Referring first to Fig. 189, which shows in simplified form a four-station K.B. lock-out line, the electromagnet is shown at 1 and the rocker arm at 2.  The ratchet 3 in this case is not a complete wheel but rather a segment thereof, and it is provided with a series of notches of different depths.  It is obvious that the depth of the notches will determine the degree of movement which the upper end of the rocker arm may have toward the left, this being dependent on the extent to which the pawl 6 is permitted to enter into the segment.  The first or normal notch, i.e., the top notch, 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