50 HANDBOOK OF MAGNETIC COMPASS 



Flinders bar in the holder and again note the compass reading. Aiiy 

 appreciable difference (2° or more) between these observed readings 

 indicates residual magnetism in the Flinders bar. Spheres or Flin- 

 ders bars which show signs of such' residual magnetism should be 

 annealed, i. e., heated to a dull red and allowed to cool slowly. 



75. Correct alignment of the lubber's line of the compass, gyro 

 repeater, and pelorus with the fore-and-aft line of the ship, is of 

 major importance. Such a misalignment will produce a constant 

 A error in the curve of deviations. Any of these instruments may 

 be aligned correctly with the fore-and-aft line of the ship by using 

 the azimuth circle and a metal tape measure. Should the instrument 

 be located on the centerline of the ship, a sight is taken on a mast 

 or other object on the centerline. In the case of an instrument off the 

 centerline, a metal tape measure is used to measure the distance from 

 the centerline of the ship to the center of the instrument. A similar 

 measurement from the centerline is made forward or abaft the sub- 

 ject instrument and reference marks are placed on the deck. Sights 

 are then taken on these marks. 



Standard compasses should always be aligned so that the lubber's 

 line of the compass is parallel to the fore-and-aft line of the ship. 

 Steering compasses may occasionally be misaligned in this respect in 

 order to correct for any magnetic A error present, as discussed in 

 article 81. 



76. In addition to the physical checks listed in chapter I, there are 

 other precautions to be observed in order to assure continued satis- 

 factory compass operation. These precautions are mentioned to bring 

 to the attention of the adjuster certain conditions which might dis- 

 turb compass operation. These precautions are listed in chapter I 

 and are discussed further in chapter IX. 



Expeditious compass adjustment is dependent upon the application 

 of the various correctors in a logical sequence so as to achieve the 

 final adjustment with a minimum number of steps. Certain adjust- 

 ments may be made conveniently at dockside so as to simplify the 

 adjustment procedures at sea. 



77. Inasmuch as the Flinders bar is subject to induction from sev- 

 eral of the other correctors and, since its adjustment is not dependent 

 on any single observation, this adjustment is logically made first. 

 This adjustment is made by one of the following methods. 



(1) Deviation data obtained at two different magnetic latitudes 

 may be utilized to calculate the proper length of Flinders bar 

 for any particular compass location. Details of the acquisition 

 of such data and the calculations involved are presented in arts. 

 95 to 99, inclusive. The utilization of vertical field "Z-loop" 

 coils is being explored as a method for simulating this procedure. 



