[Illustration:  Fig. 164.  Metallic-Circuit Series Line]

In Fig. 164 there is shown a metallic-circuit series line on which five ordinary series telephones are placed in series.  In this no ground is employed, the return being through a line wire, thus making the circuit entirely metallic.

[Illustration:  Fig. 165.  Series Party Line]

The limitations of the ordinary series party line may be best understood by reference to Fig. 165, in which the circuits of three series telephones are shown connected with a single line.  The receiver of Station A is represented as being on its hook, while the receivers of Stations B and C are removed from their hooks, as when the subscribers at those two stations are carrying on a conversation.  The hook switches of Stations B and C being in raised positions, the generators and ringers of those stations are cut out of the circuit, and only the telephone apparatus proper is included, but the hook switch of Station A being depressed by the weight of its receiver, includes the ringer of that station in circuit, and through this ringer, therefore, the voice currents of Stations B and C must pass.

The generator of Station A is not in the circuit of voice currents, however, because of the automatic shunt with which the generator is provided, as described in Chapter VIII.

A slight consideration of the series system as shown in this figure, indicates that the voice currents of any two stations that are in use, must pass (as indicated by the heavy lines) through the ringers of all the stations that are not in use; and when a great number of stations are placed upon a single line, as has been frequently the case, the impedance offered by these ringers becomes a serious barrier to the passage of the voice currents.  This defect in the series party line is fundamental, as it is obvious that the ringers must be left in the circuit of the stations which are not in use, in order that those stations may always be in such condition as to be able to receive a call.

This defect may in some measure be reduced by making the ringers of low impedance.  This is the general practice with series telephones, the ringers ordinarily having short cores and a comparatively small number of turns, the resistance being as a rule about 80 ohms.

Bridging Systems.  Very much better than the series plan of party-line connections, is the arrangement by which the instruments are placed in bridges across the line, such lines being commonly known as bridged or bridging lines.  This was first strongly advocated and put into wide practical use by J.J.  Carty, now the Chief Engineer of the American Telephone and Telegraph Company.

A simple illustration of a bridging telephone line is shown in Fig. 166, where the three telephones shown are each connected in a bridge path from the line wire to ground, a type known as a “grounded bridging line.”  Its use is very common in rural districts.