What is a fusee in a clock?

A fusee clock is a type of mechanical clock that uses a fusee to improve timekeeping by equalizing the uneven pull of the mainspring as it runs down. A fusee is a cone-shaped pulley with a helical groove around it, wound with a cord or chain attached to the mainspring barrel of antique mechanical watches and clocks.

The fusee was used from the 15th century to the early 20th century. The earliest definitely dated fusee clock was made by Jacob Zech in 1525, but the fusee actually appeared earlier, with the first spring-driven clocks in the 15th century . The verge escapement was used in the earliest spring-driven clocks, which was sensitive to changes in drive force and caused inaccurate timekeeping.

The fusee was invented to even out the torque provided by a mainspring, which decreases linearly as the spring unwinds during a clock’s running period. As the movement runs, the tapering shape of the fusee pulley continuously changes the mechanical advantage of the pull from the mainspring, compensating for the diminishing spring force. The fusee was a much more lasting idea than the stackfreed, a crude cam compensator, which added a lot of friction and was abandoned after less than a century