A better arrangement is shown in Fig. 167, which represents a metallic-circuit bridging line, three telephone instruments being shown in parallel or bridge paths across the two line wires.

The actual circuit arrangements of a bridging party line are better shown in Fig. 168.  There are three stations and it will be seen that at each station there are three possible bridges, or bridge paths, across the two limbs of the line.  The first of these bridges is controlled by the hook switch and is normally open.  When the hook is raised, however, this path is closed through the receiver and secondary of the induction coil, the primary circuit being also closed so as to include the battery and transmitter.  This constitutes an ordinary local-battery talking set.

[Illustration:  Fig. 166.  Grounded Bridging Line]

[Illustration:  Fig. 167.  Metallic Bridging Line]

[Illustration:  Fig. 168.  Metallic Bridging Line]

A second bridge at each station is led through the ringer or call-bell, and this, in most bridging telephones, is permanently closed, the continuity of this path between the two limbs of the line not being affected either by the hook switch or by the automatic switch in connection with the generator.

A third bridge path at each station is led through the generator.  This, as indicated, is normally open, but the automatic cut-in switch of the generator serves, when the generator is operated, to close its path across the line, so that it may send its currents to the line and ring the bells of all the stations.

When any generator is operated, its current divides and passes over the line wires and through all of the ringers in multiple.  It is seen, therefore, that the requirements for a bridging generator are that it shall be capable of generating a large current, sufficient when divided up amongst all the bells to ring each of them; and that it shall be capable of producing a sufficient voltage to send the required current not only to the near-by stations, but to the stations at the distant end of the line.

It might seem at first that the bridging system avoided one difficulty only to encounter another.  It clearly avoids the difficulty of the series system in that the voice currents, in order to reach distant stations, do not have to pass through all of the bells of the idle stations in series.  There is, however, presented at each station a leakage path through the bell bridged across the line, through which it would appear the voice currents might leak uselessly from one side of the line to the other and not pass on in sufficient volume to the distant station.  This difficulty is, however, more apparent than real.  It is found that, by making the ringers of high impedance, the leakage of voice currents through them from one side of the line to the other is practically negligible.