[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]