Volume and surface resistivity are two basic parameters of dielectrics.
Volume resistivity It relates to the current flowing inside insulation material under the action of an electrical field. It is always measured with the use of indirect method: volume resistivity is measured taking into account the effective area of measuring electrode and thickness of a sample. It is not easy to conduct measurements of this type due to the low flowing current. That is why while measuring volume resistivity of a given material, it is recommended to examine a lot of samples and average the results, especially when examined material is of variable structure.
Surface resistivity It relates to the current flowing on the surface of examined material. It is measured indirectly: surface resistivity is measured taking into account the effective length of measuring electrode and width of space between the electrodes. The measurement is not easy because the surrounding environment has influence on surface resistivity; this is clearly visible in case of materials having high resistivity.
It is recommended to measure volume resistivity with the use of three electrodes: measuring electrode and protective electrode on one side of dielectric, and voltage electrode on the other side. Measuring surface resistivity looks similar except that the electrodes are in different places: measuring and voltage electrodes are placed on the same side, whereas protective electrode is placed on the other side.