Abstract
A new derivation of the electromagnetic field spin angular momentum is introduced. The conservation of the spin angular momentum is found to be connected with the orthogonality property of the electromagnetic field. The equation representing this conservation is found to be gauge and Lorentz invariant. A more general spin form is found that is invariant under the Heaviside–Larmor symmetry. Dual (symmetrized) Maxwell's equations are derived. It then shown that the correct form of spin angular momentum is a consequence of the symmetrized Maxwell's equations.