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The invention of the telegraph in the middle of the last century further increased the speed at which messages could be sent. In this system electrical signals, in code, are sent along metal wires. These signals travel so fast that they could go nearly eight times round the world in one second. A special device is needed to send the code. At the other end another device is used in order to receive the code. By this method messages can be sent over distances of several hundred kilometres. With the invention of the telephone the human voice could be sent over long distances. Because of this the telephone system replaced the telegraph for quick communication over long distances. The telegraph is still used, however, by newspapers in order to send news and for other purposes too. At the beginning of this century radio was invented and in a few years communication was again improved. The main difference between radio and telephone is that radio uses no electrical signals which travel long distances along wires. Instead invisible waves, moving at the same speed as electrical signals, are used. A few years ago there were not many radio stations in the world. Today there are many hundreds of radio stations broadcasting in different languages and in all countries. The invisible radio waves can easily travel from one country to another. This means that listeners in one country can listen to programmes broadcast from another country. In this way information travels from country to country. Radio is often used by policemen to communicate with on (责任编辑：admin) |