The ring-core magnetometer ¿ A new type of second-harmonic flux-gate magnetometer

A new type of flux-gate element employing an ordinary toroidal core without air-gap has been developed. By using tape-wound or laminated nickel-iron-alloy ring cores ¿ ID-OD (inside diameter-outside diameter) ratio in the range from 0.85 to 0.98; mean diameter 0.5 to 1.5 in. (inches), or less ¿ with semicircularly wound and differentially connected second-harmonic detector windings, a sensitivity of 1,000 microamperes per oersted or 1 volt per oersted can be achieved. The possibility of utilizing such cores as field-sensitive elements in second-harmonic flux-gate magnetometers and gradiometers is revealed. Although the a-c excitation windings may be supplied either from a rotating machine or from a vacuum-tube oscillator, a preferred method is to combine the 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. Various circuit configurations and different techniques are described for extracting the second-harmonic or total-even-harmonic output information from ring-core flux-gate elements ¿ including the use of such elements in self-balancing flux-gate magnetometers and gradiometers.