Volume I Part 20 (2/2)

Non-selective telephone party lines most often employ magneto telephones. The early forms of party lines employed the ordinary series magneto telephone, the bells being of low resistance and comparatively low impedance, while the generators were provided with automatic shunting devices, so that their resistance would normally be removed from the circuit of the line.

Series Systems. The general arrangement of a series party line employing a ground return is shown in Fig. 163. In this three ordinary series instruments are connected together in series, the end stations being grounded, in order to afford a return path for the ringing and voice currents.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 pa.s.s.

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 pa.s.s (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 pa.s.sage 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 ill.u.s.tration 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.

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 const.i.tutes an ordinary local-battery talking set.

[Ill.u.s.tration: Fig. 166. Grounded Bridging Line]

[Ill.u.s.tration: Fig. 167. Metallic Bridging Line]

[Ill.u.s.tration: 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 pa.s.ses 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 pa.s.s 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 pa.s.s 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.

It is obvious that in a heavily loaded bridged line, the bell at the home station, that is at the station from which the call is being sent, will take slightly more than its share of the current, and it is also obvious that the ringing of the home bell performs no useful function.

The plan is frequently adopted, therefore, of having the operation of the generator serve to cut its own bell out of the circuit. The arrangement by which this is done is clearly shown in Fig. 169. The circuit of the bell is normally complete across the line, while the circuit of the generator is normally open. When, however, the generator crank is turned these conditions are reversed, the bell circuit being broken and the generator circuit closed, so as to allow its current all to pa.s.s the line. This feature of having the local bell remain silent upon the operation of its own generator is also of advantage because other parties at the same station are not disturbed by the ringing of the bell when a call is being made by that station.

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