A fusee is a cone-shaped pulley, with a spiral groove around which a chain (or cord) is wound. They appeared mostly in antique spring-powered mechanical watches and clocks from the 15th century to the early 20th century.
By equalizing the irregular pull of the mainspring as it ran down it regulated the movement and helped improve timekeeping. Originally the fusee cord was made from gut but from the mid 17th century chain was used instead.
G. Baillie stated of the fusee, “Perhaps no problem in mechanics has ever been solved so simply and so perfectly”.