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Henry Michael Foley and Polykarp Kusch compared atomic magnetic resonance frequencies to determine the electron magnetic moment. They selected quantum states whose orbital angular momentum and spin contributions enter with different weights. Ratios of the measured frequencies revealed a departure from the simplest electron spin g-factor of exactly two. Comparing responses in the same magnetic field reduces dependence on absolute field calibration, turning a small spectral difference into a precision test of the magnetic response.^[ref]

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.

Polykarp Kusch and Henry Michael Foley measured atomic magnetic resonance frequencies to determine the electron magnetic moment. Different quantum states weight orbital angular momentum and spin contributions differently, allowing ratios of magnetic responses to isolate the spin contribution. The inferred electron spin g-factor differs slightly from two. The Kusch-Foley magnetic moment experiment thus probes a microscopic interaction through accurately counted frequencies, rather than a mechanical measurement of an isolated electron.^[ref]

A physicist.^[ref]