Perpetual motion machine

Perpetual motion machine, concept of a machine that would work continuously without external interference, or at least with 100% efficiency. Perpetual motion machines of the first kind do work without energy being supplied, they are disallowed by the first law of thermodynamics. Those of the second kind take heat from a reservoir (such as the ocean) and convert it wholly into work, although energy is conserved, they are disallowed by the second law of thermodynamics. Those of the third kind do no work merely continuing in motion forever; they are not achievable, because some energy is always dissipated—as heat by friction, for example. However, electric current flowing in a superconducting ring continues undiminished indefinitely and is in a sense a perpetual motion machine of the third kind.

See also: Thermodynamics.