| 摘要 |
Apparatus for measuring the distance to and through magnetically permeable materials having a planar surface includes a dipole field generating magnetic field transmitting antenna and a dipole field sensitive receiver antenna. The antennae are displaced from one another in a direction parallel to the planar surface a distance d with the field axis of one antenna, preferably the transmitting antenna, parallel to the planar surface and with the axis of the field of the other antenna intersecting and perpendicular to the axis of the field of the first antenna and substantially perpendicular to the planar surface. With this arrangement, perturbations in the magnetic field caused by the magnetically permeable material induce a signal in the receiver antenna which is proportional to 1/D3 where D is the distance between the antenna and the planar surface and is of the same order as d. The effects of intervening or adjacent fixed or rotating magnetically permeable materials are nulled out geometrically and/or electrically.
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