= Stark canal-ray experiment
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In 1913 <Johannes Stark> reported that an applied <electric field> resolved the <hydrogen> $H_\beta$ and $H_\gamma$ <spectral lines> into five components at his instrument's <spectral resolving power>, viewed perpendicular to the <electric field>. Three inner components had <polarization of an electromagnetic wave> perpendicular to the <electric field>; two outer components had it parallel.<Observation of the Separation of Spectral Lines by an Electric Field by Johannes Stark>[Stark, Nature 92 (1913), p. 401, two opening paragraphs; issue dated 4 December]{ref}
In his lecture of 3 June 1920 for the 1919 prize, <Johannes Stark> describes <canal rays> emerging through a perforated <cathode> into the <electric field> between it and a supplementary <electrode>. A <spectrograph> recorded the emitted <light>. His earlier <Doppler effect> comparison used a <collimator> perpendicular to the beam and then along approaching rays. Later beam/<electric field> reversal changed component intensities.<Nobel Lectures, Physics 1901–1921/Structural and Spectral Changes of Chemical Atoms by Johannes Stark>[Nobel Lectures, Physics 1901–1921; Stark, Structural and spectral changes of chemical atoms, printed pp. 430, 433–435; lecture date p. 427]{ref}
The historical account distinguishes observation perpendicular to the <electric field>, with five resolved components, from observation parallel to it, with three. These are viewing directions, distinct from the direction of the <polarization of an electromagnetic wave>.<The Discovery of the Stark Effect and Its Early Theoretical Explanations by A. J. Kox>[Kox, The discovery of the Stark effect and its early theoretical explanations, §2 Johannes Stark, paragraph following Figure 2]{ref}
The experimental distinctions matter. Changing the viewing direction tests a <Doppler effect> contribution; changing the applied <electric field> tests the <Stark effect>; analyzing <polarization of an electromagnetic wave> separates components that may overlap in an unpolarized record. A comparison with the unperturbed <spectral line> fixes the origin of a displacement. These are different controls, and the earlier <Doppler effect> comparison should not be described as a field-off control of the later discovery. A <spectrograph>'s finite <spectral resolving power> can merge several components into one. In particular, a count of visible <spectral lines> does not by itself count all <energy eigenstates>.
\Image[/stark-canal-ray-measurement-geometry.png]
{title=Stark spectroscopy: field, beam and viewing axes}
{description=Original conceptual reconstruction of the <canal ray> source and two alternative viewing geometries. No dimensions, <electric field> strengths or measured <wavelengths> are specified. The axial optical path requires an opening or adapted <electrode>; it is not a ray passing through the solid <electrode> drawn here.}
For a modern idealization of the gap, an <electric potential difference> $\Delta V$ over separation $d$ gives $|\mathcal E|\simeq|\Delta V|/d$ only if edge effects and the <charge density> in the gas are negligible. This is a calibration model, not a historical value assigned to Stark's apparatus. Similarly, a <canal ray> beam need not be a pure beam of neutral excited <hydrogen atoms>; the <quantum mechanics> model below isolates one neutral <hydrogen atom> from the source's preparation and collision processes.
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