How to Build a Cloud Chamber

Warning : Dry ice and isopropanol can be dangerous!

What you will need to build the chamber:

Line the sides of the container near the bottom with cloth or tissue. This lining will be soaked with alcohol when you run the chamber, so do not use alcohol-soluble tape or glue to attach it.

Cover one side of the cardboard with the black electrical tape; this will make the particle tracks easier to see. Place the cardboard, tape-side up, on the sheet of metal, then cover the container with the metal and cardboard, so that the tape is facing the inside.

Use the binder clips to securely fasten the container to the metal/cardboard top. This is to prevent air leaks, so be sure it is tight. Turn the container over so that the metal is on bottom and the felt is at the top. Place the container into the box. Place the slide projector against one side of the chamber so that it shines in.

This is the "dry" configuration of the chamber. You won't see anything yet, but now you are ready to go.

What you will need to run the chamber

In order to run the chamber you will need two additional items: pure isopropanol and dry ice. (You can usually get dry ice at ice cream stores.)

Place the dry ice in the box underneath the chamber, between the box and the metal plate. Make sure that the slice of dry ice is shorter than the sides of the box.

Remove the container from the box, open it, and soak the felt with the alcohol. Also place enough alcohol on the tape so that it is covered with a thin layer of liquid. Clip the metal and cardboard back into place, then replace the chamber on top of the dry ice. Be sure that the metal plate is resting directly on the dry ice. Turn on the slide projector lamp.

What you will see

What else could you do?


Like any experiment, you may find yourself with difficulties. Here are a few common ones and their solutions.

How does this work?

Because there is so much alcohol, the chamber is saturated with alcohol vapor (the gaseous form of alcohol). The dry ice keeps the bottom very cold, while the top is at room temperature. The high temperature at the top means that the alcohol in the felt produces much vapor, which falls downwards.

The low temperature at the bottom means that once the vapor has fallen, it is supercooled. That is, it is vapor form, but at a temperature at which vapor normally can't exist. It as if you had made steam at 95 C.

Since the vapor is at a temperature where it normally can't exist, it will very easily condense into liquid form. When an electrically charged cosmic ray comes along, it ionizes the vapor--that is, tears away the electrons in some of the gas atoms along its path. This leaves these atoms positively charged (since it removed electrons, which have negative charge). Other, nearby atoms are attracted to this ionized atom. This is enough to start the condensation process. So you see little droplets forming along the path the particle took through the chamber.

There is also an exhibit on cosmic rays at the Exploratorium in San Fransisco.

You can also find a page full of links for more information on cloud chambers.

Andrew Foland / Cornell University / Wilson Lab /