CHAPTER III. THEORY OF MAGNETIC COMPASS 

 ADJUSTMENT 



15. Magnetic adjustment. — The magnetic compass when used on a 

 steel ship must be so corrected for the ship's magnetic conditions 

 that its operation would be the same as if it were on a nonmagnetic 

 ship. Ship's magnetic conditions create deviations of the magnetic 

 compass, as well as sectors of sluggishness and unsteadiness. Devia- 

 tion is defined as deflection of the card (needles) to the right or left 

 of the magnetic meridian. Adjustment of the compass is the ar- 

 ranging of magnet and soft iron correctors about the binnacle so 

 that their effects are equal and opposite to the effects of the magnetic 

 material in the ship, thus reducing the deviations and eliminating 

 the sectors of sluggishness and unsteadiness. 



The magnetic conditions in a ship which affect a magnetic compass 

 are permanent magnetism and induced magnetism^ as discussed in 

 chapter II. 



16. Permanent magnetism and its effects on the compass. — The 

 total permanent magnetic field effect at the compass may be broken 

 into three components^ mutually 90° apart, as shown in figure 7. 



Fore-and-Aft "B" Component 



Athwartship "C" 

 Component 



Total Permanent Magnetic 

 Vertical Heeling ^ pieid Across Compass 



Component 



Figure 7. — Components of permanent magnetic field at the compass. 



The effect of the vertical permanent component is the tendency to 

 tilt the compass card and, in the event of rolling or pitching of the 

 ship, to create oscillating deflections of the card. Oscillation effects 



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