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.