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 1675 by the Dutch Scientist Christiaan Huygens, which greatly increased the accuracy of portable timepieces.
