(a) Angle of declination (magnetic declamatory) θ, angle of dip δ, horizontal component of earth’s magnetic field BH are the quantities which are considered as elements of earth’s magnetic field. (b) Britain is closer to magnetic north pole hence angle of dip is much larger, nearly 70° in Britain. (c) Melbourne is closer to south pole, so north of the assumed magnet buried within earth lies inside, hence the field lines would seem to be coming out of the ground. (d) At geomagnetic north or south pole, angle of dip is 90°, where horizontal component of earth’s magnetic field BH is zero. A compass needle can only turn in horizontal plane, so it can point in any direction as BH = 0, which governs its direction. (e) Let us consider the magnetic field on surface of earth due to assumed bar magnet of dipole moment 8 × 1022 J T–1 located at centre of earth.
The magnetic field at point P equatorial position on earth can be calculated as B =
= 10−7 ×
= 0.3 × 10−4 T or 0.3 G (f) Localised magnetic dipoles can develop due to magnetised mineral deposits or movement of charged ions in atmosphere.