= Lenard cathode-ray window experiment
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= Lenard window
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The <Lenard cathode-ray window experiment> lets <cathode rays> leave a <gas-discharge tube> through a thin <aluminium> foil while preserving the tube's <vacuum>. This separates production from propagation.
<Philipp Lenard>'s lecture, delivered on 28 May 1906, describes sealing a small hole in a metal plate with <aluminium> foil. Luminous phosphor grains first rested on the window, then glowed above it in air. His improved tube separated production and observation chambers; coated paper served as a <fluorescent screen>. An aperture revealed spreading beyond a narrow straight beam. Evacuating the observation chamber reduced spreading and attenuation without interrupting production. Interposed layers and stepped foil stacks gave shadow comparisons; approximate scaling with mass had exceptions.<On Cathode Rays by Philipp Lenard>[On cathode rays, title page p. 105; pp. 108–114, Figs. 3–8: foil window, phosphor readout, aperture, layer comparisons and evacuated observation chamber]{ref}
\Image[/lenard-window-tube-and-observation-chamber.png]
{title=The <Lenard window> separates a <cathode ray> source from an observation chamber}
{id=image-lenard-window-tube-and-observation-chamber}
{description=Original functional schematic, with component sizes and paths chosen for explanation. The seal sustains a pressure difference; transmission does not mean that air can leak through. The lower panels contrast ideal straight propagation with angular redistribution by <gas>; these paths are illustrative, not reconstructed measurements. A <fluorescent screen> converts incident <Electron> <energy> into visible <light>.}
The experimental advantage is unusually clean: reducing the <gas> content in the observation chamber tests whether <gas> is needed to carry the <cathode rays>, while retaining the conditions needed to generate them. The window is a boundary that transmits some <Electrons> while confining the source environment. A spot outside establishes a transmitted effect; the aperture and changed observation environment provide additional ways to distinguish propagation from the source's behavior. This logic is useful whenever a complicated source would otherwise be changed by the very intervention intended to test the emitted beam.
Several inferences require care. A darker <fluorescent screen> can mean fewer arriving <Electrons>, less deposited <energy> per <Electron>, or redistribution by <electron scattering> beyond the observed area. A visible endpoint depends on the screen's sensitivity and background. It is not an intrinsic <Electron> range independent of initial <kinetic energy>, <gas> <density>, and observation geometry. For a modern comparison, measure a reference at fixed source conditions, use <background subtraction>, and compare the whole luminous patch as well as a small central region. A wider collection area tests redistribution; changing source-to-screen separation also changes collection geometry. These are proposed controls for interpreting a readout, not additional historical observations attributed to Lenard.
Nor is <fluorescence> a unique signature of <cathode rays>: <X-rays> can also excite it. Lenard notes that a later tube could transmit both, and that slow <cathode rays> failed to pass its window.<On Cathode Rays by Philipp Lenard>[On cathode rays, pp. 114–115 and 120–121: X-rays in the later tube; limited transmitted velocity range]{ref} In modern terms, <cathode rays> are <Electrons> carrying negative <electric charge>, whereas <X-rays> are <photons> with zero <electric charge>. Magnetic bending of the incident <Electrons>, interpreted through the <Lorentz force>, supplies a discrimination that brightness alone cannot. Passage through foil alone determines neither the sign of <electric charge> nor its magnitude, and it does not measure <charge-to-mass ratio>. The separate <Thomson charge-to-mass experiment> explains that inference. Even a measured <charge-to-mass ratio> needs an independent <electric charge> measurement to determine <mass>.
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