The 7A Rotary Telephone Exchange
The goal of this section is to describe the process flow for making a call using a 7A Rotary exchange. The author has also created a "7A explorer" page. Suggested that readers continue below for a general overview then check out the Ferrymead 7A Explorer for an interactive experience.
Preamble
Throughout this site, many aspects of telephone systems technology are discussed. Each facet is a part of a big jigsaw puzzle. When the pieces are locked together, an automatic exchange starts to breathe – see image. The “outside plant” cables, poles, and conduits are not covered here but this is a big cost in servicing each subscriber.​
Each of these pieces is discussed in the Exchange Anatomy (EA) and associated sections. So, their detailed operation won’t be repeated here. If you are new to exchange workings, and want a basic understanding, it may be best to review the EA sections before continuing here. Interesting historical aspects, notable people and intriguing inventions are also covered under EA.
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Of relevant importance are the 7A rotary switches and the 7A sequence switches (reverse dial coverage, Appendix B) with diagrams and explainer videos. The last 7A1 Rotary exchange was decommissioned in Wellington, New Zealand in 1985 [Cameron]. It's possible that other versions, such as the 7E, were switched off post 1985 elsewhere in the world. The first fully automatic Rotary exchange in Europe was installed at Darlington, England, in October 1914.
See Appendix C in the article on the Strowger switch for insights on exchange installations.
7A Rotary workings – making a call
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This section outlines how the 7A Rotary system establishes a call when one subscriber dials another in the same office.
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To get a feeling for the view inside an exchange building, see Fig 1. This is a small section of a 7A Rotary system on display at [Ferrymead]. It is hi-rez, so may be slow to load. The individual racks are ~11.5’ tall and hold Line Finders, Selectors, Registers, sequence switches, and relays. These are tied together with miles of wiring and rely on other puzzle pieces to build a working exchange.
Fig 1, 7A Rotary office, partial view at Ferrymead
Fig 2, Row of selector bays [Miller] circa 1930
Fig 2A. Hague, NL, West-General View of Automatic 7A Switch Room 1925 [Turkhud]
Fig 2B. Hague, NL, West-General View of Automatic 7A Switch Room. Top rack wiring complex. 1925 [Turkhud]
A typical exchange houses thousands of switches, and a single photo cannot capture the intricate layout of frames and support systems. It takes an immense infrastructure to support even 4K subscribers. See Appendix A below.
The Rotary system is based on the high-level diagram shown in Fig 3. This template can be tweaked and applied to the panel and crossbar systems; they share many similarities. The Strowger-type, step-by-step, system is the exception. The example to be presented does not show interoffice trunking, but this is vital in any practical system.
Fig 3, 7A Rotary system overview
The following provides a simplified call flow based on Figure 3:
• An “off-hook” (#288) subscriber triggers the line circuit causing the concentration stage switches (Line Finders) to search and find this sub.
• The caller is forwarded to an idle Register via the Reg switch. This switch's operation is more complex and simplified here. The talk path output of the concentration stage is connected to the expansion stage input.
• The Register is the brains during the calling phase and provides the functions listed in the figure. It is considered a piece of "common equipment", used momentarily then resting until needed again.
--Based on the dialed digits (calling #315), the Register, via the Reg switch, closes the talking path switches in the expansion stage.
-- Next, the Reg switch flips causing the Register to disconnect from the call. With a negative busy test, the ringing cycle starts and talking commences if answered.
• Upon call end, the talking path switch chain resets and is now available to service new callers.
The video below expands on Figure 3, adding detail and color. It’s an overview of a 7A exchange and includes the process steps for connecting two subscribers. The system workings rely on a "reverse dial" on each subscriber's phone. You may want to review Appendix B before watching the video.
The focus is on basic concepts, it’s not a click-by-click description of operations.
The 7A design scales to 2,000,000 subscribers! No other exchange type was designed to this scale. [Deakin] reports on a system supporting 560,000 subscribers.
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The 7A Rotary and its successors left a legacy of durability and reliable service. It holds a prominent position among large system designs during the golden age of electromechanical exchanges.
Digit Translation in the 7A Rotary
The process of translation is the automatic conversion of the dial’s impulses on a decimal basis (1-10), into a series of impulses for controlling selector switches built on a non-decimal basis (10 vertical x 20 horizontal).
This process is carried out by the 7A Rotary’s Register, which receives and stores the dialed digits. Since the selectors are 200 points each, the Register must modify the received digits so that the switch’s brushes stop at the desired terminals out of 200. The method maps 5 dialed digits to control the 3 selector switches. This is not a direct dial-to-switch mapping as it is in most Step-by-Step exchanges. Check out the article on Common Control for more insights.
Translation involves many steps. By example, one step involves the units digit, say a 7. Its value is modified based on the hundreds digit. For instance, 10 additional horizontal steps are needed when the hundreds digit is even (say a 4). This results in moving the selector’s brush to terminal arc position 17 (10+7). It would be 7 horizontal steps if the hundreds digit was, say, a 3.
The most important point is that translation methods enable the most economical switch size, such as the 200-point switches in the 7A Rotary. This optimizes the system size and cost for a given capacity and traffic loading.
You may enjoy a brief audio overview of the translation process.
Translation was first proposed by E. C. Molina (AT&T) in 1905. A patent application titled "Translating and Selecting System" was filed in 1906. It was granted as Patent US1,083,456 issued Jan. 6, 1916.
Quoting from this patent, "This invention relates to the selection of electrical circuits involving a change from one system [subscriber dial] of designating or grouping said circuits or associated apparatus to some different system [switch control].
References and Acknowledgements
Ferrymead Post and Telegraph Historical Society, Switch Room, Christchurch, New Zealand. This site houses a well-preserved and operational 7A Rotary system. The exhibit is supported by volunteers. I am indebted to Brian Cameron for providing raw exchange videos, Appendix A metrics, and many vital insights into the 7A's workings. Thanks Brian!
Deakin, Gerald, ELECTRICAL COMMUNICATION, a Journal of Progress in the Telephone, Telegraph and Radio Art, No. 7-A Machine Switching System – Rotary System, Jan 1925.
Turkhud, B.A., ELECTRICAL COMMUNICATION, a Journal of Progress in the Telephone, Telegraph and Radio Art, 7001 TYPE OF AUTOMATIC PRIVATE BRANCH EXCHANGE, April 1926.
Miller, Kempster, Telephone Theory and Practice, 1933, New York, McGraw Hill.
Rotary: Western Electric Machine Switching System, published by Western Electric Company Limited, London. Circa 1915.
Appendix A
How many switches?
This Appendix assumes a basic knowledge of the various types of switches used in a 7A system.
Below is an inventory of switch types to support 4,000 7A Rotary lines (5 digits required). The arrangement supports a max of 200 interoffice conversations (5%) at the same time plus other subs connected to remote exchanges.
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40 bays of 1st Line Finders (LF), (15 x 40 = 600 switches.)
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16 bays of 2nd Line Finders. This is 216 switches, for reasons not stated here. (15 x 16 = 240 minus 24 unalloted 2nd Line Finder positions.)
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20 bays of 1st Group Selectors = 216 switches. Each is paired with a 2nd LF. (11 x 20 = 220, minus 4 unalloted 1st Group Selector positions.)
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20 bays of 2nd Group Selectors = 220 switches, (11 x 20 = 220 selectors.)
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20 bays of Final Selectors = 220 switches, (10 x 20 = 200 selectors, plus 20 Test Desk Final Selectors.)
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36 Registers with associated sequence switches and relays.
The total is 1,472 Rotary switches. This is an apples plus oranges total because the switch types do differ in design. Nonetheless, each needs a home in a rack and requires a control means. Add to this total about 1,124 sequence switches (including 216 Register Choosers), Register Digit Recorders and the extent of the equipment required comes into focus."
Appendix B
The Reverse Dial
The 7A Rotary system required a "reverse dial" to operate. For the earliest Rotary exchanges the numerals on the dials were arranged 0, 1, 2. ..8, 9 in a clockwise direction
(reversed numbering). Over time, the rotary exchange logic was modified to accept standard-dial numbering 0, 9, 8. ..2, 1. Western Electric called the dial type, as "complementary" (as in 10's complement math) since dialing a 1 sent 9 pulses, a 2 sent 8 pulses and so on. See [Rotary, page 78].
Telephone with reverse dial
Why the reverse dial? The electromechanical method of recording digits inside the exchange was simplified IF the subscriber dial had reversed numbering. The subscriber was likely not aware of this oddity. For more on the reasons see Appendix B on the Sequence switch pages.
Even at this early stage of dial automation selecting a reverse dial was problematic since the de-facto world standard was a normal dial. In time, this made calling interop a major issue and begged for the Rotary system to change the design to accept a normal dial.
Appendix C
Visiting the Ferrymead 7A Rotary Switchroom, November 2024
Al Kovalick (author) and Brian Cameron (7A expert) in Switchroom
May Kovalick, visiting the Ferrymead Switchroom
Motor driven cam-based interrupter. The switches open/close to create busy, other progress tones and party-line ringing cadences.
Ferrymead display. Test set with two dials. One tests for allowable slow dialing speed, the other for allowable fast dialing speed.
Check out the 7A Rotary explorer for an interactive experience.
Appendix D
Rotary 7x Exchange at Norwegian Museum of Science and Technology
Below are some images of a hybrid 7A/7A2 Rotary exchange on display at the Museum. It is not powered and not working. The author visited the museum in Oslo Feb 2026 to examine the system and for photos. Unfortunately, the Museum experts on the system were not available.
Front view of system
These relays are part of the Register of a 7A2 design. The 7A uses individual mechanical digit counters. The 7A2 design uses relays to count digits in similar fashion to how a Panel Sender counts dialed digits.
Line Finder switches similar in design to those of a 7A system.
Motor driven interrupters for tones and party line ring codes (likely)
Display attached to Rotary System
