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                                        Keith’s Strowger Switch

Highlights

Switch category: Linear and Rotary motions
 

Inventors: A.E. Keith and the Erickson brothers, employed by the Strowger Automatic Telephone Exchange Company. 
 
Important dates: Patent US 638,249 granted 1899 (applied 1895). This version replaced Keith’s Zither switch at the La Porte (Indiana) exchange in 1895. 
 
Legacy: This “reinvention” of the original Strowger design became a
worldwide success.  The fixed terminal banks were separate from the switch body, and this improved installation and maintenance. The core mechanism became the basis for other important switch types. 

Backgrounder

 

Alexander is a famous first name in the telephone world. Especially if the last name is Bell or Keith. The work of Alexander E. Keith and his associates stands out notably in the development of automatic switching.

 

He was granted, sometimes with the Erickson brothers or others, at least 45 US and many foreign patents for “Telephone Exchange Systems” and related topics.

 

Keith became General Manager of the Automatic Electric Company (AEC) and he set it on a path to becoming a world leader in exchange automation products.

 

A few of his pivotal patents are:
 

  • Zither switch: US540,168 (linear and rotary means)

  • Improved Strowger switch (discussion herein)

  • Rotary dial: US597,062  (this was the mother of all future rotary dials)

  • Line Switch: US1,304,324  (markedly improved exchange workings)

 

Name that switch

 

The name Strowger refers to the man and his first switch.  The first automatic exchange at La Porte (Indiana) could be called a Strowger exchange because it used Strowger’s switch.

 

The first switch was rightly called a "Strowger."  Then in 1895 Keith reimagined the first design and it became the “modern Strowger”, or a “Strowger-type” or, a term used here, “Keith’s Strowger” to be more specific. Comparing Keith’s version with Strowger’s first, the many differences become apparent. 

 

This does not devalue the amazing work of Almon Strowger! But Keith also deserves his name attached to the switch that changed the way many exchanges would be built for the next ~75 years. 

1, 2, and 3 dimensional switches

 

Frankly, the first manufactured Strowger switch was a bit crude. It was a rotary, 1-dimensional, 100 point, device. However, Strowger’s patent described a 1,000 point, 2-D (linear and rotary), but that device was never built.

 

Keith took Strowger’s patent design and improved it considerably. As will be described below, the Keith version was a 1,000 point, “3-D” switch.

 Switch overview
 

The figure below from [Miller] is a skeleton of Keith’s switch invention. Let’s see how it worked. Assume the “dial” is at the subscriber’s home for this explanation. In this case, it’s just two push buttons labeled V and R and not a rotary dial.
 

The switch is shown at its home position. Assume the subscriber wanted to call #88 (#VH). First, they would press the V button 8 times, causing the vertical electromagnet to lift the shaft to level 8. Next, the subscriber presses the R button 8 times causing the rotary electromagnet and pawl to advance the ratchet 8 steps. The wipers land on terminal #88 (marked red).

 

The connection continues until the call ends and then a release electromagnet (not shown) causes the wipers to return to the left most position on row 8 (spring return), then to fall (gravity) to their home position ready for another call.

image showing how the strowger switch operates. Explains vertical and horizontal ratchets and 100 terminal banks

Fig 1, Details of Strowger mechanism

Keith’s design had the terminal banks separate from the switch body. Only one bank is shown in the figure but up to ten are cited in the patent. This idea greatly improved the installation effort and maintenance of the composite switch. Brilliant.
 

Before exploring detailed switch operations, view a gallery of three switch types. The switch continued to evolve over many years partially due to the patent licensing by AEC to others who made their own innovations. So, the images show different versions.  One improvement was the addition of control relays to make each switch do more especially after the rotary dial was introduced.
 

Below see an original Keith’s Strowger switch, with 3 banks of 100 terminals/each and 300 total. The larger, copper colored, cylinders are the vertical, rotary, and release electromagnets. Notice, one wire (one wiper) per bank.

1898 picture of Keith's Strowger switch invention

Fig 2, Original Keith’s Strowger from London Science Museum, circa 1898

Two AEC produced telephone switches installed at Epsom, UK, 1912

Fig 3, Two ATM produced switches installed at Epsom, UK, 1912

Source: britishtelephones.com

image of  Keith's strowger telephone switch made by western electric

         Fig 4, Line Finder switch (based on Keith's Strowger) with control relays [WE]

 

See the Appendix A below for a short video demonstrating two digits dialed on a modern Strowger-style switch. The video captures the switch action in slow-mo. 

Fig 4A shows a nice closeup of two 110 (10x11) position terminal banks. Note there are two terminals at each position, top and bottom. So, this combine supports 440 individual connections. The 10x10 configuration was more common. 

Fig 4A, Strowger-type, two 110 position terminal banks
Source: Wim der Kinderen 

Worldwide influence
 
The switch became internationally famous and was sold or remanufactured worldwide. The British Post Office, Australia and Japan relied on Strowger type switches. Siemens and Halske in Germany signed licensing agreements in 1909 to build switches for European countries.
 
Western Electric (WE)
licensed the design and purchased some switches from AEC until 1936. WE improved the design and began to manufacture their own version for use in most small to medium sized cities in NA. WE’s Panel and Crossbar methods ruled over most metro areas. 

In Britain, the Automatic Telephone Manufacturing Company (ATM Co) was formed in 1911. Patent rights were obtained from the Automatic Electric Company Inc. of Chicago (Almon Strowger, founder).  This enabled ATM to manufacture Strowger-type automatic switches [Young]. 

The ATM company became the Automatic Telephone and Electric Company (AT&E) in 1936. At this point the British Post Office, responsible for domestic telephone service, mandated that the AT&E type 32A Strowger switch become a "standard." It was rechristened the type 2000 and included a family of step-by-step line finders, selectors and final connectors.  During 25 years of Strowger-type switch development, the British ATM company had introduced many new improvements.  The type 2000 had diverged significantly from the 1911 "American version." [Freshwater]. 

Fig 4B. Type 2000 final connector from 1970 (in demo stand)

The Commutator

By adding a "commutator" a second degree of flexibility is added. See Fig 5 and Fig 4 (left image). This addition was typically used on Line Finders and "trunk hunting" switches.

A commutator, mounted at the right of the banks, consists of eleven vertically mounted contact terminals insulated from each other. It, together with a commutator wiper which is mounted on the shaft, provides a means for stopping the vertical stepping of the shaft when, say, a ground is found on a vertical contact.  

This method enables a single Line Finder to search for any of the 200 lines requesting service.  Say subscriber #58 goes off-hook. First, the LF searches vertically for a ground on a commutator contact (at level 5) -- then it stops. Next, the switch searches horizontally for -48 VDC (at position 8) on a bank contact -- then it stops -- line found. The subscriber line circuit and associated group relay initiate the LF actions to find the new caller.   

It's worth noting that six line finder wipers (T/R/S-1 at the top and T/R/S-2 at the bottom) establish contact across the three banks.  So, this enables 200 lines (600 terminals on 3 banks) to be searched.   


The same logic applies if a switch is configured to hunt for idle trunks or an idle selector. Some switches only do hunting and are not directly controlled by the subscriber's dial. 

explainer, step-by-step switch  Commutator and commutator wiper

Fig 5, Commutator and commutator wiper [PTT]

The Keith and Erickson 1,000 point “3D” switch

Lets return to the switch's beginnings. First, hat’s off to the draughtsman who drew the superb patent figures below.  The patent explains the switch action in mind-numbing detail; we won’t go there. However, let’s explore just one intriguing aspect of Keith’s patent: its 3D switching action. Well, not exactly 3D but close.

Compare Fig 6 to Fig 1 above. Both the vertical and rotary magnets are called out. Below the “auxiliary switch” is called out and it selects what wiper/bank is active. Close inspection shows ten connecting pins on the auxiliary switch and each of these is wired to one of ten wipers with only three drawn here. So, in theory with ten banks, this is a 1,000-point switch and could support as many subscribers. Is this practical? Well, no but it’s still a worthy idea. In time, two or three banks proved to be most useful for exchange switching purposes.  

This was a one-connection-per-talking-path switch. Of course, the patent did not limit it to one wiper per bank and soon two wipers per bank (top/ bottom) was standard practice.

explanation of alexander keith's switch patent in detail how it works

Fig 6, Keith Patent Figs 1 and 2

Okay, let’s take this out for a test drive. Assume a subscriber wants to call #295 (example given in the patent) or #HTU and has three push buttons (the dial) to do so.  First, the subscriber presses the Hundreds button 2x. This advances the auxiliary switch to the second pin thereby selecting the middle bank (#2) in the figure. So, wire 23’, wiper 22’ and bank #2 are selected.
 

Next, the Tens button is pressed 9 times. This advances the vertical shaft to level 9 of the switch. Finally, the Units button is pressed 5 times. This rotates all the wipers to terminal 5 on all banks but only bank #2 is active due to the previous action of the auxiliary switch.
 

So, the switching action has a 3-D nature since 3 digits can select one out of 1,000 terminals with three variables, H/T/U. This was ingenious and demonstrates the cleverness of Keith and his partners. Before long there was no need for the auxiliary switch and all the wipers (usually 3, 4 or 6) were active per switch.
 

The 1-wire-per-connection idea was superseded with 3-wires-per-connection (so called Tip, Ring and Sleeve) since this was already the standard in manual switchboards.

The Uber Switch

Keith’s invention was a general-purpose switch, and it could be applied to many switching needs in an exchange. See Floorplan. Switch type examples are line finders, selectors, connectors, row and level hunting switches.
 

Fortunately, by adding relays (control logic) to the switch frame, many different switch functions could be implemented with the same basic mechanics. Some modern switches have 5 relays while others may have 12 or more depending on the functions needed.
 

Often the Step-by-Step exchange type used Keith’s Strowger switches. So, Keith’s switch was foundational and by tweaking it the basic idea could be redeployed for different means. This flexibility gave the core switch mechanism staying power.

During this period AT&T was full throttle ahead for manual boards with little to show related to automation. We’ve all heard, “The early bird catches the worm”, and Strowger and Keith were early birds. The La Porte experiments provided experience and incentive to invent better switches.

Did the efforts pay off? Well, in 1955 about 65% of all automatic exchanges worldwide were served by Keith’s Strowger switches and derivatives by Western Electric and other manufactures [AEC].

References

AEC (1955). “This is Automatic Electric”. Pamphlet.

Freshwater: www.britishtelephones.com/histatm.htm     (Bob Freshwater)

Miller, Kempster, & McMeen, Samuel: Telephony; a comprehensive and detailed exposition of the theory and practice of the telephone art, 1912

PTT: Survey of Telephone Switching, Pacific Telephone,  page 38, 1956

WE: Fundamentals of Telephone Communications Systems, Western Electric, 1969

WE2: The STEP-BY-STEP Dial Telephone System, Western Electric, Lesson #3, 1958

Young J.S., THE POST OFFICE 2000 TYPE SELECTOR - ITS DEVELOPMENT

AND MECHANICAL DETAILS, The Post Office Electrical Engineers Journal (POEEJ), Vol. 28, January 1936.

Appendix A
A Strowger in Slow-Mo

The video below is a trimmed version of one originally made by Len Hicken available on YouTube. Len explains how a call progresses through the switch chain of a step-by-step system. The video was edited to show only the final two digits, 8 and 9,  operating a final connector switch. See the Step-by-Step exchange section for more videos on switch ops. 

Strowger switch demo for 2 digits in slow motion

Strowger switch demo for 2 digits in slow motion

Play Video

Strowger-type switch manufacturing at the Hawthorne Works, Western Electric plant, Cicero Illinois

Strowger-type switch manufacturing at the Hawthorne Works, Western Electric plant, Cicero Illinois
Step by Step banks wiring room, Hawthorne Works, Cicero Illinois
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