Telephone systems predate the use of cheap computers and electronic switches. Yesterday’s phone system used lots of stepping relays in a box known as a “selector.” If you worked for the phone company around 1951, you might have seen the Bell System training film shown below that covers 197 selectors.
The relays are not all the normal ones we think of today. There are slow release relays and vertical shafts that are held by a “dog.” The shaft moves to match the customer’s rotary dial input.
Be sure to check out part two to get the whole story. Actually, we think [Periscope] switched the videos, so maybe start with part two. It sort of gives an overview and more of a mechanical perspective. Part one shows the schematic and assumes you know about some things covered in what they are calling part two.
You have to wonder who designed these to start with. Seems hard enough to follow when someone is explaining it, much less dreaming it up from scratch. Like most things, many people contributed to the development of the technology, and we are pretty sure the type 197 selector wasn’t the first device to appear.
Watching the current flow through the wires in the video reminded us of the Falstad circuit simulator.
I believe step by step starts with this guy: https://en.wikipedia.org/wiki/Almon_Brown_Strowger
Yes, I came here to say this. As recently as the 1970s, they were commonly called Strowger switches.
You can read about him and others in the second to last link…
I ended up finding several stepping switches in a cabinet at work that they were cleaning out (the maintenance manager let me take what i wanted). It looked like there were used as a sequence for an industrial controller.
My question is what in the hell did they need with an early night vision tube? It was NOS in the original packaging, along with some Lafyette audio amplifiers that were in a 1964 catalog.
I moved into my current house in 1994 and had to switch my modem to pulse dial (rural UK). By 2000 we could tone dial, so modern!
We had DTMF support in our city since the 80s or so, but our PBX was pulse dial only.
So we just used the modems and modern corded phones w/ LC display in pulse dial mode.
It wasn’t that bad, actually. Pulse dial was less noisy and had something soothing to it, I think.
I really like pulse dialing and relays, just not so much the primitive telephone exchanges with their poor line quality of the day.
They were built wrong way, also, I think.
Relay counter chains and relay flip-flops would have been more elegant
than this weird record player approach that works like a juke box.
But the engineers were probably not up to date back then, so I don’t blame them.
Digitalization of the phone line (through ISDN?) at the side of the telephone exchange in the 70s/80s was a good thing.
Audio compression was lossles, latency low and it was very compatible to fax machines and modems (56k modems even wanted a “digital” connection later on).
Much better than what VoIP provides nowadays, maybe.
BT had a lot of wacky systems out in the sticks – stuff to share lines, extend lines over long distances, a miniature exchange in a roadside box that could serve a small group of houses miles away from anywhere. Most of it was fairly basic old tech and was never built with the thought that someone on a farm somewhere might want one of them there computer things.
And all of it was forgotten about when they did the BT/Openreach split and then tried to work out how the holes & poles guys were supposed to repair exchange/transmission equipment.
I love electro-mechanical relays, really, but this is a situation in which fully-electronic circuits with battery tubes, diode logic, nuvistors or transistors would have been more appropriate.
The complexity in wiring and the mechanical stress caused a lot of maintenance headaches.
No offense, really, the relay technology was fine for a little town with a few hundred inhabitants with them being occasional callers but not for a big city.
It’s good that this era of telecommunications is now bygone.
When the Step-by-step telephone exchange was invented in the 1880s, semiconductors didn’t exist. Even vacuum tube technology wasn’t invented until the 1900s.
True. I was thinking about year 1951, though.
In 1880, the number of phones was comparably still small and the population low.
The population density wasn’t so high, also, I assume.
So hand-operated telephone exchange was still possible.
Telegraphs and telegrams had ruled telecommunications at this time, I suppose.
I love these videos/articles. I do love modern times and electronics, but there is something about electro mechnical stuff, I think it is because you can see how it works (or fiddle with it manually). The clicking and tuning has its charm. On one side it may look intimidating on the other it does look inspiring and may trigger the imagination of young engineers to be. One thing is sure, it has character and it also has proven its worth in times when there simply wasn’t any other option.
For sure. But electro-mechanical stuff can get quite annoying after a short wile.
It’s good that the ear shattering teletype was replaced by glass terminal or that the steam locomotive got superseded by the diesel train (and then sadly not by the monorail train using magnetic levitation).
Steam locomotive looks so awesome, but it’s a real stinker.
Whenever a locomotive passed through a town, the cloths on washing line turned black.
The children got asthma and became sick etc.
Romantic thoughts aside those big irons also had a dark side, in short.
The electric age did clean up a lot of the dirt, I think.
Let’s just take the electric tram or subway train, it doesn’t poison it’s passengers.
It’s similar with these electro-mechanical telephone exchanges, I think.:
As a visitor, it’s fun to watch in action for 5 minutes. But working there for hours can easily be a nightmare.
Much of modern health improvements are almost solely due to the fact that we stopped living in a dystopian Charles Dickens or Upton Sinclair novel where everyone is crammed together in firestarter tenements blanketed by coal ash and open sewage
I only wrote that I appreciated the video/article as I can see the charm of old technology. I never suggested that the past was better or that the “good old times” should return. I fully agree that old technology can get annoying pretty quickly (but it worked when it needed too and it helped progress to carry on. Let us keep in mind that the technology of today will be (in about 50 years) pretty annoying too. Heck, some modern technology already is annoying (but I guess that’s just me).
Another communication use for stepper switches was the WWII Japanese “Purple” cypher machine,
https://en.wikipedia.org/wiki/Type_B_Cipher_Machine
Based on the title cards in the Youtube videos, it looks like what Periscope has labelled Part 1 is actually Part 3 of the original training film, and what’s labelled Part 2 is actually Part 2, leaving only a lost Part 1
What Periscope calls Part 1, the film itself claims is Part 3. I can’t find Part 1, but I didn’t do a thorough search. Periscope Part 2 is, in fact Part 2, so start with that.
I worked in a Mountain Bell stepper office in my younger days trying to track down a noise that customers were getting upset about. First I had to isolate it to the Central Office by visiting the location of each complaint, and doing a through test of calls. Then I moved to the outside plant (cables) the that led to the Central Office. After several weeks on the entire problem I was in the Central Office using the telephone, when I heard the loud noise on the telephone and a loud noise behind me. I had the other person lay the phone, as I did, while I got a switch man (there title at the time) and asked him to trace the call through the system. He found the defective stepper switch that dragged it’s contacts though that section off steppers.
The company moved all the complaint trouble tickets to that central office. (Hundreds). The managers were graded by the number of complaints each week. The supervisor was said to have put out a bounty on me.
So much for his next raise.
That was about 1986.
Coincidentally, a dear friend of mine just recently gifted me one of these Strowger relays. A local retro telephone buddy has a collection of restored switches and switchboards and looked at it and said “we can probably get this thing stepping pretty easily, it looks like it’s in working order” so when we get to that point, we’ll post some pics and video and other info as we can come up with it.
He’s currently trying to move most of a rural telephone company into a workshop to get it running again, so i dont know how high of a priority the Strowger relay is going to be at the moment, but I know he’s excited about putting power to it.
Between the rotation, dogs, and general appearance, they remind me of Turing’s bombes used to crack Enigma in WWII.