The Vertical Variometer

This instrument is used to record the variations of the vertical component of the magnetic induction (Bv) of the earth’s magnetic field.

It consists of two magnets N1S1 and N2S2 of pole strength m1 and m2 respectively. A is a highly polished quartz plate. The magnets are tightly fixed of two quartz rods B and C and the rods are fused to the plate A. The rod B is connected to the torsion head by the quartz fiber K. The rod C is connected to the spring S by the quartz fiber L.

In the northern hemisphere, the ends S1 and S2 of the magnets points upwards. An adjustable mass m is placed in such a position that S1 and S2 are depressed below the horizontal plane of K and L. The torsion head is now turned until the axes of the two magnets lie in the same horizontal plane. The right angles prism P is used to reflect the incident light towards the quartz plate A. The light after reflection from the plate A is reflected by the prism towards the photographic film moving at constant speed. Any change in Bv causes the magnets and the plates attached to them to rotate until the change in the torisonal balances the change in the torque acting upon the system. The change is recorded on the photographic film.