发表于2025-02-02
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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新东方·新托福考试专项进阶:高级阅读 下载 mobi pdf epub txt 电子书 格式 2025
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评分IBT 全称 Internet-based Test 的缩写,即托福网考,也就是现在的新托福,与旧托福最大的区别是多了个“网”,也就是通过网络,而不再是纸质的考试。新托福考试是以互联网为依托,取代了以计算机为依托的托福考试(CBT)。网考首先应用于美国、加拿大、法国、德国和意大利,托福网络考试始于2005年末,并在2006年里在全世界普及。2004年之前的toefl是用的PBT,也就是paper basic test,是toefl考试机构ETS对考试的形式做的巨大改变,在国内考toefl是用计算机考试的,以前是paper text,题型也发生了很大的变化,如果你对自己的英语水平没有把握特别在听力和口语方面(IBT的难点)你可以去美国考PBT,全世界只有美国本土还保留着PBT。[2]
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评分新东方·新托福考试专项进阶:高级阅读 mobi epub pdf txt 电子书 格式下载 2025