. The apparatus at the central office remains in exactly
the same position during conversation as that of Fig. 244, except that
the clearing-out drop shutter is shown as having fallen. The two
subscribers are shown as having hung up their receivers, thus cutting
out their talking apparatus, and as operating their generators for the
purpose of sending the clearing-out signals. In response to this act
the operator pulls down both the calling and the answering plug, thus
restoring them to their normal seats, and bringing both lines to the
normal condition as shown in Fig. 240. The line drops are again
brought into operative relation with their respective lines so as to
be receptive to subsequent calls and the calling generators at the
sub-stations are removed from the bridge circuits across the line by
the opening of the automatic switch contacts associated with those
generators.

_Essentials of Operation_. The foregoing sequence of operations while
described particularly with respect to magneto switchboards is, with
certain modifications, typical of the operation of nearly all manual
switchboards. In the more advanced types of manual switchboards,
certain of the functions described are sometimes done automatically,
and certain other functions, not necessary in connection with the
simple switchboard, are added. The essential mode of operation,
however, remains the same in practically all manual switchboards, and
for this reason the student should thoroughly familiarize himself with
the operation and circuits of the simple switchboard as a foundation
for the more complex and consequently more-difficult-to-understand
switchboards that will be described later on.

Commercial Types of Drops and Jacks. _Early Drops_. Coming now to
the commercial types of switchboard apparatus, the first subject that
presents itself is that of magneto line signals or drops. The very
early forms of switchboard drops had, in most cases, two-coil magnets,
the cores of which were connected at their forward ends by an iron
yoke and the armature of which was pivoted opposite the rear end of
the two cores. To the armature was attached a latch rod which
projected forwardly to the front of the device and was there adapted
to engage the upper edge of the hinged shutter, so as to hold it in
its raised or undisplayed position when the armature was unattracted.
Such a drop, of Western Electric manufacture, is shown in Fig. 246.

[Illustration: Fig. 246 Ol