FUSEE

The Fusee is a conical pulley with a helical Groove, connected to the Barrel by a chain or, originally, a gut cord, intended to equalize the torque delivered by the mainspring throughout its Letting down. Indeed, the torque of a spring decreases as it unwinds; by winding the chain onto the narrow part of the Fusee when the spring is fully wound, then gradually toward its wide part as it weakens, the lever arm compensates for the loss of force and maintains a substantially constant output torque. The actual profile of the Fusee, empirically established by clockmakers, is not a simple cone but a hyperboloid.

Appearing with the first spring-driven clocks of the 15th century, the Fusee remains of uncertain origin; the Clock of Burgundy, preserved at the Germanisches Nationalmuseum and dated around 1430, is the oldest known example. The use of the chain, more durable than the cord, spread from the middle of the 17th century onwards. In 1726, John Harrison added a constant force spring to it, allowing the piece to continue running while being wound.

Bulky and costly, the Fusee was gradually abandoned after the invention of the simple Barrel by Jean-Antoine Lépine in 1760, then after the progress of Isochronism of the Hairspring (balance spring). It survived, in the 20th century, only in marine chronometers or in certain exceptional watches.