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A current-carrying conductor is surrounded by a pair of ring cores. A sense winding on one core provides flux rate feedback to a power amplifier which drives the ampere-turn compensating current through a common compensating winding. The other core serves as a second-harmonic modulator to establish zero-flux operation and thus to ensure a perfect, temperature-independent current balance. A specially-developed burden resistor converts the compensating current into a voltage signal, which is amplified to give a 10 V output signal at the nominal value of the current to be measured. A substantial number of devices, ranging from 50 to 25000 Amperes is now operational in the beam transfer and extraction power supplies for the CERN SPS.