Прочитайте текст и выберите утверждение соответствующее содержанию текста Teletypewriters are widely...: ответ на тест 223249 - Английский язык

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Teletypewriters are widely used for high speed telegraph communications as well as for transmission of written messages. For economical transmission over long distances it is essential that only a single wire or transmission channel be required to carry the signal. Furthermore, the reliable and efficient operation can be secured by using not more than two conditions on the line, such as current and no current, or positive impulses and negative impulses.

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Teletypewriters are widely used for high speed telegraph communications as well as for transmission of written messages. For economical transmission over long distances it is essential that only a single wire or transmission channel be required to carry the signal. Furthermore, the reliable and efficient operation can be secured by using not more than two conditions on the line, such as current and no current, or positive impulses and negative impulses.

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Real telegraphs were possible until it was proved that electricity and magnetism were connected. It was found that a needle of a compass moved when it was put near а wirе with a current flowing through it. A compass needle is a magnet. If a wire carrying a current could move it, then the wire must be a magnet too.

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Only at the end of the 19th century electricity began to play its part in modern civilization. The man who achieved more in this field of practical engineering than any of his contemporaries was the American inventor, Thomas Alva Edison. He became interested in the problem of electric lighting. He knew what had been done in the field of electric lighting before his time, and he had seen some devices of his contemporaries, such as arc-lamp illuminations, which had been installed here and there. Two sticks of carbon, nearly touching, can be made to produce an electric arc which closes the circuit. Many scientists and inventors working in this field knew that only incandescent electric light - produced by some substance glowing in a vacuum so that it cannot burn up - could be used instead of gas lighting.

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Through the years other machines were invented, both to receive and transmit messages. These machines are called teleprinters. They use a code called the International 5-unit teleprinter code instead of the Morse code. It is called a "5-unit" code because a letter or a figure is made up of five impulses of electrical current, either positive or negative - three positive, two negative; one positive, four negative; and so on. All the letters are the same length in time. This is quite different from the Morse code, where E is the shortest and "nought" the longest.

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Radio was invented in Russia. The world's first receiver was constructed in 1895 by the great Russian scientist Alexander Popov. There were no transmitters then, therefore his receiver could only pick up signals produced by lightning discharges during a thunderstorm. A. Popov demonstrated a device called a "storm indicator" at a meeting of the Russian Physico-Chemical Society in St. Petersburg on May 7, 1895. This day is marked now as Radio Day. Soon Alexander Popov found a way of transmitting Morse code signals. In 1897 he sent the world's first wireless telegram over a distance of 600 m and four years later the range of transmission was increased to 150 km. Nowadays radio communication has no limits.