My Science School

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

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.

 

Picture Credit : Google

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.

 

Picture Credit : Google

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.

 

Picture Credit : Google

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.

 

Picture Credit : Google

Car tyres are not just cushions for the wheels. They are there to give the car a good grip on slippery roads, and stop it sliding about when braking or cornering.

The tread pattern running all round the tyre has thin cuts (known as sipes) in the rubber to sponge up surface water, and zigzag channels to pump the water out behind as the car rolls forward. On a wet road, a tyre has to move more than 1 gallon (5 litres) of water a second to give an adequate grip.

On a perfectly dry road, the treads are not needed. A smooth tyre gives the greatest possible area of contact with the road. But if the smooth tyres are used in wet weather, the film of water on the road builds up in front of them and underneath them and actually lifts them and off the road surface – this is known as aquaplaning. When aquaplaning occurs, the driver loses control.

Most cars have to function in all weathers, so must have tyre treads, but racing cars make comparatively few outings a year. If the track is dry, they run on smooth tyres, called slicks, to get the best grip on the roads. The extra wide tyres and wheels give more grip that the average cars. In wet weather, however, the slicks have to be changed for treaded tyres.

 

Picture Credit : Google

When you are travelling in a car, you and the car are moving at the same speed. If the car stops abruptly, your body keeps moving forward. This is an illustration of inertia – the tendency of a moving object to keep moving, or of a stationary object to remain at rest.

An inertia-reel seat belt works on the same principle. Its mechanism includes a pendulum, which hangs vertically under ordinary driving conditions. But if the car stops abruptly it swings forward, and a locking lever resting on the pendulum is released. The lever engages a toothed ratchet that locks the shaft round which the belt is wound. The locked seat belt then prevents your body from being flung forward.

When you fasten a seat belt, it winds out from the reel against slight tension from a spring. This keeps it taut during normal travelling, but allows enough free movement for a driver to reach forward as necessary. But if you tug abruptly on the belt while winding it out, the locking mechanism will engage and stop the action of the spring. Slackening the belt releases the spring and the locking lever.

 

Picture Credit : Google

The exquisite workmanship of the traditional mechanical wristwatch has given way to the magic of the microchip. In the quartz watch, a vibrating crystal has taken over the role of timekeeper from the traditional finely tuned balance wheel ad hairspring. Minute electronic circuits control its operations.

A quartz crystal vibrates at an unvarying rate when an electric current is passed through it. The man-made quartz crystals used in watches are usually designed to vibrate 32,768 times a second when stimulated by the current from a battery. These vibrations produce electric pulses, and as the pulses travel through the electronic circuits of the microchip, their rate is successively halved in a series of 15 steps. The result is one pulse per second. Each one-second pulse triggers the chip to send signals to the digital display to advance the numerals one second.

The chip also uses the pulse as a base for other counting circuits, such as those that display hour and date, and to control the alarm signal.

Many modern quartz watches display the time in digits on a liquid crystal display (LCD). The liquid crystals are sandwiched between a reflective bottom layer and a top layer of polarised glass, and transparent electrical conductors separate them into segments. Each digit is formed from segments – up to seven are normally used, all seven being used for figure 8.

The liquid crystals rearrange their molecules according to their electrical state. Where the conductors carry no charge, light through the sandwich is reflected out again, so the display is blank. When the conductors are charged by an electric pulse, the molecules in the affected segments realign and twist the light away from the reflective surface, so the segments appear dark.

 

Picture Credit : Google

Because we have eight fingers and two thumbs, it seems natural for human beings to count in tens. It is just as natural for a computer to count in twos, for it has to decide between ‘yes’ or ‘no’ for every step in a process.

In everyday numbers, the digits from 0 to 9 are read from left to right and are based on the power of ten. For example, 110 is one hundred, ne ten, and no units.

The binary system uses only two digits: 0 and 1. Numbers are read from right to left and are based on the power of two. Moving from the right, each digit doubles in value, 1, 2, 4, 8, 16, and so on. 

Words fed into a computer are stored as binary numbers. If text such as LOAD”FILE in BASIC, computer language is keyed in, the word LOAD could be processed.

 

Picture Credit : Google

In the split second between the pressing of the shuttle release and the opening of the shutter, an automatic camera measures the distance between the lens and the subject and positions the lens to give sharp focus.

Most compact cameras have a tiny electric motor driving a transmitter that emits a beam of infrared light. The transmitter is linked to the lens, which moves in or out as the beam scans – focusing from near to far. The beam reflects back from objects to the camera, and a sensor monitors its signals and stops the transmitter when the strongest signal shows that the lens is in focus. This automatically triggers the shutter.

Some instant cameras have ultrasonic focusing similar to the echolocation scanning system bats use to navigate. A gold-plated disc (the transducer) sends out ‘chirps’ too high to be heard by human ears, each lasting 1/100th of a second. The disc receives the chirp echoes from the subject, and a built-in microcomputer measures the time each chirp takes to go out and come back. From this it calculates the distance to the subject.

SLR (single-lens reflex) cameras with an auto-focus use what is known as an electronic phase detection system. In this, light entering through the lens is separated into two images. A sensor measures the distance between the two images, which are a specific distance apart when the lens is in focus. If the distance is not correct, the sensor causes a motor to move the lens.

 

Picture Credit : Google

The word ‘laser’ is made up from the initials of the words that describe its action, Light Amplification by Stimulated Emission of Radiation. An American physicist, Theodore H, Maiman, invented and first demonstrated it in 1960.

One of the earliest types of laser was the solid ruby laser, made from a ruby crystal or an artificial ruby rod. The chromium atoms in the ruby are stimulated to emit the laser light. An electronic flash tube coiled round the rod gives out intense bursts of light that excite the chromium atoms from a low-energy to a high-energy state.

After a few thousandths of a second, the atoms revert to their normal state, spontaneously emitting an energy package known as a photon. When a photon strikes another chromium atom still in a high-energy state, it stimulates it to emit an identical photon.

The parts of identical photons travel together in the same direction and exactly in step. The beam is built up by millions of them being reflected back and forth between mirrors at each end of the ruby rod. It finally emerges through a half-silvered mirror at one end, in bursts (pulses) of red light of about one-thousandth of a second.

The laser’s power lies not in the amount of its energy, but in the concentration. The beam is very straight, and the photons – all strike the same surface at the same moment. a laser beam can be powerful enough to burn a hole in a steel plate, or delicate enough to be used in eye surgery.

The smallest lasers now in use are semiconductor lasers. They produce an invisible infrared beam when charged with an electric current.

 

Picture Credit : Google