Чтение общенаучной литературы. Кытманова О.А. - 39 стр.

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40
ROADCASTING explains how radio programs are planned and produced;
RADAR explains the use of radio echoes for finding the position of distant objects,
and TELEVISION describes how radio waves are used to bring pictures into the
home.
Early Days of Radio Communication
Until the 19th century nobody knew anything about radio waves. Then in 1864
James Clerk Maxwell, a brilliant Scottish mathematician, on whom there is a
separate article, showed that in theory radio waves, which are lower in frequency
than infra-red waves, must form part of the electromagnetic spectrum. However,
22 years passed before a German scientist, Heinrich Hertz, became the first to
generate these mysterious invisible waves and show that they obeyed the optical
laws governing reflection, refraction, and interference, But neither he nor any other
scientist of the day could find a practical use for them.
Hertzian waves, as they were called, remained merely a puzzling subject for
laboratory research until 1894-5, when a way was found (.0 use them to transmit
Morse code. There is some doubt as to who first used a Morse key (see MORSE,
SAMUEL) and an antenna system in conjunction with a Hertzian transmitter. It
was either a Russian, A. S. Popov, or Guglielmo Marconi, at that time an unknown
young Italian. Popov worked for the Imperial Russian Navy and his work was
secret, and Marconi too worked in secrecy, so no firm date can be given. However,
there is no doubt that Marconi went on to become the greatest influence on the
development of radio. In 1896 Marconi arrived in England and demonstrated his
"wireless apparatus". The transmitter was similar to that used by Hertz. It consisted
of an induction coil which, when connected to a battery, developed a high voltage
across two metal spheres placed close to one another. The air gap between the
spheres broke down under the electrical strain and a stream of sparks jumped
across the gap whenever the Morse key (which Marconi had added to Hertz's
apparatus) was pressed down .
An elevated wire (antenna) was connected to one side of the spark gap and the
other side was grounded, that is, connected to the earth. The stream of sparks
produced a series of oscillations, which radiated into space from the antenna. By
pressing the Morse key for differing periods, the radiations could be made to surge
from the antenna is long or short bursts to form letters in Morse code.
To receive the signals a device called a coherer was used. This was a small glass
tube containing loosely-packed metal filings and was connected between an
antenna and the ground. In the absence of any signals on the receiving antenna, the
electrical resistance of the filings remained high But as soon as a signal arrived, the
metal filings "cohered", or came together, and the resistance dropped considerably.
Whenever the resistance dropped in this way it caused electromechanical relays to
come into operation and work a Morse printer. One disadvantage of the coherer
was that every time a signal caused the filings to cohere, they had to be tapped to
restore them to high-resistance condition. This was done automatically using the
hammer of an electric bell mechanism.
When an operator at the transmitter tapped out a message in Morse code, it was
picked up by the receiver and printed in Morse characters on a paper tape. The

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