“…Applying these to an FDTD mesh in the usual way yields (3) and (4), shown at the bottom of the next page, where δx, δy, and δz are the cell sizes, i, j, and k are the serial numbers of the cell, a x is the average of the quantity, a, along an x directed cell edge and a xy is the average of the quantity, a, over an xy cell face. The other four update equations can be obtained by rotating the coordinates.…”