Complex sentence: It is also possible to have materials in which the conductivity is anisotropic, different for currents travelling in different din this case σ is a 3 × 3 matrix (or more technically a rank-2 tensor), which is generally symmetric.
Split into simpler sentences: It is also possible to have materials in which the conductivity is anisotropic, different for currents travelling in different directions through the material. In this case σ is a 3 × 3 matrix (or more technically a rank-2 tensor), which is generally symmetric.