John Stringer finds lesson ideas in your pupils' pockets
Mobile phones are radios, but unlike the two-way device, they don't send signals to a central antenna. Instead they transmit to an aerial that can be anything up to 56km away. The areas that an antenna can cover are called "cells", hence the mobile's original name, the cellular phone. When drawn on a map, the cells have a honeycomb shape.
The concept was developed in 1947 when researchers working on car phones realised that more masts covering a smaller area would increase the number of phones that could be used simultaneously. While only about 25 people can use a radio transmitter at one time, using the cell system many more people can use mobile phones. As the user moves away from a mast, the phone seeks out another antenna to continue the call. The cells overlap, which eases the problems of too many people trying to use their phones at once.
Dr Martin Cooper, of the Motorola Company, invented the first modern portable handset. In April 1973 he made the first mobile phone call and by 1977 there was a prototype cellular system across the US - but demand quickly outstripped the ability of the system. Since then, digital technology, which compresses the amount of information used to make a call, has further improved the ability of antennae to handle multiple calls.
Nowadays, possession of a mobile phone is seen by some as almost essential.
Each new model is smaller than the last and packs in more features. Makers say phones will become smaller, faster and offer a wider range of services such as photo-messaging and email.
None of this would have been possible, however, without the invention of the telephone. The first phone call, in 1876, consisted of the words: "Mr Watson, come at once. I want you." People have been making calls ever since Alexander Graham Bell spilt battery acid on his trousers and phoned his assistant in the next room for help.
It was Bell who invented the microphone that took the vibrations made by the human voice and turned them into electrical signals. He also invented the loudspeaker that took those electrical signals and turned them back into sound.
By putting his two inventions in a handset and linking them with wire, he made it possible to speak to someone in another town or country. Modern telephones are similar. The sound waves from your voice vibrate a plastic sheet in the mouthpiece, which moves a tiny coil of copper wire close to a magnet, so sending an electrical signal down the wire. The earpiece works in the same way. The electrical signals vibrate another thin plastic sheet.
The vibrations convert the signals to sound, and you hear the voice.
The telephone's microchips remember the number you press and send it to the telephone exchange as pulses of electricity or sound tones.