and shall have occasion hereafter to speak of the plate of metal least
affected as the positive pole, this plate being regarded, in this
case, as a source whence a current of positive electricity flows along
the wire connection to the other plate, which is called the negative
pole. But I must remind the reader that this is only a convenient
way of expressing the fact that the wire assumes a certain condition
when it connects two such plates, and is capable of producing certain
effects. Whether in reality any process is taking place which can be
justly compared to the flow of a current one way or the other, or
whether a negative current flows along the circuit one way, while the
positive current flows the other way, are questions still unanswered.
We need not here enter into them, however. In fact, very little is
known about these points. Nor need we consider here the various ways in
which many pairs of plates such as I have described can be combined in
many vessels of dilute acid to strengthen the current. Let it simply
be noted that such a combination is called a battery; that when the
extreme plates of opposite kinds are connected by a wire, a current
of electricity passes along the wire from the extreme plate of that
metal which is least affected, forming the positive pole, to the other
extreme plate of that metal which is most affected and forms the
negative pole. The metals commonly employed are zinc and copper, the
former being the one most affected by the action of the dilute acid,
usually sulphuric acid. But it must here be mentioned that the chemical
process, affecting both metals, but one chiefly, would soon render a
battery of the kind described useless; wherefore arrangements are made
in various ways for maintaining the efficiency of the dilute acid and
of the metallic plates, especially the copper: for the action of the
acid on the zinc tends, otherwise, to form on the copper a deposit of
zinc. I need not describe the various arrangements for forming what are
called constant batteries, as Daniell’s, Grove’s, Bunsen’s, and others.
Let it be understood that, instead of a current which would rapidly
grow weaker and weaker, these batteries give a steady current for a
considerable time. Without this, as will presently be seen, telegraphic
communication would be impossible.
We have, then, in a galvanic battery a steady source of electricity.
This electricity is of low intensity, incompetent to produce the more
striking phenomena of frictional electricity. Let us, however, consider
how it would operate at a distance.
The current will pass along any length of conducting substance properly
insulated. Suppose, then, an insulated wire passes from the positive
pole of a battery at a station A to a station B, and thence back to the
negative pole at the station A. Then the current passes along it, and
this can be indicated at B by some action such as electricity of low
intensity can produce. If now the continuity of the wire be interrupted
close by the positive pole at A, the current ceases and the action is
no longer produced. The observer at B knows then that the continuity of
the wire has been interrupted; he has been, in fact, signalled to that
effect.
But, as I have said, the electrical phenomena which can be produced by
the current along a wire connecting the positive and negative poles
of a galvanic battery are not striking. They do not afford effective
signals when the distance traversed is very great and the battery not
exceptionally strong. Thus, at first, galvanic electricity was not more
successful in practice than frictional electricity.
It was not until the effect of the galvanic current on the magnetic
needle had been discovered that electricity became practically
available in telegraphy.
Oersted discovered in 1820 that a magnetic needle poised horizontally
is deflected when the galvanic current passes above it (parallel to
the needle’s length) or below it. If the current passes above it, the
north end of the needle turns towards the east when the current travels
from north to south, but towards the west when the current travels from
south to north; on the other hand, if the current passes below the
needle, the north end turns towards the west when the current travels
from south to north, and towards the east when the current travels from
north to south. The deflection will be greater or less according to the
power of the current. It would be very slight indeed in the case of a
needle, however delicately poised, above or below which passed a wire
conveying a galvanic current from a distant station. But the effect can
be intensified, as follows:—
[Illustration: FIG. 1.]
Suppose _a b c d e f_ to be a part of the wire from A to B, passing
above a delicately poised magnetic needle N S, along _a b_ and then
below the needle along _c d_, and then above again along _e f_ and so
to the station B. Let a current traverse the wire in the direction
shown by the arrows. Then N, the north end of the needle, is deflected
towards the east by the current passing along _a b_. But it is also
deflected to the east by the current passing along _c d_; for this
produces a deflection the reverse of that which would be produced by a
current in the same direction above the needle—that is, in direction
_b a_, and therefore the same as that produced by the current along
_a b_. The current along _e f_ also, of course, produces a deflection
of the end N towards the east. All three parts, then, _a b_, _c d_,
_e f_, conspire to increase the deflection of the end N towards the
east. If the wire were twisted once again round N S, the deflection
would be further increased; and finally, if the wire be coiled in the
way shown in Fig. 1, but with a great number of coils, the deflection
of the north end towards the east, almost imperceptible without such
coils, will become sufficiently obvious. If the direction of the
current be changed, the end N will be correspondingly deflected towards
the west.
The needle need not be suspended horizontally. If it hang vertically,
that is, turn freely on a horizontal axis, and the coil be carried
round it as above described, the deflection of the upper end will be to
the right or to the left, according to the direction of the current.
The needle actually seen, moreover, is not the one acted upon by the
current. This needle is inside the coil; the needle seen turns on the
same axis, which projects through the coil.
If, then, the observer at the station B have a magnetic needle suitably
suspended, round which the wire from the battery at A has been coiled,
he can tell by the movement of the needle whether a current is passing
along the wire in one direction or in the other; while if the needle is
at rest he knows that no current is passing.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
Now suppose that P and N, Fig. 2, are the positive and negative poles
of a galvanic battery at A, and that a wire passes from P to the
station B, where it is coiled round a needle suspended vertically
at _n_, and thence passes to the negative pole N. Let the wire be
interrupted at _a b_ and also at _c d_. Then no current passes along
the wire, and the needle _n_ remains at rest in a vertical position.
Now suppose the points _a b_ connected by the wire _a b_, and at the
same moment the points _c d_ connected by the wire _c d_, then a
current flows along P _a b_ to B, as shown in Fig. 2, circuiting the
coil round the needle _n_ and returning by _d c_ to N. The upper end
of the needle is deflected to the right while this current continues
to flow; returning to rest when the connection is broken at _a b_
and _c d_. Next, let _c b_ and _a d_ be simultaneously connected as
shown by the cross-lines in Fig. 3. (It will be understood that _a d_
and _b c_ do not touch each other where they cross.) The current will
now flow from P along _a d_ to B, circuiting round the needle _n_ in
a contrary direction to that in which it flowed in the former case,
returning by _b c_ to N. The upper end of the needle is deflected then
to the left while the current continues to flow along this course.
I need not here describe the mechanical devices by which the connection
at _a b_ and _c d_ can be instantly changed so that the current may
flow either along _a b_ and _d c_, as in Fig. 2, circuiting the needle
in one direction, or along _a d_ and _b c_, as in Fig. 3, circuiting
the needle in the other direction. As I said at the outset, this paper
is not intended to deal with details of construction, only to describe
the general principles of telegraphic communication, and especially
those points which have to be explained in order that recent inventions
may be understood. The reader will see that nothing can be easier than
so to arrange matters that, by turning a handle, either (1), _a b_ and
_c d_ may be connected, or, (2), _a d_ and _c b_, or, (3), both lines
of communication interrupted. The mechanism for effecting this is
called a _commutator_.
Two points remain, however, to be explained: First, A must be a
receiving station as well as a transmitting station; secondly, the
wire connecting B with N, in Figs. 2 and 3, can be dispensed with, for
it is found that if at B the wire is carried down to a large metal
plate placed some depth underground, while the wire at _c_ is carried
down to another plate similarly buried underground, the circuit is
completed even better than along such a return wire as is shown in the
figures. The earth either acts the part of a return wire, or else,
by continually carrying off the electricity, allows the current to
flow continuously along the single wire. We may compare the current
carried along the complete wire circuit, to water circulating in a pipe
extending continuously from a reservoir to a distance and back again to
the reservoir. Water sucked up continuously at one end could be carried
through the pipe so long as it was continuously returned to the
reservoir at the other; but it could equally be carried through a pipe
extending from that reservoir to some place where it could communicate
with the open sea—the reservoir itself communicating with the open
sea—an arrangement corresponding to that by which the return wire is
dispensed with, and the current from the wire received into the earth.
The discovery that the return wire may be dispensed with was made by
Steinheil in 1837.
The actual arrangement, then, is in essentials that represented in Fig.
4.
[Illustration: FIG. 4.]
A and B are the two stations; P N is the battery at A, P´ N´ the
battery at B; P´ P´ are the positive poles, N´ N´, the negative poles.
At _n_ is the needle of station A, at _n´_ the needle of station B.
When the handle of the commutator is in its mean position—which is
supposed to be the case at station B—the points _b´ d´_ are connected
with each other, but neither with _a´_ nor _c´_; no current, then,
passes from B to A, but station B is in a condition to receive
messages. (If _b´_ and _d´_ were not connected, of course no messages
could be received, since the current from A would be stopped at
_b´_—which does not mean that it would pass round _n´_ to _b´_, but
that, the passage being stopped at _b´_, the current would not flow at
all.) When (the commutator at B being in its mean position, or _d´ b´_
connected, and communication with _c´_ and _a´_ interrupted) the handle
of the commutator at A is turned from its mean position in _one_
direction, _a_ and _b_ are connected, as are _c_ and _d_—as shown in
the figure—while the connection between _b_ and _d_ is broken. Thus
the current passes from P by _a_ and _b_, round the needle _n_; thence
to station B, round needle _n´_, and by _b´_ and _d´_, to the earth
plate G´; and so along the earth to G, and by _d c_, to the negative
pole N. The upper end of the needle of both stations is deflected to
the right by the passage of the current in this direction. When the
handle of the commutator at A is turned in the other direction, _b_ and
_c_ are connected, as also _a_ and _d_; the current from P passes along
_a d_ to the ground plate G, thence to G´, along _d´ b´_, round the
needle _n´_, back by the wire to the station A, where, after circuiting
the needle _n_ in the same direction as the needle _n´_, it travels by
_b_ and _c_ to the negative pole N. The upper end of the needle, at
both stations, is deflected to the left by the passage of the current
in this direction.
It is easily seen that, with two wires and one battery, two needles
can be worked at both stations, either one moving alone, or the other
alone, or both together; but for the two to move differently, two
batteries must be used. The systems by which either the movements of
a single needle, or of a pair of needles, may be made to indicate the
various letters of the alphabet, numerals, and so on, need not here be
described. They are of course altogether arbitrary, except only that
the more frequent occurrence of certain letters, as _e_, _t_, _a_,
renders it desirable that these should be represented by the simplest
symbols (as by a single deflection to right or left), while letters
which occur seldom may require several deflections.
One of the inventions to which the title of this paper relates can now
be understood.
[Illustration: FIG. 5.]
In the arrangement described, when a message is transmitted, the
needle of the sender vibrates synchronously with the needle at the
station to which the message is sent. Therefore, till that message is
finished, none can be received at the transmitting station. In what
is called duplex telegraphy, this state of things is altered, the
needle at the sending station being left unaffected by the transmitted
current, so as to be able to receive messages, and in self-recording
systems to record them. This is done by dividing the current from the
battery into two parts of equal efficiency, acting on the needle at
the transmitting station in contrary directions, so that this needle
remains unaffected, and ready to indicate signals from the distant
station. The principle of this arrangement is indicated in Fig. 5. Here
_a b n_ represents the main wire of communication with the distant
station, coiled round the needle of the transmitting station in one
direction; the dotted lines indicate a finer short wire, coiled round
the needle in a contrary direction. When a message is transmitted,
the current along the main wire tends to deflect the needle at _n_ in
one direction, while the current along the auxiliary wire tends to
deflect it in the other direction. If the thickness and length of the
short wire are such as to make these two tendencies equal, the needle
remains at rest, while a message is transmitted to the distant station
along the main wire. In this state of things, if a current is sent
from the distant station along the wire in the direction indicated by
the dotted arrow, this current also circuits the auxiliary wire, but
in the direction indicated by the arrows on the dotted curve, which
is the same direction in which it circuits the main wire. Thus the
needle is deflected, and a signal received. When the direction of the
chief current at the transmitting station is reversed, so also is
the direction of the artificial current, so that again the needle is
balanced. Similarly, if the direction of the current from the distant
station is reversed, so also is the direction in which this current
traverses the auxiliary wire, so that again both effects conspire to
deflect the needle.
There is, however, another way in which an auxiliary wire may be made
to work. It may be so arranged that, when a message is transmitted,
the divided current flowing equally in opposite directions, the
instrument at the sending station is not affected; but that when the
operator at the distant station sends a current along the main wire,
this neutralizes the current coming towards him, which current had
before balanced the artificial current. The latter, being no longer
counterbalanced, deflects the needle; so that, in point of fact, by
this arrangement, the signal received at a station is produced by the
artificial current at that station, though of course the real cause of
the signal is the transmission of the neutralizing current from the
distant station.
The great value of duplex telegraphy is manifest. Not only can
messages be sent simultaneously in both directions along the wire—a
circumstance which of itself would double the work which the wire is
capable of doing—but all loss of time in arranging about the order
of outward and homeward messages is prevented. The saving of time is
especially important on long lines, and in submarine telegraphy. It is
also here that the chief difficulties of duplex telegraphy have been
encountered. The chief current and the artificial current must exactly
balance each other. For this purpose the flow along each must be
equal. In passing through the long wire, the current has to encounter
a greater resistance than in traversing the short wire; to compensate
for this difference, the short wire must be much finer than the long
one. The longer the main wire, the more delicate is the task of
effecting an exact balance. But in the case of submarine wires, another
and a much more serious difficulty has to be overcome. A land wire
is well insulated. A submarine wire is separated by but a relatively
moderate thickness of gutta-percha from water, an excellent conductor,
communicating directly with the earth, and is, moreover, surrounded by
a protecting sheathing of iron wires, laid spirally round the core,
within which lies the copper conductor. Such a cable, as Faraday long
since showed, acts precisely as an enormous Leyden jar; or rather,
Faraday showed that such a cable, without the wire sheathing, would
act when submerged as a Leyden jar, the conducting wire acting as
the interior metallic coating of such a jar, the gutta-percha as the
glass of the jar (the insulating medium), and the water acting as the
exterior metallic coating. Wheatstone showed further that such a cable,
with a wire sheathing, would act as a Leyden jar, even though not
submerged, the metal sheathing taking the part of the exterior coating
of the jar. Now, regarding the cable thus as a condenser, we see that
the transmission of a current along it may in effect be compared with
the passage of a fluid along a pipe of considerable capacity, into
which and from which it is conveyed by pipes of small capacity. There
will be a retardation of the flow of water corresponding to the time
necessary to fill up the large part of the pipe; the water may indeed
begin to flow through as quickly as though there were no enlargement of
the bore of the pipe, but the full flow from the further end will be
delayed. Just so it is with a current transmitted through a submarine
cable. The current travels instantly (or with the velocity of freest
electrical transmission) along the entire line; but it does not attain
a sufficient intensity to be recognized for some time, nor its full
intensity till a still longer interval has elapsed. The more delicate
the means of recognizing its flow, the more quickly is the signal
received. The time intervals in question are not, indeed, very great.
With Thomson’s mirror galvanometer, in which the slightest motion of
the needle is indicated by a beam of light (reflected from a small
mirror moving with the needle), the Atlantic cable conveys its signal
from Valentia to Newfoundland in about one second, while with the less
sensitive galvanometer before used the time would be rather more than
two seconds.
Now, in duplex telegraphy the artificial current must be equal to the
chief current in intensity all the time; so that, since in submarine
telegraphy the current rises gradually to its full strength and as
gradually subsides, the artificial current must do the same. Reverting
to the illustration derived from the flow of water, if we had a small
pipe the rapid flow through which was to carry as much water one way as
the slow flow through a large pipe was to carry water the other way,
then if the large pipe had a widening along one part of its long course
the short pipe would require to have a similar widening along the
corresponding part of its short course. And to make the illustration
perfect, the widenings along the large pipe should be unequal in
different parts of the pipe’s length; for the capacity of a submarine
cable, regarded as a condenser, is different along different parts of
its length. What is wanted, then, for a satisfactory system of duplex
telegraphy in the case of submarine cables, is an artificial circuit
which shall not only correspond as a whole to the long circuit, but
shall reproduce at the corresponding parts of its own length all the
varieties of capacity existing along various parts of the length of the
submarine cable.
Several attempts have been made by electricians to accomplish this
result. Let it be noticed that two points have to be considered: the
intensity of the current’s action, which depends on the resistance
it has to overcome in traversing the circuit; and the velocity of
transmission, depending on the capacity of various parts of the circuit
to condense or collect electricity. Varley, Stearn, and others have
endeavoured by various combinations of condensers with resistance coils
to meet these two requisites. But the action of artificial circuits
thus arranged was not sufficiently uniform. Recently Mr. J. Muirhead,
jun., has met the difficulty in the following way (I follow partially
the account given in the _Times_ of February 3, 1877, which the reader
will now have no difficulty in understanding):—He has formed his
second circuit by sheets of paper prepared with paraffin, and having
upon one side a strip of tinfoil, wound to and fro to represent
resistance. Through this the artificial current is conducted. On the
other side is a sheet of tinfoil to represent the sheathing,[29] and
to correspond to the capacity of the wire. Each sheet of paper thus
prepared may be made to represent precisely a given length of cable,
having enough tinfoil on one side to furnish the resistance, and on the
other to furnish the capacity. A sufficient number of such sheets would
exactly represent the cable, and thus the artificial or non-signalling
part of the current would be precisely equivalent to the signalling
part, so far as its action on the needle at the transmitting station
was concerned. “The new plan was first tried on a working scale,” says
the _Times_, “on the line between Marseilles and Bona; but it has
since been brought into operation from Marseilles to Malta, from Suez
to Aden, and lastly, from Aden to Bombay. On a recent occasion when
there was a break-down upon the Indo-European line, the duplex system
rendered essential service, and maintained telegraphic communication
which would otherwise have been most seriously interfered with.” The
invention, we may well believe, “cannot fail to be highly profitable to
the proprietors of submarine cables,” or to bring about “before long a
material reduction in the cost of messages from places beyond the seas.”
* * * * *
The next marvel of telegraphy to be described is the transmission
of actual facsimiles of writings or drawings. So far as strict
sequence of subject-matter is concerned, I ought, perhaps, at this
point, to show how duplex telegraphy has been surpassed by a recent
invention, enabling three or four or more messages to be simultaneously
transmitted telegraphically. But it will be more convenient to consider
this wonderful advance after I have described the methods by which
facsimiles of handwriting, etc., are transmitted.
Hitherto we have considered the action of the electric current
in deflecting a magnetic needle to right or left, a method of
communication leaving no trace of its transmission. We have now to
consider a method at once simpler in principle and affording means
whereby a permanent record can be left of each message transmitted.
[Illustration: FIG. 6.]