12 HANDBOOK OF MAGNETIC COMPASS 



already present in the ship, depending on how the ship is aligned in 

 the magnetic field. The softer the iron the more readily it will be 

 induced by the earth's magnetic field, and the more readily it will give 

 up its magnetism when removed from that field. 



12. The magnetism in the various structures of a ship which tends 

 to change as a result of cruising, vibration, or aging, but does not 

 alter immediately so as to be properly termed induced magnetism, 

 is called subpermanent Tfiagnetism. This magnetism at any instant 

 is recognized as part of the ship's permanent magnetism, and conse- 

 quently must be corrected as such by means of permanent magnet cor- 

 rectors. This subpermanent magnetism is the principal cause of de- 

 viation changes on a magnetic compass. Subsequent reference to per- 

 manent magnetism in this text will refer to the apparent permanent 

 magnetism which includes the existing permanent and subpermanent 

 magnetism at any given instant. 



13. A ship, then, has a combination of permanent^ subpermanent, 

 and induced magnetism, since its metal structures are of varying de- 

 grees of hardness. Thus, the apparent permanent magnetic condi- 

 tion of the ship is subject to change from deperming, excessive shocks, 

 welding, vibration, etc. ; and the induced magnetism of the ship will 

 vary with the strength of the earth's magnetic field at different mag- 

 netic latitudes, and with the alignment of the rhip in that field. 



14. Resultant induced magnetism from earth's magnetic field. — 

 The above discussion of induced magnetism and terrestrial magnetism 

 leads to the following facts. A long thin rod of soft iron in a plane 

 parallel to the earth's horizontal magnetic field, H, will have a red 

 (north) pole induced in the end toward the north geographic pole and 

 a blue (south) pole induced in the end toward the south geographic 

 pole. This same rod in a horizontal plane but at right angles to the 

 horizontal earth's field would have no magnetism induced in it, be- 

 cause its alignment in the magnetic field is such that there will be no 

 tendency toward linear magnetization and the rod is of negligible 

 cross section. Should the rod be aligned in some horizontal direction 

 between those headings which create maximum and zero induction, it 

 would be induced by an amount which is a function of the angle of 

 alignment. If a similar rod is placed in a vertical position in northern 

 latitudes so as to be aligned with the vertical earth's field, Z, it will 

 have a blue (south) pole induced at the upper end and a red (north) 

 pole induced at the lower end. These polarities of vertical induced 

 magnetization will be reversed in southern latitudes. 



The amount of horizontal or vertical induction in such rods, or in 

 ships whose construction is equivalent to combinations of such rods, 

 will vary with the intensity of H and Z, heading, and heel of the ship. 



