Volume and Surface Resistivity - ASTM D257
Volume resistivity is the resistance to leakage current through the body of an insulating material.
The ratio of the potential gradient parallel to the current in a material to the current density. In SI, volume resistivity is numerically equal to the direct-current resistance between opposite faces of a one-meter cube of the material (Ohm-m).
Surface resistivity is the resistance to leakage current along the surface of an insulating material.
The electrical resistance between two parallel electrodes in contact with the specimen surface and separated by a distance equal to the contact length of the electrodes. The resistivity is therefore the quotient of the potential gradient, in V/m, and the current per unit of electrode length, A/m. Since the four ends of the electrodes define a square, the lengths in the quotient cancel and surface resistivities are reported in ohms, although it is common to see the more descriptive unit of ohms per square.