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Pendulums Have a Swinging History

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Published: 12May2009
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We've had a look at the gearing of a clock. We've seen how this links to the Escapement. Now let's tie it all together by examining the clock's timekeeper. The Pendulum.

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. The great Christiaan Huygens took Galileo's work on from there and finally developed the pendulum clock.

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.

Apart from friction in the gear train, the main enemy of timekeeping is temperature. 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 is a good medium and often underrated. 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.

Incidentally, we'll assume that all the longcase pendulums beat at one second intervals and are known as Royal Pendulums, from 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. This consisted of a number of rods, in line abreast, made from iron and zinc. 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.

Wood is a good medium and often underrated. 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. http://www.theclockssite.com

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