A mainspring is the coiled ribbon of tempered steel that stores the energy needed to drive a spring-powered clock or watch. When the clock is wound, the spring is tightened around a central arbor, and as it slowly unwinds it turns the train of wheels that carries the hands and, in many clocks, powers the striking or chiming work. The spring is usually housed in a cylindrical brass drum known as the barrel.
Spring-driven timekeepers appeared in Europe in the fifteenth century and freed clocks from the need for hanging weights, making portable table, bracket and carriage clocks possible. Because a fully wound spring pulls harder than one that is nearly run down, makers developed the fusee, a cone-shaped pulley linked to the barrel by gut or chain, to even out its force; English bracket clocks are noted for their use of it, while many French and American movements rely on a going barrel instead. Mainsprings tire, set or break over time, and a replacement is a routine part of servicing. A sudden break can damage teeth and pivots, so a restorer will examine the whole movement, and the power should be let down carefully before any clock is dismantled.
