A weight-driven clock is a clock powered by the gradual descent of one or more weights hung on cords, chains or cables, rather than by a coiled spring. As the weight falls, it turns the gear train, and the escapement regulates the rate at which the energy is released.
Weight drive is the oldest form of mechanical clock power, used in medieval turret clocks and, from the sixteenth century, in domestic chamber and lantern clocks. It became the standard for English longcase clocks from the late seventeenth century, together with wall clocks, Vienna regulators and many continental clocks such as the Black Forest cuckoo clock. The great advantage of a falling weight is that it provides a constant force, which helps accuracy, whereas a spring loses power as it unwinds. Weight-driven clocks must be fixed or placed securely, however, and are not portable. Most longcase clocks run for either thirty hours, usually wound by pulling a chain or rope, or eight days, wound with a key through holes in the dial. Collectors check that weights, pulleys, cords and pendulum are original or appropriate to the movement, and that the case has not been altered to suit a replacement.
