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