where ρ = ∂ X ∂ x + ∂ Y ∂ y + ∂ Z ∂ z {\displaystyle \rho ={\frac {\partial X}{\partial x}}+{\frac {\partial Y}{\partial y}}+{\frac {\partial Z}{\partial z}}} , denotes 4 π {\displaystyle 4\pi } times the density of electricity, and ( u x , u y , u z ) {\displaystyle (u_{x},u_{y},u_{z})} are the velocity-components of electricity.
∂ q ∂ t + ∂ j i ∂ x i = 0 {\displaystyle {\frac {\partial q}{\partial t}}+{\frac {\partial j_{i}}{\partial x_{i}}}=0} — ўраўненне неразрыўнасці для электрычнага току.