123,053. Automatic Telephone Manufacturing Co., (Assignees of Martin, T. G.). Feb. 4, 1918, [Convention date]. Automatic exchange systems and apparatus. -Relates to systems in which an interexchange .trunk line is adapted to connect a number of levels of the selector switch at the outgoing exchange to corresponding selectors or connectors at the incoming exchange. The trunk is connected to the desired selector at the incoming exchange by means of a selecting apparatus comprising polarized relays, which are controlled by polarity currents sent over one or other of the two sides of the trunk line by contacts controlled from the levels of the selector switch at the outgoing exchange. The invention is specially adapted for work between a small branch exchange and a main exchange, and is described for a system in which a branch exchange, Fig. 1, has selector levels multipled to a repeater E, which operates over a trunk 41, 42 to the main exchange, Fig. 2. The group of relays G at the branch exchange control the relays F at the main exchange tc connect the trunk to the appropriate selector or the like H, I, J, K or to an operator seat. Inter-exchange working; selection of the desired selector switches at the incoming exchange. A call having been received over one of the levels A having access to the repeater E, say, the contacts 17, 18, and 25, the line relay 2 is energized, and a circuit is completed over the contact 25 for the relay 12. The relay 2 completes a circuit for relay 3, which connects ground to the contacts 25-28, thus causing them to test busy, and supplies a holding circuit for relay 12. The relay 3 also brings the upper winding of relay 6 and the slow-acting relay 7 into circuit. Before the latter operates, however, the closing of the contact 16 of relay 12, sends an impulse from negative pole of battery B' over the side 42 of the trunk to the polarized relays 35, 36, Fig. 2. The closing of the contact 50 brings the slowacting relays 40, 32 successively into circuit, thus connecting the trunk to the selector K, and a hold-on circuit for the relay 32 is provided by this selector as is described later. In the same way, if the call is received over the level represented by the contacts 19, 20, 26, the relay 11 is energized and an impulse from the positive pole of battery B<2> over the line 42 energizes the relay 35, and connects the trunk to the selector J appropriate to the level 19, 20, 26. Calls on the other levels send impulses over the lead 41, and by means of the relays 33, 34, connect the trunk to the units H, I, one of which may be an operator. Repeating sets. The slow-acting relay 7, referred to in the preceding paragraph, energizes soon after the selecting impulse is sent over the line 42, and thus completes a circuit for the line relay 52 of the desired selector K over the upper winding of relay 52, contact of relay 32, line 41, contact 54, 55, lower winding of back bridge relay 6, right-hand winding of relay 8, contact 58 to line 42 and to second winding of line relay 52. Relay 6 is not energized by this circuit as the currents in its two windings are in opposition. The release relay 53 provides a holding circuit for the relay 32. The number impulses are now received by the line relay 2, thus completing a circuit for the slow-acting relay 5, which energizes to complete a repeating circuit over contacts 54, 59 and spring 58. As soon as the number impulses cease, the relay 5 de-energizes, and restores the connexion through the windings 6 and 8. The wanted subscriber is called in the usual way, and upon his lifting his receiver, the connector switch reverses the connexions of its line relay, thus reversing the direction of the current flow in the lower winding of back bridge relay 6. This relay now energizes and completes a circuit for the reversing relay 4, which reverses the direction of the current flow in the calling line, and connects the high resistance winding 8 in series with the right-hand winding of relay 8 and lower winding of back bridge relay 6 to reduce the current flow over the trunk 41, 42. Release. When the calling subscriber hangs up his receiver, the line relay 2 is de-energized, thus breaking the circuits of relays 3 and 7. The former removes the ground from the private contacts 25-28, and the latter disconnects the repeater E from the trunk 41, 42. The holding circuit for the connector in the unit K is thus broken, and the relay 32 is disconnected, thus restoring the connectors and connecting the sides of the trunk again to the selecting relays F.