Then quickly release the air by suddenly opening the oil-drum valve. The moment the air was released you should have seen fog form in the top of the chamber. If a fog did not form, increase the pressure about in on the manometer and again release quickly. Repeat the process slowly increasing the pressure until fog is formed in the upper chamber. If more alcohol or water must be added to the chamber, squirt some on the sponge and velveteen using an eye dropper inserted through one of the stoppered holes.
Fog is formed because the air within the chamber is cooled when the pressure is quickly dropped.
Since warm air will hold alcohol or water vapor to the point of saturation, it becomes supersaturated when cooled and the excess vapor tends to condense out of the air. At first the excess vapor condenses on dust particles which are present in the air. These fall to the bottom of the chamber, due to the pull of gravity, and carry the dust particles with them. This accounts for the fog you see during the first 10-20 expansions. For this reason the battery-powered electric sweep field is set in the chamber to attract these scattered ions and remove them from the chamber.