Placed in an external magnetic field, nuclear spins respond by dancing a two-step: They wobble, or precess, about the field axis, and polarize in the field direction. At room-temperature in a 100-µT field—comparable to that of an ordinary bar magnet—it takes water’s spin-1⁄2 hydrogen nuclei about 10 seconds to relax, or achieve equilibrium polarization. In a 10-µT field, the spins relax in roughly 3⁄4 the time.
The difference has to do with the protons’ precession rate, or Larmor frequency, which is proportional to magnetic field intensity. As the Larmor frequency decreases, slower mechanisms can contribute to the relaxation process. At 100 µT, a proton’s Larmor frequency is about 4 kHz, and spin relaxation is facilitated mostly by thermal fluctuations. At 10 µT the lower Larmor frequency of about 400 Hz allows proton exchange between H3O+ and H2O and between H2O and OH−...
