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A detector used to indicate the direction of the earth's magnetic field.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
an instrument for indicating the field strength of an external magnetic field, as that of the earth: used in some gyrocompasses and magnetometers. Also called flux′ valve`.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.