306 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



two iron spheres 55 which are usually * placed on the two 

 sides of the compass, as shown to an exaggerated scale in Fig. 

 17. These spheres are called the quadrantal correctors and the 



practical method of adjusting 

 them is explained in Art. 16. 

 The action of the quadrantal 

 correctors may be understood 

 with the help of Figs. 1 8a and 

 1 8& as follows : When the line 

 joining the centers of the two spheres 55 is parallel to H 1 as 

 shown in Fig. 1 8a, the magnetic field at the point / is more in- 

 tense than H' ; and when the line joining the centers of the 



Fig. 1 7. 



Fig. 18a. 



Fig. 18b. 



spheres is at right angles to H' as shown in Fig. 1 %b, the magnetic 

 field at / is less intense than H' '. Now the weakening of the 

 magnetic field at the compass box by the temporary magnetism of 

 the ship when the ship heads east or west is usually greater than 

 the weakening of the field at the compass box by the temporary 

 magnetism of the ship when the ship heads north or south. That 



* In some cases, namely, when the coefficient Jt l is greater than the coefficient 

 it is necessary to place the quadrantal correctors fore and aft of the compass box. 



