IS 



Handbook: of magnetic compass 



netic fields are either in line with or perpendicular to the com- 

 pass needles only on cardinal compass headings. 



(2) Such a presentation illustrates the fact that the compass 

 card tends to float in a fixed position, in line with the magnetic 

 meridian. Deviations of the card to right or left (east or west) 

 of the magnetic meridian result from the movement of the ship 

 and its magnetic fields about the compass card. 



19. Inasmuch as a compass deviation is caused by the existence 

 of a force at the compass which is superimposed upon the normal 

 earths' directwe force^ H^ a vector analysis is helpful in determining 

 deviations or the strength of deviating fields. For example, a ship as 

 shown in figure 11 on an east magnetic heading will subject its com- 

 pass to a combination of magnetic effects; namely, the earth's hori- 

 zontal field H, and the deviating field B, at right angles to the field H. 

 The compass needle will align itself in the resultant -field which is rep- 

 resented by the vector sum of H and 5, as shown. A similar analysis 

 on the ship in figure 11 will reveal that the resulting directive force 

 at the compass would be maximum on a north heading and minimum 

 on a south heading, the deviations being zero for both conditions. 



Resultant Field in 



Iilagnitude (Directive Force) 



and Direction (Deviation) 



Earth's Field 

 H 



East 

 Magnetic 

 Heading 



Deviating Field 



tIRII 



Compass Needle 



Figure 11. — General force diagram. 



The magnitude of the deviation caused by the permanent B mag- 

 netic field will vary with different values of H; hence, deviations re- 

 sulting from permanent magnetic fields will vary with the magnetic 

 latitude of the ship. 



20. Induced magnetism and its effects on the compass. — Induced 

 magnestism varies with the strength of the surrounding field, the mass 



