A fusee is a cone-shaped pulley, grooved with a spiral track, used in spring-driven clocks and watches to even out the force of the mainspring. A chain or gut line runs from the barrel containing the mainspring to the fusee, so that when the spring is fully wound and strongest it pulls on the narrow end of the cone, and as it runs down and weakens it pulls on the wider end, giving a nearly constant driving force.
The device was in use by the fifteenth century and became a hallmark of high-quality English clock and watchmaking. English bracket clocks, wall dial clocks, carriage clocks and marine chronometers were commonly made with fusees, as were English pocket watches until the later nineteenth century. Early examples used gut lines, but fine steel chains became standard for watches and many clocks. French and American makers generally abandoned the fusee in favour of the simpler going barrel. Movements are often described by the number of fusees, so a double fusee clock has a striking train as well as a timekeeping train. Collectors check that chains and lines are sound, that the fusee stop works and that the movement has been properly serviced.
