Wilson cloud-chamber experiment 2026-10-18
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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.[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.[ref]
Figure 1.
Expansion cloud chamber: functional apparatus and order of operations
. Original functional schematic, with an invented illustrative trail. Positions, sizes and delays are schematic; this is not a historical photograph or a mechanical reconstruction. Details.
After dust removal, X-rays increased droplet formation above a lower expansion threshold; larger expansions also produced dense background cloud.[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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