-resistance shunt that is placed across the line by the
instrument that is in use, it is obvious that, in the case of a long
line, the resistance of the line wire will enter into the problem in
such a way as to tend to defeat the locking-out function in some cases.
Thus, where the first instrument to use the line is at the remote end
of the line, the shunting effect that this instrument can exert with
respect to another instrument near the central office is that due to
the resistance of the line plus the resistance of the holding coil at
the end instrument. The resistance of the line wire may be so high as
to still allow a sufficient current to flow through the high-resistance
coil at the nearer station to allow its operation, even though the more
remote instrument is already in use.

Coming now to a consideration of the complete selective-signaling
lock-out systems, wherein the selection of the party and the locking
out of the others are both inherent features, a single example of the
step-by-step, and of the broken-line selective lock-out systems will
be discussed.

Step-by-Step System. The so-called K.B. system, manufactured by the
Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio,
operates on the step-by-step principle. The essential feature of the
subscriber's telephone equipment in this system is the step-by-step
actuating mechanism which performs also the functions of a relay. This
device consists of an electromagnet having two cores, with a permanent
polarizing magnet therebetween, the arrangement in this respect being
the same as in an ordinary polarized bell. The armature of this magnet
works a rocker arm, which, besides stepping the selector segment
around, also, under certain conditions, closes the bell circuit and
the talking circuit, as will be described.

[Illustration: Fig. 189. K.B. Lock-Out System]

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,