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58. Artificial magnets are bars of steel or iron to which magnetism has been communicated or imparted by artificial means. Thus, if a bar of slightly tempered steel is held in a vertical position and struck several blows with, for instance, a wooden mallet, it will acquire the property of attracting iron filings at its two extremities and, hence, will have become an artificial magnet.
Magnetism may also be imparted from one magnetized bar of steel to a large number of similar non-magnetic bars by rubbing one-half of the length of the latter with the former; the magnetism thus imparted to the bars is then called induced magnetism, or magnetism by induction.
59. The property of attraction is not manifested equally at all points of the surface of a magnet. For instance, an ordinary bar magnet when plunged into iron filings does not
become enveloped uniformly by the filings, but, instead, they arrange themselves around the ends of the bar in feathery tufts that gradually grow smaller as the middle of the bar is approached, leaving that portion bare, as shown in Fig. 16.
The points p and p around which the filings apparently concentrate are called the poles; the central portion of the bar where there is no visible magnetic force and to which the filings refuse to adhere is called the neutral zone, and the line p p connecting the two poles the axis of magnetism.
60. Polarity.- A magnetized needle when resting upon a fine point so as to turn freely in a horizontal plane will, when not in the
vicinity of other magnets, or magnetized iron, FIG. 16
always come to rest with one end pointing toward the north and the other toward the south. The end pointing northward is therefore called the north-seeking pole and the opposite end the south-seeking pole. This tendency, called polarity, applies to all magnets.
61. Law of Magnetic Attraction and Repulsion. When two magnetized bars or needles are brought close together, two poles attract and two poles repel each other. Thus the north-seeking end, or pole, of one magnetic needle will repel the north-seeking end of the other needle, while it will attract the south-seeking end. From this the following law for magnetic attraction and repulsion may be enunciated: Poles of contrary names attract each other, while poles of the same name repel each other.
62. Magnetic Property of the Earth. - As previously stated, if a magnetized needle be suspended in such a manner as to have a perfect freedom of motion, it will assume a north-and-south direction. This fact of a suspended needle taking up a fixed position has led to the theory that the earth itself is a huge magnet having its north and south magnetic poles in the neighborhood of the geographical poles, and that the magnetic needle turns to these poles as to the poles of an original magnet, according to the law just given.
63. Considering the earth as a magnet having one pole in the northern, and the other in the southern, hemisphere, and remembering that poles which attract each other are of contrary names, it follows that the magnetic pole of the earth, which is situated in the northern hemisphere, is of contrary name to that end of the magnetized needle which points to the north; in other words, the north-seeking end of the needle is in reality its south pole. From this a great deal of confusion has arisen, but popular usage, however, calls the north-seeking end of the needle the north pole and that which points to the south the south pole, and makers of magnets usually mark the north-seeking pole with the letter N. In many works on magnetism the northseeking pole is referred to as the red pole, while the other, or the south-seeking pole, is referred to as the blue pole.
64. The Magnetic Meridian. – The direction that the horizontally balanced magnetic needle assumes when uninfluenced by local attraction is called the magnetic meridian. In general, the magnetic meridian does not coincide with the geographical meridian, and there are but few places on the earth at which the compass needle points to the true north.
VARIATION 65. The angle that the magnetic meridian makes with the geographical meridian or, what is the same, the angle that the direction of the suspended needle makes with the true meridian is called the variation or declination of
the magnetic needle, or simply the variation of the compass. Thus, in Fig. 17, if NS represents the direction of one
of the geographical e meridians, and n the
north point of the magnetic needle, the angle Non is the variation of the compass.
66. Ilow Variation Named.
Variation is named westerly if the north-seeking end of the magnetic needle is deflected to the west of the true or geographical meridian; when the north-seeking end is deflected to the east of the true meridian, the variation is named easterly.
67. The variation of the compass is different at different places on the earth's surface, and at a given place it undergoes a gradual change from year to year. At such places where the magnetic meridian coincides with the true meridian the variation is said to be zero, and lines connecting these places are called lines of no variation, or agonic lines.
In Fig. 18 the lines of "no variation" are represented by heavy thick lines, westerly variation by continuous fine lines, and easterly variation by dotted lines. A glance at this chart tells us that the variation is westerly nearly all over the North Atlantic Ocean, increasing in amount toward the
north, and that the variation is easterly in the greater part of the Pacific Ocean. Lines connecting points of equal declination are called isogonic lines.
68. Magnetic variation undergoes a progressive change in amount, and, after long periods, changes of direction, or secular changes, take place; in other words, the variation vibrates between certain limits. The line of “no variation” that now crosses the American continent, making its entry at the eastern part of Lake Superior, passed through London, England, in 1657, and through Paris, France, in 1669. At the present time it is moving toward the west at the rate of about 1° in 14 years. These changes in the variation are important to the navigator and especial attention should be given to charts used by him that they contain the latest correction in regard to this matter.
69. In addition to the progressive and secular changes just mentioned, there is a diurnal, or daily, variation, the amount of which, however, is too small to be of any importance to the mariner.
70. Magnetic Variation the Great Lakes. The amount of variation is usually recorded on charts. Thus, on the Pilot chart of the Great Lakes, the amount of variation is indicated by fine dotted curves, the space between each curve representing a change of 1° east or west. On the variation chart of the Great Lakes, Plate I, westerly variation is indicated by full lines and easterly variation by dotted lines. By examining this chart we find that near Buffalo, N. Y., the variation is nearly 5° westerly; at North Point, Lake Huron, it is 3° west; at Sault Sainte Marie it is 2° west; the line of “no variation” is found a few miles east of Grand Marais Harbor, and from there the variation is changed to easterly, being 2° east between Stannard Rock and Manitou Island, increasing in amount toward the western part of Lake Superior, until at Duluth, Minn., it is nearly 8° east. From this it is evident that the amount of change in variation between Buffalo and Duluth is 135°, nearly 14 point.
71. Another deflection of the compass needle, known as the inclination, or magnetic dip, was discovered by an English instrument maker in 1576. He found that a needle, however well balanced, would, after being magnetized, depart from the horizontal position and with its north end point downwards. Further investigations in different parts of the world disclosed the fact that in the southern hemisphere the south end of the needle has a similar tendency to be deflected downwards, but at places situated within the equatorial region this inclination is zero, or the needle assumes a horizontal position. By connecting all places where the magnetic dip is zero, a line called the magnetic equator is established.
72. The magnetic equator does not coincide with the geographical equator, but crosses it at several places, never