26 



HANDBOOK OF MAGNETIC COMPASS 



tion for all headings of the ship. The deviation method is more gen- 

 erally used because it utilizes the compass itself to indicate results, 

 rather than some additional instrument for measuring the intensity 

 of magnetic fields. 



35. Occasionally, the permanent magnetic effects at the location of 

 the compass are so large that they overcome the earth's directive force, 

 H, This condition will not only create sluggish and unsteady sectors, 

 but may even freeze the compass to one reading or to one quadrant, 

 regardless of the heading of the ship. Should the compass be so 

 frozen, the polarity of the magnetism which must be attracting the 

 compass needles is indicated ; hence, correction may be effected simply 

 by the application of permanent magnet correctors in suitable quan- 

 tity to neutralize this magnetism. Whenever such adjustments are 

 made, it would be well to have the ship placed on a heading such 

 that the unfreezing of the compass needles will be immediately evi- 



East 

 (*) 



Dev. 

 in 

 Degrees 



West 

 (-) 



Point of Point of 



Maximum Maximum 



Sluggish- Deviation 

 ness 



HEADING 



Figure 20. — Uncompensated deviation curve. 



dent. For example, a ship whose compass is frozen to a north reading 

 would require fore-and-aft B corrector magnets with the red ends for- 

 ward in order to neutralize the existing blue pole which attracted the 

 compass. If made on an east heading, such an adjustment would be 

 practically complete when the compass card was freed so as to indi- 

 cate an east heading. 



36. Listed below are several reasons for correcting the errors of the 

 magnetic compass. 



(1) It is easier to use the magnetic compass if the deviations 

 are small. 



(2) Although the belief persists that it does not matter what 

 the deviations are, as long as they are known, this belief is in 



