= Davisson–Germer experiment
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The <Davisson–Germer experiment> turned <elastic scattering> from <nickel> into evidence for a <matter wave>. The search became a deliberate wave test in 1926, with <diffraction> beams found early in 1927. The earlier crystal investigation had begun in 1925; the <de Broglie wavelength> proposal was already present in the 1924 thesis.<The discovery of electron waves by Clinton J. Davisson>[The discovery of electron waves, printed p. 390: retrospective chronology and the change to a deliberate wave test]{ref}
A heated <tungsten> ribbon supplied <electrons>; gun apertures selected a narrow beam. Two $1\,\mathrm{mm}$ apertures were $8\,\mathrm{mm}$ apart; the gun ended $7\,\mathrm{mm}$ from a <nickel> crystal cut parallel to its $\{111\}$ planes. An adjustable <electric potential difference> set the speed. The beam struck normally inside an evacuated, field-free enclosure with target and outer electrodes at one potential. A double-box collector, $11\,\mathrm{mm}$ from the target with a $1\,\mathrm{mm}$ entrance, selected directions. A retarding <electric potential difference> rejected slower <electrons>; the inner collector fed a sensitive <galvanometer>. Moving the collector scanned colatitude; rotating the crystal scanned azimuth.<Diffraction of Electrons by a Crystal of Nickel by C. Davisson and L. H. Germer>[The apparatus, printed pp. 707–710, figures 2–4: source, apertures, potentials, collector and target preparation]{ref}
The readout was collected <electric current>, not a photograph of <electron> paths. Voltage scans held the gun-plate current constant because incident-beam current could not be held constant; the overall current trend therefore needed caution. A strong feature developed at $54\,\mathrm V$ near $50^\circ$ colatitude. Surface heating distinguished crystal beams from adsorbed-gas beams; finite apertures and background shifted or broadened apparent peaks.<Diffraction of Electrons by a Crystal of Nickel by C. Davisson and L. H. Germer>[Printed p. 705, abstract; pp. 715–719, figures 9–12: voltage-scan control, 54 V feature, background subtraction and instrumental widths]{ref}
At the bright beam's colatitude, an azimuth scan revealed three companion directions, matching crystal symmetry. The test compared <wavelengths> inferred from <diffraction> angles and known crystal spacings with <wavelengths> inferred from accelerating <voltage> and <momentum>; the 1937 lecture displayed this comparison against $U^{-1/2}$. Agreement across beam directions supplied more evidence than one angular peak.<The discovery of electron waves by Clinton J. Davisson>[Printed pp. 391–393, figures 2–4: angular and azimuth scans and independent wavelength comparison]{ref}
\Image[https://upload.wikimedia.org/wikipedia/commons/a/a2/The_Scattering_of_Electrons_by_a_Single_Crystal_of_Nickel_Fig._2.jpg]
{title=Original Davisson–Germer azimuth scan at 54 volts and colatitude 50 degrees}
{description=<Clinton J. Davisson> and <Lester Halbert Germer>, Nature 119 (16 April 1927), p. 559, figure 2: <galvanometer> deflection against <azimuth>, recording scattered <electron> <electric current> at fixed <colatitude>. Scan cropped by Cirosantilli2 in 2023. Via Wikimedia Commons, https://commons.wikimedia.org/wiki/Template:PD-US-expired[public domain in the United States (PD-US-expired)].}
{source=https://commons.wikimedia.org/wiki/File:The_Scattering_of_Electrons_by_a_Single_Crystal_of_Nickel_Fig._2.jpg}
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The inference is about coherent <quantum phases> in scattering probabilities. It does not establish a wavy particle trajectory, and a collector-current experiment does not resolve individual detection events. Predicting the full intensity additionally requires the material's <scattering amplitude>, its surface condition, and instrumental angular acceptance; the <de Broglie wavelength> alone predicts none of those strengths. The following <finite-row electron diffraction model> isolates phase addition without attempting to reproduce the <nickel> measurements.
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