ELECTRICAL MEASURMENT AND MEASURING INSTRUMENTS
of the sample terminals to ground, thereby moving all involved potentials close to and in reference to ground (earth) potential.
Guarding Often, current flows in considerable amounts in a path that we do not want to be included in our measurement of admittance. If we can insert a contact in that path, we can connect it to a guard terminal. The guard terminal is usually what we can consider a zeropotential of the instrument, and it is often simply a “ground” terminal (but not always). Being at zero potential, no net current flows to this point, the currents flowing to this point from the two current terminals are canceled and not measured by the current measuring circuitry. Thus, the admittance of the guarded path is excluded from the measurement. We give two examples of the immediate use of guarding: You may want to measure the resistance of a resistor within an electronic circuit, but without removing it from the circuitry. By connecting points in parallel paths to the guard terminal they are eliminated and the remaining path measured as desired. The second example is the surface conduction of a material that may dominate the measured conductivity of a high impedance sample. If the rim of a disk sample is equipped with a guard electrode, this can be contacted to the guard terminal and all conduction over the edge surface eliminated. In this case, also some volume conduction is lost to the guard, and the effective geometric factor of the measurement may be changed and should be determined by a separate measurement under other conditions (usually higher temperature) where volume conduction dominates.
Figure 17. Guarded measurements to obtain the impedance of an element D in a circuit including other elements D’ and D’’ (left) and to eliminate the surface conduction of a solid sample using a ring electrode on a disk (right). Arrows in the latter indicate currents.
Shielding As mentioned under our discussion of noise and parasitic admittances, these can be FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-999954621
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