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Article and doubts about switchboard operation

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Hello again,

I posted photos recently of a 1940/50s Ericsson switchboard that I'm restoring and re-adapting for a theatrical project. I'm currently writing an article about the project and have some doubts about the way that the switchboard would have originally functioned. I'd like to ask members of the forum to please help clear some these doubts and I will of course acknowledge the help of the forum and any individuals if wished, in the article.

I'll attach a photo of the Ericsson switchboard used in the project for reference.

Here are my questions/doubts:

1. The top row of 10 jacks on the Ericsson board had 5 that were wired up and a label above of each still shows a unique local telephone number. Is it correct or likely that these jacks were connected to 5 different external telephone exchanges or "trunks" with the inscribed numbers and that the jacks could be used both receive calls (whereby the "supervisory" lamp above the jack would illuminate with the incoming call) and make calls or transfers to external telephones?

2. I'm a little confused about the meaning of the word "trunk". Does it mean the external exchange itself or the connection to the exchange?

3. If the exchange/trunk was manual and the telephonist wished to reach an external phone, the telephonist would speak with an operator at the exchange once the line was answered and read out the desired number for that operator to make the connection. The contact between the Ericsson and the external exchange would not require the use of the "ring key". The local telephonist would simple insert a cord in the jack of the trunk and a lamp to be illuminated on the external exchange. Correct?

4. Since the Ericsson is manual, the same process would occur in reverse when an external exchange wished to reach a telephone on the Ericsson's local network. The external operator would speak with the local operator to make the connection to the appropriate internal extension. Correct?

5. If the local operator wished to call an external number and the exchange/trunk were automatic, there would be no need to speak with an operator. The local operator would insert a cord in the jack of the trunk and use the dial to call the desired phone. I think in this case it is not necessary for the operator to use the ring key. Is that correct?

6. The ring-key would be used when the telephonist wished to call or make a connection to an internal extension. If no suitable power supply was available to ring the phone on the extension, the crank (not shown in the photo) would be used.

7. And about keys, I'm in doubt about the naming of them. If I can call for the moment the device below each cord-pair a switch, the Ericsson's switches are 3-way. Are the 3 positions of the switch referred to as "keys" or is the switch itself the key? I believe it's the former. In the case of the Ericsson, the back position of the switch is the "ring-key" and it is spring-loaded so that upon release, it returns to the central position. I've also read of the talk-listen keys. I assume that the central position is the listen key and the forward ṕosition is the talk. Are my assumptions correct?

8. On transfers and this is where I may be well off target, when an incoming call is indicated by a supervisory lamp, the operator inserts an answer cord from a free cord-pair (and with the central listen-key for the channel selected) in the corresponding jack. The operator speaks with the caller and notes the room or external telephone number. If it is for a specific person/department on an internal extension, the operator would take the calling cord of the pair, insert it in desired jack and again, using the-ring key to call the phone (room numbers are marked above the two lower rows of jacks on the Ericsson). In the case of an external number request, the operator would insert the talk cord into a trunk jack and use one of the procedures I outlined above in questions 3-5.

9. If the receiver answers and accepts the call, the operator pushes the channel switch from the central "listen-key" to the forward-position "talk-key". Correct?

10. If question 9 is correct, what happens if the operator returns the switch to the central listen-key position while the transfer is still in progress? Can they listen and speak to both parties or to one? Which one, if so?   

If forum members can answer any of the above doubts and questions, I'd be very grateful.

Many thanks,


1. Yes.
The top row is trunks, or outside lines. or direct lines to the main exchange. usually sequential numbers like 4423,4424,4425 etc.
2. The trunk is like a telephone. pick up the line, and dial an outside number, or it would lite a lite at the main exchange for manual use.
3. yes, or the op would dial the number herself.

4. yes
In your case, a lite would lite on your board above one of the trunks.
As well as some audible annoyance, such as a buzzer.
Then, the op would plug in to the lit trunk "which, should extinguish the lite", and find out what the op at the other end wanted.
Next, she would plug into the extension requested, and RING that room.
Opon the answering of said extension, the op would notify the called EXTENSION of the CALLERS request to bother them, and if they would indeed, allow the op to connect the call...Or, if she should tell the CALLER, to have a nice flight... ;)
5. yup
6. Hmm
The Ring key, is for the op. it lives only IN HOUSE.
She RINGS an EXTENSION connected to her board.
The CRANK, was used on a LOCAL batt phone, So a home could summon the local operator, and vice versa. Then, the AC generator was added.
It took priority, but the generator was kept for a while, as a back up, because it was basically  the same thing. Just drivin by hand.
7. A key, is the object you interact with. It can have any number of switches and combinations as desired.
A KEY, is the method you use to place an incoming call onto a BUS, and interact with it. usually with an OP, or an automated system using DIALS, and TRUNKS.
SWITCHES, or KEYS, are mostely 3 position. The CENTER position is NOBODY CARES. The BACK position is RING. spring loaded, so you don't forget and leave your boss a with your rendition of "OH, the ringing in my ears".
It only works when the op plugs in a cord to that extension, and connects herself to that called extension, " via the BUSS, with another KEY.
oddly enough, called the OP key on some models. Allows a local conversation between the op and an extension off of her board.
I think, the Forward position, "TALK", did the same thing on later models.
8. yup, pretty much.
9. Naw, flip that. The op needs that key to be pulled in to her, for her to be ON the line WITH the CALLER and or called party.
When both partys are connected, the op can RETURN the key to CENTER, which cuts her off the buss, but leaves the two party's on the buss,
Or she can stay on the line, and get all the new poop goin on on the hood.
10. Depending on the board configuration, The op can always stay in contact with her extensions.
11. Do not believe anything I have just said.


--- Quote from: RB on August 27, 2020, 11:24:01 PM --- .....
11. Do not believe anything I have just said.

--- End quote ---
Thanks very much RB! Just to confirm, when someone calls in, or if the op instigates a call, does s/he need to push the key to the forward position to connect his/her bus and be able to speak with the connected party? And if this is the case, my understanding is that when the op begins a transfer to a second party, the key is first pushed back to the central position, thereby disconnecting the op's bus from that of the 1st party, the talk cord is inserted into the desired jack and the ring position is engaged if that jack happens to be a local extension. My new doubt is, when the op talks with the second party to advise of the transfer request, they must push the key forward again to do so. If this is true, does not the original caller hear this conversation?
All the best!

depending on if your board has a split key, which disconnects the calling party from the bus while the op chats with the called party in private.
if not, then the calling party will stay on the bus while the called party is rung up.
then after both party's are talkin, the op returns her op key to the center position.
i believe the talk position, opposite the ring position, stays active till the call is terminated.


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