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The gyromagnetic properties of the hydrogens.' Offprint from Physical Review, Vol. 50, No. 5, September 1, 1936. PRESENTATION COPY SIGNED BY RABI AND ZACHARIAS.
- Large 8vo, pp. 472-481, horizontal crease where folded for posting. Stapled as issued. FIRST EDITION, INSCRIBED PRESENTATION OFFPRINT. From 1934 to 1940, experiments on the hydrogens - hydrogen and deuterium - occupied a centre-stage position in the molecular-beam experiments of the Rabi group. The magnetic moment of the proton was anomalously large and, as a result, the sign of this particular magnetic moment was itself an open question. Equally important, the sign of the deuteron's magnetic moment was needed so that the magnetic moment of the neutron could be inferred. If it was assumed that the deuteron was a compound nucleus made up of the proton and the neutron, and if, within the confines of the deuteron, the magnetic moments of the proton and neutron were assumed to be additive, then the magnetic moment of the neutron could be deduced from a knowledge of the magnitudes and the signs of the proton's and deuteron's magnetic moments. In 1936 Rabi showed how the signs could be determined" (Rigden, 'The two discoveries of NMR,' Rev. Mod. Phys. 58 (1986), pp. 433-4). Rabi's idea, "the one that led to the determination of the signs of magnetic moments, came to him as he walked up the hill from his home on Riverside Drive toward the campus: "One day I was walking up the hill on Claremont Avenue and I was thinking about it [the sign of the nuclear magnetic moment] kinesthetically with my body. Now, yes, I was thinking about this as follows: here’s the moment and it’s wobbling around in the direction of the field and [to find] the sign was to find out in which sense it was wobbling. To do this, I have to add another field which goes with it or against it. This is the idea, just concretely. The whole resonance method goes back to this." His intuition was sound, and atoms did reorient in such a way that the signs of their magnetic moment could be determined. But Rabi’s 1936 paper was essentially qualitative . . . the work to implement it fell to his students. In their 10th floor laboratory, Kellogg and Zacharias dismantled the molecular-beam apparatus, stretched it out a bit so that a new magnetic field—the ‘T-field’—could be inserted between the two deflecting fields. The T-field was a strange one, its configuration giving it a treelike appearance. The direction of the field went up the trunk and then fanned out to right and left like tree limbs. A beam particle entered the T-field moving against the field (from the tips of the limbs in toward the trunk of the tree); and, on departure, the particle moved with the field (from the trunk out toward the ends of the limbs). As a particle moved rapidly through the T-field, it ‘saw’ a magnetic field change quickly from one direction to another; it ‘saw’ a field that appeared to rotate. Rabi recognized that if this apparent rotation was synchro- nized with the precession rate of the magnetic moment (the Larmor frequency), the T-field would exert a tipping force on the magnetic moment and make it flop from one orienta- tion to another. These reorientations would open the way for determining the signs of nuclear magnetic moments. Kellogg and Zacharias made another addition to the apparatus. They added a barrier between the first deflecting field and the T-field. With this barrier, one of the subbeams coming out of the first deflecting field was stopped before it entered the T-field. By observing the changes in the signal level of the detector as preselected beamlets were allowed to pass through the T-field, one could infer the signs of the magnetic moments. It all worked: the T-field did its job" (Rigden, 'Rabi: Scientist and Citizen, p. 108). Isidor Isaac Rabi (1898-1988) was awarded the Nobel Prize for Physics in 1944 for developing a “resonance method for recording the magnetic properties of atomic nuclei”. [Attributes: First Edition]
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