The relation between flux density and vector potential is

A short bar magnet is placed in the magnetic meridian of the earth with North Pole pointing north. Neutral points are found at a distance of $30 \mathrm{cm}$ from the magnet on the East-West line, drawn through the middle point of the magnet. The magnetic moment of the magnet in ${\mathrm{Am}}^2$ is close to:

(Given $\frac{{\mu }_0}{4\pi }={10}^{-7}$ in SI units and $B_H=$ Horizontal component of earth's magnetic field $=3.6\times {10}^{-5}$ Tesla.)

Two identical short magnetic dipoles of magnetic moments $1.0 \mathrm{A} {\mathrm{m}}^3$ each, placed at a separation of $2 \mathrm{m}$ with their axes perpendicular to each other. The resultant magnetic field at a point midway between the dipole is :

Two short magnets of equal dipole moments $M$, are fastened perpendicularly at their centre (figure). The magnitude of the magnetic field, at a distance $d$ from the centre, on the bisector of the right angle, is 

Calculate the magnetic induction at P, for the arrangement shown in figure, when two similar short magnets of magnetic moment M are joined at the middle. So that they are mutually perpendicular