= Electron magnetic moment
{title2=$\boldsymbol\mu_s=-g_s\mu_B\mathbf S/\hbar$}
{wiki}
The <electron magnetic moment> couples an <Electron>'s <angular momentum> to a <magnetic field>. Its intrinsic <spin> contribution is
$$
\boldsymbol\mu_s=-g_s\mu_B\frac{\mathbf S}{\hbar},\qquad
\mu_B=\frac{e\hbar}{2m_e},\qquad e>0.
$$
Here $\mu_B$ is the <Bohr magneton>, and $g_s$ is the positive <electron spin g-factor> under this sign convention. The minus sign makes <magnetic dipole moment> antiparallel to <spin angular momentum>. An <Electron> in a <chemical atom> can also contribute an orbital <magnetic dipole moment>; the measured response of a whole atomic level combines these contributions.
Measuring a tiny departure of $g_s$ from $2$ tests how well a theory predicts an <Electron>'s interaction with its surroundings. The <Dirac equation> gives $g_s=2$ before <radiative corrections>; the <anomalous magnetic moment> makes a small correction experimentally accessible.<The magnetic moment of the electron by Polykarp Kusch>[pp. 299–300: spin moment and Dirac prediction]{ref} A useful strategy is to compare two atomic <energy levels> whose orbital and <spin> contributions have different weights, rather than trying to calibrate an absolute <magnetic field> more accurately.
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