What is Inductance?

The flux of a circuit’s own field through itself is proportional to its current: $\Phi_{\text{own}} = Li$, with $L > 0$ its self-inductance (henry, $\mathrm{H}$). The flux that circuit 1 sends through circuit 2 is $\Phi_{1 \to 2} = Mi_1$, and symmetrically $\Phi_{2 \to 1} = Mi_2$, with $M$ their mutual inductance (sign fixed by the orientations). The total flux of circuit 1 is then $\Phi_1 = L_1i_1 + Mi_2$.