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Polykarp Kusch and Henry Michael Foley compared hyperfine structure resonance frequencies for different electronic states of gallium, indium and sodium. Their 1948 paper reports mutually consistent departures from the ratios expected with orbital factor $1$ and spin factor $2$. Comparing several states constrained the alternative explanation of perturbed atomic levels.^[ref]

The Rabi molecular beam magnetic resonance experiment explains the readout principle: a resonant transition changes an atom's magnetic response, and hence its transmitted beam signal. Kusch describes rapidly interchangeable ovens, several radio frequency oscillators and careful field homogenization. Measurements taken during monotonic field drift were reduced to a common field.^[ref]

Real hyperfine structure lines are not simply bare electronic $g_J$ times field. Nuclear magnetic moments, known interaction constants and nonlinear field dependence enter the inference.^[ref] Field homogeneity controls line broadening; alternating species addresses drift. Neither control removes an error in the atomic model. The beauty of the comparison is the separation of two problems: cancelling a shared calibration scale and testing whether several physical systems imply the same electron spin g-factor.

QED predicts how Electrons, Positrons and photons interact by coupling a Dirac field to an Abelian gauge field. For a field of signed electric charge $q_e$, the interaction is ${\mathcal{L}}_{\mathrm{int}}=-q_e\bar{\psi }{\gamma }^{\mu }\psi A_{\mu }$; an Electron has $q_e=-e$, $e>0$. Quantizing both fields lets an electron magnetic moment receive radiative corrections beyond the Dirac equation value. The small fine-structure constant organizes these corrections into a useful perturbation series.

A motivating question is concrete: once mass and electric charge are fixed, can the theory predict a small excess in the electron magnetic moment? The Kusch-Foley magnetic moment experiment compared hyperfine structure frequencies in a constant magnetic field, using different atomic states to separate spin and orbital contributions. Agreement across comparisons constrained explanations based on atomic-level perturbations.^[ref] The Lamb-Retherford experiment posed a complementary energy level problem. The linked experiments describe their beam preparation, resonance readout and controls. Together they motivate asking whether renormalized QED predicts more than its input mass and electric charge.