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Communicating over distance

Fire emergency leaflet, 1890s

Signalling by sound : within earshot

Some types of signals are based on making a noise that can be heard a long way off. The air raid sirens of the Second World War were a way of signalling by sound - using a rising and falling siren to tell people that enemy bombers were on the way and a steady note to give the 'All Clear'.

Other types of sound signals include bells, horns, guns, or the tom-tom drums used by tribes in Africa, New Guinea and South America to pass messages.

The trouble with sound signals is that the range is so uncertain. In clear country and with no other background noise, a gun or siren could be heard several miles away. But in a noisy city, with trees or buildings in the way, the sound might not be heard at all.

South Goodwin Lightship

Visual signalling : as far as the eye can see ...

For thousands of years, people have used visual signalling systems such as flags, semaphores, lights, fires, smoke and sunlight reflected by mirrors (the heliograph).

If the signal is big or bright enough it can be seen over 20 miles away - the maximum distance is achieved when both the sender and receiver are standing on tall buildings or hills (the natural curvature of the earth is the limiting factor).

But that is on a clear day. In heavy rain, snow or fog visibility may be restricted to only a few hundred metres. (Messages sent by optical telegraph from Whitehall were often sent by horse to the first station outside London, the visibility in central London was so bad!)

Double Needle Telegraph; Cooke and Wheatstone - an artefact now held by the Museum of Science and Industry in Manchester

Signalling by electricity : along a wire

The first form of modern telecommunication - the electric telegraph - sent electrical currents along wires. That basic technology is still being used today - but in vastly more sophisticated forms.

You can send signals over hundreds or thousands of miles this way - but the range is not unlimited. The electrical resistance of the wire saps signal strength, so that eventually it will be too weak to be picked up at the receiving end.

Getting around this problem means using stronger currents, thicker wires that are more 'conductive' (allow the current to pass more easily), or amplifiers at intervals along the line to boost the signal's strength.

The other limitation with signalling in this way is that you need to put a physical piece of cable between the sender and receiver. That's almost always hard work and expensive - and sometimes impossible. The alternative is to use radio.

Ship to Shore wireless leaflet (radio communications), 1936

Signalling by radio wave : a wireless world

When you can't put a wire between two places, you can use wireless. It is possible to send signals through the air, using electromagnetic radiation in the form of radio waves. Radio can also connect people who are moving around - on a ship, on foot or in a vehicle.

In the right circumstances radio signals can be sent and received across great distances, but the range is not unlimited. Radio waves decline in strength over distance and are subject to interference from other sources.

Directing radio waves accurately is not always feasible; they tend to travel outwards along straight lines from the transmitter and may not follow the curve of the Earth.

London Teleport dishes, 1980s

Signalling by satellite : the orbiting reflector

To send a radio beam in a truly straight line across the Atlantic would need incredibly high masts; as it was Marconi had trouble building a mast 60 metres high for his first transatlantic transmission. In the end, he was able to 'bounce' his signals off the electromagnetic layers of the atmosphere. But this only works for certain wavelengths - and would be impossible on the frequencies used for TV signals, which are not reflected back to earth under normal conditions.

The way around this is to bounce the signals in a more organised way, by using satellites as mirrors out in space. The satellites take in the narrowly beamed signal and then direct it back to Earth - so that anyone with a dish and suitable receiver in the satellite's coverage 'footprint' can receive it.

Communicating over distance - Invasion game

Communicating over distance - Invasion game
fun and games

Can you beat our games? Explode equipment to see what's inside, hear the changing sounds of telecommunications, see how telecommunications designs have changed over time or send an e-postacard.

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