The extremely thin, coiled spring that controls the swings, or oscillations, of the balance. The inner end of the hairspring is attached to the balance staff and the outer end to a stud on the balance cock. The spring’s elasticity ensures that the balance swings back and forth at a regular rate. The active length of the hairspring interacts with the momentum of the balance rim to determine the duration of each beat of the balance. This is why most watches are equipped with a regulator on the balance cock that can be adjusted to vary the active length of the hairspring. Lengthening the spring causes the watch to run more slowly; shortening it makes the watch run faster.
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