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Strowger switch

The Strowger switch is an electromechanical device that automated telephone exchanges by using pulses from a rotary dial to step electromagnetic selectors through banks of contacts, enabling direct caller-to-caller connections without human operators.^[1] Invented by Almon B. Strowger, an American undertaker, in 1889 and patented on March 10, 1891 (U.S. Patent No. 447,918), it marked the first practical step-by-step switching system, capable of handling up to 99 lines in its basic form and scalable to thousands through multiple levels.^[2]^[3] Strowger's motivation stemmed from frustration in his Kansas City, Missouri, business, where he suspected a rival undertaker's wife, working as a telephone operator, was redirecting client calls to her husband; ignored complaints to the telephone company prompted him to design a "girl-less" system.^[4] He incorporated the Strowger Automatic Telephone Exchange company in 1891 and installed the world's first commercial automatic exchange in La Porte, Indiana, on November 3, 1892, serving 75 subscribers with a promotional slogan emphasizing its reliability: "girl-less, cuss-less, out-of-order-less, wait-less."^[3]^[4] At its core, the switch employed a two-motion mechanism: the first dial digit caused vertical movement to select one of 10 rows in a contact bank, while the second digit triggered horizontal rotation to choose a column, establishing the circuit; additional digits routed through successive selectors for larger networks.^[1] Early systems used push-button keys to generate these pulses by interrupting the circuit for each digit; the rotary dial, patented separately in 1892 (U.S. Patent No. 486,909) and introduced around 1896, became the standard.^[4] Early systems were limited to small independent exchanges, but by 1914, over 400,000 dial telephones were in use across the United States, accelerating the shift from manual switchboards.^[1] The Strowger switch's adoption spread globally, transforming telecommunications by reducing costs, increasing privacy, and enabling mass-scale telephony; in the United Kingdom, the first automatic exchange opened in Epsom in May 1912, with the last manual London exchange converting in 1960.^[5] It remained in use, particularly in rural and developing areas, into the 1990s, when electronic and digital switching systems supplanted it, though touch-tone dialing, introduced in 1963, gradually replaced pulse dialing but was compatible with later Strowger installations.^[3]^[1] Strowger's innovation, inducted into the National Inventors Hall of Fame in 2006, laid the foundation for modern automated networks.^[3]

History

Invention

Almon Brown Strowger, an undertaker based in Kansas City, Missouri, conceived the idea for an automatic telephone switch in 1888, driven by his suspicion that local telephone operators—predominantly women—were intentionally redirecting calls intended for his funeral business to a rival undertaker.^[3]^[4] This personal grievance highlighted the limitations of manual switchboards, where operators controlled call routing, prompting Strowger to seek a mechanical solution that would enable direct, operator-free connections.^[1] Strowger's initial prototype, developed with the assistance of his brother Arnold and a local mechanic, employed a series of four keys that users pressed in sequence to input numbers, thereby automating the switching process without human intervention. This design demonstrated the feasibility of electromechanical selection, using tapping motions to advance a switch mechanism toward the desired connection. The first successful demonstration occurred in 1889 in Wichita, Kansas, where a model built by a jeweler showcased the system's ability to handle calls automatically, convincing potential investors of its potential.^[6]^[7] Building on this success, Strowger and associates formed the Strowger Automatic Telephone Exchange Company on October 30, 1891, to commercialize the invention.^[8] The company's first commercial installation took place on November 3, 1892, in La Porte, Indiana, serving approximately 75 subscribers with a capacity for up to 99 lines.^[9]^[10] This milestone marked the practical debut of automated telephony, transitioning from prototype experimentation to real-world application.

Patents and Commercialization

Almon B. Strowger received U.S. Patent 447,918 on March 10, 1891, for an automatic telephone exchange system that enabled direct connections between callers without manual operator assistance, marking a pivotal legal protection for his invention. This patent described a mechanism using electromagnets and stepping selectors to route calls based on electrical pulses, laying the foundation for automated telephony.^[1] To advance commercialization, Strowger partnered with associates M. A. Meyer and Joseph Harris, forming the Strowger Automatic Telephone Exchange Company on October 30, 1891, in Chicago, with Harris providing key funding and business oversight as an early investor.^[8]^[11] The company encountered significant challenges, including resistance from established telephone operators wary of automation and limited initial capital, which slowed adoption and forced reliance on small independent exchanges for testing and sales.^[8] In 1896, amid health issues and funding needs, Strowger sold his patents to company associates for $1,800 to support ongoing development.^[12] Two years later, in 1898, he sold his shares in the reorganized firm for $10,000, reportedly feeling undervalued by the transaction.^[13] A notable enhancement came with the introduction of the rotary dial in 1896, patented earlier under U.S. Patent 486,909 on November 29, 1892, which replaced cumbersome key-based inputs with a user-friendly finger wheel for generating dialing pulses.^[14]^[15] By 1901, the company had restructured as the Automatic Electric Company to focus on manufacturing and marketing. In 1916, AT&T licensed the Strowger technology from the Automatic Electric Company for $2,500,000—equivalent to approximately $50 million in 2024 dollars, adjusted via GDP deflator—allowing broader use in the Bell System.^[1]

Deployment

The deployment of Strowger switches began in the United States with the installation of the world's first commercial automatic telephone exchange in La Porte, Indiana, on November 3, 1892, serving approximately 75 subscribers in a small independent telephone company.^[16] This initial rollout targeted small to medium-sized communities, where independent telephone operators sought to reduce reliance on manual switchboards and lower operational costs. By the early 1900s, improved versions of the Strowger system were entering commercial use in such settings, with adoption accelerating among non-Bell System providers.^[1] By 1910, there were 131 Strowger exchanges serving 200,000 subscribers across the U.S., primarily in rural and mid-sized towns.^[17] This expansion continued into the 1910s, reaching over 400,000 dial telephones by 1914, representing about 4% of all U.S. telephones and handling a growing volume of local calls in decentralized networks.^[1] In the United Kingdom, the General Post Office (GPO) initiated Strowger deployment in 1912 with the opening of the first public automatic exchange in Epsom, Surrey, serving around 250 subscribers and marking a shift toward automation in smaller locales.^[5] The GPO favored Strowger systems for towns and cities with populations under 50,000, installing them to manage increasing demand without expanding manual operator staff. By the mid-1920s, hundreds of such non-director Strowger exchanges were operational across the UK, supporting local dialing in provincial areas while larger urban centers retained manual or hybrid setups.^[18] These installations, often using two-motion selectors, enabled efficient call routing in communities like Epsom and expanded to over 6,000 total exchanges by the 1930s, many Strowger-based, though the peak concentration in the 1920s focused on non-metropolitan rollout.^[17] International adoption outside the U.S. and UK was uneven during the 1910s, with limited uptake in continental Europe due to entrenched manual systems and preferences for operator-mediated service among state monopolies. In contrast, Canada saw early implementation, with the first Strowger exchange operational in Whitehorse, Yukon, in 1901, followed by broader use in independent networks by the 1910s.^[17] Australia adopted Strowger technology similarly, opening its first public automatic exchange in Geelong, Victoria, in July 1912 with 800 subscribers, extending to other regional centers like Perth by 1914 to address growing telephony needs in isolated communities.^[19] Deployment challenges included the high initial capital costs of electromechanical equipment and the scarcity of trained technicians skilled in maintaining the intricate stepping mechanisms, which often led to reliability issues in early installations. These factors prompted hybrid manual-automatic systems in larger cities, where manual switchboards handled complex or long-distance calls while Strowger units managed local traffic.^[1] By the 1920s, as expertise grew and costs declined through mass production, Strowger systems peaked in usage, supporting millions of daily calls across U.S. and UK networks via thousands of exchanges, though exact volumes varied by region and were dominated by local service in smaller deployments.^[17]

Technical Design

Operating Principles

The Strowger switch operates on the fundamental principle of electromechanical stepping, where electrical pulses generated by the caller's dial control the incremental movement of switch contacts to establish a direct connection between calling and called parties, thereby automating the routing process and eliminating the need for manual operator intervention.^[20] This step-by-step mechanism relies on a series of interconnected switches that respond synchronously to the dialed digits, progressively building the call path through the exchange.^[21] In the pulse dialing system, each digit dialed by the caller produces a corresponding number of electrical interruptions, or pulses, in the loop circuit—typically at a rate of about 10 pulses per second, with digit 5 generating five pulses, for example—to drive the switches forward in discrete steps.^[20] These pulses activate electromagnets that cause the switch wipers to advance vertically to select a bank level and then rotate horizontally to select a specific contact within that level, mirroring the decimal structure of the telephone number.^[21] The system employs hierarchical switching through multiple stages to route calls efficiently: a line finder detects the calling line and connects it to an idle first selector, which responds to the initial digits to access a group of connectors; the connector then uses the final digits to reach the called line.^[20] In small exchanges, a single-motion uniselector may suffice, but larger systems cascade these stages for scalability. A basic Strowger switch handles up to 100 lines, while cascading multiple levels enables connections for thousands of subscribers in urban exchanges.^[20] For error handling, early Strowger systems incorporated busy tone circuits at the connector stage, where the switch tests the called line's sleeve lead for a ground signal indicating occupancy; if detected, a relay connects an audible busy tone (in North American systems, a 480/620 Hz interrupted signal)—to the caller, preventing connection attempts to engaged lines.^[20] This feedback mechanism ensures orderly call progression without operator involvement.^[22]

Two-Motion Mechanism

The two-motion mechanism forms the electromechanical core of the Strowger switch, enabling precise selection of telephone lines through sequential vertical and rotary movements driven by pulsed electrical signals. This design allows a single selector to access up to 100 terminations arranged in a 10x10 grid, with wiper arms bridging specific contacts to complete connections.^[2]^[23] In the vertical phase, an electromagnet energizes intermittently based on dial pulses, which correspond to the tens digit of the called number, lifting the switch shaft via a pawl engaging a ratchet. Each pulse interruption de-energizes the electromagnet, allowing a lever to advance the pawl one step along the ratchet teeth, raising the wipers to the appropriate "floor" or level—typically up to 10 levels—while a holding pawl prevents backward slippage. For instance, five pulses elevate the shaft to the fifth level.^[2]^[23]^[24] Following vertical positioning, the rotary phase activates a separate electromagnet to rotate the shaft horizontally across the selected level's contacts, corresponding to the units digit. Similar to the vertical motion, pulse interruptions control a pawl-and-ratchet system to step the wipers incrementally, up to 10 positions, until they align with the target line. A detent mechanism maintains the rotary position during operation.^[2]^[23]^[24] After connection, the switch returns to its home position through gravity for vertical descent and a spring-loaded release mechanism for rotary reset, triggered by a dedicated release electromagnet that disengages the pawls. This ensures reliable restoration without external power, minimizing wear. Reliability is further enhanced by the pawl designs, which include anti-backslip features to avoid mis-selections from vibration or incomplete steps.^[2]^[23]^[1] The selection logic follows the relation Position = (vertical steps × 10) + rotary steps, where each step matches the number of dialed pulses, enabling direct mapping to two-digit subscriber numbers in the patent's original 10x10 configuration.^[2]^[25]

Key Components

The Strowger switch system relies on several core hardware elements to facilitate automatic call routing in early telephone exchanges. Central to initiating a call is the line finder, which automatically detects and connects an off-hook subscriber line to the switching network. Upon a caller lifting the receiver, the line finder's scanning electromagnet—typically a vertical and rotary pair—steps across a bank of terminals to hunt for the off-hook signal, indicated by a ground on the sleeve lead from the line relay. This process involves a commutator that identifies the calling line's level, followed by rotation to the specific position, ensuring connection to a first selector without manual intervention.^[21] The first selector and connector switches form the primary routing mechanisms, each employing a two-motion design for digit-based selection. The first selector responds to the initial dialed digits by vertically lifting its wiper assembly to the corresponding level (one step per pulse interruption) and then rotating to an idle path toward the next selector in the exchange, thereby routing the call within the local office. In contrast, the connector switch, positioned at the end of the digit chain, interprets the final two digits to vertically select the destination line's row and rotate to its column, testing for idle status before applying ringing voltage and completing the connection upon answer.^[26]^[23] Electromagnetic relays and interrupters provide essential control and signaling functions throughout the system. Relays, such as the start relay in line finders or the slow-release pulse relay in selectors, detect dial pulses by momentary interruptions in the loop current and manage timing delays—often via copper collars on armatures for brief holds of about one-third second—to ensure reliable switch stepping and supervision. Interrupters, including rotary types mounted on the switch shaft, generate periodic pulses for hunting idle paths or producing dial and ringing tones, while also feeding supervision signals back to maintain call integrity.^[21]^[26] Wiring and contact banks enable the physical interconnections in a structured, scalable manner. The banks consist of semi-circular arrays with 10 rows (levels) of 10 contacts each, terminating up to 100 lines or trunks per switch, where flexible wiper arms—often with tinsel cord connections—make selective contact for tip, ring, and sleeve leads. In a full exchange serving hundreds of subscribers, a typical call path involves 4 to 5 switches (a line finder, one or two selectors, and a connector), with multiple such switches shared among lines in the exchange to handle traffic and minimize blocking.^[23]^[26] The entire apparatus operates on a nominal 48 V DC power supply, derived from central batteries, to energize electromagnets and relays while providing line supervision. Fuses protect against short circuits in the wiring, and voltage drops (e.g., to 12 V off-hook) aid in detection, ensuring safe and efficient power distribution across the electromechanical components.^[26]^[23]

Development and Improvements

Early Enhancements

Following the initial deployment of the Strowger switch based on its 1891 patent design, early enhancements in the late 1890s focused on improving user interaction and system usability.^[1] In 1896, engineers Alexander Keith, John Erickson, and Charles Erickson, working with the Strowger Automatic Telephone Exchange Company, patented a finger-hole rotary dial (U.S. Patent 597,062), which replaced the cumbersome sequential push-button mechanism of the original system.^[1] This innovation allowed users to generate pulses intuitively by rotating the dial to a digit and releasing it, with a maximum of 10 pulses per digit corresponding to numbers 0 through 9, thereby streamlining call initiation and reducing operator dependency.^[1] Around 1900, circuits for detecting busy lines were integrated into the Strowger system to enhance reliability and caller experience.^[27] These additions enabled the final selector switch to test the destination line; if engaged, it would route a busy tone back to the caller, preventing futile attempts to connect and minimizing user frustration during high-traffic periods.^[28] By September 1901, the system incorporated central office ringing generators to further support call completion, activating electromechanical bells at the called party's telephone via the connector switch once a line was free.^[29] Minor mechanical refinements also addressed durability issues in the core switching components during this period.^[1] Improvements to pawl designs in the selector and connector switches reduced friction and wear on the wiper arms, which engaged the contact banks, extending operational life from mere months in early prototypes to several years under continuous use.^[28] These tweaks involved stronger materials and precise alignment for the pawls that stepped the vertical and rotary motions, ensuring more consistent pulse response without frequent maintenance.^[7] Economic adjustments through standardized manufacturing processes significantly lowered implementation costs by the 1910s.^[27] The adoption of uniform components across Strowger exchanges, as promoted by the Automatic Electric Company after 1901, facilitated mass production and lowered per-line installation expenses, making the technology viable for smaller independent telephone companies.^[1] This cost efficiency contributed to wider deployment, with Strowger systems serving 4% of independent lines by 1904.^[1]

Scaling and Variations

To address the growing demands of urban telephone networks in the 1920s, the uniselector was introduced as a single-motion variant of the Strowger switch, primarily for line finder applications. Unlike the traditional two-motion selector, which required both vertical lifting and rotary movement, the uniselector employed only rotary motion across a semi-circular arc of typically 24 contacts, using wipers to hunt for free outlets and connect calling lines to group selectors. This design reduced mechanical complexity, saved space, and improved efficiency by allowing multiple lines to share selectors based on anticipated traffic, with arrangements like 8 wipers over 25 outlets or variations with 4 wipers over 50 outlets.^[30]^[31] For larger-scale deployments, Strowger systems incorporated cascading hierarchies of multi-stage selectors to enhance urban scalability. In a typical 10,000-line exchange, this involved three to four stages: line finders or first selectors handling the initial digit to route to second selectors, followed by final connectors for the last digits, often structured in frames of 1,000 lines each with 100 first selectors, 10 second selectors, and 10 connectors per frame. These setups, pioneered in patents like A. E. Keith's No. 831,876 (1905), enabled step-by-step progression through pre-wired paths, supporting 4-digit numbering for up to approximately 10,000 subscribers while minimizing blocking through trunking efficiencies.^[29]^[32] Regional adaptations emerged to suit local needs during the 1920s-1940s. In the United Kingdom, the General Post Office (GPO) adopted Strowger technology from 1922, incorporating uniselectors and motor uniselector groups in director and non-director exchanges for quieter operation and reliable rural service via compact UAX variants like the UAX13 (200-800 lines). U.S. implementations, led by Automatic Electric, emphasized speed enhancements, such as refined interrupters in selectors to accelerate pulse stepping and reduce connection times, alongside multilevel trunking for non-urban areas.^[33]^[1] Key limitations in capacity and maintenance were mitigated through targeted improvements. Power efficiency was enhanced via Keith's multilevel designs and common battery systems introduced around 1905, allowing reliable handling of up to 10,000 lines without excessive energy draw or ground-return issues. Additionally, some systems integrated registers in the Director variation—developed in the early 1920s by Automatic Electric—for abbreviated dialing, where the register translated partial codes (e.g., first two digits) into full routing instructions, streamlining calls in dense areas like London.^[29]^[34] By the 1940s, peak Strowger configurations supported exchanges of 10,000 to 15,000 lines, as seen in major installations like Leeds (ultimate capacity 15,000), with metropolitan networks in cities combining multiple exchanges for broader coverage while relying on manual assistance for directory inquiries in high-traffic scenarios.^[35]^[1]

Legacy

Impact on Telephony

The introduction of the Strowger switch revolutionized telephony by enabling automated direct dialing, which significantly expanded access to telephone service, particularly in rural and small-town areas where hiring operators was economically challenging. This automation democratized phone service, allowing subscribers to connect calls without relying on human intermediaries, thereby fostering growth in independent telephone companies that competed with larger monopolies like the Bell System. As a result, the number of telephones in the United States grew from approximately 1.35 million in 1900 to over 20 million by 1930, reflecting the scalability and affordability introduced by Strowger-based systems.^[36]^[37] Economically, the Strowger switch reduced the need for telephone operators, lowering operational costs and spurring the growth of the telecommunications industry. In cities adopting automation, the proportion of women remaining in operator roles dropped to 16 percent after a decade, compared to 24 percent in non-automated areas, as the technology displaced manual labor and encouraged workforce shifts to other sectors like secretarial work. This efficiency inspired competitors, such as the Automatic Electric Company founded in 1901, which further innovated on Strowger designs and expanded market competition. By enabling cost reductions in some exchanges through minimized staffing, the switch facilitated broader industry expansion and infrastructure investment.^[38]^[39] Socially, the Strowger switch improved business efficiency and enabled quicker emergency responses by reducing reliance on operators. This reliability transformed daily communication, enhancing privacy by eliminating operator eavesdropping and supporting the rise of telephony as an essential tool for commerce and personal use. The switch's pulse-dialing mechanism also influenced global standardization, leading to the adoption of uniform rotary dial systems by the 1920s that became the basis for international telephony protocols. By 1950, automated switching served over 75 percent of U.S. telephones, with Strowger technology playing a primary role in early adoption, handling tens of billions of calls annually and solidifying its role in modernizing communication networks.^[1]^[40]

Decline and Modern Relevance

The decline of the Strowger switch began in the late 1930s with the introduction of crossbar switches, which offered faster operation, greater reliability, and better scalability for urban networks compared to the step-by-step mechanical movements of Strowger systems.^[1] The first crossbar installations occurred in 1938 in New York City, gradually supplanting Strowger in larger exchanges across the United States during the 1940s and 1950s, though some rural Step-by-Step systems persisted into the 1980s and 1990s.^[41] By the 1970s, crossbar technology had become predominant in major North American markets, addressing Strowger's limitations in handling high traffic volumes without excessive hardware duplication.^[42] The transition accelerated in the 1960s with the advent of electronic Stored Program Control (SPC) systems, which replaced mechanical components entirely with digital logic, reducing maintenance needs and enabling more efficient call routing.^[41] The first SPC trial appeared in 1960, followed by commercial deployments like the #1ESS in 1965, leading to the decommissioning of most Strowger exchanges.^[1] In the United Kingdom, the last Strowger exchange at Crawford, Scotland, was retired on June 23, 1995, marking the end of widespread electromechanical switching in that network.^[33] Maintenance challenges, including frequent mechanical wear from constant stepping motions and the vast space required for thousands of selectors in a single exchange, hastened this obsolescence, as repairs often involved manual adjustments prone to human error and downtime.^[43] Today, the Strowger switch holds no active commercial role in telephony, fully supplanted by digital and IP-based systems, though its historical significance persists in educational and preservation contexts. Examples are maintained in institutions such as the Science Museum Group in the UK and the National Museum of American History at the Smithsonian Institution, where operational demonstrations illustrate early automation principles.^[5]^[44] Software simulations and scaled hardware replicas are used in telephony training programs to teach switching fundamentals without the logistical burdens of full mechanical setups.^[45] Rare hobbyist efforts focus on restoring vintage units for private collections, underscoring the need for digitizing archival documentation to preserve technical details like line-finder configurations amid fading expertise.

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