Broken-Line Method.  The broken-line system, like the step-by-step system, is also essentially a lock-out system and for that reason only its general features, by which the selective ringing is accomplished, will be dealt with here.

Principles.  In this system there are no tuned bells, no positively and negatively polarized bells bridged to ground on each side of the line, and no step-by-step devices in the ordinary sense, by which selective signaling has ordinarily been accomplished on party lines.  Instead of this, each instrument on the line is exclusively brought into operative relation with the line, and then removed from such operative relation until the subscriber wanted is connected, at which time all of the other instruments are locked out and the line is not encumbered by any bridge circuits at any of the instruments that are not engaged in the conversation.  Furthermore, in the selecting of a subscriber or the ringing of his bell there is no splitting up of current among the magnets at the various stations as in ordinary practice, but the operating current goes straight to the station desired and to that station alone where its entire strength is available for performing its proper work.

In order to make the system clear it may be stated at the outset that one side of the metallic circuit line is continued as in ordinary practice, passing through all of the stations as a continuous conductor.  The other side of the line, however, is divided into sections, its continuity being broken at each of the subscriber’s stations.  Fig. 186 is intended to show in the simplest possible way how the circuit of the line may be extended from station to station in such manner that only the ringer of one station is in circuit at a time.  The two sides of the line are shown in this figure, and it will be seen that limb L extends from the central office on the left to the last station on the right without a break.  The limb R, however, extends to the first station, at which point it is cut off from the extension R{x}_ by the open contacts of a switch.  For the purpose of simplicity this switch is shown as an ordinary hand switch, but as a matter of fact it is a part of a relay, the operating coil of which is shown at 6, just above it, in series with the ringer.

[Illustration:  Fig. 186.  Principle of Broken-Line System]

Obviously, if a proper ringing current is sent over the metallic circuit from the central office, only the bell at Station A will operate, since the bells at the other stations are not in the circuit.  If by any means the switch lever 2 at Station A were moved out of engagement with contact 1 and into engagement with contact 3, it is obvious that the bell of Station A would no longer be in circuit, but the limb R of the line would be continued to the extension R{x}_ and the bell of Station B would be in circuit.  Any current then sent