ORDINARY DRINKING GLASS becomes a cloud chamber, a device for viewing the trajectories of charged particles. The plastic canteen provides a reservoir of vinegar, alcohol and ink, which are used to form a fluid piston within the glass. Mounting the assembly on a wooden stand with hose clamps and a bent length of steel reinforcing bar helps to hold the glass firmly on the canteen.

When air cools, the water vapor it holds eventually condenses to form a cloud. But as scientists have long known, air can be chilled well below its normal dew point without such condensation taking place. The trick is to remove the dust particles on which the water droplets normally form. The cooled air can then become "supersaturated."

So what? Well, in 1896 a University of Cambridge physicist named C.T.R. Wilson discovered that certain subatomic particles leave visible trails when they pass through supersaturated vapors. Why? The particles convert some neutral atoms in the air into charged ions, which, like dust specks, induce droplets to form. Wilson thus was able to fashion the first "cloud chamber" to reveal the trajectories of these ionizing particles.

While in high school, I spent many frustrating hours trying--and failing--to build cloud chambers from instructions I had read in this department. Then, as a college sophomore, my interest was rekindled when I noticed that a cloud had formed in the neck of a freshly popped bottle of champagne. Within two hours I had converted that bottle into my first working cloud chamber. My design has since evolved, but it remains quite simple and inexpensive to build. The current version costs less than $30 to put together.

The "generator" (a canteen) is filled with a mixture of vinegar, alcohol and ink. It's pressurized by adding baking soda. The carbon dioxide given off forces the colored liquid out and into an attached drinking glass, where the fluid acts like a piston to squeeze the gas within. The compression heats the air and causes it to become saturated with vapor from the liquid. Opening a valve allows the fluid piston to drop, which lowers the pressure and temperature of the air, which in turn supersaturates it.

I use a one-liter plastic canteen with flat sides and a wide mouth. The cap, being slightly tapered, fits snugly inside a tall drinking glass. If you find that the cap rests so deeply inside your glass that the canteen cannot be attached, ask the folks at a local glass shop to cut off some of the rim. You might also ask them to bore an off-center hole in the base of the glass for a stopcock--or do it yourself. Surprisingly, it's not hard to drill glass. Just cut several notches in the end of a piece of brass tubing. Then put it into the chuck of an electric drill and turn the notched end against the glass while bathing the surface with a slurry of number 120 Carborundum powder and water. Apply a gentle but steady pressure. It's best to use a drill press, but the job can be done with a handheld electric drill. Wear suitable eye protection (as always, when working with power tools) and gloves, in the event the glass should shatter.

Though unlikely, it's conceivable that your glass could break when it is pressurized. So you should also wear your safety glasses when experimenting. And you can add a further level of protection by coating the glass with plastic. Ace Glass in Vineland, N.J. (800-223-4524 or 856-692-3333), sells a special plastic coating (catalogue no. 13100-10) designed to hold the glass shards together in case of a catastrophe. Half a liter costs about $30.

When your protective coating has fully dried, pass the threaded brass fitting through the hole, seal it carefully with silicone aquarium cement, secure it with a washer and nut, and add the stopcock. Also, find a supply of small ceramic magnets (Radio Shack catalogue no. 64-1883 contains five such magnets) and glue one inside the glass at the top center using silicone cement.

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