New Type of Flux-Gate Magnetometer

The use of an ordinary toroidal core without air-gap as a flux-gate magnetometer has been overlooked. A magnetometer using such a core as the field-sensitive element with semicircularly wound and differentially connected second-harmonic detector windings has been developed. The semicircle portions of a nickel-iron-alloy ring core act here like two separate cores (corresponding to the two parallel nickel-iron-alloy strips or scrolls of conventional forms of flux-gate elements), as far as second-harmonic flux components are concerned. By using tape-wound or laminated (washer-type) Supermalloy cores having i.d.-o.d. ratios in the range from 0.85 to 0.98, a sensitivity of 1000 μamp/oe or 1 v/oe can be achieved. Such ``ring-core flux-gate elements'' make it possible to: (1) use ultrathin, ⅛-mil tape and correspondingly high excitation frequencies, 10–40 kc/sec, (2) make ``point measurements'' by reducing the core diameter to 0.5 in., or less, (3) minimize the magnetizing-current requirements, (4) obtain linear characteristics, (5) eliminate memory effects, (6) facilitate matching of the magnetic characteristics of the two active parts of the flux-gate element which belong here to the same core, (7) detect very small changes in the earth's magnetic field, and (8) apply multiple detector windings on a common core. The power requirements of a portable magnetometer, operated from a 6-v battery, have been reduced to 50 mw by combining the ring-core flux-gate element with a switching-transistor magnetic-coupled multivibrator in such a way that the oscillation frequency is solely determined by the parameters of the ring core and its excitation windings.