Michael Faraday unsuccessfully attempted to measure the potential difference across the River Thames due to the flow of water (which is a good conductor due to dissolved salts) in the earth's field. At Waterloo Bridge, where Faraday made his attempt, the river is 200 mm wide and flows eastward at 1.8 m/sm/s . The earth's magnetic field here has a magnitude of 50 μTμT and is tipped 65∘∘ from the horizontal, so it has both horizontal (north) and vertical (down) components.

What is the magnitude of the potential difference?