Apparatus includes a first portion of conductive material having varying response to a generated magnetic field along a length of the conductive material, wherein the first portion of conductive material produces a varying eddy current and a varying reflected magnetic field, in response to the generated magnetic field. The apparatus further includes one or more reference portions of conductive material having a relatively invariable response to the generated magnetic field, wherein the reference portion of conductive material produces a relatively invariable eddy current and a relatively invariable reflected magnetic field in response to the generated magnetic field.