Pendulum clocks have been used to keep time since 1656, and they have not changed dramatically since then. Pendulum clocks were the first clocks made to have any sort of accuracy. When you look at a pendulum clock from the outside, you notice several different parts that are important to the mechanism of all pendulum clocks:
- There is the face of the clock, with its hour and minute hand
- There are one or more weights
- The pendulum itself. In most wall clocks that use a pendulum, the pendulum swings once per second. In small cuckoo clocks the pendulum might swing twice per second. In large grandfather clocks the pendulum swings once every two seconds.
Let's start with the weight!
The weight acts as an energy storage device so that the clock can run for relatively long periods of time unattended. When you "wind" a weight-driven clock, you pull on a cord that lifts the weight. That gives the weight "potential energy" in the Earth's gravitational field. The clock uses that potential energy as the weight falls to drive the clock's mechanism.
A means of regulating the weight was needed to adjust control to one second. The Dutch astronomer Christiaan Huygens is credited with first suggesting the use of a pendulum. Pendulums are useful because they have an extremely interesting property: The period (the amount of time it takes for a pendulum to go back and forth once) of a pendulum's swing is related only to the length of the pendulum and the force of gravity. Since gravity is constant at any given spot on the planet, the only thing that affects the period of a pendulum is the length of the pendulum. The amount of weight does not matter. Nor does the length of the arc that the pendulum swings through. Only the length of the pendulum matters.
Three and a half centuries later, we still use this simple concept for clocks that keep very accurate time!