THEORY OF COMPENSATION 101 



rather than swing ship, it can be done by carefully placing magnets so 

 as to create a deflecting field perpendicular to the bisector of the angle 

 between the original and desired position of the compass needle, as 

 illustrated in figure 53. For this procedure, the use of stronger mag- 

 nets at a greater distance from the compass makes for a more symmet- 

 rical deflecting field and more accurate compensation. Applying 

 fields in this manner will simulate heading conditions quite accurately, 

 since the directive force on the needle is not disturbed. 



E. Operation 



126. Deviation curves. — Because of the many difficulties of com- 

 pensations it is not always possible to eliminate completely degaussing 

 effects on the compass; but, compensation should generally be such 

 that deviations due to degaussing alone are below 2° on all headings. 

 For the same reasons that a deviation curve is recorded for undegaussed 

 conditions of the ship after magnetic adjustment (art. 89), a deviation 

 curve should likewise be recorded for degaussed conditions after the 

 final coil compensation. These two curves will then be available for 

 appropriate use under either condition of navigation. These two 

 deviation curves are recorded on standard Navy Forms NBS 1104 

 or 1105 as illustrated in Figure 54, and in the Compass Record BooJe, 

 NBS 1101. Article 13.9 discusses the purposes of the various NBS 

 Kecord Forms more fully. Details of coil current settings and un- 

 compensated deviations are recorded on the Navy Form CC-1, illus- 

 trated in figure 55. 



The error due to degaussing is the difference between the deviations 

 observed with degaussing on and degaussing secured, and is not the 

 total deviation of the degaussed condition curve. Therefore the de- 

 gaussed condition curve will be reliable only as long as the undegaussed 

 condition curve is reliable even though the coils compensate perfectly 

 all of the degaussing effects. 



127. It is advisable to maintain a check on the deviation curves 

 and to recompensate, if necessary, for the following reasons : 



(1) Magnetic cargo, incorrect Flinders bar adjustment, de- 

 perming, etc., may tend to change the basic magnetic adjustment 

 and thus change both deviation curves. 



(2) Compensating coils are electrical circuits and subject to 

 electrical troubles. 



(3) Changes of Flinders bar length or sphere positions can 

 change the degaussing effects or certain coil compensation effects 

 on the compass. 



(4) Structural changes, magnetic cargo, or degaussing coil 

 changes can change the degaussing effects at the compass. 



