US2305137A - Telephone or like signaling system - Google Patents

Description

Dec. 15,-1942.. ETALLv 2,305,137

' TELEPHONE OR LIKE SIGNALING SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 1 haZ FIGJ

- INVENTOR DAVlD LANGSFORD CLAY QgAYRLES J0 CUFF RD BARTLETT ATTORNEY EPHONE OR LIKE SIGNALING SYSTEM Dec. 15, 1942.

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2 Sheets-Sheet 2 Filed Oct. 7. 1940 FIG. 2

DAVTD LANGSFORD CLAY CHARLES JOHN CLIFFORD BARTLETT ATTORNEY Patented Dec. 15, 1942 TELEPHONE R David Langsford LIKE SIGNALING SYSTEM Clay and Charles John Cliflord Bartlett, London, England, assignors to Siemens Brothers & 00. British company Application October Limited, London, England, a

7, 1940', Serial No. 360,038

In Great Britain October 10, 1939 11 Claims.

The present invention relates to telephone or like signaling systems and more particularly to feeding-bridge circuits in or for such systems.

With the types of feeding-bridge circuit commonly employed in practice, trouble is often caused by undesired current surges produced by signaling currents and especially by current surges produced during and as a result of impulse repetition. Such undesired current surges may be produced at the bridge itself or may be generated at some other point. They are likely to be especially troublesome in systems where they may reach a voice-frequency signal receiver of the kind liable to be temporarily paralysed by a strong input current.

The present invention has as one main object the provision of an improved feeding-bridge circuit wherein no surges of harmful magnitude can be set up or transmitted as the result of impulse repetition. 7

According to one feature of the invention a feeding bridge circuit is of the kind in which the incoming and outgoing lines are coupled solely by the transformer action of a repeating coil and an element or elements having non-linear resistance properties is or are connected to a winding or windings of the repeating coil in such a manner as to limit the magnitude of surges present at the bridge whilst having little eiiect on communication currents.

According to another feature of the invention a biassing voltage is applied to the non-lineafrsistance element, the poling of the element being such that in conjunction with the biassing voltage damping of communication currents is negligible. This biassing voltage may be obtained by a connection to a separate source of current or it may be provided by the drop in voltage occasioned by the flow of feed current through the repeating coil winding to which the non-linear resistance element is connected. The term non-linear resistance element is herein intended to include an element of which the resistance varies with the voltage applied to it and also to an element which ofiers a different resistance to the same voltage depending on the polarity of the voltage, i. e., it possesses unidirectional conductivity. An element of the first type is suitable for use in the case in which no biassing voltage is used and an element of the second type is employed in the case in which a biassing voltage is applied to the element. A dry-plate rectifier or an element of a known compound of silicon carbide in which the resistance oiTered is high when the voltage due to communication currents is applied but falls ing of the repeating coil to a lower value due to the application of a surge voltage higher than that produced by communication currents is suitable for use as a non-linear resistance element whether a biassing voltage is applied or not, it being understood that the poling of an element possessing unidirectional conductivity will be such that the element is suited to the connection in which it is employed and that any biassing voltage applied to the element will be such that the element does not become conductive to any great extent until the surge voltage exceeds that due to the communication currents.

In one Way of carrying out the invention the non-linear resistance elements may be connected across the windings of the repeating coil connected to the line and in the case of the windings on the incoming side of the bridge it is preferable to employ a balanced arrangement in which an element is connected across each of two windings of the repeatingcoil on the incoming side of the bridge. It will be realised that.

such a device is half-wave in action and will deal effectively with surges arising at the bridge. It

' will also deal effectively with surges arising at a preceding point provided that the current at that point which gives rise to the surge flows in such a direction that the initial polarity of the surge is the same as that which would be produced by a surge at the bridge itself. In a telephone system this can usually be arranged to be the case.

In another way of carrying out the invention the non-linear resistance element or elements may be connected across a winding of the repeating coil other than a line winding and hereinafter called a third winding irrespective of the number of windings connected to the lines. The third winding may itself comprise two series connected windings. Two oppositely connected elements may be connected to the third windto provide an arrangement full-wave in its action and independent of the polarity of the line wires.

In arranging for a bias on the elements when connected across the line windings it may be found'desirable to include an external resistance, which should be non-inductive, in series with a repeating coil winding and to connect the element across this series connection. The resistance should be sufiiciently low in value to prevent its presence from unduly affecting the efficiency of the bridge. The use of feed current to produce the bias where the non-linear resistance elements are connected to the line windings of the repeating coil has the advantage that it is this current which, when interrupted, gives rise to the surge and by its interruption the bias on the nonlinear resistance element is removed rendering the arrangement more effective in dissipating the resultant surge.

In the case in which non-linear resistance elements are connected to the third winding of the repeating coil a suitable biassing voltage may be obtained either from a separate source of current or from suitable connections made to the source of feed current. However by the choice of a suitable element a biassing voltage may be dispensed with.

In an automatic telephone system the feeding bridge will include an impulse receiving relay on its incoming side. A relay having a low inductance and operating at lay described in U.S. Patent No. ,992,6 g a d February 26, 1935, to E. J. Gachet, is suitable for use as an impulse receiving relay and a contact of this relay may be arranged to repeat the impulses in the outgoing side of the bridge or it may control an impulse repeating relay which may also be of the high speed type and a contact of this latter relay may be included in the outgoin line wires.

Specific embodiments of the invention will now be described by way of example with reference to the accompanyingdrawings. In the drawings Fig. 1 shows diagrammatically an outgoing repeater with a feeding bridge in which incoming impulses are repeated as D. C. impulses over an outgoing circuit, Figs. 2 and 3 show alternative forms of a similar repeater for repeating D. C. impulses as impulses of voice frequency current in an outgoing circuit, while Figs. 4, 5 and 6 show diagrammatically modifications of the arrangements at the feeding bridge that may be employed.

Reference will first .be had toFig. 1. The relay set here shown is for connection at the outgoing end of an inter-exchange junction which serves as a link between automatic switching equipments at the two telephone exchanges concerned, and as is usual in such relay sets it has incoming positive, negative, and private wires for connection to the multiple of a group of selectors in the same exchange as itself and outgoing positive and negative wires for connection to the two wires of the junction. In the drawings the incoming wires appear at the top on the left, the private wires being designated P. The outgoing wires appear at a corresponding position on the right, the private wire connection shown in this case being intended simply for extension to a distributing frame for use for the purposes of local testing. The contacts designated T are contacts of a test jack. It will be noted that underneath the reference character for each relay there is a digit indicating the number of contacts on that relay. For example, relay G is denoted by the letter G over the digit 3; this means that relay G has three contacts, numbered Ql, g2 and 93.

The principal functions of the relay set are to supply loop current backwards towards the originating end of the connection, to control the loop circuit which extends forwards to the succeeding exchange, and to control the holding of the switches in the local exchange. In order that it may properly perform these functions it includes a transmission bridge .and must hence be adapted to repeat received impulses and supervisory signals.

The relay set has a feeding bridge of the kind a high speed such as theree in which the incoming and outgoing circuits are coupled together by a so-called repeating coil RC. This repeating coil has two windings I and III in the incoming circuit and two windings II and IV in the outgoing circuit. Windings I and III are connected in series via a 2-microfarad condenser QA and windings II and IV are connected in series via a Z-microfarad condenser QB. Connected in series with windings I and III respectively of the repeating coil RC and to the line are resistances RI and R2 and across the series connection of winding I and resistance Rl is connected a non-linear resistance element, suitably a dry plate metal rectifier, SR1. A similar element SR2 is connected across the series connection of winding III and resistance R2. The connections on the incoming side of the feeding bridge are such that the incoming positive wire is normally connected to earth via winding I of a high-speed low-impedance impulsereceiving relay A and one element of a two-element ballast resistance BRA, and the incoming negative wire is normally connected to battery via winding II of relay A and the other element of BRA.

Condenser QB on the outgoing side of the bridge has two shunts connected across it. One of these shunts includes parallel connected rectifiers MRD polarised so as normally to allow current flow over the outgoing junction during the initiation and setting up of a connection; the other shunt includes rectifier MR0 and relay D, rectifier MRC being so polarised as to allow relay D to operate only when current in the junction wires is reversed consequent on the reply of the called subscriber.

When the relay set is in circuit at an exchange and is free with its apparatus at normal, it tests free to a searching selector by reason of the absence of earth on the incoming privatev Wire. When the relay set is seized for a call by such a selector, the high-speed impulse-receiving relay A is operated in a circuit which includes the impulsing loop which is controlling the setting up of the call. The actual operating circuit is from earth over the lower element of the ballast resistance BRA, winding I of relay A, winding III of the repeating coil RC resistance R2, back contact (M3, the incoming positive wire, the impulsing loop referred to the incoming negativewire, back contact d012, resistance RI, winding I of the repeating coil, winding II- of relay A, the upper element of the ballast resistance BRA to battery. With current flowing in this circuit the potential cross winding I of the repeating coil and resistance R l in series is applied to the nonlinear resistanceelement SRI biassing it so that current through it at a voltage not higher than that due to speech in thecircuit is effectively blocked. A similar biassing potential is applied to the element SR2 by the drop in winding III of the repeating coil'and resistance R2. Relay A is preferably a relay of the kind illustrated and described in U. S. Patent No. 1,992,610, as before mentioned, and has only the one contact 12!, this being of the simple change-over type. Contact al on operation closes operating circuits for a high-speed impulse-repeating relay AA and for the release relay B of the relay set, the circuit for relay AA comprising a rectifier MRB winding I of the relay, and resistance YB. The said impulse-repeating relay is a relay of the same kind as relay A and its sole contact aal on operation closes a point in a calling loop across the outgoing positive and negative wires. A

elements SRI and SR2 providing a suitable path spark-quenching combination comprising a condenser QC in series with a resistance is connected across the make portion of contact aal. It has been found that the presence of this combination exercises a beneficial effect on impulse repetition. The release relay B is as usual a relay which is slow to release. On its operation as just described, its contact bl connects earth derived via back contact (M4 to the incoming private wirefor engaging and holding purposes and contact b2 connects a battery-connected switching relay HA to this wire and thereby brings about the operation of this switching relay. The circuit of relay HA includes a resistance YC, and the relay on operation locks up in a circuit over front contact hal and at contacts M12 and ha3 completes the calling loop over the outgoing line wires. Contact b4 closes the operating circuit of a slow releasing relay G. Relay G operates and performs preparatory guarding operations. Consequent on the operation of relay HA, current flows over the outgoing junction and the parallel-connected rectifiers MRD but not through relay D owing to the polarity of rectifier MRC. The rectifier MRB is to prevent the windingof relay B from unduly increasing the release lag of 'relay AA by acting as a shunt. The connections. f the repeating coil RC are such that during impulse repetition the currents in all four windings assist each other so far as the direct current magnetisation of the core is concerned.

Impulses are now received over the incoming line and are responded to by relay A. At the first interruption in the line current, relay A releases and at contact al opens the circuit for relay AA. Relay AA releases and at contact aal opens the outgoing loop to repeat the impulse break and closes a short-circuit across the outgoing side of the feeding bridge. Consequent on the closure of back contact al a circuit is closed over contacts (1125 and b3 and the two windings of relay C in series and that relay operates. Contact cl short circuits winding I of the .relay to render it slow in releasing and contact 02 connects a low resistance shunt across the outgoing side of the bridge. Subsequent received impulses are repeated by relay AA over the junction.

In consequence of the opening of the incomingline wires the energy in the windings of the repeating coil tends to dissipate itself in the form of a surge. As regards the energy produced in the windings I and III of the repeating coil, a path for the surge current is provided over the elements SR! and SR2 the bias on which was removed consequent on the cessation of current round the loop. The potential drop across winding I and resistance RI and winding III and resistance R2 respectively is such as to cause a current to flow through the elements SR! and SR2 and the voltage across the elements is reversed in polarity and the surge dissipates itself so that surge current of any magnitude is pre- Vented from passing through the repeating coil to the outgoing line. By controlling the bias on the non-linear resistance elements by the feed current which on interruption gives rise to the surge, the bias is removed on interruption of the feed current and a rapid dissipation of the surge energy results. It will be noted also that the production of a surge of the same polarity at a preceding point will also be prevented from passing through the bridge without attenuation, the

for the dissipation of the surge energy.

At the end of each train of impulses relay C releases removing the shunt from the outgoing side of the bridge. After the connection has been completely set up and the called party replies, relay D is operated by current reversal over the junction. The closing of contact dl initiates the slow operation of an answering supervisory relay DD having a long operation lag, and contact d2 completes a circuit for a holding winding II of the impulse-repeating relay AA. Relay DD on operation is temporarily locked up in a circuit over contacts old! and M and owing to its slow operating property serves to ensure that metering is not effected unless the reversed current condition in the outgoing line Wires persists for a reasonable time. The changing over of contact doll initiates the slow release of the metering relay G, and contacts (1012 and 11013 effect a reversal of current in the incoming line wires to repeat the answering supervisory signal in case this should be necessary. Contact ddd connects booster battery potential to the incoming private wire in place of the usual holding earth, so that if the calling party is a subscriber belonging to the exchange in which the relay set is situated, the meter of this party is operated in series with re-- sistanc YD. When the slow release of relay G is completed. contact gl opens the holding circuit of relay AA and contact 92 closes an operating circuit for a control relay H which circuit includes contacts 134, dl and hl. On the operation of relay H, make-before-break contact hl renders the circuit of the relay independent of the condition of relays D and G, contacts I22 and M restore the usual holding condition on the incoming private wire, and contact M opens a holding circuit for relay DD. Release of the switching relays connected to the private Wire is prevented during the changeover period by the presence of holding earth applied over rectifier MRA which opposes flow of booster battery current. The condition of th circuit immediately subsequent to metering is that relays A, AA, B, HA, D, DD, and H are operated.

If at the end of the call the calling party clears first, then relay A releases, this release causing the immediate release of relay AA and initiating the slow release of the release relay B. The falling back of contact aal opens the loop across the outgoing junction wires, releasing relay D and initiating the release of the apparatus at the distant end of the junction. The opening of contact all releases relay DD. The falling back of contact ddl operates the metering relay G, and the falling back of contact dd5 operates the dialling relay C. Contact g3 closes an alternative circuit for relay C. Contact ci short-circuits the low-resistance winding I of relay C to render the relay slow to release. On the subsequent completion of the slow release of the release relay B, the opening of contact bl temporarily removes the holding earth'from the incoming private wire to release the switching relay HA and the selector switching relays held locked to this earth (thus causing the immediate commencement of the release of the apparatus preceding the relay set), and the opening of contact 194 releases relay H andinitiates theslow release of relay G. The switching relay HA is arranged to release rather more slowly than the said selector switching relays, and when it does release, at contact hal for again it completes a circuit over contact 03 earthing the incoming privat Wire to guard the relay setfrom seizure by asearching selector. On the completion ofthe slowrelease of relay G, the opening of contact 913 initiates the slow release of relay C. On the completion in turn of the slow release of relay C, the guarding earth is disconnected from the incoming private wire and the relay set is ready for use on another call. Owing to the sequence of operations just described, the apparatus at the distant end of the junction is given ample time to release before the relay set tests free, while the preceding apparatus is not held for the whole of the guarding period.

If the called party clears before the calling party, the polarised back-bridge relay D is in accordance with the hitherto usual practice re leased by the usual reversal of current in the line wires of the junction, and at its contact all releases the answering supervisory relay DD.

The falling back of contact (1015 operates the metering relay G, and the falling back of contacts dd? and ddt eiiects a reversal of current in the incoming line wires to repeat'the clearing signal in case this should be necessary. When the calling party clears, relays A and AA release in turn, relay C is operated over back contact ai, and the slow release of relay B is initiated. The falling back of contact cal opens the loop across the outgoing junction wires, initiating the release of the apparatus at the distant end of the junction. On the subsequent completion of the slow release of the release relay B, release of the connection proceeds in the same manner as in the case considered in the preceding paragraph.

If the called party is not free when called, then busy tone current is received over the junction and passes via the repeating coil bridge to the calling party. When the calling party clears, relays A and AA release in turn, and release of the connection proceeds in a manner similar to that just described.

The feeding bridge shown. in Fig 2- is suitable for use in a system in which incoming D. C. impulses are to be repeated as impulses of voice frequency current. bridge is arranged in a manner similar to that shown in and described for Fig. 1. The repeating coil windings on the outgoings'ide of the bridge are connected to the outgoing junction over the back contact of a change-over spring set of a dialing relay and the front contact of a change over spring set of an impulse repeating relay AA. The front contact of the change-over spring set of the dialing relay connects a 690 ohm. termination across the line and the back contact of the changeover spring set of the impulse repeating relay connects the secondary winding of a tone ransformer TTto the line. The primary winding of the tone transformer is connected to a source of voice frequency current. The impulse repeating relay corresponds to the relay designated AA in Fig. 1 and the dialing recorresponds to the relay designated C in Fig. 1 and may be operated and controlled in well known manner. When the relay set is taken into use relay AA operates and at contact aal disconnects the tone transformer from the line wires, it understood that normally the line is held open by contacts of a switching relay which is operated to close the line circuit when the relay set is taken into use. On receipt of the first impulse break. relays A and AA release and by means of a previously prepared circuit the dialing relay operated. Relay AA releases and connects the tone transformer to the line so that The incoming side of the for thei duration of the break period a pulseof voice :irequency current is transmitted. When the impulse repeating relay again operates, the GOOchm termination is connected to the line over contact ci. The dialling relay remains operated throughout the duration of animpulse train and releases at the end of the trainin knowirrnanner.

The action of the non-linear resistance elements'SR! and SR2 is the same as that described in connection with Fig. 1. With the arrangements describedparalysation of the voice frequencycurrent receiver due to the transmission of surges can be reducedto a n'egligibleamount. Like the casev of Fig. 1 the surge limiting action is half-wave inits action and can deal'with surges'arising at the bridge and with surges arising at a preceding point if the polarity is'the same. A surge limiting result may'also be obtained by connecting a non-linear resistance element across the repeating coil windings on the outgoing side of the bridge. The element may be a rectifier and have a suitable biassing voltage applied to it. The magnitude of the biassing voltage and'the poling of the element would be so arranged that a surge arising at the bridge would be shunted away from the line Whilst communication currents would suffer little attenuation. Such an arrangement is illustrated diagrammatically in Fig. 3 in which the element is designated SR and the source of biassing voltage V. The remainder of the apparatus illustrated in this figure is similar to the-correspondingly designated apparatus in Fig. 2. An advantage of this arrangement lies in the fact that it becomes effective before the dialling relay possessing contact cl operates and therefore is effective to deal with a surge arising from the transmission of the first impulse of a train.

As an alternativ to the arrangement illustratedin Fig. 3 the element SR may be of a type which does not necessitate a biassing voltage, the resistance of the element under the communication current condition being sufiicientlyhigh to prevent appreciable damping of speech currents and under surge conditions sufficiently low for the rapid dissipation of the surge energy.

It has been found that with repeating'coil windings of 10 ohms each and non-inductive resistances RI and R2 also of 10 ohms each a suitable bias for a dry plate rectifier for use as SR! and SR2 is obtained on a line up to 800 ohms loop resistance, the value of bias to be provided-dependin on thecharacteristics of the devices SR! and SR2.

Other arrangements for providing the surge limitation coming within the invention are illustrated in Figs. 4, 5 and 6. In these figures, which are only diagrammatic, the repeating coil is shown with two line windings I and II and a third winding III to which'non-linear resistance elements are connected. The arrangements shown in these figures arefull-wave in action. In Fig. 4 two rectifiers SR! and SR2 with like-faces connected to the ends of the third winding-of the repeating coil have their other faces connected together and to one end of a source of biassin voltage V the other end of the source being connected to the mid point of the third winding III. It will be seen that when the voltage induced inwinding III exceeds the voltage of source V and opposes it the'bias on the rectifiers is overcom and they become conductive and provide a low resistance shunt in which the surge may, dissipate itself.

The bias provided by the source V is such that no appreciable shunting effect is present when communication currents are flowing in the repeating coil. It will be understood by those skilled in the art that each non-linear resistance element may be associated in oposite senses with a separate winding, the result being equivalent to the single winding with mid point connection shown in the figure. Furthermore, by the choice of a suitable element the element may be connected across the third winding of the repeating coil without the application of a, biassing voltage.

The arrangements illustrated in Figs. and 6 are similar in their action to those illustrated in Fig. 4 the source of biassing voltage being a potentiometer P'connected across a battery V. These arrangements obviate the necessity for the use of a separate biassing battery and in a telephone system V may be the exchange battery.

It will be understood that a half-wave action may be had by the connection of only one rectifier to the winding 'III and the application of a suitable biassing voltage but the full-Wave action deals not only with surges arising at the bridge but also with surges arising at a preceding point whether the polarity is the same or not.

We claim:

1. In combination, a circuit including resistance, means for impressing audio frequency upon said circuit, a source of direct current in said circuit biassing same to cause the fiow of current over said resistance always to be in a particular direction even when said resistanceis traversed by said audio frequency, a rectifier connected in shunt to said resistance, whereby the voltage drop in said resistance due to current flowing therethrough produces a difference of potential across said rectifier, said rectifier so poled with respect to said voltage drop that it will resist current flow through itself whenever the current flow through said resistance is in said particular direction, said rectifier becoming conductive whenever an abnormal potential in said circuit tends to reverse the current flow through said resistance, thereby to limit the magnitude of surges through said resistance.

2. In combination, a circuit including an inductance, means for impressing audio frequency upon said circuit, a source of direct current in said circuit biassing same to cause the now of current through said inductance always to be in a particular direction even when said inductance is traversed by audio frequency, a rectifier connected across said inductance, whereby the voltage drop in said inductance due to current flowing therethrough produces a difference of potential across said rectifier, said rectifier so poled with respect to said voltage drop that it will resist current flow through itself whenever the current flow through said inductance is in said particular direction, said rectifier becoming conductive due to the surge from said inductance in the event the current flow from said source is interrupted.

3. In a telephone or like system, a communication line divided into two sections, a repeating coil inductively linking said two line sections, a source of direct current connected to one of said line sections over a circuit including a winding of said coil, said source efiective to maintain current flowing through said circuit in a particular direction even when said winding of the repeating coil is traversed by speech currents, a rectifier connected in shunt to said circuit, whereby the voltage drop in said circuit due to said current flowing therethrough produces a difference of potential across said rectifier, said rectifier so poled with respect to said voltage drop that it is substantially non-conductive so long as said current continues to flow in said particular direction through said circuit, said rectifier becoming conductive whenever an abnormal potential on said line tends/to reverse the direction of current flow through said circuit, thereby to limit the magnitude of surges through said circuit.

4. In a telephone or like system, a battery feed bridge comprising a source of direct current, a relay having two windings, a repeating coil having a plurality of windings, two non-inductive resistors,-a line having two conductors, means connecting one terminal of said source to one of said conductors over a circuit including one winding of said relay, one winding of said coil and one of said resistors in series, means connecting the other terminal of said source to the other of said conductors over a circuit including the other winding of said relay, another Winding of said coil and the other of said resistors in series, a rectifier shunting that portion of said first circuit including said one winding of said coil and. said one resistor, another rectifier shunting that portion of said second circuit including said other winding of said coil and said other resistor, each of said rectifiers being so poled that they offer high resistance to the flow of current from said source when said two conductors are connected together.

5. Ina telephone or like system, a line divided into two sections, a repeating coil inductively linking said two line sections, a relay, a source of direct current, means connecting said source to one of said line sections over a circuit including said relay and a winding of said repeating coil in series, whereby a normal current flow is established over said relay, said winding and said one line section, said relay responsive to interruptions of said normal current flow, the inductance of said one line section and of said relay tending to cause current to surge through said winding in a direction opposite to said normal current flow upon each said interruption of said normal current flow, and a non-linear resistance connected in bridge to said winding and effective to limit the magnitude of said current surges through said winding.

6. In a system as claimed in claim 5, means controlled by said relay for repeating said interruptions as impulses over the other section of said line, and other means controlled by said relay upon commencement of said repetition for short-circuiting the winding of said repeating coil connected to said other line section.

7. In a telephone or like system wherein a com nection may be extended from certain equipment to other equipment at will, wherein said other equipment thereupon is effective to impress a direct current potential upon two conductors of said extended connection, and wherein said other equipment thereafter is effective to reverse the polarity of the potential connected to said conductors for supervisory purposes, a first circuit bridging said two conductors in said certain equipment, said circuit containing a rectifier poled to permit current to flow over said circuit only when the potential impressed upon the conductors at said other equipment is of a certain polarity, a second circuit bridging said two conductors in said certain equipment, said second circuit containing a rectifier poled to permit currentrto flow over saidisecondcircuit only. when the-potential impressed upon the conductors at said other equipment is of a difierent-polarity, and a relay included in one of said circuits and operated whenever there is a current flow over that circuit due to the potential-impressedupen the conductors atsaidother equipment.

8. In a telephone or like system, a communicationline divided into two sections, a repeating coil having a different winding connected to each of said line sections thereby to inductively coupie said two sections, said repeating coil also'having a winding other than those connected to said two line sections, a rectifier, a source of direct current, andmeans connecting said source, said rectifier and at least a portion of said other winding in a series circuit, said rectifier being so poled in said circuit that it presentsahigh resistance to the flow of current over said circuit from said source.

9. A system such as claimed in claim 8, wherein the potential impressed upon'said rectifier by said source is greater than that impressed thereupon due to the currents induced'in said'other winding when speech currents are present on said communication line.

10. In 'a telephone or like system, a communication line divided into two sections, a repeating coi1 having a diilerent winding connected to each of said line sections thereby to inductively couple said two sections, said repeating coil also having a winding other than those connected to said two line sections, a source of direct current having one terminal connected'to the center point of said other winding, a rectifier connected between the other terminal of said source and one end of said other winding, and a second sectifier connected betwen said other terminal and the other end of said other winding, each of said rectifiers being so poled that it presents high resistance to the fiow of current over half of said other winding from said source.

11. In a telephone or like system, a communication line divided into two sections, a repeating coil having a different winding connected to each of said line sections thereby to inductively couple said two sections, said repeating coil also having a winding other than those connected to said two line sections, a source of direct current, a voltage divider connected across said source, means connecting the mid-point of said'divider'to one end of said other winding, a rectifier connected between the other end of said other winding and one terminal of said-source, a second rectifier connected between said other end of other winding and the other terminal of said source, each of said rectifiers so poled that it presents high resistance to the flow of current from said source over said other winding and one half of said divider.

DAVID LANGSFORD CLAY.v CHARLES JOHN CLIFFORD BARTLETT.

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