Strowger switch

Strowger switch is an electromechanical switching device developed for automatic telephone exchanges that enabled direct dialing without operator intervention. It was invented to replace manual switchboards used by companies such as Bell Telephone Company and later implemented in central offices operated by entities like AT&T and regional carriers throughout United States, United Kingdom, and across Europe. The device influenced developments in switching technology that led to crossbar, electronic, and digital systems used by corporations such as Western Electric, Siemens, and Alcatel-Lucent.

History

Almon Brown Strowger, a funeral director in Kansas City, Missouri, patented an automatic selector in the late 19th century amid disputes with local operators tied to firms related to American Telephone and Telegraph Company interests; his work culminated in patents assigned in 1891 and 1896. Early commercial adoption occurred when entrepreneurs and companies such as Strowger Automatic Telephone Exchange Company and manufacturers like Western Electric and The Automatic Electric Company produced exchanges for municipalities and rural exchanges, competing with manual switchboard services provided by operators associated with Bell System. International deployments followed in cities including London, Paris, Berlin, Toronto and colonial networks administered by entities like British Post Office and Imperial Telegraph Company. Legal and business battles over patents and standardization engaged firms such as Northern Electric and influenced regulatory decisions involving bodies like the Federal Communications Commission and national postal administrations.

Design and operation

The apparatus used cascaded electromechanical selectors, principally the two-motion selector known as the step-by-step selector, consisting of a rising wiper and rotary movement actuated by bank of impulses from the subscriber rotary dial. Mechanical assemblies were produced by manufacturers including Western Electric, Siemens-Schuckert, and Automatic Electric Company, while material suppliers like General Electric and Union Switch & Signal provided motors and relays. Call initiation relied on line feed and loop current techniques standardized by engineers at Bell Labs and technical committees from organizations such as the Institute of Electrical and Electronics Engineers and the International Telecommunication Union. Switching logic implemented line-finder trunks, selector levels, and relay sequences analogous to routing concepts later formalized by researchers at Cold Spring Harbor Laboratory and institutions including Massachusetts Institute of Technology through studies of signal switching and network optimization.

Variants and improvements

Design evolution produced several variants: concentrated selector frames for dense urban trunks used in exchanges managed by New York Telephone Company; multi-step selectors adopted by municipal networks like Chicago Telephone Company; and rotary stepping improvements introduced by firms such as Western Electric and Siemens. Later enhancements incorporated common-control features inspired by exchanges at AT&T Long Lines and experimental electronic control from laboratories including Bell Labs and RCA Laboratories, leading to transitional technologies such as the crossbar switch developed by Western Electric and the Panel system trialed by Bell System. International adaptations appeared in equipment from Siemens, STC (Standard Telephones and Cables), and Plessey, and wartime modifications were implemented in exchanges serving armed forces logistics coordinated through organizations like the War Department and Royal Corps of Signals.

Installation and maintenance

Installation of step-by-step exchanges required central office buildings, frame wiring, and technicians trained by manufacturers and telecommunication companies such as Western Electric, Automatic Electric Company, and regional utilities like General Post Office (United Kingdom). Maintenance practices drew on manuals produced by corporate engineering departments at AT&T, training curricula at vocational schools affiliated with firms such as British Telecom predecessor agencies, and field units modeled after military signal corps procedures from United States Army Signal Corps. Spare parts logistics involved industrial partners including RCA, Macy Machine Works, and component vendors supplying contact materials and insulating ceramics. Routine tasks included contact cleaning, gear adjustment, and alignment of selectors, while diagnostics used test sets and loop simulators developed by Bell Labs and equipment labs at Siemens.

Impact and legacy

The Strowger switching concept catalyzed the transition from operator-assisted to subscriber-dialed telephony, enabling scale-up of urban networks operated by entities like AT&T, General Post Office (United Kingdom), and municipal companies in Europe. It influenced later switching generations—crossbar, electronic, and digital—used by corporations including Northern Telecom, Siemens, Alcatel-Lucent, and research groups at Bell Labs that developed stored-program control and packet-switched techniques later adopted by Cisco Systems and Internet pioneers. Cultural and technological legacies appear in museum collections at institutions like the Smithsonian Institution, Science Museum (London), and Deutsches Museum, and in historical studies by scholars associated with IEEE History Center and universities such as Stanford University and University College London. The Strowger approach remains a landmark in telecommunications history, cited in textbooks and archives maintained by organizations such as ITU and national archives.

Category:Telephony