`  == Sentence in need of attention in ``Limitations of the magnetic compass``, and some expansion. ==  Doing the expansion first, the sentence: ``Local environments may contain mineral deposits and human sources such as MRIs.`` could profitably have added to it far more common sources such as ``..., loudspeakers, computer hard drives, magnetic door catches, magnetic door seals (such as on refrigerators), television and computer monitors, and a variety of common household tools and appliances that contain electromagnets or electric motors. High-current domestic appliances, such as electric welders and the whole gamut of electric cooking and heating devices, can create significant temporary magnetic fields whilst in operation``.    The sentence  ``Vehicles may contain ferrous metals, which may pick up their own fields.`` seems to be in need of some attention. Perhaps something along the lines of: ``Vehicles may contain ferrous metals, which may influence the direction and strength of the local magnetic field. These metals can themselves become magnetised, which may influence the local field even further``.   Anyone have any problems if I make these changes?   `