Although it was known as far back as the 17th century of the possibility of speech being sent along a wire, it wasn't until the 1870's that the idea of a sound converted into an electric current then sent along a metal wire took form. The word telephone comes from the Greek words `tele' which means far and `phone' which means voice.
One inventor, a Scotish-American, Alexander Graham Bell who was an expert in the field of physiology of speech, applied for a patent in 1874, on a technique for transmitting several telegraph messages concurrently along a single wire. This gave him scope to develop his idea and begin to convert speech into electrical signals.
Sound waves from the caller's voice are turned into electrical waves by a carbon granule transmitter. This consists of a metal disc called the diaphragm and a box which contains small granules of carbon, these are in light contact with the centre of the diaphragm. The sound waves make the diaphragm vibrate and this generates a change of pressure on the carbon particles. An electric current is then passed through the carbon and each change of pressure produces a corresponding change in the flow of the current.
The receiver reverses this process. It is a diaphragm fixed around its perimeter close to two electromagnetic poles. When a changing current travels through the electromagnetic coils, the draw on the diaphragm then vibrates and reproduces the original sound waves.
Traditionally telephone signals are transmitted via a wire or optical fibre network which carry the signals in the form of voltages or light pulses. International calls are carried via undersea cables or satellite links.
