= Surface-catalysed hydrogen spin conversion experiment
The 1935 primary paper https://doi.org/10.1021/ja01310a023[The Spin-Isomerization of Hydrogen at Paramagnetic and Diamagnetic Surfaces], pp. 1251–1252, describes evacuated <catalyst> samples cooled before admitting <molecular hydrogen>. Gas samples were analyzed in a <glass> cell in <liquid air>: a heated <platinum> wire and <Wheatstone bridge> converted changes in <thermal conductivity> into <electrical resistance> readings, calibrated against <normal hydrogen>.<The Spin-Isomerization of Hydrogen at Paramagnetic and Diamagnetic Surfaces by Hugh Stott Taylor and H. Diamond>[p. 1251, Experimental Details: sample handling and heated-wire thermal-conductivity analysis]{ref}
Its comparison of <oxide> surfaces supplied a control: specimens exhibiting strong <paramagnetism> converted the gas much faster than the comparison exhibiting <diamagnetism> despite all adsorbing <molecular hydrogen>. Contact time, <pressure>, and <temperature> were recorded. This supports a role for surface magnetic properties beyond <adsorption> alone; it does not identify every microscopic surface site or directly measure nuclear-spin orientations.<The Spin-Isomerization of Hydrogen at Paramagnetic and Diamagnetic Surfaces by Hugh Stott Taylor and H. Diamond>[p. 1252, Table I description and Discussion of Results: rare-earth oxide comparison and adsorption qualification]{ref}
The inference chain is preparation, contact with a specified surface, then a calibrated bulk transport readout of composition. It is distinct from a direct <calorimetry> measurement of <heat capacity>: transport and <heat capacity> can both depend on the spin populations, but they are different measured quantities. Changing the catalyst's own surface structure, introducing magnetic impurities, or changing the detector's gas <pressure> can confound an uncalibrated comparison. The free-rotor <partition functions> predict equilibrium populations and stored <energy>, not the conversion rate on a particular surface.
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