325,392. Standard Telephones & Cables, Ltd., and Baker, J. H. E. Feb. 21, 1929. Automatic exchange systems. - In a system using Strowger switches with 4 sets of wipers as group and final selectors, and having control circuits common to a group of selectors, wiper selection in the group and in the final selector is controlled by the group selector control circuit. In the system described the first digit rouses the group selector and if it indicates a junction call, one set of wipers is switched through and the control circuit is released. If not, the selector drops back and is re-set by the 2nd. The control circuit then responds to the 3rd digit and makes two records, one depending on this digit only for effecting wiper selection in the group selector, and the other depending on this digit and on whether the 2nd digit was odd or even. This record is transferred over the talking wires to the connector control circuit to effect wiper selection as soon as the group selector is switched through. A modification is referred to in which all the wiper sets of the group selector are used for local calls the set to be used being selected in accordance with the 1st 2 digits. Selecting junction. When the group selector is seized, K pulls up over the test wire in series with E, energizes X and seizes the control circuit in which impulse relay D and holding relay G pull up. At each impulse of the 1st digit D remains disconnected until the magnet V closes its contacts VINT, and then locks up over the loop. The relapse of dialling relay F makes a circuit over test relay R to wiper TJ and energizes the rotary magnet R in a self-interrupting circuit. When a junction is found, R energizes the switching relay A and D falls back releasing the control circuit. Selecting wiper set in local final selector. Post springs NP operated by the 1st digit energize 3B which energizes release magnet Z. The relapse of off-normal relay N, energizes 3A to disconnect 3B (which is somewhat sluggish). The next digit raises the selector in the same way as the 1st. In addition, the odd impulses operate 3P in 2 steps and the even impulses similarly operate 3S which releases 3P, operates 3W its full step and falls on the relapse of 3P, 3W remaining locked until 3P is operated again. At the end of the digit F which partially energized H, allows it to operate completely to disconnect the vertical magnet, and energizes L or M aecording as 3P or 3W is energized. The 3rd digit causes the operation in turn of relays 3S, 3W, 3J, 3P, each of which unlocks the preceding one and the cycle beginning with 3S or 3J according as 3P or 3W is energized. Relay 3X which is energized partially by F at the beginning of the digit makes its full step when 3P (or 3W) is released by the 1st impulse. The operation of 3P (or 3W) by the 4th and 8th impulses causes the half-step operation of 3P, 3Z respectively, these relays making their full step when 3P (or 3W) is released by the next impulse or, if the digit is 4 or 8, on the relapse of F. At the end of the digit, I which made its half-step at the beginning of the digit makes a driving circuit for magnet R and a test circuit over relay R and the wiper selected by the relays 3X - - 3Z. When an idle and available connector is found, R energizes one or both of the wiper connecting relays B, C. Relay 2K in the connector pulls up over the test wire in series with EE to seize the control circut. The talking wires are thus connected to the junction points of the wiper choosing relays 4A, 4B and 4C, 4D. In the group selector control circuit, R disconnects D and connects ground and/or battery to the talking wires over the relays 3K, O, according as 3J, 3W, 3S or 3P is energized, to energize two of the relays 4A - - 4D. These relays lock up and energize one or both of the wiper-connecting relays 2X, 2Y, or the wiper-connecting relay 2W. O and 3K release E, K to release the control circuit and switch the calling line through to the connector. Busy signal. 11th step springs S1 energize 3C which sends busy tone from BT. Operation of connector by remaining digits. Relay 4E, energized by the relays 3A - - 4D connects the impulse relay DD in their place. DD pulls up, makes a new circuit for holding relay GG and energizes dialling relay FF which half-operates HH. The remaining digits are repeated to the magnets 2V, 2R, the impulse relay being controlled by the magnets as in the group selector to guard against short impulses. Owing to the operation of off-normal relay 4N, FF falls back at the end of the digit to allow HH to make its full step and connect up the rotary magnet. At the beginning of the units digit FF pulls up again and energizes CC which half-operates II. Busy test and signal. In the interval between the relapse of FF, C'C at the end of the digit, a current is made over RR and the test wiper and if it does not pull up CC falls and II makes its full step and connects up busy tone BT. Supply and control of ringing current and completion of connection. RR pulls up if the line is idle, holds CC and completes the ringing and ring-back tone circuits. When the called party replies, 4F energizes the selector release relay 2B which initiates the release of the control circuit and extends the loops to their respective supervisory relays 2A, 2D. 2A holds 2B and 2D locks itself independently of 2B over the called loop. Release. On the release of 2A, 2B by the calling party the group selector is released. The connector remains held until the called party releases 2D to operate the release magnet 2Z.