Question is as follows,

A copper disc, 250 mm in diameter, is rotated at 300 r/min about a horizontal axis through its centre and perpendicular to its plane. If the axis points magnetic north and south, calculate the e.m.f. between the circumference of the disc and the axis. Assume the horizontal component of the earth’s field to be 18 µT.

The answer give is for this is 4.42 µ V. How do I get this?