Rotary system

The Rotary Machine Switching System, or most commonly known as the Rotary System, was a type of automatic telephone exchange manufactured and used primarily in Europe from the 1910s. Formally named the No. 7-A Machine Switching System, it was developed in Belgium by International Western Electric, a subsidiary of American Telephone and Telegraph (AT&T), at the same time when AT&T's American engineering division, Western Electric, was developing the Panel switch in the United States.

The Rotary and Panel systems were very different systems, but both used the same newly developed component technology, such as Western Electric's latest relays, and the principles of the Lorimer system of revertive pulsing and preselection. The Rotary switches were smaller than the Panel system, and served only 200 rather than 500 stations. The initial version was the model 7A. It was succeeded by 7A1 and 7A2 and a rural system had the designation 7D.

The Rotary system used 1st and 2nd linefinders; when a customer picked up the phone all free linefinders in the group drove until one picked that customer line. Calls were switched over two, three or four group selection stages followed by a final selector. An office could start with two group selection stages for local calls (a first group level would serve 2000 lines), and be expanded to three group selection stages if it outgrew say 2000 or 4000 lines, depending on the number of first group levels required for other offices in a multi-exchange area.

While the Panel system was chosen for American large cities, the Rotary system was selected for use in Europe and manufactured at the Bell Telephone Manufacturing (Western Electric) factory in Antwerp, Belgium. The first exchanges were installed in England at Darlington (10 October 1914) and Dudley (9 September 1916). However the British Post Office standardised on the Step-by-Step (SXS) system in 1922, and subsequently the SXS Director system for London and other large cities. Six Rotary exchanges were installed in the city of Hull, between 1922 and 1929, where telephone service was supplied by Hull Corporation rather than by the BPO.

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