Steel has a substantially lower resistance in this respect than air. Consequently, free ends of plates can have a lower level of magnetism than a joint including the same plates.

The level of residual magnetism can vary throughout the joint when the distances between the steels in the joint vary. For instance, in a single-V joint, the distance between the parent metals in the joint are shortest at the root section. Therefore, there is a higher level of magnetism compared to other parts of the joint where the space between the steels is greater, see illustration below.

Magnetic fields in a single-V prepared joint.

The magnetic flux density, therefore, should be measured at the location of the next weld pass in order to determine the risk for magnetic arc blow. A hand-held gauss meter is a typical instrument used for this purpose.

The geometry in joints of larger plate thicknesses can cause an increased risk for magnetic arc blow. Welding in this condition means that a substantial fraction of the arc can be surrounded by the parent metals at a close vicinity. It can especially occur during root passes and tack welds.

Since the magnetic field flows easier through steel than air, an unsymmetrical magnetic field can be formed around the torch. This effect may cause arc blow.

Similarly, arc blow can occur when welding plates of different thicknesses, according to the illustration below. In this situation, the arc may deflect towards the thicker plate in the joint.

Arc blow due to different plate thicknesses in the joint.