To cool down our apparatus we will use crushed dry ice, with is the only part of the experiment that is not readily available. Dry ice is sold in all kinds of odd places, and I found several sources in Pasadena by simply looking in the Pasadena Yellow Pages under Dry Ice . For a single experiment you will need about 10 pounds of dry ice, although for running many experiments at once it's probably sufficient to have a few pounds per experiment. Other than being cold, it's perfectly safe, as is consists of nothing more than solid carbon dioxide. This works best on a hard surface, like concrete or asphalt.
Dry ice is much softer than water ice, and it crushes very easily.
Fill the cups to the top, and cover with a piece of cardboard cut to shape, or with some paper towel strips. Small ice crystals should begin forming on the string after 5-10 minutes, and after 20 minutes you should have a pretty good bunch of crystals. A magnifying glass is useful, but not essential for crystal viewing. When things get crowded, you can pull the top off the chamber, wipe the string clear with your fingers, and try again. One charge of dry ice will last about six hours, and more can be added as needed.