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These machines have a source of energy, albeit one which is not readily apparent, so that they only seem to violate the laws of thermodynamics.
Even machines which extract energy from long-lived sources - such as ocean currents - will run down when their energy sources inevitably do.
There is a scientific consensus that perpetual motion in an isolated system violates either the first law of thermodynamics, the second law of thermodynamics, or both.
It is widely credited as the first attempt to describe such a device in order to produce useful work, that of driving millstones.
Despite this, many attempts have been made to construct such machines, continuing into modern times.
Modern designers and proponents often use other terms, such as "over unity", to describe their inventions.
Machines which comply with both laws of thermodynamics by accessing energy from unconventional sources are sometimes referred to as perpetual motion machines, although they do not meet the standard criteria for the name.
By way of example, clocks and other low-power machines, such as Cox's timepiece, have been designed to run on the differences in barometric pressure or temperature between night and day.
A common example is devices powered by ocean currents, whose energy is ultimately derived from the Sun, which itself will eventually burn out.