Pocket watches · Winding

Fusee Pocket Watches

Verified

Watches in which the mainspring drives a tapered, grooved pulley (the fusee) through a chain or gut line, equalising torque as the spring unwinds.

Mechanism

As the mainspring weakens, the chain acts on progressively larger fusee radii, keeping torque at the great wheel roughly constant — a mechanical answer to poor early mainspring quality.

Typical characteristics

Visible chain between barrel and fusee cone under the top plate
Stop-work to prevent overwinding
Almost always key-wound through the case back or dial

Construction and interpretation

The fusee compensates for changing mainspring pull by changing leverage. At full wind the strong spring acts through the smaller radius of the fusee. As the watch runs and the chain transfers onto the barrel, its pull acts through progressively larger radii. The resulting torque at the train is more even than the spring's unmodified output; it is not a claim that torque is perfectly constant in every surviving movement.

A fusee identifies a power-transmission system, not a particular escapement. It occurs with verge work and with other constructions, including precision lever and chronometer mechanisms. Gut or cord can replace a chain in earlier forms. Maintaining power is another separate feature: when fitted it keeps the train supplied while winding temporarily interrupts the normal drive. A catalogue description should not assume that every fusee has maintaining power or that its original stop mechanism survives intact.

State or feature	Power path	Purpose
Near full wind	Spring pulls at smaller fusee radius	Reduce leverage when spring pull is strong
As spring unwinds	Pull moves to larger fusee radius	Increase leverage as spring pull weakens
Stop mechanism	Limits winding travel	Avoid excessive chain travel
Maintaining power, if fitted	Temporary power during winding	Separate from the fusee's leverage compensation