Patent GB-153340-A (GB)
Abstract: 153,340. Cross, W., (Hulfish, D. S.). May 8, 1919. Automatic exchange systems.-In a call-finder system designed to prevent wrong connexions from being made when calls are made simultaueously within the same group, two co-operating switches move simultaneously and reach preliminarv positions determined by test circuits, after which one of the switches is again advanced to engage the calling-line. The test circuits for stopping the switches in their preliminary positions are third-wire circuits of the usual kind, but the test circuit for controlling the second movement of the switch includes the calling-line loop. The invention is described as applied to systems in which the call-finder comprises two separate switches, the line-finder 101. Fig. 2, and the group-finder 102, Fig. 2, the former having several sets of brushes mechanically connected together and electrically connected to the fixed contacts of the latter. The circuits of the switches 101, 102 are controlled by the pilot switch 103, Fig. 3, with which the test relays 41, 42, 44 are associated. Fig. 1 shows three subscribers' lines, with their line and cut-off relays 130, 131. The line relays of lines belonging to the same half-level of the switch 101 are arranged to earth a group lead, such as 318, 319, which identifies the calling group in the switch 102. The line relays of lines having similar positions in the switch 101 are arranged to earth a lead, such as 316, 317, identifying the line-finder position for these lines. Operation of call-finder. Assuming line 156, Fig. 1, calls, line relay 130 is energized, earthing leads 317, 318, and energizing group relay 132, which closes circuits over wire 304 for the clutch magnets 121, 122 of switches 101, 102, the circuits including the normal contacts of test relavs 42, 44. These circuits are interrupted at each step by the movement of the displaced brushes x<1> over successive pairs of contacts. When the test brushes t, t<1> reach the grounded test contacts, relays 42, 44 are energized and open the clutch magnet circuits. When both these relays are energized, the pilot switch 103 is moved into its second position, in which, if connexion has been made with the calling line, the relay 42 is energized over the loop circuit and moves the pilot switch into its third position. If, however, two lines such as 156, 1563 call simultaneously, relays 130, 1303 earth the leads 317, 318, 319, and the switches 101, 102 are brought to rest each in their third position, in which connexion is made with a third line 1562. If this line is idle, the relay 42 is not energized over the loop circuit, and the pilot switch remains in its second position, in which the clutch magnet 121 is again energized to move on the switch 101. If the line 1562 is in use, a third relay 41 closes an alternative circuit for the clutch magnet 121. As soon as the calling line is reached, relay 42 is energized over the loop and moves the pilot switch into its third position, in which the cut-off relay 131 is energized, and a loop circuit for the calling line closed over relays 41, 42, the pilot switch moving into its fourth position. Switches of the kind described in Specification 121,471 are used.