How Pendulum Clocks Work

by Marshall Brain
Source: HowStuffWorks

Have you ever looked inside a grandfather clock or a small mechanical alarm clock, seen all the gears and springs and thought, "Wow -- that's complicated!"? While clocks normally are fairly complicated, they do not have to be confusing or mysterious. In fact, as you learn how a clock works, you can see how clock designers faced and solved a number of interesting problems to create accurate timekeeping devices. In this edition of HowStuffWorks, we'll help you understand what makes clocks tick, so the next time you look inside one you can make sense of what's happening!

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 (and sometimes even a "moon phase" dial!).

• There are one or more weights (or, if the clock is more modern, a keyhole used to wind a spring inside the clock -- we will stick with weight-driven clocks in this article).

• And of course there is 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 a second. In large grandfather clocks, the pendulum swings once every two seconds.

Let's Start with the Weight!

So let's start with the weight and see what it is doing. The idea behind the weight is to act 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. As we will see in a moment, the clock uses that potential energy as the weight falls to drive the clock's mechanism.

So let's say that we wanted to use a falling weight to create the simplest possible clock -- a clock that has just a second hand on it. We want the second hand on this simple clock to work like a normal second hand on any clock, making one complete revolution every 60 seconds. We might try to do that, as shown in the figure on the right, simply by attaching the weight's cord to a drum and then attaching a second hand to the drum as well. This, of course, would not work. In this simple mechanism, releasing the weight would cause it to fall as fast as it could, spinning the drum at about 1,000 rpm until the weight clattered on the floor.

Still, it's headed in the right direction. Let's say we put some kind of friction device on the drum -- some sort of brake pad or something that would slow the drum down. This might work. We would certainly be able to devise some scheme based on friction to get the second hand to make approximately one revolution per minute. But it would only be approximate. As the temperature and the humidity in the air changed, the friction in the device would change. Thus our second hand would not keep very good time.

So, back in the 1600s, people who wanted to create accurate clocks were trying to solve the problem of how to cause the second hand to make exactly one revolution per minute. 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.

Questions & Answers

• Watches obviously do not use pendulums, so how do they keep time?
  A pendulum is one periodic mechanical system with a precise period. There are other mechanical systems that have the same feature. For example, a weight bouncing on a spring has a precise period. Another example is a wheel with a spring on its axle. In this case, the spring causes the wheel to rotate back and forth on its axis. Most mechanical watches use the wheel/spring arrangement.

• What is the difference between a weight-driven and a spring-driven clock?
  Nothing, really. Both a weight and a spring store energy. In a spring-driven clock you wind the spring and it unwinds into the same sort of gear train found on a weight-driven clock.

• What can you do to make a clock more accurate?
  There is an excellent book entitled "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time", by Dava Sobel, that discusses the creation of extremely accurate mechanical clocks to find a ship's longitude. Creating accurate mechanical clocks that can live on a ship (unlike a pendulum clock...) was a real challenge!

• How does the moon phase dial on a grandfather clock work?
  The moon phase dial works just like the hands of the clock do. The minute hand on a clock moves at the rate of one revolution every hour. The hour hand moves at one revolution every 12 hours. The moon phase dial moves at a rate of one revolution every 56 days or so. The moon's cycle is 28 days, and the moon phase dial generally has two moons painted on it.