US2408072A - Telephone repeater circuit - Google Patents

Description

K. s. JoHNsoN 2,408,072

TELEPHONE REPEATER CIRCUIT Filed Dec. 31; 1943 -3 Sheets-Sheet 1 LMI R A QN R 58.

' NLT /NvENroR A KS. JOHNSON vBi B. C. @UVA AT TORNEI Sept. 24, 1946.

Sept. 24,1946V Kr s. JHNsoN 'A 2,403,072-

ITEpEPHoNE REFEATE'R CIRCUIT Filed Dec. s1. 194s s 'sheets-sheet 2 IN VEN TOR ATTORNEY Y' Patented Sept. 24, 1946 TELEPHONE REPEATER CIRCUIT Kenneth S. Johnson, South Grange, N. J., assignor to-Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 31, 1943, Serial No. 516,464

16 Claims. l

e This application is in part a continuation .of my ccpending application Serial N o. 509,056, led November 5, 1943, for Telephone repeater circuits.

This invention relates to systems, as for example, telephone repeater systems, involving networks that include conjugate circuits and have a branch subject to impedance variation that disturbs the conjugacy.

An object of the invention is to maintain conjlugacy in such networks. f

In one specic aspect the invention is a system comprising a cord circuit repeater for connection.

to any chosen one of a groupof subscriber loops of various impedances, and having direct current supplied from the cord circuit over the connected loop for talking or supervisory purposes, the cord circuit repeater including therrnistor means responsive to the direct current to so adjust the impedance of a balancing circuit for the loop as to establish impedance balance between the loop and the balancing circuit.

Other objects, aspects and features of the invention will be apparent from the following description and claims.

Fig. 1 shows a system in which the invention is applied to a cord circuit repeater of the 21-type;

Figs. 2 to 6 show four-terminal networks useful in systems such, for example, as those of Figs'. 1 and '7; and

Fig. 7 shows a system in which the invention is applied to a cord circuit repeater of the 22-type.

In Fig. 1 is shown a cord circuit or connecting circuit, comprising a .2l-type repeater including amplifier I and bridge transformer (hybrid coil) 2, for connecting any chosen one of a group 3 of a considerable number of subscribers loops such as the two indicated at il and 5 with anychosen one of a group 6 of a considerable vnumber of subscribers loops such as the two indicated at 'l and :8. The loops. are shown terminated in substation sets 9, I0, I I and I2, which may each have an impedance of 60 ohms, for example.

Switching means of any suitable type, as for example plug I3 for cooperation with jacks of loops 3 and plug I8 for cooperation with jacks of 'lines 6, may serve to connect the cord circuit with loops 3 and 6. Y

The loops 3 have their impedances facing the cord circuit approximately equal. For example,

they may lbe private branch exchange loops of negligible length, or may be loops in a congested business area, all of substantially the same length.

The loops 6 may have various lengths, or substantially different impedances facing the cord circuit; and, indeed, any one may be considered to include one or more interoffice trunks of considerable lengths (switched in attimes in extending the talking connection from the cord circuit to a desired subscriber station) so that its impedance facing the cord circuit suifers substantial variation.l

The cord circuit includes a link 2| connecting hybrid coil 2 with plug I3, and a link 22 connecting the hybrid coil with plug I8. The link 22 includes supervisory relay 23 and central oice wiring represented by its equivalent resistance 2li. Two thermistcrs 25 and 2S comprise temperature-dependent resistances T11 and T (which may have negative temperatures coeii'lcients of resistance) and heater elements I-I1 and H2 electrically insulated from VT11 and T. (Thermistors are described in an article by G. L. Pearson at page 106 of the Bell Laboratories Record for December 1940.) The elements H1 and H2 are in series in the link 22. Resistance T is connected in series in the link 2I through an impedance-modifying network 3I and an impedance transformer 32. The (variable) resistance of element T is designated T and that of T11 is designated T11. The resistance presented tc the transformer 32 by the network 3I terminated rby T is designated R. The resistance inserted in the link 2| by the transformer 32 is designated r. This resistance forms the series arm of a 1r-network in the link 2|, one ofV the shunt arms including a group of condensers 33' adapted to be connected in parallel by a sliding switch 34, and the other shunt arm including a group of condensers 35 adapted to be connected in parallel by the switch 34.

The switch 34 is shown drawn to its extreme right-hand position against the tension of the spring 36, by action, on its soft iron plunger 3l, of flux due to current in its operating coil 38. This crurrent maybe space current of a tube 4I, controlled by temperature-responsive resistance T11. As described hereinafter, this current can -be reduced by heating T11. Then spring 3S can draw switch 3'11 to the left. The switch in this movement to the left, can remove from circuit the condensers 33 successively, and simultaneously remove from circuit the condensers 35 successively. In the extreme left-hand position of the switch, all of the condensers 33 and 35 will be out of circuit. Y

Direct current is 'suppliedto the substationY telephone sets connected to the cord circuit from battery 45 (of 24 volts, for example) through the connecting loops. This direct current applied over the loops to the substations may be either 3 for talking and supervisory purposes, if the substation is of the common battery type with its transmitter energized from battery Q5, or for supervisory purposes only, if the substation is of the local battery type with common battery super- Vision. For the sets connected to loops 3, this direct current can be supplied Via link 2l and the.

loop 3 to which plug I3 is connected. For the sets connected to loops 5, the direct current can be'sup-` When plug E8 is disconnected from all ofthe' loops 5, no direct current flows through heaters Hz and H1. rEhen T, R, and 1' have their maximum values. T11 also has its maximum Value, so the current flowing from battery i9 throughA resistor 5i! has its minimum Value. Consequently, the` component of negative grid bias thenfurnished for tube lll by the voltage drop across resistor due to this current in 59,' has its minimum Value; and the space current Vof tube #it flowing in coil 33 has its maximum value, so switch 3d is in its extreme right-hand position', as shown.

When the kcord circuit is connected between two loops, it is'desired'that before switch El is closed to complete the repeater circuit, the link 2l automatically be given'any adjustment re'- quired for building out the impedance of the connected lcops Btc make the impedance e2 presented to terminals 53 of the hybrid 'coil equal the impedance Z2 presented to terminals 52 of the hybrid coil or balance the impedance Z2 sufliciently well to prevent the repeater from having any undue singing tendency.

This automatic adjustment is eiected by response cf thermistor 26 and network' 3l to give r the proper value, and response of thermistor 25, tube il and switch '3d to properly adjust the capacities 33 and 35 connected in circuit.

The longer the loop 6 to which plug IB is connected, the greater willv be its resistance, the less will be the current in'Hz, and the 'greater will be T, R and 1; and the shorter the loop, the less will' be its resistance, the greater will be the current in H2, and the'less will be T, R and r. The responsiveness of T and 3l must, by proper design, be made such as to always establish and maintain lwithin the limits of tolerance the balance between the resistance component of Zz and that of e2.

In some systems, as for example, where the variation of length of the loops 6, or the range of impedance variation of the loops 6, does not eX- ceed certain limits, the adjustment of resistance balance may be adequate for preventing singing, without necessity forprovision of capacities 33 and 35; and then switches 55 may be opened, or capacities 33 and 35 and switch 34 and its control circuits may be omitted. However, their pro- Vision' enables closer `impedance balance between e2 and Z2 to be obtained, especially in systems in which the Variations in length or impedance of loops 6 are great. v

If plug I8 be connected, for example, to a loop 6 that is so long or has such high resistance and effective shunt capacity (distributed capacity) that, with switch 34 inits extreme right-hand position, the degree of balance between Z2 yand z2 is within the limits of tolerance, then the re- 'spense of T11, 4I and 34 should be insufficient to disconnect any of the capacities 33 or 35; whereas, if plug i8 be connected to a loop 6 so short as to require a given reduction of the capacities 33 and 35 in circuit in order to bring the degree of balance between Z2 and e2 within the limits of tolerance, then the response of T11, 4I and 3eshould be such as to eiect such reduction.

Network 3| may be referred to as a shaping network. It controls the shape of the characteristic (not shown) of R versus T, as discussed hereinafter. In the specic form shown in Fig. l, it ccnsists of resistances A and B; but other forms may be used, as willbe made apparent hereinafter.

Any suitable number of condensers 33 and S5 in link 2lV may be used, to obtain any desired number of steps in the capacity Variation. The individual condensers 33 and 35 may have any suitable Values, either the same or different, to obtain equal or different steps of any desired magnitudes; or any suitable form of condensers may be used as for example, well-known types of ganged air condensers (not shown) with sets of Xed-and movable plates shaped to continuously vary the capacity as desired upon relative motion of the plates.

Opening switches 55 and closing switches and 6l (with switches B2 closed) replaces vthe circuit of tube 4I by a circuit comprising series inductance L and shunt capacity C1, an oscillator or other alternating current source 53 or frequency f, thermistor SS, network 61, and source of electromotive force 69. Thermistor 6% comprises a temperature-dependent resistance Te with temperature coeicient of resistance of the same sign as that of temperature dependent resistance T11, and heater H6 electrically insulated from resistance Ts. Heater He is in series with source 63 and condenser C1, which is shunted across elements L and T11 in series. Element Ta terminates network 61. Network 61 is shown as a 1r network of resistances A, B and C. It is a shaping network for controlling the shape of the characteristic of Re versus T6, where Re designates the impedance or resistance that coil 3S faces (with switches 55 open) and Ts designates the impedance or resistance of thermi'stor element T6.

With L and C1 given such Values that 211e tout) at the oscillator frequency j or lZv] becomes proportional to the reciprocal of T11. That is, at the frequency of anti-resonance the network consisting or resistance T11 and inductance L inseries, shunted by capacity C1, has its impedance increasein magnitude as the resistance T11 decreases. Thus, increase of the heating current in H1 -causesdecrease of the resistance T11, with consequent increase of [Zf/I, decrease of heater current in Hs, increase of resistance Ts, increase y of resistance Re, and decrease ci?v the current flowing from'battery 69 through vcoil 38,.v

Opening switches 50 and 62 1 and v'closing switches 1U and 12 replaces the source 6'3 and the networkv L, Ciby a source ,13' with resistance 'I5 and a lattice or bridge network 14 comprising resistances A, B, C and D proportioned so that A/B=C/D. Then, as the heating current in H1 increases, the resistance T11 becomessmaller-as compared with A-and the lattice or bridge network 14- becomes more Vnearly balanced, so the current owing from source 13 in heater He decreases and consequently the resistance T6 increases. The source 13 may be either a direct current source or an alternating current source.

The shaping networks 3| and 61 are typical of four-terminal networks designed so that if terminated in a, variable resistance, for example aV network such for instance as that'of Fig. 3, kand three preassigned values, which may be designated R1, R2 and Rs can be obtained with a threeelement ladder network such', for instance, as network 61 of Fig. 1 vor the network of Fig. 4.

The design formulae for a network of theconfiguration of network 3| can be shown to be:

A- 2 i 2 t TVT, "R112 and Y Enna- Raw f (2,

The formulae for a network ofthe form shown in Fig. 2 may be given as:

The formulae for a lattice network of the configuration shown in Fig;` 3 are:

v(ERz) would be 342 ohms.

The formulae for a ladder network of the form shown in Fig.`4 (a `1r-network) are:

lll

The 1r-network may be replaced by its equivalent T-network with the conversion relations of Figs. 28A and 28B of Appendix D of my book Transmission Circuits for Telephonie Communication/ published by the D. Van Nostrand Company, New York.

It may be noted that although Formulae Y1 to l5 were derived on the assumption that all of the circuit elements involved were pure 'resistances these formulae are also Valid if all of the circuit elements are pure reactances of the same sign. Consequently, if A, B, C, R. and T in the formulae were all replaced by corresponding induct'ances, (In, Ln, Lc, LR, and LT) kor by the reciprocal of capacitances, (I/CA, l/CB, l/Cc, l/CR, andi/Cr), the formulae would still be valid. For example, Formulae l@ to 15 would serve if, with a variable terminating inductance Lr (instead of a variable resistance T), it were desired to design a network composed of pure inductances LA, LB and Lc such that the inductance at the input terminals, LR, would have anyv three desired preassigned values.

Specific numerical examples of designs for networks of the ladder and lattice types shown in Figs. 5 and 6 are givenfin the following table:

Fig. Re Rx R2 Rs To Tx Tn Ta A B C All the circuits of the table were designed such f that (1) `when 860 ohms resistance (=Ta) was connected across the output terminals, the input resistance (E123) would be 642 ohms, and (2) when 300 ohms resistance (=T2) was connected across the output terminals, the input resistance For the cases v1n which A, B and C are all nite, thev design also assumed T1==43 ohms and R1 was preassigned to have the Values indicated. For the other cases, R1' was calculated for T1=43 ohms, and for all cases Ro was calculated when the terminal resistance Tb wasl 0 ohms. l

It may be noted that cases 1) andv (f6-)f are -not allegare,

I'J'hysicalflyv realizable sinceeither B or C f is negative. I A

Also, it is noted that the value of Ro (=102 ohms), in the case of the three-element physically realizable network, (=Case 4) lies between the values of R1 (91 and 115 ohms) given by the two-element networks -Case 2 and 5); and likewise the value of R1 =142 ohms), in the Case 4, lies between the values of R1 v(133 and 152' ohms) given by the two-element networks (=Case Zand 5) For the circuit of Fig. 2, the table below gives the calculated values of Rand r-for three values of T produced by three-values of direct current I that flows in heater H2 for three specied values of RIA-34. RL designates the variable direct current resistance of the'loop-.circuit connected to link 22 by plug le; and'iill is the Value, in ohms, assumed for the total direct current resistance of 4.the elementsi-L 2li, H1v and H2 in series in the link 22, as indicated by the resistance values appearing in Fig. l. The direct current resistance of the substation sets is assumed to be 60 ohms as indicated in Fig.V irand I Yis calculated as 154-i-RL because, as indicated in the Fig. l, the voltage of battery 45 is Yassumed to be 24 volts, the direct current resistance of they secondary winding of the amplier output transformer is assumed to be 20 ohms, and that of the hybrid coil windings traverseduby I is assumed to be 40 ohms.

RL R11-F32 I T r 0 32 0. 160 6l 20 100 132 :096 287 96 200 232 069 615 205 `300v 332 .A053 1000 333 400 432 044 1270 417 500 532 037 1470 490 As indicated by the degree toA which) approaches RL+32 the network 3L may well be omitted as unnecessary in some cases.

Y Fig. 7 illustrates application of the invention to a system comprising 'a cord circuit repeater of ZZ-type including ampliiiers i and I' and hybrid coils 2 and 2', for connecting any chosen one'of a group 6 of Subscribers loops or trunk circuits such as the two indicated at 1 and 8 with any chosen one of a group 6 of subscribers loop or trunk circuits such as the two indicated at 1 and 8'. To make such a connection, a plug I8 at one end of the cord circuit is inserted in the cooperating jack of the chosen one of lines 6 and 8 and plug I8 at the other end of the cord circuit is inserted in the cooperating jack of the chosen one ofulines 6. The linesv and also the lines G' gmayljlaveyaripus lengths or substantially dierent impedances facing` the cord circuit.

The. cord circuit includes a balancing ne"- work or circuit N attached to the network terminals 53 of the hybrid coil 2. The network N comprises k av linkvcircuit. 8i .terminated in a dummy telephonev set or an ,impedance 88 simulating the impedance of a set such as il or l2. The link 8l .as shown-fis Vlike link 2l of Fig. 1, except that fan optional blocking condenser 85 and a-resistance, ,which are referred to hereinafter, are shown in the link 8|.

The cord'circuit includes alink -82 Vconnecting the-line terminals 520i the-hybrid coil 2 with the plug I8.- The link 8 2.as-shownis like link 22 of Fig@ 1,A except that the supervisory relay 23 and centralloiiceryvirir-ig resistance 24 are omitted land aheater element H9 of an additional ther@V mistor 90, similar to the thermistors 25 and 26, isshown in the link 82. The-thermistor 80 comprisestemperature-dependent resistance T9 heatvat ed vby jI-Ig and --electrically Y insulated from H9. Thev resistance T9 may have a negative temperature coeicient of resistance, Yfor example. It is connected,` across the input circuitofamplier l, either directly or through-ashaping network 81shownjas;oi the; same type as network 61, and controlsthe gain of that amplifier as indicated.h`ereinafter. If desired, the network 8l maybe as'haping network of the type of network 3|v,ormay be of any of the types shown in Figs. 2 tov 6,'for example.` y y The lowerffhalf of the cord circuit repeater as shown in Fig. 7 isla duplicate of the-upper half. The reference characters designating the `elements in theLfloWer ,halfv-'a-r'those designating the corresponding-elements of the upper half primed. In Fig. 7 the dotted lines connecting thermistor T11 and coil 3 8 are for indicating that the connection maybe as in vl g..l. Similarly the connection betweenTn' and 38 may include the apparatus shown in Fig. l in the connection between T11 and 38. Direct currentfrom battery 45 is supplied over theI loop 8 to which plug I8 is connected, to thesubstation of that loop, either for talking and supervisory purposes if the substation islet the common battery type, or for supervisory purposes, only, if the substation is of the local battery type withV common battery supervision. Similarly, direct current is supplied over the loop 6 to which plug I8' is connected, to .the substation of that loop. The blocking condenser is of negligible-impedance at voicefrequencies. It prevents direct current flow from battery 45 to network N. This results in a maximum variation ofr the resistance T, T11 and T9 as a function of the loop length. Similarly, condenser 85 prevents Iiow of direct current from battery G5 to network N Resistance 86 serves to counterbalance the resistance of heaters H1, H2 and H9; and similarly resistance 88 counterbalances the resistance of H1', H2 and H9. l

The gain which -it is possible to obtain from a 22-type repeater depends uponthe closeness of lthe impedance balances between (a) the line in one direction and its balancing network, and (b) the line in the other direction and its balancing network. In the case of each of the two balancing networks of a 22-type repeater such as a cord circuit repeater, the impedance of the net- Work ordinarily is given some value representing an average of the impedances of the lines it is to balance, since thenA lengths of the loops or trunks differ and-.consequently the impedance works can be so constructed that'they are effec-n tively duplicate copies of the actual lines that they must balance, regardless of the lengths of these lines. Hence, as the repeater `is employed between lines of different lengths whose impedances vary, the impedances of the correspondying networks will vary in the same way, thereby maintaining a close balance between the impedance of the lines and that of their corresponding balancing networks. This 'makes it possible to obtain much greater -gains from such a repeater` than would be possible if the networks were fixed, i. e., not variable.

v In the system of Fig. '7 this automatic balancing is done by means-of thermistors 25, 26, v25v and 2,6', which have .their heater elements H1,

'Ha H1I and H2 inserted in .series with the lines and a. battery such as V155 and 45. As .the length of the line varies, the current, or power dissipated in these heaters varies 4and the resistance values of their corresponding temperature-dependent resistance elements T11, T, T11' and T' also vary, these resistance values (with the usual type of thermistor) becoming rapidly larger as the current through the heaters becomes less or as the length of the loop or trunk becomes greater.

When the cord circuit is connected between two lines, it is desired that before switches and 5lv are Aclosed to complete the repeaterr circuit, the link 8| automatically be giveny any adjustment required for building out the impedance of 1 the connected set Btoznake the Vimpedance ez presented to terminals 53 of hybrid coil `2 by network N equal the .impedance Z2 presented to terminals 52 of the hybrid coil 2 or balance 'the impedance Zz sufficiently closely, and the link 8| automatically be given any adjustmentrequired for building out the impedance of theconnected set 8B to -make the impedance z2 presented to the terminals 53 of hybrid coil 2 by network N equal the impedance Z2 presented to terminals 52 of the hybrid coil 2 or balance the impedance Z2 suiiiciently closely, so the repeater will be prevented from having any undue singing tendency or any undue limitation on its permissible gain.

The automatic adjustment of the link 8| is effected by response of thermistor 26 and network 3| to give r the proper value, and response of thermistor 25, switch 34 and the circuit `therebetween (as described in connection with Fig. 1) to properly adjust the capacities 33 and 35 connected in circuit, so the 1r network formed by the resistance r and the capacities 33 and 35 will represent closely the actual loop or trunk to which plug IB has been connected. f

The longer the line 6 to which plug I8 is con nected, the greater will be its resistance, the less will be the current in H2 and the greater will be T, Rand r. The responsiveness of T and 3| must, by proper design, be made such asalways to establish and maintain within the limits of tolerance the balance between thev resistance component of Z2 and that of z2.

The automatic adjustment of the link 8| is similar to that of link 8|.

With the system of Fig. '7, (as with `that ofV 10 Eig. 1), in cases in which the variation of length of the lines connected to the repeater, or the range of impedance Variation of the lines, does not ex- Aceed certain limits,v the adjustment of the vresistance balance may be adequate for reducing singing tendency and permitting the desired amount of repeater gain, Without necessity for ` provision ofv capacities 33 and 35 and capacities Y33 and 35'.; and then theseA capacities and their control circuits may` be omitted. However, (as in the case of the capacities33 and 35 of Fig. 1), the provision of thecapacities 33 and 35 enables closer impedance balance between Z2 vand z2 to be obtained, and similarly the provision .of capacitiesv 33' and 35" enables closer'impedance balance between Z2 and 'zz' to be obtained.

If plug .I8 be connected, for example,v to a line that is so longor has such high capacity that,

A,with switch 34 in its extreme right-hand position the degree of balance between Z2 .and .z2 is within the limits of tolerance, then the response of Tn,

s4 and the circuit connecting them (as described in connection with'Fig. 1) should be insuiiicient Whereas,v if plug |8Abe connected to 'a line so short as to require a given reduction of the capacities 33'and 35 in circuit in order to bring the degree of balance between Z2 and z2 within the limits of tolerance, then the response of Tn, 34 and their connecting circuit should be such as to effect such reduction. As indicated above, control of capacities 33 and 35 is similar to the control of the capacities 33 and 35.

When the line 6 to which plug |8 is connected is comparatively short, the direct current from battery 45 through heater H9k will be relatively large and the resistance of the heated element T9 will be comparatively small.l Since T9 is connected across the input circuit of amplier through network 87, if desired, the output of this amplifier will be reduced and the amplier gain will be smaller than were the line longer and the resistance of T9 correspondingly higher. This is desirable because the longer the line the greater will be its attenuation and the greater will be the gain required from the amplifier in order to olset this greater attenuation and stabilize the transmissionequivalent of the circuit and the levels at the terminals. If network 81 be omitted, then when the lines 6 to which plug I8 may be connected are long, the simple `shunt bridging loss of T1 may be not such as to give the required shape to the characteristic of the variation of amplier gain as a function of the line length. In this case the required shaping can be obtained with a shaping network such as 81, and if desired a transformer such as the transformer 32 shown in connection with shaping network 3|, (as the proper variation of 1' as a function of the resistance of T is obtained with network 3| and transformer 32, and as the proper variation of the current through coil 3S as a function of the resistance of Te is obtained with the aid of network 61, for eX- ample). As indicated above, the control of the gain of amplifier by thermistor is similar to the control of the .gain of amplifier by ther- K mistor 95.

In the systems of Figs. 1 and 7, the substations or telephone sets may be of any suitable type. If desired, I2, and I2 may be anti-sidetone telephone sets of any of the types disclosed in my copending application Serial No. 463,184, led October 24, 1942, entitled Telephone system, having the balancing network in the set automatically adjusted in accordance with the length or impedance of the line to which the set is connected.

What is claimed is:

l. A wave translating system comprising a line impedance which may have different values, two circuits, electric wave amplifying means having an amplier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit for said line impedance and a line circuit for two-way communication with said line impedance through said two first-mentioned circuits and said amplifying means, means comprising said balancing circuit for said line impedance for connecting saidv two first-mentioned circuits in energy transmitting relation to said line impedance and in conjugate relation to each other, and thermistor means responsive to change in the value of said line impedance for changing the impedance of said balancing circuit in substantially the same ratio to maintain conjugacy of said two first-mentioned circuits.

2. Awave translating system comprising a line impedance which may have different values, two circuits, wave amplifying means having an arnplifier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit for said line impedance and a line circuit for twoway communication with said line impedance through said two iirst-mentioned circuits and said amplifying means, means comprising said balancing circuit for said line impedance for connecting said two first-mentioned circuits in energy transmitting relation to said line impedance and in conjugate relation to each other, impedances comprising a temperature-dependent resistance in said balancing circuit adjustable for producing changes of the impedance of said balancing circuit corresponding to changes occurring in said line impedance to maintain balance of said line impedance by said balancing circuit, and means comprising a heating element for said temperature-dependent resistance responsive to said changes in said line impedance for effecting said adjustment of said adjustable impedances.

3. A wave translating system comprising lines of different resistance, two circuits, wave amplifying means having an amplifier input circuit connected to one of said two first-mentioned circuits and an amplier output circuit connected to the other of said two rst-mentioned circuits, means providing a balancing circuit and a line circuit for two-way communications with any selected one of said lines through said two rstmentioned circuits and said amplifying means, said balancing circuit being adapted to balance the impedance of the selected line, means comprising said balancing circuit for connecting said two rst-mentioned circuits in energy transmitting relation to the selected line and in conjugate relation to each other, means for producing direct current dependent in magnitude on the magnitude of the resistance of the selected line, and means comprising a variable resistance in said balancing circuit responsive to said direct current for rendering the magnitude of said variable resistance a function of the magnitude of said direct current.

4. A wave translating system comprising lines of different resistance, two circuits, wave amplifying means having an amplifier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other,

means providing a balancing circuit and a line circuit for two-way communication with any selected one of said lines through said two iirstmentioned circuits and said amplifying means, said balancing circuit being adapted to balance the impedance of the selected line, means comprising said balancing circuit for connecting said two first-mentioned circuits in energy transmitting relation to the selected line and in conjugate relation to each other, means in one of said two first-mentioned circuits for supplying direct current to the selected line, and means responsive to said current for controlling the resistance of said balancing circuit.

5. A wave translating system comprising lines of different resistance, two circuits, wave amplifying means having an amplifier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit and a line circuit for two-way communication with any selected one of said lines through said two firstmentioned circuits and said amplifying means, said balancing circuit being adapted tobalance the impedance of the selected line, means comprising said balancing circuit for connecting said two iirst-mentioned circuits in energy transmitting relation to the selected line and in conjugate relation to each other, said balancing circuit cornprising a four-terminal network having shunt capacity and series resistance, means for producing direct current in the selected line dependent in magnitude on its resistance, and means responsive to said current for varying said capacity and said resistance.

6. A telephone transmission system comprising a group of subscribers circuits of different resistances, a group of subscribers circuits of approximately a given resistance, a 21-type repeater having two pairs of terminals, a 7r-network comprising adjustable shunt capacities and a temperature-dependent resistancen for connecting one of said pairs to any chosen one of said second group of circuits, means for adjusting said capacities, temperature-dependent resistance for controlling said adjusting means, a source of direct current for energizing said subscribers circuits to condition them for operation, current responsive heating means for said temperaturedependent resistances, and a circuit comprising said heating means for connecting said other pair of terminals to any chosen one of said i'lrst group of circuits with said heating means in series with that chosen circuit and said source.

7. A wave translating system comprising a line impedance whose resistance component may have diierent magnitudes, two circuits, wave ampliiying meanshaving an amplifier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit for said line impedance and a line circuit for two-way communication lwith said line impedance through said two first-mentioned circuits and said amplifying means, means comprising said balancing circuit for said line impedance for connecting said two first-mentioned circuits in energy transmitting relation to said line impedance and in conjugate relation to each other, means for producing direct current dependent in magnitude on the magnitude of the resistance component of said line impedance, said balancing circuit comprising a four-terminal network terminated in a variable resistance, and means comprising said variable resistance `responsive t0V variation of network has different preassigned values-'for y given values of said variable resistance.

8. A wave translating system comprising a line impedance whose resistance component mayhave different magnitudes, two circuits, wave amplifying means having an amplifier input circuit vconnected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit for said line impedance and a line circuit for two-way communication with said line vimpedance through said two first-mentioned circuits and said amplifying means, means comprising said balancing circuit for said line impedance for connecting said two first-mentioned circuits in energy transmitting relation to said line impedance and in conjugate relation to each other, means for producing in said line impedance a current depending in magnitude on its resistance `component, said balancing circuit comprising a fourterminal network terminated in a temperaturedependent resistance, and a heating element for said temperature-dependent resistance responsive to said current in said line impedance, said network consisting of a plurality of resistances of such values and circuit configuration that the ratio of said temperature-dependent resistance to the input resistance of said network has different preassigned values for given Values of said temperature-dependent resistance.

9. A wave translating system comprising a line impedance whose resistance component may have different magnitudes, two circuits, wave amplifying means having an amplifier input circuit connected to one of said two circuits and an amplifier output circuit connected to the other, means providing a balancing circuit for Said line impedance and a line circuit for two-way communication with said line impedance through said two first-mentioned circuits and said amplifying means, means comprising said balancing circuit for said line impedance for connecting said two first-mentioned circuits in energy transmitting relation to said line impedance and in conjugate relation to each other, means for producing in said line impedance a current depending in magnitude on its resistance component, said balancing circuit comprising an adjustable impedance, means for adjusting said adjustable impedance, a four-terminal network connected to said adjusting means and terminated in a temperature-dependent resistance for controlling said adjusting means, and means responsive to said current in said line impedance for controlling said temperature-dependent resistance, said network consisting o-f a plurality of resistances f such values and circuit conguration that the ratio of said temperature-dependent resistance to the resistance of said network facing said adjusting means has different preassigned values for given values of said temperature-dependent resistance.

10, A wave translating system comprising lines of diierent resistance, another line, a link circuit for establishing connections of said other line with any selected one of said first-mentioned lines, a two-way repeater in said link circuit having an amplifying path for amplifying waves transmitted from the selected line to said other line and an amplifying path for amplifying waves transmitted from said 'other' line to the Yselected line, a source of electromotive force in said link circuit for supplying direct current .to the selected line, and means responsive to said direct current for increasing and decreasing-the transmission efliciency of one of said paths upon increase anddecrease, respectively, ci the resistance of the selected line by change of the line selection. Y v i 11. In a telephone system, a plurality of subscriber lines of different resist-ences, a connecting circuit for establishing connections with said lines, a two-way repeater invsaid connecting circut having an amplifying path for receiving and amplifying waves ircm'said lines and an amplifying path for amplifying waves andtransmitting the waves so amplified to said lines, a source of electromotive force in said connecting circuit for supplying direct current to the line with vwhich connection is established, means comprising a resistance of high temperature coefficient in said first path, and av heating element therefor electrically insulated therefrom included in said connecting circuit and responsive to the direct current supplied to the line. Y t

12. A wave translating system comprising a line, two circuits, means comprising a balancing circuit for said line fo-r connecting said two circuits in energy transmitting relation to said line and in conjugaterelation to each other, said bal- Aancing circuit comprising an adjustable impedance, means for adjusting said impedance, a circuit connected to said adjusting means and comprising a resistance of negative temperature coeincient for controlling said adjusting means, a heating element for said resistance electrically insulated therefrom, a second resistance of negative temperature coeiicient, means for producing decreasing current in said heating element in response to decrease of said second resistance, and a heating element for said second resistance electrically insulated therefrom and responsive to current in said line for controlling said second resistance.

13. A system for'transrnitting current changes comprising an input circuit, an output circuit, means for producing variable input current in said input circuit and means for producing correspondingly variable output current in said output circuit, said last means comprising a resistance of high temperature coefcient connected in said output circuit, a heating element therefor electrically insulated therefrom, a second resistance of high temperature coeicient of the same sign as the rst coefficient, a heating element for said second resistance connected in said input circuit, a source of constant voltage for supplying heating energy to said first heating element, and a circuit comprising said second resistance connecting said source to said first heating element to supply energy of said source thereto and responsive to magnitude change of one sign in current in said second heating element for producing magnitude change of opposite sign in current in said rst heatingelement.

14. A control circuit comprising a resistance of high temperature coefficient, a heating element therefor electrically insulated therefrom, a second resistance of high temperature coeiiicient of the same sign as the first coefcient, a heating element for said secondvresistance electrically insulated therefrom, a source of electromotive force for supplying energy to heat said first heating element, and means comprising said second resistance connecting said source to supply heating energy to said first heating element and responsive to change of current of one sense in said second heating element for producing current change of opposite sense in said iirst heating element.

l5. A system comprising a temperature-,dependent resistance, a heating element therefor electrically insulated therefrom, a second temperature-dependent resistance having its temperature coecient of resistance of the same sign as that of said rst resistance, a heating element for said second resistance electrically insulated therefrom, a source of electromotive force for supplying energy to heat said first heating element, means comprising said second resistance and having variable transmission eiciency connecting said source to said rst heating element to supply energy of said source thereto and ren sponsive t0 change of current of one sign in said second heating element for sov varying said transmission efficiency as to produce current change of opposite sign in said rst heating element, and

means comprising a second source of electrometive force in circuit with said second heating element adapted to produce therein current change of magnitude sufficient to produce temperature change of said first resistance large compared to its maximum temperature Change produced by ambient temperature.

16. In combination, a resistance of high temperature coefficient, a heating element therefor electrically insulated therefrom, a second resistance of high temperature coeflicient of the same sign as the rst coeflicient, a heating element for said second resistance electrically insulated therefrom, av source of electronic-tive force of given frequency, and a circuit comprising in series said source, said first heating element and a network Vtuned to approximately said frequency, said network comprising a capacity and in parallel therewith said second resistance and an inductance in series.

KENNETH S. JOHNSON.

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