Handbook of Magnetic Compass Adjustment - Maritime Safety ...

Figure 303a – Force diagrams for fore-and-aft permanent B magnetic field
Figure 303b – Force diagrams for athwartship permanent C magnetic field
305. Inasmuch as a compass deviation is caused by the existence of a force at the compass that is superimposed upon the
normal earth's directive force, H, a vector analysis is helpful in determining deviations or the strength of deviating fields. For
example, a ship as shown in figure 305 on an east magnetic heading will subject its compass to a combination of magnetic
effects; namely, the earth's horizontal field H, and the deviating field B, at right angles to the field H. The compass needle
will align itself in the resultant field which is represented by the vector sum of H and B, as shown. A similar analysis on the
ship in figure 305 will reveal that the resulting directive force at the compass would be maximum on a north heading and
minimum on a south heading, the deviations being zero for both conditions.
The magnitude of the deviation caused by the permanent B magnetic field will vary with different values of H; hence,
deviations resulting from permanent magnetic fields will vary with the magnetic latitude of the ship.
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