Прочитайте текст и выберите утверждение соответствующее содержанию текста Modern telegraph systems a...: ответ на тест 781527 - Английский язык

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Modern telegraph systems are based upon the fact that electric current will flow through a wire which forms a circuit. The circuit can be broken to interrupt the flow of electricity and closed again for a short time by a key to send an electrical impulse through the circuit. When the key is pressed the circuit is closed. Electricity from the battery flows through the wire and earth circuit and causes the buzzer to make a noise. Using the Morse code, or some other code, messages can be sent in this way.

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Modern telegraph systems are based upon the fact that electric current will flow through a wire which forms a circuit. The circuit can be broken to interrupt the flow of electricity and closed again for a short time by a key to send an electrical impulse through the circuit. When the key is pressed the circuit is closed. Electricity from the battery flows through the wire and earth circuit and causes the buzzer to make a noise. Using the Morse code, or some other code, messages can be sent in this way.

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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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A teleprinter is really a machine which sends a typewriter message over a telegraph circuit. In fact, it looks like a typewriter and has the same keyboard. Every time one of the keys on the keyboard is pressed it sends the five impulses that make one letter or figure. It also sends a "start" impulse. This sets the teleprinter (or any number of them that are connected) at a receiving station ready to print the letter that follows immediately. A "stop" impulse brings all the teleprinters to rest after each letter or figure.

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In the early days of the telegraph and the telephone, men observed that signals sometimes jumped across the space between wires running next to each other, so that messages sent to one person went to another. They realized that electricity could pass through the air as well as along a wire. Just as sound travels through the air in waves, so electricity travels through the air in what we call radio waves. A. S. Popov, a Russian inventor, had the idea of sending messages by radio waves, or "by wireless". He constructed a coherer detector for the study of lightning discharges and his receiver was described in 1895.

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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.