Click here to purchase the entire book in PDF format. Permeability Back at the beginning of Section 2.8, I made a very short, passing reference to the fact that an iron bar is a great conductor for magnetic lines of force. Of course, iron can also conduct electrical current, but that's a completely different issue right now. If you have a magnet, there will be lines of magnetic force around it. If you put an iron bar near (but not attached to) the magnet, the lines of force will find it easier to go through the iron than the air, so they'll concentrate into the iron. This is the intuitive way of thinking of things. The more technical way is to measure the magnetic flux density, $B$, inside the iron bar, then measure the magnetic field strength, $H$, that's applied by an external magnet. The ratio of these two is called the permeability of the iron. It's the magnetic equivalent of resistance. The higher the permeability, the easier it is for the magnetic lines of force to ``travel'' through the substance. The symbol for permeability if $\mu$ and it's measured in henries per meter (H/m).