One warning right away: making projectile lauchers is DANGEROUS.
You can easily get hurt by making something which shoots a metallic
object at high speeds. Do not point your experiment at anyone or
anything. Projectiles that go up also come down, and can injure people
or damage objects even if this is not intentional. You may want to test
your projectile launcher only in a padded, enclosed area. There is also
a problem with heat and possibly sparks -- make sure your padding is
A common way that magnets are used to make things move is to put
an iron rod inside an electromagnet coil. Have a look at a doorbell
mechanism (not one of the newer ones with an electronic synthesizer and
a speaker, but an older one with metal chimes). The rod is arranged so
that it is drawn into the solenoid when current flows in the
electromagnet, and a spring pushes it back out when the current stops.
It's often set up so that both swings of the rod end up hitting chimes
on opposite sides -- hence the "ding-dong" of a doorbell.
You can set up something similar, but instead start with a bar
magnet inside of a cylindrical coil of wire. Flowing current one way
around the wire will tend to hold the magnet firmly inside, while
flowing current the other way will tend to expel the magnet.
But how much current? It turns out that if the resistance of your
wires (and the rest of your circuit, including your battery) is low
enough, you can get quite a lot of current to flow through the coil
with even a 1.5 volt battery. It may take some time to build up the
current though. And typical batteries are not designed to supply large
amounts of current -- their voltages drop if the current gets too high
(hooking an electromagnet coil up to a battery isn't much different
from making a short circuit across the battery. And there's a limit to
how much current goes through a short-circuiting wire across an
ordinary 1.5-volt battery).
The current is limited by the resistance of the whole circuit,
whicl likely is mostly in the battery. You can get around this by
hooking up lots of batteries in parallel to supply more current to the
electromagnet. Electromagnets that are used for real-world applications
have power supplies that supply fixed, known amounts of current.
You may want to restrain your projectile inside the coil while the
current builds up, and then release it when the electromagnet is at its
strongest. Keep one end of the electromagnet closed off so that you
know which end your permanent bar magnet is going to go out.
If you open a switch to stop the current in your electromagnet
after you've fired your projectile, be aware that there will be a spark
at the switch. Electromagnets do not easily change the amount of
current flowing through them. That's why it takes some time to get the
current flowing, and to get the current to stop quickly usually
involves dumping the energy stored in the electromagnet into the
switch. This can be reduced somewhat by putting a capacitor across your
switch (pick one that can take hundreds, if not thousands, of volts).
Having more turns of wire in your electromagnet strengthens its
magnetic field for the same amount of electrical current, but increases
its resistance -- it also makes the sparking problem worse.
(published on 10/22/2007)