1. Introductory.—These notes are not a complete treatise on the deviation of the compass but are an attempt to explain, by simple laws of magnets, how deviation is produced by the iron of a ship, and, by the same laws, how the deviation may be corrected. It is an explanation of principles involved, based upon the physical laws of the attraction and repulsion of magnets.
2. Natural Magnets or Lodestones.—The name magnet, or lodestone, was given by the ancients to certain hard black stones which possessed the property of attracting to them small pieces of iron or steel.
3. Artificial Magnets.—If a piece of iron, or better still a piece of hard steel, be rubbed with a lodestone, it will be found to have also acquired the properties characteristic of the magnet; it will attract light bits of iron, and, if hung up by a thread, it will point north and south.
4. First Laws of Magnets.—If two magnets be suspended as above and bought near each other, it will be found that the north seeking end of either magnet will repel the north seeking end of the other magnet; similarly, if the south seeking end of one magnet be brought near the south seeking end of the other magnet, they will repel each other; if, however, the north seeking end of one magnet be brought near the south seeking end of the other magnet, these ends will attract each other. The ends of the magnets are called its poles. This brings to notice the facts that the character of the magnetism of one pole of a magnet is different from that of the other pole, and the important law of magnets, that like poles repel each other, and, that unlike poles attract each other.
5. Polarity Represented by Colors.—It is convenient to represent the character of the end of a magnet, i. e., its polarity, by colors. In these notes, the character of the magnetism of the north seeking end of a suspended magnet is represented by red, and the other end by blue.
6. Permanent and Temporary Magnets.—Hard and Soft Iron.—With reference to its power to retain magnetism, iron is of two kinds, hard and soft. Hard iron when once magnetized remains so permanently. Such a magnet is said to be a permanent magnet. Soft iron is iron which possesses the characteristic of losing its magnetism upon the magnetizing source being removed, or discontinued. Soft iron has another magnetic characteristic to which reference will be made later.
7. Induced Magnetism.—If one pole of a magnet be brought near a mass of iron which is not already magnetic, it will induce, in this mass, magnetism, the character of which is of the opposite kind from the pole which is presented to the mass. For example, if the red end of a magnet be presented to a non-magnetic mass of iron it will induce magnetism in it, and the mass will itself become a magnet, with blue magnetism opposite the red pole presented to it, and red magnetism on the opposite side of the mass. This presents another important law of magnetism which is as follows: Induced magnetism is of the opposite polarity to the kind inducing it. See Figs. 1 and 1 (a).
8. Earth a Magnet —The earth is a large magnet the poles of which nearly coincide with the geographical poles. There is also similarly a magnetic equator, the belt of change from one magnetism to the other, or a belt of no magnetism. This belt exists in all magnets and is at right angles to the poles. The poles of the earth attract, or repel, suspended magnets. The magnetism of the earth has also the power to induce magnetism. The direction of the line of action of the earth's magnetism is in the plane of the great circle passing through the magnet poles, i. e., in the magnetic meridian.
9. Color of North Pole of Earth.—In accordance with the law of attraction existing between unlike poles, if we color the north seeking end of the magnetic needle red, the north pole of the earth possesses blue magnetism.
1o. Compass Needle a Magnet.—The compass needle is a small permanent magnet, or a collection of needles acting as one small permanent magnet.
11. Dip, Horizontal, and Vertical Components of the Earth's Force.—At the north magnetic pole of the earth, the north seeking end of the needle will point directly downward and the needle will be vertical. At the magnetic equator the needle will rest horizontally. At points of the earth's surface between the magnetic pole and equator, the needle points at some angle to the horizontal. The angle between the horizontal and the direction in which the needle points is called the dip.
This is shown in Fig. 2, where the magnetism of the earth is represented by the large magnet, the position of the small needle being shown at .different positions corresponding to different points on the earth's surface.
For a position such as (b) in Fig. 2, we have an analysis of the force of the earth as shown in Fig. 3.
The earth's total force, or its horizontal, or its vertical component, is each capable of inducing magnetism.
12. Deviation Defined.—Under the influence of the earth's magnetism acting alone, the needle of the compass is drawn to point directly to the magnetic poles of the earth, and lies in the plane of the magnetic meridian. Under the influence of other forces opposing the action of the force of the earth alone, such as the action of the iron of a ship, or an artificial magnet of either hard or soft iron, the compass needle is caused to deviate from the vertical plane of the magnetic meridian and to lie in some other plane inclined to the magnetic meridian. The angle between this plane and the plane of the magnetic meridian is called the "deviation of the compass." If the north point of the compass is drawn to the right of the magnetic meridian (facing the north magnetic Pole of the earth) the deviation is called "easterly" and is considered positive, if the north pole of the needle is drawn to the left of magnetic north, the deviation is named "westerly" and is considered as being negative. From the above definitions it Will be seen that (having a compass heading and knowing the deviation for that heading) easterly deviation must be applied to the right hand, and westerly deviation to the left hand, of the compass heading in order to obtain the reading of the correct magnetic heading.
13. Different Parts of Deviation.—For all practical purposes, the total deviation of the compass is composed of three parts; the semi-circular, quadrantal, and constant deviations.
14. Semi-circular Deviation.—The semi-circular deviation is so called because it is easterly in one semi-circle, as the ship's head swings in azimuth, and westerly in the other semi-circle. The points of change from easterly to westerly deviation, or points of no deviation, are opposite each other, and in iron and steel built ships generally occur on those headings upon which the ship rested in building.
Semi-circular deviation is fairly regular, reaching a maximum on points about 90° from the direction of the head of the ship in building. A ship built head north, for example, with reference to its semi-circular deviation, would have approximately 0° deviation on north, increasing to a maximum on east or west and decreasing to 0° on south. If the head were north in building, the deviation will, as a general rule, be westerly for all courses between north, east, and south, and easterly for all courses between north, west, and south.
Remembering the following, one is in a position to investigate the forces producing deviation:
(1) Permanent magnets always act with the same force.
(2) Soft iron, or temporary, magnets vary in the force exerted.
(3) Like poles repel, and unlike poles attract each other.
(4) Induced magnetism is of the opposite kind to the inducing magnetism.
(5) The hard iron in the ship becomes permanently magnetized while building.
(6) The soft iron in the ship has temporary magnetism induced by the earth's magnetism.
(7) We may investigate separately the effect of each kind upon the needle.
15. Forces Producing Semi-circular Deviation.—Permanent Magnetism of Ship.—Let the heavy lines in Fig. 5 represent a ship heading north while building, so that it has assumed the character of a permanent magnet, the poles of the ship being of the same kind of magnetism as the poles of the needle, as indicated by the coloring. As the action of the poles of the magnets and of the ship obey the same laws that like poles repel and unlike poles attract, the north or red end of the needle need only be considered, the action of the forces on the south poles being entirely in harmony with that of the north pole. In Fig. 5, heavy lines, the north pole of the ship repels that of the needle, but acting through the center of, and in line with the axis of the needle has no deflecting power, consequently produces no deviation, but it does oppose the attracting force of the earth, thus weakening the directive force of the needle. In Fig. 5, suppose the ship to have swung to a NE. heading. The north pole of the ship repels the N. end of the needle and now, not acting in the line of the axis of the needle, has a deflecting power, causing a deviation which increases until the repulsion due to the ship's pole and the attraction due to the earth's force bring the north end of the compass needle to rest in a line coinciding with the resultant of these two forces. This is indicated in the dotted position of the needle. The deviation produced in this case is x° W. on the course NE.
From the foregoing, it is seen that as the ship swings in azimuth from north through east to south, the deviation increases from o°, reaches a maximum near east, and finally becomes o° on south; the opposite effect obtaining in the other semi-circle. This portion of the semi-circular deviation is produced by action of the hard iron in the ship having acquired a permanent magnetic character while the ship was building. The remaining portion of the semi-circular deviation is produced by vertical soft iron as follows:
16. Portion of Semi-circular Due to Vertical Soft Iron.—In Fig. 7, let the direction of the total force of the earth be in the line T, 6 being the angle of dip. Resolve this into its components, the vertical one Z, and the horizontal one H. Let AB be a soft iron bar. The effect of the action of the vertical component of the earth's force will be to induce magnetism in this bar of opposite polarity to the inducing force as shown by the color and ends of the bar. The effect of such a bar on the compass needle is to produce a deviation which is o° when the bar is in the line of the axis of the needle, and a maximum when at right angles, or nearly so, to the axis of the needle; the deviation produced being westerly or easterly in the eastern semi-circle, depending upon whether the arrangement is that of Fig. 8 or Fig. 9. Study the figures, and note the effect of AB as it moves around the circle.
17. Changes with Change in Latitude.—From the foregoing, it is seen that 'vertical soft iron acts exactly in a like manner to the permanent magnet of Figs. 5 and 6, and produces a semi-circular deviation. As this portion of the semi-circular deviation is produced by induction due to the vertical force of the earth, and as the value of this vertical force depends upon the angle of dip, and as the dip changes with a change in latitude, it follows that this portion of the semi-circular deviation will change with a change in latitude.
Summarizing we may say:
Semi-circular deviation is produced by the horizontal component of the settled, permanent, magnetism of the ship, and by the earth's induction in vertical soft iron of the ship. The first portion of the semi-circular deviation is practically constant, the second portion changes with a change in latitude.
18. Quadrantal Deviation.—Quadantal deviation is a deviation which is easterly in one quadrant and westerly in the next quadrant. It is regular in character and is almost invariably easterly in the NE. and SW. quadrants, and westerly in SE. and NW. quadrants. It arises from induction in horizontal soft iron, as will be seen by the study of the following: Resuming the demonstration of Fig. 7, let T, Fig. 10, be the direction of the lines of action of the earth's magnetism and, as before, Z its vertical component and H its horizontal component. Let CD be a bar of soft iron, lying in the magnetic meridian, subject to the inductive action of the horizontal component H. Then CD, while lying parallel to H, i. e., in the magnetic meridian, will assume induced magnetism, as shown by its colored ends. Now refer to Fig. 11. Let NOS be the magnetic meridian or the line of action of the horizontal component of the earth's force, i. e., the H of Fig. 10, and let CD be the bar of Fig. 10. If this bar be swung around through 18o° either way, in the horizontal plane, that is, from the first to the third position, the magnetism of the ends C and D will be found to have changed places, that is, the red of the D end will be found to have changed to blue magnetism of opposite polarity. See position 3. There is evidently one position between 1 and 3, the point of just changing from one kind of magnetism to the other, at which the magnetism of the bar is neither red nor blue, that is, zero. This position is half way between the 1st and 3d positions, or at 2d and 4th; in other words, a bar held at right angles to the magnetic meridian, or to the line of action of a magnetic force, is not magnetized. For positions intermediate between that parallel to the meridian and that at right angles to it, the amount of magnetism induced in the bar is proportional to the cosine of the angle made by the bar with the meridian. Remembering now that unlike poles attract each other, that like poles repel each other, and that the nearest poles together are the most powerful in effect we are in a position to understand the action of CD when it is swung around a compass.
Referring to Fig. 12, the bar at 1, in the magnetic meridian, is most strongly magnetized but will produce no deviation because it acts through the axis of the needle and in this position has no leverage to pull or push the needle aside and cause deviation. On the contrary, the sketch shows that the magnetism of the bar will attract the needle to keep it in the meridian and will thus assist the earth's force. In such a condition the directive force of the needle is said to be increased.
At 2 the bar will not be magnetized so strongly, but will now act upon the pole of the needle with some leverage, the blue of the C end will attract the pole of the needle, draw it to the right and an easterly deviation be produced.
At 3 the bar is at right angles to the magnetic meridian and is not magnetized, hence while the leverage on the needle is greatest, there is no deviation because the bar possesses no magnetic force to act with this leverage. Hence, in passing from 1 to 3 (north to east) the horizontal soft iron bar, under the induction of the earth's magnetism, has produced an easterly deviation attaining a maximum and returning to 0°.
Passing from 3, the ends of the bar begin to take up their new character, and at 4 the red magnetism of the C end will attract the blue pole of the needle, pulling it towards it, thus throwing the north point of the needle to the west, producing westerly deviation. At 5, the bar will not produce deviation because of its magnetic force acting through the axis of the needle and thus having no leverage. It will be seen that in this position the directive force of the needle is increased. So, from 3 to 5, there has been produced a westerly deviation starting at 0°, attaining a maximum and returning to 00.
The same method of analysis shows an easterly deviation from 5 to 7, and a westerly deviation from 7 to 1. It may be easily seen that the maximum deviation occurs near 2-4-6 and 8. See now the definition of quadrantal deviation. See Fig. 12 (a) and note to Figs. 11-12.
19. Why Quadrantal does not Change with a Change in Latitude.— The force which produces quadrantal deviation is directly dependent upon the value of the earth's horizontal force and directly proportional to it. The force which acts on the needle to keep it in the magnetic meridian is the earth's horizontal force. Hence, as a change occurs in the value of H, the force tending to cause quadrantal deviation and the force tending to keep the. compass true, vary in exactly the same proportion. This bring, us to the important fact that the quadrantal deviation does not change in value with H and consequently does not change with a change in latitude. For a particular heading, of a particular ship. the quadrantal deviation is the same in all parts of the world.
20. Constant Deviation.—The constant deviation, as its name implies, is constant for all headings. For all compasses symmetrically situated with reference to the center line of a ship the constant deviation is imaginary rather than real and arises from instrumental errors, incorrect readings, misplaced lubber's line, etc. It is always small, and in most cases for compasses situated as above, it is nearly zero. Cases where there is a real value of the constant deviation may be found where compasses are not situated in the central fore and aft line but to one side of this line such as steering compasses at each side of a hand wheel aft.
Correction of the Deviation.
21. Preliminary.—If the compass needle is pushed or pulled aside from the correct position by certain magnetic forces, the effect of these forces may be overcome by applying other magnetic forces which push or pull in opposition, and thus bring the needle back to its place as fast as those forces which produce deviation tend to push or pull it out of place. Or, to better express it, if the magnetic forces producing deviation are opposed by equal but opposite magnetic forces, the forces tending to produce deviation are neutralized, and the needle swings in its correct position under the influence of the earth alone.
As has been shown in the preceding pages, the forces causing deviation are those of the magnetism existing permanently, or temporarily in the ship's iron or steel. The neutralizing forces are produced by special magnets, permanent and temporary, placed in positions from which they oppose the forces of the ship and thus leave the compass needle free to point correctly.
Generally speaking, the permanent magnetism of the ship is opposed, or neutralized, by artificial permanent magnets, and the induced, temporary, or transient magnetism, is neutralized by soft iron correctors, so placed that the induction of the earth's magnetism in these correctors is opposite in kind or effect upon the compass from that of the soft iron of the ship itself.
22. Elements of Correction Considered Separately.—Just as the effect of each kind of iron on the compass was considered separately, so the correction of each element of the deviation may be separately considered.
23. Semi-circular.—Remembering that the semi-circular deviation is composed of two parts, the correction of that part produced by the permanent magnetism of the hard iron in the ship is made by permanent magnets, and that part produced by soft iron is corrected by soft iron correctors.
24. Correction of Portion of Semi-circular Due to Permanent Magnetism.—As introductory to this correction, it may be well to consider the expressions "the poles of the ship" and the poles of the permanent magnetism of the ship."
In Fig. 13 let the coloring represent the magnetic character of a particular ship. All the red magnetism will act together, as a whole, at one point, its resultant or center of magnetism, and this point we will call the pole of the red magnetism. The blue will act similarly as if concentrated at one point, and this point will be the blue pole of the ship. These two points are called the "poles of the ship." For a ship built under the condition shown in Fig. 13, the red pole may be indicated as at P, and the blue pole as at P'. In Fig. 13, the magnetism indicated is that of the entire set of forces of the ship, i. e., of the permanent and transient magnetism combined. We may, however, separate these and investigate either, so that for purposes of illustration Fig. 13 may be used to represent only the permanent magnetism of the ship while we study that portion. In this case, P and P' would represent the "poles of the permanent magnetism of the ship." In Figs. 14 to 19 are indicated the poles of the permanent magnetism of ships built with their heads in the following directions: North, northeast, east, southeast, south, and northwest.
Suppose it is desired to correct that portion of the deviation arising from the permanent magnetism in a ship built, for example, with her head northwest. Fig. 19 indicates the location of the poles of the element of the magnetism it is desired to correct, and it is easily seen that they may be neutralized by one artificial magnet placed as in Fig. 20, or by two such magnets placed as in Fig. 21.
In Fig. 20, known as the starboard angle method, one magnet corrects the entire force PP', while in Fig. 21, PP' is considered as resolved into its components in the fore and aft, and the 'thwartship lines, and each artificial magnet corrects one of these components. This is known as the rectangular method.
Had the ship's head been northeast in building (Fig. 15), the correction would have been as indicated in Fig. 22.
The magnets used for correction of this portion of the semicircular deviation may be solid, round, or flat, bar magnets, or bundles of small magnets bound together to act as one magnet. In some cases they are placed in specially constructed holders, or trays, in the binnacles; in other cases, they are tacked down to the deck, or secured overhead near the compass. In each and every case, the principles explained in the foregoing are strictly followed, and this element of the magnetic forces of the ship corrected.
25. Correction of that Portion of the Semi-circular Deviation Due to Vertical Soft Iron.—Referring to Fig. 8, it is evident that the effect of AB would be neutralized by placing a similar soft iron bar, as at CD, Fig. 23. And that the bar AB of Fig. 9 may be corrected by a soft iron bar placed at CD, as in Fig. 24.
The results obtained by the arrangement of Fig. 24 may be made a little clearer by looking at the needle and bars in a horizontal plane through the needle, as in Fig. 25. Here the blue dots represent the end of AB on one side, and CD on the opposite side, of the diameter of the circle. Suppose, in Fig. 25, that one position of AB and DC be chosen. It is easily seen that as AB, in moving around the circle attracts or repels the needles, CD acts equally in the opposite manner.
Such a bar as CD, of Figs. 23-25, is called a "Flinders Bar."
AB, in Fig. 24, could be neutralized by putting CD directly over it end to end, but it is to be remembered that AB is an imaginary bar representing the magnetism of the ship's vertical soft iron, while CD is an actual bar used as a corrector, and is actually placed by the compass. To put it over AB would bring it in an awkward place above the compass card, and cause considerable inconvenience in taking bearings, fitting the binnacle cover, etc.
The resultant of the forces which the Flinders bar is used to correct is generally in the center line of the ship, so that nearly all compasses using this bar have it placed in the central fore and aft line of the ship, either forward or abaft the compass; forward, to correct a condition represented by Fig. 8; abaft, to correct the condition represented by Fig. 9. See, again, Figs. 23 and 24.
In many ships this bar is not used, the entire semi-circular deviation being corrected by the permanent magnets of Figs. 20-21. It is easily seen that such a correction is perfect for one latitude only, that in which the correction was made, and that the deviation will change with a change in latitude, because the entire semicircular corrected by the one set of magnets is, as before stated, composed of two elements, one of which changes with a change of latitude and another which does not change with the latitude.
26. Correction of the Quadrantal Deviation.—The quadrantal deviation may be corrected by any arrangement of soft iron which, under the influence of the earth's magnetism, produces a result opposite to that produced by the bar CD of Figs. 11 and 12. This correction is generally made by the use of soft iron spheres, and the action of these spheres will be investigated before proceeding farther.
Let the bar CD of Fig. 11 be replaced by a sphere CD, as in Fig. 26, and let this sphere be revolved through 360°, taking the successive positions I, 2, 3, etc., to 8, Fig. 27.
As the diameter CD swings around, the character of the magnetism at the C or D will change, and at 5, Fig. 27, will have become reversed just as was that of the bar CD of Fig. 11. The shape of the sphere is, however, such that no matter in which direction the diameter CD, Fig. 27, may be, the effect is that of a ball of soft iron, always maintaining the characteristic of having its northern half of red magnetism and its southern half of blue magnetism. In other words, the northern and southern hemispheres are always of the same magnetic character, with their poles north and south, regardless of how the sphere is revolved. The effect is the same as moving around in a circle, parallel to itself, a magnet of the character shown in Fig. 27.
This magnet is produced by the induction of the earth's magnetism, and, if properly placed, its force may be used to oppose the effect upon the compass of other induced magnetism of the soft iron of the ship, i. e., it may be used to counter the effect of the bar CD of Fig. 11.
For this purpose, two spheres, one on each side of the compass needle, as shown in Fig. 28, are used. Their action may be illustrated as follows:
Take the position 2 of Fig. 12, in which the ship is heading northeast, and place the spheres in position as they would ordinarily be mounted onboard ship. See Fig. 29. As in Fig. 12, the result of CD acting alone would be to pull the needle to the position e, thus producing an easterly deviation. The result of the spheres acting by themselves would be to pull the needle to the position w, and produce a westerly deviation. The action of the spheres are thus opposed to the action of the bar CD, and if the spheres are large enough and placed close enough they may be made to entirely correct the action upon the needle of the bar CD or its equivalent
So far, this explanation has applied to the particular case represented in Fig. 29, but a clear conception of the principles involved, applied to the study of Fig. 30, will show that the spheres oppose, neutralize, or correct CD in all positions of the ship's head.
The spheres to correct quadrantal deviation, such as would be produced by conditions represented by CD of Fig. 12, the conditions ordinarily existing onboard ship, are placed one to starboard and one to port on the 'thwartship line, passing through the center of the compass needle.
If conditions represented by Fig. 12 (a) existed, the spheres would be placed on the fore and aft center line of the ship, one forward and one abaft the binnacle. Such conditions are not usually found onboard ships of present construction, so that the quadrantal correctors are usually found one to starboard, and one to port of the compass, and their effect on the compass needle, varied by moving them toward or away from the compass.
In compasses which have large and powerful needles, these needles, when close to the spheres, will induce magnetism in the spheres at the same time and in the same manner as the earth's force. In this case, the quadrantal correcting force is the resultant of the permanent force due to the induction of the needles in the spheres and the variable force due to the induction of the earth in the spheres. The resultant of these two forces is a variable force and when a given quadrantal deviation is corrected in one latitude by this force, the balance will be disturbed and the correction fail to hold good upon going to another latitude. If, however, the needles are so small, and so far from the spheres, as to not appreciably affect them, the quadrantal deviation once corrected should remain corrected, practically, for all latitudes.
27. Heeling Error.—If a ship whose compass has been corrected be heeled to one side, and while so heeled be swung in azimuth, it will be found that the equilibrium of compensation (correction) has been disturbed, and that there now exists on some headings deviations which did not exist when the ship was upright. Furthermore, it will be found that if the ship be brought to an upright position and again swung in azimuth these deviations will have disappeared. The deviation brought into existence during heeling is called the heeling deviation, or heeling error.
In all of the preceding discussion of the causes of deviation, the ship has been considered as in an upright position and the discussion has proceeded on the following assumptions: (1) That such bars as CD of Figs. 11, 12 and 12 (a) have been in a horizontal plane; (2) that the effect of vertical induction in vertical soft iron, such as AB, Figs. 8 and 9, has acted at points other than directly under the center of the compass needle; and (3) that the permanent magnetism of the ship has acted only in a horizontal plane.
As a matter of fact, that portion of the permanent magnetism which has been considered as acting only in the horizontal plane, has been merely the horizontal component of the permanent magnetism of the ship, and there is yet to be considered that vertical component of the ship's permanent magnetism which acts directly under the center of the compass.
When the ship is upright, this force has no effect in producing deviation. When the ship heels, this force no longer acts vertically beneath the center of the compass but to one side of the compass needle, and consequently in certain positions will produce deviations as long as the ship remains heeled.
This may be illustrated as follows: In Figs. 31 and 31 (a), let EF represent a vertical component of permanent magnetism acting directly under the center of the compass, the ship being in an upright position heading north. In this position EF is ineffective in producing deviation.
Now let the ship be heeled to port. The compass bowl will move from K, to K, and EF will move to E'F'. The situation is projected horizontally in Fig. 32. It is evident from an inspection of Fig. 32, that EF, now in the position E'F', is at right angles to the axis of the compass needle and exerts a pull on it producing, as shown, a deviation of the needle to the high, or windward, side of the ship.
An inspection of Fig. 33 shows a similar result when the ship is heeled to starboard, the deviation of the needle again being toward the high side of the ship, although now of an opposite sign from that in Fig. 32.
In heeling from one side to the other the deviation has changed from easterly to westerly, and if the swing in azimuth, of the needle, synchronizes with the rolling, i. e., heeling of a ship, violent oscillations of the compass card may be produced.
This is one test of the presence of a heeling error, or force producing heeling deviation.
Referring again to Figs. 32 and 33, note that with the ship heading north EF, when the ship is heeled, swings to the side of the needle at right angles to its axis, and is capable of producing on north or south courses its maximum effect.
Referring now to Fig. 34, let the ship be headed east on an even keel, and from this position be heeled first to port and then to starboard, as in Figs. 35 and 36.
An inspection of these figures shows that while heeled EF may increase or decrease the directive force of the needle, but it acts in the same plane as the axis of the needle and through its center, thus possessing no power to deflect it horizontally. On easterly and westerly courses, EF produces no deviation.
This element then produces a deviation semi-circular in character, being a maximum on north and south, and zero on east and west.
28. Vertical Induction in Vertical Soft Iron.—If we endow EF with the characteristics of soft iron instead of hard iron, Figs. 31 to 36, also illustrate the effect (in north latitude) of vertical induction in vertical soft iron immediately below the compass.
The two forces just considered are, at any one geographical position, so mixed as to make it impossible to separate them.
With reference to the coloring of EF, Fig. 31, as permanent magnetism, the upper end may be blue or red, depending upon where the ship was built, how she was heading when built, and in which part of the ship the compass is located.
With reference to its coloring as representing vertical induction in vertical soft iron, the upper end is blue in north magnetic latitudes and red in south magnetic latitudes.
29. Vertical Induction in Transverse Soft Iron.—The two forces just considered combine with a third force to produce what is known as the principal heeling error. This third force arises from vertical induction in transverse iron as, for example, the deck beams.
Suppose Fig. 37 to be an end view of a ship heading north (in north magnetic latitude) and CD a soft iron deck beam in a horizontal plane immediately below the compass. This beam is, with the ship heading north, magnetic, at right angles to the magnetic meridian, and is not magnetized (see Figs. ii and 12a), and consequently produces no effect on the compass needle which is shown end on at L.
Let the ship be heeled to port, as in Fig. 38. This beam now ceases to be horizontal, becomes magnetized, and has a vertical component DZ, which is the result of induction in CD by the vertical component of the earth's force. This force now acts at one side of the compass needle, at right angles to its axis, and produces a deviation.
Consider, again, this ship, in Fig. 40, as being upright and heading east magnetic. CD now lies in the magnetic meridian, is parallel to the compass needle, and is in the same vertical plane with it. It will be magnetized both in an upright position and while heeled, but lying in the same vertical plane will dot tend to move the needle out of that plane, hence produces no deviation. Figs. 39 and 42 represent beams not continuous, i. e., beams in wake of hatches. This force thus produces a deviation which is zero on east and west and a maximum on north and south.
30. Principal Heeling Error.—The combined action of the three forces just discussed causes the principal heeling error.
It is to be noted that they are all a maximum on north and south, are zero on east and west, and are all due to vertical forces. Being due to vertical forces, they are corrected by a vertical magnet placed directly under the center of the compass needle while the ship is upright. They are usually, in a practical sense, considered as one force and corrected by one permanent magnet, so arranged that it may be raised, lowered, or its ends reversed to meet the necessities arising in going from one magnetic hemisphere to the other.