Feynman's Ratchet (Feynman Lectures Vol.1, Ch.46) is a thought experiment: a ratchet-and-pawl connected to a vane in a gas bath. Feynman showed that if both sides are at the same temperature T, the ratchet produces NO net motion — thermal fluctuations in the pawl spring are just as likely to flip the ratchet backward. This is consistent with the second law.
However, when T₁ ≠ T₂, the system can do work — it becomes a heat engine operating between two reservoirs. The Carnot efficiency ηC = 1 − T₂/T₁ sets the maximum. This simulation shows both the ratchet potential and Brownian particles being rectified when T₁ > T₂. The key insight: asymmetry alone is insufficient — thermodynamic disequilibrium is required.