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with twice the spacing as the color fluor lines. With alternate wires at about +4200
and +4800 volts respectively, and the screen at about 18,000 volts, the narrow
incoming beam is brought to a focus.
MEANS OF TELECOMMUNICATIONS
TELECOMMUNICATIONS is the name given to the field of communications
that includes the transmission of information by such things as telegraph,
telephone, telex, facsimile, radio, and television.
Telecommunications systems fall into two categories: broadcasting and person-
to-person communications. Broadcasting, which is carried out by radio and
television companies, is the transmission of news and entertainment to the general
public. Person-to-person communications is the sending of messages by such
means as telephone or facsimile between particular people or organizations. This
article deals with person-to-person communications; the other branch of
telecommunications is dealt with in the article BROADCASTING.
The Telegraph
The telegraph was the earliest device for modern telecommunications, and
because of this the former name for the whole field of distant communications was
for a long time known as telegraphy. The electric telegraph was invented by the
American Samuel Morse during the 1830s. It was an early application of the link
between electricity and magnetism. An electric current was sent along a wire from
a battery in the form of coded bursts or "pulses" that represented the morse code.
The pulses were created by the sender who used a switch called a key to turn the
current on and off. At the receiving end was a device for making a sound (the
sounder) every time current passed through it. The sounder was easily adapted to
wireless telegraphy when radio was invented by Guglielmo Marconi in the early
1900s. Unlike the electric telegraph, wireless telegraphy needed no wires and could
be used to communicate with ships far out to sea.
For many years only one message in one direction could be sent or received at
one time. The use of multiple electrical circuits introduced in the second half of the
19th century allowed several messages to pass back and forth simultaneously. In
1872 a Frenchman named Jean-Maurice-Emile Baudot invented a so-called time-
division multiplex circuit in which a number of messages could be sent over the
same line at the same time. Baudot's method linked printing machines at the
transmitting end to similar machines at the receiving end. Each sender typed his
message on a keyboard similar to a typewriter and the corresponding keys on the
receiver's machine were made to work by an electrical pulse. The reason why
Baudot's machine was such a revolution was that it sent each character (letter) of
each message one at a time and in a strict order or rotation, so that the users
literally shared time on the line. Baudot's equipment was so arranged that each
pulse of electricity could be made to represent a unique code that matched a
character on the operator's keyboard. Baudot's system of codes is still in use today,
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