Balance spring A balance spring (also termed a hairspring) is attached to the balance wheel in timepieces. It causes the balance wheel to oscillate when the watch is running and controls the speed at the which the wheels turn, thereby regulating the movement of the hands as well. It is usually made of a spiral or spring. The addition of the balance spring to the balance wheel was invented in 1657 was the idea of Robert Hooke and Christiaan Huygens (working independently of one another) and greatly increased the accuracy of portable timepieces.
