  We use a tablet to determine experimentally the dependencies of the magnetic
field (B) on the electrical current and on the axial distance from a coil (z).
Our data shows a good precision on the inverse cubic dependence of the magnetic
field on the axial distance, $B \propto z^{-3}$. We obtain with good accuracy
the value of air permeability $\mu_{air}$. We also observe the same dependence
of $B$ on $z$ when considering a magnet instead of a coil. Although our
estimates are obtained through simple data fits, we also perform a more
sophisticated error analysis, confirming the result for $\mu_{air}$.
