5.10 A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22° with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.35 G. Determine the magnitude of the earth’s magnetic field at the place.

The horizontal component of the earth's magnetic field,
BH=0.35G
Angle made by the needle with the horizontal plane
= Angle of dip =δ=22
Earth's magnetic field strength = B We can relate B and BH
as
Bn=BcosθB=BHcosδ=0.35cos22=0.377G
Hence, the strength of the earth's magnetic field at the given location is 0.377 G.