Peaucellier's Linkage

Paucellier's linkage in general creates a circular trajectory.

However, when the crank length (b) and the base length (a) are the same, it generates a linear motion.

Invented by a French Naval officer, Paucellier's linkage was the first to convert rotary motion into true, rather than approximate, linear motion.

The radiator duct controls in the houses of parliament in London are controlled using Paucellier's Linkages.