= Giauque-MacDougall magnetic cooling experiment
{c}
{title2=1933}
<William Francis Giauque>'s 1949 lecture, Some Consequences of Low Temperature Research in Chemical Thermodynamics, credits <Peter Debye> with an independent proposal. It describes <gadolinium sulfate octahydrate> in a jacket immersed in liquid <helium>, with an external <solenoid>. Exchange gas conducts away <heat> during <magnetization>; evacuation isolates the sample before field reduction. A liquid-<nitrogen> shield reduces radiative <heat transfer>. Its photograph identifies the 1933 apparatus operated with <Duncan Peck MacDougall>. An <inductance> bridge reads the sample's <magnetic susceptibility>; measuring coils remain outside the evacuated jacket in liquid <helium>. Increasing <magnetic susceptibility> indicates cooling, but converting it to <temperature> using <Curie's law> becomes approximate as that law fails.<Some Consequences of Low Temperature Research in Chemical Thermodynamics by William Francis Giauque>[Giauque, Nobel lecture, pp. 237–245, Figs. 16–20 and 23–25]{ref}
The measurement logic deserves a separate check. A changed <inductance> alone is not an absolute <temperature> calibration: the sample geometry, applied measuring <magnetic field>, and constitutive response enter the inference. A modern control would compare the susceptibility-based <thermometer> with an independently calibrated <thermometer> over their common range, check recovery after repeated sweeps, and measure subsequent warming to bound incoming <heat>. These are proposed controls derived from the readout principle, rather than additional procedures attributed to the 1933 experiment. The historical working salt has a richer magnetic level structure than the illustrative <two-level paramagnetic cooling model>.
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