GB249284A - Improvements in or relating to telephone systems - Google Patents

Abstract

249,284. Automatic Telephone Manufacturing Co., Ltd., (Automatic Electric Co.). Feb. 11, 1925. Automatic exchange systems. - A drop-back selector is provided with shaft springs 8, 9, operated on one or more levels to prevent the selector from dropping back. The release magnet 36 controls contacts for holding open the circuit of the rotary magnet 56 during dropping back independently of the shaft springs, and for grounding the incoming c-wire after the selector is released until it is fully restored, and at the same time disconnecting the switching relay to prevent its energization by this ground. In an embryo system, Fig. 1, of up to ten offices of which only one (that reached over the fourth level of selector D) serves more than 1000 lines, the selector D is of the above kind and drops back for all first digits other than 4. Operation by a first digit other than 4. Line relay 22 and release relay 23 pull up, and the digit is repeated to vertical magnet 47. Springs 35, 37, 54 change over, and at the end of the digit the relapse of series relay 24 energizes release magnet 36 over 35, 37, 8. The release magnet locks itself over 38 and holds open the rotary circuit to make the drop back independent of springs 8, 9. When the wipers reach normal, relay 21 pulls up in series with the release magnet over its lower winding (which has hitherto been short-circuited at 35), opens the circuit, and locks itself over its upper winding. A standard double-wound relay in this position has a tendency to buzz owing to the fact that it pulls up its armatures before the flux is sufficiently built up to enable the holding winding to be effective. This is avoided by means of the shunt resistance shown or (less satisfactorily) by a series resistance. Alternatively the lower winding may have fewer turns than usual. Operation by second digit or first digit 4. Owing to the fact that the springs 8, 9 are off normal or the relay 21 is operated, the relapse of the series relay, instead of energizing the release magnet, energizes the rotary magnet 56 (stepping relay 25 having been previously energized and locked up in the usual way) and hunting takes place in known manner. When an idle line is found, switching relay 26 causes the relapse of relays 22, 23. Release. The relapse of relay 26 energizes the release magnet 36 over 45, 35. The selector is held busy over 38, 27, 45 (relay 21 having been released at the same time as relay 26), until the wipers reach normal.