The relay 14, already referred to as forming a shunt for the trunk disconnect lamp, has for its function the keeping of the talking circuit through the trunk open until such time as the relay 13 operates, this being purely an insurance against unnecessary ringing of a subscriber in case the A-operator should by mistake plug into the wrong trunk. It is not, therefore, until the A-operator has plugged into the trunk and the relay 13 has been operated to cause the energization of the relay 14 that the ringing of the called subscriber can occur, regardless of what the B-operator may have done.

The relay 9 has an additional function to that of helping to control the circuit of the ringing-key holding magnet. This is the holding of the test circuit complete until the operator has tested and made a connection and then automatically opening it. The test circuit of the B-operator's trunk may be traced, at the time of testing, from the thimble of the multiple jack under test, through the tip of the cord, thence through the uppermost pair of contacts of the relay 9 to ground through a winding of the B-operator's induction coil. After the test has been made and the plug inserted, the relay 9, which is operated by the insertion of the plug, acts to open this test circuit and at the same time complete the tip side of the cord circuit.

In the upper portion of Fig. 371 the order-wire connections, by which the A-operator and the B-operator communicate, are indicated. It must be remembered in connection with these that the A-operator only has control of this connection, the B-operator being compelled necessarily to hear whatever the A-operators have to say when the A-operators come in on the circuit.

Fig. 372. Incoming Trunk Circuit
[View full size illustration.]

The incoming trunk circuit employed by the Western Electric Company for four-party line ringing is shown in Fig. 372, it being necessarily somewhat modified from that shown in Fig. 371, which is adapted for two-party line ringing only. In addition to the provision of the four-party line ringing keys, by which positive or negative pulsating current is received over either limb of the line, and to the provision of the regular alternating current ringing key for ringing on single party lines, it is necessary in the ringing cut-off relay to provide for keeping the alternating and the pulsating ringing currents entirely separate. For this reason, the ringing cut-off relay 12 is provided with two windings, that at the right being in the path of the alternating ringing currents that are supplied to the alternating current key, and that at the left being in the ground return path for all of the pulsating ringing currents supplied to the pulsating keys. With this explanation it is believed that this circuit will be understood from what has been said in connection with Fig. 371. The operation of the holding coil 8 is the same in each case, the holding magnet in Fig. 372 serving to hold depressed any one of the five ringing keys that may have been used in calling the subscriber.

Fig. 373. Western Electric Trunk Ringing Key
[View full size illustration.]

The standard four-party line, trunk ringing key of the Western Electric Company is shown in Fig. 373. In this the various keys operate not by pressure but rather by being pulled by the finger of the operator in such a way as to subject the key shaft to a twisting movement. The holding magnet lies on the side opposite to that shown in the figure and extends along the full length of the set of keys, each key shaft being provided with an armature which is held by this magnet until the magnet is de-energized by the action of the ringing cut-off relay.