may be mentioned, in which the message is printed at the receiving
station in distinct Roman characters; and as only a single instantaneous
current is required to be sent for each letter, the speed with which a
message can be dispatched is about three times as great as with the
Morse instrument. These advantages are, however, obtained only at the
cost of great delicacy and complexity in the apparatus, so that it is
unfit for ordinary use, although it is much employed on important lines,
where competent operators and skilled mechanics and electricians are at
hand to keep it duly regulated. This machine is too complicated for a
full description in these pages, although it is the best form of
type-printing telegraph, and possesses a special feature in the fact
that the printing is done whilst the wheel carrying the types is in
rapid rotation. The reader will find full and untechnical descriptions
of this and of all the more important forms of telegraphic apparatus in
Mr. R. Sabine’s useful “History and Progress of the Electric Telegraph,”
or in Lardner’s work as edited by Sir Charles Bright.
[Illustration:
FIG. 289.—_The Step-by-step Movement._
]
[Illustration:
FIG. 290.—_Froment’s Dials._
]
From the numerous forms of dial telegraphs we select two for
description. All these instruments are characterized by what is called
the “step-by-step” movement, and differ in their mechanical details, and
in the nature of the apparatus for producing the currents, some being
driven by electro-magnets and others by galvanic batteries. Their
principle may be easily explained. Suppose that a ratchet-wheel, having
twenty-six teeth, is mounted on an axis carrying a hand over a dial
having the letters of the alphabet inscribed upon it. A simple
arrangement in connection with an electro-magnet, somewhat like the
escapement of a clock, will serve to advance the wheel by one tooth each
time a current passes. The diagram, Fig. 289, will at once make this
principle clear. E is the electro-magnet, F the armature, separated by
the spring, S, from the magnet, except when the current passes, when the
catch, C, draws down the tooth in which it is engaged, so that a tooth
passes under the point at D; and when the current ceases, the spring, S,
brings up the catch to engage the succeeding tooth, and thus the hand
moves step by step in the direction of the arrow, advancing each time
the electric circuit is closed by one twenty-sixth of a revolution. In
Fig. 290 is represented lecture-table models of a step-by-step
indicating and transmitting instrument, as constructed by M. Froment, of
Paris. These instruments are supposed to be at the extremities of a long
line of wire. The left-hand figure is the manipulator, or sending
instrument, in which the operator has merely to quickly turn round the
index in the direction of the hands of a watch, by means of the knob, P,
until it points to the desired letter, pause at the letter for an
instant, and then quickly continue the movement until his index points
to the cross at the top of the dial, where he pauses if the word is
spelt out, and, if not, continues the rotation until he arrives at the
next letter, and so on. All these movements and pauses the hand on the
indicator will accurately repeat, and the reason of this may be seen by
observing that the battery contacts are made by the projections on the
metallic wheel, R, which turn with the index. The spring, N, is always
in contact with the wheel, but the spring, M, has such a shape that
contact is alternately made and broken as the projections and spaces
pass it. It is obvious that the needle of the indicator will therefore
advance over the same letters as the index of the communicator.
[Illustration:
FIG. 291.—_Wheatstone’s Universal Dial Telegraph._
]
A very elegant dial instrument has been invented by Sir Charles
Wheatstone, in which magneto-electric currents are made use of. In Fig.
291 communicator and indicator are represented mounted in one case, or
small box. The larger dial is the communicator, and its circumference is
divided into thirty equal spaces, in which are the twenty-six letters of
the alphabet, three punctuation marks, and a +. In an inner circle are
two series of numerals and other signs. About the circumference of the
dial are thirty small buttons or projecting keys, conveniently arranged,
so as to be readily depressed by the touch of a finger. Inside of the
box a strong permanent horse-shoe magnet is fixed, and near its poles a
pair of armatures of soft iron cores with insulated wire coils revolve
when the handle, A, is turned, as in the machines described in the last
article. In this manner a series of waves or short currents of
electricity are produced in the conductors when the circuit is complete,
and the currents are alternately in opposite directions, so that fifteen
revolutions of the coils will produce fifteen currents in one direction
and fifteen in the other. A pinion on the same spindle as the coils
works with a wheel on the axis carrying the pointer on the dial, so that
the pointer makes a complete revolution as often as the handle, A, makes
fifteen turns. Each of the thirty currents will pass through the
indicator, I, and through the line to the distant station, where they
will, by a step-by-step movement, advance the needle of the indicator.
So that the hand of the dial and the needle of the indicator at the
sending station, and that of the indicator at the distant station, will
all simultaneously be pointing to the same letter on their respective
dials; and they would continue to move round these, ever pointing to the
same letter, so long as the handle, A, is turned. How, then, is the
sender to cause the needle of his correspondent’s instrument to pause at
any desired letter? Not by stopping the revolution of the handle, A, for
that could not be done so as to send just the right number of currents,
inasmuch as the rotating armatures could not be instantly stopped. The
mode of causing the indicators to pause at any required letter is as
simple as it is ingenious. It has been already mentioned that the
step-by-step movement takes place at every current which passes through
the line, including the two indicators, and that thirty such currents
pass at each revolution of the pointer of the communicator. But when
these currents no longer flow, the indicators, of course, stop; and the
stoppage of the movements is reconciled with the continuous production
of the currents by having a series of little levers, each connected with
one of the buttons, and so arranged that when one of these has been
pushed down, the lever stops the revolution when it has come round of an
arm on the same central axis as the pointer, and riding loosely on a
hollow spindle, which bears the toothed wheel, driven by the pinion
already spoken of. The projecting arm is provided with a spring, which
falls between the teeth of the wheel, so that the arm is with certainty
carried round with the wheel. But where a button has been pushed down,
its lever catches the arm, lifting its spring away from the teeth of the
wheel. So long as the key remains down, the arrested arm makes a short
metallic circuit by its contact, and no currents pass into the line, for
they take the shortest path. The key is raised only when another is
depressed, and then the arm and the pointer immediately resume their
revolution until they again become stationary at the letter
corresponding with the key which has been pushed down. Suppose the key
of +, the zero of the dial, to be down, which is the proper condition of
the apparatus when a message has to be dispatched. The operator having
rung a bell at the distant end, to call the attention of the person who
receives his message, begins to turn the handle, A, at the rate of about
two revolutions per second. In this state of affairs no current is
passing into the line, and the fingers of both his communicator and
indicator remain stationary, as does also that of the indicator at the
distant end of the line. Now, suppose he has to spell the word “FOX.” He
turns the handle A continuously with his right hand the whole time he is
sending the message; and, manipulating the keys with his left, he
depresses that opposite to the letter F. By this action the key opposite
+ is raised, for the levers are pressed into notches against a
watch-chain, which has just enough _slack_ to allow one lever to enter a
notch, and therefore the pressure of another lever always raises the key
last depressed. When the operator presses down the F key, the + rises,
the radial arm is instantly released, and with the index is carried on
to F, where it stops; and the contacts will have, during that movement,
sent six currents into the line, so that the fingers of both indicators
will also point to F. When the pointer of the communicator has made just
a visible pause at F, he pushes down the key of O, and all the three
pointers recommence their journeys towards that letter. The operator
must, of course, wait until they have reached it and paused an instant,
when he depresses the button opposite X; and when the index has pointed
at that, he pushes down the + key, whereby the fingers all arrest their
movements at that point, indicating that the word is completed. In the
case supposed the word is completed by a single revolution of the
pointers; but this is, of course, not usually the case; thus, in
indicating the syllable “PON,” nearly three complete revolutions would
be required.
This admirable little instrument was designed for the use of private
persons, and is largely used in London and elsewhere. Its great
compactness and simplicity of operation render it highly suitable for
this purpose. There is no battery required, and all the inconvenient
attention demanded by a battery is therefore dispensed with. On the
other hand, the magnets gradually lose their power, and after a time
must be re-magnetized; and the electro-motive force developed in these
instruments is insufficient for lengths of line much exceeding 100
miles. For shorter lines, and for the purposes for which they are
designed, these instruments are perfection.
Very interesting forms of telegraph are those in which a despatch is not
merely written or printed, but actually transcribed as a _facsimile_ of
the writing in the original; and in this way it is possible for a design
to be drawn telegraphically at the distance of hundreds of miles. Like
the Hughes’ printing telegraph, the instruments which produce these
apparently marvellous results require synchronous movements at the two
stations. But although they are scientifically successful, there appears
to be no public demand for these copying telegraphs. One of the best
known is Bonelli’s, which dispatches its messages automatically when
they have been set up in raised metal types precisely similar to the
Roman capitals in the type of the ordinary printer. In Bonelli’s and
most other copying telegraphs the impressions are produced by chemical
decompositions—effected at the receiving station on the paper prepared
to receive the message. By Bonelli’s instrument it is said that when the
type has been set up, messages can be sent at the extraordinary rate of
1,200 words in one minute of time! The action of this system is such
that it is proved to be possible to reproduce in a few seconds—at York,
say—the very characters of a page of type the moment before set up in
London. The limits of our space will not admit of details of this
invention; but we here place before the reader a _facsimile_ of the
letters printed by it at the receiving stations.
[Illustration: BONELLI’S CHEMICAL TELEGRAPH]
We have to describe two other forms of instruments for receiving
telegraphic signals, both contrived with consummate skill by Sir William
Thomson, and, though exhibiting no new principle in any of their parts,
both fine examples of beautiful adjustment of materials for a desired
end. In these forms of apparatus, the delicacy of the mechanical
construction, and the accurate relations of one part to another, have
produced results of the greatest practical importance. Fig. 292
represents the _mirror galvanometer_, an instrument which has not only
proved of the highest value in scientific researches, but is of the
first importance in submarine telegraphy. It is in principle nothing
more than the single-needle telegraph, and it is exceedingly simple in
construction. A very small and light magnet, such as might be formed by
a fragment of the mainspring of a watch, ⅜ths of an inch long, say, is
attached to the back of a little circular mirror, made of extremely thin
silvered glass, also about ⅜ths of an inch in diameter. The mirror and
magnet are suspended by a single cocoon-fibre, so fine as to be almost
invisible, in the centre of a coil, A, of fine silk-covered copper wire.
In front of the suspended mirror, in the axis of the coil, is placed a
lens of about four feet focal distance, and opposite to this is a screen
having a slit, B, in the centre, behind which is placed a paraffin lamp,
D. The screen is provided with a paper scale, C, divided into equal
parts, and is placed at the distance of about two feet from the little
mirror. It follows, from this arrangement, that when the light passing
through the slit falls upon the mirror, it is reflected again through
the lens, and an image of the slit is seen on the scale. This image is
immediately above the slit when the beam falls perpendicularly upon the
mirror, and this condition may be brought about by properly placing the
apparatus with regard to the magnetic meridian. The directive power of
the earth over the little suspended magnet is, however, _almost_
annulled by properly fixing the steel magnet, E, which slides upon the
upright rod, so that the suspended magnet is thus free to obey the least
force impressed upon it by a current passing through the coil. And when
the mirror is deflected through a certain angle, the image on the scale
will be deflected to twice that angle, and thus the smallest movements
of the suspended magnet are readily recognized; not only by reason of
the length of the beam of light, which forms a weightless index, but
because they are doubled by this increased angular deflection.
[Illustration:
FIG. 292.—_The Mirror Galvanometer._
]
When the signals are being rapidly transmitted through a long submarine
line, the currents at the receiving station are much enfeebled and
retarded, and the result is that the movements of a suspended needle
have by no means the decided character which is seen in the instruments
connected with land lines. The signals through a submarine cable could
not therefore be received by any apparatus which required a certain
strength of current; but the mirror galvanometer indicates every change
in the currents, and the apparently irregular motions of the spot of
light can be interpreted by a skilled clerk, who, by long experience,
recognizes, in quite dissimilar effects, the same signal sent by the
clerk at the other end in precisely the same way. Thus a first contact,
corresponding with a dot of the Morse alphabet, may cause the light to
move some distance on the scale, a second contact immediately succeeding
moves it but a little way farther, and a third may occasion a movement
hardly perceptible.
The messages sent by the mirror galvanometer must be read as they are
received; and, as a telegraphic instrument, it is wanting in the
manifest advantages attending a recording instrument. Sir W. Thomson
has, however, devised another receiving instrument of great delicacy,
which is termed the _syphon recorder_. We cannot here describe its
admirable mechanical and electrical details, but the chief feature is
that the attractions and repulsions of the currents are made to produce
oscillations in a syphon formed of an extremely fine glass tube, the
shorter branch of which dips in a trough of ink, and the longer branch
terminates opposite to, but not touching, a band of paper, which is
continuously and regularly drawn along by clockwork while the message is
being received. The tube is a mere hair-like hollow filament of glass,
and the ink, which would not itself flow from a tube of so fine a bore,
is squirted out by electrical repulsion when the insulated reservoir in
which it is contained is electrified at the receiving station by an
ordinary machine. The message as written by this instrument appears
thus:
[Illustration: _the syphon recorder_]
The reader, on comparing these signals with the Morse code on page 560,
will have no difficulty in discovering their relation to it.
_TELEGRAPHIC LINES._
It now remains to give some account of the _line_, that is, the
conductor by which the sending and receiving instruments are united, and
along which the currents flow. Overhead lines are nearly always
constructed with iron wires, which are usually ⅙ in. in diameter, and
are coated with some substance to protect them from oxidation. Zinc is
often used for this purpose, the wire being drawn through melted zinc,
by which it becomes covered with a film of this metal—a process known as
“galvanizing” iron. Another mode is to cover the wires with tar, or to
varnish them from time to time with boiled linseed oil, and this _must_
be done in populous places, where the gases in the air are liable to act
upon the zinc. Sometimes _underground_ wires are used, and these are
often made of copper, covered with gutta-percha, and are laid in wooden
troughs, or in iron pipes. They are protected by having tape or other
material, saturated with tar or bitumen, wound round them. The poles
employed to suspend the overhead wires are generally made of larch or
fir, of such a length that when securely fixed in the ground they rise
12 ft. to 25 ft. above it, and at the top have a diameter of about 5 in.
About thirty poles are required for each mile, and every tenth pole
forms a “stretching-post,” being made stronger than the others and
provided with some appliance by which the wires can be tightened when
required. The wires are attached to the posts by insulating supports;
but at every pole there is always some “leakage,” the amount of which
depends on the form, material, and condition of the insulators. Glass is
quite unsuitable, because its surface strongly attracts moisture, which
thus forms a conducting film. All things considered, porcelain is found
to be the best insulating material for this purpose, since moisture is
not readily deposited on its surface, and even rain runs off without
wetting it; and it is durable, strong, and clean. Fig. 293 shows a
telegraph post, with brown salt-glazed stoneware insulators, shaped like
hour-glasses, with a perspective view and section of one of them.
Another form of insulator, shown in Fig. 294, has a stalk or hook of
porcelain, with a notch, into which the wire is simply lifted, and is
protected above by a porcelain bell. This form, or some modification of
it, is that most generally used.
[Illustration:
FIG. 293.—_Telegraph Post and Insulators._
]
[Illustration:
FIG. 294.
]
It need hardly be remarked that only a single wire is required with most
of the modern instruments for communication between any two places. Each
of the many wires often seen attached to the telegraph posts along a
road or railway represents a distinct line of communication—that is, one
wire may connect the two termini, another may join an intermediate
station and a terminus, a third may belong to two intermediate stations,
and so on. We have already alluded to the discovery by Steinheil of the
apparent conducting power of the earth; and if we must continue to think
of complete circuits, we must regard the earth as replacing for
telegraphic purposes the second or return wire, which was at first
supposed essential. For instance, when a battery current had to be sent
from Station A, Fig. 295, which we may suppose to be London, to Station
B, which we may call Slough, it was at first thought requisite to
provide a wire for the return of the current after it had traversed the
coils at the receiving station. But now the connections are made as
shown in Fig. 296, where the return wire is dispensed with, except a
small portion at each end, which is connected with a large plate of
copper buried in the earth; the arrows show the direction of the
current, according to the commonly received notion. By this plan the
current is increased in intensity, for the “earth circuit” appears to
offer less resistance than the copper wire. The view, however, which
regards the earth not as a conductor in the same sense as the wire, but
as the great _reservoir or storehouse of electricity_, accords better
with known facts.
[Illustration:
FIG. 295.—_Wire Circuit._
]
[Illustration:
FIG. 296.—_Wire and Earth Circuit._
]
The spread of telegraph lines, and the extent to which this mode of
communication is used by the public, may be illustrated by a few
particulars regarding the Central Telegraphic Office in London. The
management of all the public telegraph lines in Great Britain is now in
the hands of the Post Office authorities, and the arrangements at the
central office in London are an admirable specimen of administrative
organization. The Central Telegraph Office occupies a very large and
handsome building opposite the General Post Office, St.
Martin’s-le-Grand. In one vast apartment in this building, containing
ranges of tables, in all three-quarters of a mile long, may be seen
upwards of six hundred telegraph instruments, besides a number of
stations for the receipt and transmission of bundles of messages by
pneumatic dispatch. The number of clerks employed in working the
instruments is 1,200, and about three-fourths of these are females. The
wires from each instrument are conducted below the floor of the
apartment to a board where they terminate in binding-screws, marked with
the number of the instrument. The same board has binding-screws, with
battery connections, and others which form the terminals of the
telegraph lines, and thus the requisite connections are readily made.
The batteries are placed in a lower room, which contains about 23,000
cells of Daniell’s construction, formed into nearly 1,000 distinct
batteries, in each of which the number of cells varies according to the
length of the line through which the current has to pass. Thus, the
battery which supplies the currents that are sent through the coils of
the instrument at Edinburgh consists of 60 cells, but one-sixth of that
number suffices for some of the short lines. The instrument almost
exclusively used is the Morse recorder, and Wheatstone’s automatic
punching machine and transmitters are in constant employment. There are
also some examples of other instruments to be seen in operation, such as
the Hughes type printing telegraph, the American sounder, a few A, B, C,
dial instruments, and a solitary specimen of a double-needle instrument.
Upwards of 30,000 messages pass through this office each day.
[Illustration:
FIG. 297.—_Submarine Cable between Dover and Calais._
]