How can a magnetic field be modified in such a way to make it smaller than its regular range of action?
well, one way is to make it used, such as takig a large magnet and putting a lot of paperclips on it. it would weaken the force by using it.
If the magnet is enclosed in an iron shell the magnetic field will be confined to that shell. If you have ever been in a medical MRI room they will tell you to remove your wallet and keys and watch. A MRI is a very powerful magnet and could easily grab your keys, ruin your watch and wipe out any credit cards, or such magnetic media. I have worked on these and have stood 25 feet away with a screwdriver and felt it jump and pull in my hand. I have also seen people forget and have to get all new credit cards. As you can imagine it would not be a good thing for somebody to walk by outside of on the ones like I was working on, that were mobile on trailers, that was unshielded, and it certainly would not look good to have cars jumping against the side as the truck drove by. Even though the shielding was only an eighth of an inch thick, standing outside there was no magnetic field.
The only effective way to reduce the effective magnetic field associated with a magnet is to use shielding that bends and confines the magnetic field lines. One of the most efficient materials for doing that is called mu metal (mu for high magnetic permeability). Another approach is to produce an external magnetic filed to cancel out the magnetic field in question. Another approach is to raise the temperature of the permanent magnet, if that is source of your field, to remove the ferromagnetic properties by destroying magnetic domain order. Bozo
