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3. Everything / Maths, Science & Technology / Inventions
3. Everything / Maths, Science & Technology / Physics

How Mechanical Clocks Work

An old mechanical clock

Clocks. Aren't they just great? Without them, the Industrial Revolution might never have happened. It was only when time could be split into pieces and rationed that we could be told what to do by Big Ben. Without the accurate measurement of time we probably wouldn't have had the Spinning Jenny and all that malarkey and we wouldn't all be bound for all eternity to a repetitive 9 to 5 life. Three cheers for the timepiece!

In order for a clock to work, constant movement has to be achieved in some way. As Einstein and Newton were all too aware, perpetual motion is what is known as, in scientific terms, 'a bit of a palaver'. In fact, it's impossible.

History

The search for a way of keeping accurate time is driven by the desire to travel. Travelling requires navigation. You can tell how far north/south you are by how high the sun gets, but there is no easy way to tell how far east/west you are. However, if you know that it's midday where you are, and you know what time it is somewhere else (let's say Greenwich for argument's sake) then you can work out that every hour difference takes you (360/24=) 15 degrees round the earth. This was probably the greatest change that accurate clocks allowed - being able to sail anywhere on the earth's surface and know where you are.

Galileo was a rather clever chap who invented modern physics1 by simply measuring the speed of falling bodies. However, using his pulse and heartbeat as a guide, he conducted a series of experiments rolling balls down the groove of an inclined plane and concluded that, since this took a number of seconds, it would be impossible to measure seconds.

One potential way of measuring time might be by setting up a drum with a cord attached and wound around it rather like a cotton reel. If a weight is attached to the end of the cord and allowed to drop, 'unwinding' the drum, then the drum will spin for a specific time. If a hand were to be attached to the top of the drum then the weight and cord could be measured so that one spin of that hand could measure out one second.

However, this crude device is still clearly nowhere near as reliable or attractive as a Swatch. The main problem is that the wheel spins too quickly and some sort of friction device, such as a brake pad, is required to slow it down to a precise measurement, such as one revolution per minute. However, friction itself is difficult to regulate and alters depending on factors such as temperature and air humidity.

The Pendulum

Galileo Galilei, a sort of Sir Clive Sinclair2 of his day, had the answer, once again. Watching swaying chandeliers in a cathedral and timing them against his pulse, he concluded that the 'pendulum'3 was a way of marking off small intervals of time more regularly. Still, it wasn't until 1657 that Dutch astronomer Christiaan Huygens would exploit Galileo's discovery by employing the pendulum in order to create a clock.

The factors that affect the operation of the drum are not at play with the pendulum. The period - how long it takes a pendulum to go back and then forth once - of the swing of a pendulum is related to two factors: the length of the pendulum and the force of gravity. Gravity is always constant, so only the length affects the period. The weight of the pendulum and the length of the arc it swings through do not matter.

Therefore, the pendulum can form the basis of an accurate clock, connected to a device called an escapement, which consists of a wheel with jagged teeth that are shaped in a way that enables them to be engaged by an attached device known as an anchor. For each swing of the pendulum back and forth, one tooth of the gear is allowed to 'escape' and so the wheel moves along at a constant speed. This 'escapement' is what creates the ticking sound of a clock or watch.

The pendulum will not swing forever, and so the escapement has a dual purpose - to impart enough energy into the pendulum to overcome friction and to allow it to keep swinging. The anchor (the device which engages the teeth) is shaped in such a way that each time it engages one of the teeth on the wheel, it gives the pendulum a nudge. This boost of energy enables the pendulum to overcome friction and keep swinging.

Therefore, if we were to give an escapement 60 teeth and attach it to the weight drum with which we began (the sort of cotton reel device with a hand on it) and using a pendulum with a period of one second we can create a one-handed clock - the hand of which spins at one revolution a minute.

However, two problems remain: this device only allows us to measure minutes, and this 'clock' would need to be wound every twenty minutes as the weight would reach the floor very quickly.

Gears

To sort out the problem of constant winding, a gear train - a series of interlinked wheels - needs to be established. If the ratio is high, the drum can apparently turn once every six to twelve hours and be rewound just once a week. A gear ratio of 500:1 is required, and the escapement gear attached to the second hand needs to have 120 teeth and the pendulum a period of half a second. Each of the gears in the weight's gear train has a ratio of 8:1, making the full train's ratio 492:1. If the escapement gear drives another gear train with a ratio of 60:1 then a minute hand can be attached to the last gear in that train. A final train with a ratio of 12:1 would have the hour hand attached.

We now have a clock but its hands are all on different axes. This problem can be remedied by having tubular shafts, aligned one inside the other, on the gears, which can then be arranged so that those driving the hands are on the same axis. This arrangement can be seen on a clock face. Finally, a setting mechanism - usually a gear that can be slipped out of the train - means that the clock can be rewound and set.

So there we have it. In a watch, the pendulum is replaced by a weight bouncing on a spring or a wheel with a spring on its axle (which causes the wheel to rotate back and forth on its axis). The weight and the spring both store energy.


1 Laying down, with effortless panache, the foundation for Newton's laws of motion and theory of universal gravitation as well as the basis of Einstein's later work.
2 Sir Clive gained quite a reputation in the latter half of the 20th Century for his reasonably-priced inventions. These ranged from the first pocket calculator, the first pocket television, and the Sinclair Spectrum Computer - but he is at least as well known for his spectacular failures, most notably the C5 electric vehicle.
3 Actually it's pretty hard to identify who actually discovered the pendulum. Some say it was Galileo, others say Mersenne, Foucault, or Christiaan Huygens.

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Entry Data
Entry ID: A734988 (Edited)

Written and Researched by:
U190181

Edited by:
U158291


Date: 28   May   2002


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Referenced Guide Entries
Time - Concepts and Perceptions
Newton's Laws of Motion
Physics and the Knowledge of Ignorance
Time Dilation
A Chronology of Time
Gravity
How to Pass Time
Mersenne Numbers
Constant-mesh and Synchromesh Gears
Galileo Galilei - Father of Modern Science


Related BBC Pages
Huygens' Clock Puzzle Solved
Cuckoo Clock Crazy
John Harrison's clocks


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