This cloud chamber was made with a gallon glass jar, an aluminum plate, silicone caulk to attach them together, a couple of sponges, and a plexiglass lid. There is a foil electrode in the jar which is not visible in the video, charged to 300 volts (AC, i think) with an external power supply. This prevents the vapor trails from lingering, and allows the new trails to be more visible.
We poured 99% isopropyl alcohol into the sponges, which are attached at the top of the jar. The jar is attached to the plate and the plate rests on 2 blocks of dry ice ($6) from the ice cream distributor down the block.
The radioactive mineral sample in the jar emits particles which leave ionization trails behind. The trails act as seeds for the supersaturated alcohol vapor -- you can see the little lines of vapor appearing in the space around the rock. It takes 15 to 30 minutes to get cold enough for trails to show, and the dry ice we used would allow it to operate for about 6 to 10 hours.
This is a cheap and easy experiment that shows you something really interesting that happens around us all the time. We will definitely be making a larger chamber with an insulated dry ice compartment and a larger viewing area.