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The Ubiquitous Line Circuit

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When a subscriber’s phone goes off-hook, they are effectively signaling to the exchange -- “Hey, I want to make a call”. Also, when a called subscriber answers a ringing phone, they are signaling to the exchange -- “Stop the ringing and let me speak.”


At the exchange edge, the line circuit was created to help with these requests.  From the early days of manual switchboards two relays (named Line and Cut Off or Cutoff) were commonly used in the line circuit. This circuit is at the interface between the subscriber telephone and the exchange switching logic. There is a dedicated line circuit for each subscriber per exchange type.


As highlighted on this site, an automatic exchange for 10,000 subscribers may have approximately 60,000 relays.  However, line circuits use 20K (1/3rd) of this count, showing its vital importance.  If you are interested to see how the L and CO relays work in a typical operation, continue below, else escape now.

explainer, basic line circuit with line and cutoff relays for exchange

The L and CO Relays


The figure, derived from [Shackleton], shows a circuit for a manual switchboard with the L and CO relays.


​​ Let’s assume the subscriber goes off-hook;

  1. Current flows from one coil of the L relay through (Tip wire) the telephone and back (Ring wire) to the other coil of the L relay.  

  2. The L relay operates and lights the Line Lamp. The operator notices the lamp and inserts a spare plug into the indicated Jack. The plug has 3 terminals, the Tip, Ring and Sleeve named after the 3-conductor structure of the plug.

  3. The Sleeve wire carries 48 volts, which activates the CO relay upon plug insertion.  This causes the L relay to release, the lamp goes off, and the operator says to the caller, “Number please”.

  4. When the call is completed, the operator removes the plug and the CO relay releases. The L and CO are now both off.


Assume the operator wants to ring the idle subscriber.  They insert a plug and it operates the CO relay. This disconnects and isolates the L relay from the talking (and ringing) Tip and Ring path. When the call is completed, the plug is removed and the CO relay releases. The line circuit is ready for another call. 


Exchange technology has transformed impressively from 1892 until the 1960’s. However, the line circuit (L + CO relays) is fundamental. Of course, for the automatic exchange there is no lamp or operator cord -- the wiring part is different.  With no operator, the L relay initiates line finders to find the new caller.  Sure, the line circuit workings have been tweaked to fit into a particular exchange type, but its general high level functions have remained intact.






Shackleton and Purcell, Relays in the Bell System, Bell System Technical Journal, Jan 1924

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