The flux leakage field is a function of the strength and uniformity of the flux density in the pipe and in the surrounding gas. The flux density depends not only on the strength and orientation of the magnets but on the entire system used to channel the magnetic flux into the pipe. So, the selection of the other magnetizing assembly materials is important.

To reduce the likelihood of flux leakage around the backing bar, materials with high magnetic permeability, such as pure iron and silicon-based transformer steels, are typically used. To further reduce stray flux leakage, the backing bars thicknesses usually exceed the maximum wall thickness of pipe. In this way, the magnetic field in the pipe is not limited by the field that can be carried by the backing bars.

The magnet assemblies and brushes are forced against the pipe wall by the magnetic force of attraction. The brushes are an integral part of the mechanical dynamics of the magnetization system, helping absorb the shock that can result at internal penetrations such as weld roots, restrictions, and dents in the pipeline. Steel brushes are typically used to couple the field from the magnet assembly into the pipe. The magnet assemblies may also be spring loaded against the pipe wall; this type of system helps center the tool in the pipe and it helps absorb and damp vibrations.

Coupling through a solid steel block is also possible but is not currently used. Brushes made of mild steel have a lower permeability than the backing bars, which have a higher permeability. So, large brushes are needed to carry the same flux density as the backing bar. Brushes provide less variation in coupling than sliding steel plates, which were used in some earlier tools, leading to more consistent flux levels in the pipe.

The pole spacing of a permanent magnet system or an electromagnet system is defined here as the distance between the brushes. The pole spacing of an MFL tool is important because it often defines the minimum bend radius that a tool can pass through. The pole spacing also defines the uniformity of the flux density in the pipe wall. Long backing bars (with wide separations between the magnet brushes) lead to uniform flux densities in the material being inspected. Uniform flux densities produce leakage fields that are easier to analyze than nonuniform densities. The effects of nonuniform flux densities are discussed elsewhere in this report.