Length contraction has an effect on charge density ρ and current density J, and time dilation has an effect on the rate of flow of charge (current), so charge and current distributions must transform in a related way under a boost. It turns out they transform exactly like the space-time and energy-momentum four-vectors, j ′ = j − − γ γ ρ ρ v n + ( γ γ − − 1 ) ( j ⋅ ⋅ n ) n ρ ρ ′ = γ γ ( ρ ρ − − j ⋅ ⋅ v n c 2 ) ,