The electric current in a circular coil of \( 2 \) turns produces a magnetic induction \( B_1 \) at its centre. The coil is unwound and is rewound into a circular coil of \( 5 \) turns and the same current produces a magnetic induction \( B_2 \) at its centre. The ratio of \( \dfrac{B_2}{B_1} \) is:
