[Illustration:  ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate Capacity 24,000 Subscribers’ Lines and 2,100 Trunk Lines.  Siemens-Halske Equipment.  Note Horizontal Disposal of Multiple Jack Field.]

No induction coil is used in this system and the impedance of the holding coil is such that incoming voice currents flow through the condenser and the receiver, which, by reference to the figure, will be seen to be in shunt with the holding coil.  The holding coil is in series with the local transmitter, thus making a circuit similar to that of the Kellogg common-battery talking circuit already discussed.

A possible defect in the use of this system is one that has been common to a great many other lock-out systems, depending for their operation on the same general plan of action.  This appears when the instruments are used on a comparatively long line.  Since the locking-out of all the instruments that are not in use by the one that is in use depends on the low-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]