Unit23telephones004127王冠清.ppt

Unit23 lesson90 Self-introduction 导入新课Introduction Unit23 lesson90 Listen to the text and Answer the following questions: Question 2 Read Paragraph 3-5 and answer the following questions. Question 4 Question 5. Lesson 90 Telephones How a telephone works is a question which not everyone can answer. When you speak into the telephone,the sound waves of your voice are turned into electrical signals.The signal is sent down? a pair of wires to the receiving telephone. There the electrical signals is changed back into sound. The earliest telephone were not popular and it took a long time before people began to hire or buy them. At the beginning it was difficult to hear people over long distances.The wires which carries the telephone conversations were thick,heavy and expensive. In those days, to make a call, you had to wind a handle at the side of your phone. This produced enough electricity to make your call.You called the operator and told her which number you wanted. The operator connected the two numbers and make a note of the length of the telephone call and the charge. The first telephone directory was printed in a town in the USA in 1878.It was just one piece of paper, printed on one side. In 1877 there were only 2,600 telephones in the world. By 1900 there were over 300 million. There have been great advances in telephone equipment in the last quarter of the century. For example, radio waves can be used to send telephone signals over long distances, often by satellite. The problem with electrical signals is that they get weaker and weaker as they travel along metal wires. They need to be strengthened every 3-4 km. Light signals, however, can travel 150 km without getting weaker, and their signal is clearer too. So nowadays, instead of using metal wires to send electrical signals, very thin glass pipes, about 1 mm wide, are used to send light signals. A persons voice is changed into electrical signals. These electrical signals are