CHAPTER IX. TRANSIENT DEVIATIONS OF THE 

 MAGNETIC COMPASS 



103. Stability. — The general treatise on compass adjustment con- 

 cerns itself only with the principles of steady-state magnetisTn; 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 com- 

 pass 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 devia- 

 tion 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 re- 

 liable under specifically stated conditions, but also point out and record 

 probable causes of unreliability which cannot be eliminated. 



104. 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. Tlie following list is presented to as- 

 sist 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. 



(5) Ready ammunition boxes. 



(c) Variable quantities of ammunition in ready boxes. 



{d) Magnetic cargo. 



{e) Hoisting booms. 



(/) Cable reels. 



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