Looking at the two expressions, ρ ρ and σ σ are the matrix inverse of each other. However, in the most general case, the individual matrix elements are not necessarily reciprocals of one another; for example, σ xx may not be equal to 1/ ρ xx. This can be seen in the Hall effect, where ρ ρ x y is nonzero. In the Hall effect, due to rotational invariance about the z -axis, ρ ρ y y = ρ ρ x x and ρ ρ y x = − − ρ ρ x y , so the relation between resistivity and conductivity simplifies to: