Perpetual motion machine  (from Latin "perpetuum mobile", "that which moves constantly"): a machine that, once activated, runs endlessly (preferably while doing actual chores); not to be confused with an Eternity Machine.

Perpetual motion machine for knife grinding,
designed by Jacopo de Strada around 1580.

Many sophisticated perpetual motion ideas have been devised on paper since the 10th century; unfortunately, none of them has been successfully realized. Leonardo da Vinci was the first to realize that this was no mere accident. The laws of nature — in particular, the laws of thermodynamics — rule out the existence of such a machine.

Impossible Mills

There are two types of perpetual motion machines, categorized according to which law of thermodynamics they violate:

A perpetual motion machine of the first kind is a machine that generates the very energy needed for its operation — for example, a water wheel that also raises the water needed for its operation by means of a pump. Since every real machine constantly loses energy due to friction loss, even when it does not perform any labor, such an apparatus violates the first law of thermodynamics (the law of conservation of energy).

A perpetual motion machine of the second kind extracts its energy from ambient heat. An example for this is a thermal mill. This is an extremely small wind mill that is designed to turn via a bolt in only one direction and is driven by the thermal vibrations of the molecules in its ambient air. Such a machine does not violate the law of conservation of energy. However, it does violate the second law of thermodynamics, for although kinetic energy can be transformed into heat, heat cannot be transformed back into kinetic energy.*

If you nevertheless want to try constructing a perpetual motion machine, please be warned: patent applications for such things tend to be rejected without scrutiny by patent offices in most states.

* The bolting mechanism of a thermal mill cannot function without an external cooling source. If the bolt has the same temperature as the molecules in the mill's ambient air, it will vibrate in harmony with them. The vibrations will prevent the bolt from snapping into place. Thus, the bolt would have to be cooled by means of an external device. Since the friction caused by the snapping process heats up the bolt, the cooling effect would have to be preserved by means of additional energy. This energy consumption, however, would be greater than the energy gained.

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■ The Museum of Unworkable Devices

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