The road to the automatic telephone exchange
Two years after A.G. Bell patented the telephone, the first manual switchboard went live in New Haven, CT, 1878. From this start, inventors and engineers dreamed about an automatic system to replace operators. In 1879, one year later, inventors began experimenting with electromechanical switching systems.
For nearly 80 years, both automatic and manual telephone exchanges coexisted, each driven by various factors. However, it wasn't until the 1960s that automatic exchanges effectively replaced manual ones. By 1965 in North America, there were 95 manual Bell System offices with an impressive count of 13,027 automatic offices with many serving up to 10,000 customers. Independent phone companies likely had similar ratios.
Baby steps
Most authors give Almon Strowger credit for inventing the first automatic exchange, installed 1892 in LaPorte, Indiana. His story is legend, “Undertaker invents first automatic telephone exchange.”
Great story even though the first exchange was removed after only two years. LaPorte became the world’s first “Exchange Lab” with 5 different exchange types installed over a 6-year period. It took several more years before practical exchanges were developed that were super reliable, cost effective, and used in small and large communities. See Strowger’s first exchange for a brief story of this famous telephone system.
There were many interesting switching developments before the first LaPorte experiment in 1892. The figure below shows Strowger’s key patent in relation to 87 others, from a variety of inventors, from 1879 to 1900. Several of these are essential patents, like Strowger’s, while others were either modifications of existing inventions or impractical designs with no future [Hill].
* Almon Strowger had at least 3 other patents issued after his famous 1891 patent, but these were not significant. Using the name “Strowger” alone usually refers to the Strowger switch or a Strowger-style switch.
Some other prolific inventors during this time include Connolly, McTighe, Westinghouse, Keith, Erickson, Lundquist, and Callender.
But why consider these patents and not the many others after 1900? Well, this was the era that defined what an automatic exchange could be. It was the first generation of the exchange in terms of architecture and devices. By 1900, some important patterns had emerged that became models for generations to follow.
From 1900 until the 1960’s there were also many important exchange switching innovations. Each advanced the state of the art. The inventions were used in Step-by-Step, Rotary, Panel and Crossbar offices. To explore 15 switch inventions that were pivotal in exchange history, go to the Switch page. The all-important first Strowger switch and the Keith upgraded switch are covered here too.
The next section reviews some key progress milestones for the telephone and central office that helped enable automatic to move beyond experimental installations. There are some light technical details so you may want to exit depending on your level of interest.
Appendix
Progress towards the automatic exchange
one invention at a time
The table below provides some highlights where a significant improvement occurred in either the telephone set or the Central Office before ~1906. Of course, there were hundreds of incremental advancements each year (worldwide), and this table selects a few of importance. These steps inched us closer to a practical first-generation automatic exchange.
The Table has separate rows for manual switchboard (S) and automatic (A). The automatic exchange and the switchboard evolved in parallel over many years. Ideas from both influenced the other. Next, each row is reviewed discussing the important advancement.
Table 1—Telephone and exchange notable improvements over time
Early switchboards (1S/2S) were often passive devices with no or a small battery for auxiliary purposes.
Progress point 1S: The first manual Switchboard. Note that the telephone set required a (1) internal 3-volt talk-battery and (2) a hand cranked magneto (AC generator) to signal the operator. On the switchboard side, a “drop” occurred when it sensed the magneto current from the telephone – “I want to make a call.”
Wall telephone with batteries and magneto
The picture below shows five subscriber lines as they appear on a switchboard. Subscriber 4’s “drop” has fallen (the electromagnet’s finger moved up causing the hinged cover to fall) and this notified the operator of a new caller.
Strip of subscriber “drops” and jacks on a switchboard, from [Miller]
Progress point 2S: The Line relay replaces the drop-electromagnet. This relay can now operate a switchboard lamp (new) or ring a bell (new) for nighttime operation. While this might seem like a minor advancement, it was indeed a significant milestone. But the Line relay was essential for what was to come -- the automatic system. So, introducing the Line relay was a significant step. For switchboards, a cranked magneto operated the Line relay instead of the drop electromagnet.
Progress point 3S: The CO Common Battery approach was invented by H.V Hayes [Chapuis]. This method eliminated both the set battery and the magneto. Now the CO battery powered the telephone set microphone, the Line relay and switching equipment for all subscribers. This was a huge advance and reduced the cost of the telephone and the annoyance of changing batteries. For large automatic systems, some CO batteries might supply thousands of amperes. See Power Systems.
The simplified and improved telephone set circa 1892 [Miller]
The figure above shows a set and switchboard but would look very similar with an automatic exchange. Now the same telephone can work with a switchboard or automatic exchange (adding a dial). See Line Circuit for more information on the Line and Cut-off relays in the figure.
Progress point 1A: This entry is for Strowger’s first automatic exchange in LaPorte, Ind. The set’s “dial” mechanism had multiple switch buttons for the subscriber to press to enter a called number. This was 16 years before the rotary dial was invented. LaPorte subscribers had five wires (plus a common ground) going to each home phone, not two as is common today. Strowger’s patent shows five wires per subscriber – very inconvenient for telephone companies to install and manage.
There were batteries in each telephone and at the CO to operate the switches. It was not a common battery design. So, the plan was not optimized for low cost or simplicity and the LaPorte installation was essentially a prototype and never went into mass production. It did, however, start a revolution.
Progress point 2A: Alexander Keith and the Erickson brothers invented the rotary dial in 1898 (at last!) and significantly improved the Strowger-type switch in 1899. These were pivotal inventions that forever changed the automatic exchange landscape. Almon Strowger had a great idea but Keith and the Erickson’s invented the future.
Final words
By 1900, the first-generation exchange blueprint was nearly complete. Exchanges with 10,000 lines were being installed using Keith’s Strowger-style switch. For example, the first ever 10K line, rotary dial, exchange went live in Chicago, Ill (the Loop district) in Feb, 1903 [Smith]. The switching hardware was provided by the Automatic Electric Company (AEC, with Keith as GM), a subsidiary of the Strowger Company.
Interestingly, at the start of 1900 Western Electric (AT&T) was lagging in the development of automatic systems compared to AEC. WE eventually licensed the AEC designs for their use and AEC supplied Strowger-like Step-by-Step switchgear to AT&T until 1936.
Of course, new generations of the exchange would emerge over the following 60 years. Table 1 could be extended to include the future generation’s advances but it’s prudent to stop here.
References
Chapuis, Robert, 100 Years of Telephone Switching, Part 1, 1982.
Hill, R.B., Early Work on Dial Telephone Systems, Bell Laboratories Record, Jan 1953
Miller, Kempster, American Telephone Practice, 1905, New York, McGraw Hill.
Smith, Arthur Bessey, The Early History of the Automatic Telephone, circa 1907
