is always of such a
depth that it will allow the rocker-arm lever _2_ to engage the
contact lever _4_, but will not permit the rocker arm to swing far
enough to the left to cause that contact to engage the bell contact
_5_. As will be shown later, the condition for the talking circuit to
be closed is that the rocker arm _2_ shall rest against the contact
_4_; and from this we see that the normal notch of each of the
segments _3_ is of such a depth as to allow the talking circuit at
each station to be closed. The next notch, _i.e._, the second one in
each disk, is always shallow, as are all of the other notches except
one. A deep notch is placed on each disk anywhere from the third to
the next to the last on the segment. This deep notch is called the
_selective notch_, and it is the one that allows of contact being made
with the ringer circuit of that station when the pawl _6_ drops into
it. The position of this notch differs on all of the segments on a
line, and obviously, therefore, the ringer circuit at any station may
be closed to the exclusion of all the others by stepping all of the
segments in unison until the deep notch on the segment of the desired
station lies opposite to the pawl _6_, which will permit the rocker
arm _2_ to swing so far to the left as to close not only the circuit
between _2_ and _4_, but also between _2_, _4_, and _5_. In this
position the talking and the ringing circuits are both closed.

The position of the deepest notch, _i.e._, the selective notch, on the
circumference of the segment at any station depends upon the number of
that station; thus, the segment of Station 4 will have a deep notch in
the sixth position; the segment for Station 9 will have a deep notch
in the eleventh position; the segment for any station will have a deep
notch in the position corresponding to the number of that station plus
two.

From what has been said, therefore, it is evident that the first, or
normal, notch on each segment is of such a depth as to allow the
moving pawl _6_ to fall to such a depth in the segment as to permit
the rocker arm _2_ to close the talking circuit only. All of the other
notches, except one, are comparatively shallow, and while they permit
the moving pawl _6_ under the influence of the rocker arm _2_ to move
the segment _3_, yet they do not permit the rocker arm _2_ to move so
far to the left as to close even the talking circuit. The exception is
the deep notch, or selective notch,