Needles $N_1, N_2$ and $N_3$ are made of a ferromagnetic, a paramagnetic and adiamagnetic substance, respectively. A magnet when brought close to them will

The strength of the Earth's magnetic field is

A jet plane having a wing-span of 25 m is travelling horizontally towards East with a speed of $3600 \mathrm{~km} / \mathrm{hr}$. If the Earth's magnetic field at the location is $4 \times 10^{-4} \mathrm{~T}$ and the angle of dip is $30^{\circ}$, then the potential difference between the ends of the wing is