Timothy Hall,毕业于美国哥伦比亚大学,英语语言学专业硕士。
Arthur H. Milch,美国新泽西学院TESOL专业教育学硕士。
Denise McCormack,美国新泽西学院TESOL专业教育学硕士。
The principles making radio possible were developed throughout the nineteenth century. The first breakthrough was by the British physicist Michael Faraday in 1831. He discovered that when an electric current passes through one wire, it produces a current in another wire even though the wires do not touch each other. In 1864, James Maxwell showed that this current——composed of electromagnetic waves——travels at the speed of light. Heinrich Hertz proved that the waves pass through solid objects. With these discoveries in place, the race was on to develop a system of wireless radio.
Among the contenders was Reginald Fessenden, a Canadian inventor. Fessenden began experimenting with radio detectors in order to explore the possibility of voice transmission. In 1900, he was the first person to transmit his voice, but the sound was unrecognizable because the waves were not continuous. He invented a barreter detector, taking its name from the French word exchanger, to receive AM (amplitude modulated) signals, but it was not sensitive enough. One day in 1901, he accidentally left a filament of wire in acid for too long until only a tip of the wire was in contact with the acid. Fessenden noticed that, with the wire in the acid, the barreter was very sensitive to nearby continuous radio waves.
Fessenden called his invention a liquid barreter, but it became known as an electrolytic detector. The detector consisted of several connected parts forming an electric circuit. A silver-coated platinum wire was dipped into a small platinum cup filled with nitric or sulfuric acid and connected to the ground. A battery was connected between the wire and the acid, prompting a current to flow in the detector. Someone wearing headphones that were hooked up to the detector could hear a hissing noise, which could be adjusted by turning a dial until the hissing noise stopped. At that point, the detector was highly sensitive to incoming radio waves.
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这本托福阅读还是不错,好好准备考试,
评分之前买了中级,感觉不错又买了高级
评分印刷清晰,内容丰富,喜欢!
评分很好,分量很足,关键就看自己有没有时间看了。
评分感觉还行,没有想象中的好用,英语还是得靠自己努力啊,加油加油
评分编写的不错,考试准备自学材料有帮助。
评分书籍很好,一般的备考书籍刷题的上来就是很标准的考试模式,这个书把难度分级,学习循序渐进,非常容易上手,而且对于考试模式也是满满的教给你,很易于接收。
评分 评分2
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