As we know, the volume resistivity Pv is the electrical resistance of a cube of 1 cm on each side of a piece of material.
The surface resistivity Ps is the resistance of a surface whose side length is 1 cm square.
The measurement of surface resistivity is generally based on the method of simultaneously measuring the current intensity passing through the test piece and the voltage applied to the test piece.
In order to measure the surface resistivity of the paint film, the electrodes shown in Figure 223 are used.
The test piece 1 is placed on the fixed (lower) electrode 2 . An electrode 3 and a retaining ring 4 are placed on the test piece. In order to obtain the same gap on the circumference, a pipe (sleeve) 5 is used between the electrode 3 and the retainer 4.
Determination of volume resistivity
The schematic diagram of the equipment for measuring the volume resistivity is shown in a of Figure 224.
The voltage from the line network can be increased by means of a transformer and then rectified by a tube rectifier; the maximum necessary DC voltage is 2500 volts.
The adjustment of the voltage is carried out with the potentiometer 2 . In order to change the polarity of the voltage supplied to the test piece, a bipolar switch 3 is used. In order to limit the current during the breakdown of the test piece, use a constant resistance 4 equal to ] x 105 ohms. The switch 5 is used to switch the current on and off. The test piece 6 is placed between the electrodes 7 and 3, and a retaining ring 9 is placed on it. Ammeter 10 is shunted with electric key (wrench) 11. If the switch 5 is in the position (a) above, the test piece is under voltage; if the switch 5 is in the position (6), the electrode 8 is grounded. (Related instrument: resistance meter)
When the electrodes shown in the sketch (Fig. 224) are connected, only the current from electrode 8 to electrode 7 is measured; the volume resistance is measured from the value of this current. The current closed on the surface by the retainer is drawn through the ammeter.
Determination of Surface Resistivity
In order to measure the surface resistivity, the sketch shown in Figure 224-6 can be used.
In this case, the retainer 9 and the upper electrode 7 can be used as electrodes; the galvanometer only measures the current passing through the surface of the test piece. The volumetric current is drawn by means of the electrode 8 through a galvanometer.
After assembling the circuit diagram according to Figure 224-6, place the test piece between the electrodes and connect the voltage. Then select the shunt coefficient in such a way that the pointer (cursor) of the galvanometer will not exceed the scale range. The current passing through the annular surface between the upper electrode 7 and the retainer 9 is measured. The surface resistivity (in ohms) can be obtained by the following formula:
