The Eccentric Geomagnetic Dipole

Abstract

Summary A rock spinner magnetometer using a coil as a detector has been designed and constructed, incorporating several fundamental improvements. A detailed study of pick-up coil design has led to an instrument which can take fast measurements on weakly-magnetized rocks. The reference signals for demodulation of the signal induced in the pick-up coil are obtained from a reference oscillator rather than from the shaft by optical or magnetic means. This allows the instrument to be calibrated directly for magnetic moment and simplifies angular calibration. It also permits the electronics to be serviced and the system to be calibrated without the motor running. A phase-locked loop has been used to lock a dc motor onto one of the reference signals provided by the reference oscillator. A dc type motor was chosen in preference to an ac type in order to reduce interference from stray fields. A careful study has been made of offset drifts and several features have been introduced to minimize these undesirable effects. Amongst these are bearings designed to absorb vibrations, and a belt drive between the motor and shaft. The performance of the magnetometer compares favourably with that of two commercially produced magnetometers, the ten degree confidence limit of the instrument being 2.2 x lo-'' A m2 (1 -8 x lo-* gauss) for a six-spin operating sequence.