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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 one another. In addition, police forces in one country can communicate with those in other countries in order to catch criminals. Ships at sea use radio so that they know exactly where they are. Aeroplanes use radio for the same reason and this makes it easier for them to find their way from place to place. In the modern world there are many methods of communication. As well as radio there is television, for example. This enables information in the form of a picture to be broadcast from one place to another. Radio is often used as part of a telegraph system where distances are very large. Of the many modern methods of communication, radio probably remains the most important. 2. How the Program Is Broadcast? We turn on the radio and a program comes to us from a broadcasting station miles and miles away. We know that words and music themselves haven’t traveled all that distance through space, but something certainly is bringing the program from the station. What is this silent carrier? The answer is radio waves. We can not see radio waves or feel them or even hear them. In fact, nobody knows exactly what they are. But we do know that they are made by electricity, and we have learned how to use them. At the broadcasting station people talk or sing, instruments (乐器) play, doors slam(砰) ,and all of these make sound waves. The sound waves reach the microphone, and here they are changed into electricity. Then from a tall tower called the broadcasting aerial (天线) ,electricity sends radio waves. The waves travel in every direction, and some of them reach our radio aerial. Now a wonderful thing happens. The radio waves start an electricity current (电流) in our aerial like the one that was first made in the broadcasting station. Finally, the loudspeaker in our set changes electricity into sound, and we hear the program. The First Period (责任编辑：admin) |