Recently, we proposed the high sensitivity ac field measurement (ACFM) technique for detecting and sizing surface cracks in metals [1–2]. This non-destructive evaluation (NDE) technique requires an inducer which possesses a field region with odd symmetry for positioning the probe. When properly orientated in this field region, a single linear probe acts as a differential probe with the added advantage of a phase contribution due to the crack. Both a rectangular coil and a rhombic wire loop can be used as inducers to provide the necessary field. In Fig.1, the positions of the probe with respect to these inducers are shown. As can be inferred from this figure, the probe is coupled to the magnetic field tangent to the metal surface.