E.M. Purcell, H.C. Torrey, and R.V. Pound.
  <b>Resonance absorption by nuclear magnetic moments in a solid</b>.
  <i>Physical Review</i>, 69:37-43, 1946.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Purcell-1946-RAN.bib">(bibtex:
  Purcell-1946-RAN)</A>.<br />