When measuring high-resistivity material (standard value Rx > 20MΩ), small current can not be applied stably. Therefore,
the constant-voltage process of applying constant voltage and measuring leak current is used. The material condition of
the surface is different from the one of the inside. It’s necessary for material control that indicating condition by each index.
The surface resistivity is results of surface condition and the volume resistivity is one of inside condition. They’re defined
on JIS K 6911-2006 or ASTM standard.