Improvements in Electric Cables.
Year: 1897

page_text: Date of Application, 7th Dec., 1897-Accepted, 8th Jan., 1898
COMPLETE SPECIFICATION.
Improvements in Electric Cables.
I, JOHN HALL KELMAN, of Pittsfield, State of Massachusetts, United States of
America, Electrical Engineer, do hereby declare the nature of this invention and
in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement:
My invention relates to cables for the transmission of electric currents, con-
sisting of several, that is, two or more conductors insulated from each other and
contained in and held together by a protecting sheath or armor. Such cables
are now in general use in telephone and telegraph circuits, and are placed in some
cases overhead and in others beneath the surface of the ground, or under water.
The particular armor to be used depends upon the conditions to which the cable
is subject. For instance, an overhead cable may have an armor consisting of a
braided sheath of cotton strands, or may have a sheath of woven steel wires, or
may have a protecting sheath of lead. Underground cables very generally have
a lead sheath or some other covering resisting the action of the earth and
moisture.
Two of the principal difficulties to contend with in the cables as now made,
are the static charge between the pair of wires, or between the wires and the
earth, and the dynamic induction on each wire, both of which difficulties are due
to the interrupted or varying currents flowing through the wires, and acting
through the dielectrics used to separate the wires from one another.
The static charge is a serious matter and depends upon the specific inductive
capacity of the dielectric used to separate the wires of the cable, and also upon
the distance between the wires, being greater the closer the wires are brought
together. The dynamic induction is also serious and increases as the wires are
separated. In a cable, therefore, in which the separating dielectric is of high
specific inductive capacity, a mean relative position for the wires must be
selected which will give the minimum interference to the proper flow of the
currents due to both the static charge and the dynamic induction.
I am aware that attempts have been made to reduce these deleterious influences
by offsetting self-induction effect against that of capacity, but it must be obvious
that any attempt of this kind can be successful for only one particular frequency,
and it may, in fact, give rise to inferior results to those yielded by the natural
circuit, should the frequency be very much altered. I, on the other hand,
reduce both elements as far as possible with a result that, however far they may
be out of balance, no serious injury can follow.
The principal object of my invention is to produce a cable in which the wires
are brought into close proximity without resulting in a serious static charge.
The close proximity results in two advantages, to-wit: less mutual induction
between the circuits, and a cable of smaller bulk. My invention has also for its
object to produce a cable of durability and flexibility having high insulating
properties and high resistance to disruptive discharges, all of which are attained
by the improvement hereinafter set forth, reference being had to the accom-
panying drawing, in which
Fig. 1 represents a perspective view of a cable embodying my invention, and
Bio 0 represents A cross section of the same

page_text: In the drawing A', A³, A³, A' are four copper conductors. Two tapes B¹, B³
are wound in opposite directions so as to firmly hold and thoroughly cover the
bunch of conductors. An exterior armor C covers the tapes. Upon each con-
ductor is a separately applied continuous coating of linseed oil varnish. This
coating in baked upon the wire in extended lengths, and adheres firmly to the
surface thereof. I use a coating consisting of pure linseed oil. The linseed
oil varnish may, however, consist of any varnish or coating material in which
the principal element is linseed oil, such, for instance, as baking japan con-
taining linseed oil, or equivalent mixtures, containing a similar drying oil. The
thickness of the coating depends upon the purpose for which the cable is used.
In most cases two or three coats of varnish, each coat separately baked, and
hardened by oxidizing, so as to be firm, will be found sufficient. Each of the
conductors being thus coated, there are between the metal portions of two con-
ductors two layers of linseed oil varnish separated by a film of air. Each of
these layers is however very thin, so that the metal portions of the conductors
come very close together, reducing to a minimum the dynamic induction heretofore
referred to. The specific inductive capacity of the coating of linseed oil varnish
is however so low that the nearness of the conductors does not result in a serious
static charge. In other words, the conductors can be brought into very close
proximity without resulting in a static charge which interferes with the use of
the cable, and with the result that the dynamic induction is very little, and that
the cable is of small bulk.
The cable thus constructed possesses the advantages of cables employing con-
ductors covered with gutta percha or rubber mixture, or paper, but not their
disadvantages. It has flexibility and durability, as well as extraordinarily high
insulating properties and resistance to disruptive discharges. In all these par-
ticulars it is equal, if not the superior, to cables employing gutta percha, rubber
mixtures or paper. It is superior to cables employing gutta percha covered, or
rubber covered conductors, since these coverings are of high specific inductive
capacity, making it necessary, in order to reduce the static charge, to separate
the conductors more than would be necessary to simply resist the stress of the
current and to select a mean compromise distance at which the static charge
and dynamic induction will cause the minimum disturbance, while the linseed.
oil varnish coating is of so low specific inductive capacity, that the conductors
may be brought into as close proximity as the insulation necessary to resist the
stress of the current will permit. It is superior to paper in that it does not
deteriorate or absorb moisture, and so weaken or permit leakage of current, but
forms a continuous coating impervious and lasting, and of little bulk. In a
word, it posses to a marked degree, the advantages of the best cables now in use
without having their faults or being open to the objections made to them.
Having now particularly described and ascertained the nature of my inven-
tion and in what manner the same is to be performed I declare that what I
claim is:-
1. A cable consisting of several conductors, each separately coated with a
continuous coating of varnish hardened by oxidization, and a covering holding
said conductors together in close proximity, substantially as described.
2. A cable consisting of several conductors, each separately coated with a
continuous coating of linseed oil varnish, and a covering holding said conductors
together in close proximity, substantially as described.
1
1007
FELL & JAMES,
Agents for the Applicant.