An instrument used for detailed studies of the Earth's magnetic field on a local basis that uses the flux-gate, which consists of two identical saturable cores of high permeability, with identical, but oppositely wound, coils. An alternating current in these coils magnetizes them first with one polarity, then in the opposite sense. If an additional field is present, such as the Earth's field, it will add to the flux in one coil while decreasing that in the other, resulting in different voltage drops across the two coils. This difference is proportional to the unvarying field, which can be measured by noting the average voltage difference between the two halves of the flux gate. In use, a part of the Earth's field is balanced out by an additional winding surrounding both cores and carrying direct current. In airborne use, the recording flux gate is kept aligned with the magnetic field by the use of two additional flux gates. When these are at right angles to the Earth's field, they generate no voltage, but if they depart from this position, they can generate voltages which operate motors returning them to proper alignment. In this fashion, the recording element is held always parallel to the total field.