Proton gyromagnetic ratio

Put a proton in a magnetic field and its magnetic moment precesses about the field like a tilted gyroscope, at an angular frequency ω = γ_p·B. Nothing else about the proton's environment enters at first order, which makes the relationship extraordinarily clean: measure the frequency, know the field, or the other way round. Proton NMR magnetometers exploit exactly that, and are the standard tool for calibrating laboratory magnets to parts per million.

Felix Bloch at Stanford and Edward Purcell at Harvard independently detected the resonance in bulk matter in 1946 — water for Bloch, paraffin for Purcell — and shared the 1952 Nobel Prize. Chemists then discovered that the precession frequency shifts by a few parts per million depending on the electrons shielding each nucleus, and NMR spectroscopy was born; medicine discovered that relaxation times differ between tissues, and MRI followed.