Hairspring

The hairspring, or balance spring, is the fine coiled spring attached to the balance wheel that makes it oscillate at a steady rate.

It's extremely sensitive: magnetism can make its coils stick together, and a knock can distort it, either of which makes a watch run badly. Many modern hairsprings use non-magnetic materials to resist this.

If a watch suddenly runs very fast, magnetised hairspring coils are a common cause, and a watchmaker can often demagnetise it quickly.

A distorted hairspring is a job for a skilled watchmaker, so a watch that runs badly after a knock should be priced with a repair in mind.