If two coils of self inductance L₁ and L₂ are coupled together, their mutual inductance becomes

If two coils of self inductance L₁ and L₂ are coupled together, their mutual inductance becomes $\mathrm{M}=\mathrm{k}\sqrt{{\mathrm{L}}_1{ \mathrm{L}}_2}$ where k = coupling constant whose value lies between 0 and 1.