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!