309.Symmetrical arrangementsof horizontal softiron may existabout thecompass inanyoneof thepatterns illustrated infigure309.Figure309-Symmetricalarrangementsofhorizontalsoftiron310.The deviation resulting from the earth's field induction of these symmetrical arrangements of horizontal soft iron areillustrated infigure310, showing theshiponvarious compass headings.Theotherheading effects maybesimilarly studiedSuch a Ddeviation curve is one of the curves in figure307b.It will be noted that theseD deviations are maximum on theintercardinalheadingsandzeroonthecardinal headings.East headingSoutheast headingNorthheadingNortheastheadingbycompassbycompassbycompassby compassE.Dev.CompassW.Dev.-Rods ofSoftIronCompass NeedleMaximumdeviationMaximum deviationNo deviationNodeviationwesterlyeasterlyDDeviationsEast(+)Deg.6oDev.90135180°225°31545West(-)CompassHeading-DegreesFigure310-EffectsofsymmetricalhorizontalDinducedmagnetism10
10 309. Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated in figure 309. Figure 309 – Symmetrical arrangements of horizontal soft iron 310. The deviation resulting from the earth's field induction of these symmetrical arrangements of horizontal soft iron are illustrated in figure 310, showing the ship on various compass headings. The other heading effects may be similarly studied. Such a D deviation curve is one of the curves in figure 307b. It will be noted that these D deviations are maximum on the intercardinal headings and zero on the cardinal headings. Figure 310 – Effects of symmetrical horizontal D induced magnetism
31l.Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those infigure311.Figure31l-Asymmetricalarrangementsof horizontal soft iron312.Thedeviations resulting fromthe earth'sfield induction of these asymmetrical arrangements of horizontal soft iron areillustrated in figure 312, showing the ship on different compass headings.The other heading effects may be similarly studied.Such an Edeviation curve is one of the curves in figure 307b.It will be observed that these Edeviations are maximum oncardinal headings and zero on the intercardinal headingsNorth headingEast headingNortheast headingSoutheast headingby compassby compassby compassbycompassW.Dev.E, Dev.CompassCompassNeedleRods ofSoftIronMaximumdeviationNo deviationMaximum deviationNo deviationeasterlywesterlyEastEDeviations(+)Deg.270°Dey.4590°135180°225315°360°0°WestCompassHeading-Degrees(-) Figure 312-Effects of asymmetrical horizontal E induced magnetism313. The quadrantal deviations will not vary with latitude changes, because the horizontal induction varies proportionallywiththedirectiveforce,H.314.The earth's field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant Adeviation curve.Themagnetic A and E errors are of smaller magnitudethan theother errors,but, when encountered, aregenerallyfound together,since they both result from asymmetrical arrangements of horizontal soft iron. In addition to thismagnetic Aerror,there are constant Adeviations resultingfrom:(1)physical misalignments of the compass, pelorus, or gyro;(2)errors in calculating the sun's azimuth, observing time, or taking bearings11
11 311. Asymmetrical arrangements of horizontal soft iron may exist about the compass in a pattern similar to one of those in figure 311. Figure 311 – Asymmetrical arrangements of horizontal soft iron 312. The deviations resulting from the earth's field induction of these asymmetrical arrangements of horizontal soft iron are illustrated in figure 312, showing the ship on different compass headings. The other heading effects may be similarly studied. Such an E deviation curve is one of the curves in figure 307b. It will be observed that these E deviations are maximum on cardinal headings and zero on the intercardinal headings. Figure 312 – Effects of asymmetrical horizontal E induced magnetism 313. The quadrantal deviations will not vary with latitude changes, because the horizontal induction varies proportionally with the directive force, H. 314. The earth's field induction in certain other asymmetrical arrangements of horizontal soft iron creates a constant A deviation curve. The magnetic A and E errors are of smaller magnitude than the other errors, but, when encountered, are generally found together, since they both result from asymmetrical arrangements of horizontal soft iron. 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
315.The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, thesymmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth's magnetic field, and the angleofdip.316.Certainheeling errors,in additionto thoseresultingfrom permanent magnetism,are created bythepresence of bothhorizontal and vertical soft iron, which experience changing induction as the ship rolls in the earth's magnetic field.This partof the heeling error will naturally change in magnitude with changes of magnetic latitudeof the ship.Oscillation effectsaccompanyingroll aremaximum on northand south headings,justas with thepermanentmagneticheelingerrors.317.Adjustments and correctors.Since somemagnetic effects remain constantfor all magnetic latitudes and others varywith changes of magnetic latitude, each individual effect should be corrected independently.Further, it is apparent that thebestmethodofadjustmentisto use(1)permanentmagnet correctorstocreateequal andoppositevectorsof permanentmagneticfields atthecompass,and (2)soft iron correctorstoassume inducedmagnetism,the effectofwhich will beequaland opposite tothe induced effects of the ship for all magnetic latitude and heading conditions.The compass binnacleprovidesforthe supportof the compass and such correctors.Study of the binnacle infigure317 will reveal that suchcorrectorsarepresent intheformof(1)Verticalpermanentheelingmagnetinthecentralvertical tube,(2)Fore-and-aft B permanent magnets in their trays,(3)AthwartshipCpermanentmagnets in theirtrays(4)Vertical softironFlindersbarinitsexternaltube,(5)SoftironspheresTheheelingmagnet is theonly correctorthat correctsfor bothpermanent and induced effects,and consequentlymust bereadjusted occasionally with radical changes in latitude of the ship. (It must be noted, however, that any movement of theheelingmagnetwillrequirereadjustmentofothercorrectors.)DegaussingFlinders BarSphereCompensatingCoilHeeling MagnetTubeFore-and-aff "B"OMagnet TraysAthwartship"C'MagnetTraysFigure317-Binnaclewithcompass and correctors12
12 315. 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. 316. 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. 317. Adjustments and correctors. Since some magnetic effects remain constant for all magnetic latitudes and others vary with changes of magnetic latitude, each individual effect should be corrected independently. Further, it is apparent that the best method of adjustment is to use (1) permanent magnet correctors to create equal and opposite vectors of permanent magnetic fields at the compass, and (2) soft iron correctors to assume induced magnetism, the effect of which will be equal and opposite to the induced effects of the ship for all magnetic latitude and heading conditions. The compass binnacle provides for the support of the compass and such correctors. Study of the binnacle in figure 317 will reveal that such correctors are present in the form of: (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 spheres. The heeling magnet is the only corrector that corrects for both permanent and induced effects, and consequently must be readjusted occasionally with radical changes in latitude of the ship. (It must be noted, however, that any movement of the heeling magnet will require readjustment of other correctors.) Figure 317 – Binnacle with compass and correctors
318. The tabular summary of "Compass Errors and Adjustments," figure 318, summarizes all the various magnetic conditions in a ship, the types ofdeviation curves they create, the correctors for each effect, and headings on which each corrector is adjusted. Correctors should be appliedsymmetrically under all but exceptional conditions (discussed in detail later) and as far away from the compass as possible to preserve uniformity ofmagnetic fields about the compass needle array.Other details of corector procedure are emphasized in chapter Vl.Fortunately, each magnetic effect hasa slightly different characteristic curve that makes identification and correction convenient. A complete deviation curve can be analyzed for its differentcomponents and, thus, the necessary corrections anticipated. A method for analyzing such curves is described in chapter V.CompassCoefficientType deviation curveCauses of such errorsCorrections for such errorsMagnetic or compassheadings ofheadings on which to applymaximumcorrectorsdeviationHuman. error in calculations.....Check methods and calculations4Constant.Any.Same on all.Physical: compass, gyro, pelorus alignment..Check alignmentsMagnetic: asymmetrical arrangements of horizontal softRare arrangement of soft iron rodsiron.....Fore-and-afft component of permanent magnetic field..Fore-and-aft B magnets.B0900Semicircular sin e.090°or 270°,Induced magnetism in asymmetrical vertical iron270°Flinders bar (forward or aff)orward.oraft.ofcompass......Athwartship component of permanent magnetic fieldAthwartship C magnets.c0000000°or180°gSemicircular cos e.Induced magnetism in asymmetrical vertical iron port or180°Flinders bar (port or starboard)starboard of compass.045°Induced magnetism in all symmetrical arrangements ofSpheres on appropriate axisDQuadrantal sin 2e.045°, 135°,225°, or 315°135°horizontal soft iron.(athwartship for +D)225*(fore and af for -D)315°See sketcha0000Induced magnetism in all asymmetrical arrangementsSpheres on appropriate axisEQuadrantal cos 20.000°,090°,180°,or270°090°(port forward, starboard aft for +E)of horizontal soft iron.180°(starboard forward, port aff for -E)270°See skeich b0007HeelingOscillations with rollHeeling magnet (must be re-adjusted090° or 270° with dipChange in the horizontal component of the induced orroll180For pitch.pemanent magnetic fields at the compass duetoneedle.for latitude changes)090Deviations with000° or 180° while rolling.rolling or pitching of the shippitch2705constant list.Deviation=A+Bsine+Ccosα+Dsin2e+Ecos2e(e=compassheading0-0-E+EX+D+D(Skach a)(Sketeh 5)O-D0+E-EOFigure318-Summaryof Compass Errors andAdjustments13
13 318. The tabular summary of "Compass Errors and Adjustments," figure 318, 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. Correctors should be applied symmetrically under all but exceptional conditions (discussed in detail later) and as far away from the compass as possible to preserve uniformity of magnetic fields about the compass needle array. Other details of corrector procedure are emphasized in chapter VI. Fortunately, each magnetic effect has a slightly different characteristic curve that makes identification and correction convenient. A complete deviation curve can be analyzed for its different components and, thus, the necessary corrections anticipated. A method for analyzing such curves is described in chapter V. Coefficient Type deviation curve Compass headings of maximum deviation Causes of such errors Corrections 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: asymmetrical arrangements of horizontal soft iron . Check methods and calculations Check alignments Rare arrangement of soft iron rods Any. B Semicircular sin ø. 090° 270° Fore-and-aft component of permanent magnetic field. Induced magnetism in asymmetrical vertical iron forward or aft of compass . Fore-and-aft B magnets. Flinders bar (forward or aft) 090° or 270°. C Semicircular cos ø. 000° 180° Athwartship component of permanent magnetic field. Induced magnetism in asymmetrical vertical iron port or starboard of compass. Athwartship C magnets Flinders bar (port or starboard) 000° or 180°. D Quadrantal sin 2ø. 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 Quadrantal cos 2ø. 000° 090° 180° 270° Induced magnetism in all asymmetrical arrangements of horizontal soft iron. Spheres on appropriate axis. (port forward, starboard aft for +E) (starboard forward, 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 pitch 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 re-adjusted for latitude changes) 090° or 270° with dip needle. 000° or 180° while rolling. Deviation = A + B sin ø + C cos ø + D sin 2 ø + E cos 2 ø (ø = compass heading) Figure 318 – Summary of Compass Errors and Adjustments
319.Compass operation.Figure319 illustrates apointabout compass operation.Not only is an uncorrected compass subjectto largedeviations, butthere will be sectors in which the compassmaysluggishlyturn with the shipand other sectors inwhich the compass is too unsteady to use.Theseperformances maybe appreciated by visualizinga ship with deviations asshown infigure319,as it swings from west through northtoward east.Throughout this easterly swingthe compass deviationis growing more easterly,and, whenever steering in this sector, the compass card sluggishly tries to follow the ship.Similarly,there is an unsteady sector from east through southto west.These sluggish and unsteadyconditions are alwayscharacterized by the positive and negative slopes in a deviation curve. These conditions may also be associated with themaximum andminimum directive force acting on thecompass (see article305).It will beobserved that the maximumdeviation occurs atthepointof averagedirectiveforce and thatthezerodeviations occuratthepoints ofmaximum andminimumdirectiveforceEast(+)Deg.270860Dev.180%90PointofPoint ofPointofMaximumMaximumMaximumSluggish-DeviationUnsteadinessnesHeading-DegreesWest(-)Figure319-Uncompensateddeviationcurve320.Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compassfor all headings of the ship. Correction could be achieved, however, by applying correctors so as to equalize the directiveforces across the compass position for all headings of the ship.The deviation method is more generally used because itutilizes the compass itself to indicateresults,rather than some additional instrumentformeasuring the intensityof magneticfields.321. Occasionally, the permanent magnetic effects at the location of the compass are so large that they overcome the earth'sdirectiveforce,H.This condition will not only create sluggish and unsteady sectors,butmayeven freeze the compass to onereading or to one quadrant, regardless of the heading of the ship.Should the compass be so frozen,the polarity of themagnetism which must be attractingthe compass needles is indicated,hence,correction may be effected simply by theapplicationof permanentmagnetcorrectors in suitablequantitytoneutralizethismagnetism.Wheneversuchadjustmentsaremade, it would bewell to havethe ship placed on a heading suchthatthe unfreezing of the compass needles will beimmediately evident.For example,a shipwhosecompass is frozen to a northreading would requirefore-and-aft B correctormagnetswiththeredendsforwardinordertoneutralizetheexistingbluepolethatattractedthecompass.Ifmadeonaneastheading, such an adjustment would be practically complete when the compass card was freed so as to indicate an eastheading.322.Listed belowareseveral reasonsforcorrectingtheerrorsofthemagneticcompass:(1)It is easier to usea magnetic compass if thedeviations are small.(2) Although a common belief is that it does not matter what the deviations are, as long as they areknown, this is inerror inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently makethe compass operationally unsatisfactory.This is the result of unequal directive forces on the compassas the shipswings inheading.(3)Furthermore, even though the deviations are known, if they are large they will be subject to appreciable change withheel andlatitudechangesoftheship.323. Subsequent chapters will deal with the methods of bringing a ship to the desired heading, and the methods of isolatingdeviation effects and ofminimizing interaction effects between correctors.Onceproperly adjusted,themagnetic compassdeviations shouldremain constantuntil there is some change in themagnetic condition of the vessel resultingfrom magnetictreatment, shockfromgunfire,vibration,repair,or structural changes.Frequently,themovementof nearbyguns,doors,gyrorepeaters,orcargoaffectsthecompassgreatly14
14 319. Compass operation. Figure 319 illustrates a point about compass operation. Not only is an uncorrected compass subject to large deviations, but there will be sectors in which the compass may sluggishly turn with the ship and other sectors in which the compass is too unsteady to use. These performances may be appreciated by visualizing a ship with deviations as shown in figure 319, as it swings from west through north toward east. Throughout this easterly swing the compass deviation is growing more easterly; and, whenever steering in this sector, the compass card sluggishly tries to follow the ship. Similarly, there is an unsteady sector from east through south to west. These sluggish and unsteady conditions are always characterized by the positive and negative slopes in a deviation curve. These conditions may also be associated with the maximum and minimum directive force acting on the compass (see article 305). It will be observed that the maximum deviation occurs at the point of average directive force and that the zero deviations occur at the points of maximum and minimum directive force. Figure 319 – Uncompensated deviation curve 320. Correction of compass errors is generally achieved by applying correctors so as to reduce the deviations of the compass for all headings of the ship. Correction could be achieved, however, by applying correctors so as to equalize the directive forces across the compass position for all headings of the ship. The deviation method is more generally used because it utilizes the compass itself to indicate results, rather than some additional instrument for measuring the intensity of magnetic fields. 321. 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 be 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 red ends forward in order to neutralize the existing blue pole that attracted the compass. If made on an east heading, such an adjustment would be practically complete when the compass card was freed so as to indicate an east heading. 322. Listed below 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) Although a common belief is that it does not matter what the deviations are, as long as they are known, this is in error inasmuch as conditions of sluggishness and unsteadiness accompany large deviations and consequently make the compass operationally unsatisfactory. This is the result of unequal directive forces on the compass as the ship swings in heading. (3) Furthermore, even though the deviations are known, if they are large they will be subject to appreciable change with heel and latitude changes of the ship. 323. Subsequent chapters will deal with the methods of bringing a ship to the desired heading, and the methods of isolating deviation effects and of minimizing interaction effects between correctors. 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