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Why the Automatic Telephone Exchange Was Invented


From the very beginning of the telephone era, manual switchboards originally invented by the Hungarian engineer Tivadar PUSK┴S, were used in telephone exchanges. Later on as I know, these switchboards had grown as large as to serve about 60,000 subscribers. They were simple to use, worked fine with the help of many operators.

Once upon a time, a fellow named Almon B. (Brown) Strowger lived in a town somewhere in Kansas, USA. He worked as an undertaker. The exact location in this story is not sure as Strowger had run his job in several nearby towns, too. His profession was a stable and solid business since once everyone dies. He had a competitor in the town but the number of old people was just enough to satisfy the two businesses. In this town there was a (manual) phone exchange, so if somebody had passed away, the relatives could call either of the two undertakers.

However, after a time Mr. Strowger noticed that his business had declined. What happened? Less people were about to die or, somebody invented the elixir for eternal life? Nonsense. Soon he understood that it was the competitor's wife who, as an operator in the phone exchange, directed the calls  „automatically” to his husband when funeral calls came in. (Early time automation meant a manual exchange.) Even more, when the operator learned an armed conflict has unfolded somewhere in the town, she immediately warned her husband to hurry with collecting the victims' bodies.

According to other sources, when Strowger’s friend had died, the body was taken by his competitor due to a similarly diverted phone call.

Finally, Mr. Strowger became so angry that he started elaborate a method to exclude swindler operators from phone calls. As was known, the basic idea was his own, but he also searched for supporters and engineers so that in the end he invented the first automatic telephone switch, named after him.

He applied for a U.S. patent in 1889, which was issued in 1891.

The Strowger Switch is a simple and logical device. In the middle of the device there is an axis (a rod) that can be lifted and rotated by steps. An arm fixed to this axis holds a metal brush (moving contact) for making electrical contacts. Around the axis there is a semicircle block containing 100 fixed contacts, arranged in 10 levels ("storeys") with 10 contacts in each level. Each contact corresponds to a subscriber’s phone. The metal brush is first lifted to the level needed, than it rotates to the selected contact there for connecting the adequate subscriber’s line. Lifting and rotating the metal brush are performed by relay-like electromagnets and ratchet mechanics where every electric pulse will cause one step. That's why it is called a step-by-step device. Since one telephone connection requires two lines, either the construct itself was doubled with a common axis or, double contacts were applied. After a call had ended, the switch/metal brush moved back to its initial state.

In practice, one switch established only one call at a time so simultaneous calls demanded several switches.

The first telephones in this system were provided with push-buttons; e.g. if someone was about to give in number 9, the button had to be pushed 9 times. Sure, it was cumbersome in practice.

If the number of subscribers exceeded 100, similar switches were to be used for further group selectors for hundreds, thousands etc. of lines, up to some ten thousand lines. In other words, this system was „scalable”.

Later, when developing the system, very complicated set of relays i.e. sequence switches etc. were added. However, the Strowger switch itself remained essential.

(The complete description of an automatic telephone exchange in operation, even if very exciting, is far beyond the scope of this paper.)

Strowger and his backers formed a company, first Strowger Automatic Telephone Exchange Company, later the Automatic Electric Company to transfer the patent into everyday use.

The company installed and opened the first commercial telephone exchange in La Porte, Indiana on November 3, 1892, with about 75 subscribers and a capacity for 99.

Under the technical leadership of Alexander E. Keith, Automatic Electric Company produced a steady series of significant advances yielding a commercially viable system. Most notable of the advances was the invention of the phone dial in 1896. Just to understand: dialing number 0 gives 10 pulses as zero pulse will cause no effects.

About 1900, the Strowger system set off in the U.S. and about 1910 in Europe, too.

Strowger himself seemed not to have taken part in further developments. He had left the company, also selling his patent and shares. His patents were sold to Bell Systems for $2.5 million, in 1916. But Strowger’s name is forever intertwined with automated telephone systems.

In 1965, Strowger was admitted to the hall of fame of the U.S. Independent Telephone Association (today the USTA).

The stepping movement of the Strowger switch caused significant wear of the ratchets and inaccuracy in finding the contacts. In spite of its inherent disadvantages, by the 1960s, the Strowger exchanges were in widespread use. 

Later on, other, more sophisticated switches and systems emerged for use in telephone exchanges.

Rotary switch: it is similar to the Strowger switch but with no level shifting. The rotational movement of the brushes (driven from a continuously rotating axis + disk) is controlled by an electromagnetic combined clutch-brake pad. 

The cross-bar switch has a very different topology, enabling bypass routes. The basic switch is a matrix of ten by ten perpendicular contact rods, ten inputs and ten outputs, with relay-like movement that establishes contact from any input to any output, one contact at a time. The mechanical movement of the switch is very tiny therefore it is very reliable. It’s widespread use was realized by the availability of the system's computer control.

Nowadays, line telephone services use semiconductor switching elements. The system is similar to the cross-bar topology.

An original and still functional rotary exchange (type 7A1) is to be seen in Budapest, Hungary, in the Museum of Telephony. Its use dates back to the period between 1928 and 1985. Now it is already out of the city network but, by dummy calls one can see the switches rotate. It is the only complete and operating remnant of this type all over the world.

Strowger switch-1
Technology background: US Patent No 447,918 / Mar. 10,  1891

- GL -
The author is a qualified electrical engineer with experience in industrial R&D and higher education.


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