Telephone and Fax

Telephone work by turning the sounds of the human voice into electrical signals. For 'land-line' phones, these electrical signals are sent along cables, and then turned back into sound at the other end. Mobile phones do the same thing using radio waves, while faxes send a stream of electrical signals, to be decoded at the other end by another fax machine.

How does a fax work?
A fax machine scans a page by dividing it into thousands of tiny squares and checking whether each square is black or white. It then sends a stream of yes/no signals that follow the pattern of black/white squares on the original, which the receiving fax then prints out. A fax does not recognise text, but sees it as a pattern of black marks.

Why must you not use a mobile phone on an aeroplane or in a hospital?
Mobile phones are small radio transmitters. They create an electrical field when they are used, and this can disturb sensitive equipment nearby, such as an aeroplane's navigation and control systems. Hospital equipment - heart monitors and kidney dialysis machines, for example - can also be affected. Since they send and receive signal even in 'standby' mode, mobile phone should always be switched off completely in aircraft and hospitals.

Making a telephone call
When you dial the phone number of a friend in another town, your call goes first to your local telephone exchange passes it on to the nearest trunk (long-distance) exchange, which sends it to the trunk exchange closest to your friend's town. This directs the call to a local exchange, which sends it straight to your friend's home. This all happens almost instantly - as your finger lifts off the last number dialled, your friend's phone is already ringing. International calls go straight from your nearest trunk exchange to an international exchange. This sends the call to its destination by undersea cable or satellite.

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Electromagnets
Some magnets work all the time, but electromagnets produce magnetism only when they are supplied with an electric current. This is useful, because it means that they can be switched off when not needed. Electromagnets operate doorbells, microphones and loudspeakers, and they are inside telephones, computer disc drives and televisions. They are also used for lifting heavy metal objects-like wrecked cars.

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How do magnets work?
In an ordinary piece of iron, the electrons spin in different directions. But in an iron magnet, they all spin in the same direction. This creates a magnetic field, an invisible region where magnetic forces are at work. Put some iron fillings on a piece of thin card, and a magnet underneath. If you tap the card, the fillings line up in the direction of the field.

Magnetic field
Magnets have two poles. The magnetic field spreads around the magnet from one pole to the other.

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Static experience
Static electricity, the kind that makes clothes crackle, is produced when particles are rubbed off one object and left on another. Touching a machine called a Van de Graaff generator, which is specially designed to make static electricity. It is quite harmless, but can produce some hair-raising effects.

How the Van de Graaff generator works?
Charging up : The Van de Graaff generator makes static electricity by creating charged particles called ions. These are atoms that have extra electrons, which give them a negative charge. The negative ions are created on needles in the base of the generator, and are carried to the base of the generator, and are carried to the top by a rubber belt. They are collected by another set of needles, and transferred to a metal domes, which takes on their negative charge.
Lift of : When touches the dome, the negative ions spread into body, to the tips of hair. Each hair becomes negative, which means that it repels every other hair, and they all stand on end.

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How do batteries store electricity?
Unlike heat, electricity cannot be stored until it is needed. Instead, it has to be made. Batteries make electricity by using chemical reactions. In a battery, the reaction creates a charge at its negative terminal, so that it is ready to produce a current. As soon as you switch on, the battery drives electrons around the circuit. It can keep the electrons moving until its supply of chemicals is used up, and it goes flat.

Harmless shocks
Static electricity builds up in houses, cars and clothes, and can sometimes give you a shock. These shocks can measure up to 50 000 volts, but they involve few electrons and are not nearly as dangerous as ones from a household electricity supply.

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