Bohr magneton
In atomic physics, the Bohr magneton (symbol μB) is a physical constant and the natural unit for expressing the magnetic moment of an electron caused by its orbital or spin angular momentum. In SI units, the Bohr magneton is defined as μ B = e ℏ 2 m e {\displaystyle \mu _{\mathrm {B} }={\frac {e\hbar }{2m_{\mathrm {e} }}}} and in the Gaussian CGS units as μ B = e ℏ 2 m e c , {\displaystyle \mu _{\mathrm {B} }={\frac {e\hbar }{2m_{\mathrm {e} }c}},} where e is the elementary charge, ħ is the reduced Planck constant, me is the electron mass, c is the speed of light.