The telegraphic communication between England and the United States
is so grand a conception, that it would be impossible to detail its
scientific and mechanical relations within the limits of the present
work. All that we shall attempt, therefore, will be to glance at a few
of the leading operations.
In the experiments made before the Atlantic Telegraph was finally
decided on, 2000 miles of subterranean and submarine telegraphic wires,
ramifying through England and Ireland and under the waters of the
Irish Sea, were specially connected for the purpose; and through this
distance of 2000 miles 250 distinct signals were recorded and printed
in one minute.
First, as to the _Cable_. In the ordinary wires by the side of
a railway the electric current travels on with the speed of
lightning--uninterrupted by the speed of lightning; but when a wire
is encased in gutta-percha, or any similar covering, for submersion
in the sea, new forces come into play. The electric excitement of the
wire acts by induction, through the envelope, upon the particles of
water in contact with that envelope, and calls up an electric force
of an opposite kind. There are two forces, in fact, pulling against
each other through the gutta-percha as a neutral medium,--that is,
the electricity in the wire, and the opposite electricity in the film
of water immediately surrounding the cable; and to that extent the
power of the current in the enclosed wire is weakened. A submarine
cable, when in the water, is virtually _a lengthened-out Leyden jar_;
it transmits signals while being charged and discharged, instead of
merely allowing a stream to flow evenly along it: it is a _bottle_
for holding electricity rather than a _pipe_ for carrying it; and
this has to be filled for every time of using. The wire being carried
underground, or through the water, the speed becomes quite measurable,
say a thousand miles in a second, instead of two hundred thousand,
owing to the retardation by induced or retrograde currents. The energy
of the currents and the quality of the wire also affect the speed.
Until lately it was supposed that the wire acts only as a _conductor_
of electricity, and that a long wire must produce a weaker effect than
a short one, on account of the consequent attenuation of the electrical
influence; but it is now known that, the cable being a _reservoir_ as
well as a conductor, its electrical supply is increased in proportion
to its length.
The electro-magnetic current is employed, since it possesses a treble
velocity of transmission, and realises consequently _a threefold
working speed_ as compared with simple voltaic electricity. Mr. Wildman
Whitehouse has determined by his ingenious apparatus that the speed
of the voltaic current might be raised under special circumstances
to 1800 miles per second; but that of the induced current, or the
electro-magnetic, might be augmented to 6000 miles per second.
Next as to a _Quantity Battery_ employed in these investigations. To
effect a charge, and transmit a current through some thousand miles of
the Atlantic Cable, Mr. Whitehouse had a piece of apparatus prepared
consisting of twenty-five pairs of zinc and silver plates about the
20th part of a square inch large, and the pairs so arranged that
they would hold a drop of acidulated water or brine between them. On
charging this Lilliputian battery by dipping the plates in salt and
water, messages were sent from it through a thousand miles of cable
with the utmost ease; and not only so,--pair after pair was dropped
out from the series, the messages being still sent on with equal
facility, until at last only a single pair, charged by one single drop
of liquid, was used. Strange to say, with this single pair and single
drop distinct signals were effected through the thousand miles of the
cable! Each signal was registered at the end of the cable in less than
three seconds of time.
The entire length of wire, iron and copper, spun into the cable amounts
to 332,500 miles, a length sufficient to engirdle the earth thirteen
times. The cable weighs from 19 cwt. to a ton per mile, and will bear a
strain of 5 tons.
The _Perpetual Maintenance Battery_, for working the cable at the
bottom of the sea, consists of large plates of platinated silver and
amalgamated zinc, mounted in cells of gutta-percha. The zinc plates in
each cell rest upon a longitudinal bar at the bottom, and the silver
plates hang upon a similar bar at the top of the cell; so that there
is virtually but a single stretch of silver and a single stretch of
zinc in operation. Each of the ten cells contains 2000 square inches
of acting surface; and the combination is so powerful, that when the
broad strips of copper-plate which form the polar extensions are
brought into contact or separated, brilliant flashes are produced,
accompanied by a loud crackling sound. The points of large pliers
are made red-hot in five seconds when placed between them, and even
screws burn with vivid scintillation. The cost of maintaining this
magnificent ten-celled Titan battery at work does not exceed a shilling
per hour. The voltaic current generated in this battery is not,
however, the electric stream to be sent across the Atlantic, but is
only the primary power used to call up and stimulate the energy of a
more speedy traveller by a complicated apparatus of “Double Induction
Coils.” Nor is the transmission-current generated in the inner wire
of the double induction coil,--and which becomes weakened when it has
passed through 1800 or 1900 miles,--set to work to print or record the
signals transmitted. This weakened current merely opens and closes the
outlet of a fresh battery, which is to do the printing labour. This
relay-instrument (as it is called), which consists of a temporary and
permanent magnet, is so sensitive an apparatus, that it may be put in
action by a fragment of zinc and a sixpence pressed against the tongue.
The attempts to lay the cable in August 1857 failed through stretching
it so tightly that it snapped and went to the bottom, at a depth of
12,000 feet, forty times the height of St. Paul’s.
This great work was resumed in August 1858; and on the 5th the first
signals were received through _two thousand and fifty miles_ of the
Atlantic Cable. And it is worthy of remark, that just 111 years
previously, on the 5th of August 1747, Dr. Watson astonished the
scientific world by practically proving that the electric current could
be transmitted through a _wire hardly two miles and a half long_.[55]
Miscellanea.