Patent GB-399875-A (GB)
Abstract: 399,875. Automatic exchange systems. AUTOMATIC ELECTRIC CO., Ltd., Strowger Works, Liverpool. April 15, 1932, No. 10876. Convention date, June 8, 1931. [Class 40 (iv).] Numerical and non-numerical switches are provided with thermionic valves to enable the usual test and holding wires to be eliminated. The invention is described with reference to two similar systems employing bidirectional finders and selectors. Call-finder, Fig. 1. Line relay 103 grounds the upper talking wire to mark the line and energizes the subgroup starting relay 107 which marks the level and energizes the starting relay 122 of the next idle finder. 122 lights the valve 114 and energizes the vertical magnet which interacts with relay 125 until the calling level is reached whereupon 123 in the plate circuit of the valve switches over to the rotary magnet 119, switches the grid from the vertical to the upper talking wiper, locks up and replaces itself in the plate circuit by a relay 124. When the line is found, 124 switches through, replaces itself in the plate circuit by 126 and locks up in series with 121 which by substituting the dropping resistance 116 for the dropping resistance 115 shifts the potential of the filament from nearly that of the grounded positive end of the main battery to nearly that of the negative end. As a result, 126 remains energized in the plate circuit of the valve after the operation of the cut-off relay in series with the selector line relay, although the grid is now no longer grounded but is at the intermediate potential characteristic of busy lines. If the finder misses the line, or in case of premature release, 11th step springs 197 release the starting relay 122 and the finder is released. Group selector, Fig. 1. Line relay 141 lights the filament of the valve 151 which is given a potential close to that of the negative end of the battery and since the grid is connected to the talking circuit which is at an intermediate potential, plate relay 143 pulls up energizing release relay 142. Dialled impulses give the grid the full negative potential and the plate relay therefore repeats them to the vertical magnet 155 through series relay 145. At the end of the digit, 145 energizes the change-over' relay 144 which connects the grid to the upper talking wiper and makes a self-interrupting circuit for the rotary magnet 156. Busy lines are at an intermediate potential which keeps the plate relay energized but when an idle connector is reached the grid receives full negative over the upper winding of the line relay and the plate relay falls and energizes the switching relay 146 which releases 144, 141. Before 142 can fall, 143 pulls up again since the grid again assumes the intermediate potential when the loop is switched through. If all trunks are busy 144 is released by 11th step springs. 146 has a stiff spring to make it slightly slow to operate. This is to guard against the seizure of a busy line carrying dialling impulses and to give time for release before the seizure of a connector that has only just become idle. Final selector, Fig. 2. Line relay 201 lights the valve 213 and plate relay 204 energizes release relay 203. 204 repeats the digits in turn to the vertical and rotary magnets 216, 217. Series relay 205 switches over after the vertical movement and parallel relay 207, energized during the rotary movement, energizes 209 and partially energizes 208. At the end of the digit 20 falls and 208 then pulls up completely before 209 has had time to fall and connects the grid of the valve to the lower talking wiper. If the line is idle, there is full negative battery on this terminal and 204 falls back to energize the switching relay 210 which releases 208, puts out the valve and energizes the cut-off relay 233. Otherwise, 204 remains energized and when 209 falls, busy tone is sent back. On the operation of the switching relay, ringing current is sent out over 206, and when the subscriber replies, 206 pulls up and completes the talking circuit energizing back-bridge relay 202 which shunts resistances 211, 212 in the calling feed circuit for supervisory or metering purposes. During conversation owing to the crossing of the talking wires in the final selector, the cut-off relays are effectively on opposite sides of the talking circuit to avoid unbalance. Release. When the calling party hangs up, the grids of the valves 114, 151 are made negative from the upper winding of the calling side feed relay, which falls back, releasing release relay 203, and the plate relays 126, 143 fall back followed by the release relays 122, 142. 2nd system, Fig. 3. This is generally similar to that described above, but reverse battery supervision is employed and the cut-off relays are balanced by coils 371, 379 in the finder and final selector respectively. Idle connectors are marked by a battery connection to the lower talking wire over an off-normal contact 381. The change-over relay 344 in the group selector is made sluggish by a shunt 378 (except for release by 11th step springs) to provide a temporary circuit after the operation of the switching relay 346 for energizing the connector line relay 301 over its upper winding and for holding the cut-off relay until the operation of release relay 303 connects up the lower winding of line relay 301.