A fusee is a conical, spiral pulley used in conjunction with a chain to regulate the force of the mainspring in a mechanical watch, ensuring consistent timekeeping.
A fusee is a cone-shaped pulley connected to the mainspring via a chain. As the mainspring unwinds and weakens, the chain moves up the fusee cone, increasing leverage to compensate. This maintains consistent torque delivery for superior accuracy in antique pocket watches and high-end timepieces.
Fusee mechanisms are expensive to manufacture, require significant space, and modern mainspring alloys deliver more consistent force naturally. Most watchmakers achieve excellent accuracy through improved escapements and materials at lower cost. Fusees remain in ultra-luxury pieces showcasing traditional craftsmanship.
The fusee chain is a precisely crafted metal link chain connecting the mainspring barrel to the conical fusee. As you wind the watch, the chain wraps onto the barrel. During operation, it unwraps and climbs the fusee cone, automatically adjusting mechanical advantage to compensate for decreasing mainspring force.