Editing Perpetual motion (section)

=== Techniques ===
{{More citations needed section|date=August 2010}}
{{Rquote|right|One day man will connect his apparatus to the very wheelwork of the universe [...] and the very forces that motivate the planets in their orbits and cause them to rotate will rotate his own machinery.|[[Nikola Tesla]]|}}

Some common ideas recur repeatedly in perpetual motion machine designs. Many ideas that continue to appear today were stated as early as 1670 by [[John Wilkins]], [[Bishop of Chester]] and an official of the [[Royal Society]]. He outlined three potential sources of power for a perpetual motion machine, "{{sic|Chy|mical}} Extractions", "Magnetical Virtues" and "the Natural Affection of Gravity".<ref name="sciam">{{cite magazine
| last = Angrist | first = Stanley
| title	= Perpetual Motion Machines
| magazine = Scientific American
| date = January 1968
| volume= 218
| issue = 1
| pages = 115–122
| doi=10.1038/scientificamerican0168-114
| bibcode= 1968SciAm.218a.114A}}</ref>

The seemingly mysterious ability of [[magnet]]s to influence motion at a distance without any apparent energy source has long appealed to inventors. One of the earliest examples of a [[magnet motor|magnetic motor]] was proposed by Wilkins and has been widely copied since: it consists of a ramp with a magnet at the top, which pulled a metal ball up the ramp. Near the magnet was a small hole that was supposed to allow the ball to drop under the ramp and return to the bottom, where a flap allowed it to return to the top again. However, if the magnet is to be strong enough to pull the ball up the ramp, it cannot then be weak enough to allow gravity to pull it through the hole. Faced with this problem, more modern versions typically use a series of ramps and magnets, positioned so the ball is to be handed off from one magnet to another as it moves. The problem remains the same.

[[Image:Perpetuum mobile villard de honnecourt.jpg|thumb|{{lang|la|Perpetuum mobile}} of [[Villard de Honnecourt]] (about 1230).]]
[[Image:overbalanced_wheel.svg|thumb|lang=simple|The "Overbalanced Wheel", annotated with distances of the weights from the centreline showing that the torques on both sides even out on average]]

[[Gravity]] also acts at a distance, without an apparent energy source, but to get energy out of a gravitational field (for instance, by dropping a heavy object, producing kinetic energy as it falls) one has to put energy in (for instance, by lifting the object up), and some energy is always dissipated in the process. A typical application of gravity in a perpetual motion machine is [[Bhaskara II|Bhaskara]]'s wheel in the 12th century, whose key idea is itself a recurring theme, often called the overbalanced wheel: moving weights are attached to a wheel in such a way that they fall to a position further from the wheel's center for one half of the wheel's rotation, and closer to the center for the other half. Since weights further from the center apply a greater [[torque]], it was thought that the wheel would rotate forever. However, since the side with weights further from the center has fewer weights than the other side, at that moment, the torque is balanced and perpetual movement is not achieved.<ref name="self-oscillation">{{Cite journal | last1 = Jenkins | first1 = Alejandro| author-link1=Alejandro Jenkins | title = Self-oscillation | doi = 10.1016/j.physrep.2012.10.007 | journal = Physics Reports | volume = 525 | issue = 2 | pages = 167–222 | year = 2013 | arxiv = 1109.6640| bibcode= 2013PhR...525..167J| s2cid = 227438422}}</ref> The moving weights may be hammers on pivoted arms, or rolling balls, or mercury in tubes; the principle is the same.

[[File:Perpetual motion wheels Vinci.jpg|thumbnail|left|Perpetual motion wheels from a drawing by [[Leonardo da Vinci]]]]
Another theoretical machine involves a frictionless environment for motion. This involves the use of [[diamagnetic]] or [[magnetic levitation|electromagnetic levitation]] to float an object. This is done in a [[vacuum]] to eliminate air friction and friction from an axle. The levitated object is then free to rotate around its center of gravity without interference. However, this machine has no practical purpose because the rotated object cannot do any work as work requires the levitated object to cause motion in other objects, bringing friction into the problem. Furthermore, a ''perfect'' vacuum is an unattainable goal since both the container and the object itself would slowly [[vaporize]], thereby degrading the vacuum.

To extract work from heat, thus producing a perpetual motion machine of the second kind, the most common approach (dating back at least to [[Maxwell's demon]]) is ''unidirectionality''. Only molecules moving fast enough and in the right direction are allowed through the demon's trap door. In a [[Brownian ratchet]], forces tending to turn the ratchet one way are able to do so while forces in the other direction are not. A diode in a heat bath allows through currents in one direction and not the other. These schemes typically fail in two ways: either maintaining the unidirectionality costs energy (requiring Maxwell's demon to perform more thermodynamic work to gauge the speed of the molecules than the amount of energy gained by the difference of temperature caused) or the unidirectionality is an illusion and occasional big violations make up for the frequent small non-violations (the Brownian ratchet will be subject to internal Brownian forces and therefore will sometimes turn the wrong way).

[[Image:Prepex2.svg|thumb|The "Float Belt". The yellow blocks indicate floaters. It was thought that the floaters would rise through the liquid and turn the belt. However, pushing the floaters into the water at the bottom takes as much energy as the floating generates, and some energy is dissipated.]]
[[Buoyancy]] is another frequently misunderstood phenomenon. Some proposed perpetual-motion machines miss the fact that to push a volume of air down in a fluid takes the same work as to raise a corresponding volume of fluid up against gravity. These types of machines may involve two chambers with pistons, and a mechanism to squeeze the air out of the top chamber into the bottom one, which then becomes buoyant and floats to the top. The squeezing mechanism in these designs would not be able to do enough work to move the air down, or would leave no excess work available to be extracted.