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 THEOREM 4

If the course of magnetic field lines in ferromagnetic material is disturbed, a stray field is generated at the point of disturbance!

It is of crucial importance, therefore, that a defect actually means a disturbance for the magnetic field lines.


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Figs. 5.1 to 5.3 show three principal possibilities of a defect. As in the previous example with the iron ring, in fig. 5.1 the defect is at right angles to the field line direction and thus causes a stray field. Thus, after application of iron powder, this defect could be detected.

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However, if the field line direction has the same orientation as the defect (fig. 5.2), this results in only a very insignificant influence on the field lines, which does not cause a stray field. A defect of this type cannot be detected. In the case of defects which are now below the surface, such a clear decision is not possible, because in this case further factors have to be taken into account.

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Basically, however, it can be seen that the field line direction and the fault direction must be transverse to each other in order to generate a stray field. The angle between the two directions should ideally be 90 degrees, although angular positions of up to a minimum of 30 degrees are also sufficient.

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