File:Tidens naturlære fig29.png

English: Drawing of a fusee from "Tidens naturlære" (Nature of time) 1903 by Poul la Cour, Ill. 29. The book is digitized at s:da:Tidens Naturlære. The fusee (F) was a conical pulley that was used in antique clocks and watches to equalize the declining pull of the mainspring as the watch ran, improving timekeeping. The fusee chain (H) wrapped around the mainspring barrel (A) that contained the mainspring, and the fusee. The pull of the spiral mainspring turned the barrel, which pulled the chain, which turned the fusee. The ring of gear teeth around the fusee turned the wheels of the watch. Just after the watch is wound up the fusee chain comes off the top narrow part of the fusee pulley, so the strong pull of the wound up spring was scaled down by the narrow lever arm, As the watch ran and the mainspring unwound so its torque was less, the force was applied to the wider lower part of the fusee (as shown), increasing the lever arm, and scaling up the torque so it remained constant over the running period of the spring. The constant force applied to the watch's escapement improved timekeeping accuracy. Fusees became obsolete in the 19th century with the wide use of the going barrel, which provided a constant force.