Volume resistance: The resistance provided to the current which flows through the material is called volume resistance. For a cube of unit dimensions this is called volume resistivity. This resistivity is the flow of current resistance offered to the flow of current through the body of the material, and is expressed in the same way as the resistivity of a conducting material. In ohm-m. Thus the volume resistance of an insulating material may be expressed as
Where ρV IS called the volume resistivity, and is expressed in terms of ohm-m. "d" is length of the current path through the material in meters, "a" is the cross section area on the current path in square-m.
Surface resistance: The resistance offered by to current I which flows over the surface of insulating material is called surface material. Surface resistivity depends upon the humidity.