Line Circuit. The line circuit is easily understood in view of what has been said concerning the Western Electric line circuit, the line relay 1 being single wound and between the live side of the battery and the ring side of the line. The cut-off relay 2 is operated whenever a plug is inserted in a jack and serves to sever the connection of the line with the normal line signaling apparatus.

Cord Circuit. The cord circuit is of the four-relay type, but employs three conductors instead of two, as in the two-wire system. The relay 3, being in series between the battery and the sleeve contact on the plug, is energized whenever a plug is inserted in the jack, its winding being placed in series with the cut-off relay of the line with which the plug is connected. This completes the circuit through the associated supervisory lamp unless the relay 4 is energized, the local lamp circuit being controlled by the back contact of relay 4 and the front contact of relay 3. It is through the two windings of the relay 4 that current is fed to the subscriber's station, and, therefore, the armature of this relay is responsive to the movements of the subscriber's hook. As the relay 3 holds the supervisory lamp circuit closed as long as a plug is inserted in a jack of the line, it follows that during a connection the relay 4 will have entire control of the supervisory lamp.

Listening Key. The listening key, as usual, serves to connect the operator's set across the talking strands of the cord circuit, and the action of this in connection with the operator's set needs no further explanation.

Ringing Keys. The ringing-key arrangement illustrated is adapted for use with harmonic ringing, the single springs 5, 6, 7, and 8 each being controlled by a separate button and serving to select the particular frequency that is to be sent to line. The two springs 9 and 10 always act to open the cord circuit back of the ringing keys, whenever any one of the selective buttons is depressed, in order to prevent interference by ringing current with the other operations of the circuit.

Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356 is an end view of the entire set. In Fig. 355 the listening key is shown at the extreme right and the four selective buttons at the left. When a button is released it rises far enough to cause the disengagement of the contacts, but remains partially depressed to serve as an indication that it was last used. The group of springs at the extreme left of Fig. 355 are the ones represented at 9 and 10 in Fig. 354 and by the anvils with which those springs co-operate.

Fig. 354. Dean Multiple Board Circuits
[View full size illustration.]

Test. The test in this Dean system is simple, and, like the Western Electric and Kellogg systems, it depends on the raising of the potential of the test thimbles of all the line jacks of a line when a connection is made with that line by a plug at any position. When an operator makes a test by applying the tip of the calling plug to the test thimble of a busy line, current passes from the test thimble through the tip strand of the cord to ground through the left-hand winding of the calling supervisory relay 4. The drop of potential through this winding causes the tip strand of the cord to be raised to a higher potential than it was before, and as a result the upper plate of the condenser 11 is thus altered in potential and this change in potential across the condenser results in a click in the operator's ear.