Summary
Three orthogonal coil systems have been constructed within a cube of side 2.5 m to cancel the geomagnetic field to less than 1 gamma over a large volume. The north-south and vertical components are cancelled by systems of four square coils whilst a square Helmholtz pair cancels the east-west component. Cancellation to 1 part in 105 (0.5 gamma in Canberra) is achieved within a sphere of diameter 35 cm, more than 20 times the field-free volume which could be obtained using conventional Helmholtz coils within the available space. Three fluxgate detectors monitor the residual field and their output, taken through a feedback circuit, allows compensation for field variations. The residual field can be maintained to less than 0.5 gamma over the working volume of about 22,000 cm3 during the critical part of a laboratory experiment.
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McElhinny, M.W., Luck, G.R. & Edwards, D. A large volume magnetic field free space for thermal demagnetization and other experiments in palaeomagnetism. PAGEOPH 90, 126–130 (1971). https://doi.org/10.1007/BF00875516
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DOI: https://doi.org/10.1007/BF00875516