balance spring

A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces. It causes the balance wheel to oscillate with a resonant frequency when the timepiece is running, which controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands. A regulator lever is often fitted, which can be used to alter the free length of the spring and thereby adjust the rate of the timepiece.