fusee

A fusee is a component found in older mechanical timepieces, such as watches. It is a conical, grooved pulley that works with the mainspring and a chain or cord. The purpose of the fusee is to compensate for the decreasing power of the mainspring as it unwinds. When the mainspring is fully wound, the chain pulls on the smallest radius of the fusee, and as the spring unwinds, the chain pulls on progressively larger radii. This arrangement helps maintain a more constant torque, ensuring more consistent timekeeping. Modern mechanical watches typically use a going barrel instead of a fusee, as improved mainsprings have reduced torque variations.12