion machine contradicts the law of conservation and transformation of energy and is not realizable. The possibility of such a machine’s operating for an unlimited time would mean obtaining energy from nothing.

The first plans for a perpetual motion machine belong to the 13th century (Villard de Honnecourt, 1245, England; Pierre de Maricourt, 1269, France). The idea of a perpetual motion machine attained wide popularity in the 16th and 17th centuries, in the era of transition to machine manufacturing; the number of plans for perpetual motion machines increased steadily until the 19th century. The idea of creating a perpetual motion machine occupied not only self-taught dreamers, little acquainted with the principles of physics, but also some scientists. Toward the end of the 18th century, as a result of the fruitlessness of centuries-long attempts to realize a perpetual motion machine, the conviction of the impossibility of its creation was strengthened among scientists; and starting in 1775 the French Academy of Sciences refused to examine plans for perpetual motion machines. In the middle of the 19th century, with establishment of the law of conservation of energy, the theoretical impossibility of realizing such a machine was proved. In spite of this, vain attempts to create one have been undertaken even recently by inventors with little knowledge.

Many plans for perpetual motion machines resort to the force of gravity. In many of these mechanisms, some heavy body travels a closed path; when it descends, the precise quantity of work that was expended on lifting it is returned. Therefore, these mechanisms can perform work only at the expense of an initial store of kinetic energy communicated to it at the outset; when, however, this store is completely spent, the perpetual motion machine will stop. More complex projects for perpetual motion machines are those in which mechanical energy is converted into other forms of energy (such as electrical energy or thermal energy). As distinct from mechanical perpetual motion machines, these are called physical ones. One example of a plan for such a machine is a combination electric motor and electromechanical generator (dynamo). But since the total quantity of energy cannot be increased by any transformations of energy, this type of perpetual motion machine is also impossible to realize.

The idea of the impossibility of realizing perpetual motion machines has often served as the point of departure for important scientific conclusions. Thus, S. Stevin in his Principles of Equilibrium (1587), considers a chain of 13 spheres thrown over a trihedral prism. If the right-hand section of two spheres were not balanced on the left by four, the chain would of itself be set into perpetual motion, which is not observed in reality. From this, Stevin deduced the law of equilibrium of forces on an inclined plane.

Besides the indicated perpetual motion machines, which are called perpetual motion machines of the first type, there are also perpetual motion machines considered to be of the second type. These are imaginary, periodically operating machines that would completely transform into work heat drawn by them from surrounding bodies (ocean, atmospheric air, or other practically inexhaustible natural sources of heat). However, perpetual motion machines of the second type are also in principle unrealizable. Although they do not formally contradict the law of conservation of energy, they are in contradiction to the second law of thermodynamics.

Perpetual motion machines must be distinguished from the “supposed” perpetual motion machines—mechanisms that operate at the expense of natural stores of energy (solar, nuclear, and the like). Such mechanisms may work for an extremely long time, but they have nothing to do with the idea of a perpetual motion machine.

REFERENCES

Planck, M. Printsip sokhraneniia energii. Moscow-Leningrad, 1938. (Translated from German.)
Kudriavtsev, P. S. Istoriia fiziki, part 1. Moscow, 1956.