Central conductor magnetisation

A central conductor is a copper or aluminium bar threaded through a hollow part and connected across the contact heads. The current flows in the bar, not in the part, so no contact is made with the part and there is no burn or arc-strike risk. The circular field encircling the bar links the part and magnetises both the bore and the outside surface, finding discontinuities that lie longitudinal and radial to the bore axis.

Field strength around a conductor falls as 1/r. A conductor centred in the bore therefore produces a uniform field around the whole circumference and magnetises the part in one shot, but the outer surface always sees a weaker field than the bore, so the current is sized on the full outside diameter of the section being examined – the bore diameter plus twice the wall.

An offset conductor is laid against the bore wall. It gives a much stronger local field, which is what large-diameter or heavy-wall parts need, but only over a limited arc. The accepted working rule is that the effective inspection band extends about four conductor diameters around the circumference, measured from the point of contact, and the current is sized on the conductor diameter plus twice the wall. The part is then rotated through a series of positions with at least 10% overlap until the whole circumference has been covered.

The conductor itself must be non-ferromagnetic and big enough to carry the current without heating. A ferromagnetic bar would take flux into itself and distort the field; an undersized copper bar will simply get hot and sag inside the part.