Below, the circuit diagram of the 332 phone as conveniently stuck to the inside of the phone’s base plate. It needs to be drawn out in a less convoluted way so that sense can be more easily made of it! (Soon!)
At the same time as I rescued the phone, I also rescued some bits from the electro-mechanical automatic telephone exchange (known as a Strowger system). All exchanges operated like this until the late 1970s when the GPO started to go over to electronics (“System X”).
The unit below (shown on its side) comprises an electro-mechanical switching system. There are three sets on switch contacts in this particular system and they can connect to other contacts stacked one on top of another (not present on this unit, unfortunately). By moving up and down to a chosen set of contacts then rotating to a particular individual contact, a large number of possible telephone connections can be made. An exchange would contain a great number of these switching units all clicking away.
The contacts were moved up and down and rotated by pawls operated by electro-magnets acting on linear and circular ratchets.
Below shows a close up of the three wiper contacts which moved up and down and clockwise and anticlockwise to make the desired connection.