Strowger switch

Strowger switch. The Strowger switch, also known as a step-by-step switch, was the first commercially successful electromechanical telephone switching system. Patented by Almon Brown Strowger in 1891, it automated the process of connecting telephone calls, eliminating the need for a human telephone operator. Its introduction marked a pivotal shift in telecommunications, enabling the rapid expansion of Bell System networks and other independent telephone companies across North America and Europe.

History and development

The invention was directly motivated by Strowger's belief that a local telephone operator in Kansas City was deliberately misdirecting his business calls to a competitor. A former undertaker, Strowger developed a prototype using a collar box and pins. He received United States Patent 447918 in 1891 for his "Automatic Telephone Exchange." To commercialize the invention, he founded the Strowger Automatic Telephone Exchange Company in La Porte, Indiana. The first commercial installation went into service in 1892, serving 75 subscribers in La Porte, Indiana. The technology was later licensed and refined by the Automatic Electric Company, which became a major supplier to independent telephone companies competing with the Bell System.

Design and operation

The fundamental component is a two-motion electromechanical selector. A bank of 100 contact points is arranged in ten levels of ten contacts each. A wiper assembly, controlled by two electromagnets, moves vertically (selecting a level) and then rotates horizontally (selecting a specific contact) in direct response to pulse dialing signals from a subscriber's telephone. This "step-by-step" process was entirely directed by the dial pulses, requiring no common control. A typical call routing used a series of switches: first a line finder, then a group selector, and finally a final selector to reach the desired subscriber line. This decentralized architecture contrasted sharply with later systems like the crossbar switch and panel switch.

Impact and legacy

The deployment of Strowger systems revolutionized the telephone exchange, dramatically increasing network capacity and reducing operational costs for companies like GTE. It ended the era of manual switchboards operated by telephone operators for local calls, though operators remained for long-distance calling. The technology facilitated the growth of independent telephone companies, fostering competition with the Bell System. Its reliability and simplicity led to its adoption worldwide, with major installations across the United Kingdom by the General Post Office and in countries like Japan and Australia. The Strowger switch established the foundational principles for automated telephony that persisted for most of the 20th century.

Technical specifications and variants

A standard Strowger selector typically provided access to 100 outlets (10 levels x 10 contacts). Early systems used 11-pulse dialing (1-9, 0, and a final "connect" pulse), which later standardized to the 1-9 and 0 scheme. Key variants included the uniselector, a simpler rotary switch used as a line finder, and the two-motion selector for routing. Manufacturers like Automatic Electric Company produced the well-known "Type 3" and "Type 4" switches. Western Electric, the manufacturing arm of the Bell System, produced its own licensed versions, such as the "7A" and "7B" switches, for use in Bell Telephone Company exchanges. Different countries developed local variants, such as those by Siemens & Halske in Germany and L.M. Ericsson in Sweden.

Obsolescence and replacement

Despite its durability, the Strowger system had limitations, including slow setup times, wear and tear on mechanical parts, and noise. It began to be superseded in the mid-20th century by more efficient and faster systems like the crossbar switch, which used a matrix of horizontal and vertical bars. The ultimate replacement came with the advent of electronic switching systems (ESS), starting with the Bell Labs-developed No. 1 ESS in 1965. These used transistors and computer-style stored program control. The last major Strowger Automatic Telephone Exchange Company exchange in the United States was retired in the 1990s, though some remained in limited service in other parts of the world into the 21st century, eventually made obsolete by digital switching and Voice over IP technologies.

Category:Telecommunications equipment Category:American inventions Category:Telephone exchanges Category:1891 introductions