CHAPTER 6MAGNETICCOMPASSADJUSTMENTGENERALPROCEDURESFORMAGNETICCOMPASSADJUSTMENT600.Introductionf.Alignment of magnets in binnacleAlignment of heeling magnet tube under pivotg.This chapter presents information and procedurespointofcompass.for magneticcompass adjustment.Sections601and613See that corrector magnets are available.hcover procedures designed to eliminate compass errorssatisfactorily.Refer toFigure 607 for condensed infor-2.Physical checks ofgyro,azimuth circle,and pelorusesAlignmentof peloruses withfore-and-aft line ofmation regardingthe various compass errors and theira.correction.ship (section610).Theterm compass adjustmentreferstoanychangeofb.Synchronize gyro repeaters with master gyro.permanent magnetorsoft iron correctorsto reducenormalc. Ensure azimuth circles and peloruses are in goodcondition.compasserrors.Thetermcompasscompensationreferstoany change inthe current slupplied to the compasscompen-3.Necessarydata.sating coils to reduce degaussing errors.a.Pasthistory or log data which might establish601.Adjustment Check-Off ListlengthofFlindersbar (sections610and623).Azimuthsfordateand observer's position (sectionb.If themagnetic adjustment necessitates (a)movement633andChapter17)of degaussing compensating coils, or (b) a change ofRanges or distant objects in vicinity if needed (lo-Ccal charts).Flinders barlength,check alsothe coil compensation persection646Correctvariation (localcharts)dExpeditiouscompass adjustmentdepends on theappli-Degaussing coil current settings forswing forresid-e.ual deviations after adjustment and compensationcation of the various correctors in an optimum sequencedesigned to minimizethe number ofcorrection steps.Cer-(ship'sDegaussingFolder).tainadjustmentsmaybe madeconveniently atdockside.4.Precautions.simplifying the at seaadjustment procedures.Determinetransient deviations of compass fromMoving thewrong correctorwastestime and upsets alla.previous adjustments,sobecareful tomakethe correctadgyrorepeaters,doors,guns,etc.(sections636and639).justments.Throughout an adjustment,special care shouldbetakentopairoff sparemagnets sothattheresultantfieldb.Secure all effective magnetic gear in normal seagoingabout them will be negligible.To make doubly sure that thepositionbeforebeginningadjustments.Make sure degaussing coils are secured before be-compass isnot affected by a sparemagnet's strayfieldCkeepthem atanappropriatedistanceuntiltheyareactuallyginning adjustments. Use reversal sequence, ifinserted into the binnacle.necessary.Whenever possible, correctors should be placedd.A.Docksidetests and adjustments.symmetricallywithrespecttothecompass.1.Physical checksonthecompassandbinnacle5.Adjustments.a.Remove any bubbles in compass bowl (sectiona.PlaceFlindersbaraccordingtobestavailableinfor-610).mation (sections 610, 622 through 625)Test for moment and sensibility of compass nee-b.b.Set spheres at mid-position, or as indicated by lastdles(section610)deviation table.Removeanyslack ingimbal arrangement.Adjust heeling magnet, using balanced dip needlec.c.dMagnetization check of spheres and Flinders barifavailable (section637).(section610)e.Alignment of compass with fore-and-aft line ofB.Adjustments at sea.Make these adjustments with the shipship (section610)on an even keel and steady on each heading. When using81
81 CHAPTER 6 MAGNETIC COMPASS ADJUSTMENT GENERAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT 600. Introduction This chapter presents information and procedures for magnetic compass adjustment. Sections 601 and 613 cover procedures designed to eliminate compass errors satisfactorily. Refer to Figure 607 for condensed information regarding the various compass errors and their correction. The term compass adjustment refers to any change of permanent magnet or soft iron correctors to reduce normal compass errors. The term compass compensation refers to any change in the current slupplied to the compass compensating coils to reduce degaussing errors. 601. Adjustment Check-Off List If the magnetic adjustment necessitates (a) movement of degaussing compensating coils, or (b) a change of Flinders bar length, check also the coil compensation per section 646. Expeditious compass adjustment depends on the application of the various correctors in an optimum sequence designed to minimize the number of correction steps. Certain adjustments may be made conveniently at dockside, simplifying the at sea adjustment procedures. Moving the wrong corrector wastes time and upsets all previous adjustments, so be careful to make the correct adjustments. Throughout an adjustment, special care should be taken to pair off spare magnets so that the resultant field about them will be negligible. To make doubly sure that the compass is not affected by a spare magnet’s stray field, keep them at an appropriate distance until they are actually inserted into the binnacle. A. Dockside tests and adjustments. 1. Physical checks on the compass and binnacle. a. Remove any bubbles in compass bowl (section 610). b. Test for moment and sensibility of compass needles (section 610). c. Remove any slack in gimbal arrangement. d. Magnetization check of spheres and Flinders bar (section 610). e. Alignment of compass with fore-and-aft line of ship (section 610). 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 peloruses with fore-and-aft line of ship (section 610). b. Synchronize gyro repeaters with master gyro. c. Ensure azimuth circles and peloruses are in good condition. 3. Necessary data. a. Past history or log data which might establish length of Flinders bar (sections 610 and 623). b. Azimuths for date and observer’s position (section 633 and Chapter 17). c. Ranges or distant objects in vicinity if needed (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. (sections 636 and 639). 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. 5. Adjustments. a. Place Flinders bar according to best available information (sections 610, 622 through 625). b. Set spheres at mid-position, or as indicated by last deviation table. c. Adjust heeling magnet, using balanced dip needle if available (section 637). B. Adjustments at sea. Make these adjustments with the ship on an even keel and steady on each heading. When using
82MAGNETIC COMPASS ADJUSTMENTthegyro,swing slowlyfromheadingtoheading andcheckFore-and-af and athwartship magnetsQuadrantial spheresFlinders barEasterly on eastWesterly on eastE, on NEW.on NE,Deviation changeandW.onWW.onE.and E.onWDeviationDeviationE.onSE,E,onSE,andwesterly onand easterly onwith latitudewhen sailing towardwhen sailing toward2W.onw,W.onSW,west.west.changeequator fromnorth equatornorthfromandE. on NW.atitdeorawayomittudeorawayfromand>MagnetsSpheresW.onNW.(+B error)(-B error)(-D error)equatortosoutiBarequator to south latitudelatitude,(+D error)VVVNo fore and aftPlace required of bar Place required amountPlace magnets redPlace magnets redNo spheres onPlace spheresPlace spheres foreaflNo bar in holder.magnets inforward.athwartship.and aft.forward.of bar aff.binnacle.binnacle.Fore and aftRaise magnets.ower magnets.Spheres atMovespheres toward Movespheresncrease amountof barDeacrease amountmagnets redathwartshipoutwards or remove.Bar forward of binnacleforward.of bar forward.compass or useforward.position.larger spheres.Fore and aftLower magnets.Spheres at fore andMove spheresMove spheres towardDecreaseof ncrease amount ofRaise magnets.amountmagnets red af.Bar aft of binnacle.bar af.aft position.outward or remove.compass or usebar aft.larger spheresE. on N,DeviationEasterly , onnorth Westerly on northDeviationW. on E. and E. on W.EonEand W.on W.个W.onE,andwesterlyonand easterly onwhen sailing towardwhensailing towardE.ons,Barsouth.south.equator fromsouthequatorfromsouthandandlatitudeorawayfromlatitude oraway fromSpheresMagnets-C error)W.on w.E.onw.Deviation changeequator4onorthequator to south latitudeV?+C error)iatitude.(+E error)(-E error)with latitudechange->1oathwartship Placeathwartship Place asthwartshipspheresHeeling magnetnPlace spheres at pont Placespheres(Adjust with changes in magnetic latitude)magnetsinmagnetsredmagnets red porLbinnacle.forward and starboardstarbcardforewardstarboard.antbinnacle.intercardiralandaffportIf compass north is atracted to high side of ship when rolling, raiscpsitions.intercardinalheheelingmagnet if red endis up and lorertheheelingmagnet if blupositionscnd is up.AthwartshipatSlewRaise magnetsLower magnets.Spheresspheres SlewsphereIf compass north is attracted to low side of ship when rolling, loweredathuartshipmagnetsclockwisethroughcounter-clockwisehe heeling magnet if red end is up and raise the heeling magnet if blucstarboard.position,throughrequired angle.requiredend isup.angle.NOTE: Any change in placement of the heeling magnet willaffect thoAthwartshipLower magnets.Raise magnets.Spheres at fore andSlew spheres counter-Slew spheresieviations on all headings.magnets red pon.clockwise throughclockwise throughaft position.required angle.required angle.Figure601.Mechanicsofmagneticcompassadjustmenthalf of any observed deviation by moving thegyro errorby sun's azimuthor ranges on eachheadingtoCmagnets.ensurea greater degree of accuracy (section631).Be sureThe cardinal heading adjustments should now begyro is set for themean speed and latitude of the vesselNoteall precautions in sectionA-4above.Flythe“OSCARcomplete.QUEBECinternational code signaltoindicatesuchwork6.Come to any intercardinal magnetic heading, e.g.is in progress. Section 631 discusses methods for placingnortheast (045°).Correct any observed deviationtheshipondesiredheadingsby moving the spheres in or out.7.Cometo the next intercardinal magneticheading.1.Adjust the heeling magnet while the ship is rollinge.g., southeast (135).Correct half of any ob-on north and southmagnetic headings until theos-served deviationbymoving the spheres.cillations of thecompass card havebeen reducedto an averageminimum.This step is not requiredThe intercardinal heading adjustments should now beifprioradjustmenthasbeenmadeusingadipnee-completealthoughmoreaccurateresultsmightbeob-dleto indicate proper placement of theheelingtained by correctingtheD error determined from themagnet.deviations onall four intercardinal headings, as discussed2.Come to a cardinal magnetic heading, e.g., eastin section615.(090°).Insertfore-and-aftBmagnets,ormovetheexistingBmagnets,toremoveall deviation.8.Secure all correctors before swinging for residual3.Come to a south (180°) magnetic heading. Insertdeviations.9.Swing for residual undegaussed deviations on asathwartship C magnets,or move the existingCmagnets, to removeall deviation.many headings as desired, although the eight car-4.Cometo a west(270°)magnetic heading.Correctdinal and intercardinal headings shouldbehalf of any observed deviation by moving thesufficient.Bmagnets.10.Should there still be any largedeviations,analyze5.Come to a north (00o°)magnetic heading.Correctthe deviation curve to determine the necessary
82 MAGNETIC COMPASS ADJUSTMENT the gyro, swing slowly from heading to heading and check gyro error by sun’s azimuth or ranges on each heading to ensure a greater degree of accuracy (section 631). Be sure gyro is set for the mean speed and latitude of the vessel. Note all precautions in section A-4 above. Fly the “OSCAR QUEBEC” international code signal to indicate such work is in progress. Section 631 discusses methods for placing the ship on desired headings. 1. Adjust the heeling magnet while the ship is rolling on north and south magnetic headings 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 a cardinal magnetic heading, e.g., east (090°). Insert fore-and-aft B magnets, or move the existing B magnets, to remove all deviation. 3. Come to a south (180°) magnetic heading. Insert athwartship C magnets, or move the existing C magnets, 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 headings, as discussed in section 615. 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 Fore-and-aft and athwartship magnets Quadrantial 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, E. on SE, W. on SW, and W. on NW. (+D error) W. on NE, E. on SE, W. on SW, andE. on NW. (-D error) Deviation change with latitude change Bar E. on E. and W. on W when sailing toward equator from north latitude or away from equator to south latitude. W. on E. and E. on W when sailing toward equator from north latitude or away from equator to south 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 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 outwards or remove. Bar forward of binnacle. Increase amount of bar forward. Deacrease 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 aft. Increase amount of bar aft. 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, W. on E, E. on S, and W. on W. (+E error) W. on N, E. on E, W. on S, and E. on W. (-E error) Bar Deviation change with latitude change W. on E. and E. on W. when sailing toward equator from south latitude or away from equator to north latitude. E. on E. and W. on W. when sailing toward equator from south latitude or away from equator to south 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 foreward and port aft intercardinal positions. Heeling magnet (Adjust with changes in magnetic latitude) If compass north is attracted to high side of ship when rolling, raise the heeling magnet if red end is up and lower the heeling magnet if blue end is up. 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 low side of ship when rolling, lower the heeling magnet if red end is up and 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. Athwartship magnets red port. Lower magnets. Raise magnets. Spheres at fore and aft position. Slew spheres counterclockwise through required angle. Slew spheres clockwise through required angle. Figure 601. Mechanics of magnetic compass adjustment
83MAGNETICCOMPASSADJUSTMENTcorrections and repeat as necessary steps 1the unlike poles will attract each other.through 9 above.Magnetism can be either permanent or induced.A11.Recorddeviations andthedetailsofcorrectorpositionsbarhavingpermanentmagnetismwill retain itsmagnetismonthedeviationcardtobepostednearthecompasswhen it is removed from themagnetizing field.A bar hav-12.Swingforresidual degausseddeviations withtheing induced magnetism will lose its magnetismwhendegaussing circuits properly energized.removed from themagnetizing field.Whetherornotabar13.Recorddeviationsfordegaussed conditions onthewill retain itsmagnetism on removal fromthemagnetizingdeviation card.field will depend on the strength ofthat field, thedegree ofhardness of the iron(retentivity),andalsoupontheamountTheabovecheck-off listdescribes a simplified methodof physical stress applied to the bar while in the magnetiz-of adjusting compasses,designed to serveas a workableing field. The harder the iron, the more permanent will beoutlineforthenovice who choosestofollowa step-by-stepthe magnetism acquired.procedure. The dockside tests and adjustments are essentialas a foundation for the adjustments at sea.Neglecting the603.Terrestrial Magnetismdocksideprocedures may lead to spurious results or need-less repetition of the procedures at sea. Give carefulConsider the earth as a huge magnet surrounded byconsideration to thesedockside checks priorto making themagnetic flux lines connecting its two magnetic poles.final adjustment.This will allowtimeto repair orreplaceThese magnetic poles are near, but not coincidental withfaulty compasses,anneal or replacemagnetized spheresorthe earth'sgeographicpoles.Since the north seekingend ofFlindersbars,realign thebinnacle,moveagyro repeater ifa compass needle is conventionally called the north pole,it isaffectingthecompass, ortomakeanyothernecessaryor positive pole, it must therefore be attracted to a southpreliminaryrepairspole, or negative pole.Expeditious compassadjustmentdependsupontheap-Figure 603a illustrates the earth and its surrounding magplicationofthevariouscorrectorsinalogicalsequencesonetic field. The flux lines enter the surface of the earth atasto achievethefinal adjustmentwithaminimum numberof steps.Theabove check-off list accomplishes thispur-different angles to the horizontal, at different magnetic ati-pose.Figure607presents thevarious compass errors andtudes. This angle is called the angle of magnetic dip, e, andtheircorrection incondensedform.Frequent, carefulobser-vations should be made to determine theconstancy ofdeviations,andresultsshouldbesvstematicallyrecordedSignificant changes in deviation will indicate the need forreadjustment.NoruTo avoid Gaussin error (section 636) when adjustingGerahand swinging shipfor residuals,the ship should be steadyPolBleon thedesiredheadingforatleast2minutespriortoobserv-Magneticing thedeviation.Polc602.The Magnetic Compass And MagnetismTheprinciple of thepresentdaymagnetic compass isnodifferentfromthatofthecompassesusedbyancientmariners.It consists of amagnetized needle,or an array ofneedles,allowed torotatein thehorizontal plane.The supe-riority of the present day compasses over ancient onesMasresultsfroma betterknowledge ofthelaws ofmagnetismwhichgovernthebehavior ofthecompass andfromgreaterprecision in construction.Any piece of metal on becoming magnetized will de-velopregionsofconcentratedmagnetismcalledpoles.AnyRedSouthsuchmagnetwill haveatleast twopoles ofoppositepolar-MagneticGeographiePoleity.Magneticforce (flux) lines connectonepoleof suchaPolemagnet with the other pole.The number of such lines perunit area represents the intensity of the magnetic field inthat area.If two such magnetic bars or magnets are placedFigure 603a.Terrestrial magnetism.close to each other,the likepoles will repel each otherand
MAGNETIC COMPASS ADJUSTMENT 83 corrections and repeat as necessary steps 1 through 9 above. 11. Record deviations and the details of corrector positions on the deviation card to be posted near the compass. 12. Swing for residual degaussed deviations with the degaussing circuits properly energized. 13. Record deviations for degaussed conditions on the deviation card. The above check-off list describes a simplified method of adjusting compasses, designed to serve as a 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. Neglecting the dockside procedures may lead to spurious results or needless repetition of the procedures at sea. Give careful consideration to these dockside checks prior to making the final adjustment. This will allow time to repair or replace faulty compasses, anneal or replace magnetized spheres or Flinders bars, realign the binnacle, move a gyro repeater if it is affecting the compass, or to make any other necessary preliminary repairs. Expeditious compass adjustment depends upon the application of the various correctors in a logical sequence so as to achieve the final adjustment with a minimum number of steps. The above check-off list accomplishes this purpose. Figure 607 presents the various compass errors and their correction in condensed form. 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 (section 636) 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. 602. The Magnetic Compass And Magnetism The principle of the present day magnetic compass is no different from that of the compasses used by ancient mariners. It consists of a magnetized needle, or an array of needles, allowed to rotate in the horizontal plane. The superiority of the present day compasses over ancient ones results from a better knowledge of the laws of magnetism which govern the behavior of the compass and from greater precision in construction. Any piece of metal on becoming magnetized will develop regions of concentrated magnetism called poles. Any such magnet will have at least two poles of opposite polarity. Magnetic force (flux) lines connect one pole of such a magnet with the other pole. 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 close to each other, the like poles will repel each other and the unlike poles will attract each other. Magnetism can be either permanent or 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. 603. Terrestrial Magnetism Consider the earth as a huge magnet surrounded by magnetic flux lines connecting its two magnetic poles. These magnetic poles are near, but not coincidental with, the earth’s geographic poles. Since the north seeking end of a compass needle is conventionally called the north pole, or positive pole, it must therefore be attracted to a south pole, or negative pole. Figure 603a 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 atitudes. This angle is called the angle of magnetic dip, θ, and Figure 603a. Terrestrial magnetism
84MAGNETICCOMPASSADJUSTMENTFigure603b.Magneticdipchart,asimplificationofchart30Figure603c.Magneticvariation chart,a simplificationof chart42
84 MAGNETIC COMPASS ADJUSTMENT Figure 603b. Magnetic dip chart, a simplification of chart 30. Figure 603c. Magnetic variation chart, a simplification of chart 42
85MAGNETICCOMPASSADJUSTMENTincreasesfrom0°,atthemagnetic equator,to90°atthemagAship,then,hasa combinationofpermanent,subperma-netic poles. The total magnetic field is generally considered asnent, and induced magnetism. Therefore, the ship's apparenthavingtwocomponents:H,thehorizontal component,andZ,permanent magnetic condition is subjecttochangefrom dep-erming,excessive shocks,welding,andvibration.Theshipstheverticalcomponent.Thesecomponentschangeastheanglee, changes, such that H is maximum at the magnetic equatorinduced magnetism will vary with the earth's magnetic fieldand decreases in the direction of eitherpole,Z is zero at theStrength and with the alignment of the ship in that field.magneticeguatorandincreasesinthedirectionofeitherpoleThevaluesofmagneticdipmaybefoundonChart30(shown605.MagneticAdjustmentsimplified inFigure603b).The values of H and Z maybefoundoncharts33and36A rod of soft iron,in aplane parallel to the earth'shor-Sincethe magnetic poles of the earth do not coincideizontal magnetic field, H, will have a north pole induced inwith thegeographic poles,a compassneedle in linewith thethe end toward the north geographic pole and a south poleearth's magnetic field will not indicatetrue north, but maginduced in the end toward the south geographic pole.Thisnetic north.The angular difference between thetrue meridiansamerod in a horizontal plane, but at right angles to the hor-(greatcircleconnectingthegeographicpoles)andthemagizontalearth'sfield,wouldhavenomagnetisminducedinitnetic meridian (direction of the lines of magnetic flux)isbecause its alignment in the magnetic field is such that therecalled variation.This variation has different values at differ-will benotendencytoward linear magnetization,and the rodentlocationsontheearth.Thesevaluesofmagneticvariationisofnegligiblecrosssection.ShouldtherodbealignedinmaybefoundonChart42(shownsimplified inFigure603c)some horizontal direction between those headings whichon pilot charts,and,on thecompass rose of navigationalcreatemaximum and zero induction,it would be induced bycharts.Thevariationformostgivenareasundergoesanan-anamountwhichisafunctionoftheangleofalignment.Ifnual change,the amount of which is also noted on chartsasimilarrodisplacedinaverticalpositioninnorthernlatitudes so as to be aligned with the vertical earth's field Z, itwill have a south pole induced at the upper end and a north604.Ship'sMagnetismpole induced at the lower end.These polarities of vertical in-duced magnetization will be reversed in southern latitudes.Aship under construction or major repair will acquireThe amount of horizontal or vertical induction in suchpermanent magnetism dueto hammering and jarring whilerods,orinshipswhoseconstructioniseguivalenttocombi-sitting stationary in the earth'smagneticfield.Afterlaunch-nations of such rods,will vary with the intensity of H andingtheshipwilllosesomeofthisoriginal magnetismasaZ, heading and heel of the shipresultofvibrationandpounding invaryingmagneticfieldsThe magnetic compass must be corrected for the ves-and will eventuallyreach amoreorless stablemagneticsel's permanent and induced magnetism so that itscondition.Themagnetismwhichremains isthepermanentoperationapproximatesthatof acompletelynonmagneticmagnetism of the ship.vessel. Ship's magnetic conditions create magnetic com-The fact that a ship has permanent magnetism does notpassdeviationsandsectorsofsluggishnessandmean that itcannot also acquire induced magnetism whenunsteadiness.Deviation isdefinedasdeflectionrightor leftplaced in the earth's magnetic field. The magnetism in-ofthemagneticmeridian.Adjustingthecompassconsistsduced in any given piece of soft iron is a function of theof arranging magnetic and soft iron correctors about thefield intensity,the alignment of the soft iron in that fieldbinnacle so that their effects are equal and opposite to theand the physical properties and dimensions of the iron. Thiseffects of the magnetic material in the ship.induced magnetismmay addto,or subtractfrom,theper-The total permanent magnetic field effect at the com-manent magnetism already present in the ship, dependingpass may be broken into three components,mutually 900on how the ship is aligned in the magnetic field.The softerapart, as shown in Figure 605a.the ironthe morereadilyit will be magnetizedbytheThe vertical permanent component tilts the compassearth's magnetic field, and the more readily it will give upcard, and, when the shiprolls or pitches, causes oscillatingitsmagnetism when removedfromthatfield.deflections of thecard.Oscillation effectswhichaccompa-Themagnetism in thevarious structures of a shipnyroll aremaximumonnorthand southcompass headingswhich tends to change as a result of cruising,vibration, orandthosewhichaccompanypitcharemaximumon eastandaging,but which does not alter immediately so asto bewestcompassheadingsproperly termed induced magnetism, is called subperma-ThehorizontalBandCcomponentsofpermanentmagnent magnetism.This magnetism, at any instant, is part ofnetism cause varying deviations of the compass as the shiptheship'spermanentmagnetism,and consequentlymustbeswings in heading on an even keel.Plotting these deviationscorrected by permanent magnet correctors.It is theprinci-against compass heading yields the sine and cosine curvespal cause of deviation changes on a magnetic compass.shown in Figure 605b.These deviation curves are calledSubsequentreferencetopermanentmagnetism will refertosemicircular curves because they reverse direction by180°theapparentpermanent magnetism which includes the ex-istingpermanentand subpermanentmagnetism.A vector analysis is helpful indetermining deviations or
MAGNETIC COMPASS ADJUSTMENT 85 increases from 0°, at the magnetic equator, to 90° at the magnetic poles. The total magnetic field is generally considered as having two components: 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. The values of magnetic dip may be found on Chart 30 (shown simplified in Figure 603b). The values of H and Z may be found on charts 33 and 36. Since the magnetic poles of the earth do not coincide with the geographic poles, 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 Chart 42 (shown simplified in Figure 603c), on pilot charts, and, 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 charts. 604. Ship’s Magnetism A ship under construction or major repair will acquire permanent magnetism due to hammering and jarring while sitting stationary in the earth’s magnetic field. After launching, the ship will lose some of this original magnetism as a result of vibration and pounding in varying magnetic fields, and will eventually reach a more or less stable magnetic condition. The magnetism which remains is the permanent magnetism of the ship. The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed in the earth’s magnetic field. The magnetism induced in any given piece of soft iron is a function of 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 magnetized by the earth’s magnetic field, and the more readily it will give up its magnetism when removed from that field. The magnetism in the various structures of a ship, which tends to change as a result of cruising, vibration, or aging, but which does not alter immediately so as to be properly termed induced magnetism, is called subpermanent magnetism. This magnetism, at any instant, is part of the ship’s permanent magnetism, and consequently must be corrected by permanent magnet correctors. It is the principal cause of deviation changes on a magnetic compass. Subsequent reference to permanent magnetism will refer to the apparent permanent magnetism which includes the existing permanent and subpermanent magnetism. A ship, then, has a combination of permanent, subpermanent, and induced magnetism. Therefore, the ship’s apparent permanent magnetic condition is subject to change from deperming, excessive shocks, welding, and vibration. The ship’s induced magnetism will vary with the earth’s magnetic field strength and with the alignment of the ship in that field. 605. Magnetic Adjustment A rod of soft iron, in a plane parallel to the earth’s horizontal magnetic field, H, will have a north pole induced in the end toward the north geographic pole and a 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 which create maximum and zero induction, it would be induced by an amount which 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 south pole induced at the upper end and a 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. The magnetic compass must be corrected for the vessel’s permanent and induced magnetism so that its operation approximates that of a completely nonmagnetic vessel. Ship’s magnetic conditions create magnetic compass deviations and sectors of sluggishness and unsteadiness. Deviation is defined as deflection right or left of the magnetic meridian. Adjusting the compass consists of arranging 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. The total permanent magnetic field effect at the compass may be broken into three components, mutually 90° apart, as shown in Figure 605a. The vertical permanent component tilts the compass card, and, when the ship rolls or pitches, causes oscillating deflections of the card. Oscillation effects which accompany roll are maximum on north and south compass headings, and those which 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 yields the sine and cosine curves shown in Figure 605b. These deviation curves are called semicircular curves because they reverse direction by 180°. A vector analysis is helpful in determining deviations or