HANDBOOK OF MAGNETICCOMPASS ADJUSTMENTXNATIONALGEOSPATIAL-INTELLIGENCE AGENCYBETHESDA.MD2004(FormerlyPub.No.226)ASORIGINALLYPUBLISHEDBYDEFENSEMAPPINGAGENCYHYDROGRAPHIC/TOPOGRAPHICCENTERWASHINGTON,D.C.1980
HANDBOOK OF MAGNETIC COMPASS ADJUSTMENT NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY BETHESDA, MD 2004 (Formerly Pub.No. 226) AS ORIGINALLY PUBLISHED BY DEFENSE MAPPING AGENCY HYDROGRAPHIC/TOPOGRAPHIC CENTER WASHINGTON, D.C. 1980
INTRODUCTIONThis document has been prepared in order to present all pertinent information regarding the practical procedures ofmagneticcompass adjustment inonetext.As such,ittreats of thebasicprinciples ofcompass deviations andtheircorrection,andnotof the details ofparticular compass equipment.Although this text is presented as a systematic treatise on compass adjustment, ship's personnel who are inexperienced withcompass correction will find sufficient informationin ChaptersI andXiVtoeliminatecompasserrors satisfactorilywithoutintensivestudyofthe entiretext.Referenceshould alsobemadetofigure318forcondensed informationregardingthevariouscompasserrorsandtheircorrectionInthishandbook,thetermcompassadjustmentreferstoanychangesofpermanentmagnetofsoft ironcorrectorswherebynormalcompasserrorsarereduced.Thetermcompasscompensationreferstoanychangeinthecurrentsuppliedtocompasscompensating coils whereby the errors due to degaussing are reduced.The basic text is the outgrowth of lecture notes prepared by Nye S.Spencer and George F.Kucera while presenting coursesof instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station,NavalOperatingBase,Norfolk,Virginia.CHAPTERIPROCEDURESFORMAGNETICCOMPASSADJUSTMENT(CHECK-OFFLIST)NOTE: If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flindersbarlength,thecoilcompensationmustbechecked.RefertoChapterXIV.101.Dockside tests and adjustmentsPhysical checks on the compass and binnacle.(a)Remove any bubbles in compass bowl (article402).(b)Testformomentandsensibilityofcompassneedles(article403)(c)Removeanyslackingimbal arrangement(d)Magnetizationcheck of spheresandFlindersbar (article404)(e)Alignment of compass withfore-and-aft line of ship (article 405).(f)Alignmentofmagnetsinbinnacle(g)Alignment of heeling magnet tubeunder pivot point ofcompass.(h)SeethatcorrectormagnetsareavailablePhysical checks ofgyro,azimuth circle,and pelorusesP(a) Alignment of all gyro repeater peloruses or dial peloruses with fore-and-aft line of ship (article 405).(b)Synchronizegyro repeaters withmastergyro.(c)Make sure azimuth circle and peloruses are in good operating condition3.Necessary data.(a)Pasthistoryorlog datawhichmightestablish lengthofFlindersbar (articles407and 607)(b)Azimuthsforgivendateand observer'sposition(ChapterVIIl)(c)Ranges or distant objects invicinity (local charts).*(d)Correctvariation(localcharts)(e)Degaussingcoil current settingsforswingforresidual deviations afteradjustmentandcompensation(ship'sDegaussing Folder).Precautions.(a)Determinetransientdeviationsofcompassfromgyrorepeaters,doors,guns,etc.(ChapterX)(b)Secureall effectivemagneticgearinnormal seagoingpositionbeforebeginningadjustments(c)Make sure degaussing coils are secured before beginning adjustments. Use reversal sequence, if necessary(d)Wheneverpossible,correctorsshouldbeplacedsymmetricallywithrespecttothecompass(articles318and 613).5.Adjustments(a)PlaceFlinders bar according tobest available information(articles407, 608and 609).(b)Setspheres at midposition,oras indicatedby last deviation table(c)Adjust heeling magnet, using balanced dip needle if available (Chapter XI).Applies when system other than gyro is used as heading reference
1 INTRODUCTION This document has been prepared in order to present all pertinent information regarding the practical procedures of magnetic compass adjustment in one text. As such, it treats of the basic principles of compass deviations and their correction, and not of the details of particular compass equipment. Although this text is presented as a systematic treatise on compass adjustment, ship's personnel who are inexperienced with compass correction will find sufficient information in Chapters I and XIV to eliminate compass errors satisfactorily without intensive study of the entire text. Reference should also be made to figure 318 for condensed information regarding the various compass errors and their correction. In this handbook, the term compass adjustment refers to any changes of permanent magnet of soft iron correctors whereby normal compass errors are reduced. The term compass compensation refers to any change in the current supplied to compass compensating coils whereby the errors due to degaussing are reduced. The basic text is the outgrowth of lecture notes prepared by Nye S. Spencer and George F. Kucera while presenting courses of instruction in adjustment and compensation during World War II at the Magnetic Compass Demonstration Station, Naval Operating Base, Norfolk, Virginia. CHAPTER I PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT (CHECK-OFF LIST) NOTE: If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders bar length, the coil compensation must be checked. Refer to Chapter XIV. 101. Dockside tests and adjustments 1. Physical checks on the compass and binnacle. (a) Remove any bubbles in compass bowl (article 402). (b) Test for moment and sensibility of compass needles (article 403). (c) Remove any slack in gimbal arrangement. (d) Magnetization check of spheres and Flinders bar (article 404). (e) Alignment of compass with fore-and-aft line of ship (article 405). (f) Alignment of magnets in binnacle. (g) Alignment of heeling magnet tube under pivot point of compass. (h) See that corrector magnets are available. 2. Physical checks of gyro, azimuth circle, and peloruses. (a) Alignment of all gyro repeater peloruses or dial peloruses with fore-and-aft line of ship (article 405). (b) Synchronize gyro repeaters with master gyro. (c) Make sure azimuth circle and peloruses are in good operating condition.* 3. Necessary data. (a) Past history or log data which might establish length of Flinders bar (articles 407 and 607) (b) Azimuths for given date and observer's position (Chapter VIII). (c) Ranges or distant objects in vicinity (local charts).* (d) Correct variation (local charts). (e) Degaussing coil current settings for swing for residual deviations after adjustment and compensation (ship's Degaussing Folder). 4. Precautions. (a) Determine transient deviations of compass from gyro repeaters, doors, guns, etc. (Chapter X). (b) Secure all effective magnetic gear in normal seagoing position before beginning adjustments. (c) Make sure degaussing coils are secured before beginning adjustments. Use reversal sequence, if necessary. (d) Whenever possible, correctors should be placed symmetrically with respect to the compass (articles 318 and 613). 5. Adjustments. (a) Place Flinders bar according to best available information (articles 407, 608 and 609). (b) Set spheres at midposition, or as indicated by last deviation table. (c) Adjust heeling magnet, using balanced dip needle if available (Chapter XI). * Applies when system other than gyro is used as heading reference
102.Adjustments at sea.These adjustments are made with the ship on an even keel and after steadying on each headingWhen using the gyro, swing from heading to heading slowly and check gyro error by sun's azimuth or ranges on each headingif desired to ensure a greater degree of accuracy (article 706). Be sure gyro is set for the mean speed and latitude of thevessel.Noteallprecautionsin article1ol(4)above."OSCARQUEBEC"international code signal shouldbeflowntoindicatesuch work is inprogress.ChapterVlldiscusses methodsforplacingthe shipondesiredheadings1.Adjust theheeling magnet,while the ship is rolling, on north and south magnetic heading until the oscillationsof the compass card havebeen reduced to an averageminimum.(This step is not required if prior adjustmenthas been made usinga dip needle to indicate properplacement of the heeling magnet.)Cometo an east(090°)cardinal magnetic heading.Insert fore-and-aft Bmagnets,or move the existingB2magnets,in such a manner as to removeall deviation3.Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, insuchamannerastoremovealldeviationCome to a west (270°)magnetic heading.Correct half of any observed deviation by moving the Bmagnets.5.Cometoanorth(O0o°)magneticheading.Correcthalfof anyobserveddeviationbymovingtheCmagnets(Thecardinalheading adjustmentsshould nowbecomplete.)Come to any intercardinal magnetic heading, e.g. northeast (045°). Correct any observed deviation by moving6.thespheres inoroutCome to the next intercardinal magnetic heading, e.g. southeast (135°). Correct half of any observed deviation7.by moving the spheres. (The intercardinal heading adjustments should now be complete, although moreaccurate results might be obtained by correcting the D error determined from the deviations on all fourintercardinal heading,as discussed inarticle501.)8.Secureall correctorsbeforeswingingforresidual deviations.Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal andyintercardinalheadingsshouldbesufficient10. Should there still be any large deviations, analyze the deviation curve to determine the necessary correctionsand repeatasnecessarysteps1through9above(ChapterV)11.Record deviations and the details of corrector positions on standard Navy Form NAVSEA 3120/4 and in theMagneticCompassRecordNAVSEA3120/3(article901)12. Swing for residual degaussed deviations with the degaussing circuits properly energized (Chapter XIV).13.RecorddeviationsfordegaussedconditionsonstandardNavyFormNAVSEA3120/4.103.Theabove check-off listdescribes a simplified method of adjustingcompasses,designed to serve as a simpleworkableoutlinefor thenovicewho chooses tofollow a step-by-stepprocedure.The"DocksideTests and Adjustments"are essentialas a foundationfor the"Adjustments at Sea"andif neglected maylead to spuriousresults or needless repetitionof theprocedure at sea.Hence, it is strongly recommended that careful considerations be given thesedockside checks prior tomakingthefinaladjustmentsoastoallowtimetorepairorreplacefaultycompasses,annealorreplacemagnetizedspheresorFlinders bar,realign binnacle,movegyro repeater if it isaffectingthe compass,orto make any othernecessary preliminaryrepairs.Itisfurtherstressedthatexpeditious compassadjustmentisdependentupontheapplicationofthevariouscorrectorsin a logical sequence soas toachievethefinal adjustment withaminimum number of steps.This sequence is incorporated intheabovecheck-offlistandbetterresultswill beobtainedif itisadheredtoclosely.Figure318presentsthevariouscompasserrors and their correction in condensed form.Thetable in figure 103 will further clarify the mechanics of placing thecorrectormagnets,spheres,andFlindersbar.ChapterIVdiscussesthemoreefficientandscientificmethodsofadjustingcompasses,in addition to a more elaboratetreatment of the items mentioned in the check-off list. Frequent,carefulobservations should be made to determinethe constancy of deviations and results should be systematically recorded.Significantchanges indeviationwill indicatetheneedforreadjustment.To avoid Gaussin error (article 1003) when adjusting and swinging ship for residuals, the ship should be steady on thedesired heading for at least2minutes prior to observing the deviation2
2 102. Adjustments at sea. These adjustments are made with the ship on an even keel and after steadying on each heading. When using the gyro, swing from heading to heading slowly and check gyro error by sun's azimuth or ranges on each heading if desired to ensure a greater degree of accuracy (article 706). Be sure gyro is set for the mean speed and latitude of the vessel. Note all precautions in article 101(4) above. "OSCAR QUEBEC" international code signal should be flown to indicate such work is in progress. Chapter VII discusses methods for placing the ship on desired headings. 1. Adjust the heeling magnet, while the ship is rolling, on north and south magnetic heading until the oscillations of the compass card have been reduced to an average minimum. (This step is not required if prior adjustment has been made using a dip needle to indicate proper placement of the heeling magnet.) 2. Come to an east (090°) cardinal magnetic heading. Insert fore-and-aft B magnets, or move the existing B magnets, in such a manner as to remove all deviation. 3. Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, in such a manner as to remove all deviation. 4. Come to a west (270°) magnetic heading. Correct half of any observed deviation by moving the B magnets. 5. Come to a north (000°) magnetic heading. Correct half of any observed deviation by moving the C magnets. (The cardinal heading adjustments should now be complete.) 6. Come to any intercardinal magnetic heading, e.g. northeast (045°). Correct any observed deviation by moving the spheres in or out. 7. Come to the next intercardinal magnetic heading, e.g. southeast (135°). Correct half of any observed deviation by moving the spheres. (The intercardinal heading adjustments should now be complete, although more accurate results might be obtained by correcting the D error determined from the deviations on all four intercardinal heading, as discussed in article 501.) 8. Secure all correctors before swinging for residual deviations. 9. Swing for residual undegaussed deviations on as many headings as desired, although the eight cardinal and intercardinal headings should be sufficient. 10. Should there still be any large deviations, analyze the deviation curve to determine the necessary corrections and repeat as necessary steps 1 through 9 above (Chapter V). 11. Record deviations and the details of corrector positions on standard Navy Form NAVSEA 3120/4 and in the Magnetic Compass Record NAVSEA 3120/3 (article 901). 12. Swing for residual degaussed deviations with the degaussing circuits properly energized (Chapter XIV). 13. Record deviations for degaussed conditions on standard Navy Form NAVSEA 3120/4. 103. The above check-off list describes a simplified method of adjusting compasses, designed to serve as a simple workable outline for the novice who chooses to follow a step-by-step procedure. The "Dockside Tests and Adjustments" are essential as a foundation for the "Adjustments at Sea", and if neglected may lead to spurious results or needless repetition of the procedure at sea. Hence, it is strongly recommended that careful considerations be given these dockside checks prior to making the final adjustment so as to allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or Flinders bar, realign binnacle, move gyro repeater if it is affecting the compass, or to make any other necessary preliminary repairs. It is further stressed that expeditious compass adjustment is dependent upon the application of the various correctors in a logical sequence so as to achieve the final adjustment with a minimum number of steps. This sequence is incorporated in the above check-off list and better results will be obtained if it is adhered to closely. Figure 318 presents the various compass errors and their correction in condensed form. The table in figure 103 will further clarify the mechanics of placing the corrector magnets, spheres, and Flinders bar. Chapter IV discusses the more efficient and scientific methods of adjusting compasses, in addition to a more elaborate treatment of the items mentioned in the check-off list. Frequent, careful observations should be made to determine the constancy of deviations and results should be systematically recorded. Significant changes in deviation will indicate the need for readjustment. To avoid Gaussin error (article 1003) when adjusting and swinging ship for residuals, the ship should be steady on the desired heading for at least 2 minutes prior to observing the deviation
Flinders barFore-and-aft and athwartship magnetsQuadrantalspheresE on NE'ly,W on NEly,Deviation changeE on E'ly and WW on Ely and EEasterly on eastWesterly on eastDeviation→Deviation→Won SEly,EonSEly,with change inon W'ly when sail-on W'ly when sail-and westerly onand easterly onE on Swly,Won SWly,ing toward equatoring toward equatorlatitude→west.west.andandfrom N latitude orfrom N latitude orMagnetsSpheresBar(+ B error)( B error)Won Nly.E on NW'ly.away from equator away from equator+V4to S latitude(+ D error)( D error)to S latitudeNo fore and aftPlace requiredPlace requiredPlace magnets redPlace magnets redNo spheres onPlace spheresPlace spheres foremagnets inNo bar in holder.amount of baraff.amount of bar aft.forward.binnacle. athwartship.and aft.binnacle.forward.Fore and aftSpheres atMove spheresMove spheresBar forward ofDecrease amountIncrease amountmagnets redRaise magnets.Lower magnets.athwartshiptoward compass oroutwardorbinnacle.of bar forward.of bar forward.forward.position.use larger spheres.remove.Move spheresMove spheresFore and aftBar afftofSpheres at foreDecrease amountIncrease amountLower magnets.Raise magnets.outward ortoward compass ormagnets red aft.binnacle.and aft position.ofbarforwardof bar forward.remove.use larger spheres.个Eon Nly,Won N'ly,WonEly and EEon E'ly and WEasterly on northWesterly on northDeviation→Deviation →Won E'ly,BarEonEly,on W'ly when sail-on Wly when sail-and westerly onand easterly onWon s'ly,Eon S'ly,ing toward equatoring toward equatorsouth.south,andDeviation changeandfrom S latitude orfrom S latitude orMagnetsSpheresWon Wly(- C error)Eon Wly(+C error)with change inaway from equatoraway from equator++(+Eerror)(E error)latitude→to N latitudeto N latitudePlace spheres atPlace spheres atNo athwartshipPlace athwartshipPlace athwartshipNo spheres onport forward andstarboard forwardHeeling magnetmagnets redmagnets inmagnets red portbinnaclestarboard aft inter-and port afft inter.(Adjust with changes in magnetic latitude)binnaclestarboardcardinal positions.cardinal positions.Slew spheresAthwartshipSpheres atSlew spheresIf compass north is attracted to high side ofship when rollingcounter-clockwisemagnets redRaise magnetsLower magnetsathwartshipclockwise throughraise the heeling magnet if red end is up or lower the heelingthrough requiredstarboardpositionrequired angle.magnet if blue end is up.angle.If compass north is attracted to low side of ship when rolling.Slew spheres Slew sphereslower the heeling magnet if red end is up or raise the heelingAthwartshipSpheres at forecounter-clockwiseLower magnetsRaise magnetsclockwise throughmagnet if blue end is up.magnets red portand aft positionthrough requiredrequired angle.NOTE: Any change in placement of the heeling magnet willangle.affect the deviations on all headings.Figure 103-Mechanics of magnetic compass adjustment3
3 Fore-and-aft and athwartship magnets Quadrantal spheres Flinders bar Deviation Magnets Easterly on east and westerly on west. (+ B error) Westerly on east and easterly on west. (– B error) Deviation Spheres E on NE'ly, W on SE'ly, E on SW'ly, and W on NW'ly. (+ D error) W on NE'ly, E on SE'ly, W on SW'ly, and E on NW'ly. (– D error) Deviation change with change in latitude Bar E on E'ly and W on W'ly when sailing toward equator from N latitude or away from equator to S latitude W on E'ly and E on W'ly when sailing toward equator from N latitude or away from equator to S latitude No fore and aft magnets in binnacle. Place magnets red forward. Place magnets red aft. No spheres on binnacle. Place spheres athwartship. Place spheres fore and aft. No bar in holder. Place required amount of bar forward. Place required amount of bar aft. Fore and aft magnets red forward. Raise magnets. Lower magnets. Spheres at athwartship position. Move spheres toward compass or use larger spheres. Move spheres outward or remove. Bar forward of binnacle. Increase amount of bar forward. Decrease amount of bar forward. Fore and aft magnets red aft. Lower magnets. Raise magnets. Spheres at fore and aft position. Move spheres outward or remove. Move spheres toward compass or use larger spheres. Bar aft of binnacle. Decrease amount of bar forward. Increase amount of bar forward. Deviation Magnets Easterly on north and westerly on south. (+ C error) Westerly on north and easterly on south. (– C error) Deviation Spheres E on N'ly, W on E'ly, E on S'ly, and W on W'ly (+ E error) W on N'ly, E on E'ly, W on S'ly, and E on W'ly (– E error) Bar Deviation change with change in latitude W on E'ly and E on W'ly when sailing toward equator from S latitude or away from equator to N latitude E on E'ly and W on W'ly when sailing toward equator from S latitude or away from equator to N latitude No athwartship magnets in binnacle Place athwartship magnets red starboard Place athwartship magnets red port No spheres on binnacle Place spheres at port forward and starboard aft intercardinal positions. Place spheres at starboard forward and port aft intercardinal positions. Heeling magnet (Adjust with changes in magnetic latitude) Athwartship magnets red starboard Raise magnets Lower magnets Spheres at athwartship position Slew spheres clockwise through required angle. Slew spheres counter-clockwise through required angle. If compass north is attracted to high side of ship when rolling, raise the heeling magnet if red end is up or lower the heeling magnet if blue end is up. Athwartship magnets red port Lower magnets Raise magnets Spheres at fore and aft position Slew spheres counter-clockwise through required angle. Slew spheres clockwise through required angle. If compass north is attracted to low side of ship when rolling, lower the heeling magnet if red end is up or raise the heeling magnet if blue end is up. NOTE: Any change in placement of the heeling magnet will affect the deviations on all headings. Figure 103 – Mechanics of magnetic compass adjustment
CHAPTER IIMAGNETISM201.Themagnetic compass.The principleof the present day magnetic compass is in no way different from that of thecompassusedbythe ancients.It consists of amagnetized needle,or arrayof needles,pivoted sothatrotationis inahorizontal plane.The superiorityof thepresentday compass results froma betterknowledge of thelaws of magnetism,whichgovernthebehaviorofthecompass,and fromgreaterprecision in construction.202.Magnetism.Any piece of metal on becoming magnetized, that is, acquiring theproperty ofattracting small particles ofironorsteel,will assumeregionsofconcentratedmagnetism,calledpoles.Anysuchmagnetwillhaveatleasttwopoles,ofunlike polarity.Magnetic lines of force(flux)connect one pole of such a magnet with the other pole as indicated in figure202.The number of such lines per unit area represents the intensity of themagnetic field in that areaIf two such magneticbars or magnets areplaced sideby side,thelikepoles will repel each other and the unlikepoles will attract each other.Lines of Force(Flux)Bar MagnetFigure202-Linesofmagneticforceaboutamagnet203.Magnetism is in general of two types, permanent and induced.A bar having permanent magnetism will retain itsmagnetismwhenitisremovedfromthemagnetizingfield.Abarhavinginducedmagnetismwill loseitsmagnetismwhenremovedfromthemagnetizingfield.Whetherornotabarwill retainitsmagnetism onremoval fromthemagnetizingfieldwilldependonthestrengthofthatfield,thedegreeofhardnessoftheiron(retentivity),andalsoupontheamountofphysicalstress applied to the bar while in the magnetizing field. The harder the iron the more permanent will be the magnetismacquired.204.Terrestrial magnetism.The accepted theory of terrestrial magnetism considers the earth as a huge magnet surroundedby lines of magneticforce that connect itstwo magnetic poles.Thesemagneticpoles arenear,but not coincidental, withthegeographic poles ofthe earth. Since the north-seeking end ofa compass needle is conventionally called a red pole, north pole,orpositive pole, it must thereforebe attracted to a pole ofopposite polarity,or toa bluepole, south pole,or negativepole.Themagneticpolenear the northgeographic pole istherefore abluepole, south pole,or negativepole, and the magnetic polenear the south geographic pole is a red pole, north pole, or positive pole.205.Figure 205 illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth atdifferent angles to the horizontal, at different magnetic latitudes. This angle is called the angle of magnetic dip, , andincreases fromzero, atthe magnetic equator,to9o°atthemagneticpoles.Thetotal magneticfield isgenerally considered ashaving two components,namely H, the horizontal component, and Z,the vertical component.These components change astheangle changes such thatH is maximum at the magnetic equator and decreases inthedirection ofeither pole;Ziszero atthemagneticequatorand increases inthedirectionofeitherpole.4
4 CHAPTER II MAGNETISM 201. The magnetic compass. The principle of the present day magnetic compass is in no way different from that of the compass used by the ancients. It consists of a magnetized needle, or array of needles, pivoted so that rotation is in a horizontal plane. The superiority of the present day compass results from a better knowledge of the laws of magnetism, which govern the behavior of the compass, and from greater precision in construction. 202. Magnetism. Any piece of metal on becoming magnetized, that is, acquiring the property of attracting small particles of iron or steel, will assume regions of concentrated magnetism, called poles. Any such magnet will have at least two poles, of unlike polarity. Magnetic lines of force (flux) connect one pole of such a magnet with the other pole as indicated in figure 202. The number of such lines per unit area represents the intensity of the magnetic field in that area. If two such magnetic bars or magnets are placed side by side, the like poles will repel each other and the unlike poles will attract each other. Figure 202 – Lines of magnetic force about a magnet 203. Magnetism is in general of two types, permanent and induced. A bar having permanent magnetism will retain its magnetism when it is removed from the magnetizing field. A bar having induced magnetism will lose its magnetism when removed from the magnetizing field. Whether or not a bar will retain its magnetism on removal from the magnetizing field will depend on the strength of that field, the degree of hardness of the iron (retentivity), and also upon the amount of physical stress applied to the bar while in the magnetizing field. The harder the iron the more permanent will be the magnetism acquired. 204. Terrestrial magnetism. The accepted theory of terrestrial magnetism considers the earth as a huge magnet surrounded by lines of magnetic force that connect its two magnetic poles. These magnetic poles are near, but not coincidental, with the geographic poles of the earth. Since the north-seeking end of a compass needle is conventionally called a red pole, north pole, or positive pole, it must therefore be attracted to a pole of opposite polarity, or to a blue pole, south pole, or negative pole. The magnetic pole near the north geographic pole is therefore a blue pole, south pole, or negative pole; and the magnetic pole near the south geographic pole is a red pole, north pole, or positive pole. 205. Figure 205 illustrates the earth and its surrounding magnetic field. The flux lines enter the surface of the earth at different angles to the horizontal, at different magnetic latitudes. This angle is called the angle of magnetic dip, θ, and increases from zero, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as having two components, namely H, the horizontal component, and Z, the vertical component. These components change as the angle θ changes such that H is maximum at the magnetic equator and decreases in the direction of either pole; Z is zero at the magnetic equator and increases in the direction of either pole
NorthGeographicpoleZBlueMagneticPoleMagW.(Mag.)RedSouthMagneticGeographicPolePoleFigure205-Terrestrialmagnetism206. Inasmuch as the magnetic poles of the earth are not coincidental with the geographic poles, it is evident that a compassneedle in line with the earth's magnetic field will not indicate true north, but magnetic north. The angular differencebetweenthetruemeridian(great circleconnectingthegeographicpoles)and themagneticmeridian(direction ofthe lines ofmagneticflux)is called variation.This variation has different values at different locations on the earthThese values of magneticvariation may befound on the compass rose of navigational charts.The variation for most given areas undergoes an annualchange,theamountofwhichisalsonotedonallcharts.Seefigure206lutluluudonfJl30wiopulohe0SMAGNETICye1w青2piVAR14°30W(1980)4LCHANGE8W-8uunliANNUALoll/Irlh1oppopopoizo8t1180Figure206-Compassroseshowingvariationandannualchange5
5 Figure 205 – Terrestrial magnetism 206. Inasmuch as the magnetic poles of the earth are not coincidental with the geographic poles, it is evident that a compass needle in line with the earth's magnetic field will not indicate true north, but magnetic north. The angular difference between the true meridian (great circle connecting the geographic poles) and the magnetic meridian (direction of the lines of magnetic flux) is called variation. This variation has different values at different locations on the earth. These values of magnetic variation may be found on the compass rose of navigational charts. The variation for most given areas undergoes an annual change, the amount of which is also noted on all charts. See figure 206. Figure 206 – Compass rose showing variation and annual change
207. Ship's magnetism. A ship, while in the process of being constructed, will acquire magnetism of a permanent natureunder the extensive hammering it receives in the earth's magnetic field. After launching, the ship will lose some of thisoriginal magnetism as a result of vibration, pounding,etc., in varying magnetic fields,and will eventually reach a moreorless stablemagneticcondition.Thismagnetismwhichremains isthepermanentmagnetismoftheship208.Thefact thata shiphas permanentmagnetismdoesnotmean that itcannot also acquire inducedmagnetismwhenplacedin a magnetic field such as the earth's field.The amount of magnetism induced in any given piece of soft iron is dependentupon the field intensity,the alignment of the soft iron in that field,and thephysical properties and dimensionsof the ironThis induced magnetism may add to or subtractfrom the permanent magnetism alreadypresent in the ship, depending onhow the ship is aligned in the magnetic field. The softer the iron, the more readily it will be induced by the earth's magneticfield and the more readily it will give up itsmagnetism whenremoved from that field.209.The magnetism in the various structures ofa ship which tends to change as a result of cruising,vibration,or aging,butdoes not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. Thismagnetism, at any instant, is recognized as part of the ship's permanent magnetism, and consequently must be corrected assuchbymeansofpermanentmagnetcorectors.Thissubpermanentmagnetismistheprincipal causeofdeviationchangesona magnetic compass. Subsequent reference to permanent magnetism in this text will refer to the apparent permanentmagnetism that includes theexistingpermanent and subpermanentmagnetism atanygiven instant.210.A ship,then, has a combination of permanent, subpermanent,and induced magnetism, since its metal structures are ofvarying degrees of hardness. Thus, the apparent permanent magnetic condition of the ship is subject to change fromdeperming, excessive shocks, welding, vibration, etc., and the induced magnetism of the ship will vary with the strength ofthe earth's magnetic field at differentmagnetic latitudes,and with the alignment ofthe ship in that field.2il.Resultantinducedmagnetismfromearth's magnetic field.The above discussion of induced magnetism andterrestrial magnetism leads to thefollowing facts.A long thin rod of soft iron in a planeparallel to the earth's horizontalmagnetic field, H, will have a red (north)pole induced in theend toward the northgeographic pole and a blue (south)poleinduced in the end toward the south geographic pole.This same rod in a horizontal plane but at rightangles to the horizontalearth's field would have no magnetism induced in it, because its alignment in themagnetic field is suchthat there will be notendency toward linearmagnetization and therod is of negligible cross section.Should the rod be aligned in some horizontaldirection between those headings that create maximum and zero induction, it would be induced by an amount that is afunction ofthe angle of alignment. Ifa similar rod isplaced in a vertical position in northern latitudes so as to be aligned withthevertical earth'sfield Z, it will have a blue (south)pole induced at the upper end and a red (north)pole induced at the lowerend.Thesepolaritiesofverticalinducedmagnetizationwillbereversedinsouthernlatitudes.Theamountofhorizontalorvertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with theintensityof Hand Z, heading,and heeloftheship
6 207. Ship's magnetism. A ship, while in the process of being constructed, will acquire magnetism of a permanent nature under the extensive hammering it receives in the earth's magnetic field. After launching, the ship will lose some of this original magnetism as a result of vibration, pounding, etc., in varying magnetic fields, and will eventually reach a more or less stable magnetic condition. This magnetism which remains is the permanent magnetism of the ship. 208. The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed in a magnetic field such as the earth's field. The amount of magnetism induced in any given piece of soft iron is dependent upon the field intensity, the alignment of the soft iron in that field, and the physical properties and dimensions of the iron. This induced magnetism may add to or subtract from the permanent magnetism already present in the ship, depending on how the ship is aligned in the magnetic field. The softer the iron, the more readily it will be induced by the earth's magnetic field and the more readily it will give up its magnetism when removed from that field. 209. The magnetism in the various structures of a ship which tends to change as a result of cruising, vibration, or aging, but does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This magnetism, at any instant, is recognized as part of the ship's permanent magnetism, and consequently must be corrected as such by means of permanent magnet correctors. This subpermanent magnetism is the principal cause of deviation changes on a magnetic compass. Subsequent reference to permanent magnetism in this text will refer to the apparent permanent magnetism that includes the existing permanent and subpermanent magnetism at any given instant. 210. A ship, then, has a combination of permanent, subpermanent, and induced magnetism, since its metal structures are of varying degrees of hardness. Thus, the apparent permanent magnetic condition of the ship is subject to change from deperming, excessive shocks, welding, vibration, etc.; and the induced magnetism of the ship will vary with the strength of the earth's magnetic field at different magnetic latitudes, and with the alignment of the ship in that field. 211. Resultant induced magnetism from earth's magnetic field. The above discussion of induced magnetism and terrestrial magnetism leads to the following facts. A long thin rod of soft iron in a plane parallel to the earth's horizontal magnetic field, H, will have a red (north) pole induced in the end toward the north geographic pole and a blue (south) pole induced in the end toward the south geographic pole. This same rod in a horizontal plane but at right angles to the horizontal earth's field would have no magnetism induced in it, because its alignment in the magnetic field is such that there will be no tendency toward linear magnetization and the rod is of negligible cross section. Should the rod be aligned in some horizontal direction between those headings that create maximum and zero induction, it would be induced by an amount that is a function of the angle of alignment. If a similar rod is placed in a vertical position in northern latitudes so as to be aligned with the vertical earth's field Z, it will have a blue (south) pole induced at the upper end and a red (north) pole induced at the lower end. These polarities of vertical induced magnetization will be reversed in southern latitudes. The amount of horizontal or vertical induction in such rods, or in ships whose construction is equivalent to combinations of such rods, will vary with the intensity of H and Z, heading, and heel of the ship
CHAPTER IIITHEORYOFMAGNETICCOMPASSADJUSTMENT301.Magnetic adjustment.The magnetic compass,when used on a steel ship,mustbe so corrected for the ship's magneticconditions that its operation approximates that of a nonmagnetic ship.Ship's magnetic conditions create deviations of themagnetic compass aswellas sectors of sluggishness and unsteadiness.Deviation is defined as deflection of the card (needles)to the right or left ofthemagnetic meridian.Adjustment of the compass is the arranging ofmagnetic and soft iron correctorsaboutthebinnacle so that their effects are equal and oppositeto the effectsofthe magnetic material in the ship,thusreducingthedeviationsandeliminatingthesectorsofsluggishnessandunsteadinessThemagnetic conditions in a ship whichaffecta magnetic compass arepermanentmagnetismand induced magnetism,asdiscussed in Chapter II.302.Permanentmagnetismand itseffectsonthecompass.Thetotal permanentmagneticfield effectat thecompass maybebroken intothreecomponentsmutually90°apart,as shown infigure 302a.The effectofthevertical permanentcomponentisthetendencytotiltthecompasscardand.intheeventofrollingorpitchingoftheshiptocreateoscillatingdeflectionsofthecard.Oscillationeffects thataccompanyroll aremaximumonnorth and southcompassheadings,and thosethataccompanypitcharemaximumoneastandwestcompassheadings.ThehorizontalBandCcomponentsofpermanentmagnetismcausevaryingdeviationsofthecompassastheshipswingsinheadingonanevenkeel.Plottingthesedeviationsagainst compass heading will produce sine and cosine curves, as shown in figure 302b.These deviation curves are calledsemicircularcurves because theyreversedirection in1800EastAthwartshipPermanent(+)Magnetic CDeviationsFore-I-aftBComponenAthwartship CComponentDegDev.MagneticRDeviationsPermanent MagneticWestVerticalHeeliFieid Across CompasComponen(-)Ship's Compass HeadingDegreesFigure302a-Componentsof permanentmagneticFigure302b-Permanentmagneticdeviation effectsfieldatthecompass303.Thepermanentmagnetic semicirculardeviations can be illustratedbya series of simple sketches,representing a shiponsuccessive compass headings, as in figures 303a and 303b.304.Theships illustrated infigures 303a and 303b are pictured on cardinal compass headings rather than on cardinalmagneticheadings,fortworeasons:(l) Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzedmathematically.This can be visualizedbynoting that the ship's component magnetic fields are either in linewith orperpendiculartothecompassneedlesonlyoncardinal compassheadings.(2)Suchapresentationillustratesthefactthatthecompasscardtendstofloatin a fixedposition,in linewiththemagnetic meridian.Deviations of thecard torightor left (eastor west)of the magneticmeridian resultfromthemovementoftheshipand itsmagneticfieldsaboutthecompasscard.7
7 CHAPTER III THEORY OF MAGNETIC COMPASS ADJUSTMENT 301. Magnetic adjustment. The magnetic compass, when used on a steel ship, must be so corrected for the ship's magnetic conditions that its operation approximates that of a nonmagnetic ship. Ship's magnetic conditions create deviations of the magnetic compass as well as sectors of sluggishness and unsteadiness. Deviation is defined as deflection of the card (needles) to the right or left of the magnetic meridian. Adjustment of the compass is the arranging of magnetic and soft iron correctors about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship, thus reducing the deviations and eliminating the sectors of sluggishness and unsteadiness. The magnetic conditions in a ship which affect a magnetic compass are permanent magnetism and induced magnetism, as discussed in Chapter II. 302. Permanent magnetism and its effects on the compass. The total permanent magnetic field effect at the compass may be broken into three components mutually 90° apart, as shown in figure 302a. The effect of the vertical permanent component is the tendency to tilt the compass card and, in the event of rolling or pitching of the ship to create oscillating deflections of the card. Oscillation effects that accompany roll are maximum on north and south compass headings, and those that accompany pitch are maximum on east and west compass headings. The horizontal B and C components of permanent magnetism cause varying deviations of the compass as the ship swings in heading on an even keel. Plotting these deviations against compass heading will produce sine and cosine curves, as shown in figure 302b. These deviation curves are called semicircular curves because they reverse direction in 180°. Figure 302a – Components of permanent magnetic Figure 302b – Permanent magnetic deviation effects field at the compass 303. The permanent magnetic semicircular deviations can be illustrated by a series of simple sketches, representing a ship on successive compass headings, as in figures 303a and 303b. 304. The ships illustrated in figures 303a and 303b are pictured on cardinal compass headings rather than on cardinal magnetic headings, for two reasons: (1) Deviations on compass headings are essential in order to represent sinusoidal curves that can be analyzed mathematically. This can be visualized by noting that the ship's component magnetic fields are either in line with or perpendicular to the compass needles only on cardinal compass headings. (2) Such a presentation illustrates the fact that the compass card tends to float in a fixed position, in line with the magnetic meridian. Deviations of the card to right or left (east or west) of the magnetic meridian result from the movement of the ship and its magnetic fields about the compass card
South heading byWest heading byEast heading byNorthheadingbycompasscompasscompasscompassW.Dev.E.Dev.CompassCompassFore-and-aft BNeedlePermanent MagneticFieldMaximum deviationNo deviationMaximumdeviationNo deviationwesterlyeasterly(change in di--(change in di-rective forcerective forceonly)only)Figure303a-Forcediagramsforfore-and-aftpermanentBmagneticfieldWest heading bySouth heading byEast heading byNorthheadingbycompasscompasscompasscompass.DevW.Dev.CompassNeedleAthwartship CPermanent MagneticFieldNo deviationMaximum deviationMaximum deviationNo deviationwesterlyeasterlyFigure 303b-Force diagrams for athwartship permanent Cmagnetic field305.Inasmuch as a compass deviation is caused bytheexistence of a forceatthe compass that is superimposed uponthenormal earth's directive force, H, a vector analysis is helpful in determining deviations or the strength of deviating fields. Forexample,a shipas shown in figure 305on an eastmagneticheading will subject its compassto a combination of magneticeffects, namely, the earth's horizontal field H, and the deviating field B, at right angles to the field H. The compass needlewill align itself in the resultant field which is represented bythe vector sum of H and B,as shown.A similar analysis on theship infigure 305 will reveal that the resulting directiveforce at the compass would bemaximum on anorth heading andminimum on a southheading,thedeviationsbeingzeroforboth conditions.The magnitude of the deviation caused by the permanent B magneticfield will vary withdifferent values of H, hence,deviationsresultingfrompermanentmagneticfields will varywiththemagneticlatitudeoftheship.8
8 Figure 303a – Force diagrams for fore-and-aft permanent B magnetic field Figure 303b – Force diagrams for athwartship permanent C magnetic field 305. Inasmuch as a compass deviation is caused by the existence of a force at the compass that is superimposed upon the normal earth's directive force, H, a vector analysis is helpful in determining deviations or the strength of deviating fields. For example, a ship as shown in figure 305 on an east magnetic heading will subject its compass to a combination of magnetic effects; namely, the earth's horizontal field H, and the deviating field B, at right angles to the field H. The compass needle will align itself in the resultant field which is represented by the vector sum of H and B, as shown. A similar analysis on the ship in figure 305 will reveal that the resulting directive force at the compass would be maximum on a north heading and minimum on a south heading, the deviations being zero for both conditions. The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H; hence, deviations resulting from permanent magnetic fields will vary with the magnetic latitude of the ship
ResultantField inMagnitude (DirectiveForce)and Direction (Deviation)Earth's FieldHEastMagneticHeadingDeviating FieldCompass NeedleBFigure305-General forcediagram306.Induced magnetism and its effects on the compass.Induced magnetism varies with the strength of the surroundingfield,themass of metal, and the alignmentof themetal inthe field.Sincethe intensity of the earth's magnetic field variesoverthe earth's surface,the induced magnetism in a ship will vary with latitude, heading,and heel ofthe ship.307. With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, willcreate deviations that plot as a semicircular deviation curve.This is true because the vertical induction changes magnitudeand polarity only with magnetic latitudeand heel and not with heading of the ship.Therefore, as long as the ship is in thesame magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass asa permanent pole swinging about the compass.Figure 307a illustrates the vertical induced poles in the structures ofa ship.Vertical InducedMagnetic B DeviationsEDeviationsEast (+)from Induetionin the Unsymmet-rical HorizontalSoftIronDeg0°ompass180°909270°360Dev.D Deviations from Inductionin the Symmetrical HorizontalSoft IronADeviations fromUnsymmetricalResultant of VerticalVertical InducedWest (-)Horizontal Soft IronInducedComponentsComponent(North Latitudes)Ship's Compass Heading-DegreesFigure307a-Ship's vertical inducedmagnetismFigure307b-InducedmagneticdeviationeffectsGenerally,this semicirculardeviationwill beaB sine curve,as shown in figure307b,sincemost ships are symmetricalabout thecenterline and have their compasses mounted on the centerline.Themagnitude of these deviations will changewithmagnetic latitudechanges because thedirectiveforce and the ship's vertical induction both change with magnetic latitude308.The masses of horizontal soft iron that are subjectto induced magnetization create characteristic deviations, as indicatedinfigure307b.TheDand Edeviation curves arecalled quadrantal curves becausetheyreversepolarity in each of thefourquadrants.9
9 Figure 305 – General force diagram 306. Induced magnetism and its effects on the compass. Induced magnetism varies with the strength of the surrounding field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the earth's magnetic field varies over the earth's surface, the induced magnetism in a ship will vary with latitude, heading, and heel of the ship. 307. With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will create deviations that plot as a semicircular deviation curve. This is true because the vertical induction changes magnitude and polarity only with magnetic latitude and heel and not with heading of the ship. Therefore, as long as the ship is in the same magnetic latitude, its vertical induced pole swinging about the compass will produce the same effect on the compass as a permanent pole swinging about the compass. Figure 307a illustrates the vertical induced poles in the structures of a ship. Figure 307a – Ship's vertical induced magnetism Figure 307b – Induced magnetic deviation effects Generally, this semicircular deviation will be a B sine curve, as shown in figure 307b, since most ships are symmetrical about the centerline and have their compasses mounted on the centerline. The magnitude of these deviations will change with magnetic latitude changes because the directive force and the ship's vertical induction both change with magnetic latitude. 308. The masses of horizontal soft iron that are subject to induced magnetization create characteristic deviations, as indicated in figure 307b. The D and E deviation curves are called quadrantal curves because they reverse polarity in each of the four quadrants