mounted on brackets nailed to the poles. Wires so carried are called
_open wires_.

In towns and cities where many wires are to be carried along the same
route, the wires are reduced in size, insulated by a covering over
each, and assembled into a group. Such a bundle of insulated wires is
called a _cable_. It may be drawn into a duct in the earth and be
called an _underground cable_; it may be laid on the bottom of the sea
or other water and be called a _submarine cable_; or it may be
suspended on poles and be called an _aërial cable_. In the most
general practice each wire is insulated from all others by a wrapping
of paper ribbon, which covering is only adequate when very dry. Cables
formed of paper-insulated wires, therefore, are covered by a seamless,
continuous lead sheath, no part of the paper insulation of the wires
being exposed to the atmosphere during the cable's entire life in
service. Telephone cables for certain uses are formed of wires
insulated with such materials as soft rubber, gutta-percha, and cotton
or jute saturated with mineral compounds. When insulated with rubber
or gutta-percha, no continuous lead sheath is essential for
insulation, as those materials, if continuous upon the wire, insulate
even when the cable is immersed in water. Sheaths and other armors can
assist in protecting these insulating materials from mechanical
injury, and often are used for that purpose. The uses to which such
cables are suitable in telephony are not many, as will be shown.

A wire supported on poles requires that it be large enough to support
its own weight. The smaller the wire, the weaker it is, and with poles
a given distance apart, the strength of the wire must be above a
certain minimum. In regions where freezing occurs, wires in the open
air can collect ice in winter and everywhere open wires are subject to