US2332451A - Telegraph trouble indicator circuit - Google Patents

Description

Oct. 19, 1943. -r. A. MARSHALL ,3

TELEGRAPH TROUBLE INDICATOR CIRCUIT Filed Nov. 20, 1940 2 Sheets-Sheet 1' FIG! ATTENDANTS SET CIRCUIT ATTENDAN 7'8 MANUAL TEL EGRAPH /NI/E/\ /TOF TAMARSHALL A TTORWEY Oct 1943- T. A. MARSHALL 2,332,451

TELEGRAPH TROUBLE INDICATOR CIRCUIT Filed Nov. 20, 1940 2 Sheets-Sheet 2 REC. T0 LINE F/G.2

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Patented Oct. 19, 1943 TELEGRAPH TROUBLE lN-DICATOR CIRCUIT Tola A. Marshall, Floral Park, N. Y.,. assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 20, 1940,;Serial No. 366,404

18 Claims.

This invention relates ,to telegraph testing equipment and more particularly to a trouble indicator circuit or to a hit indicator as it is generally known in theart.

An object of this invention is the improvement of hit indicatorcircuits for use in maintaining telegraph circuits.

A new type of telegraph operation known as inverse neutral repeater hub circuit operation .is

at present being widely applied. Inverse neutral repeaters are described in the patents to Kinkead 2,056,277, October *6, 1936, Kinkead 2,077,936, April .20, 1937 and Cummings 2,069,224, February 2, 1937. The administration of networks built up of such circuits isdescribed in the copending application of Bonorden .et 'al.;Serial No. 362,548 filed October 24, 1940. The maintenance of such circuits involves new problems. The provision of .a satisfactory hitwindicator for such circuits is one of the important problems involved in the operation of such circuits. Reference is made to the copending applications of J. A. Mahoney Serial No. 301,358, filed October 26, 1939 and of A. R. Bonorden Serial No. 317,809, filed February 8, 1940, now Patent No. 2,275,126,.granted March 3, 1942, whichdisclose arrangements for maintaining such circuits. The present invention is an. improved hit indicator for. use in the maintenance ,of inverse neutral repeater concentration groups.

The inventionwillbe understood from the following description whenread ,with reference to the associated drawings in which:

Fig. 1 shows a typical trouble indicatorcircuit of this invention designed to indicate trouble in an inverse neutral repeater concentrationgroup;

Fig. 2 shows a half duplex inverse neutral repeater concentration group including facilities for cooperating with thezcircuit of Fig. 1 so that troubles occurring-in the concentration group may be detected.

General description of an inverse neutral telegraph repeater concentration group tion jacks individual to cachet-the three circuits. -'I*hethree circuits are extended to .a con1 mon neutral electrical point knownas the hub of the concentration. One of the jacks in .each concentrationgroup is attesting jackdes'igned. to cooperate with the trouble indicator'cord circuit of this invention. The testingjack as it is known is the. bottomo'ack of-each three concentration jacks associated with each repeater of Fig. 2. These jacks are indicated .as::20l,.202..and 2%. A resistance of .smallnmagnitude is :inserted in series in each of the transmitting conductors which join the individual inverse neutral repeaters to the .hub. .These resistances are indicated as 291, 2218 and 209.v -A relay. having a winding in series with thesleevelof each of jackszZll I 29?. and 203 is controlled from thetrouble indicator cordcircuit. The relays are indicated .as 1205, 265 and2li6.

The operation .of Fig. 2. is-..described in detail in the copending applicationof Bonorden :et al. mentioned above. It will be described herein in a general way in so, far .as it is necessary to an understanding of .the manner in which it co? operates with the present invention.

The circuits are shown in the marking condition. For this condition relay 2E0, which receives receiving relay '2! I also :in the marking condition.

For this condition the leg of the inverse neutral repeater connects negative'battery to the :huboi The circuit may be traced; from the repeater. negative battery ZlZ through the bottom windings of sending relays H3 and 21M,bottom armature and make contact of .the full duplex versus half duplex control relay .2 l5; whichis operated for a halt duplex concentration, top Winding of relay 2| I, topmake contact and armature-oi relay 2.!5', right-hand or. marking contact and are mature of. relay 2, left-hand :outermost. armature and .break contact of relay 2%, test resistance 201, armature and break contact of. test relay 2.0.4, to the ;hub.of the concentration 2H.

For the marking condition the legs of: the inverse neutral repeaters in the other glineandin :the station circuit. connectnegative battery also .to the hub. Forthe .line negativebattery, 218

is connected toone end .of .the circuit which ultimately extends through test resistance 2% to .hubill. For the station negative battery 219 is connected toone end of the circuit which ulti matelYeXtends through test resistance 209 tohub 2!]. Thus -all.;.legs end in negative batter-yfof .the. same magnitude for themarking condition. No. current flows any leg and therefore there is no ohmic drop across any test resistance while the marking condition prevails.

When the distant terminal connected to the left-hand line transmits a spacing signal toward the hub, it is received by relay 240. The armature of relay 2! is actuated to engage its alternate contact. Relay 2II responds. The armature of relay 2 connects ground to the leg which extends through test resistance 201 to hub 2H. Current flows from battery H8 and battery 2H) through test resistances 203 and 200 in parallel toward the hub, and from the hub through test resistance 261 to ground on the spacing contact of relay 2| I. There is an ohmic drop in resistances 2G3 and 209. Resistances 20?, 200 and 209 are all of the same value. The combined current which fiows through resistances 2G8 and 09 toward the hub flows through resistance 281 in the reverse direction, that is to say, away from the hub. The armatures of the sending relays 220 and 22I are actuated to transrnit spacing signals to the distant connected terminal and to the local station respectively.

Attention is especially directed to the fact that there is no ohmic drop in any test resistance in the various legs of an inverse neutral concentration group while the repeaters in the group are in the marking condition, as no current flows for that condition. For the spacing condition, on a branch which is receiving from its associated distant terminal or local station and transmitting toward the hub, there is an ohmic drop in a certain direction dependent in magnitude upon the number of branches connected into the hub to which it is transmitting. In the case of any branch connected into a hub which is receiving from another branch connected to the hub, there is an ohmic drop in the reverse direction from the'drop for the condition when the branch is sending and, in the usual concentration, wherein more than two branches are interconnected in a group, the drop for the receiving condition is smaller than for the transmitting condition.

In tracing the transmission branch which interconnected the inverse neutral repeater to the hub 2 I? of the concentration group, it was shown to extend through the back contact and armature of the testing relay. The testing relay is arranged so that it requires a relatively high current to operate it. The cooperating trouble indicator cord circuit is arranged so that when observations are being made on a branch the resistance of the connected sleeve circuit is high to prevent the operation of the testing relay. When a branch is found to be causing trouble, the resistance of the sleeve circuit of the trouble indicator cord circuit is changed, by operating a key, to a lower value which permits the test relay to operate and thereby cuts the branch which is causing the trouble ofi from the concentration.

The tip of each testing jack in Fig. 2, such as 205, 202 or 203, is connected in parallel with the terminal of the test resistance such as 201, 203 or 200 which is further from the hub. The ring of the testing jack is connected in parallel with the terminal of the testing resistance which is nearer the hub. The tip and ring of each testing jack are therefore connected across each test resistance and the trouble indicator circuit per Fig. l which will now be described is arranged so that it is responsive to ohmic drops in the test resistance of a certain magnitude and in a certain direction to indicate trouble on the branch.

Receiving an indication of trouble Refer now to Fig. 1.

'lhe circuit per Fig. 1 is made ready for use by tne operation of key IOI. This operates relay i352. Relay I62 controls the connection of battery to the filament of tube I03; This circuit may be traced from battery I8 through the make contact and armature of relay 02, through the break contact of filament jack I05, through fuse i053, variable resistance Ifi'l, resistance I08 and the filament of tube I03 to ground. When plug N19 is inserted into the testing jack associated with the leg of an inverse netural telegraph repeater concentration group, such as Jacks 20!, 202 or 203 of Fig. 2, ground is connectedthrough the winding of the testing relay, such as 26 3, 2E5 or 206 of Fig. 2 to the sleeve of the testing jack and the circuit extends from this point through the sleeve of plug I09, through the bottom winding of relay IEO, through resistance l i i, through the top outer armature and break contact of relay I i2, through the top winding of relay H0 to battery. This operates relay no. The operation of relay IIO connects ground through its bottom outermost front contact and armature to supplement the ground from key Edi. The operation of relay IIO supplies ground through its bottom innermost armature and front contact for future use in connection with the starting of teletypewriter and telegraph circuits which cooperate with the trouble indicator cord circuit but which will not be described in detail in connection with the operation of this circuit. The operation of relay liil also closes the plate supply circuit of tube E63. This circuit may be traced from plate battery H3 through resistance H4, the winding of relay H5, through the bottom middle front contact and armature of relay H0, through the top back contact and armature of relay H0, through resistance II! and induction coil II8 to the plate of tube I03.

When a space signal is transmitted toward the hub of the inverse neutral concentration group circuit through the leg to which the circuit herein is connected, the tip of the test resistance, such as 201, 208 or 209 of Fig. 2 and the tip of the concentration test jack as well as the tip of plug I09 are grounded. The potential of the tip of the plug, therefore, is more positive than the potential of the ring of the plug connected to the other end of the test resistance in the same leg of the concentration jack circuit. A circuit may be traced from the tip of the plug through the top outer armature and back contact of relay H9 through the top outer armature and back contact of relay I28, through the bottom back contact and armature of relay H2 to point I2I where it is connected to two copper-oxide rectifiers or varistors, as they are otherwise known, I22 and I23, in parallel. Varistor I22 is connected so that it presents a low impedance to current of this polarity. The ring circuit may be traced from the ring of plug I09 through the top break contact and armature of key I21, through the bottom inner break contact and armature of relay M2 to point I28. When the tip of plug !09 is more positive than the ring of plug I89, current will flow from point I2I through varis tor E22 and through a three-branch parallel circuit comprising varistor I24 and the low resistance coils I25 and 126, forming the primary of the transformer I 39, to point I28. Current flows through the coils I25 and I26 in such a direction as to induce a potentialin coil Hi0 whichma'kes the gridiottube I Il3:;sufliciently positive with respect to-the -:c.athodes of tube m3 so that current will :flow in theplatecircuit'of tube 1:03,.

.If the space signal is coming .from theother legs wot the concentration group, .thearing of plug I59 will be more positivethan the ctiphf plug I89 and very little current :will'flow through the transformer, primary windings =due to thev high impedance presented .under this condition by the varistorunit- I22. Forthis condition tube I93 will not ionize.

When current flows inthe plate circuit-of tube I93, relay, I I5 operates. Theoperation zofvrelay I I5.closes a circuit from battery through .the bottom armature and front contact of: relay -I I5 and the filament of lamp 129 toground, lighting lamp-I29. The operation otzrelay- H5 alsozcloses a circuit from battery throughwindingof relay II 6 and the top front-contact and armature .of relay II5,'.through the Winding of relay I30 to ground operating relay I 30. The operationof relay I30 closes a circuit from ground through the sourceof alternating potentiall3l, winding of buzzer I32, resistance I33, throughthe armature and front contact of relay J30, back to Relay. Ill 5 I139, which Restoring the circuit to normal Following the reception of a troubletindication or the possible lighting of lamp i129 when plug IDS is inserted in the testing jack, the circuit may be restored to normal for the reception of a further hit by momentarily operating the unlock alarm key I34. The momentary oper ation ofkey I34 short-circuits the winding. of relay I311. Relay I30 releases. This in turn breaks the buzzer .circuit. Momentary operationof. key I34 also provides a direct ground which permits relay IIB tooperate. The operation of relay I I8 opens the plate battery supply for tubeHiB. 'Iube I03 deionizes. Relay IIS releases when the plate battery circuit is opened.

The release of relay H5 in turn releases relay.

1 IS. The release of relay I I6 by .openingits bottomcontactsextinguishes lamp I 29. The circuit is now ready for .the reception of a .secondtspace signal. a I

I Transmission indication The space signal which operated the indicator may havebeena telegraph transmission signal from the associated inverse neutral circuit branch instead of a hit. In thiscase, immediately after the momentary operationof 'key 134 the circuit will'reoperateand if key I34'is held operated,- lamp I29 will flash irregularly.

. The manner inwhich' the lamp is flashed is as follows: The first spacing pulse causes the tube 103' to "function in the manner described above. When-relay H5 operates to light lamp I29 it simultaneouslycloses the circuit for the operation of 'relay H6. If key I34 is held operated relay I16 operates. "Relay I I6 in operating closes a circuit "for the continued operation of lamp" I'Zlat its bottom armature and front contact. The operationofrelay I'Iii also opens the plate circuit of the tube. This releases relay l l5 which in turn opens thecircuit through the winding of relay M6. 'Relay" Me is designed-so that itacts slowly. It does not'release before lamp I29 is flashed. With relays l lfiy' 'and "M6 restored to'norm'al, and "key 134 maintained operated, the cycle may be repeated. 5

:Disabling the circuit without.- disconnecting If by the operations described in the two preceding sectionsthe circuit is foundtto be responding 'to telegraph signals coming in on the branch of the inverse'neutral concentration group 'to' which the trouble indicator cord circuit herein'is connected, further responses of the indicator circuit herein may be prevented by operating the "disconnect ,alarm'key I27. This opens the circuitrheretoforei traced through the ring conductor to one side of the input coils of the transformer. "Theoperation of key I21 further connects ground through its bottom armature'and front contact. In the event that relay II5 was operated when key. I21 was "operated,

relay I I6 will operate.

Connecting.. attendants. set

Thecircuit herein isarranged so that it may be connected to a manual telegraph setor to an attendants teletypewriter set so that manual telegraph signals or 'teletypewriter signals may be transmitted or received through the trouble indicator circuit to the inverse neutral concentration group to which the trouble indicator cord circuit herein is connected through plug me. If it is desired to connect a manual telegraph set to the trouble indicator cord circuit herein,

key MI is operated. When key I lI is operated ground is connected through. the armature and front contact of key It and the winding of relay I 20 to battery operating relay I29. This breaks the connection ofthe tip circuit heretofore traced to the input coil. of the transformer and extends the tip instead into an attendants manual telegraph set circuit (indicated bya rectangularbox) so that the attendant may transmit and receive manual telegraph signals toand from the connected circuit over conductors ti l tors I41 and I48 so that teletypewriter.signals, may be transmitted or received. The operation Y of relay H9 also disconi'lects the tip conductor of the troubleindicato'r cord circuit from the input "windings of the transformerfor the same reason-as described above in connection with the operation of relay I 26) Removin alc'g in trouble 7 If operation of the trouble indicator cord circuitherein indicates that the branch of the in verse neutral ooncentration-group'to whichit has been connected is in trouble, the leg associated i with the branch may be disconnect-ed from the concentration group "by operating key Hi9. The

operation of key: M9 closesa circuit from ground" through the bottom armature and front contact I52 and I53 and of condensers I54 andIEE to the ring of the indicator cord circuit. The operation of'relay II2 will also connect the varistor and transformer circuit acrossresistance 52 in the artificial leg in the proper direction to respond to signals from the hub. The operation of relay I I2 also reduces the resistance in the sleeve circuit of the cord by disconnecting the top winding of relay Hi) from the sleeve connection and connecting the sleeve to battery through the top outer armature and front contact of relay H2.

The object of reducing the sleeve resistance is to permit the operation of the testing relay such as 295, 295 or 206 in Fig. 2 in the concentration jack circuit which disconnects the corresponding leg circuit. The operation of key I49 also connects ground through its top armature and front contact for operating relay H6 in the event that relay II5 responds to a signal. Thus lamp I29 will flash on any transmission which is taking place on the concentration group circuit.

If the operator wishes to communicate on the leg of the inverse neutral circuit which has been split off from the inverse neutral concentration group, it may be done by operating key MI or I42 to connect the attendants manual telegraph set circuit or the attendants teletypewriter set circuit to the split-on leg through the tip of the trouble indicator cord circuit herein.

Apparatus junctions Condensers I56, I57, I53, I59 and E69 and resistances I 6| and I62 are to prevent false operation due to interference. Resistance I62 is of the order of one megohm.

Fuse I06 is intended to protect resistances I9? and I08 from overheating in the event of accidental ground.

Resistance I63 and condenser I64 provide shock protection on the sleeve of the cord.

The three copper-oxide rectifiers, or varistors as they are otherwise known, I22, I23 and 524, as is well known, have the property of presenting a low resistance to current in one direction and a high resistance to current in the opposite direction. Each of these varistors is a single copper-oxide unit and must be protected against excessive voltage applied in the direction opposite to which it is intended that current shall be passed as, if excessive voltage in the opposite direction is applied, the unit will break down.

In order, therefore, to protect against breakdown, it is arranged so that each unit, on which potentials in the reverse direction might be applied to cause breakdown, is shunted by a varistor arranged in the direction to present a low resis.ance to currents of the polarity which might cause damage.

After key I49 has been operated so as to disconnect the branch of the circuit with which the trouble indicator cord circuit herein has been functioning, there is a momentary interval, when key I49 is restored to normal while the branch of the inverse neutral concentration group is being reconnected into the group during which it is possible that the full 4.8 volts may be impressed from the group circuit on the tip or ring of the circuit herein. In order to prevent the breakdown of varistor I22 if an excessive reverse potential were to be impressed upon it, the circuit through the primary windings of the transformer and through the varistor I22 in the reverse direction is shunted by varistor I23 arranged in the proper direction to prevent the breakdown of varistor I22. The manner in which practically the full 48-volt potential may be impressed from the group circuit on the single copper-oxide rectifier unit I22 in the indicator circuit may be understood from the following.

It will be assumed that plug I09 has been connected to jack 203 and that key I49, relay H2 and relay 206 are all in the operated condition, so as to cut the branch with which relay 266 is associated off from the hub. The attendant restores key I49 to normal to reconnect the branch to the hub. The restoration of key I 49 to normal disconnects ground at the bottom armature and front contact of key I49 from the circuit extending through the winding of relay I I2. Relay II2 releases. This reestablishes the circuit which may be traced from the tip of jack 263 through the tip of plug I09, top outer armatures and back contacts of relay H9 and relay I20, bottom outer back contact and armature of relay H2, the single copper-oxide rectifier unit I22, primary windings I25 and I26 of transformer i39 in parallel, bottom inner armature and back contact of relay II2, top armature and back contact of key I21 and the ring of plug I09 to the ring of jack 203.

Normally, that is while relay 296 isreleased, the test resistance 289 is connected in shunt with the circuit just traced, across the tip and ring of jack 203 through the top contacts of relay 206. Relay 206 is operated while the branch with which it is associated is disconnected from the hub. Relay 206 has been operated due to the reduced resistance of the sleeve circuit in Fig. 1 heretofore traced. While relay II2 was operated, direct battery was connected to the sleeve circuit at the top outer front contact and armature of relay Ii2. When relay H2 releases, the alternate high resistance path through the top outer back contact of relay I I2 and the top winding of relay H0 is reestablished. This reduces the current through the bottom winding of relay 206 and relay 206 releases. During the interval while relay 206 is releasing, however, before the low valued test resistance 209 is reconnected in shunt across the tip and ring of plug I09 of the indicator circuit, an excessive potential, sufiicient to break down the single unit copper-oxide recti fier I22, may be applied in a direction, th reverse of the low resistance direction, across rectifier I22. If one of the circuits which has remained connected into the hub is transmitting while the reconnected branch is in the marking condition, a direct ground connected to the hub by the circuit which is transmitting will be connected, to the ring of the indicator circuit through the ring of jack 203. Battery of 48 volts will be connected through the bottom winding of relay HI and the marking contact and armature of the sending relay in the reconnected branch to the tip of the indicator circuit through the tip of jack 203. Due to the fact that the resistance in the high resistance direction through rectifier 22I is very high relative to the combined resistance of the other elements in the circuit, practically the Whole 48 volts would be applied across the rectifier I22 in the reverse or high resistance direction which would break down the rectifier if it were not protected. Rectifier I23 is connected so as to form a low resistance shunt for rectifier I22 to protect it against breakdown from excessive reverse voltage. In the same manner varistors I22 and I24 are arranged to protect varistor I23 from an excessive back potential. Varistor I24 which is connected in shunt with the primary windings of thetransformerboth of which windings are of low -resistance protects them from excessive current,

What is claimed is: I

1. In a telegraph system, a telegraph trouble indicator circuit, means insaid circuit for observing non-standard electricalconditions, a group of intercommunicating telegraph circuits, means for o-perativelyinterconnecting said--indicator circuit and said group and means i'nsaid indicator circuit for controlling the disconnection from said group of a particulartelegraph circuit on which anon-standard conditioned curs.

group of intercommunicating, telegraph circuits, means for interconnecting said indicatorcircui't and said group, telegraph means connected to said indicator circuit for communicating through said indicator circuit to said group, and means for restricting said communication to one circuit of said group.

3. In a telegraph system, a telegraph trouble indicator circuit, means in said circuit for'observing non-standard electrical conditions, a group of intercommunicating telegraph circuits, means for interconnecting said indicator circuit and said group, telegraph means connected to said indicator circuit for communicating simultaneously with all of said intercommunicating circuits and means for excluding from such communication a circuit in said group on which trouble is indicated.

4. In a telegraph system, a telegraph trouble indicator circuit, means in said circuit for observing non-standard electrical conditions, a group of telegraph circuits interconnected by means of individual telegraph repeaters to a common neutral electrical point, means for interconnecting said indicator circuit and said group, and I means in said indicator circuit for controlling the disconnection of an observed circuit in said group from said group.

5. A system in accordance with claim 4, and means in said indicator circuit for observing transmission on the circuits remaining in said group after the disconnection of said observed circuit without the necessity for disconnecting said indicator circuit from a connection in its original means of access to said observed circuit.

6. In combination in a telegraph system, an inverse neutral telegraph repeater concentration group, a branch connecting a repeater in said group to the hub of said concentration, a resistance in series in said branch, a trouble indicator circuit, means for connecting said indicator circuit across said resistance, and means in said indicator circuit responsive to a potential drop across said resistance in one direction only for indicating a non-standard electrical condition arising on a circuit connected to said hub through said branch.

'7. In combination in a telegraph system, an inverse neutral telegraph repeater concentration group, a branch connecting a repeater in said group to the hub of said concentration group, a resistance in series in said branch, a trouble indicator circuit, means for connecting said circuit across said resistance, and a copper-oxide rectifier in said circuit for discriminating between pctential drops in reverse directions across said resistance.

8. In a teletypewriter switching system, a plurality of telegraph circuits connected in an in- 2. Ina telegraph system, a t'clegrap'h trouble indicator circuit, means insaidcircuitfor ob serving non-standard electrical conditions, a

verse neutral telegraph repeater concentration group through a common hub point, a trouble indicator circuit, means "in said concentration group and insaid-indicator circuit for interconnectingsaid group and said indicator, 2. telegraph signal transmitting and receiving circuit, means insaid indicator circuit for extending said indicator circuit to said telegraph circuit so tha.t communication signals may be transmitted or received through" said indicator circuit to and from said concentration group; l

9. In a telegraph system, a trouble indicator circuit, means in said circuit for connecting said circuit. to an operable'telegraph path comprising a plurality of branches; means in said indicator circuit for discriminating between-communication signals and sig-rials caused' by trouble con ditions on one of saidbranches, means in said indicator circuit for controlling the disconnection of'a branch found to becausing trouble and means in saidindicator circuit, forestablishing a circuit for simulating said disconnected. branch.

10. In a telegraph system, in combination, a plurality of telegraphcircuits associated together in an inverse neutral telegraph repeater c'oncen" trationgroup, a trouble indicator cord circuit conditioned to interconnect with said concentra tion group, means in saidindicator circuit for indicating the transmission of normal communication signals through said concentration group, means in said indicator circuit for indicating the transmission of false signals caused by trouble through said concentration group, means in said indicator circuit for identifying the branch of said concentration group on which said false signals originate, and means in said indicator circuit for disconnecting the particular telegraph circuit on which said trouble arises.

11. In a telegraph system, a telegraph circuit, means connected to said circuit for communicating over said circuit in a first direction at a first time, lumped impedance means connected to said circuit for developing a potential to operate an indicator when signals are transmitted over said circuit in a second direction at said first time, a copper-oxide rectifier connected ,to said impedance means for detecting signals transmitted over said circuit in said second direction at said first time and an indicator, connected to said rectifier, responsive to said signal.

12. In a telegraph system, a telegraph trouble indicator circuit, means in said circuit for connecting said circuit to a telegraph communication circuit, means in said circuit for observing non-standard signals transmitted oversaid communication circuit While said circuits are interconnected and-means in said testing circuit for establishing in said testing circuit an artificial branch of said communication circuit.

13. In a telegraph system, a telegraph trouble indicator circuit, means in said circuit for establishing in said circuit an artificial branch of a telegraph communications circuit and means also.

in said circuit for testing said branch to identify false signals in said communications circuit.

14. In a telegraph system, an inverseneutral telegraph repeater hub circuit comprising a plurality of branches, a hit indicator circuit, a branch in said indicator circuit extending through a lumped resistance to battery of the same polarity and magnitude as that in which the various branches ofsaid hub circuit are terminated, means in said circuits for interconnecting said branch to the hub of said hub circuit and means in said indicator circuit cooperating with said resistance for observing non-standard conditions in said hub circuit.

15. In a telegraph system, a testing circuit, a first rectifier, a control means and indicator means, all in said circuit, means in said circuit for passing current of a particular polarity through said rectifier and said control means to operate said indicator to indicate an abnormal condition and a second rectifier in said circuit, connected in shunt with said first rectifier, to protect said first rectifier against damag from excessive reverse potential.

16. In a telegraph system, a direct current telegraph communication circuit, a telegraph testing circuit, an indicating device in said testing circuit, means for interconnecting said circuits, a first element in said testing circuit presenting resistances of substantially difierent magnitudes to direct current of different polarities, said element being susceptible of injury from voltages within the normal operating range and of a particular polarity, means in said circuits for impressing voltage of said particular polarity on said element and a second element in said testing circuit, substantially identical with said first element, shunting said first element and poled to present a low resistance to voltage of said particular polarity to protect said first element.

17. In a telegraph system, a direct current telegraph communication circuit, a telegraph testing circuit, means for interconnecting said circuits, means in said testing circuit for indicating an abnormal condition in said communication circuit and a pair of independent rectifiers in said testing circuit arranged to afford mutual protection against reverse potential.

18. In a telegraph system, a telegraph testing circuit, a transformer winding connected to an indicator, both in said testing circuit, means in said testing circuit for operating said indicator to indicate an abnormal operating condition im- 7 pressed on said circuit, a rectifier network con-

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