A metallic rod of $1 \,m$ length is rotated with a frequency of $50$ revolution per second, with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius $1\, m$ about an axis passing through the centre and perpendicular to the plane of the ring. $A $ constant uniform magnetic field of $1 \,T$ parallel to the axis is present everywhere. The emf between the centre and the metallic ring is