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 kind of time standard for measuring the passage of time, and
• Some kind of display for displaying the measured time.

In principle you can mix and match the two, so for the purposes of explaining basic principles it's worth considering them separately.

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 simple fact that if a mass is subject to a force tending to restore it to an equilbrium and in direct proportion to its distance from that equilibrium position, it will execute "Simple Harmonic Motion". This is characterised by an oscillation or vibration at a well defined frequency, independant of the amplitude of the motion. So in principle a wind-up clock won't run slower as it runs down.

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

Very rarely you might find a clock which uses a tuning fork kept in vibration electronically instead of a pendulum or balance wheel as the time standard.

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 piezoelectric 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 is carefully cut and ground so as to "ring" at a specific resonant frequency and is 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 halving it 15 times successively gives a 1 second tick. This is easily done with simple electronic circuits. An advantage of such a high frequency is that you just have to count the oscillations in 1 second and check there are exactly 32,768 (not even just one more or one less) to assure the accuracy of your clock to one second in 9 hours! If a clock ticks just a few 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.

Atomic clocks

These are the most accurate clocks available, typically gaining or losing no more than the equivalent of 1 second in 30 million years, but they are very expensive and usually very bulky. Chip-scale versions are now available but these are still by no means cheap.

Atomic clocks depend not on a mechanical vibration but on an atomic resonance, most usually in caesium or rubidium atoms. In fact the second is now defined as 9,192,631,770 oscillations in a caesioum-133 atom. National and international time standards are based on the averages of a number of atomic clocks, and it is these that all the remaining time standards are based on.

Radio Time Signals

In the UK, the National Physical Laboratory's time standard is based on three atomic clocks, and is broadcast as the "MSF Signal". This can be received across much of northern and western Europe. Numerous other radio time signals are available in different parts of the world.

Domestic clocks which automatically synchronise with one of these radio broadcasts are widely available, sold as "radio controlled clocks", or even "atomic clocks" though they are actually using someone else's atomic clock!

AC Mains

The AC mains supply has a nominal frequency of 50Hz (or 60Hz in some regions) and although the frequency can vary slightly, the total number of cycles in as day is very carefully controlled. The reason for this is that if demand exceeds supply, all the generators naturally tend to slow down under the load, so reducing the frequency, or speed up in the case of over-supply. Hence the frequency is used as a vital tool in matching supply to demand in the National Grid.

A useful consequence is that the mains supply can be used as a time standard with guaranteed long term accuracy, though like several others, it's in reality a 2nd hand atomic clock.

GPS

Satellite navigation depends on a constellation of satellites each with its own on-board atomic clock and transmitting a signal containing encoded time information. Consequently you can not only get your geographical position by receiving these signals, but also an extremely accurate time.

Internet Time Sources

Your computer, smartphone or tablet contains a quartz crystal from which it determines the time on a continuous basis, but this is regularly synchronised with time sources available on the Internet. NTP (Network Time Protocol) is the means by which any computer on the Internet can request the time from a chosen time server and estimate the correction required to account for the transmission delay.

NTP time sources are each given a "Stratum" classification. A Stratum 0 reference is an actual atomic clock or equally authoritative time source, and this feeds a Stratum 1 server, accessble on the Internet. An organisation might have its own Stratum 2 server which periodically queries a Stratum 1 server, and in turn serves Stratum 3 devices, which might be emplyees' PCs.

Time Displays

Analogue Displays

We all learned at an early age to tell the time from an analogue clock face with a minute and an hour hand, and for centuries, this has been the standard method of displaying the time. Imaginative novelty variations on the idea are sometimes seen.

Digital Displays

Electromechanical digital displays have been around for many years, the commonest form being the flip-down display. A series of cards flip down one after another, each card containing the top half of a digit on one side and the bottom half on the reverse. These clocks are now often collector's items or cherished heirlooms.

Nixie tubes were used as an early form of digital display. A Nixie tube consists of a glass envelope containing neon gas and 10 digits, each formed out of wire. Any one of these can be illuminated by applying a voltage to it, causing it to light up with the characteristic orange neon glow. Special driver circuits are required as Nixie tubes need around 150V to work. Nowadays these clocks are purely a novelty item, popular amongst the maker community.

Modern digital clocks generally use a familiar 7-segment display. The entire display may be formed as an LCD panel, often including additional symbols for AM/PM or an alarm indication, or each segment may be lit by an LED. LCDs and LEDs have the advantage of working on similar low voltages to the logic circuits driving them.

Vacuum flourescent displays (VFDs) are most often seen on older devices such as video recorders. The flourescent segments are housed in a glass envelope and are made to glow by a stream of electrons, often with a blueish green colour. They tend to use higher voltages than most logic circuits but not as high as Nixie tubes.

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.

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.

External links

• Watch and Clock Movements at Electropedia gives easily understood explanations of everything from pendulum clocks to atomic clocks.