A wheel with saw-shaped teeth round its rim is called a ratchet. The ratchet wheel usually engages with a tooth-shaped lever called a pawl. The purpose of a ratchet and pawl is to allow rotation in one direction only and prevent rotation in the opposite direction. Pawl and ratchet mechanisms are often used to control rotary motion. It makes an intermittent drive of a rotating shaft. The shaft is rotated by a pawl on a drive arm engaging with a ratchet wheel. The motion is in steps equal to the number of teeth on the escape wheel. A ratchet wheel can also be used to arrest motion. The diagram to the left shows a ratchet wheel fixed to a shaft and winding drum. A rope is wound round the drum. Any load force applied to the rope (the fusee chain in a clock) is unable to unwind the drum because of the action of the ratchet and pawl. It is possible to wind up the rope becasue the ratchet can be rotated in an anticlockwise direction. Notice the pawl is held in position against the ratchet by a tension spring. Take a look at the diagram to the right. Now that you have read about all of the parts and how gears work we are ready to explain how the clock works. This diagram is called the verge escapement. It consists of a crown wheel and balance, connected through gears to the clock hands. As the weight falls, it sets the crown wheel moving round in jerks or "ticks". This made the clock hands move round too. The above Photo is from: Beyond Mechanization, by Larry Hirschhorn; MIT paperback edition, copyright 1986