= Stern-Gerlach apparatus in 1922
{c}
In 1922, <Otto Stern> and <Walther Gerlach> used an electrically heated oven to emit neutral <silver> <chemical atoms>. Two platinum apertures, a pinhole and a slit $0.03$–$0.04\,\mathrm{mm}$ wide, supplied <collimation>. The beam passed along a $3.5\,\mathrm{cm}$ knife-edge pole in a nonuniform <magnetic field>. At its end, a glass plate collected the condensed <silver>. Continuous pumping maintained <vacuum>; the thin deposit required development even after eight hours.<Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld by Walther Gerlach and Otto Stern>[Experimental setup, paragraphs 1–5; original 1922 paper in Martin Bauer's translation, PDF pp. 2–3]{ref}
The field-off image established the undeflected slit width. A comparison put field-on and field-off deposits on the same plate. Microscopy revealed two deflected bands, no detectable central component, and broadening attributed to the <velocity> distribution. Alignment and distance from the knife-edge affected the pattern; precise <magnetic field> <gradient> measurements were still pending.<Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld by Walther Gerlach and Otto Stern>[Results, Figs. 2–5 and observations b–e; closing paragraph, PDF pp. 3–5]{ref}
The apertures act as a <collimator>: they constrain the initial transverse <positions> and <velocities>, so a magnetic displacement can be distinguished from an initially broad beam. <Vacuum> reduces collisions that would destroy this preparation. Deposition integrates arrivals over the exposure; a darkened band is not a time-resolved record of individual <chemical atoms>. The field-off comparison tests whether the magnet changes the spatial distribution, while alignment checks address a geometric source of apparent asymmetry. Finite sensitivity qualifies the absence of a central deposit: it is an experimental upper limit, not a proof that exactly zero atoms landed there.
\Image[/stern-gerlach-oven-to-deposit.png]
{title=From an oven to a silver deposit, and a separate ideal flight model}
{description=The upper schematic shows the historical sequence without reproducing the original image. The lower diagram introduces the hypothetical constant-gradient magnet length $\ell$ and additional drift distance $D$ used below. The historical plate was at the magnet exit; the extra drift is a modeling choice.}
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