Thermodynamics has a third law stating that as a system approaches absolute zero, its entropy approaches zero. Giauque cooled gases such as oxygen and hydrogen far below their liquid points and measured their heat capacities with great precision. This allowed him to verify the third law experimentally and to compile standard entropy tables for many substances. The adiabatic demagnetization technique he devised, in which simply weakening a magnetic field causes a sudden temperature drop, is now used to cool nuclear magnetic resonance magnets and prototype quantum computers. Because minute heat flows had to be detected, he introduced vacuum insulation and highly sensitive sensors into cryogenic setups. His engineering methods became the standard in low-temperature physics and helped conserve expensive liquid helium. These advances influence everyday technologies ranging from MRI scanners to detectors for cosmic microwave background surveys.

Giauque’s principal achievement was the precise measurement of heat capacities in the cryogenic range (0.05–20 K) and the resulting accurate determination of absolute entropies. He combined silver and AgCl resistance thermometers, gas calorimeters, and multilayer adiabatic shields, reaching temperature sensitivity of 10⁻⁴ K. By extrapolating the measured Cp data to 0 K and evaluating ΔS = ∫(Cp/T)dT, he provided strong experimental support for the third law. For molecular oxygen he observed the residual entropy arising from paramagnetic spin alignment, showing agreement with theory via field-induced demagnetization experiments. The adiabatic demagnetization refrigerator he co-developed in 1933 using radium salts achieved 0.25 K and opened the way to studies of nuclear spin systems. Giauque also isolated the oxygen isotope 18O in bulk, and from its molecular spectra and isotope effects determined precise bond energies; these data are now used in rocket oxidizer design and atmospheric chemistry calculations. His systematic entropy tables refined equilibrium constant calculations based on ΔG = ΔH − TΔS and became guidelines for chemical process engineering. Control of properties at low temperature is vital for superconductors and Bose–Einstein condensates, further underscoring the impact of his work. Thus, Giauque’s research bridged experimental technique and theoretical validation, bringing chemical thermodynamics into modern science.

Giauque refined gas-phase adiabatic calorimetry by introducing multilayer shielding that limited heat leaks below 10⁻⁶ W under cryogenic conditions. A triple barrier separating vacuum, radiative, and convective paths over a liquid-helium bath kept measurement uncertainties under 0.1 %. His thermodynamic data were used to fit constants of the Benedict–Webb–Rubin equation of state, extending it into non-ideal gas regimes. Residual entropy values for O₂, N₂, and CO matched quantum-statistical models incorporating spin statistics and rotational level degeneracy, providing benchmarks for quantum-chemistry calculations. In magnetocaloric studies employing radium-salt Gd₂(SO₄)₃·8H₂O, he achieved quantitative comparison of magnetic susceptibility and entropy changes. Giauque’s T₉₉ absolute temperature scale was later incorporated into IUPAC standards and influenced calibration points of ITS-90. His oxygen isotope separation combined staged gas diffusion and CaCO₃ exchange, delivering gram-scale 18O per day; this enabled precise isotope shift coefficients in IR and Raman spectroscopy, improving ground-state potential energy surfaces. Altogether, Giauque’s accomplishments delivered foundational data across low-temperature calorimetry, magnetic refrigeration, and isotope chemistry, forming indispensable references for subsequent theoretical and applied research.