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A physicist and professor of physics at Columbia University.^[ref] The Kusch-Foley magnetic moment experiment describes his work with Polykarp Kusch.

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.

Awarded in equal shares to Willis Eugene Lamb Jr. for discoveries concerning the fine structure of the hydrogen spectrum and Polykarp Kusch for precision determination of the Electron magnetic moment.^[ref]

In 1938, Isidor Isaac Rabi, Jerrold Zacharias, Sidney Millman and Polykarp Kusch introduced molecular-beam magnetic resonance. Their 1939 account describes a heated oven, vacuum slits, inhomogeneous A/B magnetic fields with opposite gradients, and a uniform static C magnetic field. Conductors supplied transverse oscillatory radiofrequency (RF) fields. Transitions spoiled refocusing, reducing the tungsten-filament detector signal. Alignment compared A-only and B-only attenuation; C-field sweeps at fixed RF frequency limited oscillator pickup. Matching frequency/field ratios across compounds checked nuclear assignments. Magnetic field calibration limited absolute magnetic moments; weak intervening fields caused unwanted transitions.^[ref]

The readout has a useful logical structure: A and the apertures admit trajectories correlated with a magnetic moment projection; B tests whether that projection has survived C. The two deflectors compensate for unchanged states, whereas a changed projection experiences a different force in B. A dip followed by recovery as the static magnetic field passes through resonance is therefore a population-sensitive signal. A comparison with RF absent or far from resonance is the natural baseline in this functional model; RF pickup, drift or background loss must not be mistaken for a state transition. A dip alone does not establish a pure initial quantum state or resolve an entire time-domain Rabi cycle.

This beam method measures transmitted particles after state-dependent deflection. The later bulk nuclear magnetic resonance methods of Felix Bloch and Edward Mills Purcell instead use a macroscopic sample and detect nuclear induction or RF absorption; NMR spectroscopy builds on that bulk setting. The common resonant coupling does not make their preparations or readouts interchangeable.^[ref][ref]