8 HANDBOOK OF MAGNETIC COMPASS 



it is removed from the magnetizing field. A bar having induced mag- 

 netism will lose its magnetism when removed from the magnetizing 

 field. Whether or not a bar will retain its magnetism on removal 

 from the magnetizing field will depend on the strength of that field, 

 the degree of hardness of the iron (retentivity), and also upon the 

 amount of physical stress applied to the bar while in the magnetizing 

 field. The harder the iron the more permanent will be the magnetism 

 acquired. 



7. Terrestrial magnetism. — The accepted theory of terrestrial 

 magnetism considers the earth as a huge magnet surrounded by lines 

 of magnetic force which connect its two magnetic poles. These mag- 

 netic poles are near, but not coincidental, with the geograpMc poles 

 of the earth. Since the north seeking end of a compass needle is con- 

 ventionally called a red pole, njorth pole, or positive pole, it must there- 

 fore be attracted to a pole of opposite polarity, or to a 'blue pole, south 

 pole, or negative pole. The magnetic pole near the north geographic 

 pole is therefore a blue pole, south pole, or negative pole ; and the mag- 

 netic pole near the south geographic pole is a red pole, north pole, or 

 positive pole. 



8. Figure 3 illustrates the earth and its surrounding magnetic field. 

 The flux lines enter the surface of the earth at different angles to the 

 horizontal, at different magnetic latitudes. This angle is called the 

 angle of magnetic dip, 6, and increases from zero, at the magnetic 

 equator, to 90° at the magnetic poles. The total magnetic field is gen- 

 erally considered as having two components, namely H, the horizontal 

 component, and Z, the vertical component. These components will 

 change when the angle 6 changes such that H is maximum at the mag- 

 netic equator and decreases in the direction of either pole, and Z is zero 

 at the magnetic equator and increases in the direction of either pole. 

 The values of H and Z may be found on U. S. Navy Hydrographic 

 Office charts H. 0. 1701 and H. 0. 1702 (figs. 4 and 5) . 



9. Inasmuch as the magnetic poles of the earth are not coincidental 

 with the geographic poles, it is evident that a compass needle in line 

 with the earth's magnetic field will not indicate true north, but m/ig- 

 netic north. The angular difference between the true meridian (great 

 circle connecting the geographic poles) and the magnetic meridian 

 (direction of the lines of magnetic flux) is called variation. This 

 variation has different values at different locations on the earth. 

 These variation values may be found on U. S. Navy Hydrographic 

 Office Chart H. O. 2406 (fig. 6) and, more exactly for each locality, 

 on the compass rose of navigational charts. The variation for most 

 given areas undergoes an annual change, the amount of which is also 

 noted on all charts. 



