Reluctance of continuous magnetic material is lower that non-magnetic discontinuities. Therefore, flux tends to flow through the parts with lower reluctance so that it is “crowded” and hence the flux density can be significantly increased in those parts (Fig. 1).
The effect occurs for instance in laminated cores (Fig. 2), in which the laminations within single layer are cut in order to accommodate stacking the core around the coil. For interleaved laminations, the within-lamination air gap is usually much greater than the gap between the surfaces of two neighbouring laminations. So the lamination-to-lamination reluctance is significantly lower and the flux tends to bypass the within-lamination gap (Fig. 1).1)
If the within-lamination air gap is large then most of the flux will migrate into the low-reluctance path. The local flux density can double as compared to other parts of the core, in which it will be more equalised (Fig. 1).
At higher operating flux density the local areas can be pushed further towards saturation, hence increasing the local losses and power required for magnetising such core (e.g. as compared to a wound core).