US2333472A

US2333472A - Telephone system

Info

Publication number: US2333472A
Application number: US431076A
Authority: US
Inventors: Thomas F Crocker; Howard H Smith
Original assignee: Automatic Electric Laboratories Inc
Current assignee: Automatic Electric Laboratories Inc
Priority date: 1942-02-16
Filing date: 1942-02-16
Publication date: 1943-11-02
Anticipated expiration: 1960-11-02

Description

Nov. 2, 1943. T, F, RocKER ET 2,333,472

TELEPHONE. SYSTEM Filed Feb. 16, 1942 9 Sheets-Sheet 4 j 46| CENTRAL 43| 462 OFFICE REPT 432 L 4 L m0 /463 .1420 403 I J4,s4 435 LINE 4n PAD 422E" 430 42% MQ/v-wes N J- 433 P467 404 R405 f 434 *C425 v h4 3 l I I 42777 INVENTORS THOMAS E CROCKER BY HOWARD H. SMITH ATTORNEYS Nov. 2, 1943. 1-. F. CROCKE R EIAL 253339472 TELEPHONE SYSTEM Filed Feb. 16,- 1942 9 Sheets-Sheet 5 LINE CIR. I26

FIG 5 v 552/ THOMAS F. CROCKER BY HOWARD H. SMITH ATTORNEYS 1943- E f T. F. CROCKER ET AL 2,333,472

TELEPHONE SYSTEM Filed Feb. 16, 1942 9 Sheets-Sheet 6 ADAPTER CIR. I36

ezss TEST R630 I AREA I22 1 CENTRAL 2 OFFICE A I IZB TEST Beu DESK c952, GIZL 6T0 660 UPY INVENTORS THOMAS F. CROCKER ATTORNEYS Nov. 2, 1943. 1-. F. CROCKER ETAL 2- TELEP HONE SYSTEM Filed Feb. 16, 1942 9 Sheets-Sheet '7 OPERATOR v POSITION TRANS f 735 R770 REPT 765 REPEATER REC TRANS INVENTORS THOMAS F. 'CROCKER BY HOWARD H. SMITH ATTORNEYS Nov. 2, 1943' I I 1-, CROCKER- AL 2,333,472.

TELEPHONE SYSTEM Filed Feb. 16, 1942 8|8 IND mo. FIG LSSNS i s CONTROL our 9 Shgets-Sheet 8 C8|5 T0 FIG 86| CGGI -0 2 i C822 2:22 J I I 1 80! 806 807 3 2 2 v C425 COM. COM. GEN I87 TEST I j 52) j 0777) COM. fig 9812A: @320 v C824 8|4 ALARM 8331 l *9/ SIGNAL INVENTORS THOMAS F. CROCKER BY HOWARD H. SMITH mm wm ATTORNEYS 1943 T. F. C-ROCKE'R ET AL- 2,333,472 I TELEPHONE SYSTEM 9 Sheets-Sheet 9 INVENTORS Filed Feb. 16. 1942 W 6 7 T T T T M M M m y T f 9 v him. h v .2? R w w M M M M e 2:95 3 OJ R K K K w u m G R 4 i I G 9 6 F J w A N MN MT r QDOQQ Oh R 3 2 ll DH- 3 6 Q 9 m m. w P M M E85 8 0H0 gfi IK m w M M RL 1 DKO LI 5? T W. N Q W N J v WWO m l 1|. H m T Tm m 9 C M .u ww a gm u z w w CC C w ATTORNEYS Patented Nov. 2, 1943 'TELEPHONE SYSTEM Thomas F. Cracker and Howard Smith, Chicago, 111., assignors to Automatic Electric Laboratories, Inc., a corporation of Delaware Application February 16, 1942,-SerialNo.- 431,076 44 Claims. (01. 1r9 41) The present invention relates to telephone sys tems and more particularly to conference communication networks. a

It is an object of the present vide a conference telephone system comprising an improved amplifier network, whereby thesystem may serve a relatively large number of stations.

Another object of. the invention is to provide a conferencetelephone system comprisinganimproved impedance balancing network,. whereby the system may serve a number of lines having.

impedances varying over a considerable range.

A further object of the invention is to provide in a conference telephone system, an improved arrangement for preselecting different combinations of the stations served thereby as called stations and then for signaling only the preselected combination of stationswhen a call is initiated thereon. a

A still further object of .the invention is to pro vide in a conference telephone system including an operator position, an improved arrangement for rendering complete'supervision of the system to the operator at the operator position.

Further features :Of the invention pertain to the particular arrangement of thecircuit elements thereof, whereby the above-outlined and additional operating features are attained.

The invention, both as to its organization and method of operation, together with further objectsand advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which Figure 1 illustrates the general arrangement of a telephone system embodying the present invention; Figs. 2 to 9, inclusive, taken together, illustrate the details of the system having incorporated therein the features of,

the invention, as briefly outlined above; and Fig. illustrates the mode of combining Figs. 2 to 9, inclusive, in order to form a unified system.

Description of the apparatus incorporated in the telephone system Referring now more particularly to Fig. l of the drawings the telephone system there illustrated comprises a central ofiice serving three areas X, Y and Z, the area X being adjacent the central office, the area Y being somewhat removed from the central-office and the area Z being considerably removed from thecentral ofrice. The area X serves a number of telephone stations arranged in pairs IXA, DB3 and 2XA, 2XB etc.; the area Y serves a number of teleinvention to prophone stations arranged inpairs IYA, -IYB and 2YA,'2YB etc.;-andthe'area Z serves a number of telephone stations arranged in pairs IZA, IZB and ZZA, 2ZB etc. The central oflice comprises a number of pairs of -line circuits: I05; IOB-etc. individual to the pairs of telephone stations IXA, lXB etc, each telephone station being connected by a telephone line to the line circuit individual thereto. 'More particularly, the telephone stations IXA, IXB, 2XA and ZXB are individually connected by the respective telephonelines Hit,

, Atthis point it is noted.

' 102,103 and IM to the respective line circuits I05, wit-.101 and ,l0B; while the telephone sta-. tionslYA, IYB, 2YA and 2Y3 are-individually connected; by the respective telephone lines vI'I l, H2, H3 and H4 to therespective line circuits H5, H6, H1 and I I8. Finally the telephone stations IZA, I ZB, ZZA and 2ZB are-individually connected by the respective telephone lines l2l,

I22, .'l,2 3-and I23 and respective adapter circuits I35, [36,131 and [38 tothe respective linecircuits I25, 126,121 and I28:

k that each 1 of the adaptervoircuits comprises two portions respectively disposed in the area. Z andinthe central ofiice and connected together by atrunk line.- More particularly, the twoportions of the. respec-. tive adapter circuits 135,136, 131 and l38are.

connected; together by the trunk lines 13:, I32,

- The centraL-office comprises a'pai'r: ofbooster transformers MI, and M2, a simulating'impedance network l5! 'andfa paired operator position l52,an amplifier network Ill and a test desk t6].

Theamplifier network I" is of the well-known type 2| comprising an Aloopand a B loop. The A loop of the amplifier network Ill includes an operative connection to the simulating impedance network I5 I, an operative connection. to one-v half of the number ofline circuits individual to the telephone stationsin the area Z, an operative connection to the booster transformer l4! and an operative'connection to one-half of the number of line circuits individuallto the telephone stations in the area, X; while the B loop of the amplifier network Ill includes an operative connection tothe operator positionISZ, an, operative connection to theotherfhalf .of the number of line circuits individualto the telephone stations in the area Z, an operative connection to the booster transformer I42 and an operative connection to the other half of the number of line circuits individual to the telephone. stations in the area X; Finally the booster transformer MI includes an operative connection to one-half of network I1 I.

the number of line circuits individual to the telephone stations in the area Y; while the booster transformer I42 includes an operative connection to the other half of the number of line circuits individual to the telephone stations in the area Y.

In other words, the telephone stations IXA, ZXA etc., in the area X, the telephone stations IYA, 2YA etc., in the area Y, and the telephone stations IZA, -2ZA etc., in the area Z, are operatively connected to the A loop of the amplifier network I 1|; while the telephone stations IXB,

2XB etc., in the area X, the telephone stations IYB, 2YB etc., in the area Y, and the telephone stations I ZB, 2ZB etc., in the area Z, are .operatively connected to the B loop of the amplifier Finally the simulating impedance network I 5I is operatively connected to the A loop of the amplifier network I1 I; while the operator position I52 is operatively connected to the B loop of the amplifier network I1I. In view of the facts that the telephone stations in each area are arranged in pairs and i'espec'tively'operatively connected to the two-loops of'the amplifier net work Land the simulating impedance'network I5! and the operator position I 52 are-pairedand respectively operatively connected to the two loops of the'amplifier network "I, the impedances of the two'lool s of the amplifier network I1 I are normally balanced in a'manner more fully explained hereinafter. 1 v

Moreparticularly, the amplifier network IlI comprises two repeaters 200 and205, a transmisslori. equalizer circuit-210, a transformer 22!), a potentiometer 225 two' amplifier tubes 230'a'nd 240 and a control network connected and ar-- ranged in a manner more fully explained hereinafter. I I

Preferably, each of the line circuits is substantially identical tofthe line circuit I25 indi-' vidual td tl'ie telephone station MB, in thea'rea Z, which comprises. as best shown in Figs. 4, 5 and 6, a repeater 400, a line pad 430, asimula'tin'g pad 4m and a rela grcup,inciuding a line relay R405, a hold, relay R440.-an an swer reiay R450. a pad switching relay R460, -a, rinfg cu'toff relay R510, 'arelease relay R520, 3. ring "relay- Ri530. a lookout relayR5'40, a call rlay RliSllJa cutofi relay R580 an'd'a test relay R5105; Also the line circuit I26 comprises three test jacks i420, 3516 and J511 which are utilized for a purpose more fully explained hereinafter; as well as a control networkmcre full described 'sjub equently, v

The various line circuits are-directly connected to the telephone lines extending to the'teiephone r stations in the areas X and? and to the adapter circuits individual tothe telephone stations in the area Z, as previously noted and as clearly indicated in Fig. '5. The line circuit I2Bi11 lls'trated is individual to the telephone, station I ZB the area Z; accordingly the. a dapter. ,:circu it I38 is interposed between the line circuit "I26 and" the telephone line I22 extendingito the telephone station IZB, as previously noted. I Preferably, each of the adapter circuits is identical to the adapter circuit I36 wh.ich"cofnprises. as shown-in Fig. .6, two repeaters 640 and 550 respectively disposed in the central oiiice andflin the area Z, a-simplexre'lay-R520 and a'te'st relay R630 disposed in the central ofiice,and a control network connected and arranged in a manner more fully described hereinafter.

The simulating impedance network 'I.5I comprises, a'sshownin'fFig; 3-, a" repeater 300, an impedance element I305, asimulating pad 3 I 0, a

test jack J320 and a control network connected and arranged in a manner more fully described hereinafter.

The operator position I52 comprises, as shown in Figs. '7, 8 and 9, two repeaters 100 and 165, a line pad 130, a simulating pad 1I0, two impedance elements I105 and I814 and a relay group, including a pad switching relay R110, an answer relay R830, 2. cutofi relay R840, an amplifier start relay R850, a supply relay R880, an alarm relay R810, a call relay R880 and three group ring start relays R550, R960 and R910.

Also the operator position I52 comprises a receiver.151, atransrnitter 159, a transmitter shunt key K158, a'hook switch S160 and a receiver ampliiier' network, including a transformer 135, an

amplifier tube 144 of the type 1H4G, a repeater 140, a potentiometer 143, a receiver transfer key K and a control network connected and arranged in a manner more fully described hereinafter. Further thejoperatorpositicn I52 comprises a control key set including a commonitalk key K800, ,a common ring keyK805, a common start key K8I0,' a common test key K820, a plurality of individual ring control keys K821 etc., respectively corresponding to the line circuits I26 etc., a plurality of individual talk keys K9I0 etc., respectively corresponding to the line circuits I25 "etc., a plurality of individual ring start keys K920 etc., respectively corresponding to theline circuits I 26 etc., three group ring start keys K941, K942'and- K943, respectively corresponding to turjeei mups of theline circuits I26 etc., and an all-groups ring start key K944. Further the operator position I52 comprises a set of control lamps,-includi-ng a common answer pilot lamp L820, a plurality of individual. answer lamps L8I1"etc., respectively corresponding to the line circuits I26 etc., a plurality of individual line lockout lamps L819 etc., respectively corresponding'to' the line circuits 126 etc., a plurality of individual ring lamps L93I etc., respectively corresponding to the line circuits I25 etc., and a plurality of individual call lamps L932 etc., re-

spectively corresponding to the line circuits I26 etc. Finally the operator'positi'on I52 comprises a control network connected and arranged in a manner more fully described hereinafter.

Eachdf the telephone stations 'IXA etc. compi'ises -converitional telephone station equipment meme-m 'a recel've'r, a transmitter, a hook switch, a ringer and a control network. Also, the test desk lG-I indicated in Fig. 6 comprises conventional test desk equipment.

The amplifier network I1I comprises, as shown in Fig.2, a control network-including an amplifier start slave relay R2 50, a filament alarm relay R210, a plate alarm relay R288, a monitor jack J290 andeanalarm lamp L29I. Each of the amplifier tubeslzea and 240 is of the type 251cc; comprising a plate, a' cathode, a cathode heater, a screen'g-rid and acontrol electrode. The

cathode heaters

235 and 245 of the

respective amplifier tubes

230 and 240 are normally connected in series circuit relation with the winding of the filament alarm relay R210, whereby the latter relay normally retained in its operated position and the cathodes 232' and 242 of the respective amplifier tubes 2 30 and 240 are retained in electrone 'nitting condition. Also tha mplifier network I1I comprises a cathode biasing circuit, including the resistor 253 and the

condensers

254 and 255, connected between the cathodes 232 and 242'and the

control electrodes

233 and 243 of the respective amplifier tubes 236 and 240.

The repeater 266 comprises a primary winding 20! connected in theplate circuit of the amplifier prises two impedance elementsZII and M2, a

resistor 2I3 and two condensers 2I l and 2 I5,'and is adapted to insure substantially uniform response of the amplifier tubes 232 and 2M! over the-audible frequency band. Also the transmission equalizer circuit 2 II] is connected to the primary winding 22I of the transformer 226, the secondary winding 222 of the transformer 229 being connected to the potentiometer 225. Finally the potentiometer 225 is connected by way of two resistors 25I and 252 to the control electrodes 234 and 2'44 of the respective amplifier tubes 236 and 246. Further it is noted that the output circuit of the tubes 23!! and 248 includes a source of anode potential having its negative terminal connected to the upper terminal of the relay R280 and its positive terminal connected to ground. I

Considering now the normal condition of the amplifier network Ill and the two associated loops, it is pointed out that the amplifier start slave relay R255 normally occupies its restored position, whereby the two parallel connected arc-- plifier tubes 266 and 2A!!! are normally nonconductive. Also, normally, the receiver at each telephone station occupies its on-hook position, thereby to interrupt the connection by way of the associated telephone line to the individually associated line circuit. Similarly, at the operator position I52, the receiver I51 occupies its on-hook position with respect to the associated hook switch S166, wherebyv the equipment at the operator position I52 normally occupies its released condition. Accordingly the repeaters 461' etc. in the various line circuits I26 etc. are normally unloaded; and the repeater I60 in the operator position I52 and the repeater 366 in the simulating impedance network I! are normally unloaded. At this time the A loop extends from ground by way of the secondary winding 262 of the repeater 266, the contacts 323 of the test jack J326, the primary windings 362 and 36! of the repeater 366, the contacts 322 of the test jack J 326, the primary windings of repeaters in line circuits not shown, the primary winding of the repeater in. .the line circuit I25, the primary winding of the booster transformer MI, the primary windings of repeaters in line circuits not shown, the primar winding of the repeater inthe line circuit- I25 and the windings 266 and 268 of the repeater 265 to ground; and the Bloop extends from ground by way of the secondary winding 222 of the reneater 266, the contacts 123 of the test jack J'I'Zil, the primary windings I02 and Ill! of the repeater Hill. the contacts I22 of the test jack J'IZG. the primary windings of repeaters, in line circuits not shown, the contacts 423 of the test jack M26; the primary windings 462 and MI of the repeater Me in the line circuit I26, the contacts 422 of the test jack J 420, the primary winding'or the booster ers in line circuitsnot' shown, the primary wind ing of the repeater in the line circuit, I66, and the windings 207 and 269 of the repeater-205 to ground. In View of the facts that .the'A loop ineludes the unloaded repeater- 306 in the simulating impedance network I5I and a fixed number of unloaded repeaters in a corresponding number of line circuits, and the B loop includes the unloaded repeater IIHI in the operator position I52 and, a fixed number of unloaded repeaters in a corresponding number of line'circuits, wherein the fixed numbers'are the 1 same, the impedances of thetwo loopsare identical; Accordingly the two loops are balanced with respect to the amplifier network! I I; and in the event the two amplifier tubes 236 and 242 were rendered conductive the two loops would conduct balanced currents. Hence, a stablecondition...would exist in the .re peater 265 preventing singing of the amplifier network :I'H with respect to the two associated loops. 7

As previously noted, the line circuit I25 individual to'the telephone station IZA, in area Z,'

and the line circuit I26 individual to the telephone station IZB, .in area Z, are respectively arranged in the A loop and in the B loop and are balanced-with respect to each other. More particularly, when the repeater in the line circuit I25 andthe repeater 466 in the line circuit I26 are-either unloaded or loaded the A loop and the B loop are balanced with respect to. the amplifier network Hi. This is accomplished by balancing the impedance of the primary winding of the secondary winding of the repeater 460 in the line circuit I26 when the repeaters mentioned are.

loaded. More particularly, in the line circuit I26, the effective impedance produced by the line pad 460, the secondarywindings 4ll3-and 404 ofthe repeater 40!], the upper and lower windings .of the line relay R465, the'adapter circuit I36,-.the telephone line I22 extending to the telephone station, IZB, in area Z, and the stationequipment at the telephone station I ZB is fixed at a given value when the repeater 460 is loaded by correctly, adjusting the line pad 430. Similarly, in the line circuit I25, the'effective im- .pedance produced by the line pad, the secondary windings of the repeater, the upper and lower windings of the line relay, the adapter circuit I35, the telephone line I2I extending to the telephone station IZA, in area Z, and the station equipment at the telephone station IZA is fixed at a given value when the repeater is loadedby correctly adjusting the line pad. Thus the effective impedances presented by the line circuits I26 and I25 to the amplifier network III are substantially equal throughout the range of frequencies which are effectively amplified.

- Also, in the line circuit I26, the impedance of the simulating pad M0 is initially set to equal the effective impedances of the line pad 430, the

adapter circuit I36, the telephone line I22 extending to the telephone station IZB, in area Z, and the telephone equipment at the telephone station IZB, whereby the repeater 400 is equally loaded bythe simulating pad M0 or the combination of the other impedances mentioned,

transformer I42, the primary windings of repeatphonestation IZB. Similarly, in the line circuit I25, the impedance of the simulating pad is initially set to equal the effective impedances of the line pad, the adapter circuit I35,"the telephone "line I2I extending to the telephone station IZ-A,'in area Z, and the telephone equipment at the telephone station I'ZA, whereby the repeater is equally loaded by the simulating pad or the combination of the other impedances mentioned, including the telephone equipment at the telephone station IZA.

The other pairs of line circuits individual to the other pairs of telephone stations in the Various 'areas are respectively arranged in the A loop and in the B loop and are balanced with respect to each other, in the manner explained above.

As previously noted, the simulating impedance network I5I and the operator position I52 are respectively arranged in the A loop and in the B loop and are paired with respect to each other. More "particularly, when the repeater 300 in the simulating impedance network I51 and the repealter 1-00 in the operator position 152 are either unloaded -or loaded, the A loop and the B 1001) are-balanced with respect to;the amplifier network -'I =1I. This is accomplished by balancing the impedance of the

primary windings

301 and 302 of the repeater 300 in the simulating imped'ance network I5I with respect to the impedance of the primary windings II and I02 of therepeater' I00 in the operator position I52 whenthe repeaters mentioned are unloaded, and by balancingtlie impedance connected to the

secondary windings

303 and 304 of the repeater 300 in the simulating impedance network 15I with respect to the impedance connected to the secondary windings 1G3 and I04 of the repeater I00 in the operator position I52 when the rep'eaters. mentioned are loaded. More particularly; in the operator position I52, theeffective impedance produced by the line pad I30, the secondary windings I03 and T04 of the repeater I00, the upper and lower windings of the impedance element 1105, the repeater T65 and the receiverlfii and the transmitter I59 at the operator position I52 is fixed at a given value when the repeater I00 is loaded by correctly adjusting the line pad 130. Similarly, in the simulating impedance network I5I, the effective impedance produced by the simulating pad 3I0, the

secondary windings

303 and 304 of the repeater 300 and the upper and lower windings of the impedance element I305 is fixed at a given value when the repeater 300 is loaded by correctly adjusting the simulating pad 3I0. Accordingly the repeater I00 in the operator position I52'will balance the repeater 300 in the simulating impedance network I5I when the repeaters mentioned are loaded.

Also, in the operator position I52, the impedance of the simulating pad "H0 is initially set to equal the effective impedances of the line pad I30, the repeater I55 and the receiver I51 and the transmitter IE9 at the operator position I52, whereby the repeater I00 is equally loaded by the simulating pad H0 or the combination of the other impedan'ces mentioned, including the receiver 151 and the transmitter I59 at the operator-position I52.

Accordingly the A loop and the 13 100p are balanced with respect to each other at all times due tc'the 3 relationship of the simulating imassaults including 'the telephone equipment at the tele-' pedance netwdrk 51 with respect to the 'operator position 152.

A better understanumg ot the connection and arrangementof the apparatus incorporated in the telephone system "will beiaci'litate'd from a consideration of the operation of the various pieces of apparatus incident to the extension of the various calls therein, as will appear herein'after.

Conference call vfrom the operator position Considering now the extension of a conference call from the operator position I52 to all or to selected ones of the telephone stations in each of the areas X, Yand Z, the operator first prepares the operator position I52 by removing the receiver I51 ir'om the associated switchhook, whereby the hook switch S is operated. When the hook switch S160 is thus operated there is completed, at the contacts 10I thereof, an obvious circuit for energizing the transmitter I59 in series circuit relation with the winding of the answer relay R830, thereby to cause the latter relay to operate. S160 is operated there is prepared, at the contacts I62 thereof, a circuit traced hereinafter for energizing the receiver I51. Upon operating, the answer relay R830 interrupts, at the contacts 832, a point in a circuit traced hereinafter for energizing the winding of the alarm relay R810; and prepares, at the contacts 83I, a point in a circuit traced hereinafter for energizing the winding of the cutofi relay R8 50. Also the answer relay R830 completes, at the contacts 833, a circuit, including the resistor 0M and the contacts I56 of the receiver transfer key K150, for heating the filamentary cathode I41 of the amplifier tube I44. Also it is noted that negative battery potential is applied byway of the resistor T48 to the control electrode 74% of the amplifier tube I04, thereby to establish a negative control bias for the amplifier tube 745 above its cutoff point.

The operator at the operator position I52 then operates the common start key K8I 0, thereby to complete, at the contacts 8I3 thereof, an obvious circuit for illuminating the common answer pilot lamp L820 in order to indicate to the operator that the telephone circuit is busy. Also, 'at'the contacts 8I2 of the common start key KSIII, there is completed a path for applying ground potential to the conductor C820 in order to complete obvious circuit for energizing the winding of the amplifier st'art relay R850, thereby to cau'se'th'e latter relay to operate. Finally, at the contacts 8 of the common start key Killil, there is completed an obvious path for applying ground potential to the ring start conductor C777. The application of' ground potential to the ring start conductor "C'III completes a circuit including the lower winding of the impedance element I305, the winding 304'01 the repeater 300, the resistors 3I2 and 3H, the winding 303 of the repeater 3'00 and the u per winding of the impedance element I305 for loading the repeater 300 in the simulating impedance network 'I5I. Also the application of ground potential to the ringstart conductor 61'1"! completes an obvious circuit, including the contacts 802 of 'the common talk key K800, 'io'r energizing the winding of the pad switching relay R'ITU, thereby to cause the latter relay to operate. Upon "operating, the pad switching relay RTIO 'co'in'pletes, at the contacts 113 and I15, apircuit including the upper winding of the impedance element 1105, the winding Also when the hook switch r 2,333,472 133 of "the repeater ic n, the simulating padIIEl,

the winding I04 of the repeater I and the lower winding of the impedance element 1105 for the ring start conductor C'I'II completes a plurality of multiple circuits for energizing the upper windings ofth'e pad switching relays in the various line circuits. The circuit ior energizing the upper winding of-thepad switching relay B400 in-the linecircuit I26 illustrated extends from the grounded ring start conductor C111 by way of-the contacts 552, the hold conductor C58I, the contacts l54and 443 and the upper winding of R450 to battery. When thus energized the pad switching relay R450 operates to interrupt, at the contacts 103 and 458, points in a connection traced hereinafter between the repeater 400 and the line pad 450; to interrupt, at the contacts Gill and 466, points in-a connection traced hereinafter between the line pad 250 and the adapter circuit I36 to prepare, at the

contacts

462 and 451, a circuit traced hereinafter for projecting ringing current over the adapter circuit I36; and to complete, at the contacts 454, an obvious circuit,'including the simulati'ng pad 410 and the secondary-windings '403 and 55d of the repeater 400, for energizing the upper and lower windings of the line relay R505 in series circuit relation, whereby the latter relay operates. Accordingly, at this time,'the repeater 400 is loaded. Also the pad switching relay R400 interrupts, at the contacts 459, a point in a circuit traced hereinafter for energizing the winding of the hold relay R440; prepares,*at the contacts i0I, a further point in the previously mentioned circuit for projecting ringing current over the adapter circuit I35; and completes, at the contacts 652', an obvious circuit for energizing the upper winding of the ring relay R530. Also the pad switching relay B400 completes, at the contacts 402, a multiple circuit including the contacts 525, the contactsiof individual ring control key K821, the contacts 534 and the resistor 810 for energizing the lower winding x of the ring relay R530, in the event the individual ring control key K821 occupies its operated position. Assuming for the present thattheindividual ring control key K821 occupies its restored position, the above-traced circuit for energizing the lower winding of the ring relay R530 is not completed; accordingly, the latter relay operates. At this point it is noted that the ring relay R530 is of the differential type and is not operated when both the upper and lower windings thereof are energized.

' 6 43 and'644 of the repeater 040 to induce a corresponding ringing voltage in the windings 64I and 042 of the repeater 640, whereby ringing current traverses a circuit extending from one terminal of the winding GM of the repeater 640 by way of the contacts 033, oneof' the line conductor sof the trunk line' I32, the windings "653 and-654 of the repeater 550, the other line conductor of the trunk line [82,.the contacts 630 and the winding 042 of the repeater 040 to the other terminal of the winding MI thereof. The

ringing current traversing the windings053 and 555 of the repeater'050 induces ringing voltage in the windings '05I and' 652' thereof; whereby ringing current traverses a circuit extending from one terminal of the winding 65! of the repeater 050 by wayoi oneof the line conductors of the telephone line I22, the ringer at the telephone station IZB-bridged between the two line conductors of the telephone line I2 2, the other'line conductor of the telephone line I22, the winding I 052 of the repeater 050-and the condenser 65! Upon operating, the ring relay R530 completes,

at the contacts 532, the previously mentioned circuit for projecting l ringing current over the adapter circuit I35. This circuit extends from ground by way of the contacts 462, 5 H, 55I, 5H

and ESL the winding 643 of the repeater 540,

etc.

to the other terminal of the winding 05I- of the repeater 050. Accordinglyth ringer at the telephone station I IZBis' operated inorder to indicatethe;presence of a conference call on the telephone system. V i

In "view' of the foregoing explanation or the mode of operation of the line circuit I'26-in' response to the application of ground potential to the ring start conductor'C'l'I'I, whereby the repeater 400 is loaded and ringing current isp'rojectedover the telephone line I 22 to the telephone station IZB, it will be understood that each'of the other line circuits I 05,- I05 etc. is

responsive to the a-pplicationof ground potenis projected over the associated-telephone lines IOI, I02 etc. to the telephone stations IXA, IXB

As previously noted, when the -cornmon start key KBIII was operated the amplifier start relay R850 operated. Upon operating, the amplifier start relay R850 completes, at the contacts 05L an obvious path for applying ground potential to the amplifier start conductor C8I 5," thereby to complete an obvious circuitfor energizing the winding of the amplifier star-t slave relay- R200,

whereby the latter r elay operates. Uponoperating, the amplifier-start slave relay R260'com pletes, at the contacts 262, an obvious "path for applying ground potential" to thescreen g'rids 233 and 243 of the respectiveamplifier tubes- 230 and 240 and'an operating'circuit for th two parallel connected amplifier tubes 230 and2fi0, whereby the two

amplifier tubes

230 and 240 are rendered conductive. The operating circuit for the two amplifier tubes 230 and 24!!) extends from ground by way of the contacts 252, the monitor jack .1290 and the primary winding 20I of'the repeater 200 to the plates 23f and 2d! of the respective amplifier'tubes 230 and i liijand from battery by way of the plate alarm relay'R280 andthe resistor 2-53 to the

cathodes

232 and 242 of the respective amplifier tubes 230 an'd 2f l0. Whenthe two amplifier tubes,;230and 2 l0ifare thus rendered conductive, space current traverses the two parallel connected amplifiertubes and the winding of the plate alarm relay R280 is energized. The latter relay now operates in the event the two amplifier tubes. 230 'and '240 are operating properly. Upon" operating; @the plate alarm relay'R280 interrupts, at the contraced circuit, including the simulating pad M10,

the repeater'400 and'the repeater 640-extends from ground by wayof the upper winding of the line relay R450, the winding 403 of the repeater 400, the contacts 463, the resistors 432 and 43I',

the contacts 46I, 54I, 55I, H and 63I, the winding 6430f the repeater 640, the contacts62I, the winding 644 of the repeater640, the contacts 634,

512, 553, 544 and 466, the resistors433 and. 434,.

the contacts 468, the winding 404 of the repeater 400 and the lower-winding of R405 to battery.

Whenthis operative connection between the repeater 400 and therepeater 640 is completed-the line relay R405 is-retainedin its operated posi tion.v Also the repeater 640 is operatively connected to the repeater 650 and the repeater 650 is operatively connected to the telephone-station .IZB; the two last-mentioned connections being identical to those previously traced for projecting ringing current from'the linecircuitI26 over the trunk line I32 and the telephone line I22 to the telephone station IZB. Hence the telephone station IZB is operatively connected by way of the adapter circuit I36 to the, repeater 400, which connection includes the line pad 430. Thus, at this time, the repeater 400 is loaded an amount equal to the prior loading thereof by way of the simulating pad4l0." Accordingly the impedance of'the-loop B is not changed when the callis answered at the telephone station IZB; and the A loop and the 3100p are still balanced with respect to each other. e 1 V Also, upon restoring, the pad switching relay R460 interrupts, at the

contacts

462, 461 and-46 I", points in thepreviously traced circuit'iorprojecting ringing current over the adapter circuit I35 to the telephone station IZB, whereby the previ ously traced circuit forenergizing the winding of the ring cutofi relay R5l0 is interrupted inorder to cause the latter relay to restoreshortly thereafter, the ring cutoff relay R5l0 being of the combination slow-to-operate and slow-to-release type, as previously noted. Also the pad switching relay R460 completes, at the contacts 469, a circuit, including the contacts 52I and 406, forenergizing the winding of the holdrelay R440,"therebyto cause the latter relay to operate. Also the pad switching relay R460interrupts, at the contacts-462, the previously traced circuit forenergizing the upper winding of the ring relay R530, thereby to cause the latter relay to restore. Previously, when the ring relay R530 operated it completed, at the contacts 533, an obvious circuit the ring relay R530 interrupts, at'the contacts 533, the above-mentioned circuit for illuminating the individual ring lamp L93I thereby to indicate to the operator at the operator position I52fth2tt the projection of ringing current over the telephone line I22 to the telephone station IZB has been interrupted. Further the ring relay R530 interrupts, at the contacts 532, a further point in the previously traced circuit for projectingringing current overthe adapter-circuit I36; 4

Uponoperating, the hold relay-R440 completes, at'the contacts 442,-an obvious multiple path-for applying ground potential to the hold conductor C581, thereby positively to insure the retention of ground potential upon the ring start conductor 'C'I'II. Further the hold relay R440 prepares, at

the contacts 43!, apomt in a circuit traced hereinafter for energizing the winding of the release relay'R520 andinterrupts, at the contacts 443, a further point-in the previously traced circuit for energizing the winding of the pad switching relay R460. Furtherthe hold relay R440 completes, at the contacts 444, an alternative path for applying ground potential to the conductor C826, thereby to insure that the amplifier start relay R850 is retained in its operated position. Finally, the hold relay R440 completes, at the contacts 445, the previously mentioned circuit, including the contacts 456', for illuminating the individual answer lamp L6H, thereby to indicate to the operator at theoperator position I52 that the conferencecall has" been answered at the telephone station'lZB.

After the conference call initiated at the operator position I52 has beenanswered at one of the telephone stations in order to cause operation of the hold relay in the associated line circuit, in the manner explained above, the common start keyKBIB may be returned'to its normal position. When the common start key K8I0 is thus returned to its normal position there is interrupted, at the contactstlll thereof, the previously traced original path for applying ground potential to the ring start conductor Clll. However, ground potential is retained upon the lastmentioned conductor due to the operated positionof the hold relay in the line circuit individual to the telephone station at which the conference call has been answered, as previously noted. Similarly, at the contacts 8i 2 of the common start key K8l0, there is interrupted the previously traced original path for applying ground potential to the conductor C325. However, ground potential is retained upon the last-mentioned conductor due to the operated position of the hold relay in the line circuit individual to the telephone station at whichthe conference call has been answered, as previously noted. Finally, at the contacts 8I3 of the common start key K8I0, the previously traced circuit for illuminating the common answer pilot lamp L820 is interrupted. In view of the foregoing explanation of the mode of operation of the line circuit I26 to interrupt the projection of ringing current over the telephone line I22 extendingto the telephone station IZB and to complete an operative connection between the telephone station IZB and the repeater 400, in response to the answering of the call at the telephone station IZB, it will be understood that the other line circuits are operative in a similar manner in order to accomplish similar functions in response tothe answering of the call at the corresponding telephone stations.

Accordingly, atthis time, an operative connection is completed between the operator position answered theconference call regardless of the loop including the answering telephonestation.

Now assume that the conference call initiated attheoperator position 1552 .has beenanswered atthe; telephone stations IXA, ,igYA, IZA etc! in winding 1 66 ofthe repeater 165 induces acorrepe d ng s s-eel v lta e i -the windings. a d l st t e r p ater 1- 51; wh y a c res n ing signal current traverses the previously traced circuit. including the windings 1133' and, 7-04- of the repeater T530 and the-,icon denser Hi9. The

signal current traversing the and 1.04 of the repeater 'lllilinducs a corresponding signal: voltage in the windings- 'l'flllanddlli'of the repeater 1B0, whereby a corresponding signalcurrez'itftraverses the previously traced circuit in-.

cliidi'ngfthe Bloop Thesignal current traversing the B loop, including the

windings

201 and 209 of the repeater 205, causes a difference of potential to exist between the mid taps between the windings 266', 268 and 231, 2091 ofltherepeater,

205,, whereby a corresponding s'ignal Voltage is impressed between the terminals of the transmission equalizer circuit Zlil and, consequently, across the terminals of the {primary winding 22I of the transformer 220. 'Thus, a corresponding signal current traverses the primary winding 22 l of the transformer 220" and induces a correspondi'ngisignal voltage in the secondary winding 222 thereof, whereby a corresponding signal current traverses the previously traced circuit includingthe secondary winding 222 of the transformer 220. and the potentiometer 225. The signal current traversing the potentiometer 225 produces a signal voltage change which is applied by way. of the resistors andf2'5'2' to the

control electrodes

234 and 244 oftherespective amplifier tubes 236' and1240; whereby the conductivity of the amplifier tubes 23B" and 24E) is changed. Accordingly the amplifier tubes 230 and 250' amplify the signal Voltage applied 'to the

respective control electrodes

234 and 244 thereof, whereby the amplified signal voltage is productive of an amplified signal current, traversing thepreviously traced circuit including the primary winding 20l of the repeater. 260 The amplified signal current traversing the primary winding Z'ill of the repeater 26E] induces a corresponding amplifiedsignal-voltage in thesecondary winding 202 of 'thetrepeater 203; which amplified signal voltage'causes a corresponding amplifiedsignal current to traverse the A and B loops. The amplified signal currents traversing the A and Shops respectively include the pairs of windings 268, 298 and 2B], 239 of the repeater 2H5; whereby the signal potential produced at the mid tapsbetween the pairs ofwindings 2G6, 208 and'2'il1, 2439 of the repeater 205 is canceled out. Accordingly the amplified" signal current traversingthe Aloop and theB l oopl is canceled out in the repeater fiilhQthereby preventing subsequent regeneration and the consequent singing of the amplifierj'network ll-l; I

Theamplifiedsignal current traversing the A loop includes the repeaters in the line circuits 1.5{125 etc. and the booster transformer Ml, whe eb wrre po dies ms i e sn l' re aeae sm is projected over thetelephoneline's, l0], L21v etc. to theftelephone stations IXA, IZA etc. The amplified signal current "traversing the booster transformer Hll' causes the last-mentioned transformer to inducea corresponding amplified signal-voltage in the. repeaters in the line circuits H5etc., whereby an amplified signal current is Projected over the telephone lines I l l etc. to the telephone stations IYA etc. Similarly, the amplified signal current traversing the B loop includes the, repeaters in the line circuits I06, vI26 etc. and. the boostertransformer I42, whereby a cor.- responding amplified signal current is projected .over the telephone lines 12,, I22 etc. to the tele- Dhone stations IXB, IZB etc. The amplified sign l-current traversin the booster transformer M2 eausesthe last-mentioned transformer to induce a corresponding. amplified signal voltage in the repeaters inthe line. circuits H5 etc., where.- by an amplified signal current is projectedover the telephone lines H2 etc. to the telephone stations etc; V

Also theamplified signal current traversing the B; loop includes theiwindings "llll and'102 of the repeater 10,0; whereby a corresponding amplified signal voltage is induced in the windings 103; and 7:04: of the repeater TOllin orderto cause a correspondingampli-fiedsignal currentto traverse the previouslytr-acedcircuit, including the windings l6! and T38 of the-repeater 765. A portion of the amplified signal current traversing the winding 75'! of the repeater 76.5 lay-passes; the, winding 168 thereof by way of a circuit including the condenser [63, the-contacts 'I SIof-th'e receiver transfer hey K150; the-winding 136 ofthe transformer 135 and; the contacts 154 of thereceiver transfer key K150, assuming that the receiver transfer key KISlloccupies ;its restored position at t time; The amplified signal current; traversing the winding 136- of thetransformer 1'35 induces a corresponding amplified signal voltage in the winding 'I3'l thereof, whereby acorresponding amplified signal current traverses the, previously traced circuit including the resistor 148. The amplified signal current traversing the winding-13! of the transmitter 135; and the resistor 14,8 varies the. bias applied to the control electrode 14.6 of

the amplifi r tube Hi l, whereby the conductivity of ;the lastmientioned tubeis changed in order to amplify further the signal current. The iurtheramplified signal current traverse an obvious circuit including the winding Ml of the repeater .140 and the electron-stream of the amplifier. tube T14 1, whereby a corresponding further amplified signal voltage is induced in the winding lfl of th repeater lililinprder to cause acorresponding further amplified" signal current to traverse an. obvious circuit including the potentiometer l ll The receiver 15'! at the operator position 52. is operativelyconnected-by way of the contacts 153 of the receiver transfer key K and the contact 762 of the hook switch S to. the potentiometer M3, whereby the further amplified signal current traverses the-receiver lBl in 'or'der' tocause operation thereof.

More particularly, the amplified signal current traversing the B loop includes the windings 4B! andfillZ; of the repeater- Mlmwherebya corresnqpdih mplified signal voltageis induced in thewindingsJQS and-A04 of the repeaterAflO in order to cause acorresponding amplified signal current-to: traverse the previously traced connection between the repeater 400: and the repeater till. The amplified signal current traversing the windings .5 3 and Mini, the repeater 540 induces all in the manner previously explained,

previously traced connection; mmudmgtne "-li n'e I32, extendingb'etweenthe repeater- 640 and therepeater 650i The'a'mplifiedsignaljfcurrent traversing thefwinding's 653 and 654 of *thej're- "peater 650 induces' a l'corresponding an plified I sign'alvoltage' in the windings 6 I and652'there- I of, whereby a Corresponding amplified signal cur-" zgsssyim 'fa c'orresponding amplified signal 'voltage in the I windings 6H and 642 thereoL-whereby aficorrespo'nding' amplified signal current "traverses the initiatiorfof. the present conference call from the rent traverses the'previously-traced connection,

includingthe telephone {line [22, extending-tothe telephone station IZB in-ord'erito cause operati'on I of the receiver fat the last-mentionedtelephone -s'tation, r f 3 When the person at any one'oi the telephone stations speaks a'signal current'isgenerated'by the transmitter'thereat in order to cause opera tion o'fth'e repeater'inthe associated line circuit.

This'operation oftherepeater in'the associated line circuit causes unbalahce' betweenthe A loop amine B loop, reg'ardlss of which loopincludes the repeater, whereby theJamplifier networl; III

I is controlledin order to return a corresponding v amplified signalcurrent over the and B.

' At any time .the operator at theo I rator po'sitions I52 may operate the receiver vItransier key. K150, therebyto cut outlthe local amplifier-net work and to connect the receiver [51 directly' to the'r repeater 165; When the receiver transfer the contacts I53 thereof; the previously traced operative connection between the' receiver I51 and the potentiometer 143'." Also, at the ContactsjIEB of the receiver transfer key K150, there is inter- I rupted the previously traced: circuit for heating the filamentary cathode'l'fl of the amplifierjtube 1. Further, at the

contacts

151 and 1 54 brthe receiver transfer key K150, the previouslytraced connection betweenflthe winding HS o f the transformer I and the repeater I65 is'iriterrupted;

Finally;,at the contacts I52 and I55Iof the re key-K150 is thus operated there is interrupted; at I I las t-rnentioned-frelayl remains its restored po- "sitioniri -view of the fact that the-ring relay-R530 ceiverftransfer key KTSOQan' operative, connec-' tion'is completdlbetween the receiver I51 and the'repeater I55. The last-mentioned circuit eX-' tends from the lowerte rrninal ofthe winding 1 68 I Off a D ate J' S byway o th Q t 7 5 I of thejfreceivertransfe'r key1K'I50, the receiver 'I5I,

the contacts I62 9f the hook switchSIGOi the contaots I52 of thereceiver tramsfer jkey K150 and the condenser 163 to the junction between the of' stations-"may bef'p recluded by I merely ;'oper'ating the respective individualring contrblkeys; In-th'e event the telephone station" lower terminal of thewinding I61 and the upp er terminal of,the winding 168 of the repeater 165; Furthermore, the "operatorat the operatorpos'ition I52 may at'anyjtimedisahlethe transmitter ,operator-position}.I 52. "In thisevent the operator at? the. operator po'sitiqn" I52 first operates I thej in- I v v l ri qqn m l' k' K82 i r sp ndin to thlin ma a ec equ tl i o 't eit ler .iphone station IZB. j Whe'n 'the individuahring I 1 i-''contr ol-'- key-K821 is thuspperated' there j is prepared, at the, contacts thereof; a: circuit traced "hereinafter for; energizing the "lower winding of he. v 'e e i hepb atcr n Q j lfi th proceeds to{initiatethe-extensionof the con ference call""in 'theniann'er'previously explained,

whereby;' the pa d switching: relaysin the various line circ uits-arepbtained; all in the manner pre viously described; "Inthe presentexample', when 'thefpad switchi'rig relay mo-operates in the line cirfcu'it I 26 it 'comiple t' hei n a tsil zfla mq l lew i for e a-Z n 't e; p and -lower-win'dings"of'the ring relay; R530;- This circuit extends mm. ground byway ofthe contacts 462 'and 525 a a-t upper winding of R530 it; ihatter'y rand fromground byway of the contacts F a I contacts- 534 and'theresistor SI 8 to battery. Ac-

in the line' 'circuitlz e areIenergiaedfWherebythe is or? the difier ential type; previously noted. Accordingly "the previously traced circuitfor' proe -j ectin'g 'ringing 'i'current overythe telephone r line 1-22 to th'ftelephone Z 'StatiQn IZB'fi' numbinpleted and the telephone station; IZB s not-Sig;

naledinconjuncticn with the-present'conference 1 call? In view 'oftlie foregoing-eiplaxiation of the arerangement for, p'recluding the {telephone station I ZB rroz rtemg signaledincident td thednitiation or. a= -'confei'ence call at the o-perator position I I 52, it will be readily understood thatthe signaling 10f IZB-is normallyhot signaled iniconjuncti'on with theiriitiation of conference calls; butfthe operator wishe'stosignal the last mentioned station 159 so that she may speak withoutheing" heard over the telephone system merely, by operating the transmitter shunt keyr K158. "When the transmitter shunt'key K158 islt'h s n cat nn obvious pathis completed for short-cireu'itin'g the transmitter I59 and'an obviousalternative holdi ingcircuitfor energizing the winding of the answer relay R830 is completed;

" In the'foregoingexplanation of the'modeof operation of the line circuit I28 incident to the initiation of a conference call from the loperator position I52, it was assumed that the individual ring'controlkey KBZI oc'cupied its restoredposi I tion', whereby the line circuit l 26 was' controlled orderto cause ringing current to 'be projected over thetelephone line I22-extendingtdtheftele phone station IZB. However, it 'rna'y be'desirable, for thefoperator at theop'erator position I 52: I

to initiate I a conference call inthe manner pre in conjunction with the present conferencecall; sh may? do -so'in laready" manner: by operating divi'dual ring startkey xszm even' though I Vidual-ringcontrol key K821 occu ies its opera edposition'i In the present example 'when the individual rin'g' start I, key K920 is thus op er ated there: is completed an} ohvious 'circ'uit rf'or illuminating the individual ring'lamp L93l and a I multiple path; -ginc1uding21the contacts534;th, individual ring -contr olkeyKQQTandxthe{con c ltactsi'zi and"=46 2- -for short eircuiting thelower I windingof the ring relay-R530 When the" lower I j winding of the ring relay R530is 'thus :short'ecir if cuited, while the' upper winding thereof is err-er:

gizeq the manner previously explained, the latter relay operates. Upon 'operating'fthefring re1ay' R530 completes, at the contacts 53'3,' an a1-1 ternativef path 1:61 shOTt-cirCuitin'gthe' 'f-lower j I winding thereof ,'-'this* 'circu-itincmdingthe indi d 525, theZl ower windin g iof 13,530, theeemi 'of' theindividual ring control keyKfiTl, the i y:

yidual ring ,ontrollkey Ka fl ;and the 2 contacts eluding :thejgronnded hold-conductor 1C 5 8;I ,for

' 5:25 andABZ". Also, upon ,operatmg',;t

' viously explained. The subsequent operation qf ,the'positionof the respectivein dividual ring I H e.; ef elay 530 completes, at the contacts 532, the p rev iously traced circuit .10: a proj ecting, ringing current over the telephoneline 'l 22 to the telephone station 'lZB', whereby the ringer at the last-rhentioned station is operated in the manner 5mener zi .-a

the line circuit I26,in responsetoI the-an I I Idf thecall .at the telephone station IZB, erthe telephorre line t2; same asthat previously described. I ;g ljtelephone 'statipn 423; endin- In h event he j at r m??? 7 t si tithe w in -2 1 255 iu thfir pein in sition I52 wishes to oinsure that each telephone the fipfeyiopsly thaoegcircuit fonepergizingthe jstation'lin ax firstgroup is signaled, regardless of wirighng of the ring reIay BS B B Finally I the 'r'eleas'e'relay R529'co-mp1etes, at tl 1e=c0ntrol 'keys 'in conjunction with-the presentcon;- -itactsuflfi an :obvious circuit for; energizing the 'ference call, this may be'acccmplished by operat- '1" windingfof, -pad switchingrelayfilfil),

1o ingthe first group ring start key K941, thereby 0 iihe'ibir I fcomplete anlobvioiis circuit for energizing the Upon i peiaiting the pad switching ,reie m'en il 0f the first group rt layfRqfill, Q9 Tcfbirhslt sf atithep cozitaetsffli'd, the pnexiicpsly whereupon the latter relay) operates. Upon'opgtiiac'ed Tcinciiitjincii din'g thesir nul'atin padll lfl, le'rating, the first group start relay R950 comfdfie'rieig'izjhgthe Iipper'and lower-windings 01" o pletes, at theicon'tacts 95] em, obvious baths irb'r the linel 'xelay 3265, thereby to cause-the latter Tapplyingyground potential to the 'v ariouIs \fc o hlrjelayito'gr'clprate. r o "diictors C952 etcQ eXtend iIig to-the corresponqin'g ff: Shortly 'after the ielease relay13115211 operates lincircuits J26 etc., whereby each of the li ne -cir -the hold'relay; Rffldfl restores, aspirevious'lyfnoted. un i'nentioned in the first. gronp is contro'll'e'd'in Upon xestorinljlthe'hdldlrelay Rflflinterrupts, at order to cause ringing current to be projected pyer Ythecdntatsdilia inuitiplelpointlinlt'he prefiously the associated telephone line-to the corresponding tfajceiiicilit for energizing the winding of the telephone station; "in the manner previonslyex- 50 afipliil'e 'snjergtrelaynasn; and,intrrupts, at the plained. Sir'nilarly -in the event the operator at fcbritaets'lsl 'i-fthe 'preivious'l trace'd circliitifor'filthe operator pos itio'nl 52 wishes to insure thateach illlfififiitin'g ,the individual answer lamp JLS Ill,

telephone istat'ion ins, seconcl group or-jathir-d thfbyjtolinficatlto the. operatorflatfthe opgroup is signaled, regardless of theiposition otthe eiat'or ion- 52 meme .person at the telerespective"individ'ijalring controlkeys inconjunce phone ta I n 'IZB' has fpl'aceii the receiver .of .tionflwlththelpresent conference call, this rnay be the telpliohe ik sti liri'entthereit Jlpon its asso accomplishedhyselectiyely operating the second ciatd.switcfihokiatptliis itinie. QAtIthisJ timE-Lthe and thirdgroup ring start keys K942 and K943 in confifneeZcEnnIition -is ma' lritairiellibtweenthe orderto' control, the. operation of, the vsecond-rand. opratorpo'sitioni-i 52 :andithe other telephone stative individual*ririg control keys in conjunction third group, ring start relays RSB O and R2910, in tionsi'wh-icfiareistillfconnected to the associated an; obvionsj znannjer, fiinally;in the eyent-rthe telephone lines In tiew ofithel'lfatIthatlthe bpe asoiatmeoperator position 152 wishes to-in- Si 'innIatingTpad M1] inflthe line-:circiiit '-f6-.mainsurelthat each telephonestation} in each groppris tainsTthe'ipatw A-thereinlloaded,' theiBlloop signaled, regardless of the position of the respecis' ipai'ritai'ned Lin Ta'; balancdeond'ition withj respect W 4 totheJA loopginlthe nianner,previolislyexplained.

with thejpresent conterence call, this ay beac- At any 'jtirneprior toLthelulti'mate' release of the ring;start relays R950, R950 and RSI-0 in, an ceivr'lof t complishedrmerely-lh joperating the all-groups the @co fe'rne colfinetition ,lthe'ipersc'in at the ringf-start key K944-,- ;thereby ;to' control each of telephone1station"2|ZBliriaylagainiiemovel'the re- I heteleliififie ihst'niiment'ther'eatTfiom obvious manner Y. 1; I 1 .50 its ass" 'itchhobkand -reentertheipon- ,No w assuming that-the present conference call I fergene 'c r'inc'tiim; 'Mojriipartlcularly,when'ithe progress when the person atthe-telephone V reente he tel phone trumefit gait thejtele- -station lZBIeplacesthe receiver of the telephone pheii'efistahpn -tz 1s 2lgaij1 1;iemovegi irmm JtS instrument -thereat :upon its associated -switchassociated lswltchhc'ifok tlief previously tracedfl'cfir- I hook the previously traced circuit for energizing 5-5' '(Sii'it ,forenerg'izingthe w'irifiiiig offthejfs'implex the winding of the simplex relay R620-is=-;i-nter relayI.B.i i20"is re conlpleteclithereby to oalise the. :r upted, thereby to :cause the latter relay to re I latter relay-fto'reoperate. vUpon reoperating, the

store. Upon restoring, ;the simplex relay-R520 siniplexrelaylRlfilfllcompletes, at'the contacts,

n rruptsat the contacts (52L thegpreviously thekpryiouslyfmehtioned alternative cirtraced connection extehdinghetween therrepeater 50 cuit forienergizing the ,windingof the -l i-ngi cut I QI I I and the, re eater 500, whereby the previously 01f relay'.R"5'lfl,' This.circ iiit"extends -trom ground restoring, t 1; v la 33,4132}interrupts,:wat111m the contacts-34, 572;,

traced circuit f or energizing the upper andlower byway: of the I epntacts462f5fll; '5 5] 571 andifitl, windingsof the-line; relay,R4D5 isinterrllpteddn the winding 6 33 .of the 1 'epeater-fi4fl, the- ,draer'to cause thelatter relaytorestore. Upon tacts 62!, the winding' M -cf then'epeaterilfl,

' 53, 544 and-tilhthewmdi'caiise I the latter relay. to .reop'erate.

' taqtsdflt, thepreviou sly traced circuit-.rf r?ening R510,-tlie-contacts 523 and the resistor--54 ergizing the windingofth ,hQ 1 3MB how: o ba tery; Sho;1 t y.- after the winding -of -mhe e Yer',-Tth latter relay does not immedi telyarer ring cutoff relay "R5 :is thus energizedethedatter tore as it is of-therslow-to-release type. Also the relay -reoperates; "Upon -,reoperating, the iringline relay Rfiqi completes, at thecontacts 013a .70 toif ar lasil-Rvfitfl completes,-at the contacts 45-11,,

cireuigincluding the -cOntacts -SZ-I, 469 and 41H a pa,th,-- 1' nc1uc; ingthengnt ct .ggg a m Hand d h resistor 1 f en i n he d n the erou dea hol d-reonductor aessi e rtsh the release relay R52 ll.-. when gthe release relay circniting the windin releasedrelay RHIQII as'zo'iis' thus energ i zed it operates to complete at I whereloyythe latter-relay restores. "Upon restore p I $2,333,472 ..cntacts;j..5.2 2;,':-'1the. previously. traced path i' for :.=sh01t -circ1Iiting;-the winding: thereof, and men rupts, at'the contacts 51:3,..the above-tracedal- .ternative circuit forx'energizing ethefwindin'g of athe ring cut-off; relay RSI llgfzaFurtherf the release 1relay R520 interrupts; at the "contacts 526,; the .previouslyjtracedcircuitfor energizing the lower windingofthe pad switching-relay Rtsmt ereb st'oring, Zthe pad;switching" re1aynR460 recomjpletes;;- at the-contacts. SI-{463,156 and 468, the

gprevious'lyz. traced; connection between the repeate'r 1408'. and the;- repeater; 640:1 whereby 1 the i tacts 469,: th .rpreviouslytraced circuit vfor .en-

' Ier'giz'ing"..the-xqwindin'griof the. hold relay. R440,

'etherebyt to .cause the latter? relaya toreoperate.

-pletes,*-..;at. thee-contacts 444, the Ipreviouslyl'mem- *tionedemultipleppath'forlienergizing. theiwinding of; the Zamplifier startirelay R850 l 3 and :recomipletes,gat' the contacts 445, the previously traced circuit. for. illuminating the 5; individual ;,answer I .lammLfl l] ithereby to. indicate. to: the. operator 6 previouslytraced circuit.fongenergizing-the :upp'er rand-lower windings.of:fsthe line ;rela'y';=R405J is i :recompleted,therebyrto retain the latter. relay'in :itszoperatedlposition.s;Also;Luponzrestoring; the

spad switchingrelays 'Rflifl .recompletes. ati'the concamplifier start slaverelay RZSm'thereby tccausLe --'the" latter relay to restore. r a Upon restoringi the r contacts. 262, the previously traced operating' cirq I f cuits. for. the parallel connected 'amplifierjtub'es .230. and.240,. whereby the plate alarmrelay ltnn restores. .lAt this time the amplifier networkli'l,l1

is completely released. The operator "at the op- Ieratorposition' l52then restoresthecommontalk xkey K8D0 thereby to interruptgat the contacts :80! sand'8ll3 thereof; thepreviouslytracedcon .nection extending between the reygieater; 159 and.

the: repeater 165,, whereby {the repeater "Hill is unloaded. Also,"ifwhen ground potential is removed from the ring start conductorcmjthe pree.

-viously 1-traced circuit for loading. the "repeater '300 in the. simulating impedance network l 5 l f is interrupted, whereby-the last-mentionedrepeater --is unloaded. Accordingly, atlthis time, there'- '-Upon1.-reoperating;: the hold .relayz-R ldfl'zrecom- 'peater 100 in the operator-'position l 52fithe re} peater 300: in the simulating impedance-network -|5|- and the repeaterqin each of 't he'lirie circuits" the B io'op are balanced at thisthnefas pr v cuh Y Y note'd ri l Finally the-:operator at the operator-"position ;l;52 may return thefrece'iverlfl .toits associated switchhook'," thereby. to operate the-hook switch sat-ithe operator' positioni ll52 that the .person-jat.

the telephone station lZB hasagain removedthe receiver of the. telephone 1 instrument thereat Irom'itsassociatedswitchhook, whereby; the tele- I i phone station "lZBfis again included in: the conierncemonhectioml; 1

At the conclusion of the conference connection;theperson..at each telephonestation replaces the receiver .of-athe telephone instrument .thereata upon its .associatedlswitchhook, whereupon the conferencetconnection islreleased. Now

assumingcthat .the r-person atithe telephone'; sta- .tion :IZB wasthe lastto replace the receiver. of. s the telephone instrument thereat upon. its a'ssociatedn's'witchhoohfwhen this-is done the {p e-.- viouslytraced circuit ifor energizing the wind;-

' ingwof the sim'plexrelay.R620 isinterrupted;' thereby to causegthe latterrrelay to restore. Upon 1 restoring, the simplex relay R620 effects the restoration of the-line relay R405;.theoperation ofthe'release relay-Rfii-land the "pad switching relay R460 and thereoperation of the line relay R405, .all-.- in the .manner previously explained.

S160. When the hook-switch S160 isthus opercall from a telephonestation tothe operator posi, ti on I52 and to all or selected; ones. of the other telephone;.stations in'geach of the? areas X; Y" I and Z; it is assumedathat the call is-iriitiat'ed'at.

. the telephone; station l ZB"in-. area The call is initiated'at-the telephone'station IZB in area-Z when the -person .thereat removes the receivers'of Shortlythereafterthe hold relay :RMD restores as v previouslyhoted. Upon-restoring, the hold relay' R440. interrupts, atthe contacts[442,the'last multiple. path :for applying ground potentialwto the'l'conductors' C58lriand C111. When ground potential: is removed L from the hold conductor C581 the previously traced. holding circuits forenergizing the windings' nf the answer relay,

R'ESO' and therelease relay R520, are interrupted;

thereby to cause the lattertrelays to restore; Upon-restoringfthe release relay "R520 effects the e'storation of the pad. switching rela'y R460,

in' the mariner previouslyfexplained. Also", upon restoring} the hold "relay-"R440 interruptsfat' the contacts 44'4Qthe' lastmultiple path \for applying ground potential to the conductor C826, whereby thegamplifier start relay R850 restoresP-When; 'g-roundpotential-is removed'from the ring start 1 conductor C111 'the'other line circuits are released.

amanner identical'to'the release of theline circuit l26-explained abovev" y Y v Upon restoring,-* the amplifier start relay R850 lnterruptsfiat the "contacts 51, the previously traced circuit or energizing the windin "Qf 1 1 ,.;.'T t.8 pplicationfofgroundpotentialto the! start fconduct'or c111 cempietesfthe previbusiy' the telephone instrument at the telephone=station .IZB fr m its associated switchhook; whereupon; thepreviously tracedsimplex circuit. for energiz -j ing thqyvvinding-ofthe simplex relay REM-is completed-in-order to causethelatter'relay to operate. Upon operating, the simplexrelay R620 I completes; at the contacts 162 I, the previously tracedconnectionjextendingbetweentherepeater 540 and the. repeater 409, whereupon the upper and "lower windings 0f*thei1in re1ayYR405'are' energized causing the last-mentioned "relay {to operatein the nanner previously explained.

UPO DD nthe linesfrelayiyR4fl5 completes,

at the contacts MIBQ-the previously traced-"circuit iopenergizing the winding of the hold relay 12,440,: thereby.,to cause -;.the latterrelay-F110. operate.= Upon operating, the hold. relay R 346 "completes; at.the;..contacts .442 the'j previously mentioned: path fof'r'gapplying *groundpotential toitheholdsconductor cssliaiium the ring start-conductor. c1111. Also the hold' relay Rdtlli interrupts-Jet the contacts V443,;a pointinthe previously traced circuitior ehergizingfthe upper-winding crudepad switching. relay jPAGfljfthereby positively to prevent.operationofthelatterrelay.