BIFILAR MAGNETOMETEK. 193 



Suppose an unmagnetic bar (as of brass) to be 

 suspended thus by two cords separated at the top and 

 at the bottom. The bar will take such a position that 

 the two cords will hang in one vertical plane. Let 

 the apparatus be so adjusted that the unmagnetic bar 

 takes a position in the magnetic meridian. Substitute 

 for it a magnetized steel bar; the steel magnet is in 

 the position which it would assume if perfectly free, 

 and therefore it exerts no mechanical effort to escape 

 from that position. Now turn, through a limited 

 angle in the horizontal plane, the substance to which 

 the upper ends of the two cords are attached. The 

 two cords are now no longer in one plane : and they 

 exert a force of torsion or wringing on the suspended 

 magnet. The magnet will yield to this, but not en- 

 tirely ; for, as soon as its position makes an angle with 

 the magnetic meridian, the earth's directive force tends 

 to pull it back towards the magnetic meridian, or to 

 resist the torsion-power produced by the bifilar sus- 

 pension. The magnet therefore will take a position 

 in which the torsion-power, produced by the circum- 

 stance that the two wires are not in one plane, exactly 

 balances the torsion-power produced by the action of 

 terrestrial directive force upon the magnet, now in- 

 clined to the magnetic meridian. 



Now suppose the terrestrial directive force suddenly 

 to increase. It will more than balance the torsion- 

 power of the suspension, and will draw the magnet 

 nearer to the meridian. Suppose the terrestrial 

 directive force to diminish: the torsion-power of the 



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