Dorrin K Mace, Horologost

Dorrin K Mace, Horologost
The Clock Man in a pensive moment

Thursday, September 1, 2011

How does a pendulum clock work?

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 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. Unfortunately it would only be approximate. As the temperature and the humidity in the air change, 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 Christian 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 pedulum swings through. Only the length of the pendulum matters.
Once someone noticed this fact about pendulums, it was realized that you could use the phenomenon to create an accurate clock. The figure below shows how you can create a clock's escapement using a pendulum.
In an escapement there is a gear with teeth of some special shape. There is also a pendulum, and attached to the pendulum is some sort of device to engage the teeth of the gear. The basic idea that is being demonstrated in the figure is that, for each swing of the pendulum back and forth, one tooth of the gear is allowed to "escape."
For example, if the pendulum is swinging toward the left and passes through the center position, then as the pendulum continues toward the left the left-hand stop attached to the pendulum will release its tooth. The gear will then advance one-half tooth's-width forward and hit the right-hand stop. In advancing forward and running into the stop, the gear will make a sound... "tick" or "tock" being the most common. That is where the ticking sound of a clock or watch comes from!
One thing to keep in mind is that pendulums will not swing forever. Therefore, one additional job of the escapement gear is to impart just enough energy into the pendulum to overcome friction and allow it to keep swinging. To accomplish this task, the verge (the name given to the gizmo attached to the pendulum to release the escapement gear one tooth at a time) and the teeth on the escapement gear are specially shaped. The gear's teeth escape properly, and the pendulum is given a nudge in the right direction by the anchor each time through a swing. The nudge is the boost of energy that the pendulum needs to overcome friction, so it keeps swinging.

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