# Cloud Chambers (DD-623)

> A cloud chamber makes the paths of electrically charged particles visible. Inside, the air holds more vapor than it normally could, so it is supersaturated and ready to condense. A fast charged particle knocks electrons off molecules along its path and leaves a line of ions behind it. Vapor condenses on those ions as tiny droplets, which form a thin trail. The trail doesn't glow: it shows up because a lamp lights it against a dark background, and it fades within moments. The Scottish physicist C.T.R. Wilson built the first cloud chamber at Cambridge's Cavendish Laboratory in 1911. His version used water vapor and made it supersaturated with a sudden expansion. He shared the 1927 Nobel Prize in Physics for the method. Many classroom chambers today are diffusion chambers, which keep alcohol vapor supersaturated over a very cold plate, so tracks form continuously. The shape of a track hints at the particle: alpha particles leave short, thick tracks and electrons leave thin, wandering ones. In a magnetic field, the tracks curve, which reveals charge and momentum. That is how Carl Anderson identified the positron, the first known antiparticle, in 1932.

- Department: Science (also History)
- First published: 2025-07-26
- Last updated: 2025-07-26
- Permalink: https://detailenjoyer.com/case/cloud-chambers/
- Author: Marcus Easterling (DetailEnjoyer), The Detail Department

## On the timeline

- **1911**: C.T.R. Wilson builds the first cloud chamber at Cambridge's Cavendish Laboratory
- **1927**: Wilson shares the Nobel Prize in Physics for the cloud chamber
- **1932**: Carl Anderson finds the positron with a cloud chamber

Full timeline: https://detailenjoyer.com/timeline/?case=cloud-chambers

## Quick facts

- **DD-623-F01**: Cloud-chamber tracks are droplets condensed on ions. _(Write-up · Exhibit A (Why droplets form))_
- **DD-623-F02**: C.T.R. Wilson built the first cloud chamber in 1911. _(Write-up · Exhibit C (Wilson's chamber))_

## Write-up

Radiation, made visible. Charged particles leave droplet trails in supersaturated vapor.

1. **Why droplets form**: The chamber's vapor is supersaturated. A charged particle leaves ions along its path, and the vapor condenses on them as droplets.
2. **Not glowing**: The droplet trails scatter light from a lamp. They don't give off light themselves.
3. **Wilson's chamber**: C.T.R. Wilson built the first cloud chamber in 1911. A sudden expansion cooled moist air until it was supersaturated. He shared the 1927 Nobel Prize in Physics.
4. **Diffusion chambers**: Many modern chambers keep alcohol vapor supersaturated over a cold plate instead, so tracks form all the time.
5. **Reading the tracks**: Track length and thickness hint at the particle. In a magnetic field, the curve shows charge and momentum, which is how Anderson found the positron in 1932.

## Related case files

- [Cherenkov Glow (DD-020)](https://detailenjoyer.com/case/cherenkov-glow/): Another way radiation becomes visible
- [Nuclear Fireball (DD-052)](https://detailenjoyer.com/case/nuclear-fireball/): Another case about ionizing radiation
- [Point Curie (DD-669)](https://detailenjoyer.com/case/point-curie/): Another physics demonstration

## Reel vs. record

- **Note**: 'Glowing tracks': the droplets don't glow. They show up because a lamp lights them, usually from the side against a dark background.
- **Note**: 'Track radiation': only charged particles leave tracks directly. Uncharged radiation such as gamma rays and neutrons shows up only through charged particles it knocks loose.
- **Note**: 'Determine the type, energy': this is read from track length and thickness and, when a magnet is used, from how the tracks curve. A plain chamber gives only rough clues.
- **Missing history**: C.T.R. Wilson invented the cloud chamber in 1911 and shared the 1927 Nobel Prize in Physics for it.
- **Missing**: how the vapor becomes supersaturated. Wilson used a sudden expansion of moist air. Many modern demo chambers are diffusion chambers using alcohol over a very cold plate.

## Video: Cloud Chambers: Radiation, Made Visible

- Duration: 25.4 s · Published: 2025-07-26
- File: https://detailenjoyer.com/assets/videos/DMlrdQISfLM.mp4

### Transcript

DID YOU KNOW? Cloud chambers track radiation by visualizing the paths of charged particles as they pass through a supersaturated vapor inside the chamber. When radiation, such as alpha or beta particles, travels through the chamber, it ionizes the vapor along its trajectory. The vapor condenses around these ions, forming tiny droplets that create visible, glowing tracks outlining the particle's path. By observing these trails scientists can determine the type, energy, and behavior of the radiation. NOW YOU KNOW!

## Questions & answers

**Q: What makes a cloud-chamber track?**  
A: Vapor condenses on the ions a charged particle leaves behind. _(Source: Write-up · Exhibit A (Why droplets form) · NobelPrize.org: C.T.R. Wilson, Nobel Prize in Physics 1927 (facts); C.T.R. Wilson: Nobel Lecture (1927), On the cloud method of making visible ions and the tracks of ionizing particles)_

**Q: Why are the tracks visible?**  
A: The droplets scatter light; they don't glow. _(Source: Write-up · Exhibit B (Not glowing) · C.T.R. Wilson: Nobel Lecture (1927), On the cloud method of making visible ions and the tracks of ionizing particles)_

**Q: Who built the first cloud chamber?**  
A: C.T.R. Wilson, in 1911. _(Source: Write-up · Exhibit C (Wilson's chamber) · NobelPrize.org: C.T.R. Wilson, Nobel Prize in Physics 1927 (facts); C.T.R. Wilson: Nobel Lecture (1927), On the cloud method of making visible ions and the tracks of ionizing particles)_

**Q: What do many modern demo chambers use?**  
A: They're diffusion chambers. _(Source: Write-up · Exhibit D (Diffusion chambers) · Historical track detectors (Cambridge University Press))_

**Q: Which particle did Carl Anderson find with a cloud chamber in 1932?**  
A: Curving tracks in a magnetic field revealed the positron. _(Source: Write-up · Exhibit E (Reading the tracks) · Anderson, C.D. (1933): The Positive Electron, Physical Review 43, 491)_

## Sources

- [NobelPrize.org: C.T.R. Wilson, Nobel Prize in Physics 1927 (facts)](https://www.nobelprize.org/prizes/physics/1927/wilson/facts/)
- [C.T.R. Wilson: Nobel Lecture (1927), On the cloud method of making visible ions and the tracks of ionizing particles](https://www.nobelprize.org/prizes/physics/1927/wilson/lecture/)
- [Anderson, C.D. (1933): The Positive Electron, Physical Review 43, 491](https://journals.aps.org/pr/pdf/10.1103/PhysRev.43.491)
- [Historical track detectors (Cambridge University Press)](https://doi.org/10.1017/9781009401531.009)

## Cite this case file

Easterling, Marcus. "Cloud Chambers." The Detail Department (DetailEnjoyer), case file DD-623, last updated Jul 26, 2025. https://detailenjoyer.com/case/cloud-chambers/