The instrument can record continuously the variation of the horizontal components of the magnetic induction (B_H) of the magnetic field at a place. The magnetic system is suspended by a quartz fiber from a torsion head. The torsion head is rotated until the magnetic axis of the magnet makes an angle of 90o with the magnetic meridian. A narrow beam of light is made to strike the mirror and the reflected beam is received on a photographic film rolled over a drum rotating at a uniform speed. Let M and B_H represents the magnetic moment of the suspended magnet and the horizontal component of the magnetic induction of the earth’s magnetic field.

The restoring torque on the magnet = MB_H.
Let θ be the twist in the suspension fiber. Let C be the tensional torque per unit twist. Then,

MB_H = Cθ, MδB_H = Cδθ, B_H = C/M δ θ.

If C is small and M is large then for a small change δBH, there will be large change is δθ. Due to the increase in the twist by δθ, the spot of light reflected on to the recording drum moves.


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