Nobel prize to P. Kusch awarded in 1955 "for his precision determination of the magnetic moment of the electron''. Co-winner W. E. Lamb "for his discoveries concerning the fine structure of the hydrogen spectrum''
Kusch, P.; Foley, H.M.; Precision Measurement of the Ratio of the Atomic "g Values'' in the 2P3/2 and 2P1/2 States of Gallium
Phys. Rev. 72 (1947) 1256;
Reprinted in The Physical Review - the First Hundred Years, AIP Press (1995) 152.
The measurement of the frequencies associated with the Zeeman splittings of the energy levels in two different atomic states in a constant magnetic field permits a determination of the ratio of the gj values of the atomic states. This determination involves only an accurate measurement of the frequencies, and does not require a knowledge of the value of the constant magnetic field. Using the atomic beam magnetic resonance technique we have measured six lines in the Zeeman spectrum
of the 2P1/2 state, and five lines of the 2P3/2 state of gallium at a field strength of 380 gauss. The spectrum is, of course, complicated by the level splittings produced by the nuclear magnetic moments and electric quadrupole moments. At the field strength employed in this experiment the nuclear energy level pattern is of an intermediate Paschen-Back character. The procedure employed in the observations was to make a series of alternate measurements
of the frequencies of the lines in the 2P1/2, and 2P3/2 states. In this way the effect of a drift in magnetic field was minimized ... (Extracted from the introductory part of the paper.).
Related references See also N. A. Renzetti, Phys. Rev. 57 (1940) 753;
First measurement of g-2 for the electron, preliminary result.