p _8_, back contacts of
the supervisory relay, third strand of the cord to contact _1_ of the
calling plug, and thence to ground through the thimble of the jack. The
lighting of this lamp is continued until the party at Station B responds
by removing his receiver from its hook, which completes the line
circuit, energizes relay windings _5_ and _6_, causes that relay to
attract its armature, and thus break the circuit of the lamp _8_. Both
supervisory lamps remain out as long as the two subscribers are
conversing, but when either one of them hangs up his receiver the
corresponding supervisory relay becomes de-energized and the
corresponding lamp lights. When both of the lamps become illuminated,
the operator knows that both subscribers are through talking and she
takes down the connection.

Countless variations have been worked in the arrangement of the line and
cord circuits, but the general mode of operation of this particular
circuit chosen for illustration is standard and should be thoroughly
mastered. The operation of other arrangements will be readily understood
from an inspection of the circuits, once the fundamental mode of
operation that is common to all of them is well in mind.

=Lamps.= The incandescent lamps used in connection with line and
supervisory signals are specially manufactured, but differ in no sense
from the larger lamps employed for general lighting purposes, save in
the details of size, form, and method of mounting. Usually these lamps
are rated at about one-third candle-power, although they have a somewhat
larger candle-power as a rule. They are manufactured to operate on
various voltages, the most usual operating pressures being 12, 24, and
48 volts. The 24-volt lamp consumes about one-tenth of an ampere when
fully illuminated, the lamp thus consuming about 2.4 watts. The 12- and
48-volt lamps consume about the same amount of energy and corresponding
amounts of current.

[Illustration: Fig. 315. Switchboard Lamp]

_Lamp Mounting._ The usual form of screw-threaded mounting employed in
lamps for commercial lighting was at first applied to the miniature
lamps used for switchboard work, but this was found unsatisfactory and
these lamps are now practically always provided with two contact strips,
one on each side of the glass bulb, these strips forming respectively
the terminals for the two ends of the filament within. Such a
construction of a common form of lamp is shown in Fig. 315, where thes