If a bar-magnet be suspended on a pivot so that it may turn freely,
it will (as is well known) turn with one end to the north, which is
owing to a current of natural electricity passing round the earth
in the direction of east and west, the magnet crossing the current
at a right angle; and if a coil of wire coated with silk (to keep
one part of the coil from another) be placed round, above and below
the long axis of a bar of steel as shown at fig. 4, and a current
of electricity passed through this wire, the steel becomes a magnet
and will take a direction similar to the natural magnet, more or
less at right angles to this coil, as in fig. 5, according to the
intensity of the current; and the instant this electrical current is
stopped it will resume its former direction. This fact has been made
use of to form the principal feature of all English telegraphs; in
the telegraph such a needle is mounted in an upright position, and
instead of its tendency to turn to the north, a tendency to maintain
the upright position is given to it by having one of the arms of
the magnet a little heavier than the other; such a magnet having
a coil of wire surrounding it. When the electric current passes
through the coil, it will turn out of the upright position to either
one side or the other, according to the direction of the current,
from its tendency to assume a position at an angle to the current
(fig. 6); if the current be stopped even for an instant, then the
needle or magnet will again assume its upright position. The pivot
of this magnet is brought forward and has on its front part another
needle, which being on the same pivot turns with it; this is visible
on the outside of the apparatus, and is looked at to ascertain the
movement of the one within. There is also an arrangement called a
“commutator,” so contrived, that by moving a handle to the right or
left, a connection shall be made with either end of the battery, and
thereby cause the direction of the current and needle to be changed
at pleasure; also by moving the handle into an upright position
the current shall be stopped; and finally, by a third movement, a
bell shall be rung. Now, as has already been explained, when the
current goes in one direction, the magnetic needle is deflected in
that direction; and when the current is reversed the position of the
needle is also reversed, and when the current is cut off the needle
will resume its perpendicular position. If two such needles and two
such handles be at each station, when the handles at one station
are moved, the needles at the other station will take on a similar
movement; and when the handles at that station are moved, the needles
at the first station will be moved to correspond. This constitutes
the system of communication kept up by the electric telegraphs
in England; but it remains to be shown how all the letters of the
alphabet, the numerals, &c., can be represented by the movements
of the two handles. These handles can be placed in eight positions
(besides the upright one) by a single movement of each hand, as may
be seen in fig. 7; and these eight signals if repeated, or made twice
in rapid succession will make eight more, and by being repeated three
times will constitute a third eight, making twenty-four; finally,
by a rapid motion right and left, they may be caused to signify a
fourth eight, or thirty-two signals, which are found to be sufficient
for every purpose, and by practice may be both produced and read
off with facility. Before a message is about to be delivered the
commutator is so placed as to ring a bell, which is done by the same
arrangement as in a common alarm-clock, but the action is set in
motion by a peculiar contrivance, which depends upon the property
a bar of soft iron has of becoming magnetic when a wire is wound
round it and a current of electricity passed through this wire; this
magnetic property exists only as long as the current passes, and
stops the instant it is cut off. The catch of the alarm is disengaged
by the movement of a bar of iron being drawn to the magnet while the
current passes, and forced back again by a spring when it is stopped,
thus setting in action the mechanism of the alarm, or in some cases
there is a simple contrivance for causing a rapid flow and stoppage
of the electricity, so that the bar is alternately attracted by the
magnet and released by the spring, and this motion of the bar rings
the bell as long as it is continued. The bell is always rung to give
notice that a message is about to be sent, and at the station where
the bell rings, the bell at the former station is rung in return, to
show that they are prepared to receive the message; the message is
then spelt letter by letter, by moving the handles into the proper
positions, and as the message is being sent, the eye is kept on
the dials having the needles which will communicate any message in
return from the station to which the message is being sent, such as
“repeat,” “not understood,” &c., &c., for which certain single signs
are made and recognised. The wires which convey the electricity
from station to station, are made of galvanized iron (iron coated
with zinc), and must be kept from all communication with the earth
by some substance incapable of conducting it; they are therefore
stretched between wooden poles (fig. 8), and rest upon sockets or
supports of glass or glazed earthenware, which are both substances
incapable of conducting the electricity to the earth (fig. 9). In
certain localities, as in towns, the wires are coated with gutta
percha (another non-conductor), and laid side by side in a tube under
ground; this is also done in the longer tunnels. In the cables which
conduct the electric power along the bottom of the sea as from Dover
to Calais; the wires are first coated with gutta percha, then bound
with yarn soaked in tar, and finally coated with galvanized iron
wires wound round spirally like the strands of a rope (fig. 10), the
whole forming a cable which is coiled up in the hold of a vessel, and
let out as the vessel crosses from one side to the other; in this way
the cable is deposited on the bed of the sea or channel, forming an
electrical connection from country to country. These cables are made
in one piece by machinery. That from Dover to Calais is twenty-five
miles long, contains four copper conducting wires, and weighs about
175 tons; that from Dover to Ostend contains six conducting wires, is
seventy miles long, weighs nearly 500 tons, and cost about £30,000;
its structure (the real size) is shown at fig. 10.
Illustration: ELECTRIC TIME BALL, CHARING CROSS.
The electric cable now constructed to be laid down between Ireland
and America, is composed of seven small copper wires twisted into
one, and surrounded by gutta percha; this is then surrounded by
eighteen small wire-ropes, each composed of seven small wires
twisted together, the whole being in its section not larger than a
four-penny-piece; 2000 miles of this cable are now ready to be laid
down. A plan was some time ago put in practice by which the correct
time could be kept at various places by electric communication with
the time at Greenwich; a clock thus regulated, is situated at Charing
Cross, and a ball placed at the top of the electric telegraph station
there, is caused by the same means to fall exactly at one o’clock. A
contrivance has of late been patented to work the electric telegraph
by steam, and the following account of it is extracted from the
“Times:”--
“A series of gutta percha bands, about six inches wide and a quarter
of an inch thick, are coiled on wheels on drums arranged for the
purpose. These bands are studded down both sides with a single row
of holes at short intervals apart. When a message is to be sent
the clerks wind off these bands, inserting in the holes small brass
pins, which, according to their combinations in twos or threes
(with blank holes between), represent certain words or letters. In
this manner the message is, as it were, “set up” in the bands with
great rapidity, and if the number of bands employed is sufficiently
large--say as numerous as the compositors employed in a large
printing-office--messages equal in length to five or six columns
of this journal could be set up and ready for transmission in the
course of a single hour. Of course this operation in no respect
interferes with the telegraph wire itself, which continues free for
use until the bands of messages are actually being despatched. The
gutta percha bands when full are removed to the instrument-room, a
most simple appliance preventing any derangement or falling out of
the pins while being moved about. In the instrument-room the bands
are connected with ordinary steam machinery, by which they are drawn
in regular order with the utmost rapidity between the charged poles
of an electrical machine in such a manner that, during the moment
of each pin’s passing, it forms electrical communication between
the instrument and the telegraph, and a signal is transmitted to
the other end of the wire, where the spark perforates a paper and
records the message. The only limit to the rapidity of the operation
is the rate at which the bands can be drawn, since the electrical
contact of each pin, even for the 200th part of a second, is more
than sufficient to transmit a word or signal from London and register
it in America. Of course, as the message is recorded (we will say in
America) with the same rapidity as that with which it is transmitted
from London, a number of reading clerks will be requisite in order to
translate it, by dividing it into small portions, with almost as much
facility as it has been sent.”