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 m wide and flows eastward at 2.4m/s . The earth's magnetic field here has a magnitude of 50 ? T and is tipped 65?from the horizontal, so it has both horizontal (north) and vertical (down) components.