Which is incorrect

At magnetic poles of earth, angle of dip is.....$^o$

Earth's magnetic field always has a horizontal component except at or Horizontal component of earth's magnetic field remains zero at

Assume the dipole model for earth’s magnetic field $\mathrm{B}$ which is given by

${{\rm{B}}_{\rm{v}}} = $ vertical component of magnetic field

$ = \frac{{{\mu _0}}}{{4\pi }}\frac{{2m\,\cos \theta }}{{{r^3}}}$

${{\rm{B}}_H} = $ Horizontal component of magnetic field

${{\rm{B}}_H} = \frac{{{\mu _0}}}{{4\pi }}\frac{{m\,\sin \theta }}{{{r^3}}}$

$\theta $ $= 90^{°}$ -latitude as measured from magnetic equator.

$(a)$ Find loci of points for which : dip angle is zero;

A magnetic needle oscillates in a horizontal plane with a period $T$ at a place where the angle of dip is $60^{\circ}$. When the same needle is made to oscillate in a vertical plane coinciding with the magnetic meridian, its period will be

A very small magnet is placed in the magnetic meridian with its south pole pointing north. The null point is obtained $20\, cm $ away from the centre of the magnet. If the earth's magnetic field (horizontal component) at this point be $0.3$ $gauss$, the magnetic moment of the magnet is