Abstract: An oscillator (10) supplies via a first feeding path a fixed frequency signal to the fixed inductance (L.sub.o) connected in series with a variable inductance (L). The signal at the common connection point of the two inductors (RW1) is used to provide an output signal (U.sub.act) and a control voltage (V) for controlling the amplification of an inverting amplifier (B3) in a second feeding path from the variable oscillator (10) to the inductance (L). The signal from the common terminal (RW1) is processed in a differential amplifier (30) and an amplification control circuit (40) comprising a rectifier (B4, S), a delay element (VZ1), an integrator (I) and a sample-and-hold circuit (S & H) controlled by a synchronization output of the oscillator stage (20). The controllable amplifier (B3) is the part of a multiplier (50), which may comprise analogue or digital components.