CORRECTOR EFFECTS 



69 



these correctors which will require some shuttling back and forth be- 

 tween headings while making adjustments. This situation can be 

 improved by using larger spheres further out, and by approximately 

 setting the spheres before starting adjustments, as well as by using 

 more magnets further from the spheres and compass. Magnetized 

 spheres, as Avell as magnetized Flinders bar, will not only cause some 

 difficulty during adjustment, but might introduce an unstable devia- 

 tion curve if they should undergo a shake- down or change of magnetic 

 condition. 



102. Compasses. — Compasses themselves play a very important 

 part in compass adjustment, although it is common belief that the 

 compass is only an indicating instrument, aligning itself in the result- 

 ant magnetic field. This would be essentially true if the magnetic 

 fields were uniform about the compass ; but unfortunately magnetism 

 close to the compass imposes nonuniforin -fields across the needles. In 

 other words, adjustment and compensation sometimes employ non- 

 uniforai fields to correct uniform fields. Figure 36 indicates the dif- 



Uniform magnetic field Nonuniform magnetic field 



(balanced torques). (unbalanced torques). 



Figure 36. — ^Magnetic fields across compass needle arrays. 



ference between uniform and nonuniform field effects on a compass. 

 Such unbalanced torques, arising from nonuniform magnetic fields, 

 create deviations of the compass which have higher frequency charac- 

 teristics. Compass designs include many combinations of different 

 length needles, different number of needles, and different spacings and 

 arrangements of needles — all designed to minimize the higher order 

 deviations resulting from such nonuniform magnetic fields. Although 

 compass design is rather successful in minimizing such deviations, it 

 is obvious that different compasses will be affected differently by the 

 same magnetic fields. It is further stressed that, even with proper 

 compass design, it is the duty of all adjusters to exercise care in apply- 

 ing correctors in order to create the most uniform magnetic field 

 possible. This is the basis for the rule which requires the use of 



