Telephony
I've been reading about the history of telephony. There are lots of buffs out there.
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Here's an excellent scary (very short) movie about the revenge of the PSTN on VoIP systems.
And from a Bell-System-lauding 1910 history:
In 1896 there came a most revolutionary change in switchboards. All things were made new. Instead of individual batteries, one at each telephone, a large common battery was installed in the exchange itself. This meant better signalling and better talking. It reduced the cost of batteries and put them in charge of experts. It established uniformity. It introduced the federal idea into the mechanism of a telephone system. Best of all, it saved four seconds on every call. The first of these centralizing switchboards was put in place at Philadelphia; and other cities followed suit as fast as they could afford the expense of rebuilding. Since then, there have come some switchboards that are wholly automatic. Few of these have been put into use, for the reason that a switchboard, like a human body, must be semi-automatic only. To give the most efficient service, there will always need to be an expert to stand between it and the public.