Patent GB-191309166-A (GB)
Title: Improvements in Observation Circuits of Telephone Systems.
IPC: H04M3/30
Filed: 1913-04-18
Granted: 1914-03-26
Abstract: 9166. Siemens Bros. & Co., and Christian, D. A. April 18. Exchange systems.- An observation line a, b, Fig. 1, terminating in a jack J at the desk of an observation clerk, is common to a group of subscribers' lines, such as T<1>, and is automatically connected, for example by a finding-switch W, to any line over which a call arrives or to w h i c h connexion is made by an exchange operator to complete a call. When the subscriber T' lifts down his receiver, the relays L, A are energized, and a circuit is closed over the contacts 4, 3 for the rotary magnet D, which moves the wipers of the switch W until they come on to the terminals belonging to the line T<1>, whereupon the cut-off relay F is energized over the contacts 14, 2, and a relay C is energized in series with relays G, H and lights a lamp 8. The observation clerk may refrain from inserting the plug P in the jack J, in which case, as soon as the relay B is energized by the operator answering the call, the relays G, H, C are de-energized, the circuit through these relays being connected to battery at both ends by the contacts 6, 7. This extinguishes the lamp 8 and open-circuits the cut-off relay F, which prepares the circuit of the rotary magnet D to deal with the next call. If, however, the observation clerk inserts the plug P in the jack J while the lamp 8 is glowing, the relays G, X are energized over the lead a, while the relay C is cut out and the relay H is short-circuited. The relay G locks the switch W and the relay X lights the lamp 9, which indicates, as long as it remains alight, that the calling subscriber's instrument is off its switch-hook. When the operator answers the call with the plug P<1>, the relays M, B are energized, and a circuit is closed over contact 7 for the relays H, Y. The relay Y lights the lamp 10 to indicate that the plug P<1> is in the jack J<1>. The cut-off relay F is now kept energized by contacts of the relays G and H, so that the switch W maintains the observation line a, b in connexion with the subscriber T' until both the subscriber has hung up his instrument and the operator has removed the plug P'. When the metering-key is depressed the current in the sleeve circuit is increased sufficiently to energize a relay R, which connects a signal coil N to the observation clerk's listening-set T. When the subscriber T' is receiving instead of initiating a call, the operations are similar, but the relays M, B. H, Y are energized before the relays L, A, G, X. In the arrangement shown in Fig. 2, instead of employing a finding-switch, each line T' has a relay V for connecting it to the observation line a, b, if disengaged. The relay Q, which controls the circuit of the relay V, is energized over the contacts 21, 22, and short-circuits a highresistance part of its own winding so as to prevent any other relay Q of the same group from receiving enough current through the resistance w<2> to become energized. The relays G, H are individual to the observation circuit instead of to the subscriber's line, but otherwise the arrangements are similar to those of Fig. 1.