Clocks

This page is a stub. Eventually it will cover various types of clock.

Summary

Clocks have seen many changes with evolving technology.

Safety

You may miss your bus if your clock is wrong. This could really spoil your day.

Basic Principles

All clocks can be considered to be composed of two parts:

• Some means of measuring the passage of time, and
• Some means of displaying the measured time.

Since to some extent you can mix and match the two, it's worth considering them separately for the purposes of explaining basic principles.

Time Standards

Mechanical

Pendulums and balance wheels are the oldest accurate means of measuring the passage of time. They were refined in the 18th Century most notably by John Harrison, spurred by the need for an accurate clock for navigation at sea. All these clocks rely on the fact that a mass subject to a force (of gravity, or a spring) directly proportional to its displacement from its equilibrium position executes Simple Harmonic Motion in which it oscillates or vibrates at a well defined frequency.

In practical clocks, an escapement is needed to give the pendulum or balance weel the regular kicks needed to keep it in motion indefinitely. This can be powered by a spring or in the case of a grandfather clock, by weights. Occasionally you might find an electrical mechanism in more modern clocks.

Quartz crystals

These in fact also depend on a mechanical vibration, but sustained electronically and at a much higher frequency.

Quartz is an example of a peizoelectric material, that is, an electical charge appears on opposite faces if you stress it, and conversely, applying a voltage will cause it to deform slightly. Using this property, a piece of quartz carefully cut and ground to have a specific resonant frequency can be kept in oscillation electronically.

Quartz crystals used in clocks very often have a resonant frequency of 32,768Hz. This might seem an odd number until you realise that dividing it by two 15 times (easily done with simple electronic circuits) gives a 1 second tick. An advantage of such a high frequency is that by just counting the oscillations for 1 second to see that there are exactly 32,768 and not even just one more or one less, you have already assured the accuracy of your clock to one second in 9 hours! If a clock ticks, say, only 5 times a second, you would have to count the ticks for a much longer time to achieve the same accuracy.

Quartz crystals provide a more accurate and more stable time source than a mechanical clock but nevertheless they are slightly temperature dependant. Those used in cheap clocks typically have an accuracy of a few seconds per day. Fine adjustment is possible by connecting a variable capacitor in the circuit with the crystal. With temperature compensation an accuracy of a few seconds per month is possible.

Clockwork clocks

These are the oldest type of clock and are purely mechanical. After many years of service they often stop working because of wear in the bearings and gears. Repairing these is a highly skilled operation, if possible at all.

Electrically-assisted clocks

These have a pendulum or a balance wheel like a clockwork clock, but it's kept in motion electrically instead of by a spring.

Mains synchronous clocks

These run off the AC mains supply and rely for their accuracy on the fact that although the mains supply can vary slightly from its nominal 50Hz (or 60Hz in some regions), the total number of cycles in as day is very carefully controlled. (This is because if demand exceeds supply, all the generators tend to slow down under the load, or speed up in the case of over-supply. Hence the frequency is used as a vital tool in manageing the National Grid.)

Quartz clocks

These rely on the vibrations of a crystal of quartz for their time keeping, rather than a balance wheel or pendulum.

Radio-controlled clocks

These generally have a quartz clock mechanism for back-up timekeeping, but get an accurate time reference from a radio signal, such as MSF Rugby.

Flip-down clocks

These are the earliest widely available form of digital clock. The hours and minutes and possibly the day of the week and day of the month are displayed on flip-down cards.

7 segment display clocks

These may use a quartz crystal, radio signal or the 50Hz mains supply for the time standard.

Other types

All the commoner types of clock are listed above but there are others you might occasionally come across or may have heard about. These will either have a different time reference or a different type of display, or both.

GPS clocks

There are now several satellite based global positioning systems in addition to the original US GPS system, and they all rely on highly accurate clocks. A GPS receiver can hence provide an accurate time source just as eaily as a positional fix.

Internet clocks

Time sources are available on the Internet, and it is these that your computer or smartphone uses as the reference to periodically correct its internal quartz-based time source.

Atomic clocks

These are the most accurate time standards available but they are far too expensive for domestic use, and until recently, too bulky.

Nixie tube clocks

Nixie tubes were used as digital displays before LED-based 7-segment displays became widely available. A nixie tube consists of a glass envelope containing neon gas and 10 digits formed out of wire. Any one of these can be lit up by applying a voltage to it, causing it to glow with the characteristic orange neon glow. Nowadays these clocks are purely a novelty item, but popular amongst the maker community.

External links

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