What is magnetic fringing at the air gap of an electromagnet?

Magnetic fringing at the air gap of an electromagnet refers to the phenomenon where magnetic lines of force spread out or "bulge" as they pass from a region of high permeability (the magnetic core) to one of low permeability (the air gap).

Because magnetic lines of force tend to crowd into permeable materials, they naturally diverge when exiting the core into the air. This bulging effect causes the magnetic flux density—the number of lines of force per unit area—to be reduced within the gap compared to the core. This effect becomes more pronounced as the length of the air gap increases. In engineering design, while this behavior makes precise calculations difficult, rough corrections are typically applied to account for the fringing effect when determining the necessary current to achieve a desired flux.1