Abstract: The sensor device uses the flux-gate principle and detects and cancels a second harmonic frequency (2f.sub.o) output by a sensor coil (23). The sensor device includes a master oscillator (1) that provides a stable output that is supplied to a generator (2) which generate two signals which respectively have a basic operating frequency (f.sub.o) and a double basic operating frequency (2f.sub.o or 2fc). The basic operating frequency (f.sub.o) is amplified in a power booster (3) and is connected to drive coil (22) mounted on a core (39) in the sensor head (4) to saturate the core (39). The output signal from the sensor coil (23) of sensor head (4) is filtered in a first bandpass filter (5) to obtain the second harmonic of the sensor coil, the output signal of the first bandpass filter (5) is detected in a phase detector (6) that is also connected to receive the double (2fc) frequency. The phase detector output is connected to a second low filter (7).

Abstract: A manufacturing process for use in manufacturing magnetic field sensor head cores, comprises: providing a core formed of an amorphous ferromagnetic material on which is placed an insulating layer; mounting, a first winding that functions as a drive coil for the core over said insulating layer; providing second (37) and third (38) coils on said core (31) that respectively function as a sensor and a compensating coil for the sensor head; both the second and third coils being wound in form of a hollow cylinder 36 inside of which is positioned an assembly formed by the core 31 and the drive coil 32; supplying a first electric current to the core 31 of the sensor head, said core heating up as said first current flows through said drive winding, supplying a second and alternating electric current to that the one 38 of compensating and sensor coils 37 and 38 that surrounds the core 31 and functions as the compensating winding 38; monitoring a signal induced in the other coil 37 that functions as the sensor winding a