An elegant and intuitive way to formulate Maxwell's equations is to use complex line bundles or a principal U(1)-bundle, on the fibers of which U(1) acts regularly. The principal U(1)- connection ∇ on the line bundle has a curvature F = ∇, which is a two-form that automatically satisfies d F = 0 and can be interpreted as a field strength. If the line bundle is trivial with flat reference connection d we can write ∇ = d + A and F = d A with A the 1-form composed of the electric potential and the magnetic vector potential.