Pendulums Have a Swinging History
Copyright (c) 2009 Mike Bond
The pendulum was a most remarkable step. Right up until the 1930's, and with the advent of the quartz movement, the pendulum was the timekeeper for every clock
It's thought that Galileo Galilei was the first person to receive the spark of the idea from examining the movement of a chandelier in Pisa Cathedral in 1582.
In 1602, he began work in earnest on the device and developed the Metronome, which musicians ever since have found to be so valuable.
A derivative of this also proved invaluable to a physician friend of his, and the machine was named the Pulsilogium. As the name suggests, it was used for taking his patients' pulses.
In 1641, Galileo actually designed a clock regulated by a pendulum, but the poor old chap never lived to see his design in action. It could be said that every development has been made by people standing on the shoulders of giants. Christiaan Huygens perched on Galileo.
There are many different types of pendulum, but for the purposes of this article, we'll content ourselves with those that fit in Grandfather and Longcase clocks, and have a quick look at those used for mantel clocks.
Temperature is one of the enemies of timekeeping in mechanical clocks. Friction is the other. When the pendulum rod warms up, its length is increased, thereby causing the clock to run slow. When it cools down, the opposite happens. Let's see what can be done about this.
There are four main types used for the Longcases. The simple bob hung on a wooden rod. This makes for a very adequate timekeeper, because of wood's relatively low coefficient of expansion. This means that if the clock's situated in a cold room, overnight, say, the pendulum rod will contract and the clock will gain time. Once it starts to warm up in the morning,the reverse will happen.
Wood, though not the best material for pendulums, is nevertheless a most adequate timekeeper. From there, though, we find a metal named Invar, or later, Elinvar. Now this is really first class, because the coefficient of expansion here is negligible.
The Royal Pendulum beats in seconds, and has a theoretical length of 39.14". It's named for the Royal Observatory at Greenwich. The theoretical length of such pendulums are 39.14", taken from the centre of the suspension spring from which they hang, to the centre of the bob.
The third type of pendulum came along in about 1715 and was invented by the English clockmaker, George Graham. He added a jar of mercury to the bottom of the rod. When the rod cooled down, the mercury contracted, effectively increasing the length to compensate, and of course vice-versa.
Then, in 1726, John Harrison of chronometer fame, (we'll discuss him later), came up with what became known as the Gridiron pendulum. The gridiron type, invented by John Harrison, carried rods made alternatively of iron and zinc, which expanded and contracted at different rates. Zinc expands more than iron, so the same sort of thing happened. The one medium expanded and contracted more than the other, thus keeping the length constant.
Now, high quality wall clocks may enjoy the same sorts of compensation as their bigger brothers, the grandfathers, and indeed good quality French mantel clocks very frequently have mercury pendulums fitted to them.
These, then, are the four basic types of pendulum. I haven't entered into the mathematics of pendulum calculation; there's an excellent source for that which I'll give later. My intention here has been solely to give you some idea of what the pendulum is and what function it plays in the clock.
I'm indebted to Wikipedia for some of the historical information in this article, and a lot of mathematical formulae may be found on their site.
About the Author
A metal named Invar, and later Elinvar, came on the market and this has virtually no coefficient of expansion whatever.
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