ESCAPEMENT

in mechanics, ratchet device that permits motion in steps in one direction only; also the mechanism that causes a piano hammer to rebound after striking. In a watch or clock it is the mechanism that controls the transfer of energy from the power source to the counting mechanism. The classic form for a timepiece, which made the mechanical clock possible, was the verge escapement invented in 14th-century Europe. This consists of a notched crown wheel (i.e., a gear wheel shaped like a crown) driven by a weight or pendulum and repeatedly checked by the retarding action of a pair of metal leaves that alternately catch in successive notches. The leaves are mounted on a vertical shaft, and their speed of oscillating back and forth is governed by a crossbar at the top with two small weights; moving the weights outward from the shaft slows the oscillations. The anchor escapement was an improved escapement invented in England in the 17th century. In this the wheel is braked by a flat device in the shape of an inverted anchor, lying in the same plane as the wheel. Various improvements have since been made in escapements, but the principle remains unchanged.