302 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



in the direction of which the compass needle points ; therefore 

 the angle </> is the compass error. Imagine B t Fig. 10, to rep- 

 resent the bow of the ship, and suppose the ship to start with its 

 bow due north and swing around to the east, the angle a increas- 

 ing from zero to 360. A careful consideration of Figs. 10 and 

 1 1 will show that the angle <f> has a series of westerly values 



Fig. 10. 



Ur^T 



Fig. 11. 



H' 



throughout the first quadrant (a between zero and 90), a series 

 of easterly values throughout the second quadrant (a between 

 90 and 1 80), a second series of westerly values throughout the 

 third quadrant (a between 180 and 270), and a second series 

 of easterly values throughout the fourth quadrant. The compass 

 error due to the temporary magnetism of a ship is therefore called 

 the quadrantal error. 



When the ship's compass is located on the center line of the 

 ship so that the iron of the ship is symmetrically placed on the 

 two sides of the compass, then the compass error due to the 

 ship's temporary magnetism is zero when the ship heads north, 

 east, south, or west, as may be shown as follows : When the ship 

 heads magnetic north or south, its temporary magnetism is sym- 

 metrical as shown in Fig. 12, the magnetic field at the compass 

 due to the temporary magnetism of the ship is therefore due 

 south, and consequently the compass is not deflected. Figure 

 13 shows a compass box C placed on the center line of a ship 

 of which the dissimilarity of bow and stern is greatly exaggerated. 



