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HANDBOOK OF MAGNETIC COMPASS 



strong correctors symmetrically arranged as far away from the com- 

 pass as possible, instead of weak correctors very close to the compass. 

 In general it is better to use larger spheres placed at the extremities 

 of the brackets, equally distant from the center of the compass. B and 

 C permanent magnet correctors should always be placed so as to have 

 an equal number of magnets on both sides of the compass where pos- 

 sible. They should also be centered as indicated in figure 37, if regular 

 tray arrangements are not available. The desire for symmetrical 

 magnetic fields is one reason for maintaining a sphere of specified 

 radius, commonly called the magnetiG circle^ about the magnetic com- 

 pass location. This circle is kept free of any magnetic or electrical 

 equipment. 





B magnet fields. O magnet fields. 



Figure 37. — Arrangements of coirector magnets. 



The magnetic inoment of the compass needle array is another factor 

 in compass design which ranks in importance with the proper arrange- 

 ment of needles. This magnetic moment controls the needle induction 

 in the soft iron correctors, as discussed in articles 92 and 93, and hence 

 governs the constancy of those corrector effects with changes in mag- 

 netic latitude. The TV2'' Navy No. 1 alcohol- water compass has a 

 magnetic moment of approximately 4000 cgs units, whereas the T^" 

 Navy No. 1 oil compass has a magnetic moment of approximately 1650 

 cgs units. The lower magnetic moment compass allows considerably 

 less change in quadrantal correction, although the periods are essen- 

 tially comparable because of the difference in the compass fluid 

 characteristics. 



Other factors which must be considered in compass design are period, 

 fluid, swirl, vibration, illumination, tilt, pivot friction, fluid expansion, 

 etc. These factors, however, are less important from an adjuster's 

 point of view than the magnetic moment and arrangement of needles, 

 and are therefore not discussed further in this text. 



