US1479442A - Metering telephone system - Google Patents

Description

Jail 1 9 F. ALDENDORFF METERING TELEPHONE SYSTEM Filed June 21 7 sheets-sheet 1 LA .Q31 E?" H- FL- FF i.

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F. ALDENDoRl-v-F AMETERING TELEPHONE SYSTEM Filed June` 2l 1920 '7 Sheets-Sheet 5 zzg 1 l6600 asco lV01l Vel Jan. l, 1924 ,479,442

F. ALDENDORFF METERING TELEPHONE SYSTEM Filed June` 21 1920 7 sheets-sheet 6 Jan, 1, 1924 1,479,442v

F. ALDENDORFF METERING TELEPHONE SYSTEM Filed June 2l 1920 '7 Sheets-Sheet 7 Wrri l `A y LL www Patented .l an. 1,1924.

UNITED STATES Pairlairr OFFICE.

METERING TELEPHONE SYSTEM.

Carved out of application on which Patent No. 1,319,031 was igranted October 14, 1919, and filed after grant of that patent. This application led June 21, 1920. Serial No. 390,32011.

(GRANTED UNDER THE PROVISIONS 0F THE ACT 0F MARCH 3, 1921, 41 STAT. L., 1313.)

To all whom it may concern:

Be it known that I, FRITZ ALDENDORFF, a citizen of the `,German Republic, residing at 32 Mannheimerstrasse, Wilmersdorf, Berlin, Germany, have invented'certain new and useful Improvements in Metering Telephone Systems (for which applications have been filed with my consent in Germany on the 15th of November, 1913, by the Gesellschaft fr elektromechanische Telephonapparate m. b. H., in Berlin-Wilmersdorf, and in England, by Harold Baron, on the 8th of December, 1913, and on which applications a German Patent No. 322,248 issued on 26th of June, 1920, and an English Patent No.

28,291 of 1913, was accepted on the 15th of July, 1915), and of which the following is a specification.

My invention relates to electromechanical or automatic or semi-automatic switching systems. It relates more specifically to meter arrangements in a system of the kinddescribed in my application Serial No. 814451, filed Jan. 26, 1914, Patent No. 1,319,031. The present application contains the same disclosure and is filed after the grant of said patent.

An object of the invention is to economize switches. IThis object is attained by providing switches which act according to the requirements of the moment as line finders or as connectors; in other words these switches at times connect a calling line to a free trunk leading to another switch such as a group selector or connector and other times they extend a connection which has advanced to a certain stage to the wanted subscribers connected to subscribers" lines and with a. setof movable contacts or brushes capable of being brought into contact with any of their sets of fixed contacts. From the set of brushes of each of these switches which will henceforth be called two-way switches because theyextend connections from calling subscribers to a trunk and at other times .from a trunk to a subscriber) one trunk or link circuit leads to the movable contact arms of a group selector or a selector finder and another trunk leads to the fixed contacts of a grou selector or selector finder. The set of brus ies of each two-way switch is connected Whilst the switch is in its normal position to the movable contact arm. of the group selector or selector finder with which it is associated. But as soon as the two-way switch is to be used for establishing a connection with a wanted subscriber its brushes are disconnected from the contact arm of the group selector or selector finder associated with it and are connected to the trunk extending to the fixed contacts of-the group selectors or selector finders.

The brushes of some of the two-way switches are directly connected to group selectors whilst others are connected to group selector finders which establish connections with free group selectors.-

The two-way switches are started or set in motion one after the other by starting impulses sent through a starting circuit in the order in which the calls are made by the calling stations.

The moment a subscriber calls the battery lead to the calling or line circuits of the other subscribers is opened by a call regulator until a connection is made between the calling subscriber and a group selector or conL nector. The line relayl of each subscriber has` a locking circuit which is closed the moment the subscriber calls in order to render thev calling circuit of this subscriber independent of the common battery lead which is opened immediately after the call is made.

Whenever a two-way switch is moved out of its normal position in order to establish a connection with a calling line the startinor circuit is disconnected from this switch and is immediately extended to the next two- Way. switch. This also takes place whenever a two-way switch is used for connecting with a wanted line. For these t-wo cases two special switches are provided one of which is shifted .bythe two-way switch itself as soon asit is moved out of its normal position, the other being shifted independentlyof its two-way switch by a current from a group selector over the trunk leading to the two-way switch.

The starting -circuits of the two-way switches are arranged so that the starting i of several switches by the same starting impulse is impossible. This is accomplished by arranging the switch that extends the starting circuit from one two-way switch to the next'so that the starting circuit is ex- I tended only after a starting impulse has ceased owing. The extending switch has a spring which is tensioned by a magnet excited by the starting impulse and which only exerts its :torce so as to shift the extending switch after the starting impulse has. ceased flowing.

In largeexchanges the subscribers lines are divided into groups and each group of lines is connected to a group of two-way switches. The groups of subscribers lines are sub-divided into sub-groups. Each twoway switch is composed of two finders, a prima and a secondary finder. The primary der inds the calling line and the secondary inder finds the sub-group in which the calling line is contained. In order to hasten the action of the two-way switch when finding a calling line its primary and secondary iinders are arranged to act simultaneously. Permutation wires are provided for each group .of subscribers which are connected to contacts of all the primary and secondary finders of the group and which, at the moment a subscriber calls, determine the points at which the linders of the two-way switch vused for the call shall stop in order to extend the calling line to a group selector or link circuit. ten sub-groups and ten lines in each subgroup the greatest number of steps to be taken by the primary and secondary finders will be ten and as both finders act simultaneously the maximum time required for iinding any particular line out of 100 is the time that the brushes of a finder take to travel the distance .occupied by ten adjacent sets of contacts.

YWhen a two-way switch is used to make a connection with a wanted line the circuits of the motor magnets of the primary and secondary finders of the switch, which are ordinarily connected to a common circuit, are separated and the motor circuits are then controlled separately by the person setting up the connection with the aid of a switch controller'.

The primary and secondary motor circuits of a two-way switch are separated also when restoring the switch to its normal position.

The two-way switches are each provided with two test relays, one for the primary finder and another for the secondary finder, which open the. circuits of the finder motor ymagnets the moment they find the proper permutation wire. These vtest relays are made inoperative when the two-way switch 4is used to connect to a Wanted line.

If there are Another feature of the invention consists in an rimpulse repeater located at a group selector at which impulses arising in one portion of a circuit are repeated in another portion of the circuit, the second portion being normally disconnected by open contacts from the first and the open contacts not being closed` until the called subscriber in responding to the call changes the strength of the ringing current sent to him from a group selector.

In order to simplify the circuits of the last selector in an established connection (i. e. of the selector or connector that makes connection with the wanted line) no circuits are used in this selector which act backwardly on the impulse repeater at the group selector. A called subscriber in responding to a call iniiuences the impulse repeater at the group selector without the aid of any intervening devices or circuits placed at the last selector. This is accomplished by tentatively applying a source of ringing current through a ringing control electromagnet or relay located at the group selector to leads extending through a chain of selectors to the wanted line. The arrangement of the ringing control circuits is such that if the wanted line is busy and the test relay in the last or final selector is not excited, no ringing current Hows through the said ringing con,- trol magnet on account of the ringing current path to the wanted line not having been closed by the said test relay. If the line is busy the ringing current does not flow. Whether the ringing current remains connected to the leads or not is made dependent on its flowing immediately after it is kconnected to the leads. If the ringing curits' armature until the called subscriber responds to the call and causes a strengtheniing of the ringing current. AWhen the ringing cut-olf relay is thus energized it closes the circuit of a connecting relay which closes the contacts that separate the calling half of the circuit from the called half and also prepares a meter circuit which is closed vwhen the connection is broken at theiend of the conversation.

The energization of the ringing cut-olf relay also results, in the cuttingvoif of the ringing current.

The invention is illustrated in the accompanying drawing in which Fig. l Shows the "iso ' secutively 3 of the. lower five group selectors,

general connecting plan of the new system. Fig. 2 shows the circuits of the subscribers line relays, the arrangement of the permutation or guide wires of the two-way switches and the arrangement of the call regulator. Figs. 3 and 4 illustrate the circuits of the two-way switches which perform the two functions of connecting a calling line to a group selector and of connecting a trunk leading from the fixed contacts of a group selector to a wanted line. Fig. 5 shows the circuits of the selector finders designated HS in Fig. l. Fig. 6 illustrates the circuits of a first group selector with an impulse repeater and Fig. 7 shows the circuits of a second group selector. By placing Figs. 2 to 7 beside each other with Fig.' 2 at the left and Fig. 7 at the right end a diagram of the complete system forming the subject of this invention will be obtained. Figs. 8 and 9 illustrate certain contacts used in my apparatus.

In Fig. l the three first groups of one hundred subscribers of an exchange for 1000 lines are diagrammatically indicated. To

the right of the bracket I are indicated the switching apparatus VS1, HSI and GWI of the first hundred group. The switching apparatus of the second and third hundred groups are shown at the right of the brackets II and III.

It will be assumed that there are 15 twoway switches (only ten shown) in each group which in the cases of calls coming from the subscribers S to the exchange conneet the calling subscribers to free group selectors GIV and in the cases of connections extended by group selectors GW to trunks h1, k2 etc. connect these trunks to the lines of the wanted subscribers.

Each two-way switch VS is shown by three arcs l, 2, 3 and a movable contact or brush 4. Each arc l, 2, 3 is supposed to represent a row of ten setsof fixed contacts. In reality each two-way switch has ten rows of ten sets of contacts in each but to simplify the drawing and description only three rows are indicated.

The lines of three subscribers S11, SI2, SIF-Sun SH2: Snai-smi; .Smm Sms Connect* ed to the third contact rows of the twoway switches are indicated.

Of'the l5 two-way switches in each group 5 are connected directly to group selectors GIV and 10 are connected to group selectors by means of selector finders HS to any idle one of a group of l5 group selectors. The latter group is made up of 5 groups selectors in each of the three hundred groups.'

The 10 selector finders HSI endeavour to pass calls first to the lower 5 group selectors GWI. If these are all busy they test con- GWn and if these are busy also they proceed to test 2 ofthe lower 5 selectors GVVm. In asimilar manner the selector finders idle.

HSII test first the lower 5 group selectors GW, then 3 selectors GVS7m and finally 2 selectors GVW. The selector finders HSm test first five selectors GWW, then 3 GWI and finally two GWH.-

Of the contact rows of the group selectors only three are indicated by the arcs p, s, t; eac row of contacts is multiply connected to a trunk cable h, h, which contains the trunk wires leading to the fifteen movable contacts 4 of the particular group of the two-wayv switches.

The trunk cable lz., leads to the two-way switches VSI of the first hundred group, the trunk cable k2 to the two-way switches VSII of the second hundred and h3 to the switches VS1 of the third hundred etc.

The path of a talking connection through the system will now be illustrated: Suppose a subscriber SI calls. The brush 4 of a disengaged two-way switch will immediately move into contact with the line of the calling subscriber and will connect this line to a group selector. This connection to a group selector GW will be either direct or through a selector finder HSI. The calling subscriber SI or a semi-automatic operator will now cause the movable contact 5 of the group selector to select the row of fixed contacts that corresponds to desired hundred group e. g. the row leading to the second hundred II that is connected to the trunk cable h2.

The group selector now hunts automatically for a free two-way switch VSH. When this is found the person controlling the switches causes the brush 4 of two-way switch VS1I to be'moved into contact'with the wanted subscribers line,-e. g. SH3.

As the two-way switches VS are used both `A for outgoing connections originating at the subscribers stations and for incoming connections which come to the subscribers stations from other stations and serve the double purpose of line finders and connectors, a considerable saving of switches is accomplished.

In the known systems in which separate line finders and connectors are used it will hap en `that all the line finders of a group are usy whilst ysome ofthe connectors of the group are still free. Further calling subscribers would not be able to make connections as .all the line finders are busy. But in the new system with two-way switches that will do the work of line finders or connectors accordin of the moment the scribers will still be able to make connections as long as any two-way switches are If at any moment the demand for connectors is in excess of the demand for to the re uirements f urther ca ling subline finders this demand will be supplied made in the busy hour or with the same coniecting capacity fewer switches can be use Fig. 2 indicates the circuits of the subscribers of a hundred group. Each hundred grou is sub-divided into tensl groups and represent the first two subscribers ofh the first tens group, S22, S22 the firsttwo subscribers of the second tens group and S131, S32 the first two subscribers of the third tens group. ,The other subscribers are omitted in order to simplify the drawing.

If the subscriber S12 calls, his line 121, 12b must be connected through the movable contacts ba, bb, boe-ba1, bb1, b01 of the two-way switch, Fig. 3, to a trunk line va, vb, @c which leads directl or indirectly (through a selector finder HS Fig. 5) to a first group selector GW1. 2

The brushes ba, bb, bc-b1, bb1, b'c1 of the two-way switches VS are set or guided to the positions in which they establish a connection between the trunk ea, vb, 'vc and the calling line (represented in this case by the contacts KS12) by guide wires or permutation wires pu, pt.

The guide wires are divided into two groups pu, pt one of which guides the primary finder to the contact representing the calling line, the other guiding the secondary finder to the contact representing the tens group in which the calling line is contained. When a subscriber calls, his line vrelay e. g.

11412 is energized and it earths one wire in each of the groups of guide wires pt and pu thus setting up a circuit combination which will cause the proper motions of the primary and secondary finders of the next free twoway switch in the particular hundred group for connecting the calling line to a trunk.

'The calling ofthe subscriber S12 i. e. the second subscriber in the first tens group results in the contacts 100, 101 of hisline re-A lay Z732 earthing the first wire 311 of the tens group of guide wires pt and the second wire 2.1`of the unitsgroup of guide wires. If the subscriber 11v calls his line relay contacts earth the first tens gpide wire Z11 and the iirstunitsguide wire Z1. `A call from sub` scriber No. 21 would result in the earthing of the tens wire 211 and the units wire Z1 and so forth, so that a certain combination of earthed guide wires corresponds to each of the hundred subscribers of the group.

In Fi 3 only the circuits of one two-way switch f S is shown but the permutation or guide wires pt, gm lead to all the two-way switches of the hundred group and are connected to each of these switches by branch Wn'es w1 we, we', 'wu 'ws-p1- pza 232 Q4 P5' The contact sets les of the two-way switches of a grou are`connected in multiple. I

'From t e common call e lator AR of eachgroup 'a starting wirev W extends to the two-way switch VS. As soon as Lama this switch is taken into use its starting wire SW is extended through a switch 104 or 103 to an extension starting wire SW#1 which leads to other switches similar to 103, 104 on the next two-way switch. If the second two-way switch of the group is also engaged the startin .wire is extended to the third two-way switch and so forth.

The primary finder of each two-way switch has ten sets of three brushes each ba, bb, bc and a guiding brush pb, al1 these brushes being driven so as to brush ast sets of contacts k3 and single contacts l respectively by a common driving shaft D. If the brushes are moved so as to make the guiding brush pb rest` upon the guiding contact k2, each of the sets of brushes ba, bb, bc will yrest upon the second sets of contacts las in the row of contact sets 70810, 76820, lasso, etc., that it rides over. If the brush pb were to be moved to the fourth guiding contact k2 each of the sets of brushes ba., bb, be would also be moved to the fourth set of contacts los in its particular row and so forth.

The common driving device that moves all the brushes consists of a ratchet wheel ZR fixed to a shaftJDwhich is rotated step byA which whilst the two-way switch is in its normal position opens the contacts y104, 110-105, 10S- 106, 107 and .closes the con'- tact 104, 108. The moment the two-way switch leaves its normal position the contacts 106, 107-105, 108-104, 110 are closed and the contact 104, 109 is opened.

Only one set of brushes ba, bb, bc will be connected to the trunk ca, ob, v0 when a connection has been extended through the two-v way switch.l The set of brushes to be connected to the trunk is selected by the secondary finder ZW which has ten-sets of fixed contacts Ke of four contacts each and a set of ^four movable brushes ba1, bb1, b01, sb. The set of brushes ba1, bb1, bc1, sb is driven in any suitable manner e. g. by a shaft D1 which is turned "with the aid of a ratchet wheel ZR1 by a pawl KL1 oscillated by the' armature Am1 of the secondary finder driving magnet ZM.

The chain of events which happen when a 'subscriber calls and establishes av connection` ulator, thence through 113, 114, 12a, S12, 12", 115 to earth. The line relay Z111, at-tracts its armature closes a locking circuit for itself through the contact 116. By closing its contact 117 the line relay closes the circuit of the call regulator AR (battery B, AR, 117, earth) and this Iattracts its armature and by opening thecontact 112 opens the common battery feeder of all the line relays of the hundred group to which the calling subscriber belongs, so that no other subscriber of the same group can make a call by closing his loop and energizing his line relay until a two-way switch VS has connected the first calling subscriber to a group selector.` The connecting operation of the two-way switch will, however, not require more than about 92 a second so that if the second subscriber calls immediately after the first the delay in taking up his call will be so short that he will not notice it. The purpose of the momentary cutting ofi' of the battery feed to -all the subscribers except the first to call is to prevent a mixing of the calls and a wrong s etting of the two-way switches which would be possible if more than one call were passed on to the two-way switches at a time. For

this reason the sequence in which the callsare passed on over the starting wire S'W is regulated so that the calls are transmitted over the starting wire one after the other.

By its contacts 100, 101 the line relay Z132 earths the first tens guide wire Zll and the second units guide wire 2l so that the contacts k2 and zin all the two-way switches of the group will be earthed and the points indicated where the brushes sb and sp of the particular two-way switch used are to stop.

rlhe call regulator in attracting its armature causes a momentary closure of the contact 119, 120. The armature 118 brushes past the spring 119 and momentarily presses 119 against 120. When the armature falls back into its normal position 119 is pressed away 120 and no closure of contact 119, 120, takes place.

It is thus seen that the moment the subscriber S12 calls-a current impulse is sent by the momentary closure of the contact 119, 120 from the battery B through SW. If the two-way switch VS is idle this impulse will fiow through 103, 111, 154, 104, 109, 155, 116, 156 t6 157. A1, che point 157 the starting impulse divides into three parts the one part flowing through 131, 158, motor magnet ZM of the secondary nder, 159 to earth and another part throuli 131, 160, 115, 161, 162, motor magnet E of the primary finder, 163 to earth and the third part through 164, 13'5, ah, 200, 201, 203, starting relay to earth. The magnets ZM and EM attract their armatures and each shifts its pawl (kl and loll) into the next tooth of its ratchetV wheel ZR, ZRl. When the starting impulse through contact 119, 120 ceases the pawls k1 and cbl are drawn back to their normal positions by their springs 165, 166 and the ratchetv wheels ZR,'ZRl are turned one step as a result of which the sets of brushes ba, bb, bc and the brush pb and also the brush set ball, bbl, bol, sb are shifted. The movement of the shaft D out of its normal osition also causes the off-normal switch O 1 to be shifted and the first two-way switch VS of the group to be cut-off' from the starting circuit 154 at Contact 104, 109 and to be connected to an interrupter 168. The starting circuit 154 is simultaneously extended through contacts 104, 110'andthrough SW1 to the next idle two-way switch of the group so that the starting impulse caused by the subscriber that calls next will flow through the magnets ZM and EM 0f the next idle two-way switch.

The motor magnets of the started twoway switch will now be energized by current impulses which flow from the negative pole of the exchange battery through 168, 105, 108, 155, 118, 156, ,157, 131, 158, ZM, 159 to earth and through 131, 160, 115,

161, 162, EM, 163 to earth respectively. As

soon as the brush pb reaches the' contact k2 of the second units guide wire 2l which is earthed by the armature contact 101 a c urrent will flow through the test relay pr from the negative pole through 117, 169,134, 170,

.moved at the same time as the brush sets ba, bbc, be and as soon as the test brush 8b strikes'the tens guiding contact al which is earthed by the armature contact 100, a current will flow through the secondary test relay sr from the negative pole through 117,`

169, 134, 170, 8f, 174, 8b, pl, zu, 173, 100 t6 earth. This will energize the secondary test relay which will open motor magnet circuit thus causing the set of brushes bwl, bbl, bcl, sb to stop at the first contact set Ica.

In the case assumed in which the twelfth subscriber of the hundred groupis calling the circuit of the secondary finder magnet will be opened immediately after the starting impulse has flowed, because the moment this impulse ceases the brush set bal, bbl,

bel, sb will be advanced to the first set ofcontacts'kaof the secondary finder.

The first contact set Ica is connected to 'the' first set of brushes ba, bb, bo counting from the right, so that the calling subscriber S12 will now-be connected through am, bn, 012, 70812, the first set of branches ba, bb, be, the first contact set las, brush set bal, bbl, bol

.to the trunk fva, fvb, Ivo.

regarded in connection with the line nding The moment the test ,relays pz, sz are both energized, the contacts 132, 133 will be closed and a current will liow from the negative pole through 132, 133, 125, connecting relay tz, 124 to the positive pole. The relay tz closes its contacts 141, 142, 143 and thus extends the wires fva, fvb, fvo to the wires zal, p51, zal. The relay tz also closes and opens a number of other contacts, but these are only of importance when the two-way switch is used as connector and need not be operation of the two-way switch.

It was mentioned above that the starting relay 205 of the selector linder HS, Fig. 5, was energized at the same timeas the motor magnets EM and SM by the starting impulses sent through .the starting wire SW at the moment the subscriber S12 called. When the relay 205 is energized the contacts 202, 203 are opened and the contacts 204, 208, 207 closed. By its contact 204 the relay 205 closes a locking circuit for itself which is independent of the starting circuit ah and may be traced from the negative pole through 213, 204, 205 to earth. By its contact 208 the relay 205 closes the circuit of test relay pzh and by its 207 the circuit of the turning or motor magnet 206 of the selector finder (negative pole, interrupter 218, 210, 206, 207, earth). The turning magnet 206 now rotates the brushes wa, wb, wc by a ratchet and pawl arrangement in a wellC known'manner causing them to brush over the fixed contacts connected to the trunks fvaz, p52, p02, until an'idle trunk is reached. A current then flows from the negative pole through 208, 211, pzh, we, 002, relay ez, 219 to earth. The relays pzh and ez are energized. The relay pzh opens the circuit of the motor magnet 206 at itscontact 210 so that the brushes wa, wb, we stop on the xed contacts connected to the trunk paz, wb2, l00 leading to an idle first selector GWI. y its contact 212 the test relay closes a locking circuit for itself which is independent of the contact 208 and at its contact 213 it opens the circuit of the starting relay 205. By its contacts 214, 215, 216 the test relay extends the line of the calling subscriber to the first group selector GWI. The relay pzh opens the contact 217 and thereby prevents a premature release impulse from flowing from the earth through 209, 202 to the release wire al. By the opening of contact 200 unnecessary oscillations ofthe armature of the starting relay are prevented which would be caused by impulses flowing from the interrupter 168 through 108155, 118, 156, 164, 135 and ha.

When the callin line is extended to-the group selector by the closure of the contacts 214, 215,216 the cut-olf relay ozu, of the calling subscriber is energized by a current owing'from the 'positive pole of the battery the trunkwa2, wb2, 'U02 will now not receive sullicient current to energize them so that the trunk fm2, wb2, zo, will appear engaged.

By the energization of the cut-off relay cz12 the line relay zu is cut oil' from the calling line. On the deenergization of the line relay Zzm the earth connections of the guide wires 111 and 21 are removed and at the same time the call regulator AR is deenergized by the opening of contact 117. When the call regulator is deenergized it does not close the contact 120, 119, but it reconnects the battery B to the contacts 112 of all the line relays Zz of the group so that the call of the next subscriber will now be taken and passed on to the next idle twoway switch.

The time that elapses from the moment the subscriber calls to the moment when the two-way switch VS and the trunk selector HS have established the connection between the calling subscriber and the group selector only amounts to a fraction of a second so that the likelihood of two calls waiting to be taken up is extremely remote and if it occurs the second.'v subscriber to call will not notice any delay.

As soon as the earth connections of the guides wires 111, 21 are removed through the cutting oil' of the line relay Zz12 the circuits of the test relays pz, sz will be broken and the contacts 163, 159 in the circuits of the motor magnets EM and ZM will be closed. But these circuits will now be kept open at the contact 131 of the connecting relay tz.

It has been said'that when the brushes wa, wb, we are caused to stop on the idle trunk fvaz, wb2, '002, relay ez is energized that is associated with this trunk. The relay ez' serves to close the battery circuit of the relays zz and cz. The relay cz is a circuit shifting or change over relay which changes over the group selector from its selecting to its trunk hunting or ktesting function. The relay zr is a release relay which closes the release circuits of the selectors when the subscriber replaces his re'- ceiver on its switchhook.

The relays zz and cz are energized in the periods of time during which the armature of the line control relay travels from one extreme position to another.l The relays zz and cz are energized by openingcontacts 232 and 233 and thereby opening paths of low resistance by which they are usually short-circuited. For this reason the battery circuits of these relays must be closed before the selecting impulses commence actuating the line control relay 280. This closure is effected by the contact 220 of the relay er.

On the calling line being extended to the trunk line we, @172,100.2 the line control relay 280 is immediately energized by a current flowing from the negative pole of B1 through the lower winding of 280, obz, wb, 214, bb 121, 142, bb, bbl, bb, blz, S12, 12a, au, ba, bal, oa, 141, 119, m1, 215, wa, m1 upper winding of 280 to earth.

Just before the energization of relay 280 the relay c1' was energized by a current iiowing from the negative pole through 220, 232, or to earth. In attracting its armature the relay er lets the sprin 236 press a ainst the contact spring 237. n the energlzation of the relay 280 the relay is short circuited by contact 233 and allows its armature to dro back whereupon the negative battery pole 1s' momentarily connected to the change over circuit 290. This circuit is, however, still open at the contact 238 so that so far no shifting of circuits will be caused by applying the negative battery pole to the wire 290 belonging to the change over circuit.

`When the relay 280 is energized the relay w' is energized bythe opening of its short circuit at contact 232 the current flowing through 'rr from the negative vpole through 220, 1'7, 233 to earth.

As the calling subscriber desires a connection with subscriber No. 3465 he will work his switch controller in a well known manner so as to rst cause three openings of his line loop in rapid succession. This will cause three oscillations of the armature of the line control relay 280. During these .oscillations the relays w' and 01' are short circuited only for very brief moments for the contacts 232, 233 are adjusted so as to be closed only whenever the armature of 280 has moved to its utmost limit. These brief short-circuitings of the relays w andV 07' are not suliicient for these relays to allow their armatures to fall back so that they keep then attracted.

Each time the armature of 280 drops back a current impulse is sent through the vertical magnet 246 of the up and around group selector from the negative pole through 220, 231, 234, 224, 246 to earth. The vertical magnet 246 lifts the wipers of the group selector, which will be assumed to be `of the Strowger type, to its third row of fixed contacts.

After the three thousand selecting impulses have thus been sent in by the three rapid openings of the calling subscribers line loop this loop 'remains steadily closed for a while until the subscriber sends in the next series of impulses for selecting` the wanted hundred. The steady closure of the line loop causing an attraction of the line control relay armature and the change over relay va relay 249. circuit 295 is shifted from the vert1cal magcfr will then be short circuited long enough to cause to release its armature. .During the falling of this armature the contact springs 237 and 291 will be connected for a moment.

.When the armature of or has fallen entirely normal switch 238, which is closed whenthe wipers are lifted out of their normal position, 226, 247 to earth. This energizes the rotary magnet 247 which turns the wipers ga, gb, gc of the group selector round one step and thereby causes the rotary olfnormal switches 255 and 256 to be closed. The switch 256 connects an interrupter 257 to the rota magnet so that impulses now commence owing from the negative pole through 257, 256, 229, 226, rotary magnet 247 to earth and the rotary magnet causes they brushes to travel over the sets of fixed contacts connected to trunks oob, @53, o0, until a trunk leading to an idle second group selector GW is reached when a current will flow from the positive battery pole through,

the test relay 248, test wiper gc, oe3, 300to earth.

The test relay 248 and the release relay 300 of the second group selector both attract their armatures. The former opens its contact 229 and thus interrupts the circuit of the motormagnet 247 and by its contacts 226, 227 it also shifts the change over circuit 290 from the rotary magnet to The stepping or measuring net 246 to the brush ga the connection to the brush ga being made through contacts 225 and 270.

By its contact 228 the test relay 248 short circuits va part of its winding and thereby reduces the potential on the test co3 to such an extent that no other first group selector will be able to establish a connection with the second group selector^GWH.

An idle group selector in the third thousand group now being selected and found the calling subscriber proceeds to select the desired fourth hundred group by causing four interruptions of his line loop with the aid of his switch controller. This causes four oscillations of the line control relay until this subscriber in respondin After the calling subscriber has caused the series of four interruptions of his line loop another interval ensues during which the loop is steadily closed and the line control relay armature is steadily attracted. The change over relay cr is then steadily short circuited by the contact 233 of the line control relay and allows its armature to drop back and to close the change over circuit Awhich may be traced from the negative' pole through 220, 230, 237,236, 291, 29o, 233, 227, relay 249 to earth. The relay is energized and sends a change over impulse from the negative pole through 252, 296, 277, gb, laba, 312, 313, 310, rotary lmagnet 306 of the second group selector GWII to earth. The rotary magnet immediately turns the brushes ga, gb, gc one step and thus causes the opening of off-normal contact 313 and the closing of oi-normal contact 315. The impulse sent over the wire @b3 thus causes the immediate opening of the change over circuit 296, 277, gb, @b3 etc. at the contact 313 and the closure of an independent circuit for the rotary magnet by thecontact 315.

When the change over relay 249 was energized it not only closed the change over circuit but also the circuit of a Slow-acting relay 250 (negative pole, 253, 250 to earth). This relay closes a contact 254 and when on the immediate opening of the change over circuit 290, the relay 249 is deenergized, the ringing current source 268 is applied 'through 258, 254, 251, 296, 277 to the line uba until the slow-acting relay allows its' armature to fall back so as to cut oli the ringing current at its contact 254. Although the ringing currentis applied to the line vb, no ringing current will flow because the change over circuit from 296, 277, gb, fvbs, 312, 313, 310, 306 to earth was interrupted the moment the relay 249 attracted its armature and sent an impulse magnet 306 causing the. contact 313 to be opened in the manner already explained.

The circuits are so arranged that a closed circuit through which ringing current will flow uponthe ringing machine being applied through 296 to the line @b3 will be found only after the inal selector (connector) has made connection with the wanted line and has found this line idle. Under these circumstances the ringing current machine will send ringing current to the called subscriber to the call removes his receiver from its switchhook ,and thereby causes the ringing current to be strengthened suiciently to energize a relay 264 at the group selector which then causes the ringing current to be cut o and the 4 calling and called ends of the talkin circuit to be connected together by a relay 2 3.

In the known circuit combinations f or au. -tomatically stopping the flowing of ringing' through the rotary= current the application of ringin current to the wanted line is made depen ent on the test relay. of the final selector or connector. According as the wanted line is idle or busy the test relay will cause ringing current to be applied to it or not..l If it were desired with circuits of this kind to place the ringing current source at a group selector some special circuits would have to be provided by which the influence of the test relay at the final selector is transmitted back to the group selector in order to connect the ringing current to the wanted line.

By this invention these special circuits for reacting from the test relay of the final selector upon the means at the group selector for connecting the ringing current are avoided. According to this invention the ringing current is applied momentarily to the line which is being extended to the wanted subscriber at each change over impulse.` The permanent connection of the ringing current y to the line is, however, made dependent on disconnecting of thev tentatively applied ringing machine 268 is effected by the dropping back of the armature of the slow-acting relay 254 shortly after the closure of the circuit of the rotary magnet 306 of the second group selector GrW,Il because on account of the opening of contact 313 no closed circuit exists in which a ringing current can `flow from the ringlng machine. The circuits by which the ringing machine is steadily -connected in the case of being applied to af'closed circuit will be explained later.

The rotary magnet 306 which is energized by current impulses from the negative pole through 314,v 315, 310, 306 to earth turns the wipers g, g", gc of the second group selectorcausing them to brush over sets of the fixed bank contacts leading to the fifteen two-way switches VS through trunks am, bm, 08,0 until anidle trunk is reached.

a tru aizo'o, baoo @soo is ldle the twovlfa'y switch Figs. 3 and4 4 associated therewith will be in its normal position and its contact 130 will be closed.

The trunk am bm, 0300 to which the group selector G I extends l the connection of the calling subscriberwill, of course, lead to a diferent two-way switch than the one whose brushes are connected to the calling line. But as the circuits of all the two-way switches are alike the events which' take place at this stage of the connection lll lll

of the calling subscriber may be explained with the aid of Figs. 2, 3, 4.

The two-way switches of the selected hun` dred group which are already being used for other connections will appear busy to the hunting group selector on account of their switches 130 being open. The moment the brushes of the group selector strike a free trunk a current will fiow through the test relay p1', from the negative battery pole through 301, prz, gc, 0300, converging relay SR of the idle two-way switch in the desired hundred group, closed switch 130 to earth. The relays p1, and SR are both energized. The former opens the circuit of the rotary magnet 306` at its contact 310 and by closing its contact 309 it short circuits a part of its winding and thus lowers the potential on the test wire 0300 so as to render the seized trunk busy. By closing its contacts 308, 311 the test relay p1', extends the talking circuit was, 9b, tothe two-way switch Figs. 3 and 4..

The converting-relay' SR by shifting its armature contacts converts the two-way switch from'a line finder to a nal selector. By opening its contact 111 and closing the contact 112 it disconnects the starting wire SW from the seized two-way switch and extends it to the wire SW1 leading to the next two-way switch in the hundred group so that the next call arising in this group will be assed on to thisv next two-way switch. By closing its contact 113 the con verting relay connects the change over wire bam, to the change over devices 190, 19.5 which at a later stage change the connecs tion of the selecting wire a300 from the tens or secondary motor magnet ZM to the units or primar motor magnet EM of the twoway switc By the closure of contact 114 the selecting wire a300 is connected to the. tens magnet ZM 1gthrough 139, 160, 158, ZM, 159 to earth). y opening of contact 115 the circuits of magnets ZM and EM which are ordinarily connected lto the common wire 193 leading from the interrupter 168 are separated so that ncw each of these motor magnets can be controlled separately. At the contact 116 the release wire leading to the associated selector finder or group selector is broken in order that when this wire is earthed on the breaking down-of the connection in the manner to be described switch are moved into congized. At contact 118 the battery lead from the interrupter 168 to the wire 1641s disconnected in order to prevent impulses flowing through 118,164, 135, ha, 200, 203 to the starting relay 205 of the selector finder HS on the shifting of the switch CS1 which happens when the two-way switch is moved out of its normal position. The contact 107 opens current path from the selecting wire a300 through 139, 131, 164, 135, 200 to the starting relay 205 and at contacts 119, 121, 123, the lines vwl, obl, fue, leading to the selector nder are disconnected. Through contacts 120, 121 the talking wires in the two-way switch are disconnected from the selector finder and connected to the trunk extending from the last group selector in the partly established connection of the calling subscriber.

By the opening of the contact 124 the ordinary circuit of the test relay t7' is broken and this relay is converted by the opening of they contact 125l and closure of contact 126 intc a test relay for testing the condition of the wanted line thatwill presently be selected by the two-way switch. By closing the contact 127 the converting relay SR closes a locking circuit for itself which is independent of the switch 130. Finally the converting relay SR moves on being energized the contact spring 129 but in'so doing no connection is made between the contacts 129, 128. These contacts are closed only when the r`elay SR is deenergized and they then cause the energization of a release relay rZ as will be more fully described later. The contacts 116, 129, 128 are so adjusted that the contact 116 is closed only after the contact between 129, 128 is opened so as to prevent an impulse flowing from\ earth through 128, 129, 195, 116, 217, 202 to the rotary magnet206 of the selector inder.

lVhen the converting relay SR is energized and has produced the circuit changes just described the calling subscriber sends in the tens selecting impulses by causing a number of interruptions of his line loop that corresponds to the tens digitof the wanted line which in this case is 6. line control relay 280, Fig. 6, lets its armature execute six oscillations. Each time the armature falls back an impulse flows from the negative battery pole through 220, 231, 234, 295, 225, 27o, ga, wwwa', gla, am, 114, 139, 158, tens motor magnet ZM, 159 to earth. By these six impulses the set of brushes bal, bbl, bc1 of the two-way switch is stepped round to the sixth set cz which is connected to the sixth ybrush seti. e. the set of brushes ba, bb, bc, that travels over the contact sets los in the sixth row or sixth tens group. The desired tens group is now selected. l

The

In order to select thea individual line in I the selected group of ten lines all the brush ZW is moved out of its normal position..

sets ba, bb, bc of the two-Way switch are moved five steps. The sixth brush set ba, bb, be will then rest upon the fifth set of bank contacts in its contact row this being the set of bank contacts that belongs to the Wanted line N o. 3465. The desired line is now selected.

The other nine sets of brushes ba, bb, bc Will also each be resting on the fifth contact sets lcs in their respective rows, but these brush sets have no connection with a brush set bal, bbl, bol so thatthey do not establish any connection with the subscribers lines on which they are resting.

A detailed description ofthe switching functions which take place in setting the brushes bca, bb, bo of the. two-Way switch onto the desired line Will noW be given;

After the series of tens selecting impulses is sent in the calling subscribers loop is again steadily .closed and the steady short circuiting of the change over relay 0r caused hereby results in the fallingback of the change over relay armature. A change over impulse then ows from the negative pole through 220, 230, 237, 236, 291, 238, 290, 227, relay 249 to earth. The relay 249 attracts its armature and closes a circuit eX- tending from the negative pole through 252,

296, 277, gb, lvb, 311, glb, bm, 113, Fig. 4,

152, change over relay 190 to earth. The change over relay is energized and immediately opens the change over circuit at ff 152 so that the ringing machine 268 applied to the change over circuit through 254, 251 can not send out any ringing current and Will be cut off again by the dropping back of the armature 254 of the slow-acting rela llhe change over relay 190 closes a locking circuit for itself from the negative pole through 151, 153, 190 to earth. The switch 151 is closed When the secondary finder.

At its contact 139 the change over` relay separates the selecting Wire a300 from the secondary motor magnet ZM and connects it through contact 138 and through 196, 162 to the primary motor magnet EM.

On the calling subscriber noW sending the five units selecting impulses the armature of the line control relay 28() drops back five times and ve units selecting impulses iiow through 220, 231, 234, 295, 225, 270, gtr, /v3,308, gla, a300, 114, 138, 196, 162, primary motor magnet EM, 163 to earth, these impulses causing the brush sets ba, bb, bo to be moved to the fifth bank contact sets and to Aselect the desired subscribers line in the manner already described.

After the last series of selecting impulses has gone in the line control relay 280 keeps its 'armature steadily attached so as to deenergize the change over relay c1' again by short circuiting it. 'The relay 249 is thenl momentarily energized and a test impulse Hows from the negative pole through 252, 296, 277, gb, aba, 311, gib, baco, 113, switch 150 which is closed the moment the primary linder is moved out of its normal position, 149, rela 193 to earth. The relay 193 is energize and by opening contact 149 disconnects itself from the change over Wire bm. But it also closes a. test circuit extending from the positive pole through 198, 146, 147, relay t1', 126, cv, bel, bc, test Wire 03,65 (not shown in the drawing but similar to the test Wire of the calling subscriber) cut ofi' relay c7' of the desired line No. 3465 to the negative pole of the exchange battery. If the Wanted line is free the relay t1', which now acts as a test relay, will be immediately energized and will extend the lines a300, bw, to the line of the Wanted subscriber by closing its contact 141, 142. The ringing machine, which is applied by the contact 254 of the slow-acting relay to the line bm after the change over impulse sent through contact 252 ceases, now sends out a ringing current before the Slow-acting relay lets its armature drop back. The ringing current liows from the earth through 268, relay 258, 254, 251, 296, 277, gb, bbs, 311, g-lb, bm, 121, 142, bb, bbl, bb, bm, aiebb, bell of- Wanted subscriber No. 3465, 3465a, c3465, ba, bal, aa, 141, 120, a300, gla, 308, aaa, ga, 270, 225, 295, 298 to earth.

By the immediate iiowing of the ringing current the steady connect1on of the same to Wanted line is effected by means of a relay 258 Which is energized by the ringing current and closes a ringing current path which is independent of the contact 254 of the slow-acting relay, this path leading through 259, 260, right hand Winding of relay 299, 296, 277 to the above-described ringing circuit. The relay 299 is now energized and the circuit including relay -258 is broken at 260 which causes the relay 258v to open its armature contact 259 and to permanently disconnect itself.

The yringing current is now kept connected to the wanted line by the relay 299 `Which closes the contacts 261, 262 before opening the contact 260. If the interrupter 267 which is continually running just happened to have closed its contact 265 the ringing current will flow from 268 through 265, 264, 261, right hand Winding of 299, 286, 277 to the Wanted subscribers ringing circuit. But if the interrupter happens to have closed the contact 266 at the moment the relay 258 is cut off, the ringing current will not liow through the line of the Wanted subscriber 'but through 266, 263 and the left Winding of 299. It: is thus seen that in any case the one or the other Winding of 299 is energized. If the contact 265 is closed the ringing current flows through the relay 264 and the right hand winding of 299 to the wanted line,but if the contact 266 is closed the whole ringing current flows through the left winding of 299 and` there will be a pause in the ringing of the wanted subscribers bell.

When the called'party responds by removing his receiver from his switchhook the ringing current ilowin through the relay 264 is strengthened su ciently to cause'this relay to close its armature contact 263. A current then flows from the negative pole through 263, connecting relay 273 to earth. The relay 27 3 thereupon interrupts the flow of ringing current at 277 so that the energization of the right winding of 299 ceases before the left winding is reenergized. The relay 299 is therefore deenergized and the ringing machine 2'68 is cut o at 261, 262.

By closing its contacts 271, 276 the relay 273 extends the line of the calling subscriber to that of the called subscriber and discnnects selecting and change over circuits at contacts 270 and 277.

The relay 273 closes a locking circuit for itself by its contact 272 (negative pole, 255, 272, 273, earth) and by its contact 269 it momentarily short circuits a part of the winding er and thus momentarily strengthens the current in the meter circuit (vez, v01, 123,143, ce, 601, be, cl2, 2,2, .negative pole) so as to make the meter 2,2 attract its armature and register the conversation.

In order to cause the meter circuit to be only momentarily closed by the energization of the relay 273 so as not to impair the busy condition of the test wire fue, the contact 269 is'made like the contact 119, 120 of the call regulator AR, Fig. 2. This is more clearly shown in Figs. 8 and 9. In Fig.v 8 273 represents the end of the armature that is moved in the direction indicated by the arrow 274 when the relay 273 is'energized.

` The end of the armature is round. and glides along the right hand oblique surface at the end of the. contact spring 269 so as to move 269 towards the left and against the contact spring 269. When the armature is fully attracted its end will be located above269 as indicated by the dotted circle 273". On the deenergization of the relay 273, the round end 27 3' glides along the left side of the oblique end of 269 forcing it further away from 269 instead of towards 269. -The aecting of the busy condition of the line ce, can also be 4prevented by using a contact 119, 120 which causes the momentary closure of the meter circuit on the deenergization of relay 273 at .the finish of the talking connection. In this case a similar arrangement to that illustrated in Figs 8 and 9 is used, butthe end-of the armature is arranged so as to push the spring 269 away rom1269 when it is attracted and to push it against 269 when it falls back. In other words the end of 269, instead ofrun- 271, 2 6 a circuit is closed including the relays 278, 279 and extending from earth through 278, 271, ga, was, 308, glo., a300 to the two-way switchy which has established the connection with the wanted line, 120, 141, wa, bal, ba of the sixth set of brushes, 07348 346m, Sms, 34651, 63,65, bb, 661,96, 142, 122, 6300, glb, 311, lcbm gb, 276, 279 to the negative pole. The relays 278, 279 are excited; 279 closes its armature contact 240 and 248 opens its contact 239.

The calling subscriber is now connected to the. called subscriber over the following path: S12, 12a, au, ba, ba.1 of the two-way switch which has connected to the calling line, bal, ca, 141, 119, 215, wa, fvaz, co1, 271, geni/vas, 308, gla, a300, two-Way switch VS in the called hundred group, contact 120 of the convertin relay SR of this switch, 141, oa, bal, ba o the sixth set of brushes of the two-way switch, c3465, 3465a, Sms, 34656, 634,5, bb, 661, lvb, 142, 122, 6,00, glb, 311, cbs, gb, 276, co2, wb2, wb, 214, '061, 121 of the twoway switch that has connected to the calling line, 142, fvb, bb1 6b of this switch, 612, 12b,

If the calling subscriber is the first to terminate the connection by replacing his receiver on its switchhook and thereby opening his line loopl the line control relay 280 will be deenergized and the release relay W' will be steadily short circuited. The latter relay will then allow its armature todrop back and close the release circuit (earth, 235, 245, 223, negative pole). The release magnet 245 will be energized and will cause the first group selector to be restored to its normal position. The release magnet also opens its armature contact 222 and thereby deenergizes the relays er and pril.. The

relay pril, lets its armature drop back, thus opening its locking circuit at its front contact 212 so that, until the starting relay 205 is energized again, no reenergization of the relay prh can take place. By the contact 210 the circuit of the rotary magnet 206 of the selector finder is closed (negative pole,

cuit extending :from earthl through 209, 202,

217, al, 116, 195, relay rl to the negative pole. The release relay rl of the two-way switch is thus energized and closes a .locking circuit for itself which may be traced from the negative battery pole to TZ, 137, 107 `to earth. At its contact 136 the release relay opens the locking circuit of the relay tr. This relay is then deenergized and closes at its back contact 131 the circuit of the magnet EM (negative pole, 168, 108, 155, 118, 157, 131, 115, 162, EM, earth) which remains closed until the driving device Dof the primary linder reaches its normal position and disconnects the interrupter 168 from the motor magnet EM by opening the contact 108.

The motor magnet ZM of the secondary finder is completely disconnected from the circuit of the motor magnet EM of the primary finder at contact 1951 of the restoring relay TZ in order that the two iinders may move independently of each other into their normal position.

The independent restoring circuit of the motor magnet ZM is closed by the contact 1961 of the restoring relay l'Z and extends from the negative pole to interrupter 1971, 1961, 1671, ZM, 159 and to earth. The mo- I ment the secondary-finder reaches its normal position the contact 1671 is opened and the motion of the finder is stopped through.

the interruption of the circuit of the magnet ZM.

In order to prevent interruptions of the circuits of the magnets EM, ZM. by enerizations of the test relays pr, sr caused during the travel of the brushes pb, sb into their normal positions by their striking earthed guide wires the battery lead to these relays is kept open by the contact 134 of the restoring relay rl until this relay is deenergized. The restoring relay is, however, not deenergized until the two-way switch reaches its normal position and opens the locking circuit of the restoring relay rl by severing the contacts 107 and 167.

To prevent the flowing of impulses from 168 through 108, 155, 118, 156, 164, ha, 200 to the startin relay 205 when the relay pril, is deenergize and the-contact 200 closes the armature contacts of pff-h are so adjusted that 200 remains open until shortly after the closure of 217 i. e. until shortly after the energization of rl and until the opening of 135 by rl. By this means current impulses are prevented from flowing from 168 through 108, 155, 118, 156, 164 to the starting wire hav during the restoration of the two-way switch which would cause a displacement of the selector finder HS.

The second group selector GrVVII is releasedv closes the circuit of the release magnet 304 (negative pole, 302, release magnet 304, 303,

earth) which thereupon releases the second group selector. At the moment the release circuit is closed the test circuit p72 is opened at 301 so as to prevent any energization of the test relay pr2 whilst the brush g1@ travels over the bank contacts of other trunks 0300 during the restoration of the selector.

The moment the contact arm g1c of the second group selector leaves the trunk 03,0 leading to the called'hundred group the converting relay SR of the two-way switch in this group that was used for the talking connection is deenergized and on its armature falling back the contact 128, 129 is momentarily closed. This causes the energization of the restoring relay rl associated with this two-way switch, the restoring circuit extending from earth, 128, 129, 194, rl to the negative pole.

The deenergization of the converting relay SR results in the opening of the circuit i of the test relay t1' (earth, 140, tr, 126, v0, 501, bc, 03465, @T3465 by the contact 126.

The two-way switch used Jfor establishingA the connection with the wanted subscriber 4is now restored in the same manner as the two-way switch that connected to the calling line, the restoring functions of which have already been described.

It will be remembered thatv the magnet EM is energized by impulses from the negative pole through 168, 108, 118, 157, 131, 115, 162, EM, 163 to earth and that the Vmagnet ZM receives its restoring impulses from the negative pole through 1971, 1961, 1671, ZM, 159 to earth.

If the called subscriber is the first to replace his receiver on the switchhook at the end of the conversation the restoration of the switches, selectors etc. will be initiated by the deenergization of the relays 278, 27 9. The rela 279 is slow-acting and lets its armature rop back later than the relay 278.

The deenergization of these two relays Lwill thus result in a short closureY of a release c1rvthe restoration of the switches in the connecment the two-way s'vitch is set onto thewanted line the relay tr will not be energized nefasta as previously described and hence the talking i leads of the calling subscriber will not be connected 'through the contact 141, 142' to the line of the wanted subscriber.

A feature of the meter circuits used in the present system which was not mentioned in the above description will now be' pointed out:

It was shown that the actuation of the meter of the calling subscriber is caused by strengthening the current flowing in the wire 012, this strengthening being produced by partially short circuiting the winding of relay er by means of contact 269. Now when a two-'way switch is used tdestablish a connection with a wanted line, the energization of the relay t1 of this switch and the partial short-circuiting of this relay will also produce'a strengthening of the current in the wire cM of the called subscriber. But the alteration of the current strength will be balanced by the resistances who, wm so as to prevent the called subscribers meter being actuated.

Mention will now be made of a detail regarding the ringing and change over circuits:

It will be remembered that the change over relay 249 is deenergized for a moment at the end of each series of selecting impulses and momentarily connects the negative pole through 252 and 277 to the change over wire. Immediately afterwards, on the dropping back of the armature of 249, tthe ringing current machine 268 is connected through 254, 251, 277 to the change over wire, but a current does not flow through it because the contact 313 is immediately opened by the operation of the rotarynmagnet 306. But it is possible that the group selector GLWII will find the first or second trunk a300 idle and that in such a case the .test relay pf2 will very soon extend the change over wire 'U63 to the linal two-way switch by closing the contact 311. This ex-l tension of the-change over wire would then possibly take place `before the. slow-acting relay 250 reopens its contact 254 and disconnects the ringinglnachine. This wouldresult in the steady application of the ringing machine to the change over circuit because a ringing current would flow from earth through 268, 258, 254, 251, 296, 277', gb, fvba, 311, 911), 5300, 113 (this contact would be closed by the energization of SR) 152, 190 to earth. In order to preventthis undesirable iowing of ringing current4 ar switch is preferably provided at the point 1991 indicated by the cross in the two-way switch which is closed only when the calling subscriber commences to send inhis tens selecting impulses. By that time the slowacting relay 250 will be lsure to have found time to open its contact 254. The switch at point 199 may consist of a contact compris- .to their normal idle condition.

ing two springs similar to 167 which closes the moment an impulse is sent through the tens magnet ZM.

It was assumed in the case of the above connection between a calling and a called subscriber that Ia two-way switch that is lassociated with a selector finder extended the calling subscribers line to a first group selector. If, at the time the calling subscriber makes a call, the next idle two-way switch in the group is one that is connected directly to a the establishment of the connection between the calling and called subscribers will be as described above except that the functions of the selector finder will 'be omitted. The lines fm1, fvbl, 'v01 will in this case not pass through contacts 214, 215, 216 and brushes wa, wb, wc but will be directly connected to an associated first group selector and the starting wire extending from point 157' through 135 will rbe unnecessary.

The release wire al instead of vpassing through contact 217 will be connected directly to an armature contact of the release magnet 245 of the lirst group selector and on thev` energizatio'nN ofthe release magnet thel restoration of the two-way switch will be initiated by the e'arthing of the restoring Wire al.

Another point will be mentioned in connection with regulating of the incoming calls by the call regulator. It will be remembered that the call regulator ordinarily prevents two calls from being simultaneirst group selector, all the events in ously passed on to the two-way switches of a hundred group. The call`regulator will perform its function perfectly as long as not more than two calls come in at the same or almost the same instant: This will generally be all that isrequired ina practical system. That three or more calls Will come in at the same instant is almost out of question, Even this contingency can however be provided for by arranging a special relay common to each group of subscribers which will be excited if more than one subscriber calls in the interval of about a second during which a twou way switch is in the act of making a connection with a calling line. vThe special common relay will th n cause relays, of which one is associated with each of the lines that .happens to call in the above mentioned inpossibly causing ineffective line' finding op-v erations of the two-way switches. The

special signal would cause the subscribers who sent in simultaneous calls to hang up their receivers and to thus restore their lines The subscribers would then wait a few seconds be-

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