perpetual-motion machine, device that would be able to operate continuously and supply useful work, in violation of the laws of thermodynamics. A machine that would produce more energy in the form of work than is supplied to it in the form of heat would violate the first law of thermodynamics, which is a special case of the law of conservation of energy (see conservation laws, in physics), and is known as a perpetual-motion machine of the first kind. A machine that would completely convert heat from a warm body into work, without letting any heat flow into a cooler body, would violate the second law of thermodynamics, which is concerned with entropy changes, and is known as a perpetual-motion machine of the second kind. There were a number of early attempts to design and construct various types of perpetual-motion machines; however, since the 19th cent., when the laws of thermodynamics became understood, most such attempts have been abandoned.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.