We want to find the magnetic field B due to an infinite slab of current, using Ampere's law. The figure shows a slab of current with current density $\overrightarrow{J}=J\widehat{z}$, where dimensions of $j$ are amps per square meter.  The slab of current is infinite in the x and z directions, and has thickness $d$ in the y-direction.

Magnetic field at $y<d/2$ is  $n{\mu }_{0}jy$. The value of $n$ is….