pull of the fusee unwinds the chain off the barrel and back onto the fusee, turning the barrel and winding the mainspring. The presence of the fusee means that the force required to wind up the mainspring is constant; it does not increase as the mainspring tightens. The gear on the fusee drives the movement's wheel train, usually the center wheel. There is a ratchet between the fusee and its gear (not visible, inside the fusee) which prevents the fusee from turning the clock's wheel train backwards while it is being wound up. In quality watches and many later fusee movements there is also a maintaining power spring, to provide temporary force to keep the movement going while it is being wound. This type is called a going fusee. It is usually a planetary gear mechanism (epicyclic gearing) in the base of the fusee "cone") which then provides turning power in the opposite direction to the 'winding up' direction therefore keeping the watch or clock running during winding. Most fusee clocks and watches include a 'winding stop' mechanism to prevent the mainspring and fusee from being wound up too far, possibly breaking the chain. As it is wound, the fusee chain rises toward the top of the fusee. When it reaches the top, it presses against a lever, which moves a metal blade into the path of a projection sticking out from the edge of the fusee. As the fusee turns, the projection catches on the blade, preventing further winding. The normal fusee can only be wound in one direction. Drunken fusees were developed, but rarely used, to allow the fusee to be wound in either direction. John Arnold unsuccessfully used them in a few marine chronometers. Obsolescence The fusee was a good mainspring compensator, but it was also expensive, difficult to adjust, and had other disadvantages: It was bulky and tall, and made pocket watches unfashionably thick. If the mainspring broke and had to be replaced, a frequent occurrence with early mainsprings, the fusee had to be readjusted to the new spring. If the fusee chain broke, the force of the mainspring sent the end whipping about the inside of the clock, causing damage. Achieving isochrony was recognised as a serious problem throughout the 500-year history of spring-driven clocks. Many parts were gradually improved to increase isochronism, and eventually the fusee became unnecessary in most timepieces. The invention of