My Science School

Pure silicon is an insulator — it does not conduct electric current However, if it is impure — containing certain other elements — it will conduct a weak current. So it is called a semiconductor, halfway between an insulator and conductor. Semiconductors allow the delicate control of current needed for demonic devices, such as transistors, to an extent impossible with full conductors such as metals. A semiconductor is made by adding elements - usually phosphorus or boron — to the silicon. if a small amount of the phosphorus is introduced as a gas while the silicon crystal is being formed into a chip, the phosphorus atoms bond together with some of the silicon atoms. Four electrons in the outer layers of each type of atom pair off, but one phosphorus electron is spare, so it is left free to form an electric current when a voltage is applied. Electrons are negatively charged, so this type of crystal is called an n- type (negative) semiconductor.

If small amount of boron is mixed with the silicon, there is one electron short in the bonding system, leaving a hole that attracts five electrons. Free holes create a positive charge so the crystal is called a p-type (positive) semiconductor. These two types of semiconductor are formed in sections within one crystal for most microchip components.

 

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Within an area no bigger than a shirt button, a microchip holds as many as 450,000 electronic components. They are linked into electric circuits and are visible only under a microscope.

Microchips have transformed modern life and made some of the science fiction of the past into reality. They regulate digital watches, set programs on washing machines, and beat us at video games. They also manipulate robots on car-production lines and control national defence systems.

Electronically, the circuits that make up a microchip are not particularly complex —many are just switches. Their wizardry lies in their minute size, which allows signals to flow through at lightning speed. So they can carry out up to 250 million calculations in a second.

Most microchips are made of silicon, one of the most abundant elements on earth, and easily obtained from sand and rocks. A few are made from gallium arsenide — a compound of arsenic and the metal gallium, found in minerals such as coal.

 Chips for everything

There are various kinds of microchip. A microprocessor chip can be a computer in itself - in a washing machine, for example. Or it can be the nerve centre of a larger computer, controlling all its activities.

Memory chips store information in computers on sets of identical circuits —either permanently or temporarily. Interface chips translate the signals coming into the microprocessor from outside — such as from a keyboard — into binary code so that the electronic circuits can handle it. They also translate the outgoing signals back into figures or words for the computer screen.

Clock chips provide the timing needed for all the computer circuits to process electric signals in the right sequence. Each is linked to a quartz crystal that vibrates at a precise frequency.

 

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In 1944, a child’s disappointment having to wait several days to see the photograph her father had taken led him to devise a quick method of film processing.

He was an American, Dr Edwin Land of Cambridge, Massachusetts, and in just a few months he had come up with a solution. Within three years the first instant-picture camera came on the market, capable of producing a finished black-and-white picture in about a minute. He called it the Polaroid-Land camera.

Today, a Polaroid camera can produce a black and white print in as little as ten seconds and a colour print in only a minute. The secret behind instant photography lies in the film, not in the camera. The film not only has a coating of light sensitive emulsion like a normal photographic film, but also carries the chemicals necessary to process it.

The film pack has both negative and positive sections – in a colour film each is many-layered, with dye developer layers alongside was colour-sensitive negative layer. The processing chemicals that trigger off the developing and printing process are in jelly-like form in a tiny plastic pod between the negative and positive sections.

The pod bursts when the film is removed from the camera through a pair of rollers. The chemicals are spread evenly over the film, and diffuse through it to set the picture processing in motion. The sandwich of film and print material develops in daylight outside the camera, and a positive picture is revealed when the negative and positive layers are pulled apart.

 

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Two of the mist widely used cameras are the compact and the single-lens reflex (SLR). Both use 35mm film, although a few SLRs, including the Hassrlblad, use 120 film – 2¼ in (60 mm) wide – which needs less enlarging so gives better definition.

The two types differ in two main ways. First, most compacts have one built-in lens whereas the SLR can be fitted with a variety of interchangeable lenses. Secondly. The compact has a separate viewfinder whereas the SLR has a reflex viewfinder which ‘sees’ through the camera lens.

With a separate viewfinder, the photographer’s view does not coincide exactly with that of the lens (this is known as parallax error), so some compensation is needed for close-ups. With a reflex viewfinder, the photographer can see exactly the image that will be thrown onto the film, because light entering the camera lens is reflected by a mirror through a pentaprism (a five-sided prism)

 to the viewfinder eyepiece. The pentaprism reverses the mirror image and presents it to the eye the right way round. When the shutter release is pressed, the mirror springs upwards to let the light from the image onto the film.

The compact is smaller than the SLR, is easy to operate, and has few controls. The most expensive models have automatic focusing, automatic exposure, a zoom lens, built-in flash, and motor-driven film wind-on. They can take pictures comparable in quality to many SLRs.

SLR cameras can be programmed for auto-exposure in different ways – for example, a suitable aperture is automatically chosen for a manually selected shutter speed, or the other way round. Often the exposure meter has an indicator in the viewfinder to show the combination of aperture and shutter speed being set for optimum exposure.

The latest S;R models have built-in microprocessors controlling auto-focusing, auto-exposure and motor-driven wind-on. They can be fitted with a range of interchangeable backs offering different features, such as using different film and printing various information on the film.

 

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Developing the first stage in film processing amplifiers the chemical changes begun by the light. It is done in the dark as the film is still light-sensitive.

In the darkroom, the film is immersed in developer, a fluid chemical mixture that reveals the image as a negative, so called because it is darkest where most light has reached the film. This is because the developer reduces the exposed silver halide grains to fine particles of metallic silver, which appear black. Before the developed film can be handled in the light, it has to be fixed – that is, unexposed silver salts are removed by immersing it in a chemical such as ‘hypo’ (sodium thiosulphate).

To make the negative into a positive print of the original scene, it is put in an enlarger and focused on silver halide coated light-sensitive printing paper. The enlarger projects the negative image on the paper at the size required, the exposes it to light. The paper retains the image in the same way as the film, but the darkest parts of the negative let through the least light, so the original light pattern is re-created. After exposure, the print is developed and fixed in a similar way to the negative.

 

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The film that captures the light rays is a strip of transparent plastic (polyester or triacetate) covered with a light sensitive coating. The coating is a compound of silver salts and a halogen element forming tiny silver halide grains, suspended in gelatine. Exposure to light makes the silver halides start to break down, to eventually form an image in silver.

For the best result, there must be exactly the right amount of light. Too little will result in underexposure, lacking detail because the print or slide is too dark. Too much will produce an overexposed result, lacking detail because it is too light.

Films with large light sensitive grains are quicker to react and are termed fast films. Slow films have small grains and need extra light for exposure. Films are graded for speed on the ISO (International Standards Organisation) scale. The higher the ISO number, the faster the film.

 

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With a modern camera, you need do no more than press a button to take a photograph – to snap the action of a sporting event or record the beauty of a prizewinning rose, for example –and make a permanent record of a fleeting moment.

Technology has taken the guesswork out of the picture taking, there are now computerized automatic cameras that focus themselves, set their own controls, and wind-on the film after each shot. In contrast there are also simple throwaway cameras that are disposed of once the film has been processed.

All cameras, no matter how sophisticated or how simple, work in much the same way. When you click your camera to take a picture, you are opening the shutter for a brief moment to let light through the lens to a dark interior. In this moment, the light rays from an inverted image of the scene in front of you on the light-sensitive film at the back of the camera.

Processing the film completes the chemical changes begun by the light striking the film, and printing the film provides a pictures of the scene you snapped.

 

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Sunlight is broadly made up of the three primary colours of light: blue, green and red. All colours can be made by different mixtures of the three. Pairs of primary colours produce secondary colours: magenta (blue and red), yellow (green and red), and cyan (green and blue). If secondary colours are paired, they produce the primary colours. Magenta and yellow make red, cyan and yellow make green, cyan and magenta make blue. Each of the six colours takes no part in making up the colours opposite to it in the charts. Blue and yellow are ‘opposites’, so are green and magenta, and red and cyan.

Negative and print

A colour film has three layers, each sensitive to one of the primary colours. When a photograph is taken, each layer records a primary colour but forms an image in dye of the opposite colour to the primary.

The negative is then printed by light in a darkroom on paper that contains similar colour-sensitive layers. When normal light passes through the blue wheel on the negative, the yellow dye blocks the blue rays but lets through the red and the green. The paper records the red and green cyan and magenta dyes (their ‘opposites’). When you look at the photograph the combination of cyan and magenta appears blue. All the other colours are produced in the same way.

 

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When you take a photograph the subject that you see through the viewfinder is recorded on the film during the brief moment that the shutter opens to let in light through the lens. The film is coated with an emulsion that is chemically affected by light. ‘Fast’ films are more sensitive to the light than ‘slow’ films, so can be used in duller conditions. The speed of the film is indicated on the box and the spool by the ISO number. The higher the number, the faster the speed.

The camera lens concentrates light from the subject of the photograph and projects an inverted image of it onto the film at the back of the camera.

The diaphragm has overlapping leaves that form an aperture which can be made larger or smaller. A big aperture lets more light enter the camera.

A common type of shutter has two blades that open to form a slit that crosses the film. The smaller the slit, the faster the shutter speed.

 

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Busy executives and technicians can carry their own personal buzzer – rather like a pocket electric bell – to warn them which they are wanted. Doctors on their rounds in a sprawling hospital, for example, can be called to a particular ward, or firemen on routine duty to a fire alert.

The pocket alarm, known as bleeper (or beeper) because of the sound it emits, to a battery-powered miniature radio receiver turned to one station. The bleep is made by a tiny crystal that vibrates to produce sound when electric signals are passed to it. The signals are generated in the bleeper’s electronic circuits, triggered by a radio signals transmitted at the touch of a button from the control unit.

The simplest bleeper can emit several different signals, rather like the dots and dashes of Morse code. Four long bleeps, for example, could mean ‘Ring the office’, or interspersed long and short ones ‘Come to reception’. More advanced types can display short message, or can store messages.

The system is known as radio-paging. A small network can call up to about 100 receivers, either separately or simultaneously in a group. Each receiver has a number, and the controller makes contact by sending the receiver number and then the required message.

Long-range paging services are operated by commercial companies who transmit messages to their subscribers’ bleepers from a control room. The world’s largest paging network is operated by British Telecom, who have transmitters covering various zones throughout the country.

Radio-paging systems all have to be licensed, and are allocated a frequency, generally around the 27mHz waveband in Australia. The operating range varies according to the power of the transmitter but could be 100 miles (160km).

 

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Newspaper have been using facsimile machines to send photographs (wire photographs) since 1907, when a photo from Paris was wired to the Daily Mirror in London. In 1959, a Japanese newspaper Asahi Shimbun

(‘Morning Sun’), sent whole pages from its main office in Tokyo to a printing works at Sapporo 600 miles (960 km) away. Now it sends a complete copy daily by satellite link to London, where it is printed, for sale in Europe.

In recent years, technological advances have resulted in cheaper machines able to give good quality reproduction. Newspapers and business firms are not the only users. Police forces can send each other copies of fingerprints and photo fit pictures.

The earliest fax machines took about six minutes to transmit a document the size of an A4 typing sheet. Later machines cut the time by half. Modern machines take less than 30 seconds. They code the information digitally although it is transmitted as analogue (like sound-wave) signals. Machines available in the 1990s will both code and transmit digitally, cutting A4 sheet transmission time to four or five seconds.

 

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Telephone cables carrying messages at the speed of light have given a new lease of life to telecommunications. The amount of information now transmitted – telex, fax and computer data as well as telephone calls – was straining the copper-cable system to the limit. Fibre optic cables, with thir high capacity, small size and freedom from electrical interference, are the key to development.

The first uses of optical fibres was in medicine in 1955, for lightning up parts of the inside of the body. The light loss through the fibres was at first too great for many other uses. But in 1966, Dr Charles Kao and Dr George Hockham, two scientists working in Britain at the Standard Telecommunications Laboratories, discovered that the loss was due to impurities in the glass. By 1970, due to impurities in the glass, Corning Glass, had produced fibre optics good enough to transmit telephone signals.

Fibre-optic cables are now gradually replacing copper ones between exchanges. The first transatlantic fibre optic cable, TAT-8 – jointly laid by American, French and British companies – began service in 1988, its capacity of around 40,000 simultaneous telephone calls is three times as great as the seven existing copper transatlantic cables put together.

 

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The first person to attempt to create sounds electronically was an American inventor, Thaddeus Cahill. He invented an instrument called a telharmonium in 1906, which used electric motors and telephone receivers to produce sounds, but without much success. By 1920, a Russian scientist named Leon Theremin generated electronic sounds using two radio-wave oscillators; it was played by moving the hands round its aerials. This altered the circuit tuning and produced sound – varying according to the hand position – from the loudspeakers. The instrument was called a Theremin.

The forerunner of today’s synthesizers was built for acoustical research by the Radio Corporation of America (RCA) at Princeton, New Jersey, in 1955. It was fed with punched paper tape, and the punched code activated the sound generators, filters and amplifiers. The music was recorded on tape.

Because the RCA synthesizer circuits used thermionic valves – electronic vacuum tubes – it was large enough to fill a room. In the 1960s, an American physicist, Robert Moog, developed the Moog synthesizer with circuits based on transistors. Further development in electronics bought the modern synthesizer down to portable size.

FM synthesis, the basis for the digital synthesizers of the 1980s, was invented by Dr John M. Chowning of Stanford University, California. The idea of sampling, on which most modern synthesis is based, was introduced by Australians Peter Vogel, Kim Ryrie, and Tony Furse with their Fairlight Computer Musical Instrument (CMI).

 

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The needle, or stylus, is an artificial sapphire or diamond with a rounded or elliptical tip. The groove is a different shape each side, one for the right-hand stereo signals and one for the left-hand signals.

The needle vibrates as it runs along the uneven groove walls, setting up electrical signals in the pick-up head. The signals are amplified and then converted to sound by cones (diaphragms) vibrated by electromagnets in the loudspeakers.

In a moving-magnet type of pick-up head, the needle is linked to a magnet. As the needle vibrates, the magnet’s movements induce electrical current in two wire coils, creating the signals fed to the two stereo speakers.

The stereo groove causes the moving magnet to induce different signals in each coil. If only the outer groove wall contains a signal. Then only the coil corresponding to the right-hand speaker will produce current. Or if only the inner groove wall contains a signal, only the left-hand speaker will receive current.

 

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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.

 

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