es has enabled
incandescent-lamp signals to be connected direct to lines without
relays, but compensated against too great a current by causing the
resistance in series with the lamp to be increased inversely as the
resistance of the filament. Employment of a "ballast" resistance in
this way is referred to in Chapter XI. In Fig. 27 is shown its
relation to a signal lamp directly in the line. _1_ is the
carbon-filament lamp; _2_ is the ballast. The latter's conductor is
fine iron wire in a vacuum. The resistance of the lamp falls as that
of the ballast rises. Within certain limits, these changes balance
each other, widening the range of allowable change in the total
resistance of the line.

CHAPTER IV

TELEPHONE LINES

_The line is a path over which the telephone current passes from
telephone to telephone._ The term "telephone line circuit" is
equivalent. "Line" and "line circuit" mean slightly different things
to some persons, "line" meaning the out-of-doors portion of the line
and "line circuit" meaning the indoor portion, composed of apparatus
and associated wiring. Such shades of meaning are inevitable and serve
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