What keeps a clock's pendulum swinging backwards and forwards?
Before we get around to that, here is a little story.
A little story - Ahhh!
My grandfather, who lived in Hull, England, was a keen
fisherman. On 23rd August, 1899, he went by ferry across
the River Humber to enter a fishing competition at Brigg.
He won the first prize, a pendulum wall clock. He also won
the booby prize, a bottle of whisky.
That evening he arrived home drunk, with the clock under his arm, to find that he now had another baby son, my father.
Unfortunately, the clock is no longer working. One day, shortly after my father had died, my mother was woken by a loud noise. Bravely investigating, she found that the clock was in small pieces on the floor. The joints of the wooden case of the clock, and any glue which had been holding them together, had dried out so it had collapsed and fallen on the floor, leaving the part with the eyehole still hanging from the hook on the wall. Practically all of the wooden parts of the clock had come apart and the glass front and sides had broken.
When my brother enquired about the cost of repair, it was approximately twenty times more than the clock was worth, and so expensive that it was not worth mending even considering its sentimental value. So the clock face, the mechanism and the many wooden components which encased them are now 'archived' in three large plastic bags. Perhaps one day...
Well, that was written probably in 1998. Several years later, that day finally arrived. The clock was repaired (2008) and is now working again.
How a pendulum clock works
Usually there is a weight on the end of a length of wire or
cord which is wrapped around a drum. The drum would turn
quickly, pulled by the weight, if it wasn't for an
escapement mechanism which slows the process down. Instead
of a weight on a cord, I have drawn a gremlin whose weight
does the same thing. If the escapement didn't stop the
spoked wheel from turning, the gremlin would quickly finish
on the floor if he couldn't run at an incredible speed.
The escapement mechanism consists of a pendulum with a bar fixed to the top having two teeth. The pendulum is positioned to that the two teeth alternately swing in and out of the spoked wheel. Generally speaking, when one tooth is within the circumference of the wheel, the other isn't, although in practice it is arranged that both are within the circumference for a short time.
The result is that as the pendulum swings one way, the wheel rotates until a spoke is touching one tooth. When the pendulum swings, the tooth releases the spoke and the wheel rotates until one of the other spokes is held by the other tooth. When that tooth swings out of the way to allow the wheel to turn again, the other tooth is back to stop another spoke.
What keeps the pendulum swinging?
Why doesn't friction cause the angle through which the
pendulum swings to become smaller and smaller until the
pendulum finally stops moving?
The answer is that the two teeth which alternately prevent the cog wheel from rotating correct for this. These two teeth are at slight angles so that the cog which has been stopped gives the tooth a very small push in the direction in which the tooth is moving as it releases the cog. It is this small push which offsets the effects of friction, and keeps the pendulum swinging.
I know that the escapement isn't drawn very well, particularly
the movement of the escapement teeth. I have no plans to improve
on the above.
Because the thread is set to 100mS, and some computers will be unable to draw all the graphics in this time, you may find that the pendulum will jump rather than swinging smoothly. Sorry. If the number you see at the bottom corner of the window is 18, 19 or 20, that is as good as it gets! It only just works on my Acorn RiscPC600 with 202MHz StrongArm.
If my description of an escapement is incorrect, please let me know how to change it.