« ΠροηγούμενηΣυνέχεια »
with its center on the fore-and-aft chalk line and with its red pole to starboard, as shown in figure.
The distance of the magnet must be determined by trial; begin by placing the magnet at some distance from the compass and gradually approach it until the compass shows correct magnetic north, when the magnet is secured to the deck. If the needle had been deflected to the west, it is evident that the red end, or pole, of the magnet should have been placed to the port side. In case this error is large, the ship is swung around until its bow is heading correct magnetic south, and if the compass then does not indicate south, a second magnet is placed with its center on the chalk line
abaft the compass, and with its blue pole to the side to which the south point of needle is deflected. The part of the semicircular error denoted by C is thus corrected.
92. The ship is next brought in an east-and-west direction with its head, or bow, toward the magnetic east, as in Fig. 23 (y). If the compass north is deflected to the east on this heading, as in the figure, a magnet b' r' is placed with its center on the athwartship line, with its red pole forwards and at a distance from the compass sufficient to insure the east point of the compass to show correct magnetic east. The needle being deflected to the west, the compensating magnet is reversed. A similar operation is then performed, if necessary, with the ship's head pointing toward the magnetic west. That portion of the semicircular error denoted by B is thus corrected.
93. When the compensating magnets are fastened to the deck it is necessary that their centers be exactly on the chalk lines and perpendicular to their respective lines. It matters not where the magnets are placed as long as this principle is observed. Care should also be taken that the magnets are not reversed when replaced after having been taken up for some purpose. Compensating magnets should be made of good steel, tempered and well magnetized; deck magnets, which may be from 12 to 24 inches in length and about
in. by 1. in. cross-section, should be covered with tallow and protected by a thin sheeting of metal. Before being used, their magnetism should be some months old, in order to possess permanent magnetic condition.
94. Compensating Binnacles.-At the present day, and particularly in iron ships, magnets for compensating the semicircular deviation are seldom, if ever, fastened to the deck, but are, instead, fitted to slide into horizontal fore-andaft and athwartship holes within the binnacle (see m, Fig. 12). In some binnacles the magnets are arranged in such a manner as to be moved up and down, nearer to and farther from the compass, as may be required, and then fastened by means of clamp screws that cannot be touched except by opening the door of the binnacle; in others, the movement of the magnets is controlled from the outside of the binnacle by means of a crank-key, thus enabling the adjuster to watch the compass while he is altering the position of the magnets and to move them the exact amount required; after adjusting, the crank-key is removed and the casing locked, making it impossible for any one to tamper with the magnets. Such binnacles are called compensating binnacles.
The principle of storing the magnets within the binnacle is precisely the same as when the magnets are secured to the deck, both the magnets for B and C being exactly parallel to the ship's deck or to the plane of the compass card when the ship is in an upright position.
95. Compensating the Quadrantal Deviation. After the semicircular deviation has been corrected or reduced to a minimum value, the quadrantal error is attended to. Since this error,
Magnetic which is caused by the magnetism of horizontal soft iron, is greatest on the quadrantal points, the ship is accordingly swung in the direction of one of these points; for example, NE (magnetic), as shown in Fig. 4, and since the error is caused by soft iron it is necessary to compensate it by using soft-iron correctors. For this reason magnets are not used to compensate the quadrantal deviation; instead, softiron spheres are used. These spheres are so placed in the plane of the compass card as to cause an opposite effect to the magnetism of the horizontal iron. The error to be corrected being easterly in the NE and S W quadrants and westerly in the N W and S E quadrants in almost every ship, the spheres, or globes, are placed athwartship on the same horizontal plane and at equal distances from the center of the compass. These soft-iron correctors c and d are shown in Fig. 26 and also in diagram in Fig. 24.
The distance at which the correctors are set is determined by trial, moving them from and to in their respective slits until the compass shows correctly the quadrantal point in which the ship is heading. The distance should, however, not be less than 1. times the diameter of the card.
96. According to a previous article, the quadrantal deviation is constant in all latitudes, hence its compensation remains constant everywhere. Such, however, is not the case with that part of the semicircular error caused by the induced magnetism of vertical iron. Since this magnetism depends on the magnetic dip, it is evident that the deviation resulting from it will depend on the magnetic dip also. To distinguish this latter error from that produced by subpermanent magnetism and to apply to it a proper compensation is a difficult task, requiring skill, good judgment, and an intimate knowledge of the magnetic condition of the ship. The usual method of correcting or compensating this error is by means of a vertical iron bar, called the Flinders bar, which is placed within the binnacle either immediately before or abaft the compass.
97. The Flinders bar, which received its name from its inventor, Captain Flinders, of the British Navy, is not a permanent magnet; it is made of soft iron and consequently receives its magnetism by induction from the earth. Thus, in the northern hemisphere the Flinders bar has blue polarity in its upper end and red in its lower; at the magnetic equator it loses its magnetism. But when brought into the southern hemisphere it again acquires magnetism, its upper end then possessing red polarity and the lower end blue polarity. The object, therefore, to be attained by the Flinders bar is to place it in such a position within the binnacle that the gradual change of its magnetism, produced by the change in latitude, will counterbalance the effect of the likewise varying magnetism of the vertical iron of the ship.
98. According to experiments recently made by the Superintendent of Compasses, and published in the report of the Chief of the Bureau of Equipments, Navy Department, Washington (1900), it is more convenient and furnishes better results to correct the quadrantal deviation before compensating for B and C of the semicircular deviation; it saves time and labor, since, if B and C are compensated before D, allowance has to be made for an approximate or accurate D, which always causes much unnecessary shiftings of magnets.
99. The Heeling Error. – The deviation hitherto considered has been for vessels in an upright position. But when, from some cause, the ship has a list to either side, a new error is created, which is generally known as the heeling
The principal cause of this error is illustrated in Fig. 25. When the ship heels over from the pressure of wind, shifting of cargo, or unequal trimming of bunker coals, all horizontal iron, such as the deck beams m n, tends to assume
a vertical position, and in doing so will receive magnetism by induction from the earth. Thus, for a ship in the northern hemisphere, the upper end of the beams, whether heeling to port or starboard, will acquire blue polarity and the lower ends red polarity, as shown in figure. In the southern hemisphere these conditions are reversed. As a consequence, the north end of the compass needle will be attracted by the upper ends of the beams in north magnetic latitudes and repelled in south magnetic latitudes, and the amount of this error will evidently depend on the amount of heeling.
100. As a general rule, the heeling error is greatest on northerly and southerly courses, and least on easterly and westerly courses. The compensation of the heeling error is usually accomplished by a magnet (see Fig. 25 and a b Fig. 26) that is placed at a suitable distance vertically below the center of the compass bowl, the distance being determined