As the clock runs, the chain is unwound from the fusee from top to bottom and wound on the barrel.So the strong pull of the wound up mainspring is applied to the small end of the fusee, and the torque on the fusee is reduced by the small lever arm of the fusee radius. 1), all the chain is wrapped around the fusee from bottom to top, and the end going to the barrel comes off the narrow top end of the fusee. In a fusee clock, the barrel turns the fusee by pulling on the chain, and the fusee turns the clock's gears. The force of the spring turns the barrel. The mainspring is coiled around a stationary axle ( arbor), inside a cylindrical box, the barrel. (A) mainspring arbor, (B) barrel, (C) chain, (D) attachment of chain to fusee, (E) attachment of chain to barrel, (F) fusee, (G) winding arbor, (W) output gear. Fusees designed for use with cords can be distinguished by their grooves, which have a circular cross section, where ones designed for chains have rectangular-shaped grooves.Īround 1726 John Harrison added the maintaining power spring to the fusee to keep marine chronometers running during winding, and this was generally adopted.įusee and mainspring barrel, showing operation. Gruet of Geneva is widely credited with introducing them in 1664, although the first reference to a fusee chain is around 1540. Around 1650 chains began to be used, which lasted longer. Fusees became the standard method of getting constant force from a mainspring, used in most spring-wound clocks, and watches when they appeared in the 17th century.Īt first the fusee cord was made of gut, or sometimes wire. The first fusees were long and slender, but later ones have a more squat compact shape. Clockmakers apparently empirically discovered the correct shape for the fusee, which is not a simple cone but a hyperboloid. As the movement ran, the tapering shape of the fusee pulley continuously changed the mechanical advantage of the pull from the mainspring, compensating for the diminishing spring force. The stackfreed, a crude cam compensator, added a lot of friction and was abandoned after less than a century. Two solutions to this problem appeared with the first spring driven clocks the stackfreed and the fusee. This problem is called lack of isochronism. So early spring-driven clocks slowed down over their running period as the mainspring unwound, causing inaccurate timekeeping. The primitive verge and foliot timekeeping mechanism, used in all early clocks, was sensitive to changes in drive force. 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. These early spring-driven clocks were much less accurate than weight-driven clocks. Springs were first employed to power clocks in the 15th century, to make them smaller and portable. ![]() Watch from 1500s with stackfreed (near top). 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 word fusee comes from the French fusée and late Latin fusata, 'spindle full of thread'. Drawings from the 15th century by Filippo Brunelleschi and Leonardo da Vinci show fusees. The idea probably did not originate with clockmakers, since the earliest known example is in a crossbow windlass shown in a 1405 military manuscript. The earliest definitely dated fusee clock was made by Zech in 1525, but the fusee actually appeared earlier, with the first spring driven clocks in the 15th century. Many sources erroneously credit clockmaker Jacob Zech of Prague with inventing it around 1525. Drawing of a machine incorporating a fusee, by Leonardo da Vinci around 1490.
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