One way to measure blood flow when blood vessels are exposed during surgery is to use an electromagnetic flowmeter. This device surrounds the blood vessel with an electromagnet, creating a magnetic field perpendicular to the blood flow. Since blood is a modest conductor, a motional emf develops across the blood vessel. Given vessel diameter d, magnetic field B, and voltage V measured across the vessel, show that the volume blood flow is given by πd²V/4Bd.