Cartesian diver
A clear plastic soda bottle is filled with water. A float is
placed in the water and the lid tightly closed. You squeeze the
bottle and the float slowly sinks. Quit squeezing and the float
rises. Apply the right amount of pressure and the float
remains suspended without rising or descending.
The only tricky part to setting this up is making a float
which is just barely suspended by water. There are many ways
to do that but all require some trial and error. The easiest
is probably a medicine dropper. If it will not sink get more
water in the dropper. If it will not float get more air in it.
A plastic pen cap with a ball of modeling clay stuck on the
clip end is sometimes used. Another type of float is made by
straightening a wire paper clip. Then slide an inch or so
length of plastic straw to the middle of the wire. Bend the
wire and straw sharply at the middle to form a U. Air trapped
in the straw will keep it afloat. A wire bent into a hook at
one end will assist in placing and retrieving the diver during
the trial and error period of getting the right amount of air
in the straw. You may even be able to add pinwheel vanes cut
from thin, stiff plastic so that the diver spins as it rises
and descends. Still another way to make a float is to use the
barrel of a Bic or similar ballpoint pen. Pull out the ink
tube with a pliers. The barrel can be shortened by cutting
with a hacksaw or using a sawing motion with a serrated edge
knife. Tape on a piece of coat hanger wire or a small nail as
ballast. One way to cut wire is to bend it back and forth
sharply at one spot using pliers. Fine adjustments to change
bouyancy can be made by sliding the end cap in or out
slightly. This float works best because it can contain a large
amount of air.