energized, then the current flow of the corresponding lamps
must flow through this relay winding. Therefore, this relay winding must
be of low resistance, so that the drop through its winding may not be
sufficient to interfere with the proper burning of the lamps, even
though a large number of lamps be fed simultaneously through it. The
pilot relay must be so sensitive that the current, even through one
lamp, will cause it to attract its armature. When it does attract its
armature it causes illumination of the pilot lamp in the same way that
the line relays cause the illumination of the line lamps.

The pilot lamp, which is commonly associated with a group of line lamps
that are placed on any one operator's position of the switchboard, is
located in a conspicuous place in the switchboard cabinet and is
provided with a larger lens so as to make a more striking signal. As a
result, whenever any line lamp on a given position lights, the pilot
lamp does also and serves to attract the attention, even of those
located in distant portions of the room, to the fact that a call exists
on that position of the board, the line lamp itself, which is
simultaneously lighted, pointing out the particular line on which the
call exists.

Pilot lamps, in effect, perform similar service to the night alarm in
magneto boards, but, of course, they are silent and do not attract
attention unless within the range of vision of the operator. They are
used not only in connection with line lamps, but also in connection with
the cord-circuit lamps or signals, as will be pointed out.

[Illustration: Fig. 311. Battery Supply Through Impedance Coils]

[Illustration: Fig. 312. Battery Supply through Repeating Coils]

[Illustration: Fig. 313. Battery Supply with Impedance Coils and
Condensers]

=Cord Circuit.= _Battery Supply._ Were it not for the necessity of
providing for cord-circuit signals in common-battery switchboards, the
common-battery cord circuit would be scarcely more complex than that for
magneto working. Stripped of all details, such as signals, ringing and
listening keys, and operator's equipment, cord circuits of three
different types are shown in Figs. 311, 312, and 313. These merely
illustrate the way in which the battery is associated with the cord
circuits and through them with the line circuits for supplying current
for talking purposes to the subscribers. It is thought that this matter
will be clear in view of the discussion of