Telephone switchboard cross-switching (Rankin Kennedy, Electrical Installations, Vol V, 1903)

NB - Note that this is not a crossbar switch. A crossbar (akin to this diagram) would have in and out connections (lines and instruments) running parallel, with a number of crossbars (representing the total simultaneous call capacity) running across them. In this early and simpler form, the lines and instruments are connected directly. This requires more connection points in total, for a moderately sized public exchange this soon becomes a huge number of potential connections.

This is in fact a manual crossbar. It has the capacity of four subscriber lines (top) and four horizontal talking paths (albeit not needed for four subscribers). The fifth crossbar is the operator circuit, and at the bottom is the ground bar for enabling the signal indicators for idle subscribers. Kbrose (talk) 17:19, 4 February 2019 (UTC)