Or its derivative F = Bil x cos{teta} to calculate the force exerted on a wire of length l, under a magnetic field B, in which a current of i is circulating. {teta} is the angle between the wire and the magnetic field. Mmmpf... that was in Physics classes nearly 30 years ago. But I don't think either of these formulas apply, as we are not interested in forces, but in inducted current. I would thought the technology is based on a simple transformer: two coils in parallel; one in the matt, the other in the little white thingy. I wonder what the efficiency of the device is, since the dielectric medium is air. Very low I guess...