US2003967A

US2003967A - Signaling and power supply system

Info

Publication number: US2003967A
Application number: US625610A
Authority: US
Inventors: Charles W Green; Harold S Black
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1932-07-29
Filing date: 1932-07-29
Publication date: 1935-06-04
Anticipated expiration: 1952-06-04
Also published as: FR754720A; DE652917C; GB416866A

Description

C. w. GREEN Er Al.- SIQNAL'ING AND PowER SUPPLY SYSTEM June 4, 1935;.

Filed July 2a, 1932 4 sheets-sheet i l 5 immun ux E @Elan uw o om. .Al

H .BLACK BV A TTORNE V June 4, 1935 c. w. GREEN Erm. 2,003,967

SIGNALING AND PQWER SUPPLY -SYSTEM I Filed July 29, 1932 4 Sheets-Sheet 2 /NTERMED/A TE EUlML/ZE'? WEST bWGkEE/v NI/ENTORS has-BLACK A TTOR/VE V June4, 1935. C W,REEN ETAL 2,003,967

SIGNALING AND POWER SUPPLY SYSTEM Filed July 29, 1932 4 Sheets-Sheet 3 T0 OTHE INVERTERS INVERTER .CWGREE/v NVENTORS'HsLAc/f A TTORNEV CZWGREEN HSBLACK A rToR/VEV June 4, 1935.

@wma u: E MGP* l INWNTORS f Patented June 4, 1935 UNITED `STATES SIGNALING AND POWER SUPPLY SYSTEM Application July 29, 1932, Serial No. 625,610

8 Claims.

The present invention relates Yto power supply for space discharge repeaters or other apparatus in a telephone `system or similar system.

The invention relates `specifically to provisions ".5 for energizing repeaters vin unattended repeater stations and will be described in connection with sucha system, although it is capable, in certain respects at least, of other uses as will appear from the scope of the claims.

Repeater stations on voice frequency cables are spaced about fty miles apart. When high frequency carrier channels are superposed on the cable conductors, it is necessary or desirable to use a closer repeater spacing, for example, half of the fty-mile span, or one third, or even less, depending upon conditions. With these closer repeater spacings, it is uneconomical to duplicate at each repeater point the equipment that is provided at the iifty-mile repeater stations. Such equipment includes a permanent building with normal and stand-by sources of power, testing equipment, etc., and requires one or more attendants continuously.

'I'he invention contemplates an intermediate repeater station comprising a minimum of apparatus-y requiring .no continuous attendance at the station, and capable of functioning under both the conditions of normal and emergency power supply.

An object of the invention'is an economical and thoroughly reliable power supply for line repeaters or the like located at a distance from a terminal or main repeater station.

. Aside from apparatus failure (such as failure of tubes, etc.) with which the present invention is not specifically concerned, the principal emergency condition to provide against in an unat tended repeater station is failure of the power supply, either the plate supply or the cathode heating supply. An interruption of the plate voltage supply, for example, practicallyV immediately interrupts the service. The plate voltage could be maintained by a floating storage battery, but such a storage battery located Vat an unattended repeater station would be both bulky and unreliable if unattended for long periods of time. T'he present invention uses a voltage source at the main repeater station .as the plate supply for the tubes at an unattended repeater station and supplies this current over the telephone lines. This source includes a floating battery which insures against interruption of plate voltage at the unattended repeater station.

The cathode heating power requirements are rather heavy especially where heater type tubes PATENT OFFICE 4are used. It would be expensive to supply this current continuously from the main station over the telephone circuits. The invention provides 'for a local supply of the relatively large heating current, power required, and as an emergency supply to bridge over power failures in such 1ocal supply, provides for sending an increased amount of current over the telephone lines themselves from the main station.t The thermal capacity of the cathodes permits the switch-over from regular to emergency power supply to be made without any interruption in service, even though the power failure should occur inthe `midst of a conversation. Since the time that the local power is off probably aggregates only onefor a very few hours a year, the expense item in send; ing the cathode heating `power over 'the telephone line is practically confined to the cost of the emergency apparatus involved, which is small because it can be made common to a large'number of lines.

A typical system to which the invention might be applied is a telephone toll cable providing twenty-four carrier speech channels per pair in the frequency range 8 to -100 kc. Theconnections for superposing direct current on the pairs in multiple might appear, at first sight, to produce a prohibitive amount of cross-talk. The use of small and inexpensive choke coils in the direct current' connections to lthe individual lines, however, aids in keeping the cross-talk dueto this cause down to practical limits.

A better understanding of the invention will be had fromthe following detailed descriptiony of a typical embodiment as represented in -thefaccompanying. drawings in which:

Figs. 1 and 2 are simplied diagrams showing two types of connections that may be employed with the invention, between terminal or main repeater stations and an unattended repeater station, while 1 Figs. 3, 4 and 5, when placed together .with Fig. 3 at the top, and Figs. 4 and 5 underneath Y in the order named, show in greater detail the power supply circuit for an eastward repeater and a westward repeater on an east line and west line, respectively, these being representativeof a group of repeaters and lines, the circuits of which are arranged similarly to those shown.

Referring to Fig. l, west, intermediate and east stations are shown operating on a four-wire basis with all of the west-to-east lines grouped in one cable CE and with all of the east-to-west lines grouped ina separate cable Cw. It is assumed that each of these `cables contains one hundred pairs, although the invention is not dependent upon the use of any particular number. It is also assumedthat each pair of lines may be multiplexed to transmit a number of voice modulated carrier waves, for example, twenty-four carrier waves eX- tending from about 8 kilocycles to 100 kilocycles.

'I'he west and east stations may be terminal stations but it will be assumed in the present description that they are repeater stations 'of the attended type and that the intermediate station is unattended. At each of these repeater stations there will be one repeater per line,A designated for convenience A1 to A100, of which only two,

A1 and A100, are illustrated at each of the repeaterY 'stations but is shown at Ill for the intermediate station.

The plate supply for the intermediate repeater is derived from rectifier units II and I2 located respectively at the west and east stations. These rectifier units supply rectied and iitered current over the line Wires from the attended repeater station to the unattended station. In the case of rectifier unit I I, this current, for example, extends from the positive terminal of the rectier, lead I4, choke coils I5, I5 in parallel, one for each line, the 100 pairs of line wires in the cable CE in multiple to the intermediate station; retard coils I6, I6, one for each line in multiple, bus-bar I'I, the plate circuit of each of the amplifiers A1 to A100, inclusive, in the west-to-east cable at the intermediate station over supply leads I8, I8, etc., and I9, I9, etc., bus-bar 20,. retards 2l, 2| in parallel, the 100 pairs of lines in the cable Cw,

.

retards

22, 22 in parallel and back through lead I4 to the minus pole of the rectifier unit II. Thus it is seen that the 4anode voltage for all of the repeaters at the intermediate 'station in the west-to-east cable is supplied from the rectifier unit I I of the west station.

In a similar manner, the plate voltage for all of the repeaters at the intermediate station in the east-to-west cable Cw is supplied from the recti- Iier unit I2 at the east station. The circuit for supplying the latter is entirely analogous to that previously traced and comprises the positive pole of rectifier unit I2,

retard coils

23, 23 in multiple, the 100 pairs in the cable Cw to the intermediate station,

retard coils

24, 24 in multiple, to the plate circuits of vall of the 100 repeaters A1 to A100, all of the

retards

25, 25 in multiple, the 100 pairs in the cable CE in multiple, retard coils 26, 2B in multiple and so back to the minus pole of rectier unit I2.

As already stated, the cathode heating current for all of the repeaters at the intermediate station, is supplied locally from the' supply mains IQ. In case of failure of this local supply the two

inverters

21, 28 at the intermediate station are brought into play to supply the cathode heating current from the east and west repeater stations, respectively, as will be more fully described hereinafter.

In the alternative arrangement in Fig. 2, the west-to-east lgroup of lines and the east-to-'west group of lines are all included'within the same cable sheath, and suitably shielded from each other. The lines within the cable are operated on a four-wire basis the same as in the case of Fig. l, but the oppositely directed lines are now included in the same cable instead of in two separate cables as in Fig. l. Assuming a 100 pair cable there kwill be 5G lines transmitting West to east and 50 lines transmitting east to West with 50 east repeaters and 50 west repeaters at each station. Except for this, the circuits are identical with those described in Fig. 1. In the circuit of Fig. l all of the lines within one cable will be of the same direct current potential, whereas in the circuit of Fig. 2 a difference of potential equal to the rectier voltage will exist between the east lines and the west lines as groups.

Referring to Figs. 3, 4 and 5 together, the identity of several of 'the elements with those shown on Figs l and 2 will be recognized from the use of similar reference characters. With Fig. 4 placed above Fig. 5 and Fig. 3 placed above Fig. 4, it will be noted that the apparatus is aligned in vertical columns, the elements at the left of each of these iigur'es being located at the West station (as in Fig. l), while the remainder of the apparatus is located at the intermediate station, the separation being indicated by the use of dotted lines in the circuits interconnecting these two groups of apparatus. For simplicity in Figs. 3, 4 and 5 the apparatus at the east station has been omitted. v

In Fig. 3 two of the amplifiers A1 and A100 for the west station are shown connected by the two corresponding' west-to-east lines with repeaters A1 and A100 of the intermediate station, A1 at this .station being shown in detail (at 300) as will be explained later.

The positive lead I4.connected to the retard coil I5 of Fig. 3 extends in Fig. 4 to the positive terminal of the rectierk I I.v There is also indicated onFig. 41 the inverter. 21 of the intermediate station.

In Fig. 5 a diagrammatic showing is given of inverter 28 of the intermediate station together with two of the ampliers at that station A1 and A100, connected by a corresponding east-to-west line with-the repeaters A1 and A100 of the west station. Retard coils 22, 22 are shown connected to the west terminals of these lilies and to lead I4 extendingto the negative pole of the rectier II of Fig. 4.

All of the repeaters A1 to A100 of the intermediate station may be alike so that only one amplifier 360 of Fig. 3 has been illustrated in detail. Each of these repeaters or ampliers is assumed to be designed to transmit the frequency range .8 to 100 k..'c. comprising, for example, 24 speech channels each of about 3000-cycle width with a spacing of about one kilocycle between channels. While any suitable vtype of amplifying repeater might be used in this system, it is preferred to use a type of amplifier disclosed and claimed in U. S. application for Letters-Patent of H. S. Black, Serial No. 606,871 led April 22, 1932, that illustrated being similar to the circuit of Fig. 65-A of the Black application. For ease in comparison similar reference characters will be used in describing this amplifier to those used in the Black application referred to.

This amplier is shown as comprising two pentode tubes 30|' and 302' in cascade leading into a third tube 303 of the coplanar grid type. This amplier is provided with a negative feed-back generally indicated at 29, between its nal output and itsJ initial input. The feed-back includes an equalizerSO for giving the amplifier circuit as a whole a characteristic such as to compensate for the unequal attenuation ofthe line over the frefluency range transmitted. The, amplifier may, for example, have a gain throughout the frequency-range from 8 kilocycles to 100 1nilocycles of the order of 100 db. without any feed-back, while with the negative feed-back employed the gain may be reduced at 8 kilocycles to the order of 20 db. or less and at ,100 kilocycles to the' order of 60 db. As pointed out more fully inthe Black application, this reduction in the gain greatly increases thestability of the ampliiier and also greatly reduces modulation or, in other words, improves the quality. The 40 db. difference between the 8 kiloeycle and 100 kilocycle gains with the feed-back is obtained by means of the equalizer 39.

The tubes 30| and 302' are Apreferably Western Electric Company type 7592-A having an amplification factor of approximately 1000 and a plate impedance of approximately '100,000 ohms. The tube 303 is preferably of the general type disclosed by H. A. Pidgeon and J. O. McNally in their copending application, Serial No. 368,647 led June 5, 1929, which has become Patent No. 1,923,686 or in their copending application, Serial No. 542,252 filed June 5, 1931, which has become Patent No. 1,920,274 or in a paper by Pidgeon and McNally in the'Proceedings of the Institute of Radio Engineers, volume 18, pages 226 to 293, February 1930.

It will be noted that the various points of the circuit of the ampliner 300 that would ordinarily be grounded are in this instance connected to the lead I9 which is in turn connected to bus-bar leading to the return path through the east-towest line and eventually to the west station rectier II, negative pole. A direct circuit to ground at the intermediate repeater point for any of the leads of the ampliner 300 is avoided since the anode voltage supply circuit is a full metallic circuit over the transmission lines of the West and east cables, respectively. Conductor I9 may, however, be taken as a point of reference for the various voltages existing in the amplier 300 at the intermediate repeater station.

One reason for omitting ground connections at the repeater points is to prevent trouble caused by electrolysis currents flowing back over the cable pairs. These currents might be quite large and, in some cases, might become so high'as to blow the amplier fuses. Another reason for omitting these grounds is to prevent danger to workers on the line, since without the ground ccnnections there is no possibility of a shock between ground and a cable pair. v

Various condenser-resistance combinations CR, CR., etc. are shown connected between the direct current supply leads and ground, to cooperate with the retards I5, I6, 2|, 22, etc., in reducing cross-talk. These resistances are so high as to amount merely to direct current leak paths and are not grounds in the sense discussed in the next preceding paragraph.

The cathodes of tubes 30| and 302 are connected to conductor I9 through resistors 343 and 353, respectively, each of which is bridged by a condenser 366 or 339 which serves as a by-pass for alternating current. The series resistances mentioned serve to provide bias for the operating grids. Similarly, the lament of tube 303 is connected to conductor i9. Plate potential is supplied from conductor I8 to the anodes of the three repeater tubes through resistors 368, 31| and the primary of output transformer 3 I 8. The condensers 340-and 350 .aii'ord an alternating` cur-` rent by-pass to the common conductor I9. `The screen grid of each pentodeis connected through a

corresponding resistance

360 or 36| to the anode supply lead I8 and shunting condensers 361, 310,

respectively are used to by-pass alternating cur; rent around these resistances. f r

The three stages are interconnected byvmeans of impedance type of couplings through the series condensers 354 and 365, respectively. "The plate supply for pentode 30 l. from resistance 368 asY described continues. through anetwork 3| .which together .with the other elements of the circuit including a similar network 32 for pentode 302 is designed to give the amplifier circuit as a Whole the proper gain and phase shift characteristic. One of the two grids of tube 303 is biased negatively frombattery 332 connected throughresistances 333, 3311, the former of which is shunted by condenser 312 so that it does not form a part of the coupling impedance between .the last two stages. The other grid of the coplanar tube 303 is biased positively by battery 335 connected through series resistance 336 and shunted. by capacity 313 for by-passing alternating current. With the high amplification' and large values of feed-back employed in this amplifier, the absolute value of the gain and the phase shift should be carefully controlled to avoid danger'of singing. The various circuit elements shown in the amplier are suflcient and capable with proper choice of constants to give satisfactory results. For a more detailed description of the theory of operation and design reference should be hadto the Black application above referred to. As is explained more fully in the Black application, supra, the equalizer 30, being in the feedback circuit rather than in the direct transmission path, has a characteristic similar to that of the line which is to be equalized rather than the inverse of the line characteristic as Wouldbe-the case if the equalizer were located inthe direct transmission path.

Cathode heating current for all of the cathcdes of this amplier is supplied from the transformer 33 connected to leads 34. f In Fig. 4 the rectifier |I may be any suitable type of rectifier circuit such as a full waverectiner for supplying the proper direct-current volt-.- age from its output. The rectiiier illustrated comprises tubes 35 and 36 of space `discharg'fe type connected by transformer 31 with the alternating current main 38. It will be assumed that this rectiiier as a whole develops a terminal directcurrent voltage 'ofy the order of 200` volts. A battery 39 is floated across the output'of the rectifier. This rectier circuit may also be equipped with any desired amount of lte'ring comprising series impedance and shunt impedance for delivering a smooth direct current. Aunidirectional device such as a copper oxide rectifier is shown at Mlfor enabling battery 39 to discharge into the load circuit but for limiting the current through the battery'when the rectier voltage is increased in a manner to be described. The inverter 21 shown on Fig. 4 is an adaptation to the purposes of the present invention of the Thyratron inverter known in the prior art and described. for example, in Electronics Vfor April 1931, pages 581 and 582, in an article by Baker, Fitzgerald and Whitney entitled Electronv tubes in industrial service (McGraw-Hill Publishing Company, lNew York, N.,Y.). When direct current voltage is applied tothe terminals of this inverter, the circuit' is set into oscillation and is caused to produce alternating current in its output circuit, the frequency of which is determined by the constants of the circuit. In the present case, when the alternating current supply I at the intermediate station fails, some of the direct current energy sent over the. line is applied to the inverter 21 in a manner that will be described more fully hereinafter and is converted by the inverter into alternating current which is applied to the cathode heating circuit of the amplier at the intermediate station. The number of amplifiers which one inverter 21 is capable of supplying will depend, of course, upon the power required for the amplifiers and the rating of the inverter tubes, but it will be assumed for present purposes that inverter 21 supplies cathode heating current to sixteen amplifiers. A bank of six inverters like 21' will, therefore, be required for each one hundred amphers and a small inverter unit may be provided to take care of the additional four amplifiers. To guard against possible power failure of an inverter, each bank of inverters is provided with a spare inverter, one of which is shown at 4|.

The equipment shown diagrammatically on Fig. 5 may be a duplicate of corresponding apparatus described in connection with Figs. 3 and 4 and `hence has not been illustrated in detail. The

nature and purpose of this apparatus will be made clear from the description of the operation to follow.

Operation Each of the West-to-east signaling lines of which two are shown in Fig. 3 may be assumed, as already stated, to be transmitting normally message waves of the frequency range 8 to 100 kilocycles. This band of waves in the case of each line is repeated at eachl of the repeater stations, two of which are shown in Fig. 3. All the repeaters or amplifiers at the various repeating stations may be identical with that shown in detail at 30D in Fig. 3 or they may be of any other suitable type. At the same time the east-to-west lines, two of which are shown in Fig. '5, transmit a similar range of frequencies in the westward direction which are similarly amplified at the various repeater stations. Y

Cathode heating current for the tubes of arnplier 300 as well as for each of the other amplifiers at the intermediate repeater station is supplied from mains I0. In the case of the amplifiers of Fig. 3 the circuit is from source I0, leads 42, normally closed back contacts of relay 43, leads 34, transformer 33 to the cathode heating circuit of amplifier 300 and through a transformer similar to 33 for each of the other amplifiers of the West-to-east group. Circuit 44 similarly supplies iilament heating current to the amplifiers of Fig. 5.

Rectifier II at the west station, Fig. 4, normally supplies a direct current voltage of about 200 volts to the leads I4, I4. The circuit by which this voltage is appliedto the anodes of the tubes of the west-to-east group of repeaters at the intermediate station, Fig. 3, has already been traced and extends through the retard coils I5 in parallel, all of the 100 (or 50 in the case of a circuit layout of Fig. 2) lines transmitting west to east, retards I6 in parallel, conductor I1, conductors I8 in multiple, one for each repeater, the space paths of all of the repeater tubes in parallel, conductors I9 in parallel, one for each repeater, conductor 20, retard coils 2| in parallel, the group of east-to-West lines (Fig. 5), the group of retards 22 in parallel back to conductor I4' and rectifier I I. It will be assumed that with 200 volts developed by the rectier the voltage actually available at the intermediate repeater point is 172 volts.

By an entirely analogous circuit as was explained in connection` with Fig. l, the east-towest group of repeaters in Fig. 5 are similarly supplied with plate voltage from the east station, one side of the circuit comprising the east-towest lines in parallel and the other side comprising the west-to-east lines in parallel between the intermediate station and the east station.

Bridged across conductors 42 of Fig. 4 are two relays 45 and 46 operating on alternating current and having marginal characteristics such that under normal voltage supply conditions, the armatures of both relays maintain their contacts open. In case of an abnormal condition in the atlernating current supply circuit IIJ such as an interruption of service, an over-voltage or an under-voltage condition, one or other of relays 45, 46, causes its armature to close its contacts. For example, relay 45 releases when the supply is interrupted or falls below a predetermined value, closing its back contacts. Relay 46 energizes if the voltage increases beyond a predetermined limit and closes its contacts. In either event a circuit is closed for relay 41 extending from conductor I1, winding of relay 41, one or the other of the contacts of relays 45, 46, to conductor 20 so that some of the direct current voltage projected over the lines from the west repeater station is applied to relay 41 causing it to operate. In operating, relay 41 connects the direct current supply conductors I1 and 20 directly to the direct current input leads 48 of the inverter 21. The Thyratron tubes of this inverter have heater type cathodes which, up to the moment under discussion, have been supplied with cathode heating current from leads 34 through branch circuit 49, thus holding the inverter tubes in readiness for immediate operation. At the moment when the direct current voltage is applied to terminals 43, therefore, the inverter 21 becomes operative to produce at its output terminals 50 alternating current of the same voltage and frequency as is normally supplied from circuit I0, for example, 55 volts, 6D cycles. The

presence of this alternating current in leads 50 causes operation of relay 5I. Relay 43 is at this time operated, it having been energized at the same time that relay 41 was energized, the circuit extending from conductor I1 through lower contacts of relay 41, Winding of relay 43 and upper contacts of relay 41 to conductor 20. With relays 43 and 5I both energized the alternating current produced across the output 50 of inverter 21 is applied directly to conductors 34 leading to the cathode heating circuit of the tubes of repeater 300 and the other West-to-east repeaters grouped with it, assumed in the present case to comprise sixteen repeaters supplied from inverter 21. 'Ihe remaining ones of the eastward repeaters are similarly supplied from other inverters which may also be connected to leads 42, these leads being shown as extending above inverter 21 in Fig. 4. It will be understood that the direct current supply leads I1 and 20 are also multipled to these other inverters by leads not shown in the drawings.

The foregoing operation takes place as stated for either interruption of supply in circuit I0 or for abnormal over-voltage or abnormal undervoltage and by an entirely analogous procedure inverter 28 and the other inverters associated with it continue the supply of cathode heating current to each of the east-to-west repeaters A1 to A100 of the intermediate station, Fig. 5. The cathodes of the tubes in all of these repeaters have sufficient heat capacity to bridge over momentary interruption of cathode heating .supply current without interrupting service.

When the inverters 21 were brought into operation by relay 41 as described, a considerable load was added to the normal load across conductors 1 and 2U and-therefore was put on the rectifier I I at the west station. Iffno increase were made in the voltage supplied by rectier Ii the plate voltage of the repeaters at the intermediate station may be assumed to have fallen from its normal value of 172 volts to much below 150 volts. As has been described, theirepeaters, such as repeater 300; are designed to have a highdegree of stability sothat a decrease of plate voltage to the order of |58 volts has but little, if any, effect on the operation of the repeaters and no interruption of service results.Y Rectifier circuit I I, therefore, lis designed to supply an increased output voltage under the abnormal conditions of operation that have beendescribed, suicient to provide a plate voltage of 150 volts. The` rectifier includes marginal relay 53 and relay 54 which Vare normally unoperatedbut are caused to operate when the inverters -at the intermediate station are brought into operation with consequent increase in drain on the rectifier circuit.` When relay -53 7operatesit closes the circuit for relay 54 causing the latter to shift its armature from its upper to its lower Vcontact and thus to4 increase the step-up-voltage ratio of transformer 31 by including a smaller number of turns in the primary. For example, the rectier output voltage may be increased from the normal value 200 to the order of 285 volts, thus maintaining the repeater anode voltage lat 150 volts at the intermediate station.

When the power supply conditions at the intermediate repeater return to normal, the circuits are restored to their normal condition by restoration of relays 45 and 46 which control the connection of inverter 21 through the agency of relays 41 and 43; the restoration of the inverters in turn causing release of relays 53 and 54 in rectifier circuit |I at the west station. Relay 54 in releasing restores the rectier voltage to its normal value.

The group of inverters 21 comprising the order of six or seven inverters for supplying emergency cathode heating current to the 100 west-to-east repeaters at the intermediate station, as stated, are provided with a spare inverter 4| which is brought into service in case of failure of any one of the inverters 21. For this purpose, the

leads

52 and 56 are multipled to the other inverters of the group. Under abnormal power conditions all of the relays 41 for the group of inverters are operated simultaneously as are also all of the relays 43. Whether or not any one relay 5I operates depends upon whether the corresponding inverter 21 develops alternating current across its output terminal 50 suiricient to operate the relay 5I. If relay 5| remains unoperated after corresponding relays 41 and 43 operate, a circuit is closed for relay 55. This circuit is traceable from upper contact of relay 41, winding of relay 55, lower armature and front contact of relay 43, and back contact of the relay 5| in question to the lower contacts of relay 41. The lowermost armatures of all of the relays 43 of the group of inverters 21 are multipled by means of conductor 56 so that the circuit above traced for relay 55 is closed in case any of the relays 5I remains un- .operated in thev bankfof inverters. Relay 55 in operating applies thedirect current voltage from leads I1 and 20 to the direct current terminals 51 of. the spare inverter4| causing this inverter to develop alternating current of the proper voltage and frequency across its output terminals. Relay 58 is'energized when relay 55 attracts its armatures. Relay 58 in its normal unoperated condition maintains the cathodes of the spare inverter 4I hot by supplying current from conductors 34; When energized relay 58 supplies tothe cathodes of the inverter 4|, the necessary ilament heating current developed in this inverter. The alternating current developed by the spare inverter 4I is also applied to conductors 52 which are multipled to the back vcontacts of all of the relays 5| in the bank of inverters. The alternating current output for the spare inverter is therefore over conductors 52 and back contacts of relay 5I of the particular inverter which fails to operate and thence over the front contacts of corresponding relay 43 to output circuit 34.

If all of the inverters 21 in a bank are operative when the corresponding relays 41 are energized, relay 55 remains unoperated. If one of the inverters 21 fails to develop enough alternating current power to cause its relay 5| to energize, thus causing relay 55 to attract its armatures, it will be noted that a locking circuit is closed through the extra windings of all of the relays 5| that are operated, this circuit extending through the upper armature and front contact of relay 55, conductor 69, extra windings on operated relays 5| in parallel, upper armatures and front contacts of these relays, and multiple conductor 6I to feed wire I1. This prevents putting excess load on the spare inverter 28 in case of subsequent failure of one of therinverters 21, since the relay 5| of the latter is notallowed to'release.

It was mentionedrabove that the condenserresistance circuits CR are provided to aid in reducing cross-talk. The vcondensers in conjunction with the retards I5, I6 etc., form lters which allow direct current to pass over the lines but prevent the setting up of high frequency voltages between pairs. The resistances are included as a safety measure. They serve as leaks to prevent the building up of a charge between the lines and ground such as might result in shock to a workman. Also, the resistances equalize the voltage between the cable and ground so that the maximum voltage that can exist between cable and ground is one-half the supply voltage if no direct ground is placed on one side of the direct-current supply.

What is claimed is:

1. In combination, a terminal station, a distant repeater station, a plurality of telephone pairs extending between said stations, an amplifier per pair at said repeater station, each including a space discharge device having anode and cathode, a second plurality of telephone pairs extending between said stations a source of voltage at said terminal station connected to impress a difference of potential between the first plurality of pairs in multiple and the second plurality of pairs in multiple, a circuit at said repeater station connected between said rst plurality of pairs in multiple and said second plurality of pairs in multiple, and including the space paths of said ampliers in multiple, whereby anode potentialY is supplied to said amplifiers.

2. The combination defined in claim 1 in which the connections between said source and said lines, and the connections between said lines and said circuit at said repeater station, contain impedance means preventing each line from cross-talking into other of said lines.

3. In a telephone system, an east group of lines and a west group of lines extending between stations, Space discharge repeaters at the east station for certain of said lines, each having a cathode adapted to be heated, a source of current at the west station, a circuit supplying current from said source to the space paths of said repeaters comprising said east lines in multiple as one side of the circuit and said west lines in multiple as the opposite side of the circuit and means at said West station controlled oversaid circuit and actuated in response to a change in load condition at said east station for supplying over said same circuit from said source of current at the west station current for heating the cathodes of sai-d repeaters.

4.. In a telephone system, a telephone line, a main station, a repeaterstation on said line, a space discharge amplifier at said repeater station requiring energizing current, a local supply of said energizing current at said repeater station, and means controlled over said line and automatically operative upon failure of said local supply for transmitting current from the main station over said line to energize said amplifier.

5. In a telephone system, a line, a main station and a repeater station on said line, a source of direct current voltage at the main station, a space discharge amplifier at the repeater station having an anode and a cathode requiring heating, a circuit for supplying space current to said device over said line from said source, a circuit for supplying heating current for said cathode from a local source, means operable in case of failure of said local source of supply for sending an increased amount of current over said line from said source at said main station, and means at the repeater station for utilizing such increase in current to heat said cathode.

6. A system according to claim in which said local source supplies alternating current to heat said cathode, and said last means includes an inverter for changing the increase in direct current received over said line into alternating current.

7. In a telephone system, an east group of lines and a West group of lines extending between stations, space discharge repeaters at the east station for certain of said east lines, a source of current at the west station having a connection from one of its terminals to all of said certain east lines in multiple and having a connection from its other terminal to a plurality of said west lines in multiple, connections at said east station from each of said certain east lines to a respective repeater for supplying space current thereto, the circuit for such space current being completed by Way of a connection from such repeater to said west lines in multiple, and a circuit at said east station common to said repeaters and connected between said east lines in multiple and said West lines in multiple for supplying to the cathodes of said repeaters-heating current energy received over said lines in multiple from said source at the west station.

8. In a telephone system, an east group of lines and a west group of lines extending between stations, space discharge repeaters at the east station for certain of said lines, a source of current at the west station, a circuit supplying current from said source to the space paths of said repeaters comprising said east lines in multiple as one side of the circuit and said west lines in multiple as the opposite side of the circuit, and a current-consuming circuit at the east station common to a plurality of said repeaters and adapted to be supplied with current from said source of current at the west station over said rst circuit. I

CHARLES W. GREEN. HAROLD S. BLACK.