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PurcellBloch

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The next decisive steps where made almost at the same time, and along two distinct paths, by E. M. Purcell, H.C. Torrey and R.V. Pound and by F. Bloch, W.W. Hansen and M. Packard, earning the two first authors a shared Nobel prize in 1952

We show here below the first column of the paper by Purcell, Phys. Rev. 69, 37 (1946) and the full (sic!) letter by Bloch Phys. Rev. 69, 127 (1946), side by side

Both make use of the nuclei inside a condensed matter sample (paraffin, for Purcell and water, for Bloch) immersed in a static magnetic field {$\mathbf{B}$}, both employ the detection of the nuclear magnetization by means of its inductive coupling to a circuit located around the sample: a cavity obtained with a coaxial geometry for the former and a simple coil for the latter.

At thermal equilibrium in the magnetic field a small paramagnetic nuclear magnetization is produced. The Purcell experiment measures the resonant radiofrequency power absorption of the nuclear spins, whereas Bloch directly obtains the coherent nuclear induction signal which is the heart of the modern pulsed NMR technique.

If the nuclear magnetization {$\mathbf{M}_n$} is coherently turned by suitable radiofrequency manipulation to lie at right angles to the static field, it will precess around the field and induce a measurable electromotive force across a coil with axis perpendicular to {$\mathbf{B}$}. (The wobbling of the magnetic field is just due to the author's insipience with animations, rather than being a novel type of nutational motion)

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