Automatic telephone switching is usually told as a story about replacing operators. Marcus Thornley argues that what was replaced was narrower and more interesting: the work of building a path between two lines, which an operator did with her hands and which a machine had to do with contacts.
The book starts where the constraint was, which is scale. A single manual switchboard topped out around ten thousand lines, and a large city therefore needed scores of central offices wired to each other. The operating company's own account of why it automated does not read as a labour-cost argument at all; it turns on the difficulty of recruiting enough operators and on a published test of whether the change would give better service.
Thornley then follows the mechanisms as successive answers to one question: where does the intelligence live? In the earliest patent the subscriber's number is literally a coordinate, rows and columns of contact holes, and the caller's keypresses drive a pawl and ratchet to the right hole. Step-by-step switches are moved directly by dial pulses, which is elegant and unforgiving: a system controlled by the pulses cannot try a second route when the first is busy. Crossbar lifts the control out of the contacts altogether, and the number stops being a coordinate and becomes an instruction.
Traffic gets its own chapter, treated as probability rather than volume, along with the economics that make one large group of trunks cheaper than several small ones.
Fourteen sources, mostly patents and the Bell System's technical journal, are cited. The last chapter returns to what the machines actually took over.
