No application of science has so completely realized the visions of
fancy as the Electric Telegraph. So closely, indeed, does the real of
the present day approach to the ideal of ages past, that it might be
supposed the narratives in the tales of faëry land were true records
of the inventions of former times, and that the combined efforts of
inventive genius during the last half century were but imitations and
reproductions of what had been successfully accomplished "once upon a
time." There is also an intermediate period--between the indefinite
of faëry tales and the positive of scientific history--in which
sympathetic tablets and magical loadstones, scarcely less mythical, are
stated to have been invented; and the individuals are named who thus
paved the way for instantaneous communication between all parts of the
world.
The Jesuits of the sixteenth and seventeenth centuries took the
place of the magicians of the Middle Ages. In the seclusion of their
monasteries, they speculated on the mysterious powers of Nature, then
partially revealed to them, and shadowed forth images of their possible
applications. It is to a vague speculation of this kind that we may
attribute the notice given by Strada, in his "Prolusiones Academicæ,"
of the sympathetic magnetic needles, by which two friends at a distance
were able to communicate; though the then fanciful idea has been
literally realized. A still more extraordinary foreshadowing of one
of the most recent improvements of the Electric Telegraph was the
transference of written letters from one place to another by electric
agency. This is said to have been accomplished by Kircher, who, in his
"Prolusiones Magneticæ," describes, though very vaguely, the mode of
operation. But even admitting that there were substantial foundations
for these imaginary phantasms, that would not in the least detract from
the merit of those who, following closely the footsteps of scientific
discovery, have successfully applied the principles unfolded by the
investigations of others, and by their own assiduous researches. Thus,
whilst steam navigation was facilitating the means of intercourse over
rivers and seas, and whilst railways and locomotive engines served to
bring distant cities within a few hours' journey of each other, another
source of power, infinitely more rapid in its action than steam, has
been made to transmit intelligence from place to place, and from one
country to another, with the speed of lightning.
The plan of making communications by signals has been in operation from
time immemorial; the beacon lights on hills having served in ancient
as well as in modern times to give warning of danger, or to announce
tidings of joy. Such simple signals were not capable of much variety of
expression; but even beacon lights might be made to indicate different
kinds of intelligence, by multiplying the number of the fires, and
by altering their relative positions. It was not, however, till the
invention of telegraphs that anything approaching to the means of
holding regular communication by signals was attained. The semaphore
of the brothers Chappe, of France, invented by them in 1794, was the
most perfect instrument of the kind, and was generally employed for
telegraphic purposes, until it was supplanted by the Electric Telegraph.
The semaphore consisted of an upright post, having arms on each side,
that could be readily extended, at any given angle. The extension of
these arms on one side or the other, either separately or together,
and at different angles, constituted a variety of signals sufficient
for the purposes of communication. The semaphores, erected on elevated
points, so as to be visible through telescopes, signalled intelligence
slowly from one station to another, till it reached its ultimate
destination; and thus--daylight and clear weather permitting--brief
orders could be sent from the Admiralty to Portsmouth in the course of
a few minutes. But the communication was liable to be interrupted by
fogs, as well as by nightfall.
A remarkable instance of the imperfection of sight telegraphs occurred
during the Peninsular War. A telegraphic despatch, received at the
Admiralty from Portsmouth, announced--"Lord Wellington defeated;"--and
then the communication was interrupted by a fog. This telegraphic
message caused great consternation, and the utmost anxiety was
experienced to learn the extent of the supposed disaster. When,
however, the fog dispersed, the remainder of the message gave a
completely opposite character to the news, which in its completed form
ran thus: "Lord Wellington defeated the French," &c.
Some better means of transmitting important intelligence was evidently
wanted; for not only was the semaphore liable to frequent interruptions
by the weather, but its action was very slow, and the frequent
repetitions from station to station increased the risk of blunders.
The instantaneous transmission of an electric shock suggested the
means of communicating with greatly increased rapidity; and when
it was ascertained, by experiments made by Dr. Watson at Shooter's
Hill, in 1747, that the charge of a Leyden jar could be sent through
a circuit of four miles, with velocity too great to be appreciable,
the practicability of applying electricity for conveying intelligence
became at once apparent.
Of the many means by which this object was attempted to be
accomplished, it will be only possible, in this general survey, to
notice those that mark the first steps of the invention, and the most
important of those that have accompanied its progress to the present
time.
The first method that suggested itself was to transmit signals by
means of pith-ball electrometers. When, for instance, two pith-balls
are suspended from a wire that is made to form part of an electric
circuit, the electricity communicated to the balls causes them to
diverge, and when the electricity in the wire is discharged, they
immediately collapse. This action of pith-balls, when electrified,
was the simplest mode known of making telegraphic signals, and it was
accordingly adopted by several of the early inventors of Electric
Telegraphs. The first person who proposed to apply it for that purpose
was M. Lesage, of Geneva, in 1774. His plan was to form 24 electric
circuits by as many separate wires, insulated from each other in glass
tubes; and to place in the circuit, at each communicating station,
an equal number of pith-ball electrometers. Each electrometer was to
represent a letter of the alphabet, and they were to be brought into
action by an excited glass rod. When a communication was to be made,
the wires connected with the separate galvanometers were to be charged
alternately with electricity by the excited rod of glass; and the
person at the receiving station, by noticing which of the electrometers
were successively put into action, could spell the words intended to be
communicated.
By the means thus proposed, correspondence could have taken place at
only short distances, for the charge of an excited glass rod would have
been too feeble to produce any sensible effect on the electrometers had
the length of the circuit been considerable. This difficulty might
have been overcome by substituting the charge of a Leyden jar for the
excited glass; but the more serious obstacle to the use of such a
telegraph would have been the cost, and the difficulty of insulating
the 24 wires required to work it.
Most of the early telegraphic inventors encumbered their inventions
with the same obstacle, as they seemed to consider it necessary to
have a separate circuit for each letter of the alphabet. It was not so
however, with all; for M. Lomond, a Frenchman, who ranks second in the
list of telegraphic inventors, modified the principle of M. Lesage, so
as to enable him to work with only two wires and one electrometer at
each station. With the experience since gained in the application of
the needle telegraph, such an arrangement seems very simple, and we are
inclined to wonder that it was not generally adopted, especially after
M. Lomond had shown the way.
To produce all the requisite signals with a single pith-ball
electrometer, it was necessary to vary the durations of each
divergence, and to combine several to form a single symbol. Thus,
suppose that a single divergence of the pith-balls for a second was
understood to signify the letter _A_; one divergence, followed by an
immediate collapse, by discharging the electricity, might signify _B_;
two prolonged divergences might signify _C_, and two short ones _D_;
and by thus increasing the number and varying the divergences of the
two pith-balls, all the letters of the alphabet might be indicated.
A still more direct method of representing the letters of the alphabet
was proposed by M. Reizen in 1794, by the application of the means
frequently adopted for exhibiting the light of the electric spark. The
charge of a Leyden jar was sent through strips of tin foil, pasted on
to a flat piece of glass, so as to form several lines, joined at the
ends alternately into a continuous circuit. Interruptions were made
in the foil by cutting small portions away, at which points brilliant
sparks appeared when the jar was discharged. As the interruptions
were so contrived as to form letters, and the strips of tin foil were
all arranged separately on a long pane of glass, any letter required
could be distinctly made visible by discharging the jar through that
particular circuit. To produce all the letters of the alphabet in this
manner, a separate circuit was required for each.
Another plan, far less feasible, and scarcely deserving of notice,
excepting for its peculiarity, was proposed in the following year
by M. Cavallo, who suggested the setting fire to combustibles, or
the explosion of detonating substances, as the means of signalling
intelligence. About the same time several attempts were made by
electricians in Spain to transmit signals by electricity, but their
plans were not more practicable than those already mentioned, and
depended for their effects on the discharge of Leyden jars.
The discovery of voltaic electricity at the beginning of the present
century was an important step in the progress of the Electric
Telegraph, though several years elapsed before the applicability of
the discovery for that purpose became known; and it was not fully
appreciated till within the last twenty years.
The electricity generated by the voltaic battery is far greater in
quantity than the most powerful electrical machine can excite, whilst
its intensity is so feeble that it cannot pass in a spark through the
smallest interval of air. It presents, therefore, much less difficulty
in the insulation of the wires than frictional electricity, whilst
the rapidity of its transmission is for practical purposes equally
efficient. The electricity generated by the voltaic battery being great
in quantity and feeble in intensity, it is capable also of effecting
chemical decomposition and of imparting magnetism, both of which
properties have proved eminently useful in perfecting the Electric
Telegraph.
The first application of voltaic electricity to telegraphic purposes
was made by Mr. Soemmering in 1809. The signals of his telegraph
consisted of the bubbles of gas arising from the decomposition of
water, during the action of the electric current. His apparatus
consisted of a small glass trough, filled with acidulated water,
through the bottom part of which were introduced several gold wires
corresponding to the letters of the alphabet. The instant that an
electric current was sent through any two of the wires, by making
connection with a voltaic battery at the transmitting instrument,
bubbles of hydrogen gas rose from one of the gold wires, and bubbles of
oxygen gas from another; and as the volume of hydrogen gas, liberated
during the decomposition of water, exceeds by sixteen times that of
the oxygen, it was easy to distinguish them. In this manner all the
letters of the alphabet could be indicated by using 24 wires. The
object of having gold wires in the decomposing trough was to prevent
the oxidation of the metal; for had copper, or any other metal that
combines with oxygen, been employed, the points of the wires would soon
have become corroded.
This telegraph of Soemmering's, though not adapted for practical
application in the form he presented it, on account of the number of
wires required for the purpose, was nevertheless superior to any that
had previously been invented; and by a little modification it might
have been made a perfect instrument, capable of transmitting messages
by means of only two wires. Such a modification of the instrument was
proposed by M. Schweigger, twenty years afterwards; the only thing
required being the adoption of a code of symbols, by means of which
all the letters might be indicated by combinations of the four primary
signals that are obtainable by two wires, as is at present done by the
needle telegraph in common use. At that time, however, the discovery of
the magnetic properties of the electric current, and other improvements
in the means of communicating, superseded for some years the use of
signals made by electro-chemical decomposition.
The next important step in the progress of telegraphic invention, after
that of Mr. Soemmering, was made by Mr. Ronalds, who in 1816 succeeded
in making a perfect apparatus, that transmitted every requisite signal
with the use of only a single circuit. In the agent employed, however,
there was a retrogression to frictional electricity and the pith-ball
electrometer, for at that time the property which a voltaic current
possesses of deflecting a magnetic needle had not been discovered.
Mr. Ronalds's plan was to have, at each communicating station, a good
clock with a light paper disc fixed on to the seconds wheel, on which
were marked all the letters of the alphabet, and the ten numerals. Only
so much of this disc was exposed to view as to show a single letter
at a time, through a small aperture, as the seconds wheel revolved.
The clocks at the corresponding stations were set exactly together, so
that the same letter was exposed to view at each instrument at the same
instant. A pith-ball electrometer, connected in a single circuit with
the transmitting station, was kept distended during the transmission
of a message by charging the wire from an electrical machine; and when
the letter required to be indicated appeared at the aperture of both
instruments, the operator at the transmitting instrument instantly
discharged the electricity of the wire by touching it, and thus
caused the pith-balls to collapse. In this manner the person at the
receiving station, by attentively watching the pith-balls, and noticing
the letter that appeared at the instant of collapse, could read the
messages signalled.
Mr. Ronalds so far perfected his invention, that it worked accurately,
though slowly, through eight miles of wire insulated in glass tubes.
Having thus succeeded in putting into action his single wire
telegraph, Mr. Ronalds sought the patronage of Government for its
practical adoption, such a notion as that of establishing a telegraph
for commercial purposes not being at that time entertained. For a
length of time his application received no attention, and when at
length the Lords of the Admiralty condescended to answer, they sent Mr.
Ronalds, as the reward for his ingenuity, and as compensation for the
time and money bestowed in perfecting the invention, the expression of
their opinion--that "telegraphs are of no use in time of peace, and
that during war the semaphore answered all required purposes"! This
reply, so characteristic of the manner in which Government _employés_
generally regard anything new to which their attention is solicited,
completely disheartened Mr. Ronalds. He abandoned the Electric
Telegraph to its fate; and having gone abroad, he returned some years
later to find that, notwithstanding the _dictum_ of the Lords of the
Admiralty, telegraphs are of great use in time of peace as well as
of war, and that the old semaphore had been entirely superseded by
the means of transmission he had indicated twenty years before. Mr.
Ronalds has since received a small pension, not however as a reward
for his ingenious telegraph invention, but for his services in other
departments of science.
The discovery of the magnetic property of an electric current by
Professor Œrsted, in 1818, was most important in the subsequent
progress of telegraphic invention, though it was not applied in a
practical manner till nearly twenty years afterwards. In 1820, indeed,
M. Ampère submitted to the Academy of Sciences at Paris a telegraphic
instrument for the transmission of signals by the deflection of
needles, but he adopted the impracticable plan of the earliest
inventors, of having a separate wire for each letter of the alphabet. A
much more important contribution to telegraphic invention by M. Ampère
was the discovery of electro-magnets, which act an important part in
many recent electric telegraphs.
As the magnetic properties of a voltaic current are extensively applied
in electric telegraphs, it is desirable briefly to explain the nature
of the action of voltaic batteries before proceeding farther with the
history of the invention.
To excite a current of voltaic electricity, it is usual to employ a
series of zinc and copper plates, arranged alternately in separate
jars; or, what is now most common, in cells of gutta percha, separated
from each other in a gutta percha trough. The cells are nearly filled
with diluted sulphuric acid, and a wire is attached to each end of
the trough; one being connected with the last zinc plate, and the
other with the last copper plate of the opposite ends of the trough.
When these wires are brought into contact, electricity is instantly
generated by the action of the acid on the zinc plates. The electricity
excited by the action on the zinc in one cell is carried on to the
next, and that again excites and transfers an additional quantity to
the third cell, thus increasing in intensity to the last pair of plates
in the series. The _electric current_, as it is called, passes along
the wire, and whether the wire be one yard, or whether it be a hundred
miles long, the generation of electricity takes place the instant
that the circuit is completed, and ends the instant that the circuit
is broken. There is this difference, however, in the transmission of
electricity through a long and through a short circuit, that in the
former case the increased resistance offered by the length of the wire
greatly diminishes the quantity of electricity transmitted though it
does not perceptibly retard the velocity.
When a balanced magnetic needle is held above a short thick copper wire
whilst it is transmitting an electric current, the needle is deflected
from its natural position, and inclines either to the right or to the
left, according to the direction in which the current passes. If, for
instance, the north pole of the needle be pointed towards the copper
pole of the battery, it will be deflected towards the east, but if
the direction of the battery current be reversed, the deflection will
be towards the west. The effect instantly ceases when the current is
interrupted by breaking connection with either pole of the battery. The
copper wire, though under ordinary circumstances incapable of being
rendered magnetic, thus becomes endowed with strong magnetic properties
when it is transmitting an electric current, and acts on the magnetic
needle in the same manner as if there were an immense number of small
magnets placed along the wire across its diameter.
The magnetic property of an electric current, first discovered by
Œrsted, was applied by M. Ampère to impart magnetism to iron, by
coiling a length of copper wire round a bar of iron, taking care to
cover the wire with an insulating substance, so that when an electric
current was transmitted the electricity might not pass through the
iron. Coils of copper wire, covered with cotton or silk, can thus
impart most powerful magnetism to a piece of soft iron; but it loses
its magnetic power the instant that the electric current is interrupted.
The effect of a coil of insulated wire in increasing the magnetic
power of an electric current, was applied by M. Schweigger in 1832
to increase the sensitiveness of a suspended magnetic needle. By
surrounding a compass needle with several convolutions of covered wire,
it was found that the deflections of the needle were much greater
and more active; and he thus showed the way to the construction of
those delicate galvanometers, which indicate by their deflections
the slightest disturbance of electrical equilibrium. Schweigger may,
therefore, be considered the original inventor of the Needle Telegraph;
and as he pointed out a method of impressing symbols on paper
mechanically, by means of electro-magnets, he may be considered also as
the original inventor of Recording Electric Telegraphs.
The first near approach to the needle telegraph, now used in this
country, was made by Baron de Schilling, who, in 1832, constructed
at St. Petersburg an electric telegraph consisting of five magnetic
needles. This may be considered as the precursor of the five-needle
telegraph, first patented by Professor Wheatstone in 1837. By the
separate deflection of those needles to the right hand or to the
left, by reversing the connections with the poles of the batteries,
ten primary signals could be obtained; and by bringing two into
action at the same time, many more signals might be made than were
required for indicating the letters of the alphabet, and they could
be appropriated to express several words. For the action of this very
efficient telegraph only five wires were required, and the signals
being all primary ones, the messages might have been transmitted very
quickly.[6] In a subsequent modification of the telegraph, he contrived
to make all the signals with one magnetic needle alone, by repeating
the deflections to the right and to the left, as done in the needle
telegraph now generally used in England.
Another step made by Baron de Schilling was the invention of an
alarum to call attention when a message was about to be sent. Some
contrivance of this kind was considered essential in the early days of
the practical application of the Electric Telegraph, as no one then
contemplated that telegraphic communications would be so frequent as
to require a person to be always near the instrument, waiting for the
receipt of messages.
Baron de Schilling's alarum was very simple. One of the magnetic
needles acted as a detent which held a weight suspended, and when
the needle was deflected, the weight fell upon a bell. The alarums
subsequently invented were constructed on the same principle, but
instead of employing one of the magnetic needles as a detent, an
electro-magnet was used for the purpose, and clock mechanism was
introduced to sound a bell continuously, as soon as it was set in
action by the withdrawal of the detent. At the present time alarums are
not used in the regular stations of the electric telegraph companies;
the sound of the needles, as they strike against the ivory rests on
each side, being sufficient to call the attention of the clerks, who
are in constant attendance.
We have hitherto been enabled to trace, step by step, the advances
made at intervals--years asunder--in bringing the Electric Telegraph
into practical use; but we are now approaching a time when it becomes
difficult to enumerate, and impossible to describe within reasonable
compass, the numerous inventions that were patented and otherwise made
known for giving greater efficiency to that means of communication.
In the early part of the year 1837, the electric telegraphs of Mr.
Alexander, of Edinburgh, and of Mr. Davy, were publicly exhibited
in London, and excited much attention; though, at that time it was
not supposed that it would be possible to make use of that means of
communication for general purposes. Mr. Alexander's telegraph was the
same in principle as those of M. Ampère and of Baron de Schilling,
though in some respects not so efficient as either, for its action was
slow, and it required a separate wire for each letter of the alphabet.
It was considered a great advantage of this telegraph at the time,
that it exhibited actual letters of the alphabet, instead of symbols.
This was effected by having the twenty-six letters painted on a board,
and concealed from view by a number of small paper screens, which were
attached to magnetic needles. When any of the needles was deflected
by sending an electric current through the surrounding coil, the
screen was withdrawn and exposed the letter behind. Twenty-six keys,
resembling those of a pianoforte, were ranged in connection, one with
each wire, and on pressing down any one of the keys, contact was made
between the battery and the wire connected with its associated magnetic
needle; and in this manner, messages might easily be transmitted
and read. The objections to this telegraph, in the form in which it
was exhibited, were not only the impracticability of laying down
and insulating so many wires, but the paper screens attached to the
needles impeded their action, and rendered the transmission a very slow
process. It is questionable, indeed, whether that telegraph could have
been worked at all through a circuit of many miles.
Mr. Davy's telegraph was similar to that of Mr. Alexander's, though
much more compact and better arranged. The letters were painted on
ground glass, lighted behind, so that when the screens were withdrawn
the letters were seen in transparency.
[Illustration]
Professor Wheatstone, who had for some previous years been endeavouring
to perfect a practical electric telegraph, took out his first patent
in 1837. It closely resembled in general features the telegraph of
Baron de Schilling. It consisted of five magnetic needles, ranged side
by side on a horizontal line that formed the diameter of a rhomb. The
needles were suspended perpendicularly, being kept in that position by
having the lower ends made slightly heavier than the upper. The rhomb
was divided into thirty-six equal parts by ten cross lines, and the
needles were placed at the points where the lines intersected, as shown
in the diagram.
At each intersection, and along the boundary lines of the rhomb,
letters were marked, any one of which might be pointed at by the
combined action of two of the needles. Thus, if the two extreme needles
were deflected inwards, one towards the left and the other towards the
right, they would point to the letter _A_ at the top of the rhomb.
If the extreme needle on the left and the fourth one were similarly
deflected, they would point to the letter _B_; and thus all the letters
marked on the intersections of the lines could be pointed to. A
telegraph that could be worked with five circuits came within the range
of practicability, and it was put into operation on the Great Western
Railway as far as Slough, a distance of 18 miles.
When the work of actually making communication by insulated wires
between places far apart came to be done, much difficulty arose as to
the best and cheapest mode of doing it. The plan first attempted was
to surround the wires with pitch, and to bury them in a trench in the
ground. But this was found to be attended with great inconvenience, for
the pitch cracked, and electric communication was established between
the adjacent wires. The method of suspending the wires on posts was, we
understand, suggested by Mr. Brunel, who had seen wires so suspended
for other purposes on the Continent, and he recommended it to Mr. Cooke
for the Electric Telegraph. The plan was tried with success, and
was generally adopted by the Electric Telegraph Company in extending
their lines over the country. We shall have occasion to revert to this
practical part of the subject, when describing more particularly the
means of making communication from one place to another.
In continuing the history of the invention, as regards the different
modes by which communications are transmitted along the insulated
wires, the next telegraphs that deserve notice are those of Dr.
Steinheil, which became known also in 1837. One of his telegraphs made
the signals by sounds, produced by magnetic needles striking, when
deflected, against bells of different tones. By another telegraph
of his invention the symbols where marked upon paper by small tubes
holding ink, fixed to the needles. In this manner the letters of the
alphabet were indicated by dots upon a strip of paper, kept slowly
moving by clock mechanism. This telegraph could be worked by a
single circuit; and it appears that Dr. Steinheil was the first who
discovered, or at least who practically applied, the conducting power
of the earth for the return current. Each circuit, therefore, consisted
of only a single wire; the wire that had been previously used to
complete the circuit being superseded by burying in the earth, at each
terminus, a small copper plate. Dr. Steinheil also introduced the use
of galvanized iron wire. An electric telegraph of this construction was
put into operation at Munich, through a distance of 12 miles.
[Illustration]
In the following year Messrs. Cooke and Wheatstone so far simplified
the arrangements of their needle telegraph as to make all the requisite
signals with two needles. With a single combined battery and two wires
six primary signals are thus obtained; and by repeating the deflections
and combining the action of the two needles, all the letters can
be readily and quickly indicated. A single needle instrument was
invented by Messrs. Cooke and Wheatstone, but as there are only two
primary signals, one to the right and one to the left, the deflections
are necessarily repeated more frequently, and the transmission is
consequently more slow. The accompanying diagram represents the
alphabet of the single needle instrument. The deflections for each
letter commence in the direction of the short marks, and end with
the long ones. Thus, to indicate the letter _R_, the needle is first
deflected once to the left and then once to the right; and the letter
_D_ has the deflections reversed, beginning with one to the right and
ending with one to the left. In no instance does it require more than
four deflections to indicate a single letter, yet the transmission
with the double needle is found so much quicker that the single needle
instrument is only rarely used.
At the end of each word, it is customary for the clerk at the receiving
station to indicate, by a deflection of the needle to the right, that
he understands, or by a deflection to the left, that he does not
understand, and in the latter case the word is repeated. In the early
days of the Electric Telegraph, the transmission of 40 letters a minute
with the double needle instrument was considered quick work; but the
practised clerks will now transmit one hundred letters in that time,
which is as fast as any person can write with pen and ink.
Since the invention of the double and single needle telegraphs there
have been many modifications in the instruments, to make them work
more promptly and with less vibration; but in all essential parts
the telegraphs of Messrs. Cooke and Wheatstone remain unaltered, and
continue to be generally used in this country.
Of the numerous other telegraph instruments that have been invented
since 1837, that of Mr. Morse is in most general use, especially on the
Continent and in America. Mr. Morse, indeed, claims to be the first
inventor of a practical Electric Telegraph; for, according to his
statement, he, in 1832, invented a telegraph, which was in principle
the same as the one now in use. It was not, however, till September,
1838, that he made his instrument known in Europe, by sending a
description of it with a model to the Academy of Sciences at Paris.
Mr. Jackson, an American, disputed with Mr. Morse for the honour of
the invention, and when the latter asserted that he had described his
telegraph in 1832, to some passengers on board a packet-boat, Mr.
Jackson affirmed that it was he who described it on that occasion, and
that Mr. Morse, being present, got the idea from him. It is painful
and difficult to decide when we find two claimants thus directly
in opposition to each other, and mutually preferring charges of
falsehood and fraud. The only safe guide in such cases is to refer to
the earliest published and authentic descriptions of the inventions;
and, following that guidance, the invention of what is called Morse's
telegraph must be attributed to him whose name it bears; but we must,
according to the same rule, date it several years later than 1832.
Mr. Morse's telegraph is a recording instrument, that embosses
the symbols upon paper, with a point pressed down upon it by an
electro-magnet. The symbols that form the alphabet consist of
combinations of short and long strokes, which by their repetitions and
variations, are made to stand for different letters. Thus a stroke
followed by a dot signifies the letter _A_; a stroke preceded by a
dot, the letter _B_; a single dot, the letter _E_; and in this manner
the whole alphabet is indicated, the number of repetitions in no case
exceeding four for each letter. The letters and words are distinguished
from one another by a longer space being left between them than
between each mark that forms only a part of a letter or of a word. The
annexed diagram represents the symbols for the whole alphabet.
[Illustration]
The mechanism of this telegraph instrument is very simple. The
transmitter is merely a spring key, like that of a musical instrument,
which, on being pressed down, makes contact with the voltaic battery,
and sends an electric current to the receiving station. The operator at
the transmitting station, by thus making contact, brings into action an
electro-magnet at the station he communicates with, and that pulls down
a point fixed to the soft iron lever upon a strip of paper that is kept
moving by clockwork slowly under it. The duration of the pressure on
the key, whether instantaneous or prolonged for a moment, occasions the
difference in the lengths of the lines indented on the paper. A single
circuit is sufficient for this telegraph, and a boy who is practised
in the use of the instrument will transmit nearly as many words in a
minute as can be sent by the double needle telegraph with two wires.
The working of Mr. Morse's telegraph, it will be observed, depends
altogether upon bringing into action at the receiving station an
electro-magnet of sufficient force to mechanically indent paper. Now
the resistance to the passage of electricity along the wires diminishes
the quantity transmitted so greatly, that at long distances it would
be almost impossible to obtain sufficient power for the purpose,
if it acted directly. To overcome that difficulty, an auxiliary
electro-magnet is employed. The electro-magnet which is directly in
connection with the telegraph wire is a small one, surrounded by about
500 yards of very fine wire, for the purpose of multiplying as much as
possible the effect of the feeble current that is transmitted. The soft
iron keeper, which is attracted by that magnet, is also very light,
so that it may be the more readily attracted. This highly sensitive
instrument serves to make and break contact with a local battery, which
brings into action a large electro-magnet, and as the local battery and
the magnet are close to the place where the work is to be done, any
required force may be easily obtained. By this means the marks may be
impressed on the paper at distances of 400 miles or more apart.
This is a very efficient and remarkably simple telegraph, and as it
operates with a single wire, it has completely supplanted the needle
telegraph on the Continent; though the liability to error, common to
all manipulated telegraphs, is considerably increased by this mode of
transmission, nor can unintelligible signals be indicated and corrected
so readily as by the needle instrument.
There have been several modifications of Mr. Morse's telegraph, for the
purpose of increasing the rapidity of its action and the distinctness
of the marks. The most important of these was made by Mr. Bain, who
in 1847 applied for this purpose the method of impressing the symbols
on paper by electro-chemical decomposition. Mr. Davy had, in 1843,
taken out a patent for the application of electro-chemical marks to
telegraphic purposes, but his method was not sufficiently practical
to be brought into use. Mr. Bain adopted an alphabet of short and
long strokes, similar to that of Mr. Morse; but instead of making and
breaking contact by a key pressed down by the finger, he punched holes
in a strip of paper, corresponding in lengths and positions to the
marks intended to be transmitted. A small metal spring, connected with
the voltaic battery, pressed upon a metal cylinder attached to the
telegraph wire, and when the spring and cylinder touched, an electric
current was transmitted. The strip of punched paper was placed upon the
cylinder so as to interrupt the circuit, excepting in the parts where
the apertures allowed the spring to make contact; therefore when the
strip of paper was moved along, an electric current was transmitted
through the apertures, and it was stopped when the paper intervened.
At the receiving station, paper well moistened with a solution of
prussiate of potass and nitric acid was placed upon a corresponding
cylinder to receive the message, and a piece of steel wire was kept
steadily pressed upon it as it moved along. The action of the electric
current at the parts where it was transmitted caused the acid to enter
into combination with the steel, and the consequent deposition of iron
on the paper was instantly converted by the prussiate of potass into
Prussian blue. On the parts where the electric current was interrupted
no action took place, and thus numbers of short and long marks were
made on the paper, corresponding to the lengths of the apertures
on the prepared message. A representation of the punched paper for
transmitting the word "Bain" is shown in this diagram.
[Illustration]
As electro-chemical action takes effect much more rapidly than the
mechanical movement of an indenting point, Mr. Bain's telegraph could
work much faster than Mr. Morse's. We have been informed that as many
as 1,000 letters per minute have occasionally been transmitted by
this means from Manchester to London. The disadvantage attending that
mode of transmission arises from the tedious process of punching the
message preparatory to transmission; and though circumstances may arise
in which it would be of great importance to adopt this rapid system
of transmission with a single wire, it has been yet but little used
in this country by the Electric Telegraph Company, who purchased Mr.
Bain's patent for £10,000.
Another modification of Mr. Morse's telegraph, which has been more
extensively adopted in England, consists in merely substituting marks
made on paper by electro-chemical decomposition for those indented
by pressure. It has been found desirable in practice, however, to
introduce an auxiliary electro-magnet, called a "picker," for making
and breaking contact, by which arrangement the dotted marks can be
made by a local battery, and any required amount of electric power be
obtained. The marks produced in this manner are more distinct, and
are more quickly made, than by mechanical pressure. By a more recent
application of Mr. Morse's system, the marks are made on paper with ink
flowing through a glass pen, in the same manner as in the telegraph of
M. Schweigger, already noticed. As the strip of paper is moved along,
a continuous line is thus drawn on the paper. When no signals are
being transmitted the line is straight, but when an electric current
is sent through the wire, it brings into action an electro-magnet,
which attracts the penholder on one side, and alters the direction of
the mark. The transmission is effected by making and breaking contact
with a key, and the continuance of the divergence of the mark from
its normal direction is regulated by the duration of pressure on the
key. The symbols are thus made by deviations from the straight line,
of different lengths and of varied combinations. Practical application
alone can determine whether this mode of making the marks possesses any
advantage over Mr. Morse's original plan. The patent for this telegraph
was granted to Mr. Wilkins in 1854, but a similar instrument, applied
to the notation of astronomical observations, was shown in the American
department of the Great Exhibition of 1851.
The recording telegraph instruments hitherto noticed impress on the
paper only hieroglyphical symbols, which require long practice to
decipher readily. It has, from the first practical application of the
invention, been considered highly desirable that the letters of the
alphabet should be indicated and printed in their proper forms, so that
the momentary transmission of an electric current should leave behind
a durable impression that could be read without difficulty. Professor
Wheatstone and Mr. Bain separately attempted to accomplish this desired
object by the invention of Printing Telegraphs, which print messages
from types. It is a question in dispute which of them was the first
to design a telegraph of this kind. In 1845, Mr. Bain had a printing
telegraph in operation experimentally on the South-Western Railway,
for a distance of seven miles, and we are not aware that Professor
Wheatstone ever succeeded in working his printing instruments when
separated at a distance from each other. In principle, both inventions
were similar. A wheel, into the periphery of which were inserted types
of the twenty-six letters, was made to rotate in close proximity to a
piece of paper, over which was placed a blackened surface that would
leave a mark on the paper when pressed upon. When the required letter
came opposite the paper, the type-wheel was stopped and forced against
it, so that the letter was impressed, and the black from the interposed
surface marked the form of the type. The paper was then moved forward
to leave space for the next letter, and thus a continuous message could
be printed. The objection to these instruments was the uncertainty of
stopping the type-wheel at the proper point, so as to avoid printing
wrong letters; and when the instruments became thus irregular, they
continued so till they were again adjusted. This difficulty has since
been overcome; and by the combined efforts of Mr. House in America,
and of Messrs. Brett in this country, the printing telegraph has
attained a high degree of perfection. The mechanical arrangements of
the instrument, though very complex, consist essentially, like those of
Mr. Bain and Professor Wheatstone, in having a type-wheel, which, by
the action of the operator at the transmitting instrument in making and
breaking contact, moves or stops at the required point, and the letters
are printed by forcing the paper against the type by an electro-magnet.
The movements of the type-wheel are regulated by an electro-magnet, and
one great improvement introduced by Mr. Brett prevents the continuance
of error, should any be made during transmission, by bringing the
type-wheel to its first position after printing each letter, so that
if a wrong letter be printed, the subsequent letters will not continue
erroneous. This printing telegraph works with a single wire, but its
operation is rather slow.
The last recording telegraph we shall notice is the one invented by the
author, which transmits copies of the handwriting of correspondents.
The communication to be transmitted is written upon tin foil, thinly
coated with varnish, with a pen dipped in an ink composed of caustic
soda and colouring matter. The alkali detaches the varnish, and when
the surface is washed over with a wet sponge, the metal is exposed
on those parts written upon, the writing appearing metallic on a
dark ground. The message is then placed round a metal cylinder that
is connected with the line wire from the receiving station. A brass
point, in connection with the voltaic battery, lightly presses on
the message as the cylinder rotates, so that the electric circuit is
made and broken through the message as it passes under the connecting
point, the coating of varnish on the foil being sufficient to interrupt
the electric current in those parts where the point is resting upon
it. On a corresponding cylinder in the electric circuit, at the
receiving station, paper moistened with a solution of prussiate of
potass and nitrate of soda is placed to receive the message; and it
is pressed upon by the point of a steel wire, in connection with the
communicating wire. The accompanying diagram will assist in explaining
the arrangement.
The cylinder of the instrument is shown at _a_; _b_ is the metal
style connected by the wire _g_ with one of the poles of the voltaic
battery; _o_ is the arm which holds the style and serves to insulate
it from the rest of the apparatus; _c_ is a fine screw on which that
arm traverses as the cylinder revolves; _d d_ are cog-wheels to turn
the screw. The speed of the instrument is regulated by the fan _e_; _f_
is the impelling weight, and _h_ the wire connected with the distant
instrument. The receiving and the transmitting instruments are alike,
the only difference between them being that the style of the copying
instrument is steel instead of brass wire.
[Illustration]
As the cylinder _a_ is connected by the wire _h_ with the distant
instrument, and through it with one of the poles of the voltaic
battery, the electric circuit is completed by passing from _g_
through the tin foil message, or through the paper placed on the
cylinder. This will be the case whenever the style of the transmitting
instrument is pressing on the metallic writing; and at those times the
electro-chemical action of the voltaic current will produce a blue mark
on the paper of the receiving instrument, by the deposition of iron and
its combination with the prussiate of potass. The circuit will in like
manner be interrupted whenever the point _b_ presses on those parts
of the message where the varnish is not removed; and thus, as the two
cylinders revolve, there will be a succession of small blue marks on
the parts where the writing allows the electric current to pass. As the
arms that carry the points traverse on screws, they are drawn along
as the cylinders rotate, so as to press on fresh parts of the message
and of the paper at each revolution. The steel point would therefore
draw a series of spiral lines on the paper, if the electric current
were not interrupted; but the interposition of the varnish breaks those
lines, and as the point passes over different portions of the letters
at each revolution of the cylinder, the marks and the interruptions on
the paper correspond exactly with the forms of the letters, and thus
produce a copy of the writing placed upon the receiving cylinder, in
blue characters on a yellowish ground. Or the message may be written
on unprepared tin foil with a pen dipped in varnish; in which case the
writing will be copied in white characters on a ground of dark lines,
as in the accompanying specimen, _A_ being the writing on tin foil, and
_B_ the message received.
It is essential to the perfect working of the copying telegraph that
the corresponding instruments should rotate exactly together. This is
effected by an electromagnetic regulator, which being put in action by
one instrument, governs the movements of the distant instrument with
the greatest exactness, as proved at a distance of 300 miles.
It might be supposed, as the points must traverse several times over
the same line of writing to copy it, that the process is a slow one;
but in consequence of the rapidity with which the cylinders revolve,
this is not the case. The ordinary speed is one rotation in two
seconds, and at that rate three lines of writing, containing sixty
words, would be copied in one minute, which is three times as fast as
an expeditious penman can write.
[Illustration]
The advantages proposed to be gained by the copying telegraph,
in addition to its increased rapidity of transmission, are the
authentication of telegraphic correspondence by the signatures of the
writers, freedom from the errors of transmission, and the maintenance
of secrecy. As a special means of obtaining secrecy, the messages may
be received on paper moistened with a solution of nitrate of soda
alone, in which case they would be invisible until brushed over with a
solution of prussiate of potass, to be applied by the person to whom
the communication is addressed.
Professor Wheatstone has recently contrived an improvement in his index
telegraph, which was described by Professor Faraday in a lecture at
the Royal Institution in June last. Its chief merit, however, consists
in the beauty of the mechanism, for it is essentially the same as the
index telegraphs he and others have previously invented, with the
substitution of magneto-electricity for the moving force.
Having now traced the history of the invention of the instruments by
means of which messages may be transmitted, it becomes necessary to
describe the methods employed for making the electrical connection from
one place to another. This part of the electric telegraph system is,
after all, the most essential to its efficient working, and bears the
same relation to the transmitting instruments that the structure of a
railroad does to locomotive engines in the system of railway conveyance.
The fact that an electric current might be sent through a long circuit
had been established by Dr. Watson, in conjunction with other Fellows
of the Royal Society, in 1747, when they sent the charge of a Leyden
jar through two miles of wire, supported upon short sticks driven into
the ground; the wire at each terminus being connected with the earth
for the return current. This method of insulation and conduction fully
answered the purpose, and served to determine the great velocity with
which electricity is transmitted, for no perceptible interval occurred
between the discharge of the Leyden jar at one end of the circuit, and
its effect at the other extremity.
Mr. Ronalds made the next experiment on an extensive scale, by
insulating eight miles of wire in glass tubes, the wire being carried
backwards and forwards for that distance on his lawn at Hammersmith.
That mode of insulation was found very efficient. It was, indeed, too
perfect, for the difficulty arose of discharging the electricity from
the wire after the charge had passed through it.
The length of telegraphic communication established at Munich, in 1837,
by Dr. Steinheil, was an important practical advance in the system of
extending and insulating the wires, and deserves consideration, not
only from the extent to which it was carried into practical operation,
but from the circumstance that the earth was employed to form the
return circuit. The wires appear to have been carried through the city
by extending them from the church towers and other elevated buildings.
That plan, indeed, presents so many facilities for passing telegraph
wires through towns, that it is not improbable it may be ultimately
adopted in this country.
Though the conducting power of the earth was thus early made use of for
one-half of the circuit, the fact seems to have been unknown in England
at the time of laying down the telegraph wires to Slough in 1845,
for a separate wire was then used for the return current. Some years
afterwards, indeed, Mr. Bain laid claim to the discovery; but the fact
that the conducting power of the earth had been previously applied to
the purpose by Dr. Steinheil has been incontestably proved.
In the early stages of the practical application of electric telegraphs
in this country, Mr. Cook took an active part in overcoming the
numerous difficulties attending the proper protection and insulation
of the wires. In the first instance, the plan of burying the wires in
trenches was tried, but with very indifferent success, as the asphaltum
and other resinous substances with which it was attempted to insulate
them were inadequate for the purpose, and allowed the electricity to
escape from wire to wire. The method of supporting the wires on tall
posts was then adopted by Mr. Cooke, the wires being insulated from
the posts at the points of suspension, by passing them through quills.
Various improvements have since been made in the insulators, and the
plan most in favour at present is to pass the wires through globular
earthenware or glass insulators, attached to the posts, as shown in the
annexed diagram. The wires themselves are about one-sixth of an inch in
diameter; they are made of iron coated with zinc, or galvanized, as it
is termed, to protect them from rust.
[Illustration]
Notwithstanding the great care taken to insulate the wires at the
posts, a large quantity of the electricity escapes in wet weather,
and returns to the battery without having reached the most distant
stations, and thus not unfrequently the communications are interrupted.
The author is of opinion that the loss of electricity in wet weather
is occasioned rather by communication from one wire to another
through the moist atmosphere, than by defective insulation at the
posts. In confirmation of this opinion it may be stated, that he has
experimentally determined that a working electric current might be
transmitted from London to Liverpool, if all the points of attachment
were connected by water with the surface of the ground, provided that
the rest of the wire were insulated.[7]
The use of gutta percha as an insulating covering for wire has given
rise to a new era in telegraphic communication. Gutta percha is an
excellent insulator, and wire covered with two coatings of that
material, about one-sixteenth of an inch each, is so far protected,
that 100 miles of it immersed in water transmits an electric current
from a powerful voltaic battery with very trifling loss. This
perfection in insulation has greatly facilitated the establishment of
telegraphic communication between England and the Continent. The first
attempt to establish a submarine circuit between Dover and Calais took
place on the 28th of August, 1850. A single copper wire, about the
thickness of a common bell wire, coated thickly with gutta percha, was
laid across the English Channel experimentally, without any protection.
It proved sufficient for the transmission of an electric current, and
several messages were sent through it between Dover and Calais; but
it was far too feeble to resist the action of the waves, and the
following day it was cut through by friction against the rocks, and the
communication was stopped.
[Illustration]
The plan afterwards adopted for a permanent submarine line was to
enclose five similar wires in a hollow iron wire cable. The wires
were first slightly twisted, to prevent them from being broken when
stretched. They were then covered with hempen yarn, to protect the
gutta percha from attrition, and they were thus introduced into the
hollow cable, of which they formed the core. The accompanying woodcut
represents this structure of the cable; the five twisted wires are
shown at _C_; _B_ represents the same covered with hemp yarn; and
_A_ a portion of the completed cable, constructed of thick iron wire
galvanized. This cable has now been laid down for seven years, and with
perfect success. Its strength has often been severely tested, as it has
been sometime drawn up by ships' anchors, and considerably strained;
but it has not been broken, and the insulation is almost perfect. The
success of this submarine cable has induced the extension of that means
of communicating with the Continent, and similar submarine telegraph
cables have been laid down from Dover to Ostend, from Harwich to the
Hague, from Scotland to Ireland, and across the Mediterranean Sea as
far as Malta. The weight and the cost of those cables present a serious
obstacle to their adoption in forming a telegraphic communication with
America; and when it was determined to attempt to establish electrical
connection with the New World, a different form of cable was adopted.
The conductor of the electric current in the Atlantic cable is composed
of seven strands of fine copper wire twisted together, the aggregate
thickness of which is not greater than the single copper wire of other
submarine cables. This fine copper cord is covered carefully with gutta
percha; it is then coated with tarred hemp, and is protected externally
by an iron wire rope, composed of numerous strands of fine wire. The
form and exact size of the cable are shown in the accompanying drawing
and section. The central dots in the section are the conducting wires
round which are the gutta percha and hemp, and the outer rim represents
the iron wire casing.
[Illustration]
[Illustration]
The successful laying down of so frail a cable, after many failures,
affords good ground for hoping that, with the experience already
gained, subsequent efforts will prove more satisfactory and much
less expensive than this first attempt to establish telegraphic
communication with America. The most questionable part of the problem
has, indeed, been already solved; for the transmission of electric
signals, through that length of submerged wire, was at one time
doubted; and though the communication through the present cable has
ceased, it has sufficiently established the fact, that telegraphic
communication with America is a practicable undertaking.
The excellent insulation obtained by means of gutta percha covered
wires has caused a return to the original plan of burying the wires in
trenches in the ground. The British and Submarine Telegraph Company
make all their communications by that means; the number of coated wires
required being enclosed in iron tubes, and laid in the ground along
the common roads. That plan is, however, attended with considerable
disadvantages. In the first place, the cost of the coated copper wire
is more than quadruple that of galvanized iron wire; and though copper,
compared with iron, offers only one-seventh part the resistance to the
transmission of electricity, yet the thin wire employed is scarcely
equal in conducting power to the galvanized iron wire usually supported
on posts. The quantity of electricity transmitted is therefore less,
and the comparative intensity of it is greater.
Another difficulty attending the use of insulated wires buried in the
ground arises from a very peculiar condition of electrical conduction,
that could scarcely have been anticipated. The wire, coated with
gutta percha, and surrounded externally with water or with moist
earth, becomes an elongated Leyden jar; the gutta percha representing
the glass, the wire the inside coating, and the water the conducting
surface outside. Thus, when electricity is transmitted through such a
medium, a portion of the charge is retained after connection with the
battery has been broken. This effect increases with the length of the
wire and the intensity of the current; and it materially interferes
with the working of many telegraph instruments. In some experiments
with the copying telegraph at the Gutta Percha Works in the City Road,
it was found that through a circuit of 50 miles of wire immersed in
water, the mark made by electro-chemical decomposition on paper had
a tendency to become continuous; so that instead of ceasing to mark,
when the varnish interrupted the current, a line was drawn continuously
on the paper, though the stronger marks where the current passed were
sufficient to make the writing legible. The retention of the charge
was also shown still more remarkably by the explosion of gunpowder by
the electricity retained in the wire half a minute after connection
with the battery had been broken. It is owing to the retention of the
electricity by the wire that the slowness with which the messages
through the Atlantic cable were transmitted is to be attributed, and
not to the length of the cable. The rate of one word a minute was the
average speed of transmission when the first messages were sent through
the wire. The effect of the _retardation_ of the electric current is
comparatively insignificant and were it not for the peculiar action of
the surrounding water, the messages might have been transmitted twelve
times faster than they were.
The cost of constructing a telegraphic line has greatly diminished
with the increased facilities of insulating the wires, and since the
expiration of patents, which conferred a monopoly on certain plans of
doing so. The cost to the Great Western Railway Company for a line of
six wires to Slough, was £150 per mile, with comparatively low and
slender posts and very imperfect insulation. The cost of the same
number of wires at the present day would not be one-half that sum, with
thicker wires and better insulation.
It is customary in England to restrict the suspension of telegraphic
wires to railways, from the notion that the protection of railways is
necessary to prevent wilful damage to the wires; and as the Electric
Telegraph Company have made exclusive arrangements with all the railway
companies out of London, the competing telegraph companies have
preferred to lay their wires underground rather than incur the supposed
risk of damage to the wires if suspended from posts on common roads,
though by this means the cost of construction is at least quadrupled.
The protection which railways afford is, however, more imaginary than
real, for any one inclined to interrupt the communication could easily
do so; and if on common roads proper precautions were taken in fixing
the posts, and a heavy penalty were imposed on wilful offenders, the
common roads and open fields would, there can be little doubt, offer
as safe a course for the telegraphic wires as railways.
The conducting power of the earth is now employed by all electric
telegraph companies for one-half of every circuit. Thus, whether a
communication be sent from London to Liverpool, to Edinburgh, Paris,
or Brussels, the moist earth serves to complete one-half of the
communication. In the telegraphic circuit between London and Liverpool,
for example, the insulated wire is connected at each end with the
earth by being soldered to a copper plate, which is buried a few feet
underground, so as to insure its being always surrounded with moisture.
To improve the connection of this plate with the earth, it is customary
to bury with it a quantity of sulphate of copper, the solution of which
surrounds the earth-plate with a better conducting liquid than water,
and thus extends the connecting surface. The gas pipes or water pipes
are sometimes employed for the attachment of the wires instead of an
earth-plate, but the latter is generally preferred.
In arranging a telegraphic circuit, the voltaic batteries and the
instruments are introduced at breaks in the telegraph wire. The course
of the electric current is from the copper end of the battery through
the transmitting instrument, then along the wire to the receiving
instrument; from that it passes to the earth and is thus returned to
the transmitting station, where it completes the circuit by being
conducted from the earth-plate to the zinc end of the voltaic battery.
The arrangement for completing the circuit will be more clearly
understood by reference to the accompanying diagram.
[Illustration]
The wire from _C_, which is the copper pole of the voltaic battery,
is connected with the instrument _A_; the electric current is then
transmitted along the wire _D_ to the receiving instrument _B_; thence
it is transferred to the earth-plate _E_, passes through the earth to
the corresponding plate _E´_, which is connected with _Z_, the zinc
pole of the battery. When a communication is returned from _B_ to _A_,
a similar arrangement is made; the wires connected with the instruments
being so arranged as to bring into action a voltaic battery at _B_, and
to throw out of circuit the one at _A_; for the connection with the
battery is only made when the transmitting instrument is worked.
Since all the electric telegraphs in different parts of the world are
connected with the earth, as one portion of the circuit, it might
be supposed that the various currents would mingle, and occasion a
confusion of messages; but it must be borne in mind that no electric
current is formed until a communication be made from one pole of a
voltaic battery to the other, and as such communication can only be
completed through the insulated wire, the earth-currents cannot
mingle, but each one passes to the proper terminus of its respective
battery. The accompanying diagram and explanation may serve to remove
the difficulty of understanding why the two circuits are maintained
quite distinct.
[Illustration]
The letters _A_ _B_ represent the wires making communications between
the batteries _D_ and _E_, and the telegraph instruments _I_ _O_ at the
receiving station. The electricity from the copper end of the battery
_D_ would be conducted along _A_ through the instrument _I_, and by
the wire _K_ to the earth-plate _H_. It would be then transmitted
through the earth on its return to the battery, in the direction of the
arrows, to the other earth-plate _G_, and thence it would find its way
to the zinc pole of the battery _D_, and complete the circuit. In the
same manner, the electric current from the copper end of the battery
_E_ would be transmitted through the wire _B_, and would complete its
current also by means of the earth-plates _G_ _H_, and would traverse
the course indicated by the arrows, and return to the zinc end of
_E_. Though both electric currents traverse the same wire from the
instruments _I_ _O_ to the earth-plate _H_, and are thence transmitted
through the earth to a single plate, _G_, at the transmitting station,
there is no mingling of currents, the electric current of each battery
being kept as distinct as if separate wires were used both for the
transmitted and the return current. It would, indeed, be as impossible
for the separate currents transmitted from the two batteries to be
mingled together, as it would be for the written contents of two
letters enclosed in the same mail-bag to intermix.[8]
The length of telegraph lines at present laid down by the several
telegraph companies in Great Britain, exceeds 10,000 miles. To complete
those lines required 40,000 miles of wire, and there are 3,000 persons
engaged in the transmission of telegraphic intelligence.
In North America there is a direct communication from New York to New
Orleans, a distance of 2,000 miles, through the whole length of which
wires messages can be transmitted without any break. Wires have also
been suspended on lofty posts across the Indian Peninsula, where no
railways have been yet laid down. Lines of insulated wire, partly
submerged in the sea, partly buried underground, and partly suspended
on posts in the air, place London and Vienna in direct communication;
and other telegraph lines are in the course of construction, which will
unite London with Africa: and a complete net-work of telegraph wires is
spreading over the face of Europe.
It will not be long before this system of communication is connected
with a similar one in America. The failure of the cable already laid
down has confirmed the opinion of the author, expressed in papers
read at meetings of the British Association for the Advancement of
Science, and in his work on Electricity, that the conducting wire
should be sufficiently strong to be self-protective, without requiring
an external coating of iron wire rope. A conducting copper wire, a
quarter of an inch in diameter, covered with gutta percha and tarred
hemp, would be more flexible and stronger than the combined cable;
and it being a much better conductor of electricity, the rapidity of
transmission would be greatly increased.
The effect of the establishment of competing telegraph companies in
England has been to diminish the charge for transmitting messages, in
some instances to one-fifth of the rate formerly demanded; and when
further experience in the construction of telegraphic lines, and the
adoption of more rapidly transmitting instruments, have facilitated
and improved the means of communication, we may anticipate that
correspondence by Electric Telegraph will in a great measure supersede
the transmission of letters by post.