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The previous page showed a simple Strowger switch capable of connecting one incoming line to one of ten outgoing lines. Strowger technology, though, was used in huge exchanges — some serving over 30,000 lines. To do this, the basic switch was arranged in stages.
The basic building block of a Strowger exchange was the two-motion selector: a Strowger switch like the one described previously, but instead of one arc of ten outgoing lines, it had ten arcs each with ten outgoing lines — able to select between 100 outlets.
The digit dialled caused the wiper to step up: dialling 1 stepped it once to the first-level arc, dialling 10 stepped it up ten times to the tenth-level arc. If only 100 lines needed selecting, the second digit could rotate the wiper around the arc. In the video, a 6 is dialled followed by a 5: the wiper steps up to the sixth level, then around five outlets to connect to line 65.
Of course, most exchanges have more than 100 lines — so how does the system allow for this?
The selector just demonstrated is called a Final Selector, because it accepts the last two digits of the phone number and selects the wanted line from a final group of 100. Before that stage, though, the call is routed through other selectors that narrow the choice down to just 100 options — these are called Group Selectors, since they select a group of 100 lines.
A small Strowger exchange for up to 1,000 lines would have three-digit phone numbers, with one Group Selector stage (the first digit) and ten groups of final selectors (the last two digits). Each added stage of Group Selectors gives capacity for another 1,000 lines.