The detector was a deposited image
Silver atoms emerged from a heated source, passed narrow apertures and traversed an inhomogeneous magnetic field. They condensed on a glass plate near the end of the magnet. The 1922 report compares deposited patterns with and without the field, including a long exposure that needed development to make the thin silver layer visible.
The raw image was an accumulated spatial distribution. It did not timestamp individual atoms or read a spin value from each one. The authors described two separated branches and no evidence for an undeflected component, within the sensitivity of their deposit method.
Read the evidence: M. Bauer (2023), English translation of Gerlach and Stern’s 1922 paperW. Gerlach and O. Stern (1922), Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld, Z. Phys. 9, 349–352
A gradient supplies the separating force
In a locally adiabatic description, an atom follows an internal magnetic-energy branch while moving through the field. The force is minus the spatial derivative of that branch’s energy, as shown below for the vertical coordinate. This convention avoids guessing the force from an arrow labeled spin.
A uniform field can change an internal precession rate but supplies no gradient force in this approximation. The trajectory also depends on flight speed, interaction length and geometry. The original report notes broader deflected bands from the velocity distribution and unequal attraction and repulsion near the knife-edge pole.
Read the evidence: M. Bauer (2023), English translation of Gerlach and Stern’s 1922 paper
Two branches constrained a particular classical picture
For the simple comparison of identical magnetic moments with continuously distributed orientations, one expects a continuous range of transverse deflections. Resolved branches with a depleted middle instead supported discrete magnetic projections. Collimation and the field-off image were essential for distinguishing a split beam from a broad source.
The authors openly discussed how a tiny aperture misalignment could spoil a symmetric image. They also postponed a quantitative magneton value until the field inhomogeneity could be measured more accurately. Evidence for discrete branches and a precision determination of their magnetic scale were separate achievements.
Read the evidence: M. Bauer (2023), English translation of Gerlach and Stern’s 1922 paper
Spin was not the interpretation available in 1922
Stern and Gerlach worked within the old quantum theory’s question of space quantization. Electron spin was introduced later. In the modern account, ground-state silver has a closed-shell core and one valence electron, with zero total electronic orbital angular momentum and a two-valued electronic spin projection dominating the familiar splitting.
That modern account neglects or leaves unresolved nuclear hyperfine details when reducing the experiment to an ideal two-state analyzer. It explains the result without validating the orbital picture that originally motivated it. The experiment became evidence within a revised theory rather than a photograph of an electron rotating.
Read the evidence: M. Bauer (2023), English translation of Gerlach and Stern’s 1922 paperSchmidt-Böcking et al. (2016), The Stern–Gerlach Experiment Revisited — historical research
The familiar three-filter story is a further experiment
A modern teaching sequence selects one output, rotates a second analyzer and examines a final output. Such a sequence tests preparation, incompatible axes and postselection. Those controls and counts were not all acquired by the single 1922 deposition experiment.
The existing Lab provides this ideal sequence and explicitly retains rejected counts. It also distinguishes an unread middle measurement from bypassing that measurement. Read those results as predictions of its declared projector model, not as extra historical panels hidden in the original silver photograph.
Read the evidence: M. Bauer (2023), English translation of Gerlach and Stern’s 1922 paper
The Nobel citation extends beyond the silver beam
The 1943 Stern award recognizes the molecular-ray method and the discovery of the proton’s magnetic moment. It is therefore inaccurate to rename the whole contribution as the Stern–Gerlach experiment. The proton result belongs to further molecular-beam work with different particles and inference.
This article covers the silver-beam evidence and its reinterpretation. It does not claim that the linked spin-filter Lab reproduces the proton-moment measurement. The next reading on Rabi’s resonance method follows how beam deflection was combined with a frequency-selective internal transition.
Read the evidence: Otto Stern — official 1943 award factsI. I. Rabi, J. R. Zacharias, S. Millman and P. Kusch (1938), A New Method of Measuring Nuclear Magnetic Moment, Phys. Rev. 53, 318 — original scan