Magnetic leakage flux processes
Non-destructive testing (NDT)

The theory of magnetic circuits indicates that the presence of an air gap corresponds to a strong local increase in the circuit's reluctance and therefore in the magnetic potential difference (m.p.d.), thus constituting an obstacle to magnetic flux, whose lines of force must then spread out laterally according to a leakage flux, as shown in figure 10 . This dispersion of flux lines occurs even with a very small air gap, as the reluctance ratio between air gap and circuit is inversely proportional to the relative permeability of the latter, i.e. a ratio of 500 to 1,000 for a ferromagnetic steel circuit excited below saturation.

This effect...