Category Everyday Science

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