the
operating current through the line and signal in series with a fixed
voltage at the central office being not harmfully great when the
entire line resistance is shunted out at or near the central office.
The increase of current may be as great as ten times without damage to
the winding of such a signal. In lamps, the safe margin is much less.
The current which just gives a sufficient lighting of the signal may
be about doubled with safety to the filament of the lamp. Consequently
it is not feasible to place the lamp directly in series with long
exposed lines. A short circuit of such a line near the central office
will burn it out.

[Illustration: Fig. 25. Lamp Signal Controlled by Relay]

The qualities of electromagnets and lamps in these respects are used
to advantage by the lamp signal arrangement shown in Fig. 25. A relay
is in series with the line and provides a large range of sensibility.
It is able to carry any current the central-office current source can
pass through it. The local circuit of the relay includes the lamp.
Energizing the relay lights the lamp, and the reverse; the lamp is
thus isolated from danger and receives the current best adapted to its
needs.

All lines are not long and when enclosed in cable or in well-insulated
interior wire, may be only remotely in danger of being
short-circuited. Such conditions exist in private-branch exchanges,
which are groups of telephones, usually local to limited premises,
connected to a switchboard on those premises. Such a situation
permits the omission of the line relay, the lamp being directly in the
line. Fig. 26 shows the extreme simplicity of the arrangement,
containing no moving parts or costly elements. Lamps for such service
have improved greatly since the demand began to grow. The small bulk
permitted by the need of compactness, the high filament resistance
required for simplicity of the general power scheme of the system, and
the need of considerable sturdiness in the completed thing have made
the task a hard one. The practical result, however, is a signal lamp
which is highly satisfactory.

[Illustration: Fig. 26. Lamp Signal Directly in Line]

[Illustration: Fig. 27. Lamp Signal and Ballast]

The nature of carbon and certain earths being that their conductivity
_rises_ with the temperature and that of metals being that their
conductivity _falls_ with the temperature, has enabled the Nernst lamp
to be successful. The same relation of properti