After a demonstration, my professor placed a metal plate over the two ends of a horseshoe magnet in order to prevent it from attracting other materials. I was wondering if this has to do with there being some type of "magnetic resistance" between the poles, where it is easier for the field to be setup inside the conductor than outside of it, so there would be less outside field to attraction. I asked my professor, and he explained to the effect that the magnetic field lines travel the shortest distance from pole to pole (most of the field lines are being channeled through the plate). I was wondering, why wouldn't the field lines travel the shortest distance in air, without the plate present? Is there a law that explains how the magnetic field becomes enclosed through a conductor? Any help will be greatly appreciated.