LOSS OF CHARGE METHOD

 Current-time characteristic of the discharge current is obtained by placing the switch B in position 2. The slope of these current time characteristic gives the time constant τ = CR,

where      C is the capacitance of the specimen.

The volume resistance of the specimen R can thus be obtained from the relationship by measuring τ and C.

Alternatively, a capacitance previously charged (C1) may be discharged through the specimen by inserting it in place of the battery E. By knowing the value of the capacitance C1 which is very large compared to the specimen capacitance (≈10 μF) and knowing the current-time characteristic of the discharge current, its time constant can be calculated. Hence, the resistance of the specimen is obtained from the relationship        𝑅=𝜏/𝐶1

where C1 – capacitance used

τ – time constant.

If initial voltage V0 on the capacitor is measured and the voltage V across it at any time τ Then the unknown resistance R can be calculated from the relationship

𝑅=𝑡/𝐶1𝑙𝑛𝑉0𝑉

➢ The accuracy of this method depends on the accuracies with which the values of the capacitance and the voltage are measured.

DC galvanometer of fig 1 is replaced by a dc amplifier for resistivity measurements.

➢ DC amplifier is used as a null detector and a separate potentiometer circuit is used for obtaining a signal e equal and opposite to the voltage drop across the standard resistance Rs.

➢ At balance, voltage drop across Rs is made zero and any ac voltage appearing across the standard resistance Rs is amplified only by the net gain of the amplifier which is small.

➢ Using a recording meter after the dc amplifier, the volt-ampere-time curves for long durations, of the order of several hours (day also) can be obtained. This type of information is essential to determine the relaxation time at low frequencies or with dc.

E-stabilized power supply 500 to 2000v

V-Voltmeter

Rp-Specimen

Rs-Standard Resistance (1MW)

e's-Auxiliary source

Fig .2-DC  amplifier circuit for resistivity measurements

Fig 3 Wheatstone bridge arrangement for resistivity measurements

Fig 3 shows the Wheatstone bridge network in which one of the resistances (usually RA) is

made variable for balancing the bridge. At balance, Rp is given by

𝑅𝑃 = 𝑅𝑠(Ra/Rb)