Winding the coils around these magnetic cores creates a sort of magnetic flux superhighway and greatly increases the resulting inductance values. Magnetic flux is the term used to describe the invisible magnetic field lines that compose a magnetic field. The first picture shows a representation of the magnetic flux lines generated by an air core inductor while a current is passed through the coil. The second picture represents the concentrated flux path and increased magnetic flux density generated using the same coil and current value, but with the advantage of a ferromagnetic core.