Handbook of Magnetic Compass Adjustment - Maritime Safety ...

315. The nature, magnitude, and polarity of all these induced effects are dependent upon the disposition of metal, the
symmetry or asymmetry of the ship, the location of the binnacle, the strength of the earth's magnetic field, and the angle of
dip.
316. Certain heeling errors, in addition to those resulting from permanent magnetism, are created by the presence of both
horizontal and vertical soft iron, which experience changing induction as the ship rolls in the earth's magnetic field. This part
of the heeling error will naturally change in magnitude with changes of magnetic latitude of the ship. Oscillation effects
accompanying roll are maximum on north and south headings, just as with the permanent magnetic heeling errors.
317. Adjustments and correctors. Since some magnetic effects remain constant for all magnetic latitudes and others vary
with changes of magnetic latitude, each individual effect should be corrected independently. Further, it is apparent that the
best method of adjustment is to use (1) permanent magnet correctors to create equal and opposite vectors of permanent
magnetic fields at the compass, and (2) soft iron correctors to assume induced magnetism, the effect of which will be equal
and opposite to the induced effects of the ship for all magnetic latitude and heading conditions. The compass binnacle
provides for the support of the compass and such correctors. Study of the binnacle in figure 317 will reveal that such
correctors are present in the form of:
(1) Vertical permanent heeling magnet in the central vertical tube,
(2) Fore-and-aft B permanent magnets in their trays,
(3) Athwartship C permanent magnets in their trays,
(4) Vertical soft iron Flinders bar in its external tube,
(5) Soft iron spheres.
The heeling magnet is the only corrector that corrects for both permanent and induced effects, and consequently must be
readjusted occasionally with radical changes in latitude of the ship. (It must be noted, however, that any movement of the
heeling magnet will require readjustment of other correctors.)
Figure 317 – Binnacle with compass and correctors
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