Jagdish Mehra, a physicist and historian of science, wrote that Jackson's text is not as good as the book of the same name by Julian Schwinger et al. Whereas Jackson treats the subject as a branch of applied mathematics, Schwinger integrates the two, illuminating the properties of the mathematical objects used with physical phenomena. Unlike Jackson, Schwinger employs variational methods and Green's functions extensively. Mehra took issue with the use of SI units in the third edition, which he considered to be more appropriate for engineering than for theoretical physics. More specifically, he argued that electric and magnetic fields should not have different units because they are components of the electromagnetic field strength tensor. Jackson himself responded to Mehra 's review.