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while being built was approximately north, the north end of the compass needle will be drawn to the east when the ship is heading on points between W and N, and its maximum easterly error, or deviation, will occur when the ship is heading exactly west, as in Fig. 21 (a); but when heading on points between E and N, the north end of the needle will be deflected to the west, with a maximum error when the ship is heading exactly east, as at Fig. 21 (6), since in both cases the blue polarity of the stern will attract the red polarity of the north end of the magnetic needle.
80. The foregoing appertains to compasses of ships built with their bows pointing to the north. For ships built with their bows pointing to the south, the conditions will be reversed; then a maximum easterly error, produced by the blue polarity of the bow, will occur when the ship is heading exactly east, and a maximum westerly error when heading west. In a similar manner, all compasses are affected, and the nature of the error, or deviation, produced will depend on the magnetic polarity of the ship, and, consequently, on the direction in which the ship's keel was when being built.
81. It should be remembered that no matter where the blue and red magnetic poles of the ship are situated (they are, as a rule, diametrically opposite each other) they will, when both are on the line of the magnetic meridian, produce no error whatever; but at any intermediate position they will produce an error that is maximum when the line connecting the two poles is perpendicular to the magnetic meridian.
This error, or deviation, is therefore called the semicircular error, or semicircular deviation, since its maximum value is attained in different semicircles; and the magnetism that produces it is generally known as permanent or subpermanent magnetism, because it remains constant or nearly so in all latitudes. It is the magnetism of the hard iron of the ship, imparted to it by the hammering received while building:
82. The amount of subpermanent magnetism appears to be influenced by the size of the vessel, or the quantity of iron used in its construction. Thus, in vessels of large tonnage, the semicircular error is comparatively greater than in vessels of a smaller size.
83. Let us now consider the magnetism of the soft iron of the ship. The soft iron entering into the make up of an iron vessel may be conveniently divided into two classes, viz., vertical and horizontal soft iron. To the former belong all iron running in a vertical direction, such as frames, stanchions, etc.; to the latter, all iron running horizontally, such as the keel, deck beams, etc. All vertical iron becomes magnetized by induction from the earth; this magretism, however, is only transient, changing with the latitude. Thus, for a ship north of the magnetic equator the
. upper ends of all vertical soft iron will have blue polarity and the lower ends red polarity, while south of the magnetic equator the same vertical iron will possess red polarity in their upper, and blue polarity in their lower, ends. The amount of this magnetism, therefore, varies in the same proportion as the vertical magnetic force of the earth; in other words, it changes as the magnetic dip, being minimum at the magnetic equator and maximum at the magnetic poles.
84. This magnetism will affect the compass in precisely the same way as the subpermanent magnetism of the ship; that is, its greatest effect will occur in two semicircles. Hence, the semicircular deviation is not the effect of subpermanent magnetism alone, but it is produced by the combined action of the subpermanent magnetism and the transient magnetism from the vertical soft iron of the ship.
85. The Quadrantal Deviation. -Having considered the magnetic influence of vertical soft iron, we will next turn our attention to that of horizontal soft iron. By horizontal soft iron is meant not only that which is in the immediate neighborhood of the compass, but all horizontal iron that constitutes a part of the ship's hull, above, below, and in the plane of the compass card. From Art. 77 we know that if a soft-iron bar is held in a horizontal north-and-south position, it will attain magnetism by induction from the earth, but will lose it when turned in an east-and-west position. The horizontal soft iron of a ship will receive and lose its magnetism in exactly the same way; hence, it will be readily understood that when a ship is heading north or south and east or west, or when on any of the cardinal points, there will be no error or deviation from this kind of magnetism; but when heading on any of the other points an error will be produced that is greatest when the ship is heading on any of the intercardinal or quadrantal points. For this reason the deviation produced by the transient magnetism of horizontal soft iron is called the quadrantal deviation.
86. Quadrantal deviation, which is constant in all latitudes, and which does not change with the lapse of time, is generally easterly in the N E and SW quadrants and westerly in the NW and S E quadrants.
The amount of this deviation is, as a rule, not very great (rarely exceeding 3° or 4°, except in armored ships), because when the ship is heading on any of the quadrantal points, the magnetic force of the fore-and-aft and athwartship horizontal iron will, to a certain extent, neutralize each other, and thus minimize its effect on the compass.
COMPENSATION OF COMPASSES
87. From the preceding remarks on the magnetic properties of iron ships, the student will readily understand that as soon as a compass is placed on board it is immediately subjected to serious disturbances from the magnetic forces in the iron surrounding it, and if left to their influence will be rendered totally useless. . By performing certain operations, however, the effect of these disturbances may be minimized or reduced, and under favorable (though very rare) circumstances removed altogether. This operation is called compensation, or compensating the compass.
88. The general principle of compensating compass is a to counteract the magnetic disturbances by means of magnets and soft iron placed in the immediate neighborhood of the compass, and in such positions as to cause a disturbance contrary to that caused by the iron of the ship, and thus to leave the magnetic needle comparatively free. This may be illustrated as follows: Bearing in mind that the north end of the compass needle always possesses
red polarity, assume
the needle to be deflected from the magnetic north n to n' by an iron mass d, Fig. 22. Then, in order to bring the needle back to its proper position, or, what is the same, to counteract the effect of the disturbing force d, two methods can be used, viz., either by placing near the north end of the needle, and in a suitable position, the blue pole of an artificial magnet, or by placing near the south end, in the position shown, the red pole of an artificial magnet. In either case the needle will be brought back to magnetic north.
This in brief, is the whole theory of compensating a compass, and the student will at once perceive that the whole proceeding is simply an application of the law of magnetic attraction and repulsion, which has already been described.
89. We will now proceed to give the student an insight into the methods of adjusting the different errors of the compass, well realizing the fact that, while the compensation of compasses, a rule, is made, and should be made, by professional compass adjusters, it is, nevertheless, essential that a navigator should have a knowledge of how compass compensation is performed, since occasions may arise where familiarity with this important subject will prove of great value to him.
METHODS OF COMPENSATION
90. Classification of Errors. The two principal errors to compensate are the semicircular deviation and the quadrantal deviation. The semicircular error, we know, is the combined effect of the subpermanent magnetism of the ship and the induced magnetism of vertical iron; but, as a whole, and in regard to compensation, it is convenient to divide this error into two parts and consider each of these parts as a separate force, one acting in a fore-and-aft and the other in an athwartship direction. The first part of that error, which affects the compass needle when heading on easterly and westerly courses, is usually denoted by the letter B; while the second part, which affects the needle when heading on northerly and southerly courses, is denoted by the letter C. The quadrantal deviation resulting from horizontal iron and affecting the compass when heading on any of the quadrantal points is denoted by D. When compensating a compass, the parts B and C are attended to first, the usual order of procedure in ships not equipped with a compensating binnacle being as follows:
91. Compensating the Semicircular Deviation. For the purpose of accuracy two chalk lines are drawn on deck, one in a fore-and-aft, the other in an athwartship, direction, so as to intersect, or cross each other, under the center of the compass to be compensated, as shown in Fig. 23. The ship is then swung with her head toward the magnetic north, according to some compass uninfluenced by the magnetism of the ship (for instance, by a compass placed on shore), or by permanent marks on land, the bearing between which coincides with the magnetic meridian. If the compass in this position does not show exactly north, that is, if the compass north and the lubber line do not coincide, a deviation must exist. Assume the north point to be deflected to the east, as in Fig. 23 (x). Then, to counteract the force causing this deviation an artificial magnet, or, as it is usually termed, a compensating bar magnet br, is placed athwartship,