112 ADVANCED ELECTRICITY AND MAGNETISM. 



since T a = kH' X cos a and T b = kH f X sin a, it is evident 

 that the resultant of T a and T b is always opposite to H f in 

 direction, so that the actual field at the compass box is constant 

 in value and always parallel and opposite to H 1 ', or, in other 

 words, the compass error due to the temporary magnetism of the 

 ship is zero on all headings of the ship when T L in Fig. 75 is 

 equal to T 2 in Fig. 76. 



69. Compensation of quadrantal error. The quadrantal cor- 

 rectors. The quadrantal error of the ship's compass is eliminated 



(that is to say, compensated) 

 by means of two iron spheres 

 55 which are usually* placed 

 on the two sides of the com- 

 pass, as shown to an exagger- 

 ated scale in Fig. 78. These 



spheres are called the quadrantal correctors, and the practical 

 method of adjusting them is explained in Art. 73. The action of the 

 quadrantal correctors may be understood with the help of Figs. 

 790 and 796 as follows: When the line joining the centers of the 

 two spheres SS is parallel to H' as shown in Fig. 790, the 

 magnetic field at the point p is more intense than H' ; and when 

 the line joining the centers of the spheres is at right angles to 

 H' as shown in Fig. 796, the magnetic field at p is less intense 

 than H'. Now the weakening of the magnetic field at the com- 

 pass box by the temporary magnetism of the ship is usually 

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

 heads north or south. That is to say, T is usually greater 

 in Fig. 76 than it is in Fig. 75, or the coefficient k 2 is usually 

 greater than the coefficient ki (see Art. 68). Therefore by plac- 

 ing the quadrantal correctors in the positions shown in Fig. 78 

 and moving them closer to, or farther away from the compass, 

 the weakening of the magnetic field at the compass by the 



* When the coefficient k\ in Art. 68 is greater than kz the quadrantal correctors 

 must be placed fore and aft of the compass box. This condition is, however, seldom 

 realized in practice. 



