Nuclear magneton

The nuclear magneton is the Bohr magneton with the proton mass in the denominator instead of the electron mass, so it is 1836 times smaller. That single factor explains why nuclear magnetism is a whisper next to electronic magnetism: fridge magnets come from electrons, while nuclear moments need tesla-scale fields and sensitive detection to notice at all.

Dirac's theory predicted a structureless proton would have a moment of exactly one nuclear magneton. In 1933 Otto Stern, working in Hamburg with a molecular hydrogen beam and refusing to trust the theory, measured about 2.5 magnetons instead. It was the first evidence that the proton is not elementary — a composite of quarks and gluons, though it took thirty more years to say so — and it earned Stern the 1943 Nobel Prize.