Only magnetic materials are permeable. Scientists model magnetic forces using field lines. A compass lines up along a field line, so you can trace field lines with a compass. Iron filings also line up along field lines. Field lines are defined as coming from the north pole of a magnet and returning to the south pole.
Magnets have field lines that exert a force on magnets or iron.
If there are no field lines, there is no force, the result is magnetic shielding. For more magnetic activites check out the snackbook. If you'd like to order any of the 3 Science Snackbooks that are currently available, please visit the Exploratorium Online Store .