One of the parameters not stressed enough in specifications is the importance and influence of pole piece coupling. By instrumentating both the yoke and the test pieces with encircling coils and by comparing the registered flux distributions, a better background knowledge can be achieved in this issue. The influence of a failure to achieve a complete flush seating of the poles as a result of the curvature, misalignment or the undulation of the surface to be tested can thus be evaluated.

The paper reports the flux distributions for a plate, a bar, a pipe and a node specimen and compares the results with H- measurements, surface flux density measurements (T), and pull- off force measurements.

The results show that no more than approximately one half and mostly only one third of the flux generated in the energizing coil of the yoke reaches the area to be tested which, however, is normally quite sufficient. The results also indicate that a failure to achieve a good seating on one of the pole pieces does not lower the flux density significantly, whereas a bad coupling of the flux at both pole pieces is detrimental to the flux density in the inspection zone and, consequently, to the leakage flux formation. This lowering of the useable flux often encountered in practice due to a "line" or "point" contact of the pole piece has to be taken into account while devising specifications by requiring more than two magnetizing directions for critical complicated testing geometries like for fillet welds in inclined narrow nodes.