A zero-flux current transformer is used to measure a current flowing in a conductor (so-called primary conductor). The zero-flux current transformer has an electrical detection winding with which the magnetic flux generated by the flowing current and surrounding the conductor is measured. The zero-flux current transformer further has an electrical compensation winding, with which the magnetic flux is compensated to zero. By means of a control circuit and an amplifier circuit, the current flowing through the compensation winding (compensation current) is adjusted so that the magnetic flux of the conductor is always compensated to zero (so-called zero flux condition). The magnitude of the compensation current needed to achieve the nulling condition is a measure of the magnitude of the current flowing through the conductor, i. for the size of the current to be measured.