In ancient times the only way that men could send messages from
village to village was on foot. When men learned to use the horse,
communication became much quicker. However, compared to the modern
world communication was still quite slow. Many parts of the world
had no knowledge of events in other places. Later, the post was
introduced and horse riders carried letters. This meant that
communication was further improved. Horses drawn coaches could move
people from town to town in quite a comfortable way. At the
beginning of the last century the steam train was invented and for
the first time really fast communication became possible. Not only
could letters be sent easily from one part of a country to another,
but travel was made easy, too. At about the same time, steam ships
helped communication between countries.
The invention of the telegraph in the middle of the last century
further increased the speed at which messages could be sent. In this
单元整体教案NSEFCII-U02(The first period)由www.yf1234.com收集及整理,转载请说明出处www.yf1234.com
www.yf1234.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
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单元整体教案NSEFCII-U02(The first period)