Reference / Paper · 1921
Automatic Telephone Systems, Vol. 1
British treatise on automatic systems (Strowger) circuits and apparatus.
Reference / Paper · 1921
British treatise on automatic systems (Strowger) circuits and apparatus.
AUTOMATIC TELEPHONE SYSTEMS i in WILIJAM AITKKN, M.l.H H., A.Am.l.J:.E. M ] IK l|; I II •• \l I Is l,\ > Ml- \ CIRCUITS IN M SM \l. <11 I II I ri I l.l’rtoM. ONi: AND APPARATUS AS USED THE PUBLIC SERVICES r , UN DON BENN BROTHERS LIMITED “THE El.ECrPKMAN” OEFICIiS, H, HOIA’EKIE STHEE'J', 1921 E.C. 4 ^ I < m ^ ' i[ld\ (S\ .Wl .!•:: \ri’ PREFACE Thr suhjrcl of Automatic 'l\‘lc|)h<»ny r(‘(|uircs wvy spt'cial trjMtmiMit, owiiifi; to tho is built, and the complicated circuits diaiLirams necessary infinite mass (jf del ail from which it to illustrate ^ it. In tlie past the diagrams ha\(‘ nniifU^lligihle wJkmi ( ithcr Ihm'H fra»>nH‘ntary or so rcdiic(‘d made suitahh^ for a book of ordinary sizt‘. \i\ siz(‘ as to Ix^ tA en wluai fohha* she(‘ts hWve' Inxm used. no. or oul\ slmht. att(“m])t has Ix'cn mad(‘ to simplify tla* diagrams, th(*se, in most cases, b(an;» r<‘|)rodu(‘t ions of t he manufact urers' diajirams. th(‘ expert, or one familiar with tlu‘ s\st(‘m. 'Tlu^ ]att(‘i‘ are liood for but an* r^i’cly suitabl(‘ lor the st ud(Mi1 or learner. This l)ook Jias foi* its ol',j(‘ei,s tlu^ making of this most comph x subj(‘ct intc'lli^ibh^ to any on(‘ havin<i soiiu' (‘lectrical know h‘djjj(‘. and that, not by writing (h^wn to the learner, or ^ivinc scrap diagrams for (‘ach f(‘atui’(‘, but by reairanging th(‘ <liag?ams eliminating unn(‘c(‘ssary crossing lines, simplifying the form and b\ disaling lh(‘m into s(M‘tions vvlnai nect‘ssary. or continuing tlu*m s<‘ction by s(‘cfion. in such a way as to show tlu‘ r(‘lationship and i?itej‘eomic‘ction t>f the s\st(‘m as a vvhoh*. Thj'^ result is greatly facilitat(‘d by a new UH'thod of d(‘scri))ing the diagrams, vvJiich (‘onsists in numb(‘ringa circuit from end t<) end with the satm* symbol, so that eac‘h eircuil, can ))e readih’ tracc*(l. '^Fliis tm^lhod was designc'd by the Author some Nc‘ars agc^. and lias been used in Patent Specilications. but to the best of his knowh‘dgc hav not Ixxmi uscrxl in lxx)k This aja’angcmicnt has been calh‘d /e/.s- vonthnj. as it r(‘sc‘mblcs the bus route London to show' tlu‘ I'outc* takcMi by a bus from terminus to ter'Idi(‘ circuits arc‘ taken 'I'his device' also allows of a simpler and bried'er dcsei ipt ion. foi’iii. minibei's used in ma])s of minus, intlu ordei’ of opcM’at ion and mimbeted progressively, so t hat with only cx-casional j’ch'i’CMicc' to the* t<‘\t. t lu' c'xpc'rt cjin j;(‘ad the diagram 'Fhc student will follow the t(‘\t more* carefully, tind the* ]'casoii for the* jicculiai it ics <»f t lie circuit, and tlu‘ reasons foi' th(‘ [)articular device's or ])arts. and their fmudions. This melliod of d(‘signating circuits makes unnec(‘s>aiy th(‘ sciap. <a‘ oiu' fc'aturi*. diagram. Thc'sc' are sometimes mish'ading as they eliminate the associated circuits and show the fealui'c' out of jxa’spect i\ <•. and thcM'C'by exit of |)roporlion, b(‘ing analogous to (pioting or extracting a pai-agraph or sentence' from written mattea*. aiid losing the* uiC'aning it ])ad wheai revul with the' conte*xt. The inc'rease'd size of the' page allows of the' diagrams Ix'ing drawn to a comj)araiively lai'ge scale', as j)ossible. so that they can Ix^ read with c'ast', For loiig diagrams, showing a Folch'r sheets have' systi'in from e'lid tc) he'C'n (‘liminated as far end, where* ]X)ssib]e the diagram htjs bex'ii diviTh'd into sex-tions. The breaks in the* continuation line's have their ends marked \jith the^'e-irciiit nimiber, so that the gaj) can be bridgc'd jc'adily. PREFACE Folck'is used wiicn lln* circuil iir(‘ (liaj^rani is unsuitahlo for division, and such sheets oidy fold in on(‘ diic'C'lion. The* I'clays ai(‘ drawn with the contacts away from the* windinj^ in contacts ar(‘ always kept in lim‘ many cases, hut the This j)revents the with the' c‘ore or ccMCtrc* of the* winjin^. confusing ciossinj; of linens that freepa-nt ly causcss a diagram t^ look more* complicatc'd than it The ^h<^l‘t donl»lc lin(‘. ca* single tliick line, is always t4u‘ moving spring that i(‘ally is. The moving spring thcai Un^aks from a energised. when tla* coil lie, lays slow lo^ de-energis(‘ ))v and makes with a front one, and the like. ing of cojiper on iht‘ core arc* sh<n\ n w it 1 lu^Jower part of 4 he eon* sh having a massiv Ih*la\s and magnets aiv d(*^ignat(*d hv the* cii'cnit symlk)!. which is prc'tixc'd hy thc^ letter- W oi M |(‘spect i\ el \ Tin* mannfact nr(*rs' d(‘signat ion.s arc* also givc'n in the* more traveds towards tla‘ coil hack (‘(intact, (‘ l li important diagra ins the ot Juc'iits t he* Use ot condc'iised mc'thods standaid or c‘ommc*rcial jMincipal the dc‘tail which Ohxioiisly the whole siihject of Automatic T(*li*|)hon\ piacticallx 'The* lic‘ld (‘ovc‘i‘c*d only made* pract icahle h\ is common to all and this is systc'ius ai‘c* ilhistratc*d and dc‘serihc‘(l. not sh(n\n. hut repi-es(*ntat i\ c* (*\am])i(^s are l'^ m dc'tail on (Uh* s\stc*m rind oth(‘i‘ features in mcSi’c* from (*ach. and some (halt with takc'ii . 'The im))oi tant circuits and arrange- detail on ot her systems Particulars arc* .dso gi\en ol ^ysicgns which are not so wc'll promise* that will Ik* intcM'sting also to 'ldi(*sc* ol rc'asonahh* to heliexc* in is it the studc'iit ot W(‘ll the future* the\ known, hut which gix’e such may lind a place* in juihlic iix*. or inv(*ntor w ho desiri's to (‘ntc*i‘ into tlie spirit automatic design .\rrangement s ,ind dc\ icc*s hecn designed to serx'iee. arc* also known a till known manntactnr(*rs and mxentors. r('(pm c'lnent hut h(*c'n \('t whi(’h haxc* introduced into pnhlii* show n ndec'd. the hook in addit ion to descrdiing I have not t he wc‘ll-know n s\ stc'ius cl(‘.irl\ and sueeinetlx also amis at indi(*ating the po'^sihle tr(*nd of future* (1(“\ c*lopment s is felt ft hy the Author th.it the* sulpect of Automatic 1'c‘lephon\ condition to dax th.in at an\ period ol its c‘\istc*ne(’. It is i* in a more* mistahle state of llii\, and the* ([iu*stion in a The* \\c*st(*rn Kle-etne must he* jiost ponc'd loi* sc*veral \ears. at lc*ast of “standardisation sX'^lc'in composc*(l of aOU line* •^witehc*s. eh.dlenge the world lh.-it (d.. with its " Pane'l that the* onl\' efiieie'ut is (|(*signers ynust rri'sH.s t rattie-eari x ing small switches, with e»r witlieait d’elephon\' has mc'dium foi’ the gic*at eilie-s of the world. ()th(‘i that ehallc'iige. and we* have the* battle* of large and e*\pc*nsive swilehe*^ i;ic*et '^pc*c*ial wiling sehc'ines. through twe* great phases and is (*nt(*!-ing the* sc'cond c‘omme>n hattc'ry with lamp magnc*lo and local l)attc*ry woiking alrc*a(l\ pa''St' 1 sc*elion of a third, signalling, and Automatic or machine switching, x i/. , . the* two formc*r heing manual systc'iiis 'The* first sc*ction and the* latter c*liminating the* hi man fac'tor c*\cept for s)K*eial serx ic(*s. of he 1 hire! phase is he* use* of tMt and -tM) ijne sw itches, he forme*i‘ h(*ing a dc*eimal syst(*m and the* laltc*r a non cl(‘cimal .systc'im ddu* second sc*ction is the* accc*ss to vc'iy large* groujis, t t r.c/., I t I line's Owing lo the incessant change, the c*ontinuous adv(*nt of nc*w dex c'lojimc'nt felt nec'esKHVy that a nc*w hook on this suhjc'C't, that aims at heing ke'jit . it has hc*(‘n up to elate*, must have spc'cial faeilifies for eliiiiinating ol)sol(^te ma1?tt*r, foi- putting ohso|(*seent fe‘ature*s in tliex whole*, and for readily adding new developnu'nts as th(*y their proper relationship to apj>ear. Instead of the usual (^hapfei\s the* heaik is divided into short ^A st*etioii J‘IIEFACK is ni(*r(*lv^ vii |jir^e>, and may doeribo one fealine may readily be replaced by new. if it should a division A\hicli ina\ contain one or several or a system. It will contain such maU(‘r tli^t become of littl(‘ value or la* abandoned. Th(‘ Author dc'^sir(‘s to exjax'ss his ii|<lebtedn(‘NS and thanks to idl thos(‘ w ho hav(‘ assist tal him by furnishing- intere.stin^ luattei* part i(MiIarly to Sii* W illiam Sliniio. late Kn<^ine('r in (hief, (r.JVO. Sir William Xoble. Ahauber of (ouneil. Institution «)f Kh'cti’ieal h^njiineeis, f]n^iiK‘(‘i' indjiii'f, (bP.O. o Aulomaiiit Khadi-ie Co. the Ollieials of \Vest(‘rn Kleetrie Automatic Tel('j)hone Manujaeturinu Co.. Siem<‘ns lUotheis A Co.. Ib'la\ Automatic d'clco.. Co\ entry Automatic Tch'phoncs. Ltd., and man\ IricmU w ho hav(‘ civ(‘n valuable ])F\()ne . : ; ( assisfanc(‘. ( . ‘ OUTLINE OF CONTENTS OF VOL. 11. (IN THE PRESS) KoUIPMKAT in Sr i^s< HiiiiHkS' () I'l’K E.*^. l*vi{TV-Li\E WoKK/NC. in» coimcctor -'^Kw n-parl V circiiil FnMpmiK'v With liiu's Ixiaids Mulli-paviv cimiit Fotir-pnrly <*in*ui( Kevorliii^ calls Discnmiiiatin.i^ SriiSi KiitEus" r<n; Oi i n i:s. Small 4*nnl l>i)ar<l.< I \s (*ii1 i‘(*(|ue*iK*y .soUtI* Look-tmt y dive* t<» sivty-litu*, manual sysl<‘ms llrauch uni<•r^ with dirucl aml])aiiy Disliici (itlicfs mi SupmviMou ol a k’fa in i(m.\ii(»N lo M vi\ Okkk k. otlicp. lioval cmiupct loU'- \M(h<ui( ulilisiujir trunks |o SxTm.i.rn. Oi <u: 1< sclcclor l>y F()i(ll<*ss Ixtatilh. liiam-li ami <illn*r .small ollico.s ii*MK\r FOR Sm\ll Oi'i H'ES. Fnmmumty olliccs Hraiich olhcrs m multi oIIum* .s\s(ciu Imps. lliLwrrt inslriiiiu*nis ol pliirjilily Me'lliods oi' .s^^•|lallill;;. <‘ircuit Smam- Swri ('ll ro \ i:i>s 1 Simple* c‘\l<‘iision Wilh <*all iim'Ipi*. W’iUi coin swilrhc.N main olli<‘(‘ i hranch ollice* at ihr mam Su'imms* manual to autinnatic A. 'V. '•!. t'o. manual to automatn* worKiii;; workinu - W. I’.. Co. M*nn-autoniat n* s\.stpin Snmn ns tralln- dist iiluit m,n sN.stmn Fh'inont. auto-manual svstem Automatic to manual otlicp jllin'tion.s ('his<‘d hraiicli olline* on a inanual liomson s I’all imlii’alor .Mixed services m automatic and s(‘ini automat le s\.''t(mi'> NVstem Skmi-\i idMVTir S\siEMS. I A. Si'F.i I 'I'. M. ol „ A. Iietulandei lelay im|ml.so «on\<*il(‘r .Method'- ol re|a‘atiim nnpul.'-es. Imi*i U'lNi. \i. call indicator. 'o. < |)i',\i< !•>. impulsi's InikuksIINC An om \ I le Swi'irii l)r.si<;N. 'I'. M. t’o. ci|ualisin'i duration 'rhn^e-movemumt AtlO-lnn* switch Interseet inji ha,r s\\ itedi Keith FOOO-hne 'IA\o dicit \ertieal and ndarv .seh*cior 'IAno hundr(‘d hin* St i ow j^er 'amneeMor SI liiw cm' mull i- \\ ip(‘r conin‘ctor {nnu'ea'-es si/,e ol .--y.'tem \\ithout adelitionul dial i'onneclor croup larce and lim*movenuMit) \'ertical rise and horizontal proje<-tion switeh 11 iilt man''hunl inc s\\ itch. on K rois rKuisi; or F\ils. H\ increase ol <*urrent 're'lephone unit and l’etticr(‘\\ s nn*1hod A. 'I' M. t'o. V. K. t'o.) method. Mirii.niNi; ].;iidlaw, (Irinstead { d'ni NK on 'roM -i.iNi: Workini;. on telephone \. '\\ M. t'o. <*in*uit Supe'ninpo.scd oi phantom wmkinc 'reh*jira)»h ruiiKs. 1 or Swnoiirs vni> run Like. Wmne e\|)edients to mcre'ase the elhcicncv ol trunkmjr tlver Keith line switches list rdnit ion ot circuit^ lM*t\\een primar> ami '-ecmidar\ How switchinc r(nipniL: ol pirictions., (’ahlmiiot Sw itches Kelt h \Viiinc of S\\7l<*lies (Sicnn*ns) W'lniNii 1 ( ( \.\\ -Ol r l*o\\i:n or 1*1. \N 1 t'rnvK.'^ 'rn\iin )ni)M:-wi ni. Wmukino vnd Ki:« onium. ri.vNi. llYP.Mii I’nivX'iK . . l*noi’.M.M^ 'I’nM in ) t Vi'ioMxiK s\ stem, I \ si \ i.r \'i and ot li(*rs. ION s. I’mi.wx^ Sumik'H''' s\v(cm 'I'nMin Kchn t'oNuao. '-\>U‘m>- •sirxmiv t 'ouch autophone . ' \ L 1 '•V.atV.. 9 LIST OF SECTIONS M:ii. 2 3 Soni(‘ 5^ 6 7 'Tin* Sl.ilc ot immIucI 11)11 1 4 )r(i 1 nipuUc SiMult'i " >ii>l I I )ial Adiusl iiMMil \mI1i 1 11 I ('<• . S\\ itcliiiiji M . i:j 1 (i\\ m . M) \\'1um-I 17 1 1 Kril li 12 Master Sw itch I l’ Ai . II I < li \ ( 7 IM \<i 1 < - 1< . 1 Kot ar\ lane Sw Select or ( 'll <*h sw it«'hle" 12 "lilt Suh'sw tellies- 'onnectoi ircuil hotary Connei'toi ''icuit and Secondar\ laii(‘ switch. I’ritnarN ('irciiil J‘dr-t Selector and ('oiinector. ( . ( as us(‘d l».\ 1)1 it i-h Lo-t Olhee t'ircml ()l Siii;:,lr lam* S\\i1<-li and IJotaiN . 19 (%imn*cl))! hrlwia'ii \n o linpiilsc l{r])i*atri inatK' Ollirfs, and ImMwimmi an mat ic and a Mamial Uticr . 20 22 andfirrml- .... Uinniiia;" \ Marm Sij;nallima on upt (‘1 s .Matin 'oimect Ol - Si;;nallmii on 92 ami (>ioup l-kiull 34 35 36 d'riink (HTeriii” d’est mii 'oniK'et oi 191 37 L 1^ .\ d'estmu Lomu'ctor |9() Ahitor ( (iroiip 97 . IM> \ d'runk ( 91 lit eri 1 and L l». .\ \pparalus and ircuil s Ulctmu 'onneci OI ( lA Li:\ I ( I Ki< *( '.» ^ S') - I r.M". 41 42 ruudamcntal . t .\ . Opi'iatiiit; oM Ii.‘l 48 d’lie . . . ( . ... 55 56 S\iiil)ols 72 <s s:{ 29 W'rrkin;; cliaurc(lM> -A (Jroiip Mann SiLinalliiiii on l'’ault. and 30 Faidt. I'oi- • l’n*'5r‘h‘ct or- . § • 54 .'.... ( t . I t . S\-(cm . Si^iiallip^ on 1M» 1.7.7 nr.im- Ml .Ml . t ) and (Jronp .Marm Selector- . 52 53 171 - . . d'he IJiiish t'lioo-ei and lirii-h Cain.iL:,)* Lmal Switeli -clei-tively conot a tiolled l)\ the Ivi'Uistci’ Wanteil lane 77‘-ted, IJuiil; aid ('on 'lone iii\en to hii-x if necletl. oi Callei ( 'on vei >a ion 'oniiei't or clean'd at ei' ('onneclion ie^isler<‘il al ter ( 'on ver-a ion Le^i-ter recei\ in^ Impulse- ;ind 'Idle coni rollmi: In* Seh‘ct inn ( Iperal toils A I.Otlt) hue IJotarv^ M aeliiiu'-w it cliin^ 50 t>'> 12'i Ket;ister coiit rols tlu' lluihtiie^ up ol l‘>7 the De-iied 7ui licet loll 'The ]»iu>h t'arriane revoKe- and hunt.*142 lor an Itlle I'runk 1 a I’oni' Iij^uh' i’allimi S\.>.t«*m DiNcriininat oi" loi Ihisati' l»ramdi l.\ t’ircuits I2.'l D«‘siied Line 'The llrush Spimlle ol a t.i up Switch controlleil l)\ the lleuister* 45 46 49 7.7 121 . 47 t I , . Ilt’» . 127 127 ‘i-* t'o Il.d . ol I'k'aliirc- . .79 . rrauslatioii S\-!rm. lleveitiNc m|)ulsc 'out H)I lane I’lmh'r conorv 'ircuit 17 \ planat nect' with a t'alliiij^ lam‘ <’.illin^ lam* coniu'ct' to an Idle llenister The Suhs,-iiher tlials tlu‘ Numhi-i ot the 43 44 to.'. Sw itch and Sclta-tor.'- liotarv 192 St^slcni . 26 27 28 9'» ( ( Foucr l>ri\c Introduction. alll’iiuUM- and .‘mIciI oi ppaial Us Stapiema' S\\ H eh and IJcjistci .\sM‘mhl\ and t'aMim: 38 39 40 51 Sii.Mi:\> on . 17 ot» . . ^I'lrct or 'Tiv-t iiiLi Siimall III'.; (tioiip 33 1 Antti . 'rnink Ollnmu 'inMiit 'r«*st (’Irrlv'' ('omM‘’lor t'licuit ami lhal 'I'rstinn Fajuipim'iil ( ‘oniMM'tor d r -t iiiLi I \iiio < Sprrd d7's1im> 23 24 25 v<n I'aiilt . ( ( 21 Alarm and Iiofniif :{() ''^ide ( !• — 32 W .79 1 i:\i 1 'on ne«'t Ol - . . ami S^ ' - '•* . . 'onnci'l or ii ( .27 . . \ la:- 11 1 » am* S w itch lh»t ar\ < l)l;() itnini tn'tl I'aiilt '.t .7.) I’oi in <d 31 'oihmm'I or- willi ( . aiic S\\ il Diaa i<*\i . . .nwl Snlr S\N ll i'lH'" attest ( Vi'ioMvin ApparafU's ('nvMiKol Si'lccl oi 1 Mr v«.M M v\ 'ri:i-M’iin\ i; 16 17 18 . : t'liK , < <i . ; I'lM I ruu' 1 . list niiiiriit > S'U{( 13 14 15 . (‘i(‘liaiiij,ra 1 t .Sii,Mi;\- I . Special I'catun''^ Dial 8 ^ 'lVn‘])li()nr 10 Art . , i >jaJ lln- ml inii-< IMiu(‘i|*l(‘> I'lrst I M’.e » (il 1 M(» Ms | 7ti I-'I u-t'd m l!.otar\ S\slem Dia. h)l 1 - LIST OF SECTIONS \ii ‘•Kci. Wi.Mi.itN IClki 'ikk’ (’(>.''5 SYS'iJ;^^^ i*\(;k — ...... A ln(K(KK»-linc Itolary Mafliinc-sw 58 Carria^je Call (Calleil System Lamps) .... KkI.V^ 1S!I Controlling;’ SMiti'lies 1P7 d'ransmissiou 1 Horizontal-type Sr(|iiene(‘ Switch ‘allmj; Line 'lnders 2 71 Messaiie Fe^istei mu and Metermu 22.*» 72 Cliaimmu ov M lr«mi Xutmnatic S\stem Manual 73 Lorimei 60 61 1 . Ad\ anla^fes <*laim(‘d lor t lie Fanel S\ stem Fanel S\st(‘ni proposed lor Loudon Apparat us ami 'ireiiils line l‘anel Switeli . Hank and W iper Const ru<*l ion < I .... .... a \utoniatie S\ stem . to ist; ISP PP 201 201 ‘ 75 76 . . . . 24i . S\STKM. l•’'ulolam(‘ntals ol Ciiniil System and A])paratris Operation Svstem 77 78 79 207 207 < .“itHt 'IVloplione Aiit<niinli<* Al tom MIC 'rKUlPlIONE Co.’S isn 62 63 64 65 66 67 68 69 70 ( Aiiu‘riclin 154 .Nnmher set up on ,\u\diarv ('inmits in a IJotaix System >ead Level and Line) Fanel .\ ntomat i<‘ A])parat us tJpeiatioii of Lim* eoniieetiii” Switelu^s 59 'I’lu* Co ‘s SysU'iii rot) Ini nnl. 57 PAtJE 'iKrr. (74 ol a 241) 7'liree-dif^it. . . 2o(i . . I'fiO Operation ol a Snl»serilK‘r?v' Siilccrou]) . 251 Ojieration ol a 1'hree-di«xit Final (Oonp , 2(»*{ . . Impulsi' and^pceeli l{<‘|K’a*t(*r s^sri'.M oi- Covi’ntim Apparatus a r tom a tic ..... 1 'ki,i:i*iiom:^, 2i:{ I/id. .') 1 80 2IS 81 Opmalion ol tin* Anio manual Systcmi 82 CiH'iiits relat m^i: to an 83 Iliit.sli ^257 27‘{ . 27 Autoinalie S_Mslenj r 22S Im^iiireiino Standard AssoeiaiK irs'l'niiis iiiid Ociinilions in Aulo- 2 1:1 matie 'releplnmy an : .... 2S() ) XIST OF ILLUSTRATIONS ii;. 1 IK. i*A«n ..... ...... Ten-line Automatic Switch. tory Pireuit h'Aplaua1 <2 Humlred line Swileli, K\|danatoiy .1 4 Thousand-line Svvilidi, E\planatory 1 4 Five-line Sw'itch, lv\])lauator\ NViriui; llundred'thousauddme Systiau (Strow - C. 0 7% S A..T. AI. Po. Dial (Front 1 1 . (Interior) ,, ,, Mew) Siemens Dial (Front Sulo N\\ itolilr.NS 42 Kotnrv 4^{ Priinaiy liiiu* Su i(rh, as used h\ r»rui>li I’osl 41 0 1 1 1 1 1 1 'nml i n Hf’t], (’«*. y\. 41 7 . ff<*r) I’vr. r. r. \. 4.1 41> 47 ( (Interior) ,, AV. F. Po. Dial ('I'liree N'lews) 11 .A. 0 12 T. M. (’o. Dud, \M)rkm^’ Parts (Plan) ,, ,, AN'orkm^ Parts (sec- ,, tion) Imikuise Spriu^^s 14 ,, ,, ,, Testing Pnemt 14 . 14 14 20 21 Siemens Wall lusIrumeiO Pireuil 22 24 \\\ IL Po. 10 17 IS 10 4 . ( . . 10 20 20 o1 2.1 'I'ahli* Srnow'i.Ku 27 2S 20 40 4! 42 44 :u •>•> IiistrumeiO Cireuil Hreast ddaiisuiOtei and Head lveeei\ei on Direid Line ,, . 2(i 21 . . . . 4S Olhn* Potar\ ( 'oniiccl or fch)si Othrc 4h Iinpulsc .10 Iiiifmlse .11 Trunk Ollcnim .12 'I'cst . . . , . . Keith Master Switidi, Const rind ion . 27 2S 20 40 >ial .41 44 .4(> : 2»7 41 Poiiiieetor, Latest ;Foui N'iews) 40 Itotary Swilidi (I'our Mews) 47 Huii(lred-!me Private lAelniuji’e. show in*: Potar> Line Sw itehes Line and ('ut oil Pelay, for above lint, ary Jane SwOeli (Urenit Side-swdfidiless SeleeU»r Pireuit .• / 48 40 40 1, ( Desk .Hi Test 1 .17 4’est IS 4\*st lit 'Test Pireui*s lor 4'i‘st <)0 4'estiUL’. 01 Mndlsmeii s lucomiiiii . . 02 . . <>.4 . . 04 04 Distril>uloi . .04 )esk < oniieetor >esk t 'ontieid i*r. lor usj* witluuit ..... ...... . . )isl rilnitoi Desk Key hoaid 'onueet HU'S ( OS . 01> . . 0!» . 72, • .... .70 ... .... 0<) 07 tlmnector (is 71 (Imiiector witli Hanks Sw'itih Helav Pnil Line Switch Pault .Vhirm 'ircints Sei’ond Preselector Pault. Alarm 72 Line Sw itch 7.*» Preselector hrcuii First Selector Pircai^ Sec.oud Selector Pircuit 41 71 44 7(i 14 77 . . 7.1 . 70 ...... ( 'ireuit ( Pouiiecl.or 'ireuit Alarm Ihise ( . ... ... .. ... . , . ... ., , . 77 S7 Cir w it h .Mann Si^ruals . 71 . ( cuits 74 0(i . t . 07 . ( 1 Olliei* Testej-, ; (>7 . aucs 'ireuit \o. II \ Ciieuits »m t'onueidor lor ahuve dVster No. 14 lor Selector Te m^ P«»st (»4 . Desk Keyhoard small Automatic lor t'lieuit SlI.MIAs 40 ,17 (>0 . PolaiN laueSwitidi Kid ary Line Switch (Another N’iiwv) til . . . .... 'Te^t 1 .17 ati 'ireuit 'ireuit .1.1 4.S 40 ( 1 70 ' ircuit Speed Tc.st iuj; ,14 .... 4Vst |)esk (Ireuits (id A. T. M. Po. ( .11 . lor iiso hid ween a MaAual Otlico .41 :>4 ,12 hy \is«m 1 Peju‘:fl(‘r. ’IcrkV ( 'oiiucidor ( as . .14 04 .12 use lad wren two .1.4 (>4 ,12 . . l{cj)ca1«M‘, lor Automatic and . (Imiieidoi, I'arlN Pattern ('oimeidor with last ol Worknif; Ihnts Details of Hank ('onlaels Numherm^ ol Ihink dVrmmals Four-(i}4 Ure Pireuil with Side Switidies Keith Line Sw 0<0'i, I'.arlN I'orm Present I'orm 'Jluudred line KeOli PiiO . (Irciiit, . 02 r>2 . , OlViees 2.4 , , 1 4’alde IiistrumeiO Pireuit Wall lustrumeiO (Ircuit fjO .... I 1 IS IS 10 10 TiO , . Staanularv /inr 'iiriiii Stdoclor ( 'iri'iiil as uspii hy Post Ollita* ( 'oninador ns hs(m 1 l»y IN)s1 tMliia* Post Sini/Ii* laiu' Switch, as usimI })\ Aulouiatic Ollires 1 Dial w oil hOiM'i'haii^ieahle Prow u W'heid Dial IMate with Ihmires and l e(ters 'ryjiieai Wall liisIrumeiO 'kypieiil 4'alde IiistrumeiO A. 4\ .M. 'o. Wall IiistrumeiO PireuO d’ahle IiistrumeiO PireuO 11 12 14 '(Hiiu‘(‘tni ('in*uil niVna* rirciill . 10 ( '«miK*<‘1or < 'ircuil S7 02 02 02 02 02 .01 L1ST,/)F ILLUSTKATIOXS xiv FAdK FH... W. 1!. (’<*.. Rotm.a S^ m KM-T- cc/d/a/jed. SHMIAi Motor 7U . . . ScI(*ctoT 'rruiik olVcniio SL^ IMP.X. 'oiim^ctor riicuit 'riiiiiK -ofTcrm^ t’ouiiccioi < S4 IM».\. W. ST) P. l'A<’liaii«'c { M<*m‘ral Sti Mow (M- S7 lam* >i I M LM Mn'l \in 'n. ( . . ivc \'ic\n S(‘l(‘ctoi <»l a Sclccioi \'<*i‘1ical StriuN <it 'r<iumials with !M 'arria^c and S<‘lcc1or liru^ll (’liooscr . SiMjUCIKM* Sw !l<di < HI* il,*t ’able !)4 MciiisM'V tt.7 Resist ci tMi I'roiit M(da.\ and Scicchus . Rear \'icws a I'.M'l \\ . . IlM I \'2 '* I2‘{ 'iicuil . . , . . . Mt2 \ SuliHcnher <lialliu^ the Nuiuhei ol Mu‘ with an I2(i M)4 IMush Spindle Irolleil h\ MM .\ Rej;ist(‘i ol' J(t.7 a I ( (iroup SwiMdi cou (hnunt Hie Muildiii^ up R(‘<.;iNter, 'ouiieet loll. ircuit ( S\Nlein l‘Mi . 1.4S 'inuiit to the Haller, Fireuit Homieelioii cleaied afl«*r I >i‘ad -hn eL 'iriMiit to \\ tnis I2S 1 i(‘ad-le\ els ( 'ireiiit t'o? Selectors in sain<‘ 20 1 )ead-lm(* ( 'irciiit Mtlt Hoiiiieetion Ml ( altei 1 .42 1 .44 1 4.7 1 4(i 1.47 14(i 142 14 . \\’. .... . ( . driMiit 1'. t o. Roia S^ ."Tl.M 112 Virst Line Fiiuler ( 'ir- uit 11.4 Second Jane J'iiuleT Seleidor (’ireuit, 1 , . and . ’ llh iirst 1 1.72 M»2 Hroup . 148 Mi Mil 1 Rank F.lcvalion of Magnet Structure h'vatioii ol Ma^iu*! Structure* !•’ Mar ...... \ i(‘w ol Hoiiimut aten Cross s(*et ion eif iilcrruple*!’ ,\ Fair of llrush Harrie*rs I .... .... View of Rrush Se‘t F.levatioii ed 1.72 'renuiiial Strips aiul lusiilatieuis /\ Hrusli Se‘1 (Mhui) A Hnisli Set (Side \‘ie*w’) A Hrusli Set {S(‘el8ui ) .7.7 Hank . . . MiS 2(i(i 2(i2 2(i4 2(ir) 2(i(i 2Mi 208 2(iS 210 lule*i- eu- 1.71 1 Mi.7 ...... ...... ( !i:{ Mi4 M position Kexhoard loi 'a rria^c 'alls .Switch(‘s involved III a Houiii'ct loii .Side \'u‘W ol a Fauci Sw itch and Scid ion Siele* 102 1 lN*rspee*live* 1.74 . . Dial with Letters and l*’i;;ur(‘s M-posilion Kca hoard lor .Manual W'oik- i'de*vatiu» 187 iss M.il Scipicnc<* SamIcIi, 11 oi i/.oiit al 4 'a jk* Hcneral \ icw ol a Fane! Systinn Instal- e>f S(i ; . Fenlieiii 1 Mid 1.70 L74 162 1 Fiont and Rear id Honiiniitalor lictads ol liiiish and 4'iip Device Front 147 S\ s LM 'hitch ,Ma;;ncl s in l'’nnnc iupte*r : Odd-line Sysleiii ( .Side* Mi 1.1, 'I'ci iniiialv Rjuilv ot 141 1 .1.72 . .\ 14.7 Hou\er.sa- W ISI) hhti. SoldiMin^ 'I'ah strip lor alnixi* lictail <d Faiu'l Frainc and Sidi^ctors of 147 Ho. F F-. with 'I'yincal Maud S<‘h‘clor Fiaiiic hdion 1.4! i Rejniwler Selector itli<a‘ ( '*178 Nuniher (('ailed i'licuit up on Lain])s) W. 1 reeiMviii^ IiiipulseN and eouTi'olliii^ the Seh'ctioii H}ieratiouK, Tlie Hireuit 111 'ouv<'rsalioii ...... tioii 1 re;ui;i.st<*r<*d .... ...... .... 'ai ria^e-call set 1 jSti 'rwo wir(‘ lor S(‘h‘ctor —1 ( lUli 14.7 ( (iroiip 1 ue,cted, or., MtS Socond 'rninlvs 1 lS(i Miti.titMi- 2<i 111 11.4 . Line TeNteil. Run;; and <7uiif husy. IIusn 'Tone ^iveii A\‘aiiled SystiMii ..... ..... ..... I8(i . Mid.(i(i(i-line 1 ( ( ISO Mid. (idO line 2.7 14S A lirusli t’lioosei and Uiusli 7n ria^^e ol ili<* Filial Sw itch seh*<*tiv(4y controlled IM'^istiu'- Rejiistci Hiicuit in a MMi,(i(i(i-line S\ st.(;in 124 I“»4 . < Switch 1 141 .141 . Ue;.;iNM‘r, Hroup Sel(‘clor, and I’lnal Si‘lector (( 'onnector) in a .*’»( IMusli j(“Vol\ iiij; to liiid ait Idh* .Iiiuetiiui hy the Mt7 . ISO 22» L4d i-out rolliu<i tesired Hireuil MKi <d th(‘ . 18# Miti.tidd liiu' a 'rinnl Hroiij* Stdector iu a 128 I 161 .. . in •).> 1 1.42 , ldd,ddd- S\st(Mn Idle . I)eMr<Ml Line, Mireuit tfi Second Hroup S(4(a*toi in a 1 1 t'alliu^ Re;;is1er. (’iicuit Line Finder in : ( c<uiu(‘c( lu;’ Mid,ddd- 121 12.7 . MM Line a Sy.^teiu, iueludiiij^^ S(‘coird Line 1‘Mnder 121 .124 diiliol coiiii(‘<‘t m;: with a Malliii;: I’iiulci' due, . . S\!itein in lim* Systc'Hi i'onnectin;; HiriMiit <d . M \«.i; \M^ I First ('allin «4 liiM* . Ma\ . Mi I2il Is . Rc\<m(i\<‘ IiupulKc ( Mi IMt I2n ttlt lane 1.7 I I NuiiK'iical \\ ini>j4 ol SclccMu liaidss Winiiii Irotn llu' (lioup Sclcclois MtO I ** oi 1 117 Its tl7 . . Rotary in . MM 162 162 162 S\ steni ’iicuit Siihscrihei '.N Line, Hiicuit line Sy,stein U*il>- . 1, Odd-line ..... ..... i IS I'hi’st IMii.-'li .... . ’oriL " rnnliii” Sec, uenci* . ( ( ' 1 |(M» .117 'rouuiiial Mauls itoii tM> . 8‘> tIM S(‘l<‘ctoi' Mriisli \ppa.ratus Svp^hols .114 Sw itcli witli Sc<juciic(* 17 : I’liulci •^8 \d jacent 1 M>7 . . . . . ) 1 'ir- 'omic<-lor 'ii<‘Uit ('(hiimmMoi ('ircmt t Realist er HinMiit Mi Diii^jran^s ( ...... <‘uil 8:i 1.7 .US ( Final Switidi (’iriuiit, 1 1 1 'ir- .... ) 81 ( . . . ^ H.*) . hs<Tiiniiiator l<n IM».\. \\ oHvim*' cult 'riuiiK -oHVi iiiJA iMsIriluitoi I'l'uo I . St> Alarm Sii;Mal nt<‘irupt<*r Pinih 1 t'in'uil . . 2Mi 210 2.Mi 21(i 214 214 214 214 214 0- LIST OF ILLI'StRATIOXS hl(J. ’ W. 170 177 I7S K. do. S\stkm 1*\ni:i. ..... Sc<picm*c Switfli Fdrvalion) (part Sf<‘tion, 70 ScqiuOii’f Switch J<*n)ss-s»‘rt ion ) ^S<.oin(*nc<^ Switcli, Insnlatinjr ayd •>!0 Lon 210 Sequence Switch, MomitTm; llracket tor .... .... llnish Set • 100 PoKitionin^ >evice 101, Pive-hundreil-lilM* (’all Fimfer Switch (Sid(‘J7leval ion . I io:i 104 107 00 1 View ot Prip Kod 210 2 2 lit 210 ...... Suhscrihmv' Switch Limvs on ('all lender 220 Sequen<‘e Swili'h controlling (’irenit Metering (’ircuits. Snh^ciilxus' Line Metering (’ircnits, ('all I'hmh'i" to S(‘le<*- 221 224 •>•> tor 107 #M(‘terin^ ('iri'iiit^. Selector to (’<Mim‘ctor OS Meterin*; ('iicmts, 'ontiollei and Allotter 100 ,K(*^isl(‘r ('ir<*iii( w Inai chan^pii^ fi<»in ...... 224 .\Ianna! to .\ntoinali<‘ is:{ S<*I| IS I A lsr» A A Workmi; lsr)V Si >ia^ram ol )nc Sect ion • oaKM'al A' ii‘>\ of llci<‘lor«l 170 1 171 ( 172 Plan, 17:1 Side, \'ie\i ot ( Lylinder S\Mtch. 227 220 IMsiii ol ( 1 < \ 'inmits ol , !N‘lji_v. Ke^iistcr 'riicnitost a ( 'allinji SiihscnluM ^ 'ali(Ml Sul)s(*nl>(‘r N-liiu‘ Svvilcli IlM) CinMiit, 'iivuil ( l>ia}.irain) . alliMl (’iicmi. U<‘<*«nd(*r ('inanl. IJ<‘<*ord<*r (’nrnil, MarkiM 104 100 107 ’ir«-uil iM)0 ..... I0.‘l . >fvi<‘(‘ lor Liiws 'Pnniks t(i ('irciiil. 'iriMut I liatliMN ( I (‘alliiii; .OOn lnn* ( : . and 2()0 nnip iM'ai I'nit ( .... . . CinMUl. Iiupukst* 2rt(l 'uimrrlor l{(*p(‘al(‘r . ..... ...... nptMalin^i ('ircint . . ( Iroupin^ ot 'i'rnnk.s on Kinal S\\ ilrii ( Op<‘ri#(ini»; 2»>0 2r»0 2('>o 2r>tl SiiOxcriluas -Imr <>l SnO-liump 105 100 2(i2 204 'ircuit ol a 'riiro(‘'di'iit l-'inal <7»\ Li I con- ( . 7nmfc1«n' Sn^Iohi <'ir. . \i’ i«»M \ riio\F> 2.14 . 202 . 'I'iiiim*; 102 242 242 240 212 212 . I 101 2:10 'oiiseciit 1011 Loot roller 7rcuil Decimal Indicator Siihscriher's Inslniimml ('iiemt Suhseriher s Instrument . 2.M ( < System . L>rtl .... .... Kiirrj^isiMl (I*arls) liiK' lind(‘V S\\ itch ( 250 < 200 'n.'s .2^0 (MUM’^j^isrd I (’iHMiil, 201 ('o.'s I.mu's . iss ISU 2 : 1:1 . Lxchan^c .... .... .... 1 . l>iii^rain) \ 2:17 \nn>rican Aul (nnan«* .VnuM'iran .Vntomalic <•11 d )iji'i:r;nu ) 'riiim.saiul liiu‘ 'Tninkjiiji Sclu'mt* iSinolr 204 . 'rio.i:- fi .... .... ('onnoclor ILink ('onlatds. \ ('onnoclor I'nil I'lalc SrIiM’toi an<l I : . . . . . . 207 20S 200 200 Dislrilmtin*^ S\\ itrh 201 Ondiiiooi Pro^olocl or 20.7 M nil i-r<nit act 200 Leal spring; Kclav .271 'Pw iolmiHlrcd-lun* l>a> ol Priinar\ Kijuipinont 27('ircnit, SuOsiMiOci V Lim* and Primary Lonirol 280 207 214 211 l{(‘lay . I’clav Switrli . . . 270 .271 ...... ..... . . rirciiO, PrmisO N 'I'nink and I'^irsl ScUm*lor rirciiit. L«H‘al or Sim'oihI Scloclo*! rircuil ronnccior • . . . . . 210 247 . 20:i 200 Iso : IS7 20 S ISI i.M IK(» 2:17 ( I Siil)srnlK*i-.-s' : and Mechanism above trolling Ma'inet 174 77 170 177 17S 170 M i:m I »; * ('I’n picTal <o'oup Loimmkk Timj t>f fSin^U'-Jiiie 10.7 ( 1 Suli-j^roup 1 Panel . ) 102 I.S2 1»10 . • K. A. T. ( '<». S^ ' ]iart (luctin^f Discs 1 v<;ji -rontiaunL . . . 2S0 2S0 2S0 — /' > . ... fO ^ -1 Sectipn S*' ^>\ 1 INTRODUCTION The State of the Art. is —Antonmtic switching ol tclcplioiic linrs in an oxohangn area the only ellieieiit and t^onoinical nietliod of interconnection. • Manual switching lias stTved its day well and faithfully, hut it has (h'jiended for its success as largely.on the orgaiiisatioii and drilling of the human factor as on the machine. * The trallic growth in hirgt* multi oftict* exchanges or areas tlu^ necessity for the* correct : and complicated services the increasing and highly (‘(licient service called for hy the ])ul)lic difliciilty of obtaining a good class of operator, owing to the o]>ening up of (ther spluTes of usefulness to women, among other reasons, have all hel])ed to sound the death-knell of manual working- exi*e])t in small oHices - and proclaimed the necessity for machim^ selection of suitalne circuits for transmission etlici^^ncy required : : tin' vari(*d tlu‘ jiromjit : s wife king. Automatic switching capable of meeting itself aiid that, in i'^ a long way lM‘y<md th(‘ (‘xjMTimental stagt‘. It has proved conditions of modern business and social requirenu'nts. Its untiring aetivitit*s for 24 a manmT. the manual s\stem could not touch. all th(‘ hours of the day, its reliability, its exactness and corrt^'tness of ojieration. its (‘ajiabilities meeting abnormal rush(*s of trallic. all place it on a ]»latform a])art, as the only hopi' and stay of the busin(‘SN and social coiniminity. Financial and political reasons in the jiast have greatly ham])e]’ed its devt^lopinent and that, ])articularly. in the rnited Slates of America, which is usually held nj) Ixdore us The American Telegraph and Tele])hotU‘ Co., (‘ommonly as a ])att(Tn of tcle[)honi<' efhci(‘ncy. for calk'd th(' “ B(‘ll.' has. until (luite lately, refrained from installing and k‘ft the (k'velopnu'nt system to tlu' ‘‘ ind<‘]H*ndent o])(‘rating com]>anies. The syst(‘m of tin* Automatic Electric Co., of (4iicago. the St ro\sg(‘r,'’ is jiractically the only system used in the States. Jfut th(‘ experience of that Company has been unfortuWith amazing energy and foresight it has nat(* in many respi'cts, due to external inlluenccs. develo})ed the souU‘\^hat crude and enmbersonu' systc'm, invented by A. B. Strowger iji 1<S89, into a comjiaratively simple and highly ellicient system, capabk' of meeting all traltic For this Mr. A. E Ki*ith, tlu'ir chief engineer, is larg(*ly responsible. Their conditions. syst(‘m has heen installed in many of th(‘ gn'at cath's in o]q)osition to the “ Bell ” interests, but in some, c.y.. San hVancisco. Cincago. Buffalo and Los Angeles, the opposition lias been too much for them and these automatic installations, the largest installations in the States, liavt* Ix'cn acquired by the Bell interests, and in some (;ases the* automatie equipment has of th(‘ automat i(‘ Ix'en ‘ abandoned. The Strowger system has been installed and is ajipn'ciated in nearly every country of the world. Tlu' American Telegraph and Telephone Co., while refraining from introducing the automatic system commercially, have been for some' years, through the W(‘stern Electric Co., developing a syst(un technically, and, it is believed, will shortly begin replacing the present manual plant. TJieir rotary ’’ system of machine switching i^wxdl known in this country. It is expectexl, however, that they will put their “ Panel '^y stem with 50()-line switches on the market soon. ^ The Automatic Telephone Maiy^ifacturing Co. of this (country use the Strowger (Auto^ 1/ A.T.S. INTRODUCTION 2 matio Electric Co., C’hicago) system and have lately introduced a self-propelled rotary line switch, inst€»ad of the Keith line switch, which greatly enhances the value of their system. Siemens and HalsRe, in Germany, and Siemens Brothers & Co., in this country, use a modified Strowger design. The latter companj^ have made mai^y important modifications, and now ])ossess a very original and efficient system. The Relay Automatic; Telephone Co. have made a departure from the beaten track, and advocate the use of relays anh/ for installations of all sizes. They haVIe a number of • small exchanges in operation. • ^ Th(‘ jorimer system has been in use in .some towns in Canada for a cor^sidcrable nnm))er I of years, aiid it has be(‘n fitt(‘d in Hereford in this country. The work of the North Klectric Go. is not known in this country, hut their auto-manual (s(*mi-automatic) systojn has h(‘en installed in a number of towns in the Ignited States. ('oventry' Automatic Telephones, Llel., are alxmt to e\])loit this system and have arranged to instal an exchange' in Dundee. The American Automatic 'IVlcpheme Gei.'s system is ned kne)wn in tliis country. The use e)f large' instc'ad e>f small switchers is like'ly te> he* an impe)rtant and me)mentons questiem in the near future. The' Streiwger system use's lOO-iinc swite;hes, with the line contacts in Id levels e)f Id sets ill the are. a se't e^f wij)ers being steppe^el up te» the reepnred level. The We'stern KU'ctrie (’e). use* a 2dd-liiie switch with Id leve'ls of 2d eontaets in the arc. ^ There are 10 se'ts ed hrnslies, the brush associated with the re'quired level being tri})peel inte) engageme'iit. In the W. E. Co.’s panel syslein the switch has five gron]>s of 100 sets of iernniials, with a set (d wipers associated with each. The wij)ers, of a particular s('t, arc trip})etl into ('Mgagcment, and then stepped over line contacts. The Relay Automatic Telephone (V). is advfU'ating the use of n'lays only for the connection (d a calling line to one of a grou]) of trunk lines. In the Strowger systt'in, obviously, a calling line can ('ach of Id lines of outgoing trunks. have only a(;cess to 10 suh-groiips I’he traffic, therefore, from a number of lines must he measured tg suit tlie carrying ca])acity of Id outgoing trunks. Then* is thus snh-division of the outgoing trunks into suh-grou]>s of ten lines. In the Western Electric (1),'s system there may he siih-divisam into groups of 20 out- going trunks as there are 2d eontaets in a level. In their [)anel system there are 500 contacts, and therefore five groups of 100 trunks available to each incoming line. The sub groups of outgoing lines, as above, are Kt, 20 and I0(h U) which each incoming * line has a(*cess. It is known that he larger he grou]) (d* lines, tlie greater is the tralhc carrying capacity t t of each line of the group therefore a 2t) line group is more efticient than a 10, anrl a 100-line group is very much more efticient than either. Against this efticiency there has to he placed the greater cost of the switches, and their maiutenanee, and t<m increased dilfu nlty of routing a call. It is possible tflit small switches, in combination with a special suh-division of the trunks, may he foun(^. <piite satisfactory. ^ The problem is one of the most interesting before ^-eleT)h one engineers at the moment. ; AUTOMATIC TELEPHONE SYSTEMS Section 2 A FEW DEFINITIONS • The Exchange. ~ Originally the term exchange was used to designate the intercommunication system (i^^er which a subscriber had the right of connection for a given sum The exchange was divided into switching rooms for the convenience of the exploiter, ^aid. or provider of the system, and the telephone numbers were continuous over the whole area. As the number ^)f subscribers grew this became impractical)le without using numbers of many digits, which were difficult to remember or reiieat. The capacity of the manual switch board was, thendore, adopted as the highest telephone number, and these numbers were prefixed by the names of the ditfenmt switch rooms or ollicos. The switch room, or office, was very generally known thereafter as the (‘xchange. With the advent of the automatic system of working, it seems advisable to ri-vert to th^ original definition, as the telephone numbers now run consecutively over an area, and the system is again divided into switch rooms, or offices, to suit the ecomimical distribution of the plant. We have, tluTefore, a fiuh-Malioti at the subscriber's office, which may consist of one, or a plurality of direct lines, or a small swit(;hboard. • An offica which is a sub-division of the main plant. An exchange or multi -office system which has a common numbering scheim*, a nd over which full interconnection* is obtainable for a given sum to lie paid. K.xidiaiiges are connected lim^s, for which an extra fee is to be charged. together by trunk or toll — The Plant. A preselector is a switch that automatically selects an idle line of a group when the receiver is lifted, /.c., ])rior to any digit imjmlses being sent. It is therefore sometimes called a pre-digit or non-numerical switch. It may be either a calling-line finder, ?.e., has a backward hunting motion, or a trunk finder having a forward hunting motion. Secondarg preselectors are generally used in combination with the former, wdiich are, therefore, known as primary prejielectors. The primary preseh'ctor, when one is associated with each calling line, is frequently called a Jim s milch. Ow ing to the use of conqiaratively small groups of lines and the consequent sub-division of the trunk group, the number of trunk lines and first digit switches to which tjie trunks are wired is excessive. iSecond [ireseleetors are therefore interposed to reduce the number of first selectors recpiired. The second jireselectors may be of any of the forms described for first preselectors. The 8trowger-Keith switch is a true presek^ctor, bt^cause an idle trunk has already been assigned to a potential calling line. When a trunk is taken into engagement the master switch moves all })lungcrs of calling lines to a jioint opposite the next idle trunk. The Siemens switch hunts for an idle trunk after the receiver i.s lifted. In the Western Electric CVi.’s system the preselector is of the call-finder type, and hunts for the calling line after the receiver is lifted. In. the Relay Automatic Telephone Co.'s system the relays instai^tly connect a calling line with an idle trunk. Common relays control the instaiitaneou^operation of the conf necting relays. • Selectors or (h'oivp Selectors . —^Tkese automatically select an idle line in a group of lines FEW DEFINITIONS A 4 correeponding to the impulses composing a digit sent from a dial sender. Tliesc Jiave usually 10 levels which are 10 sub-groups of trunks, correspo