US2020318A - System including repeater - Google Patents

Description

Nov; 12, 1935 o. B. JACOBS 9 2 9 Q SYSTEM INCLUDING REPEATER Filed March 6, 1934 3 Sheets-Sheet 1 uw/vrok I 0. JACOBS y iw/M Arron/var Nov..12, 1935. B A OB 2,020,318

SYSTEM INCLUDING BEPEATER Filed March a, 19:54 v s sheets-s et 2 FIG. 3

I JM/I-FNTOR v 0. 5. JACOBS NOV. 12; 1935. I Y Q B, J/ACQBSV I 2,020,318

SYSTEM INCLUDING REQPEATER Filed March 6, 1934 3 Sheets-Sheet 3 INVENTOR 0. B, JACQBS ATTORNEK Patented Nov. 12, 1935 PATENT OFFICE SYSTEM INCLUDING REPEATER Oliver B. Jacobs, .Morristown, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 6, 1934, Serial No. 714,335

9 Claims.

The present invention relates to signaling systems containing one or more repeaters between system terminals. It relates particularly to repeaters adapted for location in relatively inaccessible places in that the repeaters can be operated without immediate personal attention for long periods'of time.

An object of the invention is a repeater or repeater system capable of continued service for long periods of time without requiring the presence of an attendant at the repeater point or points.

A feature of the invention is the supply of the necessary power for the repeatersby transmitting current from a terminal station, and the control of certain circuit relations of the repeaters by means of the current so supplied. As an example, the supply current can be reversed on the line and the reversal causes the replacement of a repeater by another repeater or performs other switching functions. The invention will-be more fully understood from the following detaileddescription in connection with the attached drawings in which,

peater system according to the invention;

Fig. 2 is a simplified sketch of a line interconnecting terminal stations and including a number of intermediate repeater points; and

ing amplifiers in a repeater station.

In Fig. 1 there is shown a terminal station at the left connected by.a section of line or cable ID with a repeater station. The repeater station 35, is connected between cable section H1 and another cable section There may be several line sections similar to- II] and II with repeater stations between line sections generally similar to the repeater station shown in the drawings. The distant terminal station may be similar to that shown.

The terminal shown will'be called the west station and comprises a signal current branch including repeating coil l5 and a direct current branch leading through filter I6 to switch H which is connectible to either source H1 or IQ of directcurrent. The purpose of the direct current is to energize the repeaters at one or more repeater stations on the line.

It will be assumed that carrier currents in an upper group of frequencies are impressed, from suitable transmitting apparatus (not shown), on.

the line through the signaling branch including repeating coil l5 and that carrier ,currents in a 55 Fig. 1 is a schematic representation of a re- Figs. 3 to 6 show alternative ways of substitutv devices.

lower group of frequencies are received from the line by suitable receiving circuits (not shown) through the signaling branch, That is, the higher group is transmitted in the eastward direction over the line and the lower group is transmitted westward. These carrier currents may be 5 speech-modulated to provide a plurality of separate speech channels in each direction.

At the repeater station, a direct current branch leads through retard 2| while a signaling branch is provided by condenser 22 and repeating coil23. 10 Similar elements 3|, 32 and 33, respectively, are provided at the other side of the repeater, where connection is made to line section II.

The signaling branch connected between repeating coils 23 and 33 includes eastward amplifiers 24, 25, 26 and 21 and westward amplifiers 34,35, 36 and 31. High pass filters 28, 29 and low pass filters 38'and 39 serve to separate the upper and lower groups of frequencies to their respective repeaters.

In the circuit as shown, amplifers 24, 25, 34 and 35 are conditioned for operation. Amplifiers 26, 21, 36 and 31 are spare repeaters that may be substituted for those in use by reversal of the line current in a manner to be described.

Two direct current branches are provided between the retards 2| and 3|. One branch leads from retard 2| through valve 4|, resistance 42, the heaters (or filaments in case filamentary cathodes are used) of amplifier 24, resistance 56, retard coil 1|, the heaters (or filaments) of amplifier 34, resistance 12. retard coil 13, the oathodes or heaters of amplifier 35, resistances .14 and 48, the heaters or cathodes of amplifier 25 in parallel with resistance 15, winding of relay 40 to retard 3|. The other branch leads from retard 3|, winding of relay 48, valve 45, resistance 46, the heaters orcathodes of amplifier 26, resistance 51, retard 11, the heaters or cathodes of amplifier 35, resistance 18, retard 19, the heaters or cathodes of amplifier 31, resistance 8|, retard 82, resistance 49, the heaters or cathodes of amplifier 21 and parallel resistance 83 to the terminal of retard 2|. On account of the presence of valves 4| and 45, current can fiow through only one of these two direct current branches at a time depending on the direction of flow of the line current. These valves may conveniently be copper oxide rectiflers or other suitable unilateral With the switch H in the position shown at the west station, positive battery is connected to the line-at that station and it will be assumed that negative battery is connected to the line at the opposite terminal. Under these conditions,

the current will fiow through filter l8, line section l8, retard 2| and the first of the two direct current branches above traced, that is, through valve 4| to retard 3|, line section II, and so on to the next repeater station (it any) and to the distant terminal. Under these conditions there is no current in the other direct current branch at the repeater station and the amplifiers 28, 21, 88, 81 are inoperative since they are deenergized.

Associated with each of the series retards 1 i I8, ll, 19 and.82 is a shunt capacity to ground 84, 85, 88, 81 and 89, respectively, each of which forms, together with its series retard coil, a filter for suppressing the flow of fluctuating current in the respective cathode heating branch.

Anode voltage is derived from the same direct current branch by means of the drop of potential along series resistance in the branch. For the amplifiers 24 and 25, conductor 41 leading from one terminal of retard 2| connects through series retards 54, 54 to the anodes of these respective repeaters. Cooperating with the retards there is in each instance a shunt capacity 55 to the cathode forming, together with the respective series retard, a filter to prevent the backing up of signaling voltages or other fluctuating voltages in the common plate supply lead. Conductor 41 is also shown leading to the anodes oi repeater without the interposition of a filter although this could be inserted if desired. Plate voltage for amplifier 34 is derived by conductor 52 from a point intermediate the ends of resistance 42, the plate voltage for this amplifier being relatively small since it is an input amplifier for the lower group and for that reason is assumed to have relatively lower gain and lower output level than the later stages or than amplifiers for the high group. Anode voltage for corresponding repeater 88 is similarly derived from resistance 48 over conductor 58.

It will be observed from the foregoing circuit description that the anode voltage oi any given amplifier may be controlled by adjusting the points in the series energizing branch to which its anodes and cathodes are respectively connected. Thus the anode voltage for amplifier 84 is derived as the drop of potential across a portion of res stance 42 and across the heating circuit of amplifier 24 including bias resistor 58 and such additional resistance as may be included in retard 1| and the connecting leads. Amplifier 25, on the contrary, requires a relatively high anode voltage since it is the output amplifier section for the high group for which a high output level is required. For this reason series re.- sistance 48 is connected between its cathode lead and the conductor leading toward retard 2|. The presence of resistance 48 makes the cathodes of amplifier 25 more negative, that is, makes the anode to cathode voltage higher, assuming always that the energizing current from the terminal sources is kept constant.

Resistances l5 and 83 are for the purpose of shunting around the cathode heating circuits of amplifiers 25 and 21 the space currents of the amplifiers whose cathode heating circuits are in the same series branch but on the positive side. These space. currents become cumulative toward the negative terminal, and resistances l5 and 83 are proportioned with respect to the heating circuits which they shunt to carry this excess current.

with these illustrative uses of cathode lead and anode lead resistances, it will be obvious to control the anode voltage of any of the other amplifiers.

Grid bias may be obtained from cathode lead drop of potential. For example, the final tube of each input amplifier 24, 28 is shown provided 5 with a grid bias resistor 58 or 51. Similar bias resistors are shown for the final tube of each of the other amplifiers. For the other tubes the drop across succeeding tube heaters may be used for grid bias in a manner known in the art. 10

As described above, reversal of the line current as by throwing switch I I at each terminal station substitutes amplifiers 28, 21, 88 and 31 for the amplifiers 24, 25, 84 and 85 shown in energized condition in the drawings. 15 In the drawings and thus far in the description,

only the current supply and energizing circuits for the repeaters have been disclosed. The wave transmission paths through the amplifiers may be 01' any suitable type and preferably are of the 20 type disclosed in British Patent No. 381,831, Fig.

7. That figure discloses a two-way carrier repeater with three stages in the high group repeater and two stages in the low group repeater. While the present application drawings show 26 only two stages in the high group repeater, it will be apparent from what is shown how a third stage may be added as in the British patent and the cathode heaters or filaments of all stages connected in a series circuit and supplied from one 30 or the other direct current branch as herein described.

I As in the British patent, Fig. 7, the repeater station herein disclosed is suitable, for example, for

use at an outlying point such as Port aux Basques 35 or Grand Bank, N. F., between sections of submarine cable of the lengths and attenuations given in that patent. An advantage of the present invention is that the repeater stations operate without the presence of attendants for long periods of time. Equalizers 8i and 82 correspond respectively to the high group and low group interstage equalizers of Fig. 7 of the British patent.

Relay 48 may be provided at those stations at 45 which it may be desired to have a direct current ground. It may, for example, be provided where there is only'one repeater station, or if there are two repeater stations it may be provided at each station. If there are more than two repeater 50 stations it may be used at two of the stations, as will be made plain presently. When direct current of either polarity is sent over the line of suificient magnitude to energize the repeaters, relay 48 isactuated. At the break of its back 55 contact it opens a shunt around resistance 82, and at the make of its front contact it grounds the direct current branch through resistance 83, which is high enough to prevent a disturbing I current surge on the line. Resistance 82 aids in w reducing the surge from cable II to ground. At the front contact, relay 48 includes the winding of slow operate relay 84 in series with resistances 82 and 88. When relay 84 operates it shunts out resistance 88 and leaves a path to ground through 65 its'winding, i'ront contact of relay 48 and resistor 82.

Referring to. Fig. 2, a system is shown comprising three repeater stations 18, 88 and 88 in a cable I8, l8, II, II' connecting west and east terminal 0 stations. Each repeater station I8, 88 and 88 may be identical with that in Fig. 1 except that only two of these stations 88 and 88 are provided with relay 48. For simplicity relay 84 and resistances 82, 88 are omitted in Fig. 2 although they 15 may be used as in Fig. 1. It will be noted that relay 4|] at repeater station 90 is connected so as to place a ground on the side of the energizing current flows from'positive battery I8 through,

filter l6, over the line, through the direct current branches poled in that direction at the various repeater stations as described in connection with Fig. 1, through filter It at the east station to switch I? and negative battery l9 "and ground. Suitable regulating resistances as shown are provided at both terminal stations.

When current fiow is established in this manner overthe line, both relays 49 operateand proground to repeater 99, closed front contact of relay 49 at that repeater, the energizing circuits of the amplifiers as shown in detail on Fig. 1, retard 3|, cable filter l8 and switch to negative pole of source IS. The direct current is thus shunted out of cable section II. Moreover, the two terminal direct current supply systems now become independent of each other so long as current flows in either half of the line, and the direction of current flow ,can be reversed at either station by throwing switch I! or to its alternate position.

When the current is reversed as described a new set of repeaters is substituted for those previously operative. Thus by throwing switch H a new set of repeaters is substituted at repeater stations 10 and but not at repeater station 90. Reversal of current by throwing switch substitutes repeaters at repeater station 90.

When switch I! is opened, relay 40 at station 89 releases, but if relay 49 at station 99 already was operated, it remains so until released by. opening switch When relays 40 at both stations are released a through circuit for direct current exists and the systemcan be tested by the usual direct order in which energizing current is supplied to them, i. e. amplifiers 24, 34, 35 and 25 while the tubes 26' and 21 correspond to the amplifiers 26, 36, 31 and '21 of Fig. 1. Fig. 3, therefore, asre-v gards the manner of substituting amplifiers can be considered as equivalent to the more complete showing of Fig. 1. Plate and grid bias voltages .may be derived as in Fig. 1 or any other suitable manner.

For purposes of illustration, in Fig. 3 the equalizer 92 has been inserted between the input filter for separating direct current from the signaling voltages and the input coil 23. Current fiow :n the direction from cable section I!) to II, as in Fig. 1, passes through valve 4| and the cathodes of amplifiers 24' to 25, the other amplifiers 26 to 21. being inoperative since their energizing circuit is without current. Current fiow in theopposite direction passes through valve 4| and the cathodes of amplifiers 26 to 21 so that these amplifiers are now efiectively in circuit while the amplifiers 24' to 25' are effectively removed from the circuit.

Fig. 4 is a circuit diagram similar to Fig. 3, but includes a polarized relay 94 as the means for substituting one set of amplifiers for another in-,

stead of the unilateral devices 4| and 45 of the previous figure. Relay 94 has a high resistance winding permanently connected between the retards 2| and 3|. Current fiow in one direction over the cable sections l0 and I causes the relay 94 to make contact with its lower contact thus establishing a path for current flow through the energizing circuits of amplifiers 2,6 to 21 as illustrated. While the winding of relay 94 is in parallel with the energizing circuits of these .amplifiers, the winding is of such high resistance as to consume very little current, most of the current flowing through the amplifier circuit. Current flow in the opposite direction causes the relay to make contact with its upper contact in the drawings thus substituting amplifiers 24' to 25' forthose previously in circuit.

In' Fig; 5 a pair of neutral relays 95 and 96 together with a pair of valves 91 and 98 control the switching, between the different sets of amplifiers As illustrated, the lower set of amplifiers is in circuit and current fiow is assumed to be from cable section l0 toward cable section II. The path for this current is traceable through retard 2|, the energizing circuits of amplifiers 21 to 25', back left-hand contact of relay 96, winding of relay 95, right front contact of relay 95, back right-hand contact of relay 96,

retard 3| to cable section I l. The circuit branch through the upper amplifiers 24 to 25 is open at the back contact of relay 95. It will be noted that the above traced circuit does not include either rectifier 91 or 98. That is. when either set of amplifiers is in operation the rectifiers 91 and 98 are cut out of circuit as will be clear presently. Thus, the amplifier energizing circuit is kept free from any variable operation or possible noise which might be involved if the rectifier were connected in the circuit.

If itis desired to switch in the alternate amplifier, the current over the cable is first interrupted allowing relay 95 to restore. Current is then sent in the direction over the cable II toward the cable l0, this current passing through retard 3|, right-hand back contact of relay 96, right-hand back contact of relay 95, now closed, rectifier 99, winding of relay 95, back left-hand contact of relay 95, now closed, energizing circuits of amplifiers 24' to 25, retard 2| tocable section ID. This current causes relay 96 to energize opening at its left contact the circuit through the lower group of amplifiers and shunting out at its right front contact and thereafter opening the path through the rectifier 98.

Reference will now be made to Fig. 6. In this figure positive current received from cable section l0 passes through retard 2|, rectifier 4 i, the energizing circuits of amplifiers,- 24 to 25, upper normal contact of relay I02, normal contacts or switch lfliL'winding of polarized relay IE4, winding of battery control relay I05, retard 3| and to cable section The amplifiers 24' to 25' are thus energized and renderedcapable of serving as two-way repeaters between the line sec- 24 to 25 of Fig. 1.

A digression will be made at thispoint to describe the switch I00. This is preferably in the form of a sleeve or cylinder comprising an upper,

a middle and a lower section of conducting mate rial with insulating material between. A metallic plunger I01 is fitted inside the cylinder and is slidable lengthwise of the cylinder. This plunger I01 is soldered (in the position shown) to the' middle and lower portions of the sleeve so as to establish contact between these portions. It is separated, however, from the upper portion so that no contact exists between the middle portion and the upper portion of thecylinder. The cylinder is surrounded by a heating coil I09 adapted, when suitable current is passed through it, to heat up the cylinder and melt the solder. When this is done the spring I pushes the plunger I01 upward in the figure so that it passes out of contact with the lower segment of the cylinder and makes contact with the upper end of the cylinder thus establishing contact between the central portion and the upper portion of the cylinder and breaking contact between the central portion and the lower portion of the cylinder. In order to insure that the plunger will make good fused union with the end of the cylinder the turns of the heating coil I00 are more concentrated toward the closed end of the-cylinder so that when the solder is melted and allows the plunger I01 to move, the closed end of the cylinder will be at least as hot as the other portions. As the assembly cools down, the pin I01 is firmly secured in its uppermost position with its ends soldered to the terminals of the cylinder. The switch IOI may be identical with the switch I00 just described.

When current flow was established through rectifier 4| and amplifiers 24' to 25', as above described, polar relay I04 was actuated to close its lowermost contact, this relay being of a tym biased to central position so that when no current flows through its winding the armature holds both of its contacts open. Whenever the relay I04 is energized by current of proper direction to pass through the rectifier 4I, it closes its lower armature and current then flows from local dry battery cells I06 through the back contact of armature I05, contact of relay I04, upper winding of relay I03 to ground causing relay I03 to operate and lock itself over the back contact of relay I05. Relay I is slow to operate and slow to release. After a time, under the conditions above described, it attracts its two armatures and opens the holding circuit for relay I03 so that the operation above described of relays I03 and I04 is without effect at this time. At its upper front contact relay I05 closes a. shunt around the relatively high resistance of winding I04 so that this winding is removed from the series energizing circuit for the repeater. Under the conditions above described, neither heat coil of switch I00 or I III receives current since they together with corresponding relays I02 and I03 are shunted out by the normal contacts of the said relays.

If new the current on the line is reversed so that positive current is received from cable section II, this current passes through retard 3|, windings of relays I05 and I04, rectifier 45, energizing circuits of amplifiers 25' to 21, normal contact of relay I03, normal contacts of switch I 0I retard 2!. cablesection I0 so that the east and west amplifiers 26' to 21, inclusive,

' tions I0 and II as in the case of the amplifiers are now brought into the circuit in place of the amplifiers 24 to 25' previously assumed to be operative. Relays I04 and I05 cause the momentary actuation of relay I02 in the same manner that was described above in connection with relay I03, but this operation is without effect at this time. When relay I05 operates it shunts out the high resistance winding I04 from the series energizing path through the repeater.

The switches I00 and IN and the relays associated with Fig. 6 provide for two additional substitutions of amplifiers beyond the above-described substitutions that are made by the mere reversal of the line current. For example, assuming that the amplifiers comprising tubes 24 to 25', inclusive, have been used and proved unsatisfactory, the circuit of Fig. 6 permits tubes I24 to I25 to be brought into the circuit. Relatively weak positive current is first sent over the line in the direction from cable section I I to section I0. This causes the relatively sensitive relay I04 to close its upper contact, but relay I05 does not operate on this relatively weak current. Relay I02 is energized by current through its lower winding, upper contact of relay I04 and back contact of lower armature of relay I05, from battery I06. Relay I02 closes a locking circuit for itself independent of relay I04 through the normal contacts and lower armature of relay I05. If the line current is now interrupted, relay I04 restores to its open position, but relay I02 remains locked over the circuit previously traced. Current of normal operating strength is now sent over the cable in the direction from I0 toward II. This current passes through rectifier 4I, conductor I28, upper armature and upper winding of relay I02, heating coil I09, lowermost segment of cylinder of switch I00, plunger I01 to central segment of switch cylinder I00, winding of relay I04, winding of relay I05, retard 3I and cable section II. This strong current causes relay I05 to energize and attract both of its armatures. At its lower armature it opens the holding circuit through relay I02 while at its upper armature it places a shunt around the winding of relay I04 so that the current no longer needs to pass through the winding of that relay. Although the holding circuit for relay I02 is opened, as described, this relay remains energized on account of the current flowing through its upper winding. The strong current now flowing through the circuit as traced, causes the coil I00 to heat .up and raise the temperature of the switch I00. Other things being unchanged, this strong current continues to flow and eventually the temperature of the switch I00 will be raised to a point such that the solder around the plunger I01 fuses and allows the spring I00 to move the plunger to its extreme position where it contacts with the upper closed end of the cylinder of the switch at the same time breaking contact with the lower segment of the cylinder. When this occurs the circuit through upper winding of relay I02 and the heat coil is broken at the lower contact of switch I00 and relay I 02 restores without further effect. The current now passes through rectifier M and the energizing circuits of amplifiers I24 to I25, upper contact of switch I00, plunger I01 and middle contact of switch I00, armature and winding of relay I05, etc. as before. This current may now be adjusted, if necessary, to the proper operating value for the repeaters I24 to I25. These repeaters and repeaters 26 to 21', inclusive, may now be interchanged as often as desired by reversing the current in the normal way.

If it is subsequently desired to "replace the amplifiers 26 to 21', inclusive, the amplifiers I26 to I21 may be substitutedby first sending weak current over the line in the direction from l0 toward II to energize and lock up relay I03 and thereafter sending strong current in the opposite direction over the system and through the heating coil of the switch llll ina manner entirely analogous to that described in connection with the switch I00. It is thus seen that the circuit of Fig. 6 provides four complete repeater circuits, two of which initially are held in reserve but which may be switched into the circuit separately when it becomes desirable to replace the initial repeaters permanently. It is not possible by the circuit shown in Fig. 6 to actuate the switches I 00 and IOI in the opposite direction from the operations that have been disclosed. However, it is assumed that these switches are in the nature of emergency apparatus that would not be called on to operate until after one or both of the repeaters comprising tubes 24 to 25', inclusive, and 26' to 21, inclusive, have proved unsatisfactory in service. After the switches I00 and lill have been actuated, as described, an attendant may visit the station and manually restore the switches to the position shown in the drawings, in connection with such other servicing as may be necessary.

Various modifications may be made within the spirit and scope of the claims, the circuits that have been described being illustrative of the invention rather than restrictive of the same.

What is claimed is: v

1. The combination with a signal transmission line of a terminal-station and a distant repeater station on said line comprising a first space discharge repeater and a second space discharge repeater, said repeaters being connected to amplify signals transmitted over said line when conditioned for operation, means to transmit current of either polarity from said terminal to said repeater station, means at said repeater station separate from said repeaters to distinguish the current of one polarity from current of the opposite polarity, one such last means causing current of one polarity to condition said first repeater for operation to amplify signals on said line and another such means causing current of the opposite polarity to condition said second repeater for operation to amplify signals on said line.

2. The combination with a line divided into sections, of a pair of repeaters connected between said sections for repeating signals between said line sections when conditioned for operation, a conditioning circuit for each repeater, each conditioning circuit being responsive to current of one polarity only, means to transmit direct current of either polarity over said line to said repeaters, a pair of unilateral valves connected between said line sections and the respective conditioning circuits for said repeaters, said valves being poled such that current of one polarity conditions one of said repeaters for operation and current of the opposite polarity conditions the other repeater for operation.

3. The combination with a line divided into sections of a pair of amplifying repeaters connected between said line sections for repeating signals from one line section to the next, each repeater comprising a cathode and anode, means to transmit over said line direct current of either polarity, two direct current connections between said line sections, each connection including the direct current of one polarity on said line energizes the cathode and cathode-anode circuit of one repeater while direct current of opposite polarity on said line energizes the cathode and cathode-anode circuit of the other repeater.

4. In combination, a line having terminal stations and a repeater station intermediate said terminal stations, an amplifying repeater at said repeater station for amplifying signals transmitted over said line, a source of power current at a terminal station for transmission over said line to energize said repeater, the circuit for said power current being initially established from said source over said line to the distant terminal, and current actuated means at the repeater station efiective upon establishment of power current flow over said line to shunt the power currents away from the portion of line from said repeater station toward the terminal station remote from said source.

5. In a signaling system, a line connecting terminal stations and divided into sections, a plurality of repeaters in said line connected between respective line sections, a source of current at each terminal station for energizing said repeaters, means initially establishing current flow over said line from the source at one terminal station to the opposite terminal station, means at each of two repeater stations at opposite ends of one line section, operated by said current flow to shunt the current away from said line section whereby each of said two repeaters is thereafter supplied by current from a different terminal source only.

6. In a transmission line system, a line having a normal and a spare repeater, said normal repeater being operatively associated with said line to amplify waves thereon, a polarized relay controlling operative association of said normal repeater'and said spare repeater with said line, said relay operating in response to the flow of current of given polarity over said line to dissociate said normal repeater from the line and to operatively associate said spare repeater with said line.

'7. In a transmission line system, a line having a normal and a spare repeater, said normal repeater being operatively associated with said line to amplify waves thereon, a relay having its winding in circuit with said line and said normal repeater, a second relay having its winding in circuit with said spare repeater and line, said latter circuit being opened in the energized condition of said normal repeater, said second relay having contacts controlling the energizing cir- .cuit of said normal repeater, a valve connected between each relay winding and the line, whereby current of one polarity on the line actuates the first relay to associate the normal repeater with the line and current of opposite polarity operates the second relay to substitute the spare repeater for the normal'repeater.

8. In a transmission line system, a normal and a spare repeater, said normal repeater being operatively associated with said line to amplify waves thereon, a movable contact normally establishing operative relation between the line and normal repeater and adapted when moved to its alternate position to dissociate the normal repeater from the line and operatively associate the spare repeater therewith, said contact being held in normal position by a fusible material, a ground return, repeaters at intervals in said and means responsive to current flowof predeline supplied with energizing current from a distermined characteristic on said line for causing tant source over said line, and means at a resaid fusible material to melt and allow said conpeater controlled from current on said line for 5 tact to move to its alternate position. grounding the energizing circuit 01' said repeater. 5

9. In a. transmission line system, a line having OLIVER B. JACOBS.

Images