The volume resistivity between two electrodes in contact with a specimen is the ratio of the direct voltage to the part of the current that is distributed through the volume of the specimen (without that part of the current that is distributed across the surface). When using a sample with calculable dimensions, the volume resistivity is related to its dimensions, e.g. when using a plate, it is multiplied by the effective area of the electrodes and divided by the thickness of the sample. During measuring, the surface conductivity is eliminated by guard-ring electrodes. The procedure is carried out by measuring the voltage-current or by means of bridge circuits. The standard measuring device is a galvanometer. Electrometers are preferred when measuring very high levels of resistivity. Dry polymer materials have a volume resistivity on a scale of 1012 to more than 1018 W x cm. Prior to testing, the samples must be conditioned (see DIN 53482).