the diminishing spring force. Clockmakers apparently empirically discovered the correct shape for the fusee, which is not a simple cone but a hyperboloid. The first fusees were long and slender, but later ones have a more squat compact shape. 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. At first the fusee cord was made of gut, or sometimes wire. Around 1650 chains began to be used, which lasted longer. Gruet of Geneva is widely credited with introducing them in 1664, although the first reference to a fusee chain is around 1540. 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. Around 1726 John Harrison added the maintaining power spring to the fusee to keep marine chronometers running during winding, and this was generally adopted. How it works The mainspring is coiled around a stationary axle (arbor), inside a cylindrical box, the barrel. The force of the spring turns the barrel. In a fusee clock, the barrel turns the fusee by pulling on the chain, and the fusee turns the clock's gears. When the mainspring is wound up (Fig. 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. 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. As the clock runs, the chain is unwound from the fusee from top to bottom and wound on the barrel. As the mainspring runs down (Fig. 2), more of the chain is wrapped on the barrel, and the chain going to the barrel comes off the wide bottom grooves of the fusee. Now the weaker pull of the mainspring is applied to the larger radius of the bottom of the fusee. The greater turning moment provided by the larger radius at the fusee compensates for the weaker force of the spring, keeping the drive torque constant. To wind the clock up again, a key is fitted to the protruding squared off axle (winding arbor) of the fusee and the fusee is turned. The