US2333324A - Automatic telephone system - Google Patents

Description

Nov. 2, 1943. F. A. LOW 2,333,324

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 1 Hal z/e 14' law- Nov. '2, 1943.

F. A. Low

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 2 1943- I F. A. LOW 2,333,324

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 3 M6 05 I ga Mg 0d 1943. F. A. LOW ,33

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l5 Sheets-Sheet 7 Nov. 2, 1943. F. A. LOW 7 2,333,324

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AUTOMAT IO TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 9 M6 fl/l Nov. 2, 1943. F. A. LOW 2,333,324

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 10 llllll Nov. 2, 1943. F. A. LOW

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet 11 Nov. 2 1943. F. A. LOW 2,333,324

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet l2 fat/.2"

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Nov; 2, 1943.

F. A. LOW

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941 15 Sheets-Sheet l3 NOV. 2, 1943. I A ow 2,333,324

AUTOMATIC TELEPHONE SYSTEM Filed May 14, 1941' 15 Sheets-Sheet 15 Patented Nov. 2, 1943 UNITED STA'rgs LATENT QFFWE In vGreatliritainMay 25, 1940 6 Claims. (Cl. 17918) This invention relates to automatic telephone systems of the type comprising one ormore subexchanges to each of which a number of subscribers are connected, and a main exchange to which each of the sub-exchangesis connected by a number of connecting lines fewer in number than the number of subscribers lines connected to the sub-exchanges, and the object is to improve and cheapen this type of automatic telephone system.

I have, by way of example, illustrated ,my invention in the accompanying drawings which are mainly diagrams of connections.

In the drawings Figure 1 is a diagrammatic View of the general arrangement. In Figure 1 the area to be served is divided into four districts, north, south, east, west, and four different ways in which the outgoers may, according to the invention, be arranged are shown in these four sections respectively,

Figure 2 shows the connections which are made when calling and calledsubscribers are connected to different sub-exchanges,

Figure 3 is a similar View of the connections when the calling and called subscribers are connected to the same sub-exchange,

Figure 4 shows, to an enlarged scale, the mode of connection illustrated in the north' section of Figure 1,

Figure 5 shows, to an enlarged scale, the mode of connection illustrated in the east section of Figure 1, 1

Figure 6 and Figure 6 together show a more detailed view or the connections shown in Figure V,-

Figure 7 shows with more detail than can conveniently be shown in Figure 6 how the commoning between the respective three conductors of two commoned outgoers is effected,

Figure 8 is a similar view but dealing with three commoned outgoers,

ure 9 shows ho t e connec i n o 9 gcer trunks to su scribers uniselectors ,aremade,

a calling subscriber to a disengaged outgoer trunk of a group of outgoer trunks,

Fi ure 13.,show a further modification in he constructionof uniselector,

Figure 14 shows a modification made in the usual vQnstru :t ion of line-finder and outgoer trunk hunterwhenthis type is used .to huntfor the line of the calling subscriber and to find a disengagedoutgoer trunkpf a group of outgoer trunks.

Figure 15 shows the applieationof the invention to a non-director multi-exchange system with satellite exchanges,

scribers and a subeexchange is located in each subedistrict; ,there are, therefore, twenty-five sub-exchanges divided .into ,four groups, three of six and one of, seven sub-exchanges. Each su -excha g i co nec ed byt e r-equ te umber of separate lines .C (one only of Which is shewnin Fi 1) t h p ti ubs r ber in its sub-district, and is also connected to the main exchange by a suitable number of sub-ex change incomer trunks 15, Each outgoer trunk and incomer trunk ha's multiple conductors, the outgoer trunk usually three, and the incomer either two or three. In the drawings unless the contrary is stated, D, followed by a distinguishing letter or numeral, represents a group of the separate outgoer trunks to its associated subexchange, each of which outgoer trunks con- 'sists' of three insulated conductors til; (12," d3

(seeFigureW and Figure 9). Is represents a group of incomers to its associated sub-exchange; the ends only of incomers Is, at the main and sub-exchanges respectively, are shown, the intermediate parts being, to simplify the diagram, omitted.

The sub-exchanges are of the completely unattended type of automatic exchange. The equipment of a sub-exchange to carry out the present invention consists (see Figures 2 and 3) of disengaged outgoer trunk connectors DC and final selectors FS. The disengage-d outgoer trunk connectors may be of any of the well known organization adapted to be associated with a number of subscribers lines connected to the sub-exchanges, and a number of outgoer trunks from the sub-exchange to the main exchange, which, when a subscriber lifts his receiver from its rest, connects the subscribers line to an unengaged outgoer trunk; these may be uniselectors of known construction modified as described in connection with Figure 12 or Figure 13, in which case there would be one for each subscriber, or there may be a group of line finders with associated outgoer trunk hunters, the number required depending upon the traffic requirements of the sub-exchange. The final selectors FS may be of any of the well known designs operated by the dialling of the final two digits of the called subscribers number and giving the usual facilities. There would be one final selector for each incomer trunk to the sub-exchange, and each when operated would connect its associated incomer trunk to the line of the called subscriber. The whole of the equipment would be housed in one or more air-tight cabinets. Normally the sub-exchanges would be identical ,in' construction and would not be designed individually. Preferably they would be equipped for 100 subscribers lines with facilities for expansion to 200 lines.

A main exchange M for the area served .is pro.- vided, and to carry out the present invention has a groupselector foreach out oer trunk from the connected to the group selector GSI is connected at a sub-exchange to a calling subscribers line, and the first digit of the called number is dialled. the 1st selector GSLmoves vertically to the appropriate level and then, by angular movement, selects a disengaged connector, at that level, to the 2nd selector, and the latter. when the second di itisdialled. moves vertically to he appronriate level andby angular movement, selects a disen a ed incomertrunk to the sub-exchan e t which .the called subscriber s connected.

.Vhemtb two last digits are muse the final selector-PS associated with t e con cted i corner trunk connects the latterto the called subscribers line. g r a The main exchan e is equipped to ot p marily as a switching centre for the sub exchanges. it is; however; fltted'with the usual well known apparatus forconnection to a manual board: for producing and testing of tones and the like; and, where desired. for obtaining access to other. automatic exchanges.

Such a combination of a main and twenty-five sub-exchanges would be capable of dealing with two-thousand-five-hundred subscribers in the area, a four-digit system of numbering being adopted.

All sub-exchanges are similarly equipped, and suitably connected to subscribers and to the main exchange, and in what follows each of the several modifications will'be described with reference to a single group;'the connections in each modification' wouldgin practice, he followed in respect to each sub-exchange.

The sub-exchanges in the north section group are indicated by Na, Nb, Nc, Nd, Ne, N The other groups are correspondingly designated.

S is a satellite exchange with sub-exchange sl, s2, s3; and X-X5 are exchanges of a director system, associated with the main exchange M.

The connections made when a calling subscribed associated with one sub-exchange N calls a subscriber in another sub-exchange W are shown in Figure 2. When the calling subscriber lifts his receiver from the rest his line C is connected by the disengaged-out-goer-connector DC to the free outgoer trunk Dn from the subexchange N to the 1st group selector GSI in the main exchange; and when the 1st and 2nd group selectors have been actuated, by dialling the first two digits of the called subscribers number, the

outgoer trunk Dn will be connected to the incomer trunk Iw and by the latter to the final selector FSw in the sub-exchange W, which final selector will connect the incomer trunk Iw to the called subscriber's line C2. In Figure 3 the calling and called subscribers are both connected to the same sub-exchange N.

Ofthe three conductors (see Figures '7 and 9) dl, d2 and d3 of each outgoer trunk, two, dl and d2, are the speaking lines, and d3 is a line which is automatically earthed when the outgoer trunk with which it is associated is in use. A calling subscriber has, of course, to be connected to a disengaged outgoer, and accordingly when he lifts his receiver he operates switches which make battery and relay circuits which, in well known manner, cause the actuating magnet of the subscribers disengaged-outgoer-trunk-conmotor to move wipers (including its private wiper) in succession over the respective sets of contacts connected to the respective outgoer trunks until it'finds a set with an associated wire d3 which is not earthed, and therefore is a disengaged outgoer trunk.

In order to reduce the length of cable required for, and therefore the cost of, outgoer trunks which are-usually laid in the streets en route between the sub-exchanges and the main exchange the sets of outgoer trunks are commoned at junction points located in the line of route between a group of sub-exchanges and the .main exchange, said junction points are usually. enclosed in weather-tight underground chambers, the positions of which coincide with the points where the routesfrom the several sub-exchanges of the group approach close together .or intersect; for example, where the routes from two remote sub-exchanges of a group to the main exchange approach neartogether or intersect 'a junction point is formed at which the corresponding conductors of the two sets of outgoer trunks from the said two sub-exchanges are connected together as hereinafter described. There would be a set of outgoer trunks leaving each sub-exchange, the number of outgoer trunks in a set depending upon traflic requirements; usually a. set would, for average traffic, comprise twenty to thirty outgoers, each outgoer trunk having conductors for speech transmission and a testing conductor; the number of setsof outgoers leaving a junction is always less than the number of sets of outgoer trunks entering it. For example, the number of sets of outgoer trunks leaving each junction point may be (see the east section 01' Figure 1 and Figures 5 and 6) reduced to one half the number entering the junction, or where three sets of outgoer trunks meet at a junction point, is reduced to one-third of those entering; in Figure 6 it will be seen that though 24 outgoer trunks leave each of six sub-exchanges of the east section, that is 144 outgoer trunks, only 48 outgoer trunks enter the main exchange. It must be understood that the number of sub-exchanges in a group, which share a particular set of outgoer trunks to the main exchange, is determined by the traffic requirements of the sub-exchanges; in Figures and 6 there are three sub-exchanges per group. In this example normalaverage traffic has been assumed. The .comparisons given above apply when all the outgoer trunks in a group are commoned at each junction point; in some cases, however, certain of the outgoer trunks from each sub-exchange pass directly to the main exchange, and some are commoned at some of the junction points only (as shown in the west section of Figure 1 and in more detail in Fig.- ures and 11); in the latter arrangement the number of outgoer trunks leaving a two-to-one,

or a three-to-one junction point are respectively more than one-half or one-third of those entering as the case may be. Examples of these respective modes of commoning are described with more detail later herein.

In the modification illustrated as applied to the grouped sub-exchanges in the east section, Figure 1 and in Figures 5 and 6, each sub-exchange is connected to the main exchange by a group of incomer trunks Is, the ends only of which at the sub-exchange and main exchange are shown. There would usually be 12 such incomer trunks for an exchange of 100 subscribers. The arrangement of the outgoer trunks is as follows:

For an exchange of 100 subscribers there would usually be 24 outgoer trunks; the outgoers Da. and

Db (see Figures 5 and 6) from sub-exchanges Ea and Eb are commoned at the two-to-one junction point J I, the junction point J l is connected to the line Dab of 24 outgoer trunks to the junction point J2 where the outgoer trunks from subexchange Ec would be ,commoned, the line Dabc of 24 outgoer trunks enter the main exchange M, each connected (with great saving in length of cable required) to each of the three sub-ex.- changes Ea, Eb and E0.

In like manner the outgoer trunks Dd and De from sub-exchanges Ed and Ee are commoned at junction point J3 which is connected by the 24 outgoer line Dde to junction point J4 where the outgoer trunks Df from subeexchange Ef are commoned; the line Brief of 24 outgoers enters the main exchange.

By commoning is meant that the corresponding conductors of each outgoer in a set of outgoer trunks are connected to the corresponding conductors of the outgoer trunks in the other set; for example, at each of the commoning points indicated by a dot in Figure 6, the three conductors (see Figures '7 and 9) all, d2 and d3 of each one of the cornmoned outgoer trunks would ree,

spectively be electrically connected to the conductors dl, d2 and d3 of each one of the other set of outgoer trunks, and the joints insulated.

An example of the mode of construction is that shown in Figure 7 where the conductors d1, d2 :1."

and d3 of the respective outgoer trunks Da and Db are commoned at d1, 112 and d3 of the outgoer trunk line Dab. Figure 8 shows the mode of commoning where three sets of outgoer trunks are commoned at a junction point; the conductors dl, d2 and d3 of the outgoer trunks Dgc, Dy and D2 are connected to the conductors roll, (12 and (13 or the outgoer line Darya.

The junction points J J2, J3 and J4 would be located at points where the routes from the corresponding respective sub-exchanges to the main exchange approach close together or intersect.

The respective incomer trunks of the six sets of incomers Is to the respective sub-exchanges are, at the sub-exchange, each connected to' a final selector, and at the main exchange end are connected either directly or indirectly to the bank of the 2nd group selectors GC2 (see Figures 2 and 3).

In Figure 5 each of the lines in the diagram represents a group of outgoer trunks; in Figure 6 the mode in which the individual outgoer trunks are commoned at the junctions is shown to a larger scale; the connections will readily be followed as the same letters of reference are employed in both Figures 5 and 6.

It will be seen, in the arrangement of outgoer trunks last described, that for each group of three sub-exchanges twenty-four outgoer trunks, each with its 3 conductors, enter the main exchange M. These would each be connected to a group selector GSI. A group selector connected to, say outgoer trunk numbered I, would, therefore, be connected to each of the three sub-exchanges, thus the line I would be made an engaged line if selected by a subscriber at any one of the three sub-exchanges; the remainder of the outgoer trunks would be similar in this respect. Thus there would be no grading and the early choices will be in almost continuous use, the later choices being only used when the trafiic offered from the three sub-exchanges reaches a peak value.

To equalize the trailic on all choices and thus reduce the total number of outgoer trunks required, in the preferred arrangement according to the present invention, the outgoer trunks are graded by directly connecting some and by commoning others, so that an early choice outgoer trunk is shared by subscribers to one subexchange only, the later choices being shared by the subscribers to two, three or more subexchanges. This arrangement is described in greater detail later.

Figure 9 shows the mode in which the three conductors of each of the respective outgoer trunks are connected to the bank of disengaged outgoer trunk connectors in the respective subexchanges. It will be assumed that the disengaged-outgoer-connectors are uniselectors USI, US2-US'/Z4, one for each of say subscribers connected to each sub-exchange. Each uniselector has three sets use, ash and use of twenty-four concentrically arranged contacts corresponding with twenty-four outgoer trunks I D, 2D, -3D25D connected to the sub-exchange. usq and usr are the respective wipers for connecting the calling subscribers line to the speaking lines all and d2 of an outgoer trunk, and usp is the private wiper which is used in testing for a disengaged line. There would be as many uniselectors as there are subscribers but three only, USI, USE, USS, are shown and there would be 24 outgoer trunks, three only, ID, 2D, 31), are shown in the diagram.

In Figure 9 use, ash and use are the three sets of contacts fitted to each uniselector and usq,

the uniselectors are connected by the conductor by the conductor 52, and so on; conductors 53,

54 and 55, 56, 51 and 58 connecting the'corresponding contacts of all the uniselectors; The

three conductors dl, d2 and d3 'of outgoer trunk 2D are respectively connected to contacts 2 of sets usa, usb and use of all the uniselectors, similarly the conductors d1, (Z2 and d3 of outgoer 3D are connected to-contacts 3 of sets usa, usb and use of all the uniselectors, and so on in well known manner.

Assuming that a subscriber associated with the uniselector US! is making an outward call, the three wipers usq, usr and usp move over the respective sets of contacts. If outgoer trunk ID is engaged its conductor (13 and therefore wiper use would be automatically earthed, and the wipers would, by their driving magnet, pass on to contacts 2; if outgoer trunk 2D were disengaged the wipers would remain on contact 2 until the subscriber released the connection. This is the usual action of the uniselector.

In order to reduce the total number of outgoer trunks required by equalising the trafiic carried by earlyand late-choice outgoer trunks, it is preferred to connect the outgoers from the sub-exchanges to the main exchange so as to provide a graded system of outgoer trunks as shown in the western section of Figure 1 and in Figures and 11.

In Figure 10 the connecting lines D2, D3, D4 represent groups of outgoer trunks; in Figure 11 the lines represent single outgoer trunks to show the mode of commoning at the respective junction points; the same reference letters are employed to designate similar parts in both Figures 10 and 1.1. In this case there are four kinds of outgoer trunks per set; DI (shown in fine full line) are outgoer trunks which connect the subexchange with which it is associated directly to the main exchange; D2 are outgoer trunks (shown in heavier lines) which are connected to two sub-exchanges by being commoned at a junction point H; D3 are outgoer trunks (indicated by still heavier lines) which are connected to three sub-exchanges by being commoned at junction points J H and J l3; and D6 are outgoer trunks (indicated by heavy lines) which are connected to all sub-exchanges by being commoned at junction points Jll, J12, Jl3, JM and Jl5. The mode in which the several outgoer trunks are commoned at the junctions is illustrated in Figure 11 from which it will be seen that of outgoers entering the main exchange M some are connected to one sub-exchange only, some to 2 sub-exchanges, some to 3 sub-exchanges and some to 6 sub-exchanges. The wipers of the uniselector of a calling subscriber contacting with contacts I of the uniselector to which the directly connected outgoer DI is connected, would find a disengaged line unless another subscriber in the same sub-exchange had already appropriated it. On .the wiper moving over contacts 2, 3, 4 a disengaged line would be obtained unless these numbers were appropriated by a subscriber in the calling sub-exchange or in the other associated sub-exchange. On passing the contacts 5 to 3 the outgoer trunk would be engaged if it had been called up by a subscriber in the calling subexchange or either of the other two subexchanges; whilst on moving from contacts 9 to 24 an outgoer would be engaged if in use by any one of the 6 sub-exchanges. These conditions are typical of grading and by distributing the calls evenly over the various outgoer trunks each one has maximum efficiency and so renders it possible to reduce the number of themto a minimum for given traffic requirements.

The conductor d3 of an outgoer trunk is earthed at the sub-exchange by which it is en'- gaged; and it will be seen that if the conductor d3 of an outgoer trunk be earthed at a sub-exchange near to the main exchange by a call originating in that sub-exchange, and be tested for earth bya uniselector at a remote sub-exchange, a long length of conductor will intervene between the private wiper usp at theremote exchange and the earth" at the near exchange; and that this resistance would be varied according to the distance of the two said sub-exchanges. As this resistance may have a maximum of several hundreds of ohms the usual arrangement of a uniselector would prove unsatisfactory if employed with the usual standard battery voltage, relays and the like. It is, thereore, preferred to modify the usual form of uniselector in the manner illustrated in Figure 12. I I

In Figure 12, L i the line relay having three contact units Ll, L2 and L3 and K is the switching relay having four contacts Kl, K2, K3 and K4. In addition to the relays L and K there is provided a testing relay N having two contacts NI, N2. Of the relay contacts LI and L2, the former is arranged to make contact before L2, these two contacts are of the make type; L3 is a break contact unit; the respective functicns of L2 and L3 are to earth" the private wire 45 and to control the homing circuit. The line relay has a slow-to-release feature. Ll provides an operating circuit for the testing relay N by earthlng the home contact ush. The relay N is of the type which releases quickly, and when operated will hold in series with several hundreds of ohms resistance; and NI and N2 are make and break contact units respectively; the energising of the switching relay K disconnects the N relay. The testing relay N completes and controls the driving magnetcircuit, controls the switching relay K and tests vthe contacts in the uniselector bank. The uniselector mechanism may be of normal design and consists of four wipers usq, usr, usp and usk which are angularly moved, as one unit over a corresponding number of semi-circular arcs of contacts usa, usb, use and concentric segments usd known as the uniselector bank, by a pawl and ratchet arrangement actuated by an armature. The motive power is supplied by the driving electro-magnet DM. The interrupter springs dms are arranged as usual so that they disconnect the operating circuit of DM when the armature is operated.

The two conductors Ca and Cb connect the subscriber's line to the moving springs of contact units K2 and K3 of the line relay K. K2 connects conductor Ca to one end of the; L relay coil. K3 connects conductor Cb to earth. The

other end of L relay coil is connected to negative of earthed battery. The operating circuit for L is completed when the subscriber takes his receiver off the rest. Contacts L2 and L3 respectively earth conductor 45', and disconnect the homing circuit through usd and ask respectively. Ll connects the home contact ush of are use to earth, wiper usp normally resting on this contact and being connected to negative of earthed battery. Hence when precedent acting contact Ll is closed, relay N is operated. N2 then connects K relay coil to conductor 45'.and

provides an operating circuit for this. relay when N relay releases, NI closes and the circuit of the driving magnet DM and interrupter contacts dms is completed; the driving magnet DM will be energised over this circuit and: the wipers will be rotated until they rest on a free outlet, whereupon the relay N will release as there will be no earth on the private wiper to complete its circuit. NI contact in releasing disconnects the circuit of DM and thus prevents further rotation; N2 contact in releasing connects K relay coil to conductor 45' and thence through relay contacts L2 to earth; K relay then operates; Kl disconnects the homing circuit; K2 disconnects conductor Ca from relay coil L; and connects it through conductor ca to wiper usq and so to the subsequent selector; K3 disconnects conductor Cb from earth and connects it through Cb to wiper W and so to the subsequent selector. K4 disconnects conductor 45 from the relay N and connects it to conductor 45; thus extending conductor 45 through wiper uslc to the subsequent selector. The operating circuit for relay L is now broken and it starts to release; the earth providing the holding circuit for relay K is maintained on conductor 45' at L2 during the slow-to-release period of relay L until a holding earth has been returned on wire 45 from the subsequent selector. K relay remains operated until the holding earth is removed from conductor 45 due to the calling subscriber hanging up his receiver. The i'elease of K restores everything to normal, in preparation for another call, by completing the homing circuit at KI and re'-connecting wires Ca and Cb to the line relay L and earth respectively, an earth being connected to the driving magnet circuit by means of wiper usk through the homing arc usd, contact L3 and contact KI When this invention is 'applied to systems of telephony in which the engaged indicating conductor d3 is connected tonegative battery, the free condition of an outgoer trunk being indicated by such connection, and the engaged condition being indicated as usual by connecting the said conductor to earth, further modific'ation's are required to the subscribers uniselector to make the testing relay operate instead of releasing when a free outgoer trunk is reached.

One arrangement of the said modification is shown in Figure 13; as the diagram Figure 13 is similar in many respects to Figure 12, the same nomenclature has been used Where possible. The modification consists in combining the testing relay N and switching relay K of Figure 12 into one relay NK (see Figure 13) which combines the functions of both relays N and K, this combination being rendered possible inasmuch as the testing wire, in the case of a free outgoer trunk, is energised by connection to negative battery.

NK relay is a double coiled relaywith one high resistance winding nkh and one low resistance winding nkl; it has contact units NKI, N K2, NK3, NK4 and NK5; NKI operates when winding nkl is energised and before the other NK relay contact units. I The line relay L is of the usual construction and has the usual functions. Its operation causes the uniselector wipers usq, usr, usp, uslc to rotate over the arcs usa, usb, use, usd until a negative battery on d3 of a free outgoer trunk completes a circuit for the low resistance coil nkl of relay NK via wiper usp,

to earth. The energising of this coil causes the operation of NKI contact. NKI contact thus operated disconnects the circuit of the drivingmag'net DM and contacts rims and completes the operating circuit for the high resistance coil nkh of NK relay, which completely operates the relay, and through contacts NK2, NK3; NK I switches the subscribers line connections Ca and Ch through to the uniselector banks; NK relay holds on as in a normal switching relay, releasing upon the termination of the call.

If, instead of using subscribers uniselectors as disengaged outgoer trunk connectors, subscribers two' mo'tion line-finders with directly associated outgoer trunk hunters are used, the diificulty of testing for a free out'goer trunk, referred to above, does not exist with a testing relay used for this purpose; in which the wipers are not returned to the home position upon release; but there remains a difliculty when used in the arrangement shown in the W section of Figure l and Figures 10 and 11, where in order to obtain the advantages of grading it is necessary, that the outgoer trunks should be tested in the same order (1 to 24') each time. To do this it is necessary, in order to apply such outg'oer hunters to the present invention, for the wipers of the outgoer trunk hunter to return to a normal or home position when not in use, and also that the associated line finder shall be prevented from being taken into use while this homing action is occurring.

one method of accomplishing the object is shown in the arrangement of Figure 14 in which, instead of reproducing all the connections of the line-finder, lindinder allotter, control relay sets and other equipment in which no change is made, only those parts are shown which are directly concerned with the modification, which parts are a uniselector mechanism US3 of normal design, a relay operated contact unit Re; a mechanically operatedcontact unit Mo and two of the unisele'ctor line allotter arcs lfaand lib, and wipers lwl' and ma, associated with a line-finder allottei.

When theoutg'oer trunk hunter is. taken into use an "earth" is transmitted from the line-finder allotter circuit via wiper lwl to complete a self interrupted drive circuit for the driving magnet DM; thus the wipers usq; zisr and usp pass over their associated arcs and connect to the three conductors dl; d2 and d3 of each outgoer trunk in turn; this circuit is broken in a known manner by the operation of a testing relay (not shown) in the line-finder allotter circuit when a free outgoer trunk is connected to wiper usp. At the same time; as the outgoer trunk hunter is taken into use; its associated line-finder is taken into use also; as soon as this occurs the relay-operated contact unit Re breaks the homing circuit of the driving magnet DM; as the Wipers of the linefinder step on normal in a vertical direction the mechanically operated contact unit Me operates, further breaking the homing circuit in preparation for relay contact unit Rc making contact again when the line-finder is stepping in a rotary direction. When the line-finder wipers finally rest on the calling subscribers line, contactunit Rc breaks again and does not make until the .zcon'nection is released by the calling subscriber replacing his receiver on the rest. The linefinder wipers are restored to their normal position at this time, and the mechanically operated contact unit Mc makes contact again, thus completi'ng thev following self-interrupted drive homing circuit for the driving magnet DM; the wiper

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