The fine spring that returns a mechanical watch's balance wheel toward its resting position and helps determine its oscillation rate.
Also called a hairspring, it is a very fine coiled spring. Its inner end is held on the balance staff by a collet and its outer end on the balance cock by a stud. Each time the balance swings away from its rest point, the spring draws it back. The spring’s active length and the balance’s inertia together set how long each oscillation lasts: too short a spring makes the watch run fast, too long a spring makes it run slow.
Isochronism is central: each oscillation should take the same time whether the swing is wide, as when the watch is fully wound, or narrow, as when its power reserve runs low. No spring is perfectly isochronous, and these defects are among the main causes of rate differences between positions. Silicon springs are non-magnetic but cannot be reshaped after manufacture.
In job listings
Balance spring and hairspring appear in production watchmaker, regulation and after-sales service roles, and in apprenticeships. The work they signal is at the bench: fitting the balance and spring once the movement is assembled, then checking its oscillations on a timing machine and making fine adjustments. See current hairspring roles.
