198 ON MAGNETISM. 



This is the change of terrestrial horizontal force cor- 

 responding to a motion of 1 inch in the photographic 

 spot: by means of this value, a general scale for in- 

 terpreting the values of the spot-motion on the ordi- 

 nates of the photographic curve can be formed. 



86. Balance-magnetometer for record of the small 

 changes of magnetic vertical force, and evaluation of 

 their scale. 



Let Figure 59 represent the balance-magnetometer : 



Fig. 59. 



a magnet to which is attached a steel knife-edge C, by 

 means of which the magnet vibrates in the vertical 

 plane; its knife-edge being supported by horizontal 

 planes of hard stone. The vertical plane being trans- 

 versal to the magnetic meridian, the horizontal di- 

 rective force has no effect on the motion of the 

 magnet : it is affected only by magnetic vertical force 

 and by gravity. 



The red end of the needle is pulled downwards, 

 and the blue end is pushed upwards, by terrestrial 

 vertical magnetism. To maintain the magnet in a 

 horizontal position, its center of gravity cannot be 

 below the knife-edge C, but must really be somewhere 

 towards the blue end, as at G : the point G being 



