MEASUREMENTS OF DC RESISTIVITY

The shape of specimen and the arrangement of electrode should be such  that the resistivity can be easily calculated .For a solid specimen, a flat plate with plane and parallel surfaces usually circular are preferred. The specimens are normally in the form of disc of 5 to 10 cm diameter and 3 to 12 mm thickness .

A three -electrode arrangement as shown is used for measurement of resistance

The electrode which covers the surface of the specimen is called the ‘unguarded’ electrode and is connected to the high-voltage terminal.

➢ The third electrode surrounding other measuring electrode is connected to a suitable terminal of the measuring circuit.

➢ The width of this guard electrode must be at least twice the thickness of the specimen, and the unguarded electrode must extend to the outer edge of the guard electrode.

The gap between the guarded and guard electrodes should be as small as possible. The effective diameter of the guarded electrode is greater than the actual diameter and is given as follows.

𝑟=𝑟1+𝑔2−𝛿

Where 

                          r1, r2, and r – radii of the guarded electrode , guard electrode including the gap ,and the effective radius of the guarded electrode.  

                          g – gap width

                           t – specimen thickness.   And

                                                                    𝛿𝑡=2𝜋𝑙𝑛 cosh (𝜋𝑔4𝑡)