Handbook of Magnetic Compass Adjustment - Maritime Safety ...

CHAPTER X
TRANSIENT DEVIATIONS OF THE MAGNETIC COMPASS
1001. Stability. The general treatise on compass adjustment concerns itself only with the principles of steady-state
magnetism; i.e., the effects of permanent and induced magnetism and their appropriate correctors. This knowledge,
along with the ability to handle sun's azimuth and ship's heading, is the backbone of compass adjustment. However, a
correction may be very carefully and accurately made and still prove disastrous to the ship; for example, a compass
may have a perfect deviation curve, but when a nearby gun is trained the magnetic effects on the compass are changed.
Although a compass adjuster cannot place correctors on the binnacle for such variable effects, it is definitely his duty to
recognize and handle them in the best possible manner. If it is impossible to eliminate the source of trouble, or
impractical to relocate the binnacle, the details of alignment or excitation of the sources of error should be specified on
the deviation card. With such information, the navigator would know when or when not to rely on his magnetic
compass. In other words, a good adjuster should not only provide a good deviation curve which is reliable under
specifically stated conditions, but also point out and record probable causes of unreliability which cannot be eliminated.
1002. Sources of transient error. The magnetic circle about the magnetic compass is intended to reduce such transient
conditions, but there still are many items, both electrical and magnetic, which cause erratic effects on the compass. The
following list is presented to assist in the detection of such items. If in doubt, a test can be made by swinging any
movable object or energizing any electrical unit while observing the compass for deviations. This would best be tried
on two different headings, 90° apart, since the compass might possibly be affected on one heading and not on the other.
1. Some magnetic items which cause variable deviations if placed too close to the compass are as follows:
(a) Guns on movable mounts.
(b) Ready ammunition boxes.
(c) Variable quantities of ammunition in ready boxes.
(d) Magnetic cargo.
(e) Hoisting booms.
(f) Cable reels.
(g) Metal doors in wheelhouse.
(h) Chart table drawers.
(i) Movable gyro repeater.
(j) Windows and ports.
(k) Signal pistols racked near compass.
(l) Sound powered telephones.
(m) Magnetic wheel or rudder mechanism.
(n) Knives or ash trays near binnacle.
(o) Watches, wrist bands, spectacle frames.
(p) Hat grommets, belt buckles, metal pencils.
(q) Heating of smoke stack, or exhaust pipes.
(r) Landing boats.
2. Some electrical items which cause variable deviations if placed too close to the compass are as follows:
(a) Electric motors.
(b) Magnetic controllers.
(c) Gyro repeaters.
(d) Unmarried conductors.
(e) Loudspeakers.
(f) Electric indicators.
(g) Electric welding.
(h) Large power circuits.
(i) Searchlights.
(j) Electrical control panels or switches.
(k) Telephone headsets.
(l) Windshield wipers.
(m) Rudder position indicators, solenoid type.
(n) Minesweeping power circuits.
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