Before the manual switching system that the electronic switching replaced, phone calls were connected by switchboard operators like these women, who had to match a caller to the right line by hand.
Courtesy of National Archives
In 1960, Bell Telephone began testing its first electronic system for connecting phone calls here in Morris, Illinois. Morris’s Central Office was chosen for its size: small, but not too small. Before electronic switching systems (ESS), Bell used electromechanical switchers—meaning that as a person dialed a number, the phone sent signals through a series of wired relays, and the relays triggered machines that connected the caller to the right line. In contrast, the ESS stored instructions on memory cards. When a dialed number signaled the ESS, it followed rules written on a solid-state device and then used electrical signals to route the call to the proper line. The advantages of the ESS were that it was faster, more efficient and cheaper to maintain than electromechanical switchers, which had a lot of moving parts. [1] A 1965 video from Western Electric explains the science in more detail and provides a great behind-the-scenes look into the mid-century field of telecommunications and computing.
This map comes from the general description of the electronic switching system trial that Bell Telephone Laboratories wrote for internal use by its employees. “General description” is misleading—this was a pretty hefty manual, over 350 pages in length.
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