Category How does It works, How things work, How is it done, Curiosity

A German physicist named Heinrich Hertz first demonstrated in 1888 that it was possible to transmit electrical energy through the air.

Between 1894 act 1896, the Italian scientist Guglielmo Marconi developed a method of using Hertzian waves to send signals in Morse code – a method that became known as wireless telegraphy. By 1901 Marconi had improve the system so much that he was able to send wireless telegraph signals across the Atlantic from Cornwall to St John’s Newfoundland.

A Canadian engineer made the world‘s first public radio broadcast from Massachusetts, USA, heard by ships around hundred miles (160 km) away on Christmas eve, 1906. He was Reginald Aubrey Fessenden, who had a found a way of combining the signals from a microphone with an electromagnetic waves. The name radio was given to the method.

At first, listeners had earphones linked to receivers that used crystals to pick up the radio waves. These eventually give way to sets with loudspeakers, diode value (invented by an English man, John Ambrose Fleming, in 1904), and more powerful electronic circuits following the American Lee de Forest’s invention of the triode valve in 1907. With the earliest valves (used to amplify signals), sets had to be switched on to warm up for five minutes before the programme begins.

Regular public broadcasting did not begin until 1920, from the radio stations in Pittsburgh and Detroit. Edwin H. Armstrong, an American engineer, improved the receiver in 1924, and by the late 1950s, compact transistors were replacing bulky valves.

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Until the 1970s, most the telephone calls were transmitted as electric signals corresponding to the vibrations of the voice. These are known as analogue signals because they are analogues – similar in structure – to the sound. Electrical interference in the transmitting circuits can distort voices.

After the 1970s, the analogue system began to be replaced by a digital system which cuts out most interference and distortion. The analogue electrical signals from the microphone are changed to binary numbers in electronic circuits at the exchange and transmitted in coded form.

To do this, the wave heights of the electric current are measured thousands of times every second. The measurement is expressed as a sequence of the digits one and zero. Current is then converted to a series of pulses – a flow for 1 and a break in-flow 0 – representing the wave measurements. This is known as pulse-code modulation (PCM). As each pulse is very short, the pulses of one telephone message can be interleaved between the pulses of others.

This technique of time multiplexing allows 32 simultaneous calls to be sent along a single pair of wires, or thousands of messages to be sent at once along the same coaxial cable.

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Two wires, or conductors are needed to complete the circuit between the telephone transmitter and receiver. Some exchange cables carry thousands of pairs of wires.

 If every call needed a separate pair of conductors for transmission throughout the telephone network, the simultaneous transmission of thousands of calls from one exchange to another would be unmanageable. A pair of ordinary copper wires can be made to handle only a limited number of calls at once because they are designed for low-frequency current. Higher frequencies would allow more simultaneous calls, but unless a different design of cable is used, the signal radiates away and loses strength.

Most trunk lines between telephone exchanges are now coaxial cables, in which the signal is confined to prevent loss of strength and interference. Instead of a pair of wires, each coaxial cable has a central copper wire with an outer copper conductor that sounds it like a sleeve. They can handle high frequencies and carry thousands of calls. Built in amplifiers boost the signals about every 1¼ miles (2 km).

Using a technique known as frequency multiplexing, the electric signals corresponding to the voice sound waves are modulated – that is, combined with an electromagnetic carrier wave in the same way as radio waves. A number of carrier waves of different frequencies are then sent along the same pair of conductors.

At the receiving exchange, the signals are separated from the carrier wave by a process called demodulation. The other then filtered to the correct receiver.

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When you lift the receiver and complete the circuit to the exchange, dialling the number sends a series of electrical pulses down the line. Older telephone exchanges have automatic electromechanical switch gear, named after the American, Almon Strowger, conceived it in 1888. This has banks of fixed contacts, each in a half circle round of mobile selector arm.

The number is selected step by step. The first dialled digit sends the arm up to a bank corresponding to the digit. The arm then rotates to find a free contact – one that will connect it to the next bank ready for the next digit dialled. If no contact is free, the engaged tone is sounded. If contact is made, the next selector arm searches for that second digit, and so on. The final selector makes contact with the line of the number being called.

If the selector accidentally touches and sticks on an incorrect contact for the digit dialled, you get crossed line.

The latest telephone exchanges work electronically. Dialling sets up audible tones, and connections are made by circuits incorporating microchips that interpret the tones. Because there are no moving parts, electronic switching is silent and more reliable than electromechanical gear and crossed lines are rare.

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Over 500 million telephones are now in used throughout the world. In just over hundred years since the Scottish born inventor Alexander Graham Bell patented the first telephone in 1876 – telephones have revolutionized world communications.

Today, telephone networks relay not only voices but pictures and written information as well, by land and sea cables and through the air on microwaves, which are super-high-frequency radio waves. Calls can be made across half the world with less than a second delay in connection, and no difficulty in hearing. Multinational companies can even hold cross world video conferences, with executives speaking to each other from one screen to another.

Satellites, microchips and lasers

The modern inventions that have made this revolution possible include space satellites, microchips and laser beams. Early bird, the world’s first commercial satellite, was launched in 1965 by the International Telecommunication Satellite Organisation (INTELSAT).

Now there are about 130 satellites orbiting in space, relaying messages on microwaves from Earth Station to Earth Station. The orbit the earth at heights of about 22,500 miles (36,000 km) above the equator once every 24 hours, so appear to remain in the same place.

From the earth stations, microwaves carrying messages are beamed up to the satellites from huge dish aerials, some of which are 98ft (30 m) across. They are computer controlled so that they will always point directly at the satellite. Microwaves are not only used for satellite links – dish Aerials beam messages across land too, in straight lines from tower is located to ensure a clear path.

Microchips on the satellites amplify the relayed signals. Microchips have also brought about clearer, speedier communication by providing the fastest switching needed for sending telephone messages by digital transmission. And lasers have enabled the use of fibre optic cables – glass thread that carry digital messages at the speed of light, so fast that they could go seven times round the earth in a second.

Telecommunications services now available include fax, radiopaging, cordless telephones, car telephones and even aircraft telephones, allowing passengers to make calls while flying.

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When your clock radio starts playing music first thing in the morning, or the oven automatically comes on to cook a meal, the switch has probably been operated by a digital clock.

At the heart of the switch is a quartz crystal which vibrates at a fixed frequency when connected to a source of electrical power – battery or mains. The vibrations produce regular electrical pulses, which travels through circuits in a microchip to operate the digits on the clock.

The switch also has a memory, in the form of a microprocessor, which stores the time when the radio, oven or central heating system has to be turned on. The microprocessor constantly compares the stored time with the real time as measured by the clock.

When the turning on time comes, it sets off a low-voltage electronic signal. This signal is amplified by a transistor circuit and flows through a relay, an electronic device in which a small current causes a metal contact to move, switching on the main current.

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