A balance spring, also known as a hairspring, is a crucial component in mechanical watches and clocks. It is a fine coil spring that helps regulate the movement of the timekeeping mechanism. By expanding and contracting, the balance spring ensures that the oscillations of the balance wheel remain consistent, allowing for accurate timekeeping. The balance spring works in conjunction with the balance wheel, which swings back and forth. This oscillation creates a steady rhythm that divides time into equal parts. Together, the balance spring and balance wheel form the heart of a mechanical watch, enabling it to keep precise time without the need for batteries.