ABSTRACT
Rabi was not wrong so to regard himself and these artefacts. In the late 1920s nuclear physics was becoming widely recognized as the new fron tier5 o f physical research, and in the early 1930s American physicists, more than those of any other nation, began turning their research efforts in this direction. Over the six years 1932-1937, in which Rabis premagnetic-resonance researches were appearing in the Physical Review, nuclear physics was springing from under 10% to over 30% of the papers published in that journal. 7 Thus Rabi, a bit o f a Rutherford, was on the crest o f a wave that he himself was helping to create. Moreover, what he contributed to the making of that nuclear physics wave was in some respects unique. Nuclear moments-i. e., a nucleus’s ‘spin5 (angular momentum) and its magnetic moment-were among the very few para meters then regarded as necessary to define a nucleus as a quantum-phys ical system. W hile optical spectroscopists, through their analyses of hyperfine structure, provided most o f the experimental data on nuclear moments, Rabi and only Rabi was providing confirmation and supple m entation o f that data by an independent technique, the Breit-Rabi m ethod. 8 Those pre-magnetic resonance experiments were imaginatively conceived, skillfully performed, knowledgably analyzed-and technically demanding. Experimentalists so able and successful as Ernest Lawrence listened ‘almost with reverence5 to Rabis account of them . 9
Measuring nuclear magnetic moments was a desideratum that Rabis principal mentors had earlier emphasized: Ralph Kronig, with whom Rabi studied and collaborated at Colum bia University, January 1926 to June 1927; Wolfgang Pauli and O tto Stern, at Ham burg University, October 1927 to December 1928. 10 The question took on great im por tance through the acceptance early in 1926 o f the concept of electron spin-the attribution to every electron o f an intrinsic angular m om en tum of magnitude exactly half that possessed by an electron circulating in the lowest orbit o f a Bohr atom, and, along with that ‘spin5, the attribu tion o f an intrinsic magnetic m om ent exactly equal to that which an
electron circulating in a lowest Bohr orbit would produce electrodynami cally: ixB=eh/4irmec, the ‘Bohr m agneton. (Here ^is the electrons charge, me its mass, hl4ir its intrinsic angular m om entum , and c the velocity of light. ) Since, until neutrons came to the rescue in 1932, atomic nuclei were thought to be composed of protons and electrons, the attribution to electrons of this atomic size magnetic moment, roughly a thousand times greater than that which atomic nuclei had been thought to possess, pre sented atomic physicists with a perplexing problem. 11
This issue was further sharpened with Diracs publication early in 1928 of an equation that, applied to a particle of the electrons charge and mass, produced’ exactly the previously attributed intrinsic angular momentum and magnetic moment. W ith the success of Dirac’s equation, it came to be widely accepted in the early 1930s that the proton was describable by the same equation, with appropriate charge and massleading to the conclusion that the magnitude of the protons intrinsic magnetic moment, the nuclear magneton5, must be exactly fjbB{m Jm p)y where and mp are the masses of the electron and the proton, standing in the ratio of 1 to 1830. 12
Measurement of the magnetic m om ent o f a nucleus was Sterns cyno sure, the particular goal towards which his refinements of the molecular beam technique had pointed from the early 1920s to the early 1930s. 13 The implication of an atom-size nuclear magnetic moment, however per plexing, was enticing to the experimenter. W hen Rabi, itinerant postdoc in Europe, comm itted himself to a years work in Sterns laboratory late in 1927, he was paired with Sterns other American postdoctoral fellow, John B. Taylor. This very skilled experimentalist, to whose example and instruction Rabi would be greatly beholden, was then, at Sterns behest, searching for the large nuclear magnetic moment implied by the spinning electron. 14
Through the strong support of George B. Pegram, the perpetual head of Columbia’s physics department and occupant of various higher admin istrative positions in the university, in the autum n of 1929 Rabi came back to Columbia as faculty member. Although research funds at Colum bia were not especially ample, Pegram ensured that Rabi always had an exceptionally large share of them, as well as an exceptionally large share of the time of the Physics D epartm ent’s exceptionally wellequipped machine shop. Moreover, in the enormous, 14-storey, physics building that Columbia had completed as Rabi was completing his doc toral research in 1926, there was ample space through the 1930s for Rabi’s ever-expanding research group. 15
Rabi’s return to Colum bia in the summer o f 1929 coincided with Gregory Breit’s arrival in New York to begin teaching at New York University-indeed, to take, at twice Rabi’s salary, intellectual leadership of physics at NYU. Breit was Rabi’s age, but was his senior in every pro fessional sense, having been precocious rather than backward in finding
his way into theoretical physics. Industrious and learned, he already had a huge record o f publication, and he did much o f his work in close collab oration with experimentalists. His particular concern at this time and on through the early 1930s was the calculation o f nuclear magnetic moments and of hyperfine splittings, and comparison of the results with spectroscopic observations. 16
Soon after their simultaneous arrivals, Rabi and Breit initiated a joint Columbia-NYU seminar in theoretical physics. Thus it was inevitable that Breit and Rabi would come to discuss the problem o f atomic beam measurement of nuclear magnetic moments. Rabi was well aware o f the difficulties of such measurements after his year in Sterns laboratory: they required a precise mapping’ of the magnetic field and its gradient in that extremely narrow channel through which the beam passed between the poles o f the magnet (Figure 2), while the very smallness o f nuclear mag netic moments required the highest possible gradients, and hence the narrowest possible channels. The prospects for such measurements, and especially for such as could compete with those by optical spectroscopists studying hyperfine structures, would not have looked good to Rabi-and all the less good as he well knew that his own strength lay in conceptual tricks, not in refined, precise experimental technique.
