IMPROVEMENTS IN ELECTRICAL CABLES
Year: 1882

page_text: LETTERS PATENT to Amos Emerson Dolbear of Somerville Massachusetts
United States of America now of Queen Victoria Street in the City of London
for an invention of "IMPROVEMENTS IN ELECTRICAL CABLES."
PROVISIONAL SPECIFICATION left by the said Amos Emerson Dolbear at
the Office of the Commissioners of Patents on the 21st March 1882.
AMOS EMERSON DOLBEAR of Somerville, Massachusetts, United States of America,
now of Queen Victoria Street, in the City of London, "IMPROVEMENTS IN
ELECTRICAL CABLES
"
My invention relates to improvements in the construction of electrical cables
and is designed to render the same more efficient for the transmission of signals or
messages.
The rapidity with which a submarine electrical cable may be worked depends
upon the rate at which it may be charged and discharged and this rate of charge
and discharge depends upon the specific inductive capacity of the material used as
a coating for the conducting wire of the cable. Hitherto in such cables the
conducting wire has been tightly embedded in some non-conducting material such
as india-rubber which is enclosed in an external sheath or covering which strengthens
and protects the cable.
But according to my invention I construct cables that will be free from the
degree of electric absorption which at present limits the speed of signalling. For
this purpose I use air as the dielectric in contact with the whole, or the greater
part, of the surface of the conductor; that is to say;-My improved cables have
their conductors surrounded or nearly surrounded by an air-space instead of being
tightly embedded in a non-conducting material of high specific inductive capacity
such as india-rubber. This method of construction may be carried into practice by
making the sheath hollow or tubular so that the conductor lies in it touching only
the bottom of the tube, or by wrapping the wire or conductor with some loose
fibrous material such as hemp which by reason of its separate or open condition
will leave space for the admission of air around the conductor which space cannot
exist when the conductor is surrounded by the compact material heretofore used.
I use an outer sheath or covering properly constructed to impart the necessary.
strength to the cable and to ensure the retention of the air in the space
surrounding the conductor.

page_text: SPECIFICATION' in pursuance of the conditions of the Letters Patent filed
by the said Amos Emerson Dolbear in the Great Seal Patent Office
on the 21st September 1882.
AMOS EMERSON DOLBEAR, of Somerville, Massachusetts, United States of
America, now of Queen Victoria Street in the City of London. IMPROVEMENTS
IN ELECTRICAL CABLES
"
"
My invention relates to electrical cables and is designed to provide cables of
greater efficiency than those heretofore constructed for the transmission of messages
or signals.
In electrical cables heretofore constructed the conductor is coated or covered
with a non conductor (which is usually a substance of high specific inductive
capacity) and the amount of surface contact between the conductor and non-
conductor is at its maximum or nearly so.
The rapidity with which a cable may be worked depends upon the rate at
which it may be charged and discharged, and this rate of charge and discharge
depends upon the specific inductive capacity of the material used as a coating for
the conducting wire of the cable and upon the extent of surface contact between
the conductor and the non-conductor by which it is insulated, other things being
equal; now according to my invention I reduce this contact to a minimum; and
thereby accelerate the working of the conductor.
In the accompanying drawing
Figure 1 is a longitudinal section of a cable embodying my invention
Figure 2 is a transverse section on the line xx Figure 1 and
Figure 3 is a diagram illustrating my invention
The best possible form of cable would be a non-conducting pipe or tube with the
conducting wire held centrally in it as shown in Figure 3, the outer surface of the
wire being in contact with nothing but the air filling the pipe; and the nearer the
actual cable approximates to this ideal cable, the better will be the result.
I therefore construct my cable with a central conductor a, (of one or several
strands) surrounded by a tube or pipe d, whose inner diameter is considerably
greater than the outer diameter of the conductor a. The spiral b is a cord of
cotton or the like, preferably hard twisted, and extending in an open spiral around
the conductor a. Its office is to prevent the collapse of the pipe d by supporting
it at intervals along its interior, and, when the pipe d is sufficiently strong I
dispense with this spiral b for there is obviously more surface contact between the
conductor a and the spiral b, than there would be between the conductor a and
the inner surface of the pipe d, were the spiral 6 omitted. The layers of gutta-
percha, tarred twine g and wire , constituting the sheathing or armour may be
of any approved construction.
I find it advantageous to manufacture my improved cable as follows that is
to say,
The conductor a is composed of copper wires as usual, twisted into a strand.
The cord b is wound spirally around the conductor a, as it is fed along by.proper
mechanism; and the tube or pipe d is formed of several thicknesses of paper
rendered waterproof, and cemented by india rubber cement, to ensure strength and
toughness after the spiral b is put upon the conductor a, this spiral supporting the
inner layer of the paper constituting the tube d, and preventing its coming in
contact with the conductor a; the construction of the other parts of the cable
needs no description.
"

page_text: Having thus fully described my said invention, and the manner of performing
.the same,
I wish it understood that I claim :-
In an electric cable the hollow core or tube d containing the conductor a, whose
diameter is less than the, inner diameter of the core a substantially as, and for the
purpose described.
In witness whereof I the said Amos Emerson Dolbear have hereunto set my
hand and seal this First day of September in the year of our Lord One
thousand eight hundred and eighty two.
AMOS EMERSON DOLBEAR. (L.S.)