My Science School

Food is the fuel that our bodies need for movement. But we also need some fuel simply to maintain all the parts of our bodies. Individual cells are being renewed all the time. And even if we do not move the outside of our bodies at all, there are many parts inside that are constantly in motion. How much food we need depends on our size, age, gender and level of activity.

Nutrition is how food affects the health of the body. Food is essential—it provides vital nutrients for survival, and helps the body function and stay healthy. Food is comprised of macronutrients including protein, carbohydrate and fat that not only offer calories to fuel the body and give it energy but play specific roles in maintaining health. Food also supplies micronutrients (vitamins and minerals) and phytochemicals that don't provide calories but serve a variety of critical functions to ensure the body operates optimally.

Protein: Found in beef, pork, chicken, game and wild meats, fish and seafood, eggs, soybeans and other legumes included in traditional Central America cuisine, protein provides the body with amino acids. Amino acids are the building blocks of proteins which are needed for growth, development, and repair and maintenance of body tissues. Protein provides structure to muscle and bone, repairs tissues when damaged and helps immune cells fight inflammation and infection.

Carbohydrates: The main role of a carbohydrate is to provide energy and fuel the body the same way gasoline fuels a car. Foods such as corn, chayote, beans, plantains, rice, tortilla, potatoes and other root vegetables such as yucca, bread and fruit deliver sugars or starches that provide carbohydrates for energy.

Energy allows the body to do daily activities as simple as walking and talking and as complex as running and moving heavy objects. Fuel is needed for growth, which makes sufficient fuel especially important for growing children and pregnant women. Even at rest, the body needs calories to perform vital functions such as maintaining body temperature, keeping the heart beating and digesting food.

Fat: Dietary fat, which is found in oils, coconut, nuts, milk, cheese, meat, poultry and fish, provides structure to cells and cushions membranes to help prevent damage. Oils and fats are also essential for absorbing fat-soluble vitamins including vitamin A, a nutrient important for healthy eyes and lungs.

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There are more than 600 muscles in the human body. Over 100 of these are in our faces, which is why we can have so many different expressions. Although we can perform a great variety of movements, each muscle can only do one thing: contract. That is why muscles often work in pairs, so that one muscle can move a part of the body in one direction, while its partner can move it back again. Perhaps the most important muscle in the human body is the heart, which is contracting and relaxing all the time to pump blood around the body.

There are about 700 named skeletal muscles in the human body, including roughly 400 that no one cares about except specialists. There is just one important cardiac muscle. And there are literally countless smooth muscles (which do the work of the autonomic nervous system, mostly squeezing and squishing stuff in tubes).

It’s surprisingly hard to tell. You wouldn’t think the total number would be ambiguous, but it’s difficult to know what to include and exclude, and anatomists don’t always agree. Some muscle tissue really can’t be separated into countable muscles. And, believe it or not, the science of anatomy is still advancing. No, entirely new muscles aren’t being discovered — but novel variations in individual muscle anatomy are found more or less constantly, and supernumerary muscles — extra muscles — are not unusual. Many muscles, like the four-part quadriceps, are normally split into different parts that may or may not traditionally count as separate muscles — but then some people’s muscles are more divided than others. It makes a firm count just about impossible.

There are only about 200 to 300 muscles that anyone, even a massage therapist, might actually be interested in knowing about. When most people ask how many muscles are in the human body, they mean the serious bone-movers — Pecs, delts, lats, traps, glutes, biceps & triceps, hams & quads & let’s not forget the cloits & dloits!muscles that do real work, muscles like pecs, delts, lats, traps, glutes, biceps and triceps, hams and quads, and let’s not forget the cloits and dloits! There are maybe another hundred muscles if you include the fiddly little muscles of the hands and feet, and the major face muscles.

But that’s including about 600 muscles that, mostly, no one cares about except specialists. I am aware of a few that have clinical importance to a massage therapist, but I’m mostly just barely aware of their existence — like the smaller facial muscles, like the mess of little muscles around and under the tongue and around the voice box, like the muscles around the eyeball, or the crazy trampoline of muscles on the pelvic floor.

Our bodies are very complicated. It is impossible to think about all the processes that are going on inside them at the same time, so doctors often consider the body as being made up of several different systems, each one with different organs and mechanisms working together to perform particular functions.

Organ Systems

Different organs can work together to perform a common function, like how the parts of your digestive system break down food. We refer to an integrated unit as an organ system. Groups of organ systems work together to make complete, functional organisms like us! There are 11 major organ systems in the human body, which include the circulatory, respiratory, digestive, excretory, nervous and endocrine systems. The immune, integumentary, skeletal, muscle and reproductive systems are also part of the human body.

The Circulatory & Respiratory Systems

The circulatory system is responsible for transporting blood throughout the body. It consists of the heart and blood vessels known as veins, arteries and capillaries. Think of blood vessels as the highways of the body, bringing important cargo to and from the cells. In the circulatory system, blood is pumped from the heart to the lungs, so they'll get oxygen, and then pumped to the body's cells. Here is a diagram of the human circulatory system, including the heart and major arteries, which are in red, and veins, which are in blue.

In order for blood to provide oxygen to the body, the body must have a way of obtaining that oxygen. The respiratory system allows air to enter the lungs and for oxygen to diffuse into the blood en route to the body's tissues. The entrance to the respiratory system can be found in the nose and the mouth, where air enters the body and then travels through the larynx and pharynx in the throat to the trachea or windpipe. From the trachea, right and left branches, known as bronchi, carry oxygen to the alveoli, where oxygen moves into the blood, while carbon dioxide moves into the lungs to be exhaled.

Digestive & Excretory Systems

The digestive system is responsible for bringing food into the body and breaking it down to useable components. It starts at the mouth, where we ingest our food and use our saliva, teeth and tongue to bite and mash it. The food then travels through the esophagus into the stomach, where strong acids break it down even further. During the last two stages of digestion, nutrients and water are absorbed through the small intestine and the large intestine, respectively. Any remaining waste products are stored in the rectum and eliminated through the anus.

The urinary or excretory system is where liquid waste is eliminated as urine. The excretory system starts with the kidneys, important organs for cleaning the blood and balancing water in the body. In the excretory system, the liquid part of the blood, or plasma, enters through the kidneys, where important nutrients, like sugar and some salt, are reabsorbed into the body. Compounds we don't need, like urea or excess water, are sent to the bladder in the form of urine. Urine leaves the body through the urinary tract and exits the body at the urethra.

Nervous, Endocrine & Immune Systems

Without a master control system that tells our bodies what to do, none of the organ systems we've talked about so far would work. The organs in the human nervous system are made up of cells, called neurons that use chemicals and electricity to send messages. This system has two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and the spinal cord, which serve as the main control centers for the body and process all incoming and outgoing messages. The peripheral nervous system includes all the nerves in your body that bring messages to the central nervous system and from the CNS to the muscles.

Whereas the nervous system mainly uses electrical signals to communicate between cells, the endocrine system relies upon chemicals, called hormones, to send long distance messages through the body. The main organs found in the human endocrine system are located in the brain and include the hypothalamus, thalamus and pituitary gland. They talk to other endocrine organs, like the adrenal glands, testes and ovaries to assist with other organ systems.

There is much that we do not yet know about how the brain works, but we do know that the brain communicates with the rest of the body through a thick cord of nerves running down the middle of the spine and branching off to reach the limbs and internal organs. The nerves are pathways for messages to the brain, to inform it about what is happening elsewhere in the body, and from the brain to tell the rest of the body how to act. These messages, and the processes happening within the brain, are made up of tiny electrical impulses. By far the largest part of the brain is the cerebrum, which is divided into two halves, called hemispheres. The rest of the brain is made up of the cerebellum, the pons and the medulla, which join together at the top of the spinal cord.

With 80-100 billion nerve cells, known as neurons, the human brain is capable of some astonishing feats. Each neuron is connected to more than 1,000 other neurons, making the total number of connections in the brain around 60 trillion! Neurons are organized into patterns and networks within the brain and communicate with each other at incredible speeds.

The largest part of the human brain is the cerebrum, which is divided into two hemispheres, according to the Mayfield Clinic. Underneath lies the brainstem, and behind that sits the cerebellum. The outermost layer of the cerebrum is the cerebral cortex, which consists of four lobes: the frontal, parietal, temporal and occipital.

Like all vertebrate brains, the human brain develops from three sections known as the forebrain, midbrain and hindbrain. Each of these contains fluid-filled cavities called ventricles. The forebrain develops into the cerebrum and underlying structures; the midbrain becomes part of the brainstem; and the hindbrain gives rise to regions of the brainstem and the cerebellum.

The cerebral cortex is greatly enlarged in human brains and is considered the seat of complex thought. Visual processing takes place in the occipital lobe, near the back of the skull. The temporal lobe processes sound and language, and includes the hippocampus and amygdala, which play roles in memory and emotion, respectively. The parietal lobe integrates input from different senses and is important for spatial orientation and navigation.

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Although the helicopter in its present form is scarcely 50 years old, the principle of the rotary wing has been known for centuries. The Chinese used it some 2500 years ago for their flying top — a stick with propeller-like blades on top which was spun into the air. And Leonardo da Vinci actually sketched a helicopter in 1483. The English flight pioneer Sir George Cayley was among several people to design a model helicopter in the late 1700s, but the first man-carrying helicopter, which rose a few feet in the air, was not built until 1907 — by a Frenchman, Paul Cornu, at Lisieux. Problems with stability and other design aspects led to helicopters being abandoned for nearly 30 years in favour of fixed-wing aircraft. Many of the design problems were, however, solved by the Spanish inventor of the autogiro, Don Juan de la Cierva, in 1919. This aircraft had a large rotor that was not driven by the engine but turned freely in the airflow. It could not take off vertically — it had to taxi to get the rotor turning enough to lift it.

Not until 1936 did the German Professor Heinrich Focke, of the Focke-Wulf Company, design a practical helicopter with twin rotors. Three years later a Russian-born engineer, Igor Sikorsky, produced a successful single-rotor helicopter, the VS-300, in the USA. This was the true ancestor of the modern rotorcraft — the most versatile of aircraft.

The development of jet engines in the 1950s led to the adoption of turboshaft engines that have considerably increased the range and speeds possible. Today the helicopter is invaluable not only as a military transport, hedge-hopping patrol plane and sky crane — for lifting steeples onto churches, for example — but also for rescuing people from remote mountainsides, sinking ships and burning buildings.

 

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Submarines often travel in the ocean depths for weeks on end. They do not need to surface to check their position by the Sun, Moon or stars, because the latest navigation systems allow them to know where they are to within less than 320ft (100m) of their actual position while still submerged.

 These systems are known as inertial navigation systems and are computer-controlled. There are usually at least two on a submarine, operating independently. They are a modern version of 'dead reckoning' — calculating where you are by measuring exactly how far you have come from your starting point, and in what direction.

The inertial navigation system is held absolutely horizontal and pointing in a fixed direction by gyroscopes, whatever the attitude of the submarine.

 At the start of the voyage, the instruments are fed with the submarine's exact position. An accelerometer then measures movement in every direction, and the computer works out the overall distance and direction travelled, thus establishing the present position.

Sonar is used to determine the water depth below the vessel to prevent it running aground. Inertial systems are on the whole accurate, but the small errors they do make gradually accumulate. They have to be realigned regularly. This is done by picking up radio signals from satellites in space, which form part of the American NAV-STAR Global Positioning System (GPS). The submarine has to partly surface.

 The satellites transmit a radio message which contains precise details about their orbit, and a time signal controlled by an atomic clock. In effect, the signal says 'It is now time X'. The submarine uses its own clock to calculate how long it takes the signal to arrive. As radio waves travel at 186,000 miles (300,000km) per second, the navigators can calculate the sub-marine's distance from the satellite by the time the signal takes. By calculating the distance from three GPS satellites, the ship's position can be pinpointed on a chart.

During the 1990s, the last 04 18 satellites in the GPS system will be placed in orbit at a height of 12,500 miles ,20.000km), orbiting the Earth at 12-hourly intervals. They will ensure that at any time at least four satellites will be available for navigators to calculate their position to within 550yds (500m;

The GPS system has virtually replaced older forms of submarine navigation such as the OMEGA system, but submarines still carry it as a back-up. This system detects radio signals broadcast from eight stations dotted around the Earth's surface — in Japan. Hawaii, Australia, Argentina, North Dakota. Norway. Liberia and Reunion Island. These stations broadcast at very long wavelengths, so their signals carry all around the world. The signals are synchronised, and by measuring the time differences in their reception, a sub-marine's position can be estimated to within about miles (3.2km)

 Inertial navigation also mounted in long-range intercontinental launched from submarines .

 

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Medieval castles had all sorts of traps pitfalls to keep out intruders. Homes of today can have an equal number of defences, without having to resort boiling pitch. Modern defence devices include floodlights or alarms that are triggered when an intruder upsets a circuit monitored by hidden magnets or micro. chips, or by invisible beams.

The outer defences

The modern burglar might first have to face  a strategically placed invisible infrared detector that is affected by temperature changes caused by body heat. When anyone approaches the house, a sensor in When the detector switches on floodlights. If the caller is legitimate, the lights show the Way but anyone planning burglary will feel very exposed and less likely to continue.

The sensor is pyroelectric — that is, made from a ceramic material such as tourmaline, which, when heated, generates a voltage across it. The system is designed so that the sensor will respond to a temperature change caused by human body heat, but is less likely to be set off by changes in the weather.

 A burglar who dodges a floodlight - barrier may then face a door connected to a noisy alarm. A magnetic switch IL inserted between the door and its frame. When the door is shut, two contacts keep switch circuit closed. This switch is monitored electronically by an alarm circuit. If the door is opened arid switch circuit is broken, the alarm circuit triggers the alarm.

But a resolute burglar, out of sight of passers-by, might attack the door with a chisel or drill. This type of attack can be foiled by a vibration detector fitted to the door. This is a device in which a ball is disturbed by vibrations. The ball rests on sharp metal points wired to a microchip that is programmed to accept certain vibrations — such as those caused by wind or passing traffic — as normal. If the ball bouncing on the points sets up vibrations not in the program, it sets off the alarm.

The inner defences

If the burglar succeeds in getting through a door or window, he may face a battery of inner defences. These include pressure Dads concealed under the carpet and inked to an alarm circuit. They have two metal plates or foil sheets separated by a layer of spongy plastic. The two plates are pressed together if anyone treads on them, and this sets off the alarm.

Anyone who prowls around inside the house may be caught by a 'magic eye'. This is a photoelectric cell with an invisible infrared beam shining onto it. If the beam is interrupted, the photocell triggers an alarm.

Other types of indoor detector use either ultrasonic waves (too high pitched for humans to hear) or microwaves (high-frequency radio waves) transmitted by

devices called transducers. They transmit the waves at a certain frequency (a given number per second), and the waves are reflected back to the unit from objects in the room. If anyone moves through the room, the reflected waves get bunched up or pulled apart, so their frequency is altered. The sensor detects the frequency change and feeds signals to a microchip which assesses the speed and bulk of the intruder. Anything assessed as typically man-sized makes it set off the alarm.

A commoner type of indoor detector uses an infrared system similar to the outdoor floodlight type. in the detector. a many-faced mirror or special lens creates a number of sensitive zones. If anything moving in and out of these zones is at a different temperature to the room surroundings, it generates a voltage. The detector electronically monitors the voltage, and is designed to set off the alarm if the temperature increase is likely to be caused by human body heat.

 

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When lightning struck the roof of York Minster, one of Britain's finest medieval cathedrals, in 1984, it started a fire that was if trapped between the ceiling and the wooden roof. Firefighters could not get into the area, and the enormous heat that built up destroyed most of the roof.

If lightning should strike York Minster twice, space-age science has ensured that the same type of damage will not happen again. The new roof is fitted with trap doors to let the heat out and the firefighters in, and the doors have latches operated by springs made from memory metals, which will open automatically if they get hot.

Memory metals have two different shapes they can 'remember', and will switch from one to the other under certain conditions. The York Minster trap-door springs are made to remember a certain temperature, at which they will expand and withdraw the bolt, releasing the door.

One of the first uses of memory metals has been for hydraulic pipe couplings in aircraft, which came into use in 1971. The couplings are made too small to fit at a certain temperature, and are then cooled to well below room temperature and stretched to fit. When they warm up to their normal operating temperature, they shrink to the first shape, forming a tight joint. The same idea is used in surgery, with metal couplings to bind together broken bones. Body heat keeps them constantly tight.

Metals that change their shape under heat now have all kinds of uses, such as operating switches and valves in automatic coffee machines, and opening greenhouse windows when it is hot and closing them when it is cold. Most metals are made up of crystals (arrangements of atoms). When two or more metals are combined into an alloy, the alloy can form different crystal structures under different conditions.

Some alloys, if they are cooled rapidly, will undergo an abrupt change to a different alignment of crystals at a certain temperature. This transition temperature varies with the make-up of the alloy. The changed structure it brings about is called martensite, after the German metallurgist Adolph Martens who first identified it.

If such an alloy is shaped by heat treatment so that it becomes martensite at, for example, 122°F (50°C), it will change its shape at that martensitic temperature, but revert again at a different temperature.

Shaped for shaping

A Japanese company has found an unusual use for memory metals — as a super-elastic wire frame in brassieres. The alloys used will stretch up to ten times more than ordinary metals. When stretched in use, the bra wire gradually returns to its original bust-supporting shape.

Another use for super-elastic wire is in straightening teeth. Conventional stain-less-steel wires have to be regularly tightened, often by turning a tiny key. Super-elastic alloy wires exert a continuous gentle pressure ideal for coaxing teeth in the right direction.

 

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When someone blows into a breathalyzer bag, any alcohol in their breath is turned into acetic acid (vinegar). This chemical reaction changes the colour of the crystals in the blowing tube. The more crystals that change colour, the more alcohol they have in the body.

The first breathalyzer was developed by an American doctor, Rolla N. Harger (he called it a 'Drunkometer'), and it was introduced by the Indianapolis police in 1939. Similar breathalyzers began to be widely used by the police in many Countries in the 1960s, as a yardstick for judging a driver's ability to drive. A high Intake of alcohol dulls the nervous system and slows up coordination.

To begin with, the commonest type of breathalyzer was a plastic bag, similar to a balloon, with the crystals in the blowing tube. and the driver was asked to inflate the

bag. If the crystals changed colour as far as a level marked on the tube, the driver was possibly 'over the limit', and needed further tests. The crystals used were an orange-yellow mixture of sulphuric acid and potassium dichromate. They turned the alcohol into acetic acid (vinegar), and in doing so they were changed into colourless potassium sulphate and blue-green chromium sulphate.

The breathalyzers used by the police today, however, are usually electronic, and much more accurate than the inflatable-bag type. They use the alcohol blown in through the tube as fuel to produce electric current. The more alcohol the breath contains, the stronger the current. If it lights up a green light, the driver is below the legal limit and has passed the test. An amber light means the alcohol level is near the limit, a red light above the limit, and in both cases  the driver has failed the breath test and needs further testing.

This type of breathalyzer is about the size of a TV remote control, and contains a fuel cell that works like a battery. Breath from the tube is drawn into the cell through a valve, and meets a platinum anode (a positive plate). which is against a spongy disc impregnated with sulphuric acid. The platinum causes any alcohol in the breath to oxidise into acetic acid — that is, its molecules lose some of their electrons. This sets up an electric current through the disc, and it flows to a cathode (a negative plate) on the other side.

 

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Most drivers have experienced the frightening moment when the wheels lock and the car slides uncontrollably toward the vehicle in front. Although drivers ate taught to leave a sufficient gap for braking, and to take extra care on wet or icy roads, the huge number of rear-end collisions every year is ample evidence they do not.

Skidding and sliding happen because the behaviour of a car changes rapidly when the wheels begin to lock. Up to a point, pressing the brake pedal harder produces greater deceleration. But once the wheels have locked, their grip on the road is lost, they begin to slide instead of turn, and the driver can no longer control the car's direction. Panic follows, and the natural reaction is to stamp ever harder on the brake, which makes things worse.

Advanced driving manuals recommend cadence braking, in which the brake pedal is pumped up and down in quick succession to ensure that the wheels never lock. But, in practice, few drivers have the skill or experience to do this in an emergency.

Anti-lock brakes are designed to auto-mate the technique of cadence braking, taking the skill out of the hands and feet of the driver and entrusting it to a package of electronics and hydraulics. They consist of two parts: an electronic sensor that can detect how rapidly the wheels are decelerating, and a system for automatically controlling the hydraulic pressure on the brakes to achieve the best and safest deceleration.

The sensor consists of a slotted or toothed exciter disc attached to an axle or inside a brake drum. As the axle turns, each tooth and gap in this disc pass close to a monitor and generate a current, which varies according to the rate at which the disc is rotating.

The signals are interpreted by electronic circuits, which determine both the speed of the disc and the rate at which it is decelerating. If the disc is slowing down too rapidly and is about to lock, the circuits instruct the hydraulic controls to reduce brake pressure, preventing a skid. As the driver continues to press the brakes, pressure rises again, and the system repeats the operation until the vehicle has stopped. The system can produce up to 45 cadences a second, if required.

The details of how the electronic signals are used to control brake pressure depend on individual designs. Some of earliest non-skid brakes, in the 1960s, were fitted to trucks, which use air under pressure to activate their brakes. In these systems it is relatively simple to bleed off some of the air through a valve to reduce pressure. The air lost can easily be replaced by drawing on air stored under pressure in the vehicle.

The same simple arrangement cannot be applied to cars, which use hydraulic fluid. This is because there is little fluid in reserve, and also it would be both expensive and dangerous to spill bled-off hydraulic fluid all over the road. One alternative is to reduce pressure by briefly increasing the volume of the hydraulic system — with a piston arrangement, for example — and then to restore pressure again. Among the systems that have been developed are some that even allow sharp turns to take place safely during heavy braking.

Although anti-locking brakes were originally available only on the most expensive cars, they are increasingly becoming standard, or optional, on most new cars.

 

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The Czech playwright Karel Capek introduced the name robot in the early 1920s. He wrote a play called Rossum's Universal Robots in which an army of industrial robots became so clever that they took over the world. Capek coined the word robot from the Czech robota, which means 'slavery'. Since then men have worried not so much about robots taking over the world, but more about robots taking over their jobs.

Robots have indeed taken over some jobs — dull, mechanical, routine work. They save costs because there is no need for them to change shifts, they do not tire or lose concentration, they do not take tea or coffee breaks, they do not fall ill (although they may need repairs), and they do not go on strike. But even though American and British researchers have produced robotic four-fingered hands capable of picking a flower, robots are still a long way from having the perception, dexterity or flexibility of human beings.

 It is, however, generally accepted that the responsible use of robots in industry is beneficial because it saves people from doing dull and dangerous jobs. Robots were used to clear up the radioactive debris after the Three Mile Island nuclear accident in America in 1979. They are also being developed for inspecting and manufacturing nuclear plants, fighting fires, Felling forest trees, and acting as security guards — walking burglar alarms that can warn human guards of an intruder. And I four-legged, 72 ton robot was used to roll boulders to build up the sea wall in Tokyo Bay, Japan, in 1986 — saving 50 divers from the risky task.

Experiments are also under way with robots that can help infirm and disabled people to be independent. Researchers in Britain and America are developing robots that can respond to spoken commands. They will be able to undertake such tasks as brushing teeth, serving soup, loading a computer, opening filing cabinets and picking up mail.

 

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American motorists were able to buy petrol with oil company credit cards before the First World War, but the age of the credit card dawned in 1950 with the introduction of the Diners Club charge card by American businessman Frank McNamara. The idea came to him after dining in a New York restaurant and discovering he had mislaid his wallet. The Diners Club card is not strictly a credit card because the whole bill has to be paid when the invoice is received — most other cards carry forward a debit balance.

Today there are more than 350 million credit or charge cards in use in the United States alone. Worldwide, cards are numbered in billions.

Smart cards are likely to have a wider use than for money transactions. in the late 1980s some medical authorities in parts of Europe, the USA and Japan began trials with medical identity cards —smart cards carrying the holder's medical history. The cards save time and paperwork, as they can be consulted by doctors and chemists in computer terminals at hospitals, surgeries and pharmacies, and updated each time the patient is seen. The European trial programmes aim to produce a standardized EEC care or health smart card for use in the 1990s.

Also available are laser cards, developed in the USA, in California. They are not as smart as smart cards, but are able to carry a much larger store of personal information, contained in a pattern of tiny holes — only a thousandth of a millimetre across — on a photosensitive strip. The dots, like the pits and flats on a compact disc, can he read by a laser scanner in a special terminal.

The card can hold coded identification details, including fingerprints, signature, voice print, and even a photograph,' as well as various hidden security codes making it -virtually impossible to counterfeit Its information storage space is so vast there is plenty room for such things as bank accounts, medical history and educational attainments. Information is filed on the card under separate access codes, so the bank, for example, could read out only financial information and the doctor only medical information.

 

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Every successful shopkeeper needs to know which goods are selling well and which are going slowly, so that he can restock or phase them out, as appropriate. For the small shop, tidy bookkeeping and a glance at the shelves may give all the information necessary. But supermarkets and other big stores need quick and accurate records of a much larger flow of merchandise. That is why they use bar codes, which are printed on the packaging.

A bar code can be read by a laser scanner, which passes it to a computer. This supplies the details and price of the goods, records the sale for storekeeping, totals the bill, and feeds the information to the cash register which prints out a receipt.

Common bar codes are European Article Numbers (EAN), based on a number with 13 digits, and Universal Product Code (UPC), based on a number with 12 digits. The Australian Product Number (APN) is also based on 13 digits. Each digit is represented by a series of parallel straight lines and white spaces. The laser scanner translates the information into binary digit signals, which it feeds to the computer.

The code gives the manufacturer details of the product and the package size, and includes a security code that prevents anyone altering it or the scanner misreading it. The computer supplies the price from the product information. So the only way to change the price of an item is by altering it in the computer.

 A laser scans a bar code with a beam of light passed from one end to the other. It is sensitive enough to read from left to right or right to left. Although the bar codes are usually printed in black on a white background, a laser can read a bar code which is printed in any dark colour except red, and the background can be any pale or pastel colour. Some of the lasers used scan with red light, so cannot pick up a reflection from red.

Bar coding is faster and more accurate than other systems. Human error is limited because staff do not have to mark a price on every item, and checkout assistants do not have to key in prices at the register. However, because the computer sup-plies the prices at the checkout, the store management has to ensure that the goods on the shelves display the same prices. Also that the shelf price is changed if a computer price is altered, or a customer may appear to he charged the wrong price.

 

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Chips are produced several hundred at a time on a slice of ultra pure, artificially formed silicon crystal, so thin that it would take about 250 slices to form a piece 1in (25mm)thick. Layout diagrams for circuits are prepared on a computer, then each reduced to chip size and set out side by side on a glass plate known as a mask. Because witches and other components are built up in separate layers on the chip, a mask is made for each operation. The masks – which block out the unwanted parts – are made many times larger than the chip and reduced photographically.

The chips are built up by forming each layer – p-type or n-type layers or insulating layers of silicon dioxide – and etching out the unwanted parts. This is done by treating the layer with a coating sensitive to ultraviolet light, masking it, then exposing it to ultraviolet light. The exposed parts become resistant to acid, but the blocked-out parts do not – they are etched away when the layer is coated with acid.

Parts such as aluminium contacts are deposited in the areas etched for them as a vapour. When hardened, the aluminium is etched to add the required circuit connections, which lead to contact pads at the edges of the chip.

All completed chips on slice are tested with delicate electrical probes to check that they are working properly. About 70 per cent prove faulty. They are marked as rejects and thrown away. After testing, the slice is cut into individual chips under a microscope with a diamond-tipped cutter. The good chips are each mounted in a frame that is encased in plastic. The contact pads are linked to metal connectors are in turn linked to protruding legs, or pins, that plug into the external circuit.

 

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Transistors are the commonest components in a microchip. They are used mostly as switches, letting current through to represent the binary digit 1, or cutting it off to represent 0.

A widely used type of transistor has two islands of n-type semiconductor in a larger base of p-type. While the transistor is switched off. The free electrons from the layers drain into the p layer and are absorbed by the free holes. The transistor is switched on by applying a voltage from a separate low-power circuit to an aluminium gate above the p base. This voltage attracts the free electrons from the p base towards the gate. They then form a bridge between the two n islands and provide a path for the current through the circuit in which the switch is operating.

The transistor is switched off by cutting off the power. The free electrons then drain back to the p base and are absorbed by the free holes. Without the bridge they formed between the islands, current cannot flow through the circuit.

 

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