Fig. 374. Trunk Relay
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Fig. 375. Trunk Relay
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The standard trunk relays employed by the Western Electric Company in connection with the circuits just described are shown in Figs. 374 and 375. In each case the dust-cap or shield is also shown. The relay of Fig. 374 is similar to the regular cut-off relay and is the one used for relays 9 and 14 of Figs. 371 and 372. The relay of Fig. 375 is somewhat similar to the subscriber's line relay in that it has a tilting armature, and is the one used at 13 in Figs. 371 and 372. The trunk relay 3 in Figs. 371 and 372 is the same as the A-operator's supervisory relays already discussed.

It has been stated that under certain circumstances B-operator's trunk circuits devoid of ringing keys, and consequently of all keys, may be employed. This, so far as the practice of the Bell companies is concerned, is true only in offices where there are no party lines, or where, as in many of the Chicago offices, the party lines are worked on the "jack per station" basis. In "jack per station" working, the selection of the station on a party line is determined by the jack on which the plug is put, rather than by a ringing key, and hence the keyless trunk may be employed.

Fig. 376. Keyless Trunk
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A keyless trunk as used in New York is shown in Fig. 376. This has no manually operated keys whatever, and the relay 17, when it is operated, establishes connection between the ringing generator and the conductors of the trunk plug. The relays 3, 13, and 12 operate in a manner identical with those bearing corresponding numbers in Fig. 371. As soon as the trunk operator plugs into the multiple jack of the called subscriber, the relay 16 will operate for the same reason that the relay 9 operated in connection with Fig. 371. The trunk disconnect lamp will receive current, but if the operator has already established connection with the other end of the trunk, this lamp will not be lighted because shunted by the relay 17, due to the pulling up of the armature of the relay 13. The relay 15 plays no part in the operation so far described, because of the fact that its winding is short-circuited by its own contacts and those of relay 12, when the latter is not energized. As a result of the operation of the relay 17, ringing current is sent to line, the supply circuit including the coil of the relay 12. As soon as the subscriber responds to this ringing current, the armature of the relay 12 is pulled up, thus breaking the shunt about the relay 15, which, therefore, starts to operate in series with the relay 17, but as its armatures assume their attracted position, the relay 17 is cut out of the circuit, the coil of the relay 15 being substituted for that of the relay 17 in the shunt path around the lamp 4. The relay 17 falls back and cuts off the ringing current. The relay 15 now occupies the place with respect to the shunt around the lamp 4 that the relay 17 formerly did, the continuity of this shunt being determined by the energization of the relay 13. When the A-operator at the distant exchange withdraws the calling plug from the trunk jack, this relay 13 becomes de-energized, breaking the shunt about the lamp 4 and permitting the display of that lamp as a signal to the operator to take down the connection. It may be asked why the falling back of relay 15 will not again energize relay 17 and thus cause a false ring on the called subscriber. This will not occur because both the relays 15 and 17 depend for their energization on the closure of the contacts of the relay 13, and when this falls back the relay 17 cannot again be energized even though the relay 15 assumes its normal position.

Kellogg Trunk Circuits. The provision for proper working of trunk circuits in connection with the two-wire multiple switchboards is not an altogether easy matter, owing particularly to the smaller number of wires available in the plug circuits. It has been worked out in a highly ingenious way, however, by the Kellogg Company, and a diagram of their incoming trunk circuit, together with the associated circuits involved in an inter-office connection, is shown in Fig. 377.