ACTION OF A MAGNET ON A COMPASS NEEDLE. 123 



Pig. 39. 



tion of c the respective forces 



which destroy each other : but they 

 produce in the direction towards the 

 right the forces 



which are to be added together, pro- 

 ducing 2 a . a(a z + c 2 )~^ 



acting towards the right. This force, it is to be observed, 

 is parallel to the length of A or perpendicular to the 

 length of c, whatever be the plane containing the center 

 of the compass and the axis of A. The plane aaB may 

 be horizontal, or vertical, or inclined, but the expression 

 found above applies to every one of these cases. 



51. The disturbing magnet is end-on to the compass: 

 first, in the horizontal plane: secondly, in an inclined 

 plane, the axis of the magnet still directed to the compass. 



In Figure 40, the attraction of the blue 

 end of the magnet on the red end of the 



compass is represented by -5 , and the 



repulsion of the red end by , 2 ; the dif- 



(c + a) 



ference is 



ca 



