Historical Document · 1934
The Automatic Telephone
Museum booklet explaining automatic telephony and its history.
Historical Document · 1934
Museum booklet explaining automatic telephony and its history.
,,:=- ELEPHONE E t TEC ' ,, = PRTCE S |.{ ],.i#5 t ! l : II\TT'RODUCTION fflHE automatic telephone has developed so rapidly in recent y"urc _that_ it is apt to be regarded-as a relatively new idea. ^tr Actually the first patented pioposal ::r ^. ] 'i 1 I :r ii .- lr ,I I 11 i i I ,,- "l i I il 1i i t I t I I it il il iltt tl l! t\ -: for an automatic switchboard dates from three years after the introduction of the Bell telephone -in _1876. Messrs. Connolly and McTighe were the patentees' in America. The multiple slvitchboard was patented by Scribner earlier in t87g. The first patents of strowger,-which form the basis of the present ylep-by-step system used by the Post Office, were taken out in rg9r. There are at least two other major systems in wide use in other countries, each with.its .orvn peculiar advantages. The Strowger system was adopted in this country as being most suitable to our conditions after trials of various designs of apparatus on 25 different provincial and_rural cxchanges between rgrz and ry24, The first automatic. public exchange in this country wa. op"ned at Epsom in \g!2. .Discussion of systems. other than that adopted by the post Oflfice is outside the scope of this pamphlet. There have been numerous important developments, both mechanicaliy atd electrically, since Sirowger,s first ipparatus was made. The principle still remains. The motions of ihe switches are accomplished by the attraction of the armatures of selected electromagnets, ratchet and pawl action converting the attraction u 111o .rotary or vertical motion of the switch arirs, uu required (Fig. -S). _After many refinements the,system is now economically sound and very reliable. The possible outlets from each switcl are.arranged in relatively small groups, selected in a straightforward decimal progression as the subscribei sends a succession o."f electrical impulses from the dial on his telephone. The Science Museum is indebted to the Post Office for a comprehensive demonstration equipment which has been erected in Galiery XXVI. The following notes are intended to supplernent thl demonstratigls given on_ this equiprnent, which can ?eproduce the various conditions in making a call on the post Office syit"-. The diagrams illustrate the routing of various types of call. I Weekdays and Saturdays : rr.3o a.m., 3.r5 and I . 1" Sundays : 3.r5 and 4.3o p.m. No demonstrations are given on Bank Holidays. 4"r5 p.m. 3 THtr PR.OCESS OF MAKT}TG A CALL ar GENER.AI,. It would be uneconomic to have a complete set of automatic equipment for each subscriber, as that would only be necessary if all the subscribers wanted to participate in a call at the same time. Each subscriber therefore has the minimum amount of equipment at the exchange necessary to search for unoccupied instrurnents from the common banks of equipment. This enables a relatively small amount of equipment to be installed for the use of rvhatever subscribers happen to be trying to make a call at any one time' If more subscribers than the number of common equipments are calling at the same time, the excess number do not obtain the " dialling-tone," or obtain the " busy " tone at some later stage-in the process of getting through, and have to wait until some equjPments are freed before they can proceed with their call. T'he number of common equipments provided is such that not more than orie call in 5oo, on the average, is likely to be delayed for this reason. Subscriber's Bquipment. Telephone and bell set-At subscriber's end' L and K relays I Uniselector i At the exchange. Meter T ,} ) When the subscriber lifts his receiver to make a call the L retray completes the battery loop across his line and causes the uniselector to operate. The uniselector (Fig. z) is a rotary switch with z4 outlets each connected to one of the firsl common switches at the exchange. If other subscribers are using these switches the uniselector keeps stepping round until it finds a switch which -is- not being used. It then stops and holds this switch for the use of the caller. If all the first common switches connected to the uniselector at the exchange are in use the subscriber does not hear the dialling tone, and in these circumstances dialling is useless. EIe mr.rst then wait for the common equipment to be freed by other subscribers before he can make his ciil.- This is done by hanging up the receiver for a short time and then trying to obtain the " dialling tone " again. 5 UI'TTIL T}{E UNISEI,ECTOR FI}.TDS A FREE EQUIPMENT TI{E DIALLING TONE IS NOT HEARD. when a f,ree common circuit has been found it is held for the use of the caller by the operation of the K relay and the subscriber's ioog.. The dialling tone is nor,v heard and the caller naay proceed . to dial the number he requires. The rneter records completed calls rnade by the sutrscriber and is not operated until the distant subscriber iiftu his recei*er to answer. Common Exchange Equipment. There are far less circuits than subscribers, as explained above, and a subscriber must obtain a free circuit before he can continue with a call. ON }TEARING THX DIALLING TONE THE SUBSCRIBER KNOWS TIIA.T A FREE CIRCUIT IS AVAILABLE AND MAY THEN BEGIN TO DIAL. THE PR,OCESS OF DIALLING MAY LATER INVOLVE THE USE OF ADDITIONAL APPAITATUS WI{ICH MAY OR MAY NOT BE FREE WIIEN REQUIRED" The Dial. The dial is a device rvhich sends numbers of electricar irnourses into the. equipment, the first three trains of impulses selecting the appropriate exchange and the next four seleiting the reqirired subscriber's number out of the ro,ooo-i.e. oooo to 9g!9-subs"iib.r. who may be connected to that exchange. Dialling NIUS is the same as diaiiing 687, but it is easier ro remember MUSeurn 2333 than 687-z333. Electrically, horvever, there is no difference between the two, MUS being a sequence of 6,8and7impulses. When the dialling tone is heard it means that the following operations have taken place :-_ r. The uniselector has found a free switch out of z4 srvitches knorvn as lst Code Selectors, with which it"can maire connection. z. The rst code selector has been held for the later use of the subscriber, provided that a lurther switch is found to be free. 3. Finding a rst code selector has caused another switch to be operated. This is similar to the uniselector and is known as the '6 A " digit hunter. It has z4 outlets, each of whicli may be connected to a switch known as an ,,A,' digit selector. free it 4. If one of these '6 A " digit selectors is found to be is seized for the use of the subscriber and the ,,dialling tone " is heard. The reason for obtaining the use of all these switches before beginning to dial will be expliined later. 6 \ lTo Jace pige 6. EQUIPMENT I'IIE DIALLING IRD. r found it is held for the use relay ard the subscritrer's rnd the caller rnay proceed nade by the subscriber and :riber lifts his receiver to :ribers, as explained above, :uit before he can continue ]CRIBER KNOWS THAT A FR,EE N TO DIAL. THE FROCESS OF OF .dDDITIONAL APPAR.ATUS EE WI.IEN REQUIRBD. nbers of electrical impulses s of impulses selecting the rur selecting the required l. oooo to gggg-subscribers Frc. z.-Typical Uniselector. ng 687, but it is easier to 33. Electricail,v, horvever, MUS being a sequence of means that the following srvitch out of z4 switches r, with which it can maire eld for the later use of the ther sr,vitch is found to be aused another switch to be 4 Le uniselector and is known has z4 outlets, each of which L known as an tt A " digit :tors is found to be free it .bscriber and the " dialling f all these switches before Frc. 3.-Typical Director. 7 o face page 7 I RTLTAsI MAll'l[i A EMAT I]ff T COUELT DOG Ftc. 4.-Typical Selector. " A " Digit Selector (see Figs. 4 and 5). When an " A " digit selector has been found the dialiing tone tells the subscriber to begin to dial. If he is wanting, say, MUSeurn exchange, he first dials M, thereby sending six impulses through to the " A " digit selector. E E, t o t t s1 I t i = : eaLtASt AlAi$e'l AEI'4,1TUPT 0 --: lausrt roe PR r t[.i/{t € )a ?o LEVgLS Frc. 5.-Diagram of Selector mechanism. VNI :r'ertical magnet operating ratclTet and pawl (VR and \rP), lifting a shaft carrying the ^ swrtch wlper. RN{:rotary magnit operating a second ratchet and pawl (RR and RP) giving s rotary motion to the shaft, DD:double dog acting as a detent against gravity for the vertical ratchet duing the vertical movement. and asainst the coi]ed sorins durine rotarv molement. REL M - Release masne-t which knocks rnL aotble a& out of the ratchets when the ell is finished, SD : a stationary dog to bold the shaft against gravity during rotary action. RL = a releme link hoidine the double dog out of engagement when the release mamet opcrats. QN:" Off-nomal " contict springs closi-ng an etectriIal circuit on the fimt vertical movement of the switch and therefbre preparing a circuit for operation of the release maEpet at the end of a call. - {E<tracted fum P.O. tealmical pmpltlct E.r. I By means of relays,this switch steps up to the 6th level on its bank of contacts and then the s;it;L"'brad'e ;; *ip.r;;t.girr. oi to traver round the contacts on this rever uritl i, n"[5 , frJe outret. nf there is no free o,tret from ttr. nr.ii"" contacts the switch wiper reaches the eleventh contact which-caus". tt";; Urr;i;;;;,, to be sent to the caller. The first ten contacts on each usabre Directors which contain switche;l;r fi"ai?s rever are connected to th. ,"[""ir#.".hanges. AII exchanges beginning rvith th. i;tt"^ .. M ,, or ,, N,, are con_ nected to the set of dire-ctors whiciL are connected to the sixth rever on the " A " disit selector. Simih;iy."*"tung"r-"b.gir*.g ''r,', ur. .onrr".i.i-ro',rro." directors to rvhich*irf, the fourth level lea<Is,. and so on, it" jiorping.;;;;;;;;;;ing " to the Iettering on the subscriber,s a;rf pfutE. '.' 6,') 'l H," or Directors (Fig. :). The " A " digit serector finds a director which r. It finds a way to the wantecl exchange. does two things :-_ z. The directoi-may.take a longer_tirie to find the required exchange than it takes the"subr"rit"i a-Ji"i-irr" Ietters corresponding to this exchang.e. The subscriil"l, froro_ ever, will continue to dial the",, numeri;; ,;;;uired at that exchange._ The director if..r" ,rr*lt, ,rrtii the distant exchange has been ""r". found. ,, ,, capabilities of an A digit selector u,ith eieht usable ,IevelsTh." leading to different sets of air..io* eu"rr-*i,n'b'r';fii; outrets is cleariy to provide mcans of .eaching a toral exEg;r+;ient At present there are not .J many auromatic "f exchanges ililF^"?: rn use rn any automatic area. The directora, ho*"-r"r, ura-ro nected that thev can find..ag.y of the rrg.. corresponding to"orthe second and third letters diailecl. "*.t There are tlo ways of finding a-line. to an exchange. Nearby exchanges are often connected to ?r.rr-"it b; ;i;;;;'ii;., t,rt fo, economic reasons widely separaterl exchanges "; are not so connected. Every exchange, howevei, is provideJ ;i;h Ii"", ,o -fir;;;";; point knorvn as a Tandeni excrrange (Fig. 6j.u ""r,truiJ*t,"r,;r,g nection may be estabrished either ai.Jctry, oi """ri u ri.r"-io trr,a"* and thence by another line to the wantJ i*"t rrr[". The director economizes in the total. equilment and cabling required by a process known as ,, tra,slatirrg,,'tfr" i"tir-irip.rlr., received. It receives two trains of " l.ite,'ri-f"ir.r-rr,rorgi, ,r," selector a1d r-nay send out-any nurnber from l'{ " dlsjt,:!:]:". o,e-to six to. find u. path to tr,.-."""r,r;s;-;.q;;;j.'"rn"." :::::^:l operare switches '-31:l:". ,r'as brousht into knoivn as code ser6ctori.-- n.for" tt. drrector circuit the subscriu"r,r nua already seized a f"ree FIRS'[ "rrir.L"1o, ,r", for ,rb..q.r..rt and the direcror no*, sends out""0"-"-r"ctor a train ;i-i;p"[.;Tr"t ii.'i" ,u. switch, causing it to step up to an apfropriate level. I - \ up t0 the 6th level on its ade or " *ip er " begins to :ntil it finds a free outlet. I contacts the switch wiper es the " busy tone " to be rle level are connected to ing the required exchanges. ('M" or ttN" are con:onnected to the sixth level exchanges beginning with rose directors to which the ping corresponding to the First Code Selector. The " translated " impulses from the director step the switch up 10 any.one.of its_ ten levels, each with ten outlets. - If a nearby exchange is being called it may be directly connected to one of thesl Ie'r,-els and the contact arm searches round for a free line among the ten connected at this level. If it finds one, the director may now proceed to transmit the stored " numericals " to find the wanted subscriber at the exchange to which connection has now been made. It rnay be that the wanted exchange is not connectertr to one of these ten levels, as many more than len exchanges may have to be Iinked up. In that case the translation at the dirictor tikes the form of two or m.ore trains of impulses. r rvhich does two things :- DISTANT CALLED range. TANDAM time to find the required bscriber to dial the letters Je. The subscriber, how" numericals " required at stores these numbers until to an exchange. Nearby ier by direct lines, but for T(} ORIGINATING EXCNAh-G g IF \ ALTEENAY}VE trIRECT I..I'qE IS OUT found. ;eiector rvith eight usable each rvith Br usable outlets r total of 648 different exnany automatic exchanges tors, however, are so conrnges corresponding to the iIO DIRECY CONNECTICTI EXCHANGE ROU:TE t OF COMMISgION EXCHAItGE CALLEO EXC D$F+ECT AND TANDEM CONNECTIONS nges are not so connected. lines to a central switching (Fig. 6). Therefore con,'. or by a line to Tandern ::change. r1 equipment and cabling rting " the letter impulses er " impulses through the y number from one to six :'.changerequired. These Frc. 6.-Direct and Tandern Connections. With two trains, the first steps the rst code selector up to an appropriate level along which the switch arm searches for a free outlet to the [e selectors. Before the bscriber's uniselector had Second Code Selectors, which also have ten outlets on each of ten levels. The znd code seiector is operated by the second train of impulses from the director. The wanted exchange may be connected at one of these levels, or a third code selector and a third train of impulses from the director riate level. The rst code selector has ten levels. znd code selectors, each with ten levels, may be connected to each of the ten original levels, rctor for subsequent use, rf impulses back into this rnay be required. I giving roo connections in all. with a 3rd code selector group at the- exchange we may connect to any number from lor io ,,ooo exchanges, depending on the number of 3rd code selectors used. So far we have only considered a directly connected exchange. If there is no direct connection the call must go through randein. 'l[he translated impulses from the director whith refer"to exchanges for which there is no direct line, automatically fincl a lever at i'he code selectors which is connected to the Tandem exchange. succeeding trains of impulses from the director then operate furt{er code selectols (first, second, third and fourth rande^m selectors) at the Tandem exchange. These selectors finally find a channei out from the Tandem centre to the wanted exchang-e. The Tandem centre is jirnply a central exchange through which any incoming call can find a route to any wanted subsc-riber,s exchange" ThaI route involves cross connections at the Tandem exchange reqiriring the use of one, ttvo, three, or four Tandem selectors. WIIEN THE CODE SELECTORS HAVE FINALLY DONE THEIR WOR,K T}IE DIRECTOR THEN TRANSMITS THE .. NUMERICAL,, IMPULSES WHICH IT HAS BEEN STORING, . .At the foregoing -switch positions there are ten possible outrets which are searched fo1 by _t6e switch arms rvhen they have stepped up to therequired levels. If, at any position, all ten oltlets ar" b,isy, the switch arm carries on to an eleventh contact which causes the " -busy tone " to be given. The cali can get no farther when the " busy tone " is heard and it is necessary tJre-dial after an interval. Assumi-n_g that a free channel has been folnd at every switch position, the call has now reached the required exchang6 and the dlrector now transmits its stored numerical impulses. The first train actuates one of a bank of First Numerical Selectors, if there is one free. This selector has ten levels each corresponding to the " thousands " digits. These levels are each connecfed to a bank of Second Numerical Selectors, and if one of these is free it operates in accordlnce with the train of impulses corresponding to the " hundreds " digits. The slvitch reaches a level which selects a free Final Selector which steps up to a level corresponding to the third train of " numerical " impulses corresponding to the " tens " digits, and steps round that level to a position determined by the fourth set of " numerical " impulses corresponding to the " units." The wanted subscriber is connected at this position. When it is reached, a group of relays either give the " busy tone " if the line is engaged, or the " ringing conditions " if the iine is free. When the-called subscriber answers and lifts his receiver a relay operates the rneter at the originating subscriber's exchange equipmenf. IO 3rd code selector group at rumber from rot to r,ooo yrd code selectors used. rectly connected exchange. nust go through Tandem. rr which refer to exchanges ratically find a level at the the Tandem exchange. irector then operate further ourth Tandem selectors) tors finally find a channel d exchange. The Tandem ough which any incoming bscriber's exchange" That dem exchange requiring the ectors. Y DONE TI{EIR WOR,K TIIE ,, IMPULSES WHICE{ IT HAS 'L re are ten possible outlets rs when they have stepped .on, all ten outlets are busy, I contact which causes the r get n0 farther when the to re-dial after'an interval. nd at every switch position, exchan$e and the director npulses. The first train AI,TERNATIVE LINE.FINDER SYSTBM _ Instead of providing a uniselector for each subscriber a lineflnder, c-onsisting_of a switch with zoo outlets, each outlet being connected to a subscriber, may be used. several line-finders are provided for each group of zoo iubscribers. Each group is provicied with a distributor which finds a free line-finder, if"avaiiablJ, for the use of any calling subscriber. This alternative to providing a uniselector for each subscriber has two advantages :- r. It is economical. z. As several line-finders are grouped together the failure of one still enables a subscriber- to male a cafl via one of the others of the group. The failure of a uniselector curs the subscriber off until the fault has been traced and remedied. Provincial System (Non- director). The nurnber of exchanges in a provincial area is usually so few that the employment of directors ls u.rnecessary. Transiation is therefore not employed, the dialled impuises op.rrti.rg the exchangefinding selectors directly if there is moie than one "n"Yrurrg.. The " provincial " positions on the museum demonstration equipment are fitted with line-finders instead of uniselectors. PBX Final Selectors. levels each corresponding s are each connected to a :cordrnce with the train of :ds " digits. The sivitch These are employed for subscribers who have a number of rines. The selector hunts for a free line among those connected to the subscriber. If there are so many trines thi't they have to be accommodated on different levels at ihe switch, the contact arm, after searching unsuccessfully. along level, will release and step up to the next level where it will -one begin to search again. rt continues to search until a free line is reached, at which point it stops, or, if all lines are trusy it reaches a position frorn whiih the ,, busy tone " is obtained- ng to the third train of r the " tens " digits, and nined by the fourth set of Arnount of Apparatus Required. 'he " units." The wanteri When is reachecX, a In making a call the " A " digit selectors and directors are held _ only until the final selector has- reached the wanted subscriber's line. They are then released for the use of other subscribers. r relay operates the rneter .All other apparatus used in obtaining a line between the originating and calied subscriber is held until the call has been coripletedl N{ore of this apparatus is thus required. , it " if the line is engaged, or ; free. When the called luipment. II I The amount of apparatus installed in a typical London exchange in a part central, part suburban area is :Uniselectors 6,5oo one to each subscriber. "A"digitselectors Directors rst code selectors znd code selectors 3rd code selectors rst numerical selectors znd numerical selectors Final selectors roo 129 5gs +8: Extra directors installed at particularly busy levels. 63 8r6 xrloo l,o7B SPECIAI" CONDITIONS (r) Premature Dialling.-Tlre subscriber should not dial before dialling tone is heard, i.e. when the " A " digit hunter has seized a free " A " digit selector. Should the subJcriber diai while the hunter is hunting-Number Unobtainable (N.U.) tone u,ill be trans- mitted to the caller. (z) Delayed or Incomplete DiaIIing.-A period of between io-po seconds , is allowed for dialling. The " A " digit selector having b^een seized, the subscriber should dial within tiie specified period of 3o-6o seconds. If he fails to dial within this period, a spec!-a! " tim,e pulse " circuit is put in operation which alters the conditions of the lines and N.U. tone is transmitted to the calling subscriber. The " A " digit selector is then released for the u.e oT any other calling subscriber. (3) Directors. Forced ltelease.-When the director is seizeci, th: .trp.. pulse circuit is prepared. Should the subscriher delay (v'hich includes v-ery slor,v dialling, and not completing the operationi in the time specified), the time pulse relay will operate-, disconnecting the pulsing out loop and so cause N.U. ione to -be transmitted to thE caltring- subscriber. This is a similar case to (e) above, but applies only after the " A " digit selector has found a iree director. Spare Codes.-should the second and third digits received ^ (+)the from subscriber not correspond to an exchange co-tle, then no translation lvill be wired up. The send.er switch-will continue to step until a position is reached that causes a relay to operate, the contacts of which cause N.U. tone to be sent to the caller. The director and " A " digit selector is released. That is, all unoccupied leveis at the director are wired up to give the N.U. tone if reached. (5) Director with Manual Board Facilities.-should the subscriber dial " O," the lines from the " A " digit selector are reversed and so cause a special " I) " relay in the direclor to function. The_connecting wires aie joined to speiial manual board terminals on the translation board. According to the routing necessary, the director can now send out any number of trains of impulsej from r to 3 to connect the cail through to a manual switchboard. 12 a typical London exchange (6) Code Only Calls.-For trunks, tolls, telegrams, etc., the connections are similar to the " O " call except that relay " D " is not used. The wipers of the " A " digit selector, and the B.C. switch one to each subscriber. of the director are standing on their bank contacts according to the dialled impuises sent out by the subscriber. The contacts of the B.C. srvitch, on which the wipers are standing, are brought on to the translation field, and as no numerical digits are received the circuits are arranged so that after the translated code is transmitted, the homing of the sender and control switches is carried out, and the director and " A " digit selector released. (7) Metering.-The rst code selectors are equipped with what is known as a " vertical marking bank " traversed by an auxiliary contact as the switch steps up. The code selector is wired so that exchanges to which the tariff is a double call are connected at a level rvhich ensures a double metering pulse to be sent via the circuits connected to the marking bank. Similarly a triple tariff call would be routed through a level which woulul make the subscriber's meter Extra directors installecl at particularly busy levels. IONS :riber should not dial before ' digit hunter has seized a subscriber dial while the (N.U.) tone rvill be trans- operate three times. On the demonstration equiprnent. IdUSeum to MUSeum meters once, MUSeum to EASI meters three times. MUSeum to WESI meters twice. Ig.-A period of between The "A" digit selector I dial within the specified dial within this period, a rperation which alters the transmitted to the calling ren released for the use of (8) Dialling too quickly.-If the diaL plate is pulled back instead of being allowed to return at its olvn speed, the correct number of irnpulses is stili obtained, but usually at a speed faster than the relays can respond. The response of the relays may therefore be erratic and a wrong numtrer or an unobtainable level ivill be reached by the srvitches in trying to follow the impulses received. 'hen the director is seized, ruld the subscriher delay : completing the operations lvill operate, disconnecting ee to be transmitted to the to (z) above, but applies rd a free director. I and third digits received r exchange code, then no :r su,itch rvill continue to es a relay to operate, the sent to the caller. The . That is, all unoccupied ae N.U. tone if reached. Facilities.-Should the : "A" digit selector are in the director to function. Li manual board terminals .he routing necessary, the f trains of impulses from ual srvitchboard. Printed under the authority oi HIS IVIAJESTY's Sr.q.rronasv Orrrce by William Clowes & Sons, Ltd., London and Beccles, lO/34. (388) !Vt. 2849-3288. 2,000. 213s. W. C. & S., Ltd' Gp.30r. a i