108 ADVANCED ELECTRICITY AND MAGNETISM. 



the other at right angles to AB. The component which is at 

 right angles to AB has no perceptible magnetizing action on 

 AB, the component which is parallel to A B causes the end B 

 to become a north pole and the end A to become a south pole, 

 and the magnetic field at C due to these magnet poles is parallel 



Fig. 71. 



to BA and towards A. The magnetic field at the compass 

 box which is due to the temporary magnetism of the bar AB 

 in Fig. 71 is represented by the arrow T in Fig. 72, the earth's 

 field at the compass box is represented by H f , and the arrow R 

 represents the resultant field at the compass box in the direction 

 of which the compass needle points; therefore the angle $ is 

 the compass error. Imagine B, Fig. 71, to represent the bow 

 of the ship, and suppose the ship to start with its bow due north 

 and swing around to the east so as to increase the angle a from 

 zero to 360. A careful consideration of Figs. 71 and 72 will 

 show that the angle has a series of westerly values throughout 

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

 values throughout the second quadrant (a between 90 and 180), 

 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 



