98 HANDBOOK OF MAGNETIC COMPASS 



varied. This regulation is accomplished in the control box by means 

 of control resistors for each circuit. 'When these resistors have once 

 heen set, their settings need not he altered with current changes in the 

 degaussing circuits. 



Each compensating coil is controlled by a separate unit in the con- 

 trol box. Wiring diagrams in figures 47 to 50 show this in detail. 

 Each circuit should be properly labeled. 



It is best to check coil installations electrically and compensate at 

 dochside before the ship leaves the yard. Although accuracy of com- 

 pensation is impaired by welding, adjacent ships, and moving cranes, 

 time and trouble are still saved for the ship during final compensation 

 at sea. All this results from the fact that trouble-shooting is the 

 greater part of coil compensation. Chapter X presents details of 

 compensation procedure. 



Final compensation should be made at sea after magnetic adjustment. 

 This will correct for the changes of Flinders bar length or movement 

 of spheres, which might have been made as part of the magnetic 

 adjustment, as well as refine the approximate dockside settings of the 

 compass coils under more ideal conditions. 



123. Whether compensation is made at dockside or at sea, the prin- 

 ciple is merely one of isolating the three vector effects such that each 

 vector coil can be adjusted separately. The two 90° horizontal com- 

 ponents are isolated by obtaining proper compass headings relative 

 to the ship's axes. At sea these headings are usually obtained by 

 swinging ship; while at dockside, they may be simulated by deflec- 

 tion of the compass with permanent magnets. 



It is good practice to compensate for the heeling effects first, because 

 the heeling coil currents create additional induction effects in the 

 Flinders bar, which in turn creates B deviations on the compass. 

 The details of the heeling coil compensations are given in chapter X. 



It is pointed out here that horizontal component fields create maxi- 

 mum deviations when the fields are perpendicular (striking broad- 

 side) to the compass needle; and, compensation is achieved by 

 energizing coils which create similar fields in opposite directions. 

 Reference is made to figure 52 for an illustration of this principle. 



In figure 52 the ship is on a NE, heading and, with the degaussing 

 coils deenergized, the needles (or N.-S. line) of the compass card are 

 on an axis 45° to the left of the ship's fore-and-aft line (on the 

 NW./SE. axis). If any degaussing coil is energized, say the M 

 coil, deviation will be caused by a component of the degaussing coil 

 field which is perpendicular to the NW./SE. axis, or in line with the 



