Demonstrations of magnetic phenomena: Measuring the air permeablity   using tablets
  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 $\mu_{air}$. 
Vitor O. M. Lara, Dayanne F. Amaral, Daiara Faria, Leonardo P. Vieira
physics.ed-ph