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.

Tag :telephone
Tag :fax