US1314685A - Trunking arrangement for automatic telephone-exchanges - Google Patents

Description

T. G. MARTIN.

TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE EXCHANGES.

APPLICAHON FILED MAY 29, I905- RENEWED SHLG, 1918.

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TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE EXCHANGES.

APPLICATION FILED MAY 29.1905- RENEWED SEPT- 61 I918. 7 1,314,685, Patentedbepl. 2, 1919.

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TRUNKING ARRANGEMENT FOR AUTOMATIC TELEPHONE EXCHANGES.

APPLICATION FILED MAY 29,1905. RENEWED SEPT. 6. I918.

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APPLICATION FILED MAY 29,1905. RENEWED SEPT. 6. I918.

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APPLICATION FILED MAY 29'1905 RENEWED SEPT- 6. I918.

Patented Sept. 2,1919.

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, EIIHEUJP 000! IIIIIIIIII provement in Trunkin UNITED STATES PATENT OFFICE.

TALBOT G. MARTIN, OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS, TO AUTOMATIC ELECTRIC COMPANY, A CORPORATION OF ILLINOIS.

TBUNKIN G ARRANGEMENT FOR AUTOMATIC TELEPHONE-EXCHANGES.

Application filed May 29, 1905, Serial No. 262,728.

To all whom it may concern:

Be it known that I, TALBOT G. MARTIN, a citizen of the United States of America, and resident of Chicago, Cook county, Illinois, have invented a certain new and useful Im- Arrangements for Automatic Telephonexchanges, of which the following is a specification.

My invention relates to automatic telephone systems in general, but more particularly to telephone systems of that type in which the apparatus for establishing connection between one subscribers line and another comprises a suitable number of stepby-step switching machines, and especially to automatic step-by-step telephone switching apparatus in which the current, both for talking and signaling purposes, and also for operating the various switching machines at the exchange or'central station, is all furnished from. suitable batteries and generators, or' other sources of supply, located at the said exchange or central station.

Generally stated, the object of my invention is the provision of animproved construction and arrangement by which the subscribers of two automatic telephone exchanges, of the above general character, may be able to satisfactorily converse with each other over trunk-line-circuits established from a calling subscribers line entering one exchange to a called subscribers line entering the other exchange, notwithstanding that ordinarily this could not be done satisfactorily, particularly where the length and resistance of the trunk-lines are comparatively great.

Certain special objects are to provide an improved construction and arrangement involving a so-called repeater by which a subscriber of one exchange may connect his line with a subscriber of another exchange; to provide an improved construction and arrangement whereby talking current from the battery or other central source of supply may be furnished to the called subscribers .line from the connector, and through the repeater to the calling subscribers line; to provide improved means for preventing the energizing current for the talking circuits.

from being furnished thereto until the called subscriber has answered; to provide an improved construction and arrangement involving the use of a difierential-relay in the said repeater, and of such character that the.

Specification of Letters Patent.

Patented Sept. 2, 1919. Renewed September 6, 1918. Serial No. 2s2,972.'

calling subscriber may be able to release-.- that is, to restore all of the different switching machines, such as selectors and a connector, which may have been put in use between the two subscribers lines, and in case the called subscriber fails to ban up his receiver; to provide an improve arrangement of such character that when talking current is put on the lines, the circuit in the said repeater, and which controls the operating parts in certain portions of the system, will be opened and made non-effective through the use of a so-called brid e-cut-ofi' and pole-changing-rela to provi e an improved construction am arrangement whereby a single repeater may be provided with each trunk-line, as will hereinafter more fully appear; and to provide certain details and features of improvement, and certain novel features of construction and circuit arrangement, tending to increase the general efliciency and serviceability of an automatic telephone system of this particular character, and tendingto insure satisfactory telephone service between the subscribers of different automatic telephone exchanges.

I have already referred to switching mato obtain connection with an idle trunk-line leading to the desired group of switches, such as other selectors or connectors, according to the size of the exchange. A selector, whether it be a first, second or third, etc., is preferably so constructed that it is under the direct control of the calling subscriber for the purpose of selecting a certain group of trunk-lines and switches. but is also constructed in such manner that it ceases to be under thedircct control of the calling subscriber as soon as such selection has been accomplished, the selector then proceeding automatically, and without the direction or control of the calling subscriber, to select the first idle trunk-line which it encounters, inasmuch as any nonbusy trunk line in the selected group will answer the purpose of the calling subscriber. A oonnector,-. however, is employed for making the final connection directly with the line of the called subscriber, and is di 'rected in its operations by and under the control of the calling subscriber throughout its entire operationthat is to say, it is under the direct control of the calling subscriber during the first part of its operation in which it picks out'the particular group of subscribers lines including the called subscribers line, and is then controlled directly by the calling subscriber for the purpose of picking out the desired subscribers line. In other words, both selectors and connectors can be operated'or controlled by a calling subscriber for the purpose of picking out any particular group of lines; but the two types of switches difler in this --"respect, namely, that the selection of a particular line by a selector is strictly automatic, and a thing over ,which the calling subscriber has no control after he has caused the selector to pickout a certain group of lines; while on. the other hand a connector can. be operated by the calling subscriber for the purpose of picking out a certain group of lines, audits further operation for the purpose of picking out the particular line with which connection is desired is under the direct control of the calling subscriber, so that in operating a connector, the selection of both the group and the particular'or desired line in that .group are determined'by the calling subscriber. This is obviously practicable, and the natural method of operation, inasmuch as it matters not to the calling subscriber what particular trunk-lines are brought into use in connecting his line with the called subscribers line; but when it comes to the final operation of selecting another subscribers line, the calling subscriber then wants the connection extended to a particular subscribers line, and to no other. Con-I sequently, a, selector is always free. to

.search through the selected group oftrunklinesuntil it finds one which is idle or not in use, while the connector must proceed directly to the point where it can establish connection with the called suhscribers line, and must then either establish such connection, or, in theevent that the called subscribers line is found to-be busy, then refuse to make the connection and'restore itself automatically to its normal condition. In this way, a connector cannot be made to cutin, so to speak, on a talking circuit which has already been established between two subscribers stations.

-To the foregoing and other useful ends is a diagrammatic representation of the substation A on the upper left-hand corner, in connection with its central station, which will be referred to as central No. 1.

F g. 2 is a like representation of another sub-station B on the upper righthand cora subscriber at sub-station B of central In Fig. 1, together with sub-station A (which will be designated as station #6555), to the left of line XX is representedthat part of the switching apparatus or mechanism known as a, first-selector-switch which belongs to sub-station #6555; to the right of line XX is that repeater.

In'- Fig. 2, together with sub-station B (which will be desi ated as sub station #2220), to the left of line YY isrep-resen-ted that part of the switching apparatus E known as a second-selector-switch, while between lines YY and Z-Z is that G that belongs to sub-station #2220,

Figs. 3, 4 and 5 respectively are diagram-' part D which constitutes my inventionand is known as 'a matic perspective representations of the fol-- 1 lowing switches: first-selector C, seconds selector E and .connector- F.

Fig. 6 is a simplified diagram illustrating the method of supplying talking current to the talking-circuit" formed by connecting together any two subscribers lines.

Fig. 7 is a simplified diagram illustratin an arrangement which may be employed f, with line-trunks,

other sub-stations in this system, sub-station A isconnected with its central station 'by two metallic conductors 2 and 3 known as vertical and rotary lines, and is pro-1;

vided with the usual ringer a receiver a, transmitter a, lnduction-coil a condenser a and switch-hook a", together with a signahng-button a "and a calling device represented as having two push-buttons a and a", in connection with a number of springs and other essential details of construction. The calling device, however, may be of any suit- .[able design. In making acall, the metallic lines are alternately grounded a proper number of times, whilethe release, z. e., the restoring of all switching apparatus aftera call, occurs when both lines are grounded simultaneously for a moment. The firstselector may be of any suitable, known or approvedconstructio'n, C in Fig. 3 being a representative and well-known type. Proceeding to describe the mechanical parts, a

represents a so-called vertical-line-relay with an armature c", andc" represents a socalled rotary-line-relay having an armature a The so-called vertical-magnet a has two coils 0 a, and an armature 0 which latter has two pivots 0 and c and is always under thetension of a retractingsprin as. The armature has an arm 0 provi ed with an L-shaped piece 0", and with a so-called vertical-pawl a, subject to the tension of a retractin -spring 0 The vso-called release-ma net 0 has an armature 0 provided wit two pivots a and 0 and is always under the tension of the retracting-spring 0 This armature has attached to it a so-called release-link 0 on the other end of which there is a slot 0 The said link, when the switch is in normal position, is enga ed with the so called double-dog c y means of the lug a. This double-dog has two pivots 0 and 0 and is under the tension of the retractingspring 0 it has two bent arms 0, c and a downwardly-projecting arm a, which latter engages with the link a. The so-called rotary-magnet a has two coils a o", and an armature a, which latter has two pivots 0 and c, and which is always under the tension of the retracting-spring c. This armature also has a so-called interrupter-arm a and a rotary-armature-finger c and another arm a that carries a so-called rotary-pawl c at its extremity, which pawl is under the tension of the retracting-spring c". The so-called"privatemagnet c has an armature 0 provided with two pivots 0 0, and a retractingspring 0 This armature also has an arm 0 which is provided with a lateral arm 0 and with a saw-tooth escapement .near its end having teeth a, c, 0" and a. The shaft 0 has, near its middle, a set of circular teeth c" by means of which the verticalmagnet 0 gives it a longitudinal step-bystep motion through the mechanisms on its armature already referred to. There is a groove 0 along these circular teeth which receives an arm-c with which the circular teeth engage, so that the shaft may be supported during rotary motion. Just below the circular teeth there is a set of longitudinal teeth a by means of which the rotarymagnet 0 imparts to the shaft 0 a rotary step-by-step motion, through means on its armature, previously referred to. At the upper end of this shaft there is a coiled spring a, the outer end of which is fastened to the normal post 0, and against the tension of which the shaft is rotated by the rotary-magnet 0 and in this way the shaft returns to its normal position, when it is made free, in a manner to be hereinafter explained. Just below this spring there is a stop-arm c that prevents the shaft from falling back more than it should while it is returning to its normal position under the tension of the spring a. This arm also controls the position of the spring a" with regard to contact- points 0 and 0. Near the lower extremity of the shaft there is a switching-arm 0 or so-called privatewiper. Still farther below this are the vertical-line-wipe1" c and the rotaryline-Wiper c, separated by suitable insulation 0. Near the middle of the shaft is a cam 0 which prevents the arm a" from moving too much, at certain times, under the tension of the spring 0 through the interference of the lug 0". Near the middle of this arm 0 there are two pivots 0? and 0", while near the left extremity of the same there is a lateral arm 0 and a vertical shaft 0 On this shaft there are four arms or wipers a, 0 0 and 0 The arm a, with all the appended parts, is known as a sideswitch. When this arm a" is behind tooth a, the side-switch is in what is known as its first position, side-switch-wipers a, 0 c and a being in contact with points 0 0 0 and 0 respectively. When the side-switch-arml c is between teeth a and c, the side-switch is then in what is known as second position, the same sideswitch-wipers shifting into contact with points 0"", 0, 0 and 0 respectively; and when it disengages with tooth c, and falls against stop a, the side-switch is then in the third position, the side-switch-wipers being then in contact with points 0, 0"", 0 and 0, respectively. The so-called bridgecut-ofi-relay c has an armature 0 The so-called back-release-relay 0 has an armature 0 H representing the battery used for calling and talking purposes, and h representing its grounded terminal.

The repeater of my invention has no movable parts, such as the shafts and release mechanism used in selectors and connectors, but is made up of a number of rela s and springs and other details hereina ter pointed out. This instrument is designed to be bridged permanently across the trunk-lines leading to a distant exchange. In this repeater D (Fig. 1), (1 represents a verticaldine-relay having an armature (l by means of which impulses sent over the vertical-linc-conductor 2 are repeated to the so-called vertical-trunk-line-conductor 4 that leads to any suitable instrument at the distant station, and in this case to a second-se1ector"E at central station#2 (Fig. 2). The rotary-linc-relay (l has an armature d, by means of which impulses coming from subscriher-As station over the rotaryline-conductor 3 are repeated in the rotarytrunk-linc-conductor 5 that leads to the same instrument at central station #2. The differential-relay d, having two windings d and d, and the armature d, is of such a nature that current passing through only one &

The office of this relay is to provide means whereby the calling subscriber may be able to release, even if the subscriber whom he has called does not restore his receiver to the hook. Furthermore, the diflerential coil prevents impulses that have been repeated from the subscribers vertical-line-conductor 2 to the vertical-trunk-line-conductor4 from passing to the rotarytrunkline-conductor 5, and likewise prevents impulses coming from the subscribers rotaryline' -conductor 3, and repeated to the rotary-trunk-line-conductorV 5, i from passin over to the vertical-trunk-line-conduotor 4, in a manner that shall be hereinafter explained. The bridge-cut-ofi-relay d-the ofiice of which shall be explained laterhas an armature 01 As illustrated, d represents a dead resistance of substantially 6O ohms, while 65 and d represent two con- I densers.

The second-selector is very nearly the same as the first-selector; for in Fig. 4,

tension of the retracting-spring e 4.

release-link? e, on the forward end of e and 6' represent thevertical and rotary line-relays, e and .6 representing their respective 'armatures. The verticalmagnet e has two coils 6 and e; andthe usual armature e is provided with pivots e and e, and is under the tension-of a retracting-spring e. The extendiirgarm e? is provided with an L-shaped piece e", thev vertical-dog e, and itsretracting-spring e. The release-magnet 6 is provided with an armature e on which are the ivots e and e ,'and which operates againstthe which is a slot 6 enga es with the doubledog having pivots e and a about which it oscillates, It also has a retracting-spring e and the two angled arms e and 9 tary-magne e is provided with two mag-- net-coils e and e which act, when endog e by means of a 11g 6 said doubletogether with the straight arm e, which latter engages with the link a. 'The roergized, on the armature e, the latter hav ing pivots e and e" and being held in its normal position by a retracting-spring-e, On this armature are the usual interrupterarm a, the rotary-'armature-finger e, and the arm e that carries the rotarypa-wle with its retracting-spring e. As shown, e"? represents a private-magnet, its

The

e slotted at e to receive the end of the arm or rest e and the longitudinal teeth a. On the upper-extremity is the coiled spring 6, one end of which is fastened to thetpin 6"". Just below this spring is the stop-arm e while near the lower extremity are the private-wiper. e and the line-wipers 6" and e, the last two being separated by the insulation 6 7 Near the middle of the shaft L is a cam e whichcacts as a stop for the usual side-switch-arm e", and which latter is under the tension'of a retracting-spring 6 The stop-lu e strikes against the said cam; and an 6 represent the pivots of the side-switch. On the laterally-projecting arm a" is pivoted the link 6 and just beyond this arm is a shaft on which are the usual side-switch-wipers, a, 6 e and e, which latter engage with the contact-points e, e 6 and e, respectively, when the side-switch is in the first position; with contact-points e 6 6 and a when the side-switch is in the second position, and with contactpoints e. 0 e and 6 when it is in the third position. In addition to these, 6 represents a back-release-relay provided with an armature e".

The connector, may also-be of any suitable, known or approved construction, the mechanical details of which are similar to those of the first and second selectors. In the connector arealso found the rotary and vertical line-relaysj" and f", with armatures f and i the vertical-magnet f with its two coils f and i its armature f having pivots f and f, and retracting-spring f On this armature is the arm f. on which is found the, L-shaped piece f",near the middle-and at theend the verticalpawl f and its retracting-spring 1 7"". There is a'release-magnet f having its armature f provided with pivots f and f, and which is operated against the tension of the spring f. The releaselink f is subject to the downward pull of the retracting-spring f and said link has at one end an aperture f", while the other end of said link is-rfastened permanentlyto the switch-frame at pin i. As

.in the case of the other switches, this release-link? engages with the-double-dog f by means of a lug #9.: This double-dog has t'wo pivots f? and f, and its retractmg-spring f, and 1 is also provided with angled armsand f, and with a straight arm f, w ich latter engages with the llnk 7". There is also the usual rotarymagnet 7' composed of coils f and f,

and having the usual armature f with pivots f and f and retracting-spring f;

.the arm 7' having at its forward end the rotary-pawl 7' controlled by a retractingspring f. f represents the private-magnet; i the private magnet-armature; and f and f its pivots, and f the arm to which is attached the retracting-spring f. It has a lateral arm 7 and a familiar escapement having teeth f, f, f and f. The usual shaft f is shown having circular teeth f provided with a slot f in which engages the arm-"or rest f". Below these circular teeth are the longitudinal teeth f. On the upper end of the shaft is a coilspring f, one end of which is-fastened to the stop-post f and just below this spring there is a stop-arm i which engages with the spring f? controlling its contact with the spring f. On the lower end of this shaft there is the private-wiper f, and still farther down are the line-wipers f and separated by insulation f"; and near the center of this shaft islocated the wellknown cam f which latter controls-at times--the forward movement of the side: switch-arm f, due to the retractingspring 7, through a lug f on the side switch-arm. This arm has also the pivots and ff and the lateral arm 7", on the end of which is pivoted the link f. To the left of this lateral arm is a shaft f on which are located the usual side switchwipers f, 7', f and f which, while the side-switch is in its first position, engage with contact-points f f, f and 7', re-

spectively. \Vhen the side-switch is in second position, they engage with contactpoints 7', f f and f, respectively; and when the side-switch is drawn to its third position, the side-'switch-arm f having fallen against stop f, the respective wipers will then be in contact with the points f, f, f and In addition to these, the bridge-cut-o -relay f is provided with an armature f, while the pole-changing-relay f is provided with an armature f and the differential-coil f having two windings f and f has an armature f. The ringer-relay is shown at 1", and is provided with an armature f Having thus pointed out the mechanical details of allapparatus, it now remains to trace the circuits. Assuming, to start with, that all switches are in their normal positions, as indicated in Figs. 3', 4 and 5- should the subscriber at the sub-station designated as #6555 (Fig. 1) wish to establish connection with subscriber B of central station #2 (Fig. 2), the number of said "called subscriber'being #2220, he will first remove his receiver (L2 from its hook at (Fig. 1), then push button a twice and button a once. After this, he will push button u.

twice and button a once, and again button i and 11 will thereby be closed, the closing of the last two placing the spring 12 directly in contact with the ground at 13 through the following circuit: spring 12, conductors 14 and 15, springs 10 and 11 to ground 13. Now, when button ,a is pressed twice, thereby causing spring 16 to break contact with spring 17, and to come in contact with spring 12, the said number of times, the line-conductor 2 is grounded each time, thereby energizing the vertical-1ine-relav 0 through the following circuit: from ground 13 to spring 12, to line-conductm 2, side-switch-wiper a, contact-point e "vertical-line-relay c springs 20 and 21, conductors 22 and 24 to non-grounded terminal of battery H to ground h. Verticalline-relay 0 (Fig. 3) having thus been energized twice, actuates its armature c,

L-shaped piece 0", disengages the releaselink 0 from lug e allowing the doubledog-arms c and a to fall against the circular teeth 0 and the longitudinal teeth 0, respectively-due to the tension of the retractingspring 0 At the same time vertical-pawl a" engages with the circular teeth 0", raising the shaft one step; doubledog-arm a dropping underneath the first circular tooth and retaining the shaft in the raised position. The next impulse raises the shaft one more step, in a similar manner, the doublc-dog-arm engaging with the second circular tooth, the private-wiper 0 having been positioned opposite the first contact of the second row of private-bankterminal 0, line-wipers c" and 0 also having been raised to a position opposite the first contact of the second row of the linebank-terminals Againreferring to Fig. 1, when button a is pressed once, spring 35 .breaks from contact-point 36 and engages private-wiper 12 and 35, to line-conductor to- 'side switch-wiper 0 contact-point 0 rotaryline-relay-coils a, springs 39, 20 and 21.,

conductors 22 and 24 to nongrounded terminal of battery H to ground h. Rotaryline-relay a. (Fig. 3) being thus once energized, causes its armature 0 to bring springs 40 and 26 (Fig. 3) momentarily together, thereby energizing the private magnet 0 through the following circuit:

ground 27, springs 26 and 40, conductors 4:1 and 42, private-magnet 0 conductors 43, 35 and 24: to non-grounded terminal of bat.-

tery H to ground it. When the private- .the" points 0", 0 0 and 0 the arm resting between teeth 0 and 0 As soon as side-switch-wiper 0 engages wlth contact-point 0 the coils c and 0 of rotarymagnet 0 become energized through a circuit, as follows: ground 44, contact-point 0 side-switch-wiper 0 conductor 45, ro-

.tary-magnet-coil 0 springs 46 and 47, rotarynagnet-coil 0 conductors 34:, 35

and 24 to non-grounded terminal of battery H to ground '71.. These coils, being ener- I gized,.attract the rotary-armature 0 the rotary-pawl a engaging with the longitudinal teeth 0 and rotating the shaft one step, the arm-rest 0 now leaving slot 0- and engaging with the circular teeth,

therebysupporting the shaft during its rotation. As the shaft is rotated, the arm a of the double-dogfc engages with the longitudinal teeth, retaining the shaft in its advanced 0 with the first contact of the second row of private-bank C, and

the line-wipers c" and c with the first contacts of the second row of line-bank terminals 0 the trunk lead ng from which latter, it may be assumed, are dlsengaged at this time. At the same time that rotary-armature begins to rotate the shaft 0 as explained, the frotary-armaturefinger 0 begins to press the privatearmature 0 toward the pole-piece of the private-magnet 0, Now, as soon as the cam 0' has been rotated away from the lug c", the sidesswitch -arm 0 falls against the tooth 0, so that by the time that private-armature a strikes the privatemagnet-pole-piece, side-switch-arm a" switch into third position;

position and engaging the 7 has become disengaged from said tooth, and

has fallen against the tooth 0 By the time this occurs, the interrupter-arm 0 on the rotary-armature 0 has been drawn in far enough to separate springs 46 and 47, thus breaking the circuit that energizes the arm c" disengages from tooth 0 and falls against the stop 0 throwing the side- Said sideswitch-wiper will now be in contact with points a, 0 0" ands. The subscribers hue-conductors 2 and 3 (F1 3) are now extended by and along the bank-trunk-line as follows: subscribers vertical-line-conductor 2, side-switch-Wiper a, contactpoint a, conductor 48, vertical-line-wiper 0 .trunk-line-conductor similar to c to a disengaged repeater, back to a trunk-lineconductor similar to c", rotary-line-wiper a, conductor 49, contact-point 0*", sideswitch-wiper 0- and rotary-line-conductor 3. Side-switch-wiper 0 being in the third position and, therefore, engaged with contact-point 0 places private-wiper 0", and, therefore, the private-bank-contact with which it is engaged, in connection with ground, and with the grounded terminal of the battery H through the following path: from battery H to ground 71,, to sideswitch-ground 50, contact-point a, sideswitch-wiper 0 conductor 51, backrelease-relay 0 conductor 52, private wiper 0 to engaged contact. This establishes a guarding potential on the first contact of the second row of private-bankterminalsic, so as to prevent another subscriber from coming in on the same line, in a manner that will "be hereafter explained. Returning to that stage of the call before subscriber #6555 (Fig. 1) had ressed the button a once: The shaft 0 ad been raised two steps, and the private- Wiper 0 had been brought opposite the first contact of the second row of privatebank c; while the line- wipers 0 and 0" had been brought opposite contacts-of the second row of line-bank-terminals. 0 Now assume that thefirst nine trunks o1" trunk-linesof the second level are in use by nine. first-selectors all similar to the selector shown in Fig. 3. The switch using the first trunk of the second level will have its private-wiper engaging the first contact of the second row of its private-bank similar to privatebank -c; also, side-switch of said occupying selector is in third position, hav-v mg a'wiper, similar to-the' side-switchwiper 0, engag ng with the contact-point similar to 0 This places the first contact 'of the second row in connection with the tooth 0, allowing it to fall against tooth 0. and from which latter it in turn disengages as soon as the armature returns to its normal position; the side-switch being, therefore, brought to the second position, and side-switch-wiper 0 coming in contact with grounded terminal 0 as previously explained. The coils 0' and a of the rotary-magnet 0 will now be energized, attracting the armature c and rotating shaft 0 one step in the manner already described, and thus rotating the private-wiper 0 and engaging it with the first contact of the second row of privatebank 0; line-wipers c" and 0 being likewise cut-in on the first contact of the second row offline-bank-terminalsc. Owing to the guarding potential which has already been described, existing on said first contact as soon as the private-wiper 0 engages with the first contact, and before the rotary-armature 0 has begun to return 'to its normal position, the private-magnet of is energized through the followin circuit: from non-grounded terminal 0? battery H to conductors 24, 35 and 43, private-magnet 0', conductors 42 and 53, contact-point a, "sideswitch-wiperc con-v ductor 51, back-release-relay c ,'conductor 52, private-wiper 0 to the first contact of the second-row of private-bank 0, through an occupying switch (not shown) to ground h, to battery H Thus energized, privatemagnet a attracts its armature 0, and relieves the rotary-armature-fingerc of its care of said armature, side-switch-arm 0" now disengaging with tooth and falling upon toothc. As soon as the rotary-armature c rotates the shaft one step, the

interrupter-arm a separates springs 46 and 47, thus deenergizing the rotary-magnet c rotary-armature 0 returning to its normal position. It will be noticed, however,

that the private-armature 0* is still held" down, due to the circuit established through private-magnet a, and owing to the guarding potential on the first private-con. tact of the second level of private-bankterminal 0. As soon as the rotary-armature has returned to its normal position, the interrupter- springs 46 and 47 again resume contact with each other, again energizing rotary-magnet 0 through the same circuit as before. The rotary armature will again be attracted, rotating the shaft one more step, and thereby bringing the .private-wi er onto the second contact of the second evel of private-bank c. As it has been assumed that this contact also has a guarding potential, the private-armature 0 will still be held down. As soon as the interrupter-arm 0. again separates the interrupter- springs 46 and 47, the rotary-armature will fall back once more, allowing said springs to again engage each other. This will be repeated as often as privatewiper 0 encounters contact-points having guarding potential. When the shaft has been rotated nine steps, the private-wiper will then be found in engagement with the ninth contact of the second level, but which contact is also busy, as previousl stated. The rotary-magnet will be energized once more, and the shaft will be rotated one more step, at last placing the privatewiper 0 as well as line-wipers c and c, on the tenth contact of their respective banks. As soon as the private-wiper leaves the ninth contact, the circuit through the private-magnet is broken, there being no guarding potential on the tenth contact; private-armature 0 is restored to the charge of rotary-armature-finger 0, immediately after which the circuit through the rotary-magnet is broken for the last time by interrupter-arm c; and the rotary-armature will then fall back to its normal position, as well as the private-armature, when the side switch arm a" will disengage from the last tooth a, falling against sto a and bringing the sideswitch to its t ird position, as before. At this time, the subscribers line-conductors 2 and 3 are extended to a disengaged repeater through trunk-line- conductors 3 and 4, clearly shown in Fig. 1. Guarding potential is now established at the tenth contact of the second level of private-bank c, by the side-switch-wiper 0 again engaging with grounded contact-point 0", thus preventin anyone from cutting in on this trunk while it is in use. It will be noticed that while private-wiper 0" was passing over the contacts that have guarding potentialthat is, which are temporarily connected with the grounded or positive pole of the battery-the private-magnet was energized through a circuitwhich included the back-release-relay 0 but this relay is of such low resistance as not to interfere with the working of the private-magnet a; and at the same time, the private-magnet-coili s of such comparatively high re? sistance that the energizing current, when they are both in series, is not strong enough to draw down the armature 0 against the vtension of spring 54. The subscriber has extended the lines to a disengaged repeater (as clearly shown in Fig. 1), again presses his button a twice, thus agaln' grounding the vertical-line-conductor 2 by bringing it in contact withspring 12, as before, and

"thereby energizing vertical-line-rlay atv at repeater D twice as follows: from ground 18 at sub-station #6555, to ground-spring 12, line-conductor 2, switching-arm 0 wiper 0 vertical-trunk-conductor' c vertical-line-relay d, winding (Z of differential-relay d springs 54 and 55, c0nductor 56 to battery H and to ground k.

Thus energized, vertical-line-relay d actuates itsarmature d bringing springs 56 and 57 together twice, grounding verticalnormal trunk line conductor 4 the same number of times, as followsz'from ground 58 at the repeater, through springs 59 andv 60, conductor 61, springs 57 and 56, conductor 62, springs63 and 64 to verticaltrunk-line 4; The effect of the grounding of this conductor will be explained. The

current that energizes verticallinerelay d passes'through winding d of the 'difi'erential-relay d thereby energizing said difi'erential-relay with the result that its armature (i is attracted and contact thereby broken between spring 66 and contact-point 67 The separating of this spring from this contact "opens a bridge between the "vertical-trunk-conductor '4' 'and: rotarytrunk conductor 5, which is ordinarily closed as 'follows:' from vertical-trunklineconductor? *4. to .cond'uctorf 65,}:to

contact 4 point 67, spring- 66, relay 2?. to. conductor 68 and to rotary trunk-conduc tor 5. In this way, the,rotary-trunk-con ductor 5- is'prev'ented from being rounded line-relay 03 thus energized once, actuates through coil 03 at the same time that vertical-trunk-conductor 4 is grounded by the actuation of relay-armature '03}. After pressin button a, buttonfa is pressed once, t us groundin line-conductor, 3, momentarily, Twit spring' 12, thereby energizing rotary-line-relay d at repeater D, momentarily," through; the; fol-. lowing circuit: from ground 13' at substation #6555 to springs 12 and 35, to line-conductor 3, switching-arm a wiper a", conductor 0, r'otary-line-rela'y d,

winding d of difierential-coil d conductor 56 to battery H and to ground It. Rotaryits armature d, which. closes springs 69 and 57, thereby grounding ,rotary-trunk-corn ductor 5. The same current that energizes rotary-line-relay d passes through winding d of the differential-relay d thereby energizing it; and the armature 03 being attracted, contact is again broken vbetween spring 66 and contact-point 67, thereby again opening the bridge between trunkconductors 4 and 5. This prevents conductor 4 from being grounded at the. same time with trunk-conductor '5 each time that the-rotary-relay-armature d is actuated.

Referring to Fig. 4, when vertical-trunkconductor 4 is grounded twice, momentarily, as above explained, by the calling subscriber at sub-station #6555, by pressing button a twice the second time, ,verti- 1 cal-line-relay e is energized twice,as fol lows: from ground at the repeater to ver tical-trunk-conductor 4, as already explained, to side-switchswiper a to contact-point e conductor 70, vertical-line- 'relay e conductors, 71 and 72- to nongrounded termina'lof battery H and to ground k Vertical-line-relay 6 being thus energized, its armature 0 causes springs 73 and 74 to come together, thereby energizing the coils e 9 and e of ve'rti cal-magnet 6 through the followlng circuit: from ground 75 to springs 74 andj73, conductor 76, springs 77 and 78, conductor 79, rotarymagnet-coils e and e, conductors 8 0, '82 and 72 to non-grounded'terminal of battery j H and to ground h i Thus energized, the vertical-magnet? e actuates its armature e twice. As in the case of thefifirst T'ioo selector, this armature engagesrwith the circular teeth e? on the shaft 1 raising the" latter two steps. At the Qfirst impulse of armature e the release-link if? disen-" gages'froi'n 111g 6 on the double doge allowing the arms, e and e of said do'uble-d0g, respectively, to falljagainst the;

circular and longitudinal teeth e and eff;

' The-arm e, by engaging with the circular teeth, holds the shaft in its raised position;

The shaft having been raised two steps, the private-wiper e" on this shaft is brought to the level of the first'contact of the sec-'- ond level orrow .of private-bank e, while the line-wipers e and e are brought to 'the'level of the first contacts or terminals of thei secondlevel of,line-bank 0 Now,

when the rotary-trunk-conductor7 '5 is ,grounded once at ;-the repeater" D, as was zsholwn, the Frotary-,line-re1ay c (Fig. 4) is energized once as'follows: from ground at thefirepeater vertical-trunk- conductor I 5, 1 through side-switch-arm 6, contactupoint -6 conductor 83,- rotary-line relay e conductors 71' and 72 to non-grounded termi'nal'of battery H and to ground It. Thus momentarily energized, the rotaryline-relay e actuates its armature e causing springs 84 and 74 to come together, thus energizing private-magnet 0* through the following circuit: from ground 75 to springs Hand 84, conductors 85 and 86, privatemagnet e conductors 87, 82 and 72 to non-grounded terminal of battery H and to ground h When the private-magnet e is thus momentarily energized, it attracts its armature 6 once. As this armature is attracted, the side-switch-arm e disengages from tooth e and falls against tooth e but from which it in turn disengages as soon as the armature returns to its normal position, thereby placing the side-switch in second position. The sideswitchwipers e, e, and 6* having changed contact from points e, e, e and e to points 6?, e, e and e, respectively, any further advance of said side-switch is checked by the lug e" on the side-switcharm e striking against the cam e". Side-switch-wiper e?" having engaged with contact-point e", the coils 6* and e of rotary-magnet e become energized through a circuit identically the same as that previously described in the first-selector. These coils, being energized attract their armature e, causing the shaft 6 to rotate. Presuming that the first nine trunks of the second level are all busy, guarding potential will be found at the first nine contacts of the second row of private-bank e. As soon as private-wiper e engages with the first contacts, the private-magnet 0 will be energized, as in the first-selector, attracting its armature e and thereby causing said side-switch-arm e to disengage with tooth e, and to fall against tooth e holding the side-switch in its second position. The side-switch-wiper e" is now in contact with the grounded point e", and sideswitch-wiper 6 in contact with point e. As long as side-switch-wiper e remains in contact with point e, the rotarymagnet e will be energized, interruptedly, thus giving the shaft e a step-by-step rotary motion until the private-wiper e" is rotated onto some contact that has no guarding potential, as has already been explained in connection with the first-selector. At that moment, the privatemagnet 6 becomes deenergized, restoring its armature to the care of the finger e, which, with the rotary-armature which is returning to its normal position for the last time, allows the private-armaturee to also return to its normal position, thereby allowing the side-switch-arm e to disengage from the last tooth e This throws the side-switch into its third position, thus causing the' side-switch-wipers e", e, e and e to close with contact-points e", e, e and e, extending the subscribers line- conductors 4 and 5 along disengaged conductors e and e to a connector not in use, as follows from vertical-conductor 4 to side-switch e contact-point 6 conductor 88, verticaldine-wiper e, vertical-trunk-conductor a, through the connector and back through the rotarytrunk-conductor e, rotary-line-wiper e, conductor 89, contact-point e and sideswitch-wiper e to rotary-conductor 5. In the second-selector, as in the first, as soon as the side-switch passes to the third position,.the switch is cut off from the main line- conductors 4 and 5. the release of the switch depending on the private-trunk-line with which the privatewiper e" engages, as will hereinafter be demonstrated. Side-switch-wiper e havng engaged with contact-point e, a guarding potential is established on the tenth contact of the second row or level of private-bank-terminal e, with which privatewlper e" is engaged through the following: circuit from battery H to ground It, to ground 90 at the side-switch, contact-point e side-switch-wiper e conductor 91, back-release-rclay e, conductor 92, private-wlper e" to tenth contact of second row or level of private-bank e. The next act of the calling subscriber #6555 (Fig. 1) is to press the button a twice, again twice grounding subscribers line-conductor 2, thus energizing the verticaldine-relay (l twice, and winding d of differential-relay d the same number of times. Verticalline-relay (I, being thus energized, actuates its armature d twice. which grounds vertical-trunk-conducto1" 4 twice, in a manner already explained. The grounding of vertical-trunk-conductor 4 energizes verticalline-relay f of connector F (Fig. 2), over the following circuit: from ground 58 at repeater D (Fig. 1) through springs 59 and 60, conductor 61, springs 57 and 56, conductor 62. springs 63 and 61 to verticaltrunk-conductor 4 (Fig. 2), switchin -arm e wiper e, conductor 0, vertica -linerelay 7, winding 7" of differential-coil f springs 95 and 96. conductor 97 to nongrounded terminal of battery H and to ground h. The energizing of verticalline-relay f (Fig. 5), twice, causes its armature f to be actuated twice, bringing springs 99 and 100 together, thereby energizing coils i and i of verticalmagnet f through the following circuit: from ground 101 to springs 99 and 100, conductor 102, springs 103 and 104, conductor 105. side-switch-arm f, contact point f coils f and f" of the vertical-magnet. conductor 106, conductor 107, springs 108, 109 and 11.0, conductor 97 and 98 to the non-grounded terminal of battery H and. to ground It. Thus energdzed, on the first impulse the vertical-magnets f actuates its vertical-armature f", the vertical-dog f engaging with the circular teeth f on the shaft f, and raising the shaft one notch. At the same time, the

L-shaped piece f" disengages the releaselink f from the lug f on the doubledog f, allowing the arm f on the latter to drop into engagement with the circular. teeth 7', under the tension of the retractingspring f, to retain the shaft in its raised position. On the second impulse, the vertical-pawl f again engages the circular teeth, raising the shaft one more notch and bringing the private-wiper f on the said shaft to a level with the first contact of'the second row of "private bank-terminals f, and the rotary and vertical line-wipers f and f, also on said shaft, opposite the first contact of the second row or level of line-bank-terminals f. In the next step, the subscriber #6555 (Fig. 1) presses the button a once, momentarily, thus grounding line-conductor 3 by bringing it into contact with the grounded spring 12, thus energizing the rotary-linerelay (i and differential-relay d, as already explained. Thus energized, the rotary-line-relay d actuates its armature, which grounds the rotary-trunk-line-conductor 5, thereby energizing the rotaryline-relay of connectofiF in Fig. 2, as follows: from ground 58 at repeater D to springs 59 and 60, conductor 61, springs 57 and 69, conductor 169, rotary-trunk-conductor 5, switching-arm a, (Fig. 2), wiper e trunk-conductor e, rotary-linerelay 7", winding P of difierential-coil f conductor, 97 to non-grounded terminal of battery -H and to ground h. The

rotary-line-relay f, having thus been energized once, actuates its armature f (Fig. 5), causing springs 23 and 99 to come together. A circuit is thereby closed which energizes the private-magnet f as follows: from ground 101 to springs 99 and 23, conductor 112, private-magnet f,

conductor 113, springs 109 and 110, conductors 97 and 98 to the non-grounded tenninal of battery H to ground h. Thus ener gized, private-magnet f actuates its armature f. When the private-armature is attracted, the side-swltch-arm. f dis- I engages from tooth f, coming'into contact with tooth f, from which latter it disengages when the private-armature regains its normal position, under' the tension of the retracting-spring f. The side-switch is, therefore, moved to second position, the side-switch-wipers i f, f and f breaking with contact-points f, f, f" and f, and closing with contact-points f f f and f, respectively". The next operation on the part of subscriber #6555 (Fig. 1) is to press the button a ten times, for the last time, again energizing the vertical-line-relay d at repeater D, as Well as the differential-coil (l each ten'times. The

energizing of vertical-line-re1ay'd at repeater I) again energizes the vertical-linerelay f ten times (Fig. 2)of connector F-through the same circuit as before. Beferring to Fig. 5, the energizing .of' the vertical-line-relay f actuates its armature f ten times, energizing in turn coils f and f of rotary-magnet f ten times, through the following circuit: from ground 101 to springs 99 and 100, conductor 102, springs 103 and 104, conductor 105,.sideswitch-arm f, contact-point f, coils f and f of rotary-1nagnet f, conductors 106 and 107, springs 108, 109and 1'10, conductors 97 and 98 to non-grounded terminal of battery H ,and to ound 72. Each time the rotary-armature is attracted, the rotary-pawl f engages with the longitudinal teeth f, rotating the shaft one step against the tension of the coil-spring f and held in place at each step by Larm f of the double-dog f At the end of this operation, the line-Wipers f and f, as

well as private-wiper f, willbe found on the tenth contact of the second row of their respective banks f and f, the trunkswhich in turn energizes the rotary-1ine-re-.

lay f (Fig. 2) of connector I through a circuit as before. Rotary-line-relay" f (Fig. 5), thus energized, again actuates its armature f energizing private-magnet f as before. The private-magnet? 7' being thus energized once, actuates its armature once. This causes 'side-switch-arm f to disengage from tooth f on the downward stroke, and to fall againsttooth f. Said arm in turndisengages from the last tooth on return to normal osition,-and falls against stop This t rows the sidesWitch-wipers f, f", f and f into contact with points f, f, f and f",'respectively. This final operation wlll connect the calling subscriber with the called subscriber, through the normal-trunks f and f, as follows: from vertical-line-conductor 2 (Fig. 1) to switching-arm 0 conductor 48, Wiper c, conductor 0 condenser 03 to conductor 4 (Fig. 2), switching-arm e wiper e conductor a, condenser f thence to conductor 114 and to springs 115 and 116 to switching-arm f, contact-point 127 and f wiper f", contact-point f, switching-arm f, springs 128 and 129, conductor 130, condenser f thence to conductor e", switching-arm e switching-arm e rotary-trunk-conductor 5 (Fig. 1), condenser d, wiper a, conductor 49, switch ing-arm c rotary-line-conductor 3, spring 35, contact-point 36, condenser (1 secondary winding 131 of induction-coil a, conductor 132, receiver a conductor 133,

springs 7 and 6, conductor 134, contactwell as at all other private-contacts with which it is connected in multiplein this case being the tenth contact in the second row or level. This circuit is as follows: from battery H to ground h, side-switchground 135 to contact-point f, to sideswitch-wiper f, conductor 202, conductor 136 to private-wiper f and to tenth contact of the second row or level of privatebank f, thus protecting the line against intrusion. Referring to Fig. 2: At the same time the guarding potential is established at the tenth contact of the second row or level of private-bank f, a circuit is established by way of said contact along the private-normal 1 which energizes the socalled bridge-cut-ofl-relay g of the called subscribers first-selector-switch, through the following circuit: from ground 135 of connector F, to switching-arm f. conductor 136, wiper f, conductor 7', spring 137, contact-point 138, bridge-cut-ofi'-relay g, conductor 139 to battery H and to ground it. Thus energized, bridge-cut-ofl' relay-magnet g attracts its armature g, separating springs 140, 141 and 142, thus cutting off the bridge formed by line-relays g and across lines f and f and ordinarily closed through the following cir cuit: conductor f switching-arm g, vertical-line-relay springs 141 and 142, rotary-line-rela switching-arm to conductor f. The object of opening this bridge is twofold and will be referred to later on. The operator at the sub-station #6555 (Fig. 1) is now ready to ring the bell at sub-station #2220 (Figs. 1 and 2). He presses button a", which will cause the spring 143 to "break away from the contact 144, and close with contact 145. This will encrgize the vertical-line-relaf (l at repeater D. through the following circuit: from ground 13 at the calling subscribers station #6555 to springs 11 and to contact-point 145, to spring 143,

through springs 8 and 9 and 7 and 6 to contact-point 17, to spring 16, thence to vertical-line-conductor 2, through verticalline-relay rl at repeater D and differentialrelay cl, and to the non-grounded terminal of the battery 11, as before. Thus energized, the vcrtical-linc-relay" (I actuates its armature cl, which in turn grounds the verticaltrunk-conductoi" 4, thus energizing vertical-line-relaf of connector F, through the well-known circuit: The vertical-linerelay 7' then actuates its armature f, which closes contact between springs 99 and 100, thus energizing ringer-relay P, as follows: from ground 101 to springs 99 and 100 and to springs 103 and 104, conductor 105, sideswitch-wiper f contact-point f, conductor 14:6 to ringer- relay f conductor 147 and 107, springs 108, 109 and 110. thence to conductor 97, and to 1 the non-grounded terminal of the battery H to ground It. Thus energized, ringer-relay f attracts its armature f thereby separating spring 128 from 129, and bringing it into contact with spring 148; also, spring 116 from 115, bringing it into contact with 149. Springs 115 and 129 constitute the terminals of the substation #6555, and springs 116 and 128 the terminals of the sub-station #2220; while springs 148 and 149 constitute the terminals of the ringer machine N. It is evident that the function of the ringer-relay f is to separate the line of subscriber #2220 from the line of subscriber #6555, and to close it with the terminals of generator N as long as subscriber #6555 is pressing button (1?. In this way, the ringer or hell or signaling apparatus 122 at sub-station #2220 is energized through the following circuit: from generator N to conductors 150 and 151, springs 149 and 116, switching-arm 7. contact-point f wiper f conductor f, conductor 117, spring 118, contact-point 119, conductor 120, hook 6. spring 121. ringer or bell 122, conductor 123, condenser 1), conductor 124. contact-point 125, spring 126, conductors 127 and f, wiper f", contactpoint f, switching-arm f springs 128 and 148'to conductors 152 and and to generator N. The bell 122 at sub-station #2220 110 will now ring until the subscriber at #6555 releases button (1 thereby allowing the vertical-line-relay" (l at repeater (Z, and vertical-line-relay f at connector F, and

ringer-relay f to resume their normal 115 positions. Springs 128 and 116 again falling back upon springs 129 and 115, respectively, re'cstablish connection between substation #6555 and #2220. It will be noticed that heretofore as line-relays f 120 and f, respectively, have been energized, the

energizing current has in each case passed through one winding of the differentialrelay f thus energizing it and thereby actuating its armature; but up to the pres- 125 ent time this has had no significance. Had

not bridge-cut-ofi-relay 9 (Fig. 2) on the called subscribers first-selector G opened the bridgeacross lines 7' and f. the ringing current would have passed through the 130 bridging rotary and vertical line-relays g and g energizing them and making it possible for the shaft of switch Gto be raised off-normal because of the actuation of their armatures g and 9', thus grounding springs 154 and 155 against spring 156, as in the first-selector G, thereby actuating the rotary and vertical magnets, which would interfere with the connection. It is obvious that during conversation, and 'while voice currents are passing over conductors f and f, it is desirable that this bridge be spring 165, contact-point 164, magnet-cut-' off-relay f, conductor 114, springs'115'and 116, switching-arm f contact-point f, Wiper f, conductors and 117, spring 118, contact-point 119, conductor 120, springs 158 and 157, springs 160 and 159, transmitter 5, primary winding 163 of induction-coil b, conductor 124, contact-point 125, spring 126, line-conductor 127, conductor 7, wiper f, contact-point f switching-arm f springs 128 and 129, conductor 130, polechanging-relay f to the non-grounded terminal of the battery H to ground '72.. Thus current for talking purposes is furnished from the common battery H to transmitter b,'primary winding 163 of induction-coil '5 through the magnet-cut-oifrelay f and pole-changing-relay f When the latter becomes energized, its armature f is attracted, causing spring 95 to break with spring 96, and to engage with spring, 166, thereby energizing a circuit which contains both windings of the differential-relay f vertical and rotary line-relays f and f" at the connector F, and a bridge-cut-ofi-pole-changing-relay v d at the repeater D (Fig. 1). The circuit isas follows: from ground 167 at connector F to springs 166 and 95 to winding f? of differential-relay f vertical-line-relay f conductor 0 wiper a, conductor 88, switching-arm e vertical-trunk-conductor 4 (Fig. 1), conductor 65, contact-point 67, spring 66, bridge-cut-ofi' pole-changingrelay d, conductor 68, rotary-trunk-conductor 5 (Fig. 2), switching arm 6", conductor 89, wi er a, conductor e, rotary-linerelay 7", connector F, winding f of the difierential-relay f conductor 97 to the non-grounded terminal of battery H and to ground 11. This current, passing through both windingsof the difierential-relay f does so in such a manner that the magnetic effects are neutralized; but both the rotary and the vertical line-relays f and f become energized, thereby attracting their armatures and grounding springs 100 and 23 by bringin them into contact with the spring 99. agnet-cut ofi-relay f being energlzed, however, at the same time, attracts its armature f thereby separating springs 108, 109 and 110 for the purpose of rendering the following magnets inoperative; ringing-relay f private-magnet f and release-magnet f. It is not desirable that these magnets be energized at this stage by the actuation of armatures f and f of the Vertical and rotary linerelays f?" and 7. .At the same timenthat these last relays are energized, bridge-cutofl pole-changing-relay d at the repeater D becomes energized, actuating its' armature d, thereby causing spring 54 to'separate from spring 55 and to close contact with spring 59; spring 60 to separate from spring 59; and spring 64 to separate from spring 63.. As soon as contact closes between springs 54 and 59, talking current is furnished to the .callingsubscribers line duction-coil a, "and thetransmitter a at sub-station A, through the following cir-" 1 cuit: from ground 58 at repeater D (Fig. 1) to springs 59 and 54, winding (i of difi'erential-coil d vertical-line-relay d, conductor 0 wiper 0", conductor 48, switchingarm a, conductor 2, spring.16, contact-- point 17, conductor 134, springs 6 and 7, springs 9 and 8, to spring 143, to contactpoint 144, to transmitter a, primary winds ing 168 of induction-coil a, conductor 169, contact-point 36, spring 35, conductor 3, switchingarm c conductor 49, wiper c, conductor 0, rotary-line-relay d and winding d of the differential-coil d, conductor 56 to non-grounded terminal of battery H and to-ground h. This talking current, as in the case of a connector, passes through both windings of the difl'erentialcoil (1 in such a manner that the magnetic effects are neutralized; but the rotary and called subscriber at sub-station #2220 re- 0 which reach moves his receiver from its hook, talking battery current is furnished to his line, which energizes the pole-changing-relay f at the connector F. Amon other things, the energizing of this relay in turn causes the bridge-cut-off pole-changing-relay at the repeater D to be energized, causing spring 54 to break from spring 55 and to close with spring 59, thus furnishing talking battery current to the callin subscribers line. Referring again to t e repeater D, when the armature al of the bridge-cut-ofi pole-changing-relay-d" is actuated, it was stated that spring 64 broke contact with spring 63. This opens up a bridge between conductors 4 and5,---wh1ch, when springs 56, 57 and 69 are kept in contact by armatures d andvl, would ordinarily be closed as follows: conductor 4 to springs 64 and 63, conductor 62, s rings 56, 5 and 69 to conductors 169 an 5. It is obvious that this bridge, which amounts to a short-circuit, is not desirable, conductors 4 and 5 being, as will be seen, part of the main talking lines. Talking current having thus been furnished to the calling and the called subscribers stations, when operator at sub-station #2220 speaks into transmitter b, the resistance of the circuit through which talking current passes is varied, thereby causing the current to vary similarly. In this talkin current, as has been stated, there is inclu ed the primary winding 163 of induction-coil b. Variations in the intensity of the talking current induce currents in turn in the secondary winding 170 of induction-coil b in the usual way, receiver a at the substation A through the following circuit: from the secondary winding of induction-coil b at substation #2220 to conductor 171 to receiver 6 conductor 172, springs 157 and 158, conductor 120, contact-point 119, spring 118, conductors 117 and f wiper 7", contactpoint f, switching-arm f, springs 116 and 115, conductor 114, condenser f conductor e wiper e", con'ductor 88, switching-arm e ,-vertical trunk-conductor 4 (Fig. 2), vertical-trun.k-conductor 4 (Fig. 1), condenser d, conductor a, wiper a, conductor 48, switching-arm c, conductor 2, spr ng 16, contact-point 17, conductor 134, springs 6 and 7, conductor 133, receiver a conductor 132, secondary winding 131 of inductioncoil a, condenser a, contact-point 36, s ring 35, conductor 3, switching-arm 0?, con uctor 49, wiper a", conductor a, condenser d", rotary-trunk-line-conductor 5 (F g. 1), rotary-trunk-line-oonductor 5 (Fig, 2), switching-arm e conductor 89, wlper e", conductor e, condenser f conductor 130, springs 129 and 128, switching arm f, contact-point f", wiper f", conductor f, conductor 127, spring 126, contact-point 125, conductor 124, condenser b and back to secondary winding 170 of induction-coil b. It will thus be seen that variations originating in the transmitter I) at sub-station #2220 will reach the calling subscribers receiver a at sub-station #6555, over a comparatively clear path, through condensers f and f at connector F, and condensers d" and d" at the repeater D. In like manner, variations produced in the transmitter a at sub-station #6555 reach receiver b at substation #2220 over the same path. After one subscriber has established connection with another, the instruments may be restored to their normal conditions, or released in different ways; a calling subscriber may hang up his receiver first; the called subscriber may hang up his receiver first; a called subscriber may begin to call before the calling subscriber has restored his receiver, or the called subscriber may not have answered at all. We may suppose, now, that the subscriber at substation #6555 restores the receiver a to its hook a before subscriber at sub-station #2220 restores his. It will be remembered that bridge-cut-ofi' pole-changing-relay d at the repeater D, as well as the vertical and rotary line-relays d and d, are energized, their armatures being, therefore, at tracted. Now, as soon as the hook a is drawn down by the Weight of the receiver a, lug 173 on hook a presses spring 174 against spring 175 and then both together against spring 176 momentarily, with the result that both line-conductors 2 and 3 are grounded 100 at once, as follows: from ground 13 at substation A to springs 10 and 11, to conductor 15, to ground-spring 175, to springs 174 and 176, to conductors 169 and 134, to contactpoints 36 and 17, to springs 35 and 16, to '105 conductors 3 and 2, thereby short-circuiting the vertical-line-relay 03 at the repeater D, through ground as follows: from ground 13. at sub-station A to ground-spring -175 to spring 17 6, conductor 134, contact-point 17, 110 spring 16, conductor 2, switching-arm a, conductor 48, wiper 0", conductor 0 vertical-line-relay d, winding (1 of differentialcoil 0!, springs 54 and 59 to ground 58; the vertical-line-relay d is thereby deener- 115 gized, while the rotary-line-relay d retains its energized condition by reason of a circuit as follows: from ground 13 at substation A, through a circuit already traced, to conductor 3, switching-arm 0 conductor 120 49, wiper 0 rotary-line-relay (l winding (1 ofdifi'crential-relay d", conductor 56 to non-grounded terminalof battery H, to ground h. Under these conditions, it will be seen that current passes onl through 125 winding (1 of the difl'erential-coi d which,

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