Fusee (horology)

Used in antique spring-powered mechanical watches and clocks, a fusee is a cone-shaped pulley with a helical groove around it, wound with a cord or chain which is attached to the mainspring barrel. Fusees were used from the 15th century to the early 20th century to improve timekeeping by equalizing the uneven pull of the mainspring as it ran down. Gawaine Baillie stated of the fusee, "Perhaps no problem in mechanics has ever been solved so simply and so perfectly."

The origin of the fusee is not known. Many sources erroneously credit clockmaker Jacob Zech of Prague with inventing it around 1525, but it actually appeared with the first spring driven clocks in the 15th century. The idea probably did not originate with clockmakers, since the earliest known example is in a crossbow windlass shown in a 1405 military manuscript. Drawings from the 15th century by Filippo Brunelleschi and Leonardo da Vinci show fusees. The earliest existing clock with a fusee, also the earliest spring-powered clock, is the Burgunderuhr (Burgundy clock), a chamber clock whose iconography suggests that it was made for Phillipe the Good, Duke of Burgundy about 1430, now in the Germanisches Nationalmuseum The earliest definitely dated fusee clock was made by Zech in 1525. The word fusee comes from the French fusée and late Latin fusata, 'spindle full of thread'.

Springs were first employed to power clocks in the 15th century, to make them smaller and portable. These early spring-driven clocks were much less accurate than weight-driven clocks. Unlike a weight on a cord, which exerts a constant force to turn the clock's wheels, the force a spring exerts diminishes as the spring unwinds. The primitive verge and foliot timekeeping mechanism, used in all early clocks, was sensitive to changes in drive force. So early spring-driven clocks slowed down over their running period as the mainspring unwound. This problem is called lack of isochronism.

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