The measurement of nuclear and electron magnetic resonance on bulk materials was made possible by Felix Bloch and Edward Purcell and in 1952 they shared the Nobel Prize in Physics for their work. Until then, magnetic resonance was a measurable phenomena in which atoms were shot through a magnet as a beam. This was the work of Rabi. Therefore, the Nobel Prize quality in Bloch and Purcell's work was not in the theory of magnetic resonance itself, but in the development of instruments which would measure this phenomena in bulk material such as liquids and solids. These two laboratories were uniquely suited for this work.
Bloch was a great quantum mechanic and is credited with much quality work in the quantum mechanics of solids. Purcell is an electricity and magnetism physicist who worked on projects like radar during the war. His experience in radiofrequency electronics aided in the development of his NMR instrument. Bloch's firm grasp of quantum mechanics allowed him to look at the problem of magnetic resonance classically, while Purcell's classical background forced him to look at NMR with a quantum mechanic approach and develop a spectrometer much like the optical spectrometers. There is a quantum mechanical property of electrons and some nuclei called spin.
In the absence of an externally applied magnetic field, this property is not observable. However, even in optical experiments, this property can be observed when a magnetic field is applied. It was the appearance of split lines in spectroscopic experiments that caused the need for quantum mechanics to include this property. Purcell looked at the magnetic resonance experiment with the eyes of an optical spectroscopist. He built a double beam spectrometer.
The two channels were identical except that in one channel an external magnetic field was applied. His instrument scanned through the magnetic field and when transitions between the spin energy levels were made, he would notice that the power in the channel with the magnet would be less than that without. Thus he could precisely measure the frequency and the difference in energy between the two energy levels. This energy level picture is simple and the method of measuring correlates directly to that of double beam optical methods with which we are all familiar. However, it does not correspond well with the methods used to measure NMR today.
It does correlate with continuous wave spectrometers such as those used in EPR. However, it was a perfectly good way to measure the phenomena and one which we would have been able to invent as well with our knowledge of optical spectroscopy. The nuclei of some atoms, but not all, behave as if they were rotating, or spinning, about an axis passed through them, much as a top spins about its central axis. This is quite easily visualized in terms of the particle nature of the nucleus; it is less easily visualized in terms of the wave picture, which by now we know is another face of nature that is manifested in the atomic / molecular realm. Spin of nuclei is similar to electron spin. Examples of atoms whose nuclei spin are hydrogen, fluorine, the 13 isotope of carbon, and the 15 isotope of nitrogen..