Physicists have often classified so-called perpetual-motion machines according to which of the first two laws of thermodynamics they would need to violate in order to function. In 1992 engineering professor Kevin Kilty developed a new classification scheme based on ten recurring themes in the historical attempts to turn these impossible devices into reality.
1 Magical devices. These operate by no known principle.
2 Impossible machines. These defy logic, like the drawings of M.C. Escher, where water runs perpetually downhill in a closed cycle.
3 Real machines that people often confuse with true perpetual-motion machines. For instance, a Rube Goldberg machine is difficult to analyze and obscure of purpose, so the typical person thinks it’s probably a perpetual-motion machine. Into this class also goes any harebrained inventor’s scheme.
4 A real machine might be viewed as perpetual motion if it simply runs for a long time. Radioactive decay and rotation of the earth are examples. These are perpetual only in the sense that they outlast a person’s life or memory.
5 Frauds. These typically are any overly complicated machine running ostensibly forever, in a completely mysterious cycle of operation with hidden workings (and hidden source of energy).
6 Machines that are perpetually out of balance, so that a push start will make the machine run forever or perhaps even cause it to accelerate. If built they may become machines of type 5, especially after the disappointed inventor realizes his need to recover expenses.
7 Machines based on a misunderstanding of a measurement. An example is the motor of Garabed Giragossian, who managed to convince himself, along with a majority of Congress, that output power is the same thing as energy. This managed only to confuse everyone, including the inventor.
8 Machines without losses or friction are common designs. In reality these could operate only until their initial supply of energy, obtained from the person who set them in motion, is exhausted.
9 Machines that withdraw heat from a reservoir, perform work, and change nothing else in the universe. They have a closed cycle of operation that brings all parts back to an original state.
10 Perpetual lamps. Fortunio Liceti made a lifelong study of lamps supposedly found in old tombs, vaults, and temples. This is giving too much serious attention to a fantasy. Such lamps belong to the realm of myth, like unicorns and half-man beasts.