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.