The impedance of the two coils does not relatively change linearly with the position. All simple variable coils of general types are introduced which are non-linear in nature, and cannot be cancelled in a voltage divider. The coils must be made linear or a linearity which is controlled is introduced. The use of voltage of high- frequency is made essential by the lower values of induction involved. The output is always in A.C. current which is then transformed into D.C. current by a simple R.C. filter.