The resistivity of a substance is equal to the resistance of a rod of that substance which is 1 metre long and 1 square metre in cross-section. This can be a 1 cubic Metre cube. So we can also say that the resistivity of a substance is equal to he resistance between the opposite faces of a 1 metre cube of the substance. The resistivity of a substance does not depend on its length or thickness. It depends on the nature of the substance and temperature.Manzanar