Fusée and chain is a way of driving a movement: a chain from the mainspring barrel winds onto a cone-shaped pulley called the fusee. It keeps the torque reaching the movement even while the mainspring runs down. It dates from the earliest spring-driven clocks and is still used in a few modern wristwatches.
How does a fusée and chain work?
The fusee is a cone-shaped pulley cut with a spiral groove. A chain, or a gut cord in early examples, runs from the mainspring barrel and lies in that groove. When the spring is fully wound, the chain pulls on the small radius of the cone. As the spring unwinds and weakens, the chain pulls on ever larger radii. When the spring is nearly run down, it pulls on the largest. Because the pulley tapers, the leverage keeps changing. More leverage makes up for less spring force, so the torque passed on to the movement stays even.
Where it comes from
Nobody knows who invented the fusee. It appeared with the first spring-driven clocks in the 1400s. The earliest known example is not in a clock at all. It is in a crossbow winder drawn in a military manuscript of 1405. Early fusees used cord. Chains last longer and came into use from the mid-1600s. Gruet of Geneva is usually credited with introducing them in 1664. In 1726 John Harrison added a spring to the fusee, so the timepiece kept running while it was being wound.
Is the fusée still used today?
England kept making bulky full-plate fusee watches until about 1900. By the 1900s the fusee was used only in marine chronometers and a few exceptional watches. A. Lange & Söhne later made the first wristwatch with a fusee and chain. Breguet, Zenith and Romain Gauthier have also used one in modern watches. The parts are tiny. The chain in Lange’s fusee wristwatches is made of 636 parts and is only half a millimetre wide.
Not to be confused with
The going barrel, introduced by Jean-Antoine Lépine, works differently. It drives the watch’s gear train straight from the barrel, with no chain and no cone in between. In a fusee movement, the pull of the mainspring passes through the chain and the cone before it reaches the gear train. With a going barrel, nothing sits between the barrel and the gear train to even out the spring’s pull as it runs down.
Checked