Magnetic Field Intensity in a Core

Based on Ampere’s law, the formula for the magnetic field intensity in the magnetic circuit is:

$$\begin{array}{rl}\oint \vec{H}\cdot d\vec{l}& =I_{\text{net}}\\ Hl& =NI\\ H& =\frac{NI}{l}=\frac{\mathcal{F}}{l}\left(\frac{A.t}{m}\right)\end{array}$$

where $l$ is the mean path length of the magnetic core.

According to the relation $H=\mathcal{F}/l$, magnetic field intensity represents the magnetomotive force drop across each unit of length of the core.