CORRECTOR EFFECTS 67 



(2) Should the exact amount of correction required for vertical 

 induction in the ship at some particular magnetic dip, 6, be known, 

 figure 34 (a) will directly indicate the correct amount of Flinders 

 bar to be placed at the top of the holder. The exact amount of 

 correction would be known when one of the latitudes is the mag- 

 netic equator, and the deviations there are negligible. Then the 

 B deviation in degrees on magnetic headings at the other lati- 

 tude is the exact amount to correct by means of curves in figure 

 34 (a). 



(3) Lord Kelvin's rule for improving the Flinders bar setting 

 is: "Correct the deviations observed on east or west courses by 

 the use of fore-and-aft B magnets when the ship has arrived at 

 places of weaker vertical magnetic field, and by the use of Flinders 

 bar when she has arrived at places of stronger vertical magnetic 

 field, whether in the northern or southern hemisphere." 



98. After determining the correct amount of Flinders bar by either 

 method (1) or (2) above, the bar should then be inserted at the top 

 of the holder and the fore-and-aft B magnets readjusted to cor- 

 rect the remaining B error. Sphere adjustments should likewise be 

 refined. 



It is quite possible that on inserting the Flinders bar, no visible 

 deflection of the compass will be observed, even on an east or west 

 heading. This should cause no concern because certain additional 

 induction effects exist in the bar from : 



(1) The heeling magnet. 



(2) The existing fore-and-aft B magnets. 



(3) The vertical component of the ship's permanent magnetic 

 field. 



99. A common, but inaccurate, method of determining the approx- 

 imate Flinders bar length is the so-called 10° method. By this 

 method, Flinders bar is added on an east or west magnetic heading 

 until all deviation but 10° is visibly removed. This method errone- 

 ously assumes that the deviations due to fore-and-aft permanent mag- 

 netism are always 10°, regardless of compass location, structure, perm 

 conditions, class of ship, or different directive forces on the compass. 



Inasmuch as there are so many extraneous induction effects in the 

 Flinders bar, it is likewise stressed that the drop-in method of deter- 

 mining Flinders bar, is invalid. By this method, the amount of B 

 deviation due to vertical induction in the ship at the latitude of adjust- 

 ment would be calculated, and the Flinders bar would be then adjusted 

 on an east or west heading until a visible deflection equal to this 

 amount of B deviation was observed on the compass. Even though this 



