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Perpetual motion

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Perpetual motion machines are a class of hypothetical machines which produce energy "from nowhere." The existence of a perpetual motion machine is impossible according to current known laws of physics. Perpetual motion machines are divided into two subcategories, referred to as perpetual motion of the first kind and perpetual motion of the second kind.

A perpetual motion machine of the first kind is one which produces power without energy uptake. Such a machine would, once started, operate indefinitely. This is forbidden by the law of conservation of energy.

Note that this explicitly prohibits the existence of devices which produce more energy than their input energy, as they can trivially be converted to a perpetual motion machine of the first kind by diverting part of their output energy back to their input.

Note that heat engines with an 'efficiency' greater than one do not violate this rule: the 'efficiency' in this case is defined as the ratio of heat output to work input -- the total energy input (heat + work) is still equal to the total energy output (heat).

A perpetual motion machine of the second kind is one which converts heat completely into other forms of energy. Such a device would violate the second law of thermodynamics (see also entropy) and is would be viewed with great skepticism.

The invention of perpetual motion machines is a favourite pastime of many crackpots, who often come up with elabourate Rube Goldberg machine designs which may appear to work on paper at first glance, but which have various flaws or obfuscated external power sources that render them useless in practice. This sort of invention has become common enough that the US Patent Office has made an official policy of refusing to grant patents for perpetual motion machines.

Proponents of perpetual motion machines use a number of other terms to describe their inventions, including "free energy" and "over unity" machines.

Before its policy not to grant them, the U.S. Patent and Trademark Office has granted at least two patents for motors that run without net energy input:

  • 4,151,431 on April 24, 1979 ("Permanent Magnet Motor")
  • 4,074,153 on February 14, 1978 ("Magnetic propulsion device")

You can search for the whole text of these patents (see link below).

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