The name we give to the sending of messages over a distance is telecommunication. The messages are sent from something called the transmitter, and at the other end are picked up by the receiver. So that a message may be sent from transmitter to receiver. it first has to be changed into electrical signals. In the telegraph this was done by the Morse tapping key. In the telephone and radio it is done by a microphone: and in television by a TV camera. Once the message has been turned into an electrical signal. it is then added to a high frequency radio wave. This can transmit it most easily. The radio wave is called a carrier wave because it carries the electrical signals (our message) from the transmitter to the receiver. In radio or television. the carrier wave is usually sent through the air. The process of adding electrical signals on to a carrier wave is called modulation. It is possible to send many different frequencies of carrier wave along cables or through the air at the same time. This is called multiplexing. Multiplexing, which is very important. makes possible the sending of several thousand two-way telephone calls along one cable at the same time.
At the receiving end of any telecommunication system. special electronic circuits sort out the electrical signals, which are our message. from the carrier wave. These signals are then amplified and sent to the receiver. This might be your 800 series PMG/Telecom phone handset, a transistor radio or a plasma television screen.
The handset of an old 802 PMG telephone has both a transmitter in the mouthpiece. and a receiver in the earpiece. The mouthpiece used to contain a carbon microphone in old retro phones, especially the Bakelite GPO phones. This was a thin sheet or diaphragm at the front which is in contact with a number of lightly packed carbon grains. As you speak into the microphone. your voice vibrates the diaphragm. This packs the carbon grains more tightly. or less tightly together. When the grains are closer together. an electrical current passes through them more easily. When the grains are more loosely packed. the current does not pass through so easily. As you speak, the diaphragm moves. This in turn causes the grains to move and thus produces a varying electrical current related to the voice signals. These electrical signals may then be added to a carrier wave and sent along a cable.
At the receiving end of the telecom telephone line. the electrical signals are sorted out from the carrier wave. and enter the earpiece of the 800 series telephone and person you are calling. Here the electrical signals are turned into sound waves again. These sound waves are heard by your ear. They are an exact copy of the sounds spoken into the mouthpiece at the other end of the line.
To contact another telephone number, you first pick up the 802 ACF handset from its cradle. As the 802 PMG/Telecom handset cradle rises. a switch closes and makes an electrical connection between your telephone and the telephone exchange, at least thats how it used to happen in crossbar AXE exchanges. The Crossbar or step by step Phone exchange is a building full of electronic circuits, to which all the telephone lines in your area arc connected. It is the job of the exchange to connect telephones to one another. But how does the exchange know who you want to call? First you must rotary dial the number.
When you dial a number with a vintage rotary dial telephone the circular dial plate rotates a set of cogs inside the telephone. These open and close a pair of electrical contacts. As you dial each number. the contacts open and close and produce a number of electrical pulses. In the more modern push-button phone. an electronic circuit remembers the sequence of pulses produced and then sends them. one after the other to the exchange. When the electronic circuits in the exchange receive the pulses. they operate a series of switches which link your telephone with that of the person you are calling. Sometimes this may mean that the line is connected through several different exchanges. Modern telephone exchanges use integrated circuits to switch the telephone lines. This means that they make faster. more silent and more reliable connections. Electronic telephone exchanges can handle many thousands of calls at the same time. Most exchanges are completely automatic. even for international calls from one country to another.
Most telephone cables are put under the ground. Many have been laid beneath the oceans of the world to connect continents. The cables are usually made of copper or aluminum. These metals are becoming more difficult to obtain. and more expensive. However cables are being replaced by bundles of fibre optics.
A fibre optic is a very thin tube of silica glass. Its diameter is between thirty and one hundred millionths of a meter. This is similar to the thickness of a human hair. With a cable made of fibre optics. telephone messages would no longer be changed into varying electrical signals. Instead. they would use a light source of changing brightness. This varying beam of light would pass along the fibre optic at loomoo km per second. Not only is this extremely fast. but the light beam is not affected by the electrical noise which may spoil the normal telephone conversation. Fibre optics will greatly reduce the number of cables needed for very complicated telecommunication systems. Satellites orbiting above the Earth are being used more and more for long distance telephone communications.
