In the same chapter, Maxwell points out that the consequence of equation [ A ] is (in vector notation) ∇ ∇ ⋅ ⋅ B = 0 . Similarly, taking divergence of equation [ E ] gives conservation of electric charge, ∇ ∇ ⋅ ⋅ C = 0 , which, Maxwell points out, is true only if the total current includes the variation of electric displacement. Lastly, combining equation [ A ] and equation [ E ], the formula ∇ ∇ 2 A = 4 π π μ μ C is obtained which relates magnetic potential with current. Elsewhere in the Part I of the book, the electric potential is related to charge density as ∇ ∇ 2 Ψ Ψ = − − 4 π π K e in the absence of motion. Presciently, Maxwell also mentions that although some of the equations could be combined to eliminate some quantities, the objective of his list was to express every relation of which there was any knowledge of, rather than to obtain compactness of mathematical formulae.