= Wilson cloud-chamber experiment
{c}
<Charles Thomson Rees Wilson>'s 1927 lecture recalls <Electron> cloud trails produced with <X-rays> in 1911, then <alpha particle> trails from a radium-tipped source. The improved 1912 chamber had a wide, shallow space with a suddenly falling floor to limit stirring. A roof–floor clearing <electric field> removed unwanted <ions>. Sharp <photography> required expansion, then the ionizing <particle>, then illumination.<Nobel Lectures, Physics 1922–1941/On the Cloud Method of Making Visible Ions and the Tracks of Ionizing Particles by Charles Thomson Rees Wilson>[Nobel Lectures, Physics 1922–1941; Wilson, 1927 Nobel lecture, pp.199–201, Fig.3: apparatus and operating sequence]{ref}
The <piston> mechanism connects the space beneath the moving floor to an evacuated vessel: opening the valve lets the floor drop. <Water> in a trough supplies the <vapor>. The Smithsonian's surviving demonstration object is explicitly a 1961 replica of the 1912 apparatus.<Wilson’s Cloud Chamber, Replica of Cavendish Lab Apparatus by Smithsonian Institution>[Smithsonian Institution, Wilson's cloud chamber, replica of Cavendish Lab apparatus, object nmah_700205, “History and basic principle” and “Date made”]{ref}
\Image[/wilson-cloud-chamber-setup-and-sequence.png]
{title=<Expansion cloud chamber>: functional apparatus and order of operations}
{description=Original functional schematic, with an invented illustrative trail. Positions, sizes and delays are schematic; this is not a historical photograph or a mechanical reconstruction.}
After dust removal, <X-rays> increased droplet formation above a lower expansion threshold; larger expansions also produced dense background cloud.<Nobel Lectures, Physics 1922–1941/On the Cloud Method of Making Visible Ions and the Tracks of Ionizing Particles by Charles Thomson Rees Wilson>[Nobel Lectures, Physics 1922–1941; Wilson, 1927 Nobel lecture, pp.194–197: cloud limits and clearing controls]{ref}
The control logic separates a localized <ionization> trail from fog. Delays permit <ion> displacement and drop growth; stirring deforms the trail. A photograph alone supplies neither an absolute event time nor a calibrated <particle> <energy>.
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