Galileo discovered that as long as a pendulum stays the same length and keeps swinging, the time it takes to swing back and forth is always the same. This makes a pendulum the ideal timekeeper.
When you start a pendulum swinging, it continues to swing back and forth because the forces acting on it aren’t balanced. If you swing the pendulum one way, its weight, gravity and the tension in the string pull the pendulum back down towards the centre.
However, as the pendulum swings back, its weight and velocity send it swinging back up the other way past the centre point. As long as there is an energy source, the pendulum will keep swinging with this precision indefinitely.
So from where does a pendulum get its power? Well, a weight is suspended from a drum that is attached to a series of gears. As the weight falls it turns the gears that each drive the different clock hands – ie, the second, minute and hour hands.
To control the speed at which the gears turn, however, a special escapement device allows each swing of the pendulum to release Just one tooth of the gear at a time, giving the pendulum enough energy to overcome the friction of the swing.
When the weight reaches the end of its tether, it needs to be wound back on to the drum again using a special key.