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Cloud Chambers

Short answer: 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.

VideoWrite-up4 sources

In collections:Everyday Physics

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What should you know about Cloud Chambers?

Quick facts. Each one stands alone and has a stable ID you can cite.

  1. Cloud-chamber tracks are droplets condensed on ions.

    DD-623-F01 · Write-up · Exhibit A (Why droplets form)

  2. C.T.R. Wilson built the first cloud chamber in 1911.

    DD-623-F02 · Write-up · Exhibit C (Wilson's chamber)

Video transcript: Cloud Chambers: Radiation, Made Visible

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!

Burned-in captions, read frame by frame (see transcript.md) · 25.4s Reel ·

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Cloud Chambers: Radiation, Made Visible Instagram Reel · · 25.4s

The write-up

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

  1. Exhibit A

    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. Exhibit B

    Not glowing

    The droplet trails scatter light from a lamp. They don't give off light themselves.

  3. Exhibit C

    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. Exhibit D

    Diffusion chambers

    Many modern chambers keep alcohol vapor supersaturated over a cold plate instead, so tracks form all the time.

  5. Exhibit E

    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.

Questions people ask about Cloud Chambers

What makes a cloud-chamber track?

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

Why are the tracks visible?

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

Who built the first cloud chamber?

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

What do many modern demo chambers use?

They're diffusion chambers.

Source: Write-up · Exhibit D (Diffusion chambers) · Historical track detectors (Cambridge University Press)

Which particle did Carl Anderson find with a cloud chamber in 1932?

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

Test Your Knowledge

All quizzes →

5 questions, all from this case file. Answers are revealed instantly.

  1. What makes a cloud-chamber track?

    1. Droplets condensing on ions
    2. Light given off by the particle
    3. Sparks between wires
    4. Bubbles in boiling liquid
    Show the answer

    Answer: A. Droplets condensing on ions 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

  2. Why are the tracks visible?

    1. A lamp lights the droplets
    2. The droplets glow
    3. The vapor is radioactive
    4. The walls fluoresce
    Show the answer

    Answer: A. A lamp lights the droplets 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

  3. Who built the first cloud chamber?

    1. C.T.R. Wilson
    2. Marie Curie
    3. Ernest Rutherford
    4. Niels Bohr
    Show the answer

    Answer: A. C.T.R. Wilson 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

  4. What do many modern demo chambers use?

    1. Alcohol vapor over a cold plate
    2. Liquid hydrogen
    3. Helium balloons
    4. Salt water
    Show the answer

    Answer: A. Alcohol vapor over a cold plate They're diffusion chambers. Source: Write-up · Exhibit D (Diffusion chambers) · Historical track detectors (Cambridge University Press)

  5. Which particle did Carl Anderson find with a cloud chamber in 1932?

    1. The positron
    2. The neutron
    3. The proton
    4. The photon
    Show the answer

    Answer: A. The positron 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

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/

Permalink: https://detailenjoyer.com/case/cloud-chambers/ · Last updated

Markdown version

Cite a single fact by its ID, for example DD-623-F01 → https://detailenjoyer.com/case/cloud-chambers/#DD-623-F01

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