Magnetic gradiometry: a new method for magnetic gradient measurements

Instead of using a pair of different magnetic field sensors and differentiating their outputs in order to derive a magnetic gradient value, a single sensitive element can be used for measuring directly a magnetic gradient. This is done by the use of a stiff metallic string clamped at both ends and pumped with an AC current the frequency of which is tuned to the second eigenmode of the string. The string is excited at that eigenmode in the presence of a quasi-static magnetic gradient. Then, the corresponding mechanical displacements of the string can be measured by the use of an inductive technique with an instrumental noise envelope of less than +/- 10(-12) m, per 1 s measurement interval. (C) 2001 Elsevier Science B.V. All rights reserved.