A 20 m long uniform copper wire held horizontally is allowed to fall under the gravity ( $g=10 \mathrm{~m} / \mathrm{s}^2$ ) through a uniform horizontal magnetic field of 0.5 Gauss perpendicular to the length of the wire. The induced EMF across the wire when it travells a vertical distance of 200 m is $\_\_\_\_$ mV.
