0a. 22, 1946. 2,409,720 w. P. PLACE ETAL SELECTIVE CONTROL AND INDICATION SYSTEM Filed Nov. 4, 1943 ?“\wQ.xIr(Nu.a?GNSmQg. \QN YI.F I uQWN‘._RiGI.NW 4 Sheets-Sheet 1 Oct. 22, 1946. .Filéd Nov. 4, 1943 SWN 2,409,720 w. P. PLACE EI'AL SELECTIVE 001mm. m nwicmlon SYSTEM 4 Sheets-Sheet _2 ‘aw M@mT,?W ' INVENTORS Place 997a’ 0a. 22, 1946. w. P. PLACE ET AL. 2,409,720 SELECTIVE CONTROL AND INDICATION SYSTEM Filed Nov. 4, 1943 4 Sheets-Sheet 3 $3. 3m. lI-v'LVq“wm Ma 7.“ aQ@wag.E.Q .QSY kkrgh ?\.. mbIa.m:gw |. “WWW \ 1..“ -. (mg &‘Ta.uI swww ,F kw LENgm$ % ‘ INVENTORS’ wz'ZZardRPlaee and BYA/ THEIR ATTORNEY Get- 22, 1946- 2,409,720 w. P. PLACE ETAL SELECTIVE CONTROL AND INDICATION SYSTEM Filed Nov. 4, 1943 ' 4 Sheets-Sheet ‘4 INVENTORS 7mm TTORNEY ’ Patented ‘Oct. 22, 1946 2,69,?26 UNITED STATES PATENT OFFICE 2,409,720 SELECTIVE CONTROL AND'INDICATION SYSTEM ‘Willard 7P. ‘Place, .Penn Township, Allegheny County,'and Alfred B. Miller, Edgewood, Pa., assignors to The Union'Switeh & Signal Com ‘pany, Swissvalc, Pa., a corporation of Pennsyl vania Application November 4, 1943, Serial No. 508,948 13 Claims. (01. 177-353) 2 each such device beingtprovided-with an ‘electric Our invention relatesto selective control. and equipment that responds to a preselected ar indication systems, and more particularly to ap rangement of the currents thus supplied to the paratus' for suchsystems using diiierent combi line circuit. Preferably ,an office battery and nations of currents of different characteristics. An electric equipment or device is frequently 5 a commercial source of' alternating current, such as a 60 cycle current; are‘ connected to the line controlled and indicated througha singleline circuit to, power certain portions of the-equip circuit by using an alternatinglcurrent of one ment of each of the devices. Indication oscilla frequency‘ for ‘control and. a di?erent alternating current of a different frequency for indication. In this way a plurality of devices can be con tors and control oscillators; together-with. an op trolled and indicated selectively through-a single currents, control currents and an operating-cur line‘ circuit. The selectivity isrmore accuratev and also more reliable byiusing different combinations of alternating currents of different‘ frequencies. thereto an indication oscillator and a control os In such'systems simplicity of apparatus; freedom from ‘operation by shock excitation and foreign current, and the resetting and testing of the spective device. "The operating oscillator sup erating oscillator at the ;;o?ice supply-indication rent, respectively. "Each device has assigned cillator Which supply currents of’ different'fre quencies preselected to be;individua1 for the re plies current at a frequency different from; that of the other-oscillators, but the operating; oscil apparatus are essential features. Accordingly, a feature of our invention is ‘the ‘provision in selective control and indication sys tems of novel and improved apparatusforindi eating diiierent conditions of' anielectric. equip ment and for controlling the operation .ofvsuch equipment. Another feature'of ourinvention is the-pro vision of apparatus of the type here involved in corporating novel means for automatically. indi lator is commonto the group of devices. 20 ‘Each-indication oscillator is normally active and is connectedacross the line circuit through a ?lter tuned to create a resonant conditionwith respect tolthe oscillator and the distributed ca pacitance of the line circuit, so that each indi 25 cation oscillatortnormally supplies a line charg ing current which ‘is of. a relatively large prese eating ‘an 'abnorm‘al‘condition-l at a device ‘and for subsequently resetting the. apparatus. Again, a feature of our invention is the provi sion ‘of-apparatuslof the type? here’ contemplated lected value andwhich current can be used as an indication current. .Indication receivers. are provided at the oi?ce, one for each device, and each such receiver is coupled to the .o?ice ?lter interposed in the connection of the indication oscillator for the same device and the receiver is incorporating novel means to. control the :oper ation' of a‘ device and wherewiththe probability of operation due to shock excitation and .foreign energy is minimized. A speci?c feature of- our invention is the pro vision in a selective controllandindication sys tem for submarine mines of improved apparatus . nected to the line circuit through switchingmeans to indicate-at a shore of?ce the entrance of. a capable of connecting the control oscillator of vessel-into the mine?eld and¢to ?rethemine . any one of the‘ devicesand the operating oscil lator to the line circuit to supply control current and operating current to that device simulta neously. Such switching means‘may, be. con trolled. either manually or automatically. approached. A further speci?c feature of our ‘invention is the provision of - improved indication and. ?ring vapparatus for submarine .mines . wherewith. all sources of power are located at ashore o?ice. Other features, advantages and objectsnem bodying our inventionwill. appear-as the‘speci ~ ‘?cation progresses. To accomplish the‘? fOI‘EgOlIlg'. features, objects vand‘ ‘advantages -of.'iour\ invention We provide a ' control o?ice with a plurality of different. cur ' rent~ sources‘ which. are ‘connected .in different combinations ‘to a‘ two-conductor .line circuit: ex tending‘ from the 'o?ice to ‘each 1of. a; plurality;of 1 devices that are ‘to beindicatedzand controlled, .' excited in one sense to. actuate a' ?rstv indicator when the indication current is decreased and is excited in another sense .to: actuate. a second; in dicator when such indication current is increased. The control and operating oscillatorsare con Each device is provided With an electric'equip_ ment' which includes indication apparatus, con trol ‘and operating apparatus and testing and re set means. ‘ ' The indication apparatus of each ' equipment includes an ampli?er normally powered by the direct current-and commercial- alternating cur rent supplied tothe line circuit'at the o?‘ice, a detector unit, twoindication relays preferably of the bias polar type and aselector or. shunt cir cuit. The-indication relays are connected to 2,409,720 the output side of the ampli?er and the detector unit is connected to the input side of the ampli ?er to operate the relays one after the other in response to a predetermined condition or event at the particular device. Either one of these indi cation relays when operated alone connects the shunt or selector circuit which includes a ?lter across the line circuit. Such ?lter is tuned to ‘ resonance at the frequency of the indication oscillator of the same device. Thus when a selec tor circuit is connected to the line circuit, the circuit is detuned, asfar as the respective indication oscillator is concerned and its indica tion or line charging current is materially reduced and such reduction of the current supplied by the one oscillator is effected with substantially no change in the current supplied by any one of the 4 OF designates an omce from which a submarine mine field is controlled. Such mine ?eld will include a number of mines or a plurality of similar groups of mines, there being a sufficient number of mines to protect a given area. In Fig. .1 only two mines Ml and M2 of a mine ?eld are shown, since the mines are alike and two are sui?cient to fully illustrate our invention. Oi?ce OF may be on shore or any other place in the vicinity of the area that is protected, and the o?ice and mines are connected by a transmitting or line circuit which preferably comprises a me tallic conductor as one side of the circuit and the ground path as the other side of the circuit, although a second metallic conductor can be used. In Fig. l, a cable Ll extends from the o?ice to a distribution point DP conveniently located with respect to the mine ?eld and single conduc tor cables ML! and ML2 extend from the dis tribution point DP to mines MI and M2, respec tively. Thus a two-conductor line circuit extends from the office to the mines in multiple, one side of such circuit including the conductor Li and the other side being completed through eifect through the office receiver a second indica tion. A condenser is at times connected to the 25 the ground path as will appear from time to time during the speci?cation. It is to be noted that line circuit to be charged by the of?ce source of line conductor Ll possesses distributed capaci direct current and then at other times it is con tance with respect to ground and to visualize such nected to the indication relays to reset these capacitance a dotted line condenser CL has been relays to their normal positions due to the dis 30 added to the drawings but it is to be understood charge of the condenser. this condenser forms no physical piece of appa The control and operating apparatus of the ratus. Fuses IE1 and II are interposed in cables equipment of a device includes two control relays, ML! and MLZ, respectively, for reasons to appear one connected to the line circuit through a filler hereinafter. tuned to pass current only of the frequency of the The apparatus at of?ce OF comprises different control oscillator of that device. The other con sources of current and indication apparatus, to trol relay is connected to the line circuit through other indication oscillators. The indication relays govern the selector circuit to connect it from the line circuit when both indication relays are oper— ated, so that with both relays operated the indi ' cation current supplied by the respective indica tion oscillator is increased to its normal value to - ' ‘ > ‘ a ?lter tuned to pass current only of the fre - quency of the operating oscillator. These two gether with control, testing and cancelling means. The current sources include a battery, a source of commercial frequency alternating current and control relays govern each other and a pumping action results when control and operating cur 40 a plurality of oscillators. Battery 12 is prefer ably of a relatively high voltage, such as 250 volts, rents are supplied to the line circuit simultane suitable for energizing electron tubes, and it is ously. The equipment of each device also in connected across the line circuit by its positive ‘ cludes a condenser of relatively small capacity terminal B250 being connected to cable Ll and a condenser of relatively large capacity. The ' pumping action of the control relays connects the - small condenser to the line circuit for it to be charged by the direct current source and then connects this condenser to the large condenser to discharge the small condenser into the large con denser. After several such operations the large condenser is charged to a point where it excites an associated gas ?lled tube to the breakdown ' point and the large condenser discharges through the tube in series with an operating unit'of the device causing operation of the electric equip ment. The control relay alone is used to test the - indication apparatus. ' through an inductance I 3 and its negative ter minal N256 being connected to a ground elec trode ill. The commercial source of alternating ' current includes a transformer TP, a secondary winding of which is connected to ground electrode i4 and to line conductor Ll through a ?lter 9 turned to pass current of 60 cycles per second. The office oscillators may be of any one of several types, and preferably they are of the electron tube type powered from battery I2 or some other convenient source, and these oscillators are shown > conventionally for the sake of simplicity. A con tro-l oscillator and an indication oscillator are provided at the office for each of the mines, to We shall describe several forms of apparatus gether with a ?ring oscillator which is common embodying our invention, and shall then point out the novel features thereof in claims. 60 to the mine ?eld. Indication oscillator 18] and control oscillator CS1 for mine Mi, and a ?ring In the accompanying drawings, Figs. 1, 2 and 3 or operating oscillator FS only are shown in Fig. l are diagrammatic views showing three different in order to not unduly complicate the drawings ' forms, respectively, of apparatus each of which because these oscillators are sumcient for an un embodies our invention when used for control ‘ and indication of a submarine mine ?eld. Fig. 4 65 derstanding of our invention. These oscillators \ supply current at different frequencies and to ' is a diagrammatic view showing a modi?cation of ‘ aid in understanding the invention we shall refer - the apparatus of Fig. 3 that we may use, and to the frequencies of oscillators ISI, CS1 and FS which also embodies the invention. as being fl, I20 and ,f30, respectively. In each of the different views, like reference Indication oscillator 18! is normally active due characters are used to designate similar parts. 70 a to its input side being connected to terminals It is to be understood that our invention is not B258 and N256 of battery 12, and the output side limited to submarine mine ?elds, and this one of this oscillator is connected to conductor Ll ‘ application serves to illustrate the many cases . and ground. electrode 15 of the line circuit " where such apparatus is useful. Referring to Fig. l, the reference character 75 through a ?lter Fl comprising a transformer TI 2,409,720 7 spect to the armature 28 and the permanent mag tion, that is, it now flows in the direction indi net and other parts of the relay, armature 28 is held at this right-hand position with a relative ly strong pull, so that if winding 29 is energized cated by the arrow placed on winding 34 of relay D2. Armature 34 of relay D2 is now operated to its left-hand position, but armature 28 of relay DI remains at the right-hand position because by current flowing in the direction opposite to that indicated by the arrow, the energization is the energization effected through winding 29 is inelfective to overcome the relatively strong pull ine?ective to move armature 28 away from the of the magnetic bias of the relay. right-hand position unless the value of current Relays PI and P2 are direct current neutral is many times that required to operate the ar mature from the mid position to the right-hand 10 relays that are governed by the indication relays DI and D2 and by each other in a manner to position. Winding 38 of relay Dl is a reset wind appear shortly. ing and the number of turns of this winding and The shunt or selector circuit includes a ?lter the proportioning of the parts are such that when MFI comprising inductance and capacitance current of a given value ?ows therethrough in the direction indicated by an arrow placed on 15 tuned to resonance at the frequency of fl, that is, at the frequency of the indication oscillator ISI winding 36, the armature 28 is moved against for mine MI. This selector circuit extends from the relatively strong pull of the magnetic bias cable MLI of the line circuit through ?lter MFI, back to the mid position. Thus, a relatively weak back contacts 36 and 31 of relays P2 and PI, re energization of the relay by current flowing through winding 29 in the direction of the arrow 20 spectively, and either through contact 32-33 of relay D2 and condenser C2 to ground electrode placed on that winding is su?'icient to operate 38, or through contact 28-3I of relay DI and armature 28 from the mid to the right-hand po condenser Ci to ground electrode 38. Condensers sition but a relatively strong energization of the CI and C2 are relatively large and do not appre relay by current of a given value ?owing in wind ing 30 in the direction of the arrow placed on 25 ciably affect the tuning of the selector circuit. It follows, that with either relay DI or D2 operated that winding is required to reset the armature the shunt or selector circuit is connected to the 28 back to its mid position. Armature 28 re mains in the position to which it was last moved when the relay is deenergized. Relay D2 is of like construction, except it is arranged for its armature 32 to be moved from the mid position to a left-hand position as viewed in Fig. 1 to en gage a contact 33 when operating winding 34 of line circuit. When both relays DI and D2 are operated, the relays PI and P2 are supplied with direct current from the line circuit through inductance 39, con tact 28—-3l of relay DI, contact 32—33 of relay this relay is energized by current flowing in the direction of the arrow placed on winding 34, and 35 armature 32 is reset to the mid position when reset winding 35 is energized by current ?owing in the direction of the arrow placed on that wind ing. Windings 29 and 34 are connected in series to the two portions of secondary winding 63 of transformer 23 through contact member-8 of the coder, contact member 8 serving to rectify the electromotive force induced in secondary wind D2, winding of relay PI, back contact 4|] of relay P2, and ground electrode 38, and relay PI is ener gized. Relay PI when energized to close front contact 4I completes the connection for relay P2 to the line circuit and relay P2 is energized and picked up to open back contact 48 to deenergize relay PI, Which latter relay is released to open front contact 4| and deenergize relay P2. Thus relays PI and P2 are energized with a pumping action Whenever relays DI and D2 both occupy their operated positions. Such pumping action of relays PI and P2 serves to intermittently open the selector circuit at back contacts 36 and 31. Relays DI and D2 are reset through the means ing mine MI, a steady state condition exists for of a condenser C3 and a relay AAI. With relay ampli?er .AM and relays Di and D2 are deener AAI energized in a manner to appear hereinafter gized. When a vessel with a steel hull and other to close front contact ‘I0, a circuit path is formed magnetic and electric parts approaches, a mag for direct current to flow from cable MLI of the netic ?eld is created and magnetic flux threads ‘ - line circuit through inductance 39, resistance 'II, core 24 of unit DU, such flux being variable due a coil ‘I2, the function of which will appear here to the motion of the vessel and other conditions. inafter, front contact ‘II! of relay AAI, condenser This induces an electromotive force in coil 25, and C3 and ground electrode 15, and condenser C3 is such electromotive force is periodically inter rupted or chopped by operation of coder contact III Ct charged by the direct current of the line circuit. When relay AAI is subsequently deenergized, the member 1 and a corresponding coded electromo condenser 03 is disconnected from the line cir tive force is created in the secondary winding of cuit and is connected to the reset windings 30 and transformer 62 to be applied to tube 26, causing of relays DI and D2, respectively, through back a corresponding variation of current to appear in the plate current of the ?nal stage tube 22, with 60 contact ‘I? and ground electrodes 18 and ‘I5, and the discharge of condenser C3 ?ows through the result a corresponding electromotive force is windings 39 and 35 in the direction indicated by created in secondary winding 53 of transformer the arrows and is of a su?icient value to cause 23, This latter electromotive force is recti?ed the relays to be reset. by action of contact member 8 because contact The control and operating apparatus of the member 8 is operated in step with contact ‘mem equipment for mine MI includes control relays ber 7, and recti?ed current flows in the windings AI, BI and ABP, a gas ?lled tube I, condensers 29 and 34 of relays DI and D2, the connection C4 and C5 and a detonator DDI. Relays AI, BI being such that this current flows in the direction and lABP are neutral relays and relay AI is con of the arrow placed on winding 29 and armature 23 of relay Di is operated to the right-hand posi 70 nected to cable ML‘-I and ground electrode 42 of the line circuit through a?lter CF, a recti?er 43 tion, but armature 32 of relay D2 is retained at and front contact 44 of relay BI; and relay BI its mid position. When the vessel passes over the mine, the magnetic held about unit DU is op is connected to the line circuit through a filter positely varied so that the recti?ed current ?ow BF, a recti?er 45 and back contact 45 of relay AI. ing 63 in the well-known manner. Normally, that is, when no vessel is approach- ‘ ing in windings 29 and 34 is of the opposite direc 75 Filter CF is tuned to pass current only of the fre 2399339,. 9, 10 the mine andnpasses over the mine, the magnetic quency f2ii,that is, current only of the frequency supplied by the control oscillator CS1 formine ?eld vere'a'tec by the ‘vessel is'of the opposite nature anufinnu'é?ces 'theu'ni't‘DU ‘at ‘ampli?er AM to" Ml; Filter BF is tuned to pass current only of the frequency fSi'l, that is, current supplied by the ?ring oscillator FS. Thus, when currents from both oscillators CS1 and ES are supplied to the line circuit, relays AH and Bl are energized and deenergized with a pumping action because each relay controls the other. Relay ABP has one ter minal of its winding connected to ground elec~ trodell‘é and the other terminal of its winding connected to cable lViLl through front contact 4? of relay Al and an inductance 48, and relay ABP is operated in step with the operation of relay A! dueto being energized by the direct current from cau'?se‘current to‘?ow'in the operating'windings of relays ‘Di and D2 ‘in the oppositev direction from‘ that ?rst described, andrelay D2 is ‘operated, the ‘ first‘rela'y D! being retained‘ at it'soperatéd posii‘ ti'51'i“dué"to-the "strong magnetic bias of‘ the relay'." With both'rel'ays 'D'l 'and‘DZ' ‘at-“their operated positions 'the‘a'ssociated relays P'! vand‘ P2 are one ergi zed ‘with a *pump'ingaction i causing the‘ shunt circuit‘ to‘v be‘ ?rst‘dis'connect‘ed‘ from the line‘cir cuit ahdthen thereafter intermittently connected idisconne‘ctediro'n'i the: line circuit.‘ 'The‘?rs'ti "e the ‘shunt c'irc‘uitiis ‘disconnected ~the indical‘ current" rom oscillator 'ISI 'is‘increased to'its' the line circuit. Condenser Cél is normally charged by direct area ie/ ‘the: ’ receiver R'Al' is ' energized ‘ edu‘ir'ed‘tof'illuminate the second indicator" current‘supplied' from the line circuit due to one terminal of the condenser being connected to lamp 521,-“‘the display of lamp'E-‘l' serving asa'n'in ground electrode £9 and its other terminal being connected to cable ML! through back contact 58 of; relay ABP, resistance 5! and inductance 48. dicat'i'on to the'operatcr "of the vp‘re'sen’cerof ‘the’ ves's'el'within thede‘structing area ‘o'f'm‘in'e Ml.’ When relay ABP is picked up to open back con tact?? and close front contact 52, the condenser C4 is disconnected from the line circuit and is actionTt-ojcancel‘the illumination of lamps‘5'lv and‘v 5' "as "he cani’iif'a manner t'o'be'explained‘shortlm pumping action ofr'elaysrr and P2 'will‘ini media’télyiestore‘ the“ illumination 'of the’lamps "'It'ii's to5bc'ob‘served'that if the operator takes connected to condenser C5 through resistance 53, and condenser C'él discharges into condenser C5. Condenser Cl; is of relatively small capacity and condenser C5 is of relatively large capacity, and consequently several operations of relay ABP are required to alternately charge condenser C4 from main‘ at their‘ ope‘rated'positions "- s "For the operator to "fire mine‘ MI in response to the indicationTdispIayed by lamps 51 and 66; an’ arming'switch S‘l would ‘be moved either up the line and then discharge the condenser into ohddiriii to abs-stoma 64 and complete'a' cire condenser C5 before condenser C5 is fully charged. cu'it’ *ror ‘ energizing ' cbntrol" ‘oséillator’CS I , from ‘ Condenser C5 is connected across control. ele ment Eli and cathode 55 of tube Vi through a ‘ voltage divider consisting of resistors o5 and 53, and is connected to anode ?athrough an element 85, of the detonator 13134. The parts'are pro battery'WZ. Tmmediatély‘after the'operation of switch VS! a sécbnd‘zor ‘?ringiswitch'l'sl‘would be closed? The closing" orfcontactts or switch‘ I'SA'I gcornpletesaciréuit for energizing ?ring -‘osci1-; ' lator iFS Jfrom ' battery “l 2 far'fdT the ' closing" 'of ‘con portioned so that when condenser C5 is substan tact 6‘! of switch ISI completes a circuit‘ path tially‘ fully charged, tube Vi is excited to its breakdown point and condenser C5 discharges for eoeaeeane'tne‘ output sides" of oscillators CSl. and Yes, t6? the ‘linear-‘suit. “'With‘both'“control cu-rreriti'a'n'd ?ring 1‘ current supplied to‘ the" "line through the element 69 and the anode-cathode c'ircu , both’ control rélay's-‘Al'fan'd' Bl' are" en'e’r} path of the tube andsuch discharge current is gi'z'ed" and are ‘ operated by‘ a :pumping ‘action be ofa' value to heat element 653 to cause the detona tor to ?re the mine. 45 cause 'these'lrel’ay's' controleach'TotherJ? Relay ABP 'is operated in step ‘with irel‘ay Al "and 'cor'i-v Relay AM is connected across the output termi d'en'Ser'CB-i's gradually ‘charged 'due to the opera’, nal of recti?er 12-3 and thus is energized when con tion-of charging condenser C4 from the’lin'e' and: trol current from oscillator CS! is supplied to the ‘then '~'d_iscli_arg5lng'condenser "Cd into ' condenser Normally the line circuit is energized by direct 50 'When'th'e charge on condenser C5 builds: up td‘a predetermined pointj-th’e- tube VI breaks current from battery l2 and by 60 cycle alternat down and condenser (35' discharges through the ing ‘current from transformer TR and is also excited by each of the indication oscillators, each detonator-toi'firei'the miné.‘ ‘ I " Y- -' “"Ifh'e': j-‘i'n'g of mine Ml would tear the mine end indication oscillator supplying a normal value of line charging or indication current. At mine Ml 55 of cable’MIai loose an'd'this cable‘ would become grounded causin'g‘an excessive ?ow of current and‘ the amplifier AM is powered by the direct current fuse‘liifwould blow to v"free the system of the and by the 60 cycle alternating current and is in cable to the‘ ?red minei ’ ' > f‘ > ' > ‘ an active condition. Likewise, the ampli?er at ‘It'is to be noted; that the operator can ?re mine each'of the other mines is in an active condition. MI by operation‘ of switches st and IS! at-any Assuming a vessel enters'the mine ?eld to ap ti,me,'such" action being ‘independent of the indie proa'ch mine Ml, the magnetic ?eld created by cation'apparatus; It'is clean'however, that the the vessel in?uences unit DU and current ?ows action~ of ?ring the mine can be made'autor'natic in the operating‘windings of relays D! and D2 or "can be" made“ dependent‘ upon ‘the indication a direction that operates relay Di and con line circuit‘; " ' ' ‘ ' - ' nec‘ts the shunt circuit including'?lter MFl across 65 means of_‘ relays controlled in 'multiple with the the line circuit. The connecting of ?lter MFI “Vle'shallj next ‘assume that a vessel approaches miner/n ‘to'bring‘about the display of lamps ols'cille or IS; and the indication current supplied 5? ‘and ‘66 as described hereinbefore, and’the op-‘ byo'sciliator 153 is reduced from its normal value; such reduction of the indication current ener 70 erator desires not to‘?re the mine and wishes ‘to reset ‘relays DI and D2 and cancel the display gizfng receiver EAL in the manner required to across the line circuit detunes the line circuit for illuminate indicator lamp The illumination oi lamp serves as a warning indication to the since operator of the presence of a’vessel and for liinito'be on the alert A‘s'the‘vessel approaches of'lamps '51 and 636'. Under this condition the switch "Si “would be moved to energize control oscillator CS! and‘a reset switch IS2' would be operated‘ to the left‘ as viewed in Fig. l to close 2,409,720 ' 12 11 ent, however, from the corresponding elements contacts 68 and 69. The closing of contact 68 connects the output side of oscillator CSI to the line circuit to energize relay AAI at the mine and bring about the charging of condenser C3. The closing of contact 69 completes an obvious circuit for an o?ice relay XI, and relay XI is picked up to close a stick circuit which includes its own front contact ‘I4 and a normally closed contact SI of switch IS2. The operator would next move the switch ISZ from the left-hand 10 position to the right-hand position opening con tacts 68, 69 and 6| and closing a contact 16. The opening of contact 68 interrupts the supply of control current from oscillator CSI and relay AAI at the mine is deenergized to connect con denser C3 to the reset windings of relays DI and D2 and these relays are reset to their normal condition by the discharge of condenser C3. The of Fig. 1. It is believed that an understanding of the mine equipment of Fig. 2 can best be had from a description of the operation of the appa ratus taken in connection with a description of the apparatus of Fig. 1. Normally, that is, when no vessel is approach ing the mine ?eld of Fig. 2, the line circuit is en ergized by battery I2 and transformer TP at the o?ice, and ampli?er AM at the mine is powered and made active. Also, the line circuit is excited by the indication oscillators for the several mines, each such oscillator supplying a line charging or indication current of a given normal value. When a vessel approaches mine MI of Fig. 2, for example, the electromotive force created at unit DU is applied to ampli?er AM and relay DI is en ergized at a polarity as required to operate the relay to close its contact ‘28—-3i. The closing of opening of contacts 69 and ‘BI deenergizes relay 3! of relay Di connects the selector XI, but relay Xi is slow release in character and 20 contact 2 circuit including ?lter MF! across the line circuit, during the slow release period of relay XI a cir suchselector circuit extending from cable MLI cuit is formed from terminal B of a source not through ?lter MFI, contact 28-3I of relay DI, shown through contact 16 of switch IS2, front a condenser C6 and ground electrode 38. Con contact 19 of relay XI, and winding of relay CM denser Cfc‘ is of large capacity and does not ma to terminal C, and relay CM is energized and terially aiiect the tuning of this selector circuit, picked up to open back contact I9 and close front and consequently the indication current supplied contact 80. The opening of back contact I9 of by oscillator ISI is reduced. This sudden reduc relay CM opens the anode circuits for tubes V3 tion in the indication current energizes receiver and V4 and they are deionized so that the lamps RA! as required to illuminate lamp 66. As the 5‘! and 66 are extinguished. The closing of con vessel moves closer to pass the mine, the electro tact 89 of relay CM is ordinarily ine?ective to motive force created through unit DU results in maintain the anode circuit for the electron tube the current supplied to the operating windings of closed through the normal position of switch SI relays Di and D2 ?owing in the opposite direc because switch SI is now set at either the raised tion from that ?rst supplied, and the armature or lowered position. The contact 80 and the con, of relay D2 is operated to close contact 32-33, nection through switch SI are useful in connec relay D-l remaining in its left-hand position duev tion with the control of other mines of other to the strong magnetic bias of the relay. With groups. both relays DI and D2 operated, direct current We shall next assume that the apparatus is in ?ows from cable ML! through inductance 39, re its normal condition and the operator desires sistance BI, winding of a relay AAZ, contact 29— to test the equipment at mine MI. Switch SI 35 of relay DI, contact 32-3-3 of relay D2 and is set to energize control oscillator CSI and switch to ground electrode 38. Relay AAZ is a bias polar IS’! is moved to the left to connect the output relay and the connections are such that the cur side of oscillator CSI to the line circuit and to rent thus supplied to the relay causes the relay also energize the o?ice relay XI. Relay AAI at to be energized at a polarity to operate its con mine MI is energized by the control current and tact members 32 and 83 against a spring bias condenser C3 is charged as previously explained. force to open contact 83—84 and close contact The initial surge of charging current ?owing $2-85. It is to be noted that contact 83-84 through coil 12 associated with the unit DU in ?uences the unit DU in a manner to bring about controls a circuit by which a condenser C3 is nor operation of relay DI to connect the shunt cir mally charged by direct current supplied from the line circuit through inductance 35-), resist cuit to the line circuit so as to reduce the indica tion current from oscillator ISI and cause lamp 66 to be illuminated. The parts are proportioned so that the dying down of the current as con denser C3 reaches a fully charged condition in ?uences unit DU in a manner to bring about op eration of the second relay D2 and the indicator lamp 5'! is illuminated due to the action of relays PI and P2 to interrupt the shunt circuit to restore the indication current to its normal value. Illu mination of lamps 51 and 66 would serve to indi cate that the line circuit to mine MI is complete, ampli?er AM is active, unit DU is effective and relays DI and D2 are in an operative condition. Movement of switch IS2 to the right results in theresetting of relays DI and D2 and the can celling of the display of lamps 51 and 65 in the manner explained previously. In Fig. 2 the o?ice apparatus and the equip ment for mine MI comprises as essential ele ments, indication apparatus and control and ?r ing apparatus, together with reset and testing apparatus, the same as in Fig. 1. Several ele ments of the mine equipment of Fig. 2 are differ ance ll, contact 83—8¢l and ground electrode l5. Operation of relay AA2 to open contact 83-—8!l Oi Oi and close contact &2—85 transfers condenser C3 to the reset windings and 35 of relays D5 and D2 and the discharge current from condenser C3 resets these relays. With relays DI and D2 re set the selector circuit is opened so. that the indi 60 cation current from oscillator IE3! is restored to its normal value and receiver RAI is energized as required to illuminate lamp 5?. To ?re mine Ml of Fig. 2, the operator moves switches SI and ISI to supply both control and 65 ?ring current to the line circuit. The control current is passed by ?lter CF to a control relay A3 of the bias polar type, the connection includ ing a normally closed contact 86 of a second con trol relay B3, which is also of the bias polar type. 70 With relay A3 operated to close contact 81, the connection of relay B3 to the recti?er 45 is com pleted and relay B3 is energized by the ?ring current passed by ?lter BF, Such operation of relay B3 opens contact 86 and relay A3 is deen ergized and operated back to its normal condi 13 i4 tion to open contact . 8.1V and; cause. relay,‘ Bl3gto. be deenergized; and operated to its normal condition. _ This pumping actionpithetwo control-_~relays.A_2_1 and~B3 will continue aslong. asi-control and. ?ring». lays DI and D2 opens the, selector. circuit‘. and also the circuit for relay AAZ. The openingQof the: selector circuit causes theindication current to berestored to, its normal value to bringabout currents are supplied; to the linercircuit. Con. the illumination of lamp 51, and, the deenergizjing. denser 04* of 2 is normally chargedby. direct I current ?owing from caplet/ll; , through-induct ance 39,! normally closed contact 8,3;of;re1 y .53, resistance 89,;norma1ly closed contact,Qilrotrelay B3, condenser- C-‘t and- to" ground electrode, 45‘. 10 During-the interval the’ two relays A??? arr-d133,, are operated to close contacts; 9,! and ii‘lqrespecg tively, condenser C4 isrconnectedito cendenserqi and discharges thereinte. wrmlenserv Cir: heirs. 0f;relay, AAZpermits this relay to be. restoredrito. itsnormal position.’ To ?re mine-M! of Fig.v 3,.thteuoperator. oper-v ates the switches SI and lSlto bring aboutthe supply of both; control, currentand :?ring. current to the line circuit. Control current is passedby ?lterCFnto energize control relayLAii. Relay A3,.‘ isoperated to :open; contact stand. close contact 93. .operationyoi contacts. Biand ?dserves small as ccmraredto. condenser C5,; the sari 291.52»: 15 to connectcondcnsencl‘and coil it across the‘ inFia 1,. Consequentlyafter-sacral Operatiges. oi the-two control-relays; a charge-is builtaipon condenser C5 which is conneqtcdto tube Vii and detonator- DDE by circuits; which’ are- the; same lineicircuit-to charge condenser Cl? through coil The. initial surge § of'chargingv currentv in; fluences coil ‘i2. to bring . about. the operation of r relay Di, andas the condenser Ci :becomesiully as in Fig. 1. When thisvcha-rge on condenser-0'5 20 charged; the.v dyingv away of they chargin'gl cure reaches a’ given value’. tube Vl'breaks downr'agnd; rent in?uences unit EU} to .bring', about. op condenser 05; discharges» through the tube and the d'etonator element-Eibto?re the mine. eration- of. relayDZ. ‘Relay. AAL'Iis again; encr'; gized and then. restored- to. its. normal" position. and the closing of contactv $1.01‘ relay .AAlcdi?é To test the equipment ofymine Mr’! of‘ Fig.- 2,1the, operator moves. switches Si- and ISZ; to‘ supply 25 pletes thecorinection for- control relay B5. that relay is energized by tha?ringcurrentflow} control current to the line» circuit‘ and» operate throughr?lter BF; Relay. Ba. onceqperated relay A3 to close contact. 9,3;andopena normally closed contact 34, so thatacondenser- C'hlS-COIl-r nected; to the line circuitto be. chargcdiby-the direct current, current, ?owing- froin conductor . completesa ‘stick circuit inelu'dingits owncontact. 98 to by-pass contact Sl'ofrelay. Adi’; Vyjithrer, lay AAZ restored. to itsv normal position, andboth MLI through inductance 39; resistance ‘H,v con relays. A3. and B3 operated 'a' connectionis com; tact 9,3 of relayAB, coillZ, condenser Ci, resist-. pleted from cable MLi throughinductance ~. ,, ance, 95 andto ground electrode. 95. The initial surge of current‘through coil’ ‘l2 creates; an elec tromotive force in unit DU'that results in opera- i; tion of relay Dl'to bring about a reduction of the contact, lutof; relay AAZ,’ contact it} of relay; A3, contact it? oi relay, B3,. resistance. ititqan'd. condenser Cato ground “electrode lid, andacharge indication current and the resultant illumination isereduellrbuiltue on waders-each ceiidéelsler: C5 is connected to tubeVL andtothedetonator of lamp?s. The parts are proportioned so that as the current dies down when condenser C‘! be DD l, the..same.-as__ in . Fig. ,1, and .when. the charge, on condenser C5 becomesu?ficient,toieiiciteltube, comesfully charged, the electromotive force cre 40. VI to the. breaking downipQint/the, tube Vi? be} comes. conductive. and‘ condenserbdi discharge" ated at unit-DU isof-the polarity and character istic that bring about-operation of relay D2, and through the detonators to. the, mine, The‘ testing of the. apparatus, of Fig. 3 would 'beace with both relays Di and D2 operated, relay AAZ is energized'to open the selector circuit and the comnlisheci in- substailtielly. iheisems as explained in connection. withzt'he testing'oi the indication current is restored’ to its normal value apparatusof Fig. 2‘. ' ' " ‘ to bring about the illumination of’ lamp 5?. Fig- 4 ‘3359101568v a modi?er-tige- Of» tlfléi aFigarr'aiH? In Fig. 3, the apparatus at the o?ice and the mine equipment are the same as in Fig. 2 except of Fig. 3 that wemay use. InFig. 4i, condenser C5 of the mine equipment isnormally chargedby for the circuit connection for the control portion of the mine equipment, and the apparatus of Fig. 9 direct current from the, line circuit through a 3 Will be readily understood from an inspection of relatively high resistance, i535. Thus, when con Fig. 3' taken in connection with the description of trol relays A3 andB-iare. operated. in response the‘apparatus of Figs. 1‘ and 2. Normally,v the tocontrolgand ?ring. current, and the associated ampli?er AM'ofFig, 3 is powered by direct cur relay AAZ‘ hasbeenrestored toits normal ppsig rent and 69‘ cycle alternating current supplied 55 tion, the condenser C5 discharges through the to the line circuit at the o?lce. A vessel ap circuit connection including.’ contacts. tee,‘ 10 i. and proaching mine Ml of Fig. 3 causes the operation it; of‘ relays, Ant, A73; andBS, respectively, and of relay Di in the same manner explained in the detonator, and the detonatoris ?red in re: the previous cases and operation of relay Di connects in. turn the selector circuit including sponse to the discharge current. ?lter MFS to the line circuit to bring about a re duction in the indication current supplied by os~ cillator IS! and the resultant illumination of lamp 6%. With the vessel passing over the mine vantages that all Seems 0?; Gwen-i for indie Ml, the influence of'unit DU brings about operation of relay Dita-s previously explained. Opera tion of both relays DI and D2 of Fig. 3 result in relay AAZ being connected to the line circuit and energized by direct current from battery E2. Energizing of relay AA2 to open contact ‘til-~34 and‘ close. contact 82~—B-5 disconnects condenser C3 from the line circuit and connects to the reset windings of relays Di and D2, and these re~ lays are reset by the discharge from condenser C3‘ as previously explained. The resetting of re - ' ' ' ' i Apparatus such as here disclosed has the ad: Gating’ warming the Qf. a miss-?eld are loeatedin 2i seem 911306. a reiaiivelrmgn decree. of selectivity isobteined in the ‘*‘dica-tion of the approach of vessels to in. ua-l mines of the mice ?eld, all-01th‘? non‘ - cf; the mine 0;? the mine due toshocl; excita?on or foreign cur reniisminimizea - r ' Ahheueh We have herein Shawn and» described several forms. of; selective, control’ and indication systems embodying our inyention, it is under stood that various changes and modi?cations may be made therein within the. scope of; the 2.;:—_ pended‘ claims without departing from thespirit and scope of- our invention. 2,409,720 151' claim is: ~ ' 1. In combination, an office, an electric equip ment connected to said of?ce by a line circuit, 7 said normal value, and office indicator means coupled to the o?ice ?lter and operated to ,a?rst ora second condition according as such flow of an of?ce indication oscillator of o a preselected frequency connected to said line circuit to supply a line charging current of a given normal value,’ two indication relays at said equipment, means including a magnetic responsive unit connected to said relays to operate one relay in response to 16 tures of, said relays to their given positionsto disconnect said selector circuit andrestore the current supplied by said indication oscillator to Having thus described our invention, what we I ' indication current is reduced or restored. 4. In combination, an office, an electric equip ment connected to said o?ice by a line circuit, an 10 o?ice , indication oscillator of a preselected fre a ?rst condition ‘of a magnetic ?eld and to oper- ~ ' ate the other relay in response to 'a second con quency connected to said line'circuit to supply a line charging current of a given normal value and dition of a magnetic ?eld, a selector circuit in cluding a ?lter‘tuned to resonance at said pre-~ ,which current serves as an indication current, tion relays to be connected across said line cir cuit when either one of said relays is operated to 7 said relays to selectively operate the relays in, response to a ?rst and'a' second given magnetic, tWo indication relays at said equipment means selected frequency and governed by said indica l5 including a magnetic ?eld detector connected to varythe valueof the current supplied'by said ‘ indication oscillator to said line circuit, means ?eld condition at said equipment, a selector cir , cuit including a ?lter tunedto resonance at said controlled jointly by said relays when both relays 20 preselected frequency and connected across said, ~ line circuit through an operated position contact are operated sequentially to disconnect said se of either one of said indication relays to reduce lector circuit to restore the current supplied by the value of said indication current, two other said oscillator to its normal value, and o?ice in relays at said equipment, circuit means including dicating means I coupled to said oscillator and in series an operated position contact of each of , governed by the indication current thus supplied said indication relays and contacts of, said other by it‘to the line circuit. relays to energize said other relays with a pump '2. 'In' combination, an o?ice, an electric equip ment connected to said of?ce by 'a line circuit, ' an o?ice indication oscillator of a preselected ire ing action, a back contact of at least one of said other relays interposed in said selector circuit quencyconnected to said line circuit'to supply a ‘30 to alternately open and close the selector circuit ,to alternately reduce and restore the value of line charging current of a given normal value, said indication current, and office indicator means two indication relays at said equipment, means coupled to the connection of said oscillator to including a control, unit connected to said relays said line circuit selectively governed bysuch con to effectively energize one relay in response to a trol of the ?ow of said indication, current. ' ?rst given condition and to e?ectively, energize ' I the other relay in response to a second given con 5. In combination, an oi?ce, an electric equip- ' ment connected to said o?ioe by a line circuit, dition, a selector circuit including reactance con an of?ce indication oscillator of a preselected fre nected to said line circuit through a contact of quency and an of?ce source of direct current con either-one ofv said relays closed when‘ the relay is effectively energized to reduce the value of the 40 nected to said line circuit to supply a line charg ing current of a given normal value and which current supplied by said oscillator, means gov current serves as an indication current, two in erned by said relays to disconnect said selector dication relays at said equipment, each of said circuit from said line circuit when both relays relays having an operating and a reset winding are energized to restore the current supplied by and an armature operable to a ?rst and a second said oscillator to its normal value, and office indi position, means including a magnetic field detec cator means coupled to the connection of said tor connected to the operating windings of said oscillator to said line circuit and operated in one sense when said ?ow of indication current is re duced and in a second sense when the ?ow of the relays to selectively operate the relays in response to a ?rst and a second given magnetic ?eld con 50, dition, a selector circuit including a ?lter tuned indication current is restored. to resonance at said preselected frequency and 3. In combination, an o?ice, an electric equip connected across said line circuit through the ment connected to said o?ice by a line circuit, second position of the armature of either of said an o?ice indication oscillator of a preselected fre relays to reduce the value of said indication cur quency, a ?lter, said oscillator connected to said line circuit through said ?lter, said ?lter tuned 55 rent, another relay connected across said line circuit through the second position of the arma to create a resonant condition for said oscillator tures in series of said indication relays to ener and line circuit with its distributed capacitance gize said other relay from said direct current for the oscillator to supply a line charging cur source, a condenser connected to said line circuit rent of a given normal value, two indication re lays at said equipment, each of said relays having 60 through a deenergized position contact of said other relay to charge the condenser by direct cur an operating winding and a reset winding and rent and said condenser connected to said reset an armature movable between a given and an windings through an energized position contact operated position, means including a magnetic of said other relay to reset the indication relays ?eld detector connected to the operating Wind ings of said relays to selectively operate said re 65 by the discharge of the condenser and disconnect said selector circuit, and office indicator means lays in response to a ?rst and a second given coupled to the connection of said oscillator to the magnetic ?eld condition at said equipment, a line circuit energized by the indication current selector circuit including a ?lter tuned to reso thus supplied to the line circuit. nance at said preselected frequency, said selector 6. In combination, an o?ice, an electric equip~ circuit connected across said line circuit through 70 ment connected to said o?lce by a line circuit, an the operated position of the armature of either o?ice indication oscillator of a preselected fre one of said relays to detune the line circuit and quency connected to said line circuit to supply a reduce the value of current normally supplied line charging current of a given normal value by said indication oscillator, means connected to and which current serves as an indication cur said reset windings to at times reset the arma a 2,409,720 17 rent, an electron tube ampli?er at said equipment and including a ?rst and a second coder contact member which are interposed in the input side and output side respectively of the ampli?er, a 18 ual frequency, said direct current source and indication oscillator connected to said line cir cuit, switching means to at times connect said control and operating oscillators to the line cir magnetic ?eld detector connected to the input 5 cuit, a ?rst and a second indication relay at said equipment, indicating means connected to side of the ampli?er to supply a ?rst and a second said relays to selectively operate the relays in re_ electromotive force in response to a ?rst and a sponse to a ?rst and a second given condition at second magnetic ?eld condition at said equip the equipment, a selector circuit including a ?l ment, a ?rst and a second polar indication relay connected to the output side of the ampli?er to 10 ter tuned to resonance at the frequency of said indication oscillator and connected across the line selectively operate the relays in response to said circuit through an operated contact of either electromotive forces when thus coded and ampli ?ed, a selector circuit including a ?lter tuned to resonance at said preselected frequency and con nected to said line circuit through an operated position contact of either one of said relays to reduce the value of said indication current, means including in series an operated position contact of each of said relays to disconnect said selector circuit to restore said indication current to its normal value, and oi?ce indicator means coupled to said oscillator operated by such indication cur rent. 7. In combination, an o?ice, an electric equip ment connected to said o?ice by a line circuit, an electron tube ampli?er at said equipment and having a ?rst and a second code operated contact member which are operated in step with each other and interposed in the input side and output one of said indication relays to control the cur rent supplied by said indication oscillator, an o?ice indicator coupled to the line circuit ener gized by such indication current, another relay at said equipment connected to the line circuit through a contact of each of said indication re lays in seriesto energize that relay by said di rect current source, a ?rst and a second con trol relay at said equipment connected to said line circuit vthrough a ?rst and a second ?lter respectively, said ?rst ?lter tuned to pass cur rent only of the frequency of said control oscil lator and said second ?lter tuned to pass cur rent only of the frequency of said operating os cillator, a condenser and an operating unit of said equipment, and circuit means including con tacts of said other relay and said control relays side respectively of the ampli?er, a magnetic ?eld 30 to connect said condenser at times to said line circuit to be charged by said direct current source ' detector connected to the input side of the ampli and at times to said unit to operate the unit by ?er to supply electromotive forces varied accord the discharge of the condenser. ing to given variations in a magnetic ?eld at the equipment, a ?rst and a second indication relay connected to the output side of the ampli?er and 10. In combination, an office, a submarine mine . connected to said office by a line circuit, an o?ice indication oscillator of a preselected frequency and an o?ice direct current source connected to amplifying of said electromotive forces, means said line circuit, an ampli?er at said mine con including contacts of said indication relays and nected to said line circuit to be powered by said connected to said line circuit to cause an indica tion current to ?ow in the line circuit as prede 40 direct current source, two indication relays at said mine connected to said ampli?er, a control termined by the operated condition of said re unit responsive in a ?rst and a second sense to lays, and o?ice indicator means connected to the the magnetic ?eld created by a vessel approach line circuit governed by such indication current. ing said mine and connected to said ampli?er to 8. In combination, an o?ice, an electric equip control said relays according to the position of ment connected to said of?ce by a line circuit, an the vessel with respect to said mine, a selector office control oscillator and an office operating circuit including a ?lter tuned to resonance at oscillator, said oscillators of different preselected said preselected frequency and connected across . frequencies, switching means to at times connect said line circuit through a contact of either one said oscillators to said line circuit, a ?rst and a of said relays to decrease the ?ow of indication second relay at said equipment, said ?rst relay current from said oscillator, means including in connected to the line circuit through a connec series a contact of each of said relays t0 discon tion disposed to pass current only of the frequen nect said selector circuit to increase the flow of cy of the control oscillator, said second relay indication current, and of?ce indicators coupled connected to the line circuit through a connec to the connection of said oscillator to said line tion disposed to pass current only of the fre circuit and selectively governed to a ?rst and a quency of said operating oscillator, contacts of second condition according as said ?ow of indi said relays interposed in such line circuit con cation current is decreased or increased. nections for each relay to control the other to 11. In combination, an o?ice, a submarine mine e?ect a pumping action of the relays when both selectively operated in response to the coding and of said oscillators are connected to the line cir 60 connected to said o?ice by a line circuit, an office indication oscillator of a preselected frequency cuit, a relatively small condenser and a relatively and a source of direct current connected to said large condenser, circuit means including contacts line circuit, an ampli?er at said mine connected to said line circuit to be powered by said direct and to said large condenser to build up a charge 65 current source, two polar indication relays at said mine, each said relay having an operating and on the large condenser in response to the pump a reset winding, said operating windings con ing action of said relays, and an operating unit nected to the output terminals of said ampli?er, connected to said large condenser effectively en a magnetic ?eld detector unit connected to the ergized when a predetermined charge is built up 70 input terminals of the ampli?er to govern the on the large condenser. polarity of the current ?owing in said operating 9. In combination, an office, an electric equip windings according to a ?rst and a second mag~ ment connected to said of?ce by a line circuit; netic ?eld condition created by a vessel approach an of?ce direct current source, indication oscil ing said mine to selectively operate the relays, a lator, control oscillator and operating oscillator; each of said oscillators of a preselected individ 75 selector circuit including a ?lter tuned to res governed by said relays to alternately connect said small condenser to a source of direct current gleam 20 19 onance at said preselected frequency and con nected across said line circuit through an oper ated position contact of either one of said relays to decrease the ?ow of indication current from said oscillator, means including in series an op erated position contact of each of said relays to disconnect said selector circuit to increase the '?ow of such indication current, o?ice indication means coupled to said oscillator and selectively responsive to such changes in the indication cur rent, another office oscillator of a different pre selected frequency disposed to be connected at times to said line circuit, another relay at said mine connected to the line circuit through a ?l ter tuned to pass current only of said different preselected frequency, a condenser, and a circuit means including contacts of said other relay .to connect at times said condenser to said line cir cuit to charge the condenser by said direct cur ‘rent source and to connect at times said con denser to said reset windings to reset said relays ‘by the discharge of said condenser. 12. In combination, an o?'ice, a submarine mine connected to said of?ce by a line circuit, an o?'ice indication oscillator of a preselected frequency ent preselected frequency and connected at times to said line circuit, another relay at said mine connected to said line circuit through a ?lter tuned to pass current only of said different pre- ‘ selected frequency to energize said other relay, a coil disposed for magnetic relationship with said detector unit, a condenser; and circuit means in cluding a contact of said other relay to connect said coil and condenser in series across said line circuit to charge the condenser from said direct current source through said coil to test the oper ative condition of said line circuit, ampli?er, in dication relays and indication means. 13. In combination, an office, a submarine mine connected to said office by a line circuit, an o?ice direct current source connected to said circuit, an o?ice control oscillator and an o?ice ?ring oscil lator, said control and ?ring oscillators of a ?rst and a second preselected frequency respectively, 20 of?ce switching means to at times connect said ' oscillators to said line circuit, a ?rst and a second relay at said mine, said ?rst relay connected to said line circuit through a ?lter tuned to pass current only of said ?rst frequency, said second a selector circuit including a ?lter tuned to res onance at said preselected frequency and con relay connected to said line circuit through a ?lter tuned to pass current only of said second frequency, a back contact of said ?rst relay in terposed in the connection of said second relay and a front contact of said second relay inter posed in the connection of said ?rst relay to en ergize said relays by a pumping action when both of said oscillators are connected to the line cir cuit, a relatively small and a relatively large con denser, circuit means governed by said ?rst and second relays to connect said small condenser alternately to said line circuit and to said large nected across said line circuit through an ener condenser to build up a charge on the large con gized position contact of either of said relays to reduce the flow of indication current from said gized position contact of each of said relays to disconnect said selector circuit to restore the ?ow denser due to the pumpingaction of said relays, a gas ?lled tube, a detonator, and said large con denser connected to said tube and detonator to break down said tube and discharge said large condenser through said detonator and tube to ?re of said indication current, o?ice indicators cou the mine. and a source of direct current connected to said line circuit, an electron tube ampli?er at said mine connected to the line circuit to be powered by said direct current source, two indication re lays at said mine connected to said ampli?er, a magnetic ?eld detector unit connected to said ampli?er to selectively energize said relays ac cording to a ?rst or a second magnetic ?eld con dition created by a vessel approaching the mine, oscillator, means including in series an ener pled to said oscillator and selectively responsive to such changes in the ?ow of the indication ‘ current, another office oscillator having a differ ' WILLARD P. PLACE. ALFRED B. MILLER.