up to the present time.

_Line Lamp with Relay._ By far the most common method of associating the
line lamp with the line is to employ a relay, of which the actuating
coil is in the line circuit, this relay serving to control a local
circuit containing the battery and the lamp. This arrangement and the
way in which these parts are associated with the jack are clearly
indicated in Fig. 309. Here the relay may receive any amount of current,
from the smallest which will cause it to pull up its armature, to the
largest which will not injure its winding by overheat. Relays may be
made which will attract their armatures at a certain minimum current and
which will not burn out when energized by currents about ten times as
large, and it is thus seen that a very large range of current through
the relay winding is permissible, and that, therefore, a very great
latitude as to line resistance is secured. On the other hand, it is
obvious that the lamp circuit, being entirely local, is of uniform
resistance, the lamp always being subjected, in the arrangement shown,
to practically the full battery potential, the lamp being selected to
operate on that potential.

[Illustration: Fig. 309. Line Lamp with Relay]

_Pilot Signals._ In the circuits of Figs. 307, 308, and 309, but a
single line and its associated apparatus is shown, and it may not be
altogether clear to the uninitiated how it is that the battery shown in
those figures may serve, without interference of any function, a larger
number of lines than one. It is to be remembered that this battery is
the one which serves not only to operate the line signals, but also to
supply talking current to the subscribers and to supply current for the
operation of the cord-circuit signals after the cord circuits are
connected with the lines.

In Fig. 310 this matter is made clear with respect to the association of
this common battery with the lines for operating the line signals, and
also another important feature of common-battery work is brought out,
viz, the pilot lamp and its association with a group of line lamps.
Three subscribers' lines only are shown, but this serves clearly to
illustrate the association of any larger number of lines with the common
battery. Ignoring at first the pilot relay and the pilot lamp, it will
be seen that each of the tip-spring anvils of the jacks is connected to
a common wire _1_ which is grounded. Each of the sleeve-contact anvils
is connected through