A cloud chamber is a device used to visualize the paths of charged particles, such as those produced by radioactive decay. It contains a supersaturated vapor, usually of water or alcohol, which condenses into tiny droplets when a charged particle passes through. This creates visible trails that can be photographed and analyzed. Cloud chambers were invented by Charles Thomson Rees Wilson in the early 20th century. They played a significant role in the study of particle physics and helped scientists discover various subatomic particles, contributing to our understanding of the fundamental forces of nature.