Strowger System
Almon B. Strowger in his lateryears, fram "Telephony" tor October 15. 1949.
the central office for each subscriber line.The switch consisted essentially of a hollowcylinder with a shaft capable of both vertical and rotary motjon. This shaft carried awiper arm capable of wiping over all thehorizontal rows of contacts on the inside ofthe cylinder. All of the subscriber lines weremultipled to the rows of terminals aroundthe cylinder of every switch, while one ofthe subscriber lines was connected to thewiper of each switch. Strowger did not specify in his patent the maximum number oflilies which could 'be terminated in hisswitch. but in a circular issued in April,1891, he claimed that it would "connect telephones in aIsystem from two to ten thousand." In describing its method of operation,however, most writers have assumed tenhorizontal rows of 100 contacts each. (Eventhis number would have required a switchprohibitive in size and cost.) The verticaland rotary motions of the shaft were controlled by electromagnets actoated by impulses sent from the subscriber station. Fourpush buttons on the subscriber telephonebox each controlled a wire to the centraloffice, while the fifth wire was for talking.
To call the number Bl5, Strowger statesin his patent, the subscriber pressed his"hundreds" button (d) three times, thuslifting the shaft of his switch three notchesand bringing the wiper opposite the third ,row of terminals. He then pressed his "tens"
95
R. B. HILLGeneral Staff
of the
The Early Years
Some of the early attempts to devise antomatic telephone systems were describedbriefly inc a previons article", and it wasstated that, while some of the early inventions embodied ideas that were later devel- 'oped into important featores9f the modernautomatic art, none of the systems themselves went into commercial use. It remainedfor Almon B. Strowger, an undertak~r ofKansas City, Mo., to devise the first automatic telephone system to be used commercially. The important featore of this invention was that of imparting to the switch m;m£lrst a vertical movement, and then' a rotarymotion in a horizontal plane. Strowger is saidto have constructed the first crude model ofhis switch from a cardhoard collar box and apaper of pins. He stock the pins from theoutside of the box toward the center untilhe had ten horizontal rows of ten pins each,one row above the other, to represent tbeline terminals. The vertical shaft for carrying the switch arm was represented by alead pencil through the center of the box.
Strowger filed his patent application onMarch 12, 1889, and it was issued on March10, 1891 as patent No. 447,918. The mechanism of Strowger's patent is shown in Figure 1. While this particular arrangementwas never used commercially, it illustratesthe inventor's original ideas. As may beseen, an individual switch was required at
RECORD, January, 1955, page 22.
March, 1953loratories Record
.panies are restomers regfird,-feature, some
hods of piovid,ns and the speareas post cards,hen the serviceLOges occur. In.s are ,adequate.ate what,opel'a'ormation.nst "listen to allitY of the eight'ach subscriber,e teti:party sysriber hears £lve,'ide a 'ten-party,bscriber wouldrequire an addilis was found to
hipstre designed toch in the fundaSince they were{Ships nave beentnt $4,000 to the.hip and $1,500
"""ducted creativelading centers of"pient being freeit which he willJps are awarded)ommittee of scistaff of Bell Labare the research
1e importance ofd the likelihoodist. The ,awardshave recently rel' who are about
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button (H') once, which gave a single rotation to a 10-toothed ratchet wheel (E') attached to the shaft and moved the wiperarm to 310. Pressing the "units" button (I')!lve times operated a 100-toothed ratchetwheel (E) attached to the shaft and movedthe wiper arm five more points to 315.
Ringing was done with a magneto or battery. When through talking, the calling subscriber pressed the (p') button, which energized the release magnets and brought theshaft and wiper arm back to their normalpositions. The use of five wires and groundwas, of course, an impractical feature, butStrowger planned to later reduce the number of wires. A heavy battery was to be employed at each sub-station for operating thecentral office switch. There was nO provisionagainst the calling subscriber being connected to a busy liue.
The first company was incorporated onOctober 30, 1891, under the name of theStrowger Automatic Telephone Exchange.Early in 1892, A. E. Keith, who had previously been employed by' the Brush Electric Company, of Baltimore, entered theservice of the Strowger company and wasfor many years thereafter one of its leadingtechnicians and inventors.
On November 3, 1892, the first Strowgerexchange was opened for public service atLa Porte, Ind., with about seventy-five subscribers. This was the first automatic telephone exchange to be installed anywhere,and a considerable amount of ceremony wasattached to the affair, with a special trainrun from Chicago and a brass band on handto greet the guests. Since this system wasdesigned for less than 100 lines, there wasno need for a switch with a two-way movement, and a flat rubber disc type of switchwas employed, with but one (rotary) movement for the wiper arm and one circular rowof contacts. There were still five line wires,as in Strowger's patent, aud each telephoue.was equipped with four push buttons, although the hundreds button was not used.The battery for operating the switches wasnow located at the central office. There wasstill no provision against a subscriber beingconnected .to a busy line. The systemworked with reasonable accuracy wh'Jn thesubscribers operated their push buttons correctly and remembered to press the release
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button after a conversation was finished.In 1893, a small Strowger equipment was
exhibited at the Chicago World's Fair.About the beginning of the year 1894,
Frank A. Lundquist and the Erickson brothers, John and Charles J., entered the serviceof the Strowger company, and, with A. E.Keith, started work on an improved systememploying the so-called "piano wire" terminal banks with a Hat-faced multiple anda Switch arm capable of both a rotary and alongitudinal movement. Switches of this
.type, which were covered by patent No.540,168, were installed at La Porte, Ind., inthe fall of 1894 to replace the original equipment, and at Michigan City, Ind., in the sameyear. These new installations employed asimple form of busy signal. When the calling subscriber rang, both bells respondedif the called line was free. If it was busy,the calling subscriber bell remained silent.They also utilized the !lrst automatic release arrangement, invented by A. E. Keithin 1893 and covered by patent No. 573,884,embodying a mercury switch connected withthe subscriber switehhook, which restoredthe central office switch to its normal position upon the hanging up of the receiver.
The "piano wire" form of switch did notprove successful, and in 1895 a return wasmade to the principle of Strowger's originalpatent. Patent No. 638,249, issued to A. E.Keith and the Erickson brothers, covered aswitch which more nearly resembled themodem step-by-step switch. Twowews ofit in the patent are shown in Figure 2. Itemployed semi-circular banks and the familiar "up-and-around" motion of the selectorrod and wipers. One-hundred-line switchesof this type replaced the "piano wire"switches at La Porte in June, 1895. In August, 1895,' a similar system, with 200-line,"witches, was installed at Michigan City.
At this time, the Strowger system em,ployed., betw'een a subscriber's premises andthe central office, two hard drawn copperline wires, and a soft copper battery wirewhich branched into all the stations to supply signaling current. This arrangement continued in use until about the year 1900. .
In the summer of 1896, an important forward step was made with the introductionof the fuiger-wheel dial to replace the pushbuttons. It was covered by patent No. 597,-
Bell Laboratories RecordFig. l-:Strowger',
( March, 1953
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was finished.:quipment was_old's Fair.he year 1894,:rickson brothred the serviceld, with A. E.proved systemlilO wire" ter1 multiple anda rotary and aitches of thisby patent No.Porte, Ind.,ift
original equip"ld.) in the sam~IS employed aWhen the calIells responded[f it was busy,~mained silent.automatic re
by A. E. Keithat No. 573,884,connected withwhich restoredts normal posithe receiver.switch did not5 a retnrn wasIwger's originalissued to A. E.hers~ covered, aresembled theTwo views of
in Fignre' 2. Its and the familof the selector
·d-line switchescCpiano w:fre~~
" 1895. In Aug, with 200-lineHchigan City.;er system ernIS premises anddrawn· copper
or battery wirestations to suprangement conle year 1900.I important forile introductionlplace the push,.tent No. 597,-
"atories Record
Fig. I-Strowger'. firet automatic telephoM syetem, De dleclo.ed In hie patent No. 447,918.
March, 1953 97
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Fig. 2-Front and side "iews of the first Strowger switch to bear a general resembUzlUJe $0 themodern step.by.step switch., '
98 Bell Laboratories Record
062, issued t.o Jbrothers. As "this first formholds mountedble circular piaup a spring whwas withdrawnits normal positlimited to a m,ment ~echanisthe required nutook place to ccentral office aJ
In the fall ofStrowger Comphis health. He tdevelopment 01name, and he (ida, in May, 19
It was alsoA. E. Keith ,started work oning the trtmkiJorder to remOVEan automatic mcessity of multlines to each sw:of course, but nhad produced ,commercial USE
switches whichlines would eacllines, and enou!switches wouldvarious hundre<pr~ceded by aThe Selector wo
digitthe call to a canthe latter woul,units digits andTwo switches Vi
but they might hth"n a single la
The Keith.EIsysh,m, coveredof this characteStrowger exchanin March, 1897,this installation 1of terminals at (which it could
o RECORD, Janool
March, 1953
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Fig. 3-1';ir8t form of Strowger finger-wheel substation dial, which replaced the push buttonspreviously used.
rotating the selector were provided by thecalling subscriber, through the device of inserting a dummy "0" in the directory number following the hundreds digit. The selector was desigoed to rotate in response tothe pulses of the dummy digit until an idletrunk was encountered, after which the remaining pulses were disregarded. This planwas soon superseded by one in which thepulses were furnished by a continuously operating interrupter in the central office. Theplan could be extended to exchanges of10,000 lines by inserting another stage ofselectors in the train.
group of selectors had,.only one trunk toeach group of connectors. Because of the inefficiency of these one-trunk groups, thesystem although operable was not commercially practicable.
In the summer of 1897, however, theStrowger engineers started work on theproblem of a 1000.line truriking system em·ploying automatic trunk selection. Underthis plan the selector would have severalsets of contacts in each row, leading to theseveral connectors serving a group of 100lines, and would be arranged to connectwith the first idle trunk it found in rotatingover these contacts. In the first arrangementdesigoed for this purpose, the pulses for
062, issued to A. E. Keith aud the Ericksoubrothers. As will be seen from Figure 3,this first fopu employec;! numbered fingerholds mounted on the periphery of a rotata_
C
ble circular plate. Dialing a number woundup a spriog whose tension, when the fingerwas withdrawn, caused the dial to return toits normal position. The return rotation waslimited to a moderate speed by an escape"ment mechanism, and, during the return,the required number of circuit interruptionstook place to control the movement of thecentral office apparat,!s.
In the fall of 1896, A. B. Strowger left theStrowger Company and went to Florida forbis health. He took no part thereafter in thedevelopment of the system which bore hisname, and he died in St. Petersburg, Florida, in May; 1902. .
It was also during the year 1896 that'A. E. Keith and the Erickson brothersstarted work on a l,OOO-line system employing the trunking or transfer principle inbrder to remove the limitation on the size ofan automatic exchange imposed by the necessity of multipling all of the subscriberlines to each switch. The idea was not new' ,of course, but none of the previous workershad produced an arrangement suitable forcommercial use. Under such a plan theswitches which make connection to calledlines would each be limited to reaching 100lines, and enough groups of such connectorswitches would be provided to serve thevarious hundreds of lines. They would bepreceded by a stage of selector switches.The selector would respond to the initial orhundreds digit of the nUIflber and' extendthe call to a connector in the proper group;the latter would respond to the tens andunits digits and conuect to the desired line.Two switches would be used on each call,but they might be made smaller and cheaperthan a single large switch. .
The Keith-Erickson I,OOO~line trunkingsystem, covered by patent No. 672,942 wasof this character. It was employed in theStrowger exchange installed at Augusta, Ga.,'in March, 1897, to serve some 400 lines. Inthis installation the selector had only one setof terminals at each of the ten positions towhich it could be dialed, and thus each
• REcORD, January, 1953, page 22..
March, 1953oratories Record
~semblance to the
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Incidentally, after protracted Patent Of·fice interference proceedings, the patent(No. 776,524) covering the antomatic selection of an idle trunk was awarded toFrank A. Lundquist, who had left theStrowger Company in 1896.
The Strowger exchange at New Bedford,Mass. - the first automatic exchange to emplaya practicable form of trunking system was opened for service in November, 1900.It employed first and secqnd selectors, andwas equipped for some 3,600 lines.
Other improvements were also made intbis installation. The common battery supply wire was discarded, only the two linewires being carried back to the central office.However, the telepbone had a third wireconnected to ground, which served as acommon return, the call being made bygrounding one line or the other the required
Fig. 4-Strowger wall set with finger-wheel dialabout the year 1902. .
lOO
number of times. For this reason, this arrangement, which was standard until 1907,was usually referred to as a three-wire system. Local batteries still furnished the talki11g currentl and the subscriber rang byqperating a push button which connecteda central office ringing generator into thecircuit.
In September, 1901, a Strowger exchangevery similar to the New Bedford installationwas opened for service at Fall River, Mass.,with about 400 subscribers.
During the year 1901, the Automatic Electric Company was organized and acquiredthe United States rights to manufacture andsell Strowger equipment. The Strowger Automatic Exchange remained only a patentholding company until 1908, when its stockwas acquired by Automatic Electric.
The New Bedford and Fall River installations may be said to mark the beginning ofthe work which eventually led to the development of a Strowger system suitableJoruse in large single-office cities. There werestill many improvements to be made, themost important of which were: bridg;ngtheswitch-controlling relays across the line, instead of placing them in series with theline; commOn battery talking; automaticringing; the provision of practical forms ofmeasured rate and selective party-line service; means for eliminating the need of anexpensive first selector for each subscriberline; and the replacement of the three-wire(two-wire and ground) system with astraight two-wire system. Until these improvements were introduced, there was noprospect of the Strowger system competingon anything like even terms with the manualcommon battery system.
What some of the iipparatus of the Automatic Electric Company looked like at thisperiod (about the year 1902) is shown inaccompanying illustrations. Figure 4 showsa wall set equipp!"d with a finger-wheel dial.Figure 5 shows fropt arid side views of theselector SWitch; of Which there was one foreach subscriber line. In the upper half werethe electromagnets and their associatedequipment. The lower.half consisted ofthree banks of terminals, With a wiper foreach bank. The upper, or "busy," bank wasused to show whether a trunk was bury, andthe two lower, or Une," banks provided the
Bell Laboratories Record
talking conne(played first SE
four-digit sysisecond select<'t.er were veryselectors. Latmade it possil
'terminals intosuIted in a. sh,
After the Fexchanges we:trlct of Chie<.about 1,500 suJuly 1903, wiGrand Rapids5,300 subscribThe Chicago i
March, 1953
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Fig. 5-Front and Me 'views of Strowger selector switch a,pout the year 1902,
ger exchange to furnish any type of message rate service, the first selectors being
. equipped with a commercial form of meter(Veeder meters). These did not register abusy call, but did register all completedcalls, whether answered or not. The Daytonexchartge was the Brst in which the controlling magnets were bridged across the line,instead of being in series with it, thus effecting a considerable improvement in transmission characteristics.
The first of several Strowger branch offlees, designed to connect with and supplement the Main (independent) manual· exchange at Los Angeles, was completed inJuly, 1904. Several years later, the Mainexchange was cut over to dial operation.
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talking connections, 1'wee-digitsystemsem~ployed first selectors artd connectors, whilefour-digit systems employed first selectors,second' selectors and conrtectors. The latter were very similar in appearance to theselectors. Later~ mechanical improvementsmade it possible to incorporate the 200 lineterminals into a single line bank, and resulted in a shorter two-bank switch.
After the Fall River exchange, Strowgerexchanges were installed in the "Loop" district of Chicago in February, 1903, withabout 1,500 subscribers; at Dayton, Ohio, inJuly 1903, with 1,300 subscribers;' and atGrand Rapids, Mich., in January 1904, with5,300 subscribers, replacing a manual board.The Chicago installation was the Brst Strow-
March, 1953
) reaso:h~ this ar~
~dard until 1907,a three-wire syslrnished the talk)~criber rang bywhich connected;merator into the
:rowger excb;mge,dford installationFall River, Mass.,s.e Automatic Elec'ed and acquiredmanufacture andfhe Strowger Au.ed only a patent)8, when its stocklc Electric.call River installa-: the beginning ofI led to the (level'stem suitable for,ities. There wereto be made, the
vere: bridging.theICroSS the line, inII series with the,lking; automaticpractical forins ofve party-line serv19 the need' qf anlr each subscribert of the three-wireI system with aI. Until these im.ced, there was nosystem competing~s with the manual
tratus of the Autolooked like at this1902) is shown inlS. Figure 4 shows1 finger-whe"l dial.I side views of thethere was one fori
be upper half )Vere,:I their associatedhalf consisted of
:, with a wiper forr ''busy," bank wasrunk was busy, andbanks prOVided the
moratorie. Record
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Fig. 6-Early form of Keith line switch.
The first Strowger exchange with commonhattery talking was installed at South Bend,Ind., in May, 1905, to operate jointly with amanual common battery board in the samebuilding. It served about 700 subscribers.With the advent of commOn battery operation, it became possible to employ automatic(machine) ringing, whereby the. called subscriber bell was rung automatically as soonas the connection was made to his line. Common battery operation and machine ringingwere, of course, old in manual switchhoardpractice at this time. The Hayes repeatingcoil system of common battery supply hailbeen developed by the Bell System in the1890's and was widely used in their exchanges, while the Hibbard machine ringingsystem had been standard in Bell exchangesfor many years. The early Strowger common battery exchanges used a bridged impedance type of circuit.
An arrangement aimed at convenienceand economy in the grouping of the apparatus was covered by A. E. Keith's patent No.831,876, filed on March 9, 1905. It called forten rows of ~~n frames each, every framecontaining the switches for serving 100 subscribers, and consisting of 100 first selectors,
102
10 second selectors aJ;ld 10 connectors. Thisarrangl"ment provided f9r 10,000 lines, eachrow serving a particular 1,000 lines. Thispatent.. also covered the important featureknown as "bank, slip," wherein the trunkmultiples are wired or "slipped" in such away that if ten selectors start at the sametime to test the same trunk group, they willall at their first step test a different trunk.This arrangement shortened the time necessary for a selector to find an idle trunk, reducecj,'tl'le possibility of two selectors seizingthe same trunk, and equalized the wear onthe switches.
About the year 1905, the Automatic Electric Company dev~loped a four-party selective signaling system for use with their common battery equipment. This feature, ofcourse, was also 014 in Bell System manualexchanges, the Hibbard, system, employingoppositely biased polari>;ed bells~ havingbeen in use for many years. The Strowgerexchanges used harmonic ringing,
The earliest Strowger exchanges had allbeen eqUipped with a first selector for eachsubscriber line, and this practice had continued when the larger exchanges, employing trunking, had come into use. It involved
Bell Laboratories Record
a g~'eat wastete}' per centbe in use atalimitation onsystem was r€No. 1,304,324plunger type,issued until 11
rangement, ,around the yc90 per centeach subserilswitch, muchfirst selector, 'his,Iine autdrnas soon as hehook. The firinitial dialingnero This limconsisted essecarrying on iiwhen actuatemagnet, pres!that connectelten trunks Iehundred linechanically bywhich kept tIswitches direc
Early in 19(twocwire systceliminated, wawas accomplisOr siow-releasgized by a cmfor a short pelits winding h,Wire system, tIing' the line a
THE AUTHOR,. d~gree from Hay. tered the Engin'
can Telephone III
of that year. FOiengaged. principastudies. 'When tand Research wato it; and since fhad heen largel)economic phasesHe had been a rphone Lahoratori,Plant DevelopmeStaff ·Deparbnent
March, 1953
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a great waste of equipment, since only about·' stead of grounding one side of it. The vernten per cent of the. first selectors wo~ld cal and rotary relays of the previous threebe in use at any: one time. This very serious wire system were replaced, respectively, bylimitation on the economy of the Strowger a qUick-acting relay and a sluggish relay.system was removed by A. E. Keith's patent When a series of pulses (one dialing operaNo. 1,304,324, covering a line switch of the tion) had been received, the pause before aplunger type, filed on April 29, 1905, but not second series could be sent in allowed theissued until many years later. With this ar- sluggish relay, which was held up duringrangement, which' went into general use the dialing, to falloff, giving the signal foraround the year 1907, and eliminated about the switch arm to rotate and choose an idle90 per cent of the first selector switches, trunk. The two-wire system sinlplified theeach subscriber line terminated in a line central office wiring, elinlinated, during dialswitch, much smaller and cheaper than a ing, the disturbing influences due to differfirst selector, whose function was to connect ences in earth potential, and permitted ahis line automatically to an idle first selector considerable simplification of the substationas soon as he removed his receiver from the dial itself.hook. The first selector then received the With the two-wire common battery sysinitial dialing impulses in the usual man- tern, provision was made for measured servner. This line switch, shown in Figure 6, ice by employing a differential or polarizedconsisted essentially of a magnet and a lever service meter associated with the line switchcarrying on its end a plunger. The laUer, and operated by the reversal of the currentwhen actuated through the lever by the through the line of the calling subscribermagnet, pressed together certain springs at the time the called subscriber answeredthat connected the subscriber line to one of his telephone.ten trunks leading to first selectors. One The advent of the two-wire system, withhundred line switches were controlled me- its basic simplification of the method of sigchanieally by a cornman "master switch," naling from the station to the central office,which kept the plungets of the idle line marks the beginning of the modern phase ofswitches directed toward idle trunks. the Strowger (or "step-by-step") system.
Early in 1908, the first Strbwger straight The engineers of the Automatic Electrictwo'wire system, in which the ground was Company continued to make improvementseliminated, was installed at Pontiac, Ill. This in the step-by-step system, while the Bellwas accomplished by the use of a sluggish, engineers developed the panel type of rnaor slow-release, relay which, when ener- chine switching system. These parallel degized by a current, held its contacts closed v.clopments and the far-reaching effects offor a short period after the circuit through World War I led to the adoption by the Bellits winding had been broken. In the two- System after the war of step-by-step equipwire system, the pulses were sent by open- ment for small and medium size cities anding the line a certain number of times, in- panel in the largest ones.
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THE AUTHOR: ROGER B. HILL received a B.S.degree from Harvard University in 1911 and entered the EngineeTing Department of the American Telephone and Telegraph Company in Augustof that year. For several years thereafter he wasengaged principally in appr,aisal and depreciationstudies. When the Departt;hent of Dev~Iopment
and Research was formed in 1919, he transferredto it, and since then, until his retirement in 1951,had been largely concerned -with studies of theeconomic phases of .development and operation.He had been a member of the staff of Bell Telephone Laboratories since 1934. first in the OutsidePlant Development Department and later in theStaff Deparpnent.
March, 1953)ratories Record
i\.utomatic Elec:our-party selec,with their comLhis feature, ofSystem manual
item, employingd bells, havings. The Strowgerringing.changes had allselector for eachractice had conhanges,employI use. It involved
ounectors. This1,000 lines, each000 lines. Thisportant feature,rein the trunk,ped" in such ant at the samesroup, they willdifferent trunk.the time neces
1 idle trunk, reselectors seizing~ed the wear on
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