Reference / Paper · 1924

Principles of Automatic Telephony: Circuits and Trunking

AE book on Strowger circuits and trunking.

Author: Harry P. Mahoney (Automatic Electric Co.)
Year: 1924
Type: Reference / Paper
Credit: Telephone Collectors International (TCI) Library, telephonecollectors.info (retrieved via Internet Archive Wayback Machine)

PRINCIPLES OF Automatic Telephony Circuits and Trunking By HARRY P. MAHONEY REG. U. S. PAT. OFF'. Automatic Elmric ComIWlY CH I CAGO . HIS book is not intended to be an exhaustive treatise on Automatic Telephony. Its purpose is rather to give the student a brief yet reasonably thorough understanding of the principles underlying the operation of Strowger Automatic telephone apparatus, as manufactured by Automatic Electric Company. T .' The subj ect matter is the text used for many years by Automatic Electric Company's Operating Department in teaching the fundamentals of the Strowger Automatic system. The material has been thoroughly revised and amplified and is presented here in permanent form. In order that the circuits may be easily studied, each one has been divided into as many parts as it has functions to perform, and the circuit of each function sketched and described separately. Operating Department Automatic Electric Company Copy"ght 1924 Automatic Electric Company ", -. .- - - - - - - - - ~_~ / ~_ _ ....;<'.!L{J. I Principles of Automatic Telephony II • Contents PAGE CHAPTER I-Symbols and abbreviations .. :J 2-The telephone __ 8 .. .. 9 .. 3-The 100 line system ------.-- .. _ 4-The connector switch _ .. 12 5-The selector switch .. 6--Line switches .. 21 ._ .__ 30 7-Secondary line switches __ . .. 41 8-The impulse repeater 9-Power equipment _ .... __ ._. . 46 __ .__ .__ ------- -- .. .. ___ 49 10--Trunking _ _.. ll-Special service connectors __ . .. .__ 56 .. . 66 70 13-The switching selector repeater 7S 12-Toll equipment . 00. 14-Reverting call equipment .. .. _..__ .. .. 81 IS-Testing equipment .... ..... .. 84 .. . • ~...._'"---------------------------------------- ,~ .. '" I Principles of Automatic Telephony CHAPTER 1 Symbols and Abbreviations T o simplify telephone circuits and facilitate cir- cuit reading as much as possible, it is customary to use certain standard symbols and abbreviations to represent various pieces of apparatus. Some of the more commonly used of B.B. B.C.O. Bd. Bk. Harm. -Hannonic I.D.F. -Intermediate Distributing Frame -Incoming -Information In£. Int. -Interrupter -Armature L.D. List. --Long Distance -Listening -Bus Bar -Bridge Cut-Off Lock. L.Sw. -Locking -Line Switch Mach. Mas.Sw. -~Iaster Switch -Alternating Cunent -Answer -Board -Bank Brk. -Breaker Cct. -Circuit -Calling Device or Dial -~Iachine M.B. ?vr.D.F. J\LF. M.S. -Master Switch -Circuit Neg. -?\egative Comb. -Combination Compo Condo -Complaint N.I. No. T'\ or. -1\on-Inductive -l\umber -Kormal lu -Ohms CD. C.E.M.F. -Counter Electro Motive Force -Charge Chg. Ckt. Conn. -Condenser D.C. Diff. -Direct Current - Differential Dis. Dist. -Discharge D.T. Opel'. P. -Dial Tone -Electro rdotive Forct: Equip. -Equipment Freq. -Frequency Gen. O.M. O.N. -Distributor E.M.F. Grd. -::Uain Battery -l\Iain Distributin~ -.!\Iicra Farad -Connector -Cycle " -Group Inc. these are shown in Figs. 1 and 2. A.C. Ans. Arm. Grp. -Generator -Ground ----------------- -Open Main -Off Normal -Operator p.D.e -Private -Pull Down Coil Pos. Prim. Priv. -Positive -Primary -Private Pw. --cPower R. Rec. Reg. -Ring -Receiver -Regular 5 -.!.!l , r Principles of Automatic Telephony ___ _ _ _ _ _ _ _ _ _ _ _ _ __ Ne~. SPliilEllY po~,eAT"E:~"( _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _'TFI\".I<.IN6: CIli!!GUIT. @ 1'1!Flt-ll5l. 0..'.' 11 ' D1Pll.. ~ FUSE.. -9------E---II~- INCUCTION Co\\.- • Fig. 1. Symbols used in telephone circuit drawings 6 \ ......._ - - ~ c_ m ... • Principles of Automatic Telephony [~ ept. -I~epeatcr Rev. Cn.l1 -Reverting Call ~ , Rise. -Relca;::e Rly. -Relay Rot. -Rotary S. -Slee\"e Sec. -Secondary ScI. -Selector 51. -Slipped Sprgs. -Springs St. Supy. S\\'o -Straight -SUIJen'isory -5"'iteh T. Tel. Trail::>. Trk" T.T. -Tip -Telephone -'I' ra llSl1li t tcr -Trunk -Tel[ Tale Vert. -Yertical W.e. -""ire Chid ., Fig. 2. Automatic wall telephone 7 • Principles of Automatic Telephony CHAPTER 1 The Telephone book the circuit interruptions of the dial will be referrt'd to as "impulses." It is the impulses, sent over the line, at the will of the subscriber, which start the automatic switches in motion and cause them to establi"h connections between sub· scrihers. HERE ha\'e b~ell and still arc several kinds oi telephone circuits in lise between the central office and the !'lubscriber's station. The metallic circuit is by far the most widely used of all, and has 1I0W practically displaced all others. The melallic circuit consists of two wires between the telephone switchboard and the sllhscriber's premises. Practically all telephone bells arc rllng \\'ith alternating current. The telephone must there· fore be constructed so that in its normal can· dition a path is provided over which alternating current will flow to ring the bells, but oyer which direct current cannot pass. This condition is accomplished by a telephone condenser placed in series with the hell. ..\ telephone condenser T -<i> ;",(,;"W'" ,.......\ I Fig. 3. , ........ - Jl • '.""T , ...... Fig. 4. Calling device or dial mechanism The automati..: telephone. generally speaking, differs from the common battery manual telepholle only in this one respect, viz., the addition of the calling device or dial. Fig. 4 shows the lllechanism of a calling dey ice. ~ U<.-.. -~- Circuit of series automatic telephone • • Scril1s telephone cirwit In Fig. 5 is shown the impulse circuit of the telephone after the receiver has been lifted from the hook and the dial turned preparatory to send· placed in a circ\\it prevents the flow of direct current and permits the passage of alternating current. Lifting the receiyer disconnects the condenser and connects the twO line wires together through the transmitter and recei,·er. Automatic Series Telephone ".c _c .... ~" p' In Fig. 3 is shOwn the circuit at' an ordinary telephone of the "series" type. cquippetl with automalic calling dey icc or dial. The dial is essentially a circuit inlerrupter. \'-"hen the dial is released after being turned from its normal position, it interrupts the line circuit a number of lime!' in quick slIccei":<ion corre"pon<ling to the digit dialed. Each time the Ijne is so interrupted, what is known as an impulse is transmitted to the telephone switchboard. Throtlghout this Fig. 5. Impulse circuit of telephone ing alit impulses. Turning the dial closes the shunt springs placing a shunt around the trans· mitter and recei,·er. This not only reduces the resistance of the telephone circuit to a minimum eluring the sending of impulses, but also prevents the imp\lIses from sounding in the subscriber's ear. 8 • • Principles of Automatic Telephony /Jio/ Steed Review The <lial is adjusted to operate at a speed of 1. Describe the normal line conditions. 10 impulses per second. HowC\·er. the switd1e,:; 'rhat change is made in the telephone circuit aiLer the hook is raised? in the centr<ll office are <ldjl1sted to operale 011 impulses receiq>(] irom a 1110tor dri\'en impulse machine rnnning at a speed oi SO per cenl greater than this or 13 impulses per second. Dials will PJicri1te tIle ;;witdleS sali~jact()rily ii running" as slow as 8 Tlllpulse;; pcr secoml or as fast ;IS 12. 3, \"hat change is made III the telephone circuit after the dial is turned? 4, \\'hat is the normal speed of the dial:' CHAPTEg J The 100 Line System ELEPHOKE Xo. 33 in Fig. S is shOI"n connected to a connector switch <llld its associate(l bank of 100 sets of Ene terllli~ l1als or contacts. contacts each, <I\HI the line bank cOllsists of 100 pairs of contacts arranged in rows the same as T Fig. 7. Bank multiple cable the pri\<atc hank contacts. contact bank. fig. 6 shows a line H(llIk 1llllUberill!! Fig. 6. Line ccntact bank The 11l1mbers appearing on the bank in Fig. 8 are the numbers of the telephones connected to lhose contacts. This arrangement of numbering i" lllilue necessary because of the mechanical operation of the connector, whose wipers connect with the bank contacts. It will be observed that II is the first C011taCt of the first row and that Contact Bank The bank is in tll'O part;;. The pri\'atc bank consists of 100 single contacts in 10 rows of 10 'PRI'I,,",e. &""~IC. I"'\I.)I.."T.;O c'T~'ER:GeNt-I~. 2.!. 21 .9l ~ 21 !!i 12 ilH':> gjl ~---t.!' ~ Jl!' 1!. u ... iI========U 4--+----.".~' ~"""'="=;_=:-;;;-;;-~"'~.""--""o""',,,>l,,'"l!<.l '~""·'-=-----j.33 (,,1 ... CO.... N. ~--------'!:g L ~ga~~~B~~~ , ~ ~O~~~~~~~~ 'll===== =====90 &\==========~o 11==========10 tol= = === ====="0 SI==========h _)o.IoD. 4\--------~I if @ Q Llw'I!!. ap.'N'l<.. Fig. 8. C I ~D 11==========10 1~\"!o141' I" 11 16 I'!) c..c. Bank numbering scheme and multiple and normal connections '9 I TEH.... '" P r in ciples of Au torna tic 10 is the 10th contact of the first row. The reason for this is that when the dial is turned from "0," 10 impulses are sent out. Therefore. dialing the digit ''1'' would step the connector wipers opposite the first row of contacts and dialing the "0" would cause them to travel 10 contacts in a horizontal direction, thus coming Telephony In Fig. 9 is shown the simplest form of an automatic telephone system. Each telephone is connected direct to a connector and the capacity of such a system is limited to 100 lines or the capacity of one bank, as discussed aboFe. [~L.. ch telephone number has two digits. Bank 11lultiple :J. To provide means whereby each telephone may connect with anyone of all the other telephones, similar contacts on all banks are multiplied together. Fig. 9 shows three telephones, numbered 11, 10 and 55; these numbers wOlllel appear in exactly the same positions on the bank of all of the connectors. The physical form of the bank multiple is shown in Fig. 7. In the figure the connector of telephone "!\ o. 11 is shown with a cut-off relay which is operated when the telephone receives a call, disconnecting the line relays, etc., of the connector from the line so they \vill not interfere with ringing the called telephone. In explanation of the above, it should be said that a system employing a connector for each telephone is rarely, if ever, installed. The idea is used here merely to convey an understanding of the system. ~_~ - - - ---~----- - - - - - - - ~ il"~'\l~Te:. '&P\'N'lC.. ~---------------~--------~ Review Fig. 9. Showing connections for a simple 100 line system Fig. 8 to rest on the tenth set of contacts in the first level or on the terminals of telephone number 10. The normal wires are a multiple of the subscribers' lines from the line switches to the connector banks, and are comparable with the multiple between the answering jacks and subscribers' multiple on a manual multiple switchhoard. The private wire in an automatic system performs the same function as the sleeve wire in manual practice, while the and --- lines correspond to the tip and ring wires. 1. \Vhat are the two banks of a switch called? 2. \Vhich bank has double contacts? 3. \Vhat are the normal wires? manual equipment. Compare with 4. How many impulses are sent over the line when o is dialed? Fig. 9 5. 'What equipment is necessary for a 100 line system? 6. \Vhat is the function of the cut-off relay? + • 10 \......_-----------------------------------------------~--------------------------------------------' ----_ - ...... Principles of Automatic Telephony • R't!\"Pl'f 'lin.L ,.c.... ----t-"-.:::= (011.. N*~"1. PaST" NORMAL Pl~ ~"fT (.l,lp $"IM~ O~F "'-RM"'l SPJllM U,,"~ ~~"f'T 68'1'-\W4' 'lER.T1C,F\I,. ""1'\&" 00UeU! tlQ~ SPRIM~ • STAT\ONA~Y Do<!( Wl'.'TIC ...... Co ~ \I1!.~T\c.'O. WOTCrl.£S It!:u:..t\s't! !'IRJ"f\iUl.'!. \lER'TltP\\.. PI'WL RoE-LEMa LI~lC. ItO'TP\R."( OOlS!. RoT"'C.'( P"\H\". qUID" PeCoT. RoT~Q,Y NoTC.l\e.S $~",rr 'A.'TARY MP.6~i: • • Fig. 10. Connector switch II I Principles of Automatic Telephony CHAPTER" The Connector Switch ilE connector switch consists of sel'eral rcla~'s mounted all a base, a ftame of cast iron on which are mounted two bearin).:"s. a shaft capable of being dri\'en step by step by magnets first in a vertical direction and then being' given a step by slep rotary motiol1, and a set of banks to which are connected the lines of 100 telephones. To the shaft is auached a pair or bronze wipers for making cunnectiOIl to the lines leading to the telephones: In Fig. 10, the parts mentioned ahOI'C as well as 111an,l' others which go to make IIp the C'lll1plete conncctor will he found. An established connection between two suhscribers of an automatic s~'stem may. and u:>ually does, in\'oil'e line switches, selectors (the llumbel' of selectors in the series is determined by the size of the .<;)'stem) and a conncctor. Regardless of thc amount and kind of equipmcnt comprising an e:.:chan~e the connector is usually op~rate(1 by the dialing of the last two digits of the tde· phone number. This is true regardless of the number of digits comprising thc number. The many dUlies which the COllncctor performs arc listed below as its functions and are named in the order in which they take place in operation. The functions of the connector resemhle those of the manual operator and the conncctor corre· spoll,ls with the equipmC'llt with which she COIllplel(~s tcJephone connections. Its hanks correspond to the line jacks ill a manual board equipped wilh 100 lines. The shait of the switch corresponds to the operator·s hand and arm, th~ wiper cords to thc switchboard cord, and the shaft wipers to the plug which the hand and arm of the operator cle\'ate1i to the (lcsired number. The functions 0f the cunUcl-tor switch are a,;; follo\\'s: J. Execute the \'ertical and rotarr 1110\'Clllcnt of the shaft under the control of the Ilial. 2. Keep the wipers di,;;cunnected during rotation SO as not to interiere with lines O\'er whose terminals they rotate. 3. Test the called line to SC(' ii it is husy. This busr test cOllsi:-:t5 oi t\\'o p,HtS: Guard ag-ainst cnllllccllOll with the busy linc. (b) rrO\'ide an audihle signal to notifr the tailing l'uh"criber that the line is busy. (II) T ~. Protect both the called and calling lines trom intnl$1on. :>. Clear the called line of attachmellts. B. C. O. rela~' Fig-. 9.) , (See • • Fig. 11. Rear view of connector showing wiring 12 , . P r inc iples of Au toma tic Telephony 6. Ring the bell at the called telephone. quarter to one-half the length of the core. The length of the copper slug is determined by the length of time it is desired to have the relay remain operated after its circuit has been broken. vor instance, a relay with a slug one-half its length, would remain operated longer than one \vith a slug one-quarter of its length, other conditions being equal. 7, Supply talking current to both telephones. 8. Release itself without interfering with other lines, 9. Release other switches (selectors, associated with it in the connection. etc.) Each function will now be taken up m its proper order and explained in detail. • COR.e.~ "P;'-;:A':o"T To ?UL.1. UP - 5Ld'N To RE.L~~SE. "6"-'51..O'N Ta PULl-Up "'Ul'N To REL!:.F\'SE,.. "C~SI..OW 'To "UI-L.l,)p SI..o'N "'1'0 R1:.L.I$.F\e;,e,. 'C):. ~~&T "Tb ?OI..l.. UP ~\-O'N It.EI..i;.ll'Ic:.1=.. "e::"~LO~ To ~1.:l1..1.. UP-SLoW To ~e\.'E:.!=Io:.e:coP?'eR. Sloe-ave. 8< !!!II..U CO( Cif''''E.'S Rel..~'<' p. G-R.E.~E.~ ltF\'NGiE. OF p\t):rUc:.Tl'Is.~.,. Sl-u4t.~ ""0 Et4D "lEW, @ .LSI.I.:lGr RR~· -T EWD 'MITIoI. 'SLoe.e:.\l= ct4 C4~ . E Fig. 12. f Conventional symbols for slow acting relays Slow Acting Relays Copper slugs In the operation of some circuits, such as the impulse circuit which will be later described, it is necessary that certain relays remain operated while their circuits are momentarily broken. To accomplish this the slow acting relay shown in Fig 12 is used. This relay has a copper slug fitted tightly to the core. The slug extends from one- Placing the copper slug at the heel end of the core does not delay the armature in pulling up when the coil is energized, but does make it slow to release when de-energized. This type is illustrated in Fig 12. (A and D.) \Vhen the copper slug is placed on the armature end it will, in addition to making the arma13 .. Principles of Automatic Telephony ture slow to release, also cause it to be slow in pulling up when the relay coil is energized. This type is illustrated in Fig 12. (B and C.) The instant the shaft is elevated one step, the off normal springs are released mechanically. They now shift their positions so that springs No.1 and No.3 separate and No.2 and No.3 make contact. \Vhen the next or second impulse from the telephone again de-energizes relay A, the circuit is closed from (+) battery at its springs, through off normal springs 2 and 3, through make contact relay C, winding relay C, vertical magnet to (-) battery. Relay C, being slow acting remains energized while the impulses are passing through its winding. Each succeeding impulse of this series takes the same circuit. After this series of vertical impulses has been completed, relay C, receiving no more current, falls back and switches the impUlse circuit from the vertical to the rotary magnets through its back contact. Connector Circuit Impulse circuit Fig. 13 shows the impulse and vertical and rotary magnet circuits of a connector. Assuming that the calling subscriber's line is connected directly to the windings of relay A, when the receiver is lifted or removed from the hook, relay A will be energized by current flowing The impulses delivered to relay A by the dialing of the next digit have a similar effect except that each time relay A falls back, it completes a circuit from (+) battery through its back contact, through make contact relay B, off normal springs Nos. 2 and 3, back contacts of relays C, G and H, through the rotary magnet to (-) battery. This steps the wipers around in a horizontal direction. Fig. 13. The !,~ M. F. condenser in series with the 10 ohm resistance to ground connected to the make contact of relay B prevents undue sparking at the springs of relay A during impulsing. Impulse circuit of connector from (+) battery through one winding, the (+) side of the line, through the impulse springs at the telephone, the (-) side of the line, the other winding of relay A to (-) battery. Relay E is connected in multiple with the rotary magnet and receives a portion of the impulse current delivered by the A relay during the rotary motion. It energizes on the first rotary step and, being slow acting, remains operated during the series of rotary impulses. \Vhen operated, its springs form a shunt around the back contacts of relay G, so that in case G should operate, the rotary magnet circuit will not be opened because it is maintained through the make contacts of relay E. Relay G is the busy relay and operates each time the private wiper of the connector passes over the private contact of a busy line. Relay A, by the operation of its spring contacts energizes relay B which prepares the vertical magnet circuit to receive the impulses. The dial is now turned and impulses are sent from the telephone causing A to release a number of times in quick succession depending on the figure dialed. Relay B being slow acting holds up during the series of impulses. \Vhen the first impulse causes relay A to fall back a circuit is closed from (+) battery through back contact relay A, make contact relay B, off normal springs, through winding of series relay C, vertical magnet to (-) battery. This elevates the shaft one step. Testing for busy During the time of rotary movement relay E, due to its slow acting feature, remains operated, connecting the private wiper through to • 14 c .... Principles of Automatic Telephony 1O!1.'iE:.j'~K t '" r -~Ioola 'Zeo'" ~ I.INe. •..'1 'Zoo ONE paR. CONwEGToR. l">'~O\)i". Fig. 14. , ... • Circuit showing busy test of called line the winding of the busy relay G. Fig. 14 shows this circuit in detail. It should be noted here that whenever a line is busy, either by reason of being called or by making a call, the private bank contact of that number is connected to (+) bil-ttery. vVhen the private wiper reaches the private bank terminal of the number, a circuit is completed from (+) battery to the busy relay G tbrough the contacts of relays E and H. Relay G, operating, connects the busy tone to the calling line. Relay E, the rotary impulses being finished, now releases, transferring the circuit of relay G from (+) battery on the private bank contact to (+) battery on the springs of relay B. Locking the circuit of the busy relay in the foregoing manner assures that it will not permit the connector to cut in on the line when the called line becomes disengaged, or in other words, when (+) battery is removed from the private bank contact. Relay E is equipped with "make before break" springs so as to accomplish the transfer of this circuit from the private wiper to (+) battery of relay B, without any chance of the circuit being broken. The rotary impulse circuit is opened at relay G, (Fig. 13) when the connector is giving the busy signal to the calling line so that in case the calling subscriber operates the dial, the connector cannot step from the busy line. The only way to accomplish connection to a number which has been found busy is to hang up the receiver, allow the connector to restore to normal, and dial the number again. A line that is not busy has its private contact connected through the winding of the cut-off relay of its line switch to (-) battery. Assuming now that the called line is not busy, relay G will not energize, but, as soon as relay E has released a circuit will be closed from (-) battery at the cut-off relay, through its winding to the private contact and private wiper, through the back contact of relay E, the 125 ohm winding of relay H, the back contact of relay G, to (+) battery at the springs of relay B. This energizes the 125 ohm winding of relay H, giving it sufficient power to make its spring contacts, designated "X" in Fig. 14. This connects the 1300 ohm winding of relay H to (+) battery at the contact of relay B, operating relay Hits full stroke, and maintaining it in operation throughout the time the connector is in use. Rinping circuit Fig. ] 5 illustrates in addition to the ringing circuit, three other operations of the connector, v",hich are so closely related to the ringing and answering of the called telephone that they have been shown together. \Vhen the wipers of the connector have been elevated to the bank contacts of the called line and it is found to be idle, relay H immediately connects (+) battery to the interrupter equipment. This interrupting device operates the 15 Principles of Autom.atic Telephony F Ko"E~- CON~"'C.~"X" ON RELI'l'l' ·.,...·'To M"~" ~RST, .. Fig. 15. Ringing and ring-cut-off circuits using ground connected generator --- --- ---- - - ---- Fig. 16. Ringing and ring-cut-off circuits using battery connected generator 16 :::0:::=====" ...._~ .. _.-,-_ r' Principles of Automatic Telephony across the line (See Fig. 3) which allows the 200 ohm winding of relay F to receive direct current from the battery in series with the generator. This operates the armature of relay F far enough to bring the pair of springs marked "X" into contact, which closes the circuit of the 1300 ohm winding of relay F from (-) battery to (+) battery at the springs of relay B, thus operating the armature of relay F its full stroke, opening the generator and interrupter start circuits, and connecting the lines of the called telephone through to the windings of relay D, which operates. generator control relay, which furnishes alternating current from a ringing generator to the connector switch, through a resistance lamp, contacts of relays F and H, thence over the wiper to one side of the line, through the bell of the called telephone, back over the other side of the line and wiper, through contacts o~ relays Hand F, 200 ohm winding of relay F to (-) battery, thus completing the circuit and ringing the bell of the called telephone. The generator control relay is operated intermittently by a cam on the generator shaft or other form of interrupting device. The ringing circuit shown in Fig. 16 is the same as that described in Fig. 15 except for the following. The machine which furnishes the ringing current is in continuous operation; the generator is connected to the line through the 200 ohm winding of relay F instead of direct. The ringing induction tone is obtained by bridging a .005 M. F. condenser across the open line spring contacts on relay F so that a small amount of the generator cun-ent will pass back to the calling telephone direct instead of by induction. Since in order to obtain the ringing induction tone it is not necessary to utilize relay D, its windings are permanently connected to (-) and (+) battery respectively. During the period that the control relay is not operated, it falls away, thus disconnecting the ringing current from the connector. During this period the telephone bell will not ring. This is known as the "silent period." In the silent period the generator lead to the connector is connected direct to (+) battery through the back contact of the control relay, so that in the event the party answers during the silent period, the F relay will operate. Ringing tone The party making the call knows that the bell of the called party's telephone is ringing, becaus~ of the ringing induction tone heard in the receiver each time the control relay sends ringing current out over the line. Because the return path of the ringing current is through the 200 ohm winding of relay F, a certain amount of current is induced in the 1300 ohm winding, through the winding of relay D to the (+) side of line and thence through the calling telephone and over the (-) side of line to (-) battery at relay A. A portion of this induced current will be lost through the path to (+) battery at relay A before passing out on the line, but enough current takes the route through the telephone to give the desired tone. Cutting off the ring, reversal of battery, etc., is exactly the same in this method of ringing as described under Fig. IS. Current rez1ersal Relay D, operating reverses the battery current supply on the lines to the calling telephone. The copper sleeve and slug over the core of relay F give it a greater range of adjustment. This prevents its operation when the ringing current passes through its 200 ohm winding. The copper sleeve and slug reduce the impedance to the ringing current. When the called party answers, the removal of receiver places the transmitter and receiver Fig. 17. 17 Transmission circuit with battery to calling line reversed f Principles of Automatic Telephony .:ontacts of relays F and D to the supervisory lamp, causing the lamp to glow. Under these conditions, the release magnet circuit of the connector is open at the springs of both relays A and B. This reversal is accomplished by the springs on relay D which are of the "make before break" type and therefore operate without opening the line circuit. Opening of either one of the lines would de-energize relay A and might cause the connector to release. This method of supervision is similar to that employed On cord circuits of common battery manual switchboards, and assures that any connection being held by either the called or calling party will be instantly observed by the attendant. Transmission circuit The transmission circuit of the connector is shown in Fig. 17. Battery to the transmitter of the calling telephone is fed through relay A while battery to the called telephone is fed through relay D. These two circuits are separated by condensers which provide a path for the voice currents to pass between the two telephones. The spring contacts of the connector through which the conversation passes are shown in this figure. Battery to the supervisory lamp may be disconnected by means of the switch, in those offices where it is desired to supervise for held connections at certain times throughout the day. Release magnet circuit Normally the release magnet circuit of the connector is open at the off normal springs. vVhen the connector is seized, the release magnet circuit is also open at the springs of relay B. The first vertical step of shaft operates the off normal springs closing the release magnet circuit at that point, and makes the release of the connector dependent upOn the replacement of the receiver by both parties. The battery is reversed to the calling party for the purpose of operating a service meter or to give supervision in case the call originated at a manual board. Disconnect supervision Following a conversation, as shown by the circuit in Fig. 17, should the calling party place the receiver on the hook before the called party does, the A relay will fall away and by the (+) battery on its spring contacts, relayed through the contacts of relays Band D, cause the supervisory lamp to glow. It will be noted that the connector release magnet circuit remains open at the springs of relay D. The 500 ohm N.!. resistance wound on top of and in multiple with the winding of the release magnet prevents undue sparking at the off normal springs when the release circuit is opened. As previously explained, the connector is not released until both parties have replaced the receivers, although the remainder of the switches release immediately following the replacement of the receiver by the calling party. Should the called party replace receiver before the calling party does, relay D will fall away connecting the (+) battery from springs of relay H, Assume that the calling party is first to replace the receiver. Relay A will fall back, but relay B, being slow acting, remains operated long enough to close a circuit from (+) battery at the back con- o 1 e55{------=R~o... llo1 ~ 3~ ~ ~ Fig. 18. Fig. 19. Disconnect supervisory circuit 18 A~ Release circuit of connector • Principles of Au tom a tic Telephony Review tact of relay A, make contact relay B, off normal springs, \vinding relay E to (-) battery, thus operating relay E and disconnecting (+) battery from the release trunk. Removal of (+) battery from the release trunk permits all switches associated with the connector to release. Relay E restores to normal and places (+) battery on the release trunk again from the springs of relay H. This (+) battery protects the connector against being selected for another call while it is held in this manner by the called party. 2. vVhy are slugs sometimes placed on the heel end of the core and other times on the armature end? Replacement of the receiver by the called party allows relay D to fall back, closing the releas~ magnet circuit allowing the connector to release. S. vVhat relay vibrates in response to the impulses sent from telephone? Fig. 12 1. \Vhy are slow acting relays used? Fig. 13 3. \Vhat relay on the connector is energized when the receiver is removed from the hook? 4. \Vhat are impulses? 6. Trace the vertical impUlse circuit. " .TIt I. Trace the rotary impulse circuit. 8. Why is the C relay called the "series" relay? 9. At what point is the impulse circuit switched from the vertical to the rotary magnet? • 10. \Vill the resistance of the rotary magnets increase or decrease if they become heated from continued operation? Fig. 20. Protection of connector during interval between hanging up of calling and called subscribers ./ ~ .• Fig. 21. Complete connector circuit 19 Principles of Automatic Telephony Fig. 14 20. Can ringing be stopped during silent period? 11. Give the letter and the resistance of busy relay. 21. Describe the purpose of the two windings on ring cut-off relay F. 12. What part docs relay E play III giving the busy tone? • Fig. 16 13. What condition does the private wiper find on the private contact of a busy line? 22. How is the ringing induction tone furnished to the calling party? 14. Why is the rotary magnet circuit opened when busy relay is operated? 23. How does the ringing circuit in this figure differ from the ringing circuit showll in Fig. 15 ? IS. "Vhat is the procedure for a party who receives the busy tone? Fig. 11 24. To which telephone is the current supply reversed? ,- \\'hy are make before break springs used on -,. relay "D"? 26. \-\ihich relay feeds battery to calling party's telephone? To the called party's telephone? Give the resistance of the battery feed relays. Fig. 18 28. Describe the conditions when supervisory No.1 causes pilot lamp to glow. 29. Describc thc conditions when supervisory :\0. 2 causes pilot lamp to glow. 30. ITow does this compare with manual cord supervision? Fig. 19 31. \Nhere is the release magnet circuit open, whcn the switch shaft is at normal? Fig. 22. Connector side of 100 line unit Fig. 15 32. How call the called party hold the release magnet circuit opcn? \Vhich relay remains operated? 16. How is the ringing interrupter equipment started? 33. \Vhich party controls thc release of the connector? _ .. 17. How is the ringing induction tone furnished to the calling party? 34. \ \'hy is rclay B made slow acting? 18. Describe how the "ring cut-off" is accomplished. Fig. 20 35. Which rclay connects (+) battery to the release trunk after relay B falls back? 19. How is the ringing interrupter stopped? 20 • Principles of Automatic Telephony CHAPTER 5 , The Selector Switch HE mechanism of the selector as shown in Fig. 23 is practically the same as that of the connector; it employs the same type of shaft, wipers, banks, release mechanism, etc. The $elector differs from the connector, in that only the vertical movement is controlled by the dial, which means that only one digit is re· quired to operate it. The rotary movement is entirely automatic and scn'es to select an idle trunk from the level to which the wipers are stepped by the vertical movement of the shaft. The selector functions are as follow5: 1. Elevate the shaft by means of impulses from the dial. 2. Find and connect with an idle trunk. (a) Rotate shaft aUlOmatically. (b) Keep circuit to line wipers open while wipers are rotating over bank contacts. (c) Test each trunk. (d) Stop rotary movement at first idle tnmk found. (e) Protect the trunk selected. (f) Extend lines through to next switch and disconnect attachments. T • .." .... _..- Fig. 23. Fill:. 23A. Switc::hroom view showinll: selectors 21 Selector switch Principles of Automatic Telephony 3. Release itself before or after it has cut through to succeeding switch. the call to that position over one of the transfer trunks and the connection would be completed to the subscriber's line by the operator in that position. It was soon discovered, however, that this was not the most efficient method. Consequently it was abandoned, and the plan of providing a full multiple of all subscribers' lines accessible to each operator was adopted. Development of the trunking for the automatic system was just the reverse to that of the manual. The first plans tried in automatic were based on the principle of the full multiple, but were later abandoned in favor of the transfer trunk principle. This method is still employed in automatic trunking. 4. Release itself without interfering with other trunks. 5. Give busy tone to the subscriber if all trunks of the level called are engaged. Transfer Trunks By the enlargement of the system from 100 to 1000 lines, it is obvious that some means of trnnking must be provided to select a connector in the proper 100 group. Early development in manual tel e p h 0 n y made use of the transfer trunk to secure connection between two subscribers in different groups. A number of such trunks were provided between operators' positions. An operator receiving a call for a subscriber whose line terminated in an adjacent position would extend IOOO • line systent The trunking arrangement of a 1000 line system is shown in Fig. 24. Instead of each tele- .. ----------~r~---~ -----------H~---, ===========++t==~ --------=========~... - - - - - - - - - - -.....++~---. == =======:>--.1 =========;;::::::==---I-I-i+=:ft.:====1 = = ==== == === '::)o----4-+--I-If-...J = = == = = == == == =====t:t::tI---J ====='=====~ == = == == = == == == == =:>----+-h I~======::::::::J IS.! SE:I.., BRNK.. / ---------- -----------1 ----------0 ----- ------1 IO-TRI.lI-ll<.~ 'FlOeotw'l =~"" SEI•.'E!.C.'TQR. L.E.'IEI. "To GON~~<'TOR"'" eF E.flIC. ~ (,~O\)P. I" L=========I"""'Ni \"" 6ll,O\lP ----- -----/11 ==========~ ===== =====~ ==========<1 ===========t ==========JIl ==========ll' ==========. ---------- Fig. 24. Trunking scheme for 1000 line system (3 digit) 22 ---------========== ===-== =-===== =========== COi'-l",.~N)(' Principles of Automatic Telephony , . a 100 into a 1000 line system it is only necessary to terminate the lines on selectors instead of connectors and to provide the additional connector groups. 'When telephone number 125 is dialed, the connection is completed as foHows: Dialing the digit '"1" elevates the wipers of the calling subscribel,"'s first selector up to the first level. The selector then automatically rotates its wipers over the contacts of that level until a trunk to an idle connector of the 100 group is found. Dialing the digit "2" elevates the wipers of the connector to the second level. The digit 5 is now dialed. This extends the connector wipers to the fifth set of contacts of the second row (or K o. 25). This connects the calling party's telephone vvith the called party's telephone No. 125. phone being connected to a connector switch as in the 100 line system, the telephones are now connected to selector switches. The selector selects a connector in the proper hundreds group and the connector the tens and units as before. This makes possible the enlargement of the system from 100 to 1000 lines. In a 1000 line system of this kind, there would be 1000 first selectors. The bank contacts of these selectors would be multipled together, and from each row or level of contacts, there would be trunks to the connectors of each 100 group of subscribers' lines. For convenience we will consider these trunks to be on a straight 10 per cent basis, that is, 10 trunks from each level of contacts of the first selector banks to 10 connectors in each 100 line group. Each telephone number of a 1000 line system has 3 digits. It will be observed that to change 10000 Line System To increase the number of telephones from 1000 to 10,000 it is only necessary to introduce LoCI • ...----t.....1t ===== =====}W\R~C""I~"" - - - - - - - - - IlURFI\.. -------------_ ------------_ -- -- - - --C.OMp\'I'lINT ........ ...... - - - - - - - - --\\oI"o~""\O,TIO'" - - - - - -----"T"\loJIe, ==========~oo ========== 3000 ----------~~~~-i ---------========== ~'!Ps~ ---------- ========== ---------- ==============~:=;II ---------l1JJ ,a.o ~~gi ~!.:;: c;1lI ==========~~~~~ = ==================:::::;----, =========.=~~ \~~ "TDT Fig. 25. \.. Trunking scheme for 10,000 line system (4 digit) 23 Principles of Automatic Telephony 2nd selectors between the 1st selectors and con· nectors. A diagram of the trunking arrangement of a lO,()(X) line system is shown in Fig. 25. (firsts, seconds, etc.), added. the capacity of the exchange (in lines) is multiplied by ten. Each telephone number in a IO,C<lO line system ha!> 4 digits. In a system of this sir.e the first selector selects the desired thollsand, the secont! selector the de· sired hundred, and the connect