CHAPTER VII. PRACTICAL PROCEDURES FOR 

 MAGNETIC COMPASS ADJUSTMENT 



With an understanding of the theory of magnetic effects and their 

 corrections, the methods of analyzing deviation curves, and the meth- 

 ods of placing a ship on any desired heading, the operator is ready 

 to begin adjustment. 



71. Dockside tests and ad justments.— Chapter I, "Procedure for 

 Magnetic Compass Adjustment," is in general self-explanatory, and 

 brings to the attention of the operator many physical checks which 

 are desirable before beginning an adjustment. The theoretical ad- 

 justment is based on the premise that all the physical arrangements 

 are perfect, and much time and trouble will be. saved while at sea 

 if these checks are made before attempting the actual magnet and 

 soft iron corrector adjustments. A few of these checks are amplified 

 below. 



72. Should the compass have a small bubble, compass fluid may 

 be added by means of the filling plug on the side of the compass 

 bowl. If an appreciable amount of compass liquid has leaked out, 

 a careful check should be made on the condition of the sealing gasket 

 and filling plug. United States Navy compass liquid may be a mixture 

 of 45 percent grain alcohol and 55 percent distilled water, or a kerosene- 

 type fluid (specification AN-C-116 or AN'-W-C-551). These fluids 

 are not interchangeable. 



73. The compass should be removed from the ship and taken to 

 some place free from all magnetic influences except the earth's mag- 

 netic field for tests of Tnoment and sensibility. These tests involve 

 measurements of the time of vibration and the ability of the compass 

 card to return to a consistent reading after deflection. These tests 

 will indicate the condition of the pivot, jewel, and magnetic strength 

 of the compass needles. (See Dutton's Navigation and Nautical 

 Astronomy or NBS. 1107 for such test data on standard Navy compass 

 equipment.) 



74. A careful check should be made on the spheres and Flinders 

 bar for residual magnetism,. Move the spheres as close to the com- 

 pass as possible and slowly rotate each sphere separately. Any ap- 

 preciable deflection (2° or more) of the compass needles resulting 

 from this rotation indicates residual magnetism in the spheres. This 

 test may be made with the ship on any steady heading. The Flinders 

 bar magnetization check is preferably made with the ship on steady 

 east or west compass headings. To make this check: {a) note the 

 compass reading with the Flinders bar in the holder, (&) invert the 



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