If J f = 0 everywhere, the curl of H vanishes (Eq. 6.19), and we can express H as the gradient of a scalar potential W: H = – ∆W. According to Eq. 6.23, then, ∆2W = (∆ ∙ M), so W obeys Poisson's equation, with ∆ ∙ M as the "source."