Very nice question! Magnets are really a lot of fun for a lot of
reasons, and many of us never get tired of experimenting with them.
Magnets come in all shapes and sizes, and are used for a great
variety of purposes. The shape of a magnet determines how the magnetic
field lines are arranged outside of the magnet, which affects what the
magnet can be used for.
All of the methods you mention have been and continue to be used
to manufacture magnets. Some small ones may be stamped out of sheet
metal. Others may be cut from blocks of metal. Others still may be
poured into molds. Refrigerator magnets are probably made of molten
plastic with iron powder mixed in, poured on a flat surface, and then
cut into rectangular (or other) shapes. The magnetization procedure
usually involves heating up the magnet and then cooling it off with an
external magnetic field applied.
Some magnets are electromagnets. If you flow an electrical current
through a coiled wire, it will produce a magnetic field. The strongest
and largest magnets ever made are electromagnets. Very large magnets
are used in high-energy physics experiments and can be several stories
tall. They usually consist of many turns of very thick wire around in
the shape of a big cylinder. The wire is often superconducting, so the
whole thing has to be kept at liquid helium temperatures (we're not so
good yet at building wires out of high-temperature superconductors).
Superconductors are chosen to reduce the cost of flowing the thousands
of amperes of electrical current through the wire -- no resistance, no
energy gets lost!
That may pretty much answer your question, but as long as we're on
the topic, here are some more thoughts about strange magnet shapes.
Some of these may not be easy to understand except for readers who've
had a physics course.
Other magnets can be made in other shapes to accomplish other
things. Some magnets have more than one North pole and more than one
South pole. A simple extension is a "quadrupole", with two of each.
These tend to be square on the outside with alternating N and S poles
pointing inwards. They are used to focus beams of high energy
particles. You can make sextupoles and octupoles with three and four
each of the North and South poles. A refrigerator magnet also has many
North and South poles so the magnetic field lines can leave the magnet,
flow through your refrigerator door and come back in again so it will
stick well even if only part of the refrigerator magnet is in contact
with the door, or if the magnet is cut up in pieces, the pieces will
still stick well to the door (you can't cut it up so there's only a N
pole or just an S pole -- there will always be two!).
Even your television (if it's not one of the new plasma or LCD
kind) has an electromagnet inside to steer the electron beams across
the television screen.
Tom (w. mike)
(published on 10/22/2007)