Index-Glossary
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An informal term describing an Entraide wiring configuration where signals normally pass through two switches, but could pass through three under heavy load.
The specific high-capacity configuration of the Pentaconta switch, providing considerably more routing flexibility than standard 10x20 variants.
The massively expanded operational feature set achieved by the Number 5 Crossbar system by the year 1975.
A heavily utilized relay in Step-by-Step systems that responds directly to dial pulses and frequently required manual burnishing.
The historic event on March 10, 1876, when Alexander Graham Bell accidentally discovered the telephone after hearing Thomas Watson pluck a jammed reed over a telegraph wire.
AEC (Automatic Electric Company)
A leading equipment supplier of Step-by-Step systems from Chicago that employed Alexander Keith, cross-licensed technology with ATM, and manufactured the Director system.
An electromechanical switching architecture, heavily advocated by pioneers like Edward Clement, which operates using composite switch arrays made entirely of relays.
The famous undertaker and visionary who introduced the first practical commercial dial-operated switch and automated exchange.
American Bell Telephone Company
The 1881 renamed entity of the original Bell Telephone Company, which soon established AT&T as a subsidiary.
American Institute of Electrical Engineers (AIEE)
The professional organization where Thomas Watson delivered his 1915 address regarding the accidental discovery of the telephone.
A damaging electrical phenomenon across relay contacts that can cause pitting and erosion; a major focus of reliability improvements in exchange engineering.
Specialized relays in a toll office marker that engage to record the dialed 3-digit area code before querying a translator.
The moving mechanical part within a relay (or a telephone transmitter) that responds to or generates a magnetic field when an electromagnet is energized.
Co-author of the comprehensive 1914 treatise "Automatic Telephony," which was dedicated to Alexander E. Keith.
Automatic Telephone and Electric Company.
An author referenced regarding traffic engineering principles and the trunking plan of Glasgow, Scotland.
ATM (Automatic Telephone Manufacturing Co.)
A British company that cross-licensed the Strowger switch from AEC and installed automatic exchanges in England.
Audible ringing / busy signals
Distinct progress tones generated by interrupter mechanisms and sent back to the caller's headset to indicate the current state of their call.
Automatic Electric Company (AEC)
The prominent manufacturing company that produced automatic exchange products, guided heavily by the patents of Alexander Keith and the Erickson brothers.
A telephone switching system capable of routing calls automatically using machine logic and user-dialed digits rather than manual switchboard operators.
A set of stationary electrical contacts arranged in rows and columns, used in switches like the Strowger, Rotary, and Panel types for the moving brushes to connect with.
A prominent voice actress whose recorded telephone intercept messages are featured in the interactive demos on the Calling315 site.
Baron Pavel L’vovitch Schilling
A Russian-German diplomat, scientist, and inventor who created an early 8-wire demonstration telegraph in St. Petersburg in 1832.
The large, often 11-foot-tall iron structures used to mount relays, crossbar switches, and cabinets in an exchange, arranged side-by-side in long aisles.
The massive North American telephone network operator that set the gold standard for reliability and drove the development of the Panel, Step-by-Step, and Crossbar exchanges.
The mathematical foundation of 0s and 1s, essential for the logic circuits of relay switching and early computers.
An early telephone innovator mentioned among the significant inventors of the era who contributed to transmission methods.
A condition in a partially-blocking switch fabric where heavy call traffic prevents new input connections from successfully reaching an output.
Bell Laboratories Record. A magazine published by AT&T on telephone related topics. See BLTJ.
An English mathematician whose 1854 work laid the groundwork for Boolean algebra, which became essential for relay logic design.
Call Setup / Call Establishment
The complex, multi-step sequence of operations required to build a connection between subscribers, which was often managed by common control equipment (i.e markers) or state machines like the sequence switch.
The process of monitoring the status of a telephone connection (such as on-hook, off-hook, or answer states) to control call progression, maintain the talking path, and trigger the release of equipment when the call ends.
An early automatic exchange inventor who conceptualized the "numerical receiver" and attempted to route calls using metal balls, chutes, and runways.
The comprehensive educational website created by Al Kovalick to explore the ~80-year golden age of electromechanical telephone switching.
A Telephone Exchange Technician for 38 years from Christchurch, New Zealand, who provided invaluable videos, metrics, and insights regarding the 7A Rotary system.
The Bell Labs inventor of the Type-22 "2-wire" repeater and loading coils, which were essential for improving toll speech quality over long distances.
An ingenious electromechanical device utilized in 4A tandem offices to process 10-digit nationwide direct dialing routing.
A physics teacher at Lowell High School in San Francisco who championed DIY projects and helped procure relays for the author's 8-line telephone system.
Common Battery.
Centrum Call Seconds.
A class of small, often unattended exchanges serving local populations, such as the No. 380 and No. 43 Crossbar systems.
Central Switching Fabric (CSF)
The distributed mesh of many smaller switches that forms the core network stage of an exchange, routing inputs, outputs, and trunks.
Referenced author of "100 years of Telephone Switching (1878-1978)".
A Bell Labs engineer who famously published a 1953 paper proving how to guarantee that a 3-stage switch fabric can be completely non-blocking.
The founder of the North Electric Company in 1884, which later acquired Edward Clement's all-relay patents.
An author whose work in the Bell System Technical Journal forms the basis of the site's coverage on exchange trunking plans.
Acknowledged for providing excellent editorial comments related to the North CX all-relay systems.
A component of the Callender/Lorimer systems that utilized a continuously rotating arm until the desired dialed digits aligned.
Director of Systems Development for Bell Laboratories, pictured during the August 1943 cutover of the Philadelphia #4 Crossbar toll office.
Clement, Edward / Clement System
The inventor of a highly reliable, early all-relay exchange system from 1906 built by the North Company, which utilized relay trees for call selection.
The expensive, complex device required inside every Lorimer telephone by 1897 to facilitate their unique machine-driven dialing method.
A complex fabric design approach that improves non-blocking capabilities over simpler Entraide networks to handle heavy call traffic.
CMoS (Connections Museum of Seattle)
A museum hosting working panel, step, and crossbar systems, heavily credited for providing sounds, voices, videos, and images to the site. Also called CMS.
Common control is a highly efficient call routing architecture utilized in Panel, Rotary, and Crossbar exchanges where dialed information is temporarily stored and processed by shared equipment. Unlike progressive direct dial systems, these shared "brains" (such as registers, senders, and markers) orchestrate the switching operations and are immediately released back to a pool for reuse once the talking path is established.
A six-subscriber experimental automatic switch installed by the National Telephone Company in Great Britain in 1886.
The location where the first fully automatic Rotary exchange in Europe was installed in October 1914.
An under-recognized independent co-inventor of the relay, who patented an "electric re-newer" for telegraph signals in 1838.
Author referenced regarding the workings of the 7-A Machine Switching System.
Dial Facilities Management Practices.
An ingenious relay logic circuit using P1-P6 relays that count in sync with subscriber dial pulses to record digits.
A component within all-relay and other systems responsible for providing a dial tone and recording the subscriber's dialed digits.
One of the specific types of markers utilized in a #5 Crossbar office to prepare the equipment to receive a user's dialed digits.
The 10-digit long-distance routing system officially introduced to the North American network in 1951.
An exchange trunking layout where 2-way trunks connect each office directly to every other office, requiring up to 43,000 groups in the New York metropolitan area.
A store-and-forward common control routing device introduced in 1920 by AEC to translate dialed digits and assist in wide-area calling for Step-by-Step exchanges.
Discriminating Selector Repeater (DSR)
Specialized Step-by-Step routing equipment that discriminates the destination and selects the most direct path over the network.
A large linear switch in a Panel exchange that takes the initial connection and routes it toward an Incoming Selector.
The pseudonym of an audio recordist who captured authentic 1970s electromechanical "phone tapes" featured heavily in the site's interactive demos.
A specialized horizontal bar in the Pentaconta switch used to select one of two contact stacks per intersection, yielding up to 52 outputs.
An early, widely produced flat spring relay invented by E.B. Craft in 1915, notably utilized in Panel exchanges.
An esteemed AT&T/Bell Labs engineer who proposed digit translation in 1905 and developed mathematical formulas (Molina Formula) vital for telephone traffic engineering.
E.B. Craft (Edward Beech Craft)
A pivotal Western Electric inventor and Bell Labs Executive VP who developed the highly successful flat spring relay and the Grasshopper fuse.
Major transmission impairments encountered on long toll connections, requiring specialized suppression equipment.
An early 2-way amplifying concept utilizing one device that influenced later mechanical amplifier designs.
A legendary inventor who held 23 patents with "telephone" in the title and developed the electric motograph (mechanical amplifier) to renew signals.
An early mechanical amplifier invented by Thomas Edison utilizing an electrolyte-impregnated, hand-cranked rotating chalk cylinder.
The term utilized by Edward Davy in his 1838 patent for a device that renewed telegraph signals, historically recognized as an early relay.
The foundational term (which later became "electricity") coined by William Gilbert in his 1600 treatise De Magnete.
Electronic Switching System (ESS)
The computer-based exchange generations (beginning with Number 1 ESS in 1965) that eventually replaced the era of electromechanical switches.
Electromagnet.
Local central office exchanges (designated as Class 5) where subscriber lines are directly connected, distinct from toll-only offices.
Acknowledged as the Curator of the JKL Museum of Telephony who provided essential images and artifacts.
A mutual-aid or reciprocal overflow switch wiring configuration used in Pentaconta fabrics to improve call traffic handling.
Erickson brothers (John and Charles J.)
Innovative Swedish-American brothers from Lindsborg, Kansas, who collaborated with Lundquist and Keith to develop the rotary dial and early automatic switches.
The major Swedish telecommunications company that developed the AGF exchange series and the 500-point switch, and later absorbed the North Electric Company.
A robust motor-driven selector switch featuring a radial arm and a wire-bank with 20 triplets of contacts for each of its 25 rotary positions.
A flat, step-by-step type switch utilizing two degrees of motion, produced in partnership with Stromberg-Carlson.
Erlang, Agner Krarup / Erlang distribution
A Danish mathematician and engineer recognized as the father of telephone traffic theory, whose 1909 equations helped determine optimal switch capacities.
The unit of measurement for Traffic Units (TU), representing the average number of simultaneous conversations occurring during a specific period.
The pseudonym of an audio recordist who captured hours of authentic "phone tapes" detailing the operational sounds of electromechanical exchanges in the 1970s.
The segment of a switching plan that works inversely to a concentrator, taking outputs from the central switching fabric to route to the called subscriber line.
The distributed mesh of interconnected smaller switches (like the Central Switching Fabric) within an exchange that successfully route calls between input and output ports.
A linear "falling selector" switch categorized as a unique mechanical switching mechanism used in early exchanges.
A massive telephone exchange location in London that underwent a major remodel in 1939.
Foreign Area Translator.
A relay stage in an all-relay system that receives the final dialed digit and finishes the connection through the talking circuit to the called subscriber.
An interactive digital experience created by the author to explore the workings of the 7A Rotary system.
Ferrymead Post and Telegraph Historical Society
A working museum located in Christchurch, New Zealand, housing a preserved, operational 7A Rotary system and vintage Strowger equipment.
The highly successful relay architecture invented by E.B. Craft in 1915, replacing difficult-to-machine parts with punched spring contacts (includes E, R, and U models).
Blueprint-style layouts used by engineers (like Elsworth Goldsmith) to map out how subscribers are connected through various combinations of switches.
The large, often 11-foot-tall iron structural supports used to mount relays, crossbar switches, and wiring components inside a central office.
The author of the 1929 book "Beginnings of Telephony," which provided rare, detailed explanations of early switchboards.
Final Selectors.
A Swedish inventor and early adopter of the all-relay concept who developed vital early crossbar prototypes alongside Nils Palmgren.
General Telephone & Electronics Corporation (GT&E)
The telecommunications conglomerate that absorbed the Automatic Electric Company, acting as the largest independent telephone company in the US from 1926 until 2000.
Listed among the early innovators and contributors to switching methods and exchange design.
A Bell Labs mathematician and researcher who designed the first relay-based 1-bit adder in 1937 and directed the creation of the 450-relay Complex Number Computer in 1940. His inventions extended the computational capabilities of relays, contributing significantly to both early electromechanical computers and new telephone exchange designs.
A 16th-century English physician and author of De Magnete, who established foundational theories of magnetism essential to later relay development.
The critical manufacturing fix applied to palladium contacts to prevent the buildup of insulating organic polymers on non-arcing relays.
An AT&T engineer who delivered a highly regarded series of training lectures in 1926 explaining the operations of the Panel Type Dial Telephone System.
A subset of Quality of Service (QoS) focused specifically on capacity and the probability of call loss.
A toll office added in New York City in 1941 that allowed operators to dial directly into the tandem equipment to complete 7-digit calls.
A German mathematician who published the discovery of binary math in 1703, laying the foundational logic later used in relay computing.
British General Post Office. Responsible for telephone operations in Britan.
Author of the 1950 Bell Laboratories Record article on Equipment Arrangements for No. 5 Crossbar Markers.
One of the two large independent crossbar fabrics (alongside the Line Unit) found in a local Pentaconta exchange.
Group Selectors.
The inventor who patented the Common Battery (CB) system in 1892, enabling centralized power for subscriber microphones and line relays.
Bell's initial project intended to send multiple simultaneous telegraph symbols over a single wire using tuned metal reeds, which directly led to the telephone's accidental discovery.
Bell's early, impractical theoretical concept utilizing hundreds of vibrating reeds to reproduce the complex sounds of human speech.
Famous workplace productivity experiments conducted at Western Electric from 1924 to 1932 that revolutionized how employee attitudes and social organization were viewed.
The massive Western Electric plant in Cicero, Illinois, which housed the primary Relay Assembly Department and employed 42,000 people in the 1920s.
An electrical engineer whose work on transmission line theory influenced George Campbell's development of repeaters.
An organic vapor purposefully introduced in laboratory testing to replicate the problematic brown polymer deposits on palladium relay contacts.
The location of the first automatic telephone exchange in London, installed on November 12, 1927.
A crossbar switch component whose operation time was significantly sped up by utilizing high-voltage LXP Marker relays.
The duration of a telephone call, which averaged approximately 2.75 minutes according to a 2001 Statistica study.
The violent sound produced by early telephones when triggered by Watson's "buzzer" mechanism to announce a new incoming call.
Holding Time.
A co-founder and trustee of the original Bell Telephone Company formed in 1877.
The rapid, automatic motion of a switch's wipers searching across terminal banks to seize a free selector or idle line.
A free, available switch that is automatically seized by a Line Switch or Line Finder to provide a dial tone to a new off-hook caller.
Institute of Electrical and Electronics Engineers.
Quality-degrading factors, primarily audio signal loss, crosstalk, distortion, and echo, that must be overcome by amplifiers, equalizers, and suppressors on long toll network connections.
Independent Telephone Operators
Competing, non-Bell telephone companies that operated millions of phones until being progressively acquired by the Bell System around 1940.
A visual component mounted on top of a sequence switch to indicate its current mechanical state (usually 1 to 18 positions).
The electromagnetic phenomenon Bell relied upon where a vibrating, magnetized armature generated an electrical signal in a circuit.
A motor-driven, cam-based mechanism used to open and close electrical contacts to generate busy tones and party-line ringing cadences.
Interruptions Per Second.
The massive structural racks requiring hundreds of welders and installers to support a heavy electromechanical exchange like the Panel system.
IT&T (International Telephone and Telegraph)
The corporation that purchased Western Electric's international manufacturing arm in 1925, expanding Rotary exchanges globally.
Listed among the early innovators who contributed to crossbar switching methods and exchange design.
A Vice President of AT&T who notably praised Frederick Rhodes' chronicle of early telephony in 1929.
The scientist who discovered the electron in 1897, highlighting that early telephone inventors were building electrical devices without knowing about electrons.
Jackknife switch / Telephone jack
A standard switchboard connection port invented by C.E. Scribner in 1879, which derived its name from its resemblance to a pocket clasp knife.
The President of Bell Telephone Laboratories who notably praised the outstanding creative imagination of key inventor E.B. Craft.
An archive and physical museum heavily credited for providing rare framing photos, switch components, and historical telecommunications documents.This Museum was moved to CMoS in 2026.
The complex internal wiring paths and circuits within a Crossbar exchange that interconnect different switching stages, such as the links bridging the Line Link Frame directly to the Trunk Link Frame.
A brilliant line concentrator utilizing a master guide shaft and plunger mechanism to seize idle selectors for calling subscribers, saving immense equipment costs.
The General Manager of AEC and a prolific inventor who drastically improved the Strowger switch and co-invented the rotary dial.
Kelvin, Lord (Sir William Thomson)
A world-leading electric theory expert whose enthusiastic promotion of Bell's telephone at the 1876 Centennial Exhibition was crucial for its immediate credibility.
An American telephone engineer and prolific author who notably debated the pros and cons of automatic vs. manual exchanges in 1904.
An inventor granted an 1895 British patent for a fast-acting Line relay designed to replace more expensive polarized relays in switchboards.
An example named telephone exchange (prefix KLondike 2 / 552) historically utilized in San Francisco and observed by the author in the 1970s.
A nearly frictionless, space-saving relay integrated directly with a lamp, patented by Mark Edson in 1895 for early switchboards.
The author and creator of the Calling315 website, pictured visiting and analyzing the 7A Rotary system in Christchurch, New Zealand.
Key Telephone System.
The historic city where Almon Strowger's first automatic telephone exchange was installed in 1892, originally serving 75 customers.
A mathematician whose work, alongside Poisson and Erlang, was studied by E.C. Molina to develop traffic engineering models.
A specific mechanism in a Rotary 7A sequence switch that trips to extend three brushes to make contact with a desired row of arc contacts.
A physical failure mode, primarily affecting flat spring relays, where the contacts would physically stick together and require manual burnishing.
An essential relay sitting behind every phone number, paired with the Cutoff (CO) relay, which initiates line finders to find the new caller when a subscriber goes off-hook.
The critical interface between the subscriber's telephone and the exchange's switching logic, fundamentally comprising the Line and Cutoff relays.
A concentration stage switch that automatically searches for and connects an off-hook calling subscriber to an idle first selector to accept dialed digits.
A relay-based sub-system located near a Number 5 Crossbar marker that is essential for call establishment.
One of the two large, independent crossbar fabrics (alongside the Trunk Link Frame) utilized in a Number 5 Crossbar office.
Line Switch (LS) / Keith Plunger Line Switch
A brilliant concentrator switch invented by Alexander Keith that utilizes a master switch to thrust a plunger into a terminal bank, seizing a pre-selected idle selector.
The composite device housing the master switch and multiple individual line switches (often 50 to 100), which drastically reduced the per-line cost compared to standalone selectors.
One of the two main crossbar fabrics utilized in a Pentaconta exchange for routing calls.
The Swedish telecommunications company that developed the 500-point switch, the XY switch, and manufactured over one million crossbar switches globally.
A Swedish electromechanical switch that utilized a radial arm and wire-bank, widely installed starting in the 1940s.
Devices inserted at regular intervals on transmission lines, developed by George Campbell, to significantly improve toll speech quality over long distances.
The lowest tier in the long-distance switching hierarchy where subscriber lines are directly connected, which can be implemented using Panel, Step-by-Step, Crossbar, or ESS systems.
Lorimer Brothers (James, Hoyt, George)
Canadian inventors who developed an early motor-driven cylindrical rotary switch, featuring the first use of the "Line Finder" and revertive pulsing.
A motor-driven, 360-degree rotary switch utilizing a cylindrical terminal bank and clutch mechanisms, which heavily influenced the later Panel and 7A Rotary designs.
The San Francisco high school where physics teacher Carl Koenig assisted the author with a DIY 8-line telephone system.
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An innovator who collaborated with the Erickson brothers and Alexander Keith to develop the rotary dial and early automatic switches
A derivative number generated by translating a subscriber's dialed digits, used by the Director system to expedite remote routing.
The complex "logical thinker" of a crossbar exchange (often containing ~1,500 relays) that temporarily stores digits, analyzes them, and marks the path through the switch fabrics.
Sub-systems in a No. 5 Crossbar office that act as relay-based links during call establishment.
The central controller component of the Keith Line Switch Unit that continuously rotates a guide shaft to pre-select an idle selector for the next off-hook caller.
A prolific Western Electric inventor who developed the sequence switch and served as the driving force behind the 7A Rotary system.
An engineer from Switching Development at Bell Laboratories, pictured during the August 1943 cutover of the Philadelphia Number 4 Crossbar.
Co-inventor of an 1879 switch patent (with Connolly) that required only two wires and heavily influenced Almon Strowger's later designs.
The location where the very first Number 5 Crossbar exchange was installed in 1948.
The physical wearing down of relay contacts over time, prompting the need for burnishing or replacement to maintain exchange reliability.
The electrical components within an Ericsson 500-point switch that dictate clockwise or counter-clockwise rotation of the spindle.
A scientist whose work built upon earlier discoveries to help establish the science of electromagnetism.
The specific transmitter Alexander Graham Bell shouted into on March 10, 1876, calling for Mr. Watson.
Mathematical formulas for telephone traffic engineering developed by E.C. Molina (who also proposed digit translation) to optimize switch capacities.
Specific relays in a Crossbar office that initiated complex logic sequences and frequently required maintenance from switchmen due to heavy use.
George Mueller was an early innovator from the Kellogg Switchboard & Supply Company who made foundational contributions to exchange design and switching methods. He is most notably recognized for filing a 1907 patent for the bidirectional minor switch, which is considered the granddaddy of all rotary stepping switch styles.
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The first commercial manual telephone switchboard, installed in Connecticut in 1878, which utilized rotating metallic switch arms to manually interconnect eight subscriber lines.
An early pioneer of the electronics industry mentioned alongside other notable historical figures.
No single point of failure (NSPOF)
A reliability mentality adopted by the Bell System, heavily utilizing commercial power backups to prevent exchange downtime.
A specific model of a small, highly reliable all-relay telephone exchange manufactured by the North Electric Company.
The Number 1 Crossbar system was a metropolitan telephone exchange introduced by the Bell System in 1938 to begin replacing the motor-driven Panel system. First installed at Troy Avenue in Brooklyn, New York, it utilized a 2-dimensional coordinate switching matrix to provide scalable and reliable call routing.
A small-scale crossbar system introduced in 1974 that utilized 5-digit pulse counting and specialized register relays.
Number 4 Toll Crossbar Exchange
The Number 4 Crossbar was a specialized toll exchange introduced in Philadelphia in 1943 to meet the rapidly growing demand for long-distance calling. Operating as the world's first four-wire crossbar toll office, it functioned strictly as a routing machine connecting other offices without any directly attached subscriber lines.
The Number 5 Crossbar system was the highly advanced, final generation of the Bell System's great electromechanical exchanges, first installed in Media, Pennsylvania, in 1948. A typical 10,000-subscriber office utilized massive switching fabrics containing over 800 crossbar switches and tens of thousands of relays managed by intelligent common control markers.
A digit storage concept conceptualized by Romaine Callender and used in early automatic exchanges.
An inventor who proposed a lamp indicator circuit in 1891 to replace expensive electro-mechanical drops on early switchboards.
A design challenge in the DIY 8-line system solved by superimposing AC ring voltage with 48 VDC on the talk path.
The first three digits of a standard 7-digit phone number that a routing system or Director analyzes to identify a specific local exchange office.
A specific type of large linear switch used in Panel exchanges to extend the District Selector's reach when connecting to many remote offices.
Shared common control equipment in a Number 5 Crossbar system that temporarily delivers a dial tone and records the user's dialed digits.
Orsted (Hans Christian Ørsted)
A scientist who published a pamphlet in 1820 demonstrating the clear qualitative effect that an electric current generates a magnetic force.
Main counting relays used in a dial pulse digit counter circuit that dance in sync with subscriber dial pulses.
A Swedish inventor who collaborated with G.A. Betulander to patent an all-relay exchange system and invent the crucial "marker" routing concept.
The world's first metropolitan-class exchange, installed in 1922 in NYC, designed to handle large-volume intra-office and inter-office calls using highly mechanical vertical selectors.
A cost-saving wiring arrangement where multiple subscribers shared a single pair of wires to the central office, requiring complex, independent ringing cadences.
A famous radio broadcaster whose voice was utilized in site demos to simulate someone talking on a toll call.
The mechanical components used in Alexander Keith's early Zither switch to translate linear motion into rotary motion.
Peak Busy Hour.
A localized, private telephone system used by businesses, such as the 4-digit Step-by-Step system located at the UCSF hospital.
The famous, and still operating, 7-digit telephone number belonging to the first Panel-type exchange installed in New York City.
A highly versatile crossbar switch featuring 14 horizontal bars and a "doubling bar", capable of up to 52 separate outputs, manufactured globally by ITT.
A physical component of the Keith Line Switch that thrusts into a terminal stack to seize a selector and then retracts when a caller goes on-hook.
The British Post Office Electrical Engineers' Journal.
The mathematical formula used to predict the probability of a specific number of simultaneous calls occurring in an exchange.
A specialized relay utilized in early switchboard lamp indicator circuits, designed to respond to polarity logic such as the positive part of an AC sinewave only.
A critical reliability issue where organic vapors from insulating materials created an amorphous brown deposit on non-arcing palladium contacts.
High-quality relay contacts (with up to 2,000,000 used in a single No. 5 Crossbar office) designed to withstand billions of operations annually.
A small-scale all-relay telephone system produced by the Relay Automatic Telephone Company (R.A.T. Co.) circa 1928.
A mathematical calculation within traffic engineering (GoS) used to determine if an exchange has enough trunks to prevent blocked calls.
Progressive Direct Dial Control (PDDC)
A call control architecture, characteristic of Step-by-Step systems, where the switch at each successive stage responds directly and immediately to the user's dialed digits.
Pacific Telephone and Telegraph company.
The individual credited by Thomas Edison as being the very first person to suggest the idea of a central telephone exchange.
In telephony, this encompasses all aspects of a telephone call, including time to dial tone, noise, crosstalk, echo, and frequency response.
An early flat spring relay produced by Western Electric in the 1930s.
Relay Automatic Telephone Company.
A specific 10-step sequence switch utilized to direct a new caller's line to the first idle Register in a 7A Rotary system.
Dual 18-step state machines within a Rotary Register that manage the recording of dialed digits and the advancement of talking path switches.
The mechanical gear setup within an Ericsson XY switch that moves the wipers in the Y direction via a toothed sleeve.
The engineer who designed the pivotal relay-based dial pulse digit counter circuit replicated thousands of times in Panel exchanges.
The concept behind the Pentaconta Entraide network where primary switch outputs are partially interconnected to increase possible routing paths.
A sealed relay type famously utilized by Frank McBerty in his late-1940s all-relay exchange designs for the North Electric Company.
A specific switch utilized in the 7A Rotary system to forward a new caller to an idle Register.
The temporary common control "brains" of a 7A Rotary system that provides dial tone, records dialed digits, and translates them to control the talking path switches.
A Step-by-Step control architecture where a Register records digits and forwards them, insulating the exchange from the subscriber's dialing speed.
An electrically operated switch utilizing an electromagnet and armature to close or open contacts, serving as the central logical element in electromechanical exchanges.
A 50-year engineering pursuit by the Bell System to ensure long-term contact integrity for millions of exchange relays, aiming for a 40-year trouble-free lifespan by mitigating issues like arcing, metal erosion, and polymer formation.
Devices (such as vacuum tube triodes) spaced along trunk lines to renew voice signals and overcome the natural attenuation of long toll calls.
A peculiar rotary dial required by the 7A Rotary system where numbers are arranged clockwise from 0 to 9, sending pulses equal to 10 minus the dialed digit.
A control method first used in the Lorimer switch (and later in Panel and Rotary) where the exchange switch generates pulses that are sent backward to advance a controlling counter.
A specific party-line cadence consisting of two seconds of ringing followed by four seconds of silence.
A specific party-line cadence consisting of one second on, one second off, one second on, and three seconds off.
A pioneer whose early 1903 work at Bell stimulated the foundational traffic engineering theories of Edward C. Molina.
A highly reliable common control exchange developed by Western Electric that utilized sequence switches, optional reverse dials, and could scale to 2,000,000 subscribers.
Strowger Automatic Telephone Exchange.
The Assistant Director of the La Porte County Historical Society Museum, acknowledged for providing historical pictures of the first automatic exchange.
The architectural approach used to expand a telephone exchange, which became highly impractical and relay-heavy for early all-relay systems.
The Mayor of La Porte, Indiana, who collaborated with Almon Strowger to install the world's first automatic telephone exchange in 1892.
A highly ranked toll office in the North American network hierarchy, typically implemented using a Number 4 or Crossbar Tandem system.
A component in the Callender/Lorimer "Selector-Connector" mechanism that engages to stop the continuous rotary wiper when digits align.
A relatively expensive, multi-position talking path switch that receives dialed digits and advances to make a connection.
The mechanical operation in Frank McBerty's sequence switch where a cam and motor clutch ensure one positive rotary step per electrical pulse.
A common control component in Panel and Crossbar exchanges that records dialed digits and out-pulses them to route calls to remote offices. In Panel, it receives revertive pulses from local talking path switches to advance the call.
An energy-efficient, motor-driven auxiliary switch utilizing rotating cams. Invented by Frank McBerty to function as a mechanical state machine. Used in Panel and Rotary exchanges.
A Bell Labs mathematician whose 1941 MIT Master's Thesis, "A Symbolic Analysis of Relay Switching Circuits," revolutionized computer design and is considered by some to be the most important thesis of all time.
An inventor whose work on practical mechanical amplifiers influenced George Campbell's development of toll repeaters.
The severe audio feedback loop created if two one-way amplifiers in a 2-wire toll trunk are without proper balancing.
The third wire (alongside Tip and Ring) added to telephone connections to handle internal exchange control and signaling.
An R/C circuit sometimes attached to contacts to reduce electrical arcing and improve long-term relay reliability. Snubbers were common before diodes were used to quench inductive arcing and achieve the same results.
An enclosure utilized in the author's DIY 8-line telephone system to muffle the loud clicking sounds of the electromechanical relays.
Sequence Switch.
A logical construct that advances step by step to manage call operations, heavily implemented using sequence switches in Rotary and Panel systems.
The most popular exchange architecture worldwide for roughly 90 years, functioning via progressive direct dial control where switches respond instantly to dialed pulses.Very little if any common control logic involved.
A Bell Labs mathematician and researcher who designed the first relay-based 1-bit adder in 1937 and directed the creation of the 450-relay Complex Number Computer in 1940. His inventions extended the computational capabilities of relays, contributing significantly to both early electromechanical computers and new telephone exchange designs.
The routing technique used by the Director system, taking in digits and translating them to a machine-friendly number before routing the call.
A flat coordinate switch interoperable with Strowger types, frequently utilized within Step-by-Step exchanges.
The complex, interconnected mesh of switches and trunks that are engineered to provide simultaneous talking paths for subscribers.
Repair technicians who diagnosed, cleaned, and maintained the millions of electromechanical relays and switches within exchanges.
SxSSee Step-by-Step
Introduced in 1962, this transmission system required a repeater every 6,000 feet and carried 24 voice channels over twisted pair copper wire.
Tip, Ring, and Sleeve terminals/leads.
An electromechanical device, such as a Strowger, crossbar, or relay-based mechanism, specifically designed to physically route the two, three or four wires (or more) required to create a voice connection between callers.
A hybrid routing layout used in the New York metropolitan area requiring 8,800 trunk groups to efficiently connect offices.
A generic term for a switching system (such as the Crossbar Tandem) that strictly connects trunks to trunks, without any directly attached subscriber lines.
A centralized routing layout where all local offices connect through a central Toll Office, requiring only 20 trunk groups for 10 offices.
The parallel, stationary connection points utilized in Alexander Keith's early Zither switch, which gave the device its "Piano Wire" nickname.
Telephone Collectors International.
Tandem Exchanges. It is also used as an acronym for Traffic Engineering.
The colloquial name for a portable test set utilized by craftspeople to diagnose and test panel exchange selectors on the floor.
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Invented by Thaddeus Cahill, the Telharmonium is credited as the first electromechanical musical instrument and electronic music synthesizer, utilizing a massive array of alternators to generate additive musical tones. This highly complex, 200-ton machine was designed to serve as an early streaming service, broadcasting live music directly to subscribers' homes, hotels, and theaters over standard telephone lines
The German manufacturing company that developed the unique gravity-driven Fallwähler switch for business exchanges.
A museum housing historical exhibits of G.A. Betulander's early all-relay workshop and crossbar prototypes.
Specialized equipment located in exchanges featuring rows of jacks and meters, allowing craftspeople to test the fidelity of incoming and outgoing trunks.
A manual simulation device built by Bell Labs to mathematically study the traffic-carrying characteristics and blocking probabilities of the No. 5 crossbar exchange.
The fundamental pair of copper wires that directly connect a subscriber's telephone to the central office's line relays. The terms are derived from the physical look of a telephone jack with a Tip, Ring and Sleeve (control lead) terminals.
Telephone eXchange Crossbar (noted as the more appropriate, though unused, acronym for TXK).
Table of Contents.
The hierarchical classification of long-distance switching centers in the North American network, routing calls between end offices.
A mechanical component actuated by the YM electromagnet in an Ericsson XY switch to produce vertical motion.
The modern dual-tone multi-frequency (DTMF) dialing method referenced in Evan Doorbell's recordings of Number 5 Crossbar exchanges.
The complex science of predicting call flow using statistics and queuing theory to design cost-effective exchanges with an optimal number of trunks and switches.
A mathematical measurement of traffic load indicating the average number of simultaneous conversations occurring during a specific period.
A basic logic circuit demonstrating how multiple relays can be wired in a branching structure to route electrical connections.
The crucial automatic conversion of decimal dialed digits into non-decimal routing impulses to control non-decimal switches (like the 20-horizontal position Rotary switches).
A diagnostic machine in a Crossbar office that automatically punched holes into a physical card to document the specific relays involved in a call failure.
Punched diagnostic cards generated by crossbar trouble recorders, analyzed by maintenance staff to locate specific relay and equipment faults.
Two-wire or four-wire lines that connect different telephone offices together, acting as the connective tissue to carry talking paths and signaling across switching networks.
A pioneering mathematician and computer scientist whose foundational concepts of computational logic conceptually parallel the complex, binary relay logic networks developed to route telephone calls.
Telephone eXchange Crossbar.
A "2-wire" mechanical amplifier invented by George Campbell utilizing 2 amplifier devices to renew voice signals over long distances. Very difficult to keep balanced without ringing.
U Relay / U-type relay (U-650)
Introduced in 1938 as part of the flat spring family, the U-type relay featured twin contacts mounted on a single notched spring and was extensively utilized in #1 Crossbar-type telephone exchanges.
A standalone rotary stepper switch (sometimes called a rotary selector) frequently used as a digit counter or line switch, notably popular in the British Post Office domestic system.
A specific mechanism on the first Strowger-type switches that moved the wiper horizontally 3.6 degrees for each pulse from the subscriber's units button.
Vacuum tube amplifiers (Triodes)
Devices utilizing Audion technology that enabled early transcontinental telephone calls to reach beyond the 800-mile unamplified limit.
The capacity of carrier systems (such as the T-1, J-1, and L-1) to multiplex multiple simultaneous conversations over a single transmission line, requiring frequent signal repeating.
Co-author of the comprehensive 1914 treatise "Automatic Telephony," dedicated to Alexander E. Keith.
Alexander Graham Bell's most able assistant who built the first practical telephones and invented the telephone bell (ringer) in 1878 to replace crude, early signaling methods.
An early signaling device using an induction coil and flat steel spring that sent electrical impulses to make a called telephone violently "howl".
A crude early signaling mechanism rigged inside a box telephone that struck the side of the diaphragm to produce a thumping sound at the called end.
Western Electric Company. The manufacturing arm of AT&T.
An English electrical engineer who constructed the first practical electromagnet in 1825, able to support 9 pounds using a single battery cell.
A business associate of Alexander G. Bell and Thomas Watson who used the butt end of a lead pencil to "thump" on the diaphragm of his phone to signal an incoming call.
A highly reliable relay introduced around 1952 featuring completely independent twin contacts mounted on separate wires, significantly reducing dirt failures compared to older flat spring relays.
Wire Spring.
A 2-dimensional switch matrix, synonymous with the Crossbar switch, enabling connections at every intersection of its horizontal and vertical bars.
Short for a Crossbar switch.
The specific electromagnets in an Ericsson XY switch responsible for moving the mechanism's carriage in the X (horizontal) and Y (vertical) directions.
A flat coordinate switch utilized in some Step-by-Step systems, recognized alongside the Strowger mechanism as a key progressive direct dial control switch.
An early telephone switch invented by A. E. Keith, introduced in La Porte in 1894, whose shaft motion mechanics were later repurposed for the highly successful "Keith's Strowger Switch".
The builder of the Z2 and Z3 in Berlin, recognized as the world's first programmable relay-based digital computers.