THE TELEGRAPH
We come now to one of the most important inventions of the nineteenth
century, the electric telegraph. From very early times men have felt
the necessity for some means of rapidly communicating between two
distant points. The first really practical method of signalling was
that of lighting beacon fires on the tops of hills, to spread some
important tidings, such as the approach of an enemy. From this simple
beginning arose more complicated systems of signalling by semaphore,
flags, or flashing lights. All these methods proved incapable of
dealing with the rapidly growing requirements of commerce, for they
were far too slow in action, and in foggy weather they were of no
use at all. We are so accustomed to walking into a telegraph office,
filling up a form, and paying our sixpence or more, that it is very
difficult for us to realize the immense importance of the electric
telegraph; and probably the best way of doing this is to try to imagine
the state of things which would result if the world’s telegraphic
instruments were put out of action for a week or two.
The earliest attempts at the construction of an electric telegraph
date back to a time long before the discovery of the electric current.
As early as 1727 it was known that an electric discharge could be
transmitted to a considerable distance through a conducting substance
such as a moistened thread or a wire, and this fact suggested the
possibility of a method of electric signalling. In 1753 a writer in
_Scott’s Magazine_ brought forward an ingenious scheme based upon
the attraction between an electrified body and any light substance.
His telegraph was worked by an electric machine, and it consisted
of twenty-six separate parallel wires, every wire having a metal
ball suspended from it at each end. Close to each ball was placed a
small piece of paper upon which was written a letter of the alphabet.
When any wire was charged, the paper letters at each end of it were
attracted towards the metal balls, and in this way words and sentences
were spelled out. Many other systems more or less on the same lines
were suggested during the next fifty years, but although some of them
had considerable success in an experimental way, they were all far too
unreliable to have any commercial success.
With the invention of the voltaic cell, inventors’ ideas took a new
direction. In 1812 a telegraph based upon the power of an electric
current to decompose water was devised by a German named Sömmering.
He used a number of separate wires, each connected to a gold pin
projecting from below into a glass vessel filled with acidulated water.
There were thirty-five wires in all, for letters and numbers, and
when a current was sent along any wire bubbles of gas formed at the
pin at the end of it, and so the letters or numbers were indicated.
This telegraph, like its predecessors, never came into practical
use. Oersted’s discovery in 1829 of the production of magnetism by
electricity laid the foundation of the first really practical electric
telegraphs, but little progress was made until the appearance of the
Daniell cell, in 1836. The earlier forms of voltaic cells polarized so
rapidly that it was impossible to obtain a constant current from them,
but the non-polarizing Daniell cell at once removed all difficulty in
this respect. In the year 1837 three separate practical telegraphs
were invented: by Morse in the United States, by Wheatstone and Cooke
in England, and by Steinheil in Munich.
[Illustration: FIG. 26.--Dial of Five-Needle Telegraph.]
The first telegraph of Wheatstone and Cooke consisted of five magnetic
needles pivoted on a vertical dial. The letters of the alphabet were
marked on the dial, and the needles were deflected by currents made to
pass through wires by the depression of keys, so that two needles would
point towards the required letter. Fig. 26 is a sketch of the dial of
this apparatus. This telegraph was tried successfully on the London
and North-Western Railway, over a wire a mile and a half in length.
Wheatstone and Cooke afterwards invented a single-needle telegraph in
which the letters were indicated by movements of the needle to the
right or to the left, according to the direction of a current sent
through a coil of wire. Wheatstone subsequently produced an apparatus
which printed the letters on paper.
In the United States, Morse had thought out a scheme of telegraphy
in 1832, but it was not until 1837 that he got his apparatus into
working order. He was an artist by profession, and for a long time he
was unable to develop his ideas for lack of money. After many efforts
he succeeded in obtaining a State grant of £6000 for the construction
of a telegraph line between Baltimore and Washington, and the first
message over this line was sent in 1844, the line being thrown open
to the public in the following year. Amongst the features of this
telegraph were a receiving instrument which automatically recorded the
messages on a moving paper ribbon, by means of a pencil actuated by
an electro-magnet; and an apparatus called a relay, which enabled the
recording instrument to be worked when the current was enfeebled by the
resistance of a very long wire. Morse also devised a telegraphic code
which is practically the same as that in use to-day.
The great discovery of the German Steinheil was that a second wire for
the return of the current was not necessary, and that the earth could
be used for this part of the circuit.
In reading the early history of great inventions one is continually
struck with the indifference or even hostility shown by the general
public. In England the electric telegraph was practically ignored
until the capture of a murderer by means of it literally forced the
public to see its value. The murder was committed near Slough, and the
murderer succeeded in taking train for London. Fortunately the Great
Western Railway had a telegraph line between Slough and London, and a
description telegraphed to Paddington enabled the police to arrest the
murderer on his arrival. In the United States too there was just the
same indifference. The rate for messages on the line between Baltimore
and Washington was one cent for four words, and the total amount taken
during the first four days was one cent!
One of the simplest forms of telegraph is the single-needle
instrument. This consists of a magnetic needle fixed to a spindle at
the back of an upright board through which the spindle is passed. On
the same spindle, but in front of the board, is fixed a dial needle,
which, of course, moves along with the magnetic needle. A coil of wire
is passed round the magnetic needle, and connected to a commutator for
reversing the direction of the current. By turning a handle to the left
a current is made to flow through the coil, and the magnetic needle
moves to one side; but if the handle is turned to the right the current
flows through the coil in the opposite direction, and the needle moves
to the other side. Instead of a handle, two keys may be used, the
movement of the needle varying according to which key is pressed. A
good operator can transmit at the rate of about twenty words a minute
with this instrument. The Morse code, which consists of combinations
of dots and dashes, is used, a movement of the dial needle to the left
meaning a dot, and one to the right a dash. The code as used in the
single-needle instrument is shown in Fig. 27.
[Illustration: FIG. 27.--Code for Single-Needle Telegraph.]
Needle instruments are largely used in railway signal cabins, but for
general telegraphic work an instrument called the Morse sounder is
employed. This consists of an electro-magnet which, when a current is
passed through it, attracts a small piece of iron fixed to one end of
a pivoted lever. The other end of this lever moves between two stops.
At the transmitting station the operator closes a battery circuit by
pressing a key, when the electro-magnet of the sounder at the receiving
station attracts the iron, and the lever flies from one stop to the
other with a sharp click, returning again as soon as the circuit is
broken. A dot is signalled when the lever falls back immediately after
the click, and a dash when it makes a short stay before returning. Fig.
28 shows the code of signals for the Morse telegraph.
[Illustration: FIG. 28.--The Morse Code.]
In passing through a very long wire an electric current becomes greatly
reduced in strength owing to the resistance of the wire. If two
telegraph stations are a great distance apart the energy of the current
thus may be unequal to the task of making the electro-magnet move the
lever of the sounder so as to produce a click, but this difficulty is
overcome by the use of an ingenious arrangement called a “relay.” It
consists of a very small electro-magnet which attracts a light bar,
the movement of the bar being made to close the circuit of another
battery at the receiving station. The feeble current works the relay,
and the current in the local circuit operates the sounder.
The word “telegraph,” which is derived from the Greek _tele_, far
off, and _grapho_, I write, strictly signifies writing at a distance.
The needle instrument and the sounder do not write in any way, but by
modifying the construction of the sounder it can be made to record the
messages it receives. A small wheel is fitted to the free end of the
lever of the sounder, and an ink-well is placed so that the wheel dips
into it when the lever is in the normal position. When the circuit is
closed the lever moves just as in the ordinary sounder, but instead
of clicking against a stop it presses the inked wheel against a paper
ribbon which is kept slowly moving forward by clockwork. In this way
the wheel continues to mark a line along the paper as long as the
circuit remains closed, and according to the time the transmitting key
is kept down a short mark or dot, or a long mark or dash, is produced.
The clockwork which moves the paper ribbon is started automatically by
the current, and it continues working until the message is finished.
[Illustration: FIG. 29.--A Morse Message.
(_a_) Perforated Tape.
(_b_) Printed Tape.
TRANSLATION.
_Series of alternate dots and dashes indicating commencement of
message._
Sec (_section_) A. D. T. (_Daily Telegraph_) Fm (_from_) Berri,
Antivari.
_Then follow the letters_ G. Q., _signifying fresh line_.
They hd (_had_) bn (_been_) seen advancing in t (_the_) distance and
wr (_were_) recognised by thr (_their_) usual uniform wh (_which_)
consists o (_of_) a white fez.
_Finally double dots indicating full stop._]
A good Morse operator can maintain a speed of about thirty words a
minute, but this is far too slow for certain kinds of telegraphic
work, such as the transmission of press news, and for such work the
Wheatstone automatic transmitter is used. First of all the messages
are punched on a paper ribbon. This is done by passing the ribbon
from right to left by clockwork through a punching machine which is
provided with three keys, one for dots, one for dashes, and the other
for spaces. If the left-hand key is pressed, two holes opposite to one
another are made, representing a dot; and if the right-hand key is
pressed, two diagonal holes are punched, representing a dash. In Fig.
29, which shows a piece of ribbon punched in this way, a third line of
holes will be noticed between the outside holes representing the dots
and dashes. These holes are for the purpose of guiding the paper ribbon
steadily along through the transmitting machine. The punched ribbon
is then drawn by clockwork through a Wheatstone transmitter. In this
machine two oscillating needles, connected with one pole of a battery,
are placed below the moving ribbon. Each time a hole passes, these
needles make contact with a piece of metal connected with the other
pole of the battery, thus making and breaking the circuit with much
greater rapidity than is possible with the Morse key. At the receiving
station the messages are recorded by a form of Morse inker, coming out
in dots and dashes as though sent by hand. Below the punched ribbon
in Fig. 29 is shown the corresponding arrangement of dots and dashes.
The same punched ribbon may be used repeatedly when the message has
to be sent on a number of different lines. The Wheatstone automatic
machine is capable of transmitting at the rate of from 250 to 400
words a minute. Fig. 29 is a fragment of a _Daily Telegraph_ Balkan
War special, as transmitted to the _Yorkshire Post_ over the latter’s
private wire from London to Leeds. In the translation it will be seen
that many common words are abbreviated.
One weak point of telegraphy with Wheatstone instruments is that the
messages are received in Morse code, and have to be translated. During
recent years telegraphs have been invented which actually produce their
messages in ordinary written or printed characters. A very ingenious
instrument is the Hughes printing telegraph, which turns out messages
in typewritten form. Its mechanism is too complicated to be described
here, but in general it consists of a transmitter having a keyboard
something like that of a typewriter, by means of which currents of
electricity are made to press a sheet of paper at the right instant
against a revolving type-wheel bearing the various characters. This
telegraph has been modified and brought to considerable perfection,
and in one form or another it is used in European countries and in the
United States.
In the Pollak-Virag system of telegraphy the action of light upon
sensitized photographic paper is utilized. An operator punches special
groupings of holes on a paper ribbon about 1 inch wide, by means of
a perforating machine resembling a typewriter, and the ribbon is
then passed through a machine which transmits by brush contacts. The
receiver consists of a very small mirror connected to two vibrating
diaphragms, which control its movements according to the currents
received, one diaphragm moving the mirror in a vertical direction, and
the other in a horizontal direction. The mirror reflects a ray of light
on to photographic bromide paper in the form of a moving band about 3
inches in width, and the combined action of the two diaphragms makes
the mirror move so that the ray of light traces out the messages in
ordinary alphabetical characters. As it moves forward after being acted
upon by the light, the paper is automatically developed and fixed, and
then passed through drying rollers. Although the writing is rather
imperfect in formation it is quite legible enough for most messages,
but trouble occasionally occurs with messages containing figures, owing
to confusion arising from the similarity of the figures, 3, 5, and 8.
The whole process is carried out with such rapidity that 40,000 or even
more words can be transmitted easily in an hour.
One of the most remarkable of present-day telegraphs is the Creed
high-speed automatic printing telegraph. This has been devised to do
away with hand working as far as possible, and to substitute quicker
and more accurate automatic methods. In this system a perforated paper
tape is produced by a keyboard perforator at the sending station. This
tape is just ordinary Wheatstone tape, its perforations representing in
the Morse code the message to be transmitted; and the main advantage
of the Creed perforator over the three-key punching machine already
described lies in the ease and speed with which it can be worked. The
keyboard contains a separate key for each letter or signal of the Morse
code, and the pressing of any key brings into operation certain punches
which make the perforations corresponding to that particular letter.
The perforator can be worked by any one who understands how to use an
ordinary typewriter, and a speed of about 60 words a minute can be
maintained by a fairly skilful operator. If desired a number of tapes
may be perforated at the same time.
The tape prepared in this way is passed through a Wheatstone
transmitter, and long or short currents, according to the arrangement
of the perforations, are sent out along the telegraph line. At the
receiving station these signals operate a receiving perforator. This
machine produces another perforated tape, which is an exact copy of
the tape at the sending station, and it turns out this duplicate tape
at the rate of from 150 to 200 words a minute. There are two forms of
this receiving perforator, one worked entirely by electricity, and
the other by a combination of electricity and compressed air, both
forms serving the same purpose. The duplicate tape is then passed
through an automatic printer, which reproduces the message in large
Roman characters on a paper tape. The printer works at a speed of from
80 to about 100 words a minute, and the printed tape is pasted on a
telegraphic form by a semi-automatic process, and the message is then
ready for delivery. Plate XI. shows a specimen of the tape from the
receiving perforator, and the corresponding translation as turned out
by the printer. This message formed part of a leading article in the
_Daily Mail_. Some idea of the wonderful capabilities of the Creed
system may be gained from the fact that by means of it practically the
whole contents of the _Daily Mail_ are telegraphed every night from
London to Manchester and Paris, for publication next morning.
One of the most remarkable features about present-day telegraphy is
the ease with which two or more messages can be sent simultaneously
over one line. Duplex telegraphy, or the simultaneous transmission of
two separate messages in opposite directions over one wire, is now
practised on almost every line of any importance. At first sight duplex
telegraphy seems to be an impossibility, for if we have two stations,
one at each end of a single wire, and each station fitted with a
transmitter and a receiver, it appears as if each transmitter would
affect not only the receiver at the opposite end of the wire, but also
the receiver at its own end, thus causing hopeless confusion when both
transmitters were in use at the same time. This actually would be the
case with ordinary telegraphic methods, but by the use of a special
arrangement all confusion in working is avoided.
[Illustration: PLATE XI.
_By permission of_ _Creed, Bille & Co. Ltd._
SPECIMEN OF THE WORK OF THE CREED HIGH-SPEED PRINTING TELEGRAPH.]
We have seen that a magnetic needle is deflected by a current passing
through a coil of wire placed round it, and that the direction in which
the needle is deflected depends upon the direction of the current in
the coil. Now suppose we place round the needle two coils of wire,
wound so that the current in one flows in a direction opposite to that
of the current in the other. Then, if we pass two equal currents,
one through each coil, it is evident that they will neutralize one
another, so that the needle will not be deflected at all. In a duplex
system one end of one of these coils is connected to earth, say to a
copper plate buried in the ground, and one end of the other to the
line wire. The two remaining ends are arranged as branches leading
off from a single wire connected with the transmitting key. The whole
arrangement of coils and needle is repeated at the other end of the
line. If now the transmitting key at station A is pressed, the circuit
is closed and a current flows along the single wire, and then divides
into two where the wire branches, half of it taking the path through
one coil and half the path through the other. Equal currents thus flow
through the oppositely wound coils, and the needle at station A is not
deflected. Leaving the coils, one of these equal currents flows away to
earth, while the other passes out along the line wire. On its arrival
at station B the current is able to pass through only one of the coils
round the needle, and consequently the needle is deflected and the
signal given. In this way the transmitting operator at station A is
able to signal to station B without affecting the receiver at his own
end, and similarly the operator at station B can transmit to A without
affecting the B receiver. Thus there can be no confusion whether the
transmitters are worked at different times or simultaneously, for each
transmitter affects only the receiver at the opposite end of the line.
The diagram in Fig. 30 will help to make clearer the general principle.
K and K¹ are the two transmitting keys which close the circuit, and C
and C¹ are the points at which the current divides into two. Instead
of coils and needles, electro-magnets operating sounders may be used,
such magnets having two separate and oppositely wound coils, acting
in exactly the same way as the coils round the needles. The above
description is of course only a rough outline of the method, and
in practice matters are more complicated, owing to the necessity for
carefully adjusted resistances and for condensers. There is also
another and different method of duplexing a line, but we have not space
to describe it. Duplex telegraphy requires two operators at each end of
the line, one to send and the other to receive.
Diplex telegraphy is the simultaneous transmission of two separate
messages in the same direction over one line. Without going into
details it may be said that for this purpose two different transmitting
keys are required, one of which alters the direction, and the other
the strength of the current though the line wire. The receivers are
arranged so that one responds only to a strong current, and the other
only to a current in one particular direction. A line also may be
quadruplexed, so that it is possible to transmit simultaneously two
messages from each end, four operators being required at each station,
two to transmit and two to receive. Systems of multiplex telegraphy
have been devised by which very large numbers of messages can be sent
at once over a single wire, and the Baudot multiplex telegraph has
proved very successful.
[Illustration: FIG. 30.--Diagram to illustrate principle of Duplex
Telegraphy.]
The wires for telegraphic purposes may be conveyed either above or
below the ground. Overground wires are carried on poles by means of
insulators of porcelain or other non-conducting material, protected
by a sort of overhanging screen. The wires are left bare, and they
are generally made of copper, but iron is used in some cases. In
underground lines the wires formerly were insulated by a covering of
gutta-percha, but now paper is generally used. Several wires, each
covered loosely with thoroughly dry paper, are laid together in a
bundle, the whole bundle or cable being enclosed in a strong lead
pipe. The paper coverings are made to fit loosely so that the wires
are surrounded by an insulating layer of dry air. As many as 1200
separate wires are sometimes enclosed in one pipe. In order to keep
telegraph lines in working order frequent tests are necessary, and the
most important British Postal Telegraph lines are tested once a week
between 7.30 and 7.45 a.m. The earth is generally used for the return
circuit in telegraphy, and the ends of the return wires are connected
either to metal plates buried in the ground to a depth at which the
earth is permanently moist, or to iron gas or water pipes. The current
for telegraph working on a small scale is usually supplied by primary
cells, the Daniell cell being a favourite for this purpose. In large
offices the current is generally taken from a battery of storage cells.
During the early days of telegraphy, overhead lines were a source of
considerable danger when thunderstorms were taking place. Lightning
flashes often completely wrecked the instruments, giving severe shocks
to those in the vicinity, and in a few cases operators were killed
at their posts. Danger of this kind is now obviated by the use of
contrivances known as lightning arresters. There are several forms of
these, but only one need be mentioned. The main features of this are
two metal plates separated slightly from one another, so that there
is a small air gap between them. One plate is connected to the line
wire, and the other to earth. Almost all lightning flashes consist
of an oscillatory discharge, that is one which passes a number of
times backwards and forwards between a cloud and the earth. A very
rapidly alternating discharge of this kind finds difficulty in passing
along the line wire, being greatly impeded by the coils of wire in
the various pieces of apparatus; and although the resistance of this
air gap is very high, the lightning discharge will cross the gap
sooner than struggle along the line wire. In this way, when a flash
affects the line, the discharge jumps the gap between the plates of
the arrester and passes away harmlessly to earth, without entering
the telegraph office at all. As was mentioned in Chapter III., the
prevalence of magnetic storms sometimes renders telegraph lines quite
unworkable for a time, but although such disturbances cause great delay
and general inconvenience, they are not likely to be at all dangerous.
It is often possible to maintain telegraphic communication during
magnetic disturbances by using two lines to form a complete metallic
loop, so that there is no earth return.