192 ON MAGNETISM. 



it in its motions) must have turned through the angle 



=7 . The direction of horizontal magnetic force has 

 2/7i 



therefore changed through an angle represented by ~- . 



This change of direction would be produced by com- 

 bining, with the northerly directive force, a westerly 

 force equal to 



northerly directive force 

 2m 



Consequently a motion of the photographic spot through 

 1 inch in the direction of the ordinate of the curve will 



represent a westerly magnetic force equal to ^ of the 

 whole northerly horizontal force. 



85. Bifilar magnetometer for record of the small 

 changes of magnetic horizontal force, and evaluation of 

 their scale. 



A torsion-apparatus of any kind, which permits an 

 accurate measure to be made of the force that produces 

 any angle of torsion, would answer perfectly for this 

 object. But the kind of torsion which has been adopted 

 as most convenient is that produced by suspension by 

 means of two cords or wires, separated, both at the 

 upper place of attachment to a fixed beam or other 

 support, and at the lower place of attachment to the 

 magnet. 



