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by Shawn Carlson
Jan, 2001

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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."

Figure 1 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.

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.

Next obtain a short length of PVC pipe with an outer diameter that is just slightly smaller than the mouth of the canteen. Cut a hole in the cap to accommodate the pipe, which should reach down to about two centimeters above the bottom. Glue the top end into the hole in the cap. Then stretch some plastic window screening across the top of the pipe. The mesh reduces turbulence in the fluid, thereby reducing turbulence in the air inside the chamber. Secure the screening with a nylon cable tie. Then punch a small hole in the center of the screen. (You'll need this opening for access to the chamber.) Finally, glue the cap into the glass using plenty of silicone cement and attach the lower stopcock just as you did the upper one.

Figure 2: WINDOWS for viewing particle tracks and for adjusting the liquid level are formed by placing tape on the glass before spraying it with black paint.

Now wrap two strips of adhesive tape most of the way around the drinking glass, positioned a few centimeters from the base. Run a third strip vertically along one side. Then spray-paint the glass a flat black to improve contrast and make the particle tracks easier to see. You can now create viewing ports by removing the tape. To monitor the changing fluid level, make a photocopy of a ruler and glue your paper scale beside the vertical window.

Adding baking soda to the liquid while everything is sealed up is simpler than you might think. Just epoxy a ceramic magnet into a small bag fashioned from the toe of a nylon stocking. I suggest you begin by adding 2.5 milliliters (about half a teaspoon) of baking soda to the bag, but you'll probably need to adjust that amount after some trial and error. Place the baking soda on a small piece of paper towel and insert this makeshift holder into the bag. Affix the bag just below the neck of the bottle using a stack of two magnets on the outside; removing the outer magnets releases the baking soda into the solution. I put as many as five bags inside at once so that I can repeat the experiment without opening the canteen.

Mix two liters of liquid by combining equal parts of distilled vinegar and the most concentrated isopropyl alcohol you can find. Squirt in some ink, which (like the black spray paint) will make the tracks easier to see, and add two milliliters of salt. Fill the generator bottle with this solution to within about one centimeter of the nylon bags. Open the top stopcock and then screw the glass cloud chamber on tightly. You may need to cover the threads with Teflon tape or petroleum jelly to get a pressure-tight seal. Insert the whole assembly in its holder. Now open both stopcocks, carefully suck on the tube until liquid fills the cloud chamber about halfway, then close both stopcocks.

Position a bright light to one side of your viewing port. Drop in a bag of baking soda and monitor the fluid level in the cloud chamber, bleeding the pressure with the lower stopcock when the compression ratio rises above about 1.33. (If this ratio is below about 1.25, tracks won't materialize, and if it is above 1.38, a dense cloud forms and obscures everything.) Wait a minute or so, then rapidly open that same stopcock completely.

Particle tracks can appear only during the next brief instant, so the odds of seeing a vagabond cosmic ray are not at all good. A potent source of either alpha particles (helium nuclei) or beta particles (electrons) provides a much more satisfying show. Alpha particles produce short tracks, whereas beta particles leave long ones. You can obtain both an alpha and a beta source suitable for this project from the Society for Amateur Scientists.

Epoxy your radioactive source to the tip of a nail that has a large, flat head. Lift this assembly through the hole in the window screening and into the cloud chamber using a drinking straw and stick the head of the nail to the magnet at the top of the glass. This arrangement will hold the source securely in place until you want to swap it for another one.

Though easy to build, my cloud chamber has its limitations. The optical quality of most drinking glasses is poor, which can make the tracks hard to see. And reloading the device with packets of baking soda can be tedious. So despite the failure of my teenage attempts, you might want to consider other designs of this kind offered previously in the Amateur Scientist (check April and December of 1956). Prospective builders should also consider the so-called diffusion cloud chambers (described in the Amateur Scientist columns of September 1952 and June 1959), which require little more than dry ice and alcohol--isopropyl alcohol that is, not champagne.

The Society for Amateur Scientists
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East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

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