Longitudinal Magnetic Fields in Magnetic Particle Inspection.

Longitudinal Magnetic Fields in Magnetic Particle Inspection. 

When the length of a component is several times larger than its diameter, a longitudinal Magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a coil shot.
The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much denser inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component.