TELECOMMUNICATIONS
Telecommunications is the sending and receiving of information using electricity, radio waves or light. The information can be sound, television pictures or computer data (which itself can be numbers, words, sounds and images). Forms of telecommunications include the telephone, fax, two-way radio, television and radio broadcasting, and the Internet. Most of these forms of communication require transmitting and receiving machines, and a network to link them together.
The first telecommunications device was the telegraph. Messages travelled along wires from a sending device to a receiving device as pulses of electricity, using some sort of code that both the sender and receiver understood. Practical telegraph systems were developed in the first half of the nineteenth century, and were first used for railway signalling. Early systems needed several connecting wires, but the system that eventually became standard, developed in the USA by Samuel Morse, needed just one wire. A network of telegraph lines, including undersea cables across the Atlantic, was quickly established right around the world.
In the early 1900s the telegraph was automated so that machines turned the message into code and back again. The sender could type messages on a keyboard and they would be printed out at the receiver’s end. To send a telegraph message, people had to visit a telegraph office. The message arrived at another office and was delivered by hand to the recipient.
The next major step in the development of telecommunications was the invention of the telephone, which could transmit speech, allowing people far apart to talk to each other. The first telephone receiver (the part that you talk into and listen to) was patented in 1876 by Alexander Graham Bell. This device both turned the sound of the user’s voice into an electrical signal, and an incoming signal into sound, which meant that the user could not talk and listen at the same time.
When the telephone was invented, there was no telephone network to link telephones in different places, but one soon grew up. All the telephone lines in an area meet at a telephone exchange, where they can be connected to one another, or to a line to another area’s exchange. The first exchange, opened in 1878 in Connecticut, USA, had just 21 lines. Like all early exchanges, it was operated by hand. A subscriber had to tell the operator which line he or she wanted to be connected to. The automatic exchange, which allowed people to dial numbers, was invented in the USA by Almon Strowger, and started working in 1897. Meanwhile, complex telephone networks grew in large cities. It took longer for different cities and countries to be linked, and until the middle of the twentieth century, the telegraph was still used for long-distance communication.
HOW A TELEPHONE WORKS
All telephone receivers are linked to a telephone exchange by a telephone line. When you lift or turn on the receiver, electronic circuits at the exchange detect it and wait for a number to be dialled. As you dial the number, the receiver sends signals to the exchange, which uses them to make a connection to the line of the person you are calling. The exchange makes the other telephone ring, and when it is answered, it connects the two lines together.
When you speak into the receiver’s mouthpiece, the sound makes a thin metal plate called a diaphragm vibrate. This movement affects the strength of an electric current, creating an electrical copy of the sound, which is called a signal. The signal travels through the connections in the telephone network to the other receiver, where it operates a tiny speaker in the earpiece, recreating the sound.
The signal travels in digital form for most of its journey through the network.
Picture Credit : Google