The relative permeability of air is: 

$A$ and $B$ are two concentric circular conductors of centre $O$ and carrying current $i_1$ and $i_2$ as shown in the diagram. If ratio of their radii $1:2$ and ratio of the flux densities at $O$ due to $A$ and $B$ is $1:3$ then the value of $\frac{i_1}{i_2}$ will be-

Two thick wires and two thin wires, all of the same materials and same length form a square in the three different ways $P, Q$ and $R$ as shown in fig with current direction shown, the magnetic field at the centre of the square is zero in case

In the loops shown, all curved section are semicircles or quarter circles. All the loops carry the same current. The magnetic fields at the centres have magnitudes $B_1, B_2, B_3$ and $B_4$

Two infinitely long linear conductors are arranged perpendicular to each other and are in mutually perpendicular planes as shown in figure. If $I_1=2 \mathrm{A}$ along the $y$-axis and $I_2=3 \mathrm{A}$ along $- z$- axis and $AP=AB=1 \mathrm{cm}$. The value of magnetic field strength $\overrightarrow{B}$ at $P$ is