Measurement of D.C. Resistivity
D.C resistivity can be measured by various methods such as
- using Galvanometer
- using D.C. Amplifier and
- using wheatstone bridge.
Prior to study the above methods, let us take the idea of shape of specimen of insulation, and electrode system.
Specimen and Electrode
- For facilitating calculation of D.C. resistivity, the shape of specimen and the related electrode arrangement shall be as simple as possible.
- For solid insulations, the shape of the specimen is a flat plate with parallel surface of perfect plane. A circular specimen is as shown :
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| Figure A |
Following Figure B shows three electrodes kept in contact with the specimen described above.
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| Figure B |
Refer the above figure
r = r1+g/2 –
Where
g = air gap width
t = thickness of specimen
Materials Used for Electrodes
It is expected that the electrode should make a accurate and perfect contact with the specimen under test.
Following materials in different forms and modes can be used as electrodes.
- Lead of Aluminum foil of thickness 10 to 50 um thickness. It should be pressed and rolled on the specimen by suitable roller. For perfect sticking a petroleum jelly can be used. Sometimes silicon grease is suitable for sticking.
- Silver paint which is a very good conductor can also be used.
- Mercury can be used on which a specimen can be made to float on it.
Practical Fillings of Specimen and Electrode in a Measuring Unit
- The specimen and the electrode assembly is put in a closed chamber (cell) from which the electrical connects of unguarded electrode, guarded electrode and guard electrode are taken out for carrying out the measurement of resistivity.
- These connections are shown in the experimental diagrams by dots.