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A comparison of a ferromagnetic core coil and an air core coil as the sensor head of the induction magnetometer Ueda, Hajime


An experimental approach is made to elucidate the input-output characteristics of a ferromagnetic core coil used as a sensor head of an induction magnetometer. Analyses made here are mainly concerned with comparison of magnetograms obtained by the permalloy core coil with respect to the one acquired simultaneously by a large open loop coil, or air core coil. Ratios of the corresponding peak-to-peak values are taken to examine distortions of the output amplitude. Blackman-Tukey's method of power spectrum estimation is applied to a portion of magnetograms, and spectral peaks are compared. The results of these analyses show that no significant distortions or harmonics are recognized in data obtained by the permalloy core coil. This suggests that the response of the permalloy core coil sensor is linear providing that the air core coil sensor is linear in response. The demagnetization effect may be an explanation for the linear response of the permalloy core coil sensors. The demagnetization factor of the core material depends only on the geometry of the core. The permeability of the core material is suppressed in its effect if it has so large a value that its reciprocal value is much smaller than the demagnetization factor: under such circumstances, the sensitivity of the coil does not depend on the permeability of core but on the demagnetization factor. The demagnetization effect also suggests that the longer the core, the higher the coil sensitivity will be.

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