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Line Connections in Common-Battery Systems

Individual, two-party, and four-party services are provided by most common-battery systems. That is, a telephone user can obtain service over a line on which no other sets are connected (individual line),ref.1 or over a line used jointly with one other customer (a two-party line), or over a line used jointly with three others (a four-party line). For talking, the telephone sets are connected directly between the two wires of the subscriber lineref.1 (sometimes called the subscriber loop) leading to the central office.

The ringers (or bells) are not connected directly between line wires, the arrangement depending on the particular system and the type of ringing provided. In the selective ringing method used on two-party and four-party lines, the ringer of the called subscriber's station only is rung.ref.1 In the semiselective ringing method, the ringers of two subscribers' stations are rung, differentiation between subscribers being by a one-ring, two-ring code.ref.1

Two-Party Selective, Four-Party Semiselective Systems. One of the most common methods of connecting the subsets to the line for ringing is illustrated by Fig. 11. This system provides two-party selective ringing and four-party semiselective ringing. Two bells are connected between one line wire and ground, and two are connected between the other line wire and ground. It is possible to ring two bells over this ground connection without ringing the other two. In a four-party connection of this type, a coded signal is used, and each party hears the other party's signal.

Four-Party Selectivity, Biased Ringers. If the armature of a polarized ringer (page 348) is held against one of the electromagnet cores by a small spring, the ringer is said to be a biased ringer, and it will respond only to pulsating current of one polarity. A pulsating current in one direction will ring the bell; one in the opposite direction will merely attract the armature more strongly to the pole piece. If now the ringer is biased with a spring on the opposite side so that the armature is held against the opposite pole piece, then it will ring on a pulsating current of the opposite polarity.

Figure 11. Methods of ringing to ground giving the services indicated. The talking equipment is connected between wires.

It might be inferred that, if biased ringers were used in Fig. 11, four-party selectivity would be obtained, but such is not the case. To prevent the flow of direct current, which would operate central-office supervisory equipment, the ringers must be connected in series with capacitors, and these convert the pulsating currents into alternating currents and eliminate the polarity feature.

The connections of Fig. 12 are used with a biased ringer to obtain four-party selectivity. A sensitive high-impedance relay is connected in series with a capacitor and across the line at each of the four stations. Two oppositely biased ringers are connected between each line wire and ground through contacts which normally are open, but which may be closed by the relay. Pulsating ringing voltage is impressed between one side of the line and ground, with the other line wire grounded. The ringing current causes the armatures of each relay to close the contact, and then the properly biased bell rings. The relays open as soon as the ringing current ceases. It is evident that this system requires considerably more apparatus and maintenance than the semiselective system previously described.

From the standpoint of inductive interference between paralleling power lines and telephone circuits, it is desirable to have as high an impedance as possible between the telephone line wires and ground, and also to have these impedances the same. Grounded ringers in grounded two-party and four-party semiselective ringing may, under bad power exposure conditions, cause noisy lines (page 549). In some instances, where there is a high direct ground potential, it may be necessary to have the four-party selective ringers connected between the wires instead of to ground, and the relays and capacitors connected to ground instead of across the line.

Figure 12. A method of giving four-party selective ringing. Plus and minus signs indicate the polarity of the currents to which the biased ringers respond.

Biased ringers used on systems having alternating ringing current prevent tapping of the bells due to inductive or other electrical disturbances.

Cold-cathode gas tubes (page 277) are used in some instances instead of relays to make possible selective ringing on four-party lines.ref.12,13 As shown in Fig. 13, the tubes are connected with a resistor of about 100,000 ohms in the starter, or control anode, and the main anode is connected in series with the ringer. When party A is to be rung, line 1 is grounded and line 2 is made negative. This causes the control gaps at A and C to break down. The main gap at tube A also breaks down (page 277), and the alternating component of the pulsating ringing current causes ringer A to operate. The main gap at C does not break down and pass current because the main anode is negative with respect to the cathode. To cause ringer C to respond instead of A, line 2 is made positive, rather than negative. For ringing B or D, line 2 is grounded and negative pulsating ringing voltage is placed on line 1 to operate ringer B; positive pulsating ringing voltage is placed on line 2 to operate ringer D. This system offers many advantages over the relay method of providing four-party selective service.ref.12

Figure 13. Connections of ringers and cold-cathode gas triodes for giving four-party selective ringing. The small circular electrode is the cold cathode. The small straight electrode is the starter, or control, anode, and the large straight electrode connected to the ringer is the main anode. The black dot indicates that the tube contains gas.

Four-Party Selectivity, Harmonic Ringing. The harmonic system of providing selective service is widely used in the United States on common-battery circuits. The harmonic ringer has an armature tuned mechanically to the desired natural period of vibration. The armature and clapper rod are supported by stiff springs; and, by varying the thickness of the spring used and the mass and the position of the weights on the clapper rod, the ringers can be made to respond to

the frequency desired. In one system, one bell rings on a current of 16.66 cycles per second; another, on 33.33 cycles; another, on 50 cycles; and the fourth, on 66.66 cycles. The bells are connected through capacitors and directly across the line. These frequencies are generated with rotating equipment.

It is evident that all these currents are multiples of the 16.66-cycle current. There is accordingly a strong tendency for each ringer to respond to the current for the others and to ring or buzz lightly. The development of the vibrating ringing-current generator made practicable a system in which the frequencies were not multiples. One of these employs frequencies of 30, 42, 54, and 66 cycles per second. The fact that four-party selective service can be provided by a harmonic system without a ground connection is one of its greatest advantages. Electronic devices have been used to a limited extent to generate the harmonic ringing impulses.



Last Update: 2011-05-30