certain power, a great quantity of electricity in a
given time. Insulation is the method which
guards the wire against what (to compare the
wire with a pipe) may be called leakage. On
land and in the air a thick wire is no disadvantage,
and may be useful, and it is sufficient that
the wires be insulated by glass, porcelain, or
other non-conductors, wherever they touch a
support connected with the ground. But wires
laid in earth or water, without insulation, would
leak, like a pipe full of holes, and the earth or
water would absorb the electric current.

A submarine cable must be composed of some
insulated conductor, strengthened by other
surrounding materials, to protect it from injury
while it is being paid-out of a ship and after it
has been laid at the bottom of the sea. The
Blue-book committee report that, in the fifty
cables laid, the same general principles prevail—
viz.: 1. The central conductor is a copper wire,
or strand of wires. 2. The insulating covering
is gutta-percha. 3. The external covering when
used, consists of hemp or other fibrous material,
impregnated with pitch or some other resinous
substances, and, over that, in nearly all cases is
iron or steel wire. The whole is, in form, an
ordinary twisted rope. 4. The cables so prepared
have been paid-out over the stern of an ordinary
vessel, generally a steam-boat, with a friction-
brake to regulate the delivery, according to the
speed of the vessel. This speed has averaged
from four to six knots an hour.

The conducting wire of submarine cables has
in all cases been copper, because it has more
conducting power than any other metal, and is
very durable. It was originally used for land
telegraphs, but it stretched too much, and,
besides, offered an irresistible temptation to the
purveyors for marine store shops, where there is
a permanent demand for any quantity of copper
wire. A Prussian commissioner gravely reported
that mice eat not only gutta-percha, but copper
wire. The conducting power of copper to iron is
as one to eight, so that a copper wire one-tenth
of an inch diameter is equivalent, as an electric
conductor, to an iron wire nearly one-third of an
inch in diameter. But on land the extra size of
wire is an advantage. It is scarcely possible
to obtain perfectly pure copper, and there is no
substance which, added to pure copper,
increases its conducting power. The difference
between the conductivity of various qualities
of copper is very great. Thus experiments
made for the information of the committee
showed that, taking pure copper at one hundred,
Spanish Rio Tinto was, leaving out decimals,
only fourteen; Russian, sixty; "tough"
copper, seventy-one; Burra Burra, eighty-six and
three-quarters; a specimen cut from a piece
one and a half ton weight, nearly ninety-nine.
It is of the utmost importance that the best
and purest copper should be tested for submarine
cables. Hitherto no proper guarantee for
the purity of the copper wire has been exacted
from cable contractors.

The first cables were made of a single copper
wire, and, if this could be obtained of unlimited
length and uniform quality, it would perhaps be
the best plan; but, practically, a single wire was
found to be weak at the solderings of the joints,
and still more at places in the wire where it
was not well annealed. There, it was brittle, and
frequently broke off before it left the manufacturer's
yard or the hold of the ship. But the
ends of the wire, covered with gutta-percha,
would frequently remain in contact, and answer
any electrical tests applied, until in paying-out
a strain came on the cable, then the two ends
separated and broke the continuity of the
conductor, although externally it appeared
perfect. As an improvement on the single wire,
bundles of wire, in the form of a strand, were
used, and a number of very ingenious plans have
been devised by different persons for increasing
the strength and protecting these wires from
injury by the percolation of water through the
covering.

The insulating covering first used for electric
wires was india-rubber; but it was soon found
that, although it possessed insulating properties
of the highest order, although it is tough, highly
elastic, of less specific gravity than water, extremely
durable under water, and nearly impervious
to moisture, it had, besides minor defects,
the fatal one of rotting, or rather burning and
consuming, when exposed to light, and it was also
extremely difficult to work into joints or joinings.
After several trials, india-rubber was superseded
by gutta-percha. Gutta-percha, when perfectly
pure, and under moderate temperatures, is a
remarkably good insulator, capable of being
kneaded and drawn solidly through dies, and
although the joints required care, the difficulty
of making them was not so great as with india-
rubber. But gutta-percha in a raw state is far
from pure, and is consequently a very imperfect
insulator. Jacobi, the celebrated Russo-German
professor, in a paper read before the Academy
of Sciences of St. Petersburg, relates that,
being commanded, in 1844, to carry the
telegraph invented by Baron Schilling across a
gulf, he set to work to use the first parcel of
gutta-percha imported into Russia, "furiously,"
for the stupid officials employed to report
on the plan had rejected with ridicule the idea
of carrying the telegraph along poles in the
air. But the raw gutta-percha, although laid
on with extravagant thickness, failed utterly.
Submarine cables could not be satisfactorily
worked until machinery had been invented for
purifying and reducing it to an homogeneous mass.
Even at the present day, according to Jacobi, it
is only in England, where the manufacture is
carried on on a great scale, that gutta-percha
of a reliable quality can be obtained, and it has
often happened that a single fissure, not larger
in the commencement than a pin's point, by
admitting the sea water to the wire, has destroyed
the whole value of a submerged cable. Amongst
the most recent improvements referred to by
the Blue-book committee is that of using fine
ribands of thin gutta-percha to wind round the
wire, instead of kneading over each joint with a
thick sheet of the same material.