SUBMARINE CABLES.
The first attempts at transmitting messages through wires laid in water
were made about 1839. These early experiments were not very successful,
because the art of wire-insulation had not attained any degree of
perfection at that time. It was not until gutta-percha began to be used
as an insulator for submarine lines that any substantial progress was
made.
The first line, so history states, that was successfully laid and
operated was across the Hudson River in 1848. This line was constructed
for the use of the Magnetic Telegraph Company.
In the following year experiments with gutta-percha insulation were
successfully made, and about 1850 a cable was laid across the English
Channel between Dover and Calais (twenty-seven miles), consisting of a
single strand of wire having a covering of gutta-percha. The insulation
was destroyed in a day or two, which demonstrated the fact that all
submarine cables must be protected by some kind of armor. In 1851
another cable was laid between these two points, containing four
conductors insulated with gutta-percha, and over all was an armor of
iron wire. Twenty-one years later this cable was still working, and for
all we know is working now. After this successful demonstration other
cables were laid for longer distances.
These short-line cables served to demonstrate the relative value of
different material for insulating purposes under water, and it has been
found that gutta-percha possesses qualities superior to almost every
other material as an insulator for submarine cables, although there are
many better materials for air-line insulation. Gutta-percha when exposed
to air becomes hardened and will crack, but under water it seems to be
practically indestructible.
Ocean telegraphy really dates from the laying of the first successful
Atlantic cable. There were many problems that had to be solved, which
could be done only by the very expensive experiment of laying a cable
across the Atlantic Ocean. In the first place a survey had to be made of
the bottom of the ocean between the shores of America and Great Britain.
The most available route was discovered by Lieutenant Maury of the
United States Navy, who made a series of deep-sea soundings, and
discovered that, from Newfoundland to the west coast of Ireland the
bottom of the ocean was comparatively even, but gradually deepening
toward the coast of Ireland until it reached a depth of 2000 fathoms. It
was not so deep but that the cable could be laid on the bottom, nor so
shallow as to be in danger of the waves, icebergs or large sea-animals.
The water below a certain depth is always still and not affected by
winds or ocean currents. At many other points in crossing the ocean,
high mountains and deep valleys are encountered, possessing all the
topographical features of dry land--as the ocean bed is only a great
submerged continent.
The beginning of the laying of the first Atlantic cable was on Aug. 7,
1857. On the morning of Aug. 7, 1858, a year later, after a series of
mishaps and adverse circumstances that would have discouraged most men,
the country was electrified by a dispatch from Cyrus W. Field of New
York (to whom the final success of the Atlantic cable is mainly due),
that the cable had been successfully laid and worked. But this cable
worked only from the 10th of August to the 1st of September, having sent
in that time 271 messages. The insulation became impaired at some point,
when an attempt to force the current through by means of a large battery
only increased the difficulty.
The failure of this first cable served to teach manufacturers and
engineers how to construct cables with reference to the conditions under
which they are to be used. It was found that in the deep sea a much
smaller and less expensive cable could be used than would answer at the
shore ends, where the water is shallow. The shore ends of an ocean cable
are made very large, as compared to the deep-sea portions, so as to
resist the effect of the waves and other interfering obstacles. It was
further learned that the most successful mode of transmitting signals
through the cable was with a small battery of low voltage, and by the
use of very delicate instruments for receiving the messages. It is not
possible to employ such instruments on cables as are used on land-lines,
while it would not be a difficult feat to transmit even twice the
distance over land-lines strung on poles, using the ordinary Morse
telegraph.
The water of the ocean is a conductor, as well as the heavy armor that
surrounds the insulation of the cable. When a current is transmitted
through the conducting wires, in the center of the cable, they set up a
countercharge in the armor and the water above it, somewhat as an
electrified cloud will induce a charge in the earth under it, of an
opposite nature. This countercharge, being so close to the conducting
wire, has a retarding effect upon the current transmitted through it.
An ordinary land-line that is strung on poles that are high up from the
ground has this effect reduced to a minimum, but the greater the number
of wires clustered together on the same poles the more difficult it
becomes to send rapid signals through any one of them.
The instrument used for receiving cable messages was devised by Sir
William Thompson, now Lord Kelvin. One form consists of a very short and
delicate galvanometer-needle carrying a tiny mirror. This mirror is so
related to a beam of light thrown upon it that it reflects it upon a
graduated screen at some distance away, so that its motions are
magnified many hundred times as it appears upon the screen. An operator
sits in a dark room with his eye on the screen and his hand upon the key
of an ordinary Morse instrument. He reads the signal at sight, and with
his key transmits it to a sounder, which may be in another room, where
it is read and copied by another operator. Another form of
receiving-instrument carries, instead of the mirror, a delicate
capillary glass tube that feeds ink from a reservoir, and by this means
the movements of the needle are recorded on a moving strip of paper. The
symbols (representing letters) are formed by combinations of zigzag
lines. This instrument is the syphon-recorder.