after about a century. Surviving examples of stackfreed timepieces date from about 1530 to 1640. How it works See drawing, right. The stackfreed consists of a stiff spring arm (A) with a roller at the end (B) which presses against an eccentric cam (D); usually the roller rides in a groove in the cam's edge. The cam is shaped like a snail. It has a gear on it (E) that is turned by a gear on the mainspring arbor (C), so it makes one turn during the clock's running period. The force of the spring arm against the cam exerts a retarding force on the mainspring, reducing its torque, which varies with the thickness of the cam. When the mainspring is fully wound up, the arm presses against the wide part of the cam. Since it is far from the axis, the retarding force it exerts is maximum. As the clock runs and the mainspring unwinds, the cam rotates and the spring bears against the narrower parts of the cam, reducing the retarding force gradually, to compensate for the declining force of the mainspring. At the end of the running period there was often a steep depression in the cam that the roller pressed against, so the force of the stackfreed spring aided the weakened mainspring. Often (as shown at right) a solid uncut section in the cam's (E) gear teeth also functioned as stopwork, limiting the mainspring so it stopped before it was wound up all the way, and stopped before it unwound all the way. This restricted the mainspring to the center part of its range, further reducing force variation. Advantages and disadvantages The stackfreed was a very inefficient device. Since it worked by exerting an opposing friction force on the mainspring, it required more powerful mainsprings and higher gear ratios in watches, which may have introduced more variation in drive force. The fusee, the other mainspring compensation device, was much more efficient. The only advantages of the stackfreed were that it was easier to make and much thinner than the fusee, which, combined with the fact that it was located in unused space on the outside of the back plate, allowed stackfreed watches to be flatter. With the development of narrower, more compact fusees, the stackfreed disappeared from timepieces around 1630. See also History of watches Fusee References Timekeeping components