What unit of magnetic dipole moment is named after Niels Bohr?

The Bohr magneton is the unit of magnetic dipole moment named after Niels Bohr.

In atomic physics, it is the natural unit used especially for expressing the magnetic moments of electrons. In SI units, the Bohr magneton is defined as μB = eℏ/(2me), where e is the elementary charge, ℏ is the reduced Planck constant and me is the electron mass. Its value is approximately 9.274 × 10⁻²⁴ joules per tesla.

The quantity emerged from the early quantum theory of the atom. In 1913, while developing his model of quantised electron orbits, Bohr obtained natural units for atomic angular momentum and magnetic moment. The name “Bohr magneton” was introduced later, in 1920, by Wolfgang Pauli.

A frequent mix-up is the nuclear magneton. It is used for protons, neutrons and atomic nuclei and is much smaller because it is based on the proton mass. The Bohr radius is different again: it describes a characteristic length scale for the hydrogen atom, not magnetism. “Bohr energy” and “Bohr frequency” are related-looking terms but are not the standard name of a magnetic-moment unit.