FACT OF THE DAY

How watchmakers cheated a weakening spring before 1675

Long before the balance spring made watches genuinely isochronous, early spring-driven timepieces relied on a tapered, cone-shaped part called the fusee to tame the mainspring’s fading force. A fully wound mainspring pushes hard, then softer and softer as it unwinds, and in verge-and-balance watches without a balance spring, the rate wandered noticeably as that torque changed. The fusee, linked to the barrel by a small chain, quietly adjusted the leverage as the spring ran down, giving the movement steadier power across the whole wind, as Wikipedia notes.

Other compensators were tried, but the fusee became the common workaround until the balance spring, arriving around 1675, made watches far more regular and eventually made the fusee optional.

Sometimes the elegant fix wins.