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Our story starts with Lord Kelvin, one of the great old school physicists. You can read about his career from John Walker’s old Saturday Night Science. Actually, at the point he enters this story I don’t think Kelvin had made lord yet; he was just some bloke named Thompson. This Thompson fellow was playing around with magnets and electricity and that sort of thing. What he discovered is that you can change the resistance of a wire with a magnetic field. And furthermore that that change in resistance depends on the angle between the wire and the magnetic field.
Let’s take that a little more slowly. Change in resistance when you’re in a magnetic field? Okay, I can buy that; there’s all this nonsense about wires and magnets and whatnot that I’ve been blathering about up until this point. Angle? The resistance in your wire will vary a great deal whether it’s parallel or antiparallel to the magnetic field on your disk. (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.) If your wire is running current right-to-left and your magnetic field is pointed left-to-right then your wire’s resistance is at it’s highest because of your antiparallel configuration.