Perpetual motion is the action of a moving device that requires no input of energy to maintain it, and therefore can continue forever. This is impossible if the laws of thermodynamics hold, and despite many attempts since antiquity, such a device has yet to be demonstrated.
A great number of designs have been published that appear to allow work produced during the operation of a device to both maintain the motion of the device and provide excess work outside the system. Examples include springs, overbalancing wheels, and magnetic and electrical machines. These are considered impossible because the first law of thermodynamics states that the energy that is consumed by the device to cause its motion, is the maximum that could be delivered by that motion. Practically, because of frictional and other heat losses, the available energy would be less, causing the machine to eventually come to a halt.
Another kind of perpetual motion violates the second law of thermodynamics, which requires useful work always to be accompanied by an increase in the entropy of the Universe. Engines that are driven by the spontaneous evaporation of condensed vapors such as liquid ammonia are such devices, because the energy used to condense the vapor before the next cycle is greater than that obtained by harnessing its evaporation.
One famous example is the device suggested by Bishop John Wilkins (1614-72), best known as a founder and first secretary to the British Royal Society. His device consists of two tilted ramps, an iron ball, and a magnetic lodestone fastened at the top. The lodestone at the top (A) pulled the ball (F) up the straight ramp, where it fell through the hole (B) to the lower ramp, rolled down, and through another hole (F) to the straight ramp where it was pulled up again. Of course, the ball would simply remain at the top, held strongly against the fixed magnet, and if it did fall through the top hole, it would still be under the attractive influence of the magnet at the top.
Other famous names in perpetual motion include Anthony Zimara (self-blowing windmill, C 13th), Robert Fludde (self-powered water wheel, C 15th), Robert Boyle (self-filling cup, C 17th), J.W. Keely (fraudulent motor, C 19th), and J. Gamgee (liquid ammonia ship engine, C 19th).
A.W.J.G. Ord-Hume, Perpetual Motion : The History of an Obsession, George Allen & Unwin, London, 1979.