If magnetic lines of force are drawn by keeping magnet vertical, then number of neutral points will be

If $\theta _1$ and $\theta_2$ be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip $\theta$ is given by 

A telephone cable at a place has four long straight horizontal wires carrying a current of $1.0\; A$ in the same direction east to west. The earth's magnetic field at the place is $0.39 \;G ,$ and the angle of dip is $35^{\circ} .$ The magnetic declination is nearly zero. What are the resultant magnetic fields at points $4.0\; cm$ below and above the cable?

A magnetic compass needle oscillates $30$ times per minute at a place where the dip is $45^o$, and $40$ times per minute where the dip is $30^o$. If $B_1$ and $B_2$ are respectively the total magnetic field due to the earth at the two places, then the ratio $B_1/B_2$ is best given by 

Intensity of magnetic field due to earth at a point inside a hollow steel box is

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;