  We measure the magnetic field produced by a linear octupole formed by four
magnets placed in such a way that both their magnetic moments and their
quadrupole moments cancel each other. The magnetic field is measured with the
magnetic sensor of a smartphone together with a suitable app, using the Android
operating system. The main objective is to determine the dependence of the
x-component of the magnetic field with distance and compare with the
theoretical result that tells us that this dependence is a quintic decrease.
