Stepping switches change the direction of a magnetic flow to one of multiple channels, stepping through them incrementally. Which sounds incredibly boring until we realize they enabled the modern phone system and powered the decryption machines which morphed into the modern computer. Stepping switches were literally a step from the industrial revolution to the modern world.
Let’s step back. When you dial a phone number each digit zero’s in on the intended recipient. Take the theoretical number +1 212 345-6789. +1 indicated the US. The next set of numbers, 212, routes to the Washington, DC area. The next three numbers, 345, route your call to an exchange somewhere that used to be nearby your house. Finally, the last four digits find you.
Before stepping switches humans had to do manually. The +1 was implied (unless it wasn’t, in which case an overseas operator would reach the US). Dialing 212 is optional but, before stepping switches, if you wanted to dial long-distance an operator would have to plug your call into a long-distance line.
Finally, for the last part, an operator would always have to find you and plug the call in.
In case you’re wondering how this worked you would pick up the phone and tell the operator the number you wished to call. She (it was always a she) would then work with operators to get to the telephone you wished to reach.
If this sounds slow, clunky, annoying and expensive you’re right, it was. Therefore, Almon Stowger invented a device to do the work automatically. Rather than an operator routing the line, a series of stepping switches does the same work faster, cheaper, and more accurately.
Stepping switches were integral to the war effort. In Bletchley Park, the English code-breaking facility, they allowed the Allies to break Nazi encryption. Alan Turing, inventor of the modern computer, worked as a lead scientist.