Metal detector works on the principle of

A copper disc of radius 0.1 m rotates about an axis passing through its centre and perpendicular to its plane with 10 revolutions per second in a uniform transverse magnetic field of 0.1 T . The emf induced across the radius of the disc is

The self inductance of a coil depends on

A conducting circular coil is place in a uniform magnetic field with the magnetic field initially directed perpendicular to the plane of the coil. In step $A$, the coil is rotated from its initial position by $60^{\circ}$ about its diameter in time $t$. In step $B$, the coil is further rotated about the same axis in the same sense by another $120^{\circ}$ in time $2 t$. Ratio of emf induced in the coil in step $A$ to that in step $B$ is