The magnetic core of coil B is a better path for magnetic lines of force than the nonmagnetic core of coil A . The soft-iron magnetic core's high permeability has less reluctance to the magnetic flux, resulting in more magnetic lines of force. This increase in the magnetic lines of force increases the number of lines of force cutting each loop of the coil, thus increasing the inductance of the coil. The inductance of a coil increases directly as the permeability of the core material increases. The fifth factor is the number of layers of windings in the coil.
Inductance is increased by winding the coil in layers.
Figure 6-10 shows three cores with different amounts of layering. Coil A is a poor inductor compared to the others in Figure 6-10 because its turns are widely spaced with no layering. The flux movement, indicated by the dashed arrows, does not link effectively because there is only one layer of turns. Coil B is a more inductive coil. The turns are closely spaced, and the wire has been wound in two layers.