US657315A - Telephone switching apparatus. - Google Patents

Description

No. 657,3l5. Patented Sept. 4, I900. D. S. HULFISH. TELEPHONE SWITCHING APPARATUS.

(Application filed Apr. 12, 1900.)

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(No Model.)

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Patented Sept. 4, I900 D. S. HULFISH.

TELEPHONE SWITCHING APPARATUS. A licaciml filed Apr. 12. 1900.;

'(No Model.) 2 Sheets- Sheet 2.

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DAVID IIULFISH, OF CHICAGO, ILLINOIS, ASSIGNOR TO THE AMERICAN BELL'TELEPHONE COMPANY, OF BOSTON, MASSACHUSETTS;

TELEPHONE SWITCHING APPARATUS.

SPECIFICATION forming part of Letters Patent No. 657,315, dated September 4, 1900.

Application filed April 12, 1900.

which are connected to one another by trunk or toll circuits, and owing to the rapid development of telephonic service and to the improvements made in the systems employed it frequently happens that some of the substation and the toll circuits are provided with magneto-generators to operate call-bells and electromagnetic line-signals, while other substation and toll circuits are equipped with means such as a centralized or common battery for automatically operating bells and signals. When such is the case, both kinds of circuits may have their switching terminals and line-signals upon the samekeyboardsections at the central stations, and in order that the operators may make the switching connections between the terminals of the same or the diverse kind of circuits two or more kinds of linking or cord circuits have had to" be furnished.

The present invention relates to special means embodied in linking or cord circuits sible to connect two common-battery circuits with each other, a common-battery circuit with magneto substation or toll circuit, or two magneto-circuits with eachother, and to inductively unite the two plugs of a cord-circuitby the introduction of a repeating-coil in order that two connected circuits of the same kind may repeat into each other, and also that a magneto-circuit may be enabled Serial No. 125603. (No model.)

to repeat into a common-battery circuit, and 'vice versa. i

In carrying out the invention the magneto. generator circuits are provided at the central station with socket-switches and a line-signal and the test-rings of the sockets are grounded, and the com mom-battery circuits are provided with socket-switches, a line-signal, and also .with a cut-off relay, all in the usual manner, "the apparatus at the stations at the opposite ends of both kinds of circuits being of the ordinary character. It is necessary in order to effect the combination of circuits indicated that a cord-circuit shall be provided having disconnecting or supervisory signals adapted for each class or kind of circuit with means for associating the proper signals with the special combination of circuits interconnected and for operating the same and for suppressing the signals which are not appropriate.

for said combination, audit is also necessary to provide means whereby current from the central station may flow in the common-battery circuits to energize the transmitters located therein, also to prevent the current from passing to the magneto-circuits and to provide separate means for energizing the transmitters in the magneto-circuits, and it is also requisite that means he provided for introducing and withdrawing 'a repeatingcoil'between the conductors of the cord-circuit at the will of the operator, so that the same and diverse kinds of circuits may be conductively separated from one another, all of which relates to the within-described invention, which will now be described and claimed.

In the drawings which accompany and illustrate the specification, Figure 1 is a diagram indicating circuits of diverse character entering a central-station, together with a switch-cord circuit arranged to unite any two of either or both kinds for the formation of a circuit for through communication. Figs.

2, 3, 4, and 5 are simplified representations of the main connections of the switch-cord i placed. At the central station these lines cen-" circuit, and two inagiieto call-circuits, the latter arrangement being shown both with and without a repeating-coil.

L and L represent common battery telephone-circuits extending between their respective substations A A and a central station 0. They are provided at the substations with telephones in -a normallyopen bridge between the main conductors 1 and 2, and a standard call-bell in series with a condenser is connected in parallel with said' telephones, as usual, the regular telephoneswitch being arranged while supporting the receiving-telephone to maintain the normal discontinuity of the telephone-bridge, but to close the same when the said receiver is dis ter in a switchboard, where they are fittedeach with switch-sockets J, a line-signal S, and a normal connection with a source of current cut-0E relay R each, moreover, being also in E, such as a battery.

L L represent telephone circuits of the older class, technicallycalled magneto-circuits, which circuits have at theirsubstae tions a magneto-generator to send outgoing signals associated with their signal-receiw ing bells and alocal battery to supply cur-v rent for their transmitters, the telephoneswitch in the usual manner being adapted to transfer the line-circuit between the calling appliances and telephones, according as the receiying-telephone is supported, thereon or v These circuits also are provided with switch-sockets J and have line-signals S such as annunciators, in a manner well 1 understood.

removed.

L represents one terminal of a toll-circuit, a type of circuit usually operated likewise on the local battery plan.

.0, and its outer-terminal may be fitted with I either a switch-plug'or a switch-socket,as

preferred, at a distant central station.

cuit. comprising the elements of my invention and adapted to form an operative ,union zb'etween any two circuits of eithertype or between one circuit of either type and a second of another type, a repeating-coil be-- I j ing introduced'between the circuits orlnot, as desired "or as, required by the. conditions;

I The figu re shows the several appliances and I circuit arrangements employed in carrying I -in series with a condenser h in a bridge 7 out ,the 'inv ention for {the purposes hereinbef ore indicated in their normal relation'to one another. p

. 1 Band P? arethe. switch plugs of. ordinary structure,having a tip contact-surface pgand front and rear sleevecontact-surfaces 10 and 19 the former-of which is sometimes called I the ring. "These contact -surfac'es 'are adapted to register,- respectively, with corresponding line and local circuit-'contact-suw faces in the plug-sockets. v

S3 is an electromagnetic supervisory signal Itis provided with a switch-socket J 3 'at the central station resistance r, -Flg. 1, D represents a switch-cord cirbetween the front sleeiie or ring conductor a and the tip conductor 01' of the plug P.

I is arepeating induction-coil, and k a key controlling the relation of theswitch-cord conductors c and d with the terminals of the two windings of the said repeating-coil.

E is the source of signaling and transmitter current'and may. of course be common to a number of switch-cords. It has at one pole a connection to ground G Impedance or retardation coils r r are placed one on each side of said source in a bridge N between the tip conductor g and front sleeve conductor f of the plug P R is a relay, which by two armatures 14 and 28 or adouble armature controls the con- 'tinuity of the bridge N or is adapted to close or open the same at two different points 31 and 32. vA relay R is included in said bridge when the same is closed and controls the op- ;eration of a supervisory signal .9 associated with the plug P v The key It has two contact-springs s s,

p which normally press, respectively, on inner contact-points a: and y, but when the key is operated are separated therefrom and made to press upon outer contact-points w and 2.

In the former or normal condition of said key there is direct conductive continuity between the tip contact-surfaces p of the switchfplugs P and P and alsobet'ween the :ring contact-surfaces 1)? thereof, the same being traceable asfollows': between the tips 10 of the plugs by conductor d, key-spring s, inner contact-point 00, condnctor9, junction m and conductorg, and betweenthe rings g of said @plugs by conductor 0, jiinction point g conductor l2, inner contact-point y, key-spring s and conductor f.v The rear sleeve-contact p of one ofthe plugs (shown as plug P) has :a'conductor 6 extending to the ungrounded pole of thesource of current E and including the exciting-coils of the relay R and the The rear sleeve-contact of the other plug connects with the un grounded pole of the battery E at 21 by conductor m,which includes theresistance The object of the resistancesr and rf is to insure that the current supplied to the conductors containing them sh'all not-exceed the appropriate strength. It isto be noted that the socket framesor glands 5 ofthe magneto lines are between the tip-andring conductors of plug P is'to prevent undesired action of the signal S? from the current of' the source E The connection of the repeating coi1 I, as

hereinbefore indicated, is controlled by the key It. The winding 7; of the said coil terminates at one end by conductor 11 in the outer contact to of said key and at the other end in a junction 1 with the ring conductor 0 of the plug P. The second winding j terminates at one end in the outer contact-pointz of the key and at the other in a junction at 00 with conductor 9 leading to the tip of plug P When the key it is depressed, its springs s s are separated from their normal contacts with points my and form new ones with the outer points 20 z. The conductive union of the corresponding contact-surfaces of the two plugs is thus disest'ablished, the two conductors being respectively opened at the key-points a: and y, and in place of such union a conductive loop is now formed for each plug be tween the tip and ring contact-surfaces thereof, the repeating-coil windings being included in the said loops respectively. Thus the two plugs and substation-circuits,in whose switchsockets they may at any moment be inserted, may be inductively united for the transmission and exchange of conversation, while at the same time the battery-current, which may be bridged to the loop of one of the plugs, is prevented from obtaining passage to the other, the conductive connection having, as stated, been disestablished. The loop of plug P (the key having been depressed) is traceable from the tip 19 by conductor d, key-spriu g s, contact-point w, conductor 11, coil-winding 7;, junction-point g and cord conductor 0 to bring the ring contact-surface p of the same plug. The loop of plug P similarly may be traced from the tip 19 thereof by conductorg, junction-pointm conductor 10, coilwindingj,'condenser 77 conductor 13, outer key-point z, key-spring s and conductor f to the ring contact-surface p of the same plug.

These changes are simply illustrated by Figs. 2 to 5, inclusive, wherein Figs. 2 and 5 indicate the arrangement of the main-circuit conductors when connection is established between two substation-circuits, the key is remaining unoperated, and the two plugs consequently being conductively united, while Figs. 3 and 4 respectively represent diverse circuits and similar circuits united by the plug-circuit arrangement, when, the key 7c having been operated, the main conductors are looped to the .top plugs, respectively, and the connection between the plug-loops is made inductive only.

The key may be of any well-known selflocking construction, such as the cam-key in extensive use, which when depressed will so remain untilmanually released, this capa bility being indicated by the groove in its shank, which affords a lodgment for the spring 1 ends.

The bridge N between the main-circuit plug conductors, to which reference has previously been made as containing the source of current E and the retardation-coils r and 1*, begins at any point 20 on cord conductor fofl from plug P also.

lay R, conductor 33, the small relay R and conductor 25 to point 27 on cord conductorf. Thus as long as the relay R remains unexcited the-circuit-changers 31 and 32 are quiescent and the battery-bridge continues closed, supplying current to the main conductors of plug P and any substation-circuit united thereto, and, provided the key it is unoperated, to plug P also. If, however, the said" ikey be operated, the plug P is severed from .all connection with the battery, and when the relay R becomes operative the battery is cht This occurs when the said plug is inserted into the socket J 2 of one of the magneto-lines which have their socketframes grounded direct; but if the plug P be inserted into a socket J of a common-battery line the current will be so weakened by the resistance of the cut-oft relay R that while it is sufliciently strong to operate said cut-oft relay it is too weak to operate the bridge-relay R,Which has relatively few turns or may be strongly adjusted, and which therefore will not attract its armatures, so that under these last-named conditions the batterycurrent will continue to be supplied, as required, through the plug P to the commonbattery circuit switched thereto.

The small relay R by its armature 18 controls the local circuit of thesupervisory signal This local circuit extends from battery E by wire 17 to the armature 18 and contact-points 34 of relay R and thence to the signal s and return-wire 16. Any desired reducing resistance r may of course be included therein, or the circuit may be made to include such cells only of the battery as may be required to furnish the necessary current. In discussing the general operation of the invention it does not seem necessary tomake extended reference to the suboperations of sending and receiving call-signals, since for each of the classes of circuit under consideratiou these are performed in the ordinary way familiar to those skilled in the art, and the description will therefore be restricted to circnit organizations arranged for conversation betweentwo connected substations. 4 Assuming in the first place that two substationcircuits of the common-battery type are thus to be united--as for example, circuits L and L entering the central station from their respective substations A and A and that to this end the plugs Pand P have been inserted in the switch-sockets J of the said two lines it is manifest that the ring and tip contact-surfaces of the two plugs will engage the long aud short socket springs a b and that the rear or sleeve contact-surface thereof will register with the socket-frame pieces 15. The branch conductor m leading from the battery E through the resistance r to sleeve 13 of plug P will thus be made IIO to form part of a closed circuit to ground through cut-off relay R of line L, which, operating, will sever said line from its original terminal and battery connection, and the branch conductor e leading from the same source through relay R and resistance r to the-sleeve p of plug will in like manner have its circuit completed through c'ut-ofi relay of line L which performs the same ofiice for that line. As hereinbefore stated, the on rrent in this last circuit is under these conditions so regulated that while it is sufiiciently strong to work the cut-0E relay it cannot work the less=sensitive relay R. Hence the bridge N between points and 27 of the switch-cord mainconductors through the local contacts 31 and 32 of said relay remains unbroken, and the source of current E is, as long as the two lines are united, bridged between the.

the said source E sustains the same relation to line L as it does to line L and supplies current for the transmitter of substation A also.

wherein, though the annunciator-signal S3 remains bridged between the cord conductors c d, it is inert, because a steady battery-current only is on the line and is prevented by the condenserh from aifecting said signaL' The battery E is shown in its bridge placed between the retardation-coil r on one side and the retardation-coil r and signal-relay R- on the other, the said devices being allin series in said bridge. The ground connection G? at one pole of the battery does not under these conditions exercise any effect on the arrangement. Ordinarily the signal-relay R is excited and produces the display of the signal .9 as long as the telephone at either of;

the two substations remains displaced from itsswitch; but,.if desired, the relay may be so adjusted that it becomes operative only when both telephones are displaced from their switches and the external resistance of the battery-circuit through said relaythu's reduced. If desired, the signal .8 may be used to indicate the moment when the called sub station A answers by taking his receiver from the switch-hook. This can be done by transientlyoperating the key it, and thereby temporarily.disconnecting the source E from plug P and'restricting it to plug P. It while,

this-is done the signal .3? appears, th O tor is at once apprised that the telephone? receiver atstation A has been takenlfrom j the switch andthe main circuit L 'thereby I the plug P isplaced in the switch-socket 2 or closed;

cuit of the common or central battery class,

This main-circuit portion of this cir-. cuit arrangement is indicated by Fig. 2,.

. rents as these.

' J 3 of the former say circuit L L and the station-circuits with which they are associated. Fig. 3 indicates the electrical arrangement of the main circuit under these conditions. That portion of thecord-circuit associated as a loop with plug P will in all of its operations conform exactly to the description given of the previous arrangement illustrated by Fig. 2. The plug P makes contact by its tip with the'lower socket-spring b and line .conductor 3 and by its forward sleeve or ring .39 with the upper spring a and conductor 4, and as the plug is forced home the said socket-springs are lifted from their normal contacts and the line-signal S? is out ofi. During conversation the voice-currents originating at either substation are inductively.

this case concerns plug P and substationcircuit L alone and operates to call for a disconnection when on the conclusion of the message the receiver is replaced on its hook at substation A, while to indicate the-disconnecting signal from the stations of lines L or' L in the switchsockets of which the plug P may be placed the magneto-generator at the said stations is operated, and the current developed thereby actuates the electromagnetic signal S thecondenser h ofiering no bar to the passage of said fluctuating our- The third case is one wherein two magneto circuits are to be interconnected or wherein a magneto circuit is to be connected with a toll circuit. In such a connection one of vthe plugs P is thrust into the switch- -socket J or J of the circuit L or L as the case may be, While the other plug P is inserted into the switch-socket J 2 of the calledfor circuit L The connections of the registering contact-surfaces of plug P. have already been described,v but those of plug P require further notice. Since neitherv of the two circuits concerned ,in the present case require the use of the central source of our- .rent jintheir normal operations of signaling and talking, the said battery, with' its conis automatically put out of use, this "being accomplished by theoperationofthe relay R,

which opens the battery-bridge at the points 31- and 32. When the-plug P is inserted in its socket, its main-circuit contact-surfaces pand 19 as with plug I, register with socketsprings b and a, respectively, and by raising them from their normal contacts actto'cut 01f the line-signal S and to switch the line conductors 3- 4 intoconnectionwith the cord conductors g f. The third or local cord conductor e at the same time is connected. directly to ground by the engagement of the sleeve conductor 9 of the plug with the grounded socket-frame 5. The current in the circuit of conductor 6 thus completed is much stronger than in either of the former cases,

because the coils of the cut-off relay are no longer included therein and ,is, in fact, now strong enough to operate relay R, which, attracting its armatures, opens the bridge N at points 31 and 32, so that for this class of intercommunication the battery, being a superfluity, is disconnected from the cord-circuit, the relay R and its controlled signal 3 are put out of use, and the electromagnetic signal S is adapted to be operated over both substation-lines from both substations. Under these conditions the main-circuit connections of the cord are represented by the v diagram Fig. 5.

It is sometimes convenient to dispense with the auxiliary ground connections G and G and in lieu thereof to provide return-conductors or a common return to the complementary battery-pole. This of course is an immaterial change and may or may not be made, as preferred, by those practicing the invention. In practice it is, however, sometimes desirable, even with lineswhich are not energized by the current of a central and common source, to employ a repeating-coil placed intermediately between the circuits concerned. Obviously this is readily accom plished by means of my invention and, as in previous arrangements, requires only the depression of the key It. The cord-circuit mainline connections are then as in Fig. 4, andwhen so arranged conversation between the substations involved can be satisfactorily exchanged, and the signal is still responsive to the operation of the call-generators at both substations.

As a matter of course switchboard apparatus containing my invention will comprise as many switch-cord circuits as are necessary for the proper conduct of the business of switching lines together.

I claim- 1. In a telephone switchboard apparatus a switch plug and cord circuit comprising two terminal switch-plugs, each having contactsurfaces adapted to register with corresponding contacts in the switch-sockets of substav tion circuits; two switchboard conductors normally uniting the corresponding main-circuit contact-surfaces of said two plugs; a repeating induction-coil; and a key or switch controlling the relation of said switch-cord conductors, with the terminals of the two windings of the said repeating-coil, and adapted when operated to sever the said conductors, and to connect them in two loops from the two plugs respectively, said loops containing respectively the windings of said repeating-coil, thus dividing the said plugs conductively, and uniting them inductively; substan-- tially as set forth.

2. In a telephone switchboard apparatus, a switch plug and cord circuit comprising two terminal switch-plugs, each having contactsurfaces adapted to register with corresponding contacts in the switch-sockets of substation-circuits; two switch-cord conductors normally uniting the corresponding main-circuit contact-surfaces of the said two plugs; a repeating induction-coil; a key or switch controlling the relation of the winding-terminals thereof to the said conductors, and adapted to sever the latter, and inter-pose the repeating-coil in such manner that the two windings shall be brought into loops of the two plugs respectively, the said loops being conductively separated but inductively united; a supervisory signal controlled by a relay in a bridge of one of said loops; and a second supervisory signal and a condenser in a bridge of the other of said loops; substantially as described.

3. Ina switch plug and cord circuit connection for a telephone-switchboard, the combination of two terminal switch-plugs each having two main contact-surfaces adapted to engage corresponding switch-socket contacts, and a local contact-surface adapted to engage a third or frame-piece contact of such switchsockets; two switch-cord conductors normally uniting the corresponding main-circuit contact-surfaces of the said two plugs; a repeating induction-coil; a key or switch for severing said cord-conductors and for uniting the two ends proceeding from the plugs with the two ends of the repeating-coil windings respectively, thus forming two divisions inductively united by said coil; a bridge between the conductors of one of said divisions containing a source of current and a signal-controlling relay; a signal device controlled. by said relay; a bridge between the conductors of the other division containing an electromagnetic signal device; independent localcircuit conductors one for each of the said divisions, leading from the said source of current to the local contact-surface of the plug of its own division; and a relay in the localcircuit conductor of the first-named division, controlling the continuity of the bridge thereof, and the connection of the source of current contained therein; substantially as described.

4. In a telephone central-station switchboard apparatus, the combination with substation-circuits of two classes, one having its signaling appliancesand telephones supplied with current from a central battery, and the other having substation call-generators andlocal transmitter-batteries, each circuit having a switch-socket at the central station; of

a switch-cord circuit'adapted to switch m,

gether any two circuits of either or both classes, comprising two terminalswitch-plugs each having three contact-surfaces adapted o a v v 7 rogers to register with corresponding contacts i-nathe switch-sockets ofv the said circuits,- switchcord conductors extending between said plugs and unitingthe two forward contact-surfaces of each with the corresponding contact-surfaces of the other, a bridgecontaining the central source-of current uniting the said switch-cord conductors, and means for maintaining the continuity and connection of said I bridge when one or bothfplugsare. inserted in the switch-socket-srof circuits of thecommon-batteryclass, and for disconnectingthe same from either plug or both plugs, when in ing two terminal switch-plugseach having said circuits, two switch-cord conductors extwom'ain contact-surfaces adapted to engage corresponding switch-socket contacts, and a local-circuit contact-surface adapted to en-' .gage the frame-contacts of said switch-sockets, two strand conductors uniting the corresponding main contact-surfaces of the two switch-plugs, a bridge uniting the said strand conductors, and containing a centralized source of current, and associated impedanceco ls, a'th rd strand conductor in a local circult of saidsource leadingthrough the local contact-surface of one of the switch-plugs,

and when the said plug is inserted in a switchsocket, through the frame contact thereof,

and a relay in 'said local circuit controlling 6. In a telephone central-station switch i board'apparatus, the combination of substation-circuits of two classes, one having directly-grounded switch-socket f raines,and the other having switch-socket frames grounded through the electromagnet of the cut-01f relay; with a switch-cord circuit comprising two terminal switch-plugs each having three contact-surfaces adapted to register with corresponding contacts in the switch-sockets of tending between said plugs and uniting the two forward contact-surfaces of each with corresponding ones of. the other, a bridge uniting the said conductors, a centralized source of current, two impedance-coils one on each side of said source, and a signal-com trolling relay all included in series in said bridge, a supervisory signal controlled by said relay, a-third cord conductor in a local connection of a switch-socket wherein such plng'maybe'inseirted, arid arelay'in said local circuit'controlling the continuity of said bridge and thereby the'con nection of the source of current, and the said signal and controllingrrelay; substantially as specified. '7; In a telephone central-station switchboard apparatus, the combination of substation-circuits of two classes, one having directlygrounded switch-socket frames,and the other-having switch-socket frames grounded through the resistance of the cut-off relayexciting coils; with aswitch-cord circuit comprising two terminal switch-plugs each havingtwo-main contact-surfaces adapted to engage corresponding switch-socket contacts, and 'a local-circuit contact-surface adapted to register with. the frame-piece contact of said switch-contacts, two strand conductors normally uniting the corresponding main contact surfaces ofthe two switch-plugs, a repeating induction-coil, akey controlling the terininals'of the two windings thereof in relation to the" said strand conductors, and adapted when operated to sever the continuity of said conductors and to form them into andIcontaining the two windings-respectively of said repeating-coil, a bridge conductor between the strand conductors of one of said loops,including in series a centralized source 1 of current,impedance-coils'and a signal-controlling relay, a supervisory'signal controlled by'said relay, a third strand associated with said loop and formingjpart of a local circuit of saidsource leading through thelocal contact-surface of the plug of said loop, and on the insertion of said plug in a switch-socket .through the frame piece contact thereof, and a relay in. said local circuit controlling a the continuity ofsaid bridge, and thereby the connection-of the source of current and thesaid signal and its controlling-relay; substantially as specified; r g

8., In a telephone central-station switchboard apparatus, the combination of substa- 1 tion-circuits of two classes, that is to say 1 .common-battery circuits, and magnetocircuits; with a switch-cord circuit arrangement comprising two terminal switch-plugs I eachhavin g two main-circuit contact-surfaces adapted to register with corresponding con- ;-:tacts in'switch-sockets of the said circuits, i and a local-circuit contact-surface adapted to egister with the frame-piece contact of said ockets,two strand conductors normally unitng the corresponding main-circuit contact- I urfaces of the two plugs, a repeating induc- T tion-coil, means for severing the said strand 3 conductors and for looping the ends: of the two proceeding from each plug through the ;two repeating coil windings respectively gthereby forming the cord main circuit into two inductively-related divisions, abridge i between the conductors of one of the said divisions containing a central source of current, two impedance-coils oneon each side of said source and a signal-controlling relay all in series, a supervisory signal responsive to said relay, a third strand conductor associated with said division and forming part of a local circuit of said source leadingthrough the local-circuit contact of the plug of said division and through the frame-piece contact of a switch-socket wherein such plug may be inserted, a relay in said local circuit adapted to become operative and to disconnect said source of current and signal-relay when the said plug is inserted in a socket of a magneto-line but to maintain their connection when said plug is inserted in the socket of a common-battery line, and an electromagnetic signal device bridged with a condenser between the main conductors of said other division; substantially as and for the purposes set forth.

J. In a telephone central-station switchboard apparatus, the combination of substation circuits of two classes, that is to say, common-battery circuits, and magnetocircuits; with a switch-cord circuit arrangement comprising two terminal switch-plugs each having two main-circuit contact-surfaces adapted to register with corresponding contacts in switch-sockets of the said circuits, and a local -circuit contact-surface adapted to register with the frame-piece contact of said sockets, two strand conductors normally uniting the corresponding main-circuit contact-surfaces of the two plugs, a repeating induction-coil, means for severing the said strand conductors and for looping the ends of the two proceeding from each plug through the two repeating-coil windings respectively thereby forming the cord main circuit into two inductivelyrelated divisions, a bridge between the conductors of one of the said divisions containing a central source of current, two impedance-coils one on each side of said source, and a signal-controlling relay, all in series, a supervisory signal responsive to said relay, a third strand conductor associated with said division and forming part of a local circuit of said source leading through the local-circuit contact of the plug of said division and through the frame-piece contact of a switch-socket wherein such plug may be inserted, a relay in said local circuit adapted to become operative and disconnect said source of currentand signal-relay when the said plug is inserted in a socket of a magneto-line, but to maintain their connection when the said plug is inserted in the socket of a common-battery line, a third strand conductor forming part of a similar local circuit associated with the other division of the said cord-circuit, and leading through the local-circuit contact-surface of the plug thereof, and any switch-socket wherein it may be placed, and an electromagnetic signal device bridged with a condenser between the main conductors of said other division; substantially as and for the purposes set forth;

In testimony whereof I have signed my name to this specification, in the presence of two subscribing witnesses, this 30th day of I March, 1900.

DAVID S. HULFISH.

Witnesses:

SAMUEL G. MCMEEN, THOMAS D. LooKwooD.

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