Fusee is a device used in mechanical watches and clocks to even out the power delivery of the mainspring. It is a cone-shaped pulley with a spiral groove which winds around its circumference. The mainspring is attached to one end of the fusee, and a chain or cord is wound around the fusee, connected at the other end to the barrel holding the mainspring. The winding of the chain or cord on the fusee causes the barrel to unwind, providing a smooth and even power delivery to the escapement.
