Perpetual motion is an age-old goal of inventors: a machine which would work forever without external interference, or at least with 100% efficiency (see efficiency of a machine). No such machine has worked or can work, though many are plausible on paper.
Perpetual motion machines of the first kind are those whose efficiency exceeds 100% – they do work without energy being supplied. They are disallowed by the first law of thermodynamics. Those of the second kind are machines that 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 are machines that do no work, but merely continue in motion forever. They are approachable but not actually achievable, because some energy is always dissipated as heat by friction, etc. An example, however, of what is in a sense perpetual motion of the third kind is electric current flowing in a superconducting ring (see superconductivity) which continues undiminished indefinitely.