86MAGNETICCOMPASSADJUSTMENTFore-and-aft BComponent7AthwartshipCComponentCompassTotal PermanentMagneticVertical HeelingFieldAcross CompassComponentFigure 605a. Components of permanent magnetic field.the strength of deviating fields.For example, a ship aswould bemaximum on a northheadingand minimum onashown inFigure 605c on aneastmagnetic heading will sub-southheading because thedeviationsforboth conditions areject its compass to a combination of magnetic effects;zero.namely, the earth's horizontal field H, and the deviatingThe magnitude of the deviation caused by the perma-field B, at right angles to the field H. The compass needlenent B magnetic field will vary with different values of H:will align itself in the resultantfield which is represented byhence,deviationsresultingfrom permanent magneticfieldsthe vector sum ofHand B, as shown.A similar analysis willwill varywith themagnetic latitude of the ship.reveal that the resulting directive force on the compassAthwartshipPermanentEASTMagnetic CDeviations(4)Deg.Dev.7360*0200Fore-and-aftPermanentWESTMagnetic BDeviations()Ship'sCompassHeading-DegreesFigure605b.Permanent magneticdeviation effects
86 MAGNETIC COMPASS ADJUSTMENT the strength of deviating fields. For example, a ship as shown in Figure 605c 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 will reveal that the resulting directive force on the compass would be maximum on a north heading and minimum on a south heading because the deviations for both conditions are zero. 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. Figure 605a. Components of permanent magnetic field. Figure 605b. Permanent magnetic deviation effects
87MAGNETICCOMPASSADJUSTMENTrounding field, the mass ofmetal,and the alignmentofthe metalin the field. Since the intensity of the earth's magnetic field var-Resultant Ficld inies over the earth's surface, the induced magnetism in a ship willMagnitude (Directive Force)and Direction (Deviation)vary with latitude, heading, and heel of the ship.Withtheshiponanevenkeel,theresultant vertical induced-larth'sFicldmagnetism, if notdirected throughthe compass itself, will createAHdeviations which plot as a semicircular deviation curve. This is1truebecausethevertical induction changes magnitudeand po-1larity only with magnetic latitude and heel, and not with headingIdofthe ship.Therefore, as long as the ship is in the samemagnetic1latitude, its vertical induced pole swinging about the compasswill produce the sameeffect on thecompass as apermanent poleEastswinging about thecompass.MagneticHeadingThe earth's field induction in certain other unsymmetricalarrangementsof horizontal soft ironcreatea constantAdevia-tion curve. In addition to this magnetic A error, there areDeviating FieldBconstant A deviations resulting from: (I) physical misalign-Compass Needlements ofthe compass, pelorus, or gyro; (2) errors in calculatingthe sun's azimuth,observing time, or taking bearings.Figure605c.General force diagram.The nature, magnitude, and polarity of all these in-606.Induced MagnetismAnd Its Effects OnTheduced effects are dependent upon the disposition of metal,Compassthe symmetry or asymmetry of the ship,the locationof thebinnacle, the strength of the earth's magnetic field, and theInduced magnetism varies with the strength of the surangle of dip.CompassMagnetic or compassType deviationheadings ofCoefficientCauses of such errorsCorrectors for such errorsheadings on which tocurvemaximumapply corectorsdeviationCheck methods and calculations_ Human-error in calculations-.--.Check alignments4Constant.Same on all.Physicalompass, gyro,pelous alignmntAnyRare arrangement ofsoftironrodsMagnetic-unsymmetrical arrangements of horiz. soft ironFore-and-aft B magnctsSemicircularFore-and-aft component of permanent magnetic ficld_090*BInduced magnetism in unsymmetrical vertical iron forward orFlinders bar (forward or af)090°or270sing.270*aft of compassAthwartship C magnetsSemicircularAthwartship component of permanent magnetic field-000*cFlinders bar (port or starboard)000°or 180°Induced magnetism in unsymmetrical vertical iron port ocOsO.180*starboard of compass.Spheres on appropriate axisQuadrantralInduced magnetism in all symmetrical arrangements of(athwartship for +D)Dsin20225*045′,135,225′,or 315borizontal soft iron.(fore and aft for -D)315See skeicha000*Spheres on appropriate axis.QuadrantralInduced magnetism in all unsymmetrical arrangements ol(port fwd.-stb'd for +E)Ecos20.(stb'd fwdportaf for-E)0009080,or 2horizontal soft iron.270°See skeichb000Change in the horizontal component of the induced or permanenttHeling magnet (must be readjusted for090° or 270 with dip needle.Oseillations with roll roll180or pitchmagnetice fieldsat the compass due torolling or pitching ofthelatitude changes).000°or 180° while rlling.HeelingDeviatiors with090*j piteship.constant list.270°hDeviation = A + Bsin + Ccos+ Dsin20 +Ecos2(=compass heading）000+D(Santchb)(8bde)0Figure 607. Summary of compass errors and adjustments
MAGNETIC COMPASS ADJUSTMENT 87 606. 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. With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will create deviations which 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. The earth’s field induction in certain other unsymmetrical arrangements of horizontal soft iron create a constant A deviation curve. In addition to this magnetic A error, there are constant A deviations resulting from: (1) physical misalignments of the compass, pelorus, or gyro; (2) errors in calculating the sun’s azimuth, observing time, or taking bearings. The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, the symmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth’s magnetic field, and the angle of dip. Figure 605c. General force diagram. Coefficient Type deviation curve Compass headings of maximum deviation Causes of such errors Correctors for such errors Magnetic or compass headings on which to apply correctors A Constant. Same on all. Human-error in calculations _ _ _ _ _ _ _ _ _ _ _ _ Physical-compass, gyro, pelorus alignment _ _ _ _ _ _ _ Magnetic-unsymmetrical arrangements of horiz. soft iron. Check methods and calculations _ _ _ Check alignments _ _ _ _ _ _ _ _ Rare arrangement of soft iron rods. Any. B Semicircular 090˚ 270˚ Fore-and-aft component of permanent magnetic field_ _ _ Induced magnetism in unsymmetrical vertical iron forward or aft of compass. Fore-and-aft B magnets _ _ _ _ _ _ Flinders bar (forward or aft) _ _ _ _ 090˚ or 270˚. C Semicircular 000˚ 180˚ Athwartship component of permanent magnetic field- - - Induced magnetism in unsymmetrical vertical iron port or starboard of compass. Athwartship C magnets _ _ _ _ _ _ Flinders bar (port or starboard) _ _ _ 000˚ or 180˚. D Quadrantral 045˚ 135˚ 225˚ 315˚ Induced magnetism in all symmetrical arrangements of horizontal soft iron. Spheres on appropriate axis. (athwartship for +D) (fore and aft for -D) See sketch a 045˚, 135˚, 225˚, or 315˚. E Quadrantral 000˚ 090˚ 180˚ 270˚ Induced magnetism in all unsymmetrical arrangements of horizontal soft iron. Spheres on appropriate axis. (port fwd.-stb’d for +E) (stb’d fwd.-port aft for -E) See sketch b 000˚, 090˚, 180˚, or 270˚. Heeling Oscillations with roll or pitch. Deviations with constant list. 000˚ 180˚ 090˚ 270˚ }roll }pitc h Change in the horizontal component of the induced or permanent magnetic fields at the compass due to rolling or pitching of the ship. Heeling magnet (must be readjusted for latitude changes). 090˚ or 270˚ with dip needle. 000˚ or 180˚ while rolling. Figure 607. Summary of compass errors and adjustments. sin . φ cos . φ sin2φ . cos2φ . Deviation = 2 A B + sinφ + C cosφ + D sin φ + E cos2φ φ( ) = compass heading
88MAGNETICCOMPASS ADJUSTMENTCertain heeling errors, in addition to those resultingvertical tubefrom permanent magnetism,are created by the presence of2.Fore-and-af B permanent magnets in their trays.both horizontal and vertical soft iron which experience3.AthwartshipCpermanentmagnets in theirtrays.changinginductionastheshiprollsintheearth'smagnetic4.Vertical soft ironFlinders bar in its external tube.5. Soft iron quadrantal spheres.field.This part of the heeling error will naturally change inmagnitude with changes of magnetic latitude of the ship.Oscillationeffectsaccompanyingroll aremaximum onThe heelingmagnet is the only corrector which cornorth and south headings, just as with the permanent magrects for both permanent and induced effects.Therefore,itnetic heelingerrors.mustbeadjusted occasionallyforchanges inship's latitudeHowever,anymovement of theheelingmagnetwill require607.AdjustmentsAnd Correctorsreadjustmentofothercorrectors.Figure607summarizes all thevarious magneticcondi-Since some magnetic effects are functions of the ves-tions in a ship,thetypes ofdeviation curvestheycreate,thesel'smagneticlatitudeandothersarenoteachindividualcorrectors for each effect,and headings on which each cor-effectshould becorrected independently.Furthermore,torector is adjusted.Applythecorrectors symmetricallyandas far awayfrom the compass as possible.This preservesmakethecorrections,use(1)permanent magnet correctorsto compensateforpermanentmagneticfieldsatthecom-theuniformityofmagneticfields aboutthecompassneedlepass,and(2)soft ironcorrectorsto compensatefor inducedarray.Fortunately, each magnetic effect has a slightly differ-magnetism.The compassbinnacleprovides support forboththe compass and such correctors.Typical binnaclesent characteristic curve.This makes identification andholdthefollowing correctors:correction convenient.Analyzing a complete deviationcurve for its different components allows one to anticipate1.Vertical permanent heeling magnet in the centralthe necessary corrections.COMPASSOPERATION608.EffectsOfErrorsOnTheCompassple,ashipwhosecompassisfrozentoanorthreadingwouldrequirefore-and-afftBcorrectormagnetswiththepositiveAn uncorrected compass suffers largedeviations andends forward in order to neutralize the existing negative polesluggish,unsteady operation.These conditions may be as-which attracted the compass.If made on an east heading,sociatedwiththemaximumandminimumdirectiveforcesuch an adjustment would bepractically complete when theacting onthe compass.Themaximum deviation occurs atcompasscardwasfreedto indicatean eastheading.the point ofaveragedirective force;andthe zero deviationsoccuratthepointsofmaximumandminimumdirective609.ReasonsForCorrectingCompassforce.Applying correctors to reduce compass deviation ef-There are several reasons for correcting the errors offects compass error correction. Applying correctors tothemagnetic compass:equalizethe directiveforces acrossthecompass position1.It is easierto useamagnetic compass if the devia-couldalsoeffectcompasscorrection.Thedeviationmethodis most often used because it utilizes the compass itself astions are small.the correction indicator.Equalizing the directive forces2.Evenknownandcompensatedfordeviationintro-would require an additional piece of test and calibrationduceserrorbecausethe compass operatesequipment.Occasionally,thepermanentmagnetic effects atthelo-sluggishly and unsteadilywhen deviation iscation ofthe compass are so largethattheyovercomethepresent.earth's directiveforce,H.This condition will not only create3.Even thoughthedeviations are compensated for.sluggish and unsteady sectors,but mayevenfreeze the com-pass to one reading or to one quadrant, regardless of thethey will be subject to appreciablechange as aheading ofthe ship.Should the compass become sofrozen,function ofheel and magnetic latitude.the polarity of the magnetism which must be attracting thecompass needles is indicated;hence, correction may be efOnce properly adjusted,themagnetic compass devia-fected simply by the application of permanent magnettions should remain constant until there is some change incorrectors, in suitable quantity to neutralize this magnetism.themagneticconditionofthevessel resultingfrommagneticWhenever such adjustments aremade,it would bewell totreatment,shockfromgunfire,vibration,repair,orstructuralhavetheshipplacedon aheading suchthattheunfreezingofchanges.Frequently,the movement of nearbyguns, doors,thecompassneedleswill be immediately evident.For exam-gyrorepeaters,orcargo affectsthecompassgreatly
88 MAGNETIC COMPASS ADJUSTMENT Certain heeling errors, in addition to those resulting from permanent magnetism, are created by the presence of both horizontal and vertical soft iron which experience changing induction as the ship rolls in the earth’s magnetic field. This part of the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship. Oscillation effects accompanying roll are maximum on north and south headings, just as with the permanent magnetic heeling errors. 607. Adjustments And Correctors Since some magnetic effects are functions of the vessel’s magnetic latitude and others are not, each individual effect should be corrected independently. Furthermore, to make the corrections, use (1) permanent magnet correctors to compensate for permanent magnetic fields at the compass, and (2) soft iron correctors to compensate for induced magnetism. The compass binnacle provides support for both the compass and such correctors. Typical binnacles hold the following correctors: 1. Vertical permanent heeling magnet in the central vertical tube. 2. Fore-and-aft B permanent magnets in their trays. 3. Athwartship C permanent magnets in their trays. 4. Vertical soft iron Flinders bar in its external tube. 5. Soft iron quadrantal spheres. The heeling magnet is the only corrector which corrects for both permanent and induced effects. Therefore, it must be adjusted occasionally for changes in ship’s latitude. However, any movement of the heeling magnet will require readjustment of other correctors. Figure 607 summarizes all the various magnetic conditions in a ship, the types of deviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. Apply the correctors symmetrically and as far away from the compass as possible. This preserves the uniformity of magnetic fields about the compass needle array. Fortunately, each magnetic effect has a slightly different characteristic curve. This makes identification and correction convenient. Analyzing a complete deviation curve for its different components allows one to anticipate the necessary corrections. COMPASS OPERATION 608. Effects Of Errors On The Compass An uncorrected compass suffers large deviations and sluggish, unsteady operation. These conditions may be associated with the maximum and minimum directive force acting on the compass. The maximum deviation occurs at the point of average directive force; and the zero deviations occur at the points of maximum and minimum directive force. Applying correctors to reduce compass deviation effects compass error correction. Applying correctors to equalize the directive forces across the compass position could also effect compass correction. The deviation method is most often used because it utilizes the compass itself as the correction indicator. Equalizing the directive forces would require an additional piece of test and calibration equipment. Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth’s directive force, H. This condition will not only create sluggish and unsteady sectors, but may even freeze the compass to one reading or to one quadrant, regardless of the heading of the ship. Should the compass become so frozen, the polarity of the magnetism which must be attracting the compass needles is indicated; hence, correction may be effected simply by the application of permanent magnet correctors, in suitable quantity to neutralize this magnetism. Whenever such adjustments are made, it would be well to have the ship placed on a heading such that the unfreezing of the compass needles will be immediately evident. For example, a ship whose compass is frozen to a north reading would require fore-and-aft B corrector magnets with the positive ends forward in order to neutralize the existing negative pole which attracted the compass. If made on an east heading, such an adjustment would be practically complete when the compass card was freed to indicate an east heading. 609. Reasons For Correcting Compass There are several reasons for correcting the errors of the magnetic compass: 1. It is easier to use a magnetic compass if the deviations are small. 2. Even known and compensated for deviation introduces error because the compass operates sluggishly and unsteadily when deviation is present. 3. Even though the deviations are compensated for, they will be subject to appreciable change as a function of heel and magnetic latitude. Once properly adjusted, the magnetic compass deviations should remain constant until there is some change in the magnetic condition of the vessel resulting from magnetic treatment, shock from gunfire, vibration, repair, or structural changes. Frequently, the movement of nearby guns, doors, gyro repeaters, or cargo affects the compass greatly
89MAGNETICCOMPASSADJUSTMENTDETAILEDPROCEDURESFORCOMPASSADJUSTMENT610.DocksideTestsAndAdjustmentssection611AdjusttheFlinders barfirst becauseit is subjecttoSection 601,theAdjustment Checkoff List,gives theinduction from several of the correctors and its adjust-physical checks required before beginning an adjustment.ment isnot dependent on any single observation.ToTheadjustment procedure assumes that thesecheckshaveadjust the Flinders bar, use one of the followingbeen completed.Thenavigatorwill avoid much delaybymethods:making these checks before starting the magnet and softiron corrector adjustments.The most important of theseI.Use deviation data obtained at two different magchecksarediscussedbelow.netic latitudes to calculatethe proper length ofFlinders bar for anyparticular compass location.Shouldthecompasshaveasmall bubble,addcompassfluidthroughthefillingplugonthecompassbowl.Ifan apSections622through624containdetailson acquir-ing thedata and making the required calculations.preciable amountof compass liquid hasleaked out, checkthe sealinggasket and filling plug for leaks.2.If the above method is impractical, set the FlindersTake the compass to a place freefrom all magnetic in-fluences except the earth's magnetic field for tests ofbar length by:momentandsensibilitv.Thesetestsinvolvemeasurementsa. Using a Flinders bar length determined byof thetimeofvibrationandtheabilityofthecompasscardto return toa consistent reading after deflection.Thesetestsother ships of similar structure.will indicate thecondition ofthe pivot, jewel, and magneticb.Studying thearrangementofmasts,stacksstrengthofthecompassneedlesand other vertical structures and estimating theNext, check the spheres and Flinders bar for residualFlinders bar length required.magnetism.Movethe spheres as close to the compass aspossible and slowly rotate each sphere separately.Any ap-Ifthese methodsarenotsuitable,omittheFlindersbarpreciabledeflection (2°ormore)of the compass needlesresultingfromthisrotationindicatesresidualmagnetisminuntil the required data are acquired.The iron sections of Flinders bar shouldbe continu-the spheres. The Flinders bar magnetization check is pref-ous and placed atthe top of thetube with the longesterablymadewith theshipon an east orwestcompasssection at the top. Wooden spacers are used at the bottomheading.Tomakethis check:(a)note thecompass readingof the tube.with the Flinders bar in the holder; (b) invert the Flindersbar in theholder and again note the compass reading.AnyHaving adjusted thelength of Flinders bar,place theappreciabledifference(2°ormore)betweentheseobservedspheres onthebracketarmsatan approximateposition.Ifthe compass has been adjusted previously,placethereadings indicates residual magnetism in theFlinders barSpheres or Flinders bars which show signs of such residualspheres at the position indicated by the previous devia-magnetism shouldbe annealed,i.e.,heated toa dull red andtion table. In the event the compass has never beenallowedtocool slowlyadjusted,place the spheres at the midpoint on thebracketCorrect alignmentof the lubber's line of thecom-arms.The next adjustment is the positioning of the heelingpass,gyro repeater, and pelorus with the fore-and-aftmagnetusinga properlybalanced dipneedle.Section637line ofthe ship is important.Any misalignment will pro-duce a constant error in the deviation curve.All of thesediscussesthisprocedureinstruments may be aligned correctly with the fore-and-These three dockside adjustments (Flinders bar, qua-aft lineof the shipbyusingtheazimuthcircle and amet-drantal spheres,and heeling magnet)will properlyestablishthe conditions of mutual induction and shielding of theal tapemeasure.Should the instrument belocated on thecompass.This minimizes the steps required at sea to com-centerline of theship, asight istakenonamastor otherobject on the centerline.If the instrument is not on theplete the adjustment.centerline, measure the distance from the centerline of611.Expected Errorsthe ship to the center of the instrument. Mark this dis-tanceoff fromthecenterlineforward orabaft thecompassandplacereferencemarks onthedeck.TakeFigure 607 lists six different coefficients or types ofde-sights on these marks.viation errors with their causes and correspondingAlignthecompass sothatthecompass'lubber'sline iscorrectors.Adiscussion ofthesecoefficientsfollows:parallel to the fore-and-aft line of the ship.Steering com-TheAerroriscausedbythemiscalculation of azimuthspasses may occasionally be deliberately misaligned in orderorbyphysical misalignments ratherthan magneticeffectsofto correctfor anymagnetic Aerror present,as discussed inunsymmetrical arrangements of horizontal soft iron.Thus
MAGNETIC COMPASS ADJUSTMENT 89 DETAILED PROCEDURES FOR COMPASS ADJUSTMENT 610. Dockside Tests And Adjustments Section 601, the Adjustment Checkoff List, gives the physical checks required before beginning an adjustment. The adjustment procedure assumes that these checks have been completed. The navigator will avoid much delay by making these checks before starting the magnet and soft iron corrector adjustments. The most important of these checks are discussed below. Should the compass have a small bubble, add compass fluid through the filling plug on the compass bowl. If an appreciable amount of compass liquid has leaked out, check the sealing gasket and filling plug for leaks. Take the compass to a place free from all magnetic influences except the earth’s magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the pivot, jewel, and magnetic strength of the compass needles. Next, check the spheres and Flinders bar for residual magnetism. Move the spheres as close to the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2° or more) of the compass needles resulting from this rotation indicates residual magnetism in the spheres. The Flinders bar magnetization check is preferably made with the ship on an east or west compass heading. To make this check: (a) note the compass reading with the Flinders bar in the holder; (b) invert the Flinders bar in the holder and again note the compass reading. Any appreciable difference (2° or more) between these observed readings indicates residual magnetism in the Flinders bar. Spheres or Flinders bars which show signs of such residual magnetism should be annealed, i.e., heated to a dull red and allowed to cool slowly. Correct alignment of the lubber’s line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship is important. Any misalignment will produce a constant error in the deviation curve. All of these instruments may be aligned correctly with the fore-andaft line of the ship by using the azimuth circle and a metal tape measure. Should the instrument be located on the centerline of the ship, a sight is taken on a mast or other object on the centerline. If the instrument is not on the centerline, measure the distance from the centerline of the ship to the center of the instrument. Mark this distance off from the centerline forward or abaft the compass and place reference marks on the deck. Take sights on these marks. Align the compass so that the compass’ lubber’s line is parallel to the fore-and-aft line of the ship. Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A error present, as discussed in section 611. Adjust the Flinders bar first because it is subject to induction from several of the correctors and its adjustment is not dependent on any single observation. To adjust the Flinders bar, use one of the following methods: 1. Use deviation data obtained at two different magnetic latitudes to calculate the proper length of Flinders bar for any particular compass location. Sections 622 through 624 contain details on acquiring the data and making the required calculations. 2. If the above method is impractical, set the Flinders bar length by: a. Using a Flinders bar length determined by other ships of similar structure. b. Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length required. If these methods are not suitable, omit the Flinders bar until the required data are acquired. The iron sections of Flinders bar should be continuous and placed at the top of the tube with the longest section at the top. Wooden spacers are used at the bottom of the tube. Having adjusted the length of Flinders bar, place the spheres on the bracket arms at an approximate position. If the compass has been adjusted previously, place the spheres at the position indicated by the previous deviation table. In the event the compass has never been adjusted, place the spheres at the midpoint on the bracket arms. The next adjustment is the positioning of the heeling magnet using a properly balanced dip needle. Section 637 discusses this procedure. These three dockside adjustments (Flinders bar, quadrantal spheres, and heeling magnet) will properly establish the conditions of mutual induction and shielding of the compass. This minimizes the steps required at sea to complete the adjustment. 611. Expected Errors Figure 607 lists six different coefficients or types of deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows: The A error is caused by the miscalculation of azimuths or by physical misalignments rather than magnetic effects of unsymmetrical arrangements of horizontal soft iron. Thus
90MAGNETICCOMPASSADJUSTMENTchecking the physical alignments at dockside and makingTheB error results fromboththefore-and-aftperma-careful calculations will minimize the A error. Where an azi-nent magnetic field across the compass and a resultantunsymmetrical vertical inducedeffectforward or aftofthemuth or bearing circle is used on a standard compass todeterminedeviations,anyobservedA errorwill be solelymagcompass.Theformer is correctedbytheuseoffore-and-aftneticAerrorbecause suchreadings aretakenon thefaceoftheB magnets, and the latter is corrected by the use of thecompass card rather than at the lubber's line of the compass.Flindersbarforward oraftof thecompass.BecausetheOna steering compasswheredeviationsareobtainedbyaFlinders bar setting is a dockside adjustment, any remainingcomparison of the compass lubber's line reading with theB error is corrected by the use of fore-and-aft Bmagnets.ship's magnetic heading,as determined by pelorus or gyro,The C error results from the athwartship permanentanyobservedAerrormaybeacombinationofmagneticAandmagnetic field across the compass and a resultant unsym-mechanicalA(misalignment).Thesefacts explaintheproce-metrical vertical induced effect athwartshipofthe compassdure in which only mechanical A is corrected on the standardTheformer is corrected bytheuse of athwartshipC magcompass,byrealignmentofthebinnacle,andbothmechanicalnets, and thelatter by the use ofthe Flinders bar to port orAandmagneticAerrors arecorrected onthesteering compassstarboard of the compass.Because the vertical induced ef-byrealignmentofthebinnacle.Onthestandardcompass,thefect is very rare,the C error is corrected by athwartshipmechanicalAerrormaybe isolatedfromthemagneticAerrorCmagnets onlyby making thefollowing observations simultaneously:TheD erroris due onlyto induction in the symmetricalarrangementsofhorizontal soft iron,andrequirescorrection1Record a curveof deviationsbyusingan azimuthbyspheres,generallyathwartshipofthecompass.(or bearing) circle. Any A error found will be solelyE error of appreciablemagnitude is rare,since it ismagnetic A.caused by induction in the unsymmetrical arrangements ofhorizontal soft iron.When this error is appreciable itmay be2.Recorda curveof deviationsbycomparisonof thecorrectedbyslewingthespheres,asdescribed in section620compass lubber's line reading with the ship's mag-As stated previously,the heeling error is adjusted atnetic heading as determined by pelorus or by gyro.docksidewith a balanced dipneedle (see section637)Any A error found will bea combination of me-As the above discussion points out, certain errors arechanical Aand magnetic Arare andothers are correctedatdockside.Therefore,formostships, only the B, C, and D errors require at sea correction.3.ThemechanicalAonthestandardcompassisthenThese errors are corrected by the fore-and-aft B magnets,found by subtracting the A found in the first in-athwartshipC magnets,andquadrantal spheres respectivelystance from the total A found in thesecondinstance,andiscorrectedbyrotatingthebinnacle612. Study OfAdjustment Procedurein the proper direction by that amount. It is neitherconvenientnornecessaryto isolate thetwotypesofInspecting the B, C, and D errors pictured in FigureA on the steering compass and all Afound by usingthe pelorus or gyro may be removed by rotating the612a demonstrates a definiteisolation ofdeviation effectsbinnacle in the proper direction.on cardinal compass headings.HEast(+)Deg.Dev.West(-)90180"270360"oCompass Heading-DegreesFigure 612a. B, C, and D deviation effects
90 MAGNETIC COMPASS ADJUSTMENT checking the physical alignments at dockside and making careful calculations will minimize the A error. Where an azimuth or bearing circle is used on a standard compass to determine deviations, any observed A error will be solely magnetic A error because such readings are taken on the face of the compass card rather than at the lubber’s line of the compass. On a steering compass where deviations are obtained by a comparison of the compass lubber’s line reading with the ship’s magnetic heading, as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and mechanical A (misalignment). These facts explain the procedure in which only mechanical A is corrected on the standard compass, by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass by realignment of the binnacle. On the standard compass, the mechanical A error may be isolated from the magnetic A error by making the following observations simultaneously: 1. Record a curve of deviations by using an azimuth (or bearing) circle. Any A error found will be solely magnetic A. 2. Record a curve of deviations by comparison of the compass lubber’s line reading with the ship’s magnetic heading as determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A. 3. The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro may be removed by rotating the binnacle in the proper direction. The B error results from both the fore-and-aft permanent magnetic field across the compass and a resultant unsymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-and-aft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Because the Flinders bar setting is a dockside adjustment, any remaining B error is corrected by the use of fore-and-aft B magnets. The C error results from the athwartship permanent magnetic field across the compass and a resultant unsymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C magnets, and the latter by the use of the Flinders bar to port or starboard of the compass. Because the vertical induced effect is very rare, the C error is corrected by athwartship C magnets only. The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by spheres, generally athwartship of the compass. E error of appreciable magnitude is rare, since it is caused by induction in the unsymmetrical arrangements of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in section 620. As stated previously, the heeling error is adjusted at dockside with a balanced dip needle (see section 637). As the above discussion points out, certain errors are rare and others are corrected at dockside. Therefore, for most ships, only the B, C, and D errors require at sea correction. These errors are corrected by the fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively. 612. Study Of Adjustment Procedure Inspecting the B, C, and D errors pictured in Figure 612a demonstrates a definite isolation of deviation effects on cardinal compass headings. Figure 612a. B, C, and D deviation effects