US2055343A - Telegraphic repeater system - Google Patents

Description

Sept 22, 1936- K. ETZRODT ET AL 2,055,343

TELEGRAPHIC REPEATER SYSTEM Filed May 21, 1952 Patented Sept. 22, 1936 VPA'lENT OFFICE 7 REPEATER SYSTEM Karl Etzrodt,

Berlin- Siemensstadt,

Heinrich Flling, Berlin-Charlottenburg, and Walter Hhnle, Berlin-Siemensstadt,

Germany, as-

lsignersA to Siemens & Halske, Aktiengesell- V schaft, Siemensstadt near Berlin, Germany, a

corporation of Germany Application May 21, 1932, serial No. 612,756 In Germany May 23, 1931 16 Claims.

Our invention'relates to a telegraphic repeaterl system, and moreparticularly to a repeater system for wire telegraphy employing impulses of different polarity and telephone repeaters having amplier characteristics. Y

VIn wire telegraphy employing impulses of different polarity effects of disturbing currents are more noticeable than in ordinary telegraphy. Special attention must, in particular, be paid to the high voltages liable to injure attendants or operators. It would, in itself, be possible to render the telegraph apparatus high-tensionproof, as has been done in sectionalized telephony, for instance. -This is, however, unsuitable for telegraphy, as it would then be necessary to abandon the customary construction of the racks, in which the individual parts and relays are so arranged as to be readily accessible. Grounding provisions at the lines would indeed also be as 29, safeguard against dangerous voltages, but these,

due to the necessary ground-symmetrical arrangement, again cause diiculties in the phantom utilization of telephony. In telephonic communication it is known to subdivide the line into sections and to block or protect the apparatus at theends of the line against induced voltages by repeaters. A

`Such an expedient 'can be utilized in impulse telegraphy by sending directcurrent impulses when .closing the key in the telegraph transmitter. f- By the protecting repeater vor more properly transformer at the sending side the direct current' impulses 'are converted into pure impulses and sent over the line; at the receiving side the impulses mayfthrough the protecting transformer located here be passed to a relay adjusted for impulse reception.- Wire telegraphy of the polar-duplex or briefly called impulse type has, however, a number of drawbacks, the most prominent of which is that a comparatively small energy is sent over the lines, which consequently aremore liable to `be adversely affected by the switching `operations in neighboring high tension power lines. VThe impulse 'peaks of impulse telegraphy, furthermore, more easily cause utter phenomena-more particularly in loading coils and the like, than the currents-of the direct current telegraphy operating with smaller amphtudes;4 l A,

According to our invention these drawbacks are eliminated and yet a protection ofthe telegraph apparatus against high tension influences is obtained bythe sending and/or receiving apparats of .the telegraph system being guarded by thertransformers against disturbing voltages on the line, and by providing apparatus for sending the telegraph signals over the line in the `form of direct current impulses. This may, for instance, be effected by operating with directeurrent in front of the protective transformer (that is, on the input side) and connecting behind the transformer, which resolves each direct current impulse into impulses of opposite direction or polarity, a relay, for instance, .with aV holding curr-ent winding Vwhose local circuit sends direct current impulses over the line. Such a system has, however, still the disadvantage that the relay and the feeding battery, which 'directly generate the line currents, are still located on the side of the transformer affected by the disturbing voltage, and therefore, in order to prevent accidents,` must notbe accessible under normal conditions.

According to a further feature of our invention, the telegraph currents are keyed as alternating currents and sent over the protective transformer: Beyond the protective transformer the alternating current impulses are converted by rectifiersf into direct current impulses, which are then sent over the line. On the receiving side the direct current impulses reach the protective transformer on the input or receiving side and are there resolved into impulses which operate the receiving relay, if desired after amplification.

' In the drawing aixed hereto- Fig. 1 illustrates the principle of the invention with reference to a telegraph apparatus operatingl on the polar-duplex wire system and impulse reception; Fig. 2 is a partial diagram of another embodiment, and Fig. 3 is a diagram of a third embodiment.

Referring to Fig. 1, alternating current is ac-i cording to the invention sent, by closing thekey S, to the protecting transformer or stopper transformer U1 for which alternating current is Ysupplied to the rectier system g'. The rectifier sys-- tem effects a complete rectification. In this system direct current impulses are generated which pass through the lter Fi to the line l. Hence they are passed through a filter F2 to the stopper transformer U2, which converts them into impulses (two of opposite direction for one signal), which reach the impulse relay R. At t We have indicated the telephone connections or cables. In the repeater system according to Fig. 1 each disturbing impulse is, due to the construction of the transformer, resolved into two impulses ofl opposite direction which follow each other directly.

Contrary to Fig. 2 each disturbing impulse is,

` sufficiently accurate.

however,` dueV to the design of the transformer U2, resolved into two impulses of opposite direction, which follow each other directly, as distinguished from the telegraph currents, whose impulses ofopposite direction are separated by the duration of asignal. The effect of disturbing impulses may thus be reduced to the minimum length of the signal by adjusting the time con-L stant of the relay R.

In the system illustrated in Fig. 1 an impulse amplifier may naturally be connected up ahead of the receiving relay R. v

Such a wrong operation of the receiving relays may, however, to a greater extent be avoided by leading the direct current impulses arriving from thel line to an apparatus which converts them into alternating current impulses, which reach the receiving apparatus through the Vstopper transformer. Y

According to our invention this is preferably effected by leading the direct current impulses into a magnetic amplifier consisting of a choke or a rejector circuit, which is fed from a source of'alternating current of higher frequency than the impulse frequency. Magnetic amplifiers. have: in comparison with valve amplifiers the great advantage that they reproduce the low frequencies very well, and are thus particularly useful for the amplification of the impulsev telegraphy'signals. They are, furthermore, not so sensitive tov the replacement of circuit elements as valvefamplifiers; Valve amplifiers cannot be used simultaneously in both trafiic directions, as the: amplifiers of the transmission direction block the amplifiers of the opposite direction.

According to our invention av magnetic amplifier is employed as a transmitting and asta receiving amplifier respectively' or as an intermediate amplifier, the windings of which are so designed and so connected to the duplex line serving for the transmission of the signals in the lines leading tothe transmitting and receiving apparatus, that together with a balancing network of the transmission line it functions as a balancing repeater between the three lines. In thisway a two-wire or forked repeater for telegraphy may be built up which corresponds with the.twowire intermediate repeaters and forked repeaters in telephone systems. By the application of the iron repeaters. the above-mentioned drawbacks of the valve amplifiers are eliminated.

,Thejrepeater or transformer according to the invention isY furthermore at the same time available: for bothrdirections of traffic..v Transmitting and receiving circuits may be completely uncoupled provided the artificial balancing line is The magnetic amplifiers consist in their simplest designV of an iron-containing choke coil, which is connected in an alternating current circuit whose frequency is higher than the highest frequency to be amplified. TheY iron choke coil is so Vmagnetically "loi biased by direct current, that it varies its alternating current resistance when the low frequency volta-ges to be' amplified are applied with the frequency l of Ythese voltages. This causes a modulation of the alternating current of the higherv frequencies. After the rectification of the modulated carrier ffrequency the amplified low frequency is obtained.

By the introduction of the rectiflers, which arev necessary when Vmaking use of magnetic amplifiers, anew phenomenon appears as compared with the normal balancing connections,

T5 ,suchasLarerused in Y.telephone systems,for ini rectifier, i. e. with the cadence of the outgoing current. This phenomenon causes the incoming current to be modulated by the outgoing current.

According to. a further feature of our invention this modulation effect is reduced by the outgoing current notV being reduced to Zero and Vpreferably being at al1 times maintained larger than '1% of its maximum value.

The modulation effect is not entirely prevented in this way, it is true, but confined within limits which according to; experience generally do not cause any disturbances of the telegraph service. The desired control' of the modulation effect may, for instance, be attained in the transmitterby a bridging resistance for the transmitter key, and in the.

telephone repeater by the choice` of a correspond-v: If the modulation stillv continues to cause disturbances in spite of. theseing bias magnetization.

expedients, an attenuation memben, preferably adjustablamay according, to a. further feature of' our invention be connected behind the rectifier. the resistance of which,y is; so proportioned that the variation of the impedance of the rectifier no longer plays an important part. The .attenua-- tion member mayA at the same timer be designed as a correcting device for controlling the` frequencies of the vtelephone repeater.

The incoming currenttaken up bythe rectifier represents an additional load on the transmission line. If for any reason it is desirable to reduce this load, our invention provides for the cornpensation of this direct current by an additiona source of direct current.

As distinguished from the telephone repeaters the risk of self-excitation in the telegraph yrepeater can be reduced bythe choice of a repeater or transformer characteristic which is not linear in a definite way. Such a non-linearity of the characteristic exists in an ideal way in every contact relay. In this relay the amplification is zero up to reaching the limit of response and then rises to a very high value. at the moment. of reversal. In a repeater or transformer with such a non-linear characteristic a resonant rise-- of oscillations for self-excitation is impossible, be.- cause the current due to faulty balancing returned into the input circuit of the repeater is located below the threshold of response. In magnetic telephone repeaters a similarV characteristic may be obtained by asuitable-,biasing of' the rectifier with a direct current voltage,` fori instance, and thus the tendency to recoupling be reduced. This result may also be attained. bya shunt consisting of a one-way ,rectifier 1ocated behind the rectifier, ther one-Way rectifier; being biased in such a way that it operates in the: lower bend of its current. voltage-characteristic. A similar effect may also be attained by choosing the magnetic conditions of the telephone repeater in such a way that in case of low modulation the amplification is small, and large in arrangement according to our invention applied toi p,olarduplex .,wire telegraphy.. where; the tele--- graphcircuits are superposed on telephone circuits. As regards the transmitter the system is identical with Fig. `1.v VAt the receiving side the direct current impulses pass, however, through filters F2` to the rejector .circuit designed as a repeater, which is equipped with `two sets or groups of choke coils or windings D1, D2, protected against high voltage discharges Vor leaks from the line. In this Way the entire receiving apparatus with the exception of the filterFz may bellocated on the protected side of the repeater. The alternating current feeding the rejector circuit D2 is through a protective repeater or transformer Us supplied to a rectier G in the output of which there .are developed alternating currents which according to the design of the rectifier, are of the frequency of the fundamental oscillation ofthesource of alternating current or of higher harmonics, in addition to a direct current Vcomponent which serves for the magnetic biasing of the chokes D, D2. The filter F3 interposed between the rectifier G `and the choke D2 is adapted to permit direct current and one of the alternating current frequencies to pass. By way of a rectifier g2, the rejector circuit is connected with the impulse relay R.

v Thetransformers serving for protective purposes are preferably designed to be high-tensionproof and with grounded shields to the transformer windings, as in the example described above. l

Fig. 3 illustrates a forked transformer A installed in a telegraph station, and destined for the amplification of the `incoming currents, and an intermediate repeater B, which ampliiies the telegraph signals arriving from the forked repeater A vand passes them on to an arrangement similar to A, not shown in the drawing.

In Fig. 3 of the drawing SG indicates an alternating current generator whose current is dispatched by `closing the transmitting key ST. When the key is depressed toclose the circuit, this alternating current is supplied tothe recti- Iier arrangement G11 through the .transformer Between the transmitting or sending equipment described and the receiving apparatus designated ER is connected the vmagnetic repeater MV, which handles the transmission of the signals sent to the outgoing line L1 and the transmission-of the signals arriving over vthis line to the receiving apparatus ER. This magnetic repeater includes the iron-containing inductances W1 to We. A magnetic amplifier consists generally of biased iron choke coils connected in the circuit of a source of high frequency. The iron chokes W1 and W2 arebiased through the windings W5, We, which are traversed by the direct current of the battery `B. The high frequency supplied by the generator I-I is fed to the chokes W1 Yand W2 through the transformer U3. The signals arriving over the line L1 and to be amplified traverse the choke coils W1 and W2 in the same direction and thereby vary the alternating current resistance of the iron chokes for the high-frequency carrier oscillation. The carrier frequency is thus .modulated by the currents to be amplified. The modulated oscillations are through the windings Ws and W4 and the transformer U2 passed tothe receiving apparatus ER after rectification by means Vof the rectifier equipment G12. The transformation ratio of the transformer U2 is made variable to enable regulation of the repeater. Itwill be understood, however, that other means for controlling the degree of amplification may be used, for instance the variation of the amplitude of the carrier frequency. The line L1 is bridged for the carrier frequency by means of the condenser C2.

-To enable the magnetic amplifier MV to carry out its function as a balancing repeater, the choke coils W1 and W2 are each provided with a midpoint tap S1 and S2. To these taps are connected the lines leading to the transmitting apparatus. Under the assumption that the artiflcial balancing line N is a close simulation of the resistance connected to the terminal points A1 and A2 of the windings W1 and W2, an-d more particularly of the resistance of the transmission line L1, the currents emanating from the transmitting apparatus traverse the two halves of the windings W1 and W2 in opposite directions and in equal strength. This prevents a transmission of the sending currents to the windings W3 and W4 and thus to the receiving apparatus. Due to the design of its windings according to our inventionthe magnetic amplifier MV thus takes over the functions of a balancing repeater between the line L1 and the transmitting and the receiving apparatus. The transmitting and the receiving apparatus are uncoupled from each other.

As already pointed out it is necessary to prevent the direct current arriving over the line L1 and traversing the rectifier G11 (the incoming current), from being modulated by the outgoing current of said rectifier. For this purpose the transmitter key ST is according to our invention bridged by the adjustable bridging resistance UW. As a` further means for eliminating the said modulation there is connected in circuit behind the rectifier G11, a variable attenuation member D in the form of a bridged T-connection. The attenuation member may at the same time be used as a frequency correcting device.

The intermediate repeater contains the repeaters MV1 and MV2 destined for the two different directions of traffic. The repeater MV1 ampliiies in the direction A-B and the repeater MV2 in the opposite direction. The two magnetic repeaters are of a construction similar to that of the repeater described at A, so that a detailed description appears superfluous. They act at the same time as balancing repeaters so that the signals amplified by MV1, for instance, which are supplied to the amplifier MV2 at its points of symmetry S2, are only returned into the line L2 and not amplified by the repeater MV2 to the magnetic repeater MV1.

At D3 and D4 adjustable attenuation membersH are indicated, which are intended to partly neutralize the eifect of the variable output impedance of the vrectiiiers G11 and G12, and at the Sametime serve to control the frequencies of the repeater circuit. The direct current generators G1 and G2 supply direct current which serves for the compensation of the direct current taken by the rectifiers G11 and G12. Similarly, a direct current generator G3 is associated with the variable attenuation member D.

For the sake of completeness there is in this diagram of connections also provided a twowire intermediate repeater SV for the ampliiication of the superposed speech currents, which is connected to the two lines L1 and L2 through the-repeaters or transformers SU1 and SU2 respectively. The condensers KL1 and KL2 and the coil lines SL1, SLz serve in a well known manner for the separation of the telegraph and telephone currents. Y

To explain the mode ofoperation ofthe in-v termediate `repeater circuits the transmission of telegraph signals in the direction A-B will now be concisely described. The telegraph signals arriving over the line L1 pass over the coil line SL1 into the magnetic repeater MV1, in which takes place a modulation of the carrier oscillations supplied `by the source Vof high frequency current Vv`H1. Then modulatedV oscillations are through the transformer U1 supplied to the rectifier GL1, in whose output circuit there' eXistthe telegraph currents arriving over L1 with amplified amplitude, and which are passed through the attenuation apparatusDa to the symmetry points S2 ofthe repeater MYX2. Assuming a close simulation of the line L2 by the artificial balancing line Nz'the amplified currentsV pass from the stymmetrypoints S2 through the coil' line Sie into the line L2 Without generating amplified currents in the output Vof the magnetic repeater MVz. y ItY will rbe readily understood that as a matter ofcourse modifications in the connections described may be made without departing fro-m the ambitof ourappended claims. Thus the windings W2, W4 and Ws of the magnetic repeater shown at vA may, for instance, be omitted.' In the same'way it would be possible to replace the attenuation circuit D by asimpler connection.

We claim as our invention:

1. Telegraphic repeater system, more particu- Ylarlyfra 'Wire telegraph system, including telegraph apparatus, transformers so connected as to vprotect the telegraph apparatus against disturbing voltages on the line, said transformers being located at the sending and at the receivingends of the line, an alternating current circuit at the Vsending end of the line, a key for opening and closing said circuit, rectifier apparatus interposed between the sending transformer and I the' line, whereby the alternating current signals produced byVV operating said key will` be converted into direct current signals,

Y whichY are sent over the line, arreceiving circuit located beyond the transformerat the receiving end of the line, and a device located between said transformer and the line, for converting the direct current signals intov alternating current signals, which pass' through the said transformer into the receiving circuit.

2." 'Ielegraphic repeater system, more, particularly a wire telegraph system, including telegraph apparatus, transformers so connected as to protect the telegraph apparatus against disturbing voltages onjthe line, an alternating current circuit, means for opening and closing said circuit, rectifier apparatus interposed between thetransforrner at the sending station and the line to convert the alternating current impulses produced by the 'operation of said means into direct current signals, a magnetic amplifier including choke coils connected with the line, a

source of alternating current'connected with said coils, a transformer connected with said amplier, a receiving relay, land rectifying apparatus interposed between saidtransformer and said relay. g t Y Y 3. In a repeater system for Vwire telegraphy of the polar-duplex type, including telegraph .apparatus, transformers so connected as to protectV the telegraph apparatus against disturbing voltages on the line, a'magnetic amplifier adapted to amplify the telegraph signals, a double line' constituting a 'transmission channel common to both directions of traffic; two transmission channelseach constitutingY a one-Way line, and an artificial balancing line connected with said doubleline, Ysaidamplifier having coils so designed and connected with said lines that it will actV asa balancing repeater between the three transmission channels. v p

4. Ina repeater system for wire telegraphy of the polar-duplex type, including telegraph apparatus, transformers so connected as to protect Sthe telegraph apparatus against disturbing voltages on the line, an alternating current circuit, means for opening and closing said circuit, a rectifier vfor converting alternating. current impulses produced in said circuit, into direct current impulse telegraphA signals, a magnetic amplifier adapted to amplify the telegraph signals, yand an adjustable attenuation member connected in parallel with said rectifier.

5. In a repeater system for Wire telegraphy of the polar-duplex type, including telegraph apparatus, transformers so lconnected as to protect the telegraph apparatus against disturbing voltages on the line, a magnetic amplifier adapted to amplify the telegraph signals, and a biased one-way rectifier connected-in shunt to said amplifier. e

V6. In a telegraph system, more particularly a system of composite telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses, a transmitting circuit comprising a source of alternating current and means for transmitting telegraph signals from said source of current, a transformer inductively connecting said transmitting circuit with said line circuit and adapted to protect said transmitting circuit from disturbing voltages on said line circuit, and rectifying means interposed between said transformer and said line circuit for converting alternating current telegraph signals transmitted from said transmittingcircuit into direct current signals which are to be sent lover the line.

7, In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct currentV impulses in Aboth directions, a transmitting circuit comprising means for transmitting direct current telegraph signals into said line circuit, a receiving circuit for receiving direct current telegraph signals from said line circuit, an artificial line balancing out said line circuit, and a magnetic amplifier between said line circuit and .said receiving circuit for amplifying incoming direct currentv telegraph signals, said magnetic ampliier comprising a plurality of coils in inductive relation to each other and in connection with said artificial line operating as a hybrid coil for coupling said transmitting circuit with said line circuit and uncoupling it from said receiving circuit.

8. In a duplex` telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses in both directions, a transmitting circuit comprising means for transmitting direct current telegraph signals into said line circuit, a receiving circuit for receiving direct current telegraph signals from said line circuit, a magnetic amplifier interconnected between said three circuits, comprising ra plurality of coils operating as a hybrid coil arrangement for uncoupling the receivingcircuit from the transmitting circuit, anda source of direct current for magnetically biasing the coils of said' magnetic amplifier to sucha degree that the amplitude of the output current of said ampliiier, in the absence of telegraph signals, will not fall down to zero.

9. In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses'in both directions, a transmitting circuit comprising means for transmitting direct current telegraph signals into said line circuit, a receiving circuit for receiving direct current telegraph signals from said line circuit, a magnetic amplier interconnected between said three circuits, comprising a plurality of coils operating as a hybrid coil arrangement for uncoupling the receiving circuit from the transmitting circuit, and a source of direct current for magnetically biasing the coils of said magnetic amplier to such a degree that the amplitude of the output current of said amplifier, in the absence of telegraph signals, will be higher than zero but not higher than about one third of the maximum amplitude.

l0. In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses in both directions, a transmitting circuit comprising a source of alternating current and a sending key for producing alternating current telegraph signals from said source of current, rectifying means adapted to convert said alternating current signals into direct current signals and to send them into said line circuit, a receiving circuit for receiving direct current telegraph signals from said line circuit, a magnetic amplifier between said line circuit and said receiving circuit comprising a plurality of coils arranged as a hybrid transformer for uncoupling the receiving circuit from said transmitting circuit, and a resistance connected with said sending key, said resistance being of such a value that the output current of said transmitting circuit will never fall down to zero.

11. In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses in both directions, a transmitting circuit comprising means for producing alternating current telegraph signals, rectifying means adapted to convert said alternating current signals into direct current signals and to send them into said line circuit, an attenuation network interconnectedbetween said rectifying means and said line circuit, a receiving circuit for receiving direct current telegraph signals from said line circuit, a hybrid coil arrangement for uncoupling said receiving circuit from said transmitting circuit, and a source of direct current connected with the output of said attenuation network for compensating for a steady output current of said rectifying means.

12. In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses in both directions, a two-way repeater device interconnected between two sections of said line circuit, said two-way repeater device consisting of two magnetic one-way amplifiers, two hybrid coil arrangements, each connecting the sending circuit of one of said magnetic ampliers and the receiving circuit of the other with one of said two line sections and uncoupling said receiving circuit from said sending circuit, a source of direct current for magnetically biasing said magnetic ampliers, and means for adjusting said biasing current to such a value as to avoid back-coupling elects within the two-way repeater device.

.13. In a duplex telegraph system, more particularly a system of composite duplex telegraphy and telephony, a metallic line circuit adapted to transmit direct current impulses in both directions, a two-way repeater device interconnected between two sections of said line circuit, said two-way repeater device consisting of two magnetic one-way ampliers, two hybrid coil arrangements, each connecting the sending circuit of one of said magnetic ampliers and the receiving circuit of the other with one of said two line sections and uncoupling said receiving circuit from said sending circuit, and one-way rectiers connected in shunt to said magnetic amplifiers, said rectifiers being so biased as to reduce the amplication factor at low amplitudes and thereby avoid back-coupling effects within said two-way repeater device.

14. Telegraphic repeater system, more particularly a wire telegraph system, including telegraph apparatus, a transformer so connected as to protect the telegraph apparatus against disturbing voltages on the line, van alternating current circuit, a key for opening or closing said circuit, rectier apparatus interposed between said transformer and the line, whereby the alternating current signals produced by operating said key will be converted into direct current signals, which are sent over the line, a choking device located at the receiving end of the line, and means for feeding said device with alternating current, whereby such device will convert the direct current signals into alternating current signals.

15. A telegraph system including a conducting line for the transmission of direct current impulses, a transmitting circuit comprising a source of alternating current and means for transmitting telegraph signals from said source of current, a transformer inductively connected with said conducting line and adapted to protect said transmitting circuit from disturbing voltages on said conducting line, rectifying means interposed between said transformer and said conducting line for converting alternating current telegraph signals transmitted from said transmitting circuit into direct current signals which are to be sent over the line, means for reconverting said direct current signals at the receiving end of the line into alternating current signals, and a receiver responsive to such reconverted signals.

16. A telegraph system including a conducting line for the transmission of direct current impulses, a transmitting circuit comprising a source of alternating current and means for transmitting telegraph signals from said source of current, a transformer inductively connected with said conducting line and adapted to protect said transmitting circuit from disturbing voltages on said conducting line, rectifying means interposed Abetween said transformer and said conducting line for converting alternating current telegraph signals transmitted from said transmitting circuit into direct currentsignals which are to be sent over the line, means for reconverting said direct current signals at the receiving end of the line into alternating current signals, a receiver responsive to direct current signals, and rectifying means interposed between said reconverting means and said receiver.

KARL ETZRODT.

WALTER vHEINRICH FiimNG.

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