单元整体教案NSEFCII-U2017(The first period)

单元整体教案NSEFCII-U2017(The first period)
www.ybf100.com             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 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.

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