The 7A Rotary Switch
Highlights
Switch category: Rotary motion, fully electromechanical models 7A/1/2, 7B/1, 7D
Inventors: Frank McBerty and other Western Electric (AT&T) engineers. Mr. McBerty oversaw the Rotary exchange project and also contributed as a prolific inventor.
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Important patents by Mr. McBerty:
Patent filed 1910, granted 1,097,868. 1914, Final Selector.
Patent filed 1912, granted 1,085,454, 1914, Line Finder.
Gerald Deakin received patents on improvements.
Important dates: Research began 1906 in New York and development was transferred to Berlin in 1911. In 1912 the headquarters for development and manufacturing were established in Antwerp, Belgium [Deakin]. The first fully automatic Rotary exchange in Europe was installed at Darlington, England, in October 1914. In 1925 AT&T sold all Rotary exchange business interests to the IT&T Standard Electric Corporation.
Legacy: The lifetime of fully electromechanical Rotary went from 1914 to about 1952, in many countries worldwide. About 9.3 million lines were installed in total [Huurdeman]. New Zealand was a big user and had ~50,000 lines of equipment by May of 1925 [Deakin].
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Backgrounder
Around 1906, the Bell System formed two engineering teams tasked with solving the “big city problem” of switching telephone traffic in metro areas like New York City.
A few key project goals were to create a new automatic exchange design for: (1) replacing manual boards with up to 10K subscribers, (2) integrating with existing manual boards and (3) integrating with 150+ other "local" central offices to create a seamless metro telephone network. In the estimation of Bell’s engineers, Strowger switches and associated methods could not practically meet these goals due to the enormous interconnectivity needed between offices.
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So, the two groups set off to solve the same big city problem in a sort of competition. Both developments were influenced by the Lorimer system. Western purchased the Lorimer patents in 1903 so it's likely some engineers were starting to think seriously about metro systems at that time. Eventually, one group invented the panel exchange and the other invented the Rotary exchange, initially called 7A Rotary Automatic Machine Switching. During development a rift developed between Frank McBerty (Rotary leader) and Frank Jewett (panel leader).
The conflict was over what system would be superior for metropolitan scale (~New York City) offices. AT&T’s chief engineer J. J. Carty resolved the issue by decree [Conklin]. The Rotary would be developed for European markets and panel would be for North American markets. Impressively, the Rotary architecture could, in principal, support a single exchange of 2 million subscribers. One of this size was never built.
Apart from the individual switch designs, both exchanges shared many concepts such as using line finders, selectors, revertive pulsing, motor driven switches, common control dial pulse registers, call progress sequence switches, Western Electric relays and more. Both designs relied on the principle of common control and not direct dial control as in a Step-by-Step office.
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The respective switches, however, differed in many ways. Panel is a double-sided linear switch that can support 500 subscribers (up to 60 callers at once). On the other hand, a singlebay of 10 rotary selector switch supports switches could support 200 subscribers but only one call, 10 calls at a time. This is not a fair comparison for a few reasons but especially since the Rotary switch is much smaller than a panel switch. Let’s move on since a deeper comparison is beyond scope.
It’s important not to confuse the Rotary exchange with its namesake switch. Go here to learn more about the Rotary exchange. So, what makes the Rotary switch tick?
Examining the rotary switch
The word “rotary” is an overloaded term when used to describe telephone systems. It applies to many kinds of mechanisms that have rotary motion. One such device is shown in Fig 1. It is a standalone type made by Western Electric, Automatic Electric and others. An electromagnet advances the brushes. It was sometimes used as a digit counter and sometimes as a line finder. Commonly called a stepper switch, rotary selector, or uniselector (one direction with two wiper groups), it was widely used in the 1920-40s. Learn more about this type of switch here.
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Fig 1, Standalone type-200 Western Electric uniselector
On the other hand, the motor driven “McBerty Rotary”, our chief focus, supports more terminals and brushes (also called wipers) and was designed as a component switch in an array of switches. This switch does not advance in quantized steps but the wipers move relatively smoothly until directed to stop on the desired terminals.
Broadly, there are three types of McBerty Rotary switches: line finders (LF), group selectors (GS) and final selectors (FS). The two selectors are mostly alike, and the line finder differs in several ways. Let’s look at the bones of the selector switch first.
Fig 2 shows two final selectors ganged together by a common power (motor) drive. This picture is from [Ferrymead]. Notice the gears and drive shafts. The standard number of group or final selectors in a 11’ 6” tall equipment bay is 11.
Fig 2, Close up of two McBerty ganged Rotary 7009-type selectors
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--The Contact Arc
Fig 3 shows the contact arc (also called a terminal bank) from a switch without the brush (wiper) carriage assembly shown. This version shows 22 vertical arc blocks (for Group Selectors). The arc for a Final Selector has 20 blocks. Each block has 30 vertical contacts.
Fig 3, Rotary 7001-A contact arc for a selector type switch
Fig 4 adds more details including the brush assembly arranged as 30 vertical brushes segmented into groups of 3 (a block). Notice the brush trip spindle with clutch and the brush carriage rotation clutch. A connection is made by only 3 brushes (talking path leads) at a time. The brush trip spindle is used to extend only the 3 desired brushes to touch the contact arc. More on this aspect below. Each respective lead from the 10 brush blocks is connected in common to one of three commutator collector rings at the top of the brush carriage. This provides an electrical path to exit the switch.
Fig 4, component parts of the Rotary group selector switch
A final selector in action
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The video below shows a final selector operating during a call. The objective is to establish a talking path through the switch to the called subscriber. To do so, the switch rotates 3 brushes (out of 30) and connects them to 3 specific contacts (pins) on the contact arc.
Here is what you will see;
• Three target brushes are tripped so they are the only ones to make contact with the arc contacts.
• The brush carriage clutch engages, the brush carriage rotates and stops when the 3 brushes are resting on the called number's arc contacts.
• When the caller hangs up, the brush carriage returns home, ready for another call.
The action is swift, so replaying sections may help. Thanks to Brian Cameron for providing the raw video. The switch is part of a display at [Ferrymead]. Some of the original Rotary equipment was first installed in 1922.
In the video, there is a sequence switch on the right side of the selector switch. Its use was not explained. Sequence switches play a vital role in making calls. To learn more see the section on sequence switches.
The brush assembly is intricate. See Fig 5 below [Aitken]. The 30 brushes are all normally in a reset or home position. In this figure, latch block #4 (assuming the top block is #1) is tripped and three brushes are extended sufficiently to make contact with the desired row of arc contacts.
Fig 5, Rotary brush carriage with re/set springs
The tension of the comb springs quickly extends 3 brushes, when the brush trip spindle finger trips latch block #4. The video above showed this action. The springs are recompressed, and brushes are reset to the normal position once the call is completed. This is accomplished by the brush reset roller shown in Fig 4 as the carriage rotates past it.
Precise switch control
The motor powered spinning shafts, gears and clutches will exhibit undesirable small speed variations. So, reliable positioning and precise stopping of the trip spindle and the final selector brush carriage is not possible without real-time position feedback signaling.
External logic controls the switch actions using the three electromagnets/clutches, with two shown in Fig 4 (see Appendix). For brush trip spindle position feedback, a “revertive pulsing” switch is called out. The interrupted signal (on/off as the spindle rotates) is sent to the register controller. There is a second revertive switch (also called an interrupter in the literature), on the final selector only, located at the top of the brush carriage to provide carriage position feedback. This switch is not seen in Fig 4. The panel switch also used revertive pulsing for precise brush position control. Go here for more on revertive pulsing.
Line Finder (LF) switch
This switch differs from the selector versions discussed above, though there are similarities. Fig 6 shows a front view [Ferrymead]. The line finder function is discussed elsewhere on this site. Do a site search to discover more.
Fig 6, Western Electric 7001-type Rotary line finder
Aside from being a smaller switch, the biggest functional change from a selector switch is the brush assembly and brush carriage configuration. The brushes are arranged on three arms, 120 degrees apart, so that only 4 brushes touch the arc contacts at any time. This simplifies brush connectivity since the brush trip spindle and associated clutch are not required as with the selector model (Fig 4).
There are 12 vertical brushes, not 30 as with a selector. Depending on the initial position of the brushes, it may take ~3 seconds to find a line with a single LF. However, in a large Rotary exchange many line finders are searching for a new off-hook subscriber so this speeds up the time-to-dial tone considerably.
Here is a video showing a line finder searching for a new caller.
Line Finding Wars
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A Western Electric designed, Strowger-type, single line finder in 1928 could support 200 subscribers [Dodge]. The LF had three banks with 600 terminals total (3 wires per connection). This model became a workhorse for WE’s step-by-step installations for almost 50 years.
On the other hand, the Rotary LF had 240 terminals and supported 60 subscriber lines (4 wires per connection). So, a Rotary exchange needed more line finder devices per office of comparable size. The Rotary line finding method is more involved than for Step. For one, there are 2 levels, or tiers, of line finders. Secondly, for each new off-hook caller, many line finders compete to see who will find the line first. More coverage on this in the rotary exchange section.
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Final words
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The 7A (and other versions) Rotary design was a mainstay for Western Electric’s international business for ~10 years. It found a home in many large cities worldwide. The panel exchange was a tough sell internationally, so Bell went all in for Rotary except in North America. After Bell exited the Rotary business, others filled their shoes and sold Rotary systems worldwide.
An example of its robustness and durability was the Oamaru, New Zealand, 7A Rotary. It was installed on 28 Feb 1922 and was decommissioned in 1984, after 62 years of service. A portion of this system resides now in the switch room at [Ferrymead].
However, in 1925, due to pressures to manufacture locally and anti-trust problems, IT&T Corporation purchased AT&T’s large manufacturing subsidiary, International Western Electric, and renamed it IT&T Standard Electric Corporation. This move made IT&T a major telecommunications manufacturer in 11 countries including the USA. Purely electromechanical Rotary exchanges were installed starting from 1914 (WE) until at least 1947 with the installation of a 7A-2 exchange in Lexington, Kentucky, USA [ IT&T].
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Acknowledgments
The author extends gratitude to Brian Cameron of Christchurch, New Zealand, for his factual review, valuable suggestions, and editing assistance.
References
Aitken, William, Automatic Telephone Systems, Vol 1, 1921
Antwerp: Western Electric Machine Switching System (7A) Book, Bell Telephone Manufacturing, Antwerp, Belgium, 1910. Thanks to Remco Enthoven for the high resolutions scans.
Conklin, Roger, NORTH ELECTRIC COMPANY: THE FIRST INDEPENDENT TELEPHONE MANUFACTURER, Telephone Collectors International (TCI), Singing Wires, Pages 9 and 11, August 2004.
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Deakin, Gerald, ELECTRICAL COMMUNICATION, a Journal of Progress in the Telephone, Telegraph and Radio Art, No. 7-A Machine Switching System – Rotary, Jan 1925.
Dodge, W.L., Development of Step-by-Step Line Finders, Bell Laboratories Record, Feb 1929
Ferrymead Post and Telegraph Historical Society, switch room, Christchurch, New Zealand.
Thanks to Brian Cameron for images.
Huurdeman, Anton, Worldwide History of Telecommunications, Chapter 16, Wiley, 2003
IT&T: ELECTRICAL COMMUNICATION, Technical Journal of the INTERNATIONAL TELEPHONE AND TELEGRAPH CORPORATION, Vol 22, April 1945, page 302.
Appendix
Rotary mechanism details
This section breaks down the main components of the motor driven power train. In the figure [Aitken, Fig 86, cleaned], gear D3 couples the motor power into the rotary mechanism. The shaft D1 rotates at 31 RPM. Energizing electromagnet PG causes the underside rim of the driven disk B5 to be pulled into contact with driving disk D4, so that they rotate together. Thus, the brush carriage rotates.
One of Isaac Newton’s laws of motion is, Objects in motion, remain in motion unless acted upon by a force. To stop and hold the moving carriage precisely such that the brushes make reliable contact with the desired arc contacts requires such a force. So, the holding electromagnet HG (partially hidden) is energized immediately after the power to PG is removed. This braking action freezes the carriage motion so the three brushes make solid contact with the desired arc contacts.
To the left of the brushes is the trip spindle. Its use was explained earlier in the main body of this article. The driving disc D8 is attached to the rotating shaft D1, and the driven disc C6 is attached to the vertical spindle. When the magnet P2G is energized, D8 is attracted, and the spindle rotates. No holding magnet is needed to instantly stop the shaft.
Patent description
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The exquisite image below is Fig 9 from the McBerty US patent 1,097,868. The exploded view clearly shows the key elements. All three electromagnet coils are apparent: '55' (brush carriage motion), '64' (trip spindle motion) and '68' (brake with holding). The diagram displays several brushes, labeled as '18', which are propelled forward by springs, labeled as '22', when activated.
As mentioned earlier, two single pole switches (also called interrupters) help provide precise position control (revertive pulsing) to an off-switch controller called a register. One switch reports the brush trip spindle (46) position and includes contacts 52 and 53. The second switch reports the brush carriage position (shaft 10) over the stationary arc contacts and includes contacts 37 and 38.
The Rotary drive train and associated mechanics proved to be very reliable with predictable characteristics. This gave the “McBerty Rotary” system a long and useful life with some systems providing service until at least 1984.
Photo Gallery
Components from the 7A Rotary system
All images from [Antwerp]
Rear side of selector switch terminal bank
Brush chooser spindle for a selector [Antwerp]
Sequence switch sharing common motor drive with a selector switch. Notice the clearly marked index pointer from 1-18. [Antwerp]
Closeup of sequence switch with motor drive clutch, cams, and individual switches
Register digit counter. Notice 10 count then repeats, once. 20 positions total for round trip. See 7A exchange main body text for the logic behind this clever design.
Cabling for switch terminal bank. Notice the modern ribbon cable look, but in 1910.
