This is not a homework problem, just the musings of a bored mind. Suppose I plumbed a loop from some non-conductive pipe, say PVC. In this loop I placed a pump, also non-conductive. Let's say I place the pipe in a powerful magnetic field, perpendicular to the flow of fluid in the pipe. If I fill the plumbing with mercury and run the pump, a voltage should appear as the mercury passes through the field. My question is does the potential created only drive a current through the entire length of mercury, thereby short circuiting the whole thing, or if I place a couple of electrodes in the flow, perpendicular to the field and the flow, will I get a usable current? Part of my mind wants to say that the voltage appears at right angles to the flow and field, and should be extractable there. But another part says that because the mercury is a conductor, the whole shebang would be shorted. What do you think?