USE OF THE BIFILAR MAGNETOMETER. 2/9 



riations must be attended to^ by making continued observations 

 with a second apparatus of the same kind, or by making con- 

 tinued observ^ations of the time of vibration with a common mag- 

 netometer. 



11. The time of vibration, t, is observed in this reverse 

 position. 



12. The magnet-bar is then placed in its natural position, 

 (north towards the north,) by turning the stirrup with its alidade 

 exactly 180 degrees; the time of vibration, t, is again observed. 

 Then the magnetic directive force, M, is to the directive force 

 arising from the mode of suspension, S, in the ratio 



M : S = f^ - t" : t" + r"". 

 When this proportion deviates much from unity, the Avires must 

 be brought nearer to or moved further from each other, until the 

 altered directive force of the wires exceeds but little the magnetic 

 directive force; for instance, by about the tenth part of the 

 latter. This is the case in the Gottingen magnetometer. 

 13. Seek the angle z, the sine of which is 



sm ^ - ^2 + ^2 • 



turn the alidade of the stirrup (say in the direction of the dally 

 motion of the sun) 90° — z, and turn the alidade of the pivot 

 of the mirror in the opposite direction through the angle z. The 

 equilibrium is then disturbed : the wires can no longer remain in 

 their natural position, but must turn the circle to which they 

 are fixed (and thus the whole instrument) exactly through the 

 angle z, in the direction of the daily revolution of the sun. In 

 this new position the equilibrium may be re-established, since the 

 bar makes with its former position an angle (90° — z) + z = 90°, 

 while the wires have only been turned through the angle z at 

 their lower ends. It follows, thence, that if the wires were pre- 

 viously in their natural position, and if the magnetic axis of the 

 bar was situated in the magnetic meridian, the opposite moments 

 of rotation arising from the two forces 31 and S are to each other 

 in the proportion 



M sin 90° : S sin z. 



But as 



t^ + 



sm 90 



