useful purposes. The opening definition hereof is accurate.

A telephone line consists of two conductors. One of these conductors
may be the earth; the other always is some conducting material other
than the earth--almost universally it is of metal and in the form of a
wire. A line using one wire and the earth as its pair of conductors
has several defects, to be discussed later herein. Both conductors of
a line may be wires, the earth serving as no part of the circuit, and
this is the best practice. A line composed of one wire and the earth
is called a _grounded line_; a line composed of two wires not needing
the earth as a conductor is called a _metallic circuit_.

In the earliest telephone practice, all lines were grounded ones. The
wires were of iron, supported by poles and insulated from them by
glass, earthenware, or rubber insulators. For certain uses, such lines
still represent good practice. For telegraph service, they represent
the present standard practice.

Copper is a better conductor than iron, does not rust, and when drawn
into wire in such a way as to have a sufficient tensile strength to
support itself is the best available conductor for telephone lines.
Only one metal surpasses it in any quality for the purpose: silver is
a better conductor by 1 or 2 per cent. Copper is better than silver in
strength and price.

In the open country, telephone lines consist of bare wires of copper,
of iron, of steel, or of copper-covered steel supported on insulators
borne by poles. If the wires on the poles be many, cross-arms carry
four to ten wires each and the insulators are mounted on pins in the
cross-arms. If the wires on the poles be few, the insulators are