(3) The Earth's Magnetism
209. The Earth's Magnetic Field.—Dr. William Gilbert's famous book, De Magnete, contains many helpful and suggestive ideas, none perhaps more important than his explanation of the behavior of the compass needle. He assumed that the earth is a magnet, with a south-seeking pole near the geographical north pole, and with a north-seeking pole near the geographical south pole. This idea has since been shown to be correct. The north magnetic (or south-seeking) pole was found in 1831, by Sir James Ross in Boothia Felix, Canada. Its approximate present location as determined by Captain Amundsen in 1905 is latitude 70° 5´ N. and longitude 96° 46´ W. The south magnetic pole is in latitude 72° S., longitude 155° 16´ E. The north magnetic pole is continually changing its position. At present it is moving slowly westward.
Fig. 184.—Magnetic map of the earth for 1910. Isogonic lines ——— Isoclinic lines - - - -
210. Direction of the Earth's Magnetic Field.—Reference has been made to the fact that the compass does not always point exactly north. This indicates that the earth's magnetic field varies in its direction. Columbus discovered this fact upon his first voyage. The discovery alarmed the sailors since they feared they might come to a place where the compass would be unreliable. This variation is called declination. It is defined as the angle between the direction of the needle and the geographical meridian. Declination is due to the fact that the geographical and magnetic poles do not coincide. What is meant by a declination of 90°? Lines drawn upon a map so as to pass through places of the same declination are called isogonic lines. The line passing through points where the needle points north, without declination, is the agonic line. The agonic line is slowly moving westward. It now passes near Lansing, Michigan; Cincinnati, Ohio; and Charleston, S. Carolina. (See Fig. 184.) At all points in the United States and Canada east of the agonic line the declination is west, at points west of the agonic line the declination is east.
211. The Dipping Needle.—Mount an unmagnetized steel needle on a horizontal axis so as to be in neutral equilibrium, that is, so as to remain balanced in any position in which it is left. Upon being magnetized and placed so that it can swing in a north and south plane, the north-seeking pole will now be found to be depressed, the needle forming an angle of nearly 70° with the horizontal. (See Fig. 185.) The position assumed by the needle indicates that the earth's magnetic field instead of being horizontal in the United States dips down at an angle of about 70°. Over the magnetic pole, the dipping needle as it is called, is vertical. At the earth's equator it is nearly horizontal. The angle between a horizontal plane and the earth's magnetic lines of force is called the inclination or dip.
Fig. 185.—A dipping needle.
212. Inductive Effect of the Earth's Magnetic Field.—The earth's magnetic lines of force are to be considered as filling the space above the earth, passing through all objects on the surface and into and through the earth's interior. The direction of the earth's field is shown by the compass and the dipping needle. Magnetic lines of force tend to crowd into and follow iron and steel objects on account of their permeability. Therefore, iron or steel objects, such as posts, columns, etc., are permeated by the earth's lines of force, which in the United States enter at the top of these objects and leave at the bottom. The lines of force passing through these bodies arrange their molecules in line or magnetize the bodies. The inductive effect of the earth's magnetism indicates how lodestones or natural magnets acquire their magnetized condition. So far as is known, magnetism produces no effect upon the human body. It can therefore be studied only by observing its effects upon magnets or bodies affected by it.