Fig. 377. Inter-Office Connection—Kellogg System
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This figure illustrates a connection from a regular two-wire multiple subscriber's line in one office, through an A-operator's cord circuit there, to the outgoing trunk jacks at that office, thence through the incoming trunk circuit at the other office to the regular two-wire multiple subscriber's line at that second office. The portion of this diagram to be particularly considered is that of the B-operator's cord circuit. The trunk circuit terminates in the multipled outgoing trunk jacks at the first office, the trunk extending between offices consisting, of course, of but two wires. We will first consider the control of the calling supervisory lamp in the A-operator's cord circuit, it being remembered that this control must be from the called subscriber's station. It will be noticed that the left-hand armature of the relay 1 serves normally to bridge the winding of relay 2 across the cord circuit around the condenser 3. When, however, the relay 1 pulls up, the coil of relay 4 is substituted in this bridge connection across the trunk. The relay 2 has a very high resistance winding—about 15,000 ohms—and this resistance is so great that the tip supervisory relay of the A-operator's cord will not pull up through it. As a result, when this relay is bridged across the trunk circuit, the tip relay on the calling side of the A-operator's cord circuit is de-energized, just as if the trunk circuit were open, and this results in the lighting of the A-operator's calling supervisory lamp. The winding of the relay 4, however, is of low resistance—about 50 ohms—and when this is substituted for the high-resistance winding of the relay 2, the tip relay on the calling side of the A-operator's cord is energized, resulting in the extinguishing of the calling supervisory lamp. The illumination of the A-operator's calling supervisory lamp depends, therefore, on whether the high-resistance relay 2, or the low-resistance relay 4, is bridged across the trunk, and this depends on whether the relay 1 is energized or not. The relay 1, being bridged from the tip side of the trunk circuit to ground and serving as the means of supply of battery current to the called subscriber, is operated whenever the called subscriber's receiver is removed from its hook. Therefore, the called subscriber's hook controls the operation of this relay 1, which, in turn, controls the conditions which cause the illumination or darkness of the calling supervisory lamp at the distant office.

Assuming that the A-operator has received and answered a call, and has communicated with the B-operator, telling her the number of the called subscriber, and has received, in turn, the number of the trunk to be used, and that both operators have put up the connection, then it will be clear from what has been said that the calling supervisory lamp of the A-operator will be lighted until the called subscriber removes his receiver from its hook, because the tip relay in the A-operator's cord circuit will not pull up through the 15,000-ohm resistance winding of the relay 2. As soon as the subscriber responds, however, the relay 1 will be operated by the current which supplies his transmitter. This will substitute the low-resistance winding of the relay 4 for the high-resistance winding of the relay 2, and this will permit the energizing of the tip supervisory relay of the A-operator and put out the calling supervisory lamp at her position. As in the Western Electric circuit, therefore, the control of the A-operator's calling supervisory lamp is from the called subscriber's station and is relayed back over the trunk to the originating office.

In this circuit, manual instead of automatic ringing is employed, therefore, unlike the Western Electric circuit, means must be provided for notifying the B-operator when the calling subscriber has answered. This is done by placing at the B-operator's position a ringing lamp associated with each trunk cord, which is illuminated when the B-operator places the plug of the incoming trunk into the multiple jack of the subscriber's line, and remains illuminated until the subscriber has answered. This is accomplished in the following manner: when the operator plugs into the jack of the line called, relay 5 is energized but is immediately de-energized by the disconnecting of the circuit of this relay from the sleeve conductor of the cord when the ringing key is depressed, the selection of the ringing key being determined by the particular party on the line desired. These ringing keys have associated with them a set of springs 9, which springs are operated when any one of the ringing keys is depressed. Thus, with a ringing key depressed and the relay 5 de-energized, the ringing lamp will be illuminated by means of a circuit as follows: from the live side of the battery, through the ringing lamp 12, through the back contact and armature of the relay 6, through the armature and contact of relay 4, then through the armature and front contact of relay 2—which at this time is the relay bridged across the trunk and, therefore, energized—and thence through the back contact and armature of relay 5 to ground. When the subscriber removes his receiver from the hook, the relay 1 will become energized as previously described, and will, therefore, operate relay 6 to break the circuit of the ringing lamp. The circuit thus established by the operation of relay 1 is as follows: from the live side of battery, through the winding of relay 6, through the armature and contact of relay 1, through the armature and contact of relay 4, through the armature and front contact of relay 2, thence through the armature and back contact of relay 5 to ground. As soon as the B-operator notes that the ringing lamp has gone out, she knows that no further ringing is required on that line, thus allowing the operation of relay 5 and accomplishing the locking out of the ringing lamp during the remainder of that connection. The relay 6, after having once pulled up, remains locked up through the rear contact of the left-hand armature of relay 5 and ground, until the plug is removed from the jack.

At the end of the conversation, when the A-operator has disconnected her cord circuit on the illumination of the supervisory signals, both relays 2 and 4 will be in an unoperated condition and will provide a circuit for illuminating the disconnect lamp associated with the B-operator's cord. This circuit may be traced as follows: from battery through the disconnect lamp, through the armatures and contacts of relays 2 and 4, thence through the front contact and armature of relay 5 to ground, thus illuminating the disconnect lamp. The ringing lamp will not be re-illuminated at this time, due to the fact that it has been previously locked out by relay 6. The operator then removes the plug from the jack of the line called, and the apparatus in the trunk circuit is restored to normal condition.

In the circuit shown only keys are provided for ringing two parties. This circuit, however, is not confined to the use of two-party lines, but may be extended to four parties by simply duplicating the ringing keys and by connecting them with the proper current for selectively ringing the other stations.

The method of determining as to whether the called line is free or busy is similar to that previously described for the A-operator's cord circuit when making a local connection, and differs only in the fact that in the case of the trunk cord the test circuit is controlled through the contacts of a relay, whereas in the case of the A-operator's cord, the test circuit was controlled through the contacts of the listening key. The function of the resistance 10 and the battery connected thereto is the same as has been previously described.

The general make-up of trunking switchboard sections is not greatly different from that of the ordinary switchboard sections where no trunking is involved. In small exchanges where ring-down trunks are employed, the trunk line equipment is merely added to the regular jack and drop equipment of the switchboard. In common-battery multiple switchboards the A-boards differ in no respect from the standard single office multiple boards, except that immediately above the answering jacks and below the multiple there are arranged in suitable numbers the jacks of the outgoing trunks.