against the wide part of the cam, providing a strong opposing force, while later in the running period as the force of the mainspring decreased, the spring would bear against the narrower part of the cam and the opposing force would also decrease. The stackfreed added a lot of friction and probably reduced a clock's running time substantially; it was only used in some German timepieces and was abandoned after about a century. Fusee The fusee was a much longer-lasting innovation. This was a cone-shaped pulley that was turned by a chain wrapped around the mainspring barrel. Its curving shape continuously changed the mechanical advantage of the linkage to even out the force of the mainspring as it ran down. Fusees became the standard method of getting constant torque from a mainspring. They were used in most spring-driven clocks and watches from their first appearance until the 19th century when the going barrel took over, and in marine chronometers until the 1970s. Stopwork Another early device which helped even out the spring's force was stopwork or winding stops, which prevented the mainspring from being wound up all the way, and prevented it from unwinding all the way. The idea was to use only the central part of the spring's 'torque curve', where its force was more constant. The most common form was the Geneva stop or 'Maltese cross'. Stopwork isn't needed in modern watches. Remontoire A fourth device used in a few precision timepieces was the remontoire. This was a small secondary spring or weight which powered the timepiece's escapement, and was itself rewound periodically by the mainspring. This isolated the timekeeping element from the varying mainspring force. Going barrel The modern going barrel, invented in 1760 by Jean-Antoine Lépine, produces a constant force by simply using a longer mainspring than needed, and coiling it under tension in the barrel. In operation, only a few turns of the spring at a time are used, with the remainder pressed against the outer wall of the barrel. Mathematically, the tension creates a 'flat' section in the spring's 'torque curve' (see graph) and only this flat section is used. In addition, the outer end of the spring is often given a 'reverse' curve, so it has an 'S' shape. This stores more tension in the spring's outer turns where it is available toward the end of the running period. The result is that