Permanent magnets can lose their magnetism if they are dropped or banged on enough to bump their domains out of alignment. Can you turn something back into a magnet by banging on it in a specific way? How do you build a powerful electromagnet that will attract a large metal object from a distance of four inches away? Is the number of windings, voltage or current the most important factor in an electromagnet?
It's pretty unlikely, but not impossible, that you could bump a piece of iron and make it a magnet. To bump a piece of iron and turn it into a magnet you would have to bump it in such a way that a perfect vibration travels through the material. The reason that would be hard to bump a piece of iron and make it magnetic is because of the way vibrations propagate in the material. Vibrations radiate out from the point of impact and are going at different angles relative to a straight line - the line you would like the domains to line up with. Non-uniformities that exist in all materials also change the flow of the vibrations in a material.
There are some metal forming operations that can align the material and make a magnet. Usually, stretching a piece of iron will do this. This can happen when the metal is cold-formed or bent. Usually stainless steel is not magnetic, but if you sniff around a piece of bent stainless steel, you might find that it is lightly magnetic around the bends.
Field strength is linear with the current in magnets until the magnet saturates. That means that if you double the current, the field strength will double. After reaching the current that puts as much magnetism as the magnet's core can handle, adding more current just makes the magnet hotter. The amount of windings also are linear in relation to field strength. That means if you double the amount of winding, the magnetic field doubles. At some point, the windings get so far away from the magnet's core that their effect on the core becomes less and less. Changing voltage has a small affect on field strength. We have some of the most powerful magnets in the world at Jefferson Lab and they all operate at fairly low voltage, on the order of 10's of volts, but a few of them might go up to as much as 5,000 amperes of current!
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)