= Zeeman discovery experiment of 1896
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In August 1896, <Pieter Zeeman> observed broadened <sodium> <spectral lines> at Leiden, initially without resolved components. His lecture describes a slit and concave <diffraction grating>, then compares the broadening with the <sodium> doublet separation: about one thirtieth, roughly $0.2$ <ångström>. This supplied a spectral reference scale, not an absolute <magnetic field> calibration. Later <cadmium> photographs displayed resolved transverse triplets and axial doublets.<Light Radiation in a Magnetic Field by Pieter Zeeman>[Pieter Zeeman, Light Radiation in a Magnetic Field, Nobel lecture delivered May 2, 1903; University of Maryland mirror PDF pp. 2–3, paragraphs beginning “In August, 1896”, “The change was however very small” and the cadmium photograph discussion]{ref}
The fuller report specifies a concave <diffraction grating> of ten-foot radius with 14,438 lines per inch, an adjustable slit and a micrometer eyepiece with vertical crosswire. Salt in a flame between <electromagnet> poles supplied the <sodium> <spectral lines>. Repeated switching broadened and restored them. To test changes in flame <temperature> or vapour <density>, <Pieter Zeeman> examined <photon absorption> through a heated porcelain tube with water-cooled glass windows. The arc lamp was displaced from the magnet and its <light> redirected by a mirror to avoid disturbing the arc. Rotating a narrower tube made the absorption <spectral lines> equally broad at different heights; the <magnetic field> still broadened their whole length.<On the Influence of Magnetism on the Nature of the Light Emitted by a Substance (Part I) by Pieter Zeeman>[Pieter Zeeman, On the influence of magnetism on the nature of the light emitted by a substance, Part I, §§2–3 and 7–9, printed pp. 4–8: grating, eyepiece, arc-lamp control and rotating tube]{ref}
For the axial test, pierced <electromagnet> poles let the <light> travel along the <magnetic field>. A <quarter-wave plate> and <polarizer> selected one broadened edge; rotating the analyser by $90^\circ$ or reversing the magnet current exchanged the visible edge. The crosswire made its displacement detectable. This tested <electromagnetic polarization> as well as line width. For its rough estimate the report compared each side's widening with the <sodium> doublet separation, obtaining about one fortieth, and stated an approximate <magnetic field> intensity in historical units. These passages give no absolute field-calibration procedure; the inferred charge-to-<mass> ratio was explicitly a first approximation.<On the Influence of Magnetism on the Nature of the Light Emitted by a Substance (Part II) by Pieter Zeeman>[Pieter Zeeman, On the influence of magnetism on the nature of the light emitted by a substance, Part II, §§14, 20 and 23, printed pp. 10, 16–18; scan is a corrected reprint as its footnote to §24 explains]{ref}
<Hendrik Antoon Lorentz>'s charged-oscillator theory connected the observed splitting to moving <electric charge>. In modern language, a line-position measurement needs a <wavelength> or <frequency> calibration and an independent <magnetic field> calibration before it determines a <magnetic dipole moment>. Resolving the individual components adds the transition information used in the worked example below.
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