As a rule the independent operating companies in this country have not employed automatic ringing, and in this case the B-operators have been required to operate their ringing keys and to watch for the response of the called subscriber. In order to arrange for this, another supervisory lamp, termed the ringing lamp, is associated with each incoming trunk plug, the going out of this lamp being a notification to the B-operator to discontinue ringing.

Western Electric Trunk Circuits. The principles involved in inter-office trunking with automatic ringing, are well illustrated in the trunk circuit employed by the Western Electric Company in connection with its No. 1 relay boards. The dotted dividing line through the center of Fig. 371 represents the separating space between two offices. The calling subscriber's line in the first office is shown at the extreme left and the called subscriber's line in the second office is shown at the extreme right. Both of these lines are standard multiple switchboard lines of the form already discussed. The equipment illustrated in the first office is that of an A-board, the cord circuit shown being that of the regular A-operator. The outgoing trunk jacks connecting with the trunk leading to the other office are, it will be understood, multipled through the A-sections of the board and contain no relay equipment, but the test rings are connected to ground through a resistance coil 1, which takes the place of the cut-off relay winding of a regular line so far as test conditions and supervisory relay operation are concerned. The equipment illustrated in the second office is that of a B-board, it being understood that the called subscriber's line is multipled through both the A- and B-boards at that office. The part of the equipment that is at this point unfamiliar to the reader is, therefore, the cord circuit at the B-operator's board. This includes, broadly speaking, the means: (1) for furnishing battery current to the called subscriber; (2) for accomplishing the ringing of the called subscriber and for automatically stopping the ringing when he shall respond; (3) for performing the ordinary switching functions in connection with the relays of the called subscriber's line in just the same way that an A-operator's cord carries out these functions; and (4) for causing the operation of the calling supervisory relay of the A-operator's cord circuit in just the same manner, under control of the connected called subscriber, as if that subscriber's line had been connected directly to the A-operator's cord circuit.

Fig. 371. Inter-Office Connection—Western Electric System
[View full size illustration.]

The operation of these devices in the B-operator's cord circuit may be best understood by following the establishment of the connection. Assuming that the calling subscriber in the first office desires a connection with the subscriber's line shown in the second office, and that the A-operator at the first office has answered the call, she will then communicate by order wire with the B-operator at the second office, stating the number of the called subscriber and receiving from that operator in return the number of the trunk to be employed. The two operators will then proceed simultaneously to establish the connection, the A-operator inserting the calling plug into the outgoing trunk jack, and the B-operator inserting the trunk plug into the multiple jack of the called subscriber's line after testing. We will assume at first that the called subscriber's line is found idle and that both of the operators complete their respective portions of the work at the same time and we will consider first the condition of the calling supervisory relay at the A-operator's position.

The circuit of the calling supervisory lamp will have been closed through the resistance coil 1 connected with the outgoing trunk jacks and the lamp will be lighted because, as will be shown, it is not yet shunted out by the operation of its associated supervisory relay. Tracing the circuit of the calling supervisory relay of the A-operator's circuit, it will be found to pass from the live side of the battery to the ring side of the trunk circuit through one winding of the repeating coil of the B-operator's cord; beyond this the circuit is open, since no path exists through the condenser 2 bridged across the trunk circuit or through the normally open contacts of the relay 3 connected in the talking circuit of the trunk. The association of this relay 3 with the repeating coil and the battery of the trunk is seen to be just the same as that of a supervisory relay in the A-operator's cord, and it is clear, therefore, that this relay 3 will not be energized until the called subscriber has responded. When it is energized it will complete the path to ground through the A-operator's calling supervisory relay and operate to shunt out the A-operator's calling supervisory lamp in just the same manner as if the A-operator's calling plug had been connected directly with the line of the calling subscriber. In other words, the called subscriber in the second office controls the relay 3, which, in turn, controls the calling supervisory relay of the A-operator, which, in turn, shunts out its lamp.

The connection being completed between the two subscribers, the B-operator depresses one or the other of the ringing keys 5 or 6, according to which party on the line is called, assuming that it is a two-party line. It will be noticed that the springs of these ringing keys are not serially arranged in the talking circuit, but the cutting off of the trunk circuit back of the ringing keys is accomplished by the set of springs shown just at the left of the ringing keys, which set of springs 7 is operated whenever either one of the ringing keys is depressed. An auxiliary pair of contacts, shown just below the group of springs 7, is also operated mechanically whenever either one of the ringing keys is depressed, and this serves to close one of two normally open points in the circuit of the ringing-key holding magnet 8. This holding magnet 8 is so arranged with respect to the contacts of the ringing key that whenever any one of them is depressed by the operator, it will be held depressed as long as the magnet is energized just the same as if the operator kept her finger on the key. The other normally open point in the circuit of the holding magnet 8 is at the lower pair of contacts of the test and holding relay 9. This relay is operated whenever the trunk plug is inserted in the jack of a called line, regardless of the position of the subscriber's equipment on that line. The circuit may be traced from the live side of the battery through the trunk disconnect lamp 4, coil 9, sleeve strand of cord, and to ground through the cut-off relay of the line. The insertion of the trunk plug into the jack thus leaves the completion of the holding-magnet circuit dependent only upon the auxiliary contact on the ringing key, and, therefore, as soon as the operator presses either one of these keys, the clutch magnet is energized and the key is held down, so that ringing current continues to flow at regular intervals to the called subscriber's station.

The ringing current issues from the generator 10, but the supply circuit from it is periodically interrupted by the commutator 11 geared to the ringing-machine shaft. This periodically interrupted ringing current passes to the ringing-key contacts through the coil of the ringing cut-off relay 12, and thence to the subscriber's line. The ringing current is, however, insufficient to cause the operation of this relay 12 as long as the high resistance and impedance of the subscriber's bell and condenser are in the circuit. It is, however, sufficiently sensitive to be operated by this ringing current when the subscriber responds and thus substitutes the comparatively low resistance and impedance path of his talking apparatus for the previous path through his bell. The pulling up of the ringing cut-off relay 12 breaks a third normally closed contact in the circuit of the holding coil 8, de-energizing that coil and releasing the ringing key, thus cutting off ringing current. There is a third brush on the commutator 11 connected with the live side of the central battery, and this is merely for the purpose of assuring the energizing of the ringing cut-off relay 12, should the subscriber respond during the interval while the commutator 11 held the ringing current cut off. The relay 12 may thus be energized either from the battery, if the subscriber responds during a period of silence of his ringer, or from the generator 10, if the subscriber responds during a period while his bell is sounding; in either case the ringing current will be promptly cut off by the release of the ringing key.

The trunk operator's "disconnect lamp" is shown at 4, and it is to be remembered that this lamp is lighted only when the A-operator takes down the connection at her end, and also that this lamp is entirely out of the control of the subscribers, the conditions which determine its illumination being dependent on the positions of the operators' plugs at the two ends of the trunk. With both plugs up, the lamp 4 will receive current, but will be shunted to prevent its illumination. The path over which it receives this current may be traced from battery through the lamp 4, thence through the coil of the relay 9 and the cut-off relay of the called subscriber's line. This current would be sufficient to illuminate the lamp, but the lamp is shunted by a circuit which may be traced from the live side of battery through the contact of the relay 13, closed at the time, and through the coil of the trunk cut-off relay coil 14. The resistance of this coil is so proportioned to the other parts of the circuit as to prevent the illumination of the lamp just exactly as in the case of the shunting resistances of the lamps in the A-operator's cord. It will be seen, therefore, that the supply of current to the trunk disconnect lamp is dependent on the trunk plug being inserted into the jack of the subscriber's line and that the shunting out of this lamp is dependent on the energization of the relay 13. This relay 13 is energized as long as the A-operator's plug is inserted into the outgoing trunk jack, the path of the energizing circuit being traced from the live side of the battery at the second office through the right-hand winding of this relay, thence over the tip side of the trunk to ground at the first office. From this it follows that as long as both plugs are up, the disconnect lamp will receive current but will be shunted out, and as soon as the A-operator pulls down the connection, the relay 13 will be de-energized and will thus remove the shunt from about the lamp, allowing its illumination. The left-hand winding of the relay 13 performs no operating function, but is merely to maintain the balance of the talking circuit, it being bridged during the connection from the ring side of the trunk to ground in order to balance the bridge connection of the right-hand coil from the live side of battery to the tip side of the trunk circuit.