Category Everyday Science

All electromagnetic waves travel at the speed of light – about 186000 miles (300,000km) per second. They are called electromagnetic because they consist of both electric and magnetic fields acting at right angles to each other. The fields leapfrog each other, giving the wave its motion like the snaking of a length of rope when it is jerked.

The height of a loop half the distance between the customer and the trough is called the amplitude. Waves can also be measured by their frequency, that is, the number passing a given point each second. The longer the wavelength, the lower the frequency.

Frequencies are measured in units called hertz, named after the German, Heinrich Hertz, who in 1888 demonstrated that electric signals could be sent through the air.

He passed a high-voltage current through a loop of wire that had a metal sphere at each end, causing a spark to jump the short space between them. At the same time, another spark jumped between the spheres of a separate, similar wire loop placed on the other side of the room.

Hertz proved that the energy transmitted from one loop to another was electromagnetic radiation, which had been predicted theoretically by a British scientist, James clerk Maxwell, in 1864.

The hertz measurement of a frequency gives a number of complete waves, or cycles, per second. Frequencies are usually expressed as kilo hertz (thousands of hertz), megahertz (millions of hertz) or gigahertz (thousand millions of hertz). Light waves are extremely short. The longest, the red, measure about 36,000 to an inch (14,000 to a centimetre) and have a frequency of around 100,000,000 MHz. Radio waves used for communication, however, range in length from about 1/25 of an inch of (1 mm) to about 18 to 20 miles (30 km), and have frequencies ranging from 10,000 Hz to about 30,000 MHz.

 

Picture Credit : Google

Although television is thought of as a 20th century invention, its beginnings date back to the 1880s. The first ideas about transmitting pictures over a distance were considered in the years following the introduction of the telephone. If voices could be sent over a long distance, why not pictures?

From the beginning, it was realised that pictures could not be sent as an entity, and ways of breaking down and then reconstructing a picture were suggested by a German inventor, Paul Nipkow, in 1884. Nipkow used spinning perforated discs to dissect and then resurrect a black-and-white image.

In 1906, Russian scientist, Boris Rosing, put together the scanning principle of the  Nipkow disc and the display possibilities of the cathode ray tube invented by a German, Ferdinand Braun, in 1897 to create the first crude television system. The cathode ray tube is still the vital component of modern television.

Experimental broadcasts were begun in America in in 1928, but the first practical television system was set up by the eccentric Scottish inventor John Logie Baird in London. He opened the first television studio in 1929, and used Nipkow discs for scanning in both transmitter and receiver. Within a few years, however, Baird’s mechanical disc – scanning system was overtaken by the electronic camera invented by the Russian Vladimir Zworykin, who produced the first practical one in 1931.

The first three day a week television service began in Berlin in 1935, operated by Fernseh, a German company with which Baird was involved. Britain BBC opened the first public high-definition service in 1936, and RCA began transmission in America in 1939. Colour transmission started experimentally in the USA in 1951.

Cable television began in the United States in the 1950s, with commercial company sending programmes to subscribers along cables. This allows more channels than radio transmission. In Britain in Europe, cable television did not arrive until the 1980s.

Sometimes cable television is also partly satellite television, programmes being relayed by satellite to company dish Aerials at a central station, then sent out to homes through the cables.

Other television systems introduced or under review in the late 1980s included microwave television carrying up to 60 channels over short distance, high definition television (HDTV) using over 1200 screen lines and direct broadcasting by satellite (DBS) to small domestic dish aerials. For this the transmitting company has to code the signal so that only a subscriber with the decoder on the set can receive them.

 

Picture Credit : Google

The coming of the computer and the exploration of space sparked the need for increasingly complex yet small, durable, and often remotely operated controls. This has brought about the age of microelectronics, which began in the 1950s centred around the transistor and silicon chip. It is a silicon chip that is the heart of the remote control that you see use to switch on your television set from the armchair.

When you press the button on the remote control, the chip which contains a microelectronics circuit sets off an electronic oscillator (vibrator). This produces an infrared beam, which is made up of electromagnetic waves.

The beam carries a coded signal, the code varying according to the button pressed to switch on, change channels, or raise volume, for example. The code based on binary digits is superimposed on the beam in the same way that a radio signal is superimposed on a carrier wave.

In the television set, the coded beam is received by a device sensitive to infrared waves. The incoming signals are amplified and fed to another silicon chip that identifies the code. The chip then feeds the appropriate signal to electronic switches that carries out your instruction.

Ultrasonic remote controls can be used to open or close the garage door. They emit high-frequency sound waves that are directed to receiving microphone. This send signals to an electric motor that operates the doors. However, the ultrasonic control must be operated in the direct line to the doors, so radio control is no more often used. A hand-held radio control is a miniature transmitter that can open garage doors fitted with a receiver from anywhere in the vicinity. The radio waves switch on an electric current to the motor that operates the doors.

A more complex radio control system is used to operate model aeroplanes and boats. The hand held transmitter sends out beams of coded radio waves. A miniature receiver on the model decodes the signals, separating them from the radio waves. The decoded signals are fed to tiny electric motors, called servos (short for servo-mechanisms, which increase their power). The servos open and close the engine throttle, raise and lower the landing gear, and operate the control surfaces such as ailerons and rudder – on the wings and tail.

 

Picture Credit : Google

A home Video recorder picks up electrical signals from the television station at the same time as your television set. But instead of converting the signals directly to pictures, the video stores them on magnetic tape in the same way that a tape recorder stores sound signals. Because television signals carry pictures as well as sound, home video tape is generally four times the width of a sound cassette tape.

Is the video recorder is connected directly to the aerial, it picks up television broadcasts when switched on, whether or not to the television is on. Both can be turned to pick up different programmes at the same time.

The two main videocassette a recording system is available are Betamax, introduced by the Japanese company Sony and in 1975, and VHS (video home system) pioneered by JVC (Japan Victor company) in 1976. Each system needs different cassettes different recorders. Betamax produces slightly better quality pictures, but VHS tapes can run longer up to 4 hours. VHS has proved the more popular of the two and new Super VHS has better quality pictures than either of the standard systems.

Recording and playback

When a video tape cassette is fitted into the video recorder and the record button pressed, the machine draws a loop of tape from between the two reels in the Cassette and wraps it round a rotating drum driven by an electric motor.

The picture recording heads, usually two, or mounted on the drum facing outwards, and imprint the signals on the tape as they rotate with the drum. The heads are tiny electromagnets, and operate in the same way as for sound a tape recording.

The tape runs past the drum at an angle. The picture signals are recorded in the central area as a series of sloping tracks, and the accompanying sound signals are recorded as lengthways tracks along one edge of the tape.

As with the tape recorder, playback is a reversal of the recording process. When the tape is loaded and the play button pressed, the stored signals on the magnetic tape produced electrical signals in the playback head. This feeds the picture and sound signals to the television set, where the recording is recreated on the screen.

 

Picture Credit : Google

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.

 

Picture Credit : Google

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.

 

Picture Credit : Google