Noun
Brownian ratchet (plural Brownian ratchets)
(physics) An apparent perpetual motion machine, the subject of a thought experiment, that would consist of a tiny paddle wheel and a ratchet, and might seem to be able to extract useful work from random fluctuations (heat) in a system at thermal equilibrium, in violation of the second law of thermodynamics.
Synonym: Feynman-Smoluchowski ratchet
So, for example, the thought experiment of a Brownian ratchet as a perpetual motion machine was first discussed by Gabriel Lippmann in 1900 but it was not until 1912 that Marian Smoluchowski gave an adequate explanation for why it cannot work. Source: Internet