Permanent magnets, like bar magnets and horseshoe magnets, create
magnetic fields regardless of their surroundings. Many common
substances (water, wood, most plastics, some metals) have very weak
magnetic properties, and the magnetic fields penetrate them nearly
unchanged. Some materials affect the magnetic field quite a bit. Iron
nails, for instance, draw magnetic field lines towards them, where they
travel for a bit, and then come out the other side on their way to the
other pole of your magnet. Putting an iron bar across the poles of a
horseshoe magnet will weaken the magnetic field far away from the
magnet because more of it goes through the iron bar and less to farther
places.
Some liquids have similar effects. You can make a liquid
consisting of a suspension of small iron filings in a gooey fluid
(water won't work -- the iron will just settle out to the bottom).
Gooey oil might do the job better. This liquid acts a lot like the iron
nail above. If you put a magnet close to the surface of the liquid, a
little hill will form on the surface as bits of the liquid are
attracted to the magnet, while gravity wants to hold it down. If you
immerse the magnet in such a liquid, more of the field lines will want
to stay within the liquid than escape, so you might say that the magnet
isn't "working as well" (even though it is, it's just the liquid
getting in the way of the magnet's field far away). You may have a
tough time cleaning this liquid off of your magnet (it's gooey and the
magnet likes to stick to it).
Tom
(published on 10/22/2007)
