Rationalizing Maxwell's Equations Maxwell's equations in
Gaussian units are shown at the right. Gaussian units are based
on the cgs system of mechanical units, and were long dominant
for theoretical investigations. Absolute electrostatic units
(esu) are used for electrical quantities, absolute
electromagnetic units (emu) for magnetic quantities. The
electric field E and the magnetic flux density B are the basic
vectors; D and H are obtained from them by adding the
polarization P or subtracting the magnetization M, respectively.
If P and M are proportional to the applied field, then D and B
are related to E and H through the dielectric constant κ and the
permeability μ as shown. The sources of the fields are charges
and currents. The equivalent charge density and current due to
polarization and magnetization are ρb and Jb, where the "b"
stands for "bound." The divergence of D and the curl of H give
the "free" charge and current densities, respectively, while the
divergence of E and the curl of B give the total