Balance wheel A balance wheel, or balance, is used in mechanical watches and small clocks. It is a weighted device that is directly analogous to the pendulum in a pendulum clock. It comprises a balance spring (also called a hairspring) fitted into a metal wheel. One end of the spring is fixed to the wheel, and the other end is fixed to the body of the timepiece. The spring rotates around the axis of the wheel in an alternating clockwise and anticlockwise motion, thus differing from the back-and-forth arc motion of a pendulum. Like the pendulum, the balance spring is isochronous. The balance wheel is not as susceptible to vibration as the pendulum. Thus, balance-wheel timepieces are portable. The balance wheel was invented during the 1300s. The addition of the balance spring to the balance wheel in 1657 was the idea of Robert Hooke and Christiaan Huygens (working independently of one another) and greatly increased the accuracy of portable timepieces.
