Abstract: An apparatus, that is particularly advantageous for compensating for the earth's magnetic field at a fluxgate current sensor. The apparatus and method actively compensate for local anomalies and loop mismatch at local segments distributed around closed loops of ferromagnetic material in order to null out sources of error that arise in the presence of an external magnetic field. External flux nulling circuits null the external magnetic flux at each of a plurality of associated pairs of diagonally opposite segments of coaxial closed loop cores. Each flux nulling circuit has a pair of diagonally opposite segment sensing coils and a pair of diagonally opposite segment cancellation coils. A flux detecting circuit detects the net magnetic flux in associated, diagonally opposite segments. A negative feedback control circuit drives the segment cancellation coils with a current to drive the flux detected by the segment detecting circuit to a minimum.