A hairspring is the very thin spiral spring fixed to the balance wheel of a mechanical watch. It controls the swings of the balance, which set the beat of the watch. Working as a pair, the hairspring and balance wheel regulate timekeeping. Its arrival made pocket watches dramatically more accurate.
How does a hairspring work?
The hairspring is attached to the balance wheel, and the two never work alone. The spring controls each swing of the balance. Those swings set the beat of the watch, and the beat sets the pace at which the watch keeps time. So the spring and wheel act as a pair to regulate timing. One glossary puts it simply. It compares the hairspring to the string of a grandfather clock’s pendulum. The idea is the same in both: a regular swing that the timekeeping follows.
Where it comes from
The invention of the balance spring is often credited to Christiaan Huygens in 1675. Robert Hooke disputed that credit. He said he had come up with the device in London in 1658. Either way, the result was clear: pocket watches became dramatically more accurate. In 1795 Abraham-Louis Breguet added an important improvement, the overcoil. It is still the usual way of making a balance spring isochronous, meaning each swing takes the same length of time. The word itself, from ‘hair’ plus ‘spring’, was first recorded around 1820 to 1830.
What are hairsprings made of?
A common material is Nivarox. This is a nickel-steel alloy that resists magnetism and changes in temperature. Both qualities matter in a part this thin that sets the pace of the whole watch. A newer option arrived in the late twentieth century: hairsprings made from etched silicon. These cannot be magnetised at all. Each material aims at the same goal, a spring that keeps the balance swinging steadily.
Not to be confused with
The hairspring is easy to mix up with the mainspring, but the two do different jobs. The mainspring is a long coiled ribbon of metal kept in a barrel. It stores the power that drives the watch. The hairspring is a separate part. It regulates the watch, controlling the swings of the balance and so the beat. Put simply, one spring supplies the energy and the other keeps the time.
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