9.在人教版高一(上)第五课有这样两段话:“W hen you have fin ished your experim en t,please tidy the lab an d put everythin g back in the cupboards.”“W hat about w hen w e leave?”“Before you leave the lab,m ake sure the electricity is turn ed off an d the w in dow s are shut....”突出这两段话在学生生活中的实用性和教育性,你组织学生进行什么样的活动?___________ 。
1How was one telephone connected with another in the past?
2How is one telephone connected with another today ?
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 signal is changed back into sound.
he earliest telephones were not popular and it took a long time before people began to hire or buy them .At the befin ningit was difficult to hear people over long distances.The wires which carried 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 telephone.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 made 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 1990there were over 300 million.
There have been great advances in telephone equipment in the last quarter of the twentieth century.For example,radio waves can be used to send telephone signals over long distances,of ten by satellite.T he problem with electrical sifnals is that they fet weaker and weaker as they travel alongmetal wires.They need to be streng thened every 3-4km .Light signals,however,can travel 150 km without getting weaker,and their signal is clearer too.So now adays,in stead of using metal wires to send electrical signals,very thin flass pipes,about 1mm wide,are used to send light signals.
A person\'s voice is changed into electrical signals.These electrical signals are turned into light signals which are sent down the thin“pipe”.At the other end,the light sifnals are received and changed back into electrical signals and finally sound.作者: 默然回首 时间: 07-3-17 13:44 作者: garypro 时间: 07-3-17 17:11
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