This is the last of a series of posts about the #5 Crossbar Switch, an electro-mechanical telephone switching system that uses relay logic.

The first post touched on the evolution of telephone switching and introduced the switch fabric comprised of many 10×10 crossbar switches. The second post discussed the line and marker circuits; the latter of which implements the switching logic.

This post — the point of the series — discusses the very clever design that allows a 10×10 crossbar to use only 20 relays (here called “magnets”) to control 100 potential connection points within the switch.

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