A hairspring, or balance spring, is a very fine coiled spring attached to the balance wheel of a watch or clock, which controls the rate at which the balance oscillates and therefore regulates timekeeping. Without it, the balance would swing irregularly, and its introduction transformed portable timepieces from approximate indicators of the hour into accurate instruments.
The balance spring was developed in the 1670s, with both Christiaan Huygens and Robert Hooke associated with its invention, and it rapidly improved the accuracy of watches. Early hairsprings were short and made of steel, but over time watchmakers refined their length, shape and material. Abraham-Louis Breguet introduced the overcoil, in which the outer end is raised and curved inward, improving isochronism. Marine chronometers used helical springs, and in the twentieth century special alloys such as Elinvar and later Nivarox reduced the effect of temperature changes, the latter also resisting magnetism. Hairsprings are found in pocket watches, wristwatches, carriage clocks and other balance-controlled clocks. Because they are extremely delicate, they are easily distorted or damaged by careless handling, and a bent or rusted hairspring will cause poor timekeeping. Repair and adjustment require specialist skills, so servicing should be entrusted to an experienced watchmaker.
