Abstract: A flow metering device includes a very sensitive sensor and detection unit for accurately measuring the flow of fluids such as water through a meter housing. At least two excitation windings are wound around a toroidal core of magnetic material having a non-linear characteristic. The excitation windings are connected in series to an AC source in such a way that the magnetic effects of the windings are additive. A pair of pickup windings are also wound around the core but in series-opposition fashion. The output of the pickup windings is fed to a detector unit including a negative feedback amplifier, rectifier diodes and a summing network. The apparatus is so sensitive that it can accurately detect the movement of an internal magnetic rotor through the rotor housing while rejecting other sources of possible interference.