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A fusee is a mechanical component of traditional-style clocks and watches that run on gear-and-spring rotating actions. The fusee pulley is a cone-shaped spinning cylinder with a cord wrapped around it, which connects it to the mainspring, where the tension stabilizes the action of the mainspring as the watch winds down. The fusee watch, or fusee clock, design can be traced as far back as the 1600s and was considered a revolution in watch design early on, as it made such time pieces much more accurate than previous models. Fusees were dominant in English-made timepieces throughout their period of widespread use and began to become obsolete in the year 1760, when Jean-Antoine Lepine, a noted French watchmaker, invented the going barrel to replace them.
The principle behind how the rotation of a fusee in a watch stabilized the tension of the mainspring is based upon its cylindrical shape. When the mainspring was tightly wound, the cord from it was connected to the small diameter end of the fusee, and, as the watch tension ran down, the cord on the fusee moved down toward the larger end of the cone, providing slower rotation, yet greater tension for the watch movement. The movement of the cord was controlled by grooves in the fusee surface that kept it in place on a specific diameter of the cone, dependent on the tension level in the watch. This was a major improvement over mechanical timepiece design to the point that watches or clocks prior to fusee use were so inaccurate that they could not be considered true timekeeping devices.
The design of fusee watches and clocks were not without their disadvantages, however. Particularly for watch design, it was a bulky element that made pocket watches thick and cumbersome. The fusee watch was also a complicated mechanical device, and, if the mainspring broke or the chain on the fusee broke, it was difficult and expensive to repair the device, as damage could occur to other components in the watch in the process.
The going barrel invention by Jean-Antoine Lepine in 1760 was considered another revolutionary leap forward in watch design, and Lepine's designs for mechanical watches and clocks are still used as of 2011. The going barrel is essentially a very long mainspring that can hold potential torque energy that is never fully dissipated in the watch mechanism. This allows for a much more constant level of force for the watch movement, and a longer and thinner mainspring also was proven to be much less likely to break than those used in the fusee watch. Another advantage that the going barrel design brought to watches is that it gave them the capability of having oscillating movements of up to 18,000 oscillations per hour. This made them much less susceptible to inaccuracies caused by vibrations from riding on horseback, in coaches, or trains of the time period.